Silicon nitride (Si3N4) ring resonators are critical for a variety of photonic devices. However the intrinsically high film stress of silicon nitride has limited both the optical confinement and quality factor (Q) of ring resonators. We show that stress in Si3N4 films can be overcome by introducing mechanical trenches for isolating photonic devices from propagating cracks. We demonstrate a Si3N4 ring resonator with an intrinsic quality factor of 7 million, corresponding to a propagation loss of 4.2 dB/m. This is the highest quality factor reported to date for high confinement Si3N4 ring resonators in the 1550 nm wavelength range.
© 2013 OSA
Silicon nitride (Si3N4) ring resonators are critical for efficient and compact on chip optical routing [1–3], frequency combs [4–7], and high precision sensing [8–11], however the intrinsically high film stress of silicon nitride has limited both the optical confinement and quality factor (Q) of ring resonators. Whereas the silicon and silicon dioxide platforms generally suffer from high losses or delocalized optical modes, the Si3N4 platform provides advantages of both high confinement and high Q. High Q disks have also been demonstrated in Si3N4  but disks have larger mode volumes and are challenging to dispersion engineer for nonlinear applications. Si3N4 is also a deposited material, which enables seamless integration with other material platforms. However, the high film stress of Si3N4 prevents thick (>400 nm) films of high optical quality from being deposited; catastrophic cracking occurs, severely limiting device yield. In principle low stress nitride films can be grown thicker with plasma enhanced chemical vapor deposition (PECVD) and modified low pressure chemical vapor deposition (LPCVD) processes, but these deposition chemistries yield films with stronger material absorption caused by dangling H and O bonds with the Si and N in the films.
Thick films, limited to date by stress, would enable high confinement and high Q. Thick films lead to smaller optical mode overlap with the boundaries of the waveguides (responsible for scattering losses) leading to lower losses. This can be observed in Fig. 1 where we show the mode profile for a mode confined in two different waveguide thicknesses. Figure 1(a) shows the mode confined in a waveguide with a traditional thickness of 400 nm limited to date by stress. One can see that the mode overlaps significantly with the boundaries of the waveguide. Figure 1(b) shows the mode confined in a waveguide with a much higher thickness of 910 nm. One can see that very little of the mode overlaps with the boundary of the waveguide. Note that increasing the size of the waveguide increases the number of modes supported; however these higher order modes are only weakly excited.
Previous high Q Si3N4 ring resonators have circumvented film stress issues by exploiting either highly delocalized optical modes with extremely thin films  or highly confined modes within film stress limits . High Q ring resonators based on extremely thin Si3N4 films can avoid film stress issues, but they suffer from highly delocalized optical modes, requiring millimeter-scale bending radii and up to 15 µm of silicon oxide cladding. In addition, these resonators only support the transverse electric (TE) mode which prevents integration with devices that support the transverse magnetic (TM) mode. High confinement ring resonators based on thicker films can be achieved using high temperature deposition and anneal to relieve film stress, but films greater than 750 nm in thickness remain challenging.
In order to overcome the stress limitations of Si3N4, we strategically place mechanical trenches to isolate the photonic devices from propagating cracks. Physical shocks near the edge of the wafer, which occur often during handling of the wafer, can provide enough energy to induce cracking of the stressed film. Once initiated, these cracks originating from the edge of the wafer propagate continuously in a uniform stress field, terminating only once they encounter crack resistance at the edge of the wafer or at another crack boundary. We introduce trenches around our devices that terminate cracks before they can spread to our device region (shown in Fig. 2). A single trench does not guarantee crack termination however . Overstressed films store energy in the enhanced acceleration of the crack and the extended penetration into the substrate. With this stored energy, cracks can overcome the crack resistance of a single trench and continue propagation. To increase crack resistance, we create between two and five parallel trenches to ensure crack termination.
We define a series of trenches around the edge of the wafer before deposition of the Si3N4 film to prevent crack propagation. Beginning with a thermally oxidized 4 inch silicon wafer, we lightly scribe a series of lines into the silicon oxide surface to define a 5 cm by 5 cm rectangular region in the center of the wafer. This rectangular region will be the crack free region in which we pattern our photonic devices; elsewhere, the film will crack. Note that for ease of processing and throughput we define trenches with a diamond scribe, but in principle one can define more sophisticated trenches with photolithography and etching. Etching may actually improve the crack resistance of these trenches because of the increased roughness on the etched surfaces . In addition, photolithography could be used to define trenches only at the wafer periphery to enlarge the area of the crack free region. Following trench definition, we proceed with device fabrication as described in . After deposition of 910 nm of Si3N4, in steps of 400 nm and 510 nm, we pattern devices with electron beam lithography using ma-N 2405 resist, post exposure bake for 5 minutes at 115°C, and etch in an inductively coupled plasma reactive ion etcher (ICP RIE) using CHF3/O2 chemistry. After stripping the resist, we anneal devices at 1200°C in a nitrogen atmosphere for 3 hours. We clad devices with 250 nm of high temperature silicon dioxide (HTO) deposited at 800°C followed by 2 µm deposition of silicon dioxide using plasma enhanced chemical vapor deposition (PECVD). The fabricated device is shown in Fig. 3. Note that our trench definition method is not specific to this process; it can be applied to any highly stressed film in which cracks are initiated near the edge of the substrate.
We measure an intrinsic quality factor of 7 million, the highest quality factor reported to date for high confinement Si3N4 ring resonators; this quality factor corresponds to an ultra-low propagation loss of 4.2 dB/m. The ring resonator measured has a radius of 115 µm, a coupling gap of 680 nm, and a cross section of 910 nm tall by 1800 nm wide. We couple a tunable laser light source, transmitted through a polarization controller, into the inverse nanotaper of our device using a lensed fiber. We collect the output of the ring resonator through another inverse nanotaper and collimating lens. After passing the output through a polarizer, we monitor the output on a photodetector. In order to measure a single resonance, we finely scan the laser frequency by applying a triangular-wave voltage signal to the piezoelectric transducer of the laser, while monitoring the photodetector signal on an oscilloscope. We calibrate the voltage-frequency conversion with respect to a free space bowtie cavity. We observe some higher order mode resonances (shown in Fig. 4), but these resonances have very low extinction, suggesting that most of the optical power is in the fundamental mode. Minimal higher order mode excitation is expected because we design our waveguides to support few modes: five TE modes and four TM modes. From the single resonance scan normalized to the maximum value, we measure a resonance linewidth of 36 MHz and resonant transmission of 26% (shown in Fig. 4), corresponding to an intrinsic quality factor of 7 million for TE polarization. For TM polarization we measure a Q of 4 million. The Q for TE is higher because the TE mode has higher optical confinement and effective index, so the mode suffers less scattering and bending loss.
We calculate the propagation loss within the ring to be 4.2 dB/m and we estimate the absorption loss to be 71% of this loss, suggesting that the Q can be further improved with increased optical confinement. We calculate propagation loss using the relation 17]. Because silicon nitride has negligible nonlinear loss, we can assume that resonance shifting is dominated by thermal effects caused by material absorption. We use the resonance shift as a function of dropped power in  and simulated values for thermal conductance and thermo-optic coefficient to calculate an absorption loss of 3.0 dB/m for the devices in . Because the film deposition process in  is identical to the process used in this work, we can assume the same absorption for our devices, which accounts for 71% of the propagation loss. Therefore, the Q is not material limited and can be further improved with increased optical confinement and film deposition optimization.
We demonstrate a high quality factor of 7 million in a high confinement Si3N4 ring resonator using crack resistant trenches to overcome stress limitations of thick Si3N4 films. Our high Q devices herald advances in low loss optical routing, low power threshold nonlinear optics, and high sensitivity sensors. We have also overcome film stress limitations for Si3N4, revealing a new design space for integrated optics and microelectromechanical (MEMS) devices that has been unexplored to date.
The authors would like to thank Dr. Mohammad Soltani for his help with thermal conductance simulations and discussion. This work was supported in part by the Cornell Center for Materials Research with funding from A Graduate Traineeship in Materials for a Sustainable Future, (DGE-0903653). The authors gratefully acknowledge support from the Defense Advanced Research Projects Agency (DARPA) under award #FA8650-10-1-7064, DARPA for award # W911NF-11-1-0202 supervised by Dr. Jamil Abo-Shaeer, AFOSR for award # BAA-AFOSR-2012-02 supervised by Dr. Enrique Parra, and SRC/Intel under award #2012-IN-2378.
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