On-chip field emission electron source with silicon nanowire emitters, integrated gate electrodes, and benzocyclobutene as insulation material

Silicon nanowire field emitters were fabricated on top of microtubes on a silicon chip (dimensions including contact pads: 8 × 8 mm). There are 2437 microtubes (diameter: 8 μm, height: 30 μm, spacing: 40 μm) in the array over an area of 3.38 mm2. A process was developed to create an integrated metal gate for the extraction electrode. This involved coating the emitters with a uniform benzocyclobutene layer and fabricating a titanium/nickel extraction gate using lift-off. A plasma etching process was used to selectively remove the benzocyclobutene anisotropically around the microtubes, ensuring the preservation of the emitters and the extraction gate. The design reduces the capacitance between the cathode and gate and maximizes the length of the leakage paths between the cathode and gate. This reduces the overall risk of discharges between the two electrodes as well as the energy stored in the system for a given extraction voltage. The electron sources demonstrated transmission rates exceeding 96%, with currents reaching 0.4 mA at extraction voltages of 250 V for several hours under voltage-controlled operation. Investigations after the measurements showed that isolated discharges destroyed individual emitters during operation, but the electron sources remained functional. Consequently, these cathodes demonstrated resilience to failure caused by discharge-related damage due to the low capacity of the system. At higher voltages (300 V) and currents (2 mA); however, these samples exhibited catastrophic arcing, as at these voltages, the energy stored in the capacitance was large enough to result in the destruction of larger areas of the arrays and the formation of conductive paths between the cathode and the gate.