METHOD FOR PRODUCING A SEMICONDUCTOR DEVICE COMPRISING AN OXIDATION STEP

Abstract: A silicon wafer (3) having a tungsten and/or molybde­ num film (1) formed in its surface is heat-treated in hydrogen containing vapor. Thus, silicon can be selectively oxidized (2) without substantially oxidizing the tungsten and/or molybde­ num film (1).

Description: This invention relates to a method of producing a semiconductor device equipped with electrodes and interconnections consisting of tungsten or molybdenum. As is known well in the art, polycrystalline silicon has been used widely as a material for electrodes and interconnections of a semiconductor device. Polycrystalline silicon has been used for the following reasons. In order to miniaturize an MIS transistor, it is inevitable to employ so-called "self-alignment" techniques which form source and drain by ion implantation using the gate electrode as the mask. After ion-implantation is completed, however, annealing at high temperatures must be made by all means to remove damage of the source and drain region that has developed due to ion-implantation. Accordingly, to produce a miniature MOS transistor by self-alignment, the gate electrode must be made of a material which can withstand the heat-treatment at high temperatures, and polycrystalline silicon having a high melting point has replaced aluminum that was used widely and previously. The disadvantage of this polycrystalline silicon is, however, that its electric resistance is greater than that of metals. Since higher integration and miniaturization of semiconductor devices has made a rapid progress in recent years, the width of the electrode or interconnection becomes extremely small. Accordingly, if polycrystalline silicon is used, the resistance of the electrode or interconnection does not become sufficiently low and it is difficult to produce a miniature semiconductor device having high characteristics. To solve this problem, the use of tungsten or molybdenum has been proposed in place of polycrystalline silicon so as to form electrodes and interconnections. Since tungsten or molybdenum have a high melting point, they can withstand annealing at high temperatures and moreover, since their electric resistance is far lower than that of polycrystalline silicon, the problem described above, that occurs when polycrystalline silicon is used, does not develop even when the width of the electrode or interconnection is extremely small. However, tungsten and molybdenum have the problem that they are more easily oxidized than silicon. When heat-treatment is carried out at about 300°C or above in an oxidizing atmosphere, therefore, they are rapidly oxidized, disclosed or peeled off from the substrate. An insulating film (SiO₂ film) deposited on a semiconductor substrate is damaged or contaminated if an ion-implantation is effected using the gate as the mask to form the source and drain during the fabrication of an MOS transistor. Accordingly, the damaged or contaminated insulating film must be removed by etching after completion of gate formation and ion-implantation and the heat-treatment is carried out in an oxidizing atmosphere to re-grow an SiO₂ film on the semiconductor substrate. This process is carried out generally and widely and is an indispensable step to form a high reliability MOS transistor. (This process or treatment will be hereinafter referred to as "light oxidation"). When polycristalline silicon is used for the gate electrode and the interconnections, the light oxidation described above can be carried out smoothly without any problems. However, when tungsten or molybdenum is used, the gate electrode and interconnections are highly oxidized because tungsten and molybdenum are extremely oxidizable. Therefore, such semiconductors having high reliability and a high integration density cannot be produced. In the "Technical Digest of the International Electron Devices Meeting", December 1982, pages 564 to 567, a selective SiO₂ layer formation on a Mo electrode is disclosed. It is described that by annealing a poly-Si/MoO₂/Mo structure in an H₂ atmosphere the inside of the poly-Si film is oxidized into SiO₂ by oxygen generated in a MoO₂ reduction. This process is called "Internal Oxidation" because it is restricted to the internal reaction between the MoO₂ layer and the overlaying poly-Si. To eliminate the problems with the prior art described above, the present invention is directed to provide a method of producing a semiconductor device in which electrodes and interconnections are formed by using tungsten or molybdenum without any problems. That is, the present invention is directed to provide a method of producing a semiconductor device in which silicon is oxidized selectively without substantially oxidizing tungsten or molybdenum. To accomplish the object given above, in the method according to the invention as described in attached claim 1, there is selectively oxidized silicone alone without substantially oxidizing tungsten or molybdenum by carrying out heat-treatment in a mixed atmosphere of hydrogen and water vapor. Brief description of the drawings: Figure 1 is a diagram showing the preferred ranges of vapor-hydrogen partial pressure ratio and heating temperature; Figure 2 is a diagram showing the relation between the water temperature of a bubbler and the partial pressure ratio in the resulting hydrogen; Figure 3 is a diagram showing the relation between the partial pressure ratio in hydrogen and the thickness of the resulting SiO₂ film; Figures 4 and 5 are diagrams showing the relation between the thickness of the resulting SiO₂ film and the heating time and between the thickness of the SiO₂ film and the heating temperature, respectively; and Figures 6 through 11 are process diagrams showing other embodiments of the present invention, respectively. Description of preferred embodiments: As is well known, Si and most metals form their oxides upon reacting with water vapor. According to the examination carried out by the inventor of the present invention, however, it has been found that only Si can be selectively oxidized without the oxidation of W and/or Mo by heating Si and W and/or Mo in a mixed atmosphere consisting of the vapor and hydrogen. Though the mechanism of this reaction has not been clarified fully, it is assumed that even if W and Mo are oxidized by the vapor to their oxides, the resulting oxides are immediately reduced to the metallic state by the coexisting hydrogen, whereas Si is not reduced by hydrogen but remains as-oxidized by the vapor. It has also been found out that the selective reduction of W and Mo is significantly affected by the partial pressure ratio (which will be hereinafter represented by R). In other words, the relation represented by curves a, b and c in Figure 1 has been found existing between the partial pressure ratio R c , when the reduction of W, Mo and Si oxides starts, and various temperatures. As can be seen clearly from Figure 1, all of W, Mo and Si are reduced in the region below the curve c representing the reduction of SiO₂ but if the heat-treatment is carried out inside the region between the curve representing the reduction of WO₃ and the curve c described above, only Si can be selectively oxidized without substantially oxidizing W. Similarly, only Si can be oxidized selectively without substantially oxidizing W and Mo if the heat-treatment is carried out in the region interposed between the curve b representing the reduction of MoO₂ and the curve c described above. When the heat-treating temperature is 1,000°C, for example, only Si can be selectively oxidized without oxidizing W and Mo (with the oxides being reduced), if R is in the range of from 10⁻⁶ to about 1. If the present invention is applied to the "light oxidation" when fabricating MOS transistors, for example, the SiO₂ film can be formed on the Si substrate without oxidizing the electrodes and interconnections made of W or Mo, and the present invention is extremely advantageous for the fabrication of the MOS transistors having high integration density. If the heat-treating temperature is below about 400°C, however, the oxidation speed of Si becomes extremely slow and when it is above about 1,200°C, on the other hand, deformation of the diffusion region formed in the substrate becomes so remarkable and damage of the reaction tube becomes also great. For these reasons, the heat-treating temperature is selected in the range of from about 400 to about 1,200°C. Example 1 After an SiO₂ film was formed on a silicon wafer by a known thermal oxidation process, a 0.3 µm-thick W or Mo film was formed by sputtering on the SiO₂ film and was then heat-treated at 1,000°C for 30 minutes in an N₂ or Ar atmosphere containing 1 ppm of oxygen as an impurity. According to the procedures described above, the W and Mo films were not mostly oxidized but there were also the cases in which only the film edge portions were oxidized or the entire surface of the film was oxidized, so that a stable result could not be obtained. Incidentally, the Si surface was oxidized in all cases. Next, the sample described above was heated at 1,000°C for 30 minutes in the hydrogen/vapor atmosphere in which the partial pressure ratio R of vapor to hydrogen was changed stepwise from 1, 3 x 10⁻¹, 3 x 10⁻², ...., 1 x 10⁻⁶ so as to examine the state of oxidation of W, Mo and Si. As a result, oxidation of W and Mo was observed when R was 1 but could not be observed when R was below 3 x 10⁻¹. On the other hand, Si was oxidized in all cases. Here, the state of oxidation was examined by X-ray photoelectronic spectrometry. Example 2 This example illustrates the relation between the oxidation of Si and the partial pressure ratio R of H₂O and H₂ the atmosphere when heating is effected in the H₂/H₂O atmosphere. The vapor-containing hydrogen could be obtained by passing hydrogen through a bubbler containing pure water, and the vapor quantity in hydrogen could be adjusted to a desired value by changing the temperature of the pure water in the bubbler. Thus, heat-treatment was carried out at 950°C for 10 minutes by changing the ratio R and the thickness of the SiO₂ film formed on the silicon wafer was measured using an ellipsometer. The silicon wafer used for the measurement was washed by hydrofluoric acid before heating to remove in advance the oxide film on the wafer surface. The result obtained was shown in Figure 3. The thickness of the SiO₂ film increased substantially proportionally to the value R within the range of O < R ≦ 0.4. Figure 4 shows the result of the measurement of time dependence of the thickness of the SiO₂ film when the heating temperature was 1,000°C and R was 0.05. Similarly, Figure 5 shows the dependence of the thickness of the SiO₂ film upon the heating temperature when R was 0.05. Example 3 This example illustrates the application of the present invention to the fabrication of an MOS field effect transistor. First, as shown in Figure 6a, a tungsten film 1 and a silicon dioxide film 2' were formed sequentially in thickness of 350 nm and 60 nm, respectively, on a 20 nm-thick silicon dioxide film 2 that was formed on the surface of a silicon substrate 3. The silicon dioxide film 2' and the tungsten film 1 were then patterned sequentially into the pattern of a gate electrode by known dry etching techniques. Next, an impurity ion was implanted into the silicon substrate 3 through the silicon dioxide film 2 using the electrode consisting of the silicon dioxide film 2' and the tungsten film 1 as the mask, to form a source and drain 4 as shown in Figure 6b. The silicon oxide films 2, 2' at the portions other than the portion covered with the W film 1 were selectively removed using a hydrofluoric acid solution diluted to 1/10 by water, as shown in Figure 6c. Next, the heat-treatment was effected at 900°C for 15 minutes in hydrogen containing 5% of vapor to grow an about 10 nm-thick silicon dioxide film 2'' on the exposed silicon substrate 3 as shown in Figure 6d. Thereafter, a phosphoglass layer 5 was deposited in a thickness of about 500 nm over the entire surface and contact holes were bored by photoetching. Aluminum interconnections 6 were formed to complete the MOS transistor as shown in Figure 6e. This example corresponds to the light oxidation step in the silicon gate process, and the tungsten gate transistor produced by this step exhibited the improvement in the MOS characteristics (break-down voltage of the SiO₂ film and variance of breakdown voltage). Example 4 A 350 nm-thick tungsten film 1 was deposited and patterned on a 20 nm-thick SiO₂ film 2 that was formed on a Si single crystal substrate 3 as shown in Figure 7a. Heat-treatment was effected at 1,000°C for one hour in hydrogen passed through a bubbler of pure water (hydrogen containing about 3% of water), whereby the thickness d₁ of the SiO₂ film 2 of the portion covered with the tungsten film 1 and the thickness d₂ at the portion not covered with the tungsten film 1 increased to 30 nm and 70 nm, respectively. However, the tungsten fim 1 was not oxidized. The moisture content in hydrogen, heating temperature and heating time were increased (or decreased) in accordance with Example 2 and the thickness d₁ and d₂ of the SiO₂ film increased (or decreased) in response to the former. After the heat-treatment, the breakdown voltage of the SiO₂ film was measured using the tungsten film as the electrode. The breakdown voltage was found increased than before the heat-treatment. It was thus confirmed that the present invention could effectively prevent degradation of the characteristics of the SiO₂ film due to the heat-treatment. Example 5 A 300 nm-thick tungsten film 1 was vacuum deposited on a 20 nm-thick SiO₂ film 2 that was formed on an Si crystal substrate 3 as shown in Figure 8a, and an 80 nm-thick SiO₂ film 2' was deposited by CVD on the tungsten film 1. Unnecessary portions were removed by sequentially etching the SiO₂ film 2' and the tungsten film 1. The sample was then heated at 900 to 1,000°C for 15 minutes in hydrogen containing 3 to 20% of water, whereby the portion of the SiO₂ film 2''' not covered with the tungsten film 1 became thicker in the same way as in Example 4 but the thickness of the SiO₂ film 2 below the tungsten film 1 remained substantially unaltered, as shown in Figure 8b. As can be understood from this Example, when those materials (at least one of polycrystalline Si, PSG, SiO₂, Si₃N₄ and the like) which are generally used as a mask for the diffusion of an impurity are used for the heat-treatment on the tungsten film, the function of the mask for the prevention of oxidation can be more improved than when heat-treatment is carried out using the tungsten film alone. Example 6 A 350 nm-thick molybdenum film 8 was vacuum deposited on a polycrystalline silicon substrate 7 as shown in Figure 9a and unnecessary portions were removed by etching the film 8. The sample was heat-treated at 900°C for 30 minutes in hydrogen containing 5% of vapor. As a result, the molybdenum film 8 reacted with the polycrystalline silicon substrate 7 and a molybdenum silicide layer 9 was formed at their contact portion. On the other hand, the portion of the surface of the polycrystalline silicon substrate 7 at which the molybdenum film 8 did not exist and which was exposed was oxidized to form a thick SiO₂ film 2. According to this method, contact could be established between the molybdenum film and the polycrystalline silicon substrate and at the same time, an insulating film could be formed on the polycrystalline silicon in self-alignment with the molybdenum electrode. Substantially the same result could be obtained by use of a tungsten film in place of the molybdenum film. Example 7 Figures 10a through 10c illustrate another method of producing an MOS field effect semiconductor device to which the present invention is applied. First, as shown in Figure 10a, an about 350 nm-thick tungsten film was formed on a 20 nm-thick field insulating film (SiO₂ film) 2 (reference numeral 2''' represents a field silicon diode film formed in advance) that was formed on the surface of an Si crystal substrate 3. The tungsten film was then patterned to form a gate electrode 1. Next, the sample was heated in an oxygen atmosphere of about 400°C to form an about 50 nm-thick tungsten oxide film 10 on the surface of the tungsten film 1 as shown in Figure 10b. Using the tungsten oxide film 10 and the tungsten film 1 as the mask, an impurity was doped to the surface region of the Si substrate 3 and the sample was heat-treated at 950°C for 30 minutes in hydrogen containing 5% of vapor, thereby forming source and drain region 4. In this process, the tungsten oxide film 10 on the surface of the tungsten film 1 served as the mask for doping the impurity by ion implantation or the like, and was reduced to tungsten due to the subsequent heat-treatment in the H₂O-H₂ atmosphere, as shown in Figure 10c. Due to the heat-treatment described above, the silicon oxide film on the source-drain region 4 became thicker than the oxide film below the gate electrode. Example 8 An about 250 nm-thick molybdenum silicide film 9 was formed on the surface of a 300 nm-thick polycrystalline silicon plate 7 as shown in Figure 11a and a molybdenum film 8 was vacuum deposited on it in a thickness of about 300 nm. Unnecessary portions were removed by etching to form an electrode 8. The sample was heated at 900°C for 10 minutes in hydrogen containing 5% of water, whereby a part of the molybdenum electrode 8 was converted to its silicide, and an SiO₂ film 2 was formed on the exposed surface of the resulting molybdenum silicide film 9. The reason was assumed to the fact that the portion below the molybdenum electrode was converted to the silicide due to the supply of Si from the polycrystalline silicon film 7 as the base to the molybdenum silicide film 9 and the SiO₂ film could be formed at the exposed portion of the molybdenum silicide film. As can be understood clearly from this example, the present invention makes it possible to grow the SiO₂ film not only on Si but also on the silicide film. When tungsten silicide was used in place of molybdenum silicide, the exposed surface of tungsten silicide could also be oxidized without oxidizing molybdenum and tungsten. The same result could be obtained when silicides of molybdenum and tungsten were formed on single crystal silicon in place of polycrystalline silicon. Example 9 Next, still another method of producing an MOS field effect semiconductor device in accordance with the present invention will be described with reference to Figure 6 which shows Example 3. A 350 nm-thick tungsten film 1 was deposited on a 20 nm-thick gate SiO₂ film 2 that was formed on an Si crystal substrate 3. When etching the sample to a gate electrode pattern, there had been conventionally the problem that the SiO₂ film around the gate electrode was also damaged so that the SiO₂ film became thinner by about 10 nm and the breakdown voltage of the gate SiO₂ film got deteriorated. When the sample was heated at 900°C for 10 minutes in hydrogen containing 3% of water after etching the gate electrode of tungsten in accordance with the present invention, however, the damage of the SiO₂ film was recovered and at the same time, a fresh SiO₂ film grew. Accordingly, the breakdown voltage of the gate SiO₂ film was improved. This heat-treatment may be effected after etching and removing the SiO₂ film around the gate, and the same result could be obtained when the heat-treatment was effected without removing the SiO₂ film. Example 10 The following two kinds of wafers were prepared. First, an Si wafer having formed a tungsten film on the surface thereof was heated in an oxygen atmosphere to form a 300 nm-thick tungsten oxide film. Separately, an Si wafer was washed by hydrofluoric acid to prepare a wafer (up to 2 nm thick) hardly having any oxide film. These two kinds of wafers were heated at 1,000°C for 1 hour in hydrogen containing 3% of water and their surfaces were analyzed by X-ray photoelectronic spectrometry. As a result, the tungsten oxide was reduced to tungsten due to the heat-treatment but the Si wafers were oxidized and an SiO₂ film was formed on the surface. The resulting SiO₂ film was found to be 58 nm thick as a result of measurement by an elipsometer. As described above, silicon can be selectively oxidized without oxidizing tungsten and molybdenum during the fabrication of a semiconductor device by using H₂O/H₂ as the atmosphere of heat-treatment and by adjusting their partial pressure ratio. As a result, the so-called "light oxidation" process, that has been employed in the conventional polycrystalline silicon gate process, can be also used in the fabrication process of MOS transistors using tungsten or molybdenum for the gate. In other words, the present invention eliminates the problem of oxidation of tungsten and molybdenum during fabrication of semiconductor devices and a process approximate to the one used in the conventional polycrystalline silicon process can now be used. Moreover, the characteristics of the resulting device can be remarkably stabilized in comparison with the tungsten or molybdenum gate process not using the H₂O/H₂ heat-treatment. Example 11 In an Si gate process for fabricating an MOS transistor using a polycrystalline silicon film for a gate electrode, a so-called "glass flow" process is effected in which after the Si gate is covered with a PSG (phosphosilicate glass) which is an inter-layer insulating film, the surface of the PSG film is made smooth. When Mo or W is used for the gate, however, the oxidation of Mo or W due to oxygen will occur when heated in oxygen or nitrogen, even if Mo or W is covered with the PSG film because pin holes exist in the PSG film. When the sample was heated at 1,000°C for 30 minutes, for example, in the H₂+H₂O (5% moisture content) atmosphere in accordance with the present invention, however, the surface of the 500 nm-thick PSG film (P concentration = 12 mol%) covering W was made sufficiently flat. Accordingly, the present invention made it possible to carry out "glass flow" without the possibility of oxidation of W or Mo. As described above, the present invention makes it possible to selectively oxidize only Si and to form an SiO₂ film without oxidizing W or Mo and to remarkably improve the reliability and producibility of semiconductor devices using these materials. Particularly when W or Mo is used as the low resistance electrode of an MOS field effect semiconductor device, compatibility with the Si gate process can be improved. For instance, the "light oxidation" process becomes feasible. Since the present invention uses hydrogen containing water as the heating atmosphere, it can be easily practised using an ordinary heating apparatus consisting of a silica tube and an electric furnace and is excellent in both mass-producibility and economy.

Claims: A method of producing a semiconductor device comprising: forming a tungsten or molybdenum film (1) of a desired shape on a silicon substrate (7), on an SiO₂ film (2) disposed on a silicon substrate (3) or on a tungsten or molybdenum silicide film (9); and heating the resulting structure in a hydrogen atmosphere containing water vapor, the partial pressure ratio between the water vapor and the hydrogen gas and the heat treating temperature being selected as values falling in the range between the curve a and the curve c of Figure 1 for a tungsten film or falling in the range between the curve b and the curve c of Figure 1 for a molybdenum film, so as to selectively oxidize the surface of said silicon substrate or silicide film without oxidizing surfaces of said tungsten or molybdenum film. The method according to claim 1, wherein the heat treatment temperature is in the range from about 400°C to about 1200°C. The method according to claim 1, wherein said silicon substrate is a single crystal silicon substrate (3) or a polycrystalline silicon substrate (7). The method of producing a semiconductor device according to claim 1, including the steps of forming an SiO₂ film (2) on said silicon substrate (3); forming said tungsten or molybdenum film (1) in the desired shape on said SiO₂ film; doping an impurity having a conductivity type opposite to that of said silicon substrate (3) to the surface region of said silicon substrate using said tungsten or molybdenum film (1) as a mask; and carrying out said heat treatment. The method according to claim 4, wherein doping of said impurity is carried out by laminating a mask film (2') consisting of at least one member of a phosphosilicate glass film, an SiO₂ film and an Si₃N₄ film on said tungsten or molybdenum film (1) and then effecting ion implantation. The method according to claim 4, wherein said tungsten or molybdenum film (1) is the gate electrode of an MIS field effect semiconductor device. The method according to claim 4, wherein said heat-treatment is carried out after a portion of said SiO₂ film (2) where said tungsten or molybdenum film (1) ist not deposited is removed by etching.

Inventors: HOMMA, TEIICHI, IWATA, SEIICHI, KOBAYASHI, NOBUYOSHI, MATSUO, HITOCHI, YAMAMOTO, NAOKI
Applicants: HITACHI, LTD.
Application year: 1984
Main classification: H01L21
IPC codes: H01L21, H01L29