Crystalline Sample Lens Error Determination in SEM
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Solution Overview
Problem
Existing methods for determining lens errors in particle-optical devices, such as SEM and FIB, are limited by the inability to accurately assess astigmatism of higher orders and coma using tin ball samples, which have a minimum size of 10 nm, making it difficult to achieve resolutions below 1 nm and are not suitable for determining other lens errors.
Innovation Solution
The use of a crystalline sample with a free mono-crystal edge, which allows for the determination of lens errors by deconvolution of the image formed by the particle beam, enabling the assessment of beam shape and lens errors with higher precision, even at nanometer scales, using samples like magnesium oxide mono-crystals with right angles and varying secondary emission coefficients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If tin ball samples with minimum size of 10 nm are used, then the presence of astigmatism can be determined, but the resolution cannot achieve below 1 nm and higher order lens errors cannot be assessed
Solution Approach 1:
The patent changes the sample geometry parameter from spherical (tin balls) to crystalline structures with specific orientations. This parameter change enables the determination of higher order lens errors (3rd order astigmatism, coma) that cannot be detected with spherical samples, while achieving resolutions below 1 nm through the anisotropic response of crystalline edges to beam defocus.
Solution Approach 2:
The patent employs asymmetric crystalline sample structures with specific orientation relationships to the beam. The asymmetric response of crystalline edges at different orientations provides distinctive signatures for higher order lens errors, enabling their detection and correction while improving measurement precision without sacrificing beam focus resolution.
2Adaptability or versatility
If tin ball samples are used, then astigmatism of 1st order can be detected, but astigmatism of 3rd order and coma cannot be determined
Solution Approach 1:
The patent creates a universal sample structure (crystalline sample with specific orientation relationships) that can detect all types of lens errors including 1st order astigmatism, 3rd order astigmatism, and coma. The crystalline structure's anisotropic properties provide multiple measurement modes within a single sample, enabling comprehensive lens error assessment with high precision.
3Manufacturing precision
If the beam diameter is reduced to achieve higher resolution, then the line focus length increases, but the beam shape becomes more sensitive to lens errors
Solution Approach 1:
The patent introduces a crystalline sample as an intermediary between the particle beam and the detection system. This intermediary translates subtle beam shape changes (caused by lens errors at reduced beam diameters) into measurable variations in image intensity distribution along crystalline edges. The intermediary enables precise measurement of beam shape even when direct measurement becomes difficult at high resolutions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method allows for more accurate determination of lens errors, including astigmatism of the 3rd order and coma, with improved resolution and contrast, enabling precise correction of lens errors and achieving sharper images across different orientations and beam diameters.
Implementation Method 1
The beam is scanned over the sample, as a result of which (spatially dependent) information is released, e.g. in the form of secondary electrons
Implementation Method 2
The focus of the beam on the sample has a diameter of e.g. 1 nm. The beam is scanned over the sample
Data Source
AI summary
The invention relates to a method for determining lens errors in a Scanning Electron Microscope, more specifically to a sample that enables such lens errors to be determined. The invention describes, for example, the use of cubic MgO crystals which are relatively easy to produce as so-called ‘self-assembling’ crystals on a silicon wafer. Such crystals have almost ideal angles and edges. Even in the presence of lens errors this may give a clear impression of the situation if no lens errors are present. This enables a good reconstruction to be made of the cross-section of the beam in different under- and over-focus planes. The lens errors can then be determined on the basis of this reconstruction, whereupon they can be corrected by means of a corrector.