Sputtering of amorphous silicon nitride irradiated with energetic C60 ions: Preferential sputtering and synergy effect between electronic and collisional sputtering

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Abstract

Amorphous silicon nitride films (thickness 30 nm) deposited on Si(0 0 1) were irradiated with 30–1080 keV C60 and 100 MeV Xe ions to fluences ranging from 2 × 1011 to 1 × 1014 ions/cm2. The composition depth profiles of the irradiated samples were measured using high-resolution Rutherford backscattering spectrometry. The sputtering yields were estimated from the derived composition profiles. Pronounced preferential sputtering of nitrogen was observed in the electronic energy loss regime. In addition, a large synergy effect between the electronic and collisional sputtering was also observed. The sputtering yields were calculated using the unified thermal spike model to understand the observed results. Although the calculated results reproduced the observed total sputtering yields with a lowered sublimation energy, the observed preferential sputtering of nitrogen could not be explained. The present results suggest an additional sputtering mechanism related to the electronic energy loss.

Introduction

When solid surface is bombarded with energetic ions the surface atoms are removed. This is called sputtering, which is the basis of many applications, such as sputtering deposition, plasma etching, surface analysis and so on. For the bombardment of low energy ions the sputtering is caused by elastic collisions between the incoming ions and the atoms in the surface layers and closely linked to the nuclear energy loss [1]. On the contrary, for the bombardment of high energy ions, the elastic collision plays a minor role in the sputtering process because the kinetic energy of the high energy ion is deposited almost exclusively to the target electrons. Nevertheless, the surface erosion is observed especially with insulators [2]. This is called electronic sputtering and in some cases huge sputtering yields, more than 1000 atoms/ion, were observed. The origin of the electronic sputtering is attributed to the electron–phonon coupling. The energy deposited to the electrons (electronic energy loss) is transferred to the atomic subsystem and this causes large local heating where surface atoms are removed by thermal evaporation. Such a local heating can be described by a so-called inelastic thermal spike (i-TS) model [3], which was originally developed to explain the formation of ion tracks produced by swift heavy ions. Based on the i-TS model, the observed sputtering yields of crystalline and vitreous SiO2 irradiated with swift heavy ions were well reproduced [4].

Thus, the mechanism of sputtering is well understood in both low and high energy regimes. In the intermediate energy regime, the synergy effect between the collisional and electronic sputtering may play an important role. There is, however, almost no study on the synergy effect in the intermediate energy regime. This is partly because both the collisional and electronic sputtering yields are small in the intermediate energy regime. As a result, notable synergy effect is not expected. This is true for monoatomic ions but is not the case for the cluster ions. For the cluster ions, both the electronic and nuclear energy losses may be large enough to lead to a notable synergy effect in the intermediate energy regime. In this paper, the sputtering yields of amorphous silicon nitride (a-SiN) irradiated with 30–1080 keV C60 ions are measured to study the synergy effect. Differently from monoatomic ions, both nuclear and electronic energy losses of these C60 ions are rather large (∼10 keV/nm) in this energy regime.

Section snippets

Experimental

A wafer of Si(0 0 1) with an a-SiN film (thickness 30 nm) deposited by low pressure chemical vapor deposition (LPCVD) was purchased from Silson Ltd. The nominal density of the a-SiN film is 3 g/cm3. Beams of 30–1080 keV C60 ions were produced by the 400-kV ion implanter at JAEA/Takasaki. The a-SiN/Si(0 0 1) samples were irradiated with the C60 ion beams at normal incidence to fluences from 2 × 1012 to 1 × 1014 ions/cm2 under a vacuum of 10−5 Pa. For comparison, the a-SiN/Si(0 0 1) sample was irradiated with

Results

Fig. 1 shows an example of the observed RBS spectra. The dashed and solid lines show the random and 〈1 1 1〉 channeling spectra, respectively, of the pristine a-SiN/Si(0 0 1). The plateau seen from ∼282 to ∼323 keV corresponds to Si signals in the a-SiN film. Nitrogen signals are seen from ∼225 to ∼259 keV. There are small peaks at ∼273 and ∼241 keV, which correspond to oxygen and carbon atoms at the surface. From these spectra composition depth profiles were derived through spectrum simulations. The

Discussion

It is seen from the Table 1 that the nuclear energy loss Sn of 1080 keV C60 is smaller than that of 30 keV C60, and the electronic energy loss Se of 1080 keV C60 is smaller than that of 100 MeV Xe. This leads to the following relations because the collisional (electronic) sputtering yield Yc (Ye) increases with the nuclear (electronic) energy loss,Yc(1080keV C60)<Yc(30keV C60)<Y(30keV C60),andYe(1080keV C60)<Ye(100MeV Xe)<Y(100MeV Xe).

If there is no synergy effect between the collisional and

Conclusion

The sputtering yield of amorphous silicon nitride irradiated with 30–1080 keV C60 and 100 MeV Xe ions were measured using high-resolution RBS. The results of the measurement showed that there is a synergy effect between the collisional and electronic sputtering. Large preferential sputtering of nitrogen was also observed especially at higher energies. The sputtering yields were calculated based on the u-TS model, which includes contributions of the elastic-collision spike, the inelastic thermal

Acknowledgements

This work was performed under the shared use program of JAEA facilities. The authors are grateful to the crews of the 400-kV ion implanter at JAEA/Takasaki for irradiation of C60 ions. They are also grateful to the technical staff of the accelerator facilities at JAEA/Tokai for the irradiation of Xe ions. This work was partly supported by JSPS KAKENHI Grant (Grant Numbers 24651114 and 26246025).

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