A design method of fourier filter for semiconductor dark field defect detection

Abstract

The application discloses a design method of a Fourier filter for semiconductor dark field defect detection, and belongs to the field of semiconductor defect detection. The method is based on Fourier optical principle to solve the spatial spectrum of a wafer surface pattern, corresponding spatial spectrum shielding needles are designed through spatial spectrum distribution, and the wafer surface pattern is filtered to effectively suppress pattern noise, highlight other types of defects on the wafer surface, and assist a dark field detection system to complete defect detection of a wafer with patterns. The method is suitable for wafers with various shapes and sizes, and since spatial spectrum analysis is unique, spectrum analysis and Fourier filter design need to be performed on wafers with different patterns. In addition, the position of filter needles for band pass filtering in the filter, the interval between the filter needles and other parameters can be changed to meet the design requirements of a required modulation wave surface, and the method has a certain degree of design freedom.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of semiconductor defect detection, and more particularly relates to a design method of a Fourier filter for semiconductor dark field defect detection. BACKGROUND

[0002] Defects are inevitably generated in integrated circuit manufacturing, and different processes generate different kinds of defects. With the current integrated circuit scale getting larger and larger, the manufacturing process getting more and more complex, and the key size of integrated circuit design getting smaller and smaller, it is a very challenging task to accurately detect defects in integrated circuits. Patterned wafer refers to a wafer with a specific pattern or circuit element structure drawn or made by a specific process technology. Due to the pattern defects on the wafer and the complex topography of various materials, defect detection is more complex and challenging than bare wafer. Dark field defect detection technology is an important non-destructive testing technology, which has the advantages of high detection sensitivity, good directivity and non-destructive to materials. For patterned wafer detection, the main problem faced by dark field detection is pattern recognition: the detection system needs to accurately distinguish the light of the background pattern and the abnormal light caused by defects. SUMMARY

[0003] In view of the above defects or improvement needs of the prior art, the present application provides a design method of a Fourier filter for semiconductor dark field defect detection, which can accurately identify the defect signal of the patterned wafer.

[0004] To achieve the above-mentioned purpose, according to the first aspect of the present application, a design method of a Fourier filter for semiconductor dark field defect detection is provided, the Fourier filter is placed between the objective lens and the collection lens group of a scattering dark field detection system, and the distance from the objective lens is the focal length f of the objective lens, the method comprises:

[0005] S1, obtaining the two-dimensional spatial spectrum intensity F{U(x1,y1)} of the surface pattern signal of the wafer to be tested u,v , according to the relationship between F{U(x1,y1)} u,v , the distance d0 between the objective lens and the surface of the wafer to be tested, the focal length f of the objective lens and the complex amplitude distribution U f (x,y) on the back focal plane of the objective lens, converting the frequency point (u,v) of the wafer pattern signal to be tested into the imaging point (x,y) on the back focal plane of the objective lens;

[0006] S2, removing the imaging points corresponding to the frequency points with energy lower than the threshold value, and calculating the main maximum peak and the width and interval of each maximum value peak of the frequency spectrum plane composed of the corresponding frequency points in the horizontal direction and the vertical direction according to the coordinates of the remaining imaging points;

[0007] S3, if the wafer pattern to be measured has rotational symmetry, the Fourier filter comprises horizontal needles and vertical needles, otherwise the Fourier filter comprises horizontal needles or vertical needles;

[0008] The number of the horizontal needles or the vertical needles is the sum of the number of the primary maximum peaks and the secondary maximum peaks in the horizontal direction or the vertical direction, the width of each of the horizontal needles or the vertical needles is equal to the width of the primary maximum peak or each of the secondary maximum peaks in the horizontal direction or the vertical direction, and the interval between adjacent horizontal needles or vertical needles is equal to the interval between adjacent peaks in the horizontal direction or the vertical direction.

[0009] According to a second aspect of the present application, a Fourier filter for semiconductor dark field defect detection is provided, which is designed by the method according to the first aspect.

[0010] According to a third aspect of the present application, a semiconductor dark field defect detection system is provided, which comprises a scattering type dark field detection system and the Fourier filter according to the second aspect.

[0011] According to a fourth aspect of the present application, an electronic device is provided, which comprises a computer readable storage medium and a processor.

[0012] The computer readable storage medium is configured to store executable instructions.

[0013] The processor is configured to read the executable instructions stored in the computer readable storage medium and execute the method according to the first aspect.

[0014] According to a fourth aspect of the present application, a computer readable storage medium is provided, which is configured to store computer instructions, and the computer instructions are configured to make the processor execute the method according to the first aspect.

[0015] In general, the above technical solutions conceived by the present application can achieve the following beneficial effects compared with the prior art:

[0016] The design method provided by the application solves the spatial spectrum of the wafer surface pattern through Fourier optical principle, designs a Fourier filter in a band-pass filtering manner, places the filter on the back focal plane of the objective lens of a scattering dark field detection system, and matches the filter with the objective lens to filter out the influence of the wafer surface pattern on the detection imaging process; designs a corresponding spatial spectrum shielding structure through the spatial spectrum distribution, filters the wafer surface pattern, effectively suppresses the pattern noise, highlights other types of defects and defects on the wafer surface, and assists the dark field detection system to complete the defect detection of the patterned wafer; the method is suitable for various shapes and sizes of patterned wafers, the spatial spectrum analysis is unique, the Fourier filter needs to be designed separately for different patterned wafers, and the Fourier filter is selected according to the wafer pattern type in the use process; in addition, the design requirements of the required modulation wave surface can be achieved by changing the position of the filter pin for band-pass filtering and the interval between the filter pins, and the design has a certain degree of freedom.

[0017] The Fourier filter designed by the design method provided by the application can improve the signal-to-noise ratio of the output wafer surface image, effectively filter noise signals, and improve the speed and accuracy of optical detection. The filter designed by the method provided by the application can suppress the wafer surface pattern information in the output image and highlight the defect information, solve the influence of the surface pattern on the defect detection in the patterned wafer detection process, effectively suppress the detection noise, improve the output signal-to-noise ratio, and the Fourier filter can be designed according to different illumination wave bands, and the material selection range is wide; the wave surface can be controlled, and the Fourier filter is used as a phase filter in optical information processing, filters out the noise signals of the wafer surface pattern in the dark field defect detection, and accurately identifies the defect signals. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The placement position schematic diagram of the Fourier filter provided by the embodiment of the application in the scattering dark field detection system is shown in the figure;

[0019] Figure 2 (a) and (b) in the figure are schematic diagrams of wafer surface patterns with rotational symmetry and without rotational symmetry, respectively;

[0020] Figure 3 (a) and (b) in the figure are schematic diagrams of spatial spectrum intensity on the back focal plane of wafer surface patterns with rotational symmetry and without rotational symmetry, respectively;

[0021] Figure 4 (a) and (b) in the figure are binaryzation schematic diagrams of spatial spectrum intensity of wafer surface patterns with rotational symmetry and without rotational symmetry, respectively.

[0022] Figure 5Fig. 1 is a schematic diagram of filter size limitation when the wafer surface pattern in (a) and (b) has rotational symmetry and no rotational symmetry, respectively;

[0023] Figure 6 Fig. 1 is a schematic diagram of filter size limitation when the wafer surface pattern in (a) and (b) has rotational symmetry and no rotational symmetry, respectively;

[0024] Figure 7 Fig. 1 is a schematic diagram of filter size limitation when the wafer surface pattern in (a) and (b) has rotational symmetry and no rotational symmetry, respectively;

[0025] Figure 8 Fig. 1 is a schematic diagram of filter size limitation when the wafer surface pattern in (a) and (b) has rotational symmetry and no rotational symmetry, respectively. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0027] The present application provides a design method of a Fourier filter for semiconductor dark field defect detection, the Fourier filter is placed between an objective lens and a collection lens group of a scattering type dark field detection system, the distance from the Fourier filter to the objective lens is the focal length f of the objective lens, and the method comprises the following steps:

[0028] S1, obtaining the two-dimensional spatial spectrum intensity F{U(x1, y1)} of the wafer surface pattern signal to be detected u,v , converting the frequency point (u, v) of the wafer pattern signal to be detected into the imaging point (x, y) on the back focal plane of the objective lens according to the relationship between F{U(x1, y1)} u,v , the distance d0 between the objective lens and the wafer surface to be detected, the focal length f of the objective lens and the complex amplitude distribution U f (x, y) on the back focal plane of the objective lens;

[0029] S2, removing the imaging points corresponding to the frequency points with energy lower than the threshold value, and calculating the width and interval of the main maximum peak and each maximum value peak of the frequency spectrum plane composed of the corresponding frequency points in the horizontal direction and the vertical direction according to the coordinates of the retained imaging points; it can be understood that the width and position of the main maximum peak and each maximum value peak of the frequency spectrum plane composed of the corresponding frequency points in the horizontal direction and the vertical direction can be calculated according to the coordinates of the retained imaging points, and the interval between adjacent peaks can be calculated according to the position information.

[0030] S3, if the wafer pattern to be measured has rotational symmetry, the Fourier filter comprises horizontal needles and vertical needles, otherwise the Fourier filter comprises horizontal needles or vertical needles; the horizontal needles and the vertical needles are used for filtering the wafer surface pattern;

[0031] The number of the horizontal needles or the vertical needles is the sum of the number of the primary maximum peaks and the secondary maximum peaks in the horizontal direction or the vertical direction, the width of each of the horizontal needles or the vertical needles corresponds to and is equal to the width of each of the primary maximum peaks and the secondary maximum peaks in the horizontal direction or the vertical direction, and the interval between adjacent horizontal needles or vertical needles corresponds to and is equal to the interval between adjacent peaks in the horizontal direction or the vertical direction.

[0032] As shown in Figure 1 The method provided by the application places the Fourier filter between the objective lens and the collection lens group of the scattering dark field detection system, uses the Fourier transform property of the objective lens to generate the spatial spectrum of the object, filters the information carried by the wafer surface spatial structure through spatial spectrum measurement and analysis, and thus places the designed filter position on the objective Fourier plane (i.e. the back focal plane) of the light collection path of the dark field detection system.

[0033] The wafer surface pattern has periodicity, the spectral distribution has certain regularity, the spectrum of the defects and the flaws has randomness, the Fourier filter is designed to filter the light signals generated by the wafer surface pattern and only keep the light signals generated by the defects in the output image (i.e. the image detected by the detector), thereby improving the signal-to-noise ratio of the defect signal detection.

[0034] In the detection process, when the wafer surface has no abnormal defects, the Fourier filter filters out the light signals generated by the surface pattern, and the entire image plane presents a dark field; when the wafer surface has abnormal defects, the Fourier filter only filters out the light signals generated by the surface pattern, and the abnormal defects present high brightness in the dark field, so as to achieve the defect detection of the wafer with pattern.

[0035] The image information of the wafer surface is regarded as the superimposed noise of the input, the noise is additive, and the input field distribution can be expressed as

[0036] f(x1,y1)=s(x1,y1)+n(x1,y1) (1)

[0037] In the formula, s(x1,y1) and n(x1,y1) respectively represent the defect signal and the noise signal of the point (x1,y1) of the wafer surface to be measured.

[0038] A two-dimensional Fourier transform of the wafer surface image signal decomposes it into a superposition of several complex plane waves, which can then be processed using Fourier analysis (spectral analysis). A Fourier transform of the input field f(x1,y1) yields its spectrum F(u,v). S(u,v) and N(u,v) represent the spectra of the wafer surface defect signal (the optical signal generated by the defect surface) and the wafer surface pattern signal (the optical signal generated by the wafer surface pattern when probe light is incident on the wafer surface), respectively.

[0039] F(u,v)=S(u,v)+N(u,v) (2)

[0040] As long as the signal and noise have different distribution characteristics in the frequency domain, filters can be used to effectively suppress noise, improve the output signal-to-noise ratio, and filter out the influence of wafer surface patterns while transmitting signals.

[0041] As an example, such as Figure 2 Figures (a) and (b) show schematic diagrams of wafer surface patterns with and without rotational symmetry, respectively. Both wafer surface patterns are composed of regular rectangular line segments. Figure 2 The structural dimensions of (a) are 90nm*90nm. Figure 2 The structural dimensions of (b) are: linewidth CD = 46nm, line length 10*CD, horizontal spacing 2*CD, and vertical spacing 15*CD.

[0042] Calculate the light field distribution on specific planes one by one along the direction of light wave propagation: the complex amplitude distribution U(x1,y1) on the plane immediately behind the wafer being measured, the complex amplitude distribution U(x2,y2) on the plane immediately in front of the objective lens, the complex amplitude distribution U′(x2,y2) on the plane immediately behind the objective lens, and the complex amplitude distribution U′(x2,y2) on the back focal plane of the objective lens. f (x,y).

[0043] The change in the spectrum of the field distribution on the plane immediately in front of the objective lens can be calculated using angular spectrum theory. That is...

[0044]

[0045] In the formula, H(u,v) is the transfer function describing the Fresnel diffraction effect in the frequency domain.

[0046] H(u,v)=exp[-jπλd0(u 2 +v 2 (4)

[0047] λ is the wavelength of the probe light, and d0 is the distance between the wafer surface and the objective lens. This yields...

[0048]

[0049] Without considering the finite aperture of the objective lens

[0050]

[0051] According to the Fresnel diffraction formula, it can be calculated that...

[0052]

[0053] Substituting into equations (5) and (6) yields

[0054]

[0055] Based on the above formula analysis, when the input wafer image is placed on the front focal plane of the objective lens, the complex amplitude distribution on the rear focal plane of the objective lens is proportional to the Fourier transform of the complex amplitude distribution on the wafer surface. The frequency value is related to the coordinates of the rear focal plane position as follows:

[0056]

[0057] That is, point (x) on the back focal plane f ,y f The amplitude and phase of the light wave transmitted through the wafer surface are determined by the spatial frequency (f) of the light wave. x ,f y The amplitude and phase of the Fourier component of the π / 4 are determined by the π / 4. Therefore, using... Figure 1 The optical path shown can accurately reflect the Fourier transform of the complex amplitude distribution on the wafer surface at the back focal plane of the objective lens, and perform spatial filtering on the information it carries on this plane.

[0058] In this example, the distance between the objective lens and the wafer surface is d0 = f, and the focal length of the objective lens is f = 20 mm. Using the transformation relationship in equation (9), the frequency domain coordinates (u, v) can be converted to the back focal plane imaging coordinates (x, y). The back focal plane imaging result is as follows: Figure 3 As shown in (a) and (b), the strongest energy appears in the frequency domain (u,v)=(0,0), that is, at the image plane (x,y)=(0,0), and the spectral energy distribution has a periodic repeating pattern.

[0059] There are various design methods for using spatial filters to modify spectral components on the spectral plane. Horizontal slits can filter vertical structures in the image, and vertical slits can filter horizontal structures. Placing horizontal and vertical slits at a certain interval on the spectral plane can filter spectral information. The slit direction can be selected based on the energy distribution, choosing either a horizontal or vertical pin structure, or both directions working together for filtering.

[0060] Based on the principles of Fourier optics, when a point defect exists on the wafer surface, the intensity of each point in its spatial spectrum will change. However, when detecting this point defect, the image detail information carried by the high-frequency components can be ignored, and the purpose of defect detection can be achieved without all frequency components. Therefore, removing some points that do not appear according to a periodic pattern can not only reduce the design difficulty but also not affect the detection effect of defects. Based on this, the spectral energy distribution is analyzed and an appropriate filtering energy threshold is selected. The spatial frequency intensity is binarized, and spatial frequency points higher than the energy threshold are removed to facilitate the observation of the spectral distribution. Figure 2 The wafer surface pattern in (a) exhibits rotational symmetry. The spatial frequency energy distribution to be filtered shows the same distribution in both the horizontal and vertical directions, exhibiting periodicity. Therefore, both horizontal and vertical Fourier filters need to be installed simultaneously during filtration. For wafers lacking rotational symmetry... Figure 2 The wafer pattern in (b) exhibits different spectral distributions in the horizontal and vertical directions. Filtering can be applied to either the horizontal or vertical structure of the image. In this example, the vertical component shows a regular periodic distribution, so only vertical filtering (i.e., filtering the horizontal image structure) needs to be designed.

[0061] Filter shapes can be designed according to different spatial frequency distributions, such as... Figure 5 As shown in (a) and (b) in the figure. For Figure 4 The design of a Fourier filter structure is based on the binarization result of the wafer structure spectrum in (a) as shown in Figure 1. Figure 5 As shown in (a), the primary and secondary maxima are filtered using a multi-needle structure with horizontal and vertical dimensions. The structural dimensions are as follows: the width of the middle needle is the width of the primary maxima peak, which is 1.4 mm; the interval between the middle needle and the adjacent needles on the left and right is the interval between the primary maxima peak and the adjacent secondary maxima peaks on the left and right, which is 28.9 mm. Figure 4 As shown in (a), to the left of the secondary peak to the left of the main peak, there are three secondary peaks distributed sequentially at a spacing of 57.8 mm. Therefore, to the left of the left needle adjacent to the middle needle, there are three needle structures (not shown in the figure) distributed at a spacing of 57.8 mm. The width of these three needles corresponds to the width of the three secondary peaks mentioned above. Similarly, since to the right of the secondary peak to the right of the main peak, there are three secondary peaks distributed sequentially at a spacing of 57.8 mm, to the right of the right needle adjacent to the middle needle, there are three needle structures (not shown in the figure) distributed at a spacing of 57.8 mm.

[0062] because Figure 2 The vertical component of the spectrum of the wafer pattern in (b) exhibits a regular periodic distribution; therefore, the Fourier filter only needs to be designed for vertical filtering (i.e., filtering horizontal image structures). Accordingly, based on the... Figure 4The wafer structure spectrum binarization result of (b) in the foregoing is used to design the shielding structure (i.e., the needle structure) of the filter, since the main maximum peak and the adjacent secondary maximum peak have equal width and are periodically distributed, the Fourier filter is designed to have three needles as a basic unit and is periodically repeated, and in each basic unit, the width of each needle is 2.8 mm, and the spacing between adjacent needles is 14.6 mm; the spacing between each basic unit is 28.9 mm.

[0063] In actual use, the Fourier filter needs to be periodically repeated according to the numerical aperture NA of the objective lens and the image plane size. When the image plane size is limited, the filter can be set by taking only the spatial frequency components in the limited image plane to match the filtering effect of the objective lens NA on the spatial frequency, as shown in the red area of (a) and (b) in the foregoing. Figure 6 The filtering effect of the random defects is shown in (a) and (b) in the foregoing. Figure 7 The filtering effect of the random defects is shown in (a) and (b) in the foregoing. Figure 8 As shown in (a) and (b) in the foregoing, the wafer surface pattern information is filtered out in the imaging plane, and only the random defect information exists, and the contrast is strong, which can meet the detection requirements.

[0064] That is, when the image plane size of the dark field detection system is limited, the filter only filters the wafer surface pattern signal in the image plane range of the dark field detection system, the number of the horizontal needles or the vertical needles is the sum of the number of the main maximum peak and the secondary maximum peak in the horizontal direction or the vertical direction of the target spectrum plane, the width of the horizontal needles or the vertical needles is equal to the width of the main maximum peak and each secondary maximum peak in the horizontal direction or the vertical direction of the target spectrum plane, and the spacing between the adjacent horizontal needles or the vertical needles is equal to the spacing between the adjacent peaks in the horizontal direction or the vertical direction of the target spectrum plane; wherein the target spectrum plane is a spectrum plane composed of frequency points corresponding to the imaging points in the image plane range of the dark field detection system (the imaging points corresponding to the frequency points with energy lower than a threshold are removed).

[0065] In summary, the design method provided by the present application uses the Fourier transform method to perform spatial spectrum analysis on the wafer surface pattern, designs the filter needle position and spacing through the spatial spectrum distribution, i.e., designs the corresponding spatial spectrum shielding structure, filters the wafer surface pattern, so that the dark field background is not affected by the noise generated by the surface pattern during defect detection, to improve the signal-to-noise ratio and accuracy in the subsequent defect detection process. It has been verified that the Fourier filter can effectively filter the background pattern noise and can eliminate the wafer background pattern without affecting the display of the defect information.

[0066] The Fourier filter for semiconductor dark field defect detection is designed by the method described in any of the foregoing embodiments.

[0067] The embodiment of the present application provides a semiconductor dark field defect detection system, which comprises a scattering type dark field detection system and the Fourier filter as described in any of the above embodiments.

[0068] The embodiment of the present application provides an electronic device, which comprises a computer readable storage medium and a processor.

[0069] The computer readable storage medium is used for storing executable instructions.

[0070] The processor is used for reading the executable instructions stored in the computer readable storage medium, and performing the method as described in any of the above embodiments.

[0071] The embodiment of the present application provides a computer readable storage medium, which stores computer instructions, and the computer instructions are used for making the processor perform the method as described in any of the above embodiments.

[0072] Those skilled in the art can easily understand that the above description is only the preferred embodiment of the present application, and is not used to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method of designing a Fourier filter for semiconductor dark field defect detection, characterized in that, The Fourier filter is placed between the objective and the collection lens group of a scattering dark-field detection system at a distance of the focal length of the objective from the objective f The method comprises: S1, acquire the two-dimensional spatial spectrum intensity of the pattern signal on the surface of the wafer under test. ,according to The distance between the objective lens and the surface of the wafer under test d 0. Focal length of the objective lens f Complex amplitude distribution on the back focal plane of the objective lens Relationship The frequency points of the pattern signal on the wafer under test Imaging point on the focal plane after conversion to objective lens ;in, To detect the wavelength of light; S2, removing imaging points corresponding to frequency points with energy lower than the threshold value, and calculating the main maximum peak and the secondary maximum peak in the horizontal direction and the vertical direction, the width of the main maximum peak and the secondary maximum peak, and the interval of the main maximum peak and the secondary maximum peak of the spectrum plane composed of the retained imaging points; S3, if the wafer pattern to be measured has rotational symmetry, the Fourier filter comprises horizontal needles and vertical needles, otherwise the Fourier filter comprises horizontal needles or vertical needles; wherein the number of the horizontal needles or the vertical needles is the sum of the number of the main maximum peak and the secondary maximum peak in the horizontal direction or the vertical direction, the width of the horizontal needles or the vertical needles is equal to the width of the main maximum peak and the secondary maximum peak in the horizontal direction or the vertical direction, and the interval of the adjacent horizontal needles or the adjacent vertical needles is equal to the interval of the adjacent peaks in the horizontal direction or the vertical direction.

2. The method of claim 1, wherein, When the image plane size of the dark field detection system is limited, the number of the horizontal needles or the vertical needles is the sum of the number of the main maximum peak and the secondary maximum peak in the horizontal direction or the vertical direction, the width of the horizontal needles or the vertical needles is equal to the width of the main maximum peak and the secondary maximum peak in the horizontal direction or the vertical direction, and the interval of the adjacent horizontal needles or the adjacent vertical needles is equal to the interval of the adjacent peaks in the horizontal direction or the vertical direction. wherein the target spectrum plane is a spectrum plane composed of frequency points corresponding to imaging points in the image plane range of the dark field detection system.

3. A Fourier filter for semiconductor dark field defect detection, characterized in that The Fourier filter is designed by the method of claim 1 or 2.

4. A semiconductor dark field defect detection system, characterized by, The Fourier filter comprises a scattering type dark field detection system and a Fourier filter of claim 3.

5. An electronic device, comprising: It comprises: a computer readable storage medium and a processor; the computer readable storage medium is used to store executable instructions; the processor is used to read the executable instructions stored in the computer readable storage medium and execute the method of claim 1 or 2.

6. A computer readable storage medium characterized by, The computer readable storage medium stores computer instructions for making the processor execute the method of claim 1 or 2.

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