Fig. 1. Top view of microwells fabricated by two-step etching（The microwells presents an equilateral triangle distribution）

Fig. 2. Fabrication process schematic（The etching gas SF₆ reacts with silicon for etching， and the protective gas C₄F₈ is released to form a polymer on the surface to prevent the etching. The alternate release of protective gas and etching gas makes the sidewall tend to be vertical， and the sidewall will have a corrugated shape as observed by the local magnification， and the isotropic etching widens the internal area）

Fig. 3. Tilted 45° observation using SEM

Fig. 4. Variation of isotropic etched structure size with time observed at 45° by SEM

Fig. 5. Isotropic etching time versus height and width respectively

Fig. 6. Linear relationship between etching depth in the first step and experimentally obtained structure

Fig. 7. SEM images of relationship between angle of upper wall and side wall with different first etching depths