MOCVD system

Disclosed is an MOCVD system for forming a tantalum oxide film on a semiconductor wafer, while using pentoethoxytantalum as a liquid raw material. In the system, a raw material tank is connected to a vaporizing unit through an upstream main line with a flow control unit. The vaporizing unit is connected to the process chamber of a film-forming unit through a downstream main line. A partition wall is arranged to surround the entire system so as to isolate it from the other space in the clean room. The raw material tank, the flow control unit, and part of the upstream main line therebetween are accommodated in a constant temperature and heat insulating box all together and are kept at a temperature of from 25 to 35.degree. C.

CROSS-REFERENCE TO RELATED APPLICATIONS
 This application is based upon and claims the benefit of priority from the
 prior Japanese Patent Application No. 11-212874, filed Jul. 27, 1999, the
 entire contents of which are incorporated herein by reference.
 BACKGROUND OF THE INVENTION
 The present invention relates to an MOCVD (Metal Organic Chemical Vapor
 Deposition) system used in semiconductor processes, and particularly to a
 system for forming a metal oxide film, such as a tantalum oxide (Ta.sub.2
 O.sub.5) film, on a target substrate, while using as a liquid raw material
 a metal organic compound that is in a liquid phase at room temperature
 (about 20.degree. C.). The term "semiconductor process" used herein
 includes various kinds of processes which are performed to manufacture a
 semiconductor device or a structure having wiring layers, electrodes, and
 the like to be connected to a semiconductor device, on a target substrate,
 such as a semiconductor wafer or an LCD (Liquid Crystal Display)
 substrate, by forming semiconductor layers, insulating layers, and
 conductive layers in predetermined patterns on the target substrate.
 In order to manufacture semiconductor devices, film formation and pattern
 etching are repeatedly applied to a semiconductor wafer. As semiconductor
 devices are increasingly highly miniaturized and integrated, demands on
 film formation become more strict. For example, very thin insulating
 films, such as capacitor insulating films and gate insulating films are
 still required to be thinner and to be more insulating.
 Conventionally, silicon oxide films and silicon nitride films are used as
 the insulating films. In recent years, however, it has been proposed to
 form the insulating films from materials having more excellent insulating
 properties, such as metal oxides, e.g., tantalum oxide (Ta.sub.2 O.sub.5),
 or high-dielectric or ferroelectric bodies containing two metal elements
 or more, e.g., (Ba,Sr)TiO.sub.3, i.e., BST. These films can be formed by
 means of MOCVD, i.e., using vaporized metal organic compounds.
 Jpn. Pat. Appln. KOKAI Publication No. 2-283029 discloses an MOCVD
 technique for forming a tantalum oxide film. In the technique disclosed in
 this publication, a metal (tantalum) alkoxide, such as Ta(OC.sub.2
 H.sub.5).sub.5 (pentoethoxytantalum) is used as a liquid raw material. The
 liquid raw material is made to bubble by e.g., nitrogen gas, and is
 supplied to a process chamber preset to have a vacuum atmosphere. The
 supplied raw material is decomposed to offer a film forming material on
 the surface of a semiconductor wafer W heated to a process temperature of,
 e.g., 400.degree. C. With this film forming material, a tantalum oxide
 (Ta.sub.2 O.sub.5) film is formed on the surface of the semiconductor
 wafer W by means of deposition.
 In this system, however, it is difficult to control the supply rate of the
 raw material with a high accuracy. This is because the liquid raw material
 is supplied by means of bubbling, and thus the flow rate of the liquid raw
 material is hardly accurately controlled. As a result, the supply rate of
 the raw material finally supplied to the process chamber shifts from the
 target value, thereby lowering reproducibility in the film thickness and
 quality of a formed CVD film.
 Jpn. Pat. Appln. KOKAI Publication No. 10-79378 discloses another MOCVD
 technique for forming a tantalum oxide film, developed in light of the
 above described problem. In the technique disclosed in this publication, a
 liquid raw material the same as that of the former publication is
 delivered by the pressure of an inactive gas. The liquid raw material is
 supplied, while its flow rate is being controlled, and then is vaporized
 and turned into a process gas. The process gas is supplied to a process
 chamber and used for forming a CVD film on a semiconductor wafer.
 FIG. 5 is a structural diagram schematically showing a conventional MOCVD
 system disclosed in the KOKAI Publication No. 10-79378.
 In this system, a liquid raw material 4, such as pentoethoxytantalum, is
 stored in a raw material tank 2 and is delivered by the pressure of a
 pressurized gas, such as He gas. The stored liquid raw material 4 is
 heated by a heater 6 to a temperature of, e.g., from 20 to 50.degree. C.,
 at which the liquid raw material 4 can easily flow. The delivered liquid
 raw material 4 flows downstream, while its flow rate is being controlled
 by a flow control unit 8, and then is vaporized and turned into a process
 gas by an inactive gas, such as He gas, in a vaporizing unit 10.
 The process gas flows in a gas supply line 14, which is wrapped by, e.g., a
 tape heater 12 for preventing the process gas from being turned back to
 liquid, and enters a film-forming unit 16. The process gas is supplied
 from a showerhead 19 into a process chamber 18, and then is decomposed to
 offer a film forming material on the surface of a semiconductor wafer W
 heated to a process temperature. With this film forming material, a
 tantalum oxide film is formed on the surface of the semiconductor wafer W
 by means of deposition.
 The system of the KOKAI Publication No. 10-79378 can control the supply
 rate of the raw material more accurately than the former system. Depending
 on a situation, sometimes the latter system still lowers reproducibility
 in the film thickness and quality of a formed CVD film, sometimes as the
 case may be. Furthermore, the system includes heated members, such as the
 vaporizing unit 10, pipes, and so forth, other than the process chamber
 18, and thus thermally influences the ambient environment.
 BRIEF SUMMARY OF THE INVENTION
 An object of the present invention is to provide an MOCVD system, which can
 offer a high reproducibility in the film thickness and quality of a formed
 CVD film.
 Another object of the present invention is to provide an MOCVD system,
 which less thermally influences the ambient environment, but can offer a
 high reproducibility in the film thickness and quality of a formed CVD
 film.
 According to a first aspect of the present invention, there is provided an
 MOCVD system for forming a metal oxide film on a target substrate, while
 using as a liquid raw material a metal organic compound that is in a
 liquid phase at room temperature:
 an airtight process chamber;
 a support member disposed in the process chamber and configured to support
 the target substrate;
 an exhaust mechanism configured to exhaust the process chamber and set the
 process chamber to have a vacuum atmosphere;
 a downstream main line connected to the process chamber and configured to
 supply a process gas thereto;
 a vaporizing unit connected to the downstream main line outside the process
 chamber, and configured to heat the liquid raw material to a temperature
 higher than a vaporizing temperature of the liquid raw material and turn
 the liquid raw material into the process gas;
 an upstream main line connected to the vaporizing unit and configured to
 supply the liquid raw material thereto;
 a raw material tank connected to the upstream main line outside the process
 chamber and configured to store the liquid raw material;
 a flow control unit arranged on the upstream main line between the raw
 material tank and the vaporizing unit; and
 a temperature maintaining mechanism configured to maintain the raw material
 tank and the flow control unit at a temperature, which is lower than the
 vaporizing temperature and higher than the room temperature.
 According to a second aspect of the present invention, there is provided an
 MOCVD system installed inside a clean room, for forming a metal oxide film
 on a target substrate, while using as a liquid raw material a metal
 organic compound that is in a liquid phase at room temperature:
 an airtight process chamber;
 a support member disposed in the process chamber and configured to support
 the target substrate;
 an exhaust mechanism configured to exhaust the process chamber and set the
 process chamber to have a vacuum atmosphere;
 a downstream main line connected to the process chamber and configured to
 supply a process gas thereto;
 a vaporizing unit connected to the downstream main line outside the process
 chamber, and configured to heat the liquid raw material to a temperature
 higher than a vaporizing temperature of the liquid raw material and turn
 the liquid raw material into the process gas;
 an upstream main line connected to the vaporizing unit and configured to
 supply the liquid raw material thereto;
 a raw material tank connected to the upstream main line outside the process
 chamber and configured to store the liquid raw material;
 a flow control unit arranged on the upstream main line between the raw
 material tank and the vaporizing unit;
 a partition wall configured to form an isolated space inside the clean room
 to be isolated from the other space in the clean room, the isolated space
 enveloping the process chamber, the downstream main line, the vaporizing
 unit, the upstream main line, the raw material tank, and the flow control
 unit all together; and
 a temperature maintaining mechanism configured to maintain the raw material
 tank, the flow control unit, and part of the upstream main line
 therebetween at a temperature, which is lower than the vaporizing
 temperature and higher than the room temperature, wherein the temperature
 maintaining mechanism comprises a constant temperature and heat insulating
 box disposed in the isolated space and accommodating the raw material
 tank, the flow control unit, and part of the upstream main line
 therebetween all together.
 Additional objects and advantages of the invention will be set forth in the
 description which follows, and in part will be obvious from the
 description, or may be learned by practice of the invention. The objects
 and advantages of the invention may be realized and obtained by means of
 the instrumentalities and combinations particularly pointed out
 hereinafter.

DETAILED DESCRIPTION OF THE INVENTION
 In the process of developing the present invention, the present inventors
 conducted research on MOCVD systems of the type as show in FIG. 5, as to
 why reproducibility in the film thickness and quality of a formed CVD film
 was lowered. As a result, the present inventors have arrived at the
 finding given below.
 The system shown in FIG. 5 is constituted by gathering the parts to be
 assembled in a narrow space in a clean room, in light of the cost of the
 space of the clean room, the use efficiency of the raw material, the
 system maintenance, and so forth. Consequently, the liquid raw material
 line and the flow control unit 8 are apt to receive thermal influences
 from other units in the same system. The liquid raw material 4 has certain
 physical characteristics, such as viscosity, depending on the temperature
 given from the heater 6, when it is delivered from the tank 2. However,
 these physical characteristics may greatly vary due to the temperature of
 the flow control unit 8 side.
 Since the liquid raw material 4 is supplied to the vaporizing unit 10
 through the flow control unit 8, the supply rate of the liquid raw
 material 4 can be accurately controlled under ideal conditions. However,
 when the liquid raw material 4 has its physical characteristics changed
 due to a temperature difference between the tank 2 and the flow control
 unit 8, the supply rate of the liquid raw material 4 fluctuates, thereby
 lowering reproducibility in the film thickness and quality of a formed CVD
 film.
 Embodiments of the present invention, made on the basis of the above
 described findings, will be described hereinafter with reference to the
 accompanying drawings. In the following description, the constituent
 elements having substantially the same function and arrangement are
 denoted by the same reference numerals, and a repetitive description will
 be made only when necessary.
 FIG. 1 is a structural diagram schematically showing an MOCVD system
 according to an embodiment of the present invention. This embodiment is
 exemplified by a process of forming a tantalum oxide (Ta.sub.2 O.sub.5)
 film, as an example of a metal oxide film.
 The MOCVD system 22 is constituted of a film-forming unit 24 for subjecting
 a semiconductor wafer or target substrate W to a film-forming process, and
 a raw material supply section 26 for vaporizing the liquid raw material to
 produce a process gas and supplying the process gas to the film-forming
 unit 24.
 The raw material supply section 26 includes an airtight raw material tank
 30, which stores a liquid raw material 28 used for providing a
 film-forming material. The liquid raw material 28 consists of, e.g.,
 pentoethoxytantalum (Ta(OC.sub.2 H.sub.5).sub.5), which is a metal
 alkoxide. The raw material tank 30 has a vapor phase space, into which the
 tip of a pressurizing line 32 is introduced from the top. The pressurizing
 line 32 is arranged to supply a pressurized gas, such as He gas, into the
 vapor phase space of the tank 30. The pressurizing line 32 is provided
 with a valve 34 and a flow controller 36, such as a mass-flow controller.
 The raw material tank 30 is connected to a vaporizing unit 48 through an
 upstream main line 38a formed of, e.g., stainless steel pipes. The
 vaporizing unit 48 is connected to the film-forming unit 24 through a
 downstream main line 38b formed of, e.g., stainless steel pipes. The pipes
 of the upstream main line 38a have an inner diameter of from about 1 to 2
 mm, because the liquid flow rate is generally as small as e.g., about 5
 mg/min. On the other hand, the pipes of the downstream main line 38b have
 a large inner diameter of from about 10 to 20 mm, because the gaseous raw
 material or process gas flows therein.
 The upstream end 40 of the upstream main line 38a is dipped into the liquid
 raw material 28 down to a position near the bottom of the raw material
 tank 30. When the pressurized gas is supplied into the raw material tank
 30 through the pressuring line 32, the liquid raw material 28 is delivered
 into the line 38a by the pressure of the gas. The upstream main line 38a
 is provided with a flow control unit 44 for controlling the flow rate of
 the liquid raw material 28. For example, a Liquid Mss-Flow Controller (TM)
 of LINTEX KABUSHIKIKAISHA is used as the flow control unit 44. The
 upstream and downstream lines 38a and 38b are provided with switching
 valves 42a to 42d. Note that only representative valves are shown in FIG.
 1 for the sake of convenience, though many switching valves are actually
 arranged on the lines 38a and 38b.
 The vaporizing unit 48 is used for vaporizing the liquid raw material 28
 and turning it into the process gas. A vaporizing gas supply line 54 is
 connected to the vaporizing unit 48 and is provided with a flow controller
 56. A vaporizing gas, such as He gas heated to a temperature of from 150
 to 160.degree. C., is supplied to the vaporizing unit 48 through the line
 54 while its flow rate is being controlled. In the vaporizing unit 48, the
 He gas is mixed with the liquid raw material 28, so that the liquid raw
 material 28 is vaporized and turned into the process gas.
 The vaporizing unit 48 itself and that portion of the line 54 between the
 vaporizing unit 48 and the flow controller 56 are wrapped with a
 temperature maintaining heater 58, such as a tape heater. The vaporizing
 gas and the vaporizing unit 48 are heated and kept at a temperature of
 from 150 to 160.degree. C. by the heater 58, so as to assist vaporizing of
 the liquid raw material 28. The downstream main line 38b from the
 vaporizing unit 48 is also wrapped with a temperature maintaining heater
 60, such as a tape heater. The process gas in the line 38b is kept at a
 temperature of, e.g., from 150 to 180.degree. C., which is higher than the
 vaporizing temperature (i.e., condensing temperature) of the raw material,
 and lower than the decomposing temperature of the raw material.
 The film-forming unit 24 includes an airtight process chamber 80 formed of,
 e.g., aluminum and having a cylindrical shape. A load lock chamber 94 is
 connected to a side of the process chamber 80 through a gate valve 92,
 which is airtightly opened and closed. The semiconductor wafer or target
 substrate W is transferred between the load lock chamber 94 and the
 process chamber 80 through the gate valve 92.
 A showerhead 82 is arranged on the ceiling of the process chamber 80 and
 connected to the downstream end of the downstream main line 38b. Another
 line 81 is also connected to the showerhead 82 independently of the
 downstream main line 38b, for supplying 02 gas as a second process gas.
 The first process gas containing pentoethoxytantalum and a second gas
 containing 02 are spouted into the process space from a number of holes
 formed in the bottom of the showerhead 82.
 A worktable 84 is arranged on the bottom of the process chamber 80 and
 provided with a heater embedded therein. The semiconductor wafer or target
 substrate W is mounted on the top surface of the worktable 84. Exhaust
 ports 85 are formed in the bottom of the process chamber 80 and connected
 to a vacuum exhaust line 86. The vacuum exhaust line 86 is provided with a
 removing unit 88 for removing the residual raw material and byproducts in
 the exhaust gas, and a vacuum pump 90 for exhausting the process chamber
 80 and setting it to have a vacuum atmosphere.
 The MOCVD system shown in FIG. 1 is installed in a clean room. FIG. 2 is a
 side view schematically showing the MOCVD system of FIG. 1 installed in
 the clean room.
 In the system shown in FIG. 1, the vaporizing unit 48, the film-forming
 unit 24, and the portions therebetween are heated to a temperature of from
 150 to 180.degree. C., which is higher than the vaporizing temperature of
 the raw material, and lower than the decomposing temperature of the raw
 material. These members are surrounded by heat-insulators, but thermally
 influence the ambient atmosphere to some extent. Besides, the used raw
 material is a substance which may contaminate the work environment.
 Furthermore, in light of the use efficiency of the raw material and the
 system maintenance, the raw material supply section 26 is preferably
 disposed near the film-forming unit 24.
 Accordingly, in this embodiment, the whole MOCVD system is made compact and
 surrounded by a partition or surrounding wall 102 with a door panel in the
 clean room 100, such that the system is completely isolated from the other
 space in the clean room 100. In other words, the process chamber 80,
 downstream main line 38b, vaporizing unit 48, upstream main line 38a, raw
 material tank 30, and flow control unit 44 are enveloped all together
 within the isolated space formed by the partition wall 102. Furthermore, a
 device 104 for forcibly supplying air is disposed on the ceiling of the
 isolated space inside the partition wall 102, while exhaust holes 106 are
 formed in the floor thereof, so that the isolated space inside the
 partition wall 102 is forcibly ventilated.
 Since the whole system is enveloped by the surrounding or partition wall
 102, the temperature of the isolated space inside the partition wall 102
 becomes relatively higher with an increase in the process time period,
 even though the high temperature members are covered with heat-insulators
 and the isolated space inside the partition wall 102 is forcibly
 ventilated. If the raw material supply section 26, as a whole, is directly
 exposed to the atmosphere inside the partition wall 102, the upstream main
 line 38a and the flow control unit 44, which have a small thermal
 capacity, are easily heated, depending on the temperature of the space
 inside the partition wall 102. Under this assumption, the physical
 characteristics of the liquid raw material 28 vary while the liquid 28
 flows from the tank 30 to the flow control unit 44, thereby further
 lowering reproducibility in the film thickness and quality of a formed CVD
 film than in a case where there is no partition wall 102.
 For this reason, a temperature maintaining mechanism 62 is arranged to
 maintain the raw material tank 30, the flow control unit 44, and the part
 of the upstream main line 38a therebetween at a predetermined warming
 temperature, which is lower than the vaporizing temperature and higher
 than room temperature. More specifically, the temperature maintaining
 mechanism 62 includes a constant temperature and heat insulating box 64
 for accommodating these members all together inside the partition wall
 102. The box 64 is constituted of a shell 64a made of a metal, such as
 stainless steel, and a heat insulating layer 64b made of a non-metal
 material, such as urethane or silicone, and formed on the inner surface of
 the shell. The constant temperature and heat insulating box 64 is provided
 with seal members 66 at positions through which the pressurizing line 32
 and the upstream main line 38a penetrate, so that the box 64 is airtight.
 A supply port 68 is formed in a lower position of a sidewall of the
 constant temperature and heat insulating box 64, for introducing therein a
 temperature control gas, which has been warmed. An exhaust port 78 is
 formed in the ceiling of the box 64, for exhausting the temperature
 control gas. The supply portion 68 is connected to a temperature control
 ventilation unit 70 for forcibly ventilating the inside of the box 64 with
 the temperature control gas. The ventilation unit 70 includes a blower 70
 for blowing air taken from the outside of the surrounding or partition
 wall 102 into the box 64, and a heating chamber 76 having a resistance
 heater 74 for heating the air from the blower 72.
 The temperature maintaining mechanism 62 also has a sensor 69 disposed in
 the constant temperature and heat insulating box 64, for detecting the
 temperature inside the box 64. The temperature detected by the sensor 69
 is supplied to the temperature controller 71, which electrically controls
 the calorific value of the heater 74 on the basis of the detected
 temperature. With this arrangement, the warming temperature inside the box
 64, i.e., the temperature of the temperature control gas is kept constant.
 Where the liquid raw material 28 is pentoethoxytantalum having a melting
 point of about 22.degree. C., the warming temperature is set to fall in a
 range of 30.+-.5.degree. C., and preferably 30.+-.1.degree. C.
 An explanation will given of a mechanism for cleaning the line pipes of the
 system shown in FIG. 1.
 A cleaning liquid supply mechanism 50 used for cleaning the inside of the
 system is connected to the upstream main line 38a through lines 53 at two
 positions, i.e., the outlet of the raw material tank 30 and immediately
 before the vaporizing unit 48. The cleaning liquid supply mechanism 50 is
 designed to supply an alcohol, such as ethanol, in a pressurized state, so
 as to wash out byproducts deposited in the upstream main line 38a,
 vaporizing unit 48, and so forth.
 The downstream main line 38b has a vertical line portion 49 as the main
 portion, so that deposition of byproducts is suppressed, and non-vaporized
 parts of the raw material are not supplied to the film-forming unit 24,
 but freely fall down through a vent line 51. In other word, the downstream
 main line 38b is designed to have a pipe portion extending downward more
 than a half of its length from the vaporizing unit 48 to the film-forming
 unit 24. The vent line 51 is blanched from the vertical line portion 49,
 and vertically extends downward.
 Furthermore, if the raw material is left in the upstream and downstream
 main lines 38a and 38b when the system is cleaned, an abnormality is
 caused in a CVD film formed in the next process. In order to solve this
 problem, the upstream and downstream main lines 38a and 38b have a special
 structure as follows. Specifically, the upstream and downstream main lines
 38a and 38b consist of pipe portions and joints connecting the pipe
 portions to each other. Each of the joints 39 is constituted of a main
 body 39a and a packing 39b, which form a surface having no irregularity to
 be in contact with the liquid raw material or the process gas. With this
 arrangement, no dead spaces are formed in the upstream and downstream main
 lines 38a and 38b, so that the above described problem is prevented.
 The cleaning liquid supply mechanism 50 is operated under the control of a
 cleaning controller 52. The cleaning controller 52 is also configured to
 automatically select the opening/closing of valves in the system in
 accordance with different cleaning modes. More specifically, when the
 system is cleaned, the opening/closing of the valves 46a, 46b, and 42a to
 42e arranged on the lines 53 of the cleaning liquid supply mechanism 50,
 the upstream and downstream main lines 38a and 38b, and the vent line 51
 is selected by the cleaning controller 52.
 An explanation will be given of an MOCVD process performed in the system
 shown in FIG. 1.
 First, an unprocessed semiconductor wafer W is transferred from the load
 lock chamber 94 into the process chamber 80 in a vacuum state through the
 opened gate valve 92, and is mounted on the worktable 84. Then, the wafer
 W is kept at a predetermined process temperature of, e.g., 400.degree. C.,
 higher than the decomposing temperature of the raw material, and the
 process chamber 80 is kept at a predetermined process pressure. In this
 state, the first process gas containing pentoethoxytantalum and the second
 gas containing 02 are supplied to start film-forming.
 In the raw material supply section 26, the pressurized gas, e.g., He gas,
 is supplied, while its flow rate is controlled, into the raw material tank
 30. With this pressure, the liquid raw material 28 is delivered through
 the upstream main line 38a while its flow rate is controlled by the flow
 control unit 44. At this time, the supply rate of the pressurized gas is
 several hundreds SCCM, for example, and the supply rate of the liquid raw
 material is as small as several mg/min, for example, though they depend on
 the film-forming rate.
 The liquid raw material thus delivered by the pressure is vaporized and
 turned into a process gas in the vaporizing unit 48 by the vaporizing gas,
 e.g., He gas, which is controlled to be supplied at a flow rate of, e.g.,
 from about 200 to 500 SCCM. The process gas is supplied though the
 downstream main line 38b into the showerhead 82 in the process chamber 80.
 The raw material or process gas is spouted into the process chamber 80
 from the showerhead 82, as described above.
 The downstream main line 38a is heated to a predetermined temperature of,
 e.g., from 150 to 180.degree. C. by the heater 60, so that the raw
 material gas is prevented from being turned back to liquid or being
 thermally decomposed, and thus stably flows to the showerhead 82. On the
 other hand, the vaporizing unit 48 and the vaporizing gas are heated to an
 optimum temperature of, e.g., about 160.degree. C. by the heater 58, and
 thus the liquid raw material is efficiently and swiftly vaporized.
 The raw material tank 30, the flow control unit 44, and the part of the
 upstream main line 38a therebetween are accommodated in the constant
 temperature and heat insulating box 64 all together. The temperature
 control gas, e.g., air in this embodiment, is heated to a predetermined
 temperature, and is supplied into the box 64 from the temperature control
 ventilation unit 70. With this arrangement, the liquid raw material 28 is
 kept at a substantially constant temperature of from 25 to 35.degree. C.
 (30.+-.5.degree. C.), and preferably from 29 to 31.degree. C.
 (30.+-.1.degree. C.). Consequently, the liquid raw material 28 is
 delivered by pressure through the upstream main line 38a smoothly with a
 constant low viscosity. Furthermore, the liquid raw material 28 hardly has
 its physical characteristics, such as viscosity, changed during this time,
 and thus can have its flow rate accurately controlled by the flow control
 unit 44.
 The temperature control gas is introduced into the constant temperature and
 heat insulating box 64 from the supply port 68 formed at a lower position,
 and exhausted from the exhaust port formed at an upper position. With this
 arrangement, the temperature control gas reaches all over inside the box
 64, so that the inside of the box 64 is uniformly kept at a predetermined
 heated temperature, as described above.
 Note that, although the raw material tank 30, the flow control unit 44, and
 so forth are accommodated in the constant temperature and heat insulating
 box 64 all together in this embodiment, these members 30, 44 may have
 their temperature independently controlled. FIG. 4 is a structural diagram
 schematically showing an MOCVD system according to another embodiment of
 the present invention, constituted in light of this concept.
 In the system shown in FIG. 4, the constant temperature and heat insulating
 box 64 and the temperature control ventilation unit 70 of the system shown
 in FIG. 1 are not used. Instead, a raw material tank 30 is provided with a
 heater 98, and a flow control unit 44 and the part of an upstream main
 line 38a between the raw material tank 30 and the flow control unit 44 are
 wrapped with a temperature maintaining mechanism, such as a tape heater
 99.
 The raw material tank 30, the flow control unit 44, and the part of the
 upstream main line 38a therebetween are kept at a temperature of
 30+5.degree. C., and preferably 30.+-.1.degree. C., by the heaters 98 and
 99. With this arrangement, this system produces an effect the same as that
 of the system shown in FIG. 1. Note that, in a case where the part of the
 upstream main line 38a between the raw material tank 30 and the flow
 control unit 44 is short, with which the liquid raw material 28 does not
 have its temperature changed so much, that part of the upstream main line
 38a may not be wrapped with the tape heater 99.
 Although the above described embodiments are exemplified by a case where a
 tantalum oxide film is formed, the present invention is applicable to any
 MOCVD system for forming a metal oxide film on a target substrate, while
 using as a liquid raw material a metal organic compound that is in a
 liquid phase at room temperature. For example, the present invention may
 be applied to a process of forming a film of titanium oxide, zirconium
 oxide, barium oxide, or strontium oxide, while using the corresponding
 metal alkoxide as a liquid raw material. The present invention may be also
 applied to a process of forming a film of niobium oxide, hafnium oxide,
 yttrium oxide, or lead oxide.
 Furthermore, the present invention is not limited to a film-forming unit of
 the type shown in the embodiments. For example, the present invention may
 employ a film-forming unit of the lamp-heating type, or of the
 plasma-using type. As regards a target substrate, the present invention
 may be applied to a case where an LCD glass substrate is processed,
 instead of a semiconductor wafer.
 Additional advantages and modifications will readily occur to those skilled
 in the art. Therefore, the invention in its broader aspects is not limited
 to the specific details and representative embodiments shown and described
 herein. Accordingly, various modifications may be made without departing
 from the spirit or scope of the general inventive concept as defined by
 the appended claims and their equivalents.