We have devised an apparatus useful for and a method of removing
impurities from vaporous precursor compositions used to generate reactive
precursor vapors from which thin films/layers are formed under
sub-atmospheric conditions. The method is particularly useful when the
layer deposition apparatus provides for precise addition of quantities of
different combinations of reactants during a single step or when there
are a number of different individual steps in the layer formation
process, where the presence of impurities has a significant affect on
both the quantity of reactants being charged and the overall composition
of the reactant mixture from which the layer is deposited. The method is
particularly useful when the vapor pressure of a liquid reactive
precursor is less than about 250 Torr at atmospheric pressure.