Role in QLT Fabrication
The automated wafer-level optical prober is the gatekeeper between fabrication and packaging. Before any die is diced from the wafer — a destructive and irreversible step — the prober tests every die in-situ, generating a complete wafer map of optical performance. Dies that fail waveguide loss, MZI extinction ratio, or ring resonator Q-factor specifications are flagged and excluded from the expensive downstream packaging process, directly preventing wasted materials and assembly labor.
For QLT's quantum photonic processor, this is especially critical because even small deviations in waveguide performance (0.1 dB/cm excess loss, 2 dB extinction ratio degradation) can destroy quantum coherence and make a die unsuitable for single-photon operation. The prober couples a tunable laser (1500–1600 nm, covering the full telecom C-band and beyond) through a fiber array probe into each die's grating couplers or edge couplers, sweeps wavelength, and measures transmission spectra across all optical ports simultaneously.
- Waveguide propagation loss ● cutback or spiral structures measure dB/cm at 1550 nm
- MZI extinction ratio ● verifies interferometer contrast > 25 dB (target > 40 dB)
- Ring resonator Q-factor ● loaded Q > 10⁵ confirms low sidewall roughness
- Coupling loss ● grating or edge coupler insertion loss per port
- Spectral uniformity ● center wavelength variation across wafer
- Heater functionality ● electrical probe verifies TiN heater resistance (135–165 Ω)
- Known-Good-Die (KGD) mapping ● die-level pass/fail map drives dicing plan
Why Test Before Dicing?
Technical Specifications
Wafer Prober Station
Tunable Laser Source
Optical Power Meter / Detector Array
Fiber Array Probe
Process Integration
QLT PROCESS FLOW ● Wafer-Level Optical Prober (Step 07):
PRE-REQUISITES:
├── Wafer returned from MPW foundry (LIGENTEC AN350)
├── All in-house post-processing complete:
│ ├── Poovey window etch
│ ├── PZT/PVDF deposition
│ ├── As₂S₃ overlay (if GAP03)
│ ├── Ti/Au metallization
│ └── PECVD SiO₂ top cladding
└── Wafer cleaned and inspected visually
STEP 1: Wafer Load & Alignment
├── Load 200 mm wafer onto vacuum chuck
├── Vision system identifies wafer flat/notch → coarse alignment
├── Pattern recognition locks to first die fiducial marks
├── Auto-height: focus on grating coupler surface
└── Record wafer-level coordinate system
STEP 2: Fiber Array Probe Landing
├── Lower fiber array probe onto grating coupler array
├── Piezo fine-alignment: maximize coupled power at 1550 nm
├── Active alignment optimization: X/Y/Z + angle
├── Record reference coupling power (normalization)
└── Typical coupling: -3 to -5 dB per grating coupler
STEP 3: Wavelength Sweep — Transmission Spectra
├── Tunable laser sweeps 1500–1600 nm continuously
├── Sweep speed: 10–40 nm/s (balance throughput vs. resolution)
├── Lambda-trigger synchronizes detector sampling
├── Record transmission on all output ports simultaneously
└── Data: T(λ) for each input→output port combination
STEP 4: Automated Analysis (Per Die)
├── WAVEGUIDE LOSS:
│ ├── Compare spiral structures of different lengths
│ ├── Linear fit: loss (dB) vs. length (cm)
│ ├── Extract: propagation loss (dB/cm)
│ ├── PASS: < 0.5 dB/cm (AN350 spec)
│ └── FLAG: > 1.0 dB/cm → likely defect
│
├── MZI EXTINCTION RATIO:
│ ├── Find transmission null in MZI spectrum
│ ├── ER = peak - null (dB)
│ ├── PASS: ER > 25 dB (target > 40 dB)
│ └── FLAG: ER < 20 dB → path imbalance or defect
│
├── RING RESONATOR Q-FACTOR:
│ ├── Fit Lorentzian to ring resonance dip
│ ├── Q_loaded = λ_res / FWHM
│ ├── PASS: Q > 100,000
│ └── FLAG: Q < 50,000 → sidewall roughness issue
│
├── COUPLING LOSS:
│ ├── Reference: loopback structure (in→out, no device)
│ ├── Total IL - waveguide loss = 2 × coupling loss
│ ├── PASS: < 3 dB per coupler (grating)
│ └── PASS: < 1.5 dB per coupler (edge SSC)
│
└── HEATER RESISTANCE (electrical probe):
├── 4-wire measurement per TiN heater pad
├── PASS: 135–165 Ω (target 150 Ω)
└── FLAG: open circuit or < 100 Ω → metal defect
STEP 5: Die Stepping
├── Auto-step to next die (pattern recognition re-lock)
├── Repeat Steps 2–4 for every die on wafer
├── Typical: 10–50 dies per 200 mm wafer (5×5 mm dies)
└── Total test time: 5–25 minutes per wafer
STEP 6: Wafer Map Generation
├── Generate color-coded wafer map:
│ ├── GREEN: all parameters pass → Known-Good-Die (KGD)
│ ├── YELLOW: marginal → characterize further after dicing
│ └── RED: fail → exclude from dicing plan
├── Export: SEMI E142 standard wafer map format
├── Feed-forward: wafer map drives dicing saw cut plan
└── Archive: database for yield trending and process control
Vendor Options & Pricing
Wafer Prober Stations
Tunable Laser Sources
Optical Power Meters / Detector Systems
Total System Budget
WAFER-LEVEL OPTICAL PROBER — FULL SYSTEM:
Prober station (FormFactor CM300): $500,000–$800,000
Tunable laser (Keysight 81606A): $40,000–$70,000
Power meter (Keysight N7745A, 8-ch): $25,000–$35,000
Fiber array probes (×4): $8,000–$16,000
DC probe card (48-needle): $5,000–$10,000
Integration & software customization: $50,000–$100,000
Calibration standards: $5,000–$10,000
Installation & training: $30,000–$50,000
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ESTIMATED TOTAL: $2,000,000–$4,000,000
═══════════════════════════════════════════
BUDGET ALTERNATIVE — Semi-Auto Setup:
├── MPI TS200 semi-auto prober: $300,000–$500,000
├── Santec TSL-570 laser: $30,000–$55,000
├── Santec MPM-210H detector: $10,000–$20,000
├── Fiber probes + accessories: $15,000–$25,000
├── Total: $400,000–$650,000
└── Trade-off: manual load; lower throughput
OUTSOURCE ALTERNATIVE:
├── Use foundry wafer-probe service (LIGENTEC, imec)
├── Cost: $500–$2,000/wafer
├── Turnaround: 2–4 weeks
└── Limited to standard test structures
Safety & Handling
Consumables & Maintenance