received by the International Bureau on 29 June 2015 (29.06.2015)

Cathode arrangement (20) comprising:

- a thermionic cathode comprising an emission portion (30) provided with an emission surface (32) for emitting electrons, and a reservoir (38) for holding a material, wherein the material, when heated, releases work function lowering particles (70) that diffuse towards the emission portion and emanate at the emission surface at a first evaporation rate (<Dc);

- a focusing electrode (40) comprising a focusing surface (42) for focusing the electrons emitted from the emission surface (32) of the cathode; and

- an adjustable heat source (50) configured for keeping the focusing surface at a temperature (Te) at which accumulation of work function lowering particles on the focusing surface is prevented,

wherein

the thermionic cathode further comprises a cathode body (22) housing the emission portion and the reservoir,

the adjustable heat source (50) is configured for heating the cathode body (22), the focusing electrode (40) further comprises a heat trapping surface (52) facing at least a portion of the cathode body and arranged for receiving heat radiation (Q) emitted by the cathode body during use, the heat trapping surface being in thermal communication with the focusing surface (42),

wherein the contact between the cathode body and the focusing electrode is limited by radial spacers (59) defining a radial interspacing (58) between the heat trapping surface (52) and an outer surface (36) of the cathode body (22), and/or by spacing structures (48) provided between a non-emission surface (34) surrounding the emission surface (32) and an inner electrode surface (46) of the focusing electrode facing the emission portion, for providing a spacing between the focusing electrode and the emission portion.

2. Cathode arrangement according to claim 1, wherein three spacing structures (48) and/or three radial spacers (59) are provided.

3. Cathode arrangement (20) according to claim 1 or 2, wherein the adjustable heat source (50) is configured for keeping the focusing surface of the focusing electrode at a temperature (Te) above a threshold temperature (Te-) at which a release of work function lowering particles from the focusing surface at a second evaporation rate (Φε) equals an arrival rate of work function lowering particles at the focusing surface or equals the first evaporation rate (Oc).

4. Arrangement according to any one of the preceding claims, wherein the focusing electrode is heated mainly by heat radiation from the cathode body.

5. Arrangement according to any one of the preceding claims, wherein the adjustable heat source (50) comprises a heater cathode (50).

6. Arrangement according to any one of the preceding claims, wherein the adjustable heat source (50) is arranged for heating the reservoir (38) such that the work function lowering particles (70) diffuse towards the emission portion (30) and emanate at the emission surface (32) at the first evaporation rate (Oc).

7. Arrangement according to any one of the preceding claims, wherein the adjustable heat source (50) is arranged within the cathode body (22) or within a receptacle (25) formed by the cathode body.

8. Arrangement according to any one of the preceding claims, wherein the focusing electrode (40) comprises a shell (54) at least partly surrounding the cathode body (22), the shell being provided with an inner surface (54a) at least a portion thereof forming the heat trapping surface (52).

9. Arrangement according to claim 8, wherein the shell (54) comprises angular interspacings (56a-56c) for accommodating a confinement arrangement (64) for confining the focusing electrode (40) and/or the cathode body (22) with respect to a support structure (62).

10. Arrangement according to any one of the preceding claims, wherein the thermionic cathode and the focusing electrode are arranged such that thermal conduction from the cathode to the focusing electrode is avoided or at least minimized.

11. Arrangement according to any one of the preceding claims, wherein, in an

orientation intended during use, the cathode body is resting on the spacing elements (48) by means of gravity.

12. Arrangement according to any one of the preceding claims, further comprising a support structure (62) provided with a confining arrangement (64) for confining the focusing electrode (40) and/or the cathode body (22) with respect to the support structure.

13. Arrangement according to claim 12, wherein the confining arrangement comprises end stops (65a, 65b) each facing a surface area of the cathode arrangement.

14. Arrangement according to any one of the preceding claims, wherein the work

function lowering particles (70) comprise Barium (Ba).

15. Arrangement according to claim 14, wherein the adjustable heat source (50) is configured for keeping the focusing surface temperature (Te) above 900 K.

16. Arrangement according to claim 14 or 15, wherein the adjustable heat source (50) is further configured for keeping the focusing surface temperature below 1300K. 42

17. Arrangement according to any one of the preceding claims, wherein the focusing surface (42) is provided with a coating to suppress electron emission.

18. · Arrangement according to any one of the preceding claims, wherein the focusing electrode comprises an inner electrode surface (46) facing the emission portion and the focusing surface (42) arranged on an outer surface at an angle with respect to the inner electrode surface, whereby the focusing surface and the inner electrode surface converge at a transmission aperture (44).

Arrangement according to any one of the preceding claims, wherein the focusing electrode (40) comprises a transmission aperture (44) having a transmission perimeter (45) defining an area which is smaller than an emission surface area.

Focusing electrode (40), comprising a cylindrical shell (54) defining a cavity for accommodating a cathode body (22), and a front cover provided with a circular electron transmission aperture (44) with a focusing surface (42) on an outside, wherein a heat trapping surface (52) is provided on an inner surface of the cylindrical shell, the heat trapping surface (52) is configured for facing at least a portion of the cathode body and arranged for receiving heat radiation (Q) emitted by the cathode body during use, wherein the cylindrical shell (54) comprises angular interspacings (56a-56c) for accommodating a confining arrangement (64) for confining the focusing electrode (40) and/or the cathode body (22) with respect to a support structure (62), and

wherein the focusing electrode is configured such that the contact between the cathode body and the focusing electrode is limited to radial spacers (59) provided on an inner surface of the cylindrical shell for defining a radial interspacing (58) between the heat trapping surface (52) and an outer surface (36) of the cathode body (22), and/or by spacing structures (48) arranged on an inner electrode surface (46) of the front cover for providing a spacing between the focusing electrode and the cathode body. 43

21. Focusing electrode according to claim 20, wherein the focusing surface is oriented at an angle to an inner electrode surface (46) of the front cover, whereby the focusing surface and the inner electrode surface converge at the transmission aperture.

22. Focusing electrode according to claim 20 or 21, wherein three spacing structures (48) and/or three radial spacers (59) are provided.

23. Electron gun (2) for generating an electron beam (4), the electron gun comprising:

- a cathode arrangement (20) according to any one of claims 1-19 for generating a plurality of electrons; and

- at least one shaping electrode (6a-6c) for shaping the generated electrons into the electron beam. 24. Electron beam lithography system (1) for exposing a target (18) using at least one electron beamlet (5), the system comprising:

- a beamlet generator (2, 12, 13) for generating the at least one electron beamlet;

- a beamlet modulator (14, 15) for patterning the at least one electron beamlet to form at least one modulated beamlet;

- a beamlet projector (16, 17) for projecting the at least one modulated beamlet onto a surface of the target;

wherein the beamlet generator comprises an electron gun (2) according to claim 24.

25. Method for regulating a release of work function lowering particles (70) from a surface, wherein the method comprises:

- providing a cathode arrangement according to any one of claims 1 - 19, and

- keeping the temperature (Te) of the focusing electrode above a threshold temperature (Te-) corresponding to a release of work function lowering particles from the focusing surface (42) at an evaporation rate (Oe) that equals an evaporation rate (< c) of work function lowering particles emanating from the emission surface (32) of the cathode. 44

26. Method according to claim 25, further comprising keeping the temperature (Te) of the focusing electrode below a further threshold temperature (Te+) corresponding to a first electron current density created by emission of electrons from the focusing surface (42) that is 0.01-0.1% of a second electron current created by emission of electrons from the emission surface (32) of the cathode.

27. Method according to claim 25 or 26, wherein the work function lowering particles (70) comprise Barium (Ba), and wherein the method comprises keeping the temperature (Te) of the focusing electrode between 900 K and 1300 K during use of the cathode arrangement.