KR101566383B1 - Reflection-type Interference Apparatus using Optical Fibers for Measuring Geometrical Thickness and Refractive index - Google Patents

Abstract

The present invention provides a thickness measuring apparatus and a thickness measuring method. The thickness measuring optical device includes: a broadband laser light source oscillating at a plurality of wavelengths; A broadcaster for outputting the output light of the broadband laser light source to a second port through a first port and outputting light to a third port; A light splitter for transmitting a part of the light output from the second port of the optical circulator and reflecting the remaining light; And a mirror for reflecting light provided from the light splitter. An object to be measured is disposed between the optical splitter and the mirror, and the object to be measured reflects a part of the light transmitted through the optical splitter and transmits the remaining part to the mirror.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a reflection type optical fiber interferometer for measuring geometric thickness and refractive index,

The present invention relates to an optical apparatus for measuring an optical thickness of an object to be measured. More specifically, by using a broadband pulsed laser light source, it is possible to calculate an optical thickness with high accuracy by a simple operation than a calculation system using a continuous oscillation laser light source Technology.

BACKGROUND ART [0002] In general, as the display industry, optical communication, and precision optical devices are continuously developed, accurate measurement and evaluation techniques for properties (optical thickness, thickness, and refractive index) of a measurement object such as a wafer are required. A system has been developed to measure the thickness and refractive index of a material in a variety of ways.

Recently, a semiconductor substrate is polished on the back side of a substrate for mounting. These polished substrates are stacked and mounted on each other. Therefore, measurement of the thickness of the polished substrate is required. Further, the display element is formed on a glass substrate or a substrate made of a flexible material. Therefore, monitoring of the thickness of the glass substrate is required.

The material thickness and refractive index calculation system according to continuous oscillation laser projection has a small peak power. Therefore, the system for calculating the material thickness and the refractive index according to the continuous oscillation laser projection has lower visibility and signal to noise of the interference signal as compared with the system using the pulse laser. Therefore, the system for calculating the material thickness and the refractive index according to the continuous oscillation laser projection has a problem that the measurement of the thickness and the refractive index is impossible or the accuracy is low.

In addition, the continuous wide-band IR light source has a problem that it is difficult to calculate its characteristics by collimating the object with a wide measurement area because the space is not coherent.

In addition, since the continuous wide-band IR light source also has a small coherence distance when time coherence is reduced, interference signals of a silicon wafer having a large refractive index may be difficult to obtain.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a thickness measuring optical apparatus having a mechanical stability, a compact structure, and a high signal-to-noise ratio.

A thickness measuring optical apparatus according to an embodiment of the present invention includes: a broadband laser light source oscillating at a plurality of wavelengths; A broadcaster for outputting the output light of the broadband laser light source to a second port through a first port and outputting light to a third port; A light splitter for transmitting a part of the light output from the second port of the optical circulator and reflecting the remaining light; And a mirror for reflecting light provided from the light splitter. An object to be measured is disposed between the optical splitter and the mirror, and the object to be measured reflects a part of the light transmitted through the optical splitter and transmits the remaining part to the mirror.

In an exemplary embodiment of the present invention, the apparatus further includes a parallel optical lens that is disposed in the second port of the optical circulator and the optical splitter, and converts the light output from the second port into parallel light and provides the parallel light to the optical splitter can do.

In an embodiment of the present invention, the optical circulator may be an optical fiber circulator.

In one embodiment of the present invention, the apparatus may further comprise a spectrum analyzer connected to the third port of the broadcaster. The spectrum analyzer may measure an interference signal according to a wavelength of the wideband laser light source.

In an exemplary embodiment of the present invention, a processing unit may be provided for receiving the output signal of the spectrum analyzer, subjecting the output signal to Fourier transform, and extracting a light path difference caused by the measurement object.

In one embodiment of the present invention, the broadband laser light source may be a femtosecond pulse laser.

In one embodiment of the present invention, the apparatus may further include a movement stage for moving the measurement object.

In one embodiment of the present invention, there is provided an optical switch, comprising: an optical switch that receives light from a second port of the optical fiber circulator and successively switches over time; and a switch that converts light output from a plurality of output terminals of the optical switch into parallel light And a plurality of parallel optical lenses provided to the optical splitter.

In one embodiment of the present invention, the wavelength of the broadband laser light source may be an infrared band.

A thickness measuring apparatus according to an embodiment of the present invention includes: a broadband laser light source oscillating at a plurality of wavelengths; A broadcaster for outputting the output light of the broadband laser light source to a second port through a first port and outputting light to a third port; A light splitter for transmitting a part of the light output from the second port of the optical circulator and reflecting the remaining light; A mirror for reflecting light provided from the light splitter; And a spectrum analyzer for measuring the light output from the second port of the broadcaster according to the wavelength. An object to be measured is disposed between the optical splitter and the mirror, and the object to be measured reflects a part of the light transmitted through the optical splitter and transmits the remaining part to the mirror. The optical thickness of the measurement object is directly observed.

According to an embodiment of the present invention, there is provided a thickness measurement method including: transmitting broadband laser light oscillating at a plurality of wavelengths simultaneously to a second port through a first port of an optical circulator; Providing a broadband laser light provided through a second port of a broadcaster to a sequentially arranged parallel optical lens, a light splitter, a measurement object, and a mirror; Measuring light reflected from the optical splitter, the object to be measured, and the mirror through a spectrum analyzer connected to the third port of the parallel optical lens and the optical circulator; Fourier transforming the interference signal measured through the spectrum analyzer; And extracting information on the light thickness of the measurement object directly from the Fourier transformed signal.

The optical device according to an embodiment of the present invention can provide a compact structure using an optical fiber optical system, and can calculate a stable and precise optical thickness. More specifically, the optical thickness can be decomposed into a thickness and a refractive index.

The optical device according to an embodiment of the present invention can produce stable and precise optical thickness within 0.1 second.

The thickness measurement range of the optical device according to one embodiment of the present invention may be 0 to 300 mm. The measurement uncertainty of the thickness may be within 100 nm. In addition, the contact-type method and the measuring method of the present invention showed the same results in the range of 0.1 μm or less.

1 is a view for explaining a multi-wavelength Michelson interferometer.
2 is a view for explaining an interference signal and a Fourier transform of a multi-wavelength Michelson interferometer.
3 is a view for explaining a multi-wavelength Michelson interferometer for measuring an object to be measured.
4 is a diagram illustrating Fourier transform of the multi-wavelength Michelson interferometer of FIG.
5 is a conceptual diagram illustrating a thickness measuring optical apparatus according to an embodiment of the present invention.
Fig. 6 is a result of Fourier transforming the interference signal of the optical device of Fig.
7 is a conceptual diagram illustrating a thickness measuring optical apparatus according to another embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following drawings, like reference numerals refer to like elements, and the size of each element in the drawings may be exaggerated for clarity and convenience of explanation.

The present inventor has proposed an interferometer for simultaneous measurement of the thickness and refractive index of a silicon wafer using a femtosecond pulse laser in Korean Patent (KR 10-1105449).

Michelson interferometers can measure length using the principle of interference of light. The light from the light source is split through a beam splitter into a reference path and a measurement path.

 The phase of the interference signal is given as a function of the optical path difference between the reference path and the measurement path with respect to the optical splitter. The interference signal fluctuates periodically every time the optical path difference becomes half the wavelength. Therefore, due to the ambiguity of the phase, when the length measurement is performed, the number of interference signals can be detected while changing the measurement path.

1 is a view for explaining a multi-wavelength Michelson interferometer.

2 is a view for explaining an interference signal and a Fourier transform of a multi-wavelength Michelson interferometer.

1 and 2, a multi-wavelength interferometer 100 uses a broadband light source 110 and a Michelson interferometer, and an interference signal can be measured for each wavelength through a spectrum analyzer 150. The interference signal I (z, t) according to the wavelength is Fourier transformed. The amplitude of the Fourier transform of the interference signal may have a peak in the spatial frequency domain. At a spatial frequency corresponding to the position of the peak, the phase of the peak may provide information about the light path difference.

Here, I (z, f) is an interfering signal, I ₀ is the background light signal, z is the optical path of the reference path and measurement path coach, c is the speed of light in a vacuum, f is the frequency of light. Thus, an interference signal according to the frequency of the broadband light source is obtained. L 1 is the distance between the light splitter 120 and the reference mirror 140 and L 2 is the distance between the light splitter 120 and the measurement mirror 130. z is the optical path difference. n is the refractive index of the medium.

Meanwhile, the interference signal is measured according to the position (corresponding to the frequency) by the spectrum analyzer 150. The Fourier transform of the interference signal may have a peak at a predetermined spatial frequency. The predetermined spatial frequency may be expressed by the optical path difference z . Therefore, the spatial frequency SF_A at the peak can provide the optical path difference z .

3 is a view for explaining a multi-wavelength Michelson interferometer for measuring an object to be measured.

4 is a diagram illustrating Fourier transform of the multi-wavelength Michelson interferometer of FIG.

Referring to FIGS. 3 and 4, the multi-wavelength michelson interferometer 100a can measure the refractive index and thickness of the measurement object 10. The light source 110 may be a femtosecond pulse laser. The femtosecond pulse laser can oscillate in multiple modes. In addition, the light source may operate in a pulsed mode. Thus, the peak power of the light source can be increased.

The measurement target 10 can be placed in the measurement path. In this case, various optical path differences ( A, B, C, D ) may occur. Therefore, the Fourier transform of the interference signal shows a plurality of peaks P_A, P_B, P_C, and P_D in the spatial frequency domain. The peaks may be due to optical path differences. Specifically, the first spatial frequency SF_A of the first peak P_A corresponds to a path between the reference path (path L1 between the optical splitter and the reference mirror) and the measurement path (path between the optical splitter 120 and the measurement mirror 140) ( L2 = L1- L2 ) between the first optical path difference ( A = L2-L1 ). That is, the measurement of the interference signal is required in a state in which the measurement object is removed.

The second to fourth peaks P_B, P_C and P_D can be obtained through Fourier transform of the interference signal in a state where the object to be measured is arranged. The second spatial frequency SF_B of the second peak P_B is the difference between the second optical path difference B between the path L3 between the optical splitter 120 and the front surface of the measurement target 10 and the reference path L1 = L3-L1 ).

The third light between the third spatial frequency (SF_C) is the path (L3 + n T) between the rear surface of the beam splitter 120 and the measuring object 10 and the reference path (L1) of the third peak (P_C) Path difference ( C = L3 + nT-L1) .

The fourth spatial frequency SF_D of the fourth peak P_D is the sum of the optical path difference between the measurement path (path between the optical splitter and the measurement path mirror L2 ) and the reference path (path between the optical splitter and the reference mirror L1 ) It can be given to the (L2-L1) to the difference (D = L2-L1- (n -1) T) of the net path difference ((n-1) T) due to the thickness of the object to be measured. The fourth optical path difference D can be given by D = L2-L1- (n-1) T.

The first through fourth spatial frequencies SF_A, SF_B, SF_C and SF_D may be converted into the first through fourth optical path differences A, B, C and D , respectively.

Accordingly, the thickness T of the measurement target 10 and the refractive index n of the measurement target 10 can be given as follows.

In order to obtain the thickness T of the measurement target 10, an operation such as Equation 2 is required. The above operation increases the error or uncertainty. In addition, the Michelson interferometer structure has difficulty in aligning the reference mirror 130 and the measurement mirror 140. Therefore, the Michelson interferometer is also susceptible to vibration. The Michelson interferometer structure occupies a lot of space. Therefore, the Michelson interferometer structure is not easy for industrial applications. Accordingly, the present invention proposes a spectral interferometer of a new structure.

To overcome the difficulty of stable operation and alignment, the measuring mirrors were removed. Also, using the optical fiber, the reference path and the measurement path were designed to partially overlap each other. This has increased the space utilization of the measuring device.

The optical device according to the embodiment of the present invention can directly provide the Fourier transformed peak due to only the optical path difference of the object of measurement without operating it. Accordingly, the optical path difference or the optical thickness of the measurement object can be calculated at a high speed.

5 is a conceptual diagram illustrating a thickness measuring optical apparatus according to an embodiment of the present invention.

Fig. 6 is a result of Fourier transforming the interference signal of the optical device of Fig.

5 and 6, the thickness measuring optical system 200 includes a broadband laser light source 210 that oscillates at a plurality of wavelengths simultaneously, and a first port 220a that outputs light from the broadband laser light source 210 A second port 220b of the optical circulator 220 and a second port 220b of the optical circulator 220. The optical transmitter 220 includes a first port 220b and a second port 220b, And a mirror 230 for reflecting the light provided from the light splitter 244. The mirror 230 reflects the light from the light splitter 244, The measurement target 10 is disposed between the optical splitter 244 and the mirror 230. The measurement target 10 reflects a part of the light transmitted through the optical splitter 244 and transmits the remaining light, (230).

The broadband laser light source 210 may be a femtosecond laser or a broadband light source. The interference distance of the broadband laser light source 210 may be greater than the distance between the optical splitter 244 and the mirror 230. Accordingly, the light reflected by the light splitter 244 can interfere with the light reflected from the mirror. The wavelength of the broadband laser light source 210 may be a visible light band or an infrared band. The frequency FWHM of the gain curve of the broadband laser light source 210 may generate at least two sine waves according to the wavelength. The broadband laser light source 210 may be pulsed by a mode locking means or a swithcing means. Also, the wideband laser light source 210 may output an optical frequency comb using an optical resonator. The pulse operation of the broadband laser light source 210 may increase the peak output. Accordingly, the broadband laser light source may provide a broadband light having a plurality of frequency modes.

 The optical circulator 220 receives the output light of the broadband laser light source 210 through the first port 220a and outputs the output light to the second port 220b and the input signal of the second port 220b to the third port 220b, (220c). The optical circulator 220 may be an optical fiber circulator. Accordingly, the first port 220a may be connected to the broadband laser light source 210 through an optical fiber. The second port 220b may be connected to the parallel optical lens 242 or the optical splitter 244 through an optical fiber. The third port 220c may be coupled to the spectrum analyzer 250 through an optical fiber.

The parallel optical lens 242 may be a convex lens. One end of the optical fiber connected to the second port may be disposed at the focal point of the parallel optical lens 242. Accordingly, the light output from the optical fiber can be converted into parallel light through the parallel optical lens 242.

According to a modified embodiment of the present invention, a beam size conversion unit (not shown) may be disposed at the rear end of the parallel optical lens 242. The beam size conversion unit may reduce or enlarge the space between the beams.

The collimated light formed through the parallel optical lens 242 may be provided to the light splitter 244. The light splitter 244 may be a half-silvered mirror. The light splitter 244 may be formed through a metal coating on a transparent parallel plate. The light splitter 244 can transmit a part of the input light and reflect the other.

The light transmitted through the light splitter 244 may be reflected by the mirror 230. The mirror 230 may be a flat mirror.

One end of the optical fiber, the parallel optical lens, and the light splitter may be integrally formed. Thus, optical alignment can be facilitated.

The object to be measured 10 may be disposed between the light splitter 242 and the mirror 230. The measurement target 10 may be a silicon semiconductor substrate, a glass substrate, or a plastic substrate. The measurement object 10 may be a material through which the output light of the broadband laser light source 210 can be transmitted.

The spectrum analyzer 250 can simultaneously analyze the interference signals at a plurality of wavelengths by decomposing the input light into wavelengths. Hence, the measurement time at one measurement position may be less than 0.1 second since it does not scan the frequency of light.

When the measurement object 10 is removed between the optical splitter 242 and the mirror 230, a part of the light incident on the optical splitter 244 may be reflected to provide reference light. In addition, the light transmitted through the optical splitter 244 may be reflected by the mirror 230 to provide measurement light. The reference light and the measurement light may be provided to the spectrum analyzer 250 through the parallel optical lens and the optical circulator. The spectrum analyzer 250 may measure an interference signal due to the reference light and the measurement light. The interference signal can be measured at the same wavelength or frequency through a two-dimensional optical sensor. That is, the position of the two-dimensional optical sensor may correspond to the frequency (or wavelength) of the light source. Accordingly, the interference signal can be expressed by the intensity of the light according to the position (frequency of the light source) of the optical sensor. The interference signal may be converted into a digital signal and provided to the processing unit 270. The processing unit 270 may Fourier transform the digital interference signal in the spatial frequency domain. Accordingly, the amplitude of the Fourier-transformed interference signal can show the reference peak P_AA at a specific spatial frequency. The spatial frequency SF_AA showing the reference peak P_AA may depend on the optical path difference 2L between the reference light and the measurement light. L is the distance between the light splitter and the mirror.

When the measurement object 10 is disposed between the light splitter 244 and the mirror 230, a part of the light incident on the light splitter 244 may be reflected to provide the first reflected light W1 . The light transmitted through the light splitter 244 may be reflected on the front surface of the measurement target 10 to provide the second reflected light W2. In addition, the third reflected light W3 may be reflected on the rear surface of the object to be measured. The fourth reflected light W4 may be formed by being reflected from the mirror. The first through fourth reflected lights W1 through W4 may be provided to the spectrum analyzer 250 through the parallel optical lens 242 and the optical circulator 220. [ The first to fourth reflected lights W1 to W4 may form an interference signal. The interference signal due to the first reflected light and the fourth reflected light may show a modification reference peak (P_BB) in the spatial frequency domain. The position (SF_AA) of the reference peak (P_AA) may be different from the position (SF_BB) of the modification reference peak (P_BB). The position (P_BB) of the modification reference peak can be changed by a path difference of (n-1) T by the measurement object. n is the refractive index of the object to be measured. T is the thickness of the object to be measured. When the measurement object 10 is present, the interference signal may be Fourier transformed to show a plurality of peaks in the spatial frequency domain. The position of the peaks may depend on the structure of the interferometer and the object to be measured.

According to an embodiment of the present invention, the measurement target peak (P_WF) due to interference between the light reflected from the front surface of the measurement object and the light reflected from the back surface of the measurement object in the spatial frequency domain can be directly observed.

In the case of the Michelson interferometer structure described above, the Fourier transform of the interfering signal exhibits multiple peaks in the spatial frequency domain. However, the peak due to the interference between the light reflected from the front surface of the measurement object and the light reflected from the back surface of the measurement object was not directly observed.

However, in the case of the optical circulator structure, the Fourier transform of the interference signal can be obtained by measuring a peak to be measured P_WF due to interference between the light reflected from the front surface of the measurement target 10 and the light reflected from the back surface of the measurement target 10 see. The measurement target spatial frequency at the measurement target peak (P_WF) is SF_WF. The measurement target spatial frequency SF_WF is a result of an interference signal due to reflection on the front surface and the rear surface of the measurement object. The measurement target spatial frequency SF_WF may depend on the optical path difference ( 2nT ) between the front surface and the rear surface of the measurement object. Thus, the optical path difference of the measurement target 10 can be easily measured. Further, by using the spatial frequencies of other peaks, the refractive index and thickness of the measurement object can be calculated.

Also, the measurement target peak P_WF may have an intensity level equivalent to the correction reference peak P_BB. Therefore, a high signal-to-noise ratio can be obtained. In addition, when only the optical thickness is observed, the reference spatial frequency SF_AA measured in a state in which the measurement object is removed is not required. Therefore, the measurement time can be reduced.

If the refractive index and the thickness of the measurement target 10 are known, the position of the measurement target peak and the position of other peaks can be predicted in the spatial frequency region. Therefore, when the position of the measurement subject peak overlaps with another peak at the same position, the position of the light splitter 244 or the position of the mirror or the position of the mirror can be changed so that the peaks can be separated from each other.

The measurement target moving unit 260 can change the position of the measurement target 10. Accordingly, the measurement position of the measurement object can be scanned. The measurement subject movement unit 260 may be a two-axis movement stage. The measurement object moving unit 260 may be controlled by the processing unit 270. [

The processing unit 270 may be a computer. The processing unit may receive the measurement signal of the spectrum analyzer, perform Fourier transform, and process the Fourier-transformed signal to calculate the thickness, thickness, or refractive index of the measurement object.

According to one embodiment of the present invention, the thickness measurement range may be 0 to 300 mm. The measurement uncertainty of the thickness may be within 100 nm. In addition, the contact-type method and the measuring method of the present invention showed the same results in the range of 0.1 μm or less.

7 is a conceptual diagram illustrating a thickness measuring optical apparatus according to another embodiment of the present invention. A description overlapping with that described in Fig. 5 is omitted.

Referring to FIG. 7, an optical switch 280 may be disposed between the second port 220b of the optical circulator 200 and the parallel optical lens 242. The optical switch 280 may time-divide the optical signal input to the input port 280a and sequentially provide the optical signal to the plurality of output ports 280b. A parallel optical lens 242 may be disposed at each output end of the optical switch. Accordingly, the optical device 200a can measure optical thickness, thickness, or refractive index by time-division at a plurality of positions without mechanical scanning operation. The parallel optical lens 242 may have a microlens array structure. Each lens may be connected to an output end of the optical switch through an optical fiber. The optical switch 280 can use an optical fiber as an input port and an output port.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Accordingly, the true scope of the present invention should be determined by the technical idea of the appended claims.

210: broadband laser light source
220: Broadcasters
230: mirror
242: Parallel optical lens
244:
250: spectrum analyzer
270:

Claims (8)

A broadband laser light source oscillating at a plurality of wavelengths;
A broadcaster for outputting the output light of the broadband laser light source to a second port through a first port and outputting light to a third port;
A light splitter for transmitting a part of the light output from the second port of the optical circulator and reflecting the remaining light;
A mirror for reflecting light provided from the light splitter;
An optical switch for receiving light from a second port of the optical circulator and sequentially switching over time;
And a plurality of parallel optical lenses for respectively converting light output from a plurality of output ends of the optical switch into parallel light and providing the parallel light to the optical splitter,
A measurement object is disposed between the optical splitter and the mirror,
Wherein the measurement object reflects a part of the light transmitted through the optical splitter and transmits the remaining part to the mirror. The method according to claim 1,
And a parallel optical lens arranged in the second port of the optical circulator and the optical splitter to convert the light output from the second port into parallel light and to provide the light to the optical splitter, . The method according to claim 1,
Wherein the optical circulator is an optical fiber circulator. The method according to claim 1,
Further comprising a spectrum analyzer coupled to a third port of the broadcaster,
Wherein the spectrum analyzer measures an interference signal according to a wavelength of the broadband laser light source. 5. The method of claim 4,
And a processing unit that receives the output signal of the spectrum analyzer and performs Fourier transform on the output signal, and extracts a light path difference due to the measurement object. delete delete Transmitting broadband laser light oscillating at a plurality of wavelengths simultaneously to a second port through a first port of the optical circulator;
Receiving light from a second port of the optical circulator and successively switching the optical switch using an optical switch according to time;
Providing a broadband laser light provided through a second port of the optical circulator and the optical switch to a sequentially arranged parallel optical lens, a light splitter, a measurement object, and a mirror;
Measuring light reflected from each of the optical splitter, the object to be measured, and the mirror through a spectrum analyzer connected to the third port of the parallel optical lens and the optical circulator;
Fourier transforming the interference signal measured through the spectrum analyzer;
And directly extracting information on the light thickness of the measurement object from the Fourier transformed signal,
Wherein the plurality of parallel optical lenses convert the light output from the plurality of output ends of the optical switch into parallel light and provide the parallel light to the optical splitter, respectively .

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