Biomedical Engineering Reference
In-Depth Information
(DPN) method. This technique relies on the use of a small tip to deposit molecules
on a surface in a positive printing mode (Piner et al.
1999
). It can achieve relatively
small sizes, but currently it cannot go below 40 nm.
Similarly, scanning probe microscopes (SPMs) and atomic force microscopes
(AFMs) are often used in relation to nanolithography. They allow imaging the fi ne
details on surfaces without necessarily modifying it or inducing undesired damage.
However, SPM and AFM can also be used to achieve precise etch, write, or print on
surfaces obtaining nanometer or even single-atom resolution.
1.1.2
Dry Etching Techniques
In addition to the commonly used wet chemical etching for nanostructure fabrica-
tion, dry etching techniques (Lehmann
1991
) are also used in the processing of semi-
conductor wafers in which the material is immersed in plasmas or discharges
containing etching species. Dry etching is a process to remove material carried out in
the gas phase that uses no aqueous solutions, such as an acid or base, hence the name
“dry etching.” Dry etching techniques can be either purely chemical (plasma etch-
ing), purely physical (ion beam milling, IBM), or a combination of both (reactive
ion etching, RIE). The dry etch reactions are generally isotropic, taking place with
equal rate in any direction. They can be very selective etching techniques for removal
of one material relative to another, generally used in those applications in which
directionality (anisotropy) of etching in not required, e.g., in resist stripping.
Plasma etching, like most dry etching techniques, is carried out in chambers in
which high-energy electric and magnetic fi elds rapidly dissociate suitable feeding
gasses, forming a plasma. When the surface of a material gets in contact with the
plasma, it gets bombarded by molecules, ions, electrons, and photons which stimu-
late the expulsion of material from the surface, leading to atomically precise
removal of materials. Commonly, plasmas can be created in different ways: using
direct current (DC) electric fi elds, applying a radio frequency (RF) voltage source
or using a microwave source. One of the most common plasma etching techniques
used is RIE because of its relative simplicity and versatility. The RIE method
requires process pressures varying between 10
−3
and 10
−1
Torr. A schematic dia-
gram summarizing the differences between some of the dry etching methods is
shown in Fig.
3
(Madou
2002
) .
Another common dry etching technique is IBM. In this case, the plasma source
is decoupled from the substrate, which is placed on a separate third electrode in a
triode setup. IBM can be employed to cover relatively large areas. A commonly
used IBM method is focused ion beam (FIB), usually coupled to a high-resolution
scanning electron microscope (SEM) to provide visual guidance during the writing
process. FIB uses a beam of ions (usually Ga
2+
) that gets narrowed in a very small
area and it is employed as a direct writing tool for patterning holes, thinning sam-
ples, and depositing insulating or conductive material almost atom by atom. FIB
milling is a powerful technique, now commonly considered a “mechanical” machin-
ing technique in which the classical metal drill bit used in all machine shops is
