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Research Papers

Vacuum Packaging of MEMS by Self-Assembly

[+] Author and Article Information
Royi Naveh

Department of Mechanical Engineering,  Technion Israel Institute of Technology, Haifa 32000, Israel

Eyal Zussman1

Department of Mechanical Engineering,  Technion Israel Institute of Technology, Haifa 32000, Israelmeeyal@tx.technion.ac.il

1

Corresponding author.

J. Electron. Packag 134(2), 021003 (Jun 11, 2012) (7 pages) doi:10.1115/1.4006138 History: Received June 19, 2011; Revised February 02, 2012; Published June 11, 2012; Online June 11, 2012

The development of a solder-based vacuum bonding technique for micro-electro mechanical systems (MEMS) applications is presented. A chip with a micro-sensor was bonded to a cover plate to form a sealed cavity. The method relies on a solder-based hybridization comprising a self-assembly process that takes advantage of the surface tension and viscous forces of the solder. A model of the assembly was developed to predict the capillary instability of the solde, and the dynamic behavior of the assembled chip. Experimental results showed that a molten bead with parallel contact lines is stable when the ratio between the solder height and solder width is less than one half. Misalignment attributed to the self-assembly process was within a few microns. Fractographic analysis and leak and shear tests confirmed the predicted sealing and mechanical characteristics of the bonding. This method is especially suitable for bonding wafers in a vacuum for MEMS and other micro-devices, at low manufacturing temperatures (∼250 °C).

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Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Overview of the vacuum packaging of a MEMS chip over an optical chip

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Figure 2

Geometry of a perturbed solder drop of height h with arrested, parallel contact lines along a pad with width b and wetting angle θ

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Figure 3

A cross-section of the assembly shows two sessile drops (two-dimensional), resting on chip 1. Chip 2, the upper chip, moves under the action of surface tension forces and gravity. (a) Chip 2 at equilibrium, and (b) chip 2 tilted prior to the self-assembly process.

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Figure 4

Dependence of λ(R) for Ca=1, and Bo=0 for d2/S in the range 3.8–40

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Figure 5

Top view of fabricated chips including (a) a solder ring with rounded corners, and (b) a solder ring with straight corners

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Figure 6

The aligner apparatus with alignment marks projected on the computer monitor

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Figure 7

Solder geometry after reflow for (a) solder width b=150μm, and (b) a geometrically uniform solder with width b=600μm

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Figure 8

Assembled structure including the sapphire layer and solder width b=600μm viewed from (a) the top, and (b) zoomed-in at the upper-left corner

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Figure 9

Cavity height variation for different solder widths for experimental, numerical, and analytical models (Ro refers to geometry with rounded corners)

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Figure 10

Misalignment after self-assembly versus manual assembly processes, measured by controlled vertical force (Ro refers to geometry with rounded corners)

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Figure 11

Assembly that failed in the gross-leak test (solder width b=600μm). The contact line of the IPA droplet is observed in the sealed cavity.

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Figure 12

SEM photo of an assembly cross-section

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Figure 13

SEM magnification of a solder cross-section

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Figure 14

SEM image of a gold bump embedded in the solder

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