Monolithically Integrated Si-on-AlN Mid-Infrared Photonic Chips for Real-Time and Label-Free Chemical Sensing

Aluminum Nitride to Silicon Direct Bonding for an Alternative Silicon-On-Insulator Platform

The next generation of microelectromechanical systems (MEMS) requires new materials and platforms that can exploit the intrinsic properties of advanced materials and structures, such as materials with high thermal conductivity, broad optical transmission spectra, piezoelectric properties, and miniaturization potential. Therefore, we need to look beyond standard SiO₂-based silicon-on-insulator (SOI) structures to realize ubiquitous MEMS. This work proposes using AlN as an alternative SOI structure due to several inherent material property advantages as well as functional advantages. This work presents the results of reactively sputtered AlN films on a Si handle wafer bonded with a mirror-polished Si device wafer. Wafer bonding was achieved by using hydrophilic wafer bonding processes, which was realized by appropriate polymerization of the prebonding surfaces. Plasma activation of the AlN surface included O₂, Ar, SF₆, SF₆ + Ar, and/or SF₆ + O₂, which resulted in a change in the chemical and topography state of the surface. Changes in the AlN surface properties included enhanced hydrophilicity, reduced surface roughness, and low nanotopography, components essential for successful hydrophilic direct wafer bonding. Wafer bonding experiments were carried out using promising surface activation methods. The results showed a multilayered bonding interface of Si(Device)/SiO₂/ALON/AlN/Si(Handle) with fluorine in the aluminum oxynitride layer from the proceeding AlN surface activation process. More notably, this work provided wafer bonding tensile strength results of the AlN alternative SOI structure that compares with the traditional SiO₂ SOI counterpart, making AlN to Si direct bonding an attractive alternative SOI platform.

Benzene Derivatives Analysis Using Aluminum Nitride Waveguide Raman Sensors

Raman spectroscopy using aluminum nitride (AlN) optical waveguides was demonstrated for organic compound analysis. The AlN waveguide device was prepared by reactive sputtering deposition and complementary-metal-oxide semiconductor (CMOS) processes. A fundamental waveguide mode was observed over a broad visible spectrum and the waveguide evanescent wave was used to excite the Raman signals of the test analytes. The performance of the waveguide sensor was characterized by measuring the Raman spectra of the benzene derivative mixtures consisting of benzene, anisole, and toluene. The compositions and concentrations were resolved by correlating the obtained Raman spectrum with the characteristic Raman peaks associated with C-C, C-H, and C-O functional groups. With the advantages of real-time detection and enhanced Raman signal intensity, the AlN waveguides provided a sensor platform for nondestructive and online chemical compound monitoring.

Real-time and non-destructive hydrocarbon gas sensing using mid-infrared integrated photonic circuits

A chip-scale mid-infrared (mid-IR) sensor was developed for hydrocarbon gas detection. The sensor consisted of amorphous Si (a-Si) optical ridge waveguides that were fabricated by complementary metal–oxide–semiconductor (CMOS) processes. The waveguide exhibited a sharp fundamental mode through λ = 2.70 to 3.50 μm. Its sensing performance was characterized by measuring methane and acetylene. From the spectral mode attenuation, the characteristic C–H absorption bands associated with methane and acetylene were found at λ = 3.29–3.33 μm and λ = 3.00–3.06 μm, respectively. In addition, real-time methane and acetylene concentration monitoring was demonstrated at λ = 3.02 and 3.32 μm. Hence, the mid-IR waveguide sensor enabled an accurate and instantaneous analysis of hydrocarbon gas mixtures.

A chip-scale mid-infrared (mid-IR) sensor was developed for hydrocarbon gas detection.

All-nanoparticle layer-by-layer coatings for Mid-IR on-chip gas sensing

Functionalization of optical waveguides with submicron all-nanoparticle coatings significantly enhanced the detection of acetone. Such coatings were enabled via precise control of the substrate withdrawal speed using the layer-by-layer deposition.

Mid-Infrared Chalcogenide Waveguides for Real-Time and Nondestructive Volatile Organic Compound Detection

A mid-infrared (mid-IR) sensor chip was demonstrated for volatile organic compound (VOC) detection. The sensor consisted of As₂Se₃ optical waveguides built by microelectronic fabrication processes. The VOC sensing performance was characterized by measuring acetone and ethanol vapors at their characteristic C-H absorption from λ = 3.40 to 3.50 μm. Continuous VOC detection with <5 s response time was achieved by measuring the intensity attenuation of the waveguide mode. The miniaturized noninvasive VOC sensor can be applied to breath analysis and environmental toxin monitoring.