A negative hole is formed by etching a dielectric layer that includes at least a lower dielectric sublayer and an upper dielectric sublayer. The lower dielectric sublayer and the upper dielectric sublayer have substantially the same permittivity, and the lower dielectric sublayer may have a higher etching rate lower than the upper dielectric sublayer. The negative hole formed in the upper and lower dielectric sublayers has an etched profile with a protruded portion protruding from at least the boundary between the lower dielectric sublayer and the upper dielectric sublayer. With various embodiments of the disclosed invention, resistance between the cathode and the gate may be secured to prevent arc generation and signal distortion.