CMP System

Role in QLT Fabrication

Chemical-Mechanical Planarization (CMP) is the process that transforms a topographically rough SiO₂ cladding surface into an atomically smooth plane suitable for subsequent lithography, bonding, and heterogeneous material integration. After the LPCVD Si₃N₄ waveguides are etched, the PECVD SiO₂ upper cladding conforms to the waveguide topography, creating 350 nm steps across the wafer. CMP removes this topography, producing a globally flat surface with < 1 nm RMS roughness — a prerequisite for the proprietary ODR (As₂S₃) overlay deposition.

In the QLT fabrication flow, CMP serves three critical functions:

• Surface preparation for ODR overlay ● Sub-nm roughness on the SiO₂ surface directly above waveguides is required for low-loss evanescent coupling to the As₂S₃ chalcogenide overlay. Literature shows roughness above 3 nm causes 5× loss degradation in hybrid waveguides (Heydari et al., 2023)
• Multi-layer planarity ● Flat surfaces enable subsequent DUV lithography with tight depth-of-focus requirements (±0.3 µm for 248 nm KrF steppers)
• Waveguide thickness control ● CMP establishes the final SiO₂ upper cladding thickness above waveguides, controlling mode confinement and evanescent field penetration into the overlay region
• Dummy fill equalization ● Pattern-density-dependent removal rates are managed by incorporating dummy fill structures in the GDS layout, achieving ±5 nm uniformity across the OPC spiral area
• Stress relief ● Planarization relieves localized stress concentrations at waveguide step edges, reducing the risk of cracking in the high-stress Si₃N₄ film

CMP Process Physics

Film Requirements After CMP

Technical Specifications

Single-Wafer CMP (R&D / Low-Volume)

Production CMP Platform

Process Integration

QLT PROCESS FLOW ● CMP System (Step B4):

PRE-REQUISITES:
├── Si₃N₄ waveguides patterned and etched (Steps B2–B3)
├── PECVD SiO₂ upper cladding deposited: ~2 µm over waveguides
├── Dummy fill structures included in GDS layout
│   └── Equalize pattern density to 40–60% across all die areas
└── Wafer cleaned (solvent rinse + DI water)

STEP 1: Pre-CMP Metrology
├── Measure incoming SiO₂ thickness at 49 points (ellipsometry)
├── Map topography: profilometer scan across waveguide arrays
├── Calculate target removal amount: typically 1.0–1.5 µm
└── Select recipe based on removal target and pattern density

STEP 2: Pad Conditioning
├── Mount polyurethane pad (IC1010 or similar)
├── Diamond conditioner: break-in new pad (20 min)
├── Stabilize pad temperature with DI water flow
└── Verify slurry delivery (flow rate, pH)

STEP 3: CMP Polish — Step 1 (Bulk Removal)
├── Load wafer face-down on carrier head
├── Slurry: colloidal silica (Cabot SS-25 or Fujimi PL-7103)
├── pH: 10.5–11.0 (KOH-based)
├── Down force: 3–5 PSI
├── Platen/carrier: 80/75 RPM
├── Polish time: 2–5 min (remove ~1.0–1.5 µm SiO₂)
├── Endpoint: motor current change (topography cleared)
└── Rinse on platen with DI water (30 s)

STEP 4: CMP Polish — Step 2 (Buff / Touch-Up)
├── Transfer to buff platen (softer pad)
├── Dilute slurry or DI water only
├── Down force: 1–2 PSI
├── Polish time: 30–60 s
├── Purpose: remove micro-scratches, improve surface finish
└── Rinse with DI water

STEP 5: Post-CMP Clean
├── Brush clean: PVA roller brushes + dilute NH₄OH
├── Megasonic rinse: 950 kHz, DI water (removes particles)
├── Final rinse: cascading DI water (resistivity > 17 MΩ·cm)
├── Spin dry: N₂ assisted, 3000 RPM
└── Total clean time: 3–5 min

STEP 6: Post-CMP Metrology
├── Ellipsometry: remaining SiO₂ thickness ± 10 nm target
├── AFM: surface roughness < 0.5 nm RMS
├── Profilometer: verify global planarity < 50 nm TTV
├── Dark-field inspection: scratch and defect count
└── PASS/FAIL decision before proceeding to next step

SUBSEQUENT STEPS:
├── Lithography for cladding window opening (HF etch above OPC spirals)
└── As₂S₃ overlay deposition on planarized surface

Vendor Options & Pricing

New System Pricing

Refurbished Market

Facility Requirements

Space and Utilities

Consumables Cost

Safety & Handling

Hazard Summary

Slurry Handling Procedures

CMP SLURRY SAFETY:

STORAGE:
├── Store in original HDPE containers at 15–25°C
├── Do NOT freeze (colloidal silica irreversibly agglomerates)
├── Shelf life: 6–12 months (check manufacturer spec)
├── Agitate gently before use (do NOT shake vigorously)
└── Label all secondary containers with GHS placards

HANDLING:
├── Wear chemical splash goggles + nitrile gloves
├── Use chemical apron for bulk transfers
├── Slurry is alkaline (pH 10–11): causes skin/eye irritation
├── Flush any skin contact with water for 15 minutes
└── Eye contact: flush 15 min; seek medical attention

WASTE DISPOSAL:
├── Collect in dedicated slurry waste drums
├── Do NOT mix with acid waste streams
├── Neutralize pH to 6–9 before sewer discharge (if permitted)
├── Silica solids may require filtration before disposal
└── Follow local environmental regulations for nanoparticle waste