Abstract:
According to an aspect of an embodiment, a method for fabricating a workpiece comprises the steps of: irradiating the workpiece with a laser light to raise a temperature of the workpiece at an elevated level under which at least a part of the workpiece is melted, and maintaining at least a part of the workpiece to be melted until the part of the workpiece is separated.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method of fabricating a workpiece, such as Micro Electro Mechanical Systems (MEMS). 
       SUMMARY 
       [0002]    According to an aspect of an embodiment, a method for fabricating a workpiece comprises the steps of: irradiating the workpiece with a laser light to raise a temperature of the workpiece at an elevated level under which at least a part of the workpiece is melted, and maintaining at least a part of the workpiece to be melted until the part of the workpiece is separated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1  is a block diagram schematically showing the structure of an exemplary laser processing apparatus according to the present invention. 
           [0004]      FIG. 2  is a partially enlarged plan view schematically showing the structure of a fine workpiece. 
           [0005]      FIG. 3  is a partially enlarged perspective view schematically showing the state in which a temporary retainer is irradiated with a laser beam. 
           [0006]      FIG. 4  is a graph showing the relationship among an irradiation time of a laser beam, a temperature of a fine workpiece, and an output of the laser beam, in the exemplary laser processing apparatus according to the present invention. 
           [0007]      FIG. 5  is a partially enlarged perspective view schematically showing the state in which the temporary retainer is melted and separated. 
           [0008]      FIGS. 6A to 6E  are cross-sectional schematic views showing workpieces in an embodiment of the present invention. 
           [0009]      FIG. 7  is a graph showing the relationship among an irradiation time of a laser beam, a temperature of a fine workpiece, and an output of the laser beam, in a laser processing apparatus according to a related art configuration. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0010]    Optical switching elements are widely known. An optical switching element contains a mirror array. The mirror array contains a plurality of MEMS mirrors. The MEMS mirrors reflect light emitted from a plurality of input ports of optical fibers to a plurality of output ports of the optical fibers, thereby switching the path of optical signals. 
         [0011]    In each MEMS mirror, the front side of a fixed electrode faces the back side of a movable electrode. The movable electrode has a mirror surface on the front side thereof. The movable electrode is coupled to a movable electrode substrate such that the posture of the movable electrode can be changed. The posture of the movable electrode, that is, the posture of the mirror surface is changed around predetermined X-and-Y-axes because of electrostatic attraction that is generated between the fixed electrode and the movable electrode. 
         [0012]    The movable electrode has a fragile and fine structure. When the optical switch element is fabricated, it is desired to prevent the posture of the movable electrode from being changed during carriage of the mirror array. To prevent the posture from being changed, the movable electrode and the movable electrode substrate are fixed with a temporary retainer and inhibited from relatively moving. The temporary retainer is separate immediately before the assembly to the optical switching element. 
         [0013]    To separate the temporary retainer, the temporary retainer is intermittently irradiated with, for example, a Q-switch oscillation pulse laser beam. The pulse width of the pulse laser beam is a value with microsecond. Such a pulse laser beam has a large output, and rapidly heats the temporary retainer. As a result, extremely small dusts scatter. If the dusts adhere on the mirror surface, the reflectivity of the mirror surface may deteriorate. 
         [0014]    In addition, since the pulse laser beam has the large output, the pulse laser beam may apply an impact to the temporary retainer. The impact due to the irradiation causes the temporary retainer to be broken into small pieces. The small pieces scatter. If the scattering small pieces span the movable electrode and the movable electrode substrate, short circuit may occur between the movable electrode and the movable electrode substrate. 
         [0015]    In light of the above situation, the present invention provides a method of fabricating a fine workpiece and a laser processing apparatus, both capable of preventing dusts from scattering, and preventing the fine workpiece from being broken into small pieces. 
         [0016]    An embodiment of the present invention will be described below with reference to the attached drawings. 
         [0017]      FIG. 1  schematically shows the structure of a laser processing apparatus  11  according to an exemplary embodiment of the present invention. The laser processing apparatus  11  has a stage  12 . The stage  12  can move along a horizontal plane. 
         [0018]    The stage  12  faces an optical system  14 . The optical system  14  has an irradiation source, or a laser oscillator  15 . The laser oscillator  15  outputs a continuous-wave green laser beam. The green laser beam has a wavelength of 532 nm as is known. The laser oscillator  15  uses a light-emitting diode (LED) as a light source. The laser oscillator  15  amplifies the light of the LED. The output of the green laser beam ranges, for example, from about 1.5 to about 2.4 W. 
         [0019]    The laser oscillator  15  faces a collimator  16 . The collimator  16  converts the green laser beam output from the laser oscillator  15  into parallel light. The collimator  16  faces a reflection mirror  17 . The reflection mirror  17  reflects the laser beam output from the collimator  16  to a workpiece on the stage  12 . 
         [0020]    Two diaphragms  18  and  19  are disposed between the reflection mirror  17  and the stage  12 . The diaphragms  18  and  19  control the light quantity of the laser beam. A lens  21  is disposed between the diaphragm  19  and the stage  12 . The laser beam is emitted on the workpiece on the stage  12  and forms a predetermined spot with the lens  21 . 
         [0021]    A method of fabricating a fine workpiece using the laser processing apparatus  11  is described. A workpiece  22  is placed on the stage  12 . The workpiece  22  is a group of MEMS mirrors. As shown in  FIG. 2 , a temporary retainer  25  is formed between a movable electrode substrate  23  and a movable electrode  24 , in each of the MEMS mirrors. A mirror surface is formed on the movable electrode  24 . The temporary retainer  25  prevents the posture of the movable electrode  24  from being changed. The movable electrode substrate  23 , the movable electrode  24 , and the temporary retainer  25  are made by cutting silicon. 
         [0022]    The width W of the temporary retainer  25  is, for example, about 5 μm. The length L of the temporary retainer  25  is, for example, about 10 μm. The thickness W of the temporary retainer  25  is, for example, about 10 μm. The optical axis of the optical system  14  is positioned at the center position of the temporary retainer  25  in accordance with horizontal movement of the stage  12 . After the positioning, the laser oscillator  15  outputs the green laser beam. The output of the green laser beam is about 2.4 W. 
         [0023]    As shown in  FIG. 3 , the green laser beam  28  is irradiated on the temporary retainer  25 , the green laser beam  28  producing a spot  26  of light at the temporary retainer  25 . The size of the spot  26  is, for example, about 10 μm. Since the temporary retainer  25  is made of silicon, the temporary retainer  25  absorbs the green laser beam  28 . Hence, the temporary retainer  25  is heated, and the temperature of the temporary retainer  25  increases. The heat is transmitted from the center position of the spot  26  to the entire area of the temporary retainer  25 . A temperature-rise part  27  is heated. As shown in  FIG. 4 , the output of the green laser beam is held constant. 
         [0024]    The irradiation with the green laser beam is continued until the temperature of the temperature-rise part  27  of the temporary retainer  25  exceeds the melting point and then reaches the boiling point of the silicon. Accordingly, as shown in  FIG. 5 , the temporary retainer  25  is melted and evaporated. Thus, a part of the temporary retainer  25  is melted and separated. The irradiation with the green laser beam  28  is stopped. The irradiation time of the green laser beam  28  is about 570 milliseconds. The temperature of the temporary retainer  25  decreases. 
         [0025]      FIGS. 6A to 6E  are cross-sectional schematic views showing workpieces in an embodiment of the present invention. As shown in  FIG. 6A , a laser beam  28  is irradiated to a surface  63  of a temporary retainer  25 . Then, temperatures on the surface  63  and a vicinity of the surface  63  begin to rise. Then, a temperature-rise part  27  in which the temperature is higher than the surrounding area is formed. The temperature-rise part  27  extends from the center position of the surface  63  to the entire area of the temporary retainer  25  as shown in  FIG. 6B . Next, when the temperature of the temperature-rise part  27  exceeds a melting point, a part of the temperature-rise part  27  gathers in the center by surface tension thereof as shown in  FIG. 6C . Then, a spherical part  66  that has fluidity is formed. Peripheries  67 A and  67 B of the spherical part  66  can respectively be constricted. Next, the spherical part  66  disrupts, and melting sections  68 A and  68 B are formed as shown in  FIG. 6D . The ratio of the volumes of the melting section  68 A and  68 B depends on surface tension and gravity working at the spherical part  66 , and interfacial tension between solid and liquid phases, and the like. At this time, the temporary retainer  25  is not boiling though it is melting and evaporating. After the temporary retainer  25  is separated, the irradiation of the green laser light  28  is stopped. After the irradiation, the melting sections  68 A and  68 B coagulate. Then, the melting section  68 A integrates with the periphery  67 A, the melting section  68 A and the periphery  67 A composing a transformed part  69 A as a shown in  FIG. 6E . Similarly, the melting section  68 B integrates with the periphery  67 B, the melting section  68 B and the periphery  67 B composing a transformed part  69 B as shown in  FIG. 6E . 
         [0026]    After the temporary retainer  25  is separated, the irradiation is promptly stopped preferably because the temporary retainer  25  is prevented from boiling and another component around the temporary retainer  25  is prevented from overheating. Time from beginning the irradiation to the separation of the temporary retainer  25  can be examined beforehand so as to stop the irradiation promptly. The time can be assumed to be laser irradiation time. The method of examining the time is not especially limited. For instance, time until separating can be examined by preparing plural workpieces which are respectively irradiated in a variety of different time, taking images of temporary retainers of the workpieces, and judging whether the temporary retainers are respectively separated or not. 
         [0027]    Then, the stage  12  horizontally moves. The optical axis of the optical system  14  moves to a next temporary retainer  25  by the movement of the stage  12 . The temporary retainer  25  is melted and separated in a manner similar to the above. In this way, temporary retainers  25  are melted and separated for all MEMS mirrors. The MEMS mirrors are assembled to a mirror array. The mirror array is assembled to an optical switching element. The optical switching element is thus fabricated. 
         [0028]    With such a method of fabricating a fine workpiece, the temporary retainer  25  is continuously irradiated with the continuous-wave green laser beam during a time period from the beginning of the irradiation to the melting and separating. The temporary retainer  25  sufficiently absorbs the green laser beam. As a result, the temperature of the temporary retainer  25  gradually increases. The temperature of the temporary retainer  25  can be prevented from being rapidly changed. Thus, dusts can be prevented from being generated at the temporary retainer  25 . The dusts can be reliably prevented from scattering. In addition, since no impact is applied to the temporary retainer  25 , the temporary retainer  25  can be prevented from being broken into small pieces. The small pieces can be reliably prevented from scattering. 
         [0029]    In contrast, with the related art configuration, for example, a Q-switch oscillation pulse laser beam having, for example, a pulse width with microsecond is emitted. The pulse laser beam has an oscillation frequency with kHz. As shown in  FIG. 7 , the laser beam is intermittently emitted. The laser beam has a high output of, for example, several kW. The temperature of a temporary retainer rapidly increases every time when being irradiated with the laser beam. Dusts may be generated at the temporary retainer because of the change in temperature. In addition, an impact may be applied to the temporary retainer because of the irradiation with the laser beam. The temporary retainer may be broken into small pieces. 
         [0030]    The laser processing apparatus  11  may use a quasi-continuous-wave laser beam instead of the above-described green laser beam. The pulse width of the quasi-continuous wave is a smaller value than the pulse width of the above-mentioned pulse wave as is known. The pulse width is, for example, several tens of ps. The laser oscillation is several tens of MHz. The peak energy is several hundreds of W. In addition, the output of the quasi-continuous wave can be reduced as compared with the above-mentioned pulse wave. That is, the laser beam forms a continuous wave in a quasi manner. Alternatively, for example, a pulse-wave laser beam having a pulse width of 1 millisecond or greater may be used. The laser beam of such pulse wave may have a pulse width that is greater than the irradiation time from the beginning of the irradiation to the melting and separating. 
         [0031]    The above-mentioned embodiment can provide the method of fabricating the fine workpiece, and the laser processing apparatus, both capable of preventing the dusts from scattering and preventing the fine workpiece from being broken into small pieces.