Micro systems for sustainable society

Abstract

Wafer level adhesive bonding has been applied for the fabrication of micro systems which have heterogeneous components on LSI. Devices formed on a carrier wafer are transferred on a LSI wafer, which makes versatile heterogeneous integration possible. This has been applied to resonator, piezoelectric switch, tactile sensor, electron source and other value-added devices. Keywords-Micro system; MEMS; hetero-integration; adhesive bonding; wafer level packaging Ⅰ. INTRODUCTIONIntegrated MEMS (micro electro mechanical systems) as capacitive sensors and arrayed display have been produced successfully. MEMS as switches and filters fabricated on CMOS LSI are needed for future multi-band wireless systems, in which good mechanical properties or piezoelectric materials are required for the MEMS and state of the art for the LSI. Such hetero-integration can be performed by transferring MEMS devices or a film fabricated on a carrier wafer to a LSI wafer by adhesive bonding as shown in Fig.1 [1]. The LSI wafer is not damaged during the MEMS fabrication because the MEMS and the film are fabricated on a separated carrier wafer and the adhesive bonding can be carried out at low temperature. MEMS on LSI is encapsulated by WLP (wafer level packaging) [2] and finally diced to each chips. The wafer level batch transfer process can be low cost comparing chip level processes. For different chip size of MEMS from that of LSI selective transfer process described in Ⅱ is needed for cost-effective manufacturing. Figure 1. Concept of hetero-integration (MEMS on LSI) by adhesive bonding and wafer level packaging.Figure 2. Photograph of SAW resonators transferred to a LSI wafer. Ⅱ. HETERO-INTEGRATION BY SELECTIVE TRANSFERWafer level selective transfer from one carrier wafer to multiple target wafers are needed for cost-effective integration of different size chips. The selective transfer technology was applied to SAW (surface acoustic wave) resonators on LSI as in Fig.2 [3]. SAW resonators are fabricated on a LiNbO 3 wafer and the wafer is bonded to a glass carrier wafer using an adhesive and diced to make grooves. The flipped carrier wafer is bonded to the LSI wafer using bumps. Underfilling by polymer is carried out after debonding from the glass carrier wafer by laser.PZT (lead zirconate titanate)-actuated MEMS switches were fabricated on a LSI wafer as in Fig.3 [3]. The piezoelectric MEMS switch works at lower driving voltage and occupies smaller area than electrostatic MEMS switches. For the MEMS switch, the PZT was deposited on a Si wafer by a sol-gel method. Symmetrical structure made of two PZT layers is formed in order to prevent a bending. They are transferred to the LSI wafer using the adhesive bonding. After connecting the MEMS and the LSI using electroplated metal, the polymer was removed by O 2 plasma to release the MEMS switches. Figure 3. PZT MEMS switch on LSI