Astigmatism in electron beam probe instruments
US4180738A · kind A · utility
Assignee
Inventors
Key dates
| Filing date | Jul 27, 1978 |
| Grant date | Dec 25, 1979 |
| Priority date | — |
| Expiry date | Jul 27, 1998 |
Classification
- Technology area (CPC H)Electricity
- CPC primaryH01J37/153
- WIPO fieldElectrical machinery, apparatus, energy
- WIPO sectorElectrical engineering
Abstract
Supplies for the objective lens scanning coils and stigmator coils of a scanning electron microscope are arranged for digital control by a small computer which receives a digitized input from an imaging electron collector. For a specimen of suitable structure the computer can be programmed for automatic focusing and astigmatism correction or the operator can intervene to apply a manual correction. Astigmatism is detected by comparing the directional derivatives of intensity for corresponding points in two frames with lens settings above and below focus. By taking the difference between the derivatives for each direction of measurement the effect of structural directional features is eliminated; by summing the differences for all directions a value (S) is obtained which represents the magnitude of astigmatism. The required orientation of the stigmator field can be calculated and the excitation current scanned through a range of values to determine the setting for which S is a minimum.
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.