Tài liệu An Introduction to Microelectromechanical Systems Engineering P2 ppt

Silicon, however, remains the material of choice for microelectromechanical systems.. Silicon-Compatible Material System The silicon-compatible material system encompasses, in addition t

• Journal of Micromechanics and Microengineering (JMM): a

peer-reviewed scientific journal published by the Institute of Physics of Bristol, United Kingdom

• Sensors Magazine: a trade journal with emphasis on practical and commercial

applications It is published by Helmers Publishing, Inc., of Peterborough, New Hampshire

• MST News: a newsletter on microsystems and MEMS It is published by

VDI/VDE Technologiezentrum Informationstechnik GmbH of Teltow, Ger-many, and is available on-line

• Micro/Nano Newsletter: a publication companion to “R&D Magazine”

with news and updates on micromachined devices and nanoscale-level technologies It is published by Reed Business Information of Morris Plains, New Jersey

• Small Times Magazine: a trade journal reporting on MEMS, MST, and

nano-technology It is published by Small Times Media, LLC, a subsidiary company

of Ardesta, LLC, of Ann Arbor, Michigan

List of Conferences and Meetings

Several conferences cover advances in MEMS or incorporate program sessions on micromachined sensors and actuators The following list gives a few examples:

• International Conference on Solid-State Sensors and Actuators (Transducers):

held in odd years and rotates sequentially between North America, Asia, and Europe

• Solid-State Sensor and Actuator Workshop (Hilton-Head): held in even years

in Hilton Head Island, South Carolina, and sponsored by the Transducers Research Foundation of Cleveland, Ohio

Table 1.4 List of a Few Government and Nongovernment Organizations with Useful On-line Resources

clearinghouse

www.memsnet.org MEMS Exchange Reston, VA Intermediary broker for

foundry services

www.mems-exchange.org MEMS Industry Group Pittsburgh, PA Industrial consortium www.memsindustrygroup.org NIST Gaithersburg, MD Sponsored U.S.

government projects

www.atp.nist.gov

government projects

www.darpa.mil IDA Alexandria, VA Insertion in military

applications

mems.ida.org NEXUS Grenoble, France European MST network www.nexus-mems.com VDI/VDE – IT Teltow, Germany Association of German

Engineers

www.mstonline.de AIST – MITI Tokyo, Japan The “Micromachine Project”

in Japan

www.aist.go.jp ATIP Albuquerque, NM Asian Technology

Information Project

www.atip.org

• Micro Electro Mechanical Systems Workshop (MEMS): an international

meeting held annually and sponsored by the IEEE

• International Society for Optical Engineering (SPIE): regular conferences

held in the United States and sponsored by SPIE of Bellingham, Washington

• Micro Total Analysis Systems ( µTAS): a conference focusing on

microanalyti-cal and chemimicroanalyti-cal systems It is an annual meeting and alternates between North America and Europe

Summary

Microelectromechanical structures and systems are miniature devices that enable the operation of complex systems They exist today in many environments, espe-cially automotive, medical, consumer, industrial, and aerospace Their potential for future penetration into a broad range of applications is real, supported by strong development activities at many companies and institutions The technology consists

of a large portfolio of design and fabrication processes (a toolbox), many borrowed from the integrated circuit industry The development of MEMS is inherently inter-disciplinary, necessitating an understanding of the toolbox as well as of the end application

References

[1] Dr Albert Pisano, in presentation material distributed by the U.S DARPA, available at http://www.darpa.mil.

[2] System Planning Corporation, “Microelectromechanical Systems (MEMS): An SPC Market Study,” January 1999, 1429 North Quincy Street, Arlington, VA 22207.

[3] Frost and Sullivan, “World Sensors Market: Strategic Analysis,” Report 5509-32, February

1999, 2525 Charleston Road, Mountain View, CA 94043, http://www.frost.com [4] Frost and Sullivan, “U.S Microelectromechanical Systems (MEMS),” Report 5549-16, June 1997, 2525 Charleston Road, Mountain View, CA 94043, http://www.frost.com [5] Intechno Consulting, “Sensors Market 2008,” Steinenbachgaesslein 49, CH-4051, Basel, Switzerland, http://www.intechnoconsulting.com.

[6] In-Stat/MDR, “Got MEMS? Industry Overview and Forecast,” Report IN030601EA, August 2003, 6909 East Greenway Parkway, Suite 250, Scottsdale, AZ 85254, http://www.instat.com.

[7] WTC Wicht Technologie Consulting, “The RF MEMS Market 2002–2007,” Frauenplatz

5, D-80331 München, Germany, http://www.wtc-consult.de.

[8] Yole Développement, “World MEMS Fab,” 45 Rue Sainte Geneviève, 69006 Lyon, France, http://www.yole.fr.

[9] Public Citizen, Inc., et al v Norman Mineta, Secretary of Transportation, Docket No.

02-4237, August 6, 2003, United States Court of Appeals, Second Circuit, New York, http://www.ca2.uscourts.gov.

[10] “IC Makers Gear Up for New Tire Pressure Monitor Rule,” Electronic Engineering Times,

December 1, 2003, p 1.

[11] Roylance, L M., and J B Angell, “A Batch Fabricated Silicon Accelerometer,” IEEE

Trans Electron Devices, Vol 26, No 12, 1979, pp 1911–1917.

[12] Mercer Management Consulting, Inc., “New Technologies Take Time,” Business Week,

April 19, 1999, p 8.

Selected Bibliography

Angell, J B., S C Terry, and P W Barth, “Silicon Micromechanical Devices,” Scientific

American, Vol 248, No 4, April 1983, pp 44–55.

Gabriel, K J., “Engineering Microscopic Machines,” Scientific American, Vol 273, No 3,

September 1995, pp 150–153.

Micromechanics and MEMS: Classic and Seminal Papers to 1990, W S Trimmer (ed.),

New York: Wiley-IEEE Press, 1997.

“Nothing but Light,” Scientific American, Vol 279, No 6, December 1998, pp 17–20 Petersen, K E., “Silicon As a Mechanical Material,” Proceedings of the IEEE, Vol 70,

No 5, May 1982, pp 420–457.

Materials for MEMS

“You can’t see it, but it’s everywhere you go.”

—Bridget Booher, journalist, on silicon

If we view micromachining technology as a set of generic tools, then there is no rea-son to limit its use to one material Indeed, micromachining has been demonstrated using silicon, glass, ceramics, polymers, and compound semiconductors made of group III and V elements, as well as a variety of metals including titanium and tung-sten Silicon, however, remains the material of choice for microelectromechanical systems Unquestionably, this popularity arises from the large momentum of the electronic integrated circuit industry and the derived economic benefits, not least of which is the extensive industrial infrastructure The object of this chapter is to pres-ent the properties of silicon and several other materials, while emphasizing that the final choice of materials is determined by the type of application and economics

Silicon-Compatible Material System

The silicon-compatible material system encompasses, in addition to silicon itself, a host of materials commonly used in the semiconductor integrated circuit industry Normally deposited as thin films, they include silicon oxides, silicon nitrides, and silicon carbides, metals such as aluminum, titanium, tungsten, and copper, and polymers such as photoresist and polyimide

Silicon

Silicon is one of very few materials that is economically manufactured in single-crystal substrates This single-crystalline nature provides significant electrical and mechanical advantages The precise modulation of silicon’s electrical conductivity using impurity doping lies at the very core of the operation of electronic semi-conductor devices Mechanically, silicon is an elastic and robust material whose characteristics have been very well studied and documented (see Table 2.1) The tremendous wealth of information accumulated on silicon and its compounds over the last few decades has made it possible to innovate and explore new areas of appli-cation extending beyond the manufacturing of electronic integrated circuits It becomes evident that silicon is a suitable material platform on which electronic, mechanical, thermal, optical, and even fluid-flow functions can be integrated Ultrapure, electronic-grade silicon wafers available for the integrated circuit indus-try are common today in MEMS The relatively low cost of these substrates

13

(approximately $10 for a 100-mm-diameter wafer and $15 for a 150-mm wafer) makes them attractive for the fabrication of micromechanical components and systems

Silicon as an element exists with three different microstructures: crystalline, polycrystalline, or amorphous Polycrystalline, or simply “polysilicon,” and

amor-phous silicon are usually deposited as thin films with typical thicknesses below 5

µm Crystalline silicon substrates are commercially available as circular wafers with 100-mm (4-in) and 150-mm (6-in) diameters Larger-diameter (200-mm and 300-mm) wafers, used by the integrated circuit industry, are currently economically unjustified for MEMS Standard 100-mm wafers are nominally 525µm thick, and 150-mm wafers are typically 650µm thick Double-side-polished wafers commonly used for micromachining on both sides of the wafer are approximately 100µm thin-ner than standard thickness substrates

Visualization of crystallographic planes is key to understanding the dependence

of material properties on crystal orientation and the effects of plane-selective etch solutions (see Figure 2.1) Silicon has a diamond-cubic crystal structure that can be

Table 2.1 Properties of Selected Materials

Property a

Al2O3

Polyimide PMMA

Relative

permittivity ( εr)

Dielectric

strength

(V/cm ×106)

Electron

mobility

(cm2/V·s)

Hole mobility

(cm2/V·s)

Young’s

modulus (GPa)

Yield/fracture

strength (GPa)

Poisson’s ratio 0.22 0.17 0.25 0.16 0.14 0.10 0.31 0.31 0.34 — Density (g/cm3) 2.4 2.2 3.1 2.65 3.2 3.5 5.3 3.26 3.62 1.42 1.3 Coefficient of

thermal

expansion

(10−6/ºC)

Thermal

conductivity

at 300K

(W/m·K)

157 1.4 19 1.4 500 990–2,000 0.46 160 36 0.12 0.2

Specific heat

(J/g·K)

Melting

temperature (ºC)

1,415 1,700 1,800 1,610 1,800b 3,652b 1,237 2,470 1,800 380c 90c

a

Properties can vary with crystal direction, crystal structure, and grain size.

b

Sublimates before melting.

c

Glass transition temperature given for polymers.

discussed as if it were simple cubic In other words, the primitive unit—the smallest repeating block—of the crystal lattice resembles a cube The three major coordinate

axes of the cube are called the principal axes Specific directions and planes within the crystal are designated in reference to the principal axes using Miller indices [1], a

special notation from materials science that, in cubic crystals, includes three integers with different surrounding “punctuation.” Directions are specified by brackets; for

example [100], which is a vector in the +x direction, referred to the three principal axes (x,y,z) of the cube No commas are used between the numbers, and negative

numbers have a bar over the number rather than a minus sign Groups of directions with equivalent properties are specified with carets (e.g.,<100>, which covers the [100]= +x,[100]=−x,[010]= +y,[010]=−y,[001]= +z and [, 001]=−z

direc-tions) Parentheses specify a plane that is perpendicular to a direction with the same numbers; for example, (111) is a plane perpendicular to the [111] vector (a diagonal vector through the farthest corner of the unit cube) Braces specify all equivalent planes; for example, {111} represents the four equivalent crystallographic planes (111), (111 , () 111 , and () 111 )

(b)

(a)

z, [001]

y, [010]

x, [100]

z, [001]

y, [010]

x, [100]

z, [001]

y, [010]

x, [100]

(110)

(110)

Figure 2.1 (a) Three crystallographic planes and their Miller indices for a simple cubic crystal Two planes in the {110} set of planes are identified (b) The four planes in the {111} family Note that ( 111 is the same plane as (111) )

The determinants of plane and direction equivalence are the symmetry opera-tions that carry a crystal lattice (including the primitive unit) back into itself (i.e., the transformed lattice after the symmetry operation is complete is identical to the start-ing lattice) With some thought, it becomes evident that 90º rotations and mirror operations about the three principal axes are symmetry operations for a simple cubic

crystal Therefore, the +x direction is equivalent to the +y direction under a 90º rota-tion; the +y direction is equivalent to the –y direction under a mirror operation, and

so forth Hence, the +x, –x, +y, –y, +z, and –z directions are all equivalent Vector

algebra (using a dot product) shows that the angles between {100} and {110} planes are 45º or 90º, and the angles between {100} and {111} planes are 54.7º or 125.3º Similarly, {111} and {110} planes can intersect each other at 35.3º, 90º, or 144.7º The angle between {100} and {111} planes is of particular importance in micromachining because many alkaline aqueous solutions, such as potassium hydroxide (KOH), selectively etch the {100} planes of silicon but not the {111} planes (discussed in detail in Chapter 3) The etch results in cavities that are bounded

by {111} planes (see Figure 2.2)

Material manufacturers cut thin circular wafers from large silicon boules along specific crystal planes The cut plane—the top surface of the wafer—is known as the orientation cut The (100) wafers dominate in both MEMS and CMOS technology, but wafers are also readily available with (111) orientation and, to a lesser degree, (110) orientation It should be noted that saying that the surface of a wafer has a particular orientation such as (100) is arbitrary; any orientation within the equiva-lent {100} group of planes, such as (001), can alternatively be selected It should be further noted that when referring to the wafer surface (e.g., (100)), the group of planes (e.g., {100}) or direction normal to the surface (e.g., [100]) is often used

instead; all are intended to mean the same thing The (100) and (111) wafers, with n-and p-type doing, are produced with a minor flat at a specific location relative to a

wider, major flat, as shown in Figure 2.2

Crystalline silicon is a hard and brittle material deforming elastically until it reaches its yield strength, at which point it breaks Its tensile yield strength is 7 GPa, which is equivalent to a 700-kg (1,500-lb) weight suspended from a 1-mm2area Its Young’s modulus is dependent on crystal orientation, being 169 GPa in <110> directions and 130 GPa in<100> directions—near that of steel The dependence of the mechanical properties on crystal orientation is reflected in the way a silicon wafer preferentially cleaves along crystal planes1 While large silicon wafers tend to be fragile, individual dice with dimensions on the order of 1 cm × 1 cm or less are rugged and can sustain relatively harsh handling conditions As a direct consequence of being a single crystal, mechanical properties are uniform across wafer lots, and wafers are free of intrinsic stresses This helps to minimize the number of design iterations for silicon transducers that rely on stable mechanical properties for their operation Bulk mechanical properties of crystalline silicon are largely independent

1 A (100) silicon wafer can be cleaved by scratching the surface with a sharp diamond scribe along a <110> direction (parallel or perpendicular to the flat), clamping the wafer on one side of the scratch, and applying a bending force to the free side of the wafer Fracture occurs preferentially along <110> directions on the surface The newly exposed fracture surfaces tend to be {111} planes, which are sloped at 54.7° with respect

to the surface.

of impurity doping, but stresses tend to rise when dopant concentrations reach high levels (~ 1020cm−3)

Polysilicon is an important material in the integrated circuit industry and has been extensively studied A detailed description of its electrical properties is found

in [2] Polysilicon is an equally important and attractive material for MEMS It has been successfully used to make micromechanical structures and to integrate

electrical interconnects, thermocouples, p-n junction diodes, and many other

elec-trical devices with micromechanical structures The most notable example is the acceleration sensor available from Analog Devices, Inc., of Norwood, Massachu-setts, for automotive airbag safety systems Surface micromachining based on poly-silicon is today a well-established technology for forming thin (a few micrometers) and planar devices

The mechanical properties of polycrystalline and amorphous silicon vary with deposition conditions, but, by and large, they are similar to that of single crystal sili-con [3] Both normally have relatively high levels of intrinsic stress (hundreds of MPa) after deposition, which requires annealing at elevated temperatures (>900ºC)

(111)

(c) [100]

[010]

[001]

(111)

Surface

is (001)

Flat is along [110] direction

(111)

(111)

(110) plane

º

(b)

45

(001) plane

[110]

direction

x, [100]

y, [010]

z, [001]

(110)

(a)

Primary flat

(111) n-type

45°

90°

Primary flat

(111) p-type Secondary flat

(100) n-type

Primary flat

(100) p-type

Primary flat

No secondary flat

Secondar y flat

Figure 2.2 (a) Illustration showing the primary and secondary flats of {100} and {111} wafers for

both n-type and p-type doping (SEMI standard); (b) illustration identifying various planes in a

wafer of {100} orientation (the wafer thickness is exaggerated); and (c) perspective view of a {100} wafer and a KOH-etched pit bounded by {111} planes.

Beam structures made of polycrystalline or amorphous silicon that have not been subjected to a careful stress annealing step can curl under the effect of intrinsic stress

Silicon is a very good thermal conductor with a thermal conductivity greater than that of many metals and approximately 100 times larger than that of glass In com-plex integrated systems, the silicon substrate can be used as an efficient heat sink This feature will be revisited when we review thermal-based sensors and actuators Unfortunately, silicon is not an active optical material—silicon-based lasers do not exist Because of the particular interactions between the crystal atoms and the conduction electrons, silicon is effective only in detecting light; emission of light

is very difficult to achieve At infrared wavelengths above 1.1 µm, silicon is transparent, but at wavelengths shorter than 0.4µm (in the blue and ultraviolet por-tions of the spectrum), it reflects over 60% of the incident light (see Figure 2.3) The attenuation depth of light in silicon (the distance light travels before the intensity drops to 36% of its initial value) is 2.7µm at 633 nm (red) and 0.2 µm at 436 nm (blue-violet) The slight attenuation of red light relative to other colors is what gives thin silicon membranes their translucent reddish tint

Silicon is also well known to retain its mechanical integrity at temperatures up to about 700°C [4] At higher temperatures, silicon starts to soften and plastic defor-mation can occur under load While the mechanical and thermal properties of poly-silicon are similar to those of single crystal poly-silicon, polypoly-silicon experiences slow stress annealing effects at temperatures above 250°C, making its operation at ele-vated temperatures subject to long-term instabilities, drift, and hysteresis effects Some properties of silicon at and above room temperature are given in Table 2.2 The surface of silicon oxidizes immediately upon exposure to the oxygen in air

(referred to as native oxide) The oxide thickness self-limits at a few nanometers at

room temperature As silicon dioxide is very inert, it acts as a protective layer that prevents chemical reactions with the underlying silicon

The interactions of silicon with gases, chemicals, biological fluids, and enzymes remain the subject of many research studies, but, for the most part, silicon is considered stable and resistant to many elements and chemicals typical of daily

Wavelength ( m) µ

UV V Green Red IR

Si Ag

Ni

Pt Au

Al

0 10 20 30 40 50 60 70 80 90 100

Figure 2.3 Optical reflectivity for silicon and selected metals.

applications For example, experiments have shown that silicon remains intact in the presence of Freon™ gases as well as automotive fluids such as brake fluids Silicon has also proven to be a suitable material for applications such as valves involving the delivery of ultra-high-purity gases In medicine and biology, studies are ongoing to evaluate silicon for medical implants Preliminary medical evidence indicates that silicon is benign in the body and does not release toxic sub-stances when in contact with biological fluids; however, it appears from recent experiments that bare silicon surfaces may not be suitable for high-performance polymerase chain reactions (PCR) intended for the amplification of genetic DNA material

Silicon Oxide and Nitride

It is often argued that silicon is such a successful material because it has a stable oxide that is electrically insulating—unlike germanium, whose oxide is soluble in water, or gallium arsenide, whose oxide cannot be grown appreciably Various forms of silicon oxides (SiO2, SiOx, silicate glass) are widely used in micromachin-ing due to their excellent electrical and thermal insulatmicromachin-ing properties They are also used as sacrificial layers in surface micromachining processes because they can be preferentially etched in hydrofluoric acid (HF) with high selectivity to silicon Sili-con dioxide (SiO2) is thermally grown by oxidizing silicon at temperatures above 800°C, whereas the other forms of oxides and glass are deposited by chemical vapor deposition, sputtering, or even spin-on (the various deposition methods will

be described in the next chapter) Silicon oxides and glass layers are known to sof-ten and flow when subjected to temperatures above 700°C A drawback of silicon oxides is their relatively large intrinsic stresses, which are difficult to control This has limited their use as materials for large suspended beams or membranes Silicon nitride (SixNy) is also a widely used insulating thin film and is effective as

a barrier against mobile ion diffusion—in particular, sodium and potassium ions found in biological environments Its Young’s modulus is higher than that of silicon and its intrinsic stress can be controlled by the specifics of the deposition process Silicon nitride is an effective masking material in many alkaline etch solutions

Table 2.2 Temperature Dependence of Some Material Properties of Crystalline Silicon

Coefficient of linear

expansion (10−6K−1)

Specific heat (J/g·K) –0,00 0.713 –0,00 0.785 –0,00 0.832 –9 0.849 –9 0.866 Thermal conductivity

(W/cm·K)

Temperature coefficient

of Young’s modulus (10−6K−1)

Temperature coefficient

of piezoresistance (10−6K−1)

(doping <1018cm−3)

Temperature coefficient

of permittivity (10−6K−1)

(Source: [5].)