Abstract:
Disclosed are a laser processing method for cutting a semiconductor wafer having a metal layer formed thereon and a laser processing device. The disclosed laser processing method transmits a plurality of laser beams, which propagate coaxially, to the semiconductor wafer, thereby forming focusing points in positions adjacent to a surface of the metal layer, which constitutes a boundary with the semiconductor wafer, and to one surface of the semiconductor wafer, respectively.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a laser processing method and a laser processing apparatus, and more particularly, to a laser processing method of cutting a semiconductor wafer formed with a metal layer using laser, and a laser processing apparatus therefor. 
       BACKGROUND ART 
       [0002]    A laser processing apparatus irradiates a laser beam, which is emitted from a laser oscillator, on an object to be processed using an optical system to perform a laser processing operation, such as marking, exposure, etching, punching, scribing, dicing, etc., on the object by the irradiated laser beam. 
         [0003]    Recently, in order to prevent damage to a surface of the object, a method of forming a focusing point inside the object with a transmittance with respect to the laser beam and generating a crack to process the object is highlighted. For example, when a laser beam with a high output power is focused to form a focusing point inside a semiconductor wafer, a modified area is formed around the focusing point so that the crack is generated from the modified area. In addition, a laser beam is moved along a predetermined processing line of the semiconductor wafer to generate a crack row inside the object, and then the crack is extended to an external surface of the semiconductor wafer naturally or by using an external force so that the semiconductor wafer can be cut. 
         [0004]    However, there is a disadvantage in that it is impossible to cut a semiconductor wafer with a metal layer using the laser processing method. Since the metal layer is formed to have a thickness of about 10 μm, the laser beam cannot transmit the metal layer with the thickness and thus, the crack to be necessary for the cutting cannot be formed inside the metal layer or the semiconductor wafer. Also, since the metal layer formed on a surface of the semiconductor wafer has high ductility, it is a disadvantage in that a yield is decreased during a cutting operation of the semiconductor wafer. 
       DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem 
       [0005]    One or more exemplary embodiments provide a laser processing apparatus and a laser processing method of cutting a semiconductor wafer formed with a metal layer using a laser beam. 
       Advantageous Effects of the Invention 
       [0006]    In a laser processing method according to an embodiment, an object may be easily cut by transmitting plural laser beams proceeding along a coaxial path to cut the object having a metal layer formed on a surface of a semiconductor wafer, focusing the laser beams at a surface of the metal layer that is a boundary with the semiconductor wafer and a position adjacent to the surface of the semiconductor wafer that is a boundary with the metal layer, and forming focusing points respectively. Also, the laser processing method can be simpler since it is not necessary to perform a pre-process of coating a protective layer to protect chips and to perform a cleaning process to remove contamination material after the cutting process. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIGS. 1 through 4  are diagrams illustrating a general laser processing method of processing a semiconductor wafer with a metal layer. 
           [0008]      FIG. 5  is a block diagram illustrating a laser processing apparatus according to an example embodiment of the present invention. 
           [0009]      FIG. 6  is an enlarged view of a beam expansion unit and a condenser lens of  FIG. 5 . 
           [0010]      FIG. 7  is a view illustrating waveforms of a first laser beam and a second laser beam, which are emitted from the laser processing apparatus of  FIG. 5 , incident on an object with respect to a time axis. 
           [0011]      FIGS. 8A through 12  are views illustrating another laser processing method according to another example embodiment of the present invention. 
       
    
    
     BEST MODE 
       [0012]    Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. The embodiments set forth herein are not intended to limit the scope of the present invention. Rather, these embodiments are provided to explain aspects of the present invention to one of ordinary skill in the art. In the drawings, like reference numerals refer to like elements throughout, and sizes or thicknesses of elements may be exaggerated for clarity of explanation. It will be understood that when a material layer is referred to as being “on” a substrate or another layer, the material layer nay be directly on the substrate or the other layer, or another third layer may be present therebetween. In addition, a material referred to as being used to form each layer is just an example, and another material may also be used. 
         [0013]      FIGS. 1 through 4  are diagrams illustrating a general laser processing method of processing a semiconductor wafer  10  formed with a metal layer  20 . 
         [0014]    Referring to  FIG. 1 , an object  30  to be processed is made of a semiconductor wafer  10  and a metal layer  20  formed on top surface of the semiconductor  10 . Here, the semiconductor wafer  10  may be a substrate usable in a general semiconductor process and may include Silicon (Si), Silicon Carbide (Sic), Gallium Arsenide (GaAs) or Sappire, for example. The semiconductor wafer  10  may include one of various semiconductor materials. The metal layer  20  may be a conductive material usable in a general semiconductor process and may include Copper (Cu), Molybdenum (Mo), or Gold (Au), for example. The metal layer  20  may include one of various metal components having a high conductivity. A laser beam L is focused on the surface of the metal layer  20  to form a focusing point P using laser processing apparatus (not illustrated). Thus, a groove may be formed around the focusing point P of the surface of the metal layer  20  due to laser ablation. 
         [0015]    Referring to  FIG. 2 , when the object  30  is moved along a predetermined processing line S, the focusing point P is moved and thus a groove line  50  is formed to have a depth on the surface of the metal layer  20  along the predetermined processing line S. Referring to  FIG. 3 , a blade  70  is located on the groove line  50  and then is moved along the groove line  50  to perform a sawing operation such that the object  30  splits into a plurality of chips  30 ′ and  30 ″ as illustrated in  FIG. 4 . 
         [0016]    The above-described laser processing method forms the groove line  50  using the laser beam L and then cuts the object  30  using a mechanical apparatus, such as the blade  70 . However, this laser processing method needs a pre-coating process on a protection layer (not illustrated) to protect the chips  30 ′ and  30 ″. Also, an additional cleaning process is required to remove contamination materials after the cutting operation. Therefore, it has a problem that the laser processing method becomes complicated. 
         [0017]      FIG. 5  is a block diagram illustrating a laser processing apparatus according to an example embodiment of the present invention. 
         [0018]    Referring to  FIG. 5 , a laser processing apparatus according to the present embodiment may include a plurality of laser light sources  201  and  202  to emit a plurality of laser beams L 1  and L 2 , and an optical system to control the plurality of laser beams L 1  and L 2  to pass through a coaxial path to be incident on an object to be processed, more specifically, a semiconductor wafer  110 . The object  100  includes the semiconductor wafer  110  and a metal layer  120  formed on one surface of the semiconductor wafer  110  (that is, a lower surface of the semiconductor wafer  110  in  FIG. 5 ). Here, the semiconductor wafer  110  may be a substrate usable in a general semiconductor process and may include Silicon (Si), Silicon Carbide (Sic), Gallium Arsenide (GaAs) or Sappire, for example. The semiconductor wafer  10  may include one of various semiconductor materials. The metal layer  20  may be a conductive material usable in a general semiconductor process and may include Copper (Cu), Molybdenum (Mo), or Gold (Au), for example. The metal layer  20  may include one of various metal components having a high conductivity. 
         [0019]    The plurality of laser light sources  201  and  202  may include a first laser light source  201  to emit a first laser beam L 1  of a pulse type and a second laser light source  202  to emit a second laser beam L 2  of a pulse type. The first and second laser beams L 1  and L 2  may have a wavelength between about 900 nm to about 1700 nm, however, the present invention is not limited thereto. The first laser beam L 1  forms a first focusing point P 1  on the surface of the metal layer  120  that is a boundary with the semiconductor wafer  110 , as described later. And the second laser beam L 2  forms a second focusing point P 2  at a position disposed adjacent to the surface of the semiconductor wafer  110  (the lower surface of the semiconductor in  FIG. 5 ) that is the boundary with the metal layer  120 . The first laser beam L 1  may have a pulse width smaller than the second laser beam L 2 . For example, the first laser beam L 1  may have a pulse width of a range of femto seconds (fs), and the second laser beam L 2  may have a pulse width of a range of nano seconds (ns). More particularly, the first laser beam L 1  may have a pulse width in a range between about 50 to 200 fs, and the second laser beam L 2  may have a pulse width in a range between about 300 to 800 ns. However, the present invention is not limited thereto. The first and second laser beams L 1  and L 2  may have different pulse widths. 
         [0020]    The first laser beam L 1  emitted from the first laser light source  201  may proceed along a predetermined path, be reflected by a mirror  210 , transmit a beam splitter  220 , and then be incident on a beam expansion unit  230 . Here, an additional optical unit, for example, a half-wave plate, a polarization beam splitter (PBS), etc., may be provided on an optical path between the first laser light source  201  and the mirror  210  if necessary. The second laser beam L 2  emitted from the second laser light source  202  may proceed along a predetermined path, be reflected by the beam splitter  220 , and then be incident on the beam expansion unit  230 . Here, an additional optical unit may be provided on an optical path between the second laser light source  202  and the beam splitter  220  if necessary. 
         [0021]    The first and second laser beams L 1  and L 2  emitted from the beam splitter  220  pass through the beam expansion unit  230  and a condenser lens  240 , transmit the semiconductor wafer  110 , and then are focused to form first and second focusing points P 1  and P 2 , respectively.  FIG. 6  illustrates details of the beam expansion unit  230  and the condenser lens  240  of  FIG. 5 . Referring to  FIG. 6 , the beam expansion unit  230  may include a plurality of lenses  231 ,  232 , and  233 , and locations of the focusing points P 1  and P 2  may be changed according to distances among the lenses  231 ,  232 , and  233 . Accordingly, the first and second laser beams L 1  and L 2  incident on the beam expansion unit  230  may be controlled by the beam expansion unit  230  to adjust the locations of the first and second focusing points P 1  and P 2  formed inside the object  100 . 
         [0022]    As described above, in the laser processing apparatus, the first laser beam L 1  emitted from the first laser light source  201  is reflected by the mirror  210  to be transmitted to the beam splitter  220 , and the second laser beam L 2  emitted from the second laser light source  202  is reflected by the beam splitter  220 . Accordingly, the first and second laser beams L 1  and L 2  having passed through the beam splitter  220  proceed along a coaxial path. And then, the first and second laser beams L 1  and L 2  having passed along the coaxial path pass through the beam expansion unit  230  and the condenser lens  240  to be focused at predetermined locations in the object  100  to form the first and second focusing points P 1  and P 2 , respectively. Here, the locations of the focusing points P 1  and P 2  may be controlled according to distances among the lenses  231 ,  232 , and  233  of the beam expansion unit  230 . 
         [0023]    Meanwhile, the first and second laser beams L 1  and L 2  are incident in the object  100  with a time lag.  FIG. 7  is a view illustrating waveforms of the first laser beam L 1  and the second laser beam L 2 , which are emitted from the laser processing apparatus of  FIG. 5 , incident on the object  10  with respect to a time axis. Referring to  FIG. 7 , a predetermined time At after the first laser beam L 1  of a pulse type emitted from the first laser light source  201  is incident on the object  100 , the second laser beam L 2  of a pulse type emitted from the second laser light source  202  is incident on the object  100 . Although  FIG. 7  illustrates waveforms of the first and second laser beams L 1  and L 2  without overlapping and interfering each other, it is possible that the first and second laser beams L 1  and L 2  partially overlap such that the first and second laser beams L 1  and L 2  interfere each other. A set of the first and second laser beams L 1  and L 2  having a predetermined time difference At may be focused inside the object  100  so that the first and second focusing points P 1  and P 2  can be formed, and then after the object  100  moves by a predetermined distance, a next set of the first and second laser beams L 1  and L 2  are incident inside the object  100  so that the first and second focusing points P 1  and P 2  can be moved. 
         [0024]    Hereinafter, a process of cutting the object  100  formed with the semiconductor wafer  110  and the metal layer  120  using the laser processing apparatus illustrated in  FIG. 5  will be explained with reference to  FIGS. 8A through 12 . 
         [0025]      FIG. 8A  illustrates a case in which the first and second focusing points P 1  and P 2  are formed inside the object  100  by the first and second laser beams L 1  and L 2  emitted from the laser processing apparatus, and  FIG. 8B  is a side view of  FIG. 8A . 
         [0026]    Referring to  FIGS. 8A and 8B , the object  100  is prepared. The object  100  is made of the semiconductor wafer  110  and the metal layer  120  formed on a lower surface of the semiconductor wafer  110 . Here, the semiconductor wafer  110  and the metal layer  120  may include material usable in the semiconductor manufacturing process as described above. And, the first and second laser beams L 1  and L 2  of a pulse type having passed the condenser lens  240  of the laser processing apparatus of  FIG. 5  are transmitted to an inside of the semiconductor wafer  110  to form the first and second focusing points P 1  and P 2 , respectively. The first and second laser beams L 1  and L 2  are incident on an upper surface of the semiconductor wafer  110  where the metal layer  120  is not formed and transmit the semiconductor wafer  110 . 
         [0027]    The first and second laser beams L 1  and L 2  emitted from the laser processing apparatus proceed along the coaxial path to be incident on the semiconductor wafer  110  as stated above. The first and second laser beams L 1  and L 2  may have a wavelength in a range, for example, about 900 nm to about 1700 nm, however, the present invention is not limited thereto. Here, the first laser beam L 1  may have a pulse width smaller than the second laser beam L 2 . For example, the first laser beam L 1  may have a pulse width of a femto second range (for example, about 50 to 200 fs), and the second laser beam L 2  may have a pulse width of a nano second range (for example, about 300 to 800 ns). However, the present invention is not limited thereto, and thus the first and second laser beams L 1  and L 2  may have different pulse widths than the above-described pulse widths. 
         [0028]    The first and second laser beams L 1  and L 2  are incident on the transmittable semiconductor wafer  110  with a time difference to form the first and second focusing points P 1  and P 2 , respectively. As illustrated in  FIG. 7 , a predetermined time Δt is passed after the first laser beam L 1  is incident on the semiconductor wafer  110  to form the first focusing point P 1 , the second laser beam L 2  is incident on the semiconductor wafer  110  to form the second focusing point P 2 . Therefore, a set of the first and second laser beams L 1  and L 2  are incident on the semiconductor wafer  110  with a time difference such that the first and second focusing points P 1  and P 2  are respectively formed at predetermined positions inside the object  100 . 
         [0029]    The first focusing point P 1  is formed when the first laser beam L 1  transmits the semiconductor wafer  110  and then is focused at a surface of the metal layer  120  which forms a boundary with the semiconductor wafer  110 . Therefore, when the first focusing point P 1  is formed on the surface of the metal layer  120 , a groove can be formed on the surface of the metal layer  120  due to laser ablation. And, the second focusing point P 2  is formed when the second laser beam L 2  proceeding along the coaxial path with the first laser beam L 2  transmits the semiconductor wafer  110  and is focused at a portion disposed adjacent to the lower surface of the semiconductor wafer  110  which forms the boundary with the metal layer  120 . As such, when the second focusing point P 2  is formed at the position adjacent to the lower surface of the semiconductor wafer  110 , a modified area is formed around the second focusing point P 2  such that a crack can be formed and expanded from the modified area to a surface of the semiconductor wafer  110 . Here, the second focusing point P 2  may be formed at a higher position than the first focusing point P 1 , and thus the second focusing point P 2  may be formed without interference with the groove generated due to the formation of the first focusing point P 1 . 
         [0030]      FIG. 9A  illustrates a moving state of the first and second focusing points P 1  and P 2  after the first and second focusing points P 1  and P 2  are formed inside the object  100 .  FIG. 9B  is a cross-sectional view of a line I-I′ of  FIG. 9A . 
         [0031]    Referring to  FIGS. 9A and 9B , in a state that the first and second focusing points P 1  and P 2  are formed at the surface of the metal layer  120  and a position adjacent to the lower side of the semiconductor wafer  110 , respectively, as illustrated in  FIGS. 8A and 8B , the object  100  is moved along a predetermined processing line S, and then another set of the first and second laser beams L 1  and L 2  are incident on the object  100  with the time difference At. Accordingly, the first and second focusing points P 1  and P 2  are formed whiling moving along a predetermined direction (that is, an opposition direction to the movement of the object  100 ). Meanwhile, the movements of the first and second focusing points P 1  and P 2  may be performed by movement of the laser processing apparatus instead of the object  100 . When the first and second laser beams L 1  and L 2  are repeatedly incident inside the object  100  while the object  100  is moved as described above, the first and second focusing points P 1  and P 2  are moved along the predetermined processing line S inside the object  100 . 
         [0032]    In this process, when the first focusing point P 1  is moved, a portion of the metal layer  120  is removed by laser ablation to form an internal groove line along the predetermined processing line S on the surface of the metal layer  120  which is a boundary with the semiconductor layer  110 . And, at the position adjacent to the lower surface of the semiconductor wafer  110  which is a boundary with the metal layer  120 , a crack row is formed along the predetermined processing line S according to a movement of the second focusing point P 2  and expanded to the lower surface of the semiconductor wafer  110 . Therefore, when the object  100  stops the movement, the internal groove line  125  and the crack row  111  are formed inside the object  100  as illustrated in  FIG. 10 . 
         [0033]    Referring to  FIG. 11 , in a case where the semiconductor wafer  110  is thick, the internal groove line  125  and the crack row  111  are formed in the object  100 , and then, the second laser beam L 2  is focused inside the semiconductor wafer  110  to form another focusing point such that at least one internal crack row  112  and  113  is formed additionally by moving the object  100 . Meanwhile, it is possible that the internal crack rows  112  and  113  can be simultaneously formed by using one or more additional laser beams (not illustrated) when the internal groove line  125  and the crack row are formed. As such, the object  100  can be more easily cut according to the one or more internal crack rows  112  and  113 . Meanwhile, by repeating the process using the first laser beam L 1 , the internal groove line  125  can be formed deeper than a single process. 
         [0034]    Referring to  FIG. 12 , in a state in which the internal groove line  125  and the crack row  111  (and internal crack rows  112  and  113 ) are formed along the predetermined processing line S inside the object  100  as illustrated in  FIG. 11 , the object  100  can be split into a plurality of chips  100 ′ and  100 ″ by breaking naturally or according to an external impact on the object  100 . 
         [0035]    As described above, in the laser processing method according to the present embodiment, an object  100  may be easily cut by transmitting the plural laser beams L 1  and L 2  proceeding along the coaxial path to cut the object  100  having the metal layer  120  formed on the surface of the semiconductor wafer  110 , focusing the laser beams L 1  and L 2  at the surface of the metal layer  120  that is the boundary with the semiconductor wafer  110  and the position adjacent to the surface of the semiconductor wafer  110  that is the boundary with the metal layer  120 , and forming the first and second focusing points P 1  and P 2 , respectively. Also, the laser processing method can be simpler since it is not necessary to perform a pre-process of coating a protective layer to protect the chips  100 ′ and  100 ″ and to perform a cleaning process to remove contamination material after the cutting process. 
         [0036]    While one or more exemplary embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein. 
       MODE OF THE INVENTION 
       [0037]    According to an exemplary aspect, a laser processing method of processing an object including a semiconductor wafer and a metal layer formed on one surface of the semiconductor wafer by using laser includes passing a plurality of laser beams proceeding along a coaxial path through the semiconductor wafer, and forming focusing points at a surface of the metal layer which forms a boundary with the semiconductor wafer, and at a position disposed adjacent to the surface of the semiconductor wafer, respectively. 
         [0038]    The plurality of laser beams may include first and second laser beams of a pulse type, and the method may further include forming a first focusing point on the surface of the metal layer which forms the boundary with the semiconductor layer, by the first laser beam and forming a second focusing point at the position disposed adjacent to the surface of the semiconductor wafer which forms a boundary with the metal layer, by the second laser beam. 
         [0039]    The second laser beam may be incident on the semiconductor wafer according to a predetermined time lag from the first laser beam. Here, the first laser beam and the second laser beam may interfere each other or may not interfere each other. The first laser beam may have a smaller pulse width than the second laser beam. For example, the first laser beam may have a pulse width of a femto second range, and the second laser beam may have a pulse width of a nano second range. 
         [0040]    The method may further include moving the first focusing point and the second focusing point along predetermined processing line of the object. An internal groove line may be formed on the surface of the metal layer according to a movement of the first focusing point, and a crack row may be formed on the position disposed adjacent to the surface of the semiconductor according to a movement of the second focusing point. 
         [0041]    The plurality of laser beams may be incident on the other surface of the semiconductor which is not formed with the metal layer. The plurality of laser beams may have wavelengths in a range between 900 nm to about 1700 nm. The semiconductor wafer may include Si, Sic, GaAs, or sapphire; and the metal layer may include Cu, Mo, or Au. 
         [0042]    According to another exemplary aspect, a laser processing apparatus to process an object including a semiconductor wafer and a metal layer formed on one surface of the semiconductor wafer includes a plurality of laser sources to emit a plurality of laser beams, and an optical system to move the plurality of laser beams through a coaxial path to transmit the semiconductor wafer, and to form focusing points at a surface of the metal layer which forms a boundary with the semiconductor wafer, and at a position disposed adjacent to the surface of the semiconductor wafer, respectively. 
         [0043]    The plurality of laser sources may include a first laser source to emit a first laser beam of a pulse type to form a first focusing point on the surface of the metal layer, and a second laser source to emit a second laser beam of the pulse type to form a second focusing point on the position disposed adjacent to the surface of the semiconductor wafer.