Heterogeneous integration by adhesive bonding
© Esashi and Tanaka; licensee Springer. 2013
Received: 3 March 2013
Accepted: 2 July 2013
Published: 6 August 2013
Wafer level adhesive bonding has been applied for the fabrication of micro systems which have heterogeneous components on LSI. Films or MEMS devices formed on a carrier wafer are transferred on a LSI wafer, which makes versatile heterogeneous integration possible. Film transfer processes and device transfer processes have been developed and applied to mirror array, resonator, piezoelectric switch, IR imager, tactile sensor, electron source and so on. Selective bonding to transfer devices on one carrier wafer to multiple LSI wafers has been developed.
KeywordsIntegrated MEMS Heterogeneous integration Adhesive bonding Wafer level transfer
The wafer level transfer process can be low cost comparing chip level processes, however for different chip size of MEMS from that of LSI selective transfer process described in V is needed for cost-effective manufacturing.
Film transfer process
The process can be used to transfer a film from the carrier wafer to the LSI wafer as shown in Figure 3 (a). After the adhesive bonding, the carrier wafer is removed and the remained film is used to fabricate MEMS devices on the LSI wafer. The MEMS fabrication process on the LSI wafer has to be compatible with the integrated circuit.
A. Monocrystalline-Si mirror array on CMOS LSI
B. CMOS-FBAR voltage controlled oscillator
Device transfer process (via last)
The device transfer processes have advantages that the MEMS can be fabricated independently from the LSI wafer. This enables wide range and optimized MEMS structures on LSI. In the via-last process the electrical interconnections from the MEMS to the LSI are made by metal vias after the transfer as shown in Figure 3 (b).
C. PZT MEMS switch
D. Infrared Imager
E. Tactile sensor network
Device transfer process (via-FIRST)
The via-first device transfer process from the carrier wafer to the LSI wafer uses metal bumps for the electrical interconnection and the mechanical holding of MEMS devices as shown in Figure 3(c). The metal to metal bump bonding methods as metal compression bonding and solder bonding can be used. There are some variations of the solder bonding as eutectic bonding or TLP (transient liquid phase) bonding which is called SLID (solid-liquid Inter-diffusion bonding) as well. The via-first process does not need electrical interconnection after the device transfer.
Adhesives are not mandatory for the via-first device transfer process but are used for temporary bonding of the devices on the carrier wafer or for an underfill (mechanical support). The via-first device transfer process without adhesive bonding has been applied for manufacturing of combined three-axis accelerometers and three-axis gyroscope at large volume for consumer product . The MEMS wafer is used as a package and the process (Nasiri fabrication) uses Al/Ge eutectic bonding at 450°C for electrical interconnection and the package sealing simultaneously.
F. AFM based data storage (Millipede)
G. Active matric electron emitter for massive parallel electron beam exposure systems
Selective transfer technologies
The heterogeneous integration on LSI using the wafer level transfer with adhesives is reviewed. This technology enables low cost production of value added devices. PZT which requires higher temperature than 600°C for depositing a thin film can be applied on LSI using this technology. Selective transfer from one wafer to multiple LSI wafers are described to cost effective production of different size MEMS chip from LSI chip. There are related items to this technology as adhesives, alignment for the bonding and debonding. The adhesives can be applied by coating, stamping and film laminating. The adhesive layers can be patterned or unpatterned and can stand relatively high temperature (400°C) as polyimide if needed. Transparent glass or self-alignment is required depending on the processes. There are various debonding methods to remove the carrier wafer as polymer etching, sacrificial layer etching and etching or mechanical grinding of the carrier wafer. In order to remove the polymer used for the wafer bonding polymer etching by ozone in acetic acid was developed .
Masayoshi Esashi received the B.E. degree in electronic engineering in 1971 and the Doctor of Engineering degree in 1976 at Tohoku University. He served as a research associate from 1976 and an associate professor from 1981 at the Department of Electronic Engineering, Tohoku University. Since 1990 he has been a professor and he is now in The World Premier International Research Center Advanced Institute for Materials Research (WPI-AIMR) and concurrently in Micro System Integration Center (μSIC) (director) in Tohoku University. He has been studying microsensors, MEMS (Micro Electro Mechanical Systems) and integrated microsystems.
Shuji Tanaka received B.E., M.E. and Dr.E. degrees all in mechanical engineering from The University of Tokyo in 1994, 1996 and 1999, respectively. From 1996 to 1999, he was Research Fellow of the Japan Society for the Promotion of Science. He was Research Associate at Department of Mechatronics and Precision Engineering, Tohoku University from 1999 to 2001, Assistant Professor from 2001 to 2003, and Associate Professor at Department of Nanomechanics from 2003 to 2013. He is currently Professor at Department of Bioengineering and Robotics. He was also Fellow of Center for Research and Development Strategy, Japan Science and Technology Agency from 2004 to 2006, and is currently Selected Fellow. He was awarded The Young Scientists’ Prize, The Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology in 2009 etc. His research interests include RF MEMS, MEMS-LSI integration and Power MEMS.
This study was supported by Special Coordination Funds for Promoting Science and Technology, Formation of Innovation Center for Fusion of Advanced Technologies and Funding Program for World-Leading Innovative R&D on Science and Technology.
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