Micro-Electro-Mechanical-Systems
(MEMS)
Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through the use of micro fabrication technology.
Microelectronic integrated circuits (ICs) can be thought of as the "brains" of systems and MEMS augments (its decision-making capabilities) its "eyes" and "arms". They allow micro systems to sense and control the environment. In its most basic form, the micro sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena; then the micro electronics process the information derived from the sensors and through some decision making capability directs the micro actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby, controlling the environment for some desired outcome or purpose.
While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components are fabricated using compatible "micromachining" processes that selectively etch away parts of the silicon wafer (the micro structures)or add new structural layers to form the mechanical and electromechanical devices.
Impact of MEMS
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. MEMS technology is enabling new discoveries in science and engineering:
· A Polymerase Chain Reaction (PCR) micro-systems-for DNA amplification and identification
· Micro-machined Scanning Tunneling Microscopes (STMs), biochips-for detection of hazardous chemical and biological agents
· Micro-systems for high-throughput drug screening and selection.
· MEMS devices are emerging as product performance differentiators in numerous markets wherever electromechanical systems are currently functioning
· MEMS devices are extremely small (e.g. MEMS has enabled electrically-driven motors smaller than the diameter of a human hair to be realized), many processes from automobiles, to gas turbines, anything mechanical can have performance boosted by MEMS
A New Manufacturing Technology
MEMS technology is not just about
size, and it’s not about making things out of silicon. (Even though silicon
possesses excellent materials properties making it an attractive choice for
many high-performance mechanical applications–e.g. the strength-to-weight ratio
for silicon is higher than many other engineering materials allowing very high
bandwidth mechanical devices to be realized). Instead, MEMS is a manufacturing technology—a new way of making complex
electromechanical systems using batch fabrication techniques similar to the way
integrated circuits are made and making these electromechanical elements along
with electronics.
MEMS is an extremely diverse technology that potentially could significantly impact every category of commercial and military products. Already, MEMS is used for everything ranging from in-dwelling blood pressure monitoring to active suspension systems for automobiles. The nature of MEMS technology and its diversity of useful applications make it a far more pervasive technology than even integrated circuit microchips.
MEMS blurs the distinction between complex mechanical systems and integrated circuit electronics. Historically, sensors and actuators are the most costly and unreliable part of a macro-scale sensory-actuator-electronics system.. 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. As a breakthrough technology, allowing unparalleled synergy between hitherto unrelated fields of endeavor such as biology and microelectronics, many new MEMS applications will emerge, expanding beyond that which is currently identified or known.
MEMS technology is based on a number
of tools and methodologies, which are used to form small structures with
dimensions in the micrometer scale (one millionth of a meter).
There are three basic building blocks in MEMS technology, which are(1) the ability to deposit thin films of material on a substrate—Deposition , (2) to apply a patterned mask on top of the films by photolithographic imaging—Lithography, and (3) to etch the films selectively to the mask--Etching.
Deposition
One of the basic building blocks in MEMS processing is the ability to deposit thin films of material. Usually the thin film has a thickness between a few nanometers to about 100 micrometer. MEMS deposition technology can be classified in two groups:
(1) Depositions that happen because of a chemical reaction. These processes exploit the creation of solid materials directly from chemical reactions in gas and/or liquid compositions or with the substrate material. The solid material is usually not the only product formed by the reaction. Byproducts can include gases, liquids and even other solids.
(2) Depositions that happen because of a physical reaction. Common for all these processes are that the material deposited is physically moved on to the substrate. In other words, there is no chemical reaction which forms the material on the substrate. This is not completely correct for casting processes, though it is more convenient to think of them that way.
Lithography
Lithography in the MEMS context is typically the transfer of a pattern to a photosensitive material by selective exposure to a radiation source such as light. A photosensitive material is selectively exposed to radiation (e.g. by masking some of the radiation or not).
If the resist is placed in a developer solution after selective exposure to a light source, it will etch away one of the two regions (exposed or unexposed). If the exposed material is etched away by the developer and the unexposed region is resilient, the material is considered to be a positive resist. If the exposed material is resilient to the developer and the unexposed region is etched away, it is considered to be a negative resist.
The pattern of the radiation on the material is transferred to the material exposed, as the properties of the exposed (positive resist) and unexposed regions (negative resist) differs this technique is capable of producing fine features in an economic fashion, a photosensitive layer is often used as a temporary mask when etching an underlying layer, so that the pattern may be transferred to the underlying layer.
Etching
In order to form a functional MEMS structure on a substrate, it is necessary to etch the thin films previously deposited and/or the substrate itself. In general, there are two classes of etching processes:
1. Wet etching where the material is dissolved when immersed in a chemical solution
2. Dry etching where the material is sputtered or dissolved using reactive ions or an etching agent. The dry etching technology can split in three separate classes called reactive ion etching (RIE), sputter etching, and vapor phase etching.
Three good comprehensive web sites:
MEMS organization website:
Cornell University Website for MEMS research:
http://www.people.cornell.edu/pages/akt1/memsmain.html
http://mems.sandia.gov/scripts/index.asp
A Great Website with movies and
photos of MEMS Micromachines:
http://www.sandia.gov/mstc/micromachine/movies.html
Website of papers in field:
http://www.bioen.utah.edu/faculty/abf/publications.html
A site for the study of MEMS methodology:
http://www.ulg.ac.be/ltas-vis/research/recherche_MEMS.pdf
Gas Turbine project with MIT
http://www-mtl.mit.edu/mtlhome/6Res/AR2000/2MEM/MEMabs10.pdf
Firm specializing in simulations of technology systems
http://www.cfdrc.com/datab/Applications/MEMS/mems.html
Website for a company consulting in micro optical MEMS lithography:
http://www.memsoptical.com/techinfo/memstut1.htm
Silicon wafer scale production-a treatise on Microwafer production using Scanning Probe Microrobots:
http://www.foresight.org/Conferences/MNT05/Abstracts/Tabiabst.html
Challenges for future of MEMS
technology:
The accessibility
of companies, both small and large, to MEMS fabrication facilities needs to be
increased.
It is believed that many of the largest beneficiaries of MEMS technology
will be firms that have no capability or core competency in micro fabrication
technology and access by these companies is critically important to their
successful utilization. A mechanism allowing these organizations to have
responsive and affordable access to MEMS fabrication resources for prototyping
and manufacturing is essential.
Advanced simulation and modeling tools for
MEMS design are urgently needed. Presently, most MEMS devices are modeled
using weak analytical tools resulting in a relatively inaccurate prediction of
performance behavior. As a result, the MEMS design process is usually performed
in a trial-and-error fashion requiring several iterations before the
performance requirements of a given device are finally satisfied. This
non-ideal design methodology combined with the length of time and high cost
associated with MEMS prototyping results in a very inefficient and ineffective
scenario for commercial product development. The availability of suitable
design tools combined with computer networks to provide access to high
performance workstations and local or remote supercomputer capability
can radically alter this situation.
The packaging of MEMS devices and systems
needs to improve considerably from its current primitive state. MEMS packaging
presents unique challenges compared to IC packaging due to the diversity of
MEMS devices and the requirement that many of these devices are in continuous
and intimate contact with their environment. Presently, nearly all MEMS
development efforts must develop a new and specialized package each time a new
device is designed. Consequently, most companies find that packaging is the
single most expensive and time consuming task in their overall MEMS product
development program. As with the actual components themselves, numerical
modeling and simulation tools for MEMS packaging are virtually non-existent.
Approaches which allow designers to select from a catalog of existing
standardized packages for a new MEMS device without compromising performance
would be beneficial.
MEMS device design must be separated from
the complexities of the fabrication sequence. Currently, the designer of a
MEMS device requires a high level of fabrication knowledge in order to embody a
successful design. Further, the development of even the most mundane MEMS
device frequently requires a dedicated research effort directed at formulating
a suitable fabrication sequence. An interface which separates design from
fabrication allowing the designer to use process-independent design tools and
methodologies will reduce the amount of time and effort required to
successfully realize MEMS devices. This will permit more manufacturable
designs, done correct the first time or with fewer iterations to become
routine. Since extensive knowledge of fabrication will no longer be a
prerequisite before beginning design activities, more designers will be able to
participate in design activities and this will result in increased levels of
innovation and creativity. Further, an interface separating design from
fabrication will enable higher levels of integration without increasing
development time or costs. To be of most utility, the interface should allow
designers to have ability and know the manufacturing implications of their
designs at design time and fabrication specialists to be able to provide the
needed functions to aid designers.
Quality
control standards for MEMS technologies are needed. Frequently, the quality
of many MEMS devices fabricated at either academic or commercial facilities is
low. Part of the problem is that the technology is so new that the fabricators
do not yet know how to define quality, much less measure it. The output of
well-trained MEMS engineers and scientists from the nation's universities needs
to increase. The present output from the nation's universities of technical
persons trained in MEMS technology is much smaller than the number required to
support the projected growth of MEMS industry. Traditionally, the training of
MEMS engineers and scientists has entailed a graduate education at one of a few
research universities with the student working under the direction of an
experienced faculty member to design, fabricate, and test some hopefully new
and unique type of MEMS device. A graduate education in MEMS technology is very
costly and comparatively time-consuming. A new methodology to increase the
number of MEMS engineers and scientists