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 18 th, 2005

2. Thomas Bergauer, HEPHY Vienna2 Outline of Talk 1. Characterization of Silicon Strip Detectors for Quality Assurance 2. Characterization of “standardized” test- structures to monitor manufacturing process Characterization of Strip Detector  global measurements (IV, CV)  strip-by-strip tests (I leak, C ac, R poly and I diel) Characterization of test structures with 9 different measurements 6” wafer:

3. 1. Quality Assurance of Silicon Strip Detectors

4. Thomas Bergauer, HEPHY Vienna4 Sensor Characterization Basics  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 I strip poly-silicon resistor R poly coupling capacitance C ac dielectric current I diel

5. Thomas Bergauer, HEPHY Vienna5 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 SiO 2 + Si 3 N 4

6. Thomas Bergauer, HEPHY Vienna6 AC-coupled Silicon Strip Detector Corner of a typical CMS Silicon Strip Detector Strip Pitch 80-170μm

7. Thomas Bergauer, HEPHY Vienna7 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

8. Thomas Bergauer, HEPHY Vienna8 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

9. Thomas Bergauer, HEPHY Vienna9 Example Measurements: CV, IV  Combined voltage ramp up to 500- 800V Dark current (blue) and total capacitance (red, plotted 1/C 2 ) is recorded.  Depletion voltage is extracted

10. Thomas Bergauer, HEPHY Vienna10 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 I diel coupling capacitance C ac poly-silicon resistor R poly strip leakage current I strip  For each test, the switching matrix has to be reconfigured Full characterization of detector with 512 strips: 3h

11. Thomas Bergauer, HEPHY Vienna11 Example Results: Depletion Voltage, Dark current (Sensors for CMS) Depletion VoltageDark current @ 450V

12. Thomas Bergauer, HEPHY Vienna12 Results: Stripscan  Total number of bad strips Total = sum of I strip, R poly, C ac, I diel ) 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

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

14. Thomas Bergauer, HEPHY Vienna14 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

15. Thomas Bergauer, HEPHY Vienna15 TS-CAP sheet GCD CAP-TS-AC babydiode MOS in MOS out Standardized Set of Test Structures Company test-structures “Standard Half moon”  9 different structures  individually described in the next slides

16. Thomas Bergauer, HEPHY Vienna16 Test Structures  TS-CAP: Coupling capacitance C AC to determine oxide thickness IV-Curve: breakthrough voltage of oxide  Sheet: Aluminium resistivity p + -impant resistivity Polysilicon resistivity  GCD: Gate Controlled Diode IV-Curve to determine surface current I surface Characterize Si-SiO 2 interface  CAP-TS-AC: Inter-strip capacitance C int  Baby-Sensor: IV-Curve for dark current Breakthrough  CAP-TS-DC: Inter-strip Resistance R int  Diode: CV-Curve to determine depletion voltage V depletion Calculate resistivity of silicon bulk  MOS: CV-Curve to extract flatband voltage V flatband to characterize fixed oxide charges (details on next slide)

17. Thomas Bergauer, HEPHY Vienna17 MOS  Metal Oxide Semiconductor  Oxide composition represents configuration of Thick dielectric in inter-strip region Thin dielectric underneath strips (right)  Extraction of flatband voltage V fb Seen by sharp decrease of Capacitance (between accumulation and inversion) to determine fixed positive charges in Oxide  Limit defined experimental after test beam

18. Thomas Bergauer, HEPHY Vienna18 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

19. Thomas Bergauer, HEPHY Vienna19 PQC Setup

20. Thomas Bergauer, HEPHY Vienna20 Software  Self-developed LabVIEW program  Fully automatic measurement procedure (~30 minutes) Except alignment of Half moon and placement of probecard  Automatic extraction of parameters Before run: After run: Yellow Fields: Limits and cuts for qualification Blue Fields: Obtained results extracted from graph by linear fits (red/green lines)

21. Thomas Bergauer, HEPHY Vienna21 Examples of identified problems  Limit: R int > 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! Interstrip Resistance

22. Thomas Bergauer, HEPHY Vienna22 Examples of identified problems (2)  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 Flatband Voltage

23. Thomas Bergauer, HEPHY Vienna23 Examples of identified problems (3)  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 Aluminium resistivity

24. Thomas Bergauer, HEPHY Vienna24 Summary  Future experiments with a large tracker will require a huge number of silicon strip sensors. Compare with CMS Silicon Strip Tracker: 206 m 2 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

25. Thomas Bergauer, HEPHY Vienna25 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.

26. The End.

27. Backup Slides

28. Thomas Bergauer, HEPHY Vienna28 TS CAP  Array of 26 AC-coupled strips  Test of Coupling Capacitance Oxide Thickness can calculated  Test of dielectric breakdown Destructive !

29. Thomas Bergauer, HEPHY Vienna29 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

30. Thomas Bergauer, HEPHY Vienna30 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

31. Thomas Bergauer, HEPHY Vienna31 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

32. Thomas Bergauer, HEPHY Vienna32 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

33. Thomas Bergauer, HEPHY Vienna33 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

34. Thomas Bergauer, HEPHY Vienna34 Diode  Simple square diode  Voltage scan is used to measure Capacitance and to extract total bulk thickness Bulk resistivity