[mems-talk] DRIE etching

[Nicolas Duarte]

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?