Various Methods to Reduce Defect States in Tantalum Oxide Capacitors for DRAM Applications
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Various Methods to Reduce Defect States in Tantalum Oxide Capacitors for DRAM Applications W.S. Laua,*, G. Zhangb, L.L. Leonga, P.W. Qiana, Taejoon Hanc, J. Dasc, Nathan P. Sandlerc and P.K. Chud a Nanyang Technological University, School of Electrical and Electronic Engineering, Block S2.1, Nanyang Avenue, Singapore 639798, Republic of Singapore b Data Storage Institute, DSI Building, 5, Engineering Drive 1, Singapore 117608, Republic of Singapore c Lam Research Corporation, 4650 Cushing Parkway, Fremont, California 94538 d City University of Hong Kong, Department of Physics and Materials Science, Tat Chee Avenue, Kowloon, Hong Kong
ABSTRACT Tantalum oxide has attracted world-wide interest for DRAM (dynamic random access memory) capacitor applications because of its relative high dielectric constant compared to silicon dioxide or nitride. We would like to point out that tantalum oxide behaves very much like a large bandgap n-type semiconductor with 3 main types of donors responsible for leakage current. Native oxygen vacancies are very deep double donors with Ec – Ed = 0.8 eV approximately, where Ec is the bottom of the conduction band and Ed is the energy level of the defect state. Si-O vacancy complexes are relatively shallow single donors with Ec – Ed = 0.2-0.4 eV. C-O vacancy complexes are relatively shallow single donors with Ec – Ed = 0.5-0.6 eV. The key points regarding how to suppress these 3 types of donor defects will be discussed for the purpose of leakage current reduction.
INTRODUCTION For DRAM memory capacitor applications, the two most important requirements are high capacitance (about 30 fF/cell) to prevent the occurrence of soft errors and low leakage current to prevent the loss of the stored charge. Earlier capacitor structures were based on Si3N4 (dielectric constant εr = 7) and SiO2 (εr = 3.9) such as ONO (oxide-nitride-oxide) and ON (oxide-nitride) capacitor structures [1]-[2]. The technology trend is to migrate from ONO/ON to capacitor structures using dielectric materials with higher dielectric constant such as tantalum oxide Ta2O5 (εr = 20-25) [2]. It was predicted that, after tantalum oxide, ferroelectric materials such as SrTiO3 (εr = 200) or BaxSr1-xTiO3 (εr = 400) may be necessary [2]. Recently, Hiratani et al. pointed out that a hexagonal phase of tantalum oxide can be grown on Ru with an enhanced dielectric constant of over 50 [3]. Matsui et al. pointed out this high dielectric constant hexagonal phase can be formed at lower temperature by adding niobium (Nb) [4]. Thus tantalum oxide technology may last longer than previously expected. It is expected that traps or defect states in Ta2O5 will play an important role in its leakage current problem through the Poole-Frenkel effect (field assisted ionization of defect states) or trap assisted tunneling [5]. Previously, we demonstrated that ZBTSC (zero bias thermally stimulated current) spectroscopy could be used to detect defect states in capacitors with relatively thick Ta2O5 film (about 100 nm) and correlated with leakage curre
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