Gamma-ray Large Area Space Telescope

1. Gamma-ray Large Area Space Telescope Investigation on tray MID 063 Back

2. Mechanical characteristics of tray MID 063:all the geometrical data are very good • Mechanical tray planarity (without SSDs): • Top side 0.050mm • Bottom side 0.014mm • SSDs relative positional error: • Top side ±0.019mm • Bottom side ±0.016mm • SSDs planarity: • Top side 0.053mm • Bottom side 0.087mm • Tray total thickness 30.372mm (nominal thickness = 30.72mm) • Strips alignment to the tower (software alignment) • Top side shift= 0.013mm parallelism= -0.012mm (-0.033mrad) • Bottom side shift= -0.006mm parallelism= -0.050mm (-0.143mrad) • Top side efficiency = 99.4%, Bottom side efficiency = 99.8%

3. Statement of the problem Channels on the right side are completely dead – i.e. Preamplifiers inputs shorted to ground a fixed voltage reference. • First observed during Tower 1 TV test. • Only observed when P < 50-100 torr. • Tower 1 partially disassembled and tray pulled apart. • Found to be perfect (at least at atmospheric pressure) at a mechanical survey. • Basic ideas: • Short on between HV and bias line on ladder 0 (indirect evidence, impossible to confirm since we do not measure the leakage current inside the TV chamber). • On the right end of ladder 0 the wire bonds from the SSD to the MCM probably in contact with the silicon wafer. Channels on the left side are noisy at the electrical test.

4. Experimental setup Vacuum chamber • Tray connected to the EGSE during the whole test: • Standard electrical test possible (reading the tray from the left GTRC) • Leakage current continuosly monitored by means of breakout box + DMM. EGSE

5. March 23, 2005 • Wire bond between bias ring and pitch adapter on the right end of ladder 0 removed and restored with a smaller wire loop: • The height of the wire bond with respect to the edge of the wafer has been (roughly!) measured to be ~150 mm (more or less 300mm under normal conditions). • Tray electrically tested before going into the vacuum chamber. Wire bond Silicon wafer Tray panel MCM

6. March 23, 2005 • First run in vacuum: • While going down with pressure for the first time short was observed @ ~270 mbar • After that point the ladder consistently developed the short @ ~ 100 mbar

7. March 24, 2005 • Same setup as the day before: • The short still shows up consistently @ ~ 100 mbar

8. IV scan • Leakage current measured as a function of the bias voltage @ 50 mbar. • Consistent with a short between HV and bias (current driven by the 270 kOhm protection resistor)

9. Noise and gain measurement performed at different pressures Pressure makes a difference The number of wire bonds touching the silicon wafer increases as pressure goes down. Noise and gain measurement P ~ 0.1 mbar ~ 150 channels in contact with the edge of the wafer P ~ 50 mbar ~ 50 channels in contact with the edge of the wafer

10. Optical inspection after first tests in vacuum • Observations: • The wire bond between the bias ring and the pitch adapter did NOT break but... • ... At the optical inspection it was found to be bent • Conclusions: • The wire bond was probably stressed and bent while going down with pressure for the first time (short @ 270 mbar), reaching more or less the same height as the other wires (short @ 100 mbar from that point on).

11. Ladder height vs. pressure • Simple model: • Left corner of ladder 0 does not move with respect to the tray level. • A bubble pushes the right end of ladder 0 up – depending on the external pressure • Putting the all the information together: • The height of the right corner of ladder 0 can be evaluated as a function of the pressure. • The height of the wire bonds with respect to the silicon wafer is known with good precision. • Data from leakage current and noise measurements can be used • Aggiungere un disegnino esplicativo

12. Ladder height vs. pressure

13. March 24, 2005 • Wire bonds between channels 150 – 383 and pitch adapter + wire bond between bias ring and pitch adapter on the right end of ladder 0 removed: • Only 150 wire bonds (on the left end of the ladder) left in place. • Tray electrically tested. • Into the vacuum chamber again...

14. March 24, 2005 • First ~150 wire bonds (only) in place: • The short develops at a much lower pressure (5-10 mbar). • The wire bonds corresponding to channels 0-150 did NOT touch the silicon in the previous tests. • The wire bonds play some role in the dynamic of the ladder.

15. March 25, 2005 • First ~150 wire bonds (only) in place – tray upside down: • Failing ladder now on the TOP face. • The short develops at a lower pressure (< 1 mbar). • Gravity makes a difference.

16. March 25, 2005 • Wire bonds between channels 64-148 and pitch adapter + wire bond between bias ring and pitch adapter on the right end of ladder 0 removed: • Only 65 wire bonds (on the left end of the ladder) left in place. • Tray electrically tested. • Into the vacuum chamber again...

17. March 25, 2005 • First ~65 wire bonds (only) in place: • No short observed up to the limits of the chamber (~0.05 mbar). • Upside down?

18. Conclusions