JP2005021917A - Method for boring resin layer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a boring method by which a high-quality via hole can be formed to a resin layer of a printed wiring board by using a pulse laser beam. <P>SOLUTION: An object to be machined, in which an upper layer made of a second resin is arranged on a substrate made of a first resin, is irradiated with the pulse laser beam in a condition which makes the surface of the object to be machined to become first pulse energy density. Thus, a hole can be formed on the upper layer. The bottom face of the hole is irradiated with the pulse laser beam in a condition which makes the surface of the object to be machined to become second pulse energy density of larger pulse energy density than that of the first pulse energy density. Accordingly, the hole can be deepened so as to reach a certain depth of the substrate. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a drilling method for a resin layer, and more particularly to a method for drilling by making a pulse laser beam incident on a resin layer.
[0002]
[Prior art]
A technique for forming a via hole by making a pulse laser beam incident on a resin layer of a printed wiring board is known. As the pulse laser beam, a fundamental wave of a carbon dioxide laser (wavelength: 9.1 μm or 10.6 μm) or a harmonic of a solid-state laser such as Nd: YAG or Nd: YLF is used. When using a harmonic of a solid-state laser, a pulse laser beam of several tens to several hundred shots is normally incident to form a via hole that penetrates the resin layer.
[0003]
Patent Document 1 discloses a method of gradually reducing the power density on the surface of a workpiece to form a via hole that penetrates the resin layer and exposes the inner layer copper wiring. By gradually reducing the power density, damage to the inner layer copper wiring can be reduced.
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 2000-202668
[Problems to be solved by the invention]
As the speed of electronic devices mounted on a printed wiring board increases, it is desired to reduce the dielectric constant of the resin layer of the printed wiring board. As a resin material having a low dielectric constant, a liquid crystal resin or the like has attracted attention. A printed wiring board using a liquid crystal resin has, for example, a structure in which a lower layer made of a liquid crystal resin and an upper layer made of polyethylene terephthalate (PET) or the like are laminated on a core layer having a copper wiring formed on the surface thereof.
[0006]
The inventors of the present application have found that when a via hole is formed by making a pulse laser beam incident on the resin layer of such a printed wiring board, the quality of the via hole is deteriorated.
[0007]
An object of the present invention is to provide a drilling method capable of forming a high-quality via hole in a resin layer of a printed wiring board using a pulsed laser beam.
[0008]
[Means for Solving the Problems]
According to one aspect of the present invention, (a) a first pulse energy on the surface of a processing object is provided on a processing object in which an upper layer made of a second resin is disposed on a base made of the first resin. Irradiating a pulsed laser beam under conditions of density to form a hole in the upper layer; and (b) a second pulse energy density greater than the first pulse energy density on the surface of the workpiece. And a step of deepening the hole so that the bottom surface of the hole formed in the step (a) is irradiated with a pulse laser beam to reach a certain depth of the base.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
With reference to FIG.1 and FIG.2, the drilling method by the Example of this invention is demonstrated.
[0010]
FIG. 1A shows a cross-sectional view of a workpiece. Inner layer wiring 2 made of copper is formed on a part of the surface of the core layer 1 made of glass epoxy or the like. A base layer 3 is formed on the core layer 1 so as to cover the inner layer wiring 2. The underlayer 3 is made of a liquid crystal resin and has a thickness of 50 μm, for example. An upper layer 5 is bonded to the surface of the base layer 3 by an adhesive layer 4. The upper layer 5 is made of PET and has a thickness of 13 μm, for example. Note that the thickness of the adhesive layer 4 is, for example, 8 μm.
[0011]
As shown in FIG. 1B, a plurality of shots of the pulse laser beam 7 are incident on the surface of the upper layer 5 of the workpiece, penetrate through the upper layer 5, the adhesive layer 4, and the base layer 3, and reach the surface of the inner layer wiring 2. The reaching via hole 6 is formed. As the pulse laser beam 7, for example, a third harmonic (wavelength 355 nm) of an Nd: YAG laser can be used. Other solid-state lasers, for example, third harmonics such as Nd: YLF laser, excimer lasers, and the like can also be used.
[0012]
FIG. 2 shows the relationship between the pulse energy of the pulse laser beam irradiated during processing and the elapsed time. The horizontal axis represents elapsed time, and the vertical axis represents pulse energy in the unit “μJ”. First, 40 shots are irradiated with a pulse energy of 4 μJ. Thereafter, the pulse energy is increased stepwise. In the embodiment, the pulse energy is increased stepwise so that the pulse energy becomes 7 μJ, 11 μJ, 16 μJ, 23 μJ, 30 μJ, 34 μJ, 36 μJ, 39 μJ, 44 μJ, 46 μJ, 51 μJ, 59 μJ, and 30 shots are irradiated with each pulse energy. It was. That is, a total of 400 shots were irradiated. The pulse frequency is 20 kHz, the pulse width is 50 ns, and the diameter of the beam spot on the surface of the workpiece is 100 μm. For example, when the pulse energy is 4 μJ, the pulse energy density on the surface of the workpiece is 0.05 J / cm ² .
[0013]
When drilling was performed under the above conditions, a via hole 6 reaching the surface of the inner layer wiring 2 was formed as shown in FIG. The diameter of the opening of the via hole 6 was about 100 μm, and a slight raised portion 7 was formed around the opening. The height of the raised portion 7 was 10 μm or less.
[0014]
For comparison, a via hole was formed by irradiating 120 shots with a pulse energy of 80 μJ. FIG. 3 shows a diagram in which a micrograph of a cross section of a part of the formed via hole is sketched. Underlayer 3, adhesive layer 4, upper layer 5, and via hole 6 are observed. A raised portion 7 is formed around the opening of the via hole 6. The height of the raised portion 7 was about 27 μm.
[0015]
It can be seen that when the via hole is formed with a constant pulse energy, the raised portion 7 becomes higher. When the raised portion 7 becomes high, it becomes difficult to fill the via hole 6 with the conductive paste. On the other hand, the raised portion 7 can be lowered by lowering the initial pulse energy at the time of hole formation and gradually increasing the pulse energy as in the above embodiment.
[0016]
Hereinafter, the reason why the raised portion 7 is lowered will be considered. The light absorption coefficient of the upper layer 5 shown in FIG. 1A is smaller than the light absorption coefficient of the underlayer 3 in the wavelength region of the pulsed laser beam used for processing. For this reason, the pulse laser beam incident on the upper layer 5 is not easily absorbed by the upper layer 5, reaches the lower layer 3, and is absorbed by the lower layer 3. For this reason, the underlayer 3 may be melted or vaporized before the upper layer 5 is melted or vaporized.
[0017]
If the base layer 3 is melted or vaporized while the upper layer 5 remains, it is considered that the upper layer 5 is rolled up. It is considered that this swelled portion adheres to the surface of the upper layer 5 and forms a raised portion 7.
[0018]
In the case of the above-described embodiment, the pulse energy of the pulse laser beam irradiated at the initial stage of drilling is reduced, so that it is considered that a hole is formed in the upper layer 5 without melting the underlayer 3. After the hole penetrating the upper layer 5 and the adhesive layer 4 is formed, a pulse laser beam having a pulse energy large enough to melt or vaporize the underlayer 5 is exposed to the underlayer 5 exposed on the bottom surface of the hole. Incident and the hole deepens. For this reason, the upper layer 5 is not rolled up. Therefore, it is considered that the raised portion 7 is difficult to be formed.
[0019]
As described above, when forming a hole in at least two resin layers of the upper layer and the base layer, the pulse energy of the pulse laser beam to be irradiated is decreased and gradually increased, thereby opening the opening of the via hole. The raised part formed around can be made low. The pulse energy is preferably controlled so that the underlying layer does not melt or vaporize until a hole penetrating the upper layer is formed.
[0020]
In addition, after the hole penetrating the upper layer is formed, the pulse energy of the pulse laser beam incident on the underlayer is higher than the initial pulse energy. For this reason, the machining time can be shortened compared to the case where drilling is performed with the pulse energy fixed at a relatively low value.
[0021]
In the above embodiment, the pulse energy is increased stepwise 12 times. However, if the pulse energy is increased at least once, it is possible to suppress the occurrence of the swelled portion and shorten the machining time as in the above embodiment. It is. This effect is expected to be particularly high when the absorption coefficient of the upper layer is smaller than the absorption coefficient of the underlying layer in the wavelength region of the pulse laser beam used for processing.
[0022]
In the above embodiment, the pulse energy is increased every 30 shots after the pulse energy is increased for the first time. However, it is not always necessary to increase the pulse energy every 30 shots. For example, it may be increased for every shot, or may be increased for every other appropriate number of shots. Further, it is not necessary to make the number of shots irradiated with fixed pulse energy equal for all pulse energies.
[0023]
FIG. 4A shows a schematic view of a laser processing apparatus used in the drilling method according to the above embodiment.
[0024]
The laser light source 10 emits a pulse laser beam made of, for example, a third harmonic of an Nd: YAG laser. A pulsed laser beam emitted from the laser light source 10 enters an acoustooptic device (AOM) 11. A high frequency voltage is applied to the AOM 11 from a high frequency power supply 16. The intensity of the 0th-order diffracted light L0 and the 1st-order diffracted light L1 emitted from the AOM 11 varies depending on the magnitude of the applied high-frequency voltage.
[0025]
The zero-order diffracted light L0 is incident on the beam damper 12. The first-order diffracted light L1 enters the galvano scanner 13. The galvano scanner 13 scans an incident pulse laser beam in a two-dimensional direction. The pulse laser beam scanned by the galvano scanner 13 is incident on the workpiece 20 held on the XY stage 15 via the fθ lens 14. In the optical path of the laser beam between the AOM 11 and the galvano scanner 13, a field lens, a mask for shaping the beam cross section, and the like may be arranged as necessary.
[0026]
By changing the magnitude of the high frequency voltage applied to the AOM 11, the intensity and pulse energy of the pulse laser beam incident on the workpiece 20 can be changed. By increasing the pulse energy at least once during the formation of one via hole, the drilling process according to the above embodiment can be performed.
[0027]
In the apparatus configuration of FIG. 4A, the first-order diffracted light L1 of the AOM 11 is incident on the processing target 20, but the zero-order diffracted light L0 may be incident on the processing target 20.
[0028]
In the laser processing apparatus shown in FIG. 4A, the pulse energy is changed using the AOM 11, but it is also possible to change the pulse energy using another intensity modulator.
[0029]
FIG. 4B shows a schematic diagram of a laser processing apparatus using an electro-optic element as an intensity modulator. A linearly polarized pulsed laser beam emitted from the laser light source 10 enters an electro-optic element (EOM) 21. A voltage is applied to the EOM 21 from a variable voltage DC power supply 24. The pulse laser beam that has passed through the EOM 21 becomes elliptically polarized light. The ellipticity of the ellipse can be controlled by the voltage applied to the EOM 21.
[0030]
The pulse laser beam that has been elliptically polarized enters the polarization beam splitter 22 at a certain incident angle. As the polarization beam splitter 22, a polarization beam splitter cube, a thin film polarizer, a Glan-Thompson prism, or the like can be used. A part of the pulse laser beam is reflected by the polarization beam splitter 22, and the remaining components are transmitted through the polarization beam splitter 22. The pulse laser beam reflected by the polarization beam splitter 22 enters the beam damper 23. The pulse laser beam transmitted through the polarization beam splitter 22 is incident on the object to be processed via the optical system having the same configuration as the galvano scanner 13 and the fθ lens 14 shown in FIG.
[0031]
By changing the ellipticity of the elliptical pulse laser beam incident on the polarization beam splitter 22, the intensity and pulse energy of the pulse laser beam incident on the workpiece can be changed. Note that the pulsed laser beam reflected by the polarization beam splitter 22 may be incident on the object to be processed.
[0032]
In the apparatus shown in FIGS. 4A and 4B, the intensity of the pulse laser beam emitted from the laser light source 10 is modulated, so that the laser oscillator may be oscillated at a constant pulse frequency. it can. By adjusting the output of the laser oscillator at the pulse frequency used for actual processing, processing can be performed with desired pulse energy.
[0033]
Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.
[0034]
【The invention's effect】
As described above, according to the present invention, when forming a hole by making a pulse laser beam incident on at least two resin layers, by increasing the pulse energy density at least once during the formation of the hole, It is possible to make it difficult for the raised portion to be formed around the opening of the hole. Moreover, it can suppress that the processing time of a hole becomes long.
[Brief description of the drawings]
FIG. 1A is a cross-sectional view of a workpiece before forming a hole, and FIG. 1B is a cross-sectional view of the workpiece after forming a hole.
FIG. 2 is a graph showing temporal variation of pulse energy when forming a hole by a method according to an embodiment.
FIG. 3 is a diagram sketched from a photomicrograph of a hole formed with a constant pulse energy.
FIG. 4 is a schematic view of a laser processing apparatus used in a drilling method according to an embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Core layer 2 Inner layer wiring 3 Underlayer 4 Adhesive layer 5 Upper layer 6 Via hole 7 Raised part 10 Laser light source 11 Acoustooptic element 12, 23 Beam damper 13 Galvano scanner 14 fθ lens 15 XY stage 16, 24 Power supply 21 Electrooptic element 22 Polarized beam Splitter

Claims (5)

(A) A pulse laser beam is applied to a processing object in which an upper layer made of a second resin is disposed on a base made of the first resin, under a condition that the first pulse energy density is obtained on the surface of the processing object. And forming a hole in the upper layer,
(B) Irradiating the bottom surface of the hole formed in the step (a) with a pulse laser beam on the surface of the object to be processed under the condition that the second pulse energy density is higher than the first pulse energy density. And a step of deepening the hole so as to reach a certain depth of the base. 2. The drilling method according to claim 1, wherein a light absorption coefficient of the first resin in a wavelength region of pulse energy irradiated to the workpiece is smaller than a light absorption coefficient of the second resin. The drilling method according to claim 2, wherein the first resin is polyethylene terephthalate and the second resin is a liquid crystal resin. In the step (b), the intensity of the pulsed laser beam applied to the workpiece so that the pulse energy density on the surface of the workpiece increases from the second pulse energy density over time. The drilling method in any one of Claims 1-3 including the process of changing. The pulse frequency of the pulse laser beam irradiated in the step (a) and the step (b) is constant, and the step (a) and the step (b) determine the intensity of the pulse laser beam emitted from the laser light source. The drilling method in any one of Claims 1-4 including the process of adjusting the pulse energy density in the surface of the said workpiece by changing.

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