<< Figure 4: a) Before sealant applied (LEFT); b) After sealant dispensing (BELOW). >>
<< LEFT | Figure 6: Flow rate change over time. >>
<< Figure 7: a) Typical starting point (LEFT); b) A few variables were adjusted to minimise ‘dog- bone’ effect (RIGHT). >>
a glass or silicon wafer is aligned with the MEMS wafer. When a glass cap wafer is used with UV curable material, UV light penetrates the glass wafer to cure the adhesive [2]. Once the base and top wafers are bonded together, dicing is performed at the wafer level while the released MEMS structure is protected. Figure 3 illustrates the sealing dispensing method.
Dispensing Requirements There are many challenges to meet packaging requirements for MEMS devices. High yield and throughput are critical to realise low-cost solutions in volume manufacturing environments. The following section details three main topics. First, the choice for dispense sealant is important for its end use. The right sealant material for the end application is selected based on defined life time, reliability concerns, and functionality of the device. Second, consistent volumetric accuracy is a must to achieve a process yield required for volume production. Lastly, the throughput impact of sealing dispense will be studied using a specific case example. Throughput study will guide the
manufacturer to choose the right system and applicators based on their specific application requirements.
1) SEALANT MATERIAL Many varieties of capping technologies are used today based on reliability requirements ranging from solder bonding, anodic bonding, eutectic bonding, and adhesive bonding. Each method has different hermetic levels and different tools for bonding. The MEMS designer will choose the right material for their packaging requirements based on the lifetime of the device. Dispensing can be one option if liquid-type sealant is used, like epoxy adhesive, UV cure material, silver paste and solder paste. Adhesive may not meet the hermetic requirements for medical or military applications. Consumer devices can accept less reliable, but cost-effective approaches; however, time-to-market could be much more critical. Wafer capping is attractive in high- volume production because yield can be improved by MEMS’ immediate protection, and handling is easier during dicing, pick and place, and wire bonding.
41 | commercial micro manufacturing international Vol 6 No.5
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