The continual drive for miniaturisation in consumer electronics is resulting in an increased use of piezoelectric thin/thick films and micro-structured devices. The Functional Materials Group is developing metrology techniques using MEMS (micro-electro-mechanical systems) to match the length scales of the measurement tool and sample size. Scaling effects in ferroelectrics have led to major obstacles for the uptake of ferroelectric materials in microelectronic devices. The behaviour of ferroelectrics is, to a great extent, dictated by the dynamics of domain wall motion – in decreasing the sample size to the same length scale as the average domain size, surface tension effects and depolarising fields, along with substrate clamping effects in thin films, can alter the physical properties from those seen in bulk sample quite dramatically. In miniaturising our piezoelectric metrology kit, we aim to characterise some of these effects.
Micro/Nano Berlincourt Measurement & Blocking Force Characterisation
The measurement of piezoelectric coefficients by the direct piezoelectric effect (the charge developed across the surface of a sample as a consequence of an applied force and resultant change in internal polarisation) is known as the Berlincourt method. NPL owns several Berlincourt meters for the measurement of d33, d31 and dh (longitudinal, transverse and hydrostatic coefficients respectively). Thermal bimorph actuators, and electrostatic comb drive actuators that utilise the vertical levitation effect are used to provide a sinusoidal force of up to 10µN to the device under test. Ultra-low charge measurement techniques (down to atto-Coulombs) allow us to directly measure the d33 coefficient. Additionally, both actuators can be used in DC to measure the blocking force characteristics of micro-actuators.
MEMS thermal (left) and electrostatic (right) actuators designed for
the measurement of piezoelectric coefficients on small-scale systems
Magnetoelectric (ME) materials exhibit coupled magnetic and electrical polarisation, and are of interest because of their significant potential for novel applications in multifunctional devices and smart sensors, from high-density memory storage, to energy scavenging and wireless energy transmission. ME heterostructures exhibit magnetoelectric coupling orders of magnitude larger than those seen in single phase ME materials. MEMS processing techniques enable the coupling between magnetic and piezoelectric thin films. Our work in this area aims to develop the metrology techniques to traceably measure the strain-mediated coupling in these devices at the micro-scale, and enable the development of high-sensitivity magnetic field sensors, for example.
Laser Doppler Vibrometry Measurements of MEMS devices
The group has integrated a custom environmental chamber with a Polytec Laser Doppler Vibrometer (LDV) for the characterisation of MEMS devices under vacuum. LDV is a quick, non-contact, optical scanning technique from which a map of the out-of-plane velocity and displacement profiles across the sample can be created as a function of frequency, with a lateral resolution of 1µm and out of plane sensitivity of 10pm. Two laser beams are used in the system; the differential measurement technique eliminates any relative motion between the tested device and its supporting structure.