Precision passive alignment of wafers

Several macro-scale bench level experiments were carried out to evaluate the alignment repeatability that can be obtained through the elastic averaging principle. Based on these results, a precision passive alignment technique for wafer bonding application was developed. Wafer integral features that allow two stacked wafers to self-align were designed, fabricated and tested for wafer alignment repeatability and accuracy. Testing has demonstrated sub-micrometer repeatability and accuracy can be held using the proposed technique on 4 inch wafers. Passive alignment of the wafers is achieved when convex pyramids, supported on flexural cantlievers, and concave v-grooves patterned on the edges of the wafer engage and are preloaded. A silicon cantilever beam flexure between one of the wafers and the pyramid provides compliance to the coupling to avoid strain on the wafers and allows the surfaces of the wafers to mate. Both the concave coupling features and the convex coupling features are bulk microma-chined through wet anisotropic etch (KOH). The convex features are then release etched through a backside deep reactive ion etch (DRIE). As part of the fabrication process development, tests were performed to optimize the convex corner compensating mask structures needed to create the pyramid shaped convex coupling structures. Testing has shown that patterning two pairs of features on each of the four sides of the wafer is enough to achieve sub-micrometer repeatability.