MEMS Introduction
1. What is MEMS?
Imagine a machine that is so small that it is
invisible to the naked eye. Imagine
devices that is the size of grains with mechanical parts smaller than a dust mite (Fig. 1), and
entering a realm where the dominating physical principles is no longer gravity
and inertia, but are substituted by atomic forces and surface science. Now imagine these micro machines being
produced in batch sizes of thousands at a time, with cost of individual unit
nearing zero. Welcome to the micro
world, a place now occupied by a new technology known as MEMS, or more simply, micromachines.
The word MEMS is an acronym for Mrico-Electro-Mechanical
System and generally refers to the devices that are on a millimeter
scale with micro-resolution. MEMS is the integration
of mechanical elements, sensors, actuators and electronics on common silicon
substrate through the utilization of microfabrication
technology. MEMS promises to revolutionize nearly
every product category, thereby, making the realization of complete
system-on-a-chip.
In microsystems, microelectronic integrated circuits (ICs) can
be thought of as the “brains” of system and MEMS augment this decision-making
capability with “eyes” and “arms”, to allow microsystams
to sense and control the environment. The sensor gathers the information from
the environment through measuring mechanical, thermal, biological, chemical,
optical, and magnetic phenomena. While the electronics process the information
derived from the sensors and through some decision making capability direct the
actuators to response by moving, positioning, regulating, pumping, and
filtering, thereby, controlling the environment for
some desired outcome or purpose.
MEMS
is a new 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. 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
such as the polymerize chain reaction (PCR) microsystems
for DNA amplification and identification, the micromachined
scanning tunneling microscopes (STMS), biochips for
detection of hazardous and selection. In the industrial sector, MEMS devices
are emerging as product performance differentiates in numerous markets with a
projected market growth of over 50% per year. 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 is an extremely diverse technology that potentially could significantly
impact every category of commercial and military products. The nature of MEMS
technology and its diversity of useful applications make it potentially a far
more pervasive technology than even integrated circuits 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 macroscale
sensory-actuator-electronics system. In comparison, MEMS technology allows
these complex electromechanical systems to be manufactured using batch
fabrication techniques allowing the cost and reliability of the sensors and
actuators to be put into parity with that of integrated circuits.
Interestingly, even though the performance of MEMS devices and systems is
expected to be superior to macoscale components and
systems, the price is predicted to be much lower.
MEMS
is believed to become a hallmark 21st-century manufacturing
technology with numerous and diverse applications having a dramatic impact on
everything from aerospace technology to biotechnology. The MEMS technology now
being forged in R&D labs will generate new technological capabilities for
society, tremendous economic growth through countless commercial opportunities,
many of new products, and thousands of high-paying, high quality jobs. As
breakthrough technology allowing unparalleled synergy between hitherto
unrelated fields of endeavor such as biology and
microelectronics, MEMS is forecasted to have a commercial and denfence market growth similar to its parent IC technology.
The
MEMS is inevitably the next step in the silicon revolution involving the
integrated circuit and the need and desire of making things smaller, like in
mini robots. Thanks to the 3 decades
long research and development of higher performance IC chips, today, the world
is equipped with almost all the necessary equipment and procedures needed in
the successful making of MEMS devices. Hence making the
research and development work into the micro domain relatively easier and
economical. In fact, most of the
equipment used today in the making of micro-machines is actually obsolete
equipment formally used in making IC chips.
Thus MEMS offers a second chance to extend the life of aging IC
fabrication facilities. Since they are made by exploiting the existing
integrated circuit manufacturing infrastructure, MEMS-based devices can be made
cheaply. The usual process involves the successive deposition, photo
patterning, and etching of thin films on silicon. For the case of integrated circuits, these
patterns are formed to create small electrical devices. For the case of MEMS, these same fabrication
sequences are used to create mechanical structures.
The advances in the last few years in the field of micro devices shows
the immense potential of MEMS. These
devices have the ability to perform a variety of functions like physical and
chemical sensing, actuation, steering light and communication. Much interest in the MEMS devices centers around its 2 main
characteristic, (a) the very small size and (b) the promise of very low cost of
production which is really the driving force for MEMS-based devices.
2. Application of MEMS
To
date, only a handful of MEMS-based devices are being commercialized. This is in fact quite disheartening given the
many research facilities and research personnel involved in this field. But a closer look will reveal that given the
slightly more than 20 years of works done in this area, it is only recently
that the amount of resources involved in the research and development of MEMS
have increased dramatically. This can be
seen from the number of published works and authors from the pioneering years
to date. This recent explosion in
interest in the MEMS area could have been, in part, a result of the successful
commercialization of some high profile products like the Bubble Jet Printer
Head.
Microsensors
There
are quite a selection of MEMS-based sensors that have
been commercialized. One of the more common
applications of MEMS sensors comes in the form of an accerolometer
in the deployment of safety airbag in car.
Some examples of MEMS sensors include (a) pressure sensors, (b) strain
gauges, and (c) accerolometer for the measuring of
acceleration and (d) gyroscope for the measurement of rotation.
Figure 2. A Pressure Sensor with IC
integration
The
airbag deployment sensor is one of the earliest uses of MEMS sensors in
cars. Other possible use of the MEMS
sensors includes the controlling of the amount of vibration on a car using the accerolometer together with the suspension system. Also by measuring the rotation of the car
with the gyroscope, it is possible to judge whether the driver is losing
control of the car, and hence the deployment of the braking system. Outside the car industry, the gyroscope can
be used to check the rotations of essential machine parts so as to prevent
critical failure. Examples would be in
the turbine of engine and power plants.
Figure 3. An Accelerometer used for
activating the Airbag in Cars
Optical and micro-Mirrors
The
MEMS micro-mirrors can be used in the making of optical sensors and display
both of which involves the controlling and directing of the light band. Today, information is being transferred to
people from electronic devices through display technologies like the Cathode
Ray Tubes (CRTs) and Liquid Crystal Display (LCD). In the future, MEMS-based Micro-Mirror array
is a likely candidate to replace them as the dominant form of display
technologies. This is due to the
low-cost and high performance of the micro-mirrors. Furthermore, due to the similar processes and
facilities used in the fabrication of the MEMS micro-mirrors, it is relatively
easy to incorporate them with their controlling IC chip onto a single silicon
substrate.
An
example of a successful MEMS-based Micro-Mirror array comes in the form of the
Digital Mirror Device (DMD) from Texas Instruments. The DMD is a projection system based on a
very large array of micromachined mirrors. These mirrors are integrated with on-chip
CMOS microelectronics which control the position and
operation of the mirrors. A number of
large screen projection systems currently on the market use this DMD chip as
the heart of the system. Other
MEMS-based display systems are also under development. Work is underway at Silicon Light Machines on
the development of a new type of MEMS-based display. Instead of projecting images by tilting
mirrors, this technology operates by the vertical displacement of small
diffraction gratings. Each pixel of an
image is represented by an independently controlled diffraction grating.
Figure
4. The Digital Mirror Device and the subsequent product.
Biomedical applications
Another
rapidly developing field of MEMS falls under the biomedical category. In this area, MEMS have the great potential
in (a) the Biomedical Instruments and Analysis, and (b) Implants and Drug
Delivery. Miniaturization of surgical
and diagnostic instruments are done for reasons like
(a) cost reduction,
(b) less intrusive surgical
procedures,
(c) health concerns,
(d) reducing amount of test
sample needed, e.g. blood,
(e) speed of diagnosis,
(f) patient recovery time and,
(g) ease of usage
Miniaturization of medical instruments is of interest for a number of
reasons, dependent on the application.
In the case of surgical instruments, the decreasing size would mean a
less invasive operating procedure for patient, which also would mean a faster
rate of recovery for the patient.
Furthermore, having micron-size instruments would mean that previously
untreatable complication pertaining to neural and cell repair is fast becoming
a thing of the past. Having the ability
to shrink instruments to incredibly small sizes also means that previously time
consuming and expensive diagnostic procedures can be done by relatively
unskilled personnel on Credit-Card size devices. Similar in function to their room size cousin
but smaller in cost, these MEMS-based devices will be able to do things like
DNA testing, blood testing and many more.
And due to its size, only a small amount of test sample is needed for
the diagnosis and to top it off, the decreasing cost of the device allows it to
become disposable, hence reducing the chances of potential health hazard.
Another
area of potential beneficial application of MEMS-based devices in the
biomedical field comes in the form of implants.
The idea is to have a small drug-dispensing device implanted into
patients for the slow dispensing of drugs like antibiotics, etc. This method of slow dispensing will even out
the dosage of drugs in the body as compared to that of popping pills and
injections.
Micro and RF Switches
MEMS-based
devices can also be used to make high performance, high precision
switches. These switches can be used for
directing signals and to switch on or off micro devices. One of the commercialized switches can be
found in the Optical industry and was developed in 1999 by Marxer
and Sercalo for the directing of signals. One of the main advantages of the switch
comes from its low rate of signal loss.
Figure
6.
The Marxer and Sercalo
Optical Switch
Microactuators (micro pumps and microgrippers)
As
the name suggested, the MEMS micro-pump is a miniature version of its much
larger cousin. However, the method of
pumping fluids and gases can be very different from that of their macro
cousins. Some of the more interesting
pumping methods are (a) using bubble, (b) using sound waves and (c) thermal
expansion of the fluid. An example of a
highly successful MEMS-based pump comes in the form of the Ink Jet Print
Head. These devices comprised of an
array of MEMS-based heater elements that are positioned in small ink well,
behind simple orifices. When the heater
is turned on, a bubble is formed in the ink, which shoots ink through the
orifice. The accurate positioning of the bubble can be achieved by the
positioning of the heater element. As
time progressed, advances in the MEMS manufacturing technology has led to these
components from dispensing black ink to dispensing full color
ink. This is also accompanied by the
increased precision of the ink drop sizes hence an increase in resolution of
print.
Grasping
and manipulating small or micro-objects is required for a wide range of
important applications, such as the assembly of small parts to obtain microsystems, surgery and research in biology and
biotechnology. In fact, as most industrial
fields exhibit a clear trend towards miniturization,
the need for techniques and equipment for manipulating micro-objects with high
accuracy and speed becomes increasingly evident.
Today,
MEMS-based technology is still far from producing micro robots which can do
surgery in the human body, or several inches sized silicon satellites. This is
partially because while micromachining fabrication has already progressed to
the extent of being able to create several layers of planar structure precisely,
it normally does not permit too much assembling and thus limits the feasibility
of producing complex, especially three dimensional, microstructures. To achieve
more sophisticated structures, assembling of micro components is indispensable. Furthermore, the maintenance and modification
of such systems will also require the gripping and manipulating of the micro
parts in the systems. Hence there exists the need for a micro-gripper, which is
the purpose of this project.
3. Present MEMS Challenges
To date, only a handful of MEMS-based devices are
being commercialized. This is in fact
quite disheartening given the many research facilities and research personnel
involved in this field. But a closer
look will reveal that given the slightly more than 10 years of works done in
this area, it is only recently that the amount of resources involved in the
research and development of MEMS have increased dramatically. This can be seen from the number of published
works and authors from the pioneering years to date. This recent explosion in interest in the MEMS
area could have been, in part, a result of the successful commercialization of
some high profile products like the micro-accelerometer and Bubble Jet Printer
Head. In order to make MEMS technology a successful commercial one, a great
amount of efforts will be needed on the research and development of sensors,
actuators, materials and processing technologies.
Despite
the size and scale of MEMS research and development investments, they are small
compared to the R&D expenditures made by the integrated circuit industry.
However, the size of the MEMS industrial base is still very small and unable to
sustain large R&D expenditures. Since its inception, MEMS technology has
been able to leverage heavily from the development in the IC technologies.
However, the magnitude of this leveraging has begun to lessen due to the speed
of progress and change in the IC fabrication arena. Most industrial
commercialization of the technology will likely come from the relatively more
direct applications in the future. These include simple structural components,
where the short-term return can be readily attainable. Unfortunately, in most
cases, either the device has yet to exist, or have not even been imaged by
potential users.
The accessibility
of companies, both small and large, to MEMS fabrication facilities needs to be
increased. Currently. Most companies who wish to
explore the potential of MEMS technology have very limited options for getting
devices prototyped or manufactured. A mechanism allowing these organizations to
have responsive and affordable access to MEMS fabrication resources for
prototyping and manufacturing is essential.
The
output of well-trained MEMS engineers and scientists from universities needs to
increase. MEMS is a multidisciplinary field that consists of a wide range of
technical and design expertise ranging from chemical to electrical engineering
and from mechanical or electrical field. Depending on the area of use of the
MEMS device, knowledge of other disciplines like biology and materials might be
needed in the designing of the devices. A increasing
number of MEMS engineers and scientists is urgently needed. Also it is
necessary to gather expertise from different disciplines to work into the
development of MEMS devices for their successful applications.
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.
Other technical challenges. For example, (1) Advanced simulation and modeling tools for MEMS design are urgently needed; (2) The
packaging of MEMS devices and systems needs to improve considerably from its
current primitive states; (3) MEMS device design must be separated from the
complexities of the fabrication sequences, etc..