We report on a technology for multi-level microstructures manufacturing. Results are presented in the field
of multilevel diffractive optical elements (DOEs) fabrication. The DOEs presented as examples are Fresnel
lenses and Fourier computer generated holograms, calculated by means of a conventional Iterative Fourier
Transform Algorithm. The DOEs have a typical pixel dimension of 5x5 μm2 and are up to 512 by 512 pixels in
size.
The fabrication technique is based on polymer laser ablation through a chrome-on-quartz half-tone mask
with a demagnifying high NA lens. In our case, the mask is imaged onto the polymer with a 5x, 0.13 NA
reduction lens. The experimental results are presented and discussed.
Pulsed laser sources are widely used for the micro-processing of materials from the structuring and patterning of surfaces to the direct machining of devices. This paper discusses laser micro-processing techniques for the fabrication of microstructures with high accuracy and precision. Techniques discussed include laser mask projection techniques and direct beam micromachining using galvo-scanners and high precision motion stages, with a variety of different lasers. Examples of the application of these techniques to the manufacture of MEMS and MOEMS devices are discussed.
A facility for rapid prototyping of MEMS devices is crucial for the development of novel miniaturized components in all sectors of high-tech industry, e.g. telecommunications, information technology, micro-optics and aerospace. To overcome the disadvantages of existing techniques in terms of cost and flexibility, a new approach has been taken to provide a tool for rapid prototyping and small-scale production: Complex CAD/CAM software has been developed that automatically generates the tool paths according to a CAD drawing of the MEMS device. As laser ablation is a much more complicated process than mechanical machining, for which such software has already been in use for many years, the generation of these tool paths relies not only on geometric considerations, but also on a sophisticated simulation module taking into account various material and laser parameters and micro-effects. The following laser machining options have been implemented: cutting, hole drilling, slot cutting, 2D area clearing, pocketing and 2½D surface machining. Once the tool paths are available, a post processor translates this information into CNC commands that control a scanner head. This scanner head then guides the beam of a UV solid-state laser to machine the desired structure by direct laser ablation.
The use of ceramic cores of high dielectric constant is an essential part of a strategy to miniaturize GPS antennas for mobile telephones. The core reduces the guide wavelength of the conducting structures on the antenna, thereby creating a need for high-resolution imaging to maintain very accurate dimensions. It is for this principal reason that a novel laser imaging technology has been developed using a positive electrophoretic photoresist and UV excimer laser mask imaging to produce the conducting features on the surface of the antenna. Furthermore, a significant process challenge in producing this type of antenna concerns the reproducibility of the right-hand circular polarization performance and the bandwidth over which this can be achieved - which becomes progressively smaller as antenna size is reduce. It is therefore a vital requirement that the antennas have the point to be tuned by a laser trimming process at an automatic RF testing station. A galvanometer controlled Nd:YAG laser spot is used to trim the conductive pattern on the top of the antenna following an RF measurement to characterize the resonant frequencies of the four helical conductors. Results demonstrate the laser imaging and trimming techniques ensure a high-speed method of guaranteeing the antenna performance. The technique is appropriate for other antenna types such as GSM, Bluetooth and Wireless LAN.
With the aim of reducing the heat-affected zone to improve edge quality, we present results of drilling microholes using reshaped pulsed Gaussian laser beams. A diode-pumped, high repetition rate, nanosecond pulse duration 3rd harmonic Nd:YAG laser was reshaped such that the intensity gradient in the outer region of the focussed laser beam profile is increased. Compared to focussed Gaussian laser beams, such hard-edged intensity distributions produce smaller heat-affected zones. As a result there is less associated collateral damage, debris, remelt produced by the near-ablation threshold fluences. Specially designed spherically-aberrating Galilean telescopes are used to reshape the primary Gaussian laser beam into a quasi-tophat distribution at the mask plane. Gaussian illumination propagation simulations using Monte-Carlo ray tracing calculations compare well with measurements of reshaped distributions made with a beam profiler. Drilling trials in polymers and silicon nitride demonstrated improved edge quality, reduced debris and wall roughness and a significant reduction in the energy density required for drilling microholes of high aspect ratio.
An Exitech Microstepper exposure tool has been used to laser micromachine a variety of polymeric materials with high resolution at a wavelength of 157 nm. We have demonstrated it is possible to machine thin film materials, different photoresists and fluorine-based polymers with submicron accuracy and resolution. The tool used for this work incorporated a 36x 0.5 NA Schwarzschild lens to project submicron resolution images of binary chrome-on-CaF2 masks onto free-standing and spun-on polymer films. The beam delivery system and the illuminator includes beam shaping and homogenization optics that allow fluences of greater than 1J/cm2 to be produced at the workpiece. Details of the optical system are presented together with process parameters and the results of the materials which have been machined.
A novel method is presented to manufacture multilevel diffractive optical elements (DOEs) in polymer by single- step KrF excimer laser ablation using a halftone mask. The DOEs have a typical pixel dimension of 5 micrometers and are up to 512 by 512 pixels in size. The DOEs presented are Fresnel lenses and Fourier computer generated holograms, calculated by means of a conventional iterative Fourier transform algorithm. The halftone mask is built up as an array of 5 micrometers -square pixels, each containing a rectangular or L- shaped window on an opaque background. The mask is imaged onto the polymer with a 5x, 0.13 NA reduction lens. The pixels are not resolved by the lens, so they behave simply as attenuators, allowing spatial variation of the ablation rate via the window size. The advantages of halftone mask technology over other methods, such as pixel-by-pixel ablation and multi-mask overlay, are that it is very fast regardless of DOE size, and that no high-precision motion stages and alignment are required. The challenges are that the halftone mask is specific to the etch curve of the polymer used, that precise calibration of each grey-level is required, and that the halftone mask must be calculated specifically for the imaging lens used. This paper describes the design procedures for multilevel DOEs and halftone masks, the calibration of the various levels, and some preliminary DOE test results.
A novel method is presented to produce a high precision pattern of copper tracks on both sides of a 4-layer conformal radar antenna made of PEI polymer and shaped as a truncated pseudo-parabolic cylinder. The antenna is an active emitter-receiver so that an accuracy of a fraction of the wavelength of the microwave radiation is required. After 2D layer design in Allegro, the resulting Gerber file-format circuits are wrapped around the antenna shape, resulting in a cutter-path file which provides the input for a postprocessor that outputs G-code for robot- and laser control. A rules file contains embedded information such as laser parameters and mask aperture related to the Allegro symbols. The robot consists of 6 axes that manipulate the antenna, and 2 axes for the mask plate. The antenna can be manipulated to an accuracy of +/- 20 micrometers over its full dimensions of 200x300x50 mm. The four layers are constructed by successive copper coating, resist coating, laser ablation, copper etching, resist removal, insulation polyimide film lamination and laser dielectric drilling for microvia holes and through-holes drilling. Applications are in space and aeronautical communication and radar detection systems, with possible extensions to automotive and mobile hand-sets, and land stations.
KEYWORDS: Ions, Gas lasers, Excimer lasers, High power lasers, Pulsed laser operation, Electrons, Electrodes, Laser development, Safety, Power supplies
The current performance of the high average power CHIRP II excimer laser is described. A power of 430 W at 1100 pps has been achieved on the XeCl excimer. Particular reference is made to the undesirable high pulse repetition frequency (PRF) effects observed in the testbed CHIRP I laser, and successfully avoided in CHIRP II.
Within the Eureka EU213 excimer laser program, the Laser Technology Centre at Culham is developing high average power excimer lasers operating at pulse repetition frequencies (PRFs) of greater than 1 kHz. This paper reviews recent high PRF results obtained on the testbed CHIRP I laser in the areas of preionization and pulse power. PRF limiting mechanisms observed in preliminary multikilohertz laser operation are described.
KEYWORDS: Excimer lasers, Gas lasers, Helium, High power lasers, Interferometers, Electrodes, Pulsed laser operation, Cameras, Aerodynamics, Laser development
An experimental study is reported of the generation and behavior of discharge-induced shocks and waves in a compact high repetition rate XeCl excimer laser (CHIRP) with helium as buffer gas. Three sources of shocks and waves are identified. Longitudinal shocks are formed at the discharge boundaries, traveling up and downstream into the gas flow duct. A transverse shock is created at the cathode sheath region, and the anode mesh is the source of an expansion wave followed by a compression wave 1.5 microsec later. Transverse shocks can reflect off the cathode, but no reflections off the anode mesh have been observed.
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