As silicon has advantage of the highly developed micro-fabrication process and easiness of tip process, has been made many investigations in terms of field emitter tip material of FED(Field Emission Display), scanning tip material of AFM(Atomic Force Microscopy), and so on. However, overcome of problem related to low electrical and thermal conductivity and high responsibility of surface is requred for commercial application. In fact, the properties have influence on properties of electric field emission or uniformity. Coating of silicon tip by foreign material is required for complement of previous problem. Coating material used is chromium and molybdenium.
Silicon oxide layer to act as protective etch mask for silicon, with thickness of 2400~2600Å was formed on highly doped n-type <100> silicon wafer through wet oxidation at 1100℃. Oxide mask was made by photo-lithography and was etched by RIE(Reactive Ion Etch). Tip was etched through 2 steps.; Tip with thickness of 1300Å on the end of tip and height of 1㎛ was fabricated during isotropic etching process. It was followed by anisotropic etching process using gas mixture of He and $Cl_2$ and was obtain tip with final height of 1.5㎛, 2㎛ and tip-to-tip spacing of 5㎛. Wet oxidation was carried out at 900℃ for tip sharpening, and then, using BOE(Buffered Oxide Etchant), oxide layer was removed.
Chromium and molybdenium was sputtered on <100> silicon wafer at 350℃, pressure of 4 mtorr using RF-magnetron sputter. Thickness sputtered on silicon tip array was 150Å in order to prevent from variation of tip shape. We was able to observe no variation for tip shape by SEM.
In next step, we annealed some sample at 425℃ for 20 min to advance adhesive property of interface between silicon and metal and to stablize interface.
We measured properties of field emission using Keithley SMU(Source Measure Unit) 237 in UHV(Ultra High Vacuum) of $2×10^{-8}$ torr and compared bare silicon tip with chromium-coated tip in terms of properties of electron field emission. In this case, distance between cathode and anode was 33㎛.
The current-voltage characteristics among silicon tips not coated, coated with chromium, and annealed after coating were measured as increase from 0V to 1100V by 10 volt. A major improvement in field emission properties, that is, in terms of field emission current increase and turn-on voltage decrease, was made by tip coating and coated tip annealing. Especially, sample annealed at 425℃, 20min was showed more improvement in comparison with that not annealed. It corresponds to the fact that Schottky barrier height of $CrSi_2$ is lower that of chromium. In this experiment, turn-on voltage of silicon tip measured is respectively 710, 600, 630 V. A Fowler-Nordheim(FN) slopes of pure silicon tip array, chromium-deposited tip array, and annealed tip array calculated in FN plot are approximately -10.5, -7.8, -8.8, respectively.
The stability of field emission was compared in aspect of variation of current under constant 1000V for 20 minutes and were observed through difference of emission properties in the case of reiterated operation as voltage increased from 0V to 1100V by 10V for 6 times. We found that specimen annealed at 425℃, for 20 minutes is superior to that not annealed.
In general, the field emission currents can result in joule heating of the emitter-tip cathode. Therefore, in this case, it cannot be overlooked to interact between interfaces of silicon base and metal layer. We could confirm from previous result and AES analysis.
The current-voltage characteristics among silicon tips not coated, coated with molybdenium were measured as increase from 0V to 1100V by 10 volt. The formation of molybdenium thin layer onto microfabricated silicon field-emitter tip resulted in 50%~200% increase in the emission current. In addition to, the field emission current of the tip with 2㎛ height were advanced in comparison with that of the tip with 1.5㎛.
