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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 19 — Sep. 10, 2012
  • pp: 21341–21345
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High quality factor and high confinement silicon resonators using etchless process

Austin Griffith, Jaime Cardenas, Carl B. Poitras, and Michal Lipson  »View Author Affiliations


Optics Express, Vol. 20, Issue 19, pp. 21341-21345 (2012)
http://dx.doi.org/10.1364/OE.20.021341


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Abstract

We demonstrate high quality factor and high confinement in a silicon ring resonator fabricated by a thermal oxidation process. We fabricated a 50 μm bending radius racetrack resonator, with a 5 μm coupling region. We achieved an intrinsic quality factor of 760,000 for the fundamental TM mode, which corresponds to a propagation loss of 0.9 dB/cm. Both the fundamental TE and TM modes are highly confined in the waveguide, with effective indices of 3.0 for the TE mode and 2.9 for the TM mode.

© 2012 OSA

1. Introduction

Ridge etched waveguides have low losses, but present low lateral confinement of the optical mode [3

3. U. Fischer, T. Zinke, J.-R. Kropp, F. Arndt, and K. Petermann, “0.1 dB/cm waveguide losses in single-mode SOI rib waveguides,” IEEE Photon. Technol. Lett. 8, 647–648 (1996). [CrossRef]

], which requires resonators to have large bending radii to avoid significant radiation losses [4

4. I. Kiyat, A. Aydinli, and N. Dagli, “High-Q silicon-on-insulator optical rib waveguide racetrack resonators,” Opt. Express 13, 1900–1905 (2005). [CrossRef] [PubMed]

]. In contrast, etched strip waveguides present high lateral confinement of the mode, which enables resonators to have small bending radii down to 5 microns [5

5. S. Xiao, M. H. Khan, H. Shen, and M. Qi, “Compact silicon microring resonators with ultra-low propagation loss in the C band,” Opt. Express 15, 14467–14475 (2007). [CrossRef] [PubMed]

]. However strip waveguides suffer from high propagation losses in bends (typically 2–4 dB/cm in the highest quality factor etched ring resonators [6

6. Y. Vlasov and S. McNab, “Losses in single-mode silicon-on-insulator strip waveguides and bends,” Opt. Express 12, 1622–1631 (2004). [CrossRef] [PubMed]

]). Therefore neither the etched ridge waveguides nor the etched strip waveguides can serve as an ideal platform for high quality factor and at the same time high confinement resonators.

Ring resonators based on etched silicon high confinement waveguides are inherently limited in quality factor due to the imperfections introduced by the etching process, with the highest demonstrated Q of 420,000 [5

5. S. Xiao, M. H. Khan, H. Shen, and M. Qi, “Compact silicon microring resonators with ultra-low propagation loss in the C band,” Opt. Express 15, 14467–14475 (2007). [CrossRef] [PubMed]

]. Plasma etching introduces roughness into the waveguide side-walls, and can also induce absorption sites into the silicon [7

7. M. Borselli, T. Johnson, and O. Painter, “Beyond the Rayleigh scattering limit in high-Q silicon microdisks: theory and experiment,” Opt. Express 13, 1515–1530 (2005). [CrossRef] [PubMed]

] that will increase losses [8

8. G. S. Oehrlein, “Dry etching damage of silicon: a review,” Mater. Sci. Eng. B 4, 441–450 (1989). [CrossRef]

, 9

9. F. P. Payne and J. P. R. Lacey, “A theoretical analysis of scattering loss from planar optical waveguides,” Opt. Quantum Electron. 26, 977–986 (1994). [CrossRef]

].

Optical losses in high confinement resonators can in principle be mitigated by using a non-etching process to create the waveguide sidewalls. Of particular potential is the use of thermal oxidation to form silicon waveguides and resonators. The highest quality factors achieved in silicon ring resonators have been accomplished using etchless processes [10

10. R. Pafchek, R. Tummidi, J. Li, M. A. Webster, E. Chen, and T. L. Koch, “Low-loss silicon-on-insulator shallow-ridge TE and TM waveguides formed using thermal oxidation,” Appl. Opt. 48, 958–963 (2009). [CrossRef] [PubMed]

, 11

11. M. P. Nezhad, O. Bondarenko, M. Khajavikhan, A. Simic, and Y. Fainman, “Etch-free low loss silicon waveguides using hydrogen silsesquioxane oxidation masks,” Opt. Express 19, 18827–18832 (2011). [CrossRef] [PubMed]

]. To date, there have been two types of etchless ring resonators demonstrated: ridge waveguides made with the etchless process [10

10. R. Pafchek, R. Tummidi, J. Li, M. A. Webster, E. Chen, and T. L. Koch, “Low-loss silicon-on-insulator shallow-ridge TE and TM waveguides formed using thermal oxidation,” Appl. Opt. 48, 958–963 (2009). [CrossRef] [PubMed]

], and very thin etchless waveguides [11

11. M. P. Nezhad, O. Bondarenko, M. Khajavikhan, A. Simic, and Y. Fainman, “Etch-free low loss silicon waveguides using hydrogen silsesquioxane oxidation masks,” Opt. Express 19, 18827–18832 (2011). [CrossRef] [PubMed]

, 12

12. L.-W. Luo, G. S. Wiederhecker, J. Cardenas, C. Poitras, and M. Lipson, “High quality factor etchless silicon photonic ring resonators,” Opt. Express 19, 6284–6289 (2011). [CrossRef] [PubMed]

]. Both designs have shown intrinsic quality factors of over a million; however, this has been at the cost of ring size and optical confinement. The etchless ridge resonator was only demonstrated at a bending radius as large as 400 μm in order to avoid radiation losses. The thin etchless resonator, due to the small waveguide size, has optical modes that are large compared with the waveguide [12

12. L.-W. Luo, G. S. Wiederhecker, J. Cardenas, C. Poitras, and M. Lipson, “High quality factor etchless silicon photonic ring resonators,” Opt. Express 19, 6284–6289 (2011). [CrossRef] [PubMed]

].

2. Fabrication of high confinement etchless resonators

The device was fabricated on a silicon-on-insulator (SOI) wafer, with a top silicon layer of 500 nm, and a buried oxide of 3 μm. We used low-pressure chemical vapor deposition (LPCVD) to deposit 200 nm of silicon nitride (Si3N4) as a mask (see Fig. 1(a)). The mask layer was patterned using electron beam lithography with ma-N 2405 resist, and etched in a reactive ion etching (RIE) tool using CF4 gas (see Fig. 1(b)). The resist was then stripped, followed by wet and dry thermal oxidation of the silicon (see Fig. 1(c)). We used a process of 20 minutes of dry oxidation, followed by 85 minutes of wet oxidation, and then 20 minutes of dry oxidation. The oxidation was done at 1200 degrees C. The silicon nitride was left in place. The device was clad with 2 μm of plasma enhanced chemical vapor deposition (PECVD) silicon dioxide (see Fig. 1(d)).

Fig. 1 Process flow for high confinement etchless waveguide. (a) LPCVD deposition of 200 nm silicon nitride on top of 500 nm silicon-on-insulator (SOI) platform. (b) Patterning and etching of the silicon nitride using electron beam lithography and ma-N 2405 photoresist. (c) Waveguide after selective thermal oxidation of the silicon. (d) Deposition of 2 μm PECVD oxide as cladding.

3. Device design

The physical waveguide structure is determined by the silicon nitride width and by the wet and dry oxidation times. The SEM image of the resulting waveguide structure is shown in Fig. 2(a). We used commercial software (Silvaco Athena) to determine the dimensions of the waveguide structure; the simulated TM-polarized mode at a wavelength of 1.55 μm is shown in Fig. 2(b). The effective indices of the optical modes of the waveguide are 3.0 for the TE mode and 2.9 for the TM mode, indicating a high degree of confinement for both polarizations. The waveguide is multimode for both polarizations, with six modes for each polarization. The ring is designed to operate for the fundamental TM mode.

Fig. 2 (a) False colored SEM image of the cross-section of the silicon waveguide structure. In this image, the silicon nitride is not visible. (b) Simulated Poynting vector of the fundamental TM optical mode.

We patterned the nitride mask to a width of 1 μm for the waveguides and ring resonators, with a gap of 300 nm in the mask for the waveguide-resonator coupling regions (see Fig. 3(a)). For critical coupling of the TM mode, we designed the device with 5 μm long coupling regions.

Fig. 3 (a) Optical microscope image of the racetrack resonator. (b) SEM image of the cross-section of the silicon structure. The different thickness in the coupling region achieved by the slower oxidation rate in the gap region.

From simulation, coupling of the fundamental TE mode into the ring is significantly stronger than that of the fundamental TM mode. In the coupling region, about 0.5% of TM light is coupled into the ring, whereas 5% of TE light is coupled. The difference in coupling arises from the 140 nm silicon slab between the waveguides, as seen in Fig. 3.(b). Horizontally polarized light (TE) strongly interacts with the slab, whereas vertically polarized light (TM) does not. For this coupling length, the fundamental TE mode is very over coupled.

Coupling into and out of the waveguide is accomplished with a silicon nanotaper [14

14. V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28, 1302–1304 (2003). [CrossRef] [PubMed]

], with a patterned nitride mask width of 200 nm.

4. Results and discussion

We demonstrate a high quality factor of 760,000 in a high confinement etchless silicon ring resonator. To test the device, we couple light from a tunable laser light source through a polarization maintaining fiber into a lensed fiber. The lensed fiber couples light into a silicon nanotaper. The light is collected from the output of the chip by another lensed fiber, and its power measured by a photodetector. We observe a transmission spectrum with clear resonances, and varying resonance extinction, as seen in Fig. 4(a). We believe this is caused by wavelength dependent optical mode mixing in the coupling region shown in Fig. 3(b).

Fig. 4 (a) Normalized transmission spectrum for TM polarized light. The spectrum is normalized to the average off-resonance transmission. The resonance peaks with high extinction correspond to the fundamental TM mode. The resonant peaks with the different FSR (with extinctions less than 3 dB) correspond to the higher order TM modes. (b) Normalized transmission spectrum of the resonance at λo = 1533.063 nm

We measured an intrinsic quality factor of Qi = 760, 000 for the fundamental TM mode. The propagation loss in the ring is given by [15

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

]:
α=2πngQintλo=λoQintRFSR,
(1)
where α is the loss per unit length, R is the radius of the resonator, and FSR is the free spectral range of the resonator. Although the resonator is not a circular ring, we can approximate it as such by calculating its effective radius:
Reffective=C2π,
(2)
where C is the circumference of the racetrack resonator. Our device has an effective radius of 51.5 μm. From this, we calculate α = .9 dB/cm for the propagation loss in the ring. This is significantly lower than previously reported bent waveguide losses in etched silicon waveguides [5

5. S. Xiao, M. H. Khan, H. Shen, and M. Qi, “Compact silicon microring resonators with ultra-low propagation loss in the C band,” Opt. Express 15, 14467–14475 (2007). [CrossRef] [PubMed]

, 6

6. Y. Vlasov and S. McNab, “Losses in single-mode silicon-on-insulator strip waveguides and bends,” Opt. Express 12, 1622–1631 (2004). [CrossRef] [PubMed]

].

5. Conclusion

Acknowledgments

The authors gratefully acknowledge support from the Defense Advanced Research Projects Agency (DARPA) under award # FA8650-10-1-7064. This work was performed in part at the Cornell Nanoscale Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the NSF. Austin Griffith acknowledges the National Defense Science and Engineering Graduate Fellowship (NDSEG) for funding.

References and links

1.

R. Soref, “The past, present, and future of silicon photonics,” IEEE J. Sel. Top. Quantum Electron. 12, 1678–1687 (2006). [CrossRef]

2.

Y. Okawachi, A. Gaeta, and M. Lipson, “Breakthroughs in nonlinear silicon photonics 2011,” IEEE Photonics J. 20114, 601–606 (2012).

3.

U. Fischer, T. Zinke, J.-R. Kropp, F. Arndt, and K. Petermann, “0.1 dB/cm waveguide losses in single-mode SOI rib waveguides,” IEEE Photon. Technol. Lett. 8, 647–648 (1996). [CrossRef]

4.

I. Kiyat, A. Aydinli, and N. Dagli, “High-Q silicon-on-insulator optical rib waveguide racetrack resonators,” Opt. Express 13, 1900–1905 (2005). [CrossRef] [PubMed]

5.

S. Xiao, M. H. Khan, H. Shen, and M. Qi, “Compact silicon microring resonators with ultra-low propagation loss in the C band,” Opt. Express 15, 14467–14475 (2007). [CrossRef] [PubMed]

6.

Y. Vlasov and S. McNab, “Losses in single-mode silicon-on-insulator strip waveguides and bends,” Opt. Express 12, 1622–1631 (2004). [CrossRef] [PubMed]

7.

M. Borselli, T. Johnson, and O. Painter, “Beyond the Rayleigh scattering limit in high-Q silicon microdisks: theory and experiment,” Opt. Express 13, 1515–1530 (2005). [CrossRef] [PubMed]

8.

G. S. Oehrlein, “Dry etching damage of silicon: a review,” Mater. Sci. Eng. B 4, 441–450 (1989). [CrossRef]

9.

F. P. Payne and J. P. R. Lacey, “A theoretical analysis of scattering loss from planar optical waveguides,” Opt. Quantum Electron. 26, 977–986 (1994). [CrossRef]

10.

R. Pafchek, R. Tummidi, J. Li, M. A. Webster, E. Chen, and T. L. Koch, “Low-loss silicon-on-insulator shallow-ridge TE and TM waveguides formed using thermal oxidation,” Appl. Opt. 48, 958–963 (2009). [CrossRef] [PubMed]

11.

M. P. Nezhad, O. Bondarenko, M. Khajavikhan, A. Simic, and Y. Fainman, “Etch-free low loss silicon waveguides using hydrogen silsesquioxane oxidation masks,” Opt. Express 19, 18827–18832 (2011). [CrossRef] [PubMed]

12.

L.-W. Luo, G. S. Wiederhecker, J. Cardenas, C. Poitras, and M. Lipson, “High quality factor etchless silicon photonic ring resonators,” Opt. Express 19, 6284–6289 (2011). [CrossRef] [PubMed]

13.

B. Desiatov, I. Goykhman, and U. Levy, “Demonstration of submicron square-like silicon waveguide using optimized LOCOS process,” Opt. Express 18, 18592–18597 (2010). [CrossRef] [PubMed]

14.

V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28, 1302–1304 (2003). [CrossRef] [PubMed]

15.

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

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

ToC Category:
Optical Devices

History
Original Manuscript: June 28, 2012
Revised Manuscript: August 28, 2012
Manuscript Accepted: August 28, 2012
Published: September 4, 2012

Citation
Austin Griffith, Jaime Cardenas, Carl B. Poitras, and Michal Lipson, "High quality factor and high confinement silicon resonators using etchless process," Opt. Express 20, 21341-21345 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-19-21341


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References

  1. R. Soref, “The past, present, and future of silicon photonics,” IEEE J. Sel. Top. Quantum Electron.12, 1678–1687 (2006). [CrossRef]
  2. Y. Okawachi, A. Gaeta, and M. Lipson, “Breakthroughs in nonlinear silicon photonics 2011,” IEEE Photonics J.20114, 601–606 (2012).
  3. U. Fischer, T. Zinke, J.-R. Kropp, F. Arndt, and K. Petermann, “0.1 dB/cm waveguide losses in single-mode SOI rib waveguides,” IEEE Photon. Technol. Lett.8, 647–648 (1996). [CrossRef]
  4. I. Kiyat, A. Aydinli, and N. Dagli, “High-Q silicon-on-insulator optical rib waveguide racetrack resonators,” Opt. Express13, 1900–1905 (2005). [CrossRef] [PubMed]
  5. S. Xiao, M. H. Khan, H. Shen, and M. Qi, “Compact silicon microring resonators with ultra-low propagation loss in the C band,” Opt. Express15, 14467–14475 (2007). [CrossRef] [PubMed]
  6. Y. Vlasov and S. McNab, “Losses in single-mode silicon-on-insulator strip waveguides and bends,” Opt. Express12, 1622–1631 (2004). [CrossRef] [PubMed]
  7. M. Borselli, T. Johnson, and O. Painter, “Beyond the Rayleigh scattering limit in high-Q silicon microdisks: theory and experiment,” Opt. Express13, 1515–1530 (2005). [CrossRef] [PubMed]
  8. G. S. Oehrlein, “Dry etching damage of silicon: a review,” Mater. Sci. Eng. B4, 441–450 (1989). [CrossRef]
  9. F. P. Payne and J. P. R. Lacey, “A theoretical analysis of scattering loss from planar optical waveguides,” Opt. Quantum Electron.26, 977–986 (1994). [CrossRef]
  10. R. Pafchek, R. Tummidi, J. Li, M. A. Webster, E. Chen, and T. L. Koch, “Low-loss silicon-on-insulator shallow-ridge TE and TM waveguides formed using thermal oxidation,” Appl. Opt.48, 958–963 (2009). [CrossRef] [PubMed]
  11. M. P. Nezhad, O. Bondarenko, M. Khajavikhan, A. Simic, and Y. Fainman, “Etch-free low loss silicon waveguides using hydrogen silsesquioxane oxidation masks,” Opt. Express19, 18827–18832 (2011). [CrossRef] [PubMed]
  12. L.-W. Luo, G. S. Wiederhecker, J. Cardenas, C. Poitras, and M. Lipson, “High quality factor etchless silicon photonic ring resonators,” Opt. Express19, 6284–6289 (2011). [CrossRef] [PubMed]
  13. B. Desiatov, I. Goykhman, and U. Levy, “Demonstration of submicron square-like silicon waveguide using optimized LOCOS process,” Opt. Express18, 18592–18597 (2010). [CrossRef] [PubMed]
  14. V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett.28, 1302–1304 (2003). [CrossRef] [PubMed]
  15. P. Rabiei, W. Steier, and L. Dalton, “Polymer micro-ring filters and modulators,” J. Lightwave Technol.20, 1968–1975 (2002). [CrossRef]

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