Elizabeth K. Illy

Enhanced polymer ablation rates using high-repetition-rate ultraviolet lasers

Etch rates (/spl mu/m/pulse) for glycol-modified polyethylene terephthalate (PETG) under pulsed UV (255 nm) laser processing are measured as a function of pulse repetition frequency in the range 0.7-15 kHz. Materials removal rates (/spl mu/m/s) scale approximately linearly with pulse repetition frequency at a fluence of 0.59 J/cm/sup 2/, and there appears to be no attenuation of the ablating laser beam by the ejected material plume for pulse rates up to 15 kHz. The instantaneous etch rate for pulses in a sequence increases markedly (/spl sim/40%) for long pulse sequences (>100 pulses) at high PRF (15 kHz), an effect which can be used to increase machining rates while operating at a moderate laser fluence.

High-speed UV micro-machining of polymers with frequency-doubled copper vapor lasers

We report high-speed ultra-violet (UV) micro machining of polymers using a high beam quality frequency-doubled copper vapor laser (CTL). The characteristics of the UV laser source and beam delivery optics are described, Measurements of material removal rates have been made for a number of polymers. Results for UV-CVL micro-drilling of PETG and polyimide (kapton) show removal rates per pulse similar to other UV sources, but the high pulse repetition frequency of the UV-CVL results in much enhanced temporal material removal rates. >

High throughput scribing for the manufacture of LED components

Lasers are an important tool in the fabrication of photonic components and in particular their use in scribing for separating LED dies on sapphire substrates. This paper describes scribing and cutting of sapphire and GaN using UV lasers (355nm and 266nm harmonics of Nd:YVO4 and 255nm harmonic of CVL). Scribing of sapphire at speed of 30mm/s have been achieved and cutting of sapphire of up to 700 microns thickness has been demonstrated.

Optimization of trepanning strategies for micromachining of polymers with high-pulse-rate UV lasers

We have previously shown that the material removal rate scales linearly with pulse rate up to 15 kHz for pulsed UV-laser ablation of polymers, giving the potential for substantial gains in processing speeds in ablative micromachining using high-pulse-rate UV sources such as frequency-doubled copper vapor lasers and frequency-quadrupled diode-pumped solid-state lasers. These rapid processing speeds can be effectively utilized in direct-write UV-laser micromachining including trepanning. In this paper we present studies of machining rates for trepanning of a strongly absorbing polymer (PETG), and a weakly absorbing polymer (PMMA), aimed at establishing optimum conditions of pulse rate, linear write speed (laser spot overlap) and laser fluence for maximum machining rates and high quality of the machined structure using a high-pulse- rate (5 kHz) UV-CVL. For fixed fluence and pulse rate, machining rates for PETG are found to be independent of write speed in trepanning, however for PMMA machining rates increase for decreasing write speed (increasing laser spot overlap) where cumulative heating leads to enhanced dynamic etch rates. In the latter case, while reduced machining times can be achieved for high spot overlaps, this is generally at the expense of significant degradation in finish quality of the machined structure.

Laser processing of microfluidic components and bioMEMS

Micro-processing with lasers is an enabling technology for microfluidics and bioMEMS due to the high precision, accuracy and resolution achievable using laser sources. Visible micromachining of metals and ceramics has proved an efficient and precise way of producing spotting pins for high-throughput screening applications. UV micromachining of the polymer materials often used in metered dose inhalers (MDI) and other medical devices is efficient due to the high absorption properties of these polymers. Key to these types of devices is the accuracy and reproducibility. This paper describes the use of copper vapour lasers to produce micro-fluidic and bioMEMS type components.

Through-the-glass, double-sided laser crystallisation using copper vapour lasers for the production of thin film silicon material

Through-the-glass laser crystallisation of a-Si, on low-temperature glass, has been achieved for the first time using a copper vapour laser (CVL). The CVLs 578/511 nm output has minimal absorption in the substrate, thus allowing a simple double-sided irradiation regime. Raman spectroscopy showed that double-sided irradiation is more effective at producing full depth crystallisation, and incrementally increases the crystallisation depth with each pulse. A step-wise crystallisation concept is also introduced to explain the incremental crystallisation behaviour. Additionally, grain sizes were maintained without the need for substrate heating. These factors enhance the CVLs potential to simplify producing PV materials via laser crystallisation.

High-speed microdrilling of polymers using high PRF UV lasers

Correspondingly the different penetration depths before the experimental appearance of bulk damage above fluence threshold in Fig. 1 can be explained by the different evolution of the beams inside the transparent material as a result of self-focusing. The formation of filaments on the basis of the calculation for Fig. 2 is presented in Figs. 3a-3c where the transverse profile of the beams at the surface entrance and after two propagation distances (depth = 0.65 mm and 0.79 mm) is plotted. For comparison, a typical example ofbulk modification is presented in Fig. 3d for sapphire, experimentally demonstrating a stronger effect of beam filamentation than in a-SiO,. Laser processing was conducted with 50 shots, at an single input fluence of 1.5 J/cm2 and a pulse width of 7 = 1.1 ps. An arrangement of four to five sub-pm sized alterations of the material can be observed around the larger centered structure, verifymg the theoretical representation in Fig. 3c. We have presented results on laser-induced bulk modifications in transparent materials. The results can be explained by selffocusing that is due to Kerr effect and ionization. The narrowed beam of an ultrashort laser pulse can break up into several filaments in the bulk material. Because of the counteracting selfdefocusing by the free electrons, however, a single filament can even stabilize under certain conditions. This could establish the possibility of controlled and reproducible microstructuring of transparent materials, even into the submicron region.

UV Micromachining Using Copper Vapour Lasers

UV output powers in excess of IW at 255nm can readily be generated by SHG from single, medium-scale copper vapour lasers. The high PRF and high beam quality makes the frequency-doubled CVL an ideal source for many UV micromachining applications such as drilling micro-orifices in polymers at high production rates.

High-speed micromachining with UV-copper vapor lasers

Ablation rate characteristics (etch rates) are presented for micro-machining of polyimide (kapton) and PETG using a frequency doubled Copper Vapor Laser (uv-CVL) at 255 nm and a frequency quadrupled Nd:YLF (4*Nd:YLF) laser at 261 nm. A comparison is made of the etch rates obtained by continuous ablation at 4.25 kHz with the uv-CVL with rates obtained for bursts of pulses with 2 second intervals between bursts. These results suggest that the observed decrease in ablation depth per pulse after a number of pulses for fluences above 0.6 J/cm2 is due to attenuation of succeeding laser pulses by the plume of previously-ejected material. Preliminary results of the effect of sample temperature on ablation rates are also presented.

Effects of high pulse repetition frequency on the ablation of polymers

Measurement of ablation depths and etch rates are presented for PETG and polyimide (kapton) under pulsed UV(255nm) laser processing for a pulse repetition frequency (prf) range of 750Hz-15kHz using a frequency doubled copper vapour laser and a fixed fluence of 0.59 J/cm2. Results show that the material removal rate (μm/sec) scales approximately linearly with pulse repetition frequency, and there appears to be no attenuation of the ablating laser beam by the ejected material plume for this fluence. The etch rate (μm/pulse) increases slightly (~20%) for high prf (15kHz), an effect which is attributed to cumulative heating of the sample.Measurement of ablation depths and etch rates are presented for PETG and polyimide (kapton) under pulsed UV(255nm) laser processing for a pulse repetition frequency (prf) range of 750Hz-15kHz using a frequency doubled copper vapour laser and a fixed fluence of 0.59 J/cm2. Results show that the material removal rate (μm/sec) scales approximately linearly with pulse repetition frequency, and there appears to be no attenuation of the ablating laser beam by the ejected material plume for this fluence. The etch rate (μm/pulse) increases slightly (~20%) for high prf (15kHz), an effect which is attributed to cumulative heating of the sample.