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Characterising the SCUBA-2 superconducting bolometer arrays. Amiri, Mandana 2011

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Characterising the SCUBA-2 superconducting bolometer arrays   Dan Bintley1a, Michael J. MacIntoshb, Wayne S. Hollandb,d, Per Friberga, Craig Walthera, David Atkinsonb, Dennis Kellyb, Xiaofeng Gaob, Peter A.R. Adec, William Graingerc, Julian Housec, Lorenzo Moncelsic, Matthew I. Hollisterd,i, Adam Woodcraftd, Camelia Dunaree, William Parkese, Anthony J. Waltone, Kent D. Irwinf, Gene C. Hiltonf, Michael Niemackf, Carl D. Reintsemaf, Mandana Amirig, Bryce Burgerg, Mark Halperng, Matthew Hasselfieldg, Jeff Hillh, J. B. Kyciah, C.G.A. Mugfordh, Lauren Persaudh  aJoint Astronomy Centre, 660 N. A’ohoku Place, Hilo, HI 96720 bUK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK cSchool of Physics and Astronomy, 5 The Parade, Cardiff University, Cardiff CF24 3YB, UK dInstitute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK eScottish Microelectronics Centre, University of Edinburgh, West Mains Road, Edinburgh EH9 3JF, UK fNational Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305 gDepartment of Physics and Astronomy, University of British Columbia, British Columbia V6T 1Z1, Canada hDepartment of Physics, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada iCalifornia Institute of Technology, Pasadena, CA 91125 ABSTRACT SCUBA-2 is a state of the art 10,000 pixel submillimeter camera installed and being commissioned at the James Clerk Maxwell Telescope (JCMT) providing wide-field simultaneous imaging at wavelengths of 450 and 850 microns. At each wavelength there are four 32 by 40 sub-arrays of superconducting Transition Edge Sensor (TES) bolometers, each packaged with inline SQUID multiplexed readout and amplifier. In this paper we present the results of characterising individual 1280 bolometer science grade sub-arrays, both in a dedicated 50mk dilution refrigerator test facility and in the instrument installed at the JCMT.   Keywords: TES, transition edge sensor, low temperature detector, SQUID, SCUBA-2, submillimeter astronomy 1. INTRODUCTION SCUBA-2 is a new wide-field common user camera, being commissioned at the James Clerk Maxwell Telescope (JCMT) on the summit of Mauna Kea, Hawai’i. Operating simultaneously at two wavelengths (450μm and 850μm), with a 50 sq-arcmin field of view, SCUBA-2 is set to revolutionise submillimeter astronomy. SCUBA-2 will be an unprecedented imaging and survey instrument, capable of mapping large areas of sky to great depth (100 times faster than the original SCUBA instrument it replaces), addressing key questions about the origins of galaxies, stars and planets. The new instrument and re-imaging mirrors were mounted and aligned on the JCMT to schedule, a major engineering success. At 3.5 tonnes and with dimensions of 2.3m x 2m x 2m, SCUBA-2 is by far the largest instrument in the history of JCMT. The engineering challenges and achievements of SCUBA-2 in this regard are described in these proceedings by Craig et al. 1 SCUBA-2 came to Hawai’i with two commissioning grade detector sub-arrays with known flaws and limitations, but of sufficient quality to begin the instrument commissioning. Fully populated the two focal planes will each consist of four 1280 Transition Edge Sensor (TES) bolometer sub-arrays.  Early commissioning highlighted two problems that would undermine the performance of future science grade arrays in the instrument: Sensitivity to the motion of the antenna (and  1 d.bintley@jach.hawaii.edu; phone (808) 961-3756; fax (808) 961-6516 Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy V, edited by Wayne S. Holland, Jonas Zmuidzinas, Proc. of SPIE Vol. 7741, 774106 © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.856839 Proc. of SPIE Vol. 7741  774106-1 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms hence the instrument) rotating in azimuth and the 450µm detectors saturated (went normal) on the sky. Both problems have been successfully addressed. Firstly the magnetic shielding on the focal plane units was improved by using a custom single piece welded niobium shield, rather than a number of overlapping niobium panels. Secondly the optical loading on the 450μm focal plane was reduced by narrowing the 450μm pass band filter profile and by eliminating the potential for reflections from the inside of the focal plane covers (done for both wavelengths). In early 2009, the first pair (one of each wavelength) of science quality sub-arrays were completed and tested in a dedicated dilution fridge facility at Cardiff University, UK, before being installed in SCUBA-2 at JCMT. On sky commissioning and a short science observing campaign have followed. The first science results are very promising and will soon be published. The remaining six sub-arrays have now been completed and will be installed in the instrument later this summer (July 2010). The original pair of science grade sub-arrays have also been upgraded with new cold electronics modules (CEM), to correct an adverse thermal loading from 1K, discussed below. In this paper we focus on the dark testing and characterisation of the first pair of science grade sub-array modules both in the Cardiff test facility and installed in SCUBA-2 on the JCMT. Elsewhere in these proceedings Holland et al. 2 provide a full update on the SCUBA-2 project, including the first astronomical results, while Dempsey et al. 3 discuss the on-sky calibration of the instrument. 2. SCUBA-2 DETECTORS 2.1 Sub-array modules Individual sub-arrays are packaged as standalone low temperature modules, connected to their own dedicated MCE (Multi Channel Electronics; room temperature control and readout electronics). This modular approach allows for flexibility. We have been able to test and commission the instrument with various configurations of prototype sub-arrays installed, while detector and MUX development, testing and fabrication have continued in parallel; on occasion with simultaneous cold testing in Edinburgh, Waterloo and Cardiff.  Figure 1. SCUBA-2 science grade sub-array module in a handling jig, ready for unfolding.  The detector array and SQUID multiplexer are fabricated separately on two (actually three, as the detector wafer is two wafers bonded together to provide mechanical support during processing) silicon wafers before joining or hybridising with indium bump bonds.4 The bump bonds provide both thermal and electrical contact between the two layers. The upper detector wafer contains 1280 DC coupled molybdenum-copper TES detectors, in a 40 row, 32 column format. A silicon nitride membrane provides a weak thermal link to the thermal bath. Proc. of SPIE Vol. 7741  774106-2 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms Each detector pixel includes a resistive heater used to compensate for changes in optical power as the sky background changes, enabling detectors to be operated at a chosen bias point for a wide range of sky powers. The heaters play a fundamental role in the operation of the instrument while observing: • Detectors are individually calibrated by measuring their responsivity using a ‘fast heater ramp’, (a triangle wave on the heater of a few pW, peak-peak) at the start of and if we choose, periodically during every observation. • The optical power from the sky is directly measured by using heater tracking. A servo loop on the heater is run to keep detector output constant while opening and closing the cold shutter from dark to the sky. Periodic heater tracking transfers the slow changes in sky power to the heater setting, keeping each detector close to the original setup operating point in the superconducting transition. The incorporation of the heater into the design of the detector device and use of the heater while observing is one of the innovative features of SCUBA-2. The 450μm and 850μm detectors have a similar physical design for ease of manufacture, (the 850μm focal plane fully samples an area of sky, whereas the 450μm focal plane is under-sampled by a factor of 2). The transition temperature of the 450μm TES is made higher, to allow for greater power handling and a relaxed NEP requirement. The functional requirement at both wavelengths is that the instrument is sky background noise limited on the telescope. The lower wafer is a NIST designed and fabricated time division SQUID multiplexer (MUX) with 32 channels, each with 41 rows. The 41st row is a ‘dark row’ without any TES element. The dark row SQUID output is available to the data reduction pipeline, to remove excess noise particularly at low frequencies and magnetic pickup. It is the SQUID MUX which makes large-scale TES arrays such as SCUBA-2 practical, by vastly reducing the wire count between the detectors and the room temperature electronics. The operation and design of the time division multiplexer for SCUBA-2 is described by de Korte et al.5   Figure 2.  The circuit diagram and schematic drawing of a single TES/MUX element. The electrical and thermal contacts between the detector wafer and the SQUID MUX are made by cold welded indium bump bonds.  On the MUX wafer a first stage SQUID (SQ1) sits below each TES detector element. The coupling between the TES and the SQ1 is by magnetic flux. Current flowing through the TES generates a magnetic field at the SQ1 via an input transformer on the MUX wafer (see Figure 2).  Each channel or column of 41 SQ1s is then coupled by a summing coil to the second stage SQUID (SQ2). The signals in each channel are amplified by a 100 SQUID Series Array (SSA) located on the 1K PCB of the cold electronics module (CEM). The SCUBA-2 TES detectors are operated in an approximate voltage biased mode, using a small 5mΩ  shunt resistor located on the MUX wafer. The operation of Transition Edge Sensors in this fashion is described by Irwin6 and Irwin and Hilton7, where they summarise the TES small signal theory, applicable to the SCUBA-2 detectors. The advantage of voltage biasing the TES, is that negative electro-thermal feedback (ETF) stabilises the detector against thermal runaway. An increase in background power warms the device and causes an increase in resistance. This in turn causes the detector current to decrease, cooling the TES. Strong ETF essentially keeps the temperature of the TES constant, while providing a simple and direct relation between any applied power (optical or heater) and the current Proc. of SPIE Vol. 7741  774106-3 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms flowing through the device. Negative feedback also makes the detector self-biasing in terms of temperature in the transition. This is important for large arrays of TES such as with SCUBA-2. If a variation of transition temperatures and transition widths exists across the array and they do not all overlap, the Joule power dissipation in each TES will cause it to self heat to within its respective superconducting transition. Negative feedback can also increase the bandwidth of electrical systems including TES. Changes in the incident power are compensated for by changes in the bias current power on timescales shorter than the thermal time constant of the detector. The current flowing through each TES is measured by its SQ1. The output of a SQUID is periodic with magnetic flux from the input coil; the periodicity being given by a flux quantum. There is no unique output for a given detector current. Therefore we use the first stage SQUID as a null detector. Feedback current is applied by the room temperature electronics (the MCE) to the SQ1 feedback coil to null the field from the TES detector current in the input coil. By applying a flux locked loop (FLL), the applied feedback current is proportional to the current flowing through the TES. We refer to measurements using the flux locked loop as ‘closed loop’ measurements. In open loop measurements we take the output directly from the SQ1, with no feedback applied. The dynamic range of the detector feedback circuit is limited by the available first stage SQUID feedback current and the mutual inductance of the SQ1 input coil. These parameters have been carefully chosen to meet the stringent noise requirements of the instrument. However using the MCE we have been able to increase the dynamic range by flux jumping. The SQUID response is periodic and the period is measured as part of the sub-array set-up. It is possible for the MCE to jump or slip multiple flux quanta, while keeping count of the number jumped and remain locked in the FLL. The dynamic range is increased seamlessly, towards the physical limits of the TES power handling. With too much applied power (optical, thermal, bias) the TES becomes normal and ceases to work as a bolometer, too little and the TES becomes superconducting, to the same effect. 2.2 Upgrading to Science Grade Arrays The manufacture of the SCUBA-2 sub-array is a long process with many steps and testing at each stage. Prior to hybridisation, MUX wafers, which are by far the most complex of the component parts, are individually characterised in a dedicated 4K facility at the University of Waterloo.  Testing at this stage allows minor fabrication faults to be identified and corrected. The expected detector yield, based on the uniformity of the first stage SQUID properties is calculated for each MUX wafer.   Figure 3. The results of SQ1 Icmax measurements, done in a dedicated 4K facility, at the University of Waterloo: (Left) 850µm science grade MUX: (Right) 450µm MUX. The time based multiplexing scheme used requires a common SQ1 bias applied per row, together with a single value of SQ2 feedback per column. When setting up of the sub-arrays, these two parameters are optimised to maximise the number of working detectors. By using the same optimisation algorithm on the Waterloo 4K screening data, we can predict the expected detector yield for a particular MUX. For the latest generation of multiplexer yields exceed 95%. An example of the MUX screening results are given in Figure 3: We show the SQ1 Icmax values, which relate to the SQUID bias currents that give maximum modulation of the V-φ  characteristic, for the MUX wafers used for the first science Proc. of SPIE Vol. 7741  774106-4 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms grade arrays. with a single The detector and associate by adjusting mk temperatu The bump bo cleaning step present, chan Unfortunately This was attr edges of the producing a but only after Following h standing on a and mechani electrical che The design o between the niobium trac thermally att ceramic boar Figure 4. 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The new w  temperature g g the original  dilution refri e JCMT. The ys. The magne g the detector  instead a cryo t source fast g. The install hat there is a ed for optical  is instructive the pulse tube pport is bolted SCUBA-2 focal gth in the cent ardiff system  base temperat the applicatio heat leak but wai’i. s changed to p ire woven int roup in Floren pair). gerator facilit final batch of tic shielding i array. The test genic black b switching SPI ed filters have  liquid He bat testing. It has  to compare s, shutter mot  directly to the m plane unit (FPU re of the . But as ure was n of GE  did not hosphor o ribbon ce. This y at the detector s similar  bed has ody fills RE type  similar h and a no IVC, detector or, light ixing ) at Proc. of SPIE Vol. 7741  774106-6 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms Previous tests on prototype arrays in the Cardiff test bed have been reported by Woodcraft et al.8 The most significant recent progress in testing sub-arrays has been due to the advances in the use of the MCE. Rather than just characterising individual detectors, we are able to characterise the whole array. This is important because there are many compromises that have to be made to setup an array into the best state for observing. Not every detector can be biased into the optimum part of the transition. Compromises are also made with SQUID setup, which directly impacts the performance of individual bolometers. In previous papers9 we have described the software used for the automatic SQUID setup process. We have successfully automated the process to optimally adjust the 3000 or so parameters that control the MUX operation for the whole sub- array in less than 3 minutes. A faster setup, which only adjusts the SSA and SQ2 feedback currents and optimises the SQ1 bias, is used during observing on the telescope. The setup software continues to evolve as we acquire more experience. The quality of the SQUID setup directly affects the performance of the detectors. We observed lower noise from the system with the SSA and SQ2 biased to give maximum V-   modulation. For the SQ1, we aim to use the minimum bias after optimisation, which has each SQ1 in a row fully on (which tends to coincide with maximum modulation). The bandwidth of the MUX with the SQUIDs biased in this way has been measured using the MCE in the 50MHz fast mode and it is found to be sufficient. The setup procedure itself can be run from the command line or within a script.  Scripts can manipulate TES bias or TES heater, taking measurements after each change to simulate applying signals to either line. The instrument fridge temperature is also controllable from the command line or within a script, as is the cold shutter position and external blackbody source. The majority of the measurements to characterise a TES array have been automated in this way. 3.1 Array Characterisation, Basic Tools Using the MCE10, the ability to automate measurement of the TES array is limited mainly by the imagination of the user. At the heart of the MCE are a number of FPGAs. If a particular measurement is not possible with the standard MCE firmware, the FPGA can be reprogrammed to make it so.  One example of the MCE flexibility is a fast mode that allows measurements to be made at the 50MHz MCE clock speed. Normally this would not be possible due to limits in the maximum data throughput out of the MCE. In fast mode the MCE stores the measurements, which are then read out of the buffer using the standard MCE command. With fast mode it is possible to investigate the MUX switching and settling times to optimise the multiplexing frame rate and other FLL timing parameters. Another example is an MCE generated heater or TES bias square wave function, used to measure detector time constants.  However the most useful and commonly used measurement scripts are quite simple:  • ramp_bias: This is a script which generates a small triangle wave on the TES bias or heater. • ramp_bias_vi: This script ramps the TES bias over a (user selected) much larger range, to generate IV characteristics. Usually the ramp is from the normal state into the transition. • mceframetest: Collects a time stream of  SQ1 feedback current (or direct output if open loop) from all detectors in the array at a selected frame rate (up to  a few kHz, with the firmware version we use). By combining a measurement of responsivity (ramp_bias using the heater) with a current noise measurement, the NEP of each detector is measured. A slightly more sophisticated script is used to measure each detector’s dynamic thermal conductance (G=dP/dT) and superconducting transition temperature, using an open loop, ramping heater method.  The script simultaneously measures the heater power required to put each individual detector at the top or normal end of the superconducting transition with minimal TES bias applied. This is repeated for a range of different temperatures.  We operate the MCE in two modes; engineering mode for setup and array characterisation and science mode for observing. However the distinction between the modes is reducing, as many of the tasks that previously required engineering mode, can now be done in science mode. The fundamental difference between the modes is that in science mode the MCEs are fully integrated into the telescope control and observing system. The output from the multiple MCEs φ Proc. of SPIE Vol. 7741  774106-7 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms are controlled by an external sync box which supplies a clock signal and requests data frames at 200Hz. The data is subsequently packed into the standard starlink NDF format, which contain extensive header information for pipeline processing. In engineering mode, multiple MCE can also be run from the external sync box clock, however the control of the MCE is independent of the telescope system and data is saved in a simple text based data frame format with headers added by the control scripts.  The clock rate of the MCE (from internal or external clock) is 50MHz. We currently operate with a multiplexing frame rate of 9.53 kHz. That is we multiplex 41 rows of SQ1s and stay on each row for 128 50MHz clock cycles. After a settling period, a calculated SQ1 feedback is applied based on the previous frame measurement and then after a further waiting period, the output sampled and co-added for a further number of clock cycles. The SQ1 bias for that row is then turned off and the next energised and so on. The MCE has various data modes. We most commonly return filtered SQ1 feedback. In filtered mode, the MCE applies a digital 4-pole Butterworth low-pass filter to the SQ1 feedback for output, matched to the 200Hz science mode data output frame rate. The MCE filter response can be seen in figure 13.  Using the four scripts described above, the properties of individual TES detectors or the whole sub-array can be mapped out for different TES bias, heater and temperature setting. There are many interlinked properties that determine the successful operation of a TES array. However for the operation of SCUBA-2 on the telescope, the key performance figures are the detector NEP and NEFD (noise equivalent flux density). These properties have some distribution for the detectors in each focal plane. The modal value and distribution of the NEP and NEFD directly affect the mapping speed of the instrument. 3.2 Additional heat load from the science grade sub-arrays The 850µm science grade sub-array was cooled down first in the Cardiff system. The fridge base temperature was much higher than expected. We estimated that the sub-array was contributing an additional 36µW heat load on the mixing chamber, compared to previous prototype sub-arrays modules. Thermal modelling indicated that a thick layer of GE varnish used to insulate the bare niobium-titanium wires in the thermal breaks was the likely cause. The 450µm science grade had a thinner layer of GE varnish. With the 450µm sub-array in the system, the fridge base temperature was close to 50mk but still elevated. Following testing in Cardiff the 850µm CEM was changed to match the 450µm. Installed in SCUBA-2, the instrument base temperature is similarly elevated and the MUX temperatures of both science grade sub- arrays are 15mk warmer than the installed commissioning grade arrays. The higher MUX temperature has a significant affect on performance, particularly for the 850µm sub-array. 3.3 850µm Tc Flattening  Figure 6. The results of Tc flattening on the 850µm sub-array; (Left) no flattening, (right) with flattening. The SQ1 bias current is used to selectively apply heat to the detectors, to smooth out the Tc variation. Proc. of SPIE Vol. 7741  774106-8 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms The science grade 850µm detectors have a large spread in measured Tc, perhaps exaggerated by the excess heat leak from 1K causing uneven heating of the MUX wafer. This is a significant factor for setting up this sub-array. To smooth out the effect of the variation in Tc, a novel technique of ‘Tc flattening’ has been developed. By applying a higher SQ1 bias on selected number of rows for a few clock cycles at the start of the 128 cycles each row is on, the SQ1 are used as heaters. Tc flattening is row based, so it cannot correct for a Tc variation across the columns. For the 850 µm we are able to recover 10 or so rows (although not every column in the row), which would not otherwise have been in the transition, with no apparent adverse effects. The results of Tc flattening are shown in Figure 6. 3.4 Science grade sub-array detector characteristics and performance We describe the measured characteristics of typical science grade 450 and 850µm TES detectors. Table 1. Measured characteristics for science grade arrays.  450µm [r16c3] 850µm [r10c20] Normal State Resistance 59.7mΩ  28.2mΩ Total Power (Heater + IV) 274pW 43.2pW Transition Temperature 194.8mk 135mk Thermal conductance G (= dP/dT) 6.0nW/K 1.8nW/K Phonon limited NEP 1.12x10-16 W/√Hz 4.2x10-17 W/√Hz   Figure 7.  Measured transition temperatures (left) and thermal conductance (right), for the 450µm and 850µm science grade sub- arrays. 450µm 850µm Proc. of SPIE Vol. 7741  774106-9 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms Transition temperatures and thermal conductance are shown in Figure 7. From these measurements the intrinsic phonon noise limited 2NEP 4 BK T G= , can be estimated (assuming a 1Hz bandwidth). The phonon noise limited NEP for the 450µm detectors are shown in Figure 8, together with a comparison of the phonon noise limited NEP and NEP measured from the detector current noise in the dark for the 850µm array.  Figure 8. (Left) Phonon noise limited NEP for the 450um science grade detectors. (Right) Histogram showing the 850µm phonon noise limited NEP (red) and a typical measured (from the current noise) NEP (blue) at the start of Science Observing. The insert shows the 850µm measured NEP detector map.  The 450µm science grade sub-array achieves the goal to have the intrinsic detector NEP at least a factor of two less than the background from the sky and telescope: 992 detectors (78%) have measured NEP less than 1.2e-16 W/rtHz. For the 850µm science grade sub-array, 703 detectors (55%) meet the more stringent requirements. However in practice (see Figure 8) the variation of Tc (particularly for the 850), compromises in the SQUID setup and external sources of noise picked up by the TES influence the array performance tremendously.   Figure 9. Current-voltage (IV) characteristics for a typical 850µm detector, at two heater powers are plotted. (Right) IV power is plotted against applied detector bias for a range of heater settings.  IV characteristics for a typical 850µmTES at different heater powers are shown in figure 9. From the IV curves and by assuming that the shunt resistance is 5mΩ  (the design value), the TES resistance and IV power in the superconducting 850µm 450µm Proc. of SPIE Vol. 7741  774106-10 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms transition can heater resista heater setting The normal s (see Fig 10), intrinsic NEP is to narrow of the variat transition wit the detectors thermal feedb  Figure 10. The The TES resist  The small si responsivity   Figure 11. NE solid red line i For a fixed h the bias and grade array. voltage over  be calculated nce per detect s. tate resistance compared to th  of the scienc the width of th ion in transiti h a single app  into instabilit ack.  electrical resist ance is constant gnal theory fo in the transitio P (blue) and cur s a fit to the curr eater power w the electrical N We observe s the bias voltag Increas .  An (almost) or and any var  of the 850µm e commission e grade 850 ar e transition in on temperatur lied bias and h y, depending ance of an 850µ  in the normal s r voltage bias n: s = -1/V. rent responsivity ent responsivity e measured th EP. Figure 1 imilar results e range < 2.3µ ing Heater  constant pow iation across th  detectors on ing grade 850 ray are both to  terms of appl es across the eater setting. T upon the effe m detector is pl tate. (Right) A s ed TES predic  (red) plotted ag , for bias voltag e relation betw 0 illustrates th with the scien V, agrees wel er plateau in t e array can be the science gr µm the 450µm  specification ied detector b  array and m he reduced d ctive time con otted against app imilar plot is sh t a simple rel ainst the applie es < 2.3µV. een the TES ese relations f ce grade array l with theory. he transition is  calculated fro ade array are l  science grade . The impact o ias for each de akes it harder etector resistan stant of the T lied detector bi own the commis ation between  d TES bias volt  bias voltage a or a typical de s.  The linear  observed in F m IV and pow ess than 40%  array. The tot f the reduced n tector. This co  to setup the ces values ma ES and the s as current, for a sioning grade 8  the bias volta age, with the he nd the current tector on the 8  fit of the res igure 9. The e er curves for of the expecte al power hand ormal state re mpounds the whole sub-ar ybe also push trength of any  series of heater 50µm sub-array ge and the dc ater power const  responsivity 50µm commi ponsivity aga ffective different d values ling and sistance problem ray into some of  electro  powers. .  current ant. The and also ssioning inst bias Proc. of SPIE Vol. 7741  774106-11 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms The measured NEP (from the mean detector current noise between 2 to 10Hz) starts to increase (degrade) as the responsivity falls below 1x106 A/W. At lower bias voltages the detector became unstable and above 2.3µV the detector is becoming mostly normal (ignoring hysteresis).  One explanation for the rise in NEP as the responsivity reduces is the increasing dominance of noise from the SQUID multiplexer, which is higher on the commissioning grade array during this measurement.    Figure 12. Current responsivity and NEP are plotted against heater power, for a fixed detector bias. Two detector biases are shown. The solid lines are fits to the data. (The measurements are for a typical detector on the 850µm science grade sub-array). For a fixed detector bias and varying the heater, we found that the responsivity increases with reduced heater power. The NEP is broadly constant (once the responsivity is > 1x106A/W) to lower heater power, despite the increasing responsivity. For the science grade 850µm detector (see Figure 12), a lower detector bias resulted in a higher responsivity, but also a lower constant NEP. As the heater is reduced further the responsivity suddenly collapses as the TES goes superconducting. One aspect that is worth noting from this figure:  We show that the responsivity varies with power loading on the TES. This will include optical power. Therefore observing bright calibrator sources for example, such as Mars, may introduce a significant nonlinear response; similarly if the varying sky background is not corrected sufficiently or often enough with the heater, the responsivity may change during an observation. 3.5 Setup for optical testing and on-sky calibration To optimise the arrays for on-sky calibration and science observing the TES bias and heater settings are chosen to maximise the number of detectors that meet or exceed the NEP requirements. However out of spec detectors also contribute to the final maps. Therefore a more sophisticated optimisation is to weight all the working (i.e. in transition) detectors based on their NEP and optimise the TES bias and heater to maximise the mapping speed of the instrument. Data from unstable or variable detectors are thrown out by the data reduction pipeline but it was found best to turn these detectors off in advance via the setup, as flux jumps and out of lock elements with rapidly ramping SQ1 feedback can cause steps and increased noise on adjacent detectors. The optimised array setups for observing typically return 800 – 900 working (as determined by a linear response to a ramped signal on the heater, used to calibrate or flatfield) detectors at each wavelength with a spread or distribution of NEP as shown Figure 9 for the 850µm sub-array. The planned upgrade to the CEM is expected to increase the number and quality of working TES, particularly on the 850µm array. The effect of temperature oscillations in the dilution fridge on the output of the detectors is also expected to diminish. The 1K box is cooled directly by the dilution fridge still. The fridge oscillations are largest in the still and therefore are adversely coupled to the detector arrays via the CEM. Proc. of SPIE Vol. 7741  774106-12 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms  The low freq making flat m fridge oscilla pulse tube co The frequenc low frequenc frequency sp  Figure 13. (Le (Right) With th characteristics   Figure 14. Det second observ  A short secti observation h ‘spikes’ in th source. One extended sou uency oscillat aps. In Figur tion has a per olers (1.5Hz p y spectrums a y pickup (from ectrum constra ft) A typical tim e detector in th . ector current for ation. The ampli on of a quick ighlights the e e detector cur of the challen rces. As we sh ions from the e 13 we show iod of approxi lus higher harm re generally fl  the fridge) a in the observin e stream and sp e normal state a  a single 850µm tudes of the sign scan observat ffect of the fr rent. For muc ges for the d ow in referenc fridge as seen  a typical time mately 30 sec onics). at from about nd 1/f from th g modes for t ectrum from an nd the output sa  TES, scanning al are in units o ion of Mars, o idge oscillatio h less bright s ata reduction e [2]; we are m by the detector  stream and s onds. Also vi 1Hz until the r e MUX and d he instrument.  850µm detector mpled at a highe  over Mars. (Rig f power (pW). utput from a s ns on the scien ources the frid is to remove aking progres s, present a ch pectrum for an sible in the sp oll off due to etectors, toget   in the dark, sho r frame rate (1 k  ht) The resultan ingle TES at ce data. The m ge temperatur this backgroun s in this respe allenge to the  850µm TES ectrum is a th the MCE filter her with any d wing periodic fr Hz), to show th t scan map of M 850µm, is sho ars signals a e pickup dwar d while pres ct. data reduction in the dark. T ermal pickup f  (70Hz). How iscrete featur idge oscillation e MCE filter ars at 850µm, f wn in Figure re the large do fs the signal f erving flux fro  team in he large rom the ever the es in the s. or a 60 14. This wnward rom the m faint  Proc. of SPIE Vol. 7741  774106-13 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms 4. CONCLUSION We have described the characterisation of the first science grade arrays for SCUBA-2. The intrinsic properties of the TES arrays at both wavelengths achieve the design requirements and goals for the instrument, with much higher detector ‘pixel’ yields than we achieved on prototype or commissioning grade arrays. However the performance of the science grade arrays in the instrument has been hampered by a number of issues which are principally thermal in nature. We expect an improvement in performance with the new cold electronics modules that will be installed as part of the array upgrade this summer. The optical performance of the detector arrays and the results of on-sky calibration are discussed separately.  REFERENCES [1] Craig, S. et al., “SCUBA-2: engineering and commissioning challenges of the world largest sub-mm instrument at the JCMT”, Proc. SPIE 7741, (2010). [2] Holland, W. S. et al., “SCUBA-2: first results and on-sky performance”, Proc. SPIE 7741, (2010). [3] Dempsey, J. T. et al., “Extinction correction and on-sky calibration of SCUBA-2”, Proc. SPIE 7741, (2010). [4] Audley, M. D., Duncan, W. D., Holland, W. S. et al., “Fabrication of the SCUBA-2 detector arrays”, Nuclear Instruments and Methods in Physics Research A 520, 483–486 (2004). [5] deKorte, P.A.J., Beyer, J., Deiker, S., Hilton, G.C., Irwin, K.D., et al., “Time-division superconducting quantum interference device multiplexer for transition-edge sensors”, Rev. Sci. Instrum. 74, 3807-3815 (2003) [6] Irwin. K.D., “An application of electrothermal feedback for high resolution cryogenic particle detection", Appl. Phys. Lett., 66(15), 1998-2000 (1995). [7] Irwin K. D. and Hilton G. C., [Cryogenic Particle Detection], C. Enss (Ed.) Topics Appl. Phys. 99, Springer-Verlag Berlin Heidelberg, 63-149 (2005). [8]  Woodcraft, A. L., Ade, P.A.R., Bintley, D. et al., “Electrical and optical measurements on the first SCUBA-2 prototype 1280 pixel submillimetre superconducting bolometer array”, Review of Scientific Instruments, 78, 024502 (2007). [9] Gao, X., Kelly, D. et al., “Automatic setup of SCUBA-2 detector arrays”, Proc. SPIE, Vol. 7020, 702025 (2008) [10] Battistelli, E. S., Amiri, M., Burger, B. et al., “Functional Description of Read-out Electronics for Time-Domain Multiplexed Bolometers for Millimeter and Sub-millimeter Astronomy”, Journal of Low Temperature Physics, 151, 908- 914 (2008).  Proc. of SPIE Vol. 7741  774106-14 Downloaded from SPIE Digital Library on 13 Sep 2011 to Terms of Use:  http://spiedl.org/terms


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