Wang et al. Nanoscale Research Letters (2015) 10:141 DOI 10.1186/s11671-015-0842-2 NANO EXPRESS Open Access Silicon diffusion control in atomic-layer-deposited Al2O3/La2O3/Al2O3 gate stacks using an Al2O3 barrier layer Xing Wang, Hong-Xia Liu*, Chen-Xi Fei, Shu-Ying Yin and Xiao-Jiao Fan Abstract In this study, the physical and electrical characteristics of Al2O3/La2O3/Al2O3/Si stack structures affected by the thickness of an Al2O3 barrier layer between Si substrate and La2O3 layer are investigated after a rapid thermal annealing (RTA) treatment. Time of flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) tests indicate that an Al2O3 barrier layer (15 atomic layer deposition (ALD) cycles, approximately 1.5 nm) plays an important role in suppressing the diffusion of silicon atoms from Si substrate into the La2O3 layer during the annealing process. As a result, some properties of La2O3 dielectric degenerated by the diffusion of Si atoms are improved. Electrical measurements (C-V, J-V) show that the thickness of Al2O3 barrier layer can affect the shift of flat band voltage (VFB) and the magnitude of gate leakage current density. Keywords: Atomic layer deposition; Diffusion; Interfacial layer; Silicate; Equivalent oxide thickness Background encounter a bottleneck. During the past two decades, Microelectronics technology has developed in accord- rare earth oxides (Y2O3,La2O3,Nd2O3,etc.)usedas ance with Moore’s law for many years. The performance alternative gate dielectric materials have been exten- of metal-oxide-semiconductor field-effect transistor sively studied [4]. In particular, due to its high k value (MOSFET) has been improving with the downscaling of (approximately 27) and large band gap (approximately feature size. However, in sub-45-nm complementary 5.3 eV), lanthanum oxide (La2O3) is considered as one of metal oxide semiconductor (CMOS) technology, the the most promising alternative for HfO2 to achieve a scaling of SiO2 gate dielectric thickness leads to an more aggressive downscaling of the EOT [5]. But disad- unacceptable gate leakage current, which affects the re- vantages of La2O3 have also been reported, such as hy- liability of the device and causes an increase in static groscopicity and affinity for silicon atoms [6]. Al2O3 has power dissipation. Therefore, new kinds of dielectric also been used as high-k gate dielectric material in the materials with high permittivity are needed to replace early stage, but its further development is limited because the traditional SiO2 gate dielectric to obtain a smaller of the low k value (8 to 10). However, the combination equivalent oxide thickness (EOT) in the CMOS industry of Al2O3 and La2O3 results in an improvement in the [1,2]. Presently, the use of HfO2 (k ~ 13 to 20) as the gate characteristics of the films used as the gate dielectric. dielectric in the high-k/metal gate structure has been For example, when the La2O3 layer is in situ capped successfully applied to MOSFET fabrication and is grad- with an Al2O3 layer, the absorption of moisture which ually replacing the traditional SiO2/poly-Si gate structure gives rise to detrimental effects on the dielectric films [3]. Nevertheless, further downscaling trend makes the such as increased surface roughness and deterioration use of HfO2 as gate dielectric in the CMOS technology of the permittivity can be suppressed [7]; in addition, as a compound of Al2O3 and La2O3,LaAlO3 (LAO) has a nearly high k value (25 to 27) as La2O3 while * Correspondence: [email protected] Key Laboratory for Wide-Band Gap Semiconductor Materials and Devices of providing a high immunity against moisture absorption Education, School of Microelectronics, Xidian University, No. 2, South Taibai and a preferable thermal stability [8] during the annealing Road, Yanta Zone, Xi’an City, Shaanxi Province, 710071, People’s Republic of process. China © 2015 Wang et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Wang et al. Nanoscale Research Letters (2015) 10:141 Page 2 of 6 It turns out that an interfacial layer (IL) which exhibits and Al was sputtered as the back contact metal, followed a La-silicate composition is unavoidably formed between by annealing in forming gas ambient at 400°C for 20 min. the La2O3 film and Si substrate. Moreover, in the con- The capacitance-voltage (C-V) and leakage current ventional gate first process with a high-temperature an- density-voltage (J-V) measurements were carried out nealing treatment, the diffusion of Si atoms from the using a Keithley 590 C-V analyzer (Keithley Instruments, substrate into the dielectric results in the formation of Cleveland, OH, USA) and HP 4156B instrument (Hewlett- undesirable low-permittivity silicates in the films. Both Packard Development Company, L.P., Palo Alto, CA, of the two phenomena mentioned above can especially USA). The flat band voltages (VFB) and EOT of the capaci- be observed in low-temperature deposited films in which tors were extracted from the simulation software named the existence of large amounts of defects and disordered Hauser NCSU CVC program [12] taking into account of chemical bonds may enhance the diffusion of oxygen quantum-mechanical effects. and Si atoms and lead to a degradation of the EOT value [9]. Atomic layer deposition (ALD) is a typically low- Results and discussion temperature deposition method, but its self-limited sur- Figure 1 shows the schematic structure of Al2O3/La2O3/ face reaction mechanism makes the films deposited by Al2O3 gate stacks discussed in this paper. The thickness ALD have some outstanding properties such as atomic of La2O3 layers in samples S1 ~ S4 is 5 nm, and all the scale thickness controllability, fine uniformity, and excel- samples are in situ ALD-capped with a 2-nm Al2O3 lent conformality [10]. Regarding this, ALD is consid- layer. The thickness of the Al2O3 barrier layer between ered as one of the most appropriate way to produce La2O3 layers and Si substrate is tuned by varying the high-quality high-k gate dielectric. In this study, we number of ALD cycles, which is 0, 5, 10, and 15 cycles prepared Al2O3/La2O3/Al2O3 gate stacks by ALD to for samples S1 ~ S4 separately. Table 1 shows the total circumvent the hygroscopicity and diffusion-related thickness of the as-grown and annealed samples mea- problems of La2O3. A thickness-varied Al2O3 layer was sured by spectroscopic ellipsometry (SE). It is reported deposited between La2O3 layer and Si substrate as a bar- that La2O3 films are easily hydrated, but in this work, rier layer, and its effects on the physical and electrical the existence of capping Al2O3 layer suppresses the hy- characteristics of the films were investigated. dration reaction during the rapid thermal annealing (RTA) process, and as a result, the thickness of each Methods sample does not increase too much after the annealing Al2O3/La2O3/Al2O3 gate stacks (S1 ~ S4) were deposited treatment. However, sample S1 shows a thicker incre- on p-type Si (100) wafers using an atomic layer depos- ment about 0.4 nm when compared with the other three ition reactor (Picosun R-200, Espoo, Finland). The wa- samples, which indicates a thicker interfacial layer for- fers were treated with a diluted HF solution to remove mation in sample S1. i- the native SiO2 before deposition. La( PrCp)3 and TMA Figure 2 shows the O 1s XPS spectra and their decon- wereusedastheLaandAlprecursorwhileO3 was volution results for the RTA treated Al2O3/La2O3/Al2O3 used as the oxidant. Under the deposition temperature gate stacks. C 1s peak from adventitious carbon at of 300°C, for La2O3, a linear relation with a growth rate 284.6 eV [13] was used as an internal energy reference of approximately 0.85 Å/cycle is obtained, and the during the analysis. The O 1s XPS spectra consists of steady-state growth rate of Al2O3 filmsisapproxi- mately 0.93 Å/cycle. The fabricated nanolaminate (Al2O3/ La2O3/Al2O3) films were annealed at 700°C for 1 min in N2 ambient. Film thicknesses were measured by Woollam M2000D (Woollam Co. Inc., Lincoln, NE, USA) spectroscopic ellipsometry (SE). Cross-sectional high- resolution transmission electron microscopy (HRTEM) performed with the [100] direction [11] of the Si substrate was used to observe the microstructures of the gate stacks. The bonding structures and chemical quantitative com- position of the films were examined by X-ray photoelec- tron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (TOF-SIMS). The electrical properties of the films were measured using a metal-insulator- semiconductor (MIS) capacitor structure. Metal gate Figure 1 Schematic structure of Al2O3/La2O3/Al2O3/Si stacks. (160 nm Au/20 nm Ni) with a diameter of 300 μm was de- The thickness of Al2O3 barrier layer between Si substrate and La2O3 layer in samples S1 ~ S4 changes with the number of ALD cycles. posited by magnetron sputtering through a shadow mask, Wang et al. Nanoscale Research Letters (2015) 10:141 Page 3 of 6 Table 1 Thickness (measured by SE) of the as-grown and La-O-Si and Si-O-Si is reduced from samples S1 to S4, annealed samples discussed in this work respectively. The decreasing trend of peak II and peak Sample Number of ALD Thickness Thickness after IV suggests that, on the one hand, in sample S1 without Al2O3 cycles as-grown (nm) RTA (nm) an Al2O3 barrier layer, Si diffusion into the La2O3 layer S1 0 8.2 8.6 is enhanced during a thermal treatment, resulting in a S2 5 8.4 8.7 favorable silicate formation through the film and the S3 10 8.8 9.0 interface; on the other hand, to some extent, a uniform S4 15 9.3 9.5 and dense Al2O3 barrier layer (approximately 1.5 nm) in sample S4 produced by 15 cycles of ALD deposition suppresses the formation of IL and prevents the diffu- four peaks, which are (I) La-O-La (approximately sion of Si atoms, resulting in a silicate formation de- 529.3 eV), (II) La-O-Si (approximately 530.4 eV), (III) crease.