Sinter kinetics and interface reactions of silver thick films on aluminium nitride

Abstract

The advantages of Aluminium nitride (AlN) ceramics are a high bulk thermal conductivity, insulating resistance, dielectric strength, flexural strength and a coefficient of thermal expansion, which is well matched to semiconductor materials, like silicon carbide. AlN ceramics show high potential as substrate materials for thick film- / hybrid applications in the field of power electronics and multichip modules. These applications require metallisations with high ampacity. Herein, the direct copper bond technology (DBC) is state of the art. Nevertheless, DBC shows several disadvantages in the circuit building process, for instance the etching process for patterning the foil, what limits the structure resolution on the one hand. On the other hand, this subtractive process is expensive along with a high chemicals consumption. The use of copper thick film pastes (thick print copper-TPC) instead of DBC results in higher structure resolutions, is an easier industrial applicable, energy-saving and environmentally friendlier process [Gun16]. Due to the fact that copper thick films have to be fired in nitrogen atmosphere and the actual bottlenecks in copper production, what might rise the copper price [Wie17], thick print silver (TPS) seems to be a promising alternative to TPC. Moreover, the conductivity of silver is slightly higher than those of copper and silver can be fired in air. The aim of this study is to get insight the silver films sinter kinetics and the interface formation on AlN. Therefore, systematic variations in the paste recipe where done and the resulting film shrinkage, blister behaviour, solderability and adhesion strength where correlated with the sintering kinetics and the microstructure of the films. A significant role in this process play the inorganic components bismuth oxide and the glass. This is because of the ability of bismuth oxide solution in the glass matrix, what shifts the softening down to lower temperatures. The influence of the glass and bismuth oxide volume fractions on the glass-viscosity, sintering behaviour of silver pastes as well as the interfacial reactions with AlN were investigated by means of thermomechanical analysis, FESEM-cross sections of fired films and XRD. Phase transformation mechanisms and root causes for blister formation are investigated. It was found out that sinter kinetics in combination with the simultaneous glas softening and phase redistribution are crucial for the interface reaction, film adhesion and blistering. Ways to attain evaporation possibilities via controlled sintering retardation of the thick films are examined

The silver paste containing ZnO-B2O3-SiO2 glass sintered at high temperature with low solid content formed high performance conductive thick film on MgTiO3 microwave ceramics

“…Marcinkowski et al studied the sintering kinetics and interface formation of silver films on AlN ceramics. The conductivity of silver films prepared at 850 °C was as low as 0.93 mΩ/,, but the adhesive strength could only reach 6.7 MPa [33]. Zou et al prepared a conductive silver slurry with a solid content of up to 90% at 850 °C, resulting in a thick silver film with a block resistance of 0.9 mΩ/, but a not particularly high adhesive strength of 15.3 MPa [16].…”

“…Si [34] 800 °C/10 min 2.00 --AlN [35] 850 °C/10 min -4.00 11.7 Si [36] 800 °C/10 min -5.90 -AlN [37] 850 °C/15 min -5.30 12.7 AlN [33] 850 °C/10 min -0.93 6.7 Al 2 O 3 [28] 600 °C/15 min 2.50 -28.5 Al 2 O 3 [38] 850 °C/10 min 2.87 -14.6 AlN [16] 850 °C/15 min -0. 9, in which obvious cracks are visible at the interface between the silver layer and MgTiO 3 ceramic substrate.…”