JP5112930B2 - Thickness measuring device - Google Patents

Description

  The present invention relates to a thickness measuring apparatus.

  A device disclosed in Patent Document 1 is known as a device for measuring the thickness of a semiconductor wafer. The thickness measuring device disclosed in this document is particularly for measuring the thickness of a semiconductor wafer during the execution of wet etching using an etching solution, and is output from a light source using an optical system of a Michelson interferometer. The measured light is branched into two, one of the branched lights is incident on the reflection mirror and reflected, and the other branched light is incident on the semiconductor wafer and reflected on both sides of the semiconductor wafer. The intensity of interference light due to light interference is detected.

The thickness measuring device detects the interference light intensity by changing the optical path length of one of the branched lights, thereby obtaining a distribution of the interference light intensity with respect to the change in the optical path length. The thickness of the semiconductor wafer is calculated based on the optical path length. This thickness measuring apparatus can measure the thickness of a semiconductor wafer even if an etching solution exists on the surface of the semiconductor wafer or the thickness of the etching solution changes.
JP 2002-176087 A

  The thickness measuring device disclosed in Patent Document 1 allows measurement light to enter from the first surface side of an object whose thickness is to be measured, reflects a part of the light on the first surface, and first It is necessary to reflect the light transmitted through the surface by the second surface. Therefore, an object whose thickness can be measured by this thickness measuring apparatus needs to be able to reflect and transmit the measurement light on the first surface on which the measurement light is incident, and to be transparent at the wavelength of the measurement light. It is.

  That is, the thickness measuring device disclosed in Patent Document 1 is a semiconductor wafer having a high impurity concentration and an opaque object, an object having a metal coating film or the like formed on the first surface, and a light having a rough first surface. Thickness cannot be measured for materials with large scattering. In addition, this thickness measuring device cannot measure the thickness of the object including the opaque film when the opaque film is formed on the second surface of the object.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a thickness measuring apparatus capable of measuring the thickness of a wider range of objects with respect to materials, surface conditions, and the like.

  A thickness measuring device according to the present invention is a device for measuring the thickness of an object having a first surface and a second surface that are parallel to each other, and (1) a measurement light source that outputs measurement light; One of the two branches is provided with an end, a second end, a third end, and a fourth end, the measurement light output from the measurement light source is input to the first end, and the measurement light input to the first end is branched into two The light is output from the second end to the reference optical path, and the other branched light is output from the third end to the measurement optical path. The third light passes through the reference optical path and enters the second end and the measurement optical path. An optical coupler that outputs light input to the end from the fourth end, and (3) reflects the light that reaches the reference optical path from the second end of the optical coupler, and refers to the reflected light as reference light The light is input to the second end of the optical coupler via the optical path, and the light output from the second end of the optical coupler is reflected to reflect the optical coupler. A reference light generator that can change the optical path length of the reference optical path until it is input to the second end, and (4) input the light that reaches the measurement optical path from the third end of the optical coupler. A first beam splitter that divides the input light into two and outputs the first branched light and the second branched light; and (5) the first branched light output from the first beam splitter is used as the first surface of the object. And a first branched light path for inputting the first reflected light generated by the reflection to the first beam splitter, and (6) the second branched light output from the first beam splitter for the first of the object. A second branched optical path for inputting and reflecting the second reflected light generated by the reflection to the two surfaces, and (7) a reference light output from the fourth end of the optical coupler; Receiving each of the reflected light and the second reflected light A photodetector that detects the intensity of the received light, and (8) thickness calculation that calculates the thickness of the object based on the relationship between the light intensity detected by the photodetector and the optical path length of the reference optical path And a section. The thickness calculation unit includes an optical path length of the reference optical path when the interference light caused by the interference between the reference light and the first reflected light is received by the photodetector and the detected light intensity reaches a peak, the reference light, and the second light. The thickness of the object is calculated based on the optical path length of the reference optical path when the light intensity detected by receiving the interference light by the interference with the reflected light is received by the photodetector. .

  In the thickness measurement apparatus according to the present invention, the measurement light output from the measurement light source is input to the first end of the optical coupler, and is branched into two at this optical coupler. One of the branched lights is output from the second end of the optical coupler to the reference optical path, and the other branched light is output from the third end of the optical coupler to the measuring optical path.

  The light output from the second end of the optical coupler to the reference optical path is reflected by the reference light generation unit, and the reflected light is input as reference light to the second end of the optical coupler via the reference optical path. At this time, the optical path length of the reference optical path from when the light output from the second end of the optical coupler is reflected and input to the second end of the optical coupler is variable.

  The light output from the third end of the optical coupler to the measurement optical path is bifurcated by the first beam splitter and output as the first branched light and the second branched light. The first branched light output from the first beam splitter is incident on and reflected by the first surface of the object through the first branched optical path, and the first reflected light generated by the reflection passes through the first branched optical path. Input to one beam splitter. The second branched light output from the first beam splitter is incident on the second surface of the object through the second branched optical path and reflected, and the second reflected light generated by the reflection passes through the second branched optical path. Then, it is input to the first beam splitter. The first reflected light and the second reflected light input to the first beam splitter are input to the third end of the optical coupler through the measurement optical path.

  The reference light input to the second end of the optical coupler via the reference optical path, and the first reflected light and the second reflected light input to the third end of the optical coupler via the measurement optical path are the first of the optical coupler. Output from 4 ends. Each of the reference light, the first reflected light, and the second reflected light output from the fourth end of the optical coupler is received by the photodetector, and the intensity of the received light is detected.

  At this time, the reference light, the first reflected light, and the second reflected light received by the photodetector are reflected from the measurement light output from the common measurement light source. Accordingly, interference occurs between the reference light, the first reflected light, and the second reflected light when the optical path lengths from the measurement light source to the reflection position coincide with each other. Therefore, the intensity of light received by the photodetector is increased. growing.

  Therefore, the thickness calculation unit calculates the thickness of the object based on the relationship between the light intensity detected by the photodetector and the optical path length of the reference optical path. That is, the optical path length of the reference optical path when the detected light intensity reaches the peak when the interference light due to the interference between the reference light and the first reflected light is received by the photodetector by the thickness calculation unit, and the reference light The thickness of the object is calculated based on the optical path length of the reference optical path when the light intensity detected by receiving the interference light due to the interference with the second reflected light is received by the photodetector.

  In the thickness measurement apparatus according to the present invention, it is preferable that each of the reference optical path and the measurement optical path includes an optical fiber. In this case, handling of these optical paths becomes simple.

  Preferably, the thickness measuring apparatus according to the present invention is (a) provided in the middle of the first branching optical path, and a part of the light traveling from the first beam splitter toward the first surface of the object is branched. A second beam splitter that outputs to the first branch optical path and outputs the light that has reached through the third branch optical path to the first branch optical path; and (b) reflects the light output from the second beam splitter to the third branch optical path. And a reflecting section for inputting the third reflected light generated by the reflection to the second beam splitter. The photodetector also receives the third reflected light output from the fourth end of the optical coupler. Further, the thickness calculation unit includes an optical path length of the reference optical path when interference light due to interference between the reference light and the first reflected light is received by the photodetector and the detected light intensity reaches a peak, and the reference light The thickness of the object is provisionally calculated based on the optical path length of the reference optical path when the light intensity detected by receiving the interference light by the interference with the second reflected light is received by the photodetector. Further, the thickness calculation unit is configured to temporarily calculate the optical path length of the reference optical path when the light intensity detected when the interference light due to the interference between the reference light and the third reflected light is received by the photodetector reaches a peak. The calculated thickness of the object is corrected.

  In this case, a part of the light traveling from the first beam splitter toward the first surface of the object is branched and output to the third branch optical path by the second beam splitter provided in the middle of the first branch optical path. The third reflected light reflected by the reflecting portion and generated by the reflection at the reflecting portion is output to the first branch optical path through the third branch optical path and input to the first beam splitter. The third reflected light is input to the third end of the optical coupler through the measurement optical path, output from the fourth end of the optical coupler, and received by the photodetector, similarly to the first reflected light and the second reflected light. Is done. Also, interference occurs between the third reflected light and the reference light when the optical path lengths from the measurement light source to the reflection position coincide with each other, so that the intensity of light received by the photodetector increases.

  Therefore, the optical path length of the reference optical path when the light intensity detected when the interference light due to the interference between the reference light and the first reflected light is received by the photodetector reaches a peak by the thickness calculation unit, and the reference light The thickness of the object is provisionally calculated based on the optical path length of the reference optical path when the light intensity detected by receiving the interference light due to the interference with the second reflected light is received by the photodetector. Further, based on the optical path length of the reference optical path when the light intensity detected when the interference light due to the interference between the reference light and the third reflected light is received by the photodetector is peaked by the thickness calculation unit, The calculated thickness of the object is corrected. By doing so, even if the optical path length difference between the reference optical path and the measurement optical path varies, the influence of the optical path length difference variation is compensated, and the thickness of the object can be accurately obtained. obtain.

  The thickness measuring apparatus according to the present invention can measure the thickness of a wider range of objects with respect to materials, surface conditions, and the like.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a configuration diagram of a thickness measuring apparatus 1 according to the present embodiment. A thickness measuring apparatus 1 shown in this figure is an apparatus for measuring the thickness of an object 2 having a first surface 2a and a second surface 2b that are parallel to each other, and includes a measurement light source 11, an optical coupler 12, a probe head 13, Reference light generation unit 14, photodetector 15, thickness calculation unit 16, optical path length control unit 17, input optical fiber 11a, reference optical fiber 14a, measurement optical fiber 13a, output optical fiber 15a, first beam splitter BS1, second beam splitter BS2, and mirrors M1 to M5 are provided. The object 2 is, for example, a semiconductor wafer, a film, a sheet, or the like.

  The measurement light source 11 outputs measurement light to the input optical fiber 11a. The measurement light is low-coherent light, and the measurement light source 11 is, for example, an SLD (Super Luminescent Diode). Moreover, the wavelength of this measurement light may be arbitrary.

  The optical coupler 12 has a first end 12a, a second end 12b, a third end 12c, and a fourth end 12d. The optical coupler 12 inputs measurement light output from the measurement light source 11 and guided by the input optical fiber 11a to the first end 12a. Then, the optical coupler 12 splits the measurement light input to the first end 12a into two, and outputs one branched light from the second end 12b to the reference optical fiber 14a as a reference optical path and the other branch. The light is output from the third end 12c to the measurement optical fiber 13a as the measurement optical path. The optical coupler 12 outputs light that is input to the second end 12b via the reference optical fiber 14a and light that is input to the third end 12c via the measurement optical fiber 13a from the fourth end 12d. Output to the fiber 15a.

  The reference light generation unit 14 reflects the light output from the second end 12b of the optical coupler 12 and guided by the reference optical fiber 14a to be reflected by the mirror 14c, and uses the reflected light as reference light. The light is input to the second end 12b of the optical coupler 12 through the fiber 14a. The reference light generation unit 14 also includes an optical path length of the reference optical path 14b until the light output from the second end 12b of the optical coupler 12 is reflected by the mirror 14c and input to the second end 12b of the optical coupler 12. Can be made variable. In order to make the optical path length of the reference optical path 14b variable, the reference light generation unit 14 includes a glass substrate 14d and a galvanometer 14e.

  The glass substrate 14d is made of glass that is transparent at the wavelength of the measurement light, and the two opposing principal surfaces are parallel to each other, and is inserted in the middle of the reference optical path 14b. The galvanometer 14 e periodically changes the orientation of the glass substrate 14 d in accordance with an instruction from the reference optical path length control unit 17. Since the optical path length of the light passing through the glass substrate 14a can be changed by changing the orientation of the glass substrate 14d, the light output from the second end 12b of the optical coupler 12 is reflected by the mirror 14c. The optical path length of the reference optical path 14b until it is input to the second end 12b of the optical coupler 12 can be made variable.

  The probe head 13 is provided at the tip of the measurement optical fiber 13a. The probe head 13 outputs light output from the third end 12c of the optical coupler 12, guided by the measurement optical fiber 13a, and reaching the other end to the external space. Further, the probe head 13 inputs light reaching from the outside to the measurement optical fiber 13a and guides the light through the measurement optical fiber 13a.

  The first beam splitter BS1 inputs light that is output from the third end 12c of the optical coupler 12, is guided by the measurement optical fiber 13a, is output from the probe head 13 to the external space, and reaches the input light. The light is branched and output as first branched light and second branched light. The first beam splitter BS1 outputs the first branched light to the mirror M1, and outputs the second branched light to the mirror M3.

  The first branched light path is an optical path of the first branched light from the first beam splitter BS1 to the first surface 2a of the object 2 via the mirrors M1 and M2 and the second beam splitter BS2. The first branched light path causes the first branched light output from the first beam splitter BS1 to be incident on the first surface 2a of the object 2 and reflected, and the first reflected light generated by the reflection is reflected in the reverse path. Input to one beam splitter BS1.

  The second branched optical path is an optical path of the second branched light from the first beam splitter BS1 through the mirrors M3 and M4 to the second surface 2b of the object 2. The second branched light path causes the second branched light output from the first beam splitter BS1 to be incident on the second surface 2b of the object 2 and reflected, and the second reflected light generated by the reflection is reflected on the second path. Input to one beam splitter BS1.

  The second beam splitter BS2 is provided in the middle of the first branch optical path. The second beam splitter BS2 branches a part of the light from the first beam splitter BS1 toward the first surface 2a of the object 2, and outputs the branched light to the third branch optical path toward the mirror M5. The second beam splitter BS2 outputs the light that has reached from the mirror M5 via the third branch optical path to the first branch optical path. The mirror M5 reflects the light which has been output from the second beam splitter BS2 to the third branch optical path and arrived, and inputs the third reflected light generated by the reflection to the second beam splitter BS2.

  The photodetector 15 receives the reference light, the first reflected light, the second reflected light, and the third reflected light that are output from the fourth end 12d of the optical coupler 12 and guided by the output optical fiber 15a. Then, the intensity of the received light is detected, and an electric signal representing the light intensity is output to the thickness calculator 16. The photodetector 15 is, for example, a photodiode.

  The optical path length controller 17 periodically changes the orientation of the glass substrate 14d by controlling the galvanometer 14e in the reference light generator 14, and thereby periodically changes the optical path length of the reference optical path 14b. Further, the optical path length control unit 17 gives an electrical signal representing information related to the orientation of the glass substrate 14 d (that is, information related to the optical path length of the reference optical path 14 b) to the thickness calculation unit 16. The optical path length of the reference optical path 14b is not necessarily an absolute value and may be a relative value with respect to a certain reference value.

  The thickness calculator 16 calculates the thickness of the object 2 based on the relationship between the light intensity detected by the photodetector 15 and the optical path length of the reference optical path 14b. The thickness calculator 16 includes a signal processing circuit 16a, a raw thickness value calculator 16b, and a statistical thickness value calculator 16c.

  The signal processing circuit 16a receives an electrical signal output from the photodetector 15, and obtains information on the intensity of light received by the photodetector 15 based on the electrical signal. Further, the signal processing circuit 16a receives an electrical signal output from the optical path length control unit 17, and obtains information on the optical path length of the reference optical path 14b based on the electrical signal. The signal processing circuit 16a converts the input information into a digital value and gives it to the raw thickness value calculation unit 16b.

  The raw thickness value calculation unit 16b receives the light intensity information and the optical path length information output from the signal processing circuit 16a. At this time, the raw thickness value calculation unit 16b inputs each value of the optical path length of the reference optical path 14b and the value of the received light intensity in the photodetector 15 corresponding to this optical path length value in association with each other. Remember. Then, the raw thickness value calculation unit 16b obtains a relationship between the light intensity detected by the photodetector 15 and the optical path length of the reference optical path 14b. In other words, the raw thickness value calculation unit 16b obtains the light intensity distribution with respect to the optical path length by using the optical path length of the reference optical path 14b as one axis and the light intensity detected by the photodetector 15 as the other axis. Furthermore, the raw thickness value calculation unit 16b calculates the thickness of the object 2 by performing a necessary process based on the positions of the plurality of light intensity peaks specified by the light intensity distribution, and calculates the calculated thickness value. Is output. A method for calculating the thickness of the object 2 from the light intensity distribution will be described later.

  The statistical thickness value calculation unit 16c inputs the thickness value of the object 2 (hereinafter referred to as “raw thickness value”) output from the raw thickness value calculation unit 16b, and performs statistical processing on the input raw thickness value. The thickness value after the statistical processing (hereinafter referred to as “statistical thickness value”) is output. For example, when measuring the thickness of the target object 2 in the course of the wet etching process, and when it is expected that the thickness of the target object 2 will decrease at a constant speed with time, a statistical thickness value calculation unit 16c can obtain a statistical thickness value with higher accuracy by determining the linear function formula and using the linear function formula, assuming that the raw thickness value is expressed by a linear function formula over time. it can. For example, when it is expected that the thickness of the object 2 does not change greatly with the passage of time, or when the thickness of the object 2 is expected to change at a constant speed with the passage of time, The statistical thickness value calculation unit 16c can obtain a statistical thickness value with higher accuracy by treating the raw thickness value greatly deviating from the expectation as noise.

  Next, operation | movement of the thickness measuring apparatus 1 which concerns on this embodiment is demonstrated. The low-coherent measurement light output from the measurement light source 11 is guided by the input optical fiber 11 a, input to the first end 12 a of the optical coupler 12, and branched into two in this optical coupler 12. One branched light generated by being branched into two in the optical coupler 12 is output from the second end 12b of the optical coupler 12 to a reference optical fiber 14a as a reference optical path. The other branched light is output from the third end 12c of the optical coupler 12 to the measurement optical fiber 13a as the measurement optical path.

  The light output from the second end 12b of the optical coupler 12 is reflected by the mirror 14c and input to the second end 12b of the optical coupler 12 in the reference light generation unit 14, and the optical path of the reference optical path 14b therebetween. The length is variable. That is, the light output from the second end 12b of the optical coupler 12 is guided by the reference optical fiber 14a, output from the end face of the reference optical fiber 14a, passes through the glass substrate 14d, and is reflected by the mirror 14c. Is done. The light reflected by the mirror 14c passes through the glass substrate 14d as reference light, is input to the end face of the reference optical fiber 14a, and is guided by the reference optical fiber 14a. Input to the end 12b. At this time, the orientation of the glass substrate 14d made of transparent glass at the wavelength of the measurement light and inserted in the middle of the reference optical path 14b is periodically changed by the galvanometer 14e controlled by the reference optical path length control unit 17. By changing the orientation of the glass substrate 14d, the optical path length of the reference optical path 14b becomes variable.

  The light output from the third end 12c of the optical coupler 12 is guided to the measurement optical fiber 13a and output from the probe head 13 provided at the tip of the measurement optical fiber 13a to the external space. The light output from the probe head 13 is branched into two by the first beam splitter BS1 to be the first branched light and the second branched light.

  The first branched light output from the first beam splitter BS1 is incident on the first surface 2a of the object 2 via the first branched optical path including the mirror M1, the mirror M2, and the second beam splitter BS2, and the first branched light is inputted. Reflected on the first surface 2a. The first reflected light generated by the reflection follows the path opposite to that at the time of entering the first branch path, and is input to the first beam splitter BS1.

  The second branched light output from the first beam splitter BS1 is incident on the second surface 2b of the object 2 through the second branched optical path including the mirror M3 and the mirror M4, and is reflected on the second surface 2b. The The second reflected light generated by the reflection follows the path opposite to the incident time on the second branch path, and is input to the first beam splitter BS1.

  Further, a part of the first branched light output from the first beam splitter BS1 is branched to the third branched optical path toward the mirror M5 by the second beam splitter BS2 provided in the middle of the first branched optical path. . The branched light is reflected by the mirror M5. The third reflected light generated by the reflection is input to the second beam splitter BS2, and is input from the second beam splitter BS2 to the first beam splitter BS1 via the mirrors M2 and M1.

  The first reflected light, the second reflected light, and the third reflected light are input to the measurement optical fiber 13a via the first beam splitter BS1 and the probe head 13, guided by the measurement optical fiber 13a, and the optical coupler 12. Is input to the third end 12c. Further, the reference light is guided from the reference light generation unit 14 through the reference optical fiber 14 a and input to the second end 12 b of the optical coupler 12.

  Reference light input to the second end 12b of the optical coupler 12 through the reference optical fiber 14a, and first reflected light and second input to the third end 12c of the optical coupler 12 through the measurement optical fiber 13a The reflected light and the third reflected light are output from the fourth end 12d of the optical coupler 12, guided by the output optical fiber 15a, and received by the photodetector 15. Then, the light detector 15 detects the intensity of the received light, and an electric signal representing the light intensity is output to the thickness calculator 16.

  The reference light, the first reflected light, the second reflected light, and the third reflected light received by the photodetector 15 are reflected from the low coherent light output from the common measurement light source 11. Accordingly, interference occurs between the reference light, the first reflected light, the second reflected light, and the third reflected light when the optical path lengths from the measurement light source 11 to the reflection position coincide with each other. The intensity of received light increases. As the coherency of light output from the measurement light source 11 is lower, interference does not occur unless the optical path length difference between the two lights is small.

  FIG. 2 and FIG. 3 are diagrams showing the light intensity distribution obtained by the thickness calculator 16 while showing the state of light reflection on the object 2 in the thickness measuring apparatus 1 according to the present embodiment. Each figure (a) has shown the mode of light reflection in subject 2 in thickness measuring device 1 concerning this embodiment, and the 1st which light injects and reflects with respect to the 1st surface 2a of subject 2 is shown. A branched optical path is represented by P1, and a second branched optical path through which light is incident and reflected on the second surface 2b of the object 2 is represented by P2. Moreover, each figure (b) shows the light intensity distribution calculated | required by the thickness calculation part 16 of the thickness measuring apparatus 1 which concerns on this embodiment.

In FIG. 2, the first branched optical path is considered among the first branched optical path and the second branched optical path. In this case, when the optical path length from the optical coupler 12 to the mirror 14c is equal to the optical path length from the optical coupler 12 to the first surface 2a of the object 2, the reference light and the first reflected light interfere with each other and strengthen. Accordingly, the intensity of light detection by the light detector 15 increases. However, when the maximum value of the variable range of the optical path length from the optical coupler 12 to the mirror 14c is L, and the width of the variable range and delta _max, the optical path length from the optical coupler 12 to the first surface 2a of the object 2 L ₁ needs to be in a range of “L−Δ _max <L ₁ <L”. At this time, in the light intensity distribution shown in FIG. (B), the position of the optical path length change amount delta _P1 of the reference optical path, the light intensity peak due to interference between the reference beam and the first reflected light is obtained.

In FIG. 3, the second branch optical path is considered among the first branch optical path and the second branch optical path. In this case, when the optical path length from the optical coupler 12 to the mirror 14c is equal to the optical path length from the optical coupler 12 to the second surface 2b of the object 2, the reference light and the second reflected light interfere with each other and strengthen. Accordingly, the intensity of light detection by the light detector 15 increases. However, when the maximum value of the variable range of the optical path length from the optical coupler 12 to the mirror 14c is L, and the width of the variable range and delta _max, the optical path length from the optical coupler 12 to the second surface 2b of the object 2 L ₂ needs to be in a range of “L−Δ _max <L ₂ <L”. At this time, in the light intensity distribution shown in FIG. (B), the position of the optical path length change amount delta _P2 of the reference optical path, the light intensity peak due to the interference between the reference light and the second reflected light can be obtained.

FIG. 4 is a diagram illustrating a light intensity distribution obtained by the thickness calculation unit 16 of the thickness measuring apparatus 1 according to the present embodiment. The light obtained by the thickness calculation unit 16 as shown in FIG. 4 by adjusting the arrangement of the mirror and the object 2 so that both the optical path length L ₁ and the optical path length L ₂ satisfy the above relational expression. in the intensity distribution, obtained light intensity peak due to interference between the reference beam and the first reflected light at the position of the optical path length change amount delta _P1 of the reference light path, and at the same time, the optical path length change amount of the reference optical path delta _P2 A light intensity peak due to interference between the reference light and the second reflected light is obtained at the position.

FIG. 5 is a diagram illustrating a method for calculating the thickness of the target object 2 in the thickness calculation unit 16 of the thickness measurement apparatus 1 according to the present embodiment. Here, as shown in FIG. 5A, a reference object 2c having a known thickness t _c is used, and as shown in FIG. 5B, the thickness of the reference object 2c is increased. In the light intensity distribution obtained by the calculation unit 16, a light intensity peak due to interference between the reference light and the first reflected light is obtained at the position of the optical path length variation _ΔP1c of the reference optical path, and at the same time, for reference It is _{assumed that} a light intensity peak due to interference between the reference light and the second reflected light is obtained at the position of the optical path length variation _ΔP2c of the optical path.

If the position of the first surface 2a of the object 2 is different from the first surface of the reference object 2c as shown in FIG. 5C, as shown in FIG. A light intensity peak due to interference between the reference light and the first reflected light is obtained at a position that differs by an optical path length difference (Δ _P1 −Δ _P1c ) according to the position difference. Further, as shown in FIG. 5C, if the position of the second surface 2b of the target object 2 is different from the second surface of the reference target object 2c, as shown in FIG. A light intensity peak due to interference between the reference light and the second reflected light is obtained at a position that differs by an optical path length difference (Δ _P2 −Δ _P2c ) according to the position difference. From this, the thickness t of the object 2 is obtained by the following equation (1).

Thus, the thickness measuring device 1 according to this embodiment, the optical path length change amount of the reference optical path for the light intensity peak is obtained at a light intensity distribution with respect to reference object 2c having a known thickness t _c Δ _P1c, Δ _P2c And obtaining the optical path length variation Δ _P1 , Δ _P2 of the reference optical path from which the light intensity peak is obtained in the light intensity distribution with respect to the object 2 whose thickness is to be measured, thereby obtaining the object according to the above equation (1). A thickness t of 2 can be calculated. Further, as can be seen from the above equation (1), the same thickness t can be obtained even when the object 2 moves up and down, so that stable thickness measurement can be performed with respect to the vertical movement of the object 2.

  In the thickness measuring apparatus 1 according to the present embodiment, since the measurement light does not need to pass through the inside of the object 2, the thickness can be measured even for the object 2 that is not transparent at the wavelength of the measurement light. Thickness can also be measured for highly-concentrated and opaque semiconductor wafers, objects having a metal coating film formed on the surface, objects having a rough surface and large light scattering, and the like. Thus, the thickness measuring apparatus 1 according to the present embodiment can measure the thickness of the object in a wider range with respect to the material, the surface state, and the like. Further, in the thickness measuring apparatus 1 according to the present embodiment, the wavelength of the measurement light may be arbitrary, so that the selection range of the measurement light source 11 is wide.

  Next, the case where the optical path lengths of the reference optical fiber 14a and the measurement optical fiber 13a are changed in the thickness measurement apparatus 1 according to the present embodiment due to changes in the environment such as temperature will be described.

  If the optical path lengths of the reference optical fiber 14a and the measurement optical fiber 13a vary by the same amount, the optical path length difference between the reference light and the first reflected light does not vary, and the reference Since the optical path length difference between the light and the second reflected light does not vary, the calculation result of the thickness t according to the above equation (1) does not vary. Therefore, it is preferable that the same type of reference optical fiber 14a and the measurement optical fiber 13a are used with the same length so that the optical path length varies by the same amount even if there is a temperature variation. It is also preferable to maintain the reference optical fiber 14a and the measurement optical fiber 13a at a constant temperature.

  However, when the reference optical fiber 14a and the measurement optical fiber 13a are of different or different lengths, or even if the reference optical fiber 14a and the measurement optical fiber 13a are the same type and the same length, temperature fluctuation occurs. When they are different from each other, the variation amounts of the optical path lengths of the reference optical fiber 14a and the measurement optical fiber 13a are different from each other, and the optical path length difference between the reference optical fiber 14a and the measurement optical fiber 13a varies. Therefore, the optical path length difference between the reference light and the first reflected light varies, and the optical path length difference between the reference light and the second reflected light also varies. At this time, both the fluctuation amount of the optical path length difference between the reference light and the first reflected light and the fluctuation amount of the optical path length difference between the reference light and the second reflected light are both the same as the reference optical fiber 14a. It is equal to the fluctuation amount ΔL of the optical path length difference with the measurement optical fiber 13a.

FIG. 6 shows the light intensity obtained by the thickness calculator 16 when the optical path length difference between the reference optical fiber 14a and the measurement optical fiber 13a varies by ΔL in the thickness measurement apparatus 1 according to the present embodiment. It is a figure explaining the change of distribution. Again, as shown in FIG. 6 (a), a reference object 2c having a known thickness t _c is used, and as shown in FIG. 6 (b), the thickness of the reference object 2c is as follows. In the light intensity distribution obtained by the calculation unit 16, a light intensity peak due to interference between the reference light and the first reflected light is obtained at the position of the optical path length variation _ΔP1c of the reference optical path, and at the same time, for reference It is _{assumed that} a light intensity peak due to interference between the reference light and the second reflected light is obtained at the position of the optical path length variation _ΔP2c of the optical path.

At this time, if the optical path length difference between the reference optical fiber 14a and the measurement optical fiber 13a varies by ΔL, the light intensity distribution obtained by the thickness calculator 16 as shown in FIG. A light intensity peak due to the interference between the reference light and the first reflected light is obtained at the position of the optical path length variation Δ _P1 (= Δ _P1c + ΔL) of the reference optical path, and the optical path length variation of the reference optical path A light intensity peak due to interference between the reference light and the second reflected light is obtained at a position of Δ _P2 (= Δ _P2c + ΔL). When this result is applied to the above equation (1), the following equation (2) is obtained, and the result is as if the thickness of the reference object 2c is increased by 2ΔL.

  Next, in the thickness measuring apparatus 1 according to the present embodiment, even when the optical path length difference between the reference optical fiber 14a and the measuring optical fiber 13a varies, the influence of the optical path length difference variation is compensated. A method for accurately obtaining the thickness of the object 2 will be described.

The thickness calculation unit 16 of the thickness measurement apparatus 1 according to the present embodiment refers to when interference light caused by interference between the reference light and the first reflected light is received by the photodetector 15 and the detected light intensity reaches a peak. the optical path length delta _P1 of use light path, the reference beam and the optical path length of the reference optical path when the light intensity detected is received reaches a peak interference light by the photodetector 15 due to interference between the second reflected light delta _P2 Based on the above, the thickness of the object 2 is provisionally calculated as described above. Then, the thickness calculating unit 16, the optical path length delta _PR of the reference optical path when the reference light and the light intensity interference light is detected is received by the photodetector 15 due to the interference between the third reflected light has a peak Based on the above, the provisionally calculated thickness of the object 2 is corrected. More specific processing contents will be described below.

The optical path length Δ _PR of the reference optical path from which the light intensity peak due to the interference between the third reflected light and the reference light generated by the reflection at the mirror M5 whose position is fixed is obtained by the reference optical fiber 14a and the measurement optical fiber. It depends only on the optical path length difference ΔL from 13a. Accordingly, by using the optical path length delta _PR of the reference optical path, the effect thickness of the object 2 by compensating for the optical path length difference ΔL can be accurately obtained.

FIG. 7 shows the influence of the optical path length difference ΔL in the thickness calculator 16 even if the optical path length difference between the reference optical fiber 14a and the measurement optical fiber 13a varies in the thickness measuring apparatus 1 according to the present embodiment. It is a figure explaining the method which compensates for and calculates the thickness of the target object. As shown in FIG. 7 (a), a reference object 2c having a known thickness t _c is used, and as shown in FIG. 7 (b), the thickness calculation unit 16 in the case of the reference object 2c. In the light intensity distribution obtained in step 1, a light intensity peak due to the interference between the reference light and the first reflected light is obtained at the position of the optical path length variation _ΔP1c of the reference optical path, and the optical path length variation of the reference optical path is obtained. delta position of _P2c light intensity peak due to interference between the reference light and the second reflected light is obtained, also the interference of the reference beam at the position of the optical path length change amount delta _PRc of the reference optical path and the third reflected light It is assumed that a light intensity peak due to the above is obtained.

Further, as shown in FIG. 7, when the optical path length difference ΔL between the reference optical fiber 14a and the measurement optical fiber 13a occurs, the light intensity distribution obtained by the thickness calculation unit 16 is used for reference. light intensity peak due to interference between the reference beam and the first reflected light at the position of the optical path length change amount delta _P1 of the optical path is obtained, the reference light and the second reflection on the position of the optical path length change amount delta _P2 of the reference light path light intensity peak due to interference with the light can be obtained and the light intensity peak due to the interference of the reference beam at the position of the optical path length change amount delta _PR and the third reflected light of the reference light path is to be obtained .

Here, the difference (Δ _PR −Δ _PRc ) corresponds to only the optical path length difference ΔL between the reference optical fiber 14a and the measurement optical fiber 13a. Further, the difference (Δ _P1 −Δ _P1c ) and the difference (Δ _P2 −Δ _P2c ) respectively correspond to the thickness of the object 2 and the optical path length difference ΔL. From this, an equation for calculating the thickness t of the object 2 while compensating for the influence of the optical path length difference ΔL is given by the following equation (3). By using the equation (3), even if the optical path length difference between the reference optical fiber 14a and the measurement optical fiber 13a varies, the influence of the optical path length difference variation is compensated for. The thickness of 2 can be obtained with high accuracy.

  If the mirror M5 and the above expression (3) are not used, as described above, the reference optical fiber 14a and the measurement optical fiber 13a are managed at a constant temperature with high accuracy, or the object 2 is measured. In addition, there is a need to measure the reference sample every time or to shorten the time between the reference sample measurement and the measurement of the object 2 as much as possible. However, by using the mirror M5 and the above equation (3), even if there is a variation in the optical path length difference between the reference optical fiber 14a and the measurement optical fiber 13a, the influence of the optical path length difference variation is compensated. Thus, the thickness of the object 2 can be obtained with high accuracy.

  Next, a method for measuring the thickness of the object 2 excluding the transparent film when the transparent film exists on one surface of the object 2 will be described. For example, a protective tape is affixed to the device forming surface of a semiconductor wafer being ground. When measuring the thickness of such a wafer as the object 2, it is necessary to measure the thickness of the wafer not including the thickness of the protective tape. Thus, when a transparent film exists on one surface of the object 2, the thickness of the object 2 is measured as follows.

FIG. 8 is a diagram illustrating a method for calculating the thickness of the target object 2 when the transparent film 3 is present on one surface of the target object 2 in the thickness measuring apparatus 1 according to the present embodiment. As shown in FIG. 8A, it is assumed that a reference object 2c having a known thickness t _c is used. Here, it is assumed that the transparent film 3 does not exist on the surface of the reference object 2c. At this time, as shown in FIG. 8B, in the light intensity distribution obtained by the thickness calculator 16 in the case of the reference object 2c, the reference light is positioned at the position of the optical path length variation _ΔP1c of the reference optical path. A light intensity peak due to the interference between the reference light and the first reflected light is obtained, and a light intensity peak due to the interference between the reference light and the second reflected light is obtained at the position of the optical path length variation _ΔP2c of the reference light path. In addition, it is assumed that a light intensity peak due to interference between the reference light and the third reflected light is obtained at the position of the optical path length variation _ΔPRc of the reference optical path.

As shown in FIG. 8C, it is assumed that the transparent film 3 is pasted on the second surface of the object 2. At this time, as shown in FIG. 8 (d), interference between the light intensity distribution obtained by the thickness calculating section 16, the reference beam and the first reflected light at the position of the optical path length change amount delta _P1 of the reference light path obtained light intensity peak due to the light intensity peak due to the interference between the reference light and the second reflected light at the position of the optical path length change amount delta _P2 of the reference light path (reflected light generated at the surface of the transparent film 3) is obtained, the light intensity peak due to the interference of the reference beam and the position of the optical path length change amount delta _PR and the third reflected light of the reference light path is to be obtained. Also, the position of the optical path length change amount delta _PT of the reference optical path, due to interference with the fourth reflected light and the reference light generated at the second surface of the object 2 (the interface between the object 2 and the transparent layer 3) Assume that a light intensity peak is obtained.

From these, the sum t _all (= t + t _T ) of the thickness t of the object 2 and the thickness t _T of the transparent film 3 is calculated according to the above equation (3). Further, the optical path length variations Δ _PT and Δ _{P2 of the} reference optical path from which the light intensity peak due to the interference between the reflected light and the reference light generated on both surfaces of the transparent film 3 are obtained, and the refractive index n _{T of the} transparent film 3 are The thickness t _T of the transparent film 3 is calculated according to the following equation (4). Then, the thickness t of the object 2 is calculated according to the following equation (5).

A method for calculating the thickness t _T of the transparent film 3 in accordance with the above equation (4) is disclosed in Patent Document 1. Further, when the thickness t _T of the transparent film 3 is measured, the second reflected light and the fourth reflected light are necessary, but the first reflected light and the third reflected light are unnecessary. It is good also as providing a shutter in the middle and closing this shutter.

It is a lineblock diagram of thickness measuring device 1 concerning this embodiment. It is a figure which shows the light intensity distribution calculated | required in the thickness calculation part 16, while showing the mode of the light reflection in the target object 2 in the thickness measuring apparatus 1 which concerns on this embodiment. It is a figure which shows the light intensity distribution calculated | required in the thickness calculation part 16, while showing the mode of the light reflection in the target object 2 in the thickness measuring apparatus 1 which concerns on this embodiment. It is a figure which shows the light intensity distribution calculated | required by the thickness calculation part 16 of the thickness measuring apparatus 1 which concerns on this embodiment. It is a figure explaining the method to calculate the thickness of the target object 2 in the thickness calculation part 16 of the thickness measuring apparatus 1 which concerns on this embodiment. In the thickness measurement apparatus 1 according to the present embodiment, when the optical path length difference between the reference optical fiber 14a and the measurement optical fiber 13a varies by ΔL, the change in the light intensity distribution obtained by the thickness calculation unit 16 is changed. It is a figure explaining. In the thickness measuring apparatus 1 according to the present embodiment, even if the optical path length difference between the reference optical fiber 14a and the measuring optical fiber 13a varies, the thickness calculator 16 compensates for the influence of the optical path length difference ΔL. It is a figure explaining the method to calculate the thickness of the target object. In the thickness measurement apparatus 1 which concerns on this embodiment, it is a figure explaining the method of calculating the thickness of the target object 2 when the transparent film 3 exists in the one surface of the target object 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Thickness measuring apparatus, 2 ... Object, 11 ... Measurement light source, 11a ... Optical fiber for input, 12 ... Optical coupler, 13 ... Probe head, 13a ... Optical fiber for measurement, 14 ... Reference light production | generation part, 14a ... reference Optical fiber, 14b ... Optical path for reference, 14c ... Reflection mirror, 14d ... Glass substrate, 14e ... Galvanometer, 15 ... Photo detector, 15a ... Optical fiber for output, 16 ... Thickness calculating section, 16a ... Signal processing circuit, 16b ... Raw thickness value calculation unit, 16c ... Statistical thickness value calculation unit, 17 ... Reference optical path length control unit.

Claims (2)

An apparatus for measuring the thickness of an object having a first surface and a second surface parallel to each other,
A measurement light source that outputs measurement light;
It has a first end, a second end, a third end, and a fourth end. The measurement light output from the measurement light source is input to the first end, and the measurement light input to the first end is branched into two. Then, one branched light is output from the second end to the reference optical path, and the other branched light is output from the third end to the measuring optical path, and is input to the second end via the reference optical path. An optical coupler that outputs light and light input to the third end via the measurement optical path from the fourth end;
Reflecting the light that is output from the second end of the optical coupler and reaching the reference optical path, and inputting the reflected light as reference light to the second end of the optical coupler via the reference optical path; A reference light generation unit capable of changing the optical path length of the reference optical path from the time when the light output from the second end of the optical coupler is reflected and input to the second end of the optical coupler;
A first beam splitter that inputs light that reaches the measurement optical path from the third end of the optical coupler and that divides the input light into two to output the first branched light and the second branched light;
A first branched light path for causing the first branched light output from the first beam splitter to be incident on the first surface of the object and reflected, and to input the first reflected light generated by the reflection to the first beam splitter. When,
A second branch optical path for causing the second branched light output from the first beam splitter to be incident on the second surface of the object and reflected, and to input the second reflected light generated by the reflection to the first beam splitter. When,
Provided in the middle of the first branch optical path, part of the light traveling from the first beam splitter toward the first surface of the object is branched and output to the third branch optical path, and arrives via the third branch optical path A second beam splitter for outputting the processed light to the first branch optical path;
A reflection unit configured to reflect light output from the second beam splitter to the third branch optical path and input third reflected light generated by the reflection to the second beam splitter;
A photodetector that receives each of the reference light, the first reflected light , the second reflected light, and the third reflected light output from the fourth end of the optical coupler and detects the intensity of the received light. When,
A thickness calculator that calculates the thickness of the object based on the relationship between the light intensity detected by the photodetector and the optical path length of the reference optical path;
With
The thickness calculator is
The optical path length of the reference optical path when the light intensity detected when the interference light due to the interference between the reference light and the first reflected light is received by the photodetector reaches a peak, the reference light, and the first light The thickness of the object is provisionally calculated based on the optical path length of the reference optical path when the light intensity detected when the interference light due to the interference with the two reflected light is received by the photodetector reaches a peak. ,
The provisional calculation based on the optical path length of the reference optical path when the light intensity detected when the interference light due to the interference between the reference light and the third reflected light is received by the photodetector reaches a peak Correct the thickness of the object,
Thickness measuring device characterized by the above.   The thickness measuring apparatus according to claim 1, wherein each of the reference optical path and the measurement optical path includes an optical fiber.

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