XeF2 etcher

Overview

Standard Operating Procedure: XeF₂ Etcher

The XeF₂ etcher in the USC Nanofab lab uses XeF₂ crystals and a vacuum chamber to produce fluorine vapors which isotropically etch silicon with very high selectivity to photoresist, oxides, nitrides, and many metals. Typical silicon etch rates are 1–2 µm per minute but can vary widely due to loading effects. Samples up to 6" diameter can be accommodated. Typical applications are cantilever release etching for micro-machined structures.

Originally written by Eugene Yoon, June 2023. See also the main equipment page: XeF₂ Etcher.

Warnings

⚠ Safety Warnings

Although XeF₂ itself is not considered very toxic, the production of hydrofluoric acid (HF) vapors by exposing XeF₂ crystals to air is a serious potential health threat. Only trained personnel may handle the crystals directly. Observe all purge and pumping procedures. If any odors are detected, put the system in standby and contact staff immediately. Do NOT proceed with etching. HF fumes have a pungent, irritating, penetrating odor and can be smelled at 0.04 ppm (a relatively safe level) — but odor should not be relied on to judge safe concentration, since the odor threshold varies widely between individuals. The crystals are also very expensive and react with water vapor. The XeF₂ source chamber should never be exposed to air, and should be isolated when etching is complete to prevent the crystals from being pumped away.

In case of emergency, call USC DPS: 213-740-4321

Sample Preparation

Samples must be clean, dry, and free from oxide, otherwise etching will be slow and non-uniform. An HF dip followed by a rinse and dehydration bake may be required to ensure adequate cleanliness and dryness.

• Oxides, photoresist, and nitrides provide high-selectivity masking for this etch.
• Loading effects may be observed depending on sample size and the amount of exposed silicon.
• Always perform a test run with the exact same sample size and pattern when determining etch rate and depth.
• In general, better results are obtained by etching in one run rather than several shorter runs where the sample is removed and re-etched.

Machine Description

Refer to the LabVIEW front panel (Figure 1) for machine configuration and valving. There are 5 valves, referred to as V1–V5:

Valve	Function
V1	Main vacuum valve, allows pumping of the etching chamber. There should always be a small N₂ flow during standby mode to prevent oil backstreaming.
V2	Flows nitrogen into the etch chamber for purging (to remove XeF₂ vapors) and for back-filling the chamber so the lid can be opened.
V3	Allows flow of XeF₂ vapor from the pulsing chamber into the etch chamber where the sample resides.
V4	Flows nitrogen to the pulsing chamber.
V5	XeF₂ source valve — allows vapor from the crystals to flow into the pulse chamber.

"Process Control" section – parameter descriptions

Parameter	Description
Etching Pressure	Pressure value setpoint of the etching chamber during pulse etches.
Pumping Pressure	Base pressure when evacuating the etching chamber.
Duration Time	Amount of time spent in each etching pulse.
Purge Duration	Amount of time spent in each purging cycle.
Number of Pulses	Number of times there will be etching pulses.
N₂ Purge Cycles	Number of times there will be N₂ purge cycles.

The etcher is operated through a LabVIEW front panel (program file: USCxef210-8-07source) showing the valve control indicators and process control parameters above.

Background + Additional Info

At room temperature, XeF₂ is a solid crystalline form. Under low pressure, XeF₂ sublimates into gas. This gaseous XeF₂ etches silicon; it does not etch silica, most photoresists, oxides, nitrides, or many metals. A more detailed explanation of the etching mechanism can be found in B. Garrison and W. Goddard, "Reaction Mechanism for Fluorine Etching of Silicon," Physical Review B 36:18 (1987).

The Etching Chamber is located at the top of the etcher and can be opened and closed to load samples using the black knob. Manual operation is possible using the 5 vacuum switches and pressure display, but users must operate the tool in automatic mode, which ensures accurate operation and saves time.

The Pressure Control Valve controls the rate at which the etching chamber's pressure drops. To prevent lightweight samples from flipping over or blowing away, turn the pressure control valve to lower the rate at which the pressure drops.

If you make an error: press the red "Stop" button in the top-left corner of the LabVIEW program, then press "Run." Click "Purge" to purge the chamber.

To restart the LabVIEW program: press "Stop," then close the program entirely by clicking the red-and-white "X" in the top-right corner. Open the file called USCxef210-8-07source on the desktop, press "Run," then click "Standby" to bring the tool back to Standby mode.

Please notify staff if:

1. The XeF₂ valve was left open.
2. The system's initial etching chamber pressure is over 100 mTorr (it should be ~20 mTorr).
3. The system takes a long time to reach its etching pressure during pulse etch.

Operating Procedure

1. Check that the XeF₂ valve is closed (the arrows point towards each other: "→ ←").
2. Activate the tool on NEMO.
3. Check that the tool is in 'Standby' mode.
4. Fill in relevant information on the log sheet.
5. Click 'Remove Wafer', unscrew the black knob, and wait until the step is complete.
6. Open the chamber, load sample(s), close the chamber, then tighten the black knob.
7. Click 'Purge' and wait until the step is complete. Note: moisture from ambient air enters the chamber when loading a sample — it is important to remove this moisture with a sufficiently long purge step.
8. Open the XeF₂ valve (the arrows do not point towards each other: "↑ ←").
9. Click 'Pulse Etch' and wait until the step is complete. Note: the first few pulses will have difficulty precisely attaining the desired etching pressure, but later pulses will occur at the desired pressure.
10. Close the XeF₂ valve (arrows point towards each other: "→ ←").
11. Click 'Remove Wafer' and wait until the step is complete.
12. Unscrew the black knob and remove your sample(s).
13. If you have more samples, load them and repeat the procedure from step 6.
14. If you are finished etching, close the chamber, tighten the black knob, and click 'Standby'.
15. Wait 2 minutes.
16. Fill in the 'End Status' on the log sheet and add any comments in 'Notes' (if applicable).
17. Deactivate the tool on NEMO.

Related Pages

• XeF₂ Etcher — equipment page
• XeF₂ Quickstart Guide