KR100925592B1 - Method of measuring surface shape using moire technique - Google Patents

Abstract

Disclosed is a method of measuring a surface shape using a moiré technique capable of accurately measuring the surface shape of a light-transmitting small object using a phase shift moiré technique. The surface shape specifying method may include forming an oxide film on a surface of a shape measurement object; Projecting a grid pattern onto the shape measuring object in parallel light; Detecting a grid pattern image projected onto a shape measurement object by projection of the grid pattern; And generating a virtual grid pattern having the same spacing and different angles as the grid pattern, and forming a moire pattern by overlapping the virtual grid pattern and the grid pattern image projected on the shape measurement object. .

Phase Shift, Moire, 3D, Shape Measurement, Oxide, Coating

Description

Surface shape measurement method using moire technique {METHOD OF MEASURING SURFACE SHAPE USING MOIRE TECHNIQUE}

The present invention relates to a method of measuring the surface shape of an object by using a moiré technique, and more particularly, by using a moiré technique that can accurately measure the surface shape of a light-transmitting small object by using a phase shift moiré technique. A surface shape measuring method.

Recently, the digital convergence trend has required miniaturization and lightening of various digital products, and in particular, mobile communication terminals including camera or camcorder functions have been generalized, and miniaturization and mass production of optical components have been required due to the demand for improvement of optical functions. . Accordingly, it is necessary to build an evaluation system capable of rapidly evaluating the characteristics of components for mass production of optical components.

Of course, there is a basic performance evaluation system using an interferometer, which is a major evaluation means of optical components, but this is used as a main means of checking the characteristics of components in the final stage. Since this is not a means of evaluating the main factors of the optical component production line for mass production, there is a disadvantage that the component evaluation feedback is very delayed during production or during the construction of the line for production. In the case of the optical component to be used, there have been many difficulties in accurately measuring the surface shape information of the optical component for evaluation of a mold for producing the optical component.

Conventionally, the method of measuring the surface shape of an optical component (for example, a lens) is mainly adopted a point scan method, but this method takes a lot of time to measure the entire area of the optical component, so only the main part is scanned. After that, only to restore the shape on average, there was a problem that the actual surface shape of the optical component can not be accurately measured. In the early 1970s, a non-contact three-dimensional shape measurement method was proposed by the Moire technique, which draws contours on objects proposed by Takasaki and Meadows.

The moiré technique for determining the contour of an object is largely known as a shadow moiré technique and a projection moiré technique. The shadow moiré technique places a reference grating in front of the object and then casts light onto the grid in front of the object to create a shadow of the grid pattern on the object and to see the shadow through the reference grid at different angles. This is how moiré patterns are analyzed. In addition, the projection moiré technique involves projecting a grid pattern on an object, placing another grid in front of the image sensor for image detection, and analyzing the moire pattern seen through the grid pattern of the other grid. Algorithms that derive three-dimensional shapes by analyzing such moiré patterns have been introduced in various ways, so a detailed description thereof will be omitted (M. Kujawinska, "Use of Phase-stepping Automatic Fringe Analysis in Moire interferometry," See Applied Optics, 26 (22) 4712-4714, YB Choi, SW Kim, "Phase-shifting Grating Projection Moire Topography," Optical Engineering 37 (3) 1005-1010, SW Kim et al., "Two frequency phase- shifting projection moire topography, "see SPIE Vol. 3520, 36--42)

 However, since most of the optical parts (lenses) that are mass-produced are polymers used as the main material, it is difficult to obtain a normal moiré pattern because the transmittance of the measurement object itself is not high to form a clear grid pattern on the surface of the measurement object. . Therefore, a method of obtaining a three-dimensional surface shape by applying microparticles to the surface of an optical component was initially proposed, but there is a problem that the shape measurement of the optical component is inaccurate because the application state of the microparticles is not uniform.

Therefore, there is a need in the art for a method capable of measuring an accurate surface shape for an object such as an optical component (lens) having high transmittance.

An object of the present invention is to provide a surface shape measuring method using a moire technique capable of forming a clear lattice pattern on a surface to be measured by forming a thin oxide film on the surface of an optical component having a high transmittance and a measurement target. .

The present invention as a means for solving the above technical problem,

Forming an oxide film on the surface of the shape measurement object;

Projecting a grid pattern onto the shape measuring object in parallel light;

Detecting a grid pattern image projected onto a shape measurement object by projection of the grid pattern; And

Generating a virtual grid pattern having the same interval as the grid pattern and having different angles, and forming a moire pattern by overlapping the virtual grid pattern and the grid pattern image projected on the shape measurement object;

It provides a surface shape measurement method using a moiré technique comprising a.

Preferably, the step of forming the oxide film, the step of coating a metal thin film on the surface of the shape measurement object; And oxidizing the coated metal thin film. In particular, the coating of the metal thin film is a step of coating a metal thin film on the surface of the shape measurement object using a sputtering technique, and the oxidizing step includes oxidizing the metal thin film using a reactive ion etching chamber. Is preferably.

Preferably, the detecting may include: enlarging a grid pattern image projected onto a shape measurement object by projection of the grid pattern; And detecting the enlarged grid pattern image by using a solid-state image sensor.

According to the present invention, by forming an oxide film on the surface of the shape measurement object to improve the sharpness of the grid pattern projected on the surface there is an effect that can obtain a clear moire pattern.

In addition, this has the effect of improving the surface shape measurement accuracy for a measurement object having a high transmittance, such as an optical component (lens).

Hereinafter, with reference to the accompanying drawings will be described in detail the operating principle of the preferred embodiment of the present invention. In the following description of the present invention, detailed descriptions of well-known functions or configurations will be omitted if it is determined that the detailed description of the present invention may unnecessarily obscure the subject matter of the present invention. Terms to be described later are terms defined in consideration of functions in the present invention, and may be changed according to intention or custom of a user or an operator. Therefore, the definition should be made based on the contents throughout the specification.

1 is a flowchart illustrating a surface shape measuring method using a moire technique according to an exemplary embodiment of the present invention. Referring to Figure 1, the surface shape measurement method using the moiré technique according to an embodiment of the present invention, the step of forming an oxide film on the surface of the shape measurement object (S11), and grating in parallel light state on the shape measurement object Projecting a pattern (S12), detecting a grid pattern image projected onto the shape measurement object (S13), generating a virtual grid pattern having the same spacing and different angles as the grid pattern, and It may include a step (S14) to form a moire pattern by superimposing the virtual grid pattern and the grid pattern image projected on the shape measurement object.

Forming an oxide film on the surface of the shape measurement object (S11) may include coating a metal thin film on the surface of the shape measurement object (S111) and oxidizing the coated metal thin film (S112). Can be. The coating of the metal thin film (S111) is preferably a step of coating the metal thin film on the surface of the shape measurement object by using a sputtering technique. In addition, the oxidizing step (S112) is preferably a step of oxidizing the metal thin film using a reactive ion etching (RIE) chamber.

Projecting the grid pattern in the state of parallel light on the shape measurement object (S12) may be performed in various ways well known in the art. For example, a method of projecting a grid pattern on the measurement object by arranging a grid between the measurement object and the light source and a method of projecting the grid pattern generated by the interference phenomenon of two lights may be known. In general, the optical component (lens) applied to the mobile communication terminal is a compact lens, so the spacing of the lattice pattern should be very dense. For this purpose, in the case of the former projection technique, a lattice having a narrow lattice spacing should be used and the gap between the measurement object and the lattice should be kept narrow. As a result, the section in which the moire pattern to be actually measured is formed may be shortened, and a problem may occur in which an afterimage of the grating and the measurement object is detected. If the gap between the grating and the object is sufficiently secured to solve this problem, the entire area may be impossible to measure if the curvature of the reds is not formed and the curvature of the reds is large. Therefore, in the step S12 of projecting the lattice pattern, it is preferable to apply a technique of generating the lattice pattern using the latter interferometer.

Detecting the grid pattern image projected onto the shape measurement object by the projection of the grid pattern (S13), the step of enlarging the grid pattern image projected on the shape measurement object by the projection of the grid pattern, and the enlarged The method may include detecting the grid pattern image by using the solid state image pickup device. As described above, an embodiment of the present invention further includes a process of enlarging a grid pattern image projected onto a shape measurement object for image processing of a small optical device (lens) applied to a mobile communication terminal or the like. The captured image is detected through a solid-state image sensor. The image magnification process may be performed through a magnification lens disposed between the measurement object and the solid-state imaging device, and the focus of the magnification lens may be adjusted to be formed on an image forming surface of the solid-state imaging device.

Forming the moiré pattern (S14) after generating a virtual grid pattern having the same interval as the grid pattern projected on the measurement object, the grid direction is different from each other, the angle of the virtual grid pattern And forming a moire pattern by superimposing the grid pattern image projected on the shape measurement object. The virtual lattice pattern may be generated by a computer provided for various algorithms and calculations required for surface shape measurement. In addition, through the theoretical calculation by the computer, it is possible to generate a moire pattern generated when the virtual lattice pattern and the lattice pattern detected by the solid-state image pickup device overlap. Detailed description of the principle of moiré pattern generation will be omitted since it is a conventional level of skill in the art.

According to the present invention, a plurality of different grid patterns (for example, four) are generated by a phase shift to be projected onto a measurement object, and the measurement object detects images projecting each projected grid pattern. A virtual grid pattern is generated for each image to detect a plurality of moiré patterns of different phases. The Phase Shift Inerferometer (PSI) and Phase Unwrapping algorithms are applied to the plurality of phase-shifted moire fringes to obtain phase map information on the measurement object, and the height proportionality constant is used as the phase map. The final three-dimensional surface shape information can be obtained by multiplying the information (S15). The PSI algorithm and the phase recovery algorithm may be applied with various techniques known in the art. For the PSI algorithm and phase reconstruction algorithm, see "JE Gallagher and DR Herriott, US Patent 3,694,088 (1972)", "K. Creath, Progress in Optics. Vol. XXVI, 349 (E. Wolf, ED., Elsevier Science Publishers, Amsterdam "," JE Greivenkamp and JH Bruning, Optical Shop Testing, 2th Edition, pp518-536 ", and" JE Greivenkamp, Appl. Opt. 26, 5245 (1987) "and the like.

2 is a configuration diagram showing an example of a measuring apparatus for applying the surface shape measuring method using the moire technique according to an embodiment of the present invention.

Referring to FIG. 2, the measuring apparatus for applying the surface shape measuring method using the moire technique according to the embodiment of the present invention includes a light source 21, a beam diffuser 22, a light splitter 23, and a mirror 24a. And 24b), a mirror driver 25, a magnification lens 26, a solid-state imaging device 27, and a computer 28. The light source 21, the beam diffuser 22, the light splitter 23, the mirrors 24a and 24b and the mirror driver 25 serve as means for projecting a lattice pattern in parallel with the measurement object 30. The magnification lens 26 and the solid-state image sensor 27 are means for detecting a grid pattern image projected onto the shape measurement object, and the computer 28 generates a virtual grid pattern and the virtual grid pattern and the After forming the moiré pattern by superimposing the lattice image projected on the shape measurement object, the moiré pattern is used to restore the three-dimensional surface shape.

As described above, since it is not preferable to form a lattice pattern using a lattice on a small measurement object, the lattice pattern is formed using an interferometer (Tyman-Green interferometer or Mach-Zehnder interferometer) instead of the lattice. To this end, as shown in FIG. 2, the light output from the light source 21 such as a laser diode is formed into parallel light having a wide area using the beam diffuser 22, and then the parallel light splitter 23 is formed. Incident. The parallel light passing through the light splitter 23 is split into the first mirror 24a and the second mirror 24b and is incident, and the parallel light reflected by the first mirror 24a and the second mirror 24b are reflected. One parallel light interferes with each other to form an interference fringe like a grating. In particular, for phase shifting, the device comprises a mirror driver 25, such as a piezoelectric actuator, in the first mirror 24a, which moves the position of the first mirror 24a back and forth. The phase of the reflected light is varied.

The lattice pattern projected on the measurement object by the interferometer as described above is projected on the measurement object, and the solid-state image sensor (for example) is magnified by a predetermined magnification (for example, 4 times) by the magnification lens 26. For example, it is detected by the CCD) 27 of 2048 x 2048 pixels. As described above, the computer forms a virtual grid having different angles and different angles from the grid pattern projected onto the measurement object, and the grid pattern projected onto the measurement object detected by the solid-state image sensor 27 and the virtual grid. The moiré pattern is created by overlaying the lattice, and then the PSI algorithm and the phase reconstruction algorithm are applied to analyze the surface shape of the measurement object.

On the other hand, the present invention applies a technique of coating the surface of the measurement object with an oxide film in order to improve the measurement accuracy of the measurement object having a high transmittance, such as an optical device (lens). FIG. 3 is an image in which a bare lens is projected on a lattice pattern, and as shown in FIG. 3, a clear lattice pattern is not projected on a lens having a high transmittance. That is, when the measurement object is a lens, since the light transmittance is 90% or more in the bare state after the injection, a phenomenon in which the sharpness of the image in which the lattice pattern is projected onto the measured lens in the bare state appears. In addition, since a large amount of light is transmitted, the lens itself generates a shadow area in the diffuse reflection effect on a surface other than the surface to be measured and the moiré-type light source incident at an angle that is not perpendicular to the measurement lens. It may further cause a problem that it is impossible to obtain a normal three-dimensional surface shape by further lowering the sharpness of the. Therefore, in order to solve the diffuse reflection on the surface other than the surface to be measured and the shadow area generated by the lens itself, it is preferable to form a thin oxide film on the surface of the lens to be measured.

The process of forming the oxide film may include forming a metal thin film on the surface of the measurement object (lens) using a sputtering technique, and oxidizing the formed metal thin film using a RIE chamber.

4 is an image of a grid pattern projected onto a lens on which a metal thin film is formed at various thicknesses. As shown in FIG. 4, the image of the grid pattern projected in the state in which only the metal film is coated on the surface of the measurement object (lens) can detect the grid pattern that is relatively clear compared to the image of the soaked state. The thinner the thickness, the higher the transmittance, the lower the clarity of the lattice pattern, and the thicker the metal coating, the higher the reflectance, and the shape information of the measurement surface may be damaged by the beam reflected directly to the solid-state image pickup device. .

Therefore, in order to reduce the reflected light directly incident to the solid-state image pickup device, it is preferable to perform a process of oxidizing the surface of the metal thin film. That is, by oxidizing the surface of the metal thin film coated on the surface of the measurement object (lens), it is possible to prevent the generation of reflected light through the scattering effect on the measurement surface.

5 and 6 are images of detecting a grid pattern projected on an oxide-coated lens according to various conditions. In particular, FIG. 5 is an image of detecting a projected grid pattern after oxidizing a metal thin film having a thickness of 20 nm with different oxidation time, and FIG. 6 shows oxidation of a metal thin film having a thickness of 200 nm with different oxidation time. The image is detected after the projected grid pattern.

As shown in Figs. 5 and 6, if the metal coating is not thick enough before the oxidation process, the transmittance of the lattice pattern becomes high, and thus it is difficult to obtain a lattice pattern with sufficient sharpness to obtain a moire fringe. If the metal thin film is not coated with a sufficient thickness during the oxidation process using a reactive ion etching chamber (RIE chamber), the lens itself is distorted during the oxidation of the metal surface, and thus a normal lens surface shape cannot be obtained. When the oxide film is formed by applying various conditions as described above and the lattice pattern projected on the oxide film is coated, the metal thin film of about 100 to 200 nm is coated and then the surface of the metal thin film is oxidized through a sufficient oxidation time of about 30 minutes or more. It was confirmed that desirable lens surface shape data could be obtained.

As described above, by forming an oxide film on the surface to be measured for the measurement object having high light transmittance, the sharpness of the grid pattern projected on the surface of the measurement object when the moiré technique is applied can be significantly improved. This significantly improves the accuracy of surface shape measurements using the moiré technique.

In the foregoing detailed description of the present invention, specific embodiments have been described, but various modifications may be made without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined not only by the scope of the following claims, but also by those equivalent to the scope of the claims.

1 is a flowchart illustrating a surface shape measuring method using a moire technique according to an exemplary embodiment of the present invention.

2 is a configuration diagram showing an example of a measuring apparatus for applying the surface shape measuring method using the moire technique according to an embodiment of the present invention.

3 is an image of a grid pattern projected onto a lens in a bare state.

4 is an image of a grid pattern projected onto a lens on which a metal thin film is formed at various thicknesses.

FIG. 5 is an image of a grid pattern projected on a lens according to an oxidation time of a metal thin film having a thickness of 20 nm.

FIG. 6 is an image of a grid pattern projected on a lens according to an oxidation time of a 200 nm thick metal thin film.

Claims (4)

Forming an oxide film on the surface of the shape measurement object; Projecting a grid pattern onto the shape measuring object in parallel light; Detecting a grid pattern image projected onto a shape measurement object by projection of the grid pattern; And Generating a virtual grid pattern having the same interval as the grid pattern and having different angles, and forming a moire pattern by overlapping the virtual grid pattern and the grid pattern image projected on the shape measurement object; Surface shape measurement method using a moiré technique comprising a. The method of claim 1, wherein the forming of the oxide film comprises: Coating a metal thin film on the surface of the shape measurement object; And Surface shape measurement method using the moire technique comprising the step of oxidizing the coated metal thin film. The method of claim 2, The coating of the metal thin film is a step of coating a metal thin film on the surface of the shape measurement object by using a sputtering technique, and the oxidizing is oxidizing the metal thin film using a reactive ion etching chamber. Surface shape measurement method using a Moray method. The method of claim 1, wherein the detecting comprises: Enlarging a grid pattern image projected onto a shape measurement object by projection of the grid pattern; And And detecting the enlarged grid pattern image by using a solid-state image pickup device.

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