Laser drilling apparatuses for carrying out drilling to a resin layer of printed circuit boards by a laser beam are provided. Nowadays, laser drilling apparatuses for collectively making a plurality of holes also are provided. Referring to FIG. 1, an example thereof will be described. A pulsed laser beam from a CO2 laser oscillator 21 is incident on a homogeneous optical system 22, so that the energy density distribution in the cross-section of the beam becomes homogeneous. The laser beam emerging from the homogeneous optical system 22 is reflected downward by a reflective mirror 23 and is incident on a conversion optical system 24. The laser beam incident on the conversion optical system 24 has a circular or rectangular cross-sectional shape. The conversion optical system 24 converts the circular or rectangular cross-sectional shape into a linear laser beam having a predetermined width W and length L. Thus, the conversion optical system 24 includes a cylindrical lens system 24-1 for defining the width W and a cylindrical lens 24-2 for defining the length L. Such a conversion optical system 24 is well known, the width W being in the range of 0.1 mm to several mm and the length being about several tens of mm.
A mask stage 25 is arranged right below the conversion optical system 24 to drive a mask 26 in one axial direction. The mask 26, as described below, has many holes for defining a processing pattern for a resin layer of a work 27. An imaging lens system 28 is arranged between the mask stage 25 and the work 27. The imaging lens system 28 defines the reduction ratio of the processing pattern of the mask 26 projected onto the work 27. Herein, the reduction ratio is 1:1.
The work 27 is loaded on a work stage 29 that is movable at least in the inverse direction relative to that of the movement of the mask stage 25, along the same axial direction. In particular, the mask stage 25 and the work stage 29 are controlled with a controller, which is not shown in the drawing, so that these move synchronously.
FIG. 2 shows the relationship between the mask 26 and the cross-sectional shape (shown by hatching) of a linear laser beam, which is converted in the conversion optical system 24. The mask 26 has a processing pattern including a plurality of holes 26-1 that are arranged at random or regularly at a given pitch P with respect to the direction of the movement shown by an arrow. In the linear laser beam, the width W is larger than the diameter of the holes 26-1 and the length L is larger than the range of the processing pattern in the width direction of the mask 26.
In FIGS. 1 and 2, the mask 26 moves across the linear laser beam so that a plurality of laser beam components passing through the holes 26-1 lying in one row of the mask 26 reach the work 27 through the imaging lens 28. Since the mask stage 25 and the work stage 29 are controlled so that these move synchronously in the opposite directions along the same axial direction, holes are continuously formed in the work 27 for every row of the processing pattern of the mask 26 in sequence. Of course, the pulse frequency of the laser beam from the CO2 laser oscillator 21 is set so as to be synchronized with the movement corresponding to one pitch of the mask stage 25. For example, the holes 26-1 in the same row are irradiated with the laser beam at least one time or two times, if necessary, depending on the energy intensity of the laser beam. This means that the same region in the work 27 is irradiated with the laser beam at least one time or two times if necessary. As a result, holes are formed in the region irradiated with the laser beam.
In the above example, the laser beam is fixed while the mask stage 25 and the work stage 29 are movable. Alternatively, for the same diameter, distance, and pitch of the holes formed in the work and for a pulse width of the laser beam on the order of microseconds, as shown in FIG. 3, a mask 26′ having a row of holes 26-1′ is used. In this case, it is not necessary to drive the mask stage during laser irradiation. In other words, the linear laser beam from the conversion optical system 24 is incident on the mask 26′, which is always fixed, whereas only the work stage 29 moves in the direction of an arrow in FIG. 3.
Alternatively, the mask 26 may be fixed while the work 27 is irradiated with a linear laser beam that is scanned on the mask 26 in one axial direction by a galvano-scanner. For this case, the galvano-scanner is provided between the conversion optical system 24 and the mask 26. In this case, after the scanning by the galvano-scanner, the work stage 29 moves the subsequent processing region to a scanning region by the galvano-scanner.
In any case, the diameter of the holes formed by the above-mentioned drilling is in the range of about 30 to 300 μm. Assuming that the reduction ratio of the imaging lens system 28 is 1:1, the diameter of the holes 26-1 formed in the mask 26 is also in the range of 30 to 300 μm. The mask 26 is generally formed of a metallic material and the holes have an aspect ratio (ratio of the diameter to the depth of the hole) of about 1. When the diameter of the holes 26-1 is 50 μm, the thickness of the mask 26 is also 50 μm. The same relationship applies to the mask 26′ shown in FIG. 3.
However, in the mask having the above-mentioned thickness, an increased density of the holes 26-1 impairs heat dissipation during laser beam irradiation, resulting in the thermal deformation of the mask 26. The deformation of the mask 26 causes the difference of the positions and the distortion of the shape of the holes formed.
Accordingly, an object of the present invention is to provide a mask for laser processing which does not cause thermal deformation during irradiation with a laser beam.
Another object of the present invention is to provide a method for making the mask.
Still another object of the present invention is to provide a laser processing apparatus using the mask.