JP2015051452A - Joining method and structural body having joint part joined by the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a joining method capable of performing the joining accompanying atomic diffusion of thick materials to be joined with one side or both sides having thickness of 1.0 mm or more.SOLUTION: A microcrystalline thin film made of single metal or alloy of Au, Cu, etc. is formed on a flat surface of each of two materials to be joined having flat surfaces in a vacuum container. Thereafter, the two materials to be joined are overlapped onto each other such that the microcrystalline thin films formed on the two materials to be joined in an environment of atmospheric pressure are brought into contact with each other, at the same time, are pressed with a pressing force corresponding to a PV value as a difference of elevation between the highest point and the lowest point on the flat surface of the thick material to be joined, thereby, causes atomic diffusion on a joint interface and crystal grain boundary of the microcrystalline thin film and are joined with a joint strength of 4.0 MPa or more.

Description

  The present invention relates to a joining method and a structure having a joined portion joined by the above-described method, for example, a joining method used when joining a plurality of members, such as when joining a radiator to an electronic device, and the radiator in the method. It is related with the structure provided with the junction part joined by the said method like the electronic device etc. which joined.

  In order to obtain a structure having a desired function, assembling a structure having a predetermined structure by joining two or more different members is widely performed.

  As an example, in the field of electronic devices, such a joining technique is indispensable when mounting, integrating, packaging, and the like.

  For example, a package of electronic components such as LSI is generally provided with a heat sink for cooling a packaged electronic device, and particularly an insulated gate bipolar transistor (IGBT). In power devices such as LED, optical devices such as LEDs, etc., as a result of higher output and higher integration in order to improve their performance, more effective cooling as the amount of heat generation increases. Therefore, there is a demand for a bonding method with high thermal conductivity that can be performed.

  This type of heat radiator is conventionally attached by bonding a heat spreader called a heat spreader made of a material with good thermal conductivity by means of adhesive bonding, soldering, brazing, or the like. In addition, if necessary, a heat sink provided with heat radiating fins and the like is bonded to the heat spreader in the same manner as the heat spreader, so that heat generated in the electronic device is conducted to the heat spreader and the heat sink, and air Cooling is performed by dissipating heat through heat exchange.

  However, the above-mentioned adhesives, solders, brazing materials, etc. used for bonding such heat radiators do not necessarily have high thermal conductivity, and electronic devices and heat dissipating parts, heat dissipating parts and Since the bonding material is interposed between the heat dissipation parts with a thickness of several μm to several mm, the thickness of such a bonding material becomes a relatively non-negligible thickness as electronic devices become smaller and highly integrated. In addition, it increases the thermal resistance and hinders cooling of electronic devices.

  In addition, among the bonding methods described above, in soldering and brazing, it is necessary to heat the material to be bonded to a high temperature in order to bond the members together, which reduces the performance of electronic devices that are vulnerable to heat and defects due to thermal strain. And problems such as deterioration occur.

As a method of joining the materials to be joined by non-heating or heating at a relatively low temperature, the inventors of the present invention apply an ultimate pressure of 10 ^{− on} each smooth surface of the materials to be joined such as wafers and chips. ^In a vacuum atmosphere with a high degree of vacuum of ⁴ Pa or less, a film having a microcrystalline structure of metal or various compounds is preferably formed by a vacuum film-forming method such as sputtering or ion plating, and preferably under the generation of plasma. The film formed between the films was formed by polymerizing the films formed on the smooth surface of the material to be bonded at normal temperature while maintaining the vacuum during the film formation or after the film formation. A “room temperature bonding method” in which two materials to be bonded are bonded by bonding has already been proposed (see Patent Document 1).

In addition, the inventors of the present invention select a metal having a predetermined body diffusion coefficient and free oxide generation energy as the material of such a coating, and thus the same method as that described in Patent Document 1 described above. Even when a microcrystalline thin film is formed on the surface of the material to be joined and then the pressure is higher than 10 ⁻⁴ Pa, for example, when the material to be joined is taken out from the vacuum vessel and superposed under atmospheric pressure, In addition to finding out that the materials to be joined can be joined in the same manner as described in No. 1, this is proposed as an invention of the joining method (see Patent Document 2).

JP 2008-207221 A JP 2011-235300 A

  In any of the above-mentioned conventional room temperature bonding methods, since the film thickness of the microcrystalline film formed on the smooth surface can be formed to a thickness of about several tens to several hundreds of nanometers, the above-mentioned adhesive, soldering, and brazing The thickness of the bonding material existing at the bonding interface can be greatly reduced compared to the case of bonding by the above method, and as a result, a relatively thick bonding material layer is formed at the bonding interface. It is expected that the increase in thermal resistance can be reduced as much as possible.

  In particular, single metals included in the numerical ranges of the body diffusion coefficient and free energy of oxide defined in Patent Document 2 are gold (Au), silver (Ag), copper (Cu), palladium (Pd), platinum ( Since Pt), nickel (Ni), zinc (Zn), etc. are all metals with high thermal conductivity, when this joining method is applied to the joining of heat sinks, the thickness of the joining material layer as described above. It is expected that the thermal resistance at the joint interface can be greatly reduced in combination with the thin film.

  However, the method described in Patent Document 1 is not only based on the premise that superposition between smooth surfaces of the materials to be bonded is performed in the same vacuum together with film formation (for example, claim 1 of Patent Document 1), Even when bonding is performed in the same vacuum, “As the contamination progresses due to the reaction of the surface of the film with the impurity gas remaining in the vacuum vessel, the adhesion strength between the films decreases. As will be explained later, "(Patent Document 1," 0055 "column), in the conventional room temperature bonding method, after the formation of a film having a microcrystalline structure on the smooth surface, Superposition must be performed in the same vacuum vessel where the smooth surface treatment has been performed and with the vacuum vessel maintained in a high vacuum state, for example, after the formation of the microcrystalline thin film. The surface is exposed to atmospheric pressure air, etc. It is the recognition of those skilled in the art, including the inventors of the present invention, that the bonding itself becomes impossible once exposed, and on the premise of this recognition, the superposition of smooth surfaces is performed in a high vacuum space. The configuration performed in the above was adopted.

  For this reason, the work to superimpose the smooth surfaces of the materials to be joined is performed in a limited space in a vacuum vessel maintained at a high vacuum, under limited conditions. Not only is it difficult, but in order to realize the above-mentioned joining method, a special structure for superimposing the smooth surfaces of the materials to be joined placed in the vacuum vessel while keeping the vacuum vessel at a high vacuum is used. A robot arm, jig, and other attachment devices are required, which requires a large initial investment.

  On the other hand, in the room temperature bonding method introduced as Patent Document 2, the material to be bonded is selected from a vacuum container by selecting a metal having a predetermined body diffusion coefficient and free oxide generation energy as the material of the microcrystalline film. Even if it is taken out and superposed under atmospheric pressure as an example, since the joining can be performed, workability is greatly improved.

  Moreover, since the material of the microcrystalline thin film, that is, the bonding material in Patent Document 2, is a material having high thermal conductivity such as gold (Au), silver (Ag), copper (Cu), etc., the thermal resistance at the bonding interface is low. The occurrence is expected to be greatly reduced.

  However, when the heat sink was actually joined by the method described in Patent Document 2, it was possible to join the electronic device and the heat sink or between the heat sinks. The structure has a weak bonding strength and relatively easily peels off at the bonding interface, and when conducting heat conduction, the temperature of the smooth surface, which is the bonding interface, is not uniform, and the temperature is partially high. In other words, a large portion of thermal resistance was generated, and the high heat dissipation performance as expected could not be obtained.

  Therefore, when the cause of such a problem was examined, the reason why such a low joint strength and a partly large thermal resistance remain is that the heat sink is manufactured by cutting out a lump of metal such as aluminum or copper. In addition, it is composed of a lump of material (so-called “bulk”) that is thicker than a wafer or chip, and a relatively long period of unevenness that exists on the smooth surface that is the joining surface of this radiator. (Hereinafter referred to as “swell”).

  That is, in the conventional joining method performed by adhesive, solder, brazing, etc., as described above, between the smooth surface of the electronic device and the radiator, or between the smooth surfaces of the radiator, Since the joining material such as the brazing material is filled with a thickness of several μm to several mm, the smooth surface of the heat spreader such as a heat spreader or heat sink has a certain amount of undulation if it has a certain level of flatness. Even if accompanied, it is considered that such swells could be absorbed by the thickness of the bonding material.

  On the other hand, in the joining method described in Patent Document 2 described above, when a chip or wafer formed with a thickness of generally less than 1.0 mm is to be joined, the surface roughness of the chip or wafer is an arithmetic average. Even if the roughness Ra shows the same numerical value as that of the joining surface of the radiator, it is possible to join without applying pressure at the time of bonding, as expressed by the arithmetic average roughness Ra. It is expected that irregularities with a relatively short period are not the cause of hindering the bonding of bulk materials, but looking at the bonding interface between the bulk materials where the bonding strength could not be obtained, It is considered that the undulation of the surface, which is longer-period unevenness than the short-period unevenness appearing in the numerical value with the arithmetic average roughness Ra, inhibits the joining of the bulk materials.

  Then, in the joining of chips and wafers, since the joining object is thin, such swell can be absorbed by the deformation of the chip or wafer (warping), but one or both of the materials to be joined are In the case of a bulk having a sufficiently large thickness compared to a chip or wafer as in the case of the heat sink described above, deformation that can absorb waviness cannot occur because the bonding strength is reduced. It is believed that there is.

  In addition, in the above-described joining by solder or adhesive, it is possible to fill the joining interface with a joining material such as solder or adhesive and absorb the above-described swell by the thickness. When the radiator is bonded by the described method, the above-described swell can be absorbed by the thin film bonding material of several nm to several hundred nm interposed between the electronic device and the radiator or the bonding interface between the radiators. As a result, it is considered that the smooth surface is joined only partially, resulting in insufficient joining strength, and a gap (void) generated in the part that is not joined causes high thermal resistance.

  In order to solve the above-mentioned problems of insufficient bonding strength and thermal resistance that may occur when the joining method described in Patent Document 2 is applied to joining of a radiator, etc., smoothing of a radiator such as a heat sink or a heat spreader is possible. It is also conceivable to polish the surface to a smooth surface with a higher accuracy than required for conventional radiators.

  However, when performing such high-precision polishing, a great deal of cost is required for polishing the radiator, and this cost is reflected in the price of the product and loses price competitiveness in the market.

  As another method, the thickness of the bonding material (microcrystalline thin film) interposed at the bonding interface may be increased to a thickness that can absorb the swell generated on the surface of the radiator.

  However, it takes a long time to form such a thick film. When bonding is performed by this method, productivity decreases, and as described above, if the film thickness intervening at the bonding interface increases, An increase in thermal resistance is also expected.

 In the above description, a heat sink such as a heat sink or a heat spreader has been described as an example of a material to be bonded. However, such a bonding failure at the time of bonding is the bonding of other parts other than the heat sink. It can also be a problem.

 In other words, insufficient bonding strength due to the partial contact of the bonding interface is a wide problem regardless of the use of the structure obtained by bonding, and the smooth surface is only partially in contact. In addition, the fact that gaps (voids) are formed at the joint interface not only lowers the thermal conductivity, but also lowers the transmission characteristics of vibrations, for example, when joining vibrators. In the case of electronic parts, physical and mechanical properties such as increased electrical resistance are reduced.

  Therefore, the inventors of the present invention applied this bonding method to bonding of a relatively thick material to be bonded such as a heat radiator, based on the bonding method described in Patent Documents 1 and 2 described above. In order to solve the above-mentioned problems that may occur in some cases without improving the flattening accuracy of the smooth surface of the structural material or increasing the film thickness of the microcrystalline thin film, the existing parts are made special. It can be used without processing, etc., and it can improve physical and mechanical properties such as heat transfer performance, vibration propagation performance, electrical and magnetic properties, and obtain the required joint strength without reducing productivity. It is an object of the present invention to provide a bonding method that can perform the above and a structure having a high heat dissipation performance, including a bonding surface bonded by the above method.

In order to achieve the above object, the joining method of the present invention comprises:
A method of joining a smooth surface provided on one material to be joined and a smooth surface provided on the other material to be joined in a polymerized state,
At least one of the one or the other material to be joined is a thick material to be joined constituted by a material lump (bulk) having a thickness of 1.0 mm or more,
A smooth surface having a microcrystalline structure made of a single metal or an alloy and having a microcrystalline structure made of a single metal or an alloy on the smooth surface of one of the materials to be joined in a vacuum vessel. The one and the other two materials to be joined are overlapped so that the thin film formed on the one material to be joined is in contact with the smooth surface of the other material to be joined. Based on the PV value which is the difference in height between the highest point and the lowest point within a predetermined measurement range on the smooth surface of the bonding material, a small pressing force is applied when the PV value is small, and a large pressing force is applied when the PV value is large. The two substrates are bonded at a bonding strength of 4.0 MPa or more by causing atomic diffusion at the bonding interface with the smooth surface of the other substrate and at the crystal grain boundary (Claim 1).

A thin film having a microcrystalline structure formed on the smooth surface of the one bonded material has a body diffusion coefficient of 1 × 10 ⁻⁴⁰ (m ² / s) or more at room temperature and free energy of formation of oxide at room temperature. Is made of a single metal or alloy of -15 (kJ / mol of compounds) or more, and at least the smooth surface of the other material to be joined has a body diffusion coefficient of 1 × 10 ⁻⁴⁰ (m ² at room temperature). / s) and a microcrystalline structure composed of a single metal or alloy having a free energy of formation of oxide at room temperature of −15 (kJ / mol of compounds) or more,
The two materials to be joined may be superposed in an atmosphere having a pressure exceeding 1 × 10 ⁻⁴ Pa (Claim 2).

  The free energy of formation of oxides at room temperature for most metals is a negative value. The smaller the value (the larger the absolute value), the easier it is to oxidize. In some cases, the free energy of formation of the oxide is expressed as “large”, but the size described here is the numerical value [the numerical value including the sign (negative sign)]. say. Therefore, for example, the generation free energy of “−n (kJ / mol of compounds) or more” (n is a number) means that −n ≦ ΔG (ΔG is an oxide generation free energy), that is, oxide generation. The free energy is negative and the absolute value is “n” or less, or the energy is “0” or a positive value (hereinafter the same).

  Note that there are Au and Ag as single metals having the body diffusion coefficient and the free energy of formation of oxides.

In addition, a thin film having a microcrystalline structure formed on the smooth surface of the one material to be bonded has a body diffusion coefficient of 1 × 10 ⁻⁴⁵ (m ² / s) or more at room temperature and generates oxide at room temperature. It is formed of a single metal or alloy having a free energy of −150 (kJ / mol of compounds) or more, and at least the smooth surface of the other material to be joined has a body diffusion coefficient at room temperature of 1 × 10 ⁻⁴⁵ ( m ² / s) and a microcrystalline structure composed of a single metal or alloy having a free energy of formation of oxide at room temperature of −150 (kJ / mol of compounds) or more,
The two bonded materials may be superposed while being heated at a temperature of 100 ° C. or higher in an atmosphere having a pressure exceeding 1 × 10 ⁻⁴ Pa.

  The single metal belonging to the numerical ranges of the body diffusion coefficient and the free energy of formation of the oxide includes Cu as well as the aforementioned Au and Ag.

Still another joining method of the present invention includes:
A thin film having a microcrystalline structure formed on the smooth surface of the one material to be bonded has a body diffusion coefficient of 1 × 10 ⁻⁵⁵ (m ² / s) or more at room temperature and free energy of formation of oxide at room temperature. Is made of a single metal or alloy of −330 (kJ / mol of compounds) or more, and at least the smooth surface of the other material to be joined has a body diffusion coefficient of 1 × 10 ⁻⁵⁵ (m ² at room temperature). / s) and a microcrystalline structure composed of a single metal or alloy having a free energy of formation of oxide at room temperature of −330 (kJ / mol of compounds) or more,
The two bonded materials may be superposed while being heated at a temperature of 200 ° C. or higher in an atmosphere having a pressure exceeding 1 × 10 ⁻⁴ Pa.

  In addition, as metals belonging to the numerical ranges of the body diffusion coefficient and the free energy of formation of oxide, there are Pd, Pt, Ni and Zn in addition to Au, Ag and Cu described above.

  Both the one and the other materials to be joined can be the thick material to be joined (Claim 5).

  Further, either the one or the other material to be joined may be a thin material to be joined having a thickness of less than 1.0 mm, such as a wafer.

The above PV value _(PV 1, PV _2), the actual in the manufacturing site or the like, for example, the thickness PV value of a plurality of samples were randomly extracted from the material to be joined groups meat _(PV 1, _PV 2) The average value or the maximum value may be used.

  In any of the above configurations, the smooth surface of the other material to be bonded can be formed by a thin film having a microcrystalline structure formed on the surface of the other material to be bonded in a vacuum vessel. .

  In addition, the above-mentioned thick-walled material to be joined may be a radiator made of a single metal of Al, Cu, Ag, or Au, or an alloy mainly composed of Al, Cu, Ag, or Au. (Claim 8).

  In any of the above bonding methods, the materials to be bonded can be superposed in an atmosphere having a pressure equal to or higher than atmospheric pressure.

  In addition, the atmosphere for superimposing the materials to be joined may be air (Claim 10), and may further include an inert gas exceeding 78% (Claim 11). The term “inert gas” here includes “nitrogen”.

  Furthermore, it is preferable that the joining of the materials to be joined is performed in an atmosphere from which dust is removed, such as in a clean room or a glove box.

  Further, before forming the microcrystalline thin film, it is preferable to remove the altered layer generated on the smooth surface of the material to be joined on which the microcrystalline thin film is formed in the same vacuum as the formation of the microcrystalline thin film. (Claim 13).

  Further, one or more underlayers made of a thin film of a material different from the microcrystalline thin film are formed on the smooth surface of the material to be joined on which the microcrystalline thin film is formed, and the microcrystalline thin film is formed on the underlayer. It may be formed (claim 14).

  In this case, the underlayer is formed of any one single metal selected from the element group of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W, or selected from the element group. It can be formed of an alloy containing one or more elements (claim 15).

  Further, the single metal or alloy forming the base layer has a higher melting point than the single metal or alloy forming the microcrystalline thin film formed on the base layer, and the single metal forming the microcrystalline thin film Alternatively, a material having a large difference in melting point with respect to the alloy can be used (claim 16).

  When the material to be bonded is made of Cu or Cu alloy, it is preferable to remove the deteriorated layer generated on the smooth surface of the material to be bonded for forming the base layer before forming the base layer. Item 17).

  The film thickness of the thin film having a microcrystalline structure can be in the range of 2 nm to 1 μm.

  Furthermore, the structure of the present invention is characterized by including a joint portion joined by any of the methods described above (claim 19).

With the structure of the present invention described above, according to the bonding method of the present invention, the coating film of the bonding material interposed at the bonding interface is formed extremely thin, and the metal bonding or bonding at the atomic level is performed over the entire bonding interface. As a result of bonding by intermolecular bonding, the necessary bonding strength is obtained, and the physical and mechanical properties at the bonding interface such as improvement of thermal conductivity and reduction of electric resistance are improved, and 1 × 10 ^−4. Bonding was relatively easy in an atmosphere exceeding Pa, for example, at atmospheric pressure.

  In addition, by adjusting the pressure applied at the time of joining according to the PV value on the smooth surface of the thick material to be joined, the applied pressure can be kept to the minimum necessary, and the comparison does not damage electronic devices. In addition, the residual stress generated at the joint interface can be kept as low as possible, and the smooth surface peeling and cracking with time caused by the residual stress can be suppressed. It was difficult to make any progress.

  By superimposing the materials to be joined at a pressure (F) that satisfies the above-mentioned relational expression, the entire surface of the bonding interface can be reliably contacted, but the pressure is higher than necessary. Was able to be prevented.

  In addition, in the bonding method of the present invention, the material to be bonded can be heated at a relatively low temperature of about 200 ° C., so that the bonding can be performed under any conditions, and damage to the material to be bonded is caused by heating. We were able to minimize it.

Especially when the body diffusion coefficient at room temperature is 1 × 10 ⁻⁴⁵ (m ² / s) or more and the free energy of formation of oxide at room temperature is −150 (kJ / mol of compounds) or more, Bonding can be performed by heating the material to 100 ° C., the body diffusion coefficient at room temperature is 1 × 10 ⁻⁴⁰ (m ² / s) or more, and the free energy of formation of oxide at room temperature is −15 ( kJ / mol of compounds) or higher, the above-described atomic diffusion bonding can be performed even when bonded at room temperature without heating the bonded material, which further damages the bonded material due to heat. It was possible to reduce.

  However, even if the bonding can be performed without heating the bonded material, the bonded material temperature is set to room temperature so that the bonded material is not damaged, for example, 400 ° C. or lower, preferably 300 ° C. or lower. By heating above, the above-mentioned body diffusion coefficient can be increased, thereby increasing the diffusion rate and diffusion length of atoms, thereby improving the diffusibility of atoms at the bonding interface and the grain boundary, and more uniformly and It was possible to perform strong bonding, and in particular, due to the increase in the diffusion length of atoms, it was possible to bond even a material with a relatively rough surface.

Here, the body diffusion coefficient D is
D = D0exp (-Q / RT)
D0: Frequency term (entropy term)
Q: Activation energy R: Gas constant T: It can be expressed by an absolute temperature, and when the temperature T is raised, the body diffusion coefficient D increases exponentially.

  In addition, when the above-mentioned thick member to be joined is a radiator, this is a single metal of Al, Cu, C, Ag, or Au having a high thermal conductivity, or Al, Cu, C, Ag, or Au. High heat radiation performance was able to be demonstrated by manufacturing with the alloy which has as a main component.

  The above bonding method is possible even when the materials to be joined are overlapped in an atmosphere having a pressure equal to or higher than the atmospheric pressure (1 atm). Even in the case of using air, that is, in the case of performing the superposition while exposed to the air, it was possible to perform the bonding appropriately.

  However, about 78% of nitrogen is nitrogen and about 20% is oxygen, and it is considered that the oxygen is chemically adsorbed on the above-mentioned microcrystalline thin film and the bonding between the materials to be bonded is hindered. , In an atmosphere containing more than 78% inert gas (the term “inert gas” includes “nitrogen”), preferably an atmosphere containing 100% inert gas. This is done in an atmosphere where the amount of oxygen, which is a factor that hinders bonding, is reduced from that in the atmosphere, and the concentration of nitrogen gas, argon gas, and other inert gases that are not chemically adsorbed to the microcrystalline thin film is increased. It becomes possible to perform joining more suitably by performing.

  In particular, when superposition is performed in an atmosphere in which the concentration of the inert gas is increased, a metal having a relatively small value for the free energy of formation of an oxide as the metal constituting the microcrystalline structure (therefore, oxygen Even when a metal that easily reacts with (a) is used, strong bonding is possible.

  Furthermore, by superimposing the materials to be joined in an atmosphere from which dust is removed, such as in a “clean room” or “glove box”, it is possible to prevent poor bonding due to the presence of impurities such as dust on the smooth surface. We were able to. As an example, the cleanliness of this space should be ISO Class 5 (equivalent to Class 100 in the 1988 US Federal Standard. Number of particles of 0.5 μm or more in a cubic foot space is less than 100). More preferably, it is preferable to adjust the humidity in the atmosphere (for example, 50% or less).

  Before forming the microcrystalline thin film, the altered layer formed on the smooth surface of one or both of the bonded materials in the same vacuum as the formation of the microcrystalline thin film is removed by a dry process such as reverse sputtering. Thus, the adhesion strength of the microcrystalline thin film to the material to be joined can be improved, and the decrease in the adhesion strength between the materials to be joined due to delamination between the surface of the material to be joined and the microcrystalline thin film can be suitably prevented. I was able to.

  Further, a thin film of a material different from the microcrystalline thin film, for example, Ti, V, Cr, Zr which is an element belonging to Group 4A to 6A in the periodic table is formed on the smooth surface of the material to be joined on which the microcrystalline thin film is formed. The underlayer is formed from a thin film of any one single metal selected from the group of elements Nb, Mo, Hf, Ta, and W, or a thin film of an alloy containing one or more elements selected from the group of elements. As a result, it was possible to increase the adhesion strength of the microcrystalline thin film to the material to be joined, and to prevent separation between the material to be joined and the microcrystalline thin film.

  In particular, as a material for forming such an underlayer, a microcrystal formed on the underlayer by using a material having a high melting point and a large difference in melting point relative to the material for forming a microcrystalline thin film. The two-dimensionality of the thin film (atomic wettability during thin film growth) is improved, and it is possible to prevent the microcrystalline thin film from growing in the shape of islands. Formation of a thin film becomes easy.

  When the material to be bonded is made of Cu or Cu alloy, the above-described underlayer is easily peeled off from the surface of the material to be bonded. However, before forming the underlayer, the underlayer to be formed is formed. By removing the altered layer generated on the smooth surface of the bonding material, it was possible to prevent the occurrence of peeling between the Cu-bonded material and the underlying layer.

  In the atomic diffusion bonding method of the present invention, the atomic diffusion bonding can be suitably performed when the thickness of the microcrystalline thin film to be formed is in the range of 2 nm to 1 μm, respectively, and the movement of electrons that conduct heat conduction by a smooth surface is hindered. It was possible to provide a bonding method that was not possible.

Schematic explanatory drawing which showed an example of the joining process by the method of this invention. The correlation diagram of the applied pressure and displacement amount with respect to the board | substrate (thick to-be-joined material) of various materials. It is explanatory drawing of the term which appears in a specification, (A) is applied pressure (F), displacement amount (X), original height (H), (B) is explanatory drawing of PV value. It is explanatory drawing of the example of a joining test between Al substrates, (A) is a pressurization condition confirmation test, (B), (C) is an explanatory view of a heating condition confirmation test. It is explanatory drawing of the example of a joining test of Cu board | substrates, (A) is a confirmation test of pressurization conditions, (B) is explanatory drawing of the confirmation test of heating conditions. Explanatory drawing of PV value. Explanatory drawing of the joining test of an Al substrate and a quartz wafer. The graph which showed the state of the change of the peeling position with respect to the change of the applied pressure in joining of an Al substrate and a quartz wafer.

Outline of Bonding Method The bonding method of the present invention is a single metal or alloy having a predetermined body diffusion coefficient and free energy of formation of an oxide formed in vacuum by vacuum film formation such as sputtering or ion plating in a vacuum vessel. By superimposing thin film of microcrystalline structure made of or consisting of a thin film of microcrystalline structure and a single metal or alloy of which at least the surface has the predetermined body diffusion coefficient and oxide free energy of formation. This is a case where the superposition and pressurization are performed in an atmosphere having a pressure exceeding 1 × 10 ⁻⁴ Pa, for example, an atmosphere having a pressure higher than atmospheric pressure, by superimposing and pressing on a smooth surface having a crystal structure. However, it has been found that atomic diffusion occurs at the bonding interface and the grain boundary, and that strong bonding is performed between the two. It is those adapted for engagement, and performs joining in conditions below.

The present invention relates to a material to be joined, which is a thick material to be joined (bulk material) at least one of which has a thickness of 1.0 mm or more. Both of them may be thick materials to be joined having a thickness of 1.0 mm or more, or one of the materials to be joined is a thick material to be joined and the other is as thick as a wafer. It is good also as what joins for the thin to-be-joined material of less than 1 mm.

The material of these materials to be joined is not particularly limited. For example, the ultimate vacuum is 1 × 10 ⁻³ to 1 × 10 ⁻⁸ Pa, preferably 1 × 10 ⁻⁴ to 1 × 10, such as sputtering or ion plating. Any material can be used as long as it can form a thin film having a microcrystalline structure by vacuum film formation in a high vacuum atmosphere using a vacuum vessel having a high vacuum of ^-8 Pa.

As an example, when the thick material to be joined, which is one of the materials to be joined, is a heat spreader as a radiator, the material is made of a material having good thermal conductivity such as Au, Ag, Cu, Al, etc. The other material to be joined is a thin material to be joined such as a core or a chip accommodated in a package such as an electronic device to be cooled, for example, an IGBT module, an RF package, or an LED chip. For example, a known material used for the above-mentioned materials, for example, a sapphire substrate, a semiconductor substrate such as SiC, Si, SiO ₂ , or GaN, glass, ceramics, or the like is the material of the other material to be joined.

  Further, when one of the materials to be joined is a heat sink provided with a radiation fin or the like, and this is joined on a heat spreader joined on an electronic device, both of them are joined with a thick material to be joined.

  For example, the bonding can be performed not only between the same materials as in the case of bonding between metals but also between different materials such as metal and ceramics.

State of the smooth surface of the material to be joined Among the materials to be joined, at least the smooth surface that is the joining surface of the thick material to be joined is polished to a mirror surface with a predetermined accuracy. The smooth surface of the material to be joined is polished into a mirror surface so that the PV value is 0.1 to 2.0 μm.

  Here, as shown in FIG. 6, the PV value is the level of the highest point (Peak) and the lowest point (Valley) in the cross-sectional line obtained by measuring within a predetermined measurement range in the flat surface of the material to be joined. It shows the difference.

  Here, the smaller the PV value, the better the bonding can be. However, in general mirror polishing, the PV value is about 0.1 μm at the lower limit, and polishing is performed with high precision until the PV value is below this value. In this case, a great amount of time and time are spent on polishing the material to be joined, which increases the cost.

  On the other hand, even when the PV value exceeds 2.0 μm, it is possible to join itself by increasing the pressure applied at the time of joining, but when joining with a large pressure, the structural material to be joined by pressurization In particular, when one of the objects to be bonded is an electronic device, there is a risk of damaging the structural material that is the electronic device, and a large residual stress remains at the bonding interface, and the PV value is 0.1. It is preferable to be in the range of ~ 2.0 μm.

  In addition, in the case where any of the materials to be joined is a thick material having a thickness of 1.0 mm or more, as in the case of joining a heat sink to the heat spreader, the smooth surface of the material to be joined In any case, it is preferable to adjust the PV value in the range of 0.1 to 2.0 μm.

  The smooth surface of the material to be joined must be the surface before the formation of the microcrystalline structure thin film, or when the ground layer is provided between the microcrystalline thin film and the smooth surface of the material to be joined. It is preferable that the altered layer such as the gas adsorbed layer and the natural oxide layer is removed. For example, the aforementioned altered layer is removed by a known wet process such as cleaning with a chemical solution. In order to prevent gas adsorption or the like, a material to be joined that has been subjected to hydrogen termination or the like can be suitably used.

  In addition, the removal of the altered layer is not limited to the wet process described above, but can also be performed by a dry process. The altered layer such as a gas adsorption layer or a natural oxide layer is reverse-sputtered by bombardment of rare gas ions in a vacuum vessel. Can also be removed.

  In particular, when the altered layer is removed by the dry process as described above, the gas adsorbed on the surface of the material to be joined between the removal of the altered layer and the formation of a thin film having a microcrystalline structure or an underlayer described later. In order to prevent the occurrence of oxidation, the removal of such a deteriorated layer is performed in the same vacuum when a thin film having a microcrystalline structure described later is formed, and the thin film having a microcrystalline structure is removed following the removal of the deteriorated layer. Alternatively, it is preferable to form an underlayer, and more preferably, the deteriorated layer is removed using an ultra-high purity inert gas to prevent the oxide layer from being re-formed after the altered layer is removed. .

  It should be noted that the bonding is not particularly limited, such as single crystal, polycrystal, amorphous, glass state, etc., and various structures can be targeted for bonding. In the case where a thin film having a microcrystalline structure, which will be described later, is formed and bonding is performed without forming a thin film having a microcrystalline structure on the other material to be bonded, the other target without forming the thin film is formed. The smooth surface of the bonding material is formed with a predetermined body diffusion coefficient and a predetermined oxide so that atomic diffusion at the bonding interface and crystal grain boundary can be obtained, at least on the surface as in the case of a thin film having a microcrystalline structure described later. It is necessary to have a microcrystalline structure (including amorphous) constituted by a metal within the range of free energy.

Thin film material with microcrystalline structure As a material for forming the thin film with the above-mentioned microcrystalline structure, a thin film of the same kind as the material to be joined may be formed. A thin film having a structure may be formed, and the material of the thin film having a microcrystalline structure formed on one of the materials to be bonded is different from the material of the thin film having a microcrystalline structure formed on the other of the materials to be bonded. It is also good.

  The material of the thin film having a microcrystalline structure to be formed can be classified into the following three types according to the heating conditions at the time of bonding.

The first material is a material that can be bonded without heating, and this material has a body diffusion coefficient of 1 × 10 ⁻⁴⁰ (m ² / s) or more at room temperature and the formation of oxide at room temperature. A single metal or alloy having a free energy (kJ / mol of compounds) of -15 or more. Note that single metals belonging to such a numerical range include Au and Ag.

The second material is a group including, in addition to the first material, a material (for example, Cu) that can be bonded by heating a material to be bonded to 100 ° C. or higher at the time of bonding. A single metal or an alloy having a body diffusion coefficient of 1 × 10 ⁻⁴⁵ (m ² / s) or more at room temperature and an oxide free energy of formation (kJ / mol of compounds) at room temperature of −150 or more. is there. In addition, as a single metal which belongs to such a numerical range, there are Cu in addition to the above-mentioned Au and Ag.

In addition to the first and second materials, the third material is a group including a material that can be bonded by heating the material to be bonded to 200 ° C. or higher at the time of bonding. Is a single metal or alloy having a body diffusion coefficient of 1 × 10 ⁻⁵⁵ (m ² / s) or more and an oxide free energy (kJ / mol of compounds) of −330 or more at room temperature. As single metals belonging to such a numerical range, there are Pd, Pt, Ni and Zn in addition to the aforementioned Au, Ag and Cu.

  As for alloys, the body diffusion coefficient and the free energy of formation of oxides vary depending on the alloy composition, mixing ratio, etc., so various combinations may exist, but the single metal contained within each of the above numerical ranges. Alloys combined within the group are included in the above numerical ranges, and alloys obtained by adding other alloy components to such an extent that the chemical characteristics of each metal belonging to each group are not lost are , Can be used in the same manner as the main metal single metal.

  As described above, the temperature range is defined for the numerical range of the body diffusion coefficient and for each free energy of the oxide. In order to perform the atomic diffusion bonding of the present invention, the larger the body diffusion coefficient, the more the atoms. On the other hand, as the value of the free energy of formation of the oxide increases, the reaction with oxygen hardly occurs and it becomes difficult to form a substance that inhibits bonding such as an oxide film.

Therefore, the body diffusion coefficient at room temperature is 1 × 10 ⁻⁵⁵ (m ² / s) or more, and the free energy of formation of oxides at room temperature (kJ / mol of compounds) is −330 (kJ / mol of compounds) or more. When a thin film having a microcrystalline structure is formed of a single metal or alloy, it is necessary to heat the material to be joined at 200 ° C. or higher in order to enable bonding with all the metal thin films belonging to this numerical range. On the other hand, if the metal diffusion coefficient is limited to a metal with a high free energy for formation of oxides, the heating temperature of the material to be bonded can be lowered and the body diffusion at room temperature can be reduced. When the coefficient is 1 × 10 ⁻⁴⁵ (m ² / s) or more and the free energy of formation of oxide at room temperature is −150 (kJ / mol of compounds) or more, the material to be joined is 100 ° C. or more, and further room temperature. The body diffusion coefficient at 1 × 10 ^-40 (m ² / s) or more, and When the free energy of formation of oxide at room temperature is -15 (kJ / mol of compounds) or more, bonding with atomic diffusion can be performed even when bonding at room temperature without heating the material to be bonded.

  When the materials to be joined, in which a thin film having a microcrystalline structure is formed of these metals, are joined at each of the above-mentioned temperature conditions, the disappearance of the joining interface occurs at least partially. Recrystallization occurs between the microcrystalline thin films joined in step 1 to produce crystal grains having a grain size covering almost the entire distance between the two materials to be joined. Can be obtained.

Although the film thickness to be formed is not particularly limited, bonding can be performed even when each microcrystalline thin film is formed with a thickness of one constituent element. For example, Au microcrystal When a thin film is formed, bonding is possible even when the film thickness is 2 nm (4 nm for two layers). The thickness of the microcrystalline thin film interposed between the materials to be bonded is determined by the electron or spin current. It is possible to form with a thickness less than the mean free process.

  As a result, the layer of the microcrystalline thin film interposed between the materials to be joined does not become a barrier against the movement of electrons and the like, and exhibits extremely good thermal conductivity and electrical conductivity.

  On the other hand, as the film thickness increases, the surface roughness of the obtained microcrystalline thin film increases, making bonding difficult, and resistance to heat and electrons that pass through the bonding interface. Since the formation of the crystal thin film takes a long time and the productivity is lowered, the upper limit is about 1 μm, and about 2 nm to 1 μm is the preferred film thickness of each microcrystalline thin film in the atomic diffusion bonding method of the present invention. It is a range.

Grain size and density The microcrystalline thin film to be formed has a higher atomic diffusion rate than that of the solid of the same microcrystalline metal, and in particular, it has a microcrystalline structure with a large proportion of grain boundaries where the diffusion rate is extremely high. Preferably, the average grain size in the in-plane direction of the crystal grains may be 50 nm or less, and more preferably 20 nm or less.

  Further, in the microcrystalline thin film, the proportion of the volume occupied by the metal constituting the microcrystalline thin film, excluding the formation of voids, is 80% or more, preferably 80 to 98, with respect to the volume of space occupied by the microcrystalline thin film is 100%. It is preferable to form so that it may become%.

Forming surface of the microcrystalline thin film Further, the thin film having the microcrystalline structure may be formed on each of the two materials to be joined. However, the microcrystalline structure is formed only on one of the materials to be joined. It is possible to obtain a bond without forming a thin film having a microcrystalline structure on the other material to be bonded.

  In this case, the smooth surface of the other material to be bonded without forming a thin film having a microcrystalline structure is, as described above, at least near the surface of the smooth surface has a predetermined body diffusion coefficient and a numerical range of oxide free energy of formation. It is necessary to have a microcrystalline structure made of a single metal or alloy. However, the smooth surface of the material to be joined on which the thin film having the microcrystalline structure is not formed and the material of the thin film having the microcrystalline structure are not necessarily common.

  In addition, on the smooth surface of the material to be joined forming the thin film of the microcrystalline structure, one or more underlayers made of a thin film of a material different from the thin film of the microcrystalline structure are formed before forming the thin film of the microcrystalline structure. In particular, when the thin film having a microcrystalline structure to be formed has a relatively low adhesion strength to the material to be bonded, the formation of the underlayer is effective in improving the adhesion strength.

  Such an underlayer can be formed by a vacuum film formation technique similar to the film formation method to be described later of the microcrystalline thin film, and the material thereof is Ti, Zr which is an element belonging to Group 4A-6A of the periodic table. , Hf, V, Nb, Ta, Cr, Mo, W, the thickness of which is 0.2 to 20 nm as an example, and 5 nm in the examples described later.

  As the material of the underlayer, it is preferable to use a material having a large difference in melting point with respect to the material for forming the microcrystalline thin film formed thereon, and a high melting point for the material for forming the thin film having a microcrystalline structure. Are preferably used. As an example of a combination of materials having a large melting point difference and a high melting point with respect to a thin film having a microcrystalline structure, for example, when forming an Ag microcrystalline thin film on a Ta underlayer, the formed microcrystalline thin film Can be suitably prevented from being peeled off from the material to be bonded, and the two-dimensionality of the microcrystalline thin film formed on the underlayer (atomic wettability when forming the microcrystalline thin film) has been improved. Ag, which is a thin film having a microcrystalline structure, can be prevented from growing in an island shape, and an extremely thin microcrystalline thin film of 2 nm can be easily formed.

  When the material to be joined is made of Cu or a Cu alloy, the adhesion strength of the underlayer is weakened due to the presence of the natural oxide formed on the smooth surface, and delamination occurs at this portion, so that the improvement of the joining strength has peaked. Therefore, it is preferable to remove the altered layer generated on the smooth surface before forming the underlayer. Such removal of the deteriorated layer is not limited to the material to be bonded made of Cu or Cu alloy, but may be performed when the base layer is formed on the material to be bonded.

  Further, since the Ta underlayer formed on the smooth surface of the Cu or Cu alloy to-be-joined material is extremely reduced in strength when heated, the Cu or Cu alloy to-be-joined material is to be joined. In the case where the microcrystalline thin film is formed of a material having a relatively low diffusion coefficient such as Cu, Pd, Pt, Ni, and Zn that requires heating at the time of bonding, it is preferable to avoid the use of a Ta underlayer. .

Film Forming Method Film Forming Technology In the bonding method of the present invention, as a method for forming a microcrystalline thin film formed on a smooth surface of a material to be bonded, in addition to PVD such as sputtering and ion plating, CVD, various vapor deposition, etc., ultimate vacuum Various film forming methods for performing vacuum film formation in a vacuum atmosphere in a vacuum vessel having a high degree of vacuum of 1 × 10 ⁻⁴ to 1 × 10 ⁻⁸ Pa can be cited. For the alloy, compound, etc., a vacuum film forming method in which film formation is preferably performed under generation of plasma capable of increasing the internal stress of the formed thin film, for example, film formation by sputtering is preferable.

Degree of vacuum The pressure in the vacuum vessel during the formation of the thin film may be a vacuum atmosphere with an ultimate degree of vacuum of 1 × 10 ⁻³ to 1 × 10 ⁻⁸ Pa, and a lower pressure (high degree of vacuum) is preferable. .

Inert gas (Ar gas) pressure When the film formation method is sputtering, the pressure of the inert gas (generally Ar gas) during film formation should be a dischargeable region, for example, 0.01 Pa or more. In addition, if it exceeds 30 Pa (300 μbar), bonding may not be performed, so the upper limit is preferably about 30 Pa (300 μbar). This is because when the Ar gas pressure is increased, the surface roughness of the formed thin film is increased, the film density is significantly decreased, and the concentration of impurities such as oxygen in the film is significantly increased.

Pressure of atmosphere of superposition conditions As described above, to-be-joined materials having a microcrystalline thin film formed on their surfaces, or to-be-joined materials having a microcrystalline thin film and a to-be-joined material having a microcrystalline structure When the surfaces are superposed in an atmosphere of a pressure exceeding 1 × 10 ⁻⁴ Pa, for example, in a pressure atmosphere of atmospheric pressure or higher, atomic diffusion occurs at the bonding interface and the grain boundary, thereby The joining material is joined.

  As described above, in the method of the present invention, bonding can be performed even when the two materials to be bonded are superposed in a higher pressure atmosphere than in the vacuum in which the microcrystalline thin film is formed. It has become.

  In addition, as a case where the pressure of the atmosphere in which the materials to be bonded are superposed exceeds the atmospheric pressure, for example, the materials to be bonded are overlapped in an air supply type clean room or a glove box.

In addition, the atmosphere component when the materials to be joined are overlapped may be air (nitrogen: about 78%, oxygen: about 20%), or nitrogen gas, argon gas, or other undesired components. The active gas may be present alone or in a mixed state in an amount of more than 78%, preferably an atmosphere of 100% inert gas.

Elapsed time after formation of microcrystalline thin film Here, in an example in which an Au film is formed as a microcrystalline thin film, the Au film is exposed to air at atmospheric pressure (1 atm) for 1 to 6 hours after the Au film is formed. It has been confirmed that sufficient bonding force can be obtained even when the materials are superposed, and bonding is performed even after exposure to air at atmospheric pressure for a sufficiently long time for 1 to 6 hours. It is possible.

In addition, reducing the amount of oxygen and water in the atmosphere makes it possible to firmly bond even after a long time has elapsed after the formation of a thin film having a microcrystalline structure. Is performed in an atmosphere containing an inert gas exceeding 78% as described above, preferably in an atmosphere containing 100% inert gas, and / or in a range of pressure exceeding 1 × 10 ⁻⁴ Pa. When the pressure of the atmosphere to be set is set low, it is possible to perform strong bonding even after a long time has elapsed since the formation of the thin film having a microcrystalline structure.

  In addition, for example, as the body diffusion coefficient of the metal forming the thin film having a microcrystalline structure decreases, and as the value of free energy of formation of the oxide decreases, the content of inert gas in the bonding atmosphere is reduced. It is also possible to change the components and pressure of the atmosphere in which the materials to be joined are superposed according to the material of the microcrystalline thin film to be formed.

The cleanliness of the atmosphere It is preferable that the atmosphere in which the materials to be joined are superposed is in a space where dust is removed in order to prevent poor bonding due to the presence of dust on the smooth surface. As described above, it can be performed in a clean room or a glove box from which dust is removed.

  The cleanliness of such an atmosphere is, for example, ISO class 5 (corresponding to class 100 in the 1988 US federal standard. 0.5 μm or more and less than 100 particles in a space of 1 cubic foot). Is preferred.

  Further, as described above, moisture in the atmosphere also causes chemical adsorption on the surface of the microcrystalline thin film to form a compound, so the atmosphere in which such a smooth surface is superposed has a humidity of 50% or less. It is preferably adjusted.

As described above, depending on the body diffusion coefficient of the metal material constituting the microcrystalline thin film formed on the smooth surface and the free energy of formation of the oxide, the materials to be joined are It joins by heating to the following temperature.

  A microcrystalline structure of a single metal or alloy whose body diffusion coefficient at room temperature is 1 × 10 −40 (m 2 / s) or more and whose free energy of formation (kJ / mol of compounds) at room temperature is −15 or more. In the case of joining with a thin film, it is not necessary to heat the material to be joined to obtain the joining.

  However, even in this case, as described above, the material to be joined may be heated within a range that does not damage the material to be joined, and joining with an increased body diffusion coefficient may be performed.

A metal forming a thin film has a body diffusion coefficient of 1 × 10 ⁻⁴⁵ (m ² / s) or more at room temperature and a single oxide having a free energy of formation of oxide (kJ / mol of compounds) of −150 or more at room temperature. In the case of expanding to a metal or an alloy, the material to be joined is heated to 100 ° C. or higher in order to obtain bonding in all the metal thin films included in this condition.

Furthermore, the range of metals that form thin films has a body diffusion coefficient of 1 × 10 ⁻⁵⁵ (m ² / s) or more at room temperature, and the free energy of formation of oxides at room temperature (kJ / mol of compounds) − When expanding to the range of single metal or alloy of 330 or more, in order to obtain bonding in all metal thin films included in this condition, the materials to be bonded are heated to a temperature of 200 ° C. or higher.

  It should be noted that the body diffusion coefficient of the metal constituting the microcrystalline thin film formed on one bonded material and the free energy of formation of the oxide, and the body diffusion of the metal forming the microcrystalline structure on the surface of the other bonded material. If the coefficient and the free energy of formation of the oxide are different, the material to be joined is heated to a temperature that is applied to a metal with a low body diffusion coefficient and a small value of the free formation energy of the oxide. Do.

Pressurization at the time of joining In addition to the above-described heating according to the body diffusion coefficient at room temperature, pressurization is performed according to the PV value on the smooth surface of the thick-walled material. is necessary.

  When one or both of the materials to be joined is a thick material to be joined having a thickness of 1.0 mm or more, the reason for hindering the joining is a relatively short period unevenness that appears in the arithmetic average roughness Ra It has already been explained that it is not caused by this, but is expected to be caused by longer-period irregularities, that is, surface waviness.

  Here, if it is assumed that the cross-sectional line obtained by measuring the surface shape in the predetermined range of the joining surface of the thick-walled workpiece shows a waveform as shown in FIG. If the tangential line and the tangential wave at the bottom of the valley are entered as auxiliary lines, it can be seen that this cross-sectional line contains a “swell” component that is a long-period unevenness.

  In this way, when the “swell” component included in the surface shape of the smooth surface is made obvious, the PV value indicating the difference in height between the highest point and the lowest point within the predetermined measurement range is the “swell”. It can be seen that it roughly corresponds to the amplitude (maximum amplitude in the measurement range).

  In this way, when the state of the undulation occurring on the surface of the bulk material, in which the short-period unevenness component is discarded, is represented in a schematic diagram, as shown in FIG. Is a waveform that oscillates within the width defined by the PV value.

  Accordingly, the “swell” as schematically shown in FIG. 3 (B) is brought into contact with the smooth surface of one of the materials to be joined and the entire smooth surface of the other material to be joined. In this case, it is only necessary to apply a pressing force that can cancel out the unevenness caused by this undulation and make the entire smooth surface of the material to be joined flat.

  If this pressure is a comparison between workpieces of the same material, the larger the PV value, the larger the pressure applied during bonding, and conversely, the smaller the PV value, the pressure applied during bonding. Can be small.

  In addition, when comparing materials to be joined having the same PV value, the greater the Young's modulus of the material constituting the material to be joined, the greater the applied pressure during joining, and conversely, the lower the Young's modulus, Sometimes the applied pressure can be reduced.

  Therefore, as shown in FIG. 3B, the PV value on the smooth surface of the material to be joined is regarded as the amplitude of the swell generated on the smooth surface, and the relative relationship between this PV value and the Young's modulus of the material to be joined. It can be seen that it is preferable to change the applied pressure.

  Here, as shown in FIG. 3A, when a downward pressure (F) is applied to the upper surface of the plate-like object, the displacement (X) generated in the object is proportional to the pressure (F) (the hook Law; see Figure 2).

In the model of FIG. 3A, the Young's modulus (E) is “pressing force (F) / displacement rate”, and the displacement rate is “displacement amount (X) / original height (H)”. As shown in FIG. 3 (A), the displacement amount (X) when a pressing force (F) is applied to a plate of thickness (H) is:
X = HF / E
You can ask for it.

  Here, the amount of displacement (X) obtained by the above equation is the amount of displacement generated in the material to be joined having an ideal plane, and the actual smooth surface of the material to be joined is shown in FIG. In addition, irregularities represented by the PV value are generated.

Therefore, the stress applied to the material to be joined is not uniformly applied to the entire smooth surface, but is concentrated on the peak portion of the unevenness. Therefore, the smooth surface of the material to be joined is obtained by the above relational expression. Pressure (F) to obtain a displacement corresponding to the PV value, that is,
F = PV · E / H (PV is PV value)
When the pressurizing force lower than the pressurizing force (F) obtained by the above is brought into contact with the smooth surface of the other material to be joined, and the pressurizing force exceeding the pressurizing force (F) obtained here is applied, Both smooth surfaces can be reliably brought into contact with the entire surface.

  However, if an excessively large applied pressure is applied, the electronic device may be damaged, and a large residual stress may remain at the joint interface, causing peeling and cracks to develop over time. Is preferably adjusted to 100 MPa or less.

Example of Bonding Method An example of a bonding process by the atomic diffusion bonding method according to the present invention will be described with reference to FIG. In FIG. 1, a magnetron cathode for performing sputtering is disposed at the upper part in a vacuum vessel for forming a thin film, and a jig for placing a material to be bonded to each other is disposed at the lower part of the magnetron cathode. Then, a microcrystalline thin film is formed on the smooth surface of the material to be joined attached to the jig.

  In the illustrated embodiment, the table provided in the above-described jig is configured to be rotatable between a thin film formation position indicated by a broken line and a bonding position indicated by a solid line in the drawing on the right side of FIG. The two tables are arranged in such a manner that one end of the table on which one of the members to be joined is placed and one end of the table on which the other of the members to be joined are brought into contact with each other. Is rotated, and the other ends of both tables are lifted upward so that the smooth surfaces of the two materials to be joined placed on the tables are superposed.

  When bonding is performed by forming a thin film having a microcrystalline structure with a metal within the numerical range of the body diffusion coefficient and the oxide free energy of formation that require heating of the materials to be bonded, It is also possible to embed an electric heater or the like in this manner so that the smooth surfaces of the two materials to be joined can be superposed and heated to a predetermined temperature.

  Note that the jig for bonding the materials to be bonded is not limited to the one shown in the figure, and various shapes and structures may be used according to the shape of the materials to be bonded. Further, for example, one or both of the materials to be joined may be operated by a robot arm or the like, and the materials to be joined may be overlapped by a human hand.

  As shown in FIG. 1, the vacuum container in which such a jig is accommodated is disposed in a clean room capable of achieving the above-mentioned class 1000 or higher cleanliness, or communicated with the entrance / exit of the vacuum container. A glove box capable of realizing a cleanness of class 1000 or higher is provided.

  In the vacuum vessel in which the jig configured as described above is arranged, a microcrystalline thin film is formed on the smooth surface of the material to be bonded under the above-described conditions in a state where the jig is in the above-described film formation position. .

Then, when the microcrystalline thin film having a predetermined thickness is formed on the smooth surface of the material to be joined, the formation of the microcrystalline thin film is finished, and the pressure in the vacuum vessel exceeds 1 × 10 ⁻⁴ Pa as described above. For example, return to atmospheric pressure or the pressure in the clean room. As described above, the pressure in the vacuum vessel may be increased by introducing an inert gas such as nitrogen gas or argon gas into the vacuum vessel, or by introducing air.

In this way, after the pressure in the vacuum vessel is increased to a pressure exceeding 1 × 10 ⁻⁴ Pa, for example, atmospheric pressure or clean room pressure, the inlet / outlet of the vacuum vessel is opened, and the above-described treatment is performed from the inside of the vacuum vessel. The material to be joined is taken out together with the tool, and the table provided on the jig is rotated and bonded to the bonding position as described above, and the pressure (F) described above is applied and bonded.

  As a result, it is possible to perform bonding in which atomic diffusion is caused at the bonding interface and crystal grain boundary between the two microcrystalline thin films and the bonding strain is reduced.

  In the above description, the case where the materials to be bonded having the microcrystalline thin film made of the same material are bonded to each other is described. However, when the materials to be bonded having the microcrystalline thin film made of different materials are bonded to each other, For example, the magnetron cathode described above is arranged in each of two vacuum vessels installed in a common clean room, so that a microcrystalline thin film of a different material can be formed in each vacuum vessel. After forming microcrystalline thin films of different materials on the smooth surfaces, the materials to be joined taken out from the respective vacuum vessels may be superposed in the above-mentioned clean room and joined by pressing.

  In addition, the formation of the microcrystalline thin film on both materials to be bonded is not necessarily performed at the same time, and may be performed with a time difference.

  Further, if at least the smooth surface of the other material to be joined has a microcrystalline structure of a single metal or alloy within the numerical ranges of the above-mentioned body diffusion coefficient and oxide formation free energy, the material to be joined 2 It is possible to form the above-mentioned microcrystalline thin film only on one of the materials to be joined and to join both materials directly to the other material to be joined without forming a microcrystalline thin film. .

  Below, the example of a joining test which joined by the joining method of this invention is shown.

[Materials to be joined]
Thick materials to be joined As the thick materials to be joined, Al substrates and Cu substrates shown in Table 1 below were used.

Thin material to be joined As a thin material to be joined, a transparent quartz wafer was used to make it easy to visually confirm the joining state.

  Two types of quartz wafers were used: one with a diameter of 2 inches (5.08 cm) and a thickness of 310 μm, and one with a diameter of 1 inch (2.54 cm) and a thickness of 380 μm.

  The arithmetic average roughness Ra measured using AFM is 0.31 nm, and Rmax (JISB0610-'82) is 6.45 nm.

[Example of joining test between thick materials to be joined]
4A and 5A, after forming a Ti layer (10 nm) as an underlayer on the surface, a pair of Al having a microcrystalline thin film (150 nm) formed of Au Substrates and a Cu substrate were prepared and superposed at room temperature so that the Au microcrystalline thin films formed on both substrates overlapped.

  The pressure applied during superposition was changed, and the change in bonding strength with changes in pressure was measured. The measurement results are shown in Table 2.

  From the above results, it was confirmed that 4.0 MPa was obtained with a pressure of 20 MPa or more at the time of joining in any of the joining of Al substrates and Cu substrates in the above example.

  In particular, in bonding between Al substrates, a bonding strength of 21.9 MPa or more was obtained by increasing the pressure (F) to 50 MPa.

  On the other hand, in bonding between Cu substrates, after a bonding strength of 4.2 MPa is obtained with a pressure F of 20 MPa, the bonding strength increases only to 4.9 MPa even if the pressure F is increased to 50 MPa. Compared with the case of bonding between Al substrates, the degree of increase in bonding strength with respect to an increase in applied pressure is low.

  However, the above phenomenon in the bonding of the Cu substrate is because the Ti film formed as the underlayer in the tensile test peels off from the Cu substrate, that is, the bonding strength reaches a peak due to the adhesion strength of the underlayer. The adhesive strength at the joint interface where the films are joined is considered to be higher than 4.9 MPa.

  Note that the Cu substrate has a higher Young's modulus than the Al substrate, and its deformation requires a larger applied pressure than the Al substrate. As in the case, the required adhesion strength (4.0 MPa or more) is obtained by applying a pressure of 20 MPa.

  This is because, as shown in Table 1, the Cu substrate has a smaller PV value and higher long-period flatness than the Al substrate, so that the entire smooth surface has a smaller amount of deformation than the Al substrate. It is thought that this is because the contact was realized.

  Incidentally, the maximum PV value of the Al substrate is 0.79 μm, the Young's modulus (E) is 70.6 GPa, the thickness (H) is 3 mm, the maximum PV value of the Cu substrate is 0.46 μm, and the Young's modulus (E) is 129. Based on 8 GPa and thickness (H) 3 mm, the pressure (F) required to generate the displacement corresponding to the PV value is calculated using the relational expression “F = PV · E / H”. In the case of Al, 18.6 MPa in the case of Cu and 19.9 MPa in the case of Cu, which substantially agrees with the above experimental results. In the joining of thick materials to be joined, the pressure applied at the time of joining is “F ≧ PV · It was confirmed that it was approximately obtained by “E / H”.

Confirmation of influence of underlying layer As shown in FIGS. 4B and 5B, after a Ti, Cr, or Ta layer (10 nm) is formed on the surface as an underlying layer, the body diffusion coefficient is 1 × 10 ^−40. A pair of Al substrate and Cu substrate on which Au microcrystalline thin film (150 nm) which is a material of (m ² / s) or more is formed, and Au microcrystalline thin films formed on both substrates are overlapped with each other. At room temperature, a pressure of 50 MPa was applied to perform superposition.

  In bonding between Al substrates, in the example in which a microcrystalline thin film of Au was formed, a bonding strength of 20 MPa or more was obtained regardless of the material of the underlayer.

  On the other hand, in the example of bonding between Cu substrates by forming an Au microcrystalline thin film, any of the underlying layers peels off at the interface between the underlying layer and the Cu substrate, and the Au microcrystalline thin films are bonded together. It was not possible to measure the bonding strength at the bonding interface. Therefore, in these examples, the bonding strength of the base layer to the Cu substrate determines the bonding strength between the Cu substrates.

  Incidentally, when the Ti underlayer and the Ta underlayer are provided, the adhesion strength is about 5 MPa on average, and when the Cr underlayer is used, the average is about 4 MPa, and the necessary bonding strength of 4 MPa can be barely obtained. It was confirmed that it was necessary to improve the adhesion strength of the underlayer to improve the bonding strength.

Confirmation of body diffusion coefficient and bonding temperature
As shown in FIG. 4C, after forming a Ti, Cr, or Ta layer (10 nm) as a base layer on the surface, the body diffusion coefficient is 1 × 10 ⁻⁴⁵ to 1 × 10 ⁻⁴⁰ (m ² / s ), A pair of Al substrates on which a Cu microcrystalline thin film (150 nm), which is a substance within the range, is formed, and a pressure of 50 MPa is applied so that the Cu microcrystalline thin films formed on both substrates overlap each other. In addition to overlaying, the temperature at the time of bonding was changed to measure the change in bonding strength.

  In the example where the Al substrate is bonded by forming a microcrystalline thin film of Cu, bonding cannot be performed at room temperature regardless of the material of the underlying layer, and bonding is possible by heating at 80 ° C. After that, it was confirmed that the bonding strength increased with increasing heating temperature.

  It was confirmed that when the heating temperature was 80 ° C., the required bonding strength (4.0 MPa or higher) could not be obtained at a bonding strength of about 3 MPa, and that a bonding strength of 4.0 MPa or higher could be obtained only by heating at 100 ° C. or higher. .

  Also, when heated to 120 ° C or higher, peeling occurs at the bonding interface with the adhesive used to fix the sample to the holder of the tensile tester, making it impossible to measure the bonding strength of the Al substrate. However, it is considered that it has at least a bonding strength measured at the time of peeling of the adhesive (8.5 MPa when heated at 120 ° C., 11.8 MPa when heated at 150 ° C.).

  In this test example, the base layer was formed on the natural oxide film without removing the natural oxide film on the surface of the Al substrate. However, the adhesion strength of the base layer to the Al substrate is strong. At the same time, even when bonded under heating, the underlying layer did not peel off from the Al substrate, and a decrease in adhesion strength was not desired.

  From this, it was confirmed that when the thick material to be joined is made of Al, the necessary joining strength can be obtained even when the smooth surface is not cleaned before the formation of the underlayer.

[Example of joining test for thick and thin materials]
The results of a bonding test using the above-described Al substrate as a thick-walled material and the above-described quartz wafer as a thin-walled material are shown below.

  In the following test, as shown in FIG. 7, after forming a Ti layer (5 nm) as an underlayer, an Al substrate and a quartz wafer each formed with an Au microcrystalline thin film (50 nm) were prepared. The Al substrate and the quartz wafer were superposed at room temperature so that the Au microcrystalline thin films formed on the surface overlapped, and the bonding state was observed.

  2.5 MPa, 5 MPa, 7.5 MPa, 10 MPa, 25 MPa, and 50 MPa were applied as pressures at the time of superposition, respectively, and a structure formed by bonding an Al substrate and a quartz wafer for each pressure was obtained.

Changes in pressure and bonding state The bonding surfaces of the structures obtained as described above were visually observed. The observation was performed by visually observing the mirror-polished surface of the Al substrate through the quartz wafer from the transparent quartz wafer side.

  In the samples bonded with the applied pressure of 2.5MPa and 5MPa, the part where the light interference occurs is confirmed on the bonding surface, and it is clearly confirmed visually that there is a gap between the quartz wafer and the Al substrate. I was able to.

  In addition, it was observed that even in areas where the interference of light was not clearly observable with the naked eye, the hue was uneven, and it was confirmed that the interfaces were not completely in contact with each other.

  When the processing pressure was increased to 7.5 MPa, no obvious light interference could be observed, but the overall color was uneven and the adhesion at the joint interface was not perfect. Was confirmed.

  When the applied pressure is increased to 10 MPa, not only interference of light is not observed, but also shades of colors cannot be observed, and it can be observed that the colors are uniform throughout. Thereafter, the applied pressure is 25 MPa, Even when the pressure was increased to 50 MPa, there was no change in the bonded body that could be visually observed.

  Therefore, it is considered that in the bonding performed at a pressure of 10 MPa, a substantially perfect bonding state is realized at the bonding interface between the Al substrate and the quartz wafer.

  From the above results, it was confirmed that in the bonding example in which one of the members to be bonded is a wafer that is a thin member to be bonded, the entire bonding surface can be bonded by applying a pressure of 10 MPa or more.

  As described above, in the case where one of the materials to be bonded is a wafer, a bonding force as small as about ½ of the pressure required for bonding when both materials to be bonded are thick materials to be bonded. In this way, the bonding interface with no gap is obtained, but such a decrease in pressure is caused by deformation (warping) of the wafer, which is a thin-walled material, against the Al substrate, which is a thick-walled material. It is thought that this is because it is easy to generate.

  That is, in joining thick materials to be joined, both “swell” generated on the smooth surface of one material to be joined and “swell” produced on the smooth surface of the other material to be joined. However, as mentioned above, the undulation that occurs on the smooth surface of the wafer by using one of the bonded materials as a quartz wafer, which is a thin-walled bonded material, was necessary. Because the wafer itself is absorbed by deformation (warping), the surface waviness on the wafer side can be ignored, and only a pressing force that can deform only one of the materials to be bonded can be applied. This is probably because it is good.

  In the above example, a bonding test using a 3 mm thick Al substrate as the thick material to be bonded has been described. However, a quartz quartz wafer having a diameter of 2 inches and a 14 mm × 11 mm Al substrate having a thickness of 1 mm are used. In the bonding test in which bonding is performed at a pressure of about 50 MPa by forming a Ti underlayer (5 nm) and an Au microcrystalline thin film (50 nm), bonding can be suitably performed.

Confirmation of bonding strength Among the structures obtained by bonding the Al substrate and the quartz wafer by the above-described method, each of the samples can be suitably bonded, and the structures bonded at a pressure of 10 MPa, 25 MPa, and 50 MPa are used as samples. The bonding strength was measured with a tensile tester. The measurement results are shown in Table 3 below.

  From the above results, it was confirmed that the required bonding strength (4.0 MPa or more) can be obtained by bonding the Al substrate and the quartz wafer at a pressure of 10 MPa or more.

  In addition, the above values measured in the above-mentioned measurement of the bonding strength are all separated at the bonding interface between the adhesive and the sample used to attach the sample to the holder of the tensile tester, or quartz Since the wafer was broken, no further measurement was possible. From this, it can be said that the bonding force at the Au-Au bonding interface is at least higher than the measured value.

Changes in pressure and adhesion strength at the Au-Au joint interface Observing the fracture surface of the fractured specimen by measuring the joint strength described above reveals how the fracture occurred between specimens joined at different pressures. It was confirmed that there was a difference.

  That is, when observing the fracture surface of the sample joined by applying a pressure of 50 MPa, about 1/2 of the area of the quartz wafer was the adhesive used to fix the quartz wafer to the holder of the tensile tester. The other half was broken within the thickness of the quartz wafer, and the peeling at the bonding interface of the Au—Au film could not be observed.

  On the other hand, in the example in which the pressurizing force is set to 25 MPa and 10 MPa lower than this, peeling at the Au-Au joining interface occurs, and the pressurizing with the lower pressurizing force results in the Au-Au joining. It was confirmed that the peeling area at the interface increased.

  FIG. 8 shows the relationship between the observed change in the applied pressure and the change in the peeled area.

  From the above results, it was confirmed that the bonding strength at the Au-Au interface increases as the pressure applied during bonding increases.

  The joining method of the present invention described above is a special sensor that is expected to be used under high pressure, such as joining of ultra-low thermal expansion parts such as semiconductor exposure devices (steppers), joining of structural materials such as automobiles, and oil drilling. It can be used for various types of joining, such as joining and jewelry joining.

  In particular, in the bonding according to the method of the present invention, the bonding interface is unlikely to become a resistance to heat conduction, and it is possible to perform bonding while maintaining good heat conductivity. It can be suitably used for joining a heat radiator (heat sink, heat spreader) or the like to an electronic device such as a luminance LED.

Claims (19)

A method of joining a smooth surface provided on one material to be joined and a smooth surface provided on the other material to be joined in a polymerized state,
At least one of the one or the other material to be joined is a thick material to be joined constituted by a mass of material having a thickness of 1.0 mm or more,
In a vacuum vessel, a thin film having a microcrystalline structure made of a single metal or an alloy is formed on the smooth surface of one material to be joined, and at least the surface has a microcrystalline structure made of a single metal or an alloy. The one and the other two materials to be joined are overlapped so that the thin film formed on the one material to be joined is in contact with the smooth surface of the other material to be joined. Based on the PV value which is the difference in height between the highest point and the lowest point within a predetermined measurement range on the smooth surface of the bonding material, a small pressing force is applied when the PV value is small, and a large pressing force is applied when the PV value is large. A bonding method characterized by causing atomic diffusion at a bonding interface with the smooth surface of the other bonded material and a crystal grain boundary, and bonding the two bonded materials with a bonding strength of 4.0 MPa or more. A thin film having a microcrystalline structure formed on the smooth surface of the one bonded material has a body diffusion coefficient of 1 × 10 ⁻⁴⁰ (m ² / s) or more at room temperature and free energy of formation of oxide at room temperature. Is made of a single metal or alloy of -15 (kJ / mol of compounds) or more, and at least the smooth surface of the other material to be joined has a body diffusion coefficient of 1 × 10 ⁻⁴⁰ (m ² at room temperature). / s) and a microcrystalline structure composed of a single metal or alloy having a free energy of formation of oxide at room temperature of −15 (kJ / mol of compounds) or more,
The joining method according to claim 1, wherein the two materials to be joined are superposed in an atmosphere having a pressure exceeding 1 × 10 ⁻⁴ Pa. A thin film having a microcrystalline structure formed on the smooth surface of the one material to be bonded has a body diffusion coefficient of 1 × 10 ⁻⁴⁵ (m ² / s) or more at room temperature, and free energy of formation of oxide at room temperature. Is made of a single metal or alloy of −150 (kJ / mol of compounds) or more, and at least the smooth surface of the other material to be joined has a body diffusion coefficient of 1 × 10 ⁻⁴⁵ (m ² at room temperature). / s) and a microcrystalline structure composed of a single metal or alloy having a free energy of formation of oxide at room temperature of −150 (kJ / mol of compounds) or more,
The joining method according to claim 1, wherein the two materials to be joined are superposed while heating at a temperature of 100 ° C. or higher in an atmosphere having a pressure exceeding 1 × 10 ⁻⁴ Pa. A thin film having a microcrystalline structure formed on the smooth surface of the one material to be bonded has a body diffusion coefficient of 1 × 10 ⁻⁵⁵ (m ² / s) or more at room temperature and free energy of formation of oxide at room temperature. Is made of a single metal or alloy of −330 (kJ / mol of compounds) or more, and at least the smooth surface of the other material to be joined has a body diffusion coefficient of 1 × 10 ⁻⁵⁵ (m ² at room temperature). / s) and a microcrystalline structure composed of a single metal or alloy having a free energy of formation of oxide at room temperature of −330 (kJ / mol of compounds) or more,
The joining method according to claim 1, wherein the two materials to be joined are superposed while heating at a temperature of 200 ° C. or higher in an atmosphere having a pressure exceeding 1 × 10 ⁻⁴ Pa.   The joining method according to any one of claims 1 to 4, wherein each of the one and the other members to be joined is the thick member to be joined.   The joining method according to any one of claims 1 to 4, wherein either the one or the other material to be joined is a thin material to be joined having a thickness of less than 1.0 mm.   The smooth surface of said other to-be-joined material was formed with the thin film of the microcrystal structure formed in the surface of said other to-be-joined material in a vacuum vessel, The any one of Claims 1-6 characterized by the above-mentioned. Joining method.   The thick material to be joined is a radiator made of a single metal of Al, Cu, Ag, or Au, or an alloy mainly composed of Al, Cu, Ag, or Au. The joining method according to claim 1.   The joining method according to any one of claims 1 to 8, wherein the joining of the materials to be joined is performed in an atmosphere having a pressure equal to or higher than atmospheric pressure.   The joining method according to any one of claims 1 to 9, wherein an atmosphere in which the materials to be joined are overlapped is air.   The joining method according to any one of claims 1 to 9, wherein an atmosphere in which the materials to be joined are superposed includes an inert gas exceeding 78%.   The joining method according to any one of claims 1 to 11, wherein the joining of the materials to be joined is performed in an atmosphere from which dust is removed.   Before forming the microcrystalline thin film, an altered layer formed on a smooth surface of a material to be joined that forms the microcrystalline thin film is removed in the same vacuum as the formation of the microcrystalline thin film. The joining method according to claim 1.   One or more underlayers made of a thin film of a material different from the microcrystalline thin film are formed on the smooth surface of the material to be joined on which the microcrystalline thin film is formed, and the microcrystalline thin film is formed on the underlayer The joining method according to any one of claims 1 to 13.   The underlayer is formed of any one single metal selected from the element group of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W, or one or more selected from the element group The bonding method according to claim 14, wherein the bonding method is formed of an alloy containing any of the above elements.   The single metal or alloy forming the underlayer has a higher melting point than the single metal or alloy forming the microcrystalline thin film formed on the underlayer, and the single metal or alloy forming the microcrystalline thin film The bonding method according to claim 14 or 15, wherein a material having a large difference in melting point with respect to the surface is used.   When the material to be joined is made of Cu or Cu alloy, before the formation of the foundation layer, the altered layer generated on the smooth surface of the material to be joined for forming the foundation layer is removed. The joining method according to any one of claims 14 to 16.   The bonding method according to claim 1, wherein a thickness of the thin film having the microcrystalline structure is in a range of 2 nm to 1 μm.   A structure provided with the junction part joined by the method of any one of Claims 1-18.

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