We investigated microposition sensing of micro-electro- mechanical systems (MEMS) that is based on optical readout techniques. We determined the parameters that affect or limit the performance of optical readout techniques especially as they apply to detection of infrared radiation. Such microposition sensing schemes are very important as readout mechanisms for large arrays of microstructures which are required for imaging. In addition, we explored the performance of uncooled micromechanical IR sensors using Fresnel zone plates (FZP). This type of diffractive feature diffracts along the optical axis and not perpendicular to that axis. We found that temperature fluctuation noise and background fluctuation noise, are currently the limits to the performance of uncooled micromechanical IR detectors. The noise at the output of the optical readout includes amplified noise from the micromechanical structures and noise added by the optical readout itself. However, the added noise is negligible compared to the amplified temperature fluctuation noise inherent in the microstructures. In this context an optical readout is nearly an ideal, noiseless readout method.
The Oak Ridge National Laboratory has been instrumental in developing ultraprecision technologies for the fabrication of optical devices. We are currently extending our ultraprecision capabilities to the design, fabrication, and testing of micro-optics and MEMS devices. Techniques have been developed in our lab for fabricating micro-devices using single point diamond turning and ion milling. The devices we fabricated can be used in micro-scale interferometry, micro-positioners, micro-mirrors, and chemical sensors. In this paper, we focus on the optimization of microstructure performance using finite element analysis and the experimental validation of those results. We also discuss the fabrication of such structures and the optical testing of the devices. The performance is simulated using finite element analysis to optimize geometric and material parameters. The parameters we studied include bimaterial coating thickness effects; device length, width, and thickness effects, as well as changes in the geometry itself. This optimization results in increased sensitivity of these structures to absorbed incoming energy, which is important for photon detection or micro-mirror actuation. We have investigated and tested multiple geometries. the devices were fabricated using focused ion beam milling, and their response was measured using a chopped photon source and laser triangulation techniques. Our results are presented and discussed.
The technology of microelectronics that has evolved over the past half century is one of great power and sophistication and can now be extended to many applications (MEMS and MOEMS) other than electronics. An interesting application of MEMS quantum devices is the detection of electromagnetic radiation. The operation principle of MEMS quantum devices is based on the photoinduced stress in semiconductors, and the photon detection results from the measurement of the photoinduced bending. These devices can be described as micromechanical photon detectors. In this work, we have developed a technique for simulating electronic stresses using finite element analysis. We have used our technique to model the response of micromechanical photon devices to external stimuli and compared these results with experimental data. Material properties, geometry, and bimaterial design play an important role in the performance of micromechanical photon detectors. We have modeled these effects using finite element analysis and included the effects of bimaterial thickness coating, effective length of the device, width, and thickness.
Precision grinding of optical components is becoming an accepted practice for rapidly and deterministically fabrication optical surface to final or near-final surface finish and figure. In this paper, a comparison of grinding techniques and materials is performed. Flat and spherical surfaces were ground in three different substrate materials: BK7 glass, chemical vapor deposited silicon carbide ceramic, and sapphire. Spherical surfaces were used to determine the contouring capacity of the process, and flat surfaces were used for surfaces finish measurements. The recently developed Precitech Optimum 2800 diamond turning and grinding platform was used to grind surfaces in 40mm diameter substrates sapphire and silicon carbide substrates and 200 mm BK7 glass substrates using diamond grinding wheels. The results of this study compare the surface finish and figure for the three materials.
Uncooled infrared sensors are significant in a number of scientific and technological applications. A new approach to uncooled infrared detectors has been developed using piezoresistive microcantilevers coated with thermal energy absorbing material(s). Infrared radiation absorbed by the microcantilever detector can be sensitively detected as changes in the electrical resistance as a function of microcantilever bending. These devices have demonstrated sensitivities comparable to existing uncooled thermal detector technologies. The dynamic range of these devices is extremely large due to measurable resistance change obtained with only nanometer level cantilever displacement. Optimization of geometrical properties for selected commercially available cantilevers is presented. Additionally, we present results obtained from a modeling analysis of the thermal properties of several different microcantilever detector architectures.
The micro-sensor field is presently proliferating with designs and approaches. We have developed a micro-spectrometer for sensing applications containing five precision surfaces, including two off-axis aspheres. The entire monolith is less than six cubic centimeters in volume. This particular design contains a bandwidth of about 2 micrometers which is centered at 980 nm. Once an appropriate starting substrate was produced, the entire system was diamond turned to maintain the required surface figure, inter-surface spacing, and surface tilts. Only three diamond turned fixtures were needed to produce the monolith. The results proved to be more than adequate for many sensing applications. Slightly altered designs could easily be produced containing different bandwidths and resolutions as needed by the customer. Due to the spectrum of interest and the fabrication method, PMMA was the material chosen for this sensor. Other designs configurations incorporating BK7 and sapphire are presently being studied.