Biomedical Engineering Reference
In-Depth Information
PHYSICAL SPUTTERING
and Ion Milling
Momentum Transfer
Directional Etch
Poor selectivity
Possible radiation damage
< 100 mTorr
Higher
Excitation
Energy
RIE
Combines physical and chemical
Directional Etch
Better selectivity
~ 100 mTorr
PLASMA ETCHING
Chemical (faster)
Isotropic
More selectivity
Little radiation damage
Higher Pressure
Fig. 3
Schematic diagram of the dry etching techniques spectrum (modifi ed from Madou
2002
)
replaced by the focused beam of highly energetic ions. However, the disadvantages
of FIB are long processing time, the risk of undesired ion implantation, or damage
to the sample.
1.1.3
Anodization
Anodization is a term that stands for anodic oxidation and it is “the process of
growth of an oxide on the surface of a biased conductive solid (e.g., semiconduc-
tor wafer) immersed in a liquid electrolyte or gas. With this method, under spe-
cifi c process conditions, a thin, dense, barrier oxide of uniform thickness can be
grown on several different metals” (Alwitt
2002
). The properties of the grown
oxide layer vary as a function of the materials selected and the process conditions.
The commercial application of anodic oxidation extends to different surface coat-
ings, mostly based on aluminum, tantalum, niobium, and zirconium. Recently,
titanium has been also receiving increasing attention in bio-related applications
(Oh et al.
2005
). Aluminum in particular is commonly used because of its ease
and versatility of processing. In addition to the creation of a thin barrier oxide, the
anodization of aluminum, under specifi c conditions, can produce a thick oxide
coating with a high density of vertically oriented nanopores (see Fig.
4
) ( Asoh
et al.
2001
). This porous substrate can then function as template for other assisted
