METHOD AND APPARATUS FOR HEATING A SEMICONDUCTOR SUBSTRATE UNDER REDUCED PRESSURE

Abstract: A semiconductor substrate (9) arranged in a vacuum treatment chamber (1) is heated by using microwave radi­ ation. The chamber (1) is provided with a window plate (4) pervious to microwave radiation, and the substrate (9) is placed on the window (4). A microwave radiation generator (6) is arranged outside the chamber. The microwave radi­ ation produced by the generator (6) is guided to the window plate (4), penetrates it, and is absorbed by the semiconduc­ tor substrate (9), with the result that the substrate (9) is di­ rectly heated to a predetermined temperature in a short time. Since no microwave radiation leaks through the sub­ strate (9), no other part is heated.

Description: The present invention relates to the production of semiconductor devices, and more particularly, to a method and an apparatus for heat treating a semiconductor substrate under a reduced pressure. The present invention may be used in the formation of a thin layer of conductor, insulator or semiconductor on a semiconductor substrate. Aluminium (AI) is generally used as the material to form electrodes and conductor lines of semiconductor devices. The aluminium is usually deposited on the semiconductor substrate by a vacuum evaporation method to form a thin aluminium layer. Recently, in addition to aluminium, aluminium alloys (e.g. Al-Si, Al-Si-Cu), refractory metals (e.g. tungsten, molybdenum) and silicides of the refractory metals have come into use for the electrodes and conductor lines. The formation of a thin layer of the metals and silicides has primarily been by sputtering, since the vacuum evaporation method is unsuitable for refractory metal and an alloy composition of the thin layer cannot easily be controlled when it is formed by vacuum evaporation. When sputtering is used to deposit one of the above materials on a semiconductor substrate to form a thin layer, the substrate is heated to prevent the electrodes and conductor lines from disconnection at the shoulder portions of an uneven surface of the substrate, to decrease the resistivity, to prevent electromigration, to provide a smooth surface, and to increase the grain size of the conductor material. To increase the substrate temperature in a conventional magnetron sputtering apparatus, a heater, e.g. at least one infra red lamp is placed in the vacuum treatment chamber. In the case of a planetary type magnetron sputtering apparatus for batch treatment, the holder supporting the semiconductor substrate rotates and is heated. Therefore, the size of the planetary holder must be made larger. In practice, it is not always easy to make a heating system which increases the substrate temperature sufficiently. Even if such a heating system can be made, the heater heats parts of the apparatus that do not need to be heated, resulting in outgassing from these parts and poor operation of any moving parts. In another type of magnetron sputtering apparatus, the sputtering treatment is carried out in an in-line system. In this case, the apparatus can be made smaller. However it is difficult satisfactorily to heat the semiconductor substrates in a short time in the vacuum treatment chamber. Accordingly, the substrates have to be preheated. But even with this pretreatment it is not easy to heat the substrates and, in practice, the substrates are usually only heated to an unsatisfactory degree. According to a first aspect of this invention a method of heating a semiconductor substrate held under a reduced pressure in a vacuum treatment chamber during deposition of a film on it comprises the steps of placing a semiconductor substrate in a vacuum treatment chamber having a window transparent to microwave radiation, to expose a surface of the substrate on which a film is to be deposited to a reduced pressure within the vacuum treatment chamber and to cover the window transparent to microwave radiation with the opposite surface of the semiconductor substrate in contact with the window, and heating the semiconductor substrate by irradiating it with microwave radiation by directing a beam of microwave radiation through the window, the substrate absorbing the microwave radiation and acting as a shield to prevent the microwave radiation reaching the inside of the chamber. US-A-3814983 illustrates an apparatus for generating plasmas including a vacuum treatment chamber transparent to microwave radiation, and a microwave radiation generator outside the chamber and arranged to radiate microwave radiation into the chamber. The microwave radiation enters the inside of the vacuum treatment chamber and generates a plasma which is used to carry out a chemical reaction on a material located in the vacuum treatment chamber. FR-A-2335043 illustrates an apparatus for vacuum treatment of semiconductor substrates including a vacuum chamber having a window. According to a second aspect of this invention, an apparatus for heating a semiconductor substrate under reduced pressure, according to Claim 2, is provided. The term "microwaves" and "microwave radiation" describes electromagnetic radiation having a wavelength of between 0.1 mm and 1 m. Preferably the window transparent to microwave radiation is a plate made of quartz, glass or ceramic material. It is also preferred that a microwave generator located outside the vacuum treatment chamber is coupled to the window in the chamber by a waveguide. A particular embodiment of the present invention will now be described with reference to the accompanying drawings; in which: Figure 1 is a diagrammatic sectional elevation of a magnetron sputtering apparatus and microwave generator, and, Figure 2 is a graph showing the relationship between the temperature of a semiconductor substrate and the microwave application time. Referring to Figure 1, there is shown a magnetron sputtering apparatus for formation of a thin layer on a semiconductor substrate as an apparatus for treating and heating a semiconductor substrate according to the present invention. The apparatus comprises a vacuum treating chamber 1 with a gas inlet pipe 2 and an exhaust pipe 3, a plate 4 which covers air-tightly an opening 5 in the shell of the chamber 1, a microwave generator 6, and a conventional magnetron sputtering gun unit 7. The exhaust pipe 3 is connected to a vacuum pump (not shown). The microwave generator 6 is provided with a waveguide 8 for guiding microwaves to the substrate 9. On the plate 4 a semiconductor substrate 9 is arranged with the surface to be covered with a thin layer exposed to the inside space of the chamber 1. Since the plate 4 is used as a window transparent to microwaves, the plate 4 must be made of a material transparent to microwaves, such as quartz, glass and ceramic (e.g., Al₂O₃, ZrO₂, 3Al₂O₃ · 2SiO₂, MgO · Al₂O₃ and the like). The plate 4 should withstand an elevated temperature at which the substrate is heated. Furthermore, the size of the plate 4 should be the same as or slightly smaller than that of the substrate 9. The radiation microwaves penetrate the plate 4 and are absorbed by the substrate 9 to heat it up. The magnetron sputtering gun unit 7 comprises a target 10 made of, e.g., tungsten, a magnet 11, an electric power source (not shown), and a shutter 12. A method for forming a thin layer, e.g., a tungsten film, on a semiconductor substrate by using the above-mentioned magnetron sputtering apparatus is explained hereinafter. First, the silicon substrate 9 is mounted on the quartz plate 4. Argon gas is fed into the vacuum treating chamber 1 through the inlet pipe 2. At the same time the chamber 1 is evacuated through the exhaust pipe 3 to maintain the inside pressure at a constant level of from 10 _2 to 10 _3 Torr. Then, the microwave generator 6 is operated to radiate microwaves. The microwaves are guided by the waveguide 8 to reach the substrate 9 through the plate 4. The silicon substrate 9 absorbs the microwave power, thus being heated up to a temperature of, e.g., approximately 600°C. Parts of the apparatus which are made of metal and grounded e.g., a stainless steel shell of the chamber 1 do not absorb microwaves. Thus these parts are not heated, even if irradiated with the microwaves. If microwaves having a frequency of 2.45 GH Z (wavelength: approximately 12 cm) and a power of 500 W are applied to a silicon substrate 9 having a thickness of approximately 0.6 mm, the temperature of the substrate is increased as shown in Fig. 2. In this case, the silicon substrate 9 can be heated up to approximately 600°C in a very short time of approximately 70 seconds. Then, a negative high voltage is applied to the target 10 of tungsten, so that the tungsten of the target 10 is sputtered by argon ion bombardment. When the shutter 12 is opened, the sputtered tungsten is deposited on the surface of the silicon substrate 9 to form a thin layer. When a thin tungsten layer having the predetermined thickness is obtained, the shutter 12 is closed. The application of the negative voltage to the target 10 is then stopped, and the operation of the microwave generator 6 is stopped. After the vacuum condition is cancelled, the silicon substrate 9 covered with the thin tungsten layer is removed from the chamber 1. In accordance with a conventional process for producing a semiconductor device, the thin tungsten layer is selectively removed by a photoetching method to form a pattern of electrodes and conductor lines. According to the microwave heating of the present invention, the microwaves penetrating the plate 4 are entirely absorbed by the semiconductor 9. Therefore, just the substrate 9 can be directly heated with a good efficiency to a predetermined temperature in a short time. The heating rate can be precisely controlled. Accordingly, the heating operation becomes easy, and the heating time is remarkably reduced, as compared with a conventional heating means. Furthermore, since no microwaves leak out of the semiconductor substrate 9 into the vacuum treating chamber 1, i.e., the substrate 9 shields the inside space from microwaves, no other parts heated and the generation of plasma is prevented. Without the substrate on the plate, the microwaves might aid the generation of plasma. Since a microwave heating method is used as the heating means, it is unnecessary to locate a heater within the vacuum treating chamber. Furthermore, the microwaves can be guided by a waveguide even from a relatively long distance. Accordingly, the apparatus can be easily designed, which enables in remarkable miniaturization and simplification of the apparatus. It is possible to form a thin layer made of not only a single metal such as tungsten, but also alloys, silicides and the like in the above-mentioned magnetron sputtering apparatus. For example, a thin aluminium-silicon (Al-Si) layer having a grain size of approximately 1 to 2 µm can be formed on a silicon substrate in the apparatus illustrated in Fig. 1. In this case, the target of the magnetron sputtering unit is made of an Al-Si (10 wt%) alloy. The inside pressure of the vacuum treating chamber, i.e., the pressure of the argon gas atmosphere, is maintained at approximately 10 _3 Torr. The substrate is heated to approximately 300°C by means of the microwave heating. The Al-Si target is sputtered by argon ions to deposit sputtered Al-Si on the substrate so as to form a thin layer. With conventional sputtering apparatus and method the resultant thin Al-Si layer would have a grain size of from 0.2 to 0.5 µm, because the semiconductor substrate could not be satisfactorily heated. Furthermore, a thin tantalum silicide (TaSi₂) layer having a resistivity of from about 13 to 16 µΩ-cm can be formed on a silicon substrate in the above-mentioned apparatus. In this case, the target is made of TaSi₂. An argon gas atmosphere having a pressure of approximately 10 _3 Torr is formed within the vacuum treating chamber. The substrate is heated to approximately 600°C by means of the microwave heating. The TaSi₂ target is sputtered by argon ions to deposit sputtered TaSi₂ on the substrate so as to form the thin layer. With a conventional sputtering apparatus and method, the resultant thin TaSi₂ layer would have a resistivity of from about 25 to 30 µΩ-cm. The present invention is also applicable to a chemical vapor deposition (CVD) method. For example, a silicon dioxide (SiO₂) layer can be formed on a silicon substrate by using the above-mentioned apparatus without the sputtering unit. In this case, a mixed gas atmosphere having a pressure of from 1.3 to 2.0 Torr is formed within the vacuum treating chamber. The mixed gas consists of a silane (SiH₄) gas diluted with an argon gas (concentration of SiH₄: approximately 1%) and a nitric oxide (NO) gas. The substrate is heated to a temperature of from 750 to 850°C by means of microwaves as mentioned hereinbefore. Thus, the SiO₂ layer is grown on the substrate in accordance with the CVD process. The breakdown voltage of the formed SiO₂ layer becomes higher as the substrate temperature becomes higher. For example, the breakdown voltages of a SiO₂ layer formed at substrate temperatures of about 750°C and about 850°C are approximately 7.5 MV/cm and 8 to 9 MV/cm, respectively. The present invention is also applicable to an expitaxial growth method. For example, a single crystalline silicon layer can be epitaxial grown on a silicon substrate by using the above-mentioned apparatus without the sputtering unit. In this case, a mixed gas atmosphere having a pressure of from 0.1 to 10 Torr is formed within the vacuum treating chamber. The mixed gas consists of a SiH₄ gas diluted with a hydrogen gas (concentration of SiH₄: approximately 1%) and a hydrogen gas. The substrate is heated to a temperature of approximately 1050°C by means of microwaves as mentioned hereinbefore. Thus, the silicon single crystalline layer is formed in accordance with the epitaxial process. The dislocation density of the thus obtained silicon epitaxial layer is equal to or less than that of the silicon substrate and may be substantially zero. In the case where the formed silicon epitaxial layer has the N conductivity type, a resistivity of approximately 1 Ω-cm and a thickness of approximately 5 µm, the carrier life is from 0.5 to 1 msec. The present invention is applicable to a vacuum evaporation method in addition to the above-mentioned sputtering method, CVD method and epitaxial method. It is possible to form a thin layer for electrodes made of refractory metals such as molybdenum and titanium, silicides of the refractor metals, and alloys such as Al-Si-Cu and Al-Si, and to form a thin insulating layer made of insulator such as silicon nitride (Si₃N₄) and silicon oxynitride (SiO x N y ), in addition to the above-described materials. Furthermore, the present invention can applied to not only a batch system but also an in-line system. The plate pervious to microwaves may be arranged at the side or the top of the vacuum treating chamber shell, in place of the bottom thereof.

Claims: 1. A method of heating a semiconductor substrate held under a reduced pressure in a vacuum treatment chamber during deposition of a film on it comprises the steps of placing a semiconductor substrate in a vacuum treatment chamber having a window transparent to microwave radiation, to expose a surface of the substrate on which a film is to be deposited to a reduced pressure within the vacuum treatment chamber and to cover the window transparent to microwave radiation with the opposite surface of the semiconductor substrate in contract with the window, and heating the semiconductor substrate by irradiating it with microwave radiation by directing a beam of microwave radiation through the window, the substrate absorbing the microwave radiation and acting as a shield to prevent the microwave radiation reaching the inside of the chamber. 2. An apparatus for heating a semiconductor substrate under a reduced pressure including a vacuum treatment chamber (1) having a window (4) transparent to microwave radiation, and a microwave radiation generator (6) outside the chamber (1) and arranged to radiate microwave radiation into the chamber (1) through the window (4) wherein the vacuum treatment chamber (1) includes a semiconductor substrate (9) mounted in contact with and to cover the window (4) and means to deposit a film on a surface of the semiconductor substrate (9) remote from the window (4), in use, the substrate (9) being heated by absorbing the microwave radiation generated by the generator (6) and shielding the inside of the chamber (1) from the microwave radiation. 3. An apparatus according to claim 2, wherein the window (4) transparent to microwave radiation is a plate made from quartz, glass or ceramic material. 4. An apparatus according to claim 2 or 3, wherein the apparatus also includes a waveguide linking the microwave radiation generator (6) and the window (4) transparent to microwave radiation.

Inventors: INOUE, MINORU
Applicants: FUJITSU LIMITED
Application year: 1983
Main classification: H01L21
IPC codes: H01L21, C23C14