1. Introduction

Silicon nitride (SiN_x) films have been recognized as a fully CMOS-compatible platform for integrated photonic devices at various wavelength ranged from visible [1], telecom [2] to mid-infred [3] region. There have been very wide applications such as sensing [4], entangled photon generation [5], and hybrid photonic devices [6,7 ]. Low temperature deposition SiN_x film is of special interest for thin-film transistors (TFTs) [8], antireflection (AR) coatings [9], and III-V semiconductor devices [10]. Conventional techniques for depositing SiN_x include low-pressure chemical vapour deposition (LPCVD) and plasma-enhanced chemical vapour deposition (PECVD). Although LPCVD deposition yields materials with good optical properties, it cannot be used with certain integrated optical and electronic devices because of the high temperature involved (~700 °C). PECVD operates at a lower temperature (~300 °C), which is still too high for many emerging applications, such as those involving back-end integration or flexible/organic substrate [11,12 ], which is the primary motivation of developing an ultra-low temperature SiN_x photonic integration platform.

Several research groups have studied the low temperature deposition techniques of SiN_x films, mainly fousing on the underlying physical and chemical characteristics of materials [13–15 ]. However, little work has been reported regarding the capability of low temperature deposited SiN_x for integrated photonic applications in which the requirements for low stress, low loss and negligible hydrogen content are major concerns [16,17 ]. Historically, it has been difficult to grow low loss SiN_x layers thicker than 250 nm due to tensile film stress. A thermal cycling process combined with LPCVD deposition has been used to grow thick (644 nm) silicon nitride waveguides with losses down to 0.12 dB/cm [18]. As for PECVD grown films, although thicker films have been deposited, they were hydrogen-rich in the form of Si-H and N-H bonds, causing high optical losses around the 1550 nm band [19]. To reduce the hydrogen content in PECVD grown films, additional process steps, typically high temperature anneal or plasma treatment, have been required. These post-deposition steps are high cost, time-consuming and often not compatible with many CMOS processes. Recently, inductively coupled plasma CVD (ICP-CVD) technology has been developed to obtain SiN_x films at ultra-low temperatures with significantly decreased hydrogen concentration and controlled film stress. We have been able to obtain high-quality SiN_x films with thickness up to 2 μm in a single growth process attributing to its low stress [20]. Although this preliminarily demonstrated that ICP-CVD has very good potential for SiN_x growth, a comprehensive study on the viability of ICP-CVD SiN_x as an integrated photonic platform is needed.

In this paper, we report such a study on SiN_x deposited at ultra-low temperature using an ICP-CVD system, aimed at photonic integration purposes. Two common but challenging photonic elements, microcavities formed between two distributed Bragg reflectors (DBR) and planar integrated microring resonators, are fabricated to test the material growth process limitations and optical qualities of the films. For the former, we successfully demonstrate a vertical microcavity with a SiN_x “defect” layer sandwiched between the top and bottom DBR stacks respectively composed of 12.5 and 11.5 pairs of SiN_x/SiO₂ quarter-wave layers. For the latter, high-Q SiN_x microring resonators are fabricated, of which waveguide losses are also measured. These results compare favourably with published results summarized in Table 1 .

Table 1. Summary of propagation losses of silicon nitride waveguides deposited by LPCVD and PECVD published results, compared with that of ICP-CVD.

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2. Depositon of silicon nitride films

The ICP-CVD deposition system is a Plasmalab System 100 ICP180 from Oxford Instruments Plasma Technologies. For the deposition of the materials of interest, we use source gases of SiH₄/N₂ for SiN_x films and SiH₄/N₂O for SiO₂ films, respectively. We focus on the hydrogen content and total stress of the ICP-CVD SiN_x films as functions of the gas flow rate ratio of SiH₄/N₂ in a wide range of values. The substrate temperature has been kept constant at 75 °C. To reduce the ion bombardment during SiN_x deposition and the stress level in SiN_x films, no RF bias power is applied to the lower electrode.

Fourier transform infrared spectroscopy (FTIR) is used to analyze the nature of chemical bond groups in the films. Figure 1(a) displays the FTIR transmission spectra from 625 to 4000 cm⁻¹ for films deposited under different SiH₄/N₂ ratios (R). We can see that all the films have very low or no detectible Si-H stretching modes signals at ~2160 cm⁻¹, which is mainly attributed to the use of SiH₄/N₂ gas chemistry instead of SiH₄/NH₃. The absorption peaks of N-H bonds at ~1180 cm⁻¹ and ~3340 cm⁻¹ are observed in the SiN_x films prepared at R = 1:5 and 3:1, but not at R = 1:1 within the detection limit of the spectrometer, which is much less than the value (H >20 at.%) in SiN_x films deposited at ~300 °C using PECVD [26]. Therefore, we subsequently optimise the total stress as a function of SiH₄/N₂ flow ratio around R = 1:1, shown in Fig. 1(b). The film stress was measured by profilometer using Stoney’s principle [27] where the sample is scanned pre- and post-deposition in the same position, with SiNx film thicknesses in excess of 3 μm without cracking. In the present work, R = 1.12:1 is used to achieve the best trade-off between ultra-low film stress and hydrogen content. In addition, the refractive index of SiN_x films is determined using ellipsometry, shown in Fig. 1(c). Its refractive index is larger than 1.95 in the range from 400 to 2500 nm wavelength, much higher than that of SiO₂ (n~1.46) and thus provide high optical confinement.

 

Fig. 1 (a) Comparison of the FTIR spectra of typical example layers as functions of the gas flow rate ratio of SiH₄/N₂. (b) Total stress versus gas flow ratio of SiH₄/N₂. (c) Refractive index versus wavelength measured by ellipsometer.

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3. DBR microcavity

As the difference between the refractive indices of SiN_x and SiO₂ materials can be as large as 0.5, DBR stack structures with high optical performance could be fabricated, provided that stress can be brought under control. We have previously shown that high reflectivity DBRs can be fabricated by accurately controlling of individual layer thickness, realizing 12-period SiN_x/SiO₂ multilayers with high reflectivity up to 99% for visible to infrared wavelength ranges [20].

To further test the limit of the technology in terms of stress and thickness control, the microcavity fabricated in this work consists of a SiN_x “defect” layer inserted between two DBRs made of 12.5 and 11.5 pairs of SiN_x/SiO₂ multilayers as the top and bottom reflectors, respectively. The microcavity stack has a total thickness of about 11.5 μm, which is very demanding to the deposition process in terms of stress control. The samples are deposited on silica substrates by the above ICP-CVD processes at 75 °C. The cross-section morphology shown in Fig. 2(a) is obtained by scanning electron microscope (SEM). The dark regions correspond to the SiN_x layers and the bright ones to the SiO₂ layers. The defect layer and the two Bragg reflectors are clearly identifiable. No cracks are observed over the entire sample area due to the precision film stress control afforded by our method.

 

Fig. 2 (a) The cross-sectional SEM micrographs of the microcavity. The bright and the dark areas are SiO₂ and SiN_x layers, respectively. (b) Transmittance spectrum of the cavity with SiN_x/SiO₂ Bragg mirrors.

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The transmittance spectrum of the microcavity, obtained with a spectrophotometer operating in the 1000~2300 nm spectral range, is shown in Fig. 2(b). The measured broad stopband is observed over the range from 1470 to 1730 nm, which is predominantly derived from the reflectance spectrum of the SiN_x/SiO₂ DBR. The profile of the measured spectrum is in good agreement with the calculated result, although the stop band is not as wide as predicted. The sharp peak at 1587 nm corresponds to the microcavity resonance related to the defect layer inserted between the DBRs. The transmittance peak of the cavity resonance mode is at the same wavelength as designed, indicating precise refractive index and thickness control.

4. Fabrication of microring resonators

To demonstrate the ability of supporting photonic integration, we further fabricate SiN_x microring resonators to demonstrate its ability of supporting photonic integration. On a thermally oxidized <100> silicon wafer with an oxide thickness of 2 μm, a 600-nm SiN_x waveguide layer is deposited using the above ICP-CVD process. The device structures are defined in a 400-nm-thick negative resist (diluted AZ nLOF 2035 from Clariant Corporation, a 1.5:1 mixture of Propylene Glycol Monomethyl Ether Acetate) with a Vistec EBPG5000 ES electron-beam lithography (EBL) system at 100 kV. For as smooth as possible waveguide edges, we use a curved fracturing method to approximate the microring in the layout. Followed by the development of AZ nLOF 2035, reactive-ion-etch (RIE) (Oxford Instrument Plasmalab System 100 RIE180) with a mixture of CHF₃ and O₂ gases is applied to etch through the 600 nm SiN_x layer. The SiN_x RIE process has been optimized with regard to vertical and smooth sidewalls. A second layer of SiO₂ is then deposited as the upper cladding in the same ICP-CVD deposition chamber at 75 °C.

The optical waveguide cross-section has a width and height of 1.4 and 0.6 μm, respectively, shown in Fig. 3(a) . The effective refractive index of the waveguide is >1.76 for both TE and TM mode, as calculated by the finite element method assuming the cladding SiO₂ with refractive index of 1.46, and the core SiN_x with that of 1.95, indicating strongly confined modes. We noted that slight re-entrant profiles occurred during the deposition process, causing visible stringers on both sides of the SiN_x waveguide [28]. This could be further optimized to reduce the loss of the SiN_x waveguide. Figure 3(b) shows the scanning electron micrograph (SEM) of the fabricated microring resonator with 60 μm ring radius. The microring resonator is coupled to a straight probing waveguide of the same dimensions using evanescent directional couplers with a 400-nm gap.

 

Fig. 3 (a) SEM image of cross-section of the SiN_x waveguide with the upper and lower cladding. Inset: the mode profile of SiN_x channel waveguide calculated by the finite element method. (b) SEM image of a SiN_x microring resonator. Inset: zoom-in view SEMs of the coupling region.

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The responses of the SiN_x microring resonators are characterized using a tunable laser source with wavelengths from 1540 to 1560 nm, shown in Fig. 4(a) . The wavelength of the tunable laser is tuned with a high resolution 5 pm and is monitored with a picometer resolution wavelength meter. Two lensed fibers are used to couple light into and out of the straight probing waveguide. The average free spectral range (FSR) is 3.26 nm for the TE-polarized light. Figures 4(b) and 4(c) show the details of the responses of a microring resonator with a 3-dB bandwidth of δλ = 0.012 nm at ~1549.2 nm and of δλ = 0.01 nm at ∼1559 nm, respectively. The total quality factors (Q) of 1.28 × 10⁵ at ~1549.2 nm and of 1.56 × 10⁵ at ~1558.9 nm are achieved, respectively.

 

Fig. 4 (a) Measured transmission spectrum of the microring resonator. (b-c) Lorentzian fit to a single resonance for obtaining the quality factor (Q) of the resonator at ~1549.2 nm and ~1558.9 nm, respectively.

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5. Measurement of SiN_x waveguide propagation loss

To investigate the progation loss related to wavelength and bending radius, a series of symmetrically coupled add-drop microrings with different radii (20/40/60/80 μm) are fabricated. Using the formulae derived from [29], the propagation power loss coefficient can be determined to be ${\kappa }_{\text{p}}^2=2\pi \times (\delta {\lambda }_d)\times {(\gamma )}^{1/2}/FSR$, where $\gamma$is defined as the minimum power transmission in the through port, $\delta {\lambda }_d$ is the −3 dB bandwidth of the drop-port. Then the propagation loss in a microring resonator can be expressed as $\alpha (\text{dB}/cm)=-10\times {\log }_{10}(1-{\kappa }_{\text{p}}^2)/(2\pi r)$, where $r$is the radius of the microring resonator. Compared to the well-known cut-back method, this measurement method is independent of fiber-to-waveguide coupling or cleaved waveguide facet quality, which has been shown to be very useful in determining the low propagation loss in our waveguides and/or bends from the response of such add-drop resonators [30].

Figure 5 shows the drop-port response of a microring resonator with 60 μm radius over a broad wavelength band (1500-1600 nm). At each resonance, we further use a high resolution of 0.1 pm to ensure the accurate determination of the peak wavelength and −3 dB bandwidth. We then calculate the propagation loss of each microring at various wavelengths according to above formulae, as plotted in Fig. 6 .

 

Fig. 5 Measured transmission spectrum of the drop-port of an add-drop microring resonator with r = 60 μm.

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Fig. 6 Waveguide loss of the microring resonator at different radius versus wavelength in the range of 1500-1600 nm.

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For a microring with a small radius of 20 μm, the waveguide loss is relative high due to the bending loss. For microrings with radii larger than 40 μm, in the wavelength range of 1550-1600 nm, the average propagation loss is 0.79 ± 0.22 dB/cm. In the wavelength range of 1500-1545 nm, the loss is considerably high, mainly caused by the vibrational overtones of the Si-H bonds in SiN_x films, especially around the wavelength of 1520 nm [22]. Further optimization is needed to reduce the hydrogen content.

6. Conclusion

We have studied the properties of SiN_x films deposited at ultra-low temperature (75 °C) using ICP-CVD system. Optical microcavity composed of two SiN_x/SiO₂ DBRs and a SiN_x defect layer has been successfully fabricated in a single growth process. By fine control of film stress we have been able to reach a total of 49 alternating layers with a total stack thickness of 11.5 μm while maintaining good material quality. The transmittance spectrum shows a predicted cavity resonance centered around 1587 nm with a wide stop band from 1470 to 1730 nm. Symmetrically coupled add-drop microrings with different radii (20/40/60/80 μm) have also been fabricated and tested to obatin their waveguide loss values. An avearge value of 0.79 ± 0.22 dB/cm (1550-1600 nm) has been achieved for microrings with radii larger than 40 μm. While for the wavelength range of 1500-1545 nm, the measured waveguide loss for microrings is increased to > 10 dB/cm.

With the demonstrated control over both stress and optical loss, the ultra-low temperature ICP-CVD SiN_x/SiO₂ deposition process has significantly advantages of achieving high performance while maintaining process simplicity and feasibility, and is very widely compatible not only with CMOS processing but also other processes involving back-end integration or flexible/organic substrate. Therefore, the ultra-low temperature SiN_x materials provide a very flexible material platform that may open up many new capabilities in photonic integration applications.