During recent years, excimer lasers become to be widely used for real application due to the improvement of quality and reliability. In comparison with $CO_2$ and Nd:YAG laser, the history of excimer laser application is not so long, but nowadays industrial and scientific fields where excimer laser is effectively adopted increase drastically because they can provide material processing with low thermal damage and high accuracy due to high peak power output in short pulse at several ultraviolet wavelength. The short pulse width and high beam intensity means that excimer laser can induce heating, melting, and vaporization of material on a time scale of nanoseconds to microseconds.
Material processing by excimer laser is usually limited to the polymer, thin metal film, ceramic ablation, and laser assisted etching of semiconductors. Excimer lase processing of thin metal film has drawn great attention from many researchers in microelectronics and micromechanics for the application such as circuit patterning, mask manufacturing, and repairing.
Excimer laser ablation of thin metal films with various wavelengths, fluences, and substrates has been investigated, and three physical mechanisms describing removal process have been proposed to explain the observational results of laser irradiated surface morphology. Proposed removal mechanisms are pure vaporization from the surface of thin metal film, explosive removal due to the high gas pressure built up at the thin metal film-substrate interface, and two phase removal by the sequence of surface vaporization and liquid expulsion. Two phase removal mechanism is proposed by the researchers who investigate the hydrodynamic motion of the molten metal during the ablation process of thin metal films using high speed camera technique reflectance and scattering diagnostics of prove laser beam for the measurement of melting duration. In these efforts, energy balance equation, analytic solution, and one-dimensional heat conduction equation have been used to calculate the metal film temperature and to predict the ablation rate. Although many scientists have studied the excimer laser ablation of metal film ablation, the researches have been mostly concentrated on the investigation of the ablation mechanism. The researches to investigate the removal quality and the optimal processing conditions have been done by mostly Japanese scientists. They have investigated the removal quality and the optimal laser fluence conditions for Cu films deposited on PMMA with a KrF excimer laser, and have investigated the removal quality for Cr films deposited on quartz with second harmonic of a Nd:YAG laser.
In the eighties, many researches have been concentrated to the understanding of film removal mechanism. Although metal film cracking and peeling off phenomena has been observed in the excimer laser removal experiments of several combinations of metal film and substrate, researchers have not been concerned about the damage generation mechanism. They have simply understood it as a result of excessive thermal expansion or pressure build up at the film-substrate interface during laser heating.
In the nineties, systematic investigations of single shot excimer laser damage of W and Cr films of varying thickness on transparent substrates have been done to find the threshold fluences for visible damage and understand the film cracking and peeling off mechanism. Matthias have investigated the surface morphologies of single shot irradiation spots on 200㎚ Cr films and reported that Cr film showed a distinctly different reaction to the laser pulses which is governed by film fracture. Toth found that solid phase ablation was dominant for W while Cr was ablated in form of vapor and small molten droplets, and they observed that at early stage of ablation Cr films deforms in a wave structure by mechanical tension and finally separated from the substrates. They confirmed the removal of solid fragments from W and Cr films, which is indicative of prior fracturing. Siegel showed that brittle Cr films rupture before melting and reported that the main cause of film fracture is the brittle to ductile phase transition and which must be included in the energy balance to estimate the ablation thresholds accurately. Actually, metals with fcc structure are not expected to show brittle to ductile phase transition and, indeed, we never observed any fracture for the laser irradiated Cu, Au, and Ni films. In these efforts, they related the film fracture to the stress builds up in the metal film by thermal expansion during laser irradiation and estimated the fractional linear expansion of the film as a function of fluence with a simple equation. But the stress and strain distribution induced by laser irradiation is not simple because the thermal expansion of spot irradiated by laser is restricted by the surrounding part of film.
In the first half of this work, excimer laser ablation of Cr films on glass substrates is studied at a wide range of fluence and 0.1~0.3㎚ of film thickness with the purpose of understanding the ablation mechanism and seeking the optimal processing condition for pattering. One-dimensional heat flow calculation incorporating melting and vaporization is carried out, temperature dependent thermo-physical properties are taken into the calculation. Reflectivity of, Cr films, temporal and spatial intensity distribution of laser pulses is determined experimentally. Surface morphology of the ablated film is investigated with scanning electron microscopy. Ablation rate measured by the energy dispersive X-ray spectrometry of SEM is compared with the numerical predictions. Film damage characterized by
cracks and peel off, melting, vaporization, and melt expulsion is explained on the basis of surface morphology. Optimal processing conditions are finally established after examining several factors determining removal quality.
The measured single-shot ablation rate is found to be about two times of the result of numerical analysis based on a surface vaporization model and heat conduction theory. Surface morphology examination indicates that the Cr film is removed by the sequence of cracking-melting-surface vaporization-melt expulsion by plasma recoil, and that the outmost ripple of diffraction pattern gives a strong effect on the morphology of molten Cr during melting and vaporization process. Although removal quality is divided into film damage, partial removal, excellent removal, and glass damage, EDXS shows that about 14% of Cr remain even at the excellent removal condition. Optimal fluence range which gives excellent removal, good precision of pattern size, and few droplets and cracks, is so narrow that the process conditions for pattering should be established carefully. From this study, it is found that thinner films are well ablated by excimer lasers.
In the second half of this work, single-shot laser damage of thin Cr films on glass substrates has been studied to understand the cracking and peeling off mechanism. A mathematical model was developed for the calculation of heat transfer and thermal stresses in laser heating of Cr films deposited on glass substrates, the transient temperature and stress-strain fields were analyzed by using a three dimensional finite element model. The finite element program ABAQUS, together with user subroutines, was adopted to perform the numerical analysis. KrF excimer laser was used in experiments as a source of UV radiation. Morphological inspection of damaged Cr films was done by using SEM and the threshold fluences for visible damage were investigated for various film thicknesses. According to the numerical analysis for the experimentally determined cracking and peeling off conditions, cracking is found to be the result of tensile brittle fracture due to the excessive thermal stresses formed during the cooling process, while peeling off is found to be the combined result of film bulging from the softened glass surface at higher temperature and tensile brittle fracture cooling process.
