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Floating guard rings as high-voltage termination structures for radiation-tolerant silicon detectors
Nuclear Instruments and Methods in Physics Research A 409 (1998) 271—274
Floating guard rings as high-voltage termination structures for radiation-tolerant silicon detectors K.H. Wyllie* Cavendish Laboratory, University of Cambridge, HEP Group, Madingly Road, Cambridge CB3 OHE, UK
Abstract Multiple floating guard ring designs have been optimised for high-voltage operation of silicon X-ray detectors and microstrip detectors at hadron colliders. They have been processed on both single- and double-sided devices. Results are presented on their performance before and after being subjected to both ionising and non-ionising irradiation. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Silicon detectors in high-energy hadron collider experiments and in X-ray imaging applications will require high ('300 V) operating voltages. Bulk radiation damage sustained during an experiment increases the depletion voltage of silicon detectors and thick substrates are required to improve the detection efficiency of X-rays in the 60 keV regime. Such high-voltages create localised areas of high electric field which can cause avalanche breakdown and generate large, unstable leakage currents. One such critical area is the edge of a p—n junction. This paper describes the use of multiple floating guard rings as a junction termination structure for high-resistivity single- and double-sided silicon detectors. As an aid to design, two-dimensional
* Corresponding author. Present address: CERN, ECP Division, CH-1211 Geneva 23, Switzerland. Tel.: #41 22 767 5903; fax: #41 22 767 9355; e-mail: [email protected].
simulations were carried out using the software package ToSCA [1]. Avalanche breakdown is enhanced by high surface charge densities due to charge trapped within the SiO . The inclusion of floating guard implants 2 around the junction allows the lateral depletion region to punch through to these guards, considerably reducing the field crowding at the implant edge. Subsequent increases in bias voltage will prompt punchthrough to further guards. 2. Simulations and designs Simulations were carried out to optimise the geometry of the guard rings. Guard spacings and widths were varied systematically and ToSCA provided information on the resulting potential distributions and electric field strengths. Fig. 1 shows a profile plot of absolute electric field at the surface of a device. The sharp peaks correspond to the outer edges of the floating guard ring implants.
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 2 7 8 - 3
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Fig. 1. Profile plot of absolute electric field strength to a depth of 50 lm below the surface of the optimised guard structure at 300 V bias.
Structures with guard rings on both sides were also simulated, and bulk doping and oxide charge densities varied to simulate the effects of radiation. These simulations led to an optimised guard geometry described in Ref. [2]. A variety of guard designs, including the optimised geometry, was fabricated on 8 mm]8 mm test structures by Micron Semiconductor Ltd. The wafers of 64 structures were S1 1 1T 300 lm-thick n-type silicon of bulk carrier concentrations in the range 1—5]1011 cm~3. Devices were fabricated by single-sided or double-sided processing. Doublesided devices had n-implant guards mirroring the p-implants on the opposite side but with intervening p-stops.
3. Results The inclusion of guard rings led to large improvements in the breakdown voltage of singlesided structures, that is the reverse bias at which avalanche breakdown occurs. Fig. 2 shows current—voltage (I—») characteristics for structures
with an arbitrary guard ring design and the optimised design. For comparison, a range of breakdown voltages of un-guarded devices described in Ref. [3] is shown. Breakdown voltages of the optimised designs were consistently in excess of 650 V. This was improved to 900 V by overlapping the metal in DC contact with the guard implants across the oxide in the direction of the device edge. The potential of the metal is negative with respect to the underlying silicon and will thus help dispel accumulated electrons at the surface. Measurements of the guard potentials indicated that high bias voltages were dropped in a regular manner by the guard rings. No deterioration in breakdown performance was observed after the devices were irradiated by a Sr90 b-source to a saturated level of oxide charge. Measurements of the potential on double-sided devices showed that, once the device was fully depleted, the guards on both sides helped to drop the bias voltage. Double-sided devices were irradiated with neutrons past-type inversion to a fluence of 4] 1013 neutrons/cm2 (1 MeV equivalent). Depletion
K.H. Wyllie/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 271—274
Fig. 2. I—» characteristics for single-sided devices.
Fig. 3. I—» characteristics of a double-sided device after irradiation to 4]1013 neutrons/cm2.
voltages were 80 V when measured shortly after irradiation. The n-implant guards now dropped the majority of the bias volts. Fig. 3 shows the I—» characteristics from a neutron-irradiated optimised
273
Fig. 4. I—» characteristics of a double-sided device after irradiation to 1.5]1014 protons/cm2.
structure with a breakdown voltage in excess of 450 V. This performance was superior to that obtained with non-optimised structures which broke down at 200—250 V. There was also no deterioration after these optimised devices were irradiated with the b-source. Devices from further wafers were irradiated with 24 GeV/c protons past-type inversion to fluences of 1.5]1014 protons/cm2. Depletion voltages were around 170 V. Bias voltages were shared by the guards on both sides. Fig. 4 shows the I—» characterisitics of an optimised structure following proton irradiation. The breakdown voltage is around 400 V. The optimised structures again performed much better than the non-optimised devices, but those with metal overlaps in the outward direction had high currents due to the enhancement of accumulated electrons on the n-side and the creation of high-field regions at the edge of the p-stops.
4. Conclusions The breakdown voltages of silicon detectors have been substantially increased by the inclusion
II. TRACKING (SOLID STATE)
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K.H. Wyllie/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 271—274
of guard ring designs which can be easily manufactured using standard fabrication processes. Double-sided structures equipped with multiple guard rings have been heavily irradiated with neutrons, electrons and protons and have been able to withstand high bias voltages well in excess of operating points determined by depletion voltages. These designs have been incorporated on ATLAS prototype microstrip detectors [4] and on 1 mm thick X-ray detectors.
References [1] H. Gajewski et al., ToSCA User Guide, Institut fu¨r angewandte Analysis und Stochastik im Forschungsverbund, Berlin, 1992. [2] G.A. Beck et al., Radiation tolerant breakdown protection of silicon detectors multiple floating guard rings, Nucl. Instr. and Meth. A 396 (1997) 214. [3] G.A. Beck et al., Nucl. Instr. and Meth. A 373 (1996) 223. [4] C. Haber, these Proceedings (7th Pisa Meeting on Advanced Detectors, La Biodola, Isola d’Elba, Italy, 1997) Nucl. Instr. and Meth. A 409 (1998) 161.
Floating guard rings as high-voltage termination structures for radiation-tolerant silicon detectors K.H. Wyllie* Cavendish Laboratory, University of Cambridge, HEP Group, Madingly Road, Cambridge CB3 OHE, UK
Abstract Multiple floating guard ring designs have been optimised for high-voltage operation of silicon X-ray detectors and microstrip detectors at hadron colliders. They have been processed on both single- and double-sided devices. Results are presented on their performance before and after being subjected to both ionising and non-ionising irradiation. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Silicon detectors in high-energy hadron collider experiments and in X-ray imaging applications will require high ('300 V) operating voltages. Bulk radiation damage sustained during an experiment increases the depletion voltage of silicon detectors and thick substrates are required to improve the detection efficiency of X-rays in the 60 keV regime. Such high-voltages create localised areas of high electric field which can cause avalanche breakdown and generate large, unstable leakage currents. One such critical area is the edge of a p—n junction. This paper describes the use of multiple floating guard rings as a junction termination structure for high-resistivity single- and double-sided silicon detectors. As an aid to design, two-dimensional
* Corresponding author. Present address: CERN, ECP Division, CH-1211 Geneva 23, Switzerland. Tel.: #41 22 767 5903; fax: #41 22 767 9355; e-mail: [email protected].
simulations were carried out using the software package ToSCA [1]. Avalanche breakdown is enhanced by high surface charge densities due to charge trapped within the SiO . The inclusion of floating guard implants 2 around the junction allows the lateral depletion region to punch through to these guards, considerably reducing the field crowding at the implant edge. Subsequent increases in bias voltage will prompt punchthrough to further guards. 2. Simulations and designs Simulations were carried out to optimise the geometry of the guard rings. Guard spacings and widths were varied systematically and ToSCA provided information on the resulting potential distributions and electric field strengths. Fig. 1 shows a profile plot of absolute electric field at the surface of a device. The sharp peaks correspond to the outer edges of the floating guard ring implants.
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 2 7 8 - 3
II. TRACKING (SOLID STATE)
272
K.H. Wyllie/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 271—274
Fig. 1. Profile plot of absolute electric field strength to a depth of 50 lm below the surface of the optimised guard structure at 300 V bias.
Structures with guard rings on both sides were also simulated, and bulk doping and oxide charge densities varied to simulate the effects of radiation. These simulations led to an optimised guard geometry described in Ref. [2]. A variety of guard designs, including the optimised geometry, was fabricated on 8 mm]8 mm test structures by Micron Semiconductor Ltd. The wafers of 64 structures were S1 1 1T 300 lm-thick n-type silicon of bulk carrier concentrations in the range 1—5]1011 cm~3. Devices were fabricated by single-sided or double-sided processing. Doublesided devices had n-implant guards mirroring the p-implants on the opposite side but with intervening p-stops.
3. Results The inclusion of guard rings led to large improvements in the breakdown voltage of singlesided structures, that is the reverse bias at which avalanche breakdown occurs. Fig. 2 shows current—voltage (I—») characteristics for structures
with an arbitrary guard ring design and the optimised design. For comparison, a range of breakdown voltages of un-guarded devices described in Ref. [3] is shown. Breakdown voltages of the optimised designs were consistently in excess of 650 V. This was improved to 900 V by overlapping the metal in DC contact with the guard implants across the oxide in the direction of the device edge. The potential of the metal is negative with respect to the underlying silicon and will thus help dispel accumulated electrons at the surface. Measurements of the guard potentials indicated that high bias voltages were dropped in a regular manner by the guard rings. No deterioration in breakdown performance was observed after the devices were irradiated by a Sr90 b-source to a saturated level of oxide charge. Measurements of the potential on double-sided devices showed that, once the device was fully depleted, the guards on both sides helped to drop the bias voltage. Double-sided devices were irradiated with neutrons past-type inversion to a fluence of 4] 1013 neutrons/cm2 (1 MeV equivalent). Depletion
K.H. Wyllie/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 271—274
Fig. 2. I—» characteristics for single-sided devices.
Fig. 3. I—» characteristics of a double-sided device after irradiation to 4]1013 neutrons/cm2.
voltages were 80 V when measured shortly after irradiation. The n-implant guards now dropped the majority of the bias volts. Fig. 3 shows the I—» characteristics from a neutron-irradiated optimised
273
Fig. 4. I—» characteristics of a double-sided device after irradiation to 1.5]1014 protons/cm2.
structure with a breakdown voltage in excess of 450 V. This performance was superior to that obtained with non-optimised structures which broke down at 200—250 V. There was also no deterioration after these optimised devices were irradiated with the b-source. Devices from further wafers were irradiated with 24 GeV/c protons past-type inversion to fluences of 1.5]1014 protons/cm2. Depletion voltages were around 170 V. Bias voltages were shared by the guards on both sides. Fig. 4 shows the I—» characterisitics of an optimised structure following proton irradiation. The breakdown voltage is around 400 V. The optimised structures again performed much better than the non-optimised devices, but those with metal overlaps in the outward direction had high currents due to the enhancement of accumulated electrons on the n-side and the creation of high-field regions at the edge of the p-stops.
4. Conclusions The breakdown voltages of silicon detectors have been substantially increased by the inclusion
II. TRACKING (SOLID STATE)
274
K.H. Wyllie/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 271—274
of guard ring designs which can be easily manufactured using standard fabrication processes. Double-sided structures equipped with multiple guard rings have been heavily irradiated with neutrons, electrons and protons and have been able to withstand high bias voltages well in excess of operating points determined by depletion voltages. These designs have been incorporated on ATLAS prototype microstrip detectors [4] and on 1 mm thick X-ray detectors.
References [1] H. Gajewski et al., ToSCA User Guide, Institut fu¨r angewandte Analysis und Stochastik im Forschungsverbund, Berlin, 1992. [2] G.A. Beck et al., Radiation tolerant breakdown protection of silicon detectors multiple floating guard rings, Nucl. Instr. and Meth. A 396 (1997) 214. [3] G.A. Beck et al., Nucl. Instr. and Meth. A 373 (1996) 223. [4] C. Haber, these Proceedings (7th Pisa Meeting on Advanced Detectors, La Biodola, Isola d’Elba, Italy, 1997) Nucl. Instr. and Meth. A 409 (1998) 161.