why the primary flat in 100 silicon wafer is kept in parallel to the 110 direction? Is there any reason behind it or just a conevntion? Also, why does the wafer break easier along the cleavage planes compared to directions other than along the cleavage planes?
I think that the you have to look at the secondary flat to tell if your wafer is 100 or 111 and if there is no secondary flat the wafer is p type . The angle between 100 and 110 is 45 degrees so I am not sure what you mean by being parallel. The wafer flat choices are a matter of convention. If you look up SEMI wafer standards you will find documents that go into detail about the specification.
If you count the number of bonds per unit area, I think you will find them be be less along the cleavage planes.
Dear John Muth sir, I would like to thank you for your concern. I am also aware regarding the fact that angle between 100 and 110 is 45 degrees. But in article, "What is the Young’s Modulus of Silicon?" by Hopcroft et. al. published in IEEE Journal of Microelectromechanical Systems, VOL. 19(2), 229-238 (2010), it is mentioned that "For the most commonly encountered wafers in research (p- and n-type (100) and (111) wafers), the primary flat is aligned with the [110] direction." Hence my question is that during slicing of wafer, whether this convention provide any advantage or not and if so, how?
Well, In my previous answer I said I thought it was a matter of convention. You could choose a different convention, but it probably wouldn't make as much sense.
Practically speaking if it is not aligned along a crystallographic direction with some precision then it wouldn't be useful. So when manufacturing the wafers, my suspicion is that they use an x-ray machine to align boule before grinding the flat and slicing the wafers. This is because different applications may care about how far off-axis the surface is fo subsequent growth. Also for the case of micromachining you can use the wafer flat to know where the orientation of the crystal so when you do bulk etching or of other steps in device fabrication where the orientation is important you know where you are.
Wafer would easily cleave along this and the respective perpendicular direction
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