Why wafer is round in shape?

Answer 1

The mono-crystalline boule that wafers are sliced from is almost round because it is slowly rotated as it is slowly pulled from the melt. This prevents the crystal from growing large flat faces that might catch & bind on the crucible containing the melt and being damaged. After finishing the pull the boule has its pointy top and bottom sliced off and the 4 or 6 lengthwise rounded ridges, depending on the orientation of the seed crystal used to start the pull, polished away to make the finished boule a smooth cylinder. Polishing into a cylinder results in less waste than cutting and polishing to square or hexagonal crosssection to match the crystal lattice of boule.

Wafers are sliced from these finished polished cylindrical boules, resulting in round wafers.