Laser processing method and laser processing device

Abstract

To achieve laser processing having excellent processing performance by preventing processing results from becoming uneven due to performance of an optical system.SOLUTION: A laser processing method, which processes a work-piece (W) by irradiating the work-piece with laser light (LZ), includes: a step in which a coat (30) for processing, in which an application threshold that is a threshold at which application processing by the laser light is started is equal to or above an application threshold for the work-piece, is arranged on a surface of the work-piece; and a step in which processing for forming an opening (30a) that penetrates through the coat for processing by emitting laser light from above the coat for processing and removing a portion of the work-piece is performed.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a laser processing method and a laser processing apparatus. [Background technology]

[0002] Laser processing is widely used in the manufacture of circuit boards, etc. In laser processing, a workpiece is irradiated with high-intensity laser light to form a desired shape, such as a hole or a groove, in the workpiece by ablation.

[0003] In a laser processing device, a laser beam emitted by a laser oscillator is guided by an optical system to irradiate a workpiece. There are various types of laser processing devices with different optical systems and irradiation methods. For example, as shown in Patent Document 1, a laser processing device equipped with a galvano scanner that changes the angle of a mirror on the optical path to scan the laser beam on the imaging surface at high speed is known. Also, as shown in Patent Document 2, a laser processing device is known that has a mask in the optical path, and the laser beam transmitted through the mask is imaged on a printed circuit board by a projection lens to transfer a conductor pattern formed on the mask to the surface of the printed circuit board.

[0004] In addition, in laser processing, a technique is known in which the surface of a workpiece is covered with a coating or mask material for the purpose of improving processing quality and protecting the workpiece (for example, Patent Document 1 and Patent Document 3). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2015-150609 A [Patent Document 2] JP 2008-147242 A [Patent Document 3] JP 2016-181731 A Summary of the Invention [Problem to be solved by the invention]

[0006] In laser processing devices, optical performance such as aberration of the optical system affects the irradiation of the laser light, and the processing result may not be ideal. For example, in a photomask type laser processing device such as that in Patent Document 2, the resolution performance on the printed circuit board varies due to the aberration of the projection lens, and even if the laser is irradiated under the same conditions, the processing result varies within the irradiation range of the laser light. More specifically, the resolution performance decreases toward the periphery of the irradiation range, and blurred light with low contrast (low energy intensity) is likely to occur. Then, when forming holes or grooves on the printed circuit board, the blurred light causes the openings of the holes or grooves to have a gradually widened shape.

[0007] In Patent Document 1, the general tendency of holes formed by laser processing to have a widened shape at the top (opening) is addressed, so the laser processing is performed with the substrate covered with a mask material so that the widened portion at the top is formed in the mask material rather than the substrate. By removing the mask material after the laser processing, a well-shaped hole remains on the substrate.

[0008] However, existing inventions including Patent Document 1 are not technologies that focus on the non-uniformity of processing results caused by variations in the resolution performance of the optical system as described above, and do not present a solution based on this focus.

[0009] In order to solve the above problems, an object of the present invention is to provide a laser processing method and a laser processing apparatus which prevent non-uniformity in the processing results caused by the performance of the optical system and have excellent processing performance. [Means for solving the problem]

[0010] One aspect of the present invention is a laser processing method for irradiating a workpiece with laser light to process the workpiece, the method comprising: providing a laser beam having a thickness of 100 nm or more; and a laser beam having a thickness of 100 nm or more; And made of water-soluble resinand irradiating a laser beam from above the processing coat to form an opening penetrating the processing coat and to remove a portion of the workpiece.

[0011] The laser light is output in a pulsed manner, and the ablation threshold of the processing coat is 200 mJ / cm per pulse of the laser light. 2 More preferably, it is equal to or greater than this.

[0012] The thickness of the processing coat is preferably in the range of 2 μm to 20 μm.

[0014] The laser processing method of the present invention is particularly useful in forming holes having an opening diameter of 20 μm or less in the workpiece.

[0015] The processing coat prevents the workpiece from being processed by blurred light of the laser light having a contrast below a predetermined level.

[0016] One aspect of the present invention is a laser processing apparatus for use in the above-mentioned laser processing method, the apparatus having a laser light source that emits the laser light and a processing pattern to be performed on the workpiece, Source? and an imaging optical system that is provided on an optical path between the mask and the workpiece and that forms an image of the pattern and irradiates the laser light onto the workpiece. Effect of the Invention

[0017] According to each of the above aspects, it is possible to prevent non-uniformity in the processing results caused by the performance of the optical system, and to realize laser processing with excellent processing performance. [Brief description of the drawings]

[0018] [Figure 1] FIG. 2 is a perspective view of the laser processing device. [Diagram 2]1 is a cross-sectional view showing an example of a printed circuit board in which holes are machined by a laser machining device. [Diagram 3] 1 is a plan view showing an effective machining area in a laser machining device and an example of machining holes formed in a workpiece at different positions within the effective machining area. FIG. [Figure 4] 11 is a graph showing contrast distribution when holes are formed at different positions within an effective processing area. [Diagram 5] 1 is a diagram showing a cross section of a hole formed by disposing a machining coating on the surface of a workpiece. [Figure 6] FIG. 1 is a diagram illustrating the ablation threshold of a processing coat. [Figure 7] 1A and 1B are diagrams illustrating processing results of an example and a comparative example in plan view. [Figure 8] 1A and 1B are cross-sectional views showing the results of processing in an example and a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a laser processing apparatus 10 for carrying out laser processing according to the present invention. The laser processing apparatus 10 irradiates a workpiece W, which is a workpiece, with a laser beam LZ to carry out processing by laser ablation. Laser ablation is a phenomenon in which the surface of a solid material is cut, modified, or removed by irradiation with a laser having a high intensity (fluence) per unit area. The laser processing apparatus 10 has a laser light source 11, an illumination optical system 12, a mask 13, an imaging lens (imaging optical system) 14, and a workpiece support unit 15.

[0020] FIG. 2 shows a printed circuit board 20 as an example of a product manufactured by processing using the laser processing device 10. The printed circuit board 20 is a multi-layer board configured by laminating a plurality of insulating layers 21 made of an insulating material such as epoxy resin, and connecting conductor parts 22 made of a conductor such as copper on each insulating layer 21. As an example, each insulating layer 21 has a thickness of about 15 μm to 30 μm. Although omitted in FIG. 2, a protective film such as a solder resist is formed on the outermost layer of the printed circuit board 20 as necessary. Furthermore, the insulating layers 21 and the protective film are filled with an inorganic material.

[0021] Holes (vias) for interlayer connection are formed in the printed circuit board 20. By performing copper plating or the like with the holes drilled, the conductor parts 22 of each layer can be made conductive. Types of holes for interlayer connection include through vias 23 that penetrate across multiple layers, blind vias 24 that connect an outer layer and an inner layer, and buried vias 25 that connect two adjacent inner layers. In manufacturing circuit boards, single-layer holes such as the blind vias 24 are formed by laser processing, and holes that penetrate the entire board such as the through vias 23 are formed by mechanical means such as a drill.

[0022] Laser processing by the laser processing apparatus 10 of this embodiment is applied to the formation of holes (such as blind vias 24) for circuit connection between layers in the printed circuit board 20. Alternatively, other than holes for interlayer connection, the laser processing can also be applied to the formation of holes (openings) in a protective film such as a solder resist provided on the outermost layer of the printed circuit board 20. In the following description, these various processing objects will be collectively referred to as "holes," and the workpiece in which holes are to be formed will be referred to as the workpiece W.

[0023] During processing by the laser processing device 10, the workpiece W is supported by the workpiece support part 15. As shown in Fig. 1, the workpiece W supported by the workpiece support part 15 includes a plurality of grid-like sections, and each section is an area corresponding to one printed circuit board 20.

[0024] The laser light source 11 emits the laser light LZ by pulse oscillation operation. The laser light source 11 may be any light source as long as it can meet the ablation threshold described later, and the details are not limited. The laser light source 11 of this embodiment uses an excimer laser oscillator using a mixed gas of rare gas and halogen gas as a medium. More specifically, the laser oscillator is a laser oscillator that uses a mixed gas of krypton and fluorine as a medium to emit ultraviolet light with a wavelength of 248 nm. In addition to this, a laser oscillator that uses a mixed gas of argon and fluorine to emit ultraviolet light with a wavelength of 193 nm, a laser oscillator that uses a mixed gas of xenon and chlorine to emit ultraviolet light with a wavelength of 308 nm, or a laser oscillator that uses a mixed gas of xenon and fluorine to emit ultraviolet light with a wavelength of 355 nm may be used. The laser light source 11 outputs the laser light LZ in a pulsed form as a beam with a uniform intensity distribution.

[0025] The laser light LZ output from the laser light source 11 is collected by the illumination optical system 12 and enters the mask 13. The mask 13 is constructed by disposing a light-shielding layer that blocks the laser light LZ on one side of a glass plate. By partially removing the light-shielding layer by a method such as etching, the mask 13 is completed, which has a transparent region similar to the hole pattern to be processed (a pattern including the arrangement of multiple holes and the shape and size of each hole). The pattern formed on the mask 13 is called a master pattern.

[0026] The laser light LZ emitted from the illumination optical system 12 is prevented from traveling in the area where the light-shielding layer remains on the mask 13 (is blocked by the light-shielding layer), and passes through the transparent area of ​​the mask 13 where the light-shielding layer has been removed. The laser light LZ transmitted through the mask 13 is imaged by the imaging lens 14 having a positive power, and is irradiated onto the workpiece W supported on the workpiece support part 15. The optical axis OX of the imaging lens 14 is approximately perpendicular to the support surface of the workpiece support part 15. The relative positions of the imaging lens 14 and the workpiece support part 15 in the optical axis direction along the optical axis OX are adjusted so that an image corresponding to the master pattern of the mask 13 is formed on the workpiece W. The imaging lens 14 may be a single lens or may be composed of multiple lenses.

[0027] The workpiece W is ablated at a portion where the laser light LZ reaches after passing through the mask 13 and the imaging lens 14, and a portion of the workpiece W is removed. In this manner, a hole having a similar, reduced pattern to the master pattern on the mask 13 is formed on the workpiece W.

[0028] One section of the workpiece W is machined simultaneously by irradiating the laser light LZ once (with a predetermined number of pulses). The workpiece support part 15 is a so-called XY stage that can move in a direction along the plate surface of the workpiece W with respect to the portion from the laser light source 11 to the imaging lens 14. When machining of one section of the workpiece W is completed, the workpiece support part 15 is driven to change the position of the workpiece W, and machining of the next section is performed. By repeating this process, all sections on the workpiece W are machined. It is to be noted that not only the workpiece support part 15 but also the portion from the laser light source 11 to the imaging lens 14 may be movable, or the workpiece support part 15 may be fixed and the portion from the laser light source 11 to the imaging lens 14 may be movable.

[0029] When changing the processing content to be performed by the laser processing device 10, the mask 13 is replaced with another mask 13 having a master pattern corresponding to the processing content. For example, in addition to forming holes for interlayer connections, the laser processing device 10 can be used to perform processing to provide grooves on the insulating layer 21 for forming the conductor parts 22 of the printed circuit board 20. In this case, a mask 13 having a master pattern having a configuration corresponding to the conductor parts 22 is prepared.

[0030] FIG. 3 shows the effective machining diameter D of the laser beam LZ irradiated by the imaging lens 14. The effective machining diameter D is the diameter of a virtual circle (image circle) on the focal plane where the optical performance of the laser irradiation can be ensured by the imaging lens 14. The range of the virtual circle centered on the optical axis OX and having the effective machining diameter D as its diameter is the effective machining area E1 that can be laser-machined at one time by the laser processing device 10. When laser machining a rectangular section (an area corresponding to each printed circuit board 20) on the workpiece W, a rectangular area E2 inscribed in the circular outer periphery of the effective machining area E1 is the area that can be machined all at once. In this embodiment, the rectangular area E2 is a square with each side of about 70 mm.

[0031] Incidentally, optical elements have various kinds of aberrations, and the effects of aberrations and the like cause variations in resolution performance. In general, the optical performance of an imaging optical system such as the imaging lens 14 has the smallest aberration near the optical axis OX at the center, and high contrast is obtained. The further away from the optical axis OX, the larger the aberration becomes and the lower the contrast becomes. Due to such optical characteristics, when the laser light LZ is irradiated through the imaging lens 14, differences occur in the processing results (finishing) of the laser processing inside the effective processing area E1 and the rectangular area E2.

[0032] FIG. 4 is a graph showing the contrast distribution on the irradiation surface (focal surface) when laser irradiation is performed to form holes of the same size, about 15 μm in diameter, under the same conditions at different locations in the effective processing area E1. The vertical axis of the graph shows the contrast, and the horizontal axis shows the radial width of the hole (radius from ±0, which is the center of the hole). The first graph G1 shown by a solid line in FIG. 4 is the contrast distribution when laser irradiation for hole formation is performed near the optical axis OX, that is, near the center of the effective processing area E1 and the rectangular area E2 (first position F1 in FIG. 3). The second graph G2 shown by a dashed line in FIG. 4 is the case when laser irradiation for hole formation is performed on the outer edge of the effective processing area E1 and the rectangular area E2 (second position F2 in FIG. 3).

[0033] There is a correlation between the contrast distribution shown in Fig. 4 and the shape of the hole formed on the workpiece W. Therefore, generally, when a hole is formed using an apparatus such as the laser processing apparatus 10, a difference occurs between the bottom diameter, which is the diameter of the bottom of the hole, and the opening diameter, which is the diameter of the opening of the hole, which is the top diameter, resulting in a hole with a shape in which the opening diameter is larger than the bottom diameter. If the ratio of the opening diameter to the bottom diameter is the taper ratio, then it is desirable for holes such as blind vias 24 formed in the printed circuit board 20 to have a shape with a taper ratio close to 100% (a shape in which the inner surface of the hole is close to vertical).

[0034] In the case of laser irradiation near the center of the effective processing area E1 and the rectangular area E2, as shown in the first graph G1, the contrast is high in a wide range in the radial direction from the center of the hole to the periphery, and the contrast drops sharply at the periphery of the hole. More specifically, the contrast is high from the center of the hole to a position close to a radius of 7.5 μm (diameter 15 μm), and the contrast is extremely low outside the radius of 7.5 μm (diameter 15 μm) from the center of the hole. Therefore, a good hole shape with a small difference between the bottom diameter and the opening diameter is obtained, as shown in the image (photograph) shown in Figure 3 as a processing example F11 at the first position F1.

[0035] In contrast, in the case of laser irradiation of the effective processing area E1 and the outer edge of the rectangular area E2, as shown in the second graph G2, the contrast near the center of the hole is slightly lower than that of the first graph G1, the contrast starts to decrease at a position closer to the center of the hole than that of the first graph G1, and the rate of decrease in contrast as it progresses to the periphery of the hole is more gradual than that of the first graph G1. And, in the vicinity of a radius of 7.5 μm (diameter 15 μm) from the center of the hole, the relationship between the high and low contrasts of the first graph G1 and the second graph G2 is reversed, and the contrast of the second graph G2 is higher than that of the first graph G1 at the periphery of the hole.

[0036] In this way, when the laser is irradiated to the effective machining area E1 and the outer edge of the rectangular area E2, there is a base of contrast outside a radius of 7.5 μm (diameter 15 μm) from the center of the hole, and the irradiation components to this area act as low-contrast blurred light and affect the machining state of the surface layer of the workpiece W. In other words, depending on the width of the base of contrast, the opening diameter widens, resulting in a hole shape with so-called "droop" around the opening edge, as shown in the image (photograph) shown in Figure 3 as machining example F12 at the second position F2.

[0037] Therefore, even within the range of the effective processing area E1 and the rectangular area E2, the processing accuracy is not uniform, and there is a problem that differences are likely to occur in the finish of the opening of the hole (opening diameter). In particular, in recent years, as the performance of semiconductor chips has improved and the bump pitch has narrowed, the diameter of vias and the opening diameter of solder resists have become finer (for example, about 10 μm in diameter), and there is a growing demand for improved accuracy in terms of the stability of the opening diameter. In order to respond to such finer design, measures are being taken to improve the performance of the optical system including the imaging lens 14 of the laser processing device 10, but there are individual differences in the devices, and it is not easy to meet the required values.

[0038] Also, unlike a type of laser processing device that processes holes one by one in a time-division manner by scanning a laser beam using a galvanometer mirror, the laser processing device 10 of this embodiment that irradiates the laser beam LZ using a photomask method can process many holes simultaneously, and therefore has excellent productivity. On the other hand, the effective processing area E1 and rectangular area E2, which are the ranges that can be processed at one time, are wide in the laser processing device 10 (in the example shown in FIG. 3, each side of the rectangular area E2 is about 70 mm), and in order to make the processing quality within the effective processing area E1 and rectangular area E2 uniform, the illumination optical system 12 and the imaging lens 14 are originally required to have very high performance, so there is a problem that it is difficult to address this issue by just improving the optical system.

[0039] In order to solve the above problems, in this embodiment, a processing coat 30 (see Figure 5) having predetermined characteristics is arranged in close contact with the surface of the workpiece W, and laser light LZ is irradiated from above the processing coat 30 to form holes.

[0040] The processing coat 30 is made of a resin material (organic material). More specifically, the resin material constituting the processing coat 30 is a water-soluble resin that exhibits high heat resistance and rapid decomposition behavior in order to achieve both protection performance and processability for the workpiece W when heated by laser processing. Before the workpiece W is set on the workpiece support part 15 of the laser processing device 10, a process of disposing the processing coat 30 on the surface of the workpiece W is executed. The processing coat 30 is provided on the workpiece W with a uniform thickness. The processing coat 30 can be disposed by any method such as coating or pasting. As an example, a slit-coating type coating device that ejects the water-soluble resin, which is the material of the processing coat 30, from a slit-shaped nozzle onto the workpiece W, or a device that pastes a sheet-shaped processing coat 30 on the workpiece W by lamination processing can be used.

[0041] The thickness of the processing coat 30 disposed on the workpiece W will be described later, but is in the range of 2 μm to 20 μm, and more preferably in the range of 5 μm to 20 μm.

[0042] When the step of providing the processing coat 30 on the surface of the workpiece W is completed, the workpiece W is attached to the workpiece support part 15, and the process proceeds to the laser irradiation step (the step of processing the workpiece W). In the laser irradiation step, a laser beam LZ is irradiated from above the processing coat 30. As shown in Fig. 5, an opening 30a penetrating the processing coat 30 is formed by ablation by the laser beam LZ, and the workpiece W is removed below the opening 30a to form a hole H. The hole H is a circular hole, and the opening 30a is correspondingly circular in shape.

[0043] 5, out of the laser light LZ, a blurred light having low contrast that acts on the peripheral portion of the hole is blocked by the processing coat 30 and does not reach the workpiece W to be processed. Therefore, even if a sag occurs in the peripheral portion of the hole, widening the opening diameter, it is formed in the opening 30a of the processing coat 30, not in the hole H of the workpiece W.

[0044] After laser processing by the laser processing device 10, the processing coat 30 is washed off the workpiece W by washing with water. The processing coat 30 made of a water-soluble resin can be easily removed by washing with water. After the processing coat 30 is removed, the workpiece W is left with a well-shaped hole without sagging at the opening. Note that the present invention does not limit the material of the processing coat 30, but examples of the water-soluble resin include acrylic resin and polyester resin. Also, water washing is not limited to washing with pure water, but is a concept that includes washing with a developer, etc.

[0045] In this way, by performing laser processing with the processing coating 30 disposed on the surface of the workpiece W, the effects of blurred light that causes variations in hole diameter (especially the opening diameter) can be eliminated, and the hole diameter can be stabilized throughout the effective processing area E1 and the rectangular area E2.

[0046] In order to obtain the above-mentioned effect, the material usable as the processing coat 30 can be defined by an ablation threshold (processing threshold). The ablation threshold is defined as the threshold at which effective ablation processing is initiated by laser light on the material surface of the workpiece, and is expressed as the energy or light intensity of the laser light. In this embodiment, the ablation threshold is set based on the energy at the processing position for each pulse (per pulse) of the pulsed laser. Data on the energy value at the processing position in the laser processing device 10 can be obtained by installing a measuring device on the work support part 15 and measuring the energy at a height position corresponding to the processing portion of the workpiece W.

[0047] The concept and setting of the ablation threshold will be described with reference to FIG. 6. FIG. 6 shows the results of an experiment in which the intensity (energy at the processing position per pulse) and the number of pulses of the laser light LZ, which is an ultraviolet laser light with a wavelength of 248 nm, is changed by the laser light source 11 of the laser processing device 10, and processing marks on the processing coat 30 used in this embodiment are observed. In this experiment, the master pattern of the mask 13 is set so as to form a plurality of holes of different diameters at a predetermined interval. The image (photograph) shown in the upper part of FIG. 6 is a photograph of the surface of the processing coat 30 when 100 pulses of the laser light LZ are irradiated. The table shown in the lower part of FIG. 6 shows the measured values ​​(unit: μm) of the depth of holes with a diameter of 20 μm formed in the processing coat 30 when the irradiation conditions (energy and number of pulses) of the laser light LZ are changed.

[0048] The energy per pulse of the laser light LZ is 100 mJ / cm 2 In the case of (a), slight traces remain on the surface of the processing coat 30, but no holes are formed in the processing coat 30 even when the number of pulses is increased.

[0049] The energy per pulse of the laser light LZ is 200 mJ / cm 2 In the case of , 4 pulses produced holes with a diameter of 20 μm and a measurable depth. However, the depth of the holes was very small, and a plateau was reached where the hole depth hardly changed (less than 0.2 μm) even when the number of pulses was increased from 10 to 100.

[0050] The energy per pulse of the laser light LZ is 300 mJ / cm 2 In the case of , a hole with a diameter of 20 μm started to form from the first pulse, and the depth of the hole increased as the number of pulses increased. Then, the depth of the hole exceeded 20 μm in just under 100 pulses. Since the preferable upper limit of the thickness of the processing coat 30 is about 20 μm, at this stage, the processing coat 30, which has the upper limit thickness, is penetrated.

[0051] The energy per pulse of the laser light LZ is 350 mJ / cm 2 , 300 mJ / cm2 The results are similar to those in the case of 300 mJ / cm, where a hole with a diameter of 20 μm begins to form from the first pulse, and the depth of the hole increases with the number of pulses. 2 The hole depth exceeded 20 μm with fewer pulses than in the case of

[0052] From the above experimental results, it was found that the processing coat 30 used in this embodiment has an energy per pulse of the laser light LZ of 200 mJ / cm 2 and 300 mJ / cm 2 There is a significant difference in the processing results between the cases of 200mJ / cm 2 In the case of 300 mJ / cm, slight traces of processing are left on the processing coat 30, but even if irradiation is continued for 100 pulses (or more), which is the standard number of pulses, it does not reach a depth that penetrates the minimum thickness (about 2 μm) assumed as a specification of the processing coat 30, so it is determined that the energy is insufficient to process the processing coat 30. 2 In this case, a hole is formed from the first pulse, and the maximum thickness (about 20 μm) assumed for the processing coating 30 is penetrated in less than 100 pulses, which is the guideline number of pulses, so it can be said that this is sufficient energy to process the processing coating 30. In this way, the ablation threshold is the laser light intensity (energy per pulse) that indicates the boundary between whether the processing coating 30 can be effectively processed by ablation. Therefore, the processing coating 30 of this embodiment derived from the implementation results in FIG. 6 has an ablation threshold of 300 mJ / cm when the ablation threshold is expressed as energy per pulse. 2 It is made of a resin material.

[0053] The ablation threshold for the workpiece W can also be set in the same manner as for the processing coat 30 described above.

[0054] The ablation threshold of the processing coating 30 must be equal to or higher than the ablation threshold of the workpiece W. If the ablation threshold of the processing coating 30 is lower than the ablation threshold of the workpiece W, the effect of preventing the above-mentioned blurring light may not be sufficiently obtained.

[0055] For example, an insulating material constituting an insulating layer in a circuit board such as the printed circuit board 20 may have an ablation threshold of 50 mJ / cm 2 ~100mJ / cm 2 Such insulating materials are strongly affected by blurring light caused by the aberration of the imaging lens 14 during irradiation with the laser light LZ, and therefore differences in aperture diameter are likely to occur due to individual differences in the imaging lens 14 and differences in the resolution performance within the irradiation surface of the imaging lens 14.

[0056] In particular, when drilling holes in the printed circuit board 20, it is required to make the inner surface of the hole as close to vertical as possible, so the workpiece W is irradiated with laser light LZ having extremely high energy compared to the ablation threshold of the workpiece W. As an example, when forming a hole in an insulating layer 21 having a thickness of about 15 μm to 30 μm, the laser light LZ is irradiated with energy of 1000 mJ / cm per pulse. 2 The laser beam LZ is irradiated with about 50 pulses. Therefore, the energy per pulse is close to the ablation threshold of the insulating layer 21 (several tens of mJ / cm 2 In comparison with the case where laser irradiation is performed using a laser beam of about 1000 nm, this processing condition results in a greater tendency for variation in the hole opening diameter to occur.

[0057] From this viewpoint, it is preferable that the ablation threshold of the processing coat 30 is as high as possible within a range in which ablation is possible with the laser light LZ having an energy expected for processing the workpiece W. As a specific example, 2 ~100mJ / cm 2 Assuming that the workpiece W to be processed by the laser processing device 10 is an insulating layer, a protective film, or the like, the ablation threshold of the processing coat 30 is at least 200 mJ / cm 2It is desirable to set the ablation threshold of the processing coat 30 to 200 mJ / cm or more. 2 or more (300 mJ / cm as in the example in Figure 6). 2 ), as described above, the laser light LZ used for actual processing of the insulating layer 21 is several times stronger, so that the processing coat 30 can be easily processed and the presence of the processing coat 30 does not hinder processing of the workpiece W.

[0058] For example, the ablation threshold is 300 mJ / cm 2 When a processing coat 30 having a thickness of 20 μm is used, the energy per pulse is 1000 mJ / cm 2 The laser light LZ can penetrate the processing coat 30 in several pulses to form an opening 30a as shown in Fig. 5. On the other hand, the blurring light with energy below the ablation threshold of the processing coat 30 is blocked by the processing coat 30, and the influence of the blurring light on the workpiece W, which has a lower ablation threshold than the processing coat 30, can be reliably prevented.

[0059] In this way, by providing a processing coat 30 having an ablation threshold equal to or higher than the ablation threshold of the workpiece W and performing laser processing, it is possible to perform hole processing in the workpiece W while eliminating the influence of blurred light having a contrast below a predetermined level, and the effect of stabilizing the hole opening diameter throughout the effective processing area E1 and rectangular area E2 is obtained.

[0060] Next, the thickness of the processing coat 30 will be described. As described above, the unevenness of the resolution performance on the irradiation surface caused by the aberration of the imaging lens 14 and the like causes the hole opening diameter to vary, but as a result of experiments and research, it has been found that the effect is most noticeable up to a depth of 3 to 5 μm from the hole opening (top surface). Therefore, if the effect up to this depth is absorbed by the processing coat 30, a high effect can be obtained in suppressing the variation in the opening diameter of each hole.

[0061] According to experiments and research, when the thickness of the processing coating 30 is set to about 2 μm or more, the effect of suppressing the variation in the opening diameter of the hole in the workpiece W becomes clear. Therefore, it is preferable that the minimum thickness of the processing coating 30 is set to 2 μm or more.

[0062] Regarding the function of the processing coat 30, namely, improving the stability of the opening diameter, the upper limit of the thickness of the processing coat 30 is not particularly restricted. However, the thicker the processing coat 30, the higher the time and cost required for manufacturing the processing coat 30 itself, and the lower the productivity. In addition, the thicker the processing coat 30, the longer the time until the workpiece W is processed by the laser processing device 10. Therefore, the upper limit of the processing coat 30 is set based on the mass productivity of the processing coat 30 and the processing time of the laser processing device 10. Specifically, if the upper limit of the thickness of the processing coat 30 is set to about 20 μm, the production efficiency of the processing coat 30 and the processing efficiency of the laser processing device 10 can be improved.

[0063] From the above conditions, the thickness of the processing coating 30 is set in the range of 2 μm to 20 μm. It is more preferable to set the thickness of the processing coating 30 in the range of 5 μm to 20 μm, with a lower limit of 5 μm, which covers the upper limit of the depth at which the opening diameter is likely to vary. Furthermore, when the mass productivity of the processing coating 30 and the processing efficiency of the laser processing device 10 are important, the optimal thickness of the processing coating 30 is approximately 5 μm, which is the minimum value within this range.

[0064] In summary, the processing coating 30 is set to satisfy the following conditions. First, the ablation threshold of the processing coating 30 is equal to or higher than the ablation threshold of the workpiece W. In addition, it is preferable that the ablation threshold of the processing coating 30 is as high as possible within the range that can be processed by the performance of the laser used to process the workpiece W. Specifically, it is at least 200 mJ / cm 2 The processing coat 30 of this embodiment has an energy per pulse of 300 mJ / cm2 or more. 2As can be seen from the experimental results in FIG. 6, the ablation threshold is 350 mJ / cm or more, and more preferable effects can be obtained. 2 The above processing coat 30 may be used.

[0065] Moreover, it is preferable that the processing coat 30 has excellent processability when processing with the laser processing device 10. The processability is determined according to the ablation threshold and the thickness. 2 In the case of processing coat 30 with a thickness of 5μm to 20μm, the energy per pulse is 1000mJ / cm 2 In processing the workpiece W, which requires laser irradiation of about 50 pulses, the processing coat 30 is penetrated with just a few pulses, resulting in excellent processability, and there is almost no increase in processing time due to the placement of the processing coat 30 on the workpiece W.

[0066] Furthermore, the processing coat 30 is required to be easily disposed on the workpiece W and easily removed from the workpiece W. The water-soluble resins such as acrylic resin and polyester resin mentioned above as materials for the processing coat 30 can be spread over the surface of the workpiece W with a uniform thickness of several μm by various methods such as coating or pasting. Regarding removal from the workpiece W, the processing coat 30 made of a water-soluble resin can be easily removed without damaging the workpiece W because it can be washed away with water or a developing solution.

[0067] The processing coat 30 is also required to be made of a material that does not cause damage to the insulating layer 21 or protective film of the printed circuit board 20 when it comes into contact with the workpiece W. The water-soluble resins such as acrylic resin and polyester resin exemplified above as materials for the processing coat 30 do not cause damage to insulating materials and protective films generally used for circuit boards and meet the conditions.

[0068] 7 and 8 show an embodiment which shows the result of disposing a processing coating 30 satisfying the above conditions on a workpiece W and processing it with the laser processing device 10, and a comparative example which shows the result of processing the workpiece W with the laser processing device 10 without disposing a processing coating 30 on the workpiece W.

[0069] 7 shows the results of observing holes formed at multiple positions in the above-mentioned rectangular area E2 (within a rectangular surface with sides of approximately 70 mm). Example 1 shows the processing result when processing coat 30 is present, and Comparative Example 1 shows the processing result when processing coat 30 is not present. The irradiation conditions (energy and number of pulses) of laser light LZ in Example 1 and Comparative Example 1 are the same, and the pattern of mask 13 is set so as to form multiple holes with a diameter of 20 μm in a predetermined arrangement on rectangular area E2.

[0070] The positions of the five holes P1-P5 on the rectangular region E2 in Example 1 correspond to the positions of the five holes Q1-Q5 on the rectangular region E2 in Comparative Example 1. Holes P1 and Q1 are formed near the center of the rectangular region E2. Holes P2-P5 and holes Q2-Q5 are formed near the four corners of the rectangular region E2. Note that Fig. 7 shows enlarged images of holes P1-P5 and holes Q1-Q5, and the relationship between the actual size of the rectangular region E2 and the dimensions of each hole is different.

[0071] In Example 1, the expansion of the hole opening is suppressed not only for hole P1 near the center of rectangular region E2, but also for holes P2 to P5 near the four corners. More specifically, for holes P1 to P5, the average opening diameter (top diameter) was 19.9 μm with a standard deviation of 0.4, the average bottom diameter was 18.8 μm with a standard deviation of 0.5, and the average inner surface angle was 88.4° (when the direction parallel to the workpiece surface is set to 0°).

[0072] In Comparative Example 1, hole Q1 near the center of rectangular region E2 has a slightly wider opening than Example 1. Furthermore, holes Q2 to Q5 near the four corners of rectangular region E2 have significantly wider openings. More specifically, the machining results for holes Q1 to Q5 were as follows: the average top diameter was 21.9 μm with a standard deviation of 0.9, the average bottom diameter was 17.2 μm with a standard deviation of 0.5, and the average inner surface angle was 83.4° (when the direction parallel to the workpiece surface is set to 0°).

[0073] Thus, in Example 1, due to the effect of the processing coat 30, the variation in the opening diameter of each hole from the center to the periphery of the rectangular region E2 is improved to less than half that of Comparative Example 1. As a result, it is possible to suppress the influence of the variation in resolution performance within the processing range (effective processing area E1 and rectangular region E2) of the laser processing device 10 on the processing results, and to obtain homogenized processing performance throughout the entire processing range.

[0074] The holes P1 to P5 in Example 1 have inner surfaces that are nearly perpendicular to the work surface (upright shape) compared to the holes Q1 to Q5 in Comparative Example 1, realizing an inner surface shape suitable for blind vias 24 (see FIG. 2) of a printed circuit board 20. The processing coat 30 contributes to improving the shape of each of the holes P1 to P5. However, the technical idea of ​​the present invention is not to simply improve the shape disorder of each processed part (drooping shape of the opening part of the hole) that is likely to occur in laser processing in general, but to use the processing coat 30 as a means for processing homogenization to prevent the unevenness of the resolution performance that the optical system of the laser processing device 10 inevitably has from appearing as a variation in the processing result at each point within the processing range. In other words, the idea of ​​using a means such as the processing coat 30 to prevent the variation in the processing result within the processing range of laser processing performed with a certain degree of spread is new and unique.

[0075] FIG. 8 shows the improvement in the shape of each hole in more detail, showing the cross-sectional shape of a hole at a specific position near the outer edge of rectangular region E2 in Example 2 and Example 3 with processing coat 30 and Comparative Example 2 and Comparative Example 3 without processing coat 30. Example 2 and Comparative Example 2 are the results of setting mask 13 to form holes with a diameter of 20 μm under the same irradiation conditions (energy and number of pulses) of laser light LZ. Example 3 and Comparative Example 3 are the results of setting mask 13 to form holes with a diameter of 14 μm under the same irradiation conditions (energy and number of pulses) of laser light LZ.

[0076] 8, in both Examples 2 and 3, the spread of the hole opening is kept small, and the inner surface of the hole is nearly vertical from the bottom to the opening. Specifically, the taper ratio, which is the ratio of the opening diameter to the bottom diameter, is about 96.0% in the hole of Example 2, and the taper ratio is about 94.8% in the hole of Example 3. Therefore, even near the outer edge of the rectangular area E2 that is easily affected by aberrations of the imaging lens 14, a hole with a good shape with a small difference between the opening diameter and the bottom diameter is formed.

[0077] In contrast, in Comparative Example 2 and Comparative Example 3, the opening diameter of the hole is wider than in Examples 2 and 3, and the angle of the inner surface of the hole is smaller than in Examples 2 and 3. The taper rate of the hole in Comparative Example 2 is approximately 83.2%, while the taper rate of the hole in Comparative Example 3 is approximately 85.9%. Therefore, it can be seen that the area near the outer edge of rectangular area E2 is significantly affected by blurred light.

[0078] As described above, according to the laser processing method and laser processing apparatus of this embodiment, in processing applications requiring stability in the hole opening diameter, the processing results can be improved by arranging a processing coating 30 that satisfies predetermined conditions on the surface of the workpiece W and processing it.

[0079] In particular, it is possible to prevent unevenness in the processing accuracy caused by the aberration of the imaging lens 14 and the like from affecting the processing results of the workpiece W. Therefore, it is highly useful in the laser processing apparatus 10 having a wide effective processing area.

[0080] Furthermore, the present invention is highly useful when the holes to be formed have a small diameter. Specifically, the present invention is particularly useful when the opening diameter of the holes is 20 μm or less. This is because the smaller the diameter of the holes to be formed, the more susceptible they are to the effects of the optical performance of the optical system that irradiates the laser light, and the more likely it is that differences in the processing results, such as variations in the opening diameter, will occur. As shown in Figures 7 and 8, in Examples 1 and 2 to which the present invention is applied, a high effect is obtained in forming small holes with a diameter of 20 μm or less.

[0081] The above-described embodiment is a specific example shown to facilitate understanding of the invention, and the present invention is not limited to this embodiment. Various modifications and changes are possible without departing from the gist of the invention.

[0082] In the above embodiment, a photomask type laser processing device 10 is used in which an image of a pattern formed on a mask 13 is formed on a workpiece W by an imaging lens 14, but it is also possible to apply the present invention to a galvano scanner type laser processing device. Even in a galvano scanner type laser processing device, differences in processing results for each processing position may occur due to the optical performance and specifications of the optical system, so that laser processing using a processing coat that satisfies the conditions of the present invention is useful in order to reduce such differences in processing results.

[0083] In the above embodiment, the energy per pulse of the pulsed laser is used as the ablation threshold, but the ablation threshold is not limited to this. For example, the ablation threshold may be set as a value obtained by multiplying the energy per pulse by the number of pulses.

[0084] In the above embodiment, a pulsed laser light source 11 is used, but a continuous wave laser light source may be used. When a continuous wave laser is used, the ablation threshold may be set based on the laser light intensity or the integrated value of the laser light intensity and irradiation time. [Explanation of symbols]

[0085] 10 Laser processing equipment 11 Laser light source 12 Illumination optical system 13. Mask 14 Imaging lens (imaging optical system) 15 Work support part 20 Printed circuit board 30 Processing coat 30a opening D Effective machining diameter E1 Effective machining area E2 rectangular area H hole LZ laser light OX optical axis W Work (workpiece)

Claims

1. A laser processing method for processing a workpiece by irradiating the workpiece with laser light, comprising the steps of: a step of disposing a processing coat made of a water-soluble resin on a surface of the workpiece, the processing coat having an ablation threshold, which is a threshold at which ablation processing is initiated by the laser light, equal to or higher than the ablation threshold of the workpiece; a step of irradiating a laser beam onto the processing coat to form an opening penetrating the processing coat and to remove a portion of the workpiece; A laser processing method comprising the steps of:

2. The laser light is output in a pulsed manner, and the ablation threshold of the processing coat is 200 mJ / cm per pulse of the laser light. ２ 2. The laser processing method according to claim 1, wherein the laser processing method is performed in a manner similar to that described above.

3. 3. The laser processing method according to claim 1, wherein the thickness of the processing coat is in the range of 2 μm to 20 μm.

4. 4. The laser processing method according to claim 1, wherein a hole having an opening diameter of 20 [mu]m or less is formed in the workpiece.

5. 5. The laser processing method according to claim 1, wherein the processing coat prevents the workpiece from being processed by blurred light of the laser light having a contrast equal to or lower than a predetermined value.

6. A laser processing apparatus for use in the laser processing method according to any one of claims 1 to 5, a laser light source that emits the laser light; a mask having a processing pattern to be applied to the workpiece and selectively passing the laser light emitted from the laser light source according to the pattern; an imaging optical system that is provided on an optical path between the mask and the workpiece and that forms an image of the pattern and irradiates the laser light onto the workpiece; A laser processing apparatus comprising:

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