ABSTRACT:
As information systems increasingly leave fixed locations and appear in our pockets and palms, they are getting closer to the physical world, creating new opportunities for perceiving and controlling our machines, structures and environments. To exploit these opportunities, information systems will need to sense and act as well as compute. Investing engineered systems with the ability to sense and act is the focus for the impact microelectro mechanical systems (MEMS) will have on the nature of engineering education and practice.
Micro Electromechanical Systems, or MEMS, are micron-sized machines that can be used as mechanical, electrical, or chemical transducers. Many different fields such as the automotive and medical industries utilize MEMS because of four advantages:
1) easier to mass-produce,
2) lower cost of production,
3) easier to make part alterations, and
4) higher reliability compared to large-scale machines.
MEMS are generally made of Polycrystalline Silicon, which is the same material used to make integrated circuits (IC). To manufacture MEMS devices, a process called photolithography is used. The general steps of the two-mask photolithography process are discussed. We can produce many type of MEMS devices using photolithography. One of the most notable applications of MEMS devices is the
accelerometer (crash sensor) of the airbag deployment system in modern automobiles. MEMS crash sensor will replace the less capable large-scale device for a fraction of the cost. MEMS development has just begun and researchers as well as engineers are working hard to create more and better MEMS devices to serve us in the future.
INTRODUCTION:
What is MEMS Technology?
Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through the utilization of microfabrication technology. Using the fabrication techniques and materials of microelectronics as MEMS processes construct both mechanical and electrical components.Mechanical components in MEMS, like transistors in microelectronics, have dimensions that are measured in microns and numbers measured from a few to millions. It can be difficult for one to imagine the size of a MEMS device. The general size of MEMS is on the order of microns (10-6 meter) as shown by the illustration of the MEMS gear (see figure 1). The main characteristic of MEMS is their small size. Due to their size, MEMS cannot be seen with the unaided eye. An optical microscope is usually required for one to be able to see them.
(Source: University of Wisconsin at Madison MEMS research laboratory, 1990)
MEMS is not about any single application or device, nor is it defined by a single fabrication process or limited to a few materials. More than any thing else MEMS is a fabrication approach that conveys the advantages of miniaturization, multiple components and microelectronics to the design and construction of integrated electromechanical systems. While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micro mechanical components are fabricated using compatible "micro machining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices. MEMS promises to revolutionize nearly every product category by bringing together silicon-based microelectronics with micro machining technology, thereby, making possible the realization of complete systems-on-a-chip. MEMS is truly an enabling technology allowing the development of smart products by augmenting the computational ability of microelectronics with the perception and control capabilities of microsensors and microactuators. MEMS is also an extremely diverse and fertile technology, both in the applications as well as in how the devices are designed and manufactured. MEMS technology makes possible the integration of microelectronics with active perception and control functions, thereby, greatly expanding the design and application.
Microelectronic integrated circuits (ICs) can be thought of as the "brains" of systems and MEMS augments this decision-making capability with "eyes" and "arms", to allow Microsystems to sense and control the environment. In its most basic form, the sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena; the electronics process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby, controlling the environment for some desired outcome or purpose. Since MEMS devices are manufactured using batch fabrication techniques, similar to ICs, unprecedented levels of functionality, reliability, and sophistication can be placed on a small silicon chip at a relatively low cost. Common examples of MEMS devices are the crash sensors of the airbag deployment system on modern automobiles and the pressure sensors in medical applications.
In light of these interesting applications, this paper will discuss the field of MEMS in three parts:
General manufacturing process of MEMS devices: photolithography.
Second, it will discuss the advantages and disadvantages of using MEMS devices.
Lastly, it’ll discuss an important application and case study of MEMS devices in the automotive industry.
MANUFACTURING PROCESSES OF MEMS:
MEMS devices are fabricated using the same processes used to construct integrated circuits in semiconductors. In fact, most of the MEMS devices are made on silicon wafers or other semiconductor materials. The reason for this is two fold. The first reason is that the semiconductor material allows the use of integrated circuits and the second is that the chemical etching processes used for semiconductor materials are well known and easily adapted to produce machines with microscopic detail.
Material:
MEMS are generally made from a material called Polycrystalline Silicon (Poly-Si) which is a common material also used to make integrated circuits (IC). Frequently, Poly-Si is doped with other materials like germanium or phosphate to enhance the material’s properties. Sometimes, copper or aluminum is plated onto the Poly-Si to allow electrical conduction between different parts of the MEMS device.
Now that we know the Material used for fabrication of MEMS, we will discuss the methods used for fabrication. The various methods used in MEMS manufacturing can be enlisted as below:
Photolithography (Surface Micro machining)
LIGA (Lithographie, Galvanoformung, Abformung)
Bulk Micro machining
Photolithography (surface Micro machining):
The most frequently used manufacturing technique is called surface micro machining. The term micro machining is a little deceptive since no machining is actually done.
There are five steps in the two-mask-photolithography process as shown in figure3:
Depositing a sacrificial layer onto a Si wafer. Exposing the sacrificial layer to a UV light and etching it with plasma gas. Depositing a layer of Poly-Si onto the sacrificial layer. Exposing the Poly-Si layer to a UV light and etching it with plasma gas. Etching the sacrificial layer with Hydrofluoric acid. Elaboration of the two-mask photolithography process is as follow:
First, a thin, one-micron thick sacrificial layer of Phosphosilicate glass (PSG) is deposited onto a silicon wafer that is 525 microns thick. Before the PSG layer is exposed to UV light, a mask with a rectangular hole is used to cover some of the PSG layer. The rectangular part of the PSG layer that was not covered is then exposed to UV light. Then, a plasma etch is performed to remove the UV exposed PSG layer. At this point, we have created a hole for the Poly-Si cantilever beam from the PSG. The third step of the two-mask photolithography process is to deposit a thin, one- micron thick layer of Poly-Si onto the remaining PSG layer. A second rectangular mask is used to cover some parts of the Poly-Si layer before exposing the Poly-Si layer to UV light. A second plasma etch is then done to remove the UV exposed Poly- Si layer.
The last step is to "release" the Poly-Si cantilever beam from the PSG layer by etching the PSG layer with Hydrofluoric acid (HF) for a given amount of time. Note that adding more “depositing-exposing-etching” steps to the two-mask photolithography process can make more complicated MEMS structures such as the micro- servomotor.
Liga:
The acronym LIGA comes from the German name for the process (Lithographie, Galvanoformung, Abformung). LIGA uses lithography, electroplating, and moulding processes to produce microstructures. It is capable of creating very finely defined microstructures of up to 1000µm high. In the process as originally developed, a special kind of photolithography using X- rays (X-ray lithography) is used to produce patterns in very thick layers of photoresist. The X-rays from a synchrotron source are shone through a special mask onto a thick photoresist layer (sensitive to X-rays), which covers a conductive substrate (a). This resist is then developed (b). (See figure 4)
The pattern formed is then electroplated with metal (c). The metal structures produced can be the final product; however it is common to produce a metal mould (d). This mould can then be filled with a suitable material, such as a plastic (e), to produce the finished product in that material (f). These moulds also allow the use of materials other than silicon, such as nickel, titanium, and gold. Because of the patterning technique employed by LIGA, structures with a wide range of materials can be fabricated. These structures can have horizontal tolerances of about 0.3 microns and vertical dimensions from microns to centimeters. One disadvantage of the Liga process is the cost of X-ray radiation equipment used to develop the photopolymer. In addition to costing millions of dollars, the radiation emissions are tightly controlled by federal regulation. This limits the use of this process to only a handful of institutions in the United States.
ADVANTAGES OF MEMS:
There are four main advantages of using MEMS rather than ordinary large scale machinery.
The first advantage is the ease of production. Borrowed from the IC industry, today’s VLSI (Very Large Scale Integration) technology allows MEMS to be produced in large quantities (up to 100,000 MEMS devices per Poly-Si wafer). It is obvious that no macro-machinery production rate can even come close to this number.
The second advantage of MEMS is that they can be mass-produced and, thus, are inexpensive to make. A single MEMS device only costs a small fraction of a cent to produce.
The third advantage of MEMS over macro-machinery is the ease of parts alteration. To alter the production of a macro-machinery part, it is sometimes necessary to revise a whole production line, which involves a plant-wide shut down that can cause major loss in production time and money. A new set of MEMS can be created by making minor alterations in the manufacturing process (i.e. change of masks, change of etch time).Lastly, MEMS devices are known to have higher reliability than their macro scale counterparts.
LIMITATIONS AND DISADVANTAGES:
Due to their size, it is physically impossible for MEMS to transfer any significant power. In addition, because MEMS are made from Poly-Si (a brittle material) they cannot be loaded with large forces. This is because brittle materials can be fractured easily under high stress. Many MEMS researchers are working hard to improve MEMS’s material strength and ability to transfer mechanical power. Nevertheless, despite these limitations, MEMS still have countless numbers of applications in the real world as discussed in the next section.
APPLICATIONS:
Automotive Application:
In automobiles that are made today there are 20 to 30 odd MEMS devices. These are mostly sensors like accelerometers, pressure sensors, and gyroscopes. The accelerometers are used to sense a collision and produce the signal that deploys air bags in cars. An advantage of these devices is the fact that they can be arranged on a single chip to detect side or front impacts and deploy the corresponding air bags. These devices come complete with microscopic springs, masses, and cantilevers. Another big use is in the production of Manifold Absolute Pressure sensors, which are referred to as MAP sensors. These sensors are used to determine the concentration of oxygen going into the engine and calibrate the fuel air mixture to insure engine performance under a variety of environmental conditions. Map sensors also produce higher fuel efficiency.
Airbag deployment system:
One can find many MEMS applications in the automotive, biomedical, data storage, micro-optics, robotics and fluid control fields. But one of the most notable MEMS application is the accelerometer (a device used to measure acceleration) found in the airbag deployment system of many modern automobiles. These accelerometers are used as crash sensors in an airbag deployment system. We will now discuss how the traditional airbag deployment system works.
A traditional airbag deployment system includes macro mechanical crash sensors which detect a crash pulse, a microprocessor which processes signals from the crash sensors and an airbag deployment mechanism which physically deploys the airbag in an event of a collision. The traditional airbag deployment system uses a "ball-on-cone type" macro mechanical device as the crash sensor. When the deceleration of a vehicle exceeds a certain limit (signal of hard braking or collision), the decelerating forces would pull the ball to a position which signals the system to deploy the airbag The inflated airbag will then act as a cushion between the occupants and the dashboard thus lessens the impact force imparted to the
occupants by the crash. Airbags are designed to protect the occupants in automobile accidents against severe injuries. The airbag deployment system has been proven an effective supplemental restraint system (SRS) when used concurrently with safety seat belts. We will now discuss the advantages of using MEMS as crash sensors in the airbag deployment system rather than macro devices:
Advantages:
MEMS are used for crash sensing in newer airbag deployment systems because the traditional macro mechanical devices are not capable of meeting new standards set by the government and the auto industry. These new standards include the need of multi-directional crash sensing which cannot be achieved by the traditional crash sensors. In addition, traditional crash sensors are more expensive to make and less reliable compared to the MEMS crash sensors. We will now discuss the details of the MEMS crash-sensing device.
Two-chip accelerometer system:
The two-chip accelerometer system is used as the crash sensor in the airbag deployment system. It consists of three layers of Poly-Si as shown in figure 5.Each layer has a particular function. The first fixed layer is used to run a self- diagnostic test every time the device is powered up. The third fixed layer is a reference electrode. The second layer is a seismic mass, which is capable of moving up and down between the first and third layer when subjected to acceleration (in the direction in or out of the paper)
The sandwich of Poly-Si layers (the crash sensor) is connected to a microprocessor which is connected to the deploying mechanism of the airbag (see figure 6). At zero acceleration, a fixed capacitance (an electrical property) is measured between the second and third Poly-Si layer. The system is said to be "at rest" in this state. In an event of a collision, a great deceleration force would be transmitted to the accelerometer. This force would move the seismic mass (the second layer) with respect to the fixed third layer. The change in spacing between these two layers will cause a change in the capacitance. This change in capacitance is analyzed by the microprocessor attached to the crash sensing unit. If the change is severe enough (meaning a real collision is in progress), the microprocessor will send a signal to deploy the airbag. (See figure 6)
Medical applications:
Future uses of MEMS devices include a variety of medical devices. One of these devices is a blood gas sensor that can be inserted through a catheter 650 microns in size. It can detect the oxygen content and the pH of the blood. Other significant medical uses are the construction of neural probes to record electrical impulses on the brain. These microscopic sensors would allow researchers to place sensors close to the neurons and would allow them to map the circuits of the brain to determine how the brain truly processes and stores information. It is estimated that MEMS devices could produce a neural electronic interface. These devices could be attached directly to neurons in the brain. If the electrical impulses naturally generated by normal sensory input such as sight or hearing could be simulated by these MEMS devices, it would be possible to provide artificial sight to the blind. Other medical uses include the use of MEMS devices to combat cancer or obstructions to blood flow that produce stokes and heart disease.
Miscellaneous applications:
Consumer electronics also use MEMS devices in increasing quantities. One prominent use is in the valves and orifices that are used in ink jet printers. Many of us have MEMS devices in our homes and don't even know it. Other uses include the production of chemical sensors such as smoke detectors.
CASE STUDY:
Migration from Electromechanical Technology to MEMS/MST:
MEMS/MST offers the major advantages of cost and performance to automotive electronic systems. There is ample history of the migration of electromechanical sensors and discrete switches to MEMS/MST based sensors. Table 1 is a summary of this activity.
CONCLUSION:
MEMS devices have evolved from laboratory curiosities of the 1980s to commercial products of today. If this growth trend continues, MEMS will very likely be the next generation of machinery to service mankind for the next century. It is predicted that the MEMS market will soar to more than $34 billion by the year 2002. This prediction combined with the foregoing discussion on the advantages of MEMS over macro devices lead us to predict that MEMS will soon be integrated into our everyday life just as the computers have been. From previous sections, we saw that the manufacturing process of MEMS is not the simplest, but we believe that the advantages that come with MEMS will outweigh the complexity of the manufacturing process. As MEMS researchers strive to compensate for MEMS’s shortcomings, we can only expect to see more and better MEMS devices created in the coming years. The future designs and applications of MEMS are only limited by the imagination of the designers.
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