Princeton Micro Fab
Using Small Mechanical Devices to Advantage
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An Overview of
Micro Fabrication Techniques
 

Introduction

This white paper discusses new 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 new micro fabrication technologies.  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 etches that stop on the crystallographic planes of a silicon wafer to generate mechanical parts.  This technique 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.
 
 This 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.
 



 
 
 LIGA
(in German, Lithographie, Galvanoformung, Abformung)
 

The LIGA process exposes PMMA 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 1
 

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 2
 

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 are willing to prototype parts.
 
 
 



 
Silicon Surface Micromachining
 

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 Bells 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.

 
 
Texas Instruments has built a large array of mirrors by depositing and patterning aluminum over a sacrificial polymer layer.  This system is being developed for projection TV.  A small portion on their chip is shown below.
 
 


 
 
Silicon Bulk Micromachining
 

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.
 

 
 
 


 
EDM, Electro Discharge Machining
 

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.
 
 

Figure 11
 
 
 
 


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