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

Motivation AlGaN/GaN HEMT rf transistors are rapidly developing due to their high output power density, high operation voltage and high input impedance. Ni-based gate metallization is widely used to form the Schottky gate contacts. However, temperature and electric field levels are higher in AlGaN/GaN compared to traditional III-Vs and therefore the diffusion of Ni into AlGaN/GaN constitutes a reliability issue [1]. Gold diffusion from the gate head is another source of deterioration for HEMT performance and reliability. It has been found that gold may be diffusing via the gate foot side wall 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 metallisation 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. Approach The metallurgical barrier effect of the Schottky contact metal and hermetic sealing are accomplished by conformal deposition of the Schottky metal over the gate trench and adjacent area. Isotropic deposition methods are required for conformal growth of the Schottky metal. Thus, sputter deposition is the preferred method here. Iridium as a refractory metal with a melting point of 2460°C and simultaneously providing a high work function > 5 eV is a prime candidate. The deposition of a conformal metal layer requires an isotropic deposition method. Such methods are not compatible with traditional lift-off as a masking technique. Particularly, lift-off technologies require strict anisotropic deposition. Therefore, forming of the Schottky contact electrode has been changed from an additive structuring method, i.e. lift-off, to subtractive structuring. Refractory metals tend to build rigid layers due to their high mechanical elastic modulus. This is beneficial for a metallurgical barrier but may adversely effect performance when mechanical stress during film growth is not properly managed and may even lead to delamination of the film. Sputtering technology typically yields compressive metal films but by choosing appropriate plasma parameter stress can be adjusted from compressive to tensile. Here, iridium films are adjusted to stress levels much lower than 100 MPa. Both, RF-power as well as high voltage switching applications suffer from electronic traps that are responsible for poor performance under dynamic operation. Trap generation in the vicinity of the gate contact due to energetic particles from the sputter plasma has to be avoided by choosing suitable processing parameter. RF-data will be chosen as a benchmark of this technology against the evaporated standard.