Abstract:
A laser processing method includes the steps of irradiating a projection mask having a light transmitting area, for allowing a laser beam to be transmitted therethrough, with the laser beam; and irradiating a processing target with the laser beam transmitted through the light transmitting area. A spot of the laser beam on the projection mask is shaped so as to irradiate a portion in the vicinity of first edges of the light transmitting area, the first edges extending in one direction, and so as not to irradiate a portion in the vicinity of second edges of the light transmitting area, the second edges extending in a second direction which is different from the first direction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a laser processing method for processing a surface of a processing target using laser light, and specifically to a laser processing method preferably usable for ablating a processing target including an organic substance so as to form a flat face which is parallel or inclining with respect to an optical axis of the laser light. 
     2. Description of the Related Art 
     One method for processing an organic substance on a substrate by etching uses an ablation function of a laser beam such as, for example, an excimer laser beam. 
     FIG. 14 shows a schematic view illustrating a structure of a laser processing apparatus  200  usable for performing laser processing. The laser processing apparatus  200  is used for, for example, forming a recessed portion in a processing target  15 . The processing target  15  includes an organic sheet formed of, for example, polycarbonate (PC) or polyethylene terephthalate (PET) which is degraded when irradiated with light such as a laser beam. The laser processing apparatus  200  includes an X-Y stage  16  on which the processing target  15  is placed, and a laser oscillator  11  for emitting an excimer laser beam  12  toward the processing target  15 . The excimer laser beam  12  emitted by the laser oscillator  11  is provided with a prescribed pattern through a projection mask  13 , then is reduced in cross-sectional area by an objective optical system  14 , and is directed toward the processing target  15  fixed on the X-Y stage  16 . 
     FIG. 6 is a plan view of the projection mask  13 . The projection mask  13  is formed of a glass plate and a metal film provided on the glass plate so as to form a light shielding area  13   b . An area of the glass plate which is not covered with the metal film is a rectangular light transmitting area  13   a  through which the excimer laser beam  12  is allowed to be transmitted. 
     A spot  21   c  of the excimer laser beam  12  on the projection mask  13  is elliptical. The spot  21   c  on the projection mask  13  covers the light transmitting area  13   a  so that the entirety of the light transmitting area  13   a  is uniformly irradiated with the excimer laser beam  12 . In FIGS. 6,  7  and  8 , arrow X represents a direction of longer sides  21   a  of the rectangular light transmitting area  13   a , and arrow Y represents a direction of shorter sides  21   b  of the rectangular light transmitting area  13   a . In this specification, the direction indicated by arrow X will be described as the “X direction”, and the direction indicated by arrow Y will be described as the “Y direction”. 
     The excimer laser beam  12  which is transmitted through the light transmitting area  13   a  of the projection mask  13  is reduced in cross-sectional area by the objective optical system  14  and collected on the processing target  15  fixed on the X-Y stage  16 . Thus, an image of the rectangular light transmitting area  13   a  is projected on the processing target  15 . The image on the processing target  15  reflects the reduction ratio of the objective optical system  14 . A surface of the processing target  15  irradiated with the excimer laser beam  12  is ablated with the excimer laser beam  12 . As a result, a recessed portion defined by faces parallel to an optical axis of the excimer laser beam  12  is formed in the processing target  15 . 
     A face inclining with respect to the optical axis of the excimer laser beam  12  can be formed in the processing target  15  by moving the processing target  15  while being irradiated with the excimer laser beam  12 . 
     With respect to FIG. 12, a method for producing the inclining face will be described. 
     The processing target  15  includes a substrate  15   a  and an organic sheet  15   b  bonded to the substrate  15   a . For irradiating the processing target  15  with the excimer laser beam  12 , conditions for ablating only the organic sheet  15   b  are used. The excimer laser beam  12  transmitted through the rectangular light transmitting area  13   a  of the projection mask  13  is directed toward the processing target  15 . In this state, the processing target  15  is moved in the direction of arrow C shown in FIG. 12 at a constant speed. A surface of the substrate  15   a  is perpendicular to the optical axis. 
     While the processing target  15  is moved in this manner, the irradiation of the excimer laser beam  12  is stopped. Therefore, the total amount of the excimer laser beam  12  received by a front portion of the processing target  15  is different from the total amount of the excimer laser beam  12  received by a rear portion of the processing target  15 . The terms “front” and “rear” are defined with respect to the direction in which the processing target  15  is moved. As a result of the above-mentioned difference in the total amount of received excimer laser beam  2 , the processing target  15  is etched to a different degree in the front portion compared to the rear portion. Therefore, the inclining face which inclines downward from the rear portion toward the front portion of the processing target  15  is formed. A face inclining at any angle can be formed by adjusting the intensity of the excimer laser beam  12  and the moving speed of the processing target  15 . 
     FIG. 7 shows a profile  29  (solid line) of the recessed portion obtained by ablating the surface of the processing target  15  by the laser processing apparatus  200 . Since the entirety of the light transmitting area  13   a  is irradiated with the excimer laser beam  12 , the entirety of the profile  29  is wave-shaped, as opposed to an ideal profile  30  (dashed line) which is formed of four straight sides. 
     The reason why the profile  29  is wave-shaped is because the excimer laser beam  12  transmitted through the light transmitting area  13   a  of the projection mask  13  is diffracted by edges (i.e., both of the longer sides  21   a  and the shorter sides  21   b ; see FIG. 6) of the light transmitting area  13   a.    
     FIG. 8 shows a light intensity distribution of the excimer laser beam  12  irradiating the surface of the processing target  15  after being transmitted through the light transmitting area  13   a . Since the excimer laser beam  12  is diffracted by the edges of the light transmitting area  13   a , the light intensity received by the surface of the processing target  15  is not uniform, but portions having a higher light intensity than the rest of the surface appear in a lattice pattern as shown in FIG.  8 . Since these portions are ablated more strongly than the rest of the surface the entirety of the profile  29  is wave-shaped. 
     FIG. 9 is a graph illustrating light intensity distributions of the excimer laser beam  12  irradiating the surface of the processing target  15  along the X direction. A solid line  9   a  represents a light intensity distribution actually obtained by the laser processing apparatus  200 . A dashed line  9   b  represents a light intensity distribution obtained when the excimer laser beam  12  is not diffracted by the edges of the light transmitting area  13   a . The solid line  9   a  in FIG. 9 corresponds to the lattice shown in FIG.  8 . The solid line  9   a  has peaks having a light intensity level of higher than 1 (referred to as “overshoot”) at positions corresponding to the vicinity of the shorter sides  21   b of the light transmitting area  13   a  (FIG.  6 ). In addition, the solid line  9   a  fluctuates in a central portion thereof. 
     Since it is substantially unavoidable that the light is diffracted at the edges of the light transmitting area  13   a , it is difficult to form a recessed portion defined by flat faces as shown by the dashed line  30  in FIG.  7 . The inclining face shown in FIG. 12 is also wave-shaped, and it is difficult to form a flat inclining face for the same reason. 
     Japanese Laid-Open Publication No. 9-206974 discloses a method for improving a light intensity distribution characteristic of a laser beam irradiating a processing target after being transmitted through a light transmitting area of a projection mask. FIG. 10 is a schematic view of a laser processing apparatus  300  disclosed in Japanese Laid-Open Publication No. 9-206974. 
     Referring to FIG. 10, the laser processing apparatus  300  includes an irradiation optical system IL for uniformly irradiating a projection mask  56  with components of laser light in a superimposing manner. The irradiation optical system IL includes an excimer laser oscillator  51 , a beam shaping optical system  52 , a fly-eye lens  53  as a homogenizer HN, an aperture  54 , and a condenser lens  55 . A laser beam  61  emitted by the excimer laser oscillator  51  is enlarged in cross-sectional area by the beam shaping optical system  52  and is directed toward the fly-eye lens  53 . The fly-eye lens  53  includes a plurality of lens elements each having a longitudinal axis parallel to an optical axis AX of the laser beam  61 . Components of the laser beam  61  coming out of the fly-eye lens  53  reach the condenser lens  55  through the aperture  54  and are collimated by the condenser lens  55 . The collimated components of light irradiate the projection mask  56  in a superimposing manner. A light spot of the laser beam  61  on the projection mask  56  covers and thus uniformly irradiates the entirety of a rectangular light transmitting area of the projection mask  56 . 
     The laser beam  61  transmitted through the light transmitting area of the projection mask  56  is collected on a processing target  60  by an imaging optical system OS, which includes two lenses  57  and  58  and an aperture  59 . A recessed portion having a pattern corresponding to the light transmitting area of the projection mask  56  is formed in the processing target  60  by ablation provided by the laser beam  61 . 
     Where a numerical aperture of the irradiation optical system IL is NAc and a numerical aperture of the imaging optical system OS is NAo, the coherence factor σ is defined by expression (1). 
     
       
         σ= NAc/NAo   (1) 
       
     
     FIGS. 11A and 11B show light intensity distributions of the excimer laser beam  12  irradiating the processing target  60  along the X direction. A solid line  111  in FIG. 11A shows a light intensity distribution obtained when the coherence factor σ is 0.2, and a solid line  113  in FIG. 11B shows the light intensity distribution obtained when the coherence factor a is 0.7. A dashed line  112  in FIG. 11A and a dashed line  114  in FIG. 11B each show a light intensity distribution obtained when the laser beam  61  is not diffracted by the edges of the light transmitting area. 
     The solid line  113  in FIG. 11B exhibits smaller peaks (smaller overshoot portions) and fluctuates less in the central portion than the solid line  111  in FIG.  11 A. It is appreciated that an increase in the coherence factor a prevents the intensity of the laser beam from increasing in portions of the surface of the processing target  60  corresponding to the edges of the light transmitting area of the projection mask  56 . The increase in the coherence factor a also alleviates fluctuations in the intensity of the laser beam in a portion of the surface of the processing target  60  corresponding to a central area of the light transmitting area of the projection mask  56 . When the coherence factor σ is 0.6 or more, a recessed portion defined by flat faces parallel to the optical axis AX (FIG. 10) can be formed in the processing target  60  at relatively high precision. 
     However, it is still difficult to completely remove the influence of the diffraction of the laser beam by the edges of the light transmitting area as can be appreciated from FIG.  11 B. Thus, it is difficult to completely prevent a local increase in the intensity of the laser beam. Formation of a flat face in a processing target, whether parallel or inclining with respect to an optical axis of a laser beam, has not been realized. 
     SUMMARY OF THE INVENTION 
     A laser processing method according to the present invention includes the steps of irradiating a projection mask having a light transmitting area, for allowing a laser beam to be transmitted therethrough, with the laser beam; and irradiating a processing target with the laser beam transmitted through the light transmitting area. A spot of the laser beam on the projection mask is shaped so as to irradiate a portion in the vicinity of first edges of the light transmitting area, the first edges extending in one direction, and so as not to irradiate a portion in the vicinity of second edges of the light transmitting area, the second edges extending in a second direction which is different from the first direction. 
     In one embodiment of the invention, the light transmitting area of the projection mask has a shape of a rectangle which extends in the first direction. 
     In one embodiment of the invention, the second edges of the light transmitting area are shorter sides of the rectangle. 
     In one embodiment of the invention, the laser processing method further includes the step of moving the processing target in the first direction. 
     In one embodiment of the invention, the laser processing method further includes the step of moving the processing target in the second direction. 
     In one embodiment of the invention, the laser processing method further includes the step of reciprocating the processing target in the first direction concurrently with moving the processing target in the second direction. 
     Thus, the invention described herein makes possible the advantages of providing a low-cost laser processing method for relatively easily controlling diffraction of a laser beam at an edge of a light transmitting area of a projection mask so as to guarantee formation of a flat face in a processing target, whether parallel or inclining to an optical axis of a laser beam. 
     These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view illustrating a structure of a laser processing apparatus used for a laser processing method according to the present invention: 
     FIG. 2 is a plan view of a projection mask used for the present invention; 
     FIG. 3 is a schematic view illustrating a profile of a recessed portion formed by a laser processing method according to the present invention; 
     FIG. 4 is a schematic view illustrating a light intensity distribution irradiating a surface of a processing target in accordance with the laser processing method according to the present invention; 
     FIG. 5 is a graph illustrating a light intensity distribution irradiating the surface of the processing target in accordance with the laser processing method according to the present invention; 
     FIG. 6 is a plan view of a conventionally used projection mask: 
     FIG. 7 is a schematic view illustrating a profile of a recessed portion formed by a conventional laser processing method; 
     FIG. 8 is a schematic view illustrating a light intensity distribution irradiating a surface of a processing target in accordance with the conventional laser processing method; 
     FIG. 9 is a graph illustrating a light intensity distribution irradiating the surface of the processing target in accordance with the conventional laser processing method; 
     FIG. 10 is a schematic view illustrating a structure of a conventional laser processing apparatus; 
     FIGS. 11A and 11B show light intensity distributions of a laser beam irradiating a processing target in accordance with the conventional laser processing method, at different values of coherence factor; 
     FIG. 12 is a schematic isometric view illustrating a technique for forming an inclining face by a conventional laser processing method; 
     FIG. 13 is a schematic isometric view illustrating a technique for forming an inclining face by a laser processing method according to the present invention; and 
     FIG. 14 is a schematic view illustrating a structure of a laser processing apparatus used for a conventional laser processing method. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described by way of illustrative examples with reference to the accompanying drawings. 
     A laser processing method according to the present invention is performed by a laser processing apparatus  100  shown in FIG.  1 . 
     The laser processing apparatus  100  is used for, for example, forming a recessed portion in a processing target  5 . The processing target  5  includes an organic sheet formed of, for example, polycarbonate (PC) or polyethylene terephthalate (PET) which is degraded when irradiated with light such as a laser beam. The laser processing apparatus  100  includes an X-Y stage  6  on which the processing target  5  is placed, and a laser oscillator  1  for emitting an excimer laser beam  2  toward the processing target  5 . The excimer laser beam  2  emitted by the laser oscillator  1  is provided with a prescribed rectangular pattern through a projection mask  3 . The excimer laser beam  2  is then reduced in cross-sectional area by an objective optical system  4 , and is directed toward the processing target  5  fixed on the X-Y stage  6 . The excimer laser beam  2  emitted by the laser oscillator  1  has a rectangular cross-section, and the size of the rectangle is, for example, 25 mm×8 mm in this example. 
     FIG. 2 is a plan view of the projection mask  3 . The projection mask  3  is formed of a glass plate and a metal film provided on the glass plate so as to form a light shielding area  3   b.  An area of the glass plate which is not covered with the metal film is a rectangular light transmitting area  3   a  through which the excimer laser beam  2  is allowed to be transmitted. 
     A spot  2   a  of the excimer laser beam  2  on the projection mask  3  is elliptical. In FIG. 2, the length of a longer axis  2   b  of the spot  2   a  is represented by “Bx”, and the length of longer sides  31   a  of the light transmitting area  3   a  is represented by “Ax”. The longer sides  3   a  are parallel to the longer axis  2   b,  and the length Ax of the longer sides  31   a  is greater than the length Bx of the longer axis  2   b.  The optical axis of the excimer laser beam  2  is positioned so that the spot  2   a  does not cover the shorter sides  31   b  of the light transmitting area  3   a.  In FIGS. 2,  3  and  4 , arrow X represents the direction of longer sides  31   a  of the rectangular light transmitting area  3   a  (X direction), and arrow Y represents the direction of shorter sides  31   b  of the rectangular light transmitting area  3   a  (Y direction). 
     According to the present invention, sides of the rectangular light transmitting area  3   a,  the sides extending in one direction, are longer than the axis of the spot  2   a  extending parallel to the above-mentioned sides. In addition, the spot  2   a  does not cover the end portions of the above-mentioned sides. In the following description, the sides  31   a  of the light transmitting area  3   a  are longer than the longer axis  2   b  in the X direction, and the end portions of the sides  31   a  are not irradiated with the excimer laser beam  2 . 
     In an alternative structure, the light transmitting area  3   a  is longer in the Y direction than in the X direction, and the excimer laser beam  2  is directed so as not to cover the shorter sides of the light transmitting area  3   a.  A similar effect is provided. 
     Referring to FIG. 1, the excimer laser beam  2  which is transmitted through the light transmitting area  3   a  of the projection mask  3  is reduced in cross-sectional area by the objective optical system  4  and collected on the processing target  5  fixed on the X-Y stage  6 . Thus, an image of the rectangular light transmitting area  3   a  is projected on the processing target  5 . The image on the processing target  5  reflects the reduction ratio of the objective optical system  4 . A surface of the processing target  5  irradiated with the excimer laser beam  2  is ablated with the excimer laser beam  2 . As a result, a recessed portion defined by faces parallel to an optical axis of the excimer laser beam  2  is formed in the processing target  5 . 
     In this example, the surface of the processing target  5  and the surface of the X-Y stage  6  are perpendicular to the optical axis of the excimer laser beam  2 . Accordingly, the direction parallel to the optical axis is vertical to the surface of the processing target  5  and the surface of the X-Y stage  6 . 
     FIG. 3 shows a profile  19  (solid line) of the recessed portion obtained by ablating the surface of the processing target  5  by the laser processing apparatus  100 . A profile  20  (dashed line) is an ideal profile which is formed of four straight sides. 
     The spot  2   a  does not cover and thus is not diffracted by the shorter sides  31   b  of the light transmitting area  3   a  as shown in FIG.  2 . Accordingly, sides  19   a , corresponding to the longer sides  31   a , of the profile  19  are straight and substantially overlap the ideal profile  20 . 
     FIG. 4 shows a light intensity distribution of the excimer laser beam  2  irradiating the surface of the processing target  5  after being transmitted through the light transmitting area  3   a . Since the excimer laser beam  2  is diffracted by the longer sides  31   a  of the light transmitting area  3   a , portions having a higher light intensity than the rest of the surface appear in stripes parallel to the X direction as shown in FIG.  4 . Since the excimer laser beam  2  is not diffracted by the shorter sides  31   b  of the light transmitting area  3   a , no stripes appear parallel to the shorter sides  31   b.    
     The surface of the processing target  5  is ablated by the excimer laser beam  2  having such a light intensity distribution. As a result, the faces parallel to the X direction of the X-Y stage  6  among the faces defining the recessed portion are flat, instead of wave-shaped, as represented by the profile  19  in FIG.  3 . (The faces parallel to the X direction of the X-Y stage  6  will be referred to as the “X direction faces”.) 
     FIG. 5 is a graph illustrating light intensity distributions of the excimer laser beam  2  irradiating the surface of the processing target  5  along the X direction. A solid line  5   a  represents a light intensity distribution actually obtained by the laser processing apparatus  100 . A dashed line  5   b  represents a light intensity distribution obtained when the excimer laser beam  2  is not diffracted by any edge of the light transmitting area  3   a . The solid line  5   a  in FIG. 5 corresponds to the stripes shown in FIG.  4 . The solid line  5   a  is substantially plateau and matches the dashed line  5   b  in a central portion thereof. However, since the length Ax (FIG. 2) of the longer sides  31   a  of the light transmitting area  3   a  is longer than the length Bx of the spot  2   a , the solid line  5   a  is offset from the dashed line  5   b  at portions corresponding to the shorter edges  31   b  of the light transmitting area  3   a . 
     Use of the projection mask  3  reduces the adverse effect of the diffraction of the excimer laser beam  2  at the edges of the light transmitting area  3   a . However, when the excimer laser beam  2  has distributions in terms of light intensity in the X direction when incident on the projection mask  3 , the distribution results in wave-shaped faces defining the recessed portion. This inconvenience can be alleviated by performing scanning, i.e., by moving the processing target  5  in the direction in which the longer sides  31   a  of the light transmitting area  3   a  is longer than the longer axis  2   b  of the spot  2   a  (i.e., the X direction), in the range that the spot  2   a  does not cover the shorter sides  31   b  of the light transmitting area  3   a . Thus, the influence of the distribution of the excimer laser beam  2  in terms of light intensity is alleviated, resulting in improving the flatness of the X direction faces defining the recessed portion. The same effect is provided by reciprocating the optical axis of the excimer laser beam  2  with the processing target  5  being kept still. 
     An inclining face can also be formed by the combination of the projection mask  3  and the scanning as described below. 
     The elliptical shape of the spot  2   a  of the excimer laser beam  2  is adjusted so as not to cover the shorter sides  31   b  of the light transmitting area  3   a , and the processing target  5  is moved at a constant speed along the shorter sides  31   b  in the state of being irradiated with the excimer laser beam  2 . While the processing target  5  is moved, the irradiation of the excimer laser beam  2  is stopped. As a result, the inclining face which inclines downward from the rear portion toward the front portion of the processing target  5  is formed by the principle described above with reference to FIG.  12 . Whereas in the case of FIG. 12, the excimer laser beam  12  having the light intensity distribution causes the inclining face to have corrugations; in the case of the present invention, the inclining face is flat with no such corrugations owing to the excimer laser beam  2  having the light intensity distribution shown in FIG.  4 . 
     Even when the projection mask  3  is used, the spot  2   a  has a cyclic light intensity distribution in the Y direction as shown in FIG.  4 . Since the above-mentioned scanning is performed in the Y direction, however, the cyclic light intensity distribution does not cause any significant problem. 
     As in the case of forming faces vertical to the surface of the processing target  5 , when the excimer laser beam  2  has distributions in terms of light intensity in the X direction when incident on the projection mask  3 , the distribution results in a wave-shaped inclining face. This inconvenience can be alleviated by moving the processing target  5  in the X direction as shown by arrow C′ in FIG. 13 while moving the processing target  5  in the Y direction. Thus, the influence of the distribution of the excimer laser beam  2  in terms of light intensity is alleviated, resulting in improving the flatness of the inclining face. 
     In the above example, a recessed portion having a prescribed pattern is formed in a processing target. The present invention is applicable to formation of a through-hole having a prescribed pattern in a processing target. 
     EXAMPLES 
     Example 1 
     The laser processing method according to the present invention was performed using an excimer laser processing apparatus (KrF laser; wavelength: 248 nm; oscillation output: 270 mJ; oscillation frequency: 200 pulses/s) produced by Sumitomo Heavy Industries, Ltd. as the laser processing apparatus  100  shown in FIG.  1 . As the processing target  5 , a PET sheet having a thickness of 200 μm bonded on a silicon substrate was used. The processing target  5  was fixed on the X-Y stage  6 . The excimer laser beam  2  emitted by the light source had a cross-section of 25 mm×8 mm. The rectangular light transmitting area  3   a  of the projection mask  3  had a size of 30 mm×3 mm. 
     The longer sides  31   a  of the light transmitting area  3   a  was provided so as to be parallel to the longer axis  2   b  of the spot  2   a , and the center of the spot  2   a  was matched to the center of the light transmitting area  3   a . Since the length Bx of the longer axis  2   b  of the spot  2   a  was 25 mm and the length Ax of the longer sides  31   a  of the light transmitting area  3   a  was 30 mm, the shorter sides  31   b  of the light transmitting area  3   a  were not irradiated with the spot  2   a.    
     It was set so that the excimer laser beam  2  transmitted through the light transmitting area  3   a  was reduced in cross-sectional area to ⅓ by the objective optical system  4 . 
     A prescribed laser processing process was performed in this state for forming a recessed portion in the processing target  5 . The planar precision (or flatness) of the X direction faces defining the recessed portion was measured by an interferometer produced by Zygo Corporation. The resultant RMS value was 50 nm, which was significantly satisfactory. 
     Example 2 
     A recessed portion was formed in the processing target  5  in the same manner as in Example 1 except that while the processing target  5  was irradiated with the excimer laser beam  2 , the X-Y stage  6  having the processing target  5  fixed thereon was moved in the X direction. 
     The planar precision of the X direction faces defining the recessed portion was measured by the interferometer produced by Zygo Corporation. The resultant RMS value was 40 nm, which was more satisfactory than the RMS value in Example 1. 
     Example 3 
     A face inclining with respect to the surface of the processing target  5  was formed in the same manner as in Example 1 except that while the processing target  5  was irradiated with the excimer laser beam  2 , the X-Y stage  6  having the processing target  5  fixed thereon was moved in the Y direction. 
     The planar precision of the inclining face was measured by the interferometer produced by Zygo Corporation. The resultant RMS value was 120 nm, which was significantly satisfactory. 
     Example 4 
     A face inclining with respect to the surface of the processing target  5  was formed in the same manner as in Example 3 except that while the processing target  5  was irradiated with the excimer laser beam  2 , the X-Y stage  6  having the processing target  5  fixed thereon was reciprocated in the X direction while moved in the Y direction. 
     The planar precision of the inclining face was measured by the interferometer produced by Zygo Corporation. The resultant RMS value was 60 nm, which was more satisfactory than the RMS value in Example 3. 
     Comparative Example 1 
     A recessed portion was formed in a processing target in the same manner as in Example 1 except that a projection mask having a rectangular light transmitting area having a size of 6 mm×3 mm was used. The spot  2   a  was 25 mm×8 mm as in Example 1. 
     The planar precision of the inclining face was measured by the interferometer produced by Zygo Corporation. The resultant RMS value was 80 nm, which was inferior to the RMS values in Examples 1 and 2. 
     Comparative Example 2 
     A face inclining with respect to the surface of the processing target was formed in the same manner as in Example 3 except that a projection mask having a rectangular light transmitting area having a size of 6 mm×3 mm was used. The spot  2   a  was 25 mm×8 mm as in Example 3. 
     The planar precision of the inclining face was measured by the interferometer produced by Zygo Corporation. The resultant RMS value was 200 nm, which was inferior to the RMS values in Examples 3 and 4. 
     A laser processing method according to the present invention uses a projection mask having a transmitting area which is longer in one direction than the axis of the spot of a laser beam. Therefore, the influence of diffraction of the laser beam at the edges of the light transmitting area is alleviated. Thus, faces which are vertical or inclining to the surface of a processing target can be formed with satisfactorily high precision. 
     The planar precision of the faces is also enhanced by performing scanning in one axis, i.e., in the direction of the longer sides of the light transmitting area or the direction perpendicular thereto; and is further enhanced by performing scanning in two axis, in both of the directions concurrently. 
     The laser processing method according to the present invention, which is realized only by changing the shape of the light transmitting area of the projection mask conventionally used, is advantageous in terms of cost. 
     Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.