- Email: [email protected]
Improvement in breakdown characteristics with multiguard structures in microstrip silicon detectors for CMS
Nuclear Instruments and Methods in Physics Research A 461 (2001) 204–206
Improvement in breakdown characteristics with multiguard structures in microstrip silicon detectors for CMS N. Bacchettaa, D. Biselloa,b, A. Candeloria,b, M. Da Roldb,1, M. Descovichb,2, A. Kaminskia,*, A. Messineoc,d, F. Rizzod, G. Verzellesie,3 a
INFN, Sezione di Padova, Via Marzolo 8, 35131 Padova, Italy Dipartimento di Fisica dell’Universita" di Padova, Padova, Italy c Dipartimento di Fisica dell’Universita" di Pisa, Pisa, Italy d INFN, Sezione di Pisa, Italy e Dipartimento di Ingegneria dei Materiali dell’Universita" di Trento, Povo (TN), Italy b
Abstract To obtain full charge collection the CMS silicon detectors should be able to operate at high bias voltage. We observed that multiguard structures enhance the breakdown performance of the devices on several tens of baby detectors designed for CMS. The beneficial effects of the multiguard structures still remains after the strong neutron irradiation performed to simulate the operation at the LHC. # 2001 Elsevier Science B.V. All rights reserved. PACS: 73.40; 61.80.Hg; 61.82.F; 29.40.g Keywords: p–n junctions; Neutron radiation effects; Semiconductors; Tracking; Position sensitive detectors
For a considerable number of years the full depletion voltage of the silicon microstrip detectors of the CMS tracker will largely exceed (by hundreds of volts) the standard values of few tens of volts. Moreover, the innermost tracker layers will use low-resistivity silicon detectors, which have initial depletion voltage 200 V. Thus, high electric fields will be applied to the junction which
*Corresponding author. Tel.: +39-0498277030; fax: +390498277237. E-mail address: [email protected] (A. Kaminski). 1 Now at IMEC vwz, Leuven, Belgium. 2 Now at University of Liverpool, Physics Department, Oliver Lodge Laboratory, Liverpool, UK. 3 Now at Dipartimento di Scienze dell’Ingegneria dell’Universit"a di Modena, Modena, Italy.
can lead to impact ionisation and avalanche multiplication, i.e. to the breakdown phenomenon. The implications of the breakdown are an abrupt leakage current increase and a noise performance degradation. The junction curvature limits the breakdown voltage below the ideal case of the abrupt junction diode. Furthermore, the presence of an electron accumulation layer at the Si-SiO2 interface influences the breakdown phenomenon. The electrons at the interface prevent the silicon surface from getting depleted, leading to a spacecharge region narrowing. This makes the electric field higher at the surface and increases the probability of impact ionisation. The multiguard structures enable to modify the potential distribution in the proximity of the surface leading to a reduction of the overall
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 2 0 7 - 9
N. Bacchetta et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 204–206
breakdown voltage. They consist of a series of floating guard rings (p+ and/or n+ implants) around the main junction [1]. The optimum layout, number, type, width and spacing of the implants, were obtained by means of simulations [2] whose results were confirmed by experimental data. We found that the best layout consists of 10 p+ implants, 15 mm wide and with intra-guards increasing from 15 mm, near the main junction, to 30 mm close to the edge [3] (see Fig. 1) The total distance from the main junction to cutting edge is 700 mm. The multiguard performance on statistical grounds has been studied on 52 wafers produced by CSEM for CMS. Each 300 mm thick wafer contained two microstrip baby detectors, which differ only by the presence of the multiguard structure on one of them, while the second one has a singleguard ring. Some defects introduced in the wafer and/or detector fabrication modify the I–V curve to a smooth and continuous increase which can simulate a breakdown. We disentangled this behaviour
205
from a real breakdown by studying the adimensional function DI V KðI; VÞ ¼ DV I where I is the detector reverse current, V the applied bias voltage and DI=DV is the slope of the I–V curve. The breakdown voltage Vbd is then defined as the maximum value of Vbias for which KðI; VÞ5Kbd having in mind that KðI; VÞ ’ 1 corresponds to an ohmic resistor and KðI; VÞ 1 to a real avalanche behaviour. Kbd can be extracted from an analysis of the I–V characteristics of the detectors under test. In our study Kbd was taken to be equal to 4. It is worthy of note that we limited our study to the wafers where both, multiguard and singleguard detectors had Vbd > 150 V. The differences of the breakdown voltages, measured with the above criteria, for every pair of babies coming from the same wafer, are shown in Fig. 2. The average value of this distribution is ’ 127 V pointing out that the detectors with
Fig. 1. Multiguard design. The leftmost implant corresponds to the main junction.
Fig. 2. Distribution of the difference of the breakdown voltage for multiguard and singleguard detectors coming from the same wafer.
SECTION III.
206
N. Bacchetta et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 204–206
Fig. 3. Breakdown voltage distribution of multiguard and singleguard after irradiation.
MG multiguards have a Vbd value higher ðhVbd i¼ 607 VÞ than that of the singleguard detectors SG (hVbd i ¼ 477 V). An even better result, although statistically less significant, was obtained with the detectors with Vbd > 500 V, thus satisfying the CMS TDR requirements. After type inversion the detectors are expected to be less sensitive to the breakdown phenomena due to the different distribution of the electric field inside the bulk. To compare the performances after a high hadron irradiation the detectors were exposed at the Lubljiana Triga Mark III nuclear reactor up to a fluence of 2 1014 n=cm2 (in 1 MeV equivalent neutrons). This batch included the detectors with Vbd > 500 V, and ‘bad detectors’ with very low initial Vbd . I–V measurements on irradiated detectors indicate a general increase of the breakdown voltages. The multi-guarded detectors still have at least 150 V higher breakdown voltage than singleguarded ones. (Fig. 3). Our setup allowed us
to measure the I–V curve up to 970 V, so we considered Vbd ¼ 970 V also for all the detectors which have Vbd above this value. The introduction of the multiguard structures improves the yield of acceptable microstrip detectors improving their breakdown voltage. The breakdown performance of the multiguarded detectors is better than that of the singleguarded ones even after high irradiation. The authors wish to thank V. Cindro for the help and support given to them during detector irradiation.
References [1] B.J. Baliga, Modern Power Devices, Willey, New York, 1987, pp. 62–100. [2] M. Da Rold, High voltage devices for silicon detector operation in future high energy physics experiment, Ph.D. Thesis, Universit!a di Padova, 1997. [3] M. Da Rold et al., IEEE Trans. Nucl. Sci. 46 (1999) 1215.
Improvement in breakdown characteristics with multiguard structures in microstrip silicon detectors for CMS N. Bacchettaa, D. Biselloa,b, A. Candeloria,b, M. Da Roldb,1, M. Descovichb,2, A. Kaminskia,*, A. Messineoc,d, F. Rizzod, G. Verzellesie,3 a
INFN, Sezione di Padova, Via Marzolo 8, 35131 Padova, Italy Dipartimento di Fisica dell’Universita" di Padova, Padova, Italy c Dipartimento di Fisica dell’Universita" di Pisa, Pisa, Italy d INFN, Sezione di Pisa, Italy e Dipartimento di Ingegneria dei Materiali dell’Universita" di Trento, Povo (TN), Italy b
Abstract To obtain full charge collection the CMS silicon detectors should be able to operate at high bias voltage. We observed that multiguard structures enhance the breakdown performance of the devices on several tens of baby detectors designed for CMS. The beneficial effects of the multiguard structures still remains after the strong neutron irradiation performed to simulate the operation at the LHC. # 2001 Elsevier Science B.V. All rights reserved. PACS: 73.40; 61.80.Hg; 61.82.F; 29.40.g Keywords: p–n junctions; Neutron radiation effects; Semiconductors; Tracking; Position sensitive detectors
For a considerable number of years the full depletion voltage of the silicon microstrip detectors of the CMS tracker will largely exceed (by hundreds of volts) the standard values of few tens of volts. Moreover, the innermost tracker layers will use low-resistivity silicon detectors, which have initial depletion voltage 200 V. Thus, high electric fields will be applied to the junction which
*Corresponding author. Tel.: +39-0498277030; fax: +390498277237. E-mail address: [email protected] (A. Kaminski). 1 Now at IMEC vwz, Leuven, Belgium. 2 Now at University of Liverpool, Physics Department, Oliver Lodge Laboratory, Liverpool, UK. 3 Now at Dipartimento di Scienze dell’Ingegneria dell’Universit"a di Modena, Modena, Italy.
can lead to impact ionisation and avalanche multiplication, i.e. to the breakdown phenomenon. The implications of the breakdown are an abrupt leakage current increase and a noise performance degradation. The junction curvature limits the breakdown voltage below the ideal case of the abrupt junction diode. Furthermore, the presence of an electron accumulation layer at the Si-SiO2 interface influences the breakdown phenomenon. The electrons at the interface prevent the silicon surface from getting depleted, leading to a spacecharge region narrowing. This makes the electric field higher at the surface and increases the probability of impact ionisation. The multiguard structures enable to modify the potential distribution in the proximity of the surface leading to a reduction of the overall
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 2 0 7 - 9
N. Bacchetta et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 204–206
breakdown voltage. They consist of a series of floating guard rings (p+ and/or n+ implants) around the main junction [1]. The optimum layout, number, type, width and spacing of the implants, were obtained by means of simulations [2] whose results were confirmed by experimental data. We found that the best layout consists of 10 p+ implants, 15 mm wide and with intra-guards increasing from 15 mm, near the main junction, to 30 mm close to the edge [3] (see Fig. 1) The total distance from the main junction to cutting edge is 700 mm. The multiguard performance on statistical grounds has been studied on 52 wafers produced by CSEM for CMS. Each 300 mm thick wafer contained two microstrip baby detectors, which differ only by the presence of the multiguard structure on one of them, while the second one has a singleguard ring. Some defects introduced in the wafer and/or detector fabrication modify the I–V curve to a smooth and continuous increase which can simulate a breakdown. We disentangled this behaviour
205
from a real breakdown by studying the adimensional function DI V KðI; VÞ ¼ DV I where I is the detector reverse current, V the applied bias voltage and DI=DV is the slope of the I–V curve. The breakdown voltage Vbd is then defined as the maximum value of Vbias for which KðI; VÞ5Kbd having in mind that KðI; VÞ ’ 1 corresponds to an ohmic resistor and KðI; VÞ 1 to a real avalanche behaviour. Kbd can be extracted from an analysis of the I–V characteristics of the detectors under test. In our study Kbd was taken to be equal to 4. It is worthy of note that we limited our study to the wafers where both, multiguard and singleguard detectors had Vbd > 150 V. The differences of the breakdown voltages, measured with the above criteria, for every pair of babies coming from the same wafer, are shown in Fig. 2. The average value of this distribution is ’ 127 V pointing out that the detectors with
Fig. 1. Multiguard design. The leftmost implant corresponds to the main junction.
Fig. 2. Distribution of the difference of the breakdown voltage for multiguard and singleguard detectors coming from the same wafer.
SECTION III.
206
N. Bacchetta et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 204–206
Fig. 3. Breakdown voltage distribution of multiguard and singleguard after irradiation.
MG multiguards have a Vbd value higher ðhVbd i¼ 607 VÞ than that of the singleguard detectors SG (hVbd i ¼ 477 V). An even better result, although statistically less significant, was obtained with the detectors with Vbd > 500 V, thus satisfying the CMS TDR requirements. After type inversion the detectors are expected to be less sensitive to the breakdown phenomena due to the different distribution of the electric field inside the bulk. To compare the performances after a high hadron irradiation the detectors were exposed at the Lubljiana Triga Mark III nuclear reactor up to a fluence of 2 1014 n=cm2 (in 1 MeV equivalent neutrons). This batch included the detectors with Vbd > 500 V, and ‘bad detectors’ with very low initial Vbd . I–V measurements on irradiated detectors indicate a general increase of the breakdown voltages. The multi-guarded detectors still have at least 150 V higher breakdown voltage than singleguarded ones. (Fig. 3). Our setup allowed us
to measure the I–V curve up to 970 V, so we considered Vbd ¼ 970 V also for all the detectors which have Vbd above this value. The introduction of the multiguard structures improves the yield of acceptable microstrip detectors improving their breakdown voltage. The breakdown performance of the multiguarded detectors is better than that of the singleguarded ones even after high irradiation. The authors wish to thank V. Cindro for the help and support given to them during detector irradiation.
References [1] B.J. Baliga, Modern Power Devices, Willey, New York, 1987, pp. 62–100. [2] M. Da Rold, High voltage devices for silicon detector operation in future high energy physics experiment, Ph.D. Thesis, Universit!a di Padova, 1997. [3] M. Da Rold et al., IEEE Trans. Nucl. Sci. 46 (1999) 1215.