Journal of Crystal Growth 201/202 (1999) 652}655
Electrical characterization of AlO on Si from thermally
oxidized AlAs and Al
C.C. Liao, Albert Chin*, C. Tsai
Department of Electronics Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC
Abstract
The scaling limit for VLSI gate oxide (SiO) is 15}20 As that is determined by the large direct-tunneling leakage current. Further scaling to improve device performance can be obtained using a higher dielectric constant material. We have studied the AlO to use as an alternative gate dielectric. To ensure good quality, AlO is thermally oxidized from MBE-grown AlAs or Al on Si-substrates. Experimental results indicate that the leakage current from oxidized AlAs is larger than that from directly oxidized Al, which may be due to the weak AsO inside AlO. The leakage current of a 53 As AlO is already lower than that of SiO with an equivalent oxide thickness of 21 As. 1999 Published by Elsevier Science B.V. All rights reserved.
PACS: 73.40.Qv
Keywords: Si CMOS; Alternative gate dielectrics; High-K; AlO
1. Introduction
It has been shown that the practical scaling limit for gate oxide is due to the leakage current by direct-tunneling process [1], although 11}15 As direct-tunneling gate oxide has already been dem-onstrated [2,3]. At present, the thickness of ther-mally grown gate oxide is scaled down to 35}40 As for the 0.18-lm VLSI generation, and further scal-ing down below 15 As is required within a few years [4]. Unfortunately, the large direct-tunneling
* Corresponding author. Tel.: 35-731841; fax:
#886-35-724361; e-mail: achin@cc.nctu.edu.tw.
current precludes the use of silicon dioxide (SiO) below 15}20 As thickness. However, continuously scaling down the gate oxide is necessary to increase the current drive capability of MOSFETs and the operation speed of ICs. The relationship of drive current and gate oxide thickness is shown as
I" "12k=¸ eKA
t (<%!<2), (1)
where I" is the device saturation current, <% is the applied gate voltage and K is the dielectric constant of the gate capacitor. The only solution to over-come this di$culty and to continuously scale down the gate dielectric is to use a thicker dielectric with a higher K value. Therefore, novel high-K gate dielectric has been identi"ed as one of the most 0022-0248/99/$ } see front matter 1999 Published by Elsevier Science B.V. All rights reserved.
important R&D plans [4] for deca}nano CMOS technology. Unfortunately, because of the stringent requirements for device quality gate dielectric, no satisfactory alternatives to SiO has so far been found. Recently, aluminum oxide (AlO) has at-tracted much attention because of its very high dielectric constant (&10) that can be used for the next generation DRAM and Flash memory ap-plication [5]. Moreover, AlO has also been treated as a gate barrier in InAlAs/InGaAs MES-FET using plasma oxidation of deposited Al [6]. In this paper, we have "rst characterized the electrical property of AlO to use as an alternative gate dielectric of MOSFETs. Because extremely high quality is required for gate dielectric, we have used thermally grown AlO from oxidized AlAs or Al that were grown in an ultra-high vacuum molecular beam epitaxy (MBE) system. In comparison, sput-tering deposition and O plasma oxidation have been used in the past to deposit AlO; however, high defects can be expected from Ar> bombard-ment and plasma damage, respectively. In this work, the leakage current of a 53 As AlO from direct oxidized Al is already better than that of SiO with the equivalent thickness of 21 As [2]. This result suggests the possible application of AlO to VLSI.
2. Experimental procedure
P-type 4 [1 0 0] Si wafers with typical resistivity of 10) cm are used in this study. After modi"ed RCA cleaning, HF dipping, rinsing in DI water and spun dry, the wafer was treated by HF-vapor pas-sivation and immediately loading into the MBE chamber. The HF-vapor passivation has been used to reduce thermal budget to desorb the native oxide [7]. Then 40}80 As AlAs or 40}55 As Al were grown by MBE at 5003C. The oxidation process was per-formed in a standard furnace at 5003C for 90 min, and the dielectric thickness was measured by an ellipsometer. After oxidation, 3000 As poly-Si was deposited followed by phosphorus doping using POCl at 8503C. The wafers then received a 30-min nitrogen anneal at 8003C. This anneal was found to be very e!ective to reduce dielectric leakage cur-rent. Subsequently, 3000 As Al was deposited and
gate electrode was de"ned by patterning and wet etching. The gate dielectrics were electrically char-acterized by I}< leakage current using a semicon-ductor parameter analyzer, and the composition pro"les were measured using secondary ion mass spectroscopy (SIMS).
3. Results and discussions
Fig. 1 presents the SIMS pro"les of oxidized AlAs at 5003C, where O, Al and As are detected within the oxidized layer. The possible AlAs oxida-tion process is in the following equaoxida-tion:
AlAs#OPAlO#AsO. (2)
Therefore, both AlO and AsO are formed dur-ing AlAs oxidation. As shown in Fig. 1, signi"cant Al and As di!usion into Si is observed. The di!used As into Si will behave as a n}type dopant and change the threshold voltage, while the amount of di!used Al is dependent on the solid solubility of Si. However, none of these e!ects are desirable for MOS devices. The reduced surface concentration of As is due to the out-di!usion into ambient during oxidation.
Fig. 2 shows the SIMS pro"le of oxidized Al. The O concentration decreases to its background value as increasing depth beyond the oxidized layer. This is because the Si oxidation rate is neg-ligibly slow at this temperature. In sharp contrast to AlAs oxidation, the oxidized Al shows a much
Fig. 1. SIMS depth pro"les of oxidized AlAs at 5003C.
Fig. 2. SIMS depth pro"les of oxidized Al at 5003C.
Fig. 3. J}< characteristics of MOS capacitors with oxidized AlAs dielectric. The capacitor area is 800lm;800 lm.
sharper SIMS pro"les. Possible reason may be due to the absence of As-enhanced di!usion during oxidation. Another advantage of direct Al oxida-tion may be due to the strong bonding energy of AlO instead of AsO from oxidized AlAs. There-fore, the oxidized Al "lms may provide not only better material quality but also lower di!usion into Si during oxidation.
We have further characterized the electrical be-havior of these oxides. Fig. 3 shows the typical J}< characteristics of MOS capacitors from oxidized AlAs. The capacitor leakage current reduces with increasing dielectric thickness from 80 to 130 As . This is a typical behavior because a thicker
Fig. 4. J}< characteristics of MOS capacitors with oxidized Al dielectric. The capacitor area is 800lm;800 lm. The J}< characteristic of 21 As SiO is from Ref. [2].
insulating barrier has lower electric "eld that can block the electron transport more easily. However, this thicker insulating barrier did not successfully reduce the leakage current at low voltages less than &0.7 V. Possible reasons may be due to trap-as-sisted tunneling at low electric "eld [8]. The traps may be generated by the weak bonding strength of AsO inside AlO matrix or vacancies by As out-di!usion.
To investigate the e!ect of As-related weak AsO or vacancies, we have studied the electric characteristics of capacitors with directly oxidized Al. In comparison the J}< characteristics of &80 As oxides in Figs. 3 and 4, a much lower leakage current is observed at gate voltage below 1 V from directly oxidized Al. This leakage current at low gate electric "eld is related to the intrinsic defects [8] that is similar to stress-induced leakage current (SILC) [7,9]. Therefore, the As-related weak AsO or vacancies are responsible to the increased oxide leakage current. The almost same current at high voltages is due to the dominated tunneling mechanism. We have further compared the leakage current of AlO with SiO from the published I}< characteristic [2]. It is important to note that the leakage current of 53 As AlO is already better than the leakage current of MOS capacitors with the same equivalent thickness of 21 As thick SiO [2].
4. Conclusion
We have studied the thermally oxidized AlAs and Al to use as an alternative gate dielectric. The leakage current from AlAs oxidation is larger than that from Al oxidation at low voltages. The leakage current of a 53 As AlO is already lower than SiO with an equivalent oxide thickness of 21 As . These results suggest that scaling equivalent oxide thick-ness below 15}20 As is possible using the AlO "lms.
Acknowledgements
We would like to thank Prof. K.C. Hsieh at the University of Illinois for his great help. The work has been supported by NSC (88-2215-E-009-032) of Taiwan.
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