There are three technology categories involved in manufacturing these tiny systems – nanotechnology, microtechnology and MEMS (Micro Electro Mechanical Systems). This article will focus only on MEMS, but it’s helpful to understand all of them.
Defining the TechnologyIn order to keep a perspective on the scale these technologies work on, keep in mind that the size of a human hair ranges in diameter from 25-100 micrometers (also called microns). MEMS has made possible electrically driven motors smaller than the diameter of a human hair (Fig. 1).
Nanotechnology is a field of applied science and technology that is involved with the control of matter on a nanometer scale (1 micrometer = 1,000 nanometers) as well as the fabrication of devices on this same scale. This involves manipulating matter on the atomic level. At this time, nanotechnology fits more into the realm of scientific research than a mature technology with industrial and commercial uses.
This technology is used for manufacturing devices that have features measuring in micrometers or microns in size (one millionth of a meter, 10-6 meter or 1µm). This is 1,000 times bigger than nanotechnology, and it is the technology used to make semiconductors.
The acronym MEMS stands for Micro Electro Mechanical Systems. This refers to a subset of microtechnology that creates devices having mechanical as well as electrical/electronic elements. The term MEMS is used to describe sophisticated mechanical systems on a chip such as micro-electric motors, resonate sensors, gears, accelerometers and so on. In practice it is used to refer to any microscopic device with a mechanical function.
MEMS is the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate using microfabrication technology. The electronics are fabricated using semiconductor processes, and the micromechanical components are fabricated using compatible “micromachining” processes. MEMS draws on skills from many disciplines including chemistry, applied physics, materials science, and even mechanical and electrical engineering. Figure 2 shows a working hinge that is about 20 microns in height.
More About MEMSScientists are using what they learned from semiconductor technology to manufacture mechanical devices on a substrate. Since the process for making mechanical devices is very similar to making electronic elements, it is easy to make small devices that have both electrical and mechanical properties. These systems may be miniaturized and mass-produced, promising the same benefits to the mechanical world that integrated-circuit technology has given to the electronic world. The electronics provide the “intelligence,” while micromechanical devices can provide the sensors and actuators.
MEMS technology removes the separation between mechanical systems and electronics. MEMS allows complex electromechanical systems to be manufactured using batch fabrication techniques, dramatically reducing cost and increasing the reliability and performance of the sensors and actuators to equal those of integrated circuits. A good example of this is the shuttle motor shown in Figure 3. A shuttle is moved back and forth by electrical components, which drive a mechanical-gear assembly.
MEMS is not all about size. In addition to size, distinct advantage is derived from the low power consumed by these devices (see sidebar) and the low cost to manufacture them. The real value of MEMS is as a new manufacturing technology for making complex electromechanical systems using batch fabrication techniques similar to those used for semiconductors. These electromechanical elements are then controlled with electronics.
The power of this technology is the combination of the decision-making capability of semiconductors with sensors and actuators. Sensors gather information from the environment by measuring mechanical, thermal, biological, chemical, optical and magnetic data. The electronics then process this information and directly control the actuators to respond by moving, positioning, regulating and pumping – thereby controlling operation.
As already mentioned, MEMS devices are manufactured using technology similar to that used for semiconductors. Semiconductors are made by depositing layer upon layer of various materials onto a substrate base – called a wafer – then etching away certain portions of the material. A photo lithographic process places a pattern on the newest layer of material. This leaves a protective layer on the parts of the layer that you want to keep. The areas that don’t have this protection are then etched away. This is repeated for each layer until a complex three-dimensional device is created. Figure 4 shows how this layering/etching process builds the three-dimensional structure of a semiconductor. Each wafer can contain hundreds of identical devices called chips. These chips are then sawed into individual items.
In addition to semiconductor material, other materials including metals can be deposited on the substrate and then etched. To create moving parts such as gears and motors, a sacrificial material is laid down underneath and between the moving parts. The mechanical parts are created in a similar manner to making semiconductors. When they are completed, the sacrificial material is etched away to free the moving parts.
What’s in Store for MEMS Technology?
It’s hard to imagine the many ways MEMS will impact our lives in the future. The diversity of useful MEMS applications gives this technology the potential to be far more pervasive than even integrated-circuit microchips. This extremely diverse technology could significantly affect every category of products. MEMS devices are in use today as key components in a wide range of products such as automobile airbags, ink-jet printers, in-dwelling blood pressure monitors and optical networking systems. Other applications include temperature, pressure and flow sensors; micro motors; actuators; gear trains; pumps; accelerometers; microsystems for DNA amplification and identification; and biochips for detection of hazardous chemical and biological agents. This is only the beginning. In a few short years, medical uses alone could result in more MEMS products than are available today.
Envision microscopic sensors measuring temperature and case depth that are embedded into products or samples that are run through a furnace. Sensors that are self-powering by harvesting energy and can communicate their readings wirelessly to control and monitoring equipment. These aren’t available now but could be made a reality by MEMS technology. IH
For more information: Jim Overturf is key accounts manager for control products for Yokogawa Corp. of America, 2 Dart Road, Newnan, GA 30265; tel: 800-888-6400; office: 757-229-3979; e-mail: Jim.firstname.lastname@example.org gawa.com; web: www.yokogawa.com/us/
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