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 non-volatile random access memories (FeRAM) 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 the ferroelectric polarization, domain configurations, switching properties, and other properties such as the piezoelectric response. Pb(Zr,Ti)O3 (PZT) is one of the favorite materials for FeRAMs. Today's capacitors are poly-crystalline with sizes in the micron range. Each capacitor is composed of a large number of grains, guaranteeing the averaging of electrical properties. If only a few grains were present, the scattering of properties from one capacitor to another would take place. This can be avoided using oriented single-crystalline capacitors. An important processing issue is patterning. The switchable polarization might be reduced due to etching damage, a frequently reported current fabrication problem. There is also the question of domain configuration - processing relations. The optimal solution of (001) oriented tetragonal PZT cannot be realized in large capacitors because ferroelastic 90° domains form spontaneously upon cooling through the transition temperature and are stabilized for reasons of stress compensation. In this work, we investigated two routes to fabricate small ferroelectric structures: A subtractive route and an additive route. Patterning was done by electron beam lithography (EBL) in order to access the sub 100nm range. Dry etching was done in an electron cyclotron resonance (ECR/RF) reactor working at very low pressures and ion energy. In the subtractive route, the starting point were continuous 50 to 200nm thick Pb(Zr0.4,Ti0.6)O3 films, which were deposited on conductive, Nb-doped SrTiO3 (100) substrates. The films were c-axis oriented with an a-axis fraction of about 20%. The films were patterned by means of EBL. The positive poly-methyl-methacrylate (PMMA) resist was spun on the PZT film, a dot pattern was directly written by the e-beam, and after development a Cr layer was evaporated. The obtained Cr patterns served as a hard mask for PZT dry etching. Features with an aspect ratio of up to 2 could be cut out of a 200nm thick PZT film. We have shown that dry etching in an electron ECR/RF 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 rat