Nanotechnology-Enabled Sensors

4.9 Nanolithography and Nano-Patterning 203
3. Isotropy: Some single crystal materials, such as silicon, exhibit anisotropic
etching in certain etchants. Anisotropic etching, in contrast to
isotropic etching, means different etch rates for different directions.
As an example, different silicon crystal orientations are known to
have different etch rates. A classic example is when potassium hydroxide
(KOH) etches the silicon plane, producing a characteristic
anisotropic V-etch, in which sidewalls form a 54.7° angle with
the surface. In contrast, for the crystal plane, the etched sidewalls
appear to be vertical.
4. Selectivity: It is a very important factor as we desire selective etch for
the target materials and not the other materials exposed to the etchant.
The etching processes fall into two categories:
(a) Wet etching: In the wet etching process the material that we wish to
etch is removed when immersed in a chemical solution. It works
very well for etching thin films, and can also be utilized to etch the
substrate itself to produce structural forms onto it.
(b) Dry etching is the removal of materials from a substrate surface
through bombardment with energetic ions that carve out the surface.
The dry etching technology can be divided into three major categories:
reactive ion etching (RIE) , sputter etching, and vapor phase
etching.
To conduct the RIE process, the substrate is placed inside a reactor.
Consequently, several gases are introduced into the reactor chamber. The
gas molecules are ionized using a RF power source which result in plasma
generation. The ions are then accelerated towards the substrate that is to be
etched. Upon arriving at the surface, they react with the material and etch
it. Other gaseous materials are formed during this process, which are
known as the chemical part of reactive ion etching. If the impinging ions
have enough energy, then they can physically knock atoms out of the material
without any chemical reactions occurring. Developing dry etching
processes that balance chemical and physical etching is a delicate task, as
there are many parameters that need fine tuning. Generally the chemical
part causes an isotropic etch and the physical part produces anisotropic
etch. By controlling these parameters, we can form features ranging from
rounded to vertical, tailoring the anisotropy of the etch.
A subclass of RIE is Deep RIE, or DRIE. In DRIE, sidewalls are almost
vertical and features with very large aspect ratios (as large as 100) can be
obtained. This process is also termed the Bosch process, after the Robert