Our back-illuminated CCDs are flip chip bonded to allow mounting their front side against a stable support substrate.  The flip chip process also allows electrical interconnection from the CCD front side bonding pads to matching pads on the substrate.  Gold bumps are placed on the CCD bonding pads prior to flip chip bonding.

At left, gold stud bumps are applied to the pads of a 4kx4k device.  We use a flip chip bonding machine to align the device with the substrate and apply the necessary heat and pressure (see below).  We have two bonders capable of bonding 4" and 6" CCDs, wafers, and substrates -- one is infrared while the other has an optical probe for non-IR transparent materials such as ceramics.

A very important goal of our CCD optimization has been to produce a flat and stable imaging surface .  Our target flatness is to maintain the imaging surface to within 20 microns of a plane, peak-to-valley.  The flip chip process allows this flatness to be maintained because the CCD is forced against a custom silicon support substrate.  Interferometic measurements of thinned 2048x2048 CCDs show we can meet this flatness specification as well as exceed it when required.

After flip chip bonding, the CCD is attached to the silicon substrate only by the adhesion of the bumps.  To ensure mechanical stability, we underflow epoxy between the CCD and substrate.  

Larger devices are more difficult to underflow due to the large area involved.  We have developed ways to underflow large devices, such as 4kx4k CCDs and 150 mm wafers, without producing bubbles of air (voids) under the chip.  Air bubbles, although not visible at the underflow stage due to device thickness, become problematic after a device is thinned.