ISAT 436 Micro-/Nanofabrication and Applications MicroElectroMechanical Systems: MEMS David J. Lawrence Spring 2004 1

MEMS (MicroElectroMechanical Systems) R. C. Jaeger, Chapter 11, beginning on page 269. MEMS are one of the most exciting application areas for microfabrication. MEMS combine the signal processing and computational capability of analog and digital integrated circuits with a variety of non-electronic elements, such as sensors (e.g., pressure, temperature, and chemical sensors), actuators (e.g., micromotors, comb drives, and pumps), mechanical elements (e.g., gears, springs, and cantilevers), and optical elements (e.g., fixed and tilting mirrors).

MEMS (MicroElectroMechanical Systems) Micromachining technologies can be divided into three categories: Bulk micromachining Surface micromachining High aspect ratio electroplated structures

Mechanical Properties of Silicon Silicon is brittle. This is especially evident for large-diameter, thin wafers. It tends to fracture (cleave) along certain crystal planes. This breakage can often be traced to flaws (e.g., scratches), often near the edges of wafers.

Mechanical Properties of Silicon However, silicon is not as delicate as many people think. Its yield strength exceeds that of high-strength steel and tungsten. Silicon’s hardness is greater than that of tungsten. See Table 11.1 on page 270 of Jaeger.

Other Materials Used in MEMS Polycrystalline silicon (polysilicon). Silicon dioxide (an insulator or dielectric). Silicon nitride (another insulator) is harder than high-strength steel. Phosphosilicate glass (PSG) is another insulator. Metals (e.g., aluminum, titanium, and nickel). Others … See Table 11.1 on page 270 of Jaeger.

Deposition The materials needed for mechanical elements can be produced on silicon wafers using the techniques we have already studied: Oxidation of silicon PVD (evaporation or sputtering) CVD Plus a technique that we have not yet studied: Electrodeposition (or electroplating)

Photolithography Of course, photolithography is used to define the patterns in the materials used for MEMS

Etching Etching is an important process in MEMS microfabrication. Wet chemical etching of silicon can be isotropic or anisotropic depending upon the chemical composition of the etchant. Wet chemical etching can also be made to stop at a p-n junction. See pages 271-278 of Jaeger.

Etching Dry etching of silicon (and other materials) can be very anisotropic if the process conditions are properly chosen. For example, deep reactive-ion etching (DRIE) can be used to produce high-aspect-ratio structures. See pages 278-280 of Jaeger.

Sacrificial Layers If a mechanical element must be free to move (e.g., a cantilever beam, a spring, or a gear), it often must be deposited on top of a “sacrificial layer” or a “release layer”. The sacrificial layer is later etched out from beneath the element that is to be free to move. Phosphosilicate glass (PSG) is often used as the sacrificial layer. Cavities and channels can also be fabricated by using sacrificial layers. See pages 279-287 of Jaeger.

MEMS Application: Displays Digital Micromirror Devices (DMD) were developed by Texas Instruments. These devices are based upon electrically-controlled, tilting micromirrors. Website: www.dlp.com DLP = Digital Light Projector These devices have numerous applications: Business Home entertainment Cinema

MEMS Application: Displays Two micromirrors: one tilted to “on” position, the other tilted to “off” position. (www.dlp.com)

MEMS Application: Displays Two micromirrors: one tilted to “on” position, the other tilted to “off” position. (www.dlp.com)

(www.dlp.com)

MEMS Application: Displays

MEMS Application: Displays (www.dlp.com)

MEMS Application: Displays (www.dlp.com)