Circuit substrate comprising nitride type ceramics, method for preparing it, and metallizing composition for use in it

Abstract: Disclosed is: i) a nitride type ceramic substrate comprises; a nitride type ceramic sintered sheet; a conductive metallized layer formed on the nitride type ceramic sintered sheet; and a layer of a compound containing yttria and alumina, present in the vicinity of the interface between the nitride type ceramic sintered sheet and the metallized layer; ii) a circuit substrate comprises; a substrate comprising a nitride type ceramic sintered sheet; a metallized layer comprising molybdenum and tungsten as a main component and an activation metal added thereto formed on the substrate; and a layer of a compound containing yttria and alumina, present inside of the metallized layer and in the vicinity of the interface between the substrate and the metallized layer; iii) a surface conductive ceramic substrate which is characterized by comprising; a nitride type ceramic substrate; and a conductive metallized layer chiefly comprised of at least one of Mo and W, a group IVa active metal element and a fourth period transition metal (except for Ti), formed on the substrate; and iV) a surface conductive ceramic substrate which is characterized by comprising; a nitride type ceramic substrate; an intermediate reactive layer chiefly comprised of a compound containing a grain boundary constituent phase component of the ceramic substrate, and a compound of a group IVa active metal; and a conductive metallized layer chiefly comprised of at least one of molybdenum and tungsten, a nitride of a group IVa active metal and a composite compound comprising an oxide of a group IVa active metal and an oxide of a fourth period transition metal (except for titanium), formed on the substrate with the interposition of the intermediate reactive layer. Disclosed is also a method for preparing these substrates and metallizing composition used therefor. According to this invention, a circuit substrate having a conductive metallized layer formed thereon with a high joint strength can be obtained.

Description: BACKGROUND OF THE INVENTION This invention relates to a circuit substrate comprising nitride type ceramics, and it is concerned with an AlN substrate comprising a metallized conductive layer formed on its surface with a high joint strength. Ceramic substrates are widely used in circuit substrates, and recently used in circuit substrates for use in hybrid integrated circuits are substrates comprising a sintered body of nitride type ceramics, as exemplified by a sintered body of aluminum nitride (AlN), having a high thermal conductivity, an excellent heat dissipation property and an electric insulating property.. The nitride type ceramics are prepared generally in the following manner: A given amount of a sintering aid such as Y 2 O 3 , Sm 2 NO 3 and CaO is blended in nitride type ceramic powder with optionally further addition of an acrylic resin binder to thoroughly mix the whole, and the resulting mixture is, for example, pressure-molded to make a sintering precursor (a formed product) having a given shape, followed by sintering at a given temperature in, for example, a nitrogen atmosphere. Incidentally, in instances in which nitride type ceramic sintered sheets are used as substrates for semiconductors, it is necessary to form a conductive layer on the face on which a semiconductor is mounted. Hitherto, this layer comprises a metallized conductive layer of copper (Cu), gold (Au) or silver-palladium (Ag-Pd) formed on the surface of a nitride type ceramic sintered sheet by applying a DBC process (direct bond copper process) or a thick film process. However, this conventional substrate has the following problems. A first problem is that the joint strength between the above metallized layer thus formed and the surface of the nitride type ceramic sintered sheet is so low that a peeling phenomenon may often occur. A second problem is the problem that may be caused when a certain semiconductor device or wire is brazed on the metallized layer thus formed thereon. More specifically, in the instance of the brazing for example, it is carried out at a temperature of about 800°C or so in a hydrogen-nitrogen mixed gas. Since, however, the temperature used at the time of the baking treatment for the above metallized layer is usually a lower temperature of about 600 to 1,000°C, the joint strength between the metallized layer and the surface of the nitride type ceramic sintered sheet is extremely lowered at the time of this brazing to make it virtually impossible to carry out the brazing. A third problem is the problem based on the difference in thermal expansion coefficient between the nitride type ceramic sintered sheet and the metallized layer. More specifically, like the instances of the brazing and the high-temperature soldering, the substrate mounted with a semiconductor device such as a silicon wafer experiences a severe heat cycle of heating/cooling when used. As a result, thermal stresses based on the difference in thermal expansion coefficient between layers are produced at the respective joint areas between the nitride type ceramic sintered body/metallized layer/brazed layer (or soldered layer)/semiconductor device, causing the action of peeling them. In the case of the above metallized layer, the thermal expansion coefficient is greater as much as about 2 to 4 times than that value of the nitride type ceramic sintered sheet (thermal expansion coefficient: about 4.6 x 10 -6 /°C) and is of a value of about a half of the value equivalent to the brazed layer (or soldered layer), showing a great difference from that of the nitride type ceramics. Accordingly, at the time of the heat cycle, fine cracks are liable to be produced at the interface between the metallized layer or brazed layer and the nitride type ceramics. Such fine cracks may gradually grow with repetition of heat cycles, finally sometimes bringing about the peel of the mounted semiconductor device. Such problems are inconvenient as there may be lowered the reliability of an apparatus actually provided with the substrate mounted with a semiconductor device. A fourth problem is that the joint strength between the above metallized layer and the nitride type ceramic sintered sheet is so small as to make low the reliability, like the second problem, when used at high temperatures. For the reasons as above, it is recently practiced, in order to form the conductive layer, to form the metallized layer by coating on the surface of a ceramic substrate a paste chiefly comprised of molybdenum (Mo) or tungsten (W) and also titanium (Ti), zirconium (Zr) or hafnium (Hf) added thereto as an activation metal, followed by firing. In the resulting metallized layer, molybdenum and tungsten are thermoresistant metals that may not be oxidized by heat at the time of the firing, and titanium, zirconium and hafnium may act as the activation metal and adhere the thermoresistant metal on the surface of the ceramic substrate through the reaction with the ceramic. Accordingly, also in the circuit substrates employing the nitride type ceramic substrate, there has been taken a method in which a paste for the metallized layer, chiefly comprised of the above molybdenum or tungsten and also an oxide or nitride of titanium, zirconium or hafnium added thereto as an activation metal is coated, followed by firing and reduction to form a conductive layer, or a method in which this paste is coated on the ceramic substrate, followed by firing to adhere the metallized layer thereon by the action of the activation metal, or the like method. In the instance in which the metallized conductive layer comprises the above W layer, Mo layer or TiN layer, the above problems becomes less serious than the instance in which it comprises the Cu layer, Au layer or Ag-Pd layer previously mentioned, and it can be achieved to improve the joint strength to the nitride type ceramic sintered body and the reliability in a practical use. However, the problems as follows have arisen in the circuit substrate on which the metallized layer is formed like this by coating, on the surface of the ceramic substrate comprised of the nitride type ceramic sintered body, a paste chiefly comprised of molybdenum or tungsten and also titanium, zirconium or hafnium added thereto as an activation metal, followed by firing to form the metallized layer. That is to say, there may arise scattering in the joint strength of the metallized layer to the nitride type ceramic substrate, resulting in non-uniformity in the joint strength of the metallized layer. Also, the surface of the metallized layer is applied with plating in order to mount a semiconductor device on the metallized layer. In this occasion, however, it is sometimes impossible to surely form a coating on the surface of the metallized layer, because grain boundary phases of the AlN substrate exude to the surface of the metallized layer. Thus, an anxiety is accompanied when a semiconductor device is firmly fitted on the metallized layer. In addition, in the method employing the metallizing paste obtained by adding and mixing a nitride of the above activation metal in the high-melting metal such as Mo and W, no sufficient joint strength can be attained unless the firing temperature is raised to higher temperatures as much as 1,700°C to 1,800°C, so that warpage of the substrate may occur. Also, no metallized layer can be formed unless the activation metal is added in a large amount as much as 70 % by weight or more, and therefore, because the amount of the high-melting metal to be blended becomes small, the electric resistance becomes greater and moreover the difference in thermal expansion coefficient from that of the ceramic substrate becomes greater. Thus, there have been involved the problem that cracking is liable to occur by the application of cold and heat cycles. Another problem is also caused such that the scattering in the joint strength also occurs in regard to the AlN ceramic substrates of about 150 W/m·k having many grain boundary phases. In the method in which the metallizing paste employing the compound such as Mo and W, there have also been involved the problem that bleeding occurs at the time of the coating or firing to make it difficult to retain the shape of circuit patterns, or no sufficient surface properties. SUMMARY OF THE INVENTION This invention was made on the basis of the above circumstances, and an object thereof is to provide a circuit substrate such that a metallized layer employing molybdenum or tungsten can be firmly joined to the surface of an aluminum nitride substrate with a uniform joint strength and a coating can be firmly formed on the metallized layer, thus having a high reliability. In regard to the nitride type ceramic substrates wherein the metallized layer comprises a W, Mo or W-Mo layer, the present inventors made studies on the relationship between the state of the interface between the nitride type ceramic sintered sheet and the W, Mo or W-Mo layer, and the joint strength. As a result, they found the fact that a layer of compounds containing yttria (Y 2 O 3 ) and alumina (Al 2 O 3 ) was formed at the interface part and in the vicinity thereof in regard to the substrates having a high joint strength, and such compounds were not present in a concentrated form at this interface part and in the vicinity thereof on the contrary in regard to the substrates having a low joint strength. This phenomenon was also seen in the instances of the substrates wherein the metallized layer comprises a Mo layer or TiN layer. Based on these findings, the present inventors had an idea that the joint strength of the substrate could be improved if the compounds containing yttria and alumina are made present in a concentrated form to form a layer, at the interface between the nitride type ceramic substrate and the metallized layer and at the portion being from the interface to the metallized layer, and, having confirmed the correctness of that idea through an experimental route, developed the nitride type ceramic substrate of this invention. Namely, the nitride type ceramic substrate that can achieve the object of this invention is characterized in that a layer of a compound containing yttria and alumina is present in the vicinity of the interface between a nitride type ceramic sintered sheet and a conductive metallized layer formed on the surface of said nitride type ceramic sintered sheet. According to a first aspect, the present invention provides a nitride ceramic substrate comprising (i) a nitride ceramic sintered sheet; (ii) a conductive metallized layer comprising molybdenum, tungsten or a mixture thereof as a main component formed on said nitride ceramic sintered sheet; and(iii) a layer of a compound containing yttria and alumina at the interface between said nitride ceramic sintered sheet and said metallized layer which is uniformly distributed over the metallized layer and which penetrates into both the nitride ceramic sintered sheet and said metallized layer; and being obtainable by the steps of: (a) coating on said nitride ceramic sintered sheet a metallizing paste comprising molybdenum, tungsten or a mixture thereof as a main component, and yttria and alumina; and (b) firing the coated sheet in an inert atmosphere at a temperature of from 1600 to 1900°C. According to a second aspect, the present invention provides a nitride ceramic substrate comprising (i) a nitride ceramic sintered sheet; (ii) a conductive metallized layer comprising molybdenum, tungsten or a mixture thereof as a main component formed on said nitride ceramic sintered sheet; and (iii) a layer of a compound containing yttria and alumina at the interface between said nitride ceramic sintered sheet and said metallized layer which is uniformly distributed over the metallized layer and which penetrates into both the nitride ceramic sintered sheet and said metallized layer; and being obtainable by the steps of: (a) coating a paste comprising a compound of yttria and a compound of alumina on said nitride ceramic sintered sheet; (b) coating a metallizing paste comprising molybdenum, tungsten or a mixture thereof as a main component on the paste of the yttria and alumina compounds; and (c) firing the coated sheet in an inert atmosphere at a temperature of from 1600 to 1900°C. The present inventors further progressed their searches about the above nitride type ceramic substrate of this invention of the basis of the following aspect, i.e., i) an aspect that the above object is achieved by making uniformly present the above compounds containing yttrium Y and aluminum Al. As a result, they succeeded in leading the above aspect i) to the following embodiment i). Embodiments i) will first described below. Specifically, in instances in which the metallized layer is formed in a sintering step by firing the metallizing paste coated on the surface of the nitride type ceramic substrate, Al 2 O 3 on the nitride type ceramic reacts with the sintering aid Y 2 O 3 to produce a compound, and this compound exists at a grain boundary of the nitride type ceramic sintered body as a grain boundary substance. Meanwhile, the metallizing paste comprised of molybdenum or tungsten and further the activation metal added thereto is usually fired at a temperature of from 1700°C to 1750°C. Firing the metallizing paste at this firing temperature, the compounds of Al 2 O 3 with Y 2 O 3 present inside the nitride type ceramic substrate as the grain boundary substances turn to a liquid phase, and exude to enter into the metallized layer on the surface of the substrate, which compounds have the action to combine in the metallized layer with the activation metal, titanium, zirconium or hafnium, to cause adhesion between the metallized layer and the nitride type ceramic substrate. More specifically, when the nitride type ceramic substrate is prepared, yttria (Y 2 O 3 ) is added as a sintering aid to nitride type ceramic powder in order to enhance the sinter density, and a body fromed by use of this powder is sintered. Liquid phase sintering takes place in this sintering step, Al 2 O 3 on the nitride type ceramic reacts with the sintering aid Y 2 O 3 to produce a compound, and it follows that this compound exists at a grain boundary of the nitride type ceramic sintered body as a grain boundary substance. The compound is presumed to exist in the form of YAG (5Al 2 O 3 ·3Y 2 O 3 ), YAM (Al 2 O 3 ·Y 2 O 3 ), YAL (Al 2 O 3 ·2Y 2 O 3 ) or Y 2 O 3 . Then, when the metallizing paste coated on the surface of the nitride type ceramic substrate, i.e., the nitride type ceramic sintered body is fired at a temperature of from about 1,500 to 1,700°C, the compounds of Al 2 O 3 with Y 2 O 3 present inside the nitride type ceramic substrate as the grain boundary substances are again fused to exude to the surface of the substrate and soak into the metallized layer. As set out previously, the joint strength of the metallized layer to the nitride type ceramic substrate can be enhanced by utilizing this phenomenon. However, the grain boundary substances present in the nitride type ceramic substrate, i.e., the compounds of Al 2 O 3 with Y 2 O 3 , are distributed usually in a non-uniform state, and therefore the state in which the compounds of Al 2 O 3 with Y 2 O 3 exude from the nitride type ceramic substrate and soak into the metallized layer tends to become also non-uniform. Accordingly, scattering tends to occur in the joint strength as the joint strength is partly different between the metallized layer and nitride type ceramic substrate. Moreover, in regard to the step of preparing the nitride type ceramic sintered body, it is very difficult to make control so that the grain boundary substances, the compounds of Al 2 O 3 with Y 2 O 3 , may be made uniformly present. Now, the present inventors noted not to rely only on the grain boundary substances present in the inside of the nitride type ceramic substrate, but to supply the same compounds of Al 2 O 3 with Y 2 O 3 as the grain boundary substances from outside to form the metallized layer on the surface of the nitride type ceramic substrate. They have thus found that this method makes it possible to make control so that the compounds of Al 2 O 3 with Y 2 O 3 may be uniformly distributed over the metallized layer, whereby the metallized layer can be joined to the nitride type ceramic substrate with uniform joint strength. As a more specific method based on this view point; they found a method in which the components for the compounds are added in the metallizing paste to form the metallized layer and a method in which a layer of the compounds is formed on the surface of the nitride type ceramic substrate and the metallized layer is formed thereon, thus accomplishing the embodiment i). Accordingly, a third aspect of the present invention provides a method for preparing a ceramic substrate comprising: (a) coating a metallizing paste comprising molybdenum, tungsten or a mixture thereof as a main component, and yttria and alumina on a nitride ceramic sintered sheet forming part of a substrate; and (b) firing the coated substrate in an inert atmosphere at a temperature of from 1600 to 1900°C to prepare the ceramic substrate comprising a layer of a compound containing yttria and alumina at the interface between said nitride ceramic sintered sheet and a metallized layer formed from said metallizing paste, wherein said compound containing yttria and alumina is uniformly distributed over the metallized layer and penetrates into both the nitride ceramic sintered sheet and said metallized layer. According to a fourth aspect, the present invention provides a method for preparing a ceramic substrate comprising: (a) coating a paste comprising a compound of yttria and a compound of alumina on a nitride ceramic sintered sheet forming part of a substrate; (b) coating a metallizing paste comprising molybdenum, tungsten or a mixture thereof as a main component on the paste of the yttria and alumina compounds; and (c) firing the coated substrate in an inert atmosphere at a temperature of from 1600 to 1900°C to prepare the ceramic substrate comprising a layer of a compound containing yttria and alumina at the interface between said nitride ceramic sintered sheet and a metallized layer formed from said metallizing paste, wherein said compound containing yttria and alumina is uniformly distributed over the metallized layer and penetrates into both the nitride ceramic sintered sheet and said metallized layer. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-section illustrating a circuit substrate of this invention; and Fig. 2 is a chart for explaining a method of measuring the joint strength of a metallized layer to a substrate; DESCRIPTION OF THE PREFERRED EMBODIMENTS The circuit substrate of this invention will be described below. In the circuit substrate of this invention, preferred as the nitride type ceramic sintered sheet as exemplified by the AlN sintered sheet is a material having a thermal conductivity of 50 W/m·K or more. Such an AlN sintered sheet is prepared, for example, by mixing AlN powder having a given grain size, a sintering aid such as Y 2 O 3 , YF 3 , Sm 2 O 3 and CaCo 3 and a binder component of wax type or plastic type in a given weight ratio, forming the resulting mixture into a sheet at room temperature by pressure forming or by a doctor blade to make a formed body, and thereafter sintering the resulting formed body. At this time, the conditions such as the grain size of the AlN powder or sintering aid powder, mixing ratio of the both, forming pressure, sintering temperature and sintering time in the respective steps may be appropriately selected to determine the properties of the resulting AlN sintered sheet. Also, the conductive metallized layer formed on the surface of this AlN sintered sheet is a conductive layer containing at least one of W and Mo. Specifically, it is a layer constituted of a single component of W and Mo respectively; a layer constituted of two components like W and Mo, W and TiN, and Mo and TiN; and further a layer constituted of three components of W, Mo and TiN. When the metallized layer is constituted of multiple components, there is no particular limitation in the incorporating proportion for the respective components. In the substrate of this invention, what is most characteristic is that a layer of the compounds containing yttrium and alumina is present in the vicinity of the interface between the above nitride type ceramic sintered sheet and the metallized layer and inside the metallized layer. The mechanism by which the formation of this Al 2 O 3 - Y 2 O 3 compound layer in the vicinity of the interface and inside the metallized layer improves the joint strength is presumed that the Al 2 O 3 - Y 2 O 3 compound is penetrated into both the substrate AlN and the metallized layer. The nitride type ceramic substrate of this invention can be prepared in the following manner: First, prepared according to a conventional method is a nitride type ceramic sintered sheet having given properties, in particular, a thermal conductivity of 50 W/m·k or more. Next, on the surface of this nitride type ceramic sintered sheet, a paste containing the components for forming the intended metallized layer is coated. As the coating method, there may be applied known methods as exemplified by screen process printing, brushing, spin roller coating, etc. The paste is constituted of the components for forming the metallized layer and a medium for dispersing them. More specifically, it is constituted of powder of any one of W and Mo dispersed in a medium. The medium to be used may include, for example, ethyl cellulose, nitrocellulose, and a mixture of these with turpineol, tetralin or butyl carbitol serving as a solvent. The dispersion amount of the above components in the medium may be appropriately determined according to the relation with the viscosity of the resulting paste. An excessively large amount of the former may result in a highly viscous paste, making difficult the uniform coating on the nitride type ceramic sintered sheet. In an instance adverse thereto, the paste may have so lower viscosity that the coated paste may flow down from the surface of the nitride type ceramic sintered sheet. In usual instances, the former may be dispersed in the latter so as to give 1.0 x 10 5 to 2.5 x 10 5 poise. After coating of the paste, the whole is heated to form the metallized layer. In this heating treatment, required are an inert atmosphere of such as nitrogen gas and a temperature of from 1,600 to 1,900°C. The temperature lower than 1,600°C may result in insufficient formation of the metallized layer of good quality and at the same time so poor diffusion and penetration in the vicinity of the interface, of the compounds containing yttrium and alumina that the joint strength can be improved with insufficiency. The temperature higher than 1,900°C may bring about inconveniences as there may be caused lowering of wettability of Mo, W or the like in the metallized layer with the substrate. It may preferably range from 1,700 to 1,800°C. In this heating process, crystals of the compounds containing yttrium and alumina that were present between crystal grains of nitride type ceramics and distributed in the nitride type ceramic sintered sheet are presumed to turn to a state of a liquid phase, and move to the interface with the metallized layer, and further penetrate also into the metallized layer side across the interface. The circuit substrate according to this invention will be described below with reference to Fig. 1. In Fig. 1, the numeral 1 denotes an aluminum nitride substrate comprising a sintered body of aluminum nitride AlN. This aluminum nitride substrate 1 is prepared by forming and sintering with use of powder obtained by adding, for example, Y 2 O 3 to aluminum nitride as a sintering aid for the purpose of enhancing the sinter density. The proportion of Y 2 O 3 added to the aluminum nitride substrate ranges from 0.1 to 10 % in weight ratio. The numeral 2 denotes a metallized layer formed on and joined to the surface of the aluminum nitride substrate 1. This metallized layer 2 is formed by coating on the surface of the aluminum nitride substrate 1 a paste chiefly comprised of molybdenum or tungsten and a compound of titanium, zirconium or hafnium, specifically an oxide, nitride or chloride of titanium, zirconium or hafnium added thereto as an activation metal, followed by firing. In this metallized layer 2, molybdenum and tungsten are thermoresistant metals that may not be oxidized at the time of firing, and the activation metal is a metal that turns active at high temperatures at the time of the firing and reacts with aluminum nitride of the aluminum nitride substrate 1 to join the metallized layer 2 to the aluminum nitride substrate 1. In this metallized layer 2, also present in a uniformly dispersed state over the whole of the metallized layer 2 are compounds of Al 2 O 3 with Y 2 O 3 , namely compounds having the same components as the grain boudary substances in the aluminum nitride substrate 1. This compounds have the action of being fused at the time of the firing of the metallized layer 2 to join the metallized layer 2 to the aluminum nitride substrate 1. The proportion of the compounds of Al 2 O 3 with Y 2 O 3 present in the metallized layer 2 should range from 3 to 25 % in weight ratio. This is because an excessively large amount of the compounds present in the metallized layer 2 may result in that the metallized layer 2 is covered with the compounds of Al 2 O 3 with Y 2 O 3 , or an overly small amount of the compounds may result in a small joint strength between the metallized layer 2 and the aluminum nitride substrate 1. The surface of the metallized layer 2 is also plated with Ni, and a semiconductor device is further mounted by use of a Ag solder. Since in the circuit substrate constituted like this the compounds of Al 2 O 3 with Y 2 O 3 , the same components as in the grain boundary substances in the aluminum nitride substrate 1, are contained in the metallized layer 2, the compounds act on the aluminum nitride substrate 1 as an adhesive having a strong affinity for the aluminum nitride substrate 1, so that the metallized layer 2 can be joined to the aluminum nitride substrate 2. More specifically, the metallized layer 2 has a good affinity for the aluminum nitride substrate 1, and joins to the aluminum nitride substrate 1 with a great joint strength. Especially when the metallized layer 2 is formed, the compounds of Al 2 O 3 with Y 2 O 3 can be readily controlled to be uniformly distributed and the compounds are uniformly dispersed over the whole area of the metallized layer 2. Thus, the metallized layer 2 joins to the aluminum nitride substrate 1 over its whole area with a uniform joint strength. Also, the grain boundary substances present in the aluminum nitride substrate 1 are fused at the firing of the metallized layer 2 to exude to the surface and penetrate into the metallized layer 2. However, since the compounds of Al 2 O 3 with Y 2 O 3 have been already present in the metallized layer 2 to prescribe the joint strength of the aluminum nitride substrate 1, there occurs no non-uniformity of the joint strength because of the penetration of the grain boundary substances. A method for preparing the circuit substrate according to the present invention will be described below. This preparation method can be grouped into two ways according to the manner by which the compounds are incorporated into the metallized layer. One of them is a method in which, taking the example of aluminum nitride as an example of the nitride type ceramics, the compounds are coated on the surface of the aluminum nitride substrate, and the metallizing paste is coated thereon, followed by firing in an integral fashion. In this method, first, the compounds of Al 2 O 3 with Y 2 O 3 are powdered, and the resulting powder is formed into a paste. Next, the resulting paste or solution is coated on the surface of the aluminum nitride substrate with a uniform thickness. Thereafter, the paste chiefly comprised of Mo or W and the activation metal added thereto is coated on the surface of the layer of the above compounds. As an example for the coating thickness of the respective layers, assuming that the aluminum nitride substrate has a thickness of 0.6 mm, the compounds are with a thickness of 2 µm and the metallizing paste is with a thickness of 10 µm, so that the total thickness for the compounds and metallizing paste is 12 µm. The compound layer and the metallized layer are then fired in an integral fashion. At the time of this firing, the compound layer is fused to soak into the metallized layer and simultaneously adhere to the aluminum nitride substrate, so that the metallized layer is firmly and uniformly joined to the aluminum nitride substrate. Another method is a method in which a metallizing paste obtained by adding the compounds of Al 2 O 3 with Y 2 O 3 or each of Al 2 O 3 and Y 2 O 3 independently is coated on the surface of the aluminum nitride substrate, followed by firing. In the instance where Al 2 O 3 and Y 2 O 3 are independently added, these are added in the proportion, for example, of 5 % in weight ratio in total based on the metallizing paste, and the mutual proportion of Al 2 O 3 : Y 2 O 3 is controlled, for example, to 5 : 3. In the instance where the compounds of Al 2 O 3 with Y 2 O 3 are added, they are added in the proportion of 5 % in weight ratio based on the metallizing paste. The metallizing paste to which Al 2 O 3 and Y 2 O 3 are added like this in the state of compounds or in an independent state is thoroughly kneaded. The resulting metallizing paste is coated on the surface of the aluminum nitride substrate to carry out firing. In this occasion, it is coated with a thickness of 10 µm relative to 0.6 mm of the aluminum nitride substrate. Temperature patterns at the firing are the same as in the case of the method described previously. The Al 2 O 3 and Y 2 O 3 added independently are reacted by this firing to produce a compound. The metallized layer thus formed is firmly and uniformly joined to the aluminum nitride substrate. A method of measuring and controlling the joint strength between the aluminum nitride substrate and metallized layer to secure the quality of circuit substrates will be described below. This method has been accomplished by taking note of the fact that the X-ray diffraction intensity of the compounds of Al 2 O 3 with Y 2 O 3 and the joint strength between the aluminum nitride substrate and metallized layer are in a proportional relationship. The aluminum nitride substrate is coated with the metallizing paste according to the method previously described, followed by firing at a firing temperature, for example, of 1,800°C, 1,750°C or 1,700°C to prepare three kinds of substrates on which the metallized layer chiefly comprised of Mo or W and containing the compounds of Al 2 O 3 with Y 2 O 3 . Using an X-ray diffraction apparatus, X-ray diffraction intensities of the aluminum nitride substrate and the metallized layer are measured for each substrate. Next, tests for measuring the joint strength of the metallized layer to the aluminum nitride substrate, i.e., the pulling strength were carried out for each substrate. The test was made in such a manner that a pad of a square shape having 2 mm sides is provided on the aluminum nitride substrate and an annealed copper wire having a diameter of 0.8 mm was joined thereto by using a Pb-Sn plating, followed by pulling to a vertical direction. As a result, it was found that the relationship as shown in the chart of Fig. 2 is established between the pulling strength of the metallized layer and the Mo-W compound intensity ratio. Here, the Mo-W compound represents a compound containing molybdenum or tungsten singly or molybdenum and tungsten in combination. In the figure, peeling ocurred between Ni coatings and metallized faces at the black points denoted by *1 and *2. Accordingly, the X-ray diffraction intensity may be measured for the metallized layer of the circuit substrate and the results thus obtained may be applied to the graph of Fig. 2 to make examination, whereby the joint strength (pulling strength) of the metallized layer to the aluminum nitride substrate can be known. Also, it is found that those having high X-ray diffraction intensity becomes low in Ni-plating property when the intensity is over a certain limit. For example, when it is intended to ensure 8 kgf/mm 2 or more of joint strength of the metallized layer, there may be selected a circuit substrate having 1.0 or more of Mo-W compound intensity ratio. According to this method, the joint strength of the metallized layers of circuit substrates can be readily and precisely measured without relying on destructive tests. Reference Example 1 Mixed were 50 parts by weight of Mo powder of a grain size of from 1 to 2 µm and 50 parts by weight of TiN powder of a grain size of from 1 to 2 µm, and 100 parts by weight of the resulting mixed powder were dispersed in 7 parts by weight of ethyl cellulose to prepare a paste having a viscosity of 1.5 x 10 5 poise. This paste was coated with a thickness of 15 um by roller coating on one side of an AlN substrate containing 5 % by weight of Y 2 O 3 as a sintering aid. This substrate was sintered for about 1 hour at a temperature of 1,700°C in a dry nitrogen gas stream. Example 1 A powder comprising aluminum nitride powder and 3 % in weight ratio, of Y 2 O 3 added thereto was formed by a doctor blade process, and the resulting formed body was fired under the conditions of a nitrogen gas atmosphere, a temperature of about 1,800°C and a time of 2 hours to prepare an aluminum nitride substrate of 0.35 mm in thickness. Also, a powder mixed in the ratio of Al 2 O 3 : Y 2 O 3 = 1 : 1 was added to a paste of Mo : TiN = 1 : 1 in the proportion of 5 % in weight ratio, followed by kneading. This paste was coated on the aluminum nitride substrate to have a thickness of 10 um, and fired for 5 hours at 1,700°C in a nitrogen gas atmosphere to form a metallized layer. The X-ray diffraction intensity of the metallized layer of this circuit substrate was measured. As a result, the Mo-W compound intensity ratio was found to be 1.0. This diffraction intensity ratio was applied to the chart of Fig. 2 to examine the joint strength of the metallized layer to find that it was about 8.4 kgf/mm 2 . The metallized layer was also found to be uniformly joined to the aluminum nitride substrate. Example 2 Prepared was an aluminum nitride substrate in accordance with Reference Example 1. A paste comprising compounds of Al 2 O 3 : Y 2 O 3 = 5 : 3 was also prepared, and this paste was coated with a thickness of 2 µm by a printing process on the surface of the aluminum nitride substrate prepared under the same conditions as in Example 1. Subsequently, on the resulting paste layer, a paste comprising Mo : TiN = 1 : 1 was coated with a thickness of 10 um, followed by firing to form a metallized layer. The joint strength of this metallized layer was measured to find that it was 2.8 kgf/mm 2 . Comparative Example 1 On the surface of the aluminum nitride substrate prepared under the same conditions as in Example 1, a paste comprising Mo : TiN = 1 : 1 was coated with a thickness of 10 µm, followed by firing to form a metallized layer. The joint strength of this metallized layer was measured to find that it was 2.0 kgf/mm 2 in average, but the joint strength of the metallized layer was scattered as ± 1.5 kgf/mm 2 . As will be clear from the above description, the nitride type ceramic substrate of this invention has the Al 2 O 3 - Y 2 O 3 compound layer concentratedly formed in the vicinity of the interface between the metallized layer and the nitride type ceramic sintered sheet and inside of the metallized layer, thus achieving a high joint strength between the metallized layer and nitride type ceramic sintered sheet. Accordingly, the nitride type ceramic substrate of this invention has high reliability when put into practical use, bringing about great industrial value. According to the embodiment i) of this invention, there can be also obtained the highly reliable circuit substrate in which the metallized layer comprising Mo or W has been firmly joined to the substrate with uniform joint strength.

Claims: A nitride ceramic substrate comprising (i) a nitride ceramic sintered sheet; (ii) a conductive metallized layer comprising molybdenum, tungsten or a mixture thereof as a main component formed on said nitride ceramic sintered sheet; and (iii) a layer of a compound containing yttria and alumina at the interface between said nitride ceramic sintered sheet and said metallized layer which is uniformly distributed over the metallized layer and which penetrates into both the nitride ceramic sintered sheet and said metallized layer; and being obtainable by the steps of: (a) coating on said nitride ceramic sintered sheet a metallizing paste comprising molybdenum, tungsten or a mixture thereof as a main component, and yttria and alumina; and (b) firing the coated sheet in an inert atmosphere at a temperature of from 1600 to 1900°C. A nitride ceramic substrate comprising (i) a nitride ceramic sintered sheet; (ii) a conductive metallized layer comprising molybdenum, tungsten or a mixture thereof as a main component formed on said nitride ceramic sintered sheet; and (iii) a layer of a compound containing yttria and alumina at the interface between said nitride ceramic sintered sheet and said metallized layer which is uniformly distributed over the metallized layer and which penetrates into both the nitride ceramic sintered sheet and said metallized layer; and being obtainable by the steps of: (a) coating a paste comprising a compound of yttria and a compound of alumina on said nitride ceramic sintered sheet; (b) coating a metallizing paste comprising molybdenum, tungsten or a mixture thereof as a main component on the paste of the yttria and alumina compounds; and (c) firing the coated sheet in an inert atmosphere at a temperature of from 1600 to 1900°C. A nitride ceramic substrate according to Claim 1 or Claim 2, wherein the conductive metallized layer further comprises at least one of titanium, zirconium and hafnium as an activation metal. A method for preparing a ceramic substrate comprising: (a) coating a metallizing paste comprising molybdenum, tungsten or a mixture thereof as a main component, and yttria and alumina on a nitride ceramic sintered sheet forming part of a substrate; and (b) firing the coated substrate in an inert atmosphere at a temperature of from 1600 to 1900°C to prepare the ceramic substrate comprising a layer of a compound containing yttria and alumina at the interface between said nitride ceramic sintered sheet and a metallized layer formed from said metallizing paste, wherein said compound containing yttria and alumina is uniformly distributed over the metallized layer and penetrates into both the nitride ceramic sintered sheet and said metallized layer. A method for preparing a ceramic substrate comprising: (a) coating a paste comprising a compound of yttria and a compound of alumina on a nitride ceramic sintered sheet forming part of a substrate; (b) coating a metallizing paste comprising molybdenum, tungsten or a mixture thereof as a main component on the paste of the yttria and alumina compounds; and (c) firing the coated substrate in an inert atmosphere at a temperature of from 1600 to 1900°C to prepare the ceramic substrate comprising a layer of a compound containing yttria and alumina at the interface between said nitride ceramic sintered sheet and a metallized layer formed from said metallizing paste, wherein said compound containing yttria and alumina is uniformly distributed over the metallized layer and penetrates into both the nitride ceramic sintered sheet and said metallized layer. A method according to Claim 4 or Claim 5, wherein the metallizing paste further comprises at least one of titanium, zirconium and hafnium as an activation metal.

Inventors: ANZAI KAZUNO, ASAI HIRONORI, HATANO TSUYOSHI, MIZUNOYA NOBUYUKI, SATO HIDEKI, ANZAI, KAZUNO, ASAI, HIRONORI, HATANO, TSUYOSHI, MIZUNOYA, NOBUYUKI, SATO, HIDEKI, Anzai, Kazuno, c/o K.K. Toshiba, Asai, Hironori, c/o K.K. Toshiba, Hatano, Tsuyoshi, c/o K.K. Toshiba, Mizunoya, Nobuyuki, c/o K.K. Toshiba, Sato, Hideki, c/o K.K. Toshiba
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: C04B41
IPC codes: C04B41, H01L21, H05K1, C04B35, H01L23, H05K3