Distortion optimized multi-beam scanning system
A multi-beam charged particle inspection system and a method of operating a multi-beam charged particle inspection system for wafer inspection with high throughput and with high resolution and high reliability comprise a mechanism for reduction and compensation of a scanning induced aberration, such as a scanning distortion of a collective multi-beam raster scanner for beamlets propagating at an angle with respect to the optical axis of the multi-beam charged particle inspection system.
1 . A multi-beam charged particle microscope, comprising:
a charged-particle multi-beamlet generator configured to generate a plurality of primary charged particle beamlets;
an object irradiation unit configured to illuminate an image patch on a surface of an object in an object plane by the plurality of primary charged particle beamlets to generate a plurality of secondary electron beamlets ( 9 ) emitting from the surface;
a detection unit comprising a projection system and an image sensor, the projection system configured to image the plurality of secondary electron beamlets onto the image sensor to acquire a digital image of the image patch;
a collective multi-beam raster scanner;
a scanning distortion compensator array disposed in a propagation direction of the plurality of primary charged particles upstream of the collective multi-beam raster scanner, the scanning distortion compensator array comprising a plurality of apertures, each aperture configured to transmit a corresponding primary charged particle beamlet of the plurality of primary charged particle beamlets, the plurality of apertures comprising a plurality of first deflection elements configured to individually deflect each corresponding primary charged particle beamlet in a first direction and a plurality of second deflection elements configured to individually deflect each corresponding primary charged particle beamlet in a second direction perpendicular to the first direction, each deflection element disposed in a circumference of each aperture;
control unit configured to provide a first scanning voltage difference to the collective multi-beam raster scanner to scanningly deflect the plurality of primary charged particle beamlets in the first direction,
wherein the scanning distortion compensator array further comprises a scanning array control unit comprising a first static voltage conversion array configured to provide a plurality of first correction voltage differences to the plurality of first deflection elements and a second static voltage conversion array configured to provide a plurality of second correction voltage differences to the plurality of second deflection elements to compensate a scanning induced aberration during the scanning deflection of the plurality of primary charged particle beamlets in the first direction.
2 . The multi-beam charged particle microscope of claim 1 , wherein the first static voltage conversion array is coupled to the control unit and configured to provide to each first and second deflection element a plurality of first voltage difference components synchronized with the first scanning voltage difference.
3 . The multi-beam charged particle microscope of claim 1 , wherein the control unit is configured to provide a second scanning voltage difference to the collective multi-beam raster scanner to scanningly deflect the plurality of primary charged particle beamlets in the second direction.
4 . The multi-beam charged particle microscope of claim 3 , wherein the first and second static voltage conversion arrays are coupled to the control unit and configured to provide to each first and second deflection element a plurality of second voltage difference components synchronized with the second scanning voltage difference.
5 . The multi-beam charged particle microscope of claim 3 , wherein the first static voltage conversion array is coupled to the control unit and configured to provide to each first deflection element a first voltage difference component synchronized with the first scanning voltage difference and a second voltage difference component synchronized with the second scanning voltage difference.
6 . The multi-beam charged particle microscope of claim 1 , wherein the first or second static voltage conversion array comprises a programmable resistor array.
7 . The multi-beam charged particle microscope of claim 1 , wherein the collective multi-beam raster scanner comprises a first set of deflection electrodes and an intersection volume, the plurality of primary charged particle beamlets traverse the intersection volume during use of the multi-beam charged particle microscope, and the collective multi-beam raster scanner is configured to generate a first inhomogeneous scanning deflection field distribution in the intersection volume to reduce a scanning induced aberration of a primary charged particle beamlet incident on the intersection volume at an inclination angle deviating from an optical axis of the multi-beam charged particle microscope.
8 . The multi-beam charged particle microscope of claim 7 , wherein a deflection electrode of the first set of deflection electrodes comprises two spatially separated electrodes, the control unit is configured to provide the first scanning voltage difference and a second scanning voltage difference to the two spatially separated electrodes, and the first and second scanning voltage differences are different.
9 . The multi-beam charged particle microscope of claim 7 , wherein the collective multi-beam raster scanner comprises a second set of deflection electrodes configured to generate a second inhomogeneous scanning deflection field distribution, and the plurality of primary charged particle beamlets travers the second predetermined inhomogeneous scanning deflection field distribution in the intersection volume during use of the multi-beam charged particle microscope to scanningly deflect the plurality of primary charged particle beamlets in the second direction.
10 . The multi-beam charged particle microscope of claim 9 , wherein a shape and a geometry of first set or second set of deflection electrodes of the collective multi-beam raster scanner are adapted to a cross section of the intersection volume of the plurality of primary charged particle beamlets.
11 . The multi-beam charged particle microscope of claim 9 , wherein, in a mean direction of propagation of the plurality of primary charged particles, the first set of deflection electrodes and the second set of deflection electrodes have a different length.
12 . The multi-beam charged particle microscope of claim 11 , wherein the collective multi-beam raster scanner further comprises a first set of correction electrodes configured to generate a scanning correction field contributing to the first inhomogeneous electrostatic field distribution.
13 . The multi-beam charged particle microscope of claim 12 , wherein an electrode of the first set of correction electrodes is in a space between an electrode of the first set of deflection electrodes and an electrode of the second set of deflection electrodes.
14 . The multi-beam charged particle microscope of claim 11 , wherein the collective multi-beam raster scanner further comprises a second set of correction electrodes configured to generate a second scanning correction field contributing to the first inhomogeneous electrostatic field distribution.
15 . The multi-beam charged particle microscope of claim 7 , wherein the collective multi-beam raster scanner is configured to adjust a lateral position of the first inhomogeneous scanning deflection field distribution with respect to the intersection volume, and the control unit is configured to provide a voltage offset to the first set of deflection electrodes and/or the second set of deflection electrodes.
16 . The multi-beam charged particle microscope of claim 7 , further comprising a first static deflection system between the charged-particle multi-beamlet generator and the collective multi-beam raster scanner, wherein the first static deflection system is configured to adjust a lateral position of the plurality of primary charged particle beamlets with respect to the intersection volume.
17 . The multi-beam charged particle microscope of claim 16 , further comprising a second static deflection system between the charged-particle multi-beamlet generator and the collective multi-beam raster scanner, wherein the second static deflection system is configured to adjust a mean angle of incidence of the plurality of primary charged particle beamlets at the entrance side of the intersection volume.
18 . The multi-beam charged particle microscope of claim 1 , further comprising a scanning compensator array configured to compensate a scanning induced telecentricity aberration, wherein the scanning compensator array is in proximity to an intermediate image plane of the multi-beam charged particle microscope, the scanning compensator array comprises a plurality of deflection elements disposed at a plurality of apertures and a second scanning array control unit comprising a second static voltage conversion array configured to provide a plurality of second correction voltage differences to each deflection element to compensate a scanning induced telecentricity aberration during an image scan for each of the primary charged particle beamlets.
19 . The multi-beam charged particle microscope of claim 1 , further comprising a further scanning compensator array configured to compensate a scanning induced aberration.
20 . The multi-beam charged particle microscope of claim 19 , wherein the scanning induced aberration comprises a scanning induced astigmatism or a focus plane deviation of each beamlet of the plurality primary charged particle beamlets.