Fracture fabrication of precision single-crystal silicon surfaces for MEMS devices

Processes for creating complementary and smooth surfaces by deliberately fracturing a weakened portion of a larger single-crystal silicon structure are described.The structure was designed to refine and concentrate an externally applied load pre-fracture and to provide precision guidance post-fracture. Under the right conditions, material is not ejected from the fracture zone, and the extreme brittleness of crystalline silicon produces complementary fracture surfaces; “closed” post-fracture gaps of 20–30 nm are typical. The effects of lithographic, focused ion beam, and anisotropically etched notches as well as crystal orientation and specimen thickness were studied, and a method was developed for fabricating smooth surfaces perpendicular to the wafer plane: 10 μm square specimens oriented with the (1 1 0) plane fully notched by anisotropic etching (KOH) and fractured within a structure optimized to apply pure tension during fracture.While developing the fabrication process for a variable capacitor device, it was found that the high temperature processing associated with the thermal oxidation necessary for growing the actuator’s dielectric layer blunted the anisotropically etched notches. This blunting manifested itself as material ejecting fractures and occurred even when the notches were covered with a nitride diffusion barrier. The blunting was overcome by employing the actuator’s thermal oxide dielectric layer as a mask for etching the notch after all high temperature processing was complete, indicating it was likely due to diffusional smoothing and that high temperature processes may be useful for strengthening anisotropically etched structures.