Model Relating Thin Film Pzt Crystallographic Texture to Ferroelectric Switching Performance
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MODEL RELATING THIN FILM PZT CRYSTALLOGRAPHIC TEXTURE TO FERROELECTRIC SWITCHING PERFORMANCE G.R. FOX* and S. SUMMERFELT** *Ramtron International Corporation, Colorado Springs, CO 80921, [email protected] **Texas Instruments Corporation, Dallas, TX
ABSTRACT The fundamental physical property used for data storage in a ferroelectric non-volatile memory (FRAM) is the remanent polarization of a ferroelectric capacitor. In order to sense the data state of the capacitor, the charge pulse generated by either a switching or non-switching ferroelectric polarization is detected. Since the polarization of the ferroelectric is a vector property, the switchable polarization response of a ferroelectric capacitor is necessarily dependent on the texture of the ferroelectric material used to make the capacitor. A series of Ca, Sr, and La doped PbZr0.4Ti0.6O3 (PZT) capacitor test samples were produced with texture ranging from nearly complete {111} texture to random orientation. The volume fraction of the textured material was quantified by X-ray diffraction. An experimental correlation between texture and the switchable polarization was established using pulse switching measurements. A mathematical model was developed to explain the dependence of the switchable polarization on the crystallographic texture. It was found that the dependence of the saturated switchable polarization on texture required that there was no 90° domain reorientation. The impact of these findings will be related to the performance of FRAM. INTRODUCTION Because ferroelectricity originates from crystal anisotropy, it is evident that the crystallographic texture of a ferroelectric thin film influences its switchable spontaneous polarization.[1] Switchable polarization is the primary property exploited in non-volatile ferroelectric random access memories (FRAM); therefore, quantification of texture and its relationship to ferroelectric switching is vital to FRAM performance and process control. In order to quantitatively relate texture and thin film electrical properties, it is necessary to identify the types and volume fractions of textured material comprising a film.[2] PbZrxTi1-xO3 (PZT) is currently the only ferroelectric material used for mass production of FRAM memories. This paper presents a quantitative texture analysis of a series of PZT films and compares the measured switching performance with that predicted by symmetry arguments. The texture analysis relies on integrated X-ray diffraction peak intensity measurements from θ2θ spectra. A complete description of the measurement procedures and method for calculating textured volume fractions of PZT thin films has been previously reported.[3] EXPERIMENTAL PROCEDURE A series of PZT films with different crystallographic textures ranging from almost fully random to almost fully {111} textured, was prepared on 150 mm diameter [100] Si substrates. A film with {111} PZT texture has a population of crystallites, with {111} type planes parallel to the substrate surface, which is higher than the population in a
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