REPORT OF MEMS TECHNOLOGY
TABLE
OF CONTENTS
APPROVAL…………………………………………………………………………………………...I
DECLARATION………………………………………………………………………………..II
ACKNOWLEDGE……………………………………………………………………………..III
ABSTRACT…………………………………………………………………………………….IV
TABLE OF CONTENTS……………………………………………………………………….V
LIST OF
FIGURE…………………………………………………………………………….VII
LIST OF FIGURE
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.
MEMS are, in their most; basic forms, small 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.
After extensive development, todays
commercial MEMS – also known as Micro System Technologies (MST), Micro Machines
(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.
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.
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:
The
two most general methods of MEMS integration are:
·
Surface micro machining
·
Bulk micro machining
Surface micromachining enables the fabrication of
complex multicomponent integrated micromechanical structures that would not be
possible with traditional bulk micromachining. This technique encases specific
structural parts of a device in layers of a sacrificial material during the
fabrication process. The substrate wafer is used primarily as a mechanical
support on which multiple alternating layers of structural and sacrificial
material are deposited and patterned to realize micromechanical structures. The
sacrificial material is then dissolved in a chemical etchant that does not
attack the structural parts. The most widely used surface micromachining
technique, polysilicon surface micromachining, uses SiO2 as the
sacrificial material and polysilicon as the structural material.
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.
Figure 2 Process flow of surface micromachining
Advantages
of surface micro machining
a) Structures, especially thicknesses, can
be smaller than 10 µm in size,
b) The micro machined device footprint can
often be much smaller than bulk wet-etched devices,
c) It is easier to integrate electronics
below surface micro-structures, and
d) Surface microstructures generally have
superior tolerance compared to bulk wet-etched devices.
The primary
disadvantage is the fragility of surface microstructures to handling, particulates
and condensation during manufacturing.
Figure 3 Steps of surface micromachining
Surface Micro machining
is being used in commercial products such as accelerometers to trigger air bags
in automobiles.
Bulk micromachining is an extension of IC technology
for the fabrication of 3D structures. Bulk micromachining of Si uses wet- and
dry-etching techniques in conjunction with etch masks and etch stops to sculpt
micromechanical devices from the Si substrate. The two key capabilities that
make bulk micromachining a viable technology are:
·
Anisotropic
etchants of Si, such as ethylene-diamine and pyrocatechol (EDP), potassium
hydroxide (KOH), and hydrazine (N2H4). These
preferentially etch single crystal Si along given crystal planes.
·
Etch
masks and etch-stop techniques that can be used with Si anisotropic etchants to
selectively prevent regions of Si from being etched. Good etch masks are
provided by SiO2 and Si3N4, and some metallic
thin films such as Cr and Au (gold).
Figure 4
Process flow of bulk micromachining
A drawback of wet anisotropic etching is that the
microstructure geometry is defined by the internal crystalline structure of the
substrate. Two additional processing techniques have extended the range of
traditional bulk micromachining technology: deep anisotropic dry etching and wafer
bonding. Reactive gas plasmas can perform deep anisotropic dry etching of Si
wafers, up to a depth of a few hundred microns, while maintaining smooth
vertical sidewall profiles. The other technology, wafer bonding, permits a Si
substrate to be attached to another substrate, typically Si or glass
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 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.
There are three
basic building blocks in MEMS technology, which are,
Deposition
Process-the ability to
deposit thin films of material on a substrate,
Lithography-to apply a patterned mask on top of the films by
photolithograpic imaging.
Etching-to etch the films selectively to the mask. A MEMS
process is usually a structured sequence of these operations to form actual
devices.
Figure 6
MEMS design flow starting to end
As
with micromachining processes, many MEMS sensor-packaging techniques are the
same as, or derived from, those used in the semiconductor industry. However, the
mechanical requirements for a sensor package are typically much more stringent than
for purely microelectronic devices. Microelectronic packages are often generic with
plastic, ceramic, or metal packages being suitable for the vast majority of IC applications.
For example, small stresses and strains transmitted to a microelectronics die
will be tolerable as long as they stay within acceptable limits and do not
affect
reliability.
In the case of a MEMS physical sensor, however, such stresses and strains and
other undesirable influences must be carefully controlled in order for the
device to function correctly. Failure to do so, even when employing electronic
compensation techniques, will reduce both the sensor performance and long-term
stability.
·
Ceramic Packages
·
Plastic Packages
·
Metal Packages
A MEMS sensor packaging
must meet several requirements :
·
Protect the sensor from external
influences and environmental effects. Since MEMS inherently include some
microscale mechanical components, the integrity of the device must be protected
against physical damage arising from mechanical shocks, vibrations, temperature
cycling, and particle contamination. The electrical aspects of the device, such
as the bond wires and the electrical properties of the interconnects, must also
be protected against these external influences and environmental effects.
·
Protect the environment from the presence
of the sensor. In addition protecting the sensor, the package must prevent the
presence of the MEMS from reacting with or contaminating potentially sensitive
environments. The classic examples of this are medical devices that contain
packaged sensors that can be implanted or used within the body; these must be
biocompatible, nontoxic, and able to withstand sterilization.
·
Provide a controlled electrical,
thermal, mechanical, and/or optical interface between the sensor, its
associated components, and its environment. Not only must the package protect
both the sensor and its environment, it must also provide a reliable and
repeatable interface for all the coupling requirements of a particular application.
In the case of mechanical sensors, the interface is of fundamental importance since, by its nature, specific
mechanical coupling is essential but unwanted effects must be prevented. A
simple example would be a pressure sensor where the device must be coupled in
some manner to the pressure but isolated from, for example, thermally induced
strains. The package must also provide reliable heat transfer to enable any
heat generated to be transmitted away from the MEMS device to its environment.
In the vast majority of cases, basic
plastic, metal, or ceramic packages do not satisfy these requirements. While
the requirements for electrical connections and heat transfers paths on sensor
packages are typically much less than in the case of most ICs, it is the
mechanical interface that complicates the package design. The mechanical
interface must isolate the sensor from undesirable external stresses and
provide relief from residual stresses in the assembly while enabling the
desired mechanical effect arising from the measurand to be coupled to the
sensor. In the vast majority of practical sensor applications, each packaging
solution will be developed specifically for that particular application.
MEMS applications in
various fields are as follows.
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.
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 high-throughput drug screening and selection.
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.
Figure 8 Capacitive accelerometer’s working diagram
Figure 9 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.
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.
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.
Sports Training
Devices
Computer Peripherals
Car and Personal
Navigation Devices
Active Subwoofers
Earthquake Detection
and Gas Shutoff
Machine Health
Shock and Tilt Sensing
Tanks
Planes
Equipment for Soldiers
Figure 10 Application
of MEMS in various fields
Some
of the major challenges facing the MEMS industry include:
MEMS
companies today have very limited access to MEMS fabrication facilities, or foundries, for prototype and device
manufacture. In addition, the majority of the organizations expected to benefit
from this technology currently do not have the required capabilities and
competencies to support MEMS fabrication. For example, telecommunication
companies do not currently maintain micromachining facilities for the
fabrication of optical switches. Affordable and receptive access to MEMS
fabrication facilities is crucial for the commercialisation of MEMS.
Due
to the highly integrated and interdisciplinary nature of MEMS, it is difficult
to separate device design from the complexities of fabrication. Consequently, a
high level of manufacturing and fabrication knowledge is necessary to design a
MEMS device. Furthermore, considerable time and expense is spent during this
development and subsequent prototype stage. In order to increase innovation and
creativity, and reduce unnecessary ‘time-to-market’ costs, an interface should
be created to separate design and fabrication. As successful device development
also necessitates modelling and simulation, it is important that MEMS designers
have access to adequate analytical tools. Currently, MEMS devices use older
design tools and are fabricated on a ‘trial and error’ basis. Therefore, more
powerful and advanced simulation and modelling tools are necessary for accurate
prediction of MEMS device behaviour.
The
packaging and testing of devices is probably the greatest challenge facing the
MEMS industry. As previously described, MEMS packaging presents unique problems
compared to traditional IC packaging in that a MEMS package typically must
provide protection from an operating environment as well as enable access to
it. Currently, there is no generic MEMS packaging solution, with each device
requiring a specialized format. Consequently, packaging is the most expensive
fabrication step and often makes up 90% (or more) of the final cost of a MEMS
device.
Due
to the relatively low number of commercial MEMS devices and the pace at which
the current technology is developing,
standardization has been very difficult. To date, high quality control and
basic forms of standardization are generally only found at multi-million dollar
(or billion dollar) investment facilities. However, in 2000, progress in industry
communication and knowledge sharing was made through the formation of a MEMS
trade organization. Based in Pittsburgh, USA, the MEMS industry group (MEMS-IG)
with founding members including Xerox, Corning, Honeywell, Intel and JDS
Uniphase, grew out of study teams sponsored by DARPA that identified a need for
technology roadmapping and a source for objective statistics about the MEMS
industry. In addition, a MEMS industry roadmap, sponsored by the Semiconductor
Equipment and Materials International organization (SEMI), has also been
identified to share pre-competitive information on the processes, technology,
application and markets for MEMS. This web-based organization can be found at
http://www.roadmap.nl.
Several
other European initiatives supported by governments and the European commission
have been coordinated: Europractice (Microsystems Service for Europe), NEXUS
(Network of Excellence in Multifunctional Microsystems), aimed at enhancing
European industrial competitiveness in the global marketplace, and Netpack,
whose role is to drive the development and use of advanced packaging and
integration technologies. The networking of these smaller companies and
organizations on both a European and a global scale is extremely important and
necessary to lay the foundation for a formal standardization system.
The
complexity and interdisciplinary nature of MEMS require educated and
well-trained
scientists
and engineers from a diversity of fields and backgrounds. The current numbers
of
qualified
MEMS-specific personnel is relatively small and certainly lower than present
industry
demand. Education at graduate level is usually necessary and although the
number
of
universities offering MEMS-based degrees is increasing, gaining knowledge is an
expensive
and time-consuming process. Therefore, in order to match the projected need for
these
MEMS scientists and engineers, an efficient and lower cost education
methodology is
necessary.
One approach, for example, is industry-led (or driven) academic research
centres
offering
technology-specific programs with commercial integration, training and
technology
transfer.
The automotive industry, motivated by the need for
more efficient safety systems and the desire for enhanced performance, is the
largest consumer of MEMS-based technology. In addition to accelerometers and
gyroscopes, micro-sized tire pressure systems are now standard issues in new
vehicles, putting MEMS pressure sensors in high demand. Such micro-sized
pressure sensors can be used by physicians and surgeons in a telemetry system
to measure blood pressure at a stet, allowing early detection of hypertension and
restenosis. Alternatively, the detection of bio molecules can benefit most from
MEMS-based biosensors. Medical applications include the detection of DNA
sequences and metabolites. MEMS biosensors can also monitor several chemicals
simultaneously, making them perfect for detecting toxins in the environment.
Lastly, the dynamic range of MEMS based silicon
ultrasonic sensors have many advantages over existing piezoelectric sensors in
non-destructive evaluation, proximity sensing and gas flow measurement. Silicon
ultrasonic sensors are also very effective immersion sensors and provide
improved performance in the areas of medical imaging and liquid level
detection.
·
The
medical, wireless technology, biotechnology, computer, automotive and aerospace
industries are only a few that will benefit greatly from MEMS.
·
This
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.
·
MEMS devices are manufactured for
unprecedented levels of functionality, reliability, and sophistication can be
placed on a small silicon chip at a relatively low cost.
·
MEMS promises to revolutionize nearly
every product category by bringing together silicon-based microelectronics with
micromachining technology, making possible the realization of complete systems-on-a-chip.
MEMS will be the
indispensable factor for advancing technology in the 21st century and it promises to create entirely new categories of
products.
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