The proliferation of the use of light as the transfer medium in communications has brought about some unique technical challenges. Among these is the development of devices that route optical signals at points where optoelectronic conversions are necessary. In this article we discuss the fabrication of silicon oxide waveguides on silicon substrates, describing low-temperature plasma deposition processes and the relationship between crucial optical parameters of the films and the significant processing parameters. The waveguides are buried-ridge, single-mode waveguide structures fabricated using two deposition techniques: plasma-enhanced chemical vapor deposition and low-pressure chemical vapor deposition. We will present the plasma deposition conditions established to control the two refractive index differences (Δn) of 0.065 and 0.020 between the guiding and cladding layers for two different waveguide designs. Uniformity of the Δn as a function of plasma conditions and dopant species will be presented for the guiding layer. The analytical techniques used to evaluate the properties of the guiding layers doped with fluorine and nitrogen consisted of Auger spectroscopy, infrared spectroscopy, ellipsometry, and prism coupling techniques. In addition, we will present the plasma etching conditions required to fabricate of the waveguiding ridge structures. The etch rates and uniformities obtained in two different reactors using either SF6 or CF4 chemistries will be discussed.
Rapid thermal annealing is used to form cobalt silicide directly on unimplanted as well as B, As, and P implanted wafers. The films are characterized by sheet resistance, X-ray diffraction, SEM, SIMS, and contact resistance measurements. The direct silicidation of cobalt on Si by rapid thermal annealing yields smooth, low resistivity films with minimal dopant redistribution.
Interest in CoSi2 as a metallization for very large scale integrated circuits (VLSI) has grown rapidly since the recent demonstration of a simple self-aligned process performed by rapid thermal annealing.1-4 Using a rapid thermal anneal (RTA) to directly silicide Co on Si yields smooth low-sheet-resistance films with little or no lateral diffusion and low contact resistance. In addition, it has been shown that rapid thermal annealing can result in reasonable quality epitaxial CoSi2 on (111) Si wafers.5 An important advantage of CoSi2 over the more commonly used TiSi2 metallization is the relative simplicity of its self-aligned silicidation process. Due to the low reactivity of Co with SiO2, a simple two-step self-alignment process is possible instead of the three-step process necessary with TiSi2.6 The primary disadvantage of CoSi2 is the amount of Si consumed for equal silicide sheet resistance. For example, to yield a silicide sheet resistance of 1.5 1/LD, Van den Hove 4 finds that compared to the TiSi, process, the CoSi, process would consume an additional 24 nm of Si. (This disadvantage can be minimized if very shallow junctions can be formed under the CoSi2.)
Optical devices, such as directional couplers made from silica waveguides, are used to route optical signals at points where optoelectronic conversions are necessary. We report the fabrication of silica waveguides using low-temperature plasma deposition and etching processes. The devices are buried-ridge, single-mode waveguide structures with a refractive index difference (Δn) of 0.03 between the guiding and cladding layers. Two different plasma deposition processes with different reactant chemistries (SiH4/N2/N2O) and 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS)/O2 were used to fabricate the waveguides. The deposition rates, uniformities, film stress, and refractive index control obtained using the two techniques are discussed. The plasma etching process (CHF3/CF4/Ar chemistry) defining the guiding layer varied from a vertical profile (90°) down to 75°, to minimize void formation in the coupling regions of the devices. The step coverage obtained using the TMCTS process showed marked improvement when compared to silica films deposited using plasma-enhanced chemical-vapor deposition with SiH4/N2/N2O chemistry. The TMCTS chemistry produced plasma deposited films with increased deposition rates and the required precision for both the thickness and refractive index parameters necessary for waveguide device fabrication.
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