Particle inspection

Particles are among the most critical sources of defects in semiconductor manufacturing. A single sub-micrometer particle, if it lands on an active structure, can already cause a short circuit, an open line, or another electrical defect, rendering an entire chip non-functional. Since modern feature sizes are now in the single-digit nanometer range, particle contamination on the wafer must be monitored continuously.

Particles can originate from a wide variety of sources: from the ambient air of the cleanroom, from equipment components (e.g. flaking coatings, seal abrasion), from process gases and chemicals, or even from the wafer itself (e.g. crystal defects that emerge at the surface). To narrow down the cause and ensure cleanroom quality as well as equipment performance, wafers are checked for particle contamination after critical process steps.

The most common method for particle detection uses the interaction of light with the wafer surface, similar to some film-thickness measurement techniques. However, since particles do not form large, homogeneous layers but rather represent localized point defects, this method does not rely on the interference or polarization of reflected light, but instead on the scattering of light at small objects.

In laser scattering measurement, the wafer surface is systematically scanned with a focused laser beam. When the beam hits a flat, defect-free area, the light is reflected in a largely directional manner. If, however, a particle, a scratch, or another topographic irregularity is present at that location, the light is scattered in almost all directions.

Principle of scattered-light measurement at a particle

One or more detectors positioned outside the direct reflection path capture this scattered light. The intensity of the scattered light can be used to approximate the particle size – however, this is typically not reported as the actual physical dimension, but as the so-called Latex Sphere Equivalent (LSE) size, i.e. the size of a reference sphere (usually polystyrene latex) that would produce the same scattered-light intensity.

Because different materials (metal, photoresist, crystal fragments, etc.) scatter light to differing degrees, the real particle size often deviates from the measured LSE size. Modern systems therefore frequently use several wavelengths and detection angles simultaneously (multi-channel or multi-angle scattering) to allow initial conclusions about the particle type in addition to its size.

Modern inspection systems achieve a detection limit of roughly 20–30 nm (LSE), although actual sensitivity depends heavily on the wafer surface (film structure, roughness, reflectivity) and the wavelength used. Patterned wafers with existing device structures are inherently more difficult to inspect than unstructured (blanket) wafers, since structure edges themselves scatter light and can therefore be falsely identified as particles. However, since the regular structures of the chips are repeated several times on the wafer, they can be distinguished from randomly placed particles.