JP2993836B2 - Interferometer using coherence degree - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer and, more particularly, to an interferometer that can measure the surface shape of a surface having a step by measuring the degree of coherence of interference fringes.

[0002]

2. Description of the Related Art Interferometry has recently been improved in its measurement accuracy.
High-precision measurement of about 1/0 wavelength is possible.
However, in the interference measurement using a normal laser beam, in a portion having a discontinuous surface having a step as shown in (1) of FIG. 9, interference fringes at the discontinuous portion are shown in (2) of FIG.
As shown in (1), there is a problem that the size of the step cannot be specified and the surface shape cannot be completely measured.

[0003] In order to solve such a problem, there is an interference measurement method called a two-wavelength method using light of two adjacent wavelengths. The two-wavelength method generates a longer wavelength called a synthetic wavelength from two types of wavelengths used for measurement to expand the measurement range, and a method of generating interference fringes from a phase measured using two wavelengths of light. There is a method for determining the fringe order. By using the two-wavelength method, it is possible to expand the measurement range to a range of several tens of wavelengths. If the fringe order of the interference fringes is determined from the phase, ordinary interference measurement can be performed at each wavelength. Similar measurement accuracy is obtained.

[0004]

In the interference measurement method using the two-wavelength method, a light source that outputs light of two adjacent wavelengths is required. As such a light source, for example, two types of interference filters having different center wavelengths and a narrow bandwidth are generally used. In this case, it is necessary to replace the interference filters used during the measurement. It is not preferable to replace the interference filter manually in order to automate the measurement, and there is a problem in that the replacement of the interference filter requires a complicated mechanism.

The present invention has been made in view of the above problems, and has as its object to realize an interferometer capable of measuring the shape of a stepped surface without adding a complicated mechanism.

[0006]

FIG. 1 is a diagram for explaining the principle configuration of an interferometer according to the present invention. In FIG. 1, reference numeral 1 denotes a light source, which emits light having a predetermined degree of coherence. Reference numeral 2 denotes a collimator lens that converts light from the light source 1 into parallel light, 3 denotes a beam splitter that divides the light into two light beams, and combines and returns the reflected light and returns.
Denotes a reference surface, and 6 denotes a surface of the object to be measured, and these form an interference optical system. Reference numeral 7 denotes a photoelectric conversion unit that converts an optical signal of the formed interference fringes into an electric signal. Reference numeral 8 denotes a coherence that processes a signal from the photoelectric conversion unit 7 and detects a coherence degree of the interference light from the interference fringes on the detection surface. Degree detecting means 9 stores the degree of change in the coherence degree of the interference fringe with respect to the change in the optical path length difference between the optical path on the surface 6 of the object to be measured and the optical path on the reference surface 4 measured in advance. Degree change storage means 10 corresponds to the difference in coherence degree at a plurality of detection points on the detection surface detected by the coherence degree detection means 8 according to the degree of change of the coherence degree stored in the coherence degree change storage means 9. This is an optical path length difference calculating unit that calculates an optical path length difference at a detection point.

The light source 1 desirably has a coherence degree such that the degree of change of the coherence degree on the detection surface is such that the optical path difference becomes maximum near several wavelengths. The coherence degree detecting means 8 includes a sine wave oscillating means for sine wave oscillating the position of the reference surface 4 or the surface 6 of the device under test, and a Fourier transform calculating means for performing a Fourier transform on the output of the photoelectric conversion means 7. It is desirable to accurately detect the amplitude of the sinusoidal vibration means from the calculation result of the calculation means. FIG. 1 shows an example in which a sine wave oscillating means 5 for sine wave oscillating the position of the reference surface 4 is provided as the sine wave oscillating means.

The coherence degree detecting means detects the visibility of the interference fringes which can be easily detected instead of the coherence degree, assuming that the reflected light from the surface 6 of the object to be measured on the detection surface is equal over the entire measurement surface. You may. In order to measure the degree of complex coherence, there is provided an optical path blocking means for independently blocking an optical path on the surface 6 of the object to be measured and an optical path on the side of the reference surface 4 of the interference optical system. The intensity and the light intensity on the reference surface side are measured, and the coherence degree is calculated from the visibility of the interference fringes, the light intensity on the object side, and the light intensity on the reference surface side.

The optical path length difference calculating means 10 includes means for analyzing interference fringes on the detection surface, and calculates the coherence difference between two or more portions where the interference fringes are discontinuous on the detection surface of the interference optical system. In accordance with the degree of change of the coherence degree stored in the degree change storage means 9, it is determined from the difference in coherence degree on the detection surface detected by the coherence degree detection means, and the optical path length difference is calculated from the fringe order and the phase of the interference fringe.

The interference fringe analyzing means includes sine wave oscillating means for sine wave oscillating the position of the reference surface 4 or the surface 6 of the object to be measured, and Fourier transform calculating means for performing Fourier transform on the output of the photoelectric conversion means 7. The amplitude and phase of the sine wave vibration means are accurately detected from the calculation result of the Fourier transform calculation means. These sine wave oscillating means and Fourier transform calculating means can be shared with those of the coherence degree detecting means.

The photoelectric conversion means is preferably a one-dimensional photosensor in which a plurality of light-receiving elements are arranged in a one-dimensional direction, or a two-dimensional image pickup means. Desirably, it is imaging means.

[0012]

The coherence of interference fringes on the detection surface is 2
It changes according to the optical path length difference of the optical path. FIG. 2 is a diagram illustrating an example of how the coherence degree of the interference fringes on the detection surface changes with respect to the optical path length difference between the two optical paths. As shown in FIG. 2, if the degree of coherence of the light source is high, the change in the coherence of the interference fringe with respect to the optical path length difference is gradual. Is steep. For example, when the light source is constituted by a white light source and an interference filter, the smaller the wavelength width of the interference filter is, the higher the coherence degree of the light source is, and the larger the wavelength width of the interference filter is, the lower the coherence degree of the light source is.

Therefore, the degree of change in the coherence of the interference fringes with respect to the change in the optical path length difference is measured and stored in advance, and the optical path length difference between the two optical paths can be determined by associating the measured coherence degree of the interference fringes. . Since it is difficult to determine the optical path length difference between the two optical paths with high accuracy only by the coherence degree of the interference fringes, the coherence degree of the interference fringes on both sides of the step is used together with the interference fringe analysis means in a normal interferometer. The optical path length difference calculated from the difference is used only for determining the difference in the order of the interference fringes, and the detection of a minute optical path length difference larger than that is performed by interference fringe analysis means. By doing so, the detection range can be expanded while maintaining the detection accuracy.

Further, the coherence degree of the interference fringes on the detection surface is measured by subjecting the position of the reference surface 4 or the surface 6 of the device under test to a sine wave oscillation, and then performing a Fourier transform on the optical signal of the interference fringes at the measurement point. It is desirable to calculate the sine wave oscillation means and the Fourier transform operation means required for this, since the one used in the interference fringe analysis means in a normal interferometer can be shared, so that the apparatus may be complicated. Absent.

[0015]

FIG. 3 is a diagram showing the structure of an interferometer according to a first embodiment of the present invention. In FIG. 3, reference numeral 11 denotes a super luminescent diode (SLD), and the output light has a relatively wide spectral width as shown in FIG.
Coherence is not very high. Therefore, when the SLD is used as the light source of the interferometer, the coherence length is short, and if the optical path length difference between the two optical paths slightly changes, the contrast of the interference fringe changes abruptly.

Reference numeral 12 denotes a collimator lens that converts the light from the SLD into parallel light, 13 denotes a beam splitter that splits the parallel light and then combines the split light, and 14 denotes a high-precision plane that reflects the split light flux. Reference plane with degree 1
Reference numeral 5 denotes a first piezoelectric element made of PZT for causing the reference surface 14 to vibrate in a sine wave. Reference numeral 16 denotes an object whose surface reflects the other light beam split by the beam splitter 13. Reference numeral 161 denotes a second PZ that causes the DUT 16 to vibrate in a sine wave.
This is a T piezoelectric element. The light beam reflected by the reference surface 14 and the light beam reflected by the surface of the device under test 16 are combined by the beam splitter 13 to generate an interference image. The above configuration is the configuration of an interferometer generally known as a Twyman-Green interferometer. Here, an example in which the present invention is applied to a Twyman-Green interferometer will be described. However, the present invention can be applied to any interferometer.

Reference numeral 17 denotes a CCD (Charge C).
A TV camera 171 includes an optical device 171 and the like. An image forming unit 171 forms an image of the light beam from the reference surface 14 combined by the beam splitter 13 and the light beam reflected by the surface of the device 16 on the TV camera 17. It is an image lens. On the TV camera 17, interference fringes between the light beam reflected by the reference surface 14 and the light beam reflected by the surface of the device 16, that is, the difference between the surface shapes of the surface of the device 16 and the reference surface 14. Interference fringes are generated. The TV camera 17 converts the interference fringe image into an electric signal and outputs it. 18 is A /
The D converter converts an analog electric signal from the TV camera 17 into a digital signal.

Reference numeral 200 denotes a computer for arithmetically processing an image of the interference fringe converted into a digital signal, and includes a central processing unit (CPU) 19, a ROM 20, and a RAM 21.
, A frame memory 22, and an I / O port 23. The coherence degree detection means 8, the coherence degree change storage means 9, and the optical path length difference calculation means 10 shown in FIG. 1 are realized by this computer. A / D converter 18
Is temporarily stored in the frame memory 22 and then processed. Reference numeral 24 denotes a first PZT driver that outputs a drive signal to the first piezoelectric element 15, and reference numeral 25 denotes a second PZT driver that outputs a drive signal to the second piezoelectric element 161, which is controlled by the computer 200.

In the apparatus of the present embodiment, the coherence degree is detected and the interference fringes captured by the TV camera 17 are detected.
By processing the two-dimensional image by the computer 200, the interference fringes can be analyzed at the same time. Since the method and apparatus for analyzing interference fringes are widely known, they are omitted here. In this embodiment, the measurement of coherence degree and the analysis of interference fringes are described in "Optics" Vol.
No. (February, 1986) or the like. This method will be briefly described.

The DUT 16 is fixed, and the reference surface 14 is
The amplitude a, the angular frequency ω _c , the initial phase θ by the piezoelectric element 15
Is sine wave oscillated. Assuming that the optical path difference between the light reflected by the DUT 16 (hereinafter referred to as object light) and the light reflected by the reference surface 14 (hereinafter referred to as reference light) is L, at one point of an image captured by the TV camera 17 The AC component of the detected interference signal is given by equation (1).

[0021]

(Equation 1)

In the equation (1), I ₀ is the intensity of the object light, I _r is the intensity of the reference light, and R (L) is the optical path difference L.
₁ = is an average degree of coherence from L-2a for L ₂ = L + 2a. Now, assuming that the DC component and the AC component of Expression (1) are represented by B and S ₀ as in Expression (2), Expression (1) becomes Expression (3).

[0023]

(Equation 2)

[0024]

(Equation 3)

When the interference signal S (t) is Fourier-transformed, a result as shown in equation (4) is obtained.

[0026]

(Equation 4)

Here, F (ω _c ) and F (3ω) in equation (4)
Equation (5) is obtained from _c ).

[0028]

(Equation 5)

Therefore, the relationship between r ₃₁ and z can be obtained by calculating the Bessel function, and the relationship is shown in FIG. As can be seen from FIG. 5, if the range of z is known to some extent, r ₃₁ and z have a one-to-one correspondence in that range, and r ₃₁ is calculated according to the result of the Fourier transform of the interference signal S (t). Then, z is determined. Although z is determined by the amplitude a of the first piezoelectric element 15 as described above, z can be obtained more accurately by calculating z in this manner.

If z is obtained as described above, the phase term of F (ω _c ) in equation (4) is obtained by equation (6).

[0031]

(Equation 6)

That is, θ is determined by the sign of α with the uncertainty of π. Further, from F (ω _c ) and F (2ω _c ) in Expression (4), Expression (7) is obtained.

[0033]

(Equation 7)

That is, α is determined from F (ω _c ) and F (2ω _c ). As described above, the optical path length difference with respect to the reference plane at an arbitrary point of the interference fringe image captured by the TV camera can be accurately calculated, and the surface shape of the measured object is determined. The analysis of interference fringes is performed in this manner. From equation (4), B and S ₀ in equation (2) are represented by equation (8).

[0035]

(Equation 8)

Therefore, if z and α are known, B and S _{0 in} equation (2) are obtained. Also, from equation (2), the result of dividing S ₀ by B is as shown in equation (9), which is the visibility V of the interference fringes. I ₀ and I _r is degree of coherence Knowing R
(L) is obtained by equation (10).

[0037]

(Equation 9)

[0038]

(Equation 10)

Usually, the visibility V indicates the contrast of interference fringes. The contrast of the interference fringes is highest when the optical path lengths of the object light and the reference light match, and decreases as the optical path length difference increases. In the present invention, the difference in the optical path length is detected using the change in the visibility of the interference fringes due to the difference in the optical path length. As shown in FIG. 3, in the apparatus of the present embodiment, a second piezoelectric element 161 for moving the DUT 16 is provided. The device under test 16 is moved at predetermined intervals over a predetermined range on both sides of the position where the contrast of the interference fringes is highest, and the visibility of the interference fringes is measured and stored at each position by the above-described method. FIG. 6 shows the measurement result, which corresponds to the case where the center position coincides with the optical path length, and the visibility of the interference fringes decreases as it moves to both sides from the center position. This measurement result is stored in the ROM 20 or the RAM 21 of the computer 200.
To memorize it. Once such a measurement result is stored, the optical path length difference is determined by measuring the visibility of the interference fringes. As is clear from FIG. 6, if a light source having a wavelength characteristic as shown in FIG. 4 is used, an optical path length difference in a range of about ± 16 μm can be detected.

However, since it is difficult to determine the optical path length difference with high accuracy, such as 1/10 wavelength or 1/100 wavelength, from the visibility of the interference fringes, the continuous surface shape is measured by analyzing the interference fringes. Then, the visibility of the interference fringes is used to measure the difference between the two portions that cannot be measured by analyzing the interference fringes such as steps. Specifically, the visibility of the interference fringes is used to determine the order of the interference fringes of the two portions separated by the step, and the precise shape measurement at each portion is performed by analyzing the interference fringes. Hereinafter, a procedure for measuring an object to be measured having a step will be described.

First, an object to be measured or an object having a surface state close thereto is placed at a measurement position and moved by the second piezoelectric element 161 to measure in advance the change in the visibility of interference fringes due to the difference in optical path length. The measurement result is stored. With respect to two portions separated by the step of the device under test, the contrast of the interference fringes when moved by the second piezoelectric element 161 changes in the same direction, and one of the contrasts becomes about 0.65. Set as follows. In this state, the visibility and the phase of the interference fringe are measured at appropriate points in the two portions separated by the step. Now, the visibility and the phase are 0.63 and -2.5 radians (rad), respectively.
6 and 1.5 rad. Fig. 7 Changes in visibility
In the case as shown in the above, the optical path length difference obtained from the visibility of 0.63 and 0.56 is 15 rad. Since the interference fringe changes by 2πrad every time the order changes by 1, the largest order is 2 within 15 rad. Therefore, the phase difference between the two points in this case is expressed by the following equation.

[0042]

[Equation 11]

When 16.6 rad is divided by 2π rad and the center wavelength is multiplied by 0.84 μm, 2.2 μm is obtained.
This is the optical path length difference between the two points. The shape of each part divided by the step can be obtained by the same analysis of interference fringes as in the prior art.
The shape over the entire surface is known. As is clear from the above description, most of the means necessary for detecting the visibility can be used as they are for the analysis of interference fringes, so it is a simple operation to add for detecting the visibility. Only means and storage means. Therefore, even if the visibility is detected, the apparatus does not become complicated, and the increase in the calculation time is almost negligible.

[0044] In the above embodiment, as the intensity I ₀ of the object beam is equal throughout the measuring surface, is used visibility instead of the coherence degree, the intensity of the object light I ₀ is the surface of the object state , The measurement result is affected by the state of the surface of the measured object. Then, the coherence degree R
The second embodiment detects (L). FIG. 8 is a diagram showing a configuration of the interferometer of the second embodiment.

As shown in FIG. 8, the difference from the first embodiment is that light shielding plates 26 and 27 capable of blocking light are provided on the optical paths of object light and reference light, respectively. Has the same configuration as the first embodiment. While the light path of the object light is blocked by the light shielding plate 27, the intensity of the reference light is measured,
While the light path of the reference light is blocked by the light shielding plate 26, the intensity of the object light is measured. Then, S ₀ and B are calculated by performing the same measurement as described above, and the equations (2), (9) and (10) are calculated.
With the intensity I _r of the intensity I ₀ of the object light detected as the reference light at, degree of coherence R (L)
Can be calculated.

The embodiment of the present invention has been described above.
Changing the wavelength characteristic of the light source also changes the degree of change in the degree of visibility or coherence with respect to the change in optical path length difference. Therefore, it is desirable to select the wavelength characteristic of the light source according to the measurement target. As the light source, an interference filter may be combined with a light source that emits light in a wide wavelength range. Also,
In the above embodiment, the visibility or the coherence degree is calculated by the sine wave phase modulation interferometry, but the contrast of the interference fringes can be simply used.

[0047]

As described above, according to the present invention, an interferometer capable of measuring a stepped surface can be realized without complicating the apparatus and increasing the time required for measurement.

[Brief description of the drawings]

FIG. 1 is a diagram showing a principle configuration of an interferometer of the present invention.

FIG. 2 is a diagram illustrating an example of a change in a coherence degree with respect to a change in an optical path length difference.

FIG. 3 is a diagram illustrating a configuration of an interferometer according to a first embodiment.

FIG. 4 is a diagram showing wavelength characteristics of a light source in the first embodiment.

FIG. 5 is a diagram showing a relationship between z relating to the amplitude of sinusoidal vibration, a ratio of angular vibration frequency of Fourier transform, and a value of three times the frequency thereof in the sinusoidal phase modulation interferometry.

FIG. 6 is a diagram showing how the visibility changes with respect to changes in the optical path length difference.

FIG. 7 is a diagram for explaining a step of measuring the size of a step in the example.

FIG. 8 is a diagram illustrating a configuration of an interferometer according to a second embodiment.

FIG. 9 is a diagram illustrating a step and interference fringes when the step is measured.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Light source 2 ... Collimator lens 3 ... Beam splitter 4 ... Reference surface 5 ... 1st piezoelectric element 6 ... DUT 7 ... Photoelectric conversion means 8 ... Coherence degree detection means 9 ... Coherence degree change storage means 10 ... Optical path length difference calculation means

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-166016 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01B 9/00-11/30 102

Claims (9)

(57) [Claims] A light source for emitting light having a predetermined degree of coherence; and a light source for splitting the light from the light source, reflecting the light from a surface of an object to be measured and a reference surface, and synthesizing the light. An interferometer comprising: an interference optical system for causing the interference optical system to convert an optical signal of an interference fringe obtained on a detection surface of the interference optical system into an electric signal; Coherence degree detecting means (8) for detecting the coherence degree of the interference light from the light
A coherence degree change storage means (9) for storing a degree of change of the coherence degree of the interference fringe with respect to a change in the optical path length difference between the optical path on the object side and the optical path on the reference surface, which is measured in advance; In accordance with the degree of change of the coherence degree stored in the change storage means (9), the optical path length at the detection point corresponding to the difference in the coherence degree at a plurality of detection points on the detection surface detected by the coherence degree detection means (8) An interferometer utilizing a degree of coherence, comprising: an optical path length difference calculating means (10) for calculating a difference. 2. The interferometer according to claim 1, wherein the light source has a degree of coherence change on the detection surface such that an optical path difference becomes maximum near several wavelengths. 3. The coherence degree detecting means (8), a sine wave oscillating means (5) for sine wave oscillating the reference surface or the surface position of the device under test, and a Fourier transform of an output of the photoelectric conversion means. 2. An interferometer using a degree of coherence according to claim 1, further comprising a Fourier transform operation means, wherein the amplitude of the sinusoidal vibration means (5) is accurately detected from the operation result of the Fourier transform operation means. . 4. The coherence degree detecting means (8)
Optical path blocking means (26, 27) for independently blocking the optical path on the object side and the optical path on the reference surface side of the interference optical system,
Measuring the light intensity on the object to be measured and the light intensity on the reference surface, respectively, and calculating the degree of coherence from the visibility of the interference fringes, the light intensity on the object to be measured, and the light intensity on the reference surface The interferometer using the degree of coherence according to claim 1 or 3, wherein: 5. The coherence degree detecting means (8)
4. The interference according to claim 1, wherein, assuming that reflected light from the object to be measured on the detection surface is equal over the entire measurement surface, visibility of an easily detectable interference fringe is used instead of the coherence degree. 5. Total. 6. The optical path length difference calculating means (10) includes means for analyzing interference fringes on a detection surface, and fringe orders of two or more portions where the interference fringes are discontinuous on the detection surface of the interference optical system. Is determined from the difference in the coherence degree on the detection surface detected by the coherence degree detection means according to the degree of change of the coherence degree stored in the coherence degree change storage means, and the optical path is determined from the fringe order and the phase of the interference fringes. The interferometer according to claim 1, wherein the difference is calculated. 7. The interference fringe analyzing unit includes: a sine wave oscillating unit that sine-wave oscillates the reference surface or the surface position of the device under test; a Fourier transform operation unit that performs a Fourier transform on an output of the photoelectric conversion unit. 7. The interferometer using coherence according to claim 6, further comprising: accurately detecting an amplitude and a phase of the sine wave vibration means from a calculation result of the Fourier transform calculation means. 8. The interferometer according to claim 1, wherein said photoelectric conversion means is a one-dimensional photosensor in which a plurality of light receiving elements are arranged in a one-dimensional direction. 9. The interferometer according to claim 1, wherein the photoelectric conversion unit is a two-dimensional imaging unit.