Surface micromachining techniques for integrated microsystems.

摘要

On-chip integration of circuitry with MicroElectroMechanical Systems (MEMS) is vital to the performance of many types of microsensors. Unfortunately, the most common material for surface micromachining, LPCVD polysilicon, requires processing temperatures that are incompatible with pre-fabricated standard CMOS circuitry. Sputtered films, by contrast, can be deposited directly atop CMOS circuitry at room temperatures. Both sputtered aluminum and sputtered silicon were examined as structural layers for electrostatic surface-micromachined MEMS.; Sputtered aluminum microstructures built atop polyimide sacrificial layers warped when released in oxygen plasma. The plasma release parameters were discovered to be a dominant factor in the warping. Design and process techniques to reduce warping of electrostatic parallel plate devices were explored and evaluated. Using these techniques it was possible to build released aluminum-based variable capacitors. However, fabrication complexity increased and the techniques did not completely eliminate warping, as evidence by the inconsistent actuation voltages of the variable capacitors.; Released curvatures of sputtered silicon cantilevers depended on thickness but were more consistent than in aluminum cantilevers. A model based on surface stresses as the dominant source of the strain gradient in the films was proposed and predicted a released radius of curvature proportional to thickness squared, matching empirical results. Several properties of sputtered silicon films were investigated. Improvements in the electrical conductivity of completed structures were realized by cladding the sputtered silicon structural layers in symmetric, 50 nm thick layers of titanium-tungsten.; Sputtered silicon microstructures made using oxide sacrificial layers were wet-released in buffered HF. It was discovered that sputtered silicon was permeable to buffered HF at film thickness of up to 5 microns. Using this permeability, buried cavities were made in underlying oxide layers. Structures made using polyimide as a sacrificial layer were dry-released in oxygen plasma, which eliminated the need for critical point drying normally required to prevent stiction caused by capillary forces in wet release processes.; The dry-release process had negligible effect on underlying CMOS transistors. As a demonstration of the dry-released TiW-clad sputtered silicon process, electrostatically deflected plate structures with integrated capacitance measurement circuitry were fabricated. This work represents the first application of sputtered silicon to integrated MEMS.