Isotropic Wet Etching and Improving Surface Flatness of Ge for Etchback Ge-on-Insulator Fabrication
Authors: Shimizu Noboru , Yamamoto Keisuke , Wang Dong , Nakashima Hiroshi 
Published: December 2020

1. Introduction

Ge has been received much attention as the next generation semiconductor materials due to its attractive characteristics such as high carrier mobility[1] and narrow bandgap corresponding near infrared wavelength[2]. In order to utilize its characteristics for the applications, Ge-on-Insulator (GOI) structure is necessary. Several fabrication methods for GOI have been suggested such as wafer bonding and mechanical thinning[3], Smart-CutTM[4,5], and SiGe condensation[6]. It is well known Smart-CutTM is widely used for commercial Si-on-Insulator (SOI) fabrication. However, Ge is more sensitive to damages caused by ion implantation and difficult to recover damages completely. Therefore, the best way for GOI fabrication have not been developed.
In early stage of SOI R&D, many approaches were proposed, too[7]. Wafer bonding and etchback is one of the simplest technique, and the advantage of this method is any damages due to ion implantation or mechanical stress are not induced to the top semiconductor layer. Therefore, we focused on etchback technique for GOI fabrication. It is necessary to develop appropriate Ge etching method with moderate etching rate and keeping or improving surface flatness. In this study, we aim to develop appropriate etching method of Ge for etchback GOI fabrication.

2. Experimental and results

We used single side mirror polished Ge wafer with a thickness of 500 μm. Figure 1 shows the experimental procedure in this study. The original polished top surface was covered by photoresist to avoid etching from this side. Firstly, we confirmed HF + HNO3 mixture solution for Ge etching because it is widely known as etching solution for Si. Figure 2 shows optical microscope images for the back side (non-polished side) of (100)-oriented Ge wafer (a) before etching and (b) after etching by HF + HNO3 for 7 minutes. By etching, surface uniformity drastically improved. So, wet etching can be used for Ge thinning and planarization. However, this solution reacts handle Si substrate of GOI, too. As an alternative etching solution, we selected HF + H2O2 mixture[8]. Figure 3 shows the etching result of (100)-oriented Ge etched by HF + H2O2 + H2O (7:7:6) solution. Although Ge was etched similar to Fig. 2(b), surface uniformity was inferior to HF + HNO3 etching. To improve surface uniformity, CH3COOH was added as diluent instead of H2O because politic amount of CH3COOH into HF + HNO3 solution leads isotropic etching and mirror plane on Si surface[9]. Figures 4 shows the etching results of (100)-oriented Ge by HF + H2O2 + CH3COOH (1:1:1) solution. Surface uniformity improved than Fig. 3 and there is no orientation dependence (data not shown). Therefore, isotropic etching occurs on Ge surface. The average etching rate in the first 20 minutes calculated from the lost masses was 7.9 μm/min. It should be noted any agitation was not carried out during etching and longer etching time leads decreasing of etching rate. Possible etching reaction is expressed as[10]:
2CH3COOH + 2H2O2 → 2CH3COOOH + 2H2O (1)
Ge + 2CH3COOOH + 2e
- → Ge2+ + 2CH3COO- + 2OH- (2)
As further investigation of surface uniformity, atomic force microscope (AFM) observation was carried out. Figure 5 shows AFM images for backside of (100)-oriented Ge etched for 110 minutes. The RMS of 30×30 μm2 area is 0.48 nm. Compared with the RMS of original backside (> 5 μm), the surface flatness is improved drastically. In the conference, electrical characteristics of etched surface will be presented.

3. Conclusions

For etchback GOI fabrication, we study isotropic etching for Ge. HF + H2O2 + CH3COOH mixture solution can etch Ge isotropic and improving surface uniformity. This solution has a potential as etching solution for etchback to make GOI structure.

Acknowledgements This work was supported by JSPS KAKENHI Grant Numbers 19K15028 and 19H05616.

Appendix Original concentrations of the chemicals in this study are HF: 49 wt%, HNO3: 69 wt%, H2O2: 30 wt%, and CH3COOH: 99.7 wt%. All compounding ratios in this article are volume rate.

References [1] A. Toriumi et al., JJAP, 57, 010101 (2018). [2] G. Z. Mashanovich et al., Opt. Mat. Express, 8, 2276 (2018). [3] Z. Zheng et al., APL, 109, 023503 (2016). [4] J. Kang et al., Mat. Sci. Semicond. Process., 42, 259 (2016). [5] K. Yamamoto et al., ECS trans. 93, 73 (2019). [6] K.-W. Jo et al., APL, 114, 062101 (2019). [7] W. P. Maszara, JES, 138, 341 (1991). [8] B. Schwartz, JES, 114, 285 (1967). [9] S. Wolf et al., Silicon Processing for the VLSI Era Volume 1, Lattice Press. (1990). [10] T. K. Carns et al., JES, 142, 1260 (1995).

Figure 1