Manufacture of Aluminum Nitride Ceramics
Electrical insulation, high thermal conductivity, thermal expansion matching to silicon devices, and a low dielectric constant are all characteristics that aluminum nitride ceramics may possess in excess of other materials for electronic packaging applications. Microelectronic assemblies requiring high heat dissipation, such as multilayer metal-ceramic assemblies for high power devices, can benefit from using aluminum nitride substrates. As a result, high-power electronic devices that produce heat and metallization components must be accommodated by polymer layers and aluminum nitride ceramics for microelectronic applications.
Ceramics made from aluminum nitride powder need to achieve a specific density, at least 90% of the theoretical value, in order to have the desired properties. Densification can be accomplished at low temperature by using sintering aids. Since the sintering aid becomes liquid at temperatures below those at which the ceramic decomposes and the sintering temperature of its pure compound, it can be used to hasten densification of ceramic particles via particle rearrangement induced by capillary force between wet liquid and solid particle, followed by dissolution and precipitation. With this technique, solids (small particles) dissolve in high-curvature areas and then are redeposited in low-curvature areas (large particles). Additionally, during the subsequent stages of the liquid sintering cycle, particle growth and coalescence improve the solid’s microstructure.
The sintering aid also contributes to improving the aluminum nitride sintered body's thermal conductivity by absorbing oxygen from the powder. Because of this, an efficient sintering additive needs to create a liquid at the low temperature where it dissolves and redeposits aluminum nitride without oxidizing the material.
During the process of densification, the volume of the green body and, for multilayer structures, the volume of the thin layers of metal contained in the green body, along with the linear dimensions of the green body, will change as a function of the temperature that was experienced and the particular material that is being reduced. If metals and ceramics shrink at various rates and multiples, this uncoordinated shrinkage will cause residual stress between various sintered body components, which will distort the sintered body's final shape. Ceramics and metals must sinter roughly at the same rate in order to maintain the precise geometric tolerances necessary for multilayer ceramic assemblies in the electronics packaging industry. As a result, it is preferable to encourage the effective sintering of aluminum nitride at specific low temperatures to address issues with varying sintering rates and incompatible thermal expansion between ceramic and metallic parts of multilayer electronic assemblies.
Lower sintering temperatures may also produce less-than-ideal results for the required properties in theory. This could be as a result of the sintering aid's inability to create an efficient sintering liquid necessary for the densification of the ceramic, its inability to remove dissolved oxygen from the AlN lattice, or as a result of the development of new phases in the sintering aid or the AlN structure. The formation of additional phases in the medium resulted in the reaction product of sintering aid, aluminum, and oxygen being present in the AlN structure. Hence, this paper presents a pre-sintered mixture of aluminum nitride powder and sintering aids, as well as a low-temperature method for manufacturing aluminum nitride sintered bodies.
Low-Temperature Sintering of Aluminum Nitride Ceramics
(1) Make Green Body
The AlN powder, binder, and sintering aid composition powders are combined to make the green body, which is then processed using standard techniques like dry pressing or sheet casting.
Thin sheets of AlN green bodies are printed with metal pastes, such as those made from the refractory metals tungsten and molybdenum, to create multilayer ceramic bodies. Prior to sintering, the printed green sheets are hot pressed together to form a multilayer structure with alternating metals and ceramics.
(2) Sintering
Sintering is done in a high temperature furnace, like one made of refractory metal or graphite. The required sintering temperature is gradually increased during the sintering process from ambient temperature. Around 800-900°C, the glass component turns into a highly viscous flowable solid, and ceramic viscous sintering starts. The glass starts to crystallize as the temperature rises to between 900 and 1200 °C. The crystallized glass melts at temperatures above about 1200°C.
Further densification takes place in this liquid phase-assisted sintering region, and the reaction between oxygen from the low-temperature sintering phase, AlN particles, and crystalline oxides also starts to take place. As a result, oxygen cannot enter the AlN crystal, increasing the thermal conductivity of the final product. In this temperature range, in order to achieve efficient and uniform sintering, it is necessary to keep a vapor of the boron oxide component of the furnace atmosphere close to the AlN part.
Matters Needing Attention in Sintering
The sintering system must be set up to provide the aluminum nitride with an atmosphere that includes vapors of the various components of the sintering aid composition at appropriate temperatures, as well as a suitable sintering gas, such as nitrogen. When using a refractory metal furnace, the sintering atmosphere should also include a gas, such as hydrogen, that safeguards the furnace's components.
In order to maintain an effective amount of the high temperature liquid component for a long enough period of time for this to occur final densification, it is necessary to maintain a partial pressure of the liquid component in the furnace atmosphere close to the AlN part. This will yield the highest possible percentage of theoretical densification.
Carriers for AlN parts must not form bonds to the parts either during or after sintering, and they must not interfere with the process of densifying the parts in any way. The use of weights during sintering offers a higher level of control to lessen the curvature of the sintered part.