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
| Commonly Etched Materials | |
| Material | Selectivity to Si |
Si | 1:1 |
| Mo | 2:1 |
| Ge | Same or Faster than Si |
| SiGe | Same or Faster than Si |
Etch rates depend on the amount of exposed silicon, and will depend on a particular sample. | |
| Commonly Used Low or Non Reacting Materials | |
| Material | Selectivity to Si |
Thermal SiO2 | 1000:1 |
| Low Temperature SiO2 | 1000:1 |
| Si3N4 | >1000:1 |
| Gold | Low Amount of Attack Under Certain Conditions |
| Copper | Low Amount of Attack Under Certain Conditions |
SiC | Low Amount of Attack Under Certain Conditions |
| Non Reactive Materials | |
| Metals | Compounds |
| Al | PZT |
| Ni | MgO |
| Cr | ZnO |
| Pt | AlN |
| Ga | GaAs |
| Polymers and Organics | |
| Photo Resists | PDMS |
| C4F8 | Silica Glass |
| Dicing Tape | PP |
| PEN | PET |
| ETFE | Acrylic |
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.