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Characterization and quality control of CMS silicon microstrip sensors
Nuclear Instruments and Methods in Physics Research A 505 (2003) 144–147
Characterization and quality control of CMS silicon microstrip sensors Nicoleta Dinu*,1 INFN Sezione di Perugia, Perugia, Italy On behalf of the CMS Tracker Collaboration
Abstract This paper presents the results of quality control tests performed on single-sided, pre-production silicon microstrip detectors for the compact muon solenoid silicon strip tracker. These tests included both a global and a strip-by-strip electrical characterization of the sensors. Additionally, all sensors were optically inspected as part of the quality control process. The goal of the tests was to finalize the procedures of the CMS Quality Test Centres, to qualify the sensor production lines, and to compare measurement data provided by the manufacturers with our own. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.40.Gx Keywords: LHC; CMS; Silicon microstrip detector; Electrical characterization
1. Introduction Silicon microstrip sensors represent an important component of many systems used for particle detection in high-energy physics. The main advantages of these sensors are high detection efficiency, good spatial resolution, compactness, and very fast response time. The physics potential of their precise tracking performance has been extensively demonstrated by the LEP experiments [1,2] and by CDF [3]. *Corresponding author. Tel.: +39-075-585-2756; fax: +39075-44666. E-mail address: [email protected] (N. Dinu). 1 On leave from Institute of Space Sciences, Bucharest, Romania.
The compact muon solenoid (CMS), one of the future experiments at the CERN large hadron collider (LHC), will extensively use silicon microstrip sensors for tracking charged particles [4]. The silicon strip tracker (SST) of CMS will contain some 25,000 single-sided silicon microstrip sensors, covering a total surface area of about 210 m2. Since the sensors will be located close to the interaction point in LHC, they are expected to be heavily irradiated. In the CMS experiment, the innermost SST inner region will be subject to a fluence of 1.6 1014 1 MeV equivalent neutrons/ cm2. R&D studies performed before and after irradiation [5] have shown that sensors which satisfy precise electrical and mechanical characteristics will retain acceptable particle detection efficiency for at least 10 years of LHC running.
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)01038-6
N. Dinu / Nuclear Instruments and Methods in Physics Research A 505 (2003) 144–147
To insure the electrical and the mechanical characteristics as determined by the irradiation studies and to guarantee the quality of the sensors which will be used in the construction of the SST, CMS will both qualify the sensor production lines and monitor sensor quality throughout the production process. These steps will greatly reduce the risk of sensor damage during the LHC start-up period. The delivery of sensors for the construction of the SST has been divided in two parts: *
*
A pre-production phase, used to qualify the production line of companies and to calibrate the manufacturers’ quality criteria with that of the CMS Quality Test Centres (QTCs); The main construction phase, consisting of the majority of the detectors which will be used in the SST.
Some 330 pre-production sensors have been received by CMS and all of these have been measured by the QTCs. Results presented in this paper are based on these measurements. A description of general sensor characteristics and the quality criteria are also given.
2. General description of sensors The sensor material is FZ-grown, n-type silicon that is phosphorus doped and cut with a /1 0 0S crystal orientation. A thickness of 320 mm and a resistivity of 1.5–3.0 kO cm were chosen for the inner layers as a compromise between signal-tonoise and full depletion voltage after irradiation. The outer layers will be instrumented with 500 mm thick, 3.5–7.5 kO cm wafers. The low resistivity for the inner layers was chosen to improve the radiation hardness by increasing the fluence at which type inversion of the bulk silicon takes place. A p+ implantation is performed on the front side in order to define the strip-shaped diodes. The p+ implants are AC coupled to readout electronics through integrated capacitors made out of an arrangement of grown and deposited oxides and nitrides. This configuration minimizes the number of pinholes while at the same time allowing enough
145
capacitance to guarantee efficient charge collection. The strips are biased by a p+ implant surrounding the active area through an array of poly-silicon resistors. The choice of poly-silicon for the biasing was mainly due to its established radiation resistance. The resistor values were chosen so as to be sufficiently high to guarantee low noise and still low enough to avoid undesirable voltage drops between the strips and the bias ring. The bias ring is further surrounded by a second p+ guard ring designed to prevent the flowing of external leakage current to the active area. Surrounding the guard ring there is an n+ implantation ring at the edge of the sensor, which reduces the cut-line contribution to the leakage current.
3. Quality control of sensors The quality control of the SST silicon microstrip sensors can be divided in two categories: electrical characterization and optical inspection. Given the large number of sensors to be tested and the time required for all of the checks, the responsibility for the quality control of the sensors has been divided among four QTC’s: Karlsruhe (Germany), Vienna (Austria), Perugia (Italy) and Pisa (Italy). 3.1. Electrical characterization All the above-mentioned centers have the availability of clean rooms (purity class M5.5) with temperature and humidity control (71 C, 75% RH) and use automatic probe stations for the electrical testing. The probe stations are sufficiently flexible so that they can be used for all CMS silicon microstrip sensors regardless of sensor size and strip pitch. A PC running LabView provides control of the test station and both records and analyzes the electrical characteristics of the sensors. In order to have comparable output, all of the QTC’s use the same testing procedures. To verify the uniformity of the results, calibration sensors were measured in turn by each center. Results from this study indicated good agreement between the different centers.
146
N. Dinu / Nuclear Instruments and Methods in Physics Research A 505 (2003) 144–147
Consequently, the combined results from all four QTC’s are presented here. The electrical characterization of SST sensors is based on global and strip-by-strip tests. The global tests consist of measurements of current as a function of voltage (IV) and capacitance as a function of voltage (CV). The IV measurement was performed to determine the total leakage current of the sensor and to study the breakdown performance. A Keithley 237 (6517) high voltage supply was used to apply the bias voltage between the bias ring and the back plane of the sensor. Over the course of LHC running, radiation damage will increase the effective doping concentration in the sensors thus requiring increasingly higher bias voltages in order to maintain satisfactory charge collection efficiency. The total leakage current was measured from 0 to 550 V in 5 V steps and was required to be less than 20 mA at 450 V. The slope of the total leakage current, DIleak =DV ; was also required to be less than 100 nA/V in the range 450–550 V. This latter condition is intended to assure that the sensors will not experience breakdown and will otherwise be stable for bias levels up to 500 V. Pre-production detectors consisting of 170 rectangular-shaped sensors, with strip pitches 183 and 120 mm, and 160 trapezoidal-shaped sensors, with strip pitches 163–85 and 186–205 mm, were delivered by two companies and tested at the QTC’s. Fig. 1 shows a histogram of the total leakage current at a bias voltage of 450 V for these
sensors. Out of the 330 sensors, 300 satisfied the IV test acceptance criteria. CV measurement was performed to determine the sensor depletion voltage and the total capacitance at full depletion. Capacitance was measured at 1 kHz using an Agilent 4284A (HP 4274A) LCR meter. The full depletion voltage, Vdepletion ; was extracted in the standard way from the plot of 2 1=Cbulk versus Vbias : Total capacitance was measured in 5 V steps from 0 to 500 V. The full depletion voltage for the sensors was required to be less than 300 V. The distribution of depletion voltages for the CMS preproduction sensors is shown in Fig. 2. All preproduction sensors satisfied the CV test acceptance criteria. Strip-by-strip tests were used to identify defective strips and to determine the extent of strip-tostrip variations within the sensors. These tests consisted of measurements of individual strip leakage currents, poly-silicon resistances, coupling capacitances, and leakage current through the dielectric layer (dielectric current). All of these tests were performed at a bias voltage of 400 V. The leakage current of each strip was measured in order to identify potentially noisy strips and was required to be less than 100 nA. The poly-silicon resistance was also measured for each strip and was required to be within the range 1.0–2.0 MO. For each sensor, all poly-silicon resistor values were further required to lie within 70.3 MO of the average value. Dielectric current was measured with 10 V applied across the coupling capacitor of
Fig. 1. A histogram plot of total leakage current at 450 V for CMS pre-production sensors.
Fig. 2. Depletion voltage for CMS pre-production silicon sensors.
N. Dinu / Nuclear Instruments and Methods in Physics Research A 505 (2003) 144–147
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edge and the active area, for eight points near the corners of the sensor. (The cut tolerance was 720 mm.)
4. Conclusions
Fig. 3. Number of bad strips per sensor for pre-production CMS silicon sensors.
each strip. Strips with Idiel exceeding 1 nA were classified as defective. The coupling capacitance for each strip was also measured. This test provided a measure of the uniformity of the oxide layer and checked again for the pinholes. The measurements were performed in such a way that they would also reveal any inter-strip shorts. The number of strips on each sensor with parameters outside of these specifications was required to be less than 1%. Fig. 3 shows a plot of total number of bad strips for the pre-production CMS sensors. Seventy-five percent of the sensors satisfied the individual strip acceptance criteria for all strips. 3.2. Optical inspection The aim of the optical inspection was to verify both cut and surface quality of the sensors. Inspection equipment consisted of a computercontrolled X–Y stage equipped with a microscope, a CCD camera, and a display monitor. The acceptance of sensors was based on following observations: (a) an overall survey of sensors, by eye, for large scratches, anomalous coloration, and any other obvious defects; (b) a detailed inspection, under magnification, of sensor edges, looking for breaks or chips greater than 40 mm; and (c) a precise measurement of the distance between the
Results of quality control tests performed by CMS on pre-production silicon sensors have been presented. The full electrical characterization of sensors has allowed us to finalize the testing procedures for the CMS QTC’s, to qualify the sensor manufacturer production lines, and to understand any difference between the manufacturers’ quality criteria and our own. Analysis of the measured parameters did reveal some problems with the pre-production sensors. However, since these tests the manufacturers have improved both the electrical characteristics of sensors and the production yield. The testing procedure has been finalized and the quality control program for SST sensors is now in progress.
Acknowledgements I would like to acknowledge all my colleagues in the CMS Tracker Collaboration, in particular those from the Perugia group, who have contributed to the work described here.
References [1] B. Mours, et al., The design, construction and performance of the ALEPH silicon vertex detector, Nucl. Instr. and Meth. A 379 (1996) 101. [2] G. Anzivino, et al., The DELPHI silicon microvertex detector, Nucl. Instr. and Meth. A 263 (1998) 215. [3] D. Amidei, et al., Nucl. Instr. and Meth. A 350 (1994) 73. [4] CMS Traker Project, Techincal Design Report CMS TDR 5 CERN/LHCC), 15 April 1998, pp. 8–6. [5] A. Buffini, et al., Characterization of neutron irradiated silicon microstrip detectors, Nucl. Instr. and Meth. A 476 (2002) 734.
Characterization and quality control of CMS silicon microstrip sensors Nicoleta Dinu*,1 INFN Sezione di Perugia, Perugia, Italy On behalf of the CMS Tracker Collaboration
Abstract This paper presents the results of quality control tests performed on single-sided, pre-production silicon microstrip detectors for the compact muon solenoid silicon strip tracker. These tests included both a global and a strip-by-strip electrical characterization of the sensors. Additionally, all sensors were optically inspected as part of the quality control process. The goal of the tests was to finalize the procedures of the CMS Quality Test Centres, to qualify the sensor production lines, and to compare measurement data provided by the manufacturers with our own. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.40.Gx Keywords: LHC; CMS; Silicon microstrip detector; Electrical characterization
1. Introduction Silicon microstrip sensors represent an important component of many systems used for particle detection in high-energy physics. The main advantages of these sensors are high detection efficiency, good spatial resolution, compactness, and very fast response time. The physics potential of their precise tracking performance has been extensively demonstrated by the LEP experiments [1,2] and by CDF [3]. *Corresponding author. Tel.: +39-075-585-2756; fax: +39075-44666. E-mail address: [email protected] (N. Dinu). 1 On leave from Institute of Space Sciences, Bucharest, Romania.
The compact muon solenoid (CMS), one of the future experiments at the CERN large hadron collider (LHC), will extensively use silicon microstrip sensors for tracking charged particles [4]. The silicon strip tracker (SST) of CMS will contain some 25,000 single-sided silicon microstrip sensors, covering a total surface area of about 210 m2. Since the sensors will be located close to the interaction point in LHC, they are expected to be heavily irradiated. In the CMS experiment, the innermost SST inner region will be subject to a fluence of 1.6 1014 1 MeV equivalent neutrons/ cm2. R&D studies performed before and after irradiation [5] have shown that sensors which satisfy precise electrical and mechanical characteristics will retain acceptable particle detection efficiency for at least 10 years of LHC running.
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)01038-6
N. Dinu / Nuclear Instruments and Methods in Physics Research A 505 (2003) 144–147
To insure the electrical and the mechanical characteristics as determined by the irradiation studies and to guarantee the quality of the sensors which will be used in the construction of the SST, CMS will both qualify the sensor production lines and monitor sensor quality throughout the production process. These steps will greatly reduce the risk of sensor damage during the LHC start-up period. The delivery of sensors for the construction of the SST has been divided in two parts: *
*
A pre-production phase, used to qualify the production line of companies and to calibrate the manufacturers’ quality criteria with that of the CMS Quality Test Centres (QTCs); The main construction phase, consisting of the majority of the detectors which will be used in the SST.
Some 330 pre-production sensors have been received by CMS and all of these have been measured by the QTCs. Results presented in this paper are based on these measurements. A description of general sensor characteristics and the quality criteria are also given.
2. General description of sensors The sensor material is FZ-grown, n-type silicon that is phosphorus doped and cut with a /1 0 0S crystal orientation. A thickness of 320 mm and a resistivity of 1.5–3.0 kO cm were chosen for the inner layers as a compromise between signal-tonoise and full depletion voltage after irradiation. The outer layers will be instrumented with 500 mm thick, 3.5–7.5 kO cm wafers. The low resistivity for the inner layers was chosen to improve the radiation hardness by increasing the fluence at which type inversion of the bulk silicon takes place. A p+ implantation is performed on the front side in order to define the strip-shaped diodes. The p+ implants are AC coupled to readout electronics through integrated capacitors made out of an arrangement of grown and deposited oxides and nitrides. This configuration minimizes the number of pinholes while at the same time allowing enough
145
capacitance to guarantee efficient charge collection. The strips are biased by a p+ implant surrounding the active area through an array of poly-silicon resistors. The choice of poly-silicon for the biasing was mainly due to its established radiation resistance. The resistor values were chosen so as to be sufficiently high to guarantee low noise and still low enough to avoid undesirable voltage drops between the strips and the bias ring. The bias ring is further surrounded by a second p+ guard ring designed to prevent the flowing of external leakage current to the active area. Surrounding the guard ring there is an n+ implantation ring at the edge of the sensor, which reduces the cut-line contribution to the leakage current.
3. Quality control of sensors The quality control of the SST silicon microstrip sensors can be divided in two categories: electrical characterization and optical inspection. Given the large number of sensors to be tested and the time required for all of the checks, the responsibility for the quality control of the sensors has been divided among four QTC’s: Karlsruhe (Germany), Vienna (Austria), Perugia (Italy) and Pisa (Italy). 3.1. Electrical characterization All the above-mentioned centers have the availability of clean rooms (purity class M5.5) with temperature and humidity control (71 C, 75% RH) and use automatic probe stations for the electrical testing. The probe stations are sufficiently flexible so that they can be used for all CMS silicon microstrip sensors regardless of sensor size and strip pitch. A PC running LabView provides control of the test station and both records and analyzes the electrical characteristics of the sensors. In order to have comparable output, all of the QTC’s use the same testing procedures. To verify the uniformity of the results, calibration sensors were measured in turn by each center. Results from this study indicated good agreement between the different centers.
146
N. Dinu / Nuclear Instruments and Methods in Physics Research A 505 (2003) 144–147
Consequently, the combined results from all four QTC’s are presented here. The electrical characterization of SST sensors is based on global and strip-by-strip tests. The global tests consist of measurements of current as a function of voltage (IV) and capacitance as a function of voltage (CV). The IV measurement was performed to determine the total leakage current of the sensor and to study the breakdown performance. A Keithley 237 (6517) high voltage supply was used to apply the bias voltage between the bias ring and the back plane of the sensor. Over the course of LHC running, radiation damage will increase the effective doping concentration in the sensors thus requiring increasingly higher bias voltages in order to maintain satisfactory charge collection efficiency. The total leakage current was measured from 0 to 550 V in 5 V steps and was required to be less than 20 mA at 450 V. The slope of the total leakage current, DIleak =DV ; was also required to be less than 100 nA/V in the range 450–550 V. This latter condition is intended to assure that the sensors will not experience breakdown and will otherwise be stable for bias levels up to 500 V. Pre-production detectors consisting of 170 rectangular-shaped sensors, with strip pitches 183 and 120 mm, and 160 trapezoidal-shaped sensors, with strip pitches 163–85 and 186–205 mm, were delivered by two companies and tested at the QTC’s. Fig. 1 shows a histogram of the total leakage current at a bias voltage of 450 V for these
sensors. Out of the 330 sensors, 300 satisfied the IV test acceptance criteria. CV measurement was performed to determine the sensor depletion voltage and the total capacitance at full depletion. Capacitance was measured at 1 kHz using an Agilent 4284A (HP 4274A) LCR meter. The full depletion voltage, Vdepletion ; was extracted in the standard way from the plot of 2 1=Cbulk versus Vbias : Total capacitance was measured in 5 V steps from 0 to 500 V. The full depletion voltage for the sensors was required to be less than 300 V. The distribution of depletion voltages for the CMS preproduction sensors is shown in Fig. 2. All preproduction sensors satisfied the CV test acceptance criteria. Strip-by-strip tests were used to identify defective strips and to determine the extent of strip-tostrip variations within the sensors. These tests consisted of measurements of individual strip leakage currents, poly-silicon resistances, coupling capacitances, and leakage current through the dielectric layer (dielectric current). All of these tests were performed at a bias voltage of 400 V. The leakage current of each strip was measured in order to identify potentially noisy strips and was required to be less than 100 nA. The poly-silicon resistance was also measured for each strip and was required to be within the range 1.0–2.0 MO. For each sensor, all poly-silicon resistor values were further required to lie within 70.3 MO of the average value. Dielectric current was measured with 10 V applied across the coupling capacitor of
Fig. 1. A histogram plot of total leakage current at 450 V for CMS pre-production sensors.
Fig. 2. Depletion voltage for CMS pre-production silicon sensors.
N. Dinu / Nuclear Instruments and Methods in Physics Research A 505 (2003) 144–147
147
edge and the active area, for eight points near the corners of the sensor. (The cut tolerance was 720 mm.)
4. Conclusions
Fig. 3. Number of bad strips per sensor for pre-production CMS silicon sensors.
each strip. Strips with Idiel exceeding 1 nA were classified as defective. The coupling capacitance for each strip was also measured. This test provided a measure of the uniformity of the oxide layer and checked again for the pinholes. The measurements were performed in such a way that they would also reveal any inter-strip shorts. The number of strips on each sensor with parameters outside of these specifications was required to be less than 1%. Fig. 3 shows a plot of total number of bad strips for the pre-production CMS sensors. Seventy-five percent of the sensors satisfied the individual strip acceptance criteria for all strips. 3.2. Optical inspection The aim of the optical inspection was to verify both cut and surface quality of the sensors. Inspection equipment consisted of a computercontrolled X–Y stage equipped with a microscope, a CCD camera, and a display monitor. The acceptance of sensors was based on following observations: (a) an overall survey of sensors, by eye, for large scratches, anomalous coloration, and any other obvious defects; (b) a detailed inspection, under magnification, of sensor edges, looking for breaks or chips greater than 40 mm; and (c) a precise measurement of the distance between the
Results of quality control tests performed by CMS on pre-production silicon sensors have been presented. The full electrical characterization of sensors has allowed us to finalize the testing procedures for the CMS QTC’s, to qualify the sensor manufacturer production lines, and to understand any difference between the manufacturers’ quality criteria and our own. Analysis of the measured parameters did reveal some problems with the pre-production sensors. However, since these tests the manufacturers have improved both the electrical characteristics of sensors and the production yield. The testing procedure has been finalized and the quality control program for SST sensors is now in progress.
Acknowledgements I would like to acknowledge all my colleagues in the CMS Tracker Collaboration, in particular those from the Perugia group, who have contributed to the work described here.
References [1] B. Mours, et al., The design, construction and performance of the ALEPH silicon vertex detector, Nucl. Instr. and Meth. A 379 (1996) 101. [2] G. Anzivino, et al., The DELPHI silicon microvertex detector, Nucl. Instr. and Meth. A 263 (1998) 215. [3] D. Amidei, et al., Nucl. Instr. and Meth. A 350 (1994) 73. [4] CMS Traker Project, Techincal Design Report CMS TDR 5 CERN/LHCC), 15 April 1998, pp. 8–6. [5] A. Buffini, et al., Characterization of neutron irradiated silicon microstrip detectors, Nucl. Instr. and Meth. A 476 (2002) 734.