To increase the efficiency of field emission devices, the design of the emitters should be optimized. A simulation program using the finite element method was developed to investigate the effects of emitter geometry on the emission current and voltage for ungated emitters. It was found that the tip-anode spacing and tip radius have the most significant effects on the emitted current. The simulation predicted that a field emitter consisting of a vertical base and a cone shaped tip on the top has a higher enhancement factor and therefore a lower operating voltage. Emitters were fabricated in Si based on the optimized design. Dry etching in CF4 was used to control the slope of the SiO2 mask. The SiO2 sidewall angle decreased from 80 degrees to 51 degrees as pressure was increased from 40 to 300 mTorr. The SiO2 mask profile was also changed substantially by adding CHF3 or O-2 to CF4 during etching. An electron cyclotron resonance source was used to generate a Cl-2 plasma to etch Si field emitters. SiO2 mask was eroded during etching to control the Si emitter profile. The slope of the Si etch profile increased from 52 degrees to 78 degrees as the SiO2 slope increased from 17 degrees to 45 degrees. Radio frequency and microwave power had little influence on the Si etch profile, but they were adjusted to achieve the desired etch rate. The lateral etch rate of SiO2 decreased at higher pressure, resulting in more vertical Si profiles. Oxygen was also added to the Cl-2 plasma and the Si sidewall angle was changed between 47 degrees and 80 degrees. Using the above results, the Si etch profile was controlled and field emitter tips and arrays with different geometries were etched in Si. To comply with the optimum design, emitter tips with two different sidewall angles of 85 degrees and 70 degrees were fabricated by changing the pressure from 5 to 0.5 mTorr during the etch. Arrays of emitter tips with sidewall angle of 80 degrees, 11 mu m height, and 2.2 mu m basewidth were also fabricated at 10 mTorr with a packing density of 4x10(6) tip/cm(2)