Formation of Al x O y N z Barriers for Advanced Silver Metallization
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Formation of AlxOyNz barriers for advanced silver metallization Y. Wang and T.L. Alford Department of Chemical, Bio, and Materials Engineering NSF Center for Low Power Electronics Arizona State University, Tempe, AZ 85287-6006, USA Abstract Silver has been explored as a potential candidate for future advanced interconnects due to its lowest electrical resistivity, when compared with Al and Cu. As in the case of Cu metallization, an additional layer between the Ag film and underneath dielectric is necessary in order to improve adhesion and to block the diffusion of Ag atoms. In this study, thin aluminum oxynitride (AlxOyNz) diffusion barriers have been formed in the temperature range of 400-725 °C by annealing Ag/Al bilayers on oxidized Si substrates in ammonia ambient. Rutherford backscattering spectrometry showed that the out-diffused Al reacted with both the ammonia and oxygen in the ambient and encapsulated the Ag films. Higher process temperatures and thinner original Al layers showed to improve the resistivity of the encapsulated Ag layers. The resulting Ag resistivity values are ~1.75 ± 0.35 µΩ-cm. The thermal stability test of these diffusion barriers showed that these barriers sustained the interdiffusion between Cu and Ag up to 620 °C at least for 30 min in either vacuum or flowing He-0.5% H2. This temperature is a 200°C improvement over previously reported values for the self-encapsulated Cu and Ag films. X-ray diffraction spectra showed no formation of any high resistive intermetallic compounds, i.e., Ag3Al, Ag2Al, and AlAg3. I.
INTRODUCTION
Due to more stringent demands on interconnects for the advanced ultra-large-scaleintegration (ULSI) technologies, advanced metallization schemes, other than Al- and Cu -based ones, has been explored to achieve higher current densities and faster switching speeds1-2. Compared with Al and Cu, silver is more attractive due to its lowest bulk electrical resistivity (1.47 µΩ-cm)3 since RC delay ( inherent with the metallization schemes) will become more significant with the development of ULSI technology. But advanced silver metallization has serious shortcomings, such as poor adhesion to dielectrics and the potential degradation of Si based semiconductor devices4. To address these issues, the addition of another metal layer (e.g., Ti or Cr with Ag1-2, Ti, Al, or Mg with Cu metallization1-2, 5-8) has been explored. Using the bilayer configuration and annealing in an appropriate ambient, a metal-nitride or metal-oxide encapsulation layer was formed on top of the Ag or Cu layers. The encapsulation layer reduced the rate of Ag sulfidation1-2 or Cu oxidation1-2, 5-8 in the corrosive environments and hence ensured the structure integrity of the metal lines. Addition of Al has shown good results for Cu metallization by forming Al-oxide passivation layers on Cu layers during anneals in a low pressure of O2 ambient6. However, the use of Al underlayer has not been extensively investigated for Ag metallization. Compared with Al-oxide, Al-nitride has higher thermal conductivity (200
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