Development of a stress compensation layer for thin pixel modules 3D assembly

The pixel modules are the fundamental building blocks of the ATLAS pixel detector system used in CERN LHC facility. They consist in their basic form of a silicon sensor that is flip-chipped bonded to a CMOS read-out integrated chip (ROIC). One of the main objectives for the ATLAS experiment is to develop an approach towards low mass modules and thus reducing radiation length. From the module perspective this can be achieved by using advanced 3D technology processes that includes the formation of copper and solder micro-bumps on top of the ROIC front-side, the thinning of both the sensor and the CMOS ROIC and finally the flip chip assembly of the 2 chips. The thinning of the silicon chips leads to low bump yield at the solder reflow stage due to bad co-planarity of the two chips creating dead zones within the pixel array. In the case of the ROIC, which is thinned to 100um, the chip bow varies from − 100 μm at room temperature to + 175 μm at reflow temperature resulting of CTE mismatch between materials in the CMOS stack and the silicon substrate. Our objective is to compensate dynamically the stress of the front side stack by adding a compensating layer to the back-side of the wafer. Utilising our material thermo-mechanical database coupled with a proprietary analytical simulator and measuring the bow of the ROIC at die level we are able to reduce the bow magnitude by approximately a factor of 3 by the introduction of the compensating layer. We show that it is possible to change the sign of the bow at room temperature after deposition of a SiN/Al:Si stack. This amplitude of the correction can be manipulated by the deposition conditions of the SiN/Al:Si stack. Further development of the backside deposition conditions are on-going where the target is to control the room temperature bow close to zero and reducing the bow magnitude throughout the full solder reflow temperature range hence conserving bump yield. In keeping with a 3D process the materials used are compatible with Through Silicon Via (TSV) technology with a TSV last approach in mind should we integrate this technology in the future.