Process for manufacture of microoptomechanical structures

DETAILED DESCRIPTION Inherent thin film properties of materials limit many surface micromachining processes.
For example, variability of materials properties in polysilicon thin films (such as, for example, Young's modulus and Poisson's ratio, residual stress, and stress gradients) can prohibit manufacture of desired microstructures.
This is particularly apparent in microoptical components such as mirrors, lenses, and diffraction gratings, which must be very flat for high-optical performance, and normally have to be made in the single crystal silicon layer.
Since conventional surface micromachining requires that all components be made in polysilicon layers, optical performance can be limited.
The leading commercial microelectromechanical (MEMS) processing technologies are 1) bulk micromachining of single crystal silicon, and 2) surface micromachining of polycrystalline silicon.
Each of these processing technologies has associated benefits and barriers.
Bulk micromachining of single crystal silicon, an excellent material with well controlled electrical and mechanical properties in its pure state, has historically utilized wet anisotropic wet etching to form mechanical elements.
In this process, the etch rate is dependent on the crystallographic planes that are exposed to the etch solution, so that mechanical elements are formed that are aligned to the rate limiting crystallographic planes.
For silicon these planes are the (1,1,1) crystal planes.
The alignment of mechanical features to the crystallographic planes leads to limitations in the geometries that can be generated using this technique.
Typical geometries include v-groove trenches and inverted pyramidal structures in (1,0,0) oriented silicon wafers, where the trenches and inverted pyramids are bound by (1,1,1) crystallographic planes.
Geometries that include convex corners are not allowed unless additional measures are taken to protect etching of the crystal planes that make up the corners.
The etch rate also varies with dopant concentration, so that the etch rate can be modified by the incorporation of dopant atoms, which substitute for silicon atoms in the crystal lattice