mems (report)

Seminar on Micro-Electro-Mechanical Systems

SECTION 1 INTRODUCTION

Microelectromechanical systems (MEMS) are small integrated devices

or systems that combine electrical and mechanical components. They range in size

from the sub micrometer level to the millimeter level and there can be any number,

from a few to millions, in a particular system. MEMS extend the fabrication

techniques developed for the integrated circuit industry to add mechanical elements

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

Examples of MEMS device applications include inkjet-printer

cartridges, accelerometer, miniature robots, microengines, locks inertial sensors

microtransmissions, micromirrors, micro actuator (Mechanisms for activating process

control equipment by use of pneumatic, hydraulic, or electronic signals) optical

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

emerging as the existing technology is applied to the miniaturization and integration

of conventional devices.

These systems can sense, control, and activate mechanical processes on

the micro scale, and function individually or in arrays to generate effects on the macro

scale. The micro fabrication technology enables fabrication of large arrays of devices,

which individually perform simple tasks, but in combination can accomplish

complicated functions.

MEMS are not about any one application or device, nor are they defined

by a single fabrication process or limited to a few materials. They are a fabrication

approach that conveys the advantages of miniaturization, multiple components, and

microelectronics to the design and construction of integrated electromechanical

systems. MEMS are not only about miniaturization of mechanical systems; they are

also a new paradigm for designing mechanical devices and systems.

The MEMS industry has an estimated $10 billion market, and with a

projected 10-20% annual growth rate, it is estimated to have a $34 billion market in

2002. Because of the significant impact that MEMS can have on the commercial and

defense markets, industry and the federal government have both taken a special

interest in their development.

Department of ISE,Department of ISE, February- June: 2009 February- June: 2009 11


SECTION 1.1 WHAT IS MEMS TECHNOLOGY?

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

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

substrate through microfabrication technology. While the electronics are fabricated

using integrated circuit (IC) process sequences, the micromechanical components are

fabricated using compatible "micromachining" processes that selectively etch away

parts of the silicon wafer or add new structural layers to form the mechanical and

electromechanical devices.

Microelectronic integrated circuits can be thought of as the "brains" of a

system and MEMS augments this decision-making capability with "eyes" and "arms",

to allow microsystems to sense and control the environment. Sensors gather

information from the environment through measuring mechanical, thermal, biological,

chemical, optical, and magnetic phenomena. The electronics then process the

information derived from the sensors and through some decision making capability

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

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

Because MEMS devices are manufactured using batch fabrication techniques similar

to those used for integrated circuits, unprecedented levels of functionality, reliability,

and sophistication can be placed on a small silicon chip at a relatively low cost.



SECTION 1.2 WHAT ARE MEMS / MICROSYSTEMS?

MEMS is an abbreviation for Micro Electro Mechanical Systems. This

is a rapidly emerging technology combining electrical, electronic, mechanical, optical,

material, chemical, and fluids engineering disciplines. As the smallest commercially

produced "machines", MEMS devices are similar to traditional sensors and actuators

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

across, with individual features devices of the order of 1-100 micrometers across.

MEMS devices are manufactured either using processes based on Integrated Circuit

fabrication techniques and materials, or using new emerging fabrication technologies

such as micro injection molding. These former processes involve building the device

up layer by layer, involving several material depositions and etch steps. A typical

MEMS fabrication technology may have a 5 step process. Due to the limitations of

this "traditional IC" manufacturing process MEMS devices are substantially planar,

having very low aspect ratios (typically 5 -10 micro meters thick). It is important to

note that there are several evolving fabrication techniques that allow higher aspect

ratios such as deep x-ray lithography, electrodeposition, and micro injection molding.



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

also contain the electronics required to interact with the MEMS device. Due to the

small size and mass of the devices, MEMS components can be actuated

electrostatically (piezoelectric and bimetallic effects can also be used). The position of

MEMS components can also be sensed capacitively. Hence the MEMS electronics

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

conditioning circuitry. Connection with the macroscopic world is via wire bonding

and encapsulation into familiar BGA, MCM, surface mount, or leaded IC packages.

A common MEMS actuator is the "linear comb drive" (shown above) which consists

of rows of interlocking teeth; half of the teeth are attached to a fixed "beam", the other

half attach to a movable beam assembly. Both assemblies are electrically insulated.

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

repels the movable beam away from the fixed. Conversely, by applying opposite

polarity the parts are attracted. In this manner the comb drive can be moved "in" or

"out" and either DC or AC voltages can be applied. The small size of the parts (low

inertial mass) means that the drive has a very fast response time compared to its

macroscopic counterpart. The magnitude of electrostatic force is multiplied by the

voltage or more commonly the surface area and number of teeth. Commercial comb

drives have several thousand teeth, each tooth approximately 10 micro meters long.

Drive voltages are CMOS levels.

The linear push / pull motion of a comb drive can be converted into

rotational motion by coupling the drive to push rod and pinion on a wheel. In this

manner the comb drive can rotate the wheel in the same way a steam engine

functions!



SECTION 2 HISTORICAL BACKGROUND

The invention of the at Bell Telephone Laboratories in 1947 sparked a

fast-growing microelectronic technology. Jack Kilby of Texas Instruments built the

first Integrated circuit in 1958 using germanium (Ge) devices. It consisted of one

transistor, three Resistors, and one Capacitor. The IC was implemented on a sliver of

Ge that was glued on a glass slide. Later that same year Robert Noyce of Fairchild

Semiconductor announced the development of a Planar double-diffused Si IC. The

complete transition from the original Ge transistors with grown and alloyed junctions

to silicon (Si) planar double-diffused devices took about 10 years. The success of Si

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

(SiO2-sand), resulting in potentially lower material costs relative to other

Semiconductors

Since 1970, the complexity of ICs has doubled every two to three years.

The minimum dimension of manufactured devices and ICs has decreased from 20

microns to the sub micron levels of today. Current ultra-large-scale-integration

(ULSI) technology enables the fabrication of more than 10 million transistors and

capacitors on a typical chip.

IC fabrication is dependent upon sensors to provide input from the

surrounding environment, just as control systems need actuators in order to carry out

their desired functions. Due to the availability of sand as a material, much effort was

put into developing Si processing and characterization tools. These tools are now

being used to advance transducer technology. Today's IC technology far outstrips the

original sensors and actuators in performance, size, and cost.

Attention in this area was first focused on microsensor development.

The first microsensor, which has also been the most successful, was the Si pressure

sensor. In 1954 it was discovered that the piezoresistive effect in Ge and Si had the

potential to produce Ge and Si strain gauges with a gauge factor 10 to 20 times greater

than those based on metal films. As a result, Si strain gauges began to be developed

commercially in 1958. The first high-volume pressure sensor was marketed by

National Semiconductor in 1974. This sensor included a temperature controller for

constant-temperature operation. Improvements in this technology since then have



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

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

Around 1982, the term micromachining came into use to designate the

fabrication of micromechanical parts for Si microsensors. The micromechanical parts

were fabricated by selectively etching areas of the Si substrate away in order to leave

behind the desired geometries. Isotropic etching of Si was developed in the early

1960s for transistor fabrication. Anisotropic etching of Si then came about in 1967.

Various etch-stop techniques were subsequently developed to provide further process

flexibility.

These techniques also form the basis of the bulk micromachining

processing techniques. Bulk micromachining designates the point at which the bulk of

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

Bulk micromachining has remained a powerful technique for the fabrication of

micromechanical elements. However, the need for flexibility in device design and

performance improvement has motivated the development of new concepts and

techniques for micromachining.

Among these is the sacrificial layer technique, first demonstrated in

1965 by Nathanson and Wickstrom, in which a layer of material is deposited between

structural layers for mechanical separation and isolation. This layer is removed during

the release etch to free the structural layers and to allow mechanical devices to move

relative to the substrate. A layer is releasable when a sacrificial layer separates it from

the substrate. The application of the sacrificial layer technique to micromachining in

1985 gave rise to surface micromachining, in which the Si substrate is primarily used

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

Prior to 1987, these micromechanical structures were limited in motion.

During 1987-1988, a turning point was reached in micromachining when, for the first

time, techniques for integrated fabrication of mechanisms on Si were demonstrated.

During a series of three separate workshops on microdynamics held in 1987, the term

MEMS was coined. Equivalent terms for MEMS are microsystems-preferred in

Europe and micromachines-preferred in Japan.



SECTION 3 MEMS DESCRIPTION

MEMS technology can be implemented using a number of different

materials and manufacturing techniques; the choice of which will depend on the

device being created and the market sector in which it has to operate.

SILICON

The economies of scale, ready availability of cheap high-quality

materials and ability to incorporate electronic functionality make silicon attractive for

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

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

perfect Hookean material, meaning that when it is flexed there is virtually no

hysteresis and hence almost no energy dissipation. The basic techniques for producing

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

layers by photolithography and then etching to produce the required shapes.

POLYMERS

Even though the electronics industry provides an economy of scale for

the silicon industry, crystalline silicon is still a complex and relatively expensive

material to produce. Polymers on the other hand can be produced in huge volumes,

with a great variety of material characteristics. MEMS devices can be made from

polymers by processes such as injection moulding, embossing or stereolithography

and are especially well suited to microfluidic applications such as disposable blood

testing cartridges.

METALS

Metals can also be used to create MEMS elements. While metals do not

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

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

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

Commonly used metals include gold, nickel, aluminum, chromium, titanium,

tungsten, platinum, and silver



SECTION 4 MEMS DESIGN PROCESS

There are three basic building blocks in MEMS technology, which are,

Deposition Process-the ability to deposit thin films of material on a substrate,

Lithography-to apply a patterned mask on top of the films by photolithograpic

imaging, and Etching-to etch the films selectively to the mask. A MEMS process is

usually a structured sequence of these operations to form actual devices.

SECTION 4.1 DEPOSITION PROCESSES

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

deposit thin films of material. In this text we assume a thin film to have a thickness

anywhere between a few nanometers to about 100 micrometer

MEMS deposition technology can be classified in two groups:

1. Depositions that happen because of a chemical reaction:

o Chemical Vapor Deposition (CVD)

o Electrodeposition

o Epitaxy

o Thermal oxidation



These processes exploit the creation of solid materials directly from chemical

reactions in gas and/or liquid compositions or with the substrate material. The

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

Byproducts can include gases, liquids and even other solids.

2. Depositions that happen because of a physical reaction:

o Physical Vapor Deposition (PVD)

o Casting

Common for all these processes are that the material deposited is physically

moved on to the substrate. In other words, there is no chemical reaction which

forms the material on the substrate. This is not completely correct for casting

processes, though it is more convenient to think of them that way.

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

SECTION 4.1.1 CHEMICAL VAPOR DEPOSITION (CVD)

In this process, the substrate is placed inside a reactor to which a number

of gases are supplied. The fundamental principle of the process is that a chemical

reaction takes place between the source gases. The product of that reaction is a solid

material with condenses on all surfaces inside the reactor.

The two most important CVD technologies in MEMS are the Low

Pressure CVD (LPCVD) and Plasma Enhanced CVD (PECVD). The LPCVD process

produces layers with excellent uniformity of thickness and material characteristics.

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

600°C) and the relatively slow deposition rate. The PECVD process can operate at

lower temperatures (down to 300° C) thanks to the extra energy supplied to the gas

molecules by the plasma in the reactor. However, the quality of the films tend to be

inferior to processes running at higher temperatures. Secondly, most PECVD

deposition systems can only deposit the material on one side of the wafers on 1 to 4

wafers at a time. LPCVD systems deposit films on both sides of at least 25 wafers at a

time. A schematic diagram of a typical LPCVD reactor is shown in the figure below.



Figure 1: Typical hot-wall LPCVD reactor.

WHEN DO WE WANT TO USE CVD?

CVD processes are ideal to use when you want a thin film with good

step coverage. A 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 quality of the material varies from process to process, however

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

quality and less defects.

ELECTRODEPOSITION

This process is also known as "electroplating" and is typically restricted

to electrically conductive materials. There are basically two technologies for plating:

Electroplating and Electroless plating. In the electroplating process the substrate is

placed in a liquid solution (electrolyte). When an electrical potential is applied

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

in the liquid, a chemical redox process takes place resulting in the formation of a layer

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

In the electroless plating process a more complex 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 require any external electrical potential and contact to the substrate



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

film thickness and uniformity. A schematic diagram of a typical setup for

electroplating is shown in the figure below.

WHEN DO WE WANT TO USE ELECTRODEPOSITION?

The electrodeposition process is well suited to make films of metals

such as copper, gold and nickel. The films can be made in any thickness from ~1µm

to >100µm. The deposition is best controlled when used with an external electrical

potential, however, it requires electrical contact to the substrate when immersed in the

liquid bath. In any process, the surface of the substrate must have an electrically

conducting coating before the deposition can be done.

Figure 2: Typical setup for electrodeposition.

EPITAXY

This technology is quite similar to what happens in CVD 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. If an amorphous/polycrystalline substrate surface is used, the film will

also be amorphous or polycrystalline.



There are several technologies for creating the conditions inside a

reactor needed to support epitaxial growth, of which the most important is Vapor

Phase Epitaxy (VPE). In 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 50% of the melting point of the material to be

deposited.

An advantage of epitaxy is the high growth rate of material, which

allows the formation of films with considerable thickness (>100µm). Epitaxy is a

widely used technology for producing silicon on insulator (SOI) substrates. The

technology is primarily used for deposition of silicon. A schematic diagram of a

typical vapor phase epitaxial reactor is shown in the figure below.

Figure 3: Typical cold-wall vapor phase epitaxial reactor.

WHEN DO WE WANT TO USE EPITAXY?

This has been and continues to be an emerging process technology in

MEMS. The process can be used to form films of silicon with thicknesses of ~1µm to

>100µm. Some processes require high temperature exposure of the substrate, whereas

others do not require significant heating of the substrate. Some processes can even be

used to perform selective deposition, depending on the surface of the substrate.



THERMAL OXIDATION

This is one of the most basic deposition technologies. It is simply

oxidation of the substrate surface in an oxygen rich atmosphere. The temperature is

raised to 800° C-1100° C 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 oxygen into the substrate, which means the film

growth is actually downwards into the substrate. As the thickness of the oxidized

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

leading to a parabolic relationship between film thickness and oxidation time for films

thicker than ~100nm. This process is naturally limited to materials that can be

oxidized, and it can only form films that are oxides of that material. This is the

classical process used to form silicon dioxide on a silicon substrate. A schematic

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

WHEN DO WE WANT TO USE THERMAL OXIDATION?

Whenever you can! This is a simple process, which unfortunately

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

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

purposes later in a process sequence.

Figure 4: Typical wafer oxidation furnace.



SECTION 4.1.2 PHYSICAL VAPOR DEPOSITION (PVD)

PVD 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.

WHEN DO WE WANT TO USE PVD?

PVD comprises the standard technologies for deposition of metals. It is

far more common than CVD for metals since it can be performed at lower process

risk and cheaper in regards to materials cost. The quality of the films are inferior to

CVD, which for metals means higher resistivity and for insulators more defects and

traps. The step coverage is also not as good as CVD.

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.

EVAPORATION

In evaporation the substrate is placed inside a vacuum chamber, in

which a block (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 required to allow the molecules to evaporate freely in the chamber, and they

subsequently 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. In e-beam evaporation, an

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

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

electrically with a high current to make the material evaporate. Many materials are

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

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

transition properties of that material. A schematic diagram of a typical system for e-

beam evaporation is shown in the figure below.



Figure 5: 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. Gas plasma is struck using an RF 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 break off from the target in vapor form

and condense on all surfaces including the substrate. As 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. A

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

Figure 6: Typical RF sputtering system.



CASTING

In this process the material to be deposited is dissolved in liquid form in

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

the solvent is evaporated, a thin film of the material remains on the substrate. This is

particularly useful for polymer materials, which may be easily dissolved in organic

solvents, and it is the common method used to apply photoresist to substrates (in

photolithography). The thicknesses that can be cast on a substrate range all the way

from a single monolayer of molecules (adhesion promotion) to tens of micrometers.

In recent years, the casting technology has also been applied to form films of glass

materials on substrates. The spin casting process is illustrated in the figure below.

WHEN DO WE WANT TO USE CASTING?

Casting is a simple technology which can be used for a variety of

materials (mostly polymers). The control on film thickness depends on exact

conditions, but can be sustained within +/-10% in a wide range. If you are planning to

use photolithography you will be using casting, which is an integral part of that

technology. There are also other interesting materials such as polyimide and spin-on

glass which can be applied by casting.

Figure 7: The spin casting process as used for photoresist in photolithography.



SECTION 4.2 LITHOGRAPHY

SECTION 4.2.1 PATTERN TRANSFER

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

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 we selectively expose a

photosensitive material 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 (as shown in figure 1).

Figure 1: Transfer of a pattern to a photosensitive material.

This discussion will focus on optical lithography, which is simply lithography using a

radiation source with wavelength(s) in the visible spectrum.



In lithography for micromachining, the photosensitive material used is

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

When resist is exposed to a radiation source of a specific a wavelength, the chemical

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

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

the two regions (exposed or unexposed). If the exposed material is etched away by the

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

positive resist (shown in figure 2a). If the exposed material is resilient to the

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

resist (shown in figure 2b).

Figure 2: a) Pattern definition in positive resist, b) Pattern definition in negative resist.



Lithography is the principal mechanism for pattern definition in

micromachining. Photosensitive compounds are primarily organic, and do not

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

However, as the technique is capable of producing fine features in an economic

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

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

in figure 3a). Photoresist may also be used as a template for patterning material

deposited after lithography (shown in figure 3b). The resist is subsequently etched

away, and the material deposited on the resist is "lifted off".

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

mask. 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.

Figure 3: a) Pattern transfer from patterned photoresist to underlying layer by etching, b) Pattern transfer from patterned photoresist to overlying layer by lift-off.



Once the pattern has been transferred to another layer, the resist is

usually stripped. This is often necessary as the resist may be incompatible with further

micromachining steps. It also makes the topography more dramatic, which may

hamper further lithography steps.

SECTION 4.2.2 ALIGNMENT

In order to make useful devices the patterns for different lithography

steps that belong to a single structure must be aligned to one another. The first pattern

transferred to a wafer usually includes a set of alignment marks, which are high

precision features that are used as the reference when positioning subsequent patterns,

to the first pattern (as shown in figure 4). Often alignment marks are included in other

patterns, as the original alignment marks may be obliterated as processing progresses.

It is important for each alignment mark on the wafer to be labeled so it may be

identified, and for each pattern to specify the alignment mark to which it should be

aligned.

Figure 4: Use of alignment marks to register subsequent layers



Depending on the lithography equipment used, the feature on the mask

used for registration of the mask may be transferred to the wafer. In this case, it may

be important to locate the alignment marks such that they don't effect subsequent

wafer processing or device performance. For example, the alignment mark shown in

figure 6 will cease to exist after a through the wafer DRIE etch. Pattern transfer of the

mask alignment features to the wafer may obliterate the alignment features on the

wafer. In this case the alignment marks should be designed to minimize this effect, or

alternately there should be multiple copies of the alignment marks on the wafer, so

there will be alignment marks remaining for other masks to be registered to.

Figure 5: Transfer of mask registration feature to substrate during lithography (contact aligner)



Figure 6: Poor alignment mark design for a DRIE through the wafer etches (cross hair is released and lost).

Alignment marks may not necessarily be arbitrarily located on the

wafer, as the equipment 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 7). The region location geometry and size may also vary with the

type of alignment, so the lithographic equipment and type of alignment to be used

should be considered before locating alignment marks. Typically two alignment

marks are used to align the mask and wafer, one alignment mark is sufficient to align

the mask and wafer in x and y, but it requires two marks (preferably spaced far apart)

to correct for fine offset in rotation.

As 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 8).

Depending on the lithography equipment used, this may be done automatically, or by

manual alignment to an explicit wafer registration feature on the mask



Figure 7: Restriction of location of alignment marks based on equipment used.

.

Figure 8: Mask alignment to the wafer flat.



SECTION 4.2.3 EXPOSURE

The exposure parameters required in order to achieve accurate pattern

transfer from the mask to the photosensitive layer depend primarily on the wavelength

of the radiation source and the dose required to achieve the desired properties change

of the photoresist. Different photoresists exhibit different sensitivities to different

wavelengths. The dose required per unit volume of photoresist for good pattern

transfer is somewhat constant; however, the physics of the exposure process may

affect the dose actually received. For example a highly reflective layer under the

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

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

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

thickness.

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

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

sidewall properties.

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

overexposed, the dose received by photoresist at the edge that shouldn't be exposed

may become significant. If 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 9). If 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. If an image is severely

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

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

different polarities of resist.

If the surface being exposed is not flat, the high-resolution image of the

mask 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. High aspect ratio features also experience

problems with obtaining even resist thickness coating, which further degrades pattern

transfer and complicates the associated processing.



Figure 9: Over and under-exposure of positive resist.

SECTION 4.2.4 THE LITHOGRAPHY MODULE

Typically lithography is performed as part of a well-characterized

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

alignment of the mask and wafer, exposure, develop and appropriate resist

conditioning. The lithography process steps need to be characterized as a sequence in

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

of the mask, and has the desired sidewall profile.



A brief explanation of the standard process steps included in a

lithography module is (in sequence):

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 thickness). Makes resist more viscous.

Alignment - align pattern on mask to features on wafers.

Exposure - projection of mask image on resist causing selective chemical

property change.

Post exposure bake - baking of resist to drive off further solvent content.

Develop - selective removal of resist after exposure. Usually a wet process.

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

Descum - removal of thin layer of resist scum that may occlude open regions

in pattern helps to open up corners.

We make a few assumptions about photolithography. Firstly, we assume

that a well characterized module exists that: prepares the wafer surface, deposits the

requisite resist thickness, aligns the mask perfectly, exposes the wafer with the

optimal dosage, develops the resist under the optimal conditions, and bakes the resist

for the appropriate times at the appropriate locations in the sequence. Unfortunately,

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

and topography dependent. It 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 exposure. It is difficult to obtain a nice uniform resist coat across a surface

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

different thickness in different locations.



Figure 10: Lithography tool depth of focus and surface topology.

SECTION 4.3 ETCHING PROCESSES

In order to form a functional MEMS structure on a substrate, it is

necessary to etch the thin films previously deposited and/or the substrate itself. In

general, there are two classes of etching processes:

1. Wet etching where the material is dissolved when immersed in a chemical

solution

2. Dry etching where the material is sputtered or dissolved using reactive ions or

a vapor phase etchant

SECTION 4.3.1 WET ETCHING

This is the simplest etching technology. All it requires is a container

with a liquid solution that will dissolve the material in question. Unfortunately, there

are complications since usually a mask is desired to selectively etch the material. One

must find a mask that will not dissolve or at least etches much slower than the

material to be patterned. Secondly, some single crystal materials, such as silicon,

exhibit anisotropic etching in certain chemicals. Anisotropic etching in contrast to

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

The classic example of this is the <111> crystal plane sidewalls that appear when



etching a hole in a <100> silicon wafer in a chemical such as potassium hydroxide

(KOH). The result is a pyramid shaped hole instead of a hole with rounded sidewalls

with a isotropic etchant. The principle of anisotropic and isotropic wet etching is

illustrated in the figure below.

WHEN DO WE WANT TO USE WET ETCHING?

This is a simple technology, which will give good results if you can find

the combination of etchant and mask material to suit your application. Wet etching

works very well for etching thin films on substrates, and can also be used to etch the

substrate itself. The problem with substrate etching is that isotropic processes will

cause undercutting of the mask layer by the same distance as the etch depth.

Anisotropic processes allow the etching to stop on certain crystal planes in the

substrate, but still results in a loss of space, since these planes cannot be vertical to the

surface when etching holes or cavities. If this is a limitation for you, you should

consider dry etching of the substrate instead. However, keep in mind that the cost per

wafer will be 1-2 orders of magnitude higher to perform the dry etching

If you are making very small features in thin films (comparable to the

film thickness), you may also encounter problems with isotropic wet etching, since

the undercutting will be at least equal to the film thickness. With dry etching it is

possible etch almost straight down without undercutting, which provides much higher

resolution.

Figure 1: Difference between anisotropic and isotropic wet etching.



SECTION 4.3.2 DRY ETCHING

The dry etching technology can split in three separate classes called

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

In RIE, the substrate is placed inside a reactor in which several gases are

introduced. Plasma is struck in the gas mixture using an RF power source, breaking

the gas molecules into ions. The ion is accelerated towards, and reacts at, the surface

of the material being etched, forming another gaseous material. This is known as the

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

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

can knock atoms out of the material to be etched without a chemical reaction. It is

very complex tasks to develop dry etch processes that balance chemical and physical

etching, since there are many parameters to adjust. By changing the balance it is

possible to influence the anisotropy of the etching, since the chemical part is isotropic

and the physical part highly anisotropic the combination can form sidewalls that have

shapes from rounded to vertical. A schematic of a typical reactive ion etching system

is shown in the figure below.

A special subclass of RIE which continues to grow rapidly in popularity

is deep RIE (DRIE). In this process, etch depths of hundreds of microns can be

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

called "Bosch process", named after the German company Robert Bosch which filed

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

The first gas composition creates a polymer on the surface of the substrate, and the

second gas composition etches the substrate. The polymer is immediately sputtered

away by the physical part of the etching, but only on the horizontal surfaces and 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 3-4 times higher than

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

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

is that substrate is now subjected to the ion bombardment instead of the material

target used in sputter deposition.



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

with simpler equipment than what RIE requires. In 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 dioxide etching

using hydrogen fluoride (HF) and silicon etching using xenon diflouride (XeF2), both

of which are isotropic in nature. Usually, care must be taken 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.

WHEN DO WE WANT TO USE DRY ETCHING?

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

expensive to run compared to wet etching. If 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. If you are concerned about the price

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

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

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

which comes at a sometimes high cost.

Figure 2: Typical parallel-plate reactive ion etching system.



SECTION 5 FABRICATION TECHNOLOGIES

The three characteristic features of MEMS fabrication technologies are

miniaturization, multiplicity, and microelectronics. Miniaturization enables the

production of compact, quick-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 monolithic merger of sensors, actuators, and

logic to build closed-loop feedback components and systems. The successful

miniaturization and multiplicity of traditional electronics systems would not have

been possible without IC fabrication technology. Therefore, IC fabrication

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

the development of MEMS. Microfabrication provides a powerful tool for batch

processing and miniaturization of mechanical systems into a dimensional domain not

accessible by conventional techniques. Furthermore, microfabrication provides an

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

performance closed-loop-controlled MEMS.

Advances in IC 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,

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

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

packaging, and instrumenting these devices.

SECTION 5.1 IC FABRICATION

Any discussion of MEMS requires a basic understanding of IC

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

development of MEMS. The major steps in IC fabrication technology are:

Film growth: Usually, a polished Si wafer is used as the substrate, on which a

thin film is grown. The film, which may be epitaxial Si, SiO2, silicon nitride

(Si3N4), polycrystalline Si, or metal, is used to build both active or passive

components and interconnections between circuits.


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Doping: To modulate the properties of the device layer, a low and controllable

level of an atomic impurity may be introduced into the layer by thermal

diffusion or ion implantation.

Lithography: A pattern on a mask is then transferred to the film by means of a

photosensitive (i.e., light sensitive) chemical known as a photoresist. The

process of pattern generation and transfer is called photolithography. A typical

mask consists of a glass plate coated with a patterned chromium (Cr) film.

Etching: Next is the selective removal of unwanted regions of a film or

substrate for pattern delineation. Wet chemical etching or dry etching may be

used. Etch-mask materials are used at various stages in the removal process to

selectively prevent those portions of the material from being etched. These

materials include SiO2, Si3N4, and hard-baked photoresist.

Dicing: The finished wafer is sawed or machined into small squares, or dice,

from which electronic components can be made.

Packaging: The individual sections are then packaged, a process that involves

physically locating, connecting, and protecting a device or component. MEMS

design is strongly coupled to the packaging requirements, which in turn are

dictated by the application environment.



SECTION 5.2 BULK MICROMACHINING AND WAFER BONDING

Bulk micromachining is an extension of IC technology for the

fabrication of 3D structures. Bulk micromachining of Si uses wet- and dry-etching

techniques in conjunction with etch masks and etch stops to sculpt micromechanical

devices from the Si substrate. The two key capabilities that make bulk

micromachining a viable technology are:

Anisotropic etchants of Si, such as ethylene-diamine and pyrocatechol (EDP),

potassium hydroxide (KOH), and hydrazine (N2H4). These preferentially etch

single crystal Si along given crystal planes.

Etch masks and etch-stop techniques that can be used with Si anisotropic

etchants to selectively prevent regions of Si from being etched. Good etch

masks are provided by SiO2 and Si3N4, and some metallic thin films such as Cr

and Au (gold).

A drawback of wet anisotropic etching is that the microstructure

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

additional processing techniques have extended the range of traditional bulk



micromachining technology: deep anisotropic dry etching and wafer bonding.

Reactive gas plasmas can perform deep anisotropic dry etching of Si wafers, up to a

depth of a few hundred microns, while maintaining smooth vertical sidewall profiles.

The other technology, wafer bonding, permits a Si substrate to be attached to another

substrate, typically Si or glass

SECTION 5.3 SURFACE MICROMACHINING

Surface micromachining enables the fabrication of complex

multicomponent integrated micromechanical structures that would not be possible

with traditional bulk micromachining. This technique encases specific structural parts

of a device in layers of a sacrificial material during the fabrication process. The

substrate wafer is used primarily as a mechanical support on which multiple

alternating layers of structural and sacrificial material are deposited and patterned to

realize micromechanical structures. The sacrificial material is then dissolved in a

chemical etchant that does not attack the structural parts. The most widely used

surface micromachining technique, polysilicon surface micromachining, uses SiO2 as

the sacrificial material and polysilicon as the structural material.

At the University of Wisconsin at Madison, polysilicon surface

micromachining research started in the early 1980s in an effort to create high-

precision micro pressure sensors. The control of the internal stresses of a thin film is

important for the fabrication of microelectromechanical structures. The

microelectronic fabrication industry typically grows polysilicon, silicon nitride, and

silicon dioxide films using recipes that minimize time. Unfortunately, a deposition

process that is optimized to speed does not always create a low internal stress film. In

fact, most of these films have internal stresses that are highly compressive. A

freestanding plate of highly compressive polysilicon that is held at all its edges will

buckle. This is highly undesirable. The solution is to modify the film deposition

process to control the internal stress by making it stress-free or slightly tensile.

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

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

polysilicon. Annealing the polysilicon after deposition at elevated temperatures can

change the film to be stress-free or tensile. The annealing temperature sets the film's



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

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

properties of the material.

Deposition temperature and the film's silicon to nitride ratio can control

the stress of a silicon nitride (Si3N4) film. The films can be deposited in compression,

stress-free, or in tension.

Deposition temperature and post annealing can control silicon dioxide

(SiO2) film stress. Because it is difficult to control the stress of SiO2 accurately, SiO2

is typically not used as a mechanical material by itself, but as electronic isolation or as

a sacrificial layer under polysilicon.

SECTION 5.4 MICRO MOLDING

In the micromolding process, microstructures are fabricated using molds

to define the deposition of the structural layer. The structural material is deposited

only in those areas constituting the microdevice structure, in contrast to bulk and

surface micromachining, which feature blanket deposition of the structural material

followed by etching to realize the final device geometry. After the structural layer

deposition, the mold is dissolved in a chemical etchant that does not attack the

structural material. One of the most prominent micromolding processes is the LIGA

process. LIGA is a German acronym standing for lithographie, galvanoformung, und

abformung (lithography, electroplating, and molding). This process can be used for

the manufacture of high-aspect-ratio 3D microstructures in a wide variety of

materials, such as metals, polymers, ceramics, and glasses. Photosensitive polyimides

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

conventional photolithography, except that polyimide works as a negative resist.

Example: An insulin pump fabricated by classic MEMS technology


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1. PUMPING MEMBRANE 2. PUMPING CHAMBER

3. INLET 4. OUTLET

5. LARGE MESA 6. UPPER GLASS PLATE

7. BOTTOM GLASS PLATE 8. PATTERNED THIN LAYER

SECTION 6 CURRENT CHALLENGES

MEMS and Nanotechnology is currently used in low- or medium-

volume applications. Some of the obstacles preventing its wider adoption are:

LIMITED OPTIONS

Most companies who wish to explore the potential of MEMS and

Nanotechnology have very limited options for prototyping or manufacturing devices,

and have no capability or expertise in microfabrication technology. Few companies

will build their own fabrication facilities because of the high cost. A mechanism

giving smaller organizations responsive and affordable access to MEMS and Nano

fabrication is essential.

PACKAGING

The packaging of MEMS devices and systems needs to improve

considerably from its current primitive state. MEMS packaging is more challenging



than IC packaging due to the diversity of MEMS devices and the requirement that

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

MEMS and Nano development efforts must develop a new and specialized package

for each new device. Most companies find that packaging is the single most expensive

and time consuming task in their overall product development program. As for the

components themselves, numerical modeling and simulation tools for MEMS

packaging are virtually non-existent. Approaches which allow designers to select

from a catalog of existing standardized packages for a new MEMS device without

compromising performance would be beneficial.

FABRICATION KNOWLEDGE REQUIRED

Currently the designer of a MEMS device requires a high level of

fabrication knowledge in order to create a successful design. Often the development

of even the most mundane MEMS device requires a dedicated research effort to find a

suitable process sequence for fabricating it. MEMS device design needs to be

separated from the complexities of the process sequence.

SECTION 7 APPLICATIONS

PRESSURE SENSORS

MEMS pressure microsensors typically have a flexible diaphragm that

deforms in the presence of a pressure difference. The deformation is converted to an

electrical signal appearing at the sensor output. A pressure sensor can be used to sense

the absolute air pressure within the intake manifold of an automobile engine, so that

the amount of fuel required for each engine cylinder can be computed.

ACCELEROMETERS

Accelerometers are acceleration sensors. An inertial mass suspended by

springs is acted upon by acceleration forces that cause the mass to be deflected from

its initial position. This deflection is converted to an electrical signal, which appears

at the sensor output. The application of MEMS technology to accelerometers is a

relatively new development.



Accelerometers in consumer electronics devices such as game

controllers (Nintendo Wii), personal media players / cell phones (Apple iPhone ) and

a number of Digital Cameras (various Canon Digital IXUS models). Also used in PCs

to park the hard disk head when free-fall is detected, to prevent damage and data loss.

iPod Touch: When the technology become sensitive. MEMS-based sensors are ideal

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

industrial markets.

The consumer market has been a key driver for MEMS technology

success. For example, in a mobile phone, MP3/MP4 player or PDA, these sensors

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

In game controllers, MEMS sensors allow the player to play just moving the

controller/pad; the sensor determines the motion.

INERTIAL SENSORS

Inertial sensors are a type

of accelerometer and are one of the

principal commercial products that

utilize surface micromachining. They

are used as airbag-deployment sensors

in automobiles, and as tilt or shock

sensors. The application of these

accelerometers to inertial measurement

units is limited by the need to manually

align and assemble them into three-

axis systems, and by the resulting

alignment tolerances, their lack of in-

chip analog-to-digital conversion

circuitry, and their lower limit of


http://en.wikipedia.org/wiki/IPhone


sensitivity

.

MICROENGINES

A three-level polysilicon micromachining process has enabled the

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

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

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

locks. They can also be used in combination with a microtransmission to drive a pop-

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

SOME OTHER COMMERCIAL APPLICATIONS INCLUDE:

Inkjet printers, which use piezoelectrics or thermal bubble ejection to deposit

ink on paper.

Accelerometers in modern cars for a large number of purposes including

airbag deployment 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.


http://en.wikipedia.org/wiki/Airbag

http://en.wikipedia.org/wiki/Accelerometer

http://en.wikipedia.org/wiki/Piezoelectric

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Displays e.g. the DMD chip in a projector based on DLP technology has on its

surface several hundred thousand micromirrors.

Optical switching technology which is used for switching technology and

alignment for data communications.

Bio-MEMS applications in medical and health related technologies from Lab-

On-Chip to MicroTotalAnalysis (biosensor, chemosensor).

Interferometric modulator display (IMOD) applications in consumer

electronics. Used to create interferometric modulation - reflective display technology

as found in mirasol displays.

MEMS IC fabrication technologies have also allowed the manufacture

of advanced memory devices (nanochips/microchips).


http://en.wikipedia.org/wiki/Optical_switching

http://en.wikipedia.org/wiki/DLP


As a final example, MEMS technology has been used in fabricating

vaporization microchambers for vaporizing liquid microthrusters for nanosatellites.

The chamber is part of a microchannel with a height of 2-10 microns, made using

silicon and glass substrates

ADVANTAGES OF MEMS DISADVANTAGES OF MEMS

Minimize energy and materials

use in manufacturing

Cost/performance advantages

Improved reproducibility

Improved accuracy and

reliability

Increased selectivity and

sensitivity

Farm establishment requires

huge investments

Micro-components are Costly

compare to macro-components

Design includes very much

complex procedures

Prior knowledge is needed to

integrate MEMS devices

SECTION 8 THE FUTURE

Each of the three basic microsystems technology processes we have

seen, bulk micromachining, sacrificial surface micromachining, and



micromolding/LIGA, employs a different set of capital and intellectual resources.

MEMS manufacturing firms must choose which specific microsystems manufacturing

techniques to invest in.

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

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

preliminary stage. A thorough understanding of the properties of existing MEMS

materials is just as important as the development of new MEMS materials.

Future MEMS applications will be driven by processes enabling greater

functionality through higher levels of electronic-mechanical integration and greater

numbers of mechanical components working alone or together to enable a complex

action. Future 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 likely limit the

initial involvement to larger companies in the IC industry. Advancing from their

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

such as printers, automobiles, and biomedical diagnostic equipment, and will enable

new and improved systems.

HOW THE MEMS AND NANO EXCHANGE CAN HELP?

The MEMS and Nanotechnology Exchange provides services that can

help with some of these problems.

We make a diverse catalog of processing capabilities available to our users, so

our users can experiment with different fabrication technologies. Our users

don't have to build their own fabrication facilities, and

Our web-based interface lets users assemble process sequences and submit

them for review by the MEMS and Nanotechnology Exchange's engineers and

fabrication sites.

SECTION 9 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. In addition to accelerometers and gyroscopes, micro-sized tire

pressure systems are now standard issues in new vehicles, putting MEMS pressure

sensors in high demand. Such micro-sized pressure sensors can be used by physicians

and surgeons in a telemetry system to measure blood pressure at a stet, allowing early

detection of hypertension and restenosis. Alternatively, the detection of

bio molecules can benefit most from MEMS-based biosensors. Medical

applications include the detection of DNA sequences and metabolites.

MEMS biosensors can also monitor several chemicals simultaneously,

making them perfect for detecting toxins in the environment.

Lastly, the dynamic range of MEMS based silicon ultrasonic

sensors have many advantages over existing piezoelectric sensors in non-

destructive evaluation, proximity sensing and gas flow measurement.

Silicon ultrasonic sensors are also very effective immersion sensors and

provide improved performance in the areas of medical imaging and liquid

level detection.

The medical, wireless technology, biotechnology, computer, automotive and

aerospace industries are only a few that will benefit greatly from MEMS.

This enabling technology allowing the development of smart products,

augmenting the computational ability of microelectronics with the perception

and control capabilities of microsensors and microactuators and expanding the

space of possible designs and applications.

MEMS devices are manufactured for unprecedented levels of functionality,

reliability, and sophistication can be placed on a small silicon chip at a

relatively low cost.

MEMS promises to revolutionize nearly every product category by bringing

together silicon-based microelectronics with micromachining technology,

making possible the realization of complete systems-on-a-chip.

MEMS will be the indispensable factor for advancing technology in the 21st

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



SECTION 10 SAMPLE SLIDES

INTRODUCTION

BUILDING BLOCKS IN MEMS

14 March 2009 5

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 DEPOSITION TECHNOLOGY

14 March 2009 6

MEMS Deposition TechnologyMEMS deposition technology can be classified in two groups:

1. Depositions that happen because of a chemical reaction: Chemical Vapor Deposition (CVD)

Electrodeposition

Epitaxy

Thermal oxidation

2. Depositions that happen because of a physical reaction: Physical Vapor Deposition (PVD)

Casting

MEMS LETHOGRAPHY TECHNOLOGY



14 March 2009 7

MEMS Lithography Technology

MEMS lithography technology can be classified in two groups:1. Pattern Transfer2. Lithographic Module

a. Dehydration bake and HMDS primeb. Resist spin/spray and Soft bakec. Alignment, Exposured. Post exposure bake and Hard bakee. Descum

MEMS ETCHING TECHNOLOGY

14 March 2009 8

MEMS Etching Technology

There are two classes of etching process:

1. Wet 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 FABRICATION PROCESS

22 March 2009 11

Microfabrication Process

MEMS APPLICATION



14 March 2009 9

MEMS ApplicationsMicro-engines –Micro Reactors, Vibrating Wheel

Inertial Sensors –Virtual Reality Systems

Accelerometers –Airbag Accelerometer

Pressure Sensors –Air Pressure Sensors

Optical MEMS –Pill Camera

Fluidic MEMS -Cartridges for Printers

Bio MEMS -Blood Pressure Sensors

MEMS Memory Units-Flash Memory

ADVANTAGES AND DISADVANTAGES

14 March 2009 10

Advantages and Disadvantages

Minimize energy and materials use in manufacturing

Cost/performance advantages

Improved reproducibility

Improved accuracy and reliability

Increased selectivity and sensitivity

Farm establishment requires huge investments

Micro-components are Costly compare to macro-components

Design includes very much complex procedures

Prior knowledge is needed to integrate MEMS devices

CONCLUSION

14 March 2009 11

ConclusionThe medical, wireless technology, biotechnology, computer, automotive and aerospace industries are only a few that will benefit 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 REFERENCES

Online Resources

• BSAC http://www-bsac.eecs.berkeley.edu/

• DARPA MTO http://www.darpa.mil/mto/

• IEEE Explore http://ieeexpl ore.ieee.org/Xplore/DynWel.jsp

• Introduction to Microengineering http://www.dbanks.demon.co.uk/ueng/

• MEMS Clearinghouse http://www.memsnet.org/

• MEMS Exchange http://www.mems-exchange.org/

• MEMS Industry Group http://www.memsindustrygroup.org/

• MOSIS http://www.mosis.org/

• MUMPS http://www.memscap.com/memsrus/crmumps.html

• Stanford Centre for Integrated Systems http://www-cis.stanford.edu/

• USPTO http://www.uspto.gov/

• Trimmer http://www.trimmer.net/

• Yole Development http://www.yole.fr/pagesAn/accueil.asp

Journals

• Journal of Micromechanical Systems

• Journal of Micromechanics and Microengineering

• Micromachine Devices

• Sensors Magazine


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