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Abstract Achieving low optical loss is critical for scaling complex photonic systems. Thin‐film lithium niobate (TFLN) offers strong electro‐optic and nonlinear properties in a compact platform, making it ideal for quantum and nonlinear optics. WhileQfactors above 107are achieved, they remain below the intrinsic material limit. A systematic study of scattering losses due to roughness in TFLN racetrack cavities is presented, with isolating contributions from sidewall and interface roughness. Quality factors up to 2.7 × 107are demonstrated in waveguides with widths of 2.2λ (≈3.5 µm), where interface roughness dominates, and up to 1.2 × 107in narrower waveguides 0.8λ wide (≈1.2 µm), where sidewall roughness is the primary limitation. The modelling framework, based on 3D wave simulations informed by roughness measured by AFM, is applicable to any index contrast between waveguide core and cladding and widely applicable on integrated photonic platforms.
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Wafer-scale waveguide sidewall roughness scattering loss characterization by image processing
Photonic integrated circuits (PICs) are vital for developing affordable, high-performance optoelectronic devices that can be manufactured at an industrial scale, driving innovation and efficiency in various applications. Optical loss of modes in thin film waveguides and devices is a critical measure of their performance. Thin film growth, lithography, masking, and etching processes are imperfect processes that introduce significant sidewall and top-surface roughness and cause dominating optical losses in waveguides and photonic structures. This roughness, as perturbations couple light from guided to far-field radiation modes, leads to scattering losses that can be estimated from theoretical models. Typically, with UV-based lithography, sidewall roughness is significantly larger than wafer-top surface roughness. Atomic force microscopy (AFM) imaging measurement gives a 3D and high-resolution roughness profile, but the measurement is inconvenient, costly, and unscalable for large-scale PICs and at wafer-scale. Here, we evaluate the sidewall roughness profile based on 2D high-resolution scanning electron microscope (SEM) imaging. We characterized the loss on two homemade nitride and oxide films on 3-inch silicon wafers with 12 waveguide devices on each and correlated the scattering loss estimated from a 2D image-based sidewall profile and theoretical Payne model. The lowest loss of guided fundamental transverse electric (TE0) mode is found at 0.075 dB/cm at 633 nm across 24 devices, a record at visible wavelength. Our work shows 100% success (edge continuity span exceeding 95% of image width/height) in edge detection in image processing of all images to estimate autocorrelation function and optical mode loss. These demonstrations offer valuable insights into waveguide sidewall roughness and a comparison of experimental and 2D SEM image processing based loss estimations with applications in loss characterization at wafer-scale PICs.
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- Award ID(s):
- 2013771
- PAR ID:
- 10633271
- Publisher / Repository:
- Optica Publishing Group
- Date Published:
- Journal Name:
- Optics Express
- Volume:
- 33
- Issue:
- 10
- ISSN:
- 1094-4087
- Page Range / eLocation ID:
- 20645
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1364/OE.558186
