Hi Mustafa,
I looked into the matter and the line does seem straight to me. As straight as a pixel display can display straight lines...
Ok, trolling out. If the material is supposed to be monocrystalline silicon, there you have your answer: the crystal lattice is most densily packed in this direction. The angle is the inevitable result from etching processes to build up the void under your membrane because this plane is etched muched slower than others. Have a look into wafer fabrication if you are further interested in this topic. Hope I could help you out.
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