Apparatus for and a method of determining surface characteristics

• US 8,112,246 B2
• Filed: 12/15/2006
• Issued: 02/07/2012
• Est. Priority Date: 12/22/2005
• Status: Active Grant

• Current Assignee: Taylor Hobson Limited (Ametek Incorporated)
• Original Assignee: Taylor Hobson Limited (Ametek Incorporated)
• Inventors: Mansfield, Daniel Ian
• Primary Examiner(s): Dunn, Drew A
• Assistant Examiner(s): SUN, XIUQIN
• Application Number:
  US12/158,903
• Publication Number:
  US 20090319225A1
• Time in Patent Office: 1,880 Days
• Field of Search: 702/172, 356/497, 356/485, 356/511, 356/512, 356/487, 382/108
• US Class Current: 702/172
• CPC Class Codes: G01B 11/0675 using interferometry
• CPC Class Codes: G01B 11/303 using photoelectric detection means
• CPC Class Codes: G01B 9/02057 by using common path configuration, i.e. reference and object path almost entirely overlapping
• CPC Class Codes: G01B 9/02083 characterised by particular signal processing and presentation
• CPC Class Codes: G01B 9/02084 Processing in the Fourier or frequency domain when not imaged in the frequency domain
• CPC Class Codes: G01B 9/0209 Low-coherence interferometers

Abstract

Light reflected by a sample surface region and a reference surface interfere. A detector senses light intensity at intervals during relative movement along a scan path between the sample surface and the reference surface to provide a series of intensity values representing interference fringes. A data processor receives first intensity data comprising a first series of intensity values resulting from a measurement operation on a surface area of a substrate and second intensity data including a second series of intensity values resulting from a measurement operation on a surface area of a thin film structure. A gain is determined for each thin film of the thin film structure. Substrate and apparent thin film structure surface characteristics are determined on the basis of the first and second intensity data, respectively. The apparent thin film structure surface characteristic is modified using the substrate surface characteristic and the determined gain or gains.

Claims

1. Apparatus for determining information relating to a surface characteristic of a sample in the form of a thin film structure comprising one or more thin film layers on a substrate, the apparatus comprising:

a light source;

a beam splitter configured to direct light from the light source along a sample path towards a surface region of a said sample and along a reference path towards a reference surface such that light reflected by a sample surface region and light reflected by the reference surface interfere;

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

a sensing device configured to sense light representing the interference fringes produced by a sample surface region;

a controller configured to carry out a measurement operation by causing the sensing device to sense light intensity at a number of locations along the measurement path to provide, for each location, a corresponding intensity value; and

a data processor configured to receive first intensity data comprising a first series of intensity values resulting from a measurement operation on a surface area of a substrate and second intensity data comprising at least a second series of intensity values resulting from a measurement operation on a surface area of a thin film structure, the data processor comprising;

a gain determiner configured to determine a gain for the or each thin film of a thin film structure; and

a surface characteristic determiner configured to determine a substrate surface characteristic on the basis of the first intensity data, to determine an apparent thin film structure surface characteristic on the basis of the second intensity data, and to modify the apparent thin film structure surface characteristic using the substrate surface characteristic and the gain or gains determined by the gain determiner.

2. Apparatus according to claim 1, wherein the gain represents the rate of change of the apparent surface height with thin film thickness.

3. Apparatus according to claim 1, wherein the gain is given by

4. Apparatus according to claim 1, wherein the gain determiner is configured to determine the gain for a thin film layer by determining a value for the refractive index and mean thin film thickness of the thin film layer, calculating a surface reflectance and using the surface reflectance and refractive index to calculate the gain.

5. Apparatus according to claim 1, wherein the light source is a monochromatic light source and the gain determiner is configured to determine the gain for a thin film layer by determining a value for the refractive index and mean thin film thickness of the thin film layer, calculating a surface reflectance and using the surface reflectance and refractive index to calculate the gain.

6. Apparatus according to claim 1, wherein the light source is a monochromatic light source and the gain determiner is configured to determine gain in accordance with:

7. Apparatus according to claim 1, wherein the light source is a monochromatic light source and, where the thin film structure has a single thin film, the gain determiner is configured to determine gain in accordance with:

8. Apparatus according to claim 1, wherein the light source is a monochromatic light source and where the thin film structure has a single thin film, the gain determiner is configured to determine gain in accordance with:

9. Apparatus according to claim 1, wherein the apparatus is part of an interferometer system that has a focussing objective and the gain determiner is configured to determine gain by replacing the surface reflectance with a net field reflectance determined by integrating over the numerical aperture of the objective.

10. Apparatus according to claim 1, wherein the controller is configured to cause the locations on the measurement path to be spaced by a predetermined phase and the surface characteristic determiner is configured to determine a surface height of a surface pixel using a phase shifting algorithm.

11. Apparatus according to claim 1, wherein the surface characteristic determiner is configured to determine surface roughness.

12. Apparatus according to claim 1, wherein the surface characteristic determiner is configured to determine surface roughness in accordance with:

13. Apparatus according to claim 1, wherein, where a thin film structure has a single thin film with a surface roughness much greater than that of a substrate, the surface characteristic determiner is configured to determine surface roughness in accordance with:

Sq²≈

Sq_app²/G₁²(v_o)

14. Apparatus according to claim 1, wherein the light source is a monochromatic light source, a thin film structure has two thin films and the surface characteristic determiner is configured to determine surface roughness as:

15. Apparatus according to claim 1, wherein:

the light source is a broad band light source;

the controller is configured to carry out a measurement operation by causing the sensing device to sense light intensity at a number of locations along the measurement path to provide, for a surface region, a series of intensity values representing an interferogram in the measurement path direction; and

the data processor is configured to determine a surface height of the surface region by determining the position of a coherence peak in the intensity values for that surface region.

16. Apparatus according to claim 15, wherein the gain is given by:

17. Apparatus according to claim 15, wherein the gain determiner is configured to determine a gain value for a thin film layer by evaluating a net spectral reflectance for the thin film structure using values z_k for the layer thicknesses, using the evaluated net spectral reflectance to generate a corresponding interferogram, determining the position along the measurement path of the coherence peak in that interferogram to obtain an apparent layer thicknesses z_app, perturbing the layer thickness values z_k by ∂

z_k to determine the corresponding change ∂

z_app, in apparent layer thickness and determining the gain G_k on the basis of;

18. Apparatus according to claim 17, wherein the surface characteristic determiner is configured to determine surface roughness.

19. Apparatus according to claim 17, wherein, where a thin film structure has a single thin film with a surface roughness much greater than that of a substrate, the surface characteristic determiner is configured to determine surface roughness in accordance with:

Sq²≈

Sq_app²/G₁²(v_o)

20. Apparatus according to claim 17, wherein the surface characteristic determiner is configured, to enable determination of a surface characteristic, to determine mean thin film thickness(es) by:

determining a frequency transform ratio corresponding to a ratio between first intensity data comprising a first series of intensity values resulting from a measurement operation on a thin film layer structure surface area and second intensity data comprising a second series of intensity values resulting from a measurement operation on a surface area of a substrate; and

fitting a layer structure model having variable model parameters related to the layer thicknesses and refractive indices of the layers of a thin film structure to the ratio determined by the ratio determiner by adjusting the model parameters, thereby obtaining, for the model parameters, values representing the layer thicknesses and refractive indices of the layers of the thin film structure.

21. Apparatus according to claim 20, wherein the surface characteristic determiner is configured to determine first frequency transformed data corresponding to the first intensity data and second frequency transformed data corresponding to the second intensity data and 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 and by multiplying that ratio by a field reflectance term:

22. Apparatus according to claim 20, wherein the ratio determiner is configured to determine:

23. Apparatus according to claim 20, wherein the layer structure model is a thin film layer structure optical admittance model.

24. Apparatus according to claim 20, wherein the layer structure model is a thin film layer structure optical admittance model:

25. Apparatus according to claim 20, wherein the layer structure model is a thin film layer structure optical admittance model:

26. Apparatus according to claim 17, wherein, to enable determination of surface roughness, the surface characteristic determiner is configured to determine mean thin film thickness(es) by:

determining a frequency transform ratio, HCF, corresponding to a ratio between first intensity data comprising a first series of intensity values resulting from a measurement operation on a thin film layer structure surface area and second intensity data comprising a second series of intensity values resulting from a measurement operation on a surface area of a substrate; and

fitting a layer structure model having variable model parameters related to the layer thicknesses and refractive indices of the layers of a thin film structure to the ratio determined by the ratio determiner by adjusting the model parameters, thereby obtaining for the model parameters values representing the layer thicknesses and refractive indices of the layers of the thin film structure; and

wherein the surface characteristic determiner is configured to determine surface roughness by determining a mean of said frequency transform ratios for a number of different surface regions of the thin film structure, determining the differential of the mean frequency transform ratio as being given by;

27. Apparatus according to claim 15, wherein the surface characteristic determiner is configured to determine surface roughness in accordance with:

28. A method of determining information relating to a surface characteristic of sample in the form of a thin film structure comprising one or more thin film layers on a substrate, the method comprising:

directing light from a light source along a sample path towards a surface region of a sample and along a reference path towards a reference surface such that light reflected by the sample surface region and light reflected by the reference surface interfere;

effecting relative movement along a measurement path between a sample and the reference surface;

carrying out a measurement operation by causing a sensing device to sense light at a number of locations along the measurement path to provide, for each location, a corresponding intensity value; and

processing intensity data comprising a first series of intensity values resulting from a measurement operation on a surface area of a substrate and second intensity data comprising at least a second series of intensity values resulting from a measurement operation on a surface area of a thin film structure by determining an apparent substrate surface characteristic of the substrate on the basis of the first intensity data, determining an apparent thin film structure surface characteristic on the basis of the second intensity data, and modifying the apparent thin film structure surface characteristic using the apparent substrate surface characteristic and a determined gain or gains.

29. A non-transient storage medium carrying program instructions for programming processing means to carry out a method in accordance with claim 28.

30. Apparatus for determining information relating to a surface characteristic of sample in the form of a thin film structure comprising one or more thin firm layers on a substrate, the apparatus comprising:

light directing means for directing light from light source means along a sample path towards a surface region of a sample and along a reference path towards a reference surface such that light reflected by a said sample surface region and light reflected by the reference surface interfere;

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

sensing means for sensing light representing the interference fringes produced by a sample surface region;

control means for carrying out a measurement operation by causing the sensing means to sense light intensity at a number of locations along the measurement path to provide, for each location, a corresponding intensity value; and

data processing means for receiving first intensity data comprising a first series of intensity values resulting from a measurement operation on a surface area of a substrate and second intensity data comprising at least a second series of intensity values resulting from a measurement operation on a surface area of a thin film structure, the data processing means comprising;

gain determining means for determining a gain for the or each thin film of a thin film structure; and

surface characteristic means for determining a substrate surface characteristic on the basis of the first intensity data, for determining an apparent thin film structure surface characteristic on the basis of the second intensity data, and for modifying the apparent thin film structure surface characteristic using the substrate surface characteristic and the gain or gains determined by the gain determining means.

31. Data processing apparatus for receiving from an interferometer first intensity data comprising a first series of intensity values resulting from a measurement operation on a surface area of a substrate and second intensity data comprising at least a second series of intensity values resulting from a measurement operation on a surface area of a thin film structure, the apparatus comprising:

a gain determiner configured to determine a gain for the or each thin film of a thin film structure; and

a surface characteristic determiner configured to determine a substrate surface characteristic on the basis of the first intensity data, to determine an apparent thin film structure surface characteristic on the basis of the second intensity data, and to modify the apparent thin film structure surface characteristic using the substrate surface characteristic and the gain or gains determined by the gain determiner.