MIcrofabrication suggestion required

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A silicon wafer is 500 μm thick. The front surface is {100}. A mask consists of a rectangular window of unknown size. The sides of the window are parallel to <110>. After through-wafer etch, a window (50 μm by 80 μm) is formed on the other side of the wafer. Find the size of the original mask window assuming the etch rate of {111} surfaces is 1/100 that of {100} surfaces.

I need to solve this one. Can anyone suggest any book/website from where I can get some help regarding this one?
 

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You need to find the crystalline angle of the {111} planes, which is a constant and can be found in many places. Then you can set up an equation incorporating the slight lateral etching to find the original opening size. There are many websites and online lecture notes that discuss silicon anisotropic etching, so it shouldn't be too difficult.

Marc Madou's Fundamental of Microfabrication will tell you exactly what's going on and is a great reference for microfabrication / nanofabrication / MEMS questions.