Optical inspection system with polarization isolation of detection system reflections

An optical inspection system includes a polarizing isolator that reduces error in measurements by preventing ghost light reflected or scattered from element of a detection subsystem from re-entering the illumination and detection optical paths. The polarizing isolator may include a polarizing splitter that isolates light directionally according the a linear polarization state and two quarter-wave plates for transforming linearly polarized light to circularly polarized light.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical measurement and inspection systems, and more specifically, to an optical inspection head and system in which light reflected from a detection optical path is isolated from the illumination and other detection optical paths.

2. Background of the Invention

Optical surface inspection systems are in common use in industry for both analysis and manufacturing test operations. The optical heads used to provide measurements when scanning a surface may combine multiple types of detection. For example, U.S. Pat. No. 7,671,978, issued to the inventors of the present application, discloses optical heads that include both an interferometer and a scatterometer channel. In other applications, single channel systems are used.

Dark field detectors are sensitive to stray light sources and leakage along the optical path. In particular, scattering detectors or scatterometers, are extremely sensitive to parasitic light originating in so-called “ghost images” in the optical system, and to reflection and re-scattering of ambient light. Light reflecting from the dark-field detection subsystem, or an additional bright field detection subsystem can re-enter the optical measurement system and enter (or re-enter) the dark-field detection channel via reflections from optical system components such as lenses and beam-splitters, and also potentially from the surface under inspection.

Therefore, it would be desirable to provide a dark field scattering detection system that prevents light that enters a detection subsystem from being reintroduced to the optical inspection system.

SUMMARY OF THE INVENTION

The foregoing objectives are achieved in an optical inspection system and a method of operation of the optical inspection system. The optical inspection system includes a polarizing isolator that reduces error in measurements by preventing ghost light reflected or scattered from element of a detection subsystem from re-entering the illumination and detection optical paths.

The polarizing isolator may include a polarizing splitter that isolates light directionally according the a linear polarization state and two quarter-wave plates for transforming linearly polarized light to circularly polarized light.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

The present invention encompasses optical inspection systems in which reflections from a detection apparatus are prevented from re-entering the optical system, thereby removing sources of error from ghost reflections from components in the detection apparatus, including reflections from the detector element itself.

Referring now toFIG. 1, an optical inspection system in which an embodiment of the present invention is practiced, is shown. A scanning head10is positioned over a surface under inspection11, which is moved via a positioner28that is coupled to a signal processor18. From scanning head10, illumination I of surface under inspection11is provided by an illumination source15. A scattering detector14receives light scattered from surface under inspection11along optical path S from an illumination spot P generated by illumination I. Scatterometric optical path S gathers light from one or more non-specular angles with respect to illumination I and surface under inspection11, so that light scattered from an artifact13(which may be a surface defect or feature, or an extraneous particle) disposed on surface under inspection11, indicates the presence of the artifact. A profilometer16is included, such as an interferometer channel that interferes reflected light R returning along the illumination path, or another optical path and combines the reflected light R with light directly coupled from illumination source15to determine the height of surface under inspection11within illumination spot P.

While the illustration shows a positioner28for moving surface under inspection under scanning head10, it is understood that scanning head10can be moved over a fixed surface, or that multiple positioners may be employed, so that both scanning head10and surface under inspection11may be moved in the measurement process. Further, while scattering detector14and illumination source15are shown as included within scanning head10, optical fibers and other optical pathways may be provided for locating scattering detector14and illumination source(s)15physically apart from scanning head10.

Signal processor18includes a processor26that includes a memory26A for storing program instructions and data. The program instructions include program instructions for controlling positioner28via a positioner control circuit24, and performing measurements in accordance with the output of scatterometric detector14via scatterometer measurement circuit20A that include signal processing and analog-to-digital conversion elements as needed for receiving the output of scatterometric detector14. Profilometer channel16is coupled to a height measurement circuit20B that provides an output to processor26. A dedicated threshold detector21can be employed to indicate to processor26when scattering from an artifact13on surface under measurement11has been detected above a threshold. As an alternative, continuous data collection may be employed. Processor26is also coupled to an external storage27for storing measurement data and a display device29for displaying measurement results, by a bus or network connection. External storage27and display device29may be included in an external workstation computer or network connected to the optical inspection system of the present invention by a wired or wireless connection.

Referring now toFIG. 2, an optical system in accordance with another embodiment of the present invention is shown, which may be implemented in the optical inspection system ofFIG. 1. In the depicted embodiment, an illumination source38directs illumination I to surface under inspection30through a polarizing isolator34that first converts illumination I to plane-polarized light and includes a first quarter-wave plate35A that introduces a 45-degree shift between the polarization states. The resulting circularly polarized illumination I is further directed to surface under inspection by a polarization-neutral bending mirror33, to produce an illumination spot31on surface under inspection. Light S scattered by artifacts within illumination spot31is collected by collecting lens32, which may have a large numerical aperture. Light S collected by collecting lens32is directed to a detector36A, which may be a point detector, or an array of detection elements in one or two dimensions, a focal plane array, a linear array of individual detectors such as avalanche photodiodes, a coherent fiber optics bundle that is coupled to a detector array or individual detectors, a microchannel image intensifier plate (MCP), or another suitable optical detector or detector array, which implement scatterometric detector14ofFIG. 1.

Profilometer16ofFIG. 1is implemented in the optical system ofFIG. 2by a detector36B, which may be a bright-field interferometer, a deflectometer or another suitable measurement subsystem for measuring a characteristic of the light R specularly reflected by surface of interest30. The circularly-polarized reflected light R passes again through first quarter-wave plate35A and is transformed into linearly-polarized light r, which is reflected by polarizing isolator34toward detector36B. Since the linearly polarized light r is rotated 90 degrees with respect to illumination I (assuming that the reflection at surface under inspection30has an insubstantial imaginary component), polarizing isolator34will direct reflected light r to detector36B, rather than toward illumination source38. Reflected light r passes through a second quarter-wave plate35B, which transforms reflected light r to circularly polarized light, which is then provided to detector36B. Light r2reflected from detector36B, which includes reflections from any optical component along the optical path from second quarter-wave plate35B to the detection element, is transformed to linearly polarized light R2by second quarter-wave plate35B, and due to the rotation of light r2upon reflection from detector36B, exits polarizing isolator in the direction of optical trap37, rather than being emitted along the direction of illumination I. Thus, polarizing isolator34provides elimination of error due to ghost light reflected from optical system components, and any other reflective structures or detritus, between polarizing isolator34and detector36B. Otherwise, light r2could re-enter the optical system, reaching either collection lens32or surface under inspection30, and being scattered through collecting lens32into dark-field detector36A.

Referring now toFIG. 3A, details of an optical system in accordance with an embodiment of the invention, as may implement the optical system ofFIG. 2is shown. The detection apparatus for the profilometer channel includes an optical fiber waveguide40, which has an end shaped as a lens42for receiving reflected light r from surface under inspection30. Polarizing isolator34is constructed as shown inFIG. 2, from a partially reflective mirror39, and two 45-degree oriented quarter-wave plates35A-35B. Reflected light r2includes light reflected from lens42, as well as all of the cumulative internal reflections of optical fiber waveguide that result in light exiting lens42in the direction of polarizing isolator34, as well as any reflections from a detector46coupled to the distal end of optical fiber waveguide40that are guided back through optical fiber waveguide40and emitted from lens42in the direction of polarizing isolator34. Reflected light R2is absorbed by an optical trap37A, which may include baffles, absorbing surfaces, mirrors and other structures that prevent reflections of reflected light R2back into the optical system, i.e., reflected in the direction of polarizing isolator34.

Referring now toFIG. 3B, details of an optical system in accordance with another embodiment of the invention, as may implement the optical system ofFIG. 2is shown. The detection apparatus for the profilometer channel includes a lens system formed by lenses42A and46B that image reflected light r from surface under inspection30onto detector46. Polarizing isolator34is constructed as shown inFIG. 2, from a partially reflective mirror39, and two 45-degree oriented quarter-wave plates35A-35B. Reflected light r2includes light reflected from lenses42A-42B, as well as any reflections from a detector46. Reflected light R2is absorbed by an optical trap37B, which is illustrated as an absorbing sheet that prevent reflections of reflected light R2back into the optical system, i.e., reflected in the direction of polarizing isolator34.

While the above-illustrated examples show circularly polarized light being used to detect artifacts on surface of interest30and isolation of linearly-polarized light after transformation by quarter-wave plates35A-35B, polarization isolation can be performed on the circularly polarized light and/or linearly polarized light can be used to perform the detection of artifacts on surface of interest30, in other embodiments of the invention that use Faraday Rotators and/or bi-refringent wedge structures to operate entirely in the linear or circular polarized light domains, or in another combined circular/linear isolation scheme similar to that illustrated above.