Devices that can field-ionize gas molecules at low bias voltages are essential for many applications such as ion mobility spectrometry and highly selective porta- ble gas sensing. Field ionization consists of a valence electron of a gas atom or molecule tunneling through a potential barrier, commonly into a vacant energy state of the conduction band of a metal at the anode. Classic ion sources require extremely high positive electric fields, of the order of 108 volts per centimeter. Such fields are only achievable in the vicinity of very sharp electrodes under a large bias. Ion sources based on microwave plasma generation have demonstrated high currents and high current densities. Yet, they are bulky and require large magnetic fields. Alternatively, single or arrays of gated tip structures have been used as field ionizers, but they emit low currents (~10 nA). Early tip burn-out due to non-uniform tip distribution and low voltage breakdown are the two main causes of such low currents.
In this work, Si field ionization arrays (FIAs) with a unique device architecture that uses a high-aspect-ratio (~50:1) silicon nanowire current limiter to regulate electron flow to each field emitter tip in the array is proposed. The nanowires are 10 µm in height, 1 µm apart and 100-200 nm in diameter. A dielectric matrix of (Si3N4/SiO2) supports a poly-Si gate while a 3 μm thick dielectric holds the contacts. Current densities >100 A/cm2 and lifetime > 100 hours have already been reported. The tip radius has a log-normal distribution varying from 2 to 8 nm with a mean of 5 nm and a standard deviation of 1.5 nm, while the gate aperture is ~350 nm. Field factors, Beta, > 1 × 106 cm-1 can be achieved with these devices implying that voltages of 250-300 V (if not less) can produce D+ ions based on the tip field of 25-30 V/nm.
Breakdown at the mesa edge at voltages ~70 V was the reported by Guerrera, et al. However this has now been overcome by etching a vertical sidewall profile with a combination of both SF6 and C4F8 flowing simultaneously. I-V characterization in air demonstrates breakdown occurs within the active region possibly due to the narrow gate apertures and the short oxide thickness from the tip to the poly-Si gate. Initial results show that further etching this oxide to expose the nanowire increases the oxide separation to the gate, which in turns increases the breakdown voltage, thus enabling the Si FIAs to be operated at voltages exceeding 200 V.