Abstract:
A reduction projection aligner has a light source accommodated in an air-tight vessel, an optical filter fitted in the air-tight vessel, a reticle provided outside of the air-tight vessel and a reduction projection lens unit provided between the reticle and a photo-sensitive layer, and an evacuating system creates vacuum in the air tight vessel so that undesirable short-wavelength light component, which promotes undesirable reaction for tarnishing the reticle, is eliminated from the light before radiation from the air-tight vessel.

Description:
FIELD OF THE INVENTION 
     This invention relates to an optical aligner and, more particularly, to an optical aligner free from reaction product tarnishing photo-mask and a method for transferring a pattern image through the photo-mask. 
     DESCRIPTION OF THE RELATED ART 
     A typical example of the reduction projection aligner is disclosed in Japanese Patent Publication of Unexamined Application No. 62-86724. An air evacuating system is incorporated in the prior art reduction projection aligner for reducing undesirable particles in the wafer chamber. FIG. 1 illustrates the prior art reduction projection aligner. The reduction projection aligner comprises a light source  1 , a vessel  2  defining a vacuum chamber  2   a , a vacuum pump  3  connected to the vacuum chamber  2   a , a movable wafer stage  4  provided outside of the vessel  2  and exposure windows  5   a / 5   b  formed in the vessel  2  on the optical path from the light source  1  to the movable wafer stage  4 . A semiconductor wafer  6  is placed on the movable wafer stage  4 , and the movable wafer stage  4  two-dimensionally moves the semiconductor wafer  6 . The light source  1  is implemented by an ultra high-pressure mercury lamp, and radiates ultra-violet light through the exposure window  5   a . The vacuum pump  3  evacuates the air from the vacuum chamber  2   a.    
     The prior art reduction projection aligner further comprises a movable reticle stage  7  installed in the vacuum chamber  2   a , a reticle loader  8  provided under the exposure window  5   a , a reduction projection lens unit  10  provided between the movable reticle stage  7  and the exposure window  5   b  and an inspection unit  11  also provided in the vacuum chamber  2   a . The reticle loader  8  picks up a reticle  9  at a loading port (not shown), and moves it onto the movable reticle stage  7 . A pattern to be transferred is formed in the reticle  9 . The ultra-violet light passes the exposure window  5   a , the reticle  9 , the reduction projection lens unit  10  and the exposure window  5   b , and transfers the pattern image onto the semiconductor wafer  6 , and the reduction projection lens unit  10  demagnifies the pattern image. The inspection unit  11  checks the reticle  9  to see whether or not the reticle is contaminated with dust particles. 
     The pattern is repeatedly transferred from the reticle  9  from the semiconductor wafer  6  as follows. First, an operator supplies the reticle  9  to the loading port, and the inspection unit  11  checks the reticle  9  to see whether or not the reticle, 9  is contaminated with dust particles. If the dust particles are not found, the reticle loader  8  transfers the reticle  9  onto the movable reticle stage  7 . The vacuum pump  3  evacuates the air from the vacuum chamber  2   a  together with dust particles. For this reason, the reticle  9  is hardly contaminated in the vacuum chamber  2   a . The movable wafer stage  4  moves the semiconductor wafer  6  in such a manner as to align a small area on the semiconductor wafer with the optical path  12  of the ultra violet light. 
     The light source  1  radiates the ultra-violet light toward the exposure window  5   a . The ultra-violet light passes the exposure window  5   a  and the reticle  9 , and carries the pattern image. The image-carrying ultra-violet light passes through the reduction projection lens unit  10 , and the pattern image is reduced by the reduction projection lens unit  10 . The image-carrying ultra-violet light falls onto the small area, and forms a latent image therein. The movable wafer stage  4  aligns another small area with the optical path, and the optical image transfer is repeated. In this way, the pattern is optically transferred from the reticle to the semiconductor wafer, and the inspection unit  11  and the vacuum pump  3  prevent the semiconductor wafer  6  from undesirable defects due to the dust particles. 
     A known modification of the prior art reduction projection aligner has the reticle and the movable wafer stage installed in the vertical direction, and a pattern image is transferred from the vertical reticle to a semiconductor wafer vertically supported by the movable wafer stage. In this instance, the possibility of the dust contamination is further decreased. However, the prior art reduction projection aligner encounters a problem in that the exposure windows  5   a / 5   b  are tarnished. The tarnished exposure windows  5   a / 5   b  deteriorate the uniformity of the ultra-violet light, and decrease the throughput. 
     SUMMARY OF THE INVENTION 
     It is therefore an important object of the present invention to provide an aligner, which maintains the uniformity of light radiated through exposure windows. 
     The present inventor analyzed the contaminant on the exposure windows, and found that the contaminant was reaction product unintentionally produced from gaseous component of the air in the presence of short-wavelength light components contained in the ultra-violet light. The reaction product tarnished the exposure windows. The ultra high-pressure mercury lamp contained the short-wavelength light. The present inventor changed the light source from the ultra high-pressure mercury lamp to another kind of lamp. However, the reaction product was produced from the certain gaseous component in so far as the exposure light contained the certain short-wavelength light components, and the reaction product tarnished the exposure windows. The present inventor concluded that the undesirable short-wavelength light components had to be eliminated from the light before the reaction. Alternatively, when the optical path from the light source to the photo-mask was in vacuum, the reaction product was not produced without the filtering. 
     To accomplish the object, the present invention proposes to cause light to pass an optical filter before radiating into the air or accommodate the light path from a light source to a photo-mask in vacuum. 
     In accordance with one aspect of the present invention, there is provided an optical aligner for transferring a pattern image to a photo-sensitive layer, comprising: an illumination system including a light source producing light containing a light component available for a pattern transfer and another light component promoting reaction of a gaseous component; a photo-mask having a pattern to be transferred, and illuminated with the light; an projection optical system provided between the photo-mask and the photo-sensitive layer for forming a latent image in the photo-sensitive layer; an air-tight vessel accommodating at least a part of the illumination system containing the light source; an optical filter for eliminating the another light component from the light before the light is radiated from the air tight vessel toward the photo-mask; and an evacuating system for creating vacuum in the air-tight vessel. 
     In accordance with another aspect of the present invention, there is provided an optical aligner for transferring a pattern image to a photo-sensitive layer, comprising: an illumination system including a light source producing light containing a light component available for a pattern transfer and another light component promoting reaction of a gaseous component; a photo-mask having a pattern to be transferred, and illuminated with the light; an projection optical system provided between the photo-mask and the photo-sensitive layer for forming a latent image in the photo-sensitive layer; an air-tight vessel accommodating at least the light source and the photo-mask; and an evacuating system for creating vacuum in the air-tight vessel. 
     In accordance with yet another aspect of the present invention, there is provided a method for transferring a pattern image from a photo-mask to a photo-sensitive layer, comprising the steps of: a) providing the photo-mask in an optical path from a light source to the photo-sensitive layer; b) evacuating the air from a vacuum chamber accommodating the light source; and c) radiating light from the light source through an optical filter provided on a boundary between the vacuum chamber and the photo-mask to the photo-sensitive layer so as to eliminate a light component promoting a reaction of a gaseous component from the light, thereby forming a latent image in the photo-sensitive layer. 
     In accordance with still another aspect of the present invention, there is provided a method for transferring a pattern image from a photo-mask to a photo-sensitive layer, comprising the steps of: a) providing the photo-mask in a vacuum chamber together with an illuminating system and a projection optical system; b) evacuating the air from the vacuum chamber; and c) radiating light from the illuminating system through the photo-mask and the projection optical system to the photo-sensitive layer so as to form a latent image in the photo-sensitive layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the aligner and the method for transferring a pattern will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic view showing the structure of the prior art reduction projection aligner; 
     FIG. 2 is a schematic view showing the structure of a reduction projection aligner according to the present invention; 
     FIG. 3 is a schematic view showing the structure of another reduction projection aligner according to the present invention; and 
     FIG. 4 is a detailed flow chart of a transferring pattern image according to an embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring to FIG. 2 of the drawings, a reduction projection aligner embodying the present invention comprises a vessel  20  defining a vacuum chamber  20   a , a filter plate  21  fitted in the vessel  20 , a light source  22  accommodated in the vessel  20 , a vacuum pump  23  connected to the vacuum chamber through a gas conduit  24  and a valve unit  25  inserted into the gas conduit  24 . An ultra high-pressure mercury lamp  22   a  and elliptical reflection mirror  22   b  form in combination the light source  22 . The ultra high-pressure mercury lamp  22   a  radiates ultra-violet light  22   c , and the elliptical reflection mirror  22   b  directs the ultra-violet light  22   c  toward the filter plate  21 . The vacuum pump  23  creates vacuum in the chamber  20   a , and the vacuum is less than 1 torr to 10 −2  torr. Undesirable gaseous component are too tight in the vacuum chamber  20   a  to produce the reaction product such as (NH 4 ) 2 SO 4  and SiO 2 . When the chamber  20   a  reaches the vacuum at less than 1 torr to 10 −2  torr, the vacuum pump  23  is stopped, and the valve unit  25  cut off the connection between the vacuum chamber  20   a  and the vacuum pump  23 . The valve unit  25  maintains the vacuum in the chamber  20   a . The vacuum pump  23  may be implemented by a dry pump or a clean pump. 
     The filter plate  21  serves as an interference filter, which allows only a certain wavelength light component used for a pattern transfer to pass there-through. The undesirable short-wavelength light components have the wave-length 340 nanometers or less. For this reason, the filter plate  21  allows the light component used for the pattern transfer to pass therethrough, and eliminates the light components equal to or less than 340 nanometer wavelength from the ultra-violet light  22   c . Thus, the exposure light  22   d  does not contain the undesirable short-wavelength light components. 
     The reduction projection aligner further comprises a condenser lens  26 , a reticle holder  27 , a reduction projection lens unit  28 , a wafer stage  29  two-dimensionally movable and a driving mechanism  30  connected to the wafer stage  29 . A reticle  27   a  is mounted on the reticle holder  27 , and a pattern to be transferred is formed in the reticle  27   a . A reticle loader (not shown) may supply the reticle  27   a  onto the reticle holder  27 . The condenser lens  26  causes the reticle  27   a  to form the pattern image on the incident surface of the reduction projection lens unit  28 . The reduction projection lens unit reduces the pattern image, and the reduced pattern image is focused on a photo-sensitive layer  29   a  over a semiconductor wafer  29   b . Reference numeral  28   a  designates a reduced image carrying light from the reduction projection lens unit  28  to the photo-sensitive layer  29   a.    
     The pattern image is transferred from the reticle to the photo-sensitive layer  29   a  as follows. First, the reticle  27   a  is placed on the reticle holder  27 , and the semiconductor wafer  29   b  is mounted on the wafer stage  29 . The driving mechanism  30  aligns a small area of the photo-sensitive layer  29   a  with the optical path of the reduced image carrying light  28   a . The vacuum pump  23  creates the vacuum in the chamber  20   a , and the ultra high-pressure mercury lamp starts to radiate the ultra-violet light. The elliptical reflecting mirror  22   b  directs the ultra-violet light toward the filter plate  21 . The ultra-violet light  22   c  is propagated through the vacuum chamber  20   a , and passes through the filter plate  21 . Even though the ultra-violet light  22   c  contains the undesirable short-wavelength light components, the reaction product is never produced in the chamber  20   a , because the air has been evacuated from the chamber  20   a.    
     The filter plate  21  eliminates the undesirable short-wavelength light components from the ultra-violet light  22   c , and only the light component available for the pattern transfer is radiated from the filter plate  21  to the condenser lens  26  as the exposure light  22   d . The exposure light  22   d  passes through the reticle  27   a , and carries the pattern image to be transferred. The image-carrying exposure light  27   b  is incident onto the reduction projection lens unit  28 , and the reduction projection lens unit  28  focuses the reduced image-carrying light  28   a  on the small area of the photo-sensitive layer  29   a.    
     In this instance, the light source  22 , the condenser lens  26  as a whole constitute an illumination system, and the reduction projection lens unit  28  serves as a projection optical system. 
     As will be appreciated from the foregoing description, while the ultra-violet light  22   c  is passing through the vacuum chamber  20   a , the reactive gaseous component is too little to produce the undesirable reaction product, and the filter plate  21  is never tarnished. After the ultra-violet light  22   c  is filtered, the exposure light  22   d  does not produce the reaction product from the reactive gaseous component contained in the air. For this reason, even though the reactive gaseous component exists in the space between the filter  21  and the reticle  27   a , the reaction product is hardly produced without the short-wavelength light components, and, accordingly, the reticle  27   a  is never tarnished. This results in the enhancement of the uniformity of the exposure light, and the reduction projection aligner improves the through-put. The above described method using the herein described reduction projection aligner is summarized in the flow chart of FIG.  4 . 
     Second Embodiment 
     FIG. 3 illustrates another reduction projection aligner embodying the present invention. The reduction projection aligner implementing the second embodiment is similar in component parts to the first embodiment except for the filter plate  21  replaced with an exposure window  40 . For this reason, the other components of the second embodiment are labeled with the same references designating corresponding parts of the first embodiment without detailed description. 
     In the second embodiment, the light source  22 , the condenser lens  26 , the reticle holder  27  and the reduction projection lens unit  28  are accommodated in the vacuum chamber  20   a . Although the light  22   c  contains the undesirable short-wavelength components, the reactant gaseous component is tight in the vacuum chamber, and the reaction product is hardly produced. 
     Although a particular embodiment of the present invention has been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. 
     For example, an optical element such as, for example, a fly-eye lens may be inserted between the light source  22  and the filter plate  21 . The reticle holder  27  may be accommodated in the vacuum chamber  20   a  so as to prevent the reticle from dust particle as similar to the prior art reduction projection aligner. 
     The present invention is applicable to any kind of aligner, and, for this reason, the reduction projection lens unit is not an indispensable element. 
     The filter plate may be provided inside of the vessel. 
     Only the light source and the photo-mask may be accommodated in the vacuum chamber. The other components of the illumination system and the components of the projection optical system may be further accommodated in the vacuum chamber.