Optical inspection device and control method therefor

Abstract

Provided are an optical inspection device capable of accurately obtaining the film thickness of a film covering a sample even when there is a difference in spatial intensity distribution between irradiation light to the sample and irradiation light to a reference mirror, and a control method therefor. The optical inspection device includes: a light source that emits light; a beam splitter that splits the light emitted from the light source into first irradiation light directed toward a sample and second irradiation light directed toward a reference mirror; an interference light measurement unit that measures interference light due to object light which is reflected light of the first irradiation light from the sample and reference light which is reflected light of the second irradiation light from the reference mirror; and a control unit that controls each unit. The optical inspection device is characterized by further including: an object light measurement unit that measures the intensity of only the object light; and a storage unit that stores the intensity of the object light of a reference sample measured in advance by the object light measurement unit. The control unit is characterized by obtaining the film thickness of a film covering an observation sample on the basis of the intensity of the object light of the reference sample and the intensity of the object light of the observation sample measured by the object light measurement unit.

Description

Optical inspection apparatus and control method thereof

[0001] The present invention relates to an optical inspection apparatus for inspecting a sample by irradiating the sample with light and detecting reflected light from the sample, and a control method thereof.

[0002] For inspection of semiconductor devices with increasing miniaturization, an optical inspection apparatus is used together with a scanning electron microscope or the like. In the optical inspection apparatus, an observation image of the sample is generated by detecting reflected light from the sample irradiated with light. Further, the thickness of the transparent film is obtained based on interference light between the reflected light from the sample covered with the transparent film and the reflected light from the reference mirror. When the transparent film is thin, the thickness of the transparent film cannot be accurately obtained.

[0003] Patent Document 1 discloses a measurement method for accurately obtaining the film thickness even when the transparent film is thin. Specifically, by using each of three or more irradiation lights having different wavelengths, measuring interference light between the reflected light from the measurement target surface and the reflected light from the reference surface, and fitting a model function to the luminance signal of the interference light, the thickness of the transparent film is obtained.

[0004] Japanese Patent Application Laid-Open No. 2014-6242

[0005] However, in Patent Document 1, consideration for the case where there is a difference in the spatial intensity distribution between the irradiation light to the sample and the irradiation light to the reference mirror is insufficient. When there is a difference in the intensity distribution of the two irradiation lights due to, for example, foreign matter getting mixed in the optical path to the sample, the accuracy of the obtained film thickness decreases.

[0006] Therefore, an object of the present invention is to provide an optical inspection apparatus and a control method thereof that can accurately obtain the thickness of a film covering a sample even when there is a difference in the spatial intensity distribution between the irradiation light to the sample and the irradiation light to the reference mirror.

[0007] To achieve the above objective, the present invention provides an optical inspection apparatus comprising: a light source that emits light; a beam splitter that splits the light emitted from the light source into a first irradiation light directed toward a sample and a second irradiation light directed toward a reference mirror; an interference light measuring unit that measures interference light caused by object light, which is the reflected light from the sample of the first irradiation light, and reference light, which is the reflected light from the reference mirror of the second irradiation light; and a control unit that controls each unit, wherein the apparatus further comprises: an object light measuring unit that measures the intensity of only the object light; and a storage unit that stores the intensity of the object light of a reference sample measured in advance by the object light measuring unit, and the control unit determines the thickness of the film covering the observation sample based on the intensity of the object light of the reference sample and the intensity of the object light of the observation sample measured by the object light measuring unit.

[0008] The present invention also relates to a control method for an optical inspection apparatus comprising a light source that emits light, a beam splitter that splits the light emitted from the light source into a first irradiation light directed toward a sample and a second irradiation light directed toward a reference mirror, an interference light measuring unit that measures interference light caused by object light, which is the reflected light from the sample of the first irradiation light, and reference light, which is the reflected light from the reference mirror of the second irradiation light, and a control unit that controls each unit, characterized in that the thickness of the film covering the observation sample is determined based on the intensity of the object light of a reference sample measured in advance by an object light measuring unit that measures the intensity of only the object light, and the intensity of the object light of the observation sample measured by the object light measuring unit.

[0009] According to the present invention, it is possible to provide an optical inspection apparatus and a control method therefor that can accurately determine the thickness of a film covering a sample, even when there is a difference in the spatial intensity distribution between the light irradiated onto the sample and the light irradiated onto the reference mirror.

[0010] Figure showing an example of the overall configuration of the optical inspection apparatus in Example 1. Figure showing an example of the observation sample and reference sample. Figure showing an example of the processing flow in Example 1. Figure showing an example of the overall configuration of the optical inspection apparatus in Example 2. Figure showing an example of the overall configuration of the optical inspection apparatus in Example 3. Figure showing an example of the overall configuration of the optical inspection apparatus in Example 4.

[0011] The following describes an embodiment of the optical inspection apparatus according to the present invention with reference to the attached drawings. The optical inspection apparatus is a device that inspects a sample by irradiating the sample with light and detecting the reflected light from the sample.

[0012] An example of the overall configuration of the optical inspection apparatus of Embodiment 1 will be explained using Figure 1. The optical inspection apparatus 100 comprises a light source 101, an expander 102, a beam splitter 103, an objective lens 104, a reference mirror 106, a mirror drive unit 107, an imaging lens 108, an interference light measuring unit 109, a half mirror 110, a light measuring unit 111, and a control unit 120, and inspects a sample 105. Note that the horizontal direction of the paper is the x-axis, the depth direction is the y-axis, and the vertical direction is the z-axis.

[0013] The light source 101 emits light. The light emitted from the light source 101 is, for example, monochromatic laser light. The light source 101 may also emit multiple lights of different wavelengths.

[0014] The expander 102 is an optical element that expands the diameter of the light emitted from the light source 101. However, if the diameter of the light emitted from the light source 101 is sufficiently large, for example, if the diameter of the light irradiated onto the sample 105 is greater than or equal to the inspection area of ​​the sample 105, the expander 102 may not be necessary.

[0015] The beam splitter 103 is an optical element that splits the light emitted from the light source 101 into a first irradiation beam, which is directed toward the sample 105, and a second irradiation beam, which is directed toward the reference mirror 106. In other words, in Figure 1, the light reflected by the beam splitter 103 is the first irradiation beam, and the light transmitted through the beam splitter 103 is the second irradiation beam.

[0016] The objective lens 104 is a lens that focuses the first irradiated light, which has passed through the half mirror 110 and the imaging lens 108, onto the sample 105. The first irradiated light focused by the objective lens 104 is reflected by the sample 105. The object light, which is the reflected light from the sample 105 of the first irradiated light, reaches the interference light measuring unit 109 via the objective lens 104, the imaging lens 108, the half mirror 110, and the beam splitter 103.

[0017] The reference mirror 106 is a reflector that reflects the second irradiated light. The reference light, which is the light reflected by the reference mirror 106 of the second irradiated light, is reflected by the beam splitter 103 and reaches the interference light measurement unit 109.

[0018] The mirror drive unit 107 is a mechanism for moving the reference mirror 106. By moving the reference mirror 106 in the x direction, the optical path length of the reference light changes.

[0019] The imaging lens 108 is a lens that focuses object light onto the interference light measuring unit 109 and the light measuring unit 111.

[0020] The interference light measuring unit 109 is a measuring instrument that measures interference light caused by object light and reference light. The interference light is measured each time the optical path length of the reference light is changed by the mirror driving unit 107, and the intensity of the interference light changes according to the optical path difference between the object light and the reference light and the thickness of the film covering the sample 105. That is, the thickness of the film covering the sample 105 can be determined from the optical path difference between the object light and the reference light and the intensity of the interference light. If the interference light measuring unit 109 has multiple measurement pixels arranged in two dimensions, the intensity of the interference light is measured for each measurement pixel, and the film thickness at the position corresponding to each measurement pixel can be determined, so a film thickness distribution can be obtained.

[0021] The control unit 120 is, for example, a computer, which controls the operation of each part and determines the thickness of the film covering the sample 105 based on the measurement signal transmitted from the interference light measurement unit 109 and the optical path difference between the object light and the reference light. A storage unit, a display unit, and an input unit may be connected to the control unit 120. The storage unit is, for example, an HDD (Hard Disk Drive) or SSD (Solid State Drive), which stores various programs, data, observation images, etc. The display unit is, for example, a liquid crystal display, which displays the observation image, the results of determining the film thickness, etc. The input unit is, for example, a keyboard or mouse, which is used for data input by the user.

[0022] However, if there is a difference in the spatial intensity distribution between the first and second irradiation beams due to foreign matter getting into the path from the beam splitter 103 to the sample 105, the accuracy of the interference light intensity measured by the interference light measurement unit 109 will decrease. Consequently, the accuracy of the film thickness determined based on the interference light intensity will also decrease.

[0023] In Example 1, the intensity of object light alone is measured by using the half mirror 110 and the light measuring unit 111, and the film thickness is determined based on the measured value. More specifically, the intensity of object light for a reference sample, which is a sample not covered with a film, and the intensity of object light for an observation sample, which is a sample covered with a film, are measured, and the film thickness is determined based on the intensities of the two object lights. The half mirror 110 is a reflecting mirror placed between the sample 105 and the beam splitter 103, which reflects a portion of the object light toward the light measuring unit 111. The light measuring unit 111 is a measuring instrument that measures the intensity of the light reflected by the half mirror. In other words, the half mirror 110 and the light measuring unit 111 constitute an object light measuring unit that measures the intensity of object light alone.

[0024] An example of an observation sample and a reference sample will be explained using Figure 2. The observation sample is an opaque member 200, such as a silicon wafer, whose surface is covered with a transparent film 201, as exemplified in Figure 2(a). The refractive index of air is n0, the refractive index of the transparent film 201 is n1, the thickness of the transparent film 201 is d, and the refractive index of the opaque member 200 is n2. The reference sample is composed of an opaque member 200 with a refractive index of n2, as exemplified in Figure 2(b).

[0025] Using Figure 3, an example of the processing flow of Example 1 will be explained step by step.

[0026] (S301) The control unit 120 measures the intensity of object light from the observed sample using the object light measuring unit and measures the phase of the interference light between the object light and the reference light using the interference light measuring unit 109.

[0027] (S302) The control unit 120 reads out the object light intensity of a reference sample that has been measured in advance using the object light measurement unit. The object light intensity of the reference sample is stored in advance in a storage unit connected to the control unit 120.

[0028] (S303) The control unit 120 calculates the film thickness d based on the object light intensity of the sample 105 measured in S301 and the object light intensity of the reference sample read out in S302. The film thickness distribution d(x, y) is calculated by, for example, the following formula.

[0029]

[0030]

[0031]

[0032]

[0033] Here, λ is the wavelength of the first irradiated light, i is the number of film thickness intervals, [i / 2] is the largest integer not exceeding i / 2, n0 is the refractive index of air, n1 is the refractive index of the film, n2 is the refractive index of the opaque material, g1(x,y) is the intensity distribution of object light of the observed sample, and g0(x,y) is the intensity distribution of object light of the reference sample.

[0034] In Math 1, due to the periodicity of the cosine function, the film thickness d is not determined to a single value, and multiple candidate film thickness values ​​are calculated. Therefore, the candidate film thickness value closest to the estimated film thickness specified by the user is selected as the output film thickness value.

[0035] Furthermore, if multiple light sources of different wavelengths are emitted from the light source 101, the value closest to the candidate film thickness value obtained at other wavelengths is selected from among the multiple candidate film thickness values ​​obtained for each wavelength as the output film thickness value. For example, if the candidate film thickness values ​​at wavelength λ1 are d11, d12, and d13, at wavelength λ2 are d21, d22, and d23, and at wavelength λ3 are d31, d32, and d33, the output film thickness value is selected according to the following procedure.

[0036] First, the value closest to d11 from d21, d22, and d23 is selected as d211, and |d11-d211| is calculated. Next, the value closest to d11 from d31, d32, and d33 is selected as d311, and |d11-d311| and |d311-d211| are calculated. Then, the sum of |d11-d211|, |d11-d311|, and |d311-d211| is calculated as Δ11.

[0037] Similar to d11, for d12, the value closest to d12 among d21, d22, and d23 is selected as d212, and the value closest to d12 among d31, d32, and d33 is selected as d312. Then, the sum of |d12 - d212|, |d12 - d312|, and |d312 - d212| is calculated as Δ12. For d13 as well, the value d213 closest to d13 among d21, d22, and d23 and the value d313 closest to d13 among d31, d32, and d33 are selected, and the sum Δ13 of |d13 - d213|, |d13 - d313|, and |d313 - d213| is calculated. Finally, the film thickness candidate value corresponding to the minimum value among Δ11, Δ12, and Δ13 is selected. For example, if Δ12 is the minimum value, d12 corresponding to Δ12 is selected as the output value of the film thickness.

[0038] (S304) The control unit 120 calculates the surface height h of the observation sample based on the film thickness calculated in S303 and the phase of the interference light measured in S301. The surface height distribution h(x, y) of the observation sample is calculated, for example, by the following formula.

[0039]

[0040] Further, the height distribution of the opaque member may be obtained by subtracting the film thickness distribution d(x, y) from the surface height distribution h(x, y).

[0041] Even when there is a difference in the spatial intensity distribution between the first irradiation light, which is the irradiation light to the sample, and the second irradiation light, which is the irradiation light to the reference mirror, as exemplified by the processing flow in FIG. 3, the thickness of the film covering the observation sample can be accurately obtained.

[0042] In Example 1, the case where the object light measurement unit that measures only the intensity of the object light is composed of the half mirror 110 and the light measurement unit 111 was described. The object light measurement unit is not limited to the configuration by the half mirror 110 and the light measurement unit 111. In Example 2, by blocking the second irradiation light, the interference light measurement unit 109 measures the intensity of only the object light.

[0043] An example of the overall configuration of the optical inspection apparatus according to the second embodiment will be described with reference to FIG. 4. In FIG. 4, a shutter 400 is provided instead of the half mirror 110 and the optical measurement unit 111 in FIG. 1. Therefore, the shutter 400 will be mainly described.

[0044] The shutter 400 is a light shielding member inserted between the beam splitter 103 and the reference mirror 106, and the movement of the shutter 400 is controlled by the control unit 120. The shutter 400 inserted between the beam splitter 103 and the reference mirror 106 shields the second irradiation light, so no reference light is generated, and only the object light, which is the reflected light from the sample 105, reaches the interference light measurement unit 109. That is, by shielding the second irradiation light with the shutter 400, the intensity of only the object light is measured in the interference light measurement unit 109 with respect to the reference sample and the observation sample.

[0045] According to the optical inspection apparatus illustrated in FIG. 4, since the intensity of only the object light with respect to the reference sample and the observation sample can be measured, even when there is a difference in the spatial intensity distribution between the first irradiation light and the second irradiation light, the thickness of the film covering the observation sample can be accurately obtained. In FIG. 1, the optical measurement unit 111 is provided together with the interference light measurement unit 109, whereas in FIG. 4, only the interference light measurement unit 109 is provided, so the cost of the optical inspection apparatus can be reduced.

[0046] In the second embodiment, it has been described that the intensity of only the object light is measured by the interference light measurement unit 109 without generating reference light by shielding the second irradiation light with the shutter 400. In the third embodiment, the intensity of only the object light is measured by the interference light measurement unit 109 without generating reference light by retracting the reference mirror 106.

[0047] An example of the overall configuration of the optical inspection apparatus of Embodiment 3 will be explained using Figure 5. In Figure 5, instead of inserting the shutter 400 as in Figure 4, the mirror drive unit 107 moves in the z direction to prevent the second irradiation light from illuminating the reference mirror 106. The direction of movement of the mirror drive unit 107 is not limited to the z direction; it may also be in the y direction, or the mirror drive unit 107 may rotate around the y or z axis as the axis of rotation to prevent the second irradiation light from illuminating the reference mirror 106. When the reference mirror 106 is moved to a position where the second irradiation light does not irradiate it, no reference light is generated, and only object light, which is reflected light from the sample 105, reaches the interference light measurement unit 109. That is, by moving the reference mirror 106 by the mirror drive unit 107, the intensity of only object light is measured in the interference light measurement unit 109 for the reference sample and the observation sample.

[0048] According to the optical inspection apparatus illustrated in Figure 5, the intensity of only object light relative to the reference sample and the observation sample can be measured, so even if there is a difference in the spatial intensity distribution between the first and second illuminations, the thickness of the film covering the observation sample can be accurately determined. Furthermore, while Figure 4 requires a shutter 400, which is a light-shielding member, and a mechanism for inserting the shutter 400, Figure 5 only requires the addition of a movement direction for the mirror drive unit 107, thus further reducing the cost of the optical inspection apparatus.

[0049] In Example 1, a case was described in which the object light measuring unit, which measures the intensity of only object light, is composed of a half mirror 110 and a light measuring unit 111. The object light measuring unit is not limited to a configuration of a half mirror 110 and a light measuring unit 111. In Example 4, the light emitted by the light source 101 is separated into p-polarized and s-polarized light, one of which is the first irradiated light and the other is the second irradiated light, thereby generating interference light consisting of p-polarized and s-polarized light, and the polarization component of the object light is extracted from the interference light to measure the intensity of only the object light.

[0050] An example of the overall configuration of the optical inspection apparatus of Embodiment 4 will be explained using Figure 6. In Figure 6, instead of the half mirror 110 and light measuring unit 111 of Figure 1, a half-wave plate 601, a half mirror 610, a polarizer 611, and a light measuring unit 612 are provided, and quarter-wave plates 603, 604, and 605 are added. Furthermore, in Figure 6, the beam splitter 103 of Figure 1 is a polarizing beam splitter 602, and the interference light measuring unit 109 is a polarizing camera 606.

[0051] The half-wave plate 601 is an optical element placed between the light source 101 and the polarizing beam splitter 602, which separates the light emitted from the light source 101 into p-polarized and s-polarized light.

[0052] The polarizing beam splitter 602 splits the beam, using one of the p-polarized or s-polarized beams as the first irradiation light and the other as the second irradiation light.

[0053] The first irradiation light is directed onto the sample 105 via the quarter-wave plate 603, the imaging lens 108, and the objective lens 104, generating object light in the sample 105. The object light reaches the polarizing beam splitter 602 via the objective lens 104, the imaging lens 108, and the quarter-wave plate 603. Since the polarization of the object light passing through the quarter-wave plate 603 is rotated by 90°, it passes through the polarizing beam splitter 602 and heads towards the half mirror 610.

[0054] The second irradiation light is directed onto the reference mirror 106 via the quarter-wave plate 604, generating reference light in the reference mirror 106. The reference light reaches the polarizing beam splitter 602 via the quarter-wave plate 604. Note that the polarization of the reference light passing through the quarter-wave plate 604 is rotated by 90°, so it is reflected by the polarizing beam splitter 602 and heads towards the half-mirror 610.

[0055] The half-mirror 610 is positioned between the polarizing beam splitter 602 and the polarizing camera 606, and is a reflecting mirror that reflects a portion of the interference light, which consists of object light and reference light, toward the light measuring unit 612.

[0056] The polarizer 611 is an optical element positioned between the half mirror 610 and the light measuring unit 612, and transmits only the polarization component of the object light from the light reflected by the half mirror 610 toward the light measuring unit 612.

[0057] The light measuring unit 612 is a measuring instrument that measures the intensity of object light transmitted through the polarizer 611. In other words, the half-wave plate 601, the half-mirror 610, the polarizer 611, and the light measuring unit 612 constitute an object light measuring unit that measures the intensity of only object light.

[0058] The quarter-wave plate 605 is positioned between the half-mirror 610 and the polarizing camera 606, and is an optical element that creates a phase difference between the object light and the reference light in the interference light transmitted through the half-mirror 610.

[0059] The polarizing camera 606 detects the phase difference between object light and reference light based on the intensity of the interference light between the object light and the reference light. The phase difference detected by the polarizing camera 606 is used to calculate the surface height of the observed sample.

[0060] According to the optical inspection apparatus illustrated in Figure 6, the intensity of object light alone can be measured relative to the reference sample and the observation sample. Therefore, even if there is a difference in the spatial intensity distribution between the first and second illuminations, the thickness of the film covering the observation sample can be accurately determined. Furthermore, while the intensity of object light and the phase of interference light are measured separately in Figures 4 and 5, the intensity of object light and the phase of interference light can be measured simultaneously in Figures 6 and 1.

[0061] The embodiments of the present invention have been described above. The present invention is not limited to the embodiments described above, and the components can be modified and implemented without departing from the spirit of the invention. Furthermore, the multiple components disclosed in the above embodiments may be combined as appropriate. In addition, some components may be deleted from all the components shown in the above embodiments.

[0062] Optical inspection device 100, light source 101, expander 102, beam splitter 103, objective lens 104, sample 105, reference mirror 106, mirror drive unit 107, imaging lens 108, interference light measurement unit 109, half mirror 110, light measurement unit 111, control unit 120, opaque member 200, transparent film 201, shutter 400, half wave plate 601, polarizing beam splitter 602, quarter wave plate 603, quarter wave plate 604, quarter wave plate 605, polarizing camera 606, half mirror 610, polarizer 611, light measurement unit 612.

Claims

1. An optical inspection apparatus comprising a light source that emits light, a beam splitter that splits the light emitted from the light source into a first irradiation beam directed toward a sample and a second irradiation beam directed toward a reference mirror, an interference light measuring unit that measures interference light caused by object light, which is the reflected light from the sample of the first irradiation beam, and reference light, which is the reflected light from the reference mirror of the second irradiation beam, and a control unit that controls each unit, wherein the apparatus further comprises an object light measuring unit that measures the intensity of only the object light, and a storage unit that stores the intensity of the object light of a reference sample measured in advance by the object light measuring unit, and the control unit determines the thickness of the film covering the observation sample based on the intensity of the object light of the reference sample and the intensity of the object light of the observation sample measured by the object light measuring unit.

2. An optical inspection apparatus according to claim 1, wherein the object light measuring unit comprises a half mirror disposed between the sample and the beam splitter, and a light measuring unit for measuring the intensity of light reflected by the half mirror.

3. An optical inspection apparatus according to claim 1, wherein the object light measuring unit has a shutter inserted between the beam splitter and the reference mirror, and the measurement value of the interference light measuring unit when the shutter blocks the second irradiated light is defined as the intensity of the object light.

4. An optical inspection apparatus according to claim 1, wherein the object light measuring unit has a mirror drive unit for moving the reference mirror, and the measurement value of the interference light measuring unit when the mirror drive unit moves the reference mirror to a position where the second irradiation light does not irradiate it is defined as the intensity of the object light.

5. An optical inspection apparatus according to claim 1, wherein the beam splitter is a polarizing beam splitter, the object light measuring unit comprises a half-wave plate, a half mirror, a polarizer, and a light measuring unit, the half-wave plate is disposed between the light source and the polarizing beam splitter and separates light emitted from the light source into p-polarized and s-polarized light, one of the p-polarized and s-polarized light becomes the first irradiated light in the polarizing beam splitter and the other becomes the second irradiated light, the half mirror is disposed between the polarizing beam splitter and the interference light measuring unit, the polarizer is disposed between the half mirror and the light measuring unit and transmits the object light from the light reflected by the half mirror, and the light measuring unit measures the intensity of the object light transmitted through the polarizer.

6. An optical inspection apparatus according to claim 5, further comprising a quarter-wave plate disposed between the half-mirror and the interference light measuring unit, which creates a phase difference between the object light and the reference light, wherein the interference light measuring unit is a polarizing camera and detects the phase difference based on the intensity of the interference light.

7. A control method for an optical inspection apparatus comprising a light source that emits light, a beam splitter that splits the light emitted from the light source into a first irradiation beam directed toward a sample and a second irradiation beam directed toward a reference mirror, an interference light measuring unit that measures interference light caused by object light, which is the reflected light from the sample of the first irradiation beam, and reference light, which is the reflected light from the reference mirror of the second irradiation beam, and a control unit that controls each unit, characterized in that the thickness of the film covering the observation sample is determined based on the intensity of the object light of a reference sample measured in advance by an object light measuring unit that measures the intensity of only the object light, and the intensity of the object light of the observation sample measured by the object light measuring unit.

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