The Microelectronics Research Group (MRG) at The University of Western Australia is a key partner of the Australian Research Council Centre of Excellence for Transformative Meta-Optical Systems. In this presentation, an overview of ongoing research will be given with an emphasis on the flagship research activities of MCT-based imaging arrays and Microelectromechanical Systems (MEMS). The MCT research and development utilise a vertically integrated capability from semiconductor material growth, through device modelling and design, to focal-plane-array fabrication and packaging. In support of the detector array capability, fully integrated MEMS technology can be used to further enhance the sensor device performance through the focal plane integration of tunable filters for spectral classification and infrared spectroscopy. The combination of high-performance detector designs and tunable spectral filters provides a major differentiator for military imaging systems, particularly for those operating in complex and degraded environments. This talk will highlight several research activities that are highly relevant to defence applications including metamaterial enhanced infrared detectors, and the fabrication of infra-red focal plane arrays on flexible substrates. For the MEMS technology, both wideband and narrowband tunable spectral filters will be discussed for multispectral imaging in the SWIR, MWIR and LWIR bands, and for hyperspectral imaging and spectroscopy. Considerations on future research activities and technology trends will be presented including opportunities for the rapid development of high-performance and spectrally adaptive low SWaP sensing systems for enhanced detection and discrimination of partially concealed or camouflaged targets in cluttered backgrounds.
While agile multispectral imaging solutions presently exist, their size, weight and power (SWaP) specifications prevents deployment on small portable platforms such as drones. As much of the size and weight of existing solutions is attributed to the wavelength-selective optical subsystem, realizing low-SWaP hinges on miniaturization of this subsystem. The ultimate multispectral imaging implementation would integrate the wavelength-selective component at the imaging focal plane array. This paper presents a solution which aims to achieve such integration. Recent developments in Microelectromechanical Systems (MEMS) have realized a surface-micromachined optical tunable filter, operating in the shortwave infrared wavelength band (SWIR: 1 μm – 2.6 μm) for applications in miniature optical spectrometers. The tunable filter is a Fabry-Perot (FP) structure, composed of a fixed dielectric mirror on a silicon substrate, and a movable dielectric mirror suspended above. The separation (air gap) between these two mirrors defines the optical transmission centre-wavelength of this Fabry Perot structure. Consequently, electrostatic actuation of the top mirror towards the bottom mirror allows the gap, and thus the transmission centre-wavelength, to be controlled. This paper presents work towards integration of such a MEMS tunable filter technology directly on an infrared focal plan array. Realizing this integration relies on: (1) expanding the optical area of the MEMS Fabry Perot structure to cover a significant portion of the two-dimensional focal plan array, which is generally multi-millimetre in each of its two dimensions; and (2) devising a structure that will allow actuation of the MEMS filter with under 20 V.
We examine the use of foundry process optical waveguide couplers in making 1 × 2 optical switches. The concept involves a post-foundry process to provide a moveable dielectric load over one of the waveguides in the coupler, such that the dielectric load changes the propagation constant of the affected waveguide depending on its proximity to the waveguide. Coupled mode theory is employed to explain the operation of the switch and identify key requirements of the dielectric load. Finite difference time domain simulations are employed to verify that the concept is viable for two standard photonic integrated circuit platforms. The concept ensures that the optical signals are always constrained within the high-quality foundry process waveguides while also allowing the material and lithography requirements for the layer in which the movable load is realized to be relaxed. Results show that a contrast between the switch ports of >20 dB is possible with relaxed tolerances for the dielectric load layer, at an operating wavelength of 1550 nm. We envision that the dielectric load would be moved by a micro-electromechanical systems actuator. Having the optical signals always within the foundry waveguides will permit fabrication of high-performance mechanically switched optical systems by a wide range of facilities.
Future remote imaging systems promise spectroscopic functionalities extending well beyond the visible wavelengths. This allows real-time spectral information to be gathered from multiple wavelength bands which is highly attractive for numerous remote sensing spectroscopy/imaging applications and aids target recognition. This paper briefly presents a micro-electromechanical systems (MEMS) based electrically tuneable adaptive filter technology developed for the technologically important infrared (IR) bands of the electromagnetic spectrum and reports on the progress towards extension to the significantly longer wavelength THz band. The demonstrated concepts focus on merging MEMSenabled dynamic modulation with the spectral sensitivity and selectivity of metamaterials, as well as on the possibility of adopting the rapidly evolving 3D printing technologies.
This paper presents a proof-of-concept for microelectromechanical system (MEMS)-based fixed cavity Fabry–Pérot interferometers (FPIs) operating in the long-wavelength infrared (LWIR, 8 to 12 μm) region. This work reports for the first time on the use of low-index BaF2 thin films in combination with Ge high-index thin films for such applications. Extremely flat and stress-free ∼3-μm-thick free-standing distributed Bragg reflectors (DBRs) are also presented in this article, which were realized using thick lift-off of a trilayer structure fabricated using Ge and BaF2 optical layers. A peak-to-peak flatness was achieved for free-standing surface micromachined structures within the range of 10 to 20 nm across large spatial dimensions of several hundred micrometers. Finally, the optical characteristics of narrowband LWIR fixed cavity FPIs are also presented with a view toward the future realization of tunable wavelength MEMS-based spectrometers for spectral sensing. The measured optical characteristics of released FPIs agree with the modeled optical response after taking into consideration the fabrication-induced imperfections in the free-standing top DBR such as an average tilt of 15 nm and surface roughness of 25 nm. The fabricated FPIs are shown to have a linewidth of ∼110 nm and a suitable peak transmittance value of ∼50 % , which meets the requirements for their utilization in tunable MEMS-based LWIR spectroscopic sensing and imaging applications requiring spectral discrimination with narrow linewidth.
The anticipated feature of future generation remote infrared (IR) sensing and imaging technologies includes adding so called multi-colour capabilities. Such enhancement of the current state-of-the-art IR detector and imaging focal plane array (FPA) technologies allows real-time spectral information to be gathered from multiple wavelength bands. Multi/hyper-spectral imaging results in improved target recognition and is applicable to numerous remote sensing spectroscopy/imaging applications. In order to provide a reduced size, weight and power (SWaP) solution, a micro electromechanical systems (MEMS) based electrically tuneable adaptive filter technology has been developed for important IR bands of the electromagnetic spectrum. The adopted approach is capable of delivering on-chip remote hyper/multi-spectral sensing by obtaining narrow-band spectral sensitivity utilising a tuneable MEMS optical filter fabricated directly on a detector. This paper summarizes the performance demonstrated within the most technologically relevant bands of short-wave IR (SWIR, 1.4-2.5 µm), mid-wave IR (MWIR, 3-5 µm), and long-wave IR (LWIR, 8-12 µm). In SWIR, the demonstrated nanometer-scale uniformity in the flatness of suspended MEMS allows for spatial uniformity of the filtered peak centre wavelength and the achieved 30-35 nm spectral width to remain within single nanometers over 500µm x 500µm optical apertures. In LWIR, the spatial peak wavelength selectivity variation is achieved to be less than 1.2% across 200μm × 200μm optical imaging areas, exceeding the requirements for passive multispectral thermal imaging and validating the suitability for mechanically robust multi/hyper-spectral remote sensing and imaging applications deployable on low-SWaP field-portable platforms.
High performance distributed Bragg reflectors (DBRs) are key elements to achieving high finesse MEMS-based Fabry–Pérot interferometers (FPIs). Suitable mechanical parameters combined with high contrast between the refractive indices of the constituent optical materials are the main requirements. In this paper, Germanium (Ge) and barium fluoride (BaF2) optical thin-films have been investigated for mid-wave infrared (MWIR) and long-wave infrared (LWIR) filter applications. Thin-film deposition and fabrication processes were optimised to achieve mechanical and optical properties that provide flat suspended structures with uniform thickness and maximum reflectivity. Ge-BaF2-Ge 3-layer solid-material DBRs have been fabricated that matched the predicted simulation performance, although a degradation in performance was observed for wavelengths beyond 10 μm that is associated with optical absorption in the BaF2 material. Ge-Air-Ge 3-layer air-gap DBRs, in which air rather than BaF2 served as the low refractive index layer, were realized to exhibit layer flatness at the level of 10 to 20 nm across lateral DBR dimensions of several hundred micrometers. Measured DBR reflectance was found to be ≳90 % over the entire wavelength range of the MWIR band and for the LWIR band up to a wavelength of 11 μm. Simulations based on the measured DBR reflectance indicates that MEMS-based FPIs are able to achieve a peak transmission of ≳90 % over the entire MWIR band and up to 10 μm in the LWIR band, with a corresponding spectral passband of ≲50 nm in the MWIR and <80 nm in the LWIR.
High performance tunable absorbers for terahertz (THz) frequencies will be crucial in advancing applications such as single-pixel imaging and spectroscopy. Metamaterials provide many new possibilities for manipulating electromagnetic waves at the subwavelength scale. Due to the limited response of natural materials to terahertz radiation, metamaterials in this frequency band are of particular interest.
The realization of a high-performance tunable (THz) absorber based on microelectromechanical system (MEMS) is challenging, primarily due to the severe mismatch between the actuation range of most MEMS (on the order of 1-10 microns) and THz wavelengths on the order of 100-1000 microns. Based on a metamaterial design that has an electromagnetic response that is extremely position sensitive, we combine meta-atoms with suspended at membranes that can be driven electrostatically. This is demonstrated by using near-field coupling of the meta-atoms to create a substantial change in the resonant frequency.
The devices created in this manner are among the best-performing tunable THz absorbers demonstrated to date, with an ultrathin device thickness ( 1/50 of the working wavelength), absorption varying between 60% and 80% in the initial state when the membranes remain suspended, and with a fast switching speed ( 27 us). In the snap-down state, the resonance shifts by γ >200% of the linewidth (14% of the initial resonance frequency), and the absorption modulation measured at the initial resonance can reach 65%.
We present an experimental demonstration of a novel, integrated readout approach for measuring the suspended height of micro-electro-mechanical systems (MEMS) structures. The approach is based on creating a resonant optical cavity between the suspended MEMS structure and the substrate that the MEMS structure is anchored to. The resulting interferometric effect causes modulation of an optical laser signal which is strongly dependent on the position of the MEMS device.
This work focuses on the development of a cryogenic optical profilometry system for the measurement of material properties of thin films across a wide temperature range. A cryostat was machined and integrated with a Zygo NewView 600K optical profilometer and vacuum system. Curvature data were taken for a SiNx thin film on a GaAs substrate from 300 K down to 80 K. From the curvature data, the coefficient of thermal expansion was calculated. The cryogenic optical profilometry system was benchmarked with a three beam curvature technique, and demonstrated excellent agreement across the full temperature range from 300 K to 80 K
Recent progress in short wavelength infrared MEMS based Fabry-Pérot microspectrometers at The University of
Western Australia is presented. The original monolithic approach has been replaced with a hybrid one due to HgCdTe
restricting the thermal budget and affecting the quality of structural silicon nitride films. The spectral resolution has been
improved by introducing five layer Bragg mirrors and by limiting the electrostatically actuated top mirror bowing and
tilting using stress balancing between layers. In effect the FWHM has been reduced to 30nm at ~2.0μm in comparison to
the ideal theoretical mid-range value of 9nm. Analysis of mirror profiles shows that this difference is a result of
remaining mirror imperfections.
Thin-film MEMS are essential to realization of intelligent integrated microsystems. Of critical importance in such microsystems is the determination and control of mechanical properties in the thin films used for construction of the MEMS, which can be the decisive factor in the realization and subsequent performance, reliability, and long-term stability of the system. In future microsystems the need to fabricate MEMS on temperature sensitive, non-standard substrates will be of particular importance. In this work, mechanical properties of low-temperature (50-300°C) plasma-enhanced chemical vapour deposited silicon nitride thin films have been investigated using depth sensing indentation. Young’s modulus, E, and hardness, H, values obtained for the examined film/substrate bilayers were found to vary asymptotically from the thin film properties for shallow indents to the substrate properties for deep indents. A simple empirical formulation is shown to relate E and H obtained for the film/substrate bilayers to corresponding material properties of the constituent materials via a power-law relation. The temperature of the deposition process was found to strongly influence the thin film mechanical properties. Values of E ~ 150-160GPa and H ~ 14-15GPa were observed for depositions above 225°C. Decreasing the deposition temperature initially caused a moderate and linear decrease in E and H parameters, which was followed by an abrupt decrease in E and H once the deposition temperature was lowered below 100°C, such that E ~ 50GPa and H ~ 3.5GPa at a deposition temperature of 50°C.
Two experimental techniques have been investigated to examine residual stress in low temperature plasma enhanced chemical vapour deposited (PECVD) SiNx thin films: one that measures the stress induced substrate curvature, and the other that takes advantage of the stress induced deformation of freestanding diagnostic microstructures. A general linear dependence of residual stress on deposition temperature is observed, with the magnitude of stress changing linearly from circa 300MPa tensile stress to circa 600MPa compressive stress as the deposition temperature is decreased from 300°C to 100°C. However, the results deviate from the linear dependence by a different degree for both measurement techniques at successively lower deposition temperatures. The stress values obtained via the substrate curvature method deviate from the linear dependence for deposition temperatures below 200°C, whereas the values obtained via the diagnostic microstructures method deviate from the linear dependence for deposition temperatures below 100°C. Stress uniformity over the deposition area is also investigated.
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