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A theoretical study on initial growth mechanism of ZrO2 film using cyclopentadienyl-type precursor
Thin Solid Films 519 (2011) 3716–3721
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
A theoretical study on initial growth mechanism of ZrO2 film using cyclopentadienyl-type precursor Jie Ren a,⁎, Chengxing Cui b, Guangfen Zhou a, Yanchun Liu a, Yongqi Hu a,⁎, Baozhu Wang a a b
Colleges of Science and Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang, Henan 453003, China
a r t i c l e
i n f o
Article history: Received 26 February 2010 Received in revised form 12 January 2011 Accepted 21 January 2011 Available online 28 January 2011 Keywords: Density functional theory Dielectrics Zirconia Atomic layer deposition
a b s t r a c t The initial growth reaction of atomic layer deposition (ALD) of ZrO2 using Cp2Zr(CH3)2 (Cp_C5H5, cyclopentadienyl) as metal precursor on the hydroxylated silicon surface is investigated by using density functional theory (DFT). The ALD cycle is achieved through two types of ligand elimination reactions (i.e., CH4 and CpH elimination reactions). The possible reaction pathways are proposed in order to find the dominant initial reaction during the Cp2Zr(CH3)2 precursor pulse. DFT calculations show that the CH4 elimination reaction is energetically more favorable than CpH elimination reaction. As a result, the two CH3 ligands of Cp2Zr(CH3)2 may be dissociated prior to the two Cp rings during the metal precursor pulse. In addition, one CpH elimination may occurs sequentially following the first CH4 elimination reaction according to activation barrier analysis during the Cp2Zr(CH3)2 pulse. All the calculated results are in agreement with the experimental findings. © 2011 Elsevier B.V. All rights reserved.
1. Introduction With the continuous downscaling of metal oxide semiconductor (MOS) devices, silicon dioxide (SiO2) gate dielectric will reach its physical limits due to gate leakage and reliability issues [1,2]. Currently, high dielectric constant (high-κ) materials have been introduced in MOS devices to replace the traditional SiO2 (κ = 3.9) as gate dielectric materials, since their higher capacitances with thicker films can suppress leakage currents largely [3,4]. Among these high-κ candidates, ZrO2 is a promising material for applications due to its high permittivity (κ = 17–22), wide band gap, good thermal stability and chemical compatibility with silicon [5,6]. By far high-κ oxide films have been extensively fabricated by atomic layer deposition (ALD) technique. Due to the characteristics of self-limiting surface reactions, ALD can offer a lot of practical advantages, such as excellent conformity, accurate thickness control down to the nanometer level and uniform film properties over a large area [7,8]. ZrO2 thin films have been grown by ALD most commonly using ZrCl4 and H2O as precursor combination [9]. Unfortunately, the ZrCl4/H2O process has some distinct drawbacks, such as contamination of chlorine and release of corrosive HCl as the by-product. Therefore alternative precursors are being sought. So far, some chlorine-free precursors have been employed so as to improve the dielectric quality of ALD ZrO2 (Table 1) [9]. Especially, experiments have recently shown that the organometallic cyclopentadienyl (Cp and C5H5) compounds, e.g. Cp2Zr(CH3)2, together with H2O as an ⁎ Corresponding authors. Tel./fax: +86 311 8166 8535. E-mail addresses: [email protected] (J. Ren), [email protected] (Y. Hu). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.278
oxygen source can yield stoichiometric ZrO2 films with extremely low impurity contents (below 0.1 atom% for C and H) and promising electrical properties [21,22]. As we know that the understanding of reaction mechanism will help to optimize the process condition parameters and control the interface formed between the deposited films and the silicon substrate. Usually, the experimental investigation of the ALD mechanism for high-κ oxide can be achieved by capturing the intermediate and final products. At present, density functional theory (DFT) has already been considered as an effective theoretical method to explore the ALD mechanism. For example, the initial ALD-growth mechanisms of zirconia and hafnia, and the chemical kinetic and thermodynamic data, have always been given by DFT calculations [23–26]. These studies do provide insights into the ALD growth mechanism of high-κ oxides. However, most of these cases focus on the ALD process of zirconia and hafnia in which metal chloride is used as metal precursors, and few theoretical investigates have been made to explore the ALD growth mechanism of these high-κ oxides using the cyclopentadienyl-type precursor [27–29]. Experiments [21] show that the ALD ZrO2 using Cp2Zr(CH3)2/H2O combination is the self-limiting growth from 210 to 400 °C. The precursor pulse duration has no obvious effect on the total amount of released CH4 or CpH. Based on the growth mode of ALD type, we think that the ALD cycle is achieved through two types of ligand elimination reactions of Cp2Zr(CH3)2, i.e. CH4 and CpH elimination reactions. Therefore, the possible reactions are proposed (Scheme 1) in order to find the dominant initial reaction during the Cp2Zr(CH3)2 precursor pulse. Reaction (R1) belongs to CH4 elimination reaction, in which one CH3 group of Cp2Zr(CH3)2 firstly reacts with the surface
J. Ren et al. / Thin Solid Films 519 (2011) 3716–3721 Table 1 Recently published ALD processes for the ZrO2 thin films and the studied deposition temperature range. Metal precursor
Oxygen source
Dep. temp./°C
Ref.
Zr(NMeEt)4 Zr(NMeEt)4 Zr(NMeEt)4 (RCp)Zr(NMe2)3 (R_H, Me or Et) ZrCl2[N(SiMe3)2]2 (MeCp)2ZrMe2 or (MeCp)2ZrMe(OMe) (MeCp)2ZrMe(OMe) (MeCp)2ZrMe(OMe) (MeCp)2ZrMe2 or (MeCp)2ZrMe(OMe) (MeCp)2ZrMe(OBut) or (MeCp)2ZrMe(OMe) (MeCp)2ZrMe2 or (MeCp)2ZrMe(OMe) Cp2ZrMe2
N2O or O2 Plasma O3 O2 O3 H2O H2O D2O or O3 H2O O3 H2O
240–320 225–275 110–250 300 150–350 300–500 350 350 300 or 350 300–350
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
O3 D2O
250–500 210–440
[20] [21]
hydroxyl, and R2 belongs to CpH elimination reaction, where one Cp group of Cp2Zr(CH3)2 firstly reacts with the surface hydroxyl. Similarly, the following reactions (R3 vs R4and R5 vs R6) are also designed according to CH4 and CpH as the elimination products, respectively. 2. Computational method and models Elam et al. [17] once used hydrolysis energy calculations to predict the order of ligand loss of cyclopentadienyl-type precursor in the reactions of ALD-grown ZrO2. Instead, we employed the calculations of potential energy surface for each reaction to justify the experimental findings in this work. The hybrid DFT approach utilizing Becke's threeparameter exchange functional [30,31] with Lee–Yang–Parr correlation functional [32] (B3LYP) was used to optimize the geometries, analyze
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the frequencies and obtain the reaction energies of each stationary point. We optimized all the structures and the geometrical parameters of reactants, transition states (TSs) and products using the Berny optimization algorithm in GAUSSIAN 03 software package [33], using analytic first and second derivatives of the energy at every step. A stationary point found by a geometry optimization is a minimum (local or global following its stability) when all the vibrational frequencies are real. In contrast, it is a TS linking two minima when there is an imaginary frequency. A mixed basis set scheme was used to save computational resource. In particular, the LANL2DZ basis set and effective core potential were employed to describe zirconium atom, 6–31 G(d,p) basis set for chemical active atoms (i.e., the hydroxylated surface silicon atoms, hydroxyls, Cp and CH3 ligands), and 6–31 G for the remnant silicon and the terminated hydrogen atoms. The B3LYP method and mixed basis set scheme have shown relatively good applications in identifying the growth mechanisms and energetics for the ALD of high-κ oxides, such as HfO2 and ZrO2 [25,34–37]. Previously, to explore the surface reaction mechanism on the hydroxylated Si(100)-2 × 1 surface, Si9H12 one-dimer cluster was frequently used to simulate Si(100)-2 × 1 [23,24,34–36]. In the current study, a larger hydroxylated three-dimer model Si21H24\(OH)2 cluster model (Fig. 1a) was employed to diminish the cluster size effect. The cluster approximation method was based on the predominantly localized bonding of the Si(100)-2 × 1 surface. The Si21H24―(OH)2 three-dimer cluster consisted of four layer silicon atoms where the top six silicon atoms composed the surface dimers. The remaining silicon atoms composed three subsurface layers that were terminated by hydrogen atoms to prevent unrealistic charge transfer. The hydroxyls were used to react with the incoming Cp2Zr (CH3)2 precursor as surface sites. The surface hydroxyls used here indicated that the initial silicon surface had been pretreated by H2O firstly. Moreover, only two reactive hydroxyls were kept to avoid the other neighboring hydroxyls intruding on the title surface reactions
Scheme 1. Surface reactions (R1–R6) of Cp2Zr(CH3)2 on the hydroxylated silicon surface.
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Fig. 2. Reaction pathway and predicted energetics at 0 K for the adsorption and decomposition of Cp2Zr(CH3)2 on Si21H24―(OH)2 surface sites. The zero of energy is the sum of total energies of a precursor and a hydroxylated silicon cluster. TS1 and TS2 are the abbreviation for the first and second transition states, respectively, IP is for intermediate product, PS1 and PS2 are for the first and second physisorbed state, respectively, and FP is for final product.
Fig. 1. (a) The hydroxylated Si(100)-2 × 1 cluster modeled by Si21H24―(OH)2 threedimer cluster. The red, grey and small white balls represent oxygen, silicon and hydrogen atoms, respectively. (b) Cp2Zr(CH3)2 compound.
which could make them more complicated. The structures of these cluster models were fully optimized in all calculations. The optimized structure of the precursor Cp2Zr(CH3)2 is shown in Fig. 1b. Cp2Zr (CH3)2 contains two η5-Cp and two CH3 ligands. The Zr―CH3 bond lengths are both 2.29 Å and CH3―Zr―CH3 angle is 98.9°. The distances from zirconium ion to the centers of Cp rings (Zr―Cp) are 2.29 and 2.27 Å, respectively. Individual Zr―C(π) distances to the Cp rings are in the range of 2.56 to 2.60 Å. The Cp―Zr―Cp angle is 134.0°. All above calculated values are in close agreement with the crystal structure data [38]. 3. Results and discussion 3.1. Comparison between the reaction pathways of R1 and R2 In the precursor Cp2Zr(CH3)2, both the active groups (i.e., CH3 and Cp) can possibly react with the Si―OH* (* denotes the active site) surface sites firstly. Thus there are two possible initial reaction pathways (R1 and R2), which are shown in Fig. 2. In R1, the surface Si―OH* group transfers its hydrogen atom to one CH3 group of Cp2Zr (CH3) 2 , resulting in the elimination of CH4 and forming an intermediate product (IP). The reaction profile at 0 K and the corresponding geometries along the reaction pathway for R1 are shown in Figs. 2 and 3, respectively. As the starting structure of R1, the reactive CH3 of the precursor is placed above the OH-terminated silicon surface (Fig. 3a). After full relaxation, a physisorbed state (PS1 in Fig. 2) is obtained with a calculated adsorbed energy of 23.7 kJ/mol lower than the separated reactants. Unlike Y(Cp)3 and Mg(Cp)2 adsorbed complexes [28,29], the Cp2Zr(CH3)2 adsorbed complex is not formed through the surface hydroxyl coordinating to the metal center. As we can see in Fig. 3a, the zirconium atom cannot approach the hydroxyl oxygen atom easily due to steric repulsion. As a result,
the adsorbed energy of R1 is lower than those obtained from the adsorption of Y(Cp)3 and Mg(Cp)2 on the Si―OH* surface (36.2 and 37.7 kJ/mol, respectively) [28,29]. Moreover, the geometry of adsorbed Cp2Zr(CH3)2 has not been obviously changed, where the CH3―Zr―CH3 angle is 98.8° and Zr―CH3 bond lengths are 2.30 and 2.28 Å, respectively (Table 2). After the adsorption, one CH3 group of the adsorbed Cp2Zr(CH3)2 reacts with the H atom of Si―OH* surface and forms a four-member-ring TS composed of H―O―Zr―CH3(1) (Fig. 3b), whose energy is 47.3 kJ/mol higher than the PS1 (see Fig. 2). For the TS, Zr―CH3(1) bond length increases to 2.59 Å and CH3―Zr―CH3 angle increases to 136.0°. At the same time, Zr―O bond is forming with a calculated bond length of 2.42 Å. Over the activation barrier for hydrogen transfer, the reaction reaches its IP and releases one by-product CH4 molecule. The overall reaction is exothermic by 213.9 kJ/mol, indicating that the CH4 elimination reaction through hydrogen transfer of Si―OH* to Cp2Zr(CH3)2 is energetically favorable. As we know that weak adsorption can result in non-ALD decomposition of the precursor above the surface. Experiment has shown that non-ALD decomposition occurs at temperature exceeding 400 °C[21]. Therefore, the entropy change at 298 K (Δ S 298) is calculated in order to find whether the desorption of the adsorbed Cp2Zr(CH3)2 (PS1 in Fig. 2) in R1 is thermodynamically favorable or not. T Δ S 298 of the desorption products of the surface bound Cp2Zr (CH3)2 is 32.9 kJ/mol, 9.2 kJ/mol larger than the adsorption energy. This indicates that the desorption of the adsorbed Cp2Zr(CH3)2 is favored in the thermodynamic equilibrium state. However, recent theoretical studies showed that the activation energy of first CH4 nonALD decomposition of Cp2Zr(CH3)2 in the gas phase was 147 kJ/mol [39], far higher than the energy (47.3 kJ/mol) obtained from the ALD growth (R1) in this study. Therefore, we think that R1 is easier to occur for the ALD growth before the thermodynamic equilibrium is reached. In the case of R2, one Cp group of Cp2Zr(CH3)2 abstracts the H atom from the surface Si―OH* group, resulting in the elimination of CpH from the surface and the forming of the IP state. Similar to R1, the starting adsorbed structure is modeled by placing the Cp groups of Cp2Zr(CH3)2 above the Si―OH* surface site in R2 (cf. Fig. 4a). After full geometry optimization, the energy gain upon adsorption of Cp2Zr (CH3)2 is 18.5 kJ/mol compared to 23.7 kJ/mol for that obtained in R1,
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Fig. 3. Geometrical structures along the reaction pathway of R1, R3 and R4: (a) PS1, (b) TS1, (c) IP, (d, d′) TS2 and (e, e′) FP, where (d) and (e) are for R3 and (d′) and (e′) are for R4, respectively. The four-center TS is denoted by blue dashed line.
which suggests that there exists stronger affinity of CH3 for Si―OH* surface site. Similar to R1, the geometry of the adsorbed Cp2Zr(CH3)2 is also hardly changed, in which the Zr―Cp distances are both 2.28 Å
and the Cp(1)―Zr―Cp(2) angle is 132.9° (Table 2). After the adsorption, the reaction goes through an activation barrier with an energy value of 127.1 kJ/mol, which is substantially higher than that
Table 2 Representative bond distances (Å) and bond angles (°) in the selected state of reactions (R1–R6). State (reactions a)
PS(R1)
TS(R1)
IP(R1)
PS(R2)
TS(R2)
IP(R2)
Zr―CH3(1) Zr―CH3(2) Zr―O(1) Zr―O(2) Zr―Cp(1) b Zr―Cp(2) b CH3(1)―Zr―CH3(2) Cp(1)―Zr―Cp(2) b
2.30 2.28 4.99 5.49 2.29 2.29 98.8 133.9
2.59 2.31 2.42 4.74 2.28 2.30 136.0 129.4
–
2.29 2.29 4.24 5.19 2.28 2.28 98.1 132.9
2.28 2.25 2.26 4.89 3.09 2.27 83.3 115.3
2.25 2.25 1.99 4.18
a
2.29 2.02 4.85 2.28 2.28 – 129.9
– 2.28 102.8 –
TS(R3)
FP(R3)
TS(R4)
FP(R4, R5)
TS(R5)
–
– –
–
– 2.25 2.00 2.00 – 2.29 – –
2.50 2.25 2.01 2.31 – 2.27 82.1 –
2.64 2.11 2.32 2.29 2.32 – 125.5
2.05 2.05 2.29 2.29 – 123.9
2.25 2.04 2.17 3.12 2.29 – 113.6
TS(R6)
PS(R6)
FP(R6)
2.24 2.24 1.99 2.18
2.25 2.25 1.98 2.02
2.24 2.24 1.99 1.99
–
–
2.97 106.6 –
3.62 113.1 –
– – 109.2 –
The sequence numbers of reactions are defined in Scheme 1. Strictly speaking, the Cp ring is neither planar nor regular pentagon, so the center of Cp ring cannot be located accurately. The Zr―Cp and Cp(1)―Zr―Cp(2) is therefore only a very approximate value. b
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Fig. 4. Geometrical structures along the reaction pathway of R2, R5 and R6: (a) PS1, (b) TS1, (c) IP, (d, d′) TS2, (e′) PS2 and (e, f′) FP, where (d) and (e) are for R5 and (d′), (e′) and (f′) are for R6, respectively. The four-center TS is denoted by blue dashed line.
of R1. Therefore, the dissociating of the Zr―Cp(1) bond is more difficult for R2 than that of Zr―CH3(1) for R1 by comparing the energy barriers of R1 and R2. In addition, we can find in Table 2 that the Zr―Cp(1) distance is elongated to 3.09 Å and Cp(1)―Zr―Cp(2) angle is decreased to 115.3° in the TS. Finally, R2 finishes along with the release of one CpH gas molecule. The formation of Zr―O bond is also completed with a calculated bond length of 1.99 Å. The overall reaction is exothermic by 58.3 kJ/mol for R2, far lower than that of R1. In conclusion, the CH4 elimination reaction of Cp2Zr(CH3)2 (R1) is energetically more favorable than the CpH elimination reaction (R2) on the hydroxylated silicon surface. 3.2. Comparison between the reaction pathways of R3 and R4 After elimination of the first CH4 in R1, the formed Cp2ZrCH3 fragment can react with the neighboring Si―OH* site. We assume that the reaction would branch according to either CH3 or Cp as the starting reactive group. One branch is CH4 elimination reaction (i.e., R3 in Scheme 1). The other is the CpH elimination reaction (i.e., R4 in Scheme 1). The reaction pathways and the corresponding energies at the stationary points for R3 and R4 are shown in Fig. 2. The corresponding geometries along the pathways of R3 and R4 are depicted
in Fig. 3. Starting from the IP state, R3 goes through a four-center TS composed of H―O―Zr―CH3(2) similar to R1 (Fig. 3d). The activation barrier (TS2 in Fig. 2) is 99.5 kJ/mol relative to the IP. The Zr―O(1) and Zr―O(2) bond distances of the second TS are 2.11 and 2.32 Å, respectively, and the Zr―CH3(2) bond length increases to 2.64 Å. Finally, R3 reaches the final product (FP) with a total exothermic enthalpy (ΔH0) of 318.8 kJ/mol relative to the initial reactants. As another possible reaction, R4 is also a proton transfer reaction where one Cp ring of Cp2ZrCH3 accepts the H atom from the neighboring Si―OH* and releases one CpH molecule. The activation barrier of R4 is calculated to be 105.0 kJ/mol, which is 5.5 kJ/mol slightly higher than that of R3. Also, calculations show that R4 has a decreasing exothermicity with 205.2 kJ/mol as compared to R3. Therefore, we can deduce that it is energetically less favorable for R4 than R3. 3.3. Comparison between the reaction pathways of R5 and R6 After elimination of the first CpH in R2, the adsorbed CpZr(CH3)2 fragment reacts with the neighboring hydroxyl on silicon surface, where the reaction network can branch similarly. One alternative is CH4 elimination reaction (i.e., R5 in Scheme 1). The other is the CpH elimination reaction (i.e., R6 in Scheme 1). The reaction pathways and
J. Ren et al. / Thin Solid Films 519 (2011) 3716–3721
energetics for R5 and R6 are shown in Fig. 2. The energy of the TS relative to the IP is calculated to be 39.4 kJ/mol for R5. It could be found that the reaction is exothermic and the enthalpy change is 205.2 kJ/mol. In addition, we can see in Fig. 2 that the reaction pathway of R6 is different from R5. The only difference is that one weak bound by-product CpH is found in R6. Calculation shows that R6 must overcome a 3.9 kJ/mol adsorption well (i.e., PS2 in Fig. 2) to reach the FP state. The additional adsorption energy possibly results from the Zr―C(π) interaction between zirconium ion and Cp(2) ring. The individual Zr―C(π) distances are calculated to be in the range of 3.24 to 4.40 Å. As a comparison, the stable physisorption of the byproduct CpH on the Si―OH* surface, however, is not found in R2 and R4. Therefore, it can be deduced that the physisorption in R6 is due to the relief of steric congestion around zirconium metal center. It should be emphasized that the physisorption is relatively weak. Thus we can suggest that it is easy to purge the by-product out of the reaction chamber, and it is not necessary to use a long purge time if R6 occurs in the actual ALD process. The activation barrier for the CpH formation in R6 is calculated to be 88.1 kJ/mol, which is much higher than that of R5. Moreover, R6 is exothermic by 34.3 kJ/mol, which is far lower than that of R5. Therefore, we can conclude that R5 is energetically more favorable than R6. 3.4. Comparison between the calculations and the experimental findings As we know that the ALD process of ZrO2 consists of alternating Cp2Zr(CH3)2 precursor and oxygen source pluses. In situ experiments [21] have detected that almost all (90%) of the CH3 ligands were released during the Cp2Zr(CH3)2 precursor pulse by using quadrupole mass spectrometer. By comparing the competitive surface reactions (R1–R6), we can argue that the CH4 elimination reaction on the Si―OH* surface site has more favorable thermodynamics than the CpH elimination reaction. In other words, both R1 and R3 are the energetically more favorable pathways, i.e., the dominant pathways. Theoretically, the two CH3 ligands of Cp2Zr(CH3)2 can be dissociated prior to the two Cp rings, which is in excellent agreement with the above-mentioned experimental findings. In addition, the experiment showed that about 40% of the CpH was released during the Cp2Zr(CH3)2 pulse [21], which indicates that there exists a CpH elimination reaction. By comparison of the above-mentioned competing reactions, we find that the activation barrier of R2 is 79.8 kJ/ mol higher than that of R1, however, in the case of R4, the energy barrier is only 5.5 kJ/mol slightly higher than that of R3 (see Fig. 2). We can therefore conclude that R4 probably occurs sequentially following R1 if long pulse duration of Cp2Zr(CH3)2 precursor is applied in the ALD of ZrO2 films. 4. Conclusion The initial surface reaction mechanism of ALD-grown ZrO2 on the hydroxylated Si(100)-2 × 1 surface using Cp2Zr(CH3)2 as metal precursor has been investigated theoretically. All possible reaction pathways were proposed according to either CH3 or Cp as the starting reactive group when it reacts with the Si―OH* surface site. Calculations show that the CH4 elimination reaction on the Si―OH* surface site has more favorable thermodynamics during the Cp2Zr (CH3)2 precursor pulse. This result explains why almost all (90%) of the CH3 ligands were released during the Cp2Zr(CH3)2 precursor pulse in the actual ALD experiments. In addition, the experiment also showed that about 40% of the CpH was released during the metal precursor pulse. By comparison of the activation barriers of every reaction, we could argue that the CpH elimination reaction probably occurs sequentially following the first CH4 elimination reaction if
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long pulse duration of Cp2Zr(CH3)2 precursor is applied in the ALD of ZrO2 films. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20973052), the Natural Science Foundations of Hebei Province (No. B2010000845) and the Hebei University of Science and Technology (No. XL200910). References [1] D.A. Muller, T. Sorsch, S. Moccio, F.H. Baumann, K. Evans-Lutterodt, G. Timp, Nature (London) 399 (1999) 758. [2] P.A. Packan, Science 285 (1999) 2079. [3] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243. [4] E.P. Gusev, C. Cabral, M. Copel, C. D'Emic, M. Gribelyuk, Microelectron. Eng. 69 (2003) 145. [5] R.M. Wallace, G.D. Wilk, Crit. Rev. Solid State Mater. Sci. 28 (2003) 231. [6] J. Robertson, Rep. Prog. Phys. 69 (2006) 327. [7] M. Ritala, M. Leskelä, in: H.S. Nalwa (Ed.), Handbook of Thin Film Materials, Vol. 1, Academic Press, San Diego, 2002, p. 103. [8] M. Leskelä, M. Ritala, Angew. Chem. Int. Ed. 42 (2003) 5548. [9] L. Niinistö, J. Päiväsaari, J. Niinistö, M. Putkonen, M. Nieminen, Phys. Status Solidi A 201 (2004) 1443. [10] S.-J. Won, J.-Y. Kim, G.-J. Choi, J. Heo, C.S. Hwang, H.J. Kim, Chem. Mater. 21 (2009) 4374. [11] J.-H. Kim, V. Ignatova, P. Kücher, J. Heitmann, L. Oberbeck, U. Schröder, Thin Solid Films 516 (2008) 8333. [12] S.J. Yun, J.W. Lim, J.-H. Lee, Electrochem. Solid-State Lett. 7 (2004) F81. [13] J. Niinistö, K. Kukli, M. Kariniemi, M. Ritala, M. Leskelä, N. Blasco, A. Pinchart, C. Lachaud, N. Laaroussi, Z.Y. Wang, C. Dussarrat, J. Mater. Chem. 18 (2008) 5243. [14] W.-H. Nam, S.-W. Rhee, Chem. Vap. Deposition 10 (2004) 201. [15] C.L. Dezelah, J. Niinistö, K. Kukli, F. Munnik, J. Lu, M. Ritala, M. Leskelä, L. Niinistö, Chem. Vap. Deposition 14 (2008) 358. [16] K. Knapas, M. Ritala, Chem. Mater. 20 (2008) 5698. [17] J.W. Elama, M.J. Pellin, S.D. Elliott, A. Zydor, M.C. Faia, J.T. Hupp, Appl. Phys. Lett. 91 (2007) 253123. [18] K. Kukli, J. Niinistö, A. Tamm, J. Lu, M. Ritala, M. Leskelä, M. Putkonen, L. Niinistö, F. Song, P. Williams, P.N. Heys, Microelectron. Eng. 84 (2007) 2010. [19] M.G. Jeffrey, A.C. Jones, K. Black, P.R. Chalker, T. Leese, A. Kingsley, R. Odedra, P.N. Heys, Surf. Coat. Technol. 201 (2007) 9095. [20] J. Niinistö, K. Kukli, A. Tamm, M. Putkonen, C.L. Dezelah, L. Niinistö, J. Lu, F.Q. Song, P. Williams, P.N. Heys, M. Ritala, M. Leskelä, J. Mater. Chem. 18 (2008) 3385. [21] J. Niinistö, A. Rahtu, M. Putkonen, M. Ritala, M. Leskelä, L. Niinistö, Langmuir 21 (2005) 7321. [22] M. Putkonen, J. Niinistö, K. Kukli, T. Sajavaara, M. Karppinen, H. Yamauchi, L. Niinistö, Chem. Vap. Deposition 9 (2003) 207. [23] L. Jeloaica, A. Estève, M. Djafari Rouhani, D. Estève, Appl. Phys. Lett. 83 (2003) 542. [24] Y. Widjaja, J.H. Han, C.B. Musgrave, J. Phys. Chem. B 107 (2003) 9319. [25] J.H. Han, G. Gao, Y. Widjaja, E. Garfunkel, C.B. Musgrave, Surf. Sci. 550 (2004) 199. [26] A. Dkhissi, A. Estève, C. Mastail, S. Olivier, G. Mazaleyrat, L. Jeloaica, M. Djafari Rouhani, J. Chem. Theory Comput. 4 (2008) 1915. [27] M. Nolan, S.D. Elliott, Chem. Mater. 22 (2010) 117. [28] J. Ren, G. Zhou, Y. Hu, D.W. Zhang, Appl. Surf. Sci. 255 (2009) 7136. [29] H.-L. Lu, S.-J. Ding, D.W. Zhang, J. Phys. Chem. A 113 (2009) 8791. [30] A.D. Becke, Phys. Rev. A 38 (1988) 3098. [31] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [32] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [33] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Gaussian Inc., Pittsburgh PA, 2003. [34] J. Ren, F.Y. Liu, Y.T. Zhang, D.W. Zhang, Thin Solid Films 515 (2007) 4702. [35] J. Ren, B. Sun, D.W. Zhang, Appl. Surf. Sci. 253 (2007) 9148. [36] J. Ren, H.L. Lu, W. Chen, M. Xu, D.W. Zhang, Appl. Surf. Sci. 252 (2006) 8466. [37] H. Jeon, Y. Won, Appl. Phys. Lett. 93 (2008) 124104. [38] W.E. Hunter, D.C. Hrncir, R.V. Bynum, R.A. Penttila, J.L. Atwood, Organometallics 2 (1983) 750. [39] A. Zydor, S.D. Elliott, J. Phys. Chem. A 114 (2010) 1879.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
A theoretical study on initial growth mechanism of ZrO2 film using cyclopentadienyl-type precursor Jie Ren a,⁎, Chengxing Cui b, Guangfen Zhou a, Yanchun Liu a, Yongqi Hu a,⁎, Baozhu Wang a a b
Colleges of Science and Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang, Henan 453003, China
a r t i c l e
i n f o
Article history: Received 26 February 2010 Received in revised form 12 January 2011 Accepted 21 January 2011 Available online 28 January 2011 Keywords: Density functional theory Dielectrics Zirconia Atomic layer deposition
a b s t r a c t The initial growth reaction of atomic layer deposition (ALD) of ZrO2 using Cp2Zr(CH3)2 (Cp_C5H5, cyclopentadienyl) as metal precursor on the hydroxylated silicon surface is investigated by using density functional theory (DFT). The ALD cycle is achieved through two types of ligand elimination reactions (i.e., CH4 and CpH elimination reactions). The possible reaction pathways are proposed in order to find the dominant initial reaction during the Cp2Zr(CH3)2 precursor pulse. DFT calculations show that the CH4 elimination reaction is energetically more favorable than CpH elimination reaction. As a result, the two CH3 ligands of Cp2Zr(CH3)2 may be dissociated prior to the two Cp rings during the metal precursor pulse. In addition, one CpH elimination may occurs sequentially following the first CH4 elimination reaction according to activation barrier analysis during the Cp2Zr(CH3)2 pulse. All the calculated results are in agreement with the experimental findings. © 2011 Elsevier B.V. All rights reserved.
1. Introduction With the continuous downscaling of metal oxide semiconductor (MOS) devices, silicon dioxide (SiO2) gate dielectric will reach its physical limits due to gate leakage and reliability issues [1,2]. Currently, high dielectric constant (high-κ) materials have been introduced in MOS devices to replace the traditional SiO2 (κ = 3.9) as gate dielectric materials, since their higher capacitances with thicker films can suppress leakage currents largely [3,4]. Among these high-κ candidates, ZrO2 is a promising material for applications due to its high permittivity (κ = 17–22), wide band gap, good thermal stability and chemical compatibility with silicon [5,6]. By far high-κ oxide films have been extensively fabricated by atomic layer deposition (ALD) technique. Due to the characteristics of self-limiting surface reactions, ALD can offer a lot of practical advantages, such as excellent conformity, accurate thickness control down to the nanometer level and uniform film properties over a large area [7,8]. ZrO2 thin films have been grown by ALD most commonly using ZrCl4 and H2O as precursor combination [9]. Unfortunately, the ZrCl4/H2O process has some distinct drawbacks, such as contamination of chlorine and release of corrosive HCl as the by-product. Therefore alternative precursors are being sought. So far, some chlorine-free precursors have been employed so as to improve the dielectric quality of ALD ZrO2 (Table 1) [9]. Especially, experiments have recently shown that the organometallic cyclopentadienyl (Cp and C5H5) compounds, e.g. Cp2Zr(CH3)2, together with H2O as an ⁎ Corresponding authors. Tel./fax: +86 311 8166 8535. E-mail addresses: [email protected] (J. Ren), [email protected] (Y. Hu). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.278
oxygen source can yield stoichiometric ZrO2 films with extremely low impurity contents (below 0.1 atom% for C and H) and promising electrical properties [21,22]. As we know that the understanding of reaction mechanism will help to optimize the process condition parameters and control the interface formed between the deposited films and the silicon substrate. Usually, the experimental investigation of the ALD mechanism for high-κ oxide can be achieved by capturing the intermediate and final products. At present, density functional theory (DFT) has already been considered as an effective theoretical method to explore the ALD mechanism. For example, the initial ALD-growth mechanisms of zirconia and hafnia, and the chemical kinetic and thermodynamic data, have always been given by DFT calculations [23–26]. These studies do provide insights into the ALD growth mechanism of high-κ oxides. However, most of these cases focus on the ALD process of zirconia and hafnia in which metal chloride is used as metal precursors, and few theoretical investigates have been made to explore the ALD growth mechanism of these high-κ oxides using the cyclopentadienyl-type precursor [27–29]. Experiments [21] show that the ALD ZrO2 using Cp2Zr(CH3)2/H2O combination is the self-limiting growth from 210 to 400 °C. The precursor pulse duration has no obvious effect on the total amount of released CH4 or CpH. Based on the growth mode of ALD type, we think that the ALD cycle is achieved through two types of ligand elimination reactions of Cp2Zr(CH3)2, i.e. CH4 and CpH elimination reactions. Therefore, the possible reactions are proposed (Scheme 1) in order to find the dominant initial reaction during the Cp2Zr(CH3)2 precursor pulse. Reaction (R1) belongs to CH4 elimination reaction, in which one CH3 group of Cp2Zr(CH3)2 firstly reacts with the surface
J. Ren et al. / Thin Solid Films 519 (2011) 3716–3721 Table 1 Recently published ALD processes for the ZrO2 thin films and the studied deposition temperature range. Metal precursor
Oxygen source
Dep. temp./°C
Ref.
Zr(NMeEt)4 Zr(NMeEt)4 Zr(NMeEt)4 (RCp)Zr(NMe2)3 (R_H, Me or Et) ZrCl2[N(SiMe3)2]2 (MeCp)2ZrMe2 or (MeCp)2ZrMe(OMe) (MeCp)2ZrMe(OMe) (MeCp)2ZrMe(OMe) (MeCp)2ZrMe2 or (MeCp)2ZrMe(OMe) (MeCp)2ZrMe(OBut) or (MeCp)2ZrMe(OMe) (MeCp)2ZrMe2 or (MeCp)2ZrMe(OMe) Cp2ZrMe2
N2O or O2 Plasma O3 O2 O3 H2O H2O D2O or O3 H2O O3 H2O
240–320 225–275 110–250 300 150–350 300–500 350 350 300 or 350 300–350
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
O3 D2O
250–500 210–440
[20] [21]
hydroxyl, and R2 belongs to CpH elimination reaction, where one Cp group of Cp2Zr(CH3)2 firstly reacts with the surface hydroxyl. Similarly, the following reactions (R3 vs R4and R5 vs R6) are also designed according to CH4 and CpH as the elimination products, respectively. 2. Computational method and models Elam et al. [17] once used hydrolysis energy calculations to predict the order of ligand loss of cyclopentadienyl-type precursor in the reactions of ALD-grown ZrO2. Instead, we employed the calculations of potential energy surface for each reaction to justify the experimental findings in this work. The hybrid DFT approach utilizing Becke's threeparameter exchange functional [30,31] with Lee–Yang–Parr correlation functional [32] (B3LYP) was used to optimize the geometries, analyze
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the frequencies and obtain the reaction energies of each stationary point. We optimized all the structures and the geometrical parameters of reactants, transition states (TSs) and products using the Berny optimization algorithm in GAUSSIAN 03 software package [33], using analytic first and second derivatives of the energy at every step. A stationary point found by a geometry optimization is a minimum (local or global following its stability) when all the vibrational frequencies are real. In contrast, it is a TS linking two minima when there is an imaginary frequency. A mixed basis set scheme was used to save computational resource. In particular, the LANL2DZ basis set and effective core potential were employed to describe zirconium atom, 6–31 G(d,p) basis set for chemical active atoms (i.e., the hydroxylated surface silicon atoms, hydroxyls, Cp and CH3 ligands), and 6–31 G for the remnant silicon and the terminated hydrogen atoms. The B3LYP method and mixed basis set scheme have shown relatively good applications in identifying the growth mechanisms and energetics for the ALD of high-κ oxides, such as HfO2 and ZrO2 [25,34–37]. Previously, to explore the surface reaction mechanism on the hydroxylated Si(100)-2 × 1 surface, Si9H12 one-dimer cluster was frequently used to simulate Si(100)-2 × 1 [23,24,34–36]. In the current study, a larger hydroxylated three-dimer model Si21H24\(OH)2 cluster model (Fig. 1a) was employed to diminish the cluster size effect. The cluster approximation method was based on the predominantly localized bonding of the Si(100)-2 × 1 surface. The Si21H24―(OH)2 three-dimer cluster consisted of four layer silicon atoms where the top six silicon atoms composed the surface dimers. The remaining silicon atoms composed three subsurface layers that were terminated by hydrogen atoms to prevent unrealistic charge transfer. The hydroxyls were used to react with the incoming Cp2Zr (CH3)2 precursor as surface sites. The surface hydroxyls used here indicated that the initial silicon surface had been pretreated by H2O firstly. Moreover, only two reactive hydroxyls were kept to avoid the other neighboring hydroxyls intruding on the title surface reactions
Scheme 1. Surface reactions (R1–R6) of Cp2Zr(CH3)2 on the hydroxylated silicon surface.
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J. Ren et al. / Thin Solid Films 519 (2011) 3716–3721
Fig. 2. Reaction pathway and predicted energetics at 0 K for the adsorption and decomposition of Cp2Zr(CH3)2 on Si21H24―(OH)2 surface sites. The zero of energy is the sum of total energies of a precursor and a hydroxylated silicon cluster. TS1 and TS2 are the abbreviation for the first and second transition states, respectively, IP is for intermediate product, PS1 and PS2 are for the first and second physisorbed state, respectively, and FP is for final product.
Fig. 1. (a) The hydroxylated Si(100)-2 × 1 cluster modeled by Si21H24―(OH)2 threedimer cluster. The red, grey and small white balls represent oxygen, silicon and hydrogen atoms, respectively. (b) Cp2Zr(CH3)2 compound.
which could make them more complicated. The structures of these cluster models were fully optimized in all calculations. The optimized structure of the precursor Cp2Zr(CH3)2 is shown in Fig. 1b. Cp2Zr (CH3)2 contains two η5-Cp and two CH3 ligands. The Zr―CH3 bond lengths are both 2.29 Å and CH3―Zr―CH3 angle is 98.9°. The distances from zirconium ion to the centers of Cp rings (Zr―Cp) are 2.29 and 2.27 Å, respectively. Individual Zr―C(π) distances to the Cp rings are in the range of 2.56 to 2.60 Å. The Cp―Zr―Cp angle is 134.0°. All above calculated values are in close agreement with the crystal structure data [38]. 3. Results and discussion 3.1. Comparison between the reaction pathways of R1 and R2 In the precursor Cp2Zr(CH3)2, both the active groups (i.e., CH3 and Cp) can possibly react with the Si―OH* (* denotes the active site) surface sites firstly. Thus there are two possible initial reaction pathways (R1 and R2), which are shown in Fig. 2. In R1, the surface Si―OH* group transfers its hydrogen atom to one CH3 group of Cp2Zr (CH3) 2 , resulting in the elimination of CH4 and forming an intermediate product (IP). The reaction profile at 0 K and the corresponding geometries along the reaction pathway for R1 are shown in Figs. 2 and 3, respectively. As the starting structure of R1, the reactive CH3 of the precursor is placed above the OH-terminated silicon surface (Fig. 3a). After full relaxation, a physisorbed state (PS1 in Fig. 2) is obtained with a calculated adsorbed energy of 23.7 kJ/mol lower than the separated reactants. Unlike Y(Cp)3 and Mg(Cp)2 adsorbed complexes [28,29], the Cp2Zr(CH3)2 adsorbed complex is not formed through the surface hydroxyl coordinating to the metal center. As we can see in Fig. 3a, the zirconium atom cannot approach the hydroxyl oxygen atom easily due to steric repulsion. As a result,
the adsorbed energy of R1 is lower than those obtained from the adsorption of Y(Cp)3 and Mg(Cp)2 on the Si―OH* surface (36.2 and 37.7 kJ/mol, respectively) [28,29]. Moreover, the geometry of adsorbed Cp2Zr(CH3)2 has not been obviously changed, where the CH3―Zr―CH3 angle is 98.8° and Zr―CH3 bond lengths are 2.30 and 2.28 Å, respectively (Table 2). After the adsorption, one CH3 group of the adsorbed Cp2Zr(CH3)2 reacts with the H atom of Si―OH* surface and forms a four-member-ring TS composed of H―O―Zr―CH3(1) (Fig. 3b), whose energy is 47.3 kJ/mol higher than the PS1 (see Fig. 2). For the TS, Zr―CH3(1) bond length increases to 2.59 Å and CH3―Zr―CH3 angle increases to 136.0°. At the same time, Zr―O bond is forming with a calculated bond length of 2.42 Å. Over the activation barrier for hydrogen transfer, the reaction reaches its IP and releases one by-product CH4 molecule. The overall reaction is exothermic by 213.9 kJ/mol, indicating that the CH4 elimination reaction through hydrogen transfer of Si―OH* to Cp2Zr(CH3)2 is energetically favorable. As we know that weak adsorption can result in non-ALD decomposition of the precursor above the surface. Experiment has shown that non-ALD decomposition occurs at temperature exceeding 400 °C[21]. Therefore, the entropy change at 298 K (Δ S 298) is calculated in order to find whether the desorption of the adsorbed Cp2Zr(CH3)2 (PS1 in Fig. 2) in R1 is thermodynamically favorable or not. T Δ S 298 of the desorption products of the surface bound Cp2Zr (CH3)2 is 32.9 kJ/mol, 9.2 kJ/mol larger than the adsorption energy. This indicates that the desorption of the adsorbed Cp2Zr(CH3)2 is favored in the thermodynamic equilibrium state. However, recent theoretical studies showed that the activation energy of first CH4 nonALD decomposition of Cp2Zr(CH3)2 in the gas phase was 147 kJ/mol [39], far higher than the energy (47.3 kJ/mol) obtained from the ALD growth (R1) in this study. Therefore, we think that R1 is easier to occur for the ALD growth before the thermodynamic equilibrium is reached. In the case of R2, one Cp group of Cp2Zr(CH3)2 abstracts the H atom from the surface Si―OH* group, resulting in the elimination of CpH from the surface and the forming of the IP state. Similar to R1, the starting adsorbed structure is modeled by placing the Cp groups of Cp2Zr(CH3)2 above the Si―OH* surface site in R2 (cf. Fig. 4a). After full geometry optimization, the energy gain upon adsorption of Cp2Zr (CH3)2 is 18.5 kJ/mol compared to 23.7 kJ/mol for that obtained in R1,
J. Ren et al. / Thin Solid Films 519 (2011) 3716–3721
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Fig. 3. Geometrical structures along the reaction pathway of R1, R3 and R4: (a) PS1, (b) TS1, (c) IP, (d, d′) TS2 and (e, e′) FP, where (d) and (e) are for R3 and (d′) and (e′) are for R4, respectively. The four-center TS is denoted by blue dashed line.
which suggests that there exists stronger affinity of CH3 for Si―OH* surface site. Similar to R1, the geometry of the adsorbed Cp2Zr(CH3)2 is also hardly changed, in which the Zr―Cp distances are both 2.28 Å
and the Cp(1)―Zr―Cp(2) angle is 132.9° (Table 2). After the adsorption, the reaction goes through an activation barrier with an energy value of 127.1 kJ/mol, which is substantially higher than that
Table 2 Representative bond distances (Å) and bond angles (°) in the selected state of reactions (R1–R6). State (reactions a)
PS(R1)
TS(R1)
IP(R1)
PS(R2)
TS(R2)
IP(R2)
Zr―CH3(1) Zr―CH3(2) Zr―O(1) Zr―O(2) Zr―Cp(1) b Zr―Cp(2) b CH3(1)―Zr―CH3(2) Cp(1)―Zr―Cp(2) b
2.30 2.28 4.99 5.49 2.29 2.29 98.8 133.9
2.59 2.31 2.42 4.74 2.28 2.30 136.0 129.4
–
2.29 2.29 4.24 5.19 2.28 2.28 98.1 132.9
2.28 2.25 2.26 4.89 3.09 2.27 83.3 115.3
2.25 2.25 1.99 4.18
a
2.29 2.02 4.85 2.28 2.28 – 129.9
– 2.28 102.8 –
TS(R3)
FP(R3)
TS(R4)
FP(R4, R5)
TS(R5)
–
– –
–
– 2.25 2.00 2.00 – 2.29 – –
2.50 2.25 2.01 2.31 – 2.27 82.1 –
2.64 2.11 2.32 2.29 2.32 – 125.5
2.05 2.05 2.29 2.29 – 123.9
2.25 2.04 2.17 3.12 2.29 – 113.6
TS(R6)
PS(R6)
FP(R6)
2.24 2.24 1.99 2.18
2.25 2.25 1.98 2.02
2.24 2.24 1.99 1.99
–
–
2.97 106.6 –
3.62 113.1 –
– – 109.2 –
The sequence numbers of reactions are defined in Scheme 1. Strictly speaking, the Cp ring is neither planar nor regular pentagon, so the center of Cp ring cannot be located accurately. The Zr―Cp and Cp(1)―Zr―Cp(2) is therefore only a very approximate value. b
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Fig. 4. Geometrical structures along the reaction pathway of R2, R5 and R6: (a) PS1, (b) TS1, (c) IP, (d, d′) TS2, (e′) PS2 and (e, f′) FP, where (d) and (e) are for R5 and (d′), (e′) and (f′) are for R6, respectively. The four-center TS is denoted by blue dashed line.
of R1. Therefore, the dissociating of the Zr―Cp(1) bond is more difficult for R2 than that of Zr―CH3(1) for R1 by comparing the energy barriers of R1 and R2. In addition, we can find in Table 2 that the Zr―Cp(1) distance is elongated to 3.09 Å and Cp(1)―Zr―Cp(2) angle is decreased to 115.3° in the TS. Finally, R2 finishes along with the release of one CpH gas molecule. The formation of Zr―O bond is also completed with a calculated bond length of 1.99 Å. The overall reaction is exothermic by 58.3 kJ/mol for R2, far lower than that of R1. In conclusion, the CH4 elimination reaction of Cp2Zr(CH3)2 (R1) is energetically more favorable than the CpH elimination reaction (R2) on the hydroxylated silicon surface. 3.2. Comparison between the reaction pathways of R3 and R4 After elimination of the first CH4 in R1, the formed Cp2ZrCH3 fragment can react with the neighboring Si―OH* site. We assume that the reaction would branch according to either CH3 or Cp as the starting reactive group. One branch is CH4 elimination reaction (i.e., R3 in Scheme 1). The other is the CpH elimination reaction (i.e., R4 in Scheme 1). The reaction pathways and the corresponding energies at the stationary points for R3 and R4 are shown in Fig. 2. The corresponding geometries along the pathways of R3 and R4 are depicted
in Fig. 3. Starting from the IP state, R3 goes through a four-center TS composed of H―O―Zr―CH3(2) similar to R1 (Fig. 3d). The activation barrier (TS2 in Fig. 2) is 99.5 kJ/mol relative to the IP. The Zr―O(1) and Zr―O(2) bond distances of the second TS are 2.11 and 2.32 Å, respectively, and the Zr―CH3(2) bond length increases to 2.64 Å. Finally, R3 reaches the final product (FP) with a total exothermic enthalpy (ΔH0) of 318.8 kJ/mol relative to the initial reactants. As another possible reaction, R4 is also a proton transfer reaction where one Cp ring of Cp2ZrCH3 accepts the H atom from the neighboring Si―OH* and releases one CpH molecule. The activation barrier of R4 is calculated to be 105.0 kJ/mol, which is 5.5 kJ/mol slightly higher than that of R3. Also, calculations show that R4 has a decreasing exothermicity with 205.2 kJ/mol as compared to R3. Therefore, we can deduce that it is energetically less favorable for R4 than R3. 3.3. Comparison between the reaction pathways of R5 and R6 After elimination of the first CpH in R2, the adsorbed CpZr(CH3)2 fragment reacts with the neighboring hydroxyl on silicon surface, where the reaction network can branch similarly. One alternative is CH4 elimination reaction (i.e., R5 in Scheme 1). The other is the CpH elimination reaction (i.e., R6 in Scheme 1). The reaction pathways and
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energetics for R5 and R6 are shown in Fig. 2. The energy of the TS relative to the IP is calculated to be 39.4 kJ/mol for R5. It could be found that the reaction is exothermic and the enthalpy change is 205.2 kJ/mol. In addition, we can see in Fig. 2 that the reaction pathway of R6 is different from R5. The only difference is that one weak bound by-product CpH is found in R6. Calculation shows that R6 must overcome a 3.9 kJ/mol adsorption well (i.e., PS2 in Fig. 2) to reach the FP state. The additional adsorption energy possibly results from the Zr―C(π) interaction between zirconium ion and Cp(2) ring. The individual Zr―C(π) distances are calculated to be in the range of 3.24 to 4.40 Å. As a comparison, the stable physisorption of the byproduct CpH on the Si―OH* surface, however, is not found in R2 and R4. Therefore, it can be deduced that the physisorption in R6 is due to the relief of steric congestion around zirconium metal center. It should be emphasized that the physisorption is relatively weak. Thus we can suggest that it is easy to purge the by-product out of the reaction chamber, and it is not necessary to use a long purge time if R6 occurs in the actual ALD process. The activation barrier for the CpH formation in R6 is calculated to be 88.1 kJ/mol, which is much higher than that of R5. Moreover, R6 is exothermic by 34.3 kJ/mol, which is far lower than that of R5. Therefore, we can conclude that R5 is energetically more favorable than R6. 3.4. Comparison between the calculations and the experimental findings As we know that the ALD process of ZrO2 consists of alternating Cp2Zr(CH3)2 precursor and oxygen source pluses. In situ experiments [21] have detected that almost all (90%) of the CH3 ligands were released during the Cp2Zr(CH3)2 precursor pulse by using quadrupole mass spectrometer. By comparing the competitive surface reactions (R1–R6), we can argue that the CH4 elimination reaction on the Si―OH* surface site has more favorable thermodynamics than the CpH elimination reaction. In other words, both R1 and R3 are the energetically more favorable pathways, i.e., the dominant pathways. Theoretically, the two CH3 ligands of Cp2Zr(CH3)2 can be dissociated prior to the two Cp rings, which is in excellent agreement with the above-mentioned experimental findings. In addition, the experiment showed that about 40% of the CpH was released during the Cp2Zr(CH3)2 pulse [21], which indicates that there exists a CpH elimination reaction. By comparison of the above-mentioned competing reactions, we find that the activation barrier of R2 is 79.8 kJ/ mol higher than that of R1, however, in the case of R4, the energy barrier is only 5.5 kJ/mol slightly higher than that of R3 (see Fig. 2). We can therefore conclude that R4 probably occurs sequentially following R1 if long pulse duration of Cp2Zr(CH3)2 precursor is applied in the ALD of ZrO2 films. 4. Conclusion The initial surface reaction mechanism of ALD-grown ZrO2 on the hydroxylated Si(100)-2 × 1 surface using Cp2Zr(CH3)2 as metal precursor has been investigated theoretically. All possible reaction pathways were proposed according to either CH3 or Cp as the starting reactive group when it reacts with the Si―OH* surface site. Calculations show that the CH4 elimination reaction on the Si―OH* surface site has more favorable thermodynamics during the Cp2Zr (CH3)2 precursor pulse. This result explains why almost all (90%) of the CH3 ligands were released during the Cp2Zr(CH3)2 precursor pulse in the actual ALD experiments. In addition, the experiment also showed that about 40% of the CpH was released during the metal precursor pulse. By comparison of the activation barriers of every reaction, we could argue that the CpH elimination reaction probably occurs sequentially following the first CH4 elimination reaction if
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