XeF2 etcher
About
Films / Materials: This is a custom-built xenon difluoride (XeF₂) vapor-phase etcher that provides extremely fast, isotropic, and highly selective etching of silicon. XeF₂ reacts spontaneously with Si to form volatile SiF₄, enabling rapid Si removal without plasma, ions, or damage to non-Si materials.
Hardware: Vacuum chamber with a XeF₂ crystal source that sublimates to vapor. Controlled XeF₂ vapor delivery via pulse or continuous flow. No RF/plasma required (purely chemical etch). Substrate holder for up to 6 wafers. Temperature controlled at ~20°C (room temperature). Simple, low-damage design ideal for release etching or bulk Si removal. Custom built by the Armani group.
Gases / Source:
- XeF₂ crystals (solid source – sublimes to vapor; no carrier gas needed)
- N₂ or Ar purge/backfill (if equipped; typically high vacuum or XeF₂-only atmosphere)
Etch Properties:
- Extremely high selectivity to Si (>1000:1 vs. SiO₂, Si₃N₄, photoresist, Al, Au, etc.).
- Completely isotropic (undercut, rounded features).
- Very high etch rate (often 1–10 µm/min depending on vapor pressure and exposure).
- No plasma damage, charging, or ion-induced defects.
- Excellent for release of suspended structures or deep undercut.
Applications: MEMS structural release (e.g., removing sacrificial Si in SOI devices), bulk Si thinning or cavity formation, nanowire or freestanding structure fabrication, selective Si removal in hybrid devices, and failure analysis or delayering (Si etch without attacking metals/oxides).
Usage: Load wafer (up to 6), pump down chamber, introduce XeF₂ vapor (pulse or continuous), monitor etch progress (timed or visual), then purge and vent. Use short pulses for controlled etching and avoid over-etching due to the high rate. Follow the SOP for crystal loading and handling (XeF₂ is reactive).
Detailed Specifications
- Model: Custom XeF₂ etcher (built by the Armani group)
- Location: Deposition Bay 1
- Substrate size: Up to 6 wafers (pieces/smaller supported)
- Temperature: 20°C (room-temperature operation)
- Etch source: Solid XeF₂ crystals (sublimation-based vapor delivery)
- Etch type: Purely chemical, isotropic, plasma-free
- Features: High Si etch rate, exceptional selectivity (>1000:1 to most masks/materials), simple operation, low damage
Restrictions and Materials Allowed
| Restrictions | Materials Allowed |
|---|---|
|
|
Safety & Emergency
Required PPE: Safety glasses, cleanroom gloves, bunny suit.
Hazard: XeF₂ is a reactive solid that forms hydrofluoric acid (HF) on contact with moisture, including skin and lungs. Never handle the crystal source with bare hands or expose it to water. Do not open the chamber if a gas alarm is active.
If the tool alarms or you suspect an XeF₂/HF release:
- Press EMO (Emergency Machine Off) -> red button on the tool.
- Evacuate Deposition Bay 1 and notify staff immediately.
- Call Nanofab staff: Chandan Ramakrishnaiah, +1 (213) 551-6726.
- After hours / life-threatening: USC DPS (213) 740-4321, or 911.
HF / chemical exposure: Apply calcium gluconate gel to affected skin if trained, seek immediate medical attention, and call DPS. HF exposure is a medical emergency even if pain is delayed. Chamber vent failure: Do NOT force open. Leave tool and contact staff.
Dry Etcher Comparison
Use this table to help choose the right etching tool for your process.
| Feature | Oxford RIE | Oxford DRIE | Oxford DRIE-ALE | Oxford III-V | XeF2 Etcher |
|---|---|---|---|---|---|
| Model | PlasmaPro 80 RIE | Oxford System 100 DRIE | Oxford DRIE + ALE | PlasmaPro 100 Cobra | Custom built (Armani group) |
| Location | Etch Bay 1 | Etch Bay 1 | Etch Bay 1 | Etch Bay 1 | Deposition Bay 1 |
| Substrate Size | Up to 200 mm (8") | Up to 4" clamp; 2", 3", 4", 6", 8" possible | Up to 4" clamp; 2", 3", 4", 6", 8" possible | Up to 4" clamp; 2", 3", 4", 6", 8" possible | Up to 6" |
| Etch Type | RIE (capacitively coupled) | DRIE / Bosch (ICP) | DRIE + Atomic Layer Etch | ICP-RIE (cryo capable) | Isotropic chemical etch |
| Table RF Power | 300 W max | 300 W max | 300 W max | 1500 W max | N/A |
| ICP Power | None (RIE only) | 3000 W max | 3000 W max | 1500 W max | N/A |
| Table Temperature | 20°C | 20°C | -120°C to 200°C allowed
(-150°C to 400°C max) |
-120°C to 200°C allowed
(-150°C to 400°C max) |
20°C |
| Gases | Ar, O₂, CF₄, CHF₃, SF₆ | Ar, O₂, CF₄, CHF₃, SF₆, C₄F₈ | Ar, O₂, C₄F₈, SF₆, BCl₃, Cl₂, H₂, N₂O, CH₄, CF₄, CHF₃ | Ar, O₂, SF₆, CH₄, H₂, Cl₂, BCl₃, SiCl₄ | XeF₂ (crystal source) |
| Allowed Materials | Si, SiO₂, Si₃N₄, oxide/nitride masks, Cr | Si, SiO₂, SiN, Parylene, Polyimide, LiNbO₃, BCB | InP, InAs, GaN, AlGaAs, GaAs, InGaAsP, ITO, Si, SiC, MoS₂, WSe, Graphene | InP, InAs, GaN, AlGaAs, GaAs, InGaAsP, ITO, Si, SiC, MoS₂, WSe, Graphene | Si only |
| Disallowed | No metals except Cr; no deep polymer >1 µm | No exposed metals except Cr | No metals except Cr; no deep polymer >1 µm | No metals except Cr; no deep polymer >1 µm | None listed |
| Endpoint Detection | Optical (LEM G50) | Not Available | TBD | Optical (LEM G50) | Not Available |
| Best For | Dielectric etching, passivation removal, via openings, resist descum | Deep Si etching, high-aspect-ratio structures, Bosch process | Precise atomic-layer-level etch control | III-V semiconductors, exotic/compound materials, cryo etching | Fast isotropic Si etching, high selectivity |
| Status | Operational | Operational | Operational | Operational | Operational |
| SOP | SOP | SOP | TBD | SOP | SOP |
Documentation
- SOP
- Quickstart Guide
- Training required – contact lab staff (Chandan Ramakrishnaiah) Request form
Recipes & Data
- Standard processes: typical XeF₂ pulses for controlled Si undercut/release (e.g., 30–120 s pulses at varying pressure for 1–50 µm removal).
- Process control: measure etch depth via profilometer or SEM cross-section; check selectivity on test patterns; monitor chamber pressure and XeF₂ consumption.
- Notes: etch rate depends on XeF₂ vapor pressure and substrate temperature; use short pulse + purge cycles for precision; high selectivity makes it ideal for stopping on oxide/nitride layers.
Troubleshooting
- Etch rate too low or stalled
- XeF₂ crystal source may be depleted or chamber pressure out of range. Check source and notify staff if the crystal needs replacement.
- Etch rate too high / over-etching
- Reduce pulse length and pressure; use shorter pulse + purge cycles for better control.
- Unexpected attack on mask/stop layer
- Check for moisture/HF formation in the chamber; verify samples were fully dry before loading.
- Chamber pressure won't drop
- Possible vacuum leak or pump issue. Stop process, notify staff.
Acknowledgment
If results from this tool appear in a publication, please acknowledge the facility:
"This work was performed in part at the USC Nanofab at the University of Southern California, Michelson Center for Convergent Bioscience."
Please also email the citation to chandanr@usc.edu so we can track facility output.