Semiconductor metrology and inspection based on an x-ray source with an electron emitter array
Methods and systems for realizing a high radiance x-ray source based on a high density electron emitter array are presented herein. The high radiance x-ray source is suitable for high throughput x-ray metrology and inspection in a semiconductor fabrication environment. The high radiance X-ray source includes an array of electron emitters that generate a large electron current focused over a small anode area to generate high radiance X-ray illumination light. In some embodiments, electron current density across the surface of the electron emitter array is at least 0.01 Amperes/mm 2 , the electron current is focused onto an anode area with a dimension of maximum extent less than 100 micrometers, and the spacing between emitters is less than 5 micrometers. In another aspect, emitted electrons are accelerated from the array to the anode with a landing energy less than four times the energy of a desired X-ray emission line.
1. An x-ray illumination source, comprising:
an electron emitter array including a plurality of electron emitters each fabricated on a common substrate, each of the plurality of electron emitters comprising:
an emitter structure fabricated on the common substrate, the emitter structure extending from a surface plane of the common substrate in a direction normal to the surface plane, the emitter structure terminating at a sharp tip of the emitter structure;
an extractor electrode fabricated around the sharp tip of the emitter structure, wherein a void separates the emitter tip and at least a portion of the extractor electrode; and
a first insulating layer disposed between the emitter structure and the extractor electrode, wherein the first insulating layer electrically isolates the emitter structure from the extractor electrode, wherein a difference in electrical potential between the emitter structure and the extractor electrode stimulates an electron current flow from the sharp tip of the emitter structure;
an anode structure spatially separated from the electron emitter array, wherein a difference in electrical potential between the anode structure and the substrate accelerates the electron current flow from each of the plurality of electron emitters of the electron emitter array to the anode, wherein the incidence of the electron current flow onto the anode structure stimulates X-ray emission from the anode structure;
an electron lens located around a path of the electron current flow from the electron emitter array to the anode structure;
a first barrier separating a first portion of a vacuum chamber and a second portion of the vacuum chamber, wherein the electron emitter array is disposed in the first portion of the vacuum chamber and the electron lens is disposed in the second portion of the vacuum chamber, and wherein the first portion of the vacuum chamber is maintained at a lower pressure than the second portion of the vacuum chamber; and
a second barrier separating a third portion of the vacuum chamber and the second portion of the vacuum chamber, wherein the anode structure is disposed in the third portion of the vacuum chamber, wherein the second portion of the vacuum chamber is maintained at a lower pressure than the third portion of the vacuum chamber.
2. The x-ray illumination source of claim 1 , wherein the current density of the electron current flow generated across the electron emitter array is at least 0.01 Ampere/mm2.
3. The x-ray illumination source of claim 2 ,
wherein the electron lens is configured to focus the electron current flow onto an area of the anode structure characterized by a dimension of maximum extent of less than 100 micrometers.
4. The x-ray illumination source of claim 1 , wherein the common substrate is a silicon substrate.
5. The x-ray illumination source of claim 1 , wherein a spacing between adjacent emitter structures of the electron emitter array is less than five micrometers.
6. The x-ray illumination source of claim 1 , wherein a first subset of the extractor electrodes are electrically isolated from a second subset of the extractor electrodes, and wherein a first electrical potential is present on the first subset of extractor electrodes, and wherein a second electrical potential is present on the second subset of extractor electrodes, wherein the first electrical potential is different from the second electrical potential.
7. The x-ray illumination source of claim 1 , each of the plurality of electron emitters, further comprising:
a focusing electrode fabricated around the sharp tip of the emitter structure over the extractor electrode; and
a second insulating layer disposed between the focusing electrode and the extractor electrode, wherein the second insulating layer electrically isolates the focusing electrode from the extractor electrode, wherein a difference in electrical potential between the emitter structure and the focusing electrode shapes the electron current flow from the sharp tip of the emitter structure.
8. The x-ray illumination source of claim 1 , further comprising:
an optical light source configured to illuminate the electron emitter array with optical radiation, wherein the optical illumination enhances electron emission from the electron emitter array.
9. The x-ray illumination source of claim 1 , wherein the difference in electrical potential between the anode structure and the substrate is less than four times the energy of a desired X-ray emission line generated by the anode structure.
10. The x-ray illumination source of claim 1 , wherein a temperature of the electron emitter array is maintained in a range between 400 and 1,000 degrees Centigrade.
11. The x-ray illumination source of claim 1 , wherein the first barrier includes an aperture, and wherein the electron current flow passes through the aperture.
12. The x-ray illumination source of claim 1 , wherein the first barrier includes an opening covered by a thin, metal foil, and wherein the electron current flow passes through the thin, metal foil.
13. The x-ray illumination source of claim 1 , wherein the anode structure includes an anode material, wherein the electron current flow is incident on the anode material, and wherein the anode material is any of copper, molybdenum, and tungsten.
14. The x-ray illumination source of claim 1 , wherein the anode structure includes an anode material, wherein the electron current flow is incident on the anode material, and wherein the anode material is a solid metal or a liquid metal.
15. The x-ray illumination source of claim 1 , wherein the anode structure includes a rotating anode support structure configured to rotate about an axis of rotation at a constant angular velocity, wherein the rotating anode support structure supports a metal anode material in a fixed position with respect to the rotating anode support structure while the rotating anode support structure is rotated at the constant angular velocity, and wherein the electron current is incident on the metal anode, and wherein the metal anode is a solid metal anode or a liquid metal anode.
16. A measurement system, comprising:
an electron emitter array based x-ray illumination source configured to illuminate an area of a specimen with an incident x-ray beam, wherein the electron emitter based x-ray illumination source includes,
an electron emitter array including a plurality of electron emitters each fabricated on a common substrate, each of the plurality of electron emitters comprising:
an emitter structure fabricated on the common substrate, the emitter structure terminating at a sharp tip;
an extractor electrode fabricated around the sharp tip of the emitter structure, wherein a void separates the emitter tip and at least a portion of the extractor electrode; and
a first insulating layer disposed between the emitter structure and the extractor electrode, wherein the first insulating layer electrically isolates the emitter structure from the extractor electrode, wherein a difference in electrical potential between the emitter structure and the extractor electrode stimulates an electron current flow from the sharp tip of the emitter structure; and
an anode structure spatially separated from the electron emitter array, wherein a difference in electrical potential between the anode structure and the substrate accelerates the electron current flow from each of the plurality of electron emitters of the electron emitter array to the anode, wherein the incidence of the electron current flow onto the anode structure stimulates X-ray emission from the anode structure;
an x-ray detector configured to receive radiation from the specimen in response to the incident x-ray beam and generate signals indicative of a first property of the specimen;
an electron lens located around a path of the electron current flow from the electron emitter array to the anode structure;
a first barrier separating a first portion of a vacuum chamber and a second portion of the vacuum chamber, wherein the electron emitter array is disposed in the first portion of the vacuum chamber and the electron lens is disposed in the second portion of the vacuum chamber, and wherein the first portion of the vacuum chamber is maintained at a lower pressure than the second portion of the vacuum chamber; and
a second barrier separating a third portion of the vacuum chamber and the second portion of the vacuum chamber, wherein the anode structure is disposed in the third portion of the vacuum chamber, wherein the second portion of the vacuum chamber is maintained at a lower pressure than the third portion of the vacuum chamber.
17. The measurement system of claim 16 , wherein the measurement system is a small angle x-ray scatterometer configured to perform measurements in a transmissive or a reflective mode.
18. The measurement system of claim 16 , wherein the measurement system is configured as any of a transmission small angle x-ray scatterometry system, a grazing incidence small angle x-ray scatterometry system, a wide angle x-ray scatterometry system, a x-ray reflectometry system, a grazing incidence x-ray reflectometry system, a x-ray diffractometry system, a grazing incidence x-ray diffractometry system, a high resolution x-ray diffractometery system, a x-ray photoelectron spectrometry system, a x-ray fluorescence metrology system, a total reflection x-ray fluorescence metrology system, a grazing incidence x-ray fluorescence metrology system, a x-ray tomography system, a x-ray ellipsometry system, and a hard x-ray photoemission spectrometry system.
19. The measurement system of claim 16 , wherein the current density of the electron current flow generated across the electron emitter array is at least 0.01 Ampere/mm2.
20. The measurement system of claim 16 , wherein the electron lens is configured to focus the electron current flow onto an area of the anode structure characterized by a dimension of maximum extent of less than 100 micrometers.
21. The measurement system of claim 16 , wherein the common substrate is a silicon substrate.
22. The measurement system of claim 16 , wherein a spacing between adjacent emitter structures of the electron emitter array is less than five micrometers.
23. The measurement system of claim 16 , wherein a first subset of the extractor electrodes are electrically isolated from a second subset of the extractor electrodes, and wherein a first electrical potential is present on the first subset of extractor electrodes, and wherein a second electrical potential is present on the second subset of extractor electrodes, wherein the first electrical potential is different from the second electrical potential.
24. The measurement system of claim 16 , each of the plurality of electron emitters, further comprising:
a focusing electrode fabricated around the sharp tip of the emitter structure over the extractor electrode; and
a second insulating layer disposed between the focusing electrode and the extractor electrode, wherein the second insulating layer electrically isolates the focusing electrode from the extractor electrode, wherein a difference in electrical potential between the emitter structure and the focusing electrode shapes the electron current flow from the sharp tip of the emitter structure.
25. The measurement system of claim 16 , the electron emitter based x-ray illumination source further comprising:
an optical light source configured to illuminate the electron emitter array with optical radiation, wherein the optical illumination enhances electron emission from the electron emitter array.
26. The measurement system of claim 16 , wherein the difference in electrical potential between the anode structure and the substrate is less than four times the energy of a desired X-ray emission line generated by the anode structure.
27. The measurement system of claim 16 , wherein a temperature of the electron emitter array is maintained in a range between 400 and 1,000 degrees Centigrade.
28. The measurement system of claim 16 , wherein the anode structure includes an anode material, wherein the electron current flow is incident on the anode material, and wherein the anode material is a solid metal or a liquid metal.
29. A method comprising:
providing a first difference in electrical potential between at least one emitter structure of an electron emitter array and at least one extractor electrode corresponding to the at least one emitter structure, the first difference in electrical potential stimulating an electron current flow from a sharp tip of the at least one emitter structure, wherein each emitter structure of the electron emitter array is fabricated on a common substrate;
providing a second difference in electrical potential between the common substrate and an anode structure, the second difference in electrical potential accelerating the electron current flow from the electron emitter array to the anode structure;
focusing the electron current flow onto an area of an anode structure with an electron lens disposed in a path of the electron current between the electron emitter array and the anode structure, the incidence of the electron current flow onto the anode structure stimulating X-ray emission;
maintaining a vacuum in a first portion of a vacuum chamber, wherein the electron emitter array is disposed in the first portion of the vacuum chamber;
maintaining a vacuum in a second portion of the vacuum chamber, wherein the electron lens is disposed in the second portion of the vacuum chamber, and wherein the vacuum maintained in the first portion of the vacuum chamber is a lower pressure than the vacuum maintained in the second portion of the vacuum chamber; and
maintaining a vacuum in a third portion of the vacuum chamber, wherein the anode structure is disposed in the third portion of the vacuum chamber, and wherein the vacuum maintained in the second portion of the vacuum chamber is a lower pressure than the vacuum maintained in the third portion of the vacuum chamber.
30. The method of claim 29 , further comprising:
illuminating an area of a specimen with an incident x-ray beam comprising the X-ray emission from the anode structure;
detecting an amount of radiation from the specimen in response to the incident X-ray beam; and
generating signals indicative of a first property of the specimen based on the detected amount of radiation.
31. The method of claim 29 , wherein a current density of the electron current flow generated across the electron emitter array is at least 0.01 Ampere/mm2.
32. The method of claim 29 , wherein the area of the anode structure upon which the electron current flow is focused is characterized by a dimension of maximum extent of less than 100 micrometers.
33. The method of claim 29 , wherein the common substrate is a silicon substrate.
34. The method of claim 29 , wherein a spacing between adjacent emitter structures of the electron emitter array is less than five micrometers.