ALD System

Role in QLT Fabrication

Atomic Layer Deposition provides Angstrom-level thickness control for ultra-thin, conformal films. In QLT's quantum photonic processor, ALD deposits two critical films:

• TiN heaters (20 nm) ● Resistive heater elements atop SiO₂ cladding for thermo-optic phase control in every Mach-Zehnder interferometer (MZI) node. TiN sheet resistance of ~100 Ω/sq enables precise electrical tuning of optical phase.

• Al₂O₃ passivation (10 nm) ● Conformal encapsulation layer protecting TiN heaters from oxidation and environmental degradation.

Non-uniform heater resistance causes non-uniform phase-shift across the photonic mesh, directly degrading unitary fidelity. ALD's inherent self-limiting growth mechanism guarantees the uniformity no other deposition method can match.

Why ALD Is Superior for TiN Heaters

TiN ALD Chemistry

THERMAL ALD PROCESS ● TiCl₄ + NH₃:

Precursor A: TiCl₄ (titanium tetrachloride)
├── Volatile liquid, T_source = 25°C (room temperature)
├── Corrosive; reacts with moisture → HCl
└── Handle in fume hood; acid-rated storage

Precursor B: NH₃ (ammonia) ● gas cylinder
├── Toxic, corrosive
└── Requires gas cabinet with auto-shutoff + toxic gas monitor

Alternative Chemistry: TDMAT + N₂/H₂ plasma (for PE-ALD)
├── TDMAT (tetrakis(dimethylamido)titanium) ● metal-organic, T_source = 75°C
└── Lower resistivity TiN achievable with plasma assist

One ALD cycle:
1. Pulse TiCl₄ → adsorbs on surface (self-limiting monolayer)
2. Purge N₂ (remove excess TiCl₄ and byproducts)
3. Pulse NH₃ → reacts with adsorbed Ti → forms TiN
4. Purge N₂ (remove byproduct HCl)

Growth rate: ~0.5 Å/cycle (0.05 nm/cycle)
20 nm TiN = ~400 ALD cycles
Cycle time: ~10 seconds → Total deposition: ~67 minutes for 20 nm

BYPRODUCT: HCl gas → MUST route exhaust through dry chemical scrubber

Al₂O₃ ALD Chemistry (Passivation)

THERMAL ALD PROCESS ● TMA + H₂O:

Precursor A: TMA (trimethylaluminum) ● pyrophoric liquid, T_source = 25°C
Precursor B: H₂O (deionized water vapor)

Growth rate: ~1.0 Å/cycle
10 nm Al₂O₃ = ~100 ALD cycles
Total time: ~17 minutes

This is the SAFEST and most well-characterized ALD process ●
use it first for system qualification before attempting TiN.

Specific Depositions for QLT

Technical Specifications

Process Integration

QLT PROCESS FLOW ● ALD System (Steps B6/B7):

PRE-REQUISITES:
├── SiN chip with SiO₂ top cladding deposited (Step B8 ● PECVD)
├── TiN heater pattern defined by photolithography (Step B5)
└── Chip cleaned (O₂ plasma ash, 1 min)

STEP 1: System Preparation
├── Verify precursor levels (TiCl₄, NH₃, TMA, H₂O)
├── Warm up reactor to 350°C (for TiN) or 200°C (for Al₂O₃)
├── Run 5 conditioning cycles (empty chamber) to stabilize surface
└── Verify base pressure and gas flows

STEP 2: Load Substrate
├── Open reactor door
├── Place chip on substrate end effector (center position)
├── Close door; wait for temperature stabilization (5 min)
└── Pump chamber to base pressure

STEP 3: TiN Deposition (B6)
├── Execute GEMFlow™ recipe: 400 cycles TiCl₄/purge/NH₃/purge
├── Monitor real-time process data (pressure, temperature)
├── Total time: ~67 minutes
└── Allow 5-min purge after final cycle

STEP 4: Qualification Check
├── Unload chip
├── Ellipsometry: verify 20 nm ± 0.5 nm thickness
├── 4-point probe: verify ~100 Ω/sq sheet resistance
└── If specs met, proceed to Al₂O₃ passivation

STEP 5: Al₂O₃ Passivation (B7)
├── Cool reactor to 200°C (30 min)
├── Load chip
├── Execute recipe: 100 cycles TMA/purge/H₂O/purge
├── Total time: ~17 minutes
└── Unload and inspect

STEP 6: Post-ALD Processing
├── Pattern Al₂O₃ if needed (selective etch)
└── Proceed to metallization (Step B5/B9 ● e-beam evaporator)

Vendor Options & Pricing

New Systems

Refurbished Market

Vendor Directory

Key Research Validation

• Hamburg University: Used GEMStar XT-P for electrically conductive TiN as diffusion barrier for CNT growth ● validates TiN capability
• MIT.nano: Installed GEMStar XT-P for ALD (PE-ALD) ● institutional endorsement
• TU Delft (Kavli Nanolab): Uses Veeco Fiji G2 for thermal and plasma-assisted TiN, NbN, NbTiN ● quantum device focused
• Stanford SNF: Fiji 2 (F202) for TiN, Al₂O₃, HfO₂, and other materials ● broad process library
• University of Florida NRF: Veeco Fiji 200 for TiN, Cu, Ni, Al ● multi-metal ALD

Facility Requirements

Space and Utilities

Gas Cabinet and Safety Infrastructure

Safety & Handling

Hazard Summary

Required PPE

• Nitrile gloves (chemical resistant)
• Safety goggles (splash protection)
• Lab coat
• Face shield (for TiCl₄ bottle changes)
• N95 respirator (backup, not primary ● engineering controls are primary)