Stable Thin Film Resistors for Amorphous Silicon Integrated Circuits

Recent progress in the range of applications of amorphous silicon (a-Si:H) for large area electronics has led to a better understanding of the requirements for stable circuit operation. In particular, the application of doped a-Si:H as a material for thin film resistors has highlighted the instability of this material both due to its thermal history and its relatively high activation energy. In this paper, these limitations are addressed and alternative materials studied. The properties of thin film resistor materials for a-Si:H integrated circuits are driven by the requirements of long term stability, a high sheet resistance, a weak temperature dependence, and processing compatibility with a-Si:H.Cermet films are a promising material as the nature of the transport results in a low activation energy, though the sheet resistance of these materials is usually limited to 10–50 kohms/square. In this paper, the suitability of rf sputtered Cr/SiO2 cermet films for thin film resistors is addressed. Its electrical properties are studied as a function of target composition and its structure investigated by transmission electron microscopy. A sheet resistance of 40 Mohms/square with an 0.05 eV activation energy for a 500A thick film is demonstrated. The carrier transport is interpreted in terms of a tunneling model of electrons between Cr islands.