Efficient Detection of Oxygen Vacancy Double Donors in Capacitors with Ultra-thin Ta 2 O 5 Films for DRAM Applications b
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Efficient detection of oxygen vacancy double donors in capacitors with ultra-thin Ta2O5 films for DRAM applications by zero-bias thermally stimulated current spectroscopy W. S. Lau a), L. Zhong a) , Taejoon Han b) and Nathan P. Sandler b) a) Nanyang Technological University, School of EEE, Nanyang Avenue, Singapore 639798 b) Lam Research Corporation, 4650 Cushing Parkway, Fremont, California 94538 Abstract Previously, defect D (VO+) was barely detected in ultra-thin (physical thickness < 10 nm) Ta2O5 capacitors for DRAM applications using zero-bias thermally stimulated current (ZBTSC) spectroscopy and correlated with leakage current. Our explanation is that defect D (VO+) behaves like an electron trap with an electron-repulsive energy barrier and thus small electron capture cross section at low temperature such that it is difficult for defect D to capture electrons during ultraviolet illumination at low temperature. We modified our experimental technique to a twoscan ZBTSC technique and managed to detect oxygen vacancies much more efficiently in ultrathin Ta2O5 films for the first time. Two-scan ZBTSC can also be applied to other high-K dielectric materials for process diagnosis.
Tantalum oxide has demonstrated promise as a high-K dielectric for charge storage in Gb dynamic random access memories (DRAMs) [1]-[2]. For DRAM applications, the two most important requirements are high capacitance, limiting film thickness to < 10 nm, and low leakage current, which may be related to defect states in Ta2O5. Ta is not volatile while O is volatile; therefore the presence of O vacancies (VO) in Ta2O5 is expected. O vacancies are double donors [3], which will make Ta2O5 a very weakly n-type large bandgap semiconductor, resulting in leakage current. Thus it is important to develop a technique to detect O vacancies in capacitor structures with an ultra-thin Ta2O5 insulating film. In 1995, we explained the principle of zerobias thermally stimulated current (ZBTSC) spectroscopy and demonstrated the detection of defect states in capacitors with relatively thick Ta2O5 [4]. Subsequently, we demonstrated the application of ZBTSC to capacitors with ultra-thin Ta2O5 [5]-[6]. However, the signal from defect D (the first ionization state of the O vacancy double donor) was weak such that the detection of defect D was marginal [6]. In this paper, we tried to give a theoretical explanation why the detection of defect D is quite frequently inefficient and how the problem can be handled. Ta2O5 was deposited onto (100) p+-Si or n+-Si wafers by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). The physical thickness of the film was about 8 nm. Post-deposition anneal of Ta2O5/p+-Si or Ta2O5/n+-Si samples was done by RTP (rapid thermal processing) in O2 or N2O at 700-800oC for 30 s. ZBTSC measurements were performed at a ramp rate of 0.5 K/s as before [4]-[6]. Our old method to fill the defect states was UV illumination at about 90 K. With the UV source off, the current is then recorded as a function of temperature when the
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