A fast thin-film-deposition method shows promise for making nonlinear optical materials, and other optical materials that are currently difficult to generate, reported researchers at the American Chemical Society meeting in April (San Francisco, CA). Jim Garvey and others at the State University of New York (SUNY; Buffalo, NY) used laser-assisted molecular-beam deposition (LAMBD) to create thin films in which organic molecules are incorporated into the matrix of titanium or silicon oxide glasses. Although dye-doped glasses can be made in other ways, such as by sol-gel methods, these materials may have cavities and other flaws, and the dye molecules may cluster or decompose during high-temperature processing.
How it works
The LAMBD process is similar to molecular-beam epitaxy. A 248-nm pulse from an excimer laser ablates a target—such as titanium or silicon—in a normal-pressure area, producing a 20,000 K plasma. Reactive oxygen combines with the plasma, and high-temperature chemical reactions produce metal oxides. The new material escapes into the vacuum chamber, at which point the dye molecules are introduced into the gaseous mixture.
As the gas mixture spreads into the vacuum chamber, the pressure and temperature drop. The amorphous material—copper phthalocyanine and titanium oxide glass, for example—is deposited on the substrate at room temperature. Garvey says the method can be used to make a 1-µm-thick film with an area about the size of a US quarter in 5 min, using 20-ns, 250-mJ pulses from the excimer laser at a repetition rate of 10 Hz. The deposition method has produced organic-doped titanium and aluminum oxides (TiO2 and Al2O3) as well as metals, metal-halide films (for proof-of-principle experiments), high-temperature superconductors, copper-embedded polymers, silicon, carbon, and silicon-carbide. Spectroscopic studies of the organic-doped metal-oxide composites by Paras Prasad, also at SUNY, show that the dye is an integral part of the material—as opposed to merely resting on the surface—and that the deposition process does not decompose the dye.
The researchers plan to align the dye molecules in the gas phase, using static electric fields. They say it is reasonable to expect poling to be relatively easy to achieve while the molecules are gaseous and therefore have more degrees of freedom than in liquids or solids, but the group does not yet know if the molecules will change orientation when deposited.
Poling is also used in nonlinear optical materials made with sol-gel methods. Garvey believes that the higher efficiency of poling in the LAMBD system combined with the higher concentration of dye molecules and lower rate of decomposition of the molecules during fabrication will allow thin films made using the deposition method to be more efficient and higher quality for nonlinear optics.
The technique has promise for the semiconductor industry because of the combination of high-temperature chemistry with room-temperature substrates. One application might be in coating wafers full of nearly complete chips with a metal nitride diffusion barrier before creating ohmic contacts for the chips. A conductive diffusion barrier, such as titanium nitride, keeps the copper from diffusing into the semiconductor.
Current methods of coating the wafer with the diffusion barrier, however, require high-temperature chemical-vapor deposition and caustic environments, which can destroy some of the chips. The LAMBD process could coat a wafer with titanium nitride (TiN) without raising the substrate temperature. Because the precursors of the material are a titanium rod and nitrogen gas, the process does not require or produce hazardous byproducts.
Currently, LAMBD is a deposition method that offers faster growth but not as much control over the material as, for example, molecular-beam epitaxy. Control could be increased, however, by lowering the laser pulse powers. The researchers have determined that laser power controls the particle sizes deposited on the substrate. Garvey has entered into a partnership with Strategic Materials Inc. (Piscataway, NJ) to commercialize the system for industrial use.