The sidewall scallops come from the chemical nature of the Bosch
process.  Specifically a Bosch process involves anisotropic etching
(SF6), followed by a placement of a passivation layer on all surfaces
(from C4F8), and repeat.  Some choose to add an oxygen plasma step to
help removing the polymer that deposited on the etch surface.  Note
that there is no such thing as an RIE that is just chemical or just
physical...every RIE process is a combination of both, hence the name
Reactive Ion Etching.  If it is just physical it is called Ion
Milling and if it is just chemical then your wasting power creating a
plasma.

The reason why the Bosch process is used is that no process is 100%
anisotropic.  For various reasons (ion bowing as an example) the etch
part of the Bosch process (or any plasma etch left long enough for
that matter) will cause a sort of balloon etch.  Once the passivation
layer is placed on the walls, this balloon hole will not open up any
more, instead the bottom of the hole will go deeper during the next
etch phase and create a the balloon feature below it.  It is hard to
explain without drawings but most any MEMS textbook will explain it
in greater detail.  Also a quick search on google and I found this
page which has some good drawings to explain scalloping and notching
http://cmi.epfl.ch/etch/Talk_Cyrille_CMI2004.pdf.

So as to why the solutions I suggested:

First off higher substrate power will pull the ions down more
vertically, giving them better directionality and less of the balloon
effect.  Too much however and you start getting the balloon back
since the ions will just start milling and shooting the byproducts
back out at random directions.  With higher power also comes more
speed, more roughness at the bottom of the pit, and it is harder to
control etch depth.

As for the shorter cycles, it will make smaller balloons before
moving deeper.  The smaller the balloon, the smaller the scallops.
You will also get a much slower etch and probably use up more of your
reactant gas.

Hope this description helped.

Nicolas "Nik" Duarte
Penn State University
PhD Student under Dr Srinivas Tadigadapa

At 11:12 AM +0100 9/11/06, K A Chan wrote:
>Nik,
>Can you explain why higher substrate power and shorter cycles could
>reduce the scallops?
>What chemistries have been used in Bosch process? Is it a pure
>chemical etch or a combination of chemical + physical etch with high
>energetic bombardment of ions using high substrate power?