Investigation on tray MID 063 Back Gammaray Large

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

Mechanical characteristics of tray MID 063: all the geometrical data are very good Mechanical tray planarity (without SSDs): • Top side 0. 050 mm • Bottom side 0. 014 mm SSDs relative positional error: • Top side ± 0. 019 mm • Bottom side ± 0. 016 mm SSDs planarity: • Top side • Bottom side 0. 053 mm 0. 087 mm Tray total thickness 30. 372 mm (nominal thickness = 30. 72 mm) Strips alignment to the tower (software alignment) • Top side shift= 0. 013 mm parallelism= -0. 012 mm (-0. 033 mrad) • Bottom side shift= -0. 006 mm parallelism= -0. 050 mm (-0. 143 mrad) Top side efficiency = 99. 4%, Bottom side efficiency = 99. 8% Investigation on MID 063

Statement of the problem Channels on the right side are completely dead – i. e. Preamplifiers inputs shorted to ground 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. Investigation on MID 063

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. Investigation on MID 063

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 300 mm under normal conditions). – Tray electrically tested before going into the vacuum chamber. Wire bond Silicon wafer Tray panel MCM Investigation on MID 063

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 Investigation on MID 063

March 24, 2005 • Same setup as the day before: – The short still shows up consistently @ ~ 100 mbar Investigation on MID 063

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 k. Ohm protection resistor) Investigation on MID 063

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. Investigation on MID 063

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). Investigation on MID 063

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 Investigation on MID 063

Ladder height vs. pressure Investigation on MID 063

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. . . Investigation on MID 063

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. Investigation on MID 063

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. Investigation on MID 063

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. . . Investigation on MID 063

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? Investigation on MID 063

Conclusions Number of wire bonds in place P_short (mbar) 386 (one with a shorter loop) ~270 386 ~100 150 ~10 65 - Investigation on MID 063