Abstract:
A method of manufacturing nano crystals disclosed herein is applicable to the fabrications of memory device and solar cell. The method of manufacturing nano crystals at least comprises steps of: providing a substrate with a thin film formed thereon, and transforming the thin film into the nano crystals by laser annealing, wherein a thickness of the thin film is equal to or less than about 50 Å, and a wavelength of the laser selected for laser annealing is equal to or less than about 500 nm.

Description:
[0001]     This application claims the benefit of Taiwan application Serial No. 094130426, filed Sep. 5, 2005, the subject matter of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates in general to a method of manufacturing nano crystals and application of the same, and more particularly to the method of manufacturing nano crystals at a low temperature and application of the same.  
         [0004]     2. Description of the Related Art  
         [0005]     The crystals with nano sizes possess various advantages, for example, in the application of the memory device for being the quantum well to trap the electrons. Also, the nano crystals are featured with properties of light absorption (i.e. good absorbency index), and become one of the excellent light-absorption materials. Taking the silicon crystals as an example, the silicon crystals having regular size are able to store 30% of light energy, and the silicon crystals having nano size are able to store 50%-60% of light energy.  
         [0006]     Conventionally, the nano crystals can be formed by two methods. The first conventional method is to form the nano crystals on the substrate by chemical vapor deposition, and the processing temperature is about 650° C. at least. The second conventional method is to introduce the semiconductor such as silicon (Si) or germanium (Ge) into the silicon oxide (SiO 2 ) film by ion implantation, and then the nano Si or Ge crystals are formed in the SiO 2  film by thermal-annealing at a temperature of about 800° C. at least. Both conventional methods require high temperature procedures, which are not compatible with the process of making low-temperature poly-silicon thin film transistor (LTPS TFT).  
       SUMMARY OF THE INVENTION  
       [0007]     It is therefore an object of the present invention to provide a method of manufacturing nano crystals and application of the same. The method of the present invention utilizes a thin film and low-temperature laser annealing to produce the nano crystals; thus, it is particularly compatible with the process of making low-temperature poly-silicon thin film transistor (LTPS TFT).  
         [0008]     The present invention achieves the objects by providing a method of manufacturing nano crystals, comprising steps of:  
         [0009]     providing a substrate;  
         [0010]     forming a thin film on the substrate, and a thickness of the thin film equal to or less than about 50 Å; and  
         [0011]     subjecting the thin film under a laser annealing to transform the thin film into a plurality of nano crystals, and a wavelength of the laser selected for laser annealing equal to or less than about 500 nm.  
         [0012]     The present invention achieves the objects by providing a semiconductor structure having nano crystals. The structure comprises a substrate, and a plurality of nano crystals formed on the substrate at a low crystallizing temperature. Also, a particle size average of the nano crystals is less than about 10 nm.  
         [0013]     Other objects, features, and advantages of the present invention will become apparent from the following detailed description of the preferred but non-limiting embodiment. The following description is made with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1A  and  FIG. 1B  are cross-sectional views showing a method of manufacturing nano crystals according to the embodiment of the present invention.  
         [0015]      FIG. 2  is TEM (transmission electron microscope) result of the nano crystals manufactured by the embodiment of the present invention.  
         [0016]      FIG. 3A ˜ FIG. 3C  are cross-sectional views showing a method of manufacturing the memory device having nano crystals according to the embodiment of the present invention.  
         [0017]      FIG. 4  is a graph showing the electrical property of the memory device having nano crystals manufactured according to the method of the present invention.  
         [0018]      FIG. 5A ˜ 5 D are cross-sectional views showing a method of manufacturing the solar cell having nano crystals according to the embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     In the present embodiment of the invention, a method of manufacturing nano crystals and application of the same are disclosed. It is noted that the embodiment disclosed herein is used for illustrating the present invention, but not for limiting the scope of the present invention. Additionally, the drawings used for illustrating the embodiment and applications of the present invention only show the major characteristic parts in order to avoid obscuring the present invention. Accordingly, the specification and the drawings are to be regard as an illustrative sense rather than a restrictive sense.  
         [0020]      FIG. 1A  and  FIG. 1B  are cross-sectional views showing a method of manufacturing nano crystals according to the embodiment of the present invention. First, a substrate  11  is provided. The substrate  11  is preferably made of the material with no capability of storing laser light energy, such as glass, plastics, silicon oxide and metals. Then, a thin film  13  is formed on the substrate  11  as shown in  FIG. 1A , and a thickness of the thin film  13  is equal to or less than about 50 Å, and preferably about 15˜25 Å. Material for making the thin film  13  depends on the requirement of the practical application. Commonly, material of the thin film  13  includes silicon (Si), germanium (Ge) or SiGe.  
         [0021]     Next, the thin film  13  is subjected under a laser annealing to transform the thin film  13  into the nano crystals  131 . A wavelength of the laser selected for laser annealing is equal to or less than about 500 nm, and preferably in the range of 200 nm to 500 nm. Also, the particle size average of nano crystals  131  is about 10 nm or less.  
         [0022]     Furthermore, an insulative layer (not shown) can be formed on the substrate  11  before deposition of the thin film  13 . Examples of the insulative layer include silicon oxide, silicon nitride, and a combination thereof. However, formation of the insulative layer is not a necessary step of the method according to the present invention. Whether the insulative layer will be formed depends on the requirement of practical application.  
         [0023]     It is noted that the laser annealing step can be performed at a low temperature, such as room temperature. In other words, the nano crystals  131  can be grown on the substrate  11  at room temperature by using the method described above. Thus, the method of the present invention is particularly suitable for manufacturing the nano crystals on the substrate incapable of withstanding thermal procedure. Accordingly, the method of the present invention is compatible with the process of making low-temperature poly-silicon thin film transistor (LTPS TFT).  
         [0024]     The nano crystals manufactured by the embodiment of the present invention are further observed by transmission electron microscope (TEM), and the result is presented in  FIG. 2 . The result clearly shows that those tiny particles are nano-sized and crystallized.  
         [0025]     The nano crystals manufactured by the embodiment of the present invention possess several advantages, such as being the quantum wells and able to store higher light energy. Accordingly, two practical applications are disclosed herein for the advanced illustrations. It is, of course, understood that the present invention is applicable in many fields, and the memory device and solar cell just two of them.  
         [0000]     Application 1: Memory Device  
         [0026]      FIG. 3A ˜ FIG. 3C  are cross-sectional views showing a method of manufacturing the memory device having nano crystals according to the embodiment of the present invention. First, a substrate  30  such as a transparent glass is provided. Then, a polysilicon layer  31  is formed on the substrate  30 . Practically, an amorphous layer with a certain thickness is formed on the substrate  30  and then crystallized to form the polysilicon layer  31  by the known technique such as Excimer Laser Annealing (ELA), Continuous Grain Silicon (CGS), Sequential Lateral Solidification (SLS) or Metal Induced Lateral Crystallization (MILC). Next, a first insulative layer  32 , made of the material incapable of storing laser energy, is formed on the polysilicon layer  31 . Material of the first insulative layer  32  includes silicon oxide, silicon nitride, a combination thereof, and the like. Then, a thin film  33  (such as an amorphous silicon film) is formed on the first insulative layer  32 , as shown in  FIG. 3A . Also, a thickness of the thin film  33  is equal to or less than about 50 Å, and preferably about 15˜25 Å.  
         [0027]     Next, the thin film  33  is subjected under a laser annealing to form the numerous nano crystals  331  on the first insulative layer  32 , as shown in  FIG. 3B . A wavelength of the laser selected for laser annealing is equal to or less than about 500 nm, and preferably in the range of 200 nm to 500 nm. Also, the particle size average of nano crystals  331  could be less than about 10 nm. Afterward, a second insulative layer  35  is formed on the first insulative layer  32  to cover the nano crystals  331 . Finally, a metal gate is formed on the second insulative layer  35 , as shown in  FIG. 3C . Also, the first insulative layer  32  and the second insulative layer  35  could be made of the same or different materials.  
         [0028]     The nano crystals  331  of  FIG. 3C  function as the quantum wells of the memory device to trap the electrons.  FIG. 4  is a graph showing the electrical property of the memory device having nano crystals manufactured according to the method of the present invention. The result of  FIG. 4  indicated that the nano crystals do possess the function of quantum well.  
         [0000]     Application 2: Solar Cell  
         [0029]     The nano crystals which possess good ability to store higher light energy can be applied to the fabrication of the solar cell.  FIG. 5A ˜ 5 D are cross-sectional views showing a method of manufacturing the solar cell having nano crystals according to the embodiment of the present invention. First, a first metallic substrate  51  is provided, and a p-type silicon thin film  53  is formed on the first metallic substrate  51 , as shown in  FIG. 5A . A thickness of the p-type silicon thin film  53  is equal to or less than about 50 Å, and preferably about 15-25 Å. Then, the thin film  53  is subjected under a laser annealing to form the numerous p-type nano crystals  531  on the first metallic substrate  51 , as shown in  FIG. 5B . A wavelength of the laser selected for laser annealing is equal to or less than about 500 nm (preferably in the range of 200 nm to 500 nm). Next, an n-type silicon thin film  55  is formed on the first metallic substrate  51  to cover the nano crystals  531 . Finally, a second metallic substrate  57  is formed on the n-type silicon thin film  55  to complete the fabrication of the solar cell, as shown in  FIG. 5D .  
         [0030]     When the solar cell is exposed to radiant energy, especially light, the positive-charged carriers are moved towards the p-type nano crystals  531 , and the negative-charged carriers are moved towards the n-type silicon thin film  55 ; consequently, a voltage is produced. With an excellent ability of storing light energy of the nano crystals  531 , the photoelectric characteristic of the solar cell is advanced.  
         [0031]     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.