The objectives of reactive chemical and nonreactive thermal processing with laser radiation are outlined giving indication that processing with laser radiation is governed by a hierarchy of time constants originating from photon-matter interaction, phase transition dynamics, laser source excitation fluctuations, and optical feedback in combination with the influence of beam delivery systems, processing/shielding gas flow configurations, robotics, production lines and environment. The minimization of losses by heat flow, reflection and transmission and the stringent need for quality assurance require as first approach the control of processing, which is mainly due to the capability of laser radiation source. The current status of laser radiation sources is reported giving information on the state of the art of processing with laser radiation in combination with subsequent demonstration of future trends and developments with respect to radiation sources, beam delivery, beam shaping, materials, processing and quality assurance.
Laser cutting experiments were studied using an industrial laser cutting system including a 1 kW CO2- laser and a two axis CNC cutting table. Oxygen assisted cutting was carried out with a laser beam in fundamental mode focussed by a 5 inch ZnSe lens. The materials investigated include deep drawing steels, mild steels and stainless steels. The influence of beam polarization and cutting mode (cw, pulsed and superpulsed) on the cut quality and the development of the heat affected zone is demonstrated. Lateral accuracy, microgeometry of the cut and mechanical properties of cut samples are reported as function of process parameters. As was shown earlier, laser cut samples exhibit finished product quality and mechanical properties that fulfill the requirements of DIN and ASTM standard values. Examples of 3D cutting applications for automobile industry are given.
For high precision fabrication of fine structures and for making fine surface textures - a predefined finishing roughness on surfaces - several laser processing techniques are developed at TNO. The laser micro milling technique is applicable to make blind holes, grooves and other programmed shapes. It is a self-adjusting technique applicable e.g. in the manufacturing process of high precision parts for the tool-making industry. The removal of material is achieved by local and repeatedly evaporation of small quantities of material from the workpiece-surface. The ablation rate or milling rate is determined by the laser fluence, the powerdensity and the way the process is programmed. For circular and rectangular shapes recipes are developed, which can be combined for other shapes. A patent on this work, which is carried out in a Brite program* is in preparation. An application of the laser surface texturing technique, by which a variety of finishing roughness reproducable can be achieved, is the making of predefined surface textures on metal moulds for duplicating the texture on plastics products. A laser surfacing technique, which results in glazy layers on metal surfaces, with interesting properties will also be reported. The processes described require lasers and deflection systems which parameters are of good repeatability and constancy; such equipment is available on the market. For the reported work a Nd-YAG laser processing machine is applied while short pulses with a high peak power and high powerdensities on the workpieces surface are required. The technique described can be used for both metallic and ceramic materials.
The minimum laser pulse energy necessary to obtain a sufficient penetration depth of the welding seam has been determined as a function of pulsewidth for a variety of sheet metals. Thermophysical and optical properties like thermal conductivity, thermal diffusivity, melting temperature, latent heat, and reflectivity are suitable parameters to explain measurement results. Strength tests lead to possible ranges for suitable pulse energies, pulsewidths and pulse overlaps within adjustment can be chosen under the viewpoint of economy of operation.
Miniature fuses are provided with end wires, so called pigtails, for mounting purposes. The process of laser soldering of the pigtails directly on the endcaps of the finished fuses is developed. It is demonstrated that this process is feasible for production purpose. The process has been patented.
The process of laser soldering is influenced by different phenomena like melt dynamics of the solder, wetting of the solder joint and others. In order to gain a detailed description of process evolution the soldering process is observed by high speed photography. The results are compared with signal evolution of the surface temperature and laser power reflected from the solder joint. These signals can be used for process control.
The diagnostic device introduced in this paper can be used to determine the distribution of the intensity in the unfocussed and focussed laser beam, and, on the basis of these data, to calculate the beam diameter and the maximum intensity. The temporal behavior of laser pulses can be recorded and the pulse energy and the average power of the pulses can be determined. The most significant characteristics of the presented diagnostic device are the external triggering up to 1 kHz by the repetition rate of a pulsed laser as well as the automatic adaption of the diagnostic system to the laser pulse energy.
Laser assisted direct writing for in situ etching and deposition may find applications in integrated circuit repair or adjustment. Recent work, performed by this team, on basic processes for deposition of metals and insulators and for etching materials currently in use in microelectronics, will be reviewed. This paper is to describe the last developments of an IC repair prototype, designed to draw 2 micron wide lines on Silicon using a focused CW Argon ion laser beam. As deposition mechanism is mainly pyrolytic, deposit shapes have been evaluated starting from experimental deposition kinetics and 3D finite element temperature distribution computation. General concepts of the machine to meet the conflicting requirements of obtaining a micron size laser spot over a full 150 mm Si wafer enclosed in a controlled atmosphere will be presented. Possible processes will be outlined and applications reviewed.
An argon-ion laser with the visible cw radiation was used to deposit highly doped p-type silicon stripes on (1102) sapphire substrates covered by a 1 μm thick epitaxial silicon layer (SOS). The resistivity of the stripes deposited in an atmosphere of silane with 1% diborane was 0.5 to 2.5 mΩcm. The laser-induced pyrolytic deposition of silicon on SOS was analyzed using numerical simulations of the temperature distribution caused by a focused laser beam. Comparisons were made between experimental and calculated cross-sections of deposited stripes. The measurements of piezoresistivity were carried out by controlled bending of samples of rectangular shape upon which the stripes had been deposited. The stripes deposited along various directions on the substrate revealed anisotropic behaviour in their piezoresistivity properties. The results show that the stripes tend to adopt the same epitaxial crystalline orientation as the SOS layer. The longitudinal gauge factor along the  direction of silicon, which direction is normally used in strain sensor applications, varied between 15 and 44 depending on the laser power applied. A local temperature near the melting point of silicon seems to be needed to assure the epitaxial growth.
A process is described to selectively deposit metal tracks from ammoniacal metal salt solutions. The substrate is locally heated using a 5W CW Ar+ laser. At the irradiated area, the NH3 is thermally decomposed. The liberated electrons reduce the metal ions to the corresponding metal. Simultaneously, the focused laser beam induces local boiling phenomena, leading to a stationary jet flow with extremely efficient replenishment of reactants and correspondingly high deposition rates. In this way, Pd tracks 20 μm wide and 10 μm high have been grown at a lateral rate of 500μm s-1. At present, the process is used to rewire integrated circuits, to facilitate testing and failure analysis.
The Laser Induced Synthesis (L.I.S.) of CuInTe2 has been investigated in supported and free-standing films. The laser fluence and the speed of the laser scan for the CuinTe, formation have been determined. The morphology, cristallography and optical absorption of the obtained films were investigated. These studies reveal that the compound was formed in fine grains. The T.E.M. and x-ray studies show that the films are homogeneous and do not present any secondary phases or oxides. This confirms that L.I.S. is a good technique to prepare thin films of ternary compounds.
Laser induced pyrolytic deposition of platinum spots on glass is studied for different laser powers, beam diameters and irradiation time. Laser powers up to 500 mW give deposition rates of typically 0.5 μm/s. Temperature calculations as a function of the time allows study of the process kinetics according to the thickness and diameter of the spots. Resistivity measurements of platinum lines give comparable values to those of bulk material.
Photochemical deposition has attracted widespread interest as a technique for area selective, low temperature deposition of electronic materials. More recently, considerable attention has been focussed on the growth of compound semiconductors, and this paper will describe work which has been carried out on laser induced deposition of gallium arsenide. Stoichiometric gallium arsenide has been successfully deposited on gallium arsenide substrates using pulsed uv radiation from an excimer laser to decompose trimethyl gallium and arsine mixtures. The deposition was carried out in a modified MOCVD (Metallo-organic chemical vapour deposition) reactor using pressures of between 50-70 torr and substrate temperatures of 300-400°C. Deposition has been obtained at two laser wavelengths, 193 nm and 248 nm, with the longer wavelength yielding superior material quality and improved area selectivity. Initially, the thickness of the deposit was limited by the accumulation of an opaque deposit on the reactor cell window which attenuated the laser beam. However, modification of the cell eliminated this problem, and, as a result, it has been possible to grow layers as thick as 2μm. This has enabled a preliminary assessment of the epitaxy of the deposited layers to be made.
A systematic study on ruby laser processing of supported Ge/Se bilayer structures is reported. Depending on the sequence of the initial layers and processing parameters compound synthesis, total or partial ablation of one of the constituents or the compound formed and simultaneous transfer of the ablated material onto a separate substrate in close proximity is possible with one single laser pulse. The results establish a novel single step technique for local deposition of compound films with lateral dimensions down to the micrometer range from a multilayer structure on a transparent support as a source.
First results obtained by laser-induced chemical vapour deposition of hydrogenated amorphous silicon films are reported. Processing conditions are determined for production of films suitable to be used in photovoltaic applications.
Photoablation of Polystyrene (PS) and Polymethylmethacrylate (PMMA) by 193 nm radiation of an excimer laser was studied in two types of experiments. 1) Time resolved absorption measurements show that the ablation of PS starts within the first few nanoseconds during the laser pulse. 2) By laser-mass-spectrometry of the ablated neutral species their chemical nature and velocity distributions were measured. At low fluences, monomer molecules are the main product, at high fluences many smaller fragments are observed. The velocity distributions of the molecules are similar to that of adiabatic gas expansions. For comparison experiments with condensed CH3I were performed. Extremely sensitive detection of CH3I, CH3 and I by resonant multiphoton ionization combined with time-of-flight (TOF) mass-spectrometry allows to distinguish CH3 radicals from "direct" dissociative UV-photodesorption, "thermalized" dissociation products and thermal desorption of parent molecules at low fluences. The more efficient, "explosive" photoablation is observed at fluences above 1,5 mJ/cm2.
A new method has been developed to produce conducting materials by surface chemical processing of a chlorinated polymer assisted by a CW laser. Polyvinylchloride (PVC) was first functionalized in order to absorb the 488.1 nm emission line of an argon ion laser. A short exposure of the polymer film to a 1 W laser beam, in the presence of air, leads to a complete removal of the chlorine and hydrogen atoms from the PVC backbone, with formation of a purely carbon material. The latter was shown to contain mainly graphite-type structures which are able to carry electrons, the electrical conductivity of the final product being comparable to that of graphite. Because of the high threshold energy of this solid state reaction, conventional radiation proved unable to induce the graphitization of PVC that occured within milliseconds under laser exposure, thus illustrating the unique possibilities of lasers for performing specific chemical reactions. By focusing the laser beam down into the micronic range, complex conductive patterns were directly drawn onto the photosensitive substrate at high speed. For practical applications of this laser writing technology in microelectronics, the delicate patterns were covered by a protective coating, using a photosensitive acrylic resin that hardens instantly under laser exposure, without affecting the electrical properties of the microcircuit.
The photoablative characteristics of vapour deposited poly- ethylcyanoacrylate photoresist (PECA) and phenylsiloxane spin-on-glass (SOG) are reported. The photoablation was performed using 20 ns excimer laser pulses at 193 nm and 248 nm, and was monitored interferometrically by measuring the reflectivity of the irradiated area using a He-Ne laser. The microlithographic potential of these photoresists using photoablative etching was also investigated. Resolution of 2 μm was achieved with the SOG but there is evidence of curing by the laser pulse. Feature sizes of 3 μm were attained in PECA but this figure appears to be limited by the simple imaging system used.
A UV photoablation method with an ArF excimer laser has been experimented on Mercury Cadmium Telluride alloys. We have investigated etching rate, surface quality and surface composition under several experimental conditions. We have also obtained linear photoablated grooves by sample scanning which could be a first step to convert planar into strip waveguides.
Titanium films (120 nm) deposited on single-crystalline silicon (c-Si) and on poly-Si/Si02/c-Si substrates were subjected to Nd:glass laser irradiation in air. Ti/Si samples were also submitted to excimer laser irradiation in a nitrogen jet. From RBS analysis it follows that in the first case titanium silicide is formed on c-Si substrate after one pulse of 1.5 J/cm2 energy density. On the poly-Si substrate a lower fluence (1 J/cm2) was sufficient. The samples submitted to excimer laser irradiation in a nitrogen jet show silicide synthesis at the Ti/Si interface and titanium nitride synthesis at the metal surface. The nitride was shown to work well as an interdiffusion barrier.
Laser-synthesis of thin film compound semiconductors has been demonstrated to provide a very efficient way to produce high quality single phase materials for opto-electronic applications. Three semiconducting compounds may be formed in the Cu-Te system: CuTe, Cu7Te5 and Cu2Te with specific optical properties. In this work, these phases have all been laser-synthesized from a sandwich film containing a stack of elemental layers. The overall atomic proportion is closest to the aimed stoichiometry. Upon irradiation and depending on laser fluence, the compound is formed at and above a power density threshold corresponding to the abrupt melting of the film.
Oxidation of thin metallic films on glass substrates exposed to cw Ar+ laser irradiation is studied. The evolution of the oxide thickness, x, with time, t, is measured via the reflectivity of a probed He-Ne laser beam. Under certain conditions, the temperature of the irradiated zone is measured via the reflectivity of the glass substrate. Therefore, the x(t) and dx(T)/dt are deduced. This permits to obtain a much better kinetics of oxidation than for previous works, where oxidation rates were measured from the positions of the minima and maxima of reflectivity. Evidence for a temperature threshold for laser-assisted oxidation of vanadium film is obtained. This threshold is independent on laser fluence.
A variety of gas-solid interactions have been studied using a compact home-made excimer laser. Indium phosphide was irradiated with photons of 193 nm in CF3Br atmosphere at ambient temperature. Surface aspect and etching selectivity with SiO2 seem to be very good. First evaluations of electrical damages on etched surfaces are promising. Silicon etching was investigated in presence of chlorine gas with 308 nm or 248 nm radiations. Influence of material doping on etching conditions was noticed and etch rates depend on gas pressure and gas flow. Other experiments on aluminum films have been performed with the same gas and laser wavelengths.
We have investigated the incorporation of oxygen into implanted silicon during repetitive UV laser irradiation (λ= 193 nm) in air or in oxygen atmosphere at densities near or above the melting point. The structural and compositional properties of the grown oxide layers are analyzed by infrared spectroscopy, and Rutherford backscattering spectrometry. At low laser fluence, the oxide films are stoichiometric and present a slightly broadening of the Si-0 stretching band when compared to thermal oxide films, indicating the presence of a low degree of disorder in the films. The laser oxidation kinetic is shown to be limited by the diffusion with, however, a very high growth rate attributed to oxygen atoms created by the direct photodissociation of oxygen molecules and rapid diffusion of oxygen in molten silicon. The study of the symmetry of the Si-0 stretching band as a function of the laser fluence reveals that the increase of the bandwidth is essentially caused by the presence of a pronounced shoulder on the high-frequency side of the band, whose origin is discussed. The effects of the parameters of the implanted impurity (energy, dose and nature of the ion implanted) on the oxide quality were shown.