Microelectromechanical Systems (MEMS) Essay
Microelectromechanical system is a system in which small electrical and mechanical components are combined to form a technology. Sizes of micrometers vary depending on a particular system. The combination entails mechanical elements that are diaphragms, gears, springs, and beams while technique on fabrication is used based on electrical components. Its components are microsensors, microstructures, microelectronics, and microactuators.
History of Microelectromechanical Systems
In 1947, Jack Kilby facilitated growth of the MEMS by inventing the use of a transistor in the bell telephone. He used the Germanium device to manufacture integrated circuits in 1958. The Germanium of silver was used to glue glass slide. The bell had three resistors, one capacity, and one transistor. By 1970, advancement of the ICs doubled every three years. The ICs fabrication depended on sensors providing information from the surrounding. Sand was the only raw material used in developing silicon. Silicon was then exploited to produce microsensors. Later, it was discovered that the piezoresistive effect of silicon held a huge potential for production of silicon gauze that was more sensitive than metal films. In 1982, the term micromachining came into existence and referred to removing of some parts of silicon to leave behind the desired parts. The removing process is called etching. Micromachining has become a prominent method in fabrication of micromechanical components.
The MEMS can be more effective if they cannot be fixed with only microelectronics, but also other technologies like photonics and nanotechnology. A mechanism of emerging technologies is called heterogeneous integration. The technologies have an upper hand in the market. For instance, emergence of the MEMS with nanotechnology, with the latter being the manipulation of matter in either atomic or molecular form, is associated with magnificent products manufactured on the nano dimensional scale. The two approaches are bottom up and top down. A bottom up approach involves self-assembly, deposition, and growing technologies. In a top down approach, structures and devices are produced using the same skills like in the MEMS, but in a small size that helps with etching and photolithography methods. Nanotechnology products have a market advantage over the MEMS due to their scaling laws. Since it puts every atom or molecule in a desired place and position, it makes almost any structure compliant with physics laws and manufacturing costs do not exceed the cost of raw materials and energy used in fabrication. There are also some similarities between nanotechnology and the MEMS. First, they are interdependent on each other. Atomic force microscope, which is used in positioning of atoms and molecules of substrate, is a MEMS device. Scanning tunneling tip microscope used in detecting individual molecules and atoms on nanometer is also a MEMS device.
A major concern in MEMS fabrication is the order of building mechanical and electronic elements. High temperatures are necessary for warping of layers, but there is a problem that temperatures can damage electronic circuits that have been added. Building mechanical components requires protection when electronic circuit is fabricated. Solutions are used in this process, including putting mechanical parts in shallow trenches before electronics is fabricated and then recovering them later.
The system is manufactured using fabrication technology. It has the ability to sense and control. Many types of equipment operate using MEMS system, including optical scanners, fluid pumps, transducers, micromirrors, pressure and flow sensors, inertial sensors, locks, micro actuators, microengines, accelerometers, inkjet-printer cartridges, and miniature robots.
MEMS products have a large market share in the United States of America. Due to magnificent results of the system, in 2001 only the system was estimated to have $10 billion market share and the market was predicted to increase by 20% in 2002 (Salvatore, 2001). This amounts exactly to $34 billion. Areas that have much interest in the commodities are defense department, stock exchange, federal government, sport departments, as well as manufacturing and processing industries. Barriers for market penetration of the MEMS include a need of quality packaging materials, some inefficient models and designing equipments, and their cost as compared to other technologies.
The systems operate by sensors collecting information from various locations by measuring optical, chemical, thermal, biological, magnetic, and mechanical phenomena. Electronics is stimulated by the information from sensors by allocating a necessary action for the information. Actions taken are moving, positioning, regulating, pumping, and filtering.
The MEMS is manufactured through fabrication of components. The major technology is fabrication. Fabrication technology involves microelectronics, miniaturization, and multiplicity. Prior to microelectronics allowing monolithic merger of sensors, logics, and actuators to make closed-loop feedback systems and constituents, it provides an idea to MEMS on how to handle signals sent to it. The purpose of miniaturization is ensuring that devices are efficient in their duties. Multiplicity is the fabrication process, which initiates fabrication of many components. Miniaturization and multiplicity will not be effective in the absence of the IC fabrication technology. IC fabrication is also referred to as microfabrication.
IC fabrication is the pillar behind batch processing and miniaturization of mechanical systems, which is not applicable through the use of machining techniques. Besides, it provides a platform for integration of mechanical and electronic components. Over the last ten years, IC technology has rapidly developed. This has led to improvement in performance of micromechanical devices, which has resulted in reduction of cost of instrumenting, manufacturing, and packaging of devices. IC fabrication steps are film growth, doping, lithography, etching, dicing, and packaging.
Material Components of MEMS
The following are material components of the MEMS, including silicon, polymers, ceramics, and metals. They are discussed below.
Materials used in the manufacturing of the MEMS are silicon, ceramics, metals, and polymers. The role of silicon is to form more integrated circuits. Customers in the market appreciate many products that are manufactured with the help of silicon. Silicon is a quality material and may be found cheaply in the market. Silicon is more reliable and elastic when it is coiled in many ways so that it does not break easily. It is also durable. Silicon is deposited in thin films, making them easy to moisturize (Preeti, 2012). It can be batch-fabricated and its patterns and structures can be reproduced by lithograph. One design offers broad stability of more than ten years.
Economy favors the silicon industry, but crystalline silicon is extremely expensive to be produced.
Under given circumstances, polymers can serve as an alternative for photoresist, silicon, rubber, polycarbonate, cyclic, and similar materials. This facilitates fabrication of devices not found in ancient silicon MEMS. It is preferred in sensing gases because of adsorption and absorption properties. Besides, polymers can be produced in large quantities with greater dimension of features. MEMS products are processed by the following processes such as stereolithography, molding, embossing, injection, and molding. Polymers have the following features: biodegradability that enables disposing and environmental recycling, biocompatibility that is used in medical equipment, high thermal expansion for temperature sensing, high flexibility for electrets, and visible permeability for micro optical elements. Below are diagrams illustrating polymer processes.
The main reason why ceramics is used in MEMS fabrication is because it possesses a combination of material features. Characteristics are AIN and TiN. AIN crystallizes, making it produce pyroelectric and piezoelectric to activate sensors, while TiN is highly resistable to biocorrosion, which explains its use in biosensors and biogenic environment. The two types of ceramic pressure sensors are made at low temperatures, including piezoresistive straingauge and capacitative pressure sensor. In capacitative sensor, one electrode is movable while the other is fixable. The reason to use these sensors is to decrease power consumption. The image below shows a surface micromachined resonator.
Comparison of the Materials with Others
Gallium Arsenide is also used in MEMS fabrication and it is preferred because it is a semiconductor. Due to this property, it is used in LEDs and semiconductor laser. It also helps in development of micro-optimechanical devices. The negative aspect of Gallium Arsenide is that it becomes ductile in temperatures lower than a melting point, leading to a decrease of hardness when temperatures increase. Fabrication of Gallium Arsenide is mostly performed by micromachining through wet and dry etching. Wet etching of the material is characterized by the fact that it attacks from all angles at different directions and rates, while dry etching is a process during which ions are accelerated to the materials etched.
The table below shows characteristics of Gallium Arsenide
The image below is the molecular structure of Gallium Arsenide
Some metals are used in MEMS fabrication. These metals include nickel, silver, gold, aluminum, copper, and others. Unlike silicon and Gallium Arsenide, metals are highly ductile. This means that they will deform in case of high temperatures. For example, aluminum is alloyed to restore its structural properties. Gold is also easily deformed and becomes soft and ductile. Amorphous alloys can be used in MEMS fabrication due to the fact that it has no crystal structure
Processing Techniques of Microelectromechanical Technology
In the processing techniques, many methods have been used, but they can be explained in three dimensions: bulk micromachining, surface micromachining, and molding.
The MEMS are formed from a three-dimensional spatial structure on a substrate and mechanical blocks within that structure. Then, a system is created by the fabrication of electrical circuits that drive them into a common substrate. The technology is featured in a range of devices of different sizes. They involve precision of mechanical components, which can be formed using the semiconductor processing technology.
Devices can be used as actuators and bulky sensors with elevators of micron-scale equivalents. This has an advantage of reducing weight, bulk, and consumption of power while boosting production volume, performance, and functionality.
In the general form, the MEMS are comprised of micro sensors, micro actuators, microelectronics, and microstructures. All put into a common silicon chip, microsensors sense differences in the environment by measuring magnetic, electromagnetic, thermal, and chemical information. Then, microelectronics processes the information and links it to microactuators to react and make some changes in the environment. An example of the device is accelerometer used in air bags. In addition, micromachined components include calibration, self-testing, and conditioning circuits. The following are techniques used in the processing of the MEMS.
It is one of the most ancient micromachining technologies performed by eliminating material from the body to make cavities, holes, channels, and other desired shapes. It was first done by anisotropic wet itching of glass or silicon substrates. In this process, various chemicals were used, including potassium hydroxide and tetramethyleammonium hydroxide
It is a technique that builds mechanical elements by first beginning with a silicon wafer and then removing useful parts, leaving behind wanted parts. The wafer is photo-channeled, leaving a laminated layer on wafer parts that are desired to be kept. The wafer is later dipped into a liquid called potassium hydroxide, which removes exposed silicon. It is termed as a simple and cost-effective fabrication technology that is well-adapted for applications, which are price sensitive and simple in nature.
Bulk micromachining has been used to build most pressure sensors that have several advantages over past bulk micromachined pressure sensors. They include: easy manufacturing ability, cost-effectiveness, and high reliability. Most cars in the market are made on micromachined pressure sensors used to measure manifold pressure in the engine. The pressure sensors are also applied in various medicines.
There are emerging trends in microsystem and microsenser development to improve the ratio aspect of the device to the highest possible level. The result is better performance, for instance, by increasing device sensitivity and micro-actuator force. In addition, another technique uses a deep reactive ion plasma etcher. This method is very quick, especially when used in silicon. This makes thick materials bear small widths. Lastly, another technique is wafer bonding in bulk micromachining. After preparations, needed parts in different bodies are put together to form a channel combining many parts. Wafers are joined using a link. Glass and silicon wafers are bonded by subjecting them to high temperatures of about 1,000 degrees Celsius to between the contact places. The bond made this way is strong similarly to glass and silicon. The image below shows micromachined bulk.
Fig. 5. Titanium mirrors.
Surface micromachining is among the most popular technologies used in the processing of MEMS devices. According to this method, films are put into the parent body and are patterned by the use of photolithography to create MEMS devices. There is an alternation of films between sacrificial and structural layers, whereby MEMS devices come from structural layers. Other layers play a role in the support of components in the process. After the process, sacrificial materials are removed using wet chemicals. The resulting product is a sole MEMS part that is mobile contrary to the rigid substrate.
It involves fabrication of micromechanical structures by etching of thin structural films. This includes easy microstructures like membranes and a structure complex in nature such as linkages that are fabricated on the upper side of silicon formation of systematic polysilicon to turn silicon dioxide and microstructural material into a sacrificial layer. Surface micro-machining technology entails features, which include possession of the opportunity to integrate micromechanics, a small microstructure dimension, and microelectronics in the same structure. To achieve low costs, high volume is applied.
Surface micromachining is differentiated from bulk micromachining by the process that builds up from the wafer in a systematic manner. It is a repetitive sequence of depositing thin films on a wafer photo, patterning them and then etching the patterns. The structural level is made of silicon, while the sacrificial one contains silicon dioxide.
More fabrication is needed in surface micromachining unlike bulk micromachining, which makes it more expensive. It is used mainly to make more complex devices with a more sophisticated role.
In this process of fabrication, there are a number of challenges like presence of high sensitivity of the sacrificial layer connected to functional layers. There is a need to avoid sticking of the released microstructure to the substrate. Finally, control of the stress gradient in the structural layer is required in order to avoid bending of the released microstructure.
Thermal silicon oxide films act as a sacrificial layer that can be connected with upper selectivity against silicon by the use of hydrofluoric acid. However, capillary forces make the structures move down and link to the substrate after etching of the layer.
To finalize, MEMS layers are mostly processed by forming a mold, which may be filled to form wanted parts. Polymers have been used to make molds, which include types like photoresistant, as well as metal and deep-etched silicon wafers. A mold pattern is usually defined by photolithography. To get metal parts, the mold is filled by electroplating. Parts of polymer can be made by pouring it into the mold. After molding, the parts are removed by peeling or etching the mold.
Germany was the first country to perform micro molding called LIGA. The process required an extra source in order to show thick parts of photosensitive material, but later there has occurred development of techniques that use visible sources. The figure below shows a fabricated waveguide using LIGA.
MEMS Process Integration
This is the process of characterizing, optimizing, and understanding the relationship of processing steps in a process sequence. Customization of the MEMS process should avoid a surprise that integration is an important aspect. Moreover, developers should have good data on the properties of electrical materials and mechanical material properties. It implies enormous diversification of processing techniques used in integration. The MEMS have advantages in many aspects. Thus, they are made of integrated circuit-like formation and this enables them to put broad functionality into a sole microchip. The possibility to integrate miniaturized sensors is facilitated by the structure of actuators.
Applications of Microelectromechanical Systems
Microelectromechanical systems have various applications, which are primarily technological. Their applications help in making work easy in various industries. The following applications are among the most common.
To begin with, there is a collaborated formation in various fields such as microelectronics and biology. Many applications emanate from current trends and new technology. Biotechnology is one of the fields where the MEMS apply.
It enables new discoveries in the field of engineering and science such as chain reaction of polymerase, microsystems for identification and amplification of the DNA, electroporation capillary electrophoresis, and biochips used for detecting biological agents and chemical agents. A high input of drug screening is used by the microsystems.
Another application of the MEMS is in the field of medicine. It is one of the most successful domains of application since there are many devices and the market served is larger. There are MEMS pressure sensors, which apply in many fields for a long period. Sensors are diversified in use and are used in the following spheres.
Pressure sensors are used in monitoring of blood pressure, especially for patients in critical conditions. Devices have been introduced to replace the old ones, which were not cost effective and poor in terms of efficiency. They are also used to measure intrauterine pressure produced during birth. Any problems with blood pressure in the baby are monitored by the device. They are used in ambulances and hospitals as a device to monitor patient’s conditions, which include respiration and blood pressure. In addition, they monitor the breathing ability of a patient.
The sensor is used for eye surgery to establish and control removal of fluid in the eye for cleaning. Finally, the MEMS pressure sensors are used to regulate inflated mattresses, which are applied for removal of pain in a patient. Applications of the sensor are numerous, which is why innovations have been established. A few years ago, sensors started having a wireless interrogation capacity.
Communications is another field where the MEMS apply as they are used in high frequency circuits, turn table capacitors, and inductors, which can be improved when used with the MEMS. This will improve communication performance and they can also reduce power consumption, cost, and total circuit area. Samples of circuits are used as switches and as resonators for communication circuits employed as mechanical filters.
Inertial sensing involves gyroscopes and accelerometers, which are MEMS inertial sensors. For instance, MEMS accelerometers have gradually replaced conventional accelerometers, which are used for deployment in systems in automobiles, while previous technology used heavy accelerometers. The MEMS technology has given a possibility of change to accelerometers with single silicon, which is cost effective. MEMS accelerometers have advantages, which include reliability, portability, diversified functions, and smaller size. Rate sensors like the MEMS gyroscopes have a wide application in consumer electronics and automobile.
Fabrication of devices with varying degrees of complexity has been enabled by the polysilicon micromaching process. Microengines are used to drive wheels of microcombination locks. They can also be used with microtransmission to derive a pop up mirror out of plane. The device is called micromirror. Technologies in the fabrication of the MEMS IC allow the manufacture of a microtransmission system device. MEMS technology is used in performing vaporization of microchambers.
Recently, the MEMS technology has acquired the potential to change how people live and is compared to a computer. In the future, applications of the MEMS will be of greater functionality.
The MEMS technology has been established to increase efficiency and to reduce costs. There has been a wide range of uses and applications in various fields. The technology has been developed, leading to efficiency in work, especially in medicine, biology, and communication.
Advantages of MEMS
The MEMS allow existing devices to be miniaturized, which cannot be offered by macro-machined products. For instance, capacitive pressure sensor cannot be worked with a macro-machined capacitive diaphragm. Another advantage of the MEMS is the diversity of applications through the interdisciplinary nature of the technology, which has resulted in a variety of products and bonding across differently applied fields, for instance, optics-microelectronics and biology-microelectronics. Finally, the new technology enables to produce integrated systems, circuits, actuators, and sensors to be put in a single package. This further promotes performance improvement, reliability, and cost minimization.
Manufacturing processes of the MEMS devices have a great integration with controlling and signal processing electronics. The integration is intended to result in great performance, which is essential for reducing the cost of packaging, manufacturing, and incrementing of the device.
The MEMS borrow production techniques of various fabrications from the circuit industry and, thus, the cost of complex electromechanical systems can be reduced. Although the cost of production and each wafer can be high, this cost is spread throughout the process.
Integrated circuit fabrication techniques are put together with tremendous benefits of silicon and thin film materials in mechanical applications to ensure that reliability of electromechanical systems is gradually improved. There is an expectation that actuators will be integrated into a single chip.
Miniaturization gives rise to many beneficial aspects like increased mobility, reduction of the power consumption, and a possibility to place more functions into a smaller space, which allows the overall performance of MEMS to be improved. It translates to products of low cost and higher functionality.
Production of MEMS technology has been a great milestone for making work easier in the fields of its application. However, like other devices brought about by technology, there are some drawbacks, which do not support the use of the device. Some of the challenges facing the usage of the MEMS device include access of fabrication. Most companies want to have the knowledge about manufacturing of devices, but their exploration remains a challenge, which has been related to cost and design of devices.
Another challenge concerns packaging of devices as the MEMS devices have been of great diversity and require a simultaneous contact with their environment. The devices must meet the desired packaging to comply with requirements relating to application. Fabrication packaging is expected to be the most expensive and time-consuming process.
As seen from above listed applications of the MEMS devices, there are a number of significant benefits that explain their popularity. Therefore, these devices should be encouraged to be used, especially in hospitals. Further research should be focused on modification of the system with a view to increasing its efficiency.