Role in QLT Fabrication
The forming gas anneal (FGA) is the final thermal step in QLT's post-metallization process, performed at 425°C for 90 minutes in a 95:5 N₂/H₂ atmosphere. This anneal is specifically referenced in the GAP01 patent specification as critical for achieving ultra-low waveguide loss: hydrogen atoms from the forming gas diffuse into the Si₃N₄/SiO₂ film stack, passivating dangling silicon and nitrogen bonds at interfaces that would otherwise act as optical absorption centers.
In silicon photonics, dangling bonds at dielectric interfaces are the dominant source of mid-gap absorption states. Each unpassivated Si≡ or ≡N bond creates a localized electronic state that absorbs photons in the near-infrared telecom band (1300–1600 nm). For quantum-grade photonic circuits operating at the single-photon level, even 0.01 dB/cm of excess absorption is significant — it directly reduces photon survival probability and degrades quantum interference visibility.
The forming gas anneal serves multiple critical functions:
- Dangling bond passivation ● H₂ molecules dissociate at 400–450°C; atomic H diffuses to Si₃N₄/SiO₂ interfaces and terminates dangling bonds as Si-H and N-H
- Contact resistance reduction ● anneals Ti/Au and TiN metallization contacts, forming low-resistance ohmic interfaces to waveguide heaters
- Film stress relaxation ● relieves residual stress in PECVD SiO₂ cladding and sputtered TiN heater films
- Defect annealing ● heals point defects in SiO₂ introduced during plasma processing (RIE, PECVD)
- Optical loss reduction ● target: reduce waveguide propagation loss by 0.05–0.2 dB/cm through interface passivation
Why Forming Gas (Not Pure N₂ or H₂)
Anneal Requirements for QLT
Hydrogen Passivation Physics
FORMING GAS ANNEAL — PASSIVATION MECHANISM:
STEP 1: H₂ Dissociation (surface catalysis at 400–450°C)
H₂ → 2H• (at Si or metal surfaces)
STEP 2: Diffusion (atomic hydrogen migrates through SiO₂)
D_H(425°C) ≈ 10⁻¹⁰ cm²/s in thermal SiO₂
Diffusion length in 90 min: √(D·t) ≈ 2.3 μm
This exceeds our 1.5 μm SiO₂ cladding → H reaches Si₃N₄ interface
STEP 3: Bond Termination
Si≡ (dangling) + H• → Si-H (passivated)
≡N (dangling) + H• → N-H (passivated)
DEFECT DENSITY REDUCTION:
├── Pre-anneal interface state density: D_it ~ 10¹² cm⁻²eV⁻¹
├── Post-anneal interface state density: D_it ~ 10¹⁰ cm⁻²eV⁻¹
├── Improvement: 100× reduction in absorbing states
└── Optical impact: 0.05–0.2 dB/cm loss reduction at 1550 nm
TEMPERATURE CONSTRAINTS:
├── Minimum: 350°C (insufficient H₂ dissociation below this)
├── Optimum: 400–450°C (maximum passivation efficiency)
├── Maximum: 500°C (beyond this, H can desorb from Si-H bonds)
├── As₂S₃ constraint: T_g = 200°C → FGA must precede As₂S₃ deposition
├── PVDF constraint: T_g = 130°C → FGA must precede PVDF deposition
└── TiN heaters: stable to 600°C → no concern
N-H ABSORPTION WARNING:
├── Si-N-H bonds absorb at ~1520 nm (near telecom C-band edge)
├── This is normally a concern for LPCVD Si₃N₄ films
├── QLT Si₃N₄ is pre-annealed at 1100°C by MPW foundry
│ └── High-T anneal drives out bulk N-H; FGA only adds interface H
├── Interface N-H density is 100–1000× lower than bulk
└── Net effect: absorption reduction far exceeds any N-H addition
Technical Specifications
Lindberg/Blue M Horizontal Tube Furnace
Tempress Systems Horizontal Diffusion Furnace
Process Integration
QLT PROCESS FLOW ● Forming Gas Anneal (Step B9 — Post-Metallization):
PRE-REQUISITES:
├── All metallization complete (TiN heaters + Ti/Au contact pads)
├── PECVD SiO₂ top cladding deposited (Step B8)
├── Via openings etched through SiO₂ to Au pads (if applicable)
├── All photoresist stripped; wafer clean
└── NO temperature-sensitive materials yet deposited
├── As₂S₃ overlay: NOT YET (T_g = 200°C)
├── PVDF-TrFE: NOT YET (T_g = 130°C)
└── Polymer adhesives: NOT YET
STEP 1: Furnace Preparation
├── Set furnace to 425°C (allow 1–2 hr stabilization)
├── Purge tube with N₂ at 5 SLM for 15 min
│ └── Displaces residual O₂; prevents oxidation during anneal
├── Verify temperature profile: ± 3°C across hot zone
└── Verify O₂ level < 10 ppm (inline analyzer or gettered gas)
STEP 2: Wafer Loading
├── Load wafer(s) into quartz boat (source end of tube)
├── Slow-push into hot zone (2–3 cm/min) under N₂ flow
│ └── Prevents thermal shock; gradual ramp from 200→425°C
├── Position wafers in center of flat zone
└── Wait 5 min for temperature equilibration
STEP 3: Forming Gas Anneal
├── Switch gas from pure N₂ to 95:5 N₂/H₂ forming gas
├── Flow rate: 3 SLM through process tube
├── Soak time: 90 minutes at 425°C
├── Monitor gas bubbler (positive pressure indication)
└── Log temperature and gas flow throughout anneal
STEP 4: Cool-Down
├── Switch back to pure N₂ (3 SLM)
├── Slow-pull wafer boat out of hot zone (2–3 cm/min)
├── Cool in N₂ atmosphere to < 200°C before removal
│ └── Prevents oxidation of freshly passivated surfaces
├── Total cool-down: 30–60 min (natural, no forced air)
└── Remove wafer; store in N₂ desiccator
STEP 5: Post-Anneal Verification
├── Measure TiN heater resistance (expect 5–15% decrease from anneal)
├── Visual inspection: no discoloration, no peeling
├── Optional: waveguide loss measurement (ring resonator Q)
│ └── Compare pre-anneal vs post-anneal Q factor
├── Contact resistance measurement (TLM structures if available)
└── Proceed to As₂S₃ overlay deposition (if applicable)
PROCESS WINDOW:
├── Temperature: 400–450°C (optimum 425°C)
├── Time: 30–120 min (optimum 90 min; diminishing returns beyond)
├── Below 400°C: insufficient H₂ dissociation
├── Above 450°C: risk of H desorption from weak bonds
└── Above 500°C: TiN oxidation risk if O₂ contamination present
Vendor Options & Pricing
New System Pricing
Refurbished Market
Facility Requirements
Space and Utilities
Safety & Handling
Hazard Summary
Forming Gas Safety Notes
FORMING GAS (95:5 N₂/H₂) SAFETY:
CLASSIFICATION: NON-FLAMMABLE mixture
├── H₂ content (5%) is BELOW the Lower Explosive Limit (LEL = 4% in air)
├── Even a 100% release of forming gas into air will not reach LEL
├── Standard compressed gas cylinder handling applies
├── No flammable gas cabinet required
└── No H₂ sensor legally required (but recommended for best practice)
COMPARISON TO PURE H₂:
├── Pure H₂: LEL 4%, UEL 75%, autoignition 500°C → EXTREMELY FLAMMABLE
├── 95:5 forming gas: even undiluted, H₂ is only 5% → cannot reach LEL
├── At furnace exhaust: further diluted by room air → effectively zero risk
└── This is why forming gas is the standard industry choice for FGA
CYLINDER HANDLING:
├── Secured to wall or bench with chain/strap
├── Regulator: CGA-580 (inert gas) fitting
├── Flow: 2–5 SLM through process tube
├── Consumption: ~1 cylinder per 10–20 anneals (K-size)
├── Cost: $80–$150 per cylinder
└── Storage: standard compressed gas area; no special ventilation