Apparatus for forming a thin film and process for forming a thin film by means of the apparatus

Abstract: An apparatus for vaporizing a liquid raw material (2) comprises a nozzle (4) with an open tip end for ejecting a liquid raw material into a heated gas (31) atmosphere as liquid droplets and a heated plate (30) with a small opening (40) disposed in front of the nozzle, a space opposite to that occupied by the nozzle with respect to the plate being evacuated.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an apparatus for forming a thin film, and more particularly to an apparatus for depositing a thin film by chemical vapor deposition. Furthermore, the present invention relates to a process for forming a thin film by means of the apparatus according to the invention. Related Background Art In a step of forming a thin film in the process for producing an integrated circuit with semiconductors, it is necessary to form a desired thin film on an etched wafer surface of uneven surface level while attaining a good step coverage. In a process for producing an integrated circuit of submicron level, whose minimum processing dimension is not more than 1 µm, or of further deep submicron level, it is necessary to deposit a thin film selectively in contact holes or through holes. It is a step of forming a wiring metal film that requires the keenest selective growth in the process for producing an integrated circuit. A thin film has been so far formed widely by sputtering. Sputtering is to deposit atoms sputtered from a target substance onto a substrate, and thus its step coverage is poor, and it is also impossible to attain selective growth. On the other hand, chemical vapor deposition, which vaporizes a raw material, transports the vaporized raw material into a reaction space and deposits a desired thin film through chemical reactions of the raw material molecules on the substrate surface, has a distinguished step coverage and can attain selective growth. For example, an Aℓ thin film growth process disclosed in Applied Physics Letters, Vol. 57, No. 12 (1990), Page 1221, can deposit monocrystalline Aℓ selectively in very fine via-holes and also can change the growth mode from a selected mode to a non-selected mode to completely flatten via-holes. In the chemical vapor deposition (CVD), it is necessary to vaporize a raw material and transport the vaporized raw material into a reaction space. If the raw material exists in a gaseous state in the CVD, its transportation into the reaction space can be carried out with ease. For example, SiH 4 is a gas at room temperature and thus can be filled in a high pressure gas cylinder, and thus can be transported into the reaction space at a desired rate by providing the high pressure gas cylinder with a pressure control valve and a flow meter. However, when a raw material is a liquid at room temperature, its transportation procedure is different from that for SiH 4 . For example, dimethylaluminium, which is a liquid at room temperature, is used as a raw material in the Aℓ thin film growth. Such procedure is disclosed in detail in U.S. Patent Application S.N. 578,672 titled "Process for forming deposited film by use of alkyl aluminium hydride", filed September 7, 1990 by the present inventors; U.S. Patent Application S.N. 587,045 titled "Deposited film formation method utilizing selective deposition by use of alkyl aluminum hydride", filed September 24, 1990 by the present inventors, and U.S. Patent Application S.N. 586,877 titled "Gas feeding device and deposition film forming apparatus employing the same" filed September 24, 1990 by the present inventors. Fig. 3 shows a procedure for transporting a liquid raw material so far widely utilized, where a liquid raw material 2 is stored in a vessel 1 provided with an inlet pipe 103 and an outlet pipe 104, and the inlet pipe 103 is dipped into the liquid raw material 2. When a flow rate-controlled gas 105 is introduced from the inlet pipe 103, the gas 106 is discharged from the outlet pipe 104 through the liquid raw material 2. Since the gas 105 is passed through the liquid raw material 2, the gas 106 can be saturated with the vapor of the liquid raw material 2 and led to a reaction chamber of an apparatus for chemical vapor deposition. The amount of the transported raw material according to the foregoing procedure can be represented approximately by the following equation: Q = Q in x P V P B wherein Q: Amount of transported raw material Q in : Amount of gas introduced from the inlet pipe P V : Saturated vapor pressure of raw material gas P B : Pressure inside the vessel When the liquid raw material has a low saturation vapor pressure, the amount of transported raw material is small, as is seen from the equation (1), and in the worst case, the rate of deposition is governed by the amount of the transported raw material. When the pressure P B inside the vessel is constant, it is possible to introduce dilution gas into the reaction chamber to lower a proportion of the raw material partial pressure in the reaction chamber, but it is impossible to elevate the proportion. In order to enhance the transportation efficiency of the raw material, it is possible to elevate the temperature of the vessel to increase the saturated vapor pressure of the raw material, but the entire vessel and pipes must be heated. When the inlet gas flow rate is increased in the procedure as shown in Fig. 3, the vaporization rate of the liquid raw material cannot meet the inlet gas flow rate, and thus a sufficient amount of the raw material cannot be transported. In the procedure as shown in Fig. 3, the amount of the gas 105 introduced into the inlet pipe 103 can be exactly determined, but the amount of the raw material discharged from the outlet pipe 106 can be only calculated from the equation (1), and its actual amount cannot be detected, that is, the exact amount of transported raw material can not be determined. In the above-mentioned At CVD using dimethylaluminium hydride (DMAH), deposition of high quality Aℓ film can be conducted, but the saturated vapor pressure of DMAH at room temperature is as low as 2 Torr and there thus remains the problem that a large amount of DMAH can not be efficiently transported and no higher deposition rate can be attained. As mentioned above, it is necessary in the chemical vapor deposition using a liquid raw material in the technical field of semiconductor integrated circuits of higher speed and higher degree of integration due to the recent trends for minuaturization to transport a large amount of raw material stably and exactly, and there still remain unsolved problems for the mass production. An apparatus comprising the features of the preamble of claim 1 is known from document WO-A-87 07 848. In this known apparatus the liquid droplets are injected by the injector and impinge on a heated surface whereby the liquid droplets are vaporized. The gaseous pressure created by the vaporisation drives the vapor through a nozzle structure formed in the wall of the second chamber so as to be deposited on the surface of the substrate. SUMMARY OF THE INVENTION It is an object of the present invention to provide an apparatus comprising the features of the characterizing portion of claim 1 such that said apparatus efficiently vaporizes even a raw material having a low saturated vapor pressure at room temperature and transports a large amount of the vaporized raw material stably and exactly in CVD using a liquid raw material. Furthermore, it is an object of the present invention to provide a process for forming a thin film by means of the improved apparatus. These objects are achieved by the apparatus according to claim 1 and the process according to claim 2. In the apparatus according to the invention a large number of substrates can be treated stably. In the transportation of a large amount of a liquid raw material, it is necessary to vaporize a liquid raw material efficiently in a high temperature circumstance for elevating the saturated vapor pressure. In a bubbling system as shown in Fig. 3, a sufficiently large amount of a liquid raw material is not vaporized even by heating the vessel 1 appropriately, and it is thus quite difficult to transport a large amount of the raw material. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional view of the main part of an apparatus for vaporizing a liquid raw material. Fig. 2 is a schematic cross-sectional view of an apparatus for forming a thin film, to which the apparatus for vaporizing a liquid raw material shown in Fig. 1 is applied. Fig. 3 is a cross-sectional view of a conventional liquid raw material bubbler. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferable embodiments of the present invention will be explained in detail below, referring to the drawings. According to the present invention, a liquid raw material is transported in a liquid state and ejected into a heated gas in a reaction chamber, thereby completely incorporating a vapor of the liquid raw material having its saturated vapor pressure into the heated gas. Fig. 1 schematically shows the region for ejecting a liquid raw material and the region for vaporizing the ejected liquid low material, where an injector 4 formed by a nozzle has a tip end with a reduced cross sectional area injection opening, through which a liquid raw material 2 is transported at a controlled flow rate, and a piezoelectric vibrator 8 gives vibrations to the tip end of the injector 4. When the liquid raw material 2 is led to the nozzle tip end in this structure while giving vibrations to the nozzle tip end, the liquid raw material 2 is ejected in liquid droplets having a diameter of about 10 µm to about 50 µm from the nozzle tip end due to the nozzle vibrations. A plate member 30 having a small opening 40 is provided in front of and in the direction of the nozzle tip end. A heater (not shown in the drawing) is embedded in the plate member 30 to heat the plate member 30. An inert gas or H 2 gas or their mixed gas 31 flows into the opening 40 from the nozzle surrounding. At that time, space A is under a pressure of 500 to 760 Torr, whereas space B, which is a reaction chamber, is under a pressure for depositing a thin film, for example, 0.1 to 10 Torr. A pressure gradient develops in the opening 40 from the space A towards the space B. The space B is evacuated by an evacuating means (not shown in the drawing) and kept under the predetermined pressure. The pressures of spaces A and B are not limited to the above-mentioned values. In principle, as long as the pressure of space A is higher than that of space B, any values are available. That is, the space A can be in a pressurized state of 1 to 2 kg/cm 2 . When a liquid droplet 50 is ejected from the injector 4 into the space A, small opening 40 and space B, the liquid droplet 50 is vaporized in the small opening 40 and introduced as a gas it into the space B. When the small opening 40 is at room temperature, the liquid droplet 50 will not be vaporized, but the plate member 30 is heated and the heated gas 31 is introduced, and thus the liquid droplet ejected from the injection opening can be vaporized in the small opening. Different from the conventional procedure of heating the entire vessel and the entire pipes to increase the amount of transported liquid raw material, as shown in Fig. 3, the present vaporizing structure, as shown in Fig. 1, can exactly determine the amount of liquid raw material as transported and can increase the amount of liquid raw material up to the corresponding saturated vapor pressure at an elevated temperature. Fig. 2 schematically shows one embodiment of an apparatus for forming a thin film by chemical vapor deposition, provided with the apparatus for vaporizing a liquid raw material, shown in Fig. 1, where a supply source 1 containing a liquid raw material 2 is pressurized with a pressurizing gas from a gas inlet 11 through a valve 10 and the applied pressure is adjusted with a pressure gage 9. As the pressurizing gas, any gas may be used so long as it is unreactable with the liquid raw material 2. For example, when the liquid raw material is an organometallic compound, an inert gas such as Ar, N 2 , He, Ne, etc. can be used. When trimethylaluminium, dimethylaluminium hydride, trimethylgallium or the like is used as a liquid raw material, these organometals decompose in the presence of a hydrogen gas even at room temperature, and thus Ar, N 2 , He, Ne or the like is preferable. From the viewpoint of low cost and high purity, Ar or N 2 is more preferable. When the vessel forming the supply source 1 is pressurized by the pressurizing gas from the gas inlet 11, the liquid raw material 2 is led to a liquid flow meter 3 on the basis of the principle of siphon. A commercially available liquid mass flow controller can be used as the liquid flow meter. Any flow rate meter can be used, so long as it can exactly determine the flow rate of the liquid raw material. After the passage through the liquid flow meter 3, the liquid raw material is led to injectors 4 to 7. In Fig. 2, only four injectors are shown, but less or more than four injectors can be used. In Fig. 2, the injectors are shown as if they are disposed in a straight line, but disposition of injectors in a concentric state in a plane is desirable. Each of the injectors 4 to 7 is provided with a piezoelectric vibrator 8, and liquid droplets are ejected from the individual injector tip ends. In the embodiments shown in Figs. 1 and 2, the liquid raw material is ejected into a liquid droplet state from the individual injector tip ends by vibrations of the piezoelectric vibrators. Any other means than the piezoelectric vibrators can be used, if they can eject liquid droplets having a diameter of 10 to 100 µm. A first chamber 21 which corresponds to space B in Fig. 1 is set to an appropriate pressure of 0.1 to 10 Torr for formation of a desired deposited film, whereas a second chamber 20 which corresponds to space A in Fig. 1 is set to approximately the atmospheric pressure. Pressure difference between the chamber 20 and the chamber 21 is created by the small openings 40 provided through the plate member 30. The small openings 40 of the plate member 30 are each disposed in front of the injector tip ends. The number of the small openings 40 can be equal to or more than the number of the injectors. Disposition of the small openings to the individual injector tip ends is as in Fig. 1. Near each of the small openings and each of the injector tip ends, gas ejection outlets 14 are provided. Fig. 2 shows as if only one gas ejection outlet 14 is provided for only the tip end of injector 7, but actually gas ejection outlets 14 are provided for the individual injectors 4 to 7. The gas from the gas ejection outlets 14 is heated in advance by heaters 41. The temperature of injectors 4 to 7 is set to a temperature at which the liquid raw material may not be decomposed. Numeral 12 is a gas inlet and 13 is a gas flow rate meter. A gas may be introduced into the chamber 20 through a pipe 17 via a flow rate meter 16 from a piping inlet. As explained referring to Fig. 1, the plate member 30 is heated by a heater (not shown in the drawing), and the liquid droplets ejected from the injectors 4 to 7 are vaporized in the small openings 40 provided through the plate member 30. The plate member 30 may be of a metal or of an insulating material. In case of an organometal as a liquid raw material, decomposition reaction of the organometal is promoted on a heated metal, and thus an insulating material such as quartz or BN (boron nitride) is preferable. Furthermore, from the viewpoint of excellent processability and easy embedment of a heater for heating, BN (boron nitride) is more preferable for the plate member 30. The plate member 30 need to have a thickness such that the liquid droplets ejected from the injectors 4 to 7 can be fully vaporized in the heated spaces of the individual small openings 40. In the embodiment shown in Fig. 2, the thickness is about 5 mm to about 3 cm. Besically the size of the individual small openings 40 need to be larger than the diameter of the liquid droplet, and is about 0.5 to about 2 mm in diameter. In the reaction chamber 21, a substrate 25 is placed on a substrate holder 24. In the embodiment shown in Fig. 2, the substrate 25 is heated by a heater 23 from the back side of the substrate 25. The substrate 25 can be heated by high frequency heating or by lamp heating. The reaction chamber 21 is connected to an evacuating valve 26 and are evacuating means 27. To adjust the reaction chamber 21 to a desired pressure, the degree of opening of the valve 26 is controlled while observing the pressure gage 22. The apparatus for forming a thin film shown in Figs. 1 and 2 as one embodiment of the present invention can be utilized in all the apparatuses for chemical vapor deposition using a liquid raw material. For example, organometallic raw materials such as trimethylaluminium, dimethylaluminium hydride, triisobutylaluminium hydride, trimethylgallium, etc. as a liquid raw material or tetraethylsilane can be used as a raw material. Particularly, the present invention is effective for dimethylaluminium hydride (vapor pressure at room temperature: about 1 to about 2 Torr) and triisobutylaluminium (vapor pressure at room temperature: about 0.1 Torr) having a low vapor pressure at room temperature. Furthermore, a plurality of gases can be used as raw material gas. For example, when a GaAs thin film is formed, trimethylgallium is ejected from the nozzles, and AsH 3 is introduced into the reaction chamber 21. In the apparatus for forming a thin film as shown in Fig. 2, only one kind of the liquid raw material is given, but a GaAℓAs liquid crystal thin film can be also deposited with trimethylgallium from one vessel and trimethylaluminium from another vessel. An embodiment of a process for forming a thin film by means of the apparatus as shown in Figs. 1 and 2 will be described in detail below, where dimethylaluminium hydride was used as a liquid raw material, an inert gas Ar was used as a pressurizing gas from the gas inlet 11, and injectors 4 to 7 made of stainless steel were used. Liquid droplets having a diameter of 10 to 50 µm were ejected from the injectors 4 to 7 with the vibrators 8 at a frequency of 1 to 50 kHz. An H 2 gas was used as the gas from the gas inlets 14 and adjusted to an inlet gas temperature of about 150°C at the ejection outlets 14. The inlet gas temperature was preferably lower than the decomposition temperature of dimethylaluminium hydride, 160°C. An H 2 gas was likewise introduced from the gas inlet 12. A BN plate embedded with a heater was used as the plate member 30, and the reaction chamber 21 was adjusted to a pressure of 0.1 to 3 Torr, whereas the second chamber 20 was under a pressure of the atmospheric pressure to 500 Torr. Under these conditions, the liquid droplets ejected from the injectors 4 to 7 were vaporized in the small openings 40, and a large amount of dimethylaluminium hydride could therefore be introduced into the reaction chamber 21 to deposit a thin Aℓ film. When the substrate temperature was at about 270°C, an Aℓ film of excellent film quality and surface flatness could be deposited. When dimethylaluminium hydride was bubbled at a flow rate of 100 SCCM or more under a vessel inside pressure of the atmospheric pressure at room temperature according to the conventional system shown in Fig. 3, the amount of the transported raw material was not proportional to the bubbling flow rate due to the high viscosity and was therefore saturated. On the other hand, in the apparatuses of the present invention, as shown in Figs. 1 and 2, the dimethylaluminium hydride could be transported proportionally to the transported amount of the liquid, and a high speed deposition of 1 µm/min. could be attained on a 4-inch wafer. As explained above, a liquid raw material having even a low vapor pressure can be ejected into a high temperature atmosphere as liquid droplets and a large amount of the raw material can therefore be transported in the present invention.

Claims: An apparatus for forming a thin film, comprising: (a) a substrate holder (24) for holding a substrate (25); (b) a first chamber (21) for housing the substrate; (c) an evacuating means (27) for evacuating the first chamber (21); (d) at least one injector (4 to 7) for injecting a liquid raw material in the form of liquid droplets; (e) a supply source (1) for supplying the liquid raw material to the at least one injector (4 to 7); and (f) a second chamber (20) containing the at least one injector (4 to 7), characterized by (g) a first heater (23) for heating the substrate; (h) a liquid flow meter (3) provided between said at least one injector (4 to 7) and said supply source (1) for determining the flow rate of said liquid raw material; (i) wherein the at least one injector (4 to 7) is provided with a plurality of injection openings for injecting said liquid raw material; (j) a plate member (30) for separating and providing a difference in pressure between said first chamber (21) and said second chamber (20), said plate member being placed opposite and parallel to said substrate holder (24); (k) a second heater for heating said plate member (30); (l) wherein said plate member (30) has a plurality of openings (40) corresponding to said plurality of injection openings provided therethrough to permit communication between said first chamber (21) and said second chamber (20); and (m) wherein said plurality of injection openings of said at least one injector (4 to 7) are remote from said plate member (30), whereby the liquid droplets are rendered vaporizable in said openings of said plate member (40). A process for forming a thin film by means of the apparatus according to claim 1, wherein the temperature in said second chamber (20) is held at a temperature lower than the decomposition temperature of said liquid raw material. The process according to claim 2, wherein said plate member (30) is heated to a temperature lower than the decomposition temperature of said liquid raw material. The process according to claim 2 or 3, wherein said liquid raw material is an organometal. The process according to claim 4, wherein said liquid raw material is an organoaluminium. The process according to any one of claims 2 to 5, wherein said liquid raw material is dimethylaluminium hydride, wherein said second chamber (20) contains hydrogen, and wherein said hydrogen and said heated plate member (30) are at a temperature of about 160°C or lower.

Inventors: MASU KAZUYA, TSUBOUCHI KAZUO, MASU, KAZUYA, TSUBOUCHI, KAZUO
Applicants: TSUBOCHI KAZUO, TSUBOUCHI, KAZUO
Application year: 1992
Main classification: C23C16
IPC codes: C23C16, B01D3, B01D1, C30B25, H01L21, B01J7