Role in QLT Fabrication
The DUV (Deep Ultraviolet) stepper/scanner is the primary pattern-definition tool for QLT's photonic integrated circuits. Using a 248 nm KrF excimer laser source, it transfers photomask patterns onto photoresist-coated wafers with sub-200 nm resolution and < 50 nm overlay accuracy. Every waveguide, directional coupler, MZI arm, grating coupler, alignment mark, and dummy fill structure on the chip is defined by this tool.
At 248 nm wavelength, the theoretical resolution limit is governed by the Rayleigh criterion: R = k₁ × λ / NA, where k₁ is the process factor (0.4–0.8), λ = 248 nm, and NA is the numerical aperture (0.5–0.68 for dry KrF). This yields minimum feature sizes of 150–400 nm — well within the requirements for all QLT photonic structures where the smallest critical dimension is ~180 nm (waveguide width).
- Waveguide patterning ● Defines 800 nm × 350 nm Si₃N₄ waveguide cores with < 1 nm sidewall roughness (after RIE optimization)
- Grating couplers ● Periodic structures with 600–700 nm pitch, requiring ~300 nm line/space resolution
- MZI mesh routing ● Defines all 28 MZI arms, 32 phase shifter windows, and waveguide crossings for the 8-mode Clements mesh
- Alignment marks ● Multi-layer overlay marks for subsequent heater, metal, and cladding-open lithography steps
- OPC spiral waveguides ● Archimedean spirals (2–8 cm total length, 20 µm pitch) with Euler-curve transitions
- Poovey switch windows ● Defines trench openings (34 µm × 20 µm) for oxide undercut and switch release
Lithography Technology Comparison
Photoresist and Process Requirements
Process Integration
QLT PROCESS FLOW ● DUV Stepper (Step B3):
PRE-REQUISITES:
├── Si₃N₄ film deposited and annealed (Steps B1–B2)
├── Wafer cleaned (solvent clean + O₂ plasma descum)
├── Photomask set fabricated:
│ ├── Mask 1: Waveguide core (WG_FULL etch)
│ ├── Mask 2: Shallow etch (WG_SHALLOW, if applicable)
│ ├── Mask 3: Cladding window opening (CLAD_OPEN)
│ ├── Mask 4: Heater/metal pattern (METAL)
│ └── Mask 5: Contact pad opening (PAD_OPEN)
└── Masks: 6" × 6" × 0.25" quartz, Cr pattern, pellicle-protected
STEP 1: Resist Coating (Track System)
├── HMDS vapor prime: 150°C, 60 s (promotes resist adhesion)
├── Spin BARC: 60 nm, 3000 RPM, 30 s → bake 175°C, 60 s
├── Spin DUV resist: 600 nm, 2500 RPM, 30 s
├── Soft bake: 110°C, 90 s (removes solvent)
└── Edge bead removal: 2 mm from wafer edge
STEP 2: Alignment & Exposure
├── Load wafer onto stepper chuck (vacuum)
├── Global alignment: EGA using alignment marks from prior layer
│ └── First layer: use wafer notch/flat for coarse alignment
├── Per-field fine alignment: FIA on die marks
├── Expose: 30 mJ/cm² (dose optimized per focus-exposure matrix)
├── Step-and-repeat across wafer (field size matches die layout)
├── Typical: 20–100 fields per 150/200 mm wafer
└── Total exposure time: ~30–60 s per wafer
STEP 3: Post-Exposure Bake (PEB) & Develop
├── PEB: 115°C, 60 s (activates acid catalyst in resist)
├── Develop: 0.26N TMAH (tetramethylammonium hydroxide), 60 s puddle
├── Rinse: DI water, 30 s
├── Post-develop inspection: optical microscope + CD-SEM sampling
└── Rework possible: strip resist and re-coat if defects found
STEP 4: Pattern Transfer (Etch)
├── RIE (reactive ion etch) of Si₃N₄ using CHF₃/O₂/Ar plasma
├── Etch rate: ~50 nm/min in Si₃N₄
├── Selectivity to SiO₂: ~2:1 (BOX acts as etch stop)
├── Target: vertical sidewalls (90° ± 1°), RMS roughness < 1 nm
├── Endpoint: optical emission spectroscopy (OES)
└── Over-etch: 10–15% into BOX for margin
STEP 5: Resist Strip & Clean
├── O₂ plasma ash: 300 W, 2 min (removes bulk resist)
├── Wet strip: hot NMP or EKC-265 (removes residues)
├── DI water rinse → spin dry
└── Inspection: verify clean wafer, no resist residues
PROCESS REPEATS FOR EACH MASK LAYER (typically 3–5 litho passes)
Vendor Options & Pricing
New System Pricing
Refurbished / Used Market
Photomask Costs
Facility Requirements
DUV STEPPER FACILITY REQUIREMENTS:
SPACE:
├── Footprint: 4 m × 6 m (stepper) + 3 m × 6 m (resist track)
├── Height: ≥ 3.5 m (for optics column and crane access)
├── Weight: 8,000–15,000 kg (requires reinforced floor, ≤ 200 kg/m²)
├── Vibration: CRITICAL ● VC-D or better (≤ 6.25 µm/s RMS, 1–80 Hz)
│ └── Requires vibration-isolated concrete pedestal or air-spring mounts
└── Adjacent: resist coating track, develop station, CD-SEM
POWER:
├── 3-phase, 208/480V, 80–150A
├── Total consumption: 30–60 kW (stepper) + 10–20 kW (track)
├── UPS recommended for controller and alignment system
└── Clean power (voltage regulation ± 2%)
ENVIRONMENTAL:
├── Temperature: 22.0 ± 0.1°C (CRITICAL — lens performance)
├── Humidity: 45 ± 3% RH
├── Cleanroom: ISO 3–4 (Class 10–100) within the litho bay
│ └── Higher class than rest of fab
├── Air handling: laminar flow, ULPA-filtered, ≥ 0.3 m/s down-flow
└── Pressure: positive relative to adjacent areas
UTILITIES:
├── Cooling water: 10–25 kW chilled water (20 ± 0.5°C)
├── Compressed dry air: 6 bar, dew point < -40°C
├── N₂: high purity for purge and pneumatics
├── Vacuum: roughing pump for wafer chuck
└── Exhaust: solvent exhaust for resist track (HMDS, PGMEA, NMP)
EXCIMER LASER:
├── KrF gas: pre-mix (Kr/F₂/Ne), $500–$1,000/fill
├── Gas lifetime: 50–100 million pulses per fill
├── Laser maintenance: window replacement every 1–2 billion pulses
├── Annual laser service contract: $100,000–$300,000
└── F₂ gas: TOXIC + CORROSIVE ● requires dedicated gas cabinet
Safety & Handling
Hazard Summary
Laser Safety
EXCIMER LASER SAFETY (Class 4 laser system):
1. NEVER look into the beam path — 248 nm UV causes instant corneal burns
2. All access panels are interlocked — opening any panel shuts off laser
3. Laser enclosure windows are UV-absorbing (block 248 nm completely)
4. Wear UV-blocking safety glasses (OD ≥ 5 at 248 nm) during maintenance
5. F₂ gas is HIGHLY TOXIC and CORROSIVE:
├── TLV-TWA: 1 ppm, IDLH: 25 ppm
├── Gas cabinet with dedicated scrubber is MANDATORY
├── F₂ monitor must be interlocked to laser gas supply valve
└── Leak test all connections before and after gas fill
6. Ozone is generated in un-purged beam paths — monitor at ceiling level
7. Laser service: only by trained laser engineer with LOTO lockout
8. Annual laser safety audit per ANSI Z136.1