JP7282367B2 - differential interferometer - Google Patents

Description

The present invention relates to a differential interferometer in which the density of interference signals is constant, there is no problem in interpreting unevenness on the surface of the object to be measured, and simple and rapid observation is possible without requiring a reference surface. It is particularly suitable for differential interferometers using laser light.

Interferometers are widely used not only to measure the position and orientation of objects and the distance to objects, but also to measure surface roughness and planar shape, and to observe microorganisms. Also, for measurement and observation using a differential interferometer, there is known an optical system that employs a method of shearing along the radial direction of an objective lens, that is, a method of using two beams laterally shifted in parallel. For example, non-patent documents 1 to 3 below disclose conventional optical systems that shear along the radial direction of an objective lens.

Although the optical systems of Non-Patent Documents 1 to 3 are mutually different technologies, they split the wavefront of converging or diverging light to generate an interference signal, and split the light along the radial direction of the objective lens. A differential interferometer is realized.

A Fizeau interferometer is a representative method for measuring a planar shape. However, this Fizeau interferometer requires a reference surface with a known shape in addition to the object to be measured. As a solution to this problem, a technique using a three-sheet stacking method disclosed in Non-Patent Document 4 below is known as a conventional technique.

Special Table 2019-508721 Japanese Patent Application Laid-Open No. 2002-15987 Japanese Patent Application Laid-Open No. 2005-228846

On-machine measurement by radial shear interferometer: Tsuguo Kouno, Daishi Matsumoto et al., Journal of the Japan Society for Precision Engineering, 65(3), 1999, pp.443-446 Measurement of transient near-infrared laser pulse wavefront with high precision by radial shearing interferometer：D.Liu, Y.Yang, J.Weng, X.Zhang, B.Chen, X.Qin, Optics Communications, 275,2007, pp. 173-178 A novel wavefront reconstruction algorithm based on interpolation coefficient matrix for radial shearing interferometry: C.Zhang, D.Li, M.Li, and K.E, Optics and Lasers in Engineering, 97,2O17,pp.86-92 Algorithm for Absolute Flatness Calibration by Three-layer Method: Toshiyuki Takatsuji, Yoichi Bito, Takamitsu Osawa, Ryoaki Furuya, Journal of the Japan Society for Precision Engineering, 72(11), 2006, pp. 1368－1373

However, in the optical systems according to Non-Patent Documents 1 to 3, the shear amount changes in proportion to the radius of the objective lens. This has the drawback that it is difficult to interpret the unevenness of the surface of the object to be measured, such as the fact that the density of the obtained interference signal becomes coarse due to the difference between the distances.

On the other hand, in the technique of Non-Patent Document 4 using the three-sheet alignment method, it is necessary to prepare three or more equivalent measurement objects, and it is necessary to repeat the measurement while replacing the measurement object and the reference surface. As a result, the work is complicated and requires a great deal of time and labor for measurement.

On the other hand, the above patent documents 1 to 3 are also known. For example, Patent Literature 1 discloses a technique of detecting a distance to a substrate using a pair of axicon lenses having concave and convex surfaces facing each other. Further, Patent Document 2 discloses a technique in which reflected light is incident on a pair of prism members in which a concave conical refracting surface and a convex conical refracting surface are arranged to face each other. Furthermore, Patent Document 3 discloses a technique of irradiating an object with light through a pair of prisms in which a concave prism and a convex prism are arranged to face each other.

However, even these Patent Documents 1 to 3 cannot solve the problem that the density of the interference signal changes as the shear amount changes along the radius of the objective lens. On the other hand, there has been a growing need for a measurement technique that allows easy and precise measurement without requiring a reference surface.
SUMMARY OF THE INVENTION The present invention has been made in view of the above background. An object of the present invention is to provide a differential interferometer capable of observation.

The invention according to claim 1, which solves the above problems, is a convex axicon lens having a protruding end that protrudes from one end side in a conical shape,
a concave axicon lens having a concave end with one end recessed in a conical shape that matches the shape of the protruding end, and having the concave end arranged opposite to the protruding end;
a light source that irradiates an object to be measured with a light flux and reflects the reflected light from the object to be measured to enter the pair of axicon lenses;
a light receiving element capable of directly receiving reflected light via a pair of axicon lenses and grasping interference fringes contained in the reflected light;
A differential interferometer comprising
The differential interferometer is characterized in that the refractive indices of the pair of axicon lenses are the same, and the gap between the projecting end and the concave end is set to a constant size throughout.

According to the differential interferometer of the invention of claim 1, there is provided a convex axicon lens having a conically protruding end on one end side, and a conically recessed one end side that matches the shape of the protruding end. A concave axicon lens with a concave end was arranged with the concave end facing the projecting end. Furthermore, the refractive indices of the pair of axicon lenses are the same, and the gap between the projecting end and the concave end is made constant throughout. In addition, as the light source irradiates a light beam and reflects it off the surface of the object to be measured, the light reflected by the object to be measured enters the pair of axicon lenses.

As described above, in the differential interferometer according to the present claim, the gap between the protruding end and the concave end is constant and the refractive index is the same for a pair of axicon lenses. The reflected light passes between the protruding end and the concave end, which are formed in a conical shape. These two beams are sheared and become parallel beams parallel to each other, resulting in interference fringes.

Therefore, unlike the prior art, the shear amount does not vary with the radius of the objective lens, the density of the interference signal is constant, and there is no problem in interpreting the unevenness of the surface of the object to be measured. In addition, since a reference surface unlike the prior art is naturally not required, measurement and observation can be performed in a single measurement, and the measurement time can be shortened, thereby enabling simple and rapid observation.

In the invention according to claim 2 , the reflected light reflected by the object to be measured is incident from the other end side of the convex axicon lens or the other end side of the concave axicon lens, and the distance between the projecting end and the concave end is 2. The light receiving element capable of grasping the interference fringes receives these two lights, which are divided into two lights having a shear amount via each axicon lens while passing through the axicon lens. It is a differential interferometer.

In this way, the reflected light enters from the other end side of one of the axicon lenses, passes between the protruding end and the concave end, and passes through each of these axicon lenses. , and split into two lights. As a result, interference fringes are displayed on the other end side of the other axicon lens, and are received by the light receiving element. For this reason, it becomes easier to record, analyze, etc., the unevenness, etc., by receiving light with the light receiving element, instead of simply visually confirming the unevenness, etc. on the surface of the object to be measured.

The invention according to claim 3 is the differential interferometer according to claim 1 or claim 2, which has an adjusting mechanism for adjusting the size of the gap between the projecting end and the concave end. By adjusting the size of the gap with the adjustment mechanism in this way, it is possible to obtain measurement accuracy that matches the required resolution.

According to the fourth aspect of the invention, the size of the gap between the protruding end and the concave end can be adjusted by translating either one of the pair of axicon lenses in the direction along the central axis of the axicon lenses. A differential interferometer according to any one of claims 1 to 3. By adjusting the size of the gap between the projecting end and the recessed end in this manner, it is possible to obtain measurement accuracy that matches the required resolution, as described above.

The invention of claim 5 is the differential interferometer according to any one of claims 1 to 4, wherein the light source is a laser element that generates a coherent light beam. By using a laser element that generates a coherent light beam as the light source in this manner, the reflected light reflected by the surface of the object to be measured becomes light that is relatively easy to observe, enabling more accurate measurement.

According to the differential interferometer according to the present invention, the density of the interference signal is constant, and there is no problem in interpreting the unevenness of the surface of the object to be measured. It has an excellent effect that it becomes possible.

1 is a schematic diagram of a differential interferometer according to a first embodiment of the invention; FIG. FIG. 2 is a cross-sectional view showing a state in which the concave axicon lens and the convex axicon lens in the first embodiment are separated; 4 is an enlarged cross-sectional view illustrating refraction of light in a gap between a concave axicon lens and a convex axicon lens in the first embodiment; FIG. It is a figure which shows the interference fringe which appeared on the plane of the convex axicon lens in 1st Embodiment. FIG. 4 is a schematic diagram of a differential interferometer according to a second embodiment of the invention;

A first embodiment of a differential interferometer according to the present invention will be described in detail below with reference to the drawings.
As shown in FIG. 1, in the differential interferometer 10 of the present embodiment, a laser element 12, which is a light source for generating laser light and generates a coherent light flux, is placed next to the laser element 12 so that the laser light from the laser element 12 is parallel light. A collimator lens 14 is arranged.

A beam splitter 16 for transmitting the parallel light K downward is arranged on the optical axis C on which the parallel light K is transmitted through the collimator lens 14 . An LSI silicon wafer or the like, which is the object H to be measured, is arranged below the beam splitter 16 perpendicular to the optical axis C. According to the present embodiment, the surface roughness of the object H is measured. It is designed to be able to measure the

Therefore, the parallel light K, which is a light flux from the laser element 12 and sent downward through the beam splitter 16, is sent to the object H to be measured, is irradiated, and is reflected by the surface HS of the object H to be measured. , the reflected light R returns to the beam splitter 16 side. Above the beam splitter 16, as shown in FIG. 2, a concave axicon lens 20 having a concave end 20A with one end recessed in a conical shape and a protruding end 22A protruding conically at one end side are provided. are arranged respectively. The pair of axicon lenses 20 and 22 are made of the same glass material such as BK7 so as to have the same refractive index.

The shape of the projecting end 22A and the shape of the recessed end 20A are not only formed in a right conical shape with mutually matching apex angles, but also in a posture in which the recessed end 20A faces the projecting end 22A. are placed. Therefore, as the apex of the conically projecting end 22A and the apex of the conically recessed recessed end 20A face each other and the central axes C1 and C2 coincide with each other, these projecting ends 22A The gap G shown in FIGS. 1 and 3 between and the concave end 20A is of constant dimension throughout.

On the other hand, as shown in FIG. 2, the other end of the convex axicon lens 22 is formed into a flat plane 22B perpendicular to the center axis C2 of the protruding end 22A, and the concave axicon lens 20 is also formed. is formed into a flat plane 20B orthogonal to the central axis C1 of the concave end 20A.

As shown in FIG. 1, above the pair of axicon lenses 20 and 22, a CCD camera 24 as a light-receiving element is arranged, and an image is projected onto a plane 22A on the other end side of the convex axicon lens 22. This CCD camera 24 can take an image such as interference fringes. Also, by connecting a computer or the like to the CCD camera 24, it becomes possible to process the image captured by the CCD camera 24. FIG.

As described above, the reflected light R reflected by the surface HS of the object to be measured H is transmitted through the beam splitter 16 and is incident on the concave axicon lens 20 from the flat surface 20B on the other end side, and the incident angle θ ₁ (θ ₁ =90−α/2), the light is refracted at a refraction angle θ ₂ as it enters the gap G from the interface K1 of the conical concave end 20A. Here, α is the apex angle at the concave end 20A of the concave axicon lens 20 . Also, the apex angle at the projecting end 22A of the convex axicon lens 22 described below is the same. The apex angle is the maximum angle among the angles formed by the two generatrices B shown in FIG.

As shown in FIG. 3, part of the light emitted from the interface K1 of the concave end 20A is reflected at the interface K2 of the protruding end 22A, and the rest of the light is refracted and enters the protruding end 22A. At this time, since the refractive index and apex angle of the pair of axicon lenses 20 and 22 are equal to each other, the light that has been refracted and entered the projecting end 22A is reflected from the object to be measured H and incident on the concave axicon lens 20. It becomes transmitted light L 1 parallel to the light R and travels through the convex axicon lens 22 .

On the other hand, the light reflected by the interface K2 is reflected again by the interface K1 and then refracted by the interface K2 to enter the projecting end 22A. This light incident on the protruding end 22 A also becomes transmitted light L 2 parallel to the reflected light R incident on the concave axicon lens 20 from the object H to be measured, and travels through the convex axicon lens 22 .

As a result, the transmitted light L1 that is directly incident on the convex axicon lens 22 and the transmitted light L2 that is incident on the convex axicon lens 22 after once being reflected form a flat surface 22B on the other end side of the convex axicon lens 22. is observed, interference fringes S as shown in FIG.

Then, the interference fringes S can be photographed by a CCD camera 24 arranged on the other end side of the convex axicon lens 22 . However, the reflected light R is affected by the surface roughness of the object H to be measured, such as a silicon wafer for LSI, and changes from the original light flux. As a result, it is possible to measure the surface roughness by detecting the deformation as the interference fringes S do not form clear interference fringes S at regular intervals and are slightly deformed.

Further, the differential interferometer 10 of the present embodiment translates one of the pair of axicon lenses 20 and 22 in a direction along the central axes C1 and C2 to move the projecting end 22A. and concave end 20A.

More specifically, as shown in FIG. 1, ring-shaped brackets 30 and 32 are fixed to the pair of axicon lenses 20 and 22 near the other end side, respectively. Holding portions 30A and 32A are formed so as to protrude from the right end portion in FIG.

A lower end of a guide shaft 34 extending along the central axes C1 and C2 shown in FIGS. On the other hand, the guide shaft 34 is slidably supported by the upper holding portion 32A of the upper bracket 32 fixed to the convex axicon lens 22. As shown in FIG. Therefore, in this embodiment, the convex axicon lens 22 is relatively movable with respect to the concave axicon lens 20 .

On the other hand, a rack 36 having linear teeth is fixed to the right side of the upper holding portion 32A shown in FIG. A pinion gear 38 meshing with the rack 36 is attached to a rotating shaft 40A of a motor 40 such as a stepping motor or a servo motor. It is connected to this motor 40 . The controller 42 is also connected to a CCD camera 24 for capturing images such as interference fringes and a display 44 such as a CRT monitor or LCD for displaying the image captured by the CCD camera 24 .

As described above, the adjusting mechanism 28 of the present embodiment includes a pair of brackets 30 and 32, a guide shaft 34, a rack 36, a pinion gear 38, a motor 40, a controller 42, and the like. By controlling the operation of , the convex axicon lens 22 is slightly moved relative to the concave axicon lens 20 in the directions along the central axes C1 and C2 in FIGS. The size is adjustable.

Accompanying this, when the size of the gap G is adjusted by the adjusting mechanism 28, the transmitted light L1 shifts in the radial direction of the convex axicon lens 22 along with the change in the amount of shear of the two transmitted lights L1 and L2. , L2, and the measurement accuracy can be easily obtained to the required resolution.

Furthermore, since the CCD camera 24 is also connected to the controller 42, the image captured by the CCD camera 24 can be processed within the controller 42 to quantify the surface roughness of the object H to be measured. can also display the interference fringes themselves and numerical values of surface roughness.

Next, the light shift amount x and the optical path difference p, which are shear amounts of the two transmitted lights L1 and L2 traveling through the convex axicon lens 22, will be described. Here, the dimension of the gap G is defined as d.
First, the amount of deviation between the two transmitted lights L1 and L2 is x=2d tan θ ₂ cos θ ₁ . Also, the optical path difference p is found to be 2dn ₂ cos θ ₂ from the following equation (1).
p = ( _2dn2 / _cosθ2 ) - _2dn1 tanθ2 _sinθ1 (1 ₎ Formula = ( _2dn2 / _cosθ2 ) (1- ^sin2θ2 ₎ = _{2dn2 cosθ2}

As a result, when the surface shape such as the surface roughness of the object H to be measured is f(r), the observed interference signal I is obtained by the following equation (2). It should be noted that the wavelengths of the two transmitted lights L1 and L2 are assumed to be λ here.
I=2(f(r+2d tan θ ₂ cos θ ₁ )−f(r) + dn ₂ cos θ ₂ )/λ\och (2) Equation From this equation (2), on the plane 22B of the convex axicon lens 22, It can be seen that the height difference between two points at regular intervals along the radial direction of is obtained as the interference signal I.

For example, the apex angle α of the cone is 179 degrees, the refractive index of the pair of axicon lenses 20 and 22 made of glass is n ₁ =1.5, and the medium in the gap G is air, and the refractive index is n ₂ =1.0. , the incident angle θ ₁ is 0.5 degrees according to the above formula, and the light deviation amount x is 2dtan θ ₂ cos θ ₁ =0.026d. can know

At this time, since the interval d of the gap G is constant over the entire area, the differential of the surface shape of the object H to be measured in the radial direction can be directly observed with equal density. In other words, if the distance d is made small, the differential of the surface shape of the object H to be measured can be observed along the radial direction on the pair of axicon lenses 20 and 22 at high density. Since the interference fringes S are obtained at regular intervals, the surface shape of the object to be measured H is restored as a calculation result by the CPU or the like in the controller 42 from the interference fringes S photographed by the CCD camera 24. Measurement of the absolute shape of the object H to be measured can be easily realized.

Next, the operation of the differential interferometer 10 according to this embodiment will be described below.
According to the differential interferometer 10 of the present embodiment, the convex axicon lens 22 having the protruding end 22A that protrudes in a conical shape and the one end side is recessed in a conical shape that matches the shape of the protruding end 22A. A concave axicon lens 20 having a concave end 20A is used.

In this embodiment, not only are the axicon lenses 20 and 22 made of the same material to have the same refractive index, but also the apex of the protruding end 22A and the apex of the concave end 20A are aligned with each other. The pair of axicon lenses 20 and 22 are arranged facing each other while setting a constant gap G between the projecting end 22A and the concave end 20A over the entire length. Furthermore, the other end side of the convex axicon lens 22 is formed on a flat surface 22B perpendicular to the central axes C1 and C2 of the protruding end 22A, and the other end side of the concave axicon lens 20 is formed on the concave end 20A. It is formed on a plane 20B that is orthogonal to the central axes C1 and C2.

On the other hand, the laser element 12 that generates a coherent light beam irradiates the light beam and reflects it off the surface HS of the object H to be measured. The light is incident from the plane 20B on the other end side. As a result, the reflected light R passing through the axicon lenses 20 and 22 through the gap G between the protruding end 22A and the concave end 20A has a predetermined amount of shear, resulting in two transmitted lights L1, As a result of the division into L2, an interference fringe S is displayed on the plane 22B of the convex axicon lens 22, which is received by the CCD camera 24 to measure the surface shape of the object H to be measured.

That is, in the differential interferometer 10 according to the present embodiment, the reflected light R incident from the other end side of the concave axicon lens 20 passes through the gap G between the projecting end 22A and the concave end 20A, which are respectively formed in a conical shape. 4, the two transmitted lights L1 and L2 are separated into two transmitted lights L1 and L2 and sheared, but these two transmitted lights L1 and L2 become parallel lights to form the interference fringes S shown in FIG.

As described above, the apexes of the projecting end 22A and the recessed end 20A are opposed to each other so that the central axes C1 and C2 are aligned, and the gap G between the projecting end 22A and the recessed end 20A is set constant. Thus, regardless of which part the reflected light R passes through, two transmitted lights L1 and L2 having a constant amount of shear are reliably obtained as parallel lights. Therefore, according to this embodiment, the density of the interference signal is constant, and there is no problem in interpreting the unevenness of the surface HS of the object H to be measured. Observation becomes possible.

Furthermore, by adopting the laser element 12 that generates a coherent light beam as the light source and the CCD camera 24 as the light receiving element, the reflected light R reflected by the surface HS of the object H to be measured is relatively easy to observe. In addition, it is possible to perform highly accurate measurement and observation using the unevenness of the surface HS of the object H to be measured as an interference signal.

On the other hand, the differential interferometer 10 of this embodiment can adjust the size of the gap G by moving the convex axicon lens 22 parallel to the concave axicon lens 20 in the direction along the central axes C1 and C2. Since it also has an adjusting mechanism 28, by adjusting the size of the gap G with this adjusting mechanism 28, it becomes possible to easily obtain measurement accuracy matching the required resolution.

Next, a second embodiment of a differential interferometer according to the present invention will be described in detail with reference to the drawings.
As shown in FIG. 5, also in the differential interferometer 10 of this embodiment, a convex axicon lens 22 and a concave axicon lens 20 having exactly the same shape as those of the first embodiment are used. However, in the present embodiment, the convex axicon lens 22 is arranged on the lower side and the concave axicon lens 20 is arranged on the upper side, and the adjustment mechanism 28 is arranged with respect to the fixed convex axicon lens 22. , the concave axicon lens 20 is relatively movable.

Along with this, the recessed end 20A located on the upper side is arranged so as to face the projecting end 22A located on the lower side. Also in this embodiment, which is upside down, the apex of the conically projecting end 22A and the apex of the conically recessed recessed end 20A are opposed to each other. A gap G between the ends 20A is constant throughout.

Therefore, in this embodiment as well, as the reflected light R reflected by the object H to be measured is incident, the reflected light R passes through the pair of axicon lenses 20 and 22 and interferes with the plane 20B of the concave axicon lens 20. A fringe S is produced as in the first embodiment. Then, the reflected light R can be received by the CCD camera 24, and the density of the interference signal is also constant. Therefore, in this embodiment, there is no problem in interpreting the unevenness of the surface HS of the object H to be measured. A simple and quick observation that does not require a reference plane becomes possible.

However, in the first embodiment, the transmitted light beams L1 and L2 are sheared from the central axes C1 and C2 of the cone toward the outer periphery. The difference is that the transmitted lights L1 and L2 are sheared toward C2.

In the above-described embodiment, a lens with one end having a conical surface and the other end having a flat surface, which is called a plano-concave axicon lens or a plano-convex axicon lens depending on the unevenness of the conical surface, is used. A concave axicon lens or a convex axicon lens having one conical surface may simply be adopted. On the other hand, in the above-described embodiment, the reflected light is incident perpendicularly to the plane on the other end side of any of the axicon lenses, but the reflected light may be inclined and incident on this plane.

Furthermore, although the interference fringes can be visually confirmed, the image of the interference fringes may be recorded not only by the CCD camera as in the above embodiment, but also by a CMOS camera, an imaging device, other films, etc. The image captured by the camera may be processed by a computer or the like as described above to quantify the surface roughness of the object to be measured. Then, as in each embodiment, the interference fringes and numerical values may be displayed on a display device such as a CRT monitor or LCD.

As the object H to be measured, in addition to silicon wafers for LSI, it is possible to measure the accuracy of the spherical surface of an object formed in a spherical shape. The present invention is also suitable for absolute measurement of the shape of microorganisms, etc., using biological samples such as microbes as objects to be measured.

Also, the material of the pair of axicon lenses 20 and 22 is, for example, BK7, but other glass materials, transparent plastics, fluorite, etc. can be used. The size of the gap between the pair of axicon lenses 20 and 22 can also be varied in accordance with the application.

Although the embodiments according to the present invention have been described above, the present invention is not limited to such embodiments, and various modifications can be made without departing from the gist of the present invention.

The present invention can be used to measure the surface roughness of a material having a smooth surface such as a silicon wafer for LSI, measure the accuracy of the spherical surface of an object formed in a spherical shape, observe microorganisms, and measure the surface of various samples. Real-time observation of differentiation along the radial direction enables application to various industrial fields.

10 differential interferometer 12 laser element (light source)
16 beam splitter 20 concave axicon lens 20A concave end 20B plane 22 convex axicon lens 22A projecting end 22B plane 24 CCD camera (light receiving element)
28 Adjustment mechanism C1, C2 Central axis G Gap H Object to be measured HS Surface K1, K2 Interface L1, L2 Transmitted light S Interference fringes x Light deviation amount α Vertex angle

Claims (5)

a convex axicon lens having a protruding end protruding from one end side in a conical shape;
a concave axicon lens having a concave end with one end recessed in a conical shape that matches the shape of the protruding end, and having the concave end arranged opposite to the protruding end;
a light source that irradiates an object to be measured with a light flux and reflects the reflected light from the object to be measured to enter the pair of axicon lenses;
a light receiving element capable of directly receiving reflected light via a pair of axicon lenses and grasping interference fringes contained in the reflected light;
A differential interferometer comprising
A differential interferometer characterized in that the refractive indices of the pair of axicon lenses are the same, and the gap between the protruding end and the concave end is made constant throughout. Reflected light reflected by the object to be measured enters from the other end side of the convex axicon lens or the other end side of the concave axicon lens, passes between the protruding end and the concave end, and passes through each axicon. 2. A differential interferometer according to claim 1, wherein the light is divided into two beams having a shear amount via a lens, and a light receiving element capable of grasping interference fringes receives these two beams. 3. A differential interferometer according to claim 1, further comprising an adjusting mechanism for adjusting the size of the gap between the projecting end and the concave end. 4. Any one of claims 1 to 3, wherein the size of the gap between the protruding end and the concave end can be adjusted by translating either one of the pair of axicon lenses in a direction along the central axis of the axicon lenses. The differential interferometer according to 1. A differential interferometer according to any one of claims 1 to 4, wherein the light source is a laser element that generates a coherent light beam.

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