Thin film forming method

Abstract: This invention provides a thin film forming method together with a device for that method, which cools the substrate (30) below the temperature of the liquefaction point of the deposit species, so that the deposit species in gaseous phase can exist on the substrate surface under more stable conditions than those when the deposit species are flowing in gaseous phase.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates generally to a thin film forming method and a thin film forming apparatus, and more particularly to a method and apparatus for forming a thin film on a surface of a substrate having a trench or an unevenness thereon, e.g., a semiconductor substrate. Description of the Prior Art The processes usually used to form a thin film on a surface of a substrate, such as a semiconductor, are classified broadly into Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD). The CVD process induces a chemical reaction on the substrate surface or in gaseous phase to form a thin film on the substrate, and this process is used to form insulation films such as a silicon oxide film or a silicon nitride film. The PVD process forms a thin film utilizing collision against a substrate of depositing materials generated in the gaseous phase, and this process is mainly used for metal film forming. To achieve satisfactory results, VLSI device fabrication presently requires that thin film is deposited within a trench formed in the substrate having an aspect ratio one or more (depth/width). Figure 22 is a sectional diagram to show a typical conventional plasma CVD process of the prior art (for example, J. L. Vossen & W. Kern, Thin Film Processes: Academic Press, 1978). In this process, an insulating film 53 is formed by depositing the deposit materials 52, which is in the solid phase generated in gaseous phase within a trench 51 of high aspect ratio formed on a substrate 50, such as silicon. However, the deposit material is deposited heavily on a edge 54 of the trench 51, thereby obstructing other deposit material from entering toward a bottom 55 of the trench 51. Thus, a cavity 56 is formed within the trench 51 and there is a degradation of stage coating properties on the substrate surface. To cope with the above problem, a process called the bias sputtering process, which is one of the PVD processes, is employed (for example, T. Mogami, M. Morimoto & H. Okabayashi: Extended Abstracts 16th Conf. Solid State Devices & Materials, Kobe, 1984, p. 43). This method is to form an insulating film, such as a silicon oxide film, by physically sputtering the substrate surface with ions of argon, for example. In the application of this method, the sputtering makes it difficult to have much deposition on edges, as shown in Fig. 22, and promotes heavier deposition on the flat surface portions. Therefore, the problems of forming the cavity 56 and of the stage coating properties are reduced in comparison to the CVD process above. However, as the deposit material in gaseous phase comes into the trench on a slant, it is difficult to achieve a good filling within the trench with an aspect ratio of one or more. This method actually has a low deposition velocity, which means a very low productivity, because of the competing reactions between the removal of the deposited film and the film deposition by physical sputtering. In addition, the radiation damage is inevitable because the process is conducted in the plasma. Recently, an ECR bias sputtering method (for example, H. Oikawa; SEMI TECHNOLOGY SYM, 1986, E3-1) was proposed to reduce the oblique incident element of the deposit material within the trench. This method lessens the above-mentioned problem of the oblique incidence of the deposition material within the trench even though the deposit material is in the solid state, but it is not a complete solution. Appropriate forming of a thin film with a trench of the high aspect ratio is still difficult. Other than the processes described above, a method to form a silicon oxide film using thermal decomposition method a TEOS (R. D. Rung, T. Momose & Nagakubo; IEDM. Tech. Dig. 1982, p. 237) has been proposed. This method as shown in Figure 23 (a) has a large surface movement rate of the deposit material, which makes cavity forming difficult, and realizes good stage coating properties. However, when an oxide film 57 having trenchs formed by this method is cleaned, for example with diluted HF solution, the removal velocity of the oxide film 57 at the center of the trench 51 is extremely high, as shown in Figure. 23 (b), and as a result, the flat filling actually cannot be achieved. The reason seems to be the fact that the distortion of the oxide film grown from both sides of the trench remains around the center. It is thus considered extremely difficult to fill a trench with a high aspect ratio, even when a conformable thin film forming method is employed. In Figure 23 (a), after formation of an oxide film with impurity as a solid phase diffusing source using the thermal CVD method etc., a thermal process may be applied to diffuse the impurity around the trench of the substrate. However, when comparing an oxide film formed on the side wall of the trench and that on a flat surface, the former has less impurity density, and a desired resistivity cannot be obtained with this method. FR-2,154,664 describes a process for forming an insulating layer which is carried out at a temperature at which the substance of the layer is formed as a liquid. This is achieved by doping the oxide layer in the course of growth with an additive which, in combination with the insulating substance, induces the formation of the insulating layer in the form of a liquid. SUMMARY OF THE INVENTION It is an object of the invention to provide an improved thin film forming process and a forming apparatus for this process which enables a good filling within a trench of high aspect ratio with films of an insulating material or semiconductor material. This invention solves the problems of the prior art in thin film forming with a trench having a high aspect ratio formed on above-mentioned substrate, such as semiconductors etc., such as cavity forming in the trench, degraded stage coating properties on the substrate surface, or radiation damage against the substrate. Thus, the present invention is defined by claim 1. BRIEF DESCRIPTION OF DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompany drawings, wherein: Figure 1 is a sectional view illustrating steps of a method in accordance with the invention; Figures 2 through 4 are partial sectional views illustrating forming apparatus in accordance with several embodiments of the invention; Figures 5 and 6 are graphs illustrating functions in accordance with embodiments of the invention; Figure 7 is a state diagram illustrating relationships of pressure and temperature in accordance with the embodiment of the invention; Figure 8 is a graph illustrating the relationship of the depostion velocity and the pressure in accordance with the embodiments of the invention; Figures 9 and 10 are sectional views illustrating the functions in accordance with the embodiment of the invention; Figures 11 through 15 are graphs illustrating different relationships of parameters in accordance with the embodiment of the invention; Figures 16 and 17 are partial sectional views illustrating forming apparatus in accordance with other embodiments of the invention; Figure 18 is a graph illustrating the relationship of the deposition velocity and the substrate temperature in accordance with the invention; Figures 19 and 20 are sectional views illustrating the steps of a method in accordance with the another embodiments of the invention; Figure 21 is a sectional view illustrating a semiconductor device in accordance with another embodiment of the invention; Figure 22 is a sectional view illustrating a conventional method; Figure 23 is a sectional view illustrating steps in accordance with another conventional method; and Figure 24 is a typical state diagram. DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention provides a thin film forming method together with a device for that method, which cools the substrate not more than the liquefaction point of the deposit species, so that the deposit species in gaseous phase can exist on the substrate surface under more stable conditions than those when the deposit species are flowing in gaseous phase. Referring to Figure 1, the function of the thin film forming method of this invention is explained below. In Figure 1 (a), deposit species 32 in a gaseous phase state is deposited within a trench 31 with high aspect ratio formed on a substrate 30, such as a semiconductor. The substrate is cooled to the temperature not more than the liquefaction temperature point of the deposit species 32, so the species 32 liquefies and adheres on the substrate surface. The reference number 33 is a thin film formed within the trench 31 by the deposit species 32. By repeating this process, as shown in Figure 1 (b), thin films 33a are built up within a trench 31 to form a filling. If the process is continued after the complete filling up of the trench 31, thin film 33b is formed appropriately over the trench 31 and on the surface of the substrate 30. To explain this phenomenon, Figure 24 shows, with parameters of the temperature and the pressure, three phases (gaseous phase, liquid phase and solid phase) of on active species of a first reactive gas, a second reactive gas, or their reaction product. As the temperature of the material including the reactive gas, etc., described above, decreases, the state changes from the gaseous phase A to the liquid phase B. In a high pressure condition, transfer from gaseous phase A to liquid phase B becomes distinctive. For example, if a reactive gas with pressure P 0 is in the state of gaseous phase A, when the temperature is lowered to t 0 (the boundary between the gaseous phase and the liquid phase, i.e., the liquefaction point) or below, the state changes to the liquid phase. If the temperature drops more, the state goes to the solid phase. Therefore, this invention utilizes the temperature dependency of three phases above for the trench filling in semiconductor fabrication. When only one kind of gas is used, the substrate temperature shall be set not more than the liquefaction point of that gas. As described above, this invention provides an excellent flatness after the appropriate filling or plugging of trenchs with a high aspect ratio at low temperature, and is an optimum for VLSI fabrication. A first embodiment of the thin film forming process according to this invention is described below. Firstly, an apparatus for this process is explained. Figure 2 is a schematic configuration illustrating the apparatus to be used for this process. The configuration of this apparatus is given below. In a reaction vessel 1 is accommodated a substrate 3 located on a sample holder 2. To the reaction vessel 1, active species of a first reactive gas 6 and a second reactive gas 7 are introduced through the gas introducing pipes 4 and 5, and are exhausted through the exhaust pipe 8 connected to the exhausting system. The flow rate of the first and the second reactive gases can be adjusted with a mass-flow controller (not shown). The first reactive gas 6 is activated in a microwave discharge portion 9 connected to the above mentioned gas introducing pipe 4. The gas introducing pipe 4 is made of a quartz in this embodiment. Microwave power is supplied from a microwave power source 10 through a waveguide 11 to the discharge portion 9. Activation of the reactive gas 6 is performed with the plasma in this embodiment, but it may be done with the thermal excitation, optical excitation or electron beam excitation. Pressure in the vessel 1 is set by changing the conductance of a valve (not shown), and is measured with a diaphragm vacuum gauge (not shown) to be controlled. Within the above mentioned holder 2, a cooling means 12 is provided to cool the substrate 3, and an additional heating means 13 can be provided, if required. These means are connected to a control system (not shown) to monitor the temperature of the substrate 3 and keep it at a fixed value not more than the liquefaction point of the active species generated from the first reactive gas, the second reactive gas and their reaction products. The cooling means 12 carries a nitrogen gas coming through a liquid nitrogen to the holder 2 via a cooling pipe (not shown). The cooling means is controlled by adjusting the flow rate of the nitrogen gas with a needle valve (not shown) provided in the cooling pipe. A heater is used for the heating means 13, but the cooling and the heating means are not limited to those described above, and anything that can keep a constant temperature is acceptable. The substrate is fixed to the above mentioned sample holder so that the substrate can make good thermal contact with the sample holder. Areas of the substrate other than the reaction vessel 1 and the sample holder 2 may have a structure, for example, with an electric current heater wound around the wall of the vessel 1, to maintain the space in the reaction vessel 1. As a thin film forming apparatus relating to the method of this invention, other embodiments shown in Figure 3 and 4 may be utilized. The apparatus shown in Figure 3 is almost the same as the configuration of the apparatus of Figure 2, and the same parts are shown with the same reference numbers. The difference of this apparatus from the one in Figure 2 is that a light radiation means 16 to radiate a beam 15, e.g., electron, ion or laser beams, is provided. This light radiation means 16 enables excitation of the reactive gas with a light. By using this light excitation, as with the apparatus shown in Figure 2, the damage to the substrate 2 and other adverse influences may be reduced. Although it is not shown in the figures, the substrate 3 extends into and out of the reaction vessel 1 through another adjacent chamber. This chamber may be evacuated, or may contain the inert gas with atmospheric or higher pressure. By this "load-locking" of the reaction vessel 1, reproducibility of the process may be remarkably improved. The thin film forming apparatus shown in Figure 4 also has almost the same configuration as the one shown in Figure 2, and the same parts as the Figure 2 are indicated with the same reference numbers. These reference numbers show a thin film forming apparatus capable of post treatment after the thin film is formed. In this apparatus, after the thin film forming on the substrate 3, the substrate 3 is carried through a carrying system 17 to a thermal treatment chamber 18 by a carrying mechanism (not shown). The substrate 3 carried to the thermal treatment chamber 18 is placed on a holder 20 equipped with a heating means 19, and a thermal treatment is applied to the substrate. This heating may be conducted by raising the substrate temperature instantaneously with radiation from an infrared lamp 21. By conducting the thermal treatment above, residue, dust, etc., on the surface of the substrate 3 can be removed, and the film quality of the thin film can be improved. In the same figure, a reference number 22 indicates a gas inlet port for introducing an inert gas Z 23, and the reference number 24 is a gate valve dividing the reaction vessel 1 and the thermal treatment chamber 18. An embodiment of thin film forming process according to this invention is described below. This explanation refers specifically to the apparatus shown in Figure 2, but any of the apparatus described above can be used for this process. In this embodiment, oxygen (O 2 ) is used as a first reactive gas, and tetramethylsilane (Si(CH 3 ) 4 ; TMS) as a second reactive gas. Using a silicon substrate as a substrate, a silicon oxide film is deposited on this silicon substrate. First, oxygen gas 6, which is the first reactive gas, is introduced through the gas introducing pipe 4 and microwaves of 2.45 GHz are discharged to produce an oxygen radical (0 * ). Then, this oxygen radical is moved to the reaction vessel 1. Meanwhile, TMS is introduced to the reaction vessel 1 without discharging. The total pressure in the reaction vessel 1 is fixed at 266.6 Pa (2 Torr). The sample holder 2 has a built-in stainless steel pipe 12. Cooling nitrogen (N 2 ) gas passes through the liquid nitrogen and flows through the pipe 12 to lower the temperature of the substrate 3. Figure 5 shows the deposition velocity of the silicon oxide film at variable substrate temperatures in the above process, and the resulting configuration of a trench on the substrate. The flow rates of oxygen and TMS are 56 SCCM and 7 SCCM, respectively, and the trench has an aspect ratio of 1.5. It is seen from this graph that the deposition velocity represented by the curve A reaches the highest point when the substrate temperature is -40 °C. For the filled trench configuration, at a temperature above room temperature, the silicon oxide film formed by the reaction of the 0* radical and TMS forms a cavity as described in the prior art. On the other hand, with the decrease of the substrate temperature, the intensive deposition on the trench corners decreases and at the substrate temperature of -20 °C or less, the thin film can fully fill the trench. Figure 6 shows the deposition velocity and the deposition configuration at the different substrate temperatures when the flow ratio of oxygen to TMS is 24. Conditions other than the flow ratio are the same as in Figure 5. Figure 6 shows a tendency similar to that seen in Figure 5. It is observed that this process is optimum for the formation of an interlaminar insulation film in the multilayer wiring technique. The inventors of this invention have made a further study to find optimum conditions for forming a thin film within a trench. Details are described below. Figure 7 is a phase diagram of hexamethyldisiloxane and trimethylsilanol which can be reaction products by tetramethylsilane (TMS)-oxygen (O 2 ) active species and tetramethylsilane. As described in the embodiment above, when the interior pressure of the reaction vessel is 266.6 Pa (2 Torr), the trench interior can be filled if the substrate temperature is lowered to about -20 °C. It is assumed from this phase diagram that the liquid used in the deposition includes tetramethylsilane and/or hexamethyldisiloxane at a substrate temperature in the range of 20 °C to -100 °C and a vessel internal pressure of less than 1333 Pa (10 Torr). It is also considered that oxidation proceeds while entrapping active oxide species in the liquid layer. This assumption may be reasonable since the infrared absorption spectra of this film is nearly identical to that of the plasma polymerization film of hexamethyldisiloxane. Thus, when the internal pressure of the reaction vessel is 266.6 Pa (2 Torr), the substrate temperature is required to be not more than -22 °C where hexamethyldisiloxane is liquid and not less than -100 °C where tetramethylsilane is a solid. Consequently, it is necessary to arrange an appropriate thermal contact between the substrate and the sample holder, an uniform distribution of sample temperature, and a correctly measured and controlled temperature to make the liquefaction possible. Figure 8 is a diagram to show the deposition velocity and the deposition configuration with changes of internal pressure of the reaction vessel. The substrate temperature is set at -40 °C, and the flow rate of TMS and oxygen at 7 SCCM and 168 SCCM respectively. As a result, the deposition forms a large lump on the substrate surface around 1333 Pa (10 Torr), and the trench cannot be filled. As the pressure decreases, however, filling becomes possible and the deposition velocity increases. The shape of a globule 90 adhered on the substrate surface as a lump can be expressed by the formula below in the coordinates shown in Figure 9, using a Laplace equation: γ (1/R + Sin/r) = ρgz + ΔP 0 where, the reference number 91 indicates the substrate; the main curvature of the curved surface in the plane of the figure is r/sin, and the main curvature perpendicular to this is R, γ represents the surface tension, ρ is the liquid density and ΔP 0 is the pressure difference between the inside and the outside of the globule 90. If the globule 90 is rotation symmetry and the two main curvatures at the apexes are equally b, according to the equation above, γ·2/b=ΔP 0 This means that when the pressure difference ΔP 0 is larger with an equal surface tension γ, the radius of curvature b of the apexes becomes smaller, and results in a long shaped globule as shown in Figures 10a and 10b. As the pressure of the microwave discharge changes, the types and the quantity of the oxygen active species may change, the functional group produced on the silicon substrate surface may change, and the contact angle to the hexamethyldisiloxane or tetramethylsilane may change. For these reasons, even in the pressure and temperature range of liquefaction, it is suitable to set the pressure at the contact angle in the range where liquid flows into the inside of the trench, and it is preferable to have a pressure of 1333 Pa (10 Torr) or less for to process of this embodiment. The appropriate pressure should not be less than the temperature of the triple point of hexamethyldisiloxane or tetramethylsilane, so that they will be liquefied. Figure 11 shows the deposition velocity and deposition configuration change to the flow ratio of oxygen and tetramethylsilane. From this figure, it is seen that the deposition velocity reaches its peak when the oxygen/TMS flow ratio is around 20. The trench can be filled with an approximate oxygen/TMS flow ratio of 4, and the trenches can be filled at ratios not less than this value. The reason for this phenomenon is considered to be the fact that, in the formation of hexamethyldisiloxane from oxygen and TMS, the following reaction occurs. where, oxygen/TMS equals 2. Therefore, when tetramethylsilane is 7 SCCM, the oxygen is required to be 14 SCCM or more, and the flow ratio is preferably at least 2 or more for an ideal reaction. Figure 12 shows infrared absorption spectra of a deposited film for oxygen/TMS flow ratios of 8 (graph A), 24 (graph B) and 40 (graph C). An absorption peak of Si-O-Si is observed in the range from 1200 cm -1 to 1000 cm -1 , and the film is confirmed to be the silicon oxide film. Strength changes of the absorption peaks of Si-CH 3 , SiH, O-H specified by this Si-O-Si absorption peak to oxygen/TMS flow ratio as shown in Figure 13. From this figure, it is seen that the larger the oxygen/TMS becomes, the more the silicon oxides. Reference numbers in circles 1 through 7 correspond to the references in Figure 12. Next, a study on the characteristics of the thin film formed with the present invention is described below. As shown in Figure 14, the oxygen/TMS flow ratios at varying velocities of etching on the deposited film are determined using a solution including 6 % of HF and 30 % of NH 3 F. These determined values show that, as oxygen/TMS increases, the etching velocity decreases and the film is further nitrogenized. Thus, it is found preferable to increase the oxygen flow rate and make the oxygen/TMS flow ratio larger for a good quality deposit film. After the thin film deposition on the substrate, the substrate is thermally treated in the reaction vessel with the two methods below. (1) Maintain the substrate temperature at 300 °C in oxygen of 1333 Pa (10 Torr) for 1 hour. (2) While discharging microwaves into the oxygen of 266.6 Pa (2 Torr), maintain the substrate temperature at 300 °C for 1 hour. The absorption strength changes of Si-CH 3 , Si-H and O-H specified with the Si-O-Si peak absorption of the infrared absorption spectra for the film obtained from the above versus oxygen/TMS flow ratio is shown in Figure 15. The combination of Si-CH 3 and O-H decreases first during the thermal treatment of (1), and second during the thermal treatment in (2), and the silicon oxidization proceeds. It was found that the combination of Si-H disappears during the treatment processes of (1) and (2). In conclusion, to improve the film quality of the deposited film, the substrate temperature should be at least 300 °C. The thermal treatment in (1) and (2) above can promote oxidization of silicon, and it has been observed that, in particular, the oxidization caused by the thermal treatment process (2) is remarkable. Radiation of an ArF excimer laser with a wave length of 193 nm during the film forming activates the liquefied layer or substrate surface, resulting in a further flatness of the filled trench through experiments. Radiation energy of the ArF excimer laser is then 330 Joul/cm 2 sec. As the activation proceeds more when the energy is not less than this value, this activation will occur when the energy is 330 Joul/cm 2 or higher. This activation is performed by an impact with ions, electrons, etc., on the substrate surface, and this will enhance the surface migration of the active species to the thin film to promote the flatness of the filled trench surface. The above-mentioned thin film forming apparatus have separated vessels including one for plasma forming and another for the reaction. The invention may be also applied to the one vessel type thin film forming apparatus. Figure 16 is a schematic view showing a thin film forming apparatus according to one embodiment of this invention. In Figure 16, 101 denotes a grounded vacuum vessel forming the reaction vessel. Prescribed first and second reactive gases are introduced into the reaction vessel 101 through a gas inlet 102. The mixed gas contained in the reaction vessel is exhausted through a gas outlet 103. In the reaction vessel 101, a cathode (first electrode) 105 is disposed opposite the top wall of the vacuum vessel serving as an anode (second electrode). On the first electrode is placed a substrate 106, and a cooled nitrogen gas is passed over this electrode for cooling. A heating device (not shown) is also provided to raise the temperature. Further, the first electrode is connected to a high frequency power source 109 through a matching circuit 108. A heater 111 is coiled around a wall 110 of the reaction vessel 101, thereby preventing adhesion of the deposition film. Although it is not illustrated, another vacuum vessel which is vacuous or filled with an inert gas is disposed between the above reaction vessel 101 and the atmosphere for loading and unloading substrate 106. Thus, the reliability of the process is greatly enhanced by making the reaction vessel a so-called load-lock type. In the same drawing, 116 denotes an insulator. Further, another embodiment is shown in Figure 17. Parts identical with those shown in Figure 16 are assigned the same reference numbers. In this embodiment, a thin film was deposited on a substrate 106 in the same way as in the above embodiment and the substrate 106 was moved to another reaction vessel 112 with a carrier (not shown) and placed on a holder 113 and heated thereon by a heating means (not shown). In addition to the heating from the substrate side, this heating treatment may be effected by raising the substrate temperature instantaneously with an infrared lamp irradiated just above the substrate 106, for example. Further, an inert gas or the first reactive gas may be introduced through a gas inlet 102a during this heating treatment. Also, a high frequency power source 109 is connected via a matching circuit 108, so that the plasma can be generated during the heating treatment. In the drawing, number 115 denotes a gate valve. This invention may be applied in various ways. For example, a magnetic field may be supplied from outside between parallel plate electrodes applied with the above RF power as a means to generate the plasma, thereby generating a high-density plasma. Also, electrical discharge may be generated by ECR (Electron Cyclotron Resonance) discharge, a hollow cathode discharge, or by suppling high frequency power from outside with the substrate disposed in a vacuum vessel of an insulator, such as a quartz. Another thin film depositing process according to the present invention using the apparatus shown in Figure 16 will be described below. Oxygen and a tetramethylsilane Si(CH 3 ) 4 (TMS) as the first and second reactive gases are inserted into the reaction vessel 101. Figure 18 shows how the silicon oxide film is formed on the silicon substrate. The lateral axis indicates the temperatures of the substrate and the vertical axis shows the deposition velocity. The cross sections show different filled shapes formed within the trench of the substrate. The plasma is generated by applying an RF power of 13.56 MHz between the first and second electrodes to cause a high frequency electrical discharge. In the reaction vessel, oxygen is introduced at a ratio of 40 cc/minute and TMS 5 cc/minute. The total pressure is 0.6665 Pa (5 x 10 -3 Torr). A magnet is disposed on the second electrode side to allow high-density plasma to be obtained. It is seen from Figure 18 that the deposition velocity exhibits a maximum value with respect to changes of the substrate temperature. It is seen by observing the filled shape within the trench, that when the aspect ratio of the trench is one or more, SiO 2 produced by the reaction of the oxygen radical and TMS in the gaseous phase at a temperature above room temperature as shown in Figure 18(c) is deposited on the substrate similar to falling snow, as seen in the so-called conventional plasma CVD process, and the cavity is formed. On the other hand, it is seen that with a decrease of the substrate temperature, the deposit which is predominantly formed on the corner of the trench is reduced and when the substrate temperature is -20 °C or less the trench can be completely filled. This phenomenon is believed to take place in the following manner. The reaction products of the oxygen radical and TMS, such as hexamethylsiloxane (Si(CH 3 ) 2 ) 2 O and trimethyldisiloxane Si(CH 3 ) 3 OH, liquefy at the temperatures as shown in Figure 18 and liquid is formed as a layer on the substrate surface. This liquid layer catches therein SiO 2 species which have further reacted in the gaseous phase. It is believed that the oxidation proceeds accompanying the inclusion of the oxygen radical and the tacking in of the oxygen ions. On the other hand, the above liquid layer is finely dispersed over the substrate surface and therefore it exists most stably on the bottom corner of the trench to provide a large contact area in the substrate surface. As a result, observation of deposition overtime indicates that the deposit is first formed on the corner. Therefore, as shown in Figure 19, the deposit accumulates from the bottom of the trench upward to form a film, making it possible to fill a trench with a high aspect ratio and to provide a very even surface, which could not be done heretofore. It is possible also to form a film at a low temperature. This is effective for the formation of an interlaminar insulation film in the multilayered wiring process. Using nitrogen or NH 4 instead of oxygen, a silicon nitride film (Si 3 N 4 ) can be formed. It is needless to say that by using a material containing at least one element which is included in groups II to VI of the periodic table as the second reactive gas and suitably varying the substrate temperature, oxide and nitride can be formed readily. It is also needless to mention that the gas pressure in the reaction vessel is not limited to the above-mentioned 0.1333 Pa (10 -3 Torr) but can be selected to fall in the most effective pressure range according to the discharging method and the reactive gas used. Addition of an inert gas such as argon or helium to the first reactive gas prolongs the service life of the deposition species as a metastable active species and provides more effective deposition. The reaction gas to be used may be one type and a desired film may be deposited by a process such as thermal decomposition. It was experimentally confirmed that in the process of forming the film, the irradiation with an excimer laser having a wavelength of 193 nm, or with ions, electrons or the like, enhances the activity of the above liquefied layer, increasing the surface migration of the active species within the layer and completing the filling of the trenches, making the film completely flat. In the formation of the above silicon oxide film, when impurities such as POCl 2 , PCl 3 , PH 3 , BCl 3 , B 2 H 6 and AsH 4 are added to TMS, for example, the oxide film produced includes these impurities. This film fills the trench. Then it is heated instantaneously with a heater or a lamp, for example. This disperses the impurities into the silicon substrate. Heretofore, an oxide film containing the impurities along the walls was produced by the thermal CVD process. However, the concentration of the impurities contained in the oxide film which is formed along the side walls of the trench is lower than in the planar portion. Thus, a desired specific resistance could not be attained from the side walls. The oxide film according to the present invention includes the impurities in a very uniform amount as shown in Figure 19 describing the depositing state with the lapse of time. Therefore, after depositing, as shown in Figure 20, a thermal treatment is given to remedy the above drawbacks completely. The dispersion layer as shown in Figure 20 is essential to provide a sufficient memory capacity for large scale memory devices, such as 16M and 64M DRAMs. The above thermal treatment, when effected after the film formation, for example in situ within the treating chamber as shown in Figure 16 or Figure 17, can completely avoid the contamination by the impurities other than the prescribed impurities, such as carbon, nickel and other heavy metals. Thus, a high quality film is deposited. The inclusion of gas containing a hydrogen and a halogen element in the reactive gas reduces the methyl group contained in the TMS, and more stable CH 4 and CH 3 Cl are produced and removed, resulting in the lowering of the concentration of the carbon impurities. Thus, the film quality is much enhanced. AsH 4 is used as an impurity to be added to the second reaction gas in this example, but in case of phosphor (P) diffusion, a material such as POCl 3 , PCl 3 and PH 3 which reacts with the first or second reactive gas element to produce phosphor, may be used. In case of boron (B) diffusion, BCl 3 , B 2 H 6 , etc., may be used. For example, it was confirmed that when the organic metal compounds such as Al(CH 3 ) 3 , Ti(C 5 ) 2 , carbonyl metals, such as W(CO) 6 and Cr(CO) 6 , and halogenated metals are used together with hydrogen and nitrogen, metals can entirely fill in a space with a high aspect ratio, such as a contact hole. In addition to the above thin film forming, using for examples GeH 4 , SiH 4 , SiCl 4 , GeCl 4 , and a gas including at least silicon, the deposition of silicon and germanium can be done. Using As(CH 3 ) 3 , AsH 3 , Ga(CH 3 ) 3 and GaH 3 , GaAs and other group III-V compounds can be deposited and using a reactive gas containing indium and phosphor, InP and other group III-V compounds can be deposited. Further, using a reactive gas containing at least carbon and hydrogen, various high molecular organic films can be deposited. For example, when methylmethacrylate (MMA) is introduced and the substrate temperature is lowered to -30 °C or less, PMMA, which is used for an electron beam resist, can be formed. Proceeding now to the explanation of another embodiment relating to the thin film forming process of this invention, a macromolecule thin film forming process using hydrogen, nitrogen or gases including a halogen element, such as SiCl 4 , as the first reactive gas, and a gas at least including carbon and hydrogen as the second reactive gas, is described below. This process is basically the same as above-mentioned embodiment, and is briefly described with reference to Figure 3. The substrate used here has an unevenly configured surface provided with the trench having an aspect ratio of one or more. Nitrogen (N 2 ) gas is used as the first reactive gas, and N * radical is introduced into the reaction vessel by discharging. Methylmethacrylate (MMA) is introduced as the second reactive gas and the exhaustion is conducted. The substrate temperature is cooled to -30 °C or less. This results in the covering of the unevennness on the surface of the substrate with a film of PMMA, a polymer of MMA, for the same principle as the one illustrated in Figure 1, to achieve a super flatness. The PMMA film is, as everyone knows, widely used as an electron beam resist. Another embodiment relating to the thin film forming process of the present invention is explained below. Figure 21 is the sectional view of the final process showing the formation of the source, a drain electrode and a wiring for a MOS transistor using the process of this embodiment. A gate oxide film 71 and a gate electrode 72 are formed on a silicon substrate 70, and the source and the drain areas 73, 74 are integrally formed to the gate. After the coating of the entire surface with a silicon oxide film 75 using a CVD process, etc., the silicon oxide film 75 on the source and the drain 73, 74 is partially removed by etching to form a contact holes 76 with an aspect ratio of one or more. A wiring 77 for the source and the drain electrodes is next formed with the method of this invention. In particular hydrogen is used as the first reactive gas and Al(CH 3 ) 3 as the second reactive gas, and the substrate temperature is set at a specified value to fill the aluminium electrode wiring 77 completely into the contact hole 76 and form a super flatness by repeated deposition. After patterning of the electrode wiring 77, a silicon oxide film 78 is formed as protective film all over the surface. This protective film 78 can be formed by the process of this invention. By covering the wiring and the trenches formed by the CVD oxide film 75 with a protective film utilizing the same method as shown in the first embodiment, a flat film can be formed. In this embodiment, the mechanical vibration of the substrate 70 during the forming of the electrode wiring 77 disturbs a trench retaining layer (a stagnant layer) of the gaseous phase to promote the deposition of the alminium film on the substrate 70. Such vibration of the substrate itself, or the gaseous phase during the thin film forming is effective to increase the deposition velocity and to improve the film quality. As a means to create vibrations, a motor, etc., may be provided at the sample holder 2, as shown in Figures 2 through 4, to produce the mechanical vibration, or a supersonic wave oscillator may be incorporated in the holder 2. Although, aluminium is used for the metal filling the trenches, such as the MOS transistor contacts, etc., in this embodiment, the second reactive gas can be Ti(C 2 H 5 ) 2 , carbonyl metal, such as W(CO) 6 or Cr(CO) 6 , or halide metal, etc., instead of Al(CH 3 ) 3 . Embodiments of the present invention are explained above and some additional examples are described below. In place of the oxygen discharge in these embodiments, gases at least including an oxide, such as N 2 O, can be used. By using nitrogen or NH 3 , formation of a silicon nitride film is possible. When using a gas including at least one element of the second to sixth group in the periodic table, the oxide films can produce the nitride. The substrate temperature at that time can be set at a value not more than the liquefaction points of the active species of the first reactive gas, the second reactive gas and their reaction product, depending on the type of gases. Mixing inert gases such as argon or helium with the first reactive gas generates long-life metastable active species of these inert gases. These active species can carry the active species of the first reactive gas, and this results in high flexibility of the device design. Other than the thin film forming above, for example, silicon or germanium deposition is possible using hydrogen as the first reactive gas and using a gas containing at least germanium or silicon such as GeH 4 , SiH 4 , SiCl 4 and GeCl 4 as the second reactive gas. Group III to V compound, such as GaAs can be deposited when the second reactive gas is As(CH 3 ), AsH 3 , Ga(CH) 3 or GaH 3 , and group III-V compounds, such as InP, may be used when some reactive gas includes indium and phosphor. The thin film forming process explained above using raw material gas of the thin film as the second reactive gas (for example, Al(CH 3 ) 4 + H 2 ) can produce the same effect. In Figures 2 through 4, the rotating mechanism can be connected to the sample holder 12 for supporting the substrate 3 so that the substrate 3 is rotated at a high velocity and the reactive gas diffuses uniformly. The rotation may be at a fixed velocity, or may be intermittent to avoid a rotation of the gaseous phase with the substrate. The configuration above can increase the deposition velocity more, and reduce variations in the deposition velocity and the deposition film composition on the substrate. Such variations result from the difference of the distance from the gas inlet port 4 and 5 to the surface of the substrate 3, even in the case of a large-sized substrate, such as a silicon wafer. For radiation by electrons, ions and light, such as a laser beam, this is convenient because it can compensate for beam variations, and a large diameter beam is not necessary. A plurality of substrates can be simultaneously introduced to the reaction vessel in Figures 2 through 4. For example, with a reaction vessel in the shape of a rectangular parallelepiped, four surfaces are provided with substrates, another substrate is used for putting in and taking out of the substrates, and another surface for vacuum exhaust and the introduction of the first and the second reactive gas. In this case, too, each substrate is placed an equal distance from the gas inlet port to form a uniform thin film. The invention can achieve a better filling than with the prior art without causing radiation damage of insulation, semiconductor, metal, etc., to realize a flatness even for the trench with a high aspect ratio.

Claims: A thin film forming method, comprising the steps of: supporting a substrate (30) having a trench (31) or an unevenness thereon in a reaction vessel (1); introducing into the reaction vessel (1) oxygen as a first reactive gas (6) and a silicon containing gas as a second reactive gas (7); activating the first reactive gas (6) such that the activated first reactive gas reacts with the second reactive gas (7) to form at least one deposit species (32) which includes silicon and oxygen, the or each of said deposit species (32) being characterised by a phase diagram including a liquid phase region defined by a melting curve and an evaporation curve that intersect at a triple point; and forming a thin film (33) containing some of at least one of said deposit species (32) on the substrate (30) while retaining a pressure in the reaction vessel (1) higher than the triple point of the phase diagram of the or each of said at least one deposit species (32) and retaining the temperature of the substrate (30) within the liquid phase region of the phase diagram of the or each of said at least one deposit species (32). The thin film forming method according to claim 1, wherein the deposit material includes at least the deposit species (32), and one or more of the active species or the reaction products. The thin film forming method according to claim 1 wherein the step of activating the reactive gas (6) includes at least one of the steps of generating a plasma, thermally activating the reactive gas (6), light activating the reactive gas (6), or electron beam activating the reactive gas (6). The thin film forming method according to claim 3 wherein the step of light activating includes the step of laser beam activating the reactive gas (6). The thin film forming method according to claim 1 wherein the step of maintaining the temperature includes the step of cooling the substrate (30). The thin film forming method according to claim 1 or claim 5, wherein the step of maintaining the temperature further includes the step of heating the substrate (30). The thin film forming method according to claim 1 wherein the substrate is a semiconductor and the reactive gas (6) includes an impurity to change the conductivity of the substrate (30). The thin film forming method according to claim 1 wherein the aspect ratio of the trench (31) is one or more. The thin film forming method according to claim 1 wherein the reactive gas (6) is activated within that portion of the reaction vessel (1) having the substrate (30) supported therein or another portion. The thin film forming method according to claim 1 wherein the deposit species (32) is an insulator which is silicon oxide. The thin film forming method according to claim 1 wherein the substrate (30) and the reactive gas (6) are vibrated relatively. The thin film forming method according to any one of the preceding claims, wherein the first reactive gas also includes at least one of nitrogen, hydrogen or a gas containing at least one halogen element. The thin film forming method according to any one of the preceding claims, wherein the first reactive gas also includes at least one inert gas. The thin film forming method according to any one of the preceding claims, wherein the second reactive gas includes tetramethylsilane. The thin film forming method according to any one of the preceding claims, wherein the flow ratio of the first reactive gas to the second reactive gas is at least 2 and the temperature of the substrate from 20°C to -100°C and keeping the total internal pressure of the reaction vessel less than 1333 Pa (10 Torr). The thin film forming method according to claim 14, wherein the step of activating includes the step of radiating the laser beam with the power of 330 Joule/cm 2 sec. The thin film forming method according to claim 16, wherein the step of activating includes the step of radiating the laser beam with the wave length of 200 nm or less. The thin film forming method according to claim 15, wherein after forming the deposit species, the substrate is heated at 300°C or more with flowing oxygen or oxygen radical.

Inventors: NOGUCHI SADAHISA, OKANO HARUO, SEKINE MAKOTO, NOGUCHI, SADAHISA, OKANO, HARUO, SEKINE, MAKOTO, NOGUCHI, SADAHISA C/O PATENT DIVISION, OKANO, HARUO C/O PATENT DIVISION, SEKINE, MAKOTO C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H01L21
IPC codes: C23C16, H01L21