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Mobile ion contamination of MOS structures investigated by computer-controlled laser scanning internal photoemission and self-healing breakdown techniques Tsoi, Hak-Yam


With the present trend of silicon IC's towards very large scale integration, premature device breakdown is increasingly important. Contamination of MOS devices by mobile sodium ions is of particular interest as a source of device failure. The effects of contamination of MOS structures by mobile sodium ions were investigated by using a combination of two techniques: computer-controlled laser scanning internal photoemission and self-healing breakdown measurements. The internal photoemission measurements involved focussing UV radiation from a He-Cd laser on to a spot a few microns in diameter. The spot was then scanned across the sample surface and a map of the internal photoemission current was obtained. The scanning internal photoemission measurements were made using a PDP-8E based minicomputer-controlled set up. A special technique for focussing the UV laser beam and for measuring the light spot size with the sample in situ was developed. Reflectivity imaging was used to position the sample and to identify certain types of defects on the sample surface. The self-healing breakdown tests were made using a specially built electronic device. This allowed the operator to breakdown the sample a preset number of times in order to avoid the occurrence of both weak spot and intrinsic breakdowns on the same sample. The photocurrent is low in uncontaminated samples and also in contaminated samples before application of positive gate voltage. However, if a positive bias-temperature stress treatment is applied to the contaminated samples, very pronounced photocurrent peaks were observed. The positions on the surface at which breakdowns occurred were mostly found to coincide with strong peaks in the internal photoemission current. The internal photoemission process in MOS structures was modelled. The dependence of the quantum yield on the photon energy, on the semiconductor doping, on the semiconductor surface band bending and on the applied field were calculated. The model predicted that a large downward band bending at the surface of a heavily doped p-type semiconductor would shift the photoemission threshold to a lower value. The photoemission current imaging and the reflectivity imaging of MOS structures and defects sites which are presented show that the two techniques complement each other in revealing defect sites. The transfer of Na⁺ from aqueous solutions to the oxide before and after aluminum gate deposition was examined. It was found that the Na⁺ which was transferred after the aluminum gate deposition always distributed in a non-uniform fashion. For SiO₂ films prepared in the presence of a small amount of HCL, mobile ion neutralization at the Si-SiO₂ interface was observed. For samples which were exposed to room ambient for a prolonged period, photocurrent peaks were observed along the periphery of the aluminum gate after the application of a positive bias-temperature stress treatment. This indicated that mobile ions could diffuse into the sample along the periphery of the aluminum gate. Nucleation of breakdowns was again observed corresponding to the photocurrent peaks. This indicates that any storage of MOS devices before encapsulation is best done in an environment which does not cause sodium ion contamination, since the presence of the gates does not provide total protection.

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