1

Microelectromechanical systems

Introduction

Microelectromechanical systems (MEMS) (also written as micro-electro-mechanical, MicroElectroMechanical or microelectronic and microelectromechanical systems) is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems (NEMS) and nanotechnology. MEMS are also referred to as micromachines (in Japan), or Micro Systems Technology - MST (in Europe).

MEMS are separate and distinct from the hypothetical vision of molecular

nanotechnology or molecular electronics. MEMS are made up of components

between 1 to 100 micrometres in size (i.e. 0.001 to 0.1 mm) and MEMS

devices generally range in size from 20 micrometres (20 millionths of a metre)

to a millimetre. They usually consist of a central unit that processes data, the

microprocessor and several components that interact with the outside such as

microsensors.[1] At these size scales, the standard constructs of classical

physics are not always useful. Because of the large surface area to volume

ratio of MEMS, surface effects such as electrostatics and wetting dominate

volume effects such as inertia or thermal mass.

The potential of very small machines was appreciated before the technology

existed that could make them—see, for example, Richard Feynman's famous

1959 lecture There's Plenty of Room at the Bottom. MEMS became practical

once they could be fabricated using modified semiconductor device

fabrication technologies, normally used to make electronics. These include

molding and plating, wet etching (KOH, TMAH) and dry etching (RIE and

DRIE), electro discharge machining (EDM), and other technologies capable of

manufacturing small devices. An early example of a MEMS device is the

resonistor – an electromechanical monolithic resonator.

2

History of MEMS Technology

MEMS have developed in the past decades, especially in the last fifteen

years. In the beginning of 1990s, MEMS emerged with the aid of the

development of integrated circuit (IC) fabrication processes, in which sensors,

actuators, and control functions are cofabricated in silicon. Since then,

remarkable research progresses have been achieved in MEMS under the

strong capital promotions from both government and industries. In addition to

the commercialization of some less integrated MEMS devices, such as

microaccelerometers, inkjet printer head, micromirrors for projection, etc., the

concepts and feasibility of more complex MEMS devices have been proposed

and demonstrated for the applications in such varied fields as microfluidics,

aerospace, biomedical, chemical analysis, wireless communications, data

storage, display, optics, etc. Some branches of MEMS, appearing as

microoptoelectromechanical systems (MOEMS), micrototal analysis systems

(µTAS), etc., have attracted a great deal of research interests since their

potential applications’ market. As of the end of 1990s, most of MEMS devices

with various sensing or actuating mechanisms were fabricated using silicon

bulk micromachining, surface micromachining, and lithography,

galvanoforming, moulding (LIGA) processes. Three-dimensional (3D)

microfabrication processes incorporating more materials were presented for

MEMS recently, when some specific application requirements (e.g.,

Biomedical devices) and the microactuators with higher output power were

called for in MEMS. Micromachining has become the fundamental technology

for the fabrication of microelectromechanical devices and, in particular,

miniaturized sensors and actuators. Silicon micromachining is the most

advanced of the micromachining technologies, and it allows for the fabrication

of MEMS that have dimensions in the submillimeter range. It refers to

fashioning microscopic mechanical parts out of silicon substrate or on a

silicon substrate, making the structures three dimensional and bringing new

principles to the designers. Employing materials such as crystalline silicon,

polycrystalline silicon, silicon nitride, etc., a variety of mechanical

microstructures including beams, diaphragms, grooves, orifices, springs,

3

gears, suspensions, and a great diversity of other complex mechanical

structures have been conceived. Sometimes many microdevices can also be

fabricated using semiconductor processing technologies or stereolithography

on the polymeric multifunctional structures.

There are some very important events happened in the past half

century:

• In 1750s, First electrostatic motors (Benjamin Franklin, Andrew

Gordon).

• In 1958, Silicon strain gauges commercially available.

• December 26, 1959, At California Institute of Technology, Richard

Feynman gave a remarkably insightful lecture, “There is plenty of room

at the bottom”. He tried to spur innovative miniature fabrication

techniques for micromechannics, but he failed to generate a

fundamentally new fabrication technique.

• In 1967, Invention of surface micromachining (Nathanson, Resonant

Gate Transistor).

• In 1969, Westinghouse creates the “Resonant Gate FET” based on

new microelectronics fabrication techniques.

• In 1970s, Bulk-etched silicon wafers used as pressure sensors.

• In 1970, First silicon accelerometer demonstrated (Kulite) .

• In 1977, First capacitive pressure sensor (Stanford) .

• In 1980, Petersen, K.E., "Silicon Torsional Scanning Mirror", IBM J.

R&D, v24, p631, 1980 .

• In 1982, Kurt Petersen published “silicon as Structural Material”,

reference for material properties and etching data for silicon.

4

• In 1984, First polysilicon MEMS device (Howe, Muller ).

• In 1980s, early experiments in surface-micromachined polysilicon, first

electrostatic comb drive actuators—micropositioning disc drive heads.

• In 1989, Lateral comb drive (Tang, Nguyen, Howe).

• In late 1980s, micromachining leverages microelectronics industry,

widespread experimentation and documentation increased public

interest; Early transduction and actuation methods produce simple

actuators; Micromachining methods aimed toward improving sensors,

thermal and electrical isolation between layers with suspended

structures;

• In early 1990s, Government agencies start large MEMS support

programs, AFOSR(Air Force Office of Scientific Research) support

basic research in materials and MEMS research, DARPA creates

MUMPS foundry services with MCNC in 1993, NIST supports

commercial foundries for CMOS and MEMS.

• In our country, “micron/nanometer manufacture technology national

key laboratory” was founded in 1996.

• In 1992, Chris Pister (UCLA) creates first micromachined hinge, it’s

features open possibilities for pseudo-3D structures and assembly

• In 1992 ,MCNC starts the Multi User MEMS Process (MUMPS).

• In 1993, First surface micromachined accelerometer sold (Analog

Devices,ADXL50) .

• In 1994 , Bosch process for Deep Reactive Ion Etching is patented ;.

• In 1995, Bio-MEMS comes of age.

5

• In 1998, The premiere of Star Wars shown on TI’s Digital Mirror

Device.

• In later 1990s, actuation and fabrication methods(such as deep

reactive ion etching,laser machining, fluidics, tunneling, deep UV)

produce advanced systems.

• In 2000, MEMS Fiber switches become big business.

• Richard Feynman "There's Plenty of Room at the Bottom” Presentation

given December 26,1959 at California Institute of Technology

• Tries to spur innovative miniature fabrication techniques for

micromechanics

• Fails to generate a fundamentally new fabrication technique

Westinghouse creates the "Resonant Gate FET" in 1969

• Mechanical curiosity based on new microelectronics fabrication

• Techniques Bulk-etched silicon wafers used as pressure sensors in

1970’s

• Kurt Petersen published -Silicon as a Structural Material in 1982

• Reference for material properties and etching data for silicon

• Early experiments in surface-micromachined polysilicon in 1980’s

• First electrostatic comb drive actuators- micropositioning disc drive

Heads Micromachining leverages microelectronics industry in late

1980’s

• Widespread experimentation and documentation increases public

interest.

6

Materials for MEMS manufacturing

Silicon

Silicon is the material used to create most integrated circuits used in

consumer electronics in the modern world. 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. As well as making for highly repeatable

motion, this also makes silicon very reliable as it suffers very little fatigue and

can have service lifetimes in the range of billions to trillions of cycles without

breaking.

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 molding, 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, aluminium, copper, chromium,

titanium, tungsten, platinum, and silver.

7

MEMS basic processes

Deposition processes

One of the basic building blocks in MEMS processing is the ability to deposit

thin films of material with a thickness anywhere between a few nanometres to

about 100 micrometres.

Physical deposition

There are two types of physical deposition processes.

Physical vapor deposition (PVD)

Sputtering

Evaporation

Chemical deposition

There are 2 types of chemical deposition.

Chemical vapor deposition

LPCVD : Low Pressure CVD PECVD : Plasma Enhanced CVD

Thermal oxidation

Patterning

Patterning in MEMS is the transfer of a pattern into a material.

8

Lithography

Lithography in MEMS context is typically the transfer of a pattern into a

photosensitive material by selective exposure to a radiation source such as

light. A photosensitive material is a material that experiences a change in its

physical properties when exposed to a radiation source. If a photosensitive

material is selectively exposed to radiation (e.g. by masking some of the

radiation) the pattern of the radiation on the material is transferred to the

material exposed, as the properties of the exposed and unexposed regions

differs. This exposed region can then be removed or treated providing a mask

for the underlying substrate. Photolithography is typically used with metal or

other thin film deposition, wet and dry etching.

Photolithography

KrF ArF Immersion EUV

Electron beam lithography

Main article: Electron beam lithography

Electron beam lithography (often abbreviated as e-beam lithography) is the

practice of scanning a beam of electrons in a patterned fashion across a

surface covered with a film (called the resist), ("exposing" the resist) and of

selectively removing either exposed or non-exposed regions of the resist

("developing"). The purpose, as with photolithography, is to create very small

structures in the resist that can subsequently be transferred to the substrate

material, often by etching. It was developed for manufacturing integrated

circuits, and is also used for creating nanotechnology architectures.

The primary advantage of electron beam lithography is that it is one of the

ways to beat the diffraction limit of light and make features in the nanometer

regime. This form of maskless lithography has found wide usage in

photomask-making used in photolithography, low-volume production of

semiconductor components, and research & development.

The key limitation of electron beam lithography is throughput, i.e., the very

long time it takes to expose an entire silicon wafer or glass substrate. A long

exposure time leaves the user vulnerable to beam drift or instability which

may occur during the exposure. Also, the turn-around time for reworking or re-

9

design is lengthened unnecessarily if the pattern is not being changed the

second time.

Ion beam lithography

It is known that focused-ion-beam lithography has the capability of writing

extremely fine lines (less than 50 nm line and space has been achieved)

without proximity effect. However, because the writing field in ion-beam

lithography is quite small, largearea patterns must be created by stitching

together the small fields.

X-ray lithography

X-ray lithography, is a process used in electronic industry to selectively

remove parts of a thin film. It uses X-rays to transfer a geometric pattern from

a mask to a light-sensitive chemical photo resist, or simply "resist," on the

substrate. A series of chemical treatments then engraves the produced

pattern into the material underneath the photo resist.

Etching processes

There are two basic categories of etching processes: wet etching and dry

etching. In the former, the material is dissolved when immersed in a chemical

solution. In the latter, the material is sputtered or dissolved using reactive ions

or a vapor phase etchant for a somewhat dated overview of MEMS etching

technologies.

Wet etching

Wet chemical etching consists in selective removal of material by dipping a

substrate into a solution that dissolves it. The chemical nature of this etching

process provides a good selectivity, which means the etching rate of the

target material is considerably higher than the mask material if selected

carefully.

Isotropic etching

Etching progresses at the same speed in all directions. Long and narrow

holes in a mask will produce v-shaped grooves in the silicon. The surface of

10

these grooves can be atomically smooth if the etch is carried out correctly,

with dimensions and angles being extremely accurate.

Anisotropic etching

Some single crystal materials, such as silicon, will have different etching rates

depending on the crystallographic orientation of the substrate. This is known

as anisotropic etching and one of the most common examples is the etching

of silicon in KOH (potassium hydroxide), where Si <111> planes etch

approximately 100 times slower than other planes (crystallographic

orientations). Therefore, etching a rectangular hole in a (100)-Si wafer results

in a pyramid shaped etch pit with 54.7° walls, instead of a hole with curved

sidewalls as with isotropic etching.

HF etching

Hydrofluoric acid is commonly used as an aqueous etchant for silicon dioxide

(SiO2, also known as BOX for SOI), usually in 49% concentrated form, 5:1,

10:1 or 20:1 BOE (buffered oxide etchant) or BHF (Buffered HF). They were

first used in medieval times for glass etching. It was used in IC fabrication for

patterning the gate oxide until the process step was replaced by RIE.

Hydrofluoric acid is considered one of the more dangerous acids in the

cleanroom. It penetrates the skin upon contact and it diffuses straight to the

bone. Therefore the damage is not felt until it is too late.

Electrochemical etching

Electrochemical etching (ECE) for dopant-selective removal of silicon is a

common method to automate and to selectively control etching. An active p-n

diode junction is required, and either type of dopant can be the etch-resistant

("etch-stop") material. Boron is the most common etch-stop dopant. In

combination with wet anisotropic etching as described above, ECE has been

used successfully for controlling silicon diaphragm thickness in commercial

piezoresistive silicon pressure sensors. Selectively doped regions can be

created either by implantation, diffusion, or epitaxial deposition of silicon.

11

Dry etching

Vapor etching

Xenon difluoride etching

Xenon difluoride (XeF2) is a dry vapor phase isotropic etch for silicon originally

applied for MEMS in 1995 at University of California, Los Angeles.[7][8]

Primarily used for releasing metal and dielectric structures by undercutting

silicon, XeF2 has the advantage of a stiction-free release unlike wet etchants.

Its etch selectivity to silicon is very high, allowing it to work with photoresist,

SiO2, silicon nitride, and various metals for masking. Its reaction to silicon is

"plasmaless", is purely chemical and spontaneous and is often operated in

pulsed mode. Models of the etching action are available,[9] and university

laboratories and various commercial tools offer solutions using this approach.

Reactive ion etching (RIE)

Reactive ion etching In reactive ion etching (RIE), the substrate is placed

inside a reactor, and several gases are introduced. A plasma is struck in the

gas mixture using an RF power source, which breaks the gas molecules into

ions. The ions accelerate towards, and react with, 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 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 a very complex task 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. RIE can be deep (Deep RIE or deep reactive ion etching

(DRIE).

12

Deep RIE

(DRIE) is a special subclass of RIE that is growing in popularity. In this

process, etch depths of hundreds of micrometres are achieved with almost

vertical sidewalls. The primary technology is based on the so-called "Bosch

process",[10] named after the German company Robert Bosch, which filed the

original patent, where two different gas compositions alternate in the reactor.

Currently there are two variations of the DRIE. The first variation consists of

three distinct steps (the Bosch Process as used in the Plasma-Therm tool)

while the second variation only consists of two steps (ASE used in the STS

tool). In the 1st Variation, the etch cycle is as follows: (i) SF6 isotropic etch; (ii)

C4F8 passivation; (iii) SF6 anisoptropic etch for floor cleaning. In the 2nd

variation, steps (i) and (iii) are combined. Both variations operate similarly.

The C4F8 creates a polymer on the surface of the substrate, and the second

gas composition (SF6 and O2) etches the substrate. The polymer is

immediately sputtered away by the physical part of the etching, but only on

the horizontal surfaces and not the 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 3–6 times higher than wet

etching

13

Fabrication Techniques

Microfabrication processes capable of creating threedimensional structures in

silicon were the driving force for the emergence of early MEMS devices. The

evolution of these microfabrication processes has led to the classification of

major micromachining techniques namely, bulk micromachining, surface

micromachining, dissolved wafer process, LIGA, and electrodischarge

machining. A typical MEMS device can be realized by using any of these

processes in their most prevalent form or several variants of these processes

can be used. The choice of the fabrication process is very important in

that it defines the overall performance and cost of the micromachinedm part.

1 Bulk Micromachining.

Bulk micromachining is based on a combination of isotropic and anisotropic

etchings of singlecrystalline silicon to form micro mechanical structures from

the bulk of the silicon wafer @20#. Strongly alkaline liquids such as potassium

hydroxide ~KOH!, tetra-methyl-ammonium-hydroxide ~TMAH!, ethylene-

diamine-pyrocatechol EDP! etc. preferentially etch the 100 planes of single

crystal silicon in comparison to the 111 planes. The difference in these etch

rates can be used to create large three-dimensional structures in silicon

substrates using standard photolithography techniques in conjunction with

good masking layers such as silicon dioxide and silicon nitride layers.

However, the etch rate of silicon in these chemicals is of the order of 1

mm/min and therefore takes in excess of 8 hours to etch through a 550 mm

thick wafer. Wet etching can be used either from the front side of the wafer,

backside of the wafer or from both sides to realize an array of

micromechanical structures. In addition, these chemicals are not compatible

with CMOS fabrication processes due to the presence of heavy metal and

alkali ions. Thus any device process has to be carefully designed to prevent

the possibility of contamination. For example, wet anisotropic Si etching can

be used after completing CMOS processing for releasing the micromechanical

structures so that contamination issues can be avoided. Bulk micromachining

based on wet anisotropic etching also prevents efficient use of the silicon real

estate since large etch-windows need to be defined on one surface of the

14

silicon wafer to realize relatively small micromechanical structure on the other

surface of the silicon wafer. This leads to low device densities and high device

cost. The main attraction of anisotropic wet etching arises from the

inexpensive capital investment in realizingthis process step. For example,

KOH etch module including precision temperature control, in-situ filtering and

stirring etc. costs under $25,000. In contrast high-aspect ratio, anisotropic

etching of silicon using deep reactive ion etchers is not crystal orientation

dependent and is capable of etching rates as high as 6 mm/min with load

locked cassette to cassette wafer-handling systems. However, such systems

cost in excess of a million dollars.

2 Surface Micromachining.

Surface micromachining, on the other hand, is based on sequential deposition

and etching of thin films on the surface of a carrier substrate. One of the big

advantages of surface micromachining is that a very slightly modified CMOS

process can be used for the realization of the MEMS device. Typical structural

materials are chosen from CMOS materials such as polysilicon and silicon

nitride while the sacrificial layer is silicon dioxide. The micromechanical

structure is released from the substrate by etching away the sacrificial oxide in

HF. Initial efforts in surface micromachining were largely affected by the

stresses in the structural layers and the release processes. Micromechanical

structures upon release in the wet etchant suffered from stiction problems.

These problems have been largely solved and complex CMOS based

micromachining processes using up to 3-layers of polysilicon have been used

to fabricate very complex micromechanical structures such as microgears,

micromotors, micromirrors . Due to the thickness of the structural layer being

limited to a few microns, the one disadvantage of surface micromachining is

that micromechanical structures with large mass or dimensions are difficult to

fabricate. Such structures are required for the proof mass of accelerometers

or in nozzles for inkjet printer heads etc.

15

3 Dissolved Wafer Process.

An innovative process that combines the advantages of both surface and

bulk micromachining techniques was developed at the University of Michigan

and is called dissolved wafer process. In this process the micromechanica.

Structure is defined in a silicon wafer by boron doping

The areas, which will constitute the mechanical structure. In order to create

topographical features the silicon is first anisotropically etched and then

followed by the doping step. Once the silicon process is complete, the wafer is

bonded onto a borosilicate glass e.g. PYREX™ 7740! wafer. Thereafter, the

silicon wafer is dissolved away in EDP etchant leaving behind the boron-

doped micromechanical structures on the glass substrate. Boron-doped

micromechanical structures fabricated using this technique can range from 2–

15 mm in thickness. The main advantage of this process is that the

micromechanical structures can be fabricated at high densities and can have

higher aspect ratios as compared to surface micromachined parts. In some

applications the high dielectric constant of the glass substrate offers additional

advantages. Figure

16

4 shows the schematic diagram of dissolved wafer process.

4 LIGA Process.

LIGA process has relied upon the use of x-ray lithography to define high

aspect ratio structures in photoresist, which are then used as templates for

plating and molding processes. Electroplated nickel is typically used for

creating molds from the high-aspect ratio photoresist patterns on the silicon

wafer and subsequently used to fabricate precision parts. Current

developments in LIGA process are geared towards making it more

manufactureable and to enable precision microstructures as thick as several

mm. More recently thick optical negative photoresists such as SU-8 have

become available which can also be used for realizing high-aspect ratio

structures. Current developments in the fabrication techniques based upon

plating and molding techniques include wet developing process, thermal

stress control, materials issues, plating high-strength alloys, etc. The potential

benefit is the low-cost manufacturing of icrostructures with virtually unlimited

choices of structural materials, excellent heat transfer characteristics in the

final devices, molds for polymer medical devices, etc.

CAD for MEMS

The early success stories of MEMS products such as pressure sensors,

accelerometers, micromirror displays etc. required development efforts of

several engineers and scientists working over a period of at least 10 years

after the demonstration of the R&D prototypes in Universities and Research

Laboratories. During this mostly development phase, efforts were typically

directed towards producing a reliable, reproducible and high-yield

microelectromechanical chip. This period also saw some of the pioneering

developments in MEMS related fabrication processes, materials development

and characterization for mechanical properties, and foundries that could

handle MEMS oriented processes. However, the biggest constraint for rapid

prototyping has been the lack of a unified simulation platform, which is

capable of accurately predicting the performance of the final device including

packaging effects. The lack of such simulation software has led to tedious

17

Applications

Accelerometers are being incorporated into more and more personal

electronic devices such as media players and gaming devices. In particular,

more and more smartphones (such as Apple’siPhone and the Nokia N95) are

incorporating accelerometers for step counters, user interface control, and

switching between portrait and landscape modes. They use accelerometers

as a tilt sensor for tagging the orientation to photos taken with the built-in

camera. The Nokia 5500

sport features a 3D accelerometer that can be used for tap gestures, for

example to change to next song by tapping through clothing when the device

is in a pocket. Camcorders use accelerometers for image stabilization. Still

cameras use accelerometers for anti-blur capturing. Some digital cameras,

such as Canon’s PowerShot and Ixus range contain accelerometers to

determine the orientation of the photo being taken and also for rotating the

current picture when

viewing.

Accelerometers are also being used in new contactless game controller or

mouse. IBM and Apple have recently started using accelerometers in their

laptops to protect hard drives from damage. If you accidentally drop the

laptop, the accelerometer detects the sudden freefall, and 8 switches the hard

drive off so the heads don’t crash on the platters. In a similar fashion, high g

accelerometers are the industry standard way of detecting car crashes and

deploying airbags at just the right time. They are used to detect the rapid

negative acceleration of the vehicle to determine when a collision has

occurred. They also have a built-in self-test feature, where a micro-actuator

will simulate the effect of deceleration and allow checking the integrity of the

system every time you start up the engine. Recently the gyroscopes (they rely

on a mechanical structure that is driven into resonance and excites a

secondary oscillation in either the same structure or in a second one, due to

the Coriolis force) made their apparition for anti-skidding system and also for

navigation unit. The widespread use of accelerometers in the automotive

industry has pushed their cost down dramatically. Accelerometers have also

18

found real-time applications in controlling and monitoring military and

aerospace systems. Smart weapon systems (direct and indirect fire; aviation-

launched and ship-launched missiles, rockets, projectiles and sub munitions)

are among these applications. Some MEMS sensors have already been used

in satellite. The development of micro

(less than 100kg) and nano (about 10kg) satellites is bringing the mass and

volume advantage of MEMS to good use .

19

MEMS ACCELEROMETERS

Introduction

An accelerometer is an electromechanical device that measures acceleration

forces. These forces may be static, like the constant force of gravity pulling at

our feet, or they could be dynamic-caused by moving or vibrating the

accelerometer. There are many types of accelerometers developed and

reported in the literature. The vast majority is based on piezoelectric crystals,

but they are too big and to clumsy. People tried to develop something smaller,

that could increase applicability and started searching in the field of

microelectronics. They developed MEMS (micro electromechanical systems)

accelerometers. The first micro machined accelerometer was designed in

1979 at Stanford University, but it took over 15 years before such devices

became accepted mainstream products for large volume applications. In the

1990s MEMS accelerometers revolutionised the automotive-airbagsystem

industry. Since then they have enabled unique features and applications

ranging from hard-disk protection on laptops to game controllers. More

recently, the same sensor-core technology has become available in fully

integrated, full-featured devices suitable for industrial applications. Micro

machined accelerometers are a highly enabling technology with a huge

commercial potential. They provide lower power, compact and robust sensing.

Multiple sensors are often combined to provide multi-axis sensing and more

accurate data.

MEMS technology

What could link an inkjet printer head, a video projector DLP system, a

disposable bio-analysis chip and an airbag crash sensor - yes, they are all

MEMS, but what is MEMS? Micro Electro Mechanical Systems or MEMS is a

term coined around 1989 by Prof. R. Howand others to describe an emerging

research field, where mechanical elements, like cantilevers or membranes,

had been manufactured at a scale more akin to microelectronics circuit than

to lathe machining. It appears that these devices share the presence of

features below 100_m that are not machined using standard machining but

20

using other techniques globally called micro-fabrication technology. Of course,

this simple definition would also include microelectronics, but there is a

characteristic that electronic circuits do not share with MEMS. While electronic

circuits are inherently solid and compact structures, MEMS have holes, cavity,

channels, antilevers, membranes, etc, and, in some way, imitate ‘mechanical’

parts. The emphasis on MEMS based on silicon is clearly a result of the vast

knowledge on silicon material and on silicon based microfabrication gained by

decades of research in microelectronics. And again, even when MEMS are

based on silicon, microelectronics process needs to be adapted to cater for

thicker layer deposition, deeper etching and to introduce special steps to free

the mechanical structures. MEMS needs a completely different set of mind,

where next to electronics, mechanical and material knowledge plays a

fundamental role. Then, many more MEMS are not based on silicon and can

be manufactured in polymer, in glass, in quartz or even in metals.

The development of a MEMS component has a cost that should not be

misevaluated and the technology has the possibility to bring unique benefits.

The reasons that prompt the use of MEMS technology are for example

miniaturization of existing devices, development of new devices based on

principles that do not work at larger scale, development of new tools to

interact with the micro-world. Miniaturization reduces cost by decreasing

material consumption. It also increases applicability by reducing mass and

size allowing to place the MEMS in places where a traditional system doesn’t

fit. A typical example is brought by the accelerometer developed as a

replacement for traditional airbag triggering sensor also used in digital

cameras to help stabilize the image or even in the contact-less game

controller integrated in the latest handphones. Another advantage that MEMS

can bring relates with the system integration. Instead of having a series of

external components (sensor, inductor...) connected by wire or soldered to a

printed circuit board, the MEMS on silicon can be integrated directly with the

electronics. These so called smart integrated MEMS already include data

acquisition, filtering, data storage, communication, interfacing and networking.

As we see, MEMS technology not only makes the things smaller but often

makes them better.

21

The MEMS component currently on the market can be broadly divided in six

categories, where next to the well-known pressure and inertia sensors

produced by different manufacturer like Motorola, Analog Devices, Sensonor

or Delphi we have many other products. The micro-fluidic application are best

known for the inkjet printer head popularized by Hewlett Product category

Examples Pressure sensor Manifold pressure (MAP), tire pressure, blood

pressure..Inertia sensor Accelerometer, gyroscope, crash

sensor..Microfluidics bioMEMS Inkjet printer nozzle, micro-bio-analysis

systems,DNA chips.. Optical MEMS MOEMS

Micro-mirror array for projection (DLP), micro-grating array for projection

(GLV),

optical fiber switch, adaptive optics. RF MEMS High Q-inductor, switches,

antenna, filter. Others Relays, microphone, data storage, toys. MEMS

products examples. The MEMS component currently on the market can be

broadly divided in six categories. Packard, but they also include the growing

bioMEMS market with micro analysis system liken the capillary

electrophoresis system from Agilent or the DNA chips. Optical MEMS

(MOEMS) includes the component for the fibre optic telecommunication like

the switch based on a moving mirror produced by Sercalo. Moreover MOEMS

deals with the now rather successful optical

projection system that is competing with the LCD (liquid crystal display)

projector. RF (radio frequency) MEMS is also emerging as viable MEMS

market. Next to passive components like high-Q inductors produced on the IC

surface to replace the hybridized component as proposed by company

MEMSCAP we find RF switches and soon micromechanical filters. But the

listdoes not end here and we can find micromachined relays (MMR) produced

for example by Omron, HDD (hard disk drive) read/write head and actuator or

even toys, like the autonomous

micro-robot EMRoS produced by EPSON.

22

MEMS' World

This page introduces examples of commercial MEMS applications (and how

many are application fields) that have reached the market and are really

available at this time, meaning that are used in real life! For example, MEMS

actuators featuring electrostatic actuation that are used all around the world

will be introduced here! This page shows that MEMS don't exist only in

laboratory but everywhere around you!

MEMS devices have started when microelectronics people realize that

polysilicon, one of the most used material at the beginning of icroelectronics,

has very good mechanical properties. Since then, these properties have been

largely used and now MEMS use a lot of different materials to achieve an

increasing number of applications

Commercial MEMS Applications

You probably ignore it, but there are already MEMS around you. In your car,

maybe in your television, and in your mobile phone! I can't reference here all

of the currently available applications, but I will talk about the most common

and the most spectacular ones.

Summary

• Inertial sensors

• RF switches

• Optical switches

• Digital Mirror

Inertial sensors

Inertial sensors are mechanics sensors aiming at measuring accelerations, in

the mechanics science definition. An acceleration is a changing in the speed,

could it be translational and/or rotational.

23

There are two categories of inertial sensors: accelerometers, and gyroscopes.

The first ones measure varation of translational speed, and the last ones

measure variation of rotational speed.

Microaccelerometers were the first MEMS device to flood the market.

The microaccelerometers are already used in daily life! They are parts of the

tiny systems that try to take care of us while we ignore them. To realize it, you

should remember microaccelerometers measure variation of translational

speed. So acceleration, deceleration, even very high deceleration, like..

shock! The sensor that detects a shock and launch the airbag is a

microaccelerometer combined with a electronic circuit able to decide wether

or not the shock was an accident or just your car passing a pothole

Photos of a microaccelerometer, ADXL series, produced by Analog Device.

Copyright Analog Devices, Inc. All rights reserved

24

On these photos, you can see a microaccelerometer device and the chip

including associated electronics, made by Analog Device. This is a two axis

microaccelerometer: this means it is able to measure accelerations in two

directions at a time (in the directions of the plane).

There are a lot of other applications, like navigation, microaccelerometers can

help in increasing precision, because GPS does hardly better than several

meters, integrating accelerations and direction changing can help in

calculating a position. In industrial device monitoring, an accelerometer can

detect any changing in the vibration emitted by the device and preempts a

breakdown.

There are more and more to say about microaccelerometers, they are still the

spearhead of MEMS industry.

Microgyroscopes are newer in the market compared to microaccelerometers.

Some devices have appeared on the market for navigation applications. The

key point in these devices is sensitivity, and it has been the subject of

research and development for several years before devices become

interesting for real-life applications.

RF switches

RF switches have been under development for many 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 prefered to full-electronic switches on applications where

security, integration capabilities, power consumption and other parameters

are critical

Schematics of a R.F. electromechanical microswitch in action

25

Optical multiplexers

Texas Instruments's DMD

Micromirrors have also been studied for a long time before a device become

commercially available. Texas Instruments has developped a micromirrors'

matrix for video display: the Digital Micromirror Device. It is now the base of

high quality video projector, and you can find televisions carrying the DLP

letters, meaning they embed Texas Instrument's DMD technology

Photos of Texas Instrument's DMD

Copyright Texas Instruments, Inc. All right reserved

DLP, standing for Digital Light Processor, is made of a large matrix of

micromirrors (DMD), each mirror corresponding to a pixel. These mirrors can

change their orientation angle thanks to an electrostatic actuation. So, if you

send incident light on the matrix, the mirrors reflect a quantity of light to the

screen depending on their orientation, so orientation angle controls the

luminance for each pixel.

26

Schematics of a projection system with a single DMD chip

Copyright Texas Instrument, Inc. All right reserved

There are several kind of systems using DLP devices. Some of them use 3

DMD, one for each color. One of them uses a single DMD device, with a color

filter system. Since micromirror actuation is very fast compared to the

persistence of light on the screen, it is possible during an image cycle to

switch between each of the filters so that the mirrors send successively a

dose of each color for the same pixel without possibility for human eyes to see

the sequence.

The advantages of DLP based system compared to existing ones, like

plasma, LCD, or electronic beams are high resolution, and the best power

ratio between light source and displayed light. Microtechnology also allows a

very high productivity, reducing the cost of the devices as the market's

demand grows.

Micromirrors devices have just reached the consumer market, and they are a

promising technology that should eventually find a place in most of the display

devices, like mobile phone screens, etc

27

1. World’s Smallest Car

2. Automotive Airbag Accelerometer

Ford Microelectronics ISAAC two-chip automotive airbag accelerometer

• Sensor chip is on the right

• Signal processing and control IC is on the left

• The accelerometer structure is a bulk micromachined suspended

silicon mass over a fixed metal electrode that provides a capacitive

output as a function of acceleration

• The sensor is created by anodically bonding a micromachined silicon

wafer to a glass wafer and etching away the bulk of the silicon, leaving

only the suspended silicon mass.

28

3. Vibrating Wheel Gyro

• A wheel is driven to vibrate about its axis of symmetry

• Rotation about either in-plane axis resultsin the wheel’s tilting

• Tilting of the wheel can be detected with capacitive electrodes under

the wheel

29

4. Pill Camer

Distal esophagus with edema and erythema.

Geographic ulceration suggestive of Barret's

Esophagus.

30

Conclusion

Although some products like pressure sensors have been produced for 30

years, MEMS industry in many aspects is still a young industry. MEMS will

undoubtedly invade more and more consumer products. Size of MEMS is

getting smaller, frequency response and sense range are getting wider.

MEMS are more and more reliable and their sensitivity better every day.

Prices of MEMS accelerometers and other MEMS devices aren’t excessive,

but they still have

to drop a lot if we want to expand massive consumption. Standardization of

production, testing and packaging MEMS would certainly do a big part at it.

The relatively long and expensive development cycle for a MEMS component

is a hurdle that needs to be lowered and also less expensive micro-fabrication

method than photolithography has to be pursued.

We can be sure that the future for MEMS is bright. At least because, as R.

Feynman stated boldly in his famous 1959 talk, which inspired some of the

MEMS pioneers, because, indeed, "There’s plenty of room at the bottom!".

31

References

[1] I. Lee, G. H. Yoon, J. Park, S. Seok, K. Chun, K. Lee, Development and

analysis of the vertical capacitive accelerometer, Sensors and Actuators A

119 (2005) 8-18

[2] F. Chollet, H. Liu, A (not so) short introduction to MEMS

(http://memscyclopedia.org/introMEMS.html (18.2.2008))

[3] S. Beeby, G. Ensell, M. Kraft, N.White, MEMS mechanical sensors (Artech

house inc USA, 2004)

[4] S. E. Lyshevski, Mems and Nems: systems, devices and structures (CRC

Press LLC, USA, 2002)

[5]http://www.analog.com/UploadedFiles/Obsolete_Data_Sheets/66309706A

DXL05.pdf (10.3.2008)

[6] B. E. Boser, “Electronics for micromachined inertial sensors,” in

Transducers Dig. of Tech.Papers, pp. 1169-1172, June 1997.

[7] http://www.analog.com/en/prod/0„764_800_ADXL202%2C00.html

(10.3.2008)

[8]http://www.analog.com/en/content/0,2886,764%255F800%255F122115%2

55F0,00.html (14.2.2008)

[9] C. T. Leondes, Mems/Nems Handbook techniques and applications,

Volume 4: Sensors and actuators (Springer, USA, 2006)

[10]http://rfdesign.com/military_defense_electronics/news/accelerometer_prov

es_accurate_0509/ (14.2.2008)

[11] F. Mohn-Yasin, C. E. Korman, D. J. Nagel, Measurement of noise

characteristics of MEMSaccelerometers Solid-State Electronics 47 (2003)

357-360

[12] http://en.wikipedia.org/wiki/Accelerometer (14.2.2008)

[13] S. Beeby, G. Ensell, M. Kraft, N.White, MEMS mechanical sensors

(Artech house inc USA, 2004)

[14] http://www.sensorsmag.com/articles/0399/0399_44/main.shtml

(14.2.2008)