Field emission cathode arrays of Betsui type [K. Betsui, Technical Digest of the International Vacuum of Microelectronics Conference, Nagahama, Japan, 1991 (unpublished)] need an intermediate spacer layer with excellent insulator properties, i.e., ultrahigh resistivity, high breakdown voltage, excellent adhesion both to substrate and gate layer, low roughness, low dielectric constant, and sufficient long term stability. This study reports on an investigation of various fabrication methods for this insulating film in order to optimize its properties. Silicon and some compounds (a : Si, SiO, SiO2, SiO(x)N(y)) have been deposited by e-beam or thermal evaporation and plasma enhanced chemical vapor deposition, respectively. These layers have been annealed or oxidized before deposition of the extraction gate layer. The structures have been characterized by means of capacitance measurements. The best results have been obtained with the deposition of pure silicon by e-beam evaporation followed by an oxidization process. The method described in detail results in an insulator consisting of SiO2 which exhibits excellent insulating properties (breakdown field strength 7 MV/cm). Field emission cathodes fabricated in this manner exhibit stable emission currents up to 1 muA/tip.