SCUBA-2 Detector Technology Development at the University of Edinburgh

Current Status Low stress silicon nitride process conditions have been characterised PECVD and deep etch ICP equipment delivery expected 10th April 2001 Investigation of possible sources of indium? bump bonding continuing Technology options still to be agreed

Nitride Process

Process sequence (1) Grow oxide on 3” silicon wafer Deposit Metal A layer on back surface (400 /) Deposit 1µm PECVD SiO2 onto metal (CMP surface finish?) Etch 10 µm wide trench around perimeter of pixel Thin second wafer & polish both sides. 33 µm for 450 µm or 62 µm for 850 µm wavelength

Process sequence (2) Bond thin wafer to metallised side of thick wafer Deposit low stress silicon nitride on back face of thin wafer 8) Deposit TES layers on silicon nitride

Process sequence (3) Pattern TES structure & form heater Pattern nitride layer, if required Deposit leads & aluminium bond pads Deep etch top wafer to define pixel area Etch trench in thin wafer through nitride layer

Process sequence (4) Fabricate SQUID circuitry on bottom wafer Form bump pads & indium bumps on front of squid wafer 16) Self align bump bond SQUID wafer to sensor wafer

Spacer Option 1 Detector wafer TES 140 / 240 m Spacer wafer Plated vias SQUID wafer

Spacer Option 2 TES Detector wafer Plated vias 140 / 240 m SQUID wafer TES Plated vias 140 / 240 m

Spacer Option 3 Detector wafer TES 140 / 240 m Spacer wafer Plated vias SQUID wafer

Spacer Options Issues High apect ratio through wafer etch required Through hole plating required Weakening of waffle grid (option 2) Gap set by solder Bump bonding with membrane formed Thin spacer wafers required (cost + processing) Large solder bumps (option 3)

Bonded Technology Option Issues Bump height not critical Bump bonding before membrane formation possible Thin wafers or grinding technology required Need conductor 400 / ȱ that can withstand high wafer bonding temperature, adheres well to oxide and is not superconducting Thin polished wafers will be expensive Wafer bonding technology toolkit required Wafer bonding technology needs to be developed

Bonded Technology Toolkit Essential tools Wafer bonding tool Bond inspection kit (IR) Particulate inspection tool Class 1 area Optional tools Wafer grinding tool (thinned wafer $200) - cycle time/ cost trade-off Thickness measurement (?)