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

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 < 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.

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%

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.

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

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

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

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)

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.

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).

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

Ladder height vs. pressure

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...

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.

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.

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...

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 performed @ the minimum pressure, results are identical to those obtained in air.

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 (~500-700 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 primed @ Goddard, which never showed such failures.