Control of stress in silicon nitride thin films
Department of Electrical and Computer Engineering
Master of Science
Cornely, Roy H.
Silicon nitride thin films of varying composition and thickness were deposited on silicon substrates by reactive rf diode sputtering of a silicon target using an argon/nitrogen gas mixture. Film stoichiometry could be controlled by varying the partial pressure of nitrogen, the total sputtering gas pressure and the target rf power. Films with refractive index of 2.01, the value for stoichiometric silicon nitride, could be obtained. Film stress was measured by wafer curvature; refractive index and thickness of deposited films were obtained by ellipsometry and interferometry measurements. The etch rate in buffered HF for films with refractive index 2.05 was 29A/min; as measured by JR spectroscopy these films had relatively low oxygen and hydrogen content. On increasing the film thickness from 300 A to 1900 A, the stress decreased from 4.45 x 109/m2 to 0.56 x 109N/m2 for a film with a refractive index of 2.05. It was observed that films with silicon in excess of the stoichiometric value and films with greater thickness exhibited reduced stress. Films with higher refractive index were obtained on reducing the nitrogen partial pressure and increasing the applied rf power.
Since low temperature deposition is critical for reducing the stress developed in thin films, the substrate temperature during sputter deposition was maintained at 140°C(+/-10°C). The stress content of the unannealed NJIT films was observed to be higher by about a factor of 10 than the lowest stress value (2 x 108N/m2) obtained by plasma deposition followed by a 400°C postanneal for a 1000 A thick film. The lowest stress value obtained during this research was 0.98 x 109 N/m22, with 2.24 being the refractive index for this silicon rich film with a thickness of 700 A. The value for a 730 A thick film with index of 1.702 was 2.036 x 109 N/m2.
njit-etd1990-018 (75 pages ~ 3,146 KB pdf)
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Created June 12, 2006