Abstract:
A method and article of manufacture for three-dimensional structures having micron dimensions includes coating a substrate with layers of photo resist. Each layer of photo resist is exposed with its own two-dimensional mask defining one slice of an object. Subsequent layers of photo resist are exposed with different patterns. Once all layers have been fabricated and exposed to identify two-dimensional features for the layer, the multiple layers are developed to removed photo resist which has not been exposed. The layered structure represents a three-dimensional object where the depth dimension is defined by the photo resist layer thickness. By decreasing the wavelength of the light exposure, it is possible to confine exposure to a single layer of photo resist.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. provisional application 60/575,074, filed May 27, 2004. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to the manufacture of arbitrary three-dimensional structures on a substrate. Specifically, a method is disclosed for creating three-dimensional structures having dimensions in the micron level, as well as an article of manufacture made from the method which creates three dimensional objects having dimensions in the micron level. 
         [0003]    Photolithography has been used for many years to create circuit details for electronic devices. Specifically, the techniques are used to deposit on a substrate layers of material which can be formed into three-dimensional circuit components. A photolithography provides the ability to create from a mask circuit features in the micron level which can be created on the substrate through a combination of deposition, exposure to light through a mask and developing images formed on the substrate. The resulting structures are electrical components which function as transistors and resistor elements which are interconnected in accordance with the mask used to create the layout. 
         [0004]    In the field of photonic crystals, there is a need to create small optical (photonic) circuit elements on a substrate having periodic structures with dimensions at the micron level. Also, micro-fluidic devices have been developed which have components with micro-channels also having dimensions in the micron range. Each of these technologies could benefit from a method of manufacture of precision three-dimensional components on a substrate. 
         [0005]    In accordance with the foregoing, the present invention has been provided. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The invention provides a method, and article of manufacture made by the method, for making three-dimensional structures having micron dimensions. A substrate is coated with a first layer of photo resist. The first layer of photo resist is exposed through a mask to form two-dimensions of a structure on the exposed photo resist. Subsequent layers of photo resist are formed over the exposed first layer, and are in turn exposed through a mask to form additional two-dimensional outlines on each subsequent layer of photo resist. When a plurality of layers of photo resist having two-dimensional outlines are formed on the substrate, the structure is developed. Each exposed outline of a resist layer defines the shape of a three-dimensional object form when all the layers have been developed. The third dimension of the structure is a function of a layer thickness, which is removed for all unexposed areas of each layer. 
         [0007]    In accordance with a preferred embodiment, the photo resist is either spun or sprayed in layers on the substrate surface. Each layer is heat treated by a soft bake before it is exposed by the mask. Following exposure of a mask, the substrate and patterned layer are again heated to form the latent image defining two-dimensions of the object. Once all layers are formed with the two-dimensional information, the resist is developed and unexposed areas are removed. 
         [0008]    The result of the method is an article of manufacture which has a plurality of layers of photo resist forming objects. The objects are defined in two-dimensions by the light exposed areas of each layer, and in the third dimension by the thickness of the photo resist. Areas which were not light exposed are removed leaving only objects which comprise the exposed areas of each photo resist layer. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows a substrate  10  on which three-dimensional structures having micron dimensions are formed; 
           [0010]      FIG. 2  illustrates a first layer of photo resist  11  spin coated on the surface of substrate  10 ; 
           [0011]      FIG. 3  illustrates the formation of a first set of structures in the photo resist  11 ; 
           [0012]      FIG. 4  shows the latten images of structures which were formed in the step of  FIG. 3 ; 
           [0013]      FIG. 5  illustrates a second photo resist layer  17  over the latten images  16  formed in photo resist layer  11 ; 
           [0014]      FIG. 6  shows the formation of additional structures in the photo resist  17 ; 
           [0015]      FIG. 7  shows the image of structures formed within the photo resist layer  17  and  11 ; 
           [0016]      FIG. 8  shows the formation of another layer of photo resist  22  over the layer of photo resist  17 ; 
           [0017]      FIG. 9  shows the step for forming images in the layer of photo resist  22 ; 
           [0018]      FIG. 10  shows the exposed portions for the layers of photo resist  11 ,  17  and  22 ; 
           [0019]      FIG. 11  shows the substrate  10  after developing; 
           [0020]      FIG. 12  shows another example where still another layer of  30  of photo resist is used to create additional objects on the substrate  10 ; and 
           [0021]      FIG. 13  shows still another layer of objects  31  formed on the previous layers of objects. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]      FIGS. 1-13  illustrate the process for making arbitrarily shape 3D structures having dimensions in microns.  FIG. 1  shows a substrate  10  which can support a number of layers of photo resist. The photo resist is exposed and developed to produce objects supported on the substrate  10 . 
         [0023]    3D objects can be fabricated in accordance with the invention from a series of layers, where each layer assumes the configuration of a 2D pattern or slice of the desired three-dimensional object. Initially, if the 3D structure is computer generated using mesh graphical techniques, the object can be reduced to a series of two-dimensional patterns or slices. In accordance with the invention, each of the slices is reproduced on the substrate as a layer of photo resist, having a two-dimensional pattern which corresponds to a configuration of a slice of the object. 
         [0024]    The layers of the object are created in accordance with the process of  FIGS. 1-13 . Each layer representing a slice of the object is made from a layer of photo resist.  FIG. 2  shows the first layer of photo resist  11  which has been spin coated on a substrate  10  to a thickness of several microns. The layer  11 , after being deposited on the substrate  10 , is soft baked with a hot plate to a temperature of approximately 130° C. The spin coated photo resist layer  11  is exposed through a mask  12  to light  13  as shown in  FIG. 3 . Openings  14  in the mask result in the photo resist  11  having exposed areas defined by the mask openings  14 . As is known in the art, the photo mask will include a proper alignment marks so that when additional surface layers are patterned, the layers are in registry. The photo resist is selected so that the wavelength of the exposing light  13  is shorter than the wavelength of light with which the resist is designed. For instance, a light source of 254 nm wavelength may be used to expose an i-line resist which is designed for exposure to a wavelength of 365 nm. By mismatching the exposing light and photo resist characteristics, it is possible to confine the exposure to a single layer of photo resist. Exposed areas of the photo resist are subject to cross linking increasing the molecular weight of the exposed resist. Once the material has been subject to cross linking, the exposed areas have no solubility to solvent which is used to remove the resist in unexposed areas. The result is that each layer may have a pattern etched in the photo resist which does not effect the patterns in other layers. Other resist coating methods, or additional coating steps such as spray coating, dry film resist, chemical surface treatment, electron photoretic deposition, etc. may be employed. 
         [0025]    Once the first layer  11  has been deposited and exposed through a mask  12 , as shown in  FIG. 3 , the 2D pattern  16  is formed in the photo resist layer  11  as shown in  FIG. 4 . For layer  11 , a plurality of rectangular elements, which are parallel as defined by the mask  12 , are created in the layer  11 . 
         [0026]    The method in accordance with a preferred embodiment may utilize a second soft bake step, wherein the layer is baked to a temperature of approximately 120° C. (the post exposure bake temperature, as well as the soft bake temperature, depends on particular resist. For example, if another commonly used photoresist SU 8  is used, the two bake steps use same temperature condition (60° C. for 1 minute and 90° C. for 1 minute)) following the exposure of the photo resist  11  with a mask  12 , which will form the latent image in the event that the image has not been formed immediately after the exposure of the photo resist  11 . 
         [0027]      FIGS. 5-7  demonstrate how additional layers of photo resist  17  may be formed over layer  11 , and additional two-dimensional features may be formed in the new layer of photo resist  17 . A second mask  18  includes a plurality of parallel features  19  which are perpendicular to the features  16  of photo resist layer  11 . The process is the same, wherein the photo resist  17  is soft baked after being spin coated on surface  11 . As shown in  FIG. 7 , after being exposed by light  13  through mask  18 , a plurality of rectangular bars  21  are formed in the photo resist  17 . 
         [0028]    Exposed areas of the photo resist layers comprise cross-linked molecules due to the exposure which are not soluble by solvents conventionally used to removed photo resist. Accordingly, during the step of developing the layers of photo resist, to be described with respect to  FIGS. 11-13 , unexposed areas of resist will be removed leaving only the exposed features in each of the layers. 
         [0029]      FIG. 9  shows a subsequent layer of photo resist  22  deposited on layer  17 . In a similar matter, layer  22  is soft baked and unexposed through a mask  26  having plurality of light emitting apertures  27 . The light  13  is of the aforementioned wave length, which is selected to expose the underlying photo resist without exposing the previously configured layers of photo resist  11  and  17 . 
         [0030]    As shown in  FIG. 10 , new structures  28  defined by the two-dimensional mask  26  are formed in the layer  22 . 
         [0031]    When all of the layers for comprising the  3 D structure have been deposited and exposed by the two-dimensional masks, latent images are formed throughout the layers as shown in  FIG. 10 . The entire substrate and composite layers of photo resist are then developed, to derive the structure shown in  FIG. 11 . Photo resist which has not been exposed, and therefore does not exhibit cross-linking of the molecules, is dissolved by the developer leaving only the photo resist of each layer which had been exposed to light. 
         [0032]      FIGS. 12 and 13  show three-dimensional objects formed with additional layers of photo resist to produce additional structures  30  and  31 . 
         [0033]    The foregoing process can be used to create objects on a substrate having dimensions in microns. This is specifically applicable, but not limited to, the creation of photonic optical circuit devices as well as micro-fluidic devices. Feature size may be limited by the diffusion of photo acid upon the post exposure baking if the cross-linking mechanism is chemically amplified. Using spin coating techniques, the thickness of each layer may be varied so that the objects third dimension may be more precisely controlled. 
         [0034]    The total number of layers that can be fabricated depends on the difference of solubility between the cross-linked resist due to exposure, and the uncrossed linked resist which has not been exposed. 
         [0035]    The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention in the context of a method for making three-dimensional structures on a substrate having micron dimensions, and an article of manufacture having three-dimensional objects on a substrate with micron dimensions, but, as mentioned above, it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form or application disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.