Strain displacement

The function of the piezoelectric material in PETMEM is to generate the force required to ‘switch’ the piezoresistive material. In order to achieve this, the piezoelectric must deliver a defined displacement, but this displacement must be delivered against a spring stiffness determined by the PR material. Therefore in the material discovery phase of the project, characterisation methods must address not only the piezoelectric displacement, d33, but also the force that it can generate, the blocking force. These measurements must be capable of measuring these properties on sizes commensurate with the PETMEM requirements, down to piezoelectric devices with areal sizes ~ 200nm.

 

Accessing the pm sized displacements produced by piezoelectric films is generally through the use of optical based techniques such as laser Doppler vibrometry or interferometry, or through AFM contact methods such as PFM. Although PFM is the only technique able to image ferroelectric domains down to the nm range, there are issues with regards the traceablility and repeatability of the quantitation. This is mainly due to the unknowns regarding the electric field interaction at the tip-sample interface, there are also problems associated with the crosstalk between lateral and out of plane displacements leading to errors.

 

Interferometry overcomes these issues, the traceability of the method is underpinned by the laser wavelength, and the ultimate out of plane resolution is much greater, down to sub pm. However the use of He-Ne wavelengths means that the x-y spatial resolution is limited optically to ~ 400nm, and for practical long working distance optics used with the scanning LDV microscope this is nearer 1 micron. In order to access smaller devices it is proposed that an intermediate cantilever is used to translate the mechanical motion to a larger area, similar to Vyshatko. Here an electrically conducting cantilever is used to amplify the motion such that a 3mm diameter fibre optic probe can sense the displacement. The amplification here is simply to create a larger area of motion so that the optical probe can function, not to amplify the motion, such as in the rotation of the end of an AFM tip.

 

An actuator’s ‘blocking force’ is defined as the maximum force that a piezoelectric can generate, and is sometimes measured, but more often derived from the maximum displacement and the stiffness of the actuator. Few people have attempted these measurements on MEMS scale devices because of the small forces involved. Duval et al used piezoresistive cantilevers to measure the blocking force of PZT on silicon cantilevers. These blocking force measurements coupled with FEA of the piezoelectric cantilevers confirmed the validity of the materials properties used in the FEA. Wooldridge et al have developed an electrostatic actuator that has been used to measure the direct piezoelectric effect using a 100 micron sphere as the force probe, with a dynamic force generation capability of 33microN. For piezoelectric cantilever characterisation the d33 is less important and a more significant coefficient is the e31. Muralt et al developed a method where they measured the charge developed when the end of a piezoelectric cantilever is pushed, and the technique has been commercialised by Aixacct using a four point bend technique. This method requires a specially fabricated cantilever, and only gives material parameters relevant to in plane actuation. The PETMEM device is out of plane actuation so requires a different geometry, and this will be developed in the Metrology work package.