A close proximity self-aligned shadow mask for sputter deposition onto a membrane or cavity

In this paper we report on the fabrication of a close proximity shadow mask designed for sputtering into cavities or onto the back surface of freestanding silicon nitride (SiNx) membranes. Sputtering into a well-defined area on a fragile surface is difficult since sputter deposition through a shadow mask separated from the deposition surface typically results in spreading of the deposited material. The area of spreading beyond the desired area of deposition depends on the vertical separation between the shadow mask and the surface of the membrane. In our design, a high degree of accuracy (+/- 5 mu m) in the separation (25 mu m) between the shadow mask and the deposition surface is achieved. The shadow mask is made from SiNx-coated silicon wafers, using potassium hydroxide (KOH) etching on both sides of the wafer. The design rules chosen to maintain accuracy of the fit between the shadow mask and the deposition surface over various etch conditions, fabrication and methods used for convex-corner compensation and the alignment to the wafer crystal axis, which also contribute to an accurate fit, are discussed. Spreading of deposited material due to sputtering is limited to about 40 mu m when this shadow mask is used.