Sidewall Passivation

At room temperature etch anisotropy is always obtained thanks to the formation of a sidewall passivation layer: The sidewall passivation layer can be formed by different mechanisms (slide 1):
- Mask etch products sputtered into the plasma gas phase by energetic ion bombardment and get re-deposited on the feature sidewalls.
- Condensation of some molecules or atoms originating from the dissociation of the feed gas stock.
- Etch by products dissociation in the gas phase leading to the formation of partially volatile or non volatile etch by products which get re-deposited on the feature sidewalls.
- Direct line of sight deposition of non volatile etch by -products.

Slide 2 illustrates the various passivation mechanisms.

Different etch processes are dominated by different passivation mechanisms (slide 3). This has extremely important consequences for the response of process results like CD, profile angle and profile microloading to process parameters like pressure, source and bias power. Aluminum etch is dominated by re-deposition of resist etch products on the Al sidewalls generating a carbon-based passivation layer. In dielectric etch, condensation of CFx species from the fluorocarbon gas on the oxide sidewalls and re-deposition of resist etch products both contribute to the CFy passivation layer formation. Re-deposition of silicon etch products from the plasma gas phase and direct line of sight deposition of silicon etch products drive the passivation layer formation during silicon (gate) etch.

More plasma etch fundamentals ...

The Selfclean Concept in Silicon Gate Etching

For the traditional HBr/Cl2/O2 silicon gate etch chemistry, high concentrations of etch products are present in the gas phase. These by-products can dissociate and form non volatile species. This leads to re-deposition of silicon oxyhalogenides on the feature sidewalls and in the presence of oxygen in the plasma to oxydation to form SiOx on the sidewalls. Direct line of sight re-deposition from the bottom of the features is also possible. The same mechanisms holds to explain the formation of SiOx on the chamber walls. The chamber wall will be covered with SiOx deposits (slide 1).

When CF4 is added, the formation of silicon oxyhalogenides is suppressed and silicon fluorides or oxyfluorides are formed. The SiOx based passivation film is replaced by a thin carbon rich layer (slide 2).

XPS studies reveal that the SiOxCly based passivation film is transformed in a CFxCly based passivation film when CF4 is added to the standard HBr/Cl2/O2 chemistry (slide 3).

In the steady state etching regime, a thin halogen rich fluorocarbon film is formed on the silicon surface at the bottom of the features (slide 4).

More plasma etch applications …