Excimer Laser Micromachining Research Articles

Excimer laser micromachining of aspheric microlenses with precise surface profile control and optimal focusing capability

This paper demonstrates the unique and exceptional capability of excimer laser micromachining in fabricating aspheric microlenses with precise surface profile control. A newly developed laser scanning method is introduced for machining refractive types of microlenses, which have pre-designed surface profiles aiming at minimizing the optical focal spot sizes. The machining accuracy and machined surface roughness are examined experimentally, and very good results are obtained. Optical testing on the fabricated aspheric microlenses shows significant improvement in focusing capability and the focal spot sizes are approaching optical diffraction limits. The proposed excimer laser micromachining method is flexible, versatile, and accurate, hence can be very useful and powerful in machining 3D microstructures of complex profiles and demanding profile accuracy.

Spherical and Aspheric Microlenses Fabricated by Excimer Laser LIGA-like Process

This paper presents an effective and low-cost method for fabricating spherical and aspheric microlenses based on excimer laser LIGA-like processes. It is based on a newly developed excimer laser micromachining technique that can accurately machine a 3D microstructure with a predetermined continuous surface profile. The method is called the planetary scanning method since it is based on a combination of sample rotation and revolution and a concept of laser machining probability. Spherical and aspheric microlenses with precise and smooth surface profiles are fabricated by direct laser machining on polymer materials. Laser-machined microlenses are replicated by electroforming to obtain inverse metal molds. Finally, plastic microlenses are replicated from these metal molds using hot embossing method. The profile accuracy and surface roughness of the produced microlenses at each stage have been measured and monitored. The average surface profile accuracy is better than 1μm and average surface roughness is less than 10nm. Optical performance of the fabricated microlenses is evaluated by measuring the light intensity distribution at the focal plane and the focal length. Experimental data show that the characteristics of fabricated spherical and aspheric microlenses are well matched to the theoretical predictions, which demonstrates the controllability and accuracy of this micromachining process. Potential applications and further developments will be addressed.

Miniature piezoelectric conical transducer: Fabrication, evaluation and application

This paper reports a new type miniature-conical transducer for acoustic emission measurements. The transducer follows the basic idea and structure of a conventional NBS conical transducer, but is much compact in size and easier to use. The improvements are made possible by introducing an excimer laser micromachining method for fabricating smaller PZT conical elements, which play a key role in the transducer. Conical PZT elements with contact size less then 300 μm are laser-machined and the miniature-conical transducers are constructed. Standard quantitative acoustic emission testing is performed on a plate using the fabricated transducers and good results are observed. The transducers can be very useful in many applications involving quantitative measurements of transient elastic waves.

Parametric Investigation of Laser-Assisted Direct Imprint (LADI) Technique

Nano-imprinting Lithography (NIL) has been considered as the most promising technique for nano-scaled fabrication and patterning. Recently, a new approach known as Laser-Assisted Direct Imprinting(LADI) has been proposed and demonstrated as an even more efficient way for direct nanofabrication and nanopatterning. In this study, we focused on silicon materials and utilized a single KrF excimer laser pulse (248 nm wavelength and 30 ns pulse duration) as the heating source. Molds of micro-scaled size have been prepared using conventional photolithography techniques. A working platform based on an Excimer Laser Micro-Machining system is constructed for LADI process. The influence of laser fluence and the imprinted pressure on the resulting structures was verifying by varying the laser fluence (1.0 ~ 1.2 J/cm2) and the imprinted load (3 ~ 9kg). The results have shown that the morphology and the imprinted depth were directly related to the laser fluence and the imprinted pressure. Quantitative data are obtained and will be addressed.

Fabrication and Characterization of 3D Aspheric Microlenses with Arbitrary Surface Profiles Based on a Novel Excimer Laser Contour Scanning Method

This paper presents a new method for fabricating 3D microstructures with an excimer laser micromachining system. A novel mask contour scanning method is developed for obtaining precise 3D microstructures with pre-described continuous surface profile. Two different microlenses with spherical and aspheric surfaces profiles with dimension less than 200 μm are fabricated on polycarbonate (PC) samples. The surface profiles are measured and compared with their theoretical counterparts. Excellent agreements both in profile shapes and dimensions are achieved. The surface roughness (Ra) of the machined surfaces is also measured and is less than 10 nm. The machining profile accuracy and surface smoothness of this proposed micromachining method show great potentials in fabricating micro-optic components such as aspheric microlenses or microlens arrays.

A prototyping and microfabrication CAD/CAM tool for the excimer laser micromachining process

A CAD/CAM tool for prototyping and small-scale production of micro-electro-mechanical systems (MEMS) devices based on the excimer laser ablation process has been developed. The system’s algorithms use the 3D geometry of a microstructure, defined as an STL file exported from a CAD model, and parameters that influence the process (laser fluence, pulse repetition frequency, number of shots per area, wall angle, stitching errors) to automatically generate a precise NC part program for the excimer laser machine. The performance of the system has been verified by NC part program generation for several 3D microstructures and subsequent machining trials. An initial stitching error of 23.4±2.2-μm wide and 3.4±1.5-μm high was observed when the overlap size between adjacent volumes was zero, when ablating 100×100-μm features in polycarbonate (PC) at a fluence of 0.5 J/cm2 using a workpiece-dragging technique. When the size of the overlap was optimised by a system based on optimal process parameters determined by the Taguchi design of experiment method (DOE), and incorporated in the mask design, the maximum stitching error was reduced to 13.4±2.2-μm wide and 1.4±0.9-μm high under the same conditions. By employing a hexagonal-shaped mask with incorporated size of the image overlap, reduced horizontal-stitching errors of 2.4±0.2-μm wide and 1.4±0.2-μm high were observed. The system simplifies part program creation and is useful for excimer laser operators who currently use a tedious trial and error process to create programs and complex masks to generate microstructure parts.

2123 金属ガラスの超塑性鍛造によるマイクロインダクターの開発(S29-1 マイクロ/ナノ加工の動向と可能性(1),S29 マイクロ/ナノ加工の動向と可能性)

We designed and fabricated a micro-inductor that may be used for hybrid electric circuits such as those in a mobile phone. The component was fabricated by superplastic die-forging of Pt-based metallic glass. This material exhibits Newtonian viscous flow in the supercooled liquid temperature range between the glass transition and the crystallization point. The working temperature was 543K, so a polyimide film was selected as a die material due to its good mechanical properties at elevated temperatures and its precision machinability. The die was fabricated with an excimer laser micro-machining system using the image-projection method. Laser ablation machining was carried out using a photomask in the shape of the micro-inductor. The photomask was fabricated using a lithography system. Superplastic die-forging of the metallic glass was carried out using this die, and micro-structures of the micro-inductor were easily fabricated. These fabrication processes demonstrate the advantages of metallic glasses and superplastic microforming techniques for mass producing micro/nano machine parts or devices.

Diagnostic technique for laser micromachining of multi-layer thin films

Excimer laser micromachining was a key process for producing features by removing parts of some photoresist layers. One hurdle was that the lower substrate layer was easily attacked or damaged in laser ablation of the upper photoresist layer. To overcome this, a real-time monitoring scheme of laser micromachining process was proposed on the basis of taking acoustic emissions arising from pulsed laser–material interaction as feedback signals and employing a diagnostic program. It was showed that extracted features (e.g. the RMS values) from sensor signals were indicative of the process. In particular, the feature values tended to have an abrupt change when laser beam approached the interface of consecutive two layers. Relying on this characteristic, we proposed a fuzzy expert system as the core part of the diagnostic program. The effectiveness of the program was verified with tests.

A new excimer laser micromachining method for axially symmetric 3D microstructures with continuous surface profiles

This paper presents a new method for fabricating axially symmetric 3D microstructures based on a KrF 248 nm excimer laser micromachining system. A novel mask contour scanning method is developed for obtaining a pre-described continuous surface profile with high accuracy and fast machining speed. Both convex and concave surfaces can be obtained. Four different microstructures with spherical and parabolic surface profiles with dimension around 500 μm were experimentally tested on polycarbonate (PC) samples. The surface profiles were measured and compared with theoretical counterparts. Excellent agreements both in profile shapes and dimensions were achieved. The surface roughness of the machined surfaces was also measured and the average roughness ( R a) is less than 10 nm. The machining profile accuracy and surface smoothness of this proposed micromachining method demonstrate great potentials in fabricating micro-optic components such as aspheric microlenses or microlens arrays.

Evaluation of excimer laser ablation of thin Cr film on glass substrate by analysing acoustic emission

Excimer laser ablation of a thin Cr film on a glass substrate was evaluated through detecting acoustic emission arising from the ablation process and observing surface morphology. In the experiment, an acoustic emission transducer was coupled to the sample to acquire acoustic emissions of laser–material interaction and an oscilloscope was used to record the signals. The patterned areas were examined by using an optical microscope. Characteristics of the acoustic emissions were studied through applying a range of signal analysis tools. Several features (e.g., average power, RMS) showed a clear linkage with the ablation mechanism of thin Cr film when varying the fluence of the laser source. Moreover, the damage to the glass substrate was well discriminated by the extracted features. In sum, evaluation of acoustic emissions not only provided a tool to study the ablation mechanisms of the thin metal film but also addressed a monitoring strategy for excimer laser micromachining.

Excimer laser micromachining for 3D microstructure

A new 3D micromachining method, called Hole Area Modulation (HAM), has been introduced to enhance the current micromachining technology. In this method, information on the machining depth is converted to the sizes of holes on the mask. The machining is carried out with a simple 2D movement of the workpiece or the mask. This method can be applied for machining various kinds of microcavities in various materials. In this paper, a mathematical model for excimer laser micromachining based on HAM and determination of the optimal laser ablation conditions (hole diameter, step size, mask movement velocity, etc.) are described. The simulation and experiment of the HAM-based laser ablation were carried out successfully to create microlens.

Excimer laser micromachining for 3D microstructure

A new 3D micromachining method, called H ole A rea M odulation (HAM), has been introduced to enhance the current micromachining technology. In this method, information on the machining depth is converted to the sizes of holes on the mask. The machining is carried out with a simple 2D movement of the workpiece or the mask. This method can be applied for machining various kinds of microcavities in various materials. In this paper, a mathematical model for excimer laser micromachining based on HAM and determination of the optimal laser ablation conditions (hole diameter, step size, mask movement velocity, etc.) are described. The simulation and experiment of the HAM-based laser ablation were carried out successfully to create microlens.

An alternative method of fabricating sub-micron resolution masks using excimer laser ablation

In the work presented here, an excimer laser micromachining system has been used successfully to fabricate high-resolution projection and contact masks. The contact masks were subsequently used to produce chrome-gold circular ac electro-osmotic pump (cACEOP) microelectrode arrays on glass substrates, using a conventional contact photolithography process. The contact masks were produced rapidly (∼15 min each) and were found to be accurate to sub-micron resolution, demonstrating an alternative route for mask fabrication. Laser machined masks were also used in a laser-projection system, demonstrating that such fabrication techniques are also suited to projection lithography. The work addresses a need for quick reproduction of high-resolution contact masks, given their rapid degradation when compared to non-contact masks.

Excimer laser micromachining with half-tone masks for the fabrication of 3-D microstructures

Excimer laser micromachining by mask projection with half-tone masks has been used to define deep structures with complex surface profiles in cross-linked SU-8 photoresist. A simple algorithm has been developed to derive the required mask transmission function given the material etch characteristics and the desired surface profile. This algorithm has been shown to give reasonable profile accuracy for structures with heights of up to about 150 /spl mu/m. The technique has been used in the fabrication of the rotor blades for an axial-flow microturbine.

Design and Fabrication of All-Polymer Photonic Devices

AbstractHigh quality optical waveguides have been fabricated from fluorinated poly(arylene ether ketone) materials using a standard photolithographic process. Fabrication of waveguide devices on a polymer substrate is described, including a method of end-facet preparation using excimer laser micromachining. Material issues affecting waveguide birefringence and device performance are discussed.

Fabrication of high-aspect-ratio microstructures using excimer laser

An excimer laser micromachining system is developed to study the ablation of high-aspect-ratio microstructures. The study examines the ablation efficiency, specifically, the impact of changing major laser operating parameters on the resulting microstructural shapes and morphology. The study focuses on glass, although results on silicon and aluminum are also included for comparison. In ablating grooved structures, the ablation depth has been observed to be linearly proportional to the operating parameters, such as the pulse number and fluence. The results specifically indicate that ablation at low fluence and high repetition rates tends to form a V-shaped cross-section or profile, while a U-shaped profile can be obtained at high fluence and low repetition rate. The ablation rate or ablated volume has then been quantified based on the ablation depth measured and the ablated profile observed. The threshold fluence has also been obtained by extrapolating experimental data of ablation rate. The extrapolation accuracy has been established by the good agreement between the extrapolated value and the one predicted by Beer's law. Moreover, a one-dimensional analytical solution has been adopted to predict the ablated volume so as to compare with the experimental data. The reasonable agreement between the two indicates that a simple analytical solution can be used for guiding or controlling further laser operations in ablating glass structures. Finally, the experimental results have shown that increasing the repetition rate favors the morphology of ablated surfaces, though the effect of repetition rate on ablation depth is insignificant.

Fabrication of three-dimensional inductor coil using excimer laser micromachining

Conventional lithographic techniques are typically used for the fabrication of three-dimensional multilayer structures such as micro-inductors and microactuators. In this work a novel approach based exclusively on laser micromachining of dry film resists and electroplating is presented. The process involves seed layer deposition, lamination of dry film resist, laser micromachining and electroplating to mould the resist patterns. The process is then repeated, using intermediate plating seed layers if necessary, to realize the three-dimensional coil structure. The steps for sacrificial material removal involve laser patterning and chemical removal of resist and plating seed layers. The major advantages with this process are that it has fewer steps compared to conventional UV-based processing technology, and it utilizes the benefits of dry film resist processing using the laser. A three-layer micro-inductor coil was designed and fabricated using the new laser micromachining-based processing technology. The problems encountered during fabrication and possible actions to address them are discussed. This work reports the first multilayer structure that has been fabricated exclusively using laser ablation for the patterning of resist. Furthermore, this work reports the first structural layer that has been fabricated using laser ablation of resist to expose an underlying intermediate plating seed layer.

Three-dimensional excimer laser micromachining using greyscale masks

Greyscale masks have been successfully implemented in an excimer laser micromachining system to produce structures with a continuous profile. During this work, it was found possible to machine structures to depths of several tens of microns with no observable mask degradation. The greyscale mask transmissions were defined using a matrix of pixels whose dimension was smaller than the resolution limit of the optical system in the laser micromachining system. By reduction-projecting the greyscale mask pattern onto the workpiece, the local fluence at the workpiece could be predetermined and hence the local machining rates controlled. This enabled three-dimensional (3D) structures to be fabricated at the workpiece in a single machining operation. Under the experimental conditions used in this investigation, the rate at which material was ablated was found to depend linearly on the percentage transmission of the mask. Various test structures, including complex 3D-contoured fluidic channels, have been produced.

Microlens Array Fabricated by Excimer Laser Micromachining with Gray-tone Photolithography

We demonstrate the fabrication of a refractive microlens array by using 248 nm excimer laser micromachining with coded gray-tone mask photolithography. With pre-corrections to the nonlinear exposure process, the maximum deviation from the designed shape was below 5%. The fabricated hemispherical lens of 30 µm radius was used as a solid immersion lens (SIL) and combined with a 0.54 numerical aperture (NA) objective to achieve a 0.87 effective NA through the knife-edge scanning test. The experimental results agreed with those of the simulation. Unlike the methods such as the thermal melting process, this one-step optical exposure method with a coded mask provides a relatively fast and cost-effective way to realize a microlens array in optical data storage, information processing, and optical interconnection applications.

Excimer laser micromachining of TiN films from chromium and copper sacrificiallayers

This paper presents results on the laser micromachining of TiN films. Machiningperformance was evaluated in terms of patterning quality and the ability toremove TiN with minimal interference with an underlying sacrificial layer. TiNwas arc-deposited onto (100) silicon substrate with chromium (Cr) and copper(Cu) sacrificial layers. Films were also deposited onto bare silicon substrates underthe same conditions. These films were analysed for their compositionand structure using Rutherford backscattering spectroscopy and x-raydiffraction techniques. Laser micromachining was performed using a KrFexcimer laser at 248 nm. The effect of fluence and number of shots on themachined features has been investigated in detail. The patterned features wereexamined using optical, confocal and scanning electron microscopes. Thecharacteristics observed were analysed and compared in all three sets ofsamples. The results showed selective removal of TiN films from Cr and Cusacrificial layers under different conditions. The machining of TiN from(100) silicon showed relatively poor definition of patterned features. Theanalysis of these results indicated that laser machining of TiN from Cr andCu layers is best explained using the explosion mechanism of removal.