Silicon carbide (SiC) integrated circuits have been suggested for extreme environment operation. The challenge of a new technology is to develop process flow, circuit models and circuit designs for a wide temperature range. A bipolar technology was chosen to avoid the gate dielectric weakness and low mobility drawback of SiC MOSFETs. Higher operation temperatures and better radiation hardness have been demonstrated for bipolar integrated circuits. Both digital and analog circuits have been demonstrated in the range from room temperature to 500 °C. Future steps are to demonstrate some mixed signal circuits of greater complexity. There are remaining challenges in contacting, metallization, packaging and reliability.

This letter presents an extensive investigation by means of microscopic and chemical analyses finding Ta diffusion in Cu films but not in Ag films. This difference in Ta diffusion persists in all samples containing either Cu/Ta or Ag/Ta interfaces, wherein both a driving force for diffusion and point defects for mediation of atomic movement are present. By referring to atomistic simulation results in the literature, it is plausible that the subtle difference between the Cu/Ta and Ag/Ta interfaces plays a crucial role in differentiating them in making Ta available for diffusion. The energetically favored binding between Cu and Ta assists in liberating Ta atoms from being strongly bound by surrounding Ta atoms, as the bond strength of Cu-Ta is about one third that of Ta-Ta. Hence, the formation of the much weaker Cu–Ta bonds acts as an important intermediate step. Such a mechanism does not exist for the Ag/Ta interface.

No abstract

a b s t r a c tAlthough wide band gap devices (WBG, e.g. GaN and SiC) are eminently suitable for high temperatures and harsh environments, these properties cannot be fully taken advantage of without an appropriate interconnect metallization. In this context, silver shows promise for interconnections at high temperatures. In this work, the thermal stability of Ag with two barrier metals -Ta and TaN -was therefore investigated. Metal stacks, consisting of 100 nm of silver on 45 nm of either Ta or TaN were sputterdeposited on the substrate. Each metal system was annealed in vacuum for one hour at temperatures up to 800°C. Both systems showed stable performance up to 600°C. The system with Ta as a barrier metal was found to be more stable than the TaN system. Above 700°C, silver agglomeration led to degradation of electrical performance.

In order to ascertain the applicability of the technologically well-established Cu metallization in high-temperature circuits, the authors have investigated layered metal stacks having one Ta/Cu interface at temperatures from 400 to 700 °C. The authors have found that Ta releases from the Ta layer and moves through the Cu film to the opposite interface via the grain boundaries. In the simplest bilayer stack with Cu on top of Ta, the up-diffused Ta on the surface spreads out over the Cu grains so as to cover the Cu grains completely at 650 °C. The activation energy for the grain boundary diffusion is found to be 1.0 ± 0.3 eV. The Ta diffusion in the grain boundaries leads to stabilization of the Cu grain size at 360 nm and an increase in sheet resistance of the metal stack. The latter is in fact observed for all metal stacks having Cu in contact with Ta on one side and TaN or nothing at all on the other. The implication is that the Cu metallization with one Ta/Cu interface has to be stabilized by a preanneal at the highest anticipated operating temperature before use.