CALIBRATION APPARATUS, CALIBRATION METHOD, AND MEASUREMENT APPARATUS

The present invention provides a calibration apparatus for calibrating an optical apparatus, which has a scanning member and scans light on an object by rotating the scanning member, including a target member including a region irradiated with light from the scanning member, an obtaining unit configured to obtain a light amount of light reflected by the region, and a processing unit configured to execute processing for calculating a calibration value required to calibrate a rotation angle of the scanning member, wherein the region is configured to be nonplanar so that a light amount obtained by the obtaining unit changes according to a light irradiation position.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Note that the same reference numerals denote the same members throughout the drawings, and a repetitive description thereof will not be given.

FIG. 1is a schematic view showing the arrangement of a measurement apparatus1having a calibration apparatus according to one aspect of the present invention. The measurement apparatus1is a three-dimensional shape measurement apparatus which measures a shape of an object MT using a measurement head104including a galvanometer mirror (scanning member). The calibration apparatus is a calibration apparatus used to calibrate a rotation angle (a deviation thereof) of the galvanometer mirror of the measurement head104of the measurement apparatus1. However, the calibration apparatus is also applicable to calibration of an optical apparatus which has a galvanometer mirror and scans light on an object upon rotation of that galvanometer mirror (for example, a laser processing apparatus and the like).

The measurement apparatus1includes a surface plate101, an XYZ stage102, a rotation stage103, the measurement head104, a target member105, and a processing unit106. Note that the target member105is not a member used upon measurement of the shape of the object MT, but is a member used upon calibration of the measurement apparatus1. The processing unit106includes a CPU, memory, and the like, and controls the entire (operation of) measurement apparatus1. For example, the processing unit106executes not only processing for measuring the shape of the object MT (that is, it functions as a calculation unit for calculating the shape of the object MT based on light reflected by the object MT) but also processing for calculating a calibration value required to calibrate a rotation angle of the galvanometer mirror. In this manner, the target member105and processing unit106configure a part of the calibration apparatus in this embodiment.

The XYZ stage102is set on the surface plate101. On the XYZ stage102, the rotation stage103and measurement head104are set. The measurement head104irradiates (projects) the object MT with light, and detects light reflected or scattered by the object MT, thus measuring a distance between the measurement head104and object MT.

An example of the practical arrangement of the measurement head104will be described below with reference toFIG. 2. The measurement head104includes a fiber port201, half mirror202, reference mirror203, galvanometer mirrors204and205, detector206, and scanning unit207. The fiber port201, half mirror202, reference mirror203, and galvanometer mirrors204and205function as an irradiation unit which irradiates the object MT with light. The detector206functions as a detection unit which detects light reflected or scattered by the object MT. The scanning unit207scans light on the object MT or target member105while rotating the galvanometer mirrors204and205.

Light coming from a light source is guided to the measurement head104via a fiber or the like, and exists from the fiber port201. Light from the fiber port201enters the half mirror202, and is split into light reflected by the half mirror202and light transmitted through the half mirror202. The light reflected by the half mirror202is reflected by the reference mirror203, and enters the half mirror202again as reference light. On the other hand, the light transmitted through the half mirror202is reflected by the galvanometer mirrors204and205, and is projected onto the object MT. Light reflected or scattered by the (surface of the) object MT enters the half mirror202again as detected light.

The galvanometer mirror204has a rotation axis along a z axis, and the galvanometer mirror205has a rotation axis along a y axis. Therefore, when the galvanometer mirror204is rotated, light is scanned on the object MT in a y-axis direction, and when the galvanometer mirror205is rotated, light is scanned on the object MT in an x-axis direction. In this manner, since the measurement head104includes the two galvanometer mirrors204and205, it can irradiate the object MT with light while scanning the light two-dimensionally.

The reference light and detected light which have entered the half mirror202form interference light. The detector206detects the interference light formed by the reference light and detected light, and outputs an interference signal. The processing unit106calculates a difference (optical path length difference) between a reference light optical path length and detected light optical path length based on the interference signal output from the detector206. The optical path length difference can be calculated using a method of calculating a relative distance from a certain reference, a method of calculating an absolute distance from measurement using light beams of a plurality of wavelengths, or the like. The processing unit106calculates a position (x, y, z) of an arbitrary point (actual irradiation position) of the object MT based on a position (coordinates) of the XYZ stage102, a rotation angle of the rotation stage103, rotation angles of the galvanometer mirrors204and205, and the detected light optical path length (optical path length difference).

The principle of calibration of the rotation angles of the galvanometer mirrors204and205using the target member105will be described below with reference toFIGS. 3A and 3B.FIGS. 3A and 3Bshow only a detected light optical path in the measurement head104, and only the galvanometer mirror205of the two galvanometer mirrors204and205.

The target member105includes a region irradiated with light reflected by the galvanometer mirrors204and205, and this region is nonplanar. More specifically, this region is configured by a mirror element (reflection member)301having a curvature of a convex surface, and is laid out in a two-dimensional pattern inFIGS. 3A and 3B. However, the mirror element301may be configured to, for example, change a light amount detected by the detector206depending on a light irradiation position, and may have a curvature of a concave surface. Also, in this embodiment, a light amount of light reflected by the mirror element301is detected by the detector206. Alternatively, a detection unit which detects a light amount of light reflected by the mirror element301may be arranged independently of the detector206. In this manner, the detector206functions as an obtaining unit which obtains a light amount of light reflected by the mirror element301, and configures a part of the calibration apparatus in this embodiment.

FIGS. 3A and 3Bshow a state in which one of a plurality of mirror elements301of the target member105is irradiated with light. Light reflected by the galvanometer mirror205is reflected by the (surface of the) mirror element301, is transmitted through the half mirror202, and enters the detector206. A light amount of light reflected by the mirror element301may be calculated from a contrast of interference light detected by the detector206, or only light reflected by the mirror element301may be detected by the detector206while shielding light coming from the reference mirror203. The mirror element301is positioned to the galvanometer mirror (scanning member)205in a predetermined position in advance.

A case will be examined first wherein a center (center of curvature) C of the mirror element301is located on an extension of light reflected by the galvanometer mirror205, as shown inFIG. 3A. In this case, light reflected by the galvanometer mirror205is perpendicularly reflected by the (surface of the) mirror element301, and enters a central portion of a detection surface of the detector206. Also, a case will be examined below wherein the center C of the mirror element301is not located on the extension of light reflected by the galvanometer mirror205, as shown inFIG. 3B. In this case, since the extension of light reflected by the galvanometer mirror205deviates from the center C of the mirror element301, light reflected by the galvanometer mirror205is not perpendicularly reflected by the mirror element301, and enters a position deviated from the central portion of the detection surface of the detector206. In this manner, when the mirror element301and detector206do not have an optically conjugate relationship, a light amount detected by the detector206changes due to a change in rotation angle of the galvanometer mirror205. Therefore, the rotation angle of the galvanometer mirror205can be calculated based on the light amount detected by the detector206.

The relationship between the rotation angle of the galvanometer mirror205and the light amount (light amount value) of light, which is reflected by the mirror element301and is detected by the detector206, will be described below. In this embodiment, since light coming from a light source is guided to the measurement head104via a fiber or the like, it forms an Airy pattern on the detection surface of the detector206. Let d be a deviation amount between the center of the detection surface of the detector206and that of the Airy pattern due to rotation of the galvanometer mirror205(that is, a deviation amount between the center of the detection surface of the detector206and an incident position of light reflected by the mirror element301on the detection surface). Also, letabe a radius of an integration region of the Airy pattern detected on the detection surface of the detector206(that is, a radius of light which is reflected by the mirror element301and enters the detection surface of the detector206), and (x, y) be position coordinates on the detection surface of the detector206. In this case, the relationship between the deviation amount d between the center of the detection surface of the detector206and that of the Airy pattern and a light amount IAirydetected by the detector206is expressed by:

where J1( ) expresses a Bessel function of the first kind, which assumes that light entering the detection surface of the detector206is deviated in the y direction. Also, the deviation amount d and the radiusaof the integration region in equation (1) can be expressed by variables of an actual system. Note that the variables of the actual system include a radius of curvature R of the mirror element301, a radius (beam spot radius) BS of light entering the target member105, and a radius Rd of the detection surface of the detector206. Furthermore, the variables of the actual system also include a distance WD1 from the galvanometer mirror205to the target member105, and a distance WD2 from the detector206to the target member105.

The variables (R, BS, Rd, WD1, and WD2) of the actual system will be described below with reference toFIG. 4.FIG. 4shows a section of the mirror element301when light comes from a position immediately above the center of curvature of the mirror element301.

The relationship between the deviation amount d and the variables (R, BS, Rd, WD1, and WD2) of the actual system will be described first. A deviation amount d1 of light incident on the target member105(mirror element301) caused by a rotation angle θ of the galvanometer mirror205is expressed using the distance WD1 from the galvanometer mirror205to the target member105by:

When the radius of curvature R of the mirror element301is used, a reflection angle θtof light perpendicularly entering a position shifted by d1 from the perpendicularly reflected position with respect to the mirror element301is expressed by:

A reflection angle θr of light reflected by the mirror element301with respect to the rotation angle θ of the galvanometer mirror205is given by θr=2(2θ+θt). Therefore, a deviation amount d2 of light reflected by the mirror element301is expressed using the distance WD2 from the detector206to the target member105by:

Therefore, the deviation amount d between the center of the detection surface of the detector206and that of the Airy pattern with respect to the rotation angle θ of the galvanometer mirror205is expressed by:

Next, the radiusaof the integration region and the variables (R, BS, Rd, WD1, and WD2) of the actual system will be described below. Assume that light entering the target member105is parallel light having sufficiently small NA. Using the radius Rd of the detection surface of the detector206, a radius dAof light detected by the detector206on the target member105is expressed by:

This radius dAcan be calculated from the relationship in which light reflected by the reflection angle θt given by equation (3) matches the radius Rd of the detection surface of the detector206.

Therefore, by substituting the radius (the first dark ring of the Airy pattern) BS of light entering the target member105into equation (6), the radiusaof the integration region is expressed by:

Note that the radiusaof the integration region may be adjusted by laying out an aperture in front of (incident surface side) of the detection surface of the detector206to select only required light.

By substituting equations (5) and (7) into equation (1), an ideal value of a light amount detected by the detector206, that is, an ideal light amount IAirywith resect to the rotation angle θ of the galvanometer mirror205can be calculated.

Assuming that a rotation angle of the galvanometer mirror205when a light amount detected by the detector206is a maximum light amount is 0, a ratio e between the maximum light amount and a light amount detected by the detector206when the rotation angle of the galvanometer mirror205is θ1 is expressed by:

For example, if the variables of the actual system are set to be R=10 mm, BS=15 μm, Rd=0.5 mm, WD1=150 mm, and WD2=150 mm, the relationship between the deviation amount d in the x direction and the light amount IAiryis indicated by a solid curve inFIG. 5from equation (1).FIG. 5also shows the relationship between the deviation amount in the x direction and ideal light amount IAiryin the related art (that is, when the target member is a plane mirror (R=∞ and Rd=0.003 mm) by a dotted curve. Furthermore,FIG. 5plots the ratio ε on the abscissa, and the deviation amount d in the x direction on the ordinate.

Referring toFIG. 5, in this embodiment, the deviation amount d when the light amount detected by the detector206is halved (ε=0.5) is 0.2 μm, and the rotation angle θ of the galvanometer mirror205is 0.67 μrad. On the other hand, in the related art, the deviation amount d when the light amount detected by the detector206is halved is 6.4 μm, and the rotation angle θ of the galvanometer mirror205is 22 μrad. As can be seen from the above description, in this embodiment, a change in light amount detected by the detector206with respect to the position deviation of light projected onto the target member105is increased to about 30 times compared to the related art. Therefore, in this embodiment, since the rotation angle of the galvanometer mirror205can be precisely detected, the rotation angle (a deviation thereof) of the galvanometer mirror can be precisely calibrated.

The aforementioned principle of calibration of the rotation angle of the galvanometer mirror, that is, calibration of the galvanometer mirrors204and205using the target member105will be described in detail below with reference toFIG. 6. In this case, especially, processing for calculating calibration values required to calibrate the rotation angles of the galvanometer mirrors204and205will be explained. As shown inFIG. 6, on the target member105, cylindrical mirrors601each having a concave surface are configured on regions irradiated with light reflected by the galvanometer mirrors204and205. However, each cylindrical mirror601need only be configured to change the light amount detected by the detector206, and may be that of a convex surface. The cylindrical mirrors601are laid out in a two-dimensional pattern, and axes of the cylindrical mirrors601are directed in a plurality of directions including the x and y directions. Also, the position (coordinates) of the center of curvature of each cylindrical lens601has been precisely calibrated using a contact type three-dimensional coordinate measuring machine (CMM). Thus, the region of reflection surface of the cylindrical mirror601on the target member105is positioned to the measurement head104in a predetermined position (measurement position). In other words, the reflection surface of the cylindrical mirror601is positioned to the galvanometer mirror (scanning member)204.

Initially, the position of the target member105arranged on the surface plate101is measured, and the measurement head104is moved to the measurement position. More specifically, the measurement head104is moved to a position immediately above each plane mirror603formed on the target member105, perpendicularly irradiates that plane mirror603with light, and detects light reflected by the plane mirror603, thereby calculating a distance from the measurement head104to the plane mirror603. In this case, a distance between each of three or more plane mirrors603formed on the target member105and the measurement head104is calculated, and a position and tilt (θx, θy) of the target member105in the z direction are measured. Then, the measurement head104is moved to a position immediately above each of an X alignment mark604and Y alignment mark605formed on the target member105. In this embodiment, the X alignment mark604and Y alignment mark605are configured by steps, but they may be configured by reflection films or the like. For example, when the X alignment mark604used in alignment of the x-axis direction is irradiated with light, a light amount (length measured value) of light reflected by the X alignment mark604changes according to the position on the target member105in the x-axis direction, as shown inFIG. 7. Referring toFIG. 7, the position of the X alignment mark604can be specified from a position where a light amount of light reflected by the X alignment mark604changes largely. InFIG. 7, the abscissa plots the position on the target member105in the x-axis direction, and the ordinate plots the light amount of light reflected by the X alignment mark604. In this manner, using the X alignment mark604and Y alignment mark605, the position (that on an x-y plane) on the target member105is measured. Then, based on the position on the target member105, the measurement head104is moved to the measurement position. In this embodiment, the central position on the target member105in the x- and y-axis directions, and the position separated from the target member105by a distance WD in the z-axis direction correspond to the measurement position of the measurement head104. In this case, a distance from the target member105to the measurement head104, more specifically, that to the galvanometer mirror204is uniquely determined.

After the measurement head104is moved to the measurement position, the galvanometer mirror205is rotated so that a reference position (a position of the center of curvature) of each cylindrical mirror601of the target member105is irradiated with light. Then, in this state, a (change in) light amount of light reflected by the cylindrical mirror601is detected by the detector206.FIG. 8shows an example of the light amount of light which is reflected by the cylindrical mirror601and is detected by the detector206. InFIG. 8, the abscissa plots the position on the target member105in the x-axis direction, and the ordinate plots the light amount of light reflected by the cylindrical mirror601. Referring toFIG. 8, the light amount is maximized (a maximum light amount) at the position of the center of curvature of the cylindrical mirror601. Letting Lx1be a distance from that position (x coordinate) corresponding to the maximum light amount to the position of a reflection point of the galvanometer mirror205in the x-axis direction, a rotation angle θx1of the galvanometer mirror205is expressed by:

Also, letting Lx0be a reference distance from the position of the center of curvature of the cylindrical mirror601in the x-axis direction to the position of a reflection point of the galvanometer mirror205in the x-axis direction, a reference angle θx0of the galvanometer mirror205is expressed by:

Note that the position of the center of curvature of the cylindrical mirror601in the x-axis direction has been obtained in the aforementioned calibration using the CMM.

A difference Δθx(=θx1−θx0) between the reference angle θx0and rotation angle θx1is used as a calibration value (correction value) of the rotation angle of the galvanometer mirror205. This calibration value is calculated for all the cylindrical mirrors601formed on the target member105, thereby precisely calibrating the rotation angle of the galvanometer mirror205on a two-dimensional region irradiated with light reflected by the galvanometer mirror205. More specifically, the calibration values are held in a memory of the processing unit106, and the rotation angle of the galvanometer mirror205is calibrated for each scanning angle, thereby precisely controlling the actual irradiation position of light reflected by the galvanometer mirror205.

As described above, as processing for calculating the calibration value required to calibrate the rotation angle of the galvanometer mirror205, the following three processes (first, second, and third processes) need only be executed. In the first process, in a state in which the galvanometer mirror205is rotated so as to irradiate the reference position of each cylindrical mirror601on the target member105with light, the light amount of light reflected by the cylindrical mirror601is detected by the detector206. In the second process, an actual irradiation position actually irradiated with light with respect to the cylindrical mirror601in the first process is calculated based on the light amount detected by the detector206. In the third process, a calibration value required to calibrate the rotation angle of the galvanometer mirror205is calculated from a difference between the actual irradiation position calculated in the second process and the reference position of the cylindrical mirror601.

In this embodiment, since the cylindrical mirrors601are formed in a two-dimensional pattern, a deviation of the rotation angle generated as a combination of an error in the x-axis direction and that in the y-axis direction can also be detected. In other words, since the cylindrical mirrors601are formed in a two-dimensional pattern, the rotation angle of the galvanometer mirror205can be calibrated more precisely than a case in which the cylindrical mirrors601are formed in a one-dimensional pattern.

According to the measurement apparatus1of this embodiment, a deviation of the rotation angle of the galvanometer mirror205can be precisely calibrated, and a position of light (that reflected by the galvanometer mirror205) projected from the measurement head104onto the object MT can be precisely controlled. Therefore, the measurement apparatus1can precisely measure the shape of the object MT.

Also, a case will be examined wherein the measurement head104is rotated through 90° by the rotation stage103, as shown inFIG. 9. In this case, since the rotation stage103, measurement head104, and galvanometer mirrors204and205are set in a state different from that shown inFIG. 6due to deformations caused by self weights, the galvanometer mirrors204and205suffer different deviations of the rotation angles from the state shown inFIG. 6. Therefore, the target member105is required to be laid out parallel to an x-z plane, as shown inFIG. 9, so as to calibrate the rotation angles of the galvanometer mirrors204and205. Note that as for calibration of the rotation angles of the galvanometer mirrors204and205, the z-axis direction shown inFIG. 6is replaced by the y-axis direction shown inFIG. 9, and the x-y plane shown inFIG. 6is replaced by the x-z plane shown inFIG. 9. Hence, a detailed description thereof will not be repeated.

On the target member105, a phase shift element701and plane mirror702may be configured on a region irradiated with light reflected by the galvanometer mirrors204and205, as shown inFIG. 10. The phase shift element701generates a λ/4 phase difference by the (step having the) optical path length difference between left and right regions701aand701b. Therefore, light projected onto the target member105is transmitted through the phase shift element701and is reflected by the plane mirror702, thus generating a λ/2 phase difference between the regions701aand701bof the phase shift element701.

When light is scanned on the phase shift element701, when it reaches a central position (a boundary between the regions701aand701b) of the phase shift element701, a light amount detected by the detector206becomes zero. This is because light amounts cancel with each other by light which has passed through the region701aof the phase shift element701and that which has passed through the region701bof the phase shift element701. Therefore, the target member105on which the phase shift element701and plane mirror702are configured as shown inFIG. 10can obtain doubled sensitivity compared to the related art.

On the target member105, a mirror element801having a conical shape of a convex surface may be configured on a region irradiated with light reflected by the galvanometer mirrors204and205, as shown inFIG. 11. When light is scanned along the x axis on the target member105, a reflection angle of light largely changes depending on the positive and negative sides of the x axis with respect to the center of the mirror element801. Therefore, the mirror element801can improve sensitivity to deviations of the rotation angles of the galvanometer mirrors204and205compared to the related art as in the mirror element301having a curvature of a convex surface. Note that the mirror element801need only be configured to change the light amount detected by the detector206according to a light irradiation position, and it may have a conical shape of a concave surface or a pyramidal shape of a concave or convex surface.

Also, on the target member105, a reflection member having a reflectance distribution may be configured on the region irradiated with light reflected by the galvanometer mirrors204and205. For example, by setting a reflectance near the center of the reflection member higher than that of a peripheral portion, the light amount detected by the detector206can change according to the light irradiation position. Therefore, the reflection member having the reflectance distribution can improve sensitivity to deviations of the rotation angles of the galvanometer mirrors204and205compared to the related art as in the mirror element301having a curvature of a convex surface.

Furthermore, using a micromotion stage901which moves while holding the target member105, as shown inFIG. 12, a calibration value required to calibrate the rotation angle of the galvanometer mirror can be calculated precisely (at a high resolution).

More specifically, the target member105held by the micromotion stage901is laid out parallel to the x-y plane, and the rotation angle of the galvanometer mirror is calibrated, as described above.

Next, the micromotion stage901is moved by a small amount ΔL in the x-axis direction. A reference angle θ′x0of the galvanometer mirror in this state is expressed by:

Therefore, since a small difference is generated in the angle of the galvanometer mirror in the x-axis direction before and after movement of the micromotion stage901, a calibration value required to calibrate the rotation angle of the galvanometer mirror can be calculated as an angle at a finer interval. In other words, processing for moving the micromotion stage901which holds the target member105and that for detecting a light amount of light reflected by the mirror element301of the target member105by the detector206are alternately repeated a plurality of times, thus calculating calibration values more precisely. Likewise, by moving the micromotion stage901by a small amount in the y-axis direction, the rotation angle of the galvanometer mirror in the y-axis direction can be calculated as an angle at a finer interval.

The above embodiment has explained the case using the galvanometer mirror as a scanning member of an optical apparatus. As the scanning member, a member having a function of deflecting a light beam can be used, and a polygonal mirror or prism may be used in place of the galvanometer mirror. Also, a light beam may be deflected (scanned) using an acoustooptic element (AOM). A piezoelectric element may be used as an AOM element, and ultrasonic waves (high-frequency waves) may be applied to change a frequency, thereby scanning a light beam.

This application claims the benefit of Japanese Patent Application Nos. 2012-229243 filed on Oct. 16, 2012, and 2013-191036 filed on Sep. 13, 2013, which are hereby incorporated by reference herein in their entirety.