Reaction Chemistry during the Atomic Layer Deposition of Sc2O3 and

Jan 17, 2014 - The reaction chemistry during the atomic layer deposition (ALD) of Sc2O3 and Gd2O3 from Sc(MeCp)3, Gd(iPrCp)3, and H2O was investigated...
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Reaction Chemistry during the Atomic Layer Deposition of Sc2O3 and Gd2O3 from Sc(MeCp)3, Gd(iPrCp)3, and H2O Jeong Hwan Han,†,‡,∥ Laura Nyns,† Annelies Delabie,†,§ Alexis Franquet,† Sven Van Elshocht,† and Christoph Adelmann*,† †

Imec, B-3001 Leuven, Belgium Departement Elektrotechniek (ESAT), KU Leuven, B-3001 Leuven, Belgium § Departement Chemie, KU Leuven, B-3001 Leuven, Belgium ‡

ABSTRACT: The reaction chemistry during the atomic layer deposition (ALD) of Sc2O3 and Gd2O3 from Sc(MeCp)3, Gd(iPrCp)3, and H2O was investigated by in situ time-resolved quadrupole mass spectrometry. Despite the similarity of the ligands of the Sc and Gd precursors, the growth characteristics and ligand dissociation patterns of the Sc2O3 and Gd2O3 ALD processes showed considerably different behavior. For both processes, the precursors reacted with the hydroxylated surface by proton transfer and release of the protonated ligand. The remaining ligands were then removed by hydrolysis during the H2O pulse. However, for the Sc(MeCp)3/H2O process, ∼56% of MeCpH was released during the Sc(MeCp)3 exposure, whereas in the case of the Gd(iPrCp)3/H2O process, as much as 90% of iPrCpH was released during the Gd(iPrCp)3 pulse. The observation that almost all iPrCp ligands were removed during the initial Gd(iPrCp)3 absorption step can be ascribed to CVD-like reactions between the Gd(iPrCp)3 precursor and excess hydroxide or physisorbed H2O on the hygroscopic Gd2O3 surface. The influence of the growth temperature on the ligand exchange behavior and the resulting film properties (thickness uniformity, impurity concentration) was studied in the temperature range between 200 and 350 °C. In addition, the transient growth behavior of Gd2O3 on Sc2O3 and vice versa was studied, indicating that the hygroscopic nature of Gd2O3 also strongly influences the deposition of GdxSc1−xO3 ternary oxides.

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INTRODUCTION The deposition and the properties of rare earth oxide (REO) thin films have been extensively studied for several decades for applications as protective coatings,1,2 optics,3 and microelectronics.4−9 Sc2O3, Gd2O3, and GdScO3 thin films have recently attracted strong interest for applications in microelectronic devices (e.g., as gate dielectrics for metal-oxidesemiconductor field-effect transistors or as blocking dielectrics in nonvolatile Flash memory cells) because they possess relatively high dielectric constants (Sc2O3∼14, Gd2O3 ∼16, GdScO3 ∼23),10−12 wide band gaps (Sc2O3 ∼6.3 eV, Gd2O3 ∼5.3 eV, GdScO3 ∼5.7 eV),10,13 and excellent thermal stability. Although there have been many reports on the deposition of REOs using a variety of methods such as physical vapor deposition14,15 and chemical vapor deposition16−19 (CVD), comparatively few papers report on the atomic layer deposition (ALD) of Sc2O3, Gd2O3, and GdScO3 films due to the limited number of appropriate Sc and Gd precursors with sufficiently high vapor pressure, high reactivity, and thermal stability. Sc2O3 and Gd2O3 ALD processes using β-diketonate precursors such as Sc(thd)3 and Gd(thd)3 in combination with O3 showed ALD behavior in the temperature ranges 335−375 °C and 250−300 °C, respectively, but led to rather low growth per cycle (GPC) values of only 0.14 and 0.3 Å, respectively.20,21 The lack of overlapping ALD temperature windows also renders the deposition of ternary GdxSc1−xO3 difficult. Precursors based © 2014 American Chemical Society

on the cyclopentadienyl (Cp) ligand show a lower ligand dissociation energy and smaller size than β-diketonate precursors and have been studied by several groups.22−24 It was demonstrated that the ALD of Sc2O3 films from ScCp3 in combination with H2O exhibits a GPC of 0.75 Å, with C and H concentrations below 0.5 at. %.20 For the ALD of Gd2O3 and Gd-containing ternary oxide films such as GdAlO3, GdHfOx, and GdScO3, Gd(MeCp)3 (MeCp = methyl-cyclopentadienyl), and Gd(iPrCp)3 (iPrCp = isopropyl-cyclopentadienyl) precursors have been explored in combination with H2O as the oxidizing agent.21,23,25 Niinistö et al. reported that Gd2O3 processes using Gd(MeCp)3 and H2O at deposition temperatures of 175−300 °C showed nonideal ALD behavior.21 By contrast, Vitale et al. found that ALD behavior was observed for a Gd2O3 process when Gd(iPrCp)3 was used in combination with an O2 plasma at a reactor temperature of 250 °C.26 However, at higher temperatures above 300 °C, the growth mechanism changed to CVD. To gain more insight in the growth behavior of REO ALD processes using Cp-based precursors, the surface chemistry has been studied by density functional theory calculations. Elliot has shown that the hydrolysis reaction of Cp-based precursors Received: October 15, 2013 Revised: January 8, 2014 Published: January 17, 2014 1404

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molecules, and their effect on the measurements will be discussed below in the next section. The thicknesses of Sc2O3, Gd2O3, and GdxSc1−xO3 films were determined by spectroscopic ellipsometry using a KLA-Tencor Aleris system. The within-wafer nonuniformity (WiWNU, defined as the standard deviation of the film thickness) was extracted from 49-point wafer maps. The contamination in the Sc2O3 and Gd2O3 layers was assessed by TOF-SIMS using an IONTOF-IV instrument with a Ga+ source operating at a beam energy of 15 keV (area of 80 μm × 80 μm) for analysis− and a Xe+ beam with an energy of 350 eV (sputter area of 400 μm × 400 μm) for depth profiling.

on OH-terminated surfaces is thermodynamically more favorable than that of alkoxide precursors.27 Nolan and Elliott have calculated the reaction pathways for LaCp3 and ErCp3 on hydroxylated La2O3 and Er2O3 surfaces, respectively, and have confirmed that the most stable adsorbates of the La and Er precursors are LaCp2 and ErCp with energy gains of 0.72 and 5.6 eV, respectively.28 It was concluded that not only the intrinsic properties of the metal precursor but also the nature of the reaction surface, such as the density of reactive sites, is a crucial factor that determines the growth characteristics and ligand dissociation patterns. Despite a number of experimental and theoretical reports on the ALD of REOs from Cp-based metal precursors and H2O, there have been so far no in situ studies of the surface reactions during the ALD processes. Quadrupole mass spectrometry (QMS) has been previously used as a powerful technique to gain insight into the surface chemistry during the ALD process by analyzing the gaseous reaction byproducts.29−33 In this paper, we study the surface reactions during the steady-state ALD of Sc2O3 and Gd2O3 from Sc(MeCp)3, Gd(iPrCp)3, and H2O by time-resolved in situ QMS analysis. We show that the hygroscopicity of Gd2O3 led to additional CVD reaction pathways due to sorbed H2O and/or Gd hydroxides. The impact of the CVD component increased with increasing deposition temperature and resulted in nonuniform growth as well as high carbon incorporation. By contrast, these CVD pathways were absent for the ALD of less hygroscopic Sc2O3. Furthermore, we address the transient growth behavior of Gd2O3 on Sc2O3 and vice versa to understand the effect of the properties of the underlying surface on the ALD growth mechanism of ternary GdxSc1−xO3. It was found that both the Sc(MeCp)3/H2O as well as the Gd(iPrCp)3/H2O reaction chemistry were strongly influenced by the degree of hygroscopicity of the surface (i.e., Sc2O3- or Gd2O3-like). The influence of the substrate on the deposition processes persisted for an extended number of ALD cycles up to nominal layer thicknesses of the order of 1.5 nm, suggesting that the CVD component does not only originate from surface reactions but probably also from reactions with a near-surface hydroxide layer.

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RESULTS AND DISCUSSION Atomic Layer Deposition of Sc2O3, Gd2O3, and GdxSc1−xO3. Prior to discussing the surface reactions occurring during the Sc(MeCp)3/H2O an d Gd(iPrCp)3/H2O ALD, we will shortly describe the behavior of the ALD processes themselves. More details of the ALD processes can be found in refs 23 and 34. For deposition temperatures between 250 to 350 °C, the Sc(MeCp)3/H2O process showed ideal ALD behavior.34 This is exemplified for 300 °C in Figure 1a, which shows the variation of the GPC and the WiWNU for Sc2O3 as a function of the Sc(MeCp)3 pulse time. Self-limiting ALD growth was found for Sc(MeCp)3 pulse times of 4 s and longer with a GPC of 0.65 Å. The Sc2O3 GPC depended on the deposition temperature and decreased from 0.76 Å at 250 °C to 0.58 Å at 350 °C. At all temperatures, the process led to very uniform Sc2O3 films with WiWNU values of 1% over the 300 mm wafer for sufficiently long Sc(MeCp)3 and H2O pulse times. By contrast, the deposition of Gd2O3 from Gd(iPrCp)3 and H2O did not show any self-saturating ALD behavior, as evidenced by the dependence of the GPC on the Gd(iPrCp)3 pulse time in Figure 1b. The GPC increased with Gd pulse length but without any signs of saturation. In parallel, the deposition was very nonuniform with WiWNU values on the order of 100%. This is in consistent with previous reports, which found a CVD-like growth behavior for Gd2O3 and Gdrich ternary alloys.23,34,35 Interestingly, it was observed that the film uniformity was much improved for the ALD of Gd-containing ternary oxides such as GdScO3, GdAlO3, and GdHfOx in the same temperature range, at least when the Gd-concentration was sufficiently low.23−25,34 This is apparent in Figure 1c, which shows the variation of the WiWNU of different GdxSc1−xO3 films as a function of the Gd/Sc cycle ratio in an ALD GdScO3 supercycle used for depositing them. The GdxSc1−xO3 films exhibited low WiWNU values on the order of 1% for all compositions, except for the most Gd-rich composition with m = 3 (Gd/Sc cation ratio ∼3, see ref 34). This is again consistent with earlier reports23−25,34 and indicates that the CVD component cannot be explained by the thermal decomposition of the Gd(iPrCp)3 precursor. In keeping, pulsing Gd(iPrCp)3 only (no intermittent H2O) led to no deposition. This was previously linked to the reaction of the Gd(iPrCp)3 with sorbed H2O on the Gd2O3 surface due to the hygroscopicity of Gd2O3.23 In the following section, the differences between the Sc(MeCp)3/H2O and Gd(iPrCp)3/H2O processes in steadystate will be studied by QMS. In the subsequent section, the transient behavior of Gd(iPrCp)3/H2O on Sc2O3 and Sc(MeCp)3/H2O on Gd2O3 will be discussed, which is relevant for the ALD of ternary oxides. In Situ QMS Analysis of the Homodeposition of Sc2O3 and Gd2O3. The quantitative analysis of time-resolved QMS

EXPERIMENTAL DETAILS

All Sc2O3, Gd2O3, and GdxSc1−xO3 films were deposited by ALD on 300 mm Si(100) wafers in a hot-wall, cross-flow-type ASM Pulsar 3000 reactor, connected to a Polygon 8300 platform. Sc(MeCp)3 and Gd(iPrCp)3 were used as precursors in combination with H2O as the oxidizing agent. The reactor chamber was isothermal; that is, the susceptor and the reactor walls were set to the same temperature. The reaction chemistry of the Sc2O3 and Gd2O3 ALD processes was investigated by QMS using a Hiden Analytical HPR-20 system, connected to the pump line of the ALD reactor. The gas species were sampled via a heated quartz capillary and ionized by electron impact (ionization energy of 70 eV). A secondary electron multiplier was used as the detector. Unless mentioned otherwise, several 10 Sc2O3 or Gd2O3 ALD cycles were performed prior to QMS measurements to assess the surface chemistry of the steady-state ALD on well controlled surfaces (both of the substrate and the reactor walls). The time resolution of the QMS system (including the sampling inlet response time) was specified to be nominally about 100 ms. The QMS signals may additionally be broadened by diffusional effects in the reactor. In practice, 10−90% rise times as low as 250 ms were observed in the experimental data, which can thus be considered as an upper limit of the measurements’ time resolution. Possible background effects, including artifacts due to the dissociative ionization of gas 1405

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eliminates low vapor pressure components from the gas stream. The absence of any signals that are directly related to the presence of unreacted precursor molecules rules out artifacts from the dissociative ionization of the Sc(MeCp)3 precursor in the mass spectrometer. By contrast, weak signals were observed for m/z = 80 (MeCpH) and m/z = 66 (CpH) even without H2O pulsing. This may be ascribed to reactions of Sc(MeCp)3 with residual H2O in the reactor (including the absorber). In the quantitative analysis of the reaction patterns below, these background effects were corrected for to make the analysis more accurate. However, in all cases, the background signals amounted to less than 10% of the measured signal during the full ALD sequence and the corrections did not affect the conclusions. By contrast, no background signals were observed during Gd(iPrCp)3 precursor pulses at m/z = 481 (Gd(iPrCp)3), 373 (Gd(iPrCp)2), 265 (Gd(iPrCp)), and 108 (iPrCpH). Thus, no corrections to the signals measured during the Gd(iPrCp)3/H2O pulse sequence were necessary. We now discuss the surface reaction chemistry during both the Sc2O3 and Gd2O3 ALD processes at a fixed temperature of 300 °C. For Sc2O3, the formation of MeCpH (m/z = 80) was observed during both Sc(MeCp)3 and H2O pulses. Analogously, the formation of iPrCpH (m/z = 108) was observed for the ALD of Gd2O3 during both Gd(iPrCp)3 and H2O pulses. No other reaction products could be identified. These observations indicate that surface reactions of the Sc(MeCp)3 and Gd(iPrCp)3 precursors occur at hydroxyl surface groups by proton transfer to the ligand which is then released as a neutral molecule. The hydrolysis also proceeds by proton transfer to the ligand followed by the formation of a surface hydroxyl group. This description neglects a number of additional surface reactions that include “densification” reactions that increase the cation (Sc or Gd) coordination from 3 (as in the metalorganic precursors) to 6 as in cubic Sc2O3 or Gd2O3, as well as reactions leading to equilibrium surface hydroxyl densities, such as the formation of additional hydroxyl groups or their recombination [schematically described by M−OH + M−OH ⇌ M−O−M + H2O (g)].36,37 We remark that H2O sorption is not necessarily limited to the formation of surface hydroxyl groups for hygroscopic oxides (such as Gd2O3). This will be discussed in more detail below. However, these reactions do not produce detectable gaseous products and cannot be studied by QMS since the detection of H2O (or H2) is hindered by the large background signals in the reactor and mass spectrometer. In the following, we will focus on m/z = 80 (MeCpH, Sc2O3 process) and 108 (iPrCpH, Gd2O3 process). Figure 2a shows the variation of the MeCpH partial pressure during two Sc(MeCp)3/H2O ALD cycles at a deposition temperature of 300 °C. The used pulse sequence was 3 s Sc(MeCp)3; 10 s N2 purge; 5 s H2O; 10 s N2 purge. It is schematically indicated in the figure. These QMS signals were obtained after several 10 ALD cycles (i.e., under steady-state (homodeposition) conditions) and are free from transient effects. The formation of MeCpH was observed during both Sc(MeCp)3 and H2O pulses. The ratio of MeCpH generated during the individual Sc(MeCp)3 and H2O pulses was calculated from the ratio of the integrated peak areas followed by subtracting the background intensities, as described above. The data indicate that the ratio of MeCpH formed during the Sc(MeCp)3 half cycle and during the H2O half cycle was ∼1.27. This means that on average 56% of the MeCp ligands were released during the Sc(MeCp)3 half cycle and the remaining ∼44% of MeCp ligands were removed during the H2O half

Figure 1. ALD process characteristics. (a) Sc2O3 GPC and WIWNU vs Sc(MeCp)3 pulse time. The GPC saturated after a pulse length of 4 s, indicating ALD behavior. (b) Gd2O3 GPC and WiWNU as a function of the Gd(iPrCp)3 pulse length. No saturation behavior was observed and the WiWNU of the films remained very high even for long Gd(iPrCp)3 pulses. In both figures, the error bars indicate the total variation of the film thickness across the wafer. (c) WiWNU of GdxSc1−xO3 films as a function of the Gd(iPrCp)3/Sc(MeCp)3 cycle ratio within a supercycle. The WiWNU increased strongly for the Gdrichest film (film composition Gd1.4Sc0.6O3 for a Gd(iPrCp)3/ Sc(MeCp)3 cycle ratio of 3, see ref 34). The deposition temperature was 300 °C in all cases.

data requires the understanding of potential contributions from background signals and artifacts. Such artifacts can stem from the dissociative ionization (“cracking”) of the precursors in the mass spectrometer or from the reaction with background H2O in the reactor. We remark that thermal decomposition of the Sc(MeCp)3 and Gd(iPrCp)3 precursors was found to be negligible in the investigated temperature range 200−350 °C, as no deposition was observed on the wafer after only Sc(MeCp)3 or Gd(iPrCp)3 pulses. This is consistent with the reported typical thermal decomposition temperature of Cpligand based Sc precursors of about 450 °C.20 Hence, to better understand the background and potential artifacts, QMS signals were studied during Sc(MeCp)3 or Gd(iPrCp)3 pulses only (without intermittent H2O pulses) at reactor temperatures of 200−350 °C. During Sc(MeCp)3 pulses, no signals were found for m/z = 285, 205, and 125 [corresponding to Sc(MeCp)3, Sc(MeCp)2, and Sc(MeCp), respectively]. This may be due to the presence of a ceramic absorber in the outlet of the reactor which 1406

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width of the MeCpH partial pressure peak during the H2O step implies that the hydroxylation of Sc(MeCp)*x by H2O occurs much faster. We note that the eqs 1 and 2 are not cyclical in the sense that the final surface is equal to the initial one. This is because we neglect densification reactions as well as additional reactions with the H2O vapor that restore the equilibrium hydroxyl density. By contrast, the variation of the iPrCpH partial pressure (m/ z = 108) during the Gd(iPrCp)3/H2O homodeposition process at the same temperature of 300 °C was rather different, as shown in Figure 2b. Here, the used pulse sequence was 4 s Gd(iPrCp)3; 10 s N2 purge; 1 s H2O; 10 s N2 purge, as indicated in the figure. Again, iPrCpH was released during both the Gd(iPrCp)3 and the H2O pulse steps. However, a much higher relative amount of iPrCpH was detected during the Gd(iPrCp)3 pulse than that during the H2O pulse: the integrated intensities for the two half cycles indicate that as much as 90% of the iPrCp ligands were already removed during the Gd(iPrCp)3 half cycle and only 10% during the hydrolysis. As a result, the steady-state (homodeposition) ALD of Gd2O3 can be described 300 °C by the following quantitative reaction equations: 2.7(−OH)* + Gd(iPrCp)3 (g) → (−O−)2.7 Gd(iPrCp)*0.3 + 2.7iPrCpH(g)

Figure 2. (a) Time dependence of the MeCpH partial pressure during two Sc2O3 ALD cycles. (b) Time dependence of the iPrCpH partial pressure during two Gd2O3 ALD cycles. In both cases, the deposition temperature was 300 °C. The used ALD pulse sequences are indicated in both graphs.

(3)

(−O−)2.7 Gd(iPrCp)*0.3 + 0.3H 2O(g) → (−O−)2.7 Gd(OH)*0.3 + 0.3iPrCpH(g)

(4)

i

The formation of PrCpH during the chemisorption of Gd(iPrCp)3 appears to occur faster than the corresponding process during Sc2O3 ALD. The iPrCpH partial pressure in Figure 2b increased rapidly after the start of the Gd(iPrCp)3 pulse and reached its maximum after about 2 s. This may indicate that the energetic barrier for proton transfer to the i PrCp ligand on hydroxylated Gd2O3 surfaces is lower than to MeCp on hydroxylated Sc2O3 surfaces. During the H2O pulse, the amount of iPrCpH increased sharply to its maximum height within 0.5 s and decreased rapidly to background levels indicating that the hydroxylation of Gd(iPrCp)*x was fast. These results point to different ligand elimination schemes for the Sc2O3 and Gd2O3 ALD processes although the precursors contain similar alkyl-substituted Cp ligands. This may be due to different energetic landscapes of the proton transfer and removal of successive ligands during the chemisorption of the metalorganic precursors.28 The case of Gd(iPrCp)3 chemisorption is however rather peculiar, since it occurs by the removal of almost all (90%) ligands. Although the detailed energetics of the Gd(iPrCp)3 chemisorption on hydroxylated Gd2O 3 have not been calculated so far, calculations for the related LaCp3 and ErCp3 precursors suggest that at least the removal of the third Cp-based ligand should be energetically unfavorable. Hence, iPrCpH releases of more than 2/3 of the total amount during Gd( iPrCp)3 adsorption may require a different mechanism than the proton transfer from surface hydroxyl groups. Gd2O3 is a strongly hygroscopic oxide38 and will react into Gd(OH)3 in the presence of H2O.39 This is in stark contrast to Sc2O3, for which the hydroxide formation is thermodynamically unfavorable.39 Hence, the amount of hydroxyl moieties on or near the Gd2O3 surface may be much larger than for Sc2O3. These additional hydroxyl moieties may be present as (physi-)sorbed H2O (as

cycle. Assuming that the hydrolysis was complete and all ligands were removed after the H2O half cycle, the steady-state (homodeposition) ALD Sc2O3 at 300 °C can be described by the following (averaged) quantitative reaction equations (asterisks indicate surface groups): 1.68(−OH)* + Sc(MeCp)3 (g) → (−O−)1.68 Sc(MeCp)*1.32 + 1.68MeCpH(g)

(1)

(−O−)1.68 Sc(MeCp)*1.32 + 1.32H 2O(g) → (−O−)1.68 Sc(OH)*1.32 + 1.32MeCpH(g)

(2)

In eq 1, Sc(MeCp)3 reacts with surface hydroxyl groups and releases MeCpH after proton transfer. The chemisorption rate of Sc(MeCp)3 appeared to be rather slow because the MeCpH partial pressure peak during the Sc(MeCp)3 pulse was rather broad and reached its maximum only around 1.5 s af ter the end of the Sc(MeCp)3 pulse and continued during the purge. Only about 15% of the total quantity of MeCpH generated during the first ALD half cycle (i.e., about 0.25 MeCpH per Sc atom) were actually released during the duration of the Sc(MeCp)3 pulse itself. This suggests that the proton transfer to MeCp3 is hindered by a large energetic barrier.28 Alternatively, it is also possible that MeCpH is transferred to the surface and desorbs only slowly thereafter. It should be noted that the observation that most MeCpH was generated af ter the Sc(MeCp)3 pulse suggests that the initial step in the adsorption of Sc(MeCp)3 occurs without ligand release. Subsequently, the remaining MeCp ligands on the surface are released after proton transfer during the reaction with H2O, as described by eq 2. In contrast to the chemisorption of Sc(MeCp)3, the steep leading edge and the narrow temporal 1407

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deposition temperature of 350 °C, as much as 95% of the iPrCp ligands were removed already during the Gd(iPrCp)3 pulse. However when the reactor temperature decreased to 200 °C, the fraction decreased to 73%, much closer to the equivalent fractions observed during the Sc2O3 ALD process. The absolute amount of generated iPrCpH per cycle was approximately constant. The deposition temperature was found not only to affect the ratio of iPrCpH released during the Gd(iPrCp)3 and

argued in ref 23) or as a hydroxide on or near the Gd2O3 surface and can contribute to the deposition of Gd2O3 through CVD-like reactions. This mechanism was proposed previously by Van Elshocht et al. and Adelmann et al. to explain the considerable thickness nonuniformity of Gd2O3 and Gd-rich GdxAl1−xO3 films deposited from Gd(iPrCp)3 and H2O (see also Figure 1).23,25,37 The CVD-like reactions also well explain the removal of nearly all iPrCp ligands already during the chemisorption of Gd(iPrCp)3. Temperature Dependence of the Surface Chemistry. We now turn to the temperature dependence of the abovedescribed surface chemistry of the Sc(MeCp)3/H2O and Gd(iPrCp)3/H2O ALD processes. Figure 3a shows the

Figure 3. (a) Fraction of MeCpH released during the Sc(MeCp)3 pulse (bar graphs) with respect to the total amount of MeCpH released during a complete ALD cycle (open circles) as a function of temperature. (b) Respective graphs for the fraction of iPrCpH released during the Gd(iPrCp)3 pulse as well as the total amount during a complete ALD cycle. Figure 4. (a) Time dependence of the MeCpH partial pressure during Sc2O3 ALD for different temperature, as indicated. (b) Time dependence of the iPrCpH partial pressure during Gd2O3 ALD for different temperature, as indicated. The data are offset for clarity.

variation of the relative amount of MeCpH released during a Sc(MeCp)3 pulse with respect to the amount of MeCpH released during one complete Sc2O3 ALD cycle, as a function of the deposition temperature. The data show that the relative amount was rather independent of the deposition temperature, suggesting that the surface chemistry and the reaction pathway are as such not much affected by temperature. On the other hand, the total amount of MeCpH released during a complete Sc2O3 ALD cycle was strongly affected by the deposition temperature. The integrated intensity of MeCpH (symbols in Figure 3a) showed an almost 2-fold continuous decrease when the reactor temperature increased from 200 to 350 °C. This can be linked to a temperature dependence of the surface hydroxyl density, similar to what was observed previously for HfO2.40 The lower surface hydroxyl density at higher temperatures will result in the saturated absorption of less Sc(MeCp)3 per ALD cycle and thus in a lower GPC. The decrease of the integrated intensity with temperature is in good quantitative agreement with the experimental temperature dependence of the GPC of the ALD Sc2O3 process, which was found to decrease from 0.76 to 0.58 Å when the deposition temperature increased from 250 to 350 °C.34 By contrast, a clear trend was visible in the relative amount of i PrCpH released during the Gd(iPrCp)3 half cycle (Figure 3b) in the case of the Gd(iPrCp)3/H2O ALD process. At a

H2O half cycles. Figure 4b shows the time dependence of the i PrCpH partial pressure at the different deposition temperatures, as indicated. The integrated intensities lead to the behavior shown in Figure 3b and discussed above. At 200 °C, a considerably different shape of the iPrCpH partial pressure peak during the Gd(iPrCp)3 pulse was observed, in particular, with a much steeper rising edge. This peak shape is somewhat similar to that observed during the ALD of Gd2O3 on nonhygroscopic Sc2O3 surfaces, as discussed below. The observations for Gd2O3 can be explained by the competition of the ALD process and an additional CVD-like component, which leads to the same reaction products (iPrCpH). At lower temperature (i.e., at 200 °C), the CVD process was slow compared to the ALD process but became more dominant and faster at higher temperatures. This suggests that the CVD process is kinetically limited in the studied temperature range. By contrast, the MeCpH peak shape during the ALD of Sc2O3 was rather independent of the temperature (Figure 4a), consistent with an ALD-like process in the entire temperature window. 1408

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This CVD process can be linked to the presence of the additional hydroxide moieties due to the hygroscopicity of Gd2O3, as discussed above. As the CVD component is thermally activated, it becomes more prominent at higher temperatures and leads to the increasing relative amount of i PrCpH generated during the Gd(iPrCp)3 half cycle. We remark that the process temperatures of ≤350 °C are much lower than typical calcination temperatures for (nanoscale) Gd(OH)3 of 600 °C.41,42 CVD versus ALD of Gd2O3. As discussed above, the behavior of the Gd2O3 ALD process suggests that the deposition process becomes dominated by an additional thermally activated CVD component at higher temperature. This CVD component has strong repercussions on the resulting film properties. In particular, the thickness WiWNU of the resulting films is impacted, as the CVD deposition is strongly nonuniform in the cross-flow reactor. This is shown in Figure 5a, which depicts two film thickness maps of Gd2O3

Figure 6. ToF-SIMS C− depth profiles of Gd2O3 and Sc2O3 films deposited at temperatures as indicated.

average C concentrations were much higher for the film deposited at 200 °C than for that deposited at 300 °C. Such a behavior is typical for ALD (and CVD) processes, where a higher deposition temperatures often results in lower impurity concentrations in the film.43,44 Hence, the suppression of the CVD component at lower temperature did not result in purer films. A closer look at the profile for the Gd2O3 film deposited at 300 °C reveals that the C content in the film was not uniformly distributed in the layer but increased about 10-fold between the interface with the underlying SiO2/Si substrate and the film surface. This behavior may be explained by a gradually increasing contribution of the CVD component. This may be due to larger hygroscopicity for lower density (and/or higher C contamination) Gd2O3, similar to what has been observed for LaAlO3.45 If one assumes that the CVD component deposits a lower density and/or higher C-contaminated layer, there may exist some intrinsic positive feedback for the CVD component, leading to deteriorating film quality and increasing WiWNU as the deposition progresses. In Situ QMS Analysis during the Initial Growth of Sc2O3 on Gd2O3 and Gd2O on Sc2O3. As mentioned, the WiWNU of ternary GdxSc1−xO3 (or GdAlO3, see ref 23) films was found to be much lower than for binary Gd2O3 layers and comparable to Sc2O3, at least when the Gd/Sc cycle ratio in a supercycle was below about 3 (see Figure 1c). Since the GdxSc1−xO3 ALD process consists of a combination of Sc(MeCp)3/H2O and Gd(iPrCp)3/H2O cycles, the improved WiWNU GdScO3 can be related to a strong dependency of the surface chemistry of the Gd(iPrCp)3/H2O ALD process on the initial surface before the individual cycles. To investigate this (transient) growth chemistry further, the reaction products were measured by QMS during the initial Sc(MeCp)3/H2O cycles on a hydroxylated Gd2O3 surface (after 50 Gd(iPrCp)3/ H2O cycles, see Figure 7). Analogous measurements were performed during the initial Gd(iPrCp)3/H2O cycles on a hydroxylated Sc2O3 surface (after 50 Sc(MeCp)3/H2O cycles, see Figure 8). With respect to the steady-state homodeposition (see Figure 2a), the Sc(MeCp)3/H2O ALD on the hydroxylated Gd2O3 surface led to a much increased fraction of the amount of MeCpH released during the first Sc(MeCp)3 pulses. As a matter of fact, as much as 90% of all MeCp ligands were removed during the initial Sc(MeCp)3 pulse, very similar to the case of Gd(iPrCp)3/H2O homodeposition. This indicates that the Sc precursor reacted much stronger with the hydroxylated Gd2O3 with respect to hydroxylated Sc2O3 due to the presence of additional sorbed H2O or hydroxide on the hygroscopic Gd2O3 (or Gd(OH)3) surface. This clearly demonstrates that

Figure 5. Thickness maps of Gd2O3 films deposited by 150 ALD cycles at temperatures of (a) 300 °C and (b) 200 °C, respectively. The arrows indicate the direction of the gas flow across the wafer. (c) Dependence of the WiWNU on the deposition temperature. The WiWNU deteriorates strongly above 250 °C due to the additional CVD component.

films deposited by performing 100 ALD cycles at 200 and 300 °C, respectively. While the film deposited at 200 °C showed a WiWNU of ∼10%, the film deposited at 300 °C was much more nonuniform. The thickness pattern observed at 300 °C is consistent with a rapid depletion of the Gd(iPrCp)3 precursor already close to the gas inlet by the efficient CVD reactions. Since the CVD reaction is much less efficient at 200 °C, the ALD process prevails and leads to a much more uniform chemisorption of Gd(iPrCp)3 and thus to a much more uniform film thickness. However, even in these conditions, the CVD component is not completely absent and the uniformity is still degraded with respect to GdScOx (or Sc2O3) ALD processes. The CVD component had also a profound impact on the C contamination in the layers. Figure 6 shows C− ToF-SIMS profiles of two ∼10 nm thick Gd2O3 films grown at 200 and 300 °C, respectively. For comparison, a profile is also shown for a Sc2O3 film grown at 300 °C. The C concentrations in both the Gd2O3 films were generally much higher than in the Sc2O3 film, which can be explained by the influence of the kinetically limited CVD component leading to incomplete reaction with the ligands and thus to larger C incorporation. For Gd2O3, the 1409

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the quantitative differences of the reaction schemes of the Gd(iPrCp)3/H2O and Sc(MeCp)3/H2O processes, as given by eqs 1 to 4, are not due to differences in the properties of the Gd(iPrCp)3 and Sc(MeCp)3 precursors but solely due to different properties of the Gd2O3 and Sc2O3 surfaces. When the Sc(MeCp)3/H2O ALD progressed, the fraction of MeCpH generated during the Sc(MeCp)3 decreased. This was because less sorbed H2O or hydroxide was present on the surface as the Gd2O3 became gradually covered by less hygroscopic Sc2O3. However, although some signs of process stabilization appeared already after 5 cycles, the fraction of released ligands after 25 Sc(MeCp)3/H2O ALD cycles (77%) was still significantly larger than for the case of Sc2O3 homodeposition (56%). This indicates that even an ∼1.5 nm thick Sc2O3 film could not completely eliminate the influence of the Gd2O3 surface underneath. Although the Sc2O3 film was not necessarily fully closed for such a thickness, the persisting impact on the Sc(MeCp)3/H2O ALD process strongly suggests that not only surface OH (or H2O) but also hydroxides in a near-surface layer with nonzero thickness contribute to the CVD component. In addition, the integrated intensity of MeCpH strongly decreased during the initial 10 cycles of Sc(MeCp)3/H2O ALD deposition. A larger integrated intensity during the early stage of ALD Sc2O3 indicates that substrateenhanced reactions occurred on the initial Gd2O3 surface and the transient Sc(MeCp)3/H2O GPC was higher than that of Sc2O3 ALD in homodeposition conditions. This is in good agreement with the findings for GdAlO3,23 where an enhanced incorporation of Al was observed for Gd-rich films. The transient behavior of Gd(iPrCp)3/H2O ALD on a hydroxylated Sc2O3 surface is shown in Figure 8. The trends were exactly opposite to those described above for Sc(MeCp)3/ H2O ALD on Gd2O3. The fraction of iPrCpH released during the Gd(iPrCp)3 pulse as well as the total amount of iPrCpH generated during an entire deposition cycle increased with the number of cycles. This is consistent with a surface which becomes increasingly hygroscopic as the deposition proceeds. For the initial Gd(iPrCp)3 pulses, about 55−60% of the total amount of iPrCpH were generated during the Gd(iPrCp)3 pulse, which is very similar to what was observed for Sc(MeCp)3/H2O homodeposition (see Figure 3a). This again demonstrates the similarity of the reaction chemistries if the influence of the substrate can be eliminated. However, after ∼10 cycles, about 90% of iPrCpH was released during Gd(iPrCp)3 exposure, which was again very close to the case of homodeposition (see Figure 3b). The shape of the iPrCpH peaks during the Gd(iPrCp)3 pulses also showed a strong evolution with cycle number. In particular, the initial peak shapes were not unlike the shapes observed at low temperature (200 °C) for homodeposition (Figure 4b). Such a peaks shape including a fast component followed by a slower “shoulder” may thus be linked to the ligand removal process during a true ALD process, which appears to occur through two distinct steps with different kinetics. It is tempting to attribute such shapes to the consecutive of the first and second ligands although the quantitative separation of the tow peaks is very difficult. Further deposition beyond a few cycles led to shapes close to those for Gd2O3 homodeposition, as shown in Figure 2b; these shapes may thus be linked to the predominance of the CVD component. As a whole, these results are fully consistent with the observation of poor WiWNU for Gd-rich GdxSc1−xO3 (and

Figure 7. (a) Time dependence of the MeCpH partial pressure during the first 25 Sc2O3 ALD cycles on Gd2O3 substrate. The used ALD pulse sequences is indicated in the graph. (b) Evolution of the MeCpH fraction released during the Sc(MeCp)3 pulse and the total amount of released MeCpH. The deposition temperature was 300 °C.

Figure 8. (a) Time dependence of the iPrCpH partial pressure during the first 30 Gd2O3 ALD cycles on Sc2O3 substrate. The used ALD pulse sequences is indicated in the graph. (b) Evolution of the iPrCpH fraction released during the Gd(iPrCp)3 pulse and the total amount of released iPrCpH. The deposition temperature was 300 °C.

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GdxAl1−xO3, see ref 23) films. The hygroscopicity of the GdxSc1−xO3 will depend on the previous cycle of the ALD sequence. As the above data show, the transient behavior of Sc(MeCp)3/H2O on Gd2O3 and Gd(iPrCp)3/H2O on Sc2O3 persisted for several cycles before a more stable surface was reached. In a GdxSc1−xO3 ALD process, it is thus plausible that several consecutive Gd(iPrCp)3/H2O cycles are necessary to significantly change the hygroscopicity of the surface and induce the CVD of Sc2O3 during the subsequent Sc(MeCp)3/ H2O cycle. This is consistent with the observation that the degradation of the WiWNU appears only for a Gd/Sc cycle ration of 3 and higher.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

∥ Advanced Materials Division, Korea Research Institute of Chemical Technology, Daejeon, South Korea.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.H.H.’s research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A6A3A03039896).

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CONCLUSION In conclusion, we have studied the growth mechanisms and surface reactions during the ALD Sc2O3, Gd2O3, and GdxSc1−xO3 from Sc(MeCp)3, Gd(iPrCp)3, and H2O using in situ time-resolved QMS measurements during both the initial and steady-state stages. The Sc2O3 deposition showed ALD characteristics at all studied temperatures between 200 and 350 °C. In this process, about 55% of the MeCp ligands were removed during the Sc(MeCp)3 pulse, rather independent of the temperature. By contrast, the ligand elimination behavior of the Gd(iPrCp)3/H2O process was strongly influenced by a CVD component due to additional hydroxyl moieties (sorbed H2O, hydroxides) in or on the Gd2O3 layer due to its hygroscopicity. As a result, as much as 90% of the iPrCp ligands were already removed during the Gd(iPrCp)3 pulse. The impact of the CVD component increased with increasing deposition temperature, which indicates that the CVD was kinetically limited. Investigations on the transient growth behavior of Sc(MeCp)3/H2O on Gd2O3 and Gd(iPrCp)3/H2O on Sc2O3 revealed that the intrinsic reaction chemistry of the two processes was very similar and that the differences in the properties of the deposition processes were only related to the effect of the hygroscopicity of the surface. The influence of the substrate on the deposition processes persisted for a remarkably large number of cycles. Even 25 Sc(MeCp)3/H2O cycles, leading to a Sc2O3 film thickness of 1.5 nm, were insufficient to fully recover homodeposition behavior. We finally remark that the findings in this article are not necessarily limited to the studied Gd2O3 and GdxSc1−xO3 processes but may apply more generally to the ALD of other strongly hygroscopic oxides (such as e.g. BaO or SrO) when H2O is used as an oxidizing agent. Such processes always risk to be affected by CVD components which stem from additional sorbed H2O or surface hydroxides. When the CVD component is thermally activated, low deposition temperatures may allow for true ALD deposition. However, as shown in this paper, low ALD temperatures may lead to poor film properties (e.g., large C contamination) and thus it is not always clear if an ALD process window for high-quality films exists. At higher temperature, the C concentration profile in Figure 6 suggests that the CVD component may gradually develop as the deposition progresses when the starting surface is not hygroscopic. In such a case, these results suggest that the deposition of ultrathin high-purity ALD layers of hygroscopic oxides (such as Gd2O3) may be possible even using H2O as the oxidizing agent. Hence, such processes can be useful for applications when film thicknesses of a few nanometers only are sufficient.

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ABBREVIATIONS ALD, atomic layer deposition; CVD, chemical vapor deposition; GPC, growth per cycle; QMS, quadrupole mass spectrometry; ToF-SIMS, time-of-flight secondary-ion mass spectrometry; REO, rare earth oxide; WiWNU, within-wafer nonuniformity

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