Pyroelectric lithium niobate electron emission-based ion-pump

Abstract

This work presents a compact ion-pump that utilizes pyroelectricity for ionization of ambient molecules. The pyroelectric ionization is realized by heating a lithium niobate (LiNbO3) crystal with low voltage resistive heating (10 V drive voltage, 0.5 W Joule heating) to generate a high voltage across the poled surface of the crystal. Pyroelectrically generated electrons from the polarized surface of the crystal are accelerated using an electric field generated by both the crystal and external titanium (Ti) electrodes biased at ±300 V, which results in the ionization of molecules in the chamber. The low collector current in the pA range ensures that the power consumed due to ionization current is lower than the LiNbO3 heating power. The ionized gas molecules are accelerated toward the Ti collector electrodes where they are implanted owing to large acceleration produced by the collector electrodes. The system is configured as a sputter pump for gettering ions to reduce chamber pressure from the baseline value of 1.4 μTorr with just the external pump to 1.1 μTorr by incorporating the LiNbO3 pump. The proof-of-concept of the pyroelectric pumping mechanism is demonstrated using a 140 cm3 stainless steel vacuum chamber, with supplementary turbomolecular and diaphragm pumps and demonstrates that a 50 s thermal cycling of the crystal is optimal for the ion-pump. Pumping action was measured with a Pirani gauge and a hot cathode ion gauge. Analytical modeling and experimental results for pumping speed calculations showed a good match during high-pressure pumping.