Film thickness measurement method and film thickness measurement device

Abstract

This film thickness measurement method comprises a first image acquisition step for acquiring a BG image of an object for background correction in the presence of an intervening part, a second image acquisition step for acquiring a sample image of an object S3 to be measured in the presence of the intervening part, a conversion step for converting the BG image in the presence of the intervening part into a BG image of the object for background correction in the absence of the intervening part on the basis of the ratio of the integrated intensity of light in the presence of the intervening part and the integrated intensity of light in the absence of the intervening part, and a third image acquisition step for acquiring a film thickness distribution image by subtracting the luminance value of the BG image of the object for background correction in the absence of the intervening part from the luminance value of the sample image of the object to be measured in the presence of the intervening part.

Description

Film thickness measurement method and film thickness measurement apparatus

[0001] One aspect of this disclosure relates to a method for measuring film thickness and an apparatus for measuring film thickness.

[0002] Patent Document 1 discloses a polishing apparatus that supplies polishing liquid to a polishing pad to polish a semiconductor wafer. In this polishing apparatus, in order to detect the end point of polishing, the thickness of the film on the semiconductor wafer is determined by irradiating it with light through a transparent window and receiving reflected interference light.

[0003] Japanese Patent Publication No. 2012-004276

[0004] To accurately determine the film thickness distribution of a semiconductor wafer, it is necessary to perform background correction on the captured image (a process to appropriately obtain the brightness value of the light from the sample corresponding to the irradiated light by subtracting the light from the atmosphere contained in the captured image). However, in situations where there is some kind of intervening part, such as a transparent window, as in the apparatus described above, it is difficult to perform high-precision background correction, and therefore it is not possible to obtain the film thickness distribution of the semiconductor wafer with high precision.

[0005] One aspect of this disclosure has been made in view of the above circumstances and aims to provide a film thickness measurement method and a film thickness measurement apparatus that can obtain the distribution of film thickness of an object to be measured with high accuracy even when imaging is performed in the presence of an intervening portion.

[0006] (1) A film thickness measurement method according to one aspect of the present disclosure is a film thickness measurement method for measuring the film thickness distribution of an object to be measured, comprising: a first light irradiation step of irradiating an object to be used for background correction with light in a planar manner through an intervening part with known refractive index n, damping coefficient k, and thickness d; a first image acquisition step of obtaining a background correction image of the object to be used for background correction when an intervening part is present by separating the light from the object to be used for background correction guided through the intervening part using an optical element whose transmittance and reflectance change in a predetermined wavelength range, and imaging the separated light; a second light irradiation step of irradiating an object to be measured with light in a planar manner through an intervening part; and separating the light from the object to be measured guided through the intervening part using an optical element, and imaging the separated light. The method comprises: a second image acquisition step of acquiring a sample image of the object to be measured when an intervening part is present; a conversion step of converting a background correction image of the object to be measured when an intervening part is present to a background correction image of the object to be measured when an intervening part is absent, based on the ratio of the integrated intensity of light when an intervening part is present and the integrated intensity of light when an intervening part is absent; and a third image acquisition step of acquiring a film thickness distribution image showing the film thickness distribution of the object to be measured by subtracting the brightness value of the background correction image of the object to be measured when an intervening part is absent, acquired in the conversion step, from the brightness value of the sample image of the object to be measured when an intervening part is present, acquired in the second image acquisition step.

[0007] In a film thickness measurement method according to one aspect of this disclosure, a background correction image of the object to be measured when an intervening part is present is acquired, and based on the ratio of the integrated light intensity when the intervening part is present and when it is not, the background correction image when the intervening part is present is converted to a background correction image when the intervening part is not present. In this way, by considering the ratio of the integrated light intensity, it is possible to appropriately convert from a background correction image when the intervening part is present to a background correction image when it is not present. Then, the brightness value of the background correction image when the intervening part is not present, acquired in this way, is subtracted from the brightness value of the sample image of the object to be measured when the intervening part is present (background correction is performed), so that the influence of the background light (light from the atmosphere) that is originally intended to be subtracted is subtracted without being affected by the intervening part in background correction, and background correction can be performed with high accuracy. Then, by acquiring a film thickness distribution image after background correction, the film thickness distribution of the object to be measured can be obtained with high accuracy. As described above, according to one aspect of the present disclosure, the film thickness measurement method can be used to obtain the distribution of the film thickness of the object to be measured with high accuracy, even when imaging is performed in the presence of an intervening portion.

[0008] (2) The film thickness measurement method described in (1) above may further include a step of deriving the integrated intensity of light, which involves deriving the integrated intensity of light in the presence of an intervening part based on the theoretical reflectance of the object to be used for background correction in the presence of an intervening part, the wavelength spectrum of light irradiated from the light source, and the spectral characteristics of the entire measuring device, and also deriving the integrated intensity of light in the absence of an intervening part based on the theoretical reflectance of the object to be used for background correction in the absence of an intervening part, the wavelength spectrum of light irradiated from the light source, and the spectral characteristics of the entire measuring device. In this way, by considering the theoretical reflectance, the wavelength spectrum of light, and the spectral characteristics, the integrated intensity of light can be appropriately derived, and the above-mentioned integrated intensity ratio of light can be derived with high accuracy.

[0009] (3) In the film thickness measurement method of (1) or (2) above, the object for background correction may be configured to include a blackbody sample. Thus, by using a blackbody sample with a low reflectance (close to 0% reflectance) as the object for background correction, appropriate background correction can be performed without being affected by the object for background correction.

[0010] (4) A film thickness measurement apparatus according to an aspect of the present disclosure is a film thickness measurement apparatus that measures the film thickness distribution of a measurement object, and includes: a light source that irradiates light in a planar manner through an intervening portion with a known refractive index n, attenuation coefficient k, and thickness d; an optical element that separates light transmitted and reflected through the intervening portion by changing the transmittance and reflectance in a predetermined wavelength range; an imaging unit that images the light separated by the optical element to generate image data; and an analysis unit that derives the film thickness distribution of the measurement object based on the image data. The analysis unit is configured to: acquire a background correction image, which is image data regarding the object for background correction when the intervening portion is present, and a sample image, which is image data regarding the measurement object when the intervening portion is present; convert the background correction image when the intervening portion is present into the background correction image when the intervening portion is not present based on the ratio of the integrated intensity of light in the state where the intervening portion is present and the integrated intensity of light in the state where the intervening portion is not present; and obtain a film thickness distribution image indicating the film thickness distribution of the measurement object by subtracting the luminance value of the background correction image when the intervening portion is not present from the luminance value of the sample image.

[0011] According to an aspect of the present disclosure, even when imaging is performed in a state where an intervening portion is present, the film thickness distribution of a semiconductor wafer can be obtained with high accuracy.

[0012] FIG. 1 is a diagram schematically showing a film thickness measuring apparatus according to the present embodiment. FIG. 2 is a diagram for explaining the relationship between the characteristics of the LRG filter and the wavelength of light emitted from a light source. FIG. 3 is a diagram for explaining the spectrum of light and the characteristics of the LRG filter. FIG. 4 is a diagram for explaining the wavelength shift according to the transmitted light amount and the reflected light amount. FIG. 5 is a diagram showing the relationship between the wavelength and the film thickness. FIG. 6 is a diagram for explaining each image acquisition. FIG. 7 is a diagram for explaining a reflection model of a multilayer structure considering two layers of inclusions constituting an intervening portion. FIG. 8 is a diagram for explaining a reflection model of a multilayer structure when there is no intervening portion. FIG. 9 is a diagram for explaining a reflection model considering an intervening portion composed of glass and water. FIG. 10 is a graph showing the reflectance for each wavelength when the intervening portion is present and when it is not present. FIG. 11 is a flowchart showing the processing of the film thickness measuring method according to the present embodiment.

[0013] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In each figure, the same or corresponding parts are denoted by the same reference numerals, and redundant descriptions are omitted.

[0014] [Basic Configuration of Film Thickness Measuring Apparatus] First, referring to FIGS. 1 to 5, the basic configuration of the film thickness measuring apparatus will be described. FIG. 1 is a diagram schematically showing a film thickness measuring apparatus 1 according to the present embodiment. In FIG. 1, illustration of a part of the configuration of the film thickness measuring apparatus 1 (specifically, an intervening portion 150 (see FIG. 6) described later) is omitted. Here, the basic configuration will be described assuming that the intervening portion 150 (see FIG. 6) does not exist, and the detailed configuration including the intervening portion 150 (see FIG. 6) will be described later.

[0015] The film thickness measuring device 1 is a device that irradiates light onto a sample 100 in a planar manner and measures the thickness of a film formed on the sample 100 based on the reflected light from the sample 100. Examples of sample 100 include logic devices, memory devices, analog devices, and mixed-signal devices that combine the same, which are integrated circuits (ICs) or large-scale integrated circuits (LSIs) having PN junctions such as transistors, or power semiconductor devices (power devices) such as high-current / high-voltage MOS transistors, bipolar transistors, and IGBTs, and light-emitting devices such as LEDs and semiconductor lasers. Sample 100 has a film 100b formed on the surface of a substrate 100a. In this embodiment, it is explained that in sample 100, only one layer of film 100b is formed on the surface of the substrate 100a. Film 100b is, for example, an oxide film or a nitride film, but may be other films.

[0016] As shown in Figure 1, the film thickness measuring device 1 comprises a light source 10, a half mirror 11, a field lens 12, a camera system 20, and a computer 30 (analysis unit).

[0017] The light source 10 irradiates the sample 100 with light in a planar manner. For example, the light source 10 irradiates light in a planar manner over substantially the entire surface of the sample 100. The light source 10 is, for example, a light source capable of uniformly irradiating the surface of the sample 100, and irradiates the sample 100 with diffused light. The light source 10 may be a surface illumination unit using a white LED, SC light source, halogen lamp, or Xe lamp, etc. Alternatively, it may be a surface illumination unit using a monochromatic LD, multichromatic LD, monochromatic LED, or multichromatic LED. The light emitted from the light source 10 passes through the half mirror 11 and the field lens 12 and irradiates the sample 100 in a planar manner.

[0018] The light source 10 irradiates the sample 100 with light of a wavelength included in a predetermined wavelength range of the LRG filter 22 (details to be described later) of the camera system 20. As will be described in detail later, the LRG filter 22 is an optical element that separates light from the sample 100 by transmitting and reflecting it according to its wavelength. In the predetermined wavelength range mentioned above, the transmittance and reflectance of the LRG filter 22 change according to the wavelength.

[0019] Figure 2 illustrates the relationship between the characteristics of the LRG filter 22 and the wavelength of light emitted from the light source 10. In Figure 2, the horizontal axis represents wavelength, and the vertical axis represents the transmittance of the LRG filter 22. As shown in the characteristics X4 of the LRG filter 22 in Figure 2, in the LRG filter 22, in a predetermined wavelength range X10, the transmittance (and reflectance) of light changes gradually (monotonically or linearly) in accordance with the change in wavelength, and in wavelength ranges other than this specific wavelength range, the transmittance (and reflectance) of light remains constant regardless of the change in wavelength. As shown in Figure 2, the light source 10 emits light X20 with wavelengths included in the predetermined wavelength range X10 described above. １ X20 ２ X20 ３ These can be combined in various ways to produce output. In other words, the light source 10 can output seven different wavelength spectra of light included in a predetermined wavelength range X10. The wavelength range (interference peak wavelength) used for measurement is determined by the material of the film formed on the sample 100 and the measurement film thickness range.

[0020] Returning to Figure 1, the half-mirror 11 is a mirror that reflects the light emitted from the light source 10 towards the sample 100 (more specifically, towards the field lens 12 that guides the light to the sample 100) and transmits the light from the illuminated sample 100 (more specifically, the light that has passed from the sample 100 through the field lens 12). The field lens 12 is a lens that aligns the direction of light propagation.

[0021] The camera system 20 includes a lens 21, an LRG filter 22 (optical element), an area sensor 23 (imaging unit), and an area sensor 24 (imaging unit). The camera system 20 may also include a linear image sensor instead of the area sensors.

[0022] Lens 21 is a lens that focuses light from the sample 100 that has been incident on it through the field lens 12 and the half mirror 11. Lens 21 may be positioned upstream of the LRG filter 22, or it may be positioned in the region between the LRG filter 22 and the area sensors 23 and 24. In this embodiment, it will be described as if lens 21 is positioned upstream of the LRG filter 22. Lens 21 may be a finite focus lens or an infinite focus lens. If lens 21 is a finite focus lens, the distance from lens 21 to the area sensors 23 and 24 is set to a predetermined value. If lens 21 is an infinite focus lens, lens 21 is a collimator lens that converts light from the sample 100 into parallel light, and is aberration corrected so that parallel light is obtained. The light output from lens 21 is incident on the LRG filter 22.

[0023] The LRG filter 22 is a mirror made using a special optical material, and is an optical element that separates light from the sample 100 by transmitting and reflecting it according to its wavelength. The LRG filter 22 is configured such that the transmittance and reflectance of light change according to the wavelength in a predetermined wavelength range.

[0024] Figure 3 illustrates the characteristics of the light spectrum and the LRG filter 22. In Figure 3, the horizontal axis represents wavelength, and the vertical axis represents spectral intensity (in the case of the light spectrum) and transmittance (in the case of the LRG filter 22). As shown in the characteristic X4 of the LRG filter 22 in Figure 3, in the LRG filter 22, the transmittance (and reflectance) of light changes gradually in accordance with the change in wavelength in a predetermined wavelength range (wavelength range λ1 to λ2). On the other hand, in wavelength ranges other than the predetermined wavelength range (i.e., wavelengths lower than λ1 and wavelengths higher than λ2), the transmittance (and reflectance) of light may be considered constant regardless of the change in wavelength. In other words, in a specific wavelength band (wavelength range λ1 to λ2), the transmittance of light increases monotonically (reflectance decreases monotonically) in accordance with the change in wavelength. Since transmittance and reflectance have a negative correlation, where when one increases, the other decreases, the term "transmittance (and reflectance)" may be used below instead of simply "transmittance." Furthermore, "the transmittance of light is constant regardless of the change in wavelength" includes not only cases where it is perfectly constant, but also cases where, for example, the change in transmittance for a change of 1 nm in wavelength is 0.1% or less. At wavelengths lower than λ1, the transmittance of light may be approximately 0% regardless of the change in wavelength, and at wavelengths higher than λ2, the transmittance of light may be approximately 100% regardless of the change in wavelength. Furthermore, "the transmittance of light is approximately 0%" includes transmittances of approximately 0% + 10%, and "the transmittance of light is approximately 100%" includes transmittances of approximately 100% - 10%. In Figure 3, waveform X1 shows the waveform of light output from the light source 10. As shown in waveform X1 in Figure 3, the light output from the light source 10 is within the predetermined wavelength range (wavelength λ) of the LRG filter 22. １ ~λ ２ It contains light with wavelengths included in the specified wavelength range. The specified wavelength range is, for example, the visible wavelength range, and one example is the wavelength range from 400 nm to 700 nm.

[0025] Returning to Figure 1, the area sensors 23 and 24 image the light from the sample 100. The area sensors 23 and 24 image the light separated by the LRG filter 22. Area sensor 23 images the light transmitted through the LRG filter 22 and outputs a signal. Area sensor 24 images the light reflected by the LRG filter 22 and outputs a signal. The wavelength range in which area sensors 23 and 24 are sensitive corresponds to a predetermined wavelength range in the LRG filter 22 where the transmittance (and reflectance) of light changes according to the change in wavelength. Area sensors 23 and 24 are, for example, monochrome sensors or color sensors. The imaging results (image data) from area sensors 23 and 24 are output to the computer 30 by the signals described above.

[0026] A bandpass filter (not shown) may be placed upstream of the area sensors 23 and 24. Such a bandpass filter (not shown) may be, for example, a filter that removes light in wavelength ranges other than the predetermined wavelength range described above (the wavelength range in the LRG filter 22 where the transmittance and reflectance of light change depending on the wavelength).

[0027] The computer 30 is physically composed of memory such as RAM and ROM, a processor (arithmetic circuit) such as a CPU, a communication interface, and a storage unit such as a hard disk. The computer 30 functions by executing programs stored in memory using the CPU of the computer system. The computer 30 may also be composed of a microcontroller or an FPGA.

[0028] The computer 30 derives the film thickness of the sample 100 based on the signals from the area sensors 23 and 24 that capture light. As processing related to the derivation of film thickness, the computer 30 may, for example, perform a measurement parameter derivation process based on the signals from the area sensors 23 and 24, and a film thickness derivation process based on the measurement parameters, etc. In addition, the computer 30 may store relationship information between film thickness and measurement parameters as a prerequisite for performing the film thickness derivation process.

[0029] Based on the signals from area sensors 23 and 24 that have imaged light, computer 30 derives measurement parameters regarding sample 100 for each predetermined area. The signals from area sensors 23 and 24 indicate the transmitted light distribution and the reflected light distribution in the imaging area. The measurement parameters may be various parameters that have a correlation with the film thickness. For example, the centroid wavelength of the light from sample 100, the intensity of the light transmitted through LRG filter 22, the intensity of the light reflected by LRG filter 22, or the ratio of the intensity of the light transmitted through LRG filter 22 to the intensity of the light reflected by LRG filter 22 (I Ｔ / I Ｒ or I Ｒ / I Ｔ ), etc. may be used. In the following, it will be described assuming that the measurement parameter is the centroid wavelength of the light from sample 100. Note that the centroid wavelength is sometimes also called the wavelength centroid.

[0030] Based on the spatial distribution of the transmitted light amount (the intensity of the light transmitted through LRG filter 22) specified based on the signal from area sensor 23 indicating the imaging result in area sensor 23 and the spatial distribution of the reflected light amount (the intensity of the light reflected by LRG filter 22) specified based on the signal from area sensor 24 indicating the imaging result in area sensor 24, computer 30 may derive the centroid wavelength of the light as a measurement parameter for each predetermined area. The predetermined area may be, for example, an area corresponding to the pixels of area sensors 23 and 24, or an area corresponding to a plurality of adjacent pixels. In the following, the pixel will be described as the predetermined area. When deriving the centroid wavelength of the light, specifically, computer 30 derives the centroid wavelength of each pixel based on the following equation (1). In the following equation (1), x' is the centroid wavelength, I' Ｔ is the transmitted light amount, and I' Ｒ is the reflected light amount. x' = (I' Ｔ - I' Ｒ ) / 2(I' Ｔ + I' Ｒ ) (1)

[0031] Furthermore, the computer 30 may derive the centroid wavelength of light for each pixel by considering the center wavelength of the LRG filter 22 (the center wavelength of a predetermined wavelength range) and the width of the LRG filter 22. The width of the LRG filter 22 is, for example, the wavelength range from the wavelength at which the transmittance in the LRG filter 22 is 0% to the wavelength at which the transmittance is 100%. In this case, the computer 30 may derive the centroid wavelength of each pixel based on the following equation (2). In the following equation (2), x' is the centroid wavelength, and I' Ｔ is the amount of transmitted light, I' Ｒ λ is the amount of reflected light. ０ x' = λ, where x' is the center wavelength of the LRG filter 22 and A is the width of the LRG filter 22. ０ +A(I') Ｔ -I' Ｒ ) / 2(I' Ｔ +I' Ｒ ) (2)

[0032] Figure 4 illustrates the wavelength shift corresponding to the amount of transmitted and reflected light. When x' (centroid wavelength) is derived using equation (1) or (2) above, as shown in Figure 4, I' Ｔ (Transmitted light amount) = I' Ｒ For pixels where (reflected light amount), x' = λ ０ (This is considered to be the center wavelength of the LRG filter 22). Also, I' Ｔ <I' Ｒ For pixels where the amount of reflected light is greater than the amount of transmitted light, x' = λ １ (λ ０ It is considered to be a wavelength shorter than that. Also, I' Ｔ > I' Ｒ For pixels where the amount of transmitted light is greater than the amount of reflected light, x' = λ ２ (λ ０ This refers to wavelengths that are longer than the specified wavelength. Thus, x' (centroid wavelength) shifts (wavelength shift) based on the amount of transmitted and reflected light.

[0033] Furthermore, since the centroid wavelength correlates with film thickness, it can be used to derive film thickness. Figure 5 shows the relationship between wavelength and film thickness. In Figure 5, the horizontal axis represents wavelength and the vertical axis represents reflectance. In the examples shown in Figure 5, the relationship between wavelength and reflectance is shown for film thicknesses of 820 nm, 830 nm, and 840 nm. As shown in Figure 5, the centroid wavelength differs depending on the film thickness. Thus, since the centroid wavelength and film thickness are correlated, it is possible to estimate the film thickness by determining the centroid wavelength.

[0034] Returning to Figure 1, the computer 30 may store information relating the film thickness to a measurement parameter (in this case, the centroid wavelength). As mentioned above, there is a correlation between film thickness and the centroid wavelength. Therefore, by having information relating the film thickness to the centroid wavelength prepared in advance, the film thickness can be derived from this relationship information and the actually measured centroid wavelength. The computer 30 may store this relationship information for each type of film.

[0035] The relevant information may be derived based on the theoretical reflectance corresponding to the type of film and the spectral characteristics (spectral sensitivity) of the entire film thickness measuring device 1. The theoretical reflectance can be determined for each wavelength once the type of film (refractive index and extinction coefficient) and film thickness are determined. The spectral characteristics of the entire film thickness measuring device 1 can be determined (estimated) in advance by various methods. Below, we will illustrate an example of a method for estimating the spectral characteristics of the entire film thickness measuring device 1.

[0036] The spectral characteristics of the film thickness measuring device 1 may be estimated, for example, by accumulating the spectral characteristics (spectral sensitivity) of each optical component constituting the film thickness measuring device 1. Specifically, the spectral characteristics of the film thickness measuring device 1 may be estimated by accumulating the luminance spectrum of the light source 10, the spectral transmittance (transmittance spectrum) of the half mirror 11, the spectral transmittance (transmittance spectrum) of the field lens 12, the spectral transmittance (transmittance spectrum) of the lens 21, the spectral transmittance of the LRG filter 22, the quantum efficiency (QE) or spectral sensitivity of the area sensor 23, the quantum efficiency (QE) or spectral sensitivity of the area sensor 24, and the reflectance of a bare wafer placed in place of the sample 100.

[0037] The estimation of the spectral characteristics of the film thickness measuring device 1 does not necessarily have to be performed by the film thickness measuring device 1. That is, it is sufficient that the above-mentioned relational information is stored in the computer 30, and the spectral characteristics of the film thickness measuring device 1 used to derive said relational information can be determined in any way.

[0038] As described above, the relationship between film thickness and centroid wavelength is derived based on the theoretical reflectance corresponding to the type of film and the spectral characteristics of the film thickness measuring device 1. More specifically, the derivation of the relationship information may be performed by deriving the expected value of the centroid wavelength (measurement parameter) from the theoretical reflectance and the spectral characteristics of the film thickness measuring device 1, and then plotting the expected value of the centroid wavelength and deriving the relationship equation by curve fitting.

[0039] [Measurement of sample film thickness via intervening part] The film thickness measurement of sample 100 performed by the film thickness measuring device 1 according to this embodiment is performed via an intervening part 150, as shown in detail in Figures 6(a) to (c). The intervening part 150 is a component placed on sample 100 (on the light-irradiated side of sample 100) and is a component containing liquid such as water, a window component such as glass, and a gas. The intervening part 150 may have a two-layer structure of water and glass, a two-layer structure of water and air, or a structure of three or more layers. The intervening part 150 is a component whose refractive index n, damping coefficient k, and thickness d are known (obtained in advance). When measuring film thickness via the intervening part 150, it is necessary to determine the film thickness of sample 100 using a multilayer reflection model (described later) that takes into account the liquid layer and glass layer which constitute the intervening part 150.

[0040] Here, as a preprocessing step before measuring the film thickness, the computer 30 acquires image data from the area sensors 23 and 24 and performs predetermined correction processing. The correction processing is a process for estimating the film thickness with high accuracy, and includes, for example, background correction and reference correction (shading correction).

[0041] Background correction is a process that appropriately obtains the brightness value of light from sample 100 corresponding to the irradiated light by subtracting the light from the atmosphere (background light) contained in the image data. When a coaxial reflected light optical system is used as the optical system, for example, there is a problem in that ghosts (images) due to multiple reflections from various reflective surfaces on the optical axis appear on the measurement image. Background correction is performed to eliminate the effects of such ghosts and the dark current noise of the camera. In background correction, for example, the brightness value of the image data of the sample 100 (sample image) used as the background correction target (background image, BG image), which is sample 100 in a non-reflective state, is subtracted from the brightness value of the image data of sample 100 (sample image). Reference correction (shading correction) is a process that corrects individual differences such as unevenness in brightness and aberrations caused by the optical system by obtaining difference data between the image data of sample 100 (sample image) and a shading image for the area corresponding to the image data. In reference correction, for example, the brightness value of the sample image is divided by the brightness value of the image data of the reference sample (reference image).

[0042] Figure 6 illustrates the acquisition of images when performing background correction and reference correction. The film thickness measuring device 1 acquires images while changing the sample 100 in each of the following cases: BG image acquisition for background correction (see Figure 6(a)), reference image acquisition for reference correction (see Figure 6(b)), and sample image acquisition (see Figure 6(c)). As shown in Figure 6(a), when acquiring a BG image, the object S1 for background correction is set as sample 100. The object S1 for background correction is a sample with a light reflectivity close to 0%, and is composed of, for example, a blackbody sample. As shown in Figure 6(b), when acquiring a reference image, the reference sample S2 is set as sample 100. The reference sample S2 is a sample with a light reflectivity close to 100%. As shown in Figure 6(c), when acquiring a sample image, the object S3 for which the film thickness distribution is to be measured is set as sample 100.

[0043] As described above, the film thickness measurement of sample 100 (specifically, object S3 to be measured) according to this embodiment is performed via the intervening part 150. In this case, the image data of the object to be used for background correction (background image, BG image) will include interference light from the intervening part 150. When background correction is performed, if the brightness value of such a BG image is subtracted from the brightness value of the sample image, the component of interference light from the intervening part 150 will also be removed from the sample image. As a result, it becomes impossible to derive the film thickness of object S3 to be measured with high accuracy using a reflection model of a multilayer film that takes into account the liquid layer or glass layer, which is the intervening part 150. In this regard, the above problem does not occur if the BG image itself can be acquired without the intervening part 150, but due to structural circumstances inside the semiconductor device manufacturing equipment, it is sometimes difficult to acquire a BG image without the intervening part 150. Therefore, in the film thickness measurement of the object S3 performed by the film thickness measuring device 1 according to this embodiment, the BG image acquired via the intervening part 150 is converted into a BG image without the intervening part 150, and background correction is performed using the converted BG image, thereby appropriately correcting the background and achieving high accuracy in subsequent film thickness measurement.

[0044] A film thickness measurement method for measuring the film thickness distribution of an object S3 comprises a first light irradiation step, a first image acquisition step, a second light irradiation step, a second image acquisition step, a light integral intensity derivation step, a conversion step, and a third image acquisition step. Note that the steps related to reference correction will be omitted in the following description.

[0045] The first light irradiation step is to irradiate the object S1 for background correction with light in a planar manner through an intervening part 150 whose refractive index n, attenuation coefficient k, and thickness d are known (see Figure 6(a)). In the first light irradiation step, the light source 10 irradiates light in a planar manner through the intervening part 150.

[0046] The first image acquisition step involves separating the light from the background correction target object S1 guided through the intervening part 150 using an LRG filter 22 (optical element) whose transmittance and reflectance change in a predetermined wavelength range, and acquiring a BG image when the intervening part 150 is present by imaging the separated light with area sensors 23 and 24 (see Figure 6(a)). In the first image acquisition step, the area sensors 23 and 24 image the light separated by the LRG filter 22 to generate image data (background image, BG image).

[0047] The second light irradiation step is to irradiate the object to be measured S3 with light in a planar manner through the intervening part 150 (see Figure 6(c)). In the second light irradiation step, the light source 10 irradiates light in a planar manner through the intervening part 150.

[0048] The second image acquisition step involves separating the light from the object to be measured S3 guided through the intervening part 150 using the LRG filter 22, and capturing the separated light with area sensors 23 and 24 to acquire a sample image of the object to be measured S3 when the intervening part 150 is present (see Figure 6(c)). In the second image acquisition step, the area sensors 23 and 24 capture the light separated by the LRG filter 22 to generate image data (sample image).

[0049] The step of deriving the integrated intensity of light involves deriving the integrated intensity of light with the intervening part 150 present, based on the theoretical reflectance of the background correction target object S1 with the intervening part 150 present, the wavelength spectrum of the light irradiated from the light source 10, and the spectral characteristics (spectral sensitivity) of the entire film thickness measuring device 1, and also deriving the integrated intensity of light without the intervening part 150 present, based on the theoretical reflectance of the background correction target object S1 with the intervening part 150 absent, the wavelength spectrum of the light irradiated from the light source 10, and the spectral characteristics (spectral sensitivity) of the entire film thickness measuring device 1.

[0050] The derivation of the theoretical reflectance of the object S1 for background correction will be explained with reference to Figures 7 and 8. Figure 7 is a diagram illustrating a multilayer reflection model considering the two layers of inclusions 151 and 152 that constitute the intervening portion 150. Figure 8 is a diagram illustrating a multilayer reflection model when the intervening portion 150 is absent.

[0051] As shown in Figure 7, let's assume that the object S1 for background correction is composed of two layers: a lower substrate S12 and a thin film S11. Let's also assume that the intervening portion 150 is composed of two layers: an intervening material 152 and an intervening material 151. The layers are stacked in the order of lower substrate S12, thin film S11, intervening material 152, and intervening material 151, with the uppermost intervening material 151 in contact with air. The lower substrate S12 has a refractive index n Ｂ , damping coefficient k Ｂ , thickness d Ｂ It is composed of the following. The thin film S11 has a refractive index n １ , damping coefficient k １ , thickness d １ It is composed of the following. The inclusion 152 has a refractive index n Ｔ２ , damping coefficient k Ｔ２ , thickness d Ｔ２ It is composed of the following. The inclusion 151 has a refractive index n Ｔ１ , damping coefficient k Ｔ１ , thickness d Ｔ１ It is composed of [something]. Also, the refractive index of air is n ０ , the damping coefficient is k ０ Let's assume that this is the case.

[0052] In this case, the reflectance (theoretical reflectance) R1 (see Figure 7) of the object S1 for background correction in the presence of the intervening portion 150 is given by the following equation (3).

[0053]

[0054] Furthermore, the reflectance R2 shown in equation (3) is the reflectance of the configuration excluding the inclusion 151 shown in Figure 7, and is shown by equation (4) below. The reflectance R3 shown in equation (4) below is the reflectance derived from the reflection model of the multilayer structure when the inclusion 150 shown in Figure 8 is absent, and is shown by equation (19) described later.

[0055]

[0056] In this case, the following relationships shown in equations (5) to (18) hold true.

[0057]

[0058] As shown in Figure 8, let's assume that the object S1 for background correction is composed of two layers: a lower substrate S12 and a thin film S11. Let's also assume that there is no intervening portion 150, the lower substrate S12 is in contact with the environment (air), and the thin film S11 is in contact with the environment (water). The lower substrate S12 has a refractive index n Ｂ , damping coefficient k Ｂ , thickness d Ｂ It is composed of the following. The thin film S11 has a refractive index n １ , damping coefficient k １ , thickness d １ It is composed of the refractive index of the environment (air) n ０ , the damping coefficient is k ０ Assume that the refractive index of the environment (water) is n Ｔ２ , the damping coefficient is k Ｔ２ Let's assume that this is the case.

[0059] In this case, the reflectance (theoretical reflectance) R3 of the background correction target object S1 in the absence of the intervening portion 150 (see Figure 8) is given by the following equation (19).

[0060]

[0061] In this case, the following relationships shown in equations (20) to (29) hold true. Note that Re is a real number.

[0062]

[0063] Using the above-described relational equations, the reflectance R1 of the background correction target object S1 when the intervening part 150 is present, and the reflectance R3 of the background correction target object S1 when the intervening part is absent are derived.

[0064] Figures 9 and 10 illustrate specific examples of reflectance. Figure 9 illustrates a reflection model considering an intervening portion 150 composed of glass and water. As shown in Figure 9, the object S1 for background correction is composed of two layers: a Si substrate and a thin film (SiO2). The intervening portion 150 is composed of two layers: glass (BK7) and water. The layers are stacked in the order of Si substrate, thin film (SiO2), water, and glass (BK7), with the top layer of glass (BK7) in contact with air. The Si substrate has a refractive index n Ｂ , damping coefficient k Ｂ It is composed of the following. The thin film (SiO2) has a refractive index n １ , damping coefficient k １ It is composed of a thickness of 885 nm. Water has a refractive index n Ｔ２ , damping coefficient k Ｔ２ It is composed of a thickness of 1000 μm. The glass (BK7) has a refractive index n Ｔ１ , damping coefficient k Ｔ１ It is composed of a material with a thickness of 700 μm. Also, the refractive index of air is n ０ , the damping coefficient is k ０ Let's assume that this is the case.

[0065] Figure 10 is a graph showing the reflectance R1 when the intervening portion 150 is present (dashed line labeled "glass + water" in the legend in the figure) and the reflectance R3 when the intervening portion 150 is absent (solid line labeled "original" in the legend in the figure). In Figure 10, the horizontal axis is wavelength and the vertical axis is reflectance. As shown in Figure 10, the reflectance R1 when the intervening portion 150 is present and the reflectance R3 when the intervening portion 150 is absent for each wavelength are derived based on the conditions in Figure 9 and the relational equations described above.

[0066] In the step of deriving the integrated intensity of light, the computer 30 derives the integrated intensity of light with the intervening part 150 present, based on the theoretical reflectance of the background correction target object S1 with the intervening part 150 present (reflectance R1 as described above), the wavelength spectrum of the light irradiated from the light source 10, and the spectral characteristics (spectral sensitivity) of the entire film thickness measuring device 1. In the step of deriving the integrated intensity of light, the computer 30 also derives the integrated intensity of light without the intervening part 150 present, based on the theoretical reflectance of the background correction target object S1 with the intervening part 150 absent (reflectance R3 as described above), the wavelength spectrum of the light irradiated from the light source 10, and the spectral characteristics (spectral sensitivity) of the entire film thickness measuring device 1.

[0067] The conversion step is to convert the background image of the background correction target object S1 with the intervening part 150 present to the background image of the background correction target object S1 without the intervening part 150, based on the ratio of the integrated light intensity with the intervening part 150 present to the integrated light intensity without the intervening part 150. In the conversion step, the computer 30 performs the conversion process to the background image of the background correction target object S1 without the intervening part 150 based on the ratio of the integrated light intensity. By considering the ratio of the integrated light intensity in this way, it is possible to appropriately convert from the background image with the intervening part 150 present to the background image without it.

[0068] The third image acquisition step is to acquire a film thickness distribution image showing the film thickness distribution of the object to be measured S3 by subtracting the brightness value of the background correction image of the background correction object S1 acquired in the conversion step when the intervening part 150 is absent from the brightness value of the sample image of the object to be measured S3 when the intervening part 150 is present, which was acquired in the second image acquisition step. In the third image acquisition step, the computer 30 performs background correction by subtracting the brightness value of the background correction image of the background correction object S1 when the intervening part 150 is absent from the brightness value of the sample image of the object to be measured S3 when the intervening part 150 is present. Then, after background correction, the computer 30 derives the film thickness distribution of the object to be measured S3 based on the film thickness distribution image.

[0069] Thus, the computer 30 is configured to derive the film thickness distribution of the object to be measured S3 based on image data, and is configured to perform the following: acquire a background correction image of the object to be measured S1 when the intervening part 150 is present and a sample image of the object to be measured S3 when the intervening part 150 is present; convert the background correction image when the intervening part 150 is present to a background correction image when the intervening part 150 is absent based on the ratio of the integrated light intensity when the intervening part 150 is present and when the intervening part 150 is absent; and acquire a film thickness distribution image showing the film thickness distribution of the object to be measured S3 by subtracting the brightness value of the background correction image when the intervening part 150 is absent from the brightness value of the sample image.

[0070] Next, the process of the film thickness measurement method performed by the film thickness measuring device 1 according to this embodiment will be described with reference to Figure 11.

[0071] First, the wavelength spectrum of the light emitted from the light source 10 is acquired (step S101), and the spectral characteristics (spectral sensitivity) of the entire film thickness measuring device 1 are acquired (step S102).

[0072] Then, the reflectance (theoretical reflectance) R3 of the object S1 for background correction in the absence of the intervening part 150 is derived (step S103). Also, the reflectance (theoretical reflectance) R1 of the object S1 for background correction in the presence of the intervening part 150 is derived (step S104).

[0073] Next, a background image of the object S1 for background correction is acquired when the intervening part 150 is present (step S105).

[0074] Then, based on the reflectances R1 and R3, the wavelength spectrum of the light irradiated from the light source 10, and the spectral characteristics of the entire film thickness measuring device 1, the integral intensity ratio of light in the state with the intervening portion 150 and the state without the intervening portion 150 is derived, and based on this integral intensity ratio of light, the BG image in the state with the intervening portion 150 is converted to the BG image in the state without the intervening portion 150 (step S106). By subtracting the brightness value of the BG image in the state without the intervening portion 150 from the brightness value of the sample image of the object to be measured S3, a film thickness distribution image showing the film thickness distribution of the object to be measured S3 is obtained.

[0075] Next, the effects and advantages of the film thickness measurement method performed by the film thickness measuring device 1 according to this embodiment will be explained.

[0076] The film thickness measurement method according to this embodiment is a film thickness measurement method for measuring the film thickness distribution of an object to be measured S3, and comprises: a first light irradiation step of irradiating a background correction object S1 with light in a planar manner through an intervening part 150 whose refractive index n, attenuation coefficient k, and thickness d are known; a first image acquisition step of obtaining a BG image of the background correction object S1 in the presence of the intervening part 150 by separating the light from the background correction object S1 guided through the intervening part 150 using an LRG filter 22 whose transmittance and reflectance change in a predetermined wavelength range, and imaging the separated light; a second light irradiation step of irradiating a measurement object S3 with light in a planar manner through the intervening part 150, and separating the light from the measurement object S3 guided through the intervening part 150 using an LRG filter 22, and imaging the separated light The method includes: a second image acquisition step of acquiring a sample image of the object to be measured S3 when the intervening part 150 is present; a conversion step of converting the BG image of the object to be used for background correction when the intervening part 150 is present to the BG image of the object to be used for background correction when the intervening part 150 is absent, based on the ratio of the integrated light intensity when the intervening part 150 is present to the integrated light intensity when the intervening part 150 is absent; and a third image acquisition step of acquiring a film thickness distribution image showing the film thickness distribution of the object to be measured S3 by subtracting the brightness value of the BG image of the object to be used for background correction when the intervening part 150 is absent, acquired in the conversion step, from the brightness value of the sample image of the object to be measured S3 when the intervening part 150 is present, acquired in the second image acquisition step.

[0077] In the film thickness measurement method according to this embodiment, a background image of the object S1 for background correction is acquired when the intervening portion 150 is present, and based on the ratio of the integrated light intensity when the intervening portion 150 is present and when it is not, the background image when the intervening portion 150 is present is converted to a background image when the intervening portion 150 is not present. In this way, by considering the ratio of the integrated light intensity, the background image when the intervening portion 150 is present can be appropriately converted to a background image when it is not present. Then, the brightness value of the background image when the intervening portion 150 is not present, acquired in this way, is subtracted from the brightness value of the sample image of the object S3 to be measured when the intervening portion 150 is present (background correction is performed). This makes it possible to subtract only the effect of the background light (light from the atmosphere) that is originally intended to be subtracted, without being affected by the intervening portion 150, and background correction can be performed with high accuracy. Then, by acquiring a film thickness distribution image after background correction, the film thickness distribution of the object S3 to be measured can be acquired with high accuracy. As described above, according to the film thickness measurement method of this embodiment, even when imaging is performed with the intervening portion 150 present, the film thickness distribution of the object to be measured S3 can be obtained with high accuracy.

[0078] The above film thickness measurement method may further include a step for deriving the integrated intensity of light, which involves deriving the integrated intensity of light when the intervening portion 150 is present, based on the theoretical reflectance of the background correction target object S1 when the intervening portion 150 is present, the wavelength spectrum of the light irradiated from the light source 10, and the spectral characteristics of the entire film thickness measuring device 1, and also deriving the integrated intensity of light when the intervening portion 150 is absent, based on the theoretical reflectance of the background correction target object S1 when the intervening portion 150 is absent, the wavelength spectrum of the light irradiated from the light source 10, and the spectral characteristics of the entire film thickness measuring device 1. In this way, by considering the theoretical reflectance, the wavelength spectrum of light, and the spectral characteristics, the integrated intensity of light can be appropriately derived, and the above-mentioned integrated intensity ratio of light can be derived with high accuracy.

[0079] In the above film thickness measurement method, the object S1 for background correction may include a blackbody sample. By using a blackbody sample with low reflectivity (reflectivity close to 0%) as the object S1 for background correction, background correction can be performed appropriately without being affected by the object S1.

[0080] 1...Film thickness measuring device, 10...Light source, 22...LRG filter (optical element), 23, 24...Area sensor (imaging unit), 30...Computer (analysis unit), 150...Intervening unit, S1...Object for background correction, S3...Object to be measured.

Claims

1. A method for measuring the film thickness distribution of an object to be measured, comprising: a first light irradiation step of irradiating a background correction object with light in a planar manner through an intervening part with known refractive index n, attenuation coefficient k, and thickness d; a first image acquisition step of obtaining a background correction image of the background correction object in the presence of the intervening part by separating the light from the background correction object guided through the intervening part using an optical element whose transmittance and reflectance change in a predetermined wavelength range, and imaging the separated light; a second light irradiation step of irradiating the object to be measured with light in a planar manner through the intervening part; and a second image acquisition step of obtaining a sample image of the object to be measured in the presence of the intervening part by separating the light from the object to be measured using the optical element, and imaging the separated light. A film thickness measurement method comprising: a conversion step of converting a background correction image of the object to be background corrected when the intervening part is present to a background correction image of the object to be background corrected when the intervening part is absent, based on the ratio of the integrated intensity of light when the intervening part is present to the integrated intensity of light when the intervening part is absent; and a third image acquisition step of acquiring a film thickness distribution image showing the film thickness distribution of the object to be measured by subtracting the brightness value of the background correction image of the object to be background corrected when the intervening part is absent, acquired in the conversion step, from the brightness value of the sample image of the object to be measured when the intervening part is present, acquired in the second image acquisition step, to acquire a film thickness distribution image showing the film thickness distribution of the object to be measured.

2. The method for measuring film thickness according to claim 1, further comprising a step of deriving the integrated intensity of light in the state in which the intervening part is present, based on the theoretical reflectance of the object to be used for background correction, the wavelength spectrum of light irradiated from the light source, and the spectral characteristics of the entire measuring device, and deriving the integrated intensity of light in the state in which the intervening part is absent, based on the theoretical reflectance of the object to be used for background correction, the wavelength spectrum of light irradiated from the light source, and the spectral characteristics of the entire measuring device.

3. The method for measuring film thickness according to claim 1 or 2, wherein the object to be used for background correction includes a blackbody sample.

4. A film thickness measuring device for measuring the film thickness distribution of an object to be measured, comprising: a light source that irradiates light in a planar manner through an intervening part with known refractive index n, attenuation coefficient k, and thickness d; an optical element whose transmittance and reflectance change in a predetermined wavelength range and separates light guided through the intervening part by transmitting and reflecting it; an imaging unit that captures the light separated by the optical element and generates image data; and an analysis unit that derives the film thickness distribution of the object to be measured based on the image data, wherein the analysis unit acquires a background correction image which is the image data of the object to be measured when the intervening part is present, and a sample image which is the image data of the object to be measured when the intervening part is present, and converts the background correction image when the intervening part is present to the background correction image when the intervening part is absent, based on the ratio of the integrated intensity of light when the intervening part is present and the integrated intensity of light when the intervening part is absent. A film thickness measuring device configured to perform the following actions: obtain a film thickness distribution image showing the film thickness distribution of the object to be measured by subtracting the brightness value of the background correction image in the absence of the intervening part from the brightness value of the sample image.

Images