Patent Number: 
Section: claims

1. A gas field ion source, comprising:a housing,an electrically conductive tip within the housing,a gas supply configured to supply a gas to the housing, the gas comprising neon or a noble gas with atoms having a mass larger than neon, andan extractor electrode having a hole to permit ions generated in the neighborhood of the tip to pass through the hole,wherein a surface of the extractor electrode facing the tip comprises a material selected to have a negative secondary ion sputter rate of less than 10−5 per incident neon ion so that, during use of the gas field ion source, the extractor electrode produces a relatively low rate of negative secondary ions that are accelerated from the extractor to the tip. 2. The gas field ion source of claim 1, wherein the extractor comprises at least one material selected from the group consisting of carbon, iron, molybdenum, titanium, vanadium, tatalum. 3. The gas field ion source of claim 1, wherein at least the surface of the extractor facing the tip comprises a metal that does not form oxides. 4. The gas field ion source of claim 1, wherein the hole has a vacuum conductance of less than 1 liter per second. 5. The gas field ion source of claim 4, wherein the hole has a vacuum conductance of less than 0.2 liter per second. 6. The gas field ion source of claim 4, wherein a gas pressure on one side of the hole is in the range 10−2 to 10−3 torr, and a gas pressure on another side of the hole is in the range 10−5 to 10−7 torr. 7. The gas field ion source of claim 4, wherein a gas pressure on one side of the hole is in the range 10−2 to 10−3 mbar, and a gas pressure on another side of the hole is in the range 10−5 to 10−7 mbar. 8. The gas field ion source of claim 1, wherein a distance between the tip and the extractor electrode is such that the extractor hole subtends a half angle of 15 to 40 degrees in angle. 9. The gas field ion source of claim 8, wherein the distance between the tip and the extractor electrode is such that the extractor hole subtends a half angle of 20 to 25 degrees in angle. 10. The gas field ion source of claim 8, wherein the distance between the tip and the extractor electrode is such that the extractor hole subtends a solid angle in the range between 0.38 steradians and 0.59 steradians. 11. The gas field ion source of claim 1, further comprising a cryogenic cooling system configured to cool at least an output tube of the gas supply to a cryogenic temperature. 12. The gas field ion source of claim 11, further comprising a heater configured to heat the output tube of the gas supply. 13. The gas field ion source of claim 1, further comprising a heater configured to heat the housing. 14. The gas field ion source of claim 1, further comprising a chemical getter within the housing. 15. The gas field ion source of claim 1, further comprising a flapper valve at the housing. 16. The gas field ion source of claim 1, further comprising a bypass line of the gas delivery system to a space outside the housing. 17. A gas field ion source, comprising:an external housing,an internal housing within the external housing,an electrically conductive tip within the internal housing,a gas supply configured to supply a gas to the internal housing, the gas supply comprising a tube terminating within the internal housing,an extractor electrode having a hole to permit ions generated in the neighborhood of the tip to pass through the hole into the external housing, anda flapper valve between the internal and the external housing, the flapper valve being configured to increase a flow of gas from the internal housing to the external housing when the flapper valve is opened,wherein a surface of the extractor electrode facing the tip comprises a material selected to have a negative secondary ion sputter rate of less than 10−5 per incident neon ion so that, during use of the gas field ion source, the extractor electrode produces a relatively low rate of negative secondary ions that are accelerated from the extractor to the tip. 18. The gas field ion source of claim 17, wherein the gas supply is configured so that:in a first mode of operation of the gas field ion source, the gas supply supplies a first noble gas; andin a second mode of operation of operation of the gas field ion source, the gas supply supplies a second noble gas. 19. The gas field ion source of claim 18, wherein the first gas is helium, and the second gas is neon. 20. A method, comprising:providing a gas field ion source, comprising:an external housing,an internal housing within the external housing,an electrically conductive tip within the internal housing,a gas supply configured to supply first and second gases to the internal housing, the gas supply comprising a tube terminating within the internal housing,an extractor electrode having a hole to permit ions generated in the neighborhood of the tip to pass through the hole into the external housing, a surface of the extractor electrode facing the tip comprises a material selected to have a negative secondary ion sputter rate of less than 10−5 per incident neon ion so that, during use of the gas field ion source, the extractor electrode produces a relatively low rate of negative secondary ions that are accelerated from the extractor to the tip, anda flapper valve arranged between the internal and the external housing configured to increase a flow of gas from the internal housing to the external housing when the flapper valve is opened,operating the gas field ion source while keeping the flapper valve closed, andopening the flapper valve when changing between operation of the charged particle beam source with the first gas and the second gas,wherein the first gas is different from the second gas.