Optical surface analyzer for spinning disks and wafers: laser slope, phase and scatter channels that map and classify scratches, particles, pits and stains

01

Two oblique lasers, quadrant detectors and a scatter photodiode: how one spot gives slope, phase and scatter at once

Candela Instruments' European patent EP 1 245 922 describes the optical layout behind this class of instrument. Its point is that topographic defects (scratches, pits, mounds, particles) and non-topographic ones (stains, inclusions) need different channels, and that one scan can carry all of them.

The patent states that the miniaturised designs it describes can measure sub-micron scratches, particles, stains, pits, mounds, handling damage, wear of the carbon layer, outside-diameter damage and contamination, and the longitudinal Kerr effect. [1] On a finished disk the layers being inspected are thin: Candela gives 20 to 200 Å of carbon overcoat (typ.) and 10 to 30 Å of fluorocarbon lubricant (typ.), on a substrate of NiP-plated Al-Mg alloy or glass (typ.). [1]

Editorial calculation: from the patent's sin θ / cos² θ factor, height sensitivity at 60° is about 3.5 (0.866 ÷ 0.25) against about 1.4 at 45° (0.707 ÷ 0.5), roughly 2.4 times higher. Assuming a 95 mm platter, one pulse of a 1,024-pulse encoder spans about 0.29 mm of the outer circumference (π × 95 ÷ 1,024); the patent multiplies the encoder signal with a phase-locked loop by a selectable factor to get a finer position, and says this lets repeated revolutions be averaged without losing lateral resolution. [1]

Give the substrate (glass, NiP/Al, Si, SiC; diameter and thickness), whether it is transparent, the films on it, and which defect types must be separated; ask for the laser wavelengths, angle of incidence and the channels the instrument records.

02

Defect classes a production report uses: 11 categories on a SiC substrate, from scratch and particle to IDL and bump

What the buyer receives is a classified defect map. A 2025 study from National Yang Ming Chiao Tung University and partners inspected 150 mm 4H-SiC substrates and epi-wafers on a KLA Candela 8520 in a Class 10 mini-environment, and its tables show the classes and counts such a report carries.

After epitaxy the same tool reported a different class list, including shallow triangles, pit triangles, basal plane dislocations (BPD), carrots, BSF and SSF; the 12 µm epi-wafer showed 170 defects in total and 56 SSF. [2] Scratches the analyzer flagged on the substrate were cut by focused ion beam and measured by SEM at 99.29 nm and 136.9 nm deep, and the authors found that scratches of around 100 nm depth did not transfer into the epitaxial layer. [2] They also note that industrial specifications permit a finite level of scratches on SiC substrates depending on wafer diameter, giving cumulative scratch lengths of 50 mm or less in 200 mm production as an example. [2]

Editorial calculation: small particles make up 130 of the 139 events on the 12 µm substrate scan, about 94 %, so the class definitions and size thresholds set in the recipe govern that 94 % share of the count. [2]

Ask which defect classes the recipe reports, the size threshold of each, how scratches are measured (count, length or both), and whether the defect map exports to SEM or FIB review.

03

Defects below the surface that an optical surface analyzer does not resolve: threading dislocations in SiC

The scan reads the surface and the light it returns. A researcher choosing an analyzer for substrate grading should know where that stops.

A 2025 study from Chung Yuan Christian University and LEAP Semiconductor states that commercial surface inspection tools such as KLA Candela and Lasertec SICA, which combine optical inspection with photoluminescence channels, are used for extended defects in SiC epilayers such as stacking faults and surface topographic defects, but are ineffective for threading dislocations, which have weak or no optically detectable surface features. [3] The authors' confocal subsurface defect-PL method reaches a spatial resolution of about 1.3 µm, set by optical diffraction, against about 16 µm and an imaging contrast of about 0.4 reported for an earlier PL-quenching method. [3]

Editorial calculation: 16 µm against 1.3 µm is roughly a twelvefold difference in resolution (16 ÷ 1.3), which is why threading-dislocation grading of SiC substrates is bought as a separate PL or X-ray measurement rather than as a surface-analyzer recipe. [3]

If dislocations or other subsurface defects must be graded, specify that separately; do not expect the surface scan to report them.

04

Details for an optical surface analyzer enquiry

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Substrates: [glass or NiP/Al disk; Si; SiC; diameter; thickness; transparent or not]
Films on the surface: [carbon, lubricant, oxide; thickness]
Defects to classify: [scratch, particle, pit, stain, mound, others; size thresholds]
Channels needed: [scatter; slope/topography; phase or ellipsometric; PL]
Throughput and handling: [manual or automatic loading; parts per hour]
Environment: [cleanroom class; mini-environment]
Output: [defect map format; SEM/FIB coordinate export]

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