10.3 Sputtered Iridium Gate Module for GaN HEMT with Stress Engineering and High Reliability

A new gate module with iridium as a degradation resistant Schottky contact for GaN based HEMT devices is developed. Conformal deposition of Schottky and barrier metal in the gate trench provides sealing of the semiconductor. Sputtering is the enabling technology that provides low stress iridium contacts from low damage processing. Patterning of the gate contact is achieved by a subtractive method. Robustness and reliability of the devices were investigated by step-stress-test, IDQ-test, storage test and DC life-test. Results are compared to a conventional deposition method with evaporated platinum contacts. Introduction AlGaN/GaN HEMT RF transistors are rapidly developing due to their high output power density, high operation voltage and high input impedance. Impressive progress has been achieved in improving the performance of such devices. Many RF systems already rely on GaN transistors. Open points still remain related to current collapse and leakage current and its relation to reliability issues. The quality of the epitaxial material can have a strong impact on the transistor performance. But as the epitaxial quality has matured it has become more obvious that process modules such as gate and ohmic contacts as well as surface passivation influence and limit the power performance of GaN HEMTs significantly. Due to the combination of high electric fields and large current densities close to the drain side edge of the gate, most of the heat generation takes place in this region. Therefore, a thermally stable Schottky barrier is a key condition for acceptable reliability behavior. Generally, the gate leakage current has to remain at a low level, as excess leakage current decreases the breakdown voltage significantly. In many cases leakage current increases as the device degrades. For the traditionally used NiAu gates fabricated by physical vapor deposition a thermal instability of the metal stack has been reported by different sources [1]. This results in a gradual electric degradation of the gate contact. The mechanisms associated to this are quite complex in nature. Nickel silicide formation between Ni and the SiNx passivation and subsequent Au diffusion to the Ni-GaN interface is under debate. Gold diffusion from the gate head is another source for deterioration of HEMT performance and reliability. It has been found that gold may be diffusing via the gate foot sidewall towards the Schottky barrier [2]. The shadowing effect from directional deposition by evaporation prevents full coverage of the Schottky metal in the gate trench and in this way enables sideway diffusion. This in turn not only demands for more resistant Schottky metallization but also requests hermetic sealing of the Schottky metal interface from diffusion. Several attempts have been made to replace the Schottky gate metal by more resistant metallization schemes. In particular, refractory metals exhibiting very high melting point are promising candidates [3]. On the other hand, stress in evaporated thin films is rather high for high melting point metals. This and the extreme lateral dimensional ratio of the gate could lead to poor adhesion and roll up of the gate contact. Therefore, stress engineering is highly desirable to promote well adhering Schottky metal in the gate trench. 1 st SiNx