SECTION 1INTRODUCTIONMicroelectromechanical systems (MEMS) are small integrated devices or systems that combine electrical and mechanical components. They range in size from the sub micrometer level to the millimeter level and there can be any number, from a few to millions, in a particular system. MEMS extend the fabrication techniques developed for the integrated circuit industry to add mechanical elements such as beams, gears, diaphragms, and springs to devices. Examples of MEMS device applications include inkjet-printer cartridges, accelerometer, miniature robots, microengines, locks inertial sensors microtransmissions, micromirrors, micro actuator (Mechanisms for activating process control equipment by use of pneumatic, hydraulic, or electronic signals) optical scanners, fluid pumps, transducer, pressure and flow sensors. New applications are emerging as the existing technology is applied to the miniaturization and integration of conventional devices. These systems can sense, control, and activate mechanical processes on the micro scale, and function individually or in arrays to generate effects on the macro scale. The micro fabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks, but in combination can accomplish complicated functions. MEMS are not about any one application or device, nor are they defined by a single fabrication process or limited to a few materials. They are a fabrication approach that conveys the advantages of miniaturization, multiple components, and microelectronics to the design and construction of integrated electromechanical systems. MEMS are not only about miniaturization of mechanical systems; they are also a new paradigm for designing mechanical devices and systems. The MEMS industry has an estimated $10 billion market, and with a projected 10-20% annual growth rate, it is estimated to have a $34 billion market in 2012. Because of the significant impact that MEMS can have on the commercial and defense markets, industry and the federal government have both taken a special interest in their development.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
7/31/2019 Report Me Ms
http://slidepdf.com/reader/full/report-me-ms 1/43
SECTION 1 INTRODUCTION
Microelectromechanical systems (MEMS) are small integrated devices
or systems that combine electrical and mechanical components. They range in size
from the sub micrometer level to the millimeter level and there can be any number,
from a few to millions, in a particular system. MEMS extend the fabrication
techniques developed for the integrated circuit industry to add mechanical elements
such as beams, gears, diaphragms, and springs to devices.
Examples of MEMS device applications include inkjet-printer
microtransmissions, micromirrors, micro actuator (Mechanisms for activating processcontrol equipment by use of pneumatic, hydraulic, or electronic signals) optical
scanners, fluid pumps, transducer, pressure and flow sensors. New applications are
emerging as the existing technology is applied to the miniaturization and integration
of conventional devices.
These systems can sense, control, and activate mechanical processes on
the micro scale, and function individually or in arrays to generate effects on the macro
scale. The micro fabrication technology enables fabrication of large arrays of devices,
which individually perform simple tasks, but in combination can accomplish
complicated functions.
MEMS are not about any one application or device, nor are they defined
by a single fabrication process or limited to a few materials. They are a fabrication
approach that conveys the advantages of miniaturization, multiple components, and
microelectronics to the design and construction of integrated electromechanicalsystems. MEMS are not only about miniaturization of mechanical systems; they are
also a new paradigm for designing mechanical devices and systems.
The MEMS industry has an estimated $10 billion market, and with a
projected 10-20% annual growth rate, it is estimated to have a $34 billion market in
2012. Because of the significant impact that MEMS can have on the commercial and
defense markets, industry and the federal government have both taken a special
interest in their development.
7/31/2019 Report Me Ms
http://slidepdf.com/reader/full/report-me-ms 2/43
SECTION 1.1 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 microfabrication technology. While the electronics are fabricated
using integrated circuit (IC) process sequences, the micromechanical components are
fabricated using compatible "micromachining" processes that selectively etch away
parts of the silicon wafer or add new structural layers to form the mechanical and
electromechanical devices.
Microelectronic integrated circuits can be thought of as the "brains" of a
system and MEMS augments this decision-making capability with "eyes" and "arms",
to allow microsystems to sense and control the environment. Sensors gather
information from the environment through measuring mechanical, thermal, biological,
chemical, optical, and magnetic phenomena. The electronics then 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.
Because MEMS devices are manufactured using batch fabrication techniques similar
to those used for integrated circuits, unprecedented levels of functionality, reliability,and sophistication can be placed on a small silicon chip at a relatively low cost.
7/31/2019 Report Me Ms
http://slidepdf.com/reader/full/report-me-ms 3/43
SECTION 1.2 WHAT ARE MEMS / MICROSYSTEMS?
MEMS is an abbreviation for Micro Electro Mechanical Systems. This
is a rapidly emerging technology combining electrical, electronic, mechanical, optical,
material, chemical, and fluids engineering disciplines. As the smallest commercially
produced "machines", MEMS devices are similar to traditional sensors and actuators
although much, much smaller. E.g. Complete systems are typically a few millimeters
across, with individual features devices of the order of 1-100 micrometers across.
MEMS devices are manufactured either using processes based on Integrated Circuit
fabrication techniques and materials, or using new emerging fabrication technologies
such as micro injection molding. These former processes involve building the device
up layer by layer, involving several material depositions and etch steps. A typicalMEMS fabrication technology may have a 5 step process. Due to the limitations of
this "traditional IC" manufacturing process MEMS devices are substantially planar,
having very low aspect ratios (typically 5 -10 micro meters thick). It is important to
note that there are several evolving fabrication techniques that allow higher aspect
ratios such as deep x-ray lithography, electrodeposition, and micro injection molding.
7/31/2019 Report Me Ms
http://slidepdf.com/reader/full/report-me-ms 4/43
MEMS devices are typically fabricated onto a substrate (chip) that may
also contain the electronics required to interact with the MEMS device. Due to the
small size and mass of the devices, MEMS components can be actuated
electrostatically (piezoelectric and bimetallic effects can also be used). The position of
MEMS components can also be sensed capacitively. Hence the MEMS electronics
include electrostatic drive power supplies, capacitance charge comparators, and signal
conditioning circuitry. Connection with the macroscopic world is via wire bonding
and encapsulation into familiar BGA, MCM, surface mount, or leaded IC packages.
A common MEMS actuator is the "linear comb drive" (shown above) which consists
of rows of interlocking teeth; half of the teeth are attached to a fixed "beam", the otherhalf attach to a movable beam assembly. Both assemblies are electrically insulated.
By applying the same polarity voltage to both parts the resultant electrostatic force
repels the movable beam away from the fixed. Conversely, by applying opposite
polarity the parts are attracted. In this manner the comb drive can be moved "in" or
"out" and either DC or AC voltages can be applied. The small size of the parts (low
inertial mass) means that the drive has a very fast response time compared to its
macroscopic counterpart. The magnitude of electrostatic force is multiplied by the
voltage or more commonly the surface area and number of teeth. Commercial comb
drives have several thousand teeth, each tooth approximately 10 micro meters long.
Drive voltages are CMOS levels.
The linear push / pull motion of a comb drive can be converted into
rotational motion by coupling the drive to push rod and pinion on a wheel. In this
manner the comb drive can rotate the wheel in the same way a steam engine
functions!
7/31/2019 Report Me Ms
http://slidepdf.com/reader/full/report-me-ms 5/43
SECTION 2 HISTORICAL BACKGROUND
The invention of the at Bell Telephone Laboratories in 1947 sparked a
fast-growing microelectronic technology. Jack Kilby of Texas Instruments built the
first Integrated circuit in 1958 using germanium (Ge) devices. It consisted of one
transistor, three Resistors, and one Capacitor. The IC was implemented on a sliver of
Ge that was glued on a glass slide. Later that same year Robert Noyce of Fairchild
Semiconductor announced the development of a Planar double-diffused Si IC. The
complete transition from the original Ge transistors with grown and alloyed junctions
to silicon (Si) planar double-diffused devices took about 10 years. The success of Si
as an electronic material was due partly to its wide availability from silicon dioxide
(SiO2-sand), resulting in potentially lower material costs relative to other
Semiconductors
Since 1970, the complexity of ICs has doubled every two to three years.
The minimum dimension of manufactured devices and ICs has decreased from 20
microns to the sub micron levels of today. Current ultra-large-scale-integration
(ULSI) technology enables the fabrication of more than 10 million transistors and
capacitors on a typical chip.
IC fabrication is dependent upon sensors to provide input from the
surrounding environment, just as control systems need actuators in order to carry out
their desired functions. Due to the availability of sand as a material, much effort was
put into developing Si processing and characterization tools. These tools are now
being used to advance transducer technology. Today's IC technology far outstrips the
original sensors and actuators in performance, size, and cost.
Attention in this area was first focused on microsensor development.
The first microsensor, which has also been the most successful, was the Si pressure
sensor. In 1954 it was discovered that the piezoresistive effect in Ge and Si had the
potential to produce Ge and Si strain gauges with a gauge factor 10 to 20 times greater
than those based on metal films. As a result, Si strain gauges began to be developed
commercially in 1958. The first high-volume pressure sensor was marketed by
National Semiconductor in 1974. This sensor included a temperature controller for
constant-temperature operation. Improvements in this technology since then have
7/31/2019 Report Me Ms
http://slidepdf.com/reader/full/report-me-ms 6/43
included the utilization of ion implantation for improved control of the piezoresistor
fabrication. Si pressure sensors are now a billion-dollar industry.
Around 1982, the term micromachining came into use to designate the
fabrication of micromechanical parts for Si microsensors. The micromechanical parts
were fabricated by selectively etching areas of the Si substrate away in order to leave
behind the desired geometries. Isotropic etching of Si was developed in the early
1960s for transistor fabrication. Anisotropic etching of Si then came about in 1967.
Various etch-stop techniques were subsequently developed to provide further process
flexibility.
These techniques also form the basis of the bulk micromachiningprocessing techniques. Bulk micromachining designates the point at which the bulk of
the Si substrate is etched away to leave behind the desired micromechanical elements.
Bulk micromachining has remained a powerful technique for the fabrication of
micromechanical elements. However, the need for flexibility in device design and
performance improvement has motivated the development of new concepts and
techniques for micromachining.
Among these is the sacrificial layer technique, first demonstrated in
1965 by Nathanson and Wickstrom, in which a layer of material is deposited between
structural layers for mechanical separation and isolation. This layer is removed during
the release etch to free the structural layers and to allow mechanical devices to move
relative to the substrate. A layer is releasable when a sacrificial layer separates it from
the substrate. The application of the sacrificial layer technique to micromachining in
1985 gave rise to surface micromachining, in which the Si substrate is primarily used
as a mechanical support upon which the micromechanical elements are fabricated.
Prior to 1987, these micromechanical structures were limited in motion.
During 1987-1988, a turning point was reached in micromachining when, for the first
time, techniques for integrated fabrication of mechanisms on Si were demonstrated.
During a series of three separate workshops on microdynamics held in 1987, the term
MEMS was coined. Equivalent terms for MEMS are microsystems-preferred in
Europe and micromachines-preferred in Japan.
7/31/2019 Report Me Ms
http://slidepdf.com/reader/full/report-me-ms 7/43
SECTION 3 MEMS DESCRIPTION
MEMS technology can be implemented using a number of different
materials and manufacturing techniques; the choice of which will depend on the
device being created and the market sector in which it has to operate.
SILICON
The economies of scale, ready availability of cheap high-quality
materials and ability to incorporate electronic functionality make silicon attractive for
a wide variety of MEMS applications. Silicon also has significant advantages
engendered through its material properties. In single crystal form, silicon is an almost
perfect Hookean material, meaning that when it is flexed there is virtually no
hysteresis and hence almost no energy dissipation. The basic techniques for producing
all silicon based MEMS devices are deposition of material layers, patterning of these
layers by photolithography and then etching to produce the required shapes.
POLYMERS
Even though the electronics industry provides an economy of scale for
the silicon industry, crystalline silicon is still a complex and relatively expensive
material to produce. Polymers on the other hand can be produced in huge volumes,
with a great variety of material characteristics. MEMS devices can be made from
polymers by processes such as injection moulding, embossing or stereolithography
and are especially well suited to microfluidic applications such as disposable blood
testing cartridges.
METALS
Metals can also be used to create MEMS elements. While metals do not
have some of the advantages displayed by silicon in terms of mechanical properties,
when used within their limitations, metals can exhibit very high degrees of reliability.
Metals can be deposited by electroplating, evaporation, and sputtering processes.
Commonly used metals include gold, nickel, aluminum, chromium, titanium,
tungsten, platinum, and silver
7/31/2019 Report Me Ms
http://slidepdf.com/reader/full/report-me-ms 8/43
SECTION 4 MEMS DESIGN PROCESS
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, and Etching-to etch the films selectively to the mask. A MEMS process is
usually a structured sequence of these operations to form actual devices.
SECTION 4.1 DEPOSITION PROCESSES
One of the basic building blocks in MEMS processing is the ability to
deposit thin films of material. In this text we assume a thin film to have a thickness
anywhere 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:
o Chemical Vapor Deposition (CVD)
o Electrodeposition
o Epitaxy
o Thermal oxidation
7/31/2019 Report Me Ms
http://slidepdf.com/reader/full/report-me-ms 9/43
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:
o Physical Vapor Deposition (PVD)
o Casting
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 castingprocesses, though it is more convenient to think of them that way.
This is by no means an exhaustive list since technologies evolve continuously.
SECTION 4.1.1 CHEMICAL VAPOR DEPOSITION (CVD)
In this process, the substrate is placed inside a reactor to which a number
of gases are supplied. The fundamental principle of the process is that a chemical
reaction takes place between the source gases. The product of that reaction is a solid
material with condenses on all surfaces inside the reactor.
The two most important CVD technologies in MEMS are the Low
Pressure CVD (LPCVD) and Plasma Enhanced CVD (PECVD). The LPCVD process
produces layers with excellent uniformity of thickness and material characteristics.
The main problems with the process are the high deposition temperature (higher than
600°C) and the relatively slow deposition rate. The PECVD process can operate at
lower temperatures (down to 300° C) thanks to the extra energy supplied to the gas
molecules by the plasma in the reactor. However, the quality of the films tend to be
inferior to processes running at higher temperatures. Secondly, most PECVD
deposition systems can only deposit the material on one side of the wafers on 1 to 4
wafers at a time. LPCVD systems deposit films on both sides of at least 25 wafers at a
time. A schematic diagram of a typical LPCVD reactor is shown in the figure below.
Figure 10: Lithography tool depth of focus and surface topology.
SECTION 4.3 ETCHING PROCESSES
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
a vapor phase etchant
SECTION 4.3.1 WET ETCHING
This is the simplest etching technology. All it requires is a container
with a liquid solution that will dissolve the material in question. Unfortunately, there
are complications since usually a mask is desired to selectively etch the material. One
must find a mask that will not dissolve or at least etches much slower than the
material to be patterned. Secondly, some single crystal materials, such as silicon,
exhibit anisotropic etching in certain chemicals. Anisotropic etching in contrast to
isotropic etching means different etches rates in different directions in the material.The classic example of this is the <111> crystal plane sidewalls that appear when
The dry etching technology can split in three separate classes called
reactive ion etching (RIE), sputter etching, and vapor phase etching.
In RIE, the substrate is placed inside a reactor in which several gases are
introduced. Plasma is struck in the gas mixture using an RF power source, breaking
the gas molecules into ions. The ion is accelerated towards, and reacts at, the surface
of the material being etched, forming another gaseous material. This is known as the
chemical part of reactive ion etching. There is also a physical part which is similar in
nature to the sputtering deposition process. If the ions have high enough energy, they
can knock atoms out of the material to be etched without a chemical reaction. It is
very complex tasks to develop dry etch processes that balance chemical and physical
etching, since there are many parameters to adjust. By changing the balance it is
possible to influence the anisotropy of the etching, since the chemical part is isotropic
and the physical part highly anisotropic the combination can form sidewalls that have
shapes from rounded to vertical. A schematic of a typical reactive ion etching system
is shown in the figure below.
A special subclass of RIE which continues to grow rapidly in popularity
is deep RIE (DRIE). In this process, etch depths of hundreds of microns can be
achieved with almost vertical sidewalls. The primary technology is based on the so-
called "Bosch process", named after the German company Robert Bosch which filed
the original patent, where two different gas compositions are alternated in the reactor.
The first gas composition creates a polymer on the surface of the substrate, and the
second gas composition etches the substrate. The polymer is immediately sputtered
away by the physical part of the etching, but only on the horizontal surfaces and notthe sidewalls. Since the polymer only dissolves very slowly in the chemical part of the
etching, it builds up on the sidewalls and protects them from etching. As a result,
etching aspect ratios of 50 to 1 can be achieved. The process can easily be used to
etch completely through a silicon substrate, and etch rates are 3-4 times higher than
wet etching. Sputter etching is essentially RIE without reactive ions. The systems
used are very similar in principle to sputtering deposition systems. The big difference
is that substrate is now subjected to the ion bombardment instead of the materialtarget used in sputter deposition.
MEMS pressure microsensors typically have a flexible diaphragm that
deforms in the presence of a pressure difference. The deformation is converted to an
electrical signal appearing at the sensor output. A pressure sensor can be used to sense
the absolute air pressure within the intake manifold of an automobile engine, so that
the amount of fuel required for each engine cylinder can be computed.
ACCELEROMETERS
Accelerometers are acceleration sensors. An inertial mass suspended by
springs is acted upon by acceleration forces that cause the mass to be deflected from
its initial position. This deflection is converted to an electrical signal, which appears
at the sensor output. The application of MEMS technology to accelerometers is a
relatively new development.
Accelerometers in consumer electronics devices such as game
controllers (Nintendo Wii), personal media players / cell phones (Apple iPhone ) anda number of Digital Cameras (various Canon Digital IXUS models). Also used in PCs
to park the hard disk head when free-fall is detected, to prevent damage and data loss.
iPod Touch: When the technology become sensitive. MEMS-based sensors are ideal
for a wide array of applications in consumer, communication, automotive and
The consumer market has been a key driver for MEMS technology
success. For example, in a mobile phone, MP3/MP4 player or PDA, these sensors
offer a new intuitive motion-based approach to navigation within and between pages.
In game controllers, MEMS sensors allow the player to play just moving the
controller/pad; the sensor determines the motion.
INERTIAL SENSORS
Inertial sensors are a type
of accelerometer and are one of the
principal commercial products that
utilize surface micromachining. They
are used as airbag-deployment sensors
in automobiles, and as tilt or shock
sensors. The application of these
accelerometers to inertial measurement
units is limited by the need to manually
align and assemble them into three-
axis systems, and by the resulting
alignment tolerances, their lack of in-
chip analog-to-digital conversion
circuitry, and their lower limit of
sensitivity
.
MICROENGINES
A three-level polysilicon micromachining process has enabled the
fabrication of devices with increased degrees of complexity. The process includesthree movable levels of polysilicon, each separated by a sacrificial oxide layer, plus a
stationary level. Microengines can be used to drive the wheels of microcombination
locks. They can also be used in combination with a microtransmission to drive a pop-
up mirror out of a plane. This device is known as a micromirror.