Article pubs.acs.org/cm
Combined Ligand Exchange and Substitution Reactions in Atomic Layer Deposition of Conformal Ge2Sb2Te5 Film for Phase Change Memory Application Taeyong Eom,† Taehong Gwon,† Sijung Yoo,† Byung Joon Choi,‡ Moo-Sung Kim,§ Iain Buchanan,∥ Sergei Ivanov,∥ Manchao Xiao,∥ and Cheol Seong Hwang*,† †
Department of Materials Science and Engineering, and Inter-university Semiconductor Research Center, Seoul National University, Seoul 151-744, Republic of Korea ‡ Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 139-743, Republic of Korea § Air Products Korea, 15 Nongseo-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-920, Republic of Korea ∥ Air Products and Chemicals, Inc., 1969 Palomar Oaks Way, Carlsbad, California 92011, United States ABSTRACT: For phase change memories application, Ge− Sb−Te films were prepared by a stable and reliable atomic layer deposition (ALD) method. Ge(OC2H5)4, Sb(OC2H5)3, [(CH3)3Si]3Sb, and [(CH3)3Si]2Te were used to deposit various layers with compositions that can be described by combinations of GeTe2−Sb2Te layers including Ge2Sb2Te5 at a substrate temperature as low as 70 °C. A shift in composition of Sb−Te films from Sb2Te3 to Sb2Te composition was achieved by combining ligand exchange and substitution reaction between Sb in [(CH3)3Si]3Sb and Te in the Sb2Te3 layer. This surface-limited ALD process allowed highly conformal, smooth, and reproducible film growth over a contact hole structure, highlighting the feasibility of phase change memory applications. Ge2Sb2Te5 composition. Only films with compositions lying on a tie line connecting two pseudobinary compositions of GeTe2 and Sb2Te3, such as Ge2Sb2Te7, have been achieved.19,20 Synthesizing Ge precursors with the +2 oxidation state of Ge could be a viable option to solve such a problem, but it has certain limitations due to the general preference for the +4 state over the +2 state of Ge atoms. Meanwhile, there could be different chemical routes to form Ge2Sb2Te5. With GeTe2 and Sb2Te layers, the Ge2Sb2Te5 composition could be achieved as described in eq 1.
I. INTRODUCTION Phase change random access memory (PCRAM) is one of the highly promising next-generation memories by virtue of its nonvolatile data retention property and rapid writing and reading speeds.1−4 However, high reset current has been a major obstacle to further scaling of PCRAM,5−7 and it has been suggested that confining the phase change materials into a very narrow trench8 or hole,9 a so-called confined cell, could be a viable method to solve the high reset current problem. For such confined cell structures, it is necessary to deposit the Ge2Sb2Te5 film, which is the most suited phase change material (PCM) for memory devices,10 using a process that offers excellent conformality in terms of its thickness as well as its chemical composition, such as atomic layer deposition (ALD). Although several previous reports mostly adopted the chemical-vapor-deposition-like behavior of metal−alkyl precursors,11−13 a genuine ALD of Ge2Sb2Te5 was reported only recently by a group in Helsinki University,14−18 by using metal−silyl precursor. The present authors also reported the ALD of Ge−Sb−Te phase change materials by enabling chemistry-specific ligand exchange reaction between the (CH3)3Si ligand of the Te precursor and the OC2H5 ligand of Ge and Sb precursors.19,20 However, +4 oxidation state of the Ge in Ge precursors resulted in GeTe2 rather than GeTe, making it problematic to deposit a film with the desired © 2015 American Chemical Society
2GeTe2 + Sb2Te → Ge2Sb2Te5
(1)
Stable Sb2Te3 films are readily formed due to the +3 and −2 oxidation states of Sb and Te in their respective precursors (Sb(OC2H5)3 and [(CH3)3Si]2Te,), as reported elsewhere,19,20 but the composition can be changed toward an Sb-rich one as a consequence of the substitution reaction between [(CH3)3Si]3Sb precursor and Sb2Te3 film.21 It has been reported that the [(CH3)3Si]3Sb precursor pulsed on a previously grown Sb2Te3 layer reacts with the film forming the volatile [(CH3)3Si]2Te and elemental Sb as the reaction Received: March 2, 2015 Revised: April 13, 2015 Published: April 24, 2015 3707
DOI: 10.1021/acs.chemmater.5b00805 Chem. Mater. 2015, 27, 3707−3713
Article
Chemistry of Materials products.21 In addition, it was also found that Sb(OC2H5)3 and [(CH3)3Si]3Sb react with each other resulting in elemental Sb in an ALD manner via a reaction similar to that between Sb(OC2H5)3 and [(CH3)3Si]2Te. So this method can also be used to grow Sb-rich Sb−Te films. In this report, therefore, a stable and reliable ALD method to deposit Ge2Sb2Te5 film is reported by introducing [(CH3)3Si]3Sb during the ALD process previously demonstrated to grow Sb2Te3 and Ge2Sb2Te7 films. It was also possible to elucidate the contributions from the ALD-type Sb deposition, which is called ligand-exchange reaction, and substitution of Te with Sb by the reaction with [(CH3)3Si]3Sb, which is called substitution reaction, to the growth of Ge2Sb2Te5 films.
III. RESULTS AND DISCUSSION III-1. Growth Behaviors of SbxTe(1−x) Film. It has been previously19 demonstrated that deposition of Sb2Te3 films could readily be accomplished using the ST subcycle alone. In this work, it was possible to prepare films with composition Sb2Te from a supercycle comprising one ST subcycle followed by one SS subcycle. The ALD saturation behaviors of the precursors with respect to their pulse (tin) and purge (tprg) times for the Sb2Te film growth were examined to understand the incorporation mechanism of each precursor and possible interference effects between them. Growth rate (GR, denoted as layer density per supercycle, ng·cm−2·cycle−1) values are explained according to the experimental sequence. Figure 1a
II. EXPERIMENTAL PROCEDURES The Gb−Sb−Te films were deposited using a shower head type ALD reactor with an 8-in.-diameter wafer compatible substrate heater (CN1, Plus-200). Depositions were performed using Ge(OC2H5)4, Sb(OC2H5)3, [(CH3)3Si]3Sb, and [(CH3)3Si]2Te as precursors, contained in separate stainless steel canisters which were maintained at a temperature (Tcan) of 40 °C. The vapor pressures of Ge(OC2H5)4, Sb(OC2H5)3, [(CH3)3Si]3Sb, and [(CH3)3Si]2Te are 0.89, 1.0, 0.15, and 1.5 Torr at 40 °C, and those at 70 °C, which is the typical substrate temperature for film growth, are 5.3, 5.0, 1.1, and 8.0 Torr, respectively. The precursor vapors are carried into the process chamber with the aid of Ar carrier gas at a flow rate of 50 cm3(STP) min−1, and an additional 150 cm3(STP) min−1of Ar gas was flowed during precursor pulse to compensate for the difference in Ar gas flow rate with purge step. A 200 cm3(STP) min−1 amount of Ar gas was used to purge out the physisorbed precursors and the byproducts after each precursor pulse. The pressure of the ALD reactor was maintained at 3.0 ± 0.2 Torr during the whole deposition process. Films were deposited using supercycle combinations of three different subcycles, designated ST, SS, and GT. The ST subcycle comprises the following: Sb(OC2H5)3 pulse/purge (5 s/5 s)−[(CH3)3Si]2Te pulse/purge (1 s/ 10 s). The SS subcycle comprises the following: −Sb(OC2H5)3 pulse/ purge (2 s/5 s)−[(CH3)3Si]3Sb pulse/purge (5 s/20 s). The GT subcycle comprises the following: Ge(OC2H5)4 pulse/purge (6 s/5 s)−[(CH3)3Si]2Te pulse/purge (1 s/10 s). Using these basic processes, individual precursor pulse and purge times were varied to explore their influence on the deposited films. To change the partial pressure of [(CH3)3Si]3Sb, Tcan was varied from 40 to 60 °C Thermally grown and unpatterned 100 nm thick SiO2/Si wafers were used as substrates for growth rate studies. A contact hole substrate was fabricated in a 400 nm thick plasma-enhanced CVD SiO2 layer on a Si substrate using standard photolithography and dry etching techniques. The film layer density and compositions were measured by X-ray fluorescence spectroscopy (XRF, Thermo Scientific, ARL Quant’X), and its measurement error was expected to be less than 2%. Depth profiling was performed by Auger electron spectroscopy (AES; PerkinElmer, PHI660; detection limit ∼ 1 at. %). Glancing angle incidence modes X-ray diffraction and X-ray reflectivity (XRD, XRR; PANalytical, X’pert PRO MPD) were used to confirm the crystal structure and density of the films. Atomic force microscopy (AFM; JEOL, JSPM 5200) and scanning electron microscope (SEM; Hitachi, S-4800) were used to observe the film surface morphology. Transmission electron microscopy (TEM) specimens of the contact hole structure were prepared using focused ion beam etching (FIB; FEI, Helios 650). A 200 kV field emission transmission electron microscope (JEOL, JEM-2100F) was used for imaging, and local composition analysis was performed by energy dispersive spectroscopy (EDS; Oxford Instruments, X-maxN 80T) installed in the TEM. Resistivity−temperature (ρ−T) curves of the film were achieved by a real-time resistivity−temperature measurement system (homemade) at temperatures ranging from 100 to 250 °C with a heating rate of 8.8 °C min−1.
Figure 1. Influence of the process condition on the growth rate, presented in the order of the process cycle. (a) Sb(OC2H5)3 in ST subcycle. The adsorption type of subsequent Sb(OC2H5)3 was changed to physisorption suggesting that the active site for chemisorption was eliminated. (b) [(CH3)3Si]2Te in the ST subcycle. Typical ALD saturation behavior was observed. (c) Sb(OC2H5)3 in the SS subcycle. Typical ALD saturation behavior was observed, and the feasibility of ideal ligand exchange reaction between Sb(OC2H5)3 and [(CH3)3Si]3Sb was demonstrated. (d) [(CH3)3Si]3Sb in the SS subcycle for pulse time (left panel) and purge time for respective Tcan (right panel). The GR of Te significantly decreased with introduction of [(CH3 )3 Si] 3Sb due to the substitution reaction between [(CH3)3Si]3Sb and Te in the film.
shows the variations in GR of Sb and Te as a function of the tin and tprg of Sb(OC2H5)3 precursor in the ST subcycle for the given conditions of other precursors mentioned earlier. Wellbehaved ALD saturation behavior of this precursor for both tin and tprg times have been achieved for the growth of Sb2Te3 and Ge2Sb2Te7 films under almost identical conditions.19,20 However, the present results deviate significantly from selflimiting ALD behavior, which is highly unexpected. This can be attributed to influence of the SS subcycle prior to the second Sb(OC2H5)3 pulse step. Therefore, detailed discussions for this unexpected behavior will be made after the discussion of the SS subcycle. Figure 1b shows typical ALD saturation behavior of the [(CH3)3Si]2Te pulse and purge steps in the ST subcycle for the given conditions of other precursors. The GR’s of both Te and Sb increased up to tin = 3 s and then saturated afterward, which is typical ALD behavior. GR of Sb varies according to GR of Te which could be ascribed to adsorption of Sb(OC2H5)3 on the chemisorbed [(CH3)3Si]2Te during the following Sb(OC2H5)3 pulse step of the SS subcycle. Similar 3708
DOI: 10.1021/acs.chemmater.5b00805 Chem. Mater. 2015, 27, 3707−3713
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Chemistry of Materials
This could be ascribed to the fact that the pulse condition of [(CH3)3Si]3Sb precursor was just enough to induce saturation of the ALD process due to the lower vapor pressure of this precursor at 40 °C. To further confirm this effect, the Tcan of [(CH3)3Si]3Sb precursor was increased to 60 °C and the experiments with increasing tprg were repeated. These data are included in the right panel of Figure 1d. As expected, GR was greatly increased due to the increase of physisorbed precursors, but it eventually saturated to a saturation level when tprg became longer than ∼20 s. Another notable finding is that the GR was slightly lower for short tprg than when tprg ∼ 2 and 5 s, regardless of whether Tcan is 40 or 60 °C. This suggests that an excessively large amount of [(CH3)3Si]3Sb physisorbate remained until the next [(CH3)3Si]2Te precursor step and hinders the chemical adsorption of Te precursor. In parts b−d of Figure 1, very high GRs of the films were observed if a short tprg (2, the average slope of GR (equal to the incorporation amount per subcycle, denoted in layer density per subcycle, ng·cm−2·subcycle−1) of Te (44.6 ng·cm−2· subcycle−1) was 1.45 times higher than Sb (30.8 ng·cm−2· subcycle−1). Because the molar weights of Sb and Te are similar (121.75 g·mol−1 vs 127.60 g·mol−1), this ratio implies that the ST subcycle condition produced Sb2Te3 materials. However, when the ST subcycle number ≤ 2, the slope of GR of Sb increased to 65 ng·cm−2·subcycle−1, whereas the slope of GR of Te decreased slightly. This could be ascribed to the influence of substitution reaction during the subsequent [(CH3)3Si]3Sb step as described in Figure 1a. It has to be borne in mind that the data in Figure 2 were collected from films deposited using ST− SS supercycles, so the influence of a subsequent process could be also involved in this data set. According to the discussions for Figure 1a the Sb2Te3 formed in ST subcycles becomes Sbrich and Te-poor during the subsequent [(CH3)3Si]3Sb pulse, which is consistent with the data when the ST subcycle number was 2, the thickness (or layer density) of the Sb2Te3 layer deposited during repeated ST subcycles increases and the influence of the subsequent step, a substitution reaction limited to only the surface of the film, decreases. From the difference of slopes of GR of Sb in the two subcycle number regions, the amount of Sb incorporated from substitution reaction could be 34.2 (=65.0−30.8) ng·cm−2·subcycle−1. The composition of the resulting SbxTe1−x film can be varied from x = 0.6 (at ST subcycle number of 1) to x = 0.5 (at ST subcycle number of 8). Figure 2b shows the change of GR and composition when the SS subcycle number is increased for an ST subcycle number of 1. The GR of Sb increased linearly as the SS subcycle number increases, implying Sb is incorporated by ligand exchange as represented by eq 2, while the GR of Te remains constant. According to the discussions regarding Figure 1a, a certain fraction of the previously deposited Te must be removed. Even when the SS subcycle number increases further, no further decrease in the GR of Te was observed since the film surface is now covered with Sb and outward diffusion of Te at this temperature is not active.21 From the slope of the best linear fitted graph of GR of Sb, the incorporated amount of Sb per SS subcycle, which could be ascribed exclusively to the ligand exchange reaction, was 41.6 ng·cm−2·subcycle−1. Since two Sb-precursor pulse steps (Sb(OC 2 H 5 ) 3 and [(CH3)3Si]3Sb) were involved in one SS subcycle, the average increase in GR of Sb per Sb-precursor pulse is 20.8 ng·cm−2· subcycle−1. This is almost precisely two-thirds of the increase of GR in the ST subcycle, shown in Figure 2a. This can be understood from the schematic diagrams in Figure 2c−f, where the ALD reactions between Sb(OC2H5)3 and [(CH3)3Si]2Te, and Sb(OC2H5)3 and [(CH3)3Si]3Sb, respectively, are depicted. When the surface resulting from the ST subcycle is exposed to the initial Sb(OC2H5)3 pulse of the SS subcycle, two types of chemisorbates could be formed, depending on the environment of steric hindrance and proximity of adjacent Te sites. If the
Figure 2. Variation in the GR (left ordinate) and composition (right ordinate) as a function of (a) ST subcycle number and (b) SS subcycle number in a supercycle. With increasing ST subcycle number, the change in GR of Sb was faster at first, due to the influence of substitution reaction, but it converged to that of Sb2Te3 at ST subcycle number > 2 condition. With increasing SS subcycle number, the GR of Sb linearly increased as a consequence of ligand exchange reaction. (c) Schematic diagrams of ALD reaction between Sb(OC2H5)3 and [(CH3)3Si]2Te in ST subcycle. (d) Configurations of Sb(OC2H5)3−n chemisorbates (n = 1 and 2; 1 is A type and 2 is B type) on the Sb2Te3 surface. Schematic diagrams of reaction between Sb(OC2H5)3 and [(CH3)3Si]3Sb at (e) A type and (b) B type chemisorbates. 3710
DOI: 10.1021/acs.chemmater.5b00805 Chem. Mater. 2015, 27, 3707−3713
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Chemistry of Materials
viable process to form a GeTe2 film, although it relies on the physisorption of Ge(OC2H5)4, so it can hardly be a genuine ALD process.19,20 To confirm the change in film composition, the number of GT subcycles was varied from 0 to 10. Since the change in composition on the ternary graph was laid on the GeTe2−Sb0.61Te0.39 tie line as shown in the inset of Figure 3a, the empirical formula of the two binary materials,
steric hindrance is low, Sb(OC2H5)2 could be formed which is bonded to one Te atom below (A type), and if it is high, Sb(OC2H5)1 is formed which bridges between two Te atoms below (B type). Therefore, the configuration of the surface (excluding physisorbates) following the Sb(OC2H5)3 pulse might be represented by Figure 2d. Now, it can be understood that an A-type chemisorbate would form a SbTe2 compound and a B-type chemisorbate would form a SbTe compound during the subsequent [(CH3)3Si]2Te pulse step. The experiments showed that the film composition is always maintained at Sb2Te3, meaning that ALD proceeds involving identical portions of A- and B-type reactions. The formation of Sb2Te3, therefore, can be understood from the following reaction in eq 7. SbTe2 + SbTe → Sb2 Te3
(7)
The ALD reactions between Sb(OC 2 H 5 ) 3 and [(CH3)3Si]3Sb might be considered in a similar way, but the two-thirds of GR of Sb in the SS subcycle implies that something different is involved, as can be seen in Figure 2e,f. In Figure 2e the surface starts with the A-type site. Upon pulsing with [(CH3)3Si]3Sb, one of the two OC2H5 ligands is removed through reaction with a (CH3)3Si ligand from the incoming [(CH3)3Si]3Sb. Then, the surface site is now occupied by the [(CH3)3Si]2Sb intermediate. Two Sb atoms are now bound to one another, one bearing an OC2H5 ligand and the other bearing a (CH3)3Si ligand. These ligands can now undergo an elimination reaction and change the single bond between the two Sb atoms to double bond. The presence of SbSb double bonds in Figure 2e could be unusual in the sense of inorganic chemistry. However, it has been theoretically22 and experimentally23 proven that the double bonds can be stabilized when bulky ligands are attached to Sb ions. There now remains only one (CH3)3Si ligand on the surface Sb atom, and only one Sb(OC2H5)3 molecule could be attached on this site, whereas the A-type site is supposed to chemisorb two molecules (See Figure 2c). For the B-type surface sites, a similar reaction route can be considered as shown in Figure 2f. Here, the interaction between the remaining (CH3)3Si and OC2H5 ligands also removes one reaction site and induces chemisorption of one Sb(OC2H5)3 molecule during its pulse step. This means that two Sb atoms (one from the A-type site and one from the Btype site) are deposited during one SS subcycle according to this reaction route, whereas three Sb atoms (two from the Atype site and one from the B-type site) are deposited during one ST subcycle according to the reaction route described previously. This is in a very good accordance with the experimental results observed in Figures 2a,b. Similar interaction between Te−Si(CH3)3 and Sb−OC2H5 can be conceived of. However, the more favorable Lewis acid−base bonding nature21 of Te−Si(CH3)3 compared with that of Sb− Si(CH3)3 precludes such reactions in the ST subcycle. The red open up triangle symbol in Figure 2b revealed that the x value of SbxTe1−x can be varied from 0.6 (at SS subcycle number of 1) to 0.85 (at SS subcycle number of 6). When both ST and SS subcycle numbers are 1, a film with composition close to Sb2Te (more precisely Sb0.61Te0.39) could be achieved, which could be utilized to deposit the Ge2Sb2Te5 film according to eq 1. III-2. Growth Behaviors and Characteristics (GeTe2)y(Sb0.66Te0.33)(1−y) Films. For deposition of Ge2Sb2Te5, the GT subcycle was combined with the 1 ST−1 SS process. The previously mentioned GT subcycle was proven to be a
Figure 3. Growth characteristics of the Ge−Sb−Te layer. (a) Composition change in the GeTe2−Sb0.61Te0.39 tie line as a function of GT subcycle number. Ge2Sb1.9Te5.2(∼Ge2Sb2Te5) composition was obtained on the GeTe2−Sb0.61Te0.39 tie line 10 GT subcycle combined with 1 ST−1 SS supercycle. (b) GR and composition change as a function of the number of 1 ST−1 SS−10 GT supercycles.
(GeTe2)y(Sb0.66Te0.33)(1−y), could express the films’ composition appropriately. With the increasing subcycle number of GT, increasing amounts of the GeTe2 component was incorporated and (GeTe2)0.39(Sb0.61Te0.39)0.61 (∼Ge2Sb1.9Te5.2) could be achieved when the GT subcycle number was 10. The growth characteristics of Ge2Sb1.9Te5.2 film by the 1 ST−1 SS−10 GT supercycle is shown in Figure 3b. The process showed a fixed GR with respect to the supercycle number up to 30 involving an incubation supercycle number of ∼3, and the GR was 0.99 μg·cm−2·cycle−1. A uniform composition (open symbols in Figure 3b) was observed except for the sample with 5 supercycles which is close to the incubation supercycle number, where the film was slightly Sbrich and Te-poor. A TEM cross-section was used to examine the Ge2Sb1.9Te5.2 film grown with 20 supercycles in a 90 nm diameter and 300 nm deep contact hole structure, and the compositions in the structure were analyzed by EDS. Figure 4 shows a very conformal deposition of Ge2Sb1.9Te5.2 film in thickness and chemical composition along the depth direction within the hole as well as the top surface. As shown in Figure 4a, the physical film thickness at the top of the hole structure was measured as 30 nm which as almost identical to that at the bottom (29 nm). The chemical compositions at all
Figure 4. (a) Cross-sectional TEM images of Ge2Sb1.9Te5.2 film in contact hole structure. (b) Composition of Ge, Sb, and Te at the points marked by numbers in panel a. 3711
DOI: 10.1021/acs.chemmater.5b00805 Chem. Mater. 2015, 27, 3707−3713
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Chemistry of Materials
IV. CONCLUSION In conclusion, an ALD process was explored for depositing Ge2Sb2Te5 films at a low deposition temperature of 70 °C by combining the deposition process for Sb2Te3, Sb, and GeTe2 components. While the strong affinity between the (CH3)3Si ligand in [(CH3)3Si]3Te and the OC2H5 ligand in Sb(OC2H5)3 provide an efficient ligand exchange reaction, the [(CH3)3Si]− Sb bond in [(CH3)3Si]2Sb provided a combined deposition mechanism of ligand exchange and substitution reaction. This chemistry-specific deposition process exhibited partly selflimiting growth behavior. The films grown showed highly promising conformal, smooth, and reproducible growth over a contact hole structure. The overall film compositions were wellrepresented by SbxTe(1−x) and (GeTe2)y(Sb0.61Te0.39)(1−y), respectively, and a continuous variation in composition could also be achieved through adjustment in the relative number of Sb−Te, Sb, and Ge−Te subcycles. The material showed a reliable phase change and accompanying resistance switching behavior assuring the possibility of phase change memory application.
positions also showed negligible variations from the top to the bottom portions of the contact hole structure. As shown in the AFM and SEM images of Figure 5a, a smooth surface morphology [root-mean-squared (RMS)
Figure 5. Properties of 20 nm thick Ge2Sb1.9Te5.2 film: (a) SEM (left) and AFM (right) image; (b) AES depth profile.
roughness of ∼1.1 nm at a film thickness of 20 nm] was achieved. Figure 5b shows the AES depth profile results of the 20 nm thick layer. The film has a highly uniform composition along the thickness direction. The O- and Si-contamination level was lower than the detection limit (
