JPH11103149A - Forming method for via hole and laser beam machining apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a forming method in which the machining time of a via hole can be shortened without a complicated wet pretreatment, by a method wherein the via hole which reaches a metal film is formed, by a laser beam, in a polymer film on the metal film and a via-hole machined residue is removed by a dry treatment. SOLUTION: Respective laser beams are scanned, inside the face of an X-Y stage 18, by a first scanning mirror 13 and a second scanning mirror 14 in which two mirrors capable of being turned to mutually different directions are combined so as to correspond to a first solid-state laser oscillation part 11 used to radiate an infrared-range laser beam and to a second excimer laser oscillation part 12 used to radiate an ultraviolet or visible-range laser beam. A first converging fθ lens 15 and a second converging fθlens 16 focus the laser beams so as to be incident vertically on an object 17, to be machined, irrespective of angles of incidence of the respective laser beams from the two-axis scanning mirrors 13, 14. Then, the two-axis scanning mirrors 13, 14 are combined with the fθ lenses 15, 16, a via hole can be formed in a prescribed position on the object 17, to be machined, even when the object 17 to be machined is not moved by using the X-Y stage 18. Consequently, a machined residue can be removed simultaneously.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method and an apparatus for forming a via hole, and more particularly to a method and an apparatus for forming a 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 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.

An object of the present invention is to provide a via hole forming method which does not require a complicated post-processing step such as a wet post-processing.

Another object of the present invention is to provide a laser processing apparatus suitable for the above-described via hole forming method.

[0010]

SUMMARY OF THE INVENTION The present invention is a method for forming a via hole in a polymer layer on a metal film, the via hole reaching the metal film. A via hole is formed in the polymer layer using a laser beam. The method is characterized by including a first step of forming and a second step of removing a processing residue of the via hole by dry processing.

It is preferable that the second step is performed by irradiating the via hole with laser light having a wavelength different from that of the laser light in the first step.

According to the present invention, there is also provided a laser oscillating portion for emitting a laser beam, and a via hole reaching the metal film by irradiating the laser beam to a polymer layer on a metal film in a workpiece. A laser processing apparatus, comprising: a first optical system described above, and processing residue removing means for removing processing residue of the via hole formed in the workpiece by dry processing.

It is preferable that the first optical system includes a drive system for irradiating the laser beam to an arbitrary position on the workpiece.

In addition to holding the workpiece,
By providing an XY stage capable of moving the workpiece, the processing residue removing means can perform the XY stage.
A processing residue of the via hole formed in the workpiece is removed on a stage.

The processing residue removing means may include means for generating laser light having a different wavelength from the laser light, and a second optical system for irradiating the via hole with the laser light having a different wavelength. preferable.

The means for generating laser light having a different wavelength from the laser light can be realized by including means for converting the wavelength of the laser light from the laser oscillation section.

Further, as means for generating laser light having a wavelength different from that of the laser light, a laser oscillation part different from the laser oscillation part may be provided.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a laser processing apparatus according to an embodiment of the present invention. This laser processing apparatus is referred to as first and second laser oscillation units 11 and 12, first and second biaxial scan mirrors 13 and 14 corresponding to these laser oscillation units 11 and 12, respectively, and an fθ lens. First
And second convergence lenses 15 and 16 and an XY stage 18 on which a workpiece 17 is placed. The first laser oscillating unit 11, the first biaxial scan mirror 13, and the focusing lens 15 are for performing the first step, and the second laser oscillating unit 12, the second biaxial scan mirror 1
4 and the focusing lens 16 are for performing the second step.

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-state lasers, as well as second, third, or higher harmonic lasers of this type.

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 the second harmonic (2ω)) (further, a BBO crystal may be combined; Higher harmonics (4ω) can be used. At this time, the fundamental wave and the second harmonic may be combined to form a third harmonic (3ω). In addition, argon, copper vapor laser (oscillation wavelength 511nm
And 578 nm) can also be used.

The two-axis scan mirror is realized by combining two mirrors that can rotate in different directions from each other, and converts the laser light from the first and second laser oscillators 11 and 12 into X-rays.
-Scan within the Y plane. The fθ lens is designed such that the laser beam is focused on and vertically incident on the workpiece 17 irrespective of the incident angle of the laser beam incident from the biaxial scan mirror. Therefore, by combining the two-axis scan mirror and the fθ lens, it is possible to form a via hole at a predetermined position on the workpiece 17 without moving the workpiece 17 using the XY stage 18. it can. 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.
The laser beams from the second laser oscillators 11 and 12 can be applied to different positions. For this reason, when forming a plurality of via holes within a predetermined range, a step of forming via holes in the insulating layer using laser light from the first laser oscillation unit 11 and a step of forming laser holes from the second laser oscillation unit 12 And the step of removing the processing residue remaining in the via hole can be performed simultaneously (for different via holes). Therefore, the processing time can be greatly reduced as compared with the case of performing the processing sequentially.

Further, in the apparatus of the present embodiment, there is little need for movement by the XY stage 18 (when processing is performed over a wide range, the processing is performed for each predetermined range.
Although the movement by the Y stage 18 is necessary, it is not necessary at all within the predetermined range), so that high-speed processing can be performed. More specifically, the moving time interval of the workpiece by the XY stage is only about 10 Hz, but the moving time interval of the focal position by the biaxial scan mirror can be 100 Hz or more. Travel time becomes incomparably short.

The first and second focusing lenses 15, 16 are
The laser light focused by the first and second scan mirrors 13 and 14 is focused on the surface of the workpiece 17 so that the laser light has a predetermined diameter. The focusing lens 15 focuses infrared light, and its material is, for example, ZnSe.
Is used. Further, since the focusing lens 16 focuses visible light from ultraviolet, synthetic quartz, fused quartz, or BK7 is used as a material, for example. The surfaces of the first and second focusing lenses 15 and 16 are coated in consideration of their 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. Then, the infrared laser light emitted from the laser light oscillating unit 11 is irradiated to the insulating layer 21 inside the hole 23 via the biaxial scan mirror 13 and the focusing lens 15. The insulating layer 2 inside the hole 23 is irradiated with infrared laser light.
1 is ablated and 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, a processing residue 24 remains on the surface of the lower conductive layer 22. The hole as above
Usually, a plurality of pieces are formed in one processing area in a predetermined range.

When drilling of one processing area is completed, the multilayer printed circuit board is moved to the XY stage 1
Use 8 to move. Then, the laser light in the ultraviolet to visible range emitted from the laser light oscillating unit 12 is irradiated to the processing residue 24 in the hole 23 via the biaxial scan 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 since only a few shots of ultraviolet laser light or visible laser light are used at the end, carbon is not generated. rare. When an ultraviolet laser is used, carbon generation can be further suppressed by using oxygen as an assist gas.

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, the 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.

The YAG laser and the YLF laser are:
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 has a feature that it has a large peak power and a very high repetition frequency of 7000 Hz, but requires a warm-up time of about 30 minutes.

[0033]

According to the present invention, a hole is formed in a polymer layer by using a laser beam of a certain wavelength, and then a processing residue is removed by a laser beam of another wavelength, so that wet post-processing is performed. Do not need. In addition, since the laser beam can be scanned over the non-processed object by the two-axis scan mirror, a via hole forming method and a laser processing apparatus capable of high-speed processing can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an 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 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 unit 13, 14 Biaxial scan mirror 15, 16 Focusing lens 17 Workpiece 18 XY stage 21 Polymer insulating layer 22 Conductive layer 23 Hole 24 Processing residue

Claims (8)

[Claims] In a via hole forming method for forming a via hole reaching the metal film in a polymer layer on the metal film, a first step of forming a via hole in the polymer layer using a laser beam; And a second step of removing a processing residue of the via hole by a treatment. 2. The method of forming a via hole according to claim 1, wherein the second step is performed by irradiating the via hole with laser light having a wavelength different from the laser light in the first step. Via hole forming method. 3. A laser oscillating section for emitting laser light, and a first optical element for irradiating the laser light on a polymer layer on a metal film in a workpiece to form a via hole reaching the metal film. A laser processing apparatus comprising: a system; and a processing residue removing unit configured to remove a processing residue of the via hole formed in the workpiece by dry processing. 4. The laser processing apparatus according to claim 3, wherein the first optical system includes a drive system for irradiating the laser beam to an arbitrary position on the workpiece. Processing equipment. 5. The laser processing apparatus according to claim 4, further comprising an XY stage capable of holding said workpiece and moving said workpiece, and said processing residue removing means. A laser processing apparatus for removing processing residues of the via holes formed in the workpiece on the XY stage. 6. The laser processing apparatus according to claim 3, wherein said processing residue removing means includes means for generating a laser light having a wavelength different from said laser light, and a laser having a different wavelength from said laser light. A second method of irradiating the via hole with light;
A laser processing apparatus comprising: an optical system; 7. A laser processing apparatus according to claim 6, wherein said means for generating laser light having a wavelength different from said laser light includes means for converting the wavelength of laser light from said laser oscillation section. Laser processing equipment. 8. The laser processing apparatus according to claim 6, further comprising a laser oscillation unit different from said laser oscillation unit as means for generating laser light having a wavelength different from said laser light. Processing equipment.