Electron beam spot shape reconstruction unit

A computer-implemented method for reconstructing an electron beam spot shape, the method comprises:

• obtaining, by a processor, multiple groups of images of a sample, each group comprising images of a plurality of reference structures formed on the sample, each reference structure comprising a cylindrical body extending perpendicularly from a top surface of the sample, the cylindrical body having a circular top surface, wherein for each group of the multiple groups of images, the images within the group are associated with different polar angles relative to the electron beam spot;
• processing, by the processor, at least two of the multiple groups of images to:
• (i) identify edge regions corresponding to the circular top surfaces of the reference structures, and (ii) determine, for each group, edge width information along a first axis and along a second axis, the first and second axes being non-parallel directions within a plane of the top surface of the sample;
• and
• reconstructing, by the processor, a two-dimensional shape of the electron beam spot shape by:
• (i) mapping the first-axis and second-axis edge width information as virtual sampling points onto a coordinate grid;
• and (ii) interpolating between the virtual sampling points to generate a continuous spot shape profile.

A non-transitory computer readable medium for reconstructing an electron beam spot shape, the non-transitory computer readable medium storing instructions that once executed by a processor in a system for reconstructing an electron beam spot shape, cause the system to:

• obtain multiple groups of images of a sample, each group comprising images of a plurality of reference structures formed on the sample, each reference structure comprising a cylindrical body extending perpendicularly from a top surface of the sample, the cylindrical body having a circular top surface, wherein for each group of the multiple groups of images, the images within the group are associated with different polar angles relative to the electron beam spot;
• process at least two of the multiple groups of images to:
• (i) identify edge regions corresponding to the circular top surfaces of the reference structures, and (ii) determine, for each group, edge width information along a first axis and along a second axis, the first and second axes being non-parallel directions within a plane of the top surface of the sample;
• and
• reconstruct a two-dimensional shape of the electron beam spot shape by:
• (i) mapping the first-axis and second-axis edge width information as virtual sampling points onto a coordinate grid;
• and (ii) interpolating between the virtual sampling points to generate a continuous spot shape profile.

An electron beam spot shape reconstruction system comprising a processor and a memory unit, the memory unit storing instructions that, when executed by the processor, cause the system to:

• obtain a plurality of groups of images of circular targets of a sample, wherein each group is associated with a different polar angle;
• calculate edge width information for each group of images by:
• (i) generating derivative images from the groups of images;
• (ii) defining regions of interest related to edges of the circular targets within the derivative images;
• (iii) generating histograms of edge widths within the regions of interest;
• and (iv) identifying a peak of each histogram using a parabolic approximation algorithm to define a discrete edge width value;
• map the discrete edge width values to a coordinate grid as virtual sampling points, where each virtual sampling point is positioned on the grid based on its corresponding discrete edge width value and polar angle;
• and
• reconstruct a continuous two-dimensional electron beam spot shape profile by interpolating between the virtual sampling points on the coordinate grid.