Process Control in OVPD

Key requirements for industrial deposition of single films are the rate of deposition achievable, controllability and reproducibility, and film quality, which is important for high uptime and production yield. In contrast to VTE, in which the deposition rate is controlled by the evaporation temperature in the crucible, OVPD is kept under steady-state temperature conditions and the deposition rate is adjusted only 

These calibration curves also reveal the huge increase of deposition rate with temperature, as expected from the Clausius-Clapeyron equation. For small flows the deposition rate increases linearly with source flow whereas at flows 500 seem the increase is sub-linear. With higher flows the deposition rate is dominated by the flow restriction from the process chamber to the source container. If the evaporation temperature is increased from 306 to 312 °C the deposition rates increases approximately 47%, from 19.2 to 28.2 A s-1. 

By combining multiple separate source containers of the same organic material in a parallel configuration the deposition rate can be linearly increased, leading, for example, to deposition rates of up to 60 A s-1 for two Alq3 sources. 

The uniformity of such an OVPD film of Alq3 is shown in Fig. 9.6. Analysis by variable angle spectroscopic ellipsometry (VASE) confirmed the surface was smooth across the entire substrate area with thickness deviation of +1.7%, a standard deviation, a, of 1.0% only. Atomic force microscopic analysis of such a typical film revealed RMS values to be 6 A, i.e. thickness differences in the range of a single monolayer only, irrespective of deposition rate [20-22]. 

400 x 400 mm2 glass substrate measured by variable angle spectroscopic ellipsometry (VASE) across the diagonal (b) 

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