MEMS | ARTICLE
Sealing Dispensing for MEMS Wafer Capping
Heakyoung Park, Nordson ASYMTEK, Carlsbad, CA, USA
<< Figure 2: Bond pad exposed after capping. >>
<< Figure 1: ADI Process [1]. >>
<< Figure 3: Wafer capping dispensing method. >>
Wafer capping technology in MEMS production has gradually become popular in many MEMS devices such as inertial sensors, oscillators and micro fluidic packages. During production, MEMS release timing is very critical. Wafer dicing and die handling are difficult after release because the MEMS devices become fragile. Wafer capping provides MEMS structure protection after release during the dicing process. A cavity wafer covers the MEMS structure by bonding the cavity wafer to the MEMS die wafer, and the two bonded wafers are diced together. In addition, wafer capping can provide a vacuum or low pressure with the right material and a hermetic or almost hermetic seal can be realised.
However, there are manufacturing challenges. First, wafer capping requires precise alignment. Second, a variety of bonding methods have technical drawbacks. Stencil or screen printing is one way to deposit sealing lines to cap wafers but consumes a relatively wide area as sealing width and may not work if the sealing line must go onto the MEMS die wafer which is not a flat surface. Dispensing, an alternative to printing, can apply a conductive or non- conductive sealing line on the MEMS die wafer even if it has a fragile MEMS structure. Precise alignment is still a challenge regardless of various capping methods.
Cap wafer screen printing method Cap wafer fabrication is done to make a cavity using different etching techniques such as deep-silicon reactive ion etch (DRIE). Bonding material, like seal glass, is applied to a cap wafer as shown in figure 1[1]. The material is applied only to the bonding area. Then the MEMS wafer and cap wafer are aligned together and bonding is completed by heating them to the fusion point of 430ºC. The bonded wafers are diced together. In some cases cap structures are partially etched and thus wire bonding pads are exposed as shown in figure 2.
Wafer capping dispensing method Some structures require sealant in the MEMS Si wafer and it is difficult to use a printing method on a device wafer that contains movable parts. Dispensing is attractive due to its flexibility to apply sealant in various shapes especially for areas with geometric constraint or three dimensional structure surfaces. For example, when the getter material is applied to the cap wafer, sealant material needs to be applied on the MEMS wafer. Sealant material like silver paste, bonding adhesive, UV curable material, or low- temperature solder, is applied to the MEMS wafer side. Rectangular shape sealant is common. Then the cap wafer such as
40 | commercial micro manufacturing international Vol 6 No.5
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