ABSTRACT
The FinFET architecture leads to a more complex plasma etching
processing, especially for active fins with a critical dimension (CD)
below 25 nm targeting straight profiles, needed for controlling the
short channel effects (SCE) [1]. Patterning such silicon fins becomes
more challenging when the structure density increases and the CD
is scaled down to less than 15 nm. When a node is scaled down,
among many other parameters, the fin pitch changes, provoking
drifts between the plasma and exposed surface, i.e., etching rates
(ER) and etched profiles shapes. In this chapter we present the
evolution of the plasma etching processes from a 130 nm-like
node to 16 nm node, in other words, from a “relax” fin pitch
of 350 nm to an aggressive 64 nm pitch. In R&D or industrial
environment, the etching of fins, gates, spacers, trenches, contact
holes, etc., is generally carried out in glow discharge reactors, which
are commonly referred to as processing plasma’s, plasma etching or
dry etching. Although, glow discharges are beyond the scope of this
chapter, a general overview is needed in order to make the reader
familiar with the jargon of plasma etching. A general definition of
plasma etching would be: a process that uses a glow discharge
of an appropriate gas chemistry as a source of etching species,
which in contact with the substrate enables chemical reactions
near room temperature1 producing volatile etching products. For
example, in a glow discharge the inert molecule Cl2 is converted
into very reactive species such as Cl atoms that spontaneously
react with silicon to form SiCl4 (silicon tetra-chloride) molecules
which are very volatile at room temperature. In plasma etching,
molecular glow discharges such as CF4, CH2F2, SF6, HBr, Cl2, etc., are
widely used; these molecules can be used individually or combined
depending on the goal of the etching step [2]. A glow discharge is
formedwhenever a gas is forced to conduct electric current, creating
a collection of electrically charged and neutral particles. The density
of the negatively charged particles (electrons and negative ions) is
equal to the density of positively charge particles (positive ions).
This is true for the bulk of the plasma well away from the boundary
surfaces. Whenever a plasma is in contact with a surface, a boundary
layer known as “sheath” is formed [2, 3]. In a sheath there are more
positive charges than electrons because it tends to repel electrons
and attracts positive ions, allowing the ion bombardment for etching
materials (Figure 2.1). For example the glow discharge of CF4 can
create plasmas that are considered to contain F atoms, CF, CF2, CF3 radicals, CF3
