Patent Number: 
Section: claims

1. A method for operating a charged particle beam emitting device comprising a charged particle beam source having an emitter surface, the method comprising the steps of:(a) placing the charged particle beam source in a vacuum of a given pressure, the charged particle beam source exhibiting a high initial emission current I0 and a lower stable mean emission current IS under given operational conditions;(b) applying the given operational conditions to the charged particle beam source for emitting charged particles from the emitter surface, so that the emission current of the charged particle beam source is higher than the stable mean emission current IS;(c) performing a cleaning process by applying at least one heating pulse to the charged particle beam source for heating the emitter surface to a temperature TC, whereby the cleaning process is performed before the emission current of the charged particle beam source has declined to the lower stable mean emission value IS; and(d) repeating the cleaning process to keep the emission current of the charged particle beam source continuously above the substantially stable emission value IS. 2. The method as claimed in claim 1, wherein the cleaning process is performed at predefined time intervals or when the emission current has dropped to a predefined value IC, wherein IC is higher than the stable mean emission value IS. 3. The method as claimed in claim 2, wherein IC is defined as IC=αI0 with α>0.9. 4. The method as claimed in claim 2, wherein a predefined maximum value Imax, and a predefined minimum value Imin=IC>IS is selected, whereby a mean deviation value ΔI is defined as ΔI=Imax−Imin, and the cleaning process is performed when the emission current has fallen from the maximum emission value Imax to Imin. 5. The method as claimed in claim 4, wherein ΔI/Imax=γ, and γ is about 0.1. 6. The method as claimed in claim 1, whereinthe charged particle beam emitting device is an electron beam emitting device,the charged particle beam source is an electron emitter for emitting electrons andthe given operational conditions comprise a given pressure and a given energy applied to the emitter surface of the electron emitter to extract electrons. 7. The method as claimed in claim 6, wherein the electron emitter is a cold field emitter and the energy is an electric field. 8. The method as claimed in claim 1, wherein:the charged particle beam emitting device is an ion beam emitting device; andthe charged particle beam source is an ion emitter for emitting ions. 9. The method as claimed in claim 1, wherein step (a) comprises a master cleaning process for cleaning the emitter surface such that the charged particle beam source exhibits the high initial emission current I0 and the lower stable mean emission current IS. 10. The method as claimed in claim 9, wherein the master cleaning process comprises at least one master heating pulse to heat the emitter surface to a temperature TMC. 11. The method as claimed in claim 10, wherein TMC is about 2500 K to 2800 K. 12. The method as claimed in claim 1, wherein the charged particle beam source has an emitter tip at which the emitter surface is formed and wherein a thermal build-up process is performed to sharpen the emitter tip. 13. The method as claimed in claim 12, wherein multiple heating pulses are applied to the charged particle beam source during the build-up process. 14. The method as claimed in claim 13, wherein the heating pulses heat the emitter surface to a temperature TB, which is higher than the temperature TC of the cleaning process. 15. The method as claimed in claim 1, wherein the cleaning process is automatically synchronized with non-operational periods of the charged particle beam emitting device. 16. The method as claimed in claim 15, wherein the non-operational periods are periods during which the charged particle beam is defocused or deflected from a sample or specimen. 17. The method as claimed in claim 15, wherein the non-operational periods are triggered by an impulse of a timer, a drop of the emission current, an exchange of a sample or specimen, calibration of the charged particle beam emitting device, a sample or specimen movement, or a stage movement. 18. A method for operating an electron beam emitting device comprising a cold field emitter having an emitter surface, the method comprising the steps of:(a) placing the cold field emitter in a vacuum of a given pressure, the cold field emitter exhibiting a high initial emission current I0 and a lower stable mean emission current IS under a given electric extraction field;(b) applying the given electric extraction field to the cold field emitter for emitting electrons from the emitter surface, so that the emission current of the cold field emitter is higher than the stable mean emission current IS;(c) adjusting the strength of the electric extraction field to keep the emission current substantially stable and continuously above the mean stable emission current IS at a predefined value IC being higher than IS;(d) performing a cleaning process by applying at least one heating pulse to the cold field emitter for heating the emitter surface to a temperature TC, whereby the cleaning process is performed when the strength of the electric extraction field exceeds a predefined reference value; and(e) repeating the adjusting step (c) and the cleaning process to keep the emission current of the cold field emitter continuously above the substantially stable emission value IS. 19. The method as claimed in claim 18, wherein the predefined value IC is defined as IC=αI0, wherein α>0.9. 20. The method as claimed in claim 18, wherein TC is about 2200 K to 2500 K. 21. The method as claimed in claim 18, wherein the pulse width of the at least one heating pulse is 1 second to 2 seconds. 22. The method as claimed in claim 18, wherein each cleaning process comprises about 10 heating pulses and, in particular, 2 to 4 heating pulses being applied to the particle beam source or cold field emitter about every 1 second to 3 seconds. 23. The method as claimed in claim 18, wherein the electron beam emitting device comprises a suppressor electrode arranged around the cold field emitter, and wherein a suppressor voltage is applied to the suppressor electrode during the cleaning process. 24. The method as claimed in claim 23, wherein the suppressor voltage is between 300 V and 1000 V. 25. A method for operating a charged particle beam emitting device comprising a charged particle beam source having an emitter surface, the method comprising the steps of:(a) generating a charged particle beam;(b) focusing the generated charged particle beam onto a sample or specimen; and(c) automatically performing a cleaning process for cleaning the emitter surface upon occurrence of a triggering event, wherein the triggering event is an impulse of a timer, a decline of an emission current of the charged particle beam emitting device, an exchange of the sample or specimen, a calibration period, a sample or specimen movement, or a stage movement. 26. The method as claimed in claim 25, wherein the charged particle beam is defocused or deflected from the sample or specimen during the cleaning process. 27. A charged particle beam emitting device comprising:a charged particle beam source for emitting charged particles, the charged particle beam source comprising an emitter surface;a voltage unit configured to apply a voltage to the charged particle beam source for generating a charged particle beam;a heating element configured to heat the emitter surface; anda control unit comprising an input configured to receive a trigger signal, the control unit being operative to control the heating element to apply at least one heating pulse to the emitter surface of the charged particle beam source during the generation of the charged particle beam upon reception of a trigger signal, generated in response to a triggering event wherein the triggering event is an impulse of a timer, a decline of an emission current of the charged particle beam emitting device, an exchange of the sample or specimen, a calibration period, a sample or specimen movement, or a stage movement. 28. The charged particle beam emitting device as claimed in claim 27, wherein the charged particle beam emitting device comprises a beam blanker, the control unit being operative to control the beam blanker to deflect the generated particle beam upon reception of the trigger signal. 29. The charged particle beam emitting device as claimed in claim 27, wherein the charged particle beam emitting device comprises a focusing unit configured to focus and defocus the charged particle beam onto a sample or specimen, the control unit being operative to control the focusing unit to defocus the generated charged particle beam with respect to the sample or specimen upon reception of the trigger signal. 30. The charged particle beam emitting device as claimed in claim 27, wherein the charged particle beam emitting device is an electron beam emitting device and the charged particle beam source is a cold field electron emitter. 31. The charged particle beam emitting device as claimed in claim 30, wherein the electron beam emitting device is a scanning electron microscope (SEM), a transmission electron microscope (TEM), or a scanning transmission electron microscope (STEM). 32. The charged particle beam emitting device as claimed in claim 27, wherein the charged particle beam emitting device is an ion beam emitting device and the charged particle beam source is an ion emitter. 33. The charged particle beam emitting device as claimed in claim 27, wherein the emitter surface of the charged particle beam source has a radius of curvature of less than 250 nm. 34. The charged particle beam emitting device as claimed in claim 27, wherein the charged particle beam emitting device comprises a measuring element configured to measure the emission current of the charged particle beam source and a triggering unit adapted to provide a trigger signal when the emission current has dropped to a predefined value. 35. The charged particle beam emitting device as claimed in claim 27, wherein the charged particle beam emitting device comprises a carrying element configured to movably carry a specimen, a motion controller unit configured to control the carrying element, and a synchronizing means adapted to provide a trigger signal upon movement of the carrying element. 36. A charged particle beam emitting device comprising:a charged particle beam source for emitting charged particles, the charged particle beam source comprising an emitter surface;a voltage unit for applying a voltage to the charged particle beam source for generating a charged particle beam;a heating element configured to heat the emitter surface; anda control unit comprising an input for receiving a trigger signal, the control unit being operative to control the heating element to apply at least one heating pulse to the emitter surface of the charged particle beam source during the generation of the charged particle beam upon reception of a trigger signal;characterized in that the charged particle beam emitting device further comprises a measuring element for measuring the emission current of the charged particle beam source, and a triggering unit adapted to provide the trigger signal when the voltage applied by the voltage unit exceeds a predefined voltage value.