Why do sidewalls bend during dry etching ?

Non-uniformity of ion bombardment

Dry etching is usually a process that combines physical and chemical effects, in which ion bombardment is an important physical etching method. During the etching process, the incident angle and energy distribution of ions may be uneven.

If the ion incident angle is different at different positions on the sidewall, the etching effect of ions on the sidewall will also be different. In areas with larger ion incident angles, the etching effect of ions on the sidewall is stronger, which will cause the sidewall in this area to be etched more, causing the sidewall to bend. In addition, the uneven distribution of ion energy will also produce similar effects. Ions with higher energy can remove materials more effectively, resulting in inconsistent etching degrees of the sidewall at different positions, which in turn causes the sidewall to bend.

The influence of photoresist

Photoresist plays the role of a mask in dry etching, protecting areas that do not need to be etched. However, the photoresist is also affected by plasma bombardment and chemical reactions during the etching process, and its performance may change.

If the thickness of the photoresist is uneven, the consumption rate during the etching process is inconsistent, or the adhesion between the photoresist and the substrate is different at different locations, it may lead to uneven protection of the sidewalls during the etching process. For example, areas with thinner photoresist or weaker adhesion may make the underlying material more easily etched, causing the sidewalls to bend at these locations.

Differences in substrate material properties

The etched substrate material itself may have different properties, such as different crystal orientations and doping concentrations in different regions. These differences will affect the etching rate and etching selectivity.
For example, in crystalline silicon, the arrangement of silicon atoms in different crystal orientations is different, and their reactivity and etching rate with the etching gas will also be different. During the etching process, the different etching rates caused by the differences in material properties will make the etching depth of the sidewalls at different locations inconsistent, ultimately leading to sidewall bending.

Equipment-related factors

The performance and status of the etching equipment also have an important impact on the etching results. For example, problems such as uneven plasma distribution in the reaction chamber and uneven electrode wear may lead to uneven distribution of parameters such as ion density and energy on the wafer surface during etching.

In addition, uneven temperature control of the equipment and slight fluctuations in gas flow may also affect the uniformity of etching, leading to sidewall bending.