1. Introduction

Over the past few decades, the miniaturization of photonic waveguides has emerged as one of the most prominent technology platforms in photonic integrated circuits (PICs) [1–5]. The micro-ring resonator is a promising candidate for a variety of optical devices due to its compact size, wavelength selectivity, tunability and flexible structure [6, 7]. Based on the intra-cavity effective refractive index change or resonance shift, such micro-ring resonators have been widely utilized in optical components such as optical sensors [8, 9], de-multiplex systems [9], optical filters [10], and optical modulators [11]. The high quality factor (Q) is a main positive attribute of micro-ring resonators. High-Q micro-ring resonators can act as high sensitivity sensors, narrow pass band filters and low power consumption modulators. The value of the Q-factor depends on several waveguide parameters of the resonator. In practice, optimizing the transmission of the waveguide and coupling condition is the main method to obtain ultrahigh-Q micro-ring resonators [12, 13].

Recently, low-loss Si₃N₄ waveguide has been studied as potential candidates for photonic integration platforms. Compared with silicon waveguide, Si₃N₄ has a wide transparent wavelength bandwidth ranging from visible to near infrared wavelengths of the C-band [14–17]. Additionally, Si₃N₄ has a high Kerr nonlinearity and a negligible two photon absorption, which are preferred for the parametric oscillation application [18]. As a consequence of the high nonlinearity and low transmission loss, excellent Si₃N₄ based four wave mixing devices have been demonstrated with easy phase matching and high signal amplification over a wide wavelength range [19].

The Si₃N₄ film has been deposited by either low pressure chemical vapor deposition (LPCVD) at around 700°C or plasma enhanced chemical vapor deposition (PECVD) at around 400°C, both temperatures of which are too high for many emerging applications [20]. Moreover, the composition of Si₃N₄ deposited by these methods strongly depends on the deposition condition, and is silicon-rich or nitrogen-rich. Therefore, both PECVD and LPCVD methods lead to dangling Si-H and N-H bonds within the films due to imperfection of SiH₄ and NH₃ chemistry [20, 21]. These bonds have an intrinsic absorption centered at 1520 nm, which leads to excess propagation losses in the C-band. Furthermore, deposited Si₃N₄ films have demonstrated a high tensile stress. This tensile stress prevents a thicker film deposition greater than 250 nm-thick with low loss state [21, 22]. However, achieving suitably thick layers is critical for nonlinear optical waveguide because both high mode confinement and dispersion engineering for phase matching are required. In order to overcome these problems, we investigated the use of LSCVD to form Si₃N₄ films under low temperature (150°C) as the deposition method. Compared with LPCVD and PECVD, LSCVD is capable of producing high quality Si₃N₄ films without the presence of undesired H-N and Si-H bonds. Additionally, the refractive index of the Si₃N₄ film can be tailored by changing the ratio of deposition source and gases.

In this work, we have reported a study on micro-ring resonators using an Si₃N₄ film deposited by the LSCVD method. A ring resonator waveguide based on as-deposited Si₃N₄ was fabricated and its material growth limitations and optical qualities evaluated. We designed and fabricated Bragg grating couplers as the input and output terminals in order to effectively couple the light into the device and perform the optical test [23]. The depositions and fabrications were optimized to achieve a low loss and high Q micro-ring resonator. A propagation loss of 2.9 dB/cm and a high Q-factor of 5.2 × 10⁴ were achieved. The proposed device exemplifies its viability toward a photonic integration platform for lower energy and small-footprint applications. This study provides the method to develop high Q micro-ring resonators and their optimization by utilizing LSCVD deposition of the Si₃N₄ film.

2. Experimental

2.1 Fabrication process of the Si₃N₄ ring resonator

Recently, Si₃N₄ has been widely utilized in integrated optic devices because of its conspicuous flexibility in the refractive index around 2.0. In our fabrication (Fig. 1), the Si₃N₄ films with controlled thickness and refractive index were deposited onto the SiO₂/Si substrate through LSCVD using the liquid SiN-X source (SAMCO Inc.) with N₂ or N₂O. The measured refractive index of the deposited Si₃N₄ film was 1.99, which can be turned to between 1.66 and 1.78 as silicon oxynitride (SiO_xN_y) by mixing N₂O gas. The propagation loss of the films also demonstrated a temperature dependence of the deposition. If the temperature is set too low, the chemical reaction may be uncompleted to form Si₃N₄. We found that the optimal deposition temperature was 150°C to obtain the required low loss properties. The photonic patterns of waveguides, ring resonators, and grating couplers were transferred onto the resist layer on Si₃N₄ by using the electron beam lithography (EBL) technique. The direct write capability of EBL guarantees the small feature size and high accuracy of the device. After development, the patterned resist was hard-baked at 150°C for 5 minutes. This process is effective to improve the dry etching selectivity of the resist and Si₃N₄, and to help to achieve rectangle waveguide cross-sections. We used a mixed gas of CHF₃/O₂ for the inductively coupled plasma reactive ion etching (ICP-RIE). After etching to reach the desired depths, we striped the leftover resist using the RIE in which the O₂ plasma removes only the resist, but leaves the exposed Si₃N₄ waveguide untouched. By utilizing CVD, we deposited a top cladding of SiO₂ onto the Si₃N₄ core to form a buried ridge waveguide resonator.

 

Fig. 1 Schematic of fabrication process.

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Figure 2 shows the SEM images of the fabricated ring resonator. In the cross sectional and top views, the vertical side wall with an ideal rectangle and smooth surface with little roughness can be confirmed. Such an observation is in contrast to the previous PECVD Si₃N₄, which causes the sidewall angle to be larger than 20° and the roughness to be much greater [22, 24]. The optimized dimensions of the waveguide in this study were 2.0 μm-wide and 0.9 μm-thick. The radius of the ring was set to be 100 μm. The gap between the bus and ring was manipulated between 0.1 μm to 0.2 μm.

 

Fig. 2 SEM micrograph of the fabricated Si₃N₄ waveguide. (a) Top view; (b) Enlarged view of the coupling region; (c) Cross section.

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2.2 Optimization of coupling efficiency via Bragg grating

The grating coupler was designed by using the FDTD method as shown in Fig. 3(a). We calculated the coupling efficiency of the light from grating to a single mode fiber, assuming that the coupling efficiency from fiber to grating was the same. TE polarized light with a wavelength of 1550 nm was fed into the waveguide through the grating coupler. The output power was received by a standard single mode fiber which was placed above the grating at a 10° angle with respect to the vertical axis. The calculated coupling efficiency for grating couplers with various widths and periods are shown in Figs. 3(b) and (c). It can be observed that the coupling efficiencies are highly dependent on the width and period. The coupling efficiency reaches the maximum value of 50.1%, while the period of the grating is around 1.2 μm and the duty circle is set to be 50%. According to the same fabrication technique as the micro-rings and waveguides, the grating couplers were fabricated at the ends of the Si₃N₄ waveguide. Figure 3(d) shows the SEM image of the grating coupler as designed for the best efficiency.

 

Fig. 3 (a) Simulated field distribution of the waveguide grating coupler by FDTD; (b) Dependence of coupling efficiency on slot width; (c) Dependence of coupling efficiency on grating period; (d) SEM micrograph of fabricated waveguide grating coupler.

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3. Results and discussion

By using the LSCVD method the Si₃N₄ films were deposited on a Si/SiO₂ substrate with a several micrometers thickness without the limitation of obvious cracking. The low material absorption of Si₃N₄ realized the low loss waveguide. The fabricated waveguide with smooth sidewalls and a rectangular cross-section contributed to reduce the scattering loss also. Figure 4(a) shows the measured propagation loss of the 2.0 μm-wide and 0.9 μm-high Si₃N₄ waveguide at a wavelength of 1550 nm by using the cut-back method. The result indicates that the propagation loss of the Si₃N₄ waveguide was 2.9 dB/cm. Compared to maturely developed LPCVD and PECVD technologies, this propagation loss under LSCVD method is still in need of improvement. A subsequent thermal annealing process and modified ICP etching condition will be undertaken to help decrease the propagation loss in our next step.

 

Fig. 4 (a) Propagation loss of waveguides with a 2 μm width; (b) Measured normalized intensity of grating coupler versus wavelength.

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Based on the simulated result, we fabricated Bragg grating couplers with a period of 1.2 μm at both ends of the bus waveguide. Figure 4(b) shows the transmission spectrum of the of 10 mm-long Si₃N₄ waveguide using single mode TE polarization maintained fibers and the corresponding schematic diagram of detection system is inserted inside. The central wavelength for this grating coupler was 1575nm with a 3dB bandwidth of 61.51 nm, and coupling efficiency from fiber to grating was 3 dB. The measured spectral bandwidth can cover most of operating frequency of C-band and L-band. It ensures a broadband optical coupling and effective device transmission spectrum testing.

The transmission spectrums of the fabricated rings with various gaps were measured using the tunable laser. The wavelengths from a tunable laser were swept through and the transmission intensities were recorded by using an optical detector with a wavelength step of 1 pm. Figure 5(a) shows the normalized transmission spectra of the ring resonators having different bus-ring gap distances (G). The measured Q factors are steadily increased with increasing by increasing gap distances, while the extinction ratio was attenuated for G = 0.16 μm and 0.2 μm. Based on the theoretical equation of coupling efficiency and definition of coupling status [25], such performance implies that the coupling state of the micro-ring is gradually moving away from the critical coupling situation. It also explains why the Q factor is high even though the propagation loss is not ideally low enough. Therefore, the obtained Q factor is not limited by the material loss, and it can be further improved with an optimized optical confinement to obtain the critical coupling condition.

 

Fig. 5 (a) Transmission spectrum of the ring resonator with various gaps from 0.1 μm to 0.2 μm with a radius of 100 μm; (b) High transmission spectrum of the ring resonator (G = 0.2 μm) showing a Q factor of 5.2 × 10⁴.

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In Fig. 5(b), the high resolution spectrum of the ring resonator where G = 0.2 μm was measured. A Q factor of 5.2 × 10⁴ is obtained. The obtained resonance property of the ring resonator is high enough to achieve a high sensing and an excellent modulation function. Through the fabrication under the relatively low temperature steps, LSCVD deposited Si₃N₄ will have a significant impact on future potential application toward low-temperature, low-energy and small-footprint integrated photonics. This platform also enables the realization of devices that possess many novel nonlinear functions at telecommunication wavelengths. Thus our final goal is to fabricate a much more compact micro-ring resonator with both a lower loss and higher Q factor.

4. Summary

Among all CMOS-compatible components, Si₃N₄ based ring resonators show strong potential for future cost effective nonlinear integrated photonic circuits which offer a smaller footprint and lower energy consumption. High-Q ring resonators based on a low-loss Si₃N₄ waveguide platform prepared by LSCVD were investigated. The technique provides the merits of low temperature deposition and easy optimization of refractive index. Ring resonators with grating couplers were fabricated and exhibited a preliminary average waveguide loss of 2.9 dB/cm and a high Q factor of 5.2 × 10⁴. The low temperature deposited Si₃N₄ with a high optical quality is therefore a promising photonic integration platform for various photonic integration applications. We believe LSCVD deposited Si₃N₄ will lead to substantial achievements building on what has already been made thus far in silicon nano-photonics.