Joining method

Abstract

To bond an electrode having a gold layer on its surface and a gold wire with high bonding strength.SOLUTION: A bonding method comprising: a plasma treatment step of treating a bonding portion of at least one of two objects in which gold is exposed at the bonding portion with plasma whose raw material is water or a mixture of water and oxygen in which a proportion of oxygen in the mixture is 50% or less; and a bonding step of bringing the bonding portions into contact with each other and pressing the two objects after the plasma treatment step.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for bonding electrodes having a gold layer on the surface thereof and gold wires. [Background technology]

[0002] Gold (Au) has high electrical conductivity and corrosion resistance, and is easily formed into a film by sputtering, etc., and is therefore sometimes used as a material for electrodes on substrates and wires for wire bonding. For example, Patent Document 1 describes electrically connecting an electrode formed by laminating a copper layer, a nickel layer, and a gold layer in this order on a substrate to an electrode on a semiconductor chip mounted on this substrate with a wire made of an ultra-fine gold wire (wire bonding).

[0003] When bonding electrodes together or bonding an electrode to a wire, the surfaces of the electrodes may be treated with various plasmas to improve bonding characteristics. For example, Patent Document 2 describes that the bonding strength of the bonded portion can be improved by removing impurities such as organic matter present on the surface of the electrode with oxygen plasma (cleaning the surface of the electrode). Furthermore, Patent Document 3 describes that by treating the surface of the electrode with water plasma, highly flexible electrodes can be bonded together so that the bonded portion also has high flexibility. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 7-106363 [Patent Document 2] JP 2017-112383 A (paragraph

[0089] ) [Patent Document 3] Japanese Patent Publication No. 2022-111523 [Non-patent literature]

[0005] [Non-Patent Document 1] Yamamoto, Michitaka, Takashi Matsumae, Yuichi Kurashima, Hideki Takagi, Tadatomo Suga, Toshihiro Itoh, and Eiji Higurashi. 2019. "Comparison of Argon and Oxygen Plasma Treatments for Ambient Room-Temperature Wafer-Scale Au-Au Bonding Using Ultrathin Au Films." Micromachines 10, no. 2: 119. Summary of the Invention [Problem to be solved by the invention]

[0006] As is well known, gold has an extremely low tendency to ionize, and therefore has been thought to be resistant to oxidation (except when dissolved in aqua regia). However, it has recently been revealed that gold is oxidized when treated with oxygen plasma (Non-Patent Document 1). In an electrode having a gold layer on its surface, if the surface of the gold layer is oxidized, the electrical conductivity of the electrode may decrease, and the bonding strength between electrodes or between an electrode and a wire may decrease. Therefore, the method described in Patent Document 2 cannot bond electrodes or gold wires having a gold layer on their surface with high bonding strength.

[0007] In the method described in Patent Document 3, after the electrodes are brought into contact with each other, they are either left for a predetermined time or pressure is applied to at least one of the electrodes to bond them together. Because the electrodes described in Patent Document 3 are highly flexible, applying high pressure during bonding may cause deformation or damage to the electrodes. Therefore, the method described in Patent Document 3 does not allow high pressure to be applied during bonding, and therefore does not allow for increased bonding strength between the electrodes.

[0008] The problem to be solved by the present invention is to bond electrodes having a gold layer on the surface or gold wires with high bonding strength. [Means for solving the problem]

[0009] In order to solve the above problems, the bonding method according to the present invention includes: a plasma treatment step in which at least one of the joining points of two objects, each having gold exposed at the joining point, is treated with plasma made from water or a mixture of water and oxygen, the mixture having an oxygen content of 50% or less; a joining step of contacting the two joining points and pressing the two objects together after the plasma treatment step; Including, A bonding method characterized in that at least one of the two objects is an electrode on a substrate, and the thickness of the substrate is 50 μm or more.

[0010] The two objects may be, for example, electrodes each having a gold layer on its surface, or an electrode having a gold layer on its surface and a gold wire. [Effects of the Invention]

[0011] In the bonding method of the present invention, exposed gold areas can be cleaned without oxidation by treating with plasma made from water or a mixture of water and oxygen, where the oxygen content is 50% or less. Furthermore, in the bonding method of the present invention, exposed gold areas can be cleaned while reducing the gold oxide by treating with plasma made from water or a mixture of water and oxygen, where the oxygen content is 50% or less. Furthermore, in the bonding method of the present invention, one of the two objects to be bonded is an electrode on a substrate, and the substrate is 50 μm or thick, so that the electrode is not deformed or damaged even when high pressure is applied to the bonding area.

[0012] In the bonding method according to the present invention, the electrode, which is one of the two objects to be bonded, preferably has a substrate containing any of polycarbonate, cycloolefin, polyamide, epoxy polymer, indium phosphide, gallium arsenide, gallium nitride, silicon, and silicon carbide, so that the electrode will not be deformed or damaged even when a high pressure is applied to the bonding point.

[0013] Therefore, according to the bonding method of the present invention, electrodes and gold wires having gold layers on their surfaces can be bonded with high bonding strength. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram showing an outline of the configuration of a sample used in this experiment. [Figure 2] The processing procedures for Experiments 1 to 4 of this experiment are shown below. [Figure 3] FIG. 1 is a diagram illustrating the state of plasma processing in this experiment. [Figure 4] FIG. 10 is a diagram illustrating one of the evaluation methods of this experiment. [Figure 5] FIG. 10 shows the results of plasma treatment in this experiment. [Figure 6] FIG. 10 shows the results of plasma treatment in this experiment. [Figure 7] FIG. 10 is a diagram showing the results of the joining process of this experiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] The methods and results of various experiments conducted to confirm the effects of the present invention will be described below with reference to the drawings.

[0016] <Sample> Figure 1 shows an outline of the structure of the sample used in this experiment. Sample 1 used in this experiment was formed by sputtering an adhesion layer 12 (5 nm thick) made of titanium (Ti) and a gold layer 13 (500 nm thick) on a substrate 11 (525 μm thick) made of silicon (Si). The material and thickness of substrate 11 were determined so that sample 1 would not be deformed or damaged by the bonding process described below. That is, in addition to silicon, materials that can be used for the substrate 11 include polycarbonate (PC), cycloolefin polymer (COP) (in this specification, cycloolefin polymer (COP) is considered to include cycloolefin copolymer (COC)), polyamide, polyester, polyurethane, polysiloxane, phenolic resin, polysulfide, polyacetal, polyacrylonitrile, polyvinyl chloride, polystyrene, polymethyl methacrylate, polyvinyl acetate, polytetrafluoroethylene, polyisoprene, polyether, polyimide, polybenzimidazole, polybenzoxazole, polybenzothiazole, polyoxadiazole, polytriazole, polyquinoxaline, polyimidazopyrrolone, epoxy resin, copolymers containing an aromatic component and a component selected from vinyl and cyclobutane groups, indium phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN), silicon carbide (SiC), glass, ceramic, etc. The thickness of the substrate must be 50 μm or more. In this experiment, a 525 μm sample 1 was pressed at 0.1 MPa, as described below. However, the thickness of the substrate can be appropriately determined depending on the pressure applied to the sample 1. If the substrate is thinner than 50 μm, the substrate's hardness is low, requiring another substrate with sufficient hardness to support the substrate, resulting in poor handling. Therefore, to adequately press the two objects together during the bonding process, the substrate thickness must be 50 μm or more, and 500 μm or more is more preferable. Materials such as titanium and chromium (Cr) can be used for the adhesion layer 12. The thicknesses of the adhesion layer 12 and the gold layer 13 are not particularly limited.

[0017] In addition, in the bonding method of the present invention, one of the objects can be a gold wire. When one of the objects is a gold wire, the tip of the gold wire passed through a hollow cylindrical capillary is brought into contact with the joining point of the other object, and ultrasonic vibration or a load is applied to the tip of the gold wire to join the two objects. Since the bonding strength tends to decrease if the other object warps due to an impact load (bond load) during pressing, the thickness of the substrate constituting the other object must be 50 μm or more, and preferably 500 μm or more. Therefore, Parylene (registered trademark), obtained by chemical vapor deposition of paraxylene, is difficult to obtain a sufficient thickness and is therefore difficult to use as a substrate in the present invention. In the bonding method of the present invention, a predetermined plasma treatment process is performed to remove or reduce bonding inhibitors, such as organic matter and oxides, present on the surface of the joining point, thereby enabling a high bonding strength between the joining point and the gold wire.

[0018] <Joining method> In these experiments, we obtained results for a case where we attempted to bond Samples 1 and 1 using a method according to the present invention (Experiments 1 and 2 and part of Experiment 3) and a case where we attempted to bond Samples 1 and 1 using a method other than the present invention (Part of Experiment 3 and Experiment 4). Figure 2 shows the processing procedures for Experiments 1 to 4 of this experiment. In Experiment 1, the surfaces of the gold layers 13 of Samples 1 and 1 were treated with water plasma, and then the two samples were bonded together. In Experiment 2, the surfaces of the gold layers 13 of Samples 1 and 1 were treated with oxygen plasma, and then further treated with water plasma, and then the two samples were bonded together. In Experiment 3, the surfaces of the gold layers 13 of Samples 1 and 1 were treated with plasma using a mixed gas of water and oxygen as the raw material, and then the two samples were bonded together. In Experiment 4, the surfaces of the gold layers 13 of Samples 1 and 1 were treated with oxygen plasma, and then the two samples were bonded together.

[0019] [Plasma treatment (H2O)] These are plasma treatments for Experiments 1 and 2. For this treatment, an Aqua Plasma (registered trademark) cleaner AQ-2000 manufactured by Samco Inc. was used. FIG. 3 is a diagram illustrating the plasma treatment in this experiment. As shown in FIG. 3, Sample 1,1 was placed with the bonding surface facing up on the lower electrode (ground electrode) 42 of the parallel plate type electrode in the treatment chamber 41 of the apparatus. The plasma treatment conditions were as follows: Process gas: H2O (water vapor) Flow rate: 20sccm Pressure: approx. 5 Pa High frequency power: 500W Processing time: 180 seconds Temperature: 25℃

[0020] In the bonding method according to the present invention, the plasma treatment conditions are not limited to those described above. For example, the flow rate of the treatment gas and the high-frequency power may be set to the optimum conditions for the plasma treatment apparatus, e.g., 12 to 20 sccm and 250 to 500 W, respectively. The pressure is determined depending on the flow rate of the treatment gas. The treatment time can be determined based on the characteristics of the progress of the reduction reaction. That is, the reduction reaction does not progress for about one minute after the start of plasma treatment, and it takes about three minutes for the entire surface of the gold layer 13 to react, so the treatment time is preferably three minutes or more. Furthermore, the progress of the reduction reaction slows down after about five minutes, so the treatment time is preferably five minutes or less.

[0021] [Plasma treatment (O2)] These are plasma treatments according to Experiments 2 and 4. The treatment conditions were the same as those for the above-mentioned [Plasma Treatment (H2O)] except that the treatment gas was oxygen (O2).

[0022] [Plasma treatment (H2O+O2)] This is the plasma treatment in Experiment 3. The treatment conditions were the same as those in the above [Plasma Treatment (H2O)] except that the treatment gas was a mixed gas of water vapor and oxygen. In this experiment, the proportion of oxygen gas in the mixed gas was changed in the range of 0 to 100%, and results were obtained for each mixture ratio.

[0023] [Joining process (contact / pressure)] The bonding surfaces of Samples 1 and 1 were brought into contact with each other and pressed with a hand roller to bond the two together under the following bonding conditions. Temperature: 25℃ Pressure: 0.1 MPa Pressing time: 30 seconds

[0024] <Evaluation method> [Surface condition] The surface state of Sample 1 after plasma treatment in Experiments 1 to 4 was analyzed by X-ray photoelectron spectroscopy (XPS). For the analysis, a PHI5000 VersaProbe2 manufactured by ULVAC-PHI, Inc. was used.

[0025] [Joining strength] 4 is a diagram illustrating the crack opening method. According to the crack opening method, Young's modulus E (Pa) and thickness T w The thickness of the two wafers is T b When a blade (razor blade) of length L (m) is inserted into the crack, the surface energy of the joint surface is λ (J / m 2 ) can be expressed as follows:

number

[0026] In this experiment, the bond strength of bonded Sample 1,1 was calculated using the crack opening method. The Young's modulus and thickness of Sample 1 and the thickness of the razor blade are as follows: Young's modulus of sample 1: 160 GPa Thickness of sample 1: 525 μm Razor blade thickness: 100μm

[0027] <Evaluation results> [Evaluation results of plasma treatment] The results of the narrow-band analysis of Au-4f by XPS are shown in Figures 5 and 6(a). Figure 5 is a graph showing the surface state of the gold layer 13 immediately after deposition and after plasma treatment in Experiments 1, 2, and 4. Two peaks near 84 eV and 88 eV, which appear in common in all of the graphs in Figures 5(a) to 5(d), indicate the presence of gold.

[0028] Comparing the surface state of the gold layer 13 immediately after deposition (Figure 5(a)) with the surface state of the gold layer 13 after plasma treatment in Experiment 4 (Figure 5(b)), we see that the signal intensity around 86 eV and 89 eV is larger in the latter. This indicates that the oxygen plasma treatment oxidized the surface of the gold layer 13, forming gold oxide (Au2O3). On the other hand, comparing the surface state of the gold layer 13 immediately after deposition (Figure 5(a)) with the surface state of the gold layer 13 after plasma treatment in Experiment 1 (Figure 5(c)), we see that the two surfaces are almost identical, including the signal intensities around 86 eV and 89 eV. This indicates that the water plasma treatment was able to clean the gold layer 13 without oxidizing it. Furthermore, comparing the surface state of the gold layer 13 immediately after deposition (Figure 5(a)) with the surface state of the gold layer 13 after plasma treatment in Experiment 2 (Figure 5(d)), we see that the two surfaces are almost identical, despite the oxygen plasma treatment. This means that the gold oxide formed by the oxygen plasma treatment was reduced by the water plasma treatment.

[0029] FIG. 6 shows the surface state of the gold layer 13 after plasma treatment in Experiment 3. FIG. 6(a) shows the surface state of the gold layer 13 after plasma treatment in Experiment 3 when the oxygen content of the mixed gas used in the plasma treatment was 0% (consisting only of water vapor, corresponding to Experiment 1), 30%, 50%, 80%, and 100% (consisting only of oxygen, corresponding to Experiment 4). FIG. 6(b) shows the relationship between the oxygen content of the mixed gas and the gold to oxygen composition ratio on the surface of the gold layer 13. Referring to FIGS. 6(a) and 6(b), it can be seen that the gold layer 13 was hardly oxidized when the oxygen content of the mixed gas was 0% to 50%.

[0030] Even if the proportion of oxygen in the mixed gas is 0% to 50%, the composition ratio of oxygen on gold layer 13 is not 0% because oxygen atoms are introduced to the surface of gold layer 13 as hydroxyl groups (OH groups) rather than as gold oxides. When hydroxyl groups are introduced to the surface of gold layer 13, the wettability and adhesiveness of the surface of gold layer 13 are improved.

[0031] Furthermore, because oxygen plasma exhibits a high cleaning effect, it is believed that using a mixed gas of water vapor and oxygen as the processing gas can improve the cleaning speed compared to processing with water plasma alone. Therefore, from this perspective, it is preferable to use a mixed gas of water vapor and oxygen as the processing gas. Alternatively, as in Experiment 2, it is preferable to perform oxygen plasma treatment before processing with water plasma. In this way, organic matter on the surface of the gold layer 13 can be rapidly removed with oxygen plasma, and then the gold oxide formed by the oxygen plasma can be reduced with water plasma.

[0032] [Evaluation results of bonding process] Figure 7 shows the results of the bonding process in Experiments 1, 2, and 4. In Experiment 1 (Fig. 7(a)) and Experiment 2 (Fig. 7(b)), Sample 1 was bonded without deformation or damage due to pressing, and did not peel even when a razor blade was inserted. The surface energy was estimated using the crack opening method to be at least 3 mJ / m2 On the other hand, in experiment 4 (Fig. 7(c)), sample 1,1 was not bonded.

[0033] In this experiment, plate-shaped samples 1 each having a gold layer on its surface and a silicon substrate were bonded together, but the objects that can be bonded are not limited to these. In other words, as long as the objects have gold or gold oxide exposed at least partially at the bonding location, it is clear that the effects confirmed in this experiment can be achieved by performing the plasma treatment and bonding process according to the experiment when bonding. For example, it is possible to bond an electrode having a gold layer on its surface and a substrate made of polycarbonate or cycloolefin polymer to a gold wire with high bonding strength.

[0034] Furthermore, the results of this experiment revealed that water plasma has the effect of reducing gold under reduced pressure. Therefore, it is thought that the oxidation effect can be suppressed not only in oxygen plasma but also in other plasmas that have an oxidizing effect by adding water vapor to the raw material. [Explanation of symbols]

[0035] 1...Sample 11... Circuit board 12...Adhesion layer 13…Gold layer 41...Processing chamber 42...Lower electrode (ground electrode)

Claims

1. a plasma treatment step in which at least one of the joining points of two objects, each having gold exposed at the joining point, is treated with plasma made from water or a mixture of water and oxygen, the mixture having an oxygen content of 50% or less; a joining step of contacting the two joining points and pressing the two objects together after the plasma treatment step; Including, A bonding method characterized in that at least one of the two objects is an electrode on a substrate, and the thickness of the substrate is 50 μm or more.

2. 2. The bonding method according to claim 1, further comprising, before the plasma treatment step, performing an oxygen plasma treatment step of treating the joining portion of at least one of the two objects with oxygen plasma.

3. 2. The bonding method according to claim 1, wherein the surface of the gold exposed at the bonding point is oxidized.

4. 4. The bonding method according to claim 1, wherein the thickness of the substrate is 500 μm or more.

5. 4. The bonding method according to claim 1, wherein the substrates contain any one of polycarbonate, cycloolefin, polyamide, epoxy polymer, indium phosphide, gallium arsenide, gallium nitride, silicon, and silicon carbide.

6. 4. The bonding method according to claim 1, wherein one of the objects is a gold wire.

Images