Surface modification of Si field emitters by CHF3 plasma is investigated for damageless vacuum sealing. The emission characteristics bf nontreated Si field,emitters and CHF3-plasma-treated Si field emitters were measured in an ultra-high-vacuum chamber and in a frit-sealed vacuum package. The vacuum sealing process decreased the emission currents from the nontreated Si field emitters by a factor of 10. On the contrary, the emission current from the CHF3-plasma-treated emitters did not change before and after the vacuum sealing process. Analysis by Auger electron spectroscopy and by x-ray photoelectron spectroscopy revealed that the CHF3-plasma-exposed Si surface was covered with SIC and fluorocarbon films. These carbon-related films work as a protective material in the sealing process. By using this method, we achieved the first operation of metal-oxide-semiconductor field-effect-transistor-structured Si field emitters in a vacuum package.