JP5597946B2 - Low-temperature metal joining method - Google Patents

Description

  The present invention relates to a joining method used for joining, for example, various electronic parts such as semiconductor elements, motors, and sensors, mechanical parts, structural parts, and more specifically, more specifically than the melting point of a metal material constituting a material to be joined. The present invention relates to a low-temperature bonding method capable of bonding at a low temperature and a bonding apparatus used in the method.

  Conventionally, for example, when a semiconductor chip and a substrate wiring layer are bonded to each other, for example, when a semiconductor chip and a substrate wiring layer are bonded to each other, for example, an electric component for an automobile is generated inside. It is necessary to solve various problems caused by heat. As one of the problems, there is a thermal shock property at the time of joining the semiconductor chip and the substrate.

Further, when the density and current of the chip increase and the temperature environment becomes severe, bonding by brazing using a low melting point brazing material such as a Pb—Sn alloy cannot be adopted from the viewpoint of heat resistance. On the other hand, when a higher melting point brazing material, such as an aluminum-based brazing material, is used, the melting point is too high, heat shock during brazing joining is large, and problems such as cracking and peeling may occur during joining. .
In the first place, these brazed portions are generally poor in ductility, and even if they can be joined, they are insufficient in durability against thermal stress due to temperature cycles during use and temperature differences between parts. Moreover, it may react with Cu or Al which is a wiring layer at the time of bonding to generate a fragile intermetallic compound.

  As a bonding method at such a low temperature, for example, Patent Document 1 proposes a method of performing bonding after irradiating a bonding surface of an object to be bonded with an argon ion beam and performing a cleaning process.

Japanese Patent Application Laid-Open No. 08-118043

  However, in the method described in Patent Document 1, a large pressing force is required to secure a contact area when pressing and joining at normal temperature after cleaning the sample surface. As a result of plastic deformation, desired joint strength may not be obtained.

  The present invention has been made in view of the above-mentioned problems in joining in a low temperature range such as brazing, and the object is to obtain good joining strength at a relatively low temperature. Another object is to provide a low-temperature bonding method. Moreover, it aims at providing the joining apparatus which can be used suitably for such low temperature joining.

  As a result of intensive studies to achieve the above object, the present inventors have found that the above problem can be solved by forming fine irregularities in advance on the bonding surfaces of the materials to be bonded, The present invention has been completed.

That is, this invention is based on the said knowledge, Comprising: As for the low-temperature joining method of the metal of this invention, the distance between adjacent convex parts or the distance between recessed parts is 200- at least one of the joining surfaces in copper to- be-joined materials. The joining surfaces are abutted with each other in a state in which fine unevenness having an arbitrary value between 1000 nm and an elevation difference between adjacent unevenness portions of 40 to 300 nm is formed, and the melting point of the copper joining material The material to be joined is joined by heating to a lower temperature.

Further, the joining apparatus of the present invention is suitably used for the above-described low-temperature joining method, and includes a closed furnace capable of controlling the atmosphere containing the copper-made material to be bonded, and a heat control means for adjusting the temperature in the closed furnace. And the distance between the convex portions adjacent to at least one of the bonding surfaces of the copper workpiece or the concave portion is an arbitrary value between 200 and 1000 nm, and the height difference between the adjacent concave and convex portions is 40 to 300 nm. It is characterized by comprising a laser processing means for performing fine uneven shape processing which is an arbitrary value between and a pressurizing means for abutting the joining surfaces of copper to- be-joined materials.

  According to the present invention, since nano-order fine irregularities are formed on at least one of the bonding surfaces, the bonded materials can be bonded with good bonding strength at a temperature lower than the melting point of the bonded materials. Will be able to.

It is explanatory drawing which shows the setting state of the femtosecond laser processing apparatus used for the fine unevenness | corrugation processing in the Example of this invention. It is a SEM (scanning electron microscope) image which shows an example of the fine uneven | corrugated shape formed in the joining surface of a to-be-joined material in this invention. It is a SEM image which shows the surface state at the time of heating the pure copper material provided with the fine unevenness | corrugation to 600 degreeC (b), 500 degreeC (c), and 400 degreeC (d) before a heating (a). It is the schematic which shows an example of the joining apparatus of this invention. It is explanatory drawing which shows the arrangement | positioning condition of the to-be-joined material in Example 1, 2 of this invention. It is a SEM image of the fracture surface of the tensile test piece obtained by Example 1 of this invention. It is explanatory drawing which shows the arrangement | positioning condition of the to-be-joined material in the reference example 3 of this invention. It is a SEM image which shows the fine uneven | corrugated shape of the center part (a) in the joint surface by Example 4 of this invention, the center part (b) by high magnification, and a periphery part (c) similarly. It is a SEM image of the fracture surface in the tension test piece joined at 400 degreeC (a) and 700 degreeC (b) in Example 4 of this invention. It is explanatory drawing which shows the arrangement | positioning condition of the to-be-joined material in the comparative example 2.

  Hereinafter, the low-temperature bonding method of the present invention will be described in more detail and specifically.

  The melting start temperature of the metal particles is known to decrease as the particle size decreases, but the particle size at which such an effect starts to appear is 100 nm or less, and the effect becomes significant when the particle size is 20 nm or less. Become. In particular, depending on the metal, when the particle size is 10 nm or less, the metal melts at a temperature considerably lower than the melting point of the metal in the bulk state and bonds to each other.

This is explained thermodynamically as follows. Here, let us consider a case where a part of spherical fine particles having a radius r, a density ρ, and a surface energy Es are melted.
If the heat of fusion of the constituents of the particles is ΔHm, the entropy of fusion is ΔSm, and the surface area of dA is reduced due to dissolution of the weight of dW, the following equation (1) is established.
ΔHmdW−ΔSmTsdW−EsdA = 0 (1)
In the formula (1), Ts is a phase change temperature (melting point).

On the other hand, in the case of a bulk solid that has a negligible decrease in surface area due to melting, the energy balance at the time of melting is expressed by the following equation (2).
ΔHmdW−ΔSmTsdW = 0 (2)

  It is said that the melting point lowering phenomenon is caused by the contribution of the decrease of the surface energy in this way (refer to the Chemical Society 48 “Chemical Review 48 Ultrafine Particles—Science and Application” Society of Science Publishing Center, p. 47-56).

Furthermore, a particle sintering phenomenon occurs prior to melting, but this sintering start temperature is also significantly lower than in the case of bulk, and bonding occurs by low temperature sintering.
For example, in the case of Ag ultrafine particles with an average particle size of 20 nm, published data that sintering starts at a low temperature of 60 to 80 ° C. (Yasuo Sato “Preliminary Proceedings of the Metallurgical Society of Japan, from production to application of metal ultrafine particles”, 1975 , P.26).

Such a phenomenon is explained as follows.
When solid particles come into contact with each other, the contact area gradually increases with time under high temperature or pressure. This phenomenon is caused by a driving force (Driving force) that reduces the free energy (or chemical potential) of a system consisting of a plurality of particles and the environment surrounding them in order to shift to a more stable system as a whole. Based on. Since the surface energy of the first particle system is larger as the particle size is smaller, the driving force of sintering is said to be larger as finer raw material powder is used (“Ceramic Engineering Handbook No. 1 edited by Japan Ceramic Society) 2nd edition ", Gihodo Publishing, p. 109-121).

When fine irregularities are present on the surface of the materials to be joined, the surfaces are made less elastic, the contact between the materials to be joined is promoted, and the degree of close contact with each other is improved. Further, the increase in the contact portion increases the diffusion path of metal atoms, so that the diffusion reaction is promoted. As a result, bonding at a low temperature and in a short time becomes possible.
In addition, by providing fine irregularities, a large load is generated at the convex tip, thereby destroying the surface oxide film. Since the surface oxide film is a factor that hinders bonding, the new surfaces of the materials to be bonded come into direct contact with each other due to the destruction, and a good bonded portion is formed. Furthermore, as a secondary effect, it is possible to bring out an anchor effect due to the fact that the fine irregularities are bitten.

  When the finer shape becomes finer, it becomes possible to induce the melting point lowering phenomenon due to the effects other than the improvement of the adhesion degree due to the low elasticity and the increase of the diffusion path.

In the low-temperature bonding method of the present invention, as described above, fine unevenness having a height difference of 40 to 300 nm with a distance between tops or valleys of 200 to 1000 nm is formed on the bonding surface of at least one material to be bonded. It is formed and bonded by heating to a temperature lower than the melting point of the materials to be bonded.
That is, the low-temperature bonding method of the present invention is characterized by positively utilizing the decrease in surface energy due to the formation of fine irregularities on at least one of the opposing surfaces of the material to be bonded forming the bonding surface. The melting point lowering phenomenon and the agglomeration phenomenon are realized, and the bonding is realized at a temperature lower than the melting point. Therefore, the materials to be joined can be joined without any deformation caused by thermal deformation or pressurization during joining. Further, there is almost no altered layer (heat-affected zone) of the base material such as fusion welding.

Here, when the distance between the protrusions and the recesses in the fine unevenness (distance between adjacent tops or valleys) exceeds 1000 nm, or the height difference between adjacent uneven parts exceeds 300 nm, Such melting point lowering phenomenon and aggregation phenomenon are less likely to occur.
In addition, as an above-described distance and height difference, it is more preferable to exist in the range of 200-800 nm and 40-150 nm, respectively.

  In the low-temperature bonding method of the present invention, in order to process the fine unevenness as described above on the bonding surface of the material to be bonded, for example, femtosecond laser processing, anodic oxidation method, FIB (focused ion beam) processing, nanoimprint processing, machine Processing (cutting, grinding) or the like can be used, and two or more of these processing methods can be combined. However, the method is not limited to these as long as it is a method capable of processing the fine uneven shape as described above.

  FIG. 1 shows a setting state of a femtosecond laser processing apparatus used for such fine unevenness processing. The oscillation laser is a shutter 5, an aperture 6, a λ / 2 plate 7, a Glan laser prism 8, and an objective lens 9. (Aperture ratio: 0.13) is sequentially passed.

FIG. 2 shows an example of a fine uneven shape formed on the surface of pure copper by femtosecond laser processing using the above-mentioned apparatus. Here, according to the apparatus, the pulse width: 120 fs, wavelength: 800 nm, The processed surface was irradiated with a femtosecond laser having a pulse energy density of 1 mJ / pulse and a repetition frequency of 1 kHz. The wavelength is changed by a BBO (beta barium bolite) crystal.
As a result, it was confirmed that fine unevenness was formed on the processed surface of pure copper in the range where the distance between adjacent convex portions was in the range of 240 to 720 nm and the height difference between adjacent uneven portions was in the range of 48 to 144 nm. It was done.

3 (a) to 3 (d) show that the pure copper material provided with fine irregularities by the femtosecond laser processing is heated to 600 ° C. (b), 500 ° C. (c) and 400 ° C. (d), respectively, in a vacuum furnace. It shows the surface state when compared with (a) before heating.
As is clear from these figures, while the melting point of copper is 1083 ° C., there is a phenomenon that the fine uneven shape of the copper surface collapses even in the above temperature range, particularly 400 ° C. By forming the irregularities, it was confirmed that a melting point lowering phenomenon and an agglomeration phenomenon as observed in the fine particles during the sintering occurred.

  The kind of the material to be joined to which the low-temperature joining method of the present invention is applicable is not limited to the above-described copper. In addition, the bonding method can be applied not only to the same kind of material but also to the joining of different kinds of materials.

As specific steps for realizing the low-temperature bonding method of the present invention, a step of forming the above-described fine irregular shape on at least one bonding surface, a step of matching the bonding surfaces of the materials to be bonded, The process of heating the joining surface of a to-be-joined material is required.
At this time, it is desirable to perform one or both of the fine uneven shape forming step and the bonding surface heating step in a vacuum, a low oxygen concentration atmosphere, or a non-oxidizing atmosphere, thereby suppressing oxidation of the bonding surface. And a healthier joint can be obtained. It is more desirable to perform both steps continuously in the same atmosphere.

In the low temperature bonding method of the present invention, the above-mentioned low oxygen concentration atmosphere means a case where the oxygen concentration is about 500 ppm or less.
In addition, as a non-oxidizing gas for making a non-oxidizing atmosphere, nitrogen and argon are given as typical examples. However, as long as the gas has an action of suppressing the oxidation of the bonding surface of the material to be bonded as compared with the atmosphere. Good.

In the low temperature bonding method of the present invention, it is desirable to add a step of applying a coating for preventing oxidation to the processed surface simultaneously with the processing of the fine uneven shape or between the processing step and the step of matching the bonding surface.
As a result, it is possible to omit atmosphere control and evacuation at the time of fine unevenness processing, and the apparatus can be simplified. In addition, it becomes possible to suppress the oxidation of the surface of the fine irregularities formed on the material to be joined between the fine irregularity processing step and the bonding step, and the degree of freedom in the construction method between the processes from the processing to the bonding is increased. Become.

  Examples of the coating agent for preventing oxidation as described above include at least one resin selected from the group consisting of polyethylene (wax), polyacrylate, polyamine, polyamide, urethane, polyether, polyester and polysilicate. , Or a mixture of these resins can be used. In addition, since these coating agents are decomposed | disassembled by the heating at the time of joining, they can be used for joining with apply | coating.

The joining apparatus of the present invention contains a material to be joined, a sealed furnace capable of controlling the atmosphere in a non-oxidizing gas such as vacuum or Ar, a thermal control means for adjusting the temperature in the sealed furnace, A laser processing means for performing fine unevenness processing on the joining surface of the material and a pressing means for abutting the joining surfaces of the materials to be joined are provided, and can be suitably used for the low-temperature joining of the present invention.
Moreover, the said joining apparatus can also be provided with the coating agent injection means for coating a joining surface which gave the fine unevenness | corrugation as needed.

  FIG. 4 shows an example of the bonding apparatus of the present invention. The bonding apparatus 10 shown in the figure can control the atmosphere by evacuating the furnace or replacing it with an inert gas such as Ar. A femtosecond laser processing apparatus is set in the diffusion bonding apparatus.

That is, the joining apparatus 10 includes a sealed furnace 11 capable of controlling the atmosphere, a thermal control apparatus 12 that adjusts the temperature in the sealed furnace, and the joining surfaces of the materials 1 and 2 accommodated in the sealed furnace 11. A femtosecond laser processing apparatus 13 as laser processing means for performing fine unevenness processing and an air cylinder 14 as pressurizing means for abutting the bonding surfaces of the materials 1 and 2 to be bonded are provided.
In the bonding apparatus 10, coating agent spraying means (not shown) for coating the bonding surface at the same time as or immediately after the fine unevenness processing is disposed at the same position as the laser processing apparatus 13. May be.

  Hereinafter, the present invention will be specifically described based on examples. Needless to say, the present invention is not limited to these examples.

Example 1
As shown in FIG. 5, a material to be joined 1 made of a pure copper material and having a cylindrical shape with a diameter of 5 mm and a length of 15 mm, and a material to be joined 2 made of a pure copper material and having a cylindrical shape with a diameter of 10 mm and a length of 25 mm. Were subjected to fine concavo-convex processing by a femtosecond laser on the joint surface of the columnar workpiece 2 having a diameter of 10 mm.
That is, by using a laser processing apparatus configured as shown in FIG. 1 and projecting a femtosecond laser having a pulse width of 120 fs, a wavelength of 800 nm, a pulse energy density of 1 mJ / pulse, and a repetition frequency of 1 kHz, Fine irregularities having a distance of 240 to 720 nm and a height difference of 48 to 144 nm were formed.

And both the to-be-joined materials 1 and 2 were butt-joined by heating to 400 degreeC and 700 degreeC, and applying the pressurization force of 5 Mpa for 30 minutes. Note that the fine unevenness processing and the joining step at this time were both continuously performed in an Ar atmosphere.
After joining, a tensile test piece was cut out and subjected to a tensile test.

FIG. 6 shows the observation result of the fracture surface after the tensile test by a scanning electron microscope. The fracture surface of the joint shows a dimple shape showing ductile fracture and forms a good joint. It has been found.
Thus, according to the present invention, it was confirmed that good bonding can be realized even at a low temperature of 400 ° C., which is about ３ of the bulk solid melting point.

(Example 2)
Both the joint surfaces of the materials to be joined 1 and 2 are subjected to the same fine unevenness processing, and immediately after the processing, an antioxidant containing a urethane resin as a main component for preventing oxidation of the processed surface (thermal decomposition temperature) : 200 ° C.) The same operation as in Example 1 was repeated except that the materials to be joined 1 and 2 were joined, and then a tensile test was conducted in the same manner.
As a result, it was confirmed that good bonding could be realized even at a low temperature of 400 ° C., and the fracture surface after the test showed a dimple fracture surface similar to that of FIG.

( Reference Example 3)
As shown in FIG. 7, a thickness obtained by performing the same fine unevenness processing on both surfaces between a 5 mm-diameter workpiece 1 made of a pure copper material and a 10 mm-diameter workpiece 2 made of the same pure copper material. The same operations as those of Example 1 were repeated except that the pure nickel foil 3 having a thickness of 50 μm was sandwiched as an intermediate material, and the materials 1 and 2 were bonded via the intermediate material 3.
As a result, it was confirmed that good bonding can be realized even at a low temperature of 400 ° C. And as a result of cutting out the tensile test piece from the joint part and carrying out the tensile test in the same manner, the fracture surface of the test piece shows a similar dimple fracture surface, and even when a nickel intermediate material is sandwiched, a good joint part is obtained. It was confirmed that it was obtained.

Example 4
Fine concavities and convexities having a distance between convex portions of 800 to 1000 nm and a height difference of 150 to 300 nm were formed on the joint surface of the columnar workpiece 2 having a diameter of 10 mm by cutting using a diamond tool. In this case, the fine shape processing step was performed in an air atmosphere, and after processing, pickling was performed to remove the oxide film.
8A to 8C show the fine shape formed on the joining surface of the columnar workpiece 2 by this.

Except for the above, the same operation as in Example 1 was repeated, and after joining the materials 1 and 2 to be joined, a tensile test was carried out in the same manner.
As a result, as in Example 1, it was confirmed that good bonding was realized even at a low temperature of 400 ° C., which is about 1/3 of the melting point of the bulk solid.
Moreover, as shown in FIGS. 9A and 9B, the fracture surface of the tensile test showed a dimple fracture surface, and it was found that a good joint was formed.

(Example 5)
Except that the bonding atmosphere was a nitrogen atmosphere with an oxygen concentration of about 400 ppm, the same operation as in Example 4 was repeated to bond the materials to be bonded 1 and 2 and then the tensile test was performed in the same manner.
As a result, it was confirmed that good bonding was realized even at a low temperature of 400 ° C. as in Example 1. Moreover, the fracture surface of the tensile test showed a dimple fracture surface, and it was found that a good joint was formed.

(Comparative Example 1)
When performing butt-joining (see FIG. 5) as in Example 1 above, the surface is similarly polished with a polishing mark having irregularities with a period of 100 μm made by # 80 abrasive paper without applying fine irregularities to the joining surface. I tried joining.
As a result, only extremely low strength joints were obtained not only at 400 ° C. but also at 700 ° C. Even when the fractured surface was observed, only a part of the convex and concave portions that had been machined was joined, and most of the fractured surface was unjoined.

(Comparative Example 2)
When performing the same joining as in Reference Example 3 above, the above comparison is made without subjecting the joining surfaces of the 5 mm diameter joined material 1 made of pure copper material and the 10 mm diameter joined material 2 made of pure copper material to fine unevenness processing. In the same manner as in Example 1, polishing marks having irregularities with a period of 100 μm were formed by using # 80 polishing paper. Then, as shown in FIG. 10, between the welded material 1 and 2, by a pure nickel foil 3 without thickness 50μm of finely roughened after sandwiching the intermediate material, the same procedure as Reference Example 3, intermediate The joined materials 1 and 2 were joined via the material 3.
However, at 400 ° C., the strength value is 1 or less, which is a sufficient value of 1 as compared with the case of Reference Example 3 subjected to fine unevenness processing, and although the strength is slightly improved, the joint strength is satisfactory even at 700 ° C. A good joint could not be obtained. Moreover, many unjoined areas were observed even when the fracture surface was observed.

(Comparative Example 3)
The joining surface of the columnar workpiece 2 having a diameter of 10 mm was mirror-finished by cutting using a diamond tool. After processing, pickling was performed to remove the oxide film. Except for this, the same operation as in Example 1 was repeated to join the materials to be joined 1 and 2 and then the tensile test was carried out in the same manner. The flatness of the obtained mirror surface was 1 μm or less, the 10-point average roughness (Ra) was 0.01 μm or less, and the distance between the convex portions was 0.1 mm.
As a result, it was found that the bonding strength remained at about half the value of Reference Example 3, and good bonding was not possible.

  The joining results of the above examples and comparative examples are summarized in Table 1.

DESCRIPTION OF SYMBOLS 1, 2 To-be-joined material 10 Joining apparatus 11 Sealed furnace 12 Thermal controller (thermal control means)
13 femtosecond laser processing equipment (laser processing means)
14 Air cylinder (Pressurizing means)

Claims (6)

The distance between adjacent convex portions or concave portions is an arbitrary value between 200 and 1000 nm on at least one of the bonding surfaces in the copper workpiece, and the height difference between adjacent concave and convex portions is 40 to 300 nm. A metal low-temperature bonding method characterized in that the joining surfaces are butted together in a state in which fine irregularities having an arbitrary value are formed, and are joined by heating to a temperature lower than the melting point of the copper material to be joined.   The distance between adjacent convex portions or concave portions in the fine unevenness is an arbitrary value between 200 and 800 nm, and the height difference between adjacent uneven portions is an arbitrary value between 40 and 150 nm. The low temperature bonding method according to claim 1. Comprising the step of heating and forming at least one of copper fine irregularities on the bonding surface of the bonding material, a process to match the joining faces of the copper material to be joined, the bonding surface of the copper material to be joined The low-temperature bonding method according to claim 1 or 2, wherein: 4. The low temperature according to claim 3, wherein at least one of the step of forming the fine unevenness and the step of heating the bonding surface of the copper joining material is performed in a vacuum, a low oxygen concentration atmosphere or a non-oxidizing atmosphere. Joining method.   The low-temperature bonding method according to claim 3 or 4, wherein the processed surface is coated at the same time as or after the processing of the fine uneven shape. A closed furnace capable of controlling the atmosphere containing the material to be bonded made of copper, a thermal control means for adjusting the temperature in the closed furnace, and between the convex portions or the concave portions adjacent to at least one of the bonding surfaces of the copper bonded material Laser processing means for performing fine uneven shape processing in which the distance is an arbitrary value between 200 and 1000 nm, and the height difference between adjacent uneven portions is an arbitrary value between 40 and 300 nm, and copper to- be-joined A joining apparatus comprising pressure means for abutting joining surfaces of materials together.

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