Micromachining and micro electro mechanical systems (MEMS) technologies can be used to produce complex structure, devices, and systems on the scale of micrometres. MEMS is the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrates through microfabrification technology. MEMS is an enabling technology allowing the development of smart products, augmenting the computational ability of microelectronics with the perception and control capabilities of microsensors and microactuators and expanding the space of possible designs and applications. There are numerous possible applications for MEMS. As a breakthrough technology, allowing unparalled synergy between previously unrelated fields such as biology and microelectronics, many new MEMS applications will emerge, expanding beyond that which is currently identified or known. This paper reviews the scope of technology and the application it addresses. It includes a short analysis of future opportunities. This paper also describes the micromachining techniques used in the fabrication of MEMS.
Micromachining, Microsystems, Piezoelectric actuation, Electrostatic actuation, Thermal actuation, Magnetic- actuation.
Micromachining and Micro-Electro-Mechanical Systems (MEMS) technology can be used to produce complex structure, devices, and systems on the scale of micrometers. Micromechanical structures and systems are miniturised devices that enable the operation of complex systems. They exist today in many environments, especially medical, consumer, industrial and aerospace. Their potential for penetration into a broad range of applications is real, supported by strong developmental activities at many companies and institution. The technology consists of large portfolio of design and fabrication processes, many borrowed from integrated circuit industry. The development of MEMS is inherently interdisciplinary, necessitating an understanding of MEMS components as well as end applications. Due to the enormous breadth and diversity of the devices and systems that are being miniaturized, the acronym MEMS is not particularly apt one (i.e. the field is more than simply micro, mechanical and electrical systems). Other names for this general field of miniaturization include Microsystems technology (MST), popular in Europe, and micromachines, popular in Asia
The needs for miniaturization of various ultra precision items utilized for producing highly precision machines and equipment necessitate the development of manufacturing process capable of performing micromanufacturing activities. The emergence of MEMS has strongly enhanced the use of fewer and harder materials, brittle materials and their micromachining technologies. A set of new technologies useful for successful international competitive development is micromachining, which satisfy many of the present industrial needs in the manufacturing. According to CIRP committee of Physical and Chemical machining Processes, “machining,” means machining the dimensions in the range of 1_199 um.
Although many of the microfabrication techniques and materials used to produce MEMS have been borrowed from the IC industry, the field of MEMS also driven the development and refinement of other microfabrication processes and materials not traditionally used by the IC industry. This paper briefly reviews selected micro machining processes capable of sub-micron structure to large structure micro. Large numbers of micro machine processes or presently used for the various kinds of applications.
2.1 REVIEW OF LARGE STRUCTURE MICROMACHINING TECHNIQUES
Process utilizing tool as in conventional machining process hardly achieve small amount of hard material removal by mechanical force or shear or shear phenomenon. The machinable size is also limited because of the elastic deformation of micro tool or work piece. These processes are only suitable for 3D products and hardly achieve unit removal at atomic level. On the other hand Non-conventional Machining processes using tool or tool in the form of shape i.e. Electro Discharge Micromachining (EDMM). Laser Beam Machining (LBM), Ultrasonic Micromachining (USMM). Abrasive Jet Machining (AJM) etc. which has been very successful in the large structure micromachining inherently achieves very small unit removal. These processes have been found to be very suitable for micromachining because of having remarkable advantages. Machining force in these processes such as than that of in conventional machining processes such as cutting, milling, drilling etc. Metal chips can be removed with a very small force in processes such as EDM, USM and LBM etc
[/b]2.2 REVIEW OF SUB-MICRON MICROMACHINING TECHNIQUES
The majority of sub-micron micromachining procedures involve two types of interaction; electromagnetic radiation (e.g. optical, UV or X-ray photons) or charged particles (electronics, low energy heavy ions, high energy light ions) In general the micromachining procedures based on electromagnetic radiation require masks. In mask processes a selective pattern of radiation is transmitted through a structured mask on to a resist material, and subsequent development of the exposed resist using specific chemicals can produce microstructures. The use of charged particle techniques for micromachining therefore is essentially limited to direct write processes, where a focused charged particle beams is scanned over a material in a specific pattern to produce microstructures. Although the detect write processes has the advantage that masks are not required, it has an obvious limitation in that the production of micro components is a serial process that has greatly reduced efficiency for multiple component production(4).Optical lithography, X-ray lithography (LIG),deep UV lithography, electron beam direct writing are some of the widely used sub-micron micromachining methods. The vast majority of methods are condensed into three major categories:
1. Material deposition, including thin film deposition and bonding processes.
2. Pattern definition using lithography.
Different sub-micron micro machining techniques have been summarized in the Table 1 and 2.
3. MATERIALS FOR MEMS
The choice substrate materials for MEMS are very broad, but crystalline silicon is by far, the most common. Complementing silicon is a host of materials that can be deposited as thin films. These include polysilicon, amorphous silicon, silicon oxides and nitrides, glass, and organics polymers as well as host of metals. Crystallographic plane play important role in the design and fabrication of silicon based MEMS and also affect some mechanical properties of Silicon. Three physical effects commonly used in the micromechanical sensor and actuators: Piezoresistivity, piezoelectricity and thermoelectricity
4. MICRO SYSTEM COMPONENTS
A micro system will typically comprise of components from one or more of the three classes: microsensors, to detect changes in the system's environment; an intelligent component that makes decisions based on changes detected by the sensors; and microactuators, by which the system changes its environment. The basic sensing and actuation system vary considerably from one design to another, with significant consequences to control electronics. Design considerations are many; they include specifications of end applications, functionality, process feasibility and economics justification.
Three general categories from the total extent of the MEMS: sensors, actuators and passive structures. Sensors are the transducers that convert mechanical, thermal are any other form of energy into electrical energy; actuators do actually the opposite
4.1 TECHNIQUES FOR SENSING AND ACTUATION
Micromachining and MEMS are the technologies well suited to improve the performance, size and cost of the sensing system. For this reason the greatest commercial successes in MEMS are microsensors and they represent the majority of MEMS developed to date. Although historically, the greatest demand and most research and development activity has been on pressure sensor and accelerometers, the field of MEMS is maturing and the diversity of the applications and sensor technologies has been increased tremendously. The actuation option available in MEMS are strain gauges, capacitive position detectors, pressure sensors, inertial sensors, magnetometers, thermal sensors, chemical sensors, polymer based gas sensors, resonant sensors, electrochemical sensors, molecular -specific sensors, cell based sensors etc. although it is impossible to describe each micro sensor technology in detail the most prominent microsensors technologies are described