Techniques - dry etch

“Dry etching” generally refers to a variety of etching techniques. In the NanoFab, however, “dry etching” most commonly refers to the removal of material from a substrate through the reaction of that material with ions and free radicals created by a plasma. The material being etched is normally masked in order to create a useful pattern. The NanoFab has both reactive ion etch (RIE) systems, and inductively couple plasma (ICP) systems. The RIE systems are further subdivided into parallel plate reactors, and barrel reactors.

Not all etch systems require plasmas. For example, dry vapor phase etch systems do not use plasmas. The NanoFab has a XeF2 dry vapor phase etch system. This system is specifically designed for the isotropic etching of silicon. Etching is achieved through the room temperature reaction between XeF2 and silicon.

Etch Process Considerations
Some considerations for the user in the development a new etch process are given in this section. It is also strongly suggested that the user consult NanoFab staff prior to beginning etch process development. Staff may be able to provide additional insight into etch process development for specific materials.

Google is your friend
The probability that the user is the first person in the history of the planet to try etching a particular material is, in all likelihood, very small. A Google search for an etch process for the material of interest will probably yield several possible processes. These processes can then be further evaluated for transferability to the etch equipment in the NanoFab.

Volatility of evolved compounds (products)
The primary etch chemistries available in the NanoFab are: Fluorine, chlorine, methane/hydrogen, and oxygen. If a material forms a volatile product with any of these chemistries, then that chemistry can be used as an etchant. If a material does not form a volatile product with any of these chemistries, then dry etching in the NanoFab will not work for the material.

Choice of masking material
Photoresist is a common choice for etch masking. It is easy to pattern and serves as a satisfactory etch mask in most cases. If photoresist is not a suitable etch mask, then one may use a dielectric etch mask.vvDielectric masks must themselves be patterned, however. This is usually done with a photoresist and etch process.

Selectivity
In addition to the etch rate of the material of interest, one should also consider the etch rate of the masking material, and the buried landing material beneath the material of interest when engineering an etch process. The etch rate of the mask should be low enough to enable to material of interest to be completely etched without significant erosion of the mask. The etch rate of the landing material should also be low enough to enable over etching of the material of interest without etching significantly into the landing material.

Etch tools available at NanoFab

The following is a list of NanoFab's dry etch tools and their capabilities:

Dry vapor phase etching

• Xactix XeF2 Si Etcher – deep isotropic silicon etching (XeF2)

Inductively coupled plasma etching

• STS ASE ICP DRIE - Fluorine – Bosch Process for anisotropic deep silicon etching(SF6, C4F8, O2, Ar).
• STS AGE ICP - Chlorine – Compound semiconductor etching(BCl3, Cl2, CH4, H2, O2, Ar).

Reactive ion etching, parallel plate reactor

• PlasmaLab M80 Plus – Fluorine - Silicon, silicon dioxide and silicon nitride etching(SF6, CHF3, CF4, O2, Ar).
• PlasmaLab M80 Plus – Chlorine - Compound semiconductors and metals (BCl3, Cl2, O2, Ar, CF4, N2).

Reactive ion etching, barrel reactor

• Tegal Ashers 1 and 2 – Photoresist stripping only (O2).