- Email: [email protected]
Tests of the Silicon Strip Sensors for the CMS Tracker
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 518 (2004) 317–320
Tests of the Silicon Strip Sensors for the CMS Tracker T. Bergauer Institute for High Energy Physics, Austrian Academy of Sciences, Nikolsdorfergasse 18, A-1050 Vienna, Austria On behalf of the CMS Tracker Collaboration
Abstract The Compact Muon Solenoid (CMS) is one of the experiments at the Large Hadron Collider (LHC) currently under construction at CERN. Its inner tracking system consists of the world’s largest Silicon Strip Tracker which implements around 25 000 silicon sensors covering an active area of 206 m2 : The CMS collaboration developed a detailed testing scheme in order to ensure the functionality of all these sensors for the full LHC lifetime. The measurements performed on the sensors will be presented. Selected results from these measurements and a comparison with the data from the suppliers will be shown. r 2003 Elsevier B.V. All rights reserved. PACS: 06.60.Mr; 29.40.Gx; 29.40.Wk Keywords: Quality control; Silicon; Tracker; CMS; LHC
1. Compact Muon Solenoid (CMS) tracker design The central part of the CMS all-silicon tracker consists of four inner barrel (TIB) and six outer barrel (TOB) layers [1]. In the forward region three slices of silicon sensors are forming the inner disks (TID). Together with the two endcaps (TEC), which consist of seven rings on two times nine disks, the tracker covers a pseudorapidity range of jZjp2:5: All four layers of TIB, the three rings of TID and the four innermost rings of the forward detector are composed of silicon sensors with a thickness of 320 mm (inner region), while the other layers house sensors of 500 mm thickness (outer region). All thin silicon sensors (type IB1, IB2,
W1, W2, W3 and W4) will be produced by HPK1 using low-resistivity silicon, while all thick sensors (type OB1, OB2, W5a/b, W6a/b, W7a/b) will be produced by STM2 using high-resistivity silicon. The requirement on resistivity is 3:5-7:5 kO cm for thick sensors and 1:25-3:25 kO cm for thin sensors. By using low-resistivity silicon, the type inversion is delayed which results in a lower depletion voltage after 10 years of Large Hadron Collider operation. Each module consists of a single microstrip sensor in the inner region and two daisy-chained sensors in the outer region. While, the sensors for TIB (IB1 and IB2) and TOB (OB1 and OB2) are rectangular, the sensors for TID and TEC are wedge-shaped (W1 to W7).
E-mail address: [email protected] (T. Bergauer). 0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.008
1 2
Hamamatsu Photonics K.K., Hamamatsu-shi, Japan. ST Microelectronics, Catania, Italy.
ARTICLE IN PRESS 318
T. Bergauer / Nuclear Instruments and Methods in Physics Research A 518 (2004) 317–320
All sensors are single-sided silicon strip sensors with either 512 or 768 implanted pþ -strips with a pitch between 80 to 183 mm without intermediate strips [2]. A dielectric SiO2 layer between implant and metallization allows an AC coupled readout. A metal overhang over the pþ -strips improves the field configuration to ensure a stable detector behavior with respect to the high bias voltage up to 500 V:
2. Sensor logistics After receiving and registering the sensors at CERN they are shipped in equal percentages to the four ‘Quality Test Centers’ (QTC), which are responsible for the overall quality. These centers are located in Pisa, Perugia, Karlsruhe and Vienna. A certain percentage will be distributed to the ‘Irradiation Qualification Centers’ (IQC), to the ‘Process Quality Centers’ (PQC) and to bonding centers for further tests [3]. Upon arrival, the QTC centers check 100% of the delivered sensors with the microscope for mechanical defects like broken edges or scratches. During the pre-production phase, 100% of the sensors have been electrically tested. After these sensors have been qualified, it is the responsibility of the vendors to ensure that no change of the process or the substrate properties occurs. During the full production phase, only 5% of the sensors are electrically tested to verify the measurements done at the supplying companies.
the strips of the sensors with the possibility of contacting an AC and two adjacent DC pads. The test bench is controlled by a computer using the Labview measurement program and communicates with the instruments over the IEEE488 (GPIB) interface bus. The electrical circuit of the probe station setup is designed for voltages up to 1000 V and currents in the range of few pA. Therefore, a good shielding and insulation of all components is essential. The test setup consists of a source measure unit which is used to apply the reverse bias voltage to deplete the sensor. This is done in steps of 5 V up to 550 V: During the voltage ramp the IV curve is measured. Simultaneously the CV curve up to a voltage of 350 V is measured using a LCR meter. Typical diagrams for the CV and IV curves can be seen in Fig. 1. After the bias voltage reached 550 V; the voltage is ramped to 400 V: At this voltage, the strip scan is performed. For that purpose the crosspoint switching matrix is reconfigured. Four parameters are measured for every strip: the single strip current (Istrip ), coupling capacitance (CAC ), dielectric current (Idiel ) between AC and DC pad and the polysilicon resistor (Rpoly ). An example of a strip scan can be found in Fig. 2. After the strip scan is finished, the bias voltage is ramped down and an XML file with the results is written into the Tracker construction database.
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3. Measurement setup overview
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The four QTC centers are equipped with different instruments and test benches—however, the measurement procedures and the output data format are identical. In this article the Vienna setup is explained in particular [4]. It consists of a vacuum support carrying the sensor, which is mounted on a XYZ-table. While the micropositioner, which holds the needle for the bias ring connection, moves with the sensor support, three further micropositioners with needles are stationary. This configuration allows a strip scan over all
0.4
0.2
0.0
Fig. 1. Typical IV (solid line) and 1=CV 2 (dashed line) behavior of a silicon detector. The depletion voltage is around 170 V:
ARTICLE IN PRESS T. Bergauer / Nuclear Instruments and Methods in Physics Research A 518 (2004) 317–320
319
out in the companies measurements, while the total number of bad strips is more than the companies indicate. This is caused by the fact that the companies do not measure the single strip current. The second criterion is the resistivity, which can be calculated using the depletion voltage. The thickness of wafers, the pitch and the ratio of width/pitch for the different types leads to a depletion voltage in the range between 100 and 300 V: Virtually all of the tested sensors satisfy our requirements. The third criterion is the dark current, which must not exceed 5 mA at 300 V and 10 mA at 450 V: Histograms of the distribution of the dark currents at 450 V can be found in Fig. 3. The average dark current is 0.29 and 1:66 mA for thin and thick sensors, respectively.
Fig. 2. Example of a strip scan on a sensor with 512 strips. One pinhole at strip number 43 and one noisy strip at strip number 300 can be seen.
4. Results Up to now, 399 thin and 1163 thick sensors of all types complied to our acceptance criteria. These criteria are divided in three parts. First, the total number of bad strips (Istrip plus Rpoly plus CAC plus Idiel ) must not exceed 1%, which results in a cut of five bad strips for a sensor with 512 strips and seven bad strips for 768 strips, respectively. For the overall performance of CMS it is necessary to keep the total number of bad strips at a low level to have a good spatial resolution in all regions of the tracker volume. In total, 2.822 bad strips have been found out of 635,648 tested strips for thick sensors and 267 bad strips out of 228,864 total strips for thin sensors. This is 0.44% strip failure rate for thick (STM) sensors and 0.12% strip failure rate for thin (HPK) sensors. Almost all pinholes found at QTC were previously pointed
Fig. 3. Histograms of the dark current at 450 V for 1163 STM sensors (top) and 399 HPK sensors (bottom).
ARTICLE IN PRESS 320
T. Bergauer / Nuclear Instruments and Methods in Physics Research A 518 (2004) 317–320
Altogether, the statistical distributions give very satisfying results. Evaluation of these results give bright prospects to the future performance of the CMS tracker.
Acknowledgements We would gratefully acknowledge the work of all people who are involved in testing of the silicon sensors at the four quality control centers.
References [1] CMS Collaboration, CMS TDR Addendum, CERN/ LHCC, 2000-16. [2] J.-L. Agram, et al., Nucl. Instr. and Meth. A 517 (2004) 77. [3] F. Hartmann, Nucl. Instr. and Meth. 478 (2002) 285. [4] J. Hacker, T. Bergauer, M. Krammer, R. Wedenig, Nucl. Instr. and Meth. 485 (2002) 61.
Nuclear Instruments and Methods in Physics Research A 518 (2004) 317–320
Tests of the Silicon Strip Sensors for the CMS Tracker T. Bergauer Institute for High Energy Physics, Austrian Academy of Sciences, Nikolsdorfergasse 18, A-1050 Vienna, Austria On behalf of the CMS Tracker Collaboration
Abstract The Compact Muon Solenoid (CMS) is one of the experiments at the Large Hadron Collider (LHC) currently under construction at CERN. Its inner tracking system consists of the world’s largest Silicon Strip Tracker which implements around 25 000 silicon sensors covering an active area of 206 m2 : The CMS collaboration developed a detailed testing scheme in order to ensure the functionality of all these sensors for the full LHC lifetime. The measurements performed on the sensors will be presented. Selected results from these measurements and a comparison with the data from the suppliers will be shown. r 2003 Elsevier B.V. All rights reserved. PACS: 06.60.Mr; 29.40.Gx; 29.40.Wk Keywords: Quality control; Silicon; Tracker; CMS; LHC
1. Compact Muon Solenoid (CMS) tracker design The central part of the CMS all-silicon tracker consists of four inner barrel (TIB) and six outer barrel (TOB) layers [1]. In the forward region three slices of silicon sensors are forming the inner disks (TID). Together with the two endcaps (TEC), which consist of seven rings on two times nine disks, the tracker covers a pseudorapidity range of jZjp2:5: All four layers of TIB, the three rings of TID and the four innermost rings of the forward detector are composed of silicon sensors with a thickness of 320 mm (inner region), while the other layers house sensors of 500 mm thickness (outer region). All thin silicon sensors (type IB1, IB2,
W1, W2, W3 and W4) will be produced by HPK1 using low-resistivity silicon, while all thick sensors (type OB1, OB2, W5a/b, W6a/b, W7a/b) will be produced by STM2 using high-resistivity silicon. The requirement on resistivity is 3:5-7:5 kO cm for thick sensors and 1:25-3:25 kO cm for thin sensors. By using low-resistivity silicon, the type inversion is delayed which results in a lower depletion voltage after 10 years of Large Hadron Collider operation. Each module consists of a single microstrip sensor in the inner region and two daisy-chained sensors in the outer region. While, the sensors for TIB (IB1 and IB2) and TOB (OB1 and OB2) are rectangular, the sensors for TID and TEC are wedge-shaped (W1 to W7).
E-mail address: [email protected] (T. Bergauer). 0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.008
1 2
Hamamatsu Photonics K.K., Hamamatsu-shi, Japan. ST Microelectronics, Catania, Italy.
ARTICLE IN PRESS 318
T. Bergauer / Nuclear Instruments and Methods in Physics Research A 518 (2004) 317–320
All sensors are single-sided silicon strip sensors with either 512 or 768 implanted pþ -strips with a pitch between 80 to 183 mm without intermediate strips [2]. A dielectric SiO2 layer between implant and metallization allows an AC coupled readout. A metal overhang over the pþ -strips improves the field configuration to ensure a stable detector behavior with respect to the high bias voltage up to 500 V:
2. Sensor logistics After receiving and registering the sensors at CERN they are shipped in equal percentages to the four ‘Quality Test Centers’ (QTC), which are responsible for the overall quality. These centers are located in Pisa, Perugia, Karlsruhe and Vienna. A certain percentage will be distributed to the ‘Irradiation Qualification Centers’ (IQC), to the ‘Process Quality Centers’ (PQC) and to bonding centers for further tests [3]. Upon arrival, the QTC centers check 100% of the delivered sensors with the microscope for mechanical defects like broken edges or scratches. During the pre-production phase, 100% of the sensors have been electrically tested. After these sensors have been qualified, it is the responsibility of the vendors to ensure that no change of the process or the substrate properties occurs. During the full production phase, only 5% of the sensors are electrically tested to verify the measurements done at the supplying companies.
the strips of the sensors with the possibility of contacting an AC and two adjacent DC pads. The test bench is controlled by a computer using the Labview measurement program and communicates with the instruments over the IEEE488 (GPIB) interface bus. The electrical circuit of the probe station setup is designed for voltages up to 1000 V and currents in the range of few pA. Therefore, a good shielding and insulation of all components is essential. The test setup consists of a source measure unit which is used to apply the reverse bias voltage to deplete the sensor. This is done in steps of 5 V up to 550 V: During the voltage ramp the IV curve is measured. Simultaneously the CV curve up to a voltage of 350 V is measured using a LCR meter. Typical diagrams for the CV and IV curves can be seen in Fig. 1. After the bias voltage reached 550 V; the voltage is ramped to 400 V: At this voltage, the strip scan is performed. For that purpose the crosspoint switching matrix is reconfigured. Four parameters are measured for every strip: the single strip current (Istrip ), coupling capacitance (CAC ), dielectric current (Idiel ) between AC and DC pad and the polysilicon resistor (Rpoly ). An example of a strip scan can be found in Fig. 2. After the strip scan is finished, the bias voltage is ramped down and an XML file with the results is written into the Tracker construction database.
0.8
3. Measurement setup overview
0.6
The four QTC centers are equipped with different instruments and test benches—however, the measurement procedures and the output data format are identical. In this article the Vienna setup is explained in particular [4]. It consists of a vacuum support carrying the sensor, which is mounted on a XYZ-table. While the micropositioner, which holds the needle for the bias ring connection, moves with the sensor support, three further micropositioners with needles are stationary. This configuration allows a strip scan over all
0.4
0.2
0.0
Fig. 1. Typical IV (solid line) and 1=CV 2 (dashed line) behavior of a silicon detector. The depletion voltage is around 170 V:
ARTICLE IN PRESS T. Bergauer / Nuclear Instruments and Methods in Physics Research A 518 (2004) 317–320
319
out in the companies measurements, while the total number of bad strips is more than the companies indicate. This is caused by the fact that the companies do not measure the single strip current. The second criterion is the resistivity, which can be calculated using the depletion voltage. The thickness of wafers, the pitch and the ratio of width/pitch for the different types leads to a depletion voltage in the range between 100 and 300 V: Virtually all of the tested sensors satisfy our requirements. The third criterion is the dark current, which must not exceed 5 mA at 300 V and 10 mA at 450 V: Histograms of the distribution of the dark currents at 450 V can be found in Fig. 3. The average dark current is 0.29 and 1:66 mA for thin and thick sensors, respectively.
Fig. 2. Example of a strip scan on a sensor with 512 strips. One pinhole at strip number 43 and one noisy strip at strip number 300 can be seen.
4. Results Up to now, 399 thin and 1163 thick sensors of all types complied to our acceptance criteria. These criteria are divided in three parts. First, the total number of bad strips (Istrip plus Rpoly plus CAC plus Idiel ) must not exceed 1%, which results in a cut of five bad strips for a sensor with 512 strips and seven bad strips for 768 strips, respectively. For the overall performance of CMS it is necessary to keep the total number of bad strips at a low level to have a good spatial resolution in all regions of the tracker volume. In total, 2.822 bad strips have been found out of 635,648 tested strips for thick sensors and 267 bad strips out of 228,864 total strips for thin sensors. This is 0.44% strip failure rate for thick (STM) sensors and 0.12% strip failure rate for thin (HPK) sensors. Almost all pinholes found at QTC were previously pointed
Fig. 3. Histograms of the dark current at 450 V for 1163 STM sensors (top) and 399 HPK sensors (bottom).
ARTICLE IN PRESS 320
T. Bergauer / Nuclear Instruments and Methods in Physics Research A 518 (2004) 317–320
Altogether, the statistical distributions give very satisfying results. Evaluation of these results give bright prospects to the future performance of the CMS tracker.
Acknowledgements We would gratefully acknowledge the work of all people who are involved in testing of the silicon sensors at the four quality control centers.
References [1] CMS Collaboration, CMS TDR Addendum, CERN/ LHCC, 2000-16. [2] J.-L. Agram, et al., Nucl. Instr. and Meth. A 517 (2004) 77. [3] F. Hartmann, Nucl. Instr. and Meth. 478 (2002) 285. [4] J. Hacker, T. Bergauer, M. Krammer, R. Wedenig, Nucl. Instr. and Meth. 485 (2002) 61.