Role in QLT Fabrication
The LPCVD furnace deposits the stoichiometric silicon nitride (Si₃N₄) waveguide core layer — the single most critical film in the entire QLT photonic chip. This 350 nm layer forms every waveguide, directional coupler, Mach-Zehnder interferometer, grating coupler, and OPC spiral on the die. The optical loss of this layer directly determines quantum gate fidelity: each additional 0.1 dB/cm of propagation loss reduces single-photon survival by ~2% per centimeter of path length.
LPCVD (Low-Pressure Chemical Vapor Deposition) is the only production method capable of producing truly stoichiometric Si₃N₄ with the ultra-low hydrogen content required for telecom-wavelength photonics. The process uses dichlorosilane (DCS, SiH₂Cl₂) and ammonia (NH₃) at temperatures of 770–830°C, producing films with negligible N-H bond absorption at the critical 1520 nm wavelength.
The deposited Si₃N₄ film serves multiple functions in the QLT architecture:
- Waveguide core ● High refractive index (n ≈ 2.0 at 1550 nm) provides strong mode confinement in 800 nm × 350 nm cross-section
- Nonlinear medium ● χ³ nonlinearity (n₂ = 2.4 × 10⁻¹⁹ m²/W) enables four-wave mixing for OPC
- Mechanical structure ● Released Si₃N₄ ribs form the suspended arms of Poovey all-optical switches
- Thermal stability ● Low thermo-optic coefficient (2.5 × 10⁻⁵ K⁻¹) provides inherent phase stability
- Broadband transparency ● 400–4000 nm window supports visible herald photons and telecom signal simultaneously
Why LPCVD (Not Other Si₃N₄ Methods)
Film Requirements
LPCVD Chemistry
Si₃N₄ LPCVD PROCESS:
Precursors: SiH₂Cl₂ (dichlorosilane / DCS) + NH₃ (ammonia)
Carrier/purge: N₂
Reaction: 3 SiH₂Cl₂ + 4 NH₃ → Si₃N₄ + 6 HCl + 6 H₂
Process conditions:
├── Temperature: 770–830°C (typically 800°C)
├── Pressure: 200–500 mTorr (typically 330 mTorr)
├── DCS flow: 20–80 sccm
├── NH₃ flow: 100–500 sccm
├── DCS:NH₃ ratio: ~1:3 to 1:6 (stoichiometry control)
├── Deposition rate: 2–5 nm/min
└── Time for 350 nm: 70–175 minutes
STRESS MANAGEMENT:
├── As-deposited stoichiometric Si₃N₄: ~1.2 GPa tensile
├── Thick films (> 300 nm) prone to cracking on SiO₂
├── Solution 1: Multi-step deposition with intermediate CMP
├── Solution 2: Damascene process (fill pre-etched trenches)
├── Solution 3: Crack-stop barrier trenches around die perimeter
└── LIGENTEC AN350: uses proprietary crack-free thick-film process
POST-DEPOSITION ANNEAL (critical for photonics):
├── Temperature: 1100–1200°C in N₂ atmosphere
├── Duration: 3 hours minimum
├── Purpose: drive out residual N-H bonds (absorb at 1520 nm)
├── Effect: reduces absorption loss from ~1 dB/cm to < 0.01 dB/cm
└── This is the highest-temperature step in the entire process
Technical Specifications
Horizontal LPCVD Furnace (Batch)
Vertical LPCVD Furnace (Batch)
Process Integration
QLT PROCESS FLOW ● LPCVD Furnace (Step B2):
PRE-REQUISITES:
├── Silicon wafer (150 mm or 200 mm, (100), p-type)
├── Thermal SiO₂ BOX layer grown (Step B1): 3–4 µm
├── Wafer cleaned: RCA SC-1 + SC-2 + HF dip + DI rinse
└── BOX surface verified: < 0.3 nm RMS roughness (AFM)
STEP 1: Furnace Preparation
├── Idle temperature: 600°C (standby)
├── Ramp to 800°C under N₂ purge
├── Run conditioning deposition (dummy wafers, 50 nm Si₃N₄)
│ └── Coats tube walls uniformly; stabilizes deposition rate
└── Verify gas flows with in-line mass flow verification
STEP 2: Load Wafer Boat
├── Load 25–150 wafers in quartz boat (150 mm pitch)
├── Include monitor wafers at positions 1, 13, 25 (for QC)
├── Slow push into flat zone (1 cm/min ramp rate)
└── Stabilize temperature (10 min soak)
STEP 3: Pump Down & Gas Stabilization
├── Pump to base pressure (< 50 mTorr)
├── Introduce N₂ carrier (100 sccm, 5 min purge)
├── Open DCS and NH₃ lines; stabilize flows (2 min)
└── Set process pressure: 330 mTorr
STEP 4: Si₃N₄ Deposition
├── DCS: 40 sccm, NH₃: 160 sccm (ratio 1:4)
├── Temperature: 800°C, Pressure: 330 mTorr
├── Deposition rate: ~3.5 nm/min
├── Target thickness: 350 nm → deposition time: ~100 min
├── In-situ laser reflectance monitors thickness (if equipped)
└── End deposition; purge chamber with N₂ (5 min)
STEP 5: Cool-Down & Unload
├── Ramp down to 600°C under N₂ flow (2°C/min)
├── Slow pull out of tube (1 cm/min)
└── Transfer to measurement station
STEP 6: Film Qualification
├── Ellipsometry: n = 2.00 ± 0.01, thickness = 350 ± 2 nm
├── 49-point map: uniformity < 1%
├── Stress: ~1.2 GPa tensile (wafer bow)
├── FTIR: confirm stoichiometric composition (Si-N stretch at 840 cm⁻¹)
├── AFM: surface roughness < 0.3 nm RMS
└── Visual: no haze, cracks, or particulate contamination
STEP 7: High-Temperature Anneal
├── Transfer to anneal furnace
├── Ramp to 1150°C in N₂ atmosphere
├── Hold 3 hours (drives out residual N-H bonds)
├── Ramp down at 2°C/min
└── Re-measure FTIR: confirm N-H peak eliminated
SUBSEQUENT STEPS:
└── Wafer proceeds to DUV lithography (Step B3) for waveguide patterning
Vendor Options & Pricing
New System Pricing
Refurbished Market
Facility Requirements
Space and Utilities
Gas and Safety Infrastructure
Safety & Handling
Hazard Summary
Emergency Procedures
DCS / NH₃ LEAK RESPONSE:
1. EVACUATE the area immediately
2. Pull emergency gas shut-off (EMO) if accessible
3. Do NOT re-enter until gas monitors read safe (< 1 ppm DCS, < 5 ppm NH₃)
4. Call facility emergency response team
5. Auto-shutoff valves should engage on gas monitor alarm
6. DCS is flammable — do NOT use ignition sources near leak area
7. NH₃ is corrosive — avoid contact with skin/eyes
8. Scrubber must remain running during incident for exhaust abatement
9. After clearance: inspect gas lines, leak-check all fittings before restart