Low-temperature Preparation and Evaluation of Ferroelectric Thin Films for FeRAM by Chemical Solution Process

近年来，Bi_4Ti_3O_<12>，SrBi_2Ta_2O_9和SrBi_2Nb_2O_9等钙钛矿结构的Bi层状薄膜在非易失性随机存储器件中的应用引起了人们的广泛关注。其中，SrBi_2Ta_2O_9（SBT）薄膜由于具有上级的抗疲劳性能和较小的矫顽场Ec，在NvRAM中具有很高的应用潜力。SBT薄膜主要采用金属醇盐溶胶-凝胶法制备。为了获得优异的铁电性能，控制SBT薄膜的结构和结晶度至关重要。SBT薄膜的低温制备是其在硅器件中应用的理想条件。在本论文中，我们主要研究了利用准分子紫外光照射制备SBT薄膜及其性质。以SBT前驱体的醇盐溶液为原料，采用旋涂法在Pi（200 nm）/Ti（50 nm）/SiO_2/Si衬底上制备了SBT凝胶薄膜。凝胶薄膜在不同的温度下用受激准分子紫外灯照射5 min。紫外光处理后的薄膜在500℃下煅烧30 min，然后在750℃下氧气气氛中退火1h。SBT薄膜具有较强的（115）取向，且取向无规则。薄膜的显微结构由0.1 ~ 0.2μm的细小晶粒组成。在N_2气氛下，与O_2气氛下相比，在准分子紫外光照射的SBT凝胶膜中观察到了较强的C-H基团的红外吸收。在200℃下进行紫外光照射，可以提高薄膜的结晶度。SBT薄膜具有良好的饱和P-E磁滞曲线，其剩余极化强度为4.1μC/cm^2，矫顽场为88.7kV/cm。SBT薄膜的晶粒尺寸为500 ~ 600 nm，具有致密的微观结构

Recently, Bi-layered perovskite thin films such as Bi_4Ti_3O_<12>, SrBi_2Ta_2O_9 and SrBi_2Nb_2O_9 have been attracting much attention for use in nonvolatile random access memory (NvRAM) devices. Among them, SrBi_2Ta_2O_9 (SBT) thin films have an especially high potential for NvRAM applications, because SBT has superior fatigue-free properties and small coercive fields, Ec. SBT thin films have been fabricated predominantly by Sol-Gel method with metal alkoxides. It is very important to control the structure and crystallinity of SBT thin films in order to obtain the excellent ferroelectric properties. Low-temperature preparation of SBT thin films is desired for the application to silicon device. In this work, we focussed on the preparation and properties of SBT thin films using Excimer UV irradiation. Thin films of SBT gels were prepared on Pi (200nm)/Ti (50nm)/SiO_2/Si substrates by a spin-coating technique from alkoxide solutions of SBT precursors. The gel thin films were irradiated using Xe excimer UV lamp at various temperatures for 5min. The UV-processed films were calcined at 500℃ for 30min, and then annealed at 750℃ for 1h in an O_2 atmosphere. The SBT thin films exhibited random orientation with strong (115) orientation. The microstructure of the films consisted of fine grains of 0.1〜0.2μm. Strong IR absorption assigned to C-H group was observed in SBT gel films with Excimer UV irradiation in N_2 atmosphere, in contrast with the irradiation in O_2 atmosphere. The crystallinity of the prepared films was improved by the UV irradiation at 200℃. SBT thin films showed well-saturated P-E hysteresis curve with a remanent polarization of 4.1μC/cm^2 and coercive field of 88.7kV/cm. SBT thin films showed a dense microstructure with grain sizes of 500〜600nm