Making invisible visible
UNISERS is a process based on (i) physical coating of the test wafer and (ii) optical inspection. The UNISERS coating is necessary to improve the signal-to-noise ratio of the (particle) defects on wafers for inelastic (Raman) scattering by creating an optical resonance around the particles. We use surface-enhanced Raman scattering to detect particles and characterize their size and location. In addition, enhanced Raman scattering is used to acquire spectra of individual defects for chemical characterization.
Why on-wafer surface enhanced Raman spectra?
Existing particle characterization technologies are based on “elemental spectra.” These techniques can not speciate different kinds of “carbon” (e.g., organics originated particles). Most of the filters, pipes, valves, containers of process chemicals used in semiconductor fabrication are produced by different type of plastics. They can shed impurities which can create particle defects with organic origins. Molecular spectra, which can provide a specific chemical ID of particles on a wafer, can help fab operators find the source of particle excursion. Furthermore, existing elemental technologies can not differentiate crystalline from silicon oxide. Silicon oxide is another important particle source that can come from ultrapure water (UPW) or other chemicals such as sulfuric acid or hydrogen peroxide. Due to its chemically inert nature, it is very difficult to filter silicon oxide.