JP3114756B2 - Method of forming via hole in wiring board - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a via hole in an insulating layer constituting a multilayer wiring board. In order to reduce the size and speed of electronic devices, it is necessary to increase the degree of integration of semiconductor integrated circuits and to increase the density of wiring boards on which they are mounted.

For this reason, the wiring board has been improved in integration from an original single-layer printed board to a multi-layer printed board and surface mounting. Possible multi-chip module (M
CM) is being developed.

Here, a ceramic wiring board manufactured by a thick film method can be used as an MCM board. However, a large-sized computer or the like pursuing a high-speed signal has a lower dielectric constant than ceramic. Therefore, a thin film multilayer wiring board in which polyimide having excellent solder heat resistance is used as an insulating layer and copper (Cu) wiring is formed on the insulating layer alternately is developed, and this is being put to practical use. (For example, Nikkei Microdevices December 1989 issue)

[0004]

2. Description of the Related Art Polyimide, which is currently being put to practical use as an insulating material for wiring boards, has a heat resistance and a low dielectric constant (ε =
It is an excellent material that combines 3.4).

However, this material has a problem that the apparent dielectric constant increases and the insulating property decreases due to the moisture absorption due to the imide ring having a large polarity. Further, since the signal propagation speed is inversely proportional to ε ^1/2, a material having a lower dielectric constant than polyimide is required.

[0006] Perfluoroethylene-based polymers (for example, polytetrafluoroethylene, abbreviated as PTFE) are known as materials having low moisture absorption, heat resistance and extremely low dielectric constant.

[0007] Perfluorocarbon resin represented by PTFE is a material having an extremely low dielectric constant (ε) of about 2.0, and has excellent heat resistance due to a large binding energy between carbon and fluorine. I have.

This material has very low hygroscopicity,
Because of its excellent chemical resistance, it has already been used as a substrate for printed wiring boards. However, in the thin film method in which the wiring layer and the insulating layer are alternately laminated one by one, it is necessary to adhere the rigid support substrate made of ceramics or metal to the insulating layer, the insulating layer to the wiring metal, and the insulating layer to each other. Is not used as an insulating material for a multilayer wiring board because of its difficulty in bonding other than fusion.

Here, as a means for applying PTFE to the thin film method, after PTFE is made porous by a method such as biaxial stretching,
A method of impregnating the porous film with a resin having good adhesiveness can be considered.

That is, by compounding with PTFE,
The dielectric constant and the hygroscopicity can be reduced while maintaining the required adhesiveness and heat resistance. When using this method, the resin to be impregnated must be capable of impregnating porous PTFE and bonding by heat treatment, etc. Some bismaleimide-triazine resins (abbreviations)
A prepreg material impregnated with porous PTFE (BT resin) is commercially available as GORE-TEX GTM-051 series (Japan GORE-TEX).

[0011] Further, the heat resistance is further excellent as an impregnated resin,
It is also possible to select a thermosetting resin with a low dielectric constant, and a material impregnated with a thermosetting resin with adhesive properties similar to that of porous PTFE will achieve excellent properties as a thin film multilayer wiring film Can be.

An important technical problem in forming a thin film multilayer wiring is formation of a via hole in an interlayer insulating layer.
Here, in the thin-film multi-layer method, the use of reactive ion etching (abbreviated as RIE) is considered even when forming a via hole in order to take a step of stacking wiring layers one by one as in the case of forming a semiconductor integrated circuit.

However, in a thin film circuit, the insulating layer is one order of magnitude thicker than a semiconductor, so that in order to etch this thick insulating layer, a metal film having excellent dry etching resistance must be used as a resist layer. For this reason, formation of the via hole involves forming a metal film → photoresist coating → exposure → photoresist development → metal resist etching → insulating film etching →
Complicated processes such as metal resist removal are required.

For this reason, in a thin film circuit using a polyimide as an insulating layer, a method of imparting photosensitivity to a polyimide precursor and forming a via hole using the photosensitivity of the material has been studied in order to simplify the process. .

However, a material in which a porous PTFE is impregnated with a thermosetting resin having an adhesive property cannot form a via hole utilizing photosensitivity, so that it is necessary to use RIE which takes a complicated process.

From these facts, it is necessary to put into practical use a method of forming via holes in a simple process for porous PTFE impregnated with a thermosetting resin.

[0017]

As described above, it is impossible to form a via hole utilizing photosensitivity in a material in which porous PTFE is impregnated with an adhesive resin.

Therefore, it is an object of the present invention to commercialize a method for forming a via hole by a simple and practical method instead of this.

[0019]

The above object is achieved by a method of forming a via hole in an insulating film of a multilayer wiring board by utilizing ablation by excimer laser light, wherein the insulating film has at least a benzocyclobutene ring in the structure. a thermosetting resin having made that is impregnated into the porous polytetrafluoroethylene film, the excimer laser beam, the wavelength 19
This can be achieved by configuring a method for forming a via hole in a wiring substrate using excimer laser light having a light emission time of 1 ns or more with a light emission time of 1 ns or more per pulse.

[0020]

The feature of the present invention is that Kr is capable of high-output and stable oscillation.
Using a F or XeCl laser to form a via hole in porous PTFE impregnated with a thermosetting adhesive.

In recent years, attention has been paid to ablation processing using excimer laser light as a new processing technique. An excimer laser is a gas laser that uses excitons of a rare gas and a halogen, and can emit high-intensity ultraviolet light.

Here, a normal substance has a strong absorption in the ultraviolet region, and thus has a peak absorption of about 10% like an excimer laser beam.
When a strong light pulse reaching 0 MW / cm ² is applied, the chemical bond is destroyed in an instant, and the surface layer evaporates.

This is the ablation, and the excimer laser processing utilizes this phenomenon. Here, there are many combinations of rare gases and halogens. For industrial use, KrF laser (wavelength: 248 nm) and XeCl laser (wavelength: 308 nm) are excellent in oscillation output and stability. Are mainly considered.

Excimer laser processing is characterized in that the finished cross section is sharp and a relatively large area such as up to 10 mm ² can be collectively processed, so that a fine pattern can be easily formed by exposing through a mask. can do. (TAZnotis and others: Laser Focus, May 54, 19
87, Ishizaka: Applied Mechanical Optics, September 1990, etc.) For this reason, excimer laser processing has attracted attention as a via hole forming technology for thin-film multilayer circuits. (F. Bachmann:
Chemtronics, Sep. Vol. 4 1989) Basic research on excimer laser ablation, which does not limit the application, involves research on various resins, and reports on the decomposition of PTFE.

That is, since PTFE is transparent for wavelengths of 190 nm or more, it is necessary to use a fluorine (F ₂ ) laser (wavelength: 157 nm) having a shorter wavelength or an ultrashort pulse KrF laser of femtosecond. is there.

However, the development of these lasers has only just started, and the lasers have not reached the stage where they can be used industrially in terms of output, stability, cost and the like. Use of this laser for forming via holes requires the use of optical components such as lenses and mirrors, but practical optical components that can withstand high-intensity vacuum ultraviolet light or ultrashort pulse ultraviolet light are currently not available. Not developed.

For this reason, the object to be processed by excimer laser processing for the purpose of application to the thin film multilayer method is exclusively polyimide, and an example in which a via hole is formed in a PTFE insulating film using a commercially available KrF laser or XeCl laser. Is not known.

The present invention is directed to a porous PTFE film impregnated with a thermosetting resin containing an aromatic ring or a hetero ring, and a KrF or XeCl laser which can be used industrially for this insulating film, specifically, Emission wavelength is 190nm or more and emission time is 1ns
A via hole is formed by performing laser ablation using the above-described excimer laser.

When a porous PTFE impregnated with a thermosetting resin containing an aromatic ring or a hetero ring is irradiated with KrF or XeCl laser light, the aromatic ring or the hetero ring of the thermosetting resin absorbs light and is decomposed. It is rapidly gasified and ejected, but PTFE does not decompose because it does not absorb light.

However, when the thermosetting resin is decomposed and gasified and jetted out without decomposing the PTFE itself, the inventor of the present invention shears the fibrous PTFE and jets out and removes it together with the decomposed gas to form a via hole. Was formed.

The side surface of the hole (hole) thus formed has irregularities and is not a smooth surface. However, since the diameter of the via hole formed in the wiring board is as large as several tens of μm, the irregularities are formed on the side surface. Via connection is possible even if it exists.

Therefore, without using a fluorine (F ₂ ) laser having a short wavelength (wavelength: 157 nm) or an ultrashort pulse KrF laser of femtosecond, it is possible to form a via hole by using a normal ns emission KrF laser beam. it can.

Next, the advantage of this processing method is that the processing time is short. That is, since the PTFT can be removed by shearing instead of decomposition, it is not necessary to consider the light energy required for the decomposition of the PTFT.

For example, when the volume occupancy of PTFE is 1/3,
It can be processed at an etching rate that is about 1.5 times that of a film made of only thermosetting resin. Here, as the resin to be impregnated in the PTFE membrane, bismaleimide resin, epoxy resin, benzocyclobutene resin (abbreviated BC)
A thermosetting resin having an aromatic ring, such as B resin, can be used. In particular, BCB resin can be mentioned as the most preferable thermosetting resin.

That is, the BCB resin has a low relative dielectric constant of 2.7, is low in moisture absorption, and is excellent in heat resistance. It has been reported. (IEEE Transaction on Compone
nt, Hybrids, and Manufacturing Tec. vol.19 p.1357,19
90) However, no attempt has been made to further reduce the dielectric constant by combining BCB resin with porous PTFE.

[0036]

EXAMPLE An impregnation solution was prepared by adding 10 g of methyl isobutyl ketone to 20 g of a commercially available BCB resin solution (55% xylene solution of BCB resin, trade name Dow Chemical XU13005).

Further, as the porous PTFE, the thickness is 30 μm.
A porous material having a porosity of 75% by biaxial stretching at m 2 was used. First, a substrate with 10,000 electrodes formed on 6 inch square glass was used as a substrate.
A PTFE membrane was placed, and a fixed amount of the impregnating solution was printed thereon by a screen printing method to impregnate the porous PTFE membrane with the BCB solution.

Then, after the solvent was removed by hot air drying, the solution was placed in a nitrogen (N ₂ ) oven and heated at 240 ° C. for 1 hour.
The CB was cured to form a uniform PTFE film having a thickness of 20 μm and filled with voids.

For comparison, a film having a thickness of 20 μm was formed on the same substrate using only BCB resin. The relative dielectric constant of such a sample was measured. As a result, the dielectric constant of the BCB-impregnated porous PTFE was 2.45 (1 MHz), whereas the dielectric constant of the BCB resin alone film was 2.72.

Next, using a KrF excimer laser, via holes were formed by the "1: 1 mirror mask exposure method" according to the present invention. FIG. 1 shows the configuration of an optical system used. Laser light emitted from a KrF excimer laser is reduced by a first lens to increase the intensity per unit area, and then the cross-sectional area is 2 × by a second lens. The light was collimated into a beam of about 6 mm and irradiated with a PTFE composite membrane impregnated with BCB through a mask.

The mask is made of SiO ₂ and Y _{2 as} shown on the upper left.
A hole pattern is formed on a dielectric mirror formed by laminating O ₃ ,
This dielectric mirror is configured as a light shielding layer. After aligning the mask with the PTFE composite membrane, the mask was overlapped at a predetermined interval and simultaneously fixed on the XY stage.

Then, while oscillating the laser, the stage is moved in the X direction (the short axis direction of the beam),
The entire surface of the substrate was processed by repeating the operation of shifting by 1 mm in the direction, then moving in the X direction, and shifting by 1 mm at the end.

The processing conditions are as follows: output: 50 W, pulse width: 16 ns, light energy on the mask incident surface: 20 W (100 mJ × 200 Hz), beam reduction ratio: 4/15, hole diameter of the mask: 30 μm, and processing with the mask Distance to membrane: 0.4mm, X-direction stage moving speed: 23
mm / s.

Ablation processing is performed under these conditions, and BC
A hole with a diameter of 40 to 45 μm at the opening and a diameter of 20 to 25 μm at the electrode surface was formed on the entire surface of the substrate impregnated with B-impregnated porous PTFE membrane. In the case of a film with a thickness of 20 μm using only BCB, it was not possible to process the bottom (electrode surface) of the film at the stage moving speed described above, so that the moving speed had to be 19 mm / s or less. .

After forming the via holes, the substrate was washed with ethanol and the surface was cleaned with O ₂ plasma. next,
As a result of forming the upper electrode as shown in FIG. 2 and measuring the via connection with the lower electrode, there was no connection failure due to insufficient exposure of the base electrode due to insufficient processing or disconnection at the side of the via hole, and the via connection was performed reliably. I was able to confirm that it was done. Example 2 Bis (amino-methylphenyl) hexafluoropropane (abbreviated as BIS-AT-AF) and this bismaleimidated compound were dissolved in a mixture of N-methylpyrrolidone and tetrahydrofuran at a weight ratio of 1: 2. Let 70 × 70 × 1.1mm
The same porous material used in Example 1 on a glass substrate
After placing PTFE and impregnating it in the same manner, 350 ° C in N ₂ atmosphere
To form a film having a relative dielectric constant of 2.8.

As a result of forming a via hole in this film under the same conditions as in Example 1, it was confirmed that a hole having the same size as in Example 1 was formed.

[0047]

According to the present invention, a commercially available KrF having a light emission time of ns level is formed on a low dielectric constant insulating film in which a porous PTFE film is impregnated with a thermosetting resin having an aromatic ring or a hetero ring in the structure.
Since a via hole can be formed using a laser or a XeCl laser, a thin-film multilayer wiring board having a short propagation delay time can be put to practical use.

[Brief description of the drawings]

FIG. 1 is an exposure optical system.

FIG. 2 is a cross section of a via.

Claims (2)

(57) [Claims] 1. A method of forming a via hole in an insulating film of a multilayer wiring board using ablation by excimer laser light, wherein the insulating film comprises a thermosetting resin having at least a benzocyclobutene ring in a structure thereof. A method for forming a via hole in a wiring board, comprising: a film impregnated in a tetrafluoroethylene film , wherein the excimer laser light has a wavelength of 190 nm or more and a light emission time per pulse is 1 ns or more. 2. The excimer laser according to claim 1, wherein the excimer laser is a KrF or an excimer laser.
2. The device according to claim 1 , wherein the device is a XeCl excimer laser.
The via hole forming method of the wiring board according to the above.