Apparatus for and a method of determining surface characteristics

• US 7,970,579 B2
• Filed: 11/22/2006
• Issued: 06/28/2011
• Est. Priority Date: 11/22/2005
• Status: Expired due to Fees

• Current Assignee: Taylor Hobson Limited (Ametek Incorporated)
• Original Assignee: Taylor Hobson Limited (Ametek Incorporated)
• Inventors: Mansfield, Daniel Ian
• Primary Examiner(s): Bui; Bryan
• Application Number:
  US12/094,733
• Publication Number:
  US 20100161273A1
• Time in Patent Office: 1,679 Days
• Field of Search: 702/167, 702/183, 356/511, 356/512
• US Class Current: 702/167
• CPC Class Codes: G01B 9/02057 by using common path configuration, i.e. reference and object path almost entirely overlapping
• CPC Class Codes: G01B 9/02084 Processing in the Fourier or frequency domain when not imaged in the frequency domain
• CPC Class Codes: G01B 9/02089 Displaying the signal, e.g. for user interaction
• CPC Class Codes: G01B 9/0209 Low-coherence interferometers
• CPC Class Codes: G01B 9/04 Measuring microscopes
• CPC Class Codes: Y10T 29/49124 On flat or curved insulated base, e.g., printed circuit, etc.

Abstract

A coherence scanning interferometer (2) carries out: a coherence scanning measurement operation on a surface area (81) carrying a structure using a low numeric aperture objective so that the pitch of the surface structure elements (82) is much less that the spread of the point spread function at the surface (7) to obtain structure surface intensity data; and a coherence scanning measurement operation on a non-structure surface area (83), which may be part of the same sample or a different sample, to obtain non-structure surface intensity data. A frequency transform ratio determiner (105) determines a frequency transform ratio (the HCF function) related to the ratio between the structure surface intensity data and the non-structure surface intensity data. A structure provider (109) sets that frequency transform ratio equal to an expression which represents the electric field at the image plane of the coherence scanning interferometer in terms of surface structure element size (height or depth) and width-to-pitch ratio and derives the surface structure element size and width-to-pitch ratio using the frequency transform ratio. The structure provider (109) may also extract the surface structure element width, if the pitch is independently known.

Claims

1. Apparatus for determining information relating to a sample surface structure, the apparatus comprising:

a light director operable to direct light along a sample path towards a region of a surface of a sample and along a reference path towards a reference surface such that light reflected by the region of the sample surface and light reflected by the reference surface interfere, the light director being capable of providing light having a point spread function with a spread at a sample surface greater than a pitch of surface structure elements providing a surface structure of the sample;

a mover operable to effect relative movement along a measurement scan path between a sample and the reference surface;

a sensing device operable to sense light representing interference fringes produced by a sample surface region during relative movement along the measurement scan path;

a controller operable to carry out a measurement operation by causing the mover to effect relative movement while the sensing device senses light intensity at intervals to provide a series of intensity values representing interference fringes produced by a sample surface region during the relative movement; and

a data processor operable to receive and process i) structure intensity data comprising a set of intensity values resulting from a measurement operation on a structure surface sample area using light with a point spread function having a spread at the sample surface greater than a pitch of surface structure elements providing the surface structure of the structure surface sample area, and ii)non-structure intensity data comprising a set of intensity values resulting from a measurement operation on a non-structure surface sample area not having the structure, the data processor comprising;

a frequency transform ratio determiner operable to determine a frequency transform ratio corresponding to a ratio between the structure and non-structure intensity data; and

a structure provider operable to relate the frequency transform ratio to a relationship involving a first structure measure representing a ratio between the width and pitch of the surface structure elements and a second structure measure representing the size of the surface structure elements in a direction transverse to the sample surface so as to enable determination of a value for at least one of said first and second structure measures.

2. Apparatus according to claim 1, wherein the frequency transform ratio determiner comprises a frequency transformer operable to determine first frequency transformed data corresponding to the structure intensity data and second frequency transformed data corresponding to the non-structure intensity data and a ratio determiner operable to determine the frequency transform ratio by dividing one of the first and second frequency transformed data by the other of the first and second frequency transformed data.

3. Apparatus according to claim 1, wherein the ratio determiner is operable to determine

4. Apparatus according to claim 3, wherein the ratio determiner is operable to modify the ratio by a net electric field reflectance term or its conjugate.

5. Apparatus according to claim 1, wherein the ratio determiner is operable to determine

6. Apparatus according to claim 1, wherein the structure provider is operable to relate the frequency transform ratio to a relationship representing the electric field in the image plane of an objective of the light director.

7. Apparatus according to claim 1, wherein the structure provider is operable to relate the frequency transform ratio to the relationship:

((1−

β

)+β

e<sup>−

/+i4π

ν

H</sup>)e<sup>−

/+(i4π

ν

Δ

z HCF</sup>)

where β

represents a ratio between the width and pitch of the surface structure elements, H represents the size of the surface structure elements in a direction transverse to the sample surface, v is the frequency of a nominal mean wavelength of light provided by the light director, and Δ

Z_HCF is a surface height difference between the structure surface area and the non-structure surface area.

8. Apparatus according to claim 7, wherein the structure provider is operable to determine β

and H from an AC component of the frequency transform ratio and a DC offset of the frequency transform ratio.

9. Apparatus according to claim 1, wherein, in the event that there is no surface height difference between the structure surface sample area and the non-structure surface sample area, the structure provider is operable to relate the real part of the frequency transform ratio using the relationships:

1−

β

+β

cos 4π

ν

H

−

/+β

sin 4π

ν

H

where β

represents a ratio between the width and pitch of the surface structure elements, H represents the size of the surface structure elements in a direction transverse to the sample surface.

10. Apparatus according to claim 1, wherein the structure provider is operable to determine the mean electric field from the frequency transform ratio for a plurality of surface pixels.

11. Apparatus according to claim 10, wherein the structure provider is operable to relate the mean electric field to:

Re(Ē

_k(ν

)/r(ν

))=1−

β

+β

cos 4π

ν

H

Im(Ē

_k(ν

)/r(ν

))=+/−

β

sin 4π

ν

H.

12. Apparatus according to claim 1, wherein the structure provider is operable to determine the mean electric field from the frequency transform ratio for a plurality of surface pixels and to relate that mean electric field to the frequency ratio:

13. Apparatus according to claim 1, wherein in the event a width of the surface structure elements is comparable to a nominal mean wavelength λ

of the light provided by the light director or an aspect ratio of the surface structure elements is high, the structure provider is operable to modify the relationship by an absorption term.

14. Apparatus according to claim 13, wherein the absorption term is:

e<sup>−

α

(ν

)H</sup>

where a(v) is an absorption coefficient and H is the surface structure element size.

15. Apparatus according to claim 1, wherein in the event a width of the surface structure elements is comparable to a nominal mean wavelength λ

of the light provided by the light director or an aspect ratio of the surface structure elements is high, the structure provider is operable to modify a term of the relationship involving the size by

e<sup>−

i</sup>4π

ν

H

where a(v) is an absorption coefficient and H is the surface structure element size.

16. Apparatus according to claim 1, wherein the structure provider is operable to use a fitting procedure to enable determination of values for at least one of said first and second structure measures.

17. Apparatus according to claim 1, further comprising a pitch provider operable to provide the pitch of the structure elements, wherein the structure provider is operable to determine a width for the structure elements using the provided pitch and the value for the first structure measure.

18. Apparatus according to claim 1, wherein the frequency transform ratio determiner is operable to determine the frequency ratio such that it is equivalent to a term having an amplitude component, a phase component and a helical phase component.

19. Apparatus according to claim 1, wherein the frequency transform ratio determiner is operable to use a Fourier transform to convert data into frequency space.

20. Apparatus according to claim 1, wherein the data processor is operable to average the series of intensity values resulting from measurement operations on a number of adjacent regions of the structure surface sample area to provide the structure intensity data.

21. Apparatus according to claim 1, wherein the data processor is operable to average the series of intensity values resulting from measurement operations on a number of regions of the non-structure surface sample area to provide the non-structure intensity data.

22. Apparatus according to claim 1, wherein the structure and non-structure sample areas are areas of the same sample.

23. Apparatus according to claim 1, wherein the structure and non-structure sample areas are areas of different samples.

24. Apparatus according to claim 1, wherein the non-structure intensity data also result from a measurement operation wherein the spread of light from the interferometer at the sample surface area is greater than a pitch of surface structure elements of the surface sample area.

25. Apparatus according to claim 1, wherein the electric field corresponding to the surface structure elements of the structure sample area is given by the following:

r(ν

)psf(x,y)comb(x/x_p)F(x,y)

where represents a convolution, psf and comb are point spread and comb function, respectively, and F(x,y) is the topographic electric field.

26. A data processor for use in apparatus according to claim 1, the data processor comprising:

a receiver operable to receive structure intensity data comprising a set of intensity values resulting from a measurement operation on a structure surface sample area using light having a point spread function with a spread at the sample surface greater than a pitch of surface structure elements providing the surface structure of the structure surface sample area, and to receive non-structure intensity data comprising a set of intensity values resulting from a measurement operation on a non-structure surface sample area not having the structure;

a frequency transform ratio determiner operable to determine a frequency transform ratio corresponding to a ratio between the structure and non-structure intensity data; and

a structure provider operable to relate the frequency transform ratio to a relationship involving a first structure measure representing a ratio between the width and pitch of the surface structure elements and a second structure measure representing the size of the surface structure elements in a direction transverse to the sample surface to determine values for said first and second structure measures.

27. Apparatus for determining surface structure element characteristics, wherein the apparatus comprises a coherence scanning interferometer and a data processor, the data processor comprising a structure determiner operable:

to determine a frequency transform ratio relating to a ratio between structure intensity data and non-structure intensity data,

the structure intensity data comprising a first series of intensity values resulting from a measurement operation by the coherence scanning interferometer on a surface structure sample area using a low numerical aperture objective such that the spread of light from a light source of the coherence scanning interferometer at the sample surface is greater than a pitch of surface structure elements of the surface structure sample area; and

the non-structure intensity data comprising a second series of intensity values resulting from a measurement operation on a second sample surface area not having the surface structure;

to relate the frequency transform ratio to a relationship representing the amplitude and phase of the electric field at the image plane of an objective of the interferometer in terms of a first structure measure representing a ratio between the width and pitch of the surface structure elements and a second structure measure representing the size of the surface structure elements in a direction transverse to the sample surface; and

to determine values for at least one of said first and second structure measures as a result of relating the frequency transform ratio to the relationship.

28. Apparatus according to claim 27, wherein the structure determiner comprises a frequency transformer operable to determine first frequency transformed data corresponding to the structure intensity data and second frequency transformed data corresponding to the non-structure intensity data and a ratio determiner operable to determine the frequency transform ratio by dividing one of the first and second frequency transformed data by the other of the first and second frequency transformed data.

29. A method of determining information relating to a sample surface structure, the method comprising the steps of:

carrying out respective measurement operations on a structure surface sample area having a surface structure comprising surface structure elements having a pitch and a non-structure surface sample area not having the surface structure by, at least for the measurement operation on the structure surface sample area, using light having a point spread function with a spread at the sample surface greater than the pitch of the surface structure elements and by, in each measurement operation, directing the light along a sample path towards a region of the sample surface and along a reference path towards a reference surface such that light reflected by the region of the sample surface and light reflected by the reference surface interfere, and causing relative movement along a measurement scan path between the sample and the reference surface while a sensing device senses light intensity representing interference fringes produced by the sample surface region at intervals during the relative movement to provide structure intensity data comprising a series of intensity values resulting from a measurement operation on the structure surface sample area and non-structure intensity data comprising a series of intensity values resulting from a measurement operation on the non-structure surface sample area;

determining a frequency transform ratio corresponding to a ratio between the structure and non-structure intensity data; and

relating the frequency transform ratio to a relationship involving a first structure measure representing a ratio between the width and pitch of the surface structure elements and a second structure measure representing the size of the surface structure elements in a direction transverse to the sample surface to determine values for at least one of said first and second structure measures.

30. A method of manufacturing a product having a surface structure comprising surface structure elements having a pitch, which method comprises the steps of:

providing a design for a product having a surface structure;

producing the product; and

controlling at least one of the design and production in accordance with results of a method in accordance with claim 29.

31. A product manufactured by a method in accordance with claim 30.

32. A non-transitory computer readable storage medium carrying program instructions for programming processing means to carry out a method in accordance with claim 29.

33. Apparatus for determining information relating to a sample surface structure, the apparatus comprising:

light directing means for directing light along a sample path towards a region of a surface of a sample and along a reference path towards a reference surface such that light reflected by the region of the sample surface and light reflected by the reference surface interfere, the light directing means being operable to provide light having a point spread function with a spread at a sample surface greater than a pitch of surface structure elements providing a surface structure of the sample;

moving means for effecting relative movement along a measurement scan path between a sample and the reference surface;

sensing means for sensing light representing the interference fringes produced by a sample surface region during relative movement along the measurement scan path;

control means for carrying out a measurement operation by causing the moving means to effect relative movement while the sensing means senses light intensity at intervals to provide a series of intensity values representing interference fringes produced by a sample surface region during the relative movement; and

data processing means for receiving and processing i) structure intensity data comprising a set of intensity values resulting from a measurement operation on a structure surface sample area using light having a point spread function with a spread at the sample surface greater than a pitch of surface structure elements providing the surface structure of the structure surface sample area, and ii) non-structure intensity data comprising a set of intensity values resulting from a measurement operation on a non-structure surface sample area not having the structure, the data processing means comprising;

frequency transform ratio determining means for determining a frequency transform ratio corresponding to a ratio between the structure and non-structure intensity data; and

structure providing means for relating the frequency transform ratio to a relationship involving a first structure measure representing a ratio between the width and pitch of the surface structure elements and a second structure measure representing the size of the surface structure elements in a direction transverse to the sample surface to determine values for said first and second structure measures.