JPH08250858A - Circuit board - Google Patents

Abstract

PURPOSE: To provide a circuit board with low loss and high signal transmission speed at high frequencies. CONSTITUTION: On a nickel-plated layer 22 which is a magnetic material formed on a copper-plated layer 20, a gold-plated layer 24 which is a non-magnetic material is further formed. Thus when a circuit board is used at high frequencies, though inductance of the nickel-plated layer 22 increases, current flows to the overlying gold-plated layer 24 and to the underlying copper-plated layer 20 due to skin effects, while no current flows to the nickel-plated layer 22 made of the magnetic material located in between, thereby generating no transmission loss due to the nickel-plated layer 22.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit board, and more particularly to a circuit board used for high frequencies in which a plurality of insulating layers are formed of an organic polymer material and signal wirings are formed between them.

[0002]

2. Description of the Related Art Insulation with an organic polymer material such as a polyimide resin for an oscillator circuit and a filter for operating at a high frequency of GHz band used for satellite communication or for a digital circuit such as a CPU and a memory for operating at high speed. A circuit board in which layers are formed and copper signal wiring is provided between the insulating layers is used. This is because polyimide resin or the like has a relative permittivity of about 3, and therefore has an advantage that the signal transmission speed can be increased as compared with alumina having a relative permittivity of 9 to 10. Here, a conventional method of manufacturing a circuit board using a polyimide resin will be described with reference to FIG.

First, a ground layer 112 made of a conductor is formed on an insulating substrate 110 as shown in FIG.
A polyimide precursor is applied thereon and then heated and cured to obtain a polyimide insulating layer 114a. Then, a titanium thin film 116 is formed on the polyimide insulating layer 114a by sputtering, and a copper thin film 118 is further formed thereon by sputtering to form a base layer 117 for forming a signal wiring. After that, a photoresist is applied on the copper thin film 118, a desired pattern is formed by a photolithography technique, and then a copper plating layer 120 having a desired thickness is formed. Further, a nickel plating layer 122 which serves as a barrier for protecting copper from corrosion is provided on the copper plating layer 120 in an etching process described later, and the photoresist is removed.

After that, metal etching is carried out, and then, as shown in FIG.
As shown in (B), the titanium thin film 116 and the copper thin film 118 other than below the copper plating layer 120 are removed. Then, as shown in FIG. 8C, a polyimide insulating layer 114b is placed on the polyimide insulating layer 114a. The circuit board is formed by repeating the above processing.

[0005]

However, the circuit board having the above structure has a problem that the transmission loss is large and the signal transmission speed is low in the transmission of high frequency signals. The present inventor has come to the knowledge that the cause may be the nickel plating layer 122 that forms a barrier during metal etching. That is, nickel is excellent in corrosion resistance, suitable for protecting copper during etching, inexpensive,
Although it has harmony with copper, it has a very high relative permeability because it is a magnetic material, and its resistance is also higher than that of copper. Therefore,
When a high frequency is applied to the signal wiring composed of the copper plating layer 120 and the nickel plating layer 122 shown in FIG. 8C, the current is generated by the skin effect in the lower portion of the copper plating layer 120 near the ground layer 112 and the nickel plating. Since it mainly flows to the layer 122, it is speculated that the presence of the nickel plating layer 122 may increase the inductance and the resistance and cause a loss. For this reason, the present inventor has studied the use of gold, which is a non-magnetic material and has corrosion resistance as a barrier during metal etching. However, when gold plating is performed on the copper plating layer 120, the copper Plating layer 1
It was concluded that diffusion occurred between 20 and gold plating, and the resistance component increased conversely.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a circuit board having a signal wiring structure with low loss and high signal transmission speed at high frequencies. is there.

[0007]

To achieve the above object, a circuit board according to a first aspect of the present invention comprises a plurality of insulating layers made of an organic polymer material and signal wirings formed between the plurality of insulating layers. A copper wiring layer containing copper as a main component formed on the insulating layer, a magnetic conductor layer containing a magnetic material as a main component and having corrosion resistance formed on the copper wiring layer, and a magnetic conductor layer on the magnetic conductor layer. The gist of the present invention is to include a signal line formed of a non-magnetic conductor layer which is formed and has a non-magnetic material as a main component and which has corrosion resistance.

In the circuit board according to claim 2, the circuit board according to claim 1
In the above, the gist is that the thickness of the non-magnetic conductor layer is equal to or larger than the skin thickness defined by the frequency of the signal applied to the signal wiring.

According to a third aspect of the present invention, there is provided a circuit board according to the first aspect.
In the second or second aspect, the gist is that the organic polymer material is made of any one of a polyimide resin, a penzocyclobutene resin, and an epoxy resin.

According to the circuit board of claim 4,
3 to 3, the gist of the magnetic conductor layer is that the main component is any one of nickel, chromium and alloys thereof.

Further, in the circuit board of claim 4,
4 to 4, the gist of the non-magnetic conductor layer is that the main component is any one of silver, gold, platinum, palladium, rhodium and alloys thereof.

[0012]

In the circuit board according to the present invention, a non-magnetic material containing a non-magnetic material as a main component is formed on a magnetic conductor layer containing a magnetic material as a main material, which is formed on a copper wiring layer containing copper as a main ingredient. A magnetic conductor layer is formed. Therefore, even if a high frequency signal is applied to this circuit board, the transmission loss due to the magnetic conductor layer is reduced. This is because due to the skin effect, a current mainly flows through the non-magnetic conductor layer located at the top of the signal wiring and the copper wiring layer located at the bottom, and a small amount of current flows through the magnetic conductor layer located in the middle.

In the circuit board according to the second aspect, the thickness of the nonmagnetic conductor layer is equal to or larger than the skin thickness defined by the frequency of the signal applied to the signal wiring. For this reason,
When a high frequency current flows through the skin portions (upper and lower portions) of the signal wiring due to the skin effect, most of the current passes through the non-magnetic conductor layer and the copper wiring layer, and the magnetic conductor layer below the non-magnetic conductor layer. Therefore, the transmission loss due to the high-frequency current flowing in the magnetic conductor layer can be sufficiently reduced.

Further, in the circuit board of claim 3, the organic polymer material is polyimide resin, benzocyclobutene (BC).
B) Made of either resin or epoxy resin. Since these materials have low dielectric constants, the formation of an insulating layer can increase the signal transmission speed.

Furthermore, in the circuit board of claim 4, since the magnetic conductor layer contains nickel, chromium, or an alloy thereof as a main component, copper is used during metal etching in the process of manufacturing the circuit board. The signal wiring of can be protected.

Furthermore, in the circuit board according to the present invention, the nonmagnetic conductor layer contains silver, gold, platinum, palladium, rhodium, or any alloy thereof as a main component. These non-magnetic materials have high adhesion to nickel and chromium and high corrosion resistance during metal etching. Further, for example, even when the polyimide precursor is heated and cured, it does not react with the precursor or the like and does not oxidize.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. 1, 2 and 3 are views showing a manufacturing process of a circuit board according to a first embodiment of the present invention. The circuit board according to the present embodiment is formed by forming a polyimide resin on the upper surface of the ceramic substrate 10 as an insulating layer between signal lines. The ceramic substrate 10 is, for example, a multilayer substrate obtained by stacking a plurality of green sheets made of alumina as a main material and firing at high temperature in a hydrogen furnace in a humidified atmosphere. The process of forming a wiring board on the ceramic substrate 10 will be described below.

First, as shown in FIG. 1A, a ground layer 12a made of copper is deposited on the ceramic substrate 10. Next, on the ground layer 12a, a polyimide precursor adjusted to have a constant viscosity is applied by a rotary coating machine (spin coater) not shown, and then heated and cured to form a polyimide insulating layer 14a having a thickness of 16 μm. Form (Fig. 1
(B)).

Then, titanium, which has excellent adhesion to polyimide, is sputtered on the surface of the polyimide insulating layer 14a to form a titanium thin film 16 (thickness 1000Å). Further, on the surface of the titanium thin film 16, copper is sputtered to obtain adhesion with a copper plating layer 20 described later to form a copper thin film 18 (thickness 5000Å) (see FIG. 1).
(See (C)) and the underlayer 17.

Then, as shown in FIG. 2 (A), the underlayer 1
7 is coated with a resist 19, a desired pattern is opened by a photolithography technique, and then a base layer 17 is formed.
A copper plating layer 20 (thickness 3 μm) is formed in the exposed opening 19a by electrolytic plating to be a copper wiring layer. In this embodiment, the copper plating layer 20 has a width of 25 μm and is arranged in the depth direction in the figure (direction perpendicular to the paper surface, hereinafter referred to as X direction). Furthermore, in order to protect the copper plating layer 20 when removing the above-mentioned base layer 17 (titanium thin film 16 and copper thin film 18) by metal etching, a nickel plating layer 22 (which serves as a magnetic conductor layer) is formed on the copper plating layer 20. Thickness 1μ
m) is formed (see FIG. 2B).

Next, the resist 19 is dissolved and removed, and then metal etching is performed to remove unnecessary portions of the titanium thin film 16 and the copper thin film 18 formed except the lower part of the copper plating layer 20 (see FIG. 2C). . This nickel plating layer 2
2 protects the copper plating layer 20 during the above-described metal etching, and further, when forming a polyimide insulating layer,
This prevents the copper plating layer 20 from coming into contact with the polyimide precursor and chemically reacting with the polyimide precursor to increase the resistance value of the copper plating layer. The nickel forming the nickel plating layer 22 is excellent in corrosion resistance and is inexpensive as described above, but since it is a magnetic material as described above in the problems of the prior art, the inductance of the signal wiring increases at high frequencies. To do. In addition, the resistivity is higher than that of copper, and the resistance of the signal wiring also increases.

Therefore, as shown in FIG. 3A, gold is plated as a non-magnetic conductor layer on the nickel plated layer 22 to form a 2 μm gold plated layer 24.
The signal wiring 26X composed of three layers of the copper plating layer 20, the nickel plating layer 22, and the gold plating layer 24 is completed.
Thereafter, the polyimide precursor is applied in the same manner as described above, and then heated and cured to form a polyimide insulating layer 1 having a thickness of 16 μm.
4b is formed (see FIG. 3B).

As shown in FIG. 4, the polyimide insulating layer 1
The above steps are repeated on 4b to form a copper plating layer 20, a nickel plating layer 22, and a gold plating layer 24, and complete a signal wiring 26Y having a width of 25 μm in the horizontal direction (hereinafter referred to as Y direction) in the drawing. . After the polyimide insulating layer 14c (thickness 16 μm) is further formed on the signal wiring 26Y, the ground layer 12b is formed by copper on the uppermost part. This first
In the embodiment, the ground layer 12 is formed above and below the signal wirings 26X and 26Y through the polyimide insulating layers 14a and 14c.
By arranging a and 12b, a dual strip line type signal transmission structure is formed.

Next, with respect to the circuit board shown in FIG. 4, the result of simulating the transmission loss of the signal wiring with respect to the frequency when a signal is applied to the signal wirings 26X and 26Y will be described with reference to FIGS. 5 and 6. . FIG. 5 is a log-log graph in which the vertical axis represents transmission loss (dB / cm) and the horizontal axis represents frequency (GHz). In the figure, a circle indicates a circuit board of the present embodiment, and for comparison, a circuit board in which the signal wirings 26X and 26Y are composed of the above-described copper plating layer and nickel plating layer according to the related art is shown. ● indicates a circuit board made of gold only and Δ shows a circuit board made of copper only.

FIG. 6 is a table showing the thicknesses of the sputter layer and the plating layer constituting the signal wiring. Here, for comparison, the total thickness of copper or gold constituting the main conductor is unified to 5 μm. For example, in the circuit board of the present embodiment, the signal wiring has the copper plating layer 20 of 3 μm, the nickel plating layer 22 of 1 μm, and 2 μm as described above.
m of gold plating layer 24. In the structure of the prior art indicated by a black circle, which is composed of a copper plating layer and a nickel plating layer, a 5 μm copper plating layer is covered with a 1 μm nickel plating layer. In addition, between the copper-plated layer or the gold-plated layer and the insulating layer, a titanium thin film 1
6 and the copper thin film 18 of 5000 Å are formed in the simulation. Further, each of the polyimide insulating layers 14a, 14b and 14c for insulating the signal wirings 26X and 26Y shown in FIG. 4 has a thickness of 16 μm.
The width of the signal wiring is 25 μm, and the relative permittivity of polyimide is 3.2.

As can be seen from FIG. 5, the signal wiring 26X,
26A is composed of only gold, Δ is composed of only copper, and ○ is related to the structure of the copper plating layer, the nickel plating layer, and the gold plating layer of this embodiment. It shows almost equal transmission loss characteristics from a low frequency to a relatively high frequency of 10 GHz. On the other hand, the mark ● related to the structure including the copper-plated layer and the nickel-plated layer of the prior art has a large loss loss as the frequency becomes higher. For example, at 1 GHz, the circuit board of the present embodiment has 0.3 dB / cm,
The circuit board of the prior art is about 2 times at 0.6 dB / cm.
Also, at 10 GHz, this embodiment has 0.9 dB / cm.
In contrast, the prior art circuit board is 2.5 dB /
It is nearly tripled in cm.

As described above, the conventional circuit board has a large transmission loss at high frequencies. As described above with reference to FIG. 8C, when a high-frequency signal is applied to the signal wiring formed of the copper plating layer 120 and the nickel plating layer 122, the current is generated below the copper plating layer 120 due to the skin effect.
Mainly flows to the nickel plating layer 122. Therefore, it is presumed that loss occurs in the nickel plating layer 122 having higher inductance and resistance than the copper plating layer or the like.

On the other hand, in the circuit board of this embodiment, the gold plating layer 24 having a thickness of 2 μm is formed on the nickel plating layer 22.
(See FIG. 4). Therefore, when a high frequency signal is applied, due to the skin effect, the gold plating layer 24 located mainly on the upper side and the copper plating layer 20 having a thickness of 3 μm located on the lower side.
It is considered that a transmission loss due to the nickel plating layer 22 does not occur because a current flows through the nickel plating layer 22 of the magnetic material located in the middle and almost no current flows. Therefore, as shown in FIG. 5, the circuit board of this embodiment has no nickel plating layer and has a signal wiring of 5 .mu.m formed only by gold shown by .DELTA. In FIG. It is considered that the same transmission loss characteristics as those obtained when the signal wiring of 5 μm was formed by only the above were obtained.

Further, as described above, the transmission loss at high frequencies is due to the skin effect. Therefore, the thicker the non-magnetic metal layer (gold-plated layer 24) is, the better. However, it is desirable that the non-magnetic metal layer (gold plated layer 24) is formed thicker than the skin depth (skin depth) so that most skin current can flow therein. . The skin thickness δ is expressed by the following equation, where μ is the magnetic permeability of the nonmagnetic metal used, σ is the electrical conductivity, and ω is the angular frequency.

## EQU1 ## δ = √ (2 / μσω) Here, the value of the skin depth δ when forming a circuit board for 10 GHz by using gold as a non-magnetic metal is as follows:
Substituting the magnetic permeability and electrical conductivity of gold into the formula
4 μm is obtained. Therefore, the circuit board of the first embodiment is
When used at 0 GHz, the thickness of the gold plating layer 24 is set to 0.
It is desirable to form it to 784 μm or more. In addition, in the above-described first embodiment, three layers of polyimide insulating layers 14a,
Although the example in which the circuit board is configured by 14b and 14c has been described, it is also possible to form the circuit board by using a multilayered polyimide insulating layer.

Next, the manufacturing process of the circuit board according to the second embodiment of the present invention will be described with reference to FIG. In the second embodiment, first, as shown in FIG. 7A, a ground layer 12a made of copper is deposited on the ceramic substrate 10. Next, on the ground layer 12a, a thickness of 16μ
The m benzocyclobutene insulating layer 54a is formed. Then, titanium having excellent adhesion to benzocyclobutene is sputtered on the surface of the benzocyclobutene insulating layer 54a to form a titanium thin film 16 (thickness 1000Å), and further, on the surface of the titanium thin film 16, Copper plating layer 2
Copper for obtaining adhesiveness with 0 is sputtered to form a copper thin film 18 (thickness 5000Å), which is used as a base layer 17.

Then, as in the case of the first embodiment (see FIGS. 2A and 2B), a resist layer having a predetermined pattern is provided by the photolithography technique, and then electroless is formed at the position where the signal wiring is formed in the circuit board. A copper plating layer 20 (thickness 3 μm) to be a copper wiring layer is formed by plating. After that, when the resist is removed and the titanium thin film 16 and the copper thin film 18 are removed by metal etching, the copper plating layer 2
In order to protect 0, chromium is sputtered on the copper plating layer 20 to form a magnetic conductive layer, which is a chromium sputter layer 7
2 (thickness 1 μm) is formed. Then, metal etching is performed to remove unnecessary portions of the titanium thin film 56 and the copper thin film 18 other than the copper plating layer 20. Chromium forming the chromium sputter layer 72 is excellent in corrosion resistance and is inexpensive as described above, but since it is a magnetic material, the inductance increases in the high frequency signal region.

Therefore, platinum serving as a non-magnetic conductive layer is plated on the chromium sputter layer 72 to form a platinum plating layer 74 (thickness 2 μm) from the non-magnetic conductor layer, whereby a copper plating layer is formed. The signal wiring 26X composed of three layers of 20, the chromium sputter layer 72, and the platinum plating layer 74 is completed. After that, a benzocyclobutene insulating layer 54b having a thickness of 16 μm is further formed (see FIG. 7B). The circuit board is completed by repeating the above processing.

Also in the circuit board of the second embodiment, due to the skin effect, a current flows mainly through the upper 2 μm platinum plating layer 74 and the lower 3 μm copper plating layer 20, and is positioned in the middle. Since it is difficult for an electric current to flow in the chromium sputter layer 72 of the magnetic material, it is presumed that transmission loss due to the chromium sputter layer 72 does not occur.

As the material for forming the insulating layer of the circuit board, polyimide having a high heat resistance characteristic is used in the first embodiment, and benzocyclobutene having a low dielectric constant characteristic is used in the second embodiment. Although examples have been given, it is possible to use, for example, an inexpensive epoxy resin or another material as long as it is an organic polymer material having a lower dielectric constant than ceramics and capable of forming a circuit board.
In addition, nickel is used as the magnetic conductor layer serving as a protective layer at the time of metal etching of the copper plating layer 20 in the first embodiment.
Although chromium is used in the examples, for example, Ni-Co, Ni
As long as it can protect the copper plating layer 20 such as -P, Ni-B and Cr-P and prevent the reaction of the non-magnetic conductor layer such as gold with the underlying copper, an alloy containing nickel and chromium as main components,
Alternatively, other metals can be used.

Further, as a non-magnetic conductor layer on the magnetic conductor layer, a gold plating layer 24 is used as the second metal in the first embodiment.
Although the platinum-plated layer 74 is formed in the embodiment, if the metal has high adhesion to the magnetic conductor layer and does not react with the organic polymer material or its precursor when the insulating layers 14 and 54 are applied and cured, In addition to gold and platinum, for example, noble metals such as silver, palladium and rhodium, or Ag-Pd, gold, platinum,
It is also possible to use an alloy containing silver, palladium, or rhodium as a main component. Further, in the first and second embodiments, the copper wiring layer, the magnetic conductor layer and the non-magnetic conductor layer are provided by electrolytic plating, electroless plating and sputtering, but other means such as vapor deposition, ion plating, etc. You may form using a method. Further, in the above-described first and second embodiments, when the copper wiring layer and the magnetic conductor layer were provided, the resist was removed and the underlayer was removed by etching, and then the nonmagnetic conductor layer such as gold was provided. The underlayer may be removed by etching after the nonmagnetic conductor layer is provided before the removal.

The circuit boards of the first and second embodiments are used for an oscillator circuit for operating at a high frequency of GHz band in satellite communication or the like, a filter, and an integrated circuit driven by a clock signal of a very high frequency. It can also be used as a substrate or the like.

[0037]

As described above, in the circuit board of the present invention, when the conductor film of the magnetic material is used to protect the copper wiring layer, even if the inductance of the magnetic conductor layer increases when used at a high frequency. , Due to the skin effect, a current flows through the non-magnetic conductor layer located at the top and the copper wiring layer located at the bottom, and it is difficult for the current to flow in the magnetic conductor layer located in the middle. It is possible to prevent an increase in the transmission rate and a decrease in the transmission rate.

[Brief description of drawings]

FIG. 1 is an illustration showing a manufacturing process of a circuit board according to a first embodiment of the present invention.

FIG. 2 is an illustration showing a manufacturing process of the circuit board according to the first exemplary embodiment of the present invention.

FIG. 3 is an illustration showing a manufacturing process of the circuit board according to the first exemplary embodiment of the present invention.

FIG. 4 is a cross-sectional view showing the configuration of the circuit board according to the first example.

FIG. 5 is a graph showing the relationship between frequency and transmission loss between the circuit board according to the first embodiment and the circuit board of the related art.

FIG. 6 is a table showing the thicknesses of a sputter layer and a plating layer on the circuit board according to the first embodiment and the circuit board according to the related art.

FIG. 7 is an illustration showing a manufacturing process of a circuit board according to a second embodiment of the present invention.

FIG. 8 is a cross-sectional view showing a configuration of a conventional circuit board.

[Explanation of reference signs] 14a Polyimide insulating layer 20 Copper plating layer 22 Nickel plating layer 24 Gold plating layer 54a Benzocyclobutene insulating layer 72 Chromium sputter layer 74 Platinum plating layer

Claims (5)

[Claims] 1. A plurality of insulating layers made of an organic polymer material, a signal wiring formed between the plurality of insulating layers, and a copper wiring layer formed on the insulating layers and containing copper as a main component. A magnetic conductor layer formed on the copper wiring layer and containing a magnetic material as a main component and having corrosion resistance; a non-magnetic conductor layer formed on the magnetic conductor layer and containing a non-magnetic material as a main component and having corrosion resistance; A circuit board comprising: 2. The circuit board according to claim 1, wherein the thickness of the nonmagnetic conductor layer is equal to or larger than a skin thickness defined by a frequency of a signal applied to the signal wiring. . 3. The circuit board according to claim 1, wherein the organic polymer material is made of any one of polyimide resin, penzocyclobutene resin, and epoxy resin. 4. The circuit board according to claim 1, wherein the magnetic conductor layer contains nickel, chromium, or an alloy thereof as a main component. 5. The circuit board according to claim 1, wherein the non-magnetic conductor layer contains silver, gold, platinum, palladium, rhodium, or an alloy thereof as a main component. .