Improved Initial Growth Behavior of SrO and SrTiO3 Films Grown by

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Improved Initial Growth Behavior of SrO and SrTiO3 Films Grown by Atomic Layer Deposition Using {Sr(demamp)(tmhd)}2 as Sr-Precursor Woongkyu Lee,† Woojin Jeon,† Cheol Hyun An,† Min Jung Chung,† Han Joon Kim,† Taeyong Eom,† Sheby Mary George,‡ Bo Keun Park,‡ Jeong Hwan Han,‡ Chang Gyoun Kim,‡ Taek-Mo Chung,‡ Sang Woon Lee,§ and Cheol Seong Hwang*,† †

Department of Materials Science and Engineering and Inter-university Semiconductor Research Center, Seoul National University, Seoul 151-742, Republic of Korea ‡ Thin Film Materials Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, Republic of Korea § Department of Physics and Division of Energy Systems Research, Ajou University, Suwon, Gyeonggi-do 443-749, Republic of Korea ABSTRACT: An atomic layer deposition (ALD) process for SrTiO3 (STO) thin film growth was developed using a newly designed and synthesized heteroleptic Sr-precursor, {Sr(demamp)(tmhd)}2 (demampH = 1-{[2-(dimethylamino)ethyl](methyl)amino}-2methylpropan-2-ol, tmhdH = 2,2,6,6-tetramethyl-3,5-heptanedione), which offered an intermediate reactivity toward oxygen between Sr(tmhd)2 and Sr(iPr3Cp)2. Because of the appropriate reactivity of {Sr(demamp)(tmhd)}2 toward oxygen, the abnormal initial growth behavior (due to interaction between the Sr-precursor and active oxygen contained in the underlying oxidized Ru layer) became negligible during the growth of the SrO and STO films on the Ru electrode, which allowed the growth of the SrO and STO films to be highly controllable with a moderate growth rate. Using Ti(CpMe5)(OMe)3 as the Ti-precursor and O3 as the oxygen source in the TiO2 ALD subcycle, the ALD process of the STO film revealed a growth rate of 0.05 nm/cycle and ∼85% of step coverage in terms of the thickness and cation composition on a capacitor hole structure with an aspect ratio of 10 (opening diameter of 100 nm and depth of 1 μm). The minimum achievable equivalent oxide thickness (tox) with a low leakage current (100 for metal− insulator−metal (MIM) capacitors that contain an STO insulator, in which the insulators are thinner than 20 nm.1−6 Considering the extremely tiny three-dimensional (3D) structure of the DRAM capacitors,7 atomic layer deposition (ALD) appears to be the only feasible thin film growth technique that can fulfill the stringent requirements of thickness and composition step coverage in the DRAM capacitors. Despite the acute requirement for a suitable ALD process of the STO films, the development has been hindered for two main reasons: first, the lack of a suitable Sr-precursor for feasible STO ALD, although that for Ti is abundant; second, because of the low temperature of the ALD, ensuring a suitable crystalline quality of the STO film is challenging in general. Because such problems and possible solutions have already been extensively reviewed in previous reports from the authors’ group,3,7,8 more © 2015 American Chemical Society

Received: February 14, 2015 Revised: May 7, 2015 Published: May 7, 2015 3881

DOI: 10.1021/acs.chemmater.5b00843 Chem. Mater. 2015, 27, 3881−3891

Article

Chemistry of Materials More meticulous studies of STO ALD were later performed by the authors’ group for a decade. Kwon et al. started the work by using Sr(tmhd)2 (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedione) and TTIP as Sr- and Ti-precursors, respectively.11 Plasma-activated H2O was employed as an oxygen source for SrO and TiO2 film growth because the Sr(tmhd)2 was reported to not react with H2O vapor at reasonable ALD temperatures. They confirmed a very promising step coverage over a capacitor hole structure with an opening diameter of 130 nm and an aspect ratio of 8. Later, they found that maintaining a monomer-like molecule of the Sr(tmhd)2, by decreasing the vaporization temperature lower than its melting point (∼200 °C), enabled genuine ALD of STO film without employing the plasma process.12 Nevertheless, the deposition temperature should be lower than 250 °C because of the limited thermal stability of TTIP at higher temperatures. Therefore, post deposition annealing (PDA) at 500 °C was required to acquire the crystalline perovskite phase with a high dielectric constant (∼80). However, PDA at such a high temperature caused microcrack formation, which in turn introduced a currentleakage problem.13 After these findings, the authors’ group has focused on a high-temperature (>350 °C) ALD process by adopting precursors that can withstand high temperatures, such as Ti(OiPr)2(tmhd)2 as a Ti-precursor.1,2,14 In contrast, a research group in Interuniversity Microelectronics Centre (IMEC) focused on the low temperature ALD (∼250 °C) process to make use of the facile ALD reactions between the Sr(tBu3Cp)2 and Ti(OMe)4 with H2O, where Bu and Me correspond to butyl and methyl groups, respectively, which required PDA at a higher temperature to crystallize STO films.4,15,16 Similar to the current authors’ case, such high-temperature PDA induced micro- and nanocracking problems; as a result, they introduced a double-layer structured ALD process. In this structure, a thin TiO2 layer was deposited first, followed by a Sr-rich STO layer, which prevented the cracking problem, probably due to an in-plane compressive stress by the included Sr atoms in the crystallized STO layer. During the PDA, the thin TiO2 and the Sr-rich STO layer reacted with each other to result in a stoichiometric STO film. Because the current authors’ group undertook a hightemperature ALD process, the film growth and crystallization behaviors were quite different from the IMEC group’s results, as summarized in the previous report.3 Even with hightemperature ALD, in situ crystallization on heterogeneous substrates (typically the Ru bottom electrode) had not been achieved well, with the result that the “two-step process” was adopted to grow in situ crystallized STO films. In this case, a thin (∼3 nm) STO film was deposited as the crystalline “seed” layer, and its crystallization was accomplished by PDA at a typical temperature over 600 °C; subsequently, the main layer (∼10 nm) was deposited. Under these optimized ALD conditions, the STO films showed promising electrical properties.3,17 Nevertheless, such a high-temperature ALD process involved several nonideal ALD behaviors, which prohibited the achievement of the satisfactory electrical performance and the growth process that are essential for the mass production of sub-20 nm DRAMs. A schematic diagram on the ALD behaviors of different precursors is shown in Figure 1, which represents approximately the deposition amount of STO films as a function of the deposition cycle number for various precursor combinations.

Figure 1. Variations in the deposition amount of STO films as a function of the deposition cycle on the Ru substrate. Graphs A−D were plotted approximately on the basis of the description of the deposition rate and growth behavior in the references (A: ref 14; B: ref 1; C: refs 1 and 2; D: ref 3). Plot E is the desirable growth behavior, which was shown in this study.

Lee et al. employed Ti(O-iPr)2(tmhd)2 and Sr(tmhd)2 as Tiand Sr-precursors, respectively, which allowed the maximum deposition temperature of STO films to be increased to as high as 390 °C.14 By adopting the “two-step process”, the STO films showed promising electrical properties with a dielectric constant value of ∼108, an equivalent oxide thickness (tox) of ∼0.72 nm, and a low leakage current density of 10−7 A/cm2 at an applied bias of 0.8 V. However, a growth rate of the STO film that was too slow (0.017 nm/cycle) prevented further progress in the mass fabrication of DRAM capacitors because of the low reactivity of the Sr(tmhd)2 precursor toward oxygen (the relative deposition rate is plotted in Figure 1 as “A”). Hence, Sr(tmhd) 2 was replaced by the bis(1,2,4triisopropylcyclopentadienyl)strontium (Sr(iPr3Cp)2) precursor to achieve a higher growth rate.1 This choice was made because the energies that are required to break the bonds between the Sr ion and the ligands are smaller in Sr(iPr3Cp)2 than those in Sr(tmhd)2, which means that Sr(iPr3Cp)2 molecules are more reactive toward oxygen.18 The growth rate of the STO film was significantly enhanced by the replacement of the Sr-precursor (0.107 nm/cycle). However, Sr(iPr3Cp)2 was too reactive to achieve a stable ALD at an early stage of film growth. The pulsed Sr(iPr3Cp)2 molecules reacted with the oxygen that diffused out from the bottom Ru substrate, which was oxidized by O3 during the previous ALD step of the TiO2 subcycle. O3 was necessary for the TiO2 subcycle to achieve a complete ALD reaction and a high growth rate of the TiO2 sublayer when Ti(O-iPr)2(tmhd)2 was used as the Tiprecursor. Therefore, the oxidized Ru layer played a role as an oxygen reservoir, which induced a chemical vapor deposition (CVD)-like reaction between oxygen and Sr(iPr3Cp)2 during the following Sr(iPr3Cp)2 precursor injection step. As a result, an abnormally high initial growth of the STO film was observed with an incomplete ligand exchange reaction forming a SrCO3 phase (denoted by “B” in Figure 1). To suppress the initial high incorporation of the Sr element in the growth of the STO film, 3 nm-thick TiO21 or 1 nm-thick Al2O32 layers were used to interpose a reaction barrier between the Ru substrate and Sr(iPr3Cp)2 precursor. Although these barrier layers inhibited the abnormal initial growth of the STO films (“C” in Figure 1), they decreased the effective dielectric constant of the capacitor because TiO2 and Al2O3 have smaller dielectric constants than STO (tox values of 0.57 and 1.14 nm were achieved with a leakage current of 110 and a tox of 0.43 nm (with a leakage current of 99.99%). The layer density and cation composition of the STO films were confirmed by X-ray fluorescent spectroscopy (XRF, Themoscientific, ARL Quant’X). The physical thickness of the STO films was measured using an ellipsometer (Gaertner Scientific Corporation, L116D). The impurity concentration in the films, including carbon and nitrogen, and the depth profile were examined by Auger electron spectroscopy (AES, PerkinElmer, PHI 660). The chemical binding states of the films were determined by X-ray photoelectron spectroscopy (XPS, ThermoVG, SIGMA PROBE). The cross-sectional images of the hole structure and energy dispersive spectroscopy (EDS) analysis for cation composition was obtained by using high-resolution transmission electron microscopies (TEM, JEOL, JEM-3000F, and FEI, Tecnai F20). The crystal structure of the film was investigated by glancing 3883

DOI: 10.1021/acs.chemmater.5b00843 Chem. Mater. 2015, 27, 3881−3891

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Chemistry of Materials

Figure 3. Variations in the Sr layer densities of the SrO films as a function of the times of the precursor injection, precursor purge, O3 injection, and O3 purge time on (a) Ru, (b) Si, and (c) Pt substrates. (d) Summary of variations in the Sr layer density as a function of the precursor injection time on the Ru, Si, and Pt substrates. (e) Comparison of the growth behavior of the SrO films deposited with Sr(iPr3Cp)2 (pulse time: 3 s−5 s−2 s−5 s) and {Sr(demamp)(tmhd)}2 (pulse time: 7 s−10 s−5 s−5 s) on various substrates. angle mode X-ray diffraction (GAXRD), and the density of the films was investigated by X-ray reflectivity (XRR) using a Cu Kα X-ray source (PANalytical, X’Pert Pro). The incidence angle, scan step size, and time per step during the GAXRD measurement were 2°, 0.02°, and 1 s, respectively, and the scan step size and time per step during XRR were 0.005° and 3 s, respectively. The surface morphology of the STO films was confirmed by field-emission scanning electron microscopy (FESEM, Hitachi, S-4800). The planar-structured metal−insulator−metal capacitors were fabricated by depositing a 60 nm-thick Pt/20 nm-thick RuO2 layer as a top electrode by sputtering through a shadow mask (with a 0.3 mm hole diameter). The RuO2 layer contacted the STO film. The electrical properties, including the capacitance and leakage current, were measured using an HP4194A impedance analyzer at 10 kHz and an HP4140 picoammeter, respectively.

deposition amounts of the saturated levels were different on each substrate, as shown in Figure 3d, the necessary dosage of the Sr-precursor introduced into the process chamber was smaller, thus decreasing the saturation injection time to 5 s on the Si substrate. The lower saturation level on Si might be related to the smaller number of surface chemisorption sites compared with other metal substrates. As mentioned in the Introduction section, initial excessive Sr incorporation is a highly undesirable aspect during the ALD process of the SrO film on the Ru substrate using Sr(iPr3Cp)2 and O3. Accordingly, the growth behavior of the SrO films deposited with {Sr(demamp)(tmhd)}2 was examined as a function of the ALD cycles under the established saturated ALD conditions. Figure 3e shows a comparison of the growth behaviors of the SrO films deposited with the Sr(iPr3Cp)2 and {Sr(demamp)(tmhd)}2 precursors on various substrates. Initial excessive growths were observed not only on the Ru substrate but also on the Pt and Si substrate with the Sr(iPr3Cp)2 precursor. The origin of the initial excessive growth of the SrO layer was related to the oxidation of the Ru layer during the O3 pulse step, and the oxidized layer (RuOx) was reduced, which produced active oxygen atoms during the subsequent Sr(iPr3Cp)2 precursor pulse step. These oxygen atoms induced the formation of the SrO layer before the subsequent O3 pulse step, causing the initial excessive growth. Although the Pt substrate was not oxidized by O3 during the process, it is believed that the oxygen atoms could be incorporated into the polycrystalline Pt film along the grain boundaries during the O3 pulse step, which then enhanced the SrO growth during the subsequent Sr(iPr3Cp)2 precursor pulse step.21 Nonetheless, the supply of oxygen via such a mechanism must be lower than that via the oxidation and reduction of Ru, and thus, the degree of excessive growth on Pt is lower than that on Ru. It can be

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RESULTS AND DISCUSSION The ALD saturation behavior of the SrO film growth was first confirmed when {Sr(demamp)(tmhd)}2 and O3 were adopted as the Sr-precursor and oxygen source, respectively. Because unwanted initial excess growth was hardly observed when using {Sr(demamp)(tmhd)}2 and O3 for both SrO and STO deposition, O3 was used as the oxygen source for both the SrO and TiO2 subcycle to acquire a higher growth rate for the STO film. Figure 3a−c shows the variations in the deposition amounts (layer densities) as a function of the process step time on a Ru, Si, and Pt substrate. The basic process times for precursor injection−Ar purge−O3 injection−Ar purge were set to 10 s−10 s−7 s−10 s, and one of these four times was varied to check the ALD saturation behavior. On all of the substrates, 3, 5, and 5 s were sufficient to obtain the saturated behavior for precursor purge, O3 injection, and O3 purge, respectively. Only 5 s of precursor injection time was necessary for the Si substrate to reach the saturated deposition level, but over 7 s was required for both the Ru and Pt substrates. Because the 3884

DOI: 10.1021/acs.chemmater.5b00843 Chem. Mater. 2015, 27, 3881−3891

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Chemistry of Materials

Figure 4. (a) Maximum atomic concentration of C and N in the SrO films on the Ru, Si, and Pt substrate, (b) depth profile of the SrO film 150 cycle-deposited (9 nm) on the Ru substrate obtained by AES, and (c) XP spectra of N 1s in SrO film on the Ru, Si, and Pt substrate.

explanation for the lower growth rate of the STO films compared with the case using Sr(iPr3Cp)2. Nonetheless, the growth rate was still higher than that with Sr(tmhd)2 and H2O14 or O322 because of the higher chemical reactivity of the demamp ligand compared with the tmhd ligand. The growth rate on the Ru substrate showed the highest value, whereas that on the Si substrate showed the lowest value, which suggests that the oxygen-related excessive growth was not completely suppressed. Nevertheless, the data in Figure 3e suggest that the feasible ALD process could be achieved from the very early ALD cycles due to the appropriate chemical activity of this newly adopted Sr-precursor, of which the overall reactivity toward the adsorbed O was almost equivalent to that of the Tiprecursor, as shown below. This mechanism offered a feasible chance to grow the STO film via the genuine ALD mode from the very first ALD cycle. It must be noted that the authors attempted to make stoichiometric STO film using the CVD-like growth of TiO2 film to countervail the overgrowth of the SrO process during the early ALD cycles in the previous report.3 Meanwhile, an alternative interpretation for the different growth rate using various Sr-precursors should be considered. Although the reactant of the ALD process was only O3 in this work, a small amount of H2O might remain in the chamber after several reactions during the ALD process. The partial pressure of H2O must be different depending on the used Srprecursors; thus, the degree of hydroxylation on the SrO surface might be varied. Because {Sr(demamp)(tmhd)}2 is less reactive than Sr(iPr3Cp)2, but more reactive than Sr(tmhd)2, the amount of H2O on the SrO surface using {Sr(demamp)(tmhd)}2 might be in between the two cases using Sr(iPr3Cp)2 and Sr(tmhd)2. As surface hydroxyl groups enhance ALD, consequently, an appropriate coverage of hydroxyl group was formed on the SrO surface which allowed moderate growth rate of SrO with appropriate proton transfer from surface to ligands of precursors.23−25 Next, the physical and chemical properties of the SrO film grown by these two Sr-precursors were compared. Figure 4a shows the maximum values of carbon and nitrogen impurity of SrO films on the Ru, Si, and Pt substrates, and Figure 4b shows the depth profile of the SrO film (150 cycle deposited, thickness of ∼8 nm) deposited on the Ru substrate, which was obtained by AES analysis. Because there was no appropriate standard for AES calibration, the atomic concentrations must be considered only as relative numbers. Since the film was exposed to air prior to the AES analysis, the surface of the film was contaminated by carbon of which concentration is higher near the surface than the bulk region. Therefore, only the compositions inside the films were considered in this study. As shown in the left panel of Figure 4a, carbon contamination was

hardly imagined that a similar reaction could occur on bare Si; thus, the growth rate of up to 100 ALD cycles was much lower than those of the other two cases. Once the substrate surfaces had been sufficiently covered with the SrO layers (20 cycles for Ru and Pt and 100 cycles for the Si substrate), the growth rate reached a steady state value (0.012 μg/cm2 cycle). In contrast, the results of a similar test on the same substrates using the {Sr(demamp)(tmhd)}2 precursor were quite different. As observed in Figure 3e, the degree of initial excessive growth became much lower in this case, revealing that the main goal of this work could be accomplished by using this new Srprecursor. The reason is that the appropriate chemical reactivity of {Sr(demamp)(tmhd)}2 toward oxygen, which is higher than the too-stable Sr(tmhd)2 but lower than the too-reactive Sr(iPr3Cp)2, makes it suitable for mass production. This conclusion could be further confirmed from the intermediate steady state growth rate of the SrO layer on the Ru substrate from this new Sr-precursor (0.028 μg/cm2 cycle), while that from the Sr(iPr3Cp)2 was 0.123 μg/cm2 cycle. The ALD reaction mechanism for {Sr(demamp)(tmhd)}2 is suggested as follows. There is no known reaction mechanism for such complicated heteroleptic precursor molecules, and the authors lack any in situ monitoring tools for the growth behavior; as a result, the proposed possible mechanism is based on the ex situ observations. When the Sr-precursor was pulsed onto the O3 (or the active O radical)-terminated surface, the outermost tmhd ligand might have reacted with the O radical to be removed first. In fact, the bond between the Sr and tmhd ligands in Sr(tmhd)2 is known to be quite strong,16 which implies that this reaction might have occurred with a very low probability. However, the higher coordination of Sr atoms with the other two O atoms and two N atoms in the demamp ligands in {Sr(demamp)(tmhd)}2 might make the bond between the Sr ion and tmhd ligands weaker; thus, the breakaway of the tmhd ligand was believed to occur first. The dissociation behaviors of the two different types of ligands from {Sr(demamp)(tmhd)}2 are not well understood yet. If the demamp ligands are assumed to be dissociated first, the remaining precursor molecule must be similar to dimerized Sr(tmhd)2, but the subsequent growth behaviors are very different from those using standard Sr(tmhd)2. Therefore, it might be plausible to assume that the tmhd ligands dissociate first. Because the Sr atoms chemisorbed on the surface might still be combined with two oxygen atoms, it could be unlikely to absorb additional oxygen atoms from the underlying Ru (more precisely RuOx/Ru) layer. This could be a plausible reason for the lack of initial abnormal growth of SrO and STO films in this work. The bulkiness of ligands of {Sr(demamp)(tmhd)}2, which will induce steric hindrance effect, may be a reasonable 3885

DOI: 10.1021/acs.chemmater.5b00843 Chem. Mater. 2015, 27, 3881−3891

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Chemistry of Materials

in Figure 4b, while XPS results show solely the binding energy of strontium carbonate. In this study, the absolute value of atomic concentration of AES analyses was not confirmed and it was difficult to deconvolute accurately the energy spectra to the peaks of carbon and ruthenium with Auger electron. Therefore, it is more suitable to investigate the chemical property by XPS and to compare the relative concentration by AES. The STO films were deposited via the established SrO ALD process with {Sr(demamp)(tmhd)}2 and O3, and TiO2 ALD, as established in the previous study with Ti(CpMe5)(OMe)3 and O3.3 The STO ALD was always started with the TiO2 ALD subcycle to suppress any possible involvement of excessive growth of SrO. In the subcycle ratio of TiO2, SrO was varied to find the process condition for a stoichiometric STO film (Sr/ (Sr + Ti) = 50%) deposition. The cation compositions of the 5 nm-thick STO films on various substrates as a function of the subcycle ratio are indicated in Figure 6a. As the subcycle ratio

largely decreased using {Sr(demamp)(tmhd)}2 compared with that of Sr(iPr3Cp)2. In addition, the nitrogen concentration, which could be induced from the demamp ligand, was close to the detection limit of AES (Figure 4a, right panel) and XPS (Figure 4c) in the SrO films on every substrate. Moreover, the depth profile of the SrO films on the Ru substrate showed a uniform composition of Sr, and O had a very low C contamination (