Quality Assurance of Silicon Strip Detectors and Monitoring of Manufacturing Process

1. Quality Assurance of Silicon Strip Detectors and Monitoring of Manufacturing Process Thomas Bergauer Institute f. High Energy Physics HEPHY, Vienna SiLC meeting @ ILC Workshop Vienna, Nov 18th, 2005

2. Outline of Talk • Characterization of Silicon Strip Detectors for Quality Assurance • Characterization of “standardized” test-structures to monitor manufacturing process 6” wafer: Characterization of Strip Detector • global measurements (IV, CV) • strip-by-strip tests (Ileak, Cac, Rpoly and Idiel) Characterization of test structures with 9 different measurements Thomas Bergauer, HEPHY Vienna

3. 1. Quality Assurance of Silicon Strip Detectors

4. Silicon Sensors for future high-energy experiments will have many strips to achieve a high spatial resolution. Large Tracker will use enormous area of silicon sensors Efficient Quality Assurance mandatory Automated test system is necessary to determine the electrical parameters of each strip. What do we test? Global parameters: IV-Curve: Dark current Breakthrough CV-Curve: Depletion voltage Total Capacitance Strip Parameters strip leakage current Istrip poly-silicon resistor Rpoly coupling capacitance Cac dielectric current Idiel Sensor Characterization Basics Thomas Bergauer, HEPHY Vienna

5. AC-coupled Silicon Strip Detector • What do we test? • Si Strip Sensors for the CMS Tracker • n bulk • p+ implanted strips • connected to bias ring via polysilicon resistors • AC-coupled Aluminium readout strips • Dielectric Oxide SiO2 + Si3N4 Thomas Bergauer, HEPHY Vienna

6. AC-coupled Silicon Strip Detector Strip Pitch 80-170μm Corner of a typical CMS Silicon Strip Detector Thomas Bergauer, HEPHY Vienna

7. Sensor Test Setup • Light-tight Box, Instruments, Computer • vacuum support carrying the sensor • Mounted on freely movable table in X, Y and Z • Needles to contact sensor bias line • fixed relative to sensor • Needles to contact: • DC pad (p+ implant) • AC pad (Metal layer) • Can contact ever single strip while table with sensor is moving Thomas Bergauer, HEPHY Vienna

8. Sensor Test Schematics • Instruments (HV source, Amp-Meter, LCR-Meter,…) on the left are connected via a cross-point switching matrix to the needles which contact the sensor to perform different measurements Thomas Bergauer, HEPHY Vienna

9. Example Measurements: CV, IV • Combined voltage ramp up to 500-800V • Dark current (blue) and • total capacitance (red, plotted 1/C2) is recorded. • Depletion voltage is extracted Thomas Bergauer, HEPHY Vienna

10. Example Measurements: Stripscan • After IV-CV ramp, bias voltage is adjusted to stable value (e.g. 400 V) and stripscan is started • 4 parameters tested for each strip: • dielectric current Idiel • coupling capacitance Cac • poly-silicon resistor Rpoly • strip leakage current Istrip • For each test, the switching matrix has to be reconfigured • Full characterization of detector with 512 strips: 3h Thomas Bergauer, HEPHY Vienna

11. Example Results: Depletion Voltage, Dark current (Sensors for CMS) Depletion Voltage Dark current @ 450V Thomas Bergauer, HEPHY Vienna

12. Results: Stripscan • Total number of bad strips • Total = sum of Istrip , Rpoly, Cac, Idiel ) • Bad = outside specified cuts • CMS requires less than 1% of strips are outside cuts for at least one of the strip parameters • Average bad strips per sensor: • 0,37 Thomas Bergauer, HEPHY Vienna

13. 2. Monitoring of Manufacturing Process “PQC”…. Process Quality Control

14. Motivation and Assumptions • Full Characterization of Strip Detector has some disadvantages • takes a lot of time (if every strip is checked) • Only sample tests possible (assumption that production batch behave similar) • Some interesting parameters are not accessible on standard detector or would require destructive tests • Remedy: Doing similar measurements on standardized test-structures • Assumption: Test structures behave identical to main sensor, since produced on the same wafer • Measure many parameter, each on a dedicated test structure • Destructive Tests possible • Fast measurement possible Thomas Bergauer, HEPHY Vienna

15. baby diode MOS out TS-CAP GCD sheet CAP-TS-AC CAP-TS-AC MOS in StandardizedSet of Test Structures “Standard Half moon” • 9 different structures • individually described in the next slides Company test-structures Thomas Bergauer, HEPHY Vienna

16. TS-CAP: Coupling capacitance CAC to determine oxide thickness IV-Curve: breakthrough voltage of oxide Sheet: Aluminium resistivity p+-impantresistivity Polysilicon resistivity GCD: Gate Controlled Diode IV-Curve to determine surface current Isurface Characterize Si-SiO2 interface CAP-TS-AC: Inter-strip capacitance Cint Baby-Sensor: IV-Curve for dark current Breakthrough CAP-TS-DC: Inter-strip Resistance Rint Diode: CV-Curve to determine depletion voltage Vdepletion Calculate resistivity of silicon bulk MOS: CV-Curve to extract flatband voltage Vflatband to characterize fixed oxide charges (details on next slide) Test Structures Thomas Bergauer, HEPHY Vienna

17. MOS • Metal Oxide Semiconductor • Oxide composition represents configuration of • Thick dielectric in inter-strip region • Thin dielectric underneath strips (right) • Extraction of flatband voltage Vfb • Seen by sharp decrease of Capacitance (between accumulation and inversion) • to determine fixed positive charges in Oxide • Limit defined experimental after test beam Thomas Bergauer, HEPHY Vienna

18. Setup Description • Probe-card with 40 needles contacts all pads of test structures in parallel • Half moon fixed by vacuum • Micropositioner used for Alignment • In light-tight box with humidity and temperature control • Instruments • Source Measurement Unit (SMU) • Voltage Source • LCR-Meter (Capacitance) • Heart of the system: Crosspoint switching box • Used to switch instruments to different needles • PC with Labview used to control instruments and switching system • GPIB Bus for communication Thomas Bergauer, HEPHY Vienna

19. PQC Setup Thomas Bergauer, HEPHY Vienna

20. Yellow Fields: Limits and cuts for qualification Blue Fields: Obtained results extracted from graph by linear fits (red/green lines) Software After run: Before run: • Self-developed LabVIEW program • Fully automatic measurement procedure (~30 minutes) • Except alignment of Half moon and placement of probecard • Automatic extraction of parameters Thomas Bergauer, HEPHY Vienna

21. Examples of identified problems Interstrip Resistance • Limit: Rint > 1GΩ to have a good separation of neighbouring strips • Value started to getting below limit • We reported this to the company • Due to the long production pipeline, a significant amount of ~1000 sensors were affected • These sensors will not be used for CMS Tracker! Thomas Bergauer, HEPHY Vienna

22. Examples of identified problems (2) Flatband Voltage • Limit of 10V determined during irradiation campaign • We observed values up to 40V for early deliveries • Some batches from later deliveries suffer from contamination of production line Thomas Bergauer, HEPHY Vienna

23. Examples of identified problems (3) Aluminium resistivity • Aluminium resistivity too high for some delivered batches • Limit: <30mΩ/sq. • Affects noise behaviour of readout chip • After discovery of this issue we requested to increase thickness of Al layer => Problem disappeared Thomas Bergauer, HEPHY Vienna

24. Summary • Future experiments with a large tracker will require a huge number of silicon strip sensors. • Compare with CMS Silicon Strip Tracker: 206 m2 is equal to • 24.244 pieces of sensors and • 9.316.352 channels • Its fabrication will last many months (years) and a stable production during the whole production time is mandatory. • Strip-by-strip test of detectors is necessary but not sufficient • Slow, reduced set of parameters to test • Measurements on dedicated test-structures is a powerful possibility to monitor the fabrication process • During a long production time • Also on parameters which are not accessible on the main sensor (e.g. MOS, GCD,... ) • Destructive tests possible • Fast measurement allows high throughput Thomas Bergauer, HEPHY Vienna

25. Outlook and Future Plans • We have to optimize our test structures • We learned during the CMS QA that some things can be improved: • Smaller structures • Better design of some structures (e.g. diode, sheet) • We want to offer this standardized set of test structures to all interested groups in the future • To put it on unused space of their wafer design Thanks. Thomas Bergauer, HEPHY Vienna

26. The End.

27. Backup Slides

28. TS CAP • Array of 26 AC-coupled strips • Test of Coupling Capacitance • Oxide Thickness can calculated • Test of dielectric breakdown • Destructive ! Thomas Bergauer, HEPHY Vienna

29. Sheet • Combination of • Three polysilicon resistors • Three Aluminium Strips (10, 20, 50 um thickness) • Three p+ Strips (10, 20, 50 um thickness) • Used to determine resistivity of implant, Aluminium and polysilicon • These Parameters have influence on noise behavior of readout chip Thomas Bergauer, HEPHY Vienna

30. GCD • Gate controlled diodes • Two circles ones (not used) • Two squares ones with comb-shaped p+-Diodes and comb-shaped MOS structures alternately arranged • Used to extract surface current • by applying a constant reverse bias voltage through the diode • while varying the gate voltage of the MOS structure. • Sharp decrease of dark current in the inversion region gives the surface current • Important Parameter to monitor oxide and Si-SiO2 interface quality • Limit determined experimental by irradiation Thomas Bergauer, HEPHY Vienna

31. CAP-TS-AC • Measurement of inter-strip capacitance • Between single central strip and two neighboring ones • Outer strips on top and bottom are shorted and connected to ground (directly on the structure) • While biasing of structure is mandatory • Parameter related to noise and SNR of readout chip Thomas Bergauer, HEPHY Vienna

32. Baby Sensor • Structure with 192 AC-coupled strips • Identical to main detector • Used to measure IV-curve up to 700 V • Breakthrough voltage is determined Thomas Bergauer, HEPHY Vienna

33. CAP-TS-DC • Used to determine inter-strip resistance • Similar structure like CAP-TS-AC (used for C_int) but with exceptions • no polysilicon resistor (strips do not have a connection to bias ring) • p+ strips are directly connected to Aluminium strips • High value of inter-strip resistance necessary to have a good electrical separation of strips Thomas Bergauer, HEPHY Vienna

34. Diode • Simple square diode • Voltage scan is used to measure Capacitance and to extract • total bulk thickness • Bulk resistivity Thomas Bergauer, HEPHY Vienna