Abstract:
A method of multilevel microfabrication processing is provided. The method includes providing a planar substrate that comprises one or more material layers. A first hardmask layer placed on top of the substrate is patterned into the lithographic pattern desired for the top lithographic layer. Subsequent hardmask layers are patterned until the number of hardmask layers equals the number of lithographic layers desired. The method includes etching into the substrate and stripping the top hardmask layer. Furthermore, the method includes alternating etching into the substrate and stripping the subsequent hardmask layers until the bottom hardmask layer is stripped.

Description:
PRIORITY INFORMATION 
     This application claims priority from provisional application Ser. No. 60/630,679 filed Nov. 23, 2004, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to the field of microfabrication and nanofabrication processing and, in particular, to multilevel processing. 
     Multilevel processing has become a requirement for almost all microfabricated and nanofabricated devices. Multilevel processing relates to the formation of a plurality of lithographic layers. It generally involves successive material deposition, aligned lithography, etching, and planarization steps. Each lithographic layer requires a separate material deposition step, which has to be immediately followed by lithography, patterning, and planarization. These steps are then repeated for each lithographic layer until the desired microstructure is formed. This standard multilevel process has several drawbacks that will now be considered. 
     First, planarization is problematic as it is typically the yield-limiting step. In addition, it is costly and may generate particles that can never be completely removed. Second, all the abovementioned fabrication steps have different contamination requirements. If one keeps alternating from one type of process to the next, all the processes have to reach the cleanliness standard of the most demanding step, typically material deposition. In some cases, this may be impossible to achieve so the application of the standard multilevel fabrication technique may be prohibited entirely. Finally, the number of fabrication steps is high and contributes to making the process costly. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided a method of multilevel microfabrication processing. The method includes providing a planar substrate that comprises one or more material layers. A first hardmask layer placed on top of the substrate is patterned into the lithographic pattern desired for the top lithographic layer. Subsequent hardmask layers are patterned until the number of hardmask layers equals the number of lithographic layers desired. The method includes etching into the substrate and stripping the top hardmask layer. Furthermore, the method includes alternating etching into the substrate and stripping the subsequent hardmask layers until the bottom hardmask layer is stripped. 
     According to another aspect of the invention, there is provided a method of forming a multilayered upturned structure. The method includes providing cladding structure. A first hardmask layer placed on top of the cladding structure is patterned into the lithographic pattern desired for the top lithographic layer. Subsequent hardmask layers are patterned until the number of hardmask layers equals the number of lithographic layers desired. The method includes etching into the cladding structure and stripping the top hardmask layer. Furthermore, the method includes alternating etching into the cladding structure and stripping the subsequent hardmask layers until the bottom hardmask layer is stripped. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1H  illustrates the invention by applying it to the fabrication of a three-layer structure; 
         FIGS. 2A-2B  show the experimental demonstration of the invention through the fabrication of a two-layer optical polarization splitter and rotator; 
         FIGS. 3A-3I  illustrate the upturned invention by applying it to the fabrication of an upturned three-layer structure; and 
         FIGS. 4A-4C  illustrate the combination of the techniques used by the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is an improved and simplified multilevel fabrication technique that removes the need for planarization steps. It regroups processes of the same type. All the required material deposition is done first; all the needed lithography is done second; and all the needed patterning is done third. This is illustrated in  FIGS. 1A-1H  by presenting the fabrication a three-layer structure.  FIG. 1A  shows the deposition of a material  2  on a wafer  4 .  FIGS. 1B-1D  show the formation of a first  6 , second  8 , and third  10  hardmasks.  FIG. 1E  illustrates a first etch and the removal of one of the hardmask  6 .  FIGS. 1F and 1G  illustrate a second and third etch, forming an etched structure  12 , and successive removal of the remaining hardmasks  6  and  8 .  FIG. 1H  demonstrates the deposition of cladding materials  14  on the remaining etched structure  12 . In this embodiment, the last hard mask can be replaced by a resist mask. Moreover, the deposition of material  2  and of cladding materials  14  can be omitted in practice. 
     The technique uses a plurality of selective hardmasks, for example, one can be selectively removed without affecting the other. The material choices for the plurality of hardmasks are numerous and depend on the material to be patterned and on contamination issues. The plurality of hardmask materials have to be chosen so one is able to remove a hardmask selectively without removing the one just below it. Moreover, the hardmask materials must withstand the patterning steps employed. The inventive technique is particularly useful for fabrication of multilevel structures where every successive layer is patterned into a subgroup of the previous layer. Such structures were used in the illustrations presented in  FIGS. 1A-1H . Note the hardmasks described can include materials such as silicon dioxide, diamond-like carbon, nickel, chromium, other metals, or similar materials. 
     Any number of layers may be patterned. In all figures, the layer to be patterned is represented as a single material for simplicity. However, the present invention is not limited to patterning a single material and can be applied with no modification to pattern complex material stacks as well. The advantages of the invention over the standard multilevel fabrication technique are: planarization is avoided, lowering the cost of fabrication, reducing the particle-count, and improving the yield; the number of fabrication steps is smaller making the fabrication quicker and cheaper; cleanliness requirements are eased, which is particularly important when some of the processing steps are done in experimental rather than manufacturing facilities; and all of the lithography is done on the same level making the alignment easier and more accurate. Moreover, in the standard process, special care has to be taken to ensure that the alignment marks are visible and problems arise when some of the materials used are opaque. This is not a concern for the inventive technique. 
     The invention was used to successfully fabricate a two-layer optical polarization splitter and rotator.  FIGS. 2A-2B  shows electron micrographs of a fabricated two-layer optical polarization splitter and rotator. Two selective hardmask materials are used to form the device, and they comprise nickel and chromium. The sidewall roughness visible in the electron micrographs is due to insufficiently optimized lithography steps and not to the invention. The etch depths were accurately controlled by successive etching steps and profilometer measurements. Reactive-ion etching without a mask was shown not to alter the shape of the structure. In particular, cornering is not observed if excessive etching power is not used. The optical properties of the fabricated devices matched the rigorous optical numerical-simulations and further validated the invented fabrication technique. 
     The invention can also be applied to upturned structures as shown in  FIGS. 3A-3I . The upturned invention is particularly useful for structures where each layer is patterned into a subgroup of the subsequent layer.  FIG. 3A  shows the deposition of a cladding material  16  on a substrate  18 .  FIGS. 3B-3D  show the formation of a first  20 , second  22 , and third  24  hardmasks.  FIG. 3E  illustrates a first etch and removal of one of the hardmask  24 .  FIGS. 3F and 3G  illustrate a second and third etch of the cladding material, forming an etched structure  26 , and successive removal of the remaining hardmasks  20  and  22 .  FIG. 3H  demonstrates the deposition of a material  28  on the remaining etched structure  26 .  FIG. 31  illustrates the planaratization of the material  28  to form an upturned structured. 
     As shown in  FIGS. 4A-4C , the techniques of  FIGS. 1A-1G  can be combined with the techniques of  FIGS. 3A-3H  to create a wide variety of structures. The techniques of  FIGS. 3A-3H  are performed first.  FIG. 4A  represents the same step as  FIG. 3H  but with a thicker core material.  FIG. 4B  illustrates the planaratization of the thick core material  30  to become  32 .  FIG. 4C  illustrates the application of the techniques of  FIGS. 1A-1G  to pattern the thick core material  32  to result in a structure  34  exhibiting both an upturn structure and a three-layer structure such as the one shown in  FIGS. 1A-1G . Note that the planarization step illustrated at  FIG. 4B  can be omitted if one applies a self-planarizing deposition step such as spin-on processing or sputtering with a strong re-sputtering coefficient. 
     Dual-damascene processes are used in back-end microelectronic processing. These include patterning a given inter-metal dielectric layer twice to form a two-layer structure where the bottom-layer pattern is a sub-set of the top-layer pattern. Note that dual-damascene processes are only defined for two-layer structures while the present invention is defined for an arbitrary number of layers. Most of the dual-damascene processes alternate between a lithography and an etching step. These suffer from problems associated with performing lithography on non-planarized surfaces. Moreover, almost all dual-damascene processes require etch stops that impose considerable limitations on the types of materials that can be etched. For certain applications, such as microphotonic devices, etch stops are either undesirable or incompatible with the device functionality. 
     Some of the dual damascene processes use hardmasks. These, however, require the hardmask to be precisely etched using the second hardmask as a mask. This significantly constrains the possible hardmask materials one can use. In contrast, the invention only requires one to be able to strip one hardmask without damaging the other. This is a much less stringent requirement and allows a wide range of hardmask materials to be used. This strong improvement originates from the unique reversed ordering of the hardmasks reported in the invention (the first hardmask created corresponds to the pattern of the last layer and the last hardmask created corresponds to the pattern of the first layer). 
     Multiple etching steps and multilayer hardmasks are prior art. These are mostly used for patterning material stacks (plurality of materials stacked in a planar fashion). When the materials composing a material stack require different etching chemistries to be patterned, it is possible that a multistep etching process may be found useful. When a multistep etching process is used, it is possible that a multilayer hardmask may be found useful as it may be difficult to find a hardmask material that will resist to all the etching chemistries required to pattern the material stack. 
     The distinction between the invention and the multiple etching steps and multilayer hardmasks is that the multiple etching steps and multilayer hardmasks describe one lithographic layer patterning a plurality of material layers while the present invention describes a plurality of lithographic layers patterning either a single material or a plurality of materials. The present invention deals with improving the fabrication process when a plurality of lithographic layers is used. Each lithographic layer may be used to pattern a single material layer or a plurality of material layers. Also, the invention can be used in conjunction with inventions related to patterning of a single lithographic layer. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.