Two kinds of micromachining are commonly used to produce MEMS: (i) surface micromachining and (ii) bulk micromachining. Another popular method of MEMS fabrication involves molding processes such as LIGA.
A greater portion of this route have evolved directly from the CMOS (complementary metal-oxide-semiconductor) processes used to fabricate VLSI (very large scale integration) chips. In VLSI devices the layers are deposited, patterned and etched to yield highly integrated electron devices with very small feature sizes. In surface micromachined MEMS the layers are patterned and etched to produce electro-mechanical elements or are used as sacrificial layers to allow motion of the mechanical layers.
Example of commercial surface micromachined MEMS include micro-accelerometer chips (e.g., ADXL family by Analog Devices, Inc). These devices are used as sensing element in vibration sensors, in automobile air-bag deployment mechanism, and in controlling mirror arrays in portable projectors. Surface micromachining is typically limited to a layer thickness of ~ 5 µm.
Bulk involves etching of silicon wafers or other substrates to produce features directly onto it. Etch processes for MEMS fabrication deviate from legacy etch processes for integrated circuit industry. While many of the etching processes are based on basic scientific principles, etching for micromachining is still an art.
The objective is to selectively remove material using imaged photoresist as a masking template. Typically, the microelectronic elements are created using CMOS processes on the top side of the silicon wafer. Bulk micromachining is then commenced from the other side of the wafer to yield mechanical elements such as thin diaphragms, beams or cantilevers on the top side of the wafer.
Isotropic etchants etch uniformly in all directions, resulting in rounded cross-sectional features. Anisotropic etchants, on the other hand, etch in one direction preferentially over other directions. This process is used to produce trenches and cavities delineated by flat and well-defined surfaces. The etch medium (wet vs. dry) plays a role in selecting a suitable method.
Wet etchants in aqueous solution offer the advantage of low-cost batch fabrication, usually 20-25 wafers can be etched simultaneously [1]. Dry etching, othe other hand, involves the use of reactant gases in a low pressure plasma.
Fig. 1. Cross-sectional trench prifiles resulting from four different types of etch methods (adapted from ref. 1).
Isotropic wet etching: The most common of isotropic wet etchant is "HNA," also known as "poly-etch" because of its use in the early days of the integrated circuit industry as an etchant for polysilicon. HNA is a mixture of hydrofluoric, nitric and acetic acids. Nitric acid oxidizes silicon which is removed by hydrofluoric acid. The etch rate of silicon can vary from 1 to 5 µm/min which can be further increased to up to 20 µm/min by stirring. It is usually difficult to control the surface uniformity and etch depth by this method.
Anisotropic wet etching: Anisotropic wet etchants include hydroxides of alkali metals, simple and quarternary ammonium hydroxides and ethylenediamine mixed with pyrochatechol in water (EDP). Potassium hydroxide (KOH) is the most common etchant in this group. KOH etches {100} planes at a rate of 100 times haster than it etches {111} planes [2]. This method is commonly used to make precise V-shaped grooves and trenches delineated by {111} planes.
1. N. Maluf, "An Introduction to Microelectromechanical Systems Engineering," Artech House, Boston: 2000.
2. H. Seidel, L. Csepregi, A. Heuberger, and H. Baumgartel, "Anisotropic Etching of Crystalline Silicon in Alkaline Solutions I. Orientation Dependence and Behavior of Passivation Layers," JOURNAL OF THE ELECTROCHEMICAL SOCIETY, Vol. 137, No. 11, 1990, pp 3612-3632.