Low Temperature Plasma-enhanced Atomic Layer Deposition for SiO₂ Using Carbon Dioxide

SiO2 grown by atomic layer deposition (ALD) is a widely used material for the fabrication of microelectronic devices, due to its precise thickness control, high conformality, and excellent dielectric properties. Expanding the applications of ALD SiO2 to thermally sensitive materials requires the development of low-temperature (T< 100˚C) processes. Catalyzed ALD [1,2] and plasma-enhanced ALD (PEALD) [3,4] have been applied as effective solutions. However, the commonly used oxidants, H2O and O2, potentially degrade moisture/oxygen sensitive materials, thereby limiting the applications of low temperature SiO2. In this work, we report the successful deposition of high-quality SiO2 films by low temperature PEALD using an oxidant, which is compatible with moisture/oxygen sensitive materials. CO2 and Bis(tertiary-butylamino)silane (BTBAS) were used as ALD precursors. The processes were carried out in a Beneq TFS 200 reactor at 90 ˚C. A thorough analysis was conducted on both growth characteristics and film properties. The CO2-based PEALD SiO2 films with a saturated growth-per-cycle of 1.15 Å/cycle, a refractive index of 1.45 (at 632.8 nm) and a density of 2.1 g/cm3 were obtained. The films showed low impurity levels with bulk concentrations of ~ 3 and ~ 0.15 at. % for hydrogen and nitrogen, respectively, whereas the carbon content was found to be below the measurement limit of TOF-ERDA. A low residual stress of 30 ± 10 MPa (tensile) was measured on 50 nm-thick SiO2 films. Furthermore, good optical properties, high transparency and zero extinction coefficient, were obtained on sapphire substrates at wavelength range of 400-800 nm. In addition, the highest deposition speed of 62 nm/h was achieved, and we concluded that an increase of the deposition speed above 180 nm/h is potentially reachable by further optimization of process parameters. [1] J. W. Klaus, O. Sneh, S. M. George, Science. 1997, 278, 1934. [2] J. D. Ferguson, E. R. Smith, A. W. Weimer, S. M. George, J. Electrochem. Soc. 2004, 151, G528. [3] M. Putkonen, M. Bosund, O. M. E. Ylivaara, R. L. Puurunen, L. Kilpi, H. Ronkainen, S. Sintonen, S. Ali, H. Lipsanen, X. Liu, E. Haimi, S. P. Hannula, T. Sajavaara, I. Buchanan, E. Karwacki, M. Vähä-Nissi, Thin Solid Films 2014, 558, 93. [4] Z. Zhu, C. Modanese, P. Sippola, M. Di Sabatino, H. Savin, Phys. Status Solidi 2017, 215, 1700864.