Plasma etching/Gases

The type and ratio of gases used in a plasma etch is chosen depending on the material being etched, the masking material, and the etch stop material. Ideally, the gas mixture will react readily with the material being etched, while reacting minimally with materials that should not be etched, thus achieving high selectivity. In addition, the reaction byproducts should be highly volatile. In general, etch gases can be divided into a few categories depending on their use in plasma etching.

Active gases

These gases are the main reactant used in an etch process and should be chosen depending on the material being etched. They are typically molecules composed of highly reactive elements, many from the halogen group. These dissociate in the plasma creating a large concentration of this element in an excited state which will react with the material on the sample. Sometimes, a combination of active gases is used in order to achieve selectivity to another material by carefully balancing the concentrations of the different elements. For specific details on what active species to use, please review the material page. Some common species used in our lab are listed below.

• Fluorine species
  • SF₆, CF₄, CHF₃, C₄F₈
  • Used mainly for etching silicon based materials and a few metals
• Chlorine and bromine species
  • Cl₂, BCl₃, HBr
  • Used for etching polysilicon, compound semiconductors, and many reactive metals
• Oxygen
  • O₂ is used as a reactive species for etching polymers

Additive gases

These are gases added with the active gases (typically < 20% of the total gas mixture) in order to shift the concentration of reactive elements, improve the selectivity or directionality, enhance the reactivity, or some other function.

Reactant adjustment

These additive gases are used to adjust the concentration of free radicals (reactive species) in the plasma. These are typically used in fluorocarbon based processes to adjust the ratio of carbon to fluorine, or in SF₆ processes to enhance the fluorine concentration. Hydrogen is a fluorine acceptor, meaning it reduces the concentration of free fluorine in the plasma. Oxygen, on the other hand, will increase the concentration of fluorine radicals. A good example of using these additives is when trying to achieve selectivity between silicon dioxide and silicon nitride. Etching nitride requires significantly more free fluorine, so to achieve selectivity to oxide, oxygen could be added to the gas mixture. To achieve selectivity of oxide to nitride, add hydrogen instead.

Diluant gases

Noble gases like argon and helium are often used to dilute a gas mixture. This can be done to control etch rate or to improve other aspects of the etch. For example, etching silicon dioxide uses fluorocarbon species, which are known to also passivate the surface with teflon. In order to reduce this passivation enough to etch the oxide and achieve a vertical sidewall, a diluant is often used. Diluants concentration can range anywhere from 50% of the mixture to over 10x that of the reactive species.