A method and apparatus for generating a molecular or atomic fluorine jet or beam under vacuum for etching surfaces. The apparatus uses a hollow crystalline tube preferably fabricated from a crystal of a Group II fluoride such as magnesium fluoride. A terminal portion of the tube is heated to over 1000.degree. C. and a mixture of fluorine gas and an inert carrier gas is induced under pressure into the tube. An atomic fluorine jet is emitted from the opposite end of the tube. The jet can be collimated into a beam and can be directed at masked surfaces for selective etching of the surface. The atomic fluorine source has a very high intensity resulting in rapid etching of materials and as a beam is capable of highly anisotropic etching. The jet may be primarily molecular fluorine if the tube is heated to a lower temperature.