Gas-phase growing method for the method

Abstract: A gas-phase growing apparatus is provided with a reaction furnace (1), and a substrate (2) having a minute depression is placed inside the reaction furnace (1). In the reaction furnace (1), a reaction gas is supplied onto the substrate, so as to cause gas-phase growth of a layer of a reaction product within the depression of the substrate. The reaction gas is supplied such that the pressure in the reaction furnace is alternately changed between a first pressure and a second pressure. The first pressure is a pressure capable of producing a continuous or intermediate stream in which the collision between the molecules of the reaction gas is predominant, while the second pressure is a pressure lower than the first pressure. The gas-phase growing apparatus is also provided with a pipe (4) for introducing the reaction gas into the reaction furnace, a valve (5) for controlling the flow rate of the reaction gas, a pump (7) for discharging the unconverted reaction gas from the reaction furnace, an orifice (8) for providing flow resistance for the discharge side of the reaction furnace, a pressure meter (6) for measuring the internal pressure of the reaction furnace, and an electromagnetic valve (9) operating in association with the pressure meter.

Description: The present invention relates to a gas-phase growing method used for the manufacture of semiconductor devices. In a conventional CVD apparatus which is in general use, a mixture of a raw gas and a carrier gas is made to flow onto a heated substrate inside a reaction container at a constant rate. An unconverted raw gas is discharged from the reaction container together with the carrier gas. Normally, the CVD apparatus is operated under a constant pressure so as not to disturb the gas stream inside the reaction container. If a mixed gas containing silane and oxygen is used in the above type of CVD apparatus so as to grow silica (SiO) in a silicon submicron trench (which has come to assume importance in the field of semiconductor devices, such as DRAMs), the problem shown in Fig. 11 occurs. As is shown in Fig. 11, silica 106 grows thick at the inlet 104a of a trench 104 of a substrate 102, while it grows thin in the inside 104b of the trench 104. As a result, a void 105 is left in the inside 104b of the trench 104. The above problem is related to the size of the trench. If a trench in the substrate is small, like a submicron trench, then the molecules of a gas repeatedly collide against the trench wall when they diffuse into the inside of the trench. Even if a void is not produced, the top surface of the silica formed to fill the trench becomes uneven. A wire cannot be easily connected to such an uneven surface. In addition, H₂O, produced by the reaction between the silane and the oxygen, remains inside the trench, adversely affecting the quality of the resultant semiconductor device. Recently, it is reported that the problem shown in Fig. 11 can be solved by using a combination of both tetra ethoxyl silane (TEOS) and ozone as a raw gas. Even if this method is effective in solving the problem, the effect is limited to the case where SiO is grown. The method does not provide any guarantee of solution to the problem if a different material, such as SiN is grown. With the conventional gas-phase growing method and apparatus mentioned above, it is difficult to fill the minute trenches of an uneven-surface substrate with the same or different materials, or flatten the uneven surface of that substrate. Thin Solid Films 158 (1988) 123-131 discloses halide chemical vapour deposition technique in which first and second gases of different kinds are supplied. EP-A-0 371 854 discloses a method for selectively depositing refractory metal (for example tungsten) on the surface of a semiconductor substrate to form an electrical contact, without depositing refractory metal on an adjacent mask layer. Refractory metal halide gas and hydrogen gas are simultaneously flowed through a reaction chamber to deposit the refractory metal on the substrate surface and to a lesser extent on the mask layer. Then the flow of hydrogen is interrupted to cause the refractory metal halide to etch the refractory metal which forms on the mask layer. The deposition and etch steps are repeated until the requisite thickenss is deposited. The process may take place in a single reaction chamber, using only refractory metal halide gas and pulsed flow of hydrogen gas. An object of the present invention is to provide a gas-phase growing method which enables the minute trenches of an uneven-surface substrate to be uniformly filled with any kind of material or enable the uneven surface of the substrate to be flattened. According to one aspect of the present invention, there is provided a gas-phase growing method, wherein: first and second reaction gases of different kinds are supplied to a substrate having a minute depression and located in a reaction furnace to thereby form a layer of a reaction product within the minute depression; and said reaction gases are supplied such that pressure in the reaction furnace is alternately changed between a first pressure and a second pressure which is lower than said first pressure; characterized in that said first reaction gas is supplied continuously, and said second reaction gas is supplied in pulsation or stepwise with a pressure higher than that of the first reaction gas. According to another aspect of the present invention, there is provided a gas-phase growing method, wherein: first and second reaction gases of different kinds are supplied onto a substrate having a minute depression and located in a reaction furnace, to thereby form a layer of a reaction product within the minute depression; and said reaction gases are supplied such that pressure in the reaction furnace is alternately changed at least between a first pressure and a second pressure lower than the first pressure; characterized in that said first and second reaction gases are supplied in first and second pulsation modes, respectively, said first and second modes having the same periodic frequency, said first and second modes having first and second phases, respectively, in that when pulses of the first and second reaction gases occur during a period of said periodic frequency, said first and second phases are not equal, in that each pulse of the first mode partially overlaps with a pulse of the second mode which starts after said each pulse of the first mode, and in that the second reaction gas is supplied with a pressure higher than that of the first reaction gas. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic illustration showing a gas-phase growing apparatus; Fig. 2 is a timing chart showing an example of a gas-phase growing method; Fig. 3 is a schematic illustration showing the advantages of the method of Fig. 1; Figs. 4 to 6 are schematic illustrations showing gas-phase growing apparatuses; Fig. 7 is a schematical illustration of a gas-phase growing apparatus used in the method according to the present invention; Fig. 8 is a timing chart showing an example of a gas-phase growing method according to the present invention; Figs. 9A-9D are explanatory illustrations showing how gas-phase growth is carried out according to the present invention; Fig. 10 is a schematic illustration showing a further gas-phase growing apparatus; and Fig. 11 is an explanatory illustration showing the problems occurring in conventional gas-phase growth. Fig. 1 shows a first gas-phase growing apparatus. As is shown in Fig. 1, the apparatus comprises: a reaction furnace 1 in which a substrate 2 is placed and heated; a pipe 4 for introducing a raw gas into the reaction furnace 1; a valve 5 for controlling the flow rate of the raw gas; a pump 7 for discharging the raw gas from the reaction furnace 1; an orifice 8 for providing flow resistance for the discharge side of the reaction furnace 1; a pressure meter 6 for measuring the internal pressure of the reaction furnace 1; and an electromagnetic valve 9 which operates in association with the pressure meter 6. The orifice 8 serves to prevent a reaction product from being scattered in the reaction furnace 1 by providing flow resistance for the discharge side of the reaction furnace 1. A description will be given of the case where the above apparatus is used for growing polysilicon in a trench of the substrate 2. First, the electromagnetic valve 9 is opened and the valve 5 is closed, so as to reduce the pressure in the reaction furnace 1. Simultaneous with this, the substrate 2 is heated up to a predetermined temperature. Next, silane (i.e., a raw gas) is introduced into the reaction furnace 1, with the valve 5 gradually opened, until internal pressure P of the reaction furnace 1 becomes equal to P min (P min : an arbitrary pressure). When the length of time Δt₁ has elapsed from the time when the valve 5 is opened, the electromagnetic valve 9 is closed. Thereafter, the operation is carried out according to the timing chart shown in Fig. 2. As is shown in Fig. 2, pressure P increases from P min to P max with the lapse of time Δt₂. Time Δt₃ after pressure P increases to P max , the electromagnetic valve 9 is opened. Time Δt₄ after the electromagnetic valve 9 is opened, pressure P decreases to pressure P min , and time Δt₁ thereafter the electromagnetic valve 9 is closed. With the operations carried out up to this point being regarded as one cycle, the processing is repeated by the necessary number of cycles. The value of P max is determined by the following formula: Pmax ≧ 10kT s /(√ 2 πℓS²) where T s is the temperature of the substrate, k is a Boltzman constant, S is the area of a collision cross section of molecules, and ℓ is the width or diameter of a trench. Pressure P max expressed by the above formula is a condition of the production of a continuous stream. If pressure P max is lower than that pressure or a pressure which produces an intermediate stream, then the gas molecules cannot enter the interior of the trench without repeatedly colliding against the trench wall. If the gas molecules repeatedly collide against the trench wall, a large amount of polysilicon is deposited on the inlet portion of the trench, as in the case of a conventional constant-stream CVD apparatus. If P max is equal to or higher than 10kT s /(√ 2 πℓS²) ,the gas molecules collide against one another and can therefore swiftly reach the bottom of the trench. Strictly speaking, it takes a certain time for the gas molecules to reach the bottom of the trench, and more than a certain amount of reaction product is inevitably deposited on the inlet portion of the trench. However, if pressure P is decreased to a pressure in the range of 0.1 to 50% of pressure P max , namely to pressure P min the raw gas remaining within the trench cannot be easily discharged. The deeper the trench is, the more difficult it becomes to remove the raw gas from the trench. As a result, the raw gas stays inside the trench for a comparatively long time, and the growing speed of a reaction product is high at the bottom of the trench. Therefore, the problem of the conventional technique can be canceled to a certain extent. Further when the time needed to raise the pressure is shortened (Δt₂ + Δt₃), the raw gas is transferred to the bottom of the trench quickly so that the difference in deposition rate between the inlet portion and the bottom portion is minimized not to cause a problem in practice. Valve 5 may be closed substantially simultaneously when electromagnetic valve 9 is opened, so as to increase the pressure-reducing speed during the operation. If valve 5 is closed when electromagnetic valve 9 is opened, the reaction product can be deposited in a satisfactory manner, as is shown in Fig. 3. As described above, the condition which pressure P max should satisfy the following equation: P max ≧ 10kT s /(√ 2 πℓS²) From a different viewpoint, this condition can also be expressed as follows: K n = (λ/ℓ) ≦ 0.01 where K n is a Knudsen number, λ is a mean free path of molecules, and ℓ is the representative dimension of the opening of a trench (e.g., the diameter or width of the opening of the trench). Therefore, pressure P max is controlled to satisfy the Knudsen number expressed by the above formula. The condition of the production of an intermediate stream is expressed by: 0.01 < K n = (λ/ℓ) ≦ 0.1 Therefore, pressure P max is also controlled to satisfy the Knudsen number expressed by this formula. In the case where a continuous stream is produced, the gas molecules hardly collide against the trench wall; the gas molecules diffuse to the bottom of the trench while colliding against one another. In the case where an intermediate stream is produced, some of the gas molecules may collide against the trench wall, but most of them diffuse to the bottom of the trench while colliding against one other. In either case, the collision between the molecules of the raw gas is predominant, and due to this collision the raw gas can reach the bottom of the trench. Consequently, the advantages noted above can be obtained. Fig. 4 shows another gas-phase growing apparatus. As is shown in Fig. 4, a valve 15 and an orifice 12 are provided in a gas-introducing pipe 4, while only an orifice 8 is provided in a gas-discharging pipe (in other words, an electromagnetic valve is not provided for the gas-discharging pipe, as it is in the first embodiment shown in Fig. 1). The alternate switching of pressure is carried out by opening or closing the valve 15 provided in the gas-introducing pipe 4. The purpose for providing the orifice 12 in the gas-introducing pipe 4 is to prevent the pressure in the reaction furnace 1 from rapidly increasing when the valve 15 is opened. In this arrangement, the gas is constantly discharged from the reaction furnace 1, so that the particles of dust or the like do not flow back into the reaction furnace 1 when the pressure in the reaction furnace 1 increases. Fig. 5 shows a further gas-phase growing apparatus. As is shown in Fig. 5, a reservoir tank 20 is provided for a gas-discharge pipe such that the tank 20 is located on the downstream of an electromagnetic valve 9. When the valve 9 is closed, the tank 20 can be maintained in the vacuum state by means of a pump 7. Therefore, when the valve 9 is opened, the pressure in the reaction furnace 1 can be quickly reduced. Fig. 6 shows yet another gas-phase growing apparatus. As is shown in Fig. 6, a reservoir tank 21 is provided for a gas-introducing pipe 4 such that the tank 21 is located on the upstream of a valve 15. Since a gas to be supplied into a reaction furnace 1 can be stored in the tank 21 beforehand, the pressure in the reaction furnace 1 can be quickly increased when the valve 15 is opened. The apparatuses shown in Figs. 5 and 6 may be combined with each other. In this combination, the reservoir tanks 20 and 21 are provided for the gas-discharging pipe and gas-introducing pipe, respectively. With this structure, it is possible to quickly raise or lower the pressure in the reaction furnace 1. Fig. 7 shows an embodiment of the present invention. In the embodiment, SiO is formed within a trench by supplying silane and oxygen into a reaction furnace 1. The gas-phase growing apparatus shown in Fig. 7 comprises two gas-introducing pipes 4a and 4b, one being used for the supply of SiH₄ and the other being used for the supply of O₂. The apparatus is operated according to the timing chart shown in Fig. 8. Referring to the timing chart shown in Fig. 8, O₂ is supplied into a trench after the pressure in the reaction furnace 1 is reduced. Then, SiH₄ is supplied, with the result that the pressure in the reaction furnace 1 increases. Thereafter, the supply of both the O₂ and SiH₄ is stopped, and the reaction furnace 1 is brought nearly into a vacuum state by means of a pump 7. With the operations carried out up to this point being regarded as one cycle, the processing is repeated by the necessary number of cycles. Figs. 9A-9D show how the gas-phase growth occurs by carrying out the processing mentioned in the preceding paragraph. Fig. 9A shows the state obtained when low-pressure oxygen is supplied. Fig. 9B shows the state obtained when high-pressure SiH₄ is supplied. As is shown in Fig. 9B, the low-pressure oxygen moves into to bottom region of the trench due to the supply of the high-pressure SiH₄. Fig. 9C shows the state obtained when the SiH₄ diffuses into the oxygen and reacts therewith. As is shown in Fig. 9C, SiO₂, produced by the reaction between the SiH₄ and the O₂, is formed in the bottom of the trench. Fig. 9D shows the state where the unconverted gases are discharged from inside the trench by bringing the reaction furnace 1 nearly into a vacuum state. With this cycle repeated, SiO₂ is grown first on the bottom of the trench and then on the other portions thereof. It should be noted that the O₂ need not be supplied with its pressure alternately changed between high and low; it may be constantly supplied at a comparatively low pressure. It should be also noted that the supply of O₂ and that of SiH₄ is not be carried out in a completely alternate manner. Since these gases remain in the reaction furnace 1 for a certain time, the supply time of O₂ and the supply time of SiH₄ partially overlap with each other. Further, a time interval may be given between the supply of O₂ and that of SiH₄. It should be also noted that the order of the supply of O₂ and that of SiH₄ may be reversed, in which SiH₄ is supplied in low pressure and then O₂ is supplied in high pressure. Fig. 10 shows a further gas-phase growing apparatus. The reaction furnace 1 employed in this embodiment is a vertical diffusion type, and a plurality of substrates 2 are placed inside the reaction furnace 1 and processed simultaneously. The substrates 2 are arranged in the reaction furnace 1 while being vertically spaced from each other by a distance of 5 mm or so. In this apparatus, a raw gas supplied into the reaction furnace 1 uniformly reaches each of the substrates 2, and is discharged from the reaction furnace 1 before the diffusion of the raw gas has an adverse effect on the growth of a reaction product. With this operation repeated, the substrates 2 can be processed in a uniform manner though they are arranged at short intervals. Therefore, the processing can be carried out with high efficiency. In this apparatus, the raw gas is supplied to the substrates 2 in the direction parallel to the substrates 2. Therefore, the substrates 2 are prevented from being blown off by the raw gas introduced. In order to alternately change the pressure in the reaction furnace 1 between P max (which produces a continuous or intermediate stream) and P min (which is an arbitrary pressure lower than P max ), a pressure-regulating means made by a piston and a cylinder may be employed. As mentioned above, a raw material in the state of either a continuous stream or an intermediate stream is supplied to a minute depression of a substrate placed in a low-pressure atmosphere, so that the raw material can be uniformly fed to the minute depression, even to the bottom thereof. In the case where the conventional technique is used, a layer of reaction product is formed at high speed at the inlet portion of a minute depression and in some cases covers the depression, but this problem does not occur in the process of the invention. In addition, due to the pressure-reducing step, an auxiliary reaction product does not remain inside the depression, as it does in the conventional case. Therefore, the claimed process enable the growth of a thin film free of impurities. Further, even if a plurality of substrates are processed simultaneously, they can be processed uniformly, without reference to the shape of the reaction furnace.

Claims: A gas-phase growing method, wherein: first and second reaction gases of different kinds are supplied to a substrate having a minute depression and located in a reaction furnace to thereby form a layer of a reaction product within the minute depression; and said reaction gases are supplied such that pressure in the reaction furnace is alternately changed between a first pressure and a second pressure which is lower than said first pressure; characterized in that said first reaction gas is supplied continuously, and said second reaction gas is supplied in pulsation or stepwise with a pressure higher than that of the first reaction gas. A method according to claim 1, wherein: said first pressure is determined to satisfy K n = (λ/ℓ) ≦ 0.1 where K n is a Knudsen number, λ is a mean free path of molecules of said reaction gas, and ℓ is the representative width of the depression. A method according to claim 1, wherein: said second pressure is lower than 1/2 of said first pressure. A method according to claim 1, wherein the first gas is O₂ and the second gas is SiH₄. A gas-phase growing method, wherein: first and second reaction gases of different kinds are supplied onto a substrate having a minute depression and located in a reaction furnace, to thereby form a layer of a reaction product within the minute depression; and said reaction gases are supplied such that pressure in the reaction furnace is alternately changed at least between a first pressure and a second pressure lower than the first pressure; characterized in that said first and second reaction gases are supplied in first and second pulsation modes, respectively, said first and second modes having the same periodic frequency, said first and second modes having first and second phases, respectively, in that when pulses of the first and second reaction gases occur during a period of said periodic frequency, said first and second phases are not equal, in that each pulse of the first mode partially overlaps with a pulse of the second mode which starts after said each pulse of the first mode, and in that the second reaction gas is supplied with a pressure higher than that of the first reaction gas. A method according to claim 5, wherein: said first pressure is determined to satisfy K n = (λ/ℓ) ≦ 0.1 where K n is a Knudsen number, λ is a mean free path of molecules of said reaction gas, and ℓ is the representative width of the depression. A method according to claim 5, wherein: said second pressure is lower than 1/2 of said first pressure. A method according to claim 5, wherein the first gas is O₂ and the second gas is SiH₄. A method according to claim 5, wherein the first gas is SiH₄ and the second gas is O₂.

Inventors: OHMINE TOSHIMITSU C O INTELLEC, OHMINE, TOSHIMITSU, C/O INTELLECTUAL PROP. DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: C23C16
IPC codes: C30B25, C23C16