in a vacuum chamber 10 the electrode 13 was grounded and high frequency voltage was applied from an rf power supply 14 to the sample stage 12 to generate a plasma the sample 11 was thus surface treated by the plasma in the above the chamber 10 was evacuated to approximately 10 6 torr and then an inert gas or a mixture of inert gases was fed at the flow rate of 10 cc min at a gas pressure of 2 mtorr into the chamber 10 the frequency of a power supply 14 was 13 56 mhz the rf output was 100 w and the plasma treatment time was 1 min 1 first as shown in fig1 a semiconducting diamond film 2 was formed on a substrate 1 to a thickness of approximately 2 u03bcm by microwave plasma cvd as a reaction gas for diamond film deposition ch 4 was diluted by h 2 concentration 0 5 and a doping gas of b 2 h 6 concentration 0 01ppm was added during the cvd the substrate temperature was maintained at 800 u00b0 c and the gas pressure in the reaction chamber was 35 torr the cvd was continued for 7 hours the diamond film 2 thus obtained was then coated with a photoresist film 3 using the standard photolithography technique 2 as shown in fig1 b openings according to the specified electrode pattern were.