| AVS 61st International Symposium & Exhibition | |
| Thin Film | Monday Sessions |
| Session TF+PS-MoM |
| Session: | Atmospheric, Roll-to-Roll and other Manufacturing Advances in ALD |
| Presenter: | MoatazBellah Mousa, North Carolina State University |
| Authors: | M. Mousa, North Carolina State University C.J. Oldham, North Carolina State University G.N. Parsons, North Carolina State University |
| Correspondent: | Click to Email |
Under typical low pressure ALD conditions, ozone (O3) is reported to speed up ALD processes compared to water-based reactions because shorter purge times are needed to fully desorb ozone O3. Many high-throughput ALD processes are designed to operate at atmospheric pressure where viscous fluid transport can have significant effects. We developed an ALD process using trimethyl aluminum (TMA) and O3 in a variable-pressure flow tube reactor and measured growth rates, film composition and film uniformity in the growth zone for pressures between ~2 Torr and 760 Torr and temperature ranging from 70°C to ~250°C. We also adjusted overall gas flow rate to study the role of gas residence time. Film thickness was determined by ellipsometry and growth was monitored using an in-situ Quartz Crystal Microbalance (QCM). We observe self-limiting growth between ~150 °C and 250 °C at both ~2 Torr and 760 Torr, and larger growth rate at lower temperature. At high pressure the growth rate is ~20% larger than at low pressure, which is ascribed to slower transport of desorbing product species through the boundary layer at high pressure. We also find that longer O3 exposure times are needed compared to low pressure growth. This is consistent with a model for the ozone dissociation kinetics showing that higher pressure enhances the rate of ozone loss. The ozone depletion also predicts an observed gradient in film growth rate under sub-saturation conditions along the length of the reactor. During TMA/O3, O insertion leads to surface methoxy and formate groups at low temperature, whereas surface hydroxyls form at higher temperature. In our reactor, QCM analysis shows evidence for this temperature-dependent surface reaction mechanism at 2 Torr, and it persists at 760 Torr under saturated conditions. Under saturated growth conditions, TOF-SIMS analysis shows films deposited at 2 Torr and 760 Torr have similar composition, with some extra carbon contamination at higher pressure. Overall, similar ALD growth can be achieved for TMA/O3 at 760 Torr and ~2 Torr, where care must be taken to take into account the faster rate of O3 dissociation at higher pressures, especially at higher temperature.