Study on surface morphology of excimer laser micromachined glasses and its application in fabricating micro-tip array

Glass materials, which have been widely employed in MEMS, semi-conductor, optical communications and bio-medical technology, are superb for its high transparency, high rigidity and high chemical stability. However, the brittleness of glass materials makes them easily crack with traditional machining process. In this research, various glass materials including BK7, Soda lime and Zerodur glass, are micro-machined by ArF (λ= 193 nm) excimer laser. The relation between the surface morphology and machining parameters is studied. The results show that the main factor for the V-shape trenches on the laser-machined surface is due to the low thermal conductivity of the glass materials, which brings about the Gaussian distribution of the surface temperature with the highest temperature at the center of the machining zone. The ablation rate is reduced by the plasma shielding effect resulted from the high repetition rate– which also enlarges the thermo-erosion area. On the other hand, the microstructure of the glass is cracked with low repetition rate of the laser. When the machining depth becomes deeper, the tape angle of the trenches becomes sharper and the aspect rate becomes higher as well. When the feature size of the mask is less than 10 μm, the accuracy of the machining dimension is not easy to be controlled, because of the plasma thermo-erosion, and this makes the trenches more likely to be V-shaped. When the feature size of the mask becomes larger, the etching depth becomes deeper, and the more likely U-shaped trenches are formed. Base on this study, a novel process for fabricating micro-tip array by means of glass mode engraved by ArF excimer laser micromachining is proposed. The surface morphology and the dimension density of the glass work-piece are controlled by laser parameter, and the Ni-Co micro tip is formed by microforming technology. The micro-tip array with dimension density as high as 4901/mm2 and the radius of the tip is less than 100 nm, is successfully fabricated. Because the emission of electrons is facilitated with higher density and sharper tipping of micro-tip array, this research has a high potential in the application of field emission. A Spindt-type field emission array( FEA)of Ni-Co alloy is manufactured with the proposed techniques. In order to reduce the work function of field emission, the micro-tip array of Ni-Co alloy is deposited a layer of amorphous diamond(DLC）. The FEA emission properties under different density of micro-tip, surface morphology and surrounding temperature are then studied. The result shows that the current density is increased form 0.027 mA/cm2 to 31.42 mA/cm2–about 11000 folds amplified, when the FEA of Ni-Co alloy is coated with a DLC under the applied voltage of 110 V/μm. The FEA of Spindt-type developed in this research consists of high density and sharp tips, allowing high current density (83.48 mA/cm2), which is much higher than what can be obtained with FEA of CNTs-type. When the density of micro-tip array of FEA is elevated from 2500/mm2 to 4901/mm2, the lowest turn on field does not suppressed but the current density is increased from 21.6 mA/cm2 to 31.42 mA/cm2, an increase by over 45%. In summary, the fabricated FEA of Ni-Co alloy is very suitable for field emission device for its low turn on field and high current density.