semianarmems.doc

8/14/2019 semianarmems.doc

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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 etend the fabrication

techni!ues developed for the integrated circuit industry to add mechanical elements

such as beams, gears, diaphragms, and springs to devices.

Eamples of MEMS device applications include in"#et$printer

cartridges, accelerometer, miniature robots, microengines, loc"s inertial sensors

microtransmissions, micromirrors, micro actuator (Mechanisms for activating processcontrol e!uipment by use of pneumatic, hydraulic, or electronic signals) optical

scanners, fluid pumps, transducer, pressure and flow sensors. %ew applications are

emerging as the eisting 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 tas"s, 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 ' billion mar"et, and with a

pro#ected $*+ annual growth rate, it is estimated to have a '- billion mar"et in

**. ecause of the significant impact that MEMS can have on the commercial and

defense mar"ets, industry and the federal government have both ta"en a special

interest in their development.

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SECTION 1.1 WHATISMEMS TECHNOLOGY?

Micro$Electro$Mechanical Systems (MEMS) is the integration of

mechanical elements, sensors, actuators, and electronics on a common silicon

substrate through microfabrication technology. 5hile the electronics are fabricated

using integrated circuit (06) process se!uences, the micromechanical components are

fabricated using compatible 7micromachining7 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 7brains7 of a

system and MEMS augments this decision$ma"ing capability with 7eyes7 and 7arms7,

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 ma"ing capability

direct the actuators to respond by moving, positioning, regulating, pumping, and

filtering, thereby controlling the environment for some desired outcome or purpose.

ecause MEMS devices are manufactured using batch fabrication techni!ues 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.

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SECTION 1.2 WHATAREMEMS / 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. 8s the smallest commercially

produced 7machines7, MEMS devices are similar to traditional sensors and actuators

although much, much smaller. E.g. 6omplete systems are typically a few millimeters

across, with individual features devices of the order of $ micrometers across.

MEMS devices are manufactured either using processes based on 0ntegrated 6ircuit

fabrication techni!ues and materials, or using new emerging fabrication technologies

such as micro in#ection molding. These former processes involve building the device

up layer by layer, involving several material depositions and etch steps. 8 typicalMEMS fabrication technology may have a 9 step process. /ue to the limitations of

this 7traditional 067 manufacturing process MEMS devices are substantially planar,

having very low aspect ratios (typically 9 $ micro meters thic"). 0t is important to

note that there are several evolving fabrication techni!ues that allow higher aspect

ratios such as deep $ray lithography, electrodeposition, and micro in#ection molding.



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MEMS devices are typically fabricated onto a substrate (chip) that may

also contain the electronics re!uired to interact with the MEMS device. /ue 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. :ence the MEMS electronics

include electrostatic drive power supplies, capacitance charge comparators, and signal

conditioning circuitry. 6onnection with the macroscopic world is via wire bonding

and encapsulation into familiar ;8, M6M, surface mount, or leaded 06 pac"ages.

8 common MEMS actuator is the 7linear comb drive7 (shown above) which consists

of rows of interloc"ing teeth& half of the teeth are attached to a fied 7beam7, the otherhalf attach to a movable beam assembly. oth assemblies are electrically insulated.

y applying the same polarity voltage to both parts the resultant electrostatic force

repels the movable beam away from the fied. 6onversely, by applying opposite

polarity the parts are attracted. 0n this manner the comb drive can be moved 7in7 or

7out7 and either /6 or 86 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. 6ommercial comb

drives have several thousand teeth, each tooth approimately micro meters long.

/rive voltages are 6M

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SECTION 2 HISTORICALBACKGROUND

The invention of the at ell Telephone ?aboratories in 4-@ spar"ed a

fast$growing microelectronic technology. 2ac" Ailby of Teas 0nstruments built the

first 0ntegrated circuit in 49B using germanium (;e) devices. 0t consisted of one

transistor, three Cesistors, and one 6apacitor. The 06 was implemented on a sliver of

;e that was glued on a glass slide. ?ater that same year Cobert %oyce of 1airchild

Semiconductor announced the development of a Dlanar double$diffused Si 06. The

complete transition from the original ;e transistors with grown and alloyed #unctions

to silicon (Si) planar double$diffused devices too" about years. The success of Si

as an electronic material was due partly to its wide availability from silicon dioide

(Si


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included the utilization of ion implantation for improved control of the piezoresistor

fabrication. Si pressure sensors are now a billion$dollar industry.

8round 4B*, 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. 0sotropic etching of Si was developed in the early

4Gs for transistor fabrication. 8nisotropic etching of Si then came about in 4G@.

Harious etch$stop techni!ues were subse!uently developed to provide further process

fleibility.

These techni!ues also form the basis of the bul" micromachiningprocessing techni!ues. ul" micromachining designates the point at which the bul" of

the Si substrate is etched away to leave behind the desired micromechanical elements.

ul" micromachining has remained a powerful techni!ue for the fabrication of

micromechanical elements. :owever, the need for fleibility in device design and

performance improvement has motivated the development of new concepts and

techni!ues for micromachining.

8mong these is the sacrificial layer techni!ue, first demonstrated in

4G9 by %athanson and 5ic"strom, 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. 8 layer is releasable when a sacrificial layer separates it from

the substrate. The application of the sacrificial layer techni!ue to micromachining in

4B9 gave rise to surface micromachining, in which the Si substrate is primarily used

as a mechanical support upon which the micromechanical elements are fabricated.

Drior to 4B@, these micromechanical structures were limited in motion.

/uring 4B@$4BB, a turning point was reached in micromachining when, for the first

time, techni!ues for integrated fabrication of mechanisms on Si were demonstrated.

/uring a series of three separate wor"shops on microdynamics held in 4B@, the term

MEMS was coined. E!uivalent terms for MEMS are microsystems$preferred in

Europe and micromachines$preferred in 2apan.

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SECTION 3 MEMS DESCRIPTION

MEMS technology can be implemented using a number of different

materials and manufacturing techni!ues& the choice of which will depend on the

device being created and the mar"et sector in which it has to operate.

SILICON

The economies of scale, ready availability of cheap high$!uality

materials and ability to incorporate electronic functionality ma"e silicon attractive for

a wide variety of MEMS applications. Silicon also has significant advantages

engendered through its material properties. 0n single crystal form, silicon is an almost

perfect :oo"ean material, meaning that when it is fleed there is virtually no

hysteresis and hence almost no energy dissipation. The basic techni!ues for producing

all silicon based MEMS devices are deposition of material layers, patterning of these

layers by photolithography and then etching to produce the re!uired shapes.

POLYMERS

Even though the electronics industry provides an economy of scale for

the silicon industry, crystalline silicon is still a comple and relatively epensive

material to produce. Dolymers 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 in#ection 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. 5hile metals do not

have some of the advantages displayed by silicon in terms of mechanical properties,

when used within their limitations, metals can ehibit very high degrees of reliability.

Metals can be deposited by electroplating, evaporation, and sputtering processes.

6ommonly used metals include gold, nic"el, aluminum, chromium, titanium,

tungsten, platinum, and silver

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SECTION 4 MEMS DESIGNPROCESS

There are three basic building bloc"s in MEMS technology, which are,

Depo!"!o# P$o%e$the ability to deposit thin films of material on a substrate,

L!"&o'$(p&)$to apply a patterned mas" on top of the films by photolithograpic

imaging, and E"%&!#'$to etch the films selectively to the mas". 8 MEMS process is

usually a structured se!uence of these operations to form actual devices.

SECTION 4.1 DEPOSITION PROCESSES


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These processes eploit the creation of solid materials directly from chemical

reactions in gas and=or li!uid compositions or with the substrate material. The

solid material is usually not the only product formed by the reaction.

yproducts can include gases, li!uids and even other solids.

*. /epositions that happen because of a p&)!%(+reaction3

o Physical ,apor Deposition (DH/)

o 6asting

6ommon for all these processes are that the material deposited is physically

moved on to the substrate. 0n 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 thin" of them that way.

This is by no means an ehaustive list since technologies evolve continuously.

SECTION 4.1.1 CHEMICAL ,APOR DEPOSITION -C,D

0n 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 ta"es 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 6H/ technologies in MEMS are the Low

Pressure 6H/ (?D6H/) and Plasma Enhanced 6H/ (DE6H/). The ?D6H/ process

produces layers with ecellent uniformity of thic"ness and material characteristics.

The main problems with the process are the high deposition temperature (higher than

GI6) and the relatively slow deposition rate. The DE6H/ process can operate at

lower temperatures (down to I 6) than"s to the etra energy supplied to the gas

molecules by the plasma in the reactor. :owever, the !uality of the films tend to be

inferior to processes running at higher temperatures. Secondly, most DE6H/

deposition systems can only deposit the material on one side of the wafers on to -

wafers at a time. ?D6H/ systems deposit films on both sides of at least *9 wafers at a

time. 8 schematic diagram of a typical ?D6H/ reactor is shown in the figure below.

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!'0$e 1Typical hot-wall LPCVD reactor.

WHENDOWEWANTTOUSEC,D?

6H/ processes are ideal to use when you want a thin film with good

step coverage. 8 variety of materials can be deposited with this technology& however,

some of them are less popular with fabs because of hazardous by$products formed

during processing. The !uality of the material varies from process to process, however

a good rule of thumb is that higher process temperature yields a material with higher

!uality and less defects.

ELECTRODEPOSITION

This process is also "nown as 7electroplating7 and is typically restricted

to electrically conductive materials. There are basically two technologies for plating3

Electroplating and Electroless plating. 0n the electroplating process the substrate is

placed in a li!uid solution (electrolyte). 5hen an electrical potential is applied

between a conducting area on the substrate and a counter electrode (usually platinum)

in the li!uid, a chemical redo process ta"es place resulting in the formation of a layer

of material on the substrate and usually some gas generation at the counter electrode.

0n the electroless plating process a more comple chemical solution is

used, in which deposition happens spontaneously on any surface which forms a

sufficiently high electrochemical potential with the solution. This process is desirable

since it does not re!uire any eternal electrical potential and contact to the substrate



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during processing. nfortunately, it is also more difficult to control with regards to

film thic"ness and uniformity. 8 schematic diagram of a typical setup for

electroplating is shown in the figure below.

WHENDOWEWANTTOUSEELECTRODEPOSITION?

The electrodeposition process is well suited to ma"e films of metals

such as copper, gold and nic"el. The films can be made in any thic"ness from JKm

to LKm. The deposition is best controlled when used with an eternal electrical

potential, however, it re!uires electrical contact to the substrate when immersed in the

li!uid bath. 0n any process, the surface of the substrate must have an electrically

conducting coating before the deposition can be done.

!'0$e 2Typical setup for electrodeposition.

EPITAY

This technology is !uite similar to what happens in 6H/ processes,

however, if the substrate is an ordered semiconductor crystal (i.e. silicon, gallium

arsenide), it is possible with this process to continue building on the substrate with the

same crystallographic orientation with the substrate acting as a seed for the

deposition. 0f an amorphous=polycrystalline substrate surface is used, the film will

also be amorphous or polycrystalline.



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There are several technologies for creating the conditions inside a

reactor needed to support epitaial growth, of which the most important is ,apor

Phase Epitay (HDE). 0n this process, a number of gases are introduced in an

induction heated reactor where only the substrate is heated. The temperature of the

substrate typically must be at least 9+ of the melting point of the material to be

deposited.

8n advantage of epitay is the high growth rate of material, which

allows the formation of films with considerable thic"ness (LKm). Epitay is a

widely used technology for producing silicon on insulator (S


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THERMALOIDATION

This is one of the most basic deposition technologies. 0t is simply

oidation of the substrate surface in an oygen rich atmosphere. The temperature is

raised to BI 6$I 6 to speed up the process. This is also the only deposition

technology which actually consumes some of the substrate as it proceeds. The growth

of the film is spurned by diffusion of oygen into the substrate, which means the film

growth is actually downwards into the substrate. 8s the thic"ness of the oidized

layer increases, the diffusion of oygen to the substrate becomes more difficult

leading to a parabolic relationship between film thic"ness and oidation time for films

thic"er than Jnm. This process is naturally limited to materials that can be

oidized, and it can only form films that are oides of that material. This is the

classical process used to form silicon dioide on a silicon substrate. 8 schematic

diagram of a typical wafer oidation furnace is shown in the figure below.

WHENDOWEWANTTOUSETHERMALOIDATION?

5henever you can> This is a simple process, which unfortunately

produces films with somewhat limited use in MEMS components. 0t is typically used

to form films that are used for electrical insulation or that are used for other process

purposes later in a process se!uence.

!'0$e 4Typical wafer oxidation furnace.



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SECTION 4.1.2 PHYSICAL,APORDEPOSITION-P,D

DH/ covers a number of deposition technologies in which material is

released from a source and transferred to the substrate. The two most important

technologies are evaporation and sputtering.

WHENDOWEWANTTOUSEP,D?

DH/ comprises the standard technologies for deposition of metals. 0t is

far more common than 6H/ for metals since it can be performed at lower process

ris" and cheaper in regards to materials cost. The !uality of the films are inferior to

6H/, which for metals means higher resistivity and for insulators more defects andtraps. The step coverage is also not as good as 6H/.

The choice of deposition method (i.e. evaporation vs. sputtering) may in

many cases be arbitrary, and may depend more on what technology is available for

the specific material at the time.

E,APORATION

0n evaporation the substrate is placed inside a vacuum chamber, in

which a bloc" (source) of the material to be deposited is also located. The source

material is then heated to the point where it starts to boil and evaporate. The vacuum

is re!uired to allow the molecules to evaporate freely in the chamber, and they

subse!uently condense on all surfaces. This principle is the same for all evaporation

technologies, only the method used to the heat (evaporate) the source material differs.

There are two popular evaporation technologies, which are e$beam evaporation and

resistive evaporation each referring to the heating method. 0n e$beam evaporation, an

electron beam is aimed at the source material causing local heating and evaporation.

0n resistive evaporation, a tungsten boat, containing the source material, is heated

electrically with a high current to ma"e the material evaporate. Many materials are

restrictive in terms of what evaporation method can be used (i.e. aluminum is !uite

difficult to evaporate using resistive heating), which typically relates to the phase

transition properties of that material. 8 schematic diagram of a typical system for e$

beam evaporation is shown in the figure below.



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!'0$e Typical system for e-beam evaporation of materials.

SPUTTERING

Sputtering is a technology in which the material is released from the

source at much lower temperature than evaporation. The substrate is placed in a

vacuum chamber with the source material, named a target, and an inert gas (such as

argon) is introduced at low pressure. ;as plasma is struc" using an C1 power source,

causing the gas to become ionized. The ions are accelerated towards the surface of the

target, causing atoms of the source material to brea" off from the target in vapor form

and condense on all surfaces including the substrate. 8s for evaporation, the basic

principle of sputtering is the same for all sputtering technologies. The differences

typically relate to the manor in which the ion bombardment of the target is realized. 8

schematic diagram of a typical C1 sputtering system is shown in the figure below.

!'0$e Typical R sputterin! system.



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CASTING

0n this process the material to be deposited is dissolved in li!uid form in

a solvent. The material can be applied to the substrate by spraying or spinning.


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SECTION 4.2 LITHOGRAPHY

SECTION 4.2.1 PATTERNTRANSER

?ithography in the MEMS contet is typically the transfer of a pattern to

a photosensitive material by selective eposure to a radiation source such as light. 8

photosensitive material is a material that eperiences a change in its physical

properties when eposed to a radiation source. 0f we selectively epose a

photosensitive material to radiation (e.g. by mas"ing some of the radiation) the pattern

of the radiation on the material is transferred to the material eposed, as the properties

of the eposed and uneposed regions differs (as shown in figure ).

!'0$e 1Transfer of a pattern to a photosensitive material.

This discussion will focus on optical lithography, which is simply lithography using aradiation source with wavelength(s) in the visible spectrum.

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0n lithography for micromachining, the photosensitive material used is

typically a photoresist (also called resist, other photosensitive polymers are also used).

5hen resist is eposed to a radiation source of a specific a wavelength, the chemical

resistance of the resist to developer solution changes. 0f the resist is placed in a

developer solution after selective eposure to a light source, it will etch away one of

the two regions (eposed or uneposed). 0f the eposed material is etched away by the

developer and the uneposed region is resilient, the material is considered to be a

positive resist (shown in figure *a). 0f the eposed material is resilient to the

developer and the uneposed region is etched away, it is considered to be a negative

resist (shown in figure *b).

!'0$e 2a)Pattern definition in positive resist, b)Pattern definition in ne!ativeresist.

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?ithography is the principal mechanism for pattern definition in

micromachining. Dhotosensitive compounds are primarily organic, and do not

encompass the spectrum of materials properties of interest to micro$machinists.

:owever, as the techni!ue is capable of producing fine features in an economic

fashion, a photosensitive layer is often used as a temporary mas" when etching an

underlying layer, so that the pattern may be transferred to the underlying layer (shown

in figure a). Dhotoresist may also be used as a template for patterning material

deposited after lithography (shown in figure b). The resist is subse!uently etched

away, and the material deposited on the resist is 7lifted off7.

The deposition template (lift$off) approach for transferring a pattern

from resist to another layer is less common than using the resist pattern as an etch

mas". The reason for this is that resist is incompatible with most MEMS deposition

processes, usually because it cannot withstand high temperatures and may act as a

source of contamination.

!'0$e 3a)Pattern transfer from patterned photoresist to underlyin! layer by

etchin!, b)Pattern transfer from patterned photoresist to overlyin! layer by lift-off.



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/epending on the lithography e!uipment used, the feature on the mas"

used for registration of the mas" may be transferred to the wafer. 0n this case, it may

be important to locate the alignment mar"s such that they donFt effect subse!uent

wafer processing or device performance. 1or eample, the alignment mar" shown in

figure G will cease to eist after a through the wafer /C0E etch. Dattern transfer of the

mas" alignment features to the wafer may obliterate the alignment features on the

wafer. 0n this case the alignment mar"s should be designed to minimize this effect, or

alternately there should be multiple copies of the alignment mar"s on the wafer, so

there will be alignment mar"s remaining for other mas"s to be registered to.

!'0$e Transfer of mas# re!istration feature to substrate durin! litho!raphy%contact ali!ner&



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!'0$e Poor ali!nment mar# desi!n for a DR'( throu!h the wafer etches %crosshair is released and lost&.

8lignment mar"s may not necessarily be arbitrarily located on the

wafer, as the e!uipment used to perform alignment may have limited travel and

therefore only be able to align to features located within a certain region on the wafer

(as shown in figure @). The region location geometry and size may also vary with the

type of alignment, so the lithographic e!uipment and type of alignment to be used

should be considered before locating alignment mar"s. Typically two alignment

mar"s are used to align the mas" and wafer, one alignment mar" is sufficient to align

the mas" and wafer in and y, but it re!uires two mar"s (preferably spaced far apart)

to correct for fine offset in rotation.

8s there is no pattern on the wafer for the first pattern to align to, the

first pattern is typically aligned to the primary wafer flat (as shown in figure B).

/epending on the lithography e!uipment used, this may be done automatically, or by

manual alignment to an eplicit wafer registration feature on the mas"

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!'0$e 5Restriction of location of ali!nment mar#s based on e$uipment used.

.

!'0$e 6)as# ali!nment to the wafer flat.



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SECTION 4.2.3 EPOSURE

The eposure parameters re!uired in order to achieve accurate pattern

transfer from the mas" to the photosensitive layer depend primarily on the wavelength

of the radiation source and the dose re!uired to achieve the desired properties change

of the photoresist. /ifferent photoresists ehibit different sensitivities to different

wavelengths. The dose re!uired per unit volume of photoresist for good pattern

transfer is somewhat constant& however, the physics of the eposure process may

affect the dose actually received. 1or eample a highly reflective layer under the

photoresist may result in the material eperiencing a higher dose than if the

underlying layer is absorptive, as the photoresist is eposed both by the incident

radiation as well as the reflected radiation. The dose will also vary with resist

thic"ness.

There are also higher order effects, such as interference patterns in thic"

resist films on reflective substrates, which may affect the pattern transfer !uality and

sidewall properties.

8t the edges of pattern light is scattered and diffracted, so if an image is

overeposed, the dose received by photoresist at the edge that shouldnFt be eposed

may become significant. 0f we are using positive photoresist, this will result in the

photoresist image being eroded along the edges, resulting in a decrease in feature size

and a loss of sharpness or corners (as shown in figure 4). 0f we are using a negative

resist, the photoresist image is dilated, causing the features to be larger than desired,

again accompanied by a loss of sharpness of corners. 0f an image is severely

undereposed, the pattern may not be transferred at all, and in less sever cases the

results will be similar to those for overeposure with the results reversed for the

different polarities of resist.

0f the surface being eposed is not flat, the high$resolution image of the

mas" on the wafer may be distorted by the loss of focus of the image across the

varying topography. This is one of the limiting factors of MEMS lithography when

high aspect ratio features are present. :igh aspect ratio features also eperience

problems with obtaining even resist thic"ness coating, which further degrades patterntransfer and complicates the associated processing.

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!'0$e 7*ver and under-exposure of positive resist.

SECTION 4.2.4 THELITHOGRAPHYMODULE

Typically lithography is performed as part of a well$characterized

module, which includes the wafer surface preparation, photoresist deposition,

alignment of the mas" and wafer, eposure, develop and appropriate resist

conditioning. The lithography process steps need to be characterized as a se!uence in

order to ensure that the remaining resist at the end of the modules is an optimal image

of the mas", and has the desired sidewall profile.

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8 brief eplanation of the standard process steps included in a

lithography module is (in se!uence)3

Dehydration bake $ dehydrate the wafer to aid resist adhesion.

HMDS prime $ coating of wafer surface with adhesion promoter.

Resist spin/spray $ coating of the wafer with resist either by spinning or

spraying. Typically desire a uniform coat.

Soft bake $ drive off some of the solvent in the resist, may result in a

significant loss of mass of resist (and thic"ness). Ma"es resist more viscous.

Alignment$ align pattern on mas" to features on wafers.

Exposure $ pro#ection of mas" image on resist causing selective chemical

property change.

Post exposure bake $ ba"ing of resist to drive off further solvent content.

Develop $ selective removal of resist after eposure. sually a wet process.

Hard bake $ drive off most of the remaining solvent from the resist.

Desum$ removal of thin layer of resist scum that may occlude open regions

in pattern helps to open up corners.

5e ma"e a few assumptions about photolithography. 1irstly, we assumethat a well characterized module eists that3 prepares the wafer surface, deposits the

re!uisite resist thic"ness, aligns the mas" perfectly, eposes the wafer with the

optimal dosage, develops the resist under the optimal conditions, and ba"es the resist

for the appropriate times at the appropriate locations in the se!uence. nfortunately,

even if the module is eecuted perfectly, the properties of lithography are very feature

and topography dependent. 0t is therefore necessary for the designer to be aware of

certain limitations of lithography, as well as the information they should provide to

the technician performing the lithography.

The designer influences the lithographic process through their selections of materials,

topography and geometry. The material(s) upon which the resist is to be deposited is

important, as it affects the resist adhesion. The reflectivity and roughness of the layer

beneath the photoresist determines the amount of reflected and dispersed light present

during eposure. 0t is difficult to obtain a nice uniform resist coat across a surface

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with high topography, which complicates eposure and development as the resist has

different thic"ness in different locations.

!'0$e 18Litho!raphy tool depth of focus and surface topolo!y.

SECTION 4.3 ETCHINGPROCESSES

0n 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. 0n

general, there are two classes of etching processes3

. 5et etching where the material is dissolved when immersed in a chemical

solution

*. /ry etching where the material is sputtered or dissolved using reactive ions or

a vapor phase etchant

SECTION 4.3.1 WETETCHING

This is the simplest etching technology. 8ll it re!uires is a container

with a li!uid solution that will dissolve the material in !uestion. nfortunately, there

are complications since usually a mas" is desired to selectively etch the material.


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ehibit anisotropic etching in certain chemicals. 8nisotropic etching in contrast to

isotropic etching means different etches rates in different directions in the material.

The classic eample of this is the NL crystal plane sidewalls that appear when

etching a hole in a NL silicon wafer in a chemical such as potassium hydroide

(A


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!'0$e 1Difference between anisotropic and isotropic wet etchin!.

SECTION 4.3.2 DRYETCHING

The dry etching technology can split in three separate classes called

reactive ion etching (C0E), sputter etching, and vapor phase etching.

0n C0E, the substrate is placed inside a reactor in which several gases are

introduced. Dlasma is struc" in the gas miture using an C1 power source, brea"ing

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 "nown as the

chemical part of reactive ion etching. There is also a physical part which is similar in

nature to the sputtering deposition process. 0f the ions have high enough energy, they

can "noc" atoms out of the material to be etched without a chemical reaction. 0t is

very comple tas"s to develop dry etch processes that balance chemical and physical

etching, since there are many parameters to ad#ust. y 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. 8 schematic of a typical reactive ion etching system

is shown in the figure below.

8 special subclass of C0E which continues to grow rapidly in popularity

is deep C0E (/C0E). 0n this process, etch depths of hundreds of microns can be

achieved with almost vertical sidewalls. The primary technology is based on the so$

called 7osch process7, named after the ;erman company Cobert osch which filed

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

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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 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. 8s a result,

etching aspect ratios of 9 to can be achieved. The process can easily be used to

etch completely through a silicon substrate, and etch rates are $- times higher than

wet etching. Sputter etching is essentially C0E without reactive ions. The systems

used are very similar in principle to sputtering deposition systems. The big difference

is that substrate is now sub#ected to the ion bombardment instead of the material

target used in sputter deposition.

Hapor phase etching is another dry etching method, which can be done

with simpler e!uipment than what C0E re!uires. 0n this process the wafer to be etched

is placed inside a chamber, in which one or more gases are introduced. The material

to be etched is dissolved at the surface in a chemical reaction with the gas molecules.

The two most common vapor phase etching technologies are silicon dioide etching

using hydrogen fluoride (:1) and silicon etching using enon diflouride (Oe1*), both

of which are isotropic in nature. sually, care must be ta"en in the design of a vapor

phase process to not have bi$products form in the chemical reaction that condense on

the surface and interfere with the etching process.

WHENDOWEWANTTOUSEDRYETCHING?

The first thing you should note about this technology is that it is

epensive to run compared to wet etching. 0f you are concerned with feature

resolution in thin film structures or you need vertical sidewalls for deep etchings in

the substrate, you have to consider dry etching. 0f you are concerned about the price

of your process and device, you may want to minimize the use of dry etching. The 06

industry has long since adopted dry etching to achieve small features, but in many

cases feature size is not as critical in MEMS. /ry etching is an enabling technology,

which comes at a sometimes high cost.



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!'0$e 2Typical parallel-plate reactive ion etchin! system.

SECTION ABRICATION TECHNOLOGIES

The three characteristic features of MEMS fabrication technologies are

miniaturization, multiplicity, and microelectronics. Miniaturization enables the

production of compact, !uic"$response devices. Multiplicity refers to the batch

fabrication inherent in semiconductor processing, which allows thousands or millions

of components to be easily and concurrently fabricated. Microelectronics provides the

intelligence to MEMS and allows the monolithicmerger of sensors, actuators, and

logic to build closed$loop feedbac" components and systems. The successful

miniaturization and multiplicity of traditional electronics systems would not have

been possible without 06 fabrication technology. Therefore, 06 fabrication

technology, or microfabrication, has so far been the primary enabling technology for

the development of MEMS. Microfabrication provides a powerful tool for batchprocessing and miniaturization of mechanical systems into a dimensional domain not

accessible by conventional techni!ues. 1urthermore, microfabrication provides an

opportunity for integration of mechanical systems with electronics to develop high$

performance closed$loop$controlled MEMS.

8dvances in 06 technology in the last decade have brought about corresponding

progress in MEMS fabrication processes. Manufacturing processes allow for the

monolithic integration of microelectromechanical structures with driving, controlling,

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and signal$processing electronics. This integration promises to improve the

performance of micromechanical devices as well as reduce the cost of manufacturing,

pac"aging, and instrumenting these devices.

SECTION .1 IC ABRICATION

8ny discussion of MEMS re!uires a basic understanding of 06

fabrication technology, or microfabrication, the primary enabling technology for the

development of MEMS. The ma#or steps in 06 fabrication technology are3

!ilm gro"th3 sually, a polished Si wafer is used as the substrate, on which a

thin film is grown. The film, which may be epitaial Si, Si


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design is strongly coupled to the pac"aging re!uirements, which in turn are

dictated by the application environment.

SECTION .2 BULKMICROMACHININGANDWAERBONDING

ul" micromachining is an etension of 06 technology for the

fabrication of / structures. ul" micromachining of Si uses wet$ and dry$etching

techni!ues in con#unction with etch mas"s and etch stops to sculpt micromechanical

devices from the Si substrate. The two "ey capabilities that ma"e bul"

micromachining a viable technology are3

8nisotropic etchants of Si, such as ethylene$diamine and pyrocatechol (E/D),

potassium hydroide (A


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8 drawbac" of wet anisotropic etching is that the microstructure

geometry is defined by the internal crystalline structure of the substrate. Two

additional processing techni!ues have etended the range of traditional bul"

micromachining technology3 deep anisotropic dry etching and wafer bonding.

Ceactive 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

SECTION .3 SURACEMICROMACHINING

Surface micromachining enables the fabrication of comple

multicomponent integrated micromechanical structures that would not be possible

with traditional bul" micromachining. This techni!ue 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 attac" the structural parts. The most widely usedsurface micromachining techni!ue, polysilicon surface micromachining, uses Si


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8 better way to control the stress in polysilicon is through post

annealing, which involves the deposition of pure, fine$grained, compressive

polysilicon. 8nnealing the polysilicon after deposition at elevated temperatures can

change the film to be stress$free or tensile. The annealing temperature sets the filmFs

final stress. 8fter this, electronics can then be incorporated into polysilicon films

through selective doping, and hydrofluoric acid will not change the mechanical

properties of the material.

/eposition temperature and the filmFs silicon to nitride ratio can control

the stress of a silicon nitride (Si%-) film. The films can be deposited in compression,

stress$free, or in tension.

/eposition temperature and post annealing can control silicon dioide

(Si


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are also used for fabricating plating molds. The photolithography process is similar to

conventional photolithography, ecept that polyimide wor"s as a negative resist.

E9(*p+e A# !#0+!# p0*p :(;$!%("e< ;) %+(!% MEMS "e%o+o')

. DMD0%; MEMC8%E *. DMD0%; 6:8MEC

. 0%?ET -.


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PACKAGING

The pac"aging of MEMS devices and systems needs to improve

considerably from its current primitive state. MEMS pac"aging is more challenging

than 06 pac"aging due to the diversity of MEMS devices and the re!uirement that

many of these devices be in contact with their environment. 6urrently almost all

MEMS and %ano development efforts must develop a new and specialized pac"age

for each new device. Most companies find that pac"aging is the single most epensive

and time consuming tas" in their overall product development program. 8s for the

components themselves, numerical modeling and simulation tools for MEMS

pac"aging are virtually non$eistent. 8pproaches which allow designers to select

from a catalog of eisting standardized pac"ages for a new MEMS device without

compromising performance would be beneficial.

ABRICATIONKNOWLEDGERE=UIRED

6urrently the designer of a MEMS device re!uires a high level of

fabrication "nowledge in order to create a successful design.


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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.

8ccelerometers in consumer electronics devices such as game

controllers (%intendo 5ii), personal media players = cell phones (8pple iDhone) and

a number of /igital 6ameras (various 6anon /igital 0OS models). 8lso used in D6s

to par" the hard dis" head when free$fall is detected, to prevent damage and data loss.

iPod &ouh3 5hen the technology become sensitive. MEMS$based sensors are ideal

for a wide array of applications in consumer, communication, automotive and

industrial mar"ets.

The consumer mar"et has been a "ey driver for MEMS technology

success. 1or eample, in a mobile phone, MD=MD- player or D/8, these sensors

offer a new intuitive motion$based approach to navigation within and between pages.

0n game controllers, MEMS sensors allow the player to play #ust moving the

controller=pad& the sensor determines the motion.

INERTIALSENSORS

0nertial sensors are a type

of accelerometer and are one of the

principal commercial products thatutilize surface micromachining. They

are used as airbag$deployment sensors

in automobiles, and as tilt or shoc"

sensors. The application of theseaccelerometers to inertial measurement

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units is limited by the need to manually

align and assemble them into three$

ais systems, and by the resulting

alignment tolerances, their lac" of in$

chip analog$to$digital conversion

circuitry, and their lower limit of

sensitivity

sensitivity

.

MICROENGINES

8 three$level polysilicon micromachining process has enabled the

fabrication of devices with increased degrees of compleity. The process includes

three movable levels of polysilicon, each separated by a sacrificial oide layer, plus a

stationary level. Microengines can be used to drive the wheels of microcombination

loc"s. They can also be used in combination with a microtransmission to drive a pop$

up mirror out of a plane. This device is "nown as a micromirror.

SOMEOTHERCOMMERCIALAPPLICATIONSINCLUDE

0n"#et printers, which usepiezoelectricsor thermal bubble e#ection to deposit

in" on paper.

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8ccelerometers in modern cars for a large number of purposes including

airbagdeployment in collisions.

MEMS gyroscopes used in modern cars and other applications to detect yaw&

e.g. to deploy a roll over bar or trigger dynamic stability control.

Silicon pressure sensors e.g. car tire pressure sensors, and disposable blood

pressure sensors.

/isplays e.g. the /M/ chip in a pro#ector based on /?Dtechnology has on its

surface several hundred thousand micromirrors.


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MEMS 06 fabrication technologies have also allowed the manufacture

of advanced memory devices (nanochips=microchips).

8s a final eample, MEMS technology has been used in fabricating

vaporization microchambers for vaporizing li!uid microthrusters for nanosatellites.

The chamber is part of a microchannel with a height of *$ microns, made using

silicon and glass substrates

AD,ANTAGES O MEMS DISAD,ANTAGES O MEMS

Minimize energy and materials

use in manufacturing

6ost=performance advantages

0mproved reproducibility

0mproved accuracy and

reliability

0ncreased selectivity and

sensitivity

1arm establishment re!uires

huge investments

Micro$components are 6ostly

compare to macro$components

/esign includes very much

comple procedures

Drior "nowledge is needed to

integrate MEMS devices



42/48

SECTION 6 THE UTURE

Each of the three basic microsystems technology processes we have

seen, bul" micromachining, sacrificial surface micromachining, and

micromolding=?0;8, employs a different set of capital and intellectual resources.

MEMS manufacturing firms must choose which specific microsystems manufacturing

techni!ues to invest in.

MEMS technology has the potential to change our daily lives as much as

the computer has. :owever, the material needs of the MEMS field are at a

preliminary stage. 8 thorough understanding of the properties of eisting MEMS

materials is #ust as important as the development of new MEMS materials.

1uture MEMS applications will be driven by processes enabling greater

functionality through higher levels of electronic$mechanical integration and greater

numbers of mechanical components wor"ing alone or together to enable a comple

action. 1uture MEMS products will demand higher levels of electrical$mechanical

integration and more intimate interaction with the physical world. The high up$front

investment costs for large$volume commercialization of MEMS will li"ely limit the

initial involvement to larger companies in the 06 industry. 8dvancing from their

success as sensors, MEMS products will be embedded in larger non$MEMS systems,

such as printers, automobiles, and biomedical diagnostic e!uipment, and will enable

new and improved systems.

HOWTHEMEMS ANDNANOECHANGECANHELP?

The MEMS and %anotechnology Echange provides services that can

help with some of these problems.

5e ma"e a diverse catalog of processing capabilities available to our users, so

our users can eperiment with different fabrication technologies.


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SECTION 7 CONCLUSION

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. 0n 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,

makin them perfect for detectin to!ins in the environment.

"astly, the dynamic rane of MEMS based silicon ultrasonic

sensors have many advantaes over e!istin pie#oelectric sensors in non-

destructive evaluation, pro!imity sensin and as flo$ measurement.

Silicon ultrasonic sensors are also very effective immersion sensors and

provide improved performance in the areas of medical imain 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 epanding 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,

ma"ing possible the realization of complete )"e*>o#>(>%&!p.



44/48

MEMS will be the indispensable factor for advancing technology in the *st

century and it promises to create entirely new categories of products.

SECTION 18 SAMPLE SLIDES

INTRODUCTION

(- March *))4 ,

Introduction

What is MEMS Technology?MEMS technology is based on a number of tools and

methodologies, which are used to form small structures with

dimensions in the micrometer scale

MEMS fabrication approach that conveys the advantages of

miniaturization, multiple components, and microelectronics to the

design and construction of integrated Electromechanical systems

BUILDINGBLOCKSINMEMS

(- March *))4 9

Building Blocks In MEMS

How MEMS are prepared?

There are three basic building blocks in MEMS technology.

1. Deposition: The ability to deposit thin films of

material on a substrate.

2. Lithography: To apply a patterned mask on top of

the films by photolithograpic imaging.

3. Etching: To etch the films selectively to the mask.

MEMS DEPOSITIONTECHNOLOGY

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45/48

(- March *))4 G

MEMS Deposition Technology

MEMS deposition technology can be classified in two groups:

. !epositions that happen because of a chemical reaction:

Chemical Vapor Deposition (CVD)

Electrodeposition

Epitaxy

Thermal oxidation

". !epositions that happen because of aphysical reaction:

hysical Vapor Deposition (VD)

Casting

MEMS LETHOGRAPHYTECHNOLOGY

(- March *))4 @

MEMS Lithography Technology

MEMS lithography technology can be classified in two groups:

1. attern Trans!er

2. Lithographic "od#le

a. Dehydration bake and HMDS prime

b. Resist spin/spray and Soft bake

c. Alignment, Exposre

d. !ost exposre bake and Hard bake

e. Descm

MEMS ETCHINGTECHNOLOGY

(- March *))4 B

MEMS Etching Technology

There are two classes of etching process:

1. $et etching: The material is dissolved when immersed in a

chemical solution.

2. Dry etching: The material is sputtered or dissolved using

reactive ions or a vapor phase etchant.

MEMS ABRICATIONPROCESS

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46/48

** March *))4 ((

Microfarication !rocess

MEMS APPLICATION

(- March *))4 4

MEMS ApplicationsMicro-engines "Micro Reactors, #ibrating $heel

Inertial Sensors "#irtal Reality Systems

Accelerometers "Airbag Accelerometer

Pressure Sensors "Air !ressre Sensors

Optical MEMS "!ill %amera

Fluidic MEMS &%artridges for !rinters

Bio MEMS &'lood !ressre Sensors

MEMS Memory Units&(lash Memory

AD,ANTAGESANDDISAD,ANTAGES

(- March *))4 ()

"d#antages and Disad#antages

Minimize energy and materials

use in manufacturing

#ost$performance advantages

%mproved reproducibility

%mproved accuracy and

reliability

%ncreased selectivity and

sensitivity

&arm establishment re'uires

huge investments

Micro(components are #ostly

compare to macro(components

!esign includes very much

comple) procedures

*rior knowledge is needed to

integrate MEMS devices

CONCLUSION

/epartment of 0SE,/epartment of 0SE, 1ebruary$ 2une3 *41ebruary$ 2une3 *4 -G-G


47/48

(- March *))4 ((

$onclusion

The medical, wireless technology, biotechnology, computer,

automotive and aerospace industries are only a few that willbenefit greatly from MEMS.

This enabling technology promises to create entirely new

categories of products

MEMS will be the indispensable factor for advancing

technology in the 21st century

SECTION 11 REERENCES

O#+!#e Reo0$%e

Q S86 http3==www$bsac.eecs.ber"eley.edu=

Q /8CD8 MT< http3==www.darpa.mil=mto=

Q 0EEE Eplore http3==ieeepl ore.ieee.org=Oplore=/yn5el.#sp

Q 0ntroduction to Microengineering http3==www.dban"s.demon.co.u"=ueng=

Q MEMS 6learinghouse http3==www.memsnet.org=

Q MEMS Echange http3==www.mems$echange.org=

Q MEMS 0ndustry ;roup http3==www.memsindustrygroup.org=

Q M


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Q Micromachine /evices

Q Sensors Magazine

semianarmems.doc

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