Patent Publication Number: US-6710846-B2

Title: Environmental control apparatus for exposure apparatus

Description:
This application is a continuation of prior application Ser. No. 09/597,991, filed Jun. 19, 2000; which is a continuation of prior application Ser. No. 08/825,398 filed Mar. 28, 1997, now U.S. Pat. No. 6,208,406. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an environmental control apparatus for controlling a working environment in an exposure apparatus. More particularly, the present invention is concerned with an environmental control apparatus for an exposure apparatus for effecting exposure using exposure light having a wavelength range in which oxygen absorbs the exposure light. 
     In recent years, it has been desired to produce highly integrated semiconductor circuits. In order to produce such semiconductor circuits, exposure apparatuses have been widely used in which a circuit pattern formed on a reticle as a mask is transferred to a photosensitive substrate. Hence, there have been increasing demands for an exposure apparatus which is capable of forming an image of high resolution. As a technique for producing highly integrated semiconductor circuits, there has been known a technique which utilizes, as exposure light, light having a short wavelength, such as a g-line or i-line light emitted from a mercury lamp, KrF excimer laser light and ArF excimer laser light. Especially, as a technique for producing a DRAM (Dynamic Random Access Memory) of 1G, a technique which utilizes ArF excimer laser light having a wavelength as short as 193 nm has been proposed. 
     However, when ArF excimer laser light having a wavelength of 193 nm is used as exposure light for producing highly integrated semiconductor circuits, problems arise, such that the amount of exposure light which reaches the photosensitive substrate decreases and, therefore, becomes unstable during exposure. The causes of the above-mentioned problems are as follows: 
     (1) Light having a short wavelength, such as ArF excimer laser light of 193 nm, is absorbed by oxygen in an atmosphere. That is, ArF excimer laser light has a spectral range which includes a specific wavelength range in which oxygen absorbs the ArF excimer laser light. Therefore, in an exposure apparatus in which ArF excimer laser light is used as exposure light, when exposure is effected in a normal atmosphere comprising air having an oxygen content of about 20%, ArF excimer laser light is absorbed by oxygen during exposure. Therefore, the amount of exposure light which reaches the photosensitive substrate decreases during exposure. 
     (2) When oxygen absorbs ArF excimer laser light, ozone is produced. Since ozone also absorbs ArF excimer laser light, the amount of exposure light which reaches the photosensitive substrate progressively decreases and therefore becomes unstable during exposure, leading to difficulty in monitoring the amount of exposure light for which highly precise measurements are required. 
     Accordingly, it is presumed that the above-mentioned problems accompanying a technique which utilizes exposure light having a short wavelength can be obviated by removing oxygen in an area including a light path in an exposure apparatus in a manner such that the area is purged with an inert gas and sealed, or an inert gas is continuously supplied to the area for removing oxygen. 
     However, when the removal of oxygen in an area including a light path is conducted in such a manner as mentioned above, there are various disadvantages as follows: 
     (1) When an area including a light path is completely sealed, it is difficult to load and unload a mask and a photosensitive substrate in the light path. 
     (2) When an area including a light path is adapted to be sealable using a gate for loading and unloading a mask and a substrate, which is provided in a carrying means for the mask and substrate, it is difficult to periodically perform maintenance for a substrate stage. 
     (3) A sealing structure which has a large internal volume and is very complicated is required, due to a large degree of motion of a substrate stage. 
     (4) It is difficult to form a sealing structure having a sufficient pressure resistance against a difference in pressure between the interior and exterior thereof. 
     (5) When light is transmitted through a sealing structure having a large internal volume, the temperature distribution in the sealing structure becomes non-uniform, so that it is difficult to control the temperature of the sealing structure. 
     SUMMARY OF THE INVENTION 
     In view of the above circumstances, a primary object of the present invention is to provide an environmental control apparatus for use in an exposure apparatus for effecting exposure using exposure light having a wavelength range in which oxygen absorbs the exposure light, which has a simple construction and which suppresses a decrease in the amount of exposure light, so that the amount of exposure light can be stabilized during exposure. 
     According to the present invention, there is provided an environmental control apparatus for controlling a working environment in an exposure apparatus for effecting exposure using exposure light having a wavelength range in which oxygen absorbs the exposure light, which comprises an ozone removing filter for removing ozone in the air supplied to the exposure apparatus. 
     According to one preferred embodiment of the present invention, the environmental control apparatus further comprises an air conditioning system having an air circulation passage for supplying air having a controlled temperature to the exposure apparatus and the ozone removing filter is disposed in the air circulation passage of the air conditioning system. 
     According to another preferred embodiment of the present invention, the environmental control apparatus further comprises ozone concentration detection sensors which are disposed upstream and downstream of the ozone removing filter in a flowing direction of the air supplied to the exposure apparatus, and a device for detecting a degree of clogging of the ozone removing filter on the basis of outputs from the ozone concentration detection sensors. 
     When the environmental control apparatus of the present invention which comprises an ozone removing filter is used for an exposure apparatus for effecting exposure using exposure light having a wavelength range in which oxygen absorbs the exposure light, undesirable ozone which is produced from oxygen and absorbs the exposure light can be removed by the ozone removing filter. Therefore, the environmental control apparatus of the present invention is advantageous in that a decrease in the amount of exposure light can be suppressed and, therefore, the amount of exposure light can be stabilized during exposure. By use of the environmental control apparatus of the present invention, it has become unnecessary to form in the exposure apparatus a sealing structure for sealing an area including a light path. Therefore, the above-mentioned disadvantages accompanying a technique which utilizes a sealing structure (such as difficulties in loading and unloading a mask and a photosensitive substrate, performing maintenance, controlling a temperature of the sealing structure and the like) can be obviated. Although oxygen is relatively stable in a normal atmosphere and difficult to remove using a filter, ozone created by exposure light is relatively unstable in the same atmosphere and is highly reactive, so that it can be readily removed using a filter. 
     Since ozone has an adverse effect on a photoresist on the photosensitive substrate, the environmental control apparatus of the present invention is also advantageous in that an adverse effect on the photoresist due to ozone can be suppressed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic illustration showing a system comprising an exposure apparatus which utilizes a preferred embodiment of the environmental control apparatus according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinbelow, an embodiment of the present invention is described in detail, with reference to FIG.  1 . In this embodiment, the environmental control apparatus of the present invention is employed for a projection exposure apparatus for producing semiconductor integrated circuits. 
     FIG. 1 is a schematic illustration showing a projection exposure apparatus system which utilizes a preferred embodiment of the environmental control apparatus according to the present invention. The system generally comprises a projection exposure apparatus  10 , an air conditioning system  12  having an air circulation passage for controlling a working environment in the projection exposure apparatus  10  and a control system  14 . The projection exposure apparatus  10  comprises a light source  16  for emitting exposure light, such as ArF excimer laser light; a reticle  22  having a predetermined pattern formed thereon (not shown); an illumination optical system  18  for transmitting the exposure light emitted from the light source  16  to the reticle  22 ; a mirror  20  for guiding the transmitted exposure light from the optical system  18  to the reticle  22 ; a reticle stage  24  for receiving and shifting the reticle  22 ; a projection optical system  26  for projecting to a wafer  28  an image of the predetermined pattern (not shown) formed on the reticle  22 ; a wafer stage  30  for receiving and shifting the wafer  28 ; and a wafer loader  32  for loading the wafer  28  onto the wafer stage  30 . The light source  16  is not limited to that for emitting ArF excimer laser. Any light sources can be employed, as long as they emit exposure light having a wavelength range in which oxygen absorbs the exposure light. 
     As shown in FIG. 1, in the projection exposure apparatus  10 , ArF excimer laser light as exposure light is emitted from the light source  16  and controlled by the illumination optical system  18  so as to have a predetermined illuminance range and a predetermined illuminance distribution. The resultant exposure light is transmitted to the reticle  22  having a predetermined pattern. The exposure light passes through the reticle  22  and reaches the wafer  28  on the wafer stage  30  through the projection optical system  26 , to thereby form an image of the predetermined pattern of the reticle  22  on the wafer  28 . The wafer  28  is shifted in the X and Y directions by the wafer stage  30 . (The left and right direction in FIG. 1 is defined as the “X” direction, and the vertical direction relative to the plane of FIG. 1 is defined as the “Y” direction.) By the motion of the wafer  28 , an image of the predetermined pattern of the reticle  22  can be transferred to a predetermined portion of the wafer  28 . After completion of exposure of the wafer  28 , the exposed wafer  28  is removed for replacement from the wafer stage  30  by the wafer loader  30 . 
     The air conditioning system  12  is adapted to effect air conditioning around the wafer stage  30  in the projection exposure apparatus  10 . The air circulation passage of the air conditioning system  12  is substantially blocked from the outside. In the air conditioning system  12 , air is introduced from an air supply opening  36 . After controlling the flow rate of air by an air damper  38 , the air flows through a chemical filter  40 , which removes impurities in the air which have an adverse effect on exposure. Subsequently, the air passes through a cooler  42  and a heater  44 . After controlling the temperature of air by the cooler  42  and the heater  44 , and precisely controlling the flow rate of air to a predetermined value by a fan  46  and a damper  48 , the air then flows through a chemical filter  50 , which again removes impurities in the air. Then, the air flows through a ULPA filter  52 , which removes fine particles in the air, and passes through a wafer stage chamber  53  including the wafer stage  30  on which the wafer  28  rests. 
     Subsequently, the air flows through a ULPA filter  54  and a chemical filter  56 , and reaches an ozone removing filter  58  which is disposed downstream of the wafer stage chamber  53  as viewed in the flowing direction of the air. Ozone is produced during exposure in the wafer stage chamber  53 . The ozone is removed by the ozone removing filter  58 . The ozone removing filter  58  comprises materials, typically, such as active carbon, ion-exchange resins, and so on. However, various materials can be employed as long as they adsorb the ozone. The ULPA filter  54  which is disposed upstream of the ozone removing filter  58  is advantageously used in the case of the ozone removing filter  58  being made of a material which releases fine particles. The chemical filter  56  which is also disposed upstream of the ozone removing filter  58  is advantageously used in the case of the ozone removing filter  58  being made of a material which releases a chemical substance having an adverse effect on exposure. The flow rate of air, after it flows through the ozone removing filter  58 , is controlled by a fan  60  and a damper  62 . The air is then returned to the cooler  42 , so that recirculation of air is effected. 
     Ozone concentration detection sensors  64 ,  66  are, respectively, disposed upstream and downstream of the ozone removing filter  58  to detect ozone concentrations in the air before and after the air flows through the ozone removing filter  58 . The ozone concentration detection sensors  64 ,  66  are connected to a control apparatus  68  in the control system  14 . The control apparatus  68  detects a degree of clogging of the ozone removing filter  58  by making a comparison between outputs from the ozone concentration detection sensors  64 ,  66 , to thereby determine a timing of replacement of the ozone removing filter  58 . The control apparatus  68  is connected to a monitor  70 . Information supplied from the control apparatus  68 , such as a timing of replacement of the ozone removing filter  58 , is indicated by the monitor  70 . The control apparatus  68  also detects an amount of exposure light, and controls an output of the light source  16 , based on the detected amount of exposure light. 
     According to the above-mentioned embodiment of the present invention, the ozone which absorbs the exposure light can be removed by the ozone removing filter  58  which is disposed in the air circulation passage of the air conditioning system. Therefore, a decrease in the amount of exposure light can be effectively suppressed, so that the amount of exposure light can be stabilized during exposure and hence, the amount of exposure light can be precisely detected by the control apparatus  68 . Further, an adverse effect on a photoresist on the wafer  28  due to the ozone can be suppressed. 
     The present invention has been described above, with reference to one embodiment thereof. However, the present invention is not limited to the above-mentioned embodiment. Various embodiments are possible without departing from the scope of the present invention as defined in the appended claims. For example, the environmental control apparatus of the present invention can be applied to not only the air circulation system between the projection optical system  26  and the wafer  28  as shown in FIG. 1, but also an air circulation system between the illumination optical system  18  and the reticle  22 , or between the reticle  22  and the projection optical system  26 . Ozone removing filters may be disposed in the illumination optical system  18  at a site in the vicinity of a mobile reticle blind (not shown) disposed in a conjugate relationship to the reticle  22  and at a site in the vicinity of a revolver (not shown) for controlling a stop according to a σ value of the illumination optical system  18 . 
     The environmental control apparatus of the present invention is advantageously used when laser light having a relatively broad spectral range is utilized as exposure light. This is because when a light source which emits light having a very narrow spectral range is used as the light source  16 , it is possible to select and utilize, as exposure light, light having a wavelength which is outside the wavelength ranges in which oxygen and ozone absorb the exposure light. However, the environmental control apparatus of the present invention may be used when exposure light having a narrow spectral range is utilized. Further, the environmental control apparatus of the present invention is advantageously used for an exposure apparatus in which higher harmonics from a solid laser or light from a discharge lamp is utilized as exposure light. Additional filters may be used for removing a gas other than ozone, if exposure light used has a wavelength range in which the gas absorbs the exposure light.