Coded aperture imaging has been used for astronomical applications for several years. Typical implementations use a fixed mask pattern and are designed to operate in the X-Ray or gamma ray bands. More recent applications have emerged in the visible and infra red bands for low cost lens-less imaging systems. System studies have shown that considerable advantages in image resolution may accrue from the use of multiple different images of the same scenerequiring a reconfigurable mask.We report on work to develop a novel, reconfigurable mask based on micro-opto-electro-mechanical systems (MOEMS) technology employing interference effects to modulate incident light in the mid-IR band (3-5µm). This is achieved by tuning a large array of asymmetric Fabry-Perot cavities by applying an electrostatic force to adjust the gap between a moveable upper polysilicon mirror plate supported on suspensions and underlying fixed (electrode) layers on a silicon substrate.A key advantage of the modulator technology developed is that it is transmissive and high speed (e.g. 100kHz) -allowing simpler imaging system configurations. It is also realised using a modified standard polysilicon surface micromachining process (i.e. MUMPS-like) that is widely available and hence should have a low production cost in volume. We have developed designs capable of operating across the entire mid-IR band with peak transmissions approaching 100% and high contrast. By using a pixelated array of small mirrors, a large area device comprising individually addressable elements may be realised that allows reconfiguring of the whole mask at speeds in excess of video frame rates.
The development of a micro-opto-electro-mechanical system (MOEMS) technology employing interference effects to modulate incident light in the near-IR band (1550nm) over a wide angular range (120 degrees) is reported. Modulation is achieved by tuning a large array of Fabry-Perot cavities via the application of an electrostatic force to adjust the gap between a moveable mirror and the underlying silicon substrate.The optical design determines the layer thicknesses; however, the speed and power are determined by the geometry of the individual moveable elements. Electro-mechanical trade-offs will be presented as well as a key innovation of utilising overshoot in the device response in reduced pressure environment to reduce the drive voltage.Devices have been manufactured in a modified polysilicon surface micromachining process with anti-reflection coatings on the back of the silicon substrate. Measurements of individual mirror elements and arrays of mirrors at 1550nm show excellent uniformity across the array. This enables good response to an incident signal over a wide field of view when integrated with a silicon retroreflector in a passive optical tag. In conjunction with appropriate anti-stiction coatings, lifetimes of over 100 million cycles have been demonstrated.Key advantages of the modulator are that it is low cost being based on standard polysilicon micromachining; high speed (>100kHz) and robust due to utilising a massively parallel array of identical compact devices; low power for portable applications; and operates in transmission -allowing simple integration with a retroreflector in a passive tag for halfduplex free-space optical communications to a remote interrogator.
Further to previous work, which demonstrated the ability to engineer the in- and out-of-plane stress components in PECVD silicon nitride, this paper reports on the control of stress in a metal-nitride-metal sandwich. The addition of a symmetric metallisation to the core nitride layer provides additional functionality by enabling electrical connectivity and electrostatic transduction. This represents a widely applicable structural layer for CMOS-compatible surface micromachining.The use of advanced test structures coupled with wafer curvature measurements has allowed for detailed analysis of the stress components within the metal-nitride-metal sandwich and enables predictive engineering of the mechanical properties of the structural layer. Relationships between the in- and out-of-plane stress components of the metal-nitride-metal sandwich as a function of the nitride RF deposition power are reported and discussed. In comparison to values measured for nitride-only, a -30MPa/μm stress gradient offset is observed. A mechanism for the decrease in the stress gradient with the addition of the metallisation is proposed and compares well to modelling.The optimised metal-nitride-metal sandwich can be repeatably engineered to realise low tensile in-plane stress (100MPa) and low out-of-plane stress gradient (0 ± 10MPa/μm). The effective Young's Modulus of the metal-nitride-metal sandwich was determined to be 150GPa; and a value of 195GPa was calculated for the nitride layer using analytical modelling. Work to further reduce the in-plane stress whilst maintaining low stress gradient is in progress by independently tuning the strain of the metal layers.
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