The Development of Zero-temperature-gradient Zero-bias Thermally Stimulated Current (ZTGZBTSC) Spectroscopy Technique fo
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0917-E05-18
The Development of Zero-temperature-gradient Zero-bias Thermally Stimulated Current (ZTGZBTSC) Spectroscopy Technique for the Detection of Defect States in Ultra-thin High-k Dielectric Films Wai Shing Lau1, Kum Fai Wong1, Taejoon Han2, and N. P. Sandler2 1 School of EEE, Nanyang Technological University, NTU, School of EEE, Block S2.1, Nanyang Avenue, Singapore, Singapore, 639798, Singapore 2 Lam Research Corporation, Fremont, California, 94538
ABSTRACT Previously, we have reported our application of the zero-bias thermally stimulated current (ZBTSC) spectroscopy technique to study defect states in high-dielectric constant insulator films like tantalum oxide (Ta2O5) with much less parasitic current which can be a serious limitation for the conventional thermally stimulated current (TSC) method. However, a parasitic current can still be observed for ZBTSC because of a small parasitic temperature gradient across the sample. The thermal design of the ZBTSC system can be improved, resulting in zero-temperature-gradient ZBTSC (ZTGZBTSC) which can be used to detect deeper traps than ZBTSC.
INTRODUCTION Defect states in high-k dielectric are responsible for leakage current [1] or mobility degradation due to remote Coulomb scattering [2]. Thus there is a need to detect and understand defect states in ultra-thin high-k dielectric materials. For insulators, one of the established techniques to detect defect states is the thermally stimulated current (TSC) spectroscopy method. However, a parasitic current, which tends to increase rapidly in magnitude with temperature, is always produced by the bias voltage applied to the sample during TSC measurement. It can be easily seen that the sensitivity of the TSC technique can be significantly improved by eliminating the bias voltage applied to the sample during TSC measurement, resulting in the zero-bias thermally stimulated current (ZBTSC) technique. ZBTSC has been used by us on Ta2O5 thin films [1][3][4]. The driving force for the electrons or holes during ZBTSC can be due to a builtin electric field or a temperature gradient [1]. Experimentally, we observed that the parasitic current has become significantly smaller but not zero when we migrated from TSC to ZBTSC. Previously, we reported that the small but non-zero input offset voltage of the current-to-voltage amplifier used to measure current during ZBTSC can cause a parasitic current during ZBTSC and we also discussed how to suppress the parasitic current generated by this mechanism [5]. In this paper, we would like to point out there is another mechanism which can generate parasitic current during ZBTSC: a parasitic temperature gradient in the ZBTSC measurement system. The reduction of the parasitic current due to the second mechanism can be achieved by improving the thermal design of the ZBTSC measurement system, resulting in a new invention: zerotemperature-gradient ZBTSC (ZTGZBTSC).
EXPERIMENT Ta2O5 was deposited onto (100) n+-Si or p+-Si wafers by chemical vapor deposition as discussed before [6]. The physi
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