Partner und Internationale Organisationen
(Englisch)
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A, B, BG, CZ, DK, FIN, F, D, GR, H, IRL, I, LT, N, PL, P, RO, SK, SI, E, S, CH, TR, GB
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Abstract
(Englisch)
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Pb(Zr,Ti)O3 (PZT) is one of the favorite materials for ferroelectric non-volatile memories. The brake-through in this field is quite recent compared to mainstream microelectronics technology. Whereas standard silicon technology has already arrived at 120 nm design rule, the smallest FERAM capacitor realized to date is still 0.7 µm square. The goal of this work is the downscaling of the ferroelectric capacitor to sub-micron dimensions, and the study of ferroelectric phenomena at crystals in the range of 30 to 150 nm. In order to concentrate on fundamental problems involved in downscaling, integration on silicon was avoided for the time being and PbZr0.4Ti0.6 thin films were grown in-situ on conductive Nb-doped single crystalline (001) SrTiO3 at 600°C by a dynamic, reactive O2 sputtering process using single metal targets of Pb, Zr and Ti. Non-patterned films exhibited tetragonal symmetry with dominating c-axis orientation (i.e. (001)). The (100) planes of the minority a-domains were tilted by 2.2°. AFM measurements showed that the 50 -200 nm thick films were piezoelectric in the as grown state, exhibiting a uniform bottom-to-top polarization. Two methods have been applied for the formation of small crystals. Both were based on e-beam lithography using PMMA resist in combination with a Cr masking layer patterned by lift-off. The smallest fabricated Cr features were in the range of 40 to 60 nm. In the first method an epitaxial PZT film has been structured by dry etching in a dual frequency ECR/RF reactor using CF4/ CCl4/Ar. PZT feature sizes of 50 to 200nm have been fabricated in this way. The second method is based on the fact that PZT growth is nucleation controlled and does not nucleate easily on platinum. The STO substrate was covered with an epitaxial Pt(111) film on which 2 nm thick TiO2 seed layers have been patterned. During the subsequent PZT thin film deposition, single grains grew on the seeds only. AFM tests showed that the obtained 50 to 200nm features were still piezoelectric, and also ferroelectric as they could be switched as well. Moreover, 200 nm high features of less then 150nm width showed a strong increase of piezoelectric response, reaching a 4 times larger response at 100 nm width. This phenomenon is thought to be partially caused by the reduction of clamping. It is proposed that a-domains are disappearing or contributing to the piezoelectric response in small features additionally, because for aspect ratios > 1, 90 ° domain walls do not cross anymore the total height. The obtained features were thus mesoscopic with respect to domain configuration and mobility. Mesoscopic phenomena with respect to the existence of ferroelectricity are expected to occur at still smaller features (< 30 nm).
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