1. Field of the Invention
The present invention generally relates to discrete polarization scatterometry.
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.
Metrology processes are performed on wafers at various points in a semiconductor manufacturing process to determine a variety of characteristics of the wafers such as a width of a patterned structure on the wafer, a thickness of a film formed on the water, and overlay of patterned structures on one layer of the wafer with respect to patterned structures on another layer of the wafer. Optical critical dimension (CD) and overlay metrology are currently performed using either spectroscopic scatterometry or angle-resolved scatterometry.
Some scatterometers collect information by rotating a polarizer and/or analyzer. There are several known techniques for switching between two orthogonally-oriented linear illumination polarizations such as: 1) rotating a linearly polarizing component located in the illumination path by 90 degrees; 2) rotating a half-wave retarder in the illumination path by 45 degrees; and 3) using an electro-optical device such as a Pockels cell or liquid crystal to impose a half-wave of retardation. In addition, there are several techniques for switching between two orthogonally-oriented collection polarizations including: 1) rotating a linearly polarizing component located in the collection path by 90 degrees; 2) rotating a half-wave retarder located in the collection path by 45 degrees, which is followed by a stationary linearly-polarizing component; and 3) using an electro-optical device such as a Pockels cell or liquid crystal to impose a half-wave of retardation, which is followed by a stationary linearly-polarizing component.
Alternatively, previously used architectures separate illumination and collection paths with a non-polarizing beam splitter (non-PBS). One example of such a sensor architecture is shown in FIG. 1. In particular, FIG. 1 illustrates one example of a previously used rotating polarizer scatterometer. As shown in FIG. 1, the scatterometer includes illumination optics 10 configured to direct light to polarizing component 12, which may be a rotating polarizer. Light from polarizing component 12 is directed to non-PBS 14, which directs the light to objective lens 16. Objective lens 16 directs the light to wafer 18. Light scattered from wafer 18 is collected by objective lens 16 and directed by non-PBS 14 to polarizing component 20. Polarizing component 20 may be a rotating analyzer. Light exiting polarizing component 20 is directed to collection optics 22.
There are several disadvantages to the above-described scatterometers. For example, rotating optical components is relatively slow. In addition, precisely rotating optical components can be difficult. Furthermore, non-repeatability in the rotation of optical components can degrade system calibrations. Using separate optical components to establish illumination and collection polarizations also makes precise alignment of the collection and illumination polarization axes difficult. In addition, the polarization purity of half-wave plates and electro-optical retarders such as Pockels cells and liquid crystals is poor. Switching lasers on and off electronically also reduces stability and repeatability.
Accordingly, it would be advantageous to develop scatterometers that overcome the shortcomings of speed of measurement and ease and stability of calibration inherent in previously used architectures without reducing the information content generated by continuous motion polarizing instruments such as rotating polarizer and rotating analyzer scatterometers.