Abstract:
An apparatus for projecting a mask pattern onto a wafer has a light source, a pair of light selectors, and at least one optical element disposed between the light selectors. The first light selector selectively places an alignment filter between the light source and the optical element. The second light selector selectively places a shutter or an exposure filter in the path of light received from the optical element. The apparatus can be operated in a shut state, an alignment state, and an exposure state, the alignment filter being removed from the light path only in the exposure state. The optical element is thereby protected both from prolonged exposure to unfiltered light, and from thermal stress caused by repeated cycling between illuminated and non-illuminated conditions.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a light-exposure apparatus used in semiconductor fabrication, more particularly to the arrangement of the shutter and filters of the apparatus. 
     FIG. 1 schematically shows the structure of a conventional light-exposure apparatus  50  having a light source such as a mercury lamp  51 , and a shutter  52  mounted just below the light source. The apparatus  50  has three operating states: a shut state, an alignment state, and an exposure state. FIG. 1 illustrates the light path in the alignment state. 
     The shutter  52  moves between a shut position (indicated by dotted lines) in which it blocks the light from the mercury lamp  51 , and an open position (indicated by solid lines) in which the light is not blocked and can be used for alignment and exposure purposes. The light is refracted by the transmitting part  53   a  of a convex secondary lens  53 , reflected by a primary mirror  54 , then reflected by an aluminum coating on the reflecting part  53   b  of the secondary lens  53 . After passing through a slit  55 , the light is reflected by the reflecting surface  56   a  of a toroidal mirror  56 , then passes through one of two filters, either an exposure filter  57  that transmits light of comparatively short wavelengths, or an alignment filter  58  that transmits light of longer wavelengths. In the alignment state, these filters are positioned so that the light passes through the alignment filter  58 , as shown. 
     The elements described so far constitute the illumination system  65  of the apparatus. The light emerging from the illumination system  65  is further reflected by three relay mirrors  59 ,  60 ,  61 , then illuminates a semiconductor wafer through a mask. The wafer and mask are not shown in FIG.  1 . 
     The wafer is coated with a photoresist material that is insensitive to the wavelengths transmitted by the alignment filter  58 . This light can therefore be used for alignment of the wafer and mask. After alignment is completed, the exposure filter  57  is moved into the light path, replacing the alignment filter  58 , and light of a shorter, more energetic wavelength is used to transfer the mask pattern to the photoresist. 
     In the shut state, the shutter  52  is moved to the position indicated by dotted lines to block the light from the mercury lamp  51 , so that the light does not reach the secondary lens  53 , primary mirror  54 , slit  55 , and toroidal mirror  56  in the illumination system  65 . 
     A problem in the conventional apparatus  50  is that the short-wavelength light that interacts with the photoresist also interacts to some extent with the optical elements in the illumination system  65 , gradually clouding or darkening the coatings on their surfaces, for example. Since these optical elements are exposed to all emitted wavelengths in both the exposure state and the alignment state, they receive continuous exposure to short-wavelength light during these two states. As a result, these optical elements, more specifically the secondary lens  53 , primary mirror  54 , and toroidal mirror  56 , tend to degrade comparatively quickly. 
     Continuous exposure to the light emitted by the mercury lamp  51  during the alignment and exposure states also raises the optical elements in the illumination system  65  to a comparatively high temperature. In the shut state, in which the shutter  52  is closed, these optical elements receive no illumination, and their temperature falls back toward room temperature. As the apparatus  50  cycles repeatedly among the shut, alignment, and exposure states, the optical elements in the illumination system  65  undergo repeated large temperature swings, which affect their optical properties and contribute to the degradation thereof. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to reduce the degradation of optical elements in an apparatus for projecting a mask pattern onto a wafer. 
     A further object is to reduce temperature variations in the illumination system of the apparatus. 
     The invented apparatus has an illumination system providing light for use in alignment and exposure, and a projection system using the provided light to project a mask pattern onto a wafer. The illumination system has a light source, two light selectors, and at least one optical element disposed between the two light selectors. The first light selector has an alignment filter, which it selectively places between the light source and the optical element, in the path of the light emitted by the light source. The second light selector has at least an exposure filter and a shutter, which it selectively places between the optical element and the projection system, in the path of the light received from the optical element. 
     The apparatus can be operated in a shut state, an alignment state, and an exposure state. The alignment filter is placed in the light path in the shut state and the alignment state, and is removed from the light path in the exposure state. 
     Degradation of the optical element is reduced because the optical element is exposed to unfiltered light only in the exposure state, and not in the alignment state. 
     Temperature variations are reduced because the optical element is illuminated in the shut state, as well as in the alignment state and exposure state. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the attached drawings: 
     FIG. 1 is a schematic sectional view illustrating part of a conventional light-exposure apparatus in the alignment state; 
     FIG. 2 is a schematic sectional view of a first novel light-exposure apparatus in the shut state; 
     FIG. 3 is a schematic sectional view of the first novel light-exposure apparatus in the alignment state; 
     FIG. 4 is a schematic sectional view of the first novel light-exposure apparatus in the exposure state; and 
     FIG. 5 is a schematic sectional view of a second novel light-exposure apparatus in the exposure state. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will be described with reference to the attached drawings, in which like parts are indicated by like reference characters. 
     As a first embodiment, FIGS. 2 3 , and  4  show a novel light-exposure apparatus  1  in the shut state, the alignment state, and the exposure state, respectively. The structure of the apparatus  1  will be described with reference mainly to FIG.  2 . 
     The apparatus  1  has a housing  2  in which a mercury lamp  3  is mounted in a fixed position behind an alignment filter  4  and a secondary lens  5 . The secondary lens  5  is a plano-convex lens with a flat surface and a curved surface. The alignment filter  4  is mounted between the flat surface and the mercury lamp  3 , but can be moved as described below. The curved surface is partly coated with an aluminum film, so that the secondary lens  5  has a reflecting part  5   a  as well as a transmitting part  5   b . The mercury lamp  3  is positioned behind the transmitting part  5   b.    
     The alignment filter  4  is mounted in a first manually movable mount  20 . This mount  20  permits the alignment filter  4  to be moved between the position shown in FIGS. 2 and 3, adjacent the transmitting part  5   b  of the secondary lens  5 , and the position shown in FIG. 4, adjacent the reflecting part  5   a . The alignment filter  4  and first movable mount  20  form a first light selector. 
     The convex surface of the secondary lens  5  faces the spherically concave reflecting surface  6   a  of a primary mirror  6 . Light emitted through the transmitting part  5   b  of the secondary lens  5  is reflected from substantially all parts of the reflecting surface  6   a  of the primary mirror  6 , and converges onto the reflecting part  5   a  of the secondary lens  5 . 
     The light reflected from the reflecting part  5   a  of the secondary lens  5  passes through a slit  7  and encounters the reflecting surface  8   a  of a toroidal mirror  8  that extends in an arc perpendicular to the drawing sheet. The slit  7  and toroidal mirror  8  are disposed so that the reflecting surface  8   a  of the toroidal mirror  8  is illuminated by a substantially uniform band of light, with equal intensity at the center and both ends. This band of light is reflected from the toroidal mirror  8  toward a first relay mirror  11 . 
     Disposed between the toroidal mirror  8  and the first relay mirror  11  are an exposure filter  9  and a shutter  10 , mounted in a second manually movable mount  21 . The positions of the exposure filter  9  and shutter  10  are interchangeable: in the shut state, the shutter  10  is positioned on the light path between the toroidal mirror  8  and first relay mirror  11 , as shown in FIG. 2; in the alignment state and exposure state, the exposure filter  9  occupies this position, as shown in FIGS. 3 and 4. The exposure filter  9 , shutter  10 , and second manually movable mount  21  form a second light selector. 
     Light reaching the first relay mirror  11  is reflected to a second relay mirror  12 , then to a third relay mirror  13 . A movable carriage  14  holds a mask  15  and a wafer  16  on opposite sides of the three relay mirrors  11 ,  12 ,  13 . The light reflected by the relay mirrors  11 ,  12 ,  13  passes through the mask  15  and is transmitted by a projection guide  17  to the wafer  16 , thereby projecting an image of the mask pattern onto the surface of the wafer  16 . The carriage  14  is constructed so that the relative positions of the mask  15  and wafer  16  can be adjusted. The carriage  14  can also be moved as a whole in the direction of the arrows A and B, perpendicular to the optic axis of the incident light, by a mechanism not shown in the drawings. 
     The elements from the mercury lamp  3  to the second movable mount  21  form an illumination system  23  that furnishes the light needed for alignment and exposure. The remaining elements form a projection system  24  that projects the light through the mask  15  onto the wafer  16 . 
     The mercury lamp  3  emits light of various wavelengths, including an alignment wavelength (e.g., 546 nm) in the visible part of the spectrum, and an exposure wavelength (e.g., 365 nm) in the ultraviolet part. The alignment wavelength is used for aligning the mask  15  and wafer  16 ; the exposure wavelength is used for transferring the mask pattern to the wafer  16 . Incidentally, ‘nm’ is an abbreviation for nanometers. 
     The alignment filter  4  is an optical filter that transmits wavelengths equal to or longer than a predetermined wavelength α (e.g., 500 nm), and blocks wavelengths shorter than α. The predetermined wavelength α is disposed between the exposure wavelength and the alignment wavelength. 
     The exposure filter  9  is an optical filter that transmits wavelengths equal to or longer than a shorter predetermined wavelength β (e.g., 320 nm), and blocks wavelengths shorter than β. The predetermined wavelength β is shorter than the exposure wavelength. 
     The operation of the first embodiment in the shut state, the alignment state, and the exposure state will be described below. 
     In the shut state, the movable mounts  20 ,  21  are set manually so that the alignment filter  4  and shutter  10  are positioned on the light path, as illustrated in FIG.  2 . In the illumination system  23 , light emitted by the mercury lamp  3  travels on a first path through the alignment filter  4 , which removes wavelengths shorter than the above-mentioned wavelength α (e.g., 500 nm). The secondary lens  5 , primary mirror  6 , slit  7 , and toroidal mirror  8  reshape the transmitted light into a rectangular beam, and redirect the beam on a second path to the second movable mount  21 . There the beam is blocked by the shutter  10 . 
     In the alignment state, the movable mounts  20 ,  21  are set manually to place the alignment filter  4  and exposure filter  9  on the light path, as illustrated in FIG.  3 . Both of these filters transmit wavelengths equal to or longer than α, and the alignment filter  4  blocks wavelengths shorter than α. Light of wavelengths equal to or longer than α passes through the alignment filter  4 , is reshaped and redirected as described above, then passes through the exposure filter  9  into the projection system  24 . 
     The three relay mirrors  11 ,  12 ,  13  in the projection system  24  direct the reshaped light beam onto the mask  15 . Light passing through the mask  15  is projected onto the wafer  16 . The relative positions of the mask  15  and wafer  16  are now adjusted so that the mask pattern projected into the wafer  16  is correctly aligned with existing features or marks on the wafer  16 . The wafer  16  is coated with a photoresist that is insensitive to wavelengths equal to or longer than the above-mentioned wavelength α, so the photoresist is unaffected by the light incident on it during the alignment process. 
     In the exposure state, the movable mounts  20 ,  21  are set manually to remove the alignment filter  4  from the first light path and place the exposure filter  9  on the second light path, as illustrated in FIG.  4 . Light emitted by the mercury lamp  3  is reshaped into a rectangular beam by the secondary lens  5 , primary mirror  6 , slit  7 , and toroidal mirror  8 , without being filtered by the alignment filter  4 . Wavelengths equal to or longer than the above-mentioned wavelength β (e.g., 320 nm) are transmitted through the exposure filter  9  into the projection system  24 . The three relay mirrors  11 ,  12 ,  13  again direct the beam onto the mask  15 , thereby projecting a mask pattern onto the wafer  16 . 
     The photoresist with which the wafer  16  is coated is sensitive to the exposure wavelength, which is longer than β, so the mask pattern is transferred to the photoresist. The carriage  14  is moved at a predetermined rate in the direction of the arrows A and B, enabling the entire mask pattern to be transferred. 
     As described above, the secondary lens  5 , primary mirror  6 , and toroidal mirror  8  remain illuminated throughout the operation of the light-exposure apparatus  1 , even in the shut state, so they are not subjected to extreme temperature variations and their optical properties are comparatively unaffected by thermal stress. They are illuminated by the unfiltered light of the mercury lamp  3  only in the exposure state, however. In the alignment state, the alignment filter  4  removes wavelengths shorter than the above-mentioned wavelength α. The secondary lens  5 , primary mirror  6 , and toroidal mirror  8  therefore suffer less optical damage and last longer than in the conventional apparatus, because the time for which they are exposed to high-energy short wavelengths (less than α) is reduced. 
     Next, a second embodiment will be described. 
     FIG. 5 shows the second embodiment in the exposure state. In the light-exposure apparatus  31  in the second embodiment, the alignment filter  4  is mounted in a first automatically movable mount  32  that is driven by a driving signal s 1 . The exposure filter  9  and shutter  10  are mounted in a second automatically movable mount  33  driven by a driving signal s 2 . The driving signals s 1 , s 2  are generated by a control unit  34 . The control unit  34  may be manually operated, or may output the driving signals s 1 , s 2  according to a prestored program. 
     In the shut state, the control unit  34  generates driving signals s 1 , s 2  that automatically cause the movable mounts  32 ,  33  to position the alignment filter  4  and shutter  10  in the light path. 
     In the alignment state, the control unit  34  generates driving signals s 1 , s 2  that automatically cause the movable mounts  32 ,  33  to position the alignment filter  4  and exposure filter  9  in the light path. 
     In the exposure state, the control unit  34  generates driving signals s 1 , s 2  that automatically cause the first movable mount  32  to remove the alignment filter  4  from the light path, and the second movable mount  33  to position the exposure filter  9  in the light path. 
     The second embodiment provides the same effects as the first embodiment, and has the additional advantage of simplified operation, not requiring manual manipulation of the movable mounts  32 ,  33 . 
     In a variation of the first and second embodiments, the second movable mount  21  or  33  can be set to a position in which both the exposure filter  9  and shutter  10  are removed from the light path, and the light reflected from the toroidal mirror  8  is passed directly to the first relay mirror  11 . This position is used during alignment, so that the light is filtered only by the alignment filter  4 , and not by the exposure filter  9 . Accordingly, the exposure filter  9  does not have to transmit light of the wavelengths used for alignment purposes. 
     In another variation of the first and second embodiments, the first movable mount  20  or  32  also has an optically neutral element such as a transparent glass plate that is moved into the light path for protective purposes in the exposure state, to protect the lenses and mirrors of the illumination system from dust, for example, without attenuating the emitted light. 
     The invention is not limited to the wavelengths mentioned above or to the use of a mercury lamp as the light source; other wavelengths and other types of light sources may be used. 
     Use of the terms ‘alignment wavelength’ and ‘exposure wavelength’ does not imply that either the alignment process or the exposure process is limited to a single wavelength. 
     Those skilled in the art will recognize that further variations are possible within the scope claimed below.