JP2012046781A - Copper plating method of polytetrafluoroethylene substrate for high frequency circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily form a homogeneous and dense copper plating film on the surface of a hydrophobic PTFE (polytetrafluoroethylene) substrate for a high frequency circuit.SOLUTION: In a plating method, following processes are successively performed: a process (1) of subjecting a PTFE substrate to a plasma treatment in the presence of helium gas; a process (2) of forming a self-assembled monolayer (SAM) on the surface of the substrate by reacting the substrate with a silane coupling agent; a process (3) of subjecting the substrate having the formed SAM thereon to an activation treatment using a catalyst solution for electroless plating, which is an aqueous solution containing a palladium salt; a process (4) of reducing palladium ions fixed on the substrate into metal palladium; a process (5) of forming an electroless copper plating film on the substrate by treating the resulting substrate with an electroless copper plating solution; and a process (6) of further forming an electrolytic copper plating film on the electroless copper plating film by treating the resulting substrate with an electrolytic copper plating solution.

Description

  The present invention relates to a copper plating method for forming a uniform and dense copper plating film on a polytetrafluoroethylene (PTFE) substrate for a high frequency circuit.

  Printed circuit boards incorporated in transceivers such as satellite broadcasting or ITS (Intelligent Transport Systems) related equipment using a high frequency region of several tens of gigahertz band need excellent high frequency and low loss characteristics, and the substrate material is dielectric. PTFE, which is a fluororesin having excellent characteristics, is suitable. PTFE has a dielectric constant of 2.1 and a dielectric loss tangent of 0.0002, which is the best dielectric property among fluororesin materials, and also has excellent heat resistance and chemical resistance. In addition, PTFE is flexible and can be greatly deformed, so that it becomes a flexible printed circuit board.

  When a circuit is formed on a PTFE substrate, it is necessary to form a copper film having a thickness of several μm or less on the substrate surface. As a conventional method for forming such a copper film, there is a subtractive method in which an unnecessary portion is removed from a substrate having a copper foil (thickness: 18 μm) on the entire surface to leave a circuit. However, in order to achieve higher frequency and higher-density mounting of the transceiver in the millimeter wave band, it is possible to realize a fine pitch circuit board in which the circuit pattern wiring width / wiring interval is reduced to 10 μm / 10 μm or less. This is difficult with the subtractive method.

  Therefore, a copper film (thickness 0.1 to 1 μm) of the metal conductor is formed directly on the substrate surface, and the conductor portion where the copper pattern in the resist pattern is not to be formed is removed by etching, and the remaining circuit portion is subjected to electrolytic copper plating. A method (semi-additive method) for forming a circuit pattern by applying (thickness of several μm) has been proposed. However, at present, a copper film having a thickness of 1 μm or less is formed by a dry film forming method such as sputtering, which causes a decrease in process efficiency and a high cost.

  In order to solve these problems, it is desirable to employ an electroless copper plating process, which is a wet method, for mass production of a copper coating. However, since PTFE has high chemical stability, it is extremely difficult to adhere to other substance groups, and it is impossible to obtain strong adhesion even if the surface is subjected to electroless plating of copper. It is. Therefore, an appropriate pretreatment is required to functionalize the PTFE surface. Furthermore, from the viewpoint of high frequency response characteristics, it is desirable that the unevenness at the interface between the PTFE substrate and the copper coating is as small as possible.

  In order to improve the adhesion between the substrate and the copper coating, the surface of the fluororesin is formed by chemical etching with molten metal, which has a high environmental load, and then the plating layer is laminated to physically recycle the substrate. It is common to bond a copper film (copper plating film).

  Here, in Patent Document 1, after making the PTFE surface hydrophilic by an ultraviolet lamp or plasma treatment, a copper sulfate (—C—O—Cu) is formed by irradiating a copper sulfate aqueous solution on the PTFE surface with a vacuum ultraviolet laser. Techniques for purification are disclosed. Patent Document 2 discloses a surface modification method for the surface of a fluoropolymer molded product in which a PTFE substrate surface is irradiated with a vacuum ultraviolet laser in the presence of hydrazine vapor to introduce a hydrophilic group such as an amino group. . Patent Document 3 discloses an acrylic polymer in which gas phase polymerization is caused by vapor plasma of an acrylic monomer on a fluororesin substrate under an inert gas plasma atmosphere and atmospheric pressure by corona discharge, and graft polymerization is performed on the surface of the fluororesin substrate. Disclosed is a method for producing a surface-coated fluororesin substrate characterized in that a resin layer is obtained.

  On the other hand, there is a technique in which a self-assembled monomolecular film (SAM) is formed on a Si substrate, and a metal ion complex is chemically adsorbed on the amino group of the terminal functional group that is the outermost layer. In Non-Patent Document 1, a SAM is formed by immersing a Si substrate in an alcohol solution of (3-aminopropyl) ethoxysilane (APS), and then immersed in a palladium ion dispersion solution, whereby palladium ions are converted into SAM. A method of chemisorbing to a primary amino group of a terminal functional group is disclosed. Non-Patent Document 2 discloses that a SAM is formed by reacting the surface of a Si substrate with 3- (2-aminoethyl) -aminopropyltrimethoxysilane (AEAPS) vapor, and then immersed in a ruthenium ion dispersion. A method is disclosed in which a ruthenium ion is chemically adsorbed between a primary amino group and a secondary amino group of a terminal functional group of SAM to form a metal ion complex.

JP 2004-182516 A Japanese Patent Laid-Open No. 6-192452 JP 2008-19393 A

Lina, Xu: Thin Solid Films, 434,121 (2003). Takahiro Moriguchi: Proceedings of the Joint Conference on Applied Physics, 54, 3, 1283 (2007).

  The high frequency current is concentrated on the surface of the copper coating due to the skin effect. Therefore, from the viewpoint of high frequency response characteristics, the interface between the copper film and the substrate, which is the lower layer, is also required to be smooth. However, as disclosed in Patent Document 1 or 2, when the PTFE substrate is irradiated with a high energy density laser, the surface of the PTFE is inevitably generated. Moreover, since the processing cost is high, it lacks mass productivity.

  Even by corona discharge disclosed in Patent Document 3, it is considered that irregularities are generated on the surface of the fluororesin. Further, in the technique disclosed in Patent Document 3, an acrylic resin layer graft-polymerized on a fluororesin is formed. Such an acrylic resin layer is a functional group on the surface of the fluororesin randomly generated by corona discharge. It is considered to be in a chemically bonded state. In addition, the acrylic resin layer and the copper plating film are bonded only by physical effects without chemical bonding, and it is difficult to say that it is preferable as a lower layer for forming a dense copper plating film. Furthermore, with the method disclosed in Patent Document 3, it is difficult to perform circuit patterning before electroless copper plating.

  An object of the present invention is to provide a copper plating method for easily forming a homogeneous and dense copper plating film on the surface of a PTFE substrate which is a hydrophobic substrate for a high frequency circuit.

  The inventors of the present invention have continued intensive studies to solve the above problems. As a result, the PTFE surface is hydrophilized by plasma treatment and reacted with an aminosilane coupling agent to form a SAM, and then electroless copper plating and electrolytic copper plating are performed to obtain a uniform and dense copper plating film. As a result, the present invention has been completed.

A step (1) of plasma-treating the PTFE substrate in the presence of helium gas;
Reacting the substrate with an aminosilane coupling agent to form a self-assembled molecular film on the surface of the substrate (2);
A step (3) of activating the substrate on which the self-assembled molecular film is formed with an electroless plating catalyst solution that is an aqueous solution containing a palladium salt;
Reducing palladium ions immobilized on the substrate to metallic palladium (4);
A step (5) of forming an electroless copper plating film on the substrate by treating the substrate with an electroless copper plating solution;
A step (6) of further forming an electrolytic copper plating film on the electroless copper plating film by treating the substrate with an electrolytic copper plating solution;
The present invention relates to a copper plating method for a PTFE substrate for a high frequency circuit.

  The aminosilane coupling agent is preferably 3- (2-aminoethyl) -aminopropyltrimethoxysilane (AEAPS).

  In the step (1), the helium gas preferably contains methanol vapor or ethanol vapor in the range of 0.1% by volume to 2% by volume.

  Prior to the step (3), the step of sensitizing the substrate with an aqueous solution containing a tin salt may be omitted.

  According to the copper plating method of the PTFE substrate for high-frequency circuits of the present invention, it is possible to form a copper plating film having a uniform thickness on the PTFE substrate surface by an easy method with a small environmental load, which was impossible in the past. is there.

FIG. 1 is a conceptual diagram illustrating a copper plating method for a high-frequency circuit PTFE substrate according to the present invention. FIG. 2 is a schematic configuration diagram of the plasma processing apparatus. FIG. 3: shows the conceptual diagram explaining the process (2) of Examples 1-3. FIG. 4 is a diagram for explaining the step (2), where (a) shows hydrolysis of AEAPS molecules, and (b) shows SAM formation of AEAPS on the surface of the PTFE substrate. FIG. 5 is a spectrum showing the result of X-ray photoelectron spectroscopy (XPS) analysis of the test substrate after step (1) of Example 1. FIG. 6 is a spectrum showing the XPS analysis result of the test substrate after step (1) in Example 2. FIG. 7 is a spectrum showing the XPS analysis result of the test substrate after step (1) in Example 3. FIG. 8 is a spectrum showing the XPS analysis result of the test substrate after the step (1) of Comparative Example 1. FIG. 9 is a spectrum showing the result of XPS analysis of the test substrate of Example 2 after the step (2). FIG. 10 is a photograph of the test substrate surfaces of Example 2 and Comparative Example 2 after step (5). FIG. 7 (a) is Example 2 and FIG. 7 (b) is Comparative Example 2. FIG. 11 is a spectrum showing the XPS analysis result of the test substrate of Comparative Example 2 after the step (5). FIG. 12 is a spectrum showing the XPS analysis result of the portion of the test substrate of Example 2 where the electroless copper plating film was formed.

  Embodiments of the present invention will be described with reference to the drawings as appropriate. The present invention is not limited to the following description.

  FIG. 1 is a conceptual diagram illustrating a copper plating method for a high-frequency circuit PTFE substrate according to the present invention. In the present invention, first, the PTFE surface is plasma-treated at atmospheric pressure in the presence of helium (He) gas (step (1)). By performing plasma treatment, hydroxyl groups and the like are introduced to the surface of PTFE to make it hydrophilic (FIG. 1 (b)). The PTFE surface after plasma treatment is smooth, unlike the PTFE surface after laser irradiation or corona discharge treatment.

  Next, the substrate is reacted with an aminosilane coupling agent such as AEAPS (step (2)). Then, the aminosilane coupling agent forms SAM on the substrate surface (FIG. 1 (c)).

Next, the substrate on which the SAM is formed is activated with an electroless plating catalyst solution that is an aqueous solution containing a palladium salt (step (3)). Then, palladium ions (Pd ²⁺ ) are chemisorbed as metal ion complexes by the terminal amino group of SAM (FIG. 1 (d)). Here, since the highly basic secondary amino group has a stronger chemisorption effect than the primary amino group, the metal ion complex formed between the primary amino group and the secondary amino group of AEAPS is used as an aminosilane coupling agent. Compared with the case of using shampoo, it is considered that it does not fall off easily by rinsing or ultrasonic cleaning.

Next, the palladium ions immobilized on the substrate are reduced to metallic palladium using a reducing agent (step (4)). Further, the substrate is treated with an electroless copper plating solution (step (5)). Then, copper ions are reduced by palladium ions, and an electroless copper plating film is formed (FIG. 1 (e)). Since the electroless plating film is formed on the smooth PTFE surface, the thickness is uniform. Moreover, since the electroless plating film is chemically bonded to the PTFE surface via SAM, it has sufficient adhesion strength.
In addition, the electroless copper plating in a process (5) can use well-known methods like a commercially available reagent for electroless copper plating.

  The thickness of the electroless copper plating film is about 0.1 to 0.2 μm. However, the surface layer of the copper film on which high-frequency currents in the tens of gigahertz band are concentrated by the skin effect is 0.2 to 0.6 μm below the surface, so that the outermost surface layer is only the electroless copper plating film formed on the PTFE surface. There is a problem that cannot be configured. Moreover, impurities other than metallic copper are also contained in the copper coating, and there is a possibility that a loss in electrical resistance occurs. Therefore, finally, an electrolytic copper plating film is formed on the electroless copper plating film formed in the step (5) (step (6)). The electrolytic plating film is preferably about 1 to 5 μm thick.

[Example 1]
Process (1)
A PTFE film (Nitoflon No. 900UL: manufactured by Nitto Denko Corporation) having a thickness of 0.2 mm was cut into a size of 50 × 80 mm, and this cut piece was used as a test substrate. The test substrate was ultrasonically cleaned in acetone and then plasma treated at atmospheric pressure. For the plasma treatment, a capacitively coupled atmospheric pressure plasma generator shown in FIG. 2 was used. The conditions for the plasma treatment were as shown in Table 1.

  The capacitively coupled atmospheric pressure plasma generator used in this example is composed of a 13.56 MHz high frequency oscillation power supply 11, a matching circuit unit 12, an electrode unit, and a sample stage scanning stage (80 × 160 mm) 6. The electrode unit has a rod-like shape and has a structure in which an aluminum alloy rod 10 having a diameter of 3 mm is covered with an alumina insulating pipe 9 having an inner diameter of 3 mm and an outer diameter of 5 mm. Helium gas, which is a carrier gas, was supplied from the gas cylinder 1 through the path 2, the mass flow controller (MFC) 3, and the path 5 to the vicinity of the test substrate 7 set on the aluminum alloy sample stage 6.

  The MFC instantaneously compares the gas mass flow signal detected by the sensor unit with the flow rate setting signal and performs valve opening / closing adjustment so as to match, thereby controlling the gas flow rate. With helium gas present in the vicinity of the test substrate 7, high-frequency power was applied between the electrode unit and the gap between the aluminum alloy sample stage scanning stage 6 to generate plasma 8. This realizes glow plasma discharge under dielectric barrier discharge conditions. Then, the test substrate 7 was reciprocated within the plasma 8 a predetermined number of times. In Example 1, the gas concentration supplied to the vicinity of the test substrate was 100% by volume of helium gas. The test substrate 7 after the plasma treatment was ultrasonically cleaned in acetone.

Process (2)
As shown in FIG. 3, the test substrate 7 after cleaning and the beaker 24 containing the toluene-diluted AEAPS solution 25 are housed in a container 22 made of PFA (tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer). And sealed. The PFA container 22 was further housed in a stainless steel container 21 and sealed. Thereafter, the stainless steel container 21 was placed in an oven and heated to form an AEAPS SAM on the surface of the test substrate 7. The experimental conditions were as shown in Table 2. After the heating, the test substrate 7 was cleaned by immersing in 1 mol / L hydrochloric acid aqueous solution, distilled water, and 1 mol / L sodium hydroxide aqueous solution for 1 minute each, and finally ultrasonically cleaning with distilled water for 1 minute. After that, it was dried.

  In addition, it was confirmed that the same result as Example 1 was obtained also when it operated using only AEAPS instead of the toluene dilution AEAPS solution.

  AEAPS vapor is generated from the beaker 24 by heating. As shown in FIG. 4 (a), the methoxy group in the AEAPS molecule is hydrolyzed by reacting with moisture in the atmosphere and converted into a hydroxyl group. Thereafter, as shown in FIG. 4B, dehydration condensation occurs between the hydroxyl group on the surface of the test substrate 7 and the hydroxyl group formed in the AEAPS molecule, and the AEAPS molecule is bonded to the surface of the test substrate 7. Furthermore, dehydration condensation occurs between the hydroxyl groups of adjacent AEAPS molecules, and an AEAPS SAM is formed.

Step (3)
The cleaned test substrate was immersed in an activation treatment solution for electroless copper plating (activator solution: palladium chloride 0.1 to 0.3 g / L, hydrochloric acid concentration 1 to 3 mL / L) to perform an activation treatment. . The treatment time was 5 minutes and the treatment temperature was 25 ° C. The test substrate after the treatment was immersed in distilled water and washed.

Step (4)
The test substrate after washing was immersed in a 0.5 mol / L dimethylamine borane aqueous solution for 5 minutes. The treatment temperature was 25 ° C. The test substrate after the treatment was immersed in distilled water and washed.

Process (5)
The test substrate after washing was immersed in a commercially available electroless copper plating solution (OPC-750 electroless copper M: manufactured by Okuno Pharmaceutical Co., Ltd.) for 5 minutes. The treatment temperature was 40 ° C. The test substrate after the electroless plating treatment was immersed in distilled water, washed, and then dried.

[Example 2]
In step (1), the same operation as in Example 1 was carried out except that 1% by volume of methanol vapor was contained in the helium gas as the carrier gas to form an electroless copper plating film on the test substrate.

  A method of incorporating helium gas, which is a carrier gas, into methanol vapor, ethanol vapor, or water vapor will be described with reference to FIG. A part of the helium gas was branched from the path 2 and supplied to the path 13. The path 13 is connected to a solvent tank 15, and a solvent 16 (methanol, ethanol, or water) is stored therein. The helium gas is bubbled in the solvent 16 and then reaches the valve 4 via the path 17. Then, it is mixed with helium gas flowing through the path 5. The flow rate of methanol vapor, ethanol vapor, or water vapor contained in the helium gas after path 5 is adjusted using MFC3 and MFC14 so as to be 0.1 to 2% by volume.

  In Example 2, the solvent 16 is methanol, but in Example 3 described later, it is ethanol, and in Comparative Example 1, it is distilled water.

[Example 3]
In step (1), the same operation as in Example 1 was performed except that 1% by volume of ethanol vapor was contained in the helium gas as the carrier gas to form an electroless copper plating film on the test substrate.

[Comparative Example 1]
In step (1), the same operation as in Example 1 was carried out except that 1% by volume of water vapor was contained in the carrier gas, helium gas, to form an electroless copper plating film on the test substrate.

[Comparative Example 2]
All operations were the same as in Example 2 except that the step (4) was omitted. However, an electroless copper plating film could not be formed on the test substrate.

<Hydrophilization after atmospheric pressure plasma treatment>
XPS analysis was performed on the test substrates after Steps (1) of Examples 1 to 3 and Comparative Example 1. 5 to 8 are graphs showing the results. In addition to the main peaks F _1S and C _1S , O _1S peaks were also confirmed, and it was confirmed that hydroxyl groups and the like were generated on the surface of any test substrate. In Examples 2 and 3, the F _1S peak intensity decreased and the C _1S peak intensity increased compared to the PTFE substrate before the plasma treatment. However, in Comparative Example 1, such a tendency was hardly recognized.

  Table 3 shows the results of measuring the contact angle with respect to distilled water for the test substrates after Examples 1 to 3 and Comparative Example 1 (1). Table 3 shows that the test substrates of Examples 1 to 3 have a smaller contact angle indicating the wettability of the surface than the PTFE substrate before the plasma treatment, and the degree of hydrophilicity is high.

<SAM formation after AEAPS processing>
FIG. 9 shows the result of XPS analysis of the test substrate of Example 2 after step (2). From FIG. 9A, it was confirmed that C _1S and N _1S appeared clearly from FIG. 9B while the F _1S strength decreased. From this, it was confirmed that SAM of AEAPS was formed on the test substrate surface subjected to the hydrophilic treatment. The contact angle of distilled water was greatly reduced to 48 °.

<Formation of electroless copper plating film by electroless plating>
The photographs of the test substrate surfaces of Example 2 and Comparative Example 2 after step (5) are shown in FIGS. In Example 2, an electroless copper plating film could be formed. As a result of measuring the film thickness of the electroless copper plating film by fluorescent X-ray measurement, it was 0.14 μm. As a simple adhesion test of the electroless copper plating film, a peeling test using an adhesive tape (Scotch tape: manufactured by Sumitomo 3M Limited) was performed, and no peeling of the copper film was observed. On the other hand, in Comparative Example 2, an electroless copper plating film could not be formed. FIG. 11 shows the XPS analysis result of the test substrate of Comparative Example 2 shown in FIG. In Examples 1 and 3, the same electroless copper plating film as in Example 2 was formed.

FIG. 11B is an enlarged graph of a part of FIG. From FIG. 11 (b), it was confirmed that the Pd _{3d5 / 2} peak appeared clearly, while palladium was in an ionic state. That is, although palladium ions were chemically adsorbed on the terminal functional group (amino group) of SAM, it was considered that palladium was not metallized.

FIG. 12 shows the XPS analysis result of the portion where the electroless copper plating film of the test substrate of Example 2 was formed. From FIG. 9 (b), the Cu _{2d3 / 2} peak appeared clearly, and it was confirmed that palladium ions were reduced to metallic palladium due to the reducing action by the DMAB solution. And it was considered that the electroless copper plating film was formed by the catalytic action of metallic palladium. Similar results were obtained for the test substrates of Examples 1 and 3.

<Sensitivity treatment with tin ions>
In Examples 1 to 3, the step of sensitizing the test substrate with an aqueous solution containing a tin salt was not performed before the step (3), but a commercially available sensitizing solution for electroless copper plating was used. In the case where the sensitization treatment was performed using, an electroless copper plating film was obtained as in Examples 1 to 3. From the viewpoint of cost reduction and simplification of work, it is preferable to omit the sensitization process.

<Electrolytic copper plating>
The electrolytic copper plating treatment in step (6) can be achieved by utilizing a known treatment method. That is, an electrolytic layer plating film can be further formed on the electroless copper plating film by a known electrolytic copper plating process. In this case, a copper plating film (electroless copper plating film + electrolytic copper plating film) having a uniform thickness and strong bonding strength is formed on the PTFE substrate surface while the surface is smooth.

  The portion of the test substrate of Example 2 on which the electroless copper plating film was formed was subjected to electrolytic copper plating by a known processing method (Top Lucina 81-HL: building bath material manufactured by Okuno Pharmaceutical Co., Ltd.). As a result of measuring the film thickness of the electrolytic copper plating film by fluorescent X-ray measurement, it was 9.83 μm. As a simple adhesion test of the electrolytic copper plating film, a peeling test using an adhesive tape (Scotch tape: manufactured by Sumitomo 3M Co.) was performed, and no peeling of the copper film was observed.

  The copper plating method for a polytetrafluoroethylene substrate for a high-frequency circuit according to the present invention is useful as a method for producing a plated substrate for a high-frequency circuit. It should be noted that if the SAM of the aminosilane coupling agent is formed and then exposed to vacuum ultraviolet light through a patterned mask material, the amino group of the SAM corresponding to an unnecessary portion of the circuit can be removed, and the subsequent electroless process is performed. In the plating process and the electrolytic copper plating process, it is also possible to laminate a copper plating film on the circuit pattern conductor portion by a full additive method.

1: Gas cylinder 2: Path 3: Mass flow controller (MFC)
4: Valve 5: Path 6: Sample stage scanning stage 7: Test substrate 8: Plasma 9: Alumina insulating pipe 10: Aluminum alloy rod 11: High frequency oscillation power supply 12: Matching circuit unit 13: Path 14: Mass flow controller (MFC) )
15: solvent tank 16: solvent 17: path 21: stainless steel container 22: PFA container 23: beaker 24: toluene solution of AEAPS 25: AEAPS vapor

Claims (4)

A step (1) of plasma-treating a polytetrafluoroethylene substrate in the presence of helium gas;
Reacting the substrate with an aminosilane coupling agent to form a self-assembled monolayer on the surface of the substrate;
A step (3) of activating the substrate on which the self-assembled monomolecular film is formed with an electroless plating catalyst solution which is an aqueous solution containing a palladium salt;
Reducing palladium ions immobilized on the substrate to metallic palladium (4);
A step (5) of forming an electroless copper plating film on the substrate by treating the substrate with an electroless copper plating solution;
A step (6) of further forming an electrolytic copper plating film on the electroless copper plating film by treating the substrate with an electrolytic copper plating solution;
A method for copper plating of a polytetrafluoroethylene substrate for a high-frequency circuit, characterized in that   The copper plating method for a polytetrafluoroethylene substrate for a high-frequency circuit according to claim 1, wherein the aminosilane coupling agent is 3- (2-aminoethyl) -aminopropyltrimethoxysilane.   The copper of the polytetrafluoroethylene substrate for a high-frequency circuit according to claim 1 or 2, wherein in the step (1), the helium gas contains methanol vapor or ethanol vapor in a range of 0.1% by volume to 2% by volume. Plating method.   The polytetrafluoroethylene substrate for a high frequency circuit according to any one of claims 1 to 3, wherein a step of sensitizing the substrate with an aqueous solution containing a tin salt is not performed before the step (3). Copper plating method.