Selection of Optimized Materials for CBRAM Based on HT-XRD and Electrical Test Results

Among emerging memory technologies that rely on the bistable change of a resistor, the conductive bridging random access memory (CBRAM) is of particular interest due to its excellent scaling potential into the sub-20 nm range and low power operation. This technology utilizes electrochemical redox reactions to form nanoscale metallic filaments in an isolating amorphous solid electrolyte. Ge chalcogenides are candidate materials for high performance solid electrolytes in combination with Ag as the preferred metal showing high mobility and switching speed. Due to the thermal budget for a back end of the line (BEOL) processing, the layer stack materials must withstand temperatures in the range of 300-450°C. Pure GeS was stable up to 450°C without crystallization. For GeSe, deleterious crystallization was observed. High temperature X-ray diffraction (HT-XRD) and electrical characterization with stepwise annealing were applied to characterize the thermal stability of Ag/GeSe and Ag/GeS material systems. The higher onset temperature for solid-state reactions found with HT-XRD in the Ag/GeS system is the key for the better electrical performance compared to the Ag/GeSe system. Even after thermal annealing with a peak temperature of 300°C, excellent and stable yield numbers of more than 90% for memory elements were achieved for the sulfide, which qualifies this material system for a low temperature BEOL process.