Electron beam apparatus with a monopole-shaped magnetic field

Abstract: In an electron optical system a monopole shaped magnetic field is introduced enabling an aberration free imaging writing or detection in the system. Apparatus based on this field distribution result in improved secundary electron detection such as for chip inspection, in improved resolution in Auger electron detection for analysis, in a higher exactness in chip production in an electron beam pattern generator and in an electron beam image projector in which now also size reduction can aberration free be introduced.

Description: The invention relates to an electron beam apparatus as defined in the preamble of Claim 1. Electron beam apparatus of this kind may be provided with an object to be examined such as a specimen or a chip. Such an object may be irradiated with electrons generated by an electron source and electrons emanating from the object are, preferably energy dependent, detected. Secondary electron-, reflected electron- or especially Auger electron detecting instruments as well as chip inspecting instruments belong to this kind of apparatus. In other apparatus a target such as a semi-conductor wafer becomes irradiated with electrons in order to register integrated circuit patterns therein. Electron beam pattern generators and electron beam image projectors are examples of this kind of apparatus. In image projectors electrons are extracted from a structured device such as a master chip. For chip production with an electron beam liberated from the master chip a semi-conductor wafer is irradiated in order to image the pattern of the master chip thereupon. The electrons may be liberated from the master chip with the help of a photo emitter provided on the structure thereof. In an electron beam pattern apparatus an electron beam extracted from an electron emitting cathode tip is used for producing a chip or a chip master, the latter for subsequent use in an image projector for example. All these apparatus suffer from non-axial aberrations which deteriorate the exactness of electron beam writing, electron beam imaging or electron detection. An electron beam apparatus as defined in the preamble of Claim 1 is disclosed in the US-patent No. 3 509 503. In this document the means for providing the magnetic field comprise two pole pieces, the first pole piece having an opening being large compared with a tip diameter of the second pole piece having a sharp taper angle. Without further measures the magnetic field generated by this configuration still causes non-axial aberrations as mentioned herein before. It is an object of this invention to reduce these aberrations. To achieve this, the apparatus set forth in accordance with the invention is characterized according to the characterizing portion of Claim 1. Magnetic fields which vary monopole shaped as defined have the advantage that electrons created in such a field at a location of high field strength become, passing through that field, an adiabatic parallellisation. This results in an electron path close about a field line on which the electron has been started from the relevant surface of the object. Due to the monopole shaped field distribution such a field line is a straight line here. An object to be irradiated or examined is preferably located in a region where the magnetic field strength shows a maximum value being just in front of the second pole piece. Electrons liberated from such an object spiralize through the first magnetic pole piece structure and can be detected on a reverse side thereof as described in DE 3 590 146. A monopole field in such a system can be improved with a coil, for example wound around a liner tube in the first pole piece structure. In such a system the strength of the magnetic field, the geometry of especially the first pole piece structure and the velocity of a primary electron beam can be adapted to one another in that the primary beam forms subsequent crossovers when running to the object through the monopole shaped field. Use is thus made of the property of such a field distribution that therein subsequent crossovers, in the primary beam, can be introduced without any additional beam aberration. A monopole shaped field can also be created with a coil adapted for procuding a relatively high magnetic field strength for example a coil manufactured with superconductive coil windings. Such a coil system can be used in an image projector as indicated and more generally disclosed in US 4,902,930. In a further embodiment a beam deflection system is incorporated in the apparatus having its pivot point coinciding substantially with an effective monopole centre of a monopole field to be generated in the apparatus. A pivoted electron path than always coincides with a field line of the monopole field and thus do not suffer of aberrations there. Some embodiments of the invention will be described in more detail with reference to the accompanying drawing in which Figure 1 shows schematically a magnetic pole piece system for generating a monopole shaped magnetic field and Figure 2 shows schematically an electro magnetic coil system for generating such a field. An electron optical system indicated in Figure 1 shows an electron source 1, a beam deflection system 2 and a magnetic lens system 4 with a first pole piece structure 6 having, here arranged around a liner tube coil 8, a mean coil 10 provided with a mean magnetic jig 12 and a second magnetic jig 14 the latter being magnetically uncoupled from the main jig 12 by a non-magnetic interruption 13 providing a magnetic field flux jump there. The lens system 4 shows further a second pole piece structure 16 with a magnetic coil 18 and a magnetic jig 20 provided with a narrow central hole 21 and a sharp pointed pole conus 22 facing the first pole piece structure 6. A control hole 24 of the firt pole piece structure 6 is relatively wide compared with the hole 21. An electron beam 25 emanating from the electron source 1 is directed to an object 26 and for example Auger electrons 27 emanated from the object are directed back through the lens system 4. After passing a magnetic diaphragm 28 the electrons are directed to an electron detecting system 29 comprising for example a semi-conductor detector 30 provided with an electrode 32 for introducing an electrostatic collecting field 34. The lens system 4 is dedicated to provide a magnetic field 35 increasing running with the primary beam 25 monopole shaped up to a maximum value 36 where or in front of which the object 26 is arranged and subsequently decreasing relatively fast up to the second pole piece structure. Electrons 27 escaping from the object and thus starting in a high magnetic field strength regio are spirallized through the lens system up to the magnetic diaphragm 28 after which they are bend to the detector as described in EP 340 861. The primary beam 25 shows here an extra crossover 40, if appropriate more crossovers can be introduced in order to bridge the relatively long travelling path necessary for the relatively low energy electrons. A monopole field distrubution in such a lens structure can be used as indicated for detecting Auger electrons which are specific for the elements present in the object. It can also be used for detecting secondary electrons, reflected electrons etc. and for chip inspection with the aid of secondary electrons. Especially for chip inspection the electron detection is preferably to be made energy sensitive. In this system the beam deflecting system 2 can be used to pivot the primary electron beam about a point 42 coinciding with the effective monopole field centre of the monopole field in front of the object 26. In this kind of object scanning each electron path exactly coincides with a straight field line of the monopole field and thus is prevented from all electron optical aberrations. The structure can also be used for electron beam projection in that for example a master chip is arranged somewhere in or in front of the first pole piece structure and electrons emanating there from are imaged on a wafer located at the position of object 26. Figure 2 shows another embodiment for an image projecting system. Here a master chip 50 provided with a light source 52 and a wafer 54 are arranged in a monopole shaped magnetic field created by an electromagnetic coil system 56 provided with a coil winding distribution 58 varying in an axial direction 60 of the coil system. Electrons 62 liberated from the master chip with the aid of a photocathode 64 provided on the master chip and activated by the light source 52 are aberration free imaged on the wafer 54 through the monopole field. A substantial advantage of such a system is that adjustable size reduction can be introduced. This opens the possibility to work with a relatively large, and thus relatively easy exact to produce, master chip.

Claims: Electron beam apparatus provided with an electron collecting element (30) for collecting electrons from an electron emanating element (26) and with means ( 4 ) for generating a magnetic field in front of said electron emanating element (26), in which the means for providing the magnetic field comprise two pole pieces ( 6 , 16 ), the first pole piece ( 6 ) having an opening (24), the second pole piece ( 16 ) having a sharp taper angle ended with a sharp pointed pole conus (22) facing said first pole piece (6), said opening (24) being large compared the tip diameter of said pole conus (22), characterized in that said electron emanating element (26) is placed between said two pole pieces (6, 16) at an object plane thereof, and in that the first magnetic pole piece (6) having the wide opening (24) is provided with a non-magnetic interruption (13) in a direction transverse to the axis of the wide opening (24), so that a magnetic field is introduced in front of the second pole piece (16) which field increases relatively fast onto said object plane and subsequently decreases monopole shaped away from said object plane. Electron beam apparatus as claimed in Claim 1, characterized in that the means for providing a monopole shaped magnetic field include a coil (8). Electron beam apparatus as claimed in Claim 1 or 2, characterized in that a monopole shaped magnetic field is realised in front of an object (26) to be examined with the aid of Auger, secondary or reflected electrons (27) emanating from a surface thereof, said electrons being directed towards an electron detector (30, 32, 34) through the monopole field. Electron beam apparatus as claimed in any one of the preceding Claims, characterized in that a monopole magnetic field is realised in front of an object on which electrons generated by an electron source (1) are focused on said surface by the monopole field. Electron beam apparatus as claimed in Claim 1, characterized in that electron optical means are incorporated to image a pattern structure from a first object (50) onto a second object (54) and in that, between said objects a monopole shaped magnetic field can be introduced. Electron beam apparatus as claimed in any one of the preceding Claims, characterized in that a deflection system is incorporated having a pivot point coinciding with an effective monopole. Electron beam apparatus as claimed in any one of the preceding Claims, characterized in that within the monopole field between an electron source and an object irradiated one or more intermediate focal planes (40) are introduced. Electron beam apparatus as claimed in any one of the preceding Claims, characterized in that a detecting system comprises means for energy sensitive analyzing. Electron beam apparatus as claimed in any one of the preceding Claims, characterized in that a detection system comprises means for spatially discriminating electrons to be emitted from different parts of the emitting element and following different field lines of the monopole field.

Inventors: BLEEKER ARNO JAN, KRUIT PIETER, BLEEKER, ARNO JAN, KRUIT, PIETER
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: H01J37
IPC codes: H01J37, G01Q30, G01Q70