JP2008055466A - Through-hole machining method, through-hole machining system, and mask - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a through-hole machining method, which can simply machine a taperless through-hole using a simple optical member by a laser beam, a through-hole machining system, and a mask for use in the method. <P>SOLUTION: In this method, a through-hole is formed in a workpiece by a pulse laser beam. A workpiece 7 is irradiated with a laser beam through a mask 11 such that the profile of laser fluence of a pulse laser beam 5 shows a Gaussian distribution and the laser fluence of 1/(e<SP>1/2</SP>) of the maximum fluence in the Gaussian distribution is less than the abrasion threshold value of the workpiece. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a through hole processing method for processing a through hole with a pulse laser, a through hole processing system, and a mask used therein.

  Conventionally, through-holes have been machined in a wiring board such as a high-density multilayer wiring board using a drill or the like. However, as the density of wiring boards has increased, through-hole diameters have become smaller, and through-hole pitch intervals have become smaller, it has become difficult to cope with machining. In order to solve these problems and further improve efficiency, laser processing using a laser beam is being used. However, a through hole by a pulse laser has a tapered shape in the through hole, which causes problems such as plating defects. In order to solve such problems, it is desired to develop a high-precision through-hole forming technique using a pulse laser.

In order to meet the above requirements, laser processing methods with higher accuracy have been proposed. For example, in the case of through-hole processing with a laser, a method has been proposed in which a laser incident angle with respect to a workpiece is changed for each laser shot using a galvano mirror which is a movable mirror (Patent Document 1). According to this method, a through hole in which the diameters of the upper part (laser entry side) and the bottom part (laser exit side) of the hole are uniform can be obtained. In addition, there has been proposed a method of providing a through hole by controlling the laser intensity profile using a light intensity distribution adjusting member or a reflecting mirror to reduce the difference between the central part and the peripheral part of the laser intensity profile. (Patent Document 2). According to this method, a through hole having a large aperture ratio (laser exit side diameter / laser entering side diameter: 1 or less in laser processing) can be formed.
Japanese Patent Laid-Open No. 2004-243404 Japanese Patent No. 3717456

  However, the method disclosed in Patent Document 1 has a disadvantage that it is expensive because it uses a plurality of mirrors such as a galvanometer mirror, and has a problem that it requires a highly accurate control of the laser incident angle. Further, in the method disclosed in Patent Document 2, it is difficult to manufacture a light intensity distribution adjusting member for a small beam diameter of about 100 μm or less, and even if it can be manufactured, it takes a lot of man-hours. Further, since the strength of the central portion is lowered, the time for processing the through hole is extended.

  It is an object of the present invention to provide a through-hole processing method, a through-hole processing system, and a mask used therefor, in which a taperless through-hole can be easily processed by a laser using a simple optical member.

The through hole processing method of the present invention is a method of processing a through hole in a workpiece by a pulse laser beam. In this processing method, when a pulsed laser beam is applied to a workpiece, the laser fluence profile is assumed to match a Gaussian distribution, and a laser fluence of 1 / (e ^1/2 ) of the maximum fluence in the Gaussian distribution is applied. The through hole is processed by irradiating the workpiece with a pulsed laser beam so as to be less than the ablation threshold of the workpiece.

  With this configuration, the loose hem part of the laser fluence gradient does not contribute to the drilling of the workpiece, and the formation of a wall with a gentle inclination is suppressed. Since a hole is dug by the taperless wall, it becomes easy to form a taperless wall. The above laser beam fluence requirements may be realized by lowering the laser energy or by other methods. In the present description, the taperless need only be so small that the taper does not become an obstacle, and does not mean that there is no taper. The same applies to the following description.

Further, the workpiece can be irradiated with a pulsed laser beam through a mask, and the fluence of the laser passing through the non-opening portion of the mask can be reduced to 1 / (e ^1/2 ) or less of the maximum fluence.

  By this configuration, the taperless through hole is processed into the work piece by matching the opening shape of the mask with the desired through-hole cross-sectional shape so that the laser fluence passing through the non-opening portion is less than the ablation threshold of the work piece. Can do. In particular, the mask has an opening shape that cuts the peripheral edge of the pulse laser beam.

  Of course, the ablation threshold of the material forming the mask can be larger than the ablation threshold of the material of the workpiece. Thereby, by selecting the mask material, the through hole can be efficiently processed by reducing the replacement frequency of the mask.

  Further, the ablation threshold of the material forming the mask may be 10 times or more the ablation threshold of the material of the workpiece. Thereby, the replacement frequency of the mask can be further reduced, and the through hole can be processed more efficiently.

  The through-hole processing system of the present invention is a system for processing a through-hole in a workpiece by a pulse laser beam. This machining system includes a laser oscillator that oscillates a pulse laser, a workpiece fixing portion that fixes a workpiece, and an opening corresponding to a through hole that is disposed between the workpiece fixing portion and the laser oscillator. And a mask having. With this configuration, the taperless through-hole can be processed into a workpiece by matching the opening shape of the mask with the desired through-hole cross-sectional shape. In particular, the mask has an opening shape that cuts the peripheral edge of the pulse laser beam.

  The mask of the present invention is a mask used when processing a through hole in a workpiece with a pulse laser beam, and is characterized in that an opening corresponding to the shape of the through hole is provided.

  With the above configuration, an opening is provided in accordance with a desired through-hole cross-sectional shape, and the peripheral edge of the laser beam profile is cut by the opening to obtain a peripheral edge profile with a steepness of a predetermined value or more with a sharply cut skirt. be able to. That is, a taperless through hole can be obtained by obtaining a laser beam having an outer peripheral edge profile that rises sharply from the end and irradiating the laser beam with the laser beam.

  The ablation threshold of the material forming the mask can be made larger than the ablation threshold of the workpiece material. Thereby, the durability of the mask can be increased, the frequency of mask replacement can be suppressed, and the cost required for the mask and the through hole processing efficiency can be improved.

  In addition, the ablation threshold of the material forming the mask may be 10 times or more the ablation threshold of the material of the workpiece. Thereby, the durability of the mask can be further improved.

  By using the through-hole processing method, the through-hole processing system, and the mask of the present invention, a taperless through-hole can be easily processed by a laser using a simple optical member.

Next, embodiments of the present invention will be described with reference to the drawings.
(Principle of the present invention-Causes of tapering through holes-)
As shown in FIG. 1, the laser fluence (energy density) of the pulsed laser beam has a spatial distribution that is high at the center and low at the periphery, usually approximately Gaussian. Therefore, when O is the beam center position and A ₁ and A ₂ are positions equal to the ablation fluence threshold value, OA ₁ = OA ₂ is obtained, which is approximately axially symmetric. A hole is dug by the ablation at the center side where the laser fluence is above the threshold at which the work piece is ablated. However, since the fluence is higher at the center, it tends to be deeper at the center. For this reason, the wall surface of a hole turns into the inclined surface W, as shown in FIG. FIG. 2 is a cross-sectional view showing the workpiece 7 immediately after the hole 17 is opened. After the hole 17 has penetrated from the front surface to the back surface of the work piece 7, even the outer peripheral edge of the laser beam having a low fluence is ablated at a portion exceeding the ablation threshold value, and gradually increases as the number of pulse shots increases. It is considered that the taper angle will eventually become 0 ° (straight). In reality, however, the taper angle does not approach zero even when the number of pulse shots is increased, and a constant taper angle is maintained.

  The present inventors have insight into these phenomena, "In the above-mentioned phenomenon, an inclined wall is formed by an initial shot, but the laser fluence used for ablation is irradiated on the plane on the inclined wall. Compared to the case, the slanted wall absorbs less energy and reduces the energy directed to ablation. " According to this idea, the laser fluence used for ablation in the inclined wall decreases, and in the inclined wall, the area that does not reach the ablation threshold spreads slightly to the center side, and in the area below the ablation threshold, the number of pulse shots is increased. No hole is dug. As a result, a hole with a taper as shown in FIG. 2 is formed.

(Embodiment 1)
FIG. 3 is a diagram for explaining the through hole forming method according to the first embodiment of the present invention. In FIG. 3, a laser beam 2 oscillated from a pulse laser oscillator and whose beam shape has been adjusted becomes a focused laser beam 3 focused by a condensing lens 12 and travels toward a mask 11. The profile of the laser beam before passing through the mask 11 shows a Gaussian distribution as shown in FIG. 1, and the skirt extends to the outer peripheral side. At the hem, the laser fluence gradient is gentle (not steep). The first embodiment of the present invention is characterized in that a steep outer peripheral edge profile is formed by cutting a skirt portion with a mask 11. The laser beam 5 having a desired cross-sectional shape adjusted by the mask 11 and having a steep outer peripheral edge profile is irradiated onto the workpiece 7 to process a through hole (not shown).

  FIG. 4 is a diagram schematically showing the mask 11. The opening 11 a of the mask 11 is included in the cross-sectional shape of the laser beam 3 before passing through the mask 11. As described above, the mask shielding portion 11b cuts the skirt portion of the Gaussian distribution, and is preferably formed of a material having an ablation threshold sufficiently larger than the ablation threshold of the workpiece 7. For example, it is desirable to have an ablation threshold that is at least 10 times greater than the ablation threshold of the workpiece 7. This is because when the laser is repeatedly irradiated and ablation occurs in the mask, the dimensional accuracy of the opening 11a deteriorates and the mask 11 warps. For this reason, it is preferable to form the mask shielding part 11b with a material having high ablation resistance. However, the mask material need not be greater than the workpiece ablation threshold.

For example, when the workpiece 7 is a porous resin sheet and a conductive portion is provided on the porous resin sheet to produce an anisotropic conductive sheet, the through hole must be processed to form the conductive portion. In this case, a porous polytetrafluoroethylene film (porous PTFE film) is suitably used for the porous resin sheet, but the ablation threshold of the porous PTFE film is 0.44 J / cm ² . It is preferable that a material having an ablation threshold higher than this is used for the mask shielding part 11b. However, as described above, it is not essential.

  FIG. 5 is a diagram showing a laser fluence profile after passing through the mask 11. According to FIG. 5, the bottom part of the gentle slope of the Gaussian distribution shown in FIG. 1 is cut to show a steep outer peripheral edge profile. When such a laser beam 5 is used, a hole with a gently sloping wall is not dug in the workpiece during the first shot, and even if a sloping wall is formed for some reason, In the next shot, sufficient laser fluence is supplied to the slowly inclined wall portion, so that the slowly inclined wall is processed into a steep wall. As a result, a taperless (substantially straight) through hole can be processed.

(Embodiment 2-Influence of the slope of the skirt of the profile-)
Using a laser beam having a Gaussian distribution in the laser profile (comparative example) and a laser beam having a sharp outer peripheral edge (invention example), a processing trace when the porous PTFE film was processed was simulated. FIG. 6 shows the fluence profiles of the laser beams of the inventive example and the comparative example. In the calculation, the work piece is a porous PTFE film (ablation threshold 0.44 J / cm ² ) as described above, and a titanium sapphire laser (wavelength 800 nm, beam diameter 10 μm, 100 μJ) is used as the laser. The obtained fluororesin ablation rate was used.

  The simulation results are shown in FIG. 7 (comparative example) and FIG. 8 (invention example). In the result of the comparative example of FIG. 7, the gently sloping wall S formed by the first shot irradiation is not deeply dug by the subsequent shots and does not move. This is because, as explained in the principle of the present invention-the cause of the taper in the through hole-, the peripheral portion of the laser beam in which the wall S is formed by the first shot originally has a low fluence and a value just above the ablation threshold. It was. In the second and subsequent shots, the first flat portion is a sloped wall. When irradiating the flat part of the work piece, if the fluence is just above the ablation threshold, the work piece is ablated and drilled. The energy directed is reduced and the fluence received by the material itself falls below the ablation threshold. For this reason, after the second shot, the inclined portion S is maintained as it is without being dug.

  Such an inclined wall S is formed to extend further inward by the second shot irradiation so as to increase the gradient of the wall. For this reason, in the third shot irradiation, drilling is performed with little movement of the extended inclined wall formed by the second shot irradiation. For this reason, a substantially overlapping wall portion is generated between the second shot irradiation and the third shot irradiation. Such an inclined part is extended inward in order, and the wall part which overlaps almost by 3rd shot irradiation and 4th shot irradiation is made. As a result, a hole having a diameter of about 5 μm is dug at a depth of 80 μm after the fourth shot irradiation. An opening with a diameter of about 40 μm is processed on the inlet side. (Exit side diameter / Inlet side diameter) is (5/40), which is about 0.13. As a result, even if a through hole penetrating the workpiece is opened, a tapered through hole having a large diameter on the inlet side and a small diameter on the outlet side is processed.

  On the other hand, in the example of the present invention shown in FIG. 8, an inclined wall is not formed at the peripheral edge even in the first shot irradiation, and an almost straight hole is dug. In particular, the bottom of the hole dug by the first shot irradiation is flat and wide, and the side wall has a steep slope. Even after the second shot irradiation, the flat wide shape at the bottom of the hole and the steep slope of the side wall do not change. As a result, a hole having a diameter of about 15 μm is dug at a depth of 80 μm after the fourth shot irradiation. Since an opening having a diameter of about 20 μm is processed on the inlet side, (exit side diameter / inlet side diameter) is (15/20), which is about 0.75. From this, it can be seen that the aperture ratio is improved to 5.77 times in the present invention example as compared with the aperture ratio of 0.13 in the comparative example.

(Embodiment 3-Formation of a steep outer peripheral edge profile using a mask)
In the third embodiment of the present invention, an actual measurement example is shown in which a profile in which the outer peripheral edge is sharply cut by a mask is formed. The mask used was of the same form as the mask shown in FIG. 4, but made of a nickel-gold alloy.
This metal mask has many holes having a diameter of 10 μm. This mask was irradiated with a 20 μm diameter Gaussian laser beam, and the laser profile after passing through the mask was imaged by a CCD (Charge Coupled Device). That is, in FIG. 3, a measurement system in which a CCD is arranged instead of the workpiece 7 is used.

  FIG. 9 is a diagram showing imaging data by the CCD, and FIG. 10 is an explanatory diagram thereof. 9 and 10 show the laser fluence distribution scanned in the x and y directions through the center of the spot, along with the spot of the laser beam after passing through the mask. The x-direction scan shows a particularly sharp outer peripheral edge profile, and the y-direction scan also has a sharp outer peripheral edge profile.

9 and 10, it has been found that a steep outer peripheral edge profile can be easily formed using a mask. Conventionally, since the need for a taperless through hole has not been strong, an example of using a mask to increase the steepness of the outer peripheral edge profile of a laser beam for drilling is extremely rare. FIG. 9 and FIG. 10 show that the profile of the outer peripheral edge of the laser beam can be made sharp with a mask. That is, by using the mask to match the opening shape of the mask to the desired through-hole cross-sectional shape, the fluence of the laser passing through the non-opening portion of the mask is set to 1 / (e ^1/2 ) or less of the maximum fluence, and the non-opening portion is The laser fluence through can be less than the workpiece ablation threshold. As a result, the taperless through hole can be processed into a workpiece.

(Embodiment 4-Case of steep profile with low laser energy)
FIGS. 11A and 11B are diagrams for explaining the through-hole processing method according to the fourth embodiment of the present invention. The laser energy shown in FIG. 11 (a) is lower than the laser energy in FIG. 11 (b), but the ablation threshold of the workpiece is the same. In both figures, the laser fluence follows a Gaussian distribution, but in FIG. 11 (a), the laser energy is lowered, so that it seems that the ablation threshold of the workpiece has increased, but in reality, FIG. 11 (b) The unit scale of the energy density on the vertical axis is only increased compared to).

The hole is dug in the work piece because the irradiated laser fluence is higher than the ablation threshold, and the wall of the opening edge of the hole is formed at the portion immediately inside the radial position where it becomes equal to the ablation threshold. For this reason, the gradient of the laser fluence (distance derivative of the laser fluence) in the region immediately inside the ablation threshold of the laser beam is important. As described above, in FIG. 11A, since the laser energy is reduced, the laser fluence gradient at the above position is larger than that in FIG. 11B. That is, in FIG. 11A, the laser fluence outside the radial position that is 1 / (e ^1/2 ) of the maximum fluence in the Gaussian distribution is set to be less than the ablation threshold of the workpiece, and the gentle gradient The range effect is excluded.

In FIG. 11 (b), since the laser energy is large, the fluence gradient of the laser beam forming the wall of the opening edge of the hole is shown in FIG. Very small compared to that. That is, in FIG. 11 (b), the ablation of the workpiece is performed even if it is a portion outside the diameter that is {1 / (e ^1/2 )} of the maximum fluence of the Gaussian distribution (even if the gradient is small). It is over the threshold.

  The present embodiment is characterized in that a laser beam is used in which the fluence gradient is increased near the same diameter as the ablation threshold of the work piece or just inside thereof by reducing the laser energy. In other words, the use of a laser beam having a form as shown in FIG. 11A is characterized in that a laser beam having a steep laser fluence gradient near the ablation threshold is used although the laser energy is small. In this case, it is inevitable that the diameter of the hole to be processed becomes small.

  As mentioned above, focusing on the fact that the fluence gradient near the ablation threshold changes depending on the laser energy, through-hole processing was performed by changing the laser energy. FIG. 12 is a diagram showing the result of actually measuring the taper angle of the through-hole by changing the laser energy and performing the through-hole processing. The radius R (= 32 μm) takes a position that is 1 / e of the maximum value of the laser fluence. FIG. 13 is a diagram in which the taper angle of the same through-hole is plotted with a differential coefficient (laser profile inclination) of the laser profile intensity distribution. The workpiece was PTFE, and a titanium sapphire laser was used as the laser. According to FIG. 12, the taper angle is clearly reduced from about 13 ° to around 7 ° on the energy side lower than the laser energy of 50 μJ. Further, from FIG. 13, the lower energy side than the laser energy of 50 μJ corresponds to the range in which the inclination of the laser profile increases.

As described above, assuming that the cross-sectional fluence distribution of the laser beam follows the Gaussian distribution, the portion of the skirt portion where the fluence value is 1 / (e ^1/2 ) or less of the peak is set to be less than the ablation threshold of the workpiece. It has been found that using a laser beam is effective in reducing the taper angle of the through hole. In the above case, the laser energy was kept low in order to realize that “the range where the fluence value is 1 / (e ^1/2 ) or less of the peak is less than the ablation threshold of the workpiece” Other methods may be used.

  Although the embodiments and examples of the present invention have been described above, the embodiments and examples of the present invention disclosed above are merely examples, and the scope of the present invention is the implementation of these inventions. It is not limited to the form. The scope of the present invention is indicated by the description of the scope of claims, and further includes meanings equivalent to the description of the scope of claims and all modifications within the scope.

By using the through-hole processing method, through-hole processing system, mask and filter of the present invention, taperless through-holes can be easily processed, contributing to the production of anisotropic conductive sheets, through-hole processing of various sheets, etc. Is expected to do.

It is a spatial distribution of energy density of a laser beam cross section. It is a schematic diagram which shows the opening part shape by a laser beam qualitatively. It is a figure for demonstrating the through-hole processing method in Embodiment 1 of this invention. It is a figure which shows a mask typically. It is a figure which shows the laser profile after passing through a mask. It is a figure which shows the profile of the laser beam of 2 types (invention example, comparative example) used in the simulation of Embodiment 2 of this invention. It is a figure which shows the through-hole formation progress in a comparative example. It is a figure which shows the through-hole formation progress in the example of this invention. It is CCD data of the laser beam after passing the mask in Embodiment 3 of this invention. FIG. 10 is a schematic diagram (an explanatory diagram) of FIG. 9. It is a figure for demonstrating the through-hole processing method in Embodiment 4 of this invention, (a) is a figure when a laser energy is made low, (b) is a figure when making a laser energy higher than (a). is there. It is a figure which shows the relationship between the taper angle of a through hole, and laser energy. It is a figure which shows the relationship between the taper angle of a through hole, and the inclination of a laser profile.

Explanation of symbols

2 Laser beam, 3 Laser beam before mask, 5 Laser beam after passing through mask, 7 Workpiece, 11 Mask, 11a Opening, 11b Shielding part, 12 Condensing lens, 17 Through hole, W Inclined surface, S Loose Inclined wall part.

Claims (4)

A method of processing a through hole in a workpiece by a pulsed laser beam,
When the workpiece is irradiated with the pulsed laser beam, it is assumed that the profile of the laser fluence matches the Gaussian distribution, and the laser fluence of 1 / (e ^1/2 ) of the maximum fluence in the Gaussian distribution is the workpiece. A through-hole processing method, wherein the through-hole is processed by irradiating the workpiece with the pulsed laser beam so as to be less than the ablation threshold. The pulsed laser beam is irradiated to the workpiece through a mask, and the fluence of the laser passing through the non-opening portion of the mask is set to 1 / (e1 / ² ) or less of the maximum fluence. The through-hole processing method according to 1. A system that processes a through hole in a workpiece by a pulsed laser beam,
A laser oscillator for oscillating the pulse laser;
A workpiece fixing portion for fixing the workpiece;
A through-hole processing system comprising a mask disposed between the workpiece fixing portion and the laser oscillator and having an opening corresponding to the through-hole. A mask used when processing a through hole in a workpiece by a pulsed laser beam,
A mask provided with an opening that matches the shape of the through hole.

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