This invention relates to integrated circuits and more particularly relates to methods for manufacturing integrated circuits.
Self-aligned refractory metal-silicide technology has been recognized as one of the keys to realizing good device performance in integrated circuits as device dimensions scale down. Titanium Disilicide ("TiSi.sub.2 ") has become recognized as one of the most attractive metal-silicides, because of its low resistivity, stability, and capability for self-aligned formation.
One of the major advantages of titanium silicide technology is the availability of a self-aligned VLSI process. That is, by depositing a layer of titanium metal overall and then heating in a nitrogen atmosphere, all exposed areas of silicon (whether monocrystalline or polycrystalline) will react to form titanium silicides, and a composition dominated by titanium nitrides will be formed where the titanium metal was not in contact with silicon. This is tremendously useful, since, by performing this step after the polysilicon gate level has been patterned, silicide will be formed on the surface of exposed source/drain regions (or of other exposed substrate surface regions), on the surface of the polysilicon gate level, and no where else. This means that shallower source/drain diffusions can be made with an acceptably low sheet resistance, and also means that the sheet resistance of the polysilicon gate level can be lowered. The use of the nitrogen atmosphere in this process is critical, since otherwise silicon will out-diffuse through the growing silicide layer and permit lateral growth, so that the titanium silicide formed by this reaction will be able to bridge gaps of a one-half micron or so, e.g. between gate and source/drain of a VLSI device.
However, this self-aligned TiSi.sub.2 technology is degraded by any oxygen contamination of the nitrogen atmosphere used for the silicide react process. It has been found that the oxygen contamination during the react process can result in two problems. First, the silicide will have a high resistivity. Second, the unreacted titanium (or non-silicide material such as titanium nitride) is hard to strip off.
U.S. Pat. No. 4,690,730 issued Sep. 1, 1987 and assigned to Texas Instruments Incorporated teaches a way to avoid any possible oxygen contamination. This patent teaches that the manufacturing process places an oxide cap on top of the titanium layer before the silicide react. During the react process, this oxide is partially reduced by the adjacent layer of titanium metal. Therefore, some oxygen will be freed and can diffuse into the titanium layer. In the low temperature of the react process, the oxygen is gettered at the grain boundaries of the titanium metal and will retard the silicon atom from outdiffusion across grain boundaries. The introduced oxygen impurity apparently performs a function partially analogous to that of the nitrogen in the traditional self-aligned TiSi.sub.2 process. Another possibility for the retardation of the silicon atom from outdiffusion is the stress caused by the deposited oxide layer. The stress will prevent a volume change which is required during TiSi.sub.2 formation in the Ti layer.
However, one of the shortcomings of the oxide cap process of the patent has to do with the plasma etch employed to remove the oxide cap. More particularly at the completion of the fabrication process, a plasma etch or oxide etch is used to remove the oxide layer. However, the plasma etch is anisotropic, since it is purely a vertical etch, i.e. the plasma etches down vertically along horizontal surfaces. However, portions of the circuit geometry involve nearly vertical surfaces which are covered by the oxide layer. Because the plasma etch is required to also etch off this nearly vertical oxide layer, it requires a very long over-etch. If these portions of the vertical oxide layer are not removed by the etch, then that leaves portions of the oxide layer covering the unreacted metal titanium. Then any subsequent steps to remove the titanium (because it is covered by unremoved portions of the oxide layer) leaves portions or filaments of metal titanium which then short out the circuitry, for short metal etches, or floating pieces of oxide that increase default density, for long metal etches.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, an improved method for oxide capped titanium silicide formation to manufacture integrated circuits is provided.