Patent Number: 
Section: claims

1. A method of inspecting a substrate using electrons, the method comprising:obtaining multiple images of a region of the substrate using mirror-mode electron-beam imaging at a range of voltage differences between an electron source and a substrate, each said image corresponding to one said voltage difference;storing image data corresponding to the multiple voltage differences; andcalculating a measure of variation of an imaged aspect of a feature in the region with respect to the voltage difference between the electron source and the substrate,wherein the method provides sub-volt voltage contrast between said images without needing to use an energy-filter with sub-volt resolution. 2. The method of claim 1, wherein the imaged aspect of the feature comprises an intensity of the feature. 3. The method of claim 1, wherein the imaged aspect of the feature comprises an apparent size of the feature. 4. The method of claim 1, further comprising determining a process-related parameter based at least in part on the calculated measure of variation of the imaged aspect of the feature with respect to the voltage difference. 5. The method of claim 4, further comprising obtaining and storing image data from multiple locations on the substrate. 6. The method of claim 5, further comprising building a map showing variation of the process-related parameter over the substrate. 7. The method of claim 1, wherein the voltage difference is varied by changing a voltage applied to a stage holding the substrate. 8. The method of claim 1, wherein the voltage difference is varied by changing a voltage at the electron source. 9. The method of claim 1, wherein the substrate comprises a semiconductor wafer, and wherein the method is applied in-line during the fabrication of integrated circuits on the semiconductor wafer. 10. The method of claim 1, further comprising providing a higher-energy electron beam for substrate charge control. 11. The method of claim 1, further comprising illuminating the region with an ultraviolet light beam for substrate charge control. 12. The method of claim 1, wherein the region of the substrate being imaged includes test structures positioned in between integrated circuit dies on a semiconductor wafer. 13. An apparatus comprising:an electron beam instrument configured for obtaining multiple images of a region of the substrate using mirror-mode electron beam imaging at a range of voltage differences between an electron source and a substrate, each said image corresponding to one said voltage difference;a data processing system configured to store image data corresponding to the multiple voltage differences; andprocessor-executable code configured to calculating a measure of variation of an imaged aspect of a feature in the region with respect to the voltage difference between the electron source and the substrate,wherein the apparatus provides sub-volt voltage contrast between said images without needing to use an energy filter with sub-volt resolution. 14. The apparatus of claim 13, further comprising processor-executable code configured to determine a process-related parameter based at least in part on the calculated measure of variation of the imaged aspect of the feature with respect to the voltage difference. 15. The apparatus of claim 14, further comprising processor-executable code configured to obtain and store image data from multiple locations on the substrate. 16. The apparatus of claim 15, further comprising processor-executable code configured to build a map showing variation of the process-related parameter over the substrate. 17. The apparatus of claim 13, wherein the electron beam instrument utilizes a second electron source to provide an electron beam for substrate charge control. 18. The apparatus of claim 13, further comprising a UV light source configured to illuminate the region for surface charge control. 19. An apparatus for inspecting a substrate using a projection electron beam system, the apparatus comprising:an illumination subsystem configured to generate an incident electron beam;an objective subsystem configured to receive the incident electron beam, to focus the incident electron beam onto a region of the substrate, and to retrieve a scattered beam from the substrate;a projection subsystem configured to receive the scattered beam and to project the scattered beam onto a detector so as to detect electron images of the region; anda beam separator coupled to and interconnecting the illumination subsystem, the objective subsystem, and the projection subsystem, wherein the beam separator is configured to receive the incident beam from the illumination subsystem, bend the incident electron beam towards the objective subsystem, receive the scattered beam from the objective subsystem, and bend the scattered beam towards the projection subsystem, anda control system configured to vary a potential difference between a source of the incident electron beam and the substrate,wherein the electron images are obtained of using mirror-mode electron-beam imaging at a range potential differences between the source and the substrate, each said image corresponding to one said potential difference, andfurther wherein the apparatus provides sub-volt voltage contrast between said images without needing to use an energy-filter with sub-volt resolution. 20. The apparatus of claim 19, further comprising a data processing system configured to calculate a measure of variation of an imaged aspect of a feature with respect to the potential difference. 21. The apparatus of claim 20, wherein the imaged aspect of the feature comprises an intensity of the feature. 22. The apparatus of claim 20, wherein the imaged aspect of the feature comprises an apparent size of the feature. 23. The apparatus of claim 20, wherein the data processing system is further configured to determine a process-related parameter based at least in part on the calculated measure of variation of the imaged aspect of the feature with respect to the potential difference. 24. The apparatus of claim 23, wherein the control system is further configured to obtain image data from multiple locations on the substrate. 25. The apparatus of claim 24, wherein the data processing system is further configured to build a map showing variation of the process-related parameter over the substrate. 26. The apparatus of claim 19, wherein the control system varies the potential difference by changing a voltage applied to a stage holding the substrate. 27. The apparatus of claim 19, wherein the control system varies the potential difference by changing a voltage at the source of the incident electron beam. 28. The apparatus of claim 19, wherein the apparatus generates a second incident electron beam for substrate charge control. 29. The apparatus of claim 19, further comprising a UV light source configured to illuminate at least a portion of the substrate for surface charge control. 30. A method of inspecting a substrate using electrons, the method comprising:obtaining multiple images of a region of the substrate using mirror-mode electron-beam imaging at a range of voltage differences between an electron source and a substrate, each said image corresponding to one said voltage difference;storing mirror-mode image data corresponding to the multiple voltage differences;scattered electron imaging of the region;storing the scattered electron image data; andcomparing gray levels of a feature in the region in the mirror-mode image data to the scattered electron image data of the feature,wherein the method provides sub-volt voltage contrast between said images without needing to use an energy-filter with sub-volt resolution. 31. The method of claim 30, wherein the scattered electron imaging comprises secondary electron imaging. 32. The method of claim 30, wherein the scattered electron imaging comprises backscattered electron imaging.