Article pubs.acs.org/JPCC
Highly Conformal Amorphous W−Si−N Thin Films by PlasmaEnhanced Atomic Layer Deposition as a Diffusion Barrier for Cu Metallization Tae Eun Hong Busan Center, Korea Basic Science Institute, 1275 Jisadong, Gangseogu, Busan 618-230, Korea
Jae-Hun Jung,† Seungmin Yeo,† Taehoon Cheon,†,‡ So Ik Bae,† and Soo-Hyun Kim*,† †
School of Materials Science and Engineering, Yeungnam University, Gyeongsangbuk-do 712-749, Korea Center for Core Research Facilities, DaeguGyeongbuk Institute of Science & Technology, Daegu, Korea
‡
So Jeong Yeo, Hyo-Suk Kim, Taek-Mo Chung, Bo Keun Park, and Chang Gyoun Kim Thin Film Materials Research Group, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong, Daejeon 305-600, Republic of Korea
Do-Joong Lee School of Engineering, Brown University, Providence, Rhode Island 02912, United States ABSTRACT: Ternary and amorphous tungsten silicon nitride (W−Si−N) thin films were grown by atomic layer deposition (ALD) using a sequential supply of a new fluorine-free, silylamide-based W metallorganic precursor, bis(tert-butylimido)bis(bis(trimethylsilylamido))tungsten(VI) [W(NtBu)2{N(SiMe3)2}2], and H2 plasma at a substrate temperature of 300 °C. Here, W(NtBu)2{N(SiMe3)2}2 was prepared through a metathesis reaction of W(NtBu)2Cl2(py)2 (py = pyridine) with 2 equiv of LiN(SiMe3)2 [Li(btsa)]. The newly proposed ALD system exhibited typical ALD characteristics, such as self-limited film growth and linear dependency of the film growth on the number of ALD cycles, and showed a high growth rate of 0.072 nm/cycle on a thermally grown SiO2 substrate with a nearly zero incubation cycle. Such ideal ALD growth characteristics enabled excellent step coverage of ALD-grown W−Si−N film, ∼100%, onto nanotrenches with a width of 25 nm and an aspect ratio ∼4.5. Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy analysis confirmed that the incorporated Si and W were mostly bonded to N, as in Si−N and W−N chemical bonds. The film kept its amorphous nature until annealing at 800 °C, and crystallization happened at local areas after annealing at a very high temperature of 900 °C. An ultrathin (only ∼4 nm thick) ALD-grown W−Si−N film effectively prevented diffusion of Cu into Si after annealing at a temperature up to 600 °C.
I. INTRODUCTION Refractory metal nitrides, such as TiNx, TaNx, and WNx, have been used in many different applications because of their desirable material properties, including high melting temperatures, relatively low resistivities, chemical inertness, etc. Among those applications, the most interesting and fastest developing area nowadays is microelectronics, where these materials are being extensively studied as a diffusion barrier,1 a metal gate electrode,2,3 and a glue layer at ultrahigh-aspect-ratio contact and via holes4,5 in ultralarge-scale-integrated (ULSI) devices. Among them, WNx is particularly noteworthy because WNx films have been frequently reported to have an amorphous-like phase. This amorphous nature should be highlighted since it is contrary to the cases of TiNx and TaNx © XXXX American Chemical Society
systems, which are polycrystalline and have grain boundaries, and it can thereby give desirable properties as the diffusion barrier and the contact glue layer. Moreover, as an addition of a third element into the transition metal nitride disrupts a crystallization and leads to the formation of a stable ternary amorphous material,6 W-based ternary nitride is expected to greatly improve the diffusion barrier performance against Cu. The nearly amorphous structure and chemical inertness of these compounds give an extremely low diffusivity and high chemical stability required for many applications. In fact, it was Received: October 10, 2014 Revised: December 19, 2014
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DOI: 10.1021/jp510226g J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C reported that sputter-deposited binary WNx films failed as a diffusion barrier between Cu and Si after annealing even at 600 °C owing to the formation of copper silicide,7 while W−Si−N (W0.47Si0.41N0.12) successfully worked after annealing at 800 °C.8 The same observation was also reported in other ternary refractory nitrides, such as Ti−Si−N9,10 and Ta−Si−N.11,12 For improvement of device performances, a device size has been continuously shrunken and novel 3-dimensional structures with high aspect ratio (AR) have been integrated. For these advanced applications, a deposition technique should guarantee a conformal deposition on a high AR trench or hole structure with excellent thickness uniformity, process controllability, and large-area uniformity. In this regard, atomic layer deposition (ALD) can be a viable solution since ALD uses a self-limited film growth mode through surface-saturated reaction of precursors and enables an atomic-scale control of a film thickness and composition with excellent step coverage.13,14 Even though relatively many studies were made on the ALD growth of binary W nitrides,15−21 there are only a few reports on the W-based ternary nitride ALD processes.22−25 Binary WNx film was deposited by ALD using various Wcontaining inorganic or metallorganic precursors such as WF6,15−18 bis(tert-butylimido)bis(dimethylamido)tungsten(VI) [TBIDMW, (NtBu)2(NMe2)2W],19,20 and tungsten(III) precursor, W2(NMe2)6,21 and NH3 as a reactant at temperatures ranging from 150 to 400 °C. In particular, due to the formation of an amorphous structure, ALD-WNx showed excellent diffusion barrier performance against Cu.19 For the W-based ternary nitride, most studies focused on the ALD growth of W−C−N film. Three different kinds of reaction schemes were reported for preparing ALD W−C−N films, including the sequential supplies of (i) W-containing inorganic precursor, WF6, carbon-containing precursor, triethylboron [(CH3)3B], and NH3 as a reactant,22,23 (ii) W- and C-containing metallorganic precursor, W(NtBu)2(tBu2pz)2 (tBu2pz =3,5-ditert-butylpyrazolato) and NH3 as a reactant,24 and (iii) W-, C-, and N-containing metallorganic precursor, (5ethylcyclopentadienyl)dicarbonylnitrosyltungsten and H2 plasma as a reactant25 at temperatures ranging from 250 to 450 °C. The failure temperature of ALD W−C−N diffusion barrier (12 nm), determined by X-ray diffractomerty (XRD) and sheet resistance measurement, was as high as 700 °C for 30 min annealing owing to both high density and noncolumnar grain structure. In contrast, in the same report, the sputter-deposited Ta (12 nm) and ALD-TiN films (20 nm) failed only after annealing at 650 and 600 °C, respectively.23 Such results are a good indicative of introducing ALD-grown W-based ternary nitride for the application of microelectronics. In this study, we investigated W-based ternary nitride, W− Si−N thin films prepared by ALD using a new fluorine-free, silyl-amide-based W metallorganic precursor, bis(tertbutylimido)bis(bis(trimethylsilylamido))tungsten(VI) [W(NtBu)2{N(SiMe3)2}2, W(NtBu)2(btsa)2]. The use of F-free precursor can give advantages by excluding many issues of the conventional WF6: (i) WF6 and its byproducts such as HF are corrosive and can chemically attack many other materials exposed on substrate surfaces;16,26,27 (ii) F-containing species that are trapped inside the films may diffuse out, react with their adjacent materials such as Cu and dielectric layers, and finally degrade the device performances;28 and (iii) fluorine impurities residing on the surface of the films may impede the adhesion of Cu. In the present investigation, ALD-WSiN films were deposited using the sequential supply of W(NtBu)2(btsa)2
and H2 plasma at 300 °C, and their properties were characterized using various analysis tools. We confirmed typical ALD characteristics of the present reaction scheme, including a self-limited film growth and a linearity upon the number of ALD cycles as in an ideal ALD process, and found a perfect conformality onto very narrow trenches with an opening width of ∼25 nm and an AR of ∼4.5. Finally, ultrathin (only ∼4 nm thick) ALD W−Si−N films were evaluated as a diffusion barrier against Cu.
II. EXPERIMENTAL SECTION A. Synthesis of W Precursor. All reactions were carried out in an inert dry condition using standard Schlenk techniques or in an argon-filled glovebox. Toluene was purified by innovative technology, PS-MD-4 solvent purification system. W(NtBu)2Cl2(py)2 (py = pyridine) was prepared by the literature method.29 All other chemicals were purchased from Aldrich. Nuclear magnetic resonance spectra were recorded on Bruker 300 MHz spectrometer with C6D6 as solvent and standard. Elemental analyses were conducted on a Thermo Scientific OEA Flash 2000 analyzer or a Thermoquest EA-1110 CHNS analyzer. Synthesis of Li(btsa). n-Butyllithium (2.5 M in hexane, 7.44 mL, 18.6 mmol) was added dropwise to hexamethyldisilazane (HMDS, 3.0 g, 18.6 mmol) solution in hexane (100 mL) at 0 °C. The reaction mixture was refluxed for 24 h. After cooling the reaction mixture, solvent was removed in vacuo to obtain Li(btsa) as pale yellow powder. The crude product was sublimed at 80 °C under a reduced pressure of 10−2 Torr to give white powder (3.0 g, 95%). 1H NMR (C6D6, 300 MHz): δ 0.14 (s, 9H). Synthesis of W(NtBu)2(btsa)2. To a suspension of W(NtBu)2Cl2(py)2 (2.0 g, 3.6 mmol) in toluene (30 mL) was added Li(btsa) (1.2 g, 7.2 mmol). After stirring the mixture at room temperature for 24 h, the resultant was filtered, and then solvent of the filtrate was removed in a vacuum. The crude product was purified by sublimation at 130 °C under a reduced pressure of 10−2 Torr to obtain yellow powder (1.7 g, 75%). 1H NMR (C6D6, 300 MHz): δ 0.43 (s, 36H), 1.44 (s, 18H). FT-IR (KBr, cm−1): 2970 (s), 2924 (s), 2897 (s), 1454 (w), 1404 (w), 1356 (m), 1263 (s), 1252 (s), 1223 (s), 1211 (sh), 1138 (m), 1024 (w), 887 (s), 843 (s), 787 (w), 760 (w), 712 (m), 669 (w), 623 (w), 588 (w), 548 (w), 526 (m), 471 (w). EI-MS (m/ z): 646 [M+]. Anal. Calcd for WN3C14SH32Cl: C, 37.14; H, 8.41; N, 8.66. Found: C, 37.68; H, 8.20; N, 8.48. Thermal Analysis of W(NtBu)2(btsa)2. Thermogravimetric analysis and differential thermal analysis (TGA/DTA) for newly synthesized compounds were investigated by an SETARAM 92-18 TG-DTA apparatus. The TGA data of the compounds were obtained up to 900 °C at a heating rate of 10 °C/min under atmospheric pressure with N2 as a carrier gas. TG sampling was carried out inside an argon-filled glovebox to avoid air contact. However, the samples could be exposed to air during setting the samples for less than 1 min to TGA apparatus. B. Deposition of W−Si−N Films. W−Si−N thin films were deposited on 4 in. diameter thermally grown SiO2-covered Si wafers using a showerhead-type ALD reactor (Lucida M100, NCD Technology) with W(NtBu)2(btsa)2 as a precursor and H2 plasma as a reactant. The deposition temperature was 300 °C, and the chamber pressure was 1 Torr. The W precursor was vaporized in a bubbler kept at 210 °C and was carried into the process chamber by Ar gas with a flow rate of 50 standard cubic B
DOI: 10.1021/jp510226g J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
compound was soluble in organic solvent such as toluene, THF, hexane, diethyl ether, and benzene. W(NtBu)2(btsa)2 was easily purified by sublimation at 130 °C under vacuum (10−2 Torr). Thermogravimetric analysis (TGA) of W(NtBu)2(btsa)2 was carried out from room temperature to 900 °C under a constant flow of nitrogen gas. TGA of W(NtBu)2(btsa)2 exhibits a single weight loss (85%) which begins at about 200 °C and completes by about 350 °C (Figure 2). The weight loss
centimeters per minute (sccm). A gas line temperature was kept at 220 °C to prevent condensation of the vaporized precursor molecules during delivery. After the precursor pulsing, a purge with 200 sccm of Ar was performed for 10 s prior to reactant pulsing. H2 gas with a flow rate of 50 sccm was delivered as a reactant. During the reactant pulsing, a radio frequency (RF) power of 100 W was applied to the showerhead to ignite corresponding plasma. A capacitively coupled direct plasma was used, and the area of the powered electrode (showerhead) was ∼44 cm2 and chamber volume was ∼6 L. After the reactant pulsing, another purge step was performed for 10 s prior to the follow-up precursor pulsing. Such an sequential process, the precursor pulsing, purging, reactant pulsing, and purging, consists of a single ALD cycle. C. Characterizations of W−Si−N Films. Properties of the ALD-grown W−Si−N films were analyzed using various characterization tools. Film thicknesses were measured using cross-sectional view transmission electron microscopy (XTEM, Tecnai F20 equipped with a field emission gun operating at 200 kV) or X-ray reflectance (XRR, PANalytical X’-pert PRO MRD with Cu Kα radiation at 1.5 kW). Resistivities of the films were determined by combining sheet resistance measured by a fourpoint probe and thickness. Rutherford backscattering spectrometry (RBS) analysis was performed to characterize a film composition and to determine a density. A resonance RBS technique using He2+ with incident energy of 4.26 MeV as well as conventional RBS using He2+ with incident energy of 2 MeV was used to sensitively detect a low-mass element in the film such as nitrogen. A bonding status of the films was characterized by X-ray photoelectron spectroscopy (XPS, ESCALAP 250 XPS spectrometer in Korea Basic Science Institute Pusan, Korea). For phase identification, XRD and selected-area electron diffraction (SAD) analysis were performed. Plan-view TEM and XTEM were used for the analysis of microstructures of the W−Si−N films. Step coverages of the films were evaluated at the trench with the AR of ∼4.5 (top opening width: ∼25 nm). Thermal stability of the ALD-grown W−Si−N films was evaluated after annealing in a high vacuum (
