The recent rapid advances in the field of laser ablation have led to an important new tool for both material synthesis and material processing. Experiments in our laboratory have served as examples of both types of contributions from this new technique. The kinetic differences of growth between standard solid state synthesis and in situ growth by laser ablation have allowed the substitution of Ce, Tb and Th for Y in Y1Ba2Cu3O7 for the first time. The effects of substitution for Y on the material Tc is discussed within the framework of the existing models for the Pr system. We suggest that localization of the holes by trapping may provide a suitable mechanism to account for the observed properties of the Pr and Ce substituted compounds. As an example of new material processing, polytetrafluoroethylene (PTFE) films were deposited by the laser ablation of a bulk PTFE target using the 4th harmonic, at 266 nm, of a Nd-YAG laser. The films are found to be stoichiometric with bulk- like optical properties. We propose a model which suggests that UV absorption onsets the pyrolitic decomposition of PTFE leading to monomer as the main ablation product. The monomer, ejected from the target at high velocity, subsequently re-polymerizes on the substrate.
Diamond-like-carbon (DLC) films have been grown on various substrates at low temperatures and low pressure by ablation of carbon particles using KrF excimer laser pulses of 30 ns duration. It is shown that the film properties strongly depend on the energy density of the incident laser beam and the deposition temperature. At energy densities above 8 J/cm2 and low substrate temperatures (< 200 degree(s)C) the coatings are transparent, while at lower energy densities or higher substrate temperatures only opaque films are obtained. The thin films were characterized by optical spectroscopy, x-ray diffraction, Raman scattering, and secondary electron microscopy. In addition to film growth and characterization, the kinetic energies and masses of laser ablated carbon ions have been investigated by time-of-flight spectroscopy. We observe an almost linear relation between kinetic particle energy and laser energy density, with maximum values as high as 220 eV at 23 J/cm2, indicating a strong correlation between laser energy density, particle energies and DLC film properties.
The popularity of Pulsed Laser Deposition (PLD) as a thin film growth technique has dramatically increased over the last six years and proponents of more mature physical vapor deposition processes are quick to point out the main 'draw-back' of PLD--the apparent difficulty in scaling up the process to areas compatible with commercial applications. To date the main focus of PLD has been on its application to the growth and characterization of epitaxial thin films of a variety of chemical compounds, as well as obtaining an understanding of plume dynamics, while relatively little attention has been paid to the issue of scale-up. However, three basic approaches dealing with scaling-up the thin film growth process of pulsed-laser deposition have been reported in the literature. These include rastering the laser beam over a large area target, translation of the substrate with respect to the ablation plume, and 'off-axis' deposition. Each of these approaches has produced quality thin films of the high temperature superconducting oxides as well as other materials over substrate sizes of at least two-inches in diameter. This paper will compare these three basic approaches, and discuss the advantages and draw backs of each. Issues such as film thickness and composition profiles, reproducibility, deposition rate, substrate heating, and target resurfacing will be addressed. Projections to further scale-up of this physical vapor deposition technique will be presented.
The ejection of sub-micron size particulates from a metal target as a result of the interaction of an excimer laser with a Pt target was investigated. To study the effect of laser fluence, particulates were collected on Pt films prepared on MgO(100) substrates over a fluence range of 0.83 - 3.3 J/cm2. Films were prepared at several substrate temperatures: 25 degree(s)C, 350 degree(s)C, 450 degree(s)C, and 550 degree(s)C, and under 0.05 Torr of an inert gas (Ar). It was found that, for a substrate temperature of 450 degree(s)C, the Pt particulates would stick easily to the growing Pt film with little evidence for deformation. The size distributions and areal densities of the metal particulates were measured from low magnification scanning electron micrographs of the thin film surfaces prepared at 450 degree(s)C and compared. In general, spherical particulates were produced with diameters ranging from about 0.05 to 1.0 micrometers . The shape of the size distribution of particulates on the film surface was roughly constant radially outward from the plume center, although the magnitude of the particulates decreased from the plume center. As the fluence increased, the mean diameter of the particulates increased slightly (from 0.25 to 0.35 micrometers ) while the number density of particulates decreased by over two orders of magnitude (from 1.04 X 105 to about 1.0 X 103 particulates/cm2 per angstrom of film deposited) near the plume center.
Pulsed laser deposition of thin films is a technology that has been explored in some detail. Because of the difficulty of monitoring in real time either the ablation process itself, or thin film growth, many studies have relied on diagnostics of either the ablated plume (emission, absorption, fluorescence) or the resulting films (Tc, Jc, X-ray, RBS) to infer information about the overall process. This indirect approach has provided some vital information for improving the production of high-temperature superconductors. In this study the plume dynamics during the in-situ pulsed laser deposition of YBa2Cu3O7-(delta ) thin films are investigated. The 248 and 308 nm lines of an excimer laser were used to generate a plume from a bulk YBa2Cu3O7-(delta ) target. Variations in the plume distribution as a function of processing gas, pressure, fluence, energy, and spot size were monitored by resulting film distribution and composition and time resolved emission imaging. Results indicate that the plume distribution can be controlled to some extent by the incident laser beam size; in addition, broadening increases with increasing oxygen pressure.
Increased resolution in optical imaging is desirable for a number of important applications, including advanced integrated circuit development. In sub-micrometer optical lithography, the wavelength of the exposing radiation is a main determinant of pattern resolution, given by the Rayleigh equation R equals k1 (DOT) lambda/NA. There is an upper limit on lens numerical aperture imposed by optical design criteria; k-factors are also restricted by the physical limits of photosensitive material chemistry and practical limits of production processes; reducing the exposing wavelength is left as a logical pathway for achieving increased pattern resolution. This paper presents a novel imaging system for the use of 248 nm and 193 nm lithography. The system is designed to characterize resist materials used in advanced memory (64 Mb, 256 Mb) and high density bipolar IC manufacturing. The imaging system and its optics will be described along with process conditions used to pattern deep-UV sensitive photoresists. SEM photos of imaged wafers will be presented, and methods to further improve deep-UV pattern resolution will be discussed.
This paper is a description of the ablation of polyimide with an excimer laser ablation tool in a manufacturing line. The light source is a 150 watt, XeCl (308 nm) gas laser. Two methods of ablating via patterns with the laser tool (full-chip and scan modes) will be discussed, in addition to the tool, set-up, operation and tool/process control.
Photolithography technology is moving towards deep UV wavelengths in order to obtain smaller feature sizes on integrated circuits. Optical glasses are being replaced with fused silica because of the DUV application. There are concerns that the short pulse, high instantaneous power generated with excimer lasers can cause structural changes in the fused silica materials. The damage includes: changing absorption, observed fluorescence, and induced stress birefringence. This paper will address the laser damage testing protocol for fused silica. We will discuss the setup, equipment, and procedures for laser damaging. Precision measurements of the absorption, fluorescence, and stress birefringence are required to have a good understanding of the laser damage. The paper will also address the equipment problems associated with these measurements and initial comparisons between 193 nm and 248 nm laser damage.
Synthetic UV-grade fused silica, crystalline fluorides, and dielectric coatings have been evaluated for transparency and durability at 193 nm. Most bulk materials eventually develop color centers, and fused silica also changes its density and index of refraction. However, the rate at which these changes occur and their magnitude vary strongly with material, grade, and other more subtle details. Careful selection and possibly pretesting are recommended, in order to ensure optimal matching between the intended application and the material properties.
Recent advances in the purification and growth of high purity alkaline earth fluorides, particularly CaF2, are increasing the damage threshold and longevity of transmissive optics for use with excimer lasers. A general overview of the properties and performance of these high purity materials is presented.
Excimer laser micromachining applications have gained more and more interest in fabrication of commercial microstructure products (e.g. multi chip modules, printed circuit boards, excimer laser stripped wires and others). The unique properties of the pulsed excimer laser radiation are the deep UV wavelength and the high peak power which result in the special material interaction for high precision micromachining. The key elements in taking full advantage of the excimer laser beam and in providing reproducible and controlled micromachining is the illumination and imaging optics. Detailed investigations were performed to understand microstructure formation (e.g. wall angle and microstructure position accuracy) in regard to illumination and imaging optics parameters. Qualitative and quantitative relations were found to explain microstructure feature performance in regard to optical and processing parameters.
The affect of varying a wide range of excimer laser parameters on the average etch rate per shot of aerospace alloys (Al, Ti and Ni) has been investigated. The parameters found to most profoundly influence the etch rate were, the laser fluence (up to 70 J/cm2), pulse length (20 - 160 nsec FWHM), gas environment, beam spot size (35 - 300 micrometers ) and material thickness (0.4 - 1.8 mm). Optimization of these parameters has produced an increase in average etch rate per shot from 0.05 to 1.5 micrometers with Ti alloy (2 TA - 10). Such increases in etch rate are seen to occur above a relatively well defined 'critical' fluence for thick samples which it is postulated corresponds to the transition from a largely vaporization dominated to a vaporization/melt expulsion regime. Information is also included on the quality of the processing and on the extent of the laser affected zone around the processed area. Potential aerospace application areas identified and discussed include drilling multiple hole arrays for producing porous surfaces for drag reduction on aircraft and the cutting and profiling of alloy/glass fiber composites (GLARE).
Small, high-repetition-rate waveguide excimer lasers enable new approaches to pattern and shape generation by ultraviolet photoablation. Using a tabletop microfabrication system that integrates a waveguide excimer laser with vision and motion control systems, we have generated three dimensional structures through direct-write photoablation of polyimide substrates.
Novel uses of excimer lasers for fabricating products such as biomedical probes and sensors, fenestrated contact lenses and microelectrode sensor arrays are described. With the suppliers of these products various types of excimer laser processing techniques have been developed-- from relatively straightforward micromachining of polymers to surface modification methods and holographic recording. Results that highlight the performances of various products are presented and the excimer laser methods employed in their production are discussed.
In our work with excimer lasers, we observe a complex depth profile in processed blind holes that is energy, material, and process independent. This feature occurs when processing ceramics, polyimide, semiconductors, and metals. Further it occurs under different optical configurations. An understanding of this phenomena is essential when micron-precision features are required. We present our findings and explore explanations.
Several new applications of excimer lasers in industry and medicine have increased the requirements for control of excimer laser beam parameters. Some applications require precise control of spatial uniformity, pulse-to-pulse amplitude variations and temporal profiles. The spatial and temporal characteristics of excimer laser beams are affected by small changes in laser gas composition. The role of gas composition, as well as methods for controlling gas composition, are discussed in relation to critical beam parameters.
This paper describes recent results achieved in the development of discharge pumped excimer lasers at the Textron Defense Systems organization (formerly the Avco Research Laboratory). Included is a description of a KrF laser with more than one Joule output at 2.4% efficiency, a 200 mJ XeCl laser operating with a 500 nsec wide pulse, and a several Joule, discharge pumped, KrCl laser operating at 222 nm. All of these devices are switched using thyratrons and are therefore capable of repetitive performance. The KrF and XeCl experiments were conducted with the same laser device operating with a conventional capacitor transfer excitation circuit for the KrF experiments but modified to operate with a pulser-sustainer discharge circuit using magnetic switching for the XeCl tests. The KrCl device is a 40 liter volume system built by Northrop and also operates with a magnetically switched discharge.
Excimer lasers were commercialized in the late 1970's. The laser community thought that by the early 1980's these UV lasers would enjoy a fruitful industrial market position. CO2 and solid state lasers required almost two decades to be fully accepted as industrial machine while the excimer laser was expected to be a fast learner benefiting from the learning curve of its big brothers. In retrospect, early excimer lasers had a bad reputation for being complicated, expensive and frequently out of commission. By the late 1980's a few excimer laser manufacturers had engineered the problems to acceptable levels for successful pilot lines and small scale manufacturing to begin. At this time, the real industrial learning curves began as engineers worked to refine many subsystems and support technologies. Today, excimer lasers are being used as true industrial lasers. They have a bright future with numerous and diverse market opportunities. This paper is an overview of the technologies proven to be successful in adapting modern excimer lasers to successful full production situations.
In this experimental work, a new and very compact home made XeCl laser has been used for the ion beam generation by metallic targets. Multiply-charge heavy ion pulses have been extracted by the plasma produced by a focused laser beam at relatively low flux (approximately 30 MW/cm2) on Si, Ge, Mg and Zn targets. An output peak current of Si3+ ions of 375 mA has been recorded at an acceleration voltage of 200 V only. The insertion of a variable capacitance between the target holder and the acceleration electrode allowed a self-bunching of the ion beam. Besides, a peak current of 1.4 A of Pb3+ ions was obtained by increasing the laser flux to 86 MW/cm2 and the acceleration voltage to 500 V.
To study parameters of the diode electron gun with the current 80 - 100 kA (current density equals 1000 A/cm2), voltage 0,5 MeV, and pulse length 40 - 80 ns varied cathode materials was undertaken. Vacuum diode cathodes made of stainless steel, carbonic composite material and isotropic pyrographite are active in well-known explosion-emission regime. The cathode made of high-density pyrographite is active during all pulse-time in self-emission regime due to microspaces cooling with material anisotropic thermal and electrical conductivity.
The results of experimental investigation of factors which restrict the gas lifetime in pulsed discharge KrF laser have been presented. The correlation in between gas mixture lifetime and constant rate of fluorine concentration decrease has been observed. It has been determined that the main canal of fluorine concentration decrease is the reaction of formation of metal fluorides of electrodes. Compositions of gas phase impurities of laser mixture which are formed during the laser operating and their influence on the characteristics of laser emission have been ascertained. An absence of appreciable influence of absorption and scattering of laser emission by impurities has been revealed. The mechanism of influence of solid-state impurities on lasing energy has been suggested.
The review of known methods of application excimer lasers in the stomatology is presented and the results of investigation of possibility of KrF laser application in the stomatology are given. The versions of construction of medical stomatologic laser which includes the different types of gasifiers and purifiers are presented.
Efficient line-narrowing at 308, 248 and 193 nm is reported using intracavity etalons in commercial excimer lasers. With a single etalon the linewidth is reduced by a factor of X10 - 20 at each wavelength. The line-narrowing efficiency can then be as high as 60 - 75% of the broadband output and single pulse energies in the range of 200 - 300 mJ can be produced within a linewidth of approximately 20 pm. With two intracavity etalons the linewidth is restricted a further factor of approximately X10 with line-narrowing efficiencies of 15 - 25%. At all wavelengths single pulse energies of 60 - 100 mJ could readily be produced within 2 - 3 pm. Using an laser spectrometer with a 1-D diode array readout and PC interface, the wavelength of such a line-narrowed KrF laser has been actively locked to the stable line output from a HeNe laser.