Article pubs.acs.org/cm
Evaluation and Comparison of Novel Precursors for Atomic Layer Deposition of Nb2O5 Thin Films Timothee Blanquart,†,* Jaakko Niinistö,† Mikko Heikkila,̈ † Timo Sajavaara,‡ Kaupo Kukli,† Esa Puukilainen,† Chongying Xu,§ William Hunks,§ Mikko Ritala,† and Markku Leskela†̈ †
Department of Chemistry, P.O. Box 55, University of Helsinki, FI-00014 Helsinki, Finland Department of Physics, P.O. Box 35, University of Jyväskylä, FI-40014 Jyväskylä, Finland § ATMI, 7 Commerce Drive, Danbury, Connecticut 06810, United States ‡
ABSTRACT: Atomic layer deposition (ALD) of Nb2O5 thin films was studied using three novel precursors, namely, tBuNNb(NEt2)3, tBuNNb(NMeEt)3, and tamylN Nb(OtBu)3. These precursors are liquid at room temperature, present good volatility, and are reactive toward both water and ozone as the oxygen sources. The deposition temperature was varied from 150 to 375 °C. ALD-type saturative growth modes were confirmed at 275 °C for tBuNNb(NEt2)3 and tBuNNb(NMeEt)3 together with both oxygen sources. Constant growth rate was observed between a temperature regions of 150 and 325 °C. By contrast, amylNNb(OtBu)3 exhibited limited thermal stability and thus a saturative growth mode was not achieved. All films were amorphous in the as-deposited state and crystallized between 525−575 °C, regardless of the applied precursor and oxygen source. Time-of-flight elastic recoil detection analysis (TOF-ERDA) demonstrated the high purity of the films. Atomic force microscopy (AFM) revealed that the films were smooth and uniform. The films exhibited promising dielectric characteristics with permittivity values up to 60. KEYWORDS: ALD, niobium oxide thin film, high-k, niobium imido-amido
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INTRODUCTION Nb2O5 is a wide band gap (3.6 eV) dielectric material with a high index of refraction (n = 2.4) and permittivity (29 to 200 depending of the crystalline phase1). Due to its interesting optoelectronic properties, Nb2O5 has a wide range of applications such as capacitor dielectrics2−5 and as catalyst-supporting oxide materials.6−8 In particular, in dynamic random access memories (DRAMs), where the dielectric should possess permittivity exceeding 40, Nb2O5 has been explored as an alternative to the more widely studied SrTiO3 and the rutile phase TiO2 doped with Al. This is because the formation of high permittivity rutile phase of TiO2 requires matching oxidized ruthenium electrode material9 which can be a cost issue. In the case of the ternary SrTiO3 films the dielectric properties are sensitive to the stoichiometry which is difficult to control accurately.10 Nb2O5 thin films have mainly been deposited using physical vapor deposition techniques such as electron beam evaporation and magnetron sputtering11,12 and less frequently by pulsed laser deposition.13 Nb2O5 thin films have also been deposited using chemical techniques such as pyrolysis,14 sol−gel process,15 and chemical vapor deposition (CVD).16 ALD is an advanced variant of the CVD method, where the substrate surface is alternately exposed to the vaporized precursor fluxes.17 The reactant pulses are separated by purging periods to eliminate gas-phase reactions and remove reaction byproducts. In the case of metal oxide film growth, for instance, the complete ALD growth cycle consists of metal precursor © 2012 American Chemical Society
exposure, the first purging period, oxygen precursor exposure, and the second purging period. The main characteristic feature of the ALD growth is the self-limited adsorption of the precursor on the substrate surface, providing inherent control of the film thickness and excellent repeatability. The stepwise growth via the self-limited adsorption processes makes ALD an excellent method to deposit conformal and pinhole-free films with superior uniformity.18−20 Tantalum has similar physical and chemical properties as niobium and a large amount of work has been devoted to the development of tantalum precursors for ALD of Ta2O5 and TaNx. The most studied ALD precursors for Ta2O5 growth are the halides21−23 and Ta(OEt)5.24−26 In addition, alkylamide precursors such as Ta(NMe2)5 have also been applied.27 However, these precursors have several drawbacks such as chlorine contamination from TaCl5 and limited thermal stability of Ta(OEt)5 and the alkylamides. Thus, in the recent years research efforts have been devoted to the development of new halogen-free precursor with enhanced thermal stability, such as Ta(NtBu)(3,5-di-tert-butylpyrazolate) 3 , 28 Ta(N t Bu)( i PrNC(Me)NiPr)2(NMe2),29 and Ta(NEt)(NEt2)3,30,31 although often with a lowered growth rate. In addition, tBuNTa(NEt2)3 Received: September 8, 2011 Revised: December 2, 2011 Published: February 8, 2012 975
dx.doi.org/10.1021/cm2026812 | Chem. Mater. 2012, 24, 975−980
Chemistry of Materials
Article
inside the reactor. Evaporation temperatures were 65, 55, and 60 °C for tBuNNb(NEt2)3, tBuNNb(NMeEt)3, and tamylNNb(OtBu)3, respectively. The growth rate as a function of deposition temperature was studied in the temperature range of 150−375 °C using a pulsing sequence of 0.7/1.0/1.0/1.5 s (metal precursor pulse/ purge/oxygen precursor pulse/purge). Self-limited growth of Nb2O5 was determined by studying the growth rate as a function of the metal precursor pulse length, that is, by varying x in the pulsing sequence x/x + 0.5/1.0/1.5 s. Some of the films were annealed at 600 °C for 20 min in a tube furnace under nitrogen. Film Characterization. The thickness and crystallinity of the Nb2O5 thin films were evaluated by X-ray reflectivity (XRR) and X-ray diffraction (XRD) using a Panalytical X̀ Pert Pro MPD X-ray diffractometer, and MAUD software was used for Rietveld refinements. Hightemperature XRD (HTXRD) measurements were performed under nitrogen, 99.999%, further purified with Entegris 35KF-I-4R inert gas purifier, at temperatures ranging from 25 to 1175 °C using an AntonPaar HTK1200N oven. Film composition was analyzed by time-offlight elastic recoil detection analysis (TOF-ERDA) using the 6.8 MeV 35 3+ Cl beam from 1.7 MV Pelletron accelerators. Surface morphology was examined with a MultiMode V atomic force microscope (AFM) equipped with NanoScope V controller (Veeco Instrument) operated in the tapping mode. Samples were measured with a scanning frequency of 0.5 Hz. Several wide scan images (5 × 5 μm2) were recorded from different parts of the samples to check their uniformity. Final images were measured from a scanning area of 2 × 2 μm2. Roughness values were calculated as root mean squares (rms). Electrical characterization of the films was carried out on Al/ Nb2O5/TiN/p-Si(100)/Al capacitors with top electrodes consisting of 100−110 nm thick Al layers e-beam evaporated through a shadow mask. Capacitance−voltage (C−V) curves were recorded using a HP4284A precision LCR-meter in a two-element series circuit mode. The stair-sweep voltage step was 0.05 V. The period between the voltage steps was 0.5 s. The AC voltage applied to the capacitor was 0.05 V while the frequency of the AC signal was 1 kHz. The current−voltage (I−V) curves were measured with a Keithley 2400 Source Meter in the stair sweep voltage mode, while the voltage step was 0.05 V and the top electrodes were biased negatively in relation to the TiN/Si substrate; that is, electrons were injected from the top electrode. All measurements were performed at room temperature on samples in the as-deposited state and after annealing in N2 at 600 °C.
has been investigated in plasma-enhanced ALD of TaC,32 TaNx,33 TaCN,33 and Ta2O5.34 Different from Ta2O5, few successful Nb2O5 ALD processes have been reported. The first reference in the literature was an attempt to deposit Nb2O5 from NbCl5 and water.35 However, this process failed, reportedly due to etching caused by the formation of gaseous NbOCl3.36 The only successful Nb2O5 ALD processes reported so far use Nb(OEt)5/water,37 NbI5/ O2,38and NbF539 with either water or a combination of water and ozone. In these processes deposition temperatures were varied, respectively, between 150 and 350 °C and 400 and 600 °C and limited to 225 °C in the case of the NbF5 processes. The ALD window of these processes, if present, was narrow. In the Nb(OEt)5/water process, the maximum growth temperature at which the self-limiting ALD-type growth was confirmed was 230 °C with a low growth rate of 0.28 Å/ cycle. Furthermore, Nb(OEt)5 exists as a dimer with low volatility and decomposes when held at the >100 °C bubbler temperature needed for volatilization. With NbF5, the upper limit of the deposition temperature was restricted to 225 °C, because at higher temperatures NbF5 started to etch the growing film leading to no growth or nonuniform thickness and composition of the films. Moreover, no evidence of the selflimiting growth in the NbI5/O2 and NbF5/H2O processes was reported. According to our knowledge, no Nb2O5 ALD processes using only ozone as the oxygen source have been reported in the literature. The availability of an ozone process is important, especially for applications where the slow desorption of water would lead to problems in the form of excessively long purge times, like with high aspect ratio structures and at low deposition temperatures. Clearly, novel Nb2O5 ALD processes are needed, especially with niobium precursors having enhanced thermal stability and reactivity toward both water and ozone. In the present paper, we report a comparative study on the ALD growth of Nb2O5 thin films using three novel precursors, namely, tBuN Nb(NEt 2 ) 3 (tert-butylimido)tris(diethylamido)niobium), t BuNNb(NMeEt)3 (tert-butylimido)tris(ethylmethylamido)niobium), and t amylNNb(O t Bu) 3 ((1,1-dimethylpropylimido)tris(tert-butoxide)niobium) with either ozone or water as the oxygen source. While new to ALD deposition of Nb2O5, both tBuNNb(NEt2)3 and tBuNNb(NMeEt)3 have been investigated in plasma-enhanced ALD of NbNx for gate electrode.40
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RESULTS AND DISCUSSIONS Nb2O5 Film Growth. All the precursors are liquid at room temperature and volatile enough to be evaporated at modest temperatures (55−65 °C). As shown by the TG measurements performed under 1 atm of argon (Figure 1), the three
EXPERIMENTAL SECTION
Nb2O5 Film Deposition. The precursors evaluated for the ALD deposition of Nb2O5 thin films were tBuNNb(NEt2)3, tBuN Nb(NMeEt)3, and tamylNNb(OtBu)3 (ATMI, USA). Thermogravimetric analysis of the precursor was performed on a Netzsch STA449C instrument operating inside a nitrogen drybox. The measurements were performed under argon in open pan Pt/Rh crucibles with 5− 10 mg of sample and a heating rate of 10 K/min. Nb2O5 thin films were grown on 5 × 5 cm2 Si(100) substrates (Okmetic, Finland) in a hot-wall flow type F-120 ALD reactor (ASM Microchemistry Ltd.). For selected samples, TiN covered Si substrates were also used. The operating pressure of the reactor was 5 to 10 mbar during the deposition. As the oxygen source, either O3, produced from >99.999% O2 in an ozone generator (Wedeco Ozomatic modular 4 HC Lab Ozone, ozone concentration 100 g/m3), or water was used. Nitrogen (>99.999%) generated with Nitrox UHPN 3000−1 nitrogen generator was used as a carrier and purge gas. The air and moisture sensitive Nb2O5 precursors were handled in a glovebox and inserted into the reactor in sealed boats. Precursors were evaporated from open boats
Figure 1. TG curves of the evaluated Nb2O5 thin film precursors.
precursors show the same behavior: evaporation occurs in a single step with very low residual mass (
