Abstract:
An exposure apparatus for transferring a pattern of a mask onto a substrate. The apparatus includes a door and a sensor for detecting ozone in a space enclosed by the door. Opening of the door is prohibited when ozone concentration detected by the sensor is not less than a predetermined value.

Description:
This application is a divisional application of U.S. patent application Ser. No. 09/274,112, filed Mar. 23, 1999, now U.S. Pat. No. 6,590,631. 
    
    
     FIELD OF THE INVENTION AND RELATED ART 
     This invention relates to an exposure apparatus to be used with an exposure beam of ultraviolet rays, such as an excimer laser, for example, and also to a device manufacturing method using such an exposure apparatus. 
     In projection exposure apparatuses for the manufacture of semiconductor integrated circuits, light of various wavelength regions is projected as an exposure beam to a substrate. Examples of such an exposure beam used are e-line (wavelength λ=546 nm), g-line (λ=436 nm), h-line (λ=405 nm), i-line (λ=365 nm), KrF excimer laser (λ=248 nm), ArF excimer laser (λ=193 nm) and X-rays. 
     An exposure beam emitted from a light source goes through an illumination optical system for illuminating a mask or reticle, and a projection optical system (projection lens) for imaging a fine pattern, formed on the mask, whereby the fine pattern is lithographically transferred to a photosensitive substrate. In such exposure apparatuses, the linewidth of a pattern becomes smaller and smaller, and this has forced further improvement of throughput and resolution. Also, this requires a large power exposure beam. Simultaneously, the bandwidth of a wavelength of the exposure beam should be narrowed. 
     It is known that, when an exposure beam of i-line or a wavelength shorter than it is used, due to the band-narrowing, impurities in the air photochemically react with oxygen to cause deposition of compositions (blurring material) produced by the reaction upon optical elements (lenses or mirrors) of the optical system. This produces non-transparent “blur”. 
     A representative example of such a blurring material is ammonium sulfate (NH 4 ) 2 SO 4 , resulting from that, when sulfurous acid, for example, absorbs energies of light and is excited thereby, it reacts with oxygen in the air (i.e., oxidized). The ammonium sulfate is colored white so that, when it is deposited on an optical member such as a lens or mirror, it causes “blur”. Then, the exposure beam is scattered or absorbed by ammonium sulfate and, as a result, the transmission factor of the optical system decreases. This causes a reduction of light quantity (transmission factor) reaching the photosensitive substrate, and a decrease of throughput. 
     As a solution for this problem, Japanese Laid-Open Patent Application, Laid-Open No. 216000/1994 shows an apparatus wherein a barrel comprises a casing of a closed structure having glass members such as lenses accommodated therein, and wherein the inside of the casing is filled with an inert gas. 
     As regards an illumination optical system for illuminating a mask with a laser light source and a projection optical system for projecting the pattern of the mask in a reduced scale, enclosing optical components such as lenses by a tightly closed container and purging the inside of the container with an inert gas, may be accomplished relatively easily. However, as regards the mask and the substrate, particularly, the substrate has to be changed frequently. From the standpoint of throughput, therefore, it is practically difficult to place the substrate in a space purged by an inert gas. The space around the substrate inevitably contains air. When oxygen (O 2 ) in the air absorbs the exposure beam, ozone (O 3 ) is produced. 
     Generally, a temperature control system for the major assembly of the apparatus is structured to circulate a portion of the air conditioning gas, not to discharge all the gas outside the apparatus. Therefore, ozone successively produced in response to exposures remain in the temperature control system, and the ozone density in the apparatus gradually increases to some extent. As a result of it, the surfaces of components of the apparatus are corroded to damage the function. Further, a high ozone density environment is not good for operators. 
     In deep ultraviolet rays such as an excimer laser of having a wavelength of 250 nm or shorter than this, particularly, an ArF excimer laser having a wavelength of about 193 nm, there are plural oxygen (O 2 ) absorbing zones in the bandwidth near the wavelength described above. Thus, in response to absorption of light by oxygen, ozone is generated. The above-described problem is, therefore, quite notable in the wavelength range of 250 nm or less. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an exposure apparatus of high durability by which the problem of deterioration of components by the production of ozone, which is quite notable in the wavelength region of 250 nm, is overcome. 
     It is another object of the present invention to provide a device manufacturing method which is based on such an exposure apparatus. 
    
    
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a general structure of a projection exposure apparatus according to an embodiment of the present invention. 
     FIG. 2 is a schematic view of a modified form of the FIG. 1 embodiment. 
     FIG. 3 is a flow chart of semiconductor device manufacturing processes. 
     FIG. 4 is a flow chart of a substrate process. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in conjunction with the drawings. 
     FIG. 1 is a schematic view of a general structure of a projection exposure apparatus according to an embodiment of the present invention. Laser device  1 , which provides a light source for the exposure apparatus, is disposed separately from the exposure apparatus. The laser device comprises an excimer laser device for producing deep ultraviolet light of a wavelength region not longer than 250 nm. In this embodiment, it uses an ArF excimer laser having an emission wavelength of about 193 nm. However, in place of it, a KrF excimer laser having an emission wavelength of about 248 nm or, alternatively, EUV or soft X-rays of a much shorter wavelength may be used. 
     The laser beam emitted from the laser device  1  is introduced via mirrors  2  and  3  into the exposure apparatus major assembly. The portion around the path, including the mirrors  2  and  3 , is isolated by a light isolating tube  4  of a closed structure, from the outside gas. The introduced laser beam is directed to an illumination optical system  5  so that, through accommodated mirrors and optical elements (not shown), a predetermined region on a mask  7  (or reticle) is illuminated uniformly. A final lens  6  is disposed at the exit of the illumination optical system. The path from the emission point of the laser device  1  to the final lens  6  is formed with an integral closed structure by which communication with outside gas is intercepted. The inside of the structure is filled with an inert gas, to be described later. 
     The mask  7  is placed on a mask holder  8  and it is positioned by a mask stage. The mask  7  and the mask holder  8  are enclosed by a sealed casing  9 . Also, a mask changing means  11  is encircled by a sealed casing  12 . There is a mask library  13  for accommodating plural masks therein. Each mask can be taken out of the mask library by the mask changing means  11 , and it is conveyed to an exposure station by the mask holder  8 . Two sealed casings  9  and  12  are separated by an opening/closing means  10  of high gas-tightness, having a door. Similarly, there is another opening/closing means  14  between the sealed casing  12  and the library  13 . 
     The projection optical system includes lens groups  15 , and it functions to project a circuit pattern of the mask  7  onto a substrate (e.g., a semiconductor wafer)  17  coated with a photosensitive material, at a predetermined magnification. The lens groups  15  of the projection optical system are accommodated in a sealed casing  16 , being filled with an inert gas and being gas-tightly sealed. 
     The substrate  17  is held by a chuck  18  by vacuum attraction. The chuck  18  is mounted on a substrate stage  19  for positioning the chuck. There are a laser interferometer  20  and an alignment optical system  21 , for measuring the position of the substrate  17  held by the chuck  18 . Replacement of the substrate  17  is performed by substrate changing means  22 . It functions to change the substrate upon the chuck  18  with one of the substrates in a substrate container  23  for accommodating plural substrates therein. 
     For replacement of the substrate container  23  from the outside, a door  42  is provided at the exposure chamber side of the exposure apparatus major assembly. The door  42  is openable and closeable for the replacement. However, there is an interlocking mechanism  43  for holding the door unopenable during the exposure, for safety. Similarly, there is a door  44  for replacement of the masks in the library. There also is an interlocking mechanism  45  for holding the door  44  unopenable during the exposure. 
     Inert gas supplying means  24  supplies an inert gas, which is distributed along four channels. These channels will be described below. The first channel extends along a piping system  26 , and the gas is introduced into the light isolating tube  4  in the vicinity of the exit of the laser device  1 . The gas flows along the tube  4  and the light path of the illumination system  5  and, from the vicinity of the final lens  6  and through a piping system  27 , it is discharged toward gas discharging means  25 . The second channel extends along a piping system  28 , and the gas is introduced into the sealed casing  9  of the mask. Through a piping system  29 , the gas is discharged toward the gas discharging means  25 . The third channel extends along a piping system  30 , and the gas is supplied to one end of the sealed casing  16 . The gas flows through the lens groups  15  of the inside projection optical system and then, through a piping system connected to the other end, the gas is discharged toward the gas discharging means  25 . The fourth channel extends along a piping system  32 , and the gas is supplied into the sealed casing  12 . Via a piping system  33 , the gas is discharged toward the gas discharging means  25 . 
     Next, a description will be made of an air conditioning system of the exposure apparatus major assembly, which is an important features of this embodiment. The exposure apparatus major assembly is provided with an outside gas inlet port  34  for introducing the outside gas. Also, at the inlet from the exposure chamber to the air conditioning chamber, there is an ozone converter for converting ozone (O 3 ) into oxygen (O 2 ). The ozone converter  35  may comprise activated charcoal, for example, for converting ozone into oxygen in accordance with the conversion principle based on chemical reaction, to thereby remove ozone. The converting capacity of this ozone converter  35  is variable, to be described later, and the removing capacity can be increased when the ozone density becomes higher. 
     The flow of conditioning gas is created by operation of a fan  39 . It operates to produce circulation of the gas to be introduced from the exposure chamber via the ozone converting means  35 , and suction of gas from the outside gas inlet port  34 . A freezer  37  and a heater  38  are controlled by control means  36  in accordance with a temperature measured by a temperature monitor  41 , so as to maintain, constant, the temperature of the gas to be discharged into the exposure chamber through a filter  40 , for filtering fine particles or chemical substances. 
     There are ozone density sensors  47 ,  48  and  49  disposed at three locations inside the exposure chamber, for measuring the ozone density at respective positions. On the basis of the measurement results at the ozone density sensors  47 - 49  and in accordance with the exposure sequence, the control means  46  controls opening/closing of the interlocking mechanisms  43  and  45  for the two doors. Also, connected to the control means  46  is an ozone density display device  50  for continuously displaying the inside ozone density of the apparatus, or it operates to display the information to the operator as required. The control means  46  further functions to control and to increase the conversion capacity of the ozone converter  35  if at least one of the ozone density sensors  47 - 49  detects an increase of ozone density. 
     This embodiment provides a safety mechanism which operates so that, only when the exposure operation is not performed and when the ozone density is lower than a predetermined level, the interlocking mechanisms  43  and  45  are released to allow opening of the doors  42  and  44 . As a result, in the course of the sequence or when the ozone density is higher than the predetermined level, the interlocking action functions to prohibit access to the inside of the apparatus for enhanced safety. Since ozone is produced only during the exposure operation, the ozone density decreases gradually through the circulation system of the air conditioning system. When the ozone density is lowered to a predetermined level, the interlocking mechanisms are released, and the access to the inside of the apparatus is allowed. 
     The ozone, which is produced during the exposure operation and, particularly, from the light path adjacent to the substrate  17 , or the ozone emitted when the opening/closing means  14  is placed in a communication state, is converted by the converting means  35  into oxygen, whereby the ozone is removed. Therefore, there is no possibility of ozone remaining and staying in the gas circulation path inside the apparatus. 
     A modified form of this embodiment will now be described with reference to FIG.  2 . Like numerals are assigned to corresponding elements, and a detailed description thereof will be omitted. The distinction over the preceding embodiment resides in that the ozone converter  35  is disposed just before the freezer  37 . As a result of this, gasses from both an outside gas inlet port  34  and an air conditioning gas returning port  51  pass through the ozone converter  35 , whereby ozone is removed. Since there is a possibility that the ambience gas in a clean room of a semiconductor manufacturing factory where an exposure apparatus is placed contains a small amount of ozone, this structure is effective to remove ozone, as much as possible, in the gas to be introduced and circulated. 
     As regards the position of the ozone converter  35 , in place of those described in the preceding examples, it may be disposed at a described position inside the gas circulation system of the air conditioning system. Further, the number of the ozone converter  35  is not limited to one, and plural converters may be disposed at different locations. 
     As described above, the ozone produced in response to projection of an exposure beam is converted into oxygen whereby ozone inside the exposure apparatus is removed. Thus, deterioration of components used in the apparatus is prevented, such that an exposure apparatus of high durability is accomplished. Further, the interlocking mechanism operates in response to the detection of ozone or the information is displayed to the operator. This ensures high safety to the operator. 
     Next, an embodiment of a semiconductor device manufacturing method, which uses a projection exposure apparatus according to any one of the preceding embodiments, will be explained. 
     FIG. 3 is a flow chart of a procedure for the manufacture of microdevices such as semiconductor chips (e.g., ICs or LSIs), liquid crystal panels, CCDs, thin film magnetic heads or micro-machines, for example. 
     Step  1  is a design process for designing a circuit of a semiconductor device. Step  2  is a process for making a mask on the basis of the circuit pattern design. Step  3  is a process for preparing a wafer by using a material such as silicon. Step  4  is a wafer process, which is called a pre-process, wherein, by using the so prepared mask and wafer, circuits are practically formed on the wafer through lithography. Step  5  subsequent to this is an assembling step, which is called a post-process, wherein the wafer having been processed by step  4  is formed into semiconductor chips. This step includes an assembling (dicing and bonding) process and a packaging (chip sealing) process. Step  6  is an inspection step wherein an operation check, a durability check and so on for the semiconductor devices provided by step  5 , are carried out. With these processes, semiconductor devices are completed and they are shipped (step  7 ). 
     FIG. 4 is a flow chart showing details of the wafer process. 
     Step  11  is an oxidation process for oxidizing the surface of a wafer. Step  12  is a CVD process for forming an insulating film on the wafer surface. Step  13  is an electrode forming process for forming electrodes upon the wafer by vapor deposition. Step  14  is an ion implanting process for implanting ions to the wafer. Step  15  is a resist process for applying a resist (e.g., a photosensitive material) to the wafer. Step  16  is an exposure process for printing, by exposure, the circuit pattern of the mask on the wafer through the exposure apparatus described above. Step  17  is a developing process for developing the exposed wafer. Step  18  is an etching process for removing portions other than the developed resist image. Step  19  is a resist separation process for separating the resist material remaining on the wafer after being subjected to the etching process. By repeating these processes, circuit patterns are superposedly formed on the wafer. 
     With these processes, high density microdevices can be manufactured. 
     In an exposure apparatus according to the embodiments of the present invention described above, ozone produced in response to projection of an exposure beam can be removed, whereby deterioration of components used in the apparatus can be prevented. Thus, an exposure apparatus of high durability is accomplished. Further, detection of ozone is effective to assure enhanced safety of the apparatus. Since ozone production is quite notable for an exposure beam having a wavelength of 250 nm or shorter, the advantageous effects described above are very significant, particularly when an ArF excimer laser is used with a wavelength region of 250 nm or less. 
     Use of such an exposure apparatus enables the production of high precision devices with lower cost. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.