| AVS 59th Annual International Symposium and Exhibition | |
| Plasma Science and Technology | Thursday Sessions |
| Session PS2-ThM |
| Session: | Low Damage Processing |
| Presenter: | Y. Yanagisawa, Tohoku University, Japan |
| Authors: | Y. Yanagisawa, Tohoku University, Japan T. Kubota, Tohoku University and BEANS Project, Japan B. Altansukh, Tohoku University, Japan K. Miwa, BEANS Project, Japan S. Samukawa, Tohoku University and BEANS Project, Japan |
| Correspondent: | Click to Email |
To fabricate MEMS devices, it is necessary to etch large-scale three-dimensional (3D) structures. Plasma etching is widely used for this purpose, but plasma irradiates charged particles and high energy UV photons and they may cause problems.
Ion sheath exists between bulk plasma and the etched sample. It accelerates and collimates ions in plasma and enables vertical etching. However, when the sample has large-scale 3D structure whose height is similar to or larger than the sheath thickness, the sheath is distorted along the surface. This causes distorted acceleration of ions and results in distorted etching shape. Also, high energy UV photons from plasma generates defects and deteriorates the mechanical property. It is reported that quality factor (q factor) and resonance frequency (f) were decreased by plasma irradiation [1]. In this study, we investigated 3D, damage-free, and high aspect ratio silicon etching for MEMS application using neutral beam (NB) to solve these problems. NB source developed by Samukawa et al. [2] can achieve high neutralization efficiency and elimination of UV irradiation damage.
First, large-scale 3D structure etching was investigated by using a sample with a vertical step of up to 725 µm high. In cases of plasma etchings (Cl2 ICP etching and Bosch process), distortion of etching shape was observed. The etching shape distortion is concluded to be due to ion sheath distortion, based on the dependence on the step height and plasma parameters. On the other hands, no distortion was observed in the case of NB etching. This is because accelerated and collimated neutral particles without electric charge are used in NB etching.
Next, etching damage on mechanical property of silicon was investigated using microcantilever structure. Ar plasma and neutral beam were irradiated at the surface of the samples. Plasma irradiation degraded q factor and f of the cantilever, but neutral beam did not. This is due to crystalline defects generated by the UV irradiation from plasma. It is regarded that phonons are scattered by these defects, which causes loss of vibration energy and decrease of q factor and f.
In conclusion, MEMS process suffers both of sheath distortion and UV irradiation problems and neutral beam can solve these problems.
A part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO). This work is partly supported by Creation of Innovation Centers for Advanced Interdisciplinary Research Areas Program.
[1] M. Tomura et al., Jpn. J. Appl. Phys. 40, 04DL20 (2010).
[2] S. Samukawa et al., Jpn. J. Appl. Phys. 40, L779 (2001).