How is the crystal direction important for etching silicon surfaces?

I am a student of masters study with a background only in electrical engineering, but I have studied Miller indices and crystallographic planes and directions, personally, to help myself understand the process of silicon etching which is required in my current course of study. But, I am unable to understand the following written in my text book:



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> "When Silicon is etched anisotropically, i.e. depending on crystal direction, and a {1 0 0} surface direction is used, cavities will result which have the same {1 0 0} orientation at the bottom and are determined laterally by {1 1 1} surfaces. The reason is that {1 1 1} surfaces are etched off using a minimal etch rate and this way becomes the most resistant"
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Could someone please help me understand this concept.




Certain types of ***wet*** etching agents have differential reactivity on different crystal surfaces. Consider that the Si-Si distance and coordination environment will be quite different. 


Let's start with an isotropic etch - it just eats silicon. Well, if you started with a pinprick (i.e., just a point and not a gap in the mask) you'd etch out a hemisphere. Normally, we etch through gaps in the mask, so you'll get rounded sides.


With an [anisotropic wet etch](http://en.wikipedia.org/wiki/Etching_(microfabrication)#Anisotropic_wet_etching_.28Orientation_dependent_etching.29) you can get flat, angular side-walls defined by crystal planes.



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> For instance, potassium hydroxide (KOH) displays an etch rate selectivity 400 times higher in <100> crystal directions than in <111> directions. 
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OK, so let's consider that etch through a gap in a mask into a <100> surface. As we go down 400 atomic layers of Si, we'll clearly have some etching along the side-walls (e.g., an "undercut"). But also consider that depending on the size of the feature for the etch, you'll go down into the <100> but not completely uniformly. Different wet anisotropic etches will have different selectivity rates between <100> and <111> and thus you get different aspect ratios.


Incidentally, [dry etching](https://www.memsnet.org/mems/processes/etch.html) (e.g., RIE, plasma) can give more vertical side-walls.


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