Method for chemical vapor deposition and apparatus therefor

Abstract: A chemical vapor deposition apparatus comprises a starting material container (106) holding a starting material in a liquid state, a starting gas generating container (102) into which the liquid starting material is fed from the starting material container, a means (103) for keeping constant the liquid level of the liquid starting material held in the starting gas generating container, a means for injecting a bubbling gas (105) from the outside into the liquid starting material held in the starting gas generating container, thereby bubbling the starting gas, and a reaction chamber (101) into which a mixed gas of the starting gas and the bubbling gas are fed.

Description: BACKGROUND OF THE INVENTION Field of the invention This invention relates to an apparatus for vaporizing a liquid which becomes a starting gas (hereinafter referred to as "starting gas liquid"), and more particularly to a chemical vapor deposition apparatus capable of vaporizing a high-boiling starting gas liquid of organic metal compounds or the like in a good controllability in chemical vapor deposition (hereinafter referred to as "CVD") as used when, e.g., semiconductor devices are fabricated. This invention also relates to a method for fabricating a semiconductor device by using the chemical vapor deposition apparatus, and more particularly to a method for fabricating a semiconductor device by causing a monocrystal to selectively grow on a non-monocrystalline surface and forming an Al electrode on the monocrystal. Related Background Art Prior arts A to C: Sputtering has been hitherto used to form a metal wiring in semiconductor devices. However, the sputtering, by which atoms sputtered from a target material are deposited on a substrate, has a poor terraced-portion covering performance. It also is infeasible for selective growth. Figs. 2A and 2B are diagrammatic cross sections, the former showing an instance in which Al wiring is deposited in a contact hole having a small aspect ratio, and the latter showing an instance in which Al wiring is similarly provided in a contact hole having a large aspect ratio, according to the conventional sputtering. In Figs. 2A and 2B, reference numeral 201 denotes a silicon substrate; 202, an insulating film formed of silicon dioxide or the like; and 203, a wiring material of aluminum or the like. As shown in Fig. 2B, in the contact hole having a large aspect ratio, there is a great possibility of the occurrence of a locally thin Al wiring layer or a void to cause disconnection. Meanwhile, Fig. 3 is a diagrammatic cross section of a state in which an Al wiring layer is formed in a contact hole having a large aspect ratio by CVD. In Fig. 3, reference numeral 301 denotes a silicon substrate; 302, an insulating film formed of silicon dioxide or the like; and 303, a metallic material such as aluminum deposited by CVD; 304, aluminum wiring deposited by sputtering or CVD. As shown in Fig. 3, in the selective deposition carried out by CVD, the contact hole is completely filled up with aluminum, so that the possibility of disconnection becomes very low. Thus, since the formation of Al thin films by CVD can achieve a superior terraced-portion covering performance and besides enables selective deposition on a conductive material, it is also possible to selectively deposit Al in contact holes and thereafter carry out non-selective deposition to provide a completely flat surface without causing disconnection in contact holes. As stated above, the CVD is advantageous as a process for fabricating semiconductor devices with a fine wiring. In the CVD, however, starting materials must be converted into gases so that they can be fed into reaction space. Accordingly, in order to carry out the selective deposition using as a starting material the organic metal materials that are liquid at room temperature and normal pressure, apparatus have been hitherto constituted as follows: A basic constitution of the apparatus is shown in Fig. 4. In Fig. 4, a carrier gas such as hydrogen is brought under reduced pressure by means of a reducing valve 401 and sent to a bomb 402 provided with a bubbling mechanism. Most of starting gases feasible for selective deposition of aluminum are in a liquid state at room temperature, as exemplified by dimethylaluminum hydride (hereinafter "DMAH") and triisobutylaluminum (hereinafter "TIBA"). Hence, the bubbling is effected to make bubbles in the bomb, so that a mixed gas comprised of the carrier gas and a saturated vapor of organic aluminum compound such as DMAH or the like is pressure-forwarded at the outlet of the bomb. This mixed gas is fed into a reaction chamber 403 to cause thermal decomposition reaction on a heated semiconductor substrate, and aluminum is deposited thereon. The reaction chamber 403 is exhausted by means of a vacuum pump 404. Reference numeral 405 denotes an auxiliary pump. In an instance in which DMAH is used as the organic aluminum compound gas and hydrogen as the carrier gas, the following reaction proceeds on the surface of the semiconductor substrate. 2AlH(CH 3 ) 2 + H 2 → 2Al + 4CH 4 or 2AlH(CH 3 ) 2 → 2Al + 2CH 4 + C 2 H 6 What is questioned in the foregoing is that, although the deposition selectivity between a conductive material and a non-conductive material can be completely assured if reduction reaction attributable to hydrogen as shown in formula (1) proceeds, no deposition selectivity between a conductive material and a non-conductive material can be completely assured if simple thermal decomposition as shown in formula (2) proceeds, so that it becomes impossible to fill up the hole with aluminum as shown in Fig. 3. Hence, it follows that a molar ratio of DMAH to hydrogen in the mixed gas has a great influence on the state of deposition. In the foregoing gas feeding method, the quantity of the starting gas fed into the reaction chamber can be approximately represented as follows: Q = Q in × (P V /P B ) In formula (3), Q is a quantity of the starting material being fed, Q in is a quantity of the gas fed in from an inlet pipe, P V is a saturated vapor pressure of a starting gas, and P B is a pressure inside a reactor. As be seen from formula (3), when a starting material with a small saturated vapor pressure is used, the starting material being fed becomes smaller in quantity, so that in the worst the deposition rate is determined by the feed rate of the starting material. In such a state in which the former is determined by the latter, the following problems may occur. (1) Deposition rate differs depending on the hole diameter of contact holes. (2) The hole is filled up with aluminum in the shape having facets (as shown in Figs. 2A and 2B) and can not be filled up in a flat shape. Accordingly, in the CVD carried out using a liquid starting material, a system capable of stably feeding a large quantity of starting materials in a proper mixing ratio is considered most important among requirements. Prior art D: In recent years, as semiconductor integrated circuits are made more highly integrated, a technique by which a fine structure at a submicron level is formed on a semiconductor wafer in a good controllability has attracted notice. In particular, in the process of fabricating ICs, LSIs and so forth, an uneven structure formed on the surface of a semiconductor wafer subjected to patterning is necessarily fine and dense. Hence, in the step of forming wirings between elements, it becomes necessary to use a technique by which a conductor film serving as a wiring material can be formed on the fine and dense uneven surface in a good covering performance and at the same time fine wiring connecting openings (hereinafter referred to as "contact holes") of 1 µm or less in diameter can be filled up with the wiring material. In the formation of wirings between elements in ICs or LSIs, Al or Al alloy film formed by sputtering has also been hitherto used. However, the sputtering, in which atoms broken away from a target material by sputtering the material are deposited on the wafer surface, is known to give differences in shape of covering in accordance with unevenness of the wafer surface. Especially when contact holes are 1 µm or less in diameter and 1 µm or more in depth, it is difficult to completely fill up the contact hole by sputtering. Meanwhile, according to the film forming technique making use of CVD, the starting gas contained in a gas phase coming into contact with the wafer surface is decomposed by heat energy or the like to thereby form a desired film on the wafer surface. The CVD makes it possible to uniformly distribute a deposited film material over the wafer surface, and also makes it possible to form films with a superior terraced-portion covering performance compared with sputtering, because of the film deposition that takes place as a result of decomposition reaction of the starting material gas on the wafer surface. The CVD also makes it possible to selectively form a film only on a specific material by utilizing differences in the mechanism of decomposition reaction of starting material gases on the surfaces of various materials such as Si, metals and insulating films laid bare to the wafer surface. Hence, suitable selection of starting material gas species and conditions enables selective formation of conductor films only inside the contact holes. Accordingly, a number of studies are made on the formation of wiring material films by CVD, and it has been already ascertained that Al, W, Cu and so forth can be formed into films by CVD. However, most starting gases for wiring material films are liquid at room temperature and also commonly have a small vapor pressure. For example, DMAH used as a starting material gas of Al films has a saturated vapor pressure of about 2.66x10 2 Pa (2 Torr) at room temperature, and TIBA about 6.65x10 1 Pa (0.5 Torr). Thus, in order to supply into the reaction chamber of a CVD apparatus a starting gas liquid having a small saturated vapor pressure, it is necessary to provide a vaporizer. As the vaporizer for a starting gas liquid, a bubbling apparatus has been hitherto used. A conventional bubbling apparatus is schematically illustrated in Fig. 7. In the bubbling apparatus, a carrier gas 554 is blown from an inlet pipe 553 into a starting gas liquid 552 put in a starting gas liquid container 551, and vapor of the starting gas liquid 552 is taken into bubbles 555 of the carrier gas so that a carrier gas 557 having therein the vapor of the starting gas liquid is fed from an outlet pipe 556. Prior art E : As other prior art, development is energetically made in recent years on three-dimensional integrated circuits in which semiconductor devices are multi-layered in the direction of a normal of a substrate to provide a more highly integrated and multi-functional circuit. Development is also energentically being made year by year on large-area semiconductor devices for solar cells comprising elements arranged in an array on an inexpensive glass plate, switching transistors of liquid crystal picture elements, and so forth. What is common to the both is that they require a technique by which a semiconductor thin film is formed on an amorphous insulating substrate and electronic devices such as transistors are formed on the semiconductor thin film. In particular, what is important is a technique by which a high-quality monocrystalline semiconductor layer is formed on the amorphous insulating substrate. As the technique by which a high-quality monocrystalline semiconductor layer is formed on the amorphous insulating substrate, a crystal growth process called SENTAXY (Selective Nucleation Based Epitaxy) is proposed (Japanese Patent Application Laid-open No. 63-107016, and Applied Physics Letters, Vol. 52, page 1231, 1988). The SENTAXY is a crystal growth process in which crystal growth treatment is applied to a substrate having a free surface adjoiningly provided with i) a non-nucleation surface having a smaller nucleation density and ii) an amorphous or polycrystalline nucleation surface having a sufficient small area so as to form only a single nucleus from which a monocrystal is grown and having a larger nucleation density than that of the non-nucleation surface, thereby causing a monocrystal to grow from the single nucleus. When the monocrystal to be grown is constituted of a semiconductor material such as Si or GaAs, a semiconductor device can be made up into it. When a plurality of nucleation surfaces are arranged on the substrate, a semiconductor device array can be fabricated. Electrodes required when semiconductor devices are fabricated are usually formed by photolithography. Employment of the above crystal growth process is accompanied by formation of the facets that are peculiar to monocrystals when monocrystals are made to grow on the amorphous substrate, so that the substrate has a surface irregularity and no patterning can be carried out unless any countermeasure is taken. Then, the monocrystals on the surface are made smooth by polishing and thereafter a resist is coated to carry out patterning of electrodes by photolithography. Here, EB vacuum deposition or sputtering is used to form films of electrode materials such as Al. Problems A & B : The apparatuses in the prior art are as described above. They, however, are disadvantageous in the following points. Disadvantage (1): As stated previously, even though the mixing ratio of an organic metal compound to hydrogen is an important factor, they have a very poor controllability on the mixing ratio. Disadvantage (2): The mixing ratio of an organic metal compound to hydrogen may change depending on temperature changes in the vicinity of the bomb. Disadvantage (3): The mixing ratio of an organic metal compound to hydrogen may differ depending on the quantity of the liquid remaining in the bomb. The above three disadvantages can be further detailed as follows: taking account of the instance in which the organic metal compound is DMAH, the molar mixing ratio depends on DMAH saturated vapor pressure and hydrogen partial pressure at the outlet of the bomb. n DMAH /n H2 = P DMAH /P H2 wherein n DMAH is a molar number of DMAH gas fed into the reaction chamber; n H2 is a molar number of H 2 fed into the reaction chamber; P DMAH is a partial pressure (saturated vapor pressure) of DMAH gas at the bomb outlet; and P H2 is a partial pressure of H 2 at the bomb outlet. In the above, P DMAH unconditionally depends on temperature, and is 1.33x10 2 Pa to 2.66x10 2 Pa (1 to 2 Torr) at best at room temperature. P H2 can be controlled using the reducing valve 401 as previously described with reference to Fig. 7, but substantially depends on the precision of the reducing valve 401. When the ratio of P DMAH to P H2 should be controlled to be several times, P H2 comes to be about 1.33x10 3 Pa (10 Torr), and the reducing valve must be controlled to provide a pressure of several hundred Pascals (several Torr). However, it is very difficult to achieve this condition by reducing valve techniques of the prior art. With regard to disadvantage (2), how the molar mixing ratio may change depending on temperature changes will be shown below. As in formula (4), the molar mixing ratio changes depending on P DMAH . Since the P DMAH is merely a saturated vapor pressure, it may change depending on temperatures. Table 1 shows saturated vapor pressure of DMAH (see J. Am. Chem. Soc., vol.75, Feb. 20, pp. 835-839, 1953) Saturated vapor pressure of DMAH Temperature (°C) Vapor pressure 10 2 Pa (mmHg) (Found) Temperature (°C) Vapor pressure 10 3 Pa (mmHg) (Found) 24.7 2.4 (1.8) 61.8 2.2 (16.7) 25.9 2.8 (2.1) 66.5 3.0 (22.4) 35.8 4.8 (3.6) 69.2 3.3 (25.1) 39.4 6.4 (4.7) 72.0 3.9 (29.6) 47.5 10.2 (7.7) 75.2 4.5 (34.0) 49.1 11.2 (8.4) 80.4 5.8 (43.4) 54.6 15.6 (11.7) 86.2 7.6 (57.4 57.1 17.6 (13.2) 91.0 9.4 (70.7) Hydrogen is different from DMAH, and is gaseous at room temperature, and hence the P H2 does not change exponentially. As a result, the molar mixing ratio greatly diverge depending on the temperature changes in the vicinity of the bomb. Finally, disadvantage (3) is discussed below. The hydrogen partial pressure P H2 at the bomb outlet does not come in agreement with the hydrogen pressure at the inlet of the bomb, and they have the relationship: P H2(out) - P H2(in) = c·ρ·h wherein P H2(out) is a partial pressure of hydrogen at the bomb outlet; P H2(in) is a partial pressure of hydrogen at the bomb inlet: ρ is a specific gravity of the organic metal compound; h is a distance between the opening of a bubbling nozzle and the surface of liquid in the bomb; and c is a pressure conversion factor. In this case, the variable that can be controlled by a regulator is only the P H2(in) . Since the value of h becomes smaller as the apparatus is used, the P H2(out) changes correspondingly. As a result, in order to keep the P H2(out) constant, the P H2(in) must be corrected in accordance with the quantity of the liquid remaining in the bomb. This, however, is accompanied with a great difficulty on account of the constitution of the apparatus. As described above, the conventional apparatus has involved a great problem in controlling a starting gas in order to an optimum condition. Problem C : The CVD process of the prior art making use of a liquid starting material also has the following problems. In the case when the organic metal compound is used as a starting material as previously stated, the organic metal compound gas is usually led into the reaction chamber by a method in which the organic metal compound that is liquid at room temperature is bubbled using a carrier gas so that a mixed gas of the carrier gas and the starting gas is fed into it. Some organic metal compounds, however, have high viscosity in a liquid state, and hence usual bubbling may produce bubbles that become larger in size before they reach the surface of liquid. In such a case, the gas containing the starting material gas coming out of the surface of liquid undergoes pulsation to make it difficult to feed the material gas at a stable pressure. Under such circumstances, even the flow rate of the gas fed into the reaction chamber may become unstable, and in the worst the deposition rate can not be stable to bring about the problems that the layer thickness can not be completely controlled or the film quality can not be stable. Problem D: In the case of the prior art as shown in Fig. 7. the partial pressure of the starting gas liquid 552 which is contained in the gas fed from the outlet pipe 556 can not be higher than the saturated vapor pressure of the starting gas liquid 552 at the temperature of a gas 554, and may change depending on the shape of the bubbling apparatus, the flow rate of the carrier gas 554, the temperature of the starting gas liquid 552, etc. Hence, it has been difficult to keep constant the partial pressure ratio of the starting gas liquid 552 contained in a gas 557. In addition, in the case when the starting gas liquid 552 has a vapor pressure of several hundred Pascals (several Torr) or less, the ratio of partial pressure of the starting gas liquid 552 contained in the gas 557 to that of the carrier gas 554 comes to be 1:100 or less when bubbled under 10 5 Pa (1 atmospheric pressure). In this case, the partial pressure of the starting gas liquid 552 inside the reaction chamber comes to be 1.33 Pa (10 -2 Torr) or less when the gas pressure inside the reaction chamber of the CVD apparatus into which the gas 557 is fed is set at 1.33 x 10 2 Pa (1 Torr), thereby making it difficult to feed the starting gas liquid to the wafer surface in a large quantity. Problem E In the method of forming electrodes as described in prior art E, there is a limit to the flatness of the surface and hence there is a limit to an improvement of yield in photolithography, often bringing about an obstacle to the formation into devices. In the case when the polishing is employed, the polishing must be carried out at a high precision to hinder a cost decrease. Thus,it has been sought to make an improvement. Also in the case when the semiconductor device array formed by SENTAXY is fabricated, it is indispensable to separate devices, and a method that enables easy device separation is sought so long as the semiconductor device formed by SENTAXY is necessarily an aggregate of elements made up into a number of domains. US-A-4278458 discloses a method of fabricating optical fibres by producing a vapour which may be produced by heating or even boiling a bath of an appropriate precursor liquid, the vapour then being combined with a carrier gas. JP-A-60-248228 discloses a bubbling apparatus in which a carrier gas is bubbled through a liquid raw material stored in a bubbler vessel to form a gaseous mixture of the carrier and the gaseous raw material. An embodiment of the present invention aims to provide a chemical vapor deposition apparatus, an apparatus capable of improving the controllability of the mixing ratio of the starting gas to the carrier gas, and obtaining a stable mixing ratio without regard to any changes in temperature in the vicinity of the bomb or any changes in quantity of the liquid remaining in the bomb. An embodiment of the present invention also aims to provide a means for feeding a starting gas into a CVD reaction chamber in a good efficiency and at a stable pressure even when a starting liquid having a relatively high viscosity is bubbled. An embodiment of the present invention aims to provide, in a chemical vapor deposition apparatus, a means for feeding a starting gas liquid into a film forming chamber in a large quantity and a stable state even in the case of the starting gas liquid having a low saturated vapor pressure at room temperature in order to solve the problems of the prior art as described above. An embodiment of the present invention aims to provide a method for fabricating a semiconductor device with a high reliability at a low cost, which enables easy device separation of a semiconductor device array formed on the same substrate and can improve the yield of fabrication. A first aspect of the present invention provides a method as set out in claim 1. A semiconductor device may be produced using a chemical vapour deposition process in which a starting gas for the chemical vapour deposition process is produced by performing the method according to the first aspect of the present invention. A second aspect of the present invention provides an apparatus as set out in claim 5. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically illustrates the construction of an illustrative example of a chemical vapor deposition apparatus. Figs. 2A and 2B are diagrammatic cross sections showing the deposition state of aluminum by conventional sputtering. Fig. 3is a diagrammatic cross section showing the deposition state of aluminum by ideal CVD. Fig. 4 diagrammatically illustrates the construction of a conventional chemical vapor deposition apparatus (CVD apparatus). Fig. 5 is a diagrammatic cross section showing the deposition state of aluminum by CVD when hydrogen is in excess. Fig. 6 is a diagrammatic cross section of a liquid starting material vaporizing apparatus used in the present invention. Fig. 7 is a diagrammatic cross section of a conventional liquid starting material vaporizing apparatus. Figs. 8A to 8D are cross-sections illustrating steps of an embodiment of the semiconductor device fabrication method of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below. Figs. 8A to 8D illustrate an embodiment of a process for fabricating a semiconductor device. As shown in Fig. 8A, tungsten (W) is embedded by photolithography and sputtering in the vicinity of the surface of a substrate 805 whose surface is comprised of SiO 2 , and SiO 2 is removed from only the part at which a semiconductor monocrystal is grown, so as for a W region 804 to be laid bare, which serves as a nucleation surface. Next, as shown in Fig. 8B, a first conductivity type monocrystalline region 803 is grown from the nucleation surface of this minute W region 804 by the crystal growth process (SENTAXY) as disclosed, e.g., in Japanese Patent Application Laid-open No. 63-107016, in the presence of a first conductivity type doping gas. Next, as shown in Fig. 8C, a reverse conductivity type monocrystalline region 802 is grown in the presence of a reverse conductivity type doping gas to make up a pn junction in the monocrystal. The pn junction may also be formed by ion implantation carried out at the stage where the crystal growth has been completed. When the crystal growth is carried out in the absence of the doping gas to form an i-type monocrystal, the present invention is also applicable in instances in which a monocrystal is formed in combination of the conductivity type monocrystal with the i-type monocrystal. The crystal growth process is not limited to the SENTAXY and any process may be used so long as a monocrystal can be formed on a non-monocrystal. According to the procedure as described above, devices or a device array can be formed on a substrate such as of SiO 2 or the like. The size of individual devices can be arbitrarily selected by controlling the distance between nucleation surfaces and the time for crystal growth. For example, it is easy to grow monocrystals of about 50 to several hundred µm at intervals of 0.1 to 1 mm. When electrodes are formed on these individual devices or the device array, electrode can not be formed by conventional semiconductor fabrication processes (comprising, e.g., a combination of the step of metal film formation by EB deposition or sputtering and the step of photolithography) since, as previously stated, the surface has irregularities because of the facets peculiar to monocrystals. The specific feature of the present invention resides in carrying out the formation of Al electrodes by low-pressure CVD using an alkylaluminum hydride diluted with H 2 . Use of such an electrode forming method makes it possible, as shown in Fig. 8D, to form an Al electrode 801 only on the monocrystal facets and to separate the individual device without electrode formation on the SiO 2 portion on the surface of the substrate 805. The low-pressure CVD used in the present invention is carried out using alkylaluminum hydride diluted with H 2 , at a pressure of about 10- 1 Pa to 10 5 Pa 10 -3 to 760 Torr)and at a substrate temperature of about 160 to 450°C. A specific method therefor is disclosed in Japanese Patent Application No. 2-6558. According to this method, Al can be deposited so long as the surface is made of an electron-donative material. The electron-donative material herein refers to those in which free electrons are present in the substrate or free electrons are intentionally produced in it, as exemplified by a material having a surface capable of accelerating chemical reaction by electron transfer between the substrate surface and the starting material gas molecules having adhered to the surface. For example, metals and semiconductors are included in this material. It may also include metals or semiconductors on the surfaces of which thin oxide films are present. This is because the chemical reaction takes place by electron transfer between the substrate and the starting material molecules adhered thereto. Specifically, such a material includes semiconductors such as monocrystalline silicon, polycrystalline silicon and amorphous silicon; two-component, three-component or four-component type Groups III-V compound semiconductors comprised of a combination of any of Group III elements Ga, In and Al and Group V elements P, As and N; and metals, alloys and silicides thereof such as tungsten, molybdenum, tantalum, tungsten silicide, titanium silicide, aluminum, aluminum-silicon, titanium-aluminum, titanium nitride, copper, aluminum-silicon-copper, aluminum-palladium, titanium, molybdenum silicide and tantalum silicide. On the substrate thus constituted, Al can be deposited only by simple thermal reaction in a reaction system comprised of the starting material gas and H 2 . For example, a thermal reaction taking place in a reaction system comprised of (CH 3 ) 2 AlH (DMAH) and H 2 is considered to basically proceed as follows: The DMAH assumes a dimer structure at room temperature. Material not falling within this range, as exemplified by highly insulating materials such as SiO 2 , Si 3 N 4 , SiC, BN, diamond, Al 2 O 3 and TaO 2 , can not be deposited upon by low-pressure CVD merely using the alkylaluminum hydride diluted with H 2 . So long as the surface of a semiconductor integrated circuit or the like is covered with a layer comprised of such a non-electron-donative material, the deposition of Al can be prevented at that part. Hence, when the film formation of Al electrodes is carried out by this method, the Al electrodes can be formed only on the protruded surfaces of a plurality of devices without any patterning carried out by photolithography or the like and no electrodes adhere to the SiO 2 portion locating between the devices, thereby bringing about the advantage that the devices can be automatically or naturally separated. As the alkylaluminum hydride, dimethylaluminum hydride DMAH, (CH 3 ) 2 AlH, or monomethylaluminum hydride MMAH, CH 3 AlH 2 , may be used. The material for the substrate 805 is not limited to only SiO 2 , and any materials may be used so long as they are non-electron-donative materials capable of controlling nucleation. Fig. 1 is a diagrammatic view of the construction of an illustrative example of a chemical vapor deposition apparatus. In Fig. 1, reference numeral 101 denotes a reaction chamber; 102, an airtight container (starting gas-generating container) with a mechanism capable of keeping a liquid level constant and a mechanism for bubbling; 103, a liquid level sensor; 104, a valve; 105, a reducing valve that acts for reaction gas; 106, a bomb that holds a starting gas liquid. An example in which DMAH is used as a starting gas will be described below in detail. First, on the primary side of the bomb 106 having an opening above the surface of liquid, argon is pressure-fed into the bomb at a pressure higher than 10 5 Pa (1 atm) (atmospheric pressure). This pressure of argon causes the DMAH to flow into the airtight container 102 through the valve 104. In this airtight container 102, the liquid level sensor 103 is provided, which is of a noncontact and positive displacement type, so as to control the valve 104. More specifically, the valve 104 is kept open until the DMAH reaches to a given liquid level, and the valve 104 is kept close when the DMAH is judged by the sensor 103 to have reached a given liquid level. A chain line in Fig. 1 shows a signal system therefor. Because of the mechanism attached in this way, the DMAH liquid level in the container 102 can be always kept constant. As a result, since the DMAH is bubbled in the container 102, no liquid level variations may occur even if the DMAH has been consumed in the reaction chamber, and the value of P H2(out) - P H2(in) shown in formula (5) is kept constant. Therefore, it is unnecessary to correct P H2(in) in order to make P H2(out) constant. The constitution as described above can substantially give solution to the problem to be solved by the present invention, that is, the disadvantage (3), as described above. Next, a course extending from the container 102 in which the liquid level is kept constant to the reaction chamber 101 is kept at a temperature of 80°C. The temperature can be kept by a method in which, not shown in the drawing, the container 102 is put in a thermostatic chamber and a heater is wound around the piping that connects the reaction chamber 101 and the container 102. In both instances, the temperature is controlled at 80°C while it is measured by a thermocouple. The extent in which the temperature is controlled at 80°C is the zone defined by a dotted line in Fig. 1. Keeping the temperature at 80°C is meaningful on two points. One is to accurately control temperature so as to decrease any variations in DMAH/hydrogen mixing ratio that may be caused by variations in temperature as previously discussed as disadvantage (2). The other is to increase the vapor pressure of DMAH by setting the temperature at 80°C, which is higher than room temperature, to make it easy to control the DMAH/hydrogen mixing ratio. That is, the vapor pressure of DMAH at 80°C is, as previously shown in Table 1, comes to be abut 5.3x10 3 Pa (40 Torr). Hence, in order to set the DMAH/hydrogen mixing ratio to 1/2 to 1/5, the pressure may be controlled at 2.7x10 4 Pa to 4.3x10 4 Pa (200 Torr) to (320 Torr). This value is not so difficult for the reducing valve 105, and can be readily achieved (here, the bubbling differential pressure of hydrogen in the container 102 is set at 1.6x10 4 Pa (120 Torr). Control precision itself of the reducing valve 105 also becomes advantageous by 20 times compared with room temperature bubbling, thereby bringing about a great improvement in respect of the disadvantage (1) previously stated. Increasing the vapor pressure of DMAH also makes it possible to feed DMAH gas in a large quantity, so that it is possible to control the rate of reaction in the reaction chamber 101 by the reaction itself. In that instance, deposition is carried out under conditions of a substrate heating temperature of 270°C, a reaction chamber internal pressure of 1.6x10 2 Pa (1.2 Torr), a hydrogen primary pressure of 3.3x10 4 Pa (250 Torr) and a heating temperature of 80°C in the course extending from the container 102 to the reaction chamber 101. The mixing ratio can be more readily controlled if the heating temperature in the course extending from the container 102 to the reaction chamber 101 is further raised. However, in the case of DMAH, it is an unstable substance, which begins to thermally decompose at 130°C or above, and hence the temperature was set at 80°C from the viewpoint of safety. The constitution as described above made it possible to stably achieve the ideal filling-up with aluminum as shown by the aluminum 303 in Fig. 3. The present invention can be applied to not only the instance where DMAH is used as the starting gas but also an instance where other organic aluminum compound such as triisobutylaluminum (TIBA) or the like is used. The present invention can also be applied to not only the instance where the organic aluminum compound is used but also an instance where other CVD starting gas having a low vapor pressure at room temperature is used, which may include, for example, tetraethyl orthosilicate (TEOS) or trimethylgalium (TMG). The present invention can still also be applied to not only the instance where argon is used as the gas for pressure-feeding the liquid which becomes a starting gas but also an instance where other inert gas such as helium, neon nitrogen or the like is used. In the present invention, in order to make more accurate the mixing ratio of the organic metal compound to the reaction gas such as hydrogen, it is also effective to preheat the hydrogen or the like as a reaction gas between the reducing valve 105 and the container 102. This preheating carried out at the same temperature as of the container 102 makes the partial pressure ratio more accurately controlled. In this case, the precision viewed from the degree of variation may act neither advantageously nor disadvantageously. Since, however, the partial pressure ratio of hydrogen to the organic metal compound directly corresponds to the molar ratio, no complicated calculation becomes unnecessary when deposition conditions are changed, thereby making it easy to carry out process control. An embodiment of the present invention will be described below in detail with reference to the drawing. Fig. 6 schematically illustrates the construction of a liquid starting material-vaporizing apparatus which represents a specific feature of the present invention. In the drawing, reference numeral 511 denotes a starting gas liquid container; 512, a starting gas liquid; 513 and 514, heaters for heating the starting gas liquid container 511; 521, a gas feed pipe from which a carrier gas is fed into the starting gas liquid container 511 and whose tip is formed of a capillary tube; 522, a carrier gas feed valve; 523, a starting gas liquid-feeding pipe from which the liquid material gas is fed into the starting gas liquid container 511; and 524, a starting gas liquid-feeding valve. Reference numeral 531 denotes a reflux condenser connected to the starting gas liquid container 511; 532, a refrigerant; 533, a refrigerant inlet; 534, a refrigerant outlet; 536, a discharge valve for discharging a mixed gas of the carrier gas and the starting gas liquid; and 537, a heater for heating gas piping in its course extending from a mixed gas discharge portion 535 of the reflux condenser 531 to the use point of the mixed gas. Reference numeral 538 denotes a mixed gas of the carrier gas and the starting gas liquid, which flows out of the reflux condenser 531. First, the refrigerant temperature-conditioned to the intended temperature is led from the refrigerant inlet 533 into the reflux condenser 531, and the reflux condenser 531 is kept at a prescribed temperature. Subsequently, the starting gas liquid 512 is led from the starting gas liquid-feeding pipe 523 into the starting gas liquid container 511, and the the starting gas liquid container 511 is heated at its bottom by means of the heater 513, to a temperature higher than the boiling point of the starting gas liquid 512 held under the pressure kept in the starting gas liquid container 511. At that time, the carrier gas is flowed from the gas feed pipe 521 at the intended flow rate. Here, the tip of the gas feed pipe 521 is formed of a capillary tube, and is so provided as to come into contact with the bottom of the starting gas liquid container 511. The temperature of the heaters 514 and 537 is set at a temperature higher than the temperature of the refrigerant 532 led into the reflux condenser 531. In the condition set as described above, the starting gas liquid 512 is kept smoothly boiled. The vapor of the starting gas liquid flowed into the reflux condenser 531 is cooled by the refrigerant 532 and refluxed into the starting gas liquid container 511. At this time, the carrier gas containing the vapor of the starting gas liquid with a saturated vapor pressure corresponding to that of the starting gas liquid 512 held at the temperature of the refrigerant 532 and under the pressure inside the mixed gas discharge portion 535 is flowed out through the mixed gas discharge portion 535. The flow rate and composition of the mixed gas 538 thus flowed out can be maintained at constant values so long as the carrier gas is fed from the gas feed pipe 521 at a constant flow rate and the starting gas liquid 512 is smoothly refluxed. An example in which a liquid starting gas is actually fed will be described below while showing temperature and pressure at each component part in the case when DMAH is used as the starting gas liquid. DMAH has a boiling point of 154°C at 10 5 Pa (1 atm.) At this temperature, DMAH may gradually decompose. Hence, the reflux must be carried out at about 80°C or below at which the DMAH can be stable. More specifically, when DMAH is used, the inside of the starting gas liquid container 511 and the inside of the reflux condenser 531 are evacuated to 5.3x10 3 Pa (40 mmHg). Then, the refrigerant 532, comprising FLORINATE FX330 (available from 3M Co., U.S.A), temperature-conditioned at 65°C is led into the reflux condenser 531, and the heater 513 is set at about 90°C and the heaters 514 and 537 at 70°C each. Subsequently, hydrogen gas of 100 sccm is fed as the carrier gas from the gas feed pipe 521. In this condition, the DMAH is boiled at about 80°C and smoothly refluxed. The composition of the mixed gas 538 was examined to find that a mixed gas of 200 sccm, comprised of 50% of the carrier gas hydrogen and 50% of DMAH, was obtained. To the mixed gas of 200 sccm, containing hydrogen and DMAH in a 1:1 partial pressure ratio, hydrogen is added at 800 sccm, and the resulting mixed gas was fed into the CVD apparatus. In the present Example, portions coming into contact with DMAH, including inner walls of the starting gas liquid container 511 and the gas feed pipe 521, starting gas liquid-feeding pipe 523 and inner walls of reflux condenser 531, are made of an insulating material such as quartz or Teflon. Examples of the methods of manufacture of semiconductor devices to which the present invention may be applied will be described below with reference to Figs. 8A to 8D. A pn diode array for use in an image sensor can be formed on a transparent fused quartz substrate. First, on a fused quartz substrate 805, W (tungsten) is formed into a film by sputtering, which is then subjected to patterning by photolithography to form stripes of W electrodes of 50 µm wide in a pitch of 500 µm. Next, on this substrate, an SiO 2 layer of about 3.00 nm thick is formed by thermal CVD so as to cover the W electrodes, further followed by photolithography to partially remove SiO 2 present on the W electrodes to make windows of 2 µm square in the SiO 2 layer formed on the stripes of W electrodes. Thus, regions 804 in which W has been laid bare in a size of 2 µm are arranged at a pitch of 500 µm. Next, this substrate is put in a thermal CVD reactor to make Si crystals grow from the surfaces of the 2 µm square W electrodes as the nucleation surface. The crystal growth is carried out at a substrate temperature of 920°C and a pressure of 2.1x10 4 Pa (160 Torr),using as the starting gas a mixed gas of SiH 2 Cl 2 , HCl and H 2 , which are fed at flow rates of 0.4, 1.7 and 120 liter/minute, respectively. Thus, monocrystals of Si are grown only on the W electrodes. At this time, film formation is halfway carried out while mixing PH 3 in the gas and, after the mixing of PH 3 is stopped, film formation is further carried out while mixing B 2 H 6 , so that n-type Si monocrystals 803 are formed on the W electrodes and p-type Si monocrystals 802 are formed thereon. The growth of the n-type Si is carried out for 30 minutes and thereafter the growth of the p-type Si is carried out for 25 minutes, so that an array of Si monocrystals with pn junctions, of about 300 µm in size, can be made up on the W electrodes at a pitch of 500 µm, The respective surfaces of the crystals are all formed of p-type Si, and no n-type Si is laid bare to the surface. The respective crystals are also separated with SiO 2 of the substrate. Next, this substrate with the Si monocrystals is moved to another CVD apparatus, and Al is deposited at a substrate temperature of 250°C. As a starting gas, DMAH diluted with H 2 is used, and the deposition is carried out at a total pressure of 2x10 2 Pa (1.5 Torr) and a DMAH partial pressure of about 2.6x10 -2 Pa (2x10 -4 Torr). Thus, Al electrodes 801 are formed only on the Si monocrystal regions and no Al is deposited on the surface of SiO 2 , so that Al deposition and device separation can be completed at the same time. Contacts are attached to the respective Al electrodes by wire bonding to connect them to external circuits. Thus, a pn diode array can be formed on the transparent substrate. These devices are well separated from each other, and hence can be driven in an electrically independent form. The device thus fabricated can be used as an image sensor. A further example of the method of manufacture of a semiconductor device will now be described. In place of W, Si 3 N 4 is used as the material for the nucleation surfaces. The nucleation surfaces are arranged at a pitch of 10 µm in a size of 1 µm. Si 3 N 4 is formed into a film by LPCVD, a well known process. On the substrate, Si monocrystals are made to grow in substantially the same manner as in Example E1. The crystal growth is carried out at a substrate temperature of 950°C for 16 minutes at a pressure of 2.1x10 4 Pa (160 Torr), while feeding SiH 2 Cl 2 , HCl and H 2 at flow rates of 0.53, 1.8 and 100 liter/minute, respectively. Thus, Si monocrystals of about 5 pm in size are formed on the Si 3 N 4 nucleation surfaces. Next, this substrate with the Si monocrystals is moved to another CVD apparatus, and Al is deposited in the same manner as in Example El, i.e., at a substrate temperature of 250°C, a DMAH/H 2 total pressure of 2x10 2 Pa (1.5 Torr) and a DMAH partial pressure of about 2.66x10 -2 Pa (2x10 -2 Torr). Thus, Al can be deposited only on the crystal surfaces of about 5 µm size arranged on SiO 2 at intervals of 10 µm. Advantage A: As described above, the bubbling is carried out not in the starting material bomb but in another container in which a liquid level is kept constant, and the course extending from the container to the reaction chamber is heated to a temperature higher than room temperature and lower than the thermal decomposition temperature of the starting gas liquid. This brings about the following advantages: (i) The molar mixing ratio of the starting gas with a low vapor pressure at room temperature to the reaction gas can be controlled at a precision improved by one figure or more compared with the room temperature bubbling; (ii) no variations in mixing ratio may be caused by temperature changes in the vicinity of the apparatus; and (iii) no variation in mixing ratio may also be caused by a drop of liquid level in the bomb during the feeding of gas, so that the gas phase chemical reaction can be stably carried out. Advantage B: As also described above, when the bubbling is carried out using not the reaction gas but the inert gas, the flow rate of the reaction gas and that of the organic metal compound can be independently controlled. This brings about the advantage that the mixing ratio of the organic metal compound gas and the reaction gas can be more accurately controlled. Advantage C: As still also described above, in the feeding of the liquid starting material into the CVD apparatus, by the starting material in the bubbling container is stirred, it is possible to feed the starting gas into the reaction chamber in a good efficiency without causing the pulsation. This has made it possible to obtain a deposited reaction product with a good film quality and in a good state of deposition. Advantage D: As still also described above, according to the present invention by providing, a starting gas reflux condenser, it is possible to feed the starting gas liquid in a large quantity and at a high partial pressure ratio even when the liquid starting material has a low vapor pressure at room temperature. Advantage E: As still also described above, according to the present invention, Al can be readily deposited only on the surface of a monocrystal region comprised of an electron donative material, which is formed on the substrate having a non-electron-donative and non-monocrystalline surface.

Claims: A method of producing a starting gas for use in chemical vapour deposition, which method includes the following steps: heating within a container (511) a liquid starting material (512) to a temperature at or above its boiling point at an operating pressure; producing a vapour from said liquid starting material and a carrier gas; and condensing liquid from said vapour to output a saturated vapour as starting gas; which method is characterised by: bubbling the carrier gas at a constant flow rate through the heated liquid via a capillary tube (521) provided in contact with the bottom of the container (511). A method according to claim 1 wherein the steps of heating and condensing are carried out at a reduced pressure. A method according to claim 1 or claim 2, wherein the saturated vapour output from the condensation step is heated. A method according to any preceding claim, wherein the starting material is an organic metal compound. A method according to claim 4, wherein the organic metal compound is an organic aluminium compound. A method according to claim 4, wherein the organic metal compound is DMAH. A method according to claim 4, wherein the organic metal compound is triisobutylaluminium (TIBA). A method according to claim 4, wherein the organic metal compound is trimethyl galium (TMG). A method according to any of the preceding claims, wherein the inert gas is argon. A method according to any of claims 1 to 8, wherein the inert gas is nitrogen, helium or neon. A method of producing a semiconductor device using a chemical vapour deposition process comprising the steps of producing a starting gas for the chemical vapour deposition process by the method of any preceding claim, supplying the vapour to a reaction chamber and reacting the supplied gas to bring about chemical vapour deposition onto a substrate in said chamber. A method according to claim 11, wherein the substrate is silicon. A method of producing a semiconductor device, according to claim 11 or 12, comprising forming an electrode (804) in the vicinity of a surface of a substrate (805), forming a first conductivity type monocrystalline region (803) on a surface of the electrode (804), forming a reverse conductivity type monocrystalline region (802) on the first conductivity type monocrystalline region (803) and using the chemical vapour deposition process to form an electrode on the reverse conductivity type monocrystalline region (802). A chemical vapour deposition apparatus comprising: a container (511) for a liquid starting material (512); first heater means for heating liquid in the container (511) of a temperature equal to or greater than the boiling point at operating pressure of the liquid starting material (512); gas feed means (521,522) for introducing a carrier gas into said container (511) at a constant flow rate; and a reflux condenser (531) for cooling and condensing vapour of the liquid starting material (512) and returning condensed vapour to the container; characterised in that said gas feed means (522,521) includes a capillary tube (521) positioned in contact with the bottom of the container (511) for bubbling the carrier gas into the liquid starting material. An apparatus according to claim 14, having a second heater (537) arranged to heat the cooled starting gas from the reflux condenser (531). An apparatus according to claim 14 or claim 15, wherein those parts of the apparatus coming into contact with the liquid starting material or the starting gas, including the inner walls of said container (511), the inner walls of said reflux condenser (531), and gas feed means (521), are of quartz, teflon or other insulating material. An apparatus according to any of claims 14 to 16, including means of maintaining a reduced pressure within said container (511) and said reflux condenser (531).

Inventors: ASABA TETSUO, HAYAKAWA YUKIHIRO, KAWASUMI YASUSHI, OHMI KAZUAKI, SEKINE YASUHIRO, ASABA, TETSUO, HAYAKAWA, YUKIHIRO, KAWASUMI, YASUSHI, OHMI, KAZUAKI, ASABA, TETSUO, C/O CANON KABUSHIKI KAISHA, HAYAKAWA, YUKIHIRO, C/O CANON KABUSHIKI KAISHA, KAWASUMI, YASUSHI, C/O CANON KABUSHIKI KAISHA, OHMI, KAZUAKI, C/O CANON KABUSHIKI KAISHA, SEKINE YASUHIRO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: C23C16
IPC codes: H01L21, C23C16, H01L23