Abstract:
The present invention discloses a method of surface texturing at nano/micro-scale by aluminum-induced rapid crystallization of amorphous silicon for controlling the wettability of a surface, enhancing cell attachment to a surface, and promoting cell growth on a surface. The present invention can be used in a variety of applications, such as producing superhydrophobic or superhydrophilic surfaces for medical devices, microelectromechanical systems, and microfluidic channels.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application claim priority to U.S. Provisional Patent Application Ser. No. 61/020,014, filed Jan. 9, 2008, entitled “NANO/MICRO-TEXTURED SURFACES AND METHODS OF MAKING SAME BY ALUMINUM-INDUCED CRYSTALLIZATION OF AMORPHOUS SILICON” by Min Zou and Hengyu Wang. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates primarily to a method of surface texturing and its applications, and more specifically, to a method of surface texturing at nano-/micro-scale by aluminum-induced rapid crystallization of amorphous silicon for uses in controlling surface wetting properties, promoting cell attachment on a surface, and promoting cell growth on a surface. 
       BACKGROUND OF THE INVENTION 
       [0003]    Surface textures have wide applications: they can be used to control the wetting properties of a surface and to promote cell attachment and growth on a surface [1-4]. 
         [0004]    Superhydrophobicity and superhydrophilicity are two valuable wetting properties of a surface. A superhydrophobic surface is defined as a surface with water contact angle (WCA) of more than 150°, while the superhydrophilic surface is a surface with WCA of less than 5° within 1 second after a water droplet drops on the surface. 
         [0005]    Superhydrophobic surfaces have attracted great attention in the past decade due to their important applications in surface self-cleaning, stiction prevention and drag reduction. Extensive studies show that a superhydrophobic surface has to have surface textures. A typical example of superhydrophobic surface is a lotus leaf surface that is textured with micro-sized “bumps” with nano-sized particles on the “bumps”. With a hydrophobic wax-like coating, the micro- and nano-textured surface becomes superhydrophobic. 
         [0006]    Superhydrophilic textured surfaces have also attracted much attention due to their wide range of applications. For example, superhydrophilic surface is anti-foggy. In winters, window-glasses of vehicles and lenses of eyeglasses can become foggy because the water vapor in air condenses on these surfaces. Similarly, endoscopic lenses and the mirrors used by dentists can be fogged by the condensation of the hot moisture in patients on the cold surfaces. Foggy surfaces can create safety issues in many cases. The safety of a driver can be undermined if the windshield of a vehicle becomes blurred, and an operation may be compromised if an endoscopic lens becomes foggy. 
         [0007]    In the biomedical field, textured surfaces also have important applications. Nano- or micro-scale textures can promote cell attachment to a surface and cell growth on a surface. For example, textured surfaces are beneficial to osteoblast response and bone growth for artificial joints. Textured surfaces can also improve the attachment of living cells on glass slides, which makes cell observation and analysis easy in research on living cells under microscopes. 
         [0008]    Given the importance of micro- and nano-textured surfaces, various surface texturing methods have been investigated and developed. For example, aligned carbon nanotubes, electroless etching, electroplating, oxygen plasma etching, soft-lithography imprinting, deep reactive ion etching have been used to texture surfaces [5-7]. However, these approaches are either too complicated, or too expensive, or not being able to produce a textured surface with stable wettability. 
         [0009]    In view of the above problems, there is a need for developing a simple and low cost method that can produce nano-/micro-textured surfaces for controlling surface wetting properties, promoting cell attachment to a surface, and enhancing cell growth on a surface. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention relates to a method of surface texturing at nano- and micro-scale by aluminum-induced rapid crystallization (AIRC) of amorphous silicon (a-Si) for controlling the wettability of a surface, enhancing cell attachment to a surface, and promoting cell growth on a surface. 
         [0011]    The method comprises the steps of: (a) forming a layer of a-Si on a surface of a substrate; (b) exposing the a-Si to air to form native silicon oxide buffer layer on the top of the a-Si film; (c) forming a layer of aluminum (Al) on the layer of native silicon oxide to form a sample; (d) applying heat treatment to the sample; and (e) removing excessive Al from the sample after the heat treatment. 
         [0012]    The native silicon oxide buffer layer between the a-Si and the Al is not critical to the present invention. The buffer layer can be silicon oxide, silicon nitride, etc. The a-Si and Al layers can also contact each other directly without a buffer layer. In addition, the order of forming a-Si and Al layers is not critical. An a-Si layer can be formed before or after the formation of an Al layer. Al and a-Si can also be deposited on a substrate simultaneously to form an Al and a-Si mixed layer. The number of a-Si or Al layers is not critical to the present invention either. The most important concept of present invention is using Al to induce the crystallization of a-Si to create a nano/micro-textured surface for use in controlling surface wettability and promote cell attachment and growth on a surface. 
         [0013]    Although aluminum-induced crystallization of a-Si has been extensively studied for many years, the focus has been on growing polycrystalline silicon films with large grains at low temperatures for the applications in the fields of electronics, photovaltics, and optoelectronics [8-11]. The present invention discloses, for the first time, the use of aluminum-induced crystallization of a-Si to produce nano/micro-textured surfaces for the applications in controlling the wetting properties of a surface, enhancing cell attachment to a surface, and promoting cell growth on a surface. 
         [0014]    The present invention has the following advantages:
       1. The wettability of the nano-/micro-textured surfaces produced by the present invention is stable, which is critical for real world applications.   2. The textures are transparent, which is particularly important for producing textures on glass slides for laboratory analysis of living cells attached on the glass slides.   3. The textures produced by the present invention are bio-compatible, which is critical for bio-related applications.   4. The equipment and materials used to produce surface textures are commonly used in semiconductor industry.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The accompany drawings illustrate one or more embodiments of the present invention and serve to explain the principles of the present invention. 
           [0020]      FIG. 1  is a schematic drawing of one embodiment showing the process steps for producing nano/micro-textured surface using the AIRC of a-Si technique according to the present invention. 
           [0021]      FIG. 2  is a scanning electron microscopy (SEM) image showing the surface topography of a textured surface created by the AIRC of a-Si technique according to the present invention. 
           [0022]      FIG. 3  is an energy dispersive X-ray spectroscopy (EDS) spectrum showing the chemical elements of the textured surface created by the AIRC of a-Si technique according to the present invention. 
           [0023]      FIG. 4  is an X-ray diffraction (XRD) spectrum showing the crystalline orientation of the textures on the textured surface created by the AIRC of a-Si technique according to the present invention. 
           [0024]      FIG. 5  is an electron diffraction pattern taken from a silicon crystallite on the textured surface. 
           [0025]      FIG. 6  is a schematic drawing of another embodiment showing the process steps for producing nano/micro-textured surface using the AIRC of a-Si technique according to the present invention. 
           [0026]      FIG. 7  is a schematic drawing of yet another embodiment showing the process steps for producing nano/micro-textured surface using the AIRC of a-Si technique according to the present invention. 
           [0027]      FIG. 8  is a schematic drawing showing sample structures illustrating how to produce a superhydrophobic surface using the AIRC of a-Si technique according to the present invention. 
           [0028]      FIG. 9  is an SEM image showing the surface topography of a textured surface created by the AIRC of a-Si technique. 
           [0029]      FIG. 10  is an optical image showing superhydrophobicity of a textured surface. 
           [0030]      FIG. 11  is a schematic drawing showing sample structures illustrating how to produce a superhydrophilic surface using the AIRC of a-Si technique according to the present invention. 
           [0031]      FIG. 12  is a SEM image showing surface topography of a textured surface created by the AIRC of a-Si technique. 
           [0032]      FIG. 13  is an optical image showing superhydrophilicity of a textured surface. 
           [0033]      FIG. 14  is a schematic drawing showing sample structures illustrating how to produce a textured surface to enhance cell attachment using the AIRC of a-Si technique according to the present invention. 
           [0034]      FIG. 15  is an SEM image showing the surface topography of a textured surface created by the AIRC of a-Si technique. 
           [0035]      FIG. 16  is a high magnification optical image showing cell attachment on surfaces using the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    The present invention is more particularly described in the following embodiments and examples which are intended for illustrative purpose only since numerous modifications and variations will be apparent to those skilled in the art. The detailed description of the present invention is in no way intended to limit the invention, its application, or uses. 
       First Embodiment 
       [0037]    Referring to  FIG. 1 , a plasma-enhanced chemical vapor deposition (PECVD) system is used to deposit a layer of a-Si  102  on a silicon oxide coated silicon ( 100 ) wafer  101  to form a sample. After the deposition of a-Si  102 , the sample is removed from the PECVD system and exposed to air to form a thin layer of native oxide  103  on top of the a-Si  102 . The sample is then transferred to an evaporator for thermal deposition of a layer of Al  104  on top of the native silicon oxide  103  to form a multi-layer structure  100 . The structure  100  is annealed in air in a conventional furnace. After annealing, the excessive Al on structure  100  is removed by wet selective etching, resulting in nano/micro-textured surfaces. 
         [0038]      FIG. 2  shows an SEM image of a textured surface  200  produced by the method illustrated in  FIG. 1 . The bright spots  201  are randomly distributed nano-/micro-silicon crystallites.  FIG. 3  is an EDS spectrum taken from the textured surface  200 . The spectrum  300  contains a large amount of Si  301  and O  302  but no Al, which indicates that the textures  201  are made of silicon or silicon oxide.  FIG. 4  is an XRD spectrum  400  taken from the textured surface  200 . The peak  401  around 28.5° reveals that the textured surface  200  is made of silicon (111) crystallites.  FIG. 5  is an electron diffraction pattern collected from a silicon crystallite  201  that is detached from the textured surface  200 . It further confirms that the textures  201  on the textured surface  200  are silicon (111) crystallites. 
       Second Embodiment 
       [0039]    Referring to  FIG. 6 , a thermal evaporator is used to deposit a layer of Al  602  on a silicon oxide coated silicon (100) wafer  601  to form a sample. After the deposition of Al  602 , the sample is removed from the evaporator and exposed to air to form a thin layer of aluminum oxide  603  on top of the Al  602 . The sample is then transferred to a PECVD system for the deposition of a layer of a-Si  604  on top of the aluminum oxide  603  to form a multi-layer structure  600 . The structure  600  is annealed in air in a conventional furnace. During annealing, some Al diffuses through the a-Si layer  604  to the top surface of the sample  600 . After annealing, the Al that passed through the a-Si  604  is removed by wet selective etching, resulting in nano/micro-textured surfaces. 
       Third Embodiment 
       [0040]    Referring to  FIG. 7 , using an e-beam evaporator, a layer of a-Si and Al mixture  702  is deposited on a silicon oxide coated silicon (100) wafer  701  to form a sample  700 . The sample  700  is annealed in air in a conventional furnace. During annealing, some Al diffuses to the top surface of the sample  700 . After annealing, the excessive Al on the top surface of the structure  700  is removed by wet selective etching, resulting in nano/micro-textured surfaces. 
         [0041]    In the following examples, various process parameters will be described to illustrate how to use the technique to control the surface wettability and promote cell attachment on a surface in detail. These examples are for illustration purpose only. These specific materials, parameters, and equipment used in the examples are not meant to limit the scope of the invention. 
       EXAMPLE 1 
       [0042]    This example illustrates how to use the AIRC of a-Si technique according to the present invention to create a superhydrophobic surface. 
         [0043]    Referring to  FIG. 8 , one-side polished p-type silicon (100) wafers  801  is selected as a substrate for producing nano/micro-textured surfaces using the AIRC of a-Si technique. The silicon wafer  801  is cleaned by acetone, isopropanol, and deionized water and then wet oxidized at 950° C. for 8 hours to grow a 2 micron-thick silicon oxide film  802 . The purpose of growing the thick silicon oxide  802  prior to depositing a-Si  803  is to prevent the crystalline structure of the substrate  801  from affecting the AIRC of a-Si process. The reason for using silicon (100) wafer  801  as a substrate is that silicon (100) wafer  801  is easy to cut for sample preparation in research. 
         [0044]    A PECVD system (Plasma-Therm SLR730) is used to deposit an a-Si film  803  of thickness 100 nm on the silicon oxide layer  802  to form a structure consisting of the silicon substrate  801 , thermal silicon oxide  802 , and a-Si  803 . The radio-frequency (RF) power, chamber pressure, substrate temperature, and SiH 4  flow rate are controlled at 20 W, 1 Torr, 250° C., and 85 sccm, respectively. After a-Si  803  deposition, the structure is removed from the PECVD system and exposed to air for three days to form a thin layer of native oxide  804  on top of the a-Si  803 . The native oxide  804  increases the consistency and repeatability of the AIRC of a-Si process. An evaporator (Edward&#39;s Auto  306 ) is then used to evaporate an 800 nm-thick Al  805  on the native oxide  804  to form a sample  800 . 
         [0045]    The sample  800  is then annealed in air in a conventional furnace (Lindberg/Blue Box BF51894C) at 850° C. for 5 seconds. After annealing, the excessive Al was removed by immersing the sample  800  in etching solution “Al etchant-type D” (Transene Company, Inc., Danvers, Mass.) for 15 min while maintaining the solution at 50° C., resulting in textured surface  900  with nano/micro-structures as shown in  FIG. 9 , in which bright spots  901  are nano/micron-sized silicon crystallites. 
         [0046]    The sample  900  is cleaned by soaking in piranha solution at 20° C. for 1 hour. After piranha cleaning, the sample  900  is rinsed with deionized water and Toluene and blown dried with N 2  gas. The samples were then dipped into an Octadecyltrichlorosilane (OTS)/Toluene solution with an OTS mass concentration of 1% for 10 min to allow the OTS to uniformly self-assemble on the sample surfaces. After the process, the textured surface  900  becomes superphydrobolic with a WCA of 155°.  FIG. 10  shows a water droplet  1001  on the OTS coated textured surface  900 , in which  1001  is an optical image of a water droplet. 
       EXAMPLE 2 
       [0047]    This example illustrates how to use the AIRC of a-Si technique according to the present invention to create a superhydrophilic surface. 
         [0048]    Referring to  FIG. 1 , pre-cleaned glass micro slide from VWR International is used as substrate  1101 . The substrate  1101  is cleaned in an O 2  plasma asher (LFE APE 110 Plasma system) prior to a-Si deposition. The RF power, chamber pressure, time, and O 2  flow rate were controlled at 250 W, 500 mTorr, 5 min, and 80 sccm, respectively. After cleaning, the substrate  1101  is immediately placed in a PECVD system (Plasma-Therm SLR730) for the deposition of 100 nm-thick a-Si  1102 . The RF power, chamber pressure, substrate temperature, and SiH 4  flow rate are controlled at 20 W, 1 Torr, 250° C., and 85 sccm, respectively. The a-Si  1102  coated substrate  1101  is taken out from the PECVD system and left in air for 48 hours at 24° C. in air to grow a thin layer of native oxide  1103  on top of the a-Si  1102 . After that, a 670 nm-thick Al film  1104  is deposited on the native oxide  1103  using a thermal evaporator (Edward&#39;s Auto 306) to form a sample  1100 . 
         [0049]    The sample  1100  is annealed at 650° C. for 10 minutes in air using a Lindberg/Blue Box Furnace (Model BF51894C) and then is selectively etched to remove the residual Al in an etching solution “Al etchant—type D” (Transene Company, Inc., Danvers, Mass.) at 50° C. for 5 minutes to form a nano/micro-textured surface  1200  as shown in  FIG. 12 .  FIG. 12  is an SEM image of the textured surface  1200 . The white spots  1201  are micro-scaled textures with nano-sized spikes. Such unique structure is critical for a stable superphydrophilic surface.  FIG. 13  shows an optical image of a 0.5 μl water droplet  1302  on the textured surface  1200  0.5 seconds after a water droplet drops onto the textured surface  1200  from a needle  1303 .  FIG. 13  is taken 192 hours after the textured surface  1200  is fabricated. The WAC is still less than 5°, confirming that the textured surface  1200  is superhydrophilic and the superhydrophilicity is stable. 
       EXAMPLE 3 
       [0050]    This example illustrates how to use the AIRC of a-Si technique according to the present invention to promote cell attachment on a surface. 
         [0051]    Referring to  FIG. 14 , pre-cleaned plain glass micro slide from VWR International is selected as a substrate  1401 . The substrate  1401  is 76.2 mm long, 25.4 mm wide, and 1.2 mm thick. The primary reason for using glass slides, instead of silicon wafers, is that glass slides is biocompatible and optically transparent, which allows the adhered cells to be characterized using a phase-contrast microscope. 
         [0052]    The first step of the process is to use a PECVD system (Plasma-Therm SLR730) to deposit a 10 nm-thick a-Si film  1402  on the glass slide  1401 . The PECVD chamber pressure, RF power, substrate temperature, and SiH 4  flow rate are controlled at 0.2 torr, 20 W, 250° C., and 85 sccm, respectively. After a-Si deposition, the a-Si  1402  coated glass slide  1401  is moved into a thermal evaporation system (Edward&#39;s Auto 306) for the deposition of an 800 nm-thick Al layer  1403  on top of the a-Si  1402  to form a sample  1400 . Next, the sample  1400  is annealed in a furnace (Lindberg/Blue Box BF51894C) at 550° C. for 10 minutes to crystallize the a-Si. Finally, the Al  1403  on sample  1400  is removed by immersing the sample  1400  in an etching solution for 15 minutes while maintaining the solution at 50° C. The etching solution is Al etchant-type D from Transene Company, Inc., Danvers, Mass. 
         [0053]      FIG. 15  is an SEM image showing the textured surface  1500  of sample  1400 . The white spots  1501  are silicon crystallites. The size of crystallite ranges from nano- to micro-scale.  FIG. 16  shows high magnification optical images of live bacteria cells ( Escherichia coli  KAF95) attached on a non-textured surface and the textured surface  1500 .  FIG. 16(   a ) and  FIG. 16(   b ) are the optical images of a non-textured surface and a textured surface  1500 , respectively. The white spots  1601  in both  FIGS. 16(   a ) and  16 ( b ) are images of the bacteria cells. Compared with a non-textured surface showing in  FIG. 16(   a ), the textured surface showing in  FIG. 16(   b ) significantly improves cell attachment to a surface. The number of attached cells per unit area on the textured surface  1500  is more than 2 times higher than that on a non-textured surface. 
         [0054]    While there have been shown the preferred embodiments and several examples of the present invention, it is to be understood that the specific techniques, materials, parameters, and mechanisms that have been described are merely illustrative of the principles of the invention. Numerous modifications can be made to the method without departing from the scope and spirit of the invention. Furthermore, the embodiments and examples described above and the claims set forth below are only intended to illustrate the principles of the present invention and are not intended to limit the scope of the invention to the disclosed elements. 
       LIST OF REFERENCES 
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