Charles M. Egert

Advanced matrix-based algorithm for ion-beam milling of optical components

Control of an ion beam for milling optical surfaces is a nontrivial problem in two-dimensional deconvolution. The ion milling operation is performed by moving an ion beam gun through a grid of points over the surface of an optical workpiece. The control problem is to determine the amount of time to dwell at each point in the grid to obtain a desired surface profile. This research treats the problem in linear algebra terms. The required dwell times are the solutions to a large, sparse system of linear equations. Traditional factorization methods such as Gaussian elimination cannot be used because the linear equations are severely ill conditioned. Theoretically, a least-squares solution to this problem exists. Practical approaches to finding a minimal least-squares solution are discussed.

Roughness evolution of optical materials induced by ion-beam milling

Ion beam milling is an emerging advanced optical fabrication technology capable of deterministic figuring of optical surfaces. Much of the work in ion milling to data has emphasized figuring of glass-like materials, such as fused silica, which do not significantly roughen during ion milling. However, for ion milling to reach its full potential as an advanced optical fabrication technique it must be applicable to a broad range of materials to interest in optical fabrication including polycrystalline metals, semiconductors, and ceramics. In order to assess the feasibility of ion milling, the effect of ion dose on roughness evolution was investigated for a variety of materials including: silicon, germanium, sapphire, silicon carbide, fused silica, aluminum, and copper. Single crystal silicon, germanium and sapphire as well as polycrystalline CVD silicon carbide did not significantly roughen during ion milling. The roughness evolution of aluminum, copper and gold thin films were also studied; fine grained gold films were found to remain smooth during ion milling.

Novel photon detection based on electronically-induced stress in silicon

The feasibility of microcantilever-based optical detection is demonstrated. Specifically, we report here on an evaluation of laboratory prototypes that are based on commercially available microcantilevers. In this work, optical transduction techniques were used to measure microcantilever response to photons and study the electronic stress in silicon microcantilevers, and their temporal and photometric response. The photo-generation of free charge carriers (electrons, holes) in a semiconductor gives rise to photo-induced (electronic) mechanical strain. The excess charge carriers responsible for the photo-induced stress, were produced via photon irradiation from a diode laser with wavelength (lambda) equals 780 nm. We found that for silicon, the photo-induced stress results in a contraction of the crystal lattice due to the presence of excess electron-hole-pairs. In addition, the photo-induced stress is of opposite direction and about four times larger than the stress resulting from direct thermal excitation. When charge carriers are generated in a short time, a very rapid deflection of the microcantilever is observed (response time approximately microseconds).

Detection of infrared photons using the electronic stress in metal-semiconductor interfaces

It is well known that the work function of metals decrease when they are placed in a nonpolar liquid. A similar decrease occurs when the metal is placed into contact with a semiconductor forming a Schottky barrier. We report on a new method for detecting photon is using the stress caused by photon-electronics emitted forma metal film surface in contact with a semiconductor microstructure. The photoelectrons diffuse into the microstructure and produced an electronic stress. The photon detection results from the measurement of the photo-induced bending of the microstructure. Internal photo-emission has been sued in the past to detect photons, however, in those cases the detection was accomplished by measuring the current due to photoelectrons and not due to electronic stress. Small changes in position of microstructures are routinely measured in atomic force microscopy where atomic imaging of surface relies on the measurement of small changes in the bending of microcantilevers. In the present work we studied the photon response of Si microcantilevers with a thin film of Pt. The Si microcantilevers. In the present work we studied the photon response of Si microcantilevers with a thin film of Pt. The Si microcantilevers were 500 nm thick and had a 30 nm layer of Pt. Photons with high enough energies produce electrons from the platinum-silicon interface which diffuse into the Si and produce an electronic stress. Since the excess charge carriers cause the Si microcantilever to contact in length but not the Pt layer, the bimaterial microcantilever bends. In our present studies we used the optical detection technique to measure the photometric response of Pt-Si microcantilevers as a function of photon energy. The charge carriers responsible for the photo-induced stress in Si, were produced via internal photo-emission using a diode laser with wavelength (lambda) equals 1550 nm.

Ion beam milling of silicon carbide optical components

Silicon carbide (SiC) is emerging as an important ceramic material for optical applications requiring stiff, lightweight structures with good thermal conductivity. This report discusses the application of ion milling in the fabrication of SiC optical components. Ion beam milling combined with either ductile grinding or polishing provides an excellent approach to deterministic fabrication of SiC optical components. Results of recent roughness evolution studies for SiC samples prepared by several pre-ion milling fabrication processes suggest that ductile grinding and some polishing processes can be used to produce low-subsurface-damage components suitable for ion milling. Typical improvements with optical figures after ion milling have convergences on the order of 2 or 3. Overall, these experiments indicate that ion milling combined with other fabrication processes represents a viable, highly deterministic approach to producing high-precision SiC optical components.

Diffuse Absorbing Beryllium Coatings Produced by Magnetron Sputtering

Beryllium coatings with varying thicknesses and columnar grain sizes were deposited by low temperature magnetron sputtering and wet chemically etched to enhance diffuse absorption of light. After etching these coatings exhibited a matte black surface finish and low specular reflectance (below 2%) in the IR up to a critical wavelength dependent upon the original grain size of the coating. Extremely thick coatings (350 j.tm) with original grain sizes of 10 to 12 j.m were produced which exhibited specular reflectances below 0.5% up to 50 p.m wavelength and a Lambertian BRDF at 10.6 p.m averaging 4.3x103 ster1. Scanning electron micrographs are presented for etched and unetched beryllium coatings which showed the etching process produces roughness and porosity over several size scales simultaneously with the maximum size scale limited by the initial coating grain size and thickness. This technique for producing diffuse absorbing baffle materials has great versatility in choice of coating material and substrate and can be expected to provide optical system designers with a variety of material options for stray light management.

Infrared microcalorimetric spectroscopy using uncooled thermal detectors

We have investigated a novel IR microcalorimetric spectroscopy technique that can be used to detect the presence of trace amounts of target molecules. The chemical detection is accomplished by obtaining the IR photothermal spectra of molecules absorbed on the surface of an uncooled thermal detector. Traditional gravimetric based chemical detectors require highly selective coatings to achieve chemical specificity. In contrast, IR microcalorimetric based detection requires only moderately specific coatings since the specificity is a consequence of the photothermal spectrum. We have obtained IR photothermal spectra for trace concentrations of chemical analytes including diisopropyl methylphosphonate (DIMP), 2-mercaptoethanol and trinitrotoluene (TNT) over the wavelength region 2.5 to 14.5 micrometers . We found that in the wavelength region 2.5 to 14.5 micrometers DIMP exhibits two strong photothermal peaks. The photothermal spectra of 2-mercaptoethanol and TNT exhibit a number of peaks in the wavelength region 2.5 to 14.5 micrometers and the photothermal peaks for 2-mercaptoethanol are in excellent agreement with IR absorption peaks present in its IR spectrum. The photothermal response of chemical detectors based on microcalorimetric spectroscopy has been found to vary reproducibly and sensitively as a consequence of adsorption of small number of molecules on a detector surface followed by photon irradiation and can be used for improved chemical characterization.

Comparison of materials for use in the precision grinding of optical components

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.

Piezoresistive microcantilever optimization for uncooled infrared detection technology

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.

Dimensional variation and roughness of LIGA fabricated microstructures

We have measured the dimensional variation and sidewall roughness of features on PMMA microcomponents fabricated by deep x-ray lithography in order to assess the effect of dimensional variation on subsequent assembly operations. Dimensional measurements were made using a stylus profilometer with a repeatability in step height of better than 0.01 micrometers . Roughness measurements were made with the same profilometer scanning in a direction perpendicular to the length of the parts. 22 micrometers and 54 micrometers features exhibited dimensional variations described by a Gaussian distribution with standard deviations of 0.202 micrometers and 0.381 micrometers , respectively. This corresponds to a maximum relative variation of between 0.6% and 0.9%. Sidewall roughnesses were found to be in the range of 0.02 micrometers to 0.03 micrometers , an insignificant contribution to the total variation when compared to overall dimensional variation. Several potential sources of this variation are discussed, but no single cause was identified as the source of the significant dimensional variation observed here.