Article pubs.acs.org/cm
Study on Initial Growth Behavior of RuO2 Film Grown by Pulsed Chemical Vapor Deposition: Effects of Substrate and Reactant Feeding Time Jeong Hwan Han,† Sang Woon Lee,† Seong Keun Kim,† Sora Han,† Woongkyu Lee,† and Cheol Seong Hwang*,† †
WCU hybrid materials program, Department of Materials Science and Engineering and Inter-university Semiconductor Research Center, Seoul National University, Seoul 151-744, Korea
Christian Dussarat‡ and Julien Gatineau‡ ‡
Air Liquide, 28, Wadai, Tsukuba-Shi, Ibaraki Pref., 300-4247, Japan ABSTRACT: This study examined the nucleation behavior of RuO2 films grown by pulsed chemical vapor deposition (p-CVD) using a RuO4 solution and a N2(95%)/H2(5%) mixed gas as the Ru precursor and reactant, respectively, on various substrates such as Pt, TiN, TiO2, and SiO2 surfaces. In addition, highly doped and nondoped Si substrates were also used to understand the influence of the electrical conductivity of a given substrate material. Contrary to the nucleation behavior of atomic layer deposited and CVD Ru or RuO2 using metal−organic precursors, where oxygen supplying surfaces facilitate nucleation, the nucleation in this study was enhanced by oxygen consuming substrates such as TiN and Si. This is basically due to the thermal decomposition mechanism of the RuO4 precursor. This precursor showed fluent nucleation properties on a Pt substrate too, which is thought to come from the catalytic activity of the surface. The electrical conductivity and ionicity of the substrate had little relevance with the nucleation characteristics of the film. The addition of a H2 reducing gas improved the nucleation to a certain degree, but the enhancement was not substantial. The deposited films were pure and maintained a highly uniform composition along the film thickness direction. The films were already crystallized with the rutile structure. KEYWORDS: RuO2, RuO4, pulsed chemical vapor deposition, initial growth, nucleation
I. INTRODUCTION Thin films of noble metals and their oxides, such as Ir, Ru, IrO2, and RuO2, have a wide range of applications, spanning from electro-catalysts to microelectronics. The numerous applications of these materials lead them to have a variety of favored film types for each specific application. For example, in catalyst applications, since island-like films have a larger active surface area than a continuous and smooth film and thereby have a higher catalytic activity, they are the film of choice for a better performance of a catalytic system.1 The growth of uniformly distributed and highly dense metallic nanoparticles is also desirable for applications in charge trap flash memory technology.2,3 On the other hand, for electrode applications in semiconductor memory devices or for a diffusion barrier layer, the deposited film should have smooth, dense, and continuous surface because a rough electrode surface may induce local electric field concentrations at the interface, which would lead to a high leakage current density.4 The growth characteristics of a film, namely whether it will show a nanoscale discrete nucleation or a uniformly smooth growth behavior, largely depends on the initial growth (nucleation) step of the process. There have been many studies on the initial growth behavior of noble metals and their oxides by atomic © 2012 American Chemical Society
layer deposition (ALD) or metal−organic chemical vapor deposition (MOCVD), which are by far the most promising methods to grow films on substrates with complex surface structures.5−7 Among the various noble metal oxide materials, the electrically conducting RuO2 is a material of a great interest to researchers in the field of metal−insulator−metal capacitor for dynamic random access memory because of its low resistivity (∼35 μΩ·cm), large work function (∼5.1 eV), appropriate thermal stability, and also because it can be easily dry etched. Recent reports on the promotion of rutile phase formation in the ALD of TiO2 films, which have the highest dielectric constant among the binary dielectric oxides, by adopting RuO2 or RuO2/Ru as the bottom electrode invokes an even higher interest in this material.8−12 However, there are hardly any systematic reports regarding the initial growth of this metal oxide film. Most studies on the initial growth behavior of Ru based materials are focused on the Ru metal film itself. In addition, most of the ALD and MOCVD studies on the nucleation behavior of Ru and RuO2 used MO precursors, such Received: April 7, 2011 Revised: September 26, 2011 Published: April 9, 2012 1407
dx.doi.org/10.1021/cm200989t | Chem. Mater. 2012, 24, 1407−1414
Chemistry of Materials
Article
Figure 1. (a)-(d) Changes in the Ru layer density at various N2/H2 reactant feeding time from 1 to 5 s as a function of cycle numbers of RuO2 deposition on Pt, TiN, TiO2, and SiO2 substrates. Insets in parts (a)-(d) show the magnified plots for 10 s, 100 standard cubic centimeters per minuite (sccm)) was required to achieve the Ru phase.20 Therefore, it is probable that the initial nucleation behavior of the Ru or RuO2 films grown using the RuO4 precursor from the p-CVD process will show quite a different mechanism from the ALD and CVD of these films using the MO precursors. This is because the growth mechanism of p-CVD RuO2 is not governed by the oxidation of precursor molecules to break the chemical bonds between the metal ion and ligands.16,20 In this study, RuO2 films were
as 2,4-(dimethylpentadienyl)(ethylcyclopentadienyl)Ru (DER) or Ru(ethylcyclopentadienyl)2, as the Ru source and oxygen as the reaction agent.13−15 These are based on the oxidative decomposition (MOCVD) or chemical ligand exchange reaction (ALD) of the Ru MO precursors when the films grow in the steady state. For metallic film growth, the relatively weak bond between the Ru and O ions were broken by the more oxygen-prone organic ligands and inert gas purge action. Therefore, the fabrication of RuO2 films generally requires an environment with oxygen over pressure. The nucleation behavior of Ru on different substrates is affected by various factors that have an influence on the reactivity between the substrate and precursor. These are not necessarily identical to the factors that affect the growth behavior during the steady-state growth stage if the substrate surface is chemically distinct from the growing film. Aaltonen et al. reported that the surfaces of noble metals such as Pd, Ir, Pt, and Ru have catalytic effects for the dissociation of O2 molecules to atomic oxygen.16 It is well-known that when atomic oxygen diffuses to subsurface regions, it lowers the energy barrier for surface reactions, facilitating the initial growth of films. Kim et al. reported a detailed summary on the former reaction studies of Ru ALD and CVD.13 However, they recently found that the incubation time of ALD Ru films using DER precursor is not greatly affected by the catalytic O2 dissociation ability of the substrate but rather affected by the bonding nature of the substrate surface, i.e. whether it has metallic, ionic, or covalent character.13 Similar with the ALD case, the initial growth behavior of Ru films deposited by MOCVD using Ru MO precursors and an O2 reactant is also known to depend on substrate type and growth temperature.17,18 It has been commonly observed that the purely metallic materials that evade any intervening native oxides, such as Pt and Au, are the most favorable substrate for fluent 1408
dx.doi.org/10.1021/cm200989t | Chem. Mater. 2012, 24, 1407−1414
Chemistry of Materials
Article
and TiN substrates. The appreciable deposition of the RuO2 was achieved only after the p-CVD cycles of ∼30 and ∼50, which correspond to the incubation cycles, on TiO2 and SiO2 substrates, respectively, at the N2/H2 exposure time of 1 s. The estimated incubation cycles were only roughly estimated due to the insufficient number of data points in the region where the number of deposition cycles was >60. On SiO2 surface, several additional experiments with RuO4 feeding time of 2 s were performed to examine the influence of RuO4 feeding time on the incubation cycles. The incubation cycle was slightly reduced (from ∼30 to 20 cycles), but still there was a rather long incubation period. Although the nucleation of ALD or CVD Ru and RuO2 using MO precursors and an O2 reactant generally showed a retarded nucleation rate on the TiN surface, the pCVD RuO2 films grown in this study show a very fluent nucleation behavior on TiN, matching the growth behavior observed on metallic Pt. This accelerated nucleation on TiN substrates has also been observed for metal Ru growth using the same precursors but with longer reaction gas supply times.19 This phenomenon suggests that there is a significant discrepancy between the nucleation characteristics of the pCVD RuO2 (or Ru) reported in this study and the previously reported Ru films grown by the reaction route based on the oxidative decomposition of MO precursors. Figure 1 (a)-(d) also shows the influence of the N2/H2 feeding time on the incubation cycles of the RuO2 films. On the Pt surface, the incubation cycles of the RuO2 films are identical despite the change in reactant exposure time due to the accelerated nucleation behavior. The TiN substrate shows a similar behavior even though the N2/H2 feeding time of 1 s shows a slightly retarded growth behavior. However, on TiO2, the number of incubation cycles reduced from ∼30 cycles to ∼2 cycles when the reactant pulse time was increased from 1 to 5 s. As in the case of the TiO2 substrate, a decrement of incubation cycles was also observed on SiO2 when increasing the N2/H2 feeding time, although less significant in this case. Even with a N2/H2 feeding time of 5 s, ∼30 incubation cycles were observed on SiO2. Unlike the initial growth behavior, however, the steady state growth rates of RuO2 on the Pt, TiN, and TiO2 substrates were almost unaffected by reactant feeding. The growth rate was indirectly extracted from the variation of Ru layer density as a function of the number of p-CVD cycles performed. The slope observed when plotting the Ru layer density as a function of the number of cycles performed excluding the incubation cycles is nearly identical at different reactant feeding times on each substrate. The growth rate was ∼170−180 ng/cm2·cycle (12.1−12.9 ng/cm2·s), which corresponds to a thickness growth rate of ∼0.34 nm/cycle (0.024 nm/s). This suggests that the increased feeding of the N2/H2 influences only the initial nucleation behavior and not the steady state growth when the substrate was fully covered with RuO2. Although the Ru layer density increases abruptly after ∼60 cycles in the case of the SiO2 substrate, the limited data points do not guarantee that the growth is in the steady state. A detailed study on the initial growth behavior of films grown by ALD or CVD is essential because this step has a great influence on the surface morphology characteristics of the films such as roughness, grain size, nuclei density, and so on. It also influences the step coverage over a three-dimensional topological surface. To examine how the surface morphology of the films were influenced by the types of substrate and reactant feeding time, the peak to valley roughness (Rz) of the substrates and RuO2 films was examined using AFM. Figure 2
deposited using a RuO 4 precursor, and a systematic investigation was conducted on the nucleation behavior of these films according to substrate type and reactant feeding time.
II. EXPERIMENTAL PROCEDURE RuO2 films were deposited by p-CVD using a RuO4 precursor dissolved in a blend of organic solvents containing fluorinated solvents (ToRuS, Total Ruthenium Solution, produced by Air Liquide Co. with concentration of 1.8 M) and a 95% N2/5% H2 mixed gas (N2/H2) as the Ru precursor and reactant gas, respectively. The deposition temperature was 230 °C, and the p-CVD sequence consisted of four steps: Ru precursor injection (1 s) − Ar purge (7 s) − N2/H2 gas injection (1−5 s) − and Ar purge (5 s). The Ru solution was cooled to 3 °C to achieve an appropriate vapor pressure, and no carrier gas was used to introduce the precursor molecules into the p-CVD chamber. A more detailed description on the deposition procedure of RuO2 films is given in the previous report.20 The N2/H2 reactant exposure time was varied from 1 to 5 s to investigate the effect of the reactant feeding time on the initial growth behavior of RuO2. The N2/H2 reactant feeding time (1−5 s) corresponds to the growth of RuO2, not Ru, under the given p-CVD conditions.20 The flow rate of the N2/H2 gas was fixed to 100 sccm. Sputtered Pt, CVD TiN, ALD TiO2 (∼4 nm)/ SiO2, and thermally grown SiO2 (∼100 nm) were used as substrates to examine the relation between RuO2 nucleation and substrate characteristics. The ALD TiO2 was deposited using Ti(i-OC3H7)4 (TTIP) and O3 (250 g/m3) as the Ti precursor and oxygen source, respectively. In addition, heavily doped and nominally nondoped Si wafers were also used to understand the influence of the electrical conductivity of the substrate. The growth rate of the RuO2 films was determined by X-ray fluorescence spectroscopy (XRF, Thermoscientific, ARL Quant’X). XRF cannot detect oxygen so that the layer density of Ru estimated was used to check the amount of RuO2 deposited. This is a rather safe procedure because the growth condition was well within the RuO2 phase formation conditions.20 The film thickness can be calculated from the density estimated by X-ray reflectivity and Ru layer density estimated by XRF. To compare the peak to valley roughness (Rz) of the RuO2 films according to substrate type and reactant exposure time, atomic force microscopy (AFM, JSPM-5200) was used. The surface morphology of the RuO2 films was also observed by scanning electron microscopy (SEM, Hitachi S-4800). The cross-section microstructure of the thin RuO2(
