Heat-treatment apparatus having exhaust system

A heat-treatment apparatus of the invention includes a collective exhaust unit for constantly performing gas exhaust from a clean room to keep the room in a clean atmosphere, a heat-treatment furnace for receiving a gas and/or a liquid for forming a desired film on a surface of an object to be treated in a heating atmosphere, an exhaust path, communicating with the collective exhaust unit and the heat-treatment furnace, for introducing a gas filling the heat-treatment furnace into the collective exhaust unit, an outer air intake unit for taking in outer air in the exhaust path to adjust an exhaust pressure of the exhaust path, and a trap unit, arranged below the exhaust path, for trapping a waste liquid collected in the exhaust path.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a heat-treatment apparatus for performing 
a heat treatment of semiconductor wafers and, more particularly, to an 
exhaust system for removing exhaust gases from a heat-treatment furnace. 
2. Description of the Related Art 
Recently, in the heat treatment process of manufacturing semiconductor 
devices, the size of a semiconductor wafer to be treated is increased from 
six inches (four inches) to eight inches, and the number of wafers to be 
subjected to batch processing in a heat-treatment furnace is also 
increased from 100 pieces to 150 pieces. For this reason, the type of a 
heat-treatment furnace to be used is shifted from a horizontal type to a 
vertical type, resulting in a great increase in furnace capacity. 
In such a large, vertical type heat-treatment apparatus, since large 
amounts of exhaust gases are generated in oxidation and diffusion 
processes, an exhaust system is arranged to exhaust them out of the 
process system. 
Published Unexamined Japanese Patent Application Nos. 62-63421, 63-238281, 
and 63-304620 respectively disclose exhaust systems having flow control 
valves. In these exhaust systems, an exhaust apparatus is complicated and 
expensive. In addition, a CVD apparatus, in which reaction products tend 
to adhere to the exhaust system, requires frequent maintenance and 
inspection. 
Published Unexamined Japanese Patent Application No. 58-124226 discloses an 
exhaust system in which the pressure of exhaust gases from a process tube 
is kept constant in relation to a pressure corresponding to a liquid 
column. In this exhaust system, only when the internal pressure of an 
apparatus becomes larger than the atmospheric pressure, exhaust gases in 
the process tube are exhausted. In this system, however, if the internal 
pressure of the process tube becomes smaller than the atmospheric 
pressure, no exhaust gases are exhausted, and the flows of gases in the 
process tube become nonuniform. As a result, a film having a uniform 
thickness cannot be formed on a wafer surface. 
As shown in FIG. 1, a collective exhaust apparatus 7 is generally installed 
in a semiconductor device manufacturing factory so as to perform an 
exhausting operation of the overall factory. This collective exhaust 
apparatus 7 includes a high-power fan 8. In addition, exhaust paths 5b of 
a plurality of heat-treatment apparatuses 2 (only one apparatus is shown 
in FIG. 1 but the other apparatuses 2 are omitted) merge into a path 5c of 
the collective exhaust apparatus 7. That is, the plurality of 
heat-treatment apparatuses 2 share the single exhaust system 7. Referring 
to FIG. 1, reference numeral 3 denotes a gas reservoir; 3a, a mass flow 
controller; 4, combustion apparatus (Steam generator); 5, a process tube; 
5a, 5b, and 5c, gas pipes; and 6, a heater. Reference symbol B denotes a 
wafer boat; and W, a semiconductor wafer. 
In such a collective exhaust apparatus 7, when all the heat-treatment 
apparatuses 2 in the factory are fully operated, exhaust gases in the 
process tubes 5 can be properly and uniformly exhausted. If, however, only 
some of the heat-treatment apparatuses are to be operated, the pressure in 
each process tube 5 is difficult to control to be a desired value. More 
specifically, the internal pressure of the exhaust path 5c of each 
heat-treatment apparatus 2 becomes an excessively large negative value, 
and the pressure in the process tube 5 tends to be smaller than that in a 
normal state. For this reason, the internal pressure of the process tube 5 
does not reach a predetermined value, and a film having a desired 
thickness cannot be formed on a surface of a semiconductor wafer W. 
In addition, in the collective exhaust apparatus 7, its exhaust power 
varies due to various factors such as voltage variations, and strong and 
weak exhaust flows are produced, resulting in so called pulsating flows. 
Pulsating flows in such a collective exhaust system disturb a gas flow in 
the process tube 5 and adversely affect the manufacture of semiconductor 
devices, such as variations in thickness of films formed on the wafers W. 
Published Unexamined Japanese Patent Application No. 2-59002 discloses a 
trap unit for trapping reaction products contained in exhaust gases in a 
pipe of an exhaust system. 
Published Unexamined Japanese Patent Application No. 61-160933 discloses a 
waste liquid processing system of a semiconductor substrate developing 
apparatus. 
In the above-described exhaust system, however, an exhaust gas is cooled 
while it flows in the system, and a gas component whose temperature is 
lowered below a condensation temperature is condensed on a pipe wall. As a 
result, a waste liquid collects in the pipe. 
In an apparatus used for a special treatment among vertical type 
heat-treatment apparatuses, since a waste liquid is produced in a pipe in 
large quantities which cannot be ignored, various problems are posed. 
Especially, in a technique of forming an oxide film by oxidizing a 
semiconductor wafer surface, since steam or humidified oxygen gas (wet 
O.sub.2 gas) is used as a process gas, steam in the process gas is 
condensed on a pipe of the exhaust system, and a large amount of waste 
liquid is produced. 
Of such apparatuses, especially in a steam oxidation apparatus, the amount 
of waste liquid is large, and a waste liquid tends to collect in a pipe of 
an exhaust system. If a large amount of waste liquid collects in the pipe, 
an exhaust path of an exhaust gas is narrowed by the waste liquid, and an 
exhaust gas in a process tube cannot be sufficiently exhausted. As a 
result, a gas flow in the process tube becomes nonuniform, and the 
thicknesses of films formed on semiconductor wafers subjected to batch 
processing vary. This impairs quality stability. 
In another oxide film forming technique, in order to obtain a high-quality 
oxide film, a gas mixture obtained by mixing hydrochloric acid in a dry 
oxygen gas is sometimes used. In an exhaust system for exhausting such a 
gas, since stainless steel pipes tend to be corroded by hydrochloric acid, 
the inner surface of the pipe must be periodically cleaned. In cleaning, 
the pipes must be disassembled, and a time-consuming operation is 
required, resulting in a decrease in operating efficiency of a 
heat-treatment apparatus. 
Furthermore, in order to remove adhesive contamination in a process tube, 
an HCl gas is introduced in the process tube while no semiconductor wafers 
are processed. If such a cleaning gas component is left in a stainless 
steel pipe of the exhaust system, the pipe is corroded and must be 
frequently replaced with a new one. This decreases the operating 
efficiency of the heat-treatment apparatus. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a heat-treatment 
apparatus having an exhaust system which can reduce variations in films 
formed on wafers to be subjected to batch processing even when variations 
(pulsating flows) occur in a factory collective exhaust apparatus or when 
only some of a plurality of heat-treatment apparatuses are operated (i.e., 
all apparatuses are not operated). 
It is another object of the present invention to provide a heat-treatment 
apparatus having an exhaust system which is resistant to corrosion by an 
exhaust gas or a waste liquid and can endure long-term use. 
In order to reduce the influences of variations of the above-described 
conventional collective exhaust apparatus upon each heat-treatment 
apparatus, the present inventor examined an arrangement wherein a branch 
pipe is arranged in an exhaust pipe of an exhaust system, and a pressure 
adjusting unit is provided in this branch pipe. An adjusting valve of this 
pressure adjusting unit is vertically arranged so that when the difference 
in pressure between the atmospheric pressure and the pressure in the 
exhaust pipe exceeds a predetermined value (the internal pressure of the 
exhaust pipe is decreased), the valve plug is pushed upward, and outer air 
flows into the exhaust pipe to automatically increase the internal 
pressure of the exhaust pipe. 
In such an exhaust system, however, it is difficult to adjust the weight of 
the valve plug such that the valve is opened/closed in response to a small 
pressure difference (e.g., -1 mmH.sub.2 O) between the internal pressure 
of the exhaust pipe and the atmospheric pressure. In addition, if control 
of a factory exhaust system is disturbed and the internal pressure of a 
pipe of the exhaust system is greatly decreased as compared with a 
pressure in a normal operation, the valve is completely opened, and the 
internal pressure of the exhaust pipe cannot be held at a desired value. 
As a result, it is found that the pressure in the process tube fluctuates 
to cause variations in thickness of films formed on semiconductor wafer 
surfaces. The present invention has been made in consideration of such a 
situation. 
According to an aspect of the present invention, a heat-treatment apparatus 
comprises collective exhaust means for performing gas exhaust from a room 
to keep the room in a clean atmosphere, heat-treatment vessel means for 
receiving a gas and/or a liquid for forming a desired film on a surface of 
an object to be treated in a heating atmosphere, an exhaust path, 
communicating with the collective exhaust means and the heat-treatment 
vessel means, for introducing a gas filling the heat-treatment vessel 
means into the collective exhaust means, outer air intake means for taking 
in outer air in the exhaust path to adjust an exhaust pressure of the 
exhaust path, and trap means, arranged at downstream of the exhaust path, 
for trapping a waste liquid collected in the exhaust path. 
As the outer air intake means, an air intake unit may be employed which 
includes a switching valve designed to be automatically opened when a 
difference is caused between pressures inside and outside the exhaust 
system. 
By providing such outer air intake means in the respective exhaust paths, 
an excessive difference between pressures inside and outside the exhaust 
system can be automatically canceled. Note that the outer air intake means 
may be arranged in a vertical exhaust path or a horizontal path of the 
collective exhaust means. 
In addition, a lower exhaust unit may be arranged to exhaust a gas to the 
side of the trap means. The exhaust power of such a lower exhaust unit is 
preferably weaker than that of the upper collective exhaust unit so as to 
prevent an exhaust gas from being drawn downward during a normal operation 
and to prevent an unpleasant order from flowing upward from the trap unit 
when the upper collective exhaust unit is stopped. 
Note that the exhaust path, the outer air intake unit, and the trap unit 
are preferably composed of ethylene fluoride resins depending on 
temperatures at which they are used. 
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 
An embodiment of the present invention will be described below with 
reference to the accompanying drawings. 
As shown in FIG. 2, a plurality of vertical type heat-treatment furnaces 10 
are arranged in a clean room equipped with an air conditioner (not shown), 
and a large number of silicon wafers W loaded in a boat B are subjected to 
batch processing in each heat-treatment furnace 10 at one time. Each 
heat-treatment furnace 10 is an automatic apparatus backed up by a 
computer system including a controller. All the operations of each 
heat-treatment furnace 10 are automatically controlled by the controller. 
A process tube 15 in each heat-treatment furnace 10 has a vertical 
cylindrical shape and is fixed to a frame (not shown) by a support member 
(not shown). A heater 16 is wound around the process tube 15. A current to 
be supplied by a power source (not shown) for the heater 16 is properly 
controlled by a controller (not shown) of a computer system to set a 
desired temperature in the tube 15. Note that the heating capacity of the 
heater 16 is adjusted to set the temperature in the process tube within a 
temperature range of 800.degree. to 1,200.degree. C. 
A cover member 17 is arranged at the lower opening of the process tube 15. 
A heat insulating cylinder 14 is mounted on the cover member 17. The 
vertical boat B is mounted on the heat insulating cylinder 14. The cove 
member 17 is supported by a support member (not shown). A nut of the 
support member is threadably engaged with a ball screw of an elevating 
mechanism (not shown). 
A gas inlet port 20 is formed in an uppermost portion of the process tube 
15. A gas exhaust port 23 is formed in a lower portion of the tube 15. The 
gas inlet port 20 communicates with a plurality of gas reservoirs (GR) 11 
through pipes 21, 22 and combustion apparatus 13. Each gas reservoir 11 
has a mass flow controller (MFC) 12 so that the amount of a gas to be fed 
into the process tube 15 can be controlled. The gas reservoirs 11 
respectively store an oxygen gas, a hydrogen gas, a nitrogen gas, 
hydrochloric acid, and the like or solutions capable of being gasified. 
The gas exhaust port 23 of the process tube 15 is constituted by a quartz 
tube 24, which is connected to a branch tube 27 of an exhaust system 30. 
As shown in FIG. 3, a distal end 24a of the quartz tube 24 has a 
hemispherical shape. A spherical end portion 25a of a short quartz tube 25 
is slidably connected to the hemispherical end 24a. The short quartz tube 
25 on the downstream side of this connecting portion is inclined downward 
at an angle of about 2.degree. to 5.degree.. The branch tube 27 and a 
nipple joint 28 consist of ethylene tetrafluoride. 
As shown in FIG. 2, a collective exhaust unit 60 for exhausting gases from 
a factory is arranged on the ceiling. The collective exhaust unit 60 
includes a common duct 61 and a large fan 63, and serves to exhaust 
contaminated air and exhaust gases from the room. A path 32 of a vertical 
pipe 31 of the exhaust system 30 of each heat-treatment furnace 10 
communicates with a path 62 of the common duct 61. The large fan 63 is 
arranged at one end portion of the common duct 61 so as to discharge 
exhaust gases to the outside of the clean room through each path 32. Note 
that the common duct 61 is made of vinyl chloride resin and the like. 
The gas exhaust tube 24 of the furnace 10 is connected to the vertical pipe 
31 of the exhaust system 30 through the branch pipe 27. An air intake unit 
50 is arranged at a proper portion, of the vertical pipe 31, above each 
branch pipe 27. A pair of pressure adjusting valves 33a and 33b are 
arranged in the flow path of each vertical pipe 31 so a to sandwich the 
air intake unit 50. Differential manometers 34a and 34b are respectively 
attached to the pressure adjusting valves 33a and 33b on the upstream and 
downstream sides. Note that an air intake unit 50 is also provided for the 
common duct 61. 
A trap unit 70 is arranged at a portion, of the vertical pipe 31, below 
each branch pipe 27. Each trap unit 70 communicates with a collective pipe 
80 through a corresponding lower pipe 78. The collective pipe 80 
communicates with a tank (not shown) for waste liquid disposal. Weak 
exhaust of gases from the collective pipe 80 is performed by a blower (not 
shown) in an exhaust amount smaller than that of the upper collective 
exhaust unit. 
The air intake unit 50 will be described below with FIG. 4. 
An outer cylinder 53 of the air intake unit 50 laterally extends from the 
vertical pipe 31. A path 54 of the air intake unit 50 horizontally extends 
and is vertically bent downward. An opening 58 is formed in the lower end 
portion of the path 54. A stepped holding portion 58a is formed near the 
opening 58. The inner diameter of the holding portion 58a is larger than 
that of the path 54, and a peripheral portion of a float 56 rests on the 
upper step of the holding portion 58a. The float 56 is arranged to be 
substantially horizontal to close the opening 58 and to externally close 
the path 54 (to set the air intake unit 50 in a closed state) and to cause 
the path 54 to communicate with the outside (to set the air intake unit 50 
in an open state). 
An O-ring 57 is arranged on the lower step of the holding portion 58a. The 
O-ring 57 is brought into con tact with the peripheral portion of the 
float 56 in a closed state of the air intake unit 50 so as to ensure 
airtightness The weight of the float 56 is determined to set a proper air 
intake amount. For example, if it is required that the difference between 
the atmospheric pressure and the pressure in the exhaust pipe path 32 
becomes -10 mmH.sub.2 O, and if one atmospheric pressure corresponds to 
10,000 mmH.sub.2 O (=1 kg/cm.sup.2), the weight of the float 56 is set to 
correspond to 1 g/cm.sup.2. Note that the respective components of the air 
intake unit 50 consist of an ethylene difluoride resin. 
The trap unit 70 will be described below with reference to FIG. 5. 
The trap unit 70 is arranged at a lower portion of each vertical exhaust 
pipe 31. More specifically, the trap unit 70 is arranged at a portion 
lower than a position at which the branch pipe 27 on the side of each 
process tube 15 merges a corresponding exhaust pipe 31. The trap unit 70 
has a double casing structure consisting of inner and outer cylinders 72 
and 76. The path 32 of each vertical exhaust pipe 31 communicates with the 
interior of a corresponding inner cylinder 72. A plurality of openings 74 
are formed in an upper wall portion of the inner cylinder 72. The inner 
cylinder 72 communicates with the outer cylinder 76 through the openings 
74. In addition, a waste liquid pipe 78 extends downward from a lower 
portion of the outer cylinder 76. A path 79 of each waste liquid pipe 78 
communicates with the interior of a waste liquid tank (not shown) through 
the collective pipe 80. 
Note that the material for each component of the trap unit 70 may be 
variously changed in accordance with a temperature. For example, a member 
used at a temperature of 140.degree. C. or less consists of an ethylene 
difluoride resin (PVDF); a member used at a temperature of 140.degree. to 
260.degree. C., an ethylene tetrafluoride resin (PTFE); and a member used 
at a temperature of 260.degree. C. or more, quartz. In this case, since 
the outer cylinder 76 of each trap unit 70 is used at a temperature of 
100.degree. C. or less, transparent vinyl chloride resin may be used for 
it. A transparent ethylene fluoride resin may be used for the inner 
cylinder 72. Since the inner and outer cylinders 72 and 76 are constituted 
by transparent members, the amount of waste liquid can be visually checked 
from the outside. 
A case will be described below, in which only one of the plurality of 
heat-treatment furnaces 10 is operated while the rest of the 
heat-treatment furnaces are stopped. 
(I) Heat-treatment conditions are input beforehand in a CPU of a computer 
system through a keyboard. 150 silicon wafers W are mounted on the wafer 
boat B. The boat B is loaded in the process tube 15 of the first 
heat-treatment furnace 10 and is heat by the heater 16. At this time, a 
gas in the process tube 15 is replaced with an nitrogen gas in advance. 
The upper collective exhaust unit 60 is always operated, and hence the 
clean room is always subjected to an exhausting operation. 
(II) When the uniform temperature area of the process tube 15 sets up e.g 
1,000.degree. C., an oxygen gas, a hydrogen gas, and a small amount of 
hydrochloric acid are respectively supplied from first, second, and third 
gas reservoirs 11 into combustion apparatus 13, steam (H.sub.2 O) 
generates by apparatus 13. The steam is introduced to the process tube 15 
through the gas inlet port 20. The amount of each process gas component is 
adjusted by a corresponding MFC 12 to be desired value. 
When the process gases flow among the silicon wafers W, the upper surface 
of each wafer W is oxidized to form an SiO.sub.2 film. 
(III) Since the exhaust system 30 is set at a negative pressure by the 
upper collective exhaust unit 60, exhaust gases after oxidation reaction 
and non-reaction process gas (exhaust gas G) are drawn into the exhaust 
system 30 through the lower exhaust port 23. The temperature of the 
exhaust gas G is about 200.degree. to 400.degree. C. at the exhaust port 
23. While the exhaust gas G passes through the branch pipe 27 of the 
exhaust system 30, its heat is dissipated, and its temperature is 
decreased to liquefaction temperature of the gas G. The exhaust gas G 
flows from the branch pipe 27 into the vertical exhaust pipe 31 and 
collects in the path 62 of the upper collective pipe 61. In addition, the 
exhaust gas G is exhausted to the outside by the fan 63 through a filter 
(not shown). 
(IV) Since the rest of the heat-treatment furnaces 10 are not operated, the 
amount of the exhaust gas G flowing in the path 62 of the upper collective 
pipe 61 is smaller than that of exhaust gas flowing when the plurality of 
the heat-treatment furnaces 10 are simultaneously operated. For this 
reason, the amount of the exhaust gas G flowing in the path 62 becomes 
smaller than the normal exhaust amount of the fan 63, and the internal 
pressure of the vertical exhaust pipe 31 is excessively decreased, thus 
increasing a differential pressure .DELTA.P between the internal pressure 
and the atmospheric pressure. For example, when the differential pressure 
.DELTA.P exceeds -10 mmH.sub.2 O, the floats 56 of the respective air 
intake units 50 are simultaneously raised to be separated from the O-rings 
57, and air flows into the exhaust paths 32. As a result, the internal 
pressure of each vertical exhaust pipe 31 is increased, and the 
differential pressure .DELTA.P becomes lower than -10 mmH.sub.2 O. 
(V) When the exhaust amount of the collective exhaust unit 60 greatly 
varies due to irregular rotation, of the fan 63, caused by voltage 
variations or the like, pulsating flows may occur in the exhaust system. 
If pulsating flows occur in the exhaust system, the flow of a process gas 
(laminar flow) is disturbed. As a result a film may not be uniformly 
formed on the upper surface of each wafer W. However, if the differential 
pressure .DELTA.P temporarily exceeds -10 mmH.sub.2 O due to pulsating 
flows, air is taken in from the air intake unit 50 into the exhaust system 
only during this period, thereby adjusting the internal pressure of the 
exhaust system to decrease the differential pressure .DELTA.P. When the 
differential pressure becomes lower than the -10 mmH.sub.2 O, the 
influences of the exhaust system upon the process system are suppressed. 
(VI) In this manner, the interior of the exhaust pipe 31 is kept at about 
-10 mmH.sub.2 O instead of being set in an excessively negative state. At 
this time, the pressure adjusting valve 33a on the upstream side of the 
air intake unit 50 is adjusted to control the flow rate of the exhaust gas 
G such that a value detected by the differential manometer 34a falls 
within the range of -1 to 0 mmH.sub.2 O. In addition, the pressure 
adjusting valve 33b on the downstream side of the air intake unit 50 is 
adjusted while a differential pressure is detected by the differential 
manometer 34b in such a manner that the differential pressure between the 
internal pressure of the exhaust path 32 and the atmospheric pressure, in 
a state wherein the opening 58 is closed by the float 56, is kept to be a 
desired differential pressure .DELTA.P, e.g., within the range of -15 to 
-20 mmH.sub.2 O. Note that, in this case, feedback control may be 
performed to automatically drive the pressure adjusting valves 33a and 33b 
in accordance with outputs from the differential manometer and 34b. 
By adjusting the pair of pressure adjusting valves 33a and 33b in the 
above-described manner, automatic pressure adjustment can be optimally 
performed by the air intake unit 50. 
(VII) Meanwhile, exhaust gas components are partially condensed and 
liquified in the branch pipe 27. This waste liquid flows downward from the 
branch pipe 27 to the vertical pipe 31 and then flows from the vertical 
pipe 31 into the trap unit 70. When a waste liquid 77 collects in the 
inner cylinder 72 of the trap unit 70 and its liquid level is raised, the 
waste liquid 77 overflows from the opening 74. The overflowing waste 
liquid collects in the lower collective pipe 80 and is stored in the waste 
liquid tank (not shown). 
(VIII) after the heat treatment, the exhaust gas G is caused to flow from 
the exhaust port 23 into the exhaust pipe 31. Since other heat-treatment 
furnaces 10 are not operated at this time, no exhaust gas G flows into 
them. Therefore, the ratio of the flow rate of the exhaust gas G to the 
exhaust power of the fan 21 is decreased. 
Assume that the internal pressure of the exhaust pipe 31 is adjusted by 
only the float 56 of the air intake unit 50. In this case, even if a 
desired differential pressure between the internal pressure of the exhaust 
pipe 31 and the atmospheric pressure is -1 to 0 mmH.sub.2 O, the air 
intake unit 50 does not function until the negative pressure of the 
exhaust pipe 31 becomes very high. 
For example, the negative pressure becomes -10 mmH.sub.2 O, the float 56 is 
pushed by the atmospheric pressure and is removed from the O-ring 57. As a 
result, outer air flows into the exhaust pipe 31 through the unit 50, and 
the excessive negative pressure of the path 32 of the exhaust pipe 31 is 
released. At this time, the pressure adjusting valve 33a arranged on the 
upstream side of the air intake unit 50 is adjusted to set the internal 
pressure of a portion on the upstream side of the pressure adjusting valve 
33a to be -1 to 0 mmH.sub.2 O. The pressure adjusting valve 33b arranged 
on the downstream side of the air intake unit 50 is adjusted such that the 
differential manometer 34b always indicated -10 mmH.sub.2 O or lower, 
e.g., -20 mmH.sub.2 O, regardless of operating states of other 
heat-treatment furnaces 10. 
According to the exhaust system 30 of the above-described embodiment, a 
variation in oxidation rate of the upper surface of a silicon wafer in 
each vertical heat-treatment apparatus can be suppressed to 2 to 3%. In a 
conventional exhaust system, a variation in oxidation rate of the upper 
surface of a silicon wafer corresponds to 5 to 10%. That is, according to 
the pre sent invention, such a variation can be reduced by 2 to 8%. 
Therefore, the manufacturing yield can be greatly increased. 
According to the exhaust system 30 of the above-described embodiment, since 
the trap unit 70 is provided in lower portion of the vertical exhaust pipe 
31, even if the lower collective exhaust unit of pipe 80 is stopped, no 
unpleasant odor of the waste liquid flows upward to the upstream side of 
the trap unit 70. For this reason, reverse flows of the waste liquid and 
exhaust gas to the process system can be effectively prevented. 
The effects of the present invention will be summarized below. 
According to the exhaust system of the present invention, a variation in 
oxidation rate of the upper surface of a semiconductor wafer can be 
greatly reduced as compared with the prior art. For this reason, an oxide 
film having a desired thickness can be formed to reduce the fraction 
defective of semiconductor devices, thus greatly increasing the 
manufacturing yield. 
The present invention is not limited to a heat-treatment such as an 
oxidation furnace or a diffusion furnace. The present invention can be 
applied to any heat-apparatus as long as it is arranged in a clean room a 
collective exhaust unit, e.g a heat-treatment for performing a heat 
treatment of semiconductor at a normal pressure such as a normal pressure 
CVD apparatus or a spin quarter apparatus. 
In addition, the present invention is not limited to a type heat-treatment 
apparatus and can be applied to a horizontal type heat-treatment 
apparatus. 
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.