ATLAS SCT Detector Irradiation QA Procedures



FIRST DRAFT 28/05/99

Updated 13/07/99



Contents

  1. Overview
    1. Baby Detectors
    2. Full-size Detectors
  2. Pre-Irradiation Tests
  3. Post-Irradiation Tests
    1. Annealing
    2. IV Curve
    3. CV tests
    4. Strip Integrity
    5. Leakage Current Stability
    6. Biassing Resistance and Interstrip Resistance
    7. Interstrip Capacitance
    8. Charge Collection Efficiency
    9. Strip Noise vs Bias
  4. Equipment Required

1.0 Overview

During production it is nessary to periodically verify that detector behaviour after irradiation remains consistent with that observed during the qualification tests. Any changes from qualification data may indicate variations in processing which may only be evident after irradiation. A very small number of full-sized detectors will be selected for irradiation during production, and a thorough evaluation of these detectors will be performed for detailed comparisons with qualification data. It is anticipated that larger numbers of baby detectors (identical to the full-size detector but only 1cm2 in size with 98 8mm-long strips) will be used for irradiation tests, as their small size means that they can be irradiated more quickly and easily. It is anticipated that the measurement of the post-irradiation IV characteristics of baby detectors will provide a minimum check of processing consistency.

1.1 Baby Detectors

It is anticipated that many detectors will be delivered to ATLAS institutes with baby detectors diced from the same wafer. At this stage we would propose to use the baby detectors mainly for a check on post-irradiation IV characteristics, though many of the measurement techniques described in this document can in principle also be applied to the baby detectors. The main convenience for using these devices is their small size, and therefore scanning during the irradiation at the PS will not be necessary and the full fluence can be obtained in several hours.

1.2 Full-Size Detectors

Although full size detectors will provide the most realistic comparison with qualification data, their size requires scanning in the proton beam at the PS, and several days are required to reach the full fluence.

2.0 Pre-Irradiation Tests

On delivery the full set of detector measurements described in document ATLAS SCT/Detector FDR/99-7 should be performed to ensure the detector is fully characterised. The detectors are then glued with araldite 2011 to ceramic support cards and bonded to pitch adaptors for compatibility with readout by both binary and analogue readout electronics. During irradiation the strip metals are shorted together (via bonds on the pitch adaptor to a common rail) to simulate the condition of being bonded to readout electronics. The detector bias rail and backplane must be connected to ~3cm long leads (via bonds to the pitch adaptor and/or flexible PCB) terminating in 2-pin SIL connectors for biassing. The IV characteristics should be remeasured after gluing to ensure no deterioration has ocurred during assembly.

3.0 Post-Irradiation Tests

Unlesss where otherwise specified, all post-irradiation detector tests are performed cold (-10C) in a freezer containing dry air, and with the detector ceramics screwed to an aluminium support frame protected by an aluminium cover lid. To ensure good thermal contact, the aluminium support frame should itself be in direct contact with a large thermal mass inside the freezer. Annealing times (when the detector is brought to room temperature for measurements, bonding/soldering work etc) should be recorded in units of days at 25degC equivalent temperature.

3.1 Annealing

After irradiation the detectors should undergo a controlled beneficial anneal for 7 days at 25degC to minimise the depletion voltage

3.2 IV Curve

Aim

To measure the IV characteristic after irradiation.

Procedure

This test requires a voltage source/picoammeter (SMU) to measure the IV characteristic between the bias rail and the backplane. The current is measured every 10V step up to 500V, with a 10 second delay between steps. The temperature of the detector should be recorded (either via a PT100 on the detector ceramic, or a PT100 in contact with the large thermal mass inside the freezer). On a subsample of detectors, the IV should be remeasured at -18C to verify that thermal contact is good and current scales correctly with temperature.

Acceptance

The detector displays a characteristic at -10degC which is below ?uA at 350V, and without any significant increase in current up to 500V.

3.3 CV Measurement

Aim

To measure the CV characteristic after irradiation.

Procedure

This measurement requires a CV meter equipped (if necessary) with an external bias adaptor and a voltage source. Connect the biasrail to the AC output of the CV meter bias adaptor, and the backplane to the voltage output of the CV meter bias adaptor. Record the capacitance in 10V steps up to 350V, with a 10 second delay between steps. Use 100 Hz with CR modelled in SERIES, and with 500mV AC amplitude. Plot the data as 1/C**2 (1/nF**2) vs bias (volts).

Acceptance

Interpretation in terms of depletion voltage is ambiguous after irradiation. The data should be stored for information only.

3.4 Strip Integrity

Aim

To check for additional oxide punchthroughs caused by the irradiation.

Procedure

This test requires a volt source/picoammeter (SMU) to check for oxide punchthroughs, and a CV meter to measure capacitance anomolies due to strip metal shorts/opens and pitch adaptor scratches/shorts. The detector is warmed to room temperature and the lid of the aluminium frame is removed. Any bonds still connecting strip metals to the common ground rail of the pitch adaptor must be removed. The frame is attached to a jig on the chuck of an automatic probestation, and the planarity adjusted such that the pitch adaptor is flat relative to the probestation platen. All 512 metal pads of the pitch adaptor are then probed (in 4 groups of 128) under computer control with the light on. +10V is supplied continuously by the SMU to the backplane via the voltage output of the CV meter (or the external bias adaptor of the CV meter if applicable), with the needle connected to the AC output of the CV meter. A series resistor may be necessary to limit current if the CV meter external bias adaptor (which usually contains a large series resistance) is not used. The following technique for each strip measurement has been demonstrated to work well without any damage to the detector, and is therefore recommended, though alternative techniques are acceptable:
  1. Before the first pitch adaptor pad is probed, source +10V from the SMU and wait several seconds for the SMU current to drop to <1nA (due to the capacitor charging in the CV meter external bias adaptor)
  2. Raise the chuck to contact the pad with the probeneedle.
  3. Measure the current drawn from the SMU. Currents >> 1nA indicate an oxide punchthrough.
  4. Measure the capacitance (1kHz, CR in SERIES, 100mV amplitude).
  5. In the case of an oxide punchthrough, drop the chuck and wait several seconds for the SMU current to resettle to <1nA. If there is not oxide punchthrough, no delay is necessary.
  6. Move to the next pad, and repeast from Step 2 above.
The test (as demonstrated on a SUMMIT 10K probestation) takes about 25 minutes.

Acceptance

The number of strip defects (due to oxide punchthroughs and strip metal defects) is <2%. Care must be taken to exclude defects arising from pitch adaptor scratches/track opens etc.

3.5 Leakage Current Stability

Aim

To check that any variation in leakage current over a 24 hour period is within specifications.

Procedure

This test requires a voltage source / picoammeter (SMU), a meter for temperature monitoring, an environment chamber, and, if available, a switching matrix. The backplane and bias rail of the detector are connected to the high and grounded-low outputs of the SMU respectively. The assembly is installed in freezer containing dry air (nitrogen) maintained at -10degC. The bias is ramped to 350V, and after 60 seconds settling time the current is monitored every 15 minutes over a 24 hour period. Several detectors may be measured in parallel by use of a switching matrix.

Acceptance

Maximum increase in leakage current during 24hours is less than ?uA.

3.6 Polysilicon Bias Resistance and Interstrip Resistance

Aim

Determine the bias resistor value is within specificatons and that the interstrip isolation is sufficient when under bias.

Procedure

This test requires a voltage source and a volt source / picoammeter (SMU). This measurement yields both the polysilicon bias resistance and the interstrip resistance, and must be performed cold (-10C). The backplane and bias rail should be connected to the high and grounded-low outputs (respectively) of the voltage source. The DC contact to a strip implant and bias rail should be connected to the high and low outputs (respectively) of the SMU (it is nessary to bond from the strip DC contact to a track on the pitch adaptor. Some bonds from the strip metals to the pitch adaptor will need to be removed to provide space for this). Perform an IV (using the SMU) from -5V to +5V to determine the resistance between the strip implant and bias rail as a function of bias voltage (increase detector bias from 0 V to ~300V or until the measured resistance plateaus).

Acceptance

Interstrip resistance is sufficient if the measured resistance vs detector bias plateaus. The plateau level resistance is equivalent to the polysilicon bias resistance, and must be within 1.25 +/- 0.75 Mohm.

3.7 Interstrip Capacitance

Aim

Determine that the interstrip capacitance is within specifications.

Procedure

This test requires a CV meter and a voltage source, and requires the detector aluminium support frame to be attached to a jig to allow for bonding from the strip pads on the pitch adaptor to solderable contacts (eg a piece of PCB with appropriate gold tracking). Remove the bonds (if not already removed) connecting the detector strips to the common ground rail on the pitch adaptor. From one of the rows of 128 pads on the pitch adaptor that corresponds to 6cm strips, bond one strip pad out to a solderable contact on the PCB, and bond the neighbouring strip pad on both sides to the common ground rail of the pitch adaptor. Bond out from the common ground rail to a second solderable contact on the PCB. The central strip should then be connected (via a cable soldered to the PCB) to the AC output of the CV meter, and the two neighbouring strips to the voltage output of the CV meter (sourcing 0V). The backplane and bias rail of the detector should be connected to the high and grounded-low sides (respectively) of the voltage supply. Measure the capacitance between the central strip and its neighbour on both sides as a function of detector bias up to 500V, using 20V steps. Use 100kHz test frequency with CR modelled in parallel. Note: parasitic capacitance arising from the PCB and its cabling to the CV meter needs to be subtracted from the measured capacitance values. The best way to estimate the parasitic capacitance is to remove the bond between the pitch adaptor and PCB that connects the central strip, and remeasure capacitance.

Acceptance

Interstrip capacitance < 1.2 pF/cm at 500V bias.

3.8 Charge Collection Efficiency

Aim

To determine the onset of the plateau in charge collection efficiency vs detector bias up to 500V.

Procedure

Bond one group of 6-cm 128 channels to an analogue readout chip and measure the signal collected vs bias triggered using a Ru106 beta-source.

Acceptance

The onset of the plateau matches the value observed during qualification tests.

3.9 Strip Quality vs Bias

Aim

To determine the number of strips with excess noise due to microdischarge.

Procedure

Bond all 512 channels of the pitchadaptor to either binary or analogue readout electronics, and measure the noise per channel vs detector bias at 200V, 400V and 500V bias.

Acceptance

There is no evidence of microdischarge at 400V bias.

4.0 Equipment Required at ATLAS Institutes

A list of equipment required at ATLAS institutes for testing of irradiated detectors is shown below. Recommendations are given for equipment, but alternative equipment is acceptable if the measurement specs are comparable.

4.1 Automatic (computer controlled) probestation

eg Wentworth AWP-1050, Summit 10K, Alessi 6100 etc

Requirement

For all probestation requirements. A computer controlled high precision probestation is an absolute requirement for strip integrity tests.

4.2 Voltage source

eg Keithley 487, Keithley 6517

Requirement

For IV, CV, strip integrity, stability and general tests. Require capability of 500V or more, both polarities, compliance up to at least 1mA. Many of these tests are anticipated to take place in parallel, so at least two voltage sources are recommended.

4.3 Picoammeter

eg Keithley 487, Keithley 6517

Requirement

For IV, stability, strip integrity and general tests. Require sensitivity of 1nA (minimum) to at least 1mA. Many tests will be carried out in parallel, so at least two picoammeters are recommended.

4.4 LCR meter

eg HP4263B, Wayne Kerr 6425

Requirement

For strip integrity and CV tests.. Frequency requirement from 100Hz to 100kHz or higher. The LCR meter must have an external bias adaptor for applying bias voltages up to 400V.

4.6 Temperature/Humidity Monitor

eg dedicated monitors, or Keithley 2000/2001 equipped with Pt100/thermocouple, humidity probe etc

Requirement

Temperature monitoring during IV and stability measurements.

4.7 Switching matrix

eg Pickering 20-360, Keithley range etc

Requirement

For stability measurements.

4.8 Bonding machine

eg K&S manual or automatic

Requirement

For bonding work.

4.9 Freezers, containing large thermal mass

eg Commercial chest freezers, with sheets of >=1inch thick aluminium.

Requirement

To provide a cold stable dark environment for all tests (except for the probestation strip test). The freezers should be supplied with dry air (eg nitrogen or argon).

4.10 Networked PC with GPIB card and appropriate LabView control software and database software

Requirement

For all data-taking and updates to the ATLAS database.