The BTeV baseline pixel detector, like most other pixel systems being developed for high energy physics experiments, is based on a design relying on a hybrid approach. With this approach, the readout chip and the sensor array are developed separately, and the detector is constructed by flip-chip mating of the two. This method offers maximum flexibility in the development process, choice of fabrication technologies, and the choice of sensor materials. However, it requires the availability of a highly reliable, reasonably low cost, fine-pitch flip-chip attachment technology.
There are several bonding technologies which may be suitable for this. These include indium bumps, Pb/Sn solder bumps, fluxless solder bumps, and anisotropic conductive films (ACF) or tapes. Given the fact that we are mostly dealing with dies (for the readout chip) so far, and that our pitch is extremely fine, we chose to go with indium bumps. Even though, our recent contacts showed that Pb/Sn solder has been made to work at various places for a fine pitch, and they are a bit cheaper than indium bumps.
Both pure In and Pb/Sn solder appear to meet our requirements for providing adequate electrical and mechanical bonds at 50-microns pitch. Our task is to start qualifying vendors with facilities to provide such services to outside customers and demonstrate the quality and yield of their bump bonding services by providing bumping and bonding of a large number of dummy pixel detectors.
For this purpose, we have designed a dummy
assembly module consisting of an upper and a lower array. As much as possible,
the two arrays will be close to the size of the real sensor and readout chip
that we will have.
Picture showing wafer layout of joint BTeV/CMS daisy-chained patterns. To be fabricated at MCNC/UNITIVE.