Taejoon Han

Detection of oxygen vacancy defect states in capacitors with ultrathin Ta2O5 films by zero-bias thermally stimulated current spectroscopy

Defect state D (0.8 eV) was experimentally detected in Ta2O5 capacitors with ultrathin (physical thickness <10 nm) Ta2O5 films using zero-bias thermally stimulated current spectroscopy and correlated with leakage current. Defect state D can be more efficiently suppressed by using N2O rapid thermal annealing (RTA) instead of using O2 RTA for postdeposition annealing and by using TiN instead of Al for top electrode. We believe that defect D is probably the first ionization level of the oxygen vacancy deep double donor. Other important defects are Si/O-vacancy complex single donors and C/O-vacancy complex single donors.

Detection of defect states responsible for leakage current in ultrathin tantalum pentoxide (Ta2O5) films by zero-bias thermally stimulated current spectroscopy

Defect states responsible for leakage current in ultrathin (physical thickness <10 nm) tantalum pentoxide (Ta2O5) films were measured with a novel zero-bias thermally stimulated current technique. It was found that defect states A, whose activation energy was estimated to be about 0.2 eV, can be more efficiently suppressed by using N2O rapid thermal annealing (RTA) instead of using O2 RTA for postdeposition annealing. The leakage current was also smaller for samples with N2O RTA than those with O2 RTA for postdeposition annealing. Hence, defect states A are quite likely to be important in causing leakage current.

General theory of acceptor-oxygen-vacancy complex single donor in high-dielectric-constant metallic oxide insulators

Previously, we showed experimentally that Si∕O-vacancy complex single donors and C∕O-vacancy complex single donors in tantalum oxide films have smaller ionization energies than the first ionization energy of O-vacancy double donors [W. S. Lau, L. L. Leong, T. Han, and N. P. Sandler, Appl. Phys. Lett. 83, 2835 (2003)]. In this letter, a theory based on a larger electron orbit for acceptor O-vacancy complex single donors compared to the O-vacancy double donors is proposed to explain the physics behind our previously reported experimental observation and why Si or C may cause an increase in leakage current in tantalum oxide films.

The Superiority of N 2O Plasma Annealing over O 2 Plasma Annealing for Amorphous Tantalum Pentoxide (Ta 2O 5) Films

As-deposited tantalum pentoxide (Ta2O5) films are amorphous. The films will remain amorphous after low-temperature O2 or N2O plasma annealing. High-temperature annealing will produce polycrystalline films where grain boundaries can generate leakage current. It was found that N2O plasma annealing is superior to O2 plasma annealing in terms of leakage current reduction. This can be easily explained by the lower energy required to break the nitrogen-oxygen bond in a N2O molecule compared to the energy required to break the O=O bond in an O2 molecule. We also observed that there is less Si contamination, which may lead to leakage current, in the sample with N2O plasma annealing compared to the sample with O2 plasma annealing.

Mechanism of leakage current reduction of tantalum oxide capacitors by postmetallization annealing

In this letter, the authors will point out that defect states related to impurities or structural defects in tantalum oxide capacitors can be passivated by hydrogen during postmetallization anneal (PMA) while oxygen vacancies are enhanced by PMA such that some will observe a decrease while other may observe an increase in the leakage current after PMA. The PMA process can be tuned such that the hydrogen passivation of defect states dominates over the enhancement of oxygen vacancies, resulting in significant leakage current reduction.

Application of zero-temperature-gradient zero-bias thermally stimulated current spectroscopy to ultrathin high-dielectric-constant insulator film characterization

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 such as tantalum oxide with much less parasitic current which can be a serious limitation for the conventional thermally stimulated current 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 which can be used to detect deeper traps than those by ZBTSC.

Various methods to reduce defect states in tantalum oxide capacitors for DRAM applications

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.

Efficient Detection of Oxygen Vacancy Double Donors in Capacitors with Ultra-thin Ta 2 O 5 Films for DRAM Applications by Zero-bias Thermally Stimulated Current Spectroscopy

Previously, defect D (V O + ) was barely detected in ultra-thin (physical thickness 2 O 5 capacitors for DRAM applications using zero-bias thermally stimulated current (ZBTSC) spectroscopy and correlated with leakage current. Our explanation is that defect D (V O + ) 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 Ta 2 O 5 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 2 O 5 . Ta is not volatile while O is volatile; therefore the presence of O vacancies (VO) in Ta 2 O 5 is expected. O vacancies are double donors [3], which will make Ta 2 O 5 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 Ta 2 O 5 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 Ta 2 O 5 [4]. Subsequently, we demonstrated the application of ZBTSC to capacitors with ultra-thin Ta 2 O 5 [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. Ta 2 O 5 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 Ta 2 O 5 /p + -Si or Ta 2 O 5 /n + -Si samples was done by RTP (rapid thermal processing) in O 2 or N 2 O at 700-800°C 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 temperature is ramped from 90 K to 400 K. The energy level of the defect was estimated using ET = 23kT m , where T m is the peak temperature and k the Boltzmann constant [7]. Table I shows 3 main kinds of electron traps detected by ZBTSC: (A) the ionized Si/O vacancy complex shallow single donor (Si- -V O ++ ), (B) the ionized C/O vacancy complex shallow single donor (C- -V O ++ ) and (C) the first ionization state of the oxygen vacancy deep double donor (V O + ) which is also known as defect D [6]. Smaller leakage current can be easily correlated with lower ZBTSC signal from D (0.8 e V) [6]. We observed that C/O vacancy complex can be more easily removed by an oxidizing anneal when the Ta 2 O 5 film is very thin [6]. The physics why Si/O vacancy complex and C/O vacancy complex are shallower donors compared with the first ionization energy level of the O vacancy double donor has been explained by us in another paper [8]. The traps previously reported by Seve and Lassabetere [9] in 1974 based on their conventional thermally stimulated current (TSC) study are probably the Si/O vacancy complex and the C/O vacancy complex; they did not report about defect D. The trap previously reported by Oehrlein and Reisman [10] in 1983 based on conventional TSC is probably the C/O vacancy complex; they also did not report about defect D. Nishioka [11] pointed out that it was difficult to correlate leakage current in Ta 2 O 5 with defect states detected by conventional TSC. We suspected that the difficulty mentioned by Nishioka [11] is related to the difficulty in the detection of defect D, as discussed in the following paragraph. For example, the leakage decreases with O 2 annealing temperature while the Si/O vacancy complex TSC peak increases with O 2 annealing temperature because of Si diffusion from the Si substrate into the Ta 2 O 5 film, resulting in a wrong impression that the leakage current does not decrease with the decrease in defect states detected by TSC. After we realized that defect D (0.8 e V) is probably the first ionization energy level of the oxygen vacancy double donor, our results and the results of other scientists become more consistent with each other. For example, Sawada and Kawakami [12] estimated that the first ionization energy of the oxygen vacancy double donor in Ta 2 O 5 film is 0.8 e V below the conduction band by their theoretical calculations; their results appear to be consistent with our findings.