Abstract:
An intensity distribution management system includes a light source, a mask for receiving light therefrom and for allowing some light to propagate through and past the mask, a surface for receiving light allowed past the mask, and a diffusive element disposed between the mask and the light source for ensuring a substantially even light intensity distribution in relation to the surface. An imaging method includes emitting a light beam, manipulating the beam to have a first numerical aperture across a first divergence axis, directing the beam through a diffusive element to increase the numerical aperture of the beam, directing the beam through one or more transmissive portions of a mask, the mask being disposed relative to the diffusive element, and imaging transmitted portions of the beam to a target surface wherein the beam has a substantially ripple-free and uniform intensity distribution across the first divergence axis at the target surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Generally, the field of the present invention is laser patterning. More particularly, the present invention relates to an intensity distribution management system and method in the imaging of pixels on a mask. 
     2. Background 
     Laser systems have enjoyed application to a variety of fields for many years. As transistor and display technologies have advanced over the past few decades, transistor sizes have decreased and LED light outputs have increased, both at logarithmic rates. Laser systems, and their application to the manufacturing processes of semiconductor electroluminescent materials, have allowed the continued advancement in these areas. For example, laser systems form an important part of selective thermal transfer processes, such as laser induced thermal imaging (LITI) enjoying successful application in the flat panel display industry. 
     However, obstacles have persisted that prevent an effective and repeatable thermal imaging process. One such obstacle has been in the attempts to maintain a uniform intensity of laser light focused at the surface targeted in the imaging process. Thus, despite efforts to achieve a uniform imaging process that is also low cost and with minimal complexity, there remains a need for systems and methods without these attendant drawbacks. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, an intensity distribution management system includes a light source, such as a laser light source, and a mask for receiving light from the light source and for allowing portions of light to propagate through and past the mask. The system also includes a target surface for receiving light allowed past the mask. A diffusive element is disposed relative to the mask so that light propagating through and past the mask has a substantially even or uniform intensity distribution when incident on the targeted surface. 
     In another aspect of the invention, an intensity distribution management method includes emitting a light beam from a light source such as a laser light source, optically manipulating the light beam emitted from the light source so that the light beam has a first numerical aperture, directing the light beam through a diffusive element so as to increase the first numerical aperture to a second numerical aperture, directing the light beam through one or more transmissive portions of a mask wherein the mask is disposed relative to the diffusive element, and imaging transmitted portions of the light beam to a target surface wherein the light beam has a substantially ripple-free and uniform intensity distribution across the first divergence axis at the target surface. 
     The foregoing and other objects, features, and advantages will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a prior art laser system for laser patterning. 
         FIG. 2  is a chart of intensity distribution along a low divergence transverse axis of a laser beam incident on a target surface. 
         FIG. 3  is a block diagram of a laser system for pixel imaging according to an aspect of the present invention. 
         FIG. 4  is a side view of a mask and accompanying diffusive element according to an aspect of the present invention. 
         FIG. 5  is a zoomed-in side view of a mask and accompanying diffusive element according to an aspect of the present invention. 
         FIG. 6  is a plan view of a mask and accompanying diffusive element according to an aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a prior art laser system  10  for laser patterning is shown. Such a laser system  10  includes a light source, here in the form of a laser  12  which may be of various configurations, including pulsed or continuous wave, and in various packages, such as laser diode modules housing a plurality of laser emitters instead of just a single emitter or housing laser diode bars. In some embodiments, laser system  10  is a line generator while in others laser system  10  is capable of producing another output shape besides circular. The laser  12  is in communication with a power/control source  14  and may optionally be pumped with other lasers. The laser beam or beams  16  (hereinafter beam) emitted from the laser  12  are directed towards beam shaping optics  18  which homogenize the transverse intensity distribution across one or more axes and direct the beam  16  along a path  22 . In some embodiments the divergences across the one or more orthogonal axes of the beam  16  may be different from each other. The beam  16  is then incident on a mask  20  disposed along the beam path  22 . After mask  20 , the beam  16  propagates through projection optics  24 , such as a projection lens, before impinging on a target surface  26 . The beam  16  or the surface  26 , or both, are translatable for alignment purposes as well as for process purposes. The beam  16  incident on the mask  20  propagates through transmissive portions  50  (see  FIG. 4 ) thereof. The transmissive portions may be of various shapes including rectangular, circular, and other patterns. Optical elements  28  may also be disposed in relation to the mask  20  so as to direct portions of beam  16  not transmitted through the mask  20  to a beam dump  30 . 
     One application for a laser system  10  is laser induced thermal imaging (LITI) wherein the surface  26  is selectively targeted with the laser beam  16  such that thermal imaging can occur. A variety of surfaces may be targeted, though typically under this technique laser beams are selectively directed to a donor film comprised of a base film, a light to heat conversion layer, and a transfer layer, such as an electroluminescent layer made of small molecules or light emitting polymer. Ultra fast heating caused by the selected incidence of laser radiation on the donor film transfers the electroluminescent layer onto to an adjacent substrate. Such selective material transfer is used for pixel formation in various display technologies, such as organic LED manufacture, and is achievable with lasers having high accuracy and precision. 
     To effect a superior transfer several parameters must be carefully controlled and designed around. For example, the mask  20  and projection optics  24  must be finely matched such that the desired image is achieved at target surface  26 . Also, the thickness and composition of the donor layers must be selected to achieve adhesion between the respective layers and cohesion of the transfer layer such that suitable transfer of the electroluminescent layer on the substrate is the result. Thus, the incident laser radiation should have a generally uniform spatial luminous intensity distribution across at least one orthogonal axis such that pixels imaged at one location due to the mask will be similarly imaged at other locations. Thus, as will be further described herein, another important area of consideration is the degree of consistency of the luminous intensity distribution delivered to the targeted LITI surface. For example, a typical intensity distribution  34  for laser system  10  is shown in  FIG. 2 . As can be seen therein, across a low divergence axis  32  the intensity of electromagnetic radiation  34  at the target surface  26 , i.e., transmitted through the mask  20 , has ripples  36  near the edges  38   a ,  38   b . In view of the presence of the ripples  36  the system  10  of  FIG. 1  is unsatisfactory in terms of intensity distribution management. 
     Referring to  FIG. 3 , in an exemplary embodiment of the present invention, a laser system  100  is shown that may include many of the same components shown in  FIG. 1  and hereinbefore described. That is, a laser or lasers  12  are in communication with a power/control source  14 . A laser beam  16  is generated and directed to beam shaping optics  18  which homogenize the transverse intensity distribution across one or more orthogonal axes. The beam  16  is incident on a mask  20  disposed along a beam path  22 . After mask  20 , the beam  16  propagates through projection optics  24 , such as a projection lens, before impinging on a target surface  26 . The beam  16  or the surface  26 , or both, may be translatable for alignment purposes as well as for process purposes. The beam  16  incident on the mask  20  propagates through transmissive portions thereof. Optical elements  28  may also be disposed in relation to the mask  20  so as to direct portions of beam  16  not transmitted through the mask  20  to a beam dump  30 . 
     As mentioned before the beam shaping optics  18  homogenize the intensity distribution of the beam  16 . This is useful when a plurality of laser beams, such as a bar of laser diode emitters or an array of single emitter diodes, fiber-coupled or otherwise, is used as the laser source  12 . As shown in  FIG. 2 , even with a laser beam  16  having a substantially uniform intensity distribution across one orthogonal axis is incident on the mask  20 , a distribution  34  is seen incident on the target surface  26  across the same axis that includes ripples  36  towards the edges  38   a ,  38   b  of targeted area on the surface  26 . The ripples  36  are undesirable since they represent spatial variation in intensity distribution of portions of the laser beam  16  transmitted through the mask  20  and incident on the target surface  26 . For well controlled and repeatable laser induced thermal imaging such that predictable mechanisms are observed in and between donor film and substrate, an even and predictable intensity distribution of the beam incident on the surface  26 , such as the flat distribution  40  shown in  FIG. 2 , is desirable. 
     Accordingly, the inventors herein have developed an intensity distribution management system and method in accordance with the present invention to flatten out or eliminate ripples  36 , as seen in  FIG. 2 , near the edges  38   a ,  38   b  at target surface  26 . Referring now to  FIGS. 3 and 4 , in an exemplary embodiment of the present invention, in the laser system  100  for pixel imaging, a diffusive element  102  to enable intensity distribution management is shown disposed adjacent to and in front of mask  20 . The diffusive element  102  may be of any suitable type, such as a diffractive optical element, a lens array diffuser, or a refractive lens array. A diffusive element with fine pitch lenslets is suitable, as well as diffusive elements causing diffusion in one dimension. Other diffusive elements may be used as well, and the listing here of particular element types is not by way of limitation. By the inclusion of diffusive element  102 , the convergence angle of light propagating through and past the mask  20  is increased. The beams  106  propagating through and past the diffuser  102  are characterized by a reduced spatial coherence and result in an averaging of multiple different diffraction ripple patterns  36 . This multiple averaging causes the beam propagating past the mask  20  pattern to become incident on the target surface  26  in a more uniform way such that a flatter intensity distribution  40  is applied. 
     A more detailed side view of beam shaping optics  18  and mask  20  is shown in  FIGS. 4 and 5 . In  FIG. 4 , exemplary component beams  42  of beam  16  propagate from the laser  12  through the beam shaping optics  18  and are significantly collimated towards the mask  20 . In some embodiments, coatings are applied to the surface  48  of the mask  20  so as to allow light to propagate through some regions  50  and not through other regions  52 . The transmissive regions  50  allow the laser beam  16  to be incident on predetermined portions  54  of target surface  26 . While the diffusive element  102  shown in  FIG. 5  is spaced apart from the mask surface  48 , in some embodiments the diffusive element  102  is attached or formed on the mask  20 . 
     The incident component beams  42  typically have a low divergence of approximately 0.01 NA across axis  32  and a larger divergence of approximately 0.1 NA across a transverse axis  56 . By propagating through the diffusive element  102 , the component beams  42  expand somewhat to form component beams  106  having a larger NA across low divergence axis  32 . As seen in closer detail in  FIG. 5 , each component beam  106  can be thought to have respective principal rays  106   a  and marginal rays  106   b ,  106   c . Component beams  106   b  form a beam with different propagation angles than component beams  106   a  and  106   c . Multiple propagation angles cause different aperture diffraction patterns with different intensity ripples with the cumulative effect of a smoothing of the ripples  36 . 
     In another embodiment, shown in  FIG. 6 , the principle axis  60  of a one-dimensional diffuser  102 , having line representations  62 , is angularly displaced by an angle α in relation to a principle axis  58  of mask  20  or respective transmissive portions  50  thereof. Suitable angles may be between 0° and 5° and preferably about 3°. The slight angle change a causes an unexpected increased smoothing or reduction of intensity ripples  36  such that a more uniform intensity distribution is incident on the target surface  26 . In other embodiments the angular displacement may have a different angular range greater or less than 0 to 5, though for one-dimensional diffusers generally, the result is an effect on beam properties in the transverse axis  32 . By the addition of element  102  in both angled and perpendicular embodiments disposed relative to the mask  20 , the width of the beam  16  emitted from the laser  12  can remain within reasonable limits and aperture diffraction effects can be limited. Moreover, the beam shaping optics  18  can remain relatively simple and inexpensive. In some embodiments, the diffusive element  102  is an isotropic diffuser, such as one made with ground glass, providing diffusive effects in two dimensions instead of one. 
     It is thought that the present invention and many of the attendant advantages thereof will be understood from the foregoing description and it will be apparent that various changes may be made in the parts thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the forms hereinbefore described being merely exemplary embodiments thereof.