Sputtered TiAl-Monolayers: Optimization of Thickness Uniformity and Stress over Target Lifetime

Inhalt:
Amorphous Titanium-Aluminium (TiAl) is used for springs, beams, hinges, posts as well as core or supporting structures in the fabrication of microelectromechanical systems (MEMS). This presentation challenges the stress and planarity optimization of fundamentally required very thin 25nm and up to 2000nm thick sputtered TiAl layers over a full TiAl sputter target lifetime. The aim is to achieve a high uniformity across the whole 200mm wafer, low surface RMS values close to zero after deposition and pattering as well as at a good long-term stability. To obtain such planarity, a well-characterized and stable sputtering process is necessary, i.e. in terms of stress adjustment, uniformity and control of deposition rate. Using Zernike-polynomials for description of layer thickness distribution, the optimal target substance distance for high uniformity is calculated. Furthermore, we present a model for the prediction of the deposition rate depending on: pressure, aimed thickness, target lifetime and target substrate distance. We show that the implementation of this model in the sputter tool automation software helps to continuously optimize the TiAl film uniformity for any given target film thickness and target consumption. Additionally, we give an impression of the stability of the sputter process of the monolayers over a full TiAl-sputter target lifecycle.