K.J. Winchester

Monolithic integration of an infrared photon detector with a MEMS-based tunable filter

The monolithic integration of a low-temperature microelectromechanical system (MEMS) and HgCdTe infrared detector technology has been implemented and characterized. The MEMS-based tunable optical filter, integrated with an infrared detector, selects narrow wavelength bands in the range from 1.6 to 2.5 /spl mu/m within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The entire fabrication process is compatible with two-dimensional infrared focal plane array technology. The fabricated device consists of an HgCdTe SWIR photoconductor, two distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity, which is then removed to leave an air gap, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors shows a wide tuning range, and high percentage transmission is achieved with a tuning voltage of only 7.5 V. The full-width at half-maximum ranged from 95 to 105 nm over a tuning range of 2.2-1.85 /spl mu/m.

Determination of mechanical properties of PECVD silicon nitride thin films for tunable MEMS Fabry–Pérot optical filters

This paper reports an investigation on techniques for determining elastic modulus and intrinsic stress gradient in plasma-enhanced chemical vapor deposition (PECVD) silicon nitride thin films. The elastic property of the silicon nitride thin films was determined using the nanoindentation method on silicon nitride/silicon bilayer systems. A simple empirical formula was developed to deconvolute the film elastic modulus. The intrinsic stress gradient in the films was determined by using micrometric cantilever beams, cross-membrane structures and mechanical simulation. The deflections of the silicon nitride thin film cantilever beams and cross-membranes caused by in-thickness stress gradients were measured using optical interference microscopy. Finite-element beam models were built to compute the deflection induced by the stress gradient. Matching the deflection computed under a given gradient with that measured experimentally on fabricated samples allows the stress gradient of the PECVD silicon nitride thin films introduced from the fabrication process to be evaluated.

Effects of deposition temperature on the mechanical and physical properties of silicon nitride thin films

This study investigates the mechanical and physical properties of low-temperature plasma-enhanced chemical-vapor-deposited silicon nitride thin films, with particular respect to the effect of deposition temperature. The mechanical properties of the films were evaluated by both nanoindentation and microcantilever beam-bending techniques. The cantilever beam specimens were fabricated from silicon nitride thin films deposited on (100) silicon wafer by bulk micromachining. The density of the films was determined from quartz crystal microbalance measurements, as well as from the resonant modes of the cantilever beams, which were mechanically excited using an atomic force microscope. It was found that both the Young’s modulus and density of the films were significantly reduced with decreasing deposition temperature. The decrease in Young’s modulus is attributed to the decreasing material density. The decrease in density with decreasing deposition temperature is believed to be due to the slower diffusion rates o...

A crystallographic alignment method in silicon for deep, long microchannel fabrication

The aim of this work was to develop an alignment technique to be used in the production of long, deep, large area microchannel devices. The microchannel design specifications used for the investigation were 800 µm deep channels of 100 µm width, with a 200 µm pitch, over an area of 40 mm × 40 mm. The device was fabricated with (1 1 0) orientated silicon, to take advantage of the large wet etch ratio between the {110} and {111} planes. Silicon nitride was used as the channel etchant mask, and was patterned by reactive ion etching. The channels were wet etched in a KOH 40 wt% solution at 80 °C to minimize undercut of the silicon nitride mask, while maintaining a reasonable etch rate of 2 µm min−1. The {111} crystal plane normal to the {110} wafer surface needed to be determined with high accuracy for the fabrication of microchannels of such a large size. Investigations of several established alignment techniques revealed only one suitable technique: the use of a wet etched alignment feature that is self-aligned to the {111} crystal planes. This resulted in an silicon nitride mask undercut of 10 µm for channels 800 µm deep and 45 mm in length.

Tunable Fabry-Perot filters operating in the 3 to 5 μm range for infrared micro-spectrometer applications

In this article the design, fabrication and characterization of micro-Fabry-Perot filters operating in the mid-wavelength infrared range is presented. Using surface micromachining techniques, low temperature silicon nitride based structures with distributed Bragg mirrors made of Ge/SiO/Ge layers have been fabricated and tested, both mechanically and optically. The membrane/mirror deflection has been measured using an optical profilometer and is estimated to be of the order of 800nm with voltage bias up to 17V while still preserving good mirror parallelism. The respective optical transmission peak shifted from 4.5μm to 3.6μm. Without antireflection coating at the back of the silicon substrate ~50% maximum transmission has been measured at the resonance peaks. The FWHM was measured to be 210+/-20nm, which is ~20% larger than estimated theoretically. In agreement with theoretical modeling, after crossing 1/3 of the cavity length, the membrane/mirror structure has been found to enter into an unstable region followed by snap-down to the bottom mirror surface. In order to prevent this detrimental effect, membranes with anti-stiction bumps have been fabricated demonstrating repeatable structure recovery from the stage of full collapse.

Towards MEMS-based infrared tunable microspectrometers

Silicon nitride membrane based MEMS technologies are used in numerous optical micro-systems for many practical applications. In recent years significant effort has been invested into miniaturization of present spectrometers leading to development of compact systems based on linear detector arrays. However, further miniaturization requires the application of MEMS technology. MEMS based micro-Fabry-Perot cavity structures with a flexible mirrors operating in the visible and near infrared range have already been demonstrated. Extension of the technology to mid- (MWIR-3μm to 5μm) and long-wave infrared (LWIR - 8μm to 12μm) seems to be a natural development. However, this requires much larger deflection of the movable mirror of the Fabry-Perot cavity: ~1μm for 3μm to 5μm and ~2μm for 8μm to 12μm wavelengths range. Consequently, precise control of the intrinsic stress in the silicon nitride support film is needed. Our experiments show that suitable silicon nitride properties can be obtained by carefully controlling the process parameters during plasma enhanced CVD growth. In addition to the material requirements, the mechanical structure of the flexible mirror is also important. In order to optimize the mirror structure/shape, finite element analysis was undertaken. The results show that for certain silicon nitride support shapes and thicknesses, mirror deflections needed for both the MWIR and LWIR wavelength regions are possible.

Variable MEMS-based inductors fabricated from PECVD silicon nitride

This paper presents the implementation of inductors for on-chip integration with RF electronics. The inductors were fabricated using silicon bulk micromachining to form a micro-electro-mechanical system (MEMS) that is free from eddy-current losses found in spiral inductors formed using conventional silicon processing. By using the fabrication approach developed here, the resulting inductors are electrically tunable with the application of low DC bias voltages.

Nano-indentation characterisation of PECVD silicon nitride films

While finite element modelling (FEM) can be employed to optimise the displacement of the membrane structures to an applied electrostatic force, the accuracy of the results depend critically on the material properties of the membrane. Values for these properties can be obtained by evaluating the response of test structures such as cantilevers and beams to various loading conditions. We present in this paper an alternative approach, where measurement of Youngs modulus of thin PECVD silicon nitride films is achieved through nano-indentation of the film on a silicon substrate.

A monolithically integrated HgCdTe SWIR photodetector and tunable MEMS-based optical filter

A monolithically integrated low temperature MEMS and HgCdTe infrared detector technology has been implemented and characterised. The MEMS-based optical filter, integrated with an infrared detector, selects narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The entire fabrication process is compatible with two-dimensional infrared focal plane array technology. The fabricated device consists of an HgCdTe SWIR photoconductor, two distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity, which is then removed to leave an air-gap, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors shows a wide tuning range and high percentage transmission is achieved with a tuning voltage of only 7.5 V. The FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to determine the best geometry in terms of fill factor, voltage displacement prediction and membrane bowing.

Finite element analysis of tunable Fabry Perot MEMS structures

Finite element modelling of the mechanical response of MEMS structures is essential for device design and optimisation prior to any fabrication steps. We present finite element modelling (FEM) and optimisation of a MEMS Fabry-Perot (FP) optical filter using a commercial FEM package. A variety of membrane geometries have been characterised in order to maximise the tunable range whilst minimising any curvature of the top reflective surface. The analysis, while directed toward a FP tunable filter, is applicable to any MEMS system that relies on the deflection of a membrane or cantilever. MEMS devices such as accelerometers, pressure sensors and mechanical actuators can all be characterised, using this approach.