A overview of the recent developments in laser chemical processing is provided. The review covers advances in understanding fundamental process physics, the development of new techniques, and the implementation of applications.

Recent progress in microfabrication technologies for advanced VLSI devices, such as 16M and 64MDRAM, is presented. First, an EB delineator with a vector-scanned VSB on a moving stage has been developed for printing 0.25 μm patterns employing PMMA, high dose exposure, and 50 KeV EB. Optical lithography also has been extended toward lower submicron geometry. A Krf excimer laser reduction projection system, using a quartz/CaF2 lens, resolves successfully 0.35 μm patterns. Ga field ion beam technology has been developed with new applications in fuse-cutting of redundancy and in optimizing sense amplifier by cutting transistor gates in the SRAM device. For fine line etching technology, collimated reactive ions produced by 10-3 Torr magnetron discharge achieves deep Si trench etching and tapered Al etching by using a polymer deposition process in addition to the original thin sidewall film. Finally, a damage-free excimer laser etching process has been developed which can etch n+ poly-Si with resist mask and with pattern transfer using an optics down to 0.5 μm and 0.9 μm resolutions respectively.

The current understanding of the interaction between ultrashort laser pulses and condensed matter is demonstrated on a few selected samples. Hot electronic carrier relaxation in GaAs, plasmon aided recombination in highly excited silicon and ultrafast energy transport in metals are discussed.

Low-energy (often < 100 eV) ion bombardment during thin film deposition is commonly used in such diverse application areas as microelectronics, optical coatings, magnetic recording layers, and hard wear resistant coatings to modify the microstructure and microchemistry of films deposited by a variety of techniques (e.g. sputtering, primary ion deposition, plasma-assisted CVD. and accelerated-beam MBE). Ion irradiation has been shown to affect every phase of deposition including nucleation and growth kinetics, crystal structure and phase stability, the average grain size and degree of preferred orientation of polycrystalline films, the epitaxial temperature of single-crystal films, defect concentrations, elemental incorporation probabilities, surface segregation, and, hence, film properties. As discussed in this brief review, a detailed understanding of many of these processes is beginning to emerge. Effects such as trapping, preferential sputtering, enhanced diffusion, and collisional mixing have been used to interpret and, in some cases, model experimental results. Nevertheless, there are still large gaps in our knowledge of the role of ion bombardment on fundamental processes such as nucleation kinetics.

Surface temperatures of a laser-heated structure consisting of intersecting gold and nickel lines on an Si substrate with an SiO2 surface layer were determined experimentally and theoretically. Temperatures were measured with micron-sized thin-film thermocouples formed from intersecting gold and nickel lines. Temperature profiles were calculated using a finite difference method which took into account the optical and physical properties of the thermocouple metal lines, the silicon dioxide layer and the silicon substrate. For a 0.2 μm SiO2 layer, maximum surface temperatures were much higher with the metal lines than without because of the lower reflectivity of the gold relative to the silicon and the limited ability of the thin metal lines to conduct heat away from the irradiated region. Calculated maximum temperatures on the metal lines depended strongly on the thicknesses of the insulating SiO2 layer and the metal lines. Application of these results to the dynamics of laser-induced chemical vapor deposition is discussed.

This paper reports on a study of the nucleation phase of thin film growth during pyrolytic deposition by direct-laser writing. A Monte Carlo computer simulation is used to model the initial surface dynamics occurring during the deposition of silicon from silane (SiH4) on strongly binding substrates such as silicon, and on weakly binding substrates such as silicon dioxide (SiO2) and silicon nitride (Si3N4). Results show that for high peak temperatures (≥ 1200 K) and weakly binding substrates, the most probable location for the initiation of thin film growth is radialy displaced from the center of the locally laser-heated region of the surface. At low peak temperatures, thin film nucleation is found to begin at the center of the locally laser-heated surface for all substrates.

Polyimides are of great interest as insulators for microelectronic fabrication due to their low dielectric constant and planarizing properties. However, they are thermally sensitive films. Nonetheless, preliminary results clearly demonstrate that nickel lines readily can be laser-pyrolytically drawn over polyimide films.

While elevated laser power will damage underlying polyimide films, microelectronic quality nickel lines can be drawn using nickel carbonyl at relatively low laser intensities over polyimides with little or no change in the electrical characteristics. Polyimide's lower thermal conductivity relative to silicon dioxide facilitates increased nickel deposition rates. Self-limiting effects have been observed during the deposition that can lead to near microelectronically ideal line cross sections.

The focused output from an argon ion laser (514 nm) has been used to pattern micron-size platinum features by decomposition of spun-on metallo-organic film on quartz substrate. The role of laser power and energy density on the thermal decomposition of the film is studied using pulsed and cw laser irradiation. Transient reflectivity has been used as a probe to study the reaction steps involved in the decomposition of the metallo-organic containing film. Preliminary results on the use of the platinum features as seed layer for electroless copper plating is presented.

Electrically conducting palladium features have been produced by laser and ion beam irradiation of thin palladium acetate films. The photothermal reaction induced by scanned continuous wave Ar+ laser irradiation leads to metal lines that may exhibit periodic structure. This results from repeated propagation of “explosive” reaction fronts generated by coupling of the heat from the absorbed laser radiation with the heat of the decomposition reaction of the film. In contrast, 2 MeV He+ ion irradiation produces smooth metallic-looking features that contain up to 20% of the original carbon and 5% of the original oxygen content of the film. Films irradiated with 2 MeV Ne+ ions contain slightly lower amounts of carbon and oxygen residues, but fully exposed thick films (0.90 μm) appear black rather than metallic silver. In addition to having significantly higher purity, the laser-written features have lower resistivities than the ion beam-irradiated features. Infrared spectroscopy of the ion beam-irradiated films as a function of dose indicates a progressive loss in intensity of the characteristics acetate (COO-) vibrations. This occurs at doses lower than those associated with major C and O loss from the films. Partially ion-exposed films continue to decompose to metallic-looking material over a period of weeks after irradiation. Metallic palladium particles apparently catalyze this process.

Fine conducting features have been produced on Si and SiO2 substrates by irradiation of spin-on palladium acetate, [Pd(O2CCH3)2]3 films with a submicron focused ion beam. The exposures were made with a 20 keV Ga+, focused to a 0.2 micrometer spot. Electrical conductivity measuremnents were made on the resultant features as a function of ion dose for linewidths of one and ten micrometers. The sheet conductivity in the two cases was comparable and increased dramatically in the dose range between 2×1014 and 5×1014 ions/cm2. The conductivity of the exposed lines was further increased after heating in a hydrogen atmosphere. Measurements of carbon and oxygen content indicate that even at the highest ion doses a significant amount of organic material remains. Results are compared to those for 2 MeV He+ and Ne+ broad beam exposures. Potential applications are also discussed.

Chlorine-enhanced GaAs maskless etching using a novel focused-ion-beametching (FIBE) system has been examined for establishing high-rate and smooth FIBE. The system is composed of an air-locked ultrahigh-vacuum chamber, a 30 KeV Ga+ FIB column and two kinds of chlorine-irradiation nozzles. A fine nozzle enabled us to irradiate a high-density Cl2 flux on a desired, small area of the sample while retaining a sufficiently low surrounding-gas pressure for stable Ga+ FIB emission. Highly chemically-enhanced sputtering yields (up to 50 GaAs molecules per incident ion) were obtained. At the maximum yield, line-scanned deep-groove (6.5 um) etching with a smooth surface, capable of fabricating a laser-cavity optical mirror, was demonstrated. The chemical-enhancement effect showed high FIB-scanning-time dependence. This effect was also observed by irradiating with a plasma-dissociated Cl radicals using a novel radical beam gun. An analytical model, based on the Ga+-ion bombardment on the chlorine-adsorbed substrate surface, suggested that the maximum chemical enhancement is obtained when the Ga+-FIB scanning time is adjusted to the chlorine-coverage time, given by the Cl2-molecule or Cl-radical flux density.

KrF excimer laser (248 nm) images of mask patterns have been Iused to photochemically activate tihe surface catalytic decomposition of triisobutylaluminum (TIBA). The activation step is shown to involve the photolysis of organoaluminum surface adlayers leading to the formation of reactive Al sites. These sites serve to selectively nucleate the thermal decomposition of TIBA which results in the growth of high quality Al filns (resistivities ∼5 μΩ-cm). The growth on the chemically inert surfaces of SiO2 and Al2O3 is extremely selective and results in patterns with resolutions ∼2 μm. To evaluate the utility of this process for real circuit applications, the laser activated deposition technique has been used in conjunction with standard photolithographic processing to fabricate metal-oxide field effect transistors and Al interconnects. The successful fabrication of working devices indicates that the laser activated deposition technique is compatible with standard photolithographic patterning schemes and may provide a means for simplifying integrated circuit fabrication.

Photodissociation of trimethylaluminum molecules with a UV lamp is shown to be an effective technique for predisposing the irradiated silicon surface prior to subsequent aluminum film growth via visible laser induced pyrolysis. The Al deposits thus obtained are carbon contamination free. The UV exposure time needed for the onset of Al nucleation and growth is deduced from an in situ laser reflectometry technique. Direct laser writing is obtained using this two-step process and a microscopic analysis of the lines is made in correlation with the experimental procedure.

The laser-induced, vapor-phase deposition of gold metal from dimethyl-(2,4-pentanedionato) gold (III) and two fluorinated derivatives was examined by both photopyrolytic (LCVD) and photochemical modes of decomposition. High purity gold deposits are formed by LCVD with the rates of deposition being extremely vapor pressure dependent and thus, gold complex dependent. Photochemical deposition of gold patterns with 2 μm resolution can be achieved by irradiation of these complexes in the UV, but the deposited material is contaminated with carbon impurities.

We have been investigating deposition of tungsten and molybdenum films at lover temperatures than conventional CVD processes with the aid of ultraviolet photons. The photon source is a high intensity, medium-pressure mercury lamp. W and Mo films have been deposited from their respective hexacarbonyls. W films have also been obtained by photon enhanced reduction of tungsten hexafluoride. In all cases films were deposited over 75 mm diameter substrates, which included quartz, silicon dioxide, and silicon wafers. The deposition system, conditions and the properties of the photodeposited films are presented.

Formation of electronically excited metal atoms via excimer laser multiphoton dissociation of Cr(CO)6, C6H6Cr(CO)3, and Mo(CO)6 has been examined in the gas phase. The dissociation mechanism was studied by determining the laserfluence- dependence and buffer-gas-pressure dependence of the metal atom emission intensity. Each of these species was found to form metal atoms via two-photon and three-photon dissociation processes. The data suggest that dissociation occurs by both direct and sequential processes.

Gas phase processing rakes laser deposition over large areas possible but homogeneous nucleation of large atomic clusters must be avoided if films are to be produced. Clusters can be highly variable in size from a few atoms to significant fractions of a micrometer. If conditions do not allow for complete quenching of the clusters produced in the gas phase, these clusters can arrive at the substrate with sufficient energy to self sinter into homogeneous films which are substantially different from metallic films grown by thermal techniques. Using transmission electron microscopy (TEM), we have characterized the microstructure of thin metallic films deposited by laser breakdown chemical vapor deposition and identified a range of deposition conditions which can lead from powders to homogeneous polycrystalline films and mixed phase materials. Gas phase nucleation is dependent on reactant partial pressures and the gas phase quench rate which car be varied in part by adjusting the H2 content of the source gas. Manipulation of these parameters can vary powder size from about one micrometer to less than 2 nanometers. Variation of the quench rate during the deposition of polycrystalline films varies the grain size in the films. heating the substrate drastically changes the conditions under which the film is formed and as a consequence, can radically alter the microstructure of the film itself.

The deep ultraviolet (250 nm) photopatterning of spin-on films of polymeric Au mercaptide results in formation of adherent Au patterns. Fxcimer laser projection patterning and standard contact printing techniques give excellent pattern resolution on the micron scale. Laser direct write produces lines at very fast writing speeds. Exposed areas are less soluble than unexposed areas, i.e. the film behaves as a negative photoresist. Bakeout of developed patterns at 250°C yields good purity Au micropatterns up to 500 Å thick. Mechanistic information about pattern formation is gained from uv-visible, infrared, and mass spectrometric monitoring of the photolysis process, and from Auger analysis of films. Adherent patterns are apparently formed by photochemical cleavage of Au-S bonds followed by evaporation of a small amount of free mercaptide. The loss of ligand in the exposed areas renders them less soluble than unexposed film. Thermal decomposition of both photolyzed and unphotolyzed films has the same result of volatilizing all film material except Au.

We have demonstrated metal deposition on semiconductors immersed in electroplating solution by exposure to UV laser radiation. N-type β-SiC and Si in gold or Pd/Ni electroplating solutions were exposed to 20 ns pulses of 193 nm or 248 nm excimer laser radiation. The energy per pulse was 20–100 mJ. Au and Pd/Ni metallizations deposited in lines and circles on SiC showed leaky Schottky barrier I-V characteristics. The thickness increased with increasing pulse energy or number. Both masked and maskless deposition were demonstrated without apparent damage to the substrate. Details of the process, potential mechanisms, and sample characterization are discussed.

In the present study, thin layers of Al (<500 Å), deposited on Si under varying vacuum and substrate-cleaning conditions, have been irradiated with various wavelength photons. The films and interfacial oxide layers have been analysed with Rutherford backscattering and channeling (RBS-C), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques both before and after irradiation. Results indicate that substantial oxygen uptake, indiffusion and interface reaction can take place in some thin Al films during photon irradiation at room temperature. Such photon-induced chemical changes correlate with an increase in film adhesion.