JP3138954B2 - Via hole formation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a via hole forming method, and more particularly to a via hole forming method for forming an interstitial via hole in a multilayer printed circuit board using a laser beam.

[0002]

2. Description of the Related Art In response to the demand for higher density of printed circuit boards accompanying the miniaturization and higher density mounting of electronic devices, multilayer printed circuit boards in which a plurality of printed circuit boards are stacked have recently appeared. In such a multilayer printed board, it is necessary to electrically connect conductive layers (copper boards) between printed boards stacked one above another. Such a connection is realized by forming a hole called a via hole reaching the lower conductive layer in an insulating layer (polymer such as polyimide or epoxy resin) of a printed circuit board, and plating the inside of the hole. You.

[0003] As a method of forming via holes, a mechanical fine drill was used before. However, the diameter of the via hole is reduced due to the increase in the density of the printed circuit board, and the via hole (interstitial via) is formed only in some of the stacked layers (that is, not penetrating from the front surface to the back surface). Due to the difficulty of controlling the depth when forming a hole), a via hole forming method using ablation by a laser beam has recently been adopted instead of a fine drill.

Conventionally, in a via hole forming method using a laser of this type, an excimer laser (ultraviolet light) is mainly used.

[0005] In addition, the excimer laser is expensive and has a low running cost because the equipment price and the running cost are high and the etching rate (the processing depth per pulse) is low (the etching rate is generally proportional to the wavelength). Lasers emitting infrared light, such as a carbon dioxide gas laser and a YAG laser, which have a low cost and a high etching rate, are also being used.

[0006]

As described above, the via hole forming method using an excimer laser (ultraviolet light) has a problem that the etching rate is low. In addition, this method also has a problem in that carbon adheres and remains around the upper portion of the hole, causing insulation failure. In addition, if the energy density of the laser beam is increased to increase the etching rate, a new problem occurs that the conductor layer is damaged (depending on the reflectance of the conductor layer).

The via hole forming method using a carbon dioxide gas laser or a YAG laser is inexpensive and can be processed at a high speed. However, as shown in FIG. Conductor layer 4 to be used
However, there is a problem that a processing residue 43 which is thin (about 1 μm or less in thickness with epoxy resin and polyimide) remains on a part or the entire surface of the surface of No. 2. This processing residue cannot be completely removed even after further irradiation with laser light. This is because even if laser light is further applied to evaporate the processing residue, the surrounding polymer elutes at this time (the conductor layer is often made of copper and heat diffusion is fast), so new processing is performed. Probably because of the formation of remnants. Therefore, these methods have a problem that a wet post-treatment with a strong oxidizing agent (for example, potassium dichromate) or the like is required after the processing by the laser.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of forming a via hole which can be processed at a high speed and does not require a complicated post-processing step such as a wet post-processing. It is an object of the present invention to provide a drilling device.

[0009]

According to the present invention, there is provided a via hole forming method for forming a via hole reaching the metal film in a polymer layer laminated on the metal film, the method comprising: First to form holes in the polymer layer
And a second step of exposing the metal film by irradiating the inside of the hole with a laser beam in the ultraviolet or visible region to obtain a via hole forming method.

[0010]

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a laser processing apparatus used in one embodiment of the present invention. The laser processing apparatus includes first and second laser oscillation units 11 and 12, and these laser oscillation units 11, 1
First and second folding mirrors 1 respectively corresponding to 2
3, 14 and first and second focusing lenses 15, 16;
An XY stage 18 on which the workpiece 17 is placed is provided.

The first laser oscillating section 11 emits laser light in the infrared region. As such a laser oscillation unit 11, a carbon dioxide gas laser (TEA: CO ₂ gas laser),
YAG (Yttrium Aluminum Garnet),
Or YLF (lithium yttrium fluoride)
Solid lasers can be used.

The second laser oscillating section 12 emits laser light in the ultraviolet to visible range. This laser oscillation unit 12
As an excimer laser (for example, KrF: oscillation wavelength)
248 nm) or a combination of a YAG laser or a YAL laser and a wavelength conversion crystal (KTP, LBO, or BBO crystal; which generates a second harmonic (2ω)) (further, a BBO crystal may be combined; Harmonics (4ω) can be used. At this time, the fundamental wave and the second harmonic may be combined to form a third harmonic (3ω). Furthermore, a copper vapor laser (oscillation wavelengths 511 nm and 578 nm
) Can also be used.

First and second folding mirrors 13 and 14
Is such that the laser beams emitted from the first and second laser oscillation units 11 and 12 are perpendicular to the XY stage 18, that is, perpendicular to the workpiece 17. Is reflected. Therefore, the folding mirrors 13 and 14 are not always necessary, and the laser oscillation units 11 and 12, the focusing lenses 15 and 16, and the X-
It is appropriately used depending on the arrangement of the Y stage 18. Further, the laser light from the second laser oscillation unit 12 can be propagated in a predetermined direction by using an optical fiber instead of the folding mirror 14 (the laser light from the first laser oscillation unit 11 can also be used). And, depending on its wavelength, can be propagated in an optical fiber).

The first and second converging lenses 15, 16 are
The laser light reflected by the folding mirrors 13 and 14 is converged so that the laser light has a predetermined diameter on the surface of the workpiece 17. Since the converging lens 15 converges infrared light, for example, ZnSe is used as its material. In addition, since the focusing lens 16 focuses visible light from ultraviolet, synthetic quartz or BK7 is used as a material, for example. The surfaces of the first and second folding mirrors and the first and second converging lenses 15 and 16 are coated in consideration of their reflectance and transmittance.

A method for forming an interstitial via hole (hereinafter abbreviated as a via hole) in a multilayer printed circuit board using this apparatus will be described. First,
The multilayer printed circuit board that is the workpiece 17 will be described. As shown in FIG. 2A, the multilayer printed circuit board has a plurality of polymer insulating layers 21 such as polyimide or epoxy resin, and the surface of each insulating layer (on the lowermost substrate, both front and back surfaces) A conductive (Cu) layer (printed wiring) 22 is formed by printing. In a place where the via hole is to be formed, the upper conductive layer 22 is formed.
Is formed with a hole 23. When via holes are formed in a plurality of layers, holes are formed in all the conductive layers located on the upper layer side. The formation of via holes in such a multilayer printed circuit board is performed as follows.

First, the multilayer printed circuit board is mounted on the XY stage 18. The focusing lens 15 is inserted into the hole 23.
The position is adjusted so that the laser beam focused by the above is applied.
Then, the infrared laser light emitted from the laser light oscillating unit 11 is applied to the insulating layer 21 inside the hole 23 via the folding mirror 13 and the focusing lens 15. The insulating layer 21 inside the hole 23 is ablated by the irradiation of the infrared laser beam and is removed. Processing using infrared laser light is completed in a much shorter time than processing using conventional ultraviolet laser light. That is, the processing speed is high. Moreover, since no ultraviolet light is used, no carbon is generated. However, as shown in FIG. 2B, the processing residue 24 is formed on the surface of the lower conductive layer 22.
Remains.

Next, this multilayer printed circuit board is mounted on an X-Y
The laser beam is moved by using the stage 18 so that the laser beam focused by the focusing lens 16 falls on the hole 23. Then, the laser light in the ultraviolet to visible range emitted from the laser light oscillating unit 12 is applied to the processing residue 24 in the hole 23 via the return mirror 14 and the focusing lens 16. Then, as shown in FIG. 2C, the processing residue 24 evaporates, the lower conductive layer 22 is exposed, and the via hole is completed. Thereafter, by plating the inside of the via hole, the two conductive layers can be electrically connected.

As described above, according to the present embodiment, high-speed processing is possible, no wet post-processing is required, and only a few shots of ultraviolet laser light or visible laser light are used at the end. There is almost no.

Actually, after a hole is formed in a polymer insulating layer with a carbon dioxide laser, and this hole is irradiated with laser light from a YLF laser, a fundamental wave (1ω; wavelength of 1047 nm) of a Nd: YLF laser is obtained. (Irradiation energy of 3 mJ / pulse, one shot), the processing residue could not be removed. On the other hand, Nd: YLF
When the laser was irradiated with the second harmonic (irradiation energy: 0.8 mJ / pulse, one shot, wavelength: 523 nm), the processing residue could be completely removed. Similarly, Nd:
4th harmonic of YLF laser (irradiation energy 0.3mJ
/ Pulse, 5 shots, wavelength 266 nm), the processing residue could be removed. However, in this case, ablation appeared to have occurred in the conductive layer due to excessive irradiation of the laser beam.

The lasers that can be used for the second laser oscillation section 12 have the following features. First,
For example, in the case of a KrF: excimer laser, the energy per pulse is 800 mJ, the cross-sectional area (irradiation area) of the beam is about 3 cm ² , and the excimer laser has high energy and a wide beam area. . Therefore, considering that the energy density required for irradiating the bottom surface of the via hole having a diameter of about 0.2 to 0.5 mm is about 300 mJ / cm ² , a suitable mask is used to cover a wide area. Projection processing is possible. Other excimer laser ArF (oscillation wavelength 193 nm),
XeF (oscillation wavelength 351 nm), XeCl (oscillation wavelength 30
8 nm) and the like.

Also, the YAG laser and the YLF laser
Although the same solid-state laser is used, there is a difference that the laser light from the YAG laser is randomly polarized and the laser light from the YLF laser is linearly polarized. Since the linearly polarized component is incident on the wavelength conversion crystal, the latter has a longer wavelength. High conversion efficiency. In the latter case, the thermal lens effect is less likely to appear, and is suitable for continuous use. In these lasers, both CW oscillation and pulse oscillation are possible, but in the case of this embodiment, pulse oscillation with a large peak power is more desirable.

The copper vapor laser is characterized by a large peak power and a very high repetition frequency of 7000 Hz, but requires a warm-up time of about 30 minutes.

Next, a second embodiment of the present invention will be described.
The laser processing apparatus of the present embodiment is basically the same as the laser processing apparatus of the first embodiment. The difference between the laser processing apparatus of the present embodiment and the laser processing apparatus of the first embodiment is that
A two-axis scan mirror (not shown) such as a galvanometer is provided in place of the folding mirrors 13 and 14, and the focusing lens 15,
16 has an fθ lens (not shown).

The two-axis scan mirror is realized, for example, by combining two mirrors rotatable in mutually different directions, and scans laser beams from the laser oscillators 11 and 12 in an xy plane. The fθ lens is designed such that the laser beam is focused on the workpiece 17 and is perpendicularly incident on the workpiece 17 irrespective of the incident angle of the laser beam incident from the biaxial scan mirror. Therefore, the combination of the two-axis scan mirror and the fθ lens allows the XY stage 18
The via hole can be formed in the same manner as in the first embodiment without moving the workpiece 17 using. Further, within the predetermined range, it is not necessary to move the workpiece 17 using the XY stage 18 even when forming a plurality of via holes.

In the apparatus of this embodiment, the two two-axis scan mirrors can be controlled independently of each other, so that the laser beams from the laser oscillators 11 and 12 can be applied to different holes. For this reason, when forming a plurality of via holes within a predetermined range, a step of forming a hole in the insulating layer with the laser light from the laser oscillation unit 11 and a step of forming a hole in the hole with the laser light from the laser oscillation unit 12. The process of removing objects can be performed simultaneously (for different holes). Therefore, the processing time is greatly reduced as compared with the case of sequentially processing (substantially the same as the case where only the step of forming a hole in the insulating layer 21 with the laser light from the laser oscillation unit 11).
be able to.

In the apparatus of the present embodiment, there is little need for movement by the XY stage (when processing is performed over a wide range, the processing is performed for each predetermined range.
Although movement by the stage is required, it is not necessary at all within the predetermined range), so that high-speed processing is possible. To elaborate,
The moving time interval of the workpiece by the XY stage is 10
Although it is only about Hz, the moving time interval of the focal position by the biaxial scan mirror can be realized at 100 Hz or more, and the moving time between the processing points of the laser beam is so short as to be incomparable.

Next, a third embodiment of the present invention will be described with reference to FIG. The laser processing apparatus of the present embodiment has a first
And the second laser oscillation units 31 and 32 and the adjustment optical system 3
3, mirror 34, folding mirror 35, focusing lens 3
6 and an XY stage 38 on which the workpiece 37 is placed.

First and second laser oscillators 31, 32
Respectively correspond to the first and second laser oscillation units 11 and 12 of the first embodiment, but the arrangement may be reversed.
Further, the adjusting optical system 33 includes a focusing lens 36.
This is for correcting a focal position by eliminating a difference in chromatic aberration due to a difference in wavelength between two laser beams, and is provided corresponding to one of the first and second laser oscillation units 31 and 32. It may be provided so as to correspond to the first laser oscillation unit 31.

Whether the material and the coating material on the surface of the mirror 34 are selected so that the laser light from the first laser oscillation part 31 is transmitted and the laser light from the second laser oscillation part 32 is totally reflected. Or, it is mounted on a moving mechanism (not shown).

The surface of the folding mirror 35 is coated with a metal film or a dielectric multilayer film so as to totally reflect the laser beams from the first and second laser oscillators 31 and 32. The mirror 35 is not always necessary for the same reason as in the first embodiment.

The focusing lens 36 includes two laser oscillators 3
It is necessary to select a material that allows both of the laser beams from the first and second laser beams to pass therethrough. For example, materials that transmit both of these laser beams include NaCl and KB.
r and KI. The surface of the focusing lens 36 is coated so as to transmit both of these laser beams.

The method of forming a via hole using this laser processing apparatus is the same as in the first embodiment. However, when the mirror 34 is mounted on the moving mechanism, when irradiating the workpiece with the laser light from the first laser light oscillation unit 31, the mirror 34 is moved from the optical path to the outside of the optical path, and When the workpiece is irradiated with the laser beam from the second laser beam oscillation unit 32, the mirror 34 must be moved from outside the optical path to the optical path to perform the processing.

Further, instead of the reflecting mirror 35 and the focusing lens 36 of the present embodiment, a beam scanning device such as a galvano scan similar to that used in the second embodiment and fθ
A lens can also be used. Further, two beam scanning devices corresponding to the first and second laser oscillation units 31 and 32 are provided, similarly to the second embodiment, without using the mirror 34, and two beam scanning devices from these beam scanning devices are provided. The laser beam may be made incident on one fθ lens. In the case where chromatic aberration occurs due to the difference in the wavelength of the laser beam in the fθ lens, it is necessary to correct the angle by adjusting the angle of the scan mirror and the position of the processing point.

[0035]

According to the present invention, a hole is formed in the polymer layer by using an infrared laser beam, and then a processing residue is removed by a laser beam in a visible region , so that a wet post-treatment is required. Thus, a via hole forming method capable of high-speed processing without damaging the conductor layer can be obtained.

[0036]

[0037]

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a first embodiment of the present invention.

FIG. 2 is a process diagram for explaining a via hole forming process by the apparatus of FIG. 1;

FIG. 3 is a configuration diagram of a third embodiment of the present invention.

FIG. 4 is a perspective view of a hole formed in a polymer layer on a conductor film by infrared laser light.

[Explanation of symbols]

 11, 12 Laser oscillation part 13, 14 Folding mirror 15, 16 Focusing lens 17 Workpiece 18 XY stage 21 Polymer insulating layer 22 Conductive layer 23 Hole 24 Processing residue 31, 32 Laser oscillation part 33 Adjustment optical system 34 Mirror 35 Folding mirror 36 Focusing lens 37 Workpiece 38 XY stage 41 Hole 42 Conductive layer 43 Processing residue

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H05K 3/00 H05K 3/46 B23K 26/00

Claims (5)

(57) [Claims] 1. A via hole forming method for forming a via hole reaching the metal film in a polymer layer laminated on the metal film, wherein a first hole is formed in the polymer layer using infrared laser light. Process and visible inside the hole
A second step of exposing the metal film by irradiating the region with laser light. 2. The method according to claim 1, wherein the first step is continuously performed to form a plurality of holes, and simultaneously, the plurality of holes formed in the first step are sequentially subjected to the second step. The via hole forming method according to claim 1, wherein: 3. An excimer laser as a light source of said visible range laser light.
Via hole formation method. 4. The via hole forming method according to claim 1, wherein a harmonic laser light from an infrared laser oscillator is used as the visible laser light. 5. The method according to claim 1, wherein the metal film is a printed wiring of a first printed circuit board, and the polymer layer is an insulating substrate of a second printed circuit board laminated on the first printed circuit board. The method of forming a via hole according to claim 1, 2, 3, or 4, wherein