JP2009115503A - Method and device for measuring roughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure the roughness of a nano scale of the internal interface of a multilayer film formed on a substrate, in an optical and non-destructive/non-contact manner. <P>SOLUTION: A roughness measuring device 100 relating to one embodiment of this invention is designed for measuring the roughness of the measuring object interface between first and second layers formed on the substrate. The device has: a wavelength selecting part 24 which selects the light of an optimal wavelength on which a spectrum of reflection from a multilayer film forming surface of a specimen 30 having the first and second layers formed becomes minimal; a light source 11 which sheds the light of the optimal wavelength on the multilayer film forming surface of the specimen 30 and on a reference surface; a photodetector 22 which receives interference light formed by synthesizing the measuring light reflected on the multilayer film forming surface of the specimen 30 and reference light reflected on the reference surface; and a roughness calculating part 43 which calculates the roughness of the multilayer film forming surface of the specimen from a phase difference between the measuring light and the reference light, based on a change in the intensity of the interference light, and determines the roughness of the measuring object interface, based on the roughness of the multilayer film forming surface. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a roughness measuring method and a roughness measuring apparatus, and more particularly, optically non-destructively and non-contacting the nanoscale roughness of the inner interface of a multilayer film composed of a plurality of thin films formed on a substrate. The present invention relates to a measuring method and a measuring apparatus for measuring with the above.

An SOI (silicon on insulator) wafer has a structure in which an oxide film is embedded in a Si wafer and a two-layer thin film of SiO ₂ / Si is formed on a Si substrate. The thickness of each layer on the substrate is about several tens to several hundreds nm. By measuring the nanoscale roughness (unevenness) of the internal interface of such a multilayer film structure, it is required to discriminate between good and defective products in a short time.

Conventionally, methods for observing the structure of the internal interface of a multilayer film include, for example, a method using an optical microscope and a method using an atomic force microscope. However, each of the above methods has its own problems. That is, in a normal bright-field optical microscope, most thin films with a film thickness of several tens to several hundreds of nanometers are transparent to visible light, so that all reflected light from each interface of the multilayer film is synthesized and observed. The information on the interface to be measured cannot be extracted. Further, when the reflectance of the outermost surface is high, it is difficult to observe the internal interface. In particular, in the case of the above-described SOI structure, the reflectance of the Si film that is the outermost surface layer is higher than that of the SiO ₂ film that is the lower layer, so that the contrast at the inner interface is not obtained. Further, since the reflected light at each interface interferes with each other, interference contrast is seen depending on the film thickness and optical constant.

  In the confocal microscope, the reflected light from each interface cannot be separated unless the film thickness is several μm or more. In the case of a thin film having a thickness of each sub-micron layer such as an SOI structure, all the interfaces enter the same focal point and cannot be separated.

  In an interferometer using a white light source, the reflected light from the multilayer film has a characteristic reflection spectrum depending on the film thickness and optical constant. For this reason, the reference light and the reflection spectrum change, and a correct white interference fringe cannot be realized. Further, in an interferometer using a single wavelength light source, the reflectance varies greatly depending on the film thickness and material of the measurement object. For this reason, when the reflectance of the surface is high, it is difficult to observe the internal interface. Patent Document 1 discloses a method for measuring the surface shape of a multilayer film using a multi-wavelength interferometer, but a method for optically measuring the internal interface of the multilayer film has not been established.

In the atomic force microscope, the outermost surface can be observed. Therefore, in order to observe the internal interface, it must be removed one layer at a time from the upper layer by etching or the like, which is a destructive inspection. In scanning with a cantilever, it takes an enormous time of several tens of minutes to measure an area of about several tens of μm ² .
Japanese Patent Application Laid-Open No. 2004-286689

  The present invention has been made against the background of such circumstances, and an object of the present invention is to optically reduce the nanoscale roughness of the internal interface of a multilayer film composed of a plurality of thin films formed on a substrate. The present invention relates to a roughness measuring method and a roughness measuring apparatus that measure without breakage or contact.

  The roughness measuring method according to the first aspect of the present invention includes a first layer of a sample having a first layer formed on a substrate and a second layer formed on the first layer; A roughness measuring method for measuring the roughness of an interface to be measured with the second layer, wherein a predetermined wavelength is formed on a multilayer film forming surface on which the first layer and the second layer of the sample are formed. The illumination light of width is irradiated, the light of the first wavelength included in the wavelength width of the illumination light is selected so that the reflected light intensity from the multilayer film formation surface of the sample is near the minimum, and the multilayer film formation of the sample is performed Irradiating the surface and the reference surface with light of the first wavelength, and receiving interference light obtained by combining the measurement light reflected by the multilayer film formation surface of the sample and the reference light reflected by the reference surface; Based on the change in the intensity of the interference light, the roughness of the multilayer film formation surface of the sample is calculated from the phase difference between the measurement light and the reference light. Based on the roughness of the multilayer film forming surface of the sample to determine the roughness of the measurement object surface. Thereby, the roughness of the interface of the multilayer film can be measured in a non-destructive and non-contact manner.

  The roughness measurement method according to the second aspect of the present invention is the above measurement method, wherein the light having the first wavelength includes reflected light from an interface other than the measurement target interface, and interference light generated by the reflected light. The light having a wavelength that cancels out is selected. Thereby, the roughness of the interface of the multilayer film can be measured more accurately in a non-destructive and non-contact manner.

  The roughness measuring method according to the third aspect of the present invention is based on the film thicknesses of the first layer and the second layer and the refractive indexes of the first layer and the second layer in the above measuring method. The reflected light intensity and the interference light intensity from the first layer and the second layer are calculated, and based on the reflected light intensity and the interference light intensity, the light of the first wavelength is used as an interface other than the measurement target interface. The light having a wavelength that cancels the reflected light from the light and the interference light generated by the reflected light is selected. Thereby, the roughness of the interface of the multilayer film can be measured more accurately in a non-destructive and non-contact manner.

  The roughness measuring method according to the fourth aspect of the present invention is the above-described measuring method, wherein the reflected light intensity and the interference light intensity are 1 by the light incident on the first layer and the second layer by each interface. It is calculated by a single reflection model that is reflected once. Thereby, the wavelength of the light observed in a short time can be selected.

ここで、
Ｒｑ：前記試料の多層膜形成面の二乗平均粗さ
ＲＭＳ（ａ）：前記第２層と媒質との界面の二乗平均粗さ
ＲＭＳ（ｂ）：前記第１層と前記第２層との間の前記測定対象界面の二乗平均粗さ
ｎ_１：前記第２層の屈折率
ｎ_０：前記媒質の屈折率
とする。このように、簡単な演算式により基板上に形成された多層膜の界面の粗さを算出することができる。 The roughness measurement method according to the fifth aspect of the present invention is characterized in that, in the measurement method, the roughness of the measurement target interface is calculated based on the following equation.

here,
Rq: Root mean square roughness RMS (a) of the multilayer film forming surface of the sample: Root mean square roughness RMS (b) of the interface between the second layer and the medium: Between the first layer and the second layer The mean square roughness n _{1 of the} interface to be measured is the refractive index n ₀ of the second layer: the refractive index of the medium. Thus, the roughness of the interface of the multilayer film formed on the substrate can be calculated by a simple arithmetic expression.

  The roughness measuring method according to a sixth aspect of the present invention is the above-described measuring method, wherein the optical distance between the multilayer film forming surface of the sample and the reference surface is changed, and the measurement light and the reference light are changed. A plurality of phase differences are given between them to cause an intensity change of the interference light. Thereby, phase measurement can be performed with high resolution, and the roughness of the interface of the multilayer film can be measured more accurately.

  A roughness measuring apparatus according to a seventh aspect of the present invention includes a first layer of a sample having a first layer formed on a substrate and a second layer formed on the first layer. A roughness measuring apparatus for measuring the roughness of an interface to be measured between the second layer and reflected light from a multilayer film forming surface on which the first layer and the second layer of the sample are formed A wavelength selection unit that selects light of a first wavelength that has a minimum intensity, a light source that irradiates the multilayer film formation surface and the reference surface of the sample with light of the first wavelength, and a multilayer film formation surface of the sample. A photodetector that receives interference light obtained by combining the reflected measurement light and the reference light reflected by the reference surface, and the level of the measurement light and the reference light based on the intensity change of the interference light. The roughness of the multilayer film formation surface of the sample is calculated from the phase difference, and the measurement object is calculated based on the roughness of the multilayer film formation surface of the sample. It is intended and a calculation unit for determining the roughness of the surface. Thereby, the roughness of the interface of the multilayer film can be measured in a non-destructive and non-contact manner.

  The roughness measuring apparatus according to an eighth aspect of the present invention is the above-described measuring apparatus, wherein the wavelength selection unit includes reflected light from an interface other than the measurement target interface as the first wavelength light, and the reflected light. Light having a wavelength that cancels out interference light generated by the light is selected. Thereby, the roughness of the interface of the multilayer film can be measured more accurately in a non-destructive and non-contact manner.

  The roughness measuring apparatus according to a ninth aspect of the present invention is the roughness measuring apparatus, wherein the wavelength selecting unit includes the film thicknesses of the first layer and the second layer, the first layer, and the first layer. Based on the refractive index of two layers, the reflected light intensity and the interference light intensity from the first layer and the second layer are calculated, and the light of the first wavelength is calculated based on the reflected light intensity and the interference light intensity. The light having a wavelength that cancels out the reflected light from the interface other than the interface to be measured and the interference light generated by the reflected light is selected. Thereby, the roughness of the interface of the multilayer film can be measured more accurately in a non-destructive and non-contact manner.

  The roughness measuring apparatus according to a tenth aspect of the present invention is the above-described measuring apparatus, wherein the reflected light intensity and the interference light intensity are 1 by the light incident on the first layer and the second layer by each interface. It is calculated by a one-time reflection model that is reflected once. Thereby, the wavelength of the light observed in a short time can be selected.

ここで、
Ｒｑ：前記試料の多層膜形成面の二乗平均粗さ
ＲＭＳ（ａ）：前記第２層と媒質との界面の二乗平均粗さ
ＲＭＳ（ｂ）：前記第１層と前記第２層との間の前記測定対象界面の二乗平均粗さ
ｎ_１：前記第２層の屈折率
ｎ_０：前記媒質の屈折率
とする。このように、簡単な演算式により基板上に形成された多層膜の界面の粗さを算出することができる。 The roughness measuring apparatus according to an eleventh aspect of the present invention is characterized in that, in the above-described measuring apparatus, the calculation unit calculates the roughness of the measurement target interface based on the following equation. .

here,
Rq: Root mean square roughness RMS (a) of the multilayer film forming surface of the sample: Root mean square roughness RMS (b) of the interface between the second layer and the medium: Between the first layer and the second layer The mean square roughness n _{1 of the} interface to be measured is the refractive index n ₀ of the second layer: the refractive index of the medium. Thus, the roughness of the interface of the multilayer film formed on the substrate can be calculated by a simple arithmetic expression.

  A roughness measuring apparatus according to a twelfth aspect of the present invention is the above-described measuring apparatus, wherein the optical distance between the multilayer film forming surface of the sample and the reference surface is changed, and the measurement light and the reference light are changed. A plurality of phase differences are given between them to cause a change in the intensity of the interference light. Thereby, phase measurement can be performed with high resolution, and the roughness of the interface of the multilayer film can be measured more accurately.

  According to the present invention, there is provided a roughness measuring method and a roughness measuring apparatus for optically measuring non-destructively and non-contactingly the nanoscale roughness of the inner interface of a multilayer film composed of a plurality of thin films formed on a substrate. Can be provided.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description shows preferred embodiments of the present invention, and the scope of the present invention is not limited to the following embodiments. In the following description, the same reference numerals denote the same contents.

  The configuration of the roughness measuring apparatus 100 according to the embodiment of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a diagram schematically showing a configuration of a roughness measuring apparatus 100 according to the present embodiment. FIG. 2 is a block diagram showing the configuration of the processing apparatus 40 used in the roughness measuring apparatus 100 according to the present embodiment. In this embodiment, an example using a Michelson type phase shift interferometer will be described as an example of an interferometer. The interferometer is not particularly limited, and other two-beam interferometers such as a Mirau interferometer and a Fizeau interferometer may be used. Further, any method can be adopted as long as the method can accurately measure the phase without being limited to the phase shift method.

  As shown in FIG. 1, the roughness measuring apparatus 100 according to the present embodiment includes a light source 11, an interference filter 12, lenses 13, 21, 25, beam splitters 14, 15, 20, a measurement objective lens 16, and a stage 17. , A reference objective lens 18, a reference mirror 19, a photodetector 22, a spectrometer 23, a wavelength selection unit 24, and a processing device 40. As shown in FIG. 2, the processing apparatus 40 includes a stage driving unit 41, a wavelength conversion control unit 42, a roughness calculation unit 43, a memory 44, and the like.

The roughness measuring apparatus 100 according to the present invention measures a SIMOX (Separation by Implanted Oxygen) wafer having a structure in which a SiO ₂ / Si thin film is formed on a Si substrate by implanting oxygen into the Si wafer. set to target. Note that the SIMOX wafer is an example of an SOI wafer, and a buried oxide film is formed by performing heat treatment at a high temperature after implanting oxygen ions into the Si wafer. Of course, the wafer is not limited to a SIMOX wafer, and for example, a bonded SOI wafer can also be a measurement target. FIG. 3 shows the configuration of such a sample 30. As shown in FIG. 3, the sample 30 has a SiO ₂ layer 32 (Layer 2, the first layer in the claims) formed on the Si substrate 31 (Substrate), and an Si layer 33 (Layer 1, in the claims) on the SiO ₂ layer 32. The second layer) is formed. The surface of the Si substrate 31 on which the SiO ₂ layer 32 and the Si layer 33 are formed is defined as a multilayer film formation surface. The interface between the air (Air) and the Si layer 33 (the surface of the Si layer 33) is defined as the interface 01, the interface between the Si layer 33 and the SiO ₂ layer 32 is defined as the interface 12, and the interface between the SiO ₂ layer 32 and the Si substrate 31. Is the interface 23. Here, the interface 12 between the Si layer 33 and the SiO ₂ layer 32 is the interface to be measured.

In FIG. 3, the refractive index of air is N ₀ = n ₀ -ik ₀ , the refractive index of the Si layer 33 is N ₁ = n ₁ -ik ₁ , and the refractive index of the SiO ₂ layer 32 is N ₂ = n ₂ -ik _2. The refractive index of the Si substrate 31 is N ₃ = n ₃ −ik ₃ . Here, the thickness of each layer of the Si layer 33, the SiO ₂ layer 32, and the Si substrate is unknown, the film thickness of the Si layer 33 is d ₁ , the film thickness of the SiO ₂ layer 32 is d ₂ , and the Si substrate the thickness and _{d 3.}

  In the present invention, the reflection spectrum of a sample on which a multilayer film is formed using a white light source is measured with a spectrometer, and the wavelength at which the contrast of the measurement target interface is focused is found by analyzing the reflection spectrum. Specifically, a wavelength is found such that reflected light from the interface other than the target interface to be measured cancels out. Then, an interference contrast is generated by a two-beam interferometer using the light having the wavelength of the analysis result, and an uneven shape (roughness) is calculated from the contrast using a phase shift interferometry. When the phase shift method is applied, single wavelength illumination light must be used. In this case, it is necessary to select an optimum wavelength that increases the contrast of the interface to be measured. This is because when a non-optimal wavelength is selected, the contrast of the interference becomes low and calculation by the phase shift interferometry is practically impossible.

By calculating the apparent root mean square roughness of the multilayer film formation surface, the root mean square roughness of the interface 12 is determined. Note that the apparent root mean square roughness of the multilayer film formation surface is a phase that includes information on the interfaces (interface 01, interface 12, interface 23) of the Si substrate 31, the SiO ₂ layer 32, and the Si layer 33. From the information, it is calculated as the roughness of the interface between apparent air and solid (the multilayer film (the Si substrate 31 covered with the SiO ₂ layer 32 and the Si layer 33)). As an example, a case where air is used will be described. However, the present invention is not limited to this, and measurement may be performed in another gas such as argon gas.

The roughness measuring apparatus 100 according to the present embodiment has a configuration in which a spectrometer 23 is combined in an optical path of a phase shift interferometer capable of selecting a wavelength. In the present embodiment, the film thickness of each layer of the sample 30 is measured by the spectroscopic interference method using the spectrometer 23. In the present embodiment, the spectrometer 23 is combined in the roughness measuring apparatus 100. However, a measuring apparatus for measuring the film thickness of each layer of the sample 30 and a microscope for observing the measurement target interface are provided separately. May be. Moreover, you may obtain | require the average film thickness of each layer by methods other than spectral interference method. In addition, when the material and film thickness of each layer are known, it is not always necessary to measure the film thickness. The film thicknesses d ₁ and d ₂ may be the designed thickness of the film.

  In addition, roughness measuring apparatus 100 according to the present embodiment measures the roughness of sample 30 using an interferometer. The roughness measuring apparatus 100 includes a Michelson interferometer in the optical system. The measurement light reflected by each interface 01, 12, 23 of the sample 30 (light reflected by the multilayer film formation surface) and the reference light reflected by the reference mirror 19 serving as a reference surface are combined and received. is doing.

  As the light source 11, a white light source such as a xenon lamp is used. For example, a xenon lamp having a wide continuous spectrum in the ultraviolet to infrared region (185 nm to 2000 nm) can be used. Note that the light source may not be a light source having a continuous spectrum, but may be a light source including a plurality of bright lines in the continuous spectrum, such as a mercury xenon lamp. Of course, the light source 11 is not limited to a xenon lamp, and a white diode, a white laser, or the like may be used. As will be described later, any light source may be used as long as the wavelength can be selected. The light source 11 emits white light when the reflection spectrum is measured by the spectrometer 23. As will be described later, after the optimum wavelength for measuring the roughness of the measurement target interface is determined by analysis of the reflection spectrum, the interference filter 12 closest to the wavelength is inserted to observe the sample 30. Can be switched to single wavelength illumination.

  Note that a laser light source that emits single-wavelength laser light may be used as the light source 11, and a wavelength conversion element may be provided instead of the interference filter 12. For example, wavelength conversion of single wavelength light from the light source 11 incident on the wavelength conversion element can be performed by second harmonic generation. Further, a variable wavelength laser can be used as the light source 11. Furthermore, a plurality of light sources that emit laser beams having different wavelengths may be provided, and light having a desired wavelength among the plurality of light sources may be emitted.

  An optical system for illuminating the sample 30 and the reference mirror 19 with light from the light source 11 will be described. The light emitted from the light source 11 passes through the interference filter 12 and is converted into light having a specific wavelength. As the interference filter 12, for example, a plurality of bandpass filters that selectively transmit light of a specific wavelength can be used. Thereby, light of a specific wavelength is selectively transmitted. The wavelength conversion control unit 42 of the processing device 40 changes the wavelength of light to be transmitted by switching a plurality of bandpass filters. Then, the light having the first wavelength passes through the lens 13 and enters the beam splitter 14. The beam splitter 14 branches the light so that the amount of reflected light and transmitted light is approximately 1: 1 regardless of the polarization state. Accordingly, approximately half of the first wavelength light is reflected by the beam splitter 14.

  The light reflected by the beam splitter 14 travels downward in FIG. 1 and enters the beam splitter 15. The beam splitter 15 branches the light so that the amount of reflected light and transmitted light is approximately 1: 1 regardless of the polarization state. Accordingly, approximately half of the light of the first wavelength is transmitted through the beam splitter 15, and the remaining approximately half is reflected by the beam splitter 15. Therefore, the incident light having the first wavelength is split into two light beams by the beam splitter 15. The reflecting surface of the beam splitter 15 is inclined 45 ° with respect to the optical axis. The light transmitted through the beam splitter 15 enters the measurement objective lens 16. That is, one of the two light beams branched by the beam splitter 15 is incident on the measurement objective lens 16. The measurement objective lens 16 condenses the incident light beam and emits it to the sample 30. That is, the light beam transmitted through the beam splitter 15 is reduced and projected onto the sample 30 by the measurement objective lens 16. Thereby, the sample 30 is illuminated with the light of the first wavelength.

  The sample 30 is placed on the stage 17. The stage 17 is an XYZ stage and moves the sample 30 and the relative position of the illumination light based on a drive signal from a stage drive unit 41 provided in the processing apparatus 40. That is, the stage 17 on which the sample 30 is placed is driven using a piezoelectric element such as a piezoelectric element, and the distance between the sample 30 and the measurement objective lens 16 is changed. Accordingly, the sample 30 can be scanned in the Z direction by the light beam. That is, a plurality of phase differences can be given between the measurement light and the reference light by mechanically changing the optical distance. In other words, the two-beam interferometer according to the present embodiment is provided with a phase modulation mechanism for realizing the phase shift interferometry.

  The measurement objective lens 16 may be driven by a piezo element to change the distance between the measurement objective lens 16 and the sample 30. Further, instead of changing the distance between the measurement objective lens 16 and the sample 30, the distance between the reference objective lens 18 and the reference mirror 19 may be changed. In this way, by providing a phase difference between the measurement light and the reference light by mechanically changing the optical distance, non-polarized light can be used, and even a measurement object having optical anisotropy can be polarized. Therefore, the measurement light and the reference light can be accurately measured without causing a phase shift.

  As a method of giving a plurality of phase differences between the measurement light and the reference light, there are a method of giving a phase difference using polarization interference in addition to a method of mechanically changing the optical distance as described above. Specifically, in consideration of the rotation angle of the polarization of the measurement light and the reference light, a plurality of phase differences can be given between the measurement light and the reference light using a phase difference plate or the like. In this case, the beam splitter 15 shown in FIG. 1 is replaced with a polarization beam splitter so that the measurement light and the reference light become p-polarized light and s-polarized light, respectively. Further, a λ / 4 wavelength plate is inserted in front of the photodetector 22 so that these p-polarized light and s-polarized light interfere with each other. Note that a combination of a vibrating mirror such as a galvano mirror, an acousto-optic deflector, or the like may be used so that the illumination light scans the sample 30 in the XY direction.

  On the other hand, the light beam reflected by the beam splitter 15 enters a reference objective lens 18 provided on the side in FIG. That is, the reference objective lens 18 condenses the other light beam of the two light beams branched by the beam splitter 15 and makes it incident on the reference mirror 19. That is, the light beam reflected by the beam splitter 15 is reduced and projected onto the reference mirror 19 by the reference objective lens 18. Thereby, the reference mirror is illuminated by the light of the first wavelength. Thus, one of the light beams branched by the beam splitter 15 becomes illumination light for illuminating the sample 30, and the other becomes illumination light for illuminating the reference mirror 19.

  The reference mirror 19 is, for example, a flat mirror that is flat and has a reflectance of approximately 100%. Therefore, the reference mirror 19 regularly reflects almost all of the incident light. The reflectance of the reference mirror is preferably approximately the same as the reflectance of the measurement object. If the reflection intensity of the object to be measured and the reference mirror are equal, the contrast of the interference fringes becomes strong and measurement with less noise becomes possible. Conversely, when the difference in reflectance between the reference mirror and the measurement object is large, the contrast of the interference fringes becomes poor. Therefore, it is preferable to be able to replace the reference mirror with a different reflectance in accordance with the reflectance of the measurement object. The reference objective lens 18 and the measurement objective lens 16 are, for example, the same type of objective lens, and are equal not only in focal length and optical path length but also in aberration characteristics such as wavefront aberration characteristics.

  A focus adjustment mechanism and an angle adjustment mechanism (not shown) are attached to the back side of the reference mirror 19. The focus adjustment mechanism includes a screw or the like for adjusting the focus. By rotating this screw, the reference mirror 19 is translated along the optical axis of the reference objective lens 18. Thereby, the focus of the reference objective lens 18 can be adjusted to the surface of the reference mirror 19, and the reference mirror 19 can be arranged at the focal point position of the reference objective lens 18. In order to adjust the angle of the reference mirror 19, the angle adjustment mechanism includes, for example, screws provided on the diagonal of the reference mirror 19. By rotating this screw, the angle of the reflecting surface of the reference mirror 19 can be changed. Thereby, the density of the interference fringe pattern based on the phase difference between the measurement light and the reference light can be changed.

  Next, a detection optical system for detecting the light reflected by the sample 30 and the light reflected by the reference mirror 19 will be described. As described above, a Michelson interferometer is inserted in the roughness measuring apparatus 100 according to the present embodiment. The Michelson interferometer is provided with a measurement objective lens 16 and a reference objective lens 18.

  The measurement light reflected by the sample 30 enters the measurement objective lens 16. Then, the measurement light enters the beam splitter 15 via the measurement objective lens 16. As described above, the beam splitter 15 branches the incident light substantially 1: 1 regardless of the polarization state. Accordingly, approximately half of the incident measurement light passes through the beam splitter 15. On the other hand, the reference light reflected by the reference mirror 19 enters the reference objective lens 18. That is, the reference mirror 19 reflects the light beam collected by the reference objective lens 18 and makes it incident on the reference objective lens 18 as reference light. The reference light then enters the beam splitter 15 via the reference objective lens 18. Approximately half of the incident reference light is reflected by the beam splitter 15.

  Accordingly, the measurement light and the reference light are combined by the beam splitter 15 to form one light beam. Here, one light beam combined by the beam splitter 15 is used as combined light. That is, the measurement light and the reference light are combined by the beam splitter 15 to generate combined light. As described above, in the Michelson interferometer, the illumination light is branched by the beam splitter 15, and the measurement light reflected by the sample 30 and the reference light reflected by the reference mirror 19 are one by the beam splitter 15. Are combined into a light beam.

  The combined light emitted from the beam splitter 15 travels upward and enters the beam splitter 14. As described above, the beam splitter 14 branches the incident light approximately 1: 1 regardless of the polarization state. Accordingly, approximately half of the combined light incident on the beam splitter 14 is transmitted. That is, approximately half of the measurement light incident on the beam splitter 14 and approximately half of the reference light pass through the beam splitter 14. By this beam splitter 14, the combined light and the illumination light can be branched. That is, the combined light is separated from the optical path of the illumination light.

  The combined light separated from the illumination light enters the beam splitter 20. The beam splitter 20 branches light so that the amount of reflected light and transmitted light is approximately 1: 1 regardless of the polarization state. Almost half of the incident combined light passes through the beam splitter 20 and enters the photodetector 22 via the lens 21 via the lens 21. The lens 21 forms an image of the combined light on the light receiving surface of the photodetector 22. The photodetector 22 is, for example, a CCD sensor, and can be one provided with a light receiving element serving as a detection pixel. In the photodetector 22, signal charges corresponding to the received light amount are accumulated for each pixel. By driving the stage 17 in the Z direction and scanning the sample 30, a plurality of phase differences can be given to the measurement light and the reference light, and a plurality of interference fringe patterns can be imaged.

The photodetector 22 outputs a detection signal corresponding to the intensity of the incident light to the roughness calculation unit 43 of the processing device 40. A detection signal from the photodetector 22 is input to the processing device 40. The intensity of the detection signal output to the processing device 40 is based on the interference light intensity according to the phase difference between the measurement light and the reference light. In the present invention, the roughness of the interface of the multilayer film is determined from the root mean square roughness of the multilayer film formation surface. For example, the processing device 40 performs A / D conversion of the detection signal and stores the detection signal in the memory 44. The roughness calculation unit 43 calculates the SiO ₂ layer 32 on the Si substrate 31 of the sample 30 from the phase difference between the measurement light and the reference light corresponding to the interference light intensity corresponding to the detection signal for each pixel stored in the memory 44. And the apparent surface roughness of the multilayer film formation surface on which the multilayer film composed of the Si layer 33 is formed. Then, the roughness of the interface 12 between the SiO ₂ layer 32 and the Si layer 33 is calculated based on the apparent surface roughness of the multilayer film formation surface. A method for calculating the roughness of the interface 12 will be described in detail later. Note that the processing device 40 is not limited to a physically single device.

  On the other hand, an optical system for measuring the film thickness of the sample 30 with light from the light source 11 will be described. In the present embodiment, the film thickness of the sample 30 is measured by the spectroscopic interferometry using the spectrometer 23. Therefore, it is necessary to irradiate the sample 30 with white light emitted from the light source 11. For this reason, when measuring the film thickness of the sample 30, the interference filter 12 is moved so that the light emitted from the light source 11 does not pass through the interference filter 12. The light emitted from the light source 11 passes directly through the lens 13 and enters the beam splitter 14. The beam splitter 14 reflects substantially half of the light having a specific wavelength. Then, the light traveling downward in FIG. 1 is collected by the measurement objective lens 16 and irradiated onto the sample 30. Then, the light traveling downward in FIG. 1 is collected by the measurement objective lens 16 and irradiated onto the sample 30.

  When white light is incident on the sample 30, multiple reflection occurs inside the film. The reflected lights interfere with each other according to the phase difference. Therefore, the film thickness can be measured by measuring and analyzing the reflection spectrum from the sample 30 with the spectrometer 23. The measurement light reflected by the sample 30 passes through the measurement objective lens 16 and enters the beam splitter 14. Then, approximately half of the incident light passes through the beam splitter 14 and enters the beam splitter 20. Half of the light emitted to the beam splitter 17 passes through the lens 25 and enters the spectrometer 21. The spectrometer 23 measures the reflection spectrum from the incident light using a known spectral interference method, and measures the film thickness of the sample 30.

  In addition, you may make it provide separately the white light source for measuring the film thickness of the sample 30, and the several light source of a different wavelength for performing observation. A laser light source that emits a single-wavelength laser beam may be used as a light source for observation, and a wavelength conversion element may be provided. For example, wavelength conversion of single wavelength light incident on the wavelength conversion element can be performed by second harmonic generation. Further, a variable wavelength laser can be used as the light source 11. Furthermore, a plurality of laser light sources that emit laser beams having different wavelengths may be provided, and light having a desired wavelength among the plurality of laser light sources may be selected.

  The wavelength selection unit 24 performs a reflection simulation to observe the sample 30 with light having a wavelength at which the reflected light from the interface other than the measurement target interface of the sample 30 and the interference light from the interface other than the measurement target interface cancel each other. Select as the light of the optimum wavelength. Specifically, the wavelength selection unit 24 calculates the reflected light intensity and the interference light intensity from each interface based on the film thickness and the refractive index of each layer of the multilayer film, and calculates the reflected light intensity and the interference light intensity. Based on this, the optimum wavelength for observing the sample 30 is selected. A method for determining the optimum wavelength will be described in detail later.

  The processing device 40 changes the wavelength of light to be transmitted by switching the interference filter 12 in order to emit light having the wavelength selected by the wavelength selection unit 24. By measuring the measurement target interface of the sample 30 using the light of the selected wavelength, the influence of the reflected light from each interface other than the measurement target interface can be suppressed, and the nanoscale on the measurement target interface can be suppressed. It is possible to optimize the contrast of the unevenness. Using the light of this wavelength, an interference contrast is generated by a two-beam interferometer, and a concavo-convex shape (roughness) is calculated from the contrast using a phase shift interferometry. Thereby, the roughness of the interface of the multilayer film can be measured in a non-destructive and non-contact manner.

Here, the roughness measuring method according to the present invention will be described. The procedure of the method for measuring the roughness of the nominal interface of the multilayer film according to the present embodiment is simply shown as follows.
(1) Determination of optimum wavelength In the present embodiment, first, a spectrum is measured. The sample 30 is illuminated with white light, and the reflection spectrum thereof is dispersed by the spectrometer 23, whereby the average film thickness of the multilayer film structure of the sample 30 is measured. Then, the spectrum is analyzed using the film thickness and refractive index of each layer of the sample 30, and the reflected light intensity and the interference light intensity from each interface (interface 01, interface 12, interface 23) are separated by calculation, Determine the optimum wavelength of light for the measurement.
(2) Observation of measurement object interface The interference filter 12 is used to emit light (monochromatic light) having the optimum wavelength derived from the light source 11, and the two-beam interferometer using this wavelength light is used to measure the measurement object interface. Measure roughness.

  Thereby, the influence of the reflected light from each interface other than the measurement object interface of the sample 30 can be suppressed, and the contrast of the nanoscale unevenness on the measurement object interface can be optimized. For this reason, the nanoscale unevenness (roughness) of the internal interface of the multilayer film can be measured in a short time without contact and nondestructively, and a good / defective product can be determined.

Hereinafter, the method for measuring the roughness of the internal interface of the multilayer film according to the present embodiment will be described in detail using the roughness measuring apparatus 100 described above. As described above, the roughness measuring apparatus according to the present embodiment is a combination of the two-beam interferometer capable of selecting a wavelength and the spectrometer 23. Here, the structure of the Si layer 33 / SiO ₂ layer 32 / Si substrate 31 as shown in FIG. 2 is considered. The interface to be measured is the interface 12 between the Si layer 33 and the SiO ₂ layer 32.

First, the optimum wavelength for measuring the roughness of the measurement target interface is determined. In this example, since the film thickness of each layer is unknown, the film thickness of each layer is measured first. In the sample 30 as shown in FIG. 2, there are three interfaces (interface 01, interface 12, and interface 23). White light is incident from the surface of the sample 30 on which the Si layer 33 / SiO ₂ layer 32 is formed. The Si substrate 31 is thick enough not to be reflected from the back surface.

  Incident light is repeatedly reflected one or more times at each interface, and finally they are all combined (interfered) and emitted as reflected light. The intensity of the reflected light is determined by the incident wavelength, the film thicknesses d1 and d2, and the complex refractive index N of each layer. The complex refractive index has a refractive index n for the real part and an extinction coefficient k for the imaginary part. Here, as described above, N = n−ik. The refractive index n and the extinction coefficient k are called optical constants. In general, these optical constants are wavelength-dependent, and are databased for various substances. Therefore, the optical thickness of each layer also changes depending on the wavelength. For this reason, the interference conditions of the thin film change.

Instead of measuring the reflected light intensity for various wavelengths, the reflected light exhibits a characteristic color by using white light as the incident light. The reflected light is dispersed and the obtained spectrum is analyzed by spectral interferometry. Thereby, the film thickness d ₁ of the Si layer 33 and the film thickness d ₂ of the SiO ₂ layer 32 can be determined. The refractive index n and the extinction coefficient k can be determined simultaneously.

It is assumed that the film thickness d ₁ of the Si layer 33 and the film thickness d ₂ of the SiO ₂ layer 32 are obtained by the above film thickness measurement. The reflected light from each interface is reproduced by using data of known optical constants (refractive index n, extinction coefficient k) of each layer. From the result, the wavelength at which the nanoscale roughness of the interface 12 can be measured is determined.

  A general spectrum simulation of spectral interference reproduces the wavelength dependency of the reflectance of the entire multilayer film structure (synthesis of reflected light at each interface). Therefore, precisely, complicated numerical calculation in consideration of all the multiple reflections at each interface is required. However, to separate the reflections at each interface, the conventional complex formula for multiple reflections can calculate the final overall reflectivity, but the prospect is poor. In the present invention, in order to think in the simplest analytical manner, calculation is performed assuming that reflection occurs only once at each interface. Thereby, the time concerning the calculation for selection of the wavelength of the light to observe can be shortened.

In the present embodiment, incident light is assumed to be incident perpendicular to the surface of the sample 30 on which the Si layer 33 and the SiO ₂ layer 33 are formed, and reflection on the back surface of the Si substrate 31 is not considered. Illumination light from the light source 11 is split into two light beams by the beam splitter 15, one of which enters the sample as incident light E ₀ and is reflected at each interface (E ₁ , E ₂ , E ₃ ). The other beam is a reference beam E _R reflected by the reference mirror. The measurement light and the reference light reflected by the sample are combined to generate a so-called interference fringe (interference contrast).

FIG. 4 shows the state of reflection and transmission of light at each interface when light is incident. As shown in FIG. 4, the electric field of incident light is E ₀ , the electric field of reflected light reflected once at the interface 01 is E ₁ , the electric field of reflected light reflected once at the interface 12 is E ₂ , and once at the interface 23. the electric field of the reflected light reflected and E _3. Further, the electric field at the reference mirror 19 once reflected reflected light E _R. In FIG. 4, the incident light _{E 0} and the reflected light _E 1 for _explanation, E _2, E 3, illustrates a _{E R} diagonally.

Assuming that the traveling direction of light is z, the wave number is β, each frequency is ω, and the amplitude is A ₀ , the wave equation of incident light is expressed by the following equation (1).

Also, assuming that the phase of the reflected light at each interface is δ ₁ , δ ₂ , and δ ₃ , the wave functions of each reflected light are as shown in the following equations (2), (3), and (4).

このとき、干渉強度をＩ_ＩＦとすると、次の式（６）のように各界面からの反射振幅と参照光の反射振幅の総和の絶対値の２乗で求めることができる。

When the phase difference between the reference light E _R and E ₁ and epsilon, written as the following equation (5).

At this time, if the interference intensity is I _IF , it can be obtained by the square of the absolute value of the sum of the reflection amplitude from each interface and the reflection amplitude of the reference light as in the following equation (6).

とする。 here,

And

In the formula (6), I is the reflection spectrum of the sample 30. I _RR is considered constant regardless of wavelength. In the case where the change in the amplitude of the equations (8) to (10) depending on the wavelength is gradual, it can be regarded as almost constant. Therefore, in order to increase the contrast of I _IF , it is sufficient that the reflection spectrum I of the sample 30 is small.

Ｉ_１１は界面０１の反射光強度、Ｉ_２２は界面１２の反射光強度、Ｉ_３３は界面２３の反射光強度、Ｉ_１２は反射光Ｅ_１、Ｅ_２の干渉光強度、Ｉ_２３は反射光Ｅ_２、Ｅ_３の干渉光強度、Ｉ_３１は反射光Ｅ_１、Ｅ_３の干渉光強度を示している。 At this time, assuming that the reflection intensity of the sample 30 is I, the reflection intensity I can be obtained by the square of the absolute value of the sum of the reflection amplitudes from each interface as shown in the following equation (11).

I ₁₁ is the reflected light intensity of the interface 01, I ₂₂ is the reflected light intensity of the interface 12, I ₃₃ is the reflected light intensity of the interface 23, I ₁₂ is the interference light intensity of the reflected lights E ₁ and E ₂ , and I ₂₃ is the reflected light. The interference light intensities E ₂ and E ₃ , and I ₃₁ indicate the interference light intensities of the reflected light E ₁ and E ₃ .

Here, I can be considered as the sum of six terms as in the following equations (12)-(17).

If the amplitude and phase are calculated from the complex refractive index and film thickness, the reflection intensity I can be obtained. By calculating by changing the wavelength, the reflection spectrum can be approximately reproduced. To calculate the amplitude and phase of the reflected light, converting the incident light (i) the amplitude A ₀ Section, considered separately for the conversion of (ii) phase term.

(I) an amplitude _{A 0} of the conversion light incident amplitude _{A 0} (real number) is converted to _{A 1,} A 2, _{A 3,} respectively (complex) by reflection and transmission of each interface. This conversion is the lower layer (Layer b) determined by the complex refractive index _{N a} and _{N b} of the amplitude reflectance _{r ab} and amplitude transmittance _{t ab} (generally upper (Layer a) with respect to the traveling direction of light at each interface (It is called Fresnel coefficient). The amplitude reflectance _rab and the amplitude transmittance _tab are expressed by the following equations (18) and (19).

このとき、反射と透過で新たに位相がθだけ追加されることになる。 Therefore, the amplitude A ₀ can be converted into the following equations (20) to (22) by sequentially considering the amplitude reflectance r _ab and the amplitude transmittance t _ab .

At this time, the phase is newly added by θ by reflection and transmission.

(Ii) Phase term conversion If the phase change when passing through a film having a film thickness d having a complex refractive index N is Δ (complex number), the phase φ due to the change in optical path length and the amplitude attenuation rate γ due to absorption are taken into consideration. Thus, it is expressed as the following formula (23).

From the film thickness and complex refractive index of each layer, the phase φ and the amplitude attenuation rate γ at each interface are expressed by the following equations (24)-(29).

When the conversions of (i) and (ii) are combined, the converted amplitude and phase are expressed by equations (30)-(35).

Here, as a specific example, a multilayer film structure in which a Si film having a film thickness d ₁ = 150 nm and a SiO ₂ film having a film thickness d ₂ = 100 nm are stacked on a Si substrate having a thickness of 1 mm. The result of calculating the reflection spectrum from Equation (11) is shown in FIG. In the specific example shown in FIG. 5, calculation was performed up to wavelengths of 400 nm to 800 nm. In FIG. 5, the solid line indicates a reflection spectrum calculated with general software in consideration of multiple reflections. From this result, even when the calculation is performed with the single reflection model, it substantially matches the peak position of the reflection spectrum considering multi-order reflection.

  FIG. 6 and FIG. 7 show six terms calculated by separating the reflection spectrum calculated by the single reflection model into reflection and interference from each interface. FIG. 6 is a diagram showing the intensity of reflected light from each interface. FIG. 7 is a diagram showing the intensity of interference light at each interface. Thereby, the breakdown of the reflection spectrum of the sample 30 can be evaluated.

As shown in FIG. 6, the reflected light intensity I ₁₁ at the interface 01 is the strongest, which is considered to be an obstacle to observing the internal interface (interface 12: measurement target surface). In particular, when the wavelength is short, it is opaque to Si. Therefore, it is considered necessary to select light having a wavelength such that the reflected light from the interface 01 cancels with the interference lights I ₁₂ and I ₃₁ . That is, the wavelength at which reflected light and interference light from other interfaces (interface 01, interface 23) other than the interface 12 that is the measurement target interface cancel each other is selected. That is, it is necessary to observe with light having such a wavelength that the Si layer 33 (Layer 1) plays the role of an antireflection film.

  When this condition is simplified, the interface 12 is measured with a two-beam interferometer by selecting a wavelength in the reflection spectrum of the sample 30 where the reflection intensity is in the vicinity of a minimum (valley), more preferably minimum. be able to. From the simulation result of the reflection spectrum shown in FIG. 5, it is also possible to automatically select the optimum wavelength that minimizes the reflection intensity. Therefore, in this embodiment, for example, light having a wavelength of about 600 nm can be selected.

Assuming that the optimum wavelength determined in this manner can be obtained by combining the equation (6) as the following equation (36), by modulating the phase ε of the reference light, By calculation, the phase δ of the entire sample can be obtained.

For example, in the case of three steps in the phase shift method, the interference intensity when ε is changed by 0, π / 2, and π is I _IF1 , I _IF2 , and I _IF3 . The phase δ is calculated as follows.

とする。各項は、以下のように表される。 Where equation (6) is

And Each term is expressed as follows.

すべてのεで式が成り立つには、

であればいい。 If it takes the form of equation (36),

For all ε to hold the equation,

If it is good.

Therefore, the general relationship between δ and δ ₁ , δ ₂ , δ ₃ is applied as shown in equation (46).

Further, from the equations (44) and (45), C is applied as in the following equation (47).

となる。 From here, the case of the structure of the Si substrate 31 / SiO ₂ layer 32 / Si layer 33 will be solved.
(I) Case where C ₂ = C ₃ can be set From equations (41) and (42)

It becomes.

In the case of a transparent film that hardly absorbs like the SiO ₂ layer 32, k ₂ = 0. Therefore, C ₂ and C ₃ depend only on the film thickness d 1 of the SiO ₂ layer 32. However, from equations (28) and (29), γ ₂ = γ ₃ , so the ratio of I ₂₂ and I ₃₃ does not depend on the film thickness. The results are shown in FIG.

As shown in FIG. 8, the ratio of I ₂₂ to I ₃₃ is about 1.5. Here, C ₂ = C ₃ is handled, and the calculation proceeds analytically. The visible light wavelength is applied as shown in equation (50) regardless of the film thicknesses d1 and d2.

と置くことのできる場合について考える。この関係が成り立つ条件を探すために、以下の式（５１）と式（５２）を波長に対してプロットする。式（５１）は膜厚ｄ１で変化し、式（５２）は膜厚ｄ２で変化する。その結果をそれぞれ図９、図１０に示す。

And (ii)

Think about when you can put it. In order to search for a condition for satisfying this relationship, the following equations (51) and (52) are plotted with respect to wavelength. Expression (51) varies with the film thickness d1, and Expression (52) varies with the film thickness d2. The results are shown in FIGS. 9 and 10, respectively.

9 and 10, when the wavelength is 550 nm or more, it is considered that the formula (51) is satisfied when 0 <d1 <150 nm and the formula (52) is satisfied when 50 nm <d2 <100 nm. In this case, equation (50) can be further calculated and applied as in equation (53).

Here, since B, δ ₁ , δ ₂ , and δ ₃ do not change with respect to the modulation of ε, the relationship of Expression (36) is satisfied. Therefore, the following formula is satisfied.

Furthermore, it is assumed that there are nano-scale irregularities of h _a , h _b , and h _c on the interface 01, the interface 12, and the interface 23, respectively. If the phase change due to unevenness is Δδ, θ is constant on the same interface,

Here, the relationship with the phase change at each interface 01, interface 12, and interface 23 is expressed by equations (56) to (58).

となる。式（５５）に、式（５６）−（５９）を代入して、ｈについて整理すると、

When the phase difference Δδ obtained from the result of the interference measurement is converted as the height of the surface in the air (apparent height) is h,

It becomes. Substituting Equations (56)-(59) into Equation (55) and organizing h,

Here, the refractive index n ₀ = 1, n ₁ = 3.8, and n ₂ = 1.4 are calculated as approximate values.

  Thereby, the breakdown of the contribution of each interface 01, 12, 23 of the apparent height h obtained from the interference measurement can be understood. From the equation (61), the height distribution of the interface 12 is included 55% in the apparent height distribution. When it is considered that the outermost surface, that is, the unevenness of the interface 01 is almost flat, 80% can be measured as the unevenness information of the interface 12.

Further, when measuring the surface roughness (RMS), the RMS (a) of the outermost surface is measured in advance by AFM or the like, and the following formula (62) is calculated from the RMS calculated from the apparent height distribution: The RMS (b) of interface 12 can be calculated. Note that the contribution of the interface 23 is ignored.

From this, the root mean square roughness RMS (b) of the interface 12 that is the measurement target interface is expressed as follows.

  From the equation (63), the root mean square roughness RMS (b) at the interface 12 can be approximately obtained using the phase shift interferometry. The roughness of the interface 01 may be measured by a method other than AFM. For example, it is possible to measure at a wavelength opaque to Si such as ultraviolet rays.

と置ける場合、干渉強度Ｉ_ＩＦは、以下のように表される。

In addition,

The interference intensity I _IF is expressed as follows.

As shown in Expression (65), in this case, the form is Expression (36). Therefore, δ ₁ can be obtained from the phase shift calculation. That is, since the [delta] = [delta] _1, the Δδ = Δδ ₁ = Δφ _1. Accordingly, the height of the surface and the apparent height are as follows.

従って、式（６４）の上限が成立するならば、最表面（界面０１）の凹凸を測定することができる。このような条件について考える。

From the equations (66) and (67), the following equation is obtained.

Therefore, if the upper limit of Formula (64) is satisfied, the unevenness of the outermost surface (interface 01) can be measured. Consider these conditions.

As shown in FIG. 10, when the film thickness d ₂ of the SiO ₂ layer 32 is 150 nm, the formula (64) is satisfied near the wavelength 450 nm, and when d ₂ = 200 nm, the wavelength is near 580 nm. In this case, the apparent result represents the unevenness on the outermost surface, and the inside cannot be measured.

  Thus, according to the present invention, the influence of reflected light from each interface other than the measurement target interface of the sample 30 can be suppressed, and the contrast of the nanoscale unevenness on the measurement target interface can be optimized. it can. For this reason, the nanoscale unevenness | corrugation (roughness) of the internal interface of a multilayer film can be measured in a short time by non-contact and non-destructive.

It is a figure which shows typically the structure of the roughness measuring apparatus which concerns on embodiment. It is a figure which shows typically the structure of the processing apparatus which concerns on embodiment. It is a figure which shows typically the structure of the sample which concerns on embodiment. It is a figure which shows the state of reflection and permeation | transmission of light in each interface when light injects into a sample. It is a figure which shows the result of having calculated the reflection spectrum of the multilayer film which concerns on an Example. It is a figure which shows the reflected light intensity from each interface of the multilayer film which concerns on an Example. It is a figure which shows the interference light intensity | strength in each interface of the multilayer film which concerns on an Example. The ratio of C ₂ and C ₃ is a plot against wavelength. It is the graph which plotted the relationship of Formula (51) with respect to the wavelength. It is the graph which plotted the relationship of Formula (52) with respect to the wavelength.

Explanation of symbols

11 Light source 12 Interference filter 13, 21 Lens 14, 15, 20 Beam splitter 15 Objective lens 16 Measurement objective lens 17 Stage 18 Reference objective lens 19 Reference mirror 22 Photo detector 23 Spectrometer 24 Wavelength selection section 25 Lens 30 Sample 31 Si substrate 32 SiO ₂ film 33 Si film 40 Processing device 41 Stage drive unit 42 Wavelength conversion control unit 43 Roughness calculation unit 44 Memory 100 Roughness measurement device

Claims (12)

The roughness of the measurement target interface between the first layer and the second layer of the sample having the first layer formed on the substrate and the second layer formed on the first layer is determined. A roughness measurement method for measuring,
Irradiating illumination light having a predetermined wavelength width to the multilayer film forming surface on which the first layer and the second layer of the sample are formed,
Select the light of the first wavelength included in the wavelength width of the illumination light, the reflected light intensity from the multilayer film formation surface of the sample is near the minimum,
Irradiating the multilayer film forming surface and the reference surface of the sample with light of the first wavelength;
Receiving interference light obtained by combining the measurement light reflected by the multilayer film forming surface of the sample and the reference light reflected by the reference surface;
Based on the intensity change of the interference light, from the phase difference between the measurement light and the reference light, to calculate the roughness of the multilayer film formation surface of the sample,
A roughness measuring method for determining a roughness of the measurement target interface based on a roughness of a multilayer film forming surface of the sample.   2. The rough light according to claim 1, wherein light having a wavelength at which reflected light from an interface other than the measurement target interface and interference light generated by the reflected light cancel each other is selected as the light having the first wavelength. Measuring method. Based on the film thicknesses of the first layer and the second layer and the refractive indexes of the first layer and the second layer, the reflected light intensity and the interference light intensity from the first layer and the second layer are calculated. And
Based on the reflected light intensity and the interference light intensity, light having a wavelength at which the reflected light from the interface other than the measurement target interface and the interference light generated by the reflected light cancel each other is selected as the first wavelength light. The roughness measuring method according to claim 2.   The roughness measurement according to claim 3, wherein the reflected light intensity and the interference light intensity are calculated by a one-time reflection model in which light incident on the first layer and the second layer is reflected once by each interface. Method. The roughness measurement method according to claim 1, wherein the roughness of the measurement target interface is calculated based on the following equation.

here,
Rq: Root mean square roughness RMS (a) of the multilayer film forming surface of the sample: Root mean square roughness RMS (b) of the interface between the second layer and the medium: Between the first layer and the second layer The mean square roughness n _{1 of the} interface to be measured is the refractive index n ₀ of the second layer: the refractive index of the medium.   The optical distance between the multilayer film formation surface of the sample and the reference surface is changed, a plurality of phase differences are given between the measurement light and the reference light, and the intensity change of the interference light is caused. The roughness measuring method according to any one of claims 1 to 5. The roughness of the measurement target interface between the first layer and the second layer of the sample having the first layer formed on the substrate and the second layer formed on the first layer is determined. A roughness measuring device for measuring,
A wavelength selection unit that selects light having a first wavelength at which reflected light intensity from the multilayer film forming surface on which the first layer and the second layer of the sample are formed is in the vicinity of a minimum;
A light source that irradiates light of the first wavelength onto a multilayer film forming surface and a reference surface of the sample;
A photodetector that receives interference light obtained by combining the measurement light reflected by the multilayer film formation surface of the sample and the reference light reflected by the reference surface;
Based on the intensity change of the interference light, the roughness of the multilayer film formation surface of the sample is calculated from the phase difference between the measurement light and the reference light, and based on the roughness of the multilayer film formation surface of the sample A roughness measuring device comprising: a calculating unit that determines the roughness of the measurement target interface.   The wavelength selection unit selects, as the light of the first wavelength, light having a wavelength at which reflected light from an interface other than the measurement target interface and interference light generated by the reflected light cancel each other. 7. The roughness measuring device according to 7.   The wavelength selection unit is configured to determine the intensity of reflected light from the first layer and the second layer based on the film thicknesses of the first layer and the second layer and the refractive indexes of the first layer and the second layer. And the interference light intensity is calculated, and based on the reflected light intensity and the interference light intensity, the reflected light from the interface other than the measurement target interface and the interference light generated by the reflected light are used as the light of the first wavelength. 9. The roughness measuring apparatus according to claim 8, wherein light having a wavelength that cancels each other is selected.   The roughness measurement according to claim 9, wherein the reflected light intensity and the interference light intensity are calculated by a one-time reflection model in which light incident on the first layer and the second layer is reflected once by each interface. apparatus. The roughness calculation apparatus according to claim 7, wherein the calculation unit calculates the roughness of the measurement target interface based on the following expression.

here,
Rq: Root mean square roughness RMS (a) of the multilayer film forming surface of the sample: Root mean square roughness RMS (b) of the interface between the second layer and the medium: Between the first layer and the second layer The mean square roughness n _{1 of the} interface to be measured is the refractive index n ₀ of the second layer: the refractive index of the medium.   The optical distance between the multilayer film formation surface of the sample and the reference surface is changed, a plurality of phase differences are given between the measurement light and the reference light, and the intensity change of the interference light is caused. The roughness measuring apparatus according to any one of claims 7 to 11.

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