Projection exposure apparatus and method

A projection exposure apparatus has an exposure processing section for projecting and exposing an image of a pattern of a reticle on a photosensitive substrate, an environmental chamber for covering the exposure processing section and a fan unit for supplying a temperature-controlled gas to the environmental chamber. Exposure to the photosensitive substrate is carried out in a temperature-controlled atmosphere. The pressure of the gas in the environmental chamber is monitored by a pressure sensor. When the pressure is changed, the mixture ratio of the gas supplied to the environmental chamber is changed to keep the refractive index of the gas in the chamber constant thereby to prevent imaging characteristics of the projected image from deteriorating.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a projection exposure apparatus, e.g., 
provided in an environmental chamber connected to an air-conditioning 
mechanism for manufacturing semiconductor devices, liquid crystal display 
devices, etc. to a method of controlling optical performance in a 
projection exposure apparatus, and to a method for making a lithographic 
system. 
2. Related Background Art 
In manufacturing semiconductor devices, liquid crystal display devices, 
etc. in photolithography processes, projection exposure apparatuses are 
used in which the image of a pattern of a photomask or a reticle 
(hereinafter referred to as the reticle) is exposed on a wafer (or a glass 
plate) with photoresist applied thereto. 
FIG. 3 shows a conventional projection exposure apparatus. Exposure light 
IL from an illumination optical system 1 illuminates a pattern on a 
reticle 2 supported by a reticle stage 3 with a uniform illuminance 
distribution. A projection optical system 4 is disposed under the reticle 
stage 3. A correction optical member 7 for correcting the telecentric 
characteristic of the projection optical system 4 is mounted on the 
reticle side of the projection optical system 4 via supporting frames 5A, 
5B and driving sections 6A, 6B. The telecentric characteristic of the 
projection optical system PL can be corrected by correcting the position 
of the correction optical member 7 in the optical axis direction or the 
inclination thereof via the driving sections 6A, 6B. 
Under the exposure light IL, the pattern of the reticle 2 is projected and 
exposed via the projection optical system 4 on each shot area on a wafer 8 
supported by a wafer stage 9. At this time, the projected image on the 
wafer 8 includes various aberrations in accordance with the change in 
atmospheric pressure in the projection optical system PL. That is, the 
projection optical system 4 is designed under a condition that the 
atmospheric pressure is a predetermined value. Therefore, when the 
atmospheric pressure in the projection optical system 4 is changed from 
the predetermined value, the refractive index of the gas in each space 
between lenses constituting the projection optical system 4 is changed, 
resulting in deviations from various design conditions, i.e., changes of 
imaging characteristics (focal point position, magnification, field 
curvature, distortion, etc.). 
For avoiding this problem, conventionally, the atmospheric pressure in the 
projection optical system 4 is measured by an atmospheric pressure sensor 
10 and the measured value is constantly monitored by a control device 11. 
Then, the control device 11 adjusts the atmospheric pressure in a space 16 
between the n-th lens 14 (n: a predetermined integer) in the projection 
optical system 4 and the (n+1)-th lens 15 therein via a pressure control 
unit 12, or changes the position or inclination of the correction optical 
member 7 by driving the driving sections 6A, 6B of the projection optical 
system 4 via a drive control device 17 thereby to correct the changes of 
various aberrations of the projected image due to the change in 
atmospheric pressure. A method of changing imaging characteristics by 
controlling the pressure in a specific space between lenses in a 
projection optical system is disclosed in U.S. Pat. No. 4,666,273. Also, a 
method of changing imaging characteristics by driving a few lens elements 
in a projection optical system is disclosed in U.S. Pat. No. 5,117,255. 
In the above prior art, the problem of the change in refracting index of 
the air due to the change in atmospheric pressure is solved by changing 
the pressure in the specific space between specified lens elements in the 
projection optical system 4 to change the refractive index of a portion of 
the air in the projection optical system 4, or changing the distance 
between the reticle 2 and the wafer 8 or the distance between the lens 
elements in the projection optical system 4. There are many factors which 
cause distortion of the projected image due to the projection optical 
system 4, and allowable ranges of aberrations are limited vary narrowly. 
For example, as a factor of the distortion of the projected image due to 
the change in atmospheric pressure, there are aberrations caused by the 
defocus condition in which the focus position is changed between the 
reticle and the wafer. As the other aberrations, there are field 
curvature, comatic aberration, astigmatism, magnification, distortion, 
etc. Therefore, it becomes difficult to correct all the aberrations of the 
projected image up to presently required levels only by changing the 
distance between the reticle 2 and the wafer 8 or changing a portion of 
lens conditions in the projection optical system 4 (the pressure between 
lenses, the distance between lenses). 
Also, when a laser light source such as an excimer laser is utilized as the 
light source in the illumination optical system 1, if the wavelength of 
laser light having a narrow bandwidth is shifted, the same effect as when 
the atmospheric pressure is changed can be obtained. However, it is 
difficult to correct the aberrations of the projected image due to the 
change in atmospheric pressure by adopting this method using present 
technology. Also, as the wavelength of the laser light is changed, the 
absorbability of the photoresist on the wafer is also changed. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a projection 
exposure apparatus and method in which imaging characteristics of a 
projected image will not be deteriorated even though atmospheric pressure 
is changed. 
According to the present invention, in a projection exposure apparatus 
having an exposure processing section for forming an image of a pattern of 
a mask on a photosensitive substrate via a projection optical system, a 
chamber for covering the exposure processing section, and an 
air-conditioning mechanism for supplying a temperature-controlled gas to 
the chamber, the projection exposure apparatus performing exposure to the 
photosensitive substrate in a temperature-controlled atmosphere, there are 
a sensor for measuring the refractive index of the gas in the chamber; and 
refractive index control means for controlling the refractive index of the 
gas supplied to the chamber from the air-conditioning mechanism in 
accordance with the measurement result of the sensor. 
According to another embodiment of the present invention, in a projection 
exposure apparatus having an exposure processing section constituted of a 
light source for emitting light, an illumination optical system for 
illuminating a mask with the light and a projection optical system for 
projecting and exposing an image of a pattern of the mask on a 
photosensitive substrate, an imaging characteristic adjusting section for 
adjusting imaging characteristics of the projection optical system, a 
chamber for covering said exposure processing section and an 
air-conditioning mechanism for supplying a temperature-controlled gas to 
the chamber, there are a sensor for measuring a change in refractive index 
of the gas in said chamber; refractive index control means for controlling 
the refractive index of the gas supplied from the air-conditioning 
mechanism to the chamber in accordance with the measurement result of said 
sensor; and a control section for controlling the imaging characteristics 
adjusting section, when using the gas whose refractive index is 
controlled, so as to correct a change of the imaging characteristics of 
said projection optical system due to factors other than the change in 
refractive index of the gas. 
A projection exposure apparatus having an exposure processing section for 
forming an image of a pattern of a mask on a photosensitive substrate via 
a projection optical system and a chamber for covering said exposure 
processing section, comprising: 
means for correcting a change of imaging characteristics of said projection 
optical system due to a change in refractive index of a gas in said 
chamber by changing the refractive index of the gas; 
and 
means for correcting a change of imaging characteristics of said projection 
optical system due to factors other than the change in refractive index. 
According to still another embodiment of the present invention, in a 
projection exposure apparatus having an exposure processing section for 
forming an image of a pattern of a mask on a photosensitive substrate via 
a projection optical system and a chamber for covering the exposure 
processing section, there are means for correcting a change of imaging 
characteristics of the projection optical system due to a change in 
refractive index of a gas in the chamber by changing the refractive index 
of the gas; and means for correcting a change of imaging characteristics 
of the projection optical system due to factors other than the change in 
refractive index. 
According to the present invention, when the pressure of the gas in an 
environmental chamber (42) is changed due to the change in atmospheric 
pressure, the change in pressure is detected by pressure monitoring means 
(46) and the refractive index of the whole gas in the environmental 
chamber (42) containing the exposure processing section (44) including the 
projection optical system is changed by the refractive index control means 
(48, 26). Accordingly, the refractive index of the gas between the mask 
and the photosensitive substrate in the exposure processing section (44) 
is changed wholly to return to, e.g., the same value as the refractive 
index prior to the change in atmospheric pressure, whereby the same effect 
as when the atmospheric pressure is returned to the condition prior to its 
change can be obtained. 
That is, even though the atmospheric pressure is changed, the changes in 
aberrations of the projected image of the projection optical system can be 
limited completely to zero by changing the refractive index of the gas in 
the environmental maintaining chamber (42) so as to cancel the change in 
atmospheric pressure. More specifically, the relational expression of the 
refractive index n (P, t) of the air obtained when the atmospheric 
pressure is P (mmHg) and the temperature is t (.degree.C.) is expressed as 
follows. This is called Edlen's formula: 
##EQU1## 
As is apparent from this formula, when the atmospheric pressure P and the 
temperature t are changed by .DELTA.P and .DELTA.t respectively from the 
standard condition (atmospheric pressure: 760 mmHg, temperature: 
15.degree. C.), the refractive index n (P, t) is changed by a 
predetermined amount. Then, by changing the refractive index of the gas so 
as to cancel this change of the refractive index, it is possible to keep 
the imaging characteristics of the projected image preferable regardless 
of the change in atmospheric pressure (the error caused by the humidity is 
omitted). 
In this case, the refractive index of the gas in the environmental chamber 
(42) can be maintained to be a predetermined value more precisely by 
monitoring it actually with the refractive index monitoring means (49, 
51). 
For changing the refractive index of the gas, there is a method in which 
the gas is composed of a plurality of gases of different types and the 
mixture ratios thereof are changed. Besides this method, as known from the 
formula (1), the temperature or humidity of the gas may be changed to 
change the refractive index.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A projection exposure apparatus according to embodiment of the present 
invention will be described with respect to FIGS. 1 and 2. As the 
projection exposure apparatus of this embodiment, a conventional 
projection exposure apparatus shown in FIG. 3 is used. 
Referring to FIG. 1 showing an environmental chamber and an 
air-conditioning mechanism, a projection exposure apparatus 44 having the 
same structure as that in FIG. 3 is disposed in an environmental chamber 
42, and air cleaned and controlled in temperature is constantly supplied 
from the air-conditioning mechanism to the environmental chamber 42 such 
that the temperature in the environmental chamber 42 is kept constant. The 
air in the environmental chamber 42 is circulated such that the pressure 
thereof becomes slightly higher than that of outside air. In FIG. 1, the 
air in the environmental chamber 42 is shown as being in a closed loop, 
but many openings are formed in side walls of the environmental chamber 
42. However, as the pressure of the air in the environmental chamber 42 is 
slightly higher than that of the outside air, the inside air would escape 
through the openings but a quantity of air entering the environmental 
chamber 42 through the openings is negligible. 
In the air-conditioning mechanism of this embodiment, air whose pressure is 
slightly higher than outside air (atmospheric air) around the 
environmental chamber 42 is supplied from a first supply source 60 to a 
gas ratio adjusting chamber 23 via a pipe 22. In parallel with this 
operation, a gas (e.g., nitrogen, helium, etc.) having a different 
refractive index from that of air is supplied from a second supply source 
61 to the gas ratio adjusting chamber 23 via a pipe 21. An openable valve 
24 is provided at a connecting portion between the pipe 21 and the gas 
ratio adjusting chamber 23 while an openable valve 25 is provided at a 
connecting portion between the pipe 22 and the gas ratio adjusting chamber 
23. A gas ratio control unit 26 controls the opening and closing of the 
valves 24 and 25 to adjust the mixture ratio of gases in the gas ratio 
adjusting chamber 23, i.e., the environmental chamber 42 resultantly. The 
gases exhausted from the gas ratio adjusting chamber 23 go to a gas 
circulating chamber 27. A fan 28 for rotating blades 29 is provided in the 
gas circulating chamber 27. The operation of the fan 28 is controlled by 
an external adjusting unit 30 to keep the circulation velocity of the 
gases in the environmental chamber 42 constant. 
The gases blown by the fan 28 reach a temperature adjusting chamber 32 via 
a connecting pipe in which a temperature sensor 31 is provided. A 
detection signal from the temperature sensor 31 is supplied to a measuring 
unit 33, which then obtains the temperature of the gases from the supplied 
signal and supplies this temperature information to a control section of 
an air compressor 34. The temperature adjusting chamber 32 is provided 
with a cooling portion 35 and a heating portion 36. The air compressor 34 
controls temperatures of the cooling and heating portions and sets the 
temperature of the gases passing through the temperature adjusting chamber 
32 to a desired temperature. The temperature controlled gases flow in a 
first diffusing chamber 39 via a connecting pipe in which another 
temperature sensor 37 is provided. A detection signal from the temperature 
sensor 37 is sent to a measuring unit 38, which then obtains the 
temperature of the gases from the sent signal and sends this temperature 
information to the air compressor 37. The air compressor 34 controls the 
temperature of the gases in the temperature adjusting chamber 32 according 
to the feedback control of the temperature detected by the temperature 
sensor 37. 
Also, the gases are diffused in the first diffusing chamber 39 so as to 
prevent unevenness of a temperature distribution. The gases passed through 
the first diffusing chamber 39 flow in a second diffusing chamber 40 to 
let the gases blow toward the environmental chamber 42 uniformly. The 
gases from the second diffusing chamber 40 are blown into the 
environmental chamber 42 through a HEPA filter (High Efficiency 
Particulate Air Filter) for eliminating dust. Further, another pipe 43 is 
provided between the first diffusing chamber 39 and the projection 
exposure apparatus 44 in the environmental chamber 42. The gases passing 
through the pipe 43 are supplied to the pressure control unit 12 in FIG. 
3. In this embodiment, the pressure control unit 12 in FIG. 3 is not 
necessarily required but is provided preliminarily. 
The gases in the environmental chamber 42 are returned to the gas ratio 
adjusting chamber 23 via a heat insulating pipe 45 connected to the outlet 
of the environmental chamber 42. However, as described above, there are 
many openings in the side walls of the environmental chamber 42, so that 
all the gases supplied to the environmental chamber 42 are not returned to 
the gas ratio adjusting chamber 23. 
Also, in this embodiment, a first refractive index measuring unit 49 is 
disposed in the environmental chamber 42 at a position receiving the gases 
blown from the HEPA filter 41 and a second refractive measuring unit 51 is 
disposed in the vicinity of the outlet of environmental chamber 42 
connected to the pipe 45. Further, a pressure sensor 46 is provided in the 
vicinity of the projection exposure apparatus in the environmental chamber 
42. The refractive index measuring units 49, 51 have the same structure 
and generate signals varied in accordance with the refractive indexes of 
the gases in the vicinity of those units according to the heterodyne 
interference method. The respective signals are supplied to respective 
signal processing units 50 and 52. The respective signal processing units 
50 and 52 calculate refractive indexes based on the supplied signals and 
supply them to a control device 48. Similarly, the pressure sensor 46 
supplies a signal varied in accordance with the pressure of the gases to a 
signal processing unit 47, which then supplies the atmospheric pressure 
information in the vicinity of the projection exposure apparatus 44 to the 
control device 48. 
The control unit 48 controls the opening and closing of the valves 24, 25 
via the gas ratio control unit 36 in accordance with the pressure of the 
supplied gases to adjust the mixture ratio of the gases thereby to keep 
the refractive index of the gases in the environmental chamber 42 
constant. However, the control device 48 does not necessarily need to use 
the measurement result of the pressure sensor 46 and may control the 
opening and closing of the valves 24, 25 via the gas ratio control unit 26 
such that the average value of the refractive indexes measured by the 
refractive index measuring units 49, 51 becomes constant. Also, in this 
case, as the refractive index of the gases is changed even when not only 
the atmospheric pressure but also the temperature or humidity is changed, 
the pressure sensor 46 is used to ascertain the actual present refractive 
index. 
That is, the control unit 48 controls the gas ratio control unit 26 on the 
basis of the information from the pressure senior 46 and the refractive 
index measuring unit 49 (and 51). For example, where there is a difference 
between a temperature of a projection optical system (which is normally 
kept strictly constant) in a projection exposure apparatus and a 
temperature at a place on which the refractive index measuring unit 49 
(and 51) is mounted, the refractive index in the projection exposure 
apparatus may be actually shifted from a desired value by a degree 
corresponding to the temperature difference even if the refractive index 
is kept constant through control of the gas ratio control unit 26 based 
upon the information from the refractive index measuring unit 49 (and 51). 
In order to solve the problem, the control unit 48 obtains a refractive 
index in the projection exposure apparatus on the basis of the information 
from the pressure sensor 46, and thereafter obtains as an offset value, a 
difference between the refractive index from the refractive index 
measuring unit 49 (and 51) and the refractive index obtained through the 
information from the pressure sensor 46. Finally, the control unit 48 
controls the gas ratio control unit 26 by monitoring the refractive index 
from the refractive index measuring unit 49 (and 51), for adjusting a 
mixture ratio of gas, and obtaining an actual refractive index by 
correcting the offset. 
Next, in FIG. 2 showing the structure of the refractive index measuring 
unit 49, an interference optical system is disposed on a fixed base 55 
formed of zerodur having a small thermal expansion coefficient. A laser 
beam LB emitted from an external laser light source 53 is incident on a 
deflection beam splitter surface 54a of a prism 54 provided on the fixed 
base 55. The laser beam LB consists of laser beams LB1, LB2 having 
slightly different frequencies from each other. The respective laser beams 
LB1 and LB2 form the S and P deflections with respect to the deflection 
beam splitter surface 54a. The laser beam LB1 is reflected by the 
deflection beam splitter surface 54a and a deflection beam splitter 
surface 54b and is incident on a photoelectric detector 58 via an analyzer 
57. 
On the other hand, the laser beam LB2 transmitted through the deflection 
beam splitter surface 54a is reflected by reflecting surfaces 56a and 56b 
of a prism 56 disposed on the fixed base 55, and transmitted through a 
deflection beam splitter surface 54b of the prism 54 to be incident on the 
photoelectric detector 58 via the analyzer 57. A beat signal obtained by 
photoelectrically converting the coherent light of the laser beams LB1, 
LB2 is supplied to a signal processing unit 50, which then calculates the 
refractive index of the gases in the light path of the laser beam LB2 from 
the change in frequency of the beat signal. The other refractive index 
measuring unit 51 has the same structure. That is, the refractive index of 
the gases is measured by monitoring the difference of the optical path 
lengths of the laser beams LB1, LB2 according to the heterodyne 
interference method. 
According to this embodiment, the refractive index of the gases supplied to 
the environmental chamber 42 is changed such that the change in 
atmospheric pressure is canceled, so that changes in aberrations of the 
projected image in the projection exposure apparatus 44 can be limited to 
almost zero. Concerning this, as disclosed in Japanese Patent Application 
laid-Open No. 61-79228, there is a known system in which in order to 
change the refractive index of the air in a specific pressure room between 
two lens among a plurality of lenses, the mixture ratio of gas components 
of the air in the specific pressure room is changed. However, in this 
case, a sufficient diffusing system and a monitoring mechanism are 
required to mix the gas components of the air, which leads to the rise of 
cost. Also, all the aberrations of the projected image cannot be 
controlled to required levels only by changing the pressure in the 
specific pressure room to change the refractive index of a portion of the 
air in the projection optical system. On the other hand, in the system of 
this embodiment, the gas mixing system is introduced into a conventional 
mechanism for air-conditioning, so that the diffusing mechanism for air 
temperature control can be made to serve a double purpose. Therefore, it 
is possible to prohibit aberrations of the projected image without 
providing a new complex mechanism. 
Further, this gas mixing system may be used together with a conventional 
air pressure control mechanism in the projection optical system and a 
conventional correcting mechanism for correcting the telecentric 
characteristic of the projection optical system. For example, the gas 
mixing system for adjusting the refractive index of the gases in the 
environmental chamber 42 described in this embodiment is used to correct 
changes in imaging characteristics due to the change in atmospheric 
pressure while the pressure in the specific pressure room in the 
projection optical system is changed (disclosed in U.S. Pat. No. 
4,666,273) or a few lens elements in the projection optical system are 
driven (disclosed in U.S. Pat. No. 5,117,255) to correct changes in 
imaging characteristics due to the temperature rise in lenses caused by 
the illumination of exposure light, the switching of the illumination 
method in the illumination optical system (e.g., switching to a 
deformation light source method disclosed in Japanese Patent Application 
Laid-Open No. 4-225514. (U.S. Ser. No. 791,138, Nov. 13, 1991), i.e., the 
change in intensity distribution of light in the Fourier transform plane 
of the projection optical system with respect to the reticle pattern). 
Thus, by using the gas mixing system of this embodiment together with the 
conventional air pressure control mechanism or the like, changes in almost 
all aberrations can be prevented. Further, although two types of gases are 
mixed to keep the refractive index of the air constant thereby to control 
imaging characteristics in this embodiment, the refractive index of the 
air can be kept constant in spite of the change in atmospheric pressure by 
providing moistening and dehumidifying functions to the air-conditioning 
mechanism and thereby changing the humidity of the air or but changing the 
temperature of the air by means of the air-conditioning mechanism to such 
a degree that the system will not be adversely affected. An example of 
specific numerical values is shown below. 
Under the temperature t .degree.C.!, the pressure P mmHg! and the 
humidity R %!, the refractive index n is obtained by: 
##EQU2## 
Although this equation is derived from Edlen's formula, the change in 
refractive index n when the atmospheric pressure is varied from 720 to 770 
mmHg! is shown in the following table. (t=23.degree. C., R=40%) 
______________________________________ 
Pressure P mmHg! 
Refractive Index n 
______________________________________ 
720 1.0002544 
730 1.0002579 
740 1.0002615 
750 1.0002652 
760 1.0002686 
770 1.0002721 
______________________________________ 
On the other hand, the change in refractive index n when the humidity is 
varied from 20 to 80% is shown in the following table. 
______________________________________ 
Humidity R %! Refractive Index n 
______________________________________ 
20 1.0002688 
40 1.0002686 
80 1.0002681 
______________________________________ 
Thus, the refractive index can be adjusted slightly by changing the 
humidity, though it cannot correspond to the large change in atmospheric 
pressure. Next, refractive indexes of a plurality of gases under an above 
condition (t=23.degree. C., R=40%, P=760 mmHg) are shown. 
______________________________________ 
Gas Refractive Index n 
______________________________________ 
Air 1.0002686 
Oxygen 1.0002486 
Nitrogen 1.0002736 
Carbon Dioxide 1.0004266 
Helium 1.0000116 
______________________________________ 
Here, it is considered that air is mixed with carbon dioxide and helium 
gas, which have very different refractive indexes. When the atmospheric 
pressure is varied from 720 to 770 mmHg!, the mixture ratio of the carbon 
dioxide and helium gas for canceling the change in refractive index due to 
the change atmospheric pressure is shown below. 
______________________________________ 
Air Mixture Ratios %! 
Carbon Refractive 
PressuremmHg! 
Air Dioxide Helium 
Index 
______________________________________ 
720 91.051 8.949 0.0 1.0002686 
730 93.288 6.712 0.0 1.0002686 
740 95.525 4.475 0.0 1.0002686 
750 97.763 2.237 0.0 1.0002686 
760 100.000 0.0 0.0 1.0002686 
770 98.624 0.0 1.376 1.0002686 
______________________________________ 
(the carbon dioxide contained in the air is excluded from the mixture 
ratio) 
As above, when the mixture ratios of the gases in the chamber are changed 
to cancel the change in refractive index due to the change in atmospheric 
pressure, the apparent refractive index does not change, so that 
aberrations in the projection optical system will not be deteriorated due 
to the change in atmospheric pressure. 
Although carbon dioxide and helium gases are used here, if lenses are 
designed not in accordance with the refractive index of air but in 
accordance with the gases assumed to contain 2% helium, the same effect 
can be obtained by mixing carbon dioxide into air. In this case, with 
respect to the mixture ratio of ordinary air of nitrogen:oxygen=8:2, the 
mixture ratios of gases are nitrogen:oxygen:carbon dioxide 
.congruent.7.2:1.8:1.0 and the ratio of oxygen is not so different, so 
there is no danger. However, since Edlen's formula pertains to ordinary 
air, slight correction is necessary when changing the mixture ratio of 
air. 
According to the above embodiment, even though the pressure of the gases in 
the environmental chamber is changed, the refractive index of the gases is 
kept constant. Therefore, imaging characteristics of the projected image 
will not be deteriorated. 
Also, when refractive index monitoring means is provided and the condition 
of a predetermined gas is changed by refractive index control means in 
accordance with the measurement results of the refractive index monitoring 
means and pressure monitoring means, the change in refractive index of the 
gas can be reduced by the feedback control. 
Also, when the refractive index control means controls the refractive index 
of the predetermined gas by changing mixture ratios of a plurality of 
gases of different types, characteristics of temperature and humidity are 
kept constant. 
It will be understood that the present invention is not limited to the 
above-described embodiment and various structures can be adopted without 
departing from the spirit of the present invention.