Analysis of Ferroelectric Microcapacitors by Scanning Probe Microscope
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Analysis of Ferroelectric Microcapacitors by Scanning Probe Microscope Nobuhiro Kin and Koichiro Honda Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-0197, Japan
ABSTRACT To develop higher density FRAM requires reducing cell size. Therefore, the size effects resulting from device processing and the material’s physical properties must be measured. Therefore, analyzing the electric characteristics of a single bit cell capacitor has become important. Two known characteristics of ferroelectric material are that the Vc increases at low temperatures, and the Pr falls at high temperatures. To further evaluate the impact of temperature on ferroelectrics, we constructed a new evaluation system based on a scanning probe microscope, that can measure the electric characteristics of a single bit cell capacitor. This system can be used in the temperature range from -120 degrees to 300 degrees C. We accomplished this by circulating liquid nitrogen around a SPM stage and by using an electrical heater. We measured the electrical properties of ferroelectric microcapacitors by using a sample with IrOx/PZT/Pt structure. Our measurements revealed that 2Pr really increases at low temperatures, and Pr decreases at high temperatures. That is, we have shown that Vc increases 30% at low temperatures and Pr decreases 10% also in an actual FRAM single bit cell capacitor.
INTRODUCTION Ferroelectric thin films are currently being developed worldwide for numerous applications in low voltage, non-volatile random access memory, FRAM. FRAM is included in integrated circuit cards, etc., and its use has increasingly expanded. The ferroelectric material most widely used for FRAM is Pb(Zr,Ti)O3 [PZT]; this is because of its high remnant polarization. 1 As the capacitor size has been greatly reduced to about 1 µm x 1.5 µm with the increased integration of FRAM, size effects caused by a process or by the original physical properties of materials have begun to show up. Some examples of known problems with general ferroelectric bulk and thin films are the increase of the coercive voltage at low temperatures and the decrease of the remnant polarization at high temperatures. 2, 3 These effects cannot be predicted for a so-called parallel monotonous capacitor made up of several films. Understanding these phenomenon in regard to minute ferroelectric capacitors at the single bit level in low and high temperature is necessary. Therefore, measuring a minute capacitor that is equivalent to an actual FRAM single bit capacitor is desirable.
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Fortunately, in the past several years, scanning probe microscope (SPM) techniques have been developed for observing film surfaces at the nanoscale. 4 This technique has also the ability to evaluate electrical, optical, and magnetic properties microscopically. 5, 6 Therefore, to resolve the difficulties mentioned earlier, we built a system with sufficient accuracy to observe an electrical property of a small capacitor by using an SPM. In this system, we can control the temperature within a va
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