lEE!' PHOTO:>l'CS TECII:"(lLOr.Y LETrERS. VOL. 6.1'0. 9.SEf'TEMBER )994
`
`IlllJ
`
`Silicon Modulator Based on Mechanically-Active
`Anti-Reflection Layer with 1 Mbitlsec
`Capability for Fiber-in-the-Loop Applications
`
`K. W. Goossen, J. A. Walker, and S. C. Arney
`
`Abstract-We prescnt a micromcchanical modulator ror fiber(cid:173)
`in-the-Ioop applications with projected optical bandwidths from
`1.3 to 1.55 11m and data rates or several Mbil<;/sec. The device
`behaves as u damped oscillator. lVe have made a device with a
`ringlng frequency of 1.1 MHz and a damping time constant or
`1 liS. We indicate that ~ith an appropriate linear filter the device
`could operate digitally with data rates of 1 Mbitlsec.
`
`F OR most ncar term locul access tiber optic applications,
`
`upstream data r;lles (from the home to Ihe central office)
`of a 100 kbits-fcw Mbits!sec may be adequate in many
`situations. In particular. as networks arc installed it may be
`desirable to allow customers a reduced-cost option of simple
`audio or compressed video upstream data. with the possibility
`of later upgrade.
`In II J and 121. a s}stcm is described which converts CW
`portions of' dDwnstream
`light into upstream data. This is
`accomplished by tapping downstream light into a return fiher.
`Data is imprinted upon the n::turn light by means of a light
`modulator, thus avoiding the use of a laser at the subscriber
`terminal. For very high return data rates (e.g., hundreds of
`Mbitslsec), a lithium niobate or semiconductor modulator is
`available yet costly. As mentioned above it is desirable 10 have
`a lower-cost. lower-performance option for the modulator.
`Note that this still allows high downstream data rate,. We
`present here a modulator for this option, a micromechanical
`device fabricated on silicon using standard microelectronic
`techniques. Since this device operates surface-nom1al. tens of
`thousands may be fabricated Oil a wafer. In addition, the device
`requires only coarse:: lithography (2 miaon linewidth rules
`would suffice). Thus the cosiOI' the device (minus packaging
`costs) will be a few pennies. 'The surface-normal mode of
`operation should also make liner attachment inexpensive.
`The device hcrc is based on optical interference effects
`between a suspended. vertically moving membrane and the
`suhstrate. In Fig. I we show our device, which consists of
`a membrane supported by arms above an air gap. The air
`gap is created by complele undercut etching of a sacrificial
`
`Manust:ript received :"-pril 2g, 19Q4~ revised June 6. 1994.
`K. W. GrKls,en, and 1. A..
`\lr'alkrf are with AT&T Bell Laboratories,
`Holmdel. NJ (773) USA.
`S. C. Arnev i.\ with AT&T Be1l L~hor~lOries. MurraY HilL NJ USA
`Log Numkr 9404,36.
`•
`I Rased on the costs of chips fabricated using the MOS1S fuundry service.
`assuming an iJrca per device of 100 x 200 mit'rons. resutts in 13 cents per
`device. Wafer-scaJe CO$ts should be Ie,s.
`
`.,(cid:173) ,
`
`air gap
`
`doped silicon
`
`(b; side vieN. no bias applied
`
`... , ~
`
`-n 112
`
`n
`
`J-ig. l. Top (a) and undeflected (b) and deHected side views of our modulatur.
`
`(c) wilh bias applied
`
`byer. Similar micromechanical devices have been explored
`previously [3J, [4J, [5J. but to our knowledge ours is the first
`device to employ a simple but powerful principle that allows
`high contrast without diffraction. That principle is essentially
`t:) fabricate a vertically-moving anti-rencction coating. This
`fives specific advantages to earlier devices. In (51 a Fabry(cid:173)
`Perot modulator was made. in which the membrane comprised
`a multilayer mirror. the cavity was the air gap, and a second
`nullilayer mirror was fabricated on the substrate to form a
`tigh finesse cavity. The drawbacks of Ihis device are that it
`~. as a low optical bandwidth (-10 nm). and that precise control
`cr the mirror renectivity's must be aChieved to null the cavity.
`F:eference [4] presented a vertically-moving membrane with
`interference-induced modulation, but no particular design prin(cid:173)
`ciple was applied so only low (~2: l) contrast was achieved.
`In [5], a vertically-moving grating was made, so that device
`s",.itched between renection and diffraction. so high contra~t is
`
`1041-1 B5194$()4.00 @ 1994 IEEE
`
`FNC 1042
`
`
`
`

`
`IIlO
`
`O.B
`
`IEEE PHOTO:-lICS TECHNOLOGY LEITERS. VOL. 6. :-JO. Y. SEPTEMBER 1994
`
`, .' 'c'-~)";"..-c'Z",-",50':"I·:=.r:r.:"-,,..,-;-:--;-c.,-:-.:-.:-. ,.,rf-:.-';O""'="'.",-,-;-.-:-"l07'.,:,,O~lV~. _______ _
`
`;
`
`l~-=\--~-----------
`
`,
`I
`1-- --.: .. __ I ------ -.-----. -
`L:-:..
`.1 ' • • • • 'f·
`RI~~·(2)=q.,45Oi,l">
`
`'J..
`F .... ll(2)=9.40C1u!1
`
`---
`
`iQ11l=:;O.= 1425 ~m
`
`Iga()J2
`
`1".,=0
`OO~===---------~~~~------~---==9
`
`lJOO
`
`1400
`wavelenglh (nm)
`
`1,00
`
`1600
`
`Fig. 4. Measured temporal response of L
`!O'Iuare W3ve in atmosphere.
`
`100 11m dt:vi« 10 apptied
`
`Fig. 2. Calculated !iipCClra for different membrane positions.
`
`5 oov
`
`20.CY.'i
`
`O.OO!l
`'1"
`
`S.
`
`13V:-_--~_
`
`125-0
`
`1300
`
`1:)50
`
`1~50
`
`1400
`wavelength (nm)
`Mca~urcd spectra of L = 100 j1 m device wIth II/
`:::: 4 uJr gap.
`Fig. l
`Elchant access holes (which were not n!4uircJ ror smal1·arca ul!vil:e) t.ll!gmul!
`contrasL.
`
`'500
`
`1550
`
`1600
`
`achieved only in the diffracted order, and has the wavelength
`dependence of a grating.
`Our membrane may be fabricated out of plasma-enhanced
`chemical vapor deposition silicon nitride, whose refractive
`index may be controlled precisely in order to make it the
`square root of that of silicon (or the geometric mean of that
`of silicon and that of optical fiber, in the case where a tiber
`is attached) l6j. Thus when the membrane is brought into
`contact with the substrate (by means of electrostatic force
`resulting from bias applied between <In electrode placed on the
`membmne and the doped substrate [Fig. I j), an anti-rellection
`condition exists at a wavelength equal to four times the optical
`thickness of the membrane. This anti-reHcction condition may
`extend over an enormous bandwidth. from 1.3 to 1.55 11m
`(Fig. 2). In Fig. 2, the index and thickness of the silicon
`nitride membranc arc I.S7 and 1905 A, respectively, so that
`its optical thickness is AI" at 1425 nm. Since its thickness is
`A/1, when the air gap is also A/,I, the device forms a high
`reflectivity mirror (Fig. 2). In fact, for an air gap thickness of
`mA/4, for m evcn an anti-reflection condition exists, and for
`m. odd a high reflection exists, although the optical bandwidth
`decreases for m > 1. Hence we call the device the Mechanical
`Anti-Reflection Switch, or MARS device.
`
`I""req( D not found
`
`F"rea(2)=3'P .8kHZ
`
`Fig. 5. Same as Fig. 4 in vacuum.
`
`Here we present devices with m > I, so that upon deflection
`throu~h a quarter-wave there is still an air gap. This is
`beneficial in that possible sticking of the membrane to the
`subst·ate is avoided, but detrimental in that optical bandwidth
`is recuced. In addition, as we state below by going to TTl = 1
`designs in the future drive voltage requirements should be
`greatJ y reduced.
`A ,~uarter wave-thick layer of poly-silicon may be added to
`the membrane under the SiNx layer (this is not demonstrated
`here.) Since its index is that of the substrate, it does not
`affeci the anti-reflection situation, and slightly increases the
`reflectivity in the reflecting state. This layer could add to the
`design nexibility of the device.
`Th~ device may easily handle the optical bandwidth re(cid:173)
`quirements of fiber-in-the-loop applications. In addition it is
`cheap, has no polarization sensitivity, has wide fiber alignment
`tolen.nce, projected < I dB insertion loss (with poly-Si layer)
`and >20 dB contrast, is chemically and mechanically robust,
`and r~quires no tcmperature stahilization. We first fabricated
`an L = 100 11m device with Tn = 4 (spectra shown in Fig.
`3, nete this large area required etcham access holes which
`dcgr~ded contrast). Thc dcvicc is fabricatcd by depositing a
`film Ilf SiNx over a film of Al. A gold electrode is patterned
`over the SiNx, then openings in the SiNx are made. NaOH
`is th,;n used to selectively rcmove the AI underneath the
`acti\'<: device area. The shape of the final structure resembles a
`trampoline supported at the comers by arms. The electrode is
`patterned on the arms and around the rim of the "trampoline."
`The :;ubstrate's backside is contacted with silver paint. The
`temp,'ral characteristics of this rather large device are shown
`
`
`
`

`
`GOOSSEI'> ",,/.: SILICON ~IOJ)ULA10R HW FIBER-II'>-TI!E·LOO!'-APPLICATION~
`
`1121
`
`Fig. 6. Same as. Fi!; . .t fur I. =: 211 Illn dc..-it.:e (in armnsphereJ.
`
`T(5) =(Wo 2+cr2)/[(5+cr)2+wO 2J
`fO=0>0/2,,=1.1 MHz
`
`ldamp=1/a=1 p5
`
`2.0
`
`1.8
`
`1.6
`
`.:
`0
`15 1.4
`.:
`.2 12
`
`0.8
`
`B
`Q)
`1/1
`C 0.6
`
`0-* 1.0
`8. III 0.4
`i!!
`
`0.2
`
`0.0
`0
`
`4
`
`time ()ls)
`l.apiaCClran'ifornl model ot rct1e{'(lOn re:;;poo!i;c of L = 2U jlmdc\lice:
`fig. 7.
`W ,,"'ol1age (inset), iJild a. ... SOCLi.lIt!d re"ron~c to slep function. The agreement to
`the uala is good. Tht' i..ICV3<.:C rC:.ipon."'" ring" <Il 1.1 MHz with a (.Lamping lime
`consHml of II's.
`
`in Fig. 4. using a 12 volt drive. The risc and fall times arc
`about 10 I',s. This is, in facl, already suitabk for standard
`telephone service! (POTS), even before sC<lling down in area.
`This device's speed is limited by air resistance, which is
`demonstrated in Fig. 5. where the device is operated under
`vacuum. The device rings in vacuum at its resonant frequency
`of 350 kHz. demonstrating that the device is a mechanical
`resonator ovcrdamped ny air resistance.
`We then fabricated a scaled-down version of the device,
`with '/11. = :3 and I~ = 2() II.m. The support arms were 30 /.im
`long and 5 1"11 wide. The COll1rast of the device was 13: I
`at 1.52 I,m. This lowered contrast compared to calculated
`values is probably due to not ohtaining the design value for
`the SiNx film refractive index. For this smaller dcvice, the
`vo!tage requirements increased to approximate!y 25 volts for
`),14 deflection. These voltage requirements should decrease
`greatly by going to 111. = 1. This is because the applied force
`for a given voltage goes as the invcrse of the square of the
`air gap thickness (since both the cap,lCitanee and the electric
`
`OB
`
`0.6
`
`02
`
`0.0f---~
`
`.,
`
`R~"20 kn
`fl:2=E1 ~n
`L= 9.3mH
`C. 2 2 pF
`
`time(~s)
`Fig. 8. M(xicl of response of L = 20 11m dC"'ice [0 I liS voltage pulse.
`l..;ing function uf Fig. 6, with linear filter (inset) inserted helween voltage
`:,L)UrCe and device. The tiller rcduce~ the ringing, indicating Ihat operarion a[
`I Mblt/:;CC \s poss\hle.
`
`I,eld increase as the inverse of the gap). Therefore by going
`t·.) 1/l. = 1 in future devices the· voltage requirements should
`r.:duce by an order of magnitude .
`The tcmporal responsc of the L = 20 Im1 device to an
`applied square wave is shown in Fig. 6 (in atmosphere). The
`d,"vice is now undcrdamped. The rise time has decreased to
`250 ns, but il overshoots and rings. A Laplace transfer function
`rdating reflectivity to applied bias is given in Fig. 7, and it
`models the measured response well. It has a single pole at
`1 1 MHz and a damping time constant of I II.s. An appropriate
`Ii lIer inserted between the vollage source and the device should
`allow operation without ringing. This is shown in Fig. 8, with
`tile associated calculated response to a single 1 I's long voltage
`plJlse. The absence of ringing in the pulse indicates that digital
`o:lCralion at I Mbitfsec is possible.
`In conclusion. we present a micromechanical modulator
`for fiber-in-the-loop applications with projected optical band(cid:173)
`\\ idths from 1.3 to 1.55 I,m and datu rates of several Mbits/sec.
`The device behavcs as a damped oscillator. We have made a
`dl.:vice with a ringing frequency of 1.1 MHz and a damping
`ti lle constant of I I". We indicate that with an appropriate
`Ii lear filter the device could operate digitally wilh data rates
`01 I Mbitfsec.
`
`REFERENCES
`
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`Opt. F;h~r Commun,. 199-1- Technical Digest S~ries, postdelldiinc sessiQr..
`p. 43. voJ. 4 <Optic.1 Sociely of America, Wash .. DC. 1994).
`1'1 K. Ar.lani. P. J. French. P. M. Sarro. R ..... Wolffcnbutlcl, and S. Mid(cid:173)
`delhoek, Pmc. (~f 11::1::1:" Micro /:."lI'(·1mmnA Works/lOp, run L:tuderdaJe,
`FL. Feb. 7-10, 1993. p. 2311.
`I·~I ~'. R. Wis7.nicwski. R. E. CoHins. and D. E. Puilthurpe. 1he 7th Imanal.
`CUll/ Oil Solid-State Sensors and AcwalOr.~. (1993). p. 1027.
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`["1 J. A. Walker. K. W. Goosson, J. E. Cunningham, W. Y. Jan. and D.
`A. B. Milk-r, "Gas composilion dependence of silicon nilridc used a,
`gallium dirrusion barrier during GaAs MBE growlh on Si CMOS," 10
`.uppear in 1. Elec. Material.t, Aug. IlJ94.