Method for measuring film thickness distribution of wafers with thin films

Abstract

The present invention provides a method for measuring the film thickness distribution of a second film of a wafer, the wafer having a substrate, a first film on the surface of the substrate, and a second film on the first film, in which the height Z1 of the focus is obtained by focusing with an optical microscope, the optical microscope having an autofocus function using irradiation light of wavelength λ0, the wavelength λ1 of the irradiation light used to obtain an observation image of the second film is determined, the height of the focus is changed with Z1 as a reference, the observation image of the second film is obtained using the irradiation light of wavelength λ1, the standard deviation of the intensity distribution of reflected light in the observation image is calculated, the height Z2 corresponding to the focus at the peak position with the largest standard deviation is obtained, the difference ΔZ between Z1 and Z2 is calculated, the autofocus function is corrected with ΔZ as a correction value, focusing is performed using the corrected autofocus function, the observation image of the second film is obtained, and the film thickness distribution is calculated based on the intensity distribution of reflected light in the observation image. Thus, a method for measuring the film thickness distribution with high accuracy and stability is provided.

Description

Technical Field

[0001] The invention relates to a method for measuring film thickness distribution of a wafer with a thin film. Background Art

[0002] As a wafer with a thin film, for example, an SOI wafer is known, which has an oxide film as a BOX layer and a single crystal silicon film as an SOI layer on a substrate. Generally speaking, in the manufacturing process of an SOI wafer, the film thickness of the SOI layer and the film thickness of the BOX layer of the manufactured SOI wafer are set according to the user's specification (film thickness specification), and after the SOI wafer is manufactured with the film thickness setting value as the target value, the in-plane distribution of the film thickness of the SOI layer is evaluated in the inspection process, etc.

[0003] In particular, in FD-SOI devices with design specifications below 28nm, strict uniformity of film thickness of the SOI layer is required. This requires uniformity of film thickness not only across the entire SOI wafer with a diameter of 300mm, but also within micro-regions.

[0004] Therefore, it is necessary to evaluate not only the uniformity of the SOI film thickness of the entire wafer, but also the uniformity of the SOI layer film thickness in a micro area to improve and ensure quality. The current film thickness measurement of a large measurement area is not sufficient, and it is necessary to evaluate the film thickness of the SOI layer in a micro area at the microscope level.

[0005] The inventor of this case proposed the following evaluation method for wafers with thin films: irradiate a portion of the wafer surface with light of a single wavelength λ, detect reflected light from the region, and measure the reflected light intensity of each pixel obtained by dividing the region into a plurality of pixels, thereby obtaining the reflected light intensity distribution in the region, and calculating the film thickness distribution of the thin film in the region based on the reflected light intensity distribution (for example, patent documents 1 to 3). In addition, patent document 4 also discloses a method for measuring the thickness deviation of a layer of a multilayer semiconductor structure.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Publication No. 2011-249621 (Patent No. 5365581)

[0009] Patent Document 2: Japanese Patent Application Publication No. 2016-114506

[0010] Patent Document 3: Japanese Patent Application Publication No. 2017-125782

[0011] Patent Document 4: Japanese Patent Application Publication No. 2016-504566 Summary of the invention

[0012] 1. Technical issues to be resolved

[0013] However, even if the measurement is performed by the methods described in Patent Documents 1 to 4, there are still problems such as low measurement accuracy, large deviation and instability of the film thickness distribution in the micro-region of the SOI layer of the SOI wafer. Therefore, the inventors of this case studied various factors that affect the measurement and found that the following problems exist when using the methods described in Patent Documents 1 to 3: Since the measurement wavelength is different due to the film thickness specifications (film thickness specifications) of each SOI wafer, the measurement accuracy and stability will be different, and it is necessary to study the measurement method for each film thickness specification of each SOI wafer.

[0014] The present invention is made to solve the above-mentioned problems, and an object of the present invention is to provide a method for measuring the film thickness distribution of a wafer with a thin film, which method can measure the film thickness distribution of a micro region of a thin film with high accuracy and stability.

[0015] (II) Technical solution

[0016] The present invention is made to achieve the above-mentioned purpose and provides a method for measuring the film thickness distribution of a wafer with a thin film, wherein the wafer with a thin film has at least a first thin film formed on the surface of a substrate and a second thin film formed on the surface of the first thin film, the method measures the film thickness distribution of the second thin film of the wafer with a thin film, and includes the following steps: using the wafer with a thin film, focusing with an optical microscope to obtain a focal height Z1, the optical microscope having an automatic focusing function using irradiation light with a wavelength λ0; determining the wavelength λ1 of the irradiation light used when acquiring an observation image of the second thin film of the wafer with a thin film; and changing the focal height based on the Z1. While using the irradiation light of the wavelength λ1, an observation image of the second thin film of the thin film wafer is obtained through the optical microscope; the standard deviation of the reflected light intensity distribution in the observation image is calculated to obtain the height Z2 of the focus corresponding to the peak position where the standard deviation of the reflected light intensity distribution is the largest, and the difference ΔZ between the Z1 and the Z2 is calculated; the autofocus function of the optical microscope is corrected with the ΔZ as a correction value; and the observation image of the second thin film of the thin film wafer is obtained through the optical microscope by focusing using the corrected autofocus function, and the film thickness distribution is calculated based on the reflected light intensity distribution in the observation image.

[0017] By adopting such a method for measuring the film thickness distribution of a wafer with a thin film, it is possible to measure the film thickness distribution of a micro region of a thin film with high accuracy, stably, and simply.

[0018] At this time, the following method can be used to measure the film thickness distribution of a thin-film wafer, and the process of determining the wavelength λ1 includes the following processes: calculating a curve P1 by simulation, wherein the curve P1 represents the wavelength dependence of the reflectivity of the thin-film wafer on light in a wavelength region above the wavelength of visible light; calculating a curve P22 by simulation, wherein the curve P22 represents the wavelength dependence of the reflectivity of the thin-film wafer having a first thin film that is thinner by t [nm] or thicker by t [nm] than the set film thickness T1 of the first thin film of the thin-film wafer on light in a wavelength region above the visible light; and calculating a curve P32 (= P22-P1) of the difference between the calculated curve P1 and the curve P22, and determining the wavelength when the calculated difference curve P32 is 0 as the wavelength λ1.

[0019] This makes it possible to easily obtain the wavelength λ1 of the irradiation light for obtaining an image, and calculate the film thickness distribution of the second thin film in the irradiation area with higher spatial resolution and higher accuracy without being affected by the film thickness distribution of the first thin film.

[0020] In this case, the following method of measuring the film thickness distribution of the wafer with thin films may be used, wherein the wafer with thin films is an SOI wafer, the first thin film is a buried oxide film layer, and the second thin film is an SOI layer composed of single crystal silicon.

[0021] In this way, when the thin-film wafer to be measured is an SOI wafer, the film thickness distribution of the SOI layer in the irradiated region can be calculated with high accuracy.

[0022] In this case, the film thickness distribution of the wafer with a thin film may be measured by a method in which the wavelength of the irradiation light having the wavelength λ0 is longer than the wavelength λ1, and the wavelength λ1 is a single wavelength selected from the wavelengths of visible light.

[0023] This allows measurement with visible light using a common optical microscope (measurement system), and thus can be performed easily and at low cost.

[0024] (III) Beneficial effects

[0025] As described above, according to the method for measuring the film thickness distribution of a wafer with a thin film of the present invention, the film thickness distribution of a micro region of a thin film on a wafer with a thin film can be measured accurately, stably, and simply. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a flow chart showing the steps of the method for measuring the film thickness distribution of a wafer with a thin film according to the present invention.

[0027] Figure 2 This is a schematic diagram showing an optical microscope that can be used in the method for measuring the film thickness distribution of a wafer with a thin film according to the present invention.

[0028] Figure 3 Graph 1 shows the relationship between the standard deviation of the reflection intensity distribution obtained when calculating Z2 and the height Z of the SOI wafer used in the example.

[0029] Figure 4 Graph 1 shows the relationship between the standard deviation of the reflection intensity distribution obtained when calculating Z2 and the height Z of the SOI wafer used in the example.

[0030] Figure 5 The evaluation results of the film thickness distribution (standard deviation) of the SOI layer of the Example and the Comparative Example are shown. DETAILED DESCRIPTION

[0031] The present invention will be described in detail below, but the present invention is not limited thereto.

[0032] As described above, there is a need for a method for measuring the film thickness distribution of a wafer with a thin film, which can measure the film thickness distribution of a micro region of a thin film with high accuracy and stability.

[0033] The inventor of the present case has repeatedly conducted in-depth research on the above-mentioned subject, and as a result, has discovered a method for measuring the film thickness distribution of a wafer with a thin film. The method can be used to accurately and stably measure the film thickness distribution of a micro-region of a thin film on the wafer with a thin film, thereby completing the present invention, wherein the wafer with a thin film has at least a first thin film formed on the surface of a substrate, and a second thin film formed on the surface of the first thin film. The method measures the film thickness distribution of the second thin film of the wafer with a thin film, and includes the following steps: using the wafer with a thin film, focusing through an optical microscope with an automatic focusing function using irradiation light with a wavelength λ0, and calculating the height Z1 of the focus; determining the wavelength of the irradiation light used when obtaining an observation image of the second thin film of the wafer with a thin film. length λ1; while changing the height of the focus based on the Z1, using the irradiation light of the wavelength λ1 to obtain an observation image of the second thin film of the thin film wafer with a thin film through the optical microscope; calculating the standard deviation of the reflected light intensity distribution in the observation image, obtaining the height Z2 of the focus corresponding to the peak position where the standard deviation of the reflected light intensity distribution is the largest, and calculating the difference ΔZ between the Z1 and the Z2; correcting the autofocus function of the optical microscope using the ΔZ as a correction value; and focusing using the corrected autofocus function, obtaining an observation image of the second thin film of the thin film wafer with a thin film through the optical microscope, and calculating the film thickness distribution based on the reflected light intensity distribution in the observation image.

[0034] The following description is given with reference to the accompanying drawings.

[0035] In the method for measuring the film thickness distribution of a wafer with a thin film of the present invention, for example, Figure 2The optical microscope 2 shown. It is possible to irradiate a partial area of ​​the thin film wafer 1 to be evaluated with irradiation light emitted from a light source 3 of a common optical microscope 2 equipped with a bandpass filter 4 for wavelength selection. The optical microscope 2 has an autofocus (hereinafter sometimes referred to as "AF") function, and can automatically focus. For the irradiation light used by the AF function, a long-wavelength light with a wavelength longer than the measurement wavelength of the film thickness measurement and low energy that does not affect the measurement wavelength is used, and preferably a long wavelength of 750nm or more is used. In addition, the optical microscope 2 has a control unit, and the control unit is capable of: acquiring measurement data, analyzing and processing the acquired data, controlling the measurement according to the input measurement conditions, and correcting the action conditions based on the input data.

[0036] In addition, in the method for measuring the film thickness distribution of a wafer with a thin film of the present invention, the wafer with a thin film as the object has at least: a first thin film formed on the surface of a substrate, and a second thin film formed on the surface of the first thin film. As such a wafer with a thin film, it is preferable to use, for example, an SOI wafer having a buried oxide film layer (BOX layer) corresponding to the first thin film, and an SOI layer corresponding to the second thin film. In particular, when such an SOI wafer is used as the measurement object, the film thickness distribution of the SOI layer can be calculated with high accuracy.

[0037] Figure 1 A flow chart showing a method for measuring film thickness distribution of a wafer with a thin film according to the present invention.

[0038] like Figure 1 As shown in S1, the following process is first performed: using an optical microscope, the thin-film wafer for evaluation is used, and the AF function using the irradiation light of wavelength λ0 is used to focus, and the focal height Z1 is obtained. The thin-film wafer for evaluation is the thin-film wafer to be actually measured.

[0039] Figure 1 The process represented by S2 is a process for determining the wavelength λ1 of the irradiation light used when acquiring the observation image of the second thin film of the thin film wafer. First, using the set values ​​(specifications) of the film thickness of the second thin film and the first thin film, the relationship between the wavelength of the irradiation light and the reflectivity of the reflected light is calculated by simulation, and based on the result, the optimal wavelength λ1 of the irradiation light for measurement is selected. For the optimal wavelength λ1 corresponding to the set values ​​(specifications) of the film thickness of the second thin film and the first thin film, a wavelength is selected such that the reflectivity remains unchanged when the film thickness of the second thin film is fixed to the film thickness of the set value (specification) and the film thickness of the first thin film is changed. By selecting such an optimal wavelength λ1, the reflectivity change caused by the change in the film thickness of the first thin film is eliminated, and only the influence of the change in the film thickness of the second thin film is reflected in the reflectivity change.

[0040] Specifically, the process can be performed as described in Patent Document 2, for example. First, a curve P1 is calculated by simulation, and the curve P1 represents the wavelength dependence of the reflectivity of the evaluation object, i.e., the wafer with a thin film, on the wavelength range above the wavelength of visible light. Next, a curve P22 is calculated by simulation, and the curve P22 represents the wavelength dependence of the reflectivity of the wafer with a thin film having a first thin film thinner by t [nm] or thicker by t [nm] than the set film thickness T1 of the first thin film of the evaluation object, i.e., the wafer with a thin film, on the wavelength range above the visible light. Next, a curve P32 (= P22-P1) of the difference between the two curves P1 and P22 calculated by simulation is calculated, and the wavelength when the calculated difference curve P32 is 0, i.e., when the reflectivity difference is 0, is determined as the wavelength λ1.

[0041] For example, for SOI wafers, in the above description, the first thin film is equivalent to the BOX layer, and the second thin film is equivalent to the SOI layer. When the film thickness specifications of the SOI wafer are SOI layer: 12nm, BOX layer: 20nm, it is best to use 492nm (optimal wavelength is λ1) as the measurement wavelength λ1, and when the film thickness specifications of the SOI wafer are SOI layer: 12nm, BOX layer: 25nm, it is best to use 520nm as the measurement wavelength λ1.

[0042] In addition, the step S2 is a simulation step, and the order of the step S1 is not limited. The step S2 may be performed before the step S1, or the steps S1 and S2 may be performed simultaneously, that is, in parallel.

[0043] Figure 1 The process represented by S3 is a process of obtaining an observation image of the second thin film of the thin film wafer with an optical microscope by using the irradiation light of wavelength λ1 determined by S2 while changing the height of the focus based on Z1 obtained by S1. The pitch of the focus height position of the observation image is preferably 0.5 μm or less, and more preferably 0.2 μm or less. If it is within this range, it can be measured with higher accuracy.

[0044] Figure 1The process represented by S4 is a process of calculating the standard deviation of the reflected light intensity distribution in the observation image of the second film obtained by S3, obtaining the height Z2 of the focus corresponding to the peak position where the standard deviation of the reflected light intensity distribution is the largest, and calculating the difference ΔZ (= Z1-Z2) between Z1 and Z2. For the calculation of ΔZ, Z2 can also be input to the control unit of the optical microscope, so that the calculation of ΔZ (= Z1-Z2) can be performed in the control unit. The position Z2 where the standard deviation of the reflected light intensity distribution of the observation image of the second film obtained by S3 is the largest can be regarded as the focal position when the irradiation light of the wavelength λ1 is used. Using this Z2, ΔZ=Z1-Z2 can be obtained. In this way, ΔZ is the difference between the focal position Z1 obtained by the AF function using the irradiation light of the wavelength λ0 and the focal position Z2 when the irradiation light of the wavelength λ1 used when obtaining the observation image of the second film is used, and is used as a correction value used in the next step (S5), that is, the correction of the AF function.

[0045] Figure 1 The process represented by S5 is a process of correcting the AF function of the optical microscope using the ΔZ obtained by S4 as a correction value. The calculated ΔZ can be input as a correction value to the control unit of the optical microscope, or the ΔZ calculated in the control unit starting from the input Z2 can be read as a correction value.

[0046] Figure 1 The process represented by S6 is a process of focusing using a corrected AF function, obtaining an observation image of the second thin film of the thin film wafer through an optical microscope, and calculating the film thickness distribution based on the reflected light intensity distribution in the observation image. When using the corrected AF function, the focus position obtained by the AF function is corrected by a correction value ΔZ, so that the observation image of the second thin film is obtained at a focus position with higher accuracy when the irradiation light of wavelength λ1 is used. The film thickness distribution is obtained based on the reflection intensity distribution of the observation image obtained in this way, so that a high-precision film thickness distribution evaluation can be performed. The evaluation of the film thickness distribution is preferably performed according to the standard deviation of the film thickness distribution, for example.

[0047] Example

[0048] The present invention is described in detail below with reference to examples, but the present invention is not limited thereto.

[0049] The film thickness measurement system used in the embodiment, reference example, and comparative example, that is, the optical microscope, has an AF function and can obtain an observation image (microscope image). The wavelength of the irradiation light used by the AF function is 785 nm. In addition, a bandpass filter is used to adjust and select the wavelength of the irradiation light used to obtain the observation image.

[0050] (Example)

[0051] The following SOI wafer was used, that is, the film thickness specifications of the SOI wafer were 12nm for the SOI layer and 20nm for the BOX layer. First, the SOI wafer was placed in an optical microscope (film thickness measurement system) and focused using the AF function to obtain the focal position Z1. Z1 was obtained automatically.

[0052] Furthermore, the wavelength λ1 of the irradiation light used when acquiring the observation image of the SOI layer of the SOI wafer was obtained by simulation, and the measurement wavelength λ1 was found to be 492 nm.

[0053] Next, a bandpass filter is used to adjust the wavelength of the irradiation light used to obtain the observation image, so that the wavelength of the irradiation light is λ1 = 492nm. Then, the height of the focus is changed up and down based on Z1 to obtain the observation image, and the relationship between the standard deviation of the reflection intensity distribution of the obtained observation image and the height Z is calculated. Figure 3 The relationship between the height Z and the standard deviation of the reflection intensity distribution is shown. Figure 3 The standard deviation is shown as the height of the peak is Z2. In addition, the position of Z1 is as Figure 3 and the following Figure 4 As shown, it is located beyond the range on the right side of the graph (outside the numerical range of the horizontal axis).

[0054] Z2 obtained in this way is input to the optical microscope, a correction value ΔZ (= Z1 - Z2 ) is calculated by a control unit in the optical microscope, and the AF function is corrected using the correction value ΔZ.

[0055] Then, the focus position obtained by AF is corrected by the correction value ΔZ, and the observation image of the SOI layer of the SOI wafer is obtained, and the standard deviation of the film thickness distribution is calculated based on the reflected light intensity distribution in the observation image of the measurement area. Specifically, for 5 points in the wafer surface (1 point in the center + 4 points at positions offset 5mm inward from the outer edge), the standard deviation of the film thickness distribution of the SOI layer is calculated based on the reflected light intensity distribution in the observation image. In addition, the measurement area is 266μm×266μm (2048×2048 pixels).

[0056] (Reference example)

[0057] The following SOI wafer was used, that is, the film thickness specifications of the SOI wafer were 12nm for the SOI layer and 25nm for the BOX layer. The wavelength λ1 of the irradiation light used to obtain the observation image of the SOI layer of the SOI wafer was 520nm. As in the example, the relationship between the height Z of the obtained observation image and the standard deviation of the reflection intensity distribution was obtained, and the following was obtained: Figure 4From this result, it can be seen that if the thickness of the BOX layer is different, λ1 is different, and as a result, the correction value of the peak position Z2 based on the focus is also different.

[0058] (Comparative Example)

[0059] The following conventional film thickness distribution measurement method was used, and the film thickness distribution was measured using the same SOI wafer as in the example. The specific steps are as follows.

[0060] (1) Using a sample wafer with a pattern, the focal point height Z1 is obtained using the AF function of an optical microscope, which is a film thickness measurement system.

[0061] (2) At this position, irradiation light of wavelength λ1 is irradiated using a filter for obtaining measurement wavelength λ1, and the pattern of the microscope image is focused by visual observation to obtain the height Z2 of the focused position.

[0062] (3) Z2 is input to the optical microscope, a correction value ΔZ (= Z1 - Z2) is calculated by a control unit in the optical microscope, and the AF function is corrected using the correction value ΔZ.

[0063] (4) In the film thickness distribution measurement, the focus position adjusted by the AF function is corrected by ΔZ, an observation image is obtained, and the standard deviation of the film thickness distribution is calculated from its reflection intensity distribution.

[0064] The film thickness distribution measurement of the embodiment and the comparative example was evaluated as follows. First, the measurement was repeated three times a day over a period of 31 days using the measurement method of the comparative example. Then, the measurement method of the embodiment was switched and the measurement was repeated for another 42 days.

[0065] exist Figure 5 The evaluation results are shown in FIG. The numbers of the plotted points indicate the measurement positions shown in the wafer conceptual diagram in the figure. In addition, the order of measurement is as follows: Figure 5 The horizontal axis is from right to left. Figure 5 As shown, it can be seen that the standard deviation of the film thickness distribution of the SOI layer in the example using the measurement method of the present invention is reduced compared with the comparative example using the conventional measurement method, and a stable measurement result can be obtained.

[0066] In addition, although the value of the standard deviation of the film thickness distribution of the SOI layer of the embodiment is larger than that of the comparative example, this means that the film thickness distribution can be accurately measured with good reproducibility by accurately adjusting the focus of the measurement wavelength. For example, when measuring the surface roughness (RMS) of a specified area using an AFM (atomic force microscope), if the needle with a rounded tip is changed to a stylus with a sharp tip to measure the surface roughness (RMS) of the same area, the surface roughness (RMS) can be measured more accurately, and the result is the same phenomenon as the increase in the numerical value of the surface roughness (RMS).

[0067] The present invention is not limited to the above-mentioned embodiments. The above-mentioned embodiments are illustrative, and any scheme having substantially the same structure and achieving the same function and effect as the technical idea described in the claims of the present invention is included in the technical scope of the present invention.

Claims

1. A method for measuring film thickness distribution of a wafer with a thin film, wherein the wafer with a thin film has at least a first thin film formed on a surface of a substrate and a second thin film formed on a surface of the first thin film, wherein the method measures the film thickness distribution of the second thin film of the wafer with a thin film, characterized in that: Including the following processes: Using the thin film wafer, focusing is performed with an optical microscope to obtain a focal point height Z1, wherein the optical microscope has an automatic focusing function using irradiation light with a wavelength λ0; determining a wavelength λ1 of irradiation light used when acquiring an observation image of the second thin film of the wafer with a thin film; while changing the height of the focal point based on the Z1, using the irradiation light of the wavelength λ1, obtaining an observation image of the second thin film of the thin-film-attached wafer through the optical microscope; Calculating the standard deviation of the reflected light intensity distribution in the observed image, obtaining the height Z2 of the focus corresponding to the peak position where the standard deviation of the reflected light intensity distribution is the largest, and calculating the difference ΔZ between Z1 and Z2; Correcting the autofocus function of the optical microscope using the ΔZ as a correction value; as well as focusing using the corrected autofocus function, obtaining an observation image of the second thin film of the wafer with thin film through the optical microscope, and calculating the film thickness distribution according to the reflected light intensity distribution in the observation image, The wavelength of the irradiation light of the wavelength λ0 is longer than the wavelength λ1 which is a single wavelength selected from wavelengths of visible light.

2. The method for measuring film thickness distribution of a wafer with a thin film according to claim 1, characterized in that: The process of determining the wavelength λ1 comprises the following steps: A curve P1 is calculated by simulation, the curve P1 indicating the wavelength dependence of the reflectivity of the wafer with thin film with respect to light in a wavelength range longer than the wavelength of visible light; Calculating a curve P22 by simulation, the curve P22 representing the wavelength dependence of the reflectivity of the thin-film wafer with a first thin film thinner by t or thicker by t than the set film thickness T1 of the first thin film of the thin-film wafer with a wavelength range longer than the visible light, wherein the unit of t is nm; and A curve P32 of the difference between the calculated curve P1 and the calculated curve P22, that is, P22-P1, is calculated, and the wavelength when the calculated difference curve P32 is 0 is determined as the wavelength λ1.

3. The method for measuring film thickness distribution of a wafer with a thin film according to claim 1, characterized in that: The wafer with thin film is an SOI wafer, the first thin film is a buried oxide film layer, and the second thin film is an SOI layer composed of single crystal silicon.

4. The method for measuring film thickness distribution of a wafer with a thin film according to claim 2, characterized in that: The wafer with thin film is an SOI wafer, the first thin film is a buried oxide film layer, and the second thin film is an SOI layer composed of single crystal silicon.

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