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

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

Sample Requirements and Preparation

The Xactix E1 Xenon Difluoride etcher will accept small clean and dry pieces up to full 100 mm (4 inch) diameter wafers.  Small pieces can be placed directly on the sample surface with no mounting required.

XeF₂ etching is a chemical diffusion process, as such, the etch rate and etch profiles are dependent on loading effects. Any silicon that you do not intend to etch should be covered to prevent consumption of XeF₂ resulting in etch process variation. A test run with the exact same sample size and pattern is required to determine specific etch rate for a process.  The following effects have been observed:

1. Etch rate differences can be observed between the center and the edge for wafers with large etched areas.
2. Exposed silicon at the edge of the wafer can consume XeF₂ and lead to more etch rate non-uniformity.  This can be from photoresist edge bead removal or simply form loss of photoresist on the vertical edge of the wafer. 

3. Patterns with differences in feature sizes can show different etch rates depending on the feature size or to the proximity of two adjacent features.

Standard Operating Procedure

In general better results are obtained by etching in one run rather than several shorter runs where the sample is removed and then re-etched.

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