Low cost miniaturized spectrometer for mid infrared sensing

Field: Engineering (Physical Sciences)

Mid-infrared (MIR) spectroscopy is an attractive technology for the detection of molecules. It enables compounds to be identified rapidly. One main issue plaguing MIR sensing is that most equipment is bulky and is often laboratory-based. Hitherto, a miniaturized counterpart is facing several technological limitations: low power requirement, suitable optical filter for field use, uncooled detector operation at elevated temperature and the utilization of an appropriate packaging that confers physical and chemical protection. Because of the limitations, samples have to be brought back from the field for analysis. Often, it is challenging to survey samples with fast deterioration. To address the current limitations, a field operable miniaturized MIR spectrometer is proposed in this thesis. The scope of this thesis covers the fabrication and characterization of filters, mirrors and detectors. Specifically, as an optical filter is considered the heart of a spectrometer, it is given greater emphasis in this work. In particular, interferometric filters are discussed. Interferometric filters are arguably considered to be superior to other kinds of filters. In this thesis, linear variable optical filters (LVOF) for MIR sensing are fabricated for two different modes of operation. Normal incidence cross-plane LVOF is constructed by sandwiching photoresists (PR) pillars by two Si-SiO2 distributed Bragg reflector (DBR) coated Si substrates. By using PR with different thicknesses, the transmitted response is theoretically and experimentally proven to be affected by the taper angle. That is, an increase in taper angle results in a larger full width half maximum and greater attenuation in the transmission signal. The concept of substrates mated LVOF is further developed by replacing PR with low temperature deposited SiO2 patterned by liftoff in O2 plasma. Low pillar height variation between samples indicates excellent process control. Complementary holes, tilted at an angle, are etched in the other substrate. The mated LVOF is explored as a volatile organic contaminant sensor. The LVOF has a full width half maximum of 230 nm measured at 3500 nm. While cross-plane operation is the norm, the addition of a gas cell to the already bulky construction hampers further miniaturization. To address the challenge, in-plane LVOF, fabricated by deep etching on Si wafer, is investigated. To achieve that, low contrast SU-8 PR is patterned on Si with electron beam lithography. It has been qualified that by imposing a 2 hours post-exposure delay, a ten times improvement in resist contrast can be achieved. Thereafter, vertical Si pillars are formed by an optimized reactive etching process. Subsequently, the Si-air DBR Fabry-Perot interferometer and LVOF are subjected to poly-Si deposition. The treatment results in the broadening of Si pillars. By manipulating the increase and decrease in Si linewidth and spacing by 194 nm, respectively, the transmitted peak redshifted by 140 nm. The concept is proposed to repair erroneously made samples. It is further proposed that two wafers with a center cavity, containing the LVOF, is bonded. While the LVOF is fabricated on the bottom substrate, the top substrate contains the emitter and detector modules. Finally, the intracavity space in the wafer packaged LVOF acts as a gas cell. The in-plane LVOF located on the bottom substrate requires a parallel to the substrate light path. Without waveguides, vertical free space propagated light is generated by the emitter on the top substrate. Therefore, vertically propagated light has to be redirected. Thus, Si (110) plane as an on-chip mirror for 90o light redirection is demonstrated. KOH wet etching is carried out in the presence of tensioactive Triton X-n (n = 45, 100 and 405) molecules. Using contact angle measurement, it is shown that Triton preferentially adsorbs on (110) due to the higher packing density of Si atoms. Shorter Triton can be surface adsorbed due to lower steric repulsion between molecules. On the contrary, due to the presence of ether bonds, the long Triton hydrophilic chain is capable of forming hydrogen bonds with ionic reactants and products. This delays ions transport and slows down the rate of reaction in the following order (X-45 > X-100 > X-405). As such, the presence of long chain Triton produces smooth (110) surfaces which is essential for the fabrication of mirrors. Given that present MIR thermal detectors have a low figure of merit (FOM < 0.05), graphene as detector with better FOM (> 1) is investigated. Chemical vapor deposited grown graphene is transferred onto a SiO2/Si substrate. Subsequently, metal nanoparticles are reduced onto graphene’s surface (G-Au, Ag and Pt), assisted by ultraviolet irradiation. The fabricated device shows an increase in the thermoelectric power factor (TEPF). A decrease in sheet resistance is as follows (G-Au > G-Ag > G-Pt). While a 25% decrease in the Seebeck coefficient is observed for G-Au due to localized thermal cooling, the system offers a three times improvement in the overall TEPF. The G-Au reveals the highest TEPF amongst the tested metals. The results shown above suggest new methodologies for cross-plane and in-plane MIR LVOF fabrication. Pillars formed with PR and oxide grant the formation of precise and tunable taper angles. On the other hand, in-plane LVOF allows further miniaturization of compact spectrometers with 2D materials as thermal detectors; while cavity space can be explored as a gas cell. The concepts and design rules presented in this thesis pave ways for the manufacturing of highly precise fixed filters. The methodology presented can be leveraged upon for the creation of next-generation hyperspectral sensors with enhanced infrared sensitivity.