Tantalum based amorphous thin films as copper diffusion barrier
Date of Issue2012
School of Materials Science and Engineering
Nanyang Technological University
This dissertation presents a study of Ta-based Cu diffusion barrier for advanced semiconductor technology. With the fast development of semiconductor industry, a novel barrier is required in order to address two challenges in the back-end-of-line technology: enhancing the interconnect reliability of back-end-of-line and reducing resistance-capacitance delay faced during further scaling of semiconductor devices. Two groups of thin film barriers formed on two distinctive principles are studied in this work. The first group consists of barriers formed by Ta and inorganic elements N and Si, including Ta-N and Ta-Si-N, named as compound barriers. The second group consists of barriers formed by alloying Ta with another transition metal, to form Ta-Ni, Ta-Cr and Ta-Ti binary alloys. In this work, the properties of compound barriers, Ta-N and Ta-Si-N with a range of compositions formed by reactive sputtering, are studied. Ta-N and Ta-Si-N are stable on Si substrate at temperatures up to 800 °C and 900 °C respectively. The electrical resistance, however, is high for Ta-Si-N, up to the order of 105 µm•cm. The current work also studies the chemical bonding status of the amorphous Ta-N and Ta-Si-N film in an attempt to explain their high thermal stability. Binary alloys with amorphous structures are studied as a novel group of Cu barriers. The ability to form amorphous phase, annotated as glass formation ability or GFA, is initially predicted based on theoretical understandings. Ta-Ni, Ta-Cr and Ta-Ti films show different GFA. The experimental studies show that Ta-Ni and Ta-Cr form stable amorphous phase on Si substrate up to 800 °C, while Ta-Ti forms crystalline phase at as-deposited state first, transforms to amorphous phase at 600 °C and finally crystallizes at 800 °C. These binary alloy barriers show comparable thermal stability as Ta-N and better electrical conductivity than Ta-N and Ta-Si-N, making them potential candidates for Cu diffusion barrier. During Cu barrier performance evaluation, Ta-Si-N shows the best results among all barriers studied. It effectively blocks Cu diffusion into Si substrate after annealing in vacuum at temperatures up to 750 °C, with the formation of Cu3Si at barrier/Si interface as an indication of barrier failure. In comparison, Ta-N and binary alloy barriers fail at 700 °C. Among all Ta-based binary alloy barriers, interface integrity of Ta-Ni barrier is well maintained at initial failure temperature, while interface integrity is completely lost for Ta-Cr and Ta-Ti barriers. The diffusion profile of Cu in and failure mechanisms of the binary alloy barriers have also been investigated in order to reveal the insights behind the barrier performance of the Ta-Ni barrier. A beneficial effect of oxygen is also observed for both thin films stability and barrier performance. The current study has further advanced the understanding of the formation, stability, and diffusion mechanism of the Cu diffusion barrier. From practical point of view, it is thus proposed that Ta-Ni is the best candidate for advanced barrier application based on the barrier performance and electrical conductivity. In summary, two groups of Cu diffusion barriers, including compound barriers (Ta-N, Ta-Si-N) and binary alloy barriers (Ta-Ni, Ta-Cr and Ta-Ti), are studied. Ta-based binary amorphous alloy barriers, especially Ta-Ni barrier, show advantages over non-metallic compound barriers Ta-N and Ta-Si-N, in terms of low electrical resistance, high stability with process variation and excellent barrier performance. Guidelines of developing reliable Cu diffusion barrier are drawn base on this study.
DRNTU::Engineering::Materials::Microelectronics and semiconductor materials