Fabrication of Octagonal Structures Using Si (111) Wafers

Etch rate distribution of Si(111) wafers in KOH solution:

Similar to the (100) orientation, the structure etched from the (111) orientation has three stable planes (see red circles) and a triangular cross-section according to the rate distribution in the figure below. If the etching time is suitable, the three stable planes of the bottom side (back side) can also be preserved, so that a hexagon can be obtained. But if you want to get an octagon, it's a very difficult process. You need to design the layout carefully, incorporating some unstable planes to get the desired structure, which also means that the etch time also needs to be very precisely controlled.

Layout design:

Based on the above analysis, we designed the layout as shown below. By appropriately controlling the etching time, two unstable planes and the main six stable planes appear simultaneously, and an octagonal cross-section can be obtained. If you continue to etch, the octagon will turn into a hexagon.

Corrosion result:

The images below show the evolution of the corrosion results over time.