O₂ Plasma Asher

Role in QLT Fabrication

The O₂ plasma asher performs a critical post-etch cleaning step in QLT's fabrication flow. After the ICP-RIE opens SiO₂ cladding windows to expose Si₃N₄ waveguide surfaces (Step B2), photoresist residues, fluorocarbon polymer deposits from CHF₃ etch chemistry, and carbonaceous contamination remain on the exposed surface. These residues, even at sub-monolayer levels, cause catastrophic optical loss increases of 2–5 dB/cm and prevent reliable adhesion of subsequent deposited films.

A downstream O₂ plasma asher converts organic residues to volatile CO₂ and H₂O via radical-driven oxidation, while its remote plasma configuration avoids the direct ion bombardment that would roughen the pristine Si₃N₄ waveguide surface. This is fundamentally different from the RIE etch step — the asher operates at lower ion energies (< 5 eV at the substrate) to clean without damaging.

The O₂ plasma clean serves multiple functions:

• Resist stripping ● removes bulk photoresist (SPR-220, AZ-series) at rates of 1–5 μm/min
• Descum ● eliminates thin resist scum (< 50 nm) in pattern openings that survive wet develop
• Fluorocarbon removal ● volatilizes CF₂/CF₃ polymer sidewall passivation from ICP-RIE etch
• Surface passivation ● forms a thin, stable native oxide on exposed Si₃N₄ that improves adhesion
• Surface energy control ● increases wettability for subsequent ALD or PVD film nucleation
• Particle reduction ● removes organic particles that act as mask defects in later litho steps

Why Downstream O₂ Plasma (Not Other Cleaning Methods)

Process Requirements for QLT

O₂ Plasma Ashing Chemistry

O₂ PLASMA ASHING PROCESS:

Primary reactions (downstream mode):
O₂ + e⁻ → 2O•  (dissociation in remote plasma source)
O₂ + e⁻ → O₂⁺ + 2e⁻  (ionization — ions recombine before reaching substrate)

Organic removal:
C_xH_yO_z (resist) + O• → CO₂↑ + H₂O↑
CF_x (fluorocarbon polymer) + O• → CO₂↑ + COF₂↑

DOWNSTREAM vs DIRECT PLASMA:
├── Downstream: plasma generated remotely; only neutral radicals reach wafer
│   ├── Ion energy at substrate: < 5 eV (thermal)
│   ├── No UV radiation damage to dielectrics
│   ├── No sputtering or surface roughening
│   └── Ideal for damage-sensitive photonic waveguides
├── Direct (barrel/RIE): wafer immersed in plasma
│   ├── Ion energy: 20–300 eV (depends on bias)
│   ├── UV can cause color-center defects in SiO₂
│   └── Risk of Si₃N₄ surface roughening
└── RECOMMENDATION: downstream for all post-RIE cleans on QLT chips

OPTIONAL CF₄ ADDITIVE (5–10% of O₂ flow):
├── Adds F• radicals for enhanced inorganic residue removal
├── Helps clear stubborn fluorocarbon sidewall polymers
├── Use sparingly — excess CF₄ can etch Si₃N₄ (~1 nm/min)
└── Typical ratio: 95% O₂ / 5% CF₄ for 30–60 seconds

Technical Specifications

Nordson MARCH AP-300 Downstream Asher

PVA TePla IoN 40 Plasma System

Alternative Systems

Process Integration

QLT PROCESS FLOW ● O₂ Plasma Asher (Step B3 — Post-Etch Clean):

PRE-REQUISITES:
├── ICP-RIE SiO₂ cladding etch complete (Step B2)
│   └── 3.3 μm SiO₂ removed; Si₃N₄ surface exposed
├── Photoresist mask still present (SPR-220, ~5 μm)
├── Fluorocarbon polymer on sidewalls from CHF₃ etch
└── Wafer at room temperature

STEP 1: Resist Strip (Bulk Removal)
├── Load wafer into downstream asher
├── Recipe: O₂ 500 sccm, 600 W, 0.8 Torr, 150°C stage
├── Time: 5–8 min (strips ~5 μm SPR-220 at 1 μm/min)
├── Monitor OES endpoint (CO₂ emission drops at resist clear)
└── Visual: resist completely removed

STEP 2: Descum / Fluorocarbon Clean
├── Recipe: O₂ 400 sccm + CF₄ 20 sccm, 400 W, 0.5 Torr
├── Time: 60 seconds
├── Removes residual resist scum and CHF₃ polymer
└── CF₄ additive breaks C-F bonds in sidewall polymer

STEP 3: Surface Passivation
├── Recipe: O₂ 300 sccm, 300 W, 0.5 Torr, 30 seconds
├── Pure O₂ forms thin native oxide on Si₃N₄ surface
├── Improves wettability for subsequent ALD SiO₂ spacer
└── Surface energy: water contact angle < 10°

STEP 4: Cool-Down & Unload
├── Vent chamber with dry N₂
├── Cool to < 50°C (2 min)
└── Transfer to ALD system within 1 hour (minimize re-contamination)

STEP 5: Verification
├── Visual: no resist residue under dark-field microscope
├── Water contact angle: < 10° (hydrophilic)
├── Optional: XPS survey scan (C < 1%, F < 0.5%)
└── Proceed to ALD SiO₂ spacer deposition

ALTERNATIVE: Post-As₂S₃ Etch Clean (Step G5)
├── After CF₄/O₂ RIE patterning of As₂S₃ overlay
├── Recipe: O₂ only, 300 W, 0.5 Torr, 2 min
├── Removes organic resist without attacking As₂S₃
├── CAUTION: do NOT use CF₄ — etches As₂S₃ aggressively
└── Verify As₂S₃ surface by optical microscopy

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Operating Procedures

STANDARD OPERATING PROCEDURE — O₂ PLASMA ASHER:

STARTUP:
1. Verify gas supply pressures (O₂: 30–50 psi; CF₄ if used: 30 psi)
2. Turn on vacuum pump; verify base pressure < 100 mTorr
3. Turn on cooling water to RF/μW head
4. Power on system; load recipe from library

WAFER LOADING:
1. Open chamber (auto-vent with N₂)
2. Place wafer on stage (center position)
3. Close chamber; pump down to base pressure
4. Verify pressure reading stable

PROCESS:
1. Execute recipe (automated: gas flow → pressure stabilize → ignite plasma)
2. Monitor OES endpoint if available
3. Plasma visually confirms through viewport (blue-white O₂ glow)
4. Recipe completes automatically; chamber purges with N₂

UNLOAD:
1. Wait for temperature < 50°C (if heated stage used)
2. Vent chamber with N₂
3. Remove wafer with vacuum wand or tweezers
4. Visual inspection under microscope

SHUTDOWN:
1. Run 2-minute O₂ chamber clean (no wafer)
2. Pump down to base and backfill with N₂
3. Power off RF/μW and pump
4. Close gas supply valves