System and method for aerial image sensing

• US 20030128870A1
• Filed: 01/07/2003
• Published: 07/10/2003
• Est. Priority Date: 01/08/2002
• Status: Active Grant

• Current Assignee: ASML Netherlands BV (ASML Holding NV), Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Inventors: Ye, Jun, Pease, R. Fabian W.
• Granted Patent: US 6,906,305 B2
• Time in Patent Office: Days
• US Class Current: 382/144
• CPC Class Codes: G03F 1/84 Inspecting
• CPC Class Codes: G03F 7/70591 Testing optical components
• CPC Class Codes: G03F 7/7065 Defects, e.g. optical inspection of patterned layer for defects
• CPC Class Codes: G03F 7/70666 Aerial image, i.e. measuring the image of the patterned exposure light at the image plane of the projection system
• CPC Class Codes: G03F 7/7085 Detection arrangement, e.g. detectors of apparatus alignment possibly mounted on wafers, exposure dose, photo-cleaning flux, stray light, thermal load
• CPC Class Codes: G03F 9/7069 Alignment mark illumination, e.g. darkfield, dual focus

Abstract

In one aspect, the present invention is a technique of, and a system and sensor for sensing an aerial image produced by, for example, optical lithographic equipment used in semiconductor fabrication. In one embodiment, the apparatus includes a negative electron affinity photo-electron emission device for projecting the aerial image thereon. In response, the photo-electron emission device emits electrons with a very tight energy spread in a pattern corresponding to the light intensity distribution produced by the aerial image. Electron optics is provided for guiding and projecting the electrons to form an enlarged pattern of the pattern in which the electrons are emitted. A sensing unit senses the enlarged pattern and supplies it to a device for capturing and digitizing the enlarged pattern to obtain therefrom a digitized aerial image. The resolution increase is achieved through the enlargement of the pattern by the electron optics.

In another embodiment, the apparatus employs a photo-conducting layer for projecting the aerial image thereon. The photo-conducting layer is made of a suitable semiconducting material to render it locally conducting in correspondence to the light intensity distribution of the aerial image, which, in response to the projection of the aerial image on the photo-conducting layer, produces local charge depletion in the photo-conducting layer corresponding to the light intensity distribution. The apparatus may also include an electron source for providing electrons and a device for delivering the electrons to the photo-conducting layer to produce local re-charging currents in proportion to the local charge depletion. The apparatus may derive a pattern corresponding to the aerial image from the re-charging currents.

Claims

1. A system for sensing at an increased resolution an aerial image exhibiting a light intensity distribution at a primary resolution, the system comprising:

a photo-electron emission unit to sense the aerial image and, in response, emit electrons in a pattern corresponding to the light intensity distribution of the aerial image;

electron optics to project an enlarged pattern of the pattern of electrons emitted by the photo-electron emission unit;

a sensing unit to sense the enlarged pattern; and

an image analysis unit, coupled to the sensing unit, to digitize the enlarged pattern.

2. The system of claim 1 further comprising;

an object;

a light source to provide light;

condensing optics to illuminate the object with the light; and

an optical imaging and projection unit, wherein the optical imaging and projection unit projects the aerial image on the photo-electron emission unit.

3. The system of claim 2 wherein the system and optical imaging and projection unit are disposed in or a part of a photolithographic system.

4. The system of claim 3 wherein the photolithographic system is a stepper or a scanner.

5. The system of claim 4 wherein the object is a photolithographic mask.

6. The system of claim 5 further includes a computer to inspect the photolithographic mask using the digitized aerial image.

7. The system of claim 6 wherein the computer inspects the photolithographic mask using die-to-die inspection, die-to-database inspection, or image self-analysis techniques.

8. The system of claim 4 further includes a controller, coupled to the image analysis unit, to adjust the operation of the photolithographic stepper or the photolithographic scanner using at least a portion of the digitized aerial image.

9. The system of claim 4 wherein the light source provides a light that has a wavelength that is substantially equal to a wavelength of light that is used to expose a resist on a product wafer.

10. The system of claim 4 further includes a mechanical positioning unit wherein, the mechanical positioning unit, in response to an operator input, positions the photo-electron emission unit at the image plane.

11. The system of claim 4 further includes a mechanical positioning unit to position the photo-electron emission unit at the image plane to allow for in-situ aerial image sensing.

12. The system of claim 4 wherein the light has a wavelength in the UV range.

13. The system of claim 1 wherein the photo-electron emission unit includes an electron-multiplying cathode for amplifying the electrons.

14. The system of claim 13 wherein the electron-multiplying cathode allows the emission of substantially thermalized electrons.

15. The system of claim 1 wherein the photo-electron emission unit and the electron optics have a higher resolution than the resolution of the optical imaging and projection unit.

16. A system for sensing an aerial image exhibiting a light intensity distribution at a primary resolution, the system comprising:

a photo-conducting layer for projecting the aerial image thereon, wherein in response to the aerial image, the photo-conducting layer produces a depletion pattern corresponding to the light intensity distribution of the aerial image;

an electron source for providing electrons;

a steering device to direct the electrons to the photo-conducting layer to produce local re-charging currents in proportion to the local charge depletion;

amplifier circuitry coupled to the steering device, wherein the amplifier circuitry determines a pattern corresponding to the aerial image using the recharging currents; and

an image analysis unit, coupled to the sensing unit, to digitize the pattern.

17. The system of claim 16 further comprising:

an object;

a light source to provide light;

condensing optics to illuminate the object with the light; and

an optical imaging and projection unit to produce the aerial image on the photo-conducting layer.

18. The system of claim 17 wherein the system and the optical imaging and projection unit are disposed in or a part of a photolithographic system.

19. The system of claim 18 wherein the photolithographic system is a photolithographic stepper or a photolithographic scanner.

20. The system of claim 19 wherein the object is a photolithographic mask.

21. The system of claim 20 further includes a computer to inspect the photolithographic mask using the digitized aerial image.

22. The system of claim 21 wherein the computer inspects the photolithographic mask using die-to-die inspection, die-to-database inspection, or image self-analysis techniques.

23. The system of claim 19 further includes a controller, coupled to the image analysis unit, to adjust the operation of the photolithographic stepper or a photolithographic scanner using at least a portion of the digitized aerial image.

24. The system of claim 19 wherein the light source provides a light that has a wavelength that is substantially equal to a wavelength of light that is used to expose a resist on a product wafer.

25. The system of claim 19 further includes a mechanical positioning unit wherein, the mechanical positioning unit, in response to an operator input, positions the photo-electron emission unit at the image plane.

26. The system of claim 19 further includes a mechanical positioning unit to position the photo-electron emission unit at the image plane to allow for in-situ aerial image sensing.

27. The system of claim 19 wherein the light has a wavelength in the UV range.