Dry etching and low temperature plasma oxidation were used to sharpen Si field emitter tips. Si emitters with tip radii of 67 nm were formed by dry etching in a Cl-2 plasma at 5 mTorr. An electron cyclotron resonance (ECR) source was used to generate the plasma and erosion of a SiO2 mask was employed to control the Si emitter geometry. The Si emitters were sharpened by dry etching in the Cl-2 plasma generated at 0.7 and 5 mTorr after the SiO2 mask was completely eroded. After etching the emitters for 7 min at 0.7 mTorr, the tip radius decreased from 67 to 22 nm. The sharpening effect could be attributed to enhanced sputtering yield at sloped surfaces. At higher pressure of 5 mTorr, the sharpening effect was reduced and the tip radius only decreased from 67 to 62 nm. Optical emission spectroscopy and mass spectrometry were used to monitor the etching and sharpening of Si emitter tips. Sharp increases in the Si emission signal at 288.1 nm and mass spectrometric signal for (SiCl+)-Si-63 were observed once the SiO2 mask was completely eroded and the sharpening of Si emitters had started. Plasma oxidation was also used to sharpen the emitter tips. With the stage at room temperature, up to 257 nm of plasma oxide was grown in an O-2 plasma generated at 0.3 mTorr by the ECR source. After oxide removal, the emitters tip radius was found to decrease from 67 to 8 nm. In comparison, the same amount of oxide grown by thermal dry oxidation at 950 and 1100 degrees C only reduced the tip radius to 32 and 50 nm, respectively. Sharper emitter tips and faster oxidation rate can be obtained by plasma oxidation.