Abstract:
A laser beam direct imaging apparatus and an imaging method which can precisely determine a back-surface-side position with respect to a front-surface-side position even if any kind of photosensitive material is used. In the laser direct imaging apparatus, a laser beam is deflected toward a main scanning direction (X-axis direction) while a workpiece mounted on a table is moved in a sub-scanning direction (Y-axis direction) so that a pattern is imaged on the surface of the workpiece. Hollow pins are disposed on the table so that the tips of the hollow pins  20  project over the surface of the table by a predetermined distance. The workpiece is sucked onto the table so that indentations (indentations by the tips of the hollow pins) are formed on the back surface of the workpiece. When a pattern is imaged on the back surface, imaging is performed with reference to the indentations.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a laser direct imaging (LDI) apparatus and an imaging method for moving a workpiece in a sub-scanning direction while deflecting a laser beam, which has been modulated based on raster data, toward a main scanning direction so as to image a desired pattern on the workpiece. 
       BACKGROUND OF THE INVENTION 
       [0002]    In an LDI apparatus, CAD data used for designing a circuit pattern are converted into vector data format, and then contours are calculated from the vector data. After that, the contours are further converted into raster data for imaging. From the raster data, ON and OFF pixels for a laser beam are obtained. The ON pixels are irradiated with the laser beam. 
         [0003]      FIG. 7  is a view showing a configuration of background-art LDI apparatus. 
         [0004]    A laser source  1  is mounted on an optical table  16 . The optical table  16  is disposed on a column  17  on a bed  18 . A laser beam  5  emitted from the laser source  1  enters an acousto-optic modulator (hereinafter referred to as “AOM”)  4  reflected by mirrors  2  and an expander  3 . A laser beam  5   a  modulated by the AOM  4  is deflected by a polygon mirror  6  and enters an fθ lens  7 . The laser beam  5   a  emerged from the fθ lens  7  is deflected toward the downward direction of  FIG. 7  by a reflection mirror  8 , and enters a cylindrical lens  9 . The laser beam  5   a  emerged from the cylindrical lens  9  is incident on a workpiece  10 . A dry film resist (hereinafter referred to as “DFR”), a photo-resist or the like on the workpiece  10  is exposed to the laser beam  5   a . On this occasion, a table  12  where the workpiece  10  is mounted moves in a sub-scanning direction (Y-axis direction in  FIG. 7 ) at a constant speed. A linear motor  14  moves the table  12 . A pair of guides  13  guide the table  12 . A camera  60  is disposed above the table  12 . The camera  60  is mounted on a not-shown shifter by which the camera  60  can be positioned desirably in the X-axis direction. The camera  60  is connected to a not-shown image processor. For example, in order to determine an imaging position, the camera  60  is used for picking up images of alignment marks disposed in the surface of the workpiece  10  (Patent Document 1). 
         [0005]      FIGS. 8A and 8B  are views showing the position of a start sensor.  FIG. 8A  is a view in the X-axis direction of  FIG. 7 , and  FIG. 8B  is a view in the Y-axis direction of  FIG. 7 . 
         [0006]    A mirror  11  is disposed under the left end portion of the cylindrical lens  9  in  FIG. 7 . A start sensor  15  is disposed in the direction of reflected laser beam from the mirror  11 . In order to align the scanning start points of rows, which mean the rows of the exposed pixels by the main scanning (X-axis direction), imaging in each scan in the main scanning direction is started when a predetermined time has passed after the start sensor  15  has detected the laser beam  5   a  reflected by the mirror  11  (the distance between the detection position and the imaging start position is 10 mm in the illustrated case). Thus, the scanning start points of rows are aligned. 
         [0007]    To machine a printed circuit board, xy coordinate axes are determined with reference to alignment marks provided in the surface of the printed circuit board (workpiece) in advance before machining. It is difficult to fix the printed circuit board onto the table so that the xy coordinate axes are set in parallel with the XY coordinate axes of the driving system of the LDI apparatus. Therefore, a not-shown rotating mechanism is provided in the table  12 . By the rotating mechanism, the workpiece  10  is rotated so that the xy coordinate axes of the workpiece  10  can be set in parallel with the XY coordinate axes of the driving system of the LDI apparatus. 
         [0008]    There are various methods for manufacturing so-called multilayer boards. In a pin lamination method or a mass lamination method, double-sided boards each having conductor layers disposed on the both sides of an insulating layer are laminated to one another with insulating layers interposed. In order to improve the reliability of such a multilayer board as a product, it is necessary to precisely position patterns to be disposed on front and back surfaces of each double-sided board. 
         [0009]    When a photosensitive material such as resist is applied to surfaces of conductor layers of each double-sided board, the following technique can be used (Patent Document 1). That is, an exposure unit is provided on the back surface side. When the front surface side is being exposed to light, exposure is performed on the back surface side so as to form an alignment mark thereon. When the back surface side is exposed to light, the back surface side is machined based on the position of the alignment mark formed by the exposure in advance, as described above. According to this technique, a pattern to be disposed on the back surface side can be determined precisely with respect to a pattern disposed on the front surface side. 
         [0010]    Patent Document 1: WO 02/39794 
         [0011]    However, exposure is merely performed but development is not performed in the above-mentioned technique, that is, a latent image is used. Therefore, when some kind of photosensitive material is used, there is a case where the alignment mark cannot be identified. Development on printed circuit boards or partial development only near portions exposed to light in order to identify the alignment mark increases the number of processes of operation. Therefore such a solution cannot be employed. 
       SUMMARY OF THE INVENTION 
       [0012]    An object of the present invention is to solve the foregoing problem. Another object of the present invention is to provide a laser direct imaging apparatus and an imaging method in which a position on the back surface side can be determined precisely with respect to the front surface side even if any kind of photosensitive material is used. 
         [0013]    In order to attain the foregoing objects, a first configuration of the present invention provides a laser direct imaging apparatus for deflecting a laser beam toward a main scanning direction while moving a workpiece, which is mounted on a table, in a sub-scanning direction so as to image a pattern in a front surface of the workpiece. The laser direct imaging apparatus is characterized in that indentations forming unit is provided in the table and for forming indentations with a bottom in a back surface of the workpiece. 
         [0014]    A second configuration of the present invention provides an imaging method for deflecting a laser beam toward a main scanning direction while moving a workpiece, which is mounted on a table, in a sub-scanning direction so as to image patterns in front and back surfaces of the workpiece. The imaging method is characterized in that patterns are imaged by the following steps a to e. 
         [0000]    a. Step of mounting the workpiece on the table.
 
b. Step of imaging a pattern in the front surface of the workpiece while forming indentations with a bottom in predetermined positions of the back surface of the workpiece.
 
c. Step of turning over the workpiece, and mounting the workpiece on the table.
 
d. Step of determining a coordinate system of the front surface of the workpiece based on the positions of the indentations, and imaging a pattern in the back surface of the workpiece.
 
e. Step of removing the workpiece from the table.
 
         [0015]    The positions of the alignment marks can be known even if any kind of photosensitive material is used. Accordingly, a position on the back surface side can be determined precisely with respect to the front surface side. As a result, it is possible to improve the reliability of a multilayer board as a product and the yield of the product. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIGS. 1A-1C  are views showing a configuration of a table of a laser direct imaging apparatus according to an embodiment of the present invention; 
           [0017]      FIG. 2  is a view for explaining the configuration of a dry film; 
           [0018]      FIGS. 3A-3C  are diagrams for explaining the layout of a workpiece with respect to the table according to the embodiment of the present invention; 
           [0019]      FIGS. 4A and 4B  are views showing another embodiment of the present invention; 
           [0020]      FIGS. 5A-5D  are views showing examples of modified hollow pins; 
           [0021]      FIG. 6  is a plan view of a table showing further another embodiment of the present invention; 
           [0022]      FIG. 7  is a view showing a configuration of background-art laser direct imaging apparatus; and 
           [0023]      FIGS. 8A and 8B  are views showing the position of a start sensor. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0024]    The present invention will be described below with reference to the drawings. 
         [0025]      FIGS. 1A-1C  are views showing a configuration of a table of a laser direct imaging apparatus according to the present invention.  FIG. 1A  is a plan view,  FIG. 1B  is a side sectional view, and  FIG. 1C  is an enlarged fragmentary sectional view of a portion S in  FIG. 1B . Parts the same as or having the same functions as those in  FIG. 7  are designated with the same reference numeral and a description thereof is omitted. 
         [0026]    A large number of suction holes  22  connected to an internal hollow portion  18  are disposed like a lattice in the surface of a table  12 . The hollow portion  18  is connected to a not-shown vacuum system through joints  45 . The hollow portion  18  is divided into three blocks by not-shown partitions. The joints  45  are provided for the blocks respectively so that negative pressure can be applied to the blocks individually. Three positioning pins  21   a  to  21   c  for positioning the workpiece  10  are disposed on the surface of the table  12 . The positioning pins  21   a  to  21   c  are circular in section, and the tangent line to the positioning pins  21   a  and  21   c  are parallel to the Y axis. 
         [0027]    Four hollow pins  20  ( 20   a - 20   d ) are fixed onto the table  12 . Each hollow pin  20  is annular in section. The center of each hollow pin  20  is aligned in parallel to the X axis. Each inside of the hollow pins  20  communicates with the hollow portion  18 . As shown in  FIG. 1C , the inner edge of the tip of each hollow pin  20  is beveled to sharpen the outer edge. The hollow pins  20   a - 20   d  project over the surface of the table  12  by a distance g (here 10-50 μm). The hollow pins  20   a - 20   d  are placed out of an area where the workpiece  10  will be patterned. 
         [0028]    Here, description will be made about the workpiece  10 . The workpiece  10  is constituted by a double-sided board  50 , photosensitive resists  51  and carrier films  52 . The photosensitive resist  51  is disposed (pasted) on each side of the double-sided board  50 . The carrier film  52  is disposed (pasted) outside each photosensitive resist  51 . As shown in  FIG. 2 , the photosensitive resist  51  is kept as a dry film in which the carrier-film  52  whose material is, for example, polyester is disposed on one outer surface of the photosensitive resist  51  and a cover film  53  whose material is polyethylene is disposed on the other outer surface of the photosensitive resist  51 . The cover film  53  is removed (separated) when the photosensitive resist  51  is disposed on the double-sided board  50 . 
         [0029]    Next, description will be made about the procedure of the present invention. 
         [0030]    Here, assume that the outer shape of the workpiece is rectangular. 
         [0000]    Step 1: The workpiece  10  is mounted on the table  12 . On this occasion, the workpiece  10  is disposed so that two adjacent sides of the workpiece  10  touch the positioning pins  21   a - 21   c.  
 
Step 2: The vacuum system is operated to suck the workpiece  10  onto the table  12 . By the suction, the workpiece  10  is pressed onto the table  12  so that indentations (which are annular in plan view and V-shaped in thickness direction here. In accordance with the suction force of the vacuum system, the indentions may be formed only in the carrier film  52  or may extend to the photosensitive resist  51 .) are formed in the surface of the carrier film  52  (or the surfaces of the carrier film  52  and the photosensitive resist  51 ) by the hollow pins  20   a - 20   d.  
 
Step 3: The front side surface (The illustrated surface will be referred to as “surface F”. The back side surface will be referred to as “surface B.”.) is exposed to light. The photosensitive resist is exposed to a laser beam transmitted through the carrier film (16-25 μm thick).
 
Step 4: When the exposure of the surface F is terminated, the vacuum system is suspended. The workpiece  10  is turned over around the Y axis and mounted on the table  12 . On this occasion, two adjacent sides of the workpiece  10  are made to touch the positioning pins  21   a  to  21   c.  
 
         [0031]    Step 5: The vacuum system is operated to suck the workpiece  10  onto the table  12 . 
         [0000]    Step 6: Any two out of the indentations formed by the hollow pins  20   a  to  20   d  (here, we select an indentation  20 A formed by the hollow pin  20   a  and an indentation  20 D formed by the hollow pin  20   d ) are observed by the camera  60 , and the coordinates of the centers of the indentations are obtained by image processing.
 
Step 7: The table  12  is rotated based on the coordinates of the centers of the indentations  20 A and  20 D so that the xy axes of the pattern machined in the surface F which is now on the back side are brought into parallelism (here line) with the XY coordinate axes of the driving system of the laser direct imaging apparatus. The detail will be described later.
 
Step 8: The surface B is exposed.
 
Step 9: The workpiece  10  is removed from the table  12 .
 
         [0032]    The carrier film on each side is removed prior to a development step which is an after process. When the photosensitive material is a negative type material, the photosensitive resist which was not irradiated with (exposed to) the laser beam is removed by developer, and the portion which has been exposed forms the pattern. On the other hand, when the photosensitive material is a positive type material, the portion which was irradiated by the laser beam forms the pattern. 
         [0033]    Next, the aforementioned Step 7 will be described further in detail. 
         [0034]      FIGS. 3A-3C  are diagrams showing the procedure for positioning the back surface B.  FIG. 3A  shows the case of the aforementioned Step 2.  FIG. 3B  shows the case of the aforementioned Step 5, where the surface F has been exposed.  FIG. 3C  shows the result of the aforementioned Step 7. 
         [0035]    Here, assume that the origin O of the XY coordinate axes of the driving system of the laser direct imaging apparatus is on the optical axis of the camera  60 . The reference sign P designates the rotation center of the table  12 . The hollow pin  20   d  and the indentation  20 D are depicted as shaded portions. 
         [0036]    Here, assume that the indentations  20 A and  20 D formed in the surface B by the hollow pins  20   a  and  20   d  respectively are used. 
         [0037]    When Qa, Qd and QC designate the center of the hollow pin  20   a , the center of the hollow pin  20   d  and the midpoint of the segment QaQd respectively, the coordinates of the center Qa, the center Qd, the midpoint QC and the rotation center P are expressed by (X1R, Y1R), (X2R, Y2R), (Xcr, Ycr) and (Xt, Yt) respectively. Since the segment QaQd is parallel to the X axis, there is a relation of the form Y1R=Y2R=Ycr. 
         [0038]    As shown in  FIG. 3A , in Step 2, the indentations  20 A and  20 D are formed in the surface B by the hollow pins  20   a  and  20   d  respectively. 
         [0039]    Now assume that the coordinates of the central positions Qab and Qdb of the indentations  20 A and  20 D and the midpoint QCb (expressed with a suffix b because they are on the surface B side) measured in Step 6 are (X1, Y1), (X2, Y2) and (XC, YC) respectively as shown in  FIG. 3B . 
         [0040]    In this case, XC, YC and the tilt angle θ can be obtained by Equations 1 to 3 respectively. 
         [0000]        XC =( X 1+ X 2)/2  (Equation 1) 
         [0000]        YC =( Y 1+ Y 2)/2  (Equation 2) 
         [0000]      Δθ=tan −1 {( X 2 −X 1)/( Y 2 −Y 1)}  (Equation 3) 
         [0041]    Displacements ΔX1 and ΔY1 of the midpoint QCb from the midpoint QC in the X-axis and Y-axis directions can be obtained by Equations 4 and 5 respectively. 
         [0000]      Δ X 1= Xcr−XC   (Equation 4) 
         [0000]      Δ Y 1= Ycr−YC   (Equation 5) 
         [0042]    Here, when L designates the distance between the midpoint QCb and the rotation center P, the distance L can be obtained from Equation 6, and the tilt Δθ of the segment QCbP can be obtained from Equation 7. 
         [0000]        L =√{( Xt−XC ) 2 +( Yt−YC ) 2 }  (Equation 6) 
         [0000]      Δθ=tan −1 {( XC−Xt )/( YC−Yt )}  (Equation 7) 
         [0043]    When the table  12  is rotated by −Δθ, the center coordinates (Xch, Ych) of the corrected midpoint QCb can be expressed by Equations 8 and 9 respectively. 
         [0000]        Xch=L ×cos(θ+Δθ)  (Equation 8) 
         [0000]        Ych=L ×sin(θ+Δθ)  (Equation 9) 
         [0044]    Here, when Xg and Yg designate X-axis-direction and Y-axis-direction distances of the rotation center of the midpoint QCb from a reference point respectively, Xg and Yg can be expressed by Equations 10 and 11 respectively. 
         [0000]        Xg=XC−Xt   (Equation 10) 
         [0000]        Yg=YC−Yt   (Equation 11) 
         [0045]    The X-axis-direction and Y-axis-direction displacements of the midpoint QCb caused by the Δθ rotation can be obtained from Equations 12 and 13 respectively. 
         [0000]      Δ Xch=Xch−Xg   (Equation 12) 
         [0000]      Δ Ych=Ych−Yg   (Equation 13) 
         [0046]    Accordingly, when the table is moved in the X-axis and Y-axis directions by ΔX and ΔY expressed by Equations 14 and 15 respectively, the midpoint QCb can be brought into line with the midpoint QC as shown in  FIG. 3C . 
         [0000]      Δ X=ΔX 1+Δ Xch   (Equation 14) 
         [0000]      Δ Y=ΔY 1+Δ Ych   (Equation 15) 
         [0047]    In the aforementioned description, the indentations  20 A and  20 D were used. However, any two of the indentations  20 A- 20 D may be used. 
         [0048]      FIGS. 4A and 4B  are views showing another embodiment of the present invention.  FIG. 4A  is a plan view of a table end portion, and  FIG. 4B  is a side sectional view thereof. 
         [0049]    A pair of guide pins  24  are disposed in opposite ends of an end portion of a table  12 . An L-shaped support  27  is fixed to a side of the table  12 . A cylinder  25  is supported in a central portion of the support  27 . A piston rod  25   a  of the cylinder  25  passes through a not-shown hole formed in the support  27  and projects over the front surface of the table  12 . A plate  26  is supported on the tip of the piston rod  25   a . The plate  26  is large enough to face hollow pins  20   a - 20   d . When the piston rod  25   a  is at a standby position (at the position shown by the real line in  FIG. 4B ), a lower surface  26   b  of the plate  26  keeps away from the surface of a workpiece  10  mounted on the table  12 . When the piston rod  25   a  is at its operating position, the lower surface  26   b  is positioned rather on the front surface of the table  12 , than on the surface of the workpiece  10 . 
         [0050]    In this embodiment, the cylinder  25  may be operated at a desired time after the workpiece  10  is sucked onto the table  12  and before the workpiece  10  is released from the suction. For example, assume that each hollow pin  20  is 3 mm in its outer diameter, and a carrier sheet is 8 μm thick. In this case, when pressure of 1 kgf/cm 2  is applied, enough time for which the pressure should be applied is about 0.2 seconds. 
         [0051]    Though not shown, the hollow pins  20   a - 20   d  may be supported movably axially and designed to be able to be projected over the surface of the table  12  by a driving unit. 
         [0052]    When each hollow pins  20  are pressed to the workpiece  10 , the carrier sheet may be cracked around the portions pressed by the hollow pins  20 . When the carrier sheet is warped due to the cracks, the imaging accuracy may deteriorate. Therefore, the occurrence of cracks is undesirable. 
         [0053]      FIGS. 5A-5D  are views showing examples of modified hollow pins  20 .  FIG. 5A  shows the case where a hollow pin  20  has a rounded edge in its tip.  FIG. 5B  shows the case where the hollow pin  20  shown in  FIG. 5A  is made solid.  FIG. 5C  shows the case where the hollow pin  20  shown in  FIG. 5B  is shaped into a truncated cone.  FIG. 5D  shows the above-mentioned hollow pin  20  shown in  FIGS. 3A-3C . 
         [0054]    In the case of  FIG. 5A , due to the hollow pin  20  whose tip is rounded, it is possible to prevent the carrier sheet form being cracked around the portion pressed by the hollow pin  20 . In the case of  FIG. 5B , due to the hollow pin  20  which is solid, not only is it possible to prevent the carrier sheet form being cracked around the portion pressed by the hollow pin  20  in the same manner as in the case of  FIG. 5A , but it is also possible to make the maintenance easier because the tip of the hollow pin  20  does not wear down so quickly. 
         [0055]    Any one of the hollow pins shaped thus can be used. One which can form a clearer indentation may be used selectively in accordance with the thickness or material of the carrier sheet. 
         [0056]    When the carrier sheet in a indentation is cut off, the cut-off portion becomes waste. It is therefore practical to adjust the pressing force of the cylinder  25  to a value which is low enough to prevent the carrier sheet from being cut off in a circle. 
         [0057]      FIG. 6  is a plan view of a table showing further another embodiment of the present invention. 
         [0058]    Hollow pins  20   a  to  20   d  are disposed in a diagonal line on the surface where the workpiece  10  is mounted. If indentations can be controlled to be shallow, the hollow pins  20  can be disposed thus in an area where a pattern will be formed. In this manner, the layout of the hollow pins  20  can be less restricted. It is therefore possible to expand the intervals with which the hollow pins  20  are disposed. Thus, the positioning accuracy of the back surface can be improved further. 
         [0059]    The present invention can be also applied to a laser direct imaging apparatus which uses a laser diode as a light source and turns on/off the laser diode directly. 
         [0060]    The present invention can be also applied to a laser direct imaging apparatus which uses a spatial light modulator (DMD).