On-chip ultra-high Q silicon oxynitride optical resonators - ACS

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On-chip ultra-high Q silicon oxynitride optical resonators Dongyu Chen, Andre Kovach, Xiaoqin Shen, Sumiko Poust, and Andrea M. Armani ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00752 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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On-chip ultra-high Q silicon oxynitride optical resonators Dongyu Chen1, Andre Kovach2, Xiaoqin Shen2, Sumiko Poust3, Andrea M. Armani1,2,* 1

Ming Hsieh Department of Electrical Engineering-Electrophysics, University of Southern California, Los Angeles, California 90089, United States

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Mork Family Department of Chemical Engineering, University of Southern California, Los Angeles, California 90089, United States

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Northrop Grumman Corporation, Aerospace Systems Sector, Redondo Beach, California 90278, United States

ABSTRACT

Ultra-high quality factor (UHQ) optical resonators have enabled numerous fundamental scientific studies and advanced integrated photonic device technology. While free-standing devices can be fabricated from many different materials, only silica (SiO2) devices have been successfully integrated onto silicon wafers in large arrays. However, the UHQ factors (Q>108) are transient, gradually decaying over time due to the presence of hydroxyl groups on the silica surface that attract water. Here, we overcome this challenge by using silicon oxynitride (SiOxNy)

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instead of silica. Unlike SiO2, SiOxNy presents a mixture of –OH and –F groups to the environment, thus inhibiting the formation of a high optical loss water layer. As a result, quality factors in excess of 100 million are able to be maintained for longer than 14 days with no environmental controls on device storage. Over the same timeframe, quality factors for SiO2 devices stored in the same manner degraded by approximately an order of magnitude.

KEYWORDS : optical resonator; whispering gallery mode; silicon oxynitride; integrated photonics; microcavity


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Ultra-high quality factor (UHQ) whispering gallery mode optical resonators have impacted numerous fields across science and engineering, including label-free biosensing,1-8 Raman lasing,9-16 and frequency comb generation.17-23 One reason for their broad applicability is the long photon lifetime in the device or high quality factor (Q) which gives rise to exceptionally large circulating optical intensities with minimal input power. As a result, significant research efforts have focused on developing ways to integrate high-Q and ultra-high-Q cavities onto silicon. While many integrated devices have achieved intrinsic Q values 107, typically the Q is limited by either material loss or surface scattering loss.36-38 As such, the straightforward solution of minimizing these two factors inspired the first pair of on-chip UHQ devices: the silica toroidal and disk cavities.34-35 By leveraging the low optical loss of silica with either laserassisted reflow34 or chemical polishing methods,35 both devices achieved ultra-high-Q factors. Unfortunately, when the devices are left under ambient conditions (normal atmosphere), the Q of both device types gradually decreases over time because the surface hydroxyl groups attract water and other molecules,39 increasing the material loss of the device. Because SiO2 inherently presents hydroxyl groups to the environment,40 this represents a fundamental limitation of this approach and material system. Therefore, the performance degradation clearly impacts the reliability and robustness of the devices. An alternative material is silicon oxynitride. The refractive index of silicon oxynitride can be manipulated by changing the oxygen to nitrogen stoichiometric ratio.41 In this work, we demonstrate the fabrication of ultra-high-Q resonators from a chemical vapor-deposited SiOxNy


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wafer with a refractive index of 1.50. As a comparison, SiO2 resonators are also fabricated from thermally grown oxide following a similar protocol. We characterize the surfaces of both devices using fluorescence microscopy. Quality factors of both SiOxNy and SiO2 resonators are monitored continuously for two weeks in the visible and near-IR. Both the fluorescence and Qmeasurement results indicate that SiOxNy resonators have more stable Q and more reliable performance than the traditional SiO2 resonators.

RESULTS AND DISCUSSION Device Fabrication Optical resonators are fabricated from both SiO2 and SiOxNy layers using a similar procedure comprised of photolithography, Buffered HF and XeF2 etching, and a laser-assisted reflow.34 However, despite the similarity in the general process, there are notable differences in the procedures to accomplish each step. Before spin-coating the Shipley 1813 photoresist, a monolayer of HMDS is vapor-deposited on the SiO2. In contrast, to get the Shipley 1813 to adhere to the SiOxNy surface, it is necessary to spin coat HMDS onto the wafer, indicating that the surface properties of SiOxNy are quite different from SiO2. For both wafer types, 150 µm diameter circular pads are patterned (Figure 1a). Buffered HF is used to etch the SiOxNy and the SiO2. However, the buffered HF etching rate of SiOxNy is approximately four times faster than the etching rate of SiO2, providing further evidence that the material properties of the SiOxNy are quite different from SiO2.


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Figure 1. Rendering of the fabrication process of the optical resonant cavities. (a) Circular SiOxNy or SiO2 pads are defined on the silicon substrate using photolithography and buffered HF etching. (b) SiOxNy or SiO2 disks are supported by silicon pillars after XeF2 etching. (c) SiOxNy or SiO2 toroids with uniformly smoothed surfaces are created after the CO2 laser reflow process. (d) SEM image of an array of SiOxNy microtoroidal resonators. (e) SEM image of the SiOxNy microdisk. (f) SEM image of the SiOxNy microtoroid resonator.

Surface Characterization Based on past work, it is known that SiO2 presents a dense layer of hydroxyl (-OH) groups to the environment.39, 42 While SiOxNy also has surface –OH groups, these groups can be replaced by fluorines, depending on the post-deposition processing steps.42 Therefore, using fluorescence microscopy and a fluorescent indicator that targets –OH groups, the presence of –OH is measured on both the SiOxNy and SiO2 disk and toroid resonators. The SiOxNy and SiO2 disk and toroid device surfaces are silanated using a chemical vapor deposition method. This silanation is followed by grafting a fluorescent molecule that is attached to a coupling molecule (Figure 2). The device then is washed thoroughly with dichloromethane to remove excess non-bonded fluorescence molecules on the surface and is dried under vacuum for 1 min. The second step will result in the bonding of the fluorescence molecules, 4-(4-(1,2,2triphenylvinyl)phenyl)pyridine (TPPy)43, to any –OH groups present on the surface.


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Figure 2. Schematic of the surface functionalization process with (a)-(c) SiO2 and (d)-(f) SiOxNy devices. The impact of the initial –OH groups on the final number of attached fluorophores is clearly evident. To determine the surface coverage uniformity of the TPPy fluorophore on the devices, fluorescence microscopy is performed.39 Representative images from the fluorescence microscopy studies are shown in Figure 3a-h. The fluorescent signal is clearly present in both types of SiO2 devices (Figure 3a, c), verifying the presence of a high density of –OH groups on the silica surface. In contrast, on the SiOxNy devices (Figure 3e, g), the emission signal is significantly reduced on both device types, indicating a decrease in the density of –OH surface groups. To more quantitatively compare the two device types, fluorescent intensity maps are made by determining the intensity values radially from the center of the devices. To normalize the center location due to slight offsets, the values are plotted with respect to the center of the device, as indicated with a red + in Figure 2. As is clearly evident in Figure 2 (i) and (j), the SiO2 devices have significantly higher fluorescence intensity than the SiOxNy devices.

Figure 3. Fluorescence images of SiO2 (a) toroid cavity, (c) disk cavity and SiOxNy (e) toroid cavity and (g) disk cavity. (b), (d), (f), and (h) are the bright field images of the corresponding devices. (i)-(j) Fluorescence intensity maps of the toroids and disks taken at the dashed lines in the fluorescent images. Zero position of the horizontal axis indicates the center of the toroids as marked with a red + in (a), (c), (e), and (g). The fluorescence intensity of the both SiO2 devices is significantly higher than both SiOxNy devices, indicating the presence of a high density of hydroxyl groups on the surface.


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Device Characterization The quality factors of a series of SiOxNy and SiO2 devices are characterized using a pair of tunable lasers operating at 765 nm and 1300 nm (Figure 4a). Light is coupled from the lasers into the devices using tapered optical fiber waveguides. Tapered optical fibers are ideal for this measurement as they allow precise control over the amount of power coupled into the device, enabling the calculation of the intrinsic Q. The transmission spectrum is recorded on an oscilloscope. Numerous devices are fabricated and tested (N ≈10 for SiOxNy and N>40 for SiO2 for each wavelength), and the four highest performing devices are identified. Figure 4 (b)-(e) shows representative transmission spectra for two SiO2 and two SiOxNy devices at 765nm and 1300nm on day 1. From this data, the loaded Q is determined by fitting the spectra to a dual Lorentzian. The loaded Q includes both intrinsic and extrinsic losses, and as such, it is lower than the intrinsic cavity Q. By acquiring the spectra over a range of coupling conditions, the intrinsic cavity Q (Qo) is determined. An example of this measurement and additional details are in the Supporting Information. Due to the high quality factors of the devices, the mode-splitting shown in the transmission spectra occurs in most measurements. This behavior commonly occurs in ultra-high-Q devices because the light couples into both the clockwise and counter clockwise optical modes.44-46

Figure 4. Quality factor characterization of the SiO2 and SiOxNy devices at 765nm and 1300nm. (a) Rendering of testing set-up. inset: image of SiOxNy device coupled to tapered optical fiber waveguide. (b)-(c) Transmission spectrum of SiO2 toroids on Day 1 at 765 nm and 1300 nm, respectively. The loaded Q factors are 5.7x107 and 1.3x107. (d)-(e) Transmission


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spectrum of SiOxNy toroids on Day 1 at 765nm and 1300nm, respectively. The loaded Q factors are 8x107 and 5x107.

Environmental Stability One common issue with UHQ cavities is Q stability. Specifically, over time, the Q degrades due to an increase in the material loss (α) of the cavity (Qmat=2πn/λα where n is the refractive index). To explore the Q stability, the Q factors of the previously identified four SiOxNy and four SiO2 devices are monitored for two weeks. As observed in silica UHQ devices previously, the intrinsic Q of the SiO2 devices decreased over the 14-day window, stabilizing to approximately 107 (Figure 5 (a), (b)).38, 47 This finding agrees with the presence of a uniform –OH layer leading to the formation of a monolayer of water, as observed in the fluorescent microscopy study (Figure 3), and increasing the optical material loss of the cavity. In addition, even though more than 40 devices were fabricated for each wavelength, the results shown are for the four devices with the highest Q factors. Reproducibility of device fabrication is a rarely discussed issue in UHQ resonant cavity physics, but it is a critical concern and intimately linked with device stability as well as utility of the overall technology. In contrast, the Q factors of the SiOxNy devices does not change over the two-week timeframe within the error of the measurement, and all four devices have extremely similar Q factors that are mostly in excess of 108 (Figure 5 (c), (d)). The Q stability is only possible because the presence of the nitrogen inhibits the –OH formation thus reducing the attraction of water to the surface. Additionally, UHQ values from the visible through the near-IR were achieved due to the low material loss.


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Figure 5. Quality factor stability study of the SiO2 and SiOxNy devices at 765nm and 1300nm over 14 days. (a)-(b) Measured intrinsic quality factors of SiO2 toroids over two weeks at 765 nm and 1300 nm, respectively. As expected, the Q rapidly decreased, then stabilized to a value in the 107 range due to the absorption of water on the surface. (c)-(d) Measured intrinsic quality factors of SiOxNy toroids over two weeks at 765 nm and 1300 nm, respectively. In contrast to the SiO2 devices, the quality factors of the SiOxNy devices did not change over the course of the study.

The uniformity of device performance arises from consistency in the laser reflow step. This is most likely due to the material deposition process used. The SiO2 is grown via thermal oxidation whereas the SiOxNy is vapor deposited. In silica microspheres, the reflow process is thermodynamically governed and entirely relies on surface tension to form the device when the material is heated with a CO2 laser. However, in an integrated device, this process is slightly different, and it is hypothesized that the process may also involve contributions from the internal material stress. The stress profile in thermal oxides is less uniform than the stress profile in vapor deposited materials, thus possibly contributing to the observed variations in fabrication reproducibility.

CONCLUSION In conclusion, by combining novel materials and device fabrication techniques, we have developed and demonstrated UHQ cavities with long term Q stability. Notably, the presence of small amounts of nitrogen passivates the surface and inhibits –OH formation, eliminating the impact of –OH on the optical material absorption. In parallel, the SiOxNy is still compatible with


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a laser-assisted reflow process, reducing surface imperfections. The ability to fabricate stable, robust UHQ devices that are integrated on a chip will impact numerous fields ranging from fundamental science to applied technology including optical sensing,

1-6

Raman lasing,9-10,

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Kerr frequency comb generation,18, 21, 23 and optomechanics.49-50

METHODS Wafer preparation. The SiOxNy layer is grown on a high resistivity Si wafer in a PECVD reactor. The films were grown at 250C, using silane, ammonia, nitrogen, and nitrous oxide at 900mTorr. The RF power was 20 watts. The refractive index and thickness of the films was characterized ex-situ at 633nm using an ellipsometer and determined to be 1.50 and 1.5µm, respectively. SiO2 devices are fabricated from 4”, intrinsic silicon wafers with 2 µm thick thermally grown silica (WRS Materials). Device fabrication. The SiO2 or SiOxNy wafers are treated with hexamethyldisilazane (HMDS, Sigma Aldrich). To improve adhesion to the wafer, HMDS is spin-coated on the SiOxNy while it is vapor-deposited on the SiO2. 150µm diameter circles are defined using photolithography, and subsequently etched into the SiO2 or SiOxNy layer using buffered oxide etchant (Transene). Pulsed XeF2 dry etching is used to undercut the SiO2 or SiOxNy, forming the microdisk structure. The laser-assisted reflow is performed with a high power CO2 laser (SYNRAD 48-2KAM). After reflow, the diameter of the microdisk cavity is approximately 110 µm with a minor diameter of approximately 8 µm. Surface Chemistry. Reagents and solvents were purchased from Sigma Aldrich and used without further purification unless otherwise noted. [4-(chloromethyl)phenyl]trichlorosilane (97%) was purified by distillation. 4-(4-(1,2,2-triphenylvinyl)phenyl)pyridine

(TPPy) was


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synthesized according to a reported procedure.43 O2 plasma treatment was carried out using a SCE 104 plasma system (Anatech USA). The [4-(chloromethyl)phenyl]trichlorosilane coupling agent was deposited on the surface of the SiO2 or SiOxNy wafers using chemical vapor deposition at room temperature for 8 min, yielding a grafted CPS layer on the surface. A 4-[4-diethylamino(styryl)]pyridine solution in chloroform was first drop-casted onto the chlromethylphenylene-grafted devices to form a uniform, oriented layer of 4-[4-diethylamino(styryl)]pyridine. Then, the coated devices were heated to 110 °C under vacuum for 20 min. The devices were cooled to room temperature, rinsed thoroughly with dichlromethane, and dried under vacuum at 110 °C for 1 min, yielding a grafted 4-[4-diethylamino(styryl)] pyridinium fluorescent layer on the SiO2 or SiOxNy devices. Fluorescence microscopy. The fluorescence and bright field images of the devices are taken using a Nikon upright fluorescence microscope with a 20x objective. The fluorescence images are captured using a 510-550nm band pass optical filter under 430nm excitation. All excitation and imaging detector settings are held constant across the set of measurements. The fluorescence intensity of the image is mapped along a line bisecting the center of the devices using the Nikon imaging software. Device characterization. To determine the intrinsic cavity Q, the resonators are coupled to either a 765nm or 1300nm tunable, narrow linewidth laser using a tapered optical fiber.51-53 The tapered optical fibers are fabricated using an oxyhydride flame, and the fiber diameter is optimized for the operating wavelength. The optical fiber waveguide is positioned adjacent to the optical cavity using a 3-axis nano-positioning stage. The coupling gap is optimized by monitoring the location using a pair of machine vision systems and by recording the transmitted optical signal. By scanning the laser across a small wavelength range, the resonant wavelength


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(λ) is identified, and the transmission spectrum is recorded on an oscilloscope. The scan rate and scan range are controlled with a function generator to minimize linewidth distortion. To determine the loaded cavity linewidth (∆λ), the spectra is fit to a Lorentzian, and the loaded cavity Q (QL) is determined from QL=λ/∆λ. However, QL includes both the extrinsic and intrinsic cavity losses. By systematically varying the amount of power coupled into the cavity and using a coupled cavity model, the intrinsic cavity Q (Qo) is determined.54

ASSOCIATED CONTENT Supporting Information. Details on device fabrication and Q measurements.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. D. C. developed the fabrication method for the SiOxNy devices, fabricated all devices, and performed all SiOxNy Q characterization measurements. A. K. performed all SiO2 Q characterization measurements. X. S. functionalized the surface of the devices and performed all fluorescent microscopy measurements and analysis. S. P. grew SiOxNy films and performed characterization measurements. A. A. aided in experimental design and data analysis. All authors have given approval to the final version of the manuscript and the Supporting Information.


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Funding Sources This work was supported by the Northrop Grumman-Institute for Optical Nanomaterials and Nanophotonics and by the Office of Naval Research [N000141410374, N000141110910].

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33. Zhang, X. M.; Armani, A. M., Silica microtoroid resonator sensor with monolithically integrated waveguides. Opt. Express 2013, 21, 23592-23603. 34. Armani, D.; Kippenberg, T.; Spillane, S.; Vahala, K., Ultra-high-Q toroid microcavity on a chip. Nature 2003, 421, 925-928. 35. Lee, H.; Chen, T.; Li, J.; Yang, K. Y.; Jeon, S.; Painter, O.; Vahala, K. J., Chemically etched ultrahigh-Q wedge-resonator on a silicon chip. Nat. Photonics 2012, 6, 369-373. 36. Zhang, X.; Choi, H. S.; Armani, A. M., Ultimate quality factor of silica microtoroid resonant cavities. Appl. Phys. Lett. 2010, 96, 77. 37. Rose, B. A.; Maker, A. J.; Armani, A. M., Characterization of thermo-optic coefficient and material loss of high refractive index silica sol-gel films in the visible and near-IR. Opt. Mater. Express 2012, 2, 671-681. 38. Gorodetsky, M. L.; Savchenkov, A. A.; Ilchenko, V. S., Ultimate Q of optical microsphere resonators. Opt. Lett. 1996, 21, 453-455. 39. Hunt, H. K.; Soteropulos, C.; Armani, A. M., Bioconjugation strategies for microtoroidal optical resonators. Sensors 2010, 10, 9317-9336. 40. Hair, M. L., Hydroxyl groups on silica surface. J. Non-Cryst. Solids 1975, 19, 299-309. 41. Machorro, R.; Samano, E.; Soto, G.; Villa, F.; Cota-Araiza, L., Modification of refractive index in silicon oxynitride films during deposition. Mater. Lett. 2000, 45, 47-50. 42. Liu, L.-H.; Michalak, D. J.; Chopra, T. P.; Pujari, S. P.; Cabrera, W.; Dick, D.; Veyan, J.F.; Hourani, R.; Halls, M. D.; Zuilhof, H., Surface etching, chemical modification and characterization of silicon nitride and silicon oxide—selective functionalization of Si3N4 and SiO2. J. Phys.: Condens. Matter 2016, 28, 094014. 43. Chen, X.; Shen, X. Y.; Guan, E.; Liu, Y.; Qin, A.; Sun, J. Z.; Tang, B. Z., A pyridinylfunctionalized tetraphenylethylene fluorogen for specific sensing of trivalent cations. Chem. Commun. 2013, 49, 1503-1505. 44. Weiss, D.; Sandoghdar, V.; Hare, J.; Lefevre-Seguin, V.; Raimond, J.-M.; Haroche, S., Splitting of high-Q Mie modes induced by light backscattering in silica microspheres. Opt. Lett. 1995, 20, 1835-1837. 45. Gorodetsky, M. L.; Pryamikov, A. D.; Ilchenko, V. S., Rayleigh scattering in high-Q microspheres. J. Opt. Soc. Am. B 2000, 17, 1051-1057. 46. Zhu, J.; Ozdemir, S. K.; Xiao, Y.-F.; Li, L.; He, L.; Chen, D.-R.; Yang, L., On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator. Nat. Photonics 2010, 4, 46-49. 47. Vernooy, D. W.; Ilchenko, V. S.; Mabuchi, H.; Streed, E. W.; Kimble, H. J., High-Q measurements of fused-silica microspheres in the near infrared. Opt. Lett. 1998, 23, 247-249. 48. Jiang, X.-F.; Xiao, Y.-F.; Yang, Q.-F.; Shao, L.; Clements, W. R.; Gong, Q., Free-space coupled, ultralow-threshold Raman lasing from a silica microcavity. Appl. Phys. Lett. 2013, 103, 101102. 49. Miao, H. X.; Ma, Y. Q.; Zhao, C. N.; Chen, Y. B., Enhancing the Bandwidth of Gravitational-Wave Detectors with Unstable Optomechanical Filters. Phys. Rev. Lett. 2015, 115, 211104. 50. Brooks, D. W. C.; Botter, T.; Schreppler, S.; Purdy, T. P.; Brahms, N.; Stamper-Kurn, D. M., Non-classical light generated by quantum-noise-driven cavity optomechanics. Nature 2012, 488, 476-480. 51. Knight, J. C.; Cheung, G.; Jacques, F.; Birks, T., Phase-matched excitation of whispering-gallery-mode resonances by a fiber taper. Opt. Lett. 1997, 22, 1129-1131.


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For Table of Contents Use Only

On-chip ultra-high Q silicon oxynitride optical resonators Dongyu Chen, Andre Kovach, Xiaoqin Shen, Sumiko Poust, Andrea M. Armani

A false-colored SEM image of an ultra-high-Q SiOxNy toroidal resonator on a silicon substrate (left). The optical quality factors of the SiOxNy toroidal microcavity devices at 765nm and 1300nm measured over two weeks (right). The Q factors were stable over this duration indicating no environmental degradation.


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Figure 1. Rendering of the fabrication process of the optical resonant cavities. (a) Circular SiOxNy or SiO2 pads are defined on the silicon substrate using photolithography and buffered HF etching. (b) SiOxNy or SiO2 disks are supported by silicon pillars after XeF2 etching. (c) SiOxNy or SiO2 toroids with uniformly smoothed surfaces are created after the CO2 laser reflow process. (d) SEM image of an array of SiOxNy microtoroidal resonators. (e) SEM image of the SiOxNy microdisk. (f) SEM image of the SiOxNy microtoroid resonator. 166x90mm (300 x 300 DPI)


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Figure 2. Schematic of the surface functionalization process with (a)-(c) SiO2 and (d)-(f) SiOxNy devices. The impact of the initial –OH groups on the final number of attached fluorophores is clearly evident.

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Figure 3. Fluorescence images of SiO2 (a) toroid cavity, (c) disk cavity and SiOxNy (e) toroid cavity and (g) disk cavity. (b), (d), (f), and (h) are the bright field images of the corresponding devices. (i)-(j) Fluorescence intensity maps of the toroids and disks taken at the dashed lines in the fluorescent images. Zero position of the horizontal axis indicates the center of the toroids as marked with a red + in (a), (c), (e), and (g). The fluorescence intensity of the both SiO2 devices is significantly higher than both SiOxNy devices, indicating the presence of a high density of hydroxyl groups on the surface.

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ACS Photonics

Figure 4. Quality factor characterization of the SiO2 and SiOxNy devices at 765nm and 1300nm. (a) Rendering of testing set-up. inset: image of SiOxNy device coupled to tapered optical fiber waveguide. (b)(c) Transmission spectrum of SiO2 toroids on Day 1 at 765 nm and 1300 nm, respectively. The loaded Q factors are 5.7x107 and 1.3x107. (d)-(e) Transmission spectrum of SiOxNy toroids on Day 1 at 765nm and 1300nm, respectively. The loaded Q factors are 8x107 and 5x107.

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Quality factor stability study of the SiO2 and SiOxNy devices at 765nm and 1300nm over 14 days. (a)-(b) Measured intrinsic quality factors of SiO2 toroids over two weeks at 765 nm and 1300 nm, respectively. As expected, the Q rapidly decreased, then stabilized to a value in the 107 range due to the absorption of water on the surface. (c)-(d) Measured intrinsic quality factors of SiOxNy toroids over two weeks at 765 nm and 1300 nm, respectively. In contrast to the SiO2 devices, the quality factors of the SiOxNy devices did not change over the course of the study. 111x89mm (600 x 600 DPI)


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On-chip ultra-high Q silicon oxynitride optical resonators - ACS

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