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P. Kumar et al.
Fig. 3.6 Schematic illustration of different stages of Bosch process for etching holes or trenches in
Si: (a) placement of a polymer mask on Si surface, (b) flow of etchant plasma on exposed surface
of Si and subsequent etching of Si, (c) flowing passivation gases and subsequent formation of
passivation layer on walls of Si, and (d) erosion of polymer layer from the bottom of the Si hole
(depassivation) and subsequent further etching of Si at the bottom of the hole
and the bottom of the hole (see Fig. 3.6c) [33]. The most common passivation gas
is octa-fluoro-cyclo-butane, C 4 F 8 , which breaks down and produces the repeating
unit -[CF 2 ]- of a Teflon type polymer in presence of the plasma created in the
etching process. The polymer then gets deposited on the walls of Si hole [21].
4. In the next sequence of etching (i.e., step 1), the fast moving plasma impinges on
the bottom of the hole and abrasively erodes the polymer layer from there; this
step is called depassivation (see Fig. 3.6d). This exposes Si at the bottom of the
hole to the etchant and hence the depth of the hole further increases. On the other
hand, the passivation of the vertical sidewalls remains intact (as they are parallel to
the flow of plasma and do not collide head-on with plasma) and hence the Si walls
remain protected from the etchants. However, the plasma removes some polymer
layer from sidewalls also, especially near the bottom of the hole. This leads to
sideways etching or undercutting of the Si walls, leading to a scallopy structure
(see Fig. 3.7). Minimization of the scallopy features or surface roughness is a
challenge in Bosch process, giving rise to several innovative technologies, as will
be explained below.
P. Kumar et al.
Fig. 3.6 Schematic illustration of different stages of Bosch process for etching holes or trenches in
Si: (a) placement of a polymer mask on Si surface, (b) flow of etchant plasma on exposed surface
of Si and subsequent etching of Si, (c) flowing passivation gases and subsequent formation of
passivation layer on walls of Si, and (d) erosion of polymer layer from the bottom of the Si hole
(depassivation) and subsequent further etching of Si at the bottom of the hole
and the bottom of the hole (see Fig. 3.6c) [33]. The most common passivation gas
is octa-fluoro-cyclo-butane, C 4 F 8 , which breaks down and produces the repeating
unit -[CF 2 ]- of a Teflon type polymer in presence of the plasma created in the
etching process. The polymer then gets deposited on the walls of Si hole [21].
4. In the next sequence of etching (i.e., step 1), the fast moving plasma impinges on
the bottom of the hole and abrasively erodes the polymer layer from there; this
step is called depassivation (see Fig. 3.6d). This exposes Si at the bottom of the
hole to the etchant and hence the depth of the hole further increases. On the other
hand, the passivation of the vertical sidewalls remains intact (as they are parallel to
the flow of plasma and do not collide head-on with plasma) and hence the Si walls
remain protected from the etchants. However, the plasma removes some polymer
layer from sidewalls also, especially near the bottom of the hole. This leads to
sideways etching or undercutting of the Si walls, leading to a scallopy structure
(see Fig. 3.7). Minimization of the scallopy features or surface roughness is a
challenge in Bosch process, giving rise to several innovative technologies, as will
be explained below.