JP2015175603A - Calibration method for reference mirror surface shape of interferometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a calibration method for calibrating a geometrical error of a surface shape of a reference mirror using a reference device whose geometrical error of the surface shape is unknown.SOLUTION: The interferometer comprises a reference mirror 33 whose geometrical error of the surface shape is unknown and which is for generating interference light between the measurement object surface and it. A reference device WO whose geometrical error of the surface shape is unknown is disposed at a position of the measurement object. While parts of the measurement regions are overlapped, the reference mirror 33 is moved to scan so as to measure the surface shapes of a plurality of regions in each of a plurality of directions. With respect to each measurement region, the geometrical error of the surface shape of the reference mirror 33, the geometrical error of the surface shape of the reference device (center portion WOC), and the equation established between surface shape measurement values are solved to obtain the geometrical error of the surface shape of the reference mirror 33.

Description

  The present invention relates to a method of calibrating a reference mirror surface shape in an interferometer, and in particular, a geometric error of a surface shape suitable for use in a shape measuring apparatus using light interference such as a white light interferometer-mounted image measuring machine. The present invention relates to a method for calibrating the shape of a reference mirror surface in an interferometer, which can calibrate the geometric error of the surface shape of the reference mirror using an unknown standard.

  Conventionally, a shape measuring device using optical interference (hereinafter simply referred to as optical interference) such as a three-dimensional shape measuring device that accurately measures the three-dimensional shape of an object to be measured using luminance information of interference fringes caused by light interference. Also known as a measuring device).

  In such an optical interference measurement apparatus, the interference fringe peaks of the respective wavelengths overlap at the focus position where the optical path lengths of the reference optical path and the measurement optical path coincide with each other, and the luminance of the combined interference fringe increases. Therefore, in the optical interference measurement apparatus, an interference image showing a two-dimensional distribution of the interference light intensity is photographed by an imaging device such as a CCD camera while changing the optical path length of the reference optical path or the measurement optical path, and each measurement within the imaging field is performed. By detecting the focus position where the intensity of the interference light reaches the peak at the position, the height of the measurement target surface at each measurement position can be measured, and the three-dimensional shape of the measurement target can be measured (for example, patents) Reference 1).

  Moreover, since the shape of the reference mirror in the interferometer including the optical interference measuring apparatus as described above directly affects the measurement accuracy, high geometric accuracy (flatness in the case of a plane mirror) is required. In particular, in high-accuracy interference measurement, a technique for reducing the influence on the measurement accuracy by correcting the geometric error of the reference mirror is used.

  As a method for calibrating the geometric error of the reference mirror, there is a method in which a reference device having a sufficiently smaller geometric error than the reference mirror is subjected to interference measurement, and an error distribution from the reference device is used as a correction table (see Patent Document 2).

JP 2011-191185 A JP 2006-226918 A

  However, there is a problem that it is difficult to prepare a reference device having a sufficiently small geometric error.

  The present invention has been made to solve the above-described conventional problems, and it is an object of the present invention to be able to calibrate the geometric error of the surface shape of the reference mirror using a standard device whose geometric error of the surface shape is unknown. To do.

  The present invention relates to an interferometer having a reference mirror with an unknown surface shape geometric error for generating interference light with the surface to be measured. The reference mirror is moved and scanned while being overlapped with a part of the measurement area, and the surface shape of the plurality of areas in each direction is measured in a plurality of directions. By solving the geometric error of the surface shape, the geometric error of the surface shape of the reference unit, and the equation that holds between the surface shape measurement values, the geometric error of the surface shape of the reference mirror is obtained, thereby solving the above problem. Is.

  Here, when the sum of geometric errors of the surface shape of the reference device is known, this relationship can also be used to increase the calculation accuracy and reduce the number of measurement points.

  According to the present invention, by measuring a plurality of standard units with an interferometer, the geometric error of the standard unit and the geometric error of the reference mirror are separated, and a highly accurate reference mirror calibration that eliminates the geometric error of the standard unit is possible. It becomes possible.

Sectional drawing including a partial block diagram which shows the structure of an example of the optical interference measuring apparatus with which this invention is applied Sectional drawing which similarly shows the detailed structure of an objective lens part Flow chart showing the same measurement procedure Similarly, a plan view showing a standard device, a reference mirror, and a measurement area of one field of view. Similarly, (a) the surface shape (geometric error) of the reference mirror and (b) the plan view showing the surface shape (geometric error) of the central portion of the reference unit Similarly, the top view which shows the relationship between the reference mirror in the measurement areas 1-4 of the first half, and the center part of a standard device The top view which similarly shows the relationship between the reference mirror in the latter measurement area 5-9, and a reference | standard standard part.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited by the content described in the following embodiment and an Example. In addition, the constituent elements in the embodiments and examples described below include those that can be easily assumed by those skilled in the art, those that are substantially the same, and those in the so-called equivalent range. Furthermore, the constituent elements disclosed in the embodiments and examples described below may be appropriately combined or may be appropriately selected and used.

  First, a shape measuring apparatus which is an example of an optical interference measuring apparatus to which the present invention is applied will be described. Although a Michelson type interferometer is shown here, other iso-optical path interferometers such as a Millo type can also be used.

  As shown in FIG. 1, the shape measuring apparatus 1 includes a light emitting unit 10, an optical head unit 20, an objective lens unit 30, an imaging unit 40, an imaging lens 41, an image memory 50, and an arithmetic processing unit. 60, an input unit 70, an output unit 80, a display unit 90, and a stage S on which a measurement object (hereinafter referred to as “work”) W is placed.

  The light emitting unit 10 includes a light source that outputs a wide band light having a large number of wavelength components over a wide band and low coherency, for example, and a white light source such as a halogen or an LED (Light Emitting Diode) is used. By using white light with less coherence, the range in which interference fringes are generated can be narrowed. The light emitted from the light emitting unit 10 may have a specific wavelength (single wavelength).

  The optical head unit 20 includes a beam splitter 21 and a collimator lens 22. The light emitted from the light emitting unit 10 is irradiated in parallel to the beam splitter 21 via the collimator lens 22 from a direction perpendicular to the optical axis of the objective lens unit 30, and light along the optical axis is emitted from the beam splitter 21. It is emitted and a parallel beam is irradiated onto the objective lens unit 30 from above.

  As shown in FIG. 2, the objective lens unit 30 includes an objective lens 31, a prism 32, a reference mirror 33, and the like. In the objective lens unit 30, when a parallel beam enters the objective lens 31 from above, the incident light becomes convergent light by the objective lens 31 and enters the reflecting surface 321 inside the prism 32. Here, incident light includes reflected light (reference light) that travels along a reference optical path (broken line in the figure) having the reference mirror 33, and transmitted light (measurement light) that travels along a measurement optical path (solid line in the figure) where the workpiece W is disposed. Branch to. The reflected light is converged and reflected by the reference mirror 33, and further reflected by the reflecting surface 321 of the prism 32. On the other hand, the transmitted light is focused and reflected by the workpiece W, and passes through the reflecting surface 321 of the prism 32. The reflected light from the reference mirror 33 and the reflected light from the measurement object (work) W are combined by the reflecting surface 321 of the prism 32 to become a combined wave. The reference mirror 33 is moved and scanned in the optical axis direction by a driving unit 34 such as a piezo element under the control of the arithmetic processing unit 60. The scanning position of the reference mirror 33 is measured by the encoder 35 and input to the arithmetic processing unit 60. When the optical path lengths of the reference optical path (optical path 1 + optical path 2) and the measurement optical path (optical path 3 + optical path 4) are equal, interference fringes are generated in the combined wave.

  The synthesized wave synthesized from the reflecting surface 321 of the prism 32 becomes a parallel beam by the objective lens 31, travels upward, and enters the imaging lens 41 (a chain line in FIG. 1). The imaging lens 41 converges the combined wave and forms an interference image on the imaging unit 40.

  The imaging unit 40 is a CCD power mellar or the like made of an imaging element having a plurality of pixels arranged in a two-dimensional shape, and captures an interference image of a composite wave. The interference image is captured a plurality of times while moving and scanning the reference mirror 33. The image data of the interference image captured by the imaging unit 40 is stored in the image memory 50.

  The arithmetic processing unit 60 obtains shape measurement data of the measurement surface of the workpiece W based on the intensity of the interference light at each position on the measurement surface of the workpiece W and the scanning position of the reference mirror 33 input from the encoder 35. The input unit 70 inputs data necessary for measurement to the arithmetic processing unit 60. The output unit 80 outputs the measurement result obtained by the arithmetic processing unit 60. The display unit 90 displays information necessary for input operation and measurement results.

  FIG. 3 is a flowchart showing the shape measuring method.

  When shape measurement is started, the reference mirror 33 is moved by a predetermined amount in the optical axis direction (S1), and an interference image indicating a two-dimensional distribution of interference light intensity on the measurement surface is stored in the image memory 50 (S2). This is repeated for a predetermined number of samplings (S3), and when a predetermined number of interference images are accumulated in the image memory 50, the arithmetic processing unit 60 changes the interference light intensity according to the change in the optical path length difference at each measurement position on the measurement surface. Is detected (S4). Then, the detected peak position of each measurement position is displayed and output as the height at the measurement point (S5).

  In such a shape measuring apparatus 1, it is necessary to accurately know the geometric error of the surface shape of the reference mirror 33 (hereinafter also simply referred to as a surface shape). Even if the measurement is performed, the surface shape of the reference mirror 33 cannot be calibrated with high accuracy unless the geometric error of the surface shape of the standard device WO is sufficiently small.

  Therefore, in the present invention, as shown in FIG. 4, the reference mirror 33 whose geometric error of the surface shape is unknown is arranged at the position of the measurement object W, and the reference mirror 33 is illustrated while overlapping some measurement regions. 4, the surface shape of a plurality of regions (9 regions 1 to 9 in the drawing) is measured in a plurality of directions (left and right X directions and up and down Y directions in the drawing) as illustrated by arrows in FIG. 9, the geometrical errors Ma to Md of the surface shape of the reference mirror 33 as shown in FIG. 5A, and the geometrical errors Ra to Rd of the surface shape of the central portion WOC of the reference device WO as shown in FIG. The geometrical errors Ma to Md of the surface shape of the reference mirror 33 are obtained by solving an equation that holds between the surface shape measurement values H1 to H16.

  That is, in each measurement region 1-9, between the geometric errors Ma to Md of the surface shape of the reference mirror 33 and the geometric errors Ra to Rd of the surface shape of the central portion WOC of the reference device WO, FIGS. (A) to (i) are established. In FIG. 6A and FIG. 6B, the reference mirror 33 and the reference unit central portion WOC are slightly shifted for easy understanding, but they actually coincide.

For example, the measured value H1 at the position where the reference mirror 33 in the measurement region 1 at the start point illustrated in FIG. 6A and the reference unit central portion WOC overlap is shown in the lower right portion of the reference mirror 33 as shown in the following equation. This is the sum of the geometric error Md and the geometric error Ra of the upper left portion of the reference unit central portion WOC.
H1 = Md + Ra (1)

Further, in the state of the measurement region 2 in which the reference mirror 33 is shifted by one block in the right direction in the drawing from the state of the measurement region 1, as shown in FIG. 6A (b), the lower half of the reference mirror 33 and the reference unit central portion WOC The upper half overlaps, and the relationship between the following equation (2) and equation (3) is established between the measurement value H2 on the left side and the measurement value H3 on the right side.
H2 = Mc + Ra (2)
H3 = Md + Rb (3)

Further, in the state of the measurement region 3 in which the reference mirror 33 is further shifted by one block to the right in the drawing, as shown in FIG. 6A (c), the measured value H4 at the position where the reference mirror 33 and the reference unit central portion WOC overlap. The relationship of the following formula (4) is established.
H4 = Mc + Rb (4)

Similarly, in the state of the measurement region 4 in which the reference mirror 33 is moved downward by one block in the downward direction and returned to the left by two blocks, as shown in FIG. 6A (d), The relationship of equation (6) is established.
H5 = Mb + Ra (5)
H6 = Md + Rc (6)

Further, in the state of the measurement region 5 in which the reference mirror 33 is shifted by one block in the right direction in the drawing from the state of the measurement region 4, that is, in the state of the measurement region 5 where the reference mirror 33 and the reference unit central portion WOC completely overlap, As shown in 6B (e), the following four expressions (7) to (10) are established.
H7 = Ma + Ra (7)
H8 = Mb + Rb (8)
H9 = Mc + Rc (9)
H10 = Md + Rd (10)

Similarly, in the state of the measurement region 6 in which the reference mirror 33 is further shifted by one block in the right direction in the drawing, the relationship of the following equations (11) and (12) is established as shown in FIG. 6B (f). .
H11 = Ma + Rb (11)
H12 = Mc + Rd (12)

Further, in the state of the next measurement area 7 in which the reference mirror 33 is lowered by one block in the downward direction of the figure and returned by two blocks in the left direction, the relationship of the following equation (13) is obtained as shown in FIG. 6B (g). To establish.
H13 = Mb + Rc (13)

Further, in the state of the next measurement region 8 in which the reference mirror 33 is shifted by one block in the right direction in the figure, the relationship of the following expressions (14) and (15) is established as shown in FIG. 6B (h).
H14 = Ma + Rc (14)
H15 = Mb + Rd (15)

Further, in the state of the last measurement region 9 in which the reference mirror 33 is shifted by one block to the right in the drawing, the relationship of the following equation (16) is established as shown in FIG. 6B (i).
H16 = Ma + Rd (16)

When these are put together, a determinant as shown in Expression (17) is established.

  Here, since there are 16 unknown expressions, while there are 16 unknown expressions Ma to Md and Ra to Rd, the unknown quantities Ma to Md and Ra to Rd are calculated using, for example, the least square method. The geometrical errors Ma to Md of the surface shape of the reference mirror 33 can be obtained with high accuracy. The obtained geometric errors Ma to Md are stored in the correction table as in the conventional case and used for correction at the time of measurement.

As shown in the following equation (18), for example, when the sum of geometric errors Ra to Rd of the reference device WO is 0, this condition is converted into a determinant as shown in the following equation (19). In addition, the accuracy can be increased or the number of measurement points can be reduced.
Ra + Rb + Rc + Rd = 0 (18)

  In the above embodiment, as shown in FIG. 5, the number of sections of the reference mirror 33 and the standard unit central portion WOC is two sections in the X direction and two sections in the Y direction. The number is not limited to this. For example, it is possible to have two sections in the X direction, three sections in the Y direction, three sections in the X direction, two sections in the Y direction, or three sections in both the X direction and the Y direction. Yes, it can be divided into X sections in the X direction and n sections in the Y direction (m and n are both 2 or more). The scanning direction and the scanning order are not limited to those in the above embodiment, and for example, scanning can be performed first in the vertical Y direction.

  The application object of the present invention is not limited to an optical interference measuring device such as a white light interferometer-mounted image measuring machine, but there is a geometric error in the surface shape for generating interference light with the surface to be measured. The same applies to interferometers with unknown reference mirrors in general. The position of the reference mirror is not limited to the side of the objective lens unit, and may be between the workpiece W and below the objective lens unit.

DESCRIPTION OF SYMBOLS 1 ... Shape measuring apparatus 10 ... Light emission part 20 ... Optical head part 30 ... Objective lens part 33 ... Reference mirror 40 ... Imaging part 41 ... Imaging lens 50 ... Image memory 60 ... Arithmetic processing part 70 ... Input part 80 ... Output part 90 ... Display section S ... Stage W ... Measurement object (workpiece)
WO ... Reference device WOC ... Central portion of reference device

Claims (2)

In an interferometer with a reference mirror whose surface shape geometric error is unknown to generate interference light with the surface to be measured,
Place a reference device whose geometric error of the surface shape is unknown at the position to be measured,
While overlapping some measurement areas, move and scan the reference mirror to measure the surface shape of multiple areas in each direction for multiple directions,
In each measurement area, obtain the geometric error of the reference mirror surface shape, the standard surface geometry error, and the equation that holds between the surface shape measurement values by solving the equation. A method for calibrating the reference mirror surface shape in an interferometer.   2. The reference mirror surface shape calibration method for an interferometer according to claim 1, wherein the relationship is also used when the sum of geometric errors of the surface shape of the standard is known.

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