GB2153814A - MgO ceramics for electrically insulating substrates - Google Patents

Abstract

MgO ceramics for electrically insulating substrates, have a microstructure wherein microscopically the majority of MgO crystals has a grain size of at most 5 mu m, and a thin 2MgO.SiO2 phase is uniformly distributed at the grain boundaries of MgO crystals; the ceramics have a heat conductivity of at least 0.08 cal/cm.sec. DEG C at normal temperature, a dielectric constant at 100 KHz of at most 10 at normal temperature, an electrical insulation resistance of at least 10<10> OMEGA .cm at 300 DEG C and an excellent resistance to humidity.

Description

SPECIFICATION MgO Ceramics for electrically insulating substrates and process for the production thereof The present invention relates to MgO ceramics for electrically insulating substrates. More particularly, it relates to MgO ceramics for electrically insulating substrates, having a high heat conductivity and excellent resistance to humidity, and a process for the production thereof.

Most of the insulating materials practically employed forthe integrated circuit substrates as electronic materials, are A1203 substrates.

Whereas, MgO materials have excellent electrically insulating properties at high temperatures and they are superior to the A1203 materials also in the high frequency characteristics and the heat conductivity. Thus, they are expected to be promissing as insulators for insulating substrates and integrated circuit packages.

However, they have affinity to humidity by their nature and no adequate strength has yet been obtained.

Therefore, the above-mentioned excellent characteristics and properties have not yet been practically utilized.

With a prospect that the properties of the MgO materials will be extremely effective for future developments of high performance integrated circuit substrates, such as a development of hybrid integrated circuits or a development of multilayer packages, the present inventors have conducted various studies, and as a resu It, have succeeded in the development of ceramics for MgO substrates whereby the above-mentioned drawbacks such as affinity to humidity which used to be a stumbling block to the practical use, have been overcome.

Namely, the present invention provides MgO ceramics for electrically insulating substrates, having a microstructure wherein microscopically the majority of MgO crystals has a grain size of at most 5 wm and a thin 2MgO.SiO2 phase is uniformly distributed at the grain boundaries of MgO crystals, and having a heat conductivity of at least 0.08 callcm.sec." at normal temperature, a dielectric constant at 100 KHz of at most 10 at normal temperature, an electrical insulation resistance of at least 1010 Q.cm at 300"C and an excellent resistance to humidity.

Further, the present invention provides a process for producing such MgO ceramics which comprises thoroughly mixing a MgO powder having a MgO purity of at least 99% by weight, as chemically analyzed, and a specific surface area of at least 10 m21g in an organic solvent containing an organic silicate, followed by shaping and sintering to obtain MgO ceramics having a microstructure wherein microscopically the majority of MgO crystals has a grain size of at most 5 lim and a thin 2MgO.SiO2 phase is uniformly distributed at the grain boundaries of MgO crystals, and having a heat conductivity of at least 0.08 cal/cm.sec.C at normal temperature, a dielectric constant at 100 KHz of at most 10 at normal temperature, an electrical insulation resistance of at least 1010 R.cm at 3000C and an excellent resistance to humidity.

The MgO ceramics according to the present invention have excellent properties for insulating substrates, which are believed attributable to the microstructure of the ceramics which comprises fine MgO crystal (periclase crystal) grains and a thin layer of a phase composed mainly of 2MgO.SiO2 (forsterite) (hereinafter referred to as "2MgO.SiO2 phase") uniformly distributed at the grain boundaries.

In the past, there have been some proposals to impart water resistance by forming a coating layer of 2MgO.SiO2 phase in an attemptforthe development of MgO substrates. However, no successful results have been attained. The reason for the failure to obtain successful results in such past attempts, is not clearly understood, but appears to be in the difference in the microstructure of the ceramics, and in the difference in the process for the production which brings about such difference in the microstructure. For instance, one of the reasons may be that, as disclosed in Japanese Unexamined Patent Publication No. 217480i1983, most of the conventional attempts of this nature were based on the idea that a water resistant 2Mgo.SiO2 phase be formed by treating a preformed substrate.Namely, by such a method, no adequate water resistance was obtained, or no adequate heat conductivity was obtained. Namely, it was impossible to obtain a product having various characteristics required for a high performance substrate.

As another method, it has been proposed to mix a magnesium compound with a liquid state silicate in order to obtain fine ceramics in Japanese Unexamined Patent Publication No. 18176411983. However, no adequate resistance to humidity has been obtained. The reason for the failure to obtain adequate resistance to humidity is not clear, but the object of the proposal is to obtain ceramics having light transmitting properties and accordingly it is required to minimize the grain boundary phase such as 2MgO.SiO2, and the amount of the liquid state silicate has to be limited to a low level. Further, pre-heating treatment at a temperature of from 750 to 11 00C is required before the mixture is shaped.

Now, the ceramics of the present invention and the process for the production thereof will be described in detail.

In the accompanying drawing, Figure lisa microscopic picture of the cross section of typical ceramics of the present invention.

The ceramics of the present invention has the following structure and composition. Namely, microscopically, MgO (periclase) crystal grains are intimately sintered to one another. The majority of individual MgO crystals has a grain size of at most 5 Fm (including the portion constituting an integral thin layer of 2MgO.SiO2 phase on the surface, as will be described hereinafter). Thus, the microstructure wherein the crystal growth is suppressed, is one of the features of the ceramics of the present invention. Further, at the grain boundaries of such fine MgO crystals, a 2MgO.SiO2 (forsterite) phase is uniformly distributed throughout the entire ceramics. This distribution is not limited to the surface layer of the ceramics or its vicinity, but uniformly extends throughout the entire cross section.

From the viewpoint of resistance to humidity, an ideal structure for the ceramics of the present invention may be such that the 2MgO.SiO2 phase is present in an amount as small as possible i.e. in the form of an extremely thin layer along all grain boundaries of individual fine MgO crystal grains and over the entire surface of each crystal grain. However, for the practical purposes, the ceramics of the present invention may not completely satisfy such a condition of an ideal structure, so long as the grain surfaces of individual MgO crystal grains are covered at least partially by such a phase so that the ceramics will thereby satisfy the requirements for various desired properties.

From the viewpoint of some other natures such as heat conductivity, the presence of the 2MgO.SiO2 phase may rather be localized. Thus, the 2MgO.SiO2 phase may not necessarily be present over the entire circumferences of individual MgO crystals so long as it covers at least partially the surfaces of individual MgO crystals along the grain boundaries and it is uniformly distributed throughout the entire ceramic structure as a whole. In this specification, the term "uniformly distributed" is used in this sense for the description of such a microstructure.

Further, in such a microstructure, the grain boundaries of MgO crystals include so-called interfaces at which MgO crystals may be in contact with each other, and the same applies to such interfaces. Namely, it is preferred that a thin layer of the 2MgO.SiO2 phase is present at least partially also at such interfaces.

Thus, in the ceramics of the present invention, the 2MgO.SiO2 phase is present at the grain boundaries, (including the interfaces) of MgO crystals in the form of a thin layer as thin as possible, i.e. preferably at most 0.5 Fm, e.g. from 0.1 to 0.5 lim.

As will be apparent from the process hereinafter described, the 2MgO.SiO2 phase is formed by the reaction of fine MgO powder with the SiO2 component of an organic silicate, and accordingly it is usually present also on the surfaces of MgO crystals as an integral part thereof.

Now, the ceramics of the present invention will be described in further detail with reference to the microscopic photograph shown in Figure 1.

In Figure 1, reference numeral 1 designates MgO crystal grains and reference numerals 2 and 3 designate 2MgO.SiO2 phases.

As shown in the Figure, MgO crystal grains 1 are intimately and firmly sintered to one another via the 2MgO.SiO2 phases 2 and 3, and the majority of the MgO crystals has a grain size of at most 5 pWm. In the ceramics, crystal grains having a grain size of from 5 to 10 sLm are also present, but their amount is at most 20% by weight, usually about 10% or less.

Here, the 2MgO.SiO2 phase 2 is a 2MgO.SiO2 phase covering MgO crystals (i.e. being present at the interfaces), whereas the phase 3 is a 2MgO.SiO2 phase present at the grain boundaries between a plurality of MgO crystals.

In the present invention, such 2MgO.SiO2 phases are preferably present in a volume ratio of at least 3%, more preferably from 5 to 8%.

In order to obtain various desired properties, the ceramics of the present invention are required to be the one obtained by densely sintering high purity MgO. As their composition, it is desired that the MgO crystal grains have a MgO purity of at least 99.5% by weight, preferably at least 99.7% by weight, as a chemically analyzed value. In this specification, the purity of the MgO crystals is meant for the purity of the MgO crystal grains excluding the 2MgO.SiO2 layer formed on the grain surfaces.

As the composition of the entire ceramics, it is desirable that the ceramics contains from 0.1 to 5% by weight, preferably from 0.5 to 3% by weight, particularly from the viewpoint of resistance to humidity, of SiO2 and has a total content of SiO2 and MgO being at least 99.5% by weight, preferably at least 99.95% by weight, as chemically analyzed.

The ceramics of the present invention having the above-mentioned structure and composition may be characterized by the following various properties. In the following, the numerals in brackets ( ) indicate the preferred ranges.

Bulk density at least 3.4 (3.45) Flexural strength (normal temperature, kg/mm2) at least 20 (25) Heat conductivity (cal/cm.sec."C) Normal temperature at least 0.08 (0.10) 300"C at least 0.05 (0.06) Dielectric constant (normal temperature, 100 KHz) at most 10 Dielectric loss at most 3 x 104 (normal temperature, 100 KHz) (1 x 104) Electrical insulation resistance ( Q.cm) at least 1010 Resistance to humidity* at most 0.02% Note: *The weight increase when a test piece was held in an autoclave for 2 hours in steam under a pressure of 5 atm. at 1 50"C (the test piece is a plate having a size of 50 mm x 50 mm x 1 mm in thickness.The thickness may be negligible, and accordingly the weight increase may be represented by unit surface area.) Further, as a testing method for evaluating the reliability of an electrically insulating substrate, it is common to employ a testing method wherein a test piece is held at 120 C for 500 hours in steam under a pressure of 2 atm. The ceramics of the present invention were tested by this testing method, whereby no substantial increase in the weight was observed.

Now, a process for producing such ceramics of the present invention, particularly a preferred process of the present invention for the production of such ceramics, wil be described.

Firstly, as the Mgo component as the main starting material, MgO powder having a high purity is employed. The MgO powder to be employed here has an extremely high purity, specifically at least 99% by weight, preferably at least 99.5% by weight, as chemically analyzed, in order to obtain ceramics in which mgO crystal grains are present in a high purity.

Such a highly pure MgO powder may be obtained, for isntance, by baking highly pure Mg(OH)2 at e.g.

800"C for 2 hours.

Further, in order to obtain ceramics having fine crystal grains, the grain size of the powder used here is required to be extremely fine. Specifically, it is required to be a fine powder having a specific surface area of at least 10 m21g, preferably at least 20 m2/g.

As the SiO2 component essential to the formation of the 2MgO.SiO2 phase attributable to the resistance to humidity, it has been found desirable to use it in a total amount with MgO of at least 99.5% by weight in the ceramics, and it is desirable to use an organic silicate solution in order to meet the requirement for the formation of the 2MgO.SiO2 phase as thin as possible on the grain surfaces of individual MgO grains.

As such an organic silicate, it is preferred to use a silicon alkoxide such as ethyl silicate, mthyl silicate or butyl silicate. Among them, ethyl silicate is most preferred. When this organic silicate is mixed with the highly pure fine MgO powder, a SiO2 coating which is capable of forming a 2MgO.SiO2 phase in the subsequent sintering step, will presumably be formed on the surfaces of MgO particles in the powder form.

However, the organic silicate such as ethyl silicate is highly hygroscopic in the atmospheric air, and is likely to be hydrated to precipitate and separate SiO2, whereby it is hardly possible to accomplish the above-mentioned surface coating of MgO particles in a powder form. Therefore, for the mixing operation, it is desirable to take care not to bring ethyl silicate in contact with air. This can be readily attained by mixing the MgO powder with an organic solvent containing the organic silicate. As such an organic solvent, there may be employed an alcohol such as ethanol which will not be hydrated with MgO.

Thus, when the MgO powder is thoroughly mixed in a solvent comprising e.g. ethyl silicate and ethanol, it is believed possible to form, on the surfaces of the MgO particles, a SiO2 coating having desirable resistance to humidity.

In order to impart adequate resistance to humidity to the ceramics of the present invention, the organic silicate is used in an amount sufficient to bring the SiO2 content in the ceramics to a level of at least 0.5% by weight, as mentioned above. On the other hand, if the SiO2 content is excessive, other properties such as heat conductivity will be impaired. Therefore, the SiO2 content in the ceramics should be 5% by weight at the maximum.

The organic silicate is most preferably used in an amount sufficient to bring the SiO2 content in the ceramics to a level of from 0.8 to 3% by weight.

The proportions of the organic silicate and the organic solvent are from 50 to 10% by weight and from 50 to 90% by weight, respectively, based on the total weight of the silicate and the solvent.

Prior to the sintering, Mg(OH)2, (C2H5)4Si and C2HSOH normally constituting, on the surfaces of MgO particles, a very thin layer capable of forming a 2MgO.SiO2 phase in the subsequent sintering, are believed to react with one another and to form

Then, the mixture in such a state is shaped into a predetermined form, usually a sheet-form, by a conventional method such as a doctor blade method or a pressing method, and then sintered to obtain ceramics of the present invention. Here, a feature resides in the sintering temperature.

Namely, according to such a process of the present invention, the sintering can readily be proceeded even when the sintering temperature is extremely low, whereby the grain growth of MgO crystals can be suppressed and highly dense ceramics can be obtained even bysintering at such a low temperature.

Specifically, in order to obtain a highly dense MgO ceramics, it is usually required to conduct the sintering at a high temperature of at least 1 600 C. Whereas, according to the present invention, adequate sintering can be conducted at a temperature of not higher than 1500 C, usually not higher than 1450 C. In most cases, adequately dense ceramics can be obtained at a temperature of about 1400 C.

On the other hand, in order to attain adequate sintering, it is usually required to conduct the sintering at a temperature of at least 1 300"C, preferably at least 1350 C.

Further, according to the process of the present invention, once the mixture has been formed into a shaped product such as a sheet, the shaped product is not required to be subjected to any preheating treatment prior to the sintering, although the shaped product may be dried. Namely, the mixture can be shaped and immediately subjected to sintering at the predetermined sintering temperature.

Thus, the MgO ceramics obtained by the present invention has adequatge strength and electrical characteristics such as insulating properties required for electrically insulating MgO substrates and maintains the characteristic of the material of high heat conductivity, while substantially improving the resistance to humidity which used to be a serious weak point of the conventional ceramics, without necessity of subjecting the obtained ceramics to any additional treatment for imparting the resistance to humidity (of course, such treatment may be applied and sometimes may prove to be effective as well). The industrial value of the present invention is substantial, for example, in that it opens a way for a further development of high performance IC substrates.

Now, the present invention will be described with reference to Examples. However, it should be understood that the present invention is by no means restricted by these specific Examples.

Example 1 Mg(OH)2 having a high purity of at least 99.90% was baked at 800"C for 2 hours to obtain a MgO powder having a specific surface area of from 30 to 40 m2Jg. To 100 g of this MgO powder, 20 cc of ethyl silicate and 50 cc of ethanol were added to prepare a mixture. This mixture was mixed in a pot mill for 3 hours, then dried, shaped by pressing and sintered at 1400 C for 1 hour, whereby a ceramic sheet having a thickness of about 1 mm was obtained.

The cross sectional structure of this ceramics was observed by a microscope, and the chemical composition was analyzed. The results were as follows.

Cross sectional structure Extremely fine MgO (periclase) crystal grains were intimately and densely sintered to one another, and a 2MgO.SiO2 phase was present in most of the grain boundaries of MgO crystals and was uniformly distributed in a total volume ratio of about 5% by weight. The 2MgO.SiO2 phase was also observed partially along the interfaces of the MgO crystals. The shape of the MgO crystals was generally spherical, and the majority of them had a grain size of from 1 to 5 m. The thickness of the 2MgO.SiO2 phase was about from 0.1 to 0.5 Fm.

Chemically analyzed values (% by weight) Entire ceramics: MgO 98.9% SiO2 1.0% Further, various characteristics and properties of the ceramics were measured. The results are shown below.

Bulk density 3.52 Flexural strength (kg/mm2) Normal temperature 28 Heat conductivity (cal/cm.sec."C) Normal temperature 0.11 300"C 0.06 Dielectric constant (100 KHz) Normal temperature 9.3 Dielectric loss (100 KHz) Normal temperature 6 x 10-5 Electrical insulation resistance (Q.cm) 300" 6 x 1011 Resistance to humidity* 0.012% Note: *As defined above.

Example 2 Mg(OH)2 having a high purity of at least 99.90% was baked at 8000C for 2 hours to obtain a MgO powder having a specific surface area of from 30 to 40 m2/g.

To 100 g of this MgO powder, 20 cc of ethyl silicate and 50 cc of ethanol were added to prepare a mixture, and 100 cc of a 12% polyvinylbutyral solution was added as a binder. The mixture was thoroughly mixed and shaped into sheets. One sheet was sintered at 1450"C for 1 hour, and another sheet was sintered at 1440 C for 1 hour, whereby two kinds of ceramic sheets having a thickness of about 0.5 mm were obtained.

The cross sectional structures of these ceramics were observed by a microscope, and the compositions were chemically analyzed. The results were substantially the same as those obtained in Example 1.

Various characteristics and properties of the ceramic sheets were measured. The results are shown below.

A designates the ceramic sheet sintered at 1 450 C, and B designates the ceramic sheet sintered at 1 440"C.

The data not identified by A or B are common to A and B.

Bulk density A: 3.50 B: 3.49 Flexural strength (kg/mm2) Normal temperature 27 Heat conductivity (cal/cm.sec."C) Normal temperature 0.11 300"C 0.06 Dielectric constant (100 KHz) Normal temperature 9.3 Dielectric loss (100 KHz) Normal temperature A: 1.3 x 104, B: 2.4 x 10-4 Electrical insulation resistance (Q.cm) 300"C 6 x 1011 Resistance to humidity 0.012% Comparative Example 7 A porous MgO ceramics disclosed in Japanese Unexamined Patent Publication No. 217480/1983 was impregnated with an organic silicon compound solution and the surface was burned to form, on the surface, a coating layer composed mainly of 2MgO.SiO2. The data on the MgO ceramics thus obtained are shown below.

Bulk density 3.40 Flexural strength (kg!mm2) Normal temperature 24 Heat conductivity (cal/cm.sec."C) Normal temperature 0.07 300"C 0.04 Dielectric constant (100 KHz) Normal temperature 9.3 Dielectric loss (100 KHz) Normal temperature 2 x 10-4 Electrical insulation resistance (Q.cm) 300"C 6 x 1011 Resistance to humidity 0.03 MO Comparative Example 2 A shaped product was prepared in the same manner as in Example 1 except that the MgO powder was mixed in ethanol containing no ethyl silicate. The shaped product was not adequately sintered at a temperature of 1 400"C. Therefore, the shaped product was sintered at 1 600 C, and the ceramics thereby obtained was subjected to the measurement of resistance to humidity, whereby the ceramics disintegrated and it was impossible to conduct the measurement.

Claims (21)

1. MgO ceramics for electrically insulating substrates, having a microstructure wherein microscopically the majority of MgO crystals has a grain size of at most 5 zm and a thin 2MgO.SiO2 phase is uniformly distributed at the grain boundaries of MgO crystals, and having a heat conductivity of at least 0.08 cal/cm.sec.CC at normal temperature, a dielectric constant at 100 KHz of at most 10 at normal temperature, an electrical insulation resistance of at least 1010 Q.cm at 300'C and an excellent resistance to humidity.

2. The MgO ceramics according to Claim 1, wherein the 2MgO.SiO2 phase is also present at least partially at the grain surfaces of MgO crystals.

3. The MgO ceramics according to Claim 1, wherein the chemically analyzed value of MgO crystals is at least 99.5% by weight.

4. The MgO ceramics according to Claim 1, which has a bulk density of at least 3.4.

5. The MgO ceramics according to Claim 1, which contains from 0.5 to 5% by weight of SiO2 and has a total content of SiO2 and MgO being at least 99.5% by weight, as chemically analyzed.

6. The MgO ceramics according to Claim 1, which has a flexural strength of at least 25 kg!mm2 at normal temperature.

7. The MgO ceramics according to Claim 1, which has a heat conductivity of at least 0.1 calacm.sec. C at normal temperature.

8. The MgO ceramics according to Claim 1, which has a resistance to humidity such that when the ceramics is held in an autoclave for 2 hours in a steam atmosphere under a pressure of 5 atm. at 150"C, the weight increase due to the change of MgO to Mg(OH)2 is at most 0.02%.

9. The MgO ceramics according to Claim 1, which has a dielectric loss (tan b) at 100 KHz of at most 3 x 10-4 at normal temperature.

10. The MgO ceramics according to Claim 1, which has a dielectric loss (tan 5) at 100 KHz of at most 1 x 10-4 at normal temperature.

11. A process for producing MgO ceramics for electrically insulating substrates, which comprises thoroughly mixing a MgO powder having a MgO purity of at least 99% by weight, as chemically analyzed, and a specific surface area of at least 10 m2/g in an organic solvent containing an organic silicate, followed by shaping and sintering to obtain MgO ceramics having a microstructure wherein microscopically the majority of MgO crystals has a grain size of at most 5 llm and a thin 2MgO.SiO2 phase is uniformly distributed at the grain boundaries of MgO crystals, and having a heat conductivity of at least 0.08 calxcm.sec. C at normal temperature, a dielectric constant at 100 KHz of at most 10 at normal temperature, an electrical insulation resistance of at least 1010 Q.cm at 300"C and an excellent resistance to humidity.

12. The process according to Claim 11, wherein the MgO powder has a MgO purity of at least 99.5% by weight.

13. The process according to Claim 11, wherein the MgO powder has a specific surface area of at least 20 m2/g.

14. The process according to Claim 11, wherein the organic silicate is incorporated in the organic solvent in an amount of from 0.5 to 5% by weight as the amount of SiO2 remaining in the ceramics.

15. The process according to Claim 11, wherein the organic silicate is a silicon alkoxide.

16. The process according to Claim 15, wherein the silicon alkoxide is ethyl silicate.

17. The process according to Claim 11, wherein the organic solvent is an alcohol.

18. The process according to Claim 17, wherein the alcohol is ethanol.

19. The process according to Claim 11, wherein the sintering temperature is not higher than 1500"C.

20. The process according to Claim 11, wherein the sintering temperature is not higher than 1450"C.

21. MgO ceramics, substantially as described.