Pyro-Electro-Thermal Analysis of LiNbO3 using Microheaters

Abstract

Microheaters have been widely investigated for their sensor-based applications, such as gas sensor, flow rate sensor and other microsystems. Here, we investigate the pyroelectric effect, activate by microheaters fabricated on -Z surface of LiNbO3 crystal. Pyroelectric effect is the capability of certain crystal to produce temporary voltage during heating or cooling transient. Fabrication of microheaters on LiNbO3 crystals gives an advantage of confined temperature gradient, together with low power consumption for application-based sensors. In this paper, we study the electron emission from the -Z surface of a pyroelectric material (LiNbO3), relevant to the microheaters fabricated on the +Z surface of crystal. Different geometries of microheaters, such as Fan, Meander, Double Spiral and S- Shape were fabricated on +Z surface of a single domain LiNbO3 crystal. Thermal behavior of these microheaters was analyzed using COMSOL™ Multiphysics and compared with the experimental data obtained by FLIR SC7000 Series thermo camera. Static and time-dependent thermal analyses were performed using a voltage generator by applying DC and step voltage signals to the microheater. It was observed that the electric field resulting from the temperature induced by the microheater changes the spontaneous polarization of the LiNbO3 crystal affecting the electron emission. This pyroelectric electron emission (PEE) from -Z surface of the LiNbO3 crystal was investigated using two-probe point measurement. It was observed that the PEE from LiNbO3 is due to the perturbation, by the temperature variation, of equilibrium between spontaneous polarizations Ps in the crystal and external screening charges (qsc) on its surfaces. At equilibrium, all Ps are fully screened by qsc and no external electric field exists. Any excess or lack of screening charges (qsc) relatively to Ps, leads to the appearance of an electrostatic state from the uncompensated charges (ρ) given by ρ = Δ (Ps- qsc). Furthermore, we verified the pyroelectric emission effect in a transient condition for all the microheaters. A voltage signal at 10mHz was applied to the microheater using signal generator. A tip (diameter~0.254mm) was positioned at distance  1mm from the -Z surface of LiNbO3 crystal. The current peaks were acquired using oscilloscope that occurs due to the non-distribution of Ps on the -Z surface of LiNbO3. It was observed that microheaters with complex structure have significantly higher number of current peaks compared to a simplest structure, because complex microheater leads to a nonuniform temperature gradient on the -Z surface of the LiNbO3 crystal. It was also noted that the temporal distances between adjacent electrical peaks exponentially increases during the application of the thermal transient, showing a dependency of the rate of heating and cooling on the PEE in a transient condition of the crystal. Finally, we demonstrate two pyroelectric effect based applications: Pyro-jetting of liquid droplet (oil, OIR 906, water and PDMS) and pyro-emission lithography using microheater.