Atomic-Scale Numerical Simulations of Structural Properties in Carbon

Atomic-Scale Numerical Simulations of Structural Properties in Carbon-Based Thin Film Deposition Y. Murakami1, S. Horiguchi2, S. Hamaguchi1 1

Center for Atomic and Molecular Technologies, Osaka University 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan 2

CANON ANELVA CORPORATION

Abstract: Molecular dynamics simulations were performed to examine interaction between carbon containing gaseous radical species and an amorphous carbon (α-C) surface. In simulations charge-neutral CH3 or CH radical species are injected normally into the top surface of the α-C substrate with incident energies in the range from 2eV to 50eV. It is seen that the sticking probability of CH radicals is higher than CH3 radicals in the entire energy range. It is seen that more sp3 bonds are formed in the case of CH3 injections. This indicates that, with the availability of more hydrogen atoms, a carbon atom tends to form more diamond-like structures. Keywords: Molecular dynamics simulation; Diamond-like carbon; Deposition; Sp3 bonding 1. Introduction Diamond-like carbon (DLC) films have attracted much attention in the coating technology community [1]. For example, DLC coating films may be used as protection layers for data recoding disks. Characteristics of DLC films are generally determined by the amount of sp3 hybridized bonds present in the films [2], which may be controlled by hydrocarbon species and its injection energy used for the deposition process. Various mechanisms of formation of sp3 hybridized bonds in DLC films have been proposed [3], but some details are yet to be understood better. In this work, in an attempt to establish a high quality DLC deposition process, we have used molecular dynamics (MD) simulations to understand interaction between carbon containing gaseous radical species and an amorphous carbon (α-C) surface, and structural properties in the deposited carbon films. 2. Experiment The model used to investigate the deposition processes is a classical molecular dynamics model. This model functions for atomic interactions used in our simulation code were originally developed by Yamada et al. [4]. All atoms in the system are treated as charge-neutral and followed through space and time by integrating Newton’s equation of motion. These atoms move under the influence of the forces derived from the inter-atomic potential functions. The integration scheme used is the velocity Verlet algorithm. For the simulation, a model α-C film substrate is prepared as a cube with the edge length of 2nm. After a substrate is arranged with about 800 atoms placed at random

positions so that the density may become about 2.0g/cm3, which is set based on experimental data, the substrate is allowed to relax for 10ps at constant temperature of 300K. The substrate consisted mainly of sp2 hybridized bonds. As the periodic boundary conditions are imposed to the simulation system in the horizontal directions, the α-C cube represents an infinitely wide flat thin film with the thickness of 2 nm. The lower layer of 0.4nm in thickness is fixed (by atoms with an artificially large mass) to avoid drifting of the substrate during the simulation. In simulations, charge-neutral CH3 or CH radical species are injected 600 times (1.5×1016cm-2 dose) normally into the top surface of the substrate with incident energies in the range from 2eV to 50eV. The substrate temperature is kept at room temperature (300K) at the beginning of every injection.

Fig.1-2 Surface morphologies after CH3 (left) and CH (right) radical injections (1.5×1016cm-2 dose) with 20eV

×1023

Density （atoms/cm-3)

1.6

atoms gradually decrease until the injection dose reaches 1.0×1016cm-2 and then saturate at the sticking probability of about 0.1. On the other hand, in the case of CH radical injections, the sticking probability of C seems to take almost the same constant value, which is higher than that in the case of CH3 radical injections at the same injection energy. It may be considered that only in the case of CH3 radical injections, the number of dangling bonds on the substrate surface strongly affects the sticking probabilities of C. The sticking probability of H is almost the same values of C at the incident energy of 2eV in both cases of CH3 and CH radicals. However, at higher energies, the sticking probability of H is lower than that of C for the both cases of the radicals. It reveals that in the energy range over 20eV the injection may make a number of dangling bonds on the substrate surface or it can displace H atoms that are bonding to C. 1.0 0.8 0.6 Sticking Probability

3. Results and discussion (1) Sticking probability Surface morphologies after CH3 and CH radical injections (1.5×1016cm-2 dose) with 20eV are shown in fig.1 and fig.2, respectively. In these figures, black large spheres and grey spheres represent fixed bottom layer of C atoms and substrate C atoms, respectively. The white large spheres represent injected C atoms. Injected H atoms are represented as small black spheres. It seems that CH radicals tend to stick to the substrate easier than CH3 radicals. This was the case in all conditions examined in this study. The figures also show that both C and H atoms penetrate into the substrates. This occurs when the injection energy is larger than 20eV. Fig.3 shows the depth profile in the case of CH3 with 50eV after 1.5×1016cm-2 dose. In this figure, the x-axis shows the depth of the film measured from the lowest substrate layer. The bold solid curve (Ⅰ) and the dotted curve (Ⅱ) represent the densities of substrate C atoms and injected C atoms, respectively. The thin solid curve (Ⅲ) represents the density of injected H atoms. This figure shows clearly that C and H atoms have been penetrated deeper in the substrate. Therefore it seems that the mixing effect occurs well. It is also found that H atoms are penetrated deeper into the substrate than injected C atoms.

0.4 0.2 0.0 -0.2

H C

-0.4 -0.6

Ⅲ

1.4 1.2

substrate C(Ⅰ) injected C（Ⅱ) injected H（Ⅲ）

Ⅰ

1.0

-0.8 -1.0 0.0

0.5

1.0 Dose (cm-2 )

0.8 0.6

Fig.4. The sticking probabilities of C and H as functions of the does in the case of CH3 radical injections with the kinetic energy of 20eV

Ⅱ

0.4 0.2 0.0 0

5

10

1.5 ×1016

15

20

25

Depth(A)

Fig.3. Depth profiles in the case of CH3 injections with 50eV after 1.5×1016cm-2 dose

The deposition rates for films strongly depend on the sticking probabilities of injection particles into the substrate. The sticking probability of each radical was calculated in the method by counting the number of atoms that sticks to the surface over twenty shots. The relation between the injection dose and the sticking probability was investigated. In fig.4, the sticking probabilities of C atoms and H atoms are shown in the case of CH3 radical injections with the kinetic energy of 20eV. In this figure, the solid and dotted curves represent the sticking probabilities of C and H, respectively. The sticking probability of C

(2) Bond orders and Fraciton of sp3/sp2 In the remainder of this article the threefold coordinated atoms will be referred to as sp2 hybridized and the fourfold coordinated atoms will be referred to as sp3 hybridized. We have investigated that the ratio of the number of sp3 C atoms to that of sp2 C atoms (sp3/sp2) among deposited C atoms. Fig.5 and fig.6 show sp3/sp2 hybridization bonds as functions of the dose of CH3 and CH, respectively. In these figures, the value of sp3/sp2 is obtained from averaging by the number of substrates and injected C atoms under various conditions. For both radicals, it seems that sp3/sp2 is larger with the increasing dose, except in the case of 2eV. It is found that CH3 radicals tend to form more sp3 hybridizations than CH radicals. Especially, in the case of 50eV injection, it seems the largest sp3/sp2 is observed over 1.5×1016 dose in all our conditions. This indicates that, with the availability of more hydrogen atoms, a carbon atom tends to form more

diamond-like structures. In the case of CH3 with 50eV injection, fig.3 shows the mixing effect occurs near the top surface. Under the same conditions, sputtering due to the break of weak C bonds on the initial substrate was observed to occur. It is considered that injections under this condition cause to modify the interlayer until the dose reaches 1.0×1016cm-2 as a high density film is formed due to the high sp3 structure.

it is seen that more sp3 bonds are formed in the case of CH3 injections. This indicates that, with the availability of more hydrogen atoms, carbon atoms tend to form more diamond-like structures. The rate of formation of diamond -like structures (sp3 structures) takes an optimum value at 50eV injection energy in the case of CH3 radical. References

sp3/sp2 (Normalized by C atoms)

[1] J.Robertson, Surf.Coating Technol. 50 (1992) 185. 1.2E-04

[2] W.Jacob, W.Moller, Appl.Phys.Lett. 63 (1993) 1771. 2eV 20eV 50eV

1.0E-04 8.0E-05

[3] J.Robertson, Materials Science and Engineering, R37 (2002) 129. [4] H.Yamada and S.Hamaguchi, Plasma Phys. Control.

6.0E-05

Fusion 47 (2005) A11. 4.0E-05 2.0E-05 0.0E+00 0.0

0.5

1.0 Dose(cm-2)

1.5

2.0 ×1016

Fig.5 The ratio of sp3 to sp2 hybridization bonds as functions of the dose of CH3 injections.

sp3/sp2 (Normalized by C atoms)

1.2E-04 2eV 20eV 50eV

1.0E-04 8.0E-05 6.0E-05 4.0E-05 2.0E-05 0.0E+00 0.0

0.5

1.0 Dose(cm-2)

1.5

2.0 ×1016

Fig.6 The ratio of sp3 to sp2 hybridization bonds as functions of the dose of CH injections

4. Conclusions We have carried out MD simulations of CH3 and CH radical irradiations onto an amorphous carbon substrate to understand the formation mechanism of sp3 carbon atoms, especially in the first stage of film deposition. It is seen that the sticking probability of CH radicals is higher than CH3 radicals in the entire energy range. On the other hand,