Now that a template has been established, the next step is etching. There are two

main methods used in industry to create the features, wet etching or dry etching. Wet

etching occurs when a material is dissolved when submersed in a chemical solution. A

disadvantage of wet etching is the isotropic nature which increases the difficulty to

control critical feature dimensions [3, 5]. Isotropic etching is when a material etches at

the same rate in all directions; undercutting is often a problem as shown in Figure 2.

Anisotropic etching, when a material etches at different rates in different directions can

also be a problem. Certain crystallographic planes of some materials etch at different

rates. However, if those planes are not vertical to the surface being etched, the hole being

created will not have perfectly vertical sidewalls (Figure 2).



 Slow etching crystal plane
 / Etch mask






 Anisotropic Isotropic


Figure 2. Anisotropic etching, different etch rates in different directions, with off-vertical
sidewalls. Isotropic etching, same etch rate in all directions, with undercutting of the
mask. [5] These are extreme examples of each case; it is possible to have a combination
of anisotropic and isotropic etching characteristics.

 Dry etching occurs when a material is sputtered off or dissolved using reactive ions or

a vapor phase etchant. An advantage of dry etching is the ability to better control feature

dimensions; there is greater resolution of the features. However, the cost of dry etching

is significantly greater than for wet etching. Wet chemical etching is used extensively in