A polymer-based dynamic microlens system that can provide variable focal length and field-of-view (FOV) is fabricated and tested for its optical imaging characteristics. A flexible polydimethylsiloxane (PDMS) polymer membrane is used to form the lens surface. Two such membranes are combined with a spacer in between to form the fluidic lens chamber. The entire assembly is actuated by fluidic pressure using an external syringe pump to form either a double convex (DCX) or double concave (DCV) lens. The relationship between the focal length (f) and FOV of this dynamic lens as a function of the volume of the fluid pumped into or out of the lens chamber is investigated. The focal length of the single dynamic lens system can be tuned over the range of 75.9 to 3.1 mm and -75.9 to -3.3 mm, respectively, for the DCX and DCV lens configurations. The FOV that could be achieved using this dynamic lens system as DCX and DCV lenses is in the range of 0.12 to 61 degrees and 7 to 69 degrees, respectively. The smallest f-number (f/#) of 0.61, which corresponds to a numerical aperture of 0.64, could be achieved for a single dynamic lens system. An integrated two or three variable focal length DCV microlens system to provide wide FOV has also been fabricated and tested. The effective focal length of the integrated dynamic microlens system with two and three DCV lenses can be tuned in the range of -37.9 to -2.1 mm and -25.3 to -1.8 mm, respectively. The FOV achieved using the integrated two and three variable focal length DCV microlens systems were in the range of 8 to 76.7 degrees and 11.5 to 90.4 degrees, respectively.
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.
Masks made from graphite stock material have been demonstrated as a cost-effective and reliable method of fabricating X-ray masks for deep and ultra-deep x-ray lithography (DXRL and UDXRL, respectively). The focus on this research effort was to fabricate masks that were compatible with the requirements for deep and ultra deep X-ray lithography by using UV optical lithography and gold electroforming. The major focus was on the uniform application of a thick resist on a porous graphite substrate. After patterning the resist, gold deposition was performed to build up the absorber structures using pulsed- electroplating. In this paper we will report on the current status of the mask fabrication process and present some preliminary exposure results.
The Center for Advanced Micro structures and Devices (CAMD) at Louisiana State University supports one of the strongest programs in synchrotron radiation micro fabrication in the USA and, in particular, in deep x-ray lithography. Synchrotron radiation emitted form CAMD's bending magnets has photon energies in the range extending from the IR to approximately 20 keV. CAMD operates at 1.3 and 1.5 GeV, providing characteristic energies of 1.66 and 2.55 keV, respectively. CAMD bending magnets provide a relatively soft x-ray spectrum that limits the maximal structure height achievable within a reasonable exposure time to approximately 500 micrometers . In order to extend the x-ray spectrum to higher photon energies, a 5 pole 7T superconducting wiggler was inserted in one of the straight sections. A beam line and exposure station designed for ultra deep x-ray lithography was constructed and connected to the wiggler. First exposures into 1 mm and 2 mm thick PMMA resist using a graphite mask with 40 micrometers thick gold absorber has been completed.
The usefulness of thin (< 250 micrometers ) rigid graphite plates as x-ray mask substrates for micromachining and LIGA applications has been demonstrated. Rigid graphite offers unique properties, such as moderate x-ray absorption and optimal filtration of synchrotron radiation, relatively low cost, compatibility with additive (electroplating) and subtractive (etching, micromachining) processes for absorber patterning. The surface roughness of these substrates is associated with the inherent porosity of a commercially available rigid graphite material (typical Ra values are in the range of 1 - 2 micrometers ). The surface roughness of this rigid graphite sheet is reduced down to a 0.1 - 0.2 micrometers Ra value by polishing. To reduce surface roughness further and make the substrate usable for fine e-beam or optical absorber imaging, additional smoothing is required. In this paper, the surface characteristics of rigid graphite sheets are analyzed and a glazing technique developed to smooth the graphite surface is described. This technique employs hard baking process of novolac-based resins. An average Ra roughness value of approximately 5 nm was obtained after 5 coating using novolac-based AZ type resist.
An important aspect for the development of micromanufactured components and systems is to reduce the time and cost required to reach the prototype stage. At present, this development typically spans several years. Any fabrication approach which would reduce the cost and time-to-prototype would allow for the more rapid development of design concepts and the more rapid evolution of the design cycle. Direct fabrication of masks for X-ray lithography, by mechanical micromilling, is one potential avenue for rapid, lower cost development. The key process requirements for the fabrication of a typical X-ray mask involves the selection of both substrate and absorber materials. The substrate must provide a mechanically stable support for the patterned absorber without introducing excessive attenuation of the X- ray flux that ultimately reaches the resist surface. Frame supported, thin membranes (such as SiC, C, Si3N4, Si) are most often used as well as low atomic number bulk materials (Be). The choice of elemental composition and thickness for the absorber will be largely determined by the resist sensitivity and the X-ray wavelength used. Many process steps are required in order to define the final absorber pattern geometry and will generally involve either additive or subtractive processes. Mechanical micromilling techniques may be used with either a single bulk material which serves the dual role of both substrate and absorber or with a composite structure consisting of a thin gold layer deposited on a thick, low atomic number bulk substrate. Single material masks of aluminum and graphite have been investigated. A composite mask of graphite with a thin layer of sputtered gold has also been investigated. The paper will report on the developmental work for both types of masks and will give results for synchrotron X-ray exposure using these masks. Problems associated with using micromilling as an X- ray mask fabrication method will also be presented.
The successful application of sub-micron scaling principles to device fabrication involves an integration of tool, resist system, and process control. The precision overlay capability of a modified IBM EL-3 variable shaped beam lithography tool has been used to achieve optimized scaling of a 0.25 micrometers bipolar technology. Although the total device size is strongly coupled to linewidth control and overlay accuracy for all circuit levels, the overlay between the emitter opening and the shallow trench isolation is considered to be the most critical. We report on the integration of an advanced electron beam lithography and resist process capability with an innovative bipolar device technology to achieve emitter coupled logic (ECL) delays of 20.8 ps at a switching current of 1.1 mA. These results demonstrate the feasibility and performance leverage that can be accomplished through the aggressive scaling of conventional bipolar technologies.
In this paper we investigate the use of two negative resists, a commercially available negative tone electron beam resist (CAR), and AX-E, an IBM resist, with a 50 KeV variable shaped electron beam system. All aspects of processing and tooling were investigated to understand the impact on critical dimension control. Post applied and post exposure bake times and temperatures were examined in an effort to optimize resist contrast, resist profile, and gate linewidth variations. Proper tool set-up and proximity correction were also investigated. SEM metrology as well as high resolution SEM cross-section metrology were extensively used in the process optimization. A complete understanding of the resist, processing, tool and metrology have resulted in the fabrication of working devices and circuits with physical gate linewidth dimensions down to 0.10 micrometers with a 3(sigma) better than +/- 0.03 micrometers across a wafer.