Process for Forming TiSi.sub.2 Layers of Differing Thicknesses in a Single Integrated Circuit

This invention relates to the processes used in the fabrication of integrated circuits. More specifically this invention relates to a process for providing refractory metal silicide layers, such as TiSi.sub.2, of optimal thickness in differing portions of the integrated circuit.

The use of TiSi.sub.2 to increase the conductivity of silicon and polycrystalline silicon areas is well known in the art. Field effect transistor 10 in FIG. 1 includes TiSi.sub.2 layers 6, 7, and 8 which increase the conductivity of gate 4, source 2, and drain 3, respectively. TiSi.sub.2 layers 6, 7, and 8 are formed using the direct reaction process. This process involves depositing a layer of titanium on the surace of transistor 10 and baking transistor 10 to cause the titanium layer to react with the silicon and polysilicon areas which are in contact with the titanium layer. The unreacted titanium is then removed.

The simple direct reaction process has critical limitations when used to form TiSi.sub.2 layers in both a polysilicon layer and doped areas in a substrate when used in the fabrication of very large scale integrated circuits. In very large scale integrated circuits, doped regions generally have a depth of approximately 0.2 microns. Thus the titanium layer deposited on the surface of the integrated circuit must be limited to about 0.1 microns because the reacting titanium will diffuse into the doped region and 0.1 microns of titanium will produce a layer of TiSi.sub.2 approximately 0.15 microns deep. If a thicker layer of titanium is used, the titanium disilicide layer may diffuse through (punch-through) the doped region to the substrate and thus cause excessive leakage current from the doped region to the substrate. However, to provide a TiSi.sub.2 layer which reduces the sheet resistance of the polycrystalline silicon layer to a minimum (i.e. less than 1 ohm/sq.), a TiSi.sub.2 layer of at least 0.15 microns of titanium is necessary. Thus using the prior art direct reaction method, the polycrystalline silicon regions cannot be provided with a TiSi.sub.2 layer which minimizes the sheet resistance of the polycrystalline silicon layer in very large scale integrated circuits.

Using a process in accordance with the teachings of this invention, an integrated circuit may be fabricated providing refractory metal silicide layers, such as TiSi.sub.2, of differing thicknesses to provide optimal reductions in the sheet resistances of the regions in which refractory metal silicide layers are formed. In one embodiment of the present invention a field effect transistor having a polysilicon gate is fabricated to provide a gate having optimally minimized sheet resistance and source and drain regions having TiSi.sub.2 layers of the appropriate thickness to avoid punch-through leakage problems.

FIG. 1 is a cross-sectional diagram of a prior art field effect transistor including TiSi.sub.2 layers in its gate, source and drain formed by the simple direct reaction process;

FIGS. 2a through 2h are cross-sectional diagrams illustrating the processing steps of one embodiment of the present invention.

FIGS. 2a through 2h are cross-sectional diagrams depicting the processing steps of a preferred embodiment of this invention which are used to fabricate a field effect transistor. Field oxide regions 26 and 27 are grown using techniques well known in the art. On silicon substrate 20 a silicon dioxide layer having a thickness of 250 angstroms is formed by thermal oxidation at temperature of 900.degree. C. for approximately 30 minutes. On the surface of silicon dioxide layer 21 a polycrystalline layer having a thickness of approximately 4,000 angstroms is deposited by low pressure chemical vapor deposition (LPCVD) at a temperature of 700.degree. C. for approximately 30 minutes. On the surface of polycrystalline silicon layer 22 a silicon nitride layer having a thickness of approximately 150 angstroms is deposited by LPCVD at a temperature of 700.degree. C. for approximately 5 minutes. Silicon nitride layer 23 and polycrystalline silicon layer 22 are patterned using photolithographic techniques well known in the art to provide the structure shown in FIG. 2b. Doped regions 24 and 25 are formed by an ion implantation through silicon dioxide layer 21 of arsenic ions having an energy of 60 kiloelectron-volts at a density of 10.sup.14 ions/cm.sup.2. This ion implantation is then driven in and annealed at a temperature of 900.degree. C. for approximately 20 minutes. Sidewall silicon dioxide areas 28 of FIG. 2c are thermally grown in an oxygen ambient at a temperature of 900.degree. C. for approximately 10 minutes. This thermal oxidation step also increases the thickness of silicon dioxide layer 21. It is critical that the thickness of silicon dioxide layer 21 is increased because a 250 anstrom layer of silicon dioxide is insufficient to prevent the formation of titanium silicide in doped regions 24 and 25 during the step which forms a titanium silicide layer in polycrystalline silicon gate 22 (described below). However, silicon dioxide layer 21 must be thin enough under polycrystalline silicon gate 22 for proper operation of the field effect transistor formed in the described embodiment of this invention.

Silicon nitride layer 23 is removed using techniques well known in the art and titanium layer 29 is deposited by sputtering to a thickness of approximately 0.15 microns to provide the structure shown in FIG. 2d. Titanium layer 29 is preferably between 0.15 and 0.2 microns thick. This amount of titanium when reacted with polysilicon layer 22 will provide a sheet resistance in polysilicon layer 22 of less than 1 ohm/sq. Titanium layer 29 is reacted with polycrystalline silicon layer 22 by baking at a temperature approximately 675.degree. C. for approximately 30 minutes. Titanium layer 29 will react with polycrystalline silicon layer 22 to form TiSi.sub.2 and only with polycrystalline silicon layer 22 because all other areas of the integrated circuit formed in substrate 20 are covered by silicon dioxide which will not react with titanium layer 29. The unreacted titanium is then removed by using a mixture of hydrogen peroxide and sulphuric acid. The result is TiSi.sub.2 layer 30 as shown in FIG. 2e.

Oxide layer 31 in FIG. 2f is formed using low pressure chemical vapor deposition. Silicon dioxide layer 31 and silicon dioxide layer 21 are etched a highly anisotropic plasma etch. Source region 34 and drain region 35 are formed by implanting arsenic ions at an energy of 80 kiloelecton-volts at a density of 5.times.10.sup.15 ions/cm.sup.2 and driven in at a temperature of 975.degree. C. for approximately 10 minutes.

The process described in the present embodiment provides source and drain areas having a stepped profile in a P-channel field effect transistor which provides preferred high voltage characteristics. Although the described embodiment includes a field effect transistor having stepped source and drain regions, the scope of this invention is clearly not limited to field effect transistors let alone field effect transistors with stepped source and drain regions.

A layer of Ti (not shown) approximately 0.1 microns thick is then deposited on the surface of the structure of FIG. 2g and is reacted with source region 34 drain region 35 and polycrystalline silicon gate 22 at a temperature of 675.degree. C. for approximately 30 minutes to provide TiSi.sub.2 regions 36 and 37 and to increase the thickness TiSi.sub.2 region 30 as shown in FIG. 2h. The thick sidewalls provided by sidewall silicon dioxide regions 28, 32, and 33 prevent a TiSi.sub.2 conductor from forming between either TiSi.sub.2 region 30 and TiSi.sub.2 region 37 or TiSi.sub.2 region 30 and TiSi.sub.2 36 and formation of such a conductor is futher prevented by using the process described in copending patent application Ser. No. 492,069, Method of Forming a Metallic Silicide, which is assigned to the assignee of this application. TiSi.sub.2 regions 36 and 37 are preferrably formed from a titanium layer approximately 0.1 microns thick, thus preventing TiSi.sub.2 layers 36 and 37 from punching through source region 34 and drain region 35 respectively to substrate 20.

Although specific embodiments of the present invention are described in the specification, it is not to be construed as limiting the scope of the invention. Other embodiments of the present invention will become apparent to those skilled in the art in light of the teachings of this specification.