Tantalum oxide thin films for microelectronic applications

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Title: Tantalum oxide thin films for microelectronic applications
Author: Jiang, Fang-Xing
Abstract: There is a critical demand for new dielectric films having higher dielectric constants, higher dielectric strengths and lower leakage currents for applications such as charge storage capacitors for DRAMs in ULSI and low-inductance decoupling capacitors for the control of simultaneous switching noise (SSN) in high-speed switching ULSI chips. Among these candidates for insulators, tantalum pentoxide has received considerable attention. As earlier as in the 1960's, tantalum oxide has been used as the dielectric in discrete capacitors. Recently, several papers have been reported on the electrical properties of Ta205 films grown by various techniques. It has been reported that the electrical properties, e.g. dielectric constant, leakage current, dielectric strength as well as the nature of the Ta2Os/Si interface, are extremely sensitive to the annealing conditions. At the present time, however, the role of the as-deposited Ta2Os/Si interface is not fully understood. In the present study, a two-step process, consisting two separate depositions and annealing, has been developed to improve the physical and electrical characteristics of reactivity sputtered Ta2Os films. The reactive ion etching (RE) selectivity of Ta2Os to Si, Si02 and Ta in CHF3, CF4 and SF6 with fractions of 02, H2 and Ar has been investigated for IC process applications. The tantalum oxide films were deposited on Si wafers by reactive DC sputtering. The films were characterized for thickness and refractive index using an ellipsometer and their phase was identified using an X-ray diffractometer. The annealing effect on Ta2Oj in oxygen ambient at 800C shows that the Ta205 films crystallize into an orthorhombic phase, condensed with a decrease of thickness and an increase of refractive index. Various capacitor configurations, such as MTM ( Al/Ta205/Al ) and MIS ( Al/Ta20$/p-Si, Al/Ta205/n-Si and Al/Ta2Os/n+-Si ), were fabricated to study the nature of Ta2Os/Si interface and the I-V and C-V characteristics. The as-deposited Ta2Os film on p-type Si substrate can sustain an electric field of 3 MV/cm at a current density of 1 u.A/cm2 in the accumulation mode, which is an order higher than that on n-type substrate. The value of apparent dielectric constant of as-deposited Ta2Os film estimated from the Al/Ta205/Al capacitor is 16, however, the value varies from 6 to 10 in MIS capacitors. This shows a evidence strongly that there is a substrate sensitivity for tantalum oxide films. As a result of the two-step process, the dielectric constant of Al/Ta2Os/n+-Si capacitor increases to 21. This value is considerably close to 24 for bulk Ta2Oj. To investigate the RIE selectivity of Ta2Os to Ta, Si and Si02, the Ta2Os film was deposited onto a wafer with three other films, DC sputtered Ta, LPCVD polysilicon, and thermally grown Si02. It is revealed that in SF6 with various fractions of 20% hydrogen or argon, the Ta2Os film shows extremely low etch rate as compared with Si, Ta and Si02, and in CF4 with various fractions of 30% hydrogen or oxygen, the Ta205 film shows a lower etch rate. However, in CHF3 the etch rates of Si and Ta2Os are comparable. The absorption spectrum of deposited tantalum oxide films was also measured. This material can be used for phase shift and attenuation masks, sunglasses and light filters. The as-deposited tantalum oxide films show a high absorbency peak at 217 nm and an additional small peak. at 416 nm with two subpeaks at 286 and 510 nm using spectrophotometer. The high peak becomes broadened and the long wavelength side of the small peak is shifted to short wavelength through annealing. A model of free volume like defect and oxygen vacancy like defect is proposed to explain the change of the absorbency spectrum.
Record URI: http://hdl.handle.net/1850/11925
Date: 1995-10-31

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