1：Experimental Design and Method Selection:

The study employs a wet chemical etching method using a solution of aqueous NaOH and high molar mass PEG to fabricate Si micropyramids. The rationale is to explore the role of PEG in inducing undercutting and forming novel pyramid shapes without pre-patterned masks.

2：Sample Selection and Data Sources:

N-type Si wafers with <100> orientation, resistivity of 1–10 Ω cm, and thickness of 500 μm were purchased from University Wafer Co. Ltd. PEG powders with an average molecular weight of 8000 g/mol were used.

3：List of Experimental Equipment and Materials:

Materials include Si wafers, PEG powders, 40% NaOH solution, isopropyl alcohol (IPA), and de-ionized (DI) water. Equipment includes a Hitachi S4800 field emission scanning electron microscope (FESEM) and an Agilent 5500 stereo-atomic force microscope (AFM) for characterization.

4：Experimental Procedures and Operational Workflow:

The Si wafer was cleaned and sonicated with DI water. A 40 wt% PEG solution was prepared and heated to 50°C. A mixture of 4 ml 40 wt% NaOH, 6 ml IPA, 500 μl PEG solution, and DI water to a total volume of 200 ml was heated to 85°C. A Si wafer piece (2 cm2) was etched in this solution. After 25 min, 5 ml IPA was added, and after another 15 min, the wafer was removed, washed, and dried, resulting in octagonal pyramids. For dodecagonal pyramids, etching was extended by 10 min. Control experiments without PEG were also conducted.

5：Data Analysis Methods:

Microstructure was characterized using FESEM and AFM. Geometric characteristics, such as inclination angles, were measured from SEM and AFM images, and compared with crystallographic data to identify Miller indices.