MEMS on LSI by Adhesive Bonding and Wafer Level Packaging
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MEMS on LSI by Adhesive Bonding and Wafer Level Packaging Masayoshi Esashi and Shuji Tanaka Tohoku University, 6-6-01 Aza-Aoba Aramaki, Aobaku, Sendai, 980-8579, Japan ABSTRACT Resonators and switches are fabricated on LSI for advance wireless communication systems. In addition to surface micromachining, adhesive bonding has been applied for the fabrication. Lamb wave resonators are fabricated for multi-frequency oscillators by surface micromachining. Multi-band filters are formed by a MEMS process after bonding a Si wafer to a LSI wafer (wafer bonding first approach). SAW filters are made by bonding a MEMS wafer to a LSI wafer (wafer bonding last approach). Variable capacitors are fabricated on a piezoelectric LiNbO3 wafer. Wafer level packaging techniques are developed to encapsulate the MEMS on LSI. Open collaboration as shared wafer, prototyping facility and hands-on access fabrication facility are discussed to reduce a cost and a risk in the development of the MEMS on LSI.
INTRODUCTION Integrated 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 are required for the MEMS and small feature size for the LSI. The MEMS on LSI can be fabricated by following three methods shown in Fig.1. MEMS encapsulated by wafer level packaging and open collaboration to reduce the cost and the risk are discussed below.
Fig.1 MEMS on LSI approaches
The “deposition based approache” (surface micromachining) in Fig.1 enable high density and reduced stray capacitance and inductance comparing the wafer bonding approaches, but need damage-free fabrication process to LSI. The “wafer bonding first approach” and the “wafer bonding last approach” in Fig.1 require adhesive bonding. The polymer used for the wafer bonding has to be removed by etching. Removal of polymers by ozone in acetic acid was developed for this purpose as shown in Fig.2 [1].
Fig.2 Removal of polymers by ozone in acetic acid MEMS ON LSI Deposition-based approach (Surface micromachining) AlN Lamb wave resonators were fabricated for on-chip multi-frequency oscillators for the purpose of timing generators like quartz oscillators [2]. The resonant frequency is determined by the pitch of the electrode under the piezoelectric AlN layer and hence multi-frequency onechip oscillator can be made. The fabrication process is shown in Fig.3. SiO2 is deposited on a Ge sacrificial layer. Mo is deposited and patterned as an inter-digital transducer and then AlN is deposited at 300℃ by a reactive RF sputtering using Al as a target in nitrogen gas environment. After forming Cr-Au top electrodes the Ge is etched out using H2O2. This process does not damage the LSI. Surface-micromachined MEMS on SAW (Surface Acoustic Wave) device were fabricated for the purpose of tunable filters [3]. The fabrication process, the photograph and the tunable ladder filter circuit are shown in Fig.4. Electro
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