J. Y. Tsao

Photodeposition of Ti and application to direct writing of Ti:LiNbO3 waveguides

An ultraviolet laser photodeposition process based on the photolysis of TiCl4 has been developed. The photochemistry of this new metal‐halide system has been shown to involve a surface‐catalyzed reaction confined to adsorbed molecular layers. By using this process, Ti films have been deposited on LiNbO3 to form, after diffusion, 4‐μm‐wide single‐mode channel waveguides of comparable quality to conventionally fabricated Ti‐indiffused guides. The technique introduces new design flexibility into waveguide fabrication, permitting controlled gradations in the diffused index change and the lateral width along the guide.

Patterned photonucleation of chemical vapor deposition of Al by UV‐laser photodeposition

UV‐laser photodeposition has been used to predispose surfaces to pyrolytic chemical vapor deposition (CVD) of Al from triisobutylaluminum. Two laser beam (UV and IR) experiments indicate that the predisposition is due to the formation of a catalytic surface for heterogeneous chemistry. Time‐resolved transmission measurements indicate that a few photodeposited monolayers are sufficient to nucleate CVD growth. The technique may be generally useful for maskless patterned growth by CVD processes with large nucleation barriers.

Surface acoustic wave devices and method of manufacture thereof

A phase and amplitude compensated surface acoustic wave (SAW) structure is described in which computer controlled compensation is achieved by laser chemical etching of selective portions of a compound chemical film deposited on the surface of a piezoelectric SAW substrate in the path of propagation. The compound film comprises a layer of amplitude attenuating cermet material formed on the substrate and a phase compensating layer of molybdenum formed over the cermet material and in contact with the substrate surface.

Time‐resolved measurements of stimulated surface polariton wave scattering and grating formation in pulsed‐laser‐annealed germanium

Nanosecond resolution optical probe measurements of the growth of surface ripples in pulsed‐laser‐annealed Ge are reported. The ripples are shown to arise from stimulated scattering of the incident laser light into surface polariton waves at the air/liquid/solid interfaces which form during the optical excitation. These surface waves grow exponentially from spontaneous scattering via feedback involving modulation of the velocity of the liquid/solid interface. The final ripple structure results from the density change on melting coupled with transverse diffusional processes during the regrowth.

Laser‐controlled chemical etching of aluminum

A new technique is described for high‐spatial‐resolution (<2‐μm linewidth) etching of Al thin films. The process is based upon moderate local heating by a tightly focused Ar+ laser beam to activate an etching reaction in mixtures of phosphoric acid, nitric acid, and potassium dichromate. By chemically biasing the reaction near its passive/active transition, the laser can enhance the reaction rate by more than six orders of magnitude. The etching mechanism has been studied by etch‐rate measurements, ellipsometry, and Auger spectroscopy, and is ascribed to a competition between the formation of soluble aluminum phosphates and insoluble aluminum oxides.

Nonreciprocal laser‐microchemical processing: Spatial resolution limits and demonstration of 0.2‐μm linewidths

A simple model, which includes the important effects of ‘‘nonreciprocity’’ (nonlinear dependence on intensity), is developed to describe the spatial resolution of single‐step focused‐beam, microchemical fabrication techniques. Qualitative comparisons are made to patterning by double‐step (resist) and by projected‐image processes. At a given wavelength, linewidths are shown to be narrowest for microchemical processes. Linewidths <0.2 μm, i.e., narrower than the Rayleigh optical diffraction limit, are demonstrated for laser‐activated deposition and doping of silicon at visible wavelengths.

Submicrometer‐linewidth doping and relief definition in silicon by laser‐ controlled diffusion

Boron doping patterns characterized by very sharp lateral edge definition and 300–1000‐nm linewidths have been written in silicon by localized heating with a diffraction‐limited cw laser beam. The tightly focused beam generates free dopant atoms, and simultaneously promotes solid‐ state diffusion into the substrate. The nonlinear dependence of diffusion rates on temperature results in linewidths substantially narrower than both the temperature and the laser beam profiles on the surface. An enhancement is observed in diffusion rates under the strong thermal gradients created.

UV laser photopolymerization of volatile surface‐adsorbed methyl methacrylate

The localized UV laser photopolymerization of surface‐adsorbed methyl methacrylate has been used to deposit poly(methyl methacrylate) films for direct patterning of wet and dry chemical etching processes. Using this negative resist process, submicrometer linewidths both in the polymer deposition and in pattern transfer into Si and SiO2 have been demonstrated. The chemical kinetics of simple and catalyzed photopolymerization have been studied by dynamic lensing experiments and modeled as a dynamic equilibrium between competing surface photoreactions.

UV laser photodeposition of patterned catalyst films from adsorbate mixtures

Efficient catalysts for hydrocarbon polymerization have been prepared photochemically by reaction of mixed TiCl4 and Al2(CH3)6 adsorbate layers, in the scanned focus of a 257.2‐nm laser beam. Monolayer coverages of the photodeposited catalyst have been shown effective for in situ patterned polymerization of ethylene and acetylene at room temperature from the vapor phase organic monomers. Three‐micrometer spatial resolution has been obtained in selected‐area polymer growth. The chemical kinetics of the mixed‐adsorbate reactions and the effects of compositional changes on the catalytic activity of the laser‐photodepostied films have been studied.

Laser Fabrication of Microstructures: Effect of Geometrical Scaling on Chemical Reaction Rates

Laser chemical processing for microfabrication makes use of chemical reactions and phase changes confined to micrometer-scale dimensions at vapor/solid and liquid/solid interfaces. When conditions are established for rapid interfacial reactions, processing speeds can become limited by diffusive transport. These diffusion-limited rates, however, increase by many orders of magnitude as the microreaction geometry is reduced to micrometer lengths using well-focused visible and UV beams. A calculation of the geometric scaling of such rates is summarized here. For realistic conditions, rate increases of up to four decades are predicted relative to corresponding reactions on semi-infinite plane surfaces.