JP5729183B2 - Wafer with inspection electrode and method for measuring refractive index of electrode - Google Patents

Description

  The present invention relates to a wafer with an electrode for inspection and a method for measuring the refractive index of the electrode, and in particular, in the manufacturing process of an optical waveguide element in which an optical waveguide and an electrode such as a broadband optical modulator are formed, the refractive index of the electrode is accurately and The present invention relates to a technique for simple measurement, and relates to a wafer with an inspection electrode in which an inspection electrode is provided on a wafer forming an optical modulator, and a method for measuring the refractive index of the electrode of the wafer.

In the fields of optical communication and optical measurement, an optical modulator in which a Mach-Zehnder type optical waveguide and a traveling wave type electrode are formed on a ferroelectric substrate having an electro-optic effect has been developed and used (for example, see Patent Document 1). ). In particular, in long-distance optical communication, a baseband transmission system is the mainstream, and a broadband operation is required for the optical modulator. Currently, traveling wave light modulation using an optical waveguide for confining and controlling light on a lithium niobate (LiNbO ₃ ) substrate and a traveling wave electrode for applying a drive modulation signal (electric signal) It is the mainstream to use a vessel and is widely used.

  In order to enable the optical modulator of this configuration to operate in a wide band, it is necessary to match the speed of the optical signal propagating through the optical waveguide and the microwave traveling through the electrode. It is important to measure and manage the refractive index of the electrode with respect to microwaves.

  The process of manufacturing a broadband optical modulator using a traveling wave type electrode is generally as follows. First, a large number of optical waveguide patterns and electrode patterns are formed on a single wafer using a thin film forming process and a photolithography process. Next, the electrical characteristics of the electrode pattern in the wafer are inspected. For example, in this inspection process, as shown in Patent Document 2, a part of the electrodes of the element as a product is stretched to perform an insulation inspection between the electrodes, or as shown in Patent Document 3, an inspection electrode is used. A microwave is input to the pattern, the refractive index of the electrode with respect to the microwave is measured, and the quality of the entire wafer or each element is determined based on the obtained refractive index value. Next, the wafer is cut into individual optical modulator shapes, and each optical modulator is formed into a chip. Next, a non-defective product and a defective product of each of the optical modulators formed into chips are selected based on the electrical property inspection results before chip formation and the optical property inspection results of the optical waveguide performed after chip formation.

  Thus, measuring the refractive index of the electrode is important in the manufacturing process. In particular, it is necessary to measure the refractive index of the electrode at the portion (action portion) where the electric field formed by the electrode is applied to the optical waveguide. In the electrode pattern in which the lead wiring of the electrode is formed long as in Patent Document 2, the lead wiring portion affects the measurement of the refractive index, and it is difficult to accurately measure the refractive index of the working portion of the electrode. In general, the drawing wiring has a different drawing direction from the electrode of the working portion, and therefore, the refractive index of the drawing wiring shows a value different from the refractive index of the electrode of the working portion due to the crystal anisotropy of the wafer. For this reason, it has become more difficult to accurately measure the refractive index of the electrode at the action portion.

  Here, since the refractive index of the electrode with respect to the microwave is substantially determined in the electrode process when the wafer is formed, a specific inspection of the electrode is performed after the electrode forming process. In a next-generation optical communication system, an optical modulator that operates at an extremely high speed exceeding 40 Gbps is required. However, in the optical modulator for such an application, the management of the refractive index of the electrode is particularly important. As a method for managing such an electrode forming process, Patent Document 3 separately forms an inspection electrode as a method for evaluating and managing the refractive index of the electrode on the wafer with respect to microwaves, and its effectiveness is recognized. ing.

  However, the method of Patent Document 3 requires a plurality of electrodes having different lengths for accurate measurement of the refractive index of the electrodes, which increases the area on the wafer and cannot be ignored in terms of cost. In addition, there are problems such as a large number of measurements and a complicated inspection process.

JP-A-4-288518 JP-A-5-333297 JP 2010-256541 A

Mark Yu and AnandGopinath, “VelocityMatched Resonant Slow-Wave Structure for Optical Modulator”, Proceedings of Integrated Photonics Research (IPR), ITuH7-1, pp.365-369, PalmSprings, California, March 22, 1993 G. K. Gopalakrishman, W. K. Burns, W. Wang and C. H. Bulmer, "Electrical loss mechanism in traveling wave LiNbO3 optical modulators", Electron. Lett., Vol.28, 207-208, 1992. kleus Beilenhoff, HaraldKlingbeil, Wolfgang Heinlich and Hans L. hartnagel, "Open and short Circuits in Coplanar MMIC's", IEEE Transactions on Microwave Theory and Technologies, Vol. 40, No. 9, pp. 1534-1537, (1993)

  The problem to be solved by the present invention is to eliminate the above-mentioned problem, and in the manufacturing process of the traveling wave optical modulator, it is possible to easily and accurately measure the refractive index of the electrode. An object of the present invention is to provide a wafer with an inspection electrode capable of making a necessary inspection electrode into a smaller electrode pattern and a method for measuring the refractive index of the electrode.

In order to solve the above-mentioned problem, the invention according to claim 1 is a wafer having an electro-optic effect, wherein a plurality of optical waveguides, and signal electrodes and ground electrodes are arranged along the optical waveguides. In a wafer with an inspection electrode having a plurality of traveling wave type control electrodes for controlling light propagating through the optical waveguide by advancing an electrical signal, and an inspection electrode formed on a part of the wafer, inspection electrode is provided with a resonant electrodes extending in the same extending direction as the signal electrode, further, the resonant electrode is characterized Rukoto comprises a signal electrode and the same cross-sectional shape.

The invention according to claim 2 is the wafer with an inspection electrode according to claim 1 , wherein the signal electrode has a plurality of different cross-sectional shapes, and the inspection electrode is provided for each type of the cross-sectional shape. It is characterized by that.

According to a third aspect of the present invention, in the wafer with an inspection electrode according to the first or second aspect , a dummy optical waveguide is provided in the vicinity of the resonant electrode, as in the arrangement relationship between the signal electrode and the optical waveguide. It is formed. The “dummy optical waveguide” in the present invention means an optical waveguide used for the inspection electrode, and is different from the optical waveguide incorporated in the optical modulator.

The invention according to claim 4 is the method for measuring a refractive index of an electrode of a wafer with an inspection electrode according to any one of claims 1 to 3 , wherein an electric signal is input to the resonance electrode to measure a resonance frequency, Based on the length of the resonance electrode and the resonance frequency, a refractive index of the resonance electrode with respect to the electric signal is obtained.

  According to the invention of claim 1, a plurality of optical waveguides, and signal electrodes and ground electrodes are arranged along the optical waveguides on a wafer having an electro-optic effect. In a wafer with an inspection electrode having a plurality of traveling wave type control electrodes for controlling light propagating in an optical waveguide and an inspection electrode formed on a part of the wafer, the inspection electrode is the signal Since the resonant electrode extending in the same extending direction as the electrode is provided, the refractive index of the resonant electrode with respect to the microwave can be easily measured by determining the resonant frequency of the resonant electrode and the length of the electrode. The refractive index of the signal electrode in the traveling wave type control electrode can be easily estimated. Further, at least one resonance type electrode is sufficient, and the size of the resonance type electrode is smaller than the traveling wave type electrode shown in Patent Document 3, so that the area occupied by the inspection electrode pattern in the wafer is very large. Can be small.

In addition, according to the first aspect of the present invention, since the resonance electrode has the same cross-sectional shape as the signal electrode, it is possible to easily estimate the refractive index of the traveling wave control electrode corresponding to various shapes of the signal electrode. Become.

According to the invention of claim 2 , since the signal electrode has a plurality of different cross-sectional shapes, and the inspection electrode is provided for each type of the cross-sectional shape, the traveling wave having a plurality of cross-sectional shapes in one wafer Even if the mold control electrode is arranged, each refractive index can be measured appropriately.

According to the third aspect of the present invention, since the dummy optical waveguide is formed in the vicinity of the resonant electrode, similarly to the arrangement relationship between the signal electrode and the optical waveguide, the influence of the presence or absence of the optical waveguide on the refractive index of the electrode It is possible to measure the refractive index of the electrode more accurately.

According to the fourth aspect of the present invention, in the above-described method for measuring the refractive index of an electrode of a wafer with an inspection electrode, an electrical signal is input to the resonant electrode to measure the resonant frequency, and the length of the resonant electrode and the resonant frequency are measured. Therefore, the refractive index of the resonant electrode with respect to the electrical signal is obtained, and therefore the refractive index of the electrode can be measured easily and accurately.

1 is an overall configuration diagram of a wafer with inspection electrodes according to an embodiment of the present invention. It is an example of the resonance type electrode used for this invention. (A) shows a case where both ends of the resonance type electrode are opened, (b) shows a case where both ends of the resonance type electrode are grounded, and (c) shows a case where the resonance type electrode is formed in a ring shape. It is the figure which showed the relationship between an electrode length and a resonant frequency at the time of using the resonant electrode with which both ends were open | released as a resonant electrode. It is the figure which showed the relationship between electrode length and resonance frequency at the time of using the resonance type electrode by which both ends were short-circuited as the resonance type electrode which concerns on this invention. It is a figure which shows the example which provided the dummy optical waveguide with the resonance type electrode used for this invention.

Hereinafter, the form for implementing this invention is demonstrated in detail using a suitable example.
In the present invention, as shown in FIG. 1, a plurality of optical waveguides 2 and a signal electrode and a ground electrode are disposed along the optical waveguide on a wafer 1 having an electro-optic effect. Inspection having a plurality of traveling wave type control electrodes (not shown) for controlling light propagating through the optical waveguide and inspection electrodes (3, 31) formed on a part of the wafer In the wafer with an electrode, the inspection electrode includes a resonance electrode extending in the same extending direction as the signal electrode, and more preferably, a resonance electrode (3, 31) having the same cross-sectional shape as the signal electrode. It is characterized by providing.

  As a material having an electro-optic effect constituting the wafer 1, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), a quartz-based material, and a combination of these materials are used. Is possible. In particular, a lithium niobate (LN) crystal having a high electro-optic effect is preferably used.

  In the main area of the wafer 1, a plurality of optical waveguide patterns 2 and electrode patterns (not shown) of optical waveguide elements which are actual products are formed in a row direction or a column direction. In FIG. 1, the optical waveguide pattern 2 exemplifies a Mach-Zehnder type optical waveguide in plan view.

  As a method of forming an optical waveguide on the wafer 1, it can be formed by diffusing Ti or the like on the wafer surface by a thermal diffusion method or a proton exchange method in accordance with a predetermined optical waveguide pattern. It is also possible to use a ridge-shaped optical waveguide with a convex portion corresponding to the optical waveguide on the substrate, such as etching the wafer other than the optical waveguide or forming grooves on both sides of the optical waveguide. is there.

  The electrode pattern is not shown, but in the optical waveguide 2, a signal electrode and a ground electrode are arranged along the optical waveguide at a place where modulation of the light wave propagating through the optical waveguide is desired. . And it is a traveling wave type | mold control electrode comprised so that the electrical signal used as a modulation signal may be input into a signal electrode, and an electrical signal may advance between electrodes. In FIG. 1, a signal electrode and a ground electrode are arranged along two branch waveguides (arms) of the Mach-Zehnder type optical waveguide 2.

  In FIG. 1, one Mach-Zehnder type optical waveguide 2 and a traveling wave type electrode corresponding thereto constitute one optical modulator. The wafer 1 is cut into individual light modulators 10 to form chips. In each optical modulator 10, when a microwave is input to the traveling wave type electrode, an electric field is applied to the optical waveguide from the action part of the electrode, and the refractive index of the optical waveguide changes due to the electrooptic effect, and the Mach-Zehnder type optical waveguide is changed. The propagating light is modulated.

  In addition, the electrode pattern can be variously arranged such that the signal electrode is arranged alone, the ground electrode is arranged only on one side of the signal electrode, or the ground electrode is arranged so as to sandwich the signal electrode (coplanar electrode). It is possible to adopt a form. Moreover, various sizes can be adopted as the width and height of the signal electrode as required.

  The electrode pattern is arranged between the action part and the terminal in order to establish an electrical connection between the so-called action part of the electrode and the outside where an electric field is applied to the optical waveguide by the microwave traveling between the electrodes. It consists of a routing wiring section. Moreover, the action part of an electrode is normally arrange | positioned in parallel with an optical waveguide.

Control electrodes such as signal electrodes and ground electrodes formed on the substrate can be formed by forming a Ti / Au electrode pattern, a gold plating method, or the like. Furthermore, if necessary, a buffer layer such as dielectric SiO ₂ can be provided on the substrate surface after the optical waveguide is formed, and a control electrode can be formed on the buffer layer.

  As shown in FIG. 1, an inspection resonance electrode (3, 31) is formed on the edge or blank area of the wafer 1. Here, the “resonant electrode” in the present invention refers to a standing wave resonant electrode in which a microwave standing wave is generated at the electrode.

  2A to 2C show examples of various resonance electrodes. The resonant electrode 3 shown in FIG. 2 (a) or (b) is composed of a straight electrode portion 4 serving as a signal electrode and a power supply wiring portion 5 to the straight electrode portion 4, and the straight electrode portion 4 as required. The ground electrode 6 is arranged so as to surround the. FIG. 2A shows a both-end open type resonance electrode in which both ends of the straight electrode portion 4 are opened from the ground electrode 6, and FIG. 2B shows that both ends of the straight electrode 4 are connected to the ground electrode 6. This is a short-circuited resonance type electrode.

  The resonant electrode shown in FIG. 2C is composed of a ring electrode portion 7 and a power supply wiring portion 5. In the ring electrode portion 7, a microwave standing wave is formed by the supplied microwave depending on the length of one ring of the ring electrode 7.

  The resonant electrode has an electrode pattern of a product such as an optical modulator, in particular, a cross-sectional shape that is the same as the shape of the electrode in the action part, that is, the same film thickness (electrode height) and electrode pattern as the signal electrode in the action part It is formed to have a width. Also, when there are multiple types of optical modulators (optical waveguide elements) having different electrode patterns in the same wafer, or when the shape is different at multiple locations of electrode patterns within one optical modulator, It is necessary to measure the refractive index of the electrode according to the shape of the electrode pattern. In such a case, as shown in FIG. 1, it is preferable to prepare a plurality of types of resonant electrodes 3 and 31 corresponding to different electrode shapes.

  Further, when various films such as a buffer layer are arranged between the electrode and the wafer, the same film configuration between the signal electrode and the wafer may be formed between the inspection electrode and the wafer. preferable. Since the dielectric constant of the substrate or film placed around the electrode has a large effect on the refractive index of the electrode, the difference in measurement conditions due to the difference in the film configuration is minimized, and the electrode is refracted more accurately. It is possible to measure the rate.

  Further, as shown in FIG. 5, it is preferable to form a dummy optical waveguide 20 in the vicinity of the resonant electrode 3 in the same manner as the arrangement relationship between the signal electrode and the optical waveguide. This is because there is a difference in dielectric constant between the portion where the optical waveguide is formed and the other portion. In particular, when the optical waveguide is formed in a ridge shape, it is preferable to form such a dummy optical waveguide because the dielectric constants of the concave portions of the substrate are completely different.

  If the shape and position of the power supply wiring 5 to the resonance type electrode is a simple shape, the influence on the resonance frequency is basically negligible, so there is no need to consider it in the design. However, in order to adjust the impedance of the power supply wiring 5, when a stub structure is provided in the power supply wiring or an impedance matching circuit is incorporated, the resonance frequency is affected. For this reason, it is desirable that the power supply wiring 5 in the inspection electrode is supplied with power using a simple wiring. Needless to say, it is desirable that the feeding position for the resonating electrode such as the straight electrode portion 4 is a position where the impedance coincides with that of the feeding circuit in terms of the measurement of the resonance frequency using the network analyzer.

  In the standing wave resonance electrode as shown in FIG. 2 (a) or (c), it is shown by Non-Patent Document 1 that impedance matching with any feed line is possible by selecting the feed position. ing. For example, in a resonance electrode with both ends open as shown in FIG. 2A, the power supply position is slightly shifted from the center of the resonance electrode, and impedance matching with a power supply line having an impedance of 50Ω is obtained, thereby providing good power supply efficiency. .

  Since the size of the resonant electrode used in the present invention is smaller than that of a traveling wave electrode as shown in Patent Document 3, the area occupied on the wafer is small, and an optical modulator obtained from a single wafer ( The number of optical waveguide elements) can be increased. Also, since the size is small, it is possible to arrange a plurality of resonant electrodes, and it is also possible to evaluate and manage the refractive index of the electrode corresponding to various shapes (thickness, width, etc.) of the electrode It is.

  A method for measuring the refractive index of the electrode with respect to microwaves using the above-described resonance electrode for inspection will be described. The measurement of the refractive index of the resonance electrode for inspection may be performed in the state of a wafer, or may be performed after cutting the wafer into chips. Even if a defective electrode is found by inspection in the wafer state, it may be possible to adjust the shape of the electrode such as adjusting the gold plating processing time.

  An inspection end (probe) is connected to the power supply wiring portion of the inspection resonance electrode, and the resonance frequency of the resonance electrode is measured using a network analyzer. Further, the length L of the resonance electrode for inspection is measured under a microscope, or is calculated from the pattern length of the photomask and the projection rate of photolithography. From these two values (resonance frequency and resonance electrode length), the refractive index of the resonance electrode with respect to the microwave can be obtained.

Here, for simplicity of explanation, an example of a standing wave resonant electrode (see FIG. 2A or 2B) having a simple configuration will be described. As shown in FIG. 2A, in the case of a resonant electrode whose electrode ends are electrically open, the electrode length L is equal to the wavelength Λ _m of the microwave on the electrode, as shown in Equation 1 below. At a frequency that is 1/2 × n times (n is a natural number), a standing wave stands and enters a resonance state.
L = Λ _m × n / 2 (Formula 1)

On the other hand, as shown in FIG. 2B, in the case of a resonant electrode in which the electrode ends are electrically short-circuited, the electrode length L is equal to the wavelength Λ of the microwave on the electrode, as shown in Equation 2 below. Resonance occurs at a frequency that is 1/2 × n times _m (n is a natural number).
L = Λ _m × 1/2 · n (Expression 2)

The wavelength lambda _m the microwave to form a standing wave in the resonance type electrode, when the speed of a microwave propagating through the electrode v _m, the frequency is f _m, relationship of Equation 3 below.
v _m = f _m × Λ _m (Equation 3)
There is a relationship.

Further, the microwave velocity v _m has a relationship represented by the following formula 4 where the refractive index of the microwave of the electrode is _nm and the light velocity c _{0 in} vacuum.
v _m = c ₀ / n _m (Formula 4)

By using Equations 3 and 4, Equations 1 and 2 described above can be rewritten as Equation 5 below.
L = c ₀ / n _m / f _m × n / 2 (Formula 5)

As seen from equation 5, by measuring the resonant frequency f _m and the length L of the resonance electrodes, the refractive index n _m of the microwave resonance electrode is naturally obtained. Basically, when the resonance order n is known, the refractive index of the inspection electrode can be obtained by measuring one inspection electrode. In fact, the refractive index n _m of the electrode, since known roughly, determination of the resonant order n, the calculation of the refractive index can be easily performed. In addition, the refractive index of the electrode calculated | required here is a refractive index in a specific resonant frequency.

The measurement accuracy when measuring the refractive index of an electrode using the wafer with electrodes for inspection of the present invention will be described. Here, it is assumed that an electrode having a length of several millimeters to several tens of millimeters is used and a resonant frequency of several tens of GHz is used. The length L of the standing wave resonance electrode can be easily measured with an accuracy of 4 digits or more under a microscope. The measurement of the resonant frequency f _m is measured by a commercially available network analyzer can be measured in 5 or more digits precision. Therefore, the refractive index of the electrode can be obtained with an accuracy of 4 digits or more. As can be seen from the basic equation (Equation 1 and Equation 2) of the resonance condition, a refractive index with higher accuracy can be measured by using a measurement value having a higher resonance order n and a higher resonance frequency.

By the way, in principle, the refractive index of an electrode includes the dispersion of the dielectric constant of the material of the electrode and the wafer and the dispersion of the refractive index due to the electrode structure. Different. However, an optical modulator widely used in long-distance optical communication uses LiNbO ₃ as a wafer and gold (Au) as an electrode, but LiNbO ₃ and gold have a range of 1 GHz to several hundred GHz, There is almost no dispersion up to at least 100 GHz, and there is no need to consider the frequency dependence of the refractive index of the electrodes used in the optical modulator.

Furthermore, it is necessary to consider dispersion due to the mode of the microwave propagating through the electrode. For example, when LiNbO ₃ having a thickness of 0.5 mm is used for the wafer, it is known that there is almost no frequency dependence at 70 GHz or less. (See Non-Patent Document 2), the calculated value may be used as it is.

In actual electrode process control, it is preferable that the cross-sectional configuration of the product electrode (acting portion) and the cross-sectional configuration of the resonant electrode for inspection are the same, and the extending direction is also matched. In this case, the measured refractive index of the resonance electrode for inspection is the same as the refractive index of the product electrode. In particular, in the case of a material having dielectric anisotropy such as LiNbO ₃ , it is necessary to align the orientation of the electrode with the product and the inspection pattern.

It is also possible to preliminarily calculate the refractive index, electrode length, resonance frequency, and electrode thickness of the test pattern according to the design shape of the electrode and use it as a management index. For reference, the refractive index n of the microwave is used for the LiNbO ₃ Z-plate as a wafer and for the open-ended resonant electrode in FIG. 2A and the short-circuited resonant electrode in FIG. the relationship between the resonance electrode length L giving _m = 2.20 and the resonance frequency _{f m,} shown in FIGS.

  When the refractive index of the electrode is obtained, if it is sufficient to have an accuracy of three digits, it may be obtained simply from the resonance condition equation (Equation 1, 2, etc.) of the resonance electrode. In the case of obtaining with an accuracy of 4 digits or more, as described below, a value with higher accuracy can be obtained by considering the difference between the position of the end of the electrode and the position of the end felt by the microwave.

  As shown in Non-Patent Document 3, at both ends of the resonance electrode, the microwaves are not reflected at the end portions of the electrodes, but can be regarded as effectively reflecting further from the end portions. . In other words, since the effective length for the microwave is slightly longer than the physical length of the electrode, the change in the effective length for the microwave and the phase during reflection is increased by this amount of oozing, which is a strict evaluation. In some cases, the difference needs to be corrected. The effective leaching amount depends on the configuration of the end of the electrode, and in the case of a coplanar electrode formed on a lithium niobate wafer, it is about 5 to 20 μm. Can be obtained.

  For example, when a resonance electrode having an electrode length L of 10 mm is used, the precision of the fourth digit may be affected, and it is necessary to correct the amount of seepage according to the structure of the end portion of the inspection electrode. Of course, if it is sufficient to manage the accuracy of three digits, it can be ignored. Further, the use of an electrode pattern with both ends short-circuited is simple in handling because the effective amount of oozing is small and the actual length is close to the effective length felt by the microwave.

  As described above, according to the present invention, the refractive index of the electrode can be measured easily and accurately in the manufacturing process of the traveling wave optical modulator, and the inspection electrode required for the inspection is smaller. It is possible to provide a wafer with an inspection electrode that can be used as an electrode pattern and a method for measuring the refractive index of the electrode.

DESCRIPTION OF SYMBOLS 1 Wafer 2 Optical waveguide 3, 31 Resonance electrode 4 for inspection Linear resonance electrode part 5 Power supply part 6 Ground electrode 7 Ring-shaped resonance electrode part 10 Chip (optical modulator)
20 Dummy waveguide L Resonant electrode length

Claims (4)

For wafers with electro-optic effect,
A plurality of optical waveguides;
A plurality of traveling-wave control electrodes for controlling light propagating through the optical waveguide by advancing an electrical signal between the electrodes, wherein a signal electrode and a ground electrode are disposed along the optical waveguide;
In a wafer with an inspection electrode having an inspection electrode formed on a part of the wafer,
The inspection electrode includes a resonance electrode extending in the same extending direction as the signal electrode ,
Furthermore, the resonant electrodes, the test electrodes with the wafer, wherein Rukoto comprises a signal electrode and the same cross-sectional shape. The wafer with an inspection electrode according to claim 1 ,
The signal electrode has a plurality of different cross-sectional shapes;
A wafer with an inspection electrode, wherein the inspection electrode is provided for each type of the cross-sectional shape. In the wafer with an inspection electrode according to claim 1 or 2 ,
A wafer with an inspection electrode, wherein a dummy optical waveguide is formed in the vicinity of the resonant electrode, as in the arrangement relationship between the signal electrode and the optical waveguide. The method for measuring a refractive index of an electrode of a wafer with an inspection electrode according to any one of claims 1 to 3 ,
An electrical signal is input to the resonant electrode, a resonant frequency is measured, and a refractive index of the resonant electrode with respect to the electrical signal is obtained based on the length of the resonant electrode and the resonant frequency. A method of measuring a refractive index of an electrode of a wafer with an inspection electrode.

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