JPH0550472B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for directly bonding a metal plate to a ceramic substrate, and in particular uses alumina powder to cleverly prevent welding between the bonded metal plate and the pedestal, making it suitable for industrial mass production. The present invention relates to an improved method for joining metal and ceramic. [Prior art] An alumina substrate used as a conventional ceramic substrate,
The method of joining a copper plate, which is a metal plate, is to apply a copper paste containing a binder onto an alumina substrate, dry and fire it, and cause the glass-forming substance in the alumina substrate to react with the binder in the copper paste. There is a method of bonding by reacting the reaction product with copper, or a method of sandwiching a reactive metal (brazing material) between the alumina substrate and the copper plate, and bonding the alumina substrate and the reactive metal at a temperature higher than the melting point of the reactive metal.
Also known are methods of bonding by causing a diffusion reaction between a copper plate and a reactive metal, respectively. However, recently, semiconductor devices have been progressing toward higher power consumption, higher integration, and modularization, resulting in higher heat dissipation of ceramic substrates, simpler semiconductor packaging,
In response to the demand for higher reliability, a method of directly bonding a copper circuit board onto an alumina substrate without using the aforementioned copper paste or brazing material has come into use. The direct bonding method for copper and ceramic substrates described in British Patent Publication No. 761045 involves strongly oxidizing copper in advance, placing the copper on an alumina substrate, and heating both to a temperature higher than 1083°C (the melting point of copper). , and heated at a temperature lower than the melting point of cuprous oxide (approximately 1200°C). During the heating cycle, most of the cupric oxide is converted to cuprous oxide, so the copper plate reaches its melting point and melts, but the cuprous oxide is not melted, and a layer of cuprous oxide is formed in the boundary area between the alumina substrate and the copper. It is stated that there will be. Furthermore, in the ``Method for bonding a copper member to a ceramic substrate'' disclosed in Japanese Patent Publication No. 60-4154, before bonding and arranging a copper plate and a ceramic substrate,
200~ on the copper member surface or ceramic substrate in advance
A copper oxide layer with a thickness of 5000 Å is formed, and then the copper member and the ceramic substrate are stacked together and placed in an inert atmosphere. The temperature at which a crystalline melt is formed is 1065℃ (Cu-O eutectic temperature) and 1083℃.
A direct bonding method has been described which consists of heating to a temperature between 0.degree. C. and cooling. Furthermore, in Japanese Patent Application Laid-Open No. 63-166774 entitled "Method for manufacturing a bonded body of a copper plate and an alumina substrate," a copper plate and an alumina substrate that are in contact with each other are heated in an inert atmosphere at a temperature lower than 1083°C. By heating to a high temperature, a compound is formed between the copper and the alumina and glass-forming substance on the surface of the alumina substrate without forming a eutectic liquid phase of copper and copper oxide at the contact area, and then cooling. A direct bonding method is disclosed. In this case, when carrying the copper plate and alumina substrate to be directly bonded into a conveyor furnace, the stacked sheets are placed on a SiC pedestal, automatically carried in, and heat treated for bonding in the conveyor furnace. ing. [Problems to be Solved by the Invention] However, when using the SiC pedestal in the above-described manner, it has been found that there are the following problems. First, the production itself of the SiC pedestal, that is, the SiC
It is difficult to sinter SiC powder, and free Si and free C are generated during the manufacturing process of SiC powder and during its sintering, and these are contained in the pedestal. If the bonding process is performed in an inert atmosphere with the copper plate placed, an appropriate atmosphere cannot be maintained, especially if free Si is present,
It has been found that there are problems such as the formation of an alloy and the appearance of a golden color. In addition, this alloying makes it possible to bond with copper plates.
Since it is welded to the SiC pedestal, it becomes difficult or impossible to remove it from the pedestal, and even if it can be removed, the resulting joined body cannot be marketed as a product. On the other hand, the presence of free C creates a local reducing atmosphere in a part of the contact surface between the copper plate and the alumina substrate, which inhibits the formation of oxides that should be formed at the bonding interface. It was found that the parts tend to become non-bonded parts. Therefore, if you try to mass produce using this method,
Since the inspection process to prevent quality deterioration has become large-scale, which is a factor in high costs, some kind of solution has been needed. [Means for Solving the Problem] The inventors of the present invention conducted extensive research to solve the problem, and found that they used a ceramic pedestal made of alumina, and further coated the surface of the alumina pedestal with alumina powder. The inventors have discovered that all of the above-mentioned problems can be solved by arranging and using the device, and have achieved the present invention. That is, the present invention, in its most preferred embodiment, is a method of bonding a laminate consisting of at least one metal plate and at least one ceramic substrate stacked in contact with each other in an inert atmosphere. , the first step is to use an alumina pedestal as a pedestal to support the metal plate at the bottom of the laminate, and further place alumina powder on it; the metal plate and the ceramic substrate are brought into contact with each other on the alumina powder; A second step in which the laminate is placed and heat treated for bonding in an inert atmosphere; and a third step in which the alumina powder welded to the metal plate at the bottom of the laminate is dissolved and removed by acid treatment. The present invention provides a method for joining metal and ceramic with special characteristics. However, in the present invention, ceramic powder is placed on a ceramic pedestal, a laminate to be bonded is placed on the ceramic powder and heat treated to obtain a laminate as a bonded body, and then the metal surface of the bonded body is Since the present invention provides a method characterized in that ceramic powder welded to the metal surface is removed by acid treatment of the metal surface, it is preferable to use a ceramic pedestal and ceramic powder made of alumina, but it is not necessarily limited to alumina. It is not something that will be done. [Function] The material of the pedestal used in the method of the present invention is a stable material that does not disturb the atmosphere during bonding, that is, it does not decompose or dissociate at high temperatures, and is industrially stable. Moreover, any material may be used as long as the bonded product obtained after the bonding process can be easily removed. Non-oxide ceramic materials are thought to be appropriate as ceramic substrates that do not react with the copper plate at high temperatures, but in actual tests, many of them partially decompose or dissociate at high temperatures. Free components are often generated during the production of raw material powder and the production of ceramic sintered bodies. The present inventors have conducted numerous experiments using various non-oxide ceramic materials, and have confirmed that in many cases, the tough pitch copper used is partially reduced due to the formation of a local reducing atmosphere. . Therefore, we decided to abandon the use of non-oxide ceramic materials and consider using oxide ceramic materials. However, oxide ceramic materials are bonded to copper plates under appropriate temperature and atmospheric conditions. Therefore, if this is used as a material for a pedestal, the pedestal will be attached to the product and the product will not be finished. Moreover, since the adhesive strength is quite high, it cannot be easily separated. Therefore, the present inventors have found that by using a pedestal made of oxide-based ceramic material and further placing appropriate ceramic powder on the surface of the pedestal that should be in contact with the metal plate, the pedestal and the copper plate can be directly bonded. In addition, since the alumina powder bonded to the copper plate is separated from the pedestal, we conducted an experiment assuming that the resulting bonded body would easily separate from the pedestal, and found that the effect was as expected. This has been confirmed to be possible. In the examples, alumina powder was used as the ceramic powder used in the method of the present invention, but it is (1) inexpensive, (2) it is made of the same material as the pedestal to which the alumina powder is applied, so it has the same heat capacity, and (3) Alumina material is stable at high temperatures and does not create a reducing atmosphere. Furthermore, the reason for using an alumina pedestal as the pedestal on which alumina powder is applied is substantially the same as in (1) to (3) above. The above alumina powder has a particle size of several tens of micrometers to several micrometers.
It has been confirmed through experiments that it is preferable to use By heat treatment for bonding, the copper plate in the laminate placed on the pedestal via the ceramic powder and the alumina substrate are directly bonded, and the alumina powder placed on the pedestal also becomes part of the copper plate. Join. However, it was confirmed that the alumina pedestal itself did not bond to the alumina powder spread on it, nor did it bond to the copper material that was in contact with it via the alumina powder. Therefore, it was easy to separate the joined body from the pedestal. There are mechanical removal methods such as polishing and vibration to remove the alumina powder bonded to the copper plate, but these methods are undesirable as they give a shock to the bonded body, and they also cause scratches on the surface of the copper plate. This tends to result in unsuitable products. Therefore, the present inventors took advantage of the property that copper plates are etched by acid, and devised a method of immersing the entire bonded body in an acid solution to dissolve the entire copper surface and remove the alumina powder. In this case, the alumina powder and the copper plate are bonded in a point contact state, so when the bonded body is immersed in an acid solution, the acid quickly circulates around the point bond and dissolves the copper. Therefore, the point-bonded alumina powder could be easily removed. Although the part that is point-bonded to the alumina powder is in contact with the etching solution for a shorter period of time than the surrounding area, it was confirmed that there were no traces left in the spot-bonded part that would pose a problem as a product. Under the bonding temperature range adopted in the method of the present invention, the alumina powder is not sintered, and some of the alumina powder bonded to the copper plate is surface-to-face bonded with a wide contact surface, so etching may occur. There was nothing that was difficult to remove. Hereinafter, it will be explained in detail using examples. [Example 1] As a copper plate and an alumina substrate to be bonded to each other, a tough pitch copper plate with a thickness of 0.3 mm and a size of 28 mm x 40 mm and a 96 plate with a thickness of 0.635 mm and a size of 30 mm x 42 mm were used.
% alumina substrate was prepared. A 96% alumina plate measuring 50 mm x 50 mm x 0.635 mm <sup>t</sup> was used as the pedestal, as shown in Figure 1.
Alumina powder 3 having a particle size of 2 to 10 μm is applied onto the surface of the pedestal 4 (first step), and then a copper plate 22, an alumina substrate 1, and a copper plate 21 are placed on the pedestal from below as shown in FIG. The laminated bodies were stacked one on top of the other and transported into a conveyor furnace, where they were heated at 1063℃ under an inert atmosphere consisting of nitrogen gas with an oxygen concentration of 10ppm.
Bonding was performed under bonding conditions controlled at temperatures of 1067° C. and 1075° C. (second step). After the obtained joined body was cooled, it was taken out into the atmosphere and the state of bonding between the joined body and the pedestal was investigated. Table 1 shows the results. In this case, the bonding condition was evaluated using 20 samples of each type, and after heat treatment, the pedestal was rotated 90 degrees and stood up so that the contact surface with the laminate was vertical. The number of cases in which the body naturally separated from the pedestal was recorded in the ``Not connected to the pedestal'' column, and the other cases were recorded in the ``Connected to the pedestal'' column.

[Table] From Table 1, it can be seen that there was no bonding between the laminate and the pedestal after the bonding was completed. The cross-sectional structure of the joined body obtained in the second step is as shown in Fig. 2, with copper plates 21 and 22 of 0.3 mm ^thick on both sides of an alumina substrate of 0.635 ^mm thick.
are joined. The copper plate 22 in the figure is placed on a pedestal in the method of the present invention, and the alumina powder 3 on the pedestal is attached to the copper plate in a point-bonded state. Next, 50% of this point-bonded alumina powder 3
It was kept in a nitric acid solution for a certain period of time to dissolve and remove it (third step). In this case, we tried varying the retention time in the liquid in order to determine the appropriate retention time in the liquid, and found that
It was found that if held for 30 seconds or more, the alumina powder remaining on the surface of the copper plate 22 was completely removed, and the surface of the copper plate became healthy. When the thickness of the bonded body (copper plate 21, alumina substrate 1, and copper plate 22 integrated) that was immersed in the liquid for 30 seconds was measured with a micrometer, it was only about 2 μm thinner than the thickness before immersion. There were no problems with the dimensions of the joined product. Furthermore, although a 50% nitric acid aqueous solution was used as the acidic solution, since the alumina substrate is a substance with excellent oxidation resistance, the alumina substrate itself was not eluted. [Example 2] Similar to Example 1, a pedestal made of 96% alumina was used to conduct an experiment to investigate ceramic powder suitable for coating on the pedestal. The ceramic powders used were Al ₂ O ₃ , AlN,
There are five types: SiC, BN, and Si ₃ N ₄ , but for SiC in particular, we used commercially available products from three companies.
The symbols A, B, and C were added to SiC to distinguish them. Table 2 shows the results of a test in which a laminate of a copper plate and an alumina substrate similar to that in Example 1 was placed on the ceramic powder coated on these pedestals to obtain a laminate. The obtained bonded body was evaluated as "O" if the copper plate and alumina substrate were soundly joined, and "X" if the copper plate and alumina substrate were not soundly joined.

[Table] The grain boundaries of the copper plates that were not bonded at this time were smaller than those of the copper plates that were bonded, and when observed with an optical microscope, Cu ₂ , which should exist in the tough pitch copper, was observed. No O phase was observed.
This is SiC-C coated on an alumina pedestal.
Dissociation of ceramic powders such as AlN, BN, Si ₃ N ₄ , etc.
This is thought to be due to the deoxidation of tuff pit copper due to decomposition. This also shows that a stable atmosphere is formed during bonding when not only Al ₂ O ₃ powder is used as the ceramic powder but also alumina material is used as the material for the pedestal. This is because alumina material is a very stable substance. [Effects of the Invention] As described above, according to the present invention, a copper plate and an alumina substrate can be heated and bonded in a stable atmosphere. In addition, not only can inexpensive alumina material be used as the pedestal, but the simple method of using ceramic powder is effectively used to prevent the pedestal from joining with the copper plate. , can bring about a reduction in manufacturing costs.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an example of the arrangement of a laminate of a copper plate and an alumina substrate on a pedestal before being put into a furnace. Second
The figure is a sectional view showing an example of a bonded body of a copper plate and an alumina substrate. FIG. 3 is a schematic cross-sectional view showing the dissolved state of the surface of the copper plate after the bonded body shown in FIG. 2 is immersed in acid. Explanation of symbols: 1...Alumina substrate, 21...Copper plate, 22...Copper plate, 3...Alumina powder, 4...Alumina pedestal.

Claims (1)

[Scope of Claims] 1. A method for bonding a laminate consisting of at least one metal plate and at least one ceramic substrate alternately stacked on top of each other by holding the laminate at high temperature in an inert atmosphere, the laminate comprising: The first step is to place the ceramic powder on a pedestal for supporting the lowermost metal plate in contact with it; the laminate of the metal plate and the ceramic substrate is placed on top of the ceramic powder and placed in an inert atmosphere at a high temperature. a second step in which the metal plate and the ceramic substrate are bonded together by holding the metal plate with
A method for joining metal and ceramic, comprising the steps of: 2 The inert atmosphere in the second step is 1%
The method of joining metal and ceramic according to claim 1, characterized in that the atmosphere is an inert atmosphere containing the following oxygen: 3. The method according to claim 1 or 2, wherein the pedestal is an alumina pedestal and the ceramic powder is alumina powder.