Overview

Note: Some literature mentions XeF₂ etching of Ti, TiN, Ta, TaN, and TiW. Etching of these materials is temperature dependent, negligible etching occurs below 50 degrees C for any of these materials. Our system does not have heated chuck capability and is not suitable to etch these materials.

General Description

The Xactix E1 Xenon Difluroide etcher is an isotropic etch system, typically used for MEMS applications. XeF₂ etching shows extremely high selectivity of silicon to silicon dioxide, photoresist, silicon nitride, and aluminum. At room temperature and atmospheric pressure Xenon Difluroide (XeF₂) is a white solid material.  It sublimates directly into the required vapor-phase etchant, not requiring plasma or other activation, under vacuum at room temperature and about 4 Torr pressure.  XeF₂ etch is a dry, isotropic, etch desirable for many MEMS release applications.  Not needing plasma activation minimizes damage to other materials on the wafer and offers broad flexibility in process design. The XeF₂-Si reaction is exothermic and the process engineer should be mindful of potential thermal issues. 

For silicon, the etch proceeds as:
2XeF₂ + Si => 2Xe + SiF₄

Specifications

Etch rates: Highly dependent on sample size, from ~10 μm/min for small samples to ~0.2 μm/min for whole 6" wafers.

Selectivity:
1000:1 (Thermal/Low Temperature SiO₂:Si)
>1000:1 Si₃N₄:Si
Low attack (depending on conditions) for Gold, Copper, and Si

Nonreactive with:
Al, Ni, Cr, Pt, Ga
PZT, MgO, ZnO, AlN, GaAs,
Photoresists, PDMS, C4F8, Silica Glass, PVC dicing tape, PP, PEN, PET, ETFE, and Acrylic

Available in either pulsed or continuous flow

Technology Overview

XeF₂ exists as solid crystals, and sublimates to form a vapor-phase etchant. The process is a dry, vacuum based process that proceeds spontaneously, and thus does not rely on a plasma or other chemical activation. It proceeds as an isotropic etch for silicon, molybdenus, and germanium. The XeF2 etching rate does not depend on crystal plane, or silicon dopant content. As the XeF₂-Si reaction is exothermic, a delay step may be added to cool the wafer between etch cycles and minimize any thermal issues.

For silicon, the etch proceeds as:
2XeF₂ + Si => 2Xe + SiF₄

Sample Requirements and Preparation

For well controlled silicon etching, a BOE dip should be done directly before XeF₂ etching. XeF₂ will etch SiO₂ very slowly, so removing the native oxide layer will allow for a more uniform etch.

Whether the native oxide has been removed or not, all samples should be dehydrated directly prior to placement in etch chamber, either with IPA or a hotplate dehydration bake. The presence of adsorbed water on samples will result in the formation of both gaseous HF and a silicon flouride polymer on the sample surface. This polymer layer will reduce or completely stop etch progression, and will not be removed in either solvent soaks or O2 plasma.

Note: XeF₂ will also etch Titanium, Titanium Nitride, Tantalum, Tantalum Nitride, and Tungsten at etch rates from 0-30 nm/min, strongly depending on the local temperature.

  Silicon based samples up to 4" may be used.

Standard Operating Procedure

Questions & Troubleshooting

How long should my cycle time be?
Cycle time should be set to allow all of the XeF2 vapor to react with exposed silicon. For large areas of exposed silicon, XeF₂ will quickly react with the exposed areas, and the cycle time may be reduced. For a smaller chip, cycle times will need to somewhat longer to allow all of the XeF2 to react fully.

In order to tell that all XeF2 has reacted, the chamber pressure may be monitored. The etching reaction proceeds as 2XeF₂ + Si -> SiF₄ + 2Xe, meaning 2 moles of reactant gas will be present at the beginning of the etch step, and 3 will be present after the etch has completed. Therefore, the pressure in the chamber will increase somewhat while the etch is proceeding, and will level off after the etch has completed.

The etch rate on my sample seems nonuniform?
Uniformity is an inherent issue with XeF₂ etching. There is a limited amount of etchant released in each cycle, and the local etch rate will depend on the amount of exposed silicon and available XeF₂ vapor. In general, large exposed areas of silicon will show the worst uniformity, and regular patterns of small holes (with the rest of the wafer covered) will show the best uniformity. Minimizing unneeded exposed silicon will allow for faster and more uniform etching of the pattern.

Process Library

References

Isotropic Etching With Xenon Difluoride - Presentation from Xactix giving an overview of Xenon Difluoride etching, including selectivity and etch rate information (Internal Resource)

Unique Capabilities of Xenon Difluoride for Releasing MEMS - Xactix (Internal Resource)

"A behavior of Xenon difluoride etching on poly-silicon sacrificial layer for fabrication of MEMS device" - Takeshi Taniguchi et al., 2002. (Internal Resource)

XeF2 Etching of Silicon