MEMS

MEMS

This is the age of miniaturization. Keeping in mind the various possibilities in the near future our techno-scientists have brought a size-revolution in mechanical and electromechanical technology. In terms of technology, a device which is smallest in size is considered to be most efficient to function. The miniaturization of electro-mechanical devices is termed as microfabrication. This revolutionary technology has given birth to MEMS or Microelectromechanical Systems. The size of MEMS devices can vary from less than one micron to several millimeters. Structurally, MEMS can vary from some simple systems to highly complex electromagnetic systems. The elements which allow MEMS to function are definitely miniaturized structures like microelectronics, microsensors and microactuators. Microsensors and microactuators are typically transducers as they convert signal from one form to another.

MEMS structure

The formation of MEMS is completed only when the microelectronics, microsensors and microactuators merges with integrated circuits in a common silicon substrate. The other micromechanical devices are formulated using “micromachining” process. This process involves either selective etching of silicon wafers or depositing of extra layer for the fabrication of mechanical or electromechanical devices. MEMS can also be merged with various other technologies like photonics or nanotechnology which provides wider market opportunities.

MEMS vs Nanotechnology

Nanotechnology is the technique to manipulate the size of any matter into atomic or molecular level i.e. into nano-dimension. This manipulation is carried in two processes viz, top-down process or bottom-up process. Top-down process is the miniaturization process which involves the same technology as used in MEMS except that the size is made smaller by employing photolithography and etching methods. Besides this, the bottom-up process involves deposition, growth or self-fabrication technologies.

Though nanotechnology and MEMS technology are considered as two different technologies, yet there are no distinctive differences between the two. Both the technologies are inter-dependent on each other. For example, despite that the scanning tunneling-tip microscope is used to observe atoms and molecules of nano-dimensional size yet it is a MEMS device. Hence MEMS and nano-technology is used side-by-side for many successesful discoveries in various fields.

Material used to manufacture MEMS devices

The most important requirement to manufacture MEMS devices is the material which must be a semi-conductor and follow the basic techniques of deposition, photolithography and etching. Let us find out few suggested materials used in this process.

• SILICON is always considered as the most useful material to build the circuit oriented integrated systems as it is a perfect Hookean material which when flexed does not dissipate any energy. Manufacturing cost also goes down by using silicon material.
• POLYMERS are also one of the important materials used for building different MEMS devices because it is cheaper than crystalline silicon and it has a huge variety of material characteristics required for the technique.
• METALS can also be used for the formation of these devices because metals have high degree of reliability than silicon or polymers. But metal does not have some mechanical properties in term of silicon.
• CERAMICS are the widely used material in MEMS fabrication because of its huge variety of combined material properties. These properties are derived from the combination of different semiconductor materials like silicon, aluminum, titanium, silicon carbide and other ceramics.

MEMS Technique

Here are the basic techniques behind MEMS operation.

Deposition Process: MEMS has the ability to deposit films of material whose thickness ranges from few nanometers to 100 micrometers. There are two kinds of deposition techniques.

1. Physical Vapor Deposition  (PVD) : In PVD process the material is removed from a target and deposited on another surface by sputtering and evaporation processes.
2. Chemical Vapor Deposition (CVD): In this process vapor from the target gas is deposited into the desired substrate.

Pattern Process: This process in MEMS involves depositing a pattern into the substrate.

Lithography: A photosensitive material is one which undergoes change when exposed to any kind of radiation. Lithography technique in MEMS involves depositing a pattern on a photosensitive material with the help of radiation.

Photolithography: In this process geometric shapes are deposited on the surface of silicon wafer. The following steps are involved in the process of lithography.

• Step1: Cleaning wafer, formation of a barrier layer and application of photoresist coating.

In this process, firstly the wafer is cleaned with some chemicals to remove any impurities. Secondly, a barrier layer of silicon-dioxide is deposited on the wafer. And lastly, with the technique of spin coating, a uniform coating of photoresist is applied on the wafer.

• Step2: Positive and Negative Photoresist

Photoresist is of two types. One is positive photoresist where the resist is revealed with UV at places where the material is to be removed. Secondly is negative photoresist in which the UV exposed with resist polymerizes the resist and make it difficult to dissolve.

• Step3: Electron Beam Lithography

This technique involves scanning beam of electrons on a resist in patterned fashion, in order to remove exposed and non-exposed parts of the resist. Electron beam Lithography includes removal of resist in three different ways, viz,Ion beam Lithography, Ion track Lithography, X-Ray Lithography.

• Step4: Etching Process

Etching processes are of two types, viz, wet etching where the material is dissolved using chemical solution and dry etching where the material is dissolved using ions or vapor. Wet etching is preferred when the etchant and the substrate material suits the application. Dry etching technique has three different classes,

• Reactive Ion Etching- In this, reactive ions are moved towards the material being etched which results in the formation of other gaseous material.
• Sputter Etching- This type of etching is similar to RIE only thing is that it does not require any reactive ions.
• Vapor Phase Etching- In vapor phase etching the wafer that is to be etched is dissolved by chemical reaction of gas molecules.

Fabrication of MEMS

Fabrication of MEMS is done in following processes.

• Bulk Micromachining: This is the oldest technology used in machining. This technology is based on chemical wet etching process which involves immersing of substrate in chemical solution which etches exposed region of the substrate wherever required.
• Surface Micromachining: Surface micromachining is the commonly used technology for MEMS fabrication which is performed keeping in mind the type of material and etching combination.
• Wafer Bonding: Wafer bonding involves joining of two or more wafers to create a stack of multi-wafer. Wafer bonding is categorized in three types.Direct or fusion bonding, Anodic bonding, Intermediate layer bonding.

Applications of MEMS and Nano-technology

1.  Biotechnology: There are numerous discoveries in bio-science and engineering which have been initiated with the help of MEMS and nano-technology, both working together. Few examples are:

• Scanning tunneling-tip microscope
• Biochips for detecting chemical reactions occurring inside living body
• Polymerase Chain Reaction
• Capillary Electrophoresis and many more.

2.  Medicine: The most important example where MEMS and nanotechnology plays an important role is the formation of MEMS pressure sensors. MEMS pressure sensors are used in the following purposes.

• Intrauterine pressure during birth is measured with the MEMS sensors.
• Pressure sensors are used to monitor blood pressure, respiration and ventilation is another invention of MEMS sensors.
• It is used during eye surgery to control vacuum level.
• During kidney dialysis, MEMS pressure sensor is used to monitor the blood pressure and regulate the rate of flow of dialysis solutions.

The contribution of MEMS and nano-technology in the field of medical science is innumerable and remarkable.

3.  Inertial Sensing: MEMS inertial sensors attain great success in the market for creating sensor oriented applications like accelerometer and gyroscope. These sensor apps are nowadays also used in smartphones like Apple iPhone and Nintendo Wii.

4.  Communications: The performance of communication circuits is improving, as the high frequency circuits are gaining benefit from the discovery of RF-MEMS technology.