Micro
Electro
Mechanical
Systems
(MEMS)
Micro
Electromechanical Systems, or
MEMS, are micron-sized machines that can be used as mechanical, electrical, or
chemical transducers. Many different fields such as the automotive and medical
industries utilize MEMS because of four advantages: 1) easier to mass-produce,
2) lower cost of production, 3) easier to make part alterations, and 4) higher
reliability compared to large-scale machines. MEMS are generally made of
Polycrystalline Silicon, which is the same material used to make integrated
circuits (IC). To manufacture MEMS devices, a process called photolithography is
used. The general steps of the two-mask photolithography process are discussed.
We can produce many type of MEMS devices using photolithography. One of the most
notable applications of MEMS devices is the accelerometer (crash sensor) of the
airbag deployment system in modern automobiles. MEMS crash sensor will replace
the less capable large-scale device for a fraction of the cost. MEMS development
has just begun and researchers as well as engineers are working hard to create
more and better MEMS devices to serve us in the future.
INTRODUCTION: What is MEMS
Technology?
Micro-Electro-Mechanical
Systems (MEMS) is the integration of mechanical elements, sensors, actuators,
and electronics on a common silicon substrate through the utilization of
microfabrication technology. Using the fabrication techniques and materials of
microelectronics as MEMS processes construct both mechanical and electrical components . Mechanical components in MEMS , like
transistors in microelectronics, have dimensions that are measured in microns
and numbers measured from a few to millions. It can be difficult for one to
imagine the size of a MEMS device. The general size of MEMS is on the order of
microns (10-6 meter) as shown by the illustration of the MEMS gear
(see figure 1). The main characteristic of MEMS is their small size. Due to
their size, MEMS cannot be seen with the unaided eye. An optical microscope is
usually required for one to be able to see them.
Figure
1. A MEMS Gear in comparison to a human hair
(Source: University of
Wisconsin at Madison MEMS research laboratory, 1990)
MEMS is
not about any single application or device ,nor is it defined by a single
fabrication process or limited to a few materials. More than any thing else MEMS is a
fabrication approach that conveys the advantages of miniaturization, multiple
components and microelectronics to the design and construction of
integrated electromechanical systems. While the electronics are fabricated
using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS
processes), the micro mechanical components are fabricated using compatible
"micro machining" processes that selectively etch away parts of the silicon
wafer or add new structural layers to form the mechanical and electromechanical
devices. MEMS promises to revolutionize nearly every product category by
bringing together silicon-based microelectronics with micro machining
technology, thereby, making possible the realization of complete
systems-on-a-chip. MEMS is truly an enabling technology allowing the
development of smart products by augmenting the computational ability of
microelectronics with the perception and control capabilities of microsensors
and microactuators. MEMS is also an extremely diverse and fertile technology,
both in the applications as well as in how the devices are designed and
manufactured. MEMS technology makes possible the integration of microelectronics
with active perception and control functions, thereby, greatly expanding the
design and application.
Figure
2
Microelectronic
integrated circuits (ICs) can be thought of as the "brains" of systems and MEMS
augments this decision-making capability with "eyes" and "arms", to allow
Microsystems to sense and control the environment. In its most basic form, the
sensors gather information from the environment through measuring mechanical,
thermal, biological, chemical, optical, and magnetic phenomena; the electronics
process the information derived from the sensors and through some decision
making capability direct the actuators to respond by moving, positioning,
regulating, pumping, and filtering, thereby, controlling the environment for
some desired outcome or purpose. 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. Common examples of MEMS devices are the crash sensors of
the airbag deployment system on modern automobiles and the pressure sensors in
medical applications.
In
light of these interesting applications, this paper will discuss the field of
MEMS in three parts.
First, it will discuss a general manufacturing process of MEMS devices:
photolithography. Second, it will discuss the advantages and disadvantages of
using MEMS devices. Lastly, it’ll discuss an important application of MEMS
devices in the automotive industry.
MANUFACTURING
PROCESSES OF MEMS:
MEMS devices are fabricated using the same processes used to construct integrated circuits in semiconductors.
In fact, most of the MEMS devices are made on silicon wafers or other semiconductor materials. The reason for this is
two fold. The first reason is that the semiconductor material allows the use of integrated circuits and the second is that
the chemical etching processes used for semiconductor materials are well known and easily adapted to produce machines
with microscopic detail.
Material:
MEMS
are generally made from a material called Polycrystalline Silicon
(Poly-Si) which is a common material also used to make integrated circuits
(IC).
Frequently, Poly-Si is doped with other materials like germanium or phosphate to
enhance the material’s properties. Sometimes, copper or aluminum is plated onto
the Poly-Si to allow electrical conduction between different parts of the MEMS
device.
Now
that we know the Material used for fabrication of MEMS, we will discuss the
methods used for fabrication. The various methods used in MEMS manufacturing can
be enlisted as below:
v
Photolithography
(Surface Micro machining)
v
LIGA
(Lithographie, Galvanoformung, Abformung)
v
Bulk
Micro machining
Photolithography (surface Micro machining):
The most frequently used manufacturing technique is called surface micro machining. The term micro machining
is a little deceptive since no machining is actually done.
Figure 3:
Schematic of process steps to fabricate a cantilever beam using a two-mask
Photolithography
process
The
term micro machining is applied to a broad array of techniques that all
utilize photo chemical etching to
produce parts. This process is also referred to as photolithography.
This
process is used to mass-produce MEMS devices such as micromotors and
microvalves. The term photolithography is derived from Greek words: phos
(light), lithos (stone) and graphein (to write). The process
involves utilizing ultraviolet (UV) light to "write" images onto the surface of
a silicon wafer followed by plasma etching* to create the MEMS devices. The
general steps of the two-mask photolithography process are discussed bellow.
This process is the basis of more complex photolithography processes and is
often used to produce simple MEMS structures such as cantilever beams.
There
are five steps in the two-mask-photolithography process as shown in figure
1)
Depositing
a sacrificial layer onto a Si wafer.
2)
Exposing
the sacrificial layer to a UV light and etching it with plasma gas.
3)
Depositing
a layer of Poly-Si onto the sacrificial layer.
4)
Exposing
the Poly-Si layer to a UV light and etching it with plasma gas.
5)
Etching
the sacrificial layer with Hydrofluoric acid.
Elaboration
of the two-mask photolithography process is as follow:
First,
a thin, one-micron thick sacrificial layer of Phosphosilicate glass (PSG) is
deposited onto a silicon wafer that is 525 microns thick. Before the PSG layer
is exposed to UV light, a mask with a rectangular hole is used to cover some of
the PSG layer. The rectangular part of the PSG layer that was not covered is
then exposed to UV light. Then, a plasma etch is performed to remove the UV
exposed PSG layer.
At this
point, we have created a hole for the Poly-Si cantilever beam from the
PSG. The
third step of the two-mask photolithography process is to deposit a thin,
one-micron thick layer of Poly-Si onto the remaining PSG layer. A second
rectangular mask is used to cover some parts of the Poly-Si layer before
exposing the Poly-Si layer to UV light. A second plasma etch is then done to
remove the UV exposed Poly-Si layer.
The
last step is to "release" the Poly-Si cantilever beam from the PSG layer by
etching the PSG layer with Hydrofluoric acid (HF) for a given amount of time.
Note that adding more “depositing-exposing-etching” steps to the two-mask
photolithography process can make more complicated MEMS structures such as the
micro- servomotor.
LIGA
The
acronym LIGA comes from the German name for the process (Lithographie,
Galvanoformung, Abformung). LIGA uses lithography,
electroplating, and moulding processes to produce microstructures. It
is capable of creating very finely defined microstructures of up to 1000µm high.
In the
process as originally developed, a special kind of photolithography using X-rays
(X-ray lithography) is used to produce patterns in very thick layers of
photoresist. The X-rays from a synchrotron source are shone through a special
mask onto a thick photoresist layer (sensitive to X-rays), which covers a
conductive substrate (a). This resist is then developed (b).
The pattern formed is then electroplated with metal (c). The metal structures produced can be the final product, however
it is common to produce a metal mould (d). This mould can then be filled with a suitable material, such as a plastic (e), to
produce the finished product in that material (f). These moulds also allow the use of materials other than silicon, such as
nickel, titanium, and gold.
Figure 4: Schematic of process steps involved in
LIGA.
Because
of the patterning technique employed by LIGA, structures with a wide range of
materials can be fabricated. These structures can have horizontal
tolerances of about 0.3
microns and vertical dimensions from microns to
centimeters.
One
disadvantage of the Liga process is the cost of X-ray radiation equipment used
to develop the photopolymer. In addition to costing millions of dollars, the
radiation emissions are tightly controlled by federal regulation. This limits
the use of this process to only a handful of institutions in the United
States.
ADVANTAGES
AND DISADVANTAGES OF MEMS:
There
are four main advantages of using MEMS rather than ordinary large scale
machinery. The
first advantage is the ease of production. Borrowed from the IC industry,
today’s VLSI (Very Large Scale Integration) technology allows MEMS to be
produced in large quantities (up to 100,000 MEMS devices per Poly-Si wafer). It
is obvious that no macro-machinery production rate can even come close to this
number. The second advantage of
MEMS is
that they can be mass-produced and, thus, are inexpensive to make. A single MEMS
device only costs a small fraction of a cent to produce. The third advantage of
MEMS over macro-machinery is the ease of parts alteration. To alter the
production of a macro-machinery part, it is sometimes necessary to revise a
whole production line, which involves a plant-wide shut down that can cause
major loss in production time and money. A new set of MEMS can be created by
making minor alterations in the manufacturing process (i.e. change of masks,
change of etch time). Lastly, MEMS devices are known to have higher reliability
than their macro scale counterparts.
However,
MEMS products have their limitations and disadvantages. Due
to their size, it is physically impossible for MEMS to transfer any significant
power. In addition, because MEMS are made from Poly-Si (a brittle material) they
cannot be loaded with large forces. This is because brittle materials can be
fractured easily under high stress. Many MEMS researchers are working hard to
improve MEMS’s material strength and ability to transfer mechanical power.
Nevertheless, despite these limitations, MEMS still have countless numbers of
applications in the real world as discussed in the next
section.
APPLICATIONS:
·
Automotive
Application
In
automobiles that are made today there are 20 to 30 odd MEMS devices. These are
mostly sensors like accelerometers, pressure sensors, and gyroscopes. The
accelerometers are used to sense a collision and produce the signal that deploys
air bags in cars. An advantage of these devices is the fact that they can be
arranged on a single chip to detect side or front impacts and deploy the
corresponding air bags. These devices come complete with microscopic springs,
masses, and cantilevers. Another big use is in the production of Manifold
Absolute Pressure sensors, which are referred to as MAP sensors. These sensors
are used to determine the concentration of oxygen going into the engine and
calibrate the fuel air mixture to insure engine performance under a variety of
environmental conditions. Map sensors also produce higher fuel
efficiency.
Airbag deployment system:
One can
find many MEMS applications in the automotive, biomedical, data storage,
micro-optics, robotics and fluid control fields. But one of the most notable
MEMS
Figure
5:
Schematic representation of the three-layer micro mechanical capacitive
structure in an accelerometer system.
application
is the accelerometer (a device used to measure acceleration) found in the airbag
deployment system of many modern automobiles. These
accelerometers are used as crash sensors in an airbag deployment system. We will
now discuss how the traditional airbag deployment system
works.
A
traditional airbag deployment system includes macro mechanical crash sensors
which detect a crash pulse, a microprocessor which processes signals from the
crash sensors, and an airbag deployment mechanism which physically deploys
the airbag in an event of a collision. The traditional airbag deployment
system uses a "ball-on-cone type" macro mechanical device as the crash sensor.
When the deceleration of a vehicle exceeds a certain limit (signal of hard
braking or collision), the decelerating forces would pull the ball to a position
which signals the system to deploy the airbag . The inflated airbag will then
act as a cushion between the occupants and the dashboard thus lessens the impact
force imparted to the occupants by the crash. Airbags are designed to protect
the occupants in automobile accidents against severe injuries. The airbag
deployment system has been proven an effective supplemental restraint system
(SRS) when used concurrently with safety seat belts. We will now discuss the
advantages of using MEMS as crash sensors in the airbag deployment system rather
than macro devices.
Advantages
MEMS
are used for crash sensing in newer airbag deployment systems because the
traditional macro mechanical devices are not capable of meeting new standards
set by the government and the auto industry [5]. These
new standards include the need of multi-directional crash sensing which cannot
be achieved by the traditional crash sensors. In addition, traditional crash
sensors are more expensive to make and less reliable compared to the MEMS crash
sensors. We will now discuss the details of the MEMS crash-sensing
device.
Two-chip
accelerometer system
The
two-chip accelerometer system is used as the crash sensor in the airbag
deployment system [5]. It
consists of three layers of Poly-Si as shown in figure 3. Each layer has a
particular function. The first fixed layer is used to run a self-diagnostic test
every time the device is powered up. The third fixed layer is a reference
electrode. The second layer is a seismic mass, which is capable of moving up and
down between the first and third layer when subjected to acceleration (in the
direction in or out of the paper).
The
sandwich of Poly-Si layers (the crash sensor) is connected to a microprocessor
which is connected to the deploying mechanism of the airbag (see figure
5). At
zero acceleration, a fixed capacitance (an electrical property) is measured
between the second and third Poly-Si layer. The system is said to be "at rest"
in this state. In an event of a collision, a great deceleration force would be
transmitted to the accelerometer. This force would move the seismic mass (the
second layer) with respect to the fixed third layer. The change in spacing
between these two layers will cause a change in the capacitance. This change in
capacitance is
Figure 6:
Flowchart representation of the airbag deployment system
analyzed
by the microprocessor attached to the crash sensing unit. If the change is
severe enough (meaning a real collision is in progress), the microprocessor will
send a signal to deploy the airbag.
In
order to meet the new industrial standard of multi-directional crash sensing,
two of these small MEMS crash sensors can be placed next to each other
perpendicularly. Each sensor can therefore sense the acceleration forces from
all four directions of the automobile. The addition of multi-directional crash
sensing enables the introduction of side-impact airbags found in many new
cars
·
Medical
applications
Future
uses of MEMS devices include a variety of medical devices. One of these devices
is a blood gas sensor that can be inserted through a catheter 650 microns in
size. It can detect the oxygen content and the pH of the blood. Other
significant medical uses are the construction of neural probes to record
electrical impulses on the brain. These microscopic sensors would allow
researchers to place sensors close to the neurons and would allow them to map
the circuits of the brain to determine how the brain truly processes and stores
information. It is estimated that MEMS devices could produce a neural electronic
interface. These devices could be attached directly to neurons in the brain. If
the electrical impulses naturally generated by normal sensory input such as
sight or hearing could be simulated by these MEMS devices, it would be possible
to provide artificial sight to the blind. Other medical uses include the use of
MEMS devices to combat cancer or obstructions to blood flow that produce stokes
and heart disease.
·
Miscellaneous
applications
Consumer
electronics also use MEMS devices in increasing quantities. One prominent use is
in the valves and orifices that are used in ink jet printers. Many of us have
MEMS devices in our homes and don't even know it. Other uses include the
production of chemical sensors such as smoke detectors.
CONCLUSION:
MEMS
devices have evolved from laboratory curiosities of the 1980s to commercial
products of today. If this growth trend continues, MEMS will very likely be the
next generation of machinery to service mankind for the next century. It is
predicted that the MEMS market will soar to more than $34 billion by the year
2002 [6]. This prediction combined with the foregoing discussion on the
advantages of MEMS over macro devices lead us to predict that MEMS will soon be
integrated into our everyday life just as the computers have been. From
previous sections, we saw that the manufacturing process of MEMS is not the
simplest, but we believe that the advantages that come with MEMS will outweigh
the complexity of the manufacturing process. As MEMS researchers strive to
compensate for MEMS’s shortcomings, we can only expect to see more and better
MEMS devices created in the coming years. The future designs and applications of
MEMS are only limited by the imagination of the designers.
REFERENCES: