Metal film thickness measuring method for reducing stray capacitance effect

Abstract

The invention provides a metal film thickness measuring method capable of reducing a parasitic capacitance effect, and a device adopted by the method comprises a sensor coil, a front signal processing module, a data acquisition module, a communication module and an upper computer, the method comprises the following steps: respectively inputting alternating voltage to a sensor coil and a preposed signal processing circuit through a crystal oscillator in a signal generator, outputting a measured impedance value corresponding to the thickness of a metal film through the preposed signal processing circuit, inputting the impedance value into an upper computer through a data acquisition module and a communication module, and performing fitting operation in the upper computer to obtain the thickness of the metal film. Calculating to obtain a corresponding metal film thickness characteristic value; according to the invention, a single-coil sensor system is taken as a core part, and the measurement method adopts a characteristic slope method combined with differential evolution algorithm fitting, so that the nano-scale thickness resolution can be obtained, and the parasitic capacitance effect can be remarkably reduced.

Description

Technical Field

[0001] The invention relates to the field of measurement technology, in particular to a metal film thickness measurement method for reducing parasitic capacitance effect. Background Art

[0002] With the rapid development of integrated circuit and semiconductor manufacturing technology, the role of metal film materials in precision manufacturing has become increasingly important. Especially in the interconnection, packaging and component manufacturing process of integrated circuits, the precise control of metal film thickness is directly related to the performance and reliability of the device. Nano-scale metal films are widely used in the conductive layer, shielding layer and other functional layers inside the chip. Accurately measuring the thickness of these metal films is crucial to ensuring the performance and quality of the product. However, traditional thickness measurement methods such as mechanical contact probes and optical interferometry have low measurement accuracy and reliability when facing complex surfaces or small sizes.

[0003] The eddy current measurement method uses the electromagnetic interaction between the coil and the metal film to infer the thickness of the metal film by measuring the change in inductive impedance. Compared with traditional measurement methods, the eddy current measurement method can not only quickly and non-contactly obtain the thickness data of the metal film, but also effectively cope with the measurement challenges of high frequency, complex morphology and tiny size.

[0004] However, in practical applications, eddy current measurement technology still faces many challenges. Parasitic capacitance effects, signal noise interference and the complexity of circuit design will affect the measurement accuracy. Especially in the measurement of nano-metal films, the presence of parasitic capacitance will significantly reduce the resonant frequency, thereby affecting the measurement accuracy. Therefore, how to overcome the parasitic effects in eddy current measurement and improve the sensitivity and stability of the measurement system has become an important topic of current research. Summary of the invention

[0005] The present invention proposes a metal film thickness measurement method for reducing parasitic capacitance effect. The single-coil sensor system is used as the core part. The measurement method adopts a characteristic slope method combined with differential evolution algorithm fitting, which can obtain nanometer-level thickness resolution and significantly reduce parasitic capacitance effect.

[0006] The present invention adopts the following technical solutions.

[0007] A method for measuring the thickness of a metal film for reducing the parasitic capacitance effect, wherein the device used in the method comprises a sensor coil, a front signal processing module, a data acquisition module, a communication module and a host computer, wherein the method inputs an AC voltage to the sensor coil and the front signal processing circuit respectively through a crystal oscillator in a signal generator, the front signal processing circuit outputs a measured impedance value corresponding to the thickness of the metal film, the impedance value is input to the host computer through the data acquisition module and the communication module, a fitting operation is performed in the host computer, and a corresponding characteristic value of the thickness of the metal film is calculated, and the method comprises the following steps:

[0008] Step 1: Install the data acquisition module, communication module and host computer to ensure their normal operation;

[0009] Step 2: Pass a sweep frequency excitation signal to both ends of the sensor coil through a signal generating circuit;

[0010] Step 3: Place a wafer coated with a metal film on a wafer holder, and measure the frequency sweep value of the wafer at each lift-off height by changing the lift-off height multiple times;

[0011] Step 4: Obtain the parasitic capacitance value through calculation, and use the differential evolution algorithm to fit the true value of the impedance at each frequency to obtain the characteristic slope value. The calibration curve is obtained by multiplying the absolute value of the characteristic slope and the film thickness. The characteristic slope values ​​of multiple coated wafers with different thicknesses can also be obtained by repeating step 3;

[0012] Step 5: Repeat the above steps to complete the calibration curves of other metal films and store the calibration curves in the host computer;

[0013] Step 6: Take a wafer with known surface material to be tested, measure and fit the characteristic slope value S, and compare it with the calibration curve of the corresponding material film obtained in step 4, that is, calculate the thickness of the metal film according to the mapping relationship in table Sc.

[0014] The sensor coil is a single-coil eddy current sensor.

[0015] The single-coil eddy current sensor is an eddy current sensor fixed by a clamp and connected to the front circuit of the front signal processing module by a lead wire.

[0016] In step one, the sensor coil is located above the wafer holder and is close to the wafer holder. In step two, a sweep frequency excitation signal is sent to the sensor coil to induce an eddy current in the metal film of the wafer adjacent to the sensor coil.

[0017] In step 2, the sweep frequency range of the sweep frequency excitation signal is 0.01 MHz-50 MHz.

[0018] In step three, the lift-off height is changed multiple times within the lift-off height range of 0 mm to 3 mm, and at least two sweep frequency values ​​of the lift-off height are measured.

[0019] In step three, when the lift-off height is changed multiple times, the difference in each lift-off height is made as large as possible to measure at least one thickness of the coated wafer.

[0020] The single coil sensor is used to excite and measure the impedance change caused by different metal film thicknesses. When other measurement environment parameters remain unchanged, there is a one-to-one correspondence between the impedance change measured by the single coil sensor and the metal film thickness. When parasitic capacitance is not considered, the actual impedance value at both ends of the coil is:

[0021] Z=R+jωL Formula 1;

[0022] When there is parasitic capacitance at both ends of the coil due to the existence of the coil lead in the experimental measurement, the measured impedance value at this time is:

[0023]

[0024] When the influence of parasitic capacitance needs to be minimized to accurately measure the film thickness, the actual impedance value of the coil needs to be obtained and the characteristic slope method is derived using the formula:

[0025]

[0026] Formula 4 is the analytical solution for the impedance change. When the phase term and amplitude term in the analytical solution are separable and compared with other parts of the integrand, The rate of change with α is small, and When the characteristic frequency α0 reaches its maximum value; the approximation of the solution is calculated at α0 And proposed that the phase term of impedance change is only given by The phase term includes the conductivity, thickness and magnetic permeability of the metal film, which can be simplified by approximation:

[0027]

[0028] At this time, the peak frequency is:

[0029] The change of α0 due to the change of lift-off height is

[0030]

[0031] The change in peak frequency is:

[0032] Further we get:

[0033]

[0034] That is, the ratio of the change in peak frequency to the change in lift-off height is inversely proportional to the film thickness, and the ratio is the characteristic slope S.

[0035] In step 3, the frequency value corresponding to when the imaginary part of the inductance reaches the maximum value in the frequency sweep mode is used as the peak frequency, and the definition formula is:

[0036]

[0037] In order to obtain the accurate imaginary value of the inductance, it is necessary to obtain the precise value at the corresponding frequency, that is, the actual value of the change in the real part of the coil impedance.

[0038] From the above formula, when other conditions remain unchanged, the characteristic slope is inversely proportional to the film thickness c; when measuring the thickness of nano-scale metal films, the corresponding peak frequency is higher, and the presence of parasitic capacitance reduces the resonance frequency, making the measurement error larger, so it is necessary to reduce the influence of parasitic capacitance; when changing the lift-off height to obtain the peak frequency, the change in the lift-off height can cause a large change in the peak frequency, and a peak frequency-lift-off height relationship curve with excellent linearity can be obtained;

[0039] When the eddy current intensity in the conductor decreases exponentially with the increase of the depth into the conductor due to the skin effect, in order to accurately measure the metal film thickness, the skin depth at the corresponding working frequency f needs to be greater than the metal film thickness, that is, the electromagnetic field completely penetrates the metal film; for non-magnetic materials, the calculation formula of the skin depth is as follows:

[0040]

[0041] In order to reduce the impact of parasitic capacitance, an intelligent optimization algorithm is used to fit the true impedance value of the coil at each frequency. First, the resonant frequency of the coil in the air should be measured, and the inductance value of the coil should be measured at a low frequency. Then, the value of the parasitic capacitance is obtained by the following formula:

[0042]

[0043] The measured impedance values ​​corresponding to the film thickness and at different frequencies are obtained, such as Formula 2 and Formula 3. At a certain frequency, the parallel capacitance value, frequency, measured real part and imaginary part values ​​are known. At this time, two fitting output parameters R and L can be obtained, that is, the actual value of the coil impedance.

[0044] A global optimization algorithm needs to be selected during fitting. This method uses a differential evolution algorithm for fitting.

[0045] The specific steps of the differential evolution algorithm are as follows:

[0046] xi，0 =(R i，0 , L i，0 ) Formula 13;

[0047] R i，0 and L i，0 are the resistance and inductance values ​​of the i-th individual in the initial population, generated by random initialization:

[0048] R i，0 =R min +rand(0,1)·(R max -R min ) Formula 14;

[0049] L i，0 =L min +rand(0,1)·(L max -L min ) Formula 15;

[0050] Where R min and R max are the lower and upper bounds of resistance, L min and L max are the lower and upper bounds of the inductance, and rand(0,1) is a uniform random number between [0,1];

[0051] The mutation operation is to amplify the difference of individuals and generate new individuals; for each individual x i，G =(R i，G , L i，G ), randomly select three different individuals x from the population r1，G =(R r1，G , L r1，G ), x r2，G 、x r3，G , generate a new mutation vector v i，G :

[0052] v i，G =(R v，i，G L v，i，G )=(R r1，G , L r1，G )+F·((R r2，G , L r2，G )-(R r3，G , L r3，G )) Formula 16;

[0053] Where F is the scaling factor, controlling the magnitude of the variation;

[0054] The vector generated by the mutation operation is crossed with the current individual; for each individual x i，G =(R i，G , Li，G ), generate a new test vector u i，G =(R u，i，G , L u，i，G ):

[0055]

[0056] L u，i，G The generation and R u，i，G Similar; crossover probability C r determines the proportion of parameters inherited from the mutation vector, j is a random index that ensures that at least one component is selected from the mutation vector;

[0057] Used in the selection step to decide whether to use the trial vector u i，G Replace the original individual x i，G Enter the next generation; select according to the fitness function value, that is, the objective function value, and optimize the algorithm through multiple iterations until the termination condition is met, such as reaching the maximum number of iterations or meeting the convergence condition; set the error function to be the difference between the actual impedance measurement value and the impedance value obtained by the fitting model, that is, formula 2 and formula 3, as shown in the following formulas 18 and 19, and the objective function formula 20 is the sum of the squares of the real error and the imaginary error, and set the relative error of the objective function value to 10 -6 Stop fitting at level .

[0058] error_real=Z_measured_real-real(Z) Formula 18;

[0059] error_imag=Z_measured.imag-imaag(Z) Formula 19;

[0060] error_sum=error_real^2+error_imag^2 Formula 20.

[0061] The present invention can obtain nanometer-level thickness resolution and significantly reduce the parasitic capacitance effect.

[0062] When the method of the present invention changes the lift-off height to obtain the peak frequency, the change in the lift-off height can cause a greater change in the peak frequency, and a peak frequency-lift-off height relationship curve with excellent linearity can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0064] Attached Figure 1 It is a schematic diagram of the equivalent circuit at both ends of a single coil sensor loop when no capacitor exists;

[0065] Attached Figure 2It is a schematic diagram of the equivalent circuit at both ends of a single coil sensor loop when there is a capacitor;

[0066] Attached Figure 3 It is a schematic diagram of the optimization steps of the differential evolution algorithm;

[0067] Attached Figure 4 This is a schematic diagram of the real part of the measured impedance and the fitted value when the frequency sweep measurement is performed on a 315nm copper film at the same lift-off height;

[0068] Attached Figure 5 It is a schematic diagram of the fitted R value and the actual resistance value;

[0069] Attached Figure 6 It is a schematic diagram of the relative error of the fitting results;

[0070] Attached Figure 7 This is a schematic diagram of the relationship between the imaginary part of inductance and frequency for different film thicknesses at the same lift-off height after fitting;

[0071] Attached Figure 8 This is a diagram showing the relationship between the peak frequency and the lift-off height obtained after fitting the measurement of a 315nm copper film;

[0072] Attached Fig. 9 It is a schematic diagram of the relationship curve between the inverse of the absolute value of the characteristic slope and the film thickness;

[0073] Attached Fig.10 It is a schematic flow chart of the measuring method of the present invention;

[0074] Attached Fig.11 It is a schematic diagram of hardware connection used in the present invention. DETAILED DESCRIPTION

[0075] As shown in the figure, a method for measuring the thickness of a metal film for reducing the parasitic capacitance effect is provided. The device used in the method includes a sensor coil, a front signal processing module, a data acquisition module, a communication module and a host computer. The method inputs an AC voltage to the sensor coil and the front signal processing circuit respectively through a crystal oscillator in a signal generator, and the front signal processing circuit outputs a measured impedance value corresponding to the thickness of the metal film, and the impedance value is input to the host computer through the data acquisition module and the communication module, and a fitting operation is performed in the host computer to calculate the corresponding characteristic value of the thickness of the metal film, and the method includes the following steps;

[0076] Step 1: Install the data acquisition module, communication module and host computer to ensure their normal operation;

[0077] Step 2: Pass a sweep frequency excitation signal to both ends of the sensor coil through a signal generating circuit;

[0078] Step 3: Place a wafer coated with a metal film on a wafer holder, and measure the frequency sweep value of the wafer at each lift-off height by changing the lift-off height multiple times;

[0079] Step 4: Obtain the parasitic capacitance value through calculation, and use the differential evolution algorithm to fit the true value of the impedance at each frequency to obtain the characteristic slope value. The calibration curve is obtained by multiplying the absolute value of the characteristic slope and the film thickness. The characteristic slope values ​​of multiple coated wafers with different thicknesses can also be obtained by repeating step 3;

[0080] Step 5: Repeat the above steps to complete the calibration curves of other metal films and store the calibration curves in the host computer;

[0081] Step 6: Take a wafer with known surface material to be tested, measure and fit the characteristic slope value S, and compare it with the calibration curve of the corresponding material film obtained in step 4, that is, calculate the thickness of the metal film according to the mapping relationship in table Sc.

[0082] The sensor coil is a single-coil eddy current sensor.

[0083] The single-coil eddy current sensor is an eddy current sensor fixed by a clamp and connected to the front circuit of the front signal processing module by a lead wire.

[0084] In step one, the sensor coil is located above the wafer holder and is close to the wafer holder. In step two, a sweep frequency excitation signal is sent to the sensor coil to induce an eddy current in the metal film of the wafer adjacent to the sensor coil.

[0085] In step 2, the sweep frequency range of the sweep frequency excitation signal is 0.01 MHz-50 MHz.

[0086] In step three, the lift-off height is changed multiple times within the lift-off height range of 0 mm to 3 mm, and at least two sweep frequency values ​​of the lift-off height are measured.

[0087] In step three, when the lift-off height is changed multiple times, the difference in each lift-off height is made as large as possible to measure at least one thickness of the coated wafer.

[0088] The single coil sensor is used to excite and measure the impedance change caused by different metal film thicknesses. When other measurement environment parameters remain unchanged, there is a one-to-one correspondence between the impedance change measured by the single coil sensor and the metal film thickness. When parasitic capacitance is not considered, the actual impedance value at both ends of the coil is:

[0089] Z=R+jωL Formula 1;

[0090] When there is parasitic capacitance at both ends of the coil due to the existence of the coil lead in the experimental measurement, the measured impedance value at this time is:

[0091]

[0092]

[0093] When the influence of parasitic capacitance needs to be minimized to accurately measure the film thickness, the actual impedance value of the coil needs to be obtained and the characteristic slope method is derived using the formula:

[0094]

[0095] Formula 4 is the analytical solution for the impedance change. When the phase term and amplitude term in the analytical solution are separable and compared with other parts of the integrand, The rate of change with α is small, and When the characteristic frequency α0 reaches its maximum value; the approximation of the solution is calculated at α0 And proposed that the phase term of impedance change is only given by The phase term includes the conductivity, thickness and magnetic permeability of the metal film, which can be simplified by approximation:

[0096]

[0097] At this time, the peak frequency is: The change of α0 due to the change of lift-off height is

[0098]

[0099] The change in peak frequency is: Further we get:

[0100]

[0101] That is, the ratio of the change in peak frequency to the change in lift-off height is inversely proportional to the film thickness, and the ratio is the characteristic slope S.

[0102] In step 3, the frequency value corresponding to when the imaginary part of the inductance reaches the maximum value in the frequency sweep mode is used as the peak frequency, and the definition formula is:

[0103]

[0104] In order to obtain the accurate imaginary value of the inductance, it is necessary to obtain the precise value at the corresponding frequency, that is, the actual value of the change in the real part of the coil impedance.

[0105] From the above formula, when other conditions remain unchanged, the characteristic slope is inversely proportional to the film thickness c; when measuring the thickness of nano-scale metal films, the corresponding peak frequency is higher, and the presence of parasitic capacitance reduces the resonance frequency, making the measurement error larger, so it is necessary to reduce the influence of parasitic capacitance; when changing the lift-off height to obtain the peak frequency, the change in the lift-off height can cause a large change in the peak frequency, and a peak frequency-lift-off height relationship curve with excellent linearity can be obtained;

[0106] When the eddy current intensity in the conductor decreases exponentially with the increase of the depth into the conductor due to the skin effect, in order to accurately measure the metal film thickness, the skin depth at the corresponding working frequency f needs to be greater than the metal film thickness, that is, the electromagnetic field completely penetrates the metal film; for non-magnetic materials, the calculation formula of the skin depth is as follows:

[0107]

[0108] In order to reduce the impact of parasitic capacitance, an intelligent optimization algorithm is used to fit the true impedance value of the coil at each frequency. First, the resonant frequency of the coil in the air should be measured, and the inductance value of the coil should be measured at a low frequency. Then, the value of the parasitic capacitance is obtained by the following formula:

[0109]

[0110] The measured impedance values ​​corresponding to the film thickness and at different frequencies are obtained, such as Formula 2 and Formula 3. At a certain frequency, the parallel capacitance value, frequency, measured real part and imaginary part values ​​are known. At this time, two fitting output parameters R and L can be obtained, that is, the actual value of the coil impedance.

[0111] A global optimization algorithm needs to be selected during fitting. This method uses a differential evolution algorithm for fitting.

[0112] The specific steps of the differential evolution algorithm are as follows:

[0113] x i，0 =(R i，0 , L i，0 ) Formula 13;

[0114] R i，0 and L i，0 are the resistance and inductance values ​​of the i-th individual in the initial population, generated by random initialization:

[0115] R i，0 =R min +rand(0,1)·(R max -R min ) Formula 14;

[0116] Li，0 =L min +rand(0,1)·(L max -L min ) Formula 15;

[0117] Where R min and R max are the lower and upper bounds of resistance, L min and L max are the lower and upper bounds of the inductance, and rand(0,1) is a uniform random number between [0,1];

[0118] The mutation operation is to amplify the difference of individuals and generate new individuals; for each individual x i，G =(R i，G , L i，G ), randomly select three different individuals x from the population r1，G =(R r1，G , L r1，G ), x r2，G 、x r3，G , generate a new mutation vector v i，G :

[0119] v i，G =(R v，i，G L v，i，G )=(R r1，G , L rl，G )+F·((R r2，G , L r2，G )-(R r3，G , L r3，G )) Formula 16;

[0120] Where F is the scaling factor, controlling the magnitude of the variation;

[0121] The vector generated by the mutation operation is crossed with the current individual; for each individual x i，G =(R i，G , L i，G ), generate a new test vector u i，G =(R u，i，G , L u，i，G ):

[0122]

[0123] L u，i，G The generation and R u，i，G Similar; crossover probability C r determines the proportion of parameters inherited from the mutation vector, j is a random index that ensures that at least one component is selected from the mutation vector;

[0124] Used in the selection step to decide whether to use the trial vector u i，G Replace the original individual x i，G Enter the next generation; select according to the fitness function value, that is, the objective function value, and optimize the algorithm through multiple iterations until the termination condition is met, such as reaching the maximum number of iterations or meeting the convergence condition; set the error function to be the difference between the actual impedance measurement value and the impedance value obtained by the fitting model, that is, formula 2 and formula 3, as shown in the following formulas 18 and 19, and the objective function formula 20 is the sum of the squares of the real error and the imaginary error, and set the relative error of the objective function value to 10 -6 Stop fitting at level .

[0125] error_real=Z_measured.real-real(Z) Formula 18;

[0126] error_imag=Z_measured.imag-imag(Z) Formula 19;

[0127] error_sum=error_real^2+error_imag^2 Formula 20.

[0128] Example:

[0129] The method in this example uses the differential evolution algorithm for fitting. The key steps of the differential evolution algorithm are as follows: Figure 3 shown.

[0130] When other conditions remain unchanged, the characteristic slope is inversely proportional to the film thickness c. When measuring the thickness of nano-scale metal films, the corresponding peak frequency is higher, and the presence of parasitic capacitance reduces the resonant frequency, which increases the measurement error. Therefore, it is necessary to reduce the influence of parasitic capacitance. When this method changes the lift-off height to obtain the peak frequency, the change in the lift-off height can cause a large change in the peak frequency, and a peak frequency-lift-off height relationship curve with excellent linearity can be obtained.

[0131] Due to the skin effect, the eddy current intensity in the conductor decreases exponentially with the increase of the depth into the conductor. In order to accurately measure the thickness of the metal film, the skin depth at the corresponding operating frequency f needs to be greater than the metal film thickness, so that the electromagnetic field can completely penetrate the metal film.

[0132] Figure 4 The real part of the measured impedance and the fitted R value when the film thickness is 315nm under the same lift-off height. Figure 5 The R value after fitting and the actual resistance value at each frequency, it can be seen that the curves basically coincide; Figure 6 is the relative error between R obtained by algorithm fitting and the actual resistance value. It can be seen that the relative error is very small (within 10-11 The fitting effect is excellent.

[0133] Figure 7 This is the relationship curve between the imaginary part of inductance and frequency corresponding to different film thicknesses at the same lift-off height after fitting;

[0134] By continuously changing the lift-off height and measuring the corresponding impedance value, the peak frequency-lift-off height relationship curve can be obtained when measuring the same film thickness, as shown in the figure: Figure 8 ;

[0135] When measuring different film thicknesses, the same method as above can be used to obtain the relationship curve between the inverse of the absolute value of the characteristic slope and the film thickness, such as Fig. 9 It can be seen that the relationship curve obtained after fitting can be used to measure the film thickness.

[0136] This example is a single coil sensor measurement system designed based on the above method. Fig.11 As shown in the figure, it is mainly divided into six parts: coil sensor, signal generator, front signal processing circuit, data acquisition module, communication module and host computer. The crystal oscillator in the signal generator inputs AC voltage to the sensor coil and the front signal processing circuit respectively, and the front signal processing circuit outputs the impedance value. The impedance value is input to the host computer through the data acquisition module and the communication module, and the fitting operation is performed in the host computer to calculate the corresponding characteristic value.

[0137] Specifically in the use process, the metal film thickness measurement method of this example is as follows:

[0138] Step 1: Install the eddy current sensor through the fixture, connect the lead to the front circuit, and ensure that the data acquisition module, communication module and host computer work normally.

[0139] Step 2: A sweep frequency excitation signal is applied to both ends of the coil through a signal generating circuit, and the sweep frequency range is usually 0.01MHz-50MHz.

[0140] Step 3: Place a wafer coated with a metal film on the wafer holder, and measure at least two sweep values ​​of the lift-off height by changing the lift-off height from 0mm to 3mm. The measurement interval should be as large as possible. Measure at least one coated wafer of thickness.

[0141] Step 4: Get the parasitic capacitance value through calculation, and use the differential evolution algorithm to fit the real value of impedance at each frequency to get the characteristic slope value. The calibration curve is obtained by multiplying the absolute value of the characteristic slope and the film thickness. The characteristic slope values ​​of multiple coated wafers with different thicknesses can also be obtained by repeating step 3.

[0142] Step 5: Repeat the above steps to complete the calibration curves of other metal films and store the calibration curves in the host computer.

[0143] Step 6: Take a wafer with known surface material to be tested, measure and fit the characteristic slope value S, and compare it with the calibration curve of the corresponding material film obtained in step 4, that is, calculate the thickness of the metal film according to the mapping relationship in table Sc.

[0144] The flow chart of the measurement method is as follows Fig.10 shown.

Claims

1. A method for measuring metal film thickness by reducing parasitic capacitance effect, characterized in that: The device adopted by the method includes a sensor coil, a front signal processing module, a data acquisition module, a communication module and a host computer. The method inputs an AC voltage to the sensor coil and the front signal processing circuit respectively through a crystal oscillator in a signal generator, outputs a measured impedance value corresponding to the metal film thickness through the front signal processing circuit, inputs the impedance value to the host computer through the data acquisition module and the communication module, performs a fitting operation in the host computer, and calculates the corresponding metal film thickness characteristic value, including the following steps; Step 1: Install the data acquisition module, communication module and host computer to ensure their normal operation; Step 2: Pass a sweep frequency excitation signal to both ends of the sensor coil through a signal generating circuit; Step 3: Place a wafer coated with a metal film on a wafer holder, and measure the frequency sweep value of the wafer at each lift-off height by changing the lift-off height multiple times; Step 4: Obtain the parasitic capacitance value through calculation, and use the differential evolution algorithm to fit the true value of the impedance at each frequency to obtain the characteristic slope value. The calibration curve is obtained by multiplying the absolute value of the characteristic slope and the film thickness. The characteristic slope values ​​of multiple coated wafers with different thicknesses can also be obtained by repeating step 3; Step 5: Repeat the above steps to complete the calibration curves of other metal films and store the calibration curves in the host computer; Step 6: Take a wafer with known surface material to be tested, measure and fit the characteristic slope value S, and compare it with the calibration curve of the corresponding material film obtained in step 4, that is, calculate the thickness of the metal film according to the mapping relationship in table Sc.

2. The method for measuring metal film thickness with reduced parasitic capacitance effect according to claim 1, characterized in that: The sensor coil is a single-coil eddy current sensor.

3. The method for measuring metal film thickness with reduced parasitic capacitance effect according to claim 2, characterized in that: The single-coil eddy current sensor is an eddy current sensor fixed by a clamp and connected to the front circuit of the front signal processing module by a lead wire.

4. The method for measuring metal film thickness with reduced parasitic capacitance effect according to claim 1, characterized in that: In step one, the sensor coil is located above the wafer holder and is close to the wafer holder. In step two, a sweep frequency excitation signal is sent to the sensor coil to induce an eddy current in the metal film of the wafer adjacent to the sensor coil.

5. The method for measuring metal film thickness with reduced parasitic capacitance effect according to claim 1, characterized in that: In step 2, the sweep frequency range of the sweep frequency excitation signal is 0.01 MHz-50 MHz.

6. The method for measuring metal film thickness with reduced parasitic capacitance effect according to claim 5, characterized in that: In step three, the lift-off height is changed multiple times within the lift-off height range of 0 mm to 3 mm, and at least two sweep frequency values ​​of the lift-off height are measured.

7. The method for measuring metal film thickness with reduced parasitic capacitance effect according to claim 6, characterized in that: In step three, when the lift-off height is changed multiple times, the difference in each lift-off height is made as large as possible to measure at least one thickness of the coated wafer.

8. The method for measuring metal film thickness with reduced parasitic capacitance effect according to claim 2, characterized in that: The single coil sensor is used to excite and measure the impedance change caused by different metal film thicknesses. When other measurement environment parameters remain unchanged, there is a one-to-one correspondence between the impedance change measured by the single coil sensor and the metal film thickness. When parasitic capacitance is not considered, the actual impedance value at both ends of the coil is: Z=R+jωL Formula 1; When there is parasitic capacitance at both ends of the coil due to the existence of the coil lead in the experimental measurement, the measured impedance value at this time is: When the influence of parasitic capacitance needs to be minimized to accurately measure the film thickness, the actual impedance value of the coil needs to be obtained and the characteristic slope method is derived using the formula: Formula 4 is the analytical solution for the impedance change. When the phase term and amplitude term in the analytical solution are separable and compared with other parts of the integrand, The rate of change with α is small, and When the characteristic frequency α0 reaches its maximum value; the approximation of the solution is calculated at α0 And proposed that the phase term of impedance change is only given by The phase term includes the conductivity, thickness and magnetic permeability of the metal film, which can be simplified by approximation: At this time, the peak frequency is: The change of α0 due to the change of lift-off height is The change in peak frequency is: Further we get: That is, the ratio of the change in peak frequency to the change in lift-off height is inversely proportional to the film thickness, and the ratio is the characteristic slope S.

9. The method for measuring metal film thickness with reduced parasitic capacitance effect according to claim 8, characterized in that: In step 3, the frequency value corresponding to when the imaginary part of the inductance reaches the maximum value in the frequency sweep mode is used as the peak frequency, and the definition formula is: In order to obtain the accurate imaginary value of the inductance, it is necessary to obtain the precise value at the corresponding frequency, that is, the actual value of the change in the real part of the coil impedance. From the above formula, when other conditions remain unchanged, the characteristic slope is inversely proportional to the film thickness c; when measuring the thickness of nano-scale metal films, the corresponding peak frequency is higher, and the presence of parasitic capacitance reduces the resonance frequency, making the measurement error larger, so it is necessary to reduce the influence of parasitic capacitance; when changing the lift-off height to obtain the peak frequency, the change in the lift-off height can cause a large change in the peak frequency, and a peak frequency-lift-off height relationship curve with excellent linearity can be obtained; When the eddy current intensity in the conductor decreases exponentially with the increase of the depth into the conductor due to the skin effect, in order to accurately measure the metal film thickness, the skin depth at the corresponding working frequency f needs to be greater than the metal film thickness, that is, the electromagnetic field completely penetrates the metal film; for non-magnetic materials, the calculation formula of the skin depth is as follows: In order to reduce the impact of parasitic capacitance, an intelligent optimization algorithm is used to fit the true impedance value of the coil at each frequency. First, the resonant frequency of the coil in the air should be measured, and the inductance value of the coil should be measured at a low frequency. Then, the value of the parasitic capacitance is obtained by the following formula: The measured impedance values ​​corresponding to the film thickness and at different frequencies are obtained, such as Formula 2 and Formula 3. At a certain frequency, the parallel capacitance value, frequency, measured real part and imaginary part values ​​are known. At this time, two fitting output parameters R and L can be obtained, that is, the actual value of the coil impedance. A global optimization algorithm needs to be selected during fitting. This method uses a differential evolution algorithm for fitting.

10. The method for measuring metal film thickness with reduced parasitic capacitance effect according to claim 9, characterized in that: The specific steps of the differential evolution algorithm are as follows: x i，0 =(R i，0 , L i，0 ) Formula 13; R i，0 and L i，0 are the resistance and inductance values ​​of the i-th individual in the initial population, generated by random initialization: R i，0 =R min +rand(0,1)·(R max -R min ) Formula 14; L i，0 =L min +rand(0,1)·(L max -L min ) Formula 15; Where R min and R max are the lower and upper bounds of resistance, L min and L max are the lower and upper bounds of the inductance, and rand(0,1) is a uniform random number between [0,1]; The mutation operation is to amplify the difference of individuals and generate new individuals; for each individual x i，G =(R i，G , L i，G ), randomly select three different individuals x from the population rl，G =(R rl，G , L rl，G ), x r2，G 、x r3，G , generate a new mutation vector v i，G : v i，G =(R v，i，G L v，i，G )=(R rl，G , L rl，G )+F·((R r2，G , L r2，G )-(R r3，G , L r3，G )) Formula 16; Where F is the scaling factor, controlling the magnitude of the variation; The vector generated by the mutation operation is crossed with the current individual; for each individual x i，G =(R i，G , L i，G ), generate a new test vector u i，G =(R u，i，G , L u，i，G ): L u，i，G The generation and R u，i，G Similar; crossover probability C r It determines the proportion of parameters inherited from the mutation vector, and j is a random index that ensures that at least one component is selected from the mutation vector; Used in the selection step to decide whether to use the trial vector u i，G Replace the original individual x i，G Enter the next generation; select according to the fitness function value, that is, the objective function value, and optimize the algorithm through multiple iterations until the termination condition is met, such as reaching the maximum number of iterations or meeting the convergence condition; set the error function to be the difference between the actual impedance measurement value and the impedance value obtained by the fitting model, that is, formula 2 and formula 3, as shown in the following formulas 18 and 19, and the objective function formula 20 is the sum of the squares of the real error and the imaginary error, and set the relative error of the objective function value to 10 -6 Stop fitting at level . error_real=Z_measured.real-real(Z) Formula 18; error_imag=Z_measured.imag-imag(Z) Formula 19; error_sum=error_real^2+error_imag^2 Formula 20.

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