A photolithography alignment method based on multi-frequency heterodyne method

Abstract

The application belongs to the technical field of semiconductor integrated circuit manufacturing, and discloses a photolithography alignment method based on a multi-frequency heterodyne method, which comprises the following steps: manufacturing n measurement gratings with different frequencies on a silicon wafer, and arranging a transparent area on one side of each measurement grating on the silicon wafer; manufacturing n same measurement gratings on a mask, and arranging a reference grating on one side of each measurement grating on the mask; placing the silicon wafer and the mask in an overlapped manner, collimating and illuminating by means of laser, forming Moiré fringes with different frequencies, and collecting images of the Moiré fringes by means of a camera; inputting the collected images into a phase analysis model, and calculating alignment offset information of each group of Moiré fringes; processing the alignment offset information by means of a multi-frequency heterodyne method and a step-by-step refinement algorithm, and calculating an ultimate alignment offset; and completing alignment of the mask and the silicon wafer by means of the alignment offset. The application can realize nanometer photolithography alignment within a millimeter range.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor integrated circuit manufacturing technology, specifically relating to a photolithography alignment method based on multi-frequency heterodyne method. Background Technology

[0002] In the photolithography process, overlay error is the most significant factor leading to processing defects. As photolithography resolution increases, the requirements for alignment accuracy become increasingly stringent. In realizing this invention, the inventors discovered that existing moiré alignment techniques have at least the following problems:

[0003] (1) The photolithography alignment range is limited, which means that other methods must be used in conjunction to complete the alignment;

[0004] (2) It is impossible to achieve both high precision and large range at the same time, which limits the application areas;

[0005] (3) The measurement field of view needs to cover the entire measurement range, resulting in a complex structure;

[0006] (4) Low alignment efficiency leads to reduced production efficiency. Summary of the Invention

[0007] The present invention aims to at least partially solve the aforementioned technical problems. Therefore, the objective of the present invention is to provide a photolithography alignment method based on multi-frequency heterodyne.

[0008] The technical solution adopted in this invention is as follows:

[0009] A photolithographic alignment method based on multi-frequency heterodyne method includes the following steps:

[0010] S1. Fabricate n measurement gratings with different frequencies on a silicon wafer. Each measurement grating on the silicon wafer has a transparent area on one side. Fabricate n identical measurement gratings on a mask. Each measurement grating on the mask has a reference grating on one side.

[0011] S2. Overlap the silicon wafer and mask, collimate the illumination with a laser to form moiré fringes of different frequencies, and use a camera to capture images of the moiré fringes.

[0012] S3. Input the acquired image into the phase analysis model to calculate the alignment offset information of each group of moiré fringes;

[0013] S4. The alignment offset information is processed by multi-frequency heterodyne method and stepwise refinement algorithm to calculate the final alignment offset.

[0014] S5. Align the mask and silicon wafer by aligning the offset.

[0015] Preferably, n is greater than or equal to 3.

[0016] Preferably, the measurement grating on the silicon wafer is configured in a one-to-one correspondence with the measurement grating on the mask; the transparent area on the silicon wafer is configured in a one-to-one correspondence with the reference grating on the mask.

[0017] Preferably, the phase analysis model is a phase analysis model based on windowed Fourier transform.

[0018] Preferably, the photolithographic alignment offset information includes the offset of the silicon wafer relative to the mask in the X and Y directions.

[0019] Preferably, step S4 includes:

[0020] When the silicon wafer shifts, each set of measurement gratings will obtain a set of measurement results. For n sets of measurement results, they are sorted according to the size of the measurement range to obtain equation set one:

[0021]

[0022] Equation set one represents the results of measuring the same alignment offset using n sets of measurement gratings; where... P represents the phase difference of the measurement results for each set of gratings. mai (i = 1, 2, ..., n) represents the period of each measurement grating, Δx is the offset, and n i (i = 1, 2, ..., n) represents the number of cycles contained in each group of measurement results;

[0023] Equation set one is obtained by subtracting each pair of equations from top to bottom, and then subtracting the upper equations from the lower equations to form the first-level beat frequency signal, thus completing the first expansion of the measurement range and obtaining equation set two:

[0024]

[0025] Repeat the expansion steps from system one to system two, performing n-1 expansions, to obtain the equation:

[0026]

[0027] Let the equation The term equals 0, that is:

[0028]

[0029] n can then be determined i The values ​​of (i = 1, 2, ..., n) and Δx are used to obtain the solution to the equation, which is used to obtain the final alignment offset.

[0030] Preferably, step S5 includes: inputting the alignment offset into the control terminal, and the control terminal aligning the mask and the silicon wafer through a motion mechanism.

[0031] The beneficial effects of this invention are as follows:

[0032] The present invention provides a photolithography alignment method based on multi-frequency heterodyne method, which extends the range of photolithography moiré alignment by using multi-frequency heterodyne method and processes the data using a step-by-step refinement algorithm to achieve nanoscale photolithography alignment within the millimeter range. The multi-frequency heterodyne method increases the alignment range, allowing the photolithography alignment method to be used independently, simultaneously achieving coarse and fine alignment measurements. The step-by-step refinement algorithm can improve the measurement accuracy, achieving high-precision alignment over a large range.

[0033] Meanwhile, the measurement field of view of this invention does not need to cover the entire measurement range, reducing the complexity of the imaging system; the alignment speed is improved, thus increasing production efficiency. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of silicon wafer alignment marks.

[0035] Figure 2 This is a schematic diagram of the mask alignment marks. Detailed Implementation

[0036] The technical solutions of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0037] It should be understood that, and also noted, in the embodiments, the functions / actions may appear in a different order than those shown in the figures. For example, depending on the functions / actions involved, they may actually be performed substantially concurrently, or sometimes the two figures shown consecutively may be performed in reverse order.

[0038] like Figure 1 and Figure 2 As shown, this embodiment of a photolithography alignment method based on multi-frequency heterodyne method includes the following steps:

[0039] S1. Three measurement gratings with different frequencies are fabricated on a silicon wafer. The three measurement gratings on the silicon wafer are measurement grating 1, measurement grating 2 and measurement grating 3. A transparent area is provided on one side of each measurement grating on the silicon wafer. The three measurement gratings and the transparent area constitute the alignment mark of the silicon wafer.

[0040] Three identical measurement gratings (number 4) are fabricated on the mask. On the same side of each measurement grating (number 4) on the mask, there are reference gratings with different frequencies. The three reference gratings on the mask are reference grating 1, reference grating 2, and reference grating 3, respectively. The three measurement gratings (number 4) and the three reference gratings constitute the mask alignment marks.

[0041] The three measurement gratings on the silicon wafer are set in a one-to-one correspondence with the three measurement gratings on the mask. The measurement gratings on the silicon wafer and their corresponding measurement gratings on the mask form a set of measurement gratings. For example, measurement grating No. 1 and one of the measurement gratings No. 4 form a set of measurement gratings. The three transparent areas on the silicon wafer are set in a one-to-one correspondence with the three reference gratings on the mask.

[0042] S2. The silicon wafer and the mask are stacked and collimated by a laser. Since multiple sets of gratings of different frequencies are made on the alignment marks of the silicon wafer and the alignment marks of the mask, moiré fringes of different frequencies will be formed. The camera is used to acquire images of the moiré fringes of different frequencies.

[0043] S3. The acquired image input is based on the phase analysis model of windowed Fourier transform to calculate the alignment offset information of each set of moiré fringes; the lithographic alignment offset information includes the offset of the silicon wafer relative to the mask in the X and Y directions.

[0044] S4. The alignment offset information is processed by multi-frequency heterodyne method and stepwise refinement algorithm to calculate the final alignment offset.

[0045] Specifically, step S4 includes:

[0046] When the silicon wafer shifts, each set of measurement gratings will obtain a set of measurement results. For n sets of measurement results, they are sorted according to the size of the measurement range to obtain equation set one:

[0047]

[0048] Equation set one represents the results of measuring the same alignment offset using n sets of measurement gratings; where... P represents the phase difference of the measurement results for each set of gratings. mai (i = 1, 2, ..., n) represents the period of each measurement grating, Δx is the offset, and n i (i = 1, 2, ..., n) represents the number of cycles contained in each measurement result set; solve for n i The absolute measurement process can be completed by the values ​​of (i = 1, 2, ..., n) and Δx. So, the problem now is how to solve for n+1 parameters using the existing n equations.

[0049] Equation set one is obtained by subtracting each pair of equations from top to bottom, and then subtracting the upper equations from the lower equations to form the first-level beat frequency signal, thus completing the first expansion of the measurement range and obtaining equation set two:

[0050]

[0051] Equation set two is now composed of the existing n-1 equations. From the mathematical expression of equation set two, it can be seen that the resulting beat frequency signal will still exhibit periodic changes with different periods. Therefore, the same operation can be used to combine the first-level beat frequency signal to form a second-level beat frequency signal, completing the second expansion of the measurement range. Following this approach, combining the signals sequentially upwards can form higher-order beat frequency signals, achieving multiple expansions of the measurement range. Because each combination process reduces the number of new signals compared to the original signal by one, repeating the expansion steps from equation set one to equation set two, performing n-1 expansions, yields the following equation:

[0052]

[0053] The equation still exhibits a periodic distribution, and it remains valid for measurement within a periodic range, allowing the equation to... The term equals 0, that is:

[0054]

[0055] This equation can be referred to as the constraint equation of the measurement scheme. With the introduction of the constraint equation, n can be determined. i The values ​​of (i = 1, 2, ..., n) and Δx are used to obtain the solution to the equation, which is used to obtain the final alignment offset.

[0056] It should be noted that in this embodiment, n = 3.

[0057] S5. Align the mask and silicon wafer by aligning the offset.

[0058] Specifically, step S5 includes: inputting the alignment offset into the control terminal, and the control terminal aligning the mask and the silicon wafer through a motion mechanism. The alignment is achieved by moving the silicon wafer while the mask remains stationary.

[0059] This invention is not limited to the above-described optional embodiments. Anyone can derive other various forms of products under the guidance of this invention. However, regardless of any changes made in their shape or structure, any technical solution that falls within the scope of the claims of this invention shall be protected by this invention.

Claims

1. A photolithographic alignment method based on multi-frequency heterodyne method, characterized in that, Including the following steps: S1. Fabricate n measurement gratings with different frequencies on a silicon wafer. Each measurement grating on the silicon wafer has a transparent area on one side. Fabricate n identical measurement gratings on a mask. Each measurement grating on the mask has a reference grating on one side. S2. Overlap the silicon wafer and mask, collimate the illumination with a laser to form moiré fringes of different frequencies, and use a camera to capture images of the moiré fringes. S3. Input the acquired image into the phase analysis model to calculate the alignment offset information of each group of moiré fringes; S4. The alignment offset information is processed by multi-frequency heterodyne method and stepwise refinement algorithm to calculate the final alignment offset. S5. Align the mask and silicon wafer by aligning the offset.

2. The photolithographic alignment method according to claim 1, characterized in that: The n is greater than or equal to 3.

3. The photolithographic alignment method according to claim 1 or 2, characterized in that: The measurement gratings on the silicon wafer and the measurement gratings on the mask are set in a one-to-one correspondence; the transparent areas on the silicon wafer and the reference gratings on the mask are set in a one-to-one correspondence.

4. The photolithographic alignment method according to claim 1, characterized in that: The phase analysis model is a phase analysis model based on windowed Fourier transform.

5. The photolithographic alignment method according to claim 1 or 4, characterized in that: The alignment offset information includes the offset of the silicon wafer relative to the mask in the X and Y directions.

6. The photolithographic alignment method according to claim 1, characterized in that: Step S4 includes: When the silicon wafer shifts, each set of measurement gratings will obtain a set of measurement results. For n sets of measurement results, they are sorted according to the size of the measurement range to obtain the first set of equations: Equation set one represents the results of measuring the same alignment offset using n sets of measurement gratings; where... P represents the phase difference of the measurement results for each set of gratings. mai (i = 1, 2, ..., n) represents the period of each measurement grating, Δx is the offset, and n i (i = 1, 2, ..., n) represents the number of cycles contained in each group of measurement results; Equation set one is obtained by subtracting each pair of equations from top to bottom, and then subtracting the upper equations from the lower equations to form the first-level beat frequency signal, thus completing the first expansion of the measurement range and obtaining equation set two: Repeat the expansion steps from system one to system two, performing n-1 expansions, to obtain the equation: Let the equation The term equals 0, that is: n can then be determined i The values ​​of (i = 1, 2, ..., n) and Δx are used to obtain the solution to the equation, which is used to obtain the final alignment offset.

7. The photolithographic alignment method according to claim 1, characterized in that: Step S5 includes: inputting the alignment offset into the control terminal, and the control terminal aligning the mask and the silicon wafer through a motion mechanism.

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