Introduction to MEMS Tutorial
This MEMS tutorial discusses micro fabrication techniques for making mechanical parts. Motors, pivots, linkages, and other mechanical devices can be made to fit inside this circle O . These devices are also potentially quite inexpensive. For example, using silicon surface micromachining, a gear captivated on a pivot can be made for less than a cent.
There are a number of MEMS micro fabrication technologies discussed in this tutorial. These technologies make devices ranging in size from a dozen millimeters to a dozen microns. Silicon surface micromachining inexpensively makes completely assembled mechanical systems. Silicon bulk micromachining uses either etches that stop on the crystallographic planes of a silicon wafer or etches that act isotropically to generate mechanical parts. These techniques combined with wafer bonding and boron diffusion allows complex mechanical devices to be fabricated. The LIGA technology makes miniature parts with spectacular accuracy. Electro Discharge Machining, EDM, extends conventional machine shop technology to make sub-millimeter sized parts.
Micromechanical parts tend to be rugged, respond rapidly, use little
power, occupy a small volume, and are often much less expensive than conventional
macro parts. Belle Mead Research, BMR, specializes in helping companies
understand when it is advantageous to use micromechanical parts, and developing
products incorporating these devices.
The material below is meant to entice you with the capabilities of micro fabrication. Only a small fraction of the fabrication techniques and devices can be shown in the space available.
The LIGA process exposes PMMA (polymethyl methacrylate) plastic with synchrotron radiation through a mask. This is shown at the top of the Figure 1. (This figure is on the next page.) Exposed PMMA is then washed away, leaving vertical wall structures with spectacular accuracy. Structures a third of a millimeter high and many millimeters on a side are accurate to a few tenths of a micron. Metal is then plated into the structure, replacing the PMMA that was washed away. This metal piece can become the final part, or can be used as an injection mold for parts made out of a variety of plastics.
Figure 2 shows a electrostatic motor made using the LIGA process. This work was done by U. Wallrabe, et al in Karlsruhe, Germany (MEMS '92 page 139).
Figure 3 shows a "tinker toy" set of gears, posts, mounting fixtures, and a table for mounting parts. These structures were made by H. Guckel at University of Wisconsin, (MEMS '91 page 74).
The LIGA parts can be manufactured by the German company MicroParts.
Belle Mead Research is associated with MicroParts, and also has contacts
with other groups who will prototype parts.
Silicon surface micromachining uses the same equipment and processes as the electronics semiconductor industry. This has led to a very rapid evolution of silicon surface micromachining. Very sophisticated equipment and experienced operators are available to manufacture these devices. One company is even offering the integration of surface micromachined devices and CMOS electronics on the same chip.
This technique deposits layers of sacrificial and structural material on the surface of a silicon wafer. As each layer is deposited it is patterned, leaving material only where the designer wishes. When the sacrificial material is removed, completely formed and assembled mechanical devices are left.
Figure 4 shows the process steps to make a gear. The oxide is
the sacrificial material, and the polysilicon is structural. In Figure
5, one of the original set of gears W Trimmer and collaborators made at
Bell Laboratories is shown. These are from an article in Electron
Devices, vol 35, No 6, June 1988.
Comb drives actuators and electrostatic motors can be fabricated using this technique. Figure 6 shows a comb drive actuator. The curved white fingers are fixed to the substrate, and the gray fingers are free to move. By applying a voltage alternately to the top and bottom white fingers, the electrostatic force causes the gray structure to start to resonate. Figure 7 shows a mass attached to the comb drive resonator. Depending upon the frequency of excitation, the mass can be made to translate, or rotate. These figures are from an article by Pisano et al (MEMS '90, page 9) Numerous designs of electrostatic side drive and harmonic motors are also available.
A surprising number of structures can be made using the etch stop planes
in crystalline silicon. Figure 9 shows a mirror etched out of bulk
silicon by R. Cornely and R Marcus (in Sensors and Actuators A, Vol 29,
p 241, 1991). This mirror costs a few cents to fabricate, and can
be integrated with other structures by wafer bonding. Figure 10 shows
a complex accelerometer structure with fine tethers for supporting the
mass and capacitive sensor for detecting the motion. Similar
structures are being used by the automobile industry for air bag deployment.
Matsushita has developed a new Electro Discharge Machine with the capability
to make very small, precise parts out of almost any material that conducts
electricity. This machine uses standard machine shop tooling, and
is compatible with machine shop production techniques. Below are
a few of the impressive pictures of devices this technique can make.
In Figure 11, a small planetary gear box is show next to a mechanical pencil.