Capella 2018
`JDS Uniphase v. Capella
`IPR2015-00739
`
`1
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`US. Patent
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`May 13, 1997
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`Sheet 1 of 11
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`5,629,790
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`FIG.
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`(PRIOR ART)
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`2
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`U.S. Patent
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`May 13, 1997
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`Sheet 2 of 11
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`FIG.
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`13.
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`14
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`14
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`12 ‘
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`16
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`__:_j__
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`_ _1’:,
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`FIG. 1b
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`FIG. 1c
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`FIG. Id
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`FIG. 1e
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`3
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`US. Patent
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`May 13, 1997
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`Sheet 3 of 11
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`US. Patent
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`May 13, 1997
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`Sheet 4 of 11
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`5,629,790
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`U.S. Patent
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`May 13, 1997
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`Sheets of 11
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`5,629,790
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`FIG. 3a
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`U.S. Patent
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`May 13, 1997
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`Sheet 6 of 11
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`May 13, 1997
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`130
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`122 128
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`FIG. 7
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`May 13, 1997
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`134
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`132 K‘’)
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`I?I(:}.
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`59
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`1].
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`U.S. Patent
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`May 13, 1997
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`Sheet 11 of 11
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`FIG} 1221
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`209
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`211::
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`225”1iz'z:.1 EEW25
`I212
`203
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`209
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`2111:
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`207a
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`207b
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` ~ 221
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`~
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`FIG. 12b
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`12
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`1
`MICROMACHINED TORSIONAL SCANNER
`
`DESCRIPTION
`
`1. Technical Field
`
`beam scanners and, in
`The invention relates to
`particular, to rnicrornachined vibratory scanners.
`2. Background Art
`Beam scanners are used in digital imaging, printing, bar
`code readers, optical reading and writing systems, surface
`inspects’‘on devices’ and various scienfific and industrial
`implements. Such scanners defiect a beam of light, usually
`from a fixed source, over an angle ranging from several
`degrees to tens of degrees. The beam sweeps back and forth
`at a frequency determined in part by the mirror resonant
`frequency. A typical vibrational scanner of the prior art is
`described in U.S. Pat. No. 4,732,440 to J. Gadhok. The idea
`of making torsional scanners within a silicon body was
`proposedatanea.rlydai:ebyK.Pet.el'son,Pl‘0c. IEEE,vol.
`70, no. 5, p. 61, May 1982. See also U.S. Pat. No. 4,317,611
`to K. Peterson.
`
`FIG. 1, depicting a scanner shown in FIG. 39 of Peterson,
`Proc. EB, supra, p. 61, includes a mic-romachined tor-
`sional rnirccr 11, supported by torsion bars 13 and 15 within
`silicon body 17 (“micro scanner“ hereafter). The aforemen-
`tioned article describes typical mirror parameters, such as
`the modulus of silicon, the typical wafer thickness, the
`length of the torsion bar and the dimensions of the mirror.
`The width of the torsion bars is on the order of 500
`micrometers, while the length of the torsion bars is approxi-
`mately 0.2
`The mirror is approximately 0.22
`centimeters on a side. The out which isolates the mirror from
`the silicon body and also defines the torsion bars is approxi-
`mately 0.02 centimeters in thickness. Each cut is made by
`anisotropically etching the silicon. The silicon body rests on
`glass substrate 21 which has vapor deposited electrodes 23
`and 25. A depression 27 is etched into the glass to receive
`silicon body 17 which rests on a linear support ridge 29. A
`high voltage is applied first no one electrode then the other
`in a continuing out-of-phase sequence from a drive circuit.
`Theelecu-icfield generatedby the electrodes tiltsthemirror
`firstto one sideandthen the oti1er.'I‘herestoringforoe ofthe
`torsion bars works against each deflection. The resonant
`frequency ocl’ the mirror can be calculated with well known
`formulas cited in the above-mentioned articles, although air
`dampingcreatesan'emorinti:eresonanceheqnency.'Ihe
`substrate. electrodes and drive circuit are part of the micro
`scanner.
`
`Two dimensional rnicromacbined silicon flexure
`structures, used as gyroscopes, are known in the art. See
`U.S. Pat. No. 5,016,072 to P. Cireitf. Such structures are
`similartomicroscannersinconstructionandvibratory
`characteristics.
`
`One of the problems encountered in the prior art is in
`restricting vibrations to a single desired torsional mode. An
`object. of the invention was to devise a micro scanner which
`vibrates at a single desired mode of vibration and to be
`self-oscillating at its natural fundamental frequency.
`
`SUMMARY OF THE INVENTION
`
`The above object was achieved in a micro scanner having
`a primary vibrational mode, the torsional vibration mode,
`substantially lower in frequency from other modes by at
`least 20%. By providing the specified frequency separation.
`the micro scanner will respond primarily to the desired
`mode. Mirror thickness,
`torsion bar length. mirror
`
`5
`
`10
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`15
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`2
`dimensions, as well as drive characteristics can all define the
`vibrational mode spectra. Choices are made
`In
`contrast, while the prior artrecognized the existence of other
`vibrational modes, no attention was paid to separation of the
`frequency of the lowest mode. We have discovered that by
`separating the torsional mode frequency from other vibra-
`tional modes, energy transfer into the principal vibrational
`mode, the torsional oscillation mode. is enhanced and other
`undesired modes are suppressed.
`It has also been discovered that other silicon structures
`can complement a micro scanner. For example, a second
`mirrorwhichis arrangedto vibrate out ofphase with thefirst
`mirror can cancel torques injected into the support structure.
`Anotherexampleis nsilicondetectorwhichcanbemadeto
`surround or be adjaoentto the minor. An advantage of such
`astructureisthatthernirrorandthedetectorcanshare
`common torsion bars so that the mirror and detector always
`pointinthe same direction andthernirrorreceives aconstant
`collection angle. This solves the problem of correctly aiming
`a detector to receive a reflected beam. Another structure is
`
`a pair of plates, each supported by torsion bars within one
`silicon frame, with the plates driven in phase. Both plates
`maybemirrorsoroneplatemaybeamirrorandanother
`plate aphotsodetector, or both are combined mirror-detectqs.
`Yet another structlne is a two-dimensional scanner in which
`two sets of torsion bars are provided to two concentrically
`mounted frames supporting a single n:u'rrcr. One frame
`causes scanning at a first rate while the second frame causes
`scanning at a second rate. Such an approach would be useful
`for raster scanning because the horizontal scan rate is
`usually at a substantially higher frequency than the vertical
`scan rate.
`
`Arnicm machined containerhas been devisedhaving a
`thin tough transparent window which can maintain vacuum
`conditions inside the container, but allow the beam to be
`reflecsed from the mirror without substantial beam aberra-
`tions.
`
`40
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`Micro scanners of the present invention may be driven
`eleclrostatically from the front or back sides of the mirror, or
`both. In addition, a magnet and galvanometer type drive may ‘
`be used above or in combination with an elecuostatic drive.
`An integrated torsion sensor is used for either stimulating
`self-resonance or as an angle sensing device for feedback
`control of mirror position.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is aperspective assembly view of a micro scanner
`of the prior art.
`FIGS. la-le illustrate micro scanner mirror vibrational
`modes
`
`FIG.2is atopvicwofamicroscannadnaccordwith the
`present invention.
`FIGS. 2a, 2b, 2c and 2d are side plan views of alternative
`micro scanners in accord with the present invention.
`FIG. 3is ataopdetailview ofaportionofataorsionbarof
`a miao scanner in accord with the present invention.
`FIGS.3aand3baresideplanviews ofamethodof
`malcingamicroscannerinaccordwiththepresentinvention.
`FIG. 3cisatopp1an viewofthemicro scannershownin
`FIG. 3b.
`Flfidissnelecnicalsdreinaticofaneiectricalcircuitfor
`driving the electrodes ofthe micro scanner ofF[G. 2.
`FIG. daisawaveformdiagramforrectifiedcmrenrtoflre
`stripe electrodes in FIG. 4.
`F.[G.Saisatopplanview andFIG.5bisasidepIanview
`of a micro scanner of the present invention with a galva-
`norneter coil drive apparatus.
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`FIGS.6and7aretopplanviewsoftwodifl’erent
`embodiments of dual mirror scanners in accord with the
`present invention.
`FIG. 8 is a top view of a combined micro scanner and
`aligned photodetector in accord with the present invention.
`FIG. 9 is a plan view of the combined arrangement shown
`in FIG. 8 used in an optical scanning system.
`FIG. 10 is a top view of a dual in-phase micro scanner and
`photodetectcr combination in an optical scanning system.
`FIG. 11 is a plan view of the combined arrangement
`shown in FIG. 10 used in an optical scanning system.
`FIGS. 12a and 121: show respective top and side views of
`a two-dimensional micro scanner in accord with the present
`invention.
`
`BEST MODE FOR CARRYING OUT THE
`INVENTION
`
`With reference to FIGS. 1a—le, various vibrational modes
`of torsional scanners are shown. FIG. 1a depicts a top view
`of a desired orprincipal torsional mode of a micro scanner
`in accord with the present invention. This mode is desig-
`nated asmode 1herein.The scanningmirror 12has opposed
`axes 14 and 16 which twist in the direction shown by the
`arrows. In FIG. 1b, a vertical shaking mode is shown in a
`side view wherein the mirror 12 is moving up and down in
`the directions shown by the arrows, leaving a horizontal
`plane. This mode is designated as made 2 herein.
`FIG. 1c shows mirror 12 in a vertical rocking mode in
`which the mirror also leaves a hcrizontal support plane at the
`ends of the mirror. but not at the center. This is herein
`designated as mode 3. FIG. Id shows a lateral shaking mode
`where the mirror 12 moves first in one direction, then in an
`opposite direction within the support plane. This will be
`termed mode 4 herein. FIG. 1a shows a lateral melting mode
`in which the mirror 12 twists in one direction, then twists in
`an opposite direction. within the horizontal support plane.
`This will be termed mode 5 herein. Modes 2-5 are
`undesired, but cannot be completely eliminated. Other
`modes. called plate modes, are possible but for most
`applications, the frequency of plate modes are much higher
`and would beremoved if separation is achieved with respect
`to the modes which have been illustrated. More complex
`modes are also possible, again usually at higher frequencies.
`In the present invention, the frequency of modes 2-5 is
`separated from the frequency of mode 1 by a minimum
`frequency ratio. By maintaining an adequate separation,
`between the lower torsion mode and the next higher mode,
`less energy is transferred to these undesired modes. In the
`present invention, the frequencies of the various modes are
`shown in the following table for a typical configtnation:
`
`TABLE 1
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`
`5
`
`10
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`13
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`20
`
`35
`
`45
`
`50
`
`55
`
`adequately thick to withstand impact forces in the environ-
`ment in which the micro mirror is used. When driving a
`micromachined mirror, certain forces described below are
`applied asymmetrically. For example. spaced apart elec-
`trodes can drive one-half of the mirror and then at a later
`time the other half. On the other hand. other forces. such as
`magnetic forces, cause a symmetric application of the driv-
`ing couple. In the symmetric application. the vibrational
`h-equency or” the torsional mode may be closer to the
`undesired modes. say within twenty percent. For the asym-
`metric application. the separation should be at least thirty
`percent.
`With reference to KG. 2. a section 31 of a silicon wafer
`is shown. The wafer is etched to define a smaller wafer
`section 33 supported within the larger section 31 by means
`of torsion bars 35 and 37 which are integral to both sections.
`The surface of smaller section 33 is polished in the manner
`of commercial silicon wafers so thatit has a shiny, reflective
`surface. Mounted either below or above the wafer and
`slightly spaced therefirom are the electrodes 41 and 43,
`indicated by dashed lines. These electrodes will be alter-
`nately charged by voltages which cause electric fields which
`attract the smaller section 33. hereafter referred to as mirror
`33. which is electrically grounded through the torsion bars
`and to the surrounding larger section. Note that there is no
`fulcrum or baclrplane support as in FIG. 1. Only the torsion
`bars provide support. Apart from this, the overall design of
`the scanner mirror of the present invention, up to this point,
`may be in accordwith the prior art. However, the mass of the
`mirror and the dimensions of the torsion bars and perhaps
`othervariables are selected so that the torsional mode is well
`separated Also, the electronic circuitry associated with one
`of the torsion bars, described below, as well as torsion bar
`support radii are new.
`In FIG. Zn, the electrodes 142 and 144, corresponding to
`electrodes 41 and 43 in FIG. 2, are shown on an insulative
`substrate 45. The larger wafer section has opposite sides 131
`and 133 which are disposed on the glass substrate 45 and
`havearectangularshape sinnlartothc sectional ofFIG.2.
`The mirror 135 is supported by torsion bars from the larger
`silicon section in a position spaced above the electrodes 142
`and 144. Above the sides 131 and 133 at the larger silicon
`frame is a portion of a second wafer having opposed edges
`141 and 143. Optionally, the edges 14] and 143 support a
`vapor deposited very thin membrane window 145 (or any
`n-ansparent window) if a sealed container is desired
`The entire structure is fabricated using semiconductor
`processing techniques. Atop the dielectric substrate 45. the
`electrodes 142 and 144 are vapor deposited metal stripes
`which are patterned on the silicon dioxide coating on the
`substrate 45 using standard photolithographic techniques.
`The silicon section having sides 131 and 133 and the integral
`mirror 135 are separately fabricated by anisotnopically etch-
`ing a silicon wafer. Only opposed torsion bars snpportmirror
`135. The micrornachined silicon housing described above is
`preferred. but not necessary. A conventional box with a
`transparent top could also be used. When a membrane
`window is used. the window is made suficiently tough so
`that h-ansparent electrodes may be deposited directly on the
`membrane. With reference to FIG. 2b, electrodes 142 and
`144 arevaytltinindiumtinoctidestripesdepositedon
`window 145. The stripes may be only a few molecularlayers
`in thickness because very little current is conducted by the
`electrodes.
`
`vibrational Frequency {Hz}
`Mode 1
`Mode 2
`Mode 3
`Mode 4
`
`High
`14,100
`71,400
`12,000
`214,000
`PM
`Mid
`Freq
`Low
`92
`930
`1,600
`18,500
`Free
`
`
`1.500
`
`3,200
`
`5,300
`
`15,900
`
`In general the separations described above for the tor-
`sional mode at any given fieqnency range are achieved by
`designing the torsion bars as thin and narrow as possible, yet
`
`Thethicl:nessofthemirror12,33or 135rnaybeequalto
`the thickness of the wafer, or less. For high frequencies of
`operation, the mirror thickness is typically a fraction of the
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`wafer thickness. Minor thicltness may range from le than
`one micron to tens of microns. The preferred method of
`rnannfacture. involves use of a Simox wafer, or similar
`wafers, e.g. silicon on insulator substrates, where the mirror
`thicl:nessisdeter'minedbyanepitaxi.allayer.Singlecrystal
`silicon is preferred both for the mirror and the torsion bars
`because of its superior strength and fatigue characteristics,
`as compared to metals or polysilicon. For low fiequencies of
`scanner operation, typically below 100 Hz, if the mi.rror's
`thickness equals only that of the epitaxial layer, then the
`length of the torsion bars makes them too ffagile to with-
`stand liquid processing or shock within their working envi-
`ronments. The full thickness of the water's epitaxial layer
`should be used to form the torsion bars in this situation. The
`torsion bars would now be much broader and shorter, but
`their thickness would still be set by the epitaxial layer’s
`thickness. However, the mirror would be much thiclner
`equaling the total wafer thickness depicted in FIG. 3a. The
`wafer about the rnin'or’s mass around the center can be
`mostly etched away producing a box frame structure such as
`that illustrated fortheframe 207 depictedin FIGS. 12:! and
`121:. This aifects the resonance frequency very little, as well
`as the moment of inertia, but reduces the mass of the mirror
`and hence the forces on the torsion bars. Construction of the
`thicker section is explained below with reference to FIG. 30.
`Once completed, the larger structure has a light trans-
`sive window mounted above the scanning minor. This is
`done by taking a second silicon wafer and vapor depositing
`a layer of silicon nitride, silicon carbide or boron nitride over
`the wafer and then etching away the supporting wafer down
`totl1ethinvapordepositedfilm.AthiulayerofSi couldalso
`be used The edges 141 and 143 me sides of a second wafer
`snucturejoinedto opposing edges 131 and 133 ofthelarger
`section of a that wafer structure. The two congruent wafer
`sections are joined by a variety of processes such as anodic
`bonding, silicon to silicon bonding, solder glasses, etc. all
`done in a vacuum environment This creates vacuum con-
`ditions inside of a closed container The method of manu-
`
`facturing the thin window 145 is described in US. Pat. No.
`4,468,282 toA. Neukennans. Thepatent describes thin films
`having a thickness in the range of a few microns. The area
`ofthe window foramicl-oscannerwouldbeabouthmnxl
`mm. The advantage of such thin films is that optical aber-
`rations are eliminated. The filmwbich is selected should be
`
`substantially transmissive of light, with little absorption so
`thattheiilmwillnotbedarnagedbyanincidentlasmbeam.
`By providingavaurumcontainerforrnirror 135,damping
`dnetoairiseliminatedandthernirrorwilloseillaieto
`frequencies ranging up to sevaal tens ofthousand hertz. It
`should be noted that a vacuum enclosure is not necessary,
`but greatly helps in reducing the voltage needed for elec-
`uostatic drive, as well as for magnetic chive. Because the
`mienomaehiued mirrors are diflicult to clean, a dust cover is
`preferable. The windows, in a non-vacumn environment,
`serve as a dust cover. Large electrostatic voltages attract
`particles to the surface of the mirror and so the enclosure
`serves several purposes.
`'I'heu1in'orccnstruct:ionofFIG.2cissimilartothe
`construction of FIG. 2a. In this case, the insulative substrate
`245 supports the larger wafer section having side walls 231
`and 233. The smaller wafer section 235 is supported by
`mrsionbarswithinalramedefinedbythelargerwafer
`section, as in FIG. 2. A second wafer 2.40 has a vapor
`deposited‘ thinfilmwLndo'w245thereover which‘ is similar
`in materials and construction to the thin film window 145 in
`FIG. 2a. The second wafer 24!} has side walls 241 and 243
`andabottomwall 247 with an etched opening 249below the
`
`6
`film245. The bottom wall portion 247 overhangs mirror 235
`andhas electrode stripes 242and244 ontheundersidecfthe
`bottom wall 247 facing mirror 235. Just as previously
`mentioned, the electrode stripes 242 and 244 are conductive
`material that is vapor deposited once the bottom wall 247
`has been formed by etching the second wafer and an oxide
`coating applied. The electrode stripes 242 and 244 perform
`the same function as previously, deflecting mirror 235 by
`electrostatic force generated by alternate high voltages
`applied to the electrodes. Because of the overhang of bottom
`wall247 overthemirnor235, the opening249 will notbeas
`large as the openingprovicled in FIG. 2a where the mirror
`drive force is from the rear of the 1'I'.l.l:l:l'01'.It is possible for
`auxiliary stripes, not shown, to be placed below the mirror
`inFIGS.2aand2bsothatelectrodesarebothaboveand
`below the mirror. Driveforces are synchronized between top
`and bottom electrodes so that diagonally spaced electrodes
`are both pulling. This symmetric pull relationship between
`electrodes above and below the minor will strengthen the
`vibratory force applied to the mirror and will assist in
`principal mode selection because of the symmetry.
`InFlG.2drnirror135ismaderefiect:iveonbothsides.
`'I‘hethinwindow145'hasaeentralopening150whieh
`admits a beam 152 directed toward the center of vibration of
`themi1:ror.Asimilarbeam1S4isdirectedtothebacksurfaee
`of the mirror. In this manner, both front and back surfaces of
`the mirror can deflect diife-rent beams.
`
`InFlG. 3,adetailoftorsion bar37, suspendingrnirror33
`iron the larger section 31 of a silicon wafer. The torsion bar
`37 may be seen to have rounded corners 32, 34 formed by
`an anisotropic silicon etch. Therounding of corners removes
`stress concentrations. The radius of rounding should be at
`least equal to the thickness of the torsion and preferably it
`should be near the width of the torsion bar.
`
`In fabricating torsional scanners in accord with the
`present invention, commercially available Simox wafers are
`preferred as substrates for construction of the torsional
`scanners. Widr such Wafers, the silicon lefi
`afiea‘
`etching, is single crystal and stress free. The silicon does not
`cut], which is extremely important for mirror applications.
`Three well-controlled thicknesses of the mirror plate and
`torsion bars are obtained, giving well-controlled results over
`the entire wafer. Simox wafers have a built-in etch stop
`which greatly eases fabrication of mirror and hinges. Thick
`uniformmirrorplates andtorsionbars are madeintbis way,
`with thicknesses up to 100 microns. Germanium-
`coinpensated boron-doped silicon, and electrolytic etching
`of epitaxial layers can also be used. In FIG. 3a, a Simox
`waferisillush'atcd.'l11et:opepita::iallayer66isafewtoS0
`microns thick, the silicon dioxide layer 64 is about 2,000
`Athickandthebaselayerézistypicallyafiflo micron layer
`ofsing1eo'ystalsilieon.Acavityisetchedfxomd1ebackside
`of tire wafer, using standardandanisotropic etchants, such as
`EDP. This end: is automatically stopped at the oxide layer
`64. Subsequently, the epitaxial layer 66 of the wafer is
`patta'nedtodefineamirror"t"0andtorsionbars74.'I'he
`oxidelayer64isremovedintl1edesired.placesandthe
`exposed patterned silicon is etched in a reactive ion etch
`reactor, using, for example, chlorine as an enchant. This now
`delineatesintheepitaxiallayerfiodiemirrorilo andtaorsion
`bars '74 and produces straight walled torsion bars 74. An
`anisotropic etcbant could also be used, producing a trap-
`ezoidal cross-seetion for the torsion bars 74. After removal
`
`ofalloxides,themi1rorisfteestandingandcanbecoated
`with thin layers of metal or dielectric to enhance reflectivity.
`NotethatinFIG. 3b, thecnts72intheepita.xial1ayer66are
`made concurrently with definition of the mi::nor70 and the
`
`15
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`35
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`55
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`15
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`torsion bars 74 which support the mirror within the support-
`ing silicon wafer portion 68.
`To form the torsion bars. the front side of the wafer is
`patterned. The front mask is aligned to the back side of the
`wafer, with a two-sided aligner. Oxide layer 64 is removed
`after patterning and the exposed silicon is etched in a
`reactive ion etch reactor. using chlorine as an etchant. This
`pa-ocedIn'e gives rise to a straight walled torsion bar 74.
`Alternatively. the cuts 72 and the torsion bars 74 may be
`etched using anisotropic etchants such as K0}! or ED, or
`isotropic etchants such as I-lF.t'I-INOSII-I20. Oxide is again
`removed. leaving free standing torsion bars supporting the
`mirror.
`
`An alternate etch stop technique is the well-known elec-
`trochemical etch stop. In this method, an n-type layer is
`epitaxially grown on a p-type substrate. By applying a
`voltage to the n-layer during the etch. it is possible to etch
`the ptype substrate without etching the n-layer (see re:t”.).
`This method can be used to make n-type membranes of
`precisely determined thickness, which can then be patterned
`and etched to form mirrors.
`
`As an alternative to an etch stop layer. a plain silicon
`substrate can be time etched to form membranes of the
`desired thiclcness. which can then be patterned and etched to
`form mirrors.
`
`In achieving maximum deflection. breakage of the torsion
`bars is a risk. However, since cracks usually originate and
`propagate from the surface. the surface can be hardened by
`conversion of the surface into silicon carbide or silicon
`nitride. This is done by exposing the stniace of the bars to
`hydrocarbons or ammonia for niuidation at 900° C. This
`causes the top several thousand angstroms of silicon to be
`converted into silicon carbide or silicon nitride. See also the
`method of I. (haul and E. Wagner. Applied Physics Letters,
`21. No. 2. p. 67 (1972) relating to conversion of monocrys-
`talline silicon to polycrystalline [3-silicon carbide using
`methane. The scanner mirror must be protected with oxide
`during the silicon hardening process. The tensile strength of
`silicon carbide is approximately three times that of silicon.
`Referring again to FIG. 3. before the etching of the mirror
`andtorsionbars.afirstpairofcontactpads36and38are
`aligned along the axis of the torsion bar. A second pair of
`contactpads46 and48 are transverse tothefirstpair.Each
`of the contact pads has a respective wire lead 44 deposited
`on the torsion bar leading outwardly to the larger section of
`wafer material and to electrical connection with the circuitry
`described haein. Contact pads 36 and 38 are provided for
`the purpose of establishing a ctn-rent flow generally coaxial
`with the torsion bar axis and between the contact pads 46
`and 48. Actually, only one pad 36 could be used as a current
`injection point and the two nearest pads used as current
`sinks. Torsion in the bar then causes the ratio of the currents
`to change. Upon twisting of the torsion bar, a voltage is
`generated between pads 46 and 48. ‘The mutually orthogonal
`contact pads 36, 38 and 46, 48 may be used in two ways. In
`one mode, the torsion sensor is used for self-oscillation of
`the resonant strucun'e. ‘The generated voltage is used in a
`positive feedback scheme. by reinforcing this signal with
`force members which apply electrostatic or electromagnetic
`forcestothemirror,mah'ngthemirrorresonateatits
`principal torsional frequency. In a second mode, the trans-
`verse voltage generated by the twisting mirror is a
`transducer. measuring angular deflection of the mirror. and
`so the signal may serve to indicate the angle at which a beam
`is being deflected.
`In the situation where a DC voltage is applied across pads
`36 and 38, slight movements of the torsion bar are converted
`
`8
`to vibrations at the resonant frequency of the torsion bar. In
`this sense. the electrical sensor is a feedback mechanism
`which helps the mirror attain resonance at the mincipal
`vibrational frequcy. If an AC current is applied to the
`sensor. say 20 kHz, then the torsion signal becomes a
`modulation envelope of the imposed frequency. The benefit
`of the modulation envelope is that it is easier to detect and
`thus monitor vibrational modes. beam position or the like in
`the presence of large drive signals. The torsion bar is
`preferably aligned in the <1l0> direction for n-type silicon
`or the <100> direction for p-type silicon. These orientations
`are not only optimalfor shear stress. but these arrangements
`are nearly insensitive to uniaxial strains in these orienta-
`tions.
`
`In FIGS. 4 and 4a. an electrical circuit is shown which
`provides the drive signal for the stripe electrodes 41 and 43
`in FIG. 2. A sinusoidal or square wave low voltage input
`signal is applied to transformer primary winding 51. The
`secondary winding 53 of a ferrite core transformer steps up
`the input voltage to a higher level at a 50 to 1 turns ratio. A
`commercially available part such as Phillips 36222 PLUG-
`3Ei2Awill suffice. The secondary winding53 has a grounded
`side 55 and a hot side 57 which is rectified by one or more
`diodes 61 in a first leg and one or more reversed biased
`diodes 63 in a second leg. The diode string 61 of the first leg
`provide a rectified half wave '7]
`to electrode 43. The
`reversed biased diode suing 63 provides a rectified half
`wave 73 to electrode 41 at peak voltages as high as 1000
`volts. High Value bleed-ofi resistors 75 and 77, about 10M
`ohms. are used to discharge the plates. These voltages which
`are seen in FIG. 4a to alternate from positive to negative
`corresponding to one electrode then the other, pulling from
`one side then the other, causing mirror reciprocation.
`With reference to FIGS. 51: and 5b a torsional scanner 8]
`is shown having a central mirror 82, torsion bars 84, contact
`pads83to be usedas aposition sensorin accordwith the
`description given in relation to FIG. 3, and a circumferential
`loopcoil85.Thecoil85isaconductive1oopwhichmaybe
`formed by patterning or vapor depositing conductive mate-
`rial onto the silicon mirror 82 about the periphery thereof.
`The object is to establish a magnetic field within the coil
`perpendiculartothe mirror. The coilorloopis insulated with
`silicon dioxide or another insulator. The conduclive moss-
`over of conductor 87 is accomplished by well known
`layering techniques. Alternatively. conductor 8'? can be
`terminated prior to the crossover location and, if the under-
`lying silicon is sulliciently conductive, may be used as a
`groundreturn path. Current is broughtinto the coil by means
`of a conductor 86 on binge 84 and current is removed by
`conductor 87. An external magnetic field is applied parallel
`to the plane ofmirror 82, as indicated by arrows 90. The
`magnetic field applies a moment to the mirror when ct.u:rent
`flowsinthecoil85tendingtottn-nthemirroroutofits
`starting plane. ‘This force is balanced by torsion in the torsion
`bars and is measured by an angle transducer associated with
`the contacts 83. The coil 85 thus behaves like a ga1vanom-
`eter coil, with greater amounts of current causing greater
`amounts of deflection. Before the limit is reached, current
`may be reversed and the mirror will rotate in the opposite
`direction, In this manner, a miniature scanning mirror,
`driven by magnetic forces may be built.The signal from the
`torsion sensor 83 can be used, in the feedback loop, to
`provideany desiredscan profile, suchas alinearor sinu-
`soidal scan. Eiectrostatic forces can also be used in the
`feedback scheme, but are more prone to instabilities. Also,
`the magnetic driver described above may be combined with
`the electrostatic driver previously described
`
`10
`
`15
`
`25
`
`3D
`
`55
`
`65
`
`16
`
`16
`
`

`
`5,629,790
`
`9
`FIG. 6 shows that a single silicon name 102 may support
`apairofmicro scanners 104 and 106,eachsupported bye
`pair of torsion bars 108 and 110, respectively. Stripe elec-
`trodes 112 beneath the two mirrors provide torques in
`opposed phases so that the mir:rors reciprocate as indicated
`by the arrows A and B, La. oppotely. By applying opposite
`torques to the two mirrors, the torques transfared to the
`larger silicon section 102 cancel each other, thereby lessen-
`ing vibration which must be absorbed by the larger section.
`InFIG.7,apairofrnir1*orsl22andl24aresnpported
`within the larger silicon section 12.6. The mirrors are sup-
`ported in a closely spaced relationship by torsion bars 128
`and 130 with drive stripes 132 and 134, indicatedby dashed
`lines. below the respective mirrors. The plates drive the
`mirrors in an out-of-phase manner, as in FIG. 6, indicated by
`the arrows C and D. Once again, the out-of-phase relation-
`ship of the two mirrors removes vibration in the larger frame
`126 by nulling opposing torques. Since the mirror resonators
`are close together, as defined by a lithographic process, they
`are likely to be nearly identical in resonance charatteristics,
`and their amplitudes (and Q) are expected to be nearly
`identical. To further nine and equalize the resonance
`frequencies, laser trimming can be used either to remove
`deposited metal on the silicon surface, or from the hinge. or
`part of the silicon section itself. Hence, almost complete
`cancellation of the torques takes place. With this
`arrangement, mirrors with resonant frequency dilferences as
`small as one part in 1.000 have been made and torques have
`been cancelled to within a few percent, without any laser
`trimming of the mirrors. It should be noted that when this
`arrangt is used in a rt-y drive, then the arrangement of
`FIG. 6 is preferred. This mrangement allows for larger
`angles around the x-axis because the extent of the mint:
`arrangement is smaller in the y direction. Any combination
`of oscillating plates can be used. so long as the total torques
`cancel.
`
`InF‘IG. 8. anintegraedrnirrorandphotodiodearrange-
`ment is shown. The central mirror region 168 forzrned of
`silicon, possibly metal coated, is surrounded by a photo-
`diode array 162. For example. if the silicon is n-type, a p
`implantcanbemadeinthisregiouandp"coutact164isused
`tomalreconmcttaothe diode. N-I- contact 'l56is the other lead
`
`of the diode. Standard guard regions isolate the photodiode
`may from the central mirror region 168 and edge of the
`mirror plate. Other types of photodicdes, such as pin,
`Schotflrey,Avalanche or the like may be used, rather than the
`diodes described herein. The advantage of the structure of
`FIG. 8 is that as the central mirrorregion 168 oscillates. the
`photodetector region 162 moves with it at the same angle so