Excimer Laser Micromachining Research Articles

A new point-source/point-receiver acoustic transducer for surface wave measurement

Abstract This paper presents the development of a new acoustic transducer for measuring surface wave velocity using miniature conical PZT elements. The elements are directly fabricated out of a polarized PZT ceramic plate by excimer laser micro-machining technique. The aperture size of the machined PZT conical elements is around 200–400 μm in diameter, which allows high-frequency generation and detection of surface waves. Two of the transducers are assembled together to form a transmitter/receiver pair. Driven by a tone-burst electrical system, the transmitter acts as an acoustic point-source which launches surface acoustic waves propagating along a sample surface. The receiver then detects the surface motion. The transfer function of the transmitter/receiver system is experimentally determined. With a waveform comparison method, the surface wave velocities are obtained from the measured signal waveforms. Experimental results on several samples show that the surface wave velocity can be determined accurately. Finally, the transducer is applied on anisotropic solids. The variation of surface wave velocity as a function of propagation direction is determined.

Miniature conical transducer realized by excimer laser micro-machining technique

This paper presents the fabrication and characterization of a miniature conical transducer that can be used as an acoustic emission sensor for localized measurements. The development of the miniature conical transducer is based on the commercial NBS conical transducer and the micro-fabrication capability of an excimer laser. The conical PZT element, which is the key component in the transducer, is fabricated by an excimer laser micro-machining workstation with a specially designed laser mask and a new work piece rotation machining technique. Small conical piezoelectric elements with end surface around 200 μm in diameter are directly fabricated out of a polarized PZT ceramic plate. The miniature conical transducer is then constructed. Standard acoustic emission measurements are carried out to characterize the performance of the transducer. Very good results are observed which indicate that the miniature conical transducer can be successfully used for measuring high-frequency dynamic surface motion.

Patterning, electroplating and removal of SU-8 moulds by excimer laser micromachining

The ablation characteristics of the SU-8 photoresist (spun on Siwafers) under 248 KrF excimer pulsed laser radiation have been studied.The variation of etch rate with fluence has been investigated in the range0.05-3.01 J cm-2. The threshold fluence for ablation of SU-8 ismeasured to be about 0.05 J cm-2. The etch rate of SU-8 is found tobe higher than that of polyimide (previously reported) under similarconditions. We have investigated the effects of different prebaketemperatures (90, 110, 120 and 200 °C) on ablation characteristics,which are found to be similar for all temperatures. The effect ofincreasing the number of laser shots (from 10 to 10 000) has beenexamined at different fluences in order to understand the etch-ratevariation near the `end of film' stage of ablation. The results of ouranalysis using scanning electron microscopy, profilometry and opticalmicroscopy reveal the very smooth morphology of the etched surfaces withno significant debris, no noticeable damage to underlying silicon and thegradual build-up of a carbonaceous film outside and around the etch pits.We find SU-8 very suitable for excimer ablation lithography and havedemonstrated this by patterning a gear structure in an SU-8 resist layerwith an aspect ratio of 4.5. For the first time, we have shown that thelaser micromachining technique has the potential to cleanly remove SU-8after electroplating a microstructure with copper.

Gratings in indium oxide film overlayers on ion-exchanged waveguides by excimer laser micromachining

Relief Bragg gratings were imprinted by 248 nm interferometric excimer laser ablation on potassium ion-exchanged channel waveguides in BK-7 glass overlaid with a thin high-index InOx film. Using five pulses of energy density 60 mJ/cm2, a spectral transmittance notch of depth 66% and ΔλFWHM<0.1 nm was obtained at 1547 nm in the TE polarization for a waveguide having a nominal width of 8 μm and a 135-nm-thick InOx overlayer. In waveguides coated with 100 nm InOx, with widths increasing from 3 to 8 μm, the reflection wavelength shifted by 0.12 nm/μm and the reflectivity increased monotonically.

A quality study on the excimer laser micromachining ofelectro-thermal-compliant micro devices

The objective of this research is to improve the quality of theKrF excimer laser micromachining of metal and silicon in fabricatingelectro-thermal-compliant (ETC) micro devices. The ETC devices combine theactuator and the mechanism into one monolithic compliant continuum andenable a range of mechanical manipulation tasks at micron scale. Anefficient method for optimizing the process parameters in lasermicromachining, using the orthogonal array-based experimental designmethod, is presented in this paper. The feed rate of the XY stage, thelaser pulse frequency, the discharge voltage and the number of passes wereused as the control parameters. The roughness of the machined edge wasused as the primary indicator of cutting performance. The roughness of theedges was computed automatically from the optical image of the machinedsamples. The heat-affected zone, kerf width and rate of cutting depth(depth per one pass) were used as additional quality indicators. Theorthogonal array method enabled the optimization of the control parametersby reducing the required number of experiments compared to the traditionalfull factorial experiment. Furthermore, machining in a liquid environmentimproved the quality and eliminated more debris and recast compared tomachining in the air.

Combination of contour and half-tone masks used in laser ablation

Excimer laser micro-machining has emerged as an important manufacturing process for micro-sized mechanical and optical components. Using the projection mask technique this work presents and evaluates a new type of mask capable of fabricating a variety of micro-structures with an improved surface quality. Details of the experimental procedure and the technical requirements are discussed and examples of machined surfaces are presented.

Development of Wide Bandgap Semiconductor Photonic Device Structures by Excimer Laser Micromachining

Excimer laser ablation rates of Si (111) and AlN films grown on Si (111) and r-plane sapphire substrates were determined. Linear dependence of ablation rate of Si (111) substrate, sapphire and AlN thin films were observed. Excimer laser micromachining of the AlN thin films on silicon (111) and SiC substrates were micromachined to fabricate a waveguide structure and a pixilated structure. This technique resulted in clean precise machining of AlN with high aspect ratios and straight walls.

AlN Acoustic Wave Sensors Using Excimer Laser Micromachining Techniques

ABSTRACTAluminum Nitride (AlN) is a promising piezoelectric material for Acoustic Wave (AW) sensor application due to its high acoustic velocity, linear thermal coefficient and high electromechanical coupling coefficient. Epitaxial AlN thin films were successfully grown on the Sapphire C plane substrate by Plasma Source Molecular Beam Epitaxy (PSMBE). Standard two-port resonator structures were fabricated on the AlN/Sapphire by standard photolithography. Excimer laser micromachining techniques were utilized to fabricate microgroove gratings on the surface of thin film. Beside Surface Acoustic Wave (SAW), Surface Transverse Wave (STW) propagation was also found on the devices with those microgroove and classical metal strip gratings. Laser micromachinined microgroove gratings were found to enhance the propagation of STW. Further, sensors based on STW showed little attenuation in the liquid environment, while sensors based on SAW suffered excess loss when exposed to liquid.

Laser micro-machining of small objects for high-energy laser experiments

Targets for high-energy laser experiments are polymer or metal objects of small dimensions. These objects need high-precision processes (typical sizes of a few hundred micrometers with micrometer accuracy). In this paper, some targets for high-energy laser experiments obtained by picosecond Q-switched Nd:YAG laser and KrF excimer laser micro-machining are presented. All of the targets use an original process. Activities include cutting, drilling, and marking elements. A summary of targets made using laser processes will be presented.

Development of Wide Bandgap Semiconductor Photonic Device Structures by Excimer Laser Micromachining

AbstractExcimer laser ablation rates of Si (111) and AlN films grown on Si (111) and r-plane sapphire substrates were determined. Linear dependence of ablation rate of Si (111) substrate, sapphire and AlN thin films were observed. Excimer laser micromachining of the AlN thin films on silicon (111) and SiC substrates were micromachined to fabricate a waveguide structure and a pixilated structure. This technique resulted in clean precise machining of AlN with high aspect ratios and straight walls.

Development of biofactory-on-a-chip technology using excimer laser micromachining

Multilayer microelectrode structures, with 10 m feature sizes, have been fabricated using excimer laser ablation techniques. These structures will be incorporated into devices for the electro-manipulation and characterization of cells, microorganisms and other particles. The AC electrokinetic phenomena of dielectrophoresis, electrorotation and travelling electric field effects are utilized, all of which are dependent on the dielectric properties of the bioparticles. Examples are presented of a travelling wave junction and selective particle trap to be incorporated into a prototype biofactory-on-a-chip device. A technique for profiling the edges of via-holes has been developed to facilitate robust electrical connections through polyimide films in these multilayer devices.

Excimer laser micromachining and replication of 3D optical surfaces

We developed a new technique for the fabrication of three dimensional microstructures at low costs and in small quantities based on KrF excimer laser micromachining and inverse replication. In contrast to other production schemes where some intermediate mold processes are necessary for the master generation, we utilized a specially prepared laser machined master directly for replication. Thus, a reduction in cost and time consumption can be achieved. With this technique, we produced microprisms with individual dimensions of 300 μm width and a height of up to 100 microns in an area of an overall size of 160 mm by 160 mm. It is possible to scale up and down the dimensions of the whole array as well as the individual structural elements both laterally and vertically.

Excimer laser micromachining for rapid fabrication of diffractive optical elements

We demonstrate the fabrication of binary, multilevel, and blazed diffractive structures by a fast and flexible direct-write process by using an excimer-laser-based tabletop micromachining workstation with an integrated optical surface profiler.

Combination of excimer laser micromachining and replication processes suited for large scale production

Results are reported obtained by a process sequence comprising excimer laser ablation of microstructures, inverse replication by an electroplating process and second inverse replication by injection moulding (“Laser-LIGA”). Thus, the original microstructures, made from PMMA and novolak layers applying a commercial 193 nm excimer laser micromachining work-station and showing structural variety and aspect ratios up to 10, were replicated with high structural fidelity by a low cost process suited for large scale production.

The application of excimer laser micromachining for the fabrication of disc microelectrodes

A method is presented for the fabrication of carbon ink microelectrodes in a composite membrane or thin laminate arrangement. The scheme utilises uv excimer laser photoablation of nanosecond pulse duration at wavelength 248nm to generate the required micro-hole stucture in 12 μm PET film. At laser fluences of the order of 1.0 mJ cm −2, film ablation rates were controlled to 0.4 μm per pulse without thermal deformation of the micro-hole. From a micro-hole template a thick film printing technique is used to produce carbon ink microelectrodes with an inlaid disc geometry. A series of microelectrode devices were constructed by this process with disc diameters ranging from 38 μm down to 7 μm with fabrication precision to 1 μm. The amperometric response of the film based microelectrodes were characterized using standard electrochemical measurements and examined by SEM and optical microscopy. Applications are proposed for the new device in the assembly of electrochemical transducers for environmental or clinical sensors.