Abstract
Spindt-type field emitter arrays (FEAs) are FEAs with an integrated gate electrode fabricated using semiconductor manufacturing processes and have been applied to a variety of devices.[1] Spindt-type FEAs are fabricated by the deposition of emitter material inside a cavity through a hole formed in its ceiling, which works as the gate electrode. To fabricate sharp emitter shapes, it is important that deposited particles of emitter material must be injected along the normal to the substrate, and a vacuum evaporation technique with a long source-substrate distance has been used because it enables this. However, even in vacuum evaporation, there are particles incident on the substrate at an angle tilted from the substrate normal. Therefore, large equipment with a sufficient distance between the source and the substrate is required to increase the proportion of particles in the normal direction. This reduces the efficiency of material use and makes it hard to fabricate large-area FEAs. In contrast to vacuum evaporation, sputter deposition can fabricate a uniform film over a large area. In addition, while it is hard to deposit compounds with vacuum evaporation, sputter deposition can easily prepare compounds such as nitrides by using a metal target and reactive gases. However, most sputtered particles are electrically neutral, and the angle of incidence cannot be controlled. Therefore, we used a triode high-power pulsed magnetron sputtering (t-HPPMS), which can improve the directionality of sputtered particles. [2] In HPPMS, short pulse power with a low duty ratio is applied to the target to generate high-density plasma, which efficiently ionizes the sputtered particles for deposition. Furthermore, in t-HPPMS, the plasma potential can be controlled by the positively biased cap electrode. By increasing the plasma potential, the increased potential difference between the plasma and grounded substrate improves the directionality, enabling the fabrication of Spindt-type emitters. In this study, Hf and HfN Spindt-type emitters were fabricated by t-HPPMS, and their shapes were compared. The electron emission of Hf emitters was also measured; since the sputtering yields of HfN and Hf are different, the ionization rates, which are important for t-HPPMS, are also different. Therefore, the directionality of the sputtered particles changes and affects the emitter shape. First, HfN and Hf Spindt-type emitters were fabricated using t-HPPMS with an Hf target at an Ar gas pressure of 0.6 Pa. HfN emitters were fabricated using an Ar and N2 gas mixture. When the plasma potential is set too high, compressive stress in the deposited film becomes stronger and delamination occurs. The cap voltage was set at 10 V to form sharp emitters without exfoliation. Comparing the shape of each emitter, the Hf emitter had a higher aspect ratio. This is because the sputtered Hf particles were ionized more. Unfortunately, the HfN emitters did not work as an emission device because many particles adhered to the surface during HfN deposition. Therefore, emission measurements were performed only for Hf emitters. Emission characteristics were evaluated in an ultra-high vacuum chamber at a pressure of about 10-8 Pa. The gate electrode was positively biased and the emitter was grounded. Emission currents were collected at an anode placed above the emitter. Hf emitters started emission when 60 V was applied to the gate electrode. A maximum emission current of 6 nA was confirmed when 100 V was applied. The Fowler-Nordheim plot obtained from the measured current shows a straight line, which represents that the current is due to the field emission.
References:
[1] C. A. Spindt, J. Appl. Phys, 39, 3504-3505 (1968).
[2] Nakano, et al., JVSTB, 40, 063102-1-9 (2022).
