Abstract:
A method for maintaining a smooth surface of crystallizable material is disclosed. First, a substrate is provided. A target material layer is then formed on the substrate, with the target material being a crystallizable material. A protecting layer is subsequently formed on the target material layer. Next, an annealing treatment is implemented, with the surface of the target material layer, facing the protecting layer, being maintained in its original smooth state by the pressure and/or adhesion of the protecting layer. Finally, the protecting layer is removed to leave an open and smooth surface of the processed crystallizable material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The entire contents of Taiwan Patent Application No. 098131398, filed on Sep. 17, 2009, from which this application claims priority, are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a method for maintaining a smooth surface of material, and more particularly to a method for maintaining a smooth surface of crystallizable material. 
     2. Description of Related Art 
     Generally speaking, in the forming process of crystallizable materials, it is common to form a seed layer which is used as a crystal growth center. Then, a target crystallizable material is further formed based on the seed layer. The materials of the seed layer and the target crystallizable material can be the same or different.  FIGS. 1A-1B  show crystal-growth steps corresponding to formation of conventional zinc oxide nanorods. First, referring to  FIG. 1A , a substrate  110  is provided, and a zinc oxide seed layer  120  is formed on the substrate  110 . Then, referring to  FIG. 1B , zinc oxide nanorods  150  are formed, wherein the zinc oxide seed layer  120  is used as a crystal growth center. 
     The dimension of the zinc oxide nanorods  150  is related to the crystallization of the zinc oxide seed layer  120 . When the grain size of the zinc oxide seed layer  120  is larger, coarse zinc oxide nanorods  150  can be formed; when the grain size of the zinc oxide seed layer  120  is smaller, smaller zinc oxide nanorods  150  can be formed. In order to adjust the grain size of the zinc oxide seed layer  120 , an annealing treatment can be implemented to the zinc oxide seed layer  120  for adjusting the crystallization of the zinc oxide seed layer  120 . When the temperature of the annealing treatment is higher, the grain size of the zinc oxide seed layer  120  is bigger; when the temperature of the annealing treatment is lower, the grain size of the zinc oxide seed layer  120  is smaller. By controlling the temperature of the annealing treatment, the dimension of the zinc oxide nanorods  150  can be controlled thereby. 
     However, when the temperature of the annealing treatment is higher, the grain size of the zinc oxide seed layer  120  is larger, leading to a rough and uneven surface of the zinc oxide seed layer  120 . The subsequently formed zinc oxide nanorods  150  will not grow in (e.g., towards) substantially the same direction and will not form an ordered array. 
     For the disadvantages of the prior art mentioned above, there remains a need to provide a method for maintaining a smooth surface of material. The surface of the material should be maintained smooth after the annealing treatment. High quality nano-structures could then be formed by using the smooth surface of material as a crystal growth center. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a method for maintaining a smooth surface of material, so as to overcome the drawbacks of the prior art, while further fulfilling requirements of the relevant industry. 
     One object of the present invention is to provide a method for maintaining a smooth surface of material. The surface of material can be maintained smooth after the annealing treatment. 
     Another object of the present invention is to provide a method for maintaining a smooth surface of material. A high quality nano-structure can be formed by using the smooth surface of material as a crystal growth center. 
     According to the objects, the present invention provides a method for maintaining a smooth surface of crystallizable material. First, a substrate is provided. A target material layer is then formed on the substrate, wherein the target material is a crystallizable material. A protecting layer is subsequently formed on the target material layer. Next, an annealing treatment is implemented, wherein the surface of the target material layer, facing the protecting layer, is maintained in its original smooth state by the pressure and/or adhesion of the protecting layer. Finally, the protecting layer is removed to leave an open and smooth surface of the processed crystallizable material. 
     By the method for maintaining a smooth surface of material of the present invention, the surface of material can be maintained smooth after the annealing treatment. High quality nano-structure can be formed by using the smooth surface of material as a crystal growth center. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1B  show crystal growth steps of conventional zinc oxide nanorods; 
         FIG. 2  shows a flow diagram of a method for maintaining a smooth surface of material in accordance with one embodiment of the present invention; 
         FIGS. 3A-3E  show steps of the method for maintaining a smooth surface of material; 
         FIGS. 4A-4B  show electron microscope images of surfaces of the seed layer of the present invention and the conventional seed layer respectively; 
         FIGS. 5A-5B  show electron microscope images of ZnO nanorods of the present invention and conventional ZnO nanorods respectively; 
         FIG. 6  shows X-ray diffraction patterns of the ZnO nanorods of the present invention and the conventional ZnO nanorods; and 
         FIG. 7  shows an EDX result of the ZnO seed layer of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A detailed description of the present invention will now be provided with reference to the following embodiments, which are not intended to limit the scope of the present invention and which can be adapted for other applications. While the drawings are illustrated in detail, it is appreciated that quantities of components may be greater or less than that disclosed, except for instances expressly restricting amounts of the components. 
       FIG. 2  shows a flow diagram of a method  400  for maintaining a smooth surface of material in accordance with one embodiment of the present invention. The method  400  includes the following steps: step  410 , providing a substrate; step  420 , forming a target material layer on the substrate, wherein the target material is a crystallizable material; step  430 , forming a protecting layer on the target material layer; step  440 , implementing an annealing treatment, wherein the surface of the target material layer is maintained in its original smooth state by the pressure and/or adhesion of the protecting layer; and step  450 , removing the protecting layer. 
       FIGS. 3A-3E  show steps of the method  400  for maintaining a smooth surface of material. Referring to  FIG. 3A , a substrate  210  is provided. The material of the substrate  210  can be a metal, inorganic material, or plastic material, wherein the inorganic material can include silicon substrate, quartz, glass, or sapphire. Then, a target material layer  220  is formed on the substrate  210 . The material of the target material layer  220  is a crystallizable material. The crystallization of the crystallizable material can be adjusted by following an annealing treatment. 
     The material of the target material layer  220  can be an inorganic semiconductor material or an organic polymer material. The inorganic semiconductor material mentioned above can include at least one or any combination selected from the group consisting of: Zinc oxide (ZnO), tin dioxide (SnO 2 ), indium oxide (In 2 O 3 ), indium tin oxide (ITO), aluminum-doped zinc oxide (AZO), cadmium telluride (CdTe), titanium dioxide (TiO 2 ), zinc sulfide (ZnS), zinc selenide (ZnSe), and copper indium gallium selenium compounds (CuInGaSe 2 ). The method for forming the target material layer  220  can be spin coating, dip coating, evaporation, sputtering, atomic layer deposition, electrochemical deposition, pulsed laser deposition, or a metal-organic chemical vapor deposition method. 
     Referring to  FIG. 3B , a protecting layer  230  is formed on the target material layer  220 . The thickness of the protecting layer  230  is about 10 nm-100 μm. In this embodiment, the protecting layer  230  is formed of a metal material on the target material layer  220  by evaporation. The thickness of the metal material is about tens nm-hundreds nm. In another example, a gold layer, with thickness about 100 nm, is used as the protecting layer. The metal material should have an appropriate strength, high-temperature resistance and high-temperature stability. The metal material can be a single metal material or an alloy material used in conventional evaporation or sputtering processes, such as gold, platinum, chromium, silver, copper, zinc, gold-germanium alloy, gold-beryllium alloy, nickel, titanium, and so on, but is not limited to this. The material of the protecting layer  230  is not limited to a metal material; non-metal materials having appropriate strength, high-temperature resistance, and high-temperature stability can also be used as the material of the protecting layer  230 . The method for forming the protecting layer  230  can also be spin coating, dip coating, evaporation, sputtering, atomic layer deposition, electrochemical deposition, pulsed laser deposition, or a metal-organic chemical vapor deposition method. 
     Referring to  FIG. 3C , an annealing treatment is implemented. The temperature of the annealing treatment is about 200-2000° C. for increasing the crystallinity of the target material layer  220 . When the temperature of the annealing treatment is higher, the grain size of the target material layer  220  is larger; when the temperature of the annealing treatment is lower, the grain size of the target material layer  220  is smaller. The annealing treatment includes at least one or any combination selected from the group consisting of: rapid thermal annealing, high temperature furnace, baking oven, and laser annealing. It is noted that the surface of the target material layer  220  is maintained in its original smooth state by the pressure and/or adhesion of the protecting layer  230  during the annealing treatment. 
     Referring to  FIG. 3D , the protecting layer  230  is removed. In this embodiment, the method for removing the protecting layer  230  is a solution method, wherein the protective layer  230  can be dissolved by a solvent, and the solvent does not dissolve or damage the target material layer  220 . The solvent is selected according to the material of the protecting layer  230  and the material of the target material layer  220 . For example, if the material of the protecting layer  230  is gold, the solvent can be aqua regia, chlorine, bromine solution, potassium iodide, iodine solution, potassium cyanide, or sodium sulfide; if the material of the protecting layer  230  is silver or copper, the solvent can be nitric acid, hot concentrated sulfuric acid, or concentrated hydrochloric acid. 
     Referring to  FIG. 3E , after removing the protecting layer  230 , the target material layer  220  can be used as a seed layer for forming an inorganic micro/nano semiconductor structure on the target material layer  220 . For example, a micro/nano semiconductor array  250  is formed. The micro/nano semiconductor array  250  can be further used as a crystal growth center for forming a nitride structure. The method for forming the micro/nano semiconductor array  250  mentioned above can be hydrothermal, thermal evaporation, chemical vapor deposition, molecular beam epitaxy, a porous anodic aluminum oxide template method (AAO), or an electrochemical method. 
     The dimension of the micro/nano semiconductor array  250  is related to the crystallization of the target material layer  220 . When the grain size of the target material layer  220  is larger, coarse units of the micro/nano semiconductor array  250  can be formed; when the grain size of the target material layer  220  is smaller, smaller units of the micro/nano semiconductor array  250  can be formed. Therefore, by controlling the temperature of the annealing treatment, the dimension of the units of the micro/nano semiconductor array  250  can be controlled. Moreover, the surface of the target material layer  220  can be maintained smooth after the annealing treatment. The micro/nano semiconductor array  250 , formed on the surface of the target material layer  220  as a crystal growth center, will grow in (e.g., towards) substantially the same direction. Thus, high quality micro/nano semiconductor array  250  can be obtained. 
     According to this embodiment, each unit of the micro/nano semiconductor array  250  is a ZnO nanorod, wherein the length of the ZnO nanorod is about 10 nm to 50 μm, the lateral dimension of the ZnO nanorod is about 30 nm to 10 μm, and the pitch of the ZnO nanorod is about 10 nm to 1000 μm. 
       FIGS. 4A-4B  show electron microscope images of surfaces of the seed layer of the present invention and the conventional seed layer respectively. As shown in  FIG. 4A , the surface of the seed layer of the present invention is maintained smooth after the annealing treatment. On the other hand, as shown in  FIG. 4B , the surface of the conventional seed layer is very rough after the annealing treatment. 
       FIGS. 5A-5B  show electron microscope images of ZnO nanorods of the present invention and conventional ZnO nanorods respectively. As shown in  FIG. 5A , the ZnO nanorods of the present invention are coarse, wherein the ZnO nanorods grow in or towards substantially the same direction. On the other hand, as shown in  FIG. 5B , the conventional ZnO nanorods are small, wherein the ZnO nanorods grow in or towards irregular directions. 
       FIG. 6  shows X-ray diffraction patterns of the ZnO nanorods of the present invention and the conventional ZnO nanorods. The dashed line represents the ZnO nanorods of the present invention; the solid line represents the conventional ZnO nanorods. According to the peaks which are shown in  FIG. 6 , a conspicuous peak, which is due to ZnO (002), is observed at a 2θ value of about 34°. The peak intensity of the ZnO nanorods of the present invention is about 11 times that of the peak intensity of the conventional ZnO nanorods. This result indicates that most of the ZnO nanorods of the present invention are (e.g., are oriented) in or towards substantially the same direction. 
       FIG. 7  shows an EDX result of the ZnO seed layer of the present invention. As shown in  FIG. 7 , the ingredients of the ZnO seed layer include the substrate (Si) and ZnO seed layer (Zn, O). There is no conspicuous peak showing that the protecting layer (Au) remains on the ZnO seed layer. Thus, one can conclude that the growth of the ZnO nanorods is not related to the protecting layer (Au). 
     Although specific embodiments have been illustrated and described, it will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the present invention, which is intended to be limited solely by the appended claims.