I. What are Micromachines?
Very small gears and engines can be made on a silicon wafer with a high degree of precision
Sandia National Laboratories http://mems.sandia.gov/scripts/index.asp
As the name implies, micromachines are very tiny mechanisms. In fact, they are so small that the unaided eye cannot perceive them. Many different types of professionals, such as biologists, chemists, physicists, and engineers, are involved in the research and development of these complex devices. Micromachines can be a wide variety of different mechanisms, such as fluid channels, gears, engines, tweezers, and mirrors - all smaller than the width of a human hair. So, what are these micro-devices used for? Well, many of them are creeping into our everyday lives, in places where you may not expect them.
II. Applications for Micromachines
Micromachines are becoming more and more common in everyday life. An accelerometer is a good example of a micromachine which has made its way into most of our daily lives. An accelerometer measures acceleration, and is the device used to detect an accident in automobiles. A processor will analyse the magnitude of the acceleration and decide on whether or not to deploy the airbags in the vehicle. Many more examples exist which demonstrate the feasibility and benefit of micromachines in the world today:
III. How are Micromachines made?
Although micromachines look incredibly complex and difficult to build, the manufacturing process is simple. They are built similarly to everyday machines, that is, one piece at a time. For example, to build a micro-engine or a micro-transmission, each gear, arm or bracket can be fabricated separately, and then put together to form the device.
There are four basic ways in which these micro-devices can be formed: surface milling, surface micromachining, wet bulk micromachining, and what is known as LIGA (a German acronym for X-ray Lithography, Electrodeposition, and Molding) fabrication.
Surface milling is very similar to everyday milling that would be done for macroscopic objects such as a steel or aluminum block, except, of course, it is done with very small tools. Milling is a common process in mechanical workshops where a block of steel can be cut, welded, soldered, drilled, lathed, or ground into the desired shape.
Laser ablation is another form of surface milling that is in widespread use. This method uses a focused beam of either electrons or ions to physically bore into the device structure, blasting away unwanted silicon, and creating the desired device.
These diamond cutting disks can now cut with a precision of less that 1 nanometre.
Materials Science and Engineering, Virginia Polytechnic Institute and State University
Diamond cutting blades are widely used as cutting or grinding discs since diamonds are well known for their strength and durability. Rescaling these useful tools to the size of a dust mite has allowed a cutting precision of less than 1 nanometre, perfect for forming micro-devices. Just as a grinder in a workshop may smooth a piece of metal or cut a bracket, a miniature diamond grinder may cut a micro-gear into the correct shape.
Surface micromachining, while sounding similar to surface milling, is a very different process. Whereas, with surface milling, one could only subtract material from a device, with surface micromachining, one can both add and subtract material, or, more precisely, layers of material. Below is a full schematic of the process from a plain silicon substrate (a substrate is a circular disc of material approximately 4 inches in diameter and 1/32 inch thick, out of which micromachines are made) to a fully surface machined micromotor. Patterning is done with the use of a lithography mask; a thin film of metal encased within glass with the desired pattern on it (in this case, the shape of a micro-cantilever). Then, ultraviolet light is shone through the holes in the mask, which weakens some parts of the photoresist, allowing it to be easily washed away. Etching is a process wherein the entire substrate is immersed in a solution which "eats" material that isn't covered by photoresist. Different materials are susceptible to different etchants.
Wet bulk micromachining is very similar to surface micromachining, in that it layers different materials down one at a time, and then selectively etches them to form the desired pattern or device on the substrate. One large difference between the two is that under the wet bulk micromachining process, both sides of the substrate are patterned and etched. If done correctly, this process can then leave large features or devices that are completely removed from the original silicon substrate.
LIGA (Lithographie, Galvanoformung und Abformung, German for Lithography, Electrodeposition, and Molding) is again very similar to both wet bulk micromachining and surface micromachining, except with a few considerable differences. One of the most important differences with this method is the different photoresists used to pattern layers. Wet bulk and surface micromachining techniques both use photoresists which are susceptible to ultraviolet (UV) radiation; this is how the metal layers are patterned to form devices and structures. But under the LIGA process, a different photoresist is used, which is susceptible to X-ray radiation instead. This difference in photoresists can allow for much taller objects to be created using LIGA, ranging anywhere from microns to centimeters high.
Though the four different methods outlined above are those which are most popularly used by micro-fabrication laboratories around the globe, researchers are constantly finding new ways to fabricate micro-machines more efficiently. One such method under development is to use the ion beam mentioned above to "build up" a structure by stacking the ions on top of each other. This area of micromachine fabrication looks to be promising, but only time will tell whether this and countless other new and inventive processes will work.
IV. Advantages and Disadvantages of Micromachines
Major advantages of micromachines include:
- Diverse fields of applications
- Devices are cheap if they are mass produced
- They are very small
- Effective solutions to many problems
Micromachines do have their disadvantages. Since they are so small, they are very difficult to fix, and they can sometimes be hard to work with. Also, a micromachine fabrication facility is very costly to set up. Millions of dollars of equipment is needed to build a state of the art fabrication facility, and since many companies cannot afford this, usually facilities are created as a joint venture between industry and government. The University of Alberta Micro and Nano Fabrication Facility has over $3 million invested in process equipment. Overall, the benefits outweigh the disadvantages, and this is reflected in the burgeoning micromachine industry.
This very tiny "gear-within-a-gear" can only turn in one direction.
Sandia National Laboratories http://mems.sandia.gov/scripts/index.asp