Flip chip bonding substrates are used to mechanically support the back illuminated detectors and to provide electrical I/O connection from the device bond pads to wire bond pads which lead to the package.  We use silicon support substrates to provide an exact thermal expansion match to the thinned CCDs (see picture at right, substrates on a 150 mm wafer).  The bumps must be composed of materials which will form reliable cryogenic connections to both the metalization on the device (aluminum) and on the substrate (gold).  We have developed the gold-to-aluminum bumping process using a nickel diffusion barrier which can be done both in-house and by an outside vendor when quantity is needed.

The substrates, currently fabricated on double-side polished, ~1375 microns thick 100 or 150 mm silicon wafers, consist of 20-30 micron high indium bumps atop a nickel-plated diffusion barrier and gold traces (above, with photoresist spinner below).  Indium is used because of its excellent cryogenic characteristics.  We have developed an in-house indium and nickel plating system (left) that assures high quality bumps are grown at heights within ±2 microns across a die.  The tall bumps are needed so that a reliable electrical contact is made between CCD and substrate for very large area die.

In order to avoid the use of substandard substrates, we have implemented a quality assurance program for both internally and externally fabricated substrates.  All parts go through intensive testing to insure that good CCDs go on equally good substrates.

Close-up of substrate traces with gold trace, nickel diffusion barrier, and indium bump visible.