INCREASING ALUMINUM NITRIDE THERMAL CONDUCTIVITY VIA VAPOUR-PHASE CARBON

Abstract: A method for producing densified aluminum nitride articles having enhanced thermal conductivity is disclosed. The method comprises the steps of forming a powder compact comprising aluminum nitride alone or in combination with other ceramic compounds, adding a densification aid to the compact, and at least partially densifying the compact via sintering or hot pressing to provide a semi-dense article. This semi-dense article is thereafter exposed to an atmosphere containing vapor phase carbon for a period of time sufficient to allow the vapor phase carbon to reduce the level of oxygen contained within the article. The article thus produced is a dense aluminum nitride article having a high thermal conductivity.

Description: Increasing aluminum nitride the mal conductivity via vapour-phase carbon. Description Background of the Invention As the electronics industry advances toward higher circuit densities, efficient thermal manage¬ ment will assume increasing importance. The removal of heat from critical circuit components through the circuit substrate is directly dependent on the thermal conductivity of the substrate. Beryllium oxide (BeO) has traditionally been the ceramic of choice for applications requiring electrically insulating materials having high thermal conduc¬ tivity. Unfortunately, beryllium oxide is toxic to a small fraction of the general population, thus leading to a significant reluctance to use it. Alumina (Al 2 0_) is nontoxic and is easily fired to full density at 1500-1600 β C; however, its thermal conductivity of between about 20 to about 30 /m° is about one order of magnitude less than that of BeO (which has a thermal conductivity of about 260 W/m°K) . Additionally, the coefficients of thermal expansion (CTE) over the range of 25-400°C for alumina (6.7 x lθ "*6 /°C) and beryllia (8.0 x 10~ 6 /°C) are not well matched to those of semiconductors such as silicon (3.6 x 10~ /°C), and gallium arsenide (5.9 x 10 —6/°C). Thus, alumina and beryllia provide less than ideal results when used in applications such as integrated circuit substrates through which heat transfer is to occur. In contrast, the CTE for aluminum nitride (AlN) is 4.4 x lθ~ /°C, a value which is well matched to both of the previously described semiconductor materials. In addition to having a CTE which makes it compatible with materials such as silicon and gallium arsenide, AlN can be sintered to provide shaped ceramic articles. Additionally, AlN articles are amenable to a variety of metallization pro¬ cesses. As such, AlN has repeatedly been suggested as a ceramic substrate for semiconductor applica¬ tions. Although a variety of attempts to produce sintered AlN parts having high thermal conductivity are described in the literature, these generally have achieved limited success. There is extensive literature on the sintering of AlN using a variety of sintering or densification aids. The bulk of the literature centers around the use of oxides of either rare earth elements (i.e., yttrium and lanthanide series elements) , oxides of alkaline earth elements (i.e., the Group IIA ele¬ ments) , and mixtures thereof. These include com¬ pounds such as Y 2 ° ' La 2 ° ' Ca0 ' Ba0 / a - ~- SrO. A system using Y,0_ and carbon is described in a variety of patents, such as U.S. Patent No. 4,578,232, U.S. Patent No. 4,578,233, U.S. Patent No. 4,578,234, U.S. Patent No. 4,578,364 and U.S. Patent No. 4,578,365, each of Huseby et al.; U.S. Patents 3,930,875 and 4,097,293 of Komeya; and U.S. Patent 4,618,592 of Kura oto. Additionally, there is a wide variety of patents using Y 2 0 and YN including U.S. Patent 4,547,471 of Huseby et al. In the Huseby et al. patents which relate to the Y O_ and carbon system, described above, AlN samples which are doped with Y 2 ° 3 and cari:5θn are heated to 1500-1600°C for approximately one hour. The carbon serves to chemically reduce A1_0_ phases contained in the AlN, thereby producing additional AlN and lowering the overall oxygen level in each part. The patents state that the Y 2 ° 3 s i n-ter i n g aids are unaffected by this process. The parts are then sintered at about 1900°C. Thermal conduc¬ tivities as high as 180 /m°K have been reported for carbon treated samples produced by the methods described in these patents. Some evidence indi¬ cates, however, that these methods may introduce residual, free carbon within the sintered AlN piece, and this residual carbon can act to decrease the dielectric constant and loss throughout the piece. These effects may be undesirable in electronic applications, although acceptable in many other applications. Additionally, two other patents of Huseby et al. (U.S. Patent No. 4,478,785 and U.S. Patent No. 4,533,645) disclose a similar process that does not make use of a Y 2 ° 3 sintering aid. Other techniques for the production of sintered AlN and high thermal conductivity AlN have also been disclosed. See, for example, U.S. Patent No. 4,659,611 of Iwase et al. , U.S. Patent No. 4,642,298 of Kuramoto et al., U.S. Patent No. 4,618.592 of Kuramoto et al. , U.S. Patent No. 4,435,513 of Komeya et al., U.S. Patent No. 3,572,992 of Komeya et al., U.S. Patent No. 3,436,179 of Matsuo et al., European Patent Application No. 75,857 of Tsuge et al., and U.K. Patent Application 2,179,677 of Taniguchi et al. Thermal conductivities of up to 200 /m°K have been reported in parts sintered from mixtures of 1-5% ϊ 2 0- and an aluminum nitride powder containing a low oxygen level (for example, an oxygen content less than 1.0%). See, for example, K. Shinozaki et al. , Seramikkusu, 21(2) ;1130 (1986). In a presenta¬ tion at the 89th Annual Meeting of the American Ceramic Society, (Pittsburgh, Pennsylvania, May 1987) , Tsuge described a three stage process for increasing the thermal conductivity of sintered AlN parts. Further treatment of these parts for as long as 96 hours to remove the yttrium aluminate grain boundary phase reportedly can increase the thermal conductivity to 240 /m°K. Finally, by treating these samples to increase the average grain size, thermal conductivities which approach the theoreti¬ cal thermal conductivity of 320 /m°K have been reported. This method, however, requires lengthy, multiple, independent steps to increase the thermal conductivity of the aluminum nitride material and produces sintered parts having large grain sizes. Additionally, the ultra-high thermal conductivity samples which have been produced to date contain extremely low levels of both oxygen (less than 400 ppm) and yttrium (less than 200 ppm) . Finally, German Patent DE 3,627,317 to Taniguchi et al. describes the use of mixtures of alkaline earth and rare earth halides and oxides to produce aluminum nitride parts that are reported to have thermal conductivities as high as 250 /m°K. This technology, however, has been demonstrated only with parts which are relatively thick (e.g., 6 mm or more) . Thin samples, (on the order of 3 mm) such as those associated with circuit substrate applications exhibit significantly lower thermal conductivities, e.g. 170 - 205 W/m°K. None of the processes described above teach the production, via a simple firing program, of sintered AlN articles having a thickness below about 6 mm and a thermal conductivity above about 220 /m°K. Additionally, each of the processes described above requires either the use of solid-phase carbon or carbonaceous compounds, or extended firing schedules to increase the thermal conductivity of the final sintered part. The use of solid-phase carbon or carbonaceous compounds interfers with the ability to increase the thermal conductivity of previously sintered aluminum nitride parts, and has the potential for leaving porosity in AlN greenware following the heat treatment step. This porosity may result in non-uniform sintering. Since AlN is a material with a number of unique properties which render it particularly useful in electronic and structural applications, it is particularly desirable to develop a method for the production of high thermal conductivity aluminum nitride which is simple and does not require ex¬ tremely long firing times to produce a dense article. Summary of the Invention This invention pertains to a method for produc¬ ing dense aluminum nitride articles having high thermal conductivity and articles produced thereby. More specifically, this invention pertains to a • method for producing aluminum nitride articles having a high thermal conductivity in which vapor phase carbon is used to reduce the oxygen content of the article subsequent to densification. In a broad sense, the invention comprises the steps of provid¬ ing a powder compact of aluminum nitride and a densification aid, densifying the powder compact via sintering or hot pressing to provide a dense article and then exposing the densified article to an atmos¬ phere containing vapor-phase carbon. The exposure should be for a period of time long enough to allow the vapor-phase carbon to remove a desired amount of oxygen contained within the article. The article is then cooled and removed from the oxygen-reducing en¬ vironment. The method has been found to result in the production of dense, aluminum nitride articles having a density approaching the theoretical density of aluminum nitride, and a high thermal con¬ ductivity. The appearance of the article can vary from cream to gray to black in color; the black parts being essentially opaque to both visible and infrared radiation. The advantages of the present invention include the ability to produce dense AlN articles having high thermal conductivity from ceramic powders having oxygen present, e.g. greater than 0.5% oxygen by weight. This allows the production of thin AlN articles having higher thermal conductivity than previously obtainable while eliminating the re¬ quirement for an extended firing schedule. Ad¬ ditionally, the present invention eliminates the need to mix free carbon powder into the powder compact, thereby allowing the production of a higher quality article. Another advantage of the present invention is that the vapor phase carbothermal process (VPCP) described herein can be used to increase the thermal conductivity of densified AlN articles produced by other methods. This invention provides an AlN article which is electrically insulating and thermally conductive. The article possesses a low dielectric constant and a coefficient of thermal expansion close to that of both silicon and gallium arsenide. Such an article is suitable for use as a substrate material for electronic components. In addition, aluminum nitride articles which are both partially semi- conductive and thermally conductive can also be produced. These articles, which are typically black in color, can be provided for applications in which optical opacity is desired. Detailed Description of the Invention The thermal conductivity of dense aluminum nitride (AlN) is very sensitive to metallic impur¬ ities as well as to oxygen ions located within the crystallites of both polycrystalline and single crystal samples. In addition, AlN is difficult to densify by a sintering or hot pressing process without the addition of a densification aid. Typically, these aids form an oxide-based, inter- granular liquid phase which facilitates oxygen removal, AlN diffusion and densification. There¬ fore, it is desirable, when densifying AlN, to utilize a densification aid and process which facilitates the production of dense AlN articles having a minimum amount of oxygen present. Addi¬ tionally, the process should be one which minimizes the introduction of undesired impurities. For elec- tronics applications, the resulting sintered or hot pressed aluminum nitride articles should have thermal conductivities greater than 130 /m°K. Oxygen exerts a critical influence during the densification of AlN. Although intergranular oxide-based liquid phases are required for densi¬ fication, oxygen remaining within the AlN grains upon the completion of consolidation limits the thermal conductivity of the article. This limita¬ tion of the thermal conductivity is believed to result from aluminum vacancies in the lattice caused by the oxygen within the AlN crystallites. These vacancies limit the thermal conductivity of the densified article. The oxygen concentration contained within the article can be determined indirectly from a measurement of bulk oxygen and the level of remaining densification aid, coupled with X-ray diffraction data on the phases of densifying materials present in the article. In conventional sintering processes, AlN is mixed with a sintering aid. While these sintering aids generally comprise rare earth oxides, alkaline earth oxides and mixtures thereof, halides, sui¬ cides, nitrides, borides, hydrides and carbides of the rare earth and alkaline earth elements can be used as well. Alternatively, rare earth metals, alkaline earth metals and mixtures thereof can be used as suitable densification aids. A preferred rare earth oxide useful as a sintering aid is yttrium oxide ( γ 2 0 3 ) • The mixture of AlN and densification aid is then formed into a shape by any of a variety of techniques. Tape casting, dry pressing, roll compaction, injection molding or any other suitable method can be used. The shape can then be sintered at between about 1600 and about 2200°C to form a dense AlN article. As used herein, the term "dense" refers to an article having a density of at least about 95% of the theoretical density of AlN. The present invention relates to a method for the production of AlN articles having high thermal conductivities and which are produced from ceramic powders. The methods are further characterized by the ability to produce high thermal conductivity AlN articles without the addition of solid-phase carbon or carbon compounds to the ceramic powder compact prior to densification. Instead, vapor- phase carbon species which remove oxygen from the article sub¬ sequent to the densification process are provided. Thus, this method can be used to produce sintered or hot pressed AlN articles having high thermal con- ductivity and can also be used to increase the thermal conductivity of such articles which have previously been densified. Additionally, this treatment can be used to render the article opaque to both visible and infrared radiation. Thermal conductivities are considered "high," as that term is used herein, to indicate thermal conductivities which are significantly increased over those of dense aluminum nitride articles which have not been treated according to the present invention. Such thermal conductivities are preferably at least 25% greater than those obtained by dense articles without treatment. The term "AlN article" is used herein to include AlN composites. Such composites are formed by adding one or more ceramic powders in addition to AlN, to the powder compact. Such AlN composites can contain up to about 90%, by weight, of ceramic powders in addition to AlN (based on total ceramic powder in the compact) . Preferably such composites contain at least about 50%, by weight, of AlN. One example of an additional ceramic powder suitable for AlN composites is BN. This compound contributes machinability, and can lower the di¬ electric constant of AlN articles for use as an electronic substrate or for complex heat sink applications. Another example of an additional ceramic powder suitable for AlN composites is SiC, which adds hardness to articles including AlN and absorbs microwave energy. Thus, such an article of high thermal conductivity could be used as a cutting tool insert where hardness is important, or for radar observing applications such as for stealth aircraft. In general, those skilled in the art will be able to form dense AlN articles of high thermal conductivity exhibiting the advantageous properties of constituent ceramic materials. By proportion¬ ately mixing combinations of ceramics to form a dense AlN article of the present invention, and employing the methods of the present invention to form such dense articles of high thermal conduc¬ tivity, a balance of properties can be obtained suitable for prescribed applications of use. One preferred AlN article is formed from a powder compact wherein the ceramic powders consist essentially of AlN. Resultant dense AlN articles exhibit a balance of physical properties desired, including thermal conductivities of 130 W/m°K or more. The first step in the process is the prepara¬ tion of a powder compact through any of a variety of processes including tape casting, dry pressing, injection molding, roll compaction, etc. The powder compact can be formed of AlN or mixtures of AlN and other ceramic powders including, but not limited to, BN, SiC, B 4 C, Si 3 N 4 , iB 2 , TiC, etc. A densification aid is typically added to the compact. The densification aid increases the density and/or facilitates densification of the powder compact during densification. In the prefer- red embodiment, the densification aid comprises Y- > 0_ and is equal to less than about 5% by weight that of the powder compact. Subsequent to the formation of a semi-dense compact (i.e., an article having a density of at least about 50% of the theoretical density) , the semi-dense compact is treated to reduce its oxygen content. It should be noted herein that the the¬ oretical density is a function of sintering aid concentration. Thus, for example, when Y,0_ (having a density of about 5.01 g/cm 3) i.s added to an AlN compact in the amount of about 3% by weight, the theoretical density of the resultant AlN compact densification aid is about 3.30 g/cm 3. Various conventional sintering supports can be used. For example, the powder compact can be sintered in a boron nitride (BN) crucible within a bed of AlN or BN powder. In this example, the thermal conductivity of an AlN article after sin¬ tering will be between about 50 and about 130 W/m°K. Additionally, if an AlN powder compact and densi¬ fication aid are of high purity, the sintered articles can be translucent. The thermal conductivity of the densified articles can be increased substantially by a subsequent treatment in the presence of a vapor phase carbon source. Vapor phase carbon diffuses readily through the sample and acts to remove oxygen contained within the grain boundaries of the densified material. The use of the vapor phase carbon to reduce the oxygen content of densified AlN, referred to herein as a vapor phase carbo- thermal reduction (VPCR) , results in dense aluminum nitride parts having high thermal conductivities. The articles produced by this process are often black in color, thereby rendering them essentially opaque to both visible and infrared light. The opaque characteristic may be useful for the pro¬ tection of light sensitive materials and masking of cosmetic flaws in the interior of the ceramic. The vapor phase carbon of this carbothermal reduction can be provided by a variety of sources. In one embodiment, vapor phase carbon is introduced into the sintering chamber from an external source. Preferred vapor phase carbon sources include gasses such as CO, lower gas phase hydrocarbons and mix¬ tures thereof; however, any carbon-containing gas that can remove oxygen from dense AlN articles is suitable. CO, CH. , C 2 H., C-H g and C_H g are partic- ularly preferred. Additionally, hydrogen-containing gasses such as H , NH_, other inorganic hydrogen- containing gasses and gas phase hydrocarbons can be used as an aid in transporting the carbon-containing gas during the carbothermal reduction process. In other embodiments, the vapor phase carbon can be provided by sources contained within the sintering chamber. For example, the carbon can volatilize from the heating elements within a graphite furnace, the carbon can volatilize from setters upon which the compact is placed, the carbon can be introduced via small quantities of vacuum pump oil that are allowed to enter the densification chamber, or fine carbon can be added to the aluminum nitride or boron nitride embedding powder used to contain and support the powder compact samples during the densification or subsequent carbothermal reduction phases of the process. The present invention will now be more fully illustrated by the following exemplification. Exemplification Aluminum nitride powder having the following characteristics was used: Agglomerated particle size (urn) 1.5 Ultimate particle size (um) 0.3 2 Surface area (m /g) 3.5 Al (wt%) 65.2 N (wt%) 33.4 O (wt%) 1.0 C (wt%) 0.06 Ca (ppm) 75 Mg (ppm) 20 Fe (ppm) 20 Si (ppm) 104 Other metals (ppm) 10. The AlN powder was mixed with a sintering aid comprising 0, 1, 3, and 5% by weight 2 0 3 (99.99% pure) and 1% by weight CaO in 2-propanol and ball milled using AlN cylinders in a plastic jar. This material was dried under vacuum and maintained in a dry environment. Approximately 2 gram samples of the powders were die pressed using a 7/8 inch steel die to a density of approximately 52% of the the¬ oretical density. These samples were then sur- rounded with a low surface area, high purity AlN embedding powder in a BN crucible with a BN lid and placed in a carbon resistance furnace. Under an atmosphere of N 2 , the temperature was increased to 1900°C where the samples were held for six hours. A vacuum was then applied to the furnace for a period of six hours. The vacuum applied to the furnace facilitated the sublimation of carbon from the furnace elements. Thus, a finite amount of carbon or carbon containing compounds were present in the sintering atmosphere. The furnace was then cooled and the sintered AlN articles were removed. Sintered aluminum nitride articles serving as controls which were not subject to the application of vacuum resulted in samples that were a translu¬ cent creamy white to gray in color. The samples that were subjected to the vacuum treatment were an opaque black or gray color. The thermal conduc¬ tivities of the samples were measured via the laser flash method. The samples were coated with a thin layer of graphite to prevent transmission of laser radiation through the sample, as well as to increase the absorptivity and emissivity of the front and rear surfaces respectively. Oxygen was measured using neutron activation. The results of both are summarized in Table 1. As can be seen in Table 1, the exposure of the articles to vapor phase carbon has acted to reduce oxygen levels while simultaneously and substantially increasing the thermal conductivity of the in¬ dividual articles. The carbon level was measured by combustion in the carbother ally reduced 3% Y 0 sample to determine if carbon was incorporated in the parts. The carbon level was below the detect¬ able limit which is approximately 0.1% by weight. It was found that the thermal conductivity of the VPCR-treated articles can vary across a cross section of the article. For example, the sample containing 3% Y.,0_ was ground to approximately one-half of its original thickness and the thermal conductivity was found to have increased to 203 W/mK. The increase in thermal conductivity results from a region having greater internal conductivity which is contained within the article. The con¬ centration of yttrium in the samples, measured by X-ray fluorescence, was only slightly affected by the carbon treatment. Similar results have been obtained for articles which were vacuum treated for periods as short as about 5 minutes. Thus, the six hour treatment described in the example is likely longer than necessary to achieve desired results. Equivalents Those skilled in the art will recognize, or be able to ascertain using no more than routine ex- perimentation, many equivalents to the specific embodiment of the invention described herein. Such equivalents are intended to be encompassed in the following claims.

Claims: CLAIMS 1. A method for producing a dense AlN article having a high thermal conductivity from an oxygen-containing AlN powder compact, com- prising the steps of: a) at least partially densifying the powder compact; and thereafter b) exposing the dense AlN article to an atmosphere containing vapor phase carbon under conditions sufficient to reduce the oxygen content to a level which provides a dense AlN article having high thermal conductivity. 2. A method of Claim 1 wherein said AlN powder compact includes a densification aid. 3. A method of Claim 2 wherein the powder compact contains ceramic powder components consisting essentially of AlN. 4. A method of Claim 3 wherein the densification aid is Y 2 0 3 - 5. A method of Claim 4 wherein the AlN article is exposed in step (b) to an environment which reduces the oxygen content to a level suf¬ ficient to produce a dense AlN article having a thermal conductivity of at least 130 W/m°K. 6. A method of Claim 2 wherein the powder compact contains ceramic powder components consisting essentially of AlN and BN. 7. A method of Claim 2 wherein the powder compact contains ceramic powder, components consisting essentially of AlN and Sic. 8. A method of Claim 2 wherein the atmosphere containing vapor phase carbon is selected from the group consisting of CO, lower hydrocarbons and mixtures thereof. 9. A method for producing a dense AlN article having a high thermal conductivity from an oxygen-containing AlN powder compact, com¬ prising the steps of: a) at least partially densifying the powder compact; and thereafter b) exposing the AlN article to an atmosphere containing vapor phase carbon under conditions sufficient to reduce the oxygen content to a level which provides a dense AlN article having a thermal conductivity of at least 130 W/m°K, the AlN article formed from a powder compact containing a densification aid and having a ceramic powder component consisting essentially of AlN. 10. A method of Claim 9 wherein the densification aid is Y 2 0 3 - 11. A method of Claim 10 wherein the atmosphere containing vapor phase carbon is selected from the group consisting of CO, lower hydrocarbons and mixtures thereof. 12. A method of Claim 11 wherein the AlN article is exposed in step (b) to a temperature of between about 1,700° C to about 1,900° C under vacuum for at least one hour to reduce the oxygen content to a level which provides a dense AlN article having a thermal conductivity of at least 130 W/m°K. 13. A method of Claim 9 wherein the AlN article is exposed in step (b) to an environment which reduces oxygen content to a level sufficient to produce a dense AlN article having an oxygen content below about 0.6% by weight. 14. A method for producing a dense AlN article having a high thermal conductivity from an oxygen-containing AlN powder compact, com¬ prising the steps of: a) at least partially densifying the powder compact; and thereafter b) exposing the dense AlN article to an atmosphere containing vapor phase carbon under conditions sufficient to reduce the oxygen content to a level which provides a dense AlN article having a high thermal conductivity, the AlN article formed of an AlN powder compact containing a densifi- • cation aid and having a ceramic powder component consisting essentially of AlN and BN. 15. A method for producing a dense AlN article having a high thermal conductivity from an oxygen-containing AlN powder compact, com¬ prising the steps of: a) at least partially densifying the powder compact; and thereafter b) exposing the dense AlN article to an atmosphere containing vapor phase carbon under conditions sufficient to reduce the oxygen content to a level which provides, a dense AlN article having a high thermal conductivity, the AlN article formed of an AlN powder compact containing a densifica¬ tion aid and having a ceramic powder component consisting essentially of AlN and SiC. 16. A dense AlN article which consists essentially of AlN having a thermal conductivity of at least 130 W/m°K. 17. A dense AlN article which consists essentially of AlN and BN and having high thermal conduc¬ tivity. 18. A dense AlN article which consists essentially of AlN and SiC and having a high thermal conductivity. 19. A dense AlN article having a high thermal conductivity formed by a method of Claim 1. 20. A dense AlN article having a thermal con- ductivity of at least 130 W/m°K formed by a method of Claim 9. 21. A dense AlN article having a high thermal conductivity formed by a method of Claim 14. 22. A dense AlN article having a high thermal conductivity formed by a method of Claim 15.

Inventors: KNUDSEN ARNE K, PAQUETTE MIKE S, KNUDSEN, ARNE, K., PAQUETTE, MIKE, S.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1989
Main classification: C04B35
IPC codes: C04B35, C04B41