JP5569964B2 - Atomic diffusion bonding method - Google Patents

Description

  The present invention relates to an atomic diffusion bonding method, and more specifically, for use when firmly bonding two substrates as materials to be bonded, such as stacking IC substrates, sealing packages, and combining various devices. In the bonding method, two substrates are bonded via a microcrystalline thin film formed on the bonding surface of at least one substrate, thereby bonding atoms between the substrates by causing atomic diffusion at the bonding interface and grain boundaries. The present invention relates to a new joining method.

  In the present invention, “microcrystal” includes “polycrystal” and “amorphous”.

  In addition, the “thin film” in the present invention includes not only a continuous film but also a film having an intermittent portion such as an island structure formed in the process of nucleus growth.

  Further, the “crystal grain boundary” in the present invention refers to an intermittent part of the regularity of atomic arrangement, and in addition to the crystal grain boundary in a polycrystal (“crystal grain boundary” in a general sense), long-range order (number The above-mentioned amorphous structure having the short-range order (regularity of arrangement in an atomic group of several tens atoms or less) that has no short-range order (regularity of arrangement in an atomic group of about 10 atoms or more). ”Is the“ crystal grain boundary ”in the present invention, and when there is a void in the amorphous metal film and the volume ratio (filling rate) is lower than 100%, the interface between the void and the amorphous metal is also high. Since it is considered to have an atomic diffusion coefficient, it corresponds to the “crystal grain boundary” in the present invention as in the above-mentioned intermittent portion of the short-range order.

  Joining techniques for bonding two or more materials to be joined are used in various fields. For example, in the field of electronic components, such joining techniques are used for wafer bonding, package sealing, and the like.

  As an example, the above-described wafer bonding technique will be described as an example. In the conventional general wafer bonding technique, a method of applying high pressure and high heat between stacked wafers is generally used.

  However, in this method, it is not possible to bond or integrate substrates on which electronic devices or the like that are sensitive to heat or pressure are formed. Therefore, the materials to be bonded can be bonded to each other without causing such physical damage. There is a demand for a technique for joining the two.

  As described above, as a technique for joining the materials to be joined at room temperature and without applying pressure, each of the joining surfaces of the materials to be joined is irradiated with an ion beam such as a rare gas to form oxides or By removing organic matter, etc., the atoms on the surface of the bonding surface are brought into an active state (activation) that is easy to form a chemical bond, and in this state, heating is performed by overlapping the bonding surfaces of the materials to be bonded. There has been proposed a “normal temperature bonding method” that enables bonding at room temperature without using an adhesive or the like, for example, as a bonding method for silicon wafers or the like (see Patent Document 1).

  However, in the method described in Patent Document 1, the bonding surface of the material to be bonded is irradiated with a rare gas beam or the like to clean the bonding surface to make it active, and then the two bonding surfaces are bonded firmly. Although a sufficient bonding force can be obtained, the materials that can be bonded are limited between some metals and metals, and some metals and compounds, and the application is limited.

  In addition, when bonding is performed by the above method, even if the bonding surface is macroscopically bonded, there is a portion that is not microscopically bonded due to the roughness or waviness of the bonding surface, and at the wafer level. It cannot be used for bonding for stacking and integration.

  In this way, in order to prevent the occurrence of a part that is not partially bonded, it is conceivable to suppress the surface roughness by polishing the bonding surface, but the bonding surface that can be suppressed by polishing is also considered. There are limits to roughness and swell.

  For this reason, if the conventional room temperature bonding method is used to reduce the occurrence of unbonded parts, it is necessary to perform processing such as pressurization and pressure bonding when polymerizing the materials to be joined. Damage may occur.

  In the bonding by the above method, the surfaces of both substrates are activated as described above to generate a metal or chemical bond between atoms only at the contact interface. It does not involve atomic diffusion.

  For this reason, although the bonding itself can be performed relatively firmly, there is still a bonding interface at the bonding portion of both substrates, and an altered layer such as an oxide film is formed at the bonding interface at the time of bonding. When used as an electronic device or the like, such a bonding interface or an altered layer acts as a barrier or the like that prevents the passage of electrons, resulting in a decrease in performance.

In order to eliminate the drawbacks of the room temperature bonding method described in Patent Document 1, the inventor of the present invention provides each bonding surface of a wafer, a chip, a substrate, a package, and other various materials to be bonded. In addition, in a vacuum atmosphere with an ultimate pressure of 10 ⁻⁴ Pa or less and a high vacuum degree, for example, by using a vacuum film-forming method such as sputtering or ion plating, and preferably under the generation of plasma, fine crystals of metals and various compounds A film having a structure is formed on the bonding surface, and the films formed on the bonding surface of the material to be bonded are polymerized at room temperature while maintaining the vacuum during or after the film formation. Have already proposed a “room temperature bonding method” in which the materials to be bonded are bonded by bonding generated between the bonding surfaces (see Patent Document 2).

  According to this room temperature bonding method, the bonding surfaces of the same type or different types of coatings can be firmly bonded by metal bonding or intermolecular bonding at the atomic level without heating, pressing, voltage application, etc. , The internal stress of the coating can be relieved and the joint strain can be relaxed. The resulting joint does not peel at the interface (if it tries to peel off forcibly, the part other than the interface of the coating or the material to be joined will be destroyed. Yes) a joined state can be obtained (Patent Document 2, “0021” column).

Japanese Patent No. 2791429 JP 2008-207221 A

  Among the conventional room temperature bonding methods described above, the method described in Patent Document 1 is active by irradiating a bonding surface of a material to be bonded with a rare gas beam or the like to remove oxides or organic substances existing on the bonding surface. After making it into a state, it is intended to perform the joining by superimposing both joint surfaces.

  Therefore, in the invention described in the above-mentioned Patent Document 1, a pair of wafer holding members positioned opposite to each other in the vacuum chamber are arranged, one wafer holding member is fixed to the vacuum chamber, and the other wafer holding member is fixed. It is attached to the lower end of a push rod that can move linearly while keeping the air tightness of the vacuum chamber, and by superimposing the joint surfaces in the same vacuum vessel where the rare gas beam was irradiated (Patent Document 1, “0005” column) ), The bonding surface that has been activated by irradiation with a rare gas beam or the like is bonded again with oxygen, organic matter, or the like in the air so that the bonding is not lost.

  Further, the room temperature bonding method described in Patent Document 2 not only presupposes that the bonding surfaces of the materials to be bonded are overlapped together with film formation in the same vacuum (for example, claims in Patent Document 2). 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. In the meantime, the bonding itself can no longer be performed ”(Patent Document 2“ 0055 ”column).

  As described above, in the conventional room temperature bonding method, after the bonding surface is processed (cleaning with a rare gas beam or forming a film having a microcrystalline structure), the subsequent bonding of the bonding surfaces is performed by processing the bonding surfaces. In the same vacuum vessel and with the vacuum vessel maintained in a high vacuum state, for example, the bonding surface after cleaning with a rare gas beam or forming a microcrystalline thin film should be at atmospheric pressure. It is the recognition of those skilled in the art of the present invention, including the inventor of the present invention, that the bonding itself is impossible if exposed to air or the like. A configuration is employed in which superposition is performed in a high vacuum space.

  For this reason, it is necessary to perform the work of superimposing the joining surfaces of the materials to be joined in a limited space in a vacuum vessel maintained at a high vacuum in a limited condition. Not only is it difficult, but in order to realize the above-mentioned joining method, a special structure for superimposing the joining surfaces of the materials to be joined placed in the vacuum vessel while keeping the inside of 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.

However, in spite of the above-mentioned recognition by those skilled in the art, the inventors of the present invention have intensively studied, and as a result, a microcrystalline thin film made of a metal having specific physical properties is formed on the joint surface of the material to be joined. When bonding is performed, the overlapping of the bonding surfaces is performed at a pressure (low vacuum) higher than the vacuum (1 × 10 ⁻⁴ Pa) described in Patent Document 2 described above, for example, an atmosphere under atmospheric pressure, Furthermore, it was found that the bonding surfaces can be bonded even after exposure to atmospheric air.

The present invention has been made on the basis of the above findings obtained as a result of research by the inventors of the present invention. After the formation of a microcrystalline thin film on the bonding surface of the material to be bonded is performed under vacuum, Also used for bonding between a wide range of materials including bonding between different materials by superimposing the bonding surfaces under a pressure higher than ^10-4 Pa (low vacuum), for example, atmospheric pressure. By providing a bonding method with atomic diffusion (atomic diffusion bonding method) that can be performed without physically damaging the substrates to be bonded, it is possible to perform bonding of such bonding with atomic diffusion. The purpose is to improve workability and eliminate the need for a vacuum device or a sticking device having a special structure for performing an overlay operation.

In order to achieve the above object, the atomic diffusion bonding method of the present invention has a body diffusion coefficient at room temperature of 1 × 10 ⁻⁴⁰ (m ² / s) or more on a smooth surface of one substrate in a vacuum vessel, In addition, a thin film having a microcrystalline structure composed of a single metal or alloy having a free energy of formation of oxide of −15 (kJ / mol of compounds) or more at room temperature and an atmosphere with a pressure exceeding 1 × 10 ⁻⁴ Pa are formed. Below, a single metal having a body diffusion coefficient of 1 × 10 ⁻⁴⁰ (m ² / s) or more at room temperature and an oxide free energy of formation of −15 (kJ / mol of compounds) or more at room temperature Alternatively, the one and the other two substrates are overlapped so that the thin film formed on the one substrate contacts the smooth surface of the other substrate having a smooth surface having a microcrystalline structure made of an alloy. The thin film and the other substrate Bonding the two substrates by causing atomic diffusion at the joint interface and the crystal grain boundary between the smooth surface (excluding the bonding performed at a junction temperature of more than 0.99 ° C.) characterized by (claim 1 ).

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 is “−n (kJ / mol of compounds) or more” (n is a number) means that −n ≦ ΔG (ΔG is the generation free energy of oxide), that is, generation of oxide. 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, another atomic diffusion bonding method of the present invention has a body diffusion coefficient of 1 × 10 ⁻⁴⁵ (m ² / s) or more at room temperature on a smooth surface of one substrate in a vacuum vessel, and at room temperature. A thin film having a microcrystalline structure composed of a single metal or alloy having a free energy of formation of oxide of −150 (kJ / mol of compounds) or more is formed, and at least in an atmosphere having a pressure exceeding 1 × 10 ⁻⁴ Pa. A single metal or alloy whose surface has a body diffusion coefficient of 1 × 10 ⁻⁴⁵ (m ² / s) or more at room temperature and an oxide free energy of formation of −150 (kJ / mol of compounds) or more at room temperature. The one and the other two substrates are overlapped with each other so that the thin film formed on the one substrate is in contact with the smooth surface of the other substrate having a smooth surface having a microcrystalline structure. Said microcrystals by heating at a temperature It said cause atomic diffusion at the joint interface and grain boundaries forming a thin film bonding two substrates (except a bonding performed at a junction temperature of more than 0.99 ° C.), characterized in that (claim 2).

  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.

In any of the above construction, the smooth surface of the other substrate may be formed by a thin film of microcrystalline structure formed on a surface of the other substrate in a vacuum container (claim 3).

In any of the above atomic diffusion bonding method, superposition of the substrate may be carried out under an atmosphere of pressure above atmospheric pressure (claim 4).

The atmosphere in which the substrates are superposed may be air (Claim 5 ), or may contain more than 78% inert gas (Claim 6 ). The term “inert gas” here includes “nitrogen”.

Moreover, the superposition of the substrate, for example, is preferably performed in an atmosphere that has been removed of dust as such clean rooms and a glove box (claim 7).

Further, the strength of the overlapping of the substrates can be performed by a relatively weak force of 101 kPa or less, for example, about several kPa (Claim 8 ). However, this does not prohibit pressing with a force that does not damage the substrate or the like.

Further, before forming the microcrystalline thin film, it is preferable to remove the altered layer generated on the smooth surface of the substrate on which the microcrystalline thin film is formed in the same vacuum as the formation of the microcrystalline thin film. Item 9 ).

Furthermore, 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 substrate on which the microcrystalline thin film is formed, and the microcrystalline thin film is formed on the underlayer. It is good also as a thing (Claim 10 ).

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 11 ).

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 12 ).

The thickness of the thin film of the fine crystalline structure can be in the range of 2Nm～1myuemu (claim 13).

  With the configuration of the present invention described above, the following remarkable effects can be obtained according to the atomic diffusion bonding method of the present invention.

The above-mentioned body diffusion coefficient and the formation of a similar body diffusion coefficient and oxide on the flat surface of one substrate on which a microcrystalline coating made of a single metal or alloy having the free energy of oxide formation is formed. By superimposing the other substrate with a flat surface having a microcrystalline structure made of a single metal or alloy having free energy under the above-mentioned conditions, the superposition of the two is performed at a pressure exceeding 1 × 10 ⁻⁴ Pa. Even when performed in a relatively low-vacuum space such as an atmosphere or an atmosphere with a pressure higher than atmospheric pressure (1 atm), atomic diffusion occurs at the bonding interface and grain boundaries, thereby It is possible to firmly bond between the bonding surfaces of the microcrystalline thin film or between the microcrystalline thin film and the smooth surface of the substrate by metal bonding or intermolecular bonding at the atomic level, and release the internal stress of the thin film. We were able to relax the junction distortion. Note that the bonding obtained here is a bonding state in which peeling does not occur at the interface (a part other than the interface of the thin film or the substrate is destroyed if the peeling is forcibly).

  As a result, it is not necessary to perform bonding between substrates in a vacuum vessel maintained at a high vacuum, and as a result, the degree of freedom in bonding conditions increases, and as a result, workability when bonding by atomic diffusion can be greatly improved. did it.

  In addition, in the atomic diffusion bonding method under the above conditions, the substrate can be heated at a relatively low temperature of about 200 ° C. so that bonding can be performed under any of the above conditions, and damage to the substrate due to heating is minimized. I was able to keep it to the limit.

In particular, 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, the substrate is Bonding can be performed by heating 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 / In the case of more than mol of compounds), the above-mentioned atomic diffusion bonding can be performed even when bonding at room temperature without heating the substrate, and the damage to the substrate due to heat can be further reduced. .

However, even if the can be joined without performing heating to the substrate, it is to increase the body diffusion coefficient described above the substrate temperature in extent to prevent damage to the substrate by heating above room temperature This makes it possible to improve the diffusibility of atoms at the bonding interface and grain boundaries by increasing the diffusion rate and diffusion length of atoms, and to achieve more uniform and strong bonding. This makes it possible to bond even a substrate having a relatively rough surface.

Here, the body diffusion coefficient D is
D = D ₀ exp (−Q / RT)
D ₀ : 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.

  The atomic diffusion bonding method can be performed even when the substrates are superposed in an atmosphere having a pressure equal to or higher than atmospheric pressure (1 atm). Further, the atmosphere in which the substrates are superposed is air. In this case, that is, even when the superposition is performed in the state exposed to the air, the joining can be suitably performed.

  However, about 78% of the air is nitrogen and about 20% is oxygen, and it is considered that the oxygen is chemically adsorbed on the above-mentioned microcrystalline thin film, thereby inhibiting the bonding between the substrates. Superposition of the substrates is performed in an atmosphere containing more than 78% inert gas (the term “inert gas” includes “nitrogen”), preferably in an atmosphere containing 100% inert gas. By reducing the amount of oxygen, which is a factor that hinders bonding, from the atmosphere, bonding is performed in an atmosphere in which 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 bonding more suitably.

  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.

  In addition, by superimposing the above substrates in a “clean room” or “glove box” atmosphere where dust is removed, it is possible to prevent poor bonding due to the presence of impurities such as dust on the bonding surface. I was 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).

  According to the atomic diffusion bonding method of the present invention, it is not necessary to apply a large pressure when superposing the substrates, and the strength of the force for superposing the substrates is 101 kPa or less, for example, a pressure of about several kPa. Even if it was a case, it was able to join suitably. As a result, the substrate was preferably prevented from being damaged by the pressure applied during the bonding.

  Before forming the microcrystalline thin film, the altered layer formed on the smooth surface of one or both of the substrates in the same vacuum as the formation of the microcrystalline thin film is removed by a dry process such as reverse sputtering, The adhesion strength of the microcrystalline thin film to the substrate can be improved, and a decrease in the adhesion strength between the substrates due to the separation between the substrate surface and the microcrystalline thin film can be suitably prevented.

  Further, a thin film of a material different from that of the microcrystalline thin film, for example, Ti, V, Cr, Zr, Nb which is an element belonging to Group 4A to 6A in the periodic table is formed on the smooth surface of the substrate on which the microcrystalline thin film is formed. Forming an underlayer from a thin film of any one single metal selected from the group of elements Mo, Hf, Ta, and W, or a thin film of an alloy containing one or more elements selected from the group of elements Therefore, the adhesion strength of the microcrystalline thin film to the substrate can be increased, thereby preventing the separation between the substrate 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.

  In the atomic diffusion bonding method of the present invention, atomic diffusion bonding can be suitably performed when the film thickness of the microcrystalline thin film to be formed is in the range of 2 nm to 1 μm, respectively. In particular, it is more sufficient than the mean free process of electrons and spin currents. In addition, bonding can be performed even by forming a microcrystalline thin film with a thickness of about several tens of millimeters. Therefore, even when used for bonding silicon wafers, etc., bonding is not hindered by movement of electrons by the bonding surface. Could provide a way.

Schematic explanatory drawing which showed an example of the joining process of the base | substrate by the atomic diffusion bonding method of this invention. A cross-sectional micrograph of a Si substrate bonded after exposure to atmospheric pressure air for 5 minutes after forming a microcrystalline thin film of Au. A cross-sectional micrograph of a Si substrate bonded after exposure to atmospheric pressure air for 1 hour after formation of a microcrystalline thin film of Au. Immediately after the formation of the Au microcrystalline thin film, a cross-sectional micrograph of a Si substrate bonded in a vacuum vessel in which the microcrystalline thin film was formed (Comparative Example 1). A cross-sectional micrograph of a Si substrate that was immediately bonded at a temperature of 200 ° C. after being exposed to air at atmospheric pressure after forming a microcrystalline thin film of Pt. A cross-sectional micrograph of a Si substrate that was immediately exposed to 160 ° C. after being exposed to air at atmospheric pressure after forming a microcrystalline thin film of Pd.

Outline of Bonding Method The atomic diffusion bonding method of the present invention comprises a single metal 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, Alternatively, by superimposing microcrystalline thin films made of an alloy, or from a single metal or alloy having at least the surface having the predetermined body diffusion coefficient and oxide free energy, with the microcrystalline thin film. Even if the superposition is 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 on a flat surface having a microcrystalline structure, It was found that atomic diffusion occurred at the interface and grain boundaries, and that strong bonding was performed between them, and this was applied to bonding between substrates. Ri, and performs the joining of the base body to each other in the conditions described below.

Substrate (material to be joined)
Material As a substrate to be bonded by the atomic diffusion bonding method of the present invention, the ultimate vacuum is 1 × 10 ⁻³ to 1 × 10 ⁻⁸ Pa, preferably 1 × 10 ⁻ , for example, sputtering or ion plating. Any material can be used as long as it can form a thin film having the above-described microcrystalline structure by vacuum film formation in a high vacuum atmosphere using a vacuum vessel having a high vacuum of ⁴ to 1 × 10 ⁻⁸ Pa. It is possible to form a thin film having a microcrystalline structure by the above method, such as various kinds of pure metals and alloys, semiconductors such as Si substrate and SiO ₂ substrate, glass, ceramics, resin, oxide, etc. It can be set as the base | substrate (to-be-joined material) in invention.

  The substrate can be bonded not only between the same materials, for example, between metals, but also between different materials, such as metal and ceramics.

The state of the joining surface, etc. The shape of the substrate is not particularly limited, and for example, from flat to various complex three-dimensional shapes, various shapes can be targeted depending on the application and purpose. The portion (joint surface) to be joined with the other substrate needs to have a smooth surface formed smoothly with a predetermined accuracy.

  In addition, it is good also as what joins a several base | substrate with respect to one base | substrate by providing two or more this smooth surfaces with which another base | substrate is joined to one base | substrate.

  The surface roughness of the bonding surface is, for example, a surface roughness exceeding the maximum height (Rmax) of 5 nm (for example, 50 nm or less) when the purpose is achieved simply by bonding, such as sealing of a package. However, bonding can be performed, but Rmax is preferably 5 nm or less.

  As for the smooth surface of the substrate, it is preferable that a gas-adsorbed layer or a modified layer such as a natural oxide layer on the surface is removed before forming a thin film having a microcrystalline structure. It is possible to suitably use a substrate that has been subjected to hydrogen termination or the like in order to remove the altered layer and to prevent gas adsorption again after the altered layer is removed.

  In addition, the removal of the altered layer is not limited to the wet process described above, and can 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, gas adsorption and oxidation occur on the substrate surface after the altered layer is removed and before a thin film having a microcrystalline structure described later is formed. In order to prevent this, it is preferable to remove such a deteriorated layer in the same vacuum when a thin film having a microcrystalline structure described later is formed, and to form a thin film having a microcrystalline structure following the removal of the deteriorated layer. More preferably, the altered layer is removed using an ultra-high purity inert gas to prevent the oxide layer or the like from being re-formed after the altered layer is removed.

  The structure of the substrate is not particularly limited, such as single crystal, polycrystal, amorphous, glass state, etc., and various structures can be targeted for bonding, but only one of the two substrates will be described later. When a thin film having a microcrystalline structure is formed and bonding is performed without forming a thin film having a microcrystalline structure on the other substrate, the bonding surface of the other substrate on which the thin film is not formed is In order to obtain atomic diffusion at the junction interface and grain boundaries, at least the surface thereof is within the range of a predetermined body diffusion coefficient and a predetermined free energy of formation of an oxide, as in a thin film having a microcrystalline structure described later. It is necessary to have a microcrystalline structure (including amorphous) made of metal.

Thin film material of microcrystalline structure As a material for forming the thin film of microcrystalline structure described above, a thin film of the same kind as the base material may be formed. Further, the material of the microcrystalline thin film formed on one of the substrates may be different from the material of the microcrystalline thin film formed on the other of the substrates.

  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 the base body to 100 ° C. or higher at the time of bonding. Is a single metal or alloy having a body diffusion coefficient of 1 × 10 ⁻⁴⁵ (m ² / s) or more and an oxide free energy (kJ / mol of compounds) of −150 or more at room temperature. 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 substrate to 200 ° C. or higher at the time of bonding. It is a single metal or alloy having a diffusion coefficient of 1 × 10 ⁻⁵⁵ (m ² / s) or more and an oxide free energy of formation (kJ / mol of compounds) at room temperature of −330 or more. 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 an alloy, it is necessary to heat the substrate at 200 ° C. or higher in order to enable bonding with all metal thin films belonging to this numerical range. On the other hand, when the metal diffusion coefficient is limited to a metal having a high free energy for forming an oxide, the heating temperature of the substrate for bonding can be lowered, and the body diffusion coefficient at room temperature is 1 ×. If the free energy of formation of oxide at room temperature is ^10-45 (m ² / s) or more and -150 (kJ / mol of compounds) or more, the substrate is 100 ° C. or more, and the body diffusion coefficient is 1 at room temperature. × 10 ^-40 (m ² / s) or more and at room temperature When the free energy of formation of the oxide is -15 (kJ / mol of compounds) or more, bonding with atomic diffusion can be performed even when bonding at room temperature without heating the substrate.

  When a substrate in which a thin film having a microcrystalline structure is formed by using these metals is bonded at each of the above-described temperature conditions, the bonding interface disappears at least partially, and the bonding is performed at the disappearance portion of the bonding interface. Recrystallization occurs between the formed microcrystalline thin films, and crystal grains having a grain size covering almost the entire area between the two substrates are obtained, so that the two microcrystalline thin films are integrated by metal bonding. Can do.

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 forming a thin film, bonding is possible even when the film thickness is 2 nm (4 nm for two layers), and the thickness of the microcrystalline thin film interposed between the substrates to be bonded can be set to an average free of electrons and spin current. It is possible to form with the thickness below the process.

  As a result, the layer of the microcrystalline thin film interposed between the substrates does not become a barrier against the movement of electrons, etc., and it is possible to create a new functional device by bonding an arbitrary silicon wafer. It is possible to use an atomic diffusion bonding method.

  Note that the thin film having a microcrystalline structure to be formed does not need to continuously cover the entire bonding surface, and may partially cover this. Therefore, the term “microcrystalline thin film” as used herein includes, for example, the state of an island structure formed in the process of nuclear growth. Bonding by atomic diffusion, which will be described later, occurs at an overlapping portion between the island forming the thin film and the island forming the other thin film, and thereby the bonding is obtained.

  On the other hand, as the film thickness increases, the surface roughness of the obtained microcrystalline thin film increases and bonding becomes difficult, and it takes a long time to form a thick microcrystalline thin film, resulting in decreased productivity. Therefore, the upper limit is about 1 μm, and about 2 nm to 1 μm is a preferable film thickness range of each microcrystalline thin film in the atomic diffusion bonding method in the present invention.

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 microcrystalline thin film Furthermore, the above-mentioned thin film having a microcrystalline structure may be formed on each of two substrates to be joined, but the thin film having the microcrystalline structure is formed only on one of the substrates. However, it is possible to obtain a bond to the other substrate without forming a thin film having a microcrystalline structure.

  In this case, the bonding surface of the other substrate on which the thin film having the microcrystalline structure is not formed is, as described above, at least near the surface of the bonding surface within the predetermined range of the body diffusion coefficient and the free energy of formation of the oxide. It must have a microcrystalline structure made of a single metal or alloy. However, it is not necessary that the bonding surface of the substrate on which the microcrystalline thin film is not formed and the material of the microcrystalline thin film be the same.

  In addition, on the smooth surface of the substrate on which the microcrystalline thin film is formed, one or more underlayers made of a thin film made of a material different from the microcrystalline thin film should be formed before the microcrystalline thin film is formed. In particular, when the thin film having a microcrystalline structure to be formed has a relatively low adhesion strength to the substrate, 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 Not only can be suitably prevented from peeling off from the substrate, but the two-dimensionality of the microcrystalline thin film formed on the underlayer (atomic wettability during microcrystalline thin film formation) is improved, and microcrystals are formed during film formation. Ag, which is a thin film having a structure, can be prevented from growing in an island shape, and an extremely thin microcrystalline thin film of 2 nm can be easily formed.

Film Forming Method Film Forming Technology In the atomic diffusion bonding method of the present invention, as a method for forming a microcrystalline thin film formed on a bonding surface of a substrate which is a material to be bonded, PVD such as sputtering and ion plating, CVD, Various film-forming methods for vacuum film formation in a vacuum atmosphere in a vacuum vessel having a high vacuum degree of 1 × 10 ⁻⁴ to 1 × 10 ⁻⁸ Pa, such as vapor deposition, can be cited, and the diffusion rate is For relatively slow materials and alloys and compounds thereof, a vacuum film formation method in which film formation is preferably performed under the 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 possible, 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.

Conditions for superposition Atmospheric pressure As described above, a substrate having a microcrystalline thin film formed on the surface, or a substrate having a microcrystalline thin film and a substrate surface having a microcrystalline structure are 1 × The two substrates are bonded to each other by causing atomic diffusion at the bonding interface and the crystal grain boundary by superimposing in an atmosphere having a pressure exceeding 10 ⁻⁴ Pa, for example, a pressure atmosphere at or above atmospheric pressure.

  Thus, according to the method of the present invention, bonding can be performed even when the two substrates are superposed in a higher pressure atmosphere than in the vacuum in which the microcrystalline thin film is formed. ing.

  The case where the pressure of the atmosphere in which the substrates are superposed exceeds the atmospheric pressure includes, for example, the case where the substrates are superposed in an air supply type clean room or a glove box.

Ingredients of the atmosphere The atmosphere components when the substrates are overlapped may be air (about 78% nitrogen, about 20% oxygen), or nitrogen gas, argon gas, or other inert gas. However, it may be in a state of exceeding 78% alone or in a mixed state, preferably an atmosphere of 100% inert gas.

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

Furthermore, since the amount of oxygen and water in the atmosphere can be reduced after a long time has elapsed after the formation of a thin film having a microcrystalline structure, the substrates can be overlapped. As described above, in an atmosphere containing an inert gas exceeding 78%, preferably in an atmosphere containing 100% inert gas, and / or an atmosphere in which superposition is performed within a pressure range exceeding 1 × 10 ⁻⁴ Pa When the pressure is set low, it is possible to perform strong bonding even after a longer time has elapsed since the formation of the microcrystalline thin film.

  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 substrates 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 substrates are superposed is performed in a space from which dust is removed in order to prevent poor bonding due to the presence of dust or the like on the bonding 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 that the atmosphere for superimposing such bonding surfaces 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 bonding surface and the free energy of formation of the oxide, the substrate is subjected to the following conditions according to the following conditions. Joining by heating to temperature.

It is microcrystalline by a single metal or alloy having a body diffusion coefficient of 1 × 10 ⁻⁴⁰ (m ² / s) or more at room temperature and an oxide free energy (kJ / mol of compounds) of −15 or more at room temperature. When bonding with a thin film having a structure, heating of the substrate is not necessary to obtain bonding.

  However, even in this case, as described above, the base body may be heated within a range not damaging the base body to perform bonding with an increased body diffusion coefficient.

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. When expanded to a metal or alloy, the substrate is heated to 100 ° C. or higher in order to obtain bonding in all 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) − In the case of expanding to the range of single metal or alloy of 330 or higher, the substrate is heated to a temperature of 200 ° C. or higher in order to obtain bonding in all metal thin films included in this condition.

  It should be noted that the metal body diffusion coefficient and oxide free energy forming the microcrystalline thin film formed on one substrate, and the metal body diffusion coefficient and oxide forming the microcrystalline structure on the surface of the other substrate. In the case where the free energy of formation is different, the substrate is heated to a temperature applied to a metal having a small body diffusion coefficient and a small value of the free free energy of oxide for bonding.

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 substrates to be bonded to each other is disposed at the lower part of the magnetron cathode. A microcrystalline thin film is formed on the bonding surface of the substrate 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 one end of the table on which one of the bases is placed and the one end of the table on which the other base is placed are placed in abutment with each other, and the two tables rotate around the abutting part. Thus, the other surfaces of both tables are lifted upward so that the joint surfaces of the two substrates placed on the tables are superposed.

  When bonding by forming a thin film with a microcrystalline structure with a metal within the numerical range of the body diffusion coefficient and the free energy of formation of the oxide, which require heating of the substrates during bonding, electric heating is included in the aforementioned table. A heater or the like may be embedded so that the bonding surfaces of the two substrates can be polymerized and heated to a predetermined temperature.

  Note that the jig for bonding substrates is not limited to the one shown in the figure, and various shapes and structures can be used according to the shape of the substrate to be bonded. For example, one or both of the bases may be operated by a robot arm or the like, and the bases 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 container in which the jig configured as described above is arranged, a microcrystalline thin film is formed on the bonding surface of the substrate under the above-described conditions in a state where the jig is in the above-described film forming position.

When a microcrystalline thin film having a predetermined thickness is formed on the bonding surface of the substrate, 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, high Return to atmospheric pressure or clean room pressure. 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 manner, the pressure in the vacuum vessel is increased to a pressure exceeding 1 × 10 ⁻⁴ Pa, for example, atmospheric pressure or the pressure in the clean room, and then the inlet / outlet of the vacuum vessel is opened and the above-described treatment is performed from the inside of the vacuum vessel. The substrate is taken out together with the tool, and the table provided on the jig is rotated to the above-described bonding position to bond the substrate with a relatively weak force of 101 kPa or less, for example, about several kPa.

  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 substrates on which the microcrystalline thin film made of the same material is bonded is described. However, when the substrates on which the microcrystalline thin film made of different materials are bonded together, for example, a common clean room is used. The magnetron cathode described above is arranged in each of the two vacuum vessels installed in the inside so that a microcrystalline thin film of a different material can be formed in each vacuum vessel, and the bonding surface of each substrate is After forming microcrystalline thin films of different materials, the substrates taken out from the respective vacuum vessels may be joined by being superposed in the aforementioned clean room.

  Further, the formation of the microcrystalline thin film on both the substrates is not necessarily performed at the same time, and may be performed with a time difference.

  In addition, if at least the surface portion of the other substrate has a microcrystalline structure of a single metal or alloy in the numerical ranges of the above-described body diffusion coefficient and oxide formation free energy, two substrates to be joined It is also possible to form the above-mentioned microcrystalline thin film only on one of the substrates, and directly bond both substrates without forming the microcrystalline thin film on the other substrate.

Bonding mechanism Atomic diffusion and body diffusion coefficient In forming the present invention, a microcrystalline film made of various materials is formed on the substrate to be bonded, and the bonding between the substrates is the same as forming a microcrystalline thin film. Preliminary experiment conducted in “in vacuum”.

  From the results of this preliminary experiment, it was confirmed that the bonding of the microcrystalline thin film strongly depends on the material of the thin film, and the body diffusion coefficient of the formed microcrystalline thin film plays an important role in the bonding. Guessed.

Here, the body diffusion coefficient D of the solid substance can be expressed as follows using the Arrhenius equation.
D = D ₀ exp (−Q / RT)

In the above equation, D ₀ is the frequency term (entropy term), Q is the activation energy, R is the gas constant, and T is the absolute temperature.

Table 1 shows respective values of D ₀ and Q of the material used for forming the microcrystalline thin film in the above-described preliminary experiment and the calculated D value at 300 K (27 ° C.). In Table 1, each material is listed in order of the size of D.

The values of Ru D ₀ and Q have not been reported in the available literature, and it is assumed that the D value of Ru is approximately proportional to the melting point as in the case of other materials. It is a rough estimate. The approximate D value is shown in parentheses in the table for reference.

  As shown in Table 1, Ti has an hcp structure and has the largest D value among the materials shown in Table 1. Furthermore, the D values of Al, Au, Ag, and Cu fcc materials are larger than those of other materials. In a preliminary experiment in which thin films of these materials are joined in a vacuum, no corresponding interface remains on the surface of the original thin film. In addition, the crystal grains are formed across the original interface between the thin films, which indicates that recrystallization is promoted at the interface. Further, some crystal grains are formed over almost the entire thickness of the film, and this phenomenon is particularly remarkable in the bonding of Ti / Ti and Al / Al thin films having extremely large D values. The recrystallization that began at the original interface between each thin film has reached a distance of at least 50 nm of the bonded Al (50 nm) / Al (50 nm) thin film.

  On the other hand, in the bonding of Pt / Pt, Cr / Cr, Ru / Ru, and Ta / Ta thin films having a small D value, a clear boundary substantially corresponding to the original thin film surface remained. In particular, in the bonding of Ta / Ta and Ru / Ru thin films, the internal diffusion at the interface is not significant compared to the others, and as described above, this means that these materials have a very small D value. Both agree well.

  It should be noted that a thin amorphous-like layer was formed at the interface between the Cr / Cr and Ta / Ta thin films having the bcc crystal structure. On the other hand, in the Pt / Pt thin film junction having the fcc crystal structure, the fcc- (111) lattice is formed across the surface of the original thin film. Regarding the bonding of Co / Co, Ni / Ni, Fe / Fe, and Mo / Mo thin films, the microstructure was not observed in the preliminary test described above, but these microcrystalline structures are Pt / Pt (Co / Co and Ni / Ni), Cr / Cr (for Fe / Fe), and Ta / Ta (for Mo / Mo).

  Furthermore, it was also able to join in the W / W joining test, and it was confirmed that the structure after joining was a structure similar to Ta in the observation by the cross-sectional TEM of the joining part.

  Here, W is a material having the lowest D value among single metals. Therefore, atomic diffusion bonding with microcrystalline thin films occurs in all metal thin films (including these alloys) including W and Re when the substrates are superposed in the same vacuum as the formation of the microcrystalline thin films. It is considered a thing.

As described above, the atomic diffusion that occurs during the bonding of thin films with a microcrystalline structure becomes more pronounced as the body diffusion coefficient increases. In a material having a large coefficient, for example, a material having a body diffusion coefficient of 1 × 10 ⁻⁴⁰ (m ² / s) or more, atomic diffusion accompanied by disappearance of the bonding interface and recrystallization can occur.

  On the other hand, in such junctions involving atomic diffusion, moisture, oxygen, organic substances, etc. in the atmosphere are chemically adsorbed on the surface of the microcrystalline thin film, and compounds such as oxides are formed on the surface of the microcrystalline thin film. Conventionally, when a substrate on which a microcrystalline thin film is formed is taken out of a vacuum vessel and exposed to an atmospheric pressure atmosphere, a compound such as an oxide is immediately formed on the surface of the microcrystalline thin film. It was thought that it would be impossible to join.

  Therefore, bonding with atomic diffusion due to the formation of a microcrystalline thin film was considered to be a phenomenon that occurs only when the bonding surfaces are superimposed in the same vacuum as the microcrystalline thin film was formed. .

  However, as will be described later, when two substrates on which a thin film having a microcrystalline structure of Au is formed are exposed to air at atmospheric pressure (1 atm) for 5 minutes, bonding is performed in this case as well. It was confirmed that atomic diffusion occurred at room temperature and sufficient bonding strength was obtained.

  Similarly, when two substrates on which a thin film having a microcrystalline structure of Au is exposed after being exposed to air at atmospheric pressure (1 atm) for 1 hour, bonding is performed after further exposure for 6 hours. In this case, it was confirmed that the same degree of bonding as that obtained after the exposure for 5 minutes was obtained.

  From the above results, it can be seen that the structure of the Au microcrystalline thin film remains in any state after exposure to air at atmospheric pressure (1 atm) for 5 minutes, exposure for 1 hour, and further exposure for 6 hours. It can be inferred that no significant change has occurred.

  Here, even if the microcrystalline thin film of Au, which can be bonded after being exposed to atmospheric pressure (1 atm) air for 6 hours, is further exposed to atmospheric pressure air, Moisture, oxygen, organic matter, etc. in the material are physically or chemically adsorbed on the surface of the Au microcrystalline thin film, so that the degree of atomic diffusion at the interface decreases as the amount of adsorbed layer etc. increases, resulting in weak bonding It is guessed.

  Since such a change in the degree of atomic diffusion at the bonding interface is related to chemical reactions such as gas adsorption on the surface, it is a time-dependent change expressed statistically using the relaxation time τ. It is considered to be proportional to exp (−t / τ).

  In the above example, it is estimated that there is no significant change in the structure of the Au microcrystalline thin film regardless of whether the exposure time to atmospheric air is 5 minutes, 1 hour, or 6 hours. It means that the time is sufficiently longer than 1 hour and even 6 hours. Even if the substrate is bonded after exposing the microcrystalline thin film of Au to air for a longer time than 6 hours, atomic diffusion It is considered that the bonding can be performed.

  Even when exposed to air at atmospheric pressure for a long period of time, the fine structure of the microcrystalline thin film does not change because the surface of the microcrystalline thin film is adsorbed by physical adsorption or chemical adsorption. It is considered that the formation of layers does not occur to the extent that atomic diffusion is inhibited, that is, the Au used as the material for the microcrystalline thin film in the above example is a chemically very stable material. be able to.

  Therefore, as in the case of Au, when a thin film having a microcrystalline structure made of a chemically stable material is formed and bonded, the atmosphere under a high pressure with respect to the vacuum in which such a thin film is formed is used. Even when bonding is performed with the use of these materials, a film having a microcrystalline structure made of Ag, Cu, Pd, and Pt is formed under the prediction that bonding by atomic diffusion is possible. Similarly, it was confirmed that the substrate on which the film having the microcrystalline structure was formed could be bonded even after being exposed to air under atmospheric pressure.

  Here, the physical properties of the above-mentioned Au, Ag, Cu, Pd, and Pt that were able to be bonded even under atmospheric pressure as described above, and the metal at a relatively close position in the periodic table. If shown, it is as shown in Table 2 below.

* If there are multiple oxides, the oxide with the smaller value of free energy of formation (the one considered to be easy to oxidize) is listed.
In the table, ΔG: free energy of formation of oxide at room temperature (kJ / mol of compounds)
ΔH: Heat of oxide formation at room temperature (kJ / mol of compounds)
D: Body diffusion coefficient at room temperature (m ² / s)
Tm: Melting point (° C)
R: Electric resistance (10 ^-8 Ωm, room temperature)
Citations for physical properties: Smiths Metals Reference Book, 7th Edition, Edited by EABrandes & GBBrook, Butterworth-Heinemann.

  Due to the above physical properties, in metals that have been experimentally confirmed to be bonded by atomic diffusion, ΔG (oxide free energy of formation) is positive (that is, no oxide is formed) at room temperature, such as Au and Pt. In addition to metals with no reported value, atmospheric pressure is also applied to metals such as Ag, Cu, Pd, and Pt that have a negative ΔG value and a relatively small absolute value (a metal with a relatively large ΔG value). It has been confirmed that lower bonding is possible.

  On the other hand, as the body diffusion coefficient decreases with respect to Au and Ag, and as ΔG (oxide free energy of formation) decreases, it becomes difficult to bond at room temperature. In the example in which a thin film having a microcrystalline structure is formed, it is necessary to heat the substrate to 100 ° C. at the time of bonding, and 150 to 200 ° C. is required for the Pt thin film and 150 to 160 ° C. for the Pd thin film. It has become.

  According to such a relationship, the numerical value of ΔG (oxide free energy of formation) for the Zn having a larger body diffusion coefficient than any of the metals Au, Ag, Cu, Pd, and Pt subjected to the experiment is Among the materials that have been confirmed to be smaller than the minimum Cu of -144.9, but can be bonded by heating the substrate to about 200 ° C, which is the same as the heating temperature of the Si substrate in the case of Pt In addition, Ni, which has a larger body diffusion coefficient than Pt, which was also confirmed to be bonded, has a slightly smaller ΔG (oxide free energy of formation) of -213.1 compared to other metals. However, it is expected that bonding can be performed by heating at about 200 ° C. as in the case of Pt.

  Furthermore, when an alloy having a body diffusion coefficient similar to these metals and ΔG (oxide free energy of formation) is used as the material of the thin film having a microcrystalline structure, bonding under atmospheric pressure is possible as well. Expected to be.

  Hereinafter, the result of conducting a confirmation test of bonding conditions required when bonding is performed by forming a thin film having a microcrystalline structure on the surface of the substrate with the above-described Au, Ag, Cu, Pt, and Pd will be shown.

[Example of bonding test using Au thin film]
Bonding method Ultra high vacuum (UHV) 5 pole cathode-magnetron sputtering equipment (final vacuum 2 × 10 ^-6 Pa) for physical experiments, diameter of about 1 inch (2.7 cm) or diameter 2 An Au microcrystalline thin film was formed on each of two inches (about 5.08 cm) substrates, and a bonding test was performed. For sputtering, an ultra-high purity argon gas having an impurity concentration of 2 to 3 ppb (2 to 3 × 10 ⁻⁹ ) or less at the gas introduction part (use point) into the vacuum chamber was used.

Two types of substrates, Si substrate and SiO ₂ substrate, were prepared as the substrate, and the bonding was performed between the same type substrates on which the Au thin film was formed.

The surface roughness of the substrate as the substrate to be bonded was 0.16 nm for Ra and 1.6 nm for Rmax for the Si substrate, and 0.37 nm for Ra for the SiO ₂ substrate. In the case of the Si substrate, a natural oxide film of Si was formed on the surface, but this was used without removing it.

  An Au microcrystalline thin film formed directly on the substrate, and a Cr thin film of about 5 nm formed by sputtering on each side of the substrate before the formation of the Au microcrystalline thin film, respectively. Prepared. The Cr underlayer is inserted in order to increase the adhesion between the Au thin film and the substrate.

  After the microcrystalline thin film of Au is formed in this way, air is introduced into the vacuum chamber of the sputtering apparatus and returned to the pressure in the clean room, and then the substrate is taken out from the vacuum chamber of the sputtering apparatus, and the humidity is 50%. After being exposed to the atmosphere for a predetermined time in an ISO class 5 clean room, the two substrates are heated with a weak force of several kPa without heating both substrates so that the microcrystalline thin films formed on both substrates overlap. They were superposed and joined.

Each joining condition and joining result are shown in Table 3 below.

Comparative Example 1
Similar to the above experimental example, except that a Ta layer with a thickness of about 5 nm was formed as the underlayer, and immediately after the formation of the microcrystalline thin film, superposition was performed in the same vacuum as the microcrystalline thin film was formed. .

Test results As a result of the above-mentioned bonding test, strong bonding was obtained regardless of the conditions under which the bonding was performed. In the example in which a thin film was formed directly on the substrate, the thin film was formed on the substrate. In addition, in the structure in which the Cr underlayer was provided, the substrate was cracked without peeling at any interface.

  In bonding using a microcrystalline thin film of Au, the base is heated by superimposing the thin films with a weak force of about several kPa even after the base on which the microcrystalline thin film of Au is formed is exposed to air at atmospheric pressure. Atom diffusion bonding was possible without any problems.

  This bonding is performed in any state after a 20 nm-thick Au microcrystalline thin film is formed on each Si substrate and the Si substrate is taken out from the vacuum chamber of the sputtering apparatus and exposed to the atmosphere for 5 minutes and 1 hour. It was possible to do this, and it was possible to bond even after 6 hours of exposure.

  In addition, when the microcrystalline thin film of Au is formed and bonded immediately after the Si substrate is taken out from the vacuum chamber of the sputtering apparatus, the thickness of the microcrystalline thin film formed on each Si substrate is reduced to 2 nm. However, bonding was possible.

  2 to 4 show the cross section of the substrate bonded 5 minutes after exposure to the atmosphere (Fig. 2), the cross section of the substrate bonded after 1 hour (Fig. 3), and bonding in a vacuum vessel without exposure to the atmosphere. The cross section (FIG. 4) of the base | substrate of the comparative example 1 performed is shown, respectively.

  As shown in FIG. 4, in the joint surface of Comparative Example 1 in which the Si substrate was superposed in the same vacuum as the microcrystalline thin film was formed, it was formed on the surface portion of the original microcrystalline thin film. Example of welding after exposure to atmospheric pressure air for 5 minutes (Fig. 2), welding after 1 hour of atmospheric exposure In FIG. 3, it was confirmed that some bonding interfaces remained, but such bonding interface remained substantially unaffected by bonding.

[Joint experiment example using Ag thin film]
Table 4 shows an example of the bonding test between the substrates on which the Ag thin film was formed under the same conditions as those of the Au thin film and the results thereof. In some experiments, before the formation of the Ag microcrystalline thin film, a Ta thin film of about 5 nm was formed as an underlayer on each side of the substrate by sputtering. The Ta base film is inserted in order to increase the adhesion between the Ag thin film and the substrate.

  From the above results, even in an example where bonding was performed by forming an Ag thin film having a microcrystalline structure on any of the bonding surfaces of two substrates, strong bonding was obtained without heating the substrate, When an attempt is made to peel off the superposed substrate, the thin film is peeled off from the interface with the substrate in the case where the thin film is formed directly on the substrate. The substrate broke without.

In the examples of sample names “Ag2” and “Ag4”, since the SiO ₂ substrate is transparent, it was confirmed that an unbonded portion exists in a part of the bonded surface immediately after bonding due to the presence of impurities. However, it is confirmed that the bonding area can be expanded by pressing the unbonded portion of the substrate with a finger from the outside, and even if the bonding is performed in a space where there is a lot of dust, It was confirmed that the occurrence of unbonded portions due to the presence of impurities can be avoided by slightly pressurizing.

[Joint experiment using Cu thin film]
Table 5 shows an example of a bonding test between substrates on which a Cu thin film is formed under the same conditions as those of the Au thin film and the results thereof. In some experiments, as in the case of the Ag film experiment, a Ta thin film of about 5 nm was formed as an underlayer on each side of the substrate by sputtering before the Cu microcrystalline thin film was formed.

  As a result of the above-mentioned joining test, strong joining was obtained under any conditions, and when attempting to peel off the superposed substrate, in the example in which the thin film was formed directly on the substrate, the thin film was in contact with the substrate. In the configuration in which the layer was peeled off from the interface and provided with a Ta underlayer, the substrate was cracked without peeling off at any of the interfaces.

  From the above results, it can be confirmed that by heating the substrate to about 100 ° C., bonding can be performed in any case, and bonding can be performed even after one hour has passed after exposure to the atmosphere. It was.

  In particular, in the bonding of Si substrates, it is possible to perform bonding over the entire bonding surface without applying pressure with a bonding strength exceeding the adhesion strength of the Cu thin film to the substrate by heating the substrate to about 40 ° C. did it.

  In addition, in the bonding of the Si substrate, even if the heating of the substrate is not performed, partial bonding is possible by applying pressure, so in combination with other conditions such as pressing, Bonding can be performed under relaxed heating conditions.

[Example of bonding experiment using Pt thin film]
Table 6 shows an example of a bonding test between the substrates on which the Pt thin film is formed under the same conditions as those of the Au thin film and the results thereof.

As a result of the above, in the bonding between the substrates on which the Pt microcrystalline thin film was formed, the bonding could not be performed without heating the substrates in both cases of the Si substrate and the SiO ₂ substrate.

On the other hand, when bonding the SiO ₂ substrate, the substrate was heated to 150 ° C., and for the Si substrate, the entire bonding surface was bonded by heating the substrate to 200 ° C. FIG. 5 shows a cross section of the base material bonded with “Pt1”. Pt and Si compounds are formed under the Si natural oxide film on the Si substrate surface by heating, but there is no void at the Pt / Pt bonding interface, and the bonding can be achieved by heating at 200 ° C. I understand. From this, it was confirmed that, when bonding is performed by forming a microcrystalline thin film of Pt, bonding is possible under any conditions by heating the substrate to 200 ° C. or higher.

[Example of bonding experiment using Pd thin film]
Table 7 shows a bonding test example between the substrates on which the Pd thin film was formed under the same conditions as those of the Au thin film and the results thereof.

Even when the substrates on which the Pd thin film is formed are bonded to each other, the bonding cannot be performed without heating the substrates in both cases of the Si substrate and the SiO ₂ substrate.

In both cases where the substrate was Si or SiO ₂ and when a Ta thin film was formed as the underlayer, bonding was obtained at a substrate heating temperature of 150 to 160 ° C. FIG. 6 shows a cross section of the substrate to which “Pd2” is bonded. The compound of Pd and Si is formed under the Si natural oxide film existing on the Si substrate surface by heating, and the interface fluctuates greatly, but the bonding interface of Pd / Pd disappears completely and is bonded. I understand.

  The bonding method of the present invention described above is capable of bonding at an atomic level without heating or heating at a relatively low temperature, without applying pressure, with low interface stress after bonding, and with a microcrystalline thin film. Various new functions and high performance can be achieved because the bonding surfaces can be superposed in an atmosphere exposed to a higher pressure than the vacuum in which the material is formed, for example, in an atmosphere of atmospheric pressure. It can be easily used in applications such as creation of functional devices, miniaturization of information appliances, and high integration. Examples of these applications are as follows.

Creation of new and high-performance devices Application to creation of new functional devices stacked and integrated at the wafer level Integration of integrated circuits and short-wavelength optical devices (for example, Si devices / GaN, photonic crystals, LEDs), new Bonding when creating a functional photo-electronic conversion device.
Manufacture of spin-electronic hybrid devices. By bonding by the method of the present invention, it is possible to bond between wafers in an average free process of electrons or spin current.

Bonding between dissimilar material wafers For the purpose of forming ultra-hybrid substrates and components, manufacturing of potential barrier hybrid wafers in which semiconductor wafers are laminated with microcrystalline thin films sandwiched, and special laminated glass with microcrystalline thin films in between In the production of optical wafers (optical filtering), the method of the present invention can be used.

Increasing the brightness of a light emitting diode By the method of the present invention, a mirror layer is bonded to the light emitting diode to increase the brightness.

Lamination of quartz resonators By the method of the present invention, for example, a gold thin film is formed on a quartz crystal and the quartz crystals are bonded to each other, thereby bonding the quartz crystals that are relatively difficult to bond.

Miniaturization and high integration of information appliances Three-dimensional stacking for SiP technology, high-functionality of package substrate (three-dimensional mounting); 3D bonding of Si device and Si device stack and substrate by this bonding method To achieve higher integration.

MEMS manufacturing technology This bonding method is used for vacuum sealing of fine elements that also serve as three-dimensional wiring and for stacking with Si devices. For use in this application, there may be an interface on the joint surface, as long as the internal vacuum and joined state can be maintained.

Realization of efficient heat conduction (cooling) for higher performance and ultra-low power consumption Heat sinks and heat spreaders made of materials with a large heat dissipation coefficient such as copper, diamond, DLC, etc., semiconductor devices, etc. by the method of the present invention Bonding directly to, improve heat dissipation performance, heat diffusion performance, etc.

Others In each of the applications described above, examples of using the joining method of the present invention in the field of electric and electronic parts have been described. However, the method of the present invention is not limited to the field of use exemplified above. It can be used in various fields and applications that require joining.

Claims (13)

In a vacuum vessel, 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) on the smooth surface of one substrate. of Compounds,) or more single metal, or to form a thin film of microcrystalline structure made of an alloy, in an atmosphere of a pressure exceeding 1 × 10 ^-4 Pa, at least the surface of the body diffusion coefficient of 1 × 10 at room temperature ^- On the other hand, it has a smooth surface with a microcrystalline structure consisting of a single metal or alloy of ⁴⁰ (m ² / s) or more and an oxide free energy of formation at room temperature of −15 (kJ / mol of compounds) or more. The thin film and the smooth surface of the other substrate are joined by superimposing the one and the other two substrates so that the thin film formed on the one substrate is in contact with the smooth surface of the other substrate. Atomic diffusion occurs at interfaces and grain boundaries The joining two substrates (except a bonding performed at a junction temperature of more than 0.99 ° C.) atomic diffusion bonding wherein the By. In a vacuum vessel, 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) on the smooth surface of one substrate. a thin film having a microcrystalline structure composed of the above single metal or alloy and having a body diffusion coefficient of at least 1 × 10 ⁻⁴⁵ at room temperature in an atmosphere at a pressure exceeding 1 × 10 ⁻⁴ Pa. (m ² / s) or more, and the other provided with a smooth surface having a microcrystalline structure made of a single metal or alloy having a free energy of formation of oxide of −150 (kJ / mol of compounds) or more at room temperature. The one and the other two substrates are overlapped so that the thin film formed on the one substrate is in contact with the smooth surface of the substrate, and heated at a temperature of 100 ° C. or higher to form the thin film of the microcrystalline structure. Atoms at junction interface and grain boundaries And causing diffusing bonding the two substrates (except a bonding performed at a junction temperature of more than 0.99 ° C.) atomic diffusion bonding method, characterized in that. 3. The atomic diffusion bonding method according to claim 1, wherein the smooth surface of the other substrate is formed of a thin film having a microcrystalline structure formed on the surface of the other substrate in a vacuum vessel. The atomic diffusion bonding method according to any one of claims 1 to 3 , wherein the overlapping of the substrates is performed in an atmosphere having a pressure equal to or higher than atmospheric pressure. Claim 1-4 any one atom diffusion bonding method, wherein the atmosphere in which the superposition of the substrate is air. The atomic diffusion bonding method according to any one of claims 1 to 4, wherein an atmosphere in which the substrates are superposed includes an inert gas exceeding 78%. The superposition of the base body, according to claim 1-6 any one atom diffusion bonding method, wherein a performed under an atmosphere that is removed of dust. Claim 1-7 atomic diffusion bonding method according to any one of the preceding, wherein the strength of the force superposing said substrate is less than 101 kPa. The modified layer generated on the smooth surface of the substrate on which the microcrystalline thin film is formed is removed in the same vacuum as the formation of the microcrystalline thin film before forming the microcrystalline thin film. The atomic diffusion bonding method according to any one of 1 to 8 . On the smooth surface of the substrate on which the thin film having the microcrystalline structure is formed, one or more base layers made of a thin film of a material different from the thin film having the microcrystalline structure are formed, and the microcrystalline structure is formed on the base layer. claim 1-9 any one atom diffusion bonding method, wherein the forming the thin film. 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 atomic diffusion bonding method according to claim 10 , wherein the atomic diffusion bonding method is formed of an alloy containing any of the elements. As the single metal or alloy for forming the base layer, a thin film having a higher melting point than the single metal or alloy for forming a thin film having a microcrystalline structure formed on the base layer and having the microcrystalline structure is formed. 11. The atomic diffusion bonding method according to claim 10 , wherein a material having a large difference in melting point with respect to a single metal or an alloy is used. Claim 1-12 any one atomic diffusion bonding method according to characterized in that the thickness of the thin film of the microcrystalline structure and 2Nm～1myuemu.