SEMICONDUCTOR DEVICE HAVING A CONDUCTIVE LAYER CONSISTING OF A HIGH-MELTING POINT METAL SILICIDE AND A METHOD FOR MANUFACTURING SUCH A SEMICONDUCTOR DEVICE

Abstract: There is proposed a semiconductive device comprising a conductive layer (4) consisting of a high-melting point metal silicide having a formula MSi x (wherein M is a high-melting point metal and x is a value falling within 2.0 < × ≦ 4.0). Said conductive layer (4) has a sheet resistance of not more than 5 Ω/□. A method for manufacturing said semiconductor device is also proposed. Said method comprises the step of depositing on a semiconductor substrate (1) through an insulating film (2) a metal silicide film (4) having the formula MSi x , and annealing the metal silicide film (4).

Description: The present invention relates to a semiconductor device which uses a film of a high-melting point metal silicide which is suitable as a low-resistance electrode or wiring (to be referred to as a conductive material hereinafter). Polysilicon has widely been used as a conductive material for forming electrodes or wiring layers. However, it is very difficult to obtain a polysilicon layer having a sheet resistance of 20 Ω/□ or less even if an impurity is diffused to a high concentration (even with a film thickness of 3,000 to 4,000 Å). Meanwhile, demand for higher performance semiconductor devices is strong today. In order to achieve this, it is considered necessary to decrease the sheet resistance of an electrode material to at least 5 Ω/□ or less and preferably to about 1/10 of that of polysilicon. In an attempt to find a substance which satisfies this requirement, many studies are being made on silicide films of high-melting point metals, in particular, a molybdenum silicide film. Molybdenum silicides having three different formulae are known: MoSi₂, Mo₃Si, and Mo₅Si₃. MoSi₂ is regarded to be most stable against heat. For this reason, it has been general practice to use a molybdenum silicide film consisting of MoSi₂ as an electrode material. However, when a molybdenum silicide film having a formula of MoSi₂ is exposed to an oxidizing atmosphere at a high temperature such as 1,000°C, an SiO₂ film is formed on the surface of the molybdenum silicide film. However, anomalous oxidation locally occurs at times to form holes or cracks in the MoSi₂. In some cases, separation of the film is caused due to stress. Furthermore, although the reasons are not clear, the characteristics of a resultant MOS transistor change unexpectedly. Accordingly, the use of a molybdenum silicide film having a formula of MoSi₂ has not been necessarily suitable for the manufacture of a semiconductor device. From the article of R. S. Nowicki et al published in J. of Electrochemical Soc. Vol. 128 (1981.03) 3, p. 562-567 it is known that Molybdenum silicide films deposited by RF diode co-deposition exhibit no degredation during heat-treatment when the Mo/Si ratio is ≦0,5. The properties of MoSi 2,8 have been investigated. The present invention has been made in consideration of this and has for its object to provide a semiconductor device having a high-melting point metal DC sputtered silicide film which is stable after a high-temperature oxidizing process and which is suitable as a conductive material, and to a method for manufacturing such a semiconductor device. In order to achieve the above object, there is provided according to the present invention a semiconductor device according to claim 1. This invention can be more fully un understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a sectional view showing a MOS transistor according to the present invention; Fig. 2 is a sectional view of an apparatus for forming a high-melting point metal silicide film according to the present invention; Fig. 3 is a graph showing the sheet resistances as a function of the value of x of MoSi x of a molybdenum silicide film according to the present invention; Fig. 4 is a graph showing the relative probability of occurrence of anomalous oxidation as a function of the value of x of MoSi x of a molybdenum silicide film according to the present invention; Fig. 5 is a graph showing the thermal stress as a function of the value of x of MoSi x of a molybdenum silicide film according to the present invention; Fig. 6 is a graph showing a flat band voltage V fb as a function of the value of x of MoSi x of a molybdenum silicide film according to the present invention; Fig. 7 is a graph showing a sheet resistance as a function of the value of x of WSi x of a tungsten silicide film according to the present invention; and Fig. 8 is a graph showing thermal stress as a function of the value of x of WSi x of a tungsten silicide film according to the present invention. In a high-melting point metal silicide having a formula MSi x (where M is a high-melting point metal), when the value of x is kept within the range of 2<x≦4.0, the sheet resistance may bekept at 5 Ω/□ or less, anomalous oxidation and generation of thermal of thermal stress may be prevented, and variations in the threshold voltage of a transistor may be suppressed to the minimum. The high-melting point metal silicide as described above is suitable as a conductive material for a semiconductor device. The present invention has been established based on this finding. The method for forming a high-melting point metal silicide film comprising such a high-melting point metal silicide will now be described with reference to a case wherein a gate electrode of a MOS transistor as shown in Fig. 1 is formed. Fig. 1 shows a state wherein an impurity diffusion layer (source/drain region) 3 is formed in a surface layer of a semiconductor substrate 1. A gate electrode 4 comprising a high-melting point metal film such as a molybdenum silicide film is formed on the semiconductor substrate 1 through a gate insulating film 2. Fig. 2 shows the method for forming such a gate electrode 4. A semiconductor substrate 1 is first placed in a vacuum chamber 20. High-purity molybdenum 22 and silicon 24 are arranged next to each other at a distance of about 60 mm from the semiconductor substrate 1 in the vacuum chamber 20. After the vacuum chamber 20 is evacuated to about 1 x 10⁻⁷ Torr, argon gas (Ar) is introduced into the vacuum chamber 20 to a pressure of 2x10⁻³ Torr through a variable valve 26 so as to perform DC sputtering for 3 to 4 minutes, thereby forming a molybdenum silicide film 28 on the semiconductor substrate 1 through the gate insulating film 2. The thickness of the resultant molybdenum silicide film 28 may be controlled by controlling the sputtering time. The composition of the molybdenum silicide film formed on the semiconductor substrate 1 may be freely selected by suitably changing the amounts of the molybdenum 22 and silicon 24 placed in the vacuum chamber 20. Alternatively, the composition of the molybdenum silicide film formed on the semiconductor substrate 1 may also be selected by separately controlling power applied to the molybdenum 22 and silicon 24. In the next step, annealing is performed in an oxidizing atmosphere of about 1,000°C. Annealing is performed to cause crystallization of the point metal. In the above description, silicon and molybdenum are used as the targets and a molybdenum silicide film is formed by simultaneous sputtering using these targets. The composition and various characteristics of the high-melting point metal silicide film formed by DC sputtering as described above will now be described. The characteristics of a film when x in the formula MoSi x of molybdenum silicide will be described below. Fig. 3 shows the sheet resistance (Ω/□) as a function of the value of x of a MoSi x film having a thickness of 3,000 Å. As may be seen from the graph shown in Fig. 3, the sheet resistance increases with an increase in the amount of silicon. With recent trends for larger LSls, the wiring layer is becoming thinner, and the resistance is increasing. Since such an increase in the resistance increases delay time through such a wiring layer, the wiring resistance must be lowered. Thus, the resistance, that is, the sheet resistance of the wiring layer is preferably small. Thus, from the graph shown in Fig. 3, when a molybdenum silicide film is to be formed, the value of x of the formula MoSi x must be about 4.0 or less so as to obtain a sheet resistance of 5 Ω/□ or less. Fig. 4 shows the relative probability of occurrence of anomalous oxidation as a function of the value of x in the formula MoSi x when a molybdenum silicide film having this formula is oxidized at 1,000°C. It is seen from the graph shown in Fig. 4 that the relative probability significantly changes before and after x=2; the relative probability decreases abruptly when x becomes larger than 2. Fig. 5 shows the thermal stress as a function of the value x of the formula MoSi x of a molybdenum silicide film. Dotted curve A corresponds to a molybdenum silicide film formed by annealing in a dry 0₂ gas atmosphere at 1,000°C for 30 minutes, and solid curve 8 corresponds to a molybdenum silicide film formed by annealing in N₂ gas atmosphere at 1,000°C for 30 minutes. It is seen from the graph shown in Fig. 5 that the stress abruptly decreases when the value x is 2 or more. Since the thermal stress may be decreased significantly in this manner, peeling, separation or the like of gate electrodes or capacitor electrodes formed by conventional MoSi x can be prevented. Furthermore, the effective channel length may be designed with high precision. Fig. 6 shows the flat band voltage V FB , i.e., the threshold voltage V TH of a transistor, as a function of the value of x of the formula MoSi x of a molybdenum silicide film. As may be seen from Fig. 6, when the value of x is within the range of 2.0<x≦4.0, variations in the voltage V FB are small and stable. Accordingly, MoSi x with x falling within this range may be suitably used for a semiconductor device for channel control below a gate film of a MOS transistor or the like. It is concluded from the measurements shown in the respective graphs described above that a molybdenum silicide having a formula MoSi x with x falling within the range of 2.0<x≦4.0 may be suitably used for manufacturing a semiconductor device. It is also found out that a molybdenum silicide having a formula MoSi x with x falling within the range between 2.2 and 3.5 is particularly preferable for manufacturing a semiconductor device. Although the above description has been made with reference to a case wherein molybdenum is used as a high-melting point metal, similar results were found to be obtainable with tungsten (W), tantalum (Ta), or titanium (Ti). The relationship between the sheet resistance and the value of x of a formula WSi x of a tungsten silicide film and the relationship between the thermal stress and the value of x were examined. The obtained results are shown in Figs. 7 and 8. It is seen from the graphs shown in Figs. 7 and 8 that a tungsten silicide film having a formula WSi x with x falling within the range of 2.0<x≦4.0 may be suitably used for manufacturing a semiconductor device. The experimenting conditions for obtained the results shown in Figs. 7 and 8 were the same as those for Figs. 3 and 5. Referring to Fig. 8, dotted curve C corresponds to a tungsten silicide film annealed in a dry 0 x gas atmosphere at 1,000°C for 30 minutes and solid curve D corresponds to a tungsten silicide film annealed in a N₂ gas atmosphere at 1,000°C for 30 minutes. As can be seen from the above description, according to the present invention, a high-melting point metal silicide is used for a conductive layer. This facilitates process design since the manufacturing process for polysilicon may be used. Furthermore, the metal silicide according to the present invention has a resistance which is about 1/10 of that of polysilicon. Accordingly, when such a metal silicide is used for wiring or electrodes, a high-speed semiconductor device may be manufactured. When a high-melting point metal silicide film having the composition as defined above is used for a dynamic memory, micropatterning and low resistance for high operating speed required for a dynamic memory may be satisfied. Furthermore, the film according to the present invention may be useful as a stable conductive film even in a high-temperature oxidizing atmosphere and serves as a mask for ion-implantation. The film of the present invention thus allows self-aligned formation of an impurity diffusion region. The high-melting point metal silicide used in the present invention allows good adhesion with SiO₂ and may therefore be used for gate electrodes or capacitive electrodes. According to the present invention, a high-melting Point metal M of a high-melting point metal silicide having a formula MSi x may be a combination of at least two members selected from Mo, W, Ta, and Ti.

Claims: 1. A semiconductor device which comprises a conductive layer consisting of a high-melting point metal silicide relected from molubdenum silicide, tungsten silicide, tantalum silicide and titanium silicide characterised in that the conductive layer has a formula MSI x in which M is the high-melting point metal and x is value within the range of 2.2 ≦ x ≦ 3.5, and in that the conductive layer is a DC vacuum chamber sputtered layer subjected to annealing at a high temperature of about 1000°C in an oxidising atmosphere having a sheet resistivity of not more than 5Ω/sq, the device thus having a stable flat band voltage. 2. A semiconductor device according to claim 1, characterised in that the high-melting point metal silicide comprises at least two members selected from the group consisting of molybdenum silicide, tungsten silicide, titatium silicide and tantalum silicide. 3. A semiconductor device according to claim 1, characterised in that the conductive layer forms a gate electrode of a field-effect transistor. 4. A semiconductor device according to claim 1, characterised in that the conductive layer forms a capacitor electrode. 5. A semiconductor device according to claim 1, characterised in that the conductive layer forms a wiring electrode. 6. A method of manufacturing a semiconductor device as claimed in claims 1 or 2 comprising the step of depositing a high-melting point metal silicide film relected from molubdenum silicide, tungsten silicide, tantalum silicide and titanium silicide on an insulating film disposed in turn on a semiconductor substrate, characterised in that the high-melting point metal silicide film has a formula MSi x in which M is the high-melting point metal and x is a value within the range of 2.2 ≦ x ≦ 3.5, in that said film is DC sputtered in a vacuum chamber from separate high purity targets of said high-melting point metal and silicon and in that annealing the high-melting point metal silicide film is subsequently performed in an oxidising atmosphere at a high temperature of about 1000°C.

Inventors: KAWAGUCHI, TATSUZO, OKUMURA, KATSUYA
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1983
Main classification: H01L23
IPC codes: H01L29, H01L23, H01L21