How is the crystal direction important for etching silicon surfaces?
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 you can get flat, angular side-walls defined by crystal planes.


  For instance, potassium hydroxide (KOH) displays an etch rate selectivity 400 times higher in &lt;100> crystal directions than in &lt;111> directions. 


OK, so let's consider that etch through a gap in a mask into a &lt;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 &lt;100> but not completely uniformly. Different wet anisotropic etches will have different selectivity rates between &lt;100> and &lt;111> and thus you get different aspect ratios.

Incidentally, dry etching (e.g., RIE, plasma) can give more vertical side-walls.