Method for forming wiring on high-frequency characteristic substrate

Abstract

The present invention comprises a direct plating step for precipitating Au as an Au plating layer (4) on the surface of Cu wiring by displacement plating using an Au plating solution, wherein in the direct plating step, an affected layer (3) having electrical resistance greater than that of Au is formed between the Cu wiring (2) and the Au plating layer (4) through a chemical reaction between non-metal ions in the Au plating solution and Cu ions dissolved in the Au plating solution, and the total thickness of the affected layer (3) and the Au plating layer (4) is not more than 0.45 μm.

Description

Method for forming wiring on a high-frequency characteristic substrate

[0001] The present invention relates to a method for forming wiring on a high-frequency characteristic substrate.

[0002] A method of performing direct electroless gold plating on copper wiring of a printed wiring board is known (see, for example, Patent Document 1). On the other hand, in recent years, with the improvement of the performance of electronic devices, it has been required that the wiring of the substrate has high-frequency characteristics. Since high-frequency current has the characteristic of passing through the outer side (skin side) of the wiring, it is common to obtain good high-frequency characteristics by disposing highly conductive gold here.

[0003] Therefore, as in Patent Document 1, after performing displacement gold plating, reduction gold plating is performed to obtain the thickness of gold.

[0004] Patent No. 6803944 Specification

[0005] However, since gold plating uses gold, it is costly. Therefore, it is required to reduce the amount of gold used for gold plating as much as possible.

[0006] The present invention is considered in view of the above prior art, and an object thereof is to provide a method for forming wiring on a high-frequency characteristic substrate that can reduce the amount of gold used for gold plating and obtain high-frequency characteristics.

[0007] To achieve the above object, the present invention includes a direct plating step of depositing Au as an Au plating layer on the surface of Cu wiring by displacement plating with an Au plating solution. In the direct plating step, a non-metallic ion in the Au plating solution and a Cu ion dissolved in the Au plating solution are chemically reacted to form an influence layer having a higher electrical resistance than Au between the Cu wiring and the Au plating layer. A method for forming wiring on a high-frequency characteristic substrate is provided, wherein the total thickness of the influence layer and the Au plating layer is 0.45 μm or less.

[0008] Preferably, the thickness of the Au plating layer is 0.005 μm or more and 0.055 μm or less.

[0009] Preferably, dissolved oxygen is contained in the Au plating solution.

[0010] Preferably, the nonmetallic ion is O ２－ That is the case.

[0011] According to the present invention, since the total thickness of the influencing layer and the Au plating layer is 0.45 μm or less, this total thickness covers the skin depth through which high-frequency current flows. Therefore, the thickness of the Au plating layer can be reduced while maintaining good high-frequency characteristics.

[0012] Furthermore, since the thickness of the Au plating layer is between 0.005 μm and 0.055 μm, a thin Au plating layer that does not incur costs is sufficient under current circumstances. In other words, wiring with high-frequency characteristics can be formed by forming only an inexpensive Au plating layer.

[0013] Furthermore, because dissolved oxygen is present in the Au plating solution, this causes oxygen to be released into the Au plating solution. ２－ As a result, an influence layer is formed of CuO. Experiments have confirmed that high-frequency characteristics can be maintained even when a high-frequency current flows through such an influence layer.

[0014] This is an explanatory diagram of the wiring formation method for a high-frequency characteristic substrate according to the present invention. This is an explanatory diagram of the wiring formation method for a high-frequency characteristic substrate according to the present invention. This is an explanatory diagram of the wiring formation method for a high-frequency characteristic substrate according to the present invention. This is a table showing the frequency band of the high-frequency current and the skin depth of the Au through which the current passes. This is an explanatory diagram showing a cross-section of the wiring portion when a high-frequency current of 100 GHz is passed through. This is an explanatory diagram showing a cross-section of the wiring portion when a high-frequency current of 30 GHz is passed through. This is a graph showing the relationship between the frequency band of the high-frequency current and the transmission loss.

[0015] As shown in Figures 1 to 3, the wiring formation method for a high-frequency characteristic substrate according to the present invention forms the wiring of a printed circuit board as follows. As shown in Figure 1, first, a substrate 1 is prepared on which Cu wiring 2, which will be the wiring pattern, is formed. Conventional pattern formation techniques are used to form this Cu wiring 2. Then, direct plating is performed on this Cu wiring 2 by displacement plating. In this direct plating process, an Au plating solution containing Au is used as the plating solution. Since this is so-called direct Au plating, Au is deposited directly onto the surface of the Cu wiring 2, which is the base material. Because it is displacement plating, the surface of the underlying Cu wiring 2 dissolves as Cu ions, and the electrons released at that time combine with Au ions in the plating solution, causing Au to be deposited on the surface of the Cu wiring 2.

[0016] In this process, dissolved oxygen is present in the Au plating solution. During displacement plating, dissolved oxygen is O ２－This is what happens. These are nonmetallic ions, which react chemically with Cu ions dissolved in the Au plating solution. Specifically, as shown in Figure 2, copper oxide is formed on the surface of the Cu wiring 2. This has a higher electrical resistance than Au and acts as an influence layer 3. Then, as shown in Figure 3, the Au plating layer 4 is formed outside this influence layer 3. Therefore, although the intention is to form the Au plating layer 4 directly on the surface of the Cu wiring 2, in reality, an influence layer 3 is formed between the Cu wiring 2 and the Au plating layer 4. In this regard, the inventors have confirmed through ESCA (Electron Spectroscopy for Chemical Analysis) analysis that no matter what conditions are used for displacement plating, a rapid displacement reaction occurs somewhere, and an influence layer 3 is formed. In particular, the presence of dissolved oxygen in the Au plating solution is the cause of the rapid displacement reaction, and this causes the influence layer 3, which is copper oxide, to form between the Cu wiring 2 and the Au plating layer 4. In Figures 2 and 3, for convenience, the influence layer 3 is shown to be formed over the entire outer surface of the Cu wiring 2, but in reality, the influence layer 3 is formed locally, and its thickness is not uniform over the entire surface. As mentioned above, the Au plating layer 4 is deposited by displacement plating, but reduction plating may be used afterward to give the Au plating layer 4 thickness. Therefore, a displacement-reduction plating solution may be used as the plating solution. In the present invention, displacement plating is used, and it is sufficient if the influence layer 3 described above is formed at that time.

[0017] Furthermore, this influencing layer 3 has a higher electrical resistance than the Au plating layer 4. Therefore, there are concerns that it may interfere with high-frequency characteristics when high-frequency current flows through it. Here, since it is known that high-frequency current passes on the outside (skin side) of the wiring, Figure 4 shows how deep the high-frequency current passes through the skin relative to Au. As shown in the figure, the skin depth through which the high-frequency current passes becomes shallower as the frequency increases. In particular, at 30 GHz it is 0.45 μm, and as the frequency increases from there, the skin depth becomes even shallower, reaching 0.24 μm at 100 GHz. The present invention targets this frequency band. Therefore, in this specification, high-frequency current means 30 GHz or higher, more specifically, 30 GHz to 100 GHz.

[0018] For high-frequency currents, the current passes through a thin skin depth, so to reduce transmission loss due to increased electrical resistance, the Au plating layer 4 is usually set to a thickness that can cover this area. However, for skin depths of 0.45 μm at 30 GHz and 0.24 μm at 100 GHz, the amount of Au used is still quite large, and making the Au plating layer 4, which is made from Au, to this thickness is costly. Therefore, in the wiring formation method for a high-frequency characteristic substrate according to the present invention, the total thickness of the influence layer 3 and the Au plating layer 4 is set to 0.45 μm or less, which covers the skin depth through which high-frequency currents flow, and within that, the thickness of the Au plating layer 4 is set to 0.005 μm or more and 0.055 μm or less. As a result, the total thickness covers the skin depth through which high-frequency currents flow. Therefore, it is possible to reduce the thickness of the Au plating layer 4 while maintaining good high-frequency characteristics. Furthermore, since the thickness of the Au plating layer 4 is between 0.005 μm and 0.055 μm, a thin Au plating layer 4 that does not incur additional costs is sufficient under current circumstances. In other words, wiring with high-frequency characteristics can be formed by forming only the cost-effective Au plating layer 4. The thickness of this Au plating layer 4 is deliberately kept within a range that does not cover the skin depth of 100 GHz or 30 GHz.

[0019] As shown in Figure 5, when a high-frequency current of 100 GHz is applied, the Au plating layer 4 is between 0.005 μm and 0.055 μm thick. For example, if the maximum thickness is 0.055 μm, the high-frequency current will pass through the influence layer 3 to a depth of 0.185 μm (outside the dotted line in the figure). Also, as shown in Figure 6, when a high-frequency current of 30 GHz is applied, the combined thickness of the influence layer 3 and the Au plating layer 4 is less than 0.45 μm. Therefore, the high-frequency current will pass outside the point that roughly coincides with the outer edge of the Cu wiring 2, that is, through almost the entire influence layer 3 and the Au plating layer 4 (outside the dotted line in the figure). In other words, in the frequency band of the high-frequency current targeted by the present invention, the current will always pass through the influence layer 3 and the Au plating layer 4.

[0020] Here, two types of Au plating layers 4 were created with thicknesses of 0.055 μm and 0.550 μm, and the relationship between transmission loss and frequency bandwidth was investigated. In Figure 7, the vertical axis shows the state of transmission loss, with lower values ​​indicating less current flow (higher loss), and the horizontal axis shows the frequency bandwidth. The dashed line (upper) of the graph shows the Au plating layer 4 with a thickness of 0.550 μm (comparative example), and the solid line (lower) shows the Au plating layer 4 with a thickness of 0.055 μm (example). As shown in the graph, the transmission loss increases with increasing frequency bandwidth, but in both cases, the loss is around -0.4 dB even at 30 GHz and around -1.2 dB even at 100 GHz, indicating good results. This is not significantly different from when the thickness of the Au plating layer 4 was set to 0.24 μm (corresponding to 100 GHz) or 0.45 μm (corresponding to 30 GHz) so that high-frequency current flows only through the Au plating layer 4. Thus, it has been shown that, rather than making the Au plating layer 4 thicker to correspond to the skin depth through which high-frequency current flows, it is possible to reduce costs by making the Au plating layer 4 thinner while minimizing transmission loss in the influencing layer 3.

[0021] Furthermore, the difference in transmission loss between Au plating layer 4 with a thickness of 0.550 μm and 0.055 μm is only about 0.05 dB at both 30 GHz and 100 GHz. Therefore, the thickness of Au plating layer 4 does not particularly affect transmission loss within this range. In other words, if the thickness of Au plating layer 4 is in the range of 0.005 μm to 0.055 μm, there is not much difference in transmission loss for high-frequency currents. If the goal is to reduce costs by reducing the amount of Au used, the thickness of the gold plating layer 4 should be made as thin as possible within this range. Thus, instead of focusing solely on the Au plating layer 4 as the region through which high-frequency currents flow, even if an influence layer 3 is used, the cost benefits from reducing the amount of Au used outweigh the increased transmission loss. The present invention demonstrates that even with an influence layer 3 (copper oxide) formed by a chemical reaction between non-metallic ions and Cu ions, the amount of Au used can be reduced without significantly losing high-frequency characteristics.

[0022] Furthermore, since the thickness of the Au plating layer 4 is set to 0.005 μm or more and 0.055 μm or less, in the case of 30 GHz, which is the deepest skin depth among the high-frequency currents of 30 GHz to 100 GHz targeted by the present invention, the influence layer 3 and the Au plating layer 4 alone cover the 0.45 μm skin depth. Therefore, the thickness of the influence layer 3 is set to 0.395 μm or more and 0.445 μm or less.

[0023] 1: Substrate, 2: Cu wiring, 3: Influence layer, 4: Au plating layer

Claims

1. A method for forming wiring on a high-frequency characteristic substrate, comprising a direct plating step of depositing Au as an Au plating layer on the surface of a Cu wiring by displacement plating using an Au plating solution, wherein in the direct plating step, a chemical reaction is performed between the Cu wiring and the Au plating layer to form an influence layer having greater electrical resistance than Au by chemically reacting nonmetallic ions in the Au plating solution with Cu ions dissolved in the Au plating solution, and the total thickness of the influence layer and the Au plating layer is 0.45 μm or less.

2. The method for forming wiring on a high-frequency characteristic substrate according to claim 1, characterized in that the thickness of the Au plating layer is 0.005 μm or more and less than 0.055 μm.

3. The method for forming wiring on a high-frequency characteristic substrate according to claim 1, characterized in that dissolved oxygen is contained in the Au plating solution.

4. The nonmetallic ions are O ２－ The method for forming wiring on a high-frequency characteristic substrate according to claim 1, characterized in that it is the same.

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