MAGNETIC IMMERSION LENSES

Abstract: A magnetic immersion lens has inner and outer pole-pieces (10, 11) arranged symmetrically about a longitudinal axis X-X of the lens, the inner pole piece (10) having a through-bore (12) and the lens producing a magnetic imaging field for directing along the through-bore (12) secondary electrons emitted from a specimen (S) positioned in front of the inner pole-piece. The lens has an axially-symmetric detection arrangement (20) located within the through-bore. The detector (20) comprises a focusser electrode (21), a repeller electrode (22), an extractor electrode (23), a detector electrode (24), and a reflector electrode (25).

Description: MAGNETIC IMMF.RSTON FFNSFS This invention relates to magnetic immersion lenses In particular, the invention relates to a magnetic immersion lens of the type comprising inner and outer pole-pieces arranged symmetrically about a longitudinal axis of the lens, the inner pole-piece having a through-bore. In use, a specimen is positioned m front of the tip end of the inner pole-piece at a suitable working distance, typically about 2mm. An example of an inverted magnetic immersion lens is shown in Figure 1. In one application, the magnetic immersion lens forms part of an electron microscope, such as a scanning electron microscope. In this case, the magnetic imaging field of the lens constrains a beam of pπmary electrons to follow an axial trajectory along the through-bore and focuses the beam onto the specimen Secondary electrons emitted from the specimen surface are guided into the through-bore by the magnetic imaging field and are detected by means of a secondary electron detection arrangement located within the through-bore Due to the limited availability of space within the through-bore the detection arrangement needs to be of compact design, and this presents a significant technical problem Furthermore, a detection arrangement using electrostatic deflection fields may distort the magnetic imaging field of the lens causing, inter aha, a misalignment of the pπmary electron beam and a consequent deterioration in the electron optical properties of the microscope. This problem is particularly acute in the case of low voltage microscopy, particularly when the energy of the pπmary electrons is 1 keV or less. N known, within-the-lens, secondary electron detection arrangement has the form of a Wein energy filter and is described by M. Sato, H. Todokoro, . Kageyama - "A snorkel type conical objective lens with E cross B fields for detecting secondary electrons" SPIE, Vol 2014, Charged-Particle Optics (1993). This detection arrangement has electrostatic deflectors for producing an electrostatic field in a plane perpendicular to the axis of the primary electron beam which is used for the extraction of secondary electrons. The detection arrangement also has coils for producing a magnetic field perpendicular to, and in the same plane as, the electrostatic field. The strength and direction of the magnetic field are such that it compensates for the force exerted on the primary electron beam by the electrostatic field. Thus, there is no appreciable misalignment of the primary electron beam, even at energies as low as 1 keV. Nevertheless, the detection arrangement has a complex structure and is difficult to adjust to obtain the optimum result. According to the present invention there is provided a magnetic immersion lens comprising inner and outer pole-pieces arranged symmetrically about a longitudinal axis of the lens, the inner pole-piece having a through-bore and the lens producing a magnetic imaging field for directing, along said through-bore, charged panicles emitted from a specimen positioned in front of the inner pole-piece, and an axially-symmetric, charged- particle detection arrangement located within the through-bore for detecting charged particles being directed along the through-bore by the magnetic imaging field, the charged-particle detection arrangement comprising repeller means for deflecting said charged particles away from the longitudinal axis and detector means for detecting charged particles deflected by the repeller means. Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a longitudinal sectional view through a known magnetic immersion lens; Figure 2 is a longitudinal sectional view through part of a magnetic immersion lens according to the invention; Figure 3 gives a more detailed view of the detection arrangement used in the magnetic immersion lens of Figure 2; Figure 4 illustrates a computer simulation of the detection arrangement; and Figure 5 shows the axial distribution of electrostatic potential within the region of the magnetic immersion lens occupied by the detection arrangement. Referring to Figure 2, the magnetic immersion lens is of the snorkel type comprising inner and outer pole-pieces 10,11 arranged symmetrically about a longitudinal axis X-X of the lens, the inner pole-piece 10 having a through-bore 12. A specimen S is positioned in front of the tip end of the inner pole-piece 10 at a suitable working distance d, typically about 2mm. Charged particles emitted from the surface of specimen S are constrained by the magnetic imaging field of the lens to enter the through- bore 12 of the inner pole-piece 10 where they follow a spiral trajectory and are detected by a detection arrangement 20 located within the through-bore 12. In this particular embodiment, the magnetic immersion lens is part of a scanning electron microscope; therefore, in this case, the charged particles are secondary electrons which are caused to be emitted from the specimen surface by a beam of primary electrons directed along the longitudinal axis X-X of the lens. The detection arrangement 20 is shown in greater detail in Figure 3. It consists of a number of electrodes disposed symmetrically about the longitudinal axis X-X; namely, a focusser electrode 21 , a repeller electrode 22, an extractor electrode 23, a detector electrode 24 and a reflector electrode 25. The focusser electrode 21 consists of an annular plate electrode positioned about 20mm from the entrance to the through-bore 12 The electrode is maintained at a positive voltage V, of some + 50 volts and is effective to accelerate secondary electrons enteπng the through-bore The accelerated electrons pass through a central aperture 21' in the plate electrode and follow convergent trajectones towards the repeller electrode 22. The repeller electrode 22 has the form of a cylmdπcal electrode which is centred on the longitudinal axis X-X and is maintained at a negative voltage V r of some - 20V. The repeller electrode 22 turns the secondary electrons back towards the accelerator electrode 21 and, at the same time, deflects the electrons away from the longitudinal axis X-X. Figure 2 shows the trajectones T,,T 2 of two such electrons, the electron following trajectory T, entenng the through-bore with a different angle from the electron following trajectory T 2 . The deflected electrons are accelerated towards the detector electrode 24 by means of the extractor electrode 23 The extractor and detector electrodes 23,24 are both annular and are disposed around, but electrically isolated from, the repeller electrode 22. and they lie in respective planes perpendicular to the longitudinal axis X-X The extractor electrode 23 has the form of a mesh which is maintained at a positive voltage V e of some + 150 V, whereas the detector electrode 24 has the form of a plate electrode which is maintained at a positive voltage V d of + 500V The accelerated electrons pass through the mesh of electrode 23 to be detected by the detector electrode 24 The reflector electrode 25 is positioned betw een the focusser electrode 21 and the repeller electrode 22, and is maintained at a negative voltage V r of some -5 V The reflector electrode is so shaped as to prevent secondary electrons deflected by the repeller electrode 22 from impinging on the walls of the inner pole-piece 10 and on the focusser electrode 21 and so being lost from the flux of electrons collected by the detector electrode 24. The negative voltage V r applied to the reflector electrode 25 may be adjusted to maximise the secondary electron signal detected by the detector electrode 24. In this embodiment the reflector electrode 25 consists of an annular plate electrode 25' adjacent to the focusser electrode 21 , a cylindrical electrode 25" axially overlapping the repeller electrode 22 and a truncated, conical electrode 25'" interconnecting the electrodes 25 * ,25". The effect of the repeller electrode 22 is to prevent the secondary electron trajectories from following the longitudinal axis further and to separate them from this axis. Furthermore, because the detection arrangement 20 is axially symmetric, the secondary electrons are guided to the detector electrode 24 without causing any significant distortion of the magnetic imaging field and a consequent misalignment of the primary electron beam. The axially-symmetric detection arcangement 20 shown in more detail in Figure 3 has been modelled using computer simulation techniques. The model includes a magnetic imaging field capable of focussing a 1 keV pπmary electron beam onto the surface of a specimen supported at a working distance of 2mm. The computer simulation studies have demonstrated using a 1000 secondary electrons leaving the specimen surface at different angular trajectories and with a kinetic energy of 5eV, that over 80% of the secondary electrons are incident on the detector electrode 24. Figure 4 shows an example of this computer simulation whereby for clanty just 10 of the 1000 secondary electrons have been selected for view. Figure 5 shows that the axial distribution 0(z) of electrostatic potential within the region of detector 20 is no greater than ± 20 V. Comparing this with a typical primary electron beam energy Ep of lOOOeV, the electrostatic distribution due to the effect of the detection anangement should have little or no effect on the electro optical properties of the primary electron beam. This has been confirmed independently by other calculations which compare the lens abercation values of a high resolution electron probe formed by operating a magnetic lens with and without the detection arrangement. In the embodiment of Figures 2 and 3 the detector electrode 24 has the form of an annular plate electrode. In alternative embodiments, a different form of detector means could be used; for example, an annular microchannel plate assembly, an annular solid state diode, an annular scintillator or a detection device consisting of a number of, preferably, electrically isolated sections.

Claims: CLAIMS 1 A magnetic immersion lens compnsmg inner and outer pole-pieces ananged symmetncally about a longitudinal axis of the lens, the inner pole-piece having a through- bore and the lens producing a magnetic imaging field for directing, along said through- bore, charged particles emitted from a specimen positioned in front of the inner pole- piece, and an axially-symmetπc charged-particle detection anangement located withm the through-bore for detecting charged particles being directed along the through-bore by the magnetic imaging field, the charged-particle detection anangement compnsmg repeller means for deflecting said charged particles away from the longitudinal axis and detector means for detecting charged particles deflected by the repeller means. 2 A lens as claimed in claim 1 wherein said charged-particle detection anangement further compnses focusser means for converging said charged particles towards the repeller means 3 A lens as claimed in claim 2 w herein the charged-particle detection anangement further compnses reflector means for deflecting charged particles away from the sides of said through-bore and said focusser means 4 A lens as claimed in any one of claims 1 to 3 wherein said repeller means is ananged to cause said charged particles to move on trajectones towards the specimen before being detected by the detector means. 5. A lens as claimed in any one of claims 1 to 4 wherein the detector means comprises an annular plate electrode centred on, and lying in a plane perpendicular to the longitudinal axis. 6. A lens as claimed in claim 5 wherein the charged-particle detection anangement includes an extractor electrode for accelerating charged particles towards said annular plate electrode for detection. 7. A lens as claimed in claim 6 wherein the extractor electrode comprises an annular mesh lying in a plane parallel to, and spaced from the annular plate electrode of the detector means. 8. A lens as claimed in any one of claims 1 to 4 wherein the detector means comprises an annular microchannel plate assembly. 9. A lens as claimed in any one of claims 1 to 4 wherein the detector means comprises an annular solid state diode. 10. A lens as claimed in any one of claims 1 to 4 wherein the detector means compnses an annular scintillator. 11. A lens as claimed in any one of claims 1 to 4 wherein the detector means comprises a plurality of segments. 12. A lens as claimed in claim 1 1 wherein said segments are electrically isolated from each other. 13. A lens as claimed in any one of claims 1 to 12 wherein said repeller means comprises a cylindrical electrode ananged symmetrically about the longitudinal axis of the through-bore and having the same polarity as the charged particles. 14. A lens as claimed in claim 1 wherein said charged particle detection anangement further comprises focusser means for converging said charged particles towards the repeller means and reflector means, extending between said repeller means and said focusser means and being ananged to deflect charged particles, deflected by the repeller means, away from the focusser means and away from the sides of the lens through-bore. 15. A lens as claimed in claim 14 wherein the reflector means axially overlaps the repeller means. 16. A lens as claimed in claim 14 or claim 15 wherein the reflector means comprises an annular plate electrode adjacent to the focusser means, a cylindrical electrode adjacent to the repeller means and a truncated conical electrode interconnecting the annular plate electrode and the cylindrical electrode. 17. A lens as claimed in any one of claims 1 to 16 wherein said charged particles are secondary electrons. 18. An electron microscope including a magnetic immersion lens as claimed in any one of claims 1 to 17 and means for directing a beam of primary electrons along the through-bore of the magnetic immersion lens onto a specimen in front of the inner pole- piece of the magnetic immersion lens. 19. An electron microscope to be claimed in claim 18 in the form of a scanning electron microscope. 20. A magnetic immersion lens substantially as herein described with reference to Figures 2 to 5 of the accompanying drawings. 21. An electron microscope substantially as herein described.

Inventors: BADHEKA RANJAN KRISHNACHANDRA, MARUI TAKAO, READ FRANK HENRY, BADHEKA, RANJAN, KRISHNACHANDRA, MARUI, TAKAO, READ, FRANK, HENRY
Applicants: BADHEKA RANJAN KRISHNACHANDRA, MARUI TAKAO, READ FRANK HENRY, SHIMADZU RES LAB EUROPE LTD, BADHEKA, RANJAN, KRISHNACHANDRA, MARUI, TAKAO, READ, FRANK, HENRY, SHIMADZU RESEARCH LABORATORY (EUROPE) LTD.
Application year: 2000
Main classification: H01J37
IPC codes: H01J37