JP2004322173A - Laser beam machining method and laser beam machining apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a through hole with excellent accuracy by using laser beams. <P>SOLUTION: A substrate 1 to be machined is held by a holding stand 3. A through hole 4 from a face side 1a to a back side 1b of the substrate 1 is formed by allowing the held substrate 1 to be irradiated with laser beams L1. Then, the laser beams L1 passing through the through hole 4 are reflected at the position away from the back side 1b of the substrate 1, and the reflected beams L2 are returned to the through hole 4 and the circumference of the through hole 4. The sectional shape of the through hole 4 is faired with the returned reflected beams L2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a laser processing method and a laser processing apparatus for forming a through hole in a substrate to be processed such as a printed circuit board.
[0002]
[Prior art]
As a laser processing apparatus for forming a through hole in a substrate to be processed, there is an apparatus provided with a pulse oscillation type laser oscillator as disclosed in Patent Document 1, for example. In this apparatus, pulsed laser light emitted from a laser oscillator is guided by a mirror, a lens, or the like, and is irradiated onto the surface of a substrate to be processed held on a processing table. Thereby, the portion irradiated with the laser light on the substrate to be processed evaporates (ablation). By irradiating the substrate to be processed with a plurality of shots of laser pulses at a predetermined pulse frequency, a through hole extending from the front surface to the back surface is formed in the substrate to be processed.
[0003]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-245071 (page 2-4, FIG. 1)
[0004]
[Problems to be solved by the invention]
In the through hole formed in the substrate to be processed, it may be desired that the opening diameter (top diameter) on the substrate surface side and the opening diameter (bottom diameter) on the back surface side of the substrate are substantially equal. However, it is difficult to satisfy this requirement. Usually, the cross-sectional shape of the through hole becomes a taper shape in which the bottom diameter is smaller than the top diameter.
[0005]
Further, when the through-hole is formed using laser light, the chuck plate that holds the substrate to be processed may be damaged. For example, if an excessive laser beam is directly applied to the chuck plate after forming the through hole, a processing flaw occurs on the chuck plate. Further, when the substrate to be processed includes a metal layer such as copper, the metal melted by the laser beam may adhere to the chuck plate. When removing the deposits, the chuck plate may be damaged.
[0006]
An object of the present invention is to provide a technique for forming a through hole having a sharp side surface using a laser. Another object of the present invention is to provide a technique for preventing the generation of scratches on a substrate holding means such as a chuck plate for holding a substrate to be processed when forming a through hole in the substrate to be processed. .
[0007]
[Means for Solving the Problems]
According to one aspect of the present invention, (a) a step of holding a substrate to be processed on a holding table, and (b) irradiating the held substrate to be processed with laser light from the surface of the substrate to be processed. A step of forming a through hole extending over the back surface; and (c) reflecting the laser light passing through the formed through hole at a position away from the back surface of the substrate to be processed, and reflecting the reflected light to the through hole and the through hole. A laser processing method is provided.
[0008]
According to another aspect of the present invention, a laser optical system that irradiates a surface of a substrate to be processed with laser light, and the laser that passes through a through hole formed in the substrate to be processed by the laser light from the laser optical system. There is provided a laser processing apparatus comprising substrate holding means for reflecting light toward the through hole at a position away from the back surface of the substrate to be processed. The substrate holding means preferably includes a reflection plate that reflects the laser light, and a transmission plate that is disposed between the reflection plate and the substrate to be processed and transmits the laser light.
[0009]
When the laser light is reflected at a position away from the back surface of the substrate to be processed, the laser light spreads by diffraction after passing through the through hole, and thus the reflected light also returns to the periphery of the through hole. Therefore, processing that increases the bottom diameter of the penetrating light can be performed by using the reflected light. By interposing a transmission plate that transmits laser light between the reflection plate and the substrate to be processed, it is possible to avoid direct contact between the reflection plate and the substrate to be processed. Thereby, it is possible to prevent the reflection plate from being damaged.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a main configuration of a laser processing apparatus according to an embodiment. The laser processing apparatus includes a laser optical system 2 that irradiates a surface 1a of a substrate 1 to be processed with a laser beam L1 and a holder 3 that holds the substrate 1 to be processed. The laser optical system 2 includes a laser light source 21 and a light guide unit 22 that guides laser light emitted from the laser light source 21 to the surface 1a of the substrate 1 to be processed.
[0011]
As the laser light source 21, a gas laser such as an excimer laser or a CO ₂ laser, or a solid-state laser such as an Nd: YAG laser can be used.
[0012]
The light guide unit 22 includes a collimation lens 222 that collimates the laser beam emitted from the laser light source 21, a mask 224 that has a transmission hole that limits the beam diameter of the collimated laser beam, and a laser beam with a limited beam diameter. Galvano scanner 227 and fθ lens 228 for allowing the scanned laser light to be incident perpendicularly on the processing surface 1a. The fθ lens 228 constitutes a light collecting means.
[0013]
The light guide unit 22 includes folding mirrors 221, 223, and 226 between the laser light source 21 and the collimation lens 222, between the collimation lens 222 and the mask 224, and between the mask 224 and the galvano scanner 227, respectively. . In place of the mask 224, beam section shaping means such as a pinhole or iris can be used.
[0014]
In the light guide 22, either the light condensing method or the mask projection method can be used to guide the laser light from the laser light source 21 to the surface 1 a of the substrate 1 to be processed. When the condensing method is used, the distance between the fθ lens 228 and the substrate 1 to be processed is set so that the beam spot of the laser beam L1 is minimized on the surface 1a of the substrate 1 to be processed, that is, the substrate 1 to be processed. To adjust the focus on the surface 1a. The position where the laser beam L1 is focused will be referred to as the focal position.
[0015]
On the other hand, when the mask projection method is used, the distance between the fθ lens 228 and the substrate 1 to be processed is set so that the pattern shape of the mask 224 on the surface 1a of the substrate 1 to be processed, that is, the shape of the transmission hole of the mask 224 is desired. The image is adjusted so as to be imaged at a reduction ratio of. When the mask projection method is used, the spot shape of the laser light L1 corresponds to the pattern shape of the mask 224. A position where the mask pattern is imaged at a desired reduction ratio is called an imaging position.
[0016]
The length represents the range on the optical axis traveling without substantially changing the spot diameter at the focal point, from the viewpoint of the spot diameter, when the spot diameter at the focal point was d _0, the spot diameter is e.g. 1.05D ₀ It means the length of the following range.
[0017]
As shown in FIG. 1, the holding table 3 includes a chuck portion (substrate holding means) 31 that chucks the substrate 1 to be processed. In the present embodiment, the holding table 3 includes the chuck portion 31 and an XY table 32 that holds the chuck portion 31 so as to be movable in the biaxial direction.
[0018]
The chuck unit 31 includes a reflection plate 311 that reflects the laser light L from the laser optical system 2, a transmission plate 312 that is disposed between the reflection plate 311 and the substrate 1 to be processed, and that transmits the laser light L, and a reflection plate. 3 and a vacuum chuck plate 313 disposed between the XY stage 32.
[0019]
The reflection plate 311 is made of a material that reflects the laser light L, for example, Al. On the other hand, the transmission plate 312 is made of a material that transmits the laser light L. In particular, an inorganic material can be suitably used as the material of the transmission plate 312. An example of such a material is glass.
[0020]
As shown in FIG. 2, a suction path 313 b is formed in the vacuum chuck plate 313. One end of the suction path 313b communicates with a vacuum pump (not shown), and the other end is a suction port 313a opened on the surface of the vacuum chuck plate 313. A plurality of suction ports 313a are opened on the vacuum chuck plate 313 in plan view.
[0021]
Suction passages 311a and 312a pass through the reflection plate 311 and the transmission plate 312 at positions corresponding to the suction ports 313a, respectively. The suction paths 311a and 312a and the suction path 313b of the vacuum chuck plate 313 constitute one suction path as a whole. One end of the suction path as a whole is a suction port opened on the surface of the transmission plate 312.
[0022]
Since the suction path is evacuated by operating the vacuum pump, the back surface 1 b of the substrate 1 to be processed is in close contact with the transmission plate 312. In this way, the substrate 1 to be processed is held. That is, the surface of the transmission plate 312 is a holding surface that holds the substrate 1 to be processed.
[0023]
As described above, the vacuum chuck plate 313, the reflection plate 311, and the transmission plate 312 are laminated in this order to constitute the chuck unit 31 as a substrate holding unit. Note that the chucking method of the substrate 1 to be processed is not limited to the vacuum chuck method, and an electrostatic chuck method or the like can also be employed.
[0024]
Next, the substrate 1 to be processed as a processing target will be described. As shown in FIG. 2, the substrate 1 to be processed is a so-called two-metal material in which metal layers 12 are formed on both surfaces of a resin layer 11. The thickness of the resin layer 11 is, for example, 25 μm or 50 μm. The metal layer 12 has a thickness of 9 to 18 μm, for example.
[0025]
The resin layer 11 is made of, for example, polyimide, epoxy, phenol, tetrafluoroethylene polymer, BT resin, benzocyclobutene, or the like. The metal layer 12 is formed of, for example, a metal such as copper, aluminum, gold, silver, palladium, nickel, titanium, tungsten, platinum, molybdenum, or an alloy thereof.
[0026]
Hereinafter, the laser processing method according to the embodiment will be described. First, as shown in FIG. 1, the substrate 1 to be processed is held on a holding table 3. Since the substrate 1 to be processed is held on the transmission plate 312, the substrate 1 to be processed and the reflection plate 311 and the vacuum chuck plate 313 are not in direct contact with each other. Therefore, even when the substrate 1 to be processed includes the metal layer 12, it is possible to avoid the metal melted by the laser light L1 from adhering to the plates 311 and 313. Thereby, it can prevent that those plates 311 and 313 are damaged.
[0027]
Next, the laser optical system 2 irradiates the held substrate 1 to be processed with a laser L1, thereby forming a through hole 4 extending from the front surface 1a to the back surface 1b of the substrate 1 to be processed, as shown in FIG. Specifically, the substrate 1 to be processed is irradiated on the substrate 1 to be processed with a plurality of shots (for example, 40 shots) of ultraviolet pulse laser L1 (wavelength 355 nm) at a predetermined pulse frequency (for example, 10 kHz). The through hole 4 can be formed. The fluence per pulse of the pulse laser on the surface of the substrate 1 to be processed is, for example, 10 J / cm ² .
[0028]
Subsequently, the laser beam L1 is irradiated toward the formed through hole 4. Then, as shown in FIG. 2, after passing through the through hole 4, the laser light L <b> 1 is transmitted through the transmission plate 312, enters the reflection plate 311, and is reflected again to the through hole 4 side by the surface of the reflection plate 311. The Thus, the laser beam L1 passing through the through hole 4 is reflected at a position away from the back surface 1b of the substrate 1 to be processed by the thickness of the transmission plate 312.
[0029]
As shown in FIG. 2, when the laser beam L1 is reflected at a position away from the back surface 1b of the substrate 1 to be processed, the laser beam L1 passes through the through hole 4 and then enters the reflection plate 311. Diffracts and spreads. Therefore, as shown in FIG. 3A, the reflected light L2 from the reflection plate 311 does not pass through the through hole 4 but is incident on the region 13 around the through hole 4.
[0030]
Therefore, as shown in FIG. 3B, the region 13 around the through hole 4 is also ablated by the reflected light L2. As a result, even when the bottom diameter φb1 immediately after the through hole 4 has penetrated becomes smaller than the top diameter φt, the bottom diameter φb1 can be increased. That is, as shown in FIG. 3C, the bottom diameter φb1 can be expanded to φb2 close to the top diameter φt. In this way, the side surface of the through hole 4 can be brought close to the surface of the substrate 1 to be processed, and the formation of the through hole 4 with the side surface 4a can be realized. In addition, it is possible to accurately form a through hole having a high aspect ratio (through hole depth / hole diameter).
[0031]
In addition, if the position of the to-be-processed substrate 1 is moved with the XY stage 32, performing the formation process of the through-hole 4 demonstrated above, the cutting process of the to-be-processed substrate 1 etc. are realizable.
[0032]
Here, the energy density [J / cm ² ] per pulse in the incident region 13 of the reflected light L ² can be adjusted by, for example, the thickness of the transmission plate 312. Regardless of whether the condensing method or the mask projection method is used, the thickness of the transmission plate 312 is reflected by the reflection plate 312 after the laser light L1 passes through the focal position or the imaging position, and is again a through-hole. 4 and its surrounding length 13 (hereinafter referred to as the path length) is set to a value that is equal to or smaller than the focal depth of the laser light L1, irradiation per pulse at the focal position or the imaging position. Since the reduction of the energy density per pulse in the incident region 13 of the reflected light L2 with respect to the surface energy density can be suppressed to the minimum, the side surface of the through hole 4 can be efficiently shaped.
[0033]
Specifically, when the focal position or the imaging position is the surface 1a of the substrate 1 to be processed and the focal depth of the laser light L1 is 0.2 mm, the thickness of the substrate 1 to be processed is Ta and the transmission plate 312 is used. Assuming that the thickness of Tb is Tb, the condition that the value of Ta + 2Tb is about 0.2 mm or less can be selected. In the case of Ta = 43 μm, the above effect can be obtained by setting Tb = 0.785 mm or less.
[0034]
However, the focal position or the imaging position is not particularly limited. The focal position or the imaging position can be adjusted between the front surface 1a and the back surface 1b of the substrate 1 to be processed. Now, assuming that the focal position or the imaging position is aligned with the back surface 1b of the substrate 1 to be processed, the condition that the value of 2Tb is approximately 0.2 mm or less, that is, Tb = 0.1 mm or less, The above effects can be obtained.
[0035]
On the other hand, for example, if the thickness of the transmission plate 312 is set to such a value that the above-mentioned path length exceeds the focal depth of the laser light L, the energy density per pulse in the incident region 13 of the reflected light L2 becomes the focal point. It becomes smaller than the irradiation surface energy density per pulse at the position or imaging position.
[0036]
Since the region 13 around the through hole 4 is in a molten state when the through hole 4 penetrates, that is, at the time of FIG. 3A, the energy density per pulse in the region 13 is the focal position or imaging position. Even if it is smaller than the irradiation surface energy density per pulse in, the region 13 will be sufficiently processed.
[0037]
In practice, the laser beam L1 is focused on the surface 1a of the substrate 1 to be processed and the transmission plate 312 is a glass plate having a thickness of 1 mm so that the above-mentioned path length is more than 0.2 mm in depth of focus of the laser beam L1 When the through-hole 4 was penetrated, the top diameter was 50 μm and the bottom diameter was 35 μm. However, by shaping using the reflected light L2, the bottom diameter of the through-hole 4 was reduced to 40 μm. I was able to enlarge. In this specific example, the spot diameter of the reflected light L2 is approximately four times the focal spot diameter. Nevertheless, since favorable processing was realized, it is considered preferable that the thickness of the transmission plate 312 is 5 times (= 1 / 0.2) or less than the focal depth of the laser beam L1. Note that an optimal value can be selected for the thickness of the transmission plate 312 depending on the material of the substrate 1 to be processed, the laser beam to be used, and the like.
[0038]
Further, the width of the region 13 on which the reflected light L2 is incident can be adjusted by, for example, the surface roughness of the reflective plate 311. For example, as the surface of the reflection plate 311 is roughened, the reflected light L2 is scattered, so that the region 13 where the reflected light L2 is incident becomes wider. Thereby, reprocessing of a wide area | region is attained. In this sense, in one embodiment, the surface of the reflection plate 311 is roughened to such an extent that the laser light L2 can be scattered.
[0039]
On the other hand, the region 13 where the reflected light L2 is incident becomes narrower as the surface of the reflection plate 311 is closer to the mirror surface. In this case, reworking of a minute region is possible, and power loss of the laser beam can be minimized, and efficient reworking is possible. In this sense, in another embodiment, the surface of the reflection plate 311 is a mirror surface. The surface roughness of the reflection plate 311 can also be selected to an optimum value according to the material of the substrate 1 to be processed and the characteristics of the laser beam.
[0040]
FIG. 4 shows a configuration around the chuck portion 31 according to still another embodiment. The chuck unit 31 as a holding unit includes a vacuum chuck plate 313 and a transmission plate 312 stacked on the vacuum chuck plate 313. The vacuum chuck plate 313 also serves as the reflection plate 311 described above. The laser beam L is reflected on the surface of the vacuum chuck plate 313. The vacuum chuck plate is preferably made of a highly reflective material such as Al.
[0041]
As mentioned above, although this invention was demonstrated along the Example, this invention is not restrict | limited to these. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.
[0042]
【The invention's effect】
According to the present invention, it is possible to form a through hole having a sharp side surface using a laser. Further, according to the present invention, when the through hole is formed in the substrate to be processed, it is possible to prevent the substrate holding means such as a chuck plate that holds the substrate to be processed from being damaged.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of a main part of a laser processing apparatus according to an embodiment.
FIG. 2 is an enlarged cross-sectional view showing the periphery of a chuck portion serving as a substrate holding unit.
FIG. 3 is a schematic diagram showing how a cross-sectional shape of a through hole is shaped.
FIG. 4 is a view showing a modification of the laser processing apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Substrate 2 Laser optical system 3 Holding stand 31 Chuck part (substrate holding means)
311 Reflection plate 312 Transmission plate

Claims (10)

(A) a step of holding the substrate to be processed on a holding table;
(B) forming a through-hole extending from the front surface to the back surface of the substrate to be processed by irradiating the held substrate with the laser beam;
(C) laser processing including a step of reflecting the laser light passing through the formed through hole at a position away from the back surface of the substrate to be processed and returning the reflected light to the through hole and the periphery of the through hole. Method. 2. The laser processing method according to claim 1, wherein in the step (c), a distance between a position where the laser beam is reflected and a back surface of the substrate to be processed is 1 mm or less. In the step (b), the laser beam is focused between the front surface and the back surface of the substrate to be processed,
In the step (c), after the laser beam passes through the position of the focal point, it is reflected at a position away from the back surface of the substrate to be processed, and is again returned to the periphery of the through hole and the through hole. The laser processing method according to claim 1, wherein the path length is set to be equal to or less than a length representing a range on the optical axis along which the laser light travels without substantially changing a focal spot diameter. In the step (b), the laser beam that has passed through the transmission hole of the mask having the transmission hole is imaged at an imaging position between the front surface and the back surface of the substrate to be processed. Including the step of irradiating the substrate to be processed,
In the step (c), after the laser beam passes through the imaging position, it is reflected at a position away from the back surface of the substrate to be processed, and is again returned to the periphery of the through hole and the through hole. The laser processing method according to claim 1, wherein the path length is equal to or less than the focal depth of the laser light. The laser processing method according to claim 1, wherein in the step (c), the laser light is scattered at a position away from a back surface of the substrate to be processed. A laser optical system for irradiating the surface of the substrate to be processed with laser light;
Substrate holding means for reflecting the laser beam passing through the through hole formed in the substrate to be processed by the laser beam from the laser optical system to the through hole side at a position away from the back surface of the substrate to be processed; Laser processing apparatus provided. The substrate holding means;
A reflective plate for reflecting the laser light;
The laser processing apparatus according to claim 6, further comprising a transmission plate that is disposed between the reflection plate and the substrate to be processed and transmits the laser light. The laser processing apparatus according to claim 7, wherein a thickness of the transmission plate is 1 mm or less. The laser optical system includes condensing means for focusing the laser light between the front surface and the back surface of the substrate to be processed,
The path length from when the laser beam passes through the focal point to when it is reflected by the reflecting plate and returns to the periphery of the through hole and the through hole again substantially changes the focal spot diameter of the laser beam. The laser processing apparatus according to claim 7, wherein the laser processing apparatus has a length equal to or less than a length that represents a range on the optical axis that moves forward. The laser optical system is configured so that the laser beam that has passed through the transmission hole of the mask having the transmission hole forms an image at the imaging position between the front surface and the back surface of the substrate to be processed. Including light collecting means for irradiating the processed substrate;
The path length from when the laser light passes through the imaging position to be reflected by the reflection plate and returns to the periphery of the through-hole and the through-hole again is equal to or less than the focal depth of the laser light. Item 9. A laser processing apparatus according to Item 7 or 8.

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