Temporary wafer bonding - key technology for MEMS devices
Back thinning after temporary wafer bonding is a key technology for three dimensional (3D) integration of sensors and electronic components to obtain miniaturized systems. Ultrathin silicon wafers are difficult to handle therefore various temporary bonding approaches have been developed to temporarily stabilize and protect them during the subsequent fabrication steps. We have characterized two of these methods: The BrewerBOND (R) with a mechanical release and ZoneBOND (R) with a combined chemical/mechanical release. This paper presents a detailed description of the wafer preparation steps, temporary bonding and final debonding processes. The ZoneBOND (R) method involves the spin coating of a device wafer with an adhesive and tolerate high-temperature processes up to 250 degrees C. The carrier is coated with another adhesive at the wafer edge. The remaining area in the center of the carrier wafer is with an anti-sticking layer. The spin coating of the device and the carrier wafers is carried on the semiautomatic RCD8 spin coater from SUSS MiCROTEC. The device wafer is prepared by the deposition of a layer thickness of 15-30 mu m of an adhesive (ZoneBOND (R) 5150-30). The first step of the carrier wafer fabrication on the RCD8 is the formation of a zone on the wafer edge with the adhesive ZoneBOND (R) EM 2320-15. An exact centering of the carrier wafer on the vacuum chuck is important for the wafer edge zone to get uniform coating. The thickness of the layer is in the range of 0.5 - 3 mu m. After baking at 220 degrees C, an anti-stick layer (ZoneBOND (R) ZI 3500-02) is dispensed on the center of the carrier wafer. The subsequent adhesive wafer bonding is realized by using a SUSS MICROTEC SB8 wafer bonder. The adhesive wafer bonding is carried out under vacuum (process pressure <5 mbar) with a 170kN/m2 bonding pressure. During the bonding process, two zones are formed. In the middle of the wafer, there is a fragile adhesive region (zone 1). In contrast to the zone 1, there is a strong adhesive area on the wafer edge (zone 2). After chemical, mechanical and thermal processes (T < 250 degrees C), the edge zone of the strong adhesive area is released with limonene or mesitylene. Then, the wafer stack with the device wafer face down is mounted on a tape frame and placed in a de-bonding system (DB12T, SUSS MICROTEC). In the DB12T debonder, the mounted wafer stack is fixed on both sides by vacuum. The mechanical separation of device and carrier wafers is done by a blade over the C-cut at the wafer edge. The carrier wafer is clamped over the blade and vertically moved on a flex plate over power cylinder at room temperature. In the [ mal step, the device wafer on the mounted tape frame and the carrier wafer are cleaned. Similar to the edge release process, the cleaning is performed in the SUSS MICROTEC AR12 module with mesitylene. With this process flow, a thinned 6inch wafer down to 50 mu m can be fabricated. Beside the ZoneBOND method new material provided by Brewer Science was tested. As a release process, an easy to integrate mechanical release is used. Two silicon substrates were bonded with polished front side, without structure and with an edge trim on the front side of the device wafer. The Coating on full surface of device and carrier wafer makes this process to an easy solution for wafer thinning down to 50 mu m. Device preparation with an adhesive BrewerBOND (R) 305 saves the structures and prevents from destroying them during the bonding step. The material enables backside temperature processing at 250 degrees C - 300 degrees C. The carrier is coated with the release material BrewerBOND (R) 510, which allows a mechanical room temperature debonding process without additional laser or chemical processes.