Electrodeposition of thin film semiconductors

Laser-induced chemical processing of solid surfaces has the potential for being an important and powerful technique for fabrication of a variety of devices. Successful applications rest on a detailed understanding of the nature of laser-induced reactions and their effects on the properties of materials. In this paper fundamental studies illustrating key features of laser etching and deposition are reviewed. Topics covered include the effect of the choice of precursor and deposition conditions on film composition and morphology, self-propagation of exothermic reactions, thermal and electronic effects in laser-assisted etching of semiconductors, metals and polymers, and special aspects of laser-surface photophysics as they may affect chemical reactions.

Basic mechanisms in laser etching and deposition

Laser processing is a relatively new technique for modifying the near-surface region of materials without altering the in-bulk characteristics. A single laser can perform several functions by manipulating processing conditions such as laser power, beam diameter, and traverse speed. Lasers have shown attractive applications, such as cutting, welding, glazing, alloying, and cladding. A laser glazing process has demonstrated an improvement in the microstructure of vacuum plasma-coated copper-based alloys containing cavities, unmelted particles, and segregation. Laser glazing has also been shown to restore the degraded microstructure of components and make them equivalent to, or better than, the original wrought alloy. The laser cladding concept was used to develop nickel-based alloys for high-temperature applications that exhibited higher thermal stability than the nickel-based Rene-95 alloy. Rapid melting and quenching occurred during the laser glazing, alloying, and cladding processes resulting in a fine-grained microstructure, metastable phases and extended solid solubility of alloying additions in the matrix. Photon-assisted processing of material is a relatively new technique being explored to synthesize new materials from various substrates (solid, liquid, and gas). This process is successfully used to fabricate high-quality thin films for electronic industries. Thin films of multicomponents can be deposited with stoichiometric composition. Diamond thin films have been synthesized from liquid hydrocarbon (Benzene, C6H6) by laser-liquid hydrocarbon-substrate interaction. A laser-assisted physical vapour deposition process was found to be very successful in depositing stoichiometric compositions of multilayered thin films such as superconducting YBa2Cu3O7, ferroelectric Pb0.52Zr0.48TiO3 and other coatings such as TiN and CoSi2. This review reports some of the major advances in the understanding and engineering of new materials for electronic industries and high-temperature applications in the auto, aerospace, and turbine industries.

Laser-beam and photon-assisted processed materials and their microstructures

Dielectric breakdown of gas mixtures can be used to deposit homogeneous thin films by chemical vapor deposition with appropriate control of flow and pressure conditions to suppress gas-phase nucleation and particle formation. Using a pulsed CO2 laser operating at 10.6 μm where there is no significant resonant absorption in any of the source gases, we have succeeded in depositing homogeneous films from several gas-phase precursors by gas-phase laser pyrolysis. Nickel and molybdenum from the respective carbonyls and tungsten from the hexafluoride have been examined to date. In each case the gas precursor is buffered to reduce the partial pressure of the reactants and to induce breakdown. The films are spectrally reflective and uniform over a large area. Films have been characterized by Auger electron spectroscopy, X-ray diffraction, pull tests, and resistivity measurements. The highest quality films have resulted from the nickel depositions. Detailed X-ray diffraction analysis of these films yields a very small domain size (< 5.0 nm) consistent with rapid quenching from the gas-phase reaction zone. This analysis also shows nickel carbide formation consistent with the temperature of the reaction zone and the Auger electron spectroscopy results which show some carbon and oxygen incorporation (9% and 1%, respectively). Gas-phase transport and condensation of the molybdenum carbonyl results in substantial carbon and oxygen contamination of the molybdenum films requiring heated substrates, a requirement not consistent with the goals of the program to maximize the quench rate of the deposition. Results from tungsten deposition experiments, representing a reduction chemistry instead of the decomposition chemistry involved in the carbonyl experiments, are also reported.

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C.W. Draper

Papers

Cr, Mo and W alloy powders produced by laser-induced breakdown of metal carbonyl vapors