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.
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.
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.
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.
The influence of the place and the number of the peripheral ohmic contacts on the characteristics of a light-spot-position-sensitive photodiode based on the lateral photoeffect is investigated. Using contact pairs, a 9·3×9·3 mm2 photodiode shows a 6×6 mm2 linear area in which the x- and y-signals unambiguously depend on the co-ordinates. An extra contact ring makes it possible to make the cell light-intensity independent.
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.
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.
The EU213 EUREKA Excimer Laser project is aimed at the development of a 1kW average power excimer laser by 1992. In order to realize this goal, part of the UK programme will be to explore the possibility of creating a 1kW laser with a repetition rate approaching 10 kHz. In order to investigate the problems of, for example, gas flow, preionisation and pulse power at such a high repetition rate, an XeCl test bed laser has been assembled at Culham Laboratory: the Compact HIgh Repetition rate (CHIRP) laser. The CHIRP laser is a fast flow discharge pumped,UV corona preionised excimer laser. One of the major considerations in developing a 1 kW system is the electrical input to optical output conversion efficiency and its optimisation through the design of the pulse power system. The pulser circuit presently used uses a low impedance transmission line, and provides a high voltage prepulse for the main discharge. A voltage doubler circuit generates a synchronous pulse for the corona preioniser. Isolation of the prepulse is achieved by use of a ferrite saturable magnetic inductor. The typical stored energy per pulse is ~3.5 J (at 10 kV) for the main pulse,~1 J (at 25 kV) for the prepulse and ~ .2 J for the preioniser. Active switching is by deuterium filled hollow anode thyratrons. This paper discusses the design of the circuit, its operation at pulse repetition rates of ~1 kHz, and prospects of multikilohertz operation.
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.
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