CA2171548A1 - Aluminum nitride sintered body with high thermal conductivity and its preparation - Google Patents

Abstract

An aluminum nitride sintered product with a high thermal conductivity (at least 100 W/m.K) can be prepared at a sintering temperature of less than 1850 °C (often less than 1650 °C) using a sinterable combination of aluminum nitride powder with at least three sintering aids.
The sintering aids include a source of a rare earth metal oxide, a source of an alkaline earth metal oxide, a boron source and, optionally, a source of aluminum oxide. The sinterable combinations may also be used to prepare cofired, multilayer substrates.

Description

WO 95102563 ~1~15 ~ ~ PCT/US94/04256 .

ALUMINUM NITRIDE SINTERED BODY WITH HIGH THERMAL CONDUCTIVITY AND ITS
PREPARATION

Backqround of the Invention The invention relates to a sintered body of aluminum nitride (AIN) with high thermal conductivity and a process for preparing the sintered body at a relatively low sintering temperature. The invention relates more particularly to ternary sintering aid combinations that enable preparation of the sintered body at temperatures of 1850 Centigrade (C) or less, preferably 1650C or less. The sintered body is suitable for use in a variety of known 10 applications including integrated circuit substrates, integrated circuit heat sinks and packaging components or multichip module components. The sintered body may also be used instructural applications such as crucibles and components of armor.
Al N is an excellent material having a high thermal conductivity, insulation resistance and a low thermal expansion coefficient among its desirable properties. However, sinceAlNisacovalentbondingcompound,itisquitedifficulttoproduceapureAlNsintered product without using sintering aids or a hot-press sintering method.
Sintered AIN bodies are typically prepared by heating an admixture of AIN
powderandoneormoresinteringaidstoatemperaturewithinarangeoffrom 1500Ctoas highas2100Cinanatmospherethatpromotessintering. Thesinteringaidstypicallyinclude 20 one or more oxides of alkaline earth metals or oxides of rare earth metals. Kasori et al. (U.S.
Patent No.4,746,637) use a sintering aid combination of yttrium oxide tY2O3) and calcium oxide (CaO) to sinter AIN powder at a temperature of 1650C or above. Other sintering aids may be used in place of, or in addition to, the alkaline earth metal oxides and rare earth metal oxides. Okuno et al. (U.S. Patent No. 4,877,760) use at least one boride, carbide or nitride of 25 titanium, zirconium, hafnium, vanadium, niobium or tantalum or boride or carbide of chromium, molybdenum or tungsten and, optionally, other sintering additives such as alkaline earth metal oxides and rare earth metal oxides. The sintering aids, including optional sintering aids, should not exceed 5 parts by weight per 100 parts by weight of Al N.
JP H03-146471 discloses mixtures of at least one oxide of yttrium, scandium or a30 lanthanide and at least one of lanthanum hexaboride (LaB6), magnesium hexaboride and calci um hexaboride as sintering aids. JP H03- 197366 discloses mixtures of calcium oxide and LaB6 as sintering aids. These combinations of sintering aids lead to AIN sintered products that show a high thermal conductivity, but prefer a sintering temperature of 1900C or above. Such temperatures make it necessary to use an expensive high temperature sintering furnace and 35 fittings, such as a setter, capable of use at those temperatures. In addition, such temperatures result in high energy costs.
JP H04- 130064 discloses sintering aids that are mixtures of a boron based compound such as boron nitride, boron carbide, boron oxide or boron fluonde, at least one ~ ~ 7~8 oxide, carbide, nitride, or boride of titanium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, molybdenum, cadmium, tin or tungsten and at least one alkaline earth metal oxide or rare earth metal oxide.
Summarv of the Invention A first aspect of the invention is a process for producing an aluminum nitride (AIN) sintered productwith a high thermal conductivity at a relatively low sintering temperature. For this aspect, an AIN powder having a specific surface area in a range of about 3 to 8 m2/g, and preferably 4.5 to 7.5 m2/g, and an oxygen content between 0.5 and 1.8 wt%, is used. Optimum amounts of sintering aids are combine.. with the AIN powder. The sintering 10 aids substantially constitute a combination of three sintering aids (I), (Il) and (Ill). Sintering aid (I) is at least one selected from the group consisting of rare earth oxides and rare earth compounds. The rare earth compounds are converted to corresponding rare earth oxides during sintering. Sintering aid (I) is incorporated such that an equivalent rare earth oxide amountthereofisinarangeofO.5to10wt%,basedonweightoftheAlNsinteredproduct.
Sintering aid (Il) is at least one selected from the group consisting of alkaline earth oxides and alkaline earth compounds. The alkaline earth compounds are converted to corresponding - alkal i ne earth oxides during sintering. Sinteri ng aid (I l) is i ncorporated such that an equivalent rare earth oxide amount thereof is in a range of 0.1 to 5 wt%, based on weight of the Al N
sintered product. Sintering aid (Ill) is at least one selected from the group consisting of LaB6, 20 NbC, and WB. An additive amount of LaB6 is in a range of 0.05 to 3 wt%, based on weight of the AIN sintered product. A resulting mixture is compacted to a desired shape, and then sintered in a non-oxidative atmosphere at a sintering temperature of 1 650C or below to provideasintered productwith a highthermal conductivity~1f 120W/m Kormore.
A second aspect of the invention is a sinterable aluminum nitride powder 25 composition comprising aluminum nitride powder and a sintering aid combination that consists essentially of a rare earth metal oxide source, an alkaline earth metal oxide source, a boron source selected from the group consisting of aluminum boride, aluminum diboride, calcium boride, yttrium boride, strontium boride, barium boride, cerium boride, praseodymium boride, samarium boride and neodymium boride, and, optionally, a source of al uminum oxide.
Athird aspect of the invention is a sintered aluminum nitride body having a highthermal conductivity and comprising, based upon body weight, from about 90 to about 99.5 weight percent aluminum nitride as a primary phase, from about 0.5 to about 10 weight percent of a secondary phase selected from the group consisting of alkaline earth metal aluminates, rare earth metal aluminates, alkaline earth metal-rare earth metal aluminates, 35 complex alkaline earth metal-rare earth metai oxides and mixtures thereof, and boron at a level of from about 50 to about 5000, preferably from about 50 to about 2000 parts by weight per mlllion parts of bodyweight, asdetermined bysecondary ion mass spectrometry (SIMS).
The alkaline earth metal is preferably calcium and the rare earth metal is preferably yttrium.

3 ~ :L 7 ~ 5 d~ ~ PCT/USg4/04256 .

A fourth aspect of the invention is a process for preparing a sintered aluminum nitride body having a high thermal conductivity that comprises heating the sinterable composition of the second aspect to a temperature of from about 1 570C to about 1850C, preferably from about 1 570C to about 1 650C in a nonoxidizing atmosphere for a period of 5 time sufficient to attain a density of at least 95 percent of theoreticai density.
In an aspect related to the third and fourth aspects, the sintered aluminum nitride body can be a cofired, multilayer aluminum nitride substrate fabricated by a method compnslng:
a. preparing at least two ceramic green sheets from the sinterable composition of 10 second aspect;
b. depositing a desired pattern of a refractory metal ink on at least one major planar surface of at least one ceramic green sheet;
c. preparing a laminate of a desired number of ceramic green sheets having refractory metal ink deposited thereon;
d. heating the laminate underthe conditions of the fourth aspect to effect sintering of both the ceramic sheets and the refractory metal ink deposited on said sheets.
Description of P,e~er,ed Embodiments In the field of powder metallurgy, it has been already known that as an average particle size of a powder used for obtaining its sintered product decreases, the sintering 20 temperature can generally be lowered. For example, an AIN powder having a specific surface area within a range of from 10to 14 m2/g isdenselysintered ata temperature of 1600C or below. However, as the AIN powder becomes increasingly fine, that is, the specific surface thereof increases, oxygen content of the powder also increases. When such an AIN powder having a high oxygen content is sintered, the sintered product has a low thermal conductivity.
25 For example, when AIN powder made by a conventional carbothermal reduction method has a specific surface area of more than 10 m2/g, it also has an oxygen content of more than 1.8 wt% .
Therefore, an Al N powder with a low oxygen content should be used for produci ng the Al N
productofthefirstaspect. TheAlNpowdershouldalsohaveanoptimumsurfaceareainorder to be sintered at si nteri ng temperatures of about 1 650C or below, and preferably less than 30 1 625C. These temperatures are advantageous because a relatively inexpensive ceramic, such as aluminum oxide, can be used for fittings, such as a setter, that are placed in a furnace during sintering. A more expensive ceramic for such fittings is hexagonal boron nitride. The relatively low temperatures, compared to temperatures of 1 800C or more, lead to energy savi ngs.
With regard to the first aspect of the invention, it is preferred that the AIN
35 powder have an oxygen content between 0.5 and 1.8 wt% and a specific surface area in a range of 3 to 8 m2/g, more preferably 4.5 to 7.5 m2/g, is used for enhancing low temperature sintering thereof and improving thermal conductivity of a resulting AIN sintered product.
When the oxygen content is more than 1.8 wt%, it becomes difficult to produce an AIN

WO 95/02563 ~ PCT/US94/04256 .

sintered product with a high thermal conductivity by sintering at a temperature of about 165ûC or below. On the other hand, it is difficult and expensive to produce an AIN powder having an oxygen content less than 0.5 wt% . When the specific surface area of the Al N powder is less than 3 m2/g, the powder is not densified sufficiently at a sintering temperature of about 5 1650~C or below. An average particle size of an AIN powder suitable for use in this aspect of the invention is in a range of 0.20 micrometer (llm) to 0.46 }lm. It is also preferred that the Al N
powder be made by a carbothermal reduction method because AIN powder made by direct nitridation has an unstable aluminum oxide surface layer, so that it is possible to increase the oxygen contentoftheAlN powderduringa processforproducingtheAlN sintered product.Sinterable compositions of the second aspect of the invention are prepared by adding a sintering aid combination to aluminum nitride powder A sinterable composition is heated to a tem peratu re of from 1570C to about 1850C, preferably from about 1570C to about 1650C in a nonoxidizing atmosphere for a period of time sufficient to yield a sintered aluminum nitride body having a density of at least 95 percent of theoretical density. The resulting sintered body contains boron at a level of from about 50 to about 5000, preferably from about 50 to about 2000 parts by weight per million parts of body weight, as determi ned by SIMS.
Al N powder suitable for purposes of the invention may be of commercial or technical grade. It should not contain any i mpurities that would have a significant adverse 20 effect upon desired properties of a resulting sintered product. Although some level of impurities is present in commercial powders, that level should be less than that which produces the aforementioned adverse effect.
The Al N powder typical Iy has a bou nd oxygen cc~ntent of iess than 4 wt% . Theoxygen content is desirably less than 3 wt% and preferably less than 2 wt~/o .
The AIN powder also typically has a surface area, measured by a conventional adsorption method such as that taught by S. Brunauer, P. H. Emmett and E. Teller in Journal of the American Chemical Societv, volume 60, page 309 (1938) (herei nafter " BET"), of from 1.5 to 10 square meters per gram (m2/g). The powder surface area is desirably from 2 to 9 m2/g.
AIN powder meeting these specifications are preferably prepared either by 30 carbothermal reduction of alumina (Al2O3) ordirectnitridation of aluminum metal. AIN
powders may also be prepared by other processes using aluminum alkyls or aluminum halides.
Preferred carbothermal AIN powders are available from The Dow Chemical Company under the trade designation XUS 35544 and XUS 35548 or Tokuyama Soda under the trade designations Grade F and Grade H. Mixtures of these and other powders may also be used.
The sintering aid combination includes at least one alkaline earth metal oxide source, at least one rare earth metal oxide source, at least one boron source The si nteri ng aid combination may also include, as an optional component, at least one source of aluminum oxide (Al2O3). The alkaline earth metal oxide sources and rare eartn metal oxide sources WO 95/02563 2~ 5 ~ 8 PCT/US94/04256 .

include the oxides as well as acetates, carbonates, nitrates, hydrides, phosphates, hydroxides, aluminates, formates, oxalates and sulfates. The sources of Al2O3 include Al2O3 itself as well as aluminum acetate, aluminum hydroxide, aluminum butoxide, aluminum ethoxide, aluminum propoxide, aluminum oxalate, aluminum nitrate, aluminum phosphate and aluminum sulfate.
Sintering aid (I) is at least one of rare earth metal oxides and rare earth metal sources or compounds. The rare earth metal sources are converted to their corresponding rare earth metal oxides, rare earth metal aluminates or both during sintering. The rare earth metal oxides include yttrium oxide (Y2O3), and oxides of elements 57 through 71 of the Periodic Table of the Elernents. The oxides of said elements are lanthanum (La2O3), cerium (Ce2O3), praseodymium (Pr2O3), neodymium (Nd2O3), promethium (Pm2O3), samarium (Sm2O3), europium ~Eu2O3), gadolinium tGd2O3), terbium (Tb2O3), dysprosium (Dy2O3), holmium (Ho2O3), erbium (Er2O3), thulium (Tm2O3), ytterbium (Yb2O3), and lutetium (Lu2O3). The rare earth metal oxide is desirably Y2O3, La2O3, Ce2O2, Ce2O3, Dy2O3 or Sm2O3, preferably La2O3 or Y2O3. The rare earth metal oxide source is desirably present in an amount sufficient to provide an equivalent rare earth metal oxide content within a range of from 0.25 to 10 wt% (0.5 to 1 û
wt% for the first aspect and 0.25 to 5 wt% for all other aspects of the invention), based upon sinterable composition weight. The range is preferably from about 0.7 to about 4, more preferably from about 1 to about 3 wt%, based upon sinterable composition weight.
Sintering aid (Il) is at least one of alkaline earth metal oxides and alkaline earth 20 metal sources or compounds. The alkaline earth metal sources are converted to their corresponding alkaline earth metal oxides, alkaline earth metal aluminates or both during sintering. The alkaline earth metal oxides are magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO) and strontium oxide (SrO). Although radium is also an alkaline earth metal, its radioactivity removes it from consideration as a suitable source of a sintering aid. The 25 alkaline earth metal oxide is preferably CaO. Calcium carbonate (CaCO3) is a preferred source of CaO as it is more stable than CaO. The alkaline earth metal oxide source is desirably present in an amount sufficient to provide an equivalent al kal i ne earth metal oxide content withi n a range of from about 0.2 to 5 wt%, based upon sinterable composition weight. The range is quite satisfactory when sintering is conducted in a graphite furnace The greater amount 30 should be at least 0.5 wt%, based upon sinterable composition weight. The range is preferably from about 0.25 to about 3, more preferably from about 0.5 to about 2 wt%, based upon sinterable composition weight. If sintering is conducted in a refractory metal furnace, such as a tungsten furnace, a greater amount of alkaline earth metal oxide, especially calcium oxide, is requi red .
Sintering aid (Ill), a source of boron, is suitably: tungsten boride (WB); a compound represented as MB2 where M is a metal selected from magnesium (Mg), aluminum (Al), scandium (Sc), yttrium (Y), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum tTa), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), technetium WO 95/02~63 2 ~ 7 1 ~ 4 8 PCTJUS94/04256 (Tc), rhenium (Re), ruthenium (Ru), osmium (Os), uranium (U) and plutonium (Pu); a compound represented as M2B5 where M is a metal selected from Mo and W; a compound represented as MB4 where M is a metal selected from Ca, Y, Mo and W; MB6 where M is a metal selected from Ca, strontium (Sr), barium (Ba), Y and Lanthanides; a compound represented as MB12 where M

5 is a metal selected from Al, Sc, Y, Zr, an Actinide or a Lanthanide; a compound represented as MB66 where M is Y; ora boron rich metal oxide such as calcium borate or aluminum borate.
Other boron sources should also provide satisfactory results. Sintering aid (Ill) is capable of enhancing low temperature sintering of AIN powder and improving thermal conductivity of resulting AIN sintered products. For the first aspect of the invention only, sintering aid (Ill) may 10 be niobium carbide (NbC). The source of boron is desirably aluminum boride (AIB12), aluminum diboride (AIB2), calcium boride (CaB6), yttrium boride (YB6), lanthanum hexaboride (LaB6), strontium boride (SrB6), barium hexaboride (BaB6), cerium tetraboride (CeB4), cerium hexaboride (CeB6), praseodymium boride (PrB6), samarium boride (SmB6), WB or neodymium boride (NdB6). The source of boron is preferably AIB12, AIB2, WB, LaB6 or CaB6. The source of 15 boronforallaspectsoftheinventionotherthanthefirstaspectisdesirablypresentinanamount sufficient to provide an equivalent boron content within a range of from 0.01 to 1 wt%, based upon sinterable composition weight. The range is preferably from 0.02 to 0.5, more preferably from 0.04to 0.3 wt%, based upon sinterable composition weight.
For the first aspect of the i nvention, an optimum amount of LaB6 is i n a range of 20 0 05 to 3 wt%, based on sintered product weight. As the amount of LaB6 is increased within therange,thethermalconductivityoftheAlNsinteredproductisremarkablyimproved.
However, an amount of LaB6 in excess of 3 wt% inhibits sintering. Also with respect to the first aspect of the invention, an optimum amount of NbC or WB is in a range of 0.05 to 5 wt%, based on sintered product weight. As with LaB6, increasing amounts within the range improve 25 thermal conductivity of the Al N sintered product. However, an amount of NbC or WB in excess of 5 wt/~ inhibits sintering. In addition, the amount of sintering aid (Ill) used in the first aspect closely relates to the specific su rface area and oxygen content of the Al N powder. If the Al N
powder has a relatively smal I specific surface area and a low oxygen content, it is expected that asmallamountofsinteringaid(lll)withintherangewillbesufficienttoconverttheAlN
30 powder into a sintered product with a high thermal conductivity. On the other hand, using an AIN powder with a relatively large specific surface area and a high oxygen content requires a large amount of sintering aid (I l l) withi n the range in order to improve thermal conductivity of the sintered product. Forthe first aspect of the invention, sintering aid (Ill) should have a purity of 99.9% or more, and an average particle size of less than 10 ~m for uniformly incorporating 35 theaid intoAlN powder.
Aluminum oxide (AlzO3) may be added as a fourth component of the sintering aid combination. Preferred sources of Al2O3 include Alz03 itself and aluminum hydroxide When added, the source is desirably present in an amount sufficient to provide an equivalent Al2O3 WO 95/02563 2 1~ 15 ~ ~ PCT/US94/04256 .

content within a range of from greater than zero wt% to 2 wt%, based upon sinterable composition weight. The range is preferably from greater than zero wt% to 1 wt%, more preferably from greater than zero wt% to 0.5 wt%, based upon sinterable composition weight.
The sintering aid combination is suitably admixed with AIN powder in an amount 5 of from 0.05 wt% to 10 wt%, based upon sinterable composition weight. The amount is desirably from 0.5 wt% to 5 wt%, preferably from 0.5 to 3 wt%, based upon sinterable composition weight. Each component of the sintering aid combination suitably has a surface area similar to that of the Al N powder.
An admixture of Al N powder and the si ntering aid(s) may be prepared by 10 conventional procedures such as attrition milling and wet and dry ball milling. Wet ball milling with an appropriate solvent and suitable mi l l i ng media provides satisfactory results. Mi l l i ng media, usually in the form of cylinders or balls, should have no significant adverse effect upon admixture components or upon sintered bodies prepared from the admixture. A solvent such as ethanol, heptane or another organic liquid may be used. A suitable solvent is a blend of ethanol and chlorothene. After milling, the organic liquid may be removed by conventional procedures to yield an admixture suitable for conversion to ceramic greenware. Oven drying and spray drying produce satisfactory results.
An organic binder may be added during milling of the admixture. Suitable binders are wel I known i n the art and typical Iy comprise high molecular weight organic 20 materials that are soluble in organic solvents. Illustrative binders include polyethyloxazoline, industrial waxes such as paraffin, highly viscous polyglycols, polymethylmethacrylate and polyvinyl butyral. A blend of polyethyloxazoline in an amount of from 20 to 80, preferably from 35 to 65, wt% and polyethylene glycol in an amount of from 80 to 20, preferably from 65 to 35, wt%, based upon blend weight wherein the amounts total 100 percent, is particularly 25 suitable. The binder is suitably added to admixture components prior to milling.
Any well known dispersing aid or dispersant may also be added during milling of the admixture. Fish oil is a particularly suitable dispersant.
Ceramic greenware may be prepared by any one of several conventional procedures such as extrusion, injection molding, die pressing, isostatic pressing, slip casting, roll 30 compaction or forming or tape casting to produce a desired shape. Particularly satisfactory results are obtained by dry pressing an admixture (preferably spray dried) or tape casting a slurry.
The ceramic greenware is desirably subjected to conditions sufficient to remove the organic binder prior to sintering. Binder removal, also known as binder burn out, typically 35 occurs by heating the greenware to a temperature that ranges from 50C to 1 000C to pyrolyze, or thermally decompose, the binder. ~ suitable time and temperature combination for remo~ .g the blend polyethyloxazoline and polyethylene glycol is from 1 to 7 hours at a temperature of from 400 to 800 Centigrade (C), with four nours at a temperature of 575C

WO 95/02~i63 PCT/US94/04256 ~71548 - being particuiarly suitable. The temperature and hold time vary depending upon the binder and dimensions of the greenware. Thermal decomposition may be carried out at or near ambient pressure or in a vacuum. It may be carried out in the presence of atmospheric air or in a neutral atmosphere. The neutral atmosphere is desirably established with at least one gas 5 selected from nitrogen and a noble gases such as argon. The gas is preferably nitrogen. As a general rule, binder burn out in the presence of an inert gas such as nitrogen yields a higher residual carbon level than binder burn out in the presence of atmospheric air. Binder burnout in the presence of nitrogen is preferred for purposes of the present invention.
Conventional procedures may be used to prepare a cofired multilayer aluminum 10 nitride substrate. The procedures include using greenware in the form of sheets, depositing a conventional refractory metal ink or paste on at least one major planar surface of at least one greenware sheet, forming a laminate of a desired number of ceramic green sheets having refractory metal ink deposited thereon, and sintering the laminate to effect sintering of both the ceramic sheets and the refractory metal ink deposited on said sheets. Prior to sintering, the laminate may be subjected to a binder burnout step. A suitable refractory metal ink is a tungsten ink (Crystalero, #2003 ink).
Sintering of the greenware, after binder burnout, occurs in a nonoxidizing atmosphere established by gaseous nitrogen or a source of gaseous nitrogen and is followed by cooling in a vacuum or in a neutral atmosphere like that used forthermal debindering. The 20 source of gaseous ni lr ogen may be gaseous nitrogen, gaseous ammonia, gaseous mixtures of nitrogen and ammonia, gaseous mixtures of nitrogen, ammonia or both with an inert or noble gas such as argon, or gaseous mixtures of nitrogen, ammonia or both with hydrogen and, optionally, an inert or noble gas. A favorable sintering atmosphere may be established by placing the greenware into a crucible fabricated from a refractory material, such as boron 25 nitride, aluminum nitride, molybdenum metal or tungsten metal, prior to sintering and cooling. The greenware may also be placed on a setterwithin the crucible. The setter is preferably fabricated from the same material as the crucible. The refractory material will vary depending upon which type of furnace is used for sintering. Boron nitride and aluminum nitride are pr~rerled refractory materials for a graphite furnace, whereas molybdenum metal 30 or tungsten metal is preferred for a tungsten furnace.
Si ntering desirably occurs at a temperature of from about 1570 to about 1850C
(1650C or below for the first aspect of the invention). In accordance with the first aspect, when using intermediate surface area (4.5 to 7.5 m2/g) AIN powders and LaB6 as sintering aid (Ill), the temperature is preferably in a range of 1570C to 1640C, more preferably 1570C to 35 1625C. For all aspects of the invention and any other sintering aid (Ill), including WB or NbC
suitable for use in the first aspect of the invention, the temperature is preferably from about 1570 to about 1650C, more preferably from about 1600 to about 1650C. The sintering temperature is maintained for a period of time sufficient to attain a density of at least 95, WO 95/02563 ~ ~. 7 ~ PCT/US94/04256 preferably at least 97, percent of theoretical density. The period of time is desirably from 0.5 hour to 24 hours, preferably from 2 to 10 hours, and more preferably 6 hours. If the si nteri ng time is less than 0.5 hour, the density wil I be less than 95 percent of theoretical density unless the 5i nteri ng temperature is raised to 1 700C or higher. Although this may be done, it 5 disregards any economic or physical property advantage resulti ng from sintering at lower temperatures.
During sintering, sintering aids (I), (Il) and (Ill) react with a surface layer of aluminumoxideontheAlN powdertogenerateacomplexoxidewitharelativelylowmelting point. The complex oxide is presumably capable of removing oxygen atoms from grain 10 boundaries of the sintered AIN bodies. The removal of oxygen atoms improvesthermai conductivity of the sintered AIN bodies.
SinteredAlN bodiesoftheinvention haveathermal conductivityinexcessof 100 watts per meter-Kelvin (Wlm.K). The thermal conductivity is desirably greater than 120 W/m.K, preferably greater than 140 W/m K and more preferably greater than 150 W/m K. A
theoretical maximumthermal conductivityforsinglecrystalAlN is319W/m-K. An upperlimit for thermal conductivity is therefore 319 W/m K. An acceptable upper thermal conductivity limit for many practical applications is 230 W/m K.
Si ntered Al N bod ies of the i nvention also have a density of at I east 95 percent of theoretical density display color/translucency combinations that range from light cream and 20 translucenttodarkgrayorevenblackandopaque. Skilledartisanscanattainadesired combination of color and thermal conductivity without undue experimentation.
Sintered AIN bodies of the invention comprise, based upon body weight, from 90 to 99.5 weight percent aluminum nitride as a primary phase, from 0.5 to 10 weight percent of a secondary phase, and boron at a level of from 50 to 5000, preferably from 50 to 2000 parts by 25 weight per million parts of body weight, as determined by secondary ion mass spectrometry.
The boron is present as a boron derivative that is a secondary phase or is dispersed throughout the secondary phase(s), or is dispersed on an atom;c level within aluminum nitride's crystal lattice or a combination thereof. The secondary phase is at least Gne material seiected from yttrium aluminates, calcium-yttrium aluminates, complex calcium-yttrium oxides and mixtures 30 thereof. The secondary phase may aiso include an amount of calcium aluminate. Specific examples of secondary phase materials include Al2Y4Og, AIYO3, Al5Y3O1z, CaYAlO4, CaY2O4 and mixturesthereof.
The following examples are solely for purposes of illustration and are not to beconstrued, by implication or otherwise, as limiting the scope of the present invention.
B5 Examrole 1 Ball mill an AIN powder (1.25 wt% oxygen, specific surface area of 5.0 m2/g), si nteri ng aids and isopropyl alcohol as a solvent to prepare a m ixed powder. The si nteri ng aids are Y2O3 as (I), CaCO3 as (Il) and LaB6 as (Ill). The amounts of Y~O3, CaCO3 and LaB6 are 2.0 g WO 95/02~63 PCT/US94/04256 2171~
vvt%, 0.89 wt% and 0.1 wt%, respectively. An equivalent CaO amount of CaCO3 is 0.5 wt/~ .
The mixed powder is compacted under a pressure of 1.5 ton/cm2 (1361 kilogram (kg)/cm2) with a rubber press to a disc having a diameter of 20 mm and a height of 10 mm. The disc is set in a boron nitride setter and sintered for 4 hours at a temperature of 1 600C in a non-oxidizing 5 atmosphere i ncluding nitrogen gas to obtain a sintered product.
Examples 2-25 Repeat Example 1, but use AIN powders, sintering temperatures and sintering aid amounts as shown in Table I to obtain sintered products. Each sintered product is ground and polished to provide an Al N sintered disc having a diameter of 10 mm and a thickness of 3 mm.
10 Each disc isthen measured for relative density (% of theoretical density) and thermal conductivity (laser flash method). Results of the measurements are also shown i n Table 1.

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V~ --!t ~ tn ~ t x Q Z -- _ _ _ _ _ _ WO 95/02563 PCT/US94/042~6 2171~8 The results in Table I show that a combination of Y2O~, CaO and LaB6 provides acceptabl e Al N si ntered bod i es at tem peratu res as I ow as 1 570C with Al N powders havi ng a variety of oxygen contents and specific surface areas. Acceptable bodies have a relative density of at least 90% and a thermal conductivity of at least 100 W/m . K. The results also show that 5 while a given sinterable composition may provide unacceptable results in terms of relative density, thermal conductivity or both at a particulartemperature, a small increase in temperature produces acceptable results. Examples 17* and 18 as well as 21 * and 22 iliustrate this point.
Examales 26-35 Repeat Exampl e 1, but change at least one of the AI N powder, the si nteri ng temperature and the sintering aids as shown in Table 11. Table 11 also shows thermal conductivity measurements for resulting sintered bodies. All sintered bodies have a relative density of greater than 98% except example 37 which has a relative density of 94Ø

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WO 95/02563 21715 4 ~ PCT/US94/042S6 ~ y c ~, ~ o V

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The data in Table ll show that NbC and WB are acceptable substitutes for LaB6.
Example 39 Ball mill 100 grams (g) AIN powder (The Dow Chemical Company as XUS 35544, oxygen content of 1.1 + 0.1 wt%, carbon content of less than 0.08 wt%, both percentages being based on powder weight, and a surface area of 3.2 + 0.2 m2/g),2 9 Y2O~ powder (Unocal Molycorp,99.99% purity), 0.9 g CaCO3 powder (Fisher Scientific),0.25 9 AIB2 (Aldrich Chemical Co., Inc.) and 3.1 9 of a binder composition in 60.5 g of a solvent blend. The binder composition is a 35/65 weight ratio blend of polyethyloxazoline and polyethylene glycol 3350 (The Dow Chemical Company). The solvent blend is a 50/50 (by volume) blend of ethanol and 10 chlorothene. The binder is dissolved in the solvent blend before the Al N, Y2O3, AIB2 and CaCO3 powders are added. Ball milling continues for a period of 4 hours to provide a milled slip.
Solids contained in the milled slip are separated from most of the solvent blend using a rotary evaporator. Remaining solvent blend removal occurs via drying under vacuum at a temperature of 60C for a period of 15 hours. After drying is complete, the solids are crushed and screened through a 60 mesh (250 ~um sieve opening) sieve to provide a dried powder.
The dried powder is dry pressed into greenware using a 7/8 inch (2.2 cm) round die under uniaxial pressure of 15,000 pounds per square inch (psi) (about 103 megap~c~als (MPa)). The binder composition is removed from the greenware in the presence of flowing nitrogen (N2 BBO). Binder removal employs a heating rate of 90C/hour up to 575C, a four 20 hour hold atthattemperature and a cooling rate of 3C/min down to room temperature (25C).
After binder removal is complete, the greenware is enclosed in a boron nitride setter that is placed in a boron nitride crucible to establish a neutral environment. The crucible is placed in a graphite furnace (one cubic foot (0.028 cubic meter) capacity, Thermal Technology Model 121212G). The crucibleand itscontentsare heated, inthe presence of 25 nitrogen flowing at a rate of 2 standard cubic feet per hour (scfh) (about ~.057 standard cubic metersperhour(scmh))usingaheatingschedulethatstartswithheatingtoatemperatureof 1200Catarateof25C/min,heldat1200Cfor30minutestoensureconversionofCaCO3to CaO), heated to 1625C at a rate of 10C/min, held at 1625C for 6 hours and then cooled at a rate of 25C/mi n down to 1000C. The cruci ble contents, now sintered parts, are opaque, gray 30 in color, with a smooth surface finish. The sintered parts have a density of 3.20 g/cm3 (greater than 97% of theoretical density) and a thermal conductivity (laser flash method) of 157 W/m K.
X-ray diffraction (XRD) analysis of the sintered parts reveals yttrium aluminate (Al2Y4Og) and calcium-yttrium aluminate (CaYAlO4) as secondary phases. Boron Ka X-ray mapping using a microprobe shows a generally uniform distribution of boron containing phases. Analytical 35 transmission electron microscopy (ATEM) shows that these phases are boron nitride.
Repeati ng this example, but without the Al B2, leads to a lower density, a lower thermal conductivity, and a secondary phase chemistry that is predominantly Al2Y4Og.

WO 95102563 2 17 ~ ~ 4 8 PCT/US94/04256 .

Examples 40-56 Repeat Example 39 using the formulations shown in Table lll. The CaO amounts are equivalents resulting from higher amounts of CaC03. For example, about 0.9 wt% CaC03 results in 0.5 wt% CaO.
- 5 Table III
Den- Ther-Exam- O IB Den- sity mal Ple Y2O3 Ca A 2 sity (% Conduc-No.(9) (g) (9) (g/cm3) Theor- tivity etical) (W/m K) 40* 2.475 0.275 0 2.750 83.3 127 41 * 2 0.5 0 2.834 86.1 133 42* 1.375 1.375 0 3.025 92.1 133 43* 0.55 2.2 0 2.953 90.4 122 44* 0.275 2.475 0 2.922 89.6 115 2.25 0.25 0.25 3.159 95.9 151 46 2 0.5 0.05 3.189 96.8 148 47 2 0.5 0.25 3.205 97.4 152 48 1.75 0.75 0.25 3.159 96.1 150 49 1.25 1.25 0.25 3.112 94.9 140 0.5 2 0.25 3.004 92.0 126 51 1.375 0.275 1.1 3.081 94.1 138 52 0.825 0.825 1.1 3.120 95.4 141 53 0.275 1.375 1.1 3.003 92.2 124 54 0.275 0.275 2.2 3.060 94.2 114 1.875 0.625 0.25 3.204 97.3 148 56* 1.875 0.625 0 2.829 86.0 --* means not an example of the invention -- means not measured WO 95/02563 2 ~ PCT/US94104256 The data in Table lll, especially Exampie 55 and 56~ and 41*, 46 and 47, demonstrate that a ternary sintering aid composition ~Y203, CaO and AIB2) leads to a combination of density and thermal conductivity that exceeds the combination attainable with similar amounts of a binary sintering aid composition ~Y2O3 and CaO). The data also 5 demonstrate that a low rare earth metal oxide content, as in Examples 53 and 54, leads to a lowerthermal conductivitythan a greater rare earth metal oxide content as in Example 51.
Exam~les 57-73 Repeat Examples 40-56 at a sintering temperature of 1 650C using a tungsten furnace and enclosing the greenware in molybdenum-tungsten setters rather than boron 10 nitride setters. Data similar to that of Table lll are shown in Table IV.
Table IV
Den- Ther-Exam- y O C AlB2 Dietny~ s(iot/oy Conduc-No. (g) (9) (9) (g/cm3) Theor- tivity etical) (W/m-K) 57* 2.475 0.275 0 2.886 87.4 113 58* 2 0.5 0 2.974 90.3 122 59* 1.375 1.375 0 3.074 93.6 119 60* 0.55 2.2 0 2.866 87.7 106 61 *0.275 2.475 0 2.840 87.0 98 62* 2.25 0.25 0.25 2.714 82.3 122 63 2 0.5 0.05 3.139 95.3 142 64 2 0.5 0.25 3.19Q 96.9 149 1.75 0.75 0.25 3.181 96.8 149 66 1.25 1.25 0.25 3.135 95.6 138 67 0.5 2 0.25 3.019 92.4 122 68* 1.375 0.275 1.1 2.665 81.4 117 69 0.825 0.825 1.1 3.127 95.8 148 0.275 1.375 1.1 3.006 92.3 121 71 *0.275 0.275 2.2 2.466 75.9 80 72 1.875 0.625 0.25 3.190 96.9 152 73* 1.875 0.625 0 2.466 86.8 115 * means not an example of the invention WO 95/02563 2171~ 4 8 PCT/US94/04256 The data in Table IV, especially Example 72 in comparison with Example 73*and Example 58* in comparison with Examples 63 and 64, verify the observation made following Tablelllinthattheternarysinteringaidcompositionincludingaboronsourceprovidesbetter results than a binary sintering aid composition lacking a boron source. A comparison of Examples 45, 62*, 63, 68* and 71* shows thata metal refractory furnace may require an added amount of alkaline earth metal oxide in order to obtain performance equivalent to a graphite furnace using a boron nitride crucible. Example 71 shows that, at the constant total sintering aid level used in these examples, an excessive amount of AIB2 relative to other components of the sintering aid composition leads to unacceptably low levels of density and thermal 10 conductivity.
ExamPles 74-76 Repeat Example 55, but change the AIN powder and vary the temperature as shown in Table V. Examples 74-76 therefore contain 1.875 wt% Y2O3, 0.625 wt% CaO and 0.25 wt% AIB2. The Al N powder has a surface area of 3.8 m2/g, an oxygen content of 1.35 wt% and a carbon content of 0.09 wt% (experi menta l powder desi gnated 6419 R, The Dow Chem ica l Company). The density and thermal conductivity data are shown in Table V.
Table V
Den- Ther-Exam- Temp- Den- sity mal Ple era- sity (% Conduc-20No. tu re(g/cm3)Theor- tivity etical) (W/m K) 74 16253.203 97.3 135 16003.205 97.4 128 76 15753.149 95.7 119 Examples 77-78 Repeat Example 72, but vary the temperature as shown i n Table Vl and use the AIN powder of Examples 74-76. The density and thermal conductivity data are shown in Table Vl.

_19_ WO 95/02~63 PCT~US94/04256 4 8 ~
Table VI
Exam- Temp- Den- Ds,etny- Tmealr-Ple tera- sity (% Conduc-No.ure (g/cm3) Theor- tivity etical) (W/m K) 77 1650 3.215 97.7 136 78 1625 3.194 97.0 132 The data in Tables V and Vl show that an increase in AIN surface area improves si nterabi I ity as determi ned by mai ntai n i ng acceptabl e density at I ower tem peratu res tha n those used i n Examples 55 and 72. The data also show that an increase i n oxygen content over - that used in Examples 55 and 72 causes a minor decrease in thermal conductivity, but may also contribute to enhanced densification.
Exam~les 79-95 Repeat Exam ple 39, but use a d ifferent I ot of the Al N powder used i n Exam ple 39, change the furnace to the tungsten used in Examples 57-73, increase the sintering temperature to 1 650c and, for some examples, add an amount of Al2O3. The Al N powder has a su rface area of 3.43 m2/g, an oxygen content of 1.16 wt% and a carbon content of 0.07 wt% . The component amounts, density and thermal conductivity data are shown in Table Vll.

WO 95/02563 2 ~ 715 4 ~ PCT/US94/04256 .

Table VII
Den- Ther-Exam- y o C O AIB2 Al2O3 Den- sity Conduc-No. (9) (g) (g) (g) (g/cm3) Theor- tivity etical) (W/m K) 79 1.875 0.625 0.150 0 3.180 96.6 1 S5 2.344 0.781 0.188 0 3.183 96.5 150 81 2.813 0.938 0.225 0 3.166 95.7 146 82 1.875 0.625 0.100 0 3.200 97.2 148 83 10875 0.625 0.250 0 3.199 97.2 152 84 1.406 0.563 0.113 0 3.165 96.4 151 2.344 0.781 0.188 0.15 3.203 97.0 151 86 1.875 0.625 0.150 0.15 3.199 97.1 151 87 1.875 0.6Z5 0.100 0.15 3.200 97.2 149 88 1.000 1.000 0.150 0.15 3.176 96.9 139 89 2.344 0.781 0.188 0.25 3.197 96.8 149 1.875 0.625 0.150 0.25 3.198 97.1 151 91 1.875 0.625 0.100 0.25 3.198 97.1 151 92 1.875 0.625 O.Z50 0.25 3.194 97.0 146 93 2.344 0.781 0.188 0.40 3 191 96.6 150 94 1.875 0.625 0.150 0.40 3.205 97.3 145 1.875 0.625 0.100 0.40 3.207 97.3 148 The data in Table Vll de",on,l.dle thatthe sintering aid combination of Y203, CaO and AIB2 tolerates higher levels of oxygen without significant adverse effects as measured by either density or thermal conductivity. Substantially higher levels of oxygen that accompany 30 much higher levels of Al203 should lead to i mprovements i n density at the expense of decreases in thermal conductivity.
Examples 96-97 Repeat Examples 41 and 56 as, respectively Examples 96 and 97, but sinter in thepresence of a green body containing 2 wt% AIB2. Table Vlll shows comparative data for 35 Examples41, 56, 96and97.

WO 95/02~63 PCTIUS94104256 ~171~48. ~
Table VIII
Den- Ther-EXplem Y2O3 CaO A(lB)2 Dsjetny- s(iot/oy Concluc-No. (9) (9 9 (g/cm3) Theor- tivity etical) (W/m K) 41 * 2 0.5 0 2.834 86.1 133 96 2 0.5 0 3.140 95.4 140 56* 1.875 0.625 0 2.829 86.0 --97 1.875 0.625 0 3.137 95.3 139 * me -ns not an example of the invention -- means not measured The data in Table Vlli show that a volatile species such as boron enhances sintering of aluminum nitride compositions even when it is not part of the composition, so long 15 as it is present, for example, in a crucible used to sinter the compositions. Examples 98-1 02 Repeat Example 47, but vary the boron source to provide the data in Table IX.
Examples 47 and 98-102 all contain 2 wt% Y2O3 and 0.5 wt% CaO. Although the weight percent of the various boron sources differs, each source is present in an equivalent molar 20 percentage.
Table IX
Boron Den- Ther-Exam- Source Den- sity mal Ple sity (% Conduc-No. (g/cm3) Theor- tivity Type g etical) (W/m K) 47 AIB2 0.25 3.194 97.0 151 98 YB6 0.264 3.184 96.7 152 99 SrB6 0.260 3.203 97.3 149 100 CaB6 0.180 3.210 97.5 150 30 101 AIB12 0.134 3.193 97.0 150 102 LaB6 0.350 3.213 97.6 152 ~ 21~11 5~8 The data in Table IX show that a variety of boron sources provide satisfactory results in terms of density and thermal conductivity at 1625C in a graphite furnace.
Examples 103-108 Repeat Examples 47, 98-102, but change the sintering temperature to 1600~C to 5 provide the data in Table X.
Table X
-Boron Den- Ther-Exam- Source Den- sity mal Ple sity (% Conduc-No. (g/cm3) Theor- tivity Type g etical) (W/m K) 103 AIB2 0.25 3.097 94.1 137 104 YB6 0.264 2.984 90.6 129 105 SrB6 0.260 3.105 94.3 136 106 CaB6 0.180 3.064 93.1 134 107 AIB12 0.134 3.115 94.6 136 108 LaB6 0.350 3.133 95.2 138 The data in Table X show the same trends as in Table IX. The data for Example 105,whencomparedtothatofExamples103,104and 106-108simplyshowthatsomeboron sources provide enhanced performance over other boron sources. Grinding YB6 may provide better dispersion of the boron source and lead to improved results comparable to those of the other examples.
25 EXamDIes 109-123 Repeat Exam pl e 39, but change the heati ng sched u I e to: heat to a tem peratu re of 1400Cata rateof 10C/minute, hold at 1400Cforonehour, heatto 1850Cata rateof 2.5C/minute,holdat1850Cfortwohours,andthencooltolOOCCatarateof 10C/minute.
The formulations and resulting thermal conductivity are shown in Table Xl.

W095/02563 PCT~S94/04256 21~k~ ~
Table XI
Ther-Exam- y 0 C 0 AIB2 Al203 conduc-No. (9) (9) (g) (9) tivity (W/m- K) 109* 3.0 0.0 0.0 0.0 186 110 2.34 0.78 0.18 0.0 181 111 2.81 0.94 0.23 0.0 177 112 2.34 0.78 0.19 0.25 179 113 1.88 0.63 0.15 0.25 181 114 1.88 0.63 0.10 0.25 181 115 1.88 0.63 0.25 0.25 179 116 1.88 0.63 0.15 0.15 183 117 1.88 0.63 0.15 0.40 182 118 2.34 0.78 0.19 0.40 178 119 1.88 0.63 0.10 0.15 177 120 1.88 0.63 0.10 0.40 181 121 1.41 0.56 0.11 0.0 182 122 1.00 1.00 0.15 0.15 185 123 1.75 0.75 0.25 0.0 184 * means not an example ofthe invention ThedatainTableXlshowthatthethreecomponentsinteringcombinationof Y2O3-CaO-AlB2 (Examples 110,111, 121 and 123) and fourcomponentsintering aid combination of Y2O3-CaO-AlB2-AI2O3 (Examples 112-120 and 122), when used in sintering sinterable AIN compositions at temperatures as high as 1 850C, yield thermal conductivities thatarestatisticallyequivalenttothatprovided byastandard sinterableAlN composition using only 3 9 Y2O3 at the same temperature (Example 109*). When si ntered under equ ivalent conditions, authors writing in open literature would predict that addition of CaO, Al2O3 or both to Y2O3 would yield thermal conductivities below that provided by Y2O3 as a sole sinteringaid. ThedatainTableXlshowthatthepresenceofasmallamountofaboronsource as a sintering aid counters this prediction. Similar results are expected with other alkaline earth 35 metal sources, rare earth metal sources, boron sources and aluminum oxide sources.
Exam~le 124 Prepare a milled slip using the procedure of Example 39 and 300 9 AIN powder, 3.35 9 CaCO3 powder 5.62 9 Y2O3 powder, 0.45 9 AIB2 powder, 93 9 of toluene as the solvent, WO 95/02563 2 ~ 7 ~ 5 4 ~ PCT/US94/04256 ., ~ .
33 g of a binder/dispersant (Rohm and Haas Co, ACRYLOID'~ B-72) and 9 g of a plasticizer (Aristech Chemical Company, PX-316). The powders are the same as used in Example 39. The milled slip is converted to a cast green tape using conventional doctor blade techniques. The green tape is screen printed with a test ink pattern using a tungsten ink (Crystalero, #2003 ink).
5 Multilayertestpiecesarefabricatedbystackingandthermallylaminatingscreenprintedgreen tapepiecesunderanisostaticpressureof2000poundspersquareinch(psi)(13.8megapascals) at a temperature of 70C. Binder burnout occurs in a flowing nitrogen atmosphere using a heating rate of 90C/hour up to 750C, holding at 750C for four hours and cooling at a rate of 180C/hourto room temperature (25C).
ThemultilayertestpiecesaresinteredinthetungstenfurnaceusedinExamples 57-73 using the same sintering conditions as in Examples 57-73 to produce cofired (also known as cosintered) sub~l,dtes. The cofired substrates have a density of 3.175 g/cm3 (96.4 ~ of theoretical) and a thermal conductivity of 134 W/m-K. Test circuit patterns resulting from the inkhaveanelectricalresistancethatvariesfrom 13to26milliohms/square.
Example 125 Repeat Example 124, but reduce the amount of AIB2 powder to 0.30 9. The cofiredsub~l(dleshaveadensityof3.174g/cm3andathermalconductivityof 124W/m K. Test circuit patterns resulting from the ink have an electrical resistance that varies from 13.1 to 13.6 milliohms/square.
Examples 124 and 125 demonstrate the suitability of sinterable compositions of the invention for use in preparing multilayer, cofired substrate materials. The narrow variability of electrical resistance in Example 125 relative to Example 124 suggests that boron source levels should be kept relatively low. By way of illustration, when using AIB2 as the boron source,thelevelshouldbeatorbelowO.15wt%,basedonsinteredbodyweight,formore 25 consistent results. Similar results are expected with other sinterable compositions and AIN
powders, both of which are disclosed herein.

Claims (23)

What is claimed is:

1. A sinterable aluminum nitride powder composition comprising aluminum nitride powder and a sintering aid combination that consists essentially of a rare earth metal oxide source, an alkaline earth metal oxide source, a boron source, and, optionally, a source of aluminum oxide.

2. A sinterable aluminum nitride powder composition comprising aluminum nitride powder and a sintering aid combination that consists essentially of a rare earth metal oxide source, an alkaline earth metal oxide source, a boron source selected from the group consisting of aluminum boride, aluminum diboride, calcium boride, yttrium boride, strontium boride, barium boride, cerium boride, praseodymium boride, samarium boride and neodymium boride, and, optionally, a source of aluminum oxide.

3. A powder composition as claimed in Claim 1 or Claim 2, wherein the alkaline earth metal oxide source is present in an amount sufficient to provide an equivalent alkaline earth metal oxide content within a range of from 0.2 to 5 percent by weight, the rare earth metal oxide source is present in an amount sufficient to provide an equivalent rare earth metal oxide content within a range of from 0.25 to 5 percent by weight, and the source of boron is present in an amount sufficient to provide an equivalent boron content within a range of from 0.01 to 1 percent by weight, all percentages being based upon sinterable composition weight.

4. A powder composition as claimed in Claim 3, wherein the equivalent alkaline earth metal oxide content is within a range of from 0.5 to 5 percent by weight when the composition is to be sintered in a metal refractory furnace.

5. A powder composition as claimed in Claim 3, wherein the sintering aid combination includes a source of alumina in an amount sufficient to provide an equivalent alumina content within a range of from greater than zero percent by weight to 2 percent by weight, based upon sinterable composition weight.

6. A powder composition as claimed in Claim 5, wherein the sintering aid combination is present in an amount of from 0.5 to 10 percent by weight, based upon sinterable composition weight.

7. A powder composition as claimed in Claim 6, wherein the rare earth metal oxide source is yttrium oxide, the alkaline earth metal oxide source is calcium carbonate and the boron source is aluminum diboride or aluminum boride.

8. A sintered aluminum nitride body having a high thermal conductivity and comprising, based upon body weight, from 90 to 99.5 percent by weight aluminum nitride as a primary phase, from 0.5 to 10 percent by weight of a secondary phase selected from the group consisting of rare earth metal aluminates, alkaline earth metal-rare earth metal-aluminates, complex alkaline earth metal-rare earth metal oxides and mixtures thereof, optionally with an alkaline earth metal aluminate, and boron at a level of from 50 to 5000 parts by weight per million parts of body weight, as determined by secondary ion mass spectrometry.

9. A sintered aluminum nitride body as claimed in Claim 8, wherein the rare earth metal is yttrium and the alkaline earth metal is calcium.

10. A sintered aluminum nitride body as claimed in Claim 8, wherein the boron is present as a boron derivative that is a secondary phase or is dispersed throughout the secondary phase(s), or is dispersed on an atomic level within aluminum nitride's crystal lattice or a combination thereof.

11. The sintered aluminum nitride body as claimed in any of Claims 8- 10, wherein the secondary phase is at least one of Al2Y4O9, AlYO3, Al5Y3O12, CaYAlO4 and CaY2O4.

12. The sintered aluminum nitride body as claimed in Claim 11, wherein the body has a density of at least 95 percent of theoretical density.

13. The sintered aluminum nitride body as claimed in Claim 12, wherein the thermal conductivity is from 100 to 230 W/m.K.

14. A process for preparing a sintered aluminum nitride body having a high thermal conductivity that comprises heating a sinterable composition to a temperature of from 1570°C to 1850°C in a nonoxidizing atmosphere for a period of time sufficient to attain a density of at least 95 percent of theoretical density, the sinterable composition comprising aluminum nitride in an amount of from 90 to 99.5 percent by weight of composition and from 0.5 to 10 percent by weight of composition of a sintering aid combination that consists essentially of a rare earth metal oxide source, an alkaline earth metal oxide source and a boron source selected from the group consisting of aluminum boride, aluminum diboride, calcium boride, yttrium boride, strontium boride, barium boride, cerium boride, praseodymium boride, samarium boride and neodymium boride, and, optionally, a source of aluminum oxide.

15. A process as claimed in Claim 14, wherein the temperature is from 1570°C
to 1650°C.

16. A process as claimed in Claim 14 or Claim 15, wherein the sinterable composition comprises from 95 to 99.5 percent by weight of aluminum nitride and from 0.5 to 5 percent by weight of the sintering aid combination, both percentages being based upon sinterable composition weight and totaling 100 percent by weight.

17. A process as claimed in Claim 16, wherein the alkaline earth metal oxide source is present in an amount sufficient to provide an equivalent alkaline earth metal oxide content within a range of from 0.2 to 5 percent by weight, the rare earth metal oxide source is present in an amount sufficient to provide an equivalent rare earth metal oxide content within a range of from 0.25 to 5 percent by weight, and the source of boron is present in an amount sufficient to provide an equivalent boron content within a range of from 0.01 to 1 percent by weight, all percentages being based upon sinterable composition weight.

18. A process as claimed in Claim 17, wherein the sintering aid combination includes a source of aluminum oxide in an amount sufficient to provide an equivalent aluminum oxide content within a range of from greater than zero percent by weight to 2 percent by weight, based upon sinterable composition weight.

19. The process of Claim 14, wherein the sintered body has a thermal conductivity of from 100 to 230 W/m.K.

20. In a process for producing an aluminum nitride sintered product, which comprises the steps of adding sintering aids to AlN powder, compacting a resulting mixture and then sintering the compacted mixture at a sintering temperature in a non-oxidizing atmosphere;
said aluminum nitride powder having a specific surface area in a range of about 3 to 8 square meters per gram and an oxygen content between 0.5 and 1.8 percent by weight;
said sintering aids including:
a first sintering aid including at least one selected from the group consisting of rare earth oxides and rare earth compounds which are compounded to corresponding rare earth oxides by said sintering, said first sintering aid being incorporated such that an equivalent rare earth oxide amount thereof is in a range of 0.5 to 10 percent by weight in relation to the aluminum nitride sintered product;
a second sintering aid including at least one selected from the group consisting of alkaline earth oxides and alkaline earth compounds which are compounded to corresponding alkaline earth oxides by said sintering, said first sintering aid being incorporated such that an equivalent alkaline earth oxide amount thereof is in a range of 0.5 to 10 percent by weight in relation to the aluminum nitride sintered product; and a third sintering aid including at least one selected from the group consisting of LaB6, NbC, and WB, an additive amount of LaB6 being in a range of 0.05 to 3 percent by weight of the aluminum nitride sintered product, additive amounts of WB and NbC being respectively in a range of 0.05 to 5 percent by weight of the aluminum nitride sintered product.

21. A process as claimed in Claim 20, wherein the sintering temperature is 1650°C or less.

22. A process as claimed in Claim 20 or Claim 21, wherein the specific surface area of the aluminum nitride powder is in a range of 4.5 to 7.5 square meters per gram.

23. A method of preparing a cofired, multilayer aluminum nitride substrate, the method comprising:
a. preparing at least two ceramic green sheets from the sinterable composition of Claim 1;
b. depositing a desired pattern of a refractory metal ink on at least one major planar surface of at least one ceramic green sheet;
c. preparing a laminate of a desired number of ceramic green sheets having refractory metal ink deposited thereon;

d. heating the laminate in a non-oxidizing atmosphere, at atmospheric pressure and at a temperature of from 1570 to 1850°C to effect sintering of both the ceramic sheets and the refractory metal ink deposited on said sheets.