Automatic focusing apparatus
US4978856A · kind A · utility
Assignee
Inventor
Key dates
| Filing date | Mar 31, 1989 |
| Grant date | Dec 18, 1990 |
| Priority date | — |
| Expiry date | Mar 31, 2009 |
Classification
- Technology area (CPC H)Electricity
- CPC primaryH01J37/21
- WIPO fieldElectrical machinery, apparatus, energy
- WIPO sectorElectrical engineering
Abstract
In an automatic focusing apparatus according to the invention, a focusing lens is provided for focusing an electron beam which scans a specimen. Charged particles emanating from the specimen are then detected. The excitation current energizing the lens is varied in accordance with a sweep signal that changes linearly. The electron beam scans at each different value of the sweep signal. An evaluating signal is produced indicating the degree to which the focusing lens is focused. The excitation current supplied to the lens is set according to the output signal from the evaluating signal-producing means. The linearly swept excitation current is modulated with a periodic signal. Plural cases are determined where the evaluating signal assumes maximum values. In each case, the average value of the linearly swept signal is calculated. The excitation current is set to a value to be supplied to the focusing lens, corresponding to the average value. The focusing lens is energized with electric current having the set value to focus the beam onto the specimen.
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.