METHOD OF BEAM CENTRING

Abstract: By using, in beam path correction, a three-point adjustment of the beam wobbler, the beam path can be optimized for realizing minimum picture errors in the apparatus. For different lens energizations a picture-­ shifting correction can simply be carried out dynamically.

Description: The invention relates to a method of centring a bean of charged particles in a charged particle beam apparatus having a particle source, a system of lenses, a wobbler, an objective lens, a beam deflecting system and a central control device, and to an apparatus suitable for performing said method. Such a method is known from an article in "Ultramicroscopy", No. 3, 1978, pp . 49-60 published by F. Zemlin, et al; hereinafter referred to as the Zemlin article. In this article a method of beam-centring is described in which a tableau of diffractograms of, for example, a carbon foil, is taken at a fixed tilt angle under various azimuths. With two azimuthally diametrically located beam runs with likewise equal tilt angle and evaluation of the series of diffractograms, centring criteria for the beam are then derived in an electron-microscope. Taking and in particular also evaluation of such a series of diffractograms is comparatively time-consuming and as a result of this makes said method already less suitable for general use. A more serious disadvantage is that also after performing the method, certainty is not obtained that the beam is indeed centred correctly. On the contrary, for high resolving power adjustments, intolerable deviations in centring will be possible. This is caused by the fact that locally, for example, on the display screen, the criteria for optimum centring may also be satisfied by compensation of a directional error by a spherical aberration error of a relevant lens. The criteria specified in the Zemlin article for optimum beam centring have then been satisfied only partly. The method described is also less suitable in particular due to the time-consuming character thereof for use for a dynamic beam centring. It is the object of the invention to provide a more generally useful beam correction method which in all circumstances leads to a sufficiently exact centring. For this purpose, a method of the type mentioned in the opening paragraph is characterized according to the invention in that a first image displacement is determined for a first wobbler adjustment and a second image displacement is determined for a second wobbler adjustment with respect to an exposure beam adjustment which is situated in between the first and the second wobbler adjustment, with the aid of a position sensitive detection system said image displacements being made mutually equal by readjusting the exposure beam adjustment by controlling the beam deflecting system. Since in the method according to the invention only image displacements need be compared, the beam correction in this case requires comparatively little time and since comparison is carried out with respect to a central position the centring due to the mutual equalization of the displacements, can be carried out so that optimum centring is ensured for the whole relevant beam run. By focusing, which is not relevant to carry out the method, the images can be moved towards each other until an operative distance has been achieved which is attractive for the measurement. The method is preferably carried out in two directions extending transversely to each other. The image displacements can be performed by any desired beam tilt frequency so that this can be selected so that the displacements can simply be determined. The procedure may both be carried out visually and may be automated by means of a method suitable for that purpose. For automatic measurement of the beam displacement use can be made of either a single detector, a linear array of detectors or a linear place-sensitive detector, two linear detector arrays or linear place-sensitive detectors oriented transversely to each other, or a scanning detection system with a television circuit or a self-scanning matrix of, for example, CCD detector elements. A few preferred embodiments according to the invention will now be described in greater detail hereinafter with reference to the drawing, the sole figure of which shows an apparatus for use of the method in the form of an electron microscope. An electron microscope as shown in the drawing comprises an electron source 1 having an anode 2, a beam tilt device or wobbler 3 and a diaphragm 4, a condensor system having in this case a first condensor lens 5, a second condensor lens 6 and a condensor diaphragm 7, an objective having in this case a first objective pole 8 in which an auxiliary lens 19 has been incorporated, and a second objective pole 9, a beam-scanning system 10, an objective space 11, a diffraction lens 12 having a diffraction diaphragm 13, an intermediate lens 14, a projection system having a first projection lens 15 and a second projection lens 16, a camera 17 and an inspection screen 18. All these components are incorporated in a housing 20 having an electric supply line 21 for the source and an inspection window 22. Situated on the housing are an optical eye piece 23, a vacuum pump 24 and a space 25, for example, for a plate camera or another detection device. A central control device 26 is connected to at least the beam wobbler, an objective lens energization device 27, the camera 17 and a detection system 28 shown diagrammatically. After activating the apparatus, according to the invention, the three beam displays for three wobbler adjustments are projected on the screen or in the camera. In the case of manual operation these can be studied, for example, directly on the screen or on a monitor 29 fed by the camera 17. The images can be observed simultaneously by means of a wobbler adjusting frequency of at least a few Hz and, by readjusting the exposure direction of the beam, they can be oriented symmetrically with respect to the central image and then be moved towards each other by objective lens focusing. In essentially the same manner this treatment can be carried out automatically, in which the detector device has been constructed so that a signal is always obtained which is relevant for the image position on the screen or in the camera. Herefrom differential signals with respect to the central beam position can be derived which can be made equal mutually again by readjusting the direction of the exposure beam. The distance of the two displaced images with respect to the central image can be adjusted at a value which is most favourable for measurement anti comparison, respectively, by means of the objective lens energization. For automatic measurements it may be favourable to operate with a linear array of detectors or a linear place-sensitive detector, as a result of which a measure of mutual beam displacement can be obtained directly. Said signal determination can even be refined by means of two orthogonal arrays of linear detectors or by means of a matrix of detector elements. Since beam displacements are measured with respect to a reference adjustment, which is the central wobbler adjustment, the beam variation through the relevant part of the apparatus is optimized for as low as possible picture defects which may arise, for example, by spherical aberration of the relevant lens, in this case the objective lens. Because variation of the lens energization in the lenses most relevant for the image formation may result in a beam displacement which is not permissible for a sufficiently exact beam adjustment, the invention also provides a dynamic correction therefor. This correction can also be carried out automatically. A signal which is proportional to the energization is transmitted to the central control device from the energization device 27 for the objective lens. In this energization the beam displacements are determined and corrected and the energizations of the beam deflection device necessary therefor are related to the objective lens current. Although the beam centring described applies to all particle lenses of such apparatuses, the objective lens, in particular of, for example, electron microscopes, preferably is the subject of said correction. It is just the objective lens in this type of apparatus which is strongly image or spot-decisive and moreover the energization for said lens is very much varied. This applies to an equal extent to the objective lens, also termed terminal lens, of electron beam or ion beam writing apparatuses or inspection apparatuses operating with a beam of charged particles.

Claims: A method of centering a beam of charged particles in a charged particle beam apparatus having a particle source, a system of lenses, a wobbler, an objective lens, a beam deflecting system and a central control device, characterized in that, a first image displacement is determined for a first wobbler adjustment and a second image displacement is determined for a second wobbler adjustment with respect to an exposure beam adjustment which is situated in between the first and the second wobbler adjustment, with the aid of a position sensitive detection system said image displacements being made mutually equal by readjusting the exposure beam adjustment by controlling the beam deflecting system. A method as claimed in Claim 1, characterized in that use is made of a detection system comprising a television circuit. A method as claimed in Claim 1 or 2, characterized in that use is made of a detection system comprising a matrix of self-scanning CCD elements. An apparatus comprising a source for generation of a beam of charged particles, a system of lenses, a wobbler, a objective lens, a beam deflecting device and a central control device, characterized in that the apparatus comprises a position sensitive detection system to record image displacements of a wobbler adjustment with respect to an exposure beam adjustment to detect a beam misalignment determined according to the method as claimed in any of the preceding Claims.

Inventors: BAKKER, JOHAN GUSTAAF, VAN DER MAST, KAREL DIEDERICK
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H01J37
IPC codes: H01J37