Download MICRO ELECTRO MECHANICAL SYSTEMS

MICRO ELECTRO MECHANICAL SYSTEMS
Document By
SANTOSH BHARADWAJ REDDY
Email: [email protected]
Engineeringpapers.blogspot.com
More Papers and Presentations available on above site
Abstract:
We are poised in a revolution of microelectronics, which has
dramatically reduced the cost and increased the capability of electronics. This
has given much potential to prosper in the area of micro mechanics
encompassing MEMS (Micro Electro Mechanical Systems). MEMS promises to
revolutionize nearly every product category by bringing together silicon based
microelectronics with micro machining technology, making possible the
realization of complete systems on a chip.
The paper outlines the fabrication techniques, benefits,
applications and challenges of MEMS. MEMS, often referred to as micro systems
technology, are fabricated using modified silicon and non-silicon fabrication
technology. It reduces cost and increases reliability of the system. MEMS--scale
accelerometers, geophones and gyros are replacing some of the standard size
precursors and are establishing new markets of their own. Commercial
applications are inertial sensors, Power MEMS and RF switches. This technology
is also used in industrial, consumer and auto motive marketing.
INTRODUCTION TO MEMS:
Micro electromechanical systems (MEMS) is a technology of
miniaturization that has been largely adopted from the integrated circuit (IC) industry
and applied to the miniaturization of all systems not only electrical systems but also
mechanical, optical, fluid, magnetic, etc.
Micro Electromechanical systems or MEMS, represent an
extraordinary technology that promises to transform whole industries and drive the next
technological revolution. These devices can replace bulky actuators and sensors with
micron-scale equivalent that can be produced in large quantities by fabrication processes
used in integrated circuits photolithography. This reduces cost, bulk, weight and power
consumption while increasing performance, production volume, and functionality by
orders of magnitude. For example, one well known MEMS device is the accelerometer
(its now being manufactured using mems low cost, small size, more reliability).
Furthermore, it is clear that current MEMS products are simply precursors to greater and
more pervasive applications to come, including genetic and disease testing, guidance and
navigation systems, power generation, RF devices( especially for cell phone technology),
weapon systems, biological and chemical agent detection, and data storage. Micro mirror
based optical switches have already proven their value; several start-up companies
specializing in their development have already been sold to large network companies for
hundreds of millions of dollars. The promise of MEMS is increasingly capturing the
attention of new and old industrises alike, as more and more of their challenges are
solved with MEMS.
Background:
MEMS are, in their most; basic forms, diminutive versions of traditional
electrical and mechanical devices – such as valves, pressure sensors, hinged mirrors, and
gears with dimensions measured in microns – manufactured by techniques similar to
those used in fabricating microprocessor chips. The first MEMS products were developed
in the 1960s, when accurate hydraulic pressure sensors were needed for aircraft. Such
devices were further refined in the 1980s when implemented in fuel-injected car engines
to monitor intake manifold pressure.
In the late 80s, MEMS accelerometers for car airbags were developed as a less expensive,
more reliable, and more accurate replacement for conventional crash sensor. Taking the
spotlight today are optical MEMS (also known as Micro Opto Electro Mechanical
Systems, or MOEMS) primarily micro mirrors, which are used as digital light processors
in video projectors and as well as switches in optical network equipment. After extensive
development, todays commercial MEMS – also known as Micro System Technologies
(MST), Micro Machines (MM), or M3 (MST, MEMS & MM) – have proven to be more
manufacturable, reliable and accurate, dollar for dollar, than their conventional
counterparts. However the technical hurdles to attain these accomplishments were often
costly and time- consuming, and current advances in this technology introduce newer
challenges still. Because this field is till in its infancy, very little data on design,
manufacturing processes or liability are common or shared.
1. FABRICATION:
1.1 MATERIALS USED For the Fabrication Process:
MEMS devices are fabricated using a number of materials,
depending on the application requirements. One popular material is polycrystalline
silicon, also called “polysilicon” or “poly”. This material is sculpted with techniques such
as bulk or surface micro- machining, and Deep Reactive Ion Etching (DRIE), proving to
be fairly durable for many mechanical operations. Another is nickel, which can be shaped
by PMMA (a form of plexiglass) mask platng (LIGA), as well as by conventional
photolithographic techniques. Other materials – such as diamond, aluminum, silicon
carbide and gallium arsenide – are currently being evaluated for use in micro machines
for their desirable properties; e.g., the hardness of diamond and silicon carbide. To create
moveable parts, several layers are needed for structural and electrical interconnect
(ground plane) purposes, with so-called “sacrificial” oxide layers in between. The current
manufacturing record is five layers, making possible a variety of complex mechanical
systems. These capabilities, developed over the last several years, are beginning to
unlock the almost unlimited possibilities of MEMS applications.
1.2 Fabrication Techniques:
The methods used to integrate multiple patterned materials together to
fabricate a completed MEMS device are just as important as the individual processes
and materials themselves. Depending on the type of material used fabrication
techniques are classified as:
1.2.1 Silicon Micro fabrication:
The three most general methods of MEMS integration are:
 Surface micro machining
 Bulk micro machining
 Single point diamond machining
Here one of the above methods is explained.
Surface micro machining:
Simply stated, surface machining is a method of producing
MEMS by depositing, patterning, and etching a sequence of thin films, typically1 µm
thick. One of the most important processing steps that is required of dynamic MEMS
devices is the selective removal of an underlying film, referred to as a sacrificial layer,
without attacking an overlaying film, referred to as the structural layer, used to create the
mechanical parts. Surface micro machining has been used to produce a wide variety of
MEMS devices for many different applications. In fact, some of them are produced
commercially in large volumes.
Advantages of surface micro machining are
a)
b)
Structures, especially thicknesses, can be smaller than 10 µm in size,
The micro machined device footprint can often be much smaller than bulk wetetched devices,
c)
d)
It is easier to integrate electronics below surface micro-structures, and
Surface microstructures generally have superior tolerance compared to bulk wetetched devices.
The primary disadvantage is the fragility of surface
microstructures to handling, particulates and condensation during manufacturing.
Fig: steps
First a sacrificial silicon dioxide is deposited on the wafer
and patterned, then a structural layer of polysilicon is deposited and patterned. This
polysilicon layer will become structural elements such as gears. Other layers are then
deposited and patterned making the rest of the structure. Etching in buffered hydrofluoric
acid, HF, removes the sacrificial layers leaving the gears shown below.
Surface Micro machining is being used in commercial
products such as accelerometers to trigger air bags in automobiles.
Fig: Gears
1.2 Non-Silicon Micro fabrication:
The development of MEMS has contributed significantly to the
improvement of non-silicon micro fabrication techniques. Two prominent examples are
LIGA and plastic molding from micro machined substrates.
LIGA
LIGA is a German acronym standing for lithographie, galvanoformung (plating),
and abformung (molding). However, in practice LIGA essentially stands for a process
that combines extremely thick-film resists (often >1 mm) and x-ray lithography, which
can pattern thick resists with high fidelity and results in vertical sidewalls. Although
some applications may require only the tall patterned resist structures themselves, other
applications benefit from using the thick resist structures as plating molds (i.e., material
can be quickly deposited into the mold by electroplating). A drawback to LIGA is the
need for high-energy x-ray sources that are very expensive and rare.
The LIGA process exposes PMMA (poly methyl metha crylate) plastic with synchrotron
radiation through a mask. This is shown at the top of the Figure 1. Exposed PMMA is
then washed away, leaving vertical wall structures with spectacular accuracy. Structures
a third of a millimeter high and many millimeters on a side are accurate to a few tenths of
a micron. Metal is then plated into the structure, replacing the PMMA that was washed
away. This metal piece can become the final part, or can be used as an injection mold for
parts made out of a variety of plastics.
A cheap alternative to LIGA, with nearly the same performance, has
been developed. The solution is to use a special epoxy-resin-based optical resist, called
SU-8, that can be spun on in thick layers (>500 µm), patterned with commonly available.
2. Benefits of MEMS:
Because of increase in micromachining technology, hundreds
of MEMS can be made from a single 8-inch wafer of silicon. Below is an image which
shows how small MEMS are in comparison to a dime. Because an entire system can be
made this small and in such quantities, prices are reduced for products which incorporate
this technology. MEMS also have no moving parts, so they are much more reliable than a
macro system. Because of the reduced cost and increased reliability, there is almost no
limit to what MEMS can be used for.
3. APPLICATIONS OF MEMS:
MEMS applications in various fields are as follows.
3.1 Communications:
High frequency circuits will benefit considerably from advent of the RF-MEMS
technology. Electrical components such as inductors and tunable capacitors can be
improved significantly compared to their integrated counter parts if they are made using
MEMS technology. If the integration of such components, the performance of
communication circuits will improve, while the total circuit area, power consumption and
cost will be reduced. In addition, the mechanical switch, as developed by several research
groups, is a key component with huge potential in various micro wave circuits. The
demonstrated samples of mechanical switches have quality factors much higher than
anything previously available. The MEMS technology has the potential of replacing
many radial frequency components such as switches phase shifters, capacitors, inductors,
and filters, used in to days mobile, communication and satellite systems. RF mems
components promise superior performance in comparison with current technologies. This
also enables new functionality and system capability that are not possible with current
technologies such as nano satellites.
3.2 Biotechnology:
MEMS enabling new discoveries in science and engineering such as the
polymerase chain Reaction (PCR) Microsystems for DNA amplification and
identification, micro machined scanning Tunneling microscopes (STMs), Biochips for
detection of hazardous chemical and biological agents, and Microsystems for highthroughput drug screening and selection.
3.3 Inertial sensors:
Inertial sensors are mechanics sensors aiming at measuring
accelerations, in the mechanics science definition. There are two categories of inertial
sensors. They are, accelerometers which measures variation of rotational speed and
gyroscopes which measures variation of rotational speed.
Accelerometers:
Schematic of micro accelerometer, ADXL series, produced by Analog Device.
On these diagrams, we can see a micro accelerometer device and the
chip including associated electronics, made by Analog Device. This is a two axis micro
accelerometer. This means it is able to measure accelerations in two directions at a time
(in the directions of the plane).
Micro accelerometers were the first MEMS device to flood the market.
Micro accelerometers measure variation of translational speed. So acceleration,
deceleration, even very high deceleration, like…shock! The sensor that detects a shock
and launches the airbag is a micro accelerometer combined with a electronic circuit able
to decide wether or not the shock was an accident or just your car passing a pothole.
There are lots of applications, like navigation, micro accelerometers can help in
increasing precision. There are more and more to say about micro accelerometers, they
are still the spearhead of MEMS industry.
Gyroscopes:
Micro gyroscopes are newer in the market compared to micro accelero
meters. Some devices have appeared on the market for navigation application. The key
point in these devices is sensitivity.
3.4 RF switches:
RF switches have been under development for years, but the commercial
applications just begin to appear. The reason is the difficulty to combine high efficiency,
reproducibility and reliability. RF switches will be preferred to full electronic switches on
applications where security, integration capabilities, power consumption and other
parameters are critical.
Schematic of a R.F. electromechanical micro switch in action
3.5 Consumer Market:
Sports Training Devices
Computer Peripherals
Car and Personal Navigation Devices
Active Subwoofers
3.6 Industrial Market:
Earthquake Detection and Gas Shutoff
Machine Health
Shock and Tilt Sensing
3.7 Military:
Tanks
Planes
Equipment for Soldiers
4. CURRENT CHALLENGES:
MEMS is currently used in low or medium-volume
applications. Some of the obstacles preventing its wider adoption are:
 Limited options:
Most companies who wish to explore the potential of potential
of MEMS have very limited options for prototyping or manufacturing devices, have no
capability or expertise in micro fabrication technology. Few companies will their own
fabrication facilities because of high cost. A Mechanism giving smaller organizations
responsive and affordable access to MEMS is essential.
 Fabrication knowledge Required:
Currently designer of a MEMS device requires a high level of
fabrication knowledge in order to create a successful design. Often the development of
even the most mundane MEMS device requires a dedicated research effort to find a
suitable process sequence for fabricating it. MEMS device design needs to separated
from the complexities of the process sequence.
 Packaging:
MEMS packaging is more challenging than IC packaging due to the
diversity of MEMS devices and the requirement that many of devices be in contact with
their environment.
CONCLUSION:
MEMS is an emerging technology which will rapidly revolutionize all
the fields in the near future, if the level of fabrication knowledge of current designer of
MEMS devices and the packaging techniques are still improved.
Document By
SANTOSH BHARADWAJ REDDY
Email: [email protected]
Engineeringpapers.blogspot.com
More Papers and Presentations available on above site