Microelectromechanical systems (MEMS), also called microsystems in Europe, is typically defined as man-made mechanical components which are characterized by micron-scale size. Or say, MEMS is a functional device that utilizes mechanical mechanism to achieve a certain function (such as optical, acoustical, mechanical or electrical) with very small dimensions. MEMS is not just to fabricate a macroscopic device smaller, but to solve problems from more innovative aspects, which are more economical, more reliable, and more energy saving. A real advantage of MEMS is the manufacturing process which utilizes the batch fabrication that is originally developed for the integrated circuit technology. Batch fabrication allows thousands of identical devices to be fabricated simultaneously, on a single wafer. And that’s why a MEMS device could be a much lower cost.
The MEMS market is dominated by a few fields that all generate over $500M/year revenues. And more and more new applications are being developed to enter into MEMS market. As a part of semiconductor industry, MEMS is becoming hot rapidly.
In order to learn how a MEMS device operated functionally, it must be known firstly that what is a mechanical system. From standard equation of motion we know, the three key factors of a mechanical system are mass(m), damping(c) and spring constant(k), in which damping c is also called damping factor or damping constant γ. With these three elements there is a simplest mechanical system.
With the help of mechanical system, some signals from environment could be transferred as a motion status of mechanical system. MEMS accelerometer, one of the simplest MEMS inertial sensor, for example, converts the accelerate of the device to a mechanical movement. And once how much movement due to the accelerate is detected, how much accelerate there is will be known. A widely used detecting method is to detect the capacitance change of a set of parallel plate electrodes, since the value of capacitance is a function of distance, which might be a result of mechanical motion. By MEMS, a relationship between capacitance and accelerate is built, and by MEMS, a mechanical signal in environment is converted, through this tiny mechanical system, to a typical used electro signal such as capacitance value. That is how a MEMS device works.
The key process to fabricate a mechanical structure is surface micromachining and bulk micromachining, which really make a blank silicon wafer become thousands of functional MEMS devices.
In bulk micromachining, features are sculpted in the bulk materials like silicon, quartz, SiC, glass and some other materials in semiconductor industry. The main purpose of Bulk Micromachining is to etch material from wafer because a typical silicon wafer is around 400 µm thickness that is hard to be functional. Many MEMS structure may need somewhere to be very thin or even open as holes, for example, MEMS microphone. There is a hole on microphone for sound wave going in, which requires to be etched by etchant solution, on the wafer-made substrate.
It is bulk micromachining that three-dimensional features are etched into the bulk materials. In contrast, surface micromachining is to build up features layer by layer, on the surface of a substrate, such as the single silicon wafer. After layers build up, dry etching is used to define the surface pattern on the 2d plane. Then, wet etching releases them from the plane by undercutting.
When fabricating a MEMS device, bulk and surface micromachining should be combined to complete the structure definition. There is a brief example, as the figure illustrating. The process to form the structure above can be divided into 3 steps. First, deposit a layer of silicon dioxide right on single silicon wafer substrate. Second, use dry etching (reactive ion etching) to define a surface pattern, removing undesired portion. Third, doing bulk micromachining to etch the hole on substrate.