Sputtered Pb(Zr, Ti)O 3 Thin Films for Ferroelectric Capacitors
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THIN FILMS FOR FERROELECTRIC CAPACITORS
T. SAKODA, K. AOKI, and Y FUKUDA Texas Instruments Tsukuba R&D Center Ltd., 17 Miyukigaoka, Tsukuba, Ibaraki 305-0841, JAPAN
ABSTRACT Sputtered Pb(Zr, Ti)0 3 thin films with superior ferroelectric properties were successfully obtained by controlling the grain structure and the film compositions. We found that amorphous PbTiO3 buffer layers are effective in forming PZT thin films with fine dense grains. The sputtered PZT thin films with Ti-rich phase showed excellent ferroelectric properties. The polarization retention properties of PZT capacitors with Ti-rich phase are remarkable, and the value of the retained polarization density after 10 years is expected to be larger than 40 ýiC/cm2 . Further, 150-nm-thick PZT capacitors with Zr/Ti=30/70 showed 2Pr at 1.5 V of more than 30 pC/cm 2, and good retention property. These results indicate the potential of the lower voltage operation of sputtered PZT capacitors by optimizing the film composition and thickness.
INTRODUCTION High density, embedded ferroelectric random access memories (FeRAMs) have the potential to replace embedded Flash, embedded DRAM, and non-cache SRAM and could be a key enabler for future system-on-chip applications. Because of this appeal of FeRAMs, ferroelectric thin films, such as lead-zirconate-titanate (Pb(Zr, Ti)0 3 - PZT) and strontiumbismuth-tantalate (SrBiTa2 0 9 : SBT), have been widely studied as promising materials. In particular, PZT thin film is a candidate because of its higher remanent polarization density than that of SBT thin film. The crystallization temperature of PZT, in addition, is lower than that of SBT, and these are distinct advantages for high density FeRAMs embedded on logic LSIs [1, 2, 3]. Here, in consideration of the integration of PZT thin films in logic processes, the process temperature during the capacitor formation should be kept as low as possible. Furthermore, control of the grain size and structure of PZT thin films is very important to retain a small distribution of properties among the capacitors. For a low process temperature, we have previously reported the effects of excess Pb and deposition temperature on the crystallization processes of amorphous-PZT (a-PZT) thin films [4, 5]. For the control of PZT grain structure, some research groups have reported that a buffer layer plays an important role in the growth of high-quality PZT films [2, 6]. We reported that a 2-nm-thick Ti buffer layer enhances PZT nucleation which results in a fine dense grain structure by the sol-gel technique [7]. However, in sputtered PZT thin films, the control of PZT grain structure has not been realized with the use of Ti buffer layer. The ferroelectric properties of PZT capacitors, such as remanent polarization and switching, vary with the composition of PZT thin films [8]. Considering the compatibility with logic devices, it is desirable to have a lower saturation voltage of the polarization density. In the present study, we investigated the effects of an amorphous lead-titanate (a-PbT
