A STUDY ON HYSTERESIS EFFECT OF BARIUM STRONTIUM TITANATE THIN FILMS FOR ALTERNATIVE GATE DILECTRIC APPLICATION WEN-JIE QI, KEITH ZAWADZKI, RENEE NIEH, YONGJOO JEON, BYOUNG HUN LEE, AARON LUCAS, LAEGU KANG, JIAN-HUNG LEE, AND JACK C. LEE Microelectronics Research Center, The University of Texas at Austin, 10100 Burnet Road, Building 160, Austin, TX 78758 ABSTRACT Hysteresis effect of barium strontium titanate (BST) thin films for gate dielectric application has been studied. It is found that the "counterclockwise" hysteresis has strong sweep voltage and operating temperature dependence. It can be reduced or eliminated by proper thermal annealing or by using a barrier layer. A charge trapping and detrapping mechanism has been proposed. INTRODUCTION As metal-oxide-semiconductor (MOS) device dimensions are scaled down to 0.1 iim and beyond, conventional SiO 2 will phase out due to its excessive leakage current. High-k dielectric materials with low leakage and good interface on Si will be needed. Ferroelectric materials were discovered in 1921, and their bulk material properties have been well-characterized [3]. However, it is only recently that thin-film ferroelectric materials have been studied and compared to their well-known bulk properties. Although there has been extensive research on preventing the interfacial layer formation in order to achieve a thinner equivalent oxide thickness [1,2], the high-k materials impose other challenges, e.g. hysteresis. Ferroelectric materials have a phase transformation from ferroelectric phase to paraelectric phase at Curie temperature. Material in the paraelectric phase does not have a spontaneous polarization and hence should not have a hysteresis loop as possessed by the ferroelectric material. It is obvious that for gate dielectric application, ferroelectric materials are not acceptable because of the hysteresis. Hysteresis causes the threshold voltage to switch between two distinct values when the applied field is swept. For the BST material studied in this paper, the composition is Ba 0 . 5 Sr 0 .fTiO 3 , the Curie temperature is below room temperature. Therefore, at room temperature, it should be in paraelectric phase which will not show the hysteresis loop. Consequently, the hysteresis observed in BST film should result from other mechanisms than ferroelectricity. The literature indicates that the ultra high-k materials such as barium strontium titanate (BST) exhibit higher hysteresis [4] than Ta 2 0 5 [5] or TiO 2 [6,7]. The exact mechanism of hysteresis is not well understood. Interface charge trapping has been proposed for layered structure [2], while ferroelectricity [8,9] and mobile ion drift [10] have also been proposed. In this paper, hysteresis behavior of RF sputtered BST films as a function of temperature, sweep voltage and stack structure has been studied. It was found that the hysteresis is due to the charge trapping and detrapping, and that it can be reduced by proper thermal annealing or use of a barrier layer. The interaction at the interface between BST and Si could be the source of interface states, traps, and defects that may cause hysteresis. 269 Mat. Res. Soc. Symp. Proc. Vol. 606 0 2000 Materials Research Society