JP4987345B2 - Aluminum nitride substrate - Google Patents

Description

The present invention relates to an aluminum nitride substrate, a circuit board using the same, and a module.

Ceramic substrates have the characteristics of high electrical insulation and high thermal conductivity, so they are widely used as circuit boards, and these circuit boards are mounted on power modules and the like. Among them, an aluminum nitride substrate using an aluminum nitride sintered body is attracting attention because of its excellent thermal conductivity.

A ceramic substrate is generally manufactured by the following method. That is, an additive such as a sintering aid, a binder, a plasticizer, a dispersion medium, and a release agent is mixed with the ceramic powder, and the mixture is processed into a sheet-like molded body by extrusion molding or tape molding. Next, the molded body is heated in air or an inert gas atmosphere such as nitrogen to 350 to 700 ° C. to remove the binder (degreasing step), and then 1450 to 1900 in a non-oxidizing atmosphere such as nitrogen. Manufactured by holding at a temperature of 0.5 to 10 hours (firing step).

In the degreasing process and the firing process, the molded body may be laminated in order to increase the amount of the molded body charged into the furnace and improve the productivity. However, when laminated, there is a problem that stains and edge-like color unevenness are likely to occur on the fired ceramic substrate. Therefore, in order to eliminate the color unevenness of the ceramic sintered body, a method of laminating via a powder (Patent Literature 1), a method of alternately laminating a green substrate and a fired plate (Patent Literature 2), and a firing temperature in two stages. Attempts have been made such as a method of dividing (Patent Document 3). However, in the methods shown in Patent Documents 1 to 3, although the degree of color unevenness is reduced, there is a problem that the thermal conductivity is lowered or the productivity is inferior.
Japanese Patent Laid-Open No. 5-229872 Japanese Patent Laid-Open No. 10-59772 Japanese Patent Laid-Open No. 16-83367

An object of the present invention is to provide an aluminum nitride substrate having no color unevenness, and an aluminum nitride circuit substrate and a module using the same.

The present invention is a non-uniform aluminum nitride substrate characterized in that the difference between the maximum value and the minimum value of the oxygen content in the substrate is 0.20% by mass or less, and the aluminum nitride subjected to oxidation treatment An aluminum nitride substrate using powder as a raw material. Furthermore, it is a circuit board using an aluminum nitride substrate, and is a module using this circuit board.

ADVANTAGE OF THE INVENTION According to this invention, the aluminum nitride board | substrate without an uneven color is provided and application to a ceramic circuit board and a module is possible.

The aluminum nitride substrate manufactured according to the present invention has excellent mechanical properties and high thermal conductivity, and is therefore a material suitable for a circuit substrate used under severe usage conditions, for example, a power module circuit substrate.

As the aluminum nitride powder according to the present invention, an aluminum nitride powder produced by a known method such as a direct nitriding method or an alumina reduction method can be used, but an aluminum nitride powder produced by a direct nitriding method having excellent productivity is preferable. . Among them, it is preferable to use an aluminum nitride powder whose hydrolysis resistance is improved by heat treatment (oxidation treatment in air), a chemical oxidation method, or the like.

As a result of studies to suppress the occurrence of uneven color, it was found that the uneven color is related to the distribution of oxygen content in the aluminum nitride substrate after firing. Color unevenness occurs when the difference in oxygen content is large within one substrate, and color unevenness does not occur when the oxygen content is uniform. This is considered that when the oxygen content varies, the sintering start temperature varies, and as a result, the sintered state is affected and color unevenness occurs. The smaller the difference between the maximum value and the minimum value of the oxygen content in the substrate, the smaller the variation in the sintered state, and if the difference in oxygen content is up to 0.20% by mass, the effect is small, and color unevenness does not occur. . However, if the difference in oxygen content exceeds 0.20% by mass, a difference in translucency and color tone appears and color unevenness may occur.

It is considered that the variation in the oxygen content is caused by a hydrolysis reaction of the raw material aluminum nitride powder that occurs in the degreasing and firing processes and a reduction action by the carbon contained in the molded body. Even when a substrate is produced using aluminum nitride powder having the same oxygen content, the oxygen content distribution in the substrate varies depending on the degreasing and firing conditions. For example, moisture is more likely to flow in the central portion of the laminated molded body than in the outer peripheral portion of the molded body, and the oxygen content is locally increased. Therefore, it is preferable to use an aluminum nitride powder having improved water stability (hydrolysis resistance) by heat treatment or chemical oxidation. It is preferable that the heat treatment is performed in air at a temperature of 500 to 700 ° C. When the heating temperature is less than 500 ° C., the effect of the oxidation treatment is not sufficient, and the oxygen content may increase during degreasing and firing. On the other hand, when the temperature exceeds 700 ° C., the oxygen content rapidly increases due to the heat treatment itself, and the thermal conductivity of the aluminum nitride substrate may be lowered. The heat treatment time is preferably within 6 hours. When the treatment is performed for more than 6 hours, the oxygen content is excessively increased, so that the thermal conductivity of the aluminum nitride substrate may be lowered. As a chemical oxidation method, for example, a method of immersing in an aqueous solution of chromate, phosphoric acid-chromate, phosphoric acid-alcohol or the like, or an alkaline aqueous solution of sodium hydroxide, ammonia, amine, alcoholamine, etc. There is a way. By performing such an oxidation treatment, it is considered that an oxide film is formed on the surface of the aluminum nitride powder and the hydrolysis resistance is improved.

The manufacturing method of the aluminum nitride substrate and the circuit board of the present invention will be described.

As the sintering aid according to the present invention, a rare earth metal compound, an alkaline earth metal compound, a transition metal compound, or the like can be used. Of these, yttrium oxide and aluminum oxide are preferable. These sintering aids react with oxygen in the aluminum nitride powder, that is, aluminum oxide, to form a composite oxide liquid phase (for example, 2Y ₂ O ₃ .Al ₂ O ₃ , Y ₂ O ₃ .Al ₂ O ₃ , 3Y ₂ O ₃ · 5Al ₂ O ₃ etc.), and this liquid phase brings about a high density of the sintered body, and at the same time, extracts oxygen and the like in the aluminum nitride particles as an oxide phase at the grain boundary Segregation results in high thermal conductivity. As the liquid phase of the composite oxide, it is preferable to mainly generate Y ₂ O ₃ .Al ₂ O ₃ . When 2Y ₂ O ₃ · Al ₂ O ₃ and 3Y ₂ O ₃ · 5Al ₂ O ₃ are produced more than Y ₂ O ₃ · Al ₂ O ₃ , the thermal conductivity, bending strength, and circuit formation of the aluminum nitride substrate Sometimes the bondability is lowered. By optimizing the amount of sintering aids such as yttrium oxide and aluminum oxide according to the oxygen content in the raw aluminum nitride powder, Y ₂ O ₃ .Al ₂ O ₃ can be mainly produced.

The mixing method of the aluminum nitride powder, the sintering aid and the binder is not particularly limited, and a known mixing device such as a ball mill or a rod mill can be used. The mixed powder may be molded as it is, or may be molded after being granulated by, for example, a spray dryer method or a rolling granulation method. The molding method is not particularly limited, and can be performed by, for example, an extrusion molding method, a doctor blade molding method, a dry press molding method, a cold isostatic press molding method (CIP method), or the like. In any case, a plasticizer, a dispersion medium, a release agent, and the like can be used in combination as necessary. The binder according to the present invention is not particularly limited, but it is preferable to use a methylcellulose-based binder having plasticity or a surface-active effect or an acrylate-based binder having excellent thermal decomposability. As the plasticizer, glycerin, glycerin trioleate, diethylene glycol, and the like can be used. As the dispersion medium, ion-exchanged water, ethanol, toluene, and the like can be used. As the release agent, stearic acid, silicon, and the like can be used.

Prior to the firing treatment, the molded body is preferably heated at 350 to 700 ° C. in order to remove (defatt) the binder. The atmospheric gas for degreasing is preferably a non-oxidizing gas such as nitrogen gas. Degreasing in air may increase the oxygen content and reduce the thermal conductivity after sintering. The degreasing time is preferably 1 to 10 hours. If the degreasing time is short, the residual carbon is unevenly distributed and the oxygen content may vary. The number of laminated layers at the time of degreasing is usually preferably 10 to 20 sheets. When the number of laminated layers is small, the amount of moisture in the center of the molded body becomes light, and color unevenness is less likely to occur. However, since the amount charged into the furnace is reduced and the productivity is inferior, the number of laminated layers at the time of degreasing is appropriately determined according to the size of the molded body. The baking treatment is generally performed by holding at 1450 to 1900 ° C. for 0.5 to 10 hours in a non-oxidizing atmosphere such as nitrogen to produce an aluminum nitride substrate.

The aluminum nitride circuit board of the present invention is formed by forming a metal circuit and a heat sink on the aluminum nitride substrate surface. The method of joining the metal plate for the metal circuit and the heat sink and the aluminum nitride substrate is not particularly limited, but a brazing material joining method in which a brazing material is interposed between the aluminum nitride substrate and the metal plate, and heating and cooling are performed in a vacuum. Is preferable. Common materials for the metal circuit and the metal heat sink are copper, aluminum, tungsten, molybdenum, and alloys thereof. The brazing material may be foil or powder, but is preferably used as a paste. The paste can be prepared by adding an organic solvent and, if necessary, an organic binder to the metal component of the brazing material, and mixing with a known mixer such as a roll, a kneader, a universal mixer, a raker, or the like. The paste application method is not particularly limited, and a known method such as a screen printing method or a roll coater method can be employed.

Etching is performed after a circuit pattern is drawn on the bonded metal plate with an etching resist. A known method can be used to remove the etching resist. The etching resist is not particularly limited, and for example, a known ultraviolet curable type or thermosetting type can be used. Further, as the etchant, a suitable etchant is selected and used according to the type of the metal plate. For example, when the metal is copper, ferric chloride solution, cupric chloride solution, sulfuric acid, hydrogen peroxide solution and the like are used, and preferable examples include ferric chloride solution and cupric chloride solution.

[Example 1]
The aluminum nitride powder produced by direct nitriding was heated in air at 650 ° C. for 3 hours for oxidation treatment. The oxygen content of the oxidized aluminum nitride was 0.90% by mass. 3 parts by mass of yttrium oxide powder and 3 parts by mass of aluminum oxide powder were added to 100 parts by mass of the oxidized aluminum nitride powder, and mixed for 1 hour in a ball mill to obtain a raw material powder. To 100 parts by mass of the raw material powder, 8 parts by mass of carboxymethyl cellulose, 5 parts by mass of glycerin, 2 parts by mass of stearic acid, 2 parts by mass of oleic acid, and 7 parts by mass of ion-exchanged water are added and mixed for 1 minute with a Henschel mixer to obtain a mixture. It was. The mixture was formed into a sheet having a thickness of 1.0 mm using a single screw extruder. The molded body was punched into a size of 60 mm × 50 mm by a press machine with a mold. After applying boron nitride powder as a release agent to the molded body, 20 sheets were laminated and degreased by heating at 570 ° C. for 5 hours in a nitrogen atmosphere. After degreasing, an aluminum nitride substrate was produced by heating at 1780 ° C. for 2 hours in a nitrogen atmosphere. The obtained substrate was evaluated for oxygen content, color unevenness, and thermal conductivity. The results are shown in Table 1.

An aluminum plate was bonded to the obtained aluminum nitride substrate as a metal circuit and a metal heat sink by the following method to produce an aluminum nitride circuit substrate.
Ten sheets of aluminum alloy substrate each having a 60 mm × 50 mm × 0.2 mmt brazing alloy foil adhered thereto and a 60 mm × 50 mm × 0.2 mmt aluminum plate sandwiched between the both surfaces were laminated with a carbon spacer therebetween. After placing it on a carbon jig, it was held at 620 ° C. for 2 hours to join the aluminum nitride sintered body and the aluminum plate. In order to form a circuit pattern of a predetermined shape on one main surface of the joined body and a heat sink pattern on the other main surface, a UV curable resist ink is screen-printed and then irradiated with a UV lamp to form a resist film. Was cured. Next, the portion other than the resist-coated portion was etched with an aqueous sodium hydroxide solution, and then the resist was peeled off with an aqueous ammonium fluoride solution to produce an aluminum nitride circuit board.

In order to evaluate the reliability of the obtained circuit board, a thermal hysteresis impact test was performed. 1) Presence or absence of printed pattern deviation, 2) Presence or absence of occurrence of joint cracks between circuit surface and heat sink surface by cross-sectional observation and aluminum nitride substrate 3) After dissolving the circuit and the heat sink, the presence or absence of cracks in the aluminum nitride substrate was confirmed by an ink test. The results are shown in Table 1.

<Materials used>
Aluminum nitride powder: An aluminum powder was sprayed from the top of a tubular electric furnace heated to 1850 ° C. or more to form aluminum vapor, and reacted with nitrogen gas supplied into the tube to produce aluminum nitride by a direct nitriding method. Average particle size 1.5 μm, oxygen content 0.78%. Yttrium oxide powder: manufactured by Shin-Etsu Chemical Co., Ltd., trade name “Yttrium Oxide”
Aluminum oxide powder: Product name "AO-500" manufactured by Admatechs
Carboxymethylcellulose: Daicel Chemical Industries, trade name “CMC Daicel”
Glycerin: Product name “Exepal” manufactured by Kao Corporation
Stearic acid: manufactured by San Nopco, trade name “Nopco Sera LU-6418”
Oleic acid: Wako Pure Chemical Industries, trade name “Oleic acid”
Aluminum plate: Mitsubishi Aluminum Co., Ltd., trade name “1085”
Brazing alloy foil: product name “A2017R-H alloy foil” manufactured by Toyo Seiki Co., Ltd.
UV curable resist ink: trade name “PER-27B-6” manufactured by Kyoyo Chemical Industry Co., Ltd.

<Evaluation methods>
Oxygen content: Sampling was carried out with a dimension of □ 5 mm × 5 mm diagonally from one corner of the aluminum nitride substrate toward the center, and measured with an oxygen / nitrogen analyzer.
Thermal conductivity: An aluminum nitride sintered body processed to 10 mm × 10 mm was measured by a laser flash method.
Thermal history impact test: 3,000 ° C. thermal history is applied to the circuit board of the sample, with a cycle of 10 minutes at −25 ° C., 10 minutes at 25 ° C., 10 minutes at 125 ° C. and 10 minutes at 25 ° C. test. Presence / absence of joint cracks is determined by conducting a thermal history impact test, symbol C when joining cracks occur in less than 2000 cycles, and joining symbols B and 3000 cycles when joining cracks occur in 2000-3000 cycles. The case where no crack occurred was designated as symbol A. The reliability assurance standards for circuit boards are symbols A and B.
Productivity: Increasing the number of stacked layers increases the batch amount to be processed in a degreasing furnace and a firing furnace, thereby improving productivity. Further, when the degreasing time is shortened, the operating cycle of the degreasing furnace can be shortened, so that productivity is improved. The productivity of Example 1 was represented by symbol Δ, the case where productivity was superior to Example 1 was represented by symbol ◯, and the case where productivity was inferior to Example 1 was represented by symbol x.

[Examples 2 to 4]
An aluminum nitride substrate and an aluminum nitride circuit substrate were obtained in the same manner as in Example 1 except that the number of degreasing and the number of laminated layers during firing and the degreasing time were changed as shown in Table 1. The evaluation results are shown in Table 1.

[Examples 5 to 7]
An aluminum nitride substrate and an aluminum nitride circuit substrate were obtained in the same manner as in Example 1 except that the aluminum nitride powder was not oxidized and the number of laminated layers, degreasing and firing conditions were changed as shown in Table 1. The evaluation results are shown in Table 1.

[Comparative Example 1]
An aluminum nitride substrate and an aluminum nitride circuit substrate were obtained in the same manner as in Example 1 except that the number of degreasing and the number of laminated layers during firing and the degreasing time were changed as shown in Table 1. The evaluation results are shown in Table 1.

[Comparative Example 2]
An aluminum nitride substrate and an aluminum nitride circuit board were obtained in the same manner as in Example 1 except that the aluminum nitride powder was not oxidized. The evaluation results are shown in Table 1.

[Comparative Examples 3 and 4]
An aluminum nitride sintered body and an aluminum nitride circuit board were obtained in the same manner as in Example 2 and Example 3 except that the aluminum nitride powder was not oxidized. The evaluation results are shown in Table 1.

Since the aluminum nitride sintered body produced according to the present invention has no color unevenness and high thermal conductivity, it can be applied not only to a normal circuit board but also to a circuit board used under severe conditions such as a circuit board for a power module. It is a suitable material.

Claims (4)

Sampling with a dimension of □ 5 mm × 5 mm diagonally from one corner of the substrate to the center, and an oxygen content of 1.58% to 1.87% by weight measured with an oxygen / nitrogen analyzer From the above, an uneven aluminum nitride substrate characterized in that the difference between the maximum value and the minimum value of the oxygen content in the substrate is 0.20% by mass or less. The aluminum nitride substrate according to claim 1, wherein an aluminum nitride powder subjected to an oxidation treatment is used as a raw material. A circuit board using the aluminum nitride substrate according to claim 1. A module using the circuit board according to claim 3.