Scanning interference microscope and scanning interference measuring method

Abstract

The present invention addresses the problem of providing a technology capable of accurately measuring the film thickness of a thin film sample by using a white interference method. A scanning interference microscope according to the present invention calculates the film thickness of a sample on the basis of interference light generated by an interference objective lens, and a light source has spectral characteristics in which the Fourier transform of the wavelength spectrum of light entering the interference objective lens from the light source side becomes an exponential decay curve (see fig. 3).

Description

Scanning interference microscope and scanning interferometry method

[0001] The present invention relates to a scanning interference microscope.

[0002] One method for optically measuring the film thickness of a sample is to use an interference objective (e.g., Michelson, Mirau, or Linnik) to irradiate the sample with white light. In this method, the height positions of the sample's front and back surfaces are measured based on the position of maximum intensity of the interference fringes generated by scanning the interference objective (or the sample itself) along the optical axis (the optical axis of the light irradiating the sample, i.e., the normal direction to the sample surface).

[0003] Patent Document 1 describes a method for optically measuring the film thickness of a sample.

[0011] ...

[0004] JP 2023-170825 A US9279657B2

[0005] In the general method of measuring film thickness using white light interferometry, a short coherence length is preferable to increase the resolution of film thickness separation. If an optical system can be configured to allow light to pass through all wavelength ranges in order to shorten the coherence length, the Fourier transform of the light source spectrum will theoretically be a delta function, resulting in a coherence length of 0 μm. However, considering the wavelength range that optical microscopes can generally transmit and the spectrum of available light sources, a coherence length of up to 1 μm is realistic. Therefore, conventional methods for measuring film thickness using white light interferometry have difficulty measuring the film thickness of thin samples (e.g., less than 1 μm).

[0006] The present invention has been made in view of the above-mentioned problems, and has as its object to provide a technique that can accurately measure the film thickness of a thin film sample by white light interferometry.

[0007] The scanning interference microscope according to the present invention calculates the film thickness of a sample based on interference light generated by an interference objective lens, and the light source has spectral characteristics such that the Fourier transform of the wavelength spectrum of light incident on the interference objective lens from the light source side is an exponential decay curve.

[0008] The scanning interference microscope according to the present invention can measure the thickness of a thin film sample with high accuracy by white light interferometry. Other objects, configurations, advantages, etc. of the present invention will become apparent from the following description of the embodiments.

[0009] 5 is a schematic diagram showing the configuration of a scanning interference microscope 1 according to embodiment 1. FIG. 6 is a diagram illustrating an interference profile of interference light detected by a detector 14. FIG. 7 is a diagram illustrating the spectral characteristics of a light source 11 in embodiment 1. FIG. 8 is an example of an interferogram of a sample 2 acquired using a scanning interference microscope 1. FIG. 9 is a side view of a sample 2 having a step. FIG. 10 is an example of interference fringes created from interference light of the sample 2 shown in FIG. 5. FIG. 11 is a diagram showing an alignment of the interference fringes of FIG. 6.

[0010] 1 is a schematic diagram showing the configuration of a scanning interference microscope 1 according to a first embodiment of the present invention. The scanning interference microscope 1 is a device that optically measures the film thickness of a sample 2 using, for example, white light interferometry. The scanning interference microscope 1 includes a light source 11, a beam splitter 12, an interference objective lens 13, a detector 14, and a computing device 15.

[0011] The interference objective lens 13 includes a reference mirror and a beam splitter inside. A portion of the light incident on the interference objective lens 13 from the light source 11 passes through the beam splitter and is irradiated onto the sample 2, and the light reflected from the sample 2 passes through the beam splitter and reaches the reference mirror. The remainder of the light incident on the interference objective lens 13 from the light source 11 is reflected without passing through the beam splitter and reaches the reference mirror. Interference light is generated according to the optical path difference between these two lights.

[0012] The light source 11 emits light having a wide wavelength range. The beam splitter 12 reflects the light emitted from the light source 11 toward the interference objective lens 13 and passes the light reflected from the sample 2 toward the detector 14. The interference objective lens 13 irradiates the sample 2 with the light from the beam splitter 12 and passes the light reflected from the sample 2 toward the beam splitter 12. The detector 14 detects the light reflected from the sample 2. The computing device 15 calculates the film thickness of the sample 2 (the thickness at the position where the light is irradiated, the position of the optical interface) using the detection signal of the reflected light detected by the detector 14.

[0013] When light is irradiated onto the sample 2, reflected light is generated at the optical interfaces on the front and rear surfaces of the sample, and the reflected light interferes with a reference surface mirror (not shown) in the interference objective lens 13. The detector 14 detects the interference light.

[0014] FIG. 2 illustrates an example of an interference profile of interference light detected by the detector 14. When the interference objective lens 13 is moved in the optical axis direction, interference light is obtained at the optical interface. This is referred to as an interference profile. In FIG. 2, the horizontal axis represents the optical path difference (the difference between the optical path that passes through the beam splitter in the interference objective lens 13, is irradiated onto the sample, and reaches the reference mirror in the interference objective lens 13, and the optical path that reflects off the beam splitter and reaches the reference mirror), and the vertical axis represents the intensity of the interference light. Since the intensity change of the interference light based on the optical path difference is generally the same regardless of the sign of the optical path difference, the interference profile is generally symmetrical around an optical path difference of 0 at the optical interface. Between optical interfaces, the interference profiles overlap with each other due to a phase shift, affecting each other, and therefore the symmetry is generally lost. The interference profile is also called an interferogram.

[0015] In optical systems, the interference light from the sample front surface and the sample back surface are observed as separate peaks. By identifying these peaks in the interference profile, the height positions of the sample front surface and back surface (i.e., film thickness) can be calculated. However, if the coherence length of the light emitted from the light source 11 is long, the interference profiles from each optical interface overlap, making it impossible to clearly distinguish these peaks on the interference profile (Figure 2, top). While it is possible to obtain peaks by deconvoluting the detected signal, accuracy decreases when attempting to separate and extract small signals buried within large signals. This makes it difficult to accurately calculate the sample film thickness. This tendency is particularly pronounced for thin samples with a thickness of approximately 1 μm or less.

[0016] Therefore, in the first embodiment, a method will be described that can clearly identify interference light peaks even in such a thin film sample.

[0017] Assume that the light source 11 emits light with a wave number k. If the spectrum representing the wave number dependency of the emitted light is B(v) and the optical path difference is z, the intensity I(z) of the interference light is expressed as I(z) = ∫ ０ ∞ It is expressed by B(k)(1+cos2πkz)dk. The second term on the right-hand side corresponds to the interferogram. The interferogram can be expressed as follows by Fourier transform. F(z) represents the interferogram. B(k) is the light source spectrum, which is expressed as B(k) / 2 (k≧0) or B(-k) / 2 (k<0).

[0018] F(z) = ∫ ０ ∞ B(k) cos2πkzdk B(k)=∫ ０ ∞ F(z)cos2πkzdz

[0019] According to the above formula, it can be seen that in order to obtain the light source spectrum from the interferogram, it is sufficient to perform a Fourier transform on the interferogram. In other words, in order for the interferogram to have a clear peak, it is desirable that a clear peak appears when the light source spectrum is Fourier transformed. Focusing on this point, the first embodiment configures the spectral characteristics of the light source 11.

[0020] For the sake of convenience, the explanation will be given here in terms of the spectrum of the light source, but strictly speaking, the light irradiated onto the sample is the product of all the spectral optical characteristics of the optical systems such as lenses and cameras through which it passes after being emitted from the light source. Therefore, the spectral characteristics of the light source 11 should be configured so that the Fourier transform of the wavelength spectrum of the light incident on the interference objective lens 13 from the light source 11 side forms an exponential decay curve.

[0021] FIG. 3 is a diagram illustrating the spectral characteristics of the light source 11 in the first embodiment. The upper left of FIG. 3 shows the spectral characteristics of the light source 11, with the horizontal axis representing the wave number and the vertical axis representing the power. As shown in the figure, this spectral characteristic has a symmetrical Lorentzian function shape with respect to the wave number. If the wave number is replaced with the wavelength, it becomes as shown in the lower left of FIG. 3. If this spectral characteristic is Fourier transformed, it becomes an exponential decay curve with a symmetrical peak at the center, as shown in the right of FIG. 3. If the Lorentzian function spectrum in the upper left of FIG. 3 is converted into 2a / (a ２ +k ２ ), the exponential decay curve on the right of Figure 3 is e －ａ｜ｚ｜ It can be expressed as:

[0022] By using a light source 11 having the spectral characteristics shown in Fig. 3, a sharp peak appears clearly on the interferogram, which is thought to enable the height direction positions of the sample front surface and the sample back surface to be clearly identified. In order to realize the spectral characteristics shown in Fig. 3, it is conceivable to configure the light source 11 by combining, for example, multiple light-emitting elements (e.g., light-emitting diodes: LEDs) having different spectral characteristics. Alternatively, a phosphor having a spectrum as shown in Fig. 3 may be used.

[0023] If it were possible to construct a light source 11 with ideal spectral characteristics that have the same power across the entire wavelength range, its Fourier transform would be a delta function. However, as mentioned above, even if a light source in the visible light range realistically used in optical microscopes is used, the coherence length would realistically be 1 μm.

[0024] Even if the light source is not ideal, the Fourier transform (i.e., interferogram) of the spectral characteristics shown in Figure 3 will result in an interferogram with sharp tips at the sample interfaces (the front and back surfaces of the sample), so the tips can be said to have properties similar to those of a delta function. The present invention takes advantage of this point and configures the spectral characteristics of the light source 11 so that the interferogram has a peak shape similar to that shown in the right diagram of Figure 3 near the sample interface.

[0025] Figure 4 shows an example of an interferogram of sample 2 acquired using scanning interference microscope 1. The two positions indicated by the arrows at the top of Figure 4 correspond to the front and back surfaces of the sample, respectively. As shown in Figure 4, these peaks are clearly visible by using light source 11. Therefore, the sample film thickness can be accurately measured. Even when the film thickness is less than 1 μm and the coherence length is short, making it difficult to detect peaks, as explained in Figure 3, the spectral characteristics of light source 11 itself form an exponential decay curve on the interferogram with a sharp tip, making it easy to generate peaks. Therefore, even for thin-film samples, the peaks can be clearly identified as shown in Figure 4.

[0026] A supplementary note on the light intensity ratio of the light source 11 is provided. The light source 11 is configured, for example, with two LEDs having symmetrical spectral characteristics with respect to wavenumber, and an interferogram is created while changing the light intensity ratio. Figure 4 shows an overlay of interferograms created for each light intensity ratio. When the light source 11 shown in Figure 3 is used, the signal intensity at the sample interface exhibits characteristics similar to a delta function, so the true peak position at the optical interface does not change even when the light intensity ratio is changed. Therefore, when an interferogram is created while changing the light intensity ratio, the peak positions that do not change with the light intensity ratio represent the front and back surfaces of the sample. The computing device 15 can identify the true peak positions through the above procedure.

[0027] Second Embodiment Figure 5 is a side view of a sample 2 having a step. This sample 2 has a first portion (first thickness) in which a transparent thin film is superimposed on a first substrate, and a second portion (second thickness) formed by a second substrate disposed adjacent to the first portion. The difference in thickness between these portions is Δh. Even with this type of sample 2, by using the scanning interference microscope 1 described in the first embodiment, it is possible to clearly distinguish the signal peaks at the front and back positions of each portion, thereby enabling accurate calculation of Δh.

[0028] FIG. 6 shows an example of interference fringes created from the interference light of the sample 2 shown in FIG. 5 . The interference fringes (interferogram) exist in both the XY and Z directions. When the interference objective lens 13 is swept in the Z direction, for example, the interference fringes in the XY plane move. That is, when focusing on a certain pixel, the next signal that will appear as the interference objective lens 13 is swept is reflected in the interference fringes in the XY plane. Therefore, the outermost surface peak of the transparent thin film can be observed using the light source 11 from the interference fringes in the XY plane. In contrast, in the past, the interference signals of the outermost surface peak 61 and the thin film back surface (first substrate surface) peak 62 overlapped with each other, making it difficult to separate them.

[0029] The left side of Fig. 6 shows the interference fringes of the first portion (the left half of Fig. 5), and the right side shows the interference fringes of the second portion (the right half of Fig. 5). The interference fringes of the second portion have peaks corresponding to the front surface of the first portion, a peak corresponding to the front surface of the second portion, a peak corresponding to the front surface of the second portion, a peak corresponding to the back surface of the second portion, and so on.

[0030] The interference fringes of a sample 2 having a step as shown in Figure 5 are separated by the step portion as a boundary, as shown in Figure 6. The interference fringes of each portion are not aligned with each other, but have a deviation d as shown in Figure 6. The fringe spacing of the interference fringes is assumed to be L in the portion that does not include the transparent thin film. If it is difficult to determine L from the interference fringes in the XY plane of Figure 6, the tilt θ of the goniostage may be directly used, although with reduced accuracy (L = λ / 2 tan θ).

[0031] FIG. 7 shows an alignment of the interference fringes in FIG. 6 . As described in the first embodiment, the interface (front or back) of each portion of the sample 2 can be clearly identified by the scanning interference microscope 1. Therefore, in order to clearly identify the deviation d of the interference fringes as shown in FIG. 6 , the interference fringes of each portion are aligned with a reference plane so that they are parallel to either the horizontal or vertical plane of the screen. This allows the user to visually and accurately identify the deviation d. The calculation device 15 can also calculate the step height Δh by Δh = dλ / 2L (λ is the central wavelength of the light source 11).

[0032] <Regarding Modifications of the Present Invention> In the above-described embodiments, the arithmetic unit 15 can be configured by hardware such as a circuit device that implements its functions, or can be configured by a arithmetic unit such as a CPU (Central Processing Unit) that executes software that implements its functions.

[0033] In the above embodiment, the shape of the spectrum of the light source 11 is considered to be a Lorentzian function shape with respect to the wave number on the horizontal axis. In this case, when the central wavelength (central wave number) is also taken into consideration, the spectral shape is expressed as 2a / (a ２ +(k-k0) ２In this function, the value a at which the half-width of the exponential decay curve is 1 μm or less is 1.38×10^6.

[0034] Conventionally, a Gaussian distribution has been preferred as the light source spectral shape. This is because a Gaussian distribution remains a Gaussian distribution even when subjected to a Fourier transform, and sidebands are unlikely to arise and artifact peaks are unlikely to occur. Although a Gaussian distribution is appropriate from the perspective of the half-width (i.e., equivalent to the coherence length), it is not considered necessary to use a Gaussian distribution, given that peak separation of an interferogram is possible as long as the peaks are close to a delta function and the tip peaks can be separated. In light of this point, it should be noted that the significance of the present invention lies in the fact that the light source spectral shape is different from conventional shapes.

[0035] 1: Scanning interference microscope 11: Light source 12: Beam splitter 13: Interference objective lens 14: Detector 15: Computing device 2: Sample

Claims

1. A scanning interference microscope that measures the thickness of a sample by the interference of light irradiated onto the sample, comprising: a light source that emits the light; an interference objective lens that transmits a portion of the light irradiated onto the sample and irradiates the sample while reflecting the remainder, thereby generating interference light between the light reflected from the sample and the remainder; a detector that detects the interference light; and a computing device that calculates the thickness of the sample based on the interference light detected by the detector, wherein the light source has spectral characteristics such that the Fourier transform of the wavelength spectrum of the light incident on the interference objective lens from the light source side is an exponential decay curve.

2. A scanning interference microscope according to claim 1, wherein said light source has spectral characteristics in which the power characteristics relative to wavenumber are symmetrical with respect to wavenumber.

3. A scanning interference microscope according to claim 1, wherein said light source has spectral characteristics in which the power characteristics relative to wavenumber are of the Lorentzian type.

4. The scanning interference microscope of claim 1, characterized in that the interference objective lens moves in the optical axis direction of the light irradiated onto the sample, the arithmetic unit creates an interference profile that represents the relationship between the optical path difference of the interference light and the intensity of the interference light, and the arithmetic unit detects peaks in the intensity of the interference light in the interference profile to identify the thickness-wise position of the front surface of the sample and the thickness-wise position of the back surface of the sample.

5. The scanning interference microscope according to claim 4, characterized in that the light source comprises a first light-emitting element that emits the light and a second light-emitting element that emits the light, the arithmetic device creates the interference profile for each ratio while changing the ratio between the light intensity of the first light-emitting element and the light intensity of the second light-emitting element, and the arithmetic device identifies the thickness-wise position of the front surface and the thickness-wise position of the back surface by detecting the peak that does not change even when the ratio is changed.

6. A scanning interference microscope according to claim 4, wherein the sample includes a first portion having a first thickness and a second portion having a second thickness different from the first thickness, the computing device creates a first observation image having first interference fringes based on the interference profile in the first portion, the computing device creates a second observation image having second interference fringes based on the interference profile in the second portion, and the computing device calculates the difference between the first thickness and the second thickness based on the first observation image and the second observation image by aligning the first observation image and the second observation image so as to identify a deviation between the position of the front surface or the back surface in the first observation image and the position of the front surface or the back surface in the second observation image.

7. A scanning interference microscope according to claim 6, wherein the computing device aligns the first observation image and the second observation image by positioning the first interference fringes and the second interference fringes so that they are parallel to each other.

8. A scanning interferometry method for measuring the thickness of a sample by the interference of light irradiated onto the sample, comprising: a step of emitting the light from a light source; a step of irradiating the light onto the sample using an interference objective lens that transmits a portion of the light to irradiate the sample and reflects the remainder, thereby generating interference light between the light reflected from the sample and the remainder; a step of detecting the interference light; and a step of calculating the thickness of the sample based on the detected interference light, wherein the light source has spectral characteristics such that the Fourier transform of the wavelength spectrum of the light incident on the interference objective lens from the light source side is an exponential decay curve.

Images