We present a fully integrated photonic chip spectrometer for near-infrared tunable diode laser absorption spectroscopy of methane (CH<sub>4</sub>). The integrated photonic sensor incorporates a heterogeneously integrated III-V laser/detector chip coupled to a silicon external cavity for broadband tuning, and a long waveguide element (>20 cm) for ambient methane sensing. An on-chip sealed CH4 reference cell is utilized for in-situ wavelength calibration of the external cavity, and a real-time wavelength compensation method for laser calibration is described and demonstrated. The resulting signal is guided back to the III-V photodiodes for spectral signal readout using a custom-designed acquisition board, remotely controlled and operated by a Raspberry Pi unit. Component-level testing of the waveguide sensitivity, external cavity laser, and reference cell is demonstrated. Full-stack testing of the integrated sensor chip yields sub-100 ppmv∙Hz-<sup>1/2</sup> sensitivity, and spectral density analysis demonstrates our integrated chip sensor to have a fundamental performance within an order of magnitude of commercially available fiber-pigtailed DFB laser units. We envision our integrated photonic chip sensors to provide disruptive capability in SWaP-C (size, weight, power, and cost) limited applications, and we describe an achievable short-term pathway towards sensitivity enhancement to near-10 ppmv levels.
We present a portable optical spectrometer for fugitive emissions monitoring of methane (CH4). The sensor operation is based on tunable diode laser absorption spectroscopy (TDLAS), using a 5 cm open path design, and targets the 2ν<sub>3</sub> R(4) CH<sub>4</sub> transition at 6057.1 cm<sup>-1</sup> (1651 nm) to avoid cross-talk with common interfering atmospheric constituents. Sensitivity analysis indicates a normalized precision of 2.0 ppmv·Hz-<sup>1/2</sup>, corresponding to a noise-equivalent absorbance (NEA) of 4.4×10<sup>-6</sup> Hz<sup>-1/2</sup> and minimum detectible absorption (MDA) coefficient of αmin = 8.8×10<sup>-7</sup> cm<sup>-1</sup>·Hz<sup>-1/2</sup>. Our TDLAS sensor is deployed at the Methane Emissions Technology Evaluation Center (METEC) at Colorado State University (CSU) for initial demonstration of single-sensor based source localization and quantification of CH<sub>4</sub> fugitive emissions. The TDLAS sensor is concurrently deployed with a customized chemi-resistive metal-oxide (MOX) sensor for accuracy benchmarking, demonstrating good visual correlation of the concentration time-series. Initial angle-ofarrival (AOA) results will be shown, and development towards source magnitude estimation will be described.
The LER and LWR of subtractively patterned Si and SiN waveguides was calculated after each step in the process. It was found for Si waveguides that adjusting the ratio of CF<sub>4</sub>:CHF<sub>3</sub> during the hard mask open step produced reductions in LER of 26 and 43% from the initial lithography for isolated waveguides patterned with partial and full etches, respectively. However for final LER values of 3.0 and 2.5 nm on fully etched Si waveguides, the corresponding optical loss measurements were indistinguishable. For SiN waveguides, introduction of C4H9F to the conventional CF<sub>4</sub>/CHF<sub>3</sub> measurement was able to reduce the mask height budget by a factor of 5, while reducing LER from the initial lithography by 26%.
The need for continued device scaling along with the increasing demand for high precision have lead to the development of atomic layer etch processes in semiconductor manufacturing. We have tested this new methodology with regard to patterning applications. While these new plasma-enhanced atomic layer etch (PE-ALE) processes show encouraging results, most patterning applications are best realized by optimizations through discharge chemistry and/or plasma parameters. While PE-ALE approaches seem to have limited success for trilayer patterning applications, significant improvements were obtained when applying them to small pitch. In particular the increased selectivity to OPL seems to offer a potential benefit for patterning high aspect ratio features.