Holographic reconstruction apparatus and method

Abstract

Provided are an improved holographic reconstruction apparatus and method. A holographic reconstruction method includes: obtaining an object hologram of a measurement target object; extracting reference light information from the obtained object hologram; calculating a wavenumber vector constant of the extracted reference light information, and generating digital reference light by calculating a compensation term of the reference light information by using the calculated wavenumber vector constant; extracting curvature aberration information from the object hologram, and then generating digital curvature in which a curvature aberration is compensated for; calculating a compensated object hologram by multiplying the compensation term of the reference light information by the obtained object hologram; extracting phase information of the compensated object hologram; and reconstructing 3-dimensional (3D) shape information and quantitative thickness information of the measurement target object by calculating the quantitative thickness information of the measurement target object by using the extracted phase information of the compensated object hologram.

Claims

1. A holographic reconstruction apparatus comprising: a light source configured to emit single-wavelength light; a collimator configured to collimate the single-wavelength light emitted from the light source; a first beam splitter configured to split the single-wavelength light that passed through the collimator into object light and reference light; an object light objective lens through which the object light obtained by the first beam splitter passes; a reference light objective lens through which the reference light received by the first beam splitter passes; an optic mirror reflecting the reference light that has passed through the reference light objective lens; a recording medium configured to record an interference pattern formed when the object light reflected by a surface of a measurement target object and the reference light reflected by the optic mirror pass through the object light objective lens and the reference light objective lens, respectively, and are transmitted to the first beam splitter; and a processor configured to receive and store an image file generated when the recording medium converts the interference pattern, wherein the processor is further configured to generate digital reference light by extracting reference light information of an object hologram from the object hologram obtained from the image file, and reconstruct 3-dimensional (3D) information of the measurement target object by calculating an object hologram that is compensated for by using the object hologram and the digital reference light and extracting phase information of the compensated object hologram, and wherein the processor is further configured to extract the reference light information by: performing 2D Fourier transform on the obtained object hologram, and extracting, by using an automatic real image spot-position extraction algorithm, only a real image spot-position from a frequency spectrum that is obtained via the 2D Fourier transform and comprises spectrum information including the real image spot-position of the object hologram, spectrum information including imaginary image spot-position, and spectrum information including direct current (DC) information; and extracting the reference light information of the obtained object hologram by using the extracted real image spot-position.

2. The holographic reconstruction apparatus of claim 1, wherein the processor is further configured to generate the digital reference light by: calculating a wavenumber vector constant of the extracted reference light information; and calculating a compensation term of the extracted reference light information by using the calculated wavenumber vector constant.

3. The holographic reconstruction apparatus of claim 2, wherein the digital reference light is represented by R.sub.c(x,y)=conj[R(x,y)], wherein R.sub.c(x,y) denotes the digital reference light, R(x,y) denotes the reference light information of the obtained object hologram, and conj denotes a function for obtaining a conjugate complex number.

4. The holographic reconstruction apparatus of claim 2, wherein the processor is further configured to extract curvature aberration information from the object hologram to compensate for a curvature aberration of the object light objective lens, and then generate digital curvature that is a curvature aberration information compensation term by using an automatic frequency curvature compensation algorithm.

5. The holographic reconstruction apparatus of claim 4, wherein the compensated object hologram is calculated by multiplying the calculated compensation term of the extracted reference light information by the obtained object hologram.

6. The holographic reconstruction apparatus of claim 5, wherein the compensated object hologram is represented by U.sub.C(x,y,0)=O(x,y)R*(x,y)R.sub.C(x,y)R.sub.CA(x,y), wherein U.sub.C(x,y,0) denotes the compensated object hologram, O(x,y) and R*(x,y) respectively denote the object light and the reference light of the obtained object hologram, R.sub.C(x,y) denotes the digital reference light, and R.sub.CA(x,y) denotes the digital curvature.

7. The holographic reconstruction apparatus of claim 1, wherein the processor is further configured to convert the compensated object hologram to information of a reconstructed image plane by using an angular spectrum propagation algorithm, extract phase information of the compensated object hologram through inverse 2D Fourier transform, and reconstructs 3D shape information and quantitative thickness information of the measurement target object by calculating the quantitative thickness information of the measurement target object by using the extracted phase information.

8. The holographic reconstruction apparatus of claim 7, wherein the quantitative thickness information is represented by ΔL=λΔφ(x,y)/2πΔn(x,y), wherein ΔL denotes the quantitative thickness information, λ denotes a wavelength of the light source, Δφ(x,y) denotes the phase information of the compensated object hologram, and Δn(x,y) denotes a difference in refractive index between air and the measurement target object.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a block diagram illustrating in detail a dual-wavelength digital holographic microscopic apparatus according to the published prior art.

(2) FIG. 2A is a block diagram of a holographic reconstruction apparatus according to a first embodiment of the present disclosure.

(3) FIG. 2B is a block diagram of a holographic reconstruction apparatus according to a second embodiment of the present disclosure.

(4) FIG. 2C is a view of an object hologram of a thin-film transistor (TFT), according to an embodiment of the present disclosure.

(5) FIG. 2D is a view of digital reference light that is a compensation term obtained by calculating a wavenumber vector constant from reference light information of an object hologram extracted from an object hologram of the TFT of FIG. 2C by using an automatic real image spot-position extraction algorithm.

(6) FIG. 2E is a view of a reconstructed image of a 3-dimensional (3D) image of a measurement target object on which curvature aberration correction of an object light objective lens is not performed.

(7) FIG. 2F is a view of a reconstructed image of a 3D image of a measurement target object on which curvature aberration correction of an object light objective lens is performed.

(8) FIG. 2G is a view of a 3D shape reconstructed image of the TFT of FIG. 2C containing quantitative thickness information calculated by using extracted phase information of a compensated object hologram, according to the present disclosure.

(9) FIG. 3 is a flowchart of a holographic reconstruction method according to an embodiment of the present disclosure.

BEST MODE

(10) Hereinafter, the present disclosure will be described in detail with reference to embodiments and drawings.

(11) FIG. 2A is a block diagram of a holographic reconstruction apparatus according to a first embodiment of the present disclosure.

(12) Referring to FIG. 2A, a holographic reconstruction apparatus 1a according to the first embodiment of the present disclosure includes: a light source 10 configured to emit a single-wavelength light; a collimator 20 configured to collimate the single-wavelength light emitted from the light source 10; a first beam splitter 30 configured to split the single-wavelength light that has passed through the collimator 20 into object light O and reference light R; an object light objective lens 40 through which the object light O obtained by the first beam splitter 30 passes; a reference light objective lens 60 through which the reference light R obtained by the first beam splitter 30 passes; an optic mirror 70 reflecting the reference light R that has passed through the reference light objective lens 60; a recording medium 80 configured to record an interference pattern formed when the object light O reflected by a surface of a measurement target object 50 and the reference light R reflected by the optic mirror 70 are transmitted to the first beam splitter 30 respectively through the object light objective lens 40 and the reference light objective lens 60; and a processor 90 configured to receive and store an image file generated when the recording medium 80 converts the interference pattern, wherein the processor is further configured to generate digital reference light by extracting reference light information of an object hologram from the object hologram obtained from the image file, and reconstruct 3-dimensional (3D) information of the measurement target object 50 by calculating an object hologram that is compensated for by using the object hologram and the digital reference light and extracting phase information of the compensated object hologram.

(13) FIG. 2B is a block diagram of a holographic reconstruction apparatus according to a second embodiment of the present disclosure.

(14) Referring to FIG. 2B, a holographic reconstruction apparatus 1b according to the second embodiment of the present disclosure includes: the light source 10 configured to emit a single-wavelength light; the collimator 20 configured to collimate the single-wavelength light emitted from the light source 10; the first beam splitter 30 configured to split the single-wavelength light that passed through the collimator 20 into the object light O and the reference light R; the reference light objective lens 60 through which the reference light R obtained by the first beam splitter 30 passes; a first optic mirror 70 reflecting the reference light R that passed through the reference light objective lens 60; the object light objective lens 40 through which an object penetration light T including information of the measurement target object 50 passes after the object light O obtained by the first beam splitter 30 passes through the measurement target object 50; a second optic mirror 72 reflecting the object penetration light T that passed through the object light objective lens 40; the first beam splitter 30a second beam splitter 32 to which the reference light R reflected by the first optic mirror 70 and the object penetration light T reflected by the second optic mirror 72 are transmitted; the recording medium 80 configured to record an interference pattern formed by the reference light R and the object penetration light T, which are transmitted to the second beam splitter 32; and the processor 90 configured to receive and store an image file generated when the recording medium 80 converts the interference pattern, wherein the processor 90 is further configured to generate digital reference light by extracting reference light information of an object hologram from the object hologram obtained from the image file, and reconstruct 3D information of the measurement target object 50 by calculating an object hologram that is compensated for by using the object hologram and the digital reference light and extracting phase information of the compensated object hologram.

(15) The holographic reconstruction apparatus 1a according to the first embodiment of the present disclosure and the holographic reconstruction apparatus 1b according to the second embodiment of the present disclosure respectively shown in FIGS. 2A and 2B substantially have the same structure except that the object light O is reflected by the measurement target object 50 (the embodiment of FIG. 2A) or the object light O passes through the measurement target object 50 (the embodiment of FIG. 2B), and thus some components (for example, the second optic mirror 72 and the second beam splitter 32 in the embodiment of FIG. 2B) are additionally used and arranged, and in particular, have the same characteristics in that the interference pattern is recorded on the recording medium 80 and the digital reference light is generated from the object hologram obtained by the processor 90 in a form of the image file based on the recorded interference pattern. Accordingly, hereinafter, the holographic reconstruction apparatuses 1a and 1 b according to the first and second embodiments of the present disclosure will be collectively referred to as a holographic reconstruction apparatus 1 according to an embodiment of the present disclosure.

(16) The processor 90 of the holographic reconstruction apparatus 1 according to the embodiment of the present disclosure may be, for example, an apparatus capable of arithmetic calculation, such as a microprocessor or a personal computer (PC), and the recording medium 80 may be an image sensor, such as a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS).

(17) Also, information of the object hologram obtained by the processor 90 of the holographic reconstruction apparatus 1 according to the embodiment of the present disclosure includes a wavelength and phase information of an object, a curvature aberration of the object light objective lens 40, and may additionally include noise (for example, speckle noise according to use of photons of a laser beam).

(18) Also, the object hologram obtained by the processor 90 of the holographic reconstruction apparatus 1 according to the current embodiment of the present disclosure is a complex conjugation hologram, and may be represented by Equation 1 below.
|U.sub.o(x,y,0)|.sup.2=|O(x,y)|.sup.2+|R(x,y)|.sup.2+O*(x,y)R(x,y)+O(x,y)R*(x,y)  Equation 1:

(19) In Equation 1, x and y denote spatial coordinates, U.sub.o(x,y,0) denotes the obtained object hologram, O(x,y) and R(x,y) respectively denote the object light O and the reference light R, and O*(x,y) and R*(x,y) respectively denote complex conjugations of the object light O and the reference light R.

(20) Hereinafter, a detailed method of generating the digital reference light and a compensated object hologram from the obtained object hologram is described.

(21) First, the processor 90 of the holographic reconstruction apparatus 1 according to the embodiment of the present disclosure obtains the object hologram (see a thin-film transistor (TFT) of FIG. 2C) from the image file of the interference pattern recorded on the recording medium 80. The obtained object hologram is formed of an interference pattern of the object light O having phase information of an object and the reference light R not having the phase information of the object.

(22) Then, 2D Fourier transform is performed on the obtained object hologram to extract, from the obtained object hologram, information of the reference light R not having the phase information of the object. A frequency spectrum of the object hologram obtained via the 2D Fourier transform is separated into spectrum information including a real image spot-position, spectrum information including an imaginary image spot-position, and spectrum information including direct current (DC) information. Only the real image spot-position is extracted from the frequency spectrum by using an automatic real image spot-position extraction algorithm. The reference light information of the obtained object hologram is extracted by using the extracted real image spot-position.

(23) Then, a phase breaking phenomenon may occur every 2 π of the extracted reference light information due to a wave nature of light, and in order to compensate for the phase breaking phenomenon, the processor 90 calculates a wavenumber vector constant of the extracted reference light information by using a wavenumber algorithm. A compensation term of the extracted reference light information is calculated by using the calculated wavenumber vector constant. The compensation term of the extracted reference light information calculated from the wavenumber vector constant is a conjugate of the reference light information of the obtained object hologram. The calculated compensation term of the extracted reference light information is referred to as digital reference light (see FIG. 2D), and is represented by Equation 2.
R.sub.c(x,y)=conj[R(x,y)]  Equation 2:

(24) Here, R.sub.c(x,y) denotes the digital reference light, R(x,y) denotes the reference light information of the obtained object hologram, and conj denotes a function of obtaining a conjugate complex number.

(25) Then, in order to compensate for a curvature aberration of the object light objective lens 40 used to obtain the object hologram, the processor 90 extracts curvature aberration information from the object hologram. Then, the processor 90 generates a curvature aberration information compensation term by using an automatic frequency curvature compensation algorithm. Here, the curvature aberration information compensation term is referred to as digital curvature.

(26) Then, the processor 90 calculates the compensated object hologram by multiplying the compensation term of the extracted reference light information by the obtained object hologram. This is represented by Equation 3.
U.sub.C(x,y,0)=O(x,y)R*(x,y)R.sub.C(x,y)R.sub.CA(x,y)  Equation 3:

(27) In Equation 3, U.sub.C(x,y,0) denotes the compensated object hologram, O(x,y) and R*(x,y) respectively denote the object light and the reference light of the obtained object hologram, R.sub.C(x,y) denotes the digital reference light, and R.sub.CA(x,y) denotes the digital curvature.

(28) Then, the processor 90 converts the compensated object hologram to information of a reconstructed image plane by using an angular spectrum propagation algorithm. Here, the reconstructed image plane denotes a virtual image display plane at a location corresponding to a distance between the measurement target object 50 and the recording medium 80, and may be calculated and simulated by the processor 90. The processor 90 extracts the phase information of the compensated object hologram via inverse 2D Fourier transform. It should be noted that since the information of light in the obtained object hologram and the curvature aberration information of the objective lens are removed from the phase information extracted as such, the phase information of the extracted compensated object hologram includes only the phase information of the object.

(29) Then, the processor 90 calculates quantitative thickness information of the measurement target object 50 by using the extracted phase information of the compensated object hologram. In this case, the extracted phase information of the compensated object hologram may additionally include fine noise, such as speckle noise, due to use of photons of a laser beam, and thus the processor 90 may pre-remove the fine noise before calculating the quantitative thickness information of the measurement target object 50. In detail, the processor 90 may compensate for distorted phase information generated by the fine noise and a wrapped phase phenomenon from the extracted phase information of the compensated object hologram, by using a 2D phase unwrapping algorithm. When the distorted phase information generated by the fine noise and the wrapped phase phenomenon is removed, the quantitative thickness information of the measurement target object 50 may be further precisely calculated based on the phase information of the compensated object hologram. The quantitative thickness information of the measurement target object 50 calculated as above is represented by Equation 4.
ΔL=λΔφ(x,y)/2πΔn(x,y)  Equation 4:

(30) In Equation 4, ΔL denotes the quantitative thickness information of the measurement target object 50, λ denotes a wavelength of the light source 10 used to obtain the object hologram, (x,y) denotes the phase information of the compensated object hologram, and Δn(x,y) denotes a refractive index difference between the background (or air) and the measurement target object 50. The processor 90 reconstructs a 3D shape of the measurement target object 50 on the reconstructed image plane by using the quantitative thickness information of the measurement target object 50 calculated according to Equation 4. The reconstructed image plane reconstructed by the processor 90 may be displayed, for example, on a separate monitor (not shown).

(31) FIGS. 2E and 2F are views of a reconstructed image of a 3D shape of the measurement target object 50 on which curvature aberration correction of the object light objective lens 40 is not performed, and a reconstructed image of a 3D shape of the measurement target object 50 on which curvature aberration correction of the object light objective lens 40 is performed. Also, FIG. 2G is a view of a 3D shape reconstructed image of the TFT of FIG. 2C having the quantitative thickness information calculated by using the extracted phase information of the compensated object hologram, according to the present disclosure. Referring to the reconstructed images of FIGS. 2F and 2G, it is determined that the 3D shape of the semiconductor substrate circuit of FIG. 2C is clearly reconstructed.

(32) FIG. 3 is a flowchart of a holographic reconstruction method according to an embodiment of the present disclosure.

(33) Referring to FIG. 3 together with FIGS. 2A through 2G, a holographic reconstruction method according to an embodiment of the present disclosure includes: obtaining an object hologram of the measurement target object 50, in operation S1; extracting reference light information from the obtained object hologram, in operation S2; calculating a wavenumber vector constant of the extracted reference light information, and generating digital reference light by calculating a compensation term of the reference light information by using the calculated wavenumber vector constant, in operation S3; extracting curvature aberration information from the object hologram, and then generating digital curvature in which a curvature aberration is compensated for, in operation S4; calculating a compensated object hologram by multiplying the compensation term of the reference light information by the obtained object hologram, in operation S5; extracting phase information of the compensated object hologram, in operation S6; and reconstructing 3D shape information and quantitative thickness information of the measurement target object 50 by calculating the quantitative thickness information of the measurement target object 50 by using the extracted phase information of the compensated object hologram, in operation S7.

(34) In the holographic reconstruction method according to an embodiment of the present disclosure, operation S1 may include: splitting, by the first beam splitter 30, the single-wavelength light emitted from the light source 10 into the object light O and the reference light R; reflecting the object light O from a surface of the measurement target object 50 through the object light objective lens 40, and reflecting the reference light R at an optic mirror 70 after passing the reference light R through the reference light objective lens 60; recording the interference pattern formed when the reflected object light O and the reflected reference light R are transmitted to the first beam splitter 30 on the recording medium 80, and transmitting, to the processor 90, the image file generated by converting the interference pattern; and obtaining, by the processor 90, the object hologram from the image file.

(35) Alternatively, in the holographic reconstruction method according to an embodiment of the present disclosure, operation S1 may include: splitting, by the first beam splitter 30, the single-wavelength light emitted from the light source 10 into the object light O and the reference light R; reflecting the object penetration light T obtained by passing the object light O through the measurement target object 50 at the second optic mirror 72 after passing the object penetration light T through the object light objective lens 40, and reflecting the reference light R at the optical mirror 70 after passing the reference light R through the reference light objective lens 60; recording the interference pattern formed by transmitting the reflected object penetration light T and the reflected reference light R to the second beam splitter 32, on the recording medium 80, and transmitting the image file generated by converting the interference pattern to the processor 90; and obtaining, by the processor 90, the object hologram from the image file.

(36) Also, in the holographic reconstruction method according to an embodiment of the present disclosure, operation S2 may include: performing inverse 2D Fourier transform on the obtained object hologram; extracting, by using an automatic real image spot-position extraction algorithm, only a real image spot-position from a frequency spectrum that is obtained via the 2D Fourier transform and includes spectrum information including the real image spot-position of the object hologram, spectrum information including imaginary image spot-position, and spectrum information including DC information; and extracting the reference light information of the obtained object hologram by using the extracted real image spot-position.

(37) Also, in the holographic reconstruction method according to an embodiment of the present disclosure, the calculated compensation term in operation S3 is a conjugate of the obtained object hologram and is the digital reference light, and is represented by R.sub.C(x,y)=conj[R(x,y)], wherein R.sub.C(x,y) denotes the digital reference light, R(x,y) denotes the reference light information of the obtained object hologram, and conj denotes a function for obtaining a conjugate complex number.

(38) Also, in the holographic reconstruction method according to an embodiment of the present disclosure, the compensated object hologram calculated in operation S5 is represented by U.sub.C(x,y,0)=O(x,y)R (x,y)R.sub.C(x,y)R.sub.CA(x,y), wherein U.sub.C(x,y,0) denotes the compensated object hologram, O(x,y) and R*(x,y) respectively denote the object light and the reference light of the obtained object hologram, R.sub.C(x,y) denotes the digital reference light, and R.sub.CA(x,y) denotes the digital curvature.

(39) Also, in the holographic reconstruction method according to an embodiment of the present disclosure, the phase information of the compensated object hologram in operation S6 is extracted via inverse 2D Fourier transform, wherein the extracted phase information includes only phase information of the measurement target object as light information and curvature aberration information of an objective lens are removed from the obtained object hologram. Here, operation S6 may further include, when the phase information of the compensated object hologram includes fine noise and a wrapped phase phenomenon, removing the fine noise and the wrapped phase phenomenon by using a 2D phase unwrapping algorithm.

(40) Also, in the holographic reconstruction method according to an embodiment of the present disclosure, the quantitative thickness information calculated in operation S7 is represented by ΔL=λΔφ(x,y)/2πΔn(x,y), wherein ΔL denotes the quantitative thickness information, A denotes a wavelength of the light source, Δφ(x,y) denotes the phase information of the compensated object hologram, and Δn(x,y) denotes a difference in refractive index between air and the measurement target object.

(41) As described above, in the improved holographic reconstruction apparatus 1 and method according to the present disclosure, since the processor 90 reconstructs 3D information of the measurement target object 50 by only using the obtained object hologram and the digital reference light generated from the obtained object hologram, issues related to a complex structure of an optical apparatus required during conventional one-shot-type digital holography reconstruction using one object hologram and consequent high costs may be solved.

(42) Also, in the improved holographic reconstruction apparatus 1 and method according to the present disclosure, since the holographic reconstruction apparatus 1 additionally uses only the processor 90, an overall structure is very simple and a hologram may be reconstructed with low costs.

(43) Also, in the improved holographic reconstruction apparatus 1 and method according to the present disclosure, since the holographic reconstruction apparatus 1 substantially has the same structure as conventional reflective and transmissive hologram reconstruction apparatuses except for the processor 90, the improved holographic reconstruction apparatus 1 and method have general versatility of being applied to both the conventional reflective and transmissive hologram reconstruction apparatuses.

(44) Also, in the improved holographic reconstruction apparatus 1 and method according to the present disclosure, in particular, a reference hologram is not required during hologram reconstruction, and a quantitative 3D image of the measurement target object 50 may be reconstructed in real time.

(45) Also, in the improved holographic reconstruction apparatus 1 and method according to the present disclosure, since the quantitative 3D image of the measurement target object 50 may be reconstructed in real time without having to use the reference hologram, the improved holographic reconstruction apparatus 1 and method may be applied to defect detecting apparatuses having a ultrafine structure, such as a TFT and a semiconductor, medical devices that need to display a precise 3D image, and other detecting, determining, or displaying apparatuses in various fields including refractive index error detection of a transparent object, such as a lens.

INDUSTRIAL APPLICABILITY

(46) While one or more embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.