Comparison of Cl2 and F-Based Dry Etching Technology for High Aspect Ratio Si Microstructures with an Inductively Coupled Plasma Source

S. W. Pang and W. -C. Tian

University of Michigan, Ann Arbor, Michigan 48109-2122, USA

Inductively coupled plasma sources have been used to generate plasmas using both Cl2 and SF6/C4F8 chemistries. For bulk micromachining of microelectromechanical systems (MEMS), where large portions of the substrate must often be etched, the F-based etching can be used to form deep trenches with high etch rate and high selectivity to a mask. Conversely, Cl2 etching can be selected for etching small, high aspect ratio feature sizes with a vertical profile in both bulk and surface micromachining. As shown in Fig. 1, trenches etched using Cl2 suffered less aspect ratio dependent etching effects because the trenches can be etched at a much lower pressure than with F-based gases. A 1.4 µm wide, 65 µm deep trench can be obtained with an aspect ratio of 46 in 12 hr, as shown in Fig. 2. The average Si etch rate was 90 nm/min and the selectivity to electroplated Ni was 23. The sidewall was vertical and smooth and the trench openings were nearly the same width before and after etching.

Adjacent trenches with 0.14 µm mask opening and 2 µm line width were etched using these two etching technologies. With Cl2 etching, a wider 0.25 µm trench opening, due to the mask erosion effect, with a depth of 5.6 µm was obtained in 50 min, as shown in Fig. 3. However, the 0.33 µm undercut increased the trench opening to 0.8 µm for 10.7 µm deep trenches after the F-based etching for 55 min. The Si etch rate in a large open area using F-based etching was 1818 nm/min, which is much faster than 201 nm/min when Cl2 etching was used. However, the Si etch rate, 112 nm/min for Cl2 and 195 nm/min for F-based gases, was similar when the trench opening was decreased to submicrometer dimensions. This shows that the Cl2 etching provides better dimension and profile control with comparable Si etch rate to F-based etching when etching submicrometer trenches. The loading effect using Cl2 chemistry is less than with F-based etching. The Si etch rate was 1.74 mm/min for ~100 % Si exposed area and 3.68 mm/min when the exposed Si area was ~0 % in F-based etching. Scalloping, which is a periodic undercut near the top of the sidewalls, disappeared when using an electroplated Ni mask. The size and period of the scalloped features decreased as the Si exposed area and etch time increased.

Figure 1. Si etch rate dependence on feature size and etch depth in Cl2 and F-based dry etching.

Figure 2. Etch profile of high aspect ratio Si trenches. The trench width is 1.4 µm with etch depth of 65 µm in Cl2 etching after 12 hr.

Figure 3. No undercut was observed in submicrometer trenches when using Cl2 etching after 50 min

References

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Last Updated: November 19, 2007

E-Mail: pang@eecs.umich.edu 

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