Microwave heating is the basis of a simple technique for quickly and gently bonding two metallized dielectric or semiconductor wafers to each other. The technique can be used, for example, to bond a flat, gold-coated silicon wafer to another gold-coated silicon wafer that is flat except for a cavity, in order to hermetically seal the cavity (see figure). The technique has the potential to become a standard one for bonding in the fabrication of microelectromechanical systems (MEMS).

Test Pieces containing square recesses were bonded to form hermetically sealed rectangular parallelepiped cavities. The closeup micrograph clearly shows the fusion of metallic layers.

The predecessor of this technique is thermocompression bonding, in which two substrates to be bonded are clamped together with considerable pressure and the entire resulting assembly is heated to melt eutectic metal alloy coats on the faying substrate surfaces. (Even though elemental metals could be more desirable under some circumstances, eutectics are used because they have lower melting temperatures.) The bonding process can take as long as 24 hours. The heat and pressure can degrade the product; the degradation can include deleterious effects of clamping stresses, diffusion of the metal into the substrate material, and diffusion of substrate material into the metal bond.

In the present technique, bonding could be effected in a few seconds, with minimal or no clamping, and without heating the entire assembly. Two pieces to be bonded are simply placed (e.g., one atop the other), in a microwave cavity. The position and orientation of the pieces in the microwave cavity is chosen to optimize coupling of the metal in the bond with the electromagnetic mode that is to be excited in the microwave cavity. The microwave cavity is evacuated to prevent the formation of a plasma. A pulse of microwave power (typically a few hundred watts for a few seconds) is applied.

Because the substrates are nearly transparent to microwaves in the presence of metal layers, heating by the microwave field is concentrated in the metal layers in the bond region. More precisely, by virtue of the electromagnetic skin effect, most of the deposition of electromagnetic energy occurs within a skin depth ≈1 µm at microwave frequencies) at the surface of the metal. Thus, heating is concentrated exactly where it is needed — at the interface between the two metal layers that one seeks to melt together. By the time the pulse is turned off, the metal layers have been melted together, yet the substrates remain cool. Of course, heat is conducted from the interface to adjacent depths, but the resulting heating of the substrate is transient and minimal — not enough to cause appreciable diffusion of metal or substrate material.

The figure depicts some aspects of silicon workpieces that were fabricated and tested to demonstrate the present technique. Each piece started as 5-mm square silicon wafer. A strip 2 mm wide around the edge of one face of each piece was coated with Cr to a thickness of 150 Å, then Au to a thickness of 1,200 Å. The gold coat was to serve later as the bonding metal. Each piece was etched to form 3-by-3-mm, 100-µm-deep recess in the middle of one face; the recess was to become half of a hermetically sealed cavity. Then pairs of these pieces were bonded to form the hermetically sealed cavities. In a test of their hermeticity, the bonded pairs were found to leak at low rates comparable to the background level of a leak-measuring mass spectrometer.

This work was done by Nasser Budraa, Martin Barmatz, John Mai, Tom Pike, and Henry Jackson of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Manufacturing & Prototyping category.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel
NASA Resident Office–JPL; 818-354-4770.

Refer to NPO-20608.

This Brief includes a Technical Support Package (TSP).
Hermetic Water Bonding by Use of Microwave Heating

(reference NPO-20608) is currently available for download from the TSP library.

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This article first appeared in the May, 2001 issue of NASA Tech Briefs Magazine.

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