provide low turn-on voltage [7,8]. ITO has good transparence characteristics in the visible region of the electromagnetic spectrum while maintaining high electrical conductivity, thermal stability, and oxidation resistance [4,9]. In a field emission experiment, a nanomanipulator in a scanning electron microscope (SEM) is used in order to measure field emission while precisely controlling the distance between the anode and cathode (field emitter).
The fabrication process of the ITO-coated VANA is shown in Fig. 1. Figure 1a shows circular aperture patterns (φ = 1 μm) of a 0.1-μm-thick chrome (Cr) layer on a fused silica wafer. SU-8 50 (Microchem Co.) is deposited on the Cr layer in Fig. 1b. The backside of the fused silica wafer is exposed to UV light filtered by a narrow band-pass filter (λ = 365 nm and bandwidth = 10 nm, OptoSigma Co.) with an exposure dose of 200 mJ/cm2 in Fig. 1c. The intensity of UV light is concentrated to the central axis by diffraction in the SU-8 medium, which defines a sharp, high aspect ratio SU-8 structure . After development of SU-8 and Cr etching, the SU-8 VANA is obtained, as shown in Fig. 1d. Figure 1e shows the pyrolyzed carbon VANA obtained by a pyrolysis process in a quartz tube furnace at 900°C for 30 min . Volume shrinkage to approximately 60% occurs in this process for the SU-8 VANA. Finally, Fig. 1f shows the VANA coated with a 0.1-μm-thick ITO layer. The size of VANA is controlled by the circular aperture patterns of a Cr layer on a fused silica wafer and the aspect ratio by the UV light exposure dose.
Figure 1. Schematic view of fabrication process of ITO-coated VANA. a Circular aperture patterns (φ = 1 μm) of Cr layer on a fused silica wafer. b SU-8 50 on Cr layer. c Backside UV exposure with exposure dose of 200 mJ/cm2 filtered by a narrow band-pass filter. d SU-8 VANA after development of SU-8 and Cr etching. e Pyrolyzed carbon VANA by pyrolysis process at 900°C for 30 min. f ITO-coated VANA with 0.1-μm-thick ITO
Figures 2a–2c are the fabrication results of Fig. 1d–1f, respectively. 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, 