Surface-Micromachined Microoptical
`Elements and Systems
`
`RICHARD S. MULLER, LIFE FELLOW, IEEE AND KAM Y. LAU, FELLOW, IEEE
`
`Invited Paper
`
`Optical systems are ubiquitous in the present-day societal fabric,
`from sophisticated fiber-optic telecommunication infrastructure to
`visual information display, down to mundane chores such as bar-
`code reading at the supermarket. Most of these existing systems
`are built from bulk optical components, as they have been for
`many years. Just as miniaturization and batch-process production
`have revolutionized electronics, similar advances in optics will
`certain greatly expand its applications and markets. Production
`techniques for optical systems that employ the emerging micro-
`electromechanical systems (MEMS) technologies give promise of
`achieving this success. Simple micromechanical fabrication tech-
`niques are already employed in fiber-optic components to produce
`what is generally described as silicon-optical-bench systems. New
`developments, especially those permitting the use of microactuated
`structures, make substantial increases in system sophistication pos-
`sible. Surface micromachining, in which microoptical systems are
`batch-fabricated and placed on top of a silicon wafer, has become
`a promising approach to this progression. With the demonstration
`that surface-micromachined elements can be “folded” out from
`the plane in which they are constructed, an important new degree
`of design freedom has emerged. This paper examines some of the
`results obtained and attempts to project possibilities for surface
`micromachining in future optical systems.
`Keywords—Actuated micromirrors, bar-code reader, fiber-optic
`components, gratings, laser-fiber coupler, optical scanners.
`
`I.
`
`INTRODUCTION
`The continuing spectacular growth in optical system
`applications, as exemplified by optical-fiber telecommuni-
`cation infrastructure and display technologies, has stim-
`ulated great interest in producing miniaturized, reliable,
`inexpensive photonic devices for light-beam manipulation.
`There is strong desire for such devices to function without
`
`Manuscript received February 25, 1998; revised April 21, 1998. This
`work was supported in part by a grant from the Hewlett-Packard Science
`Centers program, in part by the Defense Advanced Research Project
`Agency, in part by the National Science Foundation, and in part by the
`Berkeley Sensor & Actuator Center membership.
`The authors are with Berkeley Sensor & Actuator Center, Department
`of Electrical Engineering and Computer Science, University of California,
`Berkeley, CA 94720-1770 USA.
`Publisher Item Identifier S 0018-9219(98)05094-4.
`
`any mechanical moving parts, out of consideration that
`these traditionally contribute to high cost and unreliability.
`For several decades, the only available control options have
`been those based on electro- or magnetooptic effects. Such
`devices, however, generally suffer from high costs and
`typically operate at low efficiencies. In most cases, optical-
`beam manipulation can be more effectively carried out with
`movable mechanical elements such as mirrors and shutters,
`if only these mechanical parts can be built reliably and
`inexpensively.
`Recent developments in the rapidly emerging discipline
`of microelectromechanical systems (MEMS) show special
`promise for providing microoptical systems to perform the
`functions described above. In particular, the technique of
`surface micromachining is interesting in that it is a planar
`process that is capable of producing large-area, high-quality
`layered structures along the plane of the substrate, which
`can then be rotated out of the surface to form large optical
`surfaces angled to the surface of the substrate. Such large
`elevated surfaces (up to millimeters in height) with high
`optical quality, and which are movable with high precision,
`are very difficult to fabricate with other micromachining
`technologies.
`At the Berkeley Sensor & Actuator Center (BSAC), a
`background in the development of surface micromachining
`has provided impetus for the development of actuated
`micromirrors for fiber-optic systems. The basis of our
`approach in microoptical systems rests on the following
`developments: methods for making pin joints and moving-
`element mechanisms [1], comb drives for oscillating sys-
`tems [2], microvibromotors as actuation elements [3], and
`folding of surface structures out of the surface plane to form
`truly three-dimensional structures [4]. Other groups are also
`active in applying surface micromachining to photonics,
`demonstrating innovative ways to use this technology [5],
`[6]. Rather than attempting to capture the full extent of the
`expanding efforts by all groups working in this field, we
`focus in this paper on examples drawn from our work at
`BSAC.
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`PROCEEDINGS OF THE IEEE, VOL. 86, NO. 8, AUGUST 1998
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`1705
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`0018–9219/98$10.00 ª
`
`1998 IEEE
`
`Petitioner Ciena Corp. et al.
`Exhibit 1048-1
`
`

`
`micromirrors for optical-alignment purposes will need both
`precision and range—involving mechanisms reminiscent of
`stepping motors. Here, we describe a specific type of motor
`that has been successfully employed to drive micromirrors
`with submicrometer precision and with a range of hundreds
`of micrometers.
`As shown in Fig. 2, each of the two sliders is actuated
`with an integrated microvibromotor [6], which consists
`of four electrostatic-comb resonators with attached impact
`arms driving a slider through oblique impact. (The mi-
`cromirror shown in Fig. 2 is similar to that shown in Fig. 1
`except that the front sliding plate is folded under the back
`support to save space.) Fig. 3 shows a detailed top view of
`the vibromotor.
`
`C. Fabrication
`The actuated microreflector was fabricated on a silicon
`substrate using silicon-surface-micromachining technology.
`The fabrication process is described in detail in [12]. An n
`polysilicon layer defines a ground plane. Three additional
`polysilicon structural layers are used to define the comb
`drives, sliders, hinges, and mirror beams. Phosphorous-
`doped silicon dioxide is used for sacrificial spacer layers
`between polysilicon layers. A special prerelease etch in
`5 : 1 hydrofluoric acid (HF) followed by a vigorous rinse
`is used to eliminate stringers. Then the structure is released
`for 10 min in concentrated HF to dissolve the oxide and
`dried using a critical-point CO drier [13] to avoid stiction.
`A 400 ˚A gold layer is evaporated onto the mirror surface
`to increase reflectivity.
`
`D. Actuated Micromirror Characterization
`To balance the forces, two opposing impacters are used
`for each direction of travel. The resonator is a capacitively
`driven mass anchored to the substrate through a folded-
`beam flexure. The spring constant of the flexure determines
`the resonant frequency and travel range of the resonator.
`The force exerted by the comb drive is proportional to the
`square of the applied voltage
`
`(1)
`
`typically has both dc and ac components. This
`where
`quadratic response produces a primary frequency driving
`term proportional to the product of the dc and ac voltages,
`effectively linearizing the resonator and increasing the im-
`pact force. The comb structures are driven at their resonance
`frequency (roughly 7.5–8.5 kHz),
`thereby achieving an
`amplification of the electrostatic force by the resonator
`quality factor (typically 30–100 in air [14], [15]). Since
`energy is transferred to the slider during impact (typically
`lasting only a few microseconds), the impacters can deliver
`short-duration forces that are large enough to overcome
`static friction in the sliders and hinges. Due to the damping
`(primarily due to air resistance [12]) in the comb structure,
`the resonators require a few initial cycles to build up
`sufficient amplitude and momentum for impact. Therefore,
`in air, slider motion is observed only after three or more
`
`Fig. 1. Basic micromirror structure for precision alignment of
`optical components. The size of the mirror measures approximately
`200  250 m.
`
`II. OPTICAL MEMS FOR PRECISION ALIGNMENT
`
`A. Folded-Micromirror Structures
`Optical MEMS designed for precision alignment are
`intended to facilitate automated packaging of optoelec-
`tronic devices and subsystems, hence substantially lowering
`the overall production cost of the modules. Examples of
`these modules include fiber-pigtailed laser transmitters and
`external-cavity continuously tunable laser diodes. These
`modules are expensive because they require submicrometer
`alignment
`tolerances that place tight constraints on the
`positional accuracy of such optoelectronics components as
`lasers, lenses, gratings, and fibers. Silicon-optical-bench
`(SOB) technology, commonly used to align optical sys-
`tems on a silicon substrate using etched v-grooves, solder
`bumps, and other integrated circuit (IC)-process-derived
`techniques to achieve
`1 m transverse alignment [7], [8],
`is insufficient for high-performance modules. Instead, we
`promote a paradigm, in which SOB technology—a hands-
`off automated process—is used for initial placement of the
`various optical components in the module, followed by an
`automated active alignment procedure using micromirrors
`that are prefabricated on the silicon substrate using MEMS
`technology [9], [10].
`Details of a movable micromirror is shown in Fig. 1. It
`consists of four polysilicon plates, interconnected by three
`sets of microhinges [11]. The two end plates can slide
`linearly and independently on the silicon-substrate surface,
`confined by hubs on the sides. This arrangement provides
`the mirror with rotational and translational freedom of
`motion and a high vertical aspect ratio in its operating
`position.
`
`B. Vibromotor Actuation
`On-chip actuation is necessary in order for the mi-
`cromachined components to function in a self-contained
`optical module. There exist numerous ways to achieve on-
`chip actuation of microstructures; however, actuation of
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`Petitioner Ciena Corp. et al.
`Exhibit 1048-2
`
`

`
`Fig. 2. Complete self-actuated micromirror. Two sets of vibromotors, comprising four comb drives
`each, drive the front and rear sliders for actuation of the mirror.
`
`m [12]. To ascertain the performance of the micromirror
`assembly driven by vibromotors, optical characterization
`was performed on the integrated microreflector system. A
`HeNe laser is reflected from the micromirror surface onto
`a charge-coupled device (CCD) camera. As the front and
`rear sliders are actuated, the beam position along each of
`the two axes is measured on the CCD and extrapolated to
`a location 200 m in front of the mirror (where a fiber
`would typically be positioned for a laser-to-fiber coupling
`application). The standard deviation of the vertical beam
`position data from a linear response depends on the selected
`slider step size. For the front slider, this deviation is roughly
`equal to the step size, while for the rear slider, the deviation
`exceeds the step size by about 50–60%. The selected
`average step sizes of 0.35 m for the front slider and
`0.42 m for the rear slider produce standard deviations of
`0.32 and 0.60 m, respectively [Fig. 4(a)]. These deviations
`are due primarily to the “play” in the hinges and the
`wobble in the slider structure. The greater length of the
`rear slider results in increased wobble and leads to a
`greater standard deviation. The horizontal beam deviation
`is 0.05 m [Fig. 4(b)] and is comparable to the 0.07
`m deviation measured in externally actuated structures.
`This precision is sufficient for laser to single-mode fiber-
`coupling applications where, due to lens magnification,
`1 m
`the beam only needs to be within approximately
`for high coupling. However, for other advanced optical
`systems such as tunable external cavity lasers, a higher
`angular precision is necessary. Since in earlier experiments
`a microreflector with no on-chip actuators and an alternate
`hub design has shown a vertical precision of 0.17 m [12],
`improving the design of the actuated slider should greatly
`improve the mirror precision.
`
`Fig. 3. Top view of the linear vibromotor.
`
`voltage cycles. The number of initiation cycles depends on
`the ambient atmosphere and decreases at lower pressures.
`When driven with a free-running resonant oscillation, the
`slider reaches a maximum velocity of over 1 mm/s. Slider
`velocity can be controlled by driving the comb drives
`with gated bursts of four to five cycles of the resonant
`waveform. Once the slider is in position, it is kept in place
`by static friction until further actuation. Similar structures
`were previously subjected to shock and vibration tests and
`showed no detectable slider motion at forces up to 500 G’s
`[12].
`Characterization of the vibromotor alone has shown the
`slider motion to have a step resolution of less than 0.3
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`MULLER AND LAU: MICROOPTICAL ELEMENTS AND SYSTEMS
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`1707
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`Petitioner Ciena Corp. et al.
`Exhibit 1048-3
`
`

`
`(a)
`
`(b)
`
`Fig. 4.
`(a) Vertical and (b) horizontal microreflector precision. The vertical position data show
`standard deviations of 0.32 and 0.6 m (for the front and rear sliders, respectively) from a linear
`response. The horizontal beam position has an average 0.05 m of in-plane wobble.
`
`To observe the actuation dynamics of the micromirror,
`a reflected HeNe beam was imaged on a position-sensitive
`detector while the angular position of the mirror was swept
`in real time. The rear vibromotor was biased at 40 V dc
`and driven with 20 V (p-p) resonant (8.2 kHz) square-
`wave pulses. The position of the beam and the drive voltage
`were then monitored on a digital oscilloscope. The resulting
`trace is shown in Fig. 5. As predicted and observed [12],
`the comb structures require a few cycles (three in this
`
`case) to build up sufficient energy for impact. The first
`significant impact occurs during the third cycle, at which
`point the mirror angle begins to change. The mirror is
`further deflected during the fourth cycle; however, sufficient
`energy is lost by the resonator to keep the next impact
`from producing significant displacement. Finally, the sixth
`cycle provides the greatest impact, resulting in a total mirror
`rotation of 0.3 . The small back-and-forth motion apparent
`in the response is most likely due to slight deformation
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`Petitioner Ciena Corp. et al.
`Exhibit 1048-4
`
`

`
`Fig. 5. Time-domain response of the microreflector to a square-wave drive voltage. An impact is
`first achieved during the third cycle, at which point the mirror angle begins to change. The motion
`concludes shortly after the last cycle, resulting in a total angular displacement of about 0.3.
`
`maximum estimated scan rate of 10.2 rad/s or a beam speed
`of 2 mm/s on a plane 200 m in front of the reflector.
`
`E. A Single-Mode-Fiber Laser Transmitter Module
`The actuated microreflector system is used in a laser-
`to-fiber coupling module, as illustrated schematically in
`Fig. 7(a). A picture of the assembled coupling module is
`shown in Fig. 7(b). An aspheric microlens was used to
`image the output of a laser diode into a single-mode fiber. A
`polysilicon micromirror with two degrees of freedom (linear
`displacement and angular position) reflects the laser beam
`at a 45 angle and provides the fine alignment. Lens, laser,
`and fiber are positioned passively on the substrate using
`etched alignment grooves and photolithographically defined
`alignment aids and held in place by low-viscosity epoxy.
`The height of the fiber core is controlled by mounting it
`in a silicon subcarrier that is attached to the foundation
`substrate. Using these techniques, the axial displacement
`and tilt of the optical components can be minimized, and
`the reduction in coupling efficiency resulting from these
`sources of misalignment is negligible. Transverse misalign-
`ment of the laser and lens, however, has a significant effect
`on the coupling, particularly because of the magnification of
`the lens system. As a consequence of our simple passive-
`alignment method, the transverse offsets of the laser and
`lens are on the order of 5–10 m, which, magnified by
`the lens, lead to roughly a 20–40 m displacement of the
`beam on the fiber plane. Therefore, the micromirror must
`has sufficient travel range to compensate for these offsets.
`This coupling module is used to couple light from a stan-
`dard telecommunications-grade 1.3 m distributed feed-
`back laser into a 9 m core single-mode fiber. The actuated
`microreflector was positioned between the lens and the fiber
`
`Fig. 6. Beam scanning with the actuated microreflector. In (a), a
`40-V dc offset was used, resulting in 0.7–1.1 m step size on the
`plane 200 m from the micromirror, while in (b), a 38-V dc offset
`produces 0.3–0.6 m steps.
`
`of the hinge joints during impact as the square pin is
`forced against the staple. The remaining “roughness” of
`the response is due to noise in the detector.
`To demonstrate the use of the actuated microreflector in
`scanning and beam-positioning applications, the mirror was
`used to scan a laser beam across the detector continuously.
`The rear vibromotor was driven with bursts of four 20 V
`(p-p) resonant (8.2 kHz) square-wave cycles spaced 10 ms
`apart, with a 40- or 38-V dc offset. The resulting output
`(Fig. 6) clearly shows the step-wise nature of the mirror
`motion. The average speed of the sweep can be changed
`by varying the spacing of the bursts. Fig. 6(a) and (b)
`also demonstrates that the size of the step itself depends
`on the applied voltage and can be controlled. With an
`average angular step size of 5 mrad, the microreflector has a
`
`MULLER AND LAU: MICROOPTICAL ELEMENTS AND SYSTEMS
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`1709
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`Petitioner Ciena Corp. et al.
`Exhibit 1048-5
`
`

`
`(a)
`
`(b)
`
`Fig. 7.
`(a) Schematic of a laser-to-fiber coupling module showing
`two-dimensional (2-D) optical alignment with two one-dimensional
`(1-D) translational degrees of freedom in the micromirror. (b)
`Scanning electron microscope (SEM) photograph of the coupling
`chip.
`
`to provide fine alignment. Fig. 8 shows the dependence of
`the coupling efficiency on the number of rear vibromotor
`steps. The step size can be varied by changing the drive
`voltage to provide various degrees of control. The data
`show a good fit
`to a Gaussian curve, with deviations
`being due to slider wobble and angular misalignment of
`the components. The coupling efficiency was limited to
`32% in this arrangement due to the “soft-focusing” optical
`arrangement; a tighter focus at the fiber plane will produce
`a higher coupling efficiency but is less forgiving in beam
`misalignment. As seen from Fig. 8, the step size in the
`actuated micromirror is sufficiently small that it can actually
`handle a much tighter focus than what is being shown. Thus,
`a much higher coupling efficiency, in the 60–80% range,
`can readily be achieved with a more aggressive optical
`design.
`
`F. Shock and Vibration Tests
`The micromirror structures are constructed from very thin
`polysilicon plates (typically 2 m thick) and have very
`low inertial masses. As a consequence, the structure can
`be maintained in position purely by frictional and stictional
`
`Fig. 8. Coupling efficiency from semiconductor laser to sin-
`gle-mode fiber using the actuated micromirror. A peak efficiency
`of 32% was achieved.
`
`forces alone. For illustration, a raised micromirror structure
`like the one shown in Fig. 1, without fixation by adhesives
`or any other means, was subjected to a random-vibration
`test (MIL-SPEC 883C) on all three axes, as well as to a 500
`G drop test. To the extent that can be determined visually
`(at a resolution of approximately 1 m), no movement
`of the structure can be detected after all of these tests.
`When the structure is subjected to a 1000 G drop test, the
`structure moved by several micrometers, but no breakage
`was observed on any of the specimens.
`In a separate experiment, a raised micromirror was
`mounted on a variable-frequency vibrating platform and
`was subjected to vibration from 5 to 100 kHz on three
`axes consecutively. The motion of the micromirror was
`monitored by optical
`interferometry through an optical
`fiber rigidly attached to the vibrating platform. There, the
`cleaved fiber facet and the micromirror form a Fabry–Perot
`interferometer and a relative motion between the fiber and
`the micromirror results in variations in the intensity of the
`reflected light. The vibrating platform is piezoelectrically
`driven, and the vibration amplitude is calibrated separately
`using a similar optical
`interferometry technique. The
`results are shown in Fig. 9. The frequency response of
`the micromirror structure can be seen to be rather complex
`and to consist of multiple resonances. This is expected
`from the relatively complex mechanical structure, which
`consists of multiple plates of different sizes interconnected
`by hinges. One conclusion that can be drawn from these
`measurements is that
`the resonance frequency in any
`excitation direction occurs well above 25 kHz, a frequency
`range beyond that of common mechanical or microphonic
`disturbances. These micromechanical devices are thus
`expected to be mechanically robust,
`in contrast
`to the
`strength of bulk-mechanical devices.
`On the other hand, the small inertial mass that contributes
`to the robustness of the microdevices also makes them
`vulnerable to disturbances from air currents. No quantitative
`tests have yet been performed on the effect of air currents,
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`PROCEEDINGS OF THE IEEE, VOL. 86, NO. 8, AUGUST 1998
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`Petitioner Ciena Corp. et al.
`Exhibit 1048-6
`
`

`
`other designs are possible and have been demonstrated
`elsewhere. For example, an electrostatically driven, linear-
`translating microactuator known sometimes as “scratch
`drive” has been demonstrated by Akiyama [16]. Scratch-
`drive microactuators employ polysilicon plates that are
`electrostatically attracted to an electrode on the silicon
`surface. The plates are made with vertical lips that impact
`the surface and are pushed forward when the plate flexes
`under the strain induced by the attractive force. When
`the force is released, the plate tends to “walk” forward.
`Hence, when a series of electrodes are laid out on the
`surface, sequentially applying bias to the electrodes makes
`it possible to walk the plate forward in a controlled fashion.
`These “scratch-drive actuators” have been demonstrated in
`optical applications at the University of California, Los
`Angeles, as effective movers for surface-micromachined
`foldout structures, including a frame to hold a separately
`machined microlens [17].
`
`III. MICROSCANNERS
`Optical scanners have long been used for scientific and
`industrial applications ranging from laser imaging and
`displays to laser surgical
`tools and appliances such as
`facsimile machines and printers. Perhaps their most familiar
`application is to bar-code scanning, a proliferating product
`area that contributes strongly to continuing progress in
`cost reduction. Production of a scanning system for bar-
`code reading or perhaps for a head-mounted raster-scanning
`display by MEMS techniques gives promise not only of cost
`but also of power reduction, as well as of portability and
`shrinkage in size when compared to present designs. The
`eventual integration of activation and detection circuits to
`produce an integrated scanner-chip set (eventually, perhaps,
`a fully integrated MEMS scanner chip) is a motivating
`longer term goal.
`MEMS microscanners built with rotating mirrors have
`been demonstrated [18], [19]. The speed, size, and deflec-
`tion angles of existing micromirrors are, however, severely
`limited by their planar structures and actuation mechanisms.
`For optical-scanning applications, the scanned-image reso-
`lution is limited by diffraction from the smallest optical
`aperture (in many cases the mirror) in the scanner. As a
`result, mirrors having lateral dimensions measured in the
`hundreds of micrometers are required to build practical
`systems. To create high-aspect-ratio optical surfaces us-
`ing processing technologies derived from IC procedures,
`polysilicon microhinges are incorporated in the scanner
`structures. These hinges allow relatively large mirrors to
`be lifted out of the plane of the substrate after planar
`processing is completed.
`Using silicon-micromachining and SOB technologies, ex-
`tremely compact optical systems incorporating low-inertia
`scanners can be built. As an example, shown in Fig. 10 is
`a sketch of such a raster-scanner microdisplay module fab-
`ricated on a chip. The light from three semiconductor light
`emitting diodes or lasers is collimated by a microlens and
`directed onto a pair of microscanners, each consisting of a
`
`(a)
`
`(b)
`
`(c)
`
`Fig. 9. Vibration testing of the micromirror structure on three
`axes. The dotted curves shows the oscillation amplitude of the
`vibration table; the dark curves show the motion recorded at the
`mirror surface.
`
`but it is clear that some form of protective packaging,
`perhaps in partial vacuum, is necessary for field deployment
`of these devices. Packaging is currently being studied.
`
`G. Alternate Means of Actuation
`Although the linear vibromotor was employed as the
`basic workhorse for our microphotonic alignment devices,
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`MULLER AND LAU: MICROOPTICAL ELEMENTS AND SYSTEMS
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`Petitioner Ciena Corp. et al.
`Exhibit 1048-7
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`

`
`Fig. 10. A 2-D head-mounted raster-scanning design using two orthogonal scanning micromirrors
`on a chip.
`
`micromirror and an actuator. Both mirrors stand vertically
`on the silicon substrate and are actuated by electrostatic
`comb drives. A planar image display is achieved by raster
`scanning the light beam in two orthogonal directions using
`the two mirrors. These out-of-plane mirrors interact with
`optical beams that propagate in a plane parallel to the
`substrate onto which other microoptical components can
`be integrated on the same chip. An integrated module like
`the one sketched in Fig. 10 can be packaged in vacuum or
`otherwise sealed to reduce air damping, mirror deformation,
`and particulate erosion of the mirror surface. Also, the
`mechanical Q-factors of the scanners increase significantly
`even under moderate vacuum. Furthermore, the mounting
`and partial alignment and assembling of the mirrors are
`carried out during the fabrication process, and large quanti-
`ties can be produced inexpensively because they are batch
`fabricated.
`
`A. Resonant Scanner Design
`In order that high-functionality optical systems such
`as the display module described above be realized, one
`needs to address basic issues concerning the optical and
`mechanical performances of microscanners. We consider
`resonant scanners, since they can be driven at high fre-
`quency at large scan angles, with small power consumption.
`Fig. 11(a) is an SEM micrograph showing one of the
`resonant microscanners we have fabricated [20]. The size
`of the mirror is 300
`400 m, and the rotation hinges
`are square torsion bars, each measuring 50 m in length
`and 4 m in cross section. Suspended by a frame that is
`connected to a hinged slider at its back, the micromirror is
`inclined 70 to the substrate. Electrostatic comb drives are
`used as the actuators for their low power consumption and
`high resonant frequencies. The electrostatic comb drive for
`this scanner system consists of 100 interdigitated fingers on
`
`(a)
`
`(b)
`
`Fig. 11. SEM photographs of two prototype resonant scanners.
`An electrostatic comb drive is attached to the bottom of the
`scanning micromirror through cross-weaving hinges, while torsion
`bars are used as the suspension and the rotation axis for the
`mirror. (a) A 300  400 m -scanning mirror, which deflects
`the laser beam in the out-of-plane direction. (b) A 440  300 m
`-scanning mirror that scans the beam in a direction parallel to
`the substrate.
`
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`Petitioner Ciena Corp. et al.
`Exhibit 1048-8
`
`

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`Table 1
`
`the shuttle comb and 101 fingers on the stationary comb.
`The comb fingers are 2 m wide and 40 m long, and
`the entire comb extends 1 mm from side to side. The
`comb drive is hinged to the bottom of the mirror through a
`bar-and-folded-spring assemblage. The folded springs have
`beams that are 2 m wide and 300 m long. The maximum
`excursion of the shuttle comb is limited to
`15 m by
`the lengths of the comb fingers. Fig. 11(b) is an SEM of
`another microscanner constructed differently with a mirror
`measuring 440 by 300 m. The mirror in Fig. 11(b) stands
`upright on the wafer, and its rotational axis is normal to the
`substrate. The maximum scan angle of the microscanner
`is determined by the relative position of the rotational
`axis and the allowed range of excursion for the shuttle
`comb. Therefore, the maximum scan angle can be increased
`simply by: 1) rearranging the position of the rotational axis
`and/or 2) increasing the range of the shuttle-comb excursion
`(lengthening the comb fingers), while the rest of the scanner
`structure remains unchanged.
`The hinges that suspend the mirror and the hinge that
`connects the mirror to the comb drive are constantly flexing
`during operation of the scanner, and the angular scanning
`precision of the mirror depends on the mechanical accuracy
`of these hinges. We have investigated pin-and-staple hinges
`similar to those described in previous sections, as well as
`torsional hinges, and found significant benefit in employing
`the latter for this application. These results will be described
`in detail below.
`
`B. Microscanners Characterization and Comparison
`We have fabricated microscanners of three different de-
`signs. Aside from the two scanning mirrors with torsion-bar
`hinges shown in Fig. 11, another scanner with a smaller
`mirror (200 by 250 m) and staple-and-pin hinges (Fig. 12)
`was also used in the experiments. The dimensions and
`relevant parameters of the three scanners are collected in
`Table 1.
`
`Fig. 12. A pin-and-staple hinge scanner. The insert shows a
`prototype design for an integrated bar-code-scanner module on a
`silicon substrate.
`
`One important issue for optical scanners is the scan-line
`repeatability from one scan to the next. To characterize
`repeatability,
`the resonant scanner was mounted in an
`optical interferometric setup, which can measure mirror
`displacement with a resolution on the order of 10 nm. The
`cleaved facet of the fiber formed a Fabry–Perot interferom-
`eter with the tip of the scanning micromirror [Fig. 13(a)].
`As the micromirror oscillates, the amount of light that is
`coupled back into the fiber decreases just as if there were a
`reduction in the reflectivity of the micromirror. The change
`in the effective reflectivity is, however, so small that it
`does not affect our measurements. We quantify the scan
`repeatability by comparing the turning points of the periodic
`interference patterns (i.e., the end points of the scan). For
`the scanner measured (scanner A), the deviation for the
`line-scan ends was measured to be on the order of 1% of
`the full scan. We believe that this inaccuracy resulted from
`the sloppiness in the journal bearings used in the design.
`To determine the dynamic accuracy of the microscanner
`
`MULLER AND LAU: MICROOPTICAL ELEMENTS AND SYSTEMS
`
`1713
`
`Petitioner Ciena Corp. et al.
`Exhibit 1048-9
`
`

`
`Fig. 13. Schematic of a Fabry–Perot interferometer formed be-
`tween the fiber front facet and the top of the scanning micromirror.
`
`over the whole scan (as opposed to only the end points),
`a position-sensing detector was used to record the position
`of the He–Ne laser beam reflected off the scanning mirrors.
`The two scanning micromirrors (scanners A and B) were
`used in the measurements. The control voltage for the comb
`drive was sinusoidal at a frequency close to mechanical
`resonance. The position of the scanning laser beam as a
`function of time is shown in Fig. 14. The dotted sinusoids
`were fitted to the measured data. For scanner A [Fig. 14(a)],
`the scan jitter, defined as the standard deviation from a
`pure sinusoid, was only 0.2 mrad (0.011 ) or
`0.06% of
`the full scan. By comparing to the results discussed in the
`previous paragraph, we conclude that the end points of the
`scan are much less repeatable than the center portion, which
`is the only part of the scan that is used in practical optical-
`scanning systems. Relative to the pixel size (see next
`section for definition), the scan jitter of scanner A was 10%.
`The scan jitter measured for scanner B [Fig. 14(b)] was 0.19
`mrad (0.01 ), or 9% of the pixel size for that mirror.
`The static-positional precision of the microscanner can
`also be measured using the same setup. By varying the
`pure dc bias voltage applied to the comb drive, we can
`statically change the position of the deflected laser beam.
`Plotted in Fig. 15 is the measured laser-beam position as
`a function of the square of the applied dc voltages (up-
`ramp and down-ramp over many voltage cycles), again
`for the two -scanning mirrors used previously. Fig. 15(a)
`shows data for scanner A, indicating an angular-positional
`accuracy of 3.3 mrad (0.19 ) in standard deviation from a
`straight line. In Fig.