1. Field of the Invention
This invention generally relates to multi-spot illumination for wafer inspection. Certain embodiments relate to illumination subsystems configured to provide multi-spot illumination for wafer inspection using light having a relatively large bandwidth.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield and thus higher profits. To increase defect detection sensitivity, some inspection systems are designed to reduce surface scattering by reducing the size of the illumination spot on the wafer and compensating for the reduced size of the spot by illuminating multiple spots on the wafer simultaneously.
In some inspection systems, a diffractive optical element (DOE) is used to split a single laser beam into multiple laser beams, and an objective lens focuses the beams to form multiple spots. Typically, the DOE is placed at the back focal plane of the objective lens, while the focused spots are formed at the front focal plane of the objective lens. For example, as shown in FIG. 1, illumination light beam 10 may be directed to DOE 12. DOE 12 separates the illumination light beam into multiple light beams 14. The multiple light beams may be directed to objective lens 16 that is configured to focus the multiple light beams to focal plane 18 as individual, spatially separated spots on the focal plane. The focal plane may be the wafer plane.
The method shown in FIG. 1 works well when the laser bandwidth is relatively small. For relatively large bandwidth lasers such as relatively high power, solid state deep ultraviolet (DUV) lasers, the laser bandwidth is large enough to cause significant blur of the focused spots because of the angular dependence on wavelength by DOE diffraction. Therefore, illumination subsystems that use DOEs are currently limited to using relatively small bandwidth lasers.
Accordingly, it would be advantageous to develop a method of splitting a light beam, which has a relatively large wavelength bandwidth, into multiple light beams to generate diffraction-limited spots for wafer inspection.