Method for Selective Deposition of Layer Structures Consisting of Silicides of HMP Metals on Silicon Substrates and Products So-Formed

1. Field of the Invention

The invention relates to integrated semiconductor circuits and somewhat more particularly to a method of selectivity depositing layer structures composed of silicides of high melting point metals on silicon substrates, such as are used in thin-film and semiconductor technology.

2. Prior Art

In VLSI (very large scale integration) technology, highly doped polysilicon gate-contact material in MOS devices is replaced with a layer combination exhibiting a lower resistance, for example, one composed of a 300 nm thick doped polysilicon layer and a 200 nm thick metal disilicide layer wherein the metal is molybdenum, tungsten or tantalum. However, problems exist in etching such double layers so that no steps or overhangs occur at the edges of the structures or tracks (or lines) composed of such metal silicides.

A technique for overcoming this problem comprises a two-stage dry etching process, or "lift-off" technique. In this regard, reference is made to a lecture by Kinsborn, Fraser and Vratny documented in the Abstracts of the Fifth International Thin-Film Congress in Herzlia (Israel) from September 21 through 25 (1981), on page 193.

Avoiding this relatively complicated involved process would effect an increase in device yield and provide a significant reduction in fabrication costs. With the use of the CVD (chemical vapor deposition) process, which deposits a material from a gaseous phase after thermal decomposition of a gaseous compound containing such material, it is possible to selectively deposit tungsten on silicon and not on silicon oxide (SiO.sub.2), see C. M. Melliar-Smith et al, J. Electrochem. Soc., Vol. 121, No. 2, (1973) pages 298-303, particularly page 299. Accordingly, it is possible to structure doped polysilicon in a problem-free manner according to hitherto known methods and to subsequently coat only the resultant polysilicon track with tungsten. A prerequisite for this, however, is that all regions of the device which are not to be coated with tungsten, must be occupied by SiO.sub.2. A disadvantage of this special technique is that tungsten forms volatile oxides so that a protective layer must be utilized. Further, tungsten reacts with a silicon base or substrate at temperatures above 600.degree. C., which produces an undesired roughness of the layer surface.

Since temperatures up to 1100.degree. C. are utilized in many VLSI processes, the advantages of a selective deposition of tungsten on silicon tracks cannot be fully exploited. Moreover, it has turned out that tantalum silicide is superior to tungsten silicide in regard to adhesion and oxidizability and forms a SiO.sub.2 passivation layer at higher temperatures.

The invention provides a method for selectively depositing a silicide of a high melting point (HMP) metal on substrates having at least some silicon regions whereby the stability of the deposited silicide relative to oxygen is insured at higher temperatures and the so-deposited silicide exhibits only a slight tendency to react with the silicon substrate, even at temperatures above 600.degree. C. Further, by practicing the principles of the invention, the production process of VLSI circuits is significantly simplified, particularly in generating contact track levels and, as a result, the yield of functional components is increased.

In accordance with the principles of the invention, layer structures composed of HMP metal silicides are selectively deposited on substrates containing at least some silicon regions, as are particularly used in thin-film and/or semiconductor technology, by thermally decomposing gaseous silicon and halogen compounds containing the HMP metal in a reaction gas and depositing the metal silicide from the gaseous phase onto a controllably heated substrate containing silicon regions while providing a gaseous hydrogen halide, such as hydrogen chloride, hydrogen bromide or hydrogen iodine, to the reaction gas and adjusting the substrate deposition temperature and composition of the reaction gas to values at which silicide nucleation in regions of the substrate other than silicon regions is suppressed during deposition from the gaseous phase due to the presence of the hydrogen halide.

In certain embodiments of the invention, the gaseous silicon compound comprises a halogenated silane which splits-off a hydrogen halide during the thermal decomposition. In other embodiments, a gaseous hydrogen halide is added to the reaction gas during the depositing process from a gaseous phase. The reaction gas can also be diluted with hydrogen and/or inert gases.

In an exemplary embodiment of the invention, a reaction gas is comprised of a gaseous HMP-metal halogenide (MeCl.sub.x) wherein Me is a high melting point metal and x is a number, hydrogen (H.sub.2) and a halogenated silane compound (SiH.sub.4-n Cl.sub.n) wherein n is a number less than 4, and which has a mixing ratio of about 1(MeCl.sub.x):10(H.sub.2):2(SiH.sub.4-n Cl.sub.n), while the substrate is maintained at a temperature in the range about 450.degree. through about 900.degree. C. at an overall gas pressure in the range of about 50 through about 1000 Pa.

In certain embodiments of the invention, where tantalum silicide structures are desired, the reaction gas is provided with about 1 part by volume of tantalum pentachloride, about 2 parts by volume of dichlorosilane and about 10 parts by volume of hydrogen.

In certain embodiments of the invention, where molybdenum silicide structures are desired, the reaction gas is provided with about 1 part by volume of molybdenum pentachloride, about 2 parts by volume of dichlorosilane and about 10 parts by volume of hydrogen.

In certain embodiments of the invention, after deposition of a desired HMP-metal silicide structure is completed, the resultant coated substrate is subjected to a surface-wide etching process, for example in a hydrofluoric acid/nitric acid mixture or in a plasma to remove a very thin silicide film that may form on substrate regions other than silicon regions.

The principles of the invention are particularly useful for producing contact tracks (or lines) composed of tantalum, tungsten or molybdenum silicide for integrated semiconductor circuit, particularly VLSI circuits.

The FIGURE is an enlarged, somewhat schematic view of an apparatus useful in the practice of the invention

The invention will now be further explained on the basis of an exemplary embodiment, for example, for selectively depositing a contact metal layer composed of tantalum disilicide (TaSi.sub.2) on a silicon substrate.

An enclosed CVD reaction housing 6 having a reaction chamber therein is provided with a gas inlet 6a connected to a controllable gas source, schematically indicated by arrow 1. The reaction gas in the gas source comprises, in the exemplary embodiment under discussion, a mixture of 1 part by volume of tantalum pentachloride (TaCl.sub.5), 10 parts by volume of hydrogen (H.sub.2) and/or an inert gas, and 2 parts by volume of dichlorosilane (SiH.sub.2 Cl.sub.2). A vacuum pump (not shown) is also connected with the reaction chamber of housing 6, such as schematically indicated by arrow 9, for evacuating the reaction chamber of undesired reaction products and for aiding in maintaining a desired pressure within the enclosed reaction chamber.

A substrate support means 8 is positioned in the interior of housing 6 and is coupled with a controllable electrical heating means 2 for maintaining a desired temperature on a substrate 5. Suitable sealing and insulating means 6b are provided at the ingresses and egresses of the various components communicating with the reaction chamber so as to maintain an enclosed atmosphere. The substrate 5 is comprised of a silicon body having a continuous insulating layer 4 thereon, which in the exemplary embodiment is composed of SiO.sub.2. Structured discrete (or discontinuous) silicon layers 3 are positioned on insulating layer 4.

During operation, the composite substrate (elements 3, 4 and 5) is heated to about 850.degree. C. via the heating means 2 and the abovedescribed reaction gas is controllably fed into the interior of the housing 6 until an overall gas pressure of about 133 Pa is attained. These temperature-pressure conditions are then maintained during the deposition process. Under these conditions, the reaction gas decomposes within the low-pressure CVD reactor housing 6 and a tantalum disilicide layer 7 is deposited only on the silicon layer 3 and not on the SiO.sub.2 layer 4, covering the underlying silicon body 5.

The growth rate under the foregoing conditions amounts to about 100 nm/min.

If desired, a different insulating layer, such as one composed of aluminum oxide (Al.sub.2 O.sub.3), tantalum pentoxide (Ta.sub.2 O.sub.5) or silicon nitride (Si.sub.3 N.sub.4) can be utilized as the insulating layer 4, instead of the SiO.sub.2 layer present in the exemplary embodiment. Further, molybdenum pentachloride or tungsten pentachloride can be utilized in the reaction gas, instead of tantalum pentachloride and trichlorosilane or monochlorosilane can be utilized in place of dichlorosilane. The deposition process parameters change in accordance with the amount of hydrogen halide, such as hydrogen chloride, formed during the thermal decomposition and are readily determined empirically.

Should very thin silicide films nonetheless incidentally form on areas other than on the free or discontinuous silicon regions 3 of a composite substrate during the HMP-metal silicide deposition, then the entire resultant structure can be subjected to a surface-wide etching process, for example, in a hydrofluoric acid/nitric acid mixture or in a plasma.

The following considerations may, at least in part, explain the substrate-associated, and, thus, the selective deposition of the metal silicides:

On the basis of thermo-dynamic calculations, tantalum disilicide, in order to be formed in accordance with the equation:

must be capable of formation at temperatures around 800.degree. C., independently of the substrate material. The fact that, with the inventive method, the foregoing reaction does not occur on, for example, SiO.sub.2, or occurs only very slowly thereon, could be connected to the fact that the reaction gas mixture releases an excess of, for example, chlorine, or hydrogen chloride, which inhibits the course of the reaction in the described manner. Hydrogen disassociating at the silicon surface could contribute to the formation of monosilane so that a reaction in accordance with the following equation:

occurs at the silicon surface. The foregoing reaction is extremely favored thermo-dynamically and could thus explain the high selectivity of HMP metal silicide deposition on silicon surface areas in accordance with the principles of the invention.

As is apparent from the foregoing specification, the present invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. For this reason, it is to be fully understood that all of the foregoing is intended to be merely illustrative, and is not to be construed or interpreted as being restrictive or otherwise limiting of the present invention, excepting as it is set forth and defined in the hereto-appended claims.