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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 22 — Nov. 4, 2013
  • pp: 26557–26563
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Athermal silicon microring resonators with titanium oxide cladding

Biswajeet Guha, Jaime Cardenas, and Michal Lipson  »View Author Affiliations


Optics Express, Vol. 21, Issue 22, pp. 26557-26563 (2013)
http://dx.doi.org/10.1364/OE.21.026557


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Abstract

We describe a novel approach for CMOS-compatible passively temperature insensitive silicon based optical devices using titanium oxide cladding which has a negative thermo-optic (TO) effect. We engineer the mode confinement in Si and TiO2 such that positive TO of Si is exactly cancelled out by negative TO of TiO2. We demonstrate robust operation of the resulting device over 35 degrees.

© 2013 OSA

1. Introduction

Integrated photonic devices are extremely sensitive to ambient temperature fluctuations which limit their integration in wavelength sensitive applications. This problem is especially severe in silicon photonics where high index contrast, large thermo-optic coefficient of Si [1

1. Y. Varshni, “Temperature dependence of the energy gap in semiconductors,” Physica 34, 149–154 (1967) [CrossRef]

] and high quality factors make the microring resonators extremely susceptible to thermal fluctuations. For example, a Si ring resonator with a quality factor of 10,000 will tune out of resonance with only 1 °C change in temperature.

Solutions to overcome the temperature sensitivity have been proposed using polymers, external temperature compensating devices or active control of device temperature. Table 1 summarizes previously reported temperature stabilization schemes and their corresponding advantages and disadvantages.

Table 1:. Summary of previously reported temperature stabilization schemes

table-icon
View This Table

Here we describe a novel approach for passive and CMOS-compatible temperature insensitive integrated optical devices using a metal-oxide cladding having negative thermo-optic effect. Metals oxides like TiO2 and SrTiO3 have been investigated in semiconductor industry as a gate dielectric [15

15. S. A. Campbell, H.-S. Kim, D. C. Gilmer, B. He, T. Ma, and W. L. Gladfelter, “Titanium dioxide (TiO2)-based gate insulators,” IBM journal of research and development 43, 383–392 (1999). [CrossRef]

]. These metal oxides also have negative thermo-optic coefficient due to presence of a soft electronic band [16

16. V. Trepakov, A. Dejneka, P. Markovin, A. Lynnyk, and L. Jastrabik, “A ‘soft electronic band’ and the negative thermooptic effect in strontium titanate,” New J. Phys. 11, 083024 (2009). [CrossRef]

] (TOTiO2 ∼ −1 × 10−4K−1, TOSrTiO3 ∼ −1 × 10−5K−1). This is in contrast to commonly available dielectrics and semiconductors which have positive thermo-optic coefficients (TOSi = 1.8 × 10−4K−1, TOSiN ∼ 2 × 10−5K−1, TOSiO2 ∼ 1 × 10−5K−1). There has been some limited effort in reducing temperature sensitivity of optical devices using TiO2 overcladding [17

17. B. Guha and M. Lipson, “Athermal silicon ring resonator with bi-material cantilever for passive thermal feedback,” in CLEO: Science and Innovations (Optical Society of America, 2013).

19

19. F. Qiu, A. M. Spring, F. Yu, and S. Yokoyama, “Complementary metaloxidesemiconductor compatible athermal silicon nitride/ titanium dioxide hybrid micro-ring resonators,” Appl. Phys. Lett. 102, 051106 (2013). [CrossRef]

]. Ref [18

18. S. S. Djordjevic, K. Shang, B. Guan, S. T. Cheung, L. Liao, J. Basak, H.-F. Liu, and S. Yoo, “CMOS-compatible, athermal silicon ring modulators clad with titanium dioxide,” Opt. Express 21, 13958–13968 (2013). [CrossRef] [PubMed]

] has done a thorough analysis of TiO2 deposition and showed close to athermal performance. Here we show that completely athermal optical devices can be realized by engineering the mode-overlap between Si based materials (Si / SiN/ SiO2) and TiO2. This scheme of temperature compensation is CMOS-compatible, lossless, does not require any extra footprint and can lead to very large temperature operating range.

2. Athermal design

Fig. 1 Resonance wavelength sensitivity to temperature ( λ0T in nm/K) for (a) TE and (b) TM polarizations. Si guiding layer is 220nm thick. Black dashed line represents the optimum waveguide width and cladding thickness for athermal operation. Energy flux density of optical modes in a 250nm wide waveguide is shown at the top. TiO2 cladding thickness is 300nm.

3. Fabrication

The athermal ring resonators were fabricated on a 220nm thick silicon-on-insulator (SOI) device layer. The waveguides were patterned using electron beam lithography and etched in chlorine chemistry in an inductively coupled reactive ion etcher. Titanium oxide was deposited on top of the waveguides using reactive sputtering of a titanium target in O2. The sputtering was performed at a pressure of 2× 10−6 Torr and 2kW power. Sputtered TiO2 films were characterized using a visible-near IR ellipsometer and Raman spectroscopy. Refractive index of ∼ 2.35 was measured at near IR wavelengths. Raman spectrum of the sputtered sample shows no visible peaks, indicating the amorphous nature of the deposited TiO2 film. AFM scan of the surface (Fig. 2(b)) indicates surfaces roughness below 2 nm RMS. Figure 2(a) shows a microscope image of a fabricated Si ring resonators with TiO2 cladding. Si waveguide width in the fabricated ring resonators was varied from 450nm to 150nm to observe the effect of mode delocalization on thermal sensitivity. Inset of Fig. 2(a) shows cross section of a waveguide with 150nm width and 200nm thick TiO2 cladding.

Fig. 2 (a) TiO2 cladded Si microring resonator. Inset shows a false colored SEM cross section of the waveguide. (b) AFM image of the TiO2 surface.

4. Experimental results

We demonstrate temperature insensitive operation of a Si microring resonator with TiO2 over-cladding. Figure 3(a) shows the temperature sensitivity of resonance wavelength of the fabricated microring resonators, for different waveguide widths and polarizations. All of these resonators were fabricated on the same chip. The temperature sensitivity was measured by collecting spectra over 10 degrees, at intervals of 2 degrees, followed by a linear fit to resonance wavelenths. Error bar corresponds to uncertainty in the linear fit. Each waveguide width corresponds to a different confinement in the Si core. Measured data is compared against numerically calculated sensitivity assuming TiO2 thickness of 200nm and λ0 = 1550nm. For the case of TE polarization (Fig. 3(a) blue line), the temperature sensitivity is around 0.09nm/K when the mode is strongly confined in Si (450nm wide waveguide). However as the optical mode is delocalized more into TiO2, temperature sensitivity decreases and becomes negative (∼ −0.03nm/K) for 150nm wide waveguide. For the case of TM polarization (Fig. 3(a) red line), the sensitivity is around 0.04nm/K for a strongly confined mode in Si and very close to zero for 150nm wide waveguide. Figure 3(b) shows the corresponding athermal transmission spectrum compared to that of a resonator without any thermal compensation. Losses in the deposited TiO2 was estimated to be around 16 dB/cm, by comparing change in quality factor of the resonances as a function of mode confinement in Si and TiO2. This loss can be reduced significantly by improving the deposition and reducing scattering at Si – TiO2 interface. Optical losses in similar material has been reported to be less than 3 dB/cm [20

20. J. T. Choy, J. D. Bradley, P. B. Deotare, I. B. Burgess, C. C. Evans, E. Mazur, and M. Loncar, “Integrated TiO2resonators for visible photonics,” Opt. Lett. 37, 539–541 (2012). [CrossRef] [PubMed]

, 21

21. J. D. Bradley, C. C. Evans, J. T. Choy, O. Reshef, P. B. Deotare, F. Parsy, K. C. Phillips, M. Loncar, and E. Mazur, “Submicrometer-wide amorphous and polycrystalline anatase TiO2waveguides for microphotonic devices,” Opt. Express 20, 23821–23831 (2012). [CrossRef] [PubMed]

].

Fig. 3 (a) Measured resonance sensitivity to temperature as a function of waveguide width, for TE and TM modes. Resonance sensitivity decreases significantly as mode is delocalized into TiO2 cladding. Theoretical curves are obtained assuming 200nm thick TiO2 layer. (b) Temperature dependence of the resonance for hybrid Si-TiO2 resonator (athermal TM mode, right) compared to that of a conventional Si resonator (left).

We demonstrate error free operation of hybrid TiO2 – Si microring resonator based optical filter over 35 degrees. The device used was similar to the one shown in Fig. 2(a) with an extra drop waveguide coupled to the ring. We transmitted 231 − 1 PRBS (pseudo random binary sequence) data at 5 Gbps through the device, centered at resonance wavelength, and varied the stage temperature (using a thermoelectric stage and temperature controller). The data was then sent to a commercial receiver (Picometrix PT15) and a bit error rate detector. Figure 4(a) shows the bit error rate (BER) and corresponding eye diagrams for both the athermal ring resonator and a conventional Si resonator with similar quality factor (where the temperature sensitivity is around 0.09 nm/K). For the conventional uncompensated device, BER becomes greater than 1E-9 after only 2 degrees (Fig. 4(a) blue line). For our athermal device, data transmission is close to error free over 35 degrees (Fig. 4(a) red line). This operating range should extend even further. Slight variations in BER is due to fluctuation in fiber to waveguide coupling with temperature. We also characterized the power penalty of the athermal optical filter for a 1°C fluctuation in temperature, and compared it to an uncompensated Si resonator based filter [7

7. K. Padmaraju, J. Chan, L. Chen, M. Lipson, and K. Bergman, “Thermal stabilization of a microring modulator using feedback control,” Opt. Express 20, 27999–28008 (2012). [CrossRef] [PubMed]

]. The 1°C temperature fluctuation was introduced by placing the sample on a temperature controlled stage and modulating the stage temperature at a rate of 1 Hz. This amount of temperature fluctuation is small enough such that the waveguide to fiber coupling is minimally perturbed, while modeling a realistic operating condition. The conventional resonator has a power penalty > 1dB, while the hybrid Si – TiO2 resonator has power penalty < 0.1dB (Fig. 4(b)).

Fig. 4 (a) BER vs. temperature for 5Gbps data transmission. (b) BER vs. received power for 1 °C temperature fluctuation.

5. Conclusion

In conclusion, we demonstrated a new technique for realizing athermal Si photonic devices using a TiO2 over cladding. This approach is CMOS compatible, low loss and yields large temperature operating range. Simulations show that athermal operation is relatively insensitive to slight variations in TiO2 thickness. Engineering the mode confinement is extremely critical for achieving athermal operation. It should be noted that this method is mainly applicable for resonators with radius > 10μm radius due to the need for a slightly delocalized mode. This method of passive athermalization can lead to practical monolithic integration of silicon photonic devices.

Acknowledgments

The authors would like to thank J. Drumheller for his help in developing TiO2 deposition recipe and C. Phare for his help in Raman measurements. All the fabrication work, including TiO2 deposition, was done at the Cornell NanoScale Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation (Grant ECCS-0335765). The authors gratefully acknowledge support from CIAN (Center for Integrated Access Networks) for funding this work Grant EEC-0812072.

References and links

1.

Y. Varshni, “Temperature dependence of the energy gap in semiconductors,” Physica 34, 149–154 (1967) [CrossRef]

2.

P. Alipour, E. Shah Hosseini, A. A. Eftekhar, B. Momeni, and A. Adibi, “Temperature-insensitive silicon microdisk resonators using polymeric cladding layers,” in Conference on Lasers and Electro-Optics (Optical Society of America, 2009).

3.

M. Han and A. Wang, “Temperature compensation of optical microresonators using a surface layer with negative thermo-optic coefficient,” Opt. Lett. 32, 1800–1802 (2007). [CrossRef] [PubMed]

4.

J. Teng, P. Dumon, W. Bogaerts, H. B. Zhang, X. G. Jian, X. Y. Han, M. S. Zhao, G. Morthier, and R. Baets, “Athermal Silicon-on-insulator ring resonators by overlaying a polymer cladding on narrowed waveguides,” Opt. Express 17, 14627–14633 (2009). [CrossRef] [PubMed]

5.

C. T. DeRose, M. R. Watts, D. C. Trotter, D. L. Luck, G. N. Nielson, and R. W. Young, “Silicon microring modulator with integrated heater and temperature sensor for thermal control,” in Conference on Lasers and Electro-Optics (Optical Society of America, 2010).

6.

P. Dong, W. Qian, H. Liang, R. Shafiiha, N.-N. Feng, D. Feng, X. Zheng, A. V. Krishnamoorthy, and M. Asghari, “Low power and compact reconfigurable multiplexing devices based on silicon microring resonators,” Opt. Express 18, 9852–9858 (2010). [CrossRef] [PubMed]

7.

K. Padmaraju, J. Chan, L. Chen, M. Lipson, and K. Bergman, “Thermal stabilization of a microring modulator using feedback control,” Opt. Express 20, 27999–28008 (2012). [CrossRef] [PubMed]

8.

K. Padmaraju, D. F. Logan, X. Zhu, J. J. Ackert, A. P. Knights, and K. Bergman, “Integrated thermal stabilization of a microring modulator,” in Optical Fiber Communication Conference (Optical Society of America, 2013).

9.

W. Zortman, A. Lentine, D. Trotter, and M. Watts, “Integrated CMOS compatible low power 10Gbps silicon photonic heater-modulator,” in Optical Fiber Communication Conference (Optical Society of America, 2012). [CrossRef]

10.

C. Qiu and Q. Xu, “Wavelength tracking with thermally controlled silicon resonators,” in CLEO: Science and Innovations (Optical Society of America, 2011).

11.

E. Timurdogan, A. Biberman, D. C. Trotter, C. Sun, M. Moresco, V. Stojanovic, and M. R. Watts, “Automated wavelength recovery for microring resonators,” in CLEO: Science and Innovations (Optical Society of America, 2012).

12.

B. Guha, A. Gondarenko, and M. Lipson, “Minimizing temperature sensitivity of silicon Mach-Zehnder interferometers,” Opt. Express 18, 1879–1887 (2010). [CrossRef] [PubMed]

13.

B. Guha, B. B. C. Kyotoku, and M. Lipson, “CMOS-compatible athermal silicon microring resonators,” Opt. Express 18, 3487–3493 (2010). [CrossRef] [PubMed]

14.

B. Guha, K. Preston, and M. Lipson, “Athermal silicon microring electro-optic modulator,” Opt. Lett. 37, 2253–2255 (2012). [CrossRef] [PubMed]

15.

S. A. Campbell, H.-S. Kim, D. C. Gilmer, B. He, T. Ma, and W. L. Gladfelter, “Titanium dioxide (TiO2)-based gate insulators,” IBM journal of research and development 43, 383–392 (1999). [CrossRef]

16.

V. Trepakov, A. Dejneka, P. Markovin, A. Lynnyk, and L. Jastrabik, “A ‘soft electronic band’ and the negative thermooptic effect in strontium titanate,” New J. Phys. 11, 083024 (2009). [CrossRef]

17.

B. Guha and M. Lipson, “Athermal silicon ring resonator with bi-material cantilever for passive thermal feedback,” in CLEO: Science and Innovations (Optical Society of America, 2013).

18.

S. S. Djordjevic, K. Shang, B. Guan, S. T. Cheung, L. Liao, J. Basak, H.-F. Liu, and S. Yoo, “CMOS-compatible, athermal silicon ring modulators clad with titanium dioxide,” Opt. Express 21, 13958–13968 (2013). [CrossRef] [PubMed]

19.

F. Qiu, A. M. Spring, F. Yu, and S. Yokoyama, “Complementary metaloxidesemiconductor compatible athermal silicon nitride/ titanium dioxide hybrid micro-ring resonators,” Appl. Phys. Lett. 102, 051106 (2013). [CrossRef]

20.

J. T. Choy, J. D. Bradley, P. B. Deotare, I. B. Burgess, C. C. Evans, E. Mazur, and M. Loncar, “Integrated TiO2resonators for visible photonics,” Opt. Lett. 37, 539–541 (2012). [CrossRef] [PubMed]

21.

J. D. Bradley, C. C. Evans, J. T. Choy, O. Reshef, P. B. Deotare, F. Parsy, K. C. Phillips, M. Loncar, and E. Mazur, “Submicrometer-wide amorphous and polycrystalline anatase TiO2waveguides for microphotonic devices,” Opt. Express 20, 23821–23831 (2012). [CrossRef] [PubMed]

OCIS Codes
(120.6780) Instrumentation, measurement, and metrology : Temperature
(130.0130) Integrated optics : Integrated optics
(130.3130) Integrated optics : Integrated optics materials

ToC Category:
Integrated Optics

History
Original Manuscript: September 3, 2013
Revised Manuscript: October 12, 2013
Manuscript Accepted: October 13, 2013
Published: October 28, 2013

Citation
Biswajeet Guha, Jaime Cardenas, and Michal Lipson, "Athermal silicon microring resonators with titanium oxide cladding," Opt. Express 21, 26557-26563 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-22-26557


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References

  1. Y. Varshni, “Temperature dependence of the energy gap in semiconductors,” Physica34, 149–154 (1967) [CrossRef]
  2. P. Alipour, E. Shah Hosseini, A. A. Eftekhar, B. Momeni, and A. Adibi, “Temperature-insensitive silicon microdisk resonators using polymeric cladding layers,” in Conference on Lasers and Electro-Optics (Optical Society of America, 2009).
  3. M. Han and A. Wang, “Temperature compensation of optical microresonators using a surface layer with negative thermo-optic coefficient,” Opt. Lett.32, 1800–1802 (2007). [CrossRef] [PubMed]
  4. J. Teng, P. Dumon, W. Bogaerts, H. B. Zhang, X. G. Jian, X. Y. Han, M. S. Zhao, G. Morthier, and R. Baets, “Athermal Silicon-on-insulator ring resonators by overlaying a polymer cladding on narrowed waveguides,” Opt. Express17, 14627–14633 (2009). [CrossRef] [PubMed]
  5. C. T. DeRose, M. R. Watts, D. C. Trotter, D. L. Luck, G. N. Nielson, and R. W. Young, “Silicon microring modulator with integrated heater and temperature sensor for thermal control,” in Conference on Lasers and Electro-Optics (Optical Society of America, 2010).
  6. P. Dong, W. Qian, H. Liang, R. Shafiiha, N.-N. Feng, D. Feng, X. Zheng, A. V. Krishnamoorthy, and M. Asghari, “Low power and compact reconfigurable multiplexing devices based on silicon microring resonators,” Opt. Express18, 9852–9858 (2010). [CrossRef] [PubMed]
  7. K. Padmaraju, J. Chan, L. Chen, M. Lipson, and K. Bergman, “Thermal stabilization of a microring modulator using feedback control,” Opt. Express20, 27999–28008 (2012). [CrossRef] [PubMed]
  8. K. Padmaraju, D. F. Logan, X. Zhu, J. J. Ackert, A. P. Knights, and K. Bergman, “Integrated thermal stabilization of a microring modulator,” in Optical Fiber Communication Conference (Optical Society of America, 2013).
  9. W. Zortman, A. Lentine, D. Trotter, and M. Watts, “Integrated CMOS compatible low power 10Gbps silicon photonic heater-modulator,” in Optical Fiber Communication Conference (Optical Society of America, 2012). [CrossRef]
  10. C. Qiu and Q. Xu, “Wavelength tracking with thermally controlled silicon resonators,” in CLEO: Science and Innovations (Optical Society of America, 2011).
  11. E. Timurdogan, A. Biberman, D. C. Trotter, C. Sun, M. Moresco, V. Stojanovic, and M. R. Watts, “Automated wavelength recovery for microring resonators,” in CLEO: Science and Innovations (Optical Society of America, 2012).
  12. B. Guha, A. Gondarenko, and M. Lipson, “Minimizing temperature sensitivity of silicon Mach-Zehnder interferometers,” Opt. Express18, 1879–1887 (2010). [CrossRef] [PubMed]
  13. B. Guha, B. B. C. Kyotoku, and M. Lipson, “CMOS-compatible athermal silicon microring resonators,” Opt. Express18, 3487–3493 (2010). [CrossRef] [PubMed]
  14. B. Guha, K. Preston, and M. Lipson, “Athermal silicon microring electro-optic modulator,” Opt. Lett.37, 2253–2255 (2012). [CrossRef] [PubMed]
  15. S. A. Campbell, H.-S. Kim, D. C. Gilmer, B. He, T. Ma, and W. L. Gladfelter, “Titanium dioxide (TiO2)-based gate insulators,” IBM journal of research and development43, 383–392 (1999). [CrossRef]
  16. V. Trepakov, A. Dejneka, P. Markovin, A. Lynnyk, and L. Jastrabik, “A ‘soft electronic band’ and the negative thermooptic effect in strontium titanate,” New J. Phys.11, 083024 (2009). [CrossRef]
  17. B. Guha and M. Lipson, “Athermal silicon ring resonator with bi-material cantilever for passive thermal feedback,” in CLEO: Science and Innovations (Optical Society of America, 2013).
  18. S. S. Djordjevic, K. Shang, B. Guan, S. T. Cheung, L. Liao, J. Basak, H.-F. Liu, and S. Yoo, “CMOS-compatible, athermal silicon ring modulators clad with titanium dioxide,” Opt. Express21, 13958–13968 (2013). [CrossRef] [PubMed]
  19. F. Qiu, A. M. Spring, F. Yu, and S. Yokoyama, “Complementary metaloxidesemiconductor compatible athermal silicon nitride/ titanium dioxide hybrid micro-ring resonators,” Appl. Phys. Lett.102, 051106 (2013). [CrossRef]
  20. J. T. Choy, J. D. Bradley, P. B. Deotare, I. B. Burgess, C. C. Evans, E. Mazur, and M. Loncar, “Integrated TiO2resonators for visible photonics,” Opt. Lett.37, 539–541 (2012). [CrossRef] [PubMed]
  21. J. D. Bradley, C. C. Evans, J. T. Choy, O. Reshef, P. B. Deotare, F. Parsy, K. C. Phillips, M. Loncar, and E. Mazur, “Submicrometer-wide amorphous and polycrystalline anatase TiO2waveguides for microphotonic devices,” Opt. Express20, 23821–23831 (2012). [CrossRef] [PubMed]

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