The effect of changing the gap width on the bulk plasmas was examined by the simple model calculation and by the diagnostics for ultrahigh frequency (UHF) (nonmagnetized) nd UHF electron cyclotron resonance (ECR) (magnetized) plasma. Higher volume/surface ratio (wider gap width at constant chamber diameter) brought higher electron density and lower electron temperature in the bulk plasma. The uniform discharge model explained the effects of the gap width change on these basic plasma parameters by the particle and energy balance. The latest spectroscopic and spectrometric measurements were applied to Ar/c-C4F8-based oxide etching plasma, and monitored the change of the electron impact reaction rate coefficients ([sigma upsilon]) for argon emission, c-C4F8 dissociation, and fluorine radical generation. The results quantitatively showed that the wider gap width (lower electron temperature) suppressed these electron impact reactions, In addition, UHF and UHF-ECR showed different gap width dependency in these [sigma upsilon]'s by the effect of magnetic confinement for the UHF-ECR plasma. Higher N-e condition was achieved at the wider gap width without changing the fluorine density. As a result, higher etching rate of the thermal oxide was achieved in the wider gap width.