Gas phase versus surface contributions to photolytic laser chemical vapor deposition rates

The rate of cwphotolytic laser chemical vapor deposition (LCVD) of platinum is measured for 350 nm as a function of the light intensity and the metalorganic vapor pressure. The growth of the metal films is studied in situ and in real time by monitoring their optical transmission. At low intensities the transmitted light decreases monotonically with time, and the LCVD process is photolytic with its rate limiting step in the surface adlayer. At higher intensities we observe two distinct time domains: Relatively slow initial photolytic deposition with its rate limiting step in the gas phase, which is followed by much faster pyrolytic LCVD. An improved method for distinguishing between adlayer and gas-phase limiting processes is demonstrated. These observations are confirmed by studying the photolytic deposition rates while varying the thickness of the adlayer.     

Growth rates and electrical conductivity of microscopic ohmic contacts fabricated by laser chemical vapor deposition of platinum

The laser chemical vapor deposition of platinum from its bishexafluoroacetylacetonate derivative is studied with a cw argon ion laser at 458 and 514 nm. The height, the width, as well as the electrical conductivity of the deposited stripes are reported as a function of the vapor pressure of the metalorganic precurser, the laser intensity, and the writing speed.     

LCVD of tungsten microstructures on quartz

The pyrolytic laser-induced chemical vapor deposition (LCVD) of tungsten microstructures in the form of spots has been investigated. Fused quartz substrates and a mixture of WF₆ and H₂ have been employed in the experiments. Different shapes of spots related to different partial pressures of WF₆ and H₂ have been revealed. The results can qualitatively be described by a combination of surface reactions and gas-phase reactions resulting in tungsten deposition and surface etching.On leave from: Reserch Group on Laser Physics of the Hungarian Academy of Sciences, H-6720 Szeged, Dòm tér 9, Hungary     

Silicon fibers produced by high-pressure LCVD

Using a unique fiber-growth control mechanism and high-reaction pressures (>1 bar), silicon fibers were grown by laser-assisted chemical vapor deposition (LCVD) from silane near the focal point of a cw Nd:YAG laser beam. Fiber-growth rates ranged from <1 to 500 m/s and fiber tip temperatures from 525 to 1412° C. At low fibertip temperatures (<600° C) silicon fibers yielding glassy fracture were obtained. Some crystallinity was observed by X-ray diffraction. Polycrystalline silicon fibers were formed at intermediate temperatures, single-crystal silicon fibers at high-laser intensities and high-tip temperatures. The single crystal LCVD silicon fibers were formed by a vapor-liquid-solid (VLS) mechanism. Single-crystal VLS-LCVD silicon fibers were also obtained from liquid silicon-metal alloys by initiating fiber growth from the end of thin palladium, gold and platinum wires.     

Dynamics of excimer laser ablation of polyimide determined by time-resolved reflectivity

The time-dependent intensity profile of pulsed KrF excimer laser radiation reflected from polyimide is determined over a range of laser fluences, from well below to above the ablation threshold. The reflected laser beam is truncated once the incident laser radiation exceeds a threshold fluence, i.e., truncation depends on the energy per unit area and not on the intensity, analogous to results for the ablation threshold and the etch depth per pulse. The threshold fluence for pulse truncation corresponds to the onset of ablation. The results indicate that the truncation is not due to laser plasma interactions at these fluences. A general mechanism is discussed involving a time dependent index of refraction.     

Searching for optimum conditions in the laser writing of small ohmic contacts

Microscopic ohmic contacts are made by laser chemical vapor deposition of platinum on a Pyrex substrate. The electrical conductivity of the deposited metal stripes is measured as a function of the laser power, the writing speed, and the organometallic vapor pressure. The latter appears to be the key parameter for producing contacts with low resistance at high writing speeds. Even on these transparent substrates there is no apparent advantage in using light at 350 nm, where photolysis may in principle play a significant role, over using visible light where photolysis is not effective.     

Femtosecond uv excimer laser ablation

Experiments on the ablation of polymethylmethacrylate (PMMA) with 300 fs uv excimer laser pulses at 248 nm are reported for the first time. With these ultrashort pulses, ablation can be done at fluences up to five times lower than the threshold fluence for 16 ns ablation of PMMA, and the surface morphology is improved, also for several other materials. A model for ablation is proposed, assuming a non-constant absorption coefficient _eff depending on the degree of incubation of the irradiated material and the intensity of the incoming excimer laser pulse. The agreement between our model and our experimental observations is excellent for 16 ns excimer laser pulses, also predicting perfectly the shape of a pulse transmitted through a thin PMMA sample under high fluence irradiation. Qualitative agreement for 300 fs excimer laser pulses is obtained so far.     

Application of molecular (static) secondary ion mass spectroscopy to detection of organic molecules in amorphous titanium carbide

Molecular, or static, secondary ion mass Spectroscopy (SIMS) is applied to the detection of organic molecules in amorphous titanium carbide films. The presence of such organic clusters is thought to stabilize the amorphous phase to higher temperatures (>1000°C) and greater thicknesses. The high corrosion resistance properties of the TiC deposits are also attributed to the inclusion of such molecular entities. The processes whereby these molecular entities in the films are transformed into secondary ions during SIMS analysis are also investigated. It is shown that the dominant ionization mechanisms in this case are electron and momentum transfer.     

Excimer-laser-induced ablation of the high-T c superconductor Bi-Ca-Sr-Cu-O

The ablation of ceramic Bi-Ca-Sr-Cu-O by XeCl-excimer-laser projection has been investigated. In both air and vacuum, etching commences at about 2.4 J/cm² and then increases with fluence within the regime investigated ( <) 20 J/cm²). At 10 J/cm² the respective etch rates are around 1 m/pulse and 1.6 m/pulse.     

Estimation of the capabilities of maskless micropatterning by laser-induced chemical etching

A three-dimensional mathematical model of laser-induced chemical etching of polysilicon films in chlorine atmosphere is presented. The model takes into account both photochemical and thermochemical mechanisms. A numerical solution of a simple process model and consequences for spatial resolution limits of the mask-less laser-induced chemical etching are presented.     

Laser-induced chemical etching of silicon in chlorine atmosphere

Laser-induced chemical etching of single-crystalline (100) Si in Cl₂ atmosphere has been investigated for continuous Ar⁺ and Kr⁺ laser irradiation at around 351 nm, and at 457.9, 488.0, 514.5, and 647.1 nm. For laser irradiances below 10⁵ W/cm² the etching mechanism is non-thermal, and is based on photo-generated electron-hole pairs within the Si surface and Cl atoms produced within the gas phase. The experimental results are compared with model calculations.     

Laser-induced surface reduction of the high-T c superconductor YBa2Cu3O7−x

The first investigations on the laser-induced reduction of YBa₂Cu₃O_7–x are presented. Here, the oxygen content of the material is diminished by local heating under cw Kr⁺ laser irradiation in H₂ atmosphere. The technique permits the superconducting properties of the material to be locally changed or destroyed. Laser-induced surface metallization may be useful for the electroding of such materials.     

Rapid thermal annealing of electrically-active defects in virgin and implanted silicon

Chemical etching of single-crystalline (100)Si induced by pulsed laser irradiation at 308, 423, and 583 nm has been investigated as a function of the laser fluence and C1₂ pressure. Without laser-induced surface melting, etching requires Cl radicals which are produced only at laser wavelengths below 500 nm. With low laser fluences ((308 nm)<100 mJ/cm²) etching is non-thermal and based on direct interactions between photocarriers and Cl radicals. For fluences which induce surface melting ((308 nm)>440 mJ/cm²) etching is thermally activated. In the intermediate region both thermal and non-thermal mechanisms contribute to the etch rate.     

Negative thermal feedback in laser-assisted oxidation of thin Zn films

The processes leading to the control of the lateral dimension of laser-assisted oxidation of Zn films on glass are examined. It is shown that it is determined by negative feedback between temperature and optical absorption.     

Laser-induced surface metallization of ceramic PLZT

Hot pressed, optically transparent ferroelectric ceramics of lanthanum modified lead zirconate titanate (PLZT) have been locally reduced and metallized by means of ultraviolet Kr⁺ laser irradiation in a hydrogen atmosphere. The technique allows maskless single-step metallic patterning of the material.On leave from Cietvielu Fizikas Institüts, Latvijas Valsts Universitäte, Riga, Latvian SSR, USSR     

Laser-assisted reaction of metals with oxygen

The present status of experimental and theoretical work on continuous wave laser-assisted reaction of metals with oxygen is presented. Differences between this and normal isothermal oxidation of metals are emphasized. Available theoretical models are discussed. They deal with roles of thermal history, feedback effects between optical absorption and reaction rate. The nature of so-called non-purely thermal effects is discussed. Hints for further research are presented.     

Ultraviolet laser ablation and etching of polymethyl methacrylate sensitized with an organic dopant

Although polymethyl methacrylate (PMMA) is essentially transparent to light of 308 or 351 nm, it can be made sensitive to photoablation and etching by excimer laser pulses (20 ns half-width) of those wavelengths by the introduction of an organic dopant. The dopant (trade name=Tinuvin^*) is actually a quencher of the first electronic excited state of PMMA and is therefore used commercially to stabilize the polymer against photodegradation. Laser etching of Tinuvin-doped PMMA can be shown to be a photochemical process in which the Tinuvin decomposes by the absorption of two or more photons and causes the ablation of the surrounding polymer.^*[2-(2-hydroxy-3,5-diisopentyl-phenyl) benzotriazole].     

Time-resolved ablation-site photography of XeCl-laser irradiated polyimid

The photoablation of polyimid was studied by time-resolved photography during the excimer laser pulse at 308 nm. Photographs showing successive instants of the formation of the etched craters demonstrate the fast-moving air-polymer interface. It is demonstrated experimentally that the polymer ablates layer by layer during the evolution of the UV laser pulse.     

Photokinetic etching of polyethylene terephthalate films by continuous wave ultraviolet laser radiation

Continuous wave laser radiation from an argonion laser in the wavelength range 275–330 nm can be used to etch polyethylene terephthalate (PET) films with as little thermal damage as from a pulsed, ultraviolet laser (248 nm or 308 nm) provided the beam is focussed to a spot of 10–100 kW/cm² of power density and is moved over the surface at speeds at which the transit time over its own diameter (which can be looked upon as a pulse width) is on the order of 10–200 s. In contrast to results which had been obtained previously on the photokinetic etching of polyimide and doped polymethyl methacrylate films under similar conditions, the sensitivity of PET to etching is >5-fold greater than either of these polymers and increases steadily with increasing pulse width. There is lateral thermal damage as the pulse widths increase to >200 s. The material that is removed is vaporized in part. More than 20% is probably ejected in a molten state and resolidifies at the edge of the cut. There is no acoustic report similar to that seen in ablative photodecomposition. The process appears to be largely thermal in nature.     

Direct writing of metal conductors with near-uv light

Deposition of micrometer-scale tungsten, molybdenum and platinum lines with the aid of the uv light from an argon-ion laser was investigated. Tungsten and molybdenum were deposited from their corresponding carbonyls and platinum from an acetylacetonate compound. High-quality metal conductors could be achieved for tungsten and platinum using a combined photolytic and pyrolytic, or hybrid, deposition scheme. The resistivity for these two deposited metals was approximately twice the bulk value. For molybdenum the deposition process was fast and dominated by the pyrolysis; the corresponding line resistivity was slightly higher than for the two other metals.