Wavelength scanning interferometer apparatus and method

Abstract

Apparatus and method for using the same, said apparatus for determining information relating to a sample surface, the apparatus comprising at least one light source, a light director means to direct light from the at least one light source 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 to produce an interferogram. An image recorder means records successive images, each image representing the interferogram produced by the reflected light; and a data processor is configured to process the images recorded by the image recorder means to produce at least one of a surface profile and / or a surface height map of at least a part of the sample surface. The apparatus includes a chromatic objective lens.

Description

[0001] Wavelength Scanning Interferometer Apparatus and Method

[0002] The present invention relates to an apparatus and a method to measure a wide range of surface features.

[0003] Surface science has become an important part of the wide engineering landscape and will play a vital role in the future development of next-generation products and technologies. The increased use of nanoscale and ultraprecision structured surfaces is wide-ranging and covers optics, silicon wafers, hard disk drives, microelectromechanical systems, and nanoelectromechanical systems (MEMS / NEMS), microfluidics, and micro molding industries. These all industries rely critically on ultraprecision surfaces. Surface measurement can be used to guarantee the functionality of high precision workpiece and production control, enabling better manufacturing throughput and cost reduction. There is, however, a fundamental limiting factor to the manufacture of such surfaces, namely, the ability to measure the product quickly and easily within the manufacturing environment.

[0004] We have introduced previously an environmentally robust surface measurement system using wavelength scanning interferometry (WSI) and active servo control techniques, described in patent application W02010 / 082066, Surface Characteristic Determining Apparatus. The WSI consists of a broadband light source filtered by an acousto-optic tunable filter. Linnik interferometer is used to generate the interference pattern. The operating principle can be summarised as the following: During the wavelength scanning, a sequence of frames is captured by the camera. All the frames are captured at focus (since the objective lenses used in the setup are corrected for chromatic aberration i.e. achromatic lenses) . Each pixel is analysed individually to determine the surface height after extracting a sinusoidal interference pattern. There are several well-known algorithms that can be used for the height calculation. The stabilisation system consists of a PZT deriven by the PI controller. The fringe oscillation due to environmental disturbances is detected by a photodetector (PD) and fedback to the stablisation system.

[0005] Although the WSI can be stabilised (under certain condition) to overcome environmental disturbances, there are still two critical limitations that constrain the use of WSI in many applications.

[0006] In particular, the measurement range is limited to the Depth of Focus (DoF) of the objectives. This range can be very short for example DoF = 1.6pm for 20x with NA=0.42. This limits the type of surfaces that can be measured especially when if the heights of surface features are larger than the DoF.

[0007] In addition, the stabilisation feature of conventional systems only functions when few fringes are generated across the entire field of view. Increasing the number of fringes across the field- of-view (FOV) degrades the signal-to-noise ratio (SNR) of the signal fed back to the active control system. The voltage variation generated by fringe oscillation on the detector surface will be averaged, producing a constant voltage. Although the photodetector used for stabilisation is usually smaller than the measured FOV, levelling the surface is still essential to achieve sufficient SNR.

[0008] It is therefore an aim of the present invention to provide an apparatus for measuring a wide range of surface features that addresses the abovementioned problems. It is a further aim of the present invention to provide a method of measuring a wide range of surface features that addresses the abovementioned problems.

[0009] It is a yet further aim of the present invention to provide an optical system that addresses the above-mentioned problems.

[0010] In a first aspect of the invention there is provided an apparatus for determining information relating to a sample surface, the apparatus comprising: at least one light source, a light director means to direct light from the at least one light source 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 to produce an interferogram; an image recorder means to record successive images, each image representing the interferogram produced by the reflected light; and a data processor to process the images recorded by the image recorder means to produce at least one of a surface profile and / or a surface height map of at least a part of the sample surface characterised in that the apparatus includes a chromatic objective lens.

[0011] Typically the light directed light the at least one light source towards a region of a surface of a sample passes through the chromatic objective.

[0012] Typically the chromatic lens is an objective lens used to scan the focus plane. Further typically the chromatic lens is used to scan the focus plane across the optical axis without any mechanical movement. As such, the chromatic objective lens achieves chromatic- aberrated wavelength scanning interferometry thereby extending the measurement range without any mechanical scanning.

[0013] Typically the image recorder means includes at least one sensor and / or a camera means.

[0014] In one embodiment at least one of the light sources is a tuneable light source. In one embodiment the light source includes an acousto-optic tuneable filter (AOTF) .

[0015] In a preferred embodiment the apparatus includes two light sources.

[0016] In one embodiment at least one of the light sources has a fixed or single wavelength. Typically the tuneable light source provides light via a multi-mode fibre. Further typically the fixed or single wavelength light source provides light via a single mode fibre.

[0017] In one embodiment the light sources include a white light source and / or a RGB light source. Further typically the light sources are coupled. Typically the light is coupled into a bundle fibre consists of a multi-mode and single-mode optical fibres. The multi-mode fibre is for the filtered white light and the singlemode or single wavelength is for the RGB illumination.

[0018] In a preferred embodiment the apparatus includes a stabilisation means. Further typically a single-mode fibre is used to illuminate a diffraction limited spot light and / or spot size which is much smaller than the spot light generated by the multi-mode fibre. Typically the single-mode illumination is used to generate an interference light feedback signal to the stabilisation system. Typically the light director means includes or provides one or more interferometers. Further typically a first interferometer that uses a multi-mode fibre to couple a white light source filtered by AOTF is used for areal surface measurement and a second interferometer that uses a single mode fibre is used to generate a feedback signal for stabilisation. Typically the first and second interferometers share an optical path comprising at least part of the reference and measurement paths. The first and second interferometers may comprise Linnik interferometers, although other interferometer configurations may be possible.

[0019] In one embodiment the surface measurement apparatus includes an interferometer system having at least one Graphics Processing Unit (GPU) to process the interferograms to enable processing of pixel data in parallel, although other computing acceleration methods such as Field Programmable Gate Arrays (FPGA) may be possible.

[0020] The light source or sources may be remote from the rest of the apparatus, for example a fibre optic coupling may be used couple the light source or light sources to the light director.

[0021] The image recorder means may comprise an image sensor which may comprise a CCD or CMOS camera.

[0022] In one embodiment the data processor is arranged to process the interferograms to produce data representing the relative surface heights of a 1 D or 2D array or sample of the surface pixels imaged by the recorder.

[0023] In an aspect, the present invention provides apparatus for determining information relating to a sample surface, the apparatus comprising: an measurement interferometer to direct light from a broadband light source along a measurement path towards a region of a surface of a sample and along a reference path towards a reference surface and to cause light reflected by the region of the sample surface and light reflected by the reference surface to interfere to produce an interferogram; a reference interferometer having a reference light source, the measurement and reference interferometers sharing a common optical path comprising at least part of the measurement and reference paths, the reference interferometer producing an output representing interference between light from the reference light source reflected by the sample surface and by the reference surface; a recorder to record successive images each image representing the interferogram produced by the sample surface; a data processor to process the images recorded by the recorder; a detector to detect the output of the reference interferometer; and a path length controller to control a length of the reference path on the basis of the output detected by the detector; a chromatic objective lens.

[0024] As used herein "beam" does not necessarily mean a continuous beam, it could be pulsed or otherwise vary in amplitude.

[0025] In a second aspect of the invention there is provided a method to extend the measurement range of WSI apparatus, characterized in that the method includes using chromatic focus variation to extend the measurement range.

[0026] In one embodiment during the wavelength scanning, the focus plane is shifted along the optical axis.

[0027] Typically a customised chromatic objective lens is employed to extend the measurement range. For example up to 100pm for 20X and NA 0.4 within the visible light spectrum. In one embodiment the method includes implementing system stabilisation using a feedback signal generated by a single point of interference on the surface.

[0028] Specific embodiments of the invention are now described with reference to the following figures wherein:

[0029] Figure 1 shows a WSI apparatus with conventional objective lenses and multiplexed light sources coupled into single core optical fibre (multi-mode);

[0030] Figure 2 shows the fringe contrast along the coherence length with corresponding FOV at different fringe density; and

[0031] Figures 3a and 3b show embodiments of the present invention wherein WSI apparatus is equipped with a chromatic objective lens and two light sources coupled into single-mode for stabilisation and multi-mode for measurement.

[0032] We have introduced previously an environmentally robust surface measurement system using wavelength scanning interferometry (WSI) and active servo control techniques, described in Figurel (patent: WO / 2010 / 082066: Surface Characteristic Determining Apparatus) . The WSI consists of a broadband light source filtered by acousto-optic tunable filter. Linnik interferometer is used to generate the interference pattern. The operation principle can be summarised as the following: During the wavelength scanning, a sequence of frames will be captured. All the frames are captured at focus (since the objective lenses used in the setup are corrected for chromatic aberration i.e. achromatic lenses) . Each pixel will be analysed individually to determine the surface height after extracting a sinusoidal interference pattern. There are several well-known algorithms that can be used for height calculation. The stabilisation system consists of PZT derived by the PI controller. The fringe oscillation due to environmental disturbances is detected by a photodetector connected to the PI controller.

[0033] Although the WSI can be stabilised (under certain condition) to overcome environmental disturbances, there are still two critical limitations that constrain the use of WSI in many applications as the following:

[0034] 1. The measurement range is limited and equal to the Depth of Focus (DoF) of the objectives. This range can be very short for example DoF = 1.6pm for 20x with NA = 0.42. This will limit the type of surfaces that can be measured especially when if the heights of surface features are larger than the DoF.

[0035] 2. The stabilisation feature of the system is only functioning when few fringes are generated across the entire field of view. Increasing the number of fringes across the field-of- view (FOV) will degrade the signal-to-noise ratio (SNR) of the signal fed back to the active control system. The voltage variation generated by fringe oscillation on the detector surface will be averaged, producing constant voltage. Although the photodetector used for stabilisation is usually smaller than the measured FOV (e.g. sensing area 0.5 mm2) , levelling the surface is still essential to achieve sufficient SNR, see Figure 2. In this figure, it can be noticed that the main light source (i.e. collimated light of the Halogen lamp filtered by AOTF) is multiplexed by an optical fibre with the SLED used for the stablisation system. Therefore, both the measurement and the stablisation spots will have the same FOV.

[0036] Operation principle: The proposed Chromatic Wavelength Scanning Interferometry 2 (CWSI), see Figures 3-5, WSI employs chromatic focus variation to extend the measurement range. So, during the wavelength scanning, the focus plane will be shifted along the optical axis.

[0037] During the wavelength scanning a sequence of frames will be captured to produce the surface topography using well-known algorithms such as Fourier transform fringe analysis methods. The novelty here is to use a chromatic objective lens 4 to extend the measurement range (for example, from few microns to 100pm for 20X and NA 0.4) . The second novelty is that the stabilisation will be operated by monitoring the interference feedback signal 6 generated from a very limited area (at the micron level) on the sample illuminated by a single mode fibre 8, making it robust to the surface slope, hence the number of fringes. In the original WSI, the feedback signal generated by a multi-mode fibre which produces a much larger spot that cover the entire field of view causing SNR degradation if the fringe density is high.

[0038] To improve the measurement resolution and remove the height ambiguity problem, this invention can be enhanced by converting the WSI to a polarised interferometer 10 to produce phase shifting mechanism at each wavelength, see Figures 4 and 5. This polarisation arrangement will form four instantaneous 90° phase shifts required to determine the surface topography with sub-nanometer precision. By operating the system at wide range of wavelength scanning a continuous linear phase slope will be produced. The phase slope can be used to obtain the surface height without any phase ambiguity problem, hence enabling measurement range equal to the shifted depth of focus.

[0039] The novel points of the proposed system include: 1) employing a customised chromatic objective lens to extend the measurement range (up to 100pm for 20X and NA 0.4), 2) implementing system stabilisation using the interference feedback signal generated by a diffraction limited spot light or spot size on the sample surface, thanks to the single-mode fibre used for illumination. In the original WSI, the feedback signal is generated by a much larger spot that covers the entire field of view, causing SNR degradation if the fringe density is high, as explained above.

[0040] As such, WSI apparatus can be modified to have a measurement range up to 100 times longer and stabilisation for inclination much more steeper than currently available.

[0041] The key feature of chromatic-aberrated WSI (this new technology) is to use a chromatic objective lens to achieve an extended measurement range, for example up to 100 um (20x, NA 0.4), without any mechanical scanning.

[0042] To improve the measurement resolution, this invention can be enhanced by using polarisation arrangement to produce phase shifting mechanism at each wavelength, see Figure 4.

[0043] A polarised beam will be propagated through a polarised interferometer and then transmitted through linear polarization elements placed onto four detection arms. This polarization arrangement will form four instantaneous 90° phase-shifts required to determine the surface topography with subnanometer precision. The four detection arms can also be replaced by single camera with pixelated phase mask placed in front of the sensor. The single camera with pixelated phase mask has been used in conventional polarising interferometers illuminated by a single wavelength or synthetic wavelength generated from maximum of four individual wavelengths. Here, we introduce the stabilised chromatic wavelength scanning interferometer to replace the conventional polarising interferometer to overcome the well-known phase ambiguity problem and extend the measurement range up to 100 times depending on the scanning range. The phase slope can be used to obtain the surface height without any phase ambiguity problem, hence enabling measurement range equal to the shifted depth of focus (targeting l OOum for 20x objective) .

[0044] In other words, the present innovation: a) Increases the depth of focus (DOF) by 100 times by using chromatic objective lens. This DoF will be shifted during the wavelength scanning to cover measurement range up to l OOum for high magnification lenses. b) The stabilisation can be operated not only for flat surfaces but for steep surfaces as well. Thanks to multiplexing a single mode fibre and multi-mode fibre intro one bundle. c) The polarization arrangement can be used to instantaneously shift phases of the wavefronts at four steps at each wavelength. This enables sub-nanometer measurement resolution for CWSI.

Claims

Claims1. Apparatus for determining information relating to a sample surface, the apparatus comprising: at least one light source, a light director means to direct light from the at least one light source 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 to produce an interferogram; an image recorder means to record successive images, each image representing the interferogram produced by the reflected light; and a data processor configured to process the images recorded by the image recorder means to produce at least one of a surface profile and / or a surface height map of at least a part of the sample surface, characterised in that the apparatus includes a chromatic objective lens.

2. Apparatus according to claim 1 wherein the light from the at least one light source directs light towards a region of a surface of a sample passes through the chromatic objective.

3. Apparatus according to claim 2 wherein the chromatic lens is an objective lens configured to scan the focus plane.

4. Apparatus according to claim 3 wherein the chromatic lens is configured to scan the focus plane across the optical axis without any mechanical movement.

5. Apparatus according to any preceding claim wherein the image recorder means includes at least one sensor and / or a camera means.

6. Apparatus according to any preceding claim wherein at least one of the light sources is a tuneable light source.

7. Apparatus according to claim 6 wherein the light source includes an acousto-optic tuneable filter (AOTF) .

8. Apparatus according to claim 1 wherein the apparatus includes two light sources.

9. Apparatus according to claims 6-8 wherein at least one of the light sources has a fixed or single wavelength.

10. Apparatus according to claims 6-9 wherein the tuneable light source provides light via a multi-mode fibre.

11. Apparatus according to claims 6-10 wherein the fixed or singlewavelength light source provides light via a single mode fibre.

12. Apparatus according to claim 1 wherein the light sources include a white light source and / or a RGB light source.

13. Apparatus according to claim 12 wherein the light sources are coupled.

14. Apparatus according to any preceding claim wherein the apparatus includes a stabilisation means.

15. Apparatus according to claims 10-14 wherein a single-mode fibre is configured to illuminate a diffraction limited spot light or spot size and the single-mode illumination is used to generate an interference light feedback signal to the stabilisation means.

16. Apparatus according to any preceding claim wherein the light director means includes or provides one or more interferometers.

17. Apparatus according to claim 16 wherein a first interferometer uses a multi-mode fibre for surface measurement and a second interferometer uses a single mode fibre to generate a feedback signal for stabilisation.

18. Apparatus according to claim 17 wherein the first and second interferometers share an optical path comprising at least part of the reference and measurement paths.

19. Apparatus according to any preceding claim wherein the image recorder means comprises an image sensor means.

20. Apparatus according to claim 19 wherein the image sensor means includes a CCD or CMOS camera.

21. Apparatus according to any preceding claim wherein the data processor is arranged to process the interferograms to produce data representing the relative surface heights of a 1 D or 2D array or sample of the surface pixels imaged by the recorder.

22. A method to extend the measurement range of WSI apparatus, characterized in that the method includes using chromatic focus variation to extend the measurement range.

23. Method according to claim 22 wherein during the wavelength scanning, the focus plane is shifted along the optical axis.

24. Method according to claim 23 wherein the method includes system stabilisation using a feedback signal generated by a single point of interference on a sample surface.

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