A wafer thickness measurement device and measurement method based on a double Fizeau interferometer

Abstract

The present invention relates to a wafer thickness measurement device and method based on a double Fizeau interferometer. The device includes an environment detection system and a central processing unit. The environment detection system is used to measure the environmental parameters around the first reference mirror and the second reference mirror. The central processing unit is used to perform Zernike fitting on the cavity topography, decompose the cavity topography into a first-order topography and a higher-order topography greater than the first order. Using the environmental parameters measured under different environments, a relationship formula between the environmental parameters and the reference mirror tilt coefficient is constructed by fitting. The first-order topography correction amount during measurement is calculated using the relationship formula, and the original first-order topography is replaced with the first-order topography correction amount, and then the thickness of each coordinate point of the wafer is calculated. By real-time monitoring the tilt changes of the two reference mirrors through the environment detection system, the tilt amount of the absolute thickness of the wafer is compensated, thereby achieving an accurate reconstruction of the absolute thickness distribution of the wafer.

Description

Technical Field

[0001] The present invention relates to the technical field of optical precision measurement, and particularly relates to a wafer thickness measurement device and a measurement method based on a double Fizeau interferometer. Background Art

[0002] The prior art provides a double-sided topography interference measurement device for a wafer (such as a silicon wafer), as Figure 1 shown. This device is a double Fizeau interferometer. One-sided Fizeau interferometer includes a reference mirror 15 / 25, a collimating mirror 14 / 24, a beam splitter 13 / 23 (such as a PBS mirror), a light source 11 / 21, a detector 16 / 26, and a computer. The computer includes a host 17 / 27 and a display 18 / 28. In addition, the two-sided Fizeau interferometers can also share one computer. A cavity is formed between the two-sided reference mirrors 15 / 25. The silicon wafer 1 is vertically placed between the cavities. The interference fringe images of the front and back surfaces of the silicon wafer 1 are respectively obtained by using the two-sided Fizeau interferometers. Methods such as wavelength phase shift or mechanical phase shift are used to obtain the front surface topography d front and the back surface topography d back . Based on the same principle, the cavity topography d cavity (i.e., the topography between the two reference mirrors when the silicon wafer 1 is not placed) is obtained by cavity interference measurement. Thus, the thickness change amount of the silicon wafer 1 can be obtained as d cavity -(d front +d back ). Combining with the result Δbias of the thickness measurement of the silicon wafer 1 by the sensor, the absolute thickness t of the silicon wafer can be obtained as t = d cavity -(d front +d back )+Δbias.

[0003] To improve the stability of the cavity topography, it is often necessary to shorten the distance between the two reference mirrors as much as possible, resulting in a reduction in the maintainability of the device. At the same time, due to the real-time change of the environment inside the double Fizeau interferometer device, such as environmental parameters such as temperature and humidity are different in each measurement, these parameters will cause elastic deformation of the reference mirror clamping mechanism of the interferometer, thus changing the tilting state of the reference mirror, and the cavity topography cannot be guaranteed to be stable for a long time, resulting in measurement errors in the absolute thickness of the wafer. Summary of the Invention

[0004] In view of the technical problems existing in the prior art, the present invention provides a wafer thickness measurement device and a measurement method based on a double Fizeau interferometer, which can monitor the tilting change of the two reference mirrors in real time through an environment detection system, and then compensate the tilting amount of the absolute thickness of the wafer, thereby realizing the accurate reconstruction of the absolute thickness distribution of the wafer.

[0005] The technical solution of the present invention to solve the above technical problems is as follows:

[0006] On the one hand, the present invention provides a wafer thickness measuring device based on a double Fizeau interferometer, comprising:

[0007] The first Fizeau interferometer and the second Fizeau interferometer are symmetrically arranged. The first Fizeau interferometer includes a first reference mirror, and the second Fizeau interferometer includes a second reference mirror. A cavity capable of placing a wafer is formed between the first reference mirror and the second reference mirror; based on the interference measurement principle, the first Fizeau interferometer and the second Fizeau interferometer acquire the front surface topography d front (x, y), the back surface topography d back (x, y) and the cavity topography d cavity (x, y);

[0008] An environment detection system for measuring the environmental parameters around the first reference mirror and the second reference mirror;

[0009] A central processing unit for performing Zernike fitting on the cavity topography d cavity (x, y), decomposing the cavity topography into a first-order topography d cavity_tilt (x, y) = ax + by and a high-order topography d' cavity (x, y) where a and b are tilt coefficients, establishing a relationship between the environmental coefficient and the tilt coefficient. The relationship is a polynomial of at least the first order. Using the environmental parameters and tilt coefficients measured under different environments, the relationship is determined by fitting, and is used to calculate the first-order topography correction amount d' cavity_tilt (x, y) during measurement, and is used to calculate the thickness t(x, y) of the coordinate point (x, y) on the wafer according to the following formula:

[0010] t(x, y) = d' cavity_tilt (x, y) + d' cavity (x, y) - d front (x, y) - d back (x, y) + Δbias

[0011] In the formula, Δbias represents the translation compensation amount obtained by measuring the actual thickness of the wafer through a thickness measuring mechanism.

[0012] Furthermore, the relationship between the environmental parameter and the tilt coefficient of the reference mirror is as follows:

[0013]

[0014] where a and b are tilt coefficients, e is an environmental parameter, a j 、b j are conversion coefficients, and j = 1, 2, 3;

[0015] The central processing unit substitutes the environmental parameters e measured under different environments and the tilt coefficients of the reference mirror obtained by Zernike fitting into the relational expression to solve for the conversion coefficients a j , b j .

[0016] Furthermore, the device further includes an isolation cavity, and the first reference mirror and the second reference mirror are arranged in the isolation cavity.

[0017] Furthermore, the environmental detection system includes a plurality of sensors, and at least two of the plurality of sensors are arranged in the isolation cavity.

[0018] Furthermore, among the sensors arranged in the isolation cavity, at least one sensor is arranged on the first reference mirror, and at least one sensor is arranged on the second reference mirror.

[0019] Furthermore, the central processing unit obtains the environmental parameter e through the method shown by the following formula:

[0020]

[0021] where I is the total number of sensors, s i is the weight of the environmental parameters collected by the sensors at different positions, e i is the environmental parameter data collected by the sensors; the environmental parameters include temperature and / or humidity.

[0022] Furthermore, the weight s of the environmental parameters i is inversely proportional to the distance of the sensor from the center of the reference mirror; the environmental parameters include temperature and / or humidity.

[0023] Furthermore, the thickness measuring mechanism includes a capacitance sensor or an infrared sensor, and Δbias is obtained by single-point thickness measurement or the average value of multi-point thickness measurement;

[0024] If the thickness at the position (x 0 , y 0 ) is measured by single-point thickness measurement as t(x 0 , y 0 ), then Δbias is obtained by the following formula:

[0025] Δbias = t(x 0 , y 0 ) - d' cavity_tilt (x 0 , y 0 ) - d' cavity (x 0 , y 0 ) + d front (x0 , y 0 ) + d back (x 0 , y 0 )。

[0026] On the other hand, the present invention also provides a method for measuring the thickness of a wafer, which is implemented based on the above-mentioned wafer thickness measuring device based on a double Fizeau interferometer, and includes:

[0027] S801, measuring the current environmental parameters, and calculating the first-order topography correction amount d' cavity_tilt (x, y) in the current measurement environment by using the relationship between the environmental parameters and the reference mirror tilt coefficient;

[0028] S802, selecting one or more reference points on the wafer to be measured, and measuring the true thickness of the wafer to be measured at the reference points by using the thickness measuring mechanism, and then calculating the translation compensation amount Δbias;

[0029] S803, vertically placing the wafer to be measured in the cavity formed by the first reference mirror and the second reference mirror and performing interference measurement, and respectively obtaining the front surface topography d front (x, y) and the back surface topography d back (x, y);

[0030] S804, calculating the thickness t(x, y) of the coordinate point (x, y) on the wafer according to the following formula:

[0031] t(x, y) = d' cavity_tilt (x, y) + d' cavity (x, y) - d front (x, y) - d back (x, y) + Δbias

[0032] where d' cavity (x, y) is the high-order cavity topography greater than the first order obtained by Zernike fitting of the cavity topography.

[0033] Furthermore, the method for constructing the relationship between the environmental parameters and the reference mirror tilt coefficient includes the following steps:

[0034] S901, performing interference measurement on the cavity formed by the first reference mirror and the second reference mirror, obtaining the topography of the cavity, and recording the current environmental parameters;

[0035] S902, performing Zernike fitting on the cavity topography, decomposing the cavity topography into the first-order tilt amount d cavity_tilt (x, y) and the high-order topography d' cavity (x, y) greater than the first order, and extracting the first-order tilt coefficients a, b, d cavity_tilt(x,y) = ax + by;

[0036] S903. Change the measurement environment and repeat steps S901 and S902 multiple times; using the environmental parameters recorded multiple times and the corresponding first-order tilt coefficients a and b extracted, construct a relationship between the environmental parameters and the tilt coefficient of the reference mirror through fitting.

[0037] Furthermore, the relationship between the environmental parameters and the tilt coefficient of the reference mirror is as follows:

[0038]

[0039] where a and b are tilt coefficients, e is the environmental parameter, a j , b j are conversion coefficients, j = 1, 2, 3;

[0040] Substitute the environmental parameter e measured under different environments and the tilt coefficient of the reference mirror obtained by Zernike fitting into the relationship to solve for the conversion coefficients a j , b j .

[0041] The beneficial effects of the present invention are as follows: By adopting the real-time environmental parameter tilt compensation scheme of the present invention, using the environmental detection system to monitor the tilt changes of the two reference mirrors caused by environmental parameters, and then compensating for the tilt amount of the absolute thickness of the wafer, combined with the translation compensation amount of the absolute thickness of the reference point wafer measured by the thickness measurement sensor, the absolute thickness distribution of the entire wafer is reconstructed, which can effectively suppress the influence of environmental interference and structural instability on the stability of wafer thickness measurement and achieve accurate reconstruction of the absolute thickness distribution of the wafer. Description of the Drawings

[0042] Figure 1 is a schematic structural diagram of a crystal thickness measurement scheme of a double Fizeau interferometer in the prior art;

[0043] Figure 2 is a schematic structural diagram of a wafer thickness measurement device based on a double Fizeau interferometer provided by an embodiment of the present invention;

[0044] Figure 3 is a schematic flowchart of a wafer thickness measurement method based on a double Fizeau interferometer provided by an embodiment of the present invention. Detailed Embodiments

[0045] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0046] As Figure 2As shown in the figure, an embodiment of the present invention provides a wafer thickness measurement device based on a double Fizeau interferometer, including: a first Fizeau interferometer, a second Fizeau interferometer, a central processing unit 2, and an environment detection system (not shown in the figure).

[0047] Among them, the first Fizeau interferometer and the second Fizeau interferometer are symmetrically arranged. The first Fizeau interferometer on the left includes a first light source 11, a first beam splitter 13, a first collimator 14, a first reference mirror 15, and a first detector 16. The second Fizeau interferometer on the right includes a second light source 21, a second beam splitter 23, a second collimator 24, a second reference mirror 25, and a second detector 26. A cavity is formed between the first reference mirror 15 and the second reference mirror 25. In this embodiment, during measurement, the wafer 1 to be measured (for example, a silicon wafer) is vertically placed in the cavity. The first Fizeau interferometer and the second Fizeau interferometer are based on the interference measurement principle to obtain the front surface topography d front (x, y), the back surface topography d back (x, y), and the cavity topography d cavity (x, y).

[0048] The environment detection system is electrically connected to the central processing unit 2, and is used to measure the environmental parameters around the first reference mirror and the second reference mirror, and send the environmental parameters to the central processing unit 2.

[0049] Since each optical system component will deform due to changes in environmental temperature, humidity, etc., the environment detection system may include a temperature sensor and a humidity sensor.

[0050] The central processing unit 2 is mainly used for: performing Zernike fitting on the cavity topography d cavity (x, y), decomposing the cavity topography into a first-order topography d cavity_tilt (x, y) = ax + by and a higher-order topography d' cavity (x, y), where a and b are tilt coefficients; using the environmental parameters measured under different environments, constructing a relationship between the environmental parameters and the tilt coefficients of the reference mirror by least squares fitting; calculating the first-order topography correction amount d' cavity_tilt (x, y) during measurement using the relationship; used to calculate the thickness t(x, y) of the coordinate point (x, y) on the wafer according to the following formula:

[0051] t(x, y) = d' cavity_tilt (x, y) + d' cavity (x, y) - d front (x, y) - d back (x, y) + Δbias

[0052] In the formula, Δbias represents the translation compensation amount obtained by measuring the actual thickness of the wafer through the thickness measurement mechanism.

[0053] Preferably, the device further includes an isolation cavity 3, and the first reference mirror and the second reference mirror are arranged in the isolation cavity 3. The isolation cavity 3 does not affect the optical path. For example, openings are provided on both sides of the isolation cavity 3 to facilitate the passage of light.

[0054] To obtain accurate environmental parameters, the sensors in the environmental detection system need to be reasonably arranged. The sensors in the environmental detection can be arranged in this isolation cavity. Multiple sensors can be arranged at symmetrical positions of the two reference mirrors to accurately obtain the surrounding environmental parameters affecting the tilt of the reference mirror, making the calibration result more accurate. However, it is not limited to being arranged on the reference mirror and can also be located outside the isolation cavity. For example, sensors can be placed on the outer wall of the isolation cavity.

[0055] The monitoring of environmental parameters is not limited to temperature or humidity. A single environmental parameter or multiple coupled environmental parameters can be used.

[0056] Based on the wafer thickness measurement device based on a dual Fizeau interferometer provided in the above embodiments, the embodiments of the present invention also provide a wafer thickness measurement method, as Figure 3 shown, which specifically includes the following steps:

[0057] S801. Measure the current environmental parameters, and calculate the first-order topography correction amount d' cavity_tilt (x,y) under the current measurement environment by using the relationship between the environmental parameters and the reference mirror tilt coefficient.

[0058] S802. Select one or more reference points on the wafer to be measured, and use the thickness measurement mechanism to measure the actual thickness of the wafer to be measured at the reference points, and then obtain the translation compensation amount Δbias; the thickness measurement can be completed by a thickness measurement mechanism such as a capacitance sensor or an infrared sensor, and single-point thickness measurement or average of multi-point thickness measurement can be used.

[0059] If the thickness at the position of (x 0 ,y 0 ) is measured by single-point thickness measurement as t(x 0 ,y 0 ), then Δbias is obtained by the following formula:

[0060] Δbias = t(x 0 ,y 0 ) - d' cavity_tilt (x 0 ,y 0 ) - d' cavity (x 0 ,y 0 ) + d front (x 0 ,y 0 ) + d back (x0 , y 0 )。

[0061] S803, vertically place the wafer to be measured in the cavity formed by the first reference mirror and the second reference mirror and perform interference measurement to respectively obtain the front surface topography d front (x, y) and the back surface topography d back (x, y).

[0062] S804, calculate the thickness t(x, y) of the coordinate point (x, y) on the wafer according to the following formula:

[0063] t(x, y) = d' cavity_tilt (x, y) + d' cavity (x, y) - d front (x, y) - d back (x, y) + Δbias

[0064] In the formula, d' cavity (x, y) is the high-order cavity topography greater than the first order obtained by Zernike fitting of the cavity topography.

[0065] As a preferred embodiment, the method for constructing the relationship between the environmental parameters and the tilt coefficient of the reference mirror includes the following steps:

[0066] S901, perform interference measurement on the cavity formed by the first reference mirror and the second reference mirror, obtain the topography of the cavity, and record the environmental parameters.

[0067] S902, perform Zernike fitting on the cavity topography, decompose the cavity topography into the first-order tilt amount d cavity_tilt (x, y) and the high-order topography d' cavity (x, y) greater than the first order, and extract the first-order tilt coefficients a, b, d cavity_tilt (x, y) = ax + by.

[0068] S903, change the measurement environment, and repeat steps S901 and S902 multiple times; then use the environmental parameters recorded multiple times and the corresponding extracted first-order tilt coefficients a, b to construct the relationship between the environmental parameters and the tilt coefficient of the reference mirror by least squares fitting.

[0069] The high-order topography d' cavity (x, y) is mainly determined by the self-processing accuracy of the reference mirror and has nothing to do with environmental changes.

[0070] Due to the real-time changes in the environment, such as environmental parameters like temperature and humidity being different in each measurement, these parameters can cause elastic deformation of the interferometer reference mirror clamping mechanism, thus changing the tilt state of the reference mirror and resulting in errors in the tilt amount of the final thickness topography distribution. Therefore, it is necessary to establish the relationship between the tilt coefficient and environmental parameters.

[0071] Considering that the change in environmental parameters and the tilt change are approximately linear, and taking into account the execution efficiency, the least squares method is used to fit and construct a second-order polynomial equation:

[0072]

[0073] This can control the error to be relatively small, where a and b are the tilt coefficients, e is the environmental parameter, a j and b j are the conversion coefficients to be determined, and j = 1, 2, 3. By changing the environmental parameters through the external environmental control system, the environmental detection system measures and records the environmental parameters, and then performs cavity topography measurement and Zernike fitting decomposition. Multiple sets of tilt coefficient values under different environmental parameters can be measured, and then a j and b j can be obtained. At this time, the conversion relationship between the tilt coefficient and the environmental parameter can be determined. During the real-time measurement process, the environmental parameters are read in real time from the environmental detection system, and then the first-order quantity d cavity_tilt (x, y) of the cavity is calculated using the calibrated conversion relationship, so as to obtain the absolute thickness distribution of the wafer with high precision.

[0074] Preferably, different weights can be set for environmental parameters of different positions and parameter types. For example, the temperature near the reference mirror inside the isolation cavity has a greater impact on the change in tilt amount, and its weight can be increased. Finally, the environmental parameter e can be expressed as

[0075]

[0076] where I is the total number of sensors, s i is the weight of the environmental parameters collected by sensors of different positions and different parameter types, and e i is the reading of the environmental parameter.

[0077] For sensors at different positions, the weight of those located inside the isolation cavity needs to be higher than that of those located outside the isolation cavity. The weights of sensors at different positions can be inversely proportional to their distances from the center of the nearest reference mirror.

[0078] By setting sensors at different positions, monitoring different types of environmental parameters, and attaching weights to environmental parameters of different positions and different types, the stability and accuracy of the system can be improved.

[0079] By adopting the real-time environmental parameter tilt compensation scheme of the present invention, it is possible to effectively suppress the influence of environmental interference and structural instability on the measurement stability of the wafer thickness, and accurately reconstruct the absolute thickness distribution of the wafer.

[0080] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they learn the basic creative concept. Therefore, the appended claims are intended to be construed as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0081] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and variations.

Claims

1. A wafer thickness measurement device based on a double Fizeau interferometer, characterized in that, it includes: A first Fizeau interferometer and a second Fizeau interferometer are symmetrically arranged. The first Fizeau interferometer includes a first reference mirror, and the second Fizeau interferometer includes a second reference mirror. A cavity capable of placing a wafer is formed between the first reference mirror and the second reference mirror. The first Fizeau interferometer and the second Fizeau interferometer are based on the interference measurement principle to obtain the front surface topography of the wafer to be measured , the back surface topography and the cavity topography ; An environmental detection system for measuring the environmental parameters around the first reference mirror and the second reference mirror; A central processing unit for the cavity topography to perform Zernike fitting, and decompose the cavity topography into a first-order topography and a high-order topography higher than the first order , , is the tilt coefficient, used to establish a relationship between the environmental parameters and the tilt coefficient. The relationship is a polynomial of at least the first order. Using the environmental parameters and the tilt coefficient measured under different environments, the relationship is determined by fitting, and is used to calculate the first-order topography correction amount during measurement , used to calculate the coordinate point on the wafer according to the following formula : wherein represents the translation compensation amount obtained by measuring the actual thickness of the wafer through the thickness measuring mechanism; An isolation cavity, where the first reference mirror and the second reference mirror are arranged in the isolation cavity; The environmental detection system includes a plurality of sensors, and at least two of the plurality of sensors are arranged in the isolation cavity.

2. The device according to claim 1, characterized in that, The relationship between the environmental parameter and the tilt coefficient of the reference mirror is shown as follows: Among them and is the inclination coefficient, is the environmental parameter, and are the conversion coefficients, ; The central processing unit substitutes the environmental parameters measured under different environments and the tilt coefficients of the reference mirror obtained by Zernike fitting into the relational expression to solve for the conversion coefficient , .

3. The device according to claim 1, characterized in that, Among the sensors arranged in the isolation cavity, at least one sensor is arranged on the first reference mirror, and at least one sensor is arranged on the second reference mirror.

4. The device according to claim 1 or 3, characterized in that, The central processing unit obtains the environmental parameters by the method shown in the following formula : wherein I is the total number of sensors, is the weight of the environmental parameters collected by the sensors at different positions, is the environmental parameter data collected by the sensors.

5. The device according to claim 4, characterized in that, The weight of the environmental parameter is inversely proportional to the distance between the sensor and the center of the reference mirror; the environmental parameter includes temperature and / or humidity.

6. The device according to claim 1, characterized in that, The thickness measurement mechanism includes a capacitance sensor or an infrared sensor, obtained by single-point thickness measurement or by averaging multi-point thickness measurements; If the thickness measured at a single point is at the position of , then it is obtained from the following formula: 。 7. A wafer thickness measurement method, which is implemented based on the wafer thickness measurement device based on a double Fizeau interferometer according to any one of claims 1-6, characterized in that, it includes: S801, measure the current environmental parameters, and calculate the first-order topography correction amount in the current measurement environment by using the relationship between the environmental parameters and the reference mirror tilt coefficient ; S802, select one or more reference points on the wafer to be measured, and use the thickness measurement mechanism to measure the true thickness of the wafer to be measured at the reference points, and then calculate the translation compensation amount ; S803, Place the wafer to be measured in the cavity formed by the first reference mirror and the second reference mirror and perform interference measurement to respectively obtain the front surface topography and the back surface topography of the wafer to be measured and the back surface topography ; S804, calculate the coordinates on the wafer according to the following formula of the thickness : where is the cavity high-order topography of orders higher than 1 obtained by Zernike fitting of the cavity topography.

8. The method according to claim 7, characterized in that, A method for constructing the relationship between the environmental parameter and the tilt coefficient of the reference mirror includes the following steps: S901, perform interference measurement on the cavity formed by the first reference mirror and the second reference mirror, obtain the topography of the cavity, and record the current environmental parameters; S902, perform Zernike fitting on the cavity morphology, decompose the cavity morphology into the first-order tilt and the high-order morphology greater than the first order, and extract the first-order tilt coefficient; ​​​​​ S903. Change the measurement environment and repeat steps S901 and S902 multiple times; use the environmental parameters recorded multiple times and the corresponding first-order tilt coefficients extracted and , and construct a relational expression between the environmental parameters and the tilt coefficient of the reference mirror by fitting: Among them , is the inclination coefficient, is the environmental parameter, , are the conversion coefficients, .

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