Improvement in the thermal conductivity of aluminumhigh thermal conductivity. AlN in a "pure" form is not only difficult to densify by pressureless sintering but also has a nitride (AlN) can be realized by additives that have a high low thermal conductivity. Densification of AlN can be easily thermodynamic affinity toward alumina (Al 2 O 3 ), as is facilitated by adding a variety of oxides (and, in some cases, clearly demonstrated in the aluminum nitride-yttria (AlNnon-oxides) as sintering aids. Rare-earth and/or alkaline-earth Y 2 O 3 ) system. A wide variety of lanthanide dopants are oxides are often added as sintering aids. 6-26 The addition of compared at equimolar lanthanide oxide:alumina (Ln 2 O 3 :sintering aids has a dual purpose: it aids in densification and Al 2 O 3 , where Ln is a lanthanide element) ratios, with enhances the thermal conductivity of the AlN-grain-boundary- samaria (Sm 2 O 3 ) and lutetia (Lu 2 O 3 ) being the dopants thatoxide-phase composite. The role of additives in enhancing thergive the highest-and lowest-thermal-conductivity AlN commal conductivity can be understood, considering the effect of posites, respectively. The choice of the sintering aid and the various parameters on thermal conductivity (in particular, the dopant level is much more important than the microstrucrole of impurities). ture that evolves during sintering. A contiguous AlN phaseThe principal impurity in AlN is oxygen. The phase diaprovides rapid heat conduction paths, even at short sintering gram 27 between AlN and alumina (Al 2 O 3 ) shows the existence times. AlN contiguity decreases slightly as the annealing times of a spinel structure with the approximate formula Al 3 O 3 N. increase in the range of 1-1000 min at 1850؇C. However, aAlN powders invariably contain some Al 2 O 3 on the surface, substantial increase in thermal conductivity results, because which spontaneously forms when AlN powder is exposed to air. of purification of AlN grains by dissolution-reprecipitationIn addition, some oxygen also is present in the AlN lattice in a and bulk diffusion. Removal of grain-boundary phases, dissolved form. Oxygen that has dissolved in the AlN lattice is with a concurrent increase in AlN contiguity, occurs at high essentially present as Al 2 O 3 that has dissolved in the lattice. As annealing temperatures or at long times and is a natural suggested by Slack and coworkers, 28,29 the most plausible mode consequence of high dihedral angles (poor wetting) in liquidof oxygen (or Al 2 O 3 ) incorporation into the AlN lattice is by the phase-sintered AlN ceramics.formation of aluminum vacancies. The corresponding defect reaction may be given as 28,29 I. IntroductionA LUMINUM NITRIDE (AlN), as a substrate material in elecwhere V Al denotes a vacant aluminum site. Mass and strain tronic packaging, has attracted considerable attention over misfits caused by the vacant aluminum site increase the scatterthe last two decades, because of its excellent properties, which ing cross section of phonons, which decreases the phonon ...
High thermal conductivity, low dielectric constant, high electrical resistivity, low density, and a thermal expansion coefficient that matches well with that of silicon are the principal attributes of AIN that have attracted much attention over the past decade. It is also now well established that oxygen as an impurity lowers the thermal conductivity of AIN. Processing techniques have been developed which not only facilitate pressureless densification of AIN but also enhance its thermal conductivity. The present work explores the thermodynamics and the kinetics of oxygen removal and the resultant enhancement of thermal conductivity. Polycrystalline AIN ceramics were fabricated with Y203, Dy, O, , Yb203, CaO, BaO, or MgO as additives. Samples were sinter/annealed at 1850°C for up to 1000 min. The AIN grain size of sintered samples ranged between 2 and 9 pm. The samples typically contained two or three phases with the predominant phase being AIN. Secondary phases in Y,O,-doped AIN consisted of yttrium aluminates which were along three grain junctions and along grain facets. The presence of Y,AI,O,,, YAIO,, M. D. Sacks-contributing editor Manuscript No. 198847.
The following figures and figure captions replace those originally published:Although the main applications of high-thermalconductivity AIN are as ceramic substrates in electronic packages, a variety of components can be formed by pressureless sintering. p. 2039Polycrystalline AIN can be sintered to near theoretical density using a variety of sintering aids The photograph shows representative samples of AIN sintered (1 850°C for 100 min) with Y 2 0 3 and various other lanthanide oxides in the molar ratio Ln2O3/AI2O3= 1 0, where Ln is La, Pr, Nd, Sm. Eu, Gd, Tb, Oy, Ho, Er, Trn, Yb, and Lu The thermal conductivities of these samples are between 150 and 190 W/(m. K) Fig. 5. Plots of In In [l/(l -x(t))] vs In t for samples sinter/annealed at 1850°C for three choices of x(m) Fig. 6. Thermal conductivity as a function of the YeOs content for samples sinrered on an AIN setter and embedded in AI2O3, AIN, and yzo3 490 Fig. 7. Schematic showing the concentration of dissolved oxygen (AI2O3) in AIN grains as a function of annealing time
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