Thin film deposition method

Abstract: A thin film deposition method consists of depositing a thin film on a wafer by supplying a reactant gas molecules toward and onto the wafer within a vacuum vessel or chamber. The pressure within the vacuum vessel is set to the pressure under which the mean free path (d) of the molecules contained in the supplied reactant gas can be longer than the shortest distance (L) between the wafer and the wall of the vacuum vessel exposed to the vacuum side, or d > L. The temperature of the wafer is set to the temperature (T sub) at which the reactant gas can cause substantially the thermally decomposing reaction. The temperature of the vacuum side-exposed wall of the vacuum vessel (T wall) is set to a temperature range having the lower limit higher than the temperature (T vap) at which the saturated vapor pressure can be maintained to be equal to the partial pressure of the molecules contained in the reactant gas, and having the upper limit lower than the temperature of the wafer (T sub), or T vap < T wall < T sub.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a thin film deposition method, and, in particular, to the process for depositing a thin film on a wafer in a vacuum chamber or vessel where any reactant gas is supplied and a thin film is deposited on the wafer under the pressure range in which the molecules in the supplied gas will have no reaction or interaction upon each other in their vapor phase. Description of the Prior Art There is a conventional thin film deposition method which is known as the "gas source epitaxial growth" process. In this process, a vacuum chamber or vessel has its inner wall covered with a shroud, and is internally placed under liquid nitrogen, allowing the shroud to be cooled to liquid nitrogen temperature. The molecules in the reactant gas from its source are diffusing as it is introduced into the vacuum chamber, and the major part of those molecules may be attracted toward the inner wall and absorbed on the shroud. During the epitaxial growth process, therefore, only a small remaining part of the molecules in the reactant gas can be flying across the space directly from the source, such as a gas nozzle and other gas sources, onto the wafer surface. In this specification, this part of the molecules will be referred to hereinafter as "primary molecules". The primary molecules which are traveling from the gas source onto the wafer surface may usually have a sticking coefficient of less than 10%. It is noted that the gaseous molecules from the source are diffusing according to the cosine law just after they have been introduced into the vacuum chamber, and the proportion of the gaseous molecules that can be flying onto the wafer surface is equal to 10% to 20% of the total gaseous molecules that have been supplied from the source. It may be understood from the above description that the conventional thin film deposition method cannot utilize the reactant gas effectively during the epitaxial growth process, whose utilization is only equal to several percents of the total reactant gas. It has been described that the reactant gas supplied from its source is diffusing according to the cosine law as described above. Similarly, when a thin film is being deposited on the wafer surface during the epitaxial growth process, it may also be influenced by the above cosine law in such a way that the film thickness distribution is affected. It is thus difficult to provide the uniform film thickness distribution across the wafer. SUMMARY OF THE INVENTION The present invention overcomes the above described problems by improving the utilization of the reactant gas. It is accordingly an object of the present invention to provide a thin film deposition method that provides a uniformly distributed thickness when a thin film is deposited on a wafer surface. In order to achieve the above object, the present invention allows the molecules in the reactant gas to be diffused, without being absorbed by the inner wall of the vacuum chamber or vessel, when the molecules are traveling across the space toward the wafer for deposition. The thin film deposition method according to the present invention consists of depositing a thin film on a wafer within a vacuum vessel or chamber by supplying reactant gas molecules for depositing the thin film on the wafer, wherein the method includes maintaining the vacuum vessel or chamber under the pressure range within which no reaction will occur in the supplied reactant gas molecules in the vapor phase, and controlling the temperature of the wall of the vacuum vessel exposed to the vacuum side to be maintained within the temperature range that has a lower temperature limit at which the saturated vapor pressure of the supplied gas molecutes is higher than its partial pressure, and an upper temperature limit lower than that of the wafer being heated. The temperature control for the wall of the vacuum vessel exposed to the vacuum side may be provided in two ways. For one, the temperature of the vacuum vessel wall itself may be controlled directly by using any heating or cooling means. For the other, a shroud may be provided along the inner wall of the vacuum vessel, through which a fluid (such as liquid nitrogen) may circulate, thereby controlling the temperature of the wall. The latter way provides a simpler means of controlling the temperature of the wall of the vacuum vessel exposed to the vacuum side, and it more commonly used. The pressure range should be maintained so that the supplied reactant gas molecules don't react in vapor phase, that is, the pressure range within which it is more probable that the supplied reactant gas molecules may strike or collide against the inner wall of the vacuum vessel (the wall exposed to the vacuum side) or the wafer, rather than the gas molecules collide each other within the vacuum vessel. In this pressure range, the probability of the supplied gas molecules striking against the wafer or the inner wall of the vacuum vessel is by far greater than that of the molecules colliding against each other, and therefore the probability of the molecules exchanging thermal energy between each other is by far smaller. Thus, the molecules can only exchange thermal energy with the wafer or the inner wall. The smaller the probability of the molecules exchanging thermal energy between each other the smaller the thermal conduction of the molecules in the vapor phase. Under such conditions, the molecules may receive enough thermal energy to allow them to cause thermal decomposition reaction (pyrolysis) from the wafer or the inner wall, but the smaller thermal conduction in the vapor phase cannot cause the thermal decomposition reaction (pyrolysis) in the overall vapor phase. This means that the reaction in the vapor phase cannot occur in the space within the vacuum vessel. Now, if the temperature of the inner wall is set to the temperature such that it cannot cause the thermal decomposition reaction (pyrolysis) in the reactant gas, it will only cause the thermal decomposition reaction (pyrolysis) on the surface of the wafer. As a result, a thin film may be deposited by causing the thermal decomposition reaction (pyrolysis) on the wafer surface only. In order to provide the pressure range within which no reaction in the vapor phase will occur, the optimum way is to set the pressure in the vacuum vessel such that the mean free path of the reactant gas molecules is longer than the shortest distance between the wafer and the inner wall of the vacuum vessel. That is, the following equation must be satisfied: d > L (1) where d is the mean free path of the reactant gas molecules, and L is the shortest distance between the wafer and the inner wall. To conform the condition of the above equation, the reactant gas molecules in the vapor phase cannot cause any thermal decomposition reaction (pyrolysis). This may be explained by the fact that when the molecules have received thermal energy from the wafer upon striking against the latter, some of the collided molecules may be reflected from the wafer (secondary molecules), but it is less probable that those secondary molecules will collide against each other, but rather it is more probable that they will strike against the inner wall of the vacuum vessel, because the mean free path of the molecules is longer than the shortest distance between the wafer and the inner wall. In order to ensure that the reactant gas molecules don't collide with each other within the vacuum vessel, it is possible that the distance between the gas source and the wafer may also be shorter than the mean free path of the reactant gas molecules. By setting the distance between the gas source and the wafer in this way, the reactant gas molecules directly supplied from the gas source (primary molecules) will not collide against each other, and therefore it is more probable that those molecules can directly reach the wafer. The only factor to be considered here is, however, the probability that the molecules given the thermal energy sufficient to cause the thermal decomposition reaction (pyrolysis) will collide with each other. It is not necessary to consider, therefore, that those molecules which have been supplied from the gas source but have not yet been given the thermal energy to cause the thermal decomposition reaction (pyrolysis) may possibly strike against each other. Thus, the only consideration is that the condition as specified by the above equation (1) should be satisfied. What is next discussed is that by setting the temperature of the wall of the vacuum vessel exposed to the vacuum side (inner wall) to the temperature at which the vapor pressure cam be maintained above the partial pressure of the supplied reactant gas molecules in the vapor phase, it is possible to ensure that the reactant gas molecules are not adsorbed on the inner wall (vacuum side-exposed wall). Usually, the vapor pressure and temperature of a substance have the relationship as represented graphically in Fig. 6. The temperature at which a given vapor pressure can be maintained is the temperature that corresponds to that saturated vapor pressure along the saturated vapor pressure curve in Fig. 6. If there is a drop in the temperature, there is the corresponding drop in the saturated vapor pressure that can be maintained, as clearly seen from the curve in Fig. 6. Any drop in the saturated vapor pressure means that the molecules flying in the vapor phase tend to be adsorbed on the inner wall that causes the corresponding drop in the temperature. If the temperature of the inner wall drops below the temperature along the saturated vapor pressure curve corresponding to the partial pressure of the reactant gas in the vapor phase, the molecules will tend to be adsorbed on the inner wall. Then, by setting the temperature of the vacuum side-exposed wall of the vacuum vessel to the temperature at which the saturated vapor pressure corresponding to the partial pressure of the reactant gas in the vapor phase, it is possible to ensure that the reactant gas molecules flying toward the wall are not adsorbed on the wall. By so doing, the reactant gas molecules can diffuse upon striking against the wall. When the reactant gas contains a mixture of several different gases, the temperature of the wall must be set to above the temperature at which the respective partial pressures of those different gases can be maintained. Another consideration is that the temperature of the vacuum side-exposed wall of the vacuum vessel must be maintained below the temperature of the wafer. The temperature of the wafer should be set to the temperature at which the reactant gas molecules can cause the thermal decomposition reaction (pyrolysis) which can deposit a thin film on the wafer. If the temperature of the wall should rise above the temperature of the wafer, the reactant gas molecules flying toward the wall will cause the thermal decomposition reaction (pyrolysis) upon striking against the wall, and a thin-film will be deposited on the wall. Thus, the utilization of the reactant gas will be reduced. Further, even if the reactant gas molecules may not thermally decompose and are reflected from the wall, the reflected molecules (secondary molecule) may diffuse as they retain enough potential thermal energy to cause the thermal decomposition reaction. It is actually possible that the molecules may cause the reaction in the vapor phase. From the above aspects, the temperature of the inner wall must be below the temperature at which the reactant gas molecules would cause no decomposition reaction. It may be concluded from the above considerations that the temperature of the vacuum side-exposed wall of the vacuum vessel should be higher than the temperature at which the saturated vapor pressure equal to the partial pressure of the reactant gas could be maintained and should be in the temperature range that is lower than the temperature of the wafer. Specifically, those requirements may be expressed by the following equation: T vap < T wall < T sub (2) where, T vap is the temperature of the saturated vapor pressure of the reactant gas; T sub is the temperature of the wafer; T wall is the temperature of the vacuum side-exposed wall. According to the thin film deposition method of the present invention, part of the reactant gas molecules supplied from the gas source is traveling directly toward the wafer. Some of the molecules that have reached the wafer will contribute to the thin film growth, and others that have not contributed to the growth will be flying back into the space within the vacuum vessel. As described, the molecules from the gas source are diffusing according to the cosine law, and several ten percents of those molecules may be traveling elsewhere other than the wafer. Those molecules which are traveling toward the inner wall strike against the inner wall, from which they are diffused. Some of the diffused molecules are traveling toward the wafer. According to the conventional method, all of those molecules that have reached the inner wall are adsorbed by the liquid nitrogen-cooled shroud. According to the present invention, those molecules are all diffusing, and can be utilized fully to allow the reactant gas to promote the epitaxial growth on the wafer. It should also be noted that the molecules flying toward the wafer includes the molecules from the gas source and governed by the cosine law (primary molecules) and the molecules diffused from the inner wall of the vacuum vessel (secondary molecules). Thus, the uniformity of the thickness distribution across a thin film formed on the wafer surface can be improved. Preferred embodiments of the invention are disclosed in dependent claims 2-28. BRIEF DESCRIPTION OF THE DRAWINGS Those and other objects, features, and advantages of the present invention will become more apparent from the detailed description of several preferred embodiments that follows with reference to the accompanying drawings, in which: Fig. 1 illustrates the schematic diagram of a preferred embodiment of the apparatus according to the present invention; Fig. 2 is a graphical representation that explains the dependence of the growth rate upon the reactant gas flow rate in the embodiment of Fig. 1; Fig. 3 is a graphical representation showing the distribution of the thin film thickness formed on a wafer in the embodiment of Fig. 1; Fig. 4 (a) and (b) are diagrams showing the SIMS-analyzed spectrums (depth profile) for the epitaxial film growth, in which (a) shows the spectrums when the water-cooled shroud is used, and (b) shows the spectrums when the liquid nitrogen-cooled shroud is used; Fig. 5 illustrates a molecular structure model for disilane gas; Fig. 6 is a graphical representation illustrating the relationships between the saturated vapor pressure and temperature of a reactant gas used in the embodiment of the present invention; Fig. 7 is a graphical diagram illustrating how the pressures change when the supply of disilane gas is switched ON/OFF in the embodiment of the present invention; Fig. 8 is a graphical diagram illustrating how the pressures change when the supply of germanium hydride is switched ON/OFF in the embodiment of the present invention; Fig. 9 is the cross sectional TEM image of Si and SiGe super lattice; Fig. 10 is an SIMS-analyzed spectrum diagram for a film formed according to the particular embodiment, by supplying a hydrogen gas containing diborane (B₂H₆) gas at regular time intervals during the silicon epitaxial growth process; Fig. 11 is an SIMS-analyzed spectrum diagram for a film formed according to the particular embodiment, by varying the rate of the supplied hydrogen gas containing diborane gas to cause variations in the doping density of boron atoms; Fig. 12 is a graphical representation illustrating the relationships between the rate of the supplied hydrogen gas containing diborane gas and the number of boron atoms in the deposited film; and Fig. 13 is an SIMS-analyzed spectrum diagram for a film formed according to the particular embodiment, by varing the temperature of the wafer while boron atoms are being doped. DESCRIPTION OF THE PREFERRED EMBODIMENTS In a first preferred embodiment, the present invention that may be applied to silicon gas source epitaxy using a disilane (Si₂H₆) gas is described. Referring to Fig. 1, there is schematically shown the general arrangement of the apparatus that may be used in the first embodiment. It should be understood that the dimensions, geometrical configurations, and the relative positons of the component parts or elements which comprise the apparatus are only shown to help understand the concept and features of the present invention. As seen from Fig. 1, there is a vacuum chamber or vessel 1 in which a gas nozzle 2 is provided, a shroud 3 is arranged along the inner wall of the vacuum vessel 1, a wafer holder 4 is disposed above the space delimited by the shroud 3, and a heater 5 is disposed to face opposite the wafer holder 4. Below the vacuum vessel 1, there are a turbo molecular pump 6 (which has a pumping speed of 1000 liters/sec.) connected to the vacuum vessel 1, and an auxiliary oil sealed rotary pump 7. These evacuate the vacuum vessel 1 to ultra high vacuum. A nude ion gauge 8 is mounted through the lateral wall of the vacuum vessel 1, and measures the pressures within the vacuum vessel 1. There is also an RHEED (Reflection High Energy Electron Diffraction) system that includes an electron gun 10 and a screen 11 and may be used for observing the surface conditions of a wafer 9 on the wafer holder 4. The gas nozzle 2 is connected by way of a three-way valve 12 to a gas inlet system 13. The gas inlet system 13 includes a mass flow controller 14, valves 15, 16, and a regulator 17 which are connected as shown, the regulator 17 being connected to a gas container 18 (which contains a disilane gas) . The three-way valve 12 provides three ways, one of which is connected to an exhaust system that includes a turbo molecular pump 19 and an auxiliary oil-sealed rotary pump 20. The exhaust system removes any residual gas from the conduits in the gas inlet system. Therefore, the vacuum vessel can also evacuate the gas inlet system. The following is provided to describe how the silicon epitaxial growth process occurs on a wafer 9 on its holder 4, by using the apparatus described above. The pressure in the vacuum vessel or chamber 1 may be measured by the nude ion gauge 8, and the temperature of the wafer 9 may be detected by a thermocouple which is not shown to be disposed between the wafer holder 4 and the wafer 9 thereon. The vacuum vessel is evacuated by the turbo molecular pump 6 and then the back-ground pressure of less than 2.66 x 10⁻⁷ Pa (2.0 x 10⁻⁹ Torr) was attained. The flow rate of Si₂H₆ may be controlled by the mass flow controller 14 in the gas inlet system 13, and then it may be supplied onto the wafer 9 through the gas nozzle 2. The shroud 3 may be cooled by circulating either liquid nitrogen or water. When the flow rate of Si₂H₆ is varied between 1 and 30 sccm, the pressure is varied between 2.66 x 10⁻⁴ and 6.66 x 10⁻³ Pa (2 x 10⁻⁶ and 5 x 10⁻⁶ Torr) for the liquid nitrogen-cooled shroud, and the pressure is varied between 7.99 x 10⁻³ and 1.99 x 10⁻¹ Pa (6 x 10⁻⁵ and 1.5 x 10⁻³ Torr) for the water-cooled shroud. When the flow of Si₂H₆ is switched by the three-way valve 12 so that it is not supplied onto the wafer 9, the turbo molecular pump 19 is then operated to remove any gas remaining in the conduits. For example, the turbo molecular pump 19 evacuates Si₂H₆ gas flown from the valve 12. A wafer 9 is a 10.16 cm (four-inch) silicon wafer, of which surface facet is (001), having a particular pattern made by CVDS i O₂, and may previously be rinsed using boiled etching solution (H₂O:H₂O₂:NH₃OH = 20:6:1). After this, the wafer may be placed on its holder 4. Then, the wafer may be thermally cleaned down at 850 °C for ten minutes within the vacuum vessel 1. After cleaning the wafer, the temperature of the wafer may be adjusted to growth temperature of 700 °C and the source gas of Si₂H₆ was supplied to be adjusted the pressure within the vacuum vessel 1 to about 1.99 x 10⁻¹ Pa (1.5 x 10⁻³ Torr), under which epitaxial Si may be grown selectively. After the growth, the oxyde layer is removed from the thin film by using hydrofluoric acid (HF) solution, and the epitaxial Si thin film is decided to measuring a step height. Fig. 2 shows Si₂H₆ gas flow rate dependent on the Si growth rate. In Fig. 2, ○ indicates the dependence when the shroud 3 is cooled by water, and  indicates the dependence when the shroud 3 is cooled by liquid nitrogen. When the water-cooled shroud is used, the growth rate is ten times as high as that when the liquid nitrogen-cooled shroud is used. ○ shows that the growth rate is constant regardless of the flow rate of above 10 sccm. This is because the growth rate is limited by the surface reaction speed. For the water-cooled shroud, the growth rate is tem times as high as that for the liquid nitrogen-cooled shroud. More clearly, this may be explained by the fact that for the liquid nitrogen-cooled shroud, it tends to adsorb the gas molecules whereas for the water-cooled shroud, it does not adsorb any source gas molecules but instead allows the gas molecules to diffuse on the surface of the shroud 3, flying again toward and onto the wafer 9 thereby contributing greatly to the growth rate. Fig. 3 illustrates the distribution of the film thickness formed into the wafer. ○ indicates the distribution of the film thickness obtained using the water-cooled shroud, and  indicates the distribution of the film thickness obtained using the liquid nitrogen-cooled shroud. For both cases, the wafer 9 is completely stationary during the growth process. For the liquid nitrogen-cooled shroud, the distribution reflects that the distribution of the molecules flowing directly from the gas nozzle 2 must be governed by the cosine law. For the water-cooled shroud, however, the distribution of the formed film thickness can be made uniform because in addition to the molecules flowing directly from the gas nozzle 2, those molecules which are diffused against the wall of the vacuum vessel 1 exposed to the vacuum side, or the inner wall of the shroud 3, and are then flying toward the wafer can also contribute to the growth rate. Fig. 4 (a) shows the results that have been obtained from the SIMS (Secondary Ion Mass Spectrometry) analysis of the Si epitaxial film formed when the water-cooled shroud is used, and Fig. 4(b) shows the similar results for the liquid nitrogen-cooled shroud. B (boron) marks the interface between the wafer and epitaxial film. Both have the oxygen concentration level below the background behind the SIMS analyzer. It is known to any person skilled in the art that the liquid nitrogen-cooled shroud provides the adsorbing effect for any impurity molecules (which contain components of O atoms, such as H₂O, CO₂) within the vacuum chamber. It has been discovered that the water-cooled shroud provides high-quality epitaxial film (which contains no impurity molecules) equivalent to that provided by the liquid nitrogen-cooled shroud, if the vacuum chamber is sufficiently baked prior to the film deposition process. The results shown in Fig. 4(a) are those obtained in the embodiment where the chamber was sufficiently baked. In the embodiment shown in Fig. 4 (a), the shroud 3 is cooled by circulating water (utilities water) or liquid nitrogen (which is filled and added as required later). As it is expected that the temperature of either of those circulating fluids may remain constant, no particular means is provided for controlling the temperature. Any suitable heat exchanger may be connected to the shroud 3 so that the temperature can remain constant, if the temperature should vary during the operation. In this embodiment, the shortest distance between the wafer 9 and the vacuum side-exposed wall (the inner side of the shroud 3) is set to 40 mm. Now, the preceding equation (1) will be mentioned again to check that the embodiment described above meets the requirements as specificed by the equation (1). In general, the mean free path d (m) for molecules may be found from the following expression: where, T is the temperature of the gas (K); P is the pressure (Pa); D is the diameter of a molecule (m). Then, the mean free path for Si₂H₆ gas molecules may be found by using the above equation (3). When the diameter D for each of Si₂H₆ gas molecules is found, the molecule structure model as shown in Fig. 5 may be used. This model is built to provide the distance between Si atoms of 2.34Å and the distance between Si atom and H atom of 1.480 Å. It may be seen that the triangular pyramid formed by one Si atom and three H atoms has an apex ϑ of 110.2°. Therefore, the height of this triangular pyramid may be obtained from the above. The result will be 0.475 Å. The diameter D between Si₂H₆ molecules may be found from (distance between Si atoms) + 2 x (height of the triangular pyramid) . The result will be D = 3.29Å, i.e, 3.29 x 10⁻¹⁰ m. Then, the terms in the equation (3) is substituted by T = 298K (assuming the room temperature of 25 °C) and P = 1.5 x 10⁻³ Torr = 0.1995 Pa, respectively. The mean free path for the Si₂H₆ may be obtained as follows: d = 42.9 x 10⁻³ m = 42.9 mm L = 40 mm, therefore it is found that d > L as required by the equation (1) is satisfied. From this, the pressure within the vacuum vessel 1 will have been established to the pressure range under which there would be no vapor phase reaction. Usually, each of the reactant gas molecules has a size (diameter D) of several angstroms (Å), and the shortest distance between the wafer and the inner wall of the vacuum vessel is generally several centimeters (cm). By setting the pressure within the vacuum vessel 1 to the order of 1.33 x 10⁻¹ Pa (1 x 10⁻³ Torr), it is possible to meet the requirements as specified by the equation (1) where no vapor phase reaction occurs. Even for the reactant gas that contains a mixture of several different gases, the mean free path for the molecules in each of those gases may be several centimeters (cm) or more if the total pressure is equal to 1.33 x 10⁻¹ Pa (1 x 10⁻³ Torr) or less. The description that has been made so far considers the probability of the reactant gas molecules striking against the wafer and/or the inner wall, and the probability of molecules colliding with each other. Macroscopically, if the reactant gas has a very small thermal conduction when it is in its vapor phase, the thermal energy cannot conduct evenly through the gas in the vapor phase, meaning that no vapor reaction can occur. In fact, the inventors of the present invention verified that no vapor phase reaction could occur when an attempt was made to maintain the vacuum vessel into which Si₂H₆ gas was supplied from the apparatus of Fig. 1 under the pressure of the order of 1.99 Pa (1.5 x 10⁻² Torr). This way verified by allowing Si to sellectively grow on a wafer whose surface has been patterned (as disclosed in the co-pending Japanese application No. 2-253004). The selective epitaxial film obtained during the above process was found to be greater in thickness than the selective epitaxial film as reported in Recannelli et al. (Appl. Phy. Lett. 58 (19), 13 May 1991, P. 2096-P. 2098 "Low-temperature selective epitaxy by ultra-high vacuum chemical vapor deposition from SiH₄ and GeH₄/H₂). This was presumably because the reactant gas did not decompose in the vapor phase, and the selectivity was not affected. This was also presumably because the Si₂H₆ gas molecules had the thermal conduction which was sufficiently small at 1.99 Pa (1.5 x 10⁻² Torr). This reason may be explained by the fact that even if it should become more probable that the molecules given the thermal energy collide with each other, there would occur no vapor phase reaction unless the thermal energy would be distributed evenly over the total reactant gas in the vapor phase. Then, an attempt was made to verify that the relationships between the temperature of Si₂H₆ gas molecules T vap, the temperature of the wafer T sub, and the temperature of the vacuum side-exposed wall T wall would satisfy the preceding equation (2). From the graphical curves in Fig. 6 showing the relationship between the vapor pressure and temperature of Si₂H₆ gas (plotted against the data from "Handbook of Chemistry and Physics" published by the Chemical Rubber Co.), the Clausius-Clapeyron equation (4) is derived as follows: where P is the vapor pressure (Torr), and T is the temperature (K). For the water-cooled shroud, the pressure within the vacuum vesel 1 was equal to any value between 7.99 x 10⁻³ and 1.99 x 10⁻¹ Pa (6 x 10⁻⁵ Torr and 1.5 x 10⁻³ Torr). The temperature T (T vap) may be obtained by substituting this value or the equation (4), as follows: T vap = 102 to 115 (K) Since the temperature of the wafer 9 was 700 °C, or 973 K, the temperature of the wall T wall must satisfy the following equation: 102 to 115 < T wall < 973 (5) The water available from the utilities service is usually at the temperature range of 10 °C and 20 °C, therefore T wall is equal to any value between 283 and 293 K. The above equation (5) can be fully satisfied. For the liquid nitrogen-cooled shroud, the pressure within the vacuum vessel was equal to any value between 2.66 x 10⁻⁴ and 6.66 x 10⁻³ Pa (2 x 10⁻⁶ Torr and 5 x 10⁻⁵ Torr). By substituting this value for the preceding equation (4), the result is as follows: T vap = 90.5 to 101.2 Thus, the temperature of the wall (T wall) must satisfy the following equation: 90.5 to 101.2 < T wall < 973 (6) As the temperature of the shroud may be expected to be 77 to 80 K when the liquid nitrogen is circulating, the value of T wall above does not satisfy the equation (6). In this case, T wall < T vap, therefore Si₂H₆ gas would be absorbed by the liquid nitrogen-cooled shroud. Investigation was made to check for the effect of any shortage of the reactant gas being supplied from the gas source (the response to any change in the pressure when the gas supply is cut). Fig. 7 shows the results obtained for the Si₂H₆ gas, and Fig. 8 shows the results for the GeH₄ (germanium hydride, simply known as "german") gas. In both figures, there are shown the pressures under which a constant rate of gas is introduced into the vacuum vessel 1 while evacuation occurs at a constant rate, and the changes in the pressure when the gas supply is cut. For either gas, when the liquid nitrogen-cooled shroud (70 to 80 K) is used, those gas molecules which have been absorbed by the inner wall are leaving slowly from the inner wall again, so the gas cut-off characteristics is affected. For the water-cooled shroud (283 to 293 K), however, the cut-off characteristics is much better for either gas. This is presumably because there are fewer gas molecules that have been absorbed by the inner wall. The pressure under which a constant rate of gas is delivered is found to be constant for the water-cooled shroud. This is also presumably because there are fewer gas molecules that have been adsorbed by the inner wall. For the liquid nitrogen-cooled shroud, it is found that a longer time is required until a constant pressure is reached. This is presumably because a large proportion of the supplied gas molecules are adsorbed by the inner wall, followed by leaving from the inner wall again, and a certain time is required until the equilibrium condition is reached following the adsorption and leaving, respectively. It may be seen from the results in Fig. 7 and Fig. 8 that the gas cut-off can be improved by maintaining both the gas suply and evacuation rates constant, and by maintaining the inner wall at the appropriate temperature, so that any remaining gas can be evacuated at high speeds. As also seen from Figs. 7 and 8, and improved system for depositing a thin film may be implemented. In Fig. 7, Si₂H₆ can be evaculated at the effective pumping speed of 500 liters/sec. The results of the actual thin film depositing may be seen in Fig. 9 and Fig. 10. Fiag. 9 shows the cross-section TEM image of super-lattice made of Si and SiGe. The monocrystalline silicon is allowed to grow by supplying a flow of Si₂H₆ gas at a constant rate. The polycrystalline germanium/silicon is allowed to grow by supplying a flow of GeH₄ gas added to Si₂H₆ gas at the respective constant rates. A multilayer structure may be provided by repeating the supply of those reactant gases. It may be understood that an expitaxial film having a flat interface may be obtained simply by switching on and off the supply of the gases repeatedly. Now, the requirements for depositing such an super-lattice structure are described below. When a monocrystalline silicon is allowed to grow, Si₂H₆ gas should be supplied at the flow rate of 6 sccm, and when a germanium/silicon is allowed to grow, Si₂H₆ gas should be supplied at the flow rate of 6 sccm and GeH₄ gas should be supplied at the flow rate of 4 sccm. The pressures under which those gases are supplied should be 3.99 x 10⁻² Pa (3 x 10⁻⁴ Torr) for Si₂H₆ gas, and 6.66 x 10⁻² Pa (5 x 10⁻⁴ Torr) for the mixture of Si₂H₆ and GeH₄ gases. The temperature of a wafer should be 580 °C and the temperature of the water-cooled shroud should be 10 °C. Those temperatures should be maintained constant. Naturally, GeH₄ should satisfy the requirements as specified by teh equation (2). The relationships between the respective vapor pressures and temperatures of the gases are shown graphically in Fig. 6. The shortest distance between the wafer and the vacuum side-exposed wall of the vacuum vessel should be 40 mm. In the multilayer structure shown in Fig. 9, the dark part represents the germanium/silicon film, and the white part represents the silicon film. The germanium/silicon film has a thickness of about 70 Å, and the silicon film has a thickness of about 120 Å. A mixture of Si₂H₆ and GeH₄ gases is made to flow for twenty-three (23) seconds when the germanium/silicon is allowed to grow. Thus, the film growth rate is 3.0Å/min. For the silicon growth, Si₂H₆ gas is made to flow for 174 seconds. Thus, the film growth rate is 0.7 Å/min. This super-lattice, multilayer structure can be built because after the supply of the reactant gas (source gas) is cut off, any residual gas that remains in the vacuum vessel can be evaculated with high speed, and the epitaxial growth that is then proceeding can be rapidly stopped. That is, no film growth due to any residual gas occurs any longer just after the gas supply is cut off. This means that if the film growth rate is known under certain conditions such as the supplied reactant gas rate, wafer temperature, etc., the thickness of the film being deposited can be controlled precisely simply by controlling the supplying period of time of the reactant gas to be supplied. In the embodiment, the super-lattice, multilayer structure that contains silicon and germanium/silicon has been described. It should be understood, however, that any compound semiconductor film having the super-lattice, multilayer structure containing GaAs, GaAlAs, InP, and so on may be provided easily in the same manner as described, by making the appropriate selection of any reactant gas. Fig. 10 shows the results obtained by analyzing the profile of the epitaxial growth silicon film on the wafer in its denth direction by using the SIMS (Secondary Ion Mass Spectrometry) technique. Those results were obtained under the particular conditions under which the temperature of the wafer was set to 680 °C, SiH₆ gas was supplied at the rate of 20 sccm, and at the same time, dihydrodiborane (B₂H₆) gas which is diluted to 1% with hydrogen (H₂) by volume was supplied at the rate of 2 sccm at every 20 seconds during the period of 120 seconds. During that period, the pressure of 1.33 x 10⁻² Pa (1 x 10⁻⁴ Torr) was maintained. The temperature of the water-cooled shroud was set to 10 °C, and the shortest distance between the wafer 9 and the vacuum side-exposed wall (inner wall of the shroud 3) was set to about 40 mm. B₂H₆ gas satisfies the equation (2), and the relationships between the vapor pressure and temperature of B₂H₆ are shown in Fig. 6. H₂ gas is the diluted gas of B₂H₆, and takes no part when B₂H₆ provides its decomposing reaction. The pressure of Si₂H₆ gas, and the pressure of the mixture of B₂H₆ gas and H₂ gas are equal to 1.33 x 10⁻² Pa (1 x 10⁻⁴ Torr), and so the partial pressure of B₂H₆ is below the above pressure (total pressure). It is therefore unnecessary to take into account the mean free path of the hydrogen gas. The thickness of the crystalline silicon film containing a doped boron (B) was found to be 2000Å, and the thickness of the monosilicon film was also found to be 2000Å. It may be seen from Fig. 10 that it contains about 10¹⁹ B atoms per cm³. An attempt was made to deposit a similar thin film by varing the rate of H₂ gas containing 1% B₂H₆ by volume to be supplied. Si₂H₆ gas was supplied at the rate of 20 sccm, and at the same time H₂ gas containing 1% B₂H₆ by volume was added every 20 seconds during the period of 120 seconds for a total of eleven times. For each time, the hydrogen gas with B₂H₆ gas and the B₂H₆ gas were increased as shown in Table 1 below. No. 1 2 3 4 5 6 7 8 9 10 11 Rate of H₂ gas with B₂H₆ (sccm) 0.03 0.06 0.07 0.12 0.22 0.5 1.0 1.0 2.0 4.9 9.9 Net B₂H₆ (sccm) 0.03 x 10⁻² 0.06 x 10⁻² 0.07 x 10⁻² 0.12 x 10⁻² 0.22 x 10⁻² 0.5 x 10⁻² 1.0 x 10⁻² 1.0 x 10⁻² 2.0 x 10⁻² 4.9 x 10⁻² 9.92 x 10⁻² The temperature of the wafer was set to 680 °C, and the temperature of the water-cooled shroud was set to 10 °C. The pressure under which a thin film was being deposited was equal to 1.33 x 10⁻¹ Pa (1 x 10⁻³ Torr). The shortest distance between the wafer 9 and the vacuum side-exposed wall (inner wall of the shroud 3) was 40 mm. The rates of the H₂ gas containing B₂H₆ were the same for the 7th and 8th times because the mass flow controller 14 was changed to a greater capacity controller at the 8th and subsequent times. The results obtained by analyzing the deposited film by the SIMS technique is shown in Fig. 11. As it may be seen from Fig. 11, the film shows the concentration gradient of the doped atoms. The B atoms presents the maximum doping concentration near the surface of the film, and it presents the minimum doping concentration near the base interface. The B atoms present the doping concentration that is decreasing from the surface toward the base interface. This means that if the rate of B₂H₆ to be supplied is varied successively as the time goes on, the doping concentration of the B atoms may be varied successively accordingly, Fig. 12 shows the relationships between the rate of B₂H₆ being supplied and the doping concentration of the B atoms, which are based upon the results in Fig. 11. It may be seen from Fig. 12 that the B atom dopes vary in proportion to the rate of B₂H₆ gas. In the embodiment, the B atoms will have their doping concentration increasing as it is nearer to the wafer surface, that is, as the film deposition progresses. Conversely, it may be possible that the doping concentration is decreasing as it is nearer to the wafer surface. It may also be possible that the doping concentration is either the maximum or the minimum in the middle of the film. The doping atom, which is a B atom in this case, may be a P (phosphorus) atom instead. The P atom may have the doping concentration gradient as desired. The method that has been described may be used to fabricate a high-speed bipolar transistor element capable of high-speed switching operation on a wafer, in such a way that any impurity substance may be doped at the base region of the bipolar transistor with the appropriate concentration gradient. The following provides the results obtained by examining the relationships that exist between the temperature of the wafer and the doping quantity of the B atoms. The examination was conducted by supplying Si₂H₆ gas at the rate of 29 sccm, and by supplying H₂ gas containing 1% B₂H₆ gas by volume at the rate of 3.6 sccm, during the periods of time as specified in Table 2. For the first time, Si₂H₆ gas was supplied for 40 seconds, and then a mixture of Si₂H₆ gas and H₂ gas containing 1% B₂H₆ gas was supplied for 40 seconds. No. 1 2 3 4 5 Temp. of Wafer (°C) 715 700 687 650 638 Supply Time of Si₂H₆ (sec) 40 40 50 50 60 60 120 120 180 180 Supply Time of B₂H₆ (sec) 0 40 0 50 0 60 0 120 0 180 In the above table, the film with doped B atoms was deposited to a thickness of about 100Å. The results obtained by the SIMS analysis on the thin films deposited are shown in Fig. 13. It is found that each thin film contains 3 or 4 x 10¹⁸ per cm³ of doped B atoms. From this, it is clear that the amount of B atoms to be doped does not depend largely on the temperature of the wafer. In all of the preferred embodiments that have been described so far, water circulates through the shroud 3 to control the temperature of the vacuum side-exposed wall. It should be understood that any other fluid that can control the temperature without trapping the reactant gas being supplied may be used instead of water. Furthermore, any combination of the reactant gases to be supplied and the fluids to circulate through the shroud may be used, depending upon the particular needs. It may he appreciated from the foregoing description that according to the present invention, the reactant gas molecules supplied into the vacuum vessel can be diffused without being adsorbed by the inner wall of the vacuum vessel, thereby improving the utilization of the reactant gas, thus increasing the epitaxial growth. The method according to the present invention allows a film thickness uniformly to be distributed into the wafer without the need of rotating the wafer. Furthermore, the present invention provides the precise controls for the thickness of a thin film being deposited during the epitaxial growth process, as well as the precise control for the concentration of any impurity atoms to be doped, by switching on and off the supply of the reactant gas. Those precise controls for the thin film thickness and doping concentration according to the present invention may be applied to the manufacture of any bipolar transistor elements having base regions, whereby a thin film may be as thin as possible and then any impurity atoms may be doped into the base region with the appropriate doping concentration gradient. Also, it is easy to change the composition ratio x of Si 1-x Ge x gradually in the SiGe thin film. Therefore, by using the method of the present invention, a high-speed bipolar transistor element that is capable of high-speed switching operation can be fabricated. In the conventional CVD thin film deposition process, the pressures are set to the viscous flow range above 1.33 x 10² Pa (1 Torr), and the vapor phase reaction is used positively. According to the present invention, however, the pressures are set to the molecule flow range below 1.33 Pa (10⁻² Torr) and the vapor chase reaction can only occur on the wafer surface. Thus, the method according to the present invention implements the conceptual design completely differently from the CVD thin film deposition implemention.

Claims: A thin film deposition method which includes depositing a thin film on a wafer surface by supplying reactant gas molecules toward and onto a wafer within a vacuum vessel or chamber, said method including: (i) evacuating the vacuum vessel or chamber until the pressure within the vacuum vessel or chamber reaches a predetermined vacuum pressure range; (ii) supplying a reactant gas containing at least one kind of molecules into the vacuum vessel; (iii) setting the pressure within the vacuum vessel to a pressure under which the mean free path "d" of the molecules contained in the reactant gas being supplied is longer than the shortest distance "L" between the wafer and the wall of the vacuum vessel or chamber exposed to the vacuum side, or d > L; (iv) setting the temperature of the wafer (T sub) to a temperature at which the reactant gas being supplied can cause thermal decomposition reaction; and (v) setting the temperature of the vacuum side-exposed wall of the vacuum vessel or chamber (T wall) to a temperature range having a lower limit higher than the temperature (T vap) at which a saturated vapor pressure can be maintained to the same value as the partial pressure of the molecules contained in the reactant gas as set in step (iii), and an upper limit of said range lower than the temperature of the wafer (T sub), or T vap < T wall < T sub. The thin film deposition method as defined in Claim 1, wherein the thin film being deposited on the wafer surface is a thin film being deposited by the epitaxial growth. The thin film deposition method as defined in Claim 1 or 2, wherein the reactant gas being supplied is Si₂H₆ gas, and wherein the thin film being deposited on the wafer surface is an epitaxial silicon film. The thin film deposition method as defined in one of claims 1-3, wherein the pressure within the vacuum vessel or chamber is maintained to about 1.99 Pa (1.5 x 10⁻² Torr) after the reactant gas has been supplied into the vacuum vessel or chamber. The thin film deposition method as defined in one of claims 1-4, wherein the temperature of the vacuum side-exposed wall of the vacuum vessel or chamber (T wall) is about 10 °C. The thin film deposition method as defined in one of claims 1-5, wherein the vacuum side-exposed wall is a shroud mounted inside the vacuum vessel or chamber, said shroud being maintained to a specific temperature by circulating water therethrough. The thin film deposition method as defined in one of claims 1-6, wherein the pressure within the vacuum vessel or chamber as set in step (i) is 2.66 x 10⁻⁷ Pa (2 x 10⁻⁹ Torr). The thin film deposition method as defined in one of claims 1-7, wherein the pressure within the vacuum vessel or chamber as set in step (i) is established during the baking process for the vacuum vessel or chamber. The thin film deposition method as defined in one of claims 1-8, wherein the reactant gas being supplied is stopped at the moment that a thin film being deposited on the wafer reaches a thickness as required after a specific period of time has elapsed. The thin film deposition method as defined in one of claims 1-8, said method further including: (vi) stopping the reactant gas being supplied in step (ii) at the moment that a thin film has reached a thickness as required, and supplying a reactant gas containing a different kind of molecules from those contained in the reactant gas supplied in step (ii); (vii) setting the pressure within the vacuum vessel to a pressure under which the mean free path "d" of the molecules contained in the reactant gas being supplied in step (vi) is longer than the shortest distance "L" between the wafer and the wall of the vacuum vessel or chamber exposed to the vacuum side, or d > L; (viii) setting the temperature of th wafer (T sub) to a temperature at which the reactant gas being supplied in step (vi) can cause thermal decomposition reaction; (ix) setting the temperature of the vacuum side-exposed wall of the vacuum vessel or chamber (T wall) to a temperature range having a lower limit higher than the temperature (T vap) at which a saturated vapor pressure can be maintained to the same value as the partial pressure of the molecules contained in the reactant gas as set in step (vii), and an upper limit of said range lower than the temperature of the wafer (T sub), or T vap < T wall < T sub; (x) stopping the reactant gas being supplied in step (vi) at the moment that a thin film has reached a thickness as required. The thin film deposition method as defined in Claim 10, wherein the steps (ii) through (x) are repeated. The thin film deposition method as defined in Claim 10 or 11, wherein the pressures being set in steps (iii) and (vii) are less than about 1.99 Pa (1.5 x 10⁻² Torr). The thin film deposition method as defined in one of claims 10-12, wherein the temperature of the vacuum side-exposed wall (T wall) being set in steps (v) and (ix) are about 10 °C. The thin film deposition method as defined in one of claims 10-13, wherein the thin film being deposited on the wafer in steps (ii) through (v) are thin films being deposited by the epitaxial growth. The thin film deposition method as defined in one of claims 10-14, wherein the thin films being deposited on the wafer in steps (vi) through (x) are polycrystalline thin films. The thin film deposition method as defined in one of claims 1-15, wherein the thin films being deposited on the wafer in steps (vi) through (x) are thin films containing impurities. The thin film deposition method as defined in one of claims 10-16, wherein the reactant gas being supplied in step (ii) is Si₂H₆ gas, and wherein the reactant gas being supplied in step (vi) is a mixture of Si₂H₆ gas and B₂H₆ gas. The thin film deposition method as defined in one of claims 10-17, wherein the reactant gas being suplied in step (ii) is Si₂H₆ gas, and wherein the reactant gas being supplied in step (vi) is a mixture of Si₂H₆ gas and H₂ gas containing B₂H₆ gas. The thin film deposition method as defined in one of claims 14-18, wherein the reactant gas being supplied in step (ii) is Si₂H₆ gas, and wherein a thin film being deposited on the wafer is an epitaxial Si thin film. The thin film deposition method as defined in one of claims 15-19, wherein the reactant gas being supplied in step (vi) is a mixture of Si₂H₆ gas and GeH₄, and wherein a thin film being deposited on the wafer is a Ge/Si thin film. The thin film deposition method as defined in one of claims 10-20, wherein the reactant gas being supplied in step (vi) is a mixture of Si₂H₆ gas and H₂ gas containing B₂H₆ gas, and wherein the thin film being deposited on the wafer is an epitaxial Si thin film containing B atoms as an impurity. The thin film deposition method as defined in one of claims 11-21, wherein the rate of the reactant as being supplied in step (vi) is varied for each repetition. The thin film deposition method as defined in Claim 22, wherein the reactant gas being supplied in step (vi) is a mixture of Si₂H₆ gas and H₂ gas containing B₂H₆ gas, and wherein the rate of H₂ gas containing B₂H₆ gas being supplied is increased or decreased for each repetition. The thin film deposition method as defined in Claim 23, wherein the thin film being deposited on the wafer is an epitaxial Si thin film containing B atoms having a concentration gradient. The thin film deposition method as defined in one of claims 10-24, wherein the thin film being deposited on the wafer is a thin film having a super-lattice structure. The thin film deposition method as defined in Claim 25, wherein the super-lattice structure includes a multilayer structure containing an epitaxial Si thin film and a Ge/Si thin film. The thin film deposition method as defined in one of claims 10-26, wherein the vacuum side-exposed wall is a shroud mounted inside the vacuum vessel or chamber, and wherein the shroud is maintained to a constant temperature by circulating water therethrough. The thin film deposition method as defined in one of claims 10-27, wherein the pressure being set in step (i) is established during the baking process for the vacuum vessel or chamber while it is being evacuated.

Inventors: AKETAGAWA KEN-ICHI C O ANELVA, MURAKAMI SHUN-ICHI C O ANELVA, MUROTA HIROYOSHI C O ANELVA CO, SAKAI JUNRO C O ANELVA CORPORA, TATSUMI TORU C O NEC CORPORATI, AKETAGAWA, KEN-ICHI, C/O ANELVA CORPORATION, MURAKAMI, SHUN-ICHI, C/O ANELVA CORPORATION, MUROTA, HIROYOSHI, C/O ANELVA CORPORATION, SAKAI, JUNRO, C/O ANELVA CORPORATION, TATSUMI, TORU, C/O NEC CORPORATION
Applicants: ANELVA CORP, NIPPON ELECTRIC CO, ANELVA CORPORATION, NEC CORPORATION
Application year: 1991
Main classification: C23C16
IPC codes: C30B25, C23C16, H01L21