Patent Publication Number: US-2003224116-A1

Title: Non-conformal overcoat for nonometer-sized surface structure

Description:
FIELD OF THE INVENTION  
       [0001] The present invention generally relates to providing a protective coating on nanometer-sized surface structures using a non-conformal deposition process.  
       BACKGROUND OF THE INVENTION  
       [0002] As micro-processing technology advances, it becomes possible to manufacture components with surface structures as small as ten to hundreds of nanometers. FIGS. 1 a - 1   c  illustrate some examples of typical nanometer-sized surface structures. More advanced components may have multiple layers of the nanometer-sized surface structures.  
       [0003] A problem with most nanometer-sized surface structure is that they are very fragile and therefore susceptible to damage. Moreover, because of their relatively small topography, nanometer-sized surface structures are easily contaminated but difficult to clean. Furthermore, optical components having nanometer-sized surface structures, such as thin film wire grid polarizers have high insertion losses in either reflection or transmission, caused by the mismatch in the index of refraction between the device&#39;s surface and its environment, typically the atmosphere.  
       [0004] Thus, there is a need for a method of depositing a non-conforming continuous overcoat layer over a surface having nanometer-sized surface structures to protect such fragile surface structures and also improve their optical characteristics.  
       SUMMARY OF THE INVENTION  
       [0005] The invention provides a method of coating nanometer-sized surface structures with a protective overcoat using a non-conformal deposition process in which an overcoat material is directed at the surface structures at an oblique deposition angle until the overcoat material forms a continuous layer of overcoat material bridging over the gaps between the nanometer-sized surface structures without filling the gaps.  
       [0006] The deposition angle is measured from an axis normal to the substrate surface bearing the nanometer-sized surface structures. The deposition angle may be selected to be between zero and 90 degrees and should be sufficiently large that the overcoat material does not deposit into the gaps between the nanometer-sized surface structure. Hence, the particular deposition angle to be used in a given application will depend on the width of the gaps between the nanometer-sized surface structures.  
       [0007] The final surface of the overcoat layer is relatively flat and smooth compared to un-coated nanometer-sized surface structures. The overcoat not only provides surface protection but also may modify or enhance the functional performance of the device, such as, its reflectivity, transmittance, operating bandwidth, acceptance angle, resonance, etc.  
       [0008] In accordance with another embodiment of the invention, a layer of seeding material is added to the top of the nanometer-sized surface structures prior to overcoating. The seed layer is added to provide such enhancements as an adhesion promoter, a diffusion barrier, and a corrosion barrier at the interface between the overcoat layer and the nanometer-sized surface structures. The seed layer may be deposited onto the nanometer-sized surface patterns in advance of the overcoat material using the same non-conformal deposition process. The seed layer may be a metal or a dielectric material and the selection of a particular material for the seed layer is determined by the purpose of the particular seed layer. The seed layer may be single-layered or multilayered. Alternatively, the seed layer is provided as part of the nanometer-sized surface structures. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] For a better understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:  
     [0010]FIGS. 1 a - 1   c  illustrate some examples of typical nanometer-sized surface structures;  
     [0011]FIG. 2 a  illustrates a cross-sectional view of an overcoat layer coated over nanometer-sized surface structures according to the present invention;  
     [0012]FIG. 2 b  illustrates a cross-sectional view of a multilayered overcoat structure with a single seed layer between the nanometer-sized surface structures and the overcoat structure;  
     [0013]FIGS. 3 a - 3   c  illustrate nanometer-sized surface structures at different stages during the process of non-conformal overcoating of the nanometer-sized surface structures according to the process of the present invention;  
     [0014]FIGS. 4 a  and  4   b  illustrate nanometer-sized surface structures at different stages during the process of bidirectional non-conformal deposition of the overcoat layer according to another embodiment of the process of the present invention;  
     [0015]FIGS. 5 a - 5   c  illustrate nanometer-sized surface structures at different stages during the process of rotational non-conformal deposition of the overcoat layer according to another embodiment of the process of the present invention;  
     [0016]FIG. 6 illustrates a two-layer seed formed during the last fabrication step of the nanometer-sized surface structures;  
     [0017]FIG. 7 a - 7   c  illustrate cross-sectional views of the seed layer being formed by different embodiments of the non-conformal deposition process according to the present invention; and  
     [0018]FIG. 8 illustrates a cross-sectional view of a single-layer overcoat deposited over a single-layer seed material where both the overcoat and the seed layer were non-conformally coated by the process according to the present invention. 
    
    
     [0019] The drawings are only schematic and are not to scale.  
     DETAILED DESCRIPTION OF THE INVENTION  
     [0020] The following detailed description of the present invention is for illustrative purposes and should not be construed to limit the invention to these examples.  
     [0021]FIGS. 1 a - 1   c  illustrate typical devices  10   a ,  10   b , and  10   c  having nanometer-sized surface structures  20   a ,  20   b , and  20   c , respectively, on which a layer of overcoat may be deposited using the method of the present invention. Typically, nanometer-sized surface structures  20   a ,  20   b , and  20   c  are formed on substrates  30   a ,  30   b , and  30   c , respectively, in a regularly spaced pattern with gaps  40   a ,  40   b , and  40   c  between each element of the pattern. The spacing of adjacent structures  20   a ,  20   b , and  20   c  is typically in the range of ten to three-hundred nanometers.  
     [0022]FIG. 2 a  illustrates a cross-sectional view of a typical device  110  of the present invention having nanometer-sized surface structures  120  onto which an overcoat layer  160  has been deposited. The structures are separated by gaps  140 . FIG. 2 b  illustrates a cross-sectional view of a device  210  having nanometer-sized surface structures  220  that is coated with an overcoat layer  260  using another embodiment of the invention where the resulting overcoat layer is a multilayered structure. Again, the elements of the nanometer-sized surface structures  220  are separated by gaps  240 . In this embodiment, a seed layer  222  is provided between the nanometer-sized surface structures  220  and the overcoat layer  260 .  
     [0023]FIGS. 3 a - 3   c  illustrate a device  310  having a nanometer-sized surface structures  320  at different stages of the process of the present invention of non-conformally overcoating the nanometer-sized surface structures  320 . Again, the elements of the structures  320  are separated by gaps  340 . In FIG. 3 a , an overcoat material  350  is directed onto the nanometer-sized surface structures  320  obliquely along a deposit direction  352 . The oblique deposition angle θ is between zero and 90 degrees with respect to the orthogonal axis  370 . Because of the oblique deposition angle, the overcoat material  350  is deposited mostly on a top portion  322  of each element of the nanometer-sized surface structures  320  with minimal deposition along sidewalls  324  of the nanometer-sized surface structures  320  facing the source of the depositing material. As illustrated, the overcoat material  350  will overhang the nanometer-sized surface structures  320  on the side facing the incoming deposition material.  
     [0024] In FIG. 3 b , the deposition process has progressed further and the deposited portions of the overcoat material  350  on the top portions  322  of the nanometer-sized surface structures  320  are now touching each other. As illustrated, because the overcoat material  350  is being deposited uni-directionally, the growth of the depositing overcoat material  350  on the top portion  322  of each nanometer-sized surface structure  320  is asymmetric.  
     [0025]FIG. 3 c  illustrates a cross-sectional view of the nanometer-sized surface pattern where the deposition has been completed so as to form an overcoat layer  360 . The interim asymmetric structures formed by deposited overcoat material  350  have now all merged to form a relatively flat and smooth surface. The desired flatness and smoothness of the overcoat layer are achieved by varying the deposition angle θ. The particular angle θ necessary will depend on the geometry of the particular nanometer-sized surface structures. More particularly, the necessary deposition angle θ will depend on the depth and the width of the gaps  340  between adjacent nanometer-sized surface structures. For example, a non-conformal overcoat of silicon oxide can be deposited over a surface bearing nanometer-sized surface structures having a periodicity of 150 nm with a gap spacing of 70 to 100 nm and depth to width aspect ratio of 10:1 using a sputter deposition method with a deposition angle θ between 5 to 10 degrees.  
     [0026] In addition, in order to form a continuous solid overcoat layer  360  that completely seals the gaps  340 , the total overcoat thickness has to reach a critical value. Typical thickness-to-spacing ratios are in the 1:1 to 3:1 range.  
     [0027] The present invention can be practiced with any of the generally known physical vapor deposition or chemical vapor deposition methods as long as the deposition material has the directional characteristics. Examples of physical vapor deposition methods are sputtering and molecular beam epitaxy. Examples of chemical vapor deposition methods are plasma assisted (enhanced) chemical vapor deposition, photo chemical vapor deposition, laser chemical deposition, and chemical beam epitaxy. The details of measuring and controlling the deposition angles in each of these illustrative deposition methods are generally known in the art and they need not be discussed here.  
     [0028] If desired, a better overcoat flatness and surface finish can be achieved by depositing the overcoat material bidirectionally. FIGS. 4 a  and  4   b  illustrate cross-sectional views of a device  410  having nanometer-sized surface structures  420  being non-conformally coated in two directions. Again, the elements of the structures  420  are separated by gaps  440 . In this embodiment, an overcoat material  450  is directed onto the nanometer-sized surface structures  420  in a first deposition direction  452  at an oblique deposition angle θ as in the first embodiment of the process described in reference to FIGS. 3 a - 3   c . The deposition angle θ is measured with respect to orthogonal axis  470  of the substrate  430 . The overcoat material  450  is deposited in this first deposition direction  452  until the overcoat material  450  has partially bridged the gaps  440  between the nanometer-sized surface structures  420  as illustrated in FIG. 4 a . The overcoat material  450  is then directed in a second deposition direction  454  that has the same deposition angle θ as the first deposition direction  452  but preferably from the opposite side of the orthogonal axis  470  of the substrate  430 . The deposition of the overcoat material  450  in θ this second deposition direction  454  is continued until the overcoat material  450  has completely bridged the gaps  440  and form an overcoat layer  460 .  
     [0029] Because of the symmetry in the deposition process, the resulting overcoat layer  460  exhibits better flatness and surface finish than the overcoat layer  360  formed by the unidirectional non-conformal deposition described in reference to FIGS. 3 a - 3   c.    
     [0030] Alternatively, a second overcoat material (not shown) different from the overcoat material  450  may be used for the deposition in the second deposition direction  454 . The resulting overcoat layer will then have a composite structure.  
     [0031] A similar improvement in the flatness and the surface finish of the overcoat layer may be achieved by another embodiment of the present invention which is illustrated in FIGS. 5 a - 5   c . Again, a device  510  has nanometer-sized surface structures  520  formed on a substrate  530 . In this embodiment, the substrate  530  is rotated about its orthogonal axis  570  while an overcoat material  550  is directed in deposition direction  552  at the deposition angle θ. Because of the radial symmetry in the process, the overcoat material  550  is deposited on the top portions of the nanometer-sized surface structures  520  in a symmetrical manner as illustrated in FIG. 5 a . As the deposition process progresses, the deposited overcoat material  550  on top of the nanometer-sized structures  520  will extend evenly in all directions until the deposited overcoat material  550  from adjacent nanometer-sized structures  520  meets as illustrated in FIG. 5 b . The deposition process is continued until a sufficient amount of the overcoat material  550  is deposited to form a substantially flat overcoat layer  560  having a desired surface finish as illustrated in FIG. 5 c.    
     [0032] Thus, the method of the present invention provides a number of options for depositing an overcoat layer onto nanometer-sized surface structures. One or more of the embodiments of the present invention described above may be utilized to select the suitable deposition method for particular nanometer-sized surface structures. For example, the rotational deposition method may not be suitable for nanometer-sized surface structures having certain patterns that lack radial symmetry, such as the surface structure illustrated in FIG. 1 c , since the rotational deposition method would deposit the overcoat material inside the gaps between the nanometer-sized surface structures. But the rotational deposition method is better suited for depositing an overcoat layer over the nanometer-sized surface structures illustrated in Figures la and lb.  
     [0033] The overcoat layer is not only used to protect the surface structures of a particular device but the overcoat layer may also be configured to modify or enhance the device&#39;s performance. This is achieved by carefully selecting the overcoat layer&#39;s structure, the number of layers within the overcoat layer, the material properties, and the particular deposition methods, etc. In optics applications, in particular, the performance parameters that may be enhanced include, but are not limited to, reflectivity, transmittance, operating bandwidth, acceptance angle, resonance, etc. of an optical component. In contrast, for surface protection purposes, materials with high hardness are desirable. Thus, selecting the appropriate overcoat material can be crucial for achieving the desired optical performance and surface durability.  
     [0034] In optics applications, it may be particularly desirable to deposit an overcoat layer having a multilayered structure. In such applications, after the first overcoat layer is non-conformally deposited utilizing one of the embodiments of the deposition process described above, additional layers of overcoat material are deposited on the first overcoat layer. The additional overcoat layers need not be deposited using the deposition process of the present invention since the subsequent layers are deposited onto a continuous and substantially flat first overcoat layer. One such application is the formation of optical coatings having multiple layers with indices of refraction that alternate in a low-high-low-high manner. Advantageously, materials with low optical losses and large differences in an optical index are used in such applications. Typical materials for optical uses are cerium oxide, hafnium oxide, silicon oxide, magnesium oxide, magnesium fluoride, and titanium oxide.  
     [0035] In some applications, one or more seed material may be provided at the interface between the nanometer-sized surface structures and the overcoat layer to provide enhancements such as improved adhesion between the nanometer-sized surface structures and the overcoat material, a diffusion barrier, or a corrosion barrier, etc. The seed material may be provided in a single or multiple layers and it may be a metallic or a dielectric material suitable for the particular application.  
     [0036] The seed material may be incorporated into the nanometer-sized surface structures as illustrated in FIG. 6. Again, a device  610  has nanometer-sized surface structures  620  formed on a substrate  630 . One or more layers  622 ,  624  of a seed material are provided as part of the nanometer-sized surface structures  620 . In this example, the seed material layers  622 ,  624  are deposited onto the nanometer-sized surface structures during the fabrication process for the nanometer-size surface structures themselves. One or more overcoat layers can be deposited onto this structure using any one of the various embodiments of the present invention described above.  
     [0037] Alternatively, the one or more seed layers may be deposited onto the nanometer-sized surface structures using the non-conformal deposition process of the present invention before the overcoat layer is deposited. FIGS. 7 a - 7   c  illustrate the three alternative methods of depositing a seed layer onto a device  710  having nanometer-sized surface structures  720  using the three embodiments of the non-conformal deposition process according to the present invention: the unidirectional deposition; the bidirectional deposition; and the rotational deposition, respectively.  
     [0038] In FIG. 7 a , a seed material is directed onto the nanometer-sized surface structures  720  in the deposit direction  752  at a deposition angle θ, thereby forming a seed layer structure  722   a . In FIG. 7 b , the seed material is directed onto the nanometer-sized surface structures  720  first in the first deposit direction  752  and then in the second deposit direction  754 , resulting in the symmetrical seed layer structure  722   b . In FIG. 7 c , the seed material is directed onto the nanometer-sized surface structures  720  in deposit direction  752  while the device  710  is rotated about the orthogonal axis  770  of the substrate  730 , resulting in the symmetrical seed layer structure  722   c.    
     [0039] An overcoat layer may then be deposited over these interim structures using the non-conformal deposition method of the present invention. FIG. 8 illustrates an example of the final structure where an overcoat layer  760  is deposited over the interim structures of FIGS. 7 b  or  7   c.    
     [0040] It will be obvious to one of ordinary skill in the art that the different embodiments of the non-conformal deposition methods described above may be used individually but they may also be practiced in combination on a given surface to produce one or more desired overcoat layers or seed layers.  
     [0041] Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as is suited to the particular use contemplated. It is intended that the scope of the invention be defined by the appended claims and their equivalents.