Abstract:
The present invention relates to a CO 2  laser-transparent material having a mark on the surface thereof and the method for making the same. The method includes the following steps: providing a first substrate, which has a top surface and a bottom surface; providing a second substrate which has a top surface; putting the bottom surface of the first substrate on the top surface of the second substrate; irradiating a CO 2  laser beam to the top surface of the second substrate by passing through the top surface and the bottom surface of the first substrate; and forming a mark on the bottom surface of the first substrate. The material of the mark is oxide of the second substrate or the same as the material of the second substrate. Whereby the cheap CO 2  laser is utilized to form the mark on the first substrate, and the mark can be erased easily by a proper chemical for recycling the first substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. Ser. No. 11/723,508, filed Mar. 20, 2007, which claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 095124898, filed in Taiwan, Republic of China on Jul. 7, 2006, the entire contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a marking method, and more particularly, to a method for forming a mark on the surface of a CO 2  laser-transparent material. 
         [0004]    2. Description of the Related Art 
         [0005]    At present, laser is a marking technique widely used in the industry, and is applied to materials such as plastic, rubber, ceramics, metal, and silicon wafer. Compared with conventional manners, for example, mechanical engraving, chemical etching, screen printing, and ink printing, laser marking has the advantages of rapid production, high flexibility, and being controllable via a computer system. In addition, a prominent characteristic of laser marking is the permanence of the mark generated by a laser on the surface of a workpiece. 
         [0006]    There are many kinds of lasers and the femtosecond laser, excimer laser, or Nd:YAG laser are mostly used in silicon wafer marking. However, these lasers are generally very expensive, and the processing mechanism thereof is ablating the surface of the silicon wafer with a laser beam of high energy, which may damage the surface structure of the silicon wafer and result in many flying minute particles, i.e., the so-called “splashing fragments”. The fragments are prone to be attached to the silicon wafer, thus becoming difficult to erase. When proceeding to the subsequent device circuit process, a grip head is used to fix the edge of the silicon wafer. However, the clamping force is easy to make the residual fragments fall off and cause another splashing, which not only contaminates the process, but also severely affects the yield and quality of the product. Moreover, these lasers remove the surface of the product to form a mark, so the mark cannot be re-made, and once marked incorrectly, the product will be abandoned for uselessness, and the material cannot be recycled. 
         [0007]    Moreover, in the conventional fabrication process of semiconductor devices, the marking process is generally performed after the silicon wafer is diced into chips. As the technique is being constantly updated and the integrated circuits are becoming lighter, thinner, and smaller, the processing technique has also evolved into dicing the wafer after marking, so as to improve the efficiency of production and operation. However, as the size of the silicon wafer is getting larger, the thickness thereof stays unchanged or becomes smaller. Therefore, when the surface of the silicon wafer is ablated with a laser beam of high energy, a large amount of stress is easily accumulated on the surface of the silicon wafer, resulting in deformation and warping thereof. Though the stress can be eliminated by high temperature annealing, the basic property of the silicon wafer is greatly affected, which is disadvantageous for the subsequent production. 
         [0008]    In view of the disadvantages of using the above lasers, ROC (TW) Patent Publication No. 350797 provides a processing method for removing particles in the semiconductor industry, and particularly for removing silicon particles generated after making a mark with a laser on the chip. In the method, the wetting and catalytic effects are achieved with the hydroxyl in the aqueous ammonia, so as to oxidize the particles. ROC (TW) Patent Publication No. 434749 provides a marking method, in which the wafer mark can be recovered after a chemical-mechanical polishing process is performed on the wafer, and no silicon particles are generated during the marking. According to the method, the photoresist is exposed with a fiber optic cable, so as to form a mark on the photoresist, and a wafer mark is formed subsequently by etching with the photoresist having a mark formed thereon as a mask. ROC (TW) Patent Publication No. 359885 provides a method, in which a mark pattern on a tape is defined with a laser beam, then the tape is adhered onto a silicon wafer, then the pattern is transferred to the wafer by a wet or dry process, and the tape is finally stripped to finish making a mark on the silicon wafer. The above method can avoid causing splashing fragments. 
         [0009]    In Japanese Patent Publication No. 11-260675, a spot-shaped mark is fabricated on a silicon wafer with a laser, and a layer of transparent thin film is formed thereon. When a laser beam passes through the transparent thin film to make the spot-shaped mark regionally melt and deformed, a plurality of spot-shaped marks can be formed. This method can prevent the splashing fragments generated during the laser processing from being attached to the silicon wafer, and the definition and visibility are ensured by the shape of these spot-shaped marks. 
         [0010]    ROC (TW) Patent Publication No. 1233197 provides a chip scale mark and a marking method of the same. According to the method, when a laser beam ablates the surface of a silicon wafer, the chip size mark is used to stably keep the laser system and the marking distance between the wafers by removing the wafer warp on the wafer support. ROC (TW) Patent Publication No. 200538304 provides a method for making a mark by forming an interference fringe on a body to be marked with a laser beam. 
         [0011]    Therefore, it is necessary to provide a method for forming a mark on the surface of a CO 2  laser-transparent material e.g. silicon to solve the above problems. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a method for forming a mark. The method includes the following steps: providing a first substrate, which has a top surface and a bottom surface; providing a second substrate, which has a top surface; disposing the bottom surface of the first substrate on the top surface of the second substrate; irradiating a CO 2  laser to the top surface of the second substrate by passing through the top surface and the bottom surface of the first substrate; and forming a mark on the bottom surface of the first substrate. The material of the mark is oxide of the second substrate or the same as the material of the second substrate. 
         [0013]    The present invention also provides a method for forming a mark. The method includes the following steps: providing a first substrate, which has a top surface and a bottom surface; providing a second substrate, which has a top surface; forming a metal film on the top surface of the second substrate; disposing the bottom surface of the first substrate on the metal film; irradiating a CO 2  laser to the metal film by passing through the top surface and the bottom surface of the first substrate; and forming a mark on the bottom surface of the first substrate. The material of the mark is a mixture. The mixture includes metal and the second substrate. 
         [0014]    The present invention further provides a substrate having a mark on the surface thereof. A substrate has a surface, wherein the material of the substrate is a CO 2  laser-transparent material; and a mark located on the surface of the substrate, wherein the mark is formed by a CO 2  laser, and the material of the mark is a CO 2  laser-absorption material. 
         [0015]    In the present invention, the CO 2  laser with a light wavelength of 10.6 μm is not absorbed by the first substrate, e.g. silicon material, solar energy wafer or solar energy chip, but can pass through the first substrate and absorbed by the metal film and the second substrate, e.g. glass, silica, metal oxide, ceramics, nitride, carbide or polymethyl methacrylate (PMMA). Thus, the mark is formed by the re-solidification of the melted and/or vaporized material of the second substrate generated by the second substrate under the input irradiation energy of the CO 2  laser. Alternatively, the mark is formed by the re-solidification of the melted and/or vaporized mixture generated by the metal film and the second substrate under the input irradiation energy of the CO 2  laser. The CO 2  laser is the cheapest laser among various lasers, so the present invention provides a method for marking the CO 2  laser-transparent material, such as the wafer, in a rapid and simple way, which costs less, consumes less energy, and has high reliability and quality. Moreover, stress can be avoided by utilizing low energy means of marking, such that the first substrate, e.g. silicon material, solar energy wafer or solar energy chip, will not be deformed or warped. Further, the present invention does not utilize the laser beam in such a way of ablation, therefore the first substrate will not be damaged after the processing, and no splashing fragments and dusts will be generated, thereby abating pollution and improving yield. Besides, it is not necessary to use a mask, and the photolithography process may not be affected, such that the capacity is improved. Additionally, when marked incorrectly with other lasers, the surface of the wafer product is usually damaged. However, in the present invention, the CO 2  laser is employed, and a common chemical can be used to erase the incorrect mark so that the first substrate, e.g. silicon material, solar energy wafer or solar energy chip, may be recycled. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a schematic diagram of a method for forming a mark according to a first embodiment of the present invention; 
           [0017]      FIG. 2  is a photograph of the silicon wafer having a mark on the surface thereof formed according to an example of the experiment in the first embodiment of the present invention, in which the mark is constituted by letters; 
           [0018]      FIG. 3  is a photograph of the silicon wafer having a mark on the surface thereof formed according to the example of the experiment in the first embodiment of the present invention, in which the mark is constituted by totems and random codes; 
           [0019]      FIG. 4  is a photograph of the first substrate having a mark on the surface thereof formed according to another example of the experiment in the first embodiment of the present invention, in which the mark is constituted by four Chinese character; 
           [0020]      FIG. 5  is a photograph showing the material composition of the mark analyzed according to the EDX analysis; 
           [0021]      FIG. 6  is a photograph showing the material of the portion without the mark analyzed according to the EDX analysis; 
           [0022]      FIG. 7  is a photograph showing the mark measured by an alpha-step profilometer; 
           [0023]      FIG. 8  is a schematic diagram of a method for forming a mark according to a second embodiment of the present invention; and 
           [0024]      FIG. 9  is a schematic diagram of a substrate having a mark on the surface thereof according to a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 1  is a schematic diagram of a method for forming a mark according to a first embodiment of the present invention. In this embodiment, firstly, a first substrate  11  is provided, which has a top surface  111  and a bottom surface  112 . In this embodiment, the material of the first substrate  11  is a CO 2  laser-transparent material. That is, the CO 2  laser transmittance of the first substrate  11  is higher than the CO 2  laser absorptance of the first substrate  11 . The CO 2  laser transmittance of the first substrate  11  can be over 50 percent. Preferably, the CO 2  laser transmittance of the first substrate  11  is over 80 percent. In other words, the absorption band at 10.6 μm of the first substrate  11  is weak in the IR spectrum due to low absorption coefficient. Herein, the first substrate  11  can be a silicon wafer, which can be pure silicon or have a multi-layered thin film. Additionally, the first substrate  11  can also be a silicon chip, a solar energy wafer or a solar energy chip. In this embodiment, the first substrate  11  is disposed with a polished surface facing upward (i.e., the top surface  111  of the first substrate  11  is a polished surface) or with a rough surface facing upward (i.e., the bottom surface  112  of the first substrate  11  is a polished surface). Alternatively, the first substrate  11  can be a double-side polished substrate (i.e., the top surface  111  and the bottom surface  112  of the first substrate  11  both are polished surfaces). Preferably, the bottom surface  112  of the first substrate  11  is a polished surface. 
         [0026]    Next, a second substrate  12  is provided, which has a top surface  121 . In this embodiment, the material of the second substrate  12  is a CO 2  laser-absorption material. That is, the CO 2  laser absorptance of the second substrate  12  is higher than the CO 2  laser transmittance of the second substrate  12 . The CO 2  laser absorptance of the second substrate  12  can be over 50 percent. Preferably, the CO 2  laser absorptance of the second substrate  12  is over 80 percent. In other words, the absorption band at 10.6 μm of the second substrate  12  is intense in the IR spectrum due to high absorption coefficient. Herein, the material of the second substrate  12  can be glass, silica, metal oxide, ceramics, nitride, carbide or polymethyl methacrylate (PMMA). Afterward, the bottom surface  112  of the first substrate  11  is disposed on the top surface  121  of the second substrate  12 , and the top surface  121  of the second substrate  12  is closely attached to the bottom surface  112  of the first substrate  11 . In this embodiment, a clamp (not shown) is used to clamp the second substrate  12  and the first substrate  11 , such that the top surface  121  of the second substrate  12  is closely attached to the bottom surface  112  of the first substrate  11 . Next, the second substrate  12  and the first substrate  11  are disposed on a support platform  18 . 
         [0027]    After that, a CO 2  laser  14  is provided by a CO 2  laser generator  13 . The CO 2  laser  14  is focused on the first substrate  11  through a focusing mechanism having a reflecting mirror  15  and a focusing lens  16 . Then, the CO 2  laser  14  is irradiated to the top surface  121  of the second substrate  12  by passing through the top surface  111  and bottom surface  112  of the first substrate  11  since the CO 2  laser  14  is not absorbed by the first substrate  11 , but the CO 2  laser  14  is absorbed by the second substrate  12 . The irradiated portion of the second substrate  12  is melted and/or vaporized, and then re-solidified on the bottom surface  112  of the first substrate  11 . Therefore, the part of the second substrate  12  is disposed on the bottom surface  112  of the first substrate  11  to form the mark. Accordingly, in this embodiment, the mark is not an-etched groove, but a protrusion. The material of the mark is an oxide of the second substrate  12  or same as the material of the second substrate  12 . The mark is formed by the re-solidification of the melted and/or vaporized material of the second substrate  12  generated by the second substrate  12  under the input irradiation energy of the CO 2  laser  14  or formed by the oxide of the second substrate  12 . 
         [0028]    In addition, the mark is formed by irradiating the CO 2  laser  14  less than five passes to prevent the CO 2  laser  14  from damaging the first substrate  11 , especially the top surface  111 . Moreover, the mark can be of any shape, such as a numeral, a letter or a totem. 
         [0029]    Further, in this embodiment, the CO 2  laser  14  can be focused on the interior, the top surface  111  or the bottom surface  112  of the first substrate  11 . The focusing position of the CO 2  laser  14  can be adjusted with the reflecting mirror  15  and the focusing lens  16 , or controlled by adjusting the direction of Z-axis of the support platform  18 , and the two methods for adjusting focusing position can be integrated. In addition, the focusing position of the CO 2  laser  14  would affect the effective energy density for melting/evaporating the second substrate  12 . Herein, the effective energy density of focusing position is higher than other divergent defocus position. In  FIG. 1 , the focusing position is on the top surface  111  of the first substrate  11  which will has higher effective energy density than the interior or the bottom surface  112  of the first substrate  11 . We can also adjust the focusing position at the bottom surface  112  of the first substrate  11  for more effective energy density for marking. In this embodiment, the CO 2  laser  14  is focused on the top surface  111  of the first substrate  11 . By adjusting appropriate laser processing parameters, such as, the power of the CO 2  laser source, scanning speed, pass number (about less than five passes), and together by scanning the laser spot or moving the support platform  18 , a desired mark shape can be achieved. 
         [0030]    Additionally, if a part or the whole of the mark is undesired or incorrect, a cleaning chemical can be used to directly erase the mark. The chemical cleaning agent can be hydrofluoric acid (HF), buffered oxide etching (BOE), or a general chemical capable of erasing oxide. 
         [0031]    Hereinafter is an example of the experiment according to this embodiment. The first substrate  11  is a silicon wafer and the second substrate  12  is a glass substrate. The processing condition of this example is: the thicknesses of the first substrate  11  and the second substrate  12  are both 500 μm; the power of the CO 2  laser  14  is 21 W and the focus point thereof is set on the top surface  111  of the first substrate  11 ; the mark is directly formed by irradiating the CO 2  laser  14  once at a scanning speed of 5 mm/sec, and the processing result is shown in  FIG. 2 . 
         [0032]    As shown in  FIG. 2 , the silicon wafer includes a silicon material  11  and a mark  17 . The silicon material  11  has a surface  112  (i.e., the bottom surface  112  in  FIG. 1 ). In this example, the surface  112  is a polished surface, and it is to be understood that the surface  112  can also be a rough surface. The mark  17  is located on the surface  112  of the silicon material  11 , and the mark  17  is silicon oxide, which can be constituted by numerals, letters, or totems. In this example, the mark  17  is constituted by letters of “NCKU.” Also, the mark is other totems or random codes, as shown in  FIG. 3 . 
         [0033]    Further, hereinafter is another example of the experiment according to this embodiment. In this example, the first substrate  11  is a silicon wafer and the second substrate  12  is a pyrex glass substrate. Then, the first substrate  11  and the second substrate  12  are irradiated and scanned by the CO 2  laser one time. As a result, a four Chinese character mark is formed on the surface of the first substrate  11 , as shown in  FIG. 4 . And, the material of the mark comprises Si, O and Na, as shown in the EDX analysis in  FIG. 5 , which consists of the primary material composition of the Pyrex glass. The material of the portion without the mark is only silicon (Si), as shown in the EDX analysis in  FIG. 6 . The profile of the mark measured by an alpha-step profilometer is shown in  FIG. 7 , which confirms that the mark protrudes from the surface of the silicon wafer. 
         [0034]      FIG. 8  is a schematic diagram of a method for forming a mark according to a second embodiment of the present invention. In this embodiment, firstly, a first substrate  21  is provided, which has a top surface  211  and a bottom surface  212 . In this embodiment, the material of the first substrate  21  is a CO 2  laser-transparent material. That is, the CO 2  laser transmittance of the first substrate  21  is higher than the CO 2  laser absorptance of the first substrate  21 . The CO 2  laser transmittance of the first substrate  21  can be over 50 percent. Preferably, the CO 2  laser transmittance of the first substrate  21  is over 80 percent. In other words, the absorption band at 10.6 μm of the first substrate  21  is weak in the IR spectrum due to low absorption coefficient. Herein, the first substrate  21  is a silicon wafer, which can be pure silicon or have a multi-layered thin film. Additionally, the first substrate  21  can also be a silicon chip, a solar energy wafer or a solar energy chip. In this embodiment, the first substrate  21  is disposed with a polished surface facing upward (i.e., the top surface  211  of the first substrate  21  is a polished surface) or a rough surface facing upward (i.e., the bottom surface  212  of the first substrate  21  is a polished surface). Alternatively, the first substrate  21  can be a double-side polished substrate (i.e., the top surface  211  and the bottom surface  212  of the first substrate  21  both are polished surfaces). Preferably, the bottom surface  212  of the first substrate  21  is a polished surface. 
         [0035]    Next, a second substrate  22  is provided, which has a top surface  221 . In this embodiment, the material of the second substrate  22  is a CO 2  laser-absorption material. Herein, the material of the second substrate  12  can be glass, silica, metal oxide, ceramics, nitride, carbide or polymethyl methacrylate (PMMA). That is, the CO 2  laser absorptance of the second substrate  22  is higher than the CO 2  laser transmittance of the second substrate  22 . The CO 2  laser absorptance of the second substrate  22  can be over 50 percent. Preferably, the CO 2  laser absorptance of the second substrate  22  is over 80 percent. In other words, the absorption band at 10.6 μm of the second substrate  22  is intense in the IR spectrum due to high absorption coefficient. Afterward, a metal film  27  is formed on the top surface  221  of the second substrate  22 . Preferably, the metal film  27  is formed on the top surface  221  of the second substrate  22  by coating, and the material of the metal film  27  can be selected from a group consisting of aluminum, titanium, chromium, tantalum, nickel, iron, cobalt, vanadium, tungsten, zirconium, zinc, copper, silver, and gold. The thickness of the metal film  27  can be between 10-1000 nm. Preferably, the material of the metal film  27  is aluminum, titanium, chromium, tantalum, nickel, iron, cobalt, vanadium, tungsten, zirconium or zinc, and the thickness thereof is between 30-80 nm. 
         [0036]    Afterward, the bottom surface  212  of the first substrate  21  is disposed on the metal film  27  coated on the second substrate  22 . And the top surface  221  of the second substrate  22 , the metal film  27 , and the bottom surface  212  of the first substrate  21  are closely attached. In this embodiment, a clamp (not shown) is used to clamp the second substrate  22  and the first substrate  21 . Next, the second substrate  22  and the first substrate  21  are disposed on a support platform  28 . 
         [0037]    Afterward, a CO 2  laser  24  is provided by a CO 2  laser generator  23 . Finally, the CO 2  laser  24  is focused on the first substrate  21  through a focusing mechanism having a reflecting mirror  25  and a focusing lens  26 . Then, the CO 2  laser  24  is irradiated to the metal film  27  by passing through the top surface  211  and bottom surface  212  of the first substrate  21  since the CO 2  laser  24  is not absorbed by the first substrate  21 , but the CO 2  laser  24  is absorbed by the metal film  27  and the second substrate  22 . When the CO 2  laser  24  irradiates to the metal film  27 , the heat caused by the CO 2  laser can be conducted to the top surface  221  of the second substrate  22  to cause the corresponding portion of the second substrate  22  melt and/or vaporize. Of course, when the metal film  27  is thin enough, the CO 2  laser  24  may pass through the metal film  27  and irradiate to the top surface  221  of the second substrate  22 . Accordingly, the irradiated portion of the metal film  27  and the corresponding portion of the second substrate  22  are melted and/or vaporized, and then re-solidified on the bottom surface  212  of the first substrate  21 . Therefore, the part of the metal film  27  and the second substrate  12  are disposed on the bottom surface  212  of the first substrate  21  to form the mark. The mark formed by growing or depositing, which is formed by utilizing the input irradiation energy of the CO 2  laser to melt and/or vaporize the metal film  27  and the top surface  221  of the second substrate  22 , and then re-solidifying the both. Thus, the mark is not an etched groove, but a protrusion. The material of the mark is a mixture, and the mixture includes metal and the second substrate. Of course, the mixture may also include metal oxide or the oxide of the second substrate  22 . 
         [0038]    Moreover, the mark can be of any shape, such as a numeral, a letter, or a totem. In addition, the mark is formed by irradiating the CO 2  laser  24  less than five passes to prevent the CO 2  laser  24  from damaging the top surface  211  of the first substrate  21 . 
         [0039]    Further, in this embodiment, the CO 2  laser  24  can be focused on the interior, the top surface  211  or the bottom surface  212  of the first substrate  21 . The focusing position of the CO 2  laser  24  can be adjusted with the reflecting mirror  25  and the focusing lens  26 , or controlled by adjusting the direction of Z-axis of the support platform  28 , and the two methods for adjusting focusing position can be integrated. In addition, the focusing position of the CO 2  laser  24  would affect the effective energy density for melting/evaporating the second substrate  22 . Herein, the effective energy density of focusing position is higher than other divergent defocus position. In  FIG. 8 , the focusing position is on the top surface  211  of the first substrate  21  which will has higher effective energy density than the interior or the bottom surface  212  of the first substrate  21 . We can also adjust the focusing position at the bottom surface  212  of the first substrate  21  for more effective energy density for marking. In this embodiment, the CO 2  laser  24  is focused on the top surface  211  of the first substrate  21 . By adjusting appropriate laser processing parameters, such as, the power of the CO 2  laser source, scanning speed pass number (about less than five passes), and together by scanning the laser spot or moving the support platform  28 , a desired mark shape can be achieved. 
         [0040]    Additionally, if a part or the whole of the mark is undesired or incorrect, a cleaning chemical can be used to directly erase the mark. The chemical cleaning agent can be hydrofluoric acid (HF), buffered oxide etching (BOE), or a general chemical capable of erasing oxide. 
         [0041]      FIG. 9  is a schematic diagram of a substrate having a mark on the surface thereof according to a third embodiment of the present invention. In this embodiment, a substrate  31  has a surface  311 , and the material of the substrate  31  is a CO 2  laser-transparent material. That is, the CO 2  laser transmittance of the substrate  31  is higher than the CO 2  laser absorptance of the substrate  31 . The CO 2  laser transmittance of the substrate  31  can be over 50 percent. Preferably, the CO 2  laser transmittance of the substrate  31  is over 80 percent. In other words, the absorption band at 10.6 μm of the substrate  31  is weak in the IR spectrum due to low absorption coefficient. A mark  32  is located on the surface  311  of the substrate  31 . Herein, the mark  32  is formed by a CO 2  laser, and the material of the mark  32  is a CO 2  laser-absorption material. That is, the CO 2  laser absorptance of the mark  32  is higher than the CO 2  laser transmittance of the mark  32 . The CO 2  laser absorptance of the mark  32  can be over 50 percent. Preferably, the CO 2  laser absorptance of the mark  32  is over 80 percent. In other words, the absorption band at 10.6 μm of the mark  32  is intense in the IR spectrum due to high absorption coefficient. 
         [0042]    In this embodiment, the material of the mark  32  can be metal, metal oxide, nitride, carbide or silicon oxide or the mixture at least selected from two of the above material, and the substrate  31  is a silicon wafer or a silicon chip. Herein, the silicon wafer can be pure silicon or has a multi-layered thin film. And, the surface  311  of the substrate  31  can be a polished surface. In the embodiment, the features and the functions of the substrate  31  and the mark  32  are same as those of the first substrate  11  and the mark in the first embodiment, and are also same as those of the first substrate  21  and the mark in the second embodiment. Those features and functions are described above. Therefore, unnecessary details are not going to be mentioned here. 
         [0043]    In the present invention, the CO 2  laser with a light wavelength of 10.6 μm is not absorbed by the first substrate, e.g. silicon material, solar energy wafer or solar energy chip, but can pass through the first substrate and absorbed by the metal film and the second substrate, e.g. glass, silica, metal oxide, ceramics, nitride, carbide or polymethyl methacrylate (PMMA). Thus, the mark is formed by the re-solidification of the melted and/or vaporized material of the second substrate generated by the second substrate under the input irradiation energy of the CO) laser or the mark is the oxide of the second substrate. Alternatively, the mark is formed by the re-solidification of the melted and/or vaporized mixture generated by the metal film and the second substrate under the input irradiation energy of the CO 2  laser. The present invention has the following advantages: 1. the CO 2  laser is the cheapest laser among various lasers; 2. the present invention provides a method for marking the wafer in a rapid and simple way, which costs less, consumes less energy, and has high reliability and quality; 3. stress can be avoided by utilizing low energy, such that the first substrate  11 , e.g. silicon material, a solar energy wafer and a solar energy chip, will not be deformed or warped; 4. the present invention does not adopt the laser beam in the manner of ablation, such that the wafer will not be damaged after the processing, and no splashing fragments and dusts will be generated, thereby abating pollution and improving the yield; 5. it is not necessary to use a mask, and the photolithography process may not be affected, such that the capacity is improved; 6. the first substrate  11  can be a silicon wafer of pure silicon, and can also be a silicon wafer with a multi-layered thin film; 7. when marked incorrectly, as other lasers are used in the conventional art, the incorrect mark cannot be erased from the surface of the damaged product (for example, an etched groove is resulted). However, in the present invention, a CO 2  laser is used to generate a mark of silicon oxide, and a common chemical can be used to erase the incorrect mark for recycling the first substrate  11 . 
         [0044]    While several embodiments of the present invention have been illustrated and described, various modifications and improvements can be made by those skilled in the art. The embodiments of the present invention are therefore described in an illustrative but not restrictive sense. It is intended that the present invention should not be limited to the particular forms as illustrated, and that all modifications which maintain the spirit and scope of the present invention are within the scope as defined in the appended claims.