Last Advances in Aqueous Processing of Aluminium Nitride (AlN) - A Review

Aluminium nitride (AlN) is a ceramic material that has been intensively studied in the last years due to its good thermal conductivity (319 W/mK, theoretical value), low dielectric losses (8.8), small dielectric consumption (4x104), a thermal expansion coefficient matching that of silicon, together with other physical properties that make AlN to be the most interesting substrate material for highly integrated microelectronic units (Greil et al., 1994; Iwase et al., 1994; Knudsen, 1995; Prohaska and Miller, 1990; Sheppard, 1990). The most recent breakthroughs were achieved in the processing science field of the AlN, namely on: (i) replacing of the traditionally used organic solvents by water; and (ii) decreasing the sintering temperatures AlN powder compacts through appropriately selecting the sintering additives and process optimization. Aqueous colloidal processing has been pursued by many authors along the most recent years as an alternative to alcoholic or other flammable and costly dispersion media. The advantages of aqueous processing are the healthier and more environmentally friend production at lower and more competitive costs, which enables to increase and diversify the applications for the nitride-based ceramics. However, nitride powders are susceptible to hydrolysis, what is particularly true in the case of aluminium nitride (AlN) (Bellosi et al., 1993; Osborne & Norton, 1998; Reetz et al., 1992). In fact, when AlN powder is hydrolysed by water, undesirable aluminium hydroxydes are formed on the surface of particles, with a concomitant increase of the oxygen content and the production and release of ammonia. Accordingly, an amorphous layer composed by AlOOH is initially formed at the surface of AlN particles, which then transforms to bayerite, Al(OH)3, according to the following reactions: