Quality Assurance of Silicon Strip Detectors and Monitoring

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

Outline of Talk 1. 2. Characterization of Silicon Strip Detectors for Quality Assurance Characterization of “standardized” teststructures 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 2

1. Quality Assurance of Silicon Strip Detectors Thomas Bergauer, HEPHY Vienna

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

AC-coupled Silicon Strip Detector ¡ What do we test? l l l Si Strip Sensors for the CMS Tracker n bulk p+ implanted strips ¡ l l connected to bias ring via polysilicon resistors AC-coupled Aluminium readout strips Dielectric Oxide Si. O 2 + Si 3 N 4 Thomas Bergauer, HEPHY Vienna 5

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

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

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 8

Example Measurements: CV, IV ¡ ¡ Combined voltage ramp up to 500800 V l Dark current (blue) and l total capacitance (red, plotted 1/C 2) is recorded. Depletion voltage is extracted Thomas Bergauer, HEPHY Vienna 9

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: ¡ l dielectric current Idiel l coupling capacitance Cac l poly-silicon resistor Rpoly l strip leakage current Istrip For each test, the switching matrix has to be reconfigured l Full characterization of detector with 512 strips: 3 h Thomas Bergauer, HEPHY Vienna 10

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

Results: Stripscan ¡ Total number of bad strips l l ¡ ¡ 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: l 0, 37 Thomas Bergauer, HEPHY Vienna 12

2. Monitoring of Manufacturing Process “PQC”…. Process Quality Control Thomas Bergauer, HEPHY Vienna

Motivation and Assumptions ¡ Full Characterization of Strip Detector has some disadvantages l l l ¡ 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 teststructures l l 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 14

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

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

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

Setup Description ¡ Probe-card with 40 needles contacts all pads of test structures in parallel l ¡ Instruments l l l ¡ Half moon fixed by vacuum Micropositioner used for Alignment In light-tight box with humidity and temperature control Source Measurement Unit (SMU) Voltage Source LCR-Meter (Capacitance) Heart of the system: Crosspoint switching box l 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 18

PQC Setup Thomas Bergauer, HEPHY Vienna 19

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

Examples of identified problems Interstrip Resistance ¡ ¡ ¡ Limit: Rint > 1 GΩ 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 21

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

Examples of identified problems (3) Aluminium resistivity ¡ ¡ Aluminium resistivity too high for some delivered batches Limit: <30 mΩ/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 23

Summary ¡ Future experiments with a large tracker will require a huge number of silicon strip sensors. l 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 l ¡ Slow, reduced set of parameters to test Measurements on dedicated test-structures is a powerful possibility to monitor the fabrication process l l 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 24

Outlook and Future Plans ¡ We have to optimize our test structures l ¡ 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 l To put it on unused space of their wafer design Thanks. Thomas Bergauer, HEPHY Vienna 25

The End. Thomas Bergauer, HEPHY Vienna

Backup Slides Thomas Bergauer, HEPHY Vienna

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

Sheet ¡ Combination of l l l ¡ ¡ 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 29

GCD ¡ Gate controlled diodes l l ¡ Used to extract surface current l l l ¡ ¡ Two circles ones (not used) Two squares ones with combshaped p+-Diodes and combshaped MOS structures alternately arranged 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-Si. O 2 interface quality Limit determined experimental by irradiation Thomas Bergauer, HEPHY Vienna 30

CAP-TS-AC ¡ Measurement of inter-strip capacitance l l l ¡ 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 31

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

CAP-TS-DC ¡ ¡ Used to determine inter-strip resistance Similar structure like CAP-TS-AC (used for C_int) but with exceptions l l ¡ 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 33

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