A method has been developed to determine the water vapor permeability of very thin dielectric films over a large range of thicknesses. It is shown that the permeability of poly(chloro‐ p ‐xylylene) unsupported thin films is the same throughout the range of thicknesses tested: 292–73,388Aå. The results agree with values reported on thick samples under similar test conditions and with thick samples obtained by standard flow methods. The results also compare favorably with the water vapor permeability of other polymeric materials. Since the permeability obtained indicates that thick ( ) films are structurally continuous, then ultrathin (<300Aå) films of poly(chloro‐ p ‐xylylene) must also be free of voids. In addition, the method is sensitive enough to detect discontinuities as small as 1µ in diameter.
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Spivack et al. (1969) studied this question.