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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 19 — Sep. 23, 2013
  • pp: 22829–22833
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Overcoming Si3N4 film stress limitations for high quality factor ring resonators

Kevin Luke, Avik Dutt, Carl B. Poitras, and Michal Lipson  »View Author Affiliations


Optics Express, Vol. 21, Issue 19, pp. 22829-22833 (2013)
http://dx.doi.org/10.1364/OE.21.022829


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Abstract

Silicon nitride (Si3N4) ring resonators are critical for a variety of photonic devices. However the intrinsically high film stress of silicon nitride has limited both the optical confinement and quality factor (Q) of ring resonators. We show that stress in Si3N4 films can be overcome by introducing mechanical trenches for isolating photonic devices from propagating cracks. We demonstrate a Si3N4 ring resonator with an intrinsic quality factor of 7 million, corresponding to a propagation loss of 4.2 dB/m. This is the highest quality factor reported to date for high confinement Si3N4 ring resonators in the 1550 nm wavelength range.

© 2013 OSA

Silicon nitride (Si3N4) ring resonators are critical for efficient and compact on chip optical routing [1

1. D. A. B. Miller, “Optical interconnects to silicon,” IEEE J. Sel. Top. Quantum Electron. 6(6), 1312–1317 (2000). [CrossRef]

3

3. R. Soref, “The Past, Present, and Future of Silicon Photonics,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1678–1687 (2006). [CrossRef]

], frequency combs [4

4. T. J. Kippenberg, R. Holzwarth, and S. A. Diddams, “Microresonator-Based Optical Frequency Combs,” Science 332(6029), 555–559 (2011). [CrossRef] [PubMed]

7

7. T. Udem, R. Holzwarth, and T. W. Hänsch, “Optical frequency metrology,” Nature 416(6877), 233–237 (2002). [CrossRef] [PubMed]

], and high precision sensing [8

8. F. Vollmer, D. Braun, A. Libchaber, M. Khoshsima, I. Teraoka, and S. Arnold, “Protein detection by optical shift of a resonant microcavity,” Appl. Phys. Lett. 80(21), 4057–4059 (2002). [CrossRef]

11

11. M. Sumetsky, R. S. Windeler, Y. Dulashko, and X. Fan, “Optical liquid ring resonator sensor,” Opt. Express 15(22), 14376–14381 (2007). [CrossRef] [PubMed]

], however the intrinsically high film stress of silicon nitride has limited both the optical confinement and quality factor (Q) of ring resonators. Whereas the silicon and silicon dioxide platforms generally suffer from high losses or delocalized optical modes, the Si3N4 platform provides advantages of both high confinement and high Q. High Q disks have also been demonstrated in Si3N4 [12

12. E. Shah Hosseini, S. Yegnanarayanan, A. H. Atabaki, M. Soltani, and A. Adibi, “High quality planar silicon nitride microdisk resonators for integrated photonics in the visible wavelength range,” Opt. Express 17(17), 14543–14551 (2009). [CrossRef] [PubMed]

] but disks have larger mode volumes and are challenging to dispersion engineer for nonlinear applications. Si3N4 is also a deposited material, which enables seamless integration with other material platforms. However, the high film stress of Si3N4 prevents thick (>400 nm) films of high optical quality from being deposited; catastrophic cracking occurs, severely limiting device yield. In principle low stress nitride films can be grown thicker with plasma enhanced chemical vapor deposition (PECVD) and modified low pressure chemical vapor deposition (LPCVD) processes, but these deposition chemistries yield films with stronger material absorption caused by dangling H and O bonds with the Si and N in the films.

In order to overcome the stress limitations of Si3N4, we strategically place mechanical trenches to isolate the photonic devices from propagating cracks. Physical shocks near the edge of the wafer, which occur often during handling of the wafer, can provide enough energy to induce cracking of the stressed film. Once initiated, these cracks originating from the edge of the wafer propagate continuously in a uniform stress field, terminating only once they encounter crack resistance at the edge of the wafer or at another crack boundary. We introduce trenches around our devices that terminate cracks before they can spread to our device region (shown in Fig. 2
Fig. 2 Microscope images in the dark field showing crack propagation terminating at a trench created with a diamond scribe. The film to the left of these trenches is crack free.
). A single trench does not guarantee crack termination however [15

15. K. H. Nam, I. H. Park, and S. H. Ko, “Patterning by controlled cracking,” Nature 485(7397), 221–224 (2012). [CrossRef] [PubMed]

]. Overstressed films store energy in the enhanced acceleration of the crack and the extended penetration into the substrate. With this stored energy, cracks can overcome the crack resistance of a single trench and continue propagation. To increase crack resistance, we create between two and five parallel trenches to ensure crack termination.

We define a series of trenches around the edge of the wafer before deposition of the Si3N4 film to prevent crack propagation. Beginning with a thermally oxidized 4 inch silicon wafer, we lightly scribe a series of lines into the silicon oxide surface to define a 5 cm by 5 cm rectangular region in the center of the wafer. This rectangular region will be the crack free region in which we pattern our photonic devices; elsewhere, the film will crack. Note that for ease of processing and throughput we define trenches with a diamond scribe, but in principle one can define more sophisticated trenches with photolithography and etching. Etching may actually improve the crack resistance of these trenches because of the increased roughness on the etched surfaces [15

15. K. H. Nam, I. H. Park, and S. H. Ko, “Patterning by controlled cracking,” Nature 485(7397), 221–224 (2012). [CrossRef] [PubMed]

]. In addition, photolithography could be used to define trenches only at the wafer periphery to enlarge the area of the crack free region. Following trench definition, we proceed with device fabrication as described in [14

14. A. Gondarenko, J. S. Levy, and M. Lipson, “High confinement micron-scale silicon nitride high Q ring resonator,” Opt. Express 17(14), 11366–11370 (2009). [CrossRef] [PubMed]

]. After deposition of 910 nm of Si3N4, in steps of 400 nm and 510 nm, we pattern devices with electron beam lithography using ma-N 2405 resist, post exposure bake for 5 minutes at 115°C, and etch in an inductively coupled plasma reactive ion etcher (ICP RIE) using CHF3/O2 chemistry. After stripping the resist, we anneal devices at 1200°C in a nitrogen atmosphere for 3 hours. We clad devices with 250 nm of high temperature silicon dioxide (HTO) deposited at 800°C followed by 2 µm deposition of silicon dioxide using plasma enhanced chemical vapor deposition (PECVD). The fabricated device is shown in Fig. 3
Fig. 3 Scanning electron microscope (SEM) image of the resonator.
. Note that our trench definition method is not specific to this process; it can be applied to any highly stressed film in which cracks are initiated near the edge of the substrate.

We measure an intrinsic quality factor of 7 million, the highest quality factor reported to date for high confinement Si3N4 ring resonators; this quality factor corresponds to an ultra-low propagation loss of 4.2 dB/m. The ring resonator measured has a radius of 115 µm, a coupling gap of 680 nm, and a cross section of 910 nm tall by 1800 nm wide. We couple a tunable laser light source, transmitted through a polarization controller, into the inverse nanotaper of our device using a lensed fiber. We collect the output of the ring resonator through another inverse nanotaper and collimating lens. After passing the output through a polarizer, we monitor the output on a photodetector. In order to measure a single resonance, we finely scan the laser frequency by applying a triangular-wave voltage signal to the piezoelectric transducer of the laser, while monitoring the photodetector signal on an oscilloscope. We calibrate the voltage-frequency conversion with respect to a free space bowtie cavity. We observe some higher order mode resonances (shown in Fig. 4
Fig. 4 (a) Transmission spectrum with the finely scanned resonance outlined in red. (b) Resonance with 36 MHz linewidth corresponding to an intrinsic Q of 7 million.
), but these resonances have very low extinction, suggesting that most of the optical power is in the fundamental mode. Minimal higher order mode excitation is expected because we design our waveguides to support few modes: five TE modes and four TM modes. From the single resonance scan normalized to the maximum value, we measure a resonance linewidth of 36 MHz and resonant transmission of 26% (shown in Fig. 4), corresponding to an intrinsic quality factor of 7 million for TE polarization. For TM polarization we measure a Q of 4 million. The Q for TE is higher because the TE mode has higher optical confinement and effective index, so the mode suffers less scattering and bending loss.

We calculate the propagation loss within the ring to be 4.2 dB/m and we estimate the absorption loss to be 71% of this loss, suggesting that the Q can be further improved with increased optical confinement. We calculate propagation loss using the relation [16

16. P. Rabiei, W. H. Steier, C. Zhang, and L. R. Dalton, “Polymer micro-ring filters and modulators,” J. Lightwave Technol. 20(11), 1968–1975 (2002). [CrossRef]

]
α=2πngQλ0=λ0QRFSR,
(1)
where ng is the group index, λ0 is the resonant wavelength, R is the radius of the ring resonator, and FSR is the free spectral range. For our device with ring radius of 115 µm, FSR of 2.0 nm, and resonant wavelength of 1554.8 nm, we calculate a propagation loss of 4.2 dB/m. This is the lowest loss reported for high confinement waveguides. We estimate the proportion of loss due to material absorption using Borselli’s method [17

17. M. Borselli, T. J. Johnson, and O. Painter, “Accurate measurement of scattering and absorption loss in microphotonic devices,” Opt. Lett. 32(20), 2954–2956 (2007). [CrossRef] [PubMed]

]. Because silicon nitride has negligible nonlinear loss, we can assume that resonance shifting is dominated by thermal effects caused by material absorption. We use the resonance shift as a function of dropped power in [18

18. Y. Okawachi, K. Saha, J. S. Levy, Y. H. Wen, M. Lipson, and A. L. Gaeta, “Octave-spanning frequency comb generation in a silicon nitride chip,” Opt. Lett. 36(17), 3398–3400 (2011). [CrossRef] [PubMed]

] and simulated values for thermal conductance and thermo-optic coefficient to calculate an absorption loss of 3.0 dB/m for the devices in [18

18. Y. Okawachi, K. Saha, J. S. Levy, Y. H. Wen, M. Lipson, and A. L. Gaeta, “Octave-spanning frequency comb generation in a silicon nitride chip,” Opt. Lett. 36(17), 3398–3400 (2011). [CrossRef] [PubMed]

]. Because the film deposition process in [18

18. Y. Okawachi, K. Saha, J. S. Levy, Y. H. Wen, M. Lipson, and A. L. Gaeta, “Octave-spanning frequency comb generation in a silicon nitride chip,” Opt. Lett. 36(17), 3398–3400 (2011). [CrossRef] [PubMed]

] is identical to the process used in this work, we can assume the same absorption for our devices, which accounts for 71% of the propagation loss. Therefore, the Q is not material limited and can be further improved with increased optical confinement and film deposition optimization.

We demonstrate a high quality factor of 7 million in a high confinement Si3N4 ring resonator using crack resistant trenches to overcome stress limitations of thick Si3N4 films. Our high Q devices herald advances in low loss optical routing, low power threshold nonlinear optics, and high sensitivity sensors. We have also overcome film stress limitations for Si3N4, revealing a new design space for integrated optics and microelectromechanical (MEMS) devices that has been unexplored to date.

Acknowledgments

The authors would like to thank Dr. Mohammad Soltani for his help with thermal conductance simulations and discussion. This work was supported in part by the Cornell Center for Materials Research with funding from A Graduate Traineeship in Materials for a Sustainable Future, (DGE-0903653). The authors gratefully acknowledge support from the Defense Advanced Research Projects Agency (DARPA) under award #FA8650-10-1-7064, DARPA for award # W911NF-11-1-0202 supervised by Dr. Jamil Abo-Shaeer, AFOSR for award # BAA-AFOSR-2012-02 supervised by Dr. Enrique Parra, and SRC/Intel under award #2012-IN-2378.

References and links

1.

D. A. B. Miller, “Optical interconnects to silicon,” IEEE J. Sel. Top. Quantum Electron. 6(6), 1312–1317 (2000). [CrossRef]

2.

M. Haurylau, G. Chen, H. Chen, J. Zhang, N. A. Nelson, D. H. Albonesi, E. G. Friedman, and P. M. Fauchet, “On-Chip Optical Interconnect Roadmap: Challenges and Critical Directions,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1699–1705 (2006). [CrossRef]

3.

R. Soref, “The Past, Present, and Future of Silicon Photonics,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1678–1687 (2006). [CrossRef]

4.

T. J. Kippenberg, R. Holzwarth, and S. A. Diddams, “Microresonator-Based Optical Frequency Combs,” Science 332(6029), 555–559 (2011). [CrossRef] [PubMed]

5.

J. S. Levy, A. Gondarenko, M. A. Foster, A. C. Turner-Foster, A. L. Gaeta, and M. Lipson, “CMOS-compatible multiple-wavelength oscillator for on-chip optical interconnects,” Nat. Photonics 4(1), 37–40 (2010). [CrossRef]

6.

P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, “Optical frequency comb generation from a monolithic microresonator,” Nature 450(7173), 1214–1217 (2007). [CrossRef] [PubMed]

7.

T. Udem, R. Holzwarth, and T. W. Hänsch, “Optical frequency metrology,” Nature 416(6877), 233–237 (2002). [CrossRef] [PubMed]

8.

F. Vollmer, D. Braun, A. Libchaber, M. Khoshsima, I. Teraoka, and S. Arnold, “Protein detection by optical shift of a resonant microcavity,” Appl. Phys. Lett. 80(21), 4057–4059 (2002). [CrossRef]

9.

S. A. Diddams, L. Hollberg, and V. Mbele, “Molecular fingerprinting with the resolved modes of a femtosecond laser frequency comb,” Nature 445(7128), 627–630 (2007). [CrossRef] [PubMed]

10.

C.-H. Li, A. J. Benedick, P. Fendel, A. G. Glenday, F. X. Kärtner, D. F. Phillips, D. Sasselov, A. Szentgyorgyi, and R. L. Walsworth, “A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s-1,” Nature 452(7187), 610–612 (2008). [CrossRef] [PubMed]

11.

M. Sumetsky, R. S. Windeler, Y. Dulashko, and X. Fan, “Optical liquid ring resonator sensor,” Opt. Express 15(22), 14376–14381 (2007). [CrossRef] [PubMed]

12.

E. Shah Hosseini, S. Yegnanarayanan, A. H. Atabaki, M. Soltani, and A. Adibi, “High quality planar silicon nitride microdisk resonators for integrated photonics in the visible wavelength range,” Opt. Express 17(17), 14543–14551 (2009). [CrossRef] [PubMed]

13.

M.-C. Tien, J. F. Bauters, M. J. R. Heck, D. T. Spencer, D. J. Blumenthal, and J. E. Bowers, “Ultra-high quality factor planar Si3N4 ring resonators on Si substrates,” Opt. Express 19(14), 13551–13556 (2011). [CrossRef] [PubMed]

14.

A. Gondarenko, J. S. Levy, and M. Lipson, “High confinement micron-scale silicon nitride high Q ring resonator,” Opt. Express 17(14), 11366–11370 (2009). [CrossRef] [PubMed]

15.

K. H. Nam, I. H. Park, and S. H. Ko, “Patterning by controlled cracking,” Nature 485(7397), 221–224 (2012). [CrossRef] [PubMed]

16.

P. Rabiei, W. H. Steier, C. Zhang, and L. R. Dalton, “Polymer micro-ring filters and modulators,” J. Lightwave Technol. 20(11), 1968–1975 (2002). [CrossRef]

17.

M. Borselli, T. J. Johnson, and O. Painter, “Accurate measurement of scattering and absorption loss in microphotonic devices,” Opt. Lett. 32(20), 2954–2956 (2007). [CrossRef] [PubMed]

18.

Y. Okawachi, K. Saha, J. S. Levy, Y. H. Wen, M. Lipson, and A. L. Gaeta, “Octave-spanning frequency comb generation in a silicon nitride chip,” Opt. Lett. 36(17), 3398–3400 (2011). [CrossRef] [PubMed]

OCIS Codes
(230.5750) Optical devices : Resonators
(230.7370) Optical devices : Waveguides

ToC Category:
Optical Devices

History
Original Manuscript: June 25, 2013
Revised Manuscript: August 19, 2013
Manuscript Accepted: August 25, 2013
Published: September 20, 2013

Citation
Kevin Luke, Avik Dutt, Carl B. Poitras, and Michal Lipson, "Overcoming Si3N4 film stress limitations for high quality factor ring resonators," Opt. Express 21, 22829-22833 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-19-22829


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References

  1. D. A. B. Miller, “Optical interconnects to silicon,” IEEE J. Sel. Top. Quantum Electron.6(6), 1312–1317 (2000). [CrossRef]
  2. M. Haurylau, G. Chen, H. Chen, J. Zhang, N. A. Nelson, D. H. Albonesi, E. G. Friedman, and P. M. Fauchet, “On-Chip Optical Interconnect Roadmap: Challenges and Critical Directions,” IEEE J. Sel. Top. Quantum Electron.12(6), 1699–1705 (2006). [CrossRef]
  3. R. Soref, “The Past, Present, and Future of Silicon Photonics,” IEEE J. Sel. Top. Quantum Electron.12(6), 1678–1687 (2006). [CrossRef]
  4. T. J. Kippenberg, R. Holzwarth, and S. A. Diddams, “Microresonator-Based Optical Frequency Combs,” Science332(6029), 555–559 (2011). [CrossRef] [PubMed]
  5. J. S. Levy, A. Gondarenko, M. A. Foster, A. C. Turner-Foster, A. L. Gaeta, and M. Lipson, “CMOS-compatible multiple-wavelength oscillator for on-chip optical interconnects,” Nat. Photonics4(1), 37–40 (2010). [CrossRef]
  6. P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, “Optical frequency comb generation from a monolithic microresonator,” Nature450(7173), 1214–1217 (2007). [CrossRef] [PubMed]
  7. T. Udem, R. Holzwarth, and T. W. Hänsch, “Optical frequency metrology,” Nature416(6877), 233–237 (2002). [CrossRef] [PubMed]
  8. F. Vollmer, D. Braun, A. Libchaber, M. Khoshsima, I. Teraoka, and S. Arnold, “Protein detection by optical shift of a resonant microcavity,” Appl. Phys. Lett.80(21), 4057–4059 (2002). [CrossRef]
  9. S. A. Diddams, L. Hollberg, and V. Mbele, “Molecular fingerprinting with the resolved modes of a femtosecond laser frequency comb,” Nature445(7128), 627–630 (2007). [CrossRef] [PubMed]
  10. C.-H. Li, A. J. Benedick, P. Fendel, A. G. Glenday, F. X. Kärtner, D. F. Phillips, D. Sasselov, A. Szentgyorgyi, and R. L. Walsworth, “A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s-1,” Nature452(7187), 610–612 (2008). [CrossRef] [PubMed]
  11. M. Sumetsky, R. S. Windeler, Y. Dulashko, and X. Fan, “Optical liquid ring resonator sensor,” Opt. Express15(22), 14376–14381 (2007). [CrossRef] [PubMed]
  12. E. Shah Hosseini, S. Yegnanarayanan, A. H. Atabaki, M. Soltani, and A. Adibi, “High quality planar silicon nitride microdisk resonators for integrated photonics in the visible wavelength range,” Opt. Express17(17), 14543–14551 (2009). [CrossRef] [PubMed]
  13. M.-C. Tien, J. F. Bauters, M. J. R. Heck, D. T. Spencer, D. J. Blumenthal, and J. E. Bowers, “Ultra-high quality factor planar Si3N4 ring resonators on Si substrates,” Opt. Express19(14), 13551–13556 (2011). [CrossRef] [PubMed]
  14. A. Gondarenko, J. S. Levy, and M. Lipson, “High confinement micron-scale silicon nitride high Q ring resonator,” Opt. Express17(14), 11366–11370 (2009). [CrossRef] [PubMed]
  15. K. H. Nam, I. H. Park, and S. H. Ko, “Patterning by controlled cracking,” Nature485(7397), 221–224 (2012). [CrossRef] [PubMed]
  16. P. Rabiei, W. H. Steier, C. Zhang, and L. R. Dalton, “Polymer micro-ring filters and modulators,” J. Lightwave Technol.20(11), 1968–1975 (2002). [CrossRef]
  17. M. Borselli, T. J. Johnson, and O. Painter, “Accurate measurement of scattering and absorption loss in microphotonic devices,” Opt. Lett.32(20), 2954–2956 (2007). [CrossRef] [PubMed]
  18. Y. Okawachi, K. Saha, J. S. Levy, Y. H. Wen, M. Lipson, and A. L. Gaeta, “Octave-spanning frequency comb generation in a silicon nitride chip,” Opt. Lett.36(17), 3398–3400 (2011). [CrossRef] [PubMed]

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