Trimming of Silicon Nitride Microring Resonators with a Polysilane Top Cladding

Microring resonators are a basic building block of photonic circuits, enabling complex functionality for optical systems. Such resonators can serve as filters for wavelength division multiplexing and demultiplexing of broadband optical signals [1], dispersion compensators for accurately controlling phase [2], lasers [3], and ultrafast all-optical switches [4]. The resonance condition for a ring resonator relates D, the diameter of the ring; λ0, the free space wavelength of resonant light; m, an integer indicating the resonator mode number; and n, the effective index of the ring (Eq. 1).

Precise control over λ 0 in each ring is critical for microphotonics integration.As ring diameters shrink to less than 10 µm, non-deterministic pattern transfer errors limit dimensional precision and cause λ 0 to shift by several nanometers for identical devices.Thus, a postproduction trimming process to modify n and control λ 0 is essential.Assuming single mode operation and continuous resonator mode number, the predicted shift of the resonance ∆λ can be derived from Eq. 1, where the subscripts 1 and 2 indicate initial and final effective indices.

  
Conventional trimming methods utilize resistive micro-heaters to induce a thermo-optic response in the core material, where the small magnitude of these thermo-optic coefficients (10 -5 -10 -4 K -1 for most dielectrics) corresponds to a feasible thermal trimming range of several nanometers.This architecture adds several steps to the fabrication of a photonic circuit, limits the density of devices to maintain thermal isolation, and requires significant power consumption to keep the rings "trimmed".
An alternative trimming method utilizes a polysilane polymer film as the cladding material and adjusts its refractive index via photo-oxidation when irradiated with UV light [6].Having a refractive index similar to silicon dioxide (SiO 2 ), the predominant material of choice for cladding layers, polysilane polymers can be integrated with Si, SiON, and Si 3 N 4 high index-contrast microring resonators.The material is transmissive over a broad range of visible and near-IR light [7,8] useful for microphotonics applications [9].
We demonstrate a vapor-phase technique for depositing an amorphous and highly cross-linked polysilane (PECVD 6M2S) top cladding layer [10] directly onto ring resonator devices.PECVD 6M2S is insoluble (unlike dip-coated materials), does not swell in organic solvents, and demonstrates good stability in ambient light, atmosphere, and temperature.Photo-

Discussion
A controllable decrease in the refractive index of the PECVD 6M2S cladding layer was achieved using photooxidation induced by UV irradiation (Fig. 1).The refractive index decreased by ~ 4%, from 1.52 to 1.46 at λ=1550 nm and was exponential in nature.UV light having a wavelength less than 300 nm causes chain scission in polysilane polymeric materials, and subsequent oxidation converts Si-Si bonds into Si-O-Si bonds [12] (Fig. 1 inset).The resulting index change is about 50% greater than the response observed with a previously reported dip-coated polysilane material [6].To compare experiment with theory, a model was formulated using the relation found in Eq. 2 and the exposure curve data in Fig. 1.The resulting theoretical resonance shifts (Fig. 2) agree well with the experimental data in terms of functional form, magnitude, and are continuous with irradiation.The overall resonance shifts, after a UV flux of 1000 µJ/cm 2 , were 12.8 nm for the TE mode and 23.5 nm for the TM mode (Fig. 2), exceeding the free spectral range (FSR) for both the TE (3.9 nm) and TM (4.5 nm) polarizations.These shifts are an order of magnitude An additional benefit of photo-oxidation trimming is the ability to focus UV light and thereby localize trimming to specified areas of a chip.This enables one to preserve the spectral response of higher order filters that require multiple rings.Differences between ring-toring and ring-to-bus coupling require localized index trimming on separate areas of the filter to keep all rings in resonance with each other.Spectral responses of the trimmed ring resonators were also measured over a range of temperatures from 25 to 70°C (Fig. 3).The thermo-optic coefficient (dn/dT) for the system was found to be -1.3x10 - K -1 for the TM mode.This value is an order of magnitude greater than oxidation trimming of PECVD 6M2S cladding provides a precise, localized, controllable, and micro-fab compatible technique for specifying the resonance condition of ring resonators.Experiment Si 3 N 4 microring resonators, designed for single mode operation at λ = 1550 nm, were fabricated on a 3 µm SiO 2 under-cladding layer on (100) Si.The microrings have a diameter of 100 µm and have crosssectional dimensions of 400 x 750 nm 2 .Next, a 1 µm thick PECVD 6M2S top cladding layer was deposited directly onto the ring resonators.Spectral characterization of the microrings was performed for the TE and TM polarizations with a JDS Uniphase tunable laser and broadband photodetector used in conjunction handheld lamp (model UVGL-25) emitting λ=254 nm UV light was used to irradiate samples with a flux of 1.7 µW/cm 2 .An identical UV irradiation process was conducted separately on a sample from the same wafer and the PECVD 6M2S film was measured after each exposure using a Woolam M-2000 variable angle spectroscopic ellipsometer (VASE).Ellipsometry data was fit to the Cauchy-Urbach model yielding the film thickness and refractive index at 1550 nm.

Figure 1 :
Figure 1: The refractive index of PECVD 6M2S versus UV flux.

Figure 2 :
Figure 2: Experimental and theoretical resonance shifts of the Si3N4 ring Resonator for the TE and TM polarizations.
can be obtained with thermal trimming for Si 3 N 4 waveguides.

SiO 2 clad
Si 3 N 4 ring resonators and enhances the potential thermal trimming/tuning range.Thermal hysteresis effects were not detected over this temperature range.