JP2024008681A - copper powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide copper powder having excellent low temperature sinterability.

SOLUTION: Copper powder with the ratio of carbon content C (mass%) to BET specific surface area (m²/g) (C/SSA) being 0.07 or less, and the peak area ratio (A2/A1) of an area A1 for a peak having a peak top in the range of 284 eV to 285 eV to an area A2 for a peak having a peak top in the range of 288 eV to 289.2 eV in the C1s spectrum by X-ray photoelectron spectroscopy being 0.5 or more.

SELECTED DRAWING: None

COPYRIGHT: (C)2024,JPO&INPIT

Description

This specification discloses technology related to copper powder.

Conductive pastes that contain copper powder and are used for forming circuits on substrates by printing, bonding semiconductor elements and substrates, etc., require a sintering process in which the copper powder contained in the paste is sintered by heating. There are cases of condensation.

A sintered conductive paste is required to sinter copper powder by heating at a relatively low temperature. This is because if the temperature during heating is high, the heat may affect the base material and the semiconductor element. Further, large thermal stress is generated in the base material or the semiconductor element during heating at high temperatures and cooling after heating, and there is also a concern that this may change the electrical characteristics of the circuit and the semiconductor element.

Furthermore, in Patent Document 1, "copper powder and copper paste for a coating film containing copper powder that can perform electroless metal plating without using an expensive catalyst such as palladium in the production of a conductive coating film," The objective is to provide a manufacturing method for efficiently forming a conductive coating film by electroless metal plating on a copper powder-containing coating film formed using the copper paste. A copper powder having a diameter of .05 to 2 μm, the BET specific surface area value (SSA) (m ² /g) and carbon content (C) (wt%) of the copper powder having a relationship according to the following formula [1]. C/SSA≦7×10 ⁻² ...[1]" has been proposed.

International Publication No. 2012/157704

Although various research and development efforts are underway regarding low-temperature sintering of copper powder, there are cases where sintering at an even lower temperature is required.

This specification discloses a copper powder that has excellent low-temperature sinterability.

The copper powder disclosed in this specification has a ratio (C/SSA) of carbon content C (mass%) to BET specific surface area (m ² /g) of 0.07 or less, and C1s by X-ray photoelectron spectroscopy. In the spectrum, the peak area ratio (A2/A1) of the area A2 of the peak having a peak top in the range of 288 eV to 289.2 eV to the area A1 of the peak having the peak top in the range of 284 eV to 285 eV is 0.5 or more. That is.

The above-mentioned copper powder has excellent low-temperature sinterability.

This is a C1s spectrum obtained by XPS of Invention Example 5.

Below, embodiments of the copper powder described above will be described in detail.
The copper powder of one embodiment has a ratio (C/SSA) of carbon content C (mass%) to BET specific surface area (m ² /g) of 0.07 or less, and has a C1s spectrum determined by X-ray photoelectron spectroscopy. The peak area ratio (A2/A1) of the area A2 of the peak having the peak top in the range of 288 eV to 289.2 eV to the area A1 of the peak having the peak top in the range of 284 eV to 285 eV is 0.5 or more. There is.

Although the details will be described later, such copper powder can be sintered at relatively low temperatures due to its small ratio (C/SSA) of carbon content C (mass%) to BET specific surface area (m ² /g). It becomes easier to do. Further, in the above copper powder, the predetermined peak area ratio (A2/A1) in the C1s spectrum in XPS is large. Here, a peak with a peak top in the range of 284 eV to 285 eV corresponds to carbon bonded to a carbon atom, and a peak with a peak top in the range of 288 eV to 289.2 eV on the higher energy side corresponds to oxygen. Corresponds to double-bonded carbon. Copper powder with a large peak area ratio (A2/A1) is recognized to contain organic matter with a high oxygen content. When organic substances with a high oxygen content are heated above several hundred degrees Celsius, CO and CO ₂ are likely to be produced due to thermal decomposition, and solid carbon is difficult to remain as a thermal decomposition residue. Therefore, it is thought that copper powder coated with an organic material with a high oxygen content is likely to be sintered easily at a relatively low temperature. However, it is not limited to this theory.

(BET specific surface area)
The BET specific surface area of the copper powder is preferably 0.5 m ² /g or more and 10.0 m ² /g or less. When the BET specific surface area exceeds 10.0 m ² /g, it is difficult to ensure oxidation resistance, and there is a concern that problems may arise in paste properties due to moisture absorption, aggregation, etc. On the other hand, if the BET specific surface area is too small, the particle size of the copper powder is large, and there is concern that it will not sinter at low temperatures and that the circuit printed with the paste or the bonding surface will not have sufficient smoothness. From this point of view, the BET specific surface area of the copper powder is preferably 0.5 m ² /g to 10.0 ^{m 2} /g, more preferably 2.0 m ² /g to 6.0 m ² /g. More preferred.

The BET specific surface area of copper powder is measured in accordance with JIS Z8830:2013. For example, after degassing the copper powder in a vacuum at a temperature of 70°C for 5 hours, It can be implemented using

(Average particle size)
The average particle size of the copper powder is preferably 0.05 μm to 2.00 μm, more preferably 0.05 μm to 1.50 μm, particularly preferably 0.1 μm to 0.5 μm. If the average particle size of the copper powder is too large, there are concerns that it will not sinter at low temperatures and that the smoothness of circuits and bonding surfaces printed with the paste will not be sufficient. Furthermore, if the average particle size of the copper powder is too small, it will be difficult to ensure oxidation resistance, and there is a concern that problems will arise in paste properties due to moisture absorption, aggregation, etc.

The average particle size of the copper powder can be calculated from the BET specific surface area value using the following formula.
D=6/(ρ×SSA)
Here, D is the average particle diameter, ρ is the true density of copper, and SSA is the BET specific surface area.

(C/SSA)
Copper powder generally contains carbon because organic reducing agents and dispersants are used in the manufacturing process. When the BET specific surface area (m ² /g) of the copper powder is "SSA" and the carbon content (mass%) of the copper powder is "C", the copper powder of the embodiment has a ratio (C/SSA) of the copper powder. ) becomes 0.07 or less. If this ratio (C/SSA) is larger than 0.07, the carbon content is too large relative to the BET specific surface area, and although excellent storage and dispersibility can be obtained, organic Carbon remains on the surface of the copper powder as a thermal decomposition residue of the reducing agent and dispersant, making it difficult for the copper powder to sinter. That is, low-temperature sinterability is impaired. From this, the above ratio is 0.07 or less, more preferably 0.05 or less.

On the other hand, if the ratio (C/SSA) of carbon content C (mass%) to BET specific surface area (m ² /g) is small, oxidation resistance will be impaired, compatibility with organic solvents will be poor, and dispersion will be reduced. There is a risk that the paste will not have excellent properties. Therefore, this ratio (C/SSA) is preferably 0.01 or more, and more preferably 0.02 or more.

The carbon content of the copper powder used to calculate the ratio (C/SSA) of carbon content C (mass %) to BET specific surface area (m ² /g) is measured by high frequency induction heating furnace combustion-infrared absorption method. Specifically, using a carbon sulfur analyzer such as LECO's CS844 model, using LECO's LECOCEL II and Fe chips as combustion improvers, and using a steel pin as a calibration curve, the carbon content of the copper powder was measured. can do.

(O/SSA)
In order for the organic matter contained in the copper powder to disappear as CO or CO ₂ through thermal decomposition in the sintering process, it is necessary for the copper powder to contain oxygen. When the BET specific surface area (m ² /g) of the copper powder is "SSA" and the oxygen content (mass %) of the copper powder is "O", the copper powder of the embodiment has a ratio (O/SSA) of the copper powder. ) is greater than 0.15. When this ratio (O/SSA) is greater than 0.15, the organic matter contained in the copper powder is easily converted into CO or CO ₂ during the sintering process, resulting in a copper powder with excellent low-temperature sinterability. . The above ratio (O/SSA) is preferably a value larger than 0.15, and more preferably 0.17 or more.

The ratio (O/SSA) of oxygen content O (mass %) to BET specific surface area (m ² /g) is preferably 0.5 or less, more preferably 0.3 or less. If this ratio (O/SSA) is too large, it means that the oxygen is not only derived from organic matter, but also that the surface of the copper powder is significantly oxidized. This is because there are effects such as unstable characteristics.

The oxygen content of the copper powder used to determine the ratio (O/SSA) of oxygen content O (mass%) to BET specific surface area (m ² /g) is measured by inert gas melting-infrared absorption method. do. Here, TC600 model manufactured by LECO was used as an oxygen nitrogen analyzer, a steel pin was used as a calibration curve, and copper powder was placed in a nickel capsule for measurement.

(XPS peak area ratio)
When copper powder is analyzed by X-ray photoelectron spectroscopy, a C1s spectrum, which is a spectrum of the C1s orbital, is obtained as a result of the analysis. In this C1s spectrum, the peak area ratio (A2/A1) is the ratio of the area A2 of the peak having a peak top in the range of 288 eV to 289.2 eV to the area A1 of the peak having the peak top in the range of 284 eV to 285 eV. is 0.5 or more.

Here, the peak between 284 eV and 285 eV corresponds to electrons in the 1s orbit of carbon that are not bonded to highly polar atoms such as oxygen. Here, it is identified as corresponding to carbon having a C--C bond. On the other hand, the peak of 288 eV to 289.2 eV, which is on the high energy side, corresponds to electrons in the 1s orbit of carbon bonded to highly polar atoms such as oxygen, and here it is assumed that it corresponds to carbon having a C-O double bond. Identified. When the copper powder contains oxygen-rich organic matter, the latter peak intensity becomes relatively high compared to the former peak intensity, and the peak area ratio (A2/A1) becomes large. When the peak area ratio (A2/A1) is 0.5 or more, when copper powder is heated, CO and CO ₂ are likely to be generated as thermal decomposition products of organic matter, and carbon as a thermal decomposition residue is absorbed into the copper powder. Does not easily remain on surfaces. From this, it is thought that sintering of copper powder progresses more easily at relatively low temperatures. From such a viewpoint, it is more preferable that the peak area ratio (A2/A1) is 0.6 or more. On the other hand, the peak area ratio (A2/A1) usually does not exceed 10 and may be 1.0 or less.

In addition, in the C1s spectrum obtained by X-ray photoelectron spectroscopy, the peak area ratio ( A3/A1) is preferably 0.3 or more.
Note that the area of the peak having the peak top at the position of 288 eV is regarded as area A2.

The peak between 286 eV and 288 eV corresponds to a carbon atom that has a single bond with oxygen. Copper powders with a peak area ratio (A3/A1) of 0.3 or more were copper powders with excellent low-temperature sinterability, although the reason is unknown. The peak area ratio (A3/A1) is preferably 0.3 to 0.7, more preferably 0.3 to 0.5.

In order to obtain the above peak area ratio (A2/A1) and peak area ratio (A3/A1), X-ray photoelectron spectroscopy (XPS) measurement and analysis were performed as follows.
Equipment: PHI 5000 Versa Probe II manufactured by ULVAC-PHI Co., Ltd. (with neutralization gun)
Excitation source: monochromatic AlKα
Output: 25.0W
Detection area: 100μmΦ
Incident angle: 90 degrees Extraction angle: 45 degrees In the measurement, copper powder was molded into a pellet shape and measured.
For data analysis after measurement, data analysis software: MultiPak manufactured by ULVAC-PHI Co., Ltd. was used. In data analysis, peak division was performed using the Voigt function, and the area of each peak was calculated. Furthermore, among the peaks that can be attributed to C1s, the position of the peak with the lowest binding energy was corrected to 284.8 eV.

(Low temperature sinterability)
Moreover, the above-mentioned copper powder can be sintered at a relatively low temperature. Such low-temperature sinterability can be confirmed as follows. Approximately 0.3 g of copper powder is filled into a cylindrical mold with a diameter of 5 mm, and then uniaxial pressure is applied to form a cylindrical mold with a height of approximately 3 mm and a density of 4.7 ± 0.1 g/cc. Produce compacted powder pellets. Thereafter, using a thermomechanical analyzer (TMA), the green powder pellets were analyzed from 25°C to 10°C in an atmosphere containing 2% by volume of hydrogen (H ₂ ) and the balance being nitrogen (N ₂ ). The temperature is raised at a rate of /min. At this time, as the temperature rises, the copper particles constituting the compact pellet are sintered, and the volume of the compact decreases, approaching the density of metallic copper (about 8.9 g/cm ³ ). If the rate of change in the cylinder height in the shrinkage direction of such a compacted powder pellet is called the linear shrinkage rate, the lower the temperature at which this linear shrinkage rate is 5%, the better the low-temperature sinterability. It can be evaluated as copper powder. In particular, it is preferable that the temperature at which the above-mentioned linear shrinkage rate becomes 5% is 350° C. or lower.

(Production method)
Copper powder as described above can be produced by, for example, using a chemical reduction method or a disproportionation method. Although the production of copper powder is not limited to these methods, details of the chemical reduction method are as follows.

When using a chemical reduction method, for example, a step of preparing a copper salt aqueous solution, an alkaline aqueous solution, a reducing agent aqueous solution, etc. as a raw material solution, a step of mixing and reacting these raw material solutions to obtain a slurry containing copper particles, A step of washing the copper particles, a step of performing solid-liquid separation, a step of drying, and a step of pulverizing as necessary are performed in this order.
In a more specific example, a copper sulfate aqueous solution is heated to an appropriate reaction temperature, the pH is adjusted with a sodium hydroxide aqueous solution or an ammonia aqueous solution, and a hydrazine aqueous solution is added all at once to carry out the reaction. It is reduced to cuprous oxide particles with a particle size of about 100 nm. After heating the slurry containing cuprous oxide particles to the reaction temperature, an aqueous solution containing sodium hydroxide and hydrazine is added dropwise, and then an aqueous hydrazine solution is added dropwise to reduce the cuprous oxide particles to copper particles. After the reaction is completed, the resulting slurry is filtered, then washed with pure water and methanol, and further dried. Thereby, copper powder is obtained.

The reducing agent such as hydrazine added to the copper sulfate aqueous solution is for reducing divalent copper to monovalent copper (cuprous oxide). At this time, if the reducing agent is added all at once, the resulting cuprous oxide particles tend to become fine as described above. After the relatively fine cuprous oxide particles are formed, the reducing agent can be added in portions. After the generation of cuprous oxide particles, the reducing agent added the first time is mainly used to generate the nucleus of metallic copper, and the reducing agent added the second time can be used for the growth of the nucleus of the metallic copper. .

In the above production, an aqueous solution of copper sulfate or nitrate can be used as the aqueous copper salt solution. Specifically, the alkaline aqueous solution may be an aqueous solution of NaOH, KOH, NH ₄ OH, or the like. Examples of the reducing agent in the aqueous reducing agent solution include organic substances such as sodium borohydride and glucose in addition to hydrazine.

If necessary, an organic substance such as a complexing agent or a dispersing agent may be added during the process of producing copper powder. For example, gelatin, ammonia, gum arabic, etc. can be added one or more times between the step of preparing a raw material solution and the step of obtaining a slurry containing copper particles.

(Application)
Copper powder produced in this way is particularly suitable for use in conductive pastes, etc., which can be mixed with resin materials, dispersion media, etc., and made into a paste, which can be used for bonding semiconductor elements and substrates and for forming wiring. Are suitable.

Next, the above-mentioned copper powder was prototyped and its effects were confirmed, which will be explained below. However, the description here is for the purpose of mere illustration, and is not intended to be limiting.

(Invention example 1)
First, 6.7 L of a mixed aqueous solution of 540 g of sodium hydroxide and 144 g of hydrazine monohydrate was mixed at once into an aqueous solution of 2,400 g of copper sulfate pentahydrate and 30 g of citric acid dissolved in 8.7 L of pure water. A slurry containing copper nanoparticles (average particle size of about 100 nm) was synthesized. Next, the slurry in which the cuprous oxide particles were suspended was heated to 50°C or higher, 4.5 L of a mixed aqueous solution of 29 g of hydrazine monohydrate and 252 g of sodium hydroxide was added dropwise, and the sodium hydroxide aqueous solution was added to adjust the pH. adjusted. Thereafter, 1.3 L of an aqueous solution of 115 g of hydrazine monohydrate was added dropwise to reduce the cuprous oxide to metallic copper. After the reaction was completed, the product was repeatedly washed with water by decantation, dried and pulverized to obtain copper powder.

(Invention example 2)
The procedure was the same as Invention Example 1 until the slurry containing cuprous oxide was synthesized. Next, 4.5 L of a mixed aqueous solution of 43 g of hydrazine monohydrate and 252 g of sodium hydroxide was added dropwise, the pH was adjusted, and 1.3 L of an aqueous solution of 101 g of hydrazine monohydrate was added dropwise to remove cuprous oxide. It was reduced to metallic copper, washed with water, dried, and crushed in the same manner.

(Invention examples 3, 4, 5)
The procedure was the same as Invention Example 1 until the slurry containing cuprous oxide was synthesized. Next, 4.5 L of a mixed aqueous solution of 72 g of hydrazine monohydrate and 252 g of sodium hydroxide was added dropwise, the pH was adjusted, and 1.3 L of an aqueous solution of 72 g of hydrazine monohydrate was added dropwise to dissolve cuprous oxide. It was reduced to metallic copper, washed with water, dried, and crushed in the same manner.

(Comparative example 1)
To an aqueous solution of 2400 g of copper sulfate pentahydrate and 30 g of citric acid dissolved in 8.7 L of pure water, 6.7 L of a mixed aqueous solution of 540 g of sodium hydroxide and 144 g of hydrazine monohydrate was added dropwise to synthesize cuprous oxide. did. Next, the slurry in which cuprous oxide was suspended was adjusted to 70°C, and the pH was adjusted to 10 by adding an aqueous sodium hydroxide solution. Thereafter, 14 g of hydrazine monohydrate and sodium hydroxide were added to start reducing some of the cuprous oxide to metallic copper. Furthermore, citric acid aqueous solution was added and then hydrazine monohydrate was added to carry out gentle reduction for several hours. After completion of the reaction, washing with water, drying, and pulverization were performed to obtain copper powder. To perform surface treatment on 600 g of this copper powder, 2 L of an aqueous solution containing 0.3 g of malonic acid with high oxygen concentration and small molecular weight was added, and the mixture was stirred at 350 rpm for 60 minutes at room temperature to coat the surface of the particles with malonic acid. was adsorbed, and then washed and dried to produce copper powder.

(Comparative example 2)
I obtained commercially available copper powder.

(evaluation)
Each of the copper powders of Invention Examples 1 to 5 and Comparative Examples 1 and 2 described above was subjected to the measurement method described above, and the BET specific surface area, particle size, carbon content, oxygen content, low temperature sinterability, and Each peak area A1, A3 and A2 was confirmed. The results are shown in Table 1. For reference, the C1s spectrum of Invention Example 5 by XPS is shown in FIG.

From what is shown in Table 1, all invention examples 1 to 5 have a C/SSA of 0.07 or less and a peak area ratio (A2/A1) of 0.5 or more, so 5% shrinkage due to TMA As can be seen from the temperature results, it had excellent low-temperature sinterability. Among them, invention examples 2 to 5 with a peak area ratio (A3/A1) greater than 0.3 had a 5% shrinkage temperature due to TMA of less than 280°C.

On the other hand, in Comparative Example 1, the C/SSA was greater than 0.07, so the 5% shrinkage temperature due to TMA was high. In Comparative Example 2, the peak area ratio (A2/A1) was smaller than 0.5 and the 5% shrinkage temperature due to TMA was high.

Therefore, it can be said that the copper powders of Invention Examples 1 to 5 have excellent low-temperature sinterability.

Claims (6)

The ratio (C/SSA) of carbon content C (mass%) to BET specific surface area (m ² /g) is 0.07 or less,
In the C1s spectrum obtained by X-ray photoelectron spectroscopy, the peak area ratio (A2/A1 ) is 0.5 or more. In the C1s spectrum obtained by X-ray photoelectron spectroscopy, the peak area ratio (A3/A1) of the area A3 of the peak having the peak top in the range of 286 eV to 288 eV to the area A1 of the peak having the peak top in the range of 284 eV to 285 eV is , 0.3 or more. The copper powder according to claim 1 or 2, wherein the peak area ratio (A2/A1) is 1.0 or less. The copper powder according to claim 1 or 2, having a BET specific surface area of 0.5 m ² /g to 10.0 m ² /g. The copper powder according to claim 1 or 2, wherein the average particle diameter calculated from the BET specific surface area is 0.05 μm to 2.00 μm. The copper powder according to claim 1 or 2, wherein the ratio (O/SSA) of oxygen content O (mass %) to BET specific surface area (m ² /g) is larger than 0.15.