화학공학소재연구정보센터
Journal of Vacuum Science & Technology A, Vol.27, No.2, 342-351, 2009
The direct injection of liquid droplets into low pressure plasmas
A much greater number of useful precursors for plasma-enhanced chemical vapor deposition (PECVD) can be dispersed in high vapor pressure solvents than can be put into the vapor phase directly. In order to enable the use of such precursors, the authors investigated a method by which one can directly inject these liquids as microdroplets into low pressure PECVD environments. The solvent evaporates first leaving behind the desired precursor in the gas/plasma. The plasma dissociates the vapor and causes the deposition of a composite film (from precursor, solvent, and plasma gas). The authors made preliminary tests using Fe nanoparticles in hexane and were able to incorporate over 4% Fe in the resulting thin films. In addition, the authors simulated the process. The time required for a droplet to fully evaporate is a function of the background pressure, initial liquid temperature, droplet-vapor interactions, and initial droplet size. A typical evaporation time for a 50 mu m diameter droplet of hexane is similar to 3 s without plasma at 100 mTorr. The presence of plasma can decrease the evaporation time by more than an order of magnitude. In addition, the model predicts that the temperature of the injected droplet first decreases by evaporative cooling (to similar to 180 K for hexane); however, once the solvent has fully evaporated/sublimated, the plasma heats any remaining solute. As a result the solute temperature can first fall to 180 K, then rise to nearly 750 K in less than 1 s.