@prefix vivo: . @prefix edm: . @prefix ns0: . @prefix dcterms: . @prefix skos: . vivo:departmentOrSchool "Science, Faculty of"@en, "Physics and Astronomy, Department of"@en ; edm:dataProvider "DSpace"@en ; ns0:degreeCampus "UBCV"@en ; dcterms:creator "Mills, David J."@en ; dcterms:issued "2010-05-28T11:49:29Z"@en, "1985"@en ; vivo:relatedDegree "Master of Applied Science - MASc"@en ; ns0:degreeGrantor "University of British Columbia"@en ; dcterms:description """The current and possible uses of semiconductor solid state detectors in nuclear physics are briefly discussed. The theory of solid state detectors is discussed with emphasis on the silicon PIN diode detector. A fabrication process for silicon surface barrier position sensitive solid state detectors has been developed at UBC based on the work of J.B.A. England. A fabrication process recipe is included. A prototype surface barrier detector system has been built and tested at UBC and TRIUMF using this process. The device has 1 mm position resolution in one direction, an active area of 40 mm in diameter and a mass thickness of 55 mg/cm². The measured efficiency for 50 MeV pions is 70% and expected rate capability is in excess if 1 MHz per strip. The detector efficiency is limited by a marginal signal-to-noise ratio."""@en ; edm:aggregatedCHO "https://circle.library.ubc.ca/rest/handle/2429/25120?expand=metadata"@en ; skos:note "FABRICATION PROCESS AND CHARACTERISTICS OF A SILICON STRIP DETECTOR by DAVID J . MILLS B.A.Sc, U n i v e r s i t y of B r i t i s h Columbia, 1983 THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Applied MASTER OF vSCIENCE in THE FACULTY OF GRADUATE STUDIES Department of E n g i n e e r i n g P h y s i c s We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1985 © David John M i l l s 1985 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of i ^ ^ g j u ^r\\TilAr^ 1/ (y/ The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 ^ t e Apr]/ - X f DE-6 (3/81) ABSTRACT The c u r r e n t and p o s s i b l e uses of semiconductor s o l i d s t a t e d e t e c t o r s in nuclear p h y s i c s are b r i e f l y d i s c u s s e d . The theory of s o l i d s t a t e d e t e c t o r s i s d i s c u s s e d with emphasis on the s i l i c o n PIN diode d e t e c t o r . A f a b r i c a t i o n process f o r s i l i c o n s u r f a c e b a r r i e r p o s i t i o n s e n s i t i v e s o l i d s t a t e d e t e c t o r s has been developed at UBC based on the work of J.B.A. England. A f a b r i c a t i o n process r e c i p e i s i n c l u d e d . A prototype s u r f a c e b a r r i e r d e t e c t o r system has been b u i l t and t e s t e d at UBC and TRIUMF using t h i s p r o c e s s . The device has 1 mm p o s i t i o n r e s o l u t i o n i n one d i r e c t i o n , an a c t i v e area of 40 mm in diameter and a mass t h i c k n e s s of 55 mg/cm2. The measured e f f i c i e n c y f o r 50 MeV pions i s 70% and expected rate c a p a b i l i t y i s i n excess i f 1 MHz per s t r i p . The de t e c t o r e f f i c i e n c y i s l i m i t e d by a marginal s i g n a l - t o - n o i s e r a t i o . TABLE OF CONTENTS CHAPTER I: INTRODUCTION . . 1 What i s a s o l i d s t a t e d e t e c t o r ? 1 General p r o p e r t i e s of SS d e t e c t o r s 3 M o t i v a t i o n for d e t e c t o r development at TRIUMF 4 CHAPTER I I : PRINCIPLES OF OPERATION 8 Ba s i c band theory of semiconductors and charge gener a t i o n 8 Bulk type SS d e t e c t o r s , r e s i s t i v i t y of semiconductors 11 Diode d e t e c t o r s , e l e c t r i c a l c h a r a c t e r i s t i c s 14 Charge c o l l e c t i o n time 20 Energy r e s o l u t i o n , n o i s e , leakage c u r r e n t ..23 Leakage c u r r e n t , r a d i a t i o n damage 26 D e p l e t i o n depth, energy r e s o l u t i o n 27 Transmission d e t e c t o r s 28 P o s i t i o n r e s o l u t i o n ...29 CHAPTER I I I : FABRICATION OF DEVICES 31 Ion Implanted SS d e t e c t o r s 31 Surface b a r r i e r SS d e t e c t o r s ....34 P r e l i m i n a r y e l e c t r i c a l c h a r a c t e r i s t i c s of s u r f a c e b a r r i e r d e t e c t o r s 39 CHAPTER IV: SOLID STATE DETECTOR SYSTEM TEST 43 P r e a m p l i f i e r e l e c t r o n i c s 43 i v Alpha source t e s t s , d e t e c t o r asymmetry 46 Beta source t e s t s 50 Observed noise of system 53 Beam t e s t design 53 Beam t e s t , p o s i t i o n r e s o l u t i o n 55 Beam t e s t , noise and e f f i c i e n c y 60 Beam t e s t , b i a s v o l t a g e and e f f i c i e n c y 67 Timing of SS d e t e c t o r s i g n a l s 70 CHAPTER V: SUMMARY AND CONCLUSION 7 2 Noise, improvements 72 Future c o n s i d e r a t i o n s 74 BIBLIOGRAPHY 7 5 APPENDICES: Appendix A: M o d i f i e d England process f o r surf a c e b a r r i e r d e t e c t o r s ....76 L I S T OF TABLES T a b l e 2.1 C h a r g e c a r r i e r m o b i l i t i e s o f s e m i c o n d u c t o r s v i L I S T OF FIGURES F i g u r e 1.1 G e n e r i c s o l i d s t a t e d e t e c t o r 2 F i g u r e 1.2 TRIUMF M13 p i o n b e a m l i n e 5 F i g u r e 1.3 P h o t o g r a p h s o f p r o t o t y p e d e t e c t o r 7 F i g u r e 2.1 C h a r g e g e n e r a t i o n i n SS d e t e c t o r 10 F i g u r e 2.2 C h a r g e c a r r i e r c o n c e n t r a t i o n i n s e m i c o n d u c t o r s 13 F i g u r e 2.3 P I N d i o d e .15 F i g u r e 2.4 Band d i a g r a m s o f pn j u n c t i o n 16 F i g u r e 2.5 E l e c t r i c a l c h a r a c t e r i s t i c s o f a r e v e r s e b i a s e d pn j u n c t i o n 18 F i g u r e 3.1 I m p l a n t a t i o n f a b r i c a t i o n t e c h n i q u e 32 F i g u r e 3.2 C r o s s s e c t i o n o f t h e s u r f a c e b a r r i e r d e t e c t o r 35 F i g u r e 3.3 C r o s s s e c t i o n o f t h e d e t e c t o r mount 38 F i g u r e 3.4 L e a k a g e c u r r e n t o f p r o t o t y p e 41 F i g u r e 4.1 P h o t o g r a p h o f d e t e c t o r p r e a m p l i f i e r s ....44 F i g u r e 4.2 C i r c u i t d i a g r a m o f d e t e c t o r p r e a m p l i f i e r s a n d b i a s v o l t a g e 45 F i g u r e 4.3 R e s p o n s e o f d e t e c t o r t o a l p h a p a r t i c l e s 47 F i g u r e 4.4 Asymmetry o f r e s p o n s e o f d e t e c t o r t o a l p h a p a r t i c l e s 48 F i g u r e 4.5 R e s p o n s e o f d e t e c t o r t o b e t a p a r t i c l e s ..51 F i g u r e 4.6 N o i s e i n d e t e c t o r / p r e a m p l i f i e r s 52 F i g u r e 4.7 D a t a a c q u i s i t i o n e l e c t r o n i c s f o r beam t e s t 54 v i i F i g u r e 4.8 Beam t e s t apparatus 56 Fi g u r e 4.9 S c a t t e r p l o t of d e t e c t o r p o s i t i o n versus wire chamber p o s i t i o n 57 Fi g u r e 4.10 S c a t t e r p l o t of d e t e c t o r p o s i t i o n versus wire chamber p o s i t i o n , noise only 58 Fi g u r e 4.11 P o s i t i o n r e s o l u t i o n of d e t e c t o r 59 Fi g u r e 4.12 Beam p r o f i l e as seen with SS d e t e c t o r ...61 F i g u r e 4.13 M u l t i p l i c i t y of d e t e c t o r events 63 F i g u r e 4.14 E f f i c i e n c y and noise of SS det e c t o r with and without noise r e j e c t i o n 64 F i g u r e 4.T5 D i s t r i b u t i o n of muon l a b angle f o r pion decay 65 Fi g u r e 4.16 Histograms of p a r t i c l e angles in the beam t e s t 66 Fi g u r e 4.17a G r a p h i c a l summary of e f f i c i e n c y and noise of the SS d e t e c t o r 68 F i g u r e 4.17b G r a p h i c a l summary of e f f i c i e n c y and noise continued 69 F i g u r e 4.18 Time spectrums of s i g n a l s from SS de t e c t o r 71 ACKNOWLEDGEMENTS I would l i k e to express my thanks to.Dr. L. Young f o r the use of the s o l i d s t a t e l a b o r a t o r y i n the E l e c t r i c a l E n g i n e e r i n g department at UBC. I would a l s o l i k e to thank Dr. P. Janega and Dr. J.B.A. England f o r t h e i r t e c h n i c a l advice i n s o l i d s t a t e f a b r i c a t i o n t e c h n i q u e s . I am indebted to J . C r e s w e l l , D. Maas, G. S h e f f e r and P. A. Amaudruz f o r t e c h n i c a l a d v i c e on e l e c t r o n i c s and nuclear experimental techniques. Many thanks to P. Amaudruz and the PISCAT group f o r time and h e l p i n the beam experiments. Thanks are due to my s u p e r v i s o r Dr. K a r l Erdman and to Dr. R.R. Johnson and Dr. D. G i l l f or encouragement and guidance d u r i n g t h i s p r o j e c t . CHAPTER I INTRODUCTION The use of s o l i d s t a t e d e t e c t o r s in nuclear p h y s i c s i s i n c r e a s i n g . The development of f a b r i c a t i o n technology has both reduced t h e i r cost and improved t h e i r q u a l i t y . What i s a s o l i d s t a t e d e t e c t o r ? The SS ( s o l i d s t a t e ) d e t e c t o r i s a s l a b of semiconducting c r y s t a l with an i n t e r n a l e l e c t r i c f i e l d generated by an e x t e r n a l v o l t a g e a p p l i e d across i t . A charged p a r t i c l e or X-ray i s det e c t e d when i t s passage through the c r y s t a l generates charge c a r r i e r s which are swept out of the c r y s t a l by the i n t e r n a l e l e c t r i c f i e l d and are dete c t e d e l e c t r o n i c a l l y as a c u r r e n t pulse (Figure 1.1). SS d e t e c t o r s can be c l a s s i f i e d as e i t h e r bulk or diode types. In bulk d e t e c t o r s the i n t r i n s i c c o n d u c t i v i t y of the semiconductor i s very low due to hig h c r y s t a l p u r i t y or charge compensation techniques. One only needs to a t t a c h ohmic c o n t a c t s to the normally i n s u l a t i n g c r y s t a l to produce a d e t e c t o r . The diode d e t e c t o r has l e s s s t r i n g e n t r e s t r i c t i o n s on the bulk p r o p e r t i e s of the semiconducting m a t e r i a l and uses the d e p l e t i o n zone c r e a t e d i n a re v e r s e b i a s e d pn j u n c t i o n as i t s a c t i v e r e g i o n . T h i s a c t i v e region can extend p a r t i a l l y or 2 FIGURE 1. 1 BIAS VOLTAGE S c h e m a t i c of a s e m i c o n d u c t o r c h a r g e d p a r t i c l e d e t e c t o r . 3 c o m p l e t e l y t h r o u g h t h e c r y s t a l s l a b . G e n e r a l p r o p e r t i e s o f SS d e t e c t o r s S o l i d s t a t e d e t e c t o r s c a n be d e s i g n e d f o r t i m i n g o f -p a r t i c l e s , e n e r g y r e s o l u t i o n , p o s t i t i o n r e s o l u t i o n o r some c o m b i n a t i o n o f t h e t h r e e . The i n t r i n s i c p r o p e r t i e s o f t h e s e d e t e c t o r s make them v e r s a t i l e d e v i c e s w i t h some u n i q u e p r o p e r t i e s . SS d e t e c t o r s d e s i g n e d f o r s m a l l c a p a c i t a n c e a n d h i g h i n t e r n a l e l e c t r i c f i e l d s h ave p u l s e r i s e t i m e s i n t h e o r d e r o f n a n o s e c o n d s w i t h p u l s e w i d t h s o f t e n s o f n a n o s e c o n d s . A c c u r a t e t i m i n g o f e v e n t s a nd h i g h r a t e a p p l i c a t i o n s a r e t h u s p o s s i b l e . P o s i t i o n r e s o l u t i o n i n SS d e t e c t o r s i s a c h i e v e d by p a t t e r n i n g t h e c h a r g e c o l l e c t i n g e l e c t r o d e s . W i t h s t a n d a r d s e m i c o n d u c t o r p h o t o l i t h o g r a p h i c t e c h n i q u e s , S i d e t e c t o r s have been b u i l t w i t h 5 um r e s o l u t i o n f o r minimum i o n i z i n g p a r t i c l e s (REF 6 ) . SS d e t e c t o r s c a n be u s e d t o d e t e r m i n e t h e e n e r g y o f s t o p p e d p a r t i c l e s o r t h e e n e r g y l o s s o f t r a n s m i t t e d p a r t i c l e s . The c h a r g e p u l s e p r o d u c e d by t h e p a r t i c l e i s d i r e c t l y p r o p o r t i o n a l t o t h e e n e r g y l o s t i n t h e a c t i v e r e g i o n o f t h e d e t e c t o r , a n d may be m e a s u r e d down t o a few p e r c e n t a c c u r a c y (REF 4 ) . H i s t o r i c a l l y , t h i s h a s been t h e most common u s e f o r SS d e t e c t o r s . A s i n g l e d e t e c t o r o r s t a c k o f t o t a l l y d e p l e t e d d e t e c t o r s i s commonly u s e d t o s t o p l o w e n e r g y p a r t i c l e s o r X - r a y s , a n d m e a s u r e t h e i r e n e r g y . The energy r e s o l u t i o n or minimum energy d e t e c t a b l e i n a s o l i d s t a t e d e t e c t o r i s l i m i t e d p r i m a r i l y by the random f l u c t u a t i o n of c u r r e n t flowing i n the absence of p a r t i c l e s . T h i s leakage c u r r e n t depends on the i n t e g r i t y of the o r i g i n a l semiconductor m a t e r i a l and on the f a b r i c a t i o n techniques used. R a d i a t i o n damage can a l s o c o n t r i b u t e to leakage c u r r e n t , and the l i f e of an SS d e t e c t o r i s l i m i t e d by the i n c r e a s e of the leakage c u r r e n t to some c r i t i c a l value. The most s i g n i f i c a n t development i n these d e t e c t o r s has been the r e d u c t i o n of leakage c u r r e n t to l e v e l s as low as 40 nA/cm2 (REF 6). These d e t e c t o r s are t y p i c a l l y l e s s than 5 cm i n diameter and g r e a t e r than 100 m t h i c k (=25 mg/cm 2). T h i s r e l a t i v e l y small area and l a r g e mass t h i c k n e s s i s l e s s of a drawback when one c o n s i d e r s that other p o s i t i o n s e n s i t i v e or timing d e t e c t o r s r e q u i r e bulky l i g h t p ipes and p h o t o m u l t i p l i e r s or gas containment systems. At present, p o s i t i o n s e n s i t i v e s o l i d s t a t e d e t e c t o r s need l a r g e a r r a y s of p r e a m p l i f i e r s , but in fu t u r e even t h i s may be avoided with a m p l i f i e r s or CCD readouts being b u i l t on the same semiconductor wafer as the d e t e c t o r . At present SS d e t e c t o r s are used e x t e n s i v e l y i n low energy p h y s i c s due to t h e i r good behavior in high vacuum. M o t i v a t i o n f o r d e t e c t o r development at TRIUMF The high r a t e c a p a b i l i t y of p o s i t i o n s e n s i t i v e SS d e t e c t o r s was the primary m o t i v a t i o n f o r b u i l d i n g them at TRIUMF. The TRIUMF M13 pion beamline can have p i + f l u x e s i n excess of 2X 1 0 6 / S . A high r a t e d e t e c t o r can be used i n t h i s 5 FIGURE 1.2 M13 p i o n beam l i n e a t TRIUMF. B1 and B2 a r e d i p o l e b e n d i n g m a g n e t s ; F l a n d F2 a r e t h e f o c u s l o c a t i o n s o f t h e momentum s e l e c t i o n s l i t s . channel as a beam p r o f i l e monitor f o r d i a g n o s t i c s or as a t o o l in s c a t t e r i n g experiments. C u r r e n t l y the momentum r e s o l u t i o n of the pion beam i s s e l e c t e d by a p a i r of s l i t s l o c a t e d at focuses downstream of a bending magnet (F1, F2 in F i g u r e 1.2). A SS d e t e c t o r l o c a t e d at one of these focuses can be used to tag the p o s i t i o n and t h e r e f o r e the momentum of each p a r t i c l e as i t t r a v e r s e s the channel. The s l i t s can then be opened to inc r e a s e the pion f l u x without degrading the momentum r e s o l u t i o n . In a s i m i l a r manner, a s o l i d s t a t e d e t e c t o r at the s c a t t e r i n g t a r g e t can be used as a p o s i t i o n monitor to improve the energy r e s o l u t i o n of the spectrometer. A p o s i t i o n s e n s i t i v e , s i l i c o n , s o l i d s t a t e d e t e c t o r has been b u i l t and t e s t e d by the PISCAT (pion s c a t t e r i n g ) experimental group at TRIUMF. The de v i c e was made using s i l i c o n s u r f a c e b a r r i e r techniques. I t has a r e s o l u t i o n of one m i l l i m e t e r i n one d i r e c t i o n and an a c t i v e area of 4 cm in diameter and i s 230 jum t h i c k (55 mg/cm2 ), ( F i g u r e 1.3). S u r f a c e b a r r i e r , p o s i t i o n s e n s i t i v e , s o l i d s t a t e d e t e c t o r manufactured at UBC. The p r e - a m p l i f i e r readout e l e c t r o n i c s are v i s i b l e . CHAPTER II PRINCIPLES OF OPERATION Basic band theory of semiconductors and charge generation A semiconductor i s a c r y s t a l with a gap in the a v a i l a b l e energy s t a t e s for e l e c t r o n s . The s t a t e s above the gap are normally empty and are c o l l e c t i v e l y known as the conduction band. S i m i l a r l y , the valence band i s the c o l l e c t i o n of normally f u l l s t a t e s below the band gap. In any c r y s t a l at zero temperature e l e c t r o n s are l y i n g in the lowest energy s t a t e s p o s s i b l e without v i o l a t i n g the P a u l i e x c l u s i o n p r i n c i p l e . At zero temperature the Fermi energy i s d e f i n e d as the energy above which no e l e c t r o n s are seen. At non-zero temperatures, the Fermi energy, E p ' d e f i n e s the thermal d i s t r i b u t i o n of e l e c t r o n s or holes (REF 8 ) : .-(E c-E F/kT) n = N e c - ( E p - E v / K T ) p = N e ^ v Equations 2.1 where n and p are the r e s p e c t i v e d e n s i t i e s of e l e c t r o n s and h o l e s , E c i s the e l e c t r o n energy in the conduction band, E v i s the h o l e energy in the valence band, N c and N v are d e n s i t i e s of s t a t e s f o r the conduction and valence bands r e s p e c t i v e l y and kT i s the temperature in energy u n i t s . Holes are e l e c t r o n v a cancies in the almost f u l l valence band and behave l i k e p o s i t i v e charged p a r t i c l e s with a mass c l o s e to the mass of the e l e c t r o n ( e f f e c t i v e mass). Both e l e c t r o n s and holes have an e f f e c t i v e mass in the c r y s t a l which depends on the s t a t e occupied by the p a r t i c l e and on i t s d i r e c t i o n of motion in the c r y s t a l l a t t i c e . E f f e c t i v e masses of charge c a r r i e r s i n semiconductors u s u a l l y are between 10% and 100% of the e l e c t r o n mass and holes can be \" h e a v i e r \" or \" l i g h t e r \" than e l e c t r o n s . I n t r i n s i c semiconductors are c h a r a c t e r i z e d by the Fermi energy l y i n g c l o s e to the c e n t r e of the band gap. I n s u l a t o r s are the same but with a l a r g e r band gap. In a s o l i d s t a t e d e t e c t o r the a c t i v e volume of semiconducting c r y s t a l normally a c t s l i k e an i n s u l a t o r . The non-conducting s t a t e has an empty conduction band (no e l e c t r o n charge c a r r i e r s ) and a f u l l valence band (no hole charge c a r r i e r s ) . When a charged p a r t i c l e passes through the c r y s t a l i t s c a t t e r s e l e c t r o n s i n t o the a v a i l a b l e energy s t a t e s in the conduction band, l e a v i n g behind empty s i t e s (holes) i n the valence band. T h i s e x c i t e d c o n f i g u r a t i o n q u i c k l y decays by phonon e x c i t a t i o n , r a d i a t i v e decay or s c a t t e r i n g from valence e l e c t r o n s (generating more e l e c t r o n - h o l e p a i r s ) u n t i l a -1 2 q u a s i - s t a b l e s t a t e i s reached (10 s ) . T h i s longer l i v e d s t a t e i s c h a r a c t e r i z e d by an equal number of e l e c t r o n s and h oles in the conduction and valence bands. These charge c a r r i e r s are f r e e to d r i f t i n the i n t e r n a l e l e c t r i c f i e l d of the d e t e c t o r and form the d e t e c t e d c u r r e n t pulse ( F i g u r e 2.1). 10 FIGURE 2 . 1 I L_. 1 _ . I\" CONDUCTION I BAND | BAND GAP I L_. L. I !<£> ( Q I , i>-I r -I 1°\" A VALENCE BAND EQUILIBRIUM AT TIME OF IONIZATION 1 ps AFTER IONIZATION > -C D cr. UJ LU Schematic of charge gener a t i o n process i n the a c t i v e r e g i o n of a s o l i d s t a t e d e t e c t o r . The s i z e of t h i s c u r r e n t p u l s e , Q, i s p r o p o r t i o n a l to the energy l o s s , AE, of the de t e c t e d p a r t i c l e i n the a c t i v e r egion and i s given by Q = qKAE/E • gap Equation 2.2 where Q i s the charge generated i n the d e t e c t o r ( e i t h e r h oles or e l e c t r o n s ) , E i s the band gap of the semiconductor (1.12 gap 3 r eV f o r S i ) and q i s the e l e c t r o n i c charge. K i s a f a c t o r l e s s than one that accounts f o r energy l o s s e s other than i o n i z a t i o n , f o r example phonon e x c i t a t i o n . K i s about 1/3 i n Si as 3.2 eV i s the average energy r e q u i r e d to produce an e l e c t r o n hole p a i r (REF 7). Gas i o n i z a t i o n chambers r e q u i r e about 30 eV per i o n i z a t i o n and o f t e n use avalanche charge m u l t i p l i c a t i o n near c o l l e c t i o n w i r e s . Avalanche charge m u l t i p l i c a t i o n a l s o occurs i n semiconductors with a high i n t e r n a l e l e c t r i c f i e l d . T h i s e f f e c t i s not e a s i l y c o n t r o l l e d , however, and o f t e n causes t o t a l breakdown, so i t i s not used in semiconductor charged p a r t i c l e d e t e c t o r s . Bulk type SS d e t e c t o r s , r e s i s t i v i t y of semiconductors Bulk SS d e t e c t o r s are made from high r e s i s t i v i t y semiconductor. An e x t e r n a l v o l t a g e , V, a p p l i e d to c o n t a c t e l e c t r o d e s generates an i n t e r n a l e l e c t r i c f i e l d i n the d e t e c t o r s l a b . For a s l a b of t h i c k n e s s d, the e l e c t r i c f i e l d s t r e n g t h i s un i f o r m l y V/d. One l i m i t a t i o n of bulk d e t e c t o r s i s the f i n i t e r e s i s t i v i t y of the c r y s t a l due to th e r m a l l y e x c i t e d charge c a r r i e r s . The leakage c u r r e n t of bulk d e t e c t o r s can be c a l c u l a t e d from semiconductor p r o p e r t i e s . In thermal e q u i l i b r i u m the c a r r i e r c o n c e n t r a t i o n i s given by Equations 2.1 or np = n^ 2 - e -E AT gap Equation 2.3 where n and p are the d e n s i t y of e l e c t r o n s and holes r e s p e c t i v e l y , n. i s the i n t r i n s i c c a r r i e r c o n c e n t r a t i o n , E ^ J i ' gap i s the semiconductor band gap, kT i s the ab s o l u t e temperature i n energy u n i t s . A p r o p o r t i o n a l i t y constant dependent on d e n s i t y of s t a t e s i s omitted. The i n t r i n s i c c a r r i e r c o n c e n t r a t i o n i s shown i n (Figure 2.2) f o r Ge, S i , GaAs. The e l e c t r o n leakage c u r r e n t per u n i t area, J n , in an i d e a l bulk d e t e c t o r (n=p=n^) i s (REF 8): J n = qn^M Equation 2.4 where q i s the e l e c t r i c charge, n^=n i s the e l e c t r o n c a r r i e r c o n c e n t r a t i o n , E i s the i n t e r n a l e l e c t r i c f i e l d and u i s the e l e c t r o n m o b i l i t y . For example, at room temperature with a 1kV/cm e l e c t r i c f i e l d , the best leakage c u r r e n t s f o r Ge, S i , GaAs bulk d e t e c t o r s are 14 A/cm 2, 3 mA/cm2 and 2 uh/cm2 r e s p e c t i v e l y . Of these three only GaAs has an acceptable behavior at room temperature. I n t r i n s i c charge c a r r i e r c o n c e n t r a t i o n of Ge, S i , and GaAs as a f u n c t i o n of i n v e r s e temperature ( a f t e r Sze Ref. 8 ) . In a non-ideal bulk d e t e c t o r , leakage c u r r e n t can be produced by the i n j e c t i o n of charges i n t o the c r y s t a l by the conta c t e l e c t r o d e s i f s p e c i a l care i s not taken to make the c o n t a c t s ohmic. Unwanted donor or acceptor i m p u r i t i e s can c o n t r i b u t e charge c a r r i e r s as can mid-band genera t i o n and recombination s i t e s caused by i m p u r i t i e s or c r y s t a l f a u l t s . F u r t h e r d i s c u s s i o n of leakage c u r r e n t i s in c l u d e d in the s e c t i o n on diode d e t e c t o r s . Recent improvements in the q u a l i t y of commercially a v a i l a b l e semiconductors are making bulk d e t e c t o r s more f e a s a b l e . Bulk d e t e c t o r s made from l a r g e band gap semiconductors such as GaAs or GdS may be common i n the f u t u r e . Diode d e t e c t o r s ' e l e c t r i c a l c h a r a c t e r i s t i c s In diode type SS d e t e c t o r s , i t i s the d e p l e t i o n zone of a reverse b i a s e d pn j u n c t i o n that p r o v i d e s the s e m i - i n s u l a t i n g a c t i v e volume f o r charged p a r t i c l e d e t e c t i o n . The e l e c t r i c a l p r o p e r t i e s of a l l j u n c t i o n SS d e t e c t o r s are approximated by the PIN diode (Figure 2.3). The p + n - n + diode w i l l be co n s i d e r e d here although the p +p~n* i s s i m i l a r . Let us co n s i d e r the i n t e r f a c e of the h i g h l y doped p + r e g i o n and the l i g h t l y doped n~ r e g i o n . T h i s pn j u n c t i o n can be made by d i f f u s i n g or implanting a c l a s s III dopant element such as boron i n t o an n\" s u b s t r a t e or by the c o n s t r u c t i o n of a su r f a c e b a r r i e r . In a p type doped semiconductor, there are fewer e l e c t r o n s per l a t t i c e s i t e than i n i n t r i n s i c m a t e r i a l FIGURE 2.3 Xo X Schematic of PIN diode and d e f i n i t i o n of c o o r d i n a t e x. 16 F I G U R E 2.4a I O N I Z E D A C C _ E P T O R _ S _ I T E S 6 § 5 e 9 g g ) e e e^ 5>5> ® Q ~ + f + + t + + + M O B I L E H O L E S qVo D E P L E T I O N ! Z O N E J M O B I L E E L E C T R O N S @ © S> ® I O N I Z E D © € D O N O R S I T E S Ef F I G U R E 2.4b Band diagrams for pn junction with (a) no external a p p l i e d voltage, (b) an a p p l i e d voltage of V a. E c and Ev are the energies of the bottom of the conduction band and the top of the valence band r e s p e c t i v e l y ; Ef i s the Fermi energy and q i s the e l e c t r o n i c charge. and the Fermi l e v e l i s consequently s h i f t e d downward i n energy toward the bottom of the band gap. S i m i l a r l y the Fermi l e v e l i s s h i f t e d upward toward the top of the band gap i n an n type semiconductor. In the absence of any e x t e r n a l f i e l d , there i s a r e d i s t r i b u t i o n of charges such that the Fermi l e v e l i s constant throughout the c r y s t a l . T h i s leaves a region d e p l e t e d of f r e e charge c a r r i e r s near the j u n c t i o n . The energy l e v e l s of the valence and conduction bands are a f u n c t i o n of the l o c a l charge d i s t r i b u t i o n and thus change acr o s s the pn j u n c t i o n ( F i g u r e 2.4). In e q u i l i b r i u m the d e p l e t i o n zone b a r r i e r i s p r e c i s e l y l a r g e enough to prevent d i f f u s i o n of holes i n t o the n region or e l e c t r o n s i n t o the p region i n the l a r g e c o n c e n t r a t i o n g r a d i e n t s . When an e x t e r n a l v o l t a g e V i s a p p l i e d a c r o s s the j u n c t i o n with the same sign as V 0 ( r e v e r s e b i a s ) , f r e e charges move away from the j u n c t i o n and leave a l a r g e r d e p l e t i o n zone; the b a r r i e r height i n c r e a s e s p r o p o r t i o n a t e l y (Figure 2.4). In a t o t a l l y d e p l e t e d PIN diode, the d e p l e t i o n zone extends acr o s s the c r y s t a l to the n + r e g i o n . In the approximation that the n\" or i n t r i n s i c region of the diode has a much lower dopant d e n s i t y than e i t h e r the p + or n + regions and assuming that the d e p l e t i o n zone i s t o t a l l y d e p l e t e d of charge c a r r i e r s , one can c a l c u l a t e the d e p l e t i o n width and i n t e r n a l e l e c t r i c f i e l d shape as a f u n c t i o n of a p p l i e d v o l t a g e V (REF 7, Fig u r e 2.5). FIGURE 2.5 CHARGE DENSITY ELECTRIC FIELD STRENGTH ELECTRIC POTENTIAL X X FULLY DEPLETED PARTIALLY DEPLETED Q u a l i t a t i v e charge d e n s i t y , e l e c t r i c f i e l d and p o t e n t i a l as a f u n c t i o n of p o s i t i o n i n a reverse b i a s e d PIN diode. Charge d e n s i t y i s p r o p o r t i o n a l to the c o n c e n t r a t i o n of uncompensated i o n i z e d dopant i o n s . For an SS d e t e c t o r that i s not f u l l y d e p l e t e d , the d e p l e t i o n width, W, i s given by (REF 7) W2 = 2 ( V 0 + V a ) e s / q p n Equation 2.5 where V 0 i s the b u i l t in d e p l e t i o n p o t e n t i a l with no a p p l i e d v o l t a g e , V i s the a p p l i e d v o l t a g e , e i s the d i e l e c t r i c a s constant of the . semiconductor, p n i s the d e n s i t y of dopant ions i n the n~ region and q i s the e l e c t r o n i c charge. The e l e c t r i c f i e l d i s c a l c u l a t e d by i n t e g r a t i n g the charge d e n s i t y i n the d e p l e t i o n zone and i s thus l i n e a r (REF 8): E(x) = (W-x)p n/e s 0 < x < W = 0 elsewhere Equations 2.5 where x i s d e f i n e d in F i g u r e 2.3. For a t o t a l l y d e p l e t e d diode the d e p l e t i o n width i s f i x e d at the width, x 0 , of the i n t r i n s i c r e g i o n . The e l e c t r i c f i e l d i s s t i l l l i n e a r with the same slope as i n a p a r t i a l l y d e p l e t e d diode, but now has maximum and minimum values (E , E . ) max min dependent on the a p p l i e d v o l t a g e (Figure 2.5). AE = E max -E min = q p f l x 0 / * s E(x) = (x 0-x)AE/x 0+E 0 = 2qBI n ^ s N Equation 2.12 where B i s the frequency bandwidth of i n t e r e s t . Thermal n o i s e i s prop'ortional to temperature and to the conductance, G, of the device (REF 8): = 4kTBG n Equation 2.13 Both these noise mechanisms have a uniform or white frequency spectrum and are t h e r e f o r e l i m i t e d by the bandwidth of i n t e r e s t . A t h i r d source of noise, f l i c k e r n o i s e , i s not important i n the high frequency band of i n t e r e s t f o r SS d e t e c t o r s due to i t s 1/f frequency spectrum. For a given leakage c u r r e n t I , the noise expected i s (REF 8) = 2qBI ( i + e ^ V / K T ) n ^ s Equat ion 2.14 where q i s the e l e c t r o n i c charge, B i s the bandwidth of i n t e r e s t , V i s the b i a s v o l t a g e and kT i s the temperature energy. The second (thermal) term has been s i m p l i f i e d using the Shockly diode equation to c a l c u l a t e conductance. E m p i r i c a l l y , one can d e t e c t a c u r r e n t pulse approximately one t e n t h of the magnitude of the leakage c u r r e n t . The leakage c u r r e n t i s p a r t i a l l y d e s c r i b e d by the t h e o r e t i c a l d i f f u s i o n c u r r e n t ( f i r s t term, Equation 2.15) and g e n e r a t i o n c u r r e n t (second term, equation 2.15) i n a pn step j u n c t i o n diode: JR = V A = q / V r p ' ( n i V N D ) + q n i W / T e Equat ion 2.15 where D i s the d i f f u s i o n constant f o r h o l e s , r and r are P P e the r e s p e c t i v e hole and e l e c t r o n l i f e t i m e s , n^ i s the i n t r i n s i c c a r r i e r c o n c e n t r a t i o n , N Q i s the donor c o n c e n t r a t i o n in the n r e g i o n , and W i s the d e p l e t i o n width. R e c a l l that the i n t r i n s i c c a r r i e r c o n c e n t r a t i o n i n c r e a s e s with temperature (Equation 2.3) while l i f e t i m e decreases; the leakage c u r r e n t thus has a strong temperature dependence. The l i f e t i m e for c a r r i e r s i s a l s o reduced by i m p u r i t i e s (generation and recombination s i t e s ) i n the c r y s t a l and by l a t t i c e f a u l t s . F u r t h e r c o n t r i b u t i o n s to leakage c u r r e n t come from i n j e c t i o n of c a r r i e r s from the back contact in a t o t a l l y depleted d e t e c t o r as w e l l as r e s i s t i v e leakage at the edges of the d e v i c e . In surface b a r r i e r d e t e c t o r s , v o l t a g e dependent Schottky emission c o n t r i b u t e s to the leakage c u r r e n t and su r f a c e s t a t e s o f t e n dominate generation c u r r e n t in the d e p l e t i o n zone. Leakage c u r r e n t i n room temperature SS d e t e c t o r s i s the primary e n g i n e e r i n g problem due to the l a r g e s u r f a c e area and d e p l e t i o n depth r e q u i r e d f o r u s e f u l d e v i c e s . Leakage c u r r e n t , r a d i a t i o n damage R a d i a t i o n c o n t r i b u t e s to leakage c u r r e n t and noise by causing l a t t i c e f a u l t s and by isotope p r o d u c t i o n . In the absence of slow neutrons, i t i s the former mechanism which dominates r a d i a t i o n damage. L a t t i c e damage causes charge t r a p s which slow d e t e c t i o n response times, reduce charge e f f i c i e n c y , and i n c r e a s e leakage c u r r e n t . Heavy ions d e p o s i t more energy in a d e t e c t o r and do p r o p o r t i o n a t e l y more damage. C r y s t a l l a t t i c e damage can sometimes be annealed out by low temperature ( l e s s than 400°C) heat treatment i n implanted or d i f f u s e d d e v i c e s . Ion implanted d e t e c t o r s have been shown to have a l i f e t i m e of 1 0 1 1 - 1 0 1 4 p a r t i c l e s / c m 2 i n a muon f l u x (REF 9). Surface b a r r i e r d e t e c t o r s tend to have s h o r t e r l i f e e x p e c t a n c i e s . In a slow neutron f l u x 3 0 S i (3% abundant) can absorb a neutron and s u b s e q u e n t a l l y /3 decay to phosphorus. Phosphorus i s a donor dopant and w i l l make the c r y s t a l more n type. In a d d i t i o n , decay of r a d i o - i s o t o p e s i n the d e t e c t o r a c t i v e r e g i o n w i l l add noise s i g n a l s . D e p l e t i o n depth, energy r e s o l u t i o n To measure the energy of stopped p a r t i c l e s i n an SS d e t e c t o r , i t i s necessary that the p a r t i c l e stop i n the a c t i v e d e p l e t i o n zone. A stack of t o t a l l y d e p l e t e d SS d e t e c t o r s can have more than 99% of t h e i r volume d e p l e t e d and are i d e a l f o r measuring the energy of medium to low energy p a r t i c l e s or X-rays. To measure low energy p a r t i c l e s , a s i n g l e s o l i d s t a t e d e t e c t o r with an a p p r o p r i a t e l y l a r g e d e p l e t i o n depth i s used. The low energy l i m i t of r e s o l u t i o n i s reached when the range of the p a r t i c l e of i n t e r e s t approaches the t h i c k n e s s of the dead zone at the s u r f a c e of the d e t e c t o r . T h i s dead zone i s c h a r a c t e r i z e d by the t h i c k n e s s of the metal c o n t a c t s on the s u r f a c e and the d i s t a n c e to the edge of the d e p l e t i o n zone. The dead zone can be reduced to hundreds of Angstroms in s u r f a c e b a r r i e r d e t e c t o r s . The ab s o l u t e t h i c k n e s s of the dead zone can be estimated by measuring the energy of a de t e c t e d p a r t i c l e as a f u n c t i o n of the angle of i n c i d e n c e f o r a c o l l i m a t e d monoenergetic source. Transmission d e t e c t o r s SS d e t e c t o r s are o f t e n used to detect p a r t i c l e s by t h e i r energy l o s s in t r a n s m i s s i o n . Assuming that the energy l o s s i s small compared to the t o t a l energy of the p a r t i c l e s , the d e t e c t e d pulse i s p r o p o r t i o n a l to the d e p l e t i o n depth. As i t i s normally d e s i r e a b l e to reduce s c a t t e r i n g , f u l l y d e p l e t e d t h i n d e t e c t o r s are used. When measuring the energy l o s s of a t r a n s m i t t e d p a r t i c l e , one i s a l s o measuring the s u b s t a n t i a l energy s t r a g g l e (^20% of energy l o s s ) . Even with such a l a r g e b u i l t - i n u n c e r t a i n t y , energy r e s o l u t i o n i s used to d i s t i n g u i s h p a r t i c l e s with d i f f e r e n t r a t e s of energy l o s s i n matter. The primary problem in a t r a n s m i s s i o n d e t e c t o r i s to d e t e c t the small s i g n a l s a s s o c i a t e d with t h i n d e t e c t o r s and p a r t i c l e s with low energy l o s s . The s i g n a l - t o - n o i s e r a t i o of t o t a l l y d e p l e t e d d e t e c t o r s can o f t e n be improved by i n c r e a s i n g the b i a s v o l t a g e across the d e v i c e . According to Equations 2.7 and 2.10, t h i s speeds up the charge c o l l e c t i o n , i n c r e a s i n g the height of the c u r r e n t p u l s e s . However, in a t o t a l l y d e p l e t e d d e t e c t o r with a l a r g e a p p l i e d b i a s v o l t a g e , i n j e c t i o n c u r r e n t at the back contact can become a problem as can breakdown e f f e c t s . The breakdown v o l t a g e s of implanted or d i f f u s e d PIN diodes are l a r g e r than s u r f a c e b a r r i e r devices and a l l types behave b e t t e r with very high r e s i s t i v i t y i n t r i n s i c r e g i o n s . Breakdown occurs i f the i n t e r n a l e l e c t r i c f i e l d exceeds a c r i t i c a l value (somewhat higher than 10 5 V/cm) or i f the d e p l e t i o n zone moves through the back contact (or f r o n t c o n t a c t in s u r f a c e b a r r i e r d e v i c e s ) . These processes are c a l l e d avalanche breakdown and punchthrough r e s p e c t i v e l y . Minimum i o n i z i n g p a r t i c l e s can be de t e c t e d i n room temperature SS d e t e c t o r s as t h i n as 100 microns i f care i s taken to reduce no i s e . T h i s corresponds to a s i g n a l of around 10\" e l e c t r o n s . It should be emphasized that the s i g n a l to noise behavior of any SS d e t e c t o r i s i n t i m a t e l y r e l a t e d to the readout e l e c t r o n i c s . It i s the behavior of the d e t e c t o r / e l e c t r o n i c s system .that should be optimized f o r any given a p p l i c a t i o n . P o s i t i o n r e s o l u t i o n P o s i t i o n s e n s i t i v e d e t e c t o r s are made by p a t t e r n i n g the charge c o l l e c t i n g e l e c t r o d e s on the surface of the d e t e c t o r . P h o t o l i t h o g r a p h i c techniques developed for the e l e c t r o n i c s i n d u s t r y can reproduce f e a t u r e s with one micron dimensions. Less e x a c t i n g techniques have been used by Kemmer et a l to produce SS s t r i p d e t e c t o r s with 5 micron r e s o l u t i o n i n one d i r e c t i o n ( R E F - 6 ) . T h i s high r e s o l u t i o n was accomplished using 20 Mm s t r i p spacing, an a m p l i f i e r on every t h i r d s t r i p and charge i n t e r p o l a t i o n between readout a m p l i f i e r s . The t h e o r e t i c a l l i m i t on p o s i t i o n r e s o l u t i o n in SS d e t e c t o r s may be the width of i o n i z a t i o n t r a c k s i n SS d e t e c t o r s or the d i f f u s i o n l e n g t h , L, of c a r r i e r s before they are c o l l e c t e d . (REF 8) L = ( D t ) l / 2 Equation 2.16a where D i s the d i f f u s i o n constant f o r charge c a r r i e r s and t i s the time of i n t e r e s t . The d i f f u s i o n constant i s r e l a t e d to c a r r i e r m o b i l i t y u and temperature (REF 8): D = MkT/q Equation 2.16b E l e c t r o n s i n s i l i c o n w i l l d i f f u s e an average of 6 /um i n 10 ns at room temperature. D e t e c t o r s made with CCD readouts or i n t e g r a t e d p r e a m p l i f i e r s c o u l d make l a r g e area high r e s o l u t i o n two dimensional d e t e c t o r s more fe a s a b l e by reducing the d e n s i t y of e l e c t r o n i c s r e q u i r e d f o r a high r e s o l u t i o n small area d e t e c t o r . In a l l types of d e t e c t o r s i t i s important to p r o t e c t ( p a s s i v a t e ) the sur f a c e of the semiconductor from i m p u r i t i e s in the environment (REF 8). In a p o s i t i o n s e n s i t i v e SS d e t e c t o r the su r f a c e should be p a s s i v a t e d between e l e c t r o d e s t r i p s . Two examples of s u r f a c e p a s s i v a t i o n techniques are used i n the two processes d e s c r i b e d i n Chapter I I I . CHAPTER III FABRICATION OF DEVICES With a recognized need for SS d e t e c t o r s at TRIUMF, i t was decided that p o s i t i o n s e n s i t i v e s i l i c o n d e v i c e s should be f a b r i c a t e d at UBC. P o s i t i o n s e n s i t i v e d e v i c e s are a v a i l a b l e commercially but they are s t i l l very expensive. The f a b r i c a t i o n work was done p r i m a r i l y in the E l e c t r i c a l E n g i n e e r i n g S o l i d State L a b o r a t o r y . The main t h r u s t of the work was to reproduce the ion i m p l a n t a t i o n techniques used by Kemmer et a l (REF 6) and the s u r f a c e b a r r i e r techniques used by England (REF 3,5,12). England's s u r f a c e b a r r i e r technique proved to be simpler and more s u c c e s s f u l , and w i l l t h e r e f o r e be emphasized in t h i s d i s c u s s i o n . Ion implanted SS d e t e c t o r s The implanted d e t e c t o r developed by Kemmer (REF 6) i n 1980 i s a standard PIN diode as d e s c r i b e d i n Chapter I I , but made in a p a t t e r n of s t r i p s so as to be p o s i t i o n s e n s i t i v e . A f a b r i c a t i o n flow c h a r t i s shown in F i g u r e 3.1. T h i s type of d e t e c t o r i s a t t r a c t i v e because of i t s very low leakage c u r r e n t (Kemmer c l a i m s 40 nA/cm 2), even when bia s e d w e l l above the t o t a l d e p l e t i o n v o l t a g e . A l s o d e s i r e a b l e i s the q u a l i t y of the s u r f a c e p a s s i v a t i o n that makes the d e t e c t o r almost immune to environmental contamination a f t e r f a b r i c a t i o n . I t was not p o s s i b l e to make a low leakage SS d e t e c t o r u s i n g t h i s FIGURE 3.1 Clean 5000K n type wafer Grow 2000A oxide (lOhrs 1040C dry oxygen) Pa t t e r n oxide with p h o t o l i t h o g r a p h y Implant Boron (15ReV 5x1u'%m 3 Arsenic (30KeV 5x1 O'Vcm'3) Anneal implant damage (30min 600C i n n i t r o g e n ) Deposit aluminum by evaporation (1000A) Pat t e r n aluminum by ph o t o l i t h o g r a p h y Kemmer's implanted d e t e c t o r f a b r i c a t i o n process. Note that the s i l i c o n s u r f a c e i s p r o t e c t e d from the environment by e i t h e r aluminum or s i l i c o n - o x i d e ( R e f . 6 ) . technique i n the development time a v a i l a b l e . The best d e t e c t o r b u i l t using a modified process had a leakage of 10 juA/cm2 and was not uniform from s t r i p to s t r i p . The most common problem in any s o l i d s t a t e f a b r i c a t i o n i s c l e a n l i n e s s . The l a r g e number of steps in t h i s process make contamination of the wafer more l i k e l y i f a h i g h l y c o n t r o l l e d c l e a n environment i s not a v a i l a b l e f o r the f a b r i c a t i o n p r o c e s s . The E.E. l a b o r a t o r y at UBC lacked t h i s q u a l i t y of c l e a n l i n e s s in both dry and wet p r o c e s s i n g steps. In an environment l i k e the E.E. l a b o r a t o r y that i s not completely c o n t r o l l e d . Wet steps l i k e p h o t o l i t h o g r a p h y can introduce i m p u r i t i e s to the s u r f a c e of the d e t e c t o r wafer and high temperature steps l i k e .oxidation and a n n e a l i n g can d r i v e i m p u r i t i e s i n t o the bulk regions of the d e t e c t o r . The i n t e g r i t y of the c r y s t a l l a t t i c e i s a l s o dependent on i t s temperature h i s t o r y . High temperature treatment can lower the r e s i s t i v i t y of the semiconductor even in a clean envi ronment. Since ion i m p l a n t a t i o n doping a l s o damages the s u r f a c e of the c r y s t a l t h i s damage must be annealed out at high temperatures. The temperature as a f u n c t i o n of time used in the anneal process i s very important. During the anneal process dopant ions d i f f u s e and are i n c o r p o r a t e d i n t o the c r y s t a l l a t t i c e . At the same time, the c r y s t a l f a u l t s heal themselves or move towards the s u r f a c e . The amount of a n n e a l i n g necessary i s dependent upon the energy and dose of the implanted dopant atoms and upon the d e s i r e d dopant \" a c t i v a t i o n \" (the percentage of dopant i n c o r p o r a t e d i n t o the c r y s t a l l a t t i c e , the r e s t remaining i n t e r s t i t i a l ) . Kemmer recommends annealing at 600°C f o r 30 minutes, but the best r e s u l t s at UBC were achieved with an 800°C anneal f o r 16 hours (REF 14). Others recommend high temperature s h o r t anneal processes (REF 13). The anneal process c e r t a i n l y a f f e c t s the leakage c u r r e n t of implanted d e t e c t o r s , and f u r t h e r experimentation i s r e q u i r e d to optimize t h i s p r o c e s s at UBC. An o p t i o n that may be c o n s i d e r e d in the f u t u r e i s sur f a c e a n n e a l i n g with high power arc lamps. The Kemmer process w i l l probably have to be mo d i f i e d before c o n s i s t e n t r e s u l t s can be . achieved i n a l a b o r a t o r y without e x t e n s i v e environmental c o n t r o l . For example, lower temperature s p u t t e r i n g or plasma techniques c o u l d be used to de p o s i t the mask/passivation oxide. A s u r f a c e e t c h before o x i d a t i o n may prove to be b e t t e r than the RCA c l e a n i n g technique used at UBC. G e n e r a l l y speaking, i t i s not always p o s s i b l e to t r a n s p l a n t a f a b r i c a t i o n process from one l a b o r a t o r y to another without c a r r y i n g out a time-consuming redevelopment procedure. Surface b a r r i e r SS d e t e c t o r s A s u r f a c e b a r r i e r f a b r i c a t i o n process was developed more than ten years ago at the U n i v e r s i t y of Birmingham by J.B.A. England (REF 10). His d e t e c t o r i s an n\" wafer with a Si / S i - o x i d e / G e tunnel diode on both s u r f a c e s ( F i g u r e 3.2). FIGURE 3.2 11OOA A l 650A Ge 20A-50A SiO, 2 30/un S i (5000.rt.cm n type) W/1777777777777777777Z Cross s e c t i o n of the s u r f a c e b a r r i e r s t r i p d e t e c t o r b u i l t at UBC (modified England technique, Ref. 3,5,10,12). Front and back are i d e n t i c a l except f o r p a t t e r n s i n aluminum. Surface e l e c t r o d e s of A l or Au are used to reduce the sheet r e s i s t a n c e of the device and to provide a p a t t e r n f o r p o s i t i o n s e n s i t i v e d e t e c t o r s . The f a b r i c a t i o n process i s short, r e q u i r i n g only one day in the l a b o r a t o r y . T h i s reduces the p r o b a b i l i t y of contamination or other mistakes during manufacture. The d e t e c t o r c r y s t a l i s never heated above 100°C in the England process and thus i t s bulk p r o p e r t i e s are not degraded. Surface b a r r i e r SS d e t e c t o r s do have s h o r t e r l i v e s in r a d i a t i o n and, i n theory, a higher leakage c u r r e n t than a s i m i l a r implanted d e v i c e . The low c o s t and simple process f o r f a b r i c a t i o n of s u r f a c e b a r r i e r d e v i c e s may make these problems unimportant. The r e c i p e d e t a i l e d i n Appendix A i s the best of s e v e r a l attempts at d u p l i c a t i n g England's process, and i s only s l i g h t l y m o d i f i e d from h i s recommendations (REF 12). Two low leakage d e t e c t o r wafers were f a b r i c a t e d u s i n g t h i s process with aluminum m e t a l i z a t i o n . S e v e r a l p a r t s of the process proved to be q u i t e important. As p r e v i o u s l y mentioned, c l e a n l i n e s s i s of primary importance. I t was found that wafers that were not etched produced diodes with l a r g e leakage c u r r e n t s , even i f they were c a r e f u l l y c l e a n e d . England a l s o c l a i m s that d e i o n i z e d f i l t e r e d water i s not c l e a n enough f o r the process and recommends only d i s t i l l e d , f i l t e r e d , d e i o n i z e d water. S i m i l a r l y , u t e n s i l s and chemicals must be very c l e a n and pure. Only a f t e r the edges are p r o t e c t e d by epoxy and the s u r f a c e i s p r o t e c t e d by germanium i s the d e t e c t o r r e l a t i v e l y safe from contamination of the s u r f a c e . The germanium prov i d e s e x c e l l e n t s u r f a c e p a s s i v a t i o n due to i t s g e t t e r i n g e f f e c t s ( a b i l i t y to t r a p i m p u r i t i e s ) ; t h i s i s one of the a t t r a c t i v e p r o p e r t i e s of the England s u r f a c e b a r r i e r d e t e c t o r . E t c h i n g of the s i l i c o n s u r f a c e p r o v i d e s an i d e a l o p p o r t u n i t y to choose the t h i c k n e s s of the f i n a l d e v i c e . However, i t was found that even with continuous a g i t a t i o n of the etch bath and c a r e f u l quick quenching of the etch, the S i s u r f a c e was l e f t m ottled or r i p p l e d . T h i s perhaps c o u l d be prevented by d i l u t i n g the etchant s l i g h t l y . With 10 um removed from each s u r f a c e (wafer reduced from 250 to 230 Mm), the uneven etch d i d not appear to e f f e c t the behavior of the d e t e c t o r . The type of epoxy used for edge p a s s i v a t i o n of these s u r f a c e b a r r i e r d e t e c t o r s i s a l s o very important. The epoxy must cover the area not p r o t e c t e d by Ge. Transene epoxy 50 and the CIBA epoxy (mentioned i n Appendix A) are designed to have a high r e s i s t i v i t y to prevent ohmic leakage. They are of low v i s c o s i t y to reduce the chance of an a i r bubble touching the s i l i c o n s u r f a c e (another source of leakage c u r r e n t (REF 3 ) ) . These are a l s o \"amine type\" epoxies, which prevent a c o n d u c t i v e s u r f a c e i n v e r s i o n l a y e r from forming underneath them i n the s i l i c o n . England recommends p a s s i v a t i n g the edges with epoxy before e v a p o r a t i n g Ge. I t was found that t h i s was not necessary as long as the evaporated f i l m s were kept away FIGURE 3.3 EVAPORATED A l CONTACT SILICON CIRCUIT EPOXY BOARD Cross s e c t i o n of the d e t e c t o r c i r c u i t board mount and edge p a s s i v a t i o n . from the wafer edges (Figure 3.2). The process developed at UBC uses a second A l eva p o r a t i o n to make contact to the mounting c i r c u i t board (Figure 3.3). I t i s p o s s i b l e , as recommended by England, to do both the c o n t a c t evaporation and the s t r i p e v a p o r a t i o n through a s i n g l e shadow mask. Care must be taken, however, to a v o i d e l e c t r i c a l l y s h o r t i n g s t r i p s together i f the shadow mask cannot touch the wafer with the epoxy ramp i n the way. U l t r a s o n i c wirebonding (used e x t e n s i v e l y in the semiconductor i n d u s t r y ) i s another technique a v a i l a b l e to make e l e c t r i c a l contact to the wafer. Wirebonding tends to be time consuming, f r a g i l e and may damage the t h i n f i l m s t r u c t u r e of a s u r f a c e b a r r i e r d e t e c t o r , and so i s not recommended. P r e l i m i n a r y e l e c t r i c a l c h a r a c t e r i s t i c s of s u r f a c e b a r r i e r d e t e c t o r s Before mounting the completed d e t e c t o r wafers, the leakage c u r r e n t and the r e s i s t a n c e between s t r i p s was measured to assess the q u a l i t y of the d e v i c e s . The r e s i s t a n c e between s t r i p s on the d e t e c t o r must be much grea t e r than the input impedance of the p r e a m p l i f i e r s to prevent c r o s s t a l k . C a p a c i t i v e c o u p l i n g can a l s o l e a d to c r o s s t a l k but i s not so e a s i l y measured or c o n t r o l l e d . The sheet r e s i s t a n c e of the germanium l a y e r i s high enoug'h that the r e s i s t a n c e between s t r i p s was measured at between 40 Kfl and 100 Kfl. T h i s i s more than adequate s t r i p i s o l a t i o n . To measure leakage c u r r e n t , a m u l t i p o i n t probe was used to apply a b i a s v o l t a g e to three adjacent s t r i p s . The c u r r e n t flowing in the middle s t r i p c o u l d then be measured in a s i t u a t i o n s i m i l a r to the o p e r a t i n g c o n f i g u r a t i o n . The d e t e c t o r s produced with the m o d i f i e d England r e c i p e had a t y p i c a l c u r r e n t v o l t a g e (I/V) behavior p i c t u r e d i n F i g u r e 3.5. The 1 M A leakage per s t r i p at -30 V was seen to be ac c e p t a b l e as the wafer was expected to be t o t a l l y d e p l e t e d at t h i s v o l t a g e . The I/V behavior was q u i t e c o n s i s t e n t a c r o s s the wafer with o n l y one or two s t r i p s being more than 50% above the average. I t was l a t e r found that these high leakage s t r i p s were more n o i s y than the r e s t i n the d e t e c t o r that was t e s t e d . One of the s u r p r i s i n g r e s u l t s of the I/V measurements i n the l a b was the asymmetry of the d e t e c t o r . In theory, the symmetric manufacturing techniques should produce a d e t e c t o r that can be b i a s e d with a v o l t a g e of e i t h e r p o l a r i t y . When a negative v o l t a g e i s a p p l i e d to the pat t e r n e d or f r o n t s i d e of the d e t e c t o r , the f r o n t s urface b a r r i e r j u n c t i o n i s reverse b i a s e d and ,the d e p l e t i o n zone^ grows from the f r o n t toward the back. The e l e c t r i c f i e l d in t h i s negative b i a s e d d e t e c t o r causes p o s i t i v e c u r r e n t p u l s e s to be seen on the d e t e c t o r s t r i p s when p a r t i c l e s are d e t e c t e d . S i m i l a r l y , a p o s i t i v e b i a s e d d e t e c t o r w i l l have the back j u n c t i o n reverse b i a s e d while the f r o n t i s forward b i a s e d ; the d e t e c t o r c u r r e n t p u l s e s w i l l now be n e g a t i v e . Symmetry i n the leakage c u r r e n t was expected but not observed. A n e g a t i v e b i a s proved to give a lower leakage c u r r e n t . T h i s asymmetry can only be caused by FIGURE 3.4 D E T E C T O R S T R I P 3 6 BIAS VOLTAGE (V) Leakage c u r r e n t of one s t r i p (1 mir,X30mm) i n the SS d e t e c t o r detScto? V ° l t a g e r e v e r s e biases the p a t t e r n e d s i d e of the the d i f f e r e n t geometry of the aluminum l a y e r s ( f r o n t i s p a t t e r n e d , back i s n o t ) , as a l l other q u a l i t i e s of the s u r f a c e s are i d e n t i c a l . A f t e r the wafers were mounted in epoxy, the asymmetry was reduced t o a f a c t o r of 3, i n d i c a t i n g that edge e f f e c t s are p a r t i a l l y r e s p o n s i b l e . Even more p u z z l i n g i s that an asymmetry was a l s o seen in charge c o l l e c t i o n i n the f i n i s h e d d e t e c t o r . T h i s charge asymmetry w i l l be f u r t h e r d i s c u s s e d in Chapter IV. Current v o l t a g e c h a r a c t e r i s t i c s show that t h i s type of s u r f a c e b a r r i e r d e t e c t o r i s not symmetric as expected. A l l s o l i d s t a t e d e t e c t o r s act to some extent l i k e a p h o t o d e t e c t o r . The England s u r f a c e b a r r i e r d e t e c t o r i s mostly s h i e l d e d from photons by the aluminum and germanium l a y e r s . There i s , however, an i n c r e a s e of about 3 ixh/cxn2 leakage c u r r e n t seen when the device i s operated i n d a y l i g h t . T h i s i n c r e a s e i s roughly constant at a l l non-zero b i a s v o l t a g e s . P r e l i m i n a r y t e s t s showed that the d e t e c t o r wafers manufactured with the m o d i f i e d England process were a c c e p t a b l e and ready f o r charged p a r t i c l e t e s t s . CHAPTER IV SOLID STATE DETECTOR SYSTEM TEST A d e t e c t o r a m p l i f i e r system was assembled- using the s u r f a c e b a r r i e r d e t e c t o r s manufactured at UBC. Pion beam t e s t s were performed on the system to measure the e f f i c i e n c y f o r d e t e c t i n g a t r a n s m i t t e d charged p a r t i c l e and the s i g n a l - t o - n o i s e r a t i o of the system. The 1 mm r e s o l u t i o n was confirmed. A l s o measured was the charge g e n e r a t i o n / c o l l e c t i o n e f f i c i e n c y f o r alpha and beta p a r t i c l e s . P r e a m p l i f i e r e l e c t r o n i c s The p r e a m p l i f i e r s used f o r the prototype system are c u r r e n t s e n s i t i v e t h i c k f i l m h y b r i d s a v a i l a b l e from Laben 1 i n I t a l y (part number MSD2). These a m p l i f i e r s were designed f o r s i l i c o n s t r i p d e t e c t o r s and are high speed (2 ns r i s e t i m e ) with a low input impedance. P a r t i c u l a r l y u s e f u l f o r s t r i p d e t e c t o r s i s t h e i r high d e n s i t y (four channels per square inch) and low noise behavior (15 nA RMS e q u i v a l e n t i n p u t ) . The gain was measured at (4±.5) mV/juA in the prototype c i r c u i t . For small s i g n a l s these a m p l i f i e r s work e q u a l l y w e l l f o r p o s i t i v e or negative p u l s e s . In summary, the a m p l i f i e r s are l i n e a r and of lower noise and higher speed than the s i l i c o n d e t e c t o r . They are good for o b s e r v i n g i t s behavior, but higher g a i n would have been u s e f u l . A photograph of the 1 D i v i s i o n e Laben, 20133 Milano, t e l e x 312451 FIGURE 4. 1 Current s e n s i t i v e p r e a m p l i f i e r a r r a y . In s p i t e of the high d e n s i t y Laben h y b r i d s , two l a y e r s of c i r c u i t boards are used to read out the 20 s t r i p s from each s i d e of the d e t e c t o r . 45 F I G U R E 4 . 2 10 K J T to Cu I—I EH W a o EH U H EH W Q B I A S \" W W ' V O L T A G E .01 MF in EH D a. EH D o L A B E N M S D 2 A M P L I F I E R S P r e a m p l i f i e r a n d b i a s v o l t a g e c i r c u i t . d e t e c t o r / p r e a m p l i f i e r system i s i n c l u d e d i n Figure 4.1, and a c i r c u i t diagram i s shown i n Fi g u r e 4.2. Alpha source t e s t s , d e t e c t o r asymmetry Charge c o l l e c t i o n and gener a t i o n c h a r a c t e r i c t i c s of the SS d e t e c t o r were measured with alpha p a r t i c l e s i g n a l s . A s t r i k i n g asymmetry was observed i n charge c o l l e c t i o n e f f i c i e n c y and time. F i g u r e 4.3 shows the p r e a m p l i f i e r output pulses seen for dete c t e d alpha p a r t i c l e s from a 2\"\"Cm source on the j u n c t i o n si d e of the d e t e c t o r with s e v e r a l negative b i a s v o l t a g e s . The c h a r a c t e r i s t i c s h o r t e n i n g of pulse width arid i n c rease i n pulse height can be seen as the b i a s v o l t a g e i s i n c r e a s e d . The charge c o l l e c t e d from the d e t e c t o r i s 230 fmC estimated from the area of the v o l t a g e pulse and the gain of the p r e a m p l i f i e r . The alpha energy i s 5.8 MeV; when reduced to 4.8 MeV by l o s s e s i n 1 cm of a i r , t h i s g i v e s a value of k=.34 in Equation 2.2 (3.3 eV per e l e c t r o n hole p a i r ) . T h i s charge ge n e r a t i o n e f f i c i e n c y i s the same as the 1/3 expected. U n c e r t a i n t i e s i n these c a l c u l a t i o n s are on the order of 20%. The c o l l e c t i o n times were seen to be as f a s t as expected from Equations 2.10; the p u l s e s p i c t u r e d i n F i g u r e 4.3 have r i s e times on the order of 7 ns. The charge c o l l e c t i o n behavior of the d e t e c t o r i n t h i s c o n f i g u r a t i o n i s as expected from theory. .Crosstalk between adjacent s t r i p s and adjacent a m p l i f i e r s was observed to be l e s s than 20%. 47 FIGURE 4.3 c) BIAS=-32V d) BIAS=-39V Curium 244 alpha p u l s e s f o r d i f f e r e n t b i a s v o l t a g e s (measured a f t e r the p r e a m p l i f i e r s ) . N o t i c e that charge c o l l e c t e d (area of pulse) i s roughly constant while the pu l s e height and r i s e time vary. ALPHA SOURCE ON FRONT SIDE ALPHA SOURCE ON BACK SIDE Response of de t e c t o r to 5.8 MeV alpha p a r t i c l e s for d i f f e r e n t b i a s p o l a r i t i e s and s u r f a c e s of entrance f o r the p a r t i c l e s . Sketches of the i n t e r n a l e l e c t r i c f i e l d and the i n i t i a l i o n i z a t i o n d i s t r i b u t i o n are i n c l u d e d . The l e f t hand s i d e of the sketches represents the fr o n t of the det e c t o r (patterned side) where the p r e a m p l i f i e r s are connected. When t h e b i a s v o l t a g e i s r e v e r s e d o r t h e a l p h a s o u r c e i s p l a c e d on t h e o p p o s i t e s i d e o f t h e d e t e c t o r f r o m t h e j u n c t i o n , t h e b e h a v i o r c h a n g e s i n a m y s t e r i o u s way. F i g u r e 4.4 shows t h e p u l s e s o b s e r v e d f o r t h e f o u r p o s s i b l e c o m b i n a t i o n s o f b i a s v o l t a g e p o l a r i t y a nd s o u r c e p o s i t i o n . A l s o shown a r e t h e q u a l i t a t i v e e l e c t r i c f i e l d a n d i o n i z a t i o n d i s t r i b u t i o n s . T h e s e o b s e r v a t i o n s c a n be u s e d t o e s t i m a t e t h e d e p l e t i o n d e p t h of t h e d e t e c t o r , t h e c h a r g e c o l l e c t i o n e f f i c i e n c y a n d t h e d r i f t t i m e o f t h e c h a r g e c a r r i e r s . The c h a r g e d e p o s i t e d i n t h e d e p l e t i o n z o n e (-area o f p u l s e ) i s s e e n t o be t h e same f o r a p a r t i c l e s e n t e r i n g e i t h e r t h e f r o n t o r t h e b a c k o f t h e d e t e c t o r ( f o r a g i v e n b i a s v o l t a g e ) . As t h e r a n g e o f t h e a i s a t most 28 Mm'in s i l i c o n , t h i s i m p l i e s t h a t t h e d e t e c t o r i s t o t a l l y d e p l e t e d a t a b i a s v o l t a g e o f ±32 V. The same r e s u l t s were n o t s e e n a t a b i a s v o l t a g e o f ±25 V, i n d i c a t i n g t h a t t h e d e t e c t o r i s o n l y s l i g h t l y o v e r - d e p l e t e d a t 32 V. The d i f f e r e n c e i n p u l s e a r e a s s e e n f o r d i f f e r e n t p o l a r i t i e s o f b i a s v o l t a g e i n d i c a t e s t h a t t h e c h a r g e c o l l e c t i o n e f f i c i e n c y f o r h o l e s ( n e g a t i v e b i a s v o l t a g e on t h e f r o n t s t r i p s i d e ) i s t h r e e t i m e s b e t t e r t h a n t h a t f o r e l e c t r o n s . T h i s e f f e c t seems t o i n d i c a t e t h e p r e s e n c e o f t r a p p i n g s i t e s f o r e l e c t r o n s e i t h e r i n t h e c r y s t a l b u l k o r i n t h e s u r f a c e . The e x t r e m e w i d e n i n g o f t h e d e t e c t o r p u l s e s e e n when t h e a p a r t i c l e s e n t e r on t h e n o n - j u n c t i o n s i d e o f t h e d e t e c t o r i s another unexpected e f f e c t . The c o l l e c t i o n time p r e d i c t e d by Equations 2.10 are at most the order of 20 ns. In a d d i t i o n , the c o l l e c t i o n times are p r e d i c t e d to be l a r g e r f o r holes than e l e c t r o n s , which i s not seen i n the data. T h i s pulse time length e n i n g e f f e c t i s c o r r e l a t e d with the a b s o l u t e s t r e n g t h of the e l e c t r i c f i e l d where the a energy i s d e p o s i t e d . The d i s c r e p a n c i e s from Equations 2.10 can p o s s i b l y be e x p l a i n e d by space charge e f f e c t s on the l o c a l e l e c t r i c f i e l d due to the l a r g e charge d e n s i t i e s ^ g e n e r a t e d by a p a r t i c l e s . Beta source t e s t s 0 p a r t i c l e s from a 1 0 6 R u source were used to observe the d e t e c t o r response to small s i g n a l t r a n s m i t t e d p a r t i c l e s . 1 0 6 R u decays to 1 0 6 R h which i n turn |3 decays with a peak energy of 3.5 MeV. The peak energy l o s t by the 0 p a r t i c l e i n 230 urn of s i l i c o n i s about 250 KeV when p a r t i c l e s from the low end of the energy spectrum are almost stopped by the d e t e c t o r . Photographs of the d e t e c t o r response to 0 p a r t i c l e s are shown in F i g u r e 4.5. The p u l s e s are about 30 ns long and show no evidence of the length e n i n g observed f o r a p a r t i c l e p u l s e s . The 1/3 charge r e d u c t i o n e f f e c t i s s t i l l seen, however, when the b i a s v o l t a g e i s switched from negative to p o s i t i v e . As a r e s u l t of the charge c o l l e c t i o n asymmetry of the prototype, the e l e c t r o n i c s f o r f u r t h e r t e s t s were designed to r e c e i v e p o s i t i v e p u l s e s from a negative b i a s e d d e t e c t o r . FIGURE 4.5 20ns per d i v i s i o n 5mV per d i v i s i o n D etector p u l s e s seen ( a f t e r p r e a m p l i f i e r s ) f o r beta p a r t i c l e s from a Ruthenium 106 source. 52 FIGURE 4.6 5mV/div 2 0 n s / d i v N o i s e s e e n a t o u t p u t o f p r e a m p l i f i e r s . Observed noise of system The noise seen on an o s c i l l i s c o p e i n the output of the p r e a m p l i f i e r was about 1 mV peak to'peak (.2 M A RMS e q u i v a l e n t input to p r e a m p l i f i e r ) ( F i g u r e 4.6). What i s i n t e r e s t i n g i s that t h i s noise d i d not change with b i a s v o l t a g e on the d e t e c t o r or with exposure of the d e t e c t o r to d a y l i g h t . The noise appeared to be mostly caused by the p r e a m p l i f i e r s ( i n s p i t e of the s p e c i f i c a t i o n s ) , as i t was only s l i g h t l y reduced when the de t e c t o r was removed. T h i s low noise behavior c o u l d only be achieved when a b a t t e r y was used f o r the d e t e c t o r b i a s v o l t a g e , as power s u p p l i e s proved to be a s i g n i f i c a n t source of n o i s e . Beam t e s t design The prototype d e t e c t o r was t e s t e d in the M13 pion beamline at TRIUMF. The e f f i c i e n c y f o r d e t e c t i n g p a r t i c l e s was measured f o r d i f f e r e n t p a r t i c l e types and energies at d i f f e r e n t s e t t i n g s of d e t e c t o r b i a s v o l t a g e . Estimates were a l s o made of the noise l e v e l of the system. It was not p o s s i b l e to measure the high ra t e performance of the d e t e c t o r as the beam f l u x was too low during the experimental time. Only one wafer was t e s t e d e x t e n s i v e l y . The d e t e c t o r system was l i n k e d to the CAMAC data a c q u i s i t i o n system as shown i n F i g u r e 4.7. The s i g n a l s from the p r e a m p l i f i e r s on the 40 d e t e c t o r s t r i p s were d i s c r i m i n a t e d i n d i v i d u a l l y by three LeCroy 2735A u n i t s . The l o g i c a l ECL SOLID STATE DETECTOR ANALOGUE SIGNALS 40 CHANNELS CD a w MALU GATES, TDC START FROM SCINTILLATOR COINCIDENCE Block diagram of de t e c t o r data for the pion beam t e s t . a q u i s i t i o n e l e c t r o n i c s o u t p u t p u l s e s f r o m t h e 2735A's were r e a d i n t h e c o u n t i n g room by CAMAC m a j o r i t y l o g i c u n i t s . The 2735A a m p l i f i e r d i s c r i m i n a t o r s do n o t g i v e an a n a l o g u e s i g n a l o u t p u t so t h e s i z e o f t h e s i g n a l p u l s e s c o u l d n o t be r e c o r d e d d i r e c t l y . I n a d d i t i o n t o t h e s t r i p d e t e c t o r , two d e l a y - l i n e m u l t i - w i r e p r o p o r t i o n a l c h a m b e r s were u s e d f o r r a y t r a c i n g a n d two s c i n t i l l a t o r s were u s e d f o r a t r i g g e r ( F i g u r e 4 . 8 ) . The d e l a y l i n e c h a m b e r s had a r e s o l u t i o n o f 1 mm i n t h e v e r t i c a l o r Y d i r e c t i o n and 2 mm i n t h e X d i r e c t i o n . The s t r i p d e t e c t o r was o r i e n t e d t o be p o s i t i o n s e n s i t i v e i n t h e Y d i r e c t i o n . The M13 b e a m l i n e was s e t a t two momentum v a l u e s d u r i n g t h e t e s t . W i t h t h e c h a n n e l s e t a t 128 MeV/c ( s l i t s open t o g i v e 10% Ap/p momentum window) t h e beam i s r o u g h l y 9 5% n*, 3% M + a n d 2% e + . H e a v i e r p a r t i c l e s a r e s t o p p e d i n t h e f i r s t s c i n t i l l a t o r . The p i o n e n e r g y a t t h i s momentum i s 50 MeV. W i t h t h e c h a n n e l s e t a t 96 MeV/c (30 MeV p i o n s ) t h e f r a c t i o n o f e l e c t r o n s and muons g o e s up t o a b o u t 8% e a c h . By u s i n g t i m e o f f l i g h t i n f o r m a t i o n , i t i s p o s s i b l e t o d i s t i n g u i s h t h e t h r e e p a r t i c l e s i n e a c h e v e n t o b s e r v e d . A r a n g e o f p a r t i c l e s f r o m minimum i o n i z i n g up t o more t h a n t w i c e minimum i o n i z i n g was a v a i l a b l e f o r t h e t e s t . A p p r o x i m a t e l y 30,000 d e t e c t o r e v e n t s were r e c o r d e d f o r e a c h d e t e c t o r c o n f i g u r a t i o n . Beam t e s t , p o s i t i o n r e s o l u t i o n M e a s u r ement of t h e SS d e t e c t o r p o s i t i o n r e s o l u t i o n was p e r f o r m e d by r a y t r a c i n g w i t h t h e w i r e c h a m b e r s . T h i s was done FIGURE 4.8 S C I N T I L L A T O R 2 W I R E C H A M B E R 2Q (1 3cmx13cm) W I R E C H A M B E R (13cmx13cm) 1, S O L I D S T A T E D E T E C T O R S C I N T I L L A T O R 1 w EH z w o FIGURE 4 . 9 e E z o in O cu K O E-CJ H ca a to 40 0 39 5 39 0 38 5 3E 0 37. 5 37 0 36 9 36 0 35 35 34 34 33 33 32 32 31 31 30 30 24 13 23 20 20 19 I—I 21 C -9. 5 : 9 0 I B 5 : 6 0 : 7 5 1 7 o : 6 s : 6 0 - . 5. 9 : 9 o : 4 9 : 4. 0 i 3. 9 1 3 o : 2 5 : 2 0 : 1 l 9 : o -I E I :iBX . . : l»K: . . . . :IBS: :I»S: :t*S :ICS. : «•«. : :Xf : :X»S : : IH, : :X««,: ::>•••: : IMi: : ft. : . : tS*.: . . . : :X««i :ICS.: : I S S I S » , : . . : l»Si: : iSS. . : . >Gi: . IS. : . . 1 I . 1 - I OO -0 50 0.00 0 .90 t o o 1 50 a. 00 3 .90 WIRE CHAMBER TRACEBACK (cm) S c a t t e r p l o t of SS d e t e c t o r p o s i t i o n ( s i n g l e or adjacent double h i t s ) versus wire chamber traceback p o s i t i o n . FIGURE 4.10 E z o co O a. a o E-< u w E-> o co CO «0 3 I I > X < : . : X XI . X i . X . S S . • X X S i X >SX». X . S X i S ' S i I I X X S S X • ISS. i i S X i i X i X i X X i S i i I n I S . I S I C . I M M . i.XS. WIRE CHAMBER TRACEBACK (cm) S c a t t e r p l o t of d e t e c t o r p o s i t i o n versus wire chamber traceback with only \" n o i s e \" i n c l u d e d . Note the excess n o i s e on s t r i p s 17 and 23. FIGURE 4.11 ' ' ' ' ' ' ' ' ' I ' ' • ' ' ' ' ' ' I ' I I I i i I i i K I I I 360. — 240. — j 120. — I WIRE CHAMBER TRACEBACK (mm) Wire chamber traceback p o s i t i o n at d e t e c t o r f o r h i t s of s t r i p 20. Note that the r e s o l u t i o n of the WC traceback o n l y 1mm at best. more to t e s t the t o t a l experimental apparatus and to measure noise than to show that 1 mm s t r i p s g i v e 1 mm r e s o l u t i o n . A s c a t t e r p l o t of wire chamber Y traceback versus d e t e c t o r p o s i t i o n i s shown i n F i g u r e 4.9. Only p a r t i c l e s that were t r a c e d back through the SS d e t e c t o r were co n s i d e r e d f o r a n a l y s i s . A s l i c e of t h i s p l o t , the c o r r e l a t i o n of s t r i p 20 and traceback p o s i t i o n , i s shown i n F i g u r e 4.11. The 1.5 mm width of the peak i n F i g u r e 4.11 i s e x a c t l y what one expects from the c o r r e l a t i o n of two 1 mm r e s o l u t i o n instruments. The events that do not l i e on the s c a t t e r p l o t d i a g o n a l (Figure 4.10) were con s i d e r e d as n o i s e . Beam t e s t , noise and e f f i c i e n c y The prototype system was found to have a low s i g n a l - t o - n o i s e r a t i o . Minimum i o n i z i n g p a r t i c l e s l o s e about 90 KeV in the d e t e c t o r . T h i s i s a s i g n a l of only .16 ULA based on a 30 ns pulse width and 3.2 eV per e l e c t r o n hole p a i r . The p r e a m p l i f i e r / d e t e c t o r RMS nois e has been measured at approximately .2 uA (Figure 4.5). P r e l i m i n a r y r e j e c t i o n of n o i s e i n the s t r i p d e t e c t o r was accomplished by r e j e c t i n g a l l d e t e c t o r events with more than one s t r i p h i t unless i t was a p a i r of adjacent s t r i p s that f i r e d . A t y p i c a l beam p r o f i l e as measured by the SS d e t e c t o r i s shown in F i g u r e 4.12. Half i n t e g e r bins represent events where adjacent s t r i p s f i r e d . Real double events were r u l e d out as the beam f l u x was too low (1 KHz); only at f l u x e s over 1 MHz i s p i l e u p a n o n - n e g l i g i b l e problem. T h i s m u l t i p l e h i t FIGURE 4.12 1 80Q\\ 1 2 0 0 . 6 0 Q . H Beam p r o f i l e as seen with the s o l i d s t a t e d e t e c t o r . Half i n t e g e r bins represent adjacent double s t r i p h i t s . noise r e j e c t i o n system w i l l i d e n t i f y a percentage of noise s i g n a l s equal to the p r o b a b i l i t y of a r e a l p a r t i c l e being d e t e c t e d . Only when no p a r t i c l e passes through the d e t e c t o r or when a r e a l p a r t i c l e pulse i s not d i s c r i m i n a t e d can a noise s i g n a l be i n t e r p r e t e d as a r e a l event. The l a r g e s t e f f e c t of the n o i s y system was to reduce the peak e f f i c i e n c y of the d e t e c t o r at low d i s c r i m i n a t o r t h r e s h o l d s . The number of s t r i p s f i r e d per p a r t i c l e i s shown in F i g u r e 4.13; events in bin zero represent p a r t i c l e s that were t r a c e d back through the d e t e c t o r but were not d i s c r i m i n a t e d . F i g u r e 4.14 shows the d e t e c t o r e f f i c i e n c y and noise versus d i s c r i m i n a t o r t h r e s h o l d with no noise r e j e c t i o n and with the r e j e c t i o n method d e s c r i b e d above. E f f i c i e n c y i s d e f i n e d as the number of p a r t i c l e s detected by the SS d e t e c t o r d i v i d e d by the number t r a c e d back through i t . The n o i s e i s d e f i n e d as d e t e c t o r events that do not c o i n c i d e with the wire chamber traceback ( F i g u r e 4.10). S t r i p s 17 and 23 show an ex c e s s i v e amount of n o i s e . The r e s i d u a l noise seen a f t e r m u l t i p l e h i t noise r e j e c t i o n i s mostly due to pion decay to muons in the t e s t apparatus. The it* l i f e t i m e , r, i s 2.6 X 10~ 8 s. The f r a c t i o n , f, of pions that decay i n a short d i s t a n c e x i s f =*X//3C7T Equation 4.1 where | 3 , 7 , are the r e l a t i v i s t i c parameters a s s o c i a t e d with the pion in the l a b frame. For 50 MeV pions, 4.2% w i l l decay i n the 30 cms between the s t r i p d e t e c t o r and the f i n a l wire FIGURE 4.13 i i i i i i i i i I i i i i i i i i i _ i i i i i i i i i I i i i i i i i i i 2 4 0 0 0 . — 2 4 0 0 0 . — 1 6 0 0 0 . 1 6 0 0 0 . — 8 0 0 0 . — L 8 0 0 0 . — 0. ~Tjn 1 1 1 1 1 1 1 5. 0 1 0. 0 MULTIPLICITY THRESHOLD=0.75V - TI 1 1 1 1 1 1 1 T T T \" 5. 0 10. MULTIPLICITY THRESHOLD=1.0V ' ' ' ' ' • ' ' ' ' ' • ' • ' ' ' i i 2 4 0 0 0 . — I 6 0 Q O . — 8 0 0 0 . 2 4 0 0 0 . — - 1 6 0 0 0 . — 8 0 0 0 . — I I I | I I I l I I I I l | -5 . 0 10. D 0 . I i ' ' • } 1 1 I 1 i i i i . TI I I l I I I I I I I I I I 5 . 0 1 0 . 0 MULTIPLICITY THRESHOLD^1. 5V MULTIPLICITY THRESHOLD=2.0V M u l t i p l i c i t y (number of s t r i p s f i r e d per event) f o r d i f f e r e n t d i s c r i m i n a t o r t h r e s h o l d s . Only s i n g l e s and some doubles are kept as good events. FIGURE 4.14 128NeV/c PARTICLES ( 9 3 % PIONS) 1 1 i i i 0.0 0.5 1. 0 1. 5 2.0 2.5 DISCRIMINATOR THRESHOLD ( V ) ' Dete c t o r e f f i c i e n c y and noise f o r a -32V b i a s v o l t a g e . Net data i n c l u d e s only s i n g l e or adjacent double SS d e t e c t o r h i t s . FIGURE 4.15 D i s t r i b u t i o n of l a b angle of muons from decay of 50MeV pi o n s . FIGURE 4.16 3600 . — I 2 O 0 . — ANGLE (mRADIANS) (ALL EVENTS) 1600. — 800. — I i I i i i i i i i i i I I I n 1111111 n 111111111 IT=T 0 40 8b 120 ANGLE (mRADIANS) (DETECTOR HITS) n 1111111 I I ] 1111 160 2D0 6 0 . — 0 4b Bb 120 160 200 ANGLE (mRADIANS) (NOISE HITS) Angle of p a r t i c l e ray through apparatus (beam c e n t r e l i n e d e f i n e d as z e r o ) . N o t i c e c o r r e l a t i o n between nois e s i g n a l s and a n g l e ; t h i s c o r r e l a t i o n i s caused by pion to muon decay i n the aparatus. chamber, the d i s t r i b u t i o n of the r e s u l t i n g muon angle i s shown in F i g u r e 4.15 (REF 11). The product muons are d e t e c t e d in the wire chambers but due to the non-zero angle to the o r i g i n a l pion path, an erroneous traceback i s c a l c u l a t e d i n most cases. A 3-4% traceback \" n o i s e \" i s expected in the p i o n t e s t data due to ir t o (i decay in the t e s t apparatus. F i g u r e 4.16 shows the c o r r e l a t i o n between angle of traceback path and n o i s e . T h i s c o r r e l a t i o n supports the TI to u n o i s e h y p o t h e s i s . Beam t e s t , b i a s v o l t a g e and e f f i c i e n c y The e f f i c i e n c y of the d e t e c t o r was measured at three d i f f e r e n t b i a s v o l t a g e s . A summary of the r e s u l t s i s represented g r a p h i c a l l y i n F i g u r e s 4.17. U n f o r t u n a t e l y , only negative b i a s e s c o u l d be used due to the f i x e d input p o l a r i t y of the 2735A d i s c r i m i n a t o r s . As was expected, the e f f i c i e n c y f o r d e t e c t i o n improves with i n c r e a s e d p a r t i c l e energy l o s s . The e f f i c i e n c y a l s o i n c r e a s e s with i n c r e a s e d b i a s v o l t a g e . In the range t e s t e d , the d e t e c t o r i s of l i t t l e use f o r d e t e c t i n g minimum i o n i z i n g e l e c t r o n s ( l e s s than 50% e f f i c i e n t , l a r g e amount of noise) and only of marginal use f o r the 50 MeV pions i t was designed f o r (70% e f f i c i e n t , low net n o i s e ) . A f u r t h e r i n c r e a s e in b i a s v o l t a g e would not be l i k e l y to improve the performance of the d e t e c t o r as t h i s would i n c r e a s e leakage c u r r e n t . Excess noise was d e t e c t e d on s t r i p s 17 and 23, and both of these s t r i p s were seen to have leakage c u r r e n t s s e v e r a l times the average ( = 10-20 /xA/cm2). FIGURE 4.17a 100 --LU 9 0 \" CO o 8 0 \" 70 -O 60 -LU M 50 -(_) U_ 40 -Ll_ W 3 0 -I— W 20 -« 10 -0 4 96MeV/c PIONS: DELTR E 190KeV I I I L 25V B i n s 32V B i n s 39V B i n s • EFFICIENCY D NOISE 1 1 1 T— 0. 0 0 . 5 1.0 1. 5 2 .0 2 .5 DISCRIMINATOR THRESHOLD ( V) 100 UJ so H co o 8 ( H 70 -O 60 H LU I—I 50 H o £ 40 Ll_ LU LU 30 -20 -10 -0 0. 0 128MeV/c PIONS: DELTA E 150KeV 25V B i n s 32V B i n s 39V B i n s • EFFICIENCY a NOISE — r ~ 1.0 —r~ 1.5 0 . 5 0 1.5 2.0 2 .5 DISCRIMINATOR THRESHOLD (V) Summary of SS de t e c t o r e f f i c i e n c i e s and noise f o r v a r i o u s d e p o s i t e d e n e r g i e s , b i a s v o l t a g e and d i s c r i m i n a t o r t h r e s h o l d s , Only s i n g l e or adjacent double h i t s are i n c l u d e d . See a lso f igure 4.17b. 69 FIGURE 4 . 17b 100 L U 9° C O 70 (_) 60 -LU 50 40 30 -20 -2 * * 10 -128MeV/c MUONS: i i DELTA E -1 1 OKeV 25V BIBS 32V BIBS 39V BIBS • EFFICIENCY • NOISE 2. 5 1 1 1 r~ 0.0 0. 5 1.0 1.5 2.0 DISCRIMINATOR THRESHOLD ( V) 100 UJ 90 H CO 70 ->-* C J 60 H L U H 50 H C J 40 -30 -20 -10 0 L U 128MeV/c ELECTRONS: 1 i i DELTA E 86KeV 2SV BIBS 32V BIBS 39V BIBS • EFFICIENCY • NOISE • 1 1 1 1— 0.0 0 .5 1. 0 1.5 2.0 DISCRIMINATOR THRESHOLD ( V) 2. 5 SS d e t e c t o r e f f i c i e n c i e s and noi s e . Timing of SS de t e c t o r s i g n a l s Timing information f o r some s t r i p s was recorded. D i s c r i m i n a t e d s i g n a l s from the s t r i p d e t e c t o r were made a v a i l a b l e by using an ECL/NIM/ECL converter (Figure 4.7). These s i g n a l s were recorded i n a TDC (time to d i g i t a l c o n v e r t e r ) s t a r t e d by the two s c i n t i l l a t o r s . A time spectrum for one s t r i p i s shown in F i g u r e 4.18. Even with the p r i m i t i v e time over t h r e s h o l d d i s c r i m i n a t i o n , the width of the time peak (time j i t t e r ) was only 10 ns. The width of the data a c q u i s i t i o n gate to the m a j o r i t y l o g i c u n i t c o u l d then be set at 40 ns, thus reducing random n o i s e . The time spectrum a l s o showed evidence of RF pickup by the d e t e c t o r . . At low d i s c r i m i n a t o r s e t t i n g s , noise peaks sepa r a t e d by 43 ns ( c h a r a c t e r i s t i c of the c y c l o t r o n ) c o u l d be seen i n the time spectrum. FIGURE 4.18 ' i i i i i i i i I i i i i i i i i i I i i i i i i i i i I I I i i i t 240. H 160. H 80. r r v v v v T r r r r r r r V V V V T ) •i i v v v •[' v \\ i' v v v v v y v v v v v v v v v y v i 320 400 80 160 240 TIME (250ps/bin) DISCRIMINATOR THRESHOLD*0.75V ' • ' • i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 180. H 1 20. —\\ 60. H I I I 1 I l\"l T I I I I I n I I I I I i^V*rf 1 T I ITI ITI I I I I T I T T T I I I I T T I 160 240 320 400 TIME (250ps/bin) DISCRIMINATOR THRESHOLDS.25V Time spectrum of s i g n a l s from one s t r i p . At low t h r e s h o l d noise i s c l e a r l y v i s i b l e with a component having a 43ns p e r i o d . CHAPTER V SUMMARY AND CONCLUSION A s u r f a c e b a r r i e r s o l i d s t a t e d e t e c t o r system was developed at TRIUMF s p e c i f i c a l l y f o r p o s i t i o n r e s o l u t i o n of 50 MeV pions at high f l u x e s . The e f f i c i e n c y of the prototype d e t e c t o r system was measured at 70% f o r these p a r t i c l e s at low r a t e s . The ra t e c a p a b i l i t y of the device i s expected to be i n excess of 1 MHz per s t r i p as the c h a r a c t e r i s t i c s i g n a l l e n g t h was 30 ns. The 1 mm r e s o l u t i o n and 40 mm a c t i v e diameter of the device are adequate f o r beam p r o f i l e monitor, a c t i v e momentum s l i t , and t a r g e t p o s i t i o n monitor a p p l i c a t i o n s i n the Ml 3 beamline. The device would be more d e s i r e a b l e i f p o s t i o n r e s o l u t i o n c o u l d be achieved i n both d i r e c t i o n s . The m u l t i p l e s c a t t e r i n g and energy s t r a g g l e a s s o c i a t e d with the 55 mg/cm2 d e t e c t o r i s acceptable f o r the a p p l i c a t i o n s proposed although t h i n n e r d e t e c t o r s would a l s o be d e s i r e a b l e . Noise, improvements The low s i g n a l to noise r a t i o and r e s u l t i n g low e f f i c i e n c y of the prototype d e t e c t o r i s due to one or more of three sources. There i s the i n t r i n s i c n o i s e from the d e t e c t o r c r y s t a l , RF ele c t r o - m a g n e t i c pickup, and e l e c t r o n i c s n o i s e . E x c e s s i v e leakage c u r r e n t noise was observed on two s t r i p s of the prototype d e t e c t o r . To remedy t h i s problem, more work must be done i n the s o l i d s t a t e l a b o r a t o r y . A l l that may be necessary i s the manufacture of s e v e r a l more d e t e c t o r wafers to smooth out problems with c l e a n l i n e s s . As with a l l f a b r i c a t i o n processes, the q u a l i t y of the product w i l l vary from run to run and the best d e v i c e s must be chosen f o r use as d e t e c t o r s . The s t r i p s of the SS d e t e c t o r act as an a e r i a l f o r ambient RF power. The pickup observed for the unshielded d e t e c t o r was small as the e l e c t r o n i c s were c a r e f u l l y s h i e l d e d and grounded. To f u r t h e r reduce t h i s problem, the d e t e c t o r / p r e a m p l i f i e r system should operate in a Faraday cage with f o i l windows, or i n s i d e the beam pipe. E l e c t r o n i c s noise appeared to be the biggest problem in the prototype system. On most of the d e t e c t o r s t r i p s , a p p l i c a t i o n of bias v o l t a g e d i d not increase the n o i s e seen on the output of the p r e a m p l i f i e r s . T h i s noise was seen to be the same s i z e as pulse expected from minimum i o n i z i n g p a r t i c l e s . The LeCroy a m p l i f i e r d i s c r i m i n a t o r s were operated very c l o s e to the minimum t h r e s h o l d p o s s i b l e (at which p o i n t o s c i l l a t i o n o c c u r s ) , and they may a l s o have c o n t r i b u t e d a p o r t i o n of the noise s i g n a l s . Higher gain low n o i s e combination p r e a m p l i f i e r and d i s c r i m i n a t o r h y b r i d s are being considered f o r the next g e n e r a t i o n of e l e c t r o n i c s . These de v i c e s , a v a i l a b l e from Newmarket Microsystems in England, a l s o have an analogue output u s e f u l f o r measuring the energy l o s s of p a r t i c l e s i n the d e t e c t o r . Future c o n s i d e r a t i o n s In the immediate fu t u r e at TRIUMF, development w i l l continue on d e v i c e s s i m i l a r to the non-integrated prototype d e t e c t o r . Two dimensional r e s o l u t i o n d e v i c e s are p o s s i b l e , but problems stemming from the asymmetry observed in the prototype device may have to be overcome. Thinner d e t e c t o r s can a l s o be made i f the s i g n a l to noise r a t i o i s i n c r e a s e d . The next step i n the development of p o s i t i o n s e n s i t i v e s o l i d s t a t e d e t e c t o r s w i l l be i n t e g r a t i o n of the f i r s t stage of a m p l i f i c a t i o n on the same wafer as the d e t e c t o r . T h i s w i l l probably be accomplished with GaAs technology. The maximum s i z e of s o l i d s t a t e d e t e c t o r s w i l l soon expand to the 8 inch diameter wafers now a v a i l a b l e . 75 REFERENCES 1 E . E . H a l l e r , H.W,Kramer, W.A.Higinbothom, Mat.Res.Soc. V o l 16, \"Nuclear R a d i a t i o n Detector M a t e r i a l s \" , H o l l a n d Press (a) J.H.Howes, J.Watling, P207 (b) A.Musa, J.P.Ponpon, M.Hage-Ali, P225 2 E.M.Lawson, Nuc.Inst.Meth. 180(1981)651 3 J.B.A.England, \"Techniques in Nuclear S t r u t u r e P h y s i c s \" , Part 1, Macmillan Press 4 A.A.Konova, Nuc.Inst.Meth. 160(1979)115 5 J.B.A.England et a l , Nuc.Inst.Meth. 196(1982)149 6 J.Kemmer et a l , Nuc.Inst.Meth. 205(1983)99 7 W.J.Price, \"Nuclear R a d i a t i o n D e t e c t i o n \" , second e d i t i o n , McGraw H i l l 8 S.M.Sze, \"Physics of Semiconductor Devices\", John Wiley and Sons 9 P.Borgeaud et a l , \"The E f f e c t of R a d i a t i o n on the Energy R e s o l u t i o n of Ion Implanted S i l i c o n D e t e c t o r s \" , Commissariat a l ' E n e r g i e Atomique Centre d'Etudes N u c l e a i r e s de Sac l a y , France 10 J.B.A.England, V.W.Hammer, Nuc ..I nst .Meth. 96(1971)81 11 R. Ta c i k , Phd T h e s i s , UBC 1984 12 J.B.A.England, P r i v a t e communication, Dec 1984 13 J.Yah-Min Lee, IEEE T r a n s a c t i o n s on E l e c t r o n Devices, Vol.ED-28,No.4, A p r i l 1981 14 P.Janega, P r i v a t e communication, May 1983 APPENDIX A MODIFIED ENGLAND PROCESS FOR SURFACE BARRIER DETECTORS S t a r t with high r e s i s t i v i t y n\" s i l i c o n wafers: p o l i s h e d both s i d e s , - 5000 ficm, c a r r i e r l i f e t i m e ^3 ms. Clean the s l i c e i n s o l v e n t (2-Propanol, Acetone) and then in d i s t i l l e d d e i o n i z e d water with u l t r a s o u n d . ( A l l wet chemicals and u t e n s i l s must be very c l e a n , a l l water should be d i s t i l l e d d e i o n i z e d ) . B o i l the s l i c e g e n t l y in con c e n t r a t e d n i t r i c a c i d f o r 5 minutes. D i l u t e and decant with d i s t i l l e d d e i o n i z e d water. B o i l i n water f o r 5 minutes, d i l u t e and decant with c o l d water. Etch the wafer, i n room temperature CP4A (40% HF: co n c e n t r a t e d a c e t i c a c i d : c o n c e n t r a t e d n i t r i c a c i d / 3 : 3 : 5 ) . The wafer must be c o n s t a n t l y a g i t a t e d and tends to form a \" r i p p l e d \" s u r f a c e anyway. CP4A etches at about 20 jim/min (per s i d e ) . At l e a s t 10 Mm should be removed. Quench the wafer i n H 20. D i l u t e and decant with water (50% d i l u t i o n , 5 t i m e s ) . O x i d i z e the wafer i n 1% potassium dicromate s o l u t i o n . P lace i t i n the c o l d s o l u t i o n and heat to 80 C, wait 10 minutes, d i l u t e and decant. At t h i s p o i n t , i f contamination can be observed as s t r e a k s under an i n f r a - r e d lamp, one may want to s t a r t over. Dry the s l i c e i n N 2 and p l a c e i n evaporator. o Evaporate 560-750 A of Germanium (p doped semiconductor grade Ge was used with a s t a r t i n g pressure of 10~ 6 Torr) onto both s i d e s of the s i l i c o n , the diode i s now formed and p a s s i v a t e d by the Ge. o o 9. Evaporate 210 A Au or 1100 A A l onto both s u r f a c e s . S t r i p p a t t e r n s are made on one s u r f a c e with a wire shadow mask in c o n t a c t with the wafer ( s t a r t at 10~ 6 Torr p r e s s u r e ) . 10. Glue the ground plane to the c i r c u i t board mounting s u r f a c e with conducting epoxy (Epotek H44). Make an epo.xy \"ramp\" with Transene epoxy 50 (or CIBA AY105:HY956, 6:1 by weight) and evaporate c o n t a c t s to the c i r c u i t board through a shadow mask (Fi g u r e 3.3). The t r a c e s on the c i r c u i t board should be g o l d . Often the evaporated contact t r a c e s must be probed with a p e n c i l p o i n t to make a good e l e c t r i c a l contact i f aluminum i s used f o r the metal e v a p o r a t i o n s . The aluminum s t r i p s form an i n s u l a t i n g oxide l a y e r very q u i c k l y in a i r . "@en ; edm:hasType "Thesis/Dissertation"@en ; edm:isShownAt "10.14288/1.0074521"@en ; dcterms:language "eng"@en ; ns0:degreeDiscipline "Engineering Physics"@en ; edm:provider "Vancouver : University of British Columbia Library"@en ; dcterms:publisher "University of British Columbia"@en ; dcterms:rights "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en ; ns0:scholarLevel "Graduate"@en ; dcterms:title "Fabrication process and characteristics of a silicon strip detector"@en ; dcterms:type "Text"@en ; ns0:identifierURI "http://hdl.handle.net/2429/25120"@en .