JP2008139177A - Method and device for measuring film thickness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for measuring film thickness and a device for measuring film thickness, capable of measuring the film thickness in a wide range, even if the relation between the film thickness of a plate member used as a reference in many cases and parameters are not used. <P>SOLUTION: The film thickness measuring method and the device therefor for measuring the thickness of a film layer included in the plate member are such that inspection light transmissible to the surface of the plate member is irradiated, while its wavelength is changed over a prescribed range; the intensity of transmitted light is detected which is acquired from the irradiated inspection light having respective wavelength, transmitted through the plate member and then exiting therefrom; a target wavelength is decided, based on a maximal point and a minimal point on the characteristic curve, showing the relation between respective wavelength of the inspection light and the intensity of the transmitted light; and the thickness of the film layer is calculated from the target wavelength decided. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a film thickness measuring method and a film thickness measuring apparatus for measuring the thickness of a film contained in a plate-like member such as a semiconductor wafer in a non-contact manner.

Conventionally, a predetermined parameter (maximum maximum value, minimum minimum value, ratio thereof, etc.) obtained from a spectral waveform of reflected light or transmitted light obtained by irradiating a semiconductor wafer (plate-like member) with probe light is used. Based on this, a method for detecting the film thickness of the surface layer of a semiconductor wafer has been proposed (Patent Document 1). This technique uses the phenomenon that the spectral waveform of reflected or transmitted light becomes substantially constant at the same film thickness. Specifically, in the process of polishing the semiconductor wafer to be measured, the parameter value obtained from the spectral waveform of the reflected or transmitted light with respect to the reference semiconductor wafer having a known film thickness is stored as a reference value. When the parameter value obtained from the spectral waveform of reflected light or transmitted light with respect to the semiconductor wafer is the same as the reference value, the film thickness of the semiconductor wafer to be measured is the film thickness of the standard semiconductor wafer. Judgment. On the other hand, if the value of the parameter is not the same as the reference value, it is determined that the film thickness of the semiconductor wafer to be measured is not the film thickness of the reference semiconductor wafer.
JP 2000-77371 A

  However, the above-described conventional method can measure only the same film thickness as the reference semiconductor wafer, and other film thicknesses are different from the reference semiconductor wafer thickness. Cannot judge. In order to be able to measure a wide range of film thickness, the film thickness of many reference semiconductor wafers and the parameters obtained from the spectral waveform must be stored in advance, and the huge storage capacity Must be secured.

  The present invention has been made in view of such circumstances of the prior art, and it is possible to measure a wide range of film thicknesses without using the relationship between the film thickness and parameters of a plate member that serves as many references. A film thickness measuring method and a film thickness measuring apparatus are provided.

  The film thickness inspection method according to the present invention is a film thickness measurement method for measuring the thickness of a film layer included in a plate-shaped member, and the inspection light that can be transmitted through the surface of the plate-shaped member has a wavelength thereof. Inspection light irradiation step of irradiating while changing over a predetermined range, transmitted light intensity detection step of detecting the intensity of the transmitted light that the inspection light of each wavelength irradiated passes through the plate member, and A wavelength of interest determination step for determining a wavelength of interest based on the maximum and minimum points in the characteristic curve representing the relationship between each wavelength of the inspection light and the intensity of the transmitted light, and the thickness of the film layer from the determined wavelength of interest. And a calculation step for calculating the thickness.

  With such a configuration, when the intensity of transmitted light is detected when the surface of the plate-like member is irradiated with inspection light while changing the wavelength over a predetermined range, each wavelength of the inspection light and the intensity of the transmitted light are The wavelength of interest is determined based on the maximum point and the minimum point in the characteristic curve representing the relationship. Then, the thickness of the film layer is calculated from the wavelength of interest.

  The maximum and minimum points of the characteristic curve are presumed to be caused by the interference between the component that directly transmits the inspection light plate-like member and the component that reflects and transmits at the boundary of the film layer. From this, it can be said that the wavelength corresponding to each of the maximum point and the minimum point in the characteristic curve depends on the thickness of the film layer. Therefore, the wavelength that can be referred to as information that most accurately represents the thickness of the film layer can be determined from the maximum point and the minimum point in the characteristic curve as the wavelength of interest, and the film thickness is calculated from the determined wavelength of interest. The

  In the film thickness inspection method according to the present invention, the wavelength of interest determination step determines the wavelength of interest based on a wavelength corresponding to at least one of the longest wavelength maximum point and minimum point in the characteristic curve. It can be configured.

  With such a configuration, when the intensity of transmitted light is detected when the surface of the plate-like member is irradiated with inspection light while changing the wavelength over a predetermined range, each wavelength of the inspection light and the intensity of the transmitted light are The wavelength of interest is determined based on the wavelength corresponding to at least one of the maximum point and the minimum point on the longest wavelength side in the characteristic curve representing the relationship.

  As described above, the wavelength corresponding to each of the maximum point and the minimum point in the characteristic curve can be said to depend on the thickness of the film layer. Further, when the wavelength becomes longer and becomes more than the film thickness, the interference action disappears, and in the characteristic curve, the maximum point and the minimum point disappear in the wavelength region above the film thickness. Therefore, it can be said that the wavelength corresponding to the maximum point and the minimum point on the longest wavelength side in the characteristic curve is information that most accurately represents the thickness of the film layer. The film from the wavelength of interest determined based on the wavelength corresponding to at least one of the maximum point and the minimum point on the longest wavelength side in the characteristic curve, which can be said to be the information that most accurately represents the thickness of the film layer It is possible to calculate the thickness.

  In the film thickness measurement method according to the present invention, the wavelength-of-interest determination step may determine the wavelength corresponding to the maximum point on the longest wavelength side in the characteristic curve as the wavelength of interest. The wavelength corresponding to the minimum point on the wavelength side can also be determined as the wavelength of interest. Furthermore, from both the wavelength corresponding to the maximum point and the wavelength corresponding to the minimum point, for example, an intermediate wavelength (such as an average value) can be determined as the wavelength of interest.

  In the film thickness measurement method according to the present invention, the wavelength of interest determination step includes a first step of generating a first approximate curve approximated as a curve passing through the maximum point of the characteristic curve, and a minimum point of the characteristic curve. A second step of generating a second approximate curve approximated as a curve passing through the first step, a third step of calculating an intersection on the long wavelength side of the first approximate curve and the second approximate curve, and the first approximate curve And determining the wavelength of interest based on a wavelength corresponding to at least one of the maximum point and the minimum point of the characteristic curve closest to the intersection of the long wavelength side of the second approximate curve and the second approximate curve on the short wavelength side It can be set as the structure which has.

  With such a configuration, the first approximate curve approximated as a curve passing through the maximum point of the characteristic curve representing the relationship between each wavelength of the inspection light and the intensity of the transmitted light is approximated as a curve passing through the minimum point of the characteristic curve. The intersection on the long wavelength side with the second approximated curve is a point where the maximum point and the minimum point of the characteristic curve coincide on the long wavelength side. That is, it is assumed that the wavelength corresponding to the intersection is in the wavelength region where the maximum point and the minimum point are eliminated in the characteristic curve. Then, the local maximum point and local minimum point of the characteristic curve closest to the intersection on the short wavelength side are the local maximum point and local minimum point on the longest wavelength side in the characteristic curve. Accordingly, the thickness of the film layer is calculated from the wavelength of interest determined based on the wavelength corresponding to at least one of the maximum point and the minimum point on the longest wavelength side in the characteristic curve.

  In the film thickness measurement method according to the present invention, the wavelength of interest determination step includes a first step of generating a first approximate curve approximated as a curve passing through the maximum point of the characteristic curve, and a minimum point of the characteristic curve. A second step of generating a second approximate curve approximated as a curve passing through the second step, and a third step of calculating an intersection on the long wavelength side of the first approximate curve and the second approximate curve. Can be configured to determine the wavelength corresponding to the wavelength of interest.

  With such a configuration, the first approximate curve approximated as a curve passing through the maximum point of the characteristic curve representing the relationship between each wavelength of the inspection light and the intensity of the transmitted light is approximated as a curve passing through the minimum point of the characteristic curve. The wavelength corresponding to the intersection on the long wavelength side with the second approximate curve is determined as the wavelength of interest. Then, the thickness of the film layer is calculated from the determined wavelength of interest.

  As described above, the first approximate curve approximated as a curve passing through the maximum point of the characteristic curve representing the relationship between each wavelength of the inspection light and the intensity of the transmitted light is approximated as a curve passing through the minimum point of the characteristic curve. The wavelength corresponding to the intersection on the long wavelength side with the second approximate curve is presumed to be in the wavelength region where the maximum point and the minimum point coincide with each other in the characteristic curve. That is, it is presumed that the interference effect caused by the film layer does not occur with the light of that wavelength. Therefore, the wavelength corresponding to the intersection can be a boundary condition as to whether or not an interference action occurs depending on the thickness of the film layer to be measured, and can be information indicating the thickness of the film layer. The thickness of the film layer can be calculated from the wavelength corresponding to the intersection point.

  Moreover, the film thickness measuring method according to the present invention may include a determination step of determining whether or not the intersection point in the third step can be calculated as the presence or absence of a defect in the film layer.

  If the film layer is normal, there are no local maximum and minimum points on the long wavelength side in the characteristic curve, that is, the first approximate curve approximated as a curve passing through the maximum point of the characteristic curve and the minimum point of the characteristic curve. There is an intersection with the second approximate curve approximated as a curve passing through. On the other hand, when there is no intersection between the first approximate curve and the second approximate curve, it can be determined that the film layer itself has a defect (peeling, void, etc.). Therefore, according to the configuration described above, in the process of measuring the thickness of the film layer, it is possible to determine the presence / absence of a defect in the film layer at the measurement site.

  Furthermore, in the film thickness measurement method according to the present invention, the characteristic curve is obtained in the same manner as the plate member with respect to the intensity of transmitted light corresponding to each wavelength of the inspection light and a predetermined reference plate member. It can be configured to represent the relationship between the difference between the transmitted light intensity and the wavelength.

  With such a configuration, the relationship between the wavelength and the value obtained by subtracting the transmitted light intensity obtained from the reference plate member from the intensity of the transmitted light obtained from the plate member to be measured is a characteristic curve. Therefore, the characteristic curve can be expressed by emphasizing the variation in the intensity of the transmitted light based on the interference phenomenon. Therefore, it is possible to easily determine the maximum point and the minimum point.

  In the film thickness measuring method according to the present invention, the characteristic curve may represent a relationship between each wavelength of the inspection light and a predetermined parameter depending on the intensity of the transmitted light.

  With such a configuration, since the parameter depends on the intensity of the transmitted light, the characteristic curve representing the relationship between each wavelength of the inspection light and the parameter results in the wavelength of the inspection light and the intensity of the transmitted light. Can be expressed as a relationship.

  In the film thickness measurement method according to the present invention, the characteristic curve is obtained in the same manner as the plate member with respect to the parameter value corresponding to each wavelength of the inspection light and a predetermined reference plate member. Further, it can be configured to represent the relationship between the difference between the parameter value and the wavelength.

  With such a configuration, the characteristic curve can be expressed by emphasizing the variation in the intensity of transmitted light based on the interference phenomenon in the same manner as described above. Therefore, it is easy to determine the maximum point and the minimum point. can do.

  In the film thickness measurement method according to the present invention, the parameter can be an absorption parameter that represents the degree of absorption of the inspection light by the plate member. The light absorption parameter is not limited as long as it is a parameter representing the degree of absorption of the inspection light by the plate-like member, and for example, the light absorbance can be used.

  Further, in the film thickness measurement method according to the present invention, the substance is based on a light absorption characteristic with respect to a predetermined substance concentration and a characteristic curve representing a relationship between each wavelength of the inspection light and the light absorption parameter of the inspection light. It can be set as the structure which has the process of calculating the density | concentration in the said plate-shaped member.

  With such a configuration, since the characteristic curve represents the relationship between each wavelength of the inspection light and the degree of absorption of the inspection light by the plate-like member, by using the light absorption characteristics with respect to a predetermined substance concentration, The concentration of the substance in the plate-like substance can be calculated. Therefore, when measuring the thickness of the film layer of the plate member, the concentration of a predetermined substance contained in the plate member can be obtained. The light absorption characteristics with respect to the concentration of the substance represent the relationship between the concentration of the substance and the degree of absorption for each wavelength of light.

  Further, in the film thickness measurement method according to the present invention, the parameter is an absorption parameter indicating a degree of absorption of the inspection light in the plate member, and an absorption characteristic with respect to a predetermined substance concentration and the inspection A substance concentration calculating step of calculating a concentration of the substance in the plate-like member based on a characteristic curve representing a relationship between each wavelength of light and the light absorption parameter of the inspection light; and included in the reference plate-like member And a correction step of correcting the characteristic curve based on the known concentration of the substance and the concentration of the substance obtained in the substance concentration calculation step.

  With such a configuration, the difference between the value of the parameter corresponding to each wavelength of the inspection light and the value of the parameter obtained in the same manner as the plate member with respect to the predetermined reference plate member, It represents the relationship with the wavelength, and when the parameter is an absorption parameter representing the degree of absorption of the inspection light in the plate-like member, it is contained in the plate-like member to be measured for film thickness. Even if the degree of light absorption varies according to the concentration of a predetermined substance, the characteristic curve is corrected according to the concentration of the substance, so that a more accurate film thickness can be measured. .

  A film thickness measuring apparatus according to the present invention is a film thickness measuring apparatus for measuring the thickness of a film layer included in a plate-shaped member, and has a wavelength of inspection light that can be transmitted through the surface of the plate-shaped member. Inspection light irradiating means for irradiating while changing over a predetermined range, transmitted light intensity detecting means for detecting the intensity of transmitted light that is transmitted through the plate-like member after the irradiated inspection light of each wavelength, and Attention wavelength determining means for determining the attention wavelength based on the maximum point and the minimum point in the characteristic curve representing the relationship between each wavelength of the inspection light and the intensity of the transmitted light, and the thickness of the film layer from the determined attention wavelength And calculating means for calculating the height.

  Moreover, in the film thickness measurement device according to the present invention, the wavelength of interest determination unit determines the wavelength of interest based on a wavelength corresponding to at least one of a maximum point and a minimum point on the longest wavelength side in the characteristic curve. It can be constituted as follows.

  Further, in the film thickness measuring apparatus according to the present invention, the wavelength of interest determination means includes first means for generating a first approximate curve approximated as a curve passing through the maximum point of the characteristic curve, and a minimum point of the characteristic curve. A second means for generating a second approximate curve approximated as a curve passing through the first approximate curve, a third means for calculating an intersection on the long wavelength side of the first approximate curve and the second approximate curve, and the first approximate curve And a second means for determining the wavelength of interest based on a wavelength corresponding to at least one of a maximum point and a minimum point of the characteristic curve closest to the intersection of the long wavelength side of the second approximate curve and the second approximate curve It can be set as the structure which has.

  Furthermore, in the film thickness measuring apparatus according to the present invention, the wavelength of interest determination means includes first means for generating a first approximate curve approximated as a curve passing through the maximum point of the characteristic curve, and a minimum point of the characteristic curve. And second means for generating a second approximate curve approximated as a curve passing through the second approximate curve, and third means for calculating an intersection on the long wavelength side of the first approximate curve and the second approximate curve. Can be configured to determine the wavelength corresponding to the wavelength of interest.

  In addition, the film thickness measuring apparatus according to the present invention may include a determination unit that determines whether or not the intersection point can be calculated by the third unit as the presence or absence of damage to the film layer.

  In the film thickness measuring apparatus according to the present invention, the characteristic curve may be configured to represent a relationship between each wavelength of the inspection light and a predetermined parameter depending on the intensity of the transmitted light.

  Further, in the film thickness measuring apparatus according to the present invention, the characteristic curve is obtained in the same manner as the plate member with respect to the parameter value corresponding to each wavelength of the inspection light and a predetermined reference plate member. Further, it can be configured to represent the relationship between the difference between the parameter value and the wavelength.

  In the film thickness measuring apparatus according to the present invention, the parameter can be configured to be an absorption parameter representing a degree of absorption of the inspection light by the plate-like member.

  Further, in the film thickness measuring apparatus according to the present invention, the substance is based on a light absorption characteristic with respect to a predetermined substance concentration and a characteristic curve representing a relationship between each wavelength of the inspection light and the light absorption parameter of the inspection light. The means for calculating the concentration in the plate member and the means for outputting the concentration can be used.

  Furthermore, in the film thickness measuring apparatus according to the present invention, the parameter is an absorption parameter indicating a degree of absorption of the inspection light in the plate member, and an absorption characteristic with respect to a predetermined substance concentration, and the inspection A substance concentration calculating means for calculating a concentration of the substance in the plate-like member based on a characteristic curve representing a relationship between each wavelength of light and the light absorption parameter of the inspection light; and included in the reference plate-like member And a correction unit that corrects the characteristic curve based on the known concentration of the substance and the concentration of the substance obtained in the substance concentration calculation step.

  According to the film thickness inspection method and the film thickness inspection apparatus according to the present invention, the intensity of transmitted light is detected when the surface of the plate-shaped member is irradiated with inspection light while changing the wavelength over a predetermined range. Since the thickness of the film layer is calculated from the wavelength of interest determined based on the maximum and minimum values in the characteristic curve representing the relationship between the wavelength and the intensity of the transmitted light, the characteristic curve for the plate member that is the reference in particular The thickness of the film layer can be measured without using or the like. Therefore, it is possible to measure a wide range of film thickness without using the relationship between the film thickness of many plate members and parameters.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  The plate-like member whose film layer thickness is to be measured according to the film thickness inspection method according to the embodiment of the present invention is, for example, as shown in FIG. This is a semiconductor wafer 100 having a so-called HOT (Hybrid-Orientation Technology) structure in which a silicon film 102 having a crystal axis (110) is formed on the surface layer.

  The inventor of the present invention applies inspection light to a semiconductor wafer 100 composed of a single silicon substrate 101 having a crystal axis (100) shown in FIG. 1 (b) over a wavelength range of 1000 nm to 2500 nm in the near infrared region. Irradiation was performed while the inspection light (intensity Ii) of each wavelength was detected, and the intensity Io of transmitted light emitted from the semiconductor wafer 100 of the silicon substrate 101 alone was detected. As a result, the relationship between each wavelength of the inspection light and the intensity Io of the transmitted light, specifically, the transmission intensity (Io / Ii) (%) corresponding to each wavelength is represented by a characteristic curve Q1 as shown in FIG. became.

  The inventor further irradiates the so-called HOT structure semiconductor wafer 100 shown in FIG. 1A with inspection light while changing the wavelength within the above range, and detects the intensity Io of the transmitted light. . As a result, the transmission intensity (Io / Ii) (%) corresponding to each wavelength was a characteristic curve Q2 as shown in FIG. When the characteristic curves Q1 and Q2 are compared, they are different in that a vibration component is generated particularly on the long wavelength side exceeding 1200 nm in the characteristic curve Q2. This difference is considered due to the presence of the silicon film 102 formed on the silicon substrate 101.

  As shown in FIG. 2, when the inspection light Rin is incident on the semiconductor wafer 100 having the silicon film 102 formed on the surface layer, the light directly transmitted through the semiconductor wafer 100, the silicon film 102 having the thickness D, and the silicon As a result of interference between light transmitted through repeated reflection at the boundary surface with the substrate 101 and the boundary surface between the silicon film 102 and air, it is presumed that a vibration component is included in the intensity of the transmitted light Rt with respect to the wavelength. Therefore, it can be said that the vibration state of the characteristic curve Q 2, specifically, the wavelength corresponding to the maximum point and the minimum point of the vibration depends on the thickness D of the silicon film 102.

  A film thickness inspection method (film thickness inspection apparatus) according to an embodiment of the present invention has a maximum point (minimum point) in a characteristic curve representing the relationship between each wavelength of inspection light and the intensity of transmitted light as described above. The thickness D of the silicon film 102 is calculated from the wavelength corresponding to. Hereinafter, the film thickness inspection method will be specifically described.

  A basic configuration of a film thickness inspection apparatus according to an embodiment of the present invention is as shown in FIG.

  In FIG. 5, the film thickness inspection apparatus includes a stage 10, a light projecting head 12, and a light receiving head 21 that are capable of two-dimensional movement in the XY plane. On the stage 10, a semiconductor wafer 100 having a HOT structure as shown in FIG. As shown in an enlarged view in FIG. 6, the light projecting head 12 is disposed on the back surface side of the semiconductor wafer 100, and the light receiving head 21 is disposed on the opposite side of the light projecting head 12 with the semiconductor wafer 100 interposed therebetween. Inspection light irradiated on the surface of the semiconductor wafer 100 in the form of a spot SP from the light projecting head 12 passes through the semiconductor wafer 100 and enters the light receiving head 21.

  The light projecting head 12 is connected via a light guide fiber 13 to a light source device 16 that outputs white light. The projection head 12 includes optical members such as a lens, a prism, and a diffraction grating, and has a wavelength within a predetermined range (for example, 1000 nm to 2500 nm) from white light input from the light source device 16 via the light guide fiber 13. The inspection light that changes in the range (1) is generated, and the surface of the semiconductor wafer 100 is irradiated with the inspection light. The light receiving head 21 receives the transmitted light emitted from the inspection light of each wavelength irradiated from the light projecting head 12 through the semiconductor wafer 100. The light receiving head 21 is connected to a detector (photoelectric conversion element) 20 through a light guide fiber 22, and transmitted light received by the light receiving head 21 enters the detector 20. The detector 20 outputs a signal (transmitted light intensity signal) having a level corresponding to the intensity of incident light, that is, the intensity of the transmitted light.

  The film thickness inspection apparatus further includes a control unit 30 and an arithmetic unit 40. The control unit 30 controls the operation of each unit such as the detector 20, the light source device 16, the light projecting head 12, and the arithmetic unit 40 described above. The arithmetic unit 40 calculates the thickness of the silicon film 102 of the semiconductor wafer 100 to be measured from the transmitted light intensity signal from the detector 20 according to the procedure described later under the control of the control unit 30.

  The camera 50 images a predetermined portion of the surface of the semiconductor wafer 100 via the optical system of the light receiving head 21. This captured image is displayed on the monitor unit 51. Thereby, the surface appearance of the semiconductor wafer 100 can be monitored together with the measurement of the thickness of the silicon film 102. Although not shown, information about the thickness of the silicon film 102 calculated by the calculation unit 40 is displayed (output) as a measurement result on the monitor unit 51.

  The control unit 30 performs processing according to the procedure shown in FIG.

  In FIG. 7, the control unit 30 sets a minimum value λmin (for example, 1000 nm) of a predetermined wavelength range (wavelength range that can be transmitted through silicon) as the wavelength λ of the irradiation light (λ = λmin) (S1), The light projecting head 12 is controlled so that the light of the set wavelength λ is irradiated onto the semiconductor wafer 100 (S2). Thereafter, the control unit 30 gives a measurement instruction to the arithmetic unit 40 at a predetermined timing (S3). When giving the measurement instruction, the control unit 30 determines whether or not the set wavelength λ is the maximum value λmax (for example, 2500 nm) of the wavelength range (S4). When it is determined that the set wavelength λ is not the maximum value λmax (NO in S4), the control unit 30 increases the wavelength λ + Δλ further increased by a predetermined amount Δλ at a predetermined timing to a new wavelength. Set as λ (S5). Thereafter, the control unit repeatedly executes the same processing (S2 to S5) as described above. In the process, when it is determined that the set wavelength λ has reached the maximum value λmax (YES in S4), the control unit 30 gives an end instruction to the arithmetic unit 40 and ends the series of processes. .

  By such processing, the inspection light is irradiated onto the surface of the semiconductor wafer 100 while the wavelength λ thereof changes little by little (Δλ) over the wavelength range that can be transmitted to silicon.

  Further, as described above, in the process in which the inspection light is irradiated onto the surface of the semiconductor wafer 100 while the wavelength λ is gradually changed (Δλ) over the wavelength range that can be transmitted to silicon, the arithmetic unit 40 is 8 and 9 are performed according to the procedure shown in FIG.

In FIG. 8, the arithmetic unit 40 is in a standby state until there is a measurement instruction (see S3 in FIG. 7) from the control unit 30 (NO in S11). Then, when there is a measurement instruction from the control unit 30 with the inspection light having the wavelength λ being applied to the semiconductor wafer 100 (YES in S11), the arithmetic unit 40 causes the inspection light to pass through the semiconductor wafer 100 and exit. The intensity of the transmitted light, that is, the level of the transmitted light intensity signal from the detector 20 representing the transmitted light intensity I _Tλ is acquired (S12).

by the way,
A (λ) = Log {(I _i −I _Tλ ) / I _i } (1)
I _i : Incident light (inspection light) intensity
I _Tλ : Absorptivity A (absorption parameter) defined by the intensity of transmitted light is a parameter depending on the intensity of transmitted light transmitted through the semiconductor wafer 100. The relationship between the absorption rate A and the wavelength λ defined by the equation (1) is the same as the relationship between the transmitted light intensity I _Tλ and the wavelength λ (see FIG. 3). It becomes like a characteristic curve Q3.

The arithmetic unit 40 calculates the absorption rate A (λ) from the acquired transmitted light intensity I _{Tλ according} to the equation (1) (S13). In addition, the arithmetic unit 40 measures the absorption rate A _R (λ) corresponding to each wavelength λ in advance and holds the reference semiconductor wafer (see FIG. 1B) as a single silicon substrate 101 in advance. Yes. The absorptance A _R (λ) with respect to the reference semiconductor wafer corresponds to the transmitted light intensity of the reference wafer (see FIG. 1B) that is the silicon substrate 101 alone.

The arithmetic unit 40 further provides a difference ΔA between the absorption rate A (λ) corresponding to the wavelength λ of the inspection light and the absorption rate A _R (λ) similarly obtained for the reference semiconductor wafer.
ΔA = A (λ) −A _R (λ) (2)
(S14), and the difference ΔA is stored in the internal memory in association with the wavelength λ (S15). Thereafter, the arithmetic unit 40 determines whether or not an end instruction (see S6 in FIG. 7) is issued from the control unit 30 (S16). If there is no end instruction (NO in S16), the arithmetic unit 40 executes the above-described processing (S11 to S16) again, and the difference ΔA (absorption ratio corresponding to the next wavelength (λ + Δλ: see S5 in FIG. 7)). λ) is calculated, and the difference is associated with the wavelength λ and stored in the internal memory.

  Thereafter, the arithmetic unit 40 repeatedly executes the above-described processing (S11 to S16) until an end instruction is given from the control unit 30. When an end instruction is given from the control unit 30 (YES in S16), the arithmetic unit 40 proceeds to the process shown in FIG. At this time, the relationship (characteristic curve) between the wavelength in a predetermined range (1000 nm to 2500 nm) and the difference ΔA (λ) of the absorption rate is stored in the arithmetic unit 40. The relationship corresponds to the difference between the waveform shown in FIG. 2 and the waveform shown in FIG. 3, and corresponds to, for example, the characteristic curve Q (ΔA-λ characteristic) shown in FIG. Thus, the characteristic curve Q is expressed using the difference ΔA in the absorption rate, so that the fluctuation component based on the interference phenomenon caused by the silicon film 102 can be emphasized in the characteristic curve Q.

  In FIG. 9, the arithmetic unit 40 detects the maximum point (peak point) and the minimum point (bottom point) in the characteristic curve Q shown in FIG. 11 from the accumulated information (S21). Then, the arithmetic unit 40 calculates a first approximate curve fp (λ) that is approximated as a curve that passes through the maximum point of the characteristic curve Q using a known mathematical method (S22). A second approximate curve fb (λ) approximated as a curve passing through the minimum point is calculated (S23). The first approximate curve fp (λ) and the second approximate curve fb (λ) are generally exponential function curves.

Next, the arithmetic unit 40 executes a process of calculating an intersection point Px between the first approximate curve fp (λ) and the second approximate curve fb (λ) (S24). Then, the arithmetic unit 40 determines whether or not the intersection point Px between the first approximate curve fp (λ) and the second approximate curve fb (λ) has been calculated (S25). When the intersection point Px is calculated (YES in S25), the arithmetic unit 40 determines the wavelength λp1 corresponding to the local maximum point closest to the intersection point Px on the short wavelength side in the characteristic curve Q as the wavelength of interest ( S26), using the wavelength λp1 (attention wavelength),
D = 2λp1 (3)
Accordingly, the film thickness D of the silicon film 102 is calculated (S27).

  The film thickness D calculated in this way is displayed on the monitor unit 51 and presented to the operator, for example. Further, the calculated film thickness D can be fed back to the film forming process.

  The wavelength corresponding to the maximum point of the characteristic curve Q closest to the intersection Px of the first approximate curve fp (λ) and the second approximate curve fb (λ) on the short wavelength side of the film thickness D of the silicon film 102 Qualitative consideration will be given to the validity of calculating as 2 times λp1 (2λp1).

  Referring to FIG. 11, the characteristic curve Q representing the relationship between the absorption rate A (λ) and the wavelength λ has a fluctuation component depending on the thickness D of the silicon film 102 as described above. The intersection Px between the first approximate curve fp (λ) approximated as a curve passing through the local maximum point and the second approximate curve fb (λ) approximated as a curve passing through the local minimum point is characteristic on the long wavelength side. In the curve Q, the maximum point and the minimum point coincide with each other. That is, it is assumed that the wavelength λp0 = λb0 = λ0 corresponding to the intersection point Px is in the wavelength region where the maximum point and the minimum point are eliminated in the characteristic curve Q.

  On the other hand, as shown in FIG. 2, light having a wavelength λ equal to or greater than the film thickness D is transmitted through the semiconductor wafer 100 without being interfered by reflected light. Accordingly, the minimum wavelength λ0 of light that is transmitted without being affected by interference (reflection order m = 0) is the film thickness D (λ0 = D). The wavelength λ1 of the light that is affected by interference by the light component that is reflected twice by the film thickness D of the silicon film 102 (reflection order m = 2) is half the wavelength λ0 (the smallest even number). Should be 1 / (2λ1 = λ0).

  Considering these, the light having the wavelength λ 0 corresponding to the intersection point Px between the primary approximate curve fp (λ) and the secondary approximate curve fb (λ) is reflected at the boundary of the silicon film 102 of the semiconductor wafer 100. However, there is no guarantee that the wavelength λ 0 will be the minimum value of the wavelength of light that is transmitted without reflecting at the boundary of the silicon film 102, that is, the film thickness D of the silicon film 102. However, in the characteristic curve Q, the wavelength λp1 corresponding to the local maximum closest to the intersection Px on the short wavelength side is the wavelength that is affected by interference for the first time on the short wavelength side from the wavelength λ0 of the film thickness D, that is, the reflection order m. It can be considered that the reflection component at = 2 is a wavelength that can be affected by interference. Therefore, λp1 corresponding to the maximum point is half of the wavelength λ0 having the same length as the film thickness D, and the film thickness D can be calculated based on the relationship (3).

  Returning to FIG. 9, when the arithmetic unit 40 determines that the intersection point Px between the first approximate curve fp (λ) and the second approximate curve fb (λ) cannot be calculated (NO in S25), It is determined that the silicon film 102 in the region where the film thickness of the semiconductor wafer 100 is being measured is damaged, such as peeling or voids (S28). For example, in a region where the silicon film 102 is damaged such as voids or peeling, the first approximate curve fp (λ) approximated as a curve passing through the maximum point obtained from the characteristic curve Q and a curve passing through the minimum point are used. The intersection point Px with the approximated second approximate curve fb (λ) does not exist as shown in FIG. By performing such processing (S25, S28), in the process of measuring the thickness D of the silicon film 102, it is possible to determine the presence or absence of defects in the silicon film 102 at the measurement site.

  By the way, the silicon substrate 101 may contain boron B (dopant). In that case, since boron B itself has a light absorption characteristic (absorption coefficient ε (λ): represents the degree of light absorption with respect to concentration), the absorption rate A (λ) obtained during the measurement of the film thickness is It is affected by the boron concentration. In general, as shown in FIG. 13, when the boron concentration B is high, the absorption rate A (characteristic QB1) is generally higher than when the boron concentration B is low (characteristic QB2).

  In such a situation, as shown in FIG. 14, in the semiconductor wafer 100 in which the boron concentration B in the silicon substrate 101 is relatively high, the basic absorptivity characteristic Q11 in a region where the absorptance is relatively high. The characteristic Q12 has a fluctuation component depending on the film thickness. Further, in the semiconductor wafer 100 having a relatively low boron concentration B in the silicon substrate 101, in a region where the absorption rate is relatively low, a characteristic in which a variation component depending on the film thickness is placed on the characteristic Q21 of the basic absorption rate. Q22. For this reason, when the difference ΔA (λ) from the absorptance obtained with a predetermined reference semiconductor wafer as described above is the characteristic curve Q, the reference semiconductor wafer having the same boron concentration as the semiconductor wafer 100 to be measured is used. Must be prepared each time.

  Since this impedes practical use, the boron concentration of the semiconductor wafer 100 to be measured is obtained from the obtained absorption rate A (λ), and the absorption rate difference ΔA representing the characteristic curve Q is corrected. Is preferred.

  Specifically, the arithmetic unit 40 performs the following process in step S14 shown in FIG.

When the absorption rate A of the semiconductor wafer 100 to be measured is obtained (S13), from the absorption rate A (λ), the absorption coefficient ε (λ) of boron B, and the thickness L of the semiconductor wafer 100,
A (λ) = Bd · ε (λ) · L
Bd = {A (λ) / (ε (λ) · L)} (4)
Then, the boron concentration Bd is calculated. Then, the difference ΔA (λ) (characteristic curve Q) is corrected according to a proportional distribution method using a ratio (Bd / Bdr) with the boron concentration Bdr already obtained from the reference semiconductor wafer.

  Thus, since the difference ΔA in the absorptance representing the characteristic curve Q is corrected according to the boron concentration B, even if the concentration of boron B (dopant) contained in the semiconductor wafer 100 to be measured varies, More accurate film thickness measurement is possible.

  Further, the boron concentration calculated in the above-described processing can be displayed (output) on the monitor unit 51. Thus, the operator can know the concentration of boron B (dopant) contained in the semiconductor wafer 100 in the process of measuring the film thickness of the semiconductor wafer 100 to be measured.

  In addition, as long as the kind of dopant has an effective absorptance in the observation wavelength range (for example, 1000 nm to 2500 nm) and satisfies the formula (4), the concentration can be measured. . On the other hand, in the case of a dopant (substance) that does not have an effective absorptivity in the observation wavelength range, its concentration cannot be known. In this case, however, it is necessary to correct the characteristic curve Q according to the concentration of the substance in the first place. Absent.

In the above-described embodiment, the intersection point Px of the first approximate curve fp (λ) approximated as a curve passing through the maximum point in the characteristic curve Q and the second approximate curve fb (λ) approximated as a curve passing through the minimum point is used. The wavelength λP1 corresponding to the maximum point of the characteristic curve Q closest to the short wavelength side is determined as the target wavelength, but the maximum point on the longest wavelength side in the characteristic curve Q is set as the target wavelength using another method. It can also be determined. Theoretically, the maximum point and the minimum point on the longest wavelength side in the characteristic curve Q are very close to each other (they coincide at the intersection point Px). Therefore, the wavelength λb1 corresponding to the minimum point on the longest wavelength side in the characteristic curve Q is set as the wavelength of interest.
D = 2 · λb1
Accordingly, the film thickness D can be calculated.

Further, based on the wavelength λp1 corresponding to the longest wavelength side maximum point in the characteristic curve Q and the wavelength λb1 corresponding to the longest wavelength side minimum point, for example, averaging may be performed to determine the target wavelength λx. it can. Then, using the wavelength of interest λx, D = 2 · λx
Accordingly, the film thickness D can be calculated.

Furthermore, a first approximate curve fp (λ) approximated as a curve passing through the maximum point of the characteristic curve Q, and a second approximate curve fb (λ) approximated as a curve passing through the minimum point of the characteristic curve Q, Alternatively, the wavelength λ0 corresponding to the intersection Px on the long wavelength side may be a target wavelength, and the film thickness D of the silicon film 102 may be calculated from the target wavelength. In particular,
D = λ0 (5)
Can be calculated as

  The light having the wavelength λ0 corresponding to the intersection Px between the first approximate curve fp (λ) and the second approximate curve fb (λ) is reflected at the boundary of the silicon film 102 of the semiconductor wafer 100 as described above. Although there is no guarantee that the wavelength λ0 is the minimum value of the wavelength of light that is transmitted without reflecting at the boundary of the silicon film 102, that is, the film thickness D of the silicon film 102. However, a method for generating the first approximate curve fp (λ) and the second approximate curve fb (λ) from the local maximum point and the local minimum point of the characteristic curve Q if the accuracy error due to this is allowed. By devising it, the wavelength λ0 can be handled as the minimum value of the wavelength of light transmitted without reflecting the film layer. In this case, the film thickness D of the silicon film 102 is calculated according to the equation (5).

In the example described above, the difference ΔA (λ) from the reference semiconductor wafer having the absorption rate A is used as the characteristic curve Q representing the relationship between each wavelength of the inspection light and the intensity of the transmitted light. However, the present invention is not limited to this. There is no particular limitation as long as it represents the relationship between the parameter depending on the intensity of the transmitted light and the wavelength of the inspection light. For example, the absorptance A (λ) can be used as the characteristic curve Q, or the transmitted light intensity I _Tλ itself can be used as the characteristic curve Q.

  In the above-described example, the semiconductor wafer 100 having the HOT structure as shown in FIGS. 1A and 1B is the object of measurement. However, by using light that can be transmitted through the plate-like member (semiconductor wafer), various kinds of light can be obtained. A plate-like member can be the measurement object. For example, the film thickness can be measured by the above-described method even in a semiconductor wafer having another HOT structure as shown in FIGS. 15 (a), (b), (c), and (d). Furthermore, even in the case of an SOI structure semiconductor wafer as shown in FIGS. 16A, 16B, and 16C, if the oxide film layer (Oxide) or the Box layer is extremely thin, the surface layer is formed by the above-described method. It is possible to measure the thickness of the film by using the light in the X-ray region that can pass through the oxide layer and the Box layer as the inspection light even if the oxide film layer and the Box layer are relatively thick. It can be measured by the method.

  In the above-described example, the thickness D of the film on the surface layer of the semiconductor wafer 100 is measured. Even in the film layer formed inside the semiconductor wafer 100, reflection occurs at the interface, If the transmitted light can be influenced by vibration by the reflected light, the thickness of the inner film can be measured according to the method described above.

  Further, in the film thickness measuring apparatus shown in FIG. 5, the semiconductor wafer 100 is configured such that even if the silicon film 102 whose thickness is to be measured is set on the stage 10 so as to face the light projecting head 12, the silicon film 102 is placed on the light receiving head. Even if it is set on the stage 10 so as to face 21, its thickness can be measured.

  Further, the processing of the control unit 30 and the arithmetic unit 40 in accordance with the procedure shown in FIGS. 7, 8, and 9 is processing for one part where the semiconductor wafer 100 is irradiated with the inspection light in the spot SP shape. For example, every time the stage 10 is moved and its measurement site is changed, the control unit 30 and the arithmetic unit 40 execute processing according to the procedure shown in FIGS. 7, 8, and 9.

  As described above, the film thickness inspection method and the film thickness inspection apparatus according to the present invention can measure a wide range of film thicknesses without using the relationship between the film thickness and parameters of the plate-like member which is a lot of standards. The film thickness measuring method and the film thickness measuring apparatus have an effect that they can be achieved, and measure the thickness of a film included in a plate-like member such as a semiconductor wafer in a non-contact manner.

FIG. 6A schematically shows a cross-sectional structure and a light transmission state of a semiconductor wafer (plate member) whose thickness is inspected according to a film thickness inspection method according to an embodiment of the present invention; FIG. 5B schematically shows a cross-sectional structure and a light transmission state of a reference semiconductor wafer (reference plate member) without a film layer. It is. It is a figure which shows typically the behavior of the inspection light which injected into the semiconductor wafer (plate-shaped member) which has the cross-sectional structure shown to Fig.1 (a). It is a figure which shows the example of a transmission intensity with respect to the wavelength in a silicon | silicone base | substrate as shown in FIG.1 (b). It is a figure which shows the example of a characteristic of the transmission intensity with respect to the wavelength in the semiconductor wafer which has a film layer as shown to Fig.1 (a). It is a figure which shows the structural example of the film thickness measuring apparatus which concerns on one Embodiment of this invention. It is a figure which expands and shows the semiconductor wafer used as the measuring object shown in FIG. 5, and its vicinity area. It is a flowchart which shows the procedure of the process in the control unit of the film thickness measuring apparatus shown in FIG. It is a flowchart (the 1) which shows the procedure of the process in the calculating unit of the film thickness measuring apparatus shown in FIG. It is a flowchart (the 2) which shows the procedure of the process in the calculating unit of the film thickness measuring apparatus shown in FIG. It is a figure which shows the example of a characteristic of the absorptance with respect to the wavelength in a semiconductor wafer. It is a figure which shows the method of calculating a film thickness from the characteristic curve Q ((DELTA) A ((lambda))). It is a figure which shows the example of the state of the characteristic curve Q in case the film | membrane is damaged. It is a figure which shows the difference in the light absorption characteristic by the difference in the boron (dopant) density | concentration contained in a silicon base. It is a figure which shows the example of the characteristic curve Q in a different boron density | concentration. It is a figure which shows the structural example of the semiconductor wafer which can become a measuring object of a film | membrane. It is a figure which shows the structural example of the semiconductor wafer which can become a measuring object of a film | membrane.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Stage 12 Light projection head 13 Light guide fiber 16 Light source device 20 Detector 21 Light receiving head 22 Light guide fiber 30 Control unit 40 Arithmetic unit 50 Camera 51 Monitor unit 100 Semiconductor wafer 101 Silicon substrate 102 Silicon film

Claims (23)

A film thickness measuring method for measuring the thickness of a film layer included in a plate-shaped member,
Inspection light irradiation step of irradiating inspection light that can be transmitted to the surface of the plate-shaped member while changing the wavelength over a predetermined range;
A transmitted light intensity detection step of detecting the intensity of transmitted light that is emitted through the plate-shaped member through which the inspection light of each wavelength irradiated is emitted;
An attention wavelength determination step of determining an attention wavelength based on a maximum point and a minimum point in a characteristic curve representing a relationship between each wavelength of the inspection light and the intensity of the transmitted light;
And a calculation step of calculating the thickness of the film layer from the determined wavelength of interest.   2. The film thickness according to claim 1, wherein the wavelength of interest determination step determines the wavelength of interest based on a wavelength corresponding to at least one of a maximum point and a minimum point on the longest wavelength side in the characteristic curve. Measuring method.   3. The film thickness measuring method according to claim 2, wherein the wavelength of interest determination step determines a wavelength corresponding to a maximum point on the longest wavelength side in the characteristic curve as the wavelength of interest.   3. The film thickness measuring method according to claim 2, wherein the wavelength of interest determination step determines a wavelength corresponding to a minimum point on the longest wavelength side in the characteristic curve as the wavelength of interest. The wavelength of interest determination step includes a first step of generating a first approximate curve approximated as a curve passing through the maximum point of the characteristic curve;
A second step of generating a second approximated curve approximated as a curve passing through the minimum point of the characteristic curve;
A third step of calculating an intersection on the long wavelength side of the first approximate curve and the second approximate curve;
The target wavelength is determined based on a wavelength corresponding to at least one of the maximum point and the minimum point of the characteristic curve that is closest to the intersection on the long wavelength side of the first approximate curve and the second approximate curve on the short wavelength side. The film thickness measuring method according to claim 2, further comprising a fourth step of determining. The wavelength of interest determination step includes a first step of generating a first approximate curve approximated as a curve passing through the maximum point of the characteristic curve;
A second step of generating a second approximated curve approximated as a curve passing through the minimum point of the characteristic curve;
A third step of calculating an intersection on the long wavelength side of the first approximate curve and the second approximate curve,
The film thickness measuring method according to claim 1, wherein a wavelength corresponding to the intersection is determined as the wavelength of interest.   7. The film thickness measurement method according to claim 5, further comprising a determination step of determining whether or not the intersection point in the third step can be calculated as the presence or absence of a defect in the film layer.   The characteristic curve includes the difference between the intensity of transmitted light corresponding to each wavelength of the inspection light and the intensity of transmitted light obtained in the same manner as the plate member with respect to a predetermined reference plate member, the wavelength, The film thickness measuring method according to claim 1, wherein:   The film thickness measurement according to claim 1, wherein the characteristic curve represents a relationship between each wavelength of the inspection light and a predetermined parameter depending on the intensity of the transmitted light. apparatus.   The characteristic curve includes a difference between a value of the parameter corresponding to each wavelength of the inspection light and a value of the parameter obtained in the same manner as the plate member with respect to a predetermined reference plate member, and a wavelength. The film thickness measuring method according to claim 9, characterized in that:   The film thickness measuring method according to claim 9 or 10, wherein the parameter is an absorption parameter indicating a degree of absorption of the inspection light by the plate member.   The concentration of the substance in the plate-like member is calculated based on a light absorption characteristic with respect to a predetermined substance concentration and a characteristic curve representing a relationship between each wavelength of the inspection light and the absorption parameter of the inspection light. The film thickness measuring method according to claim 11, further comprising a step. The parameter is an absorption parameter that represents the degree of absorption of the inspection light in the plate-shaped member,
The concentration of the substance in the plate-like member is calculated based on a light absorption characteristic with respect to a predetermined substance concentration and a characteristic curve representing a relationship between each wavelength of the inspection light and the absorption parameter of the inspection light. A substance concentration calculation step;
A correction step of correcting the characteristic curve based on the known concentration of the substance contained in the reference plate-like member and the concentration of the substance obtained in the substance concentration calculation step. The film thickness measuring method according to claim 10. A film thickness measuring device for measuring the thickness of a film layer included in a plate-shaped member,
Inspection light irradiating means for irradiating inspection light that is transmissive to the surface of the plate-shaped member while changing its wavelength over a predetermined range;
Transmitted light intensity detecting means for detecting the intensity of transmitted light that is emitted through the plate-shaped member through which the inspection light of each wavelength to be irradiated is transmitted;
Wavelength of interest determination means for determining the wavelength of interest based on the maximum and minimum points in the characteristic curve representing the relationship between each wavelength of the inspection light and the intensity of the transmitted light;
A film thickness measuring apparatus comprising: an arithmetic unit that calculates the thickness of the film layer from the determined wavelength of interest.   15. The film thickness according to claim 14, wherein the wavelength of interest determination unit determines the wavelength of interest based on a wavelength corresponding to at least one of a maximum point and a minimum point on the longest wavelength side in the characteristic curve. measuring device. The wavelength-of-interest determining unit generates a first approximate curve that is approximated as a curve that passes through the maximum point of the characteristic curve;
A second means for generating a second approximate curve approximated as a curve passing through the minimum point of the characteristic curve;
A third means for calculating an intersection on the long wavelength side of the first approximate curve and the second approximate curve;
The target wavelength is determined based on a wavelength corresponding to at least one of the maximum point and the minimum point of the characteristic curve that is closest to the intersection on the long wavelength side of the first approximate curve and the second approximate curve on the short wavelength side. 16. The film thickness measuring apparatus according to claim 15, further comprising a fourth means for determining. The wavelength-of-interest determining unit generates a first approximate curve that is approximated as a curve that passes through the maximum point of the characteristic curve;
A second means for generating a second approximate curve approximated as a curve passing through the minimum point of the characteristic curve;
A third means for calculating an intersection on the long wavelength side of the first approximate curve and the second approximate curve;
The film thickness measuring apparatus according to claim 14, wherein a wavelength corresponding to the intersection is determined as the wavelength of interest.   18. The film thickness measuring apparatus according to claim 16, further comprising: a determination unit that determines whether or not the intersection point can be calculated by the third unit as the presence or absence of damage to the film layer.   The film thickness measurement according to claim 14, wherein the characteristic curve represents a relationship between each wavelength of the inspection light and a predetermined parameter depending on the intensity of the transmitted light. apparatus.   The characteristic curve includes a difference between a value of the parameter corresponding to each wavelength of the inspection light and a value of the parameter obtained in the same manner as the plate member with respect to a predetermined reference plate member, and a wavelength. The film thickness measuring apparatus according to claim 19, wherein:   21. The film thickness measuring apparatus according to claim 19, wherein the parameter is an absorption parameter indicating a degree of absorption of the inspection light by the plate-like member. The concentration of the substance in the plate-like member is calculated based on a light absorption characteristic with respect to a predetermined substance concentration and a characteristic curve representing a relationship between each wavelength of the inspection light and the absorption parameter of the inspection light. Means,
The film thickness measuring apparatus according to claim 21, further comprising means for outputting the concentration. The parameter is an absorption parameter that represents the degree of absorption of the inspection light in the plate-shaped member,
The concentration of the substance in the plate-like member is calculated based on a light absorption characteristic with respect to a predetermined substance concentration and a characteristic curve representing a relationship between each wavelength of the inspection light and the absorption parameter of the inspection light. A substance concentration calculating means;
Correction means for correcting the characteristic curve based on the known concentration of the substance contained in the reference plate-like member and the concentration of the substance obtained in the substance concentration calculating step; The film thickness measuring device according to claim 20.

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