Kevin M. Brunson

Engineering in- and out-of-plane stress in PECVD silicon nitride for CMOS-compatible surface micromachining

This paper presents notable improvements in the ability to control and distinguish the composite stress components within plasma enhanced chemical vapour deposition (PECVD) silicon nitride. Wafer curvature measurements complemented by stress structure fabrication and characterisation has enabled detailed analysis of in- and out-of-plane stress. Analytical modelling has allowed clarification of the relative contribution to the wafer curvature attributed solely to the stress gradient, which is of the order of 10-5 microns. Therefore the measured wafer curvature (due to composite stress), can be thought as a true representation of the actual wafer curvature due solely to the in-plane stress of the deposited thin film. This work represents a considerable advance compared with our previously published stress characterisation work on PECVD silicon nitride, which relied solely on wafer curvature measurements. However, the fabricated ring-beam and fixed-fixed structures were unable to resolve the in-plane stress component in high out-of-plane stress regimes. As predicted, at the zero stress gradient point, the fixed-fixed structures did measure an in-plane longitudinal compressive stress of -50MPa, which agrees well with wafer curvature measurements. Both stress components may now be repeatably controlled to realise tensile or compressive stresses (in-plane longitudinal) and positive or negative stress gradients (out-of-plane), by varying the RF deposition power. This new methodology allows for optimisation of the material for specific applications and in addition enhances the accuracy of micromechanical device models.

A large area reconfigurable MOEMS microshutter array for coded aperture imaging systems

Coded aperture imaging has been used for astronomical applications for several years. Typical implementations used a fixed mask pattern and are designed to operate in the X-Ray or gamma ray bands. Recently applications have emerged in the visible and infra red bands for low cost lens-less imaging systems and system studies have shown that considerable advantages in image resolution may accrue from the use of multiple different images of the same scene - requiring a reconfigurable mask. Previously we reported on the early feasibility of realising such a mask based on polysilicon micr-opto-electromechanical systems (MOEMS) technology and early results in the visible and near IR bands. This employs interference effects to modulate incident light - achieved by tuning a large array of asymmetric Fabry-Perot optical cavities via an applied voltage whilst a hysteretic row/column addressing scheme is used to control the state of individual elements. In this paper we present transmission results from the target mid-IR band (3-5μm), compare them with theory and describe the scale up from a 3x3 proof-of-concept MOEMS microshutter array to a 560 x 560 element array (2cm x 2cm chip) with the associated driver electronics and embedded control - including aspects of electronic design, addressing control and integration. The resultant microsystem represents a core building block to realise much larger reconfigurable masks using a tiled approach with further integration challenges in the future.

Reconfigurable mask for adaptive coded aperture imaging (ACAI) based on an addressable MOEMS microshutter array

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 scene - requiring 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.

Large area transmissive modulator for a remotely-interrogated MEMS-based optical tag

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.