MEMS Fabrication Techniques
Bulk wet micro machining process steps
There are many techniques for micro-machining silicon; this work will concentrate on bulk wet etching. Several other techniques will also be discussed, and the design methodologies developed will not be limited to bulk wet etching. Bulk wet etching is chosen because it is an established method that has been available for many years and it is relatively easy to perform and relatively inexpensive. The review article by Petersen is an excellent summary of many MEMS fabrication methods.
The first step is to obtain silicon wafers cut along some orientation. The most common wafer orientation is (100) (the wafer is cut perpendicular to the x-axis), although other orientations such as the (110) are also available. Typical wafer sizes are one to four inches in diameter and 100 to 400 microns in thickness.
Next the wafer is cleaned and a thin masking layer is applied to the wafer.
The masking layer used in these experiments is silicon dioxide. Silicon has the advantage that its oxide is grown easily and is lattice matched, which makes for low stress masking
Oxide masking layer
layers. The bare silicon wafers are placed in a furnace at approximately 1000 to 1100 degrees Celsius for several hours. The furnace contains water vapor to enhance the oxide growth. Thicknesses of up to a micron are readily attainable, the thickness in most of our experiments is roughly 8000 °Angstroms. Other possible masking layers include silicon nitride and evaporated metals such as gold or chrome.
Next a uniform layer of photoresist of roughly micron thickness is spun onto the wafer (see Figure 2.2). The desired input mask is used to selectively expose the photoresist. When exposed to ultraviolet light the photoresist is chemically altered and the exposed portions of the photoresist can then be removed with photoresist developer. With parts of the
photoresist removed, the masking layer is now partially exposed. The exposed portions of the masking layer are etched away with an etchant which removes oxide but not photoresist or silicon. Having done so, the remaining photoresist is removed, leaving a patterned oxide masking layer.
The wafer is then placed in a container of etchant as shown. The etchant may be KOH (potassium hydroxide), EDP (ethylene diamine pyrocatecol).
Patterning oxide masking layer
Wafer etching
TMAH (tetramethyl ammonium hydroxide), or some other etchant which attacks silicon but not the masking layer. A reflux system runs cold water through the top of the container to prevent evaporation of the etchant. Many etchants are used near their boiling point since the rate of etching tends to increase greatly with temperature. The container has both a temperature feedback to maintain constant temperature and a stirrer to ensure that the etching is homogeneous. The sample is etched for a set period of time then removed. The remaining masking layer can now be stripped, leaving a three dimensional shape in the silicon wafer.
Bulk wet etching may be done alone or in conjunction with other micro machining techniques.
Disadvantages
As mentioned above, bulk wet etching is an established relatively inexpensive method, but it does have many disadvantages. Many of the etchants are very hazardous and must be handled properly. The anisotropic nature of the etchants means that certain planes dominate others and that the etched shapes are distorted versions of the original masks. The anisotropy also limits the possible output shapes and the attainable height aspect ratios.
Cesar Augusto Suarez
CI 17394384
CAF
CI 17394384
CAF
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