Apparatus and method for measuring overlay by diffraction gratings

• US 20050012928A1
• Filed: 06/02/2004
• Published: 01/20/2005
• Est. Priority Date: 07/17/2003
• Status: Active Grant

• Current Assignee: Tokyo Electron Limited
• Original Assignee: Tokyo Electron Limited
• Inventors: Sezginer, Abdurrahman, Shinagawa, Robert, Huang, Hsu-Ting
• Granted Patent: US 7,230,703 B2
• Time in Patent Office: Days
• US Class Current: 356/401
• CPC Class Codes: G01B 11/26 for measuring angles or tapers; for testing the alignment of axes
• CPC Class Codes: G01J 3/2823 Imaging spectrometer
• CPC Class Codes: G03F 7/70633 Overlay, i.e. relative alignment between patterns printed by separate exposures in different layers, or in the same layer in multiple exposures or stitching

Abstract

A method for measuring overlay in a sample includes obtaining an image of an overlay target that includes a series of grating stacks each having an upper and lower grating, each grating stack having a unique offset between its upper and lower grating. The image is obtained with a set of illumination and collection optics where the numerical aperture of the collection optics is larger than the numerical aperture of the illumination optics and with the numerical apertures of the illumination and collection optics are selected so that the unit cells of gratings are not resolved, the grating stacks are resolved and they appear to have a uniform color within the image of the overlay target.

Claims

1. An apparatus for measuring overlay within a sample, where the sample includes an overlay target that is formed as a series of grating stacks each having an upper and lower grating, each grating stack having a unique offset between its upper and lower grating, the apparatus comprising:

an illumination source that generates an optical beam;

illumination optics configured to direct a probe beam portion of the optical beam to be reflected by the target within the sample, where the target;

a detector array configured to convert an image into corresponding output signals;

collection optics configured to convey an image of the target to the detector by collecting some portion of the reflected probe beam, where the numerical aperture of the collection optics is larger than the numerical aperture of the illumination optics and with the numerical apertures of the illumination and collection optics are selected so that the unit cells of gratings are not resolved, the grating stacks are resolved and they appear to have a uniform color within the image of the overlay target; and

a processor configured to measure overlay based on the output signals of the detector.

2. An apparatus as recited in claim 1, wherein the numerical aperture of the illumination optics is less than 0.5 and the numerical aperture of the collection optics is greater than or equal to 0.5.

3. An apparatus as recited in claim 1, wherein the numerical aperture of the illumination optics is less then or equal to 0.3.

4. An apparatus as recited in claim 1, wherein an image is acquired at a wavelength that is shorter than 400 nm.

5. An apparatus as recited in claim 1, wherein the image of the overlay target is acquired as a function of wavelength.

6. An apparatus as recited in claim 5, that further comprises:

reference optics for directing a reference beam portion of the optical beam to be mixed with the reflected probe beam; and

a modulator for selectively altering the distance traveled by the reference beam.

7. An apparatus as recited in claim 5, wherein the probe beam is directed at a non-normal angle of incidence to be reflected by the target within the sample.

8. An apparatus as recited in claim 1, wherein the image of the overlay target is acquired as a function of angle of incidence of the probe beam.

9. An apparatus as recited in claim 1, wherein the image of the overlay target is acquired as a function of the azimuth angle between the plane of incidence of the probe beam and a line that lies in the plane of the overlay target.

10. An apparatus as recited in claim 1, wherein the detected image is a darkfield image.

11. An apparatus as recited in claim 1, wherein the detected image is a darkfield image wherein at least one of the angle of incidence or angle of reflection of the probe beam or the angle of reflection is off axis such that the specular reflection from the overlay target does not pass through the aperture of the collection objective.

12. An apparatus as recited in claim 1, wherein the processor is configured to compensate for the dark current of the detector array.

13. An apparatus as recited in claim 1, wherein the processor is configured to compensate for flare in the optical system.

14. An apparatus as recited in claim 1, which further comprises a photodetector positioned to measure the intensity of the intensity of the optical beam generated by the illumination source.

15. An apparatus as recited in claim 14, wherein the photodetector comprises one or more elements within the detector array.

16. A method for measuring overlay within a sample where the sample includes an overlay target that is formed as a series of grating stacks each including an upper and lower grating, each grating stack having a unique offset between its upper and lower gratings, the method comprising:

focusing an optical probe beam be reflected by the target using a set of illumination optics;

obtaining one or more images of the target using a set of collection optics, where the numerical aperture of the collection optics is larger than the numerical aperture of the illumination optics and with the numerical apertures of the illumination and collection optics are selected so that the unit cells of gratings are not resolved, the grating stacks are resolved and they appear to have a uniform color within the image of the overlay target

converting each image into corresponding output signals; and

analyzing the output signals to measure overlay within the sample.

17. A method as recited in claim 16, in which the one or more images are obtained as a function of one or more independent variables selected from the group of:

probe beam wavelength, probe beam polarization state, angle of incidence of the probe beam on the overlay target and azimuth angle of the probe beam on the overlay target.

18. A method as recited in claim 17, that further comprises:

analyzing the output signals corresponding to an image to obtain optical response information for each grating stack in the overlay target;

constructing a mapping between the respective offsets defined by the upper and lower gratings in the series of grating stacks and optical response information; and

finding an offset for which the optical response information varies in a symmetrical fashion for greater and lesser offsets.

19. A method as recited in claim 18, that further comprises:

defining a Fourier series to represent the optical response of the grating stacks, where the Fourier series is an even function of reticle offset, shifted by an unknown amount; and

solving the coefficients of the Fourier series to obtain the unknown shift amount by fitting the Fourier series to the optical response information.

20. A method as recited in claim 18, that further comprises:

generating an optical reference beam;

modulating the distance traveled by the reference beam; and

mixing the modulated reference beam into the reflected probe beam.

21. An optical apparatus configured to measure an overlay target, the target defined by a plurality of grating stacks, upper and lower gratings in each stack having a different offset, said apparatus comprising:

an illumination source for generating optical radiation;

illumination optics for directing the radiation to reflect off the target;

collection optics for collecting the light reflected from the target and imaging the reflection on to a detector array;

a detector array for monitoring the collected light and generating output signals that represent an image of the overlay target;

a controller for acquiring images of the overlay metrology target at one or more values of wavelength, angle of incidence, azimuth, or polarization state based on the detected output signals; and

a processor for determining the extent of overlay based on the images and using an algorithm wherein;

the algorithm seeks a special value of the offset between the upper and lower gratings such that observed optical response of the grating stacks are symmetric with respect to offset around said special value for all values of the independent variables of measurement; and

the algorithm returns negative of the said special value of offset as the measurement of the overlay.

22. An apparatus as recited in claim 21, wherein seeking the special point of symmetry involves representing the optical response of the grating stacks as a Fourier series with respect to the offset between the upper and lower gratings;

setting up the Fourier series so that it is an even function of reticle offset, shifted by an unknown amount;

solving the coefficients of the Fourier series and the unknown shift by fitting the said Fourier series to the measurements

23. An apparatus as recited in claim 21, wherein seeking the special point of symmetry involves taking inner product of two arrays wherein:

a. one of the arrays consists of the measured optical response of grating stacks of various reticle offsets, all measured at the same set of the independent variables of measurement;

b. the second array consists of samples of a shifted version of a predetermined odd function of reticle offset;

c. the inner product of the arrays recited in a) and b) is formed;

d. said inner products for all values of independent variables are minimized together with respect to the shift recited in b).

24. An apparatus according to claim 23, wherein sum of squares of the said inner products is minimized wherein the sum is over all independent variables of measurement

25. An apparatus as recited in claim 23, wherein the shifted odd function is the sine function:

sin((2π

/P) (r−

ξ

)), P is the period of the grating, r is the reticle offset, and ξ

is the shift.