Electron beam apparatus with dynamic focussing

Abstract: Notably for transmission measurements, an electron microscope utilizes a dynamic focussing imaging optical system so that the focussing is optimally adjusted according to scanning lines even when measurements are performed on tilted objects. To this end, the object is preferably scanned in a pattern of lines extending parallel to an object tilt axis. Any defocussing of the electron beam, occurring at the irradiating side due to the dynamic focussing, can also be instantaneously corrected.

Description: The invention relates to an electron beam apparatus according to the precharacterizing part of Claim 1. An apparatus of this kind is known from "Proceedings of the 4th Annual Scanning Electron Microscope Symposium", ITT Research Institute, Chicago, I11., US, April 1971, pages 33-40. Object tilting is desirable for many fields of examination in electron microscopy. Examples in this respect are, for example three-dimensional imaging, biological examinations, deflection microscopy crystal face measurements and the like. Notably for the comparatively large tilting angles required in such applications, for example for reducing the so-called missing cone in three-dimensional imaging, inadmissible defocusing occurs in images formed by an electron beam irradiating the object or a reflective electron beam. The focus variation within an image results in a location-dependent contrast transfer function which determines the image contrast as well as the image resolution. Consequently, the examination of structures and the like is seriously affected, notably in the case of imaging with a low dose and a small enlargement factor where comparatively large object surfaces are irradiated. Similar problems occur in high-resolution microscopy with three-dimensional imaging. The variation in focus is directly given by the local height variation of the object with respect to a focal plane, and hence directly by the beam position with respect to the optical axis or the tilt axis of the object and the tilt angle of the object. In the known electron beam apparatus defocusing errors due to object tilting are compensated for by a dynamic focusing device which is controllable in dependence of the tilt angle and the distance between the scanning beam and the tilt line. Because focusing is adapted per scanning line optimum focusing can be realized across a comparatively large image surface. For the scanning of focus-corrected straight scanning lines extending parallel to the tilt axis, the customary situation is assumed that the focal plane in the apparatus coincides with a flat plane through the tilt line transversely of the optical axis. It is an object of the invention to provide an electron beam apparatus as described in the opening paragraph in which focusing errors due to lens imperfections can also be corrected. Thereto the electron beam apparatus according to the invention is characterized in that the electron-optical system is adapted for scanning the object in semi-circular or elliptical lines, a central line of which coincides with the object tilt axis. The invention is based upon the insight that the degree of focusing compensation and the resolution are adjustable by selection of the scanning pattern, i.e. the number of scanning lines and their spacing. Such scanning facilities are usually already included in known electron microscopes, so that the use of the invention requires hardly any additional facilities. When the electron-optical system has a focal plane which is, for example spherically curved, compensation can be performed by scanning along semicircular/semi-elliptical paths with the tilt axis and the axis of the ellipse, respectively, serving as the central line. Thus, focusing errors occurring in the apparatus can also be compensated for. The procedure for dark field measurements, deflection measurements or diffraction measurements may he exactly the same. Actually, merely the position of the detector need then be adapted. All known detection means can be used, i.e. means for direct registration as well as for electronic signal processing. A preferred embodiment of the electron beam apparatus is suitable for transmission measurements, focusing being line-wise performed so that focusing is always locally optimum in images made with a beam passing through the object, a detector being suitable for the detection of transmission signals. Integral optimum imaging of the specimen is thus realized by instantaneous focusing of the image per scanning line. Customarily, focusing will always take place at the level of the irradiated object part, which object part is then instantaneously displayed on the detector. If out-of-focus measurement is desirable, the object focal point can also be selected so as to be situated in a different position, resulting in a non-optimally focused image as desired for some examinations. The invention can be used notably for examinations where a comparatively large object is irradiated, because in that case comparatively severe defocusing would occur. The need for the analysis of large objects may be imposed by the nature thereof, but it may also be advantageous to work with a comparatively large object in order to keep the local electron load of the object below a maximum permissible limit, i.e. to work with a comparatively larger spot. The comparatively small enlargement then required can be neutralized, if desired, by postenlargement of the image carrier. In a preferred embodiment, the electron beam apparatus comprises a digital control device and a signal processing device, operation being fully or partly automatic, thus enabling exact central control of dynamic focusing. If desired, control parameters can be readily stored in a memory. Some examples not being embodiments of the invention, but useful for understanding the invention, will be described in detail hereinafter with reference to the drawing. In the drawing: Fig. 1 diagrammatically shows an electron beam apparatus, and Fig. 2 shows a tilt situation for an object to be examined thereby. An electron microscope as shown in Fig. 1 is described in detail in, for example "Philips Technical Review", vol 43, No. 10, November 1987, pp. 273-291, and comprises an electron source 2, an anode 4 and a condensor system 6, a beam-scanning device 8, an objective lens system 10, an object holder 12 with an object tilting device 14 and a detector 16 for measuring an electron beam 20 which irradiates an object 18 arranged on the object holder. When the object to be examined is placed in a tilted position in such an apparatus, for example as shown in Fig. 2, in a plane 26 which encloses a tilt angle 28 with respect to a focal plane 22, the electron beam 20 will be focussed only on a tilt line 30 when the focal plane 22 coincides with a plane transversely of main axis 24 of the apparatus. Using the beam scanning device 8, which may also be arranged elsewhere in the apparatus, the object is line-wise scanned along (in this case) straight scanning lines 32 which extend parallel to the tilt axis 30. By readjusting the focussing for each scanning line, across the entire zone to be examined imaging will take place, even in the case of a tilted object, with an imaging optical system which is optimally focussed for the target of the scanning electron beam, because the optical system is instantaneously focussed for the relevant position of the irradiated object sub-region, so that scanning takes place with an optimally focussed beam at that area. To this end, a tilt angle sensor of the tilting device is connected to a central measuring and control device 34 whereto the detector 16 is also connected. A monitor 36 for display may be connected to the device 34. The central control device 34 is also coupled to a lens adjusting unit 38 for activating the refocussing and to a beam deflection control device 40 for synchronizing the focussing with the scanning position of the beam. In a lens as described in US 4,306,149, this can be realized, for example by instantaneous readjustment of the energizing of the first pole-lens 10-a of the objective lens. The beam geometry, for example the transverse dimension and the angle of aperture can thus also be adjusted for adaptation to instantaneously desired beam parameters. For beam current control, for example beam blanking, the control device may also be connected to a source control device 42 for the exposure system of the apparatus which comprises, for example a beam blanker for beam blanking. For different methods of examination and the associated beam paths, reference is made to the cited article in "Philips Technical Review". By choosing the ratio of the spot dimension to the spacing of the scanning lines, either a pattern of separate lines, a pattern of touching lines or a pattern of overlapping lines is formed. A strongly overlapping line pattern may be attractive, for example when a comparatively large spot is used with, for example non-negligibly small defocussing within the transverse dimension of the beam. By irradiating and detecting each object element a number of times, a kind of mean focussing situation can then be assumed, which situation may be considered to be the same for each pixel. As a result, reconstruction is simplified and image artefacts can be avoided without giving rise to the discontinuities inherently occurring in said known method. If the focussing of a position 46 in the focal plane 22 is shifted to a position 44 in the tilted plane 26 due to refocussing of the imaging optical system, i.e. in this case by the second pole-lens 10-b, defocussing is liable to occur in the scanning electron beam 20. Because the effect of the second pole-lens on the beam geometry is known or can be measured, in which case it is fixed, correction can also be realized for this phenomenon, for example by readjustment of the first pole-lens 10-a or the condensor system.

Claims: An electron beam apparatus, comprising: an electron source (2) for generating an electron beam (20) which extends along an optical axis (24) of the apparatus, an electron-optical system (8, 40) including scanning means (8) adapted for irradiating and scanning with the electron beam in a rectangular line pattern (32) an object (18) to be examined in the apparatus, an object holder (12) for holding the object in said electron beam comprising an object tilting device (14) for tilting the object (18) over a tilt angle (28) around a tilt axis (30), an imaging focussing device (6, 10) for focussing the electron beam (20) on the object (18) which device (6, 10) is dynamically controllable in dependence on the tilt angle (28) and the distance between the incident point of the scanning beam (20) on the object and the tilt axis (30) for compensation of defocussing errors due to object tilting, detection means (16) for detecting irradiation of said object (18) by said scanning electron beam (20), chacterized in that the electron-optical system is adapted for scanning the object in semicircular or elliptical lines, a central line of which coincides with the object tilt axis (30). An electron beam apparatus as claimed in Claim 1, characterized in that the electron source and the electron-optical system are adapted for generating a substantially coherent electron beam for scanning the object, the detection means being adapted for measuring the current of the beam part which is transmitted through the object. An electron beam apparatus as claimed in any one of the preceding Claims, characterized in that the apparatus is adapted for scanning transmission microscopy and that it is adapted for reconstructing three-dimensional images from object transmission signals detected at different angles of tilt. An electron beam apparatus as claimed in any one of the preceding Claims, characterized in that the apparatus is adapted for scanning transmission microscopy and that the beam scanning device is adapted for object scanning in a line pattern comprising overlapping lines, and for measuring a plurality of transmission signals of object elements in order to average out focussing differences for the image reconstructions. An electron beam apparatus as claimed in any one of the preceding Claims, characterized in that it comprises a digital control unit. An electron beam apparatus as claimed in any one of the preceding Claims, characterized in that it includes means for refocussing the irradiating electron beam when it is liable to be defocussed due to the dynamic focussing at the area of the object. An electron beam apparatus as claimed in any one of the preceding Claims, characterized in that it includes a beam blanker which is controllable in dependence of the object scanning. An electron beam apparatus as claimed in any one of the preceding Claims, characterized in that the focussing device comprises means for automatic control.

Inventors: DE JONG MARCUS JOHANNES CORNEL, DE JONG, MARCUS JOHANNES CORNELIS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1989
Main classification: H01J37
IPC codes: H01J37