the rie is carried out with an hbr o 2 mixed gas or an hbr cl 2 mixed gas using the resist pattern r 1 having the cured film re as a mask and the silicon nitride film 2 b as an etching stopper rie is preferably performed with an ecr ion source or an icp reactor after the etching the resist pattern r 1 and the cured film re are removed by for example ashing the patterned polycrystalline silicon film 3 is oxidized by heating at 900 u00b0 c for 30 to 60 minutes in an hcl o 2 atmosphere to form a silicon oxide film 4 s as shown in fig8 a as a result the interelectrode distance increases by the two thicknesses of the silicon oxide film 4 s this does not ruin the fineness of the electrode pattern because the gap distance before the oxide film formation has been sufficiently reduced by the relacs technique in this particular embodiment the final gap distance g see fig8 b is 0 2 u03bcm the silicon oxide film 4 s is then covered with an insulating film 4n of silicon nitride by vacuum cvd using ammonia and dichlorosilane as shown.