JP2010215465A - Aluminum nitride substrate, method for producing the same, and circuit board and semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum nitride substrate having excellent heat radiation characteristics and a high strength, a method for producing the aluminum nitride substrate, and a circuit board and a semiconductor device. <P>SOLUTION: The aluminum nitride substrate comprises a polycrystalline substance which includes a plurality of aluminum nitride crystal grains and composite oxide crystal grains including a rare-earth element and aluminum and existing at the grain boundary of the aluminum nitride crystal grains. The main component of the aluminum nitride substrate is aluminum nitride. The aluminum nitride crystal grains have an average grain diameter of 5 μm or smaller. The composite oxide crystal grains have an average grain diameter of 5 μm or smaller, a thermal conductivity of 200 W/m*K or higher and a three-point bending strength of 500 MPa or higher. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an aluminum nitride substrate and a method for manufacturing the same, and more particularly to an aluminum nitride substrate excellent in insulation characteristics and heat dissipation and used for a power transistor and the like and a method for manufacturing the same. The present invention also relates to a circuit board and a semiconductor device using these aluminum nitride substrates.

  A ceramic substrate mainly composed of aluminum nitride is excellent in insulation characteristics and heat dissipation. This ceramic substrate containing aluminum nitride as a main component is used as a heat dissipation substrate for a semiconductor by joining metal conductor layers of various thicknesses on the substrate by an active metal method, metallization method, vapor deposition or the like.

  This heat dissipation substrate is used as a circuit substrate for a high power semiconductor such as a power transistor. The power transistor forms a power transistor module (hereinafter also referred to as a module) by incorporating a plurality of power transistor chips (hereinafter also referred to as chips) in the same package.

  Since power transistors are used with high power, the amount of heat generated by the chip is large. When the chip generates heat in this way, the entire module expands thermally. At this time, since the end of the module is fixed to a heat radiating fin or the like, a bending moment is generated in the entire module when the power transistor is used. For this reason, if the strength of the ceramic substrate used to insulate the chips is weak, there is a problem that the substrate is cracked and insulation failure occurs in the module. Therefore, conventionally, a ceramic substrate with high heat dissipation and high strength has been used as a ceramic substrate for power transistors.

  In recent years, lead-free solder is often used for soldering a chip or the like to a ceramic substrate in consideration of the environment. Since lead-free solder generally has a higher melting point than lead-containing solder, the use of lead-free solder increases the soldering temperature regardless of the reflow method or flow method.

  Furthermore, the solder used for joining between the base metal of the module and the ceramic substrate is also required to be lead-free. Since the soldering area between the base metal and the ceramic substrate is the largest in the module, it is applied to the module due to the difference in linear expansion coefficient between the base metal and the ceramic substrate when the reflow temperature increases. The bending moment is also very large. As described above, the ceramic substrate is required to have higher strength than before due to the lead-free solder.

  In recent years, the size of modules and chips has been reduced, and the amount of heat generated per unit area of the module tends to increase. For this reason, higher heat dissipation characteristics are required for ceramic insulating substrates.

  Thus, ceramic substrates used in power modules are required to have higher heat dissipation characteristics and strength than before. A similar problem occurs in a ceramic substrate on which a light emitting diode is mounted.

  On the other hand, Japanese Unexamined Patent Application Publication No. 2007-63122 (Patent Document 1) discloses an aluminum nitride substrate having high thermal conductivity and excellent heat dissipation. Japanese Unexamined Patent Application Publication No. 2003-201179 (Patent Document 2) discloses an aluminum nitride substrate having high heat dissipation characteristics and mechanical strength.

JP 2007-63122 A JP 2003-201179 A

  However, the aluminum nitride substrates disclosed in Patent Document 1 and Patent Document 2 have insufficient heat dissipation characteristics and strength. For example, in Patent Document 1, although the thermal conductivity exceeds 200 W / m · K, the strength thereof is low, and in Patent Document 2, although the strength is excellent, a material having a thermal conductivity exceeding 200 W / m · K has not been obtained.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an aluminum nitride substrate, a method for manufacturing an aluminum nitride substrate, a circuit substrate, and a semiconductor device that have excellent heat dissipation characteristics and high strength.

  The present invention controls the thermal conductivity of the aluminum nitride substrate by controlling the grain size of the aluminum nitride crystal grains in the aluminum nitride substrate and controlling the grain size of the composite oxide crystal grains derived from the sintering aid. It was completed by finding out that it is possible to improve the three-point bending strength.

  An aluminum nitride substrate according to the present invention solves the above-mentioned problems, and is a composite oxide containing a plurality of aluminum nitride crystal grains and a rare earth element and aluminum that exist at the grain boundary of the aluminum nitride crystal grains. An aluminum nitride substrate comprising aluminum nitride as a main component, the aluminum nitride crystal grains having an average grain size of 5 μm or less, and the composite oxide crystal grains having an average The particle size is 5 μm or less, the thermal conductivity is 200 W / m · K or more, and the three-point bending strength is 500 MPa or more.

  Further, the method for producing an aluminum nitride substrate according to the present invention solves the above-mentioned problems. The aluminum nitride powder, the rare earth oxide powder, and the organic binder are molded, and the aluminum nitride powder and the rare earth oxide powder are formed. A first aluminum nitride molded body containing 1% by mass to 3% by mass of the rare earth oxide powder with respect to the total amount is obtained, and the second aluminum nitride molded body is degreased in a non-oxidizing atmosphere. A first degreasing step for obtaining a first sintered body by sintering the second aluminum nitride molded body at 1300 ° C. to 1500 ° C. in an inert atmosphere; And a second sintering step of obtaining an aluminum nitride substrate by sintering at 1780 ° C. to 1820 ° C. in an active atmosphere.

  Furthermore, a circuit board according to the present invention solves the above-described problems, and is characterized in that a conductor portion is provided on the aluminum nitride substrate.

  The semiconductor device according to the present invention solves the above-mentioned problems, and is characterized in that a semiconductor element is mounted on the circuit board.

  According to the aluminum nitride substrate and the manufacturing method thereof according to the present invention, an aluminum nitride substrate having high heat dissipation characteristics and strength can be obtained.

  According to the circuit board according to the present invention, a circuit board having high heat dissipation characteristics and strength can be obtained.

  According to the semiconductor device of the present invention, a semiconductor device including a circuit board having high heat dissipation characteristics and high strength can be obtained.

The SEM observation result of the torn surface of the aluminum nitride board | substrate obtained in Example 1. FIG. The SEM observation result of the surface of the aluminum nitride board | substrate obtained in Example 1. FIG.

[Aluminum nitride substrate]
An aluminum nitride substrate according to the present invention is a composite material composed mainly of aluminum nitride, comprising a polycrystal having a plurality of aluminum nitride crystal grains and a composite oxide crystal grain containing a rare earth element and aluminum. It is. In the aluminum nitride substrate according to the present invention, the complex oxide crystal grains exist at the grain boundaries of the aluminum nitride crystal grains.

  The aluminum nitride crystal grains have an average particle diameter of 5 μm or less, preferably 3 μm to 5 μm.

  Here, the average grain size of the aluminum nitride crystal grains is the average grain size of the aluminum nitride crystal grains in the aluminum nitride substrate. For example, the average grain size of the aluminum nitride crystal grains observed on the fracture surface of the aluminum nitride substrate Means. Specifically, the average grain size of the aluminum nitride crystal grains is obtained by taking an enlarged photograph of the fractured surface of the aluminum nitride substrate with an SEM (scanning electron microscope), and forming a rectangular measurement range of 50 μm × 50 μm on the fractured surface, It can be obtained by measuring the size of aluminum nitride crystal grains present in this measurement range. Since the aluminum nitride crystal grains are substantially spherical, there is a line intercept method as a simple method. The number of aluminum nitride crystal grains on a straight line of 50 μm is obtained and obtained by the formula (50 μm / number of aluminum nitride crystal grains). The average particle diameter can be calculated by performing this operation three times or more.

  If the average particle size of the aluminum nitride crystal grains is less than 3 μm, the heat conductivity of the module may be deteriorated because the thermal conductivity is lowered.

  On the other hand, if the average grain size of the aluminum nitride crystal grains exceeds 5 μm, the aluminum nitride crystal grains may become a starting point of destruction, and the three-point bending strength of the substrate may be lowered.

  The complex oxide crystal grains are complex oxide crystal grains containing a rare earth element and aluminum.

Examples of the rare earth element that forms the composite oxide include at least one selected from Y and lanthanoids such as La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, and Yb. Among rare earth elements, Y reacts with aluminum to form YAM (monoclinic structure (monoclinic structure): M ₄ N ₂ O ₉ ) or YAP (perovskite structure: M ₁ N ₁ O ₃ ). YAM and YAP are preferred because of their high bonding strength with aluminum nitride crystal grains.

  The complex oxide crystal grains of the aluminum nitride substrate are each YAM crystal grains or YAP crystal grains.

  Here, the fact that the complex oxide crystal grains of the aluminum nitride substrate are YAM or YAP is determined by, for example, the crystal structure detected by the X-ray surface analysis method on the surface of the aluminum nitride substrate.

  The complex oxide crystal grains of the aluminum nitride substrate are composed of at least one of YAM crystal grains and YAP crystal grains, and may be either a single phase or two phases.

  Of these, the composite oxide crystal grains of the aluminum nitride substrate are preferably composed of only a single phase of YAM crystal grains or two phases of YAM crystal grains and YAP crystal grains because the three-point bending strength is high.

  The complex oxide crystal grains have a characteristic that the thermal conductivity is lower than that of the aluminum nitride crystal grains.

  The composite oxide crystal grains have an average particle diameter of 5 μm or less, preferably 1 μm to 5 μm.

  Here, the average particle diameter of the composite oxide crystal grains is the average particle diameter of the composite oxide crystal grains in the aluminum nitride substrate. For example, the composite oxide crystal grains observed on the fracture surface of the aluminum nitride substrate Mean average particle size. The specific method for measuring the average grain size of the complex oxide crystal grains is that the fracture surface of the aluminum nitride substrate is polished until Ra reaches 0.05 to 0.08 μm, and the polished surface is taken with an SEM, A rectangular measurement range of 100 μm × 100 μm on the polished surface is formed, and the size of the complex oxide existing in the measurement range is measured, for example. In addition, when the surface roughness Ra of the fracture surface of the aluminum nitride substrate is less than 0.05 μm, it is not necessary to polish, and an enlarged photograph of the fracture surface may be taken as it is.

  If the average particle diameter of the complex oxide crystal grains exceeds 5 μm, the complex oxide crystal grains may become a starting point of destruction, and the three-point bending strength and thermal conductivity of the substrate may be lowered.

  Further, when the average grain size of the composite oxide crystal grains exceeds 5 μm, when the aluminum nitride circuit board is produced by bonding the aluminum nitride board and the metal plate using the active metal brazing material, the aluminum nitride circuit board Bond strength tends to be low.

  The reason why the bonding strength of the aluminum nitride circuit board tends to be low is as follows. That is, when an active metal brazing material is used to join an aluminum nitride substrate and a metal plate such as a copper plate, the interface between the aluminum nitride substrate and the active metal brazing material is mainly composed of aluminum nitride crystal grains on the surface of the aluminum nitride substrate. N is bonded to Ti, Hf, Zr, etc. in the active metal brazing material by reacting to form a nitride. For this reason, if complex oxide crystal grains that do not contribute to the reaction with Ti, Hf, Zr, etc. in the active metal brazing material exist on the surface of the aluminum nitride substrate in a large grain size, the aluminum nitride crystal grains are formed on the surface of the aluminum nitride substrate. This is because N does not exist densely and the bonding strength between the aluminum nitride substrate surface and the active brazing material layer tends to be low. In the case where a thin film of an active metal such as Ti, Hf, Zr or the like is used instead of the active brazing material, there is a possibility that the bonding strength is lowered for the same reason. The presence state of the composite oxide crystal grains on the surface of the aluminum nitride substrate can be grasped by observing the presence state of the composite oxide crystal grains on the fracture surface of the aluminum nitride substrate.

  Further, if the average grain size of the composite oxide crystal grains is less than 1 μm, the number of crystal grain boundaries that the heat flow crosses during heat dissipation of the aluminum nitride substrate increases, so that the thermal conductivity of the aluminum nitride substrate may be lowered.

  In the aluminum nitride substrate according to the present invention, the rare earth element content relative to the aluminum nitride substrate is usually 1% by mass to 3% by mass in terms of rare earth oxide.

Here, the rare earth oxide equivalent refers to the mass of rare earth elements in the aluminum nitride substrate converted to rare earth oxides. For example, when the rare earth element in the aluminum nitride substrate is Y, the oxide of the rare earth element is Y ₂ O ₃ .

  When the content of rare earth elements relative to the aluminum nitride substrate is less than 1% by mass in terms of rare earth oxides, the liquid phase component required for sintering is small and the aluminum nitride crystal grains are difficult to be densified. May be lowered.

  When the content of rare earth elements in the aluminum nitride substrate exceeds 3% by mass in terms of rare earth oxide, densification is promoted and the particle size of the composite oxide after sintering becomes too large. The point bending strength may be lowered.

  The aluminum nitride substrate according to the present invention may contain Si as an impurity. The content of Si in the aluminum nitride substrate is 50 ppm or less in terms of mass.

In the aluminum nitride substrate according to the present invention, when the strongest peak height of AlN by X-ray surface analysis is I _AlN and the strongest peak height of _YAM is I _YAM , I _YAM / I _AlN is 0.02 or more, Preferably it is 0.05-0.15.

Here, the strongest peak height _IAlN of AlN means a peak value having the strongest peak height when a plurality of AlN peaks are observed by X-ray surface analysis. The strongest peak height I _YAM of _YAM means a value determined in the same manner as the strongest peak height I _{AlN of AlN} .

I _YAM / I _AlN is an index indicating the abundance ratio of YAM type complex oxide crystal grains to aluminum nitride crystal grains on the aluminum nitride substrate surface.

If I _YAM / I _AlN is less than 0.02, it is difficult for the composite oxide crystal grains to bond the aluminum nitride crystal grains in the aluminum nitride substrate, and the three-point bending strength of the aluminum nitride substrate may be lowered.

If I _YAM / I _AlN exceeds 0.15, the thermal conductivity of the aluminum nitride substrate may be lowered.

Aluminum nitride substrate according to the present invention, in the case of the strongest peak height of YAP by X-ray surface analysis was _{I YAP,} when the _I YAM _{/ I AlN} is 0.05 to _{0.15, I YAP} / I _AlN is 0.02-0.05.

The strongest peak height I _YAP of _YAP means a value determined in the same manner as the strongest peak height I _{AlN of AlN} .

I _YAP / I _AlN is an index indicating the abundance ratio of YAP-type complex oxide crystal grains to aluminum nitride crystal grains on the surface of the aluminum nitride substrate.

When I _YAM / _{I AlN} is 0.05 to _0.15, when I YAP _{/ I AlN} is less than 0.02, it composite oxide grains in an aluminum nitride substrate is less likely to bind the aluminum nitride crystal grains, There is a possibility that the three-point bending strength of the aluminum nitride substrate is lowered.

When I _YAM / I _AlN is 0.05 to 0.15, if I _YAP / I _AlN exceeds 0.05, the thermal conductivity of the aluminum nitride substrate may be lowered.

  The aluminum nitride substrate according to the present invention is obtained through a sintering process such as a first sintering process and a second sintering process.

  The aluminum nitride substrate according to the present invention usually has a surface roughness Ra of 3 μm to 5 μm after the sintering step, that is, after polishing and in an unpolished state.

  If the aluminum nitride substrate according to the present invention smoothes the surface of the aluminum nitride substrate to a surface roughness Ra of about 1 μm by appropriately performing a polishing step after the first sintering step and the second sintering step, 3 Since the point bending strength becomes high, it is preferable.

  The aluminum nitride substrate according to the present invention has a thermal conductivity of 200 W / m · K or more, preferably 200 W / m · K to 220 W / m · K, with a surface roughness Ra of 1 μm or less.

  Here, the thermal conductivity means the thermal conductivity measured by the laser flash method.

  The aluminum nitride substrate according to the present invention has a three-point bending strength of 500 MPa or more, preferably 500 MPa to 550 MPa with a surface roughness Ra of 1 μm.

  The aluminum nitride substrate according to the present invention is efficiently manufactured, for example, by the following method for manufacturing an aluminum nitride substrate according to the present invention.

[Method of manufacturing aluminum nitride substrate]
The method for manufacturing an aluminum nitride substrate according to the present invention includes a degreasing step, a first sintering step, and a second sintering step.

(Degreasing process)
The degreasing step is a step of degreasing a first aluminum nitride molded body having a specific composition obtained by molding an aluminum nitride powder, a rare earth oxide powder and an organic binder in a non-oxidizing atmosphere to obtain a second aluminum nitride molded body. It is.

  Specifically, the degreasing step includes a first aluminum nitride molded body production step for obtaining a first aluminum nitride molded body having a specific composition by molding an aluminum nitride powder, a rare earth oxide powder, and an organic binder, and a first aluminum nitride molded body. And a second aluminum nitride molded body producing step for obtaining a second aluminum nitride molded body by degreasing in a non-oxidizing atmosphere.

<First aluminum nitride molded body production process>
In the degreasing step, first, a first aluminum nitride molded body manufacturing step is performed.

  The first aluminum nitride molded body production step is a step of obtaining a first aluminum nitride molded body having a specific composition by molding aluminum nitride powder, rare earth oxide powder and organic binder.

The aluminum nitride powder, for example, the average particle diameter _{D 50} of generally 0.5Myuemu～2myuemu, can be preferably used ones 0.7Myuemu～1.5Myuemu. Here, D ₅₀ means a cumulative 50% particle size. The aluminum nitride powder generates aluminum nitride crystal grains after the first and second sintering steps.

As the rare earth oxide powder, for example, an oxide powder of at least one rare earth element selected from Y and lanthanoids such as La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, and Yb is used. Used. Specifically, examples of the rare earth oxide powder include Y ₂ O ₃ powder.

  When the rare earth oxide powder undergoes the first and second sintering steps, the rare earth oxide powder reacts with the aluminum nitride powder to produce a composite oxide containing a rare earth element and aluminum. The crystal grains of the composite oxide exist at the grain boundaries of the aluminum nitride crystal grains, and strongly adjoin the adjacent aluminum nitride crystal grains.

Among rare earth element oxide powders, Y ₂ O ₃ powder is preferable because it reacts with aluminum to form YAG (yttrium, aluminum, garnet), and this YAG has a high bonding force with aluminum nitride crystal grains.

Examples of the rare earth element oxide powder, for example, the average particle diameter _{D 50} of generally 0.7Myuemu～2myuemu, can be preferably used ones 0.9Myuemu～1.5Myuemu.

  Examples of the organic binder include PVB (polyvinyl butyral). The organic binder combines the aluminum nitride powder and the rare earth element oxide powder to produce the first aluminum nitride molded body.

  When the aluminum nitride powder, rare earth element oxide powder and organic binder are mixed in an organic solvent such as ethanol, toluene or ketone, a slurry is produced.

  The mixture of the slurry including the aluminum nitride powder, the rare earth element oxide powder and the organic binder contains 1% by mass to 3% by mass of the rare earth oxide powder with respect to the total amount of the aluminum nitride powder and the rare earth oxide powder. To.

  When the organic solvent is removed from the slurry, a molded body in which the aluminum nitride powder and the rare earth element oxide powder are fixed with an organic binder is obtained. For example, if a slurry is shape | molded using a doctor blade method and an organic solvent is removed, a sheet-like molded object (green sheet) will be obtained.

  When the doctor blade method is used, the thickness of the green sheet can be reduced and the degreasing of the green sheet can be easily performed, so that the amount of carbon in the obtained aluminum nitride substrate can be reduced.

  The sheet-like molded body is left as it is or is cut into a desired size as necessary, whereby a first aluminum nitride molded body is obtained.

  The first aluminum nitride molded body may be in a form other than a sheet form. Further, when the first aluminum nitride molded body is in the form of a sheet, the molding method is not limited to the doctor blade method as long as the method can form the sheet-like aluminum nitride molded body.

  The first aluminum nitride molded body contains 1% by mass to 3% by mass of rare earth oxide powder with respect to the total amount of aluminum nitride powder and rare earth oxide powder. When the rare earth oxide powder is contained so as to have a content within this range, the three-point bending strength of the obtained aluminum nitride substrate is increased.

<Second aluminum nitride molded body production process>
In the degreasing step, the second aluminum nitride molded body manufacturing step is performed after the first aluminum nitride molded body manufacturing step.

  The second aluminum nitride molded body manufacturing step is a step of obtaining the second aluminum nitride molded body by degreasing the first aluminum nitride molded body in a non-oxidizing atmosphere.

  The second aluminum nitride molded body is obtained by removing the organic binder from the first aluminum nitride molded body by degreasing. The second aluminum nitride molded body is a molded body substantially free of carbon and made of aluminum nitride powder and rare earth oxide powder.

  The degreasing of the first aluminum nitride molded body is performed by heat treatment in a non-oxidizing atmosphere. Here, as the non-oxidizing atmosphere, for example, nitrogen gas or argon gas is used. Of these, nitrogen gas is preferable because it is inexpensive.

  By performing the heat treatment in a non-oxidizing atmosphere, the aluminum nitride molded body is prevented from being oxidized.

  The heat treatment conditions for degreasing are usually 600 ° C to 850 ° C. When the heat treatment condition for degreasing is 600 ° C. to 850 ° C., the organic binder is efficiently lost. If the organic binder remains more than necessary, the amount of carbon in the molded body increases, which is not preferable.

(First sintering step)
The first sintering step is a step of obtaining the first sintered body by sintering the second aluminum nitride molded body at 1300 ° C. to 1500 ° C. in an inert atmosphere.

  By sintering the second aluminum nitride molded body, a porous body comprising aluminum nitride crystal grains and composite oxide crystal grains present at the grain boundaries of the aluminum nitride crystal grains and containing a rare earth element and aluminum The 1st sintered compact which is a body is obtained.

  As the inert atmosphere, for example, nitrogen gas or argon gas is used. Of these, nitrogen gas is preferable because it is inexpensive.

  The inert atmosphere is usually 1 atm to 100 atm. When the pressure of the inert atmosphere exceeds 1 atm, the crystal structure of the aluminum nitride substrate becomes dense.

  By performing the first sintering step in an inert atmosphere, carbon-containing components such as an organic binder are removed during sintering, and the amount of carbon in the aluminum nitride crystal grains and the composite oxide crystal grains is reduced. Can do.

  The processing temperature of the first sintering step is 1300 ° C to 1500 ° C.

  When the treatment temperature in the first sintering step is less than 1300 ° C., the aluminum nitride crystal grains and the composite oxide crystal grains are not sufficiently sintered, and the three-point bending strength of the aluminum nitride substrate tends to be low.

  When the processing temperature of the first sintering step exceeds 1500 ° C., the densification of aluminum nitride crystal grains and composite oxide crystal grains is promoted too much, and the thermal conductivity of the aluminum nitride substrate tends to be low.

  The processing time of the first sintering step is usually 2 hours to 5 hours.

  When the treatment time is less than 2 hours, the sintering of aluminum nitride crystal grains and composite oxide crystal grains becomes insufficient, and the three-point bending strength of the aluminum nitride substrate tends to be low.

  When the treatment time exceeds 5 hours, the growth of aluminum nitride crystal grains and composite oxide crystal grains is promoted too much, and the three-point bending strength of the aluminum nitride substrate tends to be lowered.

(Second sintering step)
The second sintering step is a step of obtaining the aluminum nitride substrate by sintering the first sintered body at 1780 ° C. to 1820 ° C. in an inert atmosphere.

  By sintering the first sintered body, aluminum nitride crystal grains and composite oxide crystal grains grow and become dense, and an aluminum nitride substrate is obtained.

  As the inert atmosphere, for example, nitrogen gas or argon gas is used. Of these, nitrogen gas is preferable because it is inexpensive.

  The inert atmosphere is usually 1 atm to 100 atm. When the pressure of the inert atmosphere exceeds 1 atm, the crystal structure of the aluminum nitride substrate becomes dense.

  By performing the second sintering step in an inert atmosphere, an appropriate amount of deoxygenation is performed from the aluminum nitride substrate, and aluminum nitride crystal grains and complex oxide crystal grains grow and become dense.

The processing temperature of the second sintering step is 1780 ° C to 1820 ° C. Thereby, a high-density aluminum nitride sintered body having a relative density of 99% or more is obtained. The relative density is a value obtained by (actual value / theoretical density) × 100 (%), the actual value is the Archimedes method, and the theoretical density is obtained by a simple method using a value obtained by converting the sintering aid component into an oxide. It's okay. For example, in the case of an aluminum nitride sintered body of 3 mass% Y in terms of Y ₂ O ₃ and the remaining aluminum nitride, (0.97 × 3.3 g / cm ³ + 0.03 × 5.03 g / cm ³ ) = 3 .35 g / cm ³ is the theoretical density. The theoretical density of aluminum nitride (AlN) of 3.3 g / cm ³ and the theoretical density of yttrium oxide (Y ₂ O ₃ ) of 5.03 g / cm ³ were respectively quoted from “Iwanami Physical and Chemical Dictionary 5th Edition”.

  When the treatment temperature in the second sintering step is less than 1780 ° C., the aluminum nitride crystal grains and the complex oxide crystal grains are insufficiently densified, and the three-point bending strength of the aluminum nitride substrate tends to be low.

  When the processing temperature in the second sintering step exceeds 1820 ° C., the growth of aluminum nitride crystal grains and composite oxide crystal grains is promoted too much, and the three-point bending strength of the aluminum nitride substrate tends to be lowered.

  The processing time of the second sintering step is usually 2 hours to 5 hours.

  When the treatment time is less than 2 hours, densification of aluminum nitride crystal grains and composite oxide crystal grains becomes insufficient, and the three-point bending strength of the aluminum nitride substrate tends to be lowered.

  When the treatment time exceeds 5 hours, the growth of aluminum nitride crystal grains and composite oxide crystal grains is promoted too much, and the three-point bending strength of the aluminum nitride substrate tends to be lowered.

  The aluminum nitride substrate obtained through the second sintering step is the aluminum nitride substrate according to the present invention.

  That is, the obtained aluminum nitride substrate is a polycrystalline body provided with aluminum nitride crystal grains and composite oxide crystal grains that are disposed in the grain boundary space of the aluminum nitride crystal grains and include a rare earth element and aluminum. An aluminum nitride substrate comprising: an aluminum nitride crystal grain having an average grain size of 5 μm or less; a composite oxide crystal grain having an average grain size of 5 μm or less; and a thermal conductivity of 200 W / m · K or more. The three-point bending strength is 500 MPa or more.

  In addition, the aluminum nitride substrate obtained through the second sintering step usually has a composite oxide crystal grain having a YAM single phase or YAM and YAP crystal structure detected by X-ray surface analysis (XRD). And consists of only two phases.

  The method for producing an aluminum nitride substrate according to the present invention may further include a polishing step if necessary.

(Polishing process)
The polishing step is a step of polishing the surface of the aluminum nitride substrate obtained in the second sintering step to a surface roughness Ra of 1 μm or less.

  The aluminum nitride substrate obtained in the second sintering step usually has a surface roughness Ra of 3 μm to 5 μm in an unpolished state after baking.

  When the surface of the aluminum nitride substrate is polished to a surface roughness Ra of 1 μm or less in this step, the three-point bending strength of the aluminum nitride substrate is increased.

  Examples of the polishing method for polishing the surface of the aluminum nitride substrate to a surface roughness Ra of 1 μm or less include buff polishing and lapping.

[Aluminum nitride circuit board]
In the aluminum nitride substrate according to the present invention, an aluminum nitride circuit substrate can be produced by providing a conductor portion on the substrate.

  As a conductor part, the active metal thin film which consists of 1 type or more chosen from metal conductors, such as copper, and Ti, Zr, and Hf, for example is mentioned.

  When the conductor portion is a metal conductor such as copper, the aluminum nitride circuit board is obtained by, for example, joining a metal plate to the surface of the aluminum nitride substrate via an active metal brazing material layer and performing appropriate etching or the like on the metal plate. It can be produced by forming a conductor circuit.

  The active metal brazing material layer is a layer made of an active metal brazing material. An active metal brazing material is a brazing material containing at least one of Ti, Hf, and Zr, and can braze ceramics and metal directly without performing metallization or surface treatment. . The active brazing material is bonded to the aluminum nitride substrate by reaction of Ti, Hf, Zr, etc. in the brazing material with N of the aluminum nitride crystal grains on the surface of the aluminum nitride substrate.

  Examples of the active metal brazing material include Ag-Cu-Ti-In and Ag-Cu-Ti.

  An example of the metal plate is a copper plate.

  Since the aluminum oxide substrate according to the present invention has small complex oxide crystal grains exposed on the surface, the bonding strength between the aluminum nitride substrate and the active metal brazing material is high.

  When the conductor part is an active metal thin film made of one or more selected from Ti, Zr and Hf, the thin film conductor part made of the active metal thin film has a three-layer structure such as Ti / Pt / Au. It is done.

[Semiconductor device]
The aluminum nitride semiconductor device according to the present invention can be obtained by mounting a semiconductor element on a circuit board.

  The semiconductor element is mounted on the circuit board by, for example, mounting the semiconductor element on a conductor portion made of a metal plate or a thin film via a solder layer. Examples of the semiconductor element include a power element such as an IGBT and a light emitting diode (LED). Further, if necessary, wiring connection can be made by wire bonding.

  Examples are shown below, but the present invention is not construed as being limited thereto.

[Example 1]
(Preparation of aluminum nitride substrate)
<Degreasing process>
An AlN powder having an average particle size of 0.9 μm and a Y ₂ O ₃ powder having an average particle size of 1.1 μm are mixed in an organic solvent (ethanol) at a ratio shown in Table 1, and then PVB (polyvinyl butyral). Was added to prepare a slurry. The amount of AlN powder shown in Table 1 is the balance obtained by subtracting the amount of Y ₂ O ₃ powder from 100% by mass, with the total amount of AlN powder and Y ₂ O ₃ powder being 100% by mass. Next, a sheet was formed from this slurry by a doctor blade method. The obtained green sheet was cut to produce a sheet-shaped first molded body of 50 mm × 45 mm × 1 mm.

  The first molded body was degreased by heating at 800 ° C. for 4 hours in a nitrogen gas atmosphere to obtain a second molded body.

<First sintering step>
The 2nd molded object was heated at 1400 degreeC in 1 atm nitrogen gas atmosphere for 4 hours, and the 1st sintered compact was obtained.

<Second sintering step>
The first sintered body was heated in a 1 atm nitrogen gas atmosphere at 1820 ° C. for 5 hours to obtain an aluminum nitride substrate. When the surface roughness Ra of the baked surface was measured for the obtained aluminum nitride substrate, there was variation depending on the measurement location, and it was in the range of 3 μm to 5 μm.

<Polishing process>
The surface of the aluminum nitride substrate was polished by buffing until the surface roughness Ra became 1 μm.

(Evaluation of aluminum nitride substrate)
<Thermal conductivity>
The thermal conductivity of the polished aluminum nitride substrate was measured by a laser flash method.

<Average grain size of AlN crystal grains>
About the obtained aluminum nitride board | substrate, the average particle diameter of the AlN crystal grain inside an aluminum nitride board | substrate was calculated | required. The fracture surface obtained by manually rupturing an aluminum nitride substrate is taken with an SEM (scanning electron microscope) at a magnification of 2000 times, and a rectangular measurement range of 50 μm × 50 μm is formed on the fracture surface. And the particle size of the AlN crystal grain which exists in this measurement range was measured, and the average value was computed.

<Average particle diameter of complex oxide crystal grains>
About the obtained aluminum nitride board | substrate, the average particle diameter of the complex oxide crystal grain in the surface after grinding | polishing of the fracture surface of an aluminum nitride board | substrate was calculated | required. The fracture surface obtained by manually rupturing the aluminum nitride substrate was polished until Ra reached 0.08 μm, and an enlarged photograph of the magnification of 1000 times was taken with an SEM on the fracture surface after polishing. A rectangular measurement range of 100 μm × 100 μm in the subsequent fracture surface was formed, and the average particle size of the complex oxide crystal grains existing in this measurement range was determined.

  In the enlarged photograph of the fracture surface of the aluminum nitride substrate and the fracture surface after polishing, the AlN crystal grains appear in gray and the complex oxide crystal grains appear in white, so that they can be identified with the naked eye. Also, in the enlarged photograph of the surface of the aluminum nitride substrate, the AlN crystal grains appear in gray and the complex oxide crystal grains appear in white, so that they can be identified with the naked eye.

  FIG. 1 shows the SEM observation result of the fracture surface of the aluminum nitride substrate, and FIG. 2 shows the SEM observation result of the polished surface of the aluminum nitride substrate. In FIG. 1 and FIG. 2, the part that appears white is the complex oxide crystal grain, and the part that appears black is the AlN crystal grain.

<Strongest peak intensity ratio by X-ray diffraction method>
The surface after polishing of the obtained aluminum nitride substrate, an X-ray diffraction method (X-ray surface analysis techniques), AlN, complex oxide grains YAM (monoclinic type structure (monoclinic structure): M ₄ N ₂ The respective peaks of O ₉ ) and YAP (perovskite structure: M ₁ N ₁ O ₃ ) of the composite oxide crystal grains were measured.

Next, among the peaks of AlN, YAM, and YAP, the peak intensities having the maximum intensity are I _AlN , I _YAM, and I _YAP, and I _YAM / I _AlN and I _YAP / I _AlN are calculated as the strongest peak intensity ratios. did.

<Relative density>
The aluminum nitride substrate according to the example had a relative density of 99.8%.

<3-point bending strength>
With respect to the aluminum nitride substrate whose surface was polished, the three-point bending strength was measured under the conditions of a span of 30 mm and a crosshead speed of 0.5 mm / min according to JIS-R-1601.

Tables 1 to 3 show the manufacturing conditions and measurement results of the aluminum nitride substrate.

(Preparation of aluminum nitride circuit board)
An aluminum nitride substrate having a surface polished to a surface roughness Ra of 0.08 μm is used, an active metal brazing material (Ag—Cu—Ti—In) is applied to both surfaces of the substrate, and a copper plate is joined to the coated surface to form an aluminum nitride circuit. A substrate was produced.

(Evaluation of aluminum nitride circuit board)
<Joint strength>
With respect to the obtained aluminum nitride circuit board, the bonding strength between the aluminum nitride board and the copper plate was measured. The bonding strength is a value measured by peeling the aluminum nitride substrate from the copper plate using a tensile tester.

Table 4 shows the bonding strength of the aluminum nitride circuit board.

[Examples 2-5, Comparative Examples 1-2]
An aluminum nitride substrate and an aluminum nitride circuit substrate were produced in the same manner as in Example 1 except that the manufacturing conditions were changed as shown in Tables 1 and 2.

  In Comparative Example 2, the AlN crystal grains were not densified, and an aluminum nitride substrate having sufficient strength could not be produced.

  The obtained aluminum nitride substrate showed variations in the surface roughness Ra of the baked surface of 3 μm to 5 μm, but the polishing step was performed in the same manner as in Example 1 to set Ra to 0.05 μm.

  The aluminum nitride circuit board was produced using the aluminum nitride substrate after the polishing step, as in Example 1.

  The obtained aluminum nitride substrate and aluminum nitride circuit substrate were evaluated in the same manner as in Example 1.

  Tables 1 to 3 show the manufacturing conditions and measurement results of the aluminum nitride substrate. Table 4 shows the bonding strength of the aluminum nitride circuit board.

  The AlN substrate according to this example (Examples 1 to 5) was found to have a thermal conductivity of 200 W / m · K or more and a three-point bending strength of 500 MPa or more. The relative density of the aluminum nitride substrates according to Examples 2 to 5 was 99.2% or higher.

  It was also found that the AlN circuit board according to the present example has good bonding strength. As can be seen from this result, in this example, an aluminum nitride substrate having a strength of 500 MPa or more can be obtained in an aluminum nitride substrate of 200 W / m · K or more, particularly 200 W / m · K to 220 W / m · K. Moreover, since the semiconductor device using the AlN circuit board according to the first to fifth embodiments has a good bonding strength of the conductor portion, a highly reliable semiconductor device can be provided.

Claims (13)

A plurality of aluminum nitride crystal grains;
Present in the grain boundary of the aluminum nitride crystal grains, composite oxide crystal grains containing rare earth elements and aluminum,
An aluminum nitride substrate composed mainly of aluminum nitride,
The aluminum nitride crystal grains have an average grain size of 5 μm or less;
The composite oxide crystal grains have an average particle size of 5 μm or less,
The thermal conductivity is 200 W / m · K or more,
An aluminum nitride substrate having a three-point bending strength of 500 MPa or more. 2. The aluminum nitride substrate according to claim 1, wherein the complex oxide crystal grains have a crystal structure detected by an X-ray surface analysis method consisting of only a single phase of YAM or two phases of YAM and YAP. 3. The aluminum nitride substrate according to claim 1, wherein a content of the rare earth element with respect to the aluminum nitride substrate is 1% by mass to 3% by mass in terms of a rare earth oxide equivalent. The strongest peak height of AlN by X-ray surface analysis method _I AlN, when the strongest peak height of YAM and the _{I YAM,} claims 1, wherein the I YAM _{/ I AlN} is 0.02 or more 4. The aluminum nitride substrate according to any one of 3 above. The strongest peak height of AlN by X-ray surface analysis method _I AlN, the strongest peak of YAM height _I YAM, when the strongest peak height of YAP was _{I YAP,} the I YAM _{/ I AlN} .05 to 0 The aluminum nitride substrate according to any one of claims 1 to 3, wherein 0.15 and I _YAP / I _AlN are 0.02 to 0.05. The aluminum nitride substrate according to any one of claims 1 to 5, wherein the surface roughness of the substrate surface is Ra 1 µm or less. The aluminum nitride substrate according to claim 6, wherein the three-point bending strength is a value measured on a substrate having a surface roughness Ra of 1 μm or less. First aluminum nitride molding comprising molding aluminum nitride powder, rare earth oxide powder and organic binder, and containing 1% by mass to 3% by mass of said rare earth oxide powder with respect to the total amount of said aluminum nitride powder and rare earth oxide powder. A degreasing step of obtaining a second aluminum nitride molded body by degreasing the first aluminum nitride molded body in a non-oxidizing atmosphere;
A first sintering step of obtaining a first sintered body by sintering the second aluminum nitride molded body at 1300 ° C. to 1500 ° C. in an inert atmosphere;
A second sintering step of obtaining an aluminum nitride substrate by sintering the first sintered body at 1780 ° C. to 1820 ° C. in an inert atmosphere;
The manufacturing method of the aluminum nitride board | substrate characterized by the above-mentioned. The aluminum nitride substrate is made of a polycrystalline body including a plurality of aluminum nitride crystal grains, and a composite oxide crystal grain containing a rare earth element and aluminum present at a grain boundary of the aluminum nitride crystal grains, and is nitrided An aluminum nitride substrate mainly composed of aluminum,
9. The aluminum nitride substrate according to claim 8, wherein the complex oxide crystal grains have a crystal structure detected by an X-ray surface analysis method consisting of only a single phase of YAM or two phases of YAM and YAP. Production method. The method for manufacturing an aluminum nitride substrate according to claim 8 or 9, wherein the first aluminum nitride molded body is formed by a doctor blade method. The aluminum nitride substrate according to any one of claims 8 to 10, further comprising a polishing step of polishing the surface of the aluminum nitride substrate obtained in the second sintering step to a surface roughness Ra of 1 µm or less. Manufacturing method.   A circuit board comprising a conductor portion provided on the aluminum nitride substrate according to any one of claims 1 to 7.   13. A semiconductor device comprising a semiconductor element mounted on the circuit board according to claim 12.