CN102096348B - Digital Moire fringe method for improving quality of Moire fringe images in imprinting alignment process - Google Patents

Abstract

The invention discloses a digital moiré method for improving the quality of moire fringe images in the imprinting alignment process, comprising (1) mark design; (2) acquisition of grating images; (3) grating image preprocessing; (4) ) synthesizing moiré fringe images; (5) preprocessing of moiré fringe signals. In the present invention, a CCD is used to collect grating images, and a computer-synthesized digital moire fringe scheme is used to perform nano-imprint alignment instead of directly using the CCD to collect moiré fringe images for alignment. From the perspective of moiré fringe formation, it solves the problems of the traditional method due to the asymmetry of the process layer coverage, the multiple reflections of the alignment beam at the process layer interface, the decrease in the contrast of the moiré fringe during the alignment process, and the problem of excessive light The alignment image error problem caused by secondary interference and diffraction improves the quality of the moiré fringe alignment image during the imprinting process, thereby improving the accuracy of the phase extraction of the moiré fringe during alignment, and further improving the imprint alignment precision.

Description

A Digital Moiré Method for Improving Moiré Image Quality During Imprint Alignment

technical field

The invention belongs to the technical field of nano-imprint multi-layer overlay alignment, and relates to a method for solving the problem of image quality degradation caused by moiré fringe alignment signals caused by photoresist process layers, especially a method for improving imprint alignment Digital moiré methods for in-process moiré image quality.

Background technique

Imprint lithography is one of the representatives of the next-generation 16nm node lithography technology due to its high resolution, high efficiency and low cost (the difference between its process route and traditional lithography is shown in Figure 1), which can effectively solve the traditional Photolithography has size limitations due to pattern diffraction issues. Although imprint lithography has the advantages of high resolution, high efficiency and low cost in pattern transfer, there are still many key issues to be solved in order to make it a practical and competitive micro-nano manufacturing technology, many of which Layer positioning and alignment techniques are key to determining the ultimate application of imprinting in the semiconductor manufacturing industry.

In nanoimprint multilayer overlay alignment, the most feasible alignment scheme is based on the moiré fringe phase matching method. The measurement accuracy of this method depends on the extraction accuracy of the spatial phase of the interference fringe image. In the state of no photoresist filling, the alignment measurement resolution can reach sub-nanometer level, and the alignment accuracy is below 10nm.

However, in high-precision alignment measurement, since the alignment marks are covered with process layers such as photoresist and metal thin film, the quality of the alignment signal extracted by the CCD is severely degraded. In fact, due to the asymmetry of the coverage of the process layer and the multiple reflections of the alignment beam at the interface of the process layer, the contrast of the Moiré fringe drops significantly during the alignment process, which affects the alignment accuracy. The specific process may include: the incident light passes through the template grating to mark the interference between multiple stripes; the refraction of the glue material; reaches the embossed base mark, forms a moiré fringe image through interference and diffraction; is reflected back by the embossed base, and repeats The above process projects a moiré fringe image from the template. These processes determine that the contrast of the moiré fringe alignment image in imprinting is low, which affects the alignment process, and this problem must be solved.

Contents of the invention

The object of the present invention is to provide a digital moiré method for improving the image quality of moiré alignment in embossing under the condition that photoresist is introduced. In high-precision alignment measurement, since the alignment marks are covered with process layers such as photoresist and metal thin film, the quality of the alignment signal extracted by the CCD is seriously degraded. In this method, moiré fringe images are synthesized by collecting grating-marked images, instead of directly extracting moiré fringe alignment images. The method of synthesizing digital moiré fringes by computer solves the problem that the image quality of moiré fringe alignment seriously drops due to the introduction of photoresist process layer and multiple interference diffraction. By reducing the error of the moiré fringe alignment signal, the imprint alignment accuracy is further improved.

The purpose of the present invention is solved by the following technical solutions:

The digital moiré method for improving the image quality of moire fringes in the imprint alignment process comprises the following steps:

(1) Marking design

Design a group of combined grating pairs on the substrate and the template respectively, so that the template grating and the substrate grating do not overlap during imprinting; set the periods of the combined grating pairs on the substrate as P1 and P2 respectively, and the periods of the combined grating pairs on the template are respectively for P2 and P1;

(2) Acquisition of raster images

Use CCD to extract four grating area images of equal size on the grating marks of different periods on the template and the substrate at the common designated point position along the grating direction, and make the acquisition area as large as possible under the premise of ensuring that the acquisition areas do not overlap;

(3) Raster image preprocessing

One-dimensionalize the image of the grating area collected by the CCD, and then filter it to remove the noise component, and obtain four preprocessed grating signals;

(4) Synthetic moire fringe image

The above grating signal is synthesized into a moiré fringe signal, that is, the grating signal with a period of P1 on the template and the grating signal with a period of P2 on the substrate are synthesized into a group of moiré fringes; the grating signal with a period of P2 on the template and the period on the substrate are The raster signal of P1 synthesizes another set of moiré fringes.

(5) Preprocessing of Moiré fringe signal

The moiré fringe signal is filtered to obtain the moiré fringe signal required for imprint alignment.

The above method is applied in the fine alignment process of nanoimprinting, that is, the alignment error obtained in the rough alignment of imprinting is within one moiré fringe grating period.

The above-mentioned imprint lithography alignment adopts the alignment principle based on moiré fringe phase matching, and performs alignment through a group of combined grating pairs existing on the substrate and the template respectively; during the alignment process, the period of the combined grating pairs on the template The grating of P1 overlaps the grating with period P2 in the combined grating pair on the substrate to form a group of Moiré fringes; the grating with period P2 in the combined grating pair on the template overlaps with the grating with period P1 in the combined grating pair on the substrate to form another A set of moiré fringes.

In the above step 2), the image extracted by the CCD is: starting from the position of the commonly specified point along the grating direction, the image is extracted on the template grating with periods p ₁ and p ₂ and the base grating images with periods p ₁ and p ₂ respectively Four equally sized raster regions.

In the above step 3), the extracted four regional images are superimposed along the direction of the grating to generate four one-dimensional grayscale signals, and then a low-pass filter is used to filter out high-frequency components other than the fundamental frequency of the grating to obtain four cosine grayscale signals .

In step 4) above, the method for synthesizing moiré fringe images is: multiplying the cosine grayscale signal corresponding to the template grating with a period of p ₁ and the cosine grayscale signal corresponding to the base grating with a period of p ₂ to obtain a set of Superimposed signals, the cosine grayscale signal corresponding to the template grating with a period of p ₂ is multiplied by the cosine grayscale signal corresponding to the silicon wafer grating with a period of p ₁ to obtain another set of superimposed signals; two sets of moiré fringe signals are obtained.

In the above step 5), the preprocessing of the moiré fringe signal refers to the use of a low-pass filter to filter out the f _{1 ,} f ₂ and f ₁ + f ₂ frequency components other than the frequency f 1 -f ₂ to obtain two groups of one Vimor signal, and then utilize the phase matching method to process the signal; said f ₁ is the frequency of the grating whose period is p ₁ , i.e. 1/p ₁ ; said f ₂ is the frequency of the grating whose period is p ₂ , i.e. 1/ _p2 .

In the present invention, a CCD is used to collect grating images, and a computer-synthesized digital moire fringe scheme is used to perform nano-imprint alignment instead of directly using the CCD to collect moiré fringe images for alignment. From the perspective of moiré fringe formation, it solves the problems of the traditional method due to the asymmetry of the process layer coverage, the multiple reflections of the alignment beam at the process layer interface, the decrease in the contrast of the moiré fringe during the alignment process, and the problem of excessive light The alignment image error problem caused by secondary interference and diffraction improves the quality of the moiré fringe alignment image during the imprinting process, thereby improving the accuracy of the phase extraction of the moiré fringe during alignment, and further improving the imprint alignment precision.

In addition, compared with the traditional alignment method, the method used in the present invention has a simple process and does not require redundant processes. Thereby promoting the practical application of the Moiré fringe alignment scheme.

Description of drawings

Fig. 1 Principle of nanoimprint multi-layer overlay Moiré fringe alignment;

Figure 2 Digital moiré fringe grating mark;

Figure 3 One-dimensional processing of moiré fringes;

Fig. 4 Synthesis of digital moiré fringes and one-dimensional moiré signals.

Detailed ways

The digital moiré method for improving the image quality of moire fringes in the imprinting alignment process of the present invention is applied in the fine alignment process of nanoimprinting, that is, the alignment error obtained in the imprinting coarse alignment is within one moiré grating period. The method specifically includes the following steps:

(1) Marking design

Design a group of combined grating pairs on the substrate and the template respectively, so that the template grating and the substrate grating do not overlap during imprinting; set the periods of the combined grating pairs on the substrate as P1 and P2 respectively, and the periods of the combined grating pairs on the template are respectively for P2 and P1.

During the alignment of imprint lithography, the alignment principle based on moiré fringe phase matching is adopted, and the alignment is performed through a group of combined grating pairs existing on the substrate and the template respectively; during the alignment process, the period of the combined grating pairs on the template is The grating of P1 overlaps with the grating with period P2 in the combined grating pair on the substrate to form a group of moiré fringes; the grating with period P2 in the combined grating pair on the template overlaps with the grating with period P1 in the combined grating pair on the substrate to form another Group of moiré fringes. Alignment is performed by comparing the phases of two sets of moiré fringes.

(2) Acquisition of raster images

Using CCD to extract four grating area images of equal size on the grating marks of different periods on the template and substrate at the common designated point position along the grating direction, and make the acquisition area as large as possible under the premise of ensuring that the acquisition areas do not overlap.

When performing imprint alignment, the imprint template and the imprint substrate are coincident. During the extraction process of the imprint alignment grating signal, it must be ensured that the grating marks on the template flaw and the grating marks on the substrate do not coincide, so that the CCD can Acquisition of different raster regions. Therefore, the template mark and the base mark are not overlapped even after the imprint alignment is completed at the time of design. The image required for CCD acquisition refers to extracting four equal-sized grating regions on the template grating with periods p ₁ and p ₂ and the base grating image with periods p ₁ and p ₂ , respectively, starting from the specified point along the grating direction . Under the premise of ensuring that the collection areas do not overlap, the collection area should be as large as possible.

(3) Raster image preprocessing

One-dimensionalize the image of the grating area collected by the CCD, and then filter it to remove the noise component, and obtain four preprocessed grating signals;

(4) Synthetic moire fringe image

Synthesize the above grating signal into a Moiré fringe signal; the pre-processing of the grating image refers to superimposing the extracted four regional images along the direction of the grating to generate four one-dimensional grayscale signals, and then using a low-pass filter to filter out the fundamental frequency of the grating other than high-frequency components, four cosine grayscale signals are obtained. Specifically:

The cosine grayscale signal corresponding to the template grating with a period of p ₁ is multiplied by the cosine grayscale signal corresponding to the silicon wafer grating with a period of p ₂ to obtain a set of superimposed signals, and the cosine grayscale corresponding to the template grating with a period of p ₂ The grayscale signal is multiplied by the cosine grayscale signal corresponding to the silicon grating with a period of p ₁ to obtain another set of superimposed signals. Two sets of Moiré fringe signals are obtained, which are ready for the alignment of the following phase matching scheme.

(5) Preprocessing of Moiré fringe signal

Filter the moiré fringe signal to obtain the moiré fringe signal required for imprint alignment, specifically: use a low-pass filter to filter out f _{1 ,} f ₂ and f ₁ + f ₂ frequencies other than f ₁ -f 2 component to obtain two sets of one-dimensional moiré signals. Then use the phase matching method to process the signal; said f ₁ is the frequency of the grating with period p ₁ , namely 1/p ₁ ; said f ₂ is the frequency of the grating with period p ₂ , namely 1/p ₂ .

Below in conjunction with accompanying drawing, the above-mentioned method of the present invention is further described:

Referring to FIG. 1 , it is a schematic diagram of the principle of imprinting multi-layer overlay alignment. At present, the most commonly used alignment scheme for imprinting multilayer overlay is the alignment scheme based on moiré fringe phase matching. There are two groups of gratings with periods P1 and P2 on the substrate and the template, respectively. During alignment, the gratings with periods P1 on the template and the gratings with periods P2 on the substrate overlap to form a set of moiré fringes; the gratings with periods P2 on the template and The overlapping gratings with period P1 on the substrate form another set of moiré fringes. Alignment is performed by extracting the phases of the two sets of moiré fringes.

As shown in Fig. 2, four grating areas of equal size are extracted from the images of template grating p ₁ , template grating p ₂ , silicon wafer grating p ₁ and silicon wafer grating p ₂ by CCD at the position of a common designated point along the grating direction. .

Referring to Figure 3, the extracted four regional images are superimposed along the direction of the grating to generate four one-dimensional grayscale signals, and then a low-pass filter is used to filter out high-frequency components other than the fundamental frequency of the grating to obtain four cosine grayscale signals .

Referring to Figure 4, the cosine grayscale signal corresponding to the template grating _p1 is multiplied by the cosine grayscale signal corresponding to the silicon wafer grating _p2 to obtain a set of superimposed signals, and the cosine grayscale signal corresponding to the template grating _p2 and The cosine grayscale signals corresponding to the silicon grating p ₁ are multiplied to obtain another set of superimposed signals, and two sets of moiré fringe signals are obtained.

Claims (3)

1. A digital moiré method for improving moiré image quality in imprint alignment process, is characterized in that, comprises the following steps: (1) Marking design Design a group of combined grating pairs on the substrate and the template respectively, so that the template grating and the substrate grating do not overlap during imprinting; suppose the periods of the combined grating pairs on the substrate are p ₁ and p ₂ respectively, and the combined grating pairs on the template The periods are p ₂ and p ₁ respectively, and it is characterized in that the imprint alignment adopts the alignment principle based on moiré fringe phase matching, and the alignment is performed through a group of combined grating pairs existing on the substrate and the template respectively; During the calibration process, the grating with a period of p ₁ in the combined grating pair on the template overlaps with the grating with a period p ₂ in the combined grating pair on the substrate to form a group of Moiré fringes; the grating with a period of p ₂ in the combined grating pair on the template Overlapping with the grating with a period of p ₁ in the combined grating pair on the substrate forms another set of moiré fringes; in this marking design, we use a combined grating pair with p ₁ of 3 μm and p ₂ of 3.1 μm; (2) Acquisition of raster images Use CCD to extract four grating area images of equal size on the grating marks of different periods on the template and the substrate at the common designated point position along the grating direction, and make the acquisition area as large as possible under the premise of ensuring that the acquisition areas do not overlap; (3) Raster image preprocessing One-dimensionalize the grating area images collected by the CCD, and then filter and filter out the noise components to obtain four preprocessed grating signals, which are characterized in that four one-dimensional grayscale signal, and then use a low-pass filter to filter out high-frequency components other than the grating fundamental frequency to obtain four cosine grayscale signals; (4) Synthetic moire fringe image Synthesize the above grating signal into a Moiré fringe signal, that is, the grating signal with a period of p ₁ on the template and the grating signal with a period of p ₂ on the substrate to synthesize a group of Moiré fringes; the grating signal with a period of p ₂ on the template and the substrate The grating signal whose upper period is p ₁ synthesizes another group of moiré fringes; (5) Preprocessing of Moiré fringe signal Filtering the moiré fringe signal to obtain the moiré fringe signal required for imprint alignment, characterized in that the preprocessing of the moiré fringe signal refers to the use of a low-pass filter to filter out frequency f ₁ -f ₂ f ₁ , f ₂ and f ₁ + f ₂ frequency components to obtain two groups of one-dimensional Moiré signals, and then use the phase matching method to process the signals; the f ₁ is the frequency of the grating whose period is p ₁ , that is, 1/ p ₁ ; said f ₂ is the frequency of the grating with period p ₂ , ie 1/p ₂ . 2. The digital moiré method for improving the moiré image quality in the imprinting alignment process according to claim 1, wherein the method is applied in the fine alignment process of nanoimprinting, that is, in the embossing The alignment error obtained in fine alignment is within one moiré grating period. 3. The digital moiré method for improving the quality of moiré images in the imprint alignment process according to claim 1, characterized in that, in step (4), the method for synthesizing moiré images is as follows: the period is The cosine grayscale signal corresponding to the template grating of p ₁ is multiplied by the cosine grayscale signal corresponding to the base grating with a period of p ₂ to obtain a set of superimposed signals, and the cosine grayscale signal corresponding to the template grating with a period of p ₂ and The cosine grayscale signals corresponding to the base grating with a period of p ₁ are multiplied to obtain another set of superimposed signals; two sets of moiré fringe signals are obtained.

Images