A solid state electrochemical source was built that allows fluorine dosing at pressures below 8X10(-11) Torr. The cell consists of a CaF2 crystal that is clamped between two Au plates that function as the cathode and anode. An applied voltage at elevated temperature produces an ionic current due to fluorine migration;to the anode where it desorbs. This cell allows submonolayer halogen exposures, and it is ideal for studies of halogen etching because it minimizes degradation of the vacuum system and avoids toxic chemicals.