"DuPont des Matériaux" Award 2005 - Simon Bühlmann
31.12.05 - Patterned and self-assembled ferroelectric nano-structures obtained by epitaxial growth and E-beam lithography. Thesis EPFL, n° 3047 (2004). Dir.: Prof. N. Setter
For a significant contribution to the understanding of selective growth processes of functional oxide nano-structures.
Patterned and self-assembled ferroelectric nano-structures obtained by epitaxial growth and E-beam lithography
The continuous downscaling of microelectronic circuits combined with increasing interest in ferroelectric thin films for high-density non-volatile memories is drawing great attention to small ferroelectric thin film structures. There are various challenges and open questions related to processing and theoretical understanding. The processing must assure damage-free ferroelectric capacitors of reproducible properties. More theoretical understanding is necessary to estimate the impact of size effects on the stability of ferroelectric polarization, domain configurations, switching properties, and other properties such as the piezoelectric response.
Two routes to fabricate small ferroelectric structures have been investigated: A subtractive route and an additive route. Patterning was done by electron beam lithography (EBL) in order to access the sub 100nm range, in conjunction with an electron cyclotron resonance (ECR/RF) reactor working at very low pressures and ion energy.
In the subtractive route, we started processing from continuous 50 to 200nm thick PZT films, which were deposited on conductive, Nb-doped SrTiO3 (100). The films were c-axis oriented with an a-axis fraction of about 20%. EBL patterning was done using a PMMA resist, Cr hard masking by evaporation and lift-off. After dry etching and removal of the remaining Cr, features with an aspect ratio of up to two could be cut out of a 200nm thick PZT film. We have shown that dry etching in an electron cyclotron resonance reactor provides damage-free single crystalline sub-200nm features, which were still ferroelectric. The smallest features obtained had a lateral size of 80nm. We found that the resolution of the EBL was limited by the backscattering of electrons from the high density PZT layer. Piezoelectric sensitive atomic force microscopy (PAFM) revealed an increase in the piezoelectric response when the feature's aspect ratio is increased. The measured piezoelectric coefficient increased by a factor of more than three compared to the continuous film. Un-clamping was found to account only for a portion of the increase. As major contribution, we suppose a change in the domain configuration from a to c, and unpinning of domains.
It is also important to investigate non-destructive fabrication processes to reduce the negative impact of interface defects and grain boundaries. This was achieved using an additive route. The fact, that PZT nucleation on Pt surfaces is difficult but easy on TiO2 was used for the site controlled nucleation of PZT single crystals on patterned 2nm thick TiO2 layers serving as affinity spots. This route was carried out on Pt (111) and (100) surfaces. A 50nm thick Pt layer was epitaxially grown on single crystalline SrTiO3 (111) and MgO (100) substrates, and then covered by a 2nm thick TiO2 layer. On both substrates, the TiO2 was found to be (100) oriented. In this route, EBL was used to pattern the TiO2 layer into seed islands. As only 2nm of TiO2 layer have to be etched, a new type of negative organic mask could be used, which circumvented the lift off process. Direct deposition of PZT on the seed island covered Pt (111) surfaces led to the epitaxial growth of triangular shaped crystals only on the seeds and not elsewhere. The lateral size of the triangles was between 30 and 150nm. Applying a 1nm thick PbTiO3 starting layer prior to PZT deposition increased the nucleation density. A 2nm thick PbTiO3 starting layer prior to PZT deposition led to square shaped crystals on small TiO2 seeds implying the growth of (001) PZT. For this deposition, the squares showed a uniform size distribution, but their in-plane orientation was random. In all cases, the nucleus density was found to be 60 times less on Pt than on the affinity spots. The nucleation-controlled character was expressed in an observable depletion layer around larger TiO2 seeds. A theoretical nucleation model was able to well explain this behavior and the relevant parameters such as activation energies of diffusion and desorption could be derived from experiments. PAFM measurements revealed that all PZT crystals obtained by the additive route were ferroelectric. The ferroelectric features have been investigated by means of PAFM. Complex domain structures have been identified, where the smallest observed domains showed diameters of 15nm. The thinnest investigated feature showing ferroelectricity was 6nm thick.
An attempt was made to compare experimental findings with the Landau-Devonshire-Ginzburg phenomenological theory. The d33 loop together with the CV-loop of a 600nm thick, mostly c-oriented film could be fairly well described by the theory. Deviations could be identified as domain wall contributions. The critical thickness for ferroelectricity was estimated using in addition literature data for domain wall energies. The critical thickness was calculated as 1 to 2nm, which does not contradict experimental findings.