Probe station needle height measurement method based on monocular ranging

Abstract

The present invention relates to a probe station needle height measurement method based on monocular ranging, including: S1, accurately positioning the wafer on the slide stage so that the crystal grain of the wafer is located near the focusing position of the fixed-focus camera; S2, lifting the slide stage, changing the object distance, taking multiple wafer images, and finding the clearest wafer image by the square gradient function as the focused image; S3, keeping the slide stage at a height at which the focused image is obtained, slightly changing the image distance of the fixed-focus camera, and taking at least two defocused images of different degrees; S4, based on the parameters of two of the defocused images, calculating the object distance u of the focused image by the point spread function of Gaussian distribution; S5, obtaining the needle height based on the object distance u of the focused image and the height difference between the probe free end and the focusing camera. The present invention has high measurement accuracy, so that the probe station can accurately align the needle.

Description

Technical Field

[0001] The present invention relates to the field of integrated circuit testing, and in particular to a method for measuring the height of a probe needle of a probe station based on monocular ranging. Background Art

[0002] With the development of the semiconductor and integrated circuit industries, the development of probe stations has become increasingly important across various sectors. Probe stations are widely used in the research and development of precision electrical measurements for complex, high-speed devices, reducing R&D time and device manufacturing costs while ensuring quality and reliability. As semiconductor processing advances, probe stations are also developing towards higher precision to meet production requirements. Efficient and high-precision positioning has become an increasingly important performance evaluation indicator for probe test equipment.

[0003] Probe stations are primarily used in semiconductor, integrated circuit, and package testing, primarily to ensure product quality and reduce manufacturing costs. Probe stations can secure wafers or chips and precisely position the object under test. Probe stations use probes to directly contact the die on the wafer, eliciting signals and achieving the test. Probe stations are categorized by operation: manual, semi-automatic, and fully automatic. In a manual probe station, the user installs the probe arm and probes into a manipulator and uses a microscope to position the probe tips on the object under test. Once all probe tips are correctly positioned, testing can begin. In both semi-automatic and fully automatic probe stations, this operation is automated using a mechanized worktable and machine vision. Currently, probe height measurement accuracy in semi-automatic and fully automatic probe stations is low, resulting in probes failing to contact the die or over-contacting the die. Therefore, there is an urgent need for a highly accurate probe height measurement method. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the defects of the prior art and provide a method for measuring the height of a probe needle based on monocular ranging, which has high measurement accuracy and enables the probe station to accurately align the needle.

[0005] In order to solve the above technical problems, the technical solution of the present invention is: a probe station needle height measurement method based on monocular ranging, comprising:

[0006] S1, accurately positioning the wafer on the stage so that the die of the wafer is located near the focusing position of the fixed-focus camera;

[0007] S2, raising and lowering the wafer stage, changing the object distance, taking multiple wafer images, and finding the clearest wafer image as the focused image through the square gradient function;

[0008] S3, maintaining the stage at a height at which the focused image is obtained, slightly changing the image distance of the fixed-focus camera, and capturing at least two defocused images with different degrees of defocus;

[0009] S4, based on the parameters of the two defocused images, calculate the object distance of the focused image through the point spread function of the Gaussian distribution ;

[0010] S5, object distance based on the aggregated image The height difference between the free end of the probe and the focusing camera is used to obtain the needle height.

[0011] Further, step S2 specifically includes:

[0012] The wafer stage moves upward by a preset distance each time to change the object distance. Each time it moves, the fixed-focus camera captures a wafer image.

[0013] The clarity of each wafer image is quantitatively evaluated using a square gradient function, and the clearest wafer image is used as the aggregate image.

[0014] Further, step S4 specifically includes:

[0015] Bundle The point is viewed as a point light source, and the fixed-focus camera is viewed as a thin convex lens. After passing through the thin convex lens, the focused image is obtained. ; Assume that the focal length of the thin convex lens is , the distance between the point light source and the thin convex lens, that is, the object distance, is , the distance between the focused image and the lens is the image distance , the principle of point light source imaging is expressed by formula (2):

[0016] (2)

[0017] According to the defocus imaging principle, a defocused image is an image that cannot be focused, that is, A fixed-focus camera is used to obtain a

[0018] A blurry spot of light Rather than a focused point, since the lens is circular, the resulting blurred light spot is also circular. According to formula (2) and the defocus imaging principle, we can get:

[0019] (3)

[0020] Where D is the diameter of the fixed-focus camera lens, The focal length of a fixed-focus camera, is the distance between the imaging surface and the lens, For object distance, is the diffuse spot radius, i.e. the blurred spot radius;

[0021] The imaging system uses a two-dimensional Cauchy distribution function as the point spread function, which is:

[0022] (4)

[0023] in is the point spread function, and Represents the coordinates of the pixel point, is the Gaussian standard deviation, which is used to indicate the degree of blur, i.e. the size of the light spot;

[0024] and Proportional, assuming ;Will Bringing this into the defocus imaging principle, we get:

[0025] (5)

[0026] in .

[0027] The parameters of one of the defocused images are expressed as 、 、 , the parameters of the other defocused image are expressed as 、 、 , since the imaging system is a linear system, we have:

[0028] (6)

[0029] in is the power spectral density of the aggregate image, is the Fourier transform function of the aggregate image, for conjugation of;

[0030] According to formula (5), we can get:

[0031] (7)

[0032] According to formula (6), we can get:

[0033] (8)

[0034] That is: (9)

[0035] This season (10)

[0036] In order to reduce errors and improve stability, the average value of a region is used in the calculation, which is expressed as:

[0037] (11)

[0038] in Expressed as the average value of the region, Indicates not including frequency region;

[0039] According to formula (5) and formula (7), we can get:

[0040] (12)

[0041] According to formula (9), formula (10) and formula (12), we can get:

[0042] (13)

[0043] Solving the equation yields: (14).

[0044] After adopting the above technical solution, the present invention has the following beneficial effects:

[0045] (1) Autofocus evaluates image clarity by introducing a square gradient function, which has good single peak performance and makes focusing faster and more effective;

[0046] (2) Through the defocus ranging algorithm, the point spread function is introduced to calculate the object distance of the focused image, and the needle height is obtained based on the object distance, thereby improving the ranging accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 The present invention is a flow chart of a probe station needle height measurement method based on monocular ranging. DETAILED DESCRIPTION

[0048] In order to make the contents of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments in conjunction with the accompanying drawings.

[0049] like Figure 1 As shown, a probe station needle height measurement method based on monocular ranging includes:

[0050] S1, accurately positioning the wafer on the stage so that the die of the wafer is located near the focusing position of the fixed-focus camera;

[0051] S2, raising and lowering the wafer stage, changing the object distance, taking multiple wafer images, and finding the clearest wafer image as the focused image through the square gradient function;

[0052] S3, maintaining the stage at a height at which the focused image is obtained, slightly changing the image distance of the fixed-focus camera, and capturing at least two defocused images with different degrees of defocus;

[0053] S4, based on the parameters of the two defocused images, calculate the object distance of the focused image through the point spread function of the Gaussian distribution ;

[0054] S5, object distance based on the aggregated image The height difference between the free end of the probe and the focusing camera is used to obtain the needle height.

[0055] The automatic focusing of the present invention evaluates the image clarity by introducing the square gradient function, which has good single peak and makes focusing faster and more effective; the object distance of the focused image is calculated by introducing the point spread function through the defocus ranging algorithm. , based on object distance The needle height is obtained to improve the ranging accuracy.

[0056] In this embodiment, when the free end of the probe and the focusing camera are at the same height, the object distance calculated is , that is, the needle height.

[0057] In this embodiment, in step S1, the precise positioning of the wafer on the wafer stage also includes determining the position of the wafer and the fixed-focus camera. , The relative position in the direction, that is, the wafer , The coordinates of the wafer and the diameter of the wafer are determined, and then the center of the wafer on the stage is moved to just below the fixed-focus camera through the control system.

[0058] The wafer stage moves upward by a preset distance each time to change the object distance. Each time it moves, the fixed-focus camera captures a wafer image.

[0059] The clarity of each wafer image is quantitatively evaluated using a square gradient function, and the clearest wafer image is used as the aggregate image.

[0060] The preset distance can be 1 micron. A wafer image is taken every time 1 micron is moved, and the image stops after 20 microns. The square gradient function is introduced to evaluate image clarity. The square gradient function is calculated by summing the square difference between the grayscale values ​​of each adjacent column and the square difference between the grayscale values ​​of each adjacent row in the image. The clearer the image, the greater the difference between adjacent pixels, and the greater the square gradient. The square gradient function can be used to quantitatively evaluate image clarity, and the clearest image, i.e., the focused image, is found through the square gradient function. At this point, the entire autofocus process is completed, and the square gradient function has better unimodality. In order to take into account both real-time and accuracy in the focusing process, the square gradient function can be used to highlight the focused area, reduce the amount of calculation, and improve detection efficiency. The specific expression of the square gradient function is:

[0061] (1)

[0062] in For the image at point The gray value at . The larger it is, the clearer the image.

[0063] In this embodiment, step S4 specifically includes:

[0064] Process 1: First analyze the principle of point light source imaging, The point is viewed as a point light source, and the fixed-focus camera is viewed as a thin convex lens. After passing through the thin convex lens, the focused image is obtained. ; Assume that the focal length of the thin convex lens is , the distance between the point light source and the thin convex lens, that is, the object distance, is , the distance between the focused image and the lens is the image distance , the principle of point light source imaging is expressed by formula (2):

[0065] (2)

[0066] By formula (2) and focal length and image distance If both are known, the object distance can be determined. .

[0067] Then according to the defocus imaging principle, the defocused image is an image that cannot be focused, that is, The point gets a blurred spot through the fixed focus camera Rather than a focused point, since the lens is circular, the resulting blurred light spot is also circular. According to formula (2) and the defocus imaging principle, we can get:

[0068] (3)

[0069] Where D is the diameter of the fixed-focus camera lens, The focal length of a fixed-focus camera, is the distance between the imaging surface and the lens, For object distance, is the diffuse spot radius, i.e. the blurred spot radius;

[0070] Considering the non-ideal situation of lens imaging, it is inaccurate to use the cylindrical distribution point spread function to describe the defocus point spread distribution function. The main reason is that The boundary of the point on the imaging surface is not clear but a light spot with gradually blurred edges. Therefore, the two-dimensional Cauchy distribution function is used as the point spread function, and the point spread function is:

[0071] (4)

[0072] in is the point spread function, and Represents the coordinates of the pixel point, is the Gaussian standard deviation, which is used to indicate the degree of blur, i.e. the size of the light spot;

[0073] The larger the Gaussian standard deviation, the greater the blur. The larger the Gaussian standard deviation is, the better the focusing degree is. The smaller it is, the smaller it is. is a fuzzy parameter; from the above we can conclude and In direct proportion, we can assume that: ; Substituting this into the defocus imaging principle, we can get:

[0074] (5)

[0075] in .

[0076] The parameters of one of the defocused images are expressed as 、 、 , the parameters of the other defocused image are expressed as 、 、 , since the imaging system is a linear system, we have:

[0077] (6)

[0078] in is the power spectral density of the aggregate image, is the Fourier transform function of the aggregate image, for conjugation of;

[0079] According to formula (5), we can get:

[0080] (7)

[0081] According to formula (6), we can get:

[0082] (8)

[0083] That is: (9)

[0084] This season (10)

[0085] In order to reduce errors and improve stability, the average value of a region is used in the calculation, which is expressed as:

[0086] (11)

[0087] in Expressed as the average value of the region, Indicates not including frequency region;

[0088] According to formula (5) and formula (7), we can get:

[0089] (12)

[0090] According to formula (9), formula (10) and formula (12), we can get:

[0091] (13)

[0092] Solving the equation yields: (14).

[0093] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A probe station needle height measurement method based on monocular ranging, characterized in that: include: S1, accurately positioning the wafer on the stage so that the die of the wafer is located near the focusing position of the fixed-focus camera; S2, raising and lowering the wafer stage, changing the object distance, taking multiple wafer images, and finding the clearest wafer image as the focused image through the square gradient function; S3, maintaining the stage at a height at which the focused image is obtained, slightly changing the image distance of the fixed-focus camera, and capturing at least two defocused images with different degrees of defocus; S4, based on the parameters of two of the defocused images, calculate the object distance of the focused image through the point spread function of the Gaussian distribution ; S5, object distance based on the aggregated image The height difference between the free end of the probe and the focusing camera is used to obtain the needle height.

2. The method for measuring the needle height of a probe station based on monocular ranging according to claim 1, wherein Step S2 specifically includes: The wafer stage moves upward by a preset distance each time to change the object distance. Each time it moves, the fixed-focus camera captures a wafer image. The clarity of each wafer image is quantitatively evaluated using a square gradient function, and the clearest wafer image is used as the aggregate image.

3. The method for measuring the needle height of a probe station based on monocular ranging according to claim 1, wherein Step S4 specifically includes: Bundle The point is viewed as a point light source, and the fixed-focus camera is viewed as a thin convex lens. After passing through the thin convex lens, the focused image is obtained. ; Assume that the focal length of the thin convex lens is , the distance between the point light source and the thin convex lens, that is, the object distance, is , the distance between the focused image and the lens is the image distance , the principle of point light source imaging is expressed by formula (2): （2） According to the defocus imaging principle, a defocused image is an image that cannot be focused, that is, The point gets a blurred spot through the fixed focus camera Rather than a focused point, since the lens is circular, the resulting blurred light spot is also circular. According to formula (2) and the defocus imaging principle, we can get: （3） Where D is the diameter of the fixed-focus camera lens, The focal length of a fixed-focus camera, is the distance between the imaging surface and the lens, For object distance, is the diffuse spot radius, i.e. the blurred spot radius; The imaging system uses a two-dimensional Cauchy distribution function as the point spread function, which is: （4） in is the point spread function, and Represents the coordinates of the pixel point, is the Gaussian standard deviation, which is used to indicate the degree of blur, i.e. the size of the light spot; and Proportional, assuming ;Will Bringing this into the defocus imaging principle, we get: （5） in ; The parameters of one of the defocused images are expressed as 、 、 , the parameters of the other defocused image are expressed as 、 、 , since the imaging system is a linear system, we have: （6） in is the power spectral density of the aggregate image, is the Fourier transform function of the aggregate image, for conjugation of; According to formula (5), we can get: （7） According to formula (6), we can get: （8） That is: (9) This season (10) In order to reduce errors and improve stability, the average value of a region is used in the calculation, which is expressed as: （11） in Expressed as the average value of the region, Indicates not including frequency region; According to formula (5) and formula (7), we can get: （12） According to formula (9), formula (10) and formula (12), we can get: （13） Solving the equation yields: (14).

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