PECVD System

Role in QLT Fabrication

PECVD deposits the SiO₂ top cladding layer at low temperature (< 350°C), which is essential because higher-temperature processes (thermal oxidation at 1000°C, LPCVD at 700–900°C) would destroy the TiN heaters, metal interconnects, and any temperature-sensitive layers already on the chip.

The SiO₂ top cladding serves multiple functions:

• Optical isolation ● confines guided modes within the Si₃N₄ waveguide core
• Mechanical protection ● prevents surface damage during subsequent handling and packaging
• Environmental barrier ● protects waveguides from humidity and contamination
• Heater insulation ● electrically isolates TiN heaters from Au contact pads
• Stress control ● film stress must be controlled to prevent wafer bowing and waveguide birefringence

Why PECVD (Not Other SiO₂ Methods)

Film Requirements

PECVD Chemistry

SiO₂ PECVD PROCESS:

Precursors: SiH₄ (silane) + N₂O (nitrous oxide)
Carrier/purge: N₂
Temperature: 300°C (substrate)
Pressure: 0.5–1.5 Torr
RF power: 13.56 MHz, 50–200 W (typical); or 300-460 kHz LF

Reaction: SiH₄ + 2N₂O → SiO₂ + 2N₂ + 2H₂

Deposition rate: 50–100 nm/min
Time for 1.5 μm: 15–30 minutes

STRESS CONTROL:
├── Higher RF power → more compressive stress
├── Higher pressure → more tensile stress
├── Dual-frequency (HF + LF) → independent stress tuning
├── Oxford RF switching capability allows precise stress control
└── Target: < 200 MPa compressive for flat wafer

HYDROGEN MANAGEMENT (for photonic applications):
├── N-H bonds absorb at ~1500 nm (interfere with telecom C-band)
├── Optimize N₂O/SiH₄ ratio to minimize hydrogen incorporation
├── Post-deposition anneal: 300°C under N₂ (2 hr) drives out H₂
└── Use N₂O-rich conditions (ratio > 10:1) for lowest H content

Technical Specifications

Process Integration

QLT PROCESS FLOW ● PECVD System (Step B8):

PRE-REQUISITES:
├── SiN chip with all waveguide processing complete
├── TiN heaters deposited and patterned (Steps B6-B7)
├── Al₂O₃ passivation over heaters (if applicable)
└── Chip cleaned (solvent clean + O₂ plasma descum)

STEP 1: System Warm-Up
├── Heat substrate electrode to 300°C (15 min)
├── Run conditioning wafer (dummy Si) with SiO₂ recipe
│   └── Coats chamber walls uniformly; reduces particle shedding
└── Verify gas flows and pressure control

STEP 2: Load Substrate
├── Place chip on electrode (center position)
├── For Minilock-Orion III: load through loadlock
└── Wait for temperature stabilization (2 min)

STEP 3: Pre-Clean (Optional)
├── Ar/O₂ plasma clean: 50 W, 30 s
└── Removes surface contaminants for better adhesion

STEP 4: SiO₂ Deposition
├── Execute recipe: SiH₄ + N₂O + N₂ at 1 Torr, 100 W
├── Deposit 1.5 μm (~19 min at 80 nm/min)
├── Monitor in-situ (if laser endpoint available)
└── End deposition; purge chamber with N₂

STEP 5: Cool-Down & Unload
├── Reduce electrode temp to 100°C (or use loadlock for fast swap)
├── Vent with dry N₂
└── Remove chip

STEP 6: Chamber Clean
├── Run NF₃ or CF₄/O₂ plasma clean (every 10-50 runs)
├── Oxford systems: in-situ plasma clean with endpoint detection
└── Trion: manual CF₄/O₂ clean recipe

STEP 7: Film Qualification
├── Ellipsometry: n = 1.46 ± 0.02, thickness 1.5 ± 0.1 μm
├── Stress measurement: < 200 MPa compressive
├── FTIR: confirm low N-H content
└── Visual inspection: no haze, particles, or delamination

Vendor Options & Pricing

New System Pricing

Refurbished Market

Vendor Directory

Facility Requirements

CRITICAL SAFETY ● SiH₄ (Silane) Handling

⚠️  SILANE (SiH₄) IS PYROPHORIC ● SPONTANEOUSLY IGNITES IN AIR  ⚠️

This is the MOST DANGEROUS gas in our entire fabrication process.

SiH₄ safety infrastructure is MANDATORY before PECVD can be installed:

├── Gas cabinet (pyrophoric/toxic rated): $3,000–$8,000
│   └── Coaxial tubing; sprinkler head; exhaust; flow-limiting orifice
├── Toxic/combustible gas monitor (SiH₄ sensor): $2,000–$5,000
│   └── TLV-TWA = 5 ppm; alarm at 1 ppm; auto-shutoff at 2 ppm
├── Auto-shutoff valve (pneumatic + solenoid): $1,000–$2,000
│   └── Interlocked to gas monitor and fire alarm
├── Fire suppression (gas cabinet integral): Included
├── Emergency shut-off panel: $500–$1,000
└── Exhaust ventilation: continuous when cylinder connected

SAFETY INFRASTRUCTURE TOTAL: $6,500–$16,000

ALTERNATIVE: Use Trion Orion III with <20% SiH₄ premix in N₂:
├── Premixed silane is NOT pyrophoric below 4.3% (LEL)
├── <20% premix still requires gas cabinet but simpler/cheaper
├── Significantly reduces fire/explosion risk
└── Recommended for startup labs without dedicated safety engineer

Space and Utilities

Gas and Safety Infrastructure Itemized

Safety & Handling

Hazard Summary

Emergency Procedures

SiH₄ LEAK RESPONSE:

1. EVACUATE the area immediately (do NOT attempt to close valves)
2. Pull emergency shut-off if accessible WITHOUT entering gas plume
3. Call emergency services (facility fire department)
4. Auto-shutoff should engage if gas monitor detects > 2 ppm
5. SiH₄ burns with INVISIBLE FLAME ● assume fire if you hear hissing
6. Do NOT use water on SiH₄ fire (forms SiO₂ dust; water can cause
hydrogen explosion in confined space)
7. Allow gas to burn out; ventilate area thoroughly before re-entry