espectively. Figure 2a shows a SEM image of a SU-8 VANA with a diameter of 1 μm produced with an UV exposure dose of 200 mJ/cm2. The aspect ratio of the structure is more than 7. Figure 2b shows the pyrolyzed carbon VANA after pyrolysis, where the volume shrinkage is 60% and the aspect ratio is increased to more than 9. Figure 2c shows the ITO-coated VANA with 0.1-μm-thick ITO layer. Utsumi concluded that the best field emitter should be high aspect ratio rounded whisker like this ITO-coated VANA . Figure 2d presents energy dispersive spectrum (EDS) results showing the chemical composition of the ITO-coated VANA, which comprises In, Sn, O, C, and Si.
Figure 2. SEM images of a SU-8 VANA, b pyrolyzed carbon VANA, and c ITO-coated VANA. d EDS result of ITO-coated VANA
Figure 3a presents a schematic view of the experimental setup for investigating the field emission characteristics with a Zyvex nanomanipulator (Zyvex Instrument) operating inside a SEM vacuum chamber. The nanomanipulator controls the distance between the ITO-coated VANA, the field emitter, and the tungsten tip A, the counter electrode. The tungsten tip A, the anode, is connected to the (+) of a Keithley 4200 (Keithley Instrument Inc.), while the tungsten tip B, the cathode, on the surface of the wafer is connected to the (-) of the instrument via a feed through to apply voltage and to sense current. Figure 3b shows a SEM image of the ITO VANA and the tungsten tip A in a SEM vacuum chamber of 1.59 × 10−5 torr.
Figure 3. a Schematic view of the experimental setup, where a Zyvex nanomanipulator is employed in a SEM vacuum chamber. b SEM image of the tungsten tip A and ITO-coated VANA with distance control by the nanomanipulator
With the shortest distance, d, of 0.6 μm, the current is measured by the Keithley 4200 using the voltage sweep function. As the voltage and the distance are known, the I-E curve of the ITO-coated VANA can be determined, as shown in Fig. 4a. As the electric field intensity (E) is increased, the current (I) increases exponentially, following the behavior of the Fowler–Nordheim (F–N) equation. The field emission starts around 2.0 V/μm and the maximum current is 4.0 × 10−6A. Figure 4b shows the F–N curve obtained from the I-E curve of the ITO-coated VANA. This curve shows a linear relationship after turn-on electric field intensity, following the F–N equation, as indicated by the red line. The turn-on electric field intensity is 2.0/μm, which can be estimated where the slope of the F–N curve changes. Figure 4c shows the field emission current stability of the ITO-coated VANA over a period of 3 min, measured under a vacuum of 1.59 × 10−5 torr when the electric field intensity (E) is 2.5 V/μm.
Figure 4. a Field emission I-E curve of the ITO-coated VANA measured under a vacuum of 1.59 × 10−5 torr. b Corresponding F–N curve obtained from I-E curve. c Field emission current stability of ITO-coated VANA at 1.59 × 10−5 torr
where I, E, β, φ, A, ℏ, and m are the current, electric field intensity, field enhancement factor, work function, area, reduced Planck constant, and electron mass, respectively. This equation explains the shape of the I-E curve in Fig. 4a and can be modified as follows,
Note that −21.18 is the slope of the red line and −5.76 is the y-intercept of the red line in Fig. 4b. The estimated field enhancement factor of the ITO-coated VANA, β, is 3,078 when the work function of ITO is 4.5 eV . The measured value of the field enhancement factor is comparable with previous research results of field emitters [4,14,15].
In summary, an ITO-coated VANA was fabricated by a top–down method using modified conventional UV lithography, and the field emission characteristics were evaluated using a Zyvex nanomanipulator. The top–down method offers many advantages including an economical process, good fabrication reliability, and suitability for mass production. The turn-on electric field intensity of the ITO-coated VANA is about 2.0 V/μm, and the estimated field enhancement factor β is 3,078. These results show that the ITO-coated VANA is a very prom