Formation of thick spin-on glass (SOG) sacrificial layer for capacitive accelerometer encapsulation

This paper presents a method to form thick spin-on glass (SOG) sacrificial layer for accelerometer encapsulation fabrication. SOG is chosen as the sacrificial material because it is easy to apply, has good thickness uniformity, and can be easily etched back before densification. Siloxane type SOG is applied on blank wafers and accelerometer patterns by multiple spin, bake, and cure processes. A series of gradual hot plate baking up to 250°C are experimented for each spun layer. After multiple spin and bake, the SOG layers are etched back in hydrofluoric acid (HF) solution of various concentrations to form rectangular encapsulation bases. 25 samples are prepared for SOG thickness uniformity characterization. Center thickness and four perimeter thickness measurements are taken for each sample using thin-film mapper. These five measurements are averaged to get sample thickness. Two surface profiler measurements are taken for each sample perpendicularly to each other using Tencor surface profiler. The minimum reading is subtracted from the maximum reading to get sample variation. Upon SEM inspection, mildly sloped etched walls from HF etching are observed. No surface cracking was visible. Shallow trench patterns are apparent on SOG deposited on accelerometer pattern. The average sample thickness is 5 μm with 3.7% thickness variation across samples. The average variation within each sample is 0.14 μm with an average of 2.6% thickness variation within sample. These thickness variations are acceptable for encapsulation structure deposition.