JP2007504444A - Transmission and reflection type spatial heterodyne interferometry (SHIRT) measurement - Google Patents

Abstract

Systems and methods for transmission and reflection spatial heterodyne interferometry (SHIRT) measurements are described. The method digitally records a first spatially heterodyne hologram using a first reference beam and a target beam and uses a second reference beam and target beam to perform a second spatial heterodyne. Digitally record the converted hologram. The method also Fourier analyzes the digitally recorded first spatially heterodyne hologram to define a first analyzed image and to define a second analyzed image. Fourier-analyzing a second spatially heterodyned hologram recorded digitally, digitally filtering the first analyzed image to define a first result, and defining a second result The second analyzed image is digitally filtered and a first inverse Fourier transform is applied to the first result and a second inverse Fourier transform is applied to the second result.

Description

(Statement of the right of invention under federal sponsored research or development)
The present invention is based on UT-Battelle, L. L. C. With the support of the United States government under the prime contract number DE-AC05-00OR22725.

(Technical field)
The present invention relates generally to the field of spatial-heterodyne interferometry (SHI). In particular, the invention relates to a method and apparatus for providing transmissive spatial heterodyne interferometry (SHIFT) measurements and reflective and transmissive spatial heterodyne interferometry (SHIRT) measurements.

  US Pat. Nos. 6,078,392 and 6,525,821 relate to direct-to-direct holography (DDH). In DDH, the reflected target wavefront is combined with a reference wavefront at a small angle on the surface of the digital imaging device. The small angle forms a set of linear fringes that spatially heterodynize the reflected wavefront of interest. Then, Fourier analysis is used to isolate the image at the heterodyne frequency to reproduce the composite wavefront, Voelkl (1999).

  DDH is an implementation of spatial heterodyne interferometry using Fourier reconstruction to capture a complex wavefront reflected at the surface of an object. When the wavefront hits the surface of the object, the shape of the surface is incorporated into the phase of the wavefront, and the reflectivity of the surface is included in the intensity of the reflected wave. This reflected wave is combined in the digital imaging device to interfere with the reference wave and form a linear fringe set. These linear interference fringes include information on the phase and amplitude of the target wave. In Fourier space, this target wave information appears around the center of the spatial frequency of the interference fringes. The recording of wave phase and amplitude information at non-zero frequencies is known as “heterodyning”. However, DDH does not provide information about the interior of the object, but only provides information about the surface of the object.

  Phase contrast microscopy (PCM), on the other hand, is a well-known technique that is commonly used to image biological samples. PCM is particularly useful when the biological sample contains phase distinguishable features with similar transmittance. However, the limitation of PCM is that no composite wavefront information is provided, i.e. the provision of phase information from the PCM represented only by amplitude.

  Recently, Jacob (2002) reported a technique in which transmission phase shift interferometry is used to measure the phase difference between two points on a photolithographic mask. While this technique can measure phase changes, it requires 30 seconds for each height measurement, and thus a slow scan is needed to measure phase changes through the interior of the object. In particular, very long scans may be required to measure the phase change of the entire mask. Therefore, what is needed is an approach that can quickly provide complex wavefront information about the interior of an object.

  Previously, both requirements of providing complex wavefront information about the interior of an object and providing that information quickly have not been met. What is needed is a solution that solves both of these problems simultaneously.

  There is a need for the following aspects of the invention. Of course, the present invention is not limited to these aspects.

  In accordance with one aspect of the present invention, the process digitally records a spatially heterodyne hologram including spatial heterodyne fringes for Fourier analysis using the reference beam and the object beam, and the image to be analyzed. To define the original origin of the hologram to overlay the original origin of the spatially heterodyned hologram digitally recorded on the spatial telodyne carrier frequency defined by the angle between the reference beam and the target beam Fourier-analyze digitally recorded spatially heterodyned holograms by moving the origin, isolate the signal around the original origin, and digitally filter the analyzed image to define the result And applying an inverse Fourier transform to the result, the beam of interest is Without even passing through the object is a partially translucent. According to another aspect of the invention, a machine is optically coupled to a coherent light energy source, a reference beam subassembly optically coupled to the coherent light source, and a coherent light source. A target beam subassembly, a beam splitter optically coupled to both the reference beam subassembly and the target beam subassembly, and a pixelated detection device optically coupled to the beam splitter. The subassembly includes an object that is at least partially translucent, and transparently the object is optically coupled between the coherent light energy source and the beam splitter.

  According to another aspect of the present invention, the process uses a first reference beam and a first object beam to process a first spatially heterodyne hologram that includes spatial heterodyne fringes for Fourier analysis. Digitally recording, digitally recording a second spatially heterodyne hologram including a spatial heterodyne fringe for Fourier analysis using a second reference beam and a second target beam; To digitally record a first digitally recorded first frequency on a first spatial telodyne carrier frequency defined by a first angle between a first reference beam and a first target beam. By moving the first original origin of the first hologram to overlap the first original origin of the spatially heterodyned hologram of Fourier-analyze the first spatially heterodyned hologram recorded and a second reference beam and a second object beam to define a second analyzed image. The second hologram second to superimpose the second original origin of the second spatially heterodyned hologram digitally recorded on the second spatial telodyne carrier frequency defined by the angle of two. Fourier-analyze the digitally recorded second spatially heterodyned hologram by moving the original origin of the signal, decouple the signal around the first original origin, and define the first result In order to digitally filter the first analyzed image, to separate the signal around the second original origin and to define the second result, the second analyzed Digitally filtering the image, subjecting the first result to a first inverse Fourier transform, and subjecting the second result to a second inverse Fourier transform, wherein the first target beam is at least partially And the second object beam is reflected at the object. According to another aspect of the invention, an apparatus includes a coherent light energy source, a transmission reference beam subassembly optically coupled to the coherent light source, and a reflective transmission optically coupled to the coherent light source. A reference beam subassembly and a target beam subassembly optically coupled to a coherent light source, the target beam subassembly including a transmission target beam path and a reflection target beam path, the target beam subassembly, and a transmission reference A transmitted beam splitter optically coupled to both the beam subassembly and the target beam subassembly; a reflected beam splitter optically coupled to both the reflected reference beam subassembly and the target beam subassembly; and a transmitted beam A group consisting of a splitter and a reflective beam splitter A pixelated detection device optically coupled to at least one element selected from the group, wherein the object beam subassembly includes an object that is at least partially translucent, the object being i) transparently , Optically coupled between the coherent optical energy source and the transmission beam splitter, and ii) optically coupled reflectively between the coherent optical energy source and the reflective beam splitter.

  These and other aspects of the invention will be better understood and appreciated in conjunction with the following description and the accompanying drawings. While various embodiments of the present invention and numerous specific details thereof are set forth, the following description is provided by way of illustration and not limitation. Numerous alternatives, modifications, additions and / or rearrangements may be made within the scope of the present invention without departing from the spirit of the invention, and the present invention may include all such alternatives, modifications, additions and / or Includes relocation.

  The drawings, which form a part of the specification and are included, are included to depict certain aspects of the present invention. The clear concept of the present invention, the clear concept of the components, and the operation of the system provided with the present invention will be more readily clarified by reference to the exemplary and thus non-limiting embodiments shown in the drawings. Will. In the drawings, the same reference number (if present in more than one figure) indicates equivalent components. The invention may be better understood with reference to the combination of the description provided herein and one or more of the drawings. Note that the features shown in the drawings do not have to be reduced to scale.

  The invention and the details of the various features and advantages of the invention are more fully described with reference to the non-limiting embodiments that are illustrated in detail in the following description, which is illustrated in the accompanying drawings. Description of well-known starting materials, processing techniques, components, and equipment is omitted so as not to unnecessarily obscure the present invention. However, it is understood that the detailed description or specific examples given while demonstrating the best mode of the present invention are illustrative and not limiting. Various substitutions, modifications, additions and / or rearrangements within the spirit and / or scope of the underlying inventive concept will be apparent to those skilled in the art from this disclosure.

  In this application, references are made to multiple publications, which are indicated in parentheses by the name of the main author followed by the year of publication. Their full description, as well as other publications, are presented immediately before the claims and at the end of the best mode for carrying out the invention under the references. The entire disclosures of all these publications are incorporated herein for the purpose of illustrating the background of the invention and the state of the art.

  The following US patents and US patent applications disclose embodiments that are used for the intended purpose. U.S. Pat. No. 6,078,392, issued on June 20, 2000, Clearance E.I. Thomas, Larry R .; Baylor, Gregory R .; Hanson, David A.M. Rassen, Edgar Voelkl, James Castracane, Michele Sumkulet and Lawrence Clow, "Direct-to-Digital Holography, Holographic Interferometry, and Holyon". U.S. Pat. No. 6,525,821, issued on Feb. 25, 2003; Thomas and Gregory R.M. The entire content of “Acquisition and Replay Systems for Direct-to-Digital Holography and Holography” by Hanson is practically incorporated. U.S. Application Nos. 10 / 234,042, 10 / 234,043, 10 / 234,044, 10 / 349,579, 10 / 421,444, 10 The entire contents of / 607,824 and 10 / 607,840 are incorporated by reference. This application includes disclosure contained in co-pending US application Ser. No. 10 / 649,474 (Attorney Docket No. UBAT 1520) filed on the same day as this application, the entire contents of which are hereby incorporated by reference. .

(Overview)
Spatial heterodyne hologram image formation processing can be fully described using scalar diffraction logic. In scalar times析論sense, the reference wavefront _U R (x, y) and the target wave front _U o (x, y) is expressed as follows.

For simplification, the reference wavefront is in a planar shape with a phase of 0 and the light intensity is constant. Thus _U R (x, y) = become _{A R.} In generating spatial heterodyne holography (SHH), the reference wavefront is tilted at an angle of x (θ _x ) and y (θ _y ) with respect to the target wavefront. When two wavefronts are combined at the surface of an intensity measuring device (eg, film pixelated detector, charge coupled device (CCD) camera, complementary metal oxide silicon (CMOS) imager, etc.), the resulting intensity is

Where λ represents the irradiation wavelength. Thus, the recorded image includes not only the intensity of the two waves but also the relative phase between the two waves. The angle between the two waves sets (defines) the carrier frequency of the system and recovers the amplitude A _O (x, y) and phase φ (x, y) using the Fourier frequency analysis method described below. Help.

1A and 1B show an example of a spatial heterodyne hologram recorded on a chrome-on-glass target. FIG. 1B, which is an inset, shows an interference fringe of a linear sine wave pattern represented by (θ _x / λ, θ _y / λ). The interference fringes are a function of the phase φ (x, y) and vary depending on the surface structure.

Once the spatial heterodyne hologram is formed on the surface of the CCD and transferred to the computer as a scalar value data matrix, the goal is to recover the amplitude and phase of the object wave from this scalar value data matrix. This can be accomplished by computer computation by subjecting the hologram to a Fourier transform and separating one of the sideband structures of the composite spectrum. The localized sideband structure is the result of the linear fringes of most of the image and the cosine function of the hologram. The transformation of the hologram results in positioning of the complex wavefront information about discrete points at frequency positions (θ _x / λ, θ _y / λ) in the Fourier domain. This position is shifted to the origin in the frequency domain and selected using a low pass filter to give the function U _F (u, ν) represented below.

W _B (u, v) is a low-pass filter, (u, v) is a frequency variable, and the exponential function is the origin (0, 0) of the sideband structure from (θ _x / λ, θ _y / λ). ).

This is shown in FIGS. 2A and 2B. FIG. 2A represents the magnitude of the full frequency spectrum of the hologram, and FIG. 2B includes a composite wavefront guess required to determine A _O (x, y) and φ (x, y), with a low frequency centered Fig. 4 shows a filtered sideband structure.

  The result of the shift and filtering operation is an approximate determination of the original composite wavefront. That means

It is.

Once the original composite wavefront U _F (x, y) guess is determined, the amplitude and phase are

As determined. Re {•} is a real element of U _F (x, y), and Im {•} is an imaginary element of U _F (x, y).

FIG. 3A shows the resulting amplitude A _R (x, y) and FIG. 3B shows the phase φ (x, y) from the SHH of the chrome-on-glass target used to obtain FIGS. 1A and 1B. Show.

  As an overview of the description so far, two wavefronts, a reference wavefront and a target wavefront, are combined, recorded, stored, and generated to generate a spatial heterodyne hologram on the surface of the imaging device and the amplitude of the target wavefront and Fourier analysis was performed to recover the phase. It is important to note that the wavefront of interest can be not only the wavefront reflected at the surface of the object, but also the result of the wavefront transmitted through the object. In each of these two cases, the information contained in the amplitude and phase of the recovered object wavefront provides useful and different object information.

  The following description emphasizes the transmitted target wavefront. A wave that passes through an object has an amplitude that varies with the reflectivity of all the surfaces that pass through and the absorption of the material. In this description, a transparent material is assumed such that the absorption is negligible. Using the Fresnel equation, the reflectance in normal illumination when passing from one medium to another is

Where R is the reflectivity and N ₁ and N ₂ are the refractive indices of the surface of each side of the material. For irradiation with an angle,

θ = angle from normal incidence, θ ′ = angle from normal incidence in the second medium, and μ ₁ and μ ₂ are the magnetic permeability of each side of the material. From the reflectance, the transmittance can be calculated as T = 1−R.

  While the amplitude of the transmitted wavefront of the object can provide information about the object, the amplitude can be determined without using interferometry. More importantly, an interesting information that can be provided by the target wavefront transmitted in this way (via analysis of the corresponding SHH) is the phase of the target wavefront. It is important to note that the phase of the transmitted object wavefront includes information about the material that the object has and the thickness and refractive index of the material that the transmitted object wavefront has passed. The refractive index is the ratio of the speed of light (c) in a vacuum to the speed of light (ν) in a material, as given by N = c / ν. The time (t) required for the wave to pass through the width (d) of the object is t = d / ν. Therefore, the phase difference (Δθ) between when the wavefront enters and exits the material is

Where λ is the wavelength of the irradiated wavefront. Thus, it can be appreciated that the phase portion of the target wavefront includes information about the thickness and refractive index of the target.

  4A-4C show three specific measurements obtained using phase information reconstructed from transmission SHH. Of course, the present invention is not limited to the exemplary measurements depicted in FIGS.

  FIG. 4A shows a wavefront passing through an object at two different thickness positions. The equation at the bottom of FIG. 4A shows that this thickness difference can be calculated from the phase difference if the refractive indices of the object and the surrounding material are known. The surrounding material can be a material layer or a surrounding material (matrix material) that is at least partially translucent (eg, polymerized photoresist), partial vacuum (eg, ultra-high vacuum) or air. In particular, the present invention may include calculating a thickness difference (δ) between a first passing section of the object and a second passing section of the object, the calculation comprising:

It is. Δθ is the phase difference, lambda is the wavelength of the source of coherent light energy, N ₁ is the refractive index of the peripheral, N ₂ is the refractive index of the object.

  FIG. 4B shows one example that can be used to distinguish between two materials with the same thickness of phase change. The equation at the bottom of FIG. 4B makes it possible to calculate the expected phase difference of the two substances to help determine the expected contrast of the two substances in the phase image. In particular, the present invention may include calculating a phase difference (Δθ) between a first portion of the object and a second portion of the object, the calculation comprising:

It is. d is the thickness of both the first part of the object and the second part of the object, λ is the wavelength of the source of coherent light energy, and N ₂ is the refractive index of the first part of the object. N ₃ is the refractive index of the second part of the object.

FIG. 4C graphically shows that a sample of known thickness can be used to determine the refractive index of a material. The equation below FIG. 4C allows the refractive index to be calculated. In particular, the present invention may include calculating a refractive index (N ₂ ) that characterizes a portion of the object, the calculation comprising:

It is. Δθ is the phase difference, λ is the wavelength of the source of coherent light energy, and N ₁ is the peripheral refractive index.

  The present invention may include a method for obtaining a fast transmission spatial heterodyne interferometry measurement of an object. The object is at least partially translucent with respect to the planar cross section. The present invention may include an apparatus that provides for fast transmission spatial heterodyne interferometry of an object that is at least partially translucent to a planar cross section. This embodiment of the invention is described in detail as Example Set 1 below.

  The present invention relates to a method for obtaining a high-speed transmission measurement and a method for obtaining a high-speed reflection measurement using spatial heterodyne interferometry for the complete inspection / measurement of an object that is at least partially translucent to a planar cross section Can be combined. The present invention includes an apparatus that provides high-speed transmission and high-speed reflection measurements using spatial heterodyne interferometry to complete inspection / measurement of objects that are at least partially translucent to a planar cross-section. These measurements are optionally simultaneous. This embodiment of the invention is described in detail as Example Set 2 below.

  Specific embodiments of the present invention will now be further described by way of non-limiting examples showing various features in some detail below. The following examples are included to facilitate an understanding of how the invention can be practiced. It should be understood that the examples that follow represent embodiments that have been found to function properly in the practice of the present invention, and therefore can be considered to constitute the best mode in the practice of the present invention. However, it should be understood that numerous variations may be used in the disclosed exemplary embodiments, thereby achieving equivalent or equivalent results without departing from the spirit and scope of the invention. Accordingly, the examples should not be construed as limiting the scope of the invention.

(Example set 1)
The present invention includes a method of measuring a wavefront using spatial heterodyne interferometry (SHI) (in a single high-speed digital image capture) after the composite wavefront of electromagnetic waves has passed through at least partially opaque material. Measuring the composite wavefront provides the absorption and phase shift received by the electromagnetic wave as it passes through the target material. Transmitted spatial heterodyne interferometry (SHIFT) measurements use a small angle between the beam of interest and a reference to generate a set of linear fringes that cause a spatial heterodyne effect in the transmitted wavefront. The inventor understands the importance of measuring complex wavefronts transmitted through objects such as translucent materials, lithographic masks and biological samples. In contrast to traditional phase-shifting interferometry, transmissive spatial heterodyne interferometry (SHIFT) measurements enable faster reconstruction of the measured wavefront by capturing the entire composite wavefront of a single digital image To. SHIFT allows millions of phase measurements (decomposes a region into pixels) and reconstruction in a single high-speed digital image capture. SHIFT also measures the total composite wavefront that provides amplitude data as well as phase data. This reconfiguration can be done instantaneously on a desktop computer and may be much faster than if it were incorporated as a hardware algorithm implementation. In the biological microworld, many biological samples have low reflectivity, but their refractive index varies significantly between the main elements of the sample. When transmitted through these samples, multiple refractive indices cause different phase changes in the transmitted wavefront. Ideal for biological samples because of direct phase measurement by SHIFT. If the material with the object is translucent or transparent to the irradiation wavelength, the transmittance of the material is included in the transmitted wave intensity, and the combination of the thickness and refractive index of the material depends on the phase of the transmitted wave. included. Transmission-type spatial heterodyne interferometry (SHIFT) has been developed to capture the transmitted composite wavefront. The reference and object waves are combined at a small angle on the surface of the imaging device to form a complex wave spatial heterodyne hologram (SHH) image. This single image is processed using Fourier analysis Voelkl (1999) to reconstruct the phase and intensity of the composite wavefront.

  5A-5D depict some examples of the effect of material thickness and refractive index on the transmitted object wavefront, and thus some properties can be examined or measured by SHIFT. For clarity, the entire material is completely transparent so that the intensity of the transmitted image is the same as the irradiation intensity.

FIG. 5A shows a transformation from an irradiated wave 510 to a transmitted wave 512 through a single refractive index N ₁ object 514 having a surface topology 516. FIG. 5A shows that the transmitted wavefront provides a magnitude of thickness change. FIG. 5B shows a transformation from an irradiation wave 520 to a transmitted wave 522 through the layered object 524. The layered object 524 comprises two layers 526, 528 having the same width, the two layers having two different refractive indices N ₁ , N ₂ . FIG. 5B shows that a boundary between two materials that is not accessible to any surface scan measurement can be measured and / or inspected using the transmitted wavefront. FIG. 5C shows from the irradiated wave 530 (with an equation representing the general form of the transmitted wave) through the segmented object 534 to the transmitted wave 532 (with an equation representing the general form of the transmitted wave). The deformation of is shown. Its segmented object 534 includes one region 539 having two regions 536 and 538 and the refractive index _{N 2} having a refractive index _{N 1.} FIG. 5C shows that two materials with different refractive indices are evident in the phase image. FIG. 5D shows a deformation from the irradiation wave 540 to the transmitted wave 542 via the mixed object 544. The mixed object 544 includes a portion 546 having a _first refractive index N ₁ embedded in a matrix 538 having a _second refractive index N ₂ . FIG. 5D shows that the material change inside the imaged object is evident in a single transmission phase image. If this internal material change is a pure phase object, the material change does not appear in the intensity image. This is often seen in microscopic biological samples.

  6 and 7 show two basic optical designs for transmissive spatial heterodyne interferometry. Of course, the present invention is not limited to these two embodiments. Referring to FIG. 6, a first implementation of a transmission measurement system using spatial heterodyne interferometry is shown. Laser 610 is optically coupled to first beam splitter 620. The first beam splitter 620 is optically coupled to the first irradiation lens 625. The first illumination lens 625 is optically coupled to the object 630 under examination. The object 630 under examination is optically coupled to the first imaging lens 635. First imaging lens 635 is optically coupled to first mirror 640. First mirror 640 is optically coupled to second beam splitter 650. The first beam splitter 620 is also optically coupled to the second irradiation lens 626. Second illumination lens 626 is optically coupled to second imaging lens 636. Second imaging lens 636 is optically coupled to second mirror 641. Second mirror 641 is optically coupled to second beam splitter 650. The charge coupled device camera 660 is optically coupled to the second beam splitter 650. Note that the target side optics are reproduced at the reference side so that the wavefronts of the target side and the reference side match in the CCD.

  FIG. 6 shows a generic SHIFT arrangement in which the target beam and the reference beam match. The laser beam is divided into two by the first beam splitter. Both beams pass through the illumination lens. The object beam passes through the object under examination, while the reference beam continues to travel without encountering the object. In this situation, the object under examination is compared to free space, but the reference can be any object or substance that is compared to the object under examination. The imaging lens collects the object and reference beams and passes through them towards the CCD camera. A beam splitter located in front of the CCD combines the object and reference beams and gives a small angular difference between the two beams to form a spatial heterodyne fringe.

  With reference to FIG. 7, a second implementation of a transmission measurement system using spatial heterodyne interferometry is shown. Laser 710 is optically coupled to first beam splitter 720. The first beam splitter 720 is optically coupled to the object 730 under inspection. The object 730 under examination is optically coupled to the imaging lens 735. Imaging lens 735 is optically coupled to first mirror 740. First mirror 740 is optically coupled to relay lens 745. Relay lens 745 is optically coupled to second beam splitter 750. First beam splitter 720 is also optically coupled to second mirror 741. Second mirror 741 is optically coupled to illumination lens 746. The illumination lens 746 is optically coupled to the second beam splitter 750. The charge coupled device camera 760 is optically coupled to the second beam splitter 750. Note that in this implementation, a simplified optical path is used. Rather than matching the optics of the target side at the reference side, the illumination lens at the reference side is selected to match the wavefront with the target side at the CCD.

  FIG. 7 shows a simplified transmission arrangement. In that arrangement, the target and reference beams do not have matching optical paths, but an illumination lens at the reference side is used to match the target and reference beams in the CCD. Any discrepancy in these two wavefronts is constant. As long as this discrepancy is kept small so that the image can be included in the required region of the frequency domain, the discrepancy can be removed through image processing.

(Example set 2)
The present invention provides a method and apparatus for obtaining both reflected and transmitted composite wavefronts of electromagnetic waves incident on a translucent material for full material inspection / measurement using spatial heterodyne interferometry (SHI). Including. Also included is a method and apparatus for simultaneously measuring and combining a transmitted composite wavefront and a reflected composite wavefront. When electromagnetic waves (from a laser) are incident on a translucent target surface, some of the wave energy is transferred to the material and some is reflected at the material surface. A system capable of acquiring a composite wavefront of both transmitted and reflected waves can be used to analyze and inspect both composite wavefronts.

  Reflective and transmissive spatial heterodyne interferometry (SHIRT) measurements use small angle interference to form an interference pattern, but the method and apparatus uses a transmitted wavefront after passing through the object being imaged. The reflection wavefront has been modified to capture both. Capturing both wavefronts in two simultaneous digital images (or even one image) allows for faster surface and volumetric inspection of translucent objects. The combination and measurement of both transmission and reflection to fully characterize a translucent object is very valuable for inspection of lithographic masks where both (opaque) surface features and thickness or material variations are present Is. Another application is the inspection of biological samples where the information provided to the wavefront is only due to refractive index variations. Measurement of both the phase shift of the transmitted wave and the phase shift of the reflected wave (reflection occurs at each surface between two materials having different refractive indices) images and characterizes the characteristics of the sample Strengthen abilities.

  Since spatial frequency interference fringes are used to heterodyne the wave information, this is called “spatial-heterodyning”. The imaged composite wavefront is called a “hologram”, so the recorded wavefront is a “spatial heterodyne hologram” or SHH. If the imaged material is opaque or transmissive for the wavelengths it irradiates, the material transmission is included in the transmitted wave intensity, and the combination of material thickness and refractive index is included in the transmitted wave phase. The reflected wave contains not only information from reflections that occur in the material (at the boundary between materials of different refractive indices) but also information on the surface topology of the material. Thus, capturing both reflected and transmitted composite waves provides a more complete feature of the object being inspected and a method for capturing the composite wavefront of both reflected and transmitted waves And equipment is required. Reflective and transmissive spatial heterodyne interferometry (SHIRT) can simultaneously capture both reflected and transmitted composite wavefronts.

FIGS. 8A and 8B depict some examples of the effect of material surface characteristics, thickness and refractive index on the transmitted and reflected wavefronts from a transmitted object, and therefore some properties have been examined by SHIRT. Or can be measured. FIG. 8A shows the deformation from the irradiated wave 810 to the transmitted wave 512 and the deformation from the irradiated wave 810 to the reflected wave 814 through a single refractive index N ₁ object 816 having a surface topology 818. FIG. 8B shows the deformation from the irradiation wave 520 to the transmitted wave 822 and the deformation from the irradiation wave 520 to the reflected wave 824 through the layered object 825. Layered object 825 comprises two opaque surface features 826, 827 and two layers 828, 829 having the same width, the two layers having two different refractive indices N ₁ , N having _two.

  Once the transmitted and reflected signals are obtained for a translucent object, this method of using a more complete display of the object will vary from application to application. For example, in a biological sample, some parts of the sample may be well reflected, easy to see in the intensity of the reflected wavefront, and other parts may be the phase of the transmitted wavefront due to refractive index variations. Can look good on. By normalizing these two images and adding them together (addition, multiplication, etc.), a more detailed image of the sample can be made. For metrology of photolithographic masks, the reflected wavefront provides surface features, particularly height information for opaque (chrome) areas, while the transmitted image is a phase shift through the mask that is critical for high resolution masks. Provides direct measurement. If the phase shift on the mask is caused by surface etching and the mask material is known, the reflection also provides an indirect measurement of the phase shift produced by the mask. In this case, both transmission and reflection provide a measurement of phase shift, and this redundancy reduces measurement noise and provides improved reliability. For transparent film metrology, the reflected wavefront can be used to measure surface variation and deflection of the material, while transmission provides a measure of the thickness and / or refractive index of the material. Regardless of the measurement scenario, the combined method of reflection and transmission ensures a continuous method that ensures a series of depictions of the surface and interior features of the object under test.

  Figures 9 and 10 show two basic optical designs of reflective and transmissive spatial heterodyne interferometry. Of course, the present invention is not limited to these embodiments.

  Referring to FIG. 9, a first implementation of a transmission / reflection measurement system using spatial heterodyne interferometry is shown. Laser 910 is optically coupled to first beam splitter 915. The first beam splitter 915 is optically coupled with the second beam splitter 920. The second beam splitter 920 is optically coupled to the first irradiation lens 925. The first irradiation lens 925 is optically coupled to the third beam splitter 930. The third beam splitter 930 is optically coupled with the first imaging lens 935. The first imaging lens 935 is optically coupled to the object 940 under examination. Second beam splitter 920 is also optically coupled to second illumination lens 926. Second illumination lens 926 is optically coupled to fourth beam splitter 950. The fourth beam splitter 950 is optically coupled with the second imaging lens 955. Second imaging lens 955 is optically coupled to reference mirror 960. Fourth beam splitter 950 is also optically coupled to fifth beam splitter 970. The charge coupled device camera 980 is optically coupled to the fifth beam splitter 970. The first beam splitter 915 is also optically coupled to the third irradiation lens 927. Third illumination lens 927 is optically coupled to mirror 945. Mirror 945 is optically coupled to fourth illumination lens 928. The fourth irradiation lens 928 is optically coupled to the fifth irradiation lens 929. The fifth irradiation lens 929 is optically coupled to the sixth beam splitter 975. The object 940 under examination is optically coupled to the second imaging lens 936. Second imaging lens 936 is optically coupled to sixth beam splitter 975. Another charge coupled device camera 990 is optically coupled to the sixth beam splitter 975. Note that the optics of the target side are formed at the reference side so that the wavefronts of the target side and the reference side match in the CCD.

  FIG. 9 shows a typical SHIRT arrangement in which the target and reference beams are coincident. The first beam splitter 915 splits a part of the laser beam to generate a transmitted wave reference beam. The second beam splitter 920 splits a part of the laser beam to generate a reflected wave reference beam. The third beam splitter 930 directs the target irradiation beam toward the target through the imaging lens. The reflected portion of the irradiation wave becomes a reflected target beam and is transmitted to the CCD camera 980 via the third beam splitter 930 and the fifth beam splitter 970. The reflected wave reference beam is incident on the reference mirror via the fourth beam splitter 950 and is combined with the reflected target beam by the fifth beam splitter 970 with a small angular difference. This small angular difference is formed by the arrangement of the fifth beam splitter 970. The reflected object beam and reference beam interfere at the CCD camera 980 to form a reflected wave SHH. Part of the irradiation wave that passes through the object becomes a transmitted object beam, is collected by the second imaging lens, and is transmitted to the CCD camera 990 by the sixth beam splitter 975. The reference beam generated by the first beam splitter 915 is combined with the transmitted target wave by the sixth beam splitter 975 via the same optical path as the transmitted target beam. The arrangement of the sixth beam splitter 975 provides a small angle for these two beams so that SHH is generated when these two beams interfere at the CCD camera 990. If the reference object is not air or “vacuum”, the reference object can be inserted between two illumination lenses at the reference side.

  FIG. 10 shows a second implementation of a transmission / reflection measurement system using spatial heterodyne interferometry. Laser 1010 is optically coupled to first beam splitter 1020. First beam splitter 1020 is optically coupled to second beam splitter 1025. Second beam splitter 1025 is optically coupled to first illumination lens 1030. First irradiation lens 1030 is optically coupled to third beam splitter 1035. Third beam splitter 1035 is optically coupled to first imaging lens 1040. The first imaging lens 1040 is optically coupled to the object 1045 under examination. The object 1045 under examination is optically coupled to the second imaging lens 1050. Second beam splitter 1025 is optically coupled to first mirror 1055. First mirror 1055 is optically coupled to second illumination lens 1060. Second illumination lens 1060 is optically coupled to fourth beam splitter 1065. Fourth beam splitter 1065 is optically coupled to first charge coupled device camera 1070. The first beam splitter 1020 is also optically coupled to the third irradiation lens 1031. Third illumination lens 1031 is optically coupled to second mirror 1056. Second mirror 1056 is optically coupled to fourth illumination lens 1058. Fourth illumination lens 1058 and second imaging lens 1050 are both optically coupled to fifth beam splitter 1080. The fifth beam splitter 1080 is optically coupled to the second charge coupled device camera 1090. Note that in this implementation, a simplified optical path is used. The optical paths are not matched. Rather than matching the optics of the target side at the reference side, the illumination lens at the reference side is selected to match the wavefront with the target side at the CCD.

  FIG. 10 shows a simplified transmission arrangement in which illumination lenses 1031, 1058 and 1060 are used at the reference edge to match the beam wavefronts in CCD cameras 1070 and 1090, although the object and reference beams do not coincide. Indicates. Any discrepancy in the two wavefronts is constant. As long as this discrepancy is kept small so that the image can be included in the required region of the frequency domain, the discrepancy can be removed through image processing.

  Possible alternatives to the method and apparatus described above are different spatial heterodynes for each of the reflected and transmitted wavefront holograms defined by two different small angles between two corresponding sets of object and reference beams. Recording at frequency is to record both reflected wavefront and transmitted wavefront holograms on one CCD image. The optical system can be designed to allow the reflected wavefront and transmitted wavefront to be carried to the same CCD. Furthermore, each of these reference beams can be carried to the CCD so that the angle of incidence that can be made between the respective target beam and the reference beam can be varied. For example, two beam splitters that function as a combiner may be coupled to another additional beam splitter that routes the two beam sets to the same CCD. The two spatial heterodyne holograms may then be recorded simultaneously on the same CCD at different spatial heterodyne frequencies. The reflected reference beam and the reflected beam defining the angle between the reflected reference beam and the reflected beam are not coherent with the transmitted reference beam and the transmitted beam defining the angle between the transmitted and transmitted beam. Is useful. In any case, this single CCD alternative has the advantage of greatly reducing the computational requirements for analyzing two holograms. Techniques for recording multiple spatial heterodyne holograms in one digital image are described in more detail in US application Ser. Nos. 10 / 421,444, 10 / 607,824 and / or 10 / 607,840. Is done.

(Practical application of the present invention)
Practical applications of transmissive spatial heterodyne interferometry (SHIFT) include photolithographic mask metrology and inspection, biological microscopy inspection, refractive index measurement, and transmitted object thickness measurement. Practical applications of reflective and transmissive spatial heterodyne interferometry (SHIRT) include metrology and inspection of photolithographic masks, biological microscopic inspection, and refractive index, thickness and surface characteristics of transmitted objects. Includes simultaneous measurements. There are substantially many uses of the present invention, all of which need not be detailed here.

(Usefulness of the present invention)
Transmission-type spatial heterodyne interferometry (SHIFT) representing embodiments of the present invention can be useful and economical for at least the reasons described below. Transmission-type spatial heterodyne interferometry enables the capture of a composite wavefront that has passed through an object. Transmission-type spatial heterodyne interferometry allows for faster acquisition of composite transmitted wavefronts due to the fact that only a single image is required. Transmission-type spatial heterodyne interferometry enables measurement of phase changes (inhomogeneities, mixtures, material changes, etc.) through the material.

  Reflective and transmissive spatial heterodyne interferometry (SHIRT) representing embodiments of the present invention can be useful and economical for at least the reasons described below. Reflective and transmissive spatial heterodyne interferometry allows simultaneous capture of a composite wavefront transmitted through an object and a composite wavefront reflected at the surface of the object. The present invention improves quality and / or reduces costs compared to previous approaches.

  The term “a” or “an” as used herein is defined as one or more. As used herein, the term “plurality” is defined as two or more. The term “other” as used herein is defined as at least a second or more. As used herein, the terms “comprising” (comprising), “including” (including), and / or “having” (having Open language (i.e., those that require what is listed after the word, but are not specified, include procedures, configurations, and / or components, even if it is a major part) ) Is defined as having no restriction on the inclusion of. As used herein, “consisting” (constructing) and / or “composing” (assembling, assembling) is a method, apparatus or The configuration is closed to the generally described procedures, configurations, and / or inclusions of procedures, configurations, and / or components excluding appendages, accessories, and / or impurities associated with the components. The term “essentially” used in conjunction with the terms “constructing” or “assembling” means that the described method, apparatus, and / or component is a fundamental new feature of the configuration. It is not restricted to unspecified procedures, configurations, and / or components that do not significantly affect. The term coupled as used herein is defined as being connected, but need not be directly or mechanically connected. The term about as used herein is defined as being at least close to a predetermined value (eg, preferably within 10%, more preferably within 1%, even more preferably within 0.1%). The term substantially used herein is defined as large but not necessarily all that it has been specified. The term generally used herein is defined as being at least close to a predetermined state. The term deploying as used herein is defined as designing, building, shipping, installing, and / or working. The term means used herein is defined as hardware, firmware, and / or software for achieving the results. The term program or computer program as used herein is defined as a series of instructions designed for enforcement in a computer system. A program or computer program is a subroutine, function, procedure, purpose method, purpose implementation, enforceable application, applet, servlet, source code, purpose code, shared library / dynamic load designed for enforcement in a computer or computer system Defined as a library and / or other series of instructions.

  All disclosed embodiments of the invention disclosed herein can be made and used without undue experimentation in light of the present disclosure. The present invention is not limited by the logical statements described herein. Although performing the best mode of the present invention contemplated by the inventor is disclosed, the utility of the present invention is not so limited. Accordingly, those skilled in the art will appreciate that the invention is not limited to what has been specifically described herein, but is otherwise practical.

  It will be apparent that various changes, modifications, additions and / or rearrangements of the features of the present invention may be made without departing from the spirit and / or scope of the basic inventive idea. The spirit and / or scope of the basic inventive idea defined by the appended claims and their equivalents shall cover all such alternatives, modifications, additions and / or rearrangements. To be judged. All disclosed elements and features of each disclosed embodiment may be combined or substituted for, unless the disclosed elements or features in all other disclosed embodiments conflict with each other. It may be used as Changes can be made in the steps or series of steps that make up the methods described herein.

  The interferometer described herein may be a separate module, but it will be apparent that the interferometer may be integrated into the associated system (such as a photolithographic inspector). Each component need not form the disclosed form or be combined in the disclosed configuration, and can be provided in virtually any form, and / or combined in substantially all configurations. Can be done.

  The appended claims are not to be construed as including means plus function regulation, unless expressly recited in a given claim with the expression “means for” and / or “step for” . Somewhat general embodiments of the invention are set out in the accompanying independent claims and their equivalents. Particular embodiments of the present invention are distinguished by the appended dependent claims and their equivalents.

(References)
E. Voelkl et al., "Introduction to Electron Holography", Kluwer Academics / Plenum Publishers 1999, 133ff.
J. et al. Jacob, T .; Litvin and A.M. Merria High-Resolution Phototransmission Transmission and Phase Measurement Tool, Inspection, and Process Control for MicroLithography XVI, SPIR Conference, and Ms.
C. E. Thomas et al., "Direct to Digital Holography for Semiconductor Wafer Detect Detection and Review", SPIE Bulletin Vol. 4692, pages 180-194, 2002, promulgated on June 20, 2000, US Pat. No. 6,078,392, Clearance E., et al. Thomas, Larry R .; Baylor, Gregory R .; Hanson, David A.M. Rasmussen, Edgar Voelkl, James Castracane, Michele Sumkulet and Lawrence Clow, "Direct-to-Digital Holography, Holographic Interferometry, and Holovis"
February 25, 2003, U.S. Pat. No. 6,525,821, Clarence E. et al. Thomas and Gregory R.M. “Acquisition and Replay Systems for Direct-to-Digital Holography and Holography” by Hanson

1A-1B illustrate one embodiment of the present invention, which is an example of an intensity hologram formed in a CCD (Charge Coupled Device) sensor from a chrome-on-glass target, and FIG. It is an expansion area | region showing the linear sine interference fringe pattern changed with the characteristic. 2A-2B illustrate one embodiment of the present invention, FIG. 2A illustrates the magnitude of the full frequency spectrum of the hologram, and FIG. 2B illustrates a low-pass filtered sideband located in the center of the hologram. 3A and 3B illustrate one embodiment of the present invention, FIG. 3A illustrates the resulting amplitude of a portion of the chrome-on-glass target, and FIG. 3B illustrates the phase reconstruction of a portion of the chrome-on-glass target. . 4A-4C show schematics of three transmissive examples illustrating one embodiment of the present invention, FIG. 4A shows thickness calculations when given refractive index, and FIG. 4B shows different thicknesses of the same thickness. The phase difference between materials is shown, and FIG. 4C shows the technique for calculating the refractive index of a material of known thickness. 5A-5D show schematic diagrams of transformations from an illuminating wave to a transmitted wave affected by four different objects, illustrating an embodiment of the present invention. FIG. 6 shows a schematic diagram of a first basic optical design of transmissive spatial heterodyne interferometry, illustrating one embodiment of the present invention. FIG. 7 shows a schematic diagram of a second basic optical design of transmissive spatial heterodyne interferometry, illustrating one embodiment of the present invention. 8A-8B illustrate an embodiment of the present invention and are schematic diagrams of transformations from an illuminating wave to both transmitted and reflected waves of two different objects. FIG. 9 shows a schematic diagram of a first basic optical design of transmissive and reflective spatial heterodyne interferometry, illustrating an embodiment of the present invention. FIG. 10 shows a schematic diagram of a second basic optical design of transmissive and reflective spatial heterodyne interferometry illustrating an embodiment of the present invention.

Claims (26)

Digitally recording a first spatially heterodyne hologram including spatial heterodyne fringes for Fourier analysis using a first reference beam and a first target beam;
Digitally recording a second spatially heterodyned hologram including spatial heterodyne interference fringes for Fourier analysis using a second reference beam and a second target beam;
The digital over the first spatial telodyne carrier frequency defined by a first angle between the first reference beam and the first target beam to define a first analyzed image. The digitally recorded first spatially heterodyned by moving the first original origin of the hologram to overlap the original origin of the recorded first spatially heterodyned hologram Fourier analysis of the generated hologram,
In order to define a second analyzed image, the digital above the second spatial telodyne carrier frequency defined by a second angle between the second reference beam and the second target beam The digitally recorded second spatially heterodyned by moving the second original origin of the hologram to overlap the original origin of the recorded second spatially heterodyned hologram Fourier analysis of the generated hologram,
Digitally filtering the first analyzed image to decouple the signal around the first original origin and define a first result;
Digitally filtering the second analyzed image to decouple the signal around the second original origin and define a second result;
Subjecting the first result to a first inverse Fourier transform;
Applying a second inverse Fourier transform to the second result, comprising:
The method wherein the first object beam is transmitted through an object (940, 1045) that is at least partially translucent and the second object beam is reflected at the object.   The method of claim 1, wherein a first digital image comprises the first spatially heterodyned hologram and a second digital image comprises the second spatially heterodyned hologram.   The method of claim 1, wherein the first digital image is generated by a first pixelated detection device and the second digital image is generated by a second pixelated detection device.   The first angle is not equal to the second angle, and a single digital image is both the first spatially heterodyned hologram and the second spatially heterodyned hologram The method of claim 1 comprising:   Both the first spatially heterodyned hologram and the second spatially heterodyned hologram are digitally recorded by a single pixelated detection device, and the single digital image is The method of claim 4, wherein the method is generated by a pixelated detection device.   6. The method of claim 5, wherein the first reference beam and the second target beam are not coherent with respect to the second reference beam and the second target beam.   7. The spatial heterodyne interference pattern of the first spatially heterodyne hologram is substantially orthogonal to the spatial heterodyne interference pattern of the second spatially heterodyne hologram. The method described in 1. The thickness difference (δ) between the first passing section of the object and the second passing section of the object is
Where Δθ is the phase difference, λ is the wavelength of the coherent light energy source, N ₁ is the refractive index of the periphery, and N ₂ is the refractive index of the object. The method of claim 1 further comprising. The phase difference (Δθ) between the first part of the object and the second part of the object
(Where d is the thickness of both the first portion of the object and the second portion of the object, λ is the wavelength of the coherent optical energy source, and N ₂ is the object. 2. The method of claim 1, further comprising: calculating the refractive index of the first portion of the object and N ₃ is the refractive index of the second portion of the object. Refractive index (N ₂ ) characterizing a part of the object
The method of claim 1, further comprising calculating as: where Δθ is the phase difference, λ is the wavelength of the coherent optical energy source, and N ₁ is the peripheral refractive index.   An axis defined by the first target beam after digitally recording the first spatially heterodyned hologram and the second spatially heterodyned hologram; and the second target The method of claim 1, further comprising moving the object in a plane that is substantially perpendicular to both axes defined by the beam.   The method further comprises digitally recording both a third spatially heterodyned hologram and a fourth spatially heterodyned hologram after moving the object in the plane. 11. The method according to 11.   The method of claim 1, wherein the reference beam and the target beam are generated by a laser operating in a pulsed mode.   A photolithographic mask inspection process comprising the method of claim 1.   A measurement process comprising the method according to claim 1. A coherent light energy source (910, 1010);
A transmitted reference beam subassembly optically coupled to the coherent light source;
A reflected reference beam subassembly optically coupled to the coherent light source;
A target beam subassembly optically coupled to the coherent light source, the target beam subassembly including a transmitted target beam path and a reflected target beam path;
A transmitted beam splitter (975, 1080) optically coupled to both the transmitted reference beam subassembly and the target beam subassembly;
A reflective beam splitter (970, 1065) optically coupled to both the reflected reference beam subassembly and the target beam subassembly;
A pixelated detection device (980, 990, 1070, 1090) optically coupled to at least one element selected from the group consisting of the transmission beam splitter and the reflection beam splitter,
The object beam subassembly includes an object (940, 1045) that is at least partially translucent; i) the object is transparently transmitted between the coherent optical energy source and the transmitted beam splitter; An apparatus that is optically coupled and ii) the object is reflectively coupled optically between the coherent light energy source and the reflective beam splitter. i) another transmission beam splitter coupled between the coherent optical energy source and the transmitted reference beam subassembly; ii) another transmission beam splitter coupled between the coherent optical energy source and the target beam subassembly;
ii) another reflective beam splitter coupled between the coherent optical energy source and the reflected reference beam subassembly, and ii) another reflective beam splitter coupled between the coherent optical energy source and the target beam subassembly. The apparatus of claim 16 further comprising:   The apparatus of claim 16, wherein the transmitted reference beam subassembly includes an illumination lens.   The apparatus of claim 16, wherein the reflected reference beam subassembly includes an illumination lens.   The apparatus of claim 16, wherein the reflected reference beam subassembly includes a reference mirror.   The apparatus of claim 16, wherein the target beam subassembly includes an imaging lens.   The apparatus of claim 16, wherein the target beam subassembly includes an illumination lens.   The apparatus of claim 16, further comprising another pixelated detection device optically coupled to at least one element selected from the group consisting of the transmission beam splitter and the reflection beam splitter.   The apparatus of claim 16, wherein the coherent optical energy source comprises a laser operating in a pulsed mode.   A photolithographic mask inspection apparatus comprising the apparatus according to claim 16.   A measuring instrument comprising the apparatus according to claim 16.