An Introduction to mems (Micro-electromechanical Systems)

Wet etching describes the removal of material through the immersion of a material (typically 

a silicon wafer) in a liquid bath of a chemical etchant.  These etchants can be isotropic or 

anisotropic. 

Isotropic etchants etch the material at the same rate in all directions, and consequently remove 

as undercutting (Figure 19 a and b).  The most common form of isotropic silicon etch is HNA, 

which comprises a mixture of hydrofluoric acid (HF), nitric acid (HNO

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) and acetic acid 

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COOH).  Isotropic etchants are limited by the geometry of the structure to be etched.  

can stop due to diffusion limiting factors.  However, this effect can be minimized by agitation 

of the etchant, resulting in structures with near perfect and rounded surfaces (Figure 19a) [4].  

Anisotropic etchants etch faster in a preferred direction.  Potassium hydroxide (KOH) is the 

substrate are dependent on the crystal orientation of the substrate or wafer.  Most such 

anisotropic etchants progress rapidly in the crystal direction perpendicular to the (110) plane 

and less rapidly in the direction perpendicular to the (100) plane.  The direction perpendicular 

to the (111) plane etches very slowly if at all.  Figures 19c and 19d shows examples of 

anisotropic etching in (100) and (110) silicon.  Silicon wafers, originally cut from a large 

ingot of silicon grown from single seed silicon, are cut according to the crystallographic 

plane.  They can be supplied in terms of the orientation of the surface plane.   

Dopant levels within the substrate can affect the etch rate by KOH, and if levels are high 

a diffusion process.  This can be used to selectively etch regions in the silicon leaving doped 

areas unaffected. 

Figure 19.  Isotropic etching with (a) and without (b) 

silicon (c and d respectively) [9]. 

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