Method for producing sintered copper object

Abstract

The purpose of the present invention is to produce a sintered copper object having a decreased electrical resistivity, via an oxidized sintered copper object having a dense structure.　A method for producing a sintered copper object according to the present disclosure includes a first heating step, in which a copper paste comprising copper particles having an average particle diameter D50 of 100 nm to 1.2 μm and a binder resin is heated at 350-600°C in an oxidizing atmosphere containing an oxidizing gas in an amount exceeding 2,000 ppm by volume, and a second heating step, in which the sintered copper-oxide object is heated in a reducing atmosphere.

Description

Manufacturing method of sintered copper body

[0001] The present invention relates to a method for producing a copper sintered body.

[0002] Electronic components exhibit their functions by connecting wiring to elements made of semiconductors, magnetic materials, dielectric materials, etc. A well-known method for forming wiring is to print a conductive metal paste on a substrate using a screen printing method and then bake it. Examples of metal pastes used in screen printing include silver paste and copper paste.

[0003] Among these, silver paste is currently widely used because it has excellent oxidation resistance and does not oxidize even when fired in air. However, silver is expensive and has the problem of being prone to migration defects in fine-pitch wiring, so copper paste is being considered as an alternative to silver paste.

[0004] As a method for forming a conductor by firing a copper paste placed on a substrate surface, for example, Patent Document 1 discloses a method in which copper particles in the copper paste are oxidized and fired at a temperature of 350° C. or higher and 500° C. or lower in a nitrogen gas atmosphere containing an oxidizing gas at a volume ratio of 500 ppm to 2000 ppm to obtain a copper oxide sintered body (hereinafter also referred to as an "oxidized sintered body"). The oxidized sintered body is then reduced to a copper sintered body at a temperature of 400° C. or higher and 550° C. or lower in a nitrogen gas atmosphere containing a reducing gas at a volume ratio of 1% or more.

[0005] JP 2016-004671 A

[0006] The reason for setting the oxidizing gas concentration to 2000 ppm or less in the oxidation firing described in Patent Document 1 is explained as follows: If the oxidizing gas concentration exceeds 2000 ppm, an extremely dense copper oxide film forms only on the surface of the copper paste. As a result, oxygen does not reach the interior of the copper paste, and the paste components before firing remain. As a result, resin and solvent remain inside the sintered body after reduction, inhibiting the formation of a copper sintered body and increasing the electrical resistivity.

[0007] Therefore, in Patent Document 1, an atmosphere in which the oxygen concentration is controlled to a certain low range is used to suppress the formation of a dense layer due to rapid oxidation of the paste surface, and oxygen is distributed throughout the paste to form a sintered body.

[0008] However, if the formation of a dense oxidized sintered body is realized throughout the paste, rather than being limited to the paste surface, the copper sintered body after reduction firing is expected to have a lower resistivity than that obtained under conventional oxidation / reduction firing conditions.

[0009] An object of the present disclosure is to produce an oxidized copper sintered body having a dense structure and lower electrical resistivity.

[0010] The present inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that a copper sintered body obtained by a production method including: heating a copper paste containing copper particles with an average particle size D50 of 100 nm to 1.2 μm and a binder resin at 350° C. to 600° C. in an oxidizing atmosphere containing more than 2000 ppm by volume of oxidizing gas to obtain a copper oxide sintered body; and then heating the copper oxide sintered body in a reducing atmosphere in a second heating step. This discovery led to the completion of the present disclosure. That is, the present disclosure provides the following.

[0011] (1) A method for producing a copper sintered body, comprising: a first heating step of heating a copper paste containing copper particles having an average particle diameter D50 of 100 nm or more and 1.2 μm or less and a binder resin at 350° C. or more and 600° C. or less in an oxidizing atmosphere containing an oxidizing gas at a volume ratio of more than 2000 ppm to obtain a copper oxide sintered body; and a second heating step of heating the copper oxide sintered body in a reducing atmosphere.

[0012] (2) The method for producing a copper sintered body according to (1), wherein the heating temperature in the second heating step is 130°C or higher and 600°C or lower.

[0013] (3) The method for producing a copper sintered body according to (1) or (2), wherein the maximum thickness of the copper paste before heating in the first heating step is 50 μm or less.

[0014] According to the present disclosure, a method for producing a copper sintered body having low electrical resistivity can be provided.

[0015] Hereinafter, the method for producing a copper sintered body according to the present disclosure will be described with reference to specific embodiments, but the method for producing a copper sintered body according to the present disclosure is not limited to the following embodiments.

[0016] <<Method for Manufacturing a Copper Sintered Body>> A method for manufacturing a copper sintered body according to an embodiment of the present disclosure includes a first heating step of heating a copper paste containing copper particles having an average particle diameter D50 of 100 nm or more and 1.2 μm or less and a binder resin at 350° C. or more and 600° C. or less in an oxidizing atmosphere containing more than 2000 ppm by volume of an oxidizing gas, and a second heating step of heating the copper oxide sintered body in a reducing atmosphere.

[0017] It has been found that the copper sintered body obtained by this method for producing a copper sintered body is thoroughly sintered throughout, even to the interior, and has a very dense structure, unlike the findings of Patent Document 1 mentioned above. The reason for this is currently unclear, and while not limited to a specific theory, the inventors believe as follows: When copper particles having an average particle diameter D50 of 100 nm or more and 1.2 μm or less are used, cracks occur on the surface of the copper paste during the oxidative firing of the copper paste in the first heating step. These cracks then serve as a path for introducing oxidizing gas into the copper paste during firing, allowing sufficient oxidative sintering of the copper particles and decomposition of the binder resin to occur within the paste, ultimately sintering the cracked portions and resulting in a dense structure for the entire oxidized sintered body. Subsequently, in the second heating step, the copper sintered body obtained by reducing the oxidized sintered body also maintains a dense structure similar to that of the oxidized sintered body.

[0018] The copper sintered body thus obtained has a dense sintered structure throughout the copper sintered body, and has low electrical resistivity, so it can be suitably used as wiring or electrodes.

[0019] [Materials] Hereinafter, a substrate and a copper paste that can be used to manufacture a copper sintered body will be described.

[0020] [Substrate] Although not an essential component, the copper paste described below can be applied to a substrate. Specifically, the substrate is used as a base material for forming a copper sintered layer by applying the copper paste to its surface. Specifically, the substrate can be a substrate on which electronic components are mounted, a printed wiring board, a board with through holes, or the like.

[0021] The material constituting the substrate is not particularly limited, but examples thereof include silicon substrates, oxide substrates such as silicate glass, alumina, and quartz, nitride substrates such as silicon nitride and aluminum nitride, carbide substrates such as silicon carbide and titanium carbide, and resin substrates.

[0022] The shape of the substrate is not particularly limited, and a substrate of an appropriate shape can be used depending on the application, etc.

[0023] [Copper Paste] The copper paste contains copper particles having an average particle diameter D50 of 100 nm or more and 1.2 μm or less and a binder resin.

[0024] (Copper particles) The copper particles are not particularly limited as long as they have an average particle diameter D50 of 100 nm or more and 1.2 μm or less. The average particle diameter D50 is preferably 150 nm or more and 200 nm or more. On the other hand, the average particle diameter D50 is preferably 1.15 μm or less, 1.1 μm or less, 1 μm or less, 950 nm or less, 850 nm or less, 750 nm or less, or 700 nm or less. In the present disclosure, "average particle diameter D50" refers to the particle diameter at which the cumulative volume from the small diameter side in a volume-based particle size distribution obtained using a laser scattering diffraction particle size distribution measuring device reaches 50%.

[0025] The content of the copper particles is not particularly limited, but is preferably 72% by mass or more, or 74% by mass or more, relative to the total amount of the paste, and is preferably 95% by mass or less, or 90% by mass or less, or 87% by mass or less, or 85% by mass or less.

[0026] (Binder Resin) The binder resin chemically interacts with the copper particles contained in the paste, thereby imparting viscoelasticity to the paste. The binder resin is not particularly limited as long as it is a resin that is decomposed by some treatment, such as heating, during paste firing. Examples of binder resins that can be used include cellulose resins such as methyl cellulose, ethyl cellulose, and carboxymethyl cellulose, butyral resins, and alkyd resins. Among these, it is preferable to use a cellulose-based resin that tends to react with trace amounts of oxygen or carbon monoxide contained in the firing atmosphere and easily disappear from the paste. Among cellulose-based resins, it is more preferable to use ethyl cellulose.

[0027] The content of the binder resin is not particularly limited, but is preferably 0.05 mass% or more, 0.1 mass% or more, 0.5 mass% or more, or 1 mass% or more relative to the content of the copper particles, while the content of the binder resin is preferably 3 mass% or less, 2.5 mass% or less, or 2 mass% or less.

[0028] (Solvent) The solvent contained in the copper paste is not particularly limited as long as it has an appropriate boiling point, vapor pressure, and viscosity. Specifically, hydrocarbon solvents, cyclic ether solvents, amide solvents, ketone solvents, alcohol compounds, polyhydric alcohol ester solvents, polyhydric alcohol ether solvents, terpene solvents, and mixtures thereof can be used as the solvent. Among these, Texanol, butyl carbitol, butyl carbitol acetate, and terpineol, which have boiling points around 200°C, are preferred. Depending on the relationship between the ease of volatilization of the solvent in the copper paste and the drying conditions, the copper paste may be completely free of solvent when adhered to the substrate, or the amount of solvent may be reduced compared to the composition immediately before application to the substrate.

[0029] (Glass Frit) In one embodiment, the copper paste may contain glass frit to better adhere the copper sintered body to the substrate.

[0030] When the copper paste contains glass frit, the content of the glass frit is not particularly limited, but is preferably 0.5 mass% or more, 0.7 mass% or more, 1 mass% or more, 1.5 mass% or more, 2 mass% or more, or 2.5 mass% or more relative to the mass of the copper particles. By having the glass frit content be equal to or greater than the required value, the adhesion between the copper sintered body and the substrate can be improved. On the other hand, the content of the glass frit is preferably 6 mass% or less, 5 mass% or less, 4.2 mass% or less, 3.2 mass% or less, or 3 mass% or less. By having the glass frit content be equal to the required value, the increase in resistance due to the inclusion of glass frit can be suppressed.

[0031] (Method for producing copper paste) A copper paste can be produced by mixing a binder resin and a solvent, adding copper particles, and kneading the mixture using a device such as a planetary mixer. If necessary, a three-roll mill can be used to improve the dispersibility of the particles.

[0032] (Method of Applying Copper Paste) In one embodiment, the copper paste may be printed on a substrate using a method such as screen printing to form the desired shape of the wiring or electrode. In this case, the sintered body is obtained as one arranged on the substrate.

[0033] The paste applied onto the substrate as described above is then dried using a drying oven, a hot plate, or the like.

[0034] The drying temperature is not particularly limited, but is preferably, for example, 80° C. or higher, or 90° C. or higher. On the other hand, the drying temperature is preferably 150° C. or lower, or 140° C. or lower, or 130° C. or lower.

[0035] The drying time is not particularly limited, but is preferably, for example, 2 minutes or more, 3 minutes or more, or 4 minutes or more, while the drying temperature is preferably 15 minutes or less, 14 minutes or less, 13 minutes or less, or 12 minutes or less.

[0036] The atmosphere for drying is not particularly limited, and may be, for example, air.

[0037] The maximum thickness of the copper paste before heating in the first heating step is not particularly limited, but is preferably, for example, 50 μm or less, 30 μm or less, 20 μm or less, or 10 μm or less. Having a maximum thickness equal to or less than the required value can promote oxidative sintering of copper particles and thermal decomposition of resin deep into the copper paste. On the other hand, the maximum thickness of the copper paste before heating in the first heating step is preferably 5 μm or more, or 7 μm or more. Having a maximum thickness equal to or greater than the required value can prevent peeling between the copper sintered body obtained from such a copper paste and the substrate after firing. In the present disclosure, "maximum thickness" refers to the maximum thickness of the entire copper sintered body when the entire copper sintered body on the substrate is three-dimensionally scanned using a laser microscope. In the present disclosure, "copper paste before heating in the first heating step" refers to the paste immediately before being subjected to heating in the first heating step. For example, if copper paste is applied to a substrate and then dried, "copper paste before heating in the first heating step" refers to the copper paste after drying; if some treatment is performed on the copper paste between the application of the paste and the first heating, it refers to the copper paste after that treatment.

[0038] [First Heating Step] The first heating step is a step of heating a copper paste containing at least copper particles and a binder resin at 350°C or higher and 600°C or lower in an oxidizing atmosphere containing more than 2000 ppm by volume of an oxidizing gas.

[0039] The oxidizing gas is not particularly limited as long as it has an oxidizing effect on metallic copper. Specifically, the oxidizing gas may be, for example, one or more selected from the group consisting of oxygen, ozone, sulfur dioxide, and nitrogen dioxide.

[0040] The concentration of the oxidizing gas is not particularly limited as long as it is greater than 2000 ppm, but is preferably, for example, 3000 ppm or more, 10000 ppm or more, 50000 ppm or more, or 100000 ppm or more. By ensuring that the concentration of the oxidizing gas is greater than the required amount, the sintering of the copper particles and the decomposition of the binder resin are promoted, thereby expanding the sintered region and densifying the sintered body structure, thereby reducing the electrical resistivity. On the other hand, the concentration of the oxidizing gas may be 1,000,000 ppm (100%) or less, 500,000 ppm or less, or 250,000 ppm or less.

[0041] In one embodiment, air can be used as a source of oxidizing gas, since air contains about 21% (about 210,000 ppm) of oxygen as an oxidizing gas.

[0042] The heating temperature in the first heating step is not particularly limited as long as it is 350°C or higher and 600°C or lower, but is preferably 370°C or higher. By setting the heating temperature at a required value or higher, the sintered region can be expanded, the sintered body structure can be densified, and the electrical resistivity of the copper sintered body can be reduced. On the other hand, the heating temperature in the first heating step is preferably 550°C or lower, 500°C or lower, 450°C or lower, or 400°C or lower. Setting the heating temperature at a required value or lower can suppress damage to the substrate (e.g., when a resin or the like is used) and electronic components mounted on the substrate. Furthermore, when a silicon substrate is used as the substrate, setting the heating temperature at a required value or lower can suppress an increase in resistance due to compounds generated by the reaction between copper and silicon.

[0043] [Second Heating Step] The second heating step is a step of heating the copper sintered body obtained in the first heating step in a reducing atmosphere.

[0044] Here, the term "under a reducing atmosphere" refers to an atmosphere containing a reducing gas. The reducing gas is not particularly limited as long as it exhibits a reducing effect on at least copper oxide, but examples thereof include hydrogen, carbon monoxide, formic acid, and ammonia.

[0045] The reducing gas concentration is not particularly limited, and is preferably, for example, 0.5% or more, 1.5% or more, or 3% or more by volume ratio in the atmospheric gas. By having the reducing gas concentration at a required value or higher, the reduction of copper oxide can be promoted, and a copper sintered body with lower electrical resistivity can be formed. On the other hand, the reducing gas concentration may be 100% or less, 80% or less, 50% or less, or 30% or less.

[0046] The heating temperature in the second heating step is not particularly limited, but is preferably, for example, 180°C or higher, 190°C or higher, 200°C or higher, 210°C or higher, 220°C or higher, 230°C or higher, 240°C or higher, 250°C or higher, 260°C or higher, 270°C or higher, 280°C or higher, 290°C or higher, 300°C or higher, 310°C or higher, 320°C or higher, 330°C or higher, 340°C or higher, 350°C or higher, 360°C or higher, or 370°C or higher. By setting the heating temperature at a required value or higher, the reduction of copper oxide can be promoted, resulting in the formation of a copper sintered body with lower electrical resistivity. On the other hand, the heating temperature in the second heating step is preferably 600°C or lower, 570°C or lower, 550°C or lower, 520°C or lower, 500°C or lower, 470°C or lower, 450°C or lower, 420°C or lower, or 400°C or lower. By keeping the heating temperature at or below a required value, it is possible to prevent damage to the substrate (for example, when a resin substrate is used) and electronic components mounted on the substrate. Furthermore, when a silicon substrate is used as the substrate, by keeping the heating temperature at or below a required value, it is possible to prevent an increase in resistance due to a compound produced by the reaction between copper and silicon.

[0047] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0048] [Preparation of Samples] Copper sintered samples were produced by the following method.

[0049] (Preparation of Copper Paste Sample) A copper paste sample was prepared by kneading 80% by mass of copper particles, 6% by mass of ethyl cellulose, and 14% by mass of Texanol.

[0050] (Preparation of copper sintered body sample) A p-type silicon wafer was etched with phosphoryl chloride (POCl ３By placing the silicon wafer in a high-temperature atmosphere containing fluorine, phosphorus is diffused onto the surface of the light-receiving side of the silicon wafer, forming a high-concentration region of phosphorus. ＋ This resulted in a silicon wafer having a pn junction.

[0051] The silicon wafer was immersed in a KOH solution to form a pyramidal texture with a (111) surface orientation on both sides, and one side of the silicon wafer was polished to remove the texture and make it flat.

[0052] A silicon nitride film having a thickness of 50 nm was formed on the textured surface of the silicon substrate by plasma CVD, and this film was used as an anti-reflection film.

[0053] Using a metal mask with ten parallel linear openings, each 3 cm long and 60 μm wide, spaced 2 mm apart from the center of the lines, ten copper paste lines were printed on the surface of the anti-reflective coating by screen printing. The printed substrate was placed on a hot plate and dried in air at 110°C for 10 minutes.

[0054] After drying, the substrate was placed in a firing furnace and subjected to a heat treatment (oxidation sintering) for 5 minutes in an oxidizing gas atmosphere. Next, a heat treatment (reduction) was performed at 500°C for 5 minutes in a nitrogen gas atmosphere containing 5% hydrogen gas by volume. The furnace was then cooled to room temperature to obtain a copper sintered body sample, which will be described later.

[0055] [Evaluation of Samples] The copper sintered body samples were evaluated by the following methods.

[0056] (Electrical Resistivity) The electrical resistivity of the copper wiring of the test specimen was determined by a DC four-probe method. Four needle-shaped electrodes (probes) arranged at 1 mm intervals along the longitudinal direction of the copper wiring surface were brought into pressure contact, a current (I) was passed between the two outer probes, and the potential difference (V) between the two inner probes was measured. The electrical resistivity (ρ) was calculated by ρ = (V / I) × L / S. Here, S is the cross-sectional area of ​​the wiring, which was determined by observing the wiring cross section with a scanning electron microscope. L is the distance between the voltage terminals (1 mm).

[0057] (Adhesion) In the tape test, adhesive tape (3M (registered trademark) Scotch tape, manufactured by 3M Japan Ltd.) was attached to the top of the wiring and rubbed strongly with a finger to adhere, and when the tape was peeled off, it was evaluated whether or not the copper wiring peeled off from the substrate. All 10 screen-printed copper sintered bodies were evaluated, and if peeling did not occur in any of the 10 copper sintered bodies, it was evaluated as "no peeling", and if peeling occurred in even one copper sintered body, it was evaluated as "peeling".

[0058] [Evaluation Results] [Test 1: Effect of Average Particle Diameter D50 of Copper Particles] Air (oxygen concentration: approximately 210,000 ppm) was used as the oxidizing gas, the heating temperature in the oxidizing gas atmosphere was set to 400°C, and copper particles having an average particle diameter D50 shown in Table 1 were used for the copper paste. Table 1 shows the evaluation results of the electrical resistivity and the presence or absence of peeling of these samples.

[0059]

[0060] [Test 2: Effect of Oxidizing Gas Content in First Heating Step] Samples with an average particle diameter D50 of 400 nm were used, and the heating temperature in the oxidizing gas atmosphere was set to 400°C. In Examples 2-1 to 2-3 and Comparative Example 2-1, nitrogen containing oxygen at the concentration shown in Table 2 was used as the oxidizing gas. On the other hand, in Example 2-4, air (approximately 210,000 ppm) was used. Table 2 shows the evaluation results of the electrical resistivity and the presence or absence of peeling of these samples.

[0061]

[0062] [Test 3: Effect of Heating Temperature in First Heating Step] Copper particles with an average particle diameter D50 of 400 nm were used, air (oxygen concentration: approximately 210,000 ppm) was used as the oxidizing gas, and the heating temperature in the oxidizing gas atmosphere (heating temperature in the first heating step) was set to the temperature shown in Table 3. Table 3 shows the evaluation results for the electrical resistivity and the presence or absence of peeling of these samples.

[0063]

[0064] From the above, it was found that a copper sintered body produced by a production method including: heating a copper paste containing copper particles having an average particle size D50 of 100 nm or more and 1.2 μm or less and a binder resin at 350° C. or more and 600° C. or less in an oxidizing atmosphere containing more than 2000 ppm by volume of oxidizing gas to obtain a copper oxide sintered body; and then heating the copper oxide sintered body in a reducing atmosphere in a second heating step; the copper sintered body does not peel off from the substrate and has a low electrical resistivity of less than 4.0 μΩcm.

Claims

1. A method for producing a copper sintered body, comprising: a first heating step of heating a copper paste containing copper particles having an average particle size D50 of 100 nm or more and 1.2 μm or less and a binder resin at 350° C. or more and 600° C. or less in an oxidizing atmosphere containing more than 2000 ppm by volume of an oxidizing gas to obtain a copper oxide sintered body; and a second heating step of heating the copper oxide sintered body in a reducing atmosphere.

2. The method for producing a copper sintered body according to claim 1, wherein the heating temperature in the second heating step is 180°C or higher and 600°C or lower.

3. The method for producing a copper sintered body according to claim 1 or 2, wherein the maximum thickness of the copper paste before heating in the first heating step is 50 μm or less.

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