Keiji Ebata

Optical properties of ZnSe diffractive optical elements for spot array generation

The diffractive optical element (DOE) is a revolutionary technology for sophisticated optical systems. It has recently been launched within the optical industry, which is constantly seeking improvements over conventional optics. We have designed and fabricated three types of binary-phase DOE for array generation. The surface relief of a ZnSe substrate was patterned and etched with each intended phase distribution by using photolithography and reactive ion etching (RIE) technologies. The optical properties of anti-reflection coated samples were then examined by measuring the intensity distribution of their converging diffractive beams and the results compared with the calculated beam propagation.

Advanced laser optics for laser material processing

The demand for uniform intensity distribution is rising rapidly in the field of thermal processing. In this study we propose beam homogenizers with aspheric lenses or diffractive optical elements (DOE) that can convert a non-uniform Gaussian distribution into a top-hat-shaped uniform intensity distribution. The circular beam homogenizer consists of two aspheric lenses. And we propose several types of beam homogenizer, namely, rectangular and linear using DOE technology. Especially, we present a spot array generation homogenizer that can anneal several points simultaneously. This paper suggests possibilities of advance laser optics for new types of laser material processing.

Characteristic of diffractive optical element for arbitrary pattern beam shaping

Laser materials processing has been used increasingly over the wide area of electronic industries, especially for drilling microvias in printed circuit boards, and poly-silicon annealing for thin film transistor of liquid crystal display. Intensity distribution of laser beam is usually a non-uniform gaussian profile. Therefore, the demand for uniform intensity distribution is rising rapidly in some applications of heat processing. To obtain higher uniformity, beam homogenizer of a diffractive optical element (DOE) has recently been developed and introduced to some promising applications. Through the improvement of optical design algorithms and micro-fabrication techniques of a phase pattern of DOE, it becomes possible to convert a non-uniform gaussian distribution not only into a simple distribution like a square and a line but also into a complicated distribution like a distribution of printed circuit pattern. In this study, we introduce a design and fabrication result of beam shaper of DOE that can convert a gaussian distribution into the distribution of a printed circuit pattern, and present the possibility and the point at issue of new laser material processing by using such optics.

Diffractive/refractive hybrid f-theta lens for laser drilling of multilayer printed circuit boards

A new type of f-theta lens has recently been developed for microvia laser drilling of multilayer printed circuit boards. It employs a diffractive/refractive hybrid lens which has a blazed surface-relief microstructure on an aspheric surface. By introducing that hybrid lens for CO2 laser system, and by stopping the use of germanium that is optically much sensitive to temperature, the f-theta lens that consists of all zinc selenide lenses is obtained with its optical performance stable on temperature. Achromatic properties against the wavelength fluctuations of actual lasers are also achieved. A prototype is fabricated through the development of single point diamond turning of hybrid surfaces. The performance of the lens is first examined by measuring wavefront error with a tunable infrared interferometer. The results show diffraction-limited performance at all conditions, including different temperatures (up to 50°C) and wavelengths. The temperature dependence of the focal length of the lens is also measured and found to be 5 times as insensitive to temperature as that of a conventional one. Laser drilling experiments are performed for a polymide film on copper foil. The result shows good uniformity of hole size and circularity all over the 50×50 mm2 scan field.

Design and performance of multilevel phase fan-out diffractive optical elements for laser materials processing

A multilevel phase fan-out diffractive optical element (DOE) has been developed and introduced into various kinds of laser materials processing such as drilling, cutting, welding, and soldering. The larger the number of arrayed spots the DOE generates on the surface of the workpiece, the more sensitive the intensity uniformity of the spots becomes to fabrication errors, which are deviations between designed and fabricated surface microstructures. Errors in etch depth have, in particular, a significant effect on the intensity uniformity. A new design method has been developed for increasing the tolerance to the etch depth error, and applied to the design of a 16-level phase 7×7 fan-out element. The result indicates a uniformity less sensitive to etch depth error. The effect of a linewidth error due to the side etch introduced during a plasma etching process is also evaluated by computing high-resolution graphics data representing the phase of the DOE with the line width errors. Mask alignment errors, the s...

Development of adaptive mirror for CO2 laser

Material processing using CO2 laser is one of the most highly industrialized laser technologies. Recently, there have been growing demands to improve the processing quality by flexibly controlling the beam properties. Development of the adaptive mirror with variable curvature function, which is one of the vital optics to realization of these demands, has been extensively pursued. We developed the AM. It maintains the same accuracy of surface figure (PV value: 1 micrometer or below) as the conventional mirrors even when the curvature is changed, and is capable of high speed curvature control (maximum frequency: 1 kHz) aiming at application to beam sawing. Further, we applied the AM to laser cutting of mild steel as an example of its practical use, and studied smoothing of the cut surface by beam sawing.

Focussing properties of 45 degree-off-axis parabolic mirrors

The focussing properties of 45 degree-off-axis parabolic mirrors were examined using a Prometec Laser Scope, with comparison to those of 90, 60 and 30 degree-off-axis parabolic mirrors. These mirrors are all SPDT (Single Point Diamond Turning)-treated, Mo-coated surface and their focal lengths are 254mm. We examined (1) The focussing properties (focal spot size, spot elliptical shape and ellipticity) differ depending on misalignment and mode (Power). (2) With attention to the fact that ZnSe window is frequently used with parabolic mirrors, and the affect on focussing properties of parabolic mirror by the thermally induced optical distortion of this window. And (3) The condition of the parabolic mirrors under irradiation of 5kw laser was simulated.The focussing properties of 45 degree-off-axis parabolic mirrors were examined using a Prometec Laser Scope, with comparison to those of 90, 60 and 30 degree-off-axis parabolic mirrors. These mirrors are all SPDT (Single Point Diamond Turning)-treated, Mo-coated surface and their focal lengths are 254mm. We examined (1) The focussing properties (focal spot size, spot elliptical shape and ellipticity) differ depending on misalignment and mode (Power). (2) With attention to the fact that ZnSe window is frequently used with parabolic mirrors, and the affect on focussing properties of parabolic mirror by the thermally induced optical distortion of this window. And (3) The condition of the parabolic mirrors under irradiation of 5kw laser was simulated.

Developments in SiO2 multistep diffractive optical element for beam homogenizing

Diffractive Optical Element (DOE) is an advanced optics which utilizes the optical diffraction phenomena by a microstructure on its surface and can realize various applications such as homogenizing, shaping and splitting in laser material processing. The optical property of DOE is influenced by the accuracy of its microstructure formed by photolithography and the dry etching technique. The development of the reproducible dry etching technique of fused silica to obtain high depth precision in a microstructure by inductively coupled plasma reactive ion etching (ICP-RIE) is described. In ICP-RIE depth is controlled by the time determined by the etching rate of the previous batch. To stabilize the etching rate which is determined by ion energy, the ICP power is reduced to decrease the range of ion energy distribution and the thickness of grease for cooling the substrate is controlled between each batch. Depth precision of less than 10nm has been obtained. Good depth uniformity of less than 30nm P-V at the 1183nm target depth in the 50mm diameter area is also obtained using gas flow simulation. With these improvements a beam-homogenizer DOE with a 16-step microstructure for 532nm YAG-SHG is produced. Intensity is changed by this DOE from the Gaussian beam to the flat top beam whose optical intensity uniformity is less than 10% in the 1.0×0.5 mm region.

Beam-splitting ZnSe diffractive optical element

ZnSe Diffractive Optical Element (DOE) is one of the advanced optics which utilizes the optical diffraction phenomena by fabricating a micron order pattern on polished mirror-like surface of ZnSe polycrystal substrate. Various applications for a carbon dioxide (CO 2 ) laser material processing such as beam-splitting, beam-shaping and beam-homogenizing are available. The micro pattern of ZnSe DOE is fabricated by the photolithography and reactive ion etching (RIE) technique. Its optical property is highly dependent on the depth precision of microfabricated pattern. In RIE by using BCl3 as the etchant gas we have achieved an etching technique to maintain the smooth surface of the ZnSe polycrystal with minimal etching rate dependency on the crystal orientation of each crystal grain. The surface roughness is 2nm Ra before etching and 5 nm Ra after about 4 microns depth etching. This good roughness brings better depth precision. With these etching technique beam-splitting ZnSe DOE with less than 10% intensity uniformity of splitted beams is successfully obtained and it can be put to use for practical CO 2 laser hole drilling.

Characteristics of ZnSe aspheric beam shaper for CO2 laser

CO2 laser material processing is now being used increasingly in the area of electronics, specifically, for drilling micro holes (min. (phi) 50 micrometers ) in printed circuit. This has become possible through the use of galvanometer mirrors and Aspheric ZnSe f-theta lenses. ZnSe is a good infrared material because of wide transmission. We have succeeded in producing aspheric lenses by using a Single Point Diamond Turning (SPDT) lathe. Distribution of laser beam intensity mainly based on Gaussian distribution is non- uniform. Therefore, the rise great demand for uniform intensity distribution in the fields of heat processing. To obtain higher uniformity, attempts have to be made to convert non-uniform Gaussian distribution into top-hat shaped uniform intensity distribution for smoothly bending laser beams. In this study we propose it is possible to allow the surfaces of a lens to perform more than one function (the center portion functions as a concave lens and the rim portion functions as a convex lens). The Aspheric ZnSe beam shaper consists of two aspheric lenses. First one converts Gaussian profile to uniform irradiation, Second one performs phase matching (compensation). We design this optical component with our special method based on the optical ray tracing. The most important optical property, wave front distortion were measured by infrared interferometer. And we also show the intensity distribution through after beam shaper with high power CO2 laser (>100 W).