Investigations are being carried out to improve the quality of laser micromachining of glass and semiconductor materials and to achieve submicron finished tolerances. Experiments have been carried out mainly with wavelengths ranging from 248 to 800 nm and pulse lengths of 130 to 400 fs. Comparisons are also being made with machining using 10 ns excimer laser pulses. Laser ablation thresholds, incubation coefficients and ablation rates are measured using single and multiple shot irradiation over a range of incident fluences with well controlled gaussian beams. New techniques for debris removal and crack minimization are being investigated. One technique for debris removal uses a sacrificial thin film of sputtered tungsten on top of the substrate before micromachining. After ablation, the tungsten film and deposited debris may be etched away with hydrogen peroxide. This technique has shown promising results in leaving a much cleaner surface. In order to reduce the amount of cracking of the substrate during laser drilling of glass, we have also been investigating the use of preheated substrates. By raising the temperature of glass before drilling, the sample is more ductile and less prone to cracking.
Laser micromachining may be used for a variety of applications including drilling holes or creating trenches in dielectric materials. Cracking around the ablated features can be a significant problem for many applications, particularly when micromachining glass. One possible method for crack reduction, investigated here, involves heating of the substrate during ablation. This leads to a more ductile material that is more able to withstand the thermal shock of the ablation process. In order to increase the ductility, the glass targets are heated by physical contact with an electric heating element. The results of micromachining are analyzed using an optical microscope. The amount of cracking is quantified in terms of the number of visible radial cracks. For nanosecond micromachining, a reduction in the number of cracks and an improvement in the quality of the holes are observed as the glass is heated. The relative improvement using heated substrates and nanosecond pulses is also compared to femtosecond ablation of room temperature substrates.
Laser micromachining is a flexible technique for precision patterning of surfaces in microelectronics, microelectromechanical
devices and integrated optical devices. Typical applications include drilling of holes, cutting of conducting
lines or shaping of micro component surfaces. The resolution, edge finish and residual damage to the surrounding and
underlying structures depend on a variety of parameters including laser energy, intensity, pulse width and wavelength.
Femtosecond pulses are of particular interest because the limited time of interaction limits the lateral expansion of the plasma
and the inward propagation of the heat front. Thus, very small spot size can be achieved and minimal heating and damage of
underlying layers can be obtained. An additional advantage of femtosecond pulses is that multiphoton absorption leads to
efficient coupling of energy to many materials independent of the linear reflectivity of the surface. Thus metals and
transmitting dielectrics, which are difficult to micromachine, may be machined with such pulses. The coupling is improved
further by employing ultraviolet wavelength laser pulses where the linear absorption typically is much higher than for visible
and infrared laser pulses. To explore these advantages, we have initiated a study of the interaction of 250nm femtosecond
laser pulses with metals. The laser pulses are obtained by generating the third harmonic from a femtosecond Ti:sapphire laser
operating at 750nm. The pulses are focused to various intensities in the range of 1010Wcm2 to 1015 Wcm2 using reflective
and refractive microscope objectives and ablation thresholds and ablation rates have been determined for a few metals. In
addition the ability to control feature size and produce submicron holes and lines have been investigated. The results are
presented and compared to results obtained using infrared and visible femtosecond laser pulses.
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