Electron emission from a patterned substrate is investigated in order to achieve the uniform current density. Square mesas of 10 mu m X 10 mu m and 100 mu m X 100 mu m sizes are formed on (100) GaAs surface by a photolithography technique and wet etching. Each square mesa is 3 Am high and its sidewalls are vertical. The field enhancement factor of the patterned substrate is estimated to be around 240. Field emission characteristics are compared between boron nitride (BN) films deposited on flat and patterned substrates. The threshold electric field is estimated to be 17 and 10 V/mu m for BN films on flat and patterned substrates, respectively. It is demonstrated that the introduction of patterned substrates is effective in not only reducing the threshold electric field but also improving the uniformity of the electron emission. (c) 2005 American Vacuum Society.