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Thermal atomic layer deposition of device-quality SiO2 thin films under 100 °C using an aminodisilane precursor Dae Hyun Kim, Han Jin Lee, Heonjong Jeong, Bonggeun Shong, Woo-Hee Kim, and Tae Joo Park Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01107 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019
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Chemistry of Materials
Thermal atomic layer deposition of device-quality SiO2 thin films under 100 °C using an aminodisilane precursor Dae Hyun Kim,† Han Jin Lee,† Heonjong Jeong,‡ Bonggeun Shong,*,§ Woo-Hee Kim,*,∥ and Tae Joo Park,*, †,∥ †Department ‡Wonik
of Advanced Materials Engineering, Hanyang University, Ansan 15588, Korea
Materials Co. Ltd., Cheongju 28116, Korea
§Department
of Chemical Engineering, Hongik University, Seoul 04066, Korea
∥Department
of Materials Science and Chemical Engineering, Hanyang University, Ansan 15588, Korea
ABSTRACT: SiO2 is one of the most important dielectric materials that is widely used in the microelectronics industry, but its growth or deposition requires high thermal budgets. Herein, we report a low temperature thermal atomic layer deposition (ALD) process to fabricate SiO2 thin films using a novel aminodisilane precursor with a Si-Si bond, 1,2-bis(diisopropylamino)disilane (BDIPADS), together with ozone. To compare film quality, ALD SiO2 films grown at various temperatures from 250 down to 50 °C were systematically investigated. Our data suggest that even without the aid of plasma-enhanced or catalyzed surface reactions, highquality SiO2 films with relatively high growth rates, high film densities, and low impurity contents compared to conventional Si precursors can be attained through our process at a low growth temperature (~ 50 °C). Chemical analyses via Auger electron spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy confirm the formation of stoichiometric SiO2 films without noticeable impurity contents of nitrogen and carbon, regardless of the growth temperature. However, lowtemperature growth of SiO2 film (≤80 °C) results in a slight ingress of SiH-related moieties during the ALD processes that is not observed at temperatures over 80 ° C. Density functional theory calculations show that the Si-Si bond present in the BDIPADS precursor is easier to be oxidized compared to the Si-H bonds. Through electrical characterization of the SiO2 films grown at different temperatures, we confirm only slight degradation in the dielectric constants, leakage currents, and breakdown fields with decreasing growth temperature, which may be due to the slightly decreased film density and the increased defect density of SiH-related bonds.
1. Introduction Silicon dioxide, SiO2, is the most widely adopted dielectric material in the semiconductor industry for electronic and optoelectronic device applications by virtue of its excellent insulating properties, such as a large band gap of 8.9 eV, low leakage current, low impurity concentration and low interface state density in conjunction with silicon. In microelectronics, SiO2 is a common choice in various dielectric applications, including metal-oxide-semiconductor field-effect transistors, memory capacitors, and pre-metal and interlayer dielectrics for back-end interconnections, despite some recent adoptions of high-k materials in the modern nanoelectronics era.1-4 SiO2 is currently being used as a gate spacer and hard mask for double patterning for nanoelectronics, and as a passivation layer for electronic blocking or against air humidity in several optoelectronic devices.5-11 For instance, the introduction of SiO2 films on polymers can be advantageous as a gas diffusion barrier for organic light emitting devices vulnerable to the high permeability of H2O and O2 through the polymer.10,11 High-quality SiO2 growth on Si substrates has conventionally been possible through thermal oxidation under high temperatures over 900 °C. Other conventional deposition techniques that have been developed for the formation of SiO2
films include thermal chemical vapor deposition (CVD) and plasma-enhanced CVD.12-15 However, such high temperatures and damage-inducing processes limit the versatility of SiO2, particularly for back-end interconnects and thermally delicate substrates with strict temperature constraints, such as polymers. Further, with the ongoing demand for complex and threedimensional (3D) structures in advanced device architectures, there is a strong requirement for the deposition of ultrathin and conformal SiO2 films.16 As such, it is essential to satisfy the process conditions of lower deposition temperature, damagefree processing, excellent uniformity and step coverage, which conventional deposition methods have been lacking. Atomic layer deposition (ALD) based on self-limited surface reactions is the most attractive method for tackling these challenges.17-19 It is thus not surprising that numerous studies on ALD SiO2 have already been carried out both experimentally and theoretically with the implementation of numerous Si precursors and oxidizing agents.20-26 The chemical structures of the precursors have been found to play a crucial role in the formation of ALD SiO2 films.27 For ALD of SiO2 film, a number of volatile silanes with halides, alkyls, alkoxides, and aminosilanes are available.26,28-31 Among such precursors, aminosilanes show the most promise in that they are available
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in a wide range of structural variations and have higher safety and chemical reactivity, thus circumventing the generation of reactive byproducts, particles, and chlorine impurities.26 In recent years, low-temperature ALD (LT-ALD) growth of SiO2 using many different aminosilane precursors, such as bis(dimethylaminosilane) (BDMAS),32 tris(dimethylaminosilane) (TDMAS),27,33 bis(ethylmethylaminosilane) (BEMAS),7,24 bis(diethylaminosilane) (BDEAS), 20,23,27,28 and di(isopropylaminosilane) (DIPAS),25,26,34 has been demonstrated through both thermal and plasma-enhanced processes. However, only the LT-ALD of SiO2 with DIPAS has been demonstrated with temperatures as low as 80 °C with ozone and as low as 50 °C with O2 plasma. Capacitively coupled (direct) plasma-enhanced ALD is able to reduce the process temperature, but caused serious plasma damage to the substrate. In addition, it led to non-uniformity of the film on 3D structures due to the anisotropic nature of direct plasma, which either requires the development of remote plasma-enhanced ALD or returns it back to a ‘thermal’ ALD process. Although the process temperature of SiO2 ALD using SiCl4 and H2O can be further decreased to room temperature using amine catalysts, such as pyridine (C5H5N) and ammonia (NH3), a reaction product is HCl, which may impede SiO2 film growth or etch adjacent films.35-37 These persistent challenges prompted our investigation of a thermal ALD process for SiO2 films that can be accomplished below 80 °C using a novel aminodisilane precursor without the aid of plasma or catalyzed reactions. In this work, we report an LT-ALD process for SiO2 thin films using 1,2-bis(diisopropylamino)disilane (BDIPADS) and ozone (O3). To the best of our knowledge, high-quality SiO2 film growth at temperatures as low as 50 °C was achieved without introducing plasma or catalytic reactions for the first time. Growth characteristics and film properties of ALD SiO2 films grown at a wide range of temperatures from 50 to 250 °C are systematically investigated, and chemical and electrical characterization is also performed. We further investigate the surface chemistry and the bond dissociation energy of the BDIPADS precursor molecule through quantum mechanical calculations and determine the relationships between the chemical and electrical properties with the growth temperature. 2. Experimental procedure ALD of SiO2 thin films. Boron-doped p-type Si(100) substrates with resistivities of 10–12 Ω cm were cleaned in a dilute HF solution ( ∼ 10%) and rinsed in deionized water. Then, SiO2 films were grown on the treated Si wafer substrates in a 4-in. traveling wave type thermal ALD reactor (CN-1 Co.). High purity N2 (99.999%) was used as a carrier gas (300 sccm). For this study, a novel Si precursor, BDIPADS (Wonik materials Co.), was employed with ~180 g/m3 O3 (Ozonetech Co.) as an oxygen source for the purpose of enabling the low temperature deposition of SiO2 films. The feeding times of BDIPADS and O3 achieving ALD mode with self-limited growth characteristics were determined to be 3 s and 20 s, respectively. The purging time following both BDIPADS and O3 exposures was maintained at 60 s, which is sufficient to avoid vapor reactions between the precursor and oxygen source. To investigate the temperature dependence of the ALD process, the process temperature was varied from 50 °C to 250 °C. Analytical methods and quantum mechanical calculations. The film thickness was measured using a
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spectroscopic ellipsometer (Nano View Co. SE MG-1000) with a spectral range of 380−900 nm at three different angles of incidence (65°, 70°, and 75°) with the polarizer set to 45°. The thickness, density, and roughness of the films were investigated via X-ray reflectivity (XRR) with Cu Kα radiation (BRUKER Co. D8 ADVANCE with DAVINCI). The surface morphology of the films was studied via atomic force microscopy (AFM, Park system, XE-100) in tapping mode, and root-mean-squared (RMS) roughness values were extracted from the standard deviation of the height values in the plan-view AFM images. The elemental composition of the films was acquired by Auger electron spectroscopy (AES) depth profile analysis. The chemical binding state as well as the chemical composition of the films were evaluated by X-ray photoelectron spectroscopy (XPS) with a detection limit of ~100 ppm (XPS-ESCALAB 220i) using an Al Kα monochromatic source of 1486.6 eV. The chemical bonding features of the SiO2 films were further characterized by Fourier transform-infrared (FT-IR) spectrophotometry (Thermo Co. Is50), for which 20 nm-thick SiO2 films were employed to obtain signals with sufficient intensity. Density functional theory (DFT) calculations were carried out using the Gaussian 09 software package38 at the B97-D3/def2-TZVP level of theory. For surface chemistry of the precursors, a Si9H12 cluster resembling the Si(100) surface with two –OH groups on each surface atom was employed. The bond dissociation energies (BDE) of Si precursors were defined as energy changes according to homolytic cleavages of each bond. All reported energy values are electronic energies without zero-point correction. Electrical characterizations. The dielectric properties of the SiO2 films were evaluated using metal-oxide-silicon (MOS) capacitors: TiN/ALD SiO2/p-Si, wherein a TiN top electrode (100 nm-thick) was deposited through a shadow mask using DC magnetron sputtering. Then, forming gas annealing (H2 5%–N2 95%) was carried out at 400 °C for 30 min to reduce the interface trap charge densities and improve the metal to dielectric contact. Finally, In-Ga eutectic alloy was used to form a backside ohmic contact. The capacitance-voltage (C-V) and leakage current density-voltage (J-V) characteristics of MOS devices were examined using an Agilent E4980A precision LCR meter and 4156A precision semiconductor parameter analyzer, respectively. 3. Results and discussion First, we investigated the growth per cycle (GPC) of ALD SiO2 films on a Si substrate at 150 °C as a function of the BDIPADS and O3 dosing times. A self-saturated GPC of ~0.14 nm/cycle, which is a typical ALD characteristic, was achieved at ≥ 3 s and ≥ 20 s of BDIPADS and O3 dosing times, respectively (Figure 1). Under growth saturation conditions, the thickness non-uniformity of the SiO2 thin films grown on a Si wafer coupon at a size of 4 cm ×4 cm was determined to be
