Ferroelectric Thin and Ultrathin Films for MEMS Applications

By Bastani, Yaser

Georgia Institute of Technology

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Advisors: Nazanin Bassiri-Gharb, F. Levent Degertekin, Peter Hesketh, Kenneth Sandhage, Hamid Garmestani, Todd Sulchek

The advent of ferroelectric thin films with strong piezoelectric response has enabled the development of new nano- and micro-electromechanical systems (NEMS/MEMS) capable of large displacements at low voltage levels, aiming to be compatible with complementary metal oxide semiconductor industry. Key to all of these applications is the ability to process ferroelectric materials with maximized electromechanical coupling and to integrate them into the devices. With the continuous drive towards miniaturization of devices for piezoelectric and electronic applications, processing of ultrathin ferroelectric films with maintained large electromechanical coupling is essential to the development of high performance NEMS and MEMS. The piezoelectric response of ferroelectric thin films is profoundly affected by the texture and microstructural characteristics of the material and is severely reduced at sub-micron thickness ranges. For the first time, reproducible synthesis of dense, highly textured and phase-pure PZT thin films was achieved via chemical solution deposition. The consistent processing of ferroelectric thin films resulted in the elimination of the coupling effects of crystallographic anisotropy, porosity and in general microstructural characteristics on the functional properties of the films. This enabled effective study of the key parameters influencing the electromechanical response of the ferroelectric thin films, such as crystallite size (thickness dependence), chemical heterogeneities and substrate clamping. Reproducible synthesis of highly (100)-textured PZT ultrathin films enabled the study of the size effects on the dielectric and piezoelectric response of these films in the thicknesses ranging from 20 up to 260nm. Dielectric and piezoelectric responses of the films monotonically decreased in thinner films. For PZT films at MPB, a critical thickness, ~50nm was observed below which the extrinsic contributions to the dielectric responses of the films are heavily suppressed. After the study and acknowledgment of the severe reduction of the piezoelectric response in ferroelectric ultrathin film, several factors affecting piezoelectric response of ferroelectric films were studied in order to maximize the response especially at low film thickness ranges: chemical homogeneity, residual stresses and substrate clamping as well as using alternative material systems; relaxor ferroelectrics. In particular, a major part of the piezoelectric (and dielectric) response of the PZT has extrinsic sources such as domain or phase boundary motion and vibrations. Special attention was paid throughout this investigation into understanding extrinsic origins in PZT thin films and different approaches was utilized to further activate and enhance their contributions. Focusing on the chemical homogeneity of the ferroelectric films, Different routes were used to process ultrathin films (<200nm) with maintained functional properties. Superior piezoelectric properties - 40% higher piezoelectric response than in conventionally processed films - were achieved in highly (100)-oriented PZT superlattice-like films with controlled compositional gradient centered around MPB composition on Si substrates. Superlattice (SL) or heterolayered ferroelectric thin films consist of alternate layers of ferroelectric materials, or phases, with a compositional gradient normal to the substrate. The dynamic motion of "artificially created" phase boundaries between layer to layer tetragonal and rhombohedral phases participated in the extrinsic contributions to the films' dielectric and piezoelectric response. This approach led to processing of 200 nm SL films with d33,f values as high as some of the best previously reported data for 1 to 2 um-thick PZT films. Furthermore, comprehensive processing optimization was carried out on relaxor-ferroelectric PMN-PT thin films. Dense, highly (100)-textured PMN-PT films were synthesized exhibiting the highest d33,f coefficients reported so far in the literature (210pm/V) for corresponding thickness ranges. Control of the microstructural characteristics - texture and density - throughout the whole film thickness was necessary to obtain films with maximized functional properties. To study the effect of substrate clamping on the piezoelectric performance of the films, the Si substrate in PZT and PMN-PT films were back-side etched via dry etching in an inductively coupled plasma reactor. This approach is similar the final state of the films for MEMS applications, where the Si substrate is mostly removed in order to have a free-standing or semi-free standing ferroelectric membrane or cantilever. A giant enhancement in the piezoelectric d33,f coefficient of the substrate-released samples was observed with respect to the films on the virgin substrate. The response increased by at least one order of magnitude from ~75-200 pm/V (for different PZT film thicknesses ranging from 300nm to 1 um) to ~1500 to 4500 pm/V at reduced Si thickness. Experimental observations in macroscopic dielectric and piezoelectric characterization and microscopic piezo-response force microscopy of the samples indicate larger extrinsic contributions, -possibly with domain dynamic source- to the functional responses of the films in back-side etched samples. A fundamental change in the pattern of the electromechanical activity of the grains between the released and clamped films was observed in the band-excitation piezo-force microscopy investigations; A breakdown of the clustered pattern in the electromechanical activity of the grains in the PZT film. This giant enhancement promises a new pathway for greatly improved electromechanical properties which has a huge potential to enable high performance future device applications.

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Researchers should cite this work as follows:

  • Bastani, Yaser (2015), "Ferroelectric Thin and Ultrathin Films for MEMS Applications," https://matin.gatech.edu/resources/2104.

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