A SILICON NITRID

S.OxideMEMS L

ABSTRACT We present an integrated chip-scale Rdriven Opto-Mechanical Oscillator (RPnitride with excellent close-to-carrier illustrate a process to micro-fabricateresonators, waveguides and grating conitride and demonstrate an RP-OM41.95MHz, with phase noise of -85dBc/HThe phase noise does not show 1/f3 or oslopes all the way down to 10Hz ofUsing a lower optical quality factodemonstrate improvement of 6dB in pha INTRODUCTION Overview of chip-scale oscillators

Quartz oscillators in tens of MHz rafar-from-carrier phase noise, but have hRecent progress in MEMS based oscillalot of attention owing to their potentialcrystals as reference oscillators. Choscillators spanning various transductibeen shown – capacitive electrostatic [1[3] and piezoresistive [4, 5]. Barrinoscillators are not self-sustained and amplifier. The amplifier’s flicker noisenoise for close-to-carrier offsets therebterm stability. Communication with suggests the self-oscillations have large j

Review of radiation pressure in micro

Recent advances in photonics have of mechanical and optical resonances inCoupling of high quality factor (Q) mecthese microcavities to its optical mointeresting experiments in cavity optomnotable contribution that has come ouresearch is the manifestation of paramesuch resonators, resulting in mechanical thereby oscillation of the mechanical mby radiation pressure (RP) of light.

RP driven oscillations have been ex[6] and such oscillations have been obteams [6, 7]. The coupling between thoptical modes of the cavity can be coupled differential equation system [6]

siaaridtda

exex τττ111)(

0

+⎟⎟⎠

⎞⎜⎜⎝

⎛+−Δ=

eff

RPmech

mech

mech

mF

mtF

dtdr

Qdtrd )(

22

2

2

+=Ω+Ω

+

Here, a denotes the field circulating

and s denotes the launched pump fie

E OPTOMECHANICAL OSCILLAZERO FLICKER NOISE

. Tallur, S. Sridaran and S.A. Bhave Laboratory, Cornell University, Ithaca, NY – U

Radiation-Pressure P-OMO) in silicon

phase noise. We e opto-mechanical ouplers in silicon

MO operating at Hz at 1kHz offset. other higher order

ffset from carrier. or resonance, we ase noise.

ange offer superior high flicker noise. ators has gained a l to replace quartz hip scale MEMS on schemes have

1, 2], piezoelectric ng [4], the other

need a feedback shows up as 1/f3

by degrading long authors [4, 5]

jitter.

cavities enabled co-design

n the same device. chanical modes of odes has enabled mechanics [6]. A

ut of this area of etric instability in amplification and

mode driven purely

xtensively studied served by various

he mechanical and expressed via a

(1)

eff

L

mtF )( (2)

g inside the cavity eld, r denotes the

amplitude of radial displacemeand Δ(r) denotes the optical dcoupling of equation (1) to expressed as a function of the Δ0 + λ0/R, where Δ0 is thwavelength from the cavity resis the radius of the micro-ring.

Figure 1: Illustration of shifmotion of the microring resona

At an intuitive level, this

understood as follows. optomechanical resonator evwaveguide as shown in Figureoptical pump laser wavelengthoptical resonance wavelengthinside the cavity exert a radiacavity. This force can be expwhere neff is the effective reframode in the cavity, F is the circulating inside the cavity power in the waveguide. proportional to the optical quproportional to the radius of the

ineff

circeff

RP Pc

FnP

cn

F22

==π

The force induces deform

changes the optical resonmicrocavity through a change An increase in radius thus lopump power and hence the rada phase relationship betwemicrocavity deformation resufrom the optical pump laser to transfer manifests itself mathegain, for the mechanical corresponding threshold opticarate exceeds the intrinsic mechacirculating light inside the camechanical oscillations. Interesonator with photons insiamplitude modulation of the puof the resonator. This impr

ATOR WITH

USA

ent of the microring cavity, detuning that describes the (2). The detuning can be change in radius as Δ(r) =

he detuning of the laser sonant wavelength λ0 and R

ft in detuning with radial ator periphery

s excitation process can be Consider a micro-ring

vanescently coupled to a e 1. Assuming first that the h is detuned from the cavity h, the circulating photons ation pressure force on the pressed as in equation (3), active index for the optical finesse, Pcirc is the power and Pin is the input laser The finesse is directly

uality factor and inversely e ring.

(3)

mation of the cavity and nant wavelength of the

in the optical path length. owers the coupled optical diation pressure force. Such en optical pressure and

ults in net power transfer the mechanical mode. This

ematically as a mechanical oscillations. Beyond a

al power, the amplification anical damping rate and the

avity sets up self-sustained eraction of the vibrating de the cavity results in

ump laser light at the output rinting of the mechanical

978-1-4673-0325-5/12/$31.00 ©2012 IEEE 19 MEMS 2012, Paris, FRANCE, 29 January - 2 February 2012

oscillations on the laser light is converted into RF output power at the photodetector. Prior art in RP-OMOs

RP-OMOs based on strong coupling between high-Q optical and mechanical modes of a microtoroid resonator have been demonstrated before [6, 7]. However making this device requires an unconventional post-fabrication CO2 laser reflow step. The phase noise numbers reported [7] are far worse than those reported for CMOS oscillators in similar frequency ranges. The phase noise also shows higher order slopes at close to carrier frequencies that are unexplained but are often attributed to environmental noise. Theoretical analysis for OMOs [7, 8] indicates that these problems can be overcome by having a robust device with high mechanical and optical quality factors. Silicon nitride is a promising material for such an application owing to its high optical and mechanical quality factors. Being CMOS compatible, we can design resonators in silicon nitride using MEMS based processes.

Chip-scale opto-mechanical resonators in silicon nitride with integrated waveguides have been shown [9]; however no self-oscillations have been reported. We present a fabrication process to design released opto-mechanical resonators, grating couplers and integrated waveguides in silicon nitride. The next section describes the device design and the fabrication process.

DEVICE DESIGN Device design

We choose a ring geometry for the microresonator owing to its high optical Q. The ring is designed to be wide (6μm) and with a large outer radius (40μm).

Figure 2: Simulated fundamental radial expansion mode shape for the ring resonator (radius 40μm, 6μm wide, 500nm wide spokes) at 41.1MHz. Deformed geometry for the expanded ring is illustrated here.

We design grating couplers to couple light into the

waveguide. The waveguide width at the grating couplers is 15μm and it is tapered down to 800nm close to the resonator. The grating couplers were simulated in Lumerical Solutions FDTD and are designed to be broadband and have maximum transmission at 1550nm for light incident at an angle of 20° to the vertical. The ring resonator has a fundamental radial expansion mode at 41.1MHz as estimated from FEM simulations performed in COMSOL, as shown in Figure 2. Process flow

Figure 3 illustrates the devised process flow schematically. We start with silicon wafers that have 4μm SiO2 thermally grown and deposit 300nm Si3N4 using low pressure chemical vapor deposition (LPCVD). The

devices are defined using electron beam lithography. The pattern is then etched into the nitride device layer in a CHF3/O2 reactive ion etch (RIE) step. We then deposit SiO2 cladding using a plasma enhanced chemical vapor deposition (PECVD) system, to clad the gratings and waveguide with oxide. This is done to reduce losses at the grating couplers (simulated -10dB per coupler without cladding; -6dB per coupler with cladding). A second mask is then used to pattern release windows near the resonator using contact photolithography. This is followed by a partial etch into the cladding in a CHF3/O2 RIE step.

Figure 3: Illustration of process flow for micro-fabricating optomechanical components in silicon nitride

We then perform a timed release etch in buffered

oxide etchant to undercut the devices, to enable opto-mechanics. The samples are then dried using a critical point dryer to prevent stiction. The resulting devices have cladding over the gratings, and the tapered section of the waveguide. Figure 4 shows an optical micrograph and a scanning electron micrograph (SEM) of the device.

Figure 4: Micrograph (top) and SEM (bottom) of the device. The gap between the ring and waveguide at point of nearest approach is 50nm. EXPERIMENTAL RESULTS Optical characterization

We couple light from a tunable laser into the grating coupler and monitor the output using an optical power meter. The laser wavelength is swept to identify a high Q optical resonance. Figure 5 shows an optical resonance at 1550.474nm that has a total loaded optical quality factor

20

of 195,500 and an intrinsic optical Qgrating couplers introduce a loss of 8dB

Figure 5: Optical spectrum showresonance. The input laser power was -1 Measurement of self-oscillations

We use the setup illustrated in Figuoptomechanics in the silicon nitride Oavalanche photodiode (APD) with a gaithe photodetector in our system. An spectrum analyzer is used to study the ouand an Agilent E5052B signal source ameasure phase noise of the oscillations.

Figure 6: Schematic illustration of the probe the mechanical resonance of the d

For high input laser power, the reso

to absorption, resulting in a characteristispectrum as shown in Figure 7.

Figure 7: Shark fin optical resonance input laser power. The extinction at reso

This thermo-optical nonlinearity ren

detune the pump laser from the cavitymechanical resonance frequency. In thbeneficial to fix the laser wavelengcorresponds to a 3dB drop in opticalshown in Figure 7. This results amplification and hence increased optself-oscillation [4], but with an improvem

Q > 500,000. The per coupler.

wing the high-Q 15dBm.

ure 6 to study the OMO. We use an n of 6,000V/W as Agilent E4445A

utput RF spectrum analyzer is used to

test setup used to

device

nator heats up due ic shark fin optical

shape for 15dBm

onance is 20.2dB

nders it difficult to y resonance at the his situation, it is gth such that it l transmission, as

in sub-optimal tical threshold for ment in extinction

at the RF output (measured voltage to minimum voltage fo

Figure 8: Radiation pressinstability in the silicon nitride

Figure 8 shows the variatthe photodetector with the inpulaser powers, the input light modulated by the Browniafundamental radial expansio41.95MHz. The mechanical Q air is 2,000. As we increase theoscillations are observed aboveThe sharp threshold behavior ipressure induced parametric imeasured RF output power abo

Figure 9:Measured RF outpu(Brownian noise motion) and a(self-oscillation of the radial ex

The phase noise slope is -

10Hz offset from carrier as implies that the RP-OMO has be attributed to absence of an integrated on-chip waveguide low frequency environmental nmeasured phase noise and expected values as shown in Tais calculated at the waveguide,input grating coupler.

As shown in Figure 11extinction (defined as ratio of mminimum output voltage at Rfor a laser wavelength that cotransmission. This correspondsthe optical mode shown in Figu

as the ratio of maximum r the RF output).

sure induced parametric OMO.

tion of RF output power at ut laser power. At low input coupled into the cavity is

an noise motion of the n mode of the ring at for this mode measured in

e laser power, self-sustained e the input threshold power. is characteristic of radiation instability. Figure 9 shows ove and below threshold.

ut power for subthreshold above threshold laser power xpansion mode)

20dB/decade even down to shown in Figure 10. This no flicker noise. This can external amplifier and the

that eliminates coupling of noise to the oscillator. The

threshold power match able 1. The threshold power , discounting the loss at the

1, we achieve maximum maximum output voltage to

RF output of photodetector) orresponds to 3dB drop in s to complete modulation of ure 7.

21

Figure 10: Phase noise for the siliconwith RF output signal power of -11.37dB

Table 1: Comparison of measured phasthreshold power to expected simulated v

MeasurePhase noise at 1kHz offset -85dBc/HThreshold power 3dBm

Figure 11: Extinction of oscillation wavthe photodetector versus relative dedetuning to the resonance linewidth) Improving phase noise of the OMO

Phase noise in OMOs depends stquality factor [7]. Choosing a lower opwhile achieving complete modulationphase noise at a cost of higher thresemployed an optical mode with a total o(and an intrinsic Q of 200,000) and obsin phase noise by 6dB as shown in Figur

Figure 12: Improved phase noise for tha lower optical Q resonance

nitride RP-OMO

Bm.

se noise [10] and values. ed SimulatedHz -83dBc/Hz

3.6dBm

veform at output of etuning (ratio of

trongly on optical ptical Q resonance n can give better shold power. We ptical Q of 84,500 served a reduction re 12.

he RP-OMO using

CONCLUSION We have presented a

fabricating integrated optomecthese high optical Q resonatorOMO operating at 41.95MHonly 1/f2 phase noise and hastable oscillator for chip scale low bias drift oscillations for gythe tradeoff between optical improve the phase noise by modulation of the pump laser Q resonance.

ACKNOWLEDGEMEN

The authors wish to thahelping design the experimensupported by the DARPA ORAcademic Research Office. That the Cornell NanoScale Facility. REFERENCES [1] W. -T. Hsu, and K. Cioffi,

Micromechanical Oscilla2004, pp. 1927-1930.

[2] H. M. Lavasani, F. Ayazphase-noise capacitive oscillator," IEEE IEDM, D

[3] C. Zuo, G. Piazza, et Oscillator Based OPiezoelectric AlN Reson2009, pp. 381-384.

[4] P. G. Steeneken, et al., "and refrigerator," Nature P

[5] A. Rahafrooz, and S. Poudual-mode thermal-piezorIFCS, May 2011, pp. 1-4.

[6] T. J. Kippenberg and optomechanics: Back-actScience 29, 321 (5893), pp

[7] M. Hossein-Zadeh, K"Characterization of a micromechanical oscillator(2006).

[8] S. Tallur, et al., "Phase mechanical oscillators," IE268-272.

[9] G. S. Wiederhecker, M. Lphotonic structures using o633636 (2009).

[10] S. Tallur, et al., "A modriven, low phase noimechanical oscillator," O24529 (2011).

CONTACT

*S. Tallur; sgt28@cornell.

process flow for micro-hanical components. Using rs, we demonstrate an RP-z. This oscillator exhibits

as promise as a long term atomic clock (CSAC) and

yroscopes. We demonstrate Q and laser power and 6dB achieving complete

light using a lower optical

NTS ank David Hutchison for

ntal setups. This work was RCHID program and Intel he devices were fabricated Science and Technology

"Low Phase-Noise 70MHz ators," Digest IMS, June

zi, et al., "A 145MHz low silicon micromechanical

Dec. 2008, pp. 1-4. al., "1.05 GHz MEMS

On Lateral-Field-Excited ators," IEEE IFCS, April

"Piezoresistive heat engine Physics 7, 354–359 (2011). urkamali, "High frequency resistive oscillators," IEEE

K. J. Vahala, "Cavity tion at the mesoscale,"

p. 1172-1176, (2008). K. J. Vahala, et al.,

radiation-pressure-driven r," Phys. Rev. A 74, 023813

noise modeling of opto-EEE IFCS, June 2010, pp.

Lipson, et al., “Controlling optical forces,” Nature 462,

onolithic radiation-pressure ise silicon nitride opto-

Optics Express 19, 24522-

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