Surface micromachining is the fabrication of micromechanical structures by deposition and etching of thin structural and sacrificial films. Thus, simple microstructures like beams or membranes as well as complex structures like linkages or encapsulated resonators can be fabricated on top of a silicon substrate. A processing sequence using polysilicon as micro-structural material and silicon dioxide as sacrificial layer is shown in the figure below.
The main features of the surface micro-machining technology are the small microstructure dimensions and the opportunity to integrate micromechanics and microelectronics on the same chip. By use of VLSI compatible batch processing, low cost microstructure fabrication can be achieved for high volume applications.
There are three key challenges in fabrication of microstructures using surface micromachining:
- Control of stress and stress gradient in the structural layer to avoid bending or buckling of the released microstructure
- High selectivity of the sacrificial layer etchant to functional layers
- Avoidance of sticking of the released microstructure to the substrate
CVD and thermal silicon oxide films are used as sacrificial layer, which can be etched with high selectivity against silicon using hydroflouric acid. However, after wet-etching of the sacrificial layer, rinsing and drying the microstructures causes the structures to be pulled down and to stick to the substrate by capillary forces.
Two methods to prevent this unwanted situation are used at microfab: the first process uses gaseous hydroflouric acid, combined with a temperature controlled substrate heater. Thus, moisture precipitation can be avoided resulting in a high yield microscopic release. The second process uses the supercritical phase transition of carbon dioxide above the critical point. After etching the sacrificial layer and rinsing, the rinsing liquid is exchanged by liquid carbon dioxide, which is subsequently transferred in the supercritical state. The phase transition liquid-gaseous is thereby avoided, no capillary forces can occur. The apparatus for supercritical drying installed at microfab for the first time allows semi-automated batch processing of up to three 4" wafers including wet etching and rinsing steps.