JPH1082615A - Microscopic step-measuring apparatus of shearing interference contrast method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a microscopic step-measuring apparatus of shearing interference contrast method which can measure a minimal step difference highly accurately and easily in a non-contact manner, is resistive to external vibrations, can be made compact easily, and shows high resolution. SOLUTION: The apparatus is provided with a collimator lens 19 turning a light from a light source to a converging light or parallel light, an objective lens 23 turning the light from the collimator lens to a parallel light or converging light, and a double refraction element 22 shearing the light from the collimator lens to two wavefronts. The two wavefronts are projected to an object 24 to be measured, and reflected wavefronts from the object are superposed by the double refraction element thereby to bring about a shearing interference. An intensity change of interference fringes when a phase is changed is measured. Moreover, a phase difference is applied to the two wavefronts by moving the double refraction element in a direction parallel to a shear direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shearing interference contrast method microscope step difference measuring apparatus.

[0002]

2. Description of the Related Art Techniques for observing and measuring phase objects have recently become more important in various fields such as optics, precision machinery, physical physics, biology, and medicine. For example, semiconductor integrated circuits that have been miniaturized in recent years,
Or, opaque and transparent phase objects such as crystals and thin films, and step measurements of various objects are required. Conventionally, when measuring a very thin film coated on the surface of a lens or the like, for example, a film thickness measuring device using a stylus as shown in FIG. 5 is used, and a diamond stylus 201 and a skid 202 as shown in FIG. Of the glass surface 20 as shown in FIG.
3 is moved and traversed across the membrane 204, and the change is electrically amplified and detected. For example, when the membrane diameter on the sample is l0, a measurement result is obtained as shown in FIG. 5c. It is.

[0003]

[Problems to be Solved by the Invention] However, such a mechanoelectric method is not always easy to measure with high accuracy due to the electrical stability of the reference potential because of the differential detection. Because of this configuration, an optical element having a curved surface cannot be measured unless the radius of curvature is sufficiently large. In addition, the surface of the sample is broken due to the stylus type, and the measurement result remains questionable. In addition, three-dimensional observation and measurement using a differential interference microscope have not been performed. As a technique capable of three-dimensional observation and measurement using a differential interference microscope, there is a shearing interference contrast method step measurement device (see Patent Application Publication No. 45082 in 1992) previously proposed by the present inventors. Here, the resolution could not be made extremely high because the two wavefronts illuminating the sample overlapped. For this reason, there is a demand for the development of a measurement technique capable of measuring an ultra-thin step having a fine shape and a high resolution. The present invention has been made in view of the above-mentioned circumstances, and it is possible to easily measure an ultrathin film step with high accuracy and without contact, and furthermore, since it is a common optical path optical system, It is an object of the present invention to provide a shearing interference contrast method microscope step measuring apparatus which is resistant to vibration, can be easily miniaturized, and has high resolution.

[0004]

[Means for solving the problem] In response to this purpose,
The shearing interference contrast microscope microscope step measurement device of the present invention is a collimator lens that converts light from the light source to convergent light or parallel light, an objective lens that converts light from the collimator lens to parallel light or convergent light, A birefringent element for shearing light from a collimator lens to two wavefronts, illuminating the object to be measured with the two wavefronts, and superposing the two object wavefronts reflected from the object to be measured by the birefringent element. And the intensity of the interference fringes when the phase is changed is measured, and the birefringent element is moved in the direction parallel to the shear direction to move to the two wavefronts. It is characterized by providing a phase difference.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the drawings showing an embodiment. In the shearing interference contrast method microscope step difference measuring apparatus according to the present invention, the light from the light source is moved through a collimator lens to illuminate the sample with two lights: parallel light (two-beam illumination) and convergent light (confocal double spot illumination). You can make it a system. First, the first method comprises a collimator lens that converts light from a light source into convergent light, an objective lens that converts light into parallel light, and a birefringent element that shears convergent light into two wavefronts. The object is illuminated, and the two object lights reflected from the object to be measured are superposed on the ordinary ray and the extraordinary ray by the birefringent element to cause shearing interference, and the intensity change of the interference fringe when the phase is changed is measured. By moving the birefringent element in a direction parallel to the shear direction, a phase difference is provided between the two wavefronts.

Hereinafter, the present invention will be described in detail with reference to an embodiment. In FIG. 1, reference numeral 1 denotes a microscope step measurement device. The microscope step measuring device 1 is composed of a He—Ne laser light source 11.
Along the optical path of the laser beam generated from the He-Ne laser light source, the beam expander 12, the mirror
13, objective lens 14, pinhole 15, beam splitter 16, collimator lens 19, polarizer-2
0, a beam splitter-21, a birefringent element 22, and an objective lens 23. Reference numeral 24 denotes an object to be measured positioned in front of the objective lens 23, which is composed of a substrate a and a phase object b. An analyzer 25, a beam splitter 26, and a lens 28 are sequentially located on the transmission side of the beam splitter 21 along the optical path. Further, the beam splitter 26
The eyepiece 27 is located on the reflection side of. On the transmission side of the lens 28, a CCD camera 29 is located.
Reference numeral 9 is connected to an image processor 30 and observes an image on a display 31. Signals from the image processor 30 are connected to a processing device 35 via an interface 34. A white light source 17 and a lens 18 are located on the incident side of the beam splitter 16. As the birefringent element 22, a Nomarski prism, a Savart plate, a Wollaston prism, calcite, or the like can be used.

Next, the operation of the microscope level difference measuring device 1 configured as described above will be described. First, the principle of the shearing interference contrast method is described. In the contrast method using shearing interference, the wavefront of an object is divided into two and sheared with each other, and an appropriate phase difference is given between the two wavefronts to cause interference and observation. .

[0008] The laser beam from the laser light source 11 is collimated by a lens 12, the optical path is changed by a mirror 13, then spatial frequency filtered by an objective lens 14 and a pinhole 15, and the optical path is changed by a beam splitter 16. After that, the light is converged by the collimator lens 19, becomes linearly polarized light after passing through the polarizer 20, and is changed in optical path by the beam splitter 21. This light beam passes through a birefringent element 22 composed of a Nomarski prism,
It becomes two rays separated into an ordinary ray and an extraordinary ray. After passing through the localized surface of the interference fringes and the objective lens 23, the two parallel light beams sheared to illuminate the DUT 24. The object light reflected by the object to be measured 24 follows the original optical path, and the birefringent element 22
Pass through the beam splitter-21 and enter the analyzer-25. The two transmitted light beams interfere with each other, and after passing through the beam splitter 26 and the lens 28, an interference image of two points on the measured object is formed on the photoelectric surface of the CCD camera 29.
The intensity of each of the imaged two-point wavefronts is measured by a CCD camera 29.
And an image is displayed on the display 31 via the image processor. Each spot data is subjected to data processing by the processing device 35 via the interface 34. The phase of the two wavefronts is realized by periodically changing the birefringent element 22 by the electrostrictive element 32.
In addition, illumination can be performed by a normal white light source 17, and the object to be measured can be observed by a normal interference contrast method.

Next, in the second method, a beam from the light source is converted to convergent light by moving the lens from the He-Ne laser light source 11 toward the He-Ne laser light source 11 side. As a result, the focal point of the collimator lens 19 is set at the position of the pinhole 15 of the divergent light subjected to the spatial filtering. The white light source 17 lamp is set far away with the movement of the collimator lens 19 so as to be Koehler-illuminated on the surface to be measured of the object 24 to be measured. The He-Ne laser beam is converted into a parallel light beam by the collimator lens 19, passes through the birefringent element, is separated into two wavefronts of an ordinary ray and an extraordinary ray, and is separated on the surface to be detected by the objective lens.
Illuminated by two red spots. Since the light beam from the white light source is illuminated by Koehler, two red spots of the He-Ne laser are observed in the center of the green background at this time. These two spot lights are reflected by the object to be measured, then collimated by the objective lens, and the ordinary light beam and the extraordinary light beam are superimposed on each other by the birefringent element to cause shearing interference. , And by moving the birefringent element in a direction parallel to the shear direction, a phase difference is given between the two wavefronts.

Next, the principle of the shearing interference contrast method will be described. The phase difference between the two wavefronts Σ1 and Σ2 is π /
2. The shear amount is ΔS, and the phase magnitude of the object is δ.
The phase distribution of the two wavefronts after passing through the DUT 24 is as shown in FIG. For simplicity, FIG. 2 uses a phase object having a phase δ. First, a phase object is illuminated by wavefronts Σ1 and Σ2, and divided into two wavefronts. These wavefronts are sheared laterally to each other by a shear amount of ΔS, and their phase differences are given by the values shown in FIG. 2a. FIG. C shows the phase and the intensity distribution of the interference fringes, and the curve shows the phase converted to the intensity. Here, ΔI indicates an intensity change with respect to the phase change Δ. It can be easily understood from FIG. 2C that the intensity distribution of light obtained from the interference wavefronts of Σ1 and Σ2 after passing through the object to be measured is as shown in FIG. 2B. Here, the phase contrast image has a light intensity (0.5 + I, FIG. 2c) and a dark intensity (−0.5 + Δ).
I, converted to the opposite slope of FIG. 2c) and observed.

Next, a method of obtaining a step by the shearing interference contrast method will be described. The object to be measured is set as shown in FIGS. 3 and 4, and the intensity of each of the two wavefronts observed on the observation surface is represented by IA.
, IB, the intensity is represented by I = (X / 2) [1 + C0S ｛(2π / T) X + ψ｝] (1) Here, T is the period, X is the amount of movement of the birefringent element in the direction perpendicular to the optical axis, and ψ is the phase. When two points are obtained for the wavefronts Σ1 and Σ2, respectively, the period T and the phase ψ are expressed by the equations (2) and (3). T = 2π (X1−X2) / {COS ⁻¹ (2I1 −1−COS ⁻¹ (2I2−1)} (2) ψ = COS ⁻¹ (2I1 −1) −2πX / T ..... (3) a) Transmission type (when both substrate a and phase object b are transparent)
(FIG. 3a) The phase Δψ is Δψ = (2π / λ) (n−1) d (4) Δψ = (ψA−ψB) λ / {2π (n−1)} (5) Therefore, the level difference d is obtained by d = ({A− {B}) λ / {2π (n−1)} (6). Here, n represents the refractive index of the phase object.
This is obtained from the wavefronts of (π / 2) + δ and (π / 2) -δ, but can also be obtained from the wavefronts of π / 2 and (π / 2) + δ or (π / 2) -δ. In this case, d = ({A · ψB) λ / {2π (n−1)} (7)

B) Reflection type A (transparent phase object b) (FIG. 3b) Δψ = (4π / λ) (n−1) d (8) Δψ = (ψA · ψB) / 2 (9) Therefore, the step d is d = ({A−ψB) λ / {8π (n−1)} (10) Is determined by π / 2 and (π / 2) + δ or (π /
2) When it is determined from the wavefront of one δ, it is determined by d = (ψA-ψB) λ / {4π (n-1)} (11)

C) Reflection type B (opaque phase object) (FIG. 3C) The phase Δψ is Δψ = (4π / λ) d (12) Δψ = (ψA−ψB) / 2. (13) Accordingly, the step d is obtained from d = (ψA-ψB) λ / 8π (14). π / 2 and (π / 2) + δ or (π / 2)
In the case of obtaining from the wavefront of −δ, it is obtained by d = (ψA−ψB) λ / 4π (15). In the case where one point is obtained for each of the wavefronts Σ1 and Σ2, the period T and the phase 求 め have been obtained from the two points obtained for each of the two wavefronts, but then one step is obtained for each wavefront to derive the step. It shows about. In equation (1), the origin is set to X = 0. IA = IOA, I
Assuming that B = IOB, each phase ψOA, は OB is as shown in FIG. 4 は OA = C0S ⁻¹ (2IOA−1) (16) ψOB = C0S ⁻¹ (2IOB− 1)... (17) Hereinafter, the calculation of the step is the same as in the case of.

As described above, since the light intensity of the interference fringes generally changes sinusoidally when the phase changes, the difference in the light intensity of the two wavefronts is converted into a phase difference to determine the thickness of the phase object. Also, the phase can be obtained from the period of the interference fringes. Assuming that the phase between the two wavefronts at the position of the interference fringe period T and π / 2 is θ, the step d is: d = λθ / 2T (n−1) (18) ).

[0015]

[Experimental example]

(1) A sample was prepared, and the sample was measured by the microscope step measuring apparatus of the present invention, and the measurement result was compared with the result measured by an existing film thickness monitor. (2) As shown in FIG. 6a, a 40 μm × 40 μm SiO 2 deposited film was arranged in a matrix on a 30 mm × 30 mm substrate as shown in FIG. 6b. (3) The sample was measured by the microscope step measuring device of the present invention. In this case, I = (１ ／) [1 + C0S ｛(2π / T) X + ψA｝] (19) In the above equation, the origin is set to X = 0. IA = IOA, IB = IOB (20) ψOA = C0S ^-1 (2IOA-1) (21) ψOB = C0S ^-1 (2IOB-1) (22) Therefore, the step d is Δψ = (ψOA-ψOB) / 2 (23). In the case of a reflective phase object, d = Δ ＝ · λ / {4π (n-1)} (24) In this case, the birefringent element is moved manually. Was. The results are shown below. (4) Measurement result In the case of the W spot illumination method The measured value of the microscope step measuring device of the present invention 84.0 nm The measured value of the film thickness monitoring monitor 74.5 nm In the case of the two-beam illumination method The measured value of the microscope step measuring device of the present invention 42.0 nm Measured value by film thickness monitor 46.0 nm As described above, in both the case of the W spot illumination method and the case of the two-beam illumination method, the measured value of the microscope step measuring device and the measured value of the film thickness monitor were used. Are in good agreement, and it can be confirmed that the microscope step measurement device of the present invention can detect an extremely small step with high accuracy.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a microscope step difference measuring device.

FIG. 2 is a graph showing the phase and intensity of two wavefronts.

FIG. 3 is an enlarged longitudinal sectional view showing various types of objects to be measured.

FIG. 4 is a graph showing the intensity of two wavefronts.

FIG. 5 is an explanatory view showing a conventional stylus type step difference measuring device.

FIG. 6 is an explanatory diagram showing materials.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Microscope step measuring device 11 He-Ne laser light source 12 Beam expander 13 Mirror 14 Objective lens 15 Pinhole 16 Beam splitter 17 White light source 18 Lens 19 Collimator lens 20 Polarizer 21 Beam splitter 22 Birefringent element 23 Objective lens 24 DUT 25 Analyzer 26 Beam splitter 27 Eyepiece 28 Lens 29 CCD camera 30 Image processor 31 Display 32 Electrostrictive element driving device 33 Electrostrictive element driving amplifier 34 Interface 35 Processing unit a Substrate b Phase object

Claims (4)

[Claims] 1. A collimator lens for converting light from a light source into convergent light or parallel light, an objective lens for converting light from the collimator lens into parallel light or convergent light, and two light beams from the collimator lens. A birefringent element shearing on the wavefront, illuminating the two wavefronts on the object to be measured, and causing shearing interference by superimposing the two object wavefronts reflected from the object to be measured by the birefringent element, thereby changing the phase. The apparatus is configured to measure a change in the intensity of the interference fringes when changed, and to provide a phase difference to the two wavefronts by moving the birefringent element in a direction parallel to the shear direction. A shearing interference contrast microscope microscope step measuring device characterized by the above-mentioned. 2. A collimator lens for converting light from a light source into convergent light, an objective lens for converting light from the collimator lens into parallel light, and a birefringence for shearing light from the collimator lens into two wavefronts. The bi-wavefront illuminates the object under test, and superimposes the two-object wavefront reflected from the object under test by the birefringent element to cause shearing interference, thereby causing interference when the phase is changed. Shearing interference configured to measure a change in the intensity of the fringes, and configured to apply a phase difference to the two wavefronts by moving the birefringent element in a direction parallel to the shear direction. Contrast method microscope step measurement device. 3. A collimator lens for converting light from a light source into parallel light, an objective lens for converting light from the collimator lens to convergent light, and a birefringence for shearing light from the collimator lens into two wavefronts. The bi-wavefront illuminates the object under test, and superimposes the two-object wavefront reflected from the object under test by the birefringent element to cause shearing interference, thereby causing interference when the phase is changed. Shearing interference configured to measure a change in the intensity of the fringes, and configured to apply a phase difference to the two wavefronts by moving the birefringent element in a direction parallel to the shear direction. Contrast method microscope step measurement device. 4. A collimator lens whose position is variable on an optical axis for converting light from a light source to convergent light or parallel light; an objective lens for converting light from the collimator lens to parallel light or convergent light; A birefringent element for shearing light from the meter lens into two wavefronts, illuminating the object to be measured with the two wavefronts, and superposing the two object wavefronts reflected from the object to be measured by the birefringent element. The shearing interference is caused by changing the phase, and the change in the intensity of the interference fringes when the phase is changed is measured, and the birefringent element is moved in a direction parallel to the shear direction.
What is claimed is: 1. A shearing interference contrast method microscope step measurement device, wherein a phase difference is given to a wavefront.