The field of microoptics has grown into an important technology in the past decade as evidenced by the growing range of applications using microoptic compontents. The development of large-volume, batch-processed diffractive and refractive microooptics component manufacturing has created a technology that promises to revolutionize many electro-optical systems. Microoptics is an enabling technology for applications that cannot be addressed using conventional optics and is currently playing a significant role in numerous applications, including lightwave communications, optical interconnects, detector arrays, visible and infrared imagers, beam steering, and display systems. The trend toward miniaturization and integration of conventional optical systems will accelerate the adoption of microoptics technology in commercial, space and military systems. Since microoptic processes are compativle with integrated circuits, a broader range of applications is expected as microoptics become integrated with MEMS devices. Other more complex microsystems that use microoptic devices such as microspectrometers, microinterferometers and miniature on-machine-inspection subsystems are being investigated. This new technology enables high performance optical components that are lighter, easier to produce, more efficient, and less expensive than conventional components. We present a broad survey of developments in microoptics, including design, fabrication, and applications, in the last decade.
Recording holographic optical elements usually requires a good illumination uniformity as well as a spherical or plane phase. To fulfill the uniformity demand, an expansion of the Gaussian beam is necessary. This leads to a loss of intensity connected with an essential extension of the recording time. Alternatively, the recording efficiency can be increased by using a beam transformation the beam from a Gaussian into a top hat intensity distribution. We designed, realized and tested a setup for a more efficient hologram recording which can be used for full color application. The heart of the setup is a refractive beam shaping element fabricated by a gray tone lithography and proportional transfer into quartz glass. The beam shaping element shows a conversion efficiency of $GTR99,5% (like a refractive lens) in the whole visible spectral range and an intensity uniformity <5%RMS.
We present a comparison of three different technologies for the fabrication of micro-optical elements with arbitrary surfaces. We used direct laser writing in photoresist, binary mask lithography in combination with reactive ion etching in fused silica, and High-Energy- Beam-Sensitive (HEBS) glass graytone lithography in photoresist. We analyzed the efficiencies and the deflection angles of different elements in order to quantify the performance of the different technologies. We found that higher effencies can be achieved with refractive type elements, while precise deflection angles can be obtained more easily with diffractive elements.
Tetraethoxyorthosilicate and methacryloxypropyl trimethoxy silane are used to form inorganic and organic networks, respectively. Photosensitive agent is added to initiate free-radical cross-linking polymerization of unsaturated carbon bonds and thus makes the material act as a negative tone photoresist. The sensitivity of the composite material is investigated with the help of real-time diffraction for different exposure time and concentration of photoinitiator. Vibrational spectroscopy is used to provide insight into the structure changes that occur when films are exposed with UV light. Developed in dilute base solution, microoptical element, such as lenses and gratings, were fabricated by contract copy with UV-exposure. Shrinkage effect is investigated after optical elements obtained.
Although, by using gray-tone lithography and common technologies in standard IC fabrication it's easy to obtain an arbitrarily 3-D shaping of positive thick resists, there are some limitations too. E-beam writing implies a maximum of only 200 gray-levels on the project retilce, and the limited focus depth of the projection objective gives a poor lateral resolution. That's why we applied a new approach to enhance the 3-D resolution of gray-tone lithography. By combining gray-tone lithography with binary masking technique, it was possible to obtain a high resolution (vertical and horizontal directions) into thick resist. Considering it as a primary mold, a metallic variable absorber mask for deep X-ray lithography may be processed. Previously, it's necessary to transform the resist surface into a conductive layer as follows: conditioning, nucleation and electroless plating, respectively. After that, a metallic deposit is obtained by electroplating at a desired thickness, resulting in a complementary shape of the mold resist. The original design and fabrication method for the gray-tone test reticle were supported by preliminary experiments showing the main advantage of this new technology: the 3-D structuring of thick resists in a single exposure step and also a very promising aspect ratio obtained of over 9:1.
A process fabricate 100 m high aspect ratio micro-optical structures by direct X-ray exposure and development of polymethylsilsesquoixane spin- on glass (GR 650) is presented. This process is an advance over the previous process of fabrication micro-optical components by molding GR 650 using polymethylmethacrylate (PMMA) molds patterned by deep X-ray lithography (DXRL). The process presented in this article utilizes GR 650 as a DXRL resist. The polymethylsilsesquoixane is converted to silica on the surface exposed to air, and cross linked throughout the bulk. X-ray irradiated regions are then selectively retained by development in an organic solvent. A technique to cast 100 m thick GR 650 films was established. Although the height of the structures fabricated was 100 m, this technique can be extended to larger structural heights. An alternative positive tone process was also developed in which the irradiated regions of GR 650 films are etched in buffered HF. The structural height achieved by positive tone processing, however, was limited to 15 (mu) m, which is the depth of conversion to silica. Surface and bulk compositions of the irradiated films were measured by XPS and Fourier Transform infrared spectroscopy.
Optical components, such as miniature spectrometer gratings working in the infrared range for environmental monitoring or physical analytics, contribute appeciably to the price of Micro Electro Opto Mechanical Systems (MOEMS). These optical components could be a part of a miniature functional package produced with an alternative fabrication technology based on cold forming metals. The cost-efficient fabrication of these components, for example by implementation of forming technology, appears promising. With this technology, high quality embossing of optical structures for high precision requirements in a batch process is possible. In this way the system costs can be reduced. In this paper aluminum forming by cold embossed grating for the fabrication of gratings was investigated. Experiments with different geometries of the embossed grating were carried out. The quality of the embossed structures is primarily determined by the precision and surface quality of the die. Therefore we used a single crystalline silicon tool made by etching as a die. Quality criteria for the review of the formed optical grating were the geometry of surfaces and the surface roughness as well as optical properties of the total structure.
Calcium Fluoride microlens arrays have been produced with the help of an ion milling transfer of photoresist lenses which have been fabricated by two different techniques (i) by a melting resist technique and (ii) gray scale lithography. Major technical problems connected with the ion milling transfer of the photoresist lenses in the CaF2 material are surface roughness enhancement and variation of the lens shape. We studied the ion-induced decomposition of CaF2 and the surface roughness equatio in dependence on different milling conditions. For the microlens patter transfer the ratio of etching rates of the photoresist and the CaF2 have been adjusted by gas admixture of nitrogen to the Ar sputtering gas. The angle dependence of the etching rates leads also to a changing of the lens profile. The effect of ion beam induced surface roughness and the accuracy of the transfer process is discussed.
One of the most important factors limiting the optical efficiency of LEDs is total internal reflection of generated light, where photons incident to the surface at angles greater than the critical angle are reflected back into the semiconductor and absorbed. Most semiconductors have a large refractive index and hence a small critical angle. Narrow gap semiconductors, such as InSb, have particularly large refractive indexes and corresponding smaller critical angles. Additionally, strong absorption of light in the 3-5(mu) m range means that epoxy immersion lenses, which are used for GaAs Ir LEDs, cannot be used in InSb based IR LEDs. We have therefore used a novel micromachining technique to fabricate optical concentrators inInSb and HgCdTe layers. Inductively coupled plasma (ICP) etching is used to alternatively eatch the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with parabolic profiles. Continuing optimization of the process to reach the theoretical limits of optical gain is described together with some of the main issues associated with the fabrication process.
A new method has been developed to fabricate refractive microlens by etching ammonium dichromate gelatin (ADG) with enzyme solution. Unlike previous methods which are used to fabricate refractive microlens with photoresist, the process of fabricating microlens by etching ADG with enzyme solution doesn't require the use of expensive equipment, and it isn't sophisticated and time consuming. The light exposes ADG through a high contrast binary mask, then the exposed parts of ADS generate cross- linking reaction. Usually, the relief achieved by water developing is very shallow (<1um) when nonpre-harden gelatin is used, so we compound a certain concentration enzyme solution, and because of surface tension, ADG turns to spherical structure after developing. The optimum technique parameters of this process are presented. Results are presented for experiments and evaluated by profile meter and interference microscope.
We report on the fabrication of high quality microlens arrays on 4', 6' and 8'-fused silica wafers. Refractive, plano-convex microlenses are fabricated by using photolithography; a reflow or melting resist technique and reactive ion etching. A diffraction-limited optical performance (p-v wave aberrations of < (lambda) /8, Strehl ratio $GTR 0.97) is achieved. Aspherical lens profiles are obtained by varying the etch parameters during the reactive ion etching transfer. The microlens arrays are used for Microlens Projection Lithography (MPL) and within UV-light illumination systems. Microlens Projection Lithography is an innovative technique using KARL SUSS Mask Aligners equipped with an ultra-flat microlens-based projection system. The projection system consists of 500.000 identical micro-objectives side- by-side. Each micro-objective consists of 3 to 4 microlenses. A fully symmetrical optical design eliminates coma, distortion and lateral color. The lens system is frontal- and backside telecentric to provide a unit magnification (+1) over the whole depth of focus. Each micro- objective images a small part of the photomask pattern onto the wafer. The partial images from different channels overlap consistently and form a complete aerial image of the photomask. Microlens Projection Lithography provides an increased depth of focus ($GTR 50 microns) at a larger working distance ($GTR 1 mm)than standard proximity printing. Microlens Projection Lithography allows photolithography on curved on non-planar substrates, in V-grooves, holes, etc. using a KARL SUSS Mask Aligner.
The potential and limits of micromoulding technology for the wafer scale hybrid integration of micro-optic elements on top of arbitrary substrates like glass, Silicon, III/V-semiconductors by a process which is performed in a modified contact mask aligner. The elements are characterised by high precision and stability, temperature stable and precise pitch, index, homogeneity, uniformity across the wafer, and they fulfill additional requirements for a practical application (AR-coating, separation in a dicing saw). Additionally, the technology for a two- sided replication of elements has been developed. The precision of lens arrays fabricated by reflow and UV-moulding is investigated, and the high performance of these arrays in the collimation of fiber arrays is shown. Steps toward a wafer scale integration of lens arrays with vertical cavity surface emitting lasers (VCSELs) by locally selective replication are demonstrated.
Photoemission electron microscopy (PEEM) has turned out to be one of the most promising methods for surface analysis in the recent years. It is a full field imaging technique based on the emission of secondary electrons by far ultraviolet light or X-rays. The emission intensity of secondary electrons is critically dependent upon the acceptance angle of the incident radiation. However, the size of the microscope restricts this angle substantially. Miniaturizing the objective lens of the microscope reduces the restriction of the acceptance angle and improves the performance of the PEEM considerably. We report on the fabrication of a miniaturized objective lens containing the extraction electrode, the electron column, the contrast aperture and the electron optical correction system for a PEEM. The extraction electrode as well as the electron column have been manufactured using precision milling techniques and electron discharge micromachining. For the fabrication of the correction system (stigmator / bending unit), a process combining aligned photolithography into a thick SU-8 resist and electroforming has been used. All electrodes were made in gold with a height of 150 (mu) m. After attaching a FOTURAN substrate to the electrode and etching under the electrodes, free standing apertures in an octupole and quadruple arrangement were obtained. The outer diameter of the electrodes is 5 mm and the inner diameter is 1 mm, respectively. Each electrode is connect individually to the external power supply which controls their operation. The overall size of the miniaturized objective lens is 23 mm, which has reduced the size of the lens by one order of magnitude when compared to commercially available instruments.
We report on the realization of an diffractive optical isolator for use at 543 nm by the combination of two binary high frequency gratings, corrugated into the surface of a quartz substrate. A single-order grating acts as a polarizing beam splitter with a measured diffraction efficiency of greater 95%. The other grating is a zero-order diffraction with 290 nm period and 1300 nm depth, acting as a quarterwave plate for conical incidence. A good correlation between theoretical and experimental results is demonstrated.
A micro-reducer characterized in its small size, high resolution ration and high reliability has been developed to achieve sufficient performance after 5,000,000 high-speed rotations1. A 3K-type mechanical paradox planetary gear reduction system was chosen in mechanical design. The size of a reducing part is 3 x 3.8 x 1.3 mm, and its reduction ratio is about 200. The module of the fine gears is 0.03. Alloy tool steel and WC-Ni-Cr super hard alloy were selected for the materials. The mechanical, thermal and environmental attributes were investigated by the properties of the materials suitable for micro fabrication on the specific strength and resistance to thermal distortion as the functional performance, and energy content of the material as the environmental impact. Micro-EDM process was optimized to accurately shape such a microscopic components. Surface modification by DLC, CrN, and MoS+-2) thin film was applied by rotating deposition technique to improve the surface-based attributes such as hardness, friction coefficient and resistance to wear. Several kinds of lubrication and bearing systems were evaluated to understand their internal energy dissipation and durability. This report presents such a synthetic approach for the micro-reducer to be steady, efficient and durable.
Silicon micromechanics in an emerging field which is beginning to impact almost every area of science and technology. In areas as diverse as the chemical, automotive, aeronautical, cellular and optical communication industries, Silicon micromachines are becoming the solution of choice for many problems. In this paper we will describe what they are, how they are built, and show how they have the potential to revolutionize lightwave systems. Devices such as optical switches, variable attenuators, active equalizers, add/drop multiplexers, optical crossconnects, gain tilt equalizers, data transmitters and many others are beginning to find ubiquitous application in advanced lightwave systems. We will show examples of these devices and describe some of the challenges in attacking the billions of dollars in addressable markets for this technology.
Silicon bulk micromachining which is based on a silicon etching and a glass-silicon anodic bonding plays important roles to make micro sensors and micro actuators. Three dimensional microfabrication of other functional materials as piezoelectric materials are also important to develop high performance microactuators, micro energy source and so on. Vacuum sealing is required to prevent a viscous dumping for packages micromechanical sensors. Extremely small structures as microprobe are required for high resolution, high sensitivity and quick response. As sophisticated microsystems which are made of many sensors, circuits and actuators are required for example for maintenance tools used in a narrow space. Developments for those required will be described.
The continuous progress in micro- and nano-system technologies has allowed the successful development of many innovative products in process control, environmental monitoring, healthcare, automotive and aerospace as well as information processing systems. In this paper on overview will be given of current progress in micro- and nanofabrication process technologies, such as deep reactive ion etching, micro-electro discharge machining, thick photoresistant processing and plating. The availibility of these micro- and nanofabrication processes will be illustrated with examples of new generations of silicon-based sensors, actuators and Microsystems with a particular emphasis on real applications of these components and systems.