Deep Through Silicon Via in Laser-Ablated CMOS Multi-Project Wafer for Surface-Mountable Integrated MEMS

Inhalt:
A surface-mountable MEMS integrated with a signal-processing LSI by wafer bonding technology is one of the most advanced configurations of MEMS. This configuration was applied to a bus-network tactile sensor, where band pads were located an the backside of the LSI via through-silicon vias (TSVs). As shown in Fig. 1 (a), the tactile sensor is directly surface-mounted an a flexible bus line. Therefore, the bonding pads must be located an the backside, because the topside is a sensing side and must be free from bonding wires. In addition, the tactile sensor is physically touched and thus must be mechanically robust. Ta answer these requirements, the device structure using through silicon vias (TSV) in a CMOS LSI shown in Fig. 1 (b) is preferable. At the R&D stage, the production of a CMOS LSI in a foundry is expensive, and a multiproject wafer (MPW) service is a realistic way to obtain an originally-designed CMOS LSI. The MPW contains multiple kinds of LSI from different users. Each user can receive its own LSI an full wafers, where the other LSls are erased by laser ablation, as shown in Fig. 2. Therefore, we need to fabricate TSV in such a laser-ablated MPW and then integrate it with MEMS by wafer bonding. Actually, our sensor platform LSI for the tactile sensor was prototyped on originally 0.18 µm and then 0.13 µm TSMC MPWs. Our challenges to fabricate TSVs in the MPW include that the MPW is laser-ablated to destroy the other customers' LSI and the surface is heavily damaged. Another challenge is that deep TSVs are necessary because the LSI must be as thick as 300 µm for the mechanical robustness of the tactile sensor. This paper discusses the later challenge.