Micromachine technology is a very promising technology: downsizing and high integration of functional components may bring about a breakthrough in industrial engineering and medical techniques in the 21st century. The Micromachine Center has carried out a range of activities for promotion of micromachines, including `Micromachine Technology Project' delegated by the government's Agency of Industrial Science and Technology. The ultimate purpose of this project is to establish technology applicable to the realization of micromachine systems. These systems are composed of microminiature functional elements, locomotive in very narrow spaces in complex equipment or inside of the human body, can perform sophisticated work autonomously, and help realize a micro factory for small industrial products. These themes include Micro-Opto-Electro-Mechanical systems as inspection modules, optical tactile sensor for medical applications, and so forth. In this paper, light driven micropump, 2D silicon micromachined optical scanner, optical tactile sensor, micro photovoltaic device, and optical processing by scanning probe microscope are described.
Micromirror arrays are being developed that can have up to tens of thousands of micromirror elements, each as small as 20 microns on a side, each spaced relative to neighbors so that optical efficiency exceeds 90 percent, and each individually controlled with response times as small as 10 microseconds for piston-like phase-mostly displacements that cover more than one- half optical wavelength. These arrays may be well suited for active aberration control of the focused coherent beams used in many applications, including optical disk storage, optical scanning, and laser radar systems. Active aberration control requires determination of the voltage supplied to the micromirror array elements so that constructive and destructive interference in light reflected from many elements yields the desired result. This paper discussed an approach in which the voltages are determined off-line by simulated annealing optimization and stored for real-time use.
Silicon can be subjected to plasmaless isotropic etching in mixtures of elemental bromine and fluorine. BrF3 is generated in the etching process. This ensures a high etching rate on smooth surfaces. The addition of noble gases, e.g. xenon, allows extremely smooth surfaces to be etched. Thermally oxidized SiO2 layers are applied as the etching mask. Among other applications, this technique can be used to manufacture microlenses. As a consequence of the complete isotropy of the etching process, spherical depressions of 100 to 500 micrometers in diameter are produced in the silicon when small circular holes of 5 to 50 micrometers are underetched in the SiO2 mask. After removal of the SiO2 mask the silicon sample can be used as a mold insert for plastic molding. The molded microlenses have been checked dimensionally and verified optically. The microlenses are planned for technical use in a miniaturized endoscope. This requires further processing of the silicon sample. As no hemispherical recesses but calotte shells are needed, the silicon surface must be machine prior to molding. This is done by microgrinding with variable-grain diamond tools on CNC high- precision machines. To generate adjusting devices, stoppers, and holding structures, the ground silicon sample and a mechanically microstructured perforated plate are combined in a modular multi-level mold insert. The microlenses molded by hot embossing or injection molding are separated mechanically. They can then be integrated in the endoscope with a holding unit manufactured independently.
Using the principle of focussing an incoming beam onto a plane with special optical microstructures, novel miniaturized switching elements can be built up for different applications. As switching is achieved by a lateral movement of the microstructures, only small displacements (about 10 micrometers ) are sufficient for efficient beam manipulation. In this paper, we present the results of both theoretical and experimental investigations on a concept of multichannel beam deflection by microprisms located in the focal plane of an incoming beam. This concept is suitable for singlemode fiber switches. It is shown that nearly aberration free operation can be achieved by choosing the right substrate thickness and the axial focal position with respect to the microprisms for the cases of placing them on the front or rear substrate surface. A trade-off must be made between the numerical aperture of the focussed beam, the prism angles and the number of output channels of the deflecting element in order to achieve sufficient angular separation of the deflected beams. Two different techniques have been tested for microprism fabrication: wet anisotropic etching in silicon and a new method of mask projection onto scanned photoresist layers. Microprisms with sufficient optical quality have been fabricated with both methods. In experimental investigations, we showed that for a singlemode fiber 1 X 9-switch low insertion loss (< 1 dB) and low cross-talk between the output channels (-50 dB) can be obtained.
The growing availability of commercial foundry processes allows easy implementation of micro-opto-electro-mechanical systems (MOEMS) for a variety of applications. Such applications go beyond single devices to include whole optical systems on a chip, comprising mirrors, gratings, Fresnel lenses, shutters, and actuators. Hinged and rotating structures, combined with powerful and compact thermal actuators, provide the means for positioning and operating these components. This paper presents examples of such systems built in a commercial polycrystalline silicon surface-micromachining process, the ARPA-sponsored multi-user MEMS process. Examples range from optical subcomponents to large mirror arrays and micro-interferometers. Also presented are linear arrays for combining the output of laser diode sources and for holographic data storage systems. Using the examples discussed in this paper, a designer can take advantage of commercially available surface-micromachining processes to design and develop MOEMS without the need for extensive in-house micromachining capabilities.
A 4 mm by 5 mm, magnetically actuated scanning MEMS mirror is fabricated by integration of bulk silicon micromachining and magnetic thin film head techniques. Large mirror deflection angles (0 - 70 degree(s)) are achieved. The MEMS mirror is demonstrated as a laser beam scanner in both conventional and compact holographic data storage system configurations.
Two types of polysilicon surface-micromachined actuators designed for moving hinged micromirrors are described. An electrostatic comb-drive actuator comprised of interdigitated capacitors has been used to move a mirror at frequencies of at least a kHz. Impact-actuated linear vibromotors allow mirrors to travel over large (> 100 micrometers ) ranges with submicron positioning.
Beam steering using a phased array of emitters is quite common in radar applications. However, the concept of phased array is equally valid at optical wavelengths. At these wavelengths, the beam steerers can consist of arrays of phase modulators which are space-fed; that is, a uniform-phase optical beam is incident on the array, and the modulators impart the required phase shift to effect steering. One potential method of implementing such an optical phased array is through the use of flexure-beam micromirrors. We present an analysis of examples of such arrays. We compare the steering efficiency of two different arrays, one of which is an array of square mirrors, and one of which is an array of a smaller number of rectangular mirrors. The effects of deviation of the mirrors from a pure piston phase shift is also discussed. Optical beam steering using micromirror arrays is compared to other beam steering techniques.
The relatively large detector size of conventional focal plane arrays often acts as a limiting source of noise currents and requires these devices to run at undesirably low temperatures. To reduce the detector size without reducing the detector's quantum efficiency, we have developed efficient on-focal plane collection optics consisting of arrays of thin film binary optic microlenses on the back surface of hybrid detector array structures. P/n polarity photodiodes of an unusual `planar-mesa' geometry were fabricated in epitaxial HgCdTe deposited by molecular beam epitaxy on the `front' side of a CdZnTe substrate. Diffractive (8 - 16 phase level) Ge microlenses were deposited on 48 micrometers centers in a registered fashion (using an IR mask aligner and appropriate marks on the front surface of the CdZnTe) on the back side of the substrate using a lifting process. The lifting process circumvents some of the process limitations of the more conventional chemical etching method to diffractive microlens processing, allowing them to approach more closely their theoretical efficiency limit of > 95%. Prior to microlens deposition, but after diode fabrication, the test structures were flip- chip bonded or `hybridized' using indium interconnections to metallic strip lines which had been photolithographically deposited on sapphire dice (a process equally compatible with a silicon integrated circuit readout). After hybridization, the CdZnTe was thinned to equal the focal length of the lenses in the CdZnTe material. Optical characterization has demonstrated that the microlenses combined with the detector mesas concentrate light sufficiently to increase the effective collection area. The optical size of the mesa detectors being larger than the theoretical diffraction limit of the microlenses precludes determining whether the lenses themselves produce the theoretical diffraction-limited gain, but they clearly decrease required detector area by at least 3 - 6X. To our knowledge, this is the first successful demonstration of IR detectors and binary optics microlens integration.
The Honeywell Technology Center, in collaboration with the University of Wisconsin and the Mobil Corporation, and under funding from this ARPA sponsored program, are developing a new type of `hybrid' micromachined silicon/fiber optic sensor that utilizes the best attributes of each technology. Fiber optics provide a noise free method to read out the sensor without electrical power required at the measurement point. Micromachined silicon sensor techniques provide a method to design many different types of sensors such as temperature, pressure, acceleration, or magnetic field strength and report the sensor data using FDM methods. Our polysilicon resonant microbeam structures have a built in Fabry-Perot interferometer that offers significant advantages over other configurations described in the literature. Because the interferometer is an integral part of the structure, the placement of the fiber becomes non- critical, and packaging issues become considerably simpler. The interferometer spacing are determined by the thin-film fabrication processes and therefore can be extremely well controlled. The main advantage, however, is the integral vacuum cavity that ensures high Q values. Testing results have demonstrated relaxed alignment tolerances in packaging these devices, with an excellent Signal to Noise Ratio. Networks of 16 or more sensors are currently being developed. STORM (Strain Transduction by Optomechanical Resonant Microbeams) sensors can also provide functionality and self calibration information which can be used to improve the overall system reliability. Details of the sensor and network design, as well as test results, are presented.
The microjet printing method of micro-optical element fabrication is being used to make arrays of high-performance hemi-elliptical and hemi-cylindrical microlenses for potential use in applications such as collimation of edge-emitting diode laser array beams. The printing method enables both the fabrication of very fast (e.g., f/0.75) microlenses and the potential for reducing costs and increasing flexibility in micro-optics manufacture. The process for fabricating anamorphic microlenses, including those of square or rectangular shape, involves the dispensing and placing of precisely sized microdroplets of optical material onto optical substrates, and then controlling their coalescence and solidification. By varying the number, diameter and spacing of adjacent microdroplets of optical materials deposited at elevated temperatures onto heated substrate, both the dimensional aspect ratios and the ratio of `fast'- to-`slow' focal lengths of a printed hemi-elliptical microlens may be varied over a very wide range. Arrays of hemi-elliptical and hemi-cylindrical microlenses on the order of 100 - 300 micrometers in width and 150 micrometers to 20 mm long, with focal length ratios (fast/slow) from 1 (circular) to 0 (cylindrical), have been printed. A model for predicting printed hemi-elliptical microlens focal lengths from printed lenslet geometry is illustrated, along with an interferometric method of detecting lenslet defects and aberrations.
The micro-sensor field is presently proliferating with designs and approaches. We have recently been involved with several mini/micro optical systems which have pointed out several trends in design and fabrication that are somewhat more important to these smaller optical systems. These include; material choices, alignment strategies, fabrication methods, and freedom and complexity of the optical designs. Our recent experience indicates that since mini/micro optical systems are likely to be produced in much higher number, that traditional fabrication methods could prove exorbitantly expensive. Deterministic fabrication methods employing inherently self-aligning features are well worth investigating. This is particularly true for monolithic systems that fall into that `grey' area between mini and truly micro optical systems.
Two modeling approaches for analyzing micro-size ball lenses will be described. Due to the low divergence angle of the light coming out of a single mode fiber (SMF), a Gaussian Optics analysis, integrated with ray tracing, is needed to design the optical subsystems such as fiber collimators. On the other hand, due to the large divergence angle of the light coming out of the laser diode (LD), an exact solution of Maxwell's equation which can be obtained by spherical harmonic expansion, is needed in order to predict the coupling efficiency from a LD to a SMF accurately. These models were applied to the cases of forward coupling and back reflections with various arrangements of the optical elements. Excellent agreement was found between the predictions of these two models and the experimental results. These models are very important for assemblies using micro-machined micro-optical parts since they have little or no allowance for alignment adjustments.
Using the silver-sodium ion exchange process in a special optical glass, different types of gradient-index cylindrical microlenses and microlens arrays have been fabricated. High numerical aperture single cylindrical lenses are produced in thin glass slabs without any diffusion masks. Depending on the desired index profile accuracy, a two- or three-step ion exchange process is used, the maximum numerical aperture which can be achieved is about 0.6. Contrary to other lens fabrication techniques, lenses of different focal length can be produced very easily, and both focussing and diverging lenses are possible. A series of systems, combining different lenses, have been realized for anamorphotic single-mode laser diode beam transformation and high-brightness laser diode beam forming. Arrays of cylindrical lenses of moderate numerical aperture (about 0.2) have been realized by electrical field assisted ion exchange through 1D mask structures. These lens arrays have been successfully applied to multiple-stripe pulsed laser diodes for efficient reduction of the output beam divergency, thus enabling a more efficient coupling of the output power to multimode fibers.
We introduce the moving-mask method for fabrication of continuous relief grating with parabolic section profile in this paper. The depth of the grating is nearly equal to 4 micrometers , the diffraction orders from -10 to 10 has uniform intensity, such a grating can be used as multi-splitter. The grating can be concave or convex. The method presented in this paper is not only used to fabricate parabolic grating, but also has been used to generate other continuous relief patterns with line-symmetry and rotation-symmetry. Interesting experimental results are given. It is proved that this method is useful for the making of microoptics elements with continuous relief.
Fabrication of refractive microlens arrays on several infrared (IR) transmissive materials was studied. The fabrication process consists of forming photoresist microlenses by thermal reflow of photoresist islands, and transferring this pattern into the IR substrate by ion milling. Microlens arrays having a wide range of F-numbers (F/0.3 - F/12) and sizes were fabricated using a modified ion milling process, where background oxygen and ion energy were used to control the photoresist and substrate erosion rates, respectively. This approach enabled a large range of milling selectivity (e.g. 0.6 - 12 for CdTe) and hence accurate control of lens sag heights. This is important since photoresist microlenses can be successfully fabricated only within a limited range of F-numbers. The etch selectivity also enabled fabrication of nonspherical shapes, starting from spherical photoresist preforms, by judicious control of sputter selectivity during the milling process. Microlens arrays were fabricated in several IR materials, including CdTe, ZnS, Ge, Si, GaAs, InP, GaP and Al2O3. Among these materials GaP and ZnS are also attractive visible and near IR wavelength microlens materials, where their high refractive index results in much lower sag heights than quartz lenses of comparable F-number.
It is well known that aspherical surfaces (AS) can solve many problems in optics which are practically unsolvable now. These problems relate to producing high quality microlenses for endoscopy, microscopy, optical recording etc. But probably the most important direction for AS application today is transformation of astigmatic radiation of diode lasers, lines and matrixes. Well—known pecularities of semiconductor laser sources (SLS) are: 1) different numerical apertures at fast and slow axis and correspondingly elliptical cross-section of laserbeam with semi—axis ratio from 2-4 till 10 and 2) axial astigmatism connected with different positions of emitting planes (up to 20-80 JLm) for fast and slow axes [11. Due to these pecularities of SLS most optical devices with SLS require beam collimation and (or) symmetrization components. There were many new optical solutions suggested in the last years for this purpose. Refractive optical components like optical cubes , anamorphic optical elements , GRIN rod—lenses , diffractive optical elements  etc one can find among them. From many points of view, aspherization of refractive optical components or the use of combined aspherical and GRIN lenses looks preferable to improve the quality of SLS radiation [6, 7]. The problem of aspherical optics creation can be divided into three tasks : 1) calculation of AS, 2) fabrication and 3) measuring and testing. The first of them can be solved now practically for any optical device due to the progress in computer hardware and software. The third task is not new and is very difficult and very specific for every AS. But most complicated till this time is the task of fabrication of AS — it requires a universal, exactly controlled method of removal of material from the surface of an optical component. In this paper, based on many years experience of laser treatment [2, 8, 9], a new conception of aspherical optical surfaces fabrication is suggested. The main idea of this work is combination of two laser—assisted processes in time and space, such as laser shaping by removal of material and smoothing by laser thermal polishing. The general approach to the problem of AS fabrication is described by the example of the aspherization of cylindrical surfaces, most important for SLS. And this only addresses the task of beam collimation in the fast axis. In this case, the SLS beam after optical component should be transformed into a diffraction limited beam at the fast axis and it remains at its initial divergence at the slow axis.
We report the first fabrication of a spherical microlens monolithically integrated on a surface- micromachined supporting plate standing perpendicular to the substrate. The focusing and collimating abilities of the lens are successfully demonstrated.