PURPOSE: To enhance the withstand voltage property between an upper and a lower electrode using a thin insulation layer and increase the max. electron emission amount by oxidating the specific A1 surface formed on a base board, forming an insulation layer on the under lying metal and making the interface with the overlying metal especially flat and smooth.

CONSTITUTION: SiO2 is vacuum deposited on a Si (100) plane base board 1, and an insulation thin film 2 having opening of 160μm is produced through photo-lithographic etching. This is heated in a reaction tube together with dimethyl A1 hydride gas and H2 gas, and A1 (111) mono-crystals are deposited on the opening part. Then the A1 surface is anode oxided in H3BO4 electrolytic solution, and an insulation layer 4 consisting of Al2O3 is formed. This is followed by Au deposition through two runs of vacuum deposited and liftoff process in such an arrangement that the central part is solely made thin while the other parts are thick, and thus a overlying metal 5 is formed. This eliminates likelihood of easily destroying the MIM structure consisting of the layers 3, 4, 5 even through the voltage is increased or impressed for a long period of time.