An Introduction to MEMs Engineering - Nadim Maluf and Kirt Williams Part 10 potx

Achromatic Variable Optical Attenuation A variable optical attenuator VOA is a dynamic optical component used in fiber-optical telecommunications to adjust the intensity of light inside

Only the fringing radial component of magnetic field (Br) contributes to the angular and piston (vertical) displacement of the micromirror A counterclockwise current in a drive coil interacting with the radial field results in a Lorentz force that is normal to the plane of the coil and acting to pull the coil towards the magnet [see Figure 5.17(b)]—the peripheral portion of the coil contributes to the force, whereas the radial portions have little effect Switching the polarity of the current results in

an opposite force that pushes the coil away from the magnet It thus becomes evident that two adjacent coils carrying currents in opposite directions induce a torque around an axis of symmetry that divides them Torques of arbitrary magnitude can

be generated around the two axes of symmetry by the proper selection of the current direction and magnitude in each of the coils Furthermore, an additional vertical (piston) motion can be induced by driving all four coils simultaneously with a cur-rent in the same direction For example, a clockwise curcur-rent in all coils moves the mirror away from the surface of the magnet

The differential drive of the coils provides an added benefit: the developed torque stays relatively constant throughout the full range of motion of ±5º As the mirror tilts, the side that is closer to the magnet develops a larger downward force, whereas the side that is farther from the magnet develops a smaller upward force The two effects are offsetting, resulting in a minimal increase in the torque (<0.2%) over the full mirror travel This linear behavior greatly minimizes cross coupling between the two axes of rotation (<0.1% in displacement cross coupling)

The drive coils play an additional role as sense coils to detect the angular posi-tion of the mirror A multiturn planar coil deposited on the ceramic substrate that holds the silicon micromirror acts as the primary winding of a transformer, with the four drive coils as the secondary An ac signal at a frequency of approximately 5 MHz in the primary produces a corresponding sense voltage in each of the four coils

B

I

B

I

F=IL×B

F

F

F = 0

F = 0

n

r

The Lorentz force is planar to the mirror for the normal field,Bn.

The Lorentz force is normal to the plane of the mirror for the radial fringing field,Br.

Permanent magnet

Mirror structure

F=IL×B

Bn

Br

Flux lines

Figure 5.17 (a) An illustration of the rare-Earth magnet and the four independent drive coils The

magnetic flux density outside of the magnet has a normal component, Bn, and a fringing radial

component, Br (b) The normal magnetic component interacts with a counterclockwise current to induce a Lorentz force that is in the plane of the coils The radial component of the magnetic field results in a force that is normal to the plane of the coil.

through mutual inductance coupling (the mirror does not respond to this high fre-quency) This coupling is a strong function of the position and orientation of the coils relative to the primary coil These sense voltages then become a direct measure

of the angular position of the mirror and are used in a closed-loop electronic circuit

to spatially lock the mirror

The details of the fabrication process are not available, but, once again, one can design a fabrication sequence that can produce a similar device The starting material

is a SOI substrate polished on both sides The first fabrication steps cover the forma-tion of the drive coils and corresponding interconnects on the front side of the SOI wafer A gold seed layer, typically 50 to 100 nm thick, is sputtered on both sides of the wafer, then followed by standard lithography on the front side to delineate the coil layout The thin gold layer on the back side will ultimately serve as the reflecting surface of the mirror Electroplating 5–20 microns of gold on the front side forms the coils and bond pads The next step is the delineation of the torsional hinges, also on the front side of the wafer This is completed using standard lithography, followed

by standard RIE It may be necessary to delineate the suspension hinges just prior to the electroplating if the thickness of the gold is more than 5µm in order to avoid the deposition of resist over the thick topographical features of the gold coils The fabri-cation is completed by etching from the back side of the wafer the contour of the mir-ror and using the embedded silicon oxide layer as an etch stop Either DRIE or wet anisotropic etching (e.g., KOH or TMAH) can be used The very last step is the removal of the exposed silicon oxide layer using hydrofluoric acid

It is evident from this process that the thickness of the suspension is determined

by the thickness of the top SOI layer, typically a few micrometers thick As a result, the mechanical properties of the suspension are very predictable and well con-trolled Similarly, the thickness of the mirror is determined by the thickness of the handle layer (thick bottom layer) of the SOI wafer and is uniform—the measured surface flatness over the 3-mm diameter mirror is less than 15 nm RMS with local roughness of approximately 2 nm The gold layer on the back side of the wafer pro-vides a very high reflectivity in the near infrared spectrum

Achromatic Variable Optical Attenuation

A variable optical attenuator (VOA) is a dynamic optical component used in fiber-optical telecommunications to adjust the intensity of light inside the fiber A VOA typically maintains the power below 20 mW, which corresponds to the onset of nonlinear effects such as four-wave mixing, Brillouin scattering, and Raman scatter-ing [40, 41] Key characteristics of a VOA are spectral range (typically between 1,528 to 1,620 nm), insertion loss (a measure of light lost within the component exclusive of the required attenuation, typically less than 1 dB), polarization-dependent loss (a measure of the difference in loss between the two orthogonal polarizations, typically less than 0.5 dB), wavelength dependence of attenuation (typically less than 0.3 dB over the spectral range), and finally size (a volume less than 1 cm3

is highly desirable) All loss parameters are measured in dB

Numerous implementations using MEMS technology have emerged in the past few years The following example is a product by Lightconnect, Inc., of Newark, California, that utilizes a principle of operation and a structure that are identical to the GLV discussed earlier in this chapter [42] The basic concept is to use diffraction

to shift energy away (and thus attenuate) from the main undiffracted beam into higher order beams (see Figure 5.18), attenuating the incident beam (attenuation is equivalent to creating a continuum of gray shades) The closely spaced suspended reflective ribbons used for the GLV form the elements of an adjustable-phase grat-ing When the ribbons are coplanar, incident light is reflected back into the aperture without attenuation When alternating ribbons are pulled down using electrostatic actuation by one quarter of a wavelength (λ/4) relative to their adjacent ribbons, the incident energy diffracts into higher orders that are directed outside the aperture, and the incident beam is completely attenuated When the separation is less thanλ/4, the incident beam is partially attenuated, as some energy is shifted into the higher diffracted orders

While the VOA derives its basic principle of operation from the GLV, it must also address a number of specifications that are particular to fiber-optical telecom-munications The first one relates to the chromatic dependence of the diffraction grating Displays have to manipulate only three basic colors: red, green, and blue But VOAs must manipulate a nearly continuous spectrum of wavelengths from 1,528 nm to 1,610 nm without a chromatic dependence The second specification is polarization-dependent loss A difference in attenuation between the two polariza-tions that is larger than 0.5 dB greatly increases the risk of data errors during trans-mission The design from Lightconnect adapts the GLV diffractive technology with two key modifications to applications in fiber-optical telecommunications

In order to understand the basic operation of the achromatic design, one needs

to refer to the use of phasors for time-varying electric fields [43] In the case of the GLV, two phasors—one for each of the fixed and moveable ribbons—affect the

Undeflected Partial deflection Full deflection

λ/4

< /4 λ

Zeroth order

Higher orders

Intensity

Diffraction angle

No attenuation Partial attenuation Full attenuation Zeroth order First order

Aperture

Figure 5.18 An illustration of the basic principle of operation of the variable optical attenuator from Lightconnect, Inc A set of suspended ribbons act as an adjustable grating When alternating ribbons are pulled down by λ/4, the structure becomes a phase grating and diverts the incident energy into higher diffraction orders, thus providing full attenuation of the incident beam When all of the ribbons are coplanar or separated by a half wavelength, the surface acts as a reflector When the separation between adjacent ribbons is less than λ/4, there is light in all orders and the incident beam is only partially attenuated.

reflected wave [see Figure 5.19(a)] The difference in angle between the two phasors

is equal to 4πd/λ, where d is the physical separation between the ribbons and λ is the wavelength When the two phasors areπ radians apart (i.e., the total vector sum of the phasors is zero), there is complete diffraction of light into the higher orders However, this condition is satisfied only at one wavelength, which depends on the

separation d For all other wavelengths, the angle difference between the phasors is

less thanπ (the vector sum is nonzero), thus allowing light to be reflected in both the zeroth (undiffracted) and higher-order diffraction modes To correct for this dependence, the design introduces another phasor such that the sum of all three vec-tors is null over a broad range of wavelengths [see Figure 5.19(a)]

The basic repetitive cell consists of three reflective ribbons [see Figure 5.19(b)]: one moveable ribbon, a reference “ribbon,” and a compensating “ribbon,” with the latter two being spatially fixed and separated by an integer multiple of half the

center wavelength (Nλ0/2) whereλ0is typically around 1,550 nm (i.e., their phasors will be in phase only at the center wavelength) In the nominal undeflected state, all three phasors have the same orientations at the center wavelengthλ0and add con-structively to reflect the light without diffraction (no attenuation by the VOA) Pull-ing the moveable ribbon down byλ0/4 adds a round trip phase ofπ at the center

N

2

λ 0

4

λ 0

εc

εr

εm

Moveable ribbon

Compensating ribbon

Reference ribbon

(b)

εc

Re

εm

εr

ε m + ε r + ε c = 0 for all λ is satisfied when:

Re

Im

εm

εr

at λ λ = 0

(a)

A r+ 2A c=A mand A A c

m 2N

1

πλ o

λ 2Nπλλ0

at λ ≠ λ 0

Im

2 εc The three phasors add to the null vector

2 εc

Figure 5.19 (a) Phasor description of the diffractive operation of the variable optical attenuator.

At the center wavelength, the phasors add to the null vector At other wavelengths, the compen-sating ribbon introduces an error vector that cancels the error vector introduced by the moveable ribbon, thus providing broadband achromatic operation [42] (b) A schematic illustration of the achromatic implementation of the variable optical attenuator The structure consists of groups of three ribbons, one of which is moveable and two of which are spatially fixed The latter two are

vertically separated by Nλ/2 where λ is the center wavelength and N is an integer.

wavelength to the light reflected by this ribbon Schematically, the corresponding phasor,εm, rotates in the complex plane by 180º At the center wavelength,εc, the phasor corresponding to the compensating ribbon remains in the same orientation

asεr, the phasor for the reference ribbon The three phasors now add destructively to

a null vector [see Figure 5.19(a)] at the center wavelength, and thus light diffracts into higher orders, causing maximum attenuation of the main undiffracted order At

a wavelengthλ different than λ0, the phasorεmrotates by an amountπλ0/λ radians (less or more thanπ), causing an error vector relative to the phasor at λ0 Simultane-ously, the phasor εc rotates by 2Nπλ0/λ, causing an error vector in the opposite direction—εcrotates pastεmby an additionalπλ0/λ (if N = 1), placing it in an oppo-site quadrant toεm As the magnitudes of the phasors are proportional to the areas of the ribbons, the two error vectors can be made to cancel each other out under certain

geometrical conditions Analytical calculations show that if A m , A r , and A care the respective areas of the moveable, reference and compensating ribbons, then there are

two conditions that must be satisfied: A r +2A c = A m and A c /A m = 1/2N The first

con-dition ensures equality of the magnitudes of the phasors that are out of phase The second condition follows from matching the phases of the error vectors As a result, the total phasor is null (εm+εmr+εc= 0) over a wide range of wavelengths

Extending the achromatic design to also eliminate polarization dependence entails mapping the linear geometry (linear ribbons) into one with cylindrical sym-metry (circular discs), making the device effectively a two-dimensional phase grating (see Figure 5.20) The reference ribbon becomes a reference circular post; the move-able ribbon becomes a membrane with circular cut outs suspended by anchor points

on the edges; and the achromatic compensating ribbons become annular rings around the reference posts The membrane incorporates minute release holes that assist in the fast and uniform removal of the sacrificial layer during fabrication The dimensions of the gaps remain unchanged

In a typical design, N equals 3, the center wavelength is 1,550 nm,

correspond-ing to a height difference between the moveable membrane and compensatcorrespond-ing annuli

of 2.32µm The periodicity of the repeating diffractive element is typically between

20 and 200µm [42] The widths of the reference post, as well as the gap between the post and membrane, are typically a few micrometers The resulting variable optical

Silicon substrate

Anchor to substrate

Array of fixed posts

Release holes

Reflecting membrane

Reflecting surface Achromatic compensator

d=N

2

λ 0

Figure 5.20 A cross-sectional schematic of the variable optical attenuator The architecture incorporates achromatic compensation and cylindrical symmetry to ensure low dependence on polarization [42].

attenuator from Lightconnect has a dynamic range (attenuation range) of 30 dB, a wavelength dependence of attenuation of 0.25 dB, and a polarization-dependent loss of 0.2 dB The total insertion loss, which includes losses from fiber coupling, is 0.7 dB The response time of the device is, as expected from the GLV, quite fast, measuring 40µs The actuation voltage between the membrane and substrate is less than 8V The company also provides a specification for reliability: in excess of 100 billion cycles for wear out While wear out is very subjective and not quantified, it reflects the projected reliability of this device where displacements are very small (λ0/4⬇400 nm) and friction is nonexistent

The fabrication is very similar to that of the GLV with a few exceptions First, lithography followed by an etch defines the reference posts with a height of 2.32

µm A thin (20–60 nm) layer of silicon dioxide is thermally grown A layer of sacrifi-cial polysilicon or amorphous silicon is deposited This layer must be optically smooth, as any defects will subsequently imprint the moveable membrane Holes are etched through the sacrificial layer to allow for the anchor points to the sub-strate Silicon nitride is then deposited as the membrane material It may be stochio-metric or silicon rich A lithographic step followed by an etch step pattern the nitride layer into the desired membrane layout Finally, xenon difluoride (XeF2) removes the sacrificial layer of silicon to release the membrane A subsequent evaporation step deposits a thin gold layer across the entire surface, ensuring high reflectivity in the infrared

Summary

This chapter reviewed a number of commercially available products with applica-tions in imaging, displays, and fiber-optical telecommunicaapplica-tions The applicaapplica-tions are very diverse but share the common use of MEMS technology to manipulate light While MEMS have proven to be vital for the operation of the aforementioned products, it remains an enabling technology and a means to an end It is impera-tive to understand the final application in order to assess the importance and applicability of MEMS for that particular application

References

[1] Cole, B E., R E Higashi, and R A Wood, “Monolithic Two-Dimensional Arrays of

Micromachined Microstructures for Infrared Applications,” in Integrated Sensors, Micro-actuators, & Microsystems (MEMS), K D Wise (ed.), Proceedings of the IEEE, Vol 86,

No 8, August 1998, pp 1679–1686.

[2] Van Kessel, P F., et al., “A MEMS-Based Projection Display,” in Integrated Sensors, Microactuators, & Microsystems (MEMS), K D Wise (ed.), Proceedings of the IEEE, Vol.

86, No 8, August 1998, pp 1687–1704.

[3] Bloom, D M., “The Grating Light Valve: Revolutionizing Display Technology,” Proc SPIE, Projection Displays III, Vol 3013, San Jose, CA, February 10–12, 1997,

pp 165–171.

[4] Chen, Y., et al., “Metro Optical Networking,” Bell Labs Technical Journal, January–

March 1999, pp 163–186.

[5] Tomsu, P., and C Schmutzer, Next Generation Optical Networks, Upper Saddle River, NJ:

Prentice Hall, 2002, pp 68–70.

[6] Saleh, B E A., and M C Teich, Fundamentals of Photonics, New York: Wiley, 1991,

pp 461–466, 494–503.

[7] Siegman, A E., Lasers, Mill Valley, CA: University Science Books, 1986, pp 1–80 [8] Coldren, L A., and S W Corzine, Diode Lasers and Photonic Integrated Circuits, New

York: Wiley, 1995, pp 1–9, 111–116.

[9] Saleh, B E A., and M C Teich, Fundamentals of Photonics, New York: Wiley, 1991,

pp 310–317.

[10] Jerman, H., and J D Grade, “A Mechanically-Balanced, DRIE Rotary Actuator for a

High-Power Tunable Laser,” Tech Digest Solid-State Sensor and Actuator Workshop,

Hil-ton Head Island, SC, June 2–6, 2002.

[11] Pezeshki, B., et al., “20 mW Widely Tunable Laser Module Using DFB Array and MEMs

Selection,” IEEE Photonics Technology Letters, Vol 14, October 2002, pp 1457–1459.

[12] Littman, M G., and H J Metcalf, “Spectrally Narrow Pulsed Dye Laser Without Beam

Expander,” Applied Optics, Vol 17, No 14, 1978, pp 2224–2227.

[13] Agrawal, G P., and N K Dutta, Semiconductor Lasers, Boston, MA: Kluwer Academic

Publishers, 1993, pp 269–275.

[14] Coldren, L A., and S W Corzine, Diode Lasers and Photonic Integrated Circuits, New

York: Wiley, 1995, pp 17–24, 393–398.

[15] Klein, M V., Optics, New York: Wiley, 1970, pp 342–346.

[16] Klein, M V., Optics, New York: Wiley, 1970, pp 338–341.

[17] Liu, K., and M G Littman, “Novel Geometry for Single-Mode Scanning of Tunable

Lasers,” Optics Letters, Vol 6, No 3, 1981, pp 117–118.

[18] U.S Patent 6,469,415, October 22, 2002.

[19] Smith, S T., and D G Chetwynd, Foundations of Ultraprecision Mechanism Design (Developments in Nanotechnology), London, UK: Taylor and Francis, 1992, p 119 [20] Tang, W C., et al., “Electrostatic-Comb Drive of Lateral Polysilicon Resonators,” Sensors and Actuators, Vol A21, Nos 1–3, February 1990, pp 328–331.

[21] Berger, J D., and D Anthon, “Tunable MEMS Devices for Optical Networks,” Optics & Photonics News, March 2003, pp 43–49.

[22] Kogelnik, H., and C V Shank, “Coupled-Wave Theory of Distributed Feedback Lasers,”

Journal of Applied Physics, Vol 43, 1972, pp 2327–2335.

[23] Data sheet for CQF935/508 series, JDS Uniphase Corporation, 1768 Automation Parkway, San Jose, CA 95131, http://www.jdsu.com.

[24] Ghafouri-Shiraz, H., Distributed Feedback Laser Diodes and Optical Tunable Filters, New

York: Wiley, 2003.

[25] Amann, M -C., and J Buus, Tunable Laser Diodes, Norwood, MA: Artech House, 1998,

pp 40–51.

[26] Pezeshki, B., et al., “Twelve Element Multi-Wavelength DFB Arrays for Widely Tunable

Laser Modules,” Tech Digest of the Optical Fiber Communication Conference, Anaheim,

CA, March 17–22, 2002, pp 711–712.

[27] Saleh, B E A., and M C Teich, Fundamentals of Photonics, New York: Wiley, 1991,

pp 316–317.

[28] Plomteux, O., “DFL-5720 Digital Frequency-Locking System: Simplifying Wavelength-Locker Testing,” Application Note 083, EXFO Electro-Optical Engineering, Inc., Vanier, Quebec, Canada, http://documents.exfo.com/appnotes/anote083-ang.pdf.

[29] Dames, M P., et al., “Efficient Optical Elements to Generate Intensity Weighted Spot

Arrays: Design and Fabrication,” Applied Optics, Vol 30, No 19, July 1, 1991,

pp 2685–2691.

[30] Farn, M W., “Agile Beam Steering Using Phase-Array Like Binary Optics,” Applied Optics,

Vol 33, No 22, August 1, 1994, pp 5151–5158.

[31] Hecht, J., Understanding Fiber Optics, 3rd ed., Upper Saddle River, NJ: Prentice Hall,

1999, pp 133–134, 320–325, 373–374, 455.

[32] Marxer, C., et al., “Vertical Mirrors Fabricated by Deep Reactive Ion Etching for

Fiber-Optic Switching Applications,” Journal of Microelectromechanical Systems, Vol 6, No 3,

September 1997, pp 185–277.

[33] Hecht, J., Understanding Fiber Optics, 3rd ed., Upper Saddle River, NJ: Prentice Hall,

1999, pp 62–72.

[34] Zou, J., et al., “Optical Properties of Surface-Micromachined Mirrors with Etch Holes,”

Journal of Microelectromechanical Systems, Vol 8, No 4, December 1999, pp 506–513.

[35] Iannone, E., and R Sabella, “Optical Path Technologies: A Comparison Among Different

Cross-Connect Architectures,” Journal of Lightwave Technology, Vol 14, No 10,

Octo-ber 1996, pp 2184–2196.

[36] U.S Patents 5,629,790, May 13, 1997; 6,480,320 B2, November 12, 2002; and 6,628,041 B2, September 30, 2003.

[37] Burns, B., et al., “Electromagnetically Driven Integrated 3D MEMS Mirrors for Large Scale

PXCs,” in Proceedings of National Fiber Optics Engineers Conference, NFOEC 2002,

Dal-las, TX, September 15–19, 2002.

[38] Saleh, B E A., and M C Teich, Fundamentals of Photonics, New York: Wiley, 1991,

pp 81–105.

[39] Temesvary, V., et al., “Design, Fabrication, and Testing of Silicon Microgimbals for

Super-Compact Rigid Disk Drives,” Journal of Microelectromechanical Systems, Vol 4, No 1,

March 1995, pp 18–27.

[40] Hecht, J., Understanding Fiber Optics, 3rd ed., Upper Saddle River, NJ: Prentice Hall,

1999, pp 99–100.

[41] Agrawal, G., Nonlinear Fiber Optics, 2nd ed., San Diego, CA: Academic Press, 1995,

pp 239–243, 316–399.

[42] U.S Patents 6,169,624, January 2, 2001, and 6,501,600, December 31, 2002.

[43] Halliday, D., and R Resnick, Physics, 3rd ed extended, New York: Wiley, 1988,

pp 907–910.

Selected Bibliography

Buser, P., and M Imbert (translated by R H Kay), Vision, Cambridge, MA: The MIT

Press, 1992.

Hecht, J., Understanding Fiber Optics, 3rd ed., Upper Saddle River, NJ: Prentice Hall,

1999.

MacDonald, L W., and A C Lowe (eds.), Display Systems: Design and Applications, West

Sussex, England: Wiley, 1997.

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

York: IEEE, 1997.

Wise, K D (ed.), “Special Issue on Integrated Sensors, Microactuators, and Microsystems

(MEMS),” Proceeding of the IEEE, Vol 86, No.8, August 1998.

.

C H A P T E R 6

MEMS Applications in Life Sciences

“Jim, you’ve got to let me go in there! Don’t leave him in the hands of Twentieth-Century medicine.”

—Dr Leonard McCoy speaking to Captain James Kirk,

in the movie Star Trek IV: The Voyage Home, 1986 The “medical tricorder” in the famed Star Trek television series is a purely fictional

device for the remote scanning of biological functions in living organisms The device remains futuristic, but significant advances in biochemistry have made it pos-sible to decipher the genetic code of living organisms Today, dozens of companies are involved in biochemical analysis at the microscale, with a concentration of them involved in genomics, proteomics, and pharmacogenics Their successes have already had a positive impact on the health of the population; examples include faster analysis of pathogens responsible for illness and of agricultural products as well as more rapid sequencing of the human genome Systems expected in the near future will detect airborne pathogens responsible for illness (such as Legionnaire’s disease or anthrax in a terrorist attack) with a portable unit, give on-demand genetic diagnostics for the selection of drug therapies, be able to test for food pathogens

such as E coli on site, and more rapidly test for bloodborne pathogens.

Conventional commercial instruments for biochemical and genetic analysis, such as those available from Applied Biosystems of Foster City, California, perform

a broad range of analytical functions but are generally bulky The concept of micro total analysis system (µTAS), which aims to miniaturize all aspects of biochemical analysis, with its commensurate benefits, was introduced in 1989 by Manz [1] This chapter begins with an introduction to microfluidics, followed by descriptions of the state of the art of some of the microscale methods used in DNA analysis Finally, electrical probe techniques and some applications are presented A common theme will be the use of glass and plastic substrates, in contrast to most of the devices in other chapters of this book

Microfluidics for Biological Applications

The biological applications of MEMS (bio-MEMS) and microfluidics are inextrica-bly linked because the majority of devices in systems for biological and medical analysis work with samples in liquid form Outside of biological analysis, microflu-idics have applications in chemical analysis, drug synthesis, drug delivery, and point-of-use synthesis of hazardous chemicals In this section, we discuss common pumping methods in bio-MEMS and the issue of mixing

169