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

2. Statement of the problem
Channels on the right side are completely dead – i.e. Preamplifiers inputs shorted to a fixed voltage reference.
Channels on the left side are noisy at the electrical test.
First observed during Tower 1 TV test.
Only observed when P < 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.

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

4. Experimental setup Vacuum chamber EGSE
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.

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 measured to be ~150 mm (more or less 300 mm under normal conditions).
Tray electrically tested before going into the vacuum chamber.
Wire bond

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

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

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

9. 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
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.

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 270 mbar), reaching more or less the same height as the other wires 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
Wires in contact with the wafer
Nominal height of the wirebonds h
Ladder 0
Ladder 1
Tray panel

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 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 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).
Complete electrical test the minimum pressure, results are identical to those obtained in air.

18. Number of wire bonds in place
Conclusions
Number of wire bonds in place
P_short (mbar)
386 (one with a shorter loop)
~270
386
~100
150
~10 65 -
Evidence for a large movement (~ um) of the right corner of ladder 0, increasing as pressure goes down
The ladder close to the one failing does not seem affected by the problem
Indication that the primer of a tungsten tile is probably failing
The tungsten tiles used for this tray belong to the first batch etched in Torino and primed in Plyform. One tray equipped with tiles from the same batch failed the TV tests at the level of the bare panel.
The trays currently under production are equipped with tiles etched and Goddard, which never showed such failures.