On the Dissociative Chemisorption of Tris(dimethylamino) - American

May 11, 2009 - Chemicals, Inc., 1969 Palomar Oaks Way, Carlsbad, California 92011. ReceiVed: January 6, 2009; ReVised Manuscript ReceiVed: April 20, ...
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J. Phys. Chem. C 2009, 113, 9731–9736

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On the Dissociative Chemisorption of Tris(dimethylamino)silane on Hydroxylated SiO2(001) Surface Jiaye Li,† Jinping Wu,*,† Chenggang Zhou,† Bing Han,‡ Eugene J. Karwacki,‡ Manchao Xiao,§ Xinjian Lei,§ and Hansong Cheng*,‡ Engineering Research Center of Nano-Geomaterials of Ministry of Education and Institute of Theoretical Chemistry and Computational Materials Science, China UniVersity of Geosciences (Wuhan), Wuhan, China P. R. 43007, Air Products and Chemicals, Inc., 7201 Hamilton BouleVard, Allentown, PennsylVania 18195, and Air Products and Chemicals, Inc., 1969 Palomar Oaks Way, Carlsbad, California 92011 ReceiVed: January 6, 2009; ReVised Manuscript ReceiVed: April 20, 2009

We present a first-principles theoretical study on the dissociative chemisorption of tris(dimethylamino)silane (TDMAS) on a hydroxylated SiO2(001) surface. The thermochemical energies and activation barriers associated with the elementary surface reaction processes have been calculated. Our results indicate that sequential dissociation of TDMAS can occur only up to the second step with dimethylaminosilylenyl groups anchored on the surface. Further dissociation of these surface species is virtually energetically forbidden under typical atomic layer deposition processing conditions. This would result in an inherently low density and compositionally impure SiO2 film with a low deposition rate. I. Introduction Fabricating a gate structure with advanced dielectrics and sidewall spacers is one of the major technological challenges for front-end-of-line (FEOL) applications.1,2 Low-temperature deposition of silicon nitride provides a number of benefits for this type of structure. The deposited silicon dioxide and silicon nitride films are compatible with the metal silicide applications and are employed for sidewall spacers in reducing the junction leakage between the gate and the source/drain.3,4 In addition to uses as a mask material for controlling ion implantation profiles, these films are also utilized to strain the gate structure for enhancing device performance. Within memory manufacturing, dielectrics are investigated for potential new uses such as sacrificial films to assist in patterning, protection layers for other films, etching-stop layers, and diffusion barriers. These applications cannot all be addressed by just one composition of oxide or nitride; there are emerging needs for oxide- and nitride-based films with variable dielectric constants, varying wet/dry etching rates, and/or stress that is either compressive or tensile.5-8 Compositionally pure, highly conformal silicon oxide films are essential to ensure superior device performance. It has been widely accepted that, for the next generation of semiconductor devices, atomic layer deposition (ALD) is a preferred method for depositing silicon oxide films at temperatures lower than 400 °C.8 The conventional film fabrication process has been carried out by first site-selective deposition of silicon nitride precursors followed by surface oxidation to produce well-controlled SiO2 layers. However, the quality of the SiO2 films is largely dictated by the physicochemical properties of the precursors. Tris(dimethylamino)silane (TDMAS), as well as other alkylaminosilanes, has been used as precursor for SiO2 film deposition in an ALD process with ozone as the oxidation agent.9,10 These precursors first undergo * Corresponding authors. E-mail: [email protected] (J.W.), chengh@ airproducts.com (H.C.). † China University of Geosciences (Wuhan). ‡ Air Products and Chemicals, Inc., Allentown, PA. § Air Products and Chemicals, Inc., Carlsbad, CA.

sequential dissociative chemisorption on a hydroxylated SiO2 surface, resulting in anchoring of the Si atom of the precursors on the surface and release of amines to the gas phase. Ozone or oxygen plasma is then used to oxidize the surface intermediates to produce hydroxyl groups for subsequent deposition of the next layer. This process has been demonstrated for depositing SiO2 films on Si(001) surface, and it has been shown that dissociative chemisorption of TDMAS occurs preferentially on surface hydroxyl sites.9 TDMAS was found to be highly reactive with hydroxyl groups, even at room temperature, producing free amines and other surface species. In this paper, we present a first-principles study using density functional theory to model the dissociative chemisorption of TDMAS on a reconstructed and fully hydroxylated SiO2(001) surface. The main objective is to better understand the thermochemistry and kinetics involved in the surface reactions and to further examine whether the SiO2 film developed via an ALD process using TDMAS as the deposition precursor would exhibit the required conformity with low impurity. Obviously, dissociative chemisorption of TDMAS is the key step for growing the SiO2 films via ALD, since the oxidation step using ozone or oxygen plasma is energetically highly favorable.11 Therefore, we will not include the oxidation step in the present study. It is expected that a mechanistic understanding of the precursor deposition processes will enable us to design better materials for developing conformal, dense, and high-purity SiO2 films. II. Surface Model and Computational Method The reconstructed and fully hydroxylated SiO2(001) surface was modeled with a slab containing six layers alternating with two layers of O atoms and one layer of Si atoms, which is the preferred surface orientation for silica12 (Figure 1A). The top two layers of O atoms are all terminated by H atoms, representing the fully hydroxylated surface. There is a vacuum between adjacent slabs with 10.4 Å separation. The selected supercell contains 8 Si atoms, 20 O atoms, and 8 H atoms, in addition to a TDMAS molecule, with the total number of atoms being up to 65 atoms. Prior to TDMAS dissociative chemi-

10.1021/jp900119b CCC: $40.75  2009 American Chemical Society Published on Web 05/11/2009

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Figure 2. The calculated electron density map of the hydroxylated SiO2(001) surface.

Figure 1. (A and B) The side and top views of the hydroxylated SiO2 (001) surface: yellow balls, Si atoms; red balls, O atoms; white balls, H atoms; black square, the selected surface supercell. (C) Schematic view of the optimized hydroxylated SiO2(001) surface. O1 and O2 represent two types of surface hydroxyl groups.

TABLE 1: Selected Bond Parameters Compared with Reported Values bond parameters r(O-H)/Å r(Si-O)/A ∠(O-H · · · O) ∠(Si-O-H)

in this study

ref 12

EXPT12

0.98 1.60 1.66 166.9 175.8 112.0

0.98 1.64 1.66 165.8 172.0 113.0

1.60 1.61 -

sorption, the surface was first equilibrated. The optimized main structural parameters, shown in Table 1, are in good agreement with the experimental values and previous DFT calculations.12 All calculations were performed using density functional theory (DFT) with the exchange-correlation functional proposed by Perdew and Wang.13-15 The projector augmented wave (PAW) method was used to describe the core electrons of atoms, and the valence orbitals were represented with a plane wave basis set with a cutoff energy of 396 eV. Electronic energies were calculated with the SCF tolerance of 10-4 eV. Structural optimizations were performed for all atoms of the initial, intermediate, and final state structures, including the adsorbates but excluding the bottom three layers of the substrate, which were kept fixed throughout, until the total energy of the system was converged to less than 10-3 eV. Allowing the atoms of two more bottom layers to relax results in only marginal changes in the surface structure, with the differences in bond lengths and bond angles being less than 0.02 Å and 3°, respectively. Transition state structures were obtained using the nudged elastic band (NEB) method16,17 to calculate the minimum energy profile

along the prescribed dissociation pathways with the initial and final states chosen based on the optimized structures. The number of images was chosen to achieve smooth curves. The Brillouin zone integration was sampled within a 4 × 4 × 2 Monkhorst-Pack k point mesh,18 and electron smearing was employed using the Methfessel-Paxton technique19 with a width of 0.1 eV to minimize the errors in the Hellmann-Feynman forces. All calculations were performed with the Vienna ab initio simulation package (VASP).20 For analysis of TDMAS electrostatic properties, we also performed DFT calculations using the same exchange-correlation functional with a double numerical basis set augmented with polarization functions as implemented in the DMol3 package.21,22 III. Results and Discussion As shown in the optimized surface structure (Figure 1B,C), there are two types of hydroxyl groups.12 One (labeled as O1) is more exposed to the precursor with a short O-H distance of 0.980 Å, and another (labeled as O2) is slightly embedded in the top O layer with a slightly longer distance of 1.004 Å arising from the H-bonding with O1. The calculated electron density map shown in Figure 2 indicates a higher charge concentration on the O atoms at the O1 site. This makes the OH groups at O1 more accessible for electrophilic attack by TDMAS to form a Si-O-Si moiety with release of a dimethylamine molecule. The fully optimized structure of TDMAS is shown in Figure 3, where the electrostatic potential is also displayed. The positive charges on the molecule are concentrated on the Si atom (Mulliken charge, 1.336 e) as well as on the H atoms that form strong covalent bonds with C atoms. While these H atoms do not participate in the surface reaction, the Si atom should aim at the O atom of the hydroxyl group upon arrival at the surface. The H atom associated with the silicon atom is negatively charged but not involved in the dissociative chemisorption of TDMAS, since the electron-rich dimethylamine group (Mulliken charge on N: -0.157 e) is expected to be a better leaving group upon surface reaction to pick up an H atom from the O1-type hydroxyl group. We next performed minimum energy path calculations to examine the elementary steps of TDMAS surface reaction. The sequential dissociative chemisorption is broken into three steps, each of which leads to formation of a Si-O bond and release of a dimethylamine molecule. For step 1, the fully optimized structures of the initial state, transition state, and final state are shown in Figure 4. The main optimized structural parameters are shown in Table 2. Here, TDMAS diffuses to the SiO2 surface with the N atom from a dimethylamino group attacking the H

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Figure 3. (A) The optimized TDMAS structure. (B) The calculated TDMAS electrostatic potential.

Figure 4. Schematic partial dissociation of TDMAS to form a bis(dimethylamino)silylenyl fragment on SiO2(001): (i) the Si-N distance in TDMAS, (ii) the Si-Osurface distance, (iii) the Osurface-H distance, and (iv) the N-H distance.

TABLE 2: Main Optimized Bond Parameters of Step 1 distances

R1 (Å)

TS1 (Å)

P1 (Å)

(i) Si-N in TDMAS (ii) Si-Osurface (iii) Osurface-H (iv) N-H

1.791 3.410 1.058 1.604

2.851 2.012 2.172 1.026

3.429 1.739 2.649 1.022

atom of the O1 type OH group, while the Si atom aims at the O1 atom of the same OH group. This reaction undergoes a transition involving a loose Si-N bond from TDMAS and a significantly weakened O-H bond on the surface. The calculated Si-O distance is 2.012 Å at the transition state, significantly longer than its equilibrium distance of 1.739 Å at the final state. The reaction is highly concerted with Si-N and O-H, forming a four-membered ring, leading to partial dissociation of TDMAS. As a consequence, a bis(dimethylamino)silylenyl species is adsorbed on the SiO2(001) surface and a dimethylamine molecule is released into the gas phase. The reaction gives rise to a strong Si-O bond with a bond distance of 1.739 Å. The adsorbate then reorients itself vertically with an O2-type OH group on each side, ready to proceed to the next reaction step. The dissociative chemisorption gives rise to only slight surface relaxation with the Si-O1 bond distance stretched by only approximately 0.03 Å, but the neighboring Si-O2 bond length hardly changed. The only notable change of the surface is the reorientation of the hydroxyl group near the reaction site to form an OH · · · N hydrogen bond with dimethylamine with a distance of 1.604 Å. Two possible attacking sites for further dissociation of the surface species then emerge for the second step of TDMAS dissociative chemisorption. First, one side of the bis(dimethylamine)silylenyl fragment can react with the OH group associated with the same Si atom, and the adsorbate undergoes a similar reaction pathway (step 2a), as shown in Figure 5a, where the optimized structures of the initial, final, and transition states are displayed. This bonding configuration represents a deviation

from the (001) orientation of the SiO2 film, since the (001) crystalline surface requires that the O atoms that form bonds with a Si atom come from closest neighboring Si atoms. As a consequence, this type of bonding network likely leads to growth of amorphous SiO2 films. The calculated main structural parameters are shown in Table 3. The reaction along this route results in losing another dimethylamine molecule to the gas phase and anchoring of the dimethylaminosilylenyl fragment on the surface. Compared with the transition state (TS1) structure in step 1, this reaction undergoes a six-membered ring transition state structure. The Si-O distance becomes slightly shorter, but the N-H distance is slightly longer. This is due to a reduction of the steric effect in step 1 that prevents the Si atom from getting too close to the surface hydroxyl group. The structure of the final product (P2a), in which the surface SiO2 forms a four-membered ring with the Si atom of TDMAS, exhibits strong geometric strain with a short Si-Si distance of 2.444 Å. The Si-O-Si and O-Si-O angles are 92.4° and 85.5°, respectively. These bond angles deviate significantly from the typical angle of 109° expected for sp3 hybridization, indicating that thermochemically the structure might not be stable. Again, only slight surface relaxation occurs upon the second step surface reaction. Alternatively, the bis(dimethylamine)silylenyl fragment on the surface may also undergo a second reaction pathway (step 2b) with another dimethylaminosilylenyl group attacking the O atom of the OH group associated with the adjacent Si atom, as shown in Figure 5b. The calculated main structural parameters are also shown in Table 3. The optimized transition state structure (TS2b) exhibits clear separation between the dimethylamine and dimethylaminosilylenyl group with the Si-N distance of 2.778 Å, arising from the difficulty in extracting the H atom of the OH group located far away from the adsorption site. The structure of the product (P2b) appears to be significantly relaxed, with the Si atom of the dimethylaminosi-

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Figure 5. Formation of surface dimethylaminosilylenyl species. The main optimized bond parameters are listed in the tables: (i) the Si-N distance in TDMAS, (ii) the Si-Osurface distance, (iii) the Osurface-H distance, and (iv) the N-H distance.

TABLE 3: Main Optimized Bond Parameters of Step 2 Step 2a distances (i) Si-N in TDMAS (ii) Si-Osurface (iii) Osurface-H (iv) N-H

R2 (Å)

TSa2 (Å)

Pa2 (Å)

1.785 3.261 1.044 1.626

2.113 3.142 2.159 1.067

3.964 1.716 3.183 1.025

Step 2b distances

R2 (Å)

TSb2 (Å)

Pb2 (Å)

(i) Si-N in TDMAS (ii) Si-Osurface (iii) Osurface-H (iv) N-H

1.781 3.329 1.046 1.568

2.778 1.976 2.107 1.023

3.626 1.642 2.467 1.021

lylenyl group anchored by two adjacent O atoms. This bonding arrangement would allow the SiO2 film to continue growing along the (001) orientation. Finally, we continued examining the dissociation pathways of the anchored dimethylaminosilylenyl group on the surface (step 3) to complete the sequential dissociative chemisorption that gives rise to complete desorption of dimethylamine molecules and anchoring of a SiH species on the surface. The calculated structures of the initio, final, and transition states are shown in Figure 6 with the optimized main bond parameters shown in Table 4. There are two possible reaction pathways. One is for the Si atom chelated by two O atoms from the same Si atom to shift horizontally toward the O atom of the nearest OH group (step 3a). The move inherently results in severe stress in the adsorption structure. Indeed, we found that the transition state as well as the final product structures are highly strained. The dimethylaminosilylenyl group has to shift over approximately 3.5 Å to pick up the nearest surface H atom to desorb from the surface. The Si-H group is then locked by three O atoms. Again, surface reaction along path a likely promotes growth of amorphous films. The second pathway is for the dimethylaminosilylenyl group anchored by two O atoms from two neighboring Si atoms to shift to the nearest OH group with

the initial Si-O distance to be as far apart as nearly 5 Å to release the last dimethylamine molecule and to form the third Si-O bond with the surface (step 3b). Once again, the shift leads to significant structural stress. The calculated reaction energy diagram of the elementary steps of the sequential dissociative chemisorption process for TDMAS is shown in Figure 7. For step 1, it is moderately endothermic with the calculated thermochemical energy of 10.0 kcal/mol. The calculated activation barrier of 19.4 kcal/mol is also relatively high, indicating that the dissociative chemisorption is inherently a high-temperature process. In step 2, the reaction path a is kinetically more favorable than in step 1, with a barrier height of 14.1 kcal/mol. Thermochemically, it is slightly endothermic (5.0 kcal/mol) due to the structural strains. Path b is kinetically more favorable than path a with an activation barrier of 9.8 kcal/mol and becomes thermochemically exothermic (-9.7 kcal/mol). The smaller barrier and more favorable thermochemical energy in path b arises from a closer Si-O distance (ii) at the transition state and from the significantly relaxed product structure,since the two O atoms that form covalent bonds with the Si atom of the precursor bind with the two neighboring surface Si atoms, which significantly relaxes the geometric strains compared with the transition state and product structures in Step 2a (Figure 5). The results indicate that TDMAS can readily lose the second dimethylaminosilylenyl group along the reaction path b upon successful deposition in sep 1. Finally, our results show that the path a of step 3 is both kinetically and thermochemically extremely difficult due to the enormous stress of the transition state and final adsorption structures. The calculated thermochemical energy of 21.0 kcal/ mol is significantly endothermic with a huge activation barrier of 94.1 kcal/mol, indicating that the reaction would virtually be forbidden under typical ALD process conditions. The reaction path b in step 3 is also energetically difficult with extremely unfavorable thermodynamics of 58.2 kcal/mol. The significantly higher thermochemical energy reflects the fact that the reactant

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Figure 6. Dissociative chemisorption of dimethylaminosilylenyl on hydroxylated SiO2(001) surface: (i) the Si-N distance in TDMAS, (ii) the Si-Osurface distance, (iii) the Osurface-H distance, and (iv) the N-H distance.

TABLE 4: Main Optimized Bond Parameters of Step 3 Step 3a distances

Ra3 (Å)

TSa3(Å)

Pa3 (Å)

(i) Si-N in TDMAS (ii) Si-Osurface (iii) Osurface-H (iv) N-H

1.683 4.511 0.972 3.876

4.924 3.580 1.907 1.027

7.259 1.676 3.363 1.023

Step 3b distances

Rb3 (Å)

TSb3(Å)

Pb3 (Å)

(i) Si-N in TDMAS (ii) Si-Osurface (iii) Osurface-H (iv) N-H

1.702 4.994 0.971 3.670

3.482 3.329 2.234 1.025

3.927 1.718 2.862 1.020

of path b is considerably more stable than that of path a by nearly 15 kcal/mol. The calculated activation barrier of 71.3 kcal/mol is also substantial. The results again indicate that the reaction is essentially energetically forbidden. Our computational results suggest that sequential dissociative chemisorption of TDMAS on hydroxylated SiO2 surfaces can only occur up to the second step at elevated temperatures and the final chemisorption species is likely a dimethylaminosilylenyl species on surfaces. These findings are consistent with the previous experimental study by Kimoshita et al., which reported that the dissociative chemisorption of TDMAS occurs in the form of dimethylaminosilylenyl group adsorbed on the substrate surface.23 IV. Summary We have performed extensive DFT calculations on the elementary processes of sequential dissociative chemisorption of TDMAS on the hydroxylated SiO2(001) surface. This is the key step for growing SiO2 thin film via an ALD process, since the subsequent oxidation to form a SiO2 film is energetically facile. Complete dissociative chemisorption of precursors is

Figure 7. Energy diagram of the elementary processes of TDMAS dissociative chemisorption on hydroxylated SiO2(001) surface. Both steps 2 and 3 include two elementary processes with the associated transition states and products labeled as TSa2, TSb2, TSa3, TSb3 and Pa2, Pb2, Pa3, Pb3, respectively. Note that Ra3 and Rb3 correspond to different initial states for step 3, with the structures obtained from Pa2 and Pb2 of step 2 excluding a dimethylamine molecule. To facilitate the energy comparison, the energy levels of R2, Ra3, and Rb3 were shifted to 0 from their respective energy levels of the products P1, Pa2, and Pb2 upon excluding the leaving dimethylamine molecules.

essential to grow SiO2 films in a well-controlled fashion to ensure the film with minimum impurity. Several likely dissociative chemisorption pathways were explored sequentially. It was found that the paths with the Si atom of the precursor linked with two O atoms from the same surface Si atom likely lead to growth of amorphous SiO2 films. Our results indicate that the sequential dissociative chemisorption of TDMAS occurs only up to the second step with dimethylaminosilylenyl fragment anchored on the surface. Further dissociation of the species is virtually energetically forbidden at typical ALD process conditions. These conclusions are in good agreement with experimental observations. We believe that the presence of these residual dimethylaminosi-

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lylenyl groups on the surface could present a serious issue for device performance, since oxidation of these fragments would inevitably give rise to surface impurity, such as carbon or nitrogen contamination. In addition, the bulky size of the TDMAS precursor residues presents severe steric hindrance upon surface deposition and thus prevents dense packing during the deposition process. This could lead to low density films. Acknowledgment. We gratefully acknowledge Dr. T. P. M. Goumans for sharing his optimized SiO2(001) and hydroxylated SiO2(001) surface models with us. The work is supported by the National Natural Science Foundation of China (No. 20873127) and by Air Products and Chemicals, Inc. References and Notes (1) Wong, B. P.; Mittal, A.; Cao, Y.; Starr, G. CMOS Device and Process Technology. In Nano-CMOS Circuit and Physical Design; Wiley: New York, 2005; p 24. (2) Ritala, M.; Kukli, K.; Rahtu, A.; Raisanen, P. I.; Leskela, M.; Sajavaara, T.; Keinonen, J. Science 2000, 288, 319. (3) Shekhar, P.; Ming-Ren, L.; Qi, X. (Advanced Micro Devices, Inc.) Low resistance metal contact technology. US Patent 6,165,902, 2000. (4) Pin-shyne, C. (Taiwan Semiconductor Manufacturing Co.) Process sequence and mask layout to reduce junction leakage for a dual gate MOSFET device. US Patent 7,244,641, 2007. (5) Barcz, A.; Turos, A.; Wieluski, L.; Skrzynecka, I. Phys. Status Solidi A 2006, 28, 293.

Li et al. (6) Dreer, S.; Wilhartitz, P.; Mersdorf, E.; Friedbacher, G. Microchim. Acta 1999, 130, 281. (7) Okada, K.; Kameshima, Y. J. Am. Ceram. Soc. 1995, 78, 2021. (8) Nakajima, A.; Khosru, Q. D. M.; Yoshimoto, T.; Yokoyama, S. Microelectron. Reliability 2002, 42, 1823. (9) Kinoshita, Y.; Hirose, F.; Miya, H.; Hirahara, K.; Kimura, Y.; Niwano, M. Electrochem. Solid-State Lett. 2007, 10, G80. (10) Kamiyama, S.; Miura, T.; Nara, Y. Thin Solid Films 2006, 515, 1517. (11) Ichimura, S.; Kurokawa, A.; Nakamura, K.; Itoh, H.; Nonaka, H.; Koike, K. Thin Solid Films 2000, 377-378, 518. (12) Goumans, T. P. M.; Wander, A.; Brown, W. A.; Catlow, C. R. A. Phys. Chem. Chem. Phys. 2007, 9, 2146. (13) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (14) Delley, B. J. Chem. Phys. 2000, 113, 7756. (15) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169. (16) Jo´nsson, H.; Mills, G.; Jacobsen, K. W. Nudged Elastic Band Method for Finding Minimum Energy Paths of Transitions. In Classical and Quantum Dynamics in Condensed Phase Simulations; Berne, B. J., Ciccotti, G., Coker, D. F. , Eds.; World Scientific: River Edge, NJ, 1998. (17) Mills, G.; Jo´nsson, H.; Schenter, G. K. Surf. Sci. 1995, 324, 305. (18) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (19) Methfessel, M.; Paxton, A. T. Phys. ReV. B 1989, 40, 3616. (20) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 48, 13115. (21) Delley, B. J. Chem. Phys. 1990, 92, 508. (22) Delley, B. J. Chem. Phys. 2000, 113, 7756. (23) Kinoshita, Y.; Hirose, F.; Miya, H.; Hirahara, K.; Kimura, Y.; Niwano, M. Electrochem. Solid-State Lett. 2007, 10, G80.

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