Plasma-Processed Biomimetic Nanoand Micro-Structures - Biomimetic Architectures by Plasma Processing - page 94

3.2 Silicon Nanotips Formation
The published silicon nanotips can be classified into two categories.
In the first “organised” category, the geometrical properties
(e.g., arrangement and density) are already determined by the
nanolithography process. Even though it is an expensive process
due to the utilisation of nanolithography process, the key advantage
is the controllability over the arrangement and density of silicon
nanotips [7,8]. In the second “self-organised” or “self-masked”
category, there is no involvement of nanolithography process
because the geometries are given by the utilised process [9,10]. This
is a cost-efficient method as an alternative way to microfabricate
silicon nanotips.
Silicon nanotip is a modified surface consisting of a high density
array of needle-like silicon nanostructures. In the area between the
nanotips, the incoming light will be caught and cannot leave the
silicon surface any more. Thus, all the light is collected by the etched
surface and turns black. Because of the microscopic appearance and
the low reflectivity in the visible spectrum, the terms “silicon grass”
and “black silicon” were used. However, in this chapter the term
“silicon nanotips” is used.
This phenomenon was originally reported as an unwanted side
effect of reactive ion etching (RIE) plasma etching process because
it strongly slows down the etch rate of silicon and prevents the
accuracy of the geometries [11]. Thus, most published papers dealt
with strategies to prevent the formation of the silicon nanotips.
For example, Jansen et al. published guidelines to optimise highly
anisotropic RIE process—the black silicon method. Black silicon
is used as an indicator to find the optimal process conditions for
anisotropic etching [12]. However, new applications of black silicon
emerge over time such as in optoelectronics (e.g., silicon solar cells,
antireflection surface), microfluidics or biosensors, wafer bonding,
and so on [10,13,14]. Thus, the formation of a reliable silicon nanotip
process having features of both simplicity and reproducibility is of
great interest.
In this chapter, three different techniques of forming the silicon
nanotips without utilising nanolithography will be introduced. The
first method is composed of two different processing steps which