Critical conditions of epitaxy, mixing and sputtering growth on Cu(100) surface using molecular dynamics

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Critical conditions of epitaxy, mixing and sputtering growth on

Cu(1 0 0) surface using molecular dynamics

Zheng-Han Hong

a

, Shun-Fa Hwang

a,*

, Te-Hua Fang

b

a_{Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology,}

123 University Road, Section 3, Douliu 640, Taiwan, ROC

b

Institute of Mechanical and Electromechanical Engineering, National Formosa University, 64 Wunhua Road, Huwei 632, Taiwan, ROC Received 29 January 2007; received in revised form 6 March 2007; accepted 8 March 2007

Available online 8 May 2007

Abstract

Molecular dynamics is employed to investigate the film growth process and the sputtering process at different deposition conditions of incident energy, substrate temperature, incident angle, and deposition rate. Tight-binding-second-momentum-approach (TB-SMA) many-body potential is used to model the interaction. Instead of sputtering yield, an accumulation ratio that is defined as the ratio of the number of the accumulated atoms of the substrate to the total number of the incident atoms and substrate atoms is used. The results indicate that when the deposition rate is 5 atom/ps, the epitaxy mode of film growth and film mixing mode are observed at the incident energy of 0.3–5 eV. When the incident energy is larger than 10 eV, the sputtering phenomenon is significant. As the depo-sition rate is decreased to 2.5 atom/ps, the sputtering phenomenon is clear only if the incident energy is larger than 20 eV. Furthermore, both the substrate temperature and the incident angle seem to have little effect on the accumulation ratio as the epitaxy mode of film growth and the mode of film mixing is concerned. However, one could have better sputtering process by using high substrate temperature and/or high incident angle.

Keywords: Molecular dynamics; Ion beam assisted deposition (IBAD); Film growth; Sputtering; Many-body potential; Accumulation ratio

1. Introduction

The ion beam assisted deposition (IBAD) process is one popular process to produce a thin film on a substrate for further applications. To produce a thin film, the IBAD pro-cess could be applied in two aspects. One is to enhance the mobility of the deposited atoms and to promote the film growth. The other is to bombard the surface of a solid sub-strate and to remove its atoms for further deposition. To have these two aspects of applications, it is necessary to adjust the ion incident energy, the ion incident angle, and the ion assisted ratio that is the ratio of ions to neutral atoms. Furthermore, these parameters of IBAD process

will aﬀect the quality and morphology of the deposited thin ﬁlm.

To analyze the morphology of the deposited thin film in detail and to understand the growing mechanisms, molecu-lar dynamics (MD) is always adopted instead of experi-ments. From the view point of MD, it is possible to examine the film growth process, such as epitaxy, film mix-ing that represents the mixmix-ing of the deposition atoms and the substrate atoms, and the sputtering process, by mainly considering the ion incident energy and the deposition rate. In general, the film growth process may occur at a lower level of incident energy, while the sputtering process may occur at a higher level of incident energy. Smith et al. [1,2]investigated the film growth process by using MD at a lower level of incident energy and observed the film growth with various conditions of substrate temperature and incident angle. In these studies, they employed a

*

Corresponding author. Tel.: +886 5 5342601x4143; fax: +886 5 5312062.

E-mail address:[email protected](S.-F. Hwang).

www.elsevier.com/locate/commatsci

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substrate temperature, the kinetic energies of the atoms on the substrate after the deposition simulation are shown in Fig. 12a–d for different substrate temperatures with the incident energy of 30 eV, the deposition rate of 5 atom/ ps, and the incident angle of 0. From the kinetic energy, it is easy to differentiate the incident atoms from the sub-strate atoms. As shown in these figures, as the subsub-strate temperature is increased from 300 to 750 K, the incident atoms penetrate into the substrate from two layers to seven layers. Hence, it may imply that when the substrate temper-ature is higher, the atoms of the substrate are easier to bomb away by the incident atoms because the substrate atoms are more energetic.

4. Conclusion

This paper investigates in detail the film growth process and the sputtering process by controlling the deposition parameters including incident energy, substrate tempera-ture, deposition rate, and incident angle in the system of Cu on Cu(0 0 1) substrate using MD. From the above dis-cussion, when the deposition rate is 5 atom/ps, the accumu-lation ratio is not good at the incident energy of 0.01 eV. At the energy of 0.3–0.5 eV, the epitaxy mode of film growth is observed, but the surface of the film growth is not smooth and vacancies and voids occur almost everywhere. When the incident energy is increased to 1 eV, it is at the epitaxy mode, the surface of the film growth is smooth, and vacan-cies and voids are less observed. The mixing mode of film growth clearly occurs at the incident energy from 3 to 5 eV. When the incident energy is larger than 10 eV, the sputtering phenomenon is significant. As the deposition rate is 2.5 atom/ps, the epitaxy mode of film growth is observed below 5 eV, film mixing mode occurs around 5– 20 eV, and the sputtering process may be clear only after 20 eV. Furthermore, the substrate temperature and the

incident angle seem to have little eﬀect on the accumulation ratio at the epitaxy mode and mixing mode of ﬁlm growth. However, higher substrate temperature will result in higher intermixing between the incident atoms and the substrate atoms. Also, one could have better sputtering process by using high substrate temperature and/or high incident angle.

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