Reduction of RIE Induced Ge Surface Roughness by SF6-CF4 Cyclic Etching Method

Introduction

Ge is one of the most promising channel materials to further improve the performance of Si CMOS due to its high hole and electron mobility ^[1-3]. In order to overcome the current Si FinFET technology, 3D channel structure is mandatory for Ge MOSFETs. To fabricate Ge Fins, CF₄- or SF₆-based RIE etching is usually employed for Ge etching [Ref. needed]. However, both CF₄ and SF₆ RIE etching results in large surface roughness which strongly degrades the performance of Ge FinFETs. On the other hand, it is necessary to realize high selectivity of anisotropic etching of Ge which is important to obtain superior static electrical control of Fin channel. In this research, we developed a SF₆-CF₄ cyclic RIE etching method for Ge Fin formation with suppressed surface roughness. The impact of inserting SF₆-O₂ etching steps into CF₄-O₂ etching process on the surface roughness of Ge was systematically investigated, as well as the ratio of vertical etching to horizontal etching of Ge. It is found that with a time proportion of 60% for SF₆-O₂etching steps, a minimum RMS of 1.1 nm is obtained for Ge Fin etching.

Experiments

  The CF₄-O₂ and SF₆-O₂ RIE cyclic etching were carried out for (100) Ge surfaces with the same pressure, plasma power and total gas flow. Different time proportions of 0%, 20%, 40%, 60%, 80% and 100%, were used for SF₆-O₂ etching steps during the CF₄-O₂etching, with a total etching time of each cycle for 20 s. The selectivity of vertical and horizontal etching of Ge was examined using SEM, and the surface roughness of Ge after etching was evaluated using AFM.

Results and discussion

Figure 1 shows the etching rate of CF₄-O₂ and SF₆-O₂ cyclic etching with different time proportion of SF₆-O₂ step. It is observed that the etching rate significantly increases with increase of SF₆-O₂ etching proportion, due to the high reactivity of SF₆ with Ge ^[4]. The RMS of Ge surfaces after CF₄-O₂ and SF₆-O₂ cyclic etching for 20 s is evaluated by AFM (Figure 2). It is confirmed that large amount of surface roughness is generated for Ge surface after both CF₄-O₂ and SF₆-O₂ etching. With introducing SF₆-O₂ etching steps into CF₄-O₂ etching, the surface roughness of Ge is obviously reduced. The RMS values of Ge surface after CF₄-O₂ and SF₆-O₂ cyclic etching are summarized in Figure 3 as a function of the time proportion of SF₆-O₂ etching. It is found that with a time proportion of 60% for the SF₆-O₂ step, a minimum RMS of 1.1 nm is obtained for the Ge surface. The impact of SF₆-O₂ proportion to the ratio of vertical and horizontal etching for Ge is also examined, as shown in Figure 3. It is found that the selectivity of vertical etching decreases with increasing the SF₆-O₂ proportion. However, a relatively high selectivity of vertical etching of over 4 is still maintained with a time proportion of 60% for SF₆-O₂etching, resulting in Ge Fins with superior morphology (Figure 4).

Conclusion

  A CF₄-O₂ and SF₆-O₂ cyclic etching method is proposed for Ge Fin formation. It is found that with a time proportion of 60% for SF₆-O₂etching step, superior morphology is achieved for Ge Fins with sufficiently suppressed surface roughness of 1.1 nm.

References

[1] S. Takagi, et al. IEDM, 2003.

[2] H. Shang, et al. IBM. J. Res. Dev., 2006.

[3] K. C. Saraswat, et al., Microelectro. Eng., 2008.

[4] A. Campo, et al. J. Vac. Sci. Technol. B, 1995.