METHOD AND SYSTEM OF OPTICAL NANOSCALE ALIGNMENT

Abstract

A method and a system of optical nanoscale alignment, a sample support comprises a first holder for a first chip and a second holder for a second chip; the first and second chips are positioned in a facing relationship on a first and a second surface of the sample respectively, and a dual light source casts a coherent light beam on the sample from a first surface and an incoherent beam on the sample from a second, opposite, surface thereof; a positioning unit controls the movement of the first chip and controls the movement of the second chip according to isotropic interference fringes generated by the dual light in reflection mode in images captured by an imaging unit, indicating of pitch and yaw angle, light from the dual light source in transmission mode providing bright field illumination while x, y, and rotation alignments are performed.

Description

FIELD OF THE INVENTION

The present invention relates to optical nanoscale alignment. More specifically, the present disclosure is concerned with a method and system of Moiré fringes based optical nanoscale alignment.

BACKGROUND OF THE INVENTION

A Fresnel zone plate (FZP) is a diffractive optical element comprising concentric rings, referred to as Fresnel zones, alternating high-Z materials (absorbing material), such as gold for example, and low-Z materials (transparent to X-rays), such as Si₃N₄ for example. Light hitting the zone plate diffracts around the opaque zones. The zones can be spaced so that the diffracted light constructively interferes at the desired focus, creating an image there.

FZPs are used as X-ray beam focusing or magnifying optical devices, for X-ray microscopy, X-ray holography, X-ray spectroscopy, and astronomy. High diffraction efficiency is required to minimize exposure times and hence the radiation load on the specimens under study. Increasing the thickness of the FZP grating structures, between about 0.7 and about 0.8 μm for example, significantly improves the peak diffraction efficiency and the corresponding photon energy level, in particular in the high-energy region, typically above 20 keV, due to the high penetration of hard X-ray photons. Meanwhile to maintain a high resolution of the FZP, the width of the outermost zone width is limited Currently, due to fabrication limitations of aspect ratios, high-resolution zone plates only have a peak diffraction efficiency of about 10% in the hard X-ray range. When stacking two or more FZPs to increase the thickness, an accuracy of below ⅓ outermost zone width is required in the alignment process, in order to maintain the outer zone accuracy of the stacked FZPs. A high-precision alignment system to produce stacked FZP is thus needed.

The near-field stacking technique was developed in 2007 under the X-ray imaging facilities, using the Moiré fringes method for alignment in lithography. Stacking was performed with the European Synchrotron Radiation Facility (ESRF) BM05 beamline. Alignment of the two FZPs was achieved by X-ray phase contrast imaging by looking at the high-resolution CCD camera image of the beam. Two sets of silicon chips were stacked and glued for focusing 15-KeV and 50-keV X-rays. To achieve the spatial resolution of 50 nm in the energy range 6-15 keV, longitudinal proximity in the order of 20 μm was required. Intermediate-field stacking technique, later implemented in 2014, was shown to allow stacking up to five FZPs at 10, 11.8, and 25 keV energies.

Attempts were made to stack FZPs under an optical system using laser illumination. While under X-ray, the chips with different periods of grating patterns can be put within the near field region of the fine grating, under visible light illumination, the second chip is placed at integer multiples of half the Talbot distance to minimize the loss of high-frequency information. Microbeads between the zone plates determine the gap between them to align the lateral displacement with a resolution of less than 5 nm. The alignment is done by moving the sample with manual control, showing promising possibilities for stacking two high-resolution zone plates.

Other methods to improve the diffraction efficiency of high-resolution zone plates at high energies besides the stacking technique include the sputtered-sliced FZP method and direct patterning of two identically aligned FZPs on double sides of a membrane. The sputtered-sliced Fresnel zone plate method creates a Fresnel zone plate with a small aperture, which limits its applicability. Directly patterning two identically aligned Fresnel zone plates has high technical restrictions and does not offer the possibility of increasing the thickness three or more times.

Still, when stacking zone plates, alignment remains a challenge. X-ray alignment facilities such as the ESRF BM05 beamline are accessible for leasing at the European Synchrotron Radiation Facility providing a training period. For industrial production, the price of purchasing an X-ray microscope would be over $200,000 USD, not to mention the difficulty and danger of modifying this equipment.

A method and system for stacking Fresnel zone plates (FZPs) under an optical system, accessible to a general company or a research institution is still needed. Firstly, available systems do not consider rotation misalignment. Second, microbeads are used to provide parallelism in the stack and a tedious and unstable preparation is required. Third, the alignment process is controlled manually, which compromises the operation sensitivity and the alignment accuracy. Finally, the stacked Fresnel zone plates (FZPs) are not glued, which is inconvenient for further operation.

There is still a need in the art for a method and system of optical nanoscale alignment.

SUMMARY OF THE INVENTION

More specifically, in accordance with the present invention, there is provided a system of optical nanoscale alignment, comprising a sample support comprising a first holder for a first chip, and a second holder for a second chip; the first and second chips being positioned in the respective first and second chip holders in a facing relationship on the first and second surfaces of the sample respectively, a dual light source casting a coherent light beam on a sample from a first surface and an incoherent beam on the sample from a second, opposite, surface thereof; a positioning unit controlling a movement of the first chip and controlling a movement of the second chip according to isotropic interference fringes generated by a reflection mode of the dual light source in images captured by an imaging unit, indicating of pitch and yaw angle, light from the dual light source in transmission mode providing bright field illumination while x, y, and rotation alignments are performed.

There is further provided a method of optical nanoscale alignment, comprising mounting a first chip supported by a first holder and a second chip supported by a second holder in a facing relationship on a first and a second surfaces of a sample respectively; providing a positioning unit controlling movements of the first and of the second chips respectively; illuminating the first surface of the sample using a first light beam and the second surface of the sample with a second light beam of a dual light source; capturing images of the first and second chips using an imaging unit; monitoring tilt and yaw of the chips to a misalignment of at most 0.3 mrad; monitoring x and y directions and rotation of the chips to a misalignment of ±0.5 μm in and of ±1.25 mrad respectfully; and monitoring the x and y directions and rotation of the chips to a misalignment of an outermost ring of the chips, wherein the movements of the first chip and of the second chip are controlled by the positioning unit according to isotropic interference fringes generated by a reflection mode of the dual light source in the images captured by the imaging unit, indicating of pitch and yaw angles, light from the dual light source in transmission mode providing bright field illumination while x, y, and rotation alignments are performed.

Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended drawings:

FIG. 1A is a schematic view of a system according to an embodiment of an aspect of the present disclosure;

FIG. 1B is a schematical view of chips configuration according to an embodiment of an aspect of the present disclosure;

FIG. 1C shows a top chip holder according to an embodiment of an aspect of the present disclosure;

FIG. 1D is shows a bottom chip holder according to an embodiment of an aspect of the present disclosure;

FIG. 1E is a perspective view of a plate positioning the top and bottom chips in the nano-positioning stage of FIG. 1A according to an embodiment of an aspect of the present disclosure;

FIG. 2A shows top chip configuration according to an embodiment of an aspect of the present disclosure;

FIG. 2B shows bottom chip configuration according to an embodiment of an aspect of the present disclosure;

FIG. 3A shows images of parallelism under laser illumination according to an embodiment of an aspect of the present disclosure;

FIG. 3B shows misaligned chips according to an embodiment of an aspect of the present disclosure;

FIG. 3C shows aligned chips according to an embodiment of an aspect of the present disclosure;

FIG. 4A is a side-view schematic; FIG. 4B is a top-view schematic of the designed patterns on the top FZP chip and FIG. 4C is a top-view schematic of the designed patterns on the bottom FZP chip, with P_1i, P_2i, P_1o, P_2o, periods of gratings in the inner and outer areas; FIG. 4D shows scanning electron microscopy images of a fabricated bottom FZP chip with magnification ratios of 86× (top view) and two local areas marked by the blur and oranges boxes in the top image with the magnification ratio of 377× (bottom left view) and 383× (bottom right view; FIG. 4E shows optical microscopy images of a local area of the fabricated grating on the FZP chips in reflection mode (left) and transmission mode (right);

FIG. 5A is a system schematic view, with. L1-L6, lenses; M1-M7, mirrors; FIG. 5B shows the holders for the top chip; FIG. 5C shows the holders for the bottom chip; FIG. 5D is a side view of the chip holding configuration; FIG. 5E shows the loaded top chip; FIG. 5F shows the loaded bottom chip;

FIG. 6A shows images of the top (left handside) and bottom (right handside) chips under incoherent illumination; FIG. 6B shows an image of an edge (left panel) image of an edge; averaged edge spread function, with error bar: standard deviation (middle panel); line spread function with Gaussian interpolation; with FWHM, full width at half maximum (right panel);

FIG. 7A shows interference fringes produced by misalignment in both tilt and yaw; FIG. 7B shows interference fringes produced by misalignment only in yaw; FIG. 7C shows interference pattern after completing the tilt-yaw alignment;

FIG. 8A is a representative image of two stacked FZP chips during the alignment, with l_i-l_viii:line profiles of the crosses and anti-crossesl₁-l₄:line profiles of the main scales; l₅-l₈:line profiles of the Vernier scales; FIG. 8B shows analysis of line profiles l_i-l_viii; FIG. 8C shows analysis of spatial differentiation of the line profiles l₁-l₈ in pairs;

FIG. 9A shows two stacked chips during fine alignment, with insets: zoom-in views of two cropped grating areas A_oa and A_ob in the left region showing moiré fringes; FIG. 9B shows line profiles used in the fine alignment analysis for the selected areas in FIG. 9A, with l_{oa_ave} and l_{ob_ave}, averaged line profiles; l_{oa_fil} and l_{ob_fil}, line profiles by low-pass-filtering l_{oa_ave} and l_{ob_ave}; l_{oa_fit} and l_{ob_fit}, line profiles by using sinusoidally fitting of l_{oa_fil} and l_{ob_fil} without the DC term; FIG. 9C shows Fourier spectra of l_{oa_fil} and l_{ob_fil} with the employed low-pass filter;

FIG. 10A shows two aligned chip images; A_ia and A_ib indicating the selected regions from the inner area for moiré fringe analysis; FIG. 10B shows cross-sections of the corner crosses; FIG. 10C shows spatial differentiation of the lines of main scales and the Vernier scales marked in FIG. 10A; FIG. 10D shows fringes extractions of the inner and outer areas. l_{ia_fil}, l_{ib_fil}, l_{ia_fit}, and l_{ib_fit} have the same meaning as l_{oa_fil}, l_{ob_fil}, l_{oa_fit}, and l_{ob_fit} in FIG. 6 but are for selected regions from the inner area;

FIG. 11 is a table of specifications of the grating pairs designed for the FZP chips (j=i or o);

FIG. 12 is a table of experimentally determined mismatch in each region using moiré-fringe analysis;

FIG. 13 is a table of calculated misalignments of x, y, and rotation;

FIG. 14A shows the graphic user interface developed in Matlab;

FIG. 14B shows convergence of misalignments in x, y, and rotation in an iterative alignment process; and

FIG. 15 shows the sensing capability of moiré fringes to 15-nm-step movement, with error bar: standard deviation.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention is illustrated in further detail by the following non-limiting examples.

A system according to an embodiment of an aspect of the present disclosure as illustrated in FIG. 1A comprises a dual light source, positioning stages and a sample support. L1-L5 are lenses and M1-M7 are mirrors.

Experiments 350-900 nm with a dual light source comprising a 532-nm continuous-wave laser 12 (CNI Laser, green-532-200 mw) working in reflection mode illumination and an orange-color visible mounted light emitting diode LED 14 (Thorlabs, M617L5) working in transmission mode. The green laser beam from the laser 12 is expanded and reflected by a pellicle beam splitter 16 (Thorlabs, CM1-BP145B1), collected by a 10× long working distance Mitutoyo objective lens 18 (Thorlabs, MY10X-803), and then cast on the sample O from the top, as the beam from the light emitting diode LED 14 is collimated and penetrates the sample O from the bottom; the two beams are modulated by the sample O, then collected by the objective lens 18 and the beam splitter 16, and captured by a high-resolution monochromatic camera 20 (FLIR, BFS-U3-200S6M-C).

The-positioning stages as illustrated comprise a 6-axis nano-positioning stage 32 (Thorlabs, MAX603D) selected for controlling the motion of a bottom chip 40 and a micro-positioning stage 34 selected for controlling the motion of a top chip 42 (see FIG. 1B); the top and bottom chips 42, 40 are positioned in a respective top chip holder 43 and bottom chip holder 41 (shown FIG. 1C and FIG. 1D respectively) and then in a facing relationship. A plate 50, shown in FIG. 1E, comprising an aperture 52 is positioned with the bottom chip 40 thereon in the nano-positioning stage 32 (see the circled horizontal plane in FIG. 1A). The micro-positioning stage 34 as illustrated comprises a four-axis rotation mount (Thorlabs, KS1RS) connected to XYZ translation stages (Keenso, SEMX80-AS) on a 95 mm rail 35 (Thorlabs, XT95SP-500) selected and configured to move the top chip holder 43 (FIG. 1C).

The sample support formed of the top and bottom chip holders 43, 41 was machined precisely according to the size of the top chip 42 (4×4 mm²) and the size of the bottom chip 40 (5×5 mm²), in aluminum using a 3D printer for example. The top chip 42 was glued to a bottom surface of the top chip holder 43 using double-sided adhesive 45, and the bottom chip 40 was glued to a top surface of the bottom chip holder 41 using low-viscosity double-sided adhesive 46, such as UV glue for example (see FIG. 1B). The adhesives to glue the FZP chips together are selected so as not to interfere with the beams on the chips plane (see the circled horizontal plane in FIG. 1B), and to maintain the system's numerical aperture to clearly image each grating used in the FZP chips, of pitches of about 2 μm and with the objective lens of numerical aperture of about 0.3.

FIG. 2A shows the top and bottom Fresnel zone plate (FZP) chip patterns according to an embodiment of an aspect of the present disclosure. Two FZPs (one marked in FIG. 2B), with a diameter of 310 μm and outermost zone width of 100 nm are placed in the center of the top and bottom chip patterns (see FIG. 2A). On the free space around the FZPs, there are two grating areas namely inner area and outer areas (both pointed out in FIG. 2A). Two sets of diffraction gratings with 2.42 and 2.58 μm period were fabricated aside in each four direction of the inner area, and two sets of diffraction gratings with 2.46 and 2.54 μm period were fabricated aside in each four directions of the outer area, and used as reference to minimize calculation error. In each direction of each area, the periods of two sets of grating on the top and bottom chips switched. The superimposed gratings with different periods at the distance of 10 μm, as at the bottom grating's Talbot distance, generate Moiré fringe patterns of 39 μm in the inner area, and of 78 μm in the outer area (see FIG. 3B). Before alignment, laser interferometry is performed to make the two chips parallel in pitch and yaw directions (see FIG. 3A). Alignment markers, indicated by corner crosses in FIG. 2A and Vernier scales indicated in FIG. 2B are used for linear misalignment under 0.5 μm and rotation misalignment under 0.5 μrad. For further alignment, linear and rotation misalignment of the two center FZPs on the top and bottom chips is obtained by calculating the relative displacement of two opposite directionally shifted moiré patterns in each direction. An image of misaligned chips is shown in FIG. 3B, and an image of aligned chips is shown in FIG. 3C. Moiré fringes are used to achieve an accuracy of about 30 nm in alignment between the two FZP, allowing maintaining ⅓ of FZP resolution (as outermost zone width) of 100 nm. Moiré fringes are used to achieve an accuracy of about 30 nm in alignment between the two FZP, allowing maintaining ⅓ of FZP resolution (as outermost zone width) of 100 nm. The method and the system, using a dual light source optical imaging, mechanical nano-positioning motion control, and precise support of the sample, thus shown to achieve high-precision alignment of the two chips, may be used to produce stacked FZP.

In the system, the laser 12 in the range between about 350 nm and about 900 nm, or a superluminescent diode or a laser diode for example, is selected to provide coherent illumination for laser interferometry and the light-emitting diode 14, or a superluminescent diode for example, is selected in the range between about 350 nm and about 900 nm to provide incoherent illumination for moiré-fringe generation, as the light source. The nano-positioning stage 32 is selected with x,y,z movement of at most 10 nm; and yaw, tilt and rotation movement of at most 1 μrad and the micro-positioning stage 34 is selected with x,y,z movement of at most 25 nm; and yaw, tilt and rotation movement of at most 0.1 degrees, to move the FZP chip in 6 degrees of freedom. The microscope with a long working-distance objective lens 18 selected with a spatial resolution of about 1 μm is used to image the FZP chips. Alignment markers on the FZP chips, with cross/anti-cross in the range between about 5 μm and about 15 and vernier/main scale in the range between about 10 μm and about 20, and gratings of a pitch in the range between about 1 μm and about 5 μm are used to guide the alignment process, as shown in FIG. 2. The images of the FZP chips are analysed for alignment using a computer software and resulting signals are transmitted to move the nanopositioning stage 32.

By integrating the dual light source into the system, parallelism may be achieved by using isotropic interference fringes generated by the reflection mode for pitch and yaw angle adjustment. The transmission mode LED provides bright field illumination while the x, y, and rotation alignments are performed.

The method comprises tilt-yaw alignment for adjusting and aligning the tilt and yaw of the FZP chips with tilt and yaw misalignments less than 0.3 mrad; coarse alignment for x, y, and rotation to adjust and align the x, y, and rotation of the FZP chip with misalignments in x and y within ±0.5 μm. The rotation misalignment within ±1.25 mrad, fine alignment for x, y, and rotation to adjust and align the x, y, and rotation of the FZP chip to misalignment reduced to ⅓ of the outmost ring width of the FZP chips (30 nm in the example illustrated herein.

The scope of the claims should not be limited by the embodiments set forth in the examples but should be given the broadest interpretation consistent with the description as a whole.

Claims

1. A system of optical nanoscale alignment, comprising: a dual light source;a positioning unit;a sample support;a positioning unit; andan imaging unit;wherein:the dual light source comprises a coherent light source and an incoherent light source; a first beam from the coherent light source being cast on the sample from a first surface thereof, a second beam from the incoherent light source being cast on the sample from a second, opposite, surface thereof;the sample support comprises a first holder for a first chip, and a second holder for a second chip; the first and second chips being positioned in the respective first and second chip holders in a facing relationship on the first and second surfaces of the sample respectively;the imaging unit captures images of the first and second chips;the positioning unit comprises a micro-positioning stage controlling a movement of the first chip and a nano-positioning positioning stage controlling a movement of the second chip according to isotropic interference fringes generated by a reflection mode of the dual light source in images captured by the imaging unit, indicating of pitch and yaw angle, light from the dual light source in transmission mode providing bright field illumination while x, y, and rotation alignments are performed.

2. The system of claim 1, wherein the coherent light source is one of a laser, a superluminescent diode and a laser diode; and the incoherent light source is one of a light-emitting diode and a laser diode.

3. The system of claim 1, wherein the first and second beams are in a range between 350 nm and 900 nm.

4. The system of claim 1, wherein the nano-positioning stage is selected for controlling x,y,z movements of at most 10 nm and yaw, tilt and rotation movements of at most 1 μrad of the second chip; and the micro-positioning stage is selected for controlling x,y,z movements of at most 25 nm and yaw, tilt and rotation movements of at most 0.1 degrees of the second chip.

5. The system of claim 1, comprising a computer, wherein the computer determines a misalignment of the first and of the second chips in the images of the first and of the second chips and sends driving signals to the positioning stage to operate the positioning unit until a target alignment of the first and second chips.

6. The system of claim 1, wherein the imaging unit comprises a microscope and a long working-distance objective lens.

7. The system of claim 1, wherein the imaging unit comprises a microscope and a long working-distance objective lens, and has a spatial resolution of about 1 μm.

8. The system of claim 1, wherein the chips comprises alignment markers.

9. The system of claim 1, wherein the chips comprises alignment markers, with ones of: crosses and anti-crosses in a range between 5 μm and 15 μm, vernier/main scale between 10μm and 20 μm, and gratings with pitch between 1 μm and 5 μm.

10. A method of optical nanoscale alignment, comprising: mounting a first chip supported by a first holder and a second chip supported by a second holder in a facing relationship on a first and a second surfaces of a sample respectively;providing a positioning unit controlling movements of the first and of the second chips respectively;illuminating the first surface of the sample using a first light beam and the second surface of the sample with a second light beam of a dual light source;capturing images of the first and second chips using an imaging unit;monitoring tilt and yaw of the chips to a misalignment of at most 0.3 mrad;monitoring x and y directions and rotation of the chips to a misalignment of ±0.5 μm in and of ±1.25 mrad respectfully; andmonitoring the x and y directions and rotation of the chips to a misalignment of an outermost ring of the chips.wherein the movements of the first chip and of the second chip are controlled by the positioning unit according to isotropic interference fringes generated by a reflection mode of the dual light source in the images captured by the imaging unit, indicating of pitch and yaw angles, light from the dual light source in transmission mode providing bright field illumination while x, y, and rotation alignments are performed.

11. The method of claim 10, wherein the dual light source comprises one of a laser, a superluminescent diode and a laser diode; and one of a light-emitting diode and a laser diode.

12. The method of claim 10, wherein the first and second light beams are in a range between 350 nm and 900 nm.

13. The method of claim 10, wherein the positioning unit comprises a nano-positioning stage selected for controlling x,y,z movements of at most 10 nm and yaw, tilt and rotation movements of at most 1 μrad of the second chip; and a micro-positioning stage selected for controlling x,y,z movements of at most 25 nm and yaw, tilt and rotation movements of at most 0.1 degrees of the second chip.

14. The method of claim 10, comprising, using a computer, determining misalignment between the first and of the second chips in the images of the first and of the second chips and sending signals to the positioning unit to operate the positioning unit until a target alignment of the first and second chips.

15. The method of claim 10, wherein the imaging unit comprises a microscope and a long working-distance objective lens.

16. The method of claim 10, wherein the imaging unit comprises a microscope and a long working-distance objective lens, and has a spatial resolution of about 1 μm.

17. The method of claim 10, wherein the chips comprises alignment markers.

18. The system of claim 1, wherein the chips comprises alignment markers, with ones of: crosses and anti-crosses in a range between 5 μm and 15 μm, vernier/main scale between 10μm and 20 μm, and gratings with pitch between 1 μm and 5 μm.