JP2019191310A - Projector and spatial frequency component reduction method using the same - Google Patents

Abstract

To provide a projector that reduces a contrast on a side high in spatial frequency in the projector for an inspection of a three-dimensional shape of a measurement object and the like, and suppresses occurrence of a black line in a projection surface arising from a gap between mirrors of a DMD.SOLUTION: A three-dimensional measurement-purpose projector, which includes a DMD (a digital mirror device) forming a fringe pattern alternatively having a bright part and a dark part, has an optical low-pass filter 60 that is provided in an optical path between the DMD 20 and a projection lens 40. The optical low-pass filter 60 has a laminate structure that is configured to have a plurality of pieces of birefringent plates and quarter wave plates of the same number of pieces as the birefringent plate alternatively laminated. In the birefringent plate, a direction of an optical crystal axis of the birefringent plate is set so as to separate light from the DMD 20 along a direction Dp parallel with a fringe of the fringe pattern 70, and further, the optical crystal axis forms an angle of 45° with respect to an optical axis of the light from the DMD 20.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a three-dimensional measurement projector used for industrial inspection and the like, and a spatial frequency component attenuation method using the projector.

  2. Description of the Related Art Conventionally, a technique for inspecting a three-dimensional shape or the like of a measurement object by projecting a predetermined pattern of light onto the measurement object using a projector has been used. Such a projector uses a digital mirror device (DMD) as an image element in addition to optical elements such as a condenser lens, a total reflection prism, and a projection lens. Light from a light source is spatially modulated by the DMD element. A predetermined pattern light is generated, and a predetermined pattern (stripe pattern) of light is projected onto the projection surface (measurement object). Then, applying a phase shift method that temporally changes the light of a predetermined pattern projected on the projection surface, the reflected light from the projection surface is imaged at an angle different from the irradiation direction to the projection surface, and the pattern deviation is analyzed By doing so, it is possible to detect defects such as irregularities (three-dimensional shape) and warpage of the measurement object.

  As a surface inspection apparatus for inspecting unevenness of a measurement object, for example, there is a surface inspection apparatus as described in Patent Document 1. In this surface inspection apparatus, a DMD is used as a spatial modulation means, and projection light from the DMD is irradiated onto the surface of a semiconductor device, which is a measurement object, at a predetermined irradiation angle. The reflected light reflected from the surface of the semiconductor device forms an image with a luminance corresponding to the irradiation angle and is received by a light receiving element such as a CCD. Under such a configuration, the surface of the semiconductor device is inspected by switching the position of the mirror that projects light in the DMD and changing the irradiation angle to the measurement object.

JP 2007-114087 A

  In a projector configured with a DMD, there is a very small gap on the order of submicrons between the mirrors constituting the DMD so that the electrically driven mirrors do not interfere with each other. Due to this gap, there is a problem that a “black line” corresponding to the gap is included in the striped pattern projected on the projection surface. This problem is particularly noticeable when the resolution of the projection lens is increased.

  In general, an MTF (Modulation Transfer Function) curve, which is an index for evaluating lens performance, maintains a high contrast state from a low spatial frequency side to a high spatial frequency side. The reason for the occurrence of the “black line” corresponding to the gap is that when the projection lens is used without performing optical processing, the contrast on the high spatial frequency side is high. This is because the image is projected to the projection plane “sharply” up to the gap. Therefore, in order to suppress the occurrence of “black lines”, in the projector for three-dimensional measurement, the contrast on the higher spatial frequency side is greatly reduced so that the “black lines” do not appear.

  In view of the problems as described above, the present invention reduces the contrast on the high spatial frequency side in the projector for inspecting the three-dimensional shape or the like of the measurement object, and thereby the gap between the mirrors of the DMD. It is an object of the present invention to provide a projector that can more effectively suppress the occurrence of “black lines” on the projection plane.

  In order to solve the above problems, a projector according to the present invention forms a striped pattern having alternating bright and dark portions with a DMD (digital mirror device), and the projection lens projects the pattern onto the projection surface of the measurement object. A projector for three-dimensional measurement that projects toward the projector, and has an optical low-pass filter provided in an optical path between the DMD and the projection lens, and the optical low-pass filter is connected to the DMD from the DMD. A birefringent optical element (for example, the birefringent plates 60a and 60c in the embodiment) that refracts and separates a part of the light in a predetermined direction, and the birefringent optical element receives the light from the DMD. The orientation of the optical crystal axis is set so as to be separated along a direction substantially parallel to the stripe of the stripe pattern (direction Dp parallel to the stripe in the embodiment). There.

  In the projector having the above configuration, it is preferable that an optical crystal axis of the birefringent optical element forms an angle of about 45 ° with respect to an optical axis of light from the DMD.

  In the projector having the above-described configuration, the optical low-pass filter includes a quarter-wave plate, and an optical crystal axis of the quarter-wave plate is substantially the polarization direction of ordinary light that has passed through the birefringent optical element. An angle of 45 ° is preferred.

  In the projector having the above-described configuration, the optical low-pass filter includes a plurality of the birefringent optical elements and the same number of quarter-wave plates as the birefringent optical elements. It is preferable to have a laminated structure in which refractive optical elements and the quarter-wave plates are alternately laminated.

  In the projector having the above configuration, when the plate thickness of the X-th birefringent optical element from the side closest to the DMD is Tx, the plate thickness Tx is set to satisfy Tx = T1 / X. Is preferred.

  In the projector having the above configuration, the number X of the birefringent optical elements is defined as D1 as a separation width between normal rays and extraordinary rays separated by passing through the birefringent optical element closest to the DMD. When the length of one side of one mirror constituting the DMD and L is set, it is preferable to set so as to satisfy the expression D1 = (1/2) * X * L.

In the projector having the above configuration, the number X of the birefringent optical elements is 2 ^X <L / d, where d is the size of the gap between the mirror constituting the DMD and the mirror adjacent thereto. It is preferable to set so as to satisfy the following formula.

  Further, in order to solve the above problem, a stripe pattern having alternating bright and dark portions is formed by a DMD (digital mirror device), and the pattern is projected toward the projection surface of the measurement object by the projection lens. A fringe formed by the DMD by an optical low-pass filter including a birefringent optical element that refracts and separates a part of the light from the DMD in a predetermined direction using a projector for three-dimensional measurement. A method of attenuating a high spatial frequency component of a pattern includes: a plurality of the birefringent optical elements; and a plurality of quarter-wave plates alternately stacked with the birefringent optical elements. (A) providing the optical low-pass filter in an optical path between the DMD and the projection lens, the light from the DMD being Setting the orientation of the optical crystal axis of the birefringent optical element so as to be separated along a direction substantially parallel to the stripes of the striped pattern; and (b) the DMD on the birefringent optical element closest to the DMD. (C) allowing the normal and extraordinary rays to pass through a quarter-wave plate stacked on the nearest birefringent optical element. Thus, the step of converting both the normal ray and the extraordinary ray from linearly polarized light into substantially circularly polarized light, and (d) the X-th birefringent optical element and the quarter wavelength plate laminated thereon, b) and the step of repeating the step (c).

  According to the projector having the above configuration, the optical low-pass filter including the birefringent optical element is provided in the optical path between the DMD and the projection lens, and the birefringent optical element constituting the optical low-pass filter fringes the light from the DMD. By setting the direction of the optical crystal axis so as to be separated along a direction substantially parallel to the pattern stripes, it is projected onto the projection surface by the light that has passed through the optical low-pass filter and separated in a predetermined direction. The light pattern (striped pattern) has high resolution in the direction perpendicular to the striped pattern (strip width direction) and is kept sharp and parallel to the striped pattern (longitudinal direction of the striped pattern) The resolution is low and blurry. That is, by passing through the optical low-pass filter, the contrast of the component having a high spatial frequency is reduced, and the influence of a minute gap (about submicron) between DMD mirrors can be eliminated. As a result, it is possible to prevent the stripe pattern projected on the projection surface from including a “black line” corresponding to a minute gap between DMD mirrors while keeping the stripe boundary sharp. Is possible.

FIG. 1 is a schematic diagram showing components of a projector according to the present invention. FIG. 2 is a diagram for explaining the separation of light by the birefringent optical element constituting the projector. FIG. 3 is a diagram showing the “ON” state (white square) and the “OFF” state (black square) of the mirror when the DMD is arranged so that the DMD mirror group has a honeycomb arrangement. FIG. 4 shows a pattern (striped pattern) projected on the projection surface when light from the DMD in the state of FIG. 3 is projected onto the projection surface through the birefringent optical element. FIG. 5 shows the “ON” state (white square) and “OFF” state (black square) of the mirror when the DMD is rotated by 45 ° about the rotation axis with respect to the state of FIG. FIG. FIG. 6 shows a pattern (striped pattern) projected on the projection surface when light from the DMD in the state of FIG. 5 is projected onto the projection surface through the birefringent optical element.

  An embodiment of a projector for three-dimensional measurement according to the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing components of a projector according to the present invention. The projector 100 according to the present invention includes at least a light source 1, (several) condenser lenses 10, a DMD 20 that is an image element, a total reflection prism 30, a projection lens 40, and an optical low-pass filter 60. The In the illustrated embodiment, the light 2 from the light source 1 composed of an LED or the like is collected by the condenser lens 10 and transmitted through the total reflection prism 30, and then enters the DMD 20 and is reflected by the DMD 20. ) Is guided to the low pass filter 60 by passing through the total reflection prism 30 again, and obliquely projected onto the projection plane 50 after passing through the optical low pass filter 60 and several projection lenses 40. With this configuration, the stripe pattern 70 having a predetermined pattern alternately formed with the bright and dark portions formed by the DMD 20 is enlarged by the projection lens 40 and projected onto the projection surface 50 of the measurement object. Since the projection lens 40 obliquely projects toward the projection surface 50, the projection lens 40 is disposed so as to satisfy the Scheinproof condition. Thus, the light 3 including the predetermined stripe 70 is focused on both the front side (side closer to the projection lens 40) and the back side (side far from the projection lens 40) of the projection surface 50.

  The DMD 20 that generates the pattern of light to be projected (stripe pattern 70) has a plurality of minute mirrors arranged in a two-dimensional manner with a spacing of submicron order (for example, 0.5 μm). It is possible to drive individual mirrors by electrically controlling the mirrors independently. Thereby, the light 2 emitted from the light source 1 is spatially modulated by the DMD 20 into light 3 including a predetermined pattern such as a striped pattern, and various patterns (striped pattern 70) are projected onto the projection surface 50.

  The optical low-pass filter 60 of the present embodiment has two birefringent optical elements whose refractive index changes depending on the polarization state, and two quarter-wave plates. These two birefringent optical elements and two quarter-wave plates are alternately laminated to form an optical low-pass filter 60 as a unit. Examples of the material of the birefringent optical element include quartz and calcite.

  FIG. 2 shows the separation of light by the birefringent optical element constituting the optical low-pass filter 60. As shown in FIG. 2, the horizontal direction on the paper is the X direction, the vertical direction on the paper is the Y direction, and the optical axis direction of the light 3 is the Z direction. These directions are orthogonal to each other. The birefringent optical element and the quarter-wave plate constituting the optical low-pass filter 60 will be described here as having a cubic shape. Of the stacked birefringent optical elements, the birefringent optical element closest to the total reflection prism 30 (hereinafter referred to as “birefringent plate 60a”) does not receive light even when light is incident thereon, as shown in FIG. The optical crystal axis (thick solid double arrow) that is not separated is on the XZ plane of the birefringent plate 60a, is at an angle of 45 ° with respect to the optical axis direction of the light 3, and the plate thickness is T1. It consists of the elements cut out like this. In the light 3 passing through the birefringent plate 60a, since the optical crystal axis and the optical axis are inclined, an extraordinary ray that does not follow Snell's law is refracted in the direction of the optical crystal axis, and becomes an ordinary ray and an extraordinary ray. They are separated (lights 3a and 3b in FIG. 2). Here, the optical crystal axis of the birefringent plate 60a is projected onto the XY plane so that the direction in which the ordinary ray and the extraordinary ray are separated and the direction Dp parallel to the stripe of the stripe pattern formed by the DMD 20 coincide. The direction of the line (a double-pointed arrow of the thick chain line) is set to be parallel to the direction Dp parallel to the stripe (the X direction). The thin arrows shown on the lights 3a and 3b in FIG. 2 indicate the vibration directions of the respective linearly polarized light. Here, since each polarization component is equal, it is possible to separate an ordinary ray and an extraordinary ray with the same intensity.

  When the light 3a and 3b are separated, the stripe formed by the DMD 20 passes through the birefringent plate 60a, and the direction of the line (X direction) in which the optical crystal axis of the birefringent plate 60a is projected onto the XY plane. Slightly blurry. Here, the blurred stripe pattern has high resolution in the direction Dv (stripe width direction) perpendicular to each stripe and remains sharp, and the resolution in the direction Dp (longitudinal direction of the stripe) parallel to the stripes. Is low and blurry.

  The birefringent plate 60a is laminated by joining a quarter-wave plate 60b. The optical crystal axis (a thick solid double arrow) of the quarter-wave plate 60b is on the XY plane, and forms an angle of 45 ° with respect to the polarization direction of the ordinary ray (the vertical direction of the paper). . Moreover, the direction of the line which projected the optical crystal axis of the quarter wavelength plate 60b on the XZ plane is an X direction (a double-dashed double arrow). The light 3a, 3b that has passed through the birefringent plate 60a generates a phase difference by passing through the quarter-wave plate 60b, and both the light 3a and the light 3b are converted from linearly polarized light to circularly polarized light. The circle shown in FIG. 2 indicates circularly polarized light. Thus, once converted from linearly polarized light to circularly polarized light, the light 3a and the light 3b can be further separated as follows.

  The quarter-wave plate 60b has an optical crystal axis (a thick solid double-pointed arrow) on the XZ plane, and is cut out so that the plate thickness is T2 at an angle of 45 ° with respect to the optical axis direction of the light 3. The birefringent plates 60c are joined. Further, the direction of the line (thick double-chain double arrow) obtained by projecting the optical crystal axis of the birefringent plate 60c onto the XY plane is parallel to the fringe pattern formed by the DMD 20 as in the case of the birefringent plate 60a. Parallel to the direction Dp (X direction). Thereby, the light 3a that passes through the birefringent plate 60c is separated into light 3a ′ that is ordinary light and 3c that is extraordinary light, and light 3b that passes through the birefringent plate 60c is light 3b ′ that is ordinary light. And 3d which is an extraordinary ray. These lights 3a ', 3b', 3c and 3d are in the state of linear polarization. The thin double arrows shown on the lights 3a ', 3b', 3c, and 3d in FIG. 2 indicate the vibration directions of the respective linearly polarized light. When the stripe formed by the DMD 20 passes through the birefringent plate 60c, the stripe is quadruple with respect to the original stripe in the direction of the line (X direction) projected on the XY plane of the optical crystal axis of the birefringent plate 60c. It will shift.

  Further, a quarter-wave plate 60d is joined to the birefringent plate 60c. The optical crystal axis (a thick solid double arrow) of the quarter-wave plate 60d is on the XY plane, and forms an angle of 45 ° with respect to the polarization direction of the ordinary ray (up and down direction on the paper surface). . Moreover, the direction of the line which projected the optical crystal axis of the quarter wavelength plate 60b on the XZ plane is an X direction (a double-dashed double arrow). The light 3a ′, 3b ′, 3c, and 3d that has passed through the birefringent plate 60c undergoes a phase difference again by passing through the quarter-wave plate 60d, and any of the light 3a ′, 3b ′, 3c, and 3d. Are converted from linearly polarized light to circularly polarized light. Thus, the light 3a ', 3b', 3c, 3d that has passed through the quarter-wave plate 60d can be separated by passing it through another birefringent plate (not shown).

  In this way, the light 3 incident on the birefringent plate 60a is finally separated into the light 3a ′, 3b ′, 3c, and 3d, so that the stripe pattern formed by the DMD 20 has a direction perpendicular to the stripe. The resolution decreases in the direction Dp (longitudinal direction of the stripe) parallel to the stripe while maintaining a high resolution in Dv (width direction of the stripe). Therefore, the optical low-pass filter 60, which is a birefringent optical element, can constitute an optical low-pass filter having predetermined characteristics (characteristics in which the contrast on the higher spatial frequency side is greatly reduced) with respect to the spatial frequency. Become.

  In the present embodiment, two birefringent plates (birefringent plates) in which the optical crystal axis is on the XZ plane and the optical crystal axis is cut at an angle of 45 ° with respect to the optical axis direction of the light 3 as described above. 60a, 60c), and the light 3 incident from the DMD 20 through the total reflection prism 30 is separated into an ordinary ray and an extraordinary ray twice (two steps) by the birefringent plates 60a, 60c. When the number of times that the light 3 is separated into an ordinary ray and an extraordinary ray (the number of birefringent plates provided) is X, and the thickness of the first (birefringent plate 60a) is T1, the thickness of each birefringent plate. The length Tx is configured to have a relationship of Tx = T1 / x. That is, the thickness of the second sheet (birefringent plate 60c) is half the thickness of the birefringent plate 60a.

  The number of birefringent plates is not limited to two in this embodiment, and may be one or three or more. As the number of birefringent plates increases, that is, as the value of X increases, the “black line” generated on the projection surface 50 due to the gap between the mirrors of the DMD 20 becomes thinner (the contrast becomes lower). Smooth). When X birefringent plates are used, the optical low-pass filter is configured by alternately laminating X birefringent plates and X quarter-wave plates from the DMD 20 incident on the optical low-pass filter. The light of, is separated into ordinary rays and extraordinary rays by passing through the birefringent plate, and conversion from linearly polarized light to circularly polarized light by passing through the quarter-wave plate is repeated, resulting in multiple stripes A pattern will be formed.

Here, the width (separation width) D1 between the ordinary ray and the extraordinary ray shown in FIG. 2 is proportional to the thickness T1 of the birefringent plate 60a, and ne and no are the ordinary ray and extraordinary ray, respectively. Is a refractive index when passing through the birefringent plate 60a, and θ is an angle formed by the optical crystal axis and the optical axis (θ = 45 ° in the embodiment), the following equation is established.
D1 = T1 * ((2ne-2no) SinθCosθ) / (2neSin2θ + 2noCos2θ) (Formula 1)
Therefore, by setting the thickness of the birefringent plate 60a, it is possible to control the separation width between ordinary rays and extraordinary rays.

The number X of the birefringent plates is between the length L of one side of one mirror of D1 and DMD20,
D1 = (1/2) * X * L (Formula 2)
It is desirable to set so that this relationship is established.

Further, the number X of the birefringent plates is such that d is the distance between the mirrors of the DMD 20 in order to eliminate the “black line” generated on the projection surface 50.
2 ^X <L / d (Formula 3)
It is desirable to set a range that satisfies the above formula.

  Here, when one side L of one mirror (square) of the DMD 20 is 10 μm and the distance d between the mirrors is 0.8 μm, the range satisfying (Expression 3) by applying the above (Expression 3) When the maximum X is selected, the number X of birefringent plates is 3, and the optical low-pass filter 60 includes three birefringent plates and three quarter-wave plates alternately stacked on the birefringent plates. It is appropriate to configure. Further, when one side L of one mirror of the DMD 20 is 10 μm and the distance d between the mirrors is 0.8 μm, the above (Equation 3) is applied and the maximum X is selected in the same way. It is appropriate to set the number of refracting plates X to 4 (the optical low-pass filter 60 is composed of four birefringent plates and four quarter-wave plates alternately stacked on the birefringent plates). .

  3 to 6 show that the influence of the gap between the mirrors of the DMD can be suppressed by the projector using the birefringent optical element according to the present invention. FIG. 3 shows the “ON” state (white square) and “OFF” state (black square) of the mirror when the DMD is arranged so that the mirror group of the DMD 20 has a honeycomb arrangement. 4 shows the projection surface 50 when the light from the DMD 20 in the state of FIG. 3 is projected onto the projection surface 50 through the birefringent optical element 60 using two birefringent plates (the number X is 2). A pattern (striped pattern 70) shown in FIG. As shown in FIG. 4, in the direction parallel to the stripes, the portion corresponding to the gap between the mirrors in the “ON” state is white or blurred, and “black” caused by the gap The line is clearly thinner. On the other hand, high resolution is maintained in the direction perpendicular to the stripes, and the border between the bright and dark portions of the stripes is clear without being blurred.

  On the other hand, FIG. 5 shows the “ON” state (white square) and “OFF” state (black square) of the mirror when the DMD 20 is rotated by 45 ° about the rotation axis with respect to the state of FIG. Indicates. Also in this case, as shown in FIG. 6, in the direction parallel to the stripes, the portion corresponding to the gap between the mirrors in the “ON” state is white or blurred. In particular, in the case of FIG. 6, the “black line” has disappeared remarkably, and the influence of the gap between the mirrors of the image element 20 can be suppressed more effectively than in the case of FIG. 4. Also in this case, the boundary between the bright part and the dark part of the stripe is clear without being blurred.

1 Light source 10 Condenser lens 20 DMD
DESCRIPTION OF SYMBOLS 30 Total reflection prism 40 Projection lens 50 Projection surface 60 Optical low-pass filter 60a, 60c Birefringent plate (birefringent optical element)
60b, 60d 1/4 wavelength plate 70 Striped pattern 100 Projector

Claims (8)

A projector for three-dimensional measurement, in which a striped pattern having alternating bright and dark portions is formed by a DMD (digital mirror device), and the pattern is projected onto a projection surface of a measurement object by a projection lens. ,
An optical low-pass filter provided in the optical path between the DMD and the projection lens;
The optical low-pass filter includes a birefringent optical element that refracts and separates a part of the light from the DMD in a predetermined direction;
The birefringent optical element has a direction of an optical crystal axis set so as to separate light from the DMD along a direction substantially parallel to the stripes of the stripe pattern. The projector according to claim 1, wherein
The projector according to claim 1, wherein an optical crystal axis of the birefringent optical element forms an angle of about 45 ° with respect to an optical axis of light from the DMD. The projector according to claim 1 or 2,
The optical low-pass filter includes a quarter-wave plate;
The projector according to claim 1, wherein an optical crystal axis of the quarter-wave plate forms an angle of about 45 ° with respect to a polarization direction of ordinary light that has passed through the birefringent optical element. In the projector as described in any one of Claims 1-3,
The optical low-pass filter includes a plurality of the birefringent optical elements and the same number of quarter-wave plates as the birefringent optical elements, and the birefringent optical elements and the 1/4 A projector having a laminated structure in which wave plates are alternately laminated. The projector according to claim 4, wherein
The projector is characterized in that when the plate thickness of the X-th birefringent optical element closest to the DMD is Tx, the plate thickness Tx is set to satisfy Tx = T1 / X. The projector according to claim 5, wherein
The number X of the birefringent optical elements is such that a separation width between a normal ray and an extraordinary ray separated by passing through the birefringent optical element closest to the DMD is D1, and one DM constituting the DMD A projector characterized by being set to satisfy an expression of D1 = (1/2) * X * L, where L is the length of one side of the mirror. The projector according to claim 5, wherein
The number X of the birefringent optical elements is set so as to satisfy the expression 2 ^X <L / d, where d is the size of the gap between the mirror constituting the DMD and the mirror adjacent thereto. A projector characterized by that. Using a three-dimensional measurement projector that forms a stripe pattern having alternating bright and dark portions with a DMD (digital mirror device) and projects the pattern onto a projection surface of a measurement object using a projection lens A high spatial frequency component of the stripe pattern formed by the DMD is attenuated by an optical low-pass filter including a birefringent optical element that refracts and separates a part of the light from the DMD in a predetermined direction. A method,
The optical low-pass filter includes a plurality of the birefringent optical elements and a plurality of quarter-wave plates alternately stacked with the birefringent optical elements,
(A) A step of providing the optical low-pass filter in an optical path between the DMD and the projection lens so as to separate light from the DMD along a direction substantially parallel to the stripes of the stripe pattern. Setting the orientation of the optical crystal axis of the birefringent optical element;
(B) separating light into normal rays and extraordinary rays by passing light from the DMD through the birefringent optical element closest to the DMD;
(C) By passing the normal ray and the extraordinary ray through the quarter-wave plate laminated on the nearest birefringent optical element, both the normal ray and the extraordinary ray are converted from linearly polarized light to substantially circularly polarized light. Steps,
(D) repeating the step (b) and the step (c) for the Xth birefringent optical element and the quarter wavelength plate laminated thereon;
A method characterized by comprising.