MEMS refers to the technologies and practice of making three dimensional structures and devices with dimensions in the order of micrometers. The two constructional technologies of MEMS are microelectronics and micromachining. Microelectronics, producing electronic circuitry on silicon chips, is a very well developed technology. Micromachining is the name for the techniques used to produce the structures and moving parts of microengineered devices.
One of the main goals of Microengineering is to be able to integrate microelectronic circuitry into micromachined structures, to produce completely integrated systems (microsystems). Such systems could have the same advantages of low cost, reliability and small size as silicon chips produced in the microelectronics industry.
When considering such small devices, a number of physical effects have different significance on the micrometer scale compared to macroscopic scales. Interest in microengineering has spawned or renewed interest in a number of areas dealing with the study of these effects on microscopic scales. This includes such topics as micro-mechanics, which deals with the moving parts of microengineered devices, and micro-fluidics, etc.
Silicon micromachining is given most prominence, since this is one of the better-developed micromachining techniques. Silicon is the primary substrate material used in the production microelectronic circuitry (i.e., better silicon chips), and so is the most suitable candidate for the eventual production of microsystems.
The Excimer laser is an ultraviolet laser, which can be, used to micromachine a number of materials without heating them, unlike many other lasers which remove material by burning or vaporizing it. The Excimer laser lends itself particularly to the machining of organic materials (polymers, etc.).
LIGA is a technique that can be used to produce moulds for the fabrication of micromachined components. Microengineered components can be made from a variety of materials using this technique, however it does suffer the disadvantage that currently the technique requires X-rays from a synchrotron source.
Photolithography is the basic technique used to define the shape of micromachined structures in the three techniques outlined below. The technique is essentially the same as that used in the microelectronics industry, which will be described here. The differences in the photolithographic techniques for Excimer laser micromachining and LIGA will be outlined in the relevant sections.
Firstly a mask is produced. This will typically be a chromium pattern on a glass plate. The wafer is then coated with a polymer, which is sensitive to ultraviolet light, called a photoresist. Ultraviolet light is then shone through the mask onto the photoresist. The photoresist is then developed which transfers the pattern on the mask to the photoresist layer.
There are two types of photoresist, termed positive and negative. Where the ultraviolet light strikes the positive resist it weakens the polymer, so that when the image is developed the resist is washed away where the light struck it - transferring a positive image of the mask to the resist layer. The opposite occurs with negative resist. Where the ultraviolet light strikes negative resist it strengthens the polymer, so when developed the resist that was not exposed to ultraviolet light is washed away- a negative image of the mask is transferred to the resist.
A chemical (or some other method) is then used to remove the oxide where it is exposed through the openings in the resist. Finally the resist is removed leaving the patterned oxide.Micromachining-Technologies include:
- Etching techniques
- Wet etching
- Dry etching
- Material bonding
- Wafer bonding
- Fusion bonding
- Ultrasonic micromachining
Additional information on MEMS and MEMS-Materials:
- Introduction to MEMS
- A (not so) short introduction to MEMS A one chapter introduction to MEMS
- Thick photo-resist SU-8
- Material Properties
- Silicon Dioxide
- Oxidation Calculator
- Thermal Diffusion Calculator
- Ion Implantation Calculator
- Shape Memory Alloys
- Process Theory