1.  Field of the Invention
The present invention relates to an exposure apparatus for making semiconductor circuits or the like by photo-lithography, and in particular to an exposure apparatus suitable for exposure of photosensitive substrates coated with chemically amplified resist. 
2.  Description of the Related Art
A projection exposure apparatus, which is called a stepper, is used in a photo-lithographic process for making semiconductor circuits, liquid crystal substrates, or the like.  Because the apparatus needs to control its temperature with a high precision, it has an air-conditioner for this purpose.  Because the air-conditioner needs to control the temperature within a very strict range, which may be .+-.0.1 centigrade, with respect to a set temperature, the air-conditioning system is constructed as a circulatory system.  The air-conditioner needs a fan for air circulation.  To avoid vibration and other problems, the air-conditioner including a fan needs to be independent of the main body including a chamber, which houses an exposure unit.  FIG.  12 is a schematic top plan of a conventional exposure apparatus with an air-conditioner 1 independent of a main body 2.  The air-conditioning system in this type of apparatus is described below with reference to FIG.  12. 
The main body 2 has a chamber 3 housing an exposure unit, which includes a reticle R and a projection lens PL.  Because an apparatus for making semiconductors needs to keep clean, there is a need for the pressure in the chamber 3 to be always positive relatively to that outside the chamber.  The pressure difference between the inside and outside of the chamber 3 makes air in the chamber leak out.  The leakage needs compensating by supplying air from the outside.  Formed for this purpose is an outside air (OA) inlet 6 for natural intake through it to normally take in outside air. 
The air-conditioner 1 includes a cooler 7 having radiation fins (not shown), a heater 8 and a fan 9.  The air returning from the chamber 3 enters the air-conditioner 1 together with the air supplied from the outside.  The air having entered the air-conditioner 1 is first cooled by the cooler 7.  On the radiation fins condenses the superfluous moisture brought from the outside by taking in outside air.  The condensate is removed as stated later.  The cooled air is heated to a desired temperature by the heater 8.  The heated air is sent to the main body 2 by the fan 9.  A temperature sensor 11 is fitted downstream from the fan 9.  The values detected by the sensor 11 are sent to a controller (not shown).  Monitoring the output from the sensor 11, the controller controls the cooler 7 and heater 8 so that the difference between the output value and a target temperature is zero.  Thus, the temperature is adjusted by feedback control. 
The main body 2 also includes a HEPA (high efficiency particle air) box 13 having a HEPA filter 10.  To maintain the cleanliness of the air in the chamber 3, the air having passed through the air-conditioner 1 is cleaned by the filter 10 before supplied to the chamber 3. 
Positioned below the cooler 7 is a drain pan 4, to which a drain pipe 5 is connected.  The moisture condensed by the cooler 7 is collected by the pan 4.  The collected water is drained through the pipe 5 from the apparatus. 
Recently, it has been found that a very small amount of gas in a clean room atmosphere affects the reduction projection exposure apparatus or other apparatus for making semiconductors in the atmosphere.  This is explained below more specifically. 
Sometimes insufficient is the luminance of the light sources of excimer laser exposure apparatus with an excimer laser used as the light source, X-ray exposure apparatus, electron beam exposure apparatus, etc.  To cope with the insufficiency by virtue of the high sensitivity of resist, chemically amplified sensitive resist is used.  This resist contains an acid-generating agent as the photosensitive agent in it.  The acid generated by exposing the resist induces catalytic reaction in the succeeding heat treatment (PEB).  This accelerates the insolubilization (negative type) or the solubilization (positive type) in developer of the resist.  For example, if a chemically amplified resist is positive, a very small amount of basic gas of a ppb level in the atmosphere neutralizes the acid catalyst generated on the resist surface, and forms a layer which makes the surface slightly soluble.  After development by exposure, the resist cross section, which should otherwise be rectangular, has T-shaped eaves.  Because a chemically amplified resist, which is a sensitive resist, cannot be used as it is, it may need overcoating, so that the throughput lowers. 
As the wavelength of exposure light has been shorter and the illuminance has been higher, such a problem has been occurring that a very small amount of gas in the atmosphere deposits or precipitates as a clouding substance on the surfaces of illumination system members.  This is due to the photochemical interaction between the exposure light and the small amount of gas in the atmosphere.  The reactant may be ammonia gas or sulfur oxide in the air, an organosilicic compound, or the like.  The clouding of illumination system members results in remarkably low illuminance, so that the throughput is reduced. 
U.S. Pat.  No.  5,430,303 discloses a chemical filter for removing chemical substances, as means for removing a very small amount of gaseous impurities in a clean room atmosphere. 
Because a very small amount of such gas exists originally in a clean room, it is desired to remove the gas by some means before the gas enters an exposure apparatus.  The removal is difficult, however, due to the structure of the air-conditioner of the apparatus.  Specifically, the air-conditioner is open to the outside through the drain piping, which is essential for temperature control.  In addition, the heater and cooler of a typical exposure apparatus are positioned immediately upstream from the fan because of the apparatus structure.  Because of the fan characteristics, the pressure in the drain pan of the apparatus is negative (about -30 mmAq.)  relatively to the atmospheric pressure.  As a result, the following disadvantages arise. 
1.  The drain piping is connected finally to other devices in the semiconductor manufacturing factory where the apparatus is installed.  The pressure in the drain pan is negative relatively to the atmospheric pressure.  When no water stays in the pan, gaseous impurities from a manufacturing device in the factory may flow through the piping into the apparatus. 
2.  Chemical substances are produced in the atmospheres of other manufacturing devices by chemical processes of alkaline, acid or other treatment in the devices.  Very small amounts of the substances may dissolve in the air-conditioner drainage of the devices.  Such substances of a ppb level may probably flow through the drain piping into the exposure apparatus.  As far as piping systems are not drains for drainage containing highly concentrated chemical substances, chemical contamination of apparatus has been given no particular consideration in many of them.  The air-conditioner drain piping system of an exposure apparatus may be connected to a clean room atmosphere outside the apparatus. 
3.  Depending on apparatus installation conditions, condensate from the cooler of an exposure apparatus may always stay in the drain pan.  In such a case, although no outside air directly enters the apparatus through factory piping, impurities in the outside air may indirectly enter through the staying condensate.  A problem arises any way. 
4.  The condensate staying in the drain pan does not flow out for a long time due to the negative pressure.  Therefore, a very small amount of gas in the circulating air dissolves in the staying condensate.  After the solution is concentrated, microorganisms etc.  may propagate in it.  Thus, the condensate may be a secondary source of gas affecting the exposure apparatus, even if the influence of factory piping is small.