Role in QLT Fabrication
Electron-beam lithography (EBL) is a maskless direct-write patterning technique that uses a focused beam of electrons to expose resist with sub-10 nm resolution. In the QLT fabrication flow, EBL serves two distinct roles: patterning the proprietary ODR (Optically-Directed Routing) waveguide structures that require sub-50 nm edge placement precision — geometries far below the resolution limit of the DUV stepper — and writing the photomasks used by the DUV stepper for all other layers.
The ODR structures are QLT's most dimensionally sensitive features. These nano-scale waveguide geometries control evanescent coupling ratios, grating coupler periods, and phononic crystal lattice parameters for APBG SBS suppression. Dimensional errors as small as ±5 nm can shift phase-matching conditions or coupling ratios outside acceptable bounds, directly degrading quantum gate fidelity.
- ODR waveguide patterning ● Direct-write of proprietary nano-scale features (sub-50 nm linewidth) in the As₂S₃ overlay and cladding-open layers that define the hybrid nonlinear interaction zone
- Phononic crystal trenches ● APBG lateral trench arrays with sub-wavelength pitch requiring < 10 nm placement accuracy for acoustic bandgap engineering
- Adiabatic taper gateways ● 3D tiered modal transition zones with continuously varying widths from 300 nm to 800 nm over 200 µm length — smooth enough to achieve < 0.01 dB conversion loss
- Prototype mask generation ● Writes Cr-on-quartz photomasks for the DUV stepper at 4× magnification, eliminating dependence on external mask shops for design iterations
- Waveguide width modulation ● Longitudinal width variation (650–1200 nm) along OPC spirals for SBS threshold broadening — requires smooth, continuous geometry control
- Test structure R&D ● Rapid prototyping of new waveguide geometries, ring resonators, and coupling structures without mask NRE costs
E-Beam vs. DUV: Complementary Roles
Write Requirements for QLT Structures
Technical Specifications
Elionix ELS-G100 (Alternative)
Process Integration
QLT PROCESS FLOW ● E-Beam Lithography:
═══ MODE A: ODR WAVEGUIDE PATTERNING ═══
PRE-REQUISITES:
├── CMP-planarized wafer with SiO₂ cladding windows opened
├── ALD SiO₂ spacer deposited (25 nm) over exposed Si₃N₄
├── As₂S₃ overlay deposited (500 nm) by thermal evaporation
└── Wafer cleaned: solvent rinse + N₂ blow dry (no O₂ plasma on As₂S₃!)
STEP 1: Resist Coating
├── Spin e-beam resist: ZEP520A or PMMA (for lift-off)
│ └── ZEP520A: higher resolution, better etch selectivity
│ └── PMMA: standard, well-characterized
├── Thickness: 200–400 nm (depends on etch depth)
├── Bake: 180°C, 3 min (ZEP) or 180°C, 5 min (PMMA)
└── Optional: conductive layer (10 nm Al or Espacer) for charge dissipation
STEP 2: Pattern Design Preparation
├── Convert GDS layout to e-beam format (V30 / BIC / GDSII fracture)
├── Proximity effect correction (PEC): Monte Carlo simulation
│ └── Critical for As₂S₃ (high-Z substrate = strong backscatter)
├── Assign dose modulation map for varying pattern densities
├── Define alignment marks (use pre-existing DUV alignment marks)
└── Fracture: 2–5 nm address grid for critical features
STEP 3: Loading & Alignment
├── Load wafer on e-beam stage (laser interferometer positioning)
├── Automatic alignment to global marks (DUV layer fiducials)
├── Mark detection: backscatter electron imaging
├── Calculate rotation, scale, offset corrections
└── Alignment accuracy: < 15 nm to DUV-defined features
STEP 4: E-Beam Exposure
├── Voltage: 100 kV (minimizes proximity effect in As₂S₃)
├── Beam current: 1–10 nA (balance speed vs. resolution)
├── Dose: 200–500 µC/cm² (ZEP520A); 300–800 µC/cm² (PMMA)
├── Write field: 500 µm × 500 µm (stitching between fields)
├── Write order: serpentine scan across OPC spiral region
├── Total write time: 4–12 hours per die (pattern-dependent)
└── Stage temperature: 22.0 ± 0.1°C (drift control)
STEP 5: Development
├── ZEP520A: amyl acetate, 90 s → IPA rinse → N₂ dry
├── PMMA: MIBK:IPA (1:3), 60 s → IPA rinse → N₂ dry
├── Inspect: optical microscope + SEM sampling
└── CD measurement: SEM on test structures
STEP 6: Pattern Transfer
├── Etch As₂S₃: CF₄/O₂ or Cl₂-based RIE plasma
├── Selectivity (resist:As₂S₃): ~2:1 for ZEP520A
├── Endpoint: time-based (known etch rate) or OES
└── Strip resist: warm NMP or O₂ plasma (gentle)
═══ MODE B: PHOTOMASK FABRICATION ═══
STEP 1: Prepare mask blank (Cr-on-quartz, 6" × 6")
STEP 2: Spin e-beam resist on Cr surface
STEP 3: Write mask pattern at 4× magnification (22 mm field → 88 mm on mask)
STEP 4: Develop resist
STEP 5: Etch Cr (wet: ceric ammonium nitrate, or dry: Cl₂/O₂ RIE)
STEP 6: Strip resist, inspect mask (die-to-database inspection)
STEP 7: Mount pellicle → mask ready for DUV stepper
TURNAROUND: 1–2 days (vs. 2–4 weeks from external mask shop)
Vendor Options & Pricing
New System Pricing
Refurbished / Used Market
Facility Requirements
E-BEAM LITHOGRAPHY FACILITY REQUIREMENTS:
SPACE:
├── Footprint: 3 m × 4 m (main column + chamber + control rack)
├── Height: ≥ 3.0 m (column height ~2.5 m)
├── Weight: 3,000–6,000 kg (reinforced floor required)
├── Vibration: EXTREMELY CRITICAL ● VC-E or better
│ ├── ≤ 3.1 µm/s RMS (1–80 Hz)
│ ├── Ground floor or isolated foundation pedestal MANDATORY
│ ├── No HVAC fans, pumps, or heavy traffic within 10 m
│ └── Typically the most vibration-sensitive tool in the fab
├── EMF shielding: < 0.3 mG AC magnetic field at column
│ └── May require mu-metal enclosure or active compensation
└── Acoustic isolation: < 50 dB(A) in write chamber area
POWER:
├── 3-phase, 208V, 30–60A (total: 10–25 kW)
├── UPS: MANDATORY for electron column and stage
│ └── 15 kVA minimum; 30+ kVA recommended
├── Isolated ground: dedicated clean ground for column electronics
└── Voltage stability: ± 1% (critical for HV supply stability)
ENVIRONMENTAL:
├── Temperature: 22.0 ± 0.1°C (CRITICAL — thermal drift shifts patterns)
├── Humidity: 40 ± 5% RH
├── Cleanroom: ISO 5–6 (Class 100–1000)
└── Airflow: laminar flow, but avoid direct drafts on column
UTILITIES:
├── Cooling water: 5–10 kW chilled water (20 ± 0.5°C)
├── Compressed dry air: 6 bar (stage pneumatics)
├── N₂: high purity (resist processing)
├── Vacuum pumps: ion pump + turbo pump + roughing pump
│ └── May require separate pump room (vibration isolation)
└── Exhaust: resist solvent exhaust from coating/develop area
DATA INFRASTRUCTURE:
├── Pattern generator: high-speed data path (10 Gbps+)
├── CAD workstation: GDS fracture + PEC computation
│ └── Proximity effect correction requires significant CPU/RAM
├── Storage: 1–10 TB per design revision (fractured pattern data)
└── Network: isolated from fab LAN for IP security (ODR patterns)
Safety & Handling
Hazard Summary
Radiation Safety
E-BEAM RADIATION SAFETY:
X-RAY GENERATION:
├── 100 kV electrons striking metal/substrate generate Bremsstrahlung X-rays
├── Dose rate at chamber wall: typically < 0.1 mR/hr (well below limits)
├── Lead shielding (2–5 mm Pb equivalent) is integral to chamber design
├── NEVER operate with radiation shielding panels removed
└── All interlock defeats must be logged and approved by RSO
REGULATORY COMPLIANCE:
├── Register as radiation-generating device with state agency
├── Initial radiation survey by certified health physicist
├── Annual survey and interlock verification
├── Dosimetry badges for operators (TLD or OSL)
│ └── Expected dose: well below 100 mrem/year (ALARA)
├── Radiation safety officer (RSO) oversight required
└── Signage: "CAUTION — X-ray producing equipment" at entry
HIGH-VOLTAGE SAFETY:
├── 100 kV power supply contains lethal stored energy
├── Capacitor discharge time: > 5 minutes after power-off
├── LOTO procedure MANDATORY for any service access to column
├── Only JEOL/Raith trained service engineers inside HV enclosure
└── Emergency stop buttons on all four sides of instrument