Description:A METHOD FOR OBTAINING WAVELENGTH DEPENDENT MATERIAL INFORMATION OF A SAMPLE
FIELD OF THE INVENTION
The invention generally relates to a label-free method for obtaining material information in microscopic volume using an interferometry. In particular, the invention discloses a method to extract optical spectroscopy signals from White Light Interferometer (WLI) or Full Field Optical Coherence Tomography (FFOCT) data to generate three dimensional color maps.
BACKGROUND OF THE INVENTION
An ideal microscope should provide a three-dimension view of the sample, including sub surface images, with high resolution in all directions along with material information and be fast at acquiring data. Different microscopy techniques suffer from various limitations. Optical microscopy provides resolution in the micrometre to sub micrometre scale. The lateral (xy) resolution is determined by the diffraction limit (λ/(2×NA)). The axial (z) resolution is typically several times poorer. To identify different molecules with optical microscopy, fluorescence techniques are integrated that involve tagging molecules with fluorescent labels. Confocal microscopy improves the axial resolution by about 2x using a pin hole and can be used to see through semi-transparent surfaces. Hyper spectral cameras can be used with optical microscopes to obtain spectroscopy data. But typically, these require heavy data processing and are restricted to the top surface. Raman spectroscopy is a well-known method for identifying chemical species. Some optical microscopes integrate additional components to obtain localized Raman signals. One way to enhance Raman effect is to use a sharp probe near the sample. An Atomic Force Microscope (AFM) and Raman combined systems is capable of obtaining localized Raman signal. Some novel methods use AFM and Raman or IR spectroscopy combinations to obtain nanometre resolution in xyz and correlated material information. AFMs require scanning and are often slow and therefore not suitable for large sample areas. They are also restricted to the top surface of the sample. Electron microscopy can provide nanometre lateral resolution, but only provides indirect indication of axial information. Material identification is possible by adding Energy Dispersive X-ray (EDX) method, which can provide elemental composition.
White light interferometry (WLI) also provides a non-contact optical method for surface height measurement on 3D structures. WLI is a powerful technique to rapidly measure large areas with precision down to nanometers. One such method known in the art makes use of white light interferometry to extract the profile of a sample surface with varying accuracies. However, the method does not make use of the wavelength information to extract sample surface properties. Another method known in the art provides a color image of a sample from interference data captured with a color camera. The irradiance of each color on the respective photo-sensor represents the sum of DC components received from the object and the reference surface and a modulated interference component. The color is determined at each pixel by removing the interference component and the reference-surface component from the irradiance data. The color map so derived is then combined with the height map produced with the same data to yield a true-color 3D map of the sample. The arrangement makes use of an RGB detector or color camera with white light interferometry. However, the accuracy of the said method is restricted by the accuracy of the RGB camera. Yet another method known in the art discloses a device including a storage section and a calculation section. The storage section stores information for measuring a light path difference of two light paths relating to interference of a white light, from a color appearing due to the interference. The calculation section measures the information represented by a color obtained from a plurality of pixel in an image. The calculation section determines the light path difference relating to each of the pixels, based on at least the information stored in the storage section. However, the interference data is captured by a color camera in the said device.
The existing methods or arrangements provide topography information and require a color camera to obtain the sample information. Therefore, there is a need in the art for a method and an arrangement to provide material information of a sample.
SUMMARY
The present invention overcomes the drawbacks of the prior art by providing a method for obtaining wavelength dependent material information of a sample using a White light interferometry or Full field Optical Coherence Tomography (FFOCT). The method includes obtaining plurality of interferograms by traversing the sample both along in-plane and axial direction. To each of the interferogram obtained, a voxel is assigned to determine a signal specific to each of the voxel. Subsequently, signals associated with each voxel are transformed to obtain a wavelength data, and generate a three-dimensional color map from the wavelength data. The color map corresponds to the material information of the sample.
Another aspect of the invention is to provide a system to generate three-dimensional color maps from wavelength data obtained from WLI or FFOCT.
BRIEF DESCRIPTION OF DRAWINGS
So that the manner in which the recited features of the invention can be understood in detail, some of the embodiments are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
Fig. 1 represents the steps of the method involved in obtaining the material information of a sample by generating a three-dimensional colour map, according to an embodiment of the invention.
Fig. 2 shows the graphical representation of the method to obtain material information of the sample, according to an embodiment of the invention.
Fig. 3 shows the processing of the interferogram at a voxel to obtain a three-dimensional colour map, according to an example of the invention.
Fig. 4 shows the colour map displaying material information, obtained by the method of the invention.
Fig. 5 shows the WLI arrangement for obtaining material information, according to an embodiment of the invention.
DETALED DESCRIPTION OF THE INVENTION
The definitions, terms and terminology adopted in the disclosure have their usual meaning and interpretations, unless otherwise specified.
The term “Interferogram” refers to the graph of intensity plotted as a function of z, for a particular pixel.
Various embodiment of the invention discloses a method for obtaining material information of a sample. Fig. 1 represents the steps of the method involved in obtaining the material information of a sample by generating a three-dimensional color map, according to an embodiment of the invention. Initially, a plurality of interferograms are obtained by traversing the sample both along in-plane (x,y) and axial (z) direction. In one example of the invention, the interferograms are gray scale interferograms. To each of the interferogram obtained, voxel of a predetermined dimension are assigned. The assigning of voxel enables the determination of the location of the voxel. At each of the voxel, a signal is determined. The signal obtained is then transformed to obtain the location information of the voxel. The location of each voxel in z is estimated using techniques that include but is not limited to Hilbert’s transform, gradient maxima method, and Hilbert transform with phase correction. The wavelength data for each voxel is obtained by using the corresponding interferogram. Specifically, in one embodiment of the invention, the wavelength data is obtained by Fourier transform of the interferogram. The data corresponding to each wavelength is specific to an identified voxel. Each voxel is assigned a specific color. The wavelength data associated with the voxel is used to generate a three-dimension color map. In one example of the invention, the representative color is obtained by identifying the dominant wavelength. Alternatively, the representative color can also be obtained by calculating a weighted average of the wavelength data. Alternatively, the Fourier coefficient or power, at a specific wavelength, is used to create a volume material map. The wavelength data can be utilized in different ways to represent the material information of the microscopic xyz volume contained within the identified voxel. The color map corresponds to the material information of the sample. The material information includes component distribution in the sample and precise location of each of the component in the sample. The method of obtaining the material information by generating a three-dimensional color map described in brief herein shall be explained in detail herein below.
Fig.2 shows the graphical representation of the method to obtain material information of the sample, according to an embodiment of the invention. Interferograms are obtained for every pixel along the x, y axis with uniform shift in the z axis, shifting the sample by Δz, where Δz is a fraction of the center wavelength of the white light source. In one example of the invention, Δz = ƛ/10, where ƛ is the center wavelength. A plurality of images is obtained with Δz shifts. Fig. 2(A), shows the sequence of raw images acquired. The intensity at each pixel (i,j) is plotted as a function of z to form the interferograms shown in Fig. 2(B). Signal filtering on each pixel (i,j), I (z) is performed by removing average intensity and applying bandpass filter to reduce noise in the interferogram. A Hilbert transform is applied to identify the center and extent of the interferogram as shown in Fig. 2 (C). Later, the wavelength data at voxel, given by the pixel (i,j) and the center of the interferogram, is obtained displaying the dominant wavenumber for each voxel as shown in Fig.2(D). The Fourier transform is performed on the interferogram to get the spectrum P vs k, where P is the power at the wavenumber k. The spectrum represents the wavelength data of the corresponding voxel. The wavelength data is further used to assign a unique material code to the corresponding voxel. In one embodiment, the material code is the dominant wavenumber which is used to assign a color. The wavelength data is used to assign material information to plurality of voxel in the sample volume. The materials are basically differentiated using colors in the image volume. The voxels are depicted at their respective locations in the volume as shown in Fig. 2(E).
The process of extracting material information from the raw data is described in detail herein below.
The sample is scanned in z direction, with a defined z step size, to acquire a series of raw images. At any x,y pixel, the intensity graph as a function of z is used to obtain the interferogram. The interferogram is a superposition of interference signals between all the light frequencies/wavelengths that are reflected from the sample and reference mirror. The oscillatory or AC part of the interferogram is represented as shown below.
S(x,y,z)= ∑_λ▒〖I(λ)〗
I(λ)=√(I_ref (λ) I_sam (λ)) × e^(〖((-Δz)/l_c )〗^2 ) × cos⁡(4πΔz/λ)
Here Δz is the movement of the sample in z, l_c is the coherence length and λ is the wavelength of light.
The Fourier transform of the signal S(x,y,z) carries the wavelength data I_sam (λ). The decay due to coherence length leads to a broadening of the Fourier transform signal at λ which limits the wavelength resolution. A single z scan typically provides correlated position and material (wavelength) data for more than a million voxels. In one example of the invention, the dimensions of each voxel are in the order of 1 µm x 1µm x 50 nm.
The method ensures that only the light scattered from the surface-in-focus dominates and there is minimum cross-talk between spatially distant points on the sample. A material code based on the wavelength data enables the extraction of material information for the generation of a three-dimensional color map with component distribution and location. A single scan through the entire volume is sufficient to completely map the location and material information. Basically each wavelength data corresponds to a representative color and each color correspond to a specific component in the sample. Thus, through the wavelength data complete material information is obtained for generating a three-dimensional color map.
Fig. 3 shows the processing of the interferogram at a voxel to obtain a three-dimensional colour map, according to an example of the invention. The example shows two sample points with varying spectra. The location of the points is estimated from the center of the WLI interferograms. The wavelength data is used to assign material information to the points. The raw data from the interferogram comprising a plurality of pixel is depicted in Fig. 3(a). Hilbert’s transform is applied to separate the plurality of pixels. The separated interferograms for each pixel is depicted in Fig. 3(b). Hilbert’s transformation aids in identifying the center of each of the separated interferogram which corresponds to the voxel location. Further, application of a Fast Fourier transform on the separate interferograms provides the wavelength data, as shown in Fig.3(c), where the dominant wavenumbers are indicated. The average wavelength is used to color the voxels. The three-dimensional color map shows the position and color of the voxels simultaneously as shown in Fig. 3(d). From such color maps, sub-surface layers can be picked out with submicron axial resolution.
Fig. 4 shows the color map displaying material information, obtained by the method of the invention. Each of the image shown is a representative material information, as obtained from the method of the invention. Fig 4(a) show an example color map of the top layer of a microfabricated semiconductor chip with a large array of features. A magnified view is given below depicting the various materials present in the semiconductor chip. Fig. 4(b) shows the corresponding topographic view of the microfabricated semiconductor chip, where the color scales depicts only height. The color map and topography provide complementary information about the microfabricated semiconductor chip.
The invention additionally provides an arrangement for obtaining wavelength dependent material information of a sample. The arrangement includes an optical microscope provided with one or more beam splitters. The additional beam splitter is used to separate the light into a reference and a measurement beam. The beam emitted from the sample is the measurement beam and the beam obtained from the reference mirror is called the reference beam. The resultant fringes are then recorded by the camera for generating the material information.
The arrangement that enables the method, as described herein above, is provided, in an embodiment of the invention. The arrangement includes a white light interferogram for obtaining a material information in a sample volume. Fig. 5 shows the WLI arrangement for obtaining material information, according to an embodiment of the invention. The arrangement includes a sample stage (1), plurality of objective lens (5), reference mirror (7), plurality of beam splitter (9), light source (11) and a camera (13). The sample stage (1) is actuated in both in-plane (x,y axis) and axial direction (z axis) for movement of a sample (3) across the three degrees of freedom. The illumination from the light source (11) passes through the objective lens (5) positioned in front of sample (3). In one example of the invention, the light source is a Broadband LED. The reference mirror (7) and the sample (3) are positioned at the focus of their respective objective lens (5). Reflection from the reference mirror (7) and the reflection / scattering from the sample (3) are collected by an objective lens (5) and guided towards the camera (13). The additional beam splitter (9) facilitates to capture the reference beam. Once the two optical paths, i.e. the measurement beam and the reference beam match precisely, to the coherence length, interference fringes are formed on the camera. The light source such as the LED or halogen lamp has coherence lengths of a few micrometers. The sample is mounted on a precision xyz stage. In one example of the invention, a piezo scanner is used to facilitate the movement of the sample along the x, y and z axis. The sample is scanned in z axis with precise steps of a few nanometres, typically a fraction of the wavelength of light. The image obtained on the camera for each z step is recorded along with the z height information. The images thus obtained are further processed to obtain material information. The arrangement described herein above is integrated with plurality of filtering tools for generating the wavelength information from the raw images captured by the camera. The filtering techniques include but not limited to Hilbert’s transform, gradient maxima method, Hilbert transform with phase correction and Fourier Transform.
The arrangement in the present invention does not employ any color sensors such as the RGB camera, hyperspectral camera or optical filters for providing enhanced wavelength resolution. The camera used in the arrangement measures the intensity of reflected light. In one example, the arrangement for obtaining material information makes use of Full field Optical Coherence Tomography (FFOCT).
The present invention provides a label-free method for obtaining material information in microscopic volume using an interferometry. The invention makes use of wavelength data to obtain material information for generating color map by processing sample images obtained in a gray-scale camera. Therefore, the invention does not require special color camera or RGB camera to extract material data with a sample volume. Since grey-scale cameras are typically more sensitive compared to color cameras, better signal quality is achieved.
The foregoing description of the invention has been set for merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the scope and substance of the invention may occur to a person skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. , C , C , C , Claims:WE CLAIM:

1. A method for obtaining wavelength dependent material information of a sample comprising:
obtaining plurality of interferograms by traversing the sample along in-plane and axial direction;
assigning voxel to each of the interferogram to determine a signal specific to each of the voxel;
transforming signals associated with each voxel to obtain a wavelength data; and
generating a three-dimensional color map from the wavelength data,
wherein the color map corresponds to the material information of the sample.
2. The method as claimed in claim 1, wherein the interferograms are acquired from a gray-scale camera.
3. The method as claimed in claim 1, wherein the assigning of voxel requires removal of average signal and noise at each voxel.
4. The method as claimed in claim 1, wherein the voxel location is estimated by Hilbert’s transform or gradient maxima method or Hilbert transform with phase correction.
5. The method as claimed in claim 1, wherein each assigned voxel corresponds to an interferogram.
6. The method as claimed in claim 1, wherein the wavelength data for each voxel is obtained by first identifying the centre of the corresponding interferogram using its voxel location and subsequently isolating the extent of the interferogram.
7. The method as claimed in claim 1, wherein the wavelength data is obtained by Fourier transform of the interferogram.
8. The method as claimed in claim 1, wherein each wavelength data corresponds to a representative color, wherein each color corresponds to a specific component in the sample.
9. The method as claimed in claim 1, wherein material information includes distribution of specific components in the sample and precise location of each of the specific component in the sample.

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