Semiconductor treatment apparatus

A CVD apparatus in which a process gas containing a carrier gas and a raw material gas is supplied to a process chamber through a supply line. A first part of the carrier gas is supplied from a primary line through a bubbling line and passed through a raw material liquid to derive the raw material gas, and then sent to the supply line. A second part of the carrier gas is supplied from the primary line through a bypass line directly to the supply line. Electromagnetic valves are provided on each of the bubbling line and the bypass line and flow sensors are provided on each of the primary line and the supply line. Each flow sensor has a reference element and a heated element, each of which has a thermally-sensitive and electrically conductive wire. Signals from the sensors are supplied to a flow controller, which measures the flow rate of the raw material gas by comparing the measured value with a reference value, and adjusts the opening degrees of the electromagnetic valves on the basis of the comparison.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a semiconductor treatment apparatus, and more 
particularly to a treatment apparatus for supplying a process chamber with 
a process gas formed of a mixture of a carrier gas and a raw material gas. 
2. Description of the Related Art 
In semiconductor treatment apparatuses such as a thermal treatment 
apparatus, a diffusion apparatus, and a CVD apparatus, a process gas 
formed of a mixture of a carrier gas and a raw material gas is supplied 
into a process chamber to treat a semiconductor wafer or an LCD substrate. 
In the case of the CVD apparatus, for example, a silicon-base compound 
such as tetraethylorthosilicate (TEOS), or an organic metal compound such 
as triisobutylauminium are used as the raw material gas in many cases. 
The raw material gas is reserved in a reservoir generally in the form of 
liquid as a raw material liquid. The raw material liquid is bubbled with 
the use of the carrier gas to be vaporized. The reservoir is located in a 
constant-temperature bath. The process gas formed of a mixture thus 
obtained is transferred through a supply line into a process chamber. The 
flow of the process gas is controlled by a mass flow controller while it 
is transferred therethrough. To prevent a raw material gas from being 
liquefied during the transfer, the supply line is covered with a heating 
member such as a heating tape. 
The mass flow controller has a bypass pipe branching from a main pipe 
connected to the supply line. Part of the process gas flowing through the 
bypass line is heated, and the flow of the entire gas mixture is measured 
on the basis of the movement of heat. However, the bypass pipe has a very 
small inner diameter, for example, of 1 mm or less, and hence can easily 
be choked while such a gas is passed therethrough. Therefore, measuring 
errors are apt to occur and the flow control of the process gas cannot be 
performed with accuracy. 
It is considered that the bypass pipe is choked with by-products resulting 
from liquefaction of the raw material gas or decomposition of the raw 
material gas heated in the pipe. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a semiconductor treatment 
apparatus capable of accurately and reliably controlling the flow of a 
process gas supplied to a process chamber. 
According to a first aspect of the invention, there is provided an 
apparatus for treating a substrate while supplying a process chamber with 
a process gas including first and second gases, comprising: the process 
chamber; a support member arranged in the process chamber for supporting 
the substrate; a supply line for supplying the process gas into the 
process chamber; a tank for containing a raw material liquid, from which 
the second gas is derived, and having a vapor space defined therein above 
the raw material liquid, the supply line being connected to the tank such 
that it communicates with the vapor space; a primary line for supplying 
the first gas to the supply line, at least part of the first gas being 
passed through the raw material liquid in the tank in the form of bubble, 
thereby to derive the second gas from the raw material liquid and send the 
second gas to the supply line through the vapor space; a first flow sensor 
provided on the primary line and comprising a first heated element, the 
first heated element having a thermally-sensitive and electrically 
conductive wire, whose resistance varies in accordance with a variation in 
temperature; a second flow sensor provided on the supply line and 
comprising a second heated element, the second heated element having a 
thermally-sensitive and electrically conductive wire, whose resistance 
varies in accordance with a variation in temperature; and a flow 
controller connected to the first and second flow sensors for controlling 
a flow rate of the second gas contained in the process gas, the flow 
controller generating a first electric signal representing a flow rate of 
the first gas on the basis of a variation in the resistance of the 
thermally-sensitive and electrically conductive wire of the first heated 
element, also generating a second electric signal representing a flow rate 
of the process gas on the basis of a variation in the resistance of the 
thermally-sensitive and electrically conductive wire of the second heated 
element, and calculating a mesured value of the flow rate of the second 
gas on the basis of the difference between the first and second electric 
signals. 
According to a second aspect of the invention, there is provided an 
apparatus for treating a substrate while supplying a process chamber with 
a process gas including first and second gases, comprising: the process 
chamber; a support member arranged in the process chamber for supporting 
the substrate; a supply line for supplying the process gas into the 
process chamber; a delivery line for sending a first part of the first gas 
and the second gas to the supply line; a bypass line for sending a second 
part of the first gas to the supply line; a tank for containing a raw 
material liquid, from which the second gas is derived, and having a vapor 
space defined therein above the raw material liquid, the delivery line 
being connected to the tank such that it communicates with the vapor 
space; a bubbling line for sending the first part of the first gas into 
the raw material liquid in the tank, the first part of the first gas being 
passed through the raw material liquid in the form of bubbles, thereby to 
derive the second gas from the raw material liquid and send the second gas 
to the delivery line through the vapor space; first and second 
electromagnetic valves provided on the bubbling line and the bypass line, 
respectively; a primary line for supplying the first and second parts of 
the first gas to the bubbling line and the bypass line, respectively; a 
first flow sensor provided on the primary line and comprising a first 
heated element, the first heated element having a thermally-sensitive and 
electrically conductive wire, whose resistance varies in accordance with a 
variation in temperature; a second flow sensor provided on the supply line 
comprising a second heated element, the second heated element having a 
thermallysensitive and electrically conductive wire, whose resistance 
varies in accordance with a variation in temperature; and a flow 
controller, connected to the first and second flow sensors and the first 
and second electromagnetic valves, for controlling a flow rate of the 
second gas contained in the process gas, the flow controller generating a 
first electric signal representing a flow rate of the first gas on the 
basis of a variation in the resistance of the thermally-sensitive and 
electrically conductive wire of the first heated element, also generating 
a second electric signal representing a flow rate of the process gas on 
the basis of a variation in the resistance of the thermally-sensitive and 
electrically conductive wire of the second heated element, calculating a 
measured value of the flow rate of the second gas on the basis of the 
difference between the first and second electric signals, comparing the 
measured value with a preset reference value, and adjusting the opening 
degrees of the first and second electromagnetic valves to change the ratio 
of the first part of the first gas to the second part of the same on the 
basis of the comparison. 
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 in the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A CVD apparatus shown in FIG. 1 and according to an embodiment of the 
invention is used to form a silicon oxide film on a semiconductor wafer. 
Tetraethylorthosilicate (TEOS: Si(OC.sub.2 H.sub.5).sub.4), for example, 
is used as a raw material gas. This gas is mixed with nitrogen gas to 
constitute a process gas, which is supplied into a process chamber 12. The 
raw material gas in the process gas is thermally decomposed in the chamber 
12, and a resulting substance is deposited on a semiconductor wafer, 
thereby forming a silicon oxide film. 
A work table 14 on which a semiconductor wafer W is to be placed is located 
in the process chamber 12. An exhaustion pipe 16 is connected to the 
process chamber 12 to exhaust the chamber and set the pressure therein to 
a vacuum value. A heater 18 is provided on the peripheral surface of the 
process chamber for heating the same. 
Tetraethylorthosilicate as the raw material gas is contained in a tank 22 
in the form of liquid as a raw material liquid RL. The tank 22 is received 
in a constant temperature unit 24 and has its internal temperature kept at 
such a predetermined value as aids the raw material liquid RL to be 
vaporized. A bubbling nozzle 26 for guiding a carrier gas into the raw 
material liquid RL is inserted into a deep portion of the raw material 
liquid in the tank 22. The nozzle 26 is U-shaped, and its open end is 
directed upward. When the raw material liquid RL has been bubbled by the 
carrier gas, the raw material gas is created and lead to a space above the 
liquid RL, and then guided through a delivery line 28 to the outside of 
the tank 22. 
Nitrogen, which is used as the carrier gas, is contained in a source 32 
located outside the constant temperature unit 24. A primary line 34 
connected to the source 32 is guided into the constant temperature unit 
24. A temperature adjusting section 36 is provided on the primary line 34 
in the constant temperature unit 24 near the inlet of the same. The 
carrier gas is heated by the temperature adjusting section 36 to the set 
temperature of the interior of the unit 24. 
A first flow sensor 38, which will be described later, is provided on the 
primary line 34 on the downstream side of the temperature adjusting 
section 36. A pressure gauge 37 is connected to that portion of the 
primary line 34 which is located between the temperature adjusting section 
36 and the first flow sensor 38. The primary line 34 branches into a 
bubbling line 42 and a bypass line 44 at a location downstream of the 
first flow sensor 38. Electromagnetic valves 46 and 48 are provided on the 
lines 42 and 44, respectively. The bubbling line 42 extends through an 
upper wall portion of the tank 22 in an airtight manner, and is connected 
to the nozzle 26. The bypass line 44 bypasses the tank 22 and is connected 
along with the delivery line 28 to a supply line 52 for supplying the 
process gas into the process chamber 12. 
The delivery line 28 extends through an upper wall portion of the tank 22 
in an airtight manner into the space above the raw material liquid RL, and 
opens downward in the space such that it does not touch the liquid. The 
delivery line 28 is connected to the supply line 52 for supplying the 
process gas into the process chamber 12. A second flow sensor 54, which 
will be described later, is located on the supply line 52. A pressure 
gauge 53 is connected to the supply line 52 in the vicinity of the second 
flow sensor 54. That portion of the supply line 52 which is located 
between the constant-temperature unit 24 and the process chamber 12 is 
covered with a heating member 56 such as a heating tape to prevent 
liquefaction of the raw material gas during transfer. 
A flow controller 58 is provided to receive detection signals supplied from 
the first flow sensor 38 and the pressure gauge 37 located on the primary 
line 34 for the carrier gas, and also detection signals supplied from the 
second flow sensor 54 and the pressure gauge 53 located on the supply line 
52 for the process gas consisting of the raw material gas and the carrier 
gas. The opening degrees of the electromagnetic valves 46 and 48 arranged 
on the bubbling line 42 and the bypass line 44 are controlled by the flow 
controller 58. As will be explained later, the flow controller 58 measures 
the flow rate of the raw material gas, compares the measured value of the 
flow rate with a set reference value of the flow rate of the raw material 
gas, and adjusts the opening degrees of the electromagnetic valves 46 and 
48, on the basis of signals from the flow sensors 38, 54 and the pressure 
gauges 37 and 53. 
A line 64 connected to a raw material liquid-replenishing tank 62 is 
arranged in the liquid tank 22 such that it reaches a deep portion of the 
raw material liquid RL. An electromagnetic valve 66 is provided on the 
line 64 directly under the replenishing tank 62. A liquid level gauge 68 
for the raw material liquid RL is provided in the liquid tank 22. The 
electromagnetic valve 66 and the liquid level gauge 68 are connected to a 
liquid level controller 72. As will be described later, the liquid level 
controller 72 detects the liquid level of the raw material liquid RL on 
the basis of signals from the liquid level gauge 68, and adjusts the 
opening degree of the electromagnetic valve 66. 
As is shown in FIG. 2, the first flow sensor 38 provided on the primary 
line 34 for the carrier gas has two thermally-sensitive elements 76a and 
76b. The element 76a functions as a reference element for detecting an 
ambient temperature. The other element 76b functions as a heated element 
for detecting temperature variation caused by the flow of the carrier gas. 
However, both the elements 76a and 76b have substantially the same 
structure and are made of substantially the same material. 
Similarly, as is shown in FIG. 3, the second flow sensor 54 provided on the 
supply line 52 for the process gas has two thermally-sensitive elements 
76c and 76d. The element 76c functions as a reference element for 
detecting an ambient temperature. The other element 76d functions as a 
heated element for detecting temperature variation caused by the flow of 
the process gas. The elements 76c and 76d have substantially the same 
structure as the elements 76a and 76b and are made of substantially the 
same material as them. 
FIG. 7 shows in detail a thermal element 76 to be used as the element 76a, 
76b, 76c or 76d. The element 76 has a slim tube 82 inserted in the pipe of 
the line 34 or 52 in an airtight manner. The tube 82 has an open end 84, a 
closed end and a mirror-finished surface, and is formed of a metal having 
a high anti-corrosion and a high thermal conductivity, such as a stainless 
steel. The tube 82 has a thickness of 0.01 to 1.0 mm and an inner diameter 
of 1 to 2.5 mm. The portion of the tube 82 which is inserted in the pipe 
of the line 34 or 52 has a length of 15 mm or more. If that portion of the 
tube 82 is shorter than 15 mm, it is highly possible that the thermal 
influence of a substance other than an object to be measured is increased. 
Therefore, it is preferable to keep that portion of the tube 82 longer 
than 15 mm. 
A platinum resistor wire 86 as a thermally-sensitive, 
electrically-conductive and self-heating wire, which has superior 
temperature characteristics and linearity, is provided in the tube 82 such 
that it extends through the overall length thereof. The wire 86 is folded 
at an innermost portion of the tube 82, and a platinum resistor plate 88 
is welded to the folded end of the wire 86. The wire 86 is supported by a 
plurality of insulating supporting members 92 provided in the tube 82 at 
regular intervals. Further, the tube 82 is filled with a heat medium 94 
having a superior thermal conductivity, such as silicon grease, for 
enabling thermal conduct between the tube 82, and the platinum resistor 
wire 86 and the platinum resistor plate 88. The resistor wire 86 and the 
resistor plate 88 have temperature-resistance characteristics wherein 
their resistances vary, for example, by 0.3 to 0.4%/.degree. C. The both 
opposite ends of the platinum resistor wire 86 are drawn from the opening 
end of the tube 82 and connected to the flow controller 58. 
Preferably, the thermal elements 76a to 76d are inserted in respective bent 
pipes of the lines 34 and 52 in an airtight manner, as is shown in FIGS. 2 
and 3. The tube 82 is located parallel to the direction of the flow of 
gas, and has its closed end 84 located upstream of the gas flow. The 
element pair 76a and 76b are located symmetrical to each other with 
respect to the axis of the pipe of the line 34, and the element pair 76c 
and 76d are located symmetrical to each other with respect to the axis of 
the pipe of the line 52. 
As is shown in FIG. 5, a constant current circuit 102 incorporated in the 
flow controller 58 is connected to the platinum resistor wire 86 of the 
reference element 76a of the first flow sensor 38, for supplying the wire 
with a predetermined small constant current, for example, of 0.1 to 1.0 mA 
at the time of measurement. The small constant current is set to a value 
falling within a range in which the platinum resistor wire 86 of the 
reference element 76a generates substantially no heat. Since the reference 
element 76a is in a nonheated state in the constant temperature unit 24, 
it has a temperature substantially the same as the carrier gas flowing 
through the line 34. On the other hand, a constant current circuit 104 
incorporated in the flow controller 58 is connected to the platinum 
resistor wire 86 of the heated element 76b, for supplying the wire with a 
predetermined large constant current, for example, of 4.0 to 12.0 mA at 
the time of measurement. As a result, the resistor wire 86 of the element 
76b generates heat and the element 76b is heated. 
Since the reference element 76a in the non-heated state has the same 
temperature as the carrier gas in the line 34, the amount of heat 
exchanged between the reference element 76a and the carrier gas is always 
substantially zero irrespective of the flow rate of the carrier gas. In 
other words, even if the flow rate of the carrier gas varies, the 
temperature of the reference element 76a does not vary. Thus, the 
resistance of the platinum resistor wire 86 of the reference element 76a 
is kept at a value determined by the temperature of the carrier gas 
flowing through the line 34. 
On the other hand, there is a large temperature difference between the 
heated element 76b and the carrier gas flowing through the line 34. 
Therefore, when the flow rate of the carrier gas has varied, the amount of 
heat deprived of from the heated element 76b varies, and accordingly the 
temperature of the same varies. This means that the amount of heat 
exchanged between the heated element 76b and the carrier gas varies 
depending upon the flow rate of the carrier gas. 
As is shown in FIG. 5, the output terminals of the constant current 
circuits 102 and 104 for the thermal elements 76a and 76b are connected to 
the plus and minus input terminals of a differential circuit 112, 
respectively. The differential circuit 112 calculates a difference in 
voltage between the elements 76a and 76b which varies in accordance with 
variations in the resistances of the platinum resistor wires 86 thereof. 
The voltage of the resistor wire 86 of the heated element 76b represents 
the flow rate of the carrier gas on the basis of the temperature of the 
carrier gas. The reference voltage of the resistor wire 86 of the 
reference element 76a represents the temperature of the carrier gas. Thus, 
a signal representing only the flow rate of the carrier gas can be 
obtained by subtracting the reference voltage of the reference element 76a 
from the voltage of the heated element 76b. 
When the ambient temperature of the first flow sensor 38 has varied, for 
example, when the temperature of the carrier gas has varied, the reference 
voltage of the reference element 76a varies accordingly. Thus, the 
differential circuit 112 can eliminate the influence caused by variations 
in ambient temperature by subtracting the reference voltage of the 
reference element 76a from the voltage of the heated element 76b. 
Similarly, a constant current circuit 106 incorporated in the flow 
controller 58 is connected to the resistor wire 86 of the reference 
element 76c of the second flow sensor 54, for supplying the wire with a 
predetermined small constant current, for example, of 0.1 to 1.0 mA at the 
time of measurement. The small constant current is set to a value falling 
within a range in which the platinum resistor wire 86 of the reference 
element 76c generates substantially no heat. Since the reference element 
76c is in a non-heated state in the constant temperature unit 24, it has a 
temperature substantially the same as the process gas flowing through the 
line 52. On the other hand, a constant current circuit 108 incorporated in 
the flow controller 58 is connected to the platinum resistor wire 86 of 
the heated element 76d, for supplying the wire with a predetermined large 
constant current, for example, of 4.0 to 12.0 mA at the time of 
measurement. As a result, the resistor wire 86 of the element 76d 
generates heat and the elements 76d is heated. 
Since the reference element 76c in the non-heated state has the same 
temperature as the process gas in the line 52, the amount of heat 
exchanged between the reference element 76c and the process gas is always 
substantially zero irrespective of the flow rate of the process gas. In 
other words, even if the flow rate of the process gas varies, the 
temperature of the reference element 76c does not vary. Thus, the 
resistance of the platinum resistor wire 86 of the reference element 76c 
is kept at a value determined by the temperature of the process gas 
flowing through the line 52. 
On the other hand, there is a large temperature difference between the 
heated element 76d and the process gas flowing through the line 52. 
Therefore, when the flow rate of the process gas has varied, the amount of 
heat deprived of from the heated element 76d varies, and accordingly the 
temperature of the same varies. This means that the amount of heat 
exchanged between the heated element 76d and the process gas varies 
depending upon the flow rate of the process gas. 
As is shown in FIG. 5, the output terminals of the constant current 
circuits 106 and 108 for the thermal elements 76c and 76d are connected to 
the plus and minus input terminals of a differential circuit 114, 
respectively. The differential circuit 114 calculates a difference in 
voltage between the elements 76c and 76d which varies in accordance with 
variations in the resistances of the platinum resistor wires 86 thereof. 
The voltage of the resistor wire 86 of the heated element 76d represents 
the flow rate of the process gas on the basis of the temperature of the 
process gas. The reference voltage of the resistor wire 86 of the 
reference element 76c represents the temperature of the process gas. Thus, 
a signal representing only the flow rate of the process gas can be 
obtained by subtracting the reference voltage of the reference element 76c 
from the voltage of the heated element 76d. 
When the ambient temperature of the second flow sensor 54 has varied, for 
example, when the temperature of the process gas has varied, the reference 
voltage of the reference element 76c varies accordingly. Thus, the 
differential circuit 114 can eliminate the influence caused by variations 
in ambient temperature by subtracting the reference voltage of the 
reference element 76c from the voltage of the heated element 76d. 
The output terminals of the differential circuits 112 and 114 are connected 
to the plus and minus input terminals of a differential circuit 116. The 
differential circuit 116 calculates the flow rate of the raw material gas 
from the difference in output voltage between the differential circuits 
112 and 114. In other words, the output voltages of the differential 
circuits 112 and 114 indicate the flow rates of the carrier gas and the 
process gas, respectively. Accordingly, the difference between the output 
voltages represents the flow rate of the raw material gas, which is 
obtained by subtracting the flow rate of the carrier gas from that of the 
process gas. If, for example, the relationship between the output voltage 
of the differential circuit 116 and the flow rate of the raw material gas, 
which is indicated by a calibration curve, is beforehand input in the flow 
controller 58, the flow rate of the raw material gas can be detected from 
the output voltage of the differential circuit 116. Further, if the 
relationships between the output voltages of the differential circuits 112 
and 114 and the flow rates of the carrier gas and the process gas, 
respectively, which are indicated by calibration curves, are beforehand 
input in the flow controller 58, the flow rates of the carrier gas and the 
process gas can be detected from the output values of the differential 
circuits 112 and 114, respectively. 
Where it is highly possible that the pressure in the lines 34 and 52 varies 
significantly, it is desirable to detect the pressure in the lines 34 and 
52 with the use of the pressure gauges 37 and 53, respectively, and then 
to input the detected values in the flow controller 58. The gas flow rates 
can be detected more accurately by correcting the output voltages of the 
differential circuits 112 and 114 on the basis of the detected values. 
Constant voltage circuits can be used in place of the constant current 
circuits 102 to 108. In this case, the differential circuits 112 and 114 
calculate the difference in current between the reference element 76a and 
the heated element 76b and that between the reference element 76c and the 
heated element 76d, respectively. These differences in current vary 
depending upon variation in the resistance of each platinum resistor wire 
86. 
The output terminal of the differential circuit 116 is connected to the 
plus input terminal of a comparator circuit 118. Resistors 122 and 124 are 
connected to the minus terminal of the comparator circuit 118, for setting 
a reference value of the flow rate of the raw material gas. At least one 
of the resistors 122 and 124 consists of a variable resistor which enables 
the reference value of the flow rate to be changed. The comparator circuit 
118 compares the detected flow rate of the raw material gas obtained from 
the differential circuit 116, with the reference value, and adjusts the 
opening degree of the electromagnetic valves 46 and 48 provided on the 
lines 42 and 44 for the carrier gas, on the basis of the comparison 
result. 
To control the flow rate of the raw material gas, the flow controller 58 
adjusts the opening degrees of the electromagnetic valves 46 and 48 and 
changes the ratio (M1/M2) of the flow M1 of a first part of the carrier 
gas passing through the bubbling line 42 to the flow M2 of a second part 
of the carrier gas passing through the bypass line 44. Here, control may 
be performed so as to keep (M1+M2) at a constant value, or so as to keep 
constant the flow rate of the process gas detected by the flow sensor 54. 
Such control facilitates the control of process conditions such as the 
pressure and temperature in the process chamber. 
A method for changing a factor such as the flow rate of the overall carrier 
gas, the temperature of the liquid tank 22 or the level of the raw 
material liquid RL can be employed as another method for controlling the 
flow rate of the raw material gas. However, changing the flow rate of the 
overall carrier gas may greatly change the pressure in the process 
chamber. Changing the temperature of the tank 22 may reduce the response 
speed in the control since it requires a relatively long time, or cause an 
erroneous operation of the flow sensor 38 or 54. Changing the level of the 
raw material liquid RL may reduce the response speed in the control since 
it requires a relatively long time, or may make it difficult to accurately 
perform the control. 
As is shown in FIG. 6, the liquid level gauge 68 provided in the liquid 
tank 22 has a lower limit sensor 132 and an upper limit sensor 134. These 
sensors 132 and 134 are contained in a cylinder 136 having opening at its 
lower and upper ends. A plurality of holes 138 are formed in a portion 
thereof which does not face the carrier gas nozzle 26, for preventing the 
carrier gas from the nozzle 26, from adversely affecting the sensors 132 
and 134. 
The lower limit sensor 132 has thermally-sensitive elements 132a and 132b, 
and the upper limit sensor 134 has thermally-sensitive elements 134a and 
134b. The thermally-sensitive elements 132a and 134a of the sensots 132 
and 134 function as reference elements for detecting the ambient 
temperature, while the thermally-sensitive elements 132b and 134b function 
as heated elements for detecting variations in temperature due to 
variations in liquid level. The elements 132a, 132b, 134a and 134b are 
made of substantially the same material and have substantially the same 
structure as those shown in FIG. 7, except for the length thereof. 
Therefore, no detailed explanation is given thereof. 
The platinum resistor wires 86 of the reference elements 132a and 134a are 
supplied with a predetermined small constant current, for example, of 0.1 
to 1.0 mA from a constant current circuit (not shown). The small constant 
current is set to a value falling within a range in which the platinum 
resistor wires 86 of the reference elements 132a and 132b generate 
substantially no heat. On the other hand, the platinum resistor wires 86 
of the heated elements 132b and 134b generate heat and the elements 132b 
and 134b are supplied with a predetermined large constant current, for 
example, of 4.0 to 12.0 mA from a constant current circuit (not shown). As 
a result, the resistor wires 86 of the elements 132b and 134b generate 
heat and the elements 132b and 134b are heated. Like the flow sensors 38 
and 54 explained with reference to FIG. 5, each of the sensors 132 and 134 
is connected to a differential circuit (not shown), which enables the 
calculation of the difference in voltage between the reference element 
132a and the heated element 132b and that between the reference element 
134a and the heated element 134b. 
The lower end of each of the thermal elements 132a and 12b of the lower 
limit sensor 132 is located at a level (i.e., a lower limit level) higher 
than the level of the open end of the nozzle 26. when the raw material 
liquid RL has been reduced and its level has become lower than the lower 
ends of the elements 132a and 132b, the thermal capacity of substances 
which contact the elements significantly varies. Accordingly, the 
temperature of the heated element 132b varies, so that there is provided 
information indicating that the level of the liquid has become lower than 
the lower limit level. The liquid level controller 72 opens the 
electromagnetic valve 66 on the basis of the information, causing the 
replenishing tank 62 to replenish the tank 22 with the raw material 
liquid. 
The lower end each of the thermal elements 134a and 134b of the upper limit 
sensor 134 is located at a level (i.e., an upper limit level) lower than 
the level of the open end of the delivery line 28. When the raw material 
liquid RL has been replenished and its level has become higher than the 
upper ends of the elements 134a and 134b, the thermal capacity of 
substances which contact the elements significantly varies. Accordingly, 
the temperature of the heated element 134b varies, so that there is 
provided information indicating that the level of the liquid has become 
higher than the upper limit level. The liquid surface controller closes 
the electromagnetic valve 66 on the basis of the information, ceasing the 
supply of the raw material liquid from the replenishing tank 62 to the 
tank 22. 
The operation of the CVD apparatus shown in FIG. 1 will now be explained, 
referring to the case of forming a silicon oxide film on a semiconductor 
wafer. 
First, the relationship between the output voltage of the differential 
circuit 116 and the flow rate of the raw material gas, which is indicated 
by a calibration curve, is input to the flow controller 58. Further, if 
necessary, the relationships between the output voltages of the 
differential circuits 112 and 114 and the flow rates of the carrier gas 
and the gas, respectively, which are indicated by calibration curves, are 
also input to the flow controller 58. Moreover, a reference value for the 
flow rate of the raw material gas is selected and input to the controller 
58. 
The raw material gas such as Tetraethylorthosilicate is contained in the 
tank 22 in the form of liquid and supplied from the replenishing tank 62 
to the tank 22. The interior of the constant temperature unit 24 
containing the tank 22 is kept at a predetermined temperature, for 
example, of 60.degree. C. to facilitate the vaporization of the raw 
material liquid RL. 
A semiconductor wafer W to be treated is placed onto a work table 14 in the 
process chamber 12. Then, the process chamber 12 is exhausted in an 
airtight state, thereby setting the interior of the chamber under vacuum 
pressure, for example, of 2 Tort. Further, the wafer W in the chamber 12 
is heated to a predetermined temperature, for example, of 650.degree. C. 
The carrier gas is discharged from the source 32 to the line 34 with a 
predetermined flow rate of e.g. 7.5 i/min. A first part of the carrier gas 
is supplied to the liquid tank 22 through the line 42 and the nozzle 26. 
The first part of the carrier gas bubbles the raw material liquid RL to 
derive the raw material gas therefrom with a flow rate of e.g. 100 sccm. 
The first part of the carrier gas and the raw material gas are introduced 
into the supply line 52 through the delivery line 28. The remaining second 
part of the carrier gas is introduced into the supply line 52 through the 
bypass line 44. The thus obtained process gas consisting of the carrier 
gas and the raw material gas are supplied to the process chamber 12 
through the supply line 52. 
Tetraethylorthosilicate as the raw material gas is thermally decomposed in 
the process chamber 12, and part of resultant substances is deposited on 
the semiconductor wafer to form a silicon oxide film. Gaseous substances 
including the carrier gas, which are not the component of the silicon 
oxide film, are exhausted from the process chamber 12. 
During forming the film, the flow controller 58 compares the flow rate of 
the raw material gas calculated using the flow sensors 38 and 54, with the 
reference value, and adjusts the opening degrees of the electromagnetic 
valves 42 and 44 on the basis of the differences therebetween. The control 
of the flow rate of the raw material gas by the flow controller 58 is 
performed by changing the ratio of the flow rate of the first part of the 
carrier gas passing through the bubbling line 42, to that of the second 
part of the carrier gas passing through the bypass line 44. 
During forming the silicon oxide film, the level of the raw material liquid 
RL in the tank 22 is monitored by the liquid level gauge 68. The liquid 
level controller 72 adjusts the opening degree of the electromagnetic 
valve 66 on the basis of a signal from the gauge 68, and maintains the 
level of the liquid RL within a range set by the lower and upper limit 
sensors 132 and 134. 
FIG. 4 shows modifications of the flow sensors 38 and 54. In the 
modifications, the flow sensors 38 and 54 have only the heated elements 
76b and 76d, respectively, and no reference elements 76a and 76c. Those 
portions of the lines 34 and 52 at which the flow sensors 38 and 54 are 
provided are located adjacent to each other. The sensors 38 and 54 are 
arranged in a common temperature adjustment section 78. The section 78 
forcibly maintains these portions of lines 34 and 52 containing the flow 
sensors 38 and 54 at a constant temperature. The portions of the lines 34 
and 52 which are located in the section 78 are long enough on the upstream 
sides of the flow sensors 38 and 54, respectively, so that the gas flowing 
through each of the lines 34 and 52 can be set at the constant 
temperature. 
As a result, the flow sensors 38 and 54 according to the modification shown 
in FIG. 4 own a common and constant ambient temperature as the reference 
temperature. This being so, the reference elements 76a and 76c required in 
the structure shown in FIGS. 2 and 3 are not necessary. Further, the 
differential circuits 112 and 114 shown in FIG. 5, required to calculate 
the differences in voltage between the reference elements 76a and 76c and 
the heated elements 76b and 76d, are also not necessary. In this case, 
instead of using the circuits 112 and 114, the heated elements 76b and 76d 
are directly connected to the input terminals of the differential circuit 
116. 
FIGS. 8 to 10 show modifications of the thermal element 76 used as the 
heated element. In these modifications, a member serving only as a heater 
is used as well as the platinum resistor wire 86 as a thermallysensitive 
body. Although these modifications inevitably have complicated structures, 
they are advantageous in that only a small amount of current is necessary 
for the platinum resistor wire 86, and a higher sensitivity to heat can be 
attained. 
A thermal element 142 shown in FIG. 8 is obtained by providing in the tube 
82 shown in FIG. 7 a resistance heating wire 144 serving as a heater along 
with the platinum resistor wire 86. One of the twofold portions of the 
resistor wire 86 is used as a common line to supply voltage to the 
resistance heating wire 144. 
A thermal element 152 shown in FIG. 9 has a covering tube which consists of 
a pair of resistance heating parts 156 adhered to each other with an 
insulating layer 154 interposed therebetween. The resistance heating parts 
156 are directly connected to each other only at the closed end of the 
tube, and power supply wires 158 are connected to the parts 156 at the 
open end of the tube. In other words, the tube itself serves as a heater. 
A thermal element 162 shown in FIG. 10 is obtained by winding a heating 
coil 164 on the tube 82 shown in FIG. 7 and containing the tube 82 with 
the heating coil 164 in a tube 166 made of a metal having a high corrosion 
resistance and a high heat conductivity, such as stainless steel. The 
power supply wires 168 of the coil 164 are drawn from the open end of the 
tube 166. 
Although the CVD apparatus is used in the embodiment, the invention is not 
limited to this apparatus, but also applicable to a thermal treatment 
apparatus, a diffusion apparatus, an etching apparatus, etc., which have a 
raw material gas supply system. 
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 devices 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.