JPH0711413B2 - Non-contact type surface profile measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention is a non-contact type surface shape measurement for measuring the surface of a spherical or aspherical object to be measured with ultrahigh accuracy by using light interference. It relates to the device.

[Prior Art] Conventionally, a highly accurate non-contact type surface profile measuring apparatus using an optical interferometric method such as a Twyman-Green interferometer has been developed. Among this kind of interferometric method, the fringe scanning method using a plurality of interference fringes obtained by gradually changing the optical path length has a particularly high precision, and the surface of a mold, an optical lens, a semiconductor wafer or the like with high precision Used to measure shape. By the way, since the interferometric method irradiates the surface of the object to be measured with coherent light and utilizes the interference between the reflected light and the reference light, it is necessary to make the light incident vertically on the surface of the object to be measured. is there. Therefore, an optical path correction lens is placed in front of the object to be measured, and even if the surface of the object to be measured is spherical or aspherical, light is irradiated almost perpendicularly to the surface of the object to be measured, and measurement is performed by light interference. Has been proposed (see Japanese Patent Application No. 63-258904). The interference fringes generated at this time are those obtained by measuring the spherical or aspherical shape of the measured object as a deviation from the surface shape of the optical path correction lens.

[Problems to be Solved by the Invention] In the above-mentioned conventional example, if the setting of the optical path correction lens is improper, the number of interference fringes becomes too large or too small to make it difficult to measure the surface shape. was there. Therefore, in the past, the optical path correction lens was set at the optimum position based on the intuition and experience of the measurer. However, in that case,
There is a problem that it is difficult to determine whether the setting of the optical path correction lens is displaced to the plus side or the minus side, and the accuracy of the setting differs depending on the measurer, and there is a problem that the error is large.

The present invention has been made in view of such a point, and an object thereof is to measure a spherical surface or an aspherical surface in a non-contact type surface profile measuring apparatus using a fringe scanning method by light interference. It is to facilitate the positioning of the optical path correction lens for measuring an object.

[Means for Solving the Problems] In the present invention, in order to solve the above problems, the first
As shown in the figure, a light source 1 that emits coherent light, and a light source 1
Optical systems 2 to 4 for converting the coherent light from the light into parallel rays,
A beam splitter 7 that divides a parallel light beam from the optical system 1 into a first light beam and a second light beam, an object 6 that reflects the first light beam toward the beam splitter 7, and a second light beam. A reference mirror 9 for reflecting the beam toward the beam splitter 7,
The optical path correction lens 5 interposed between the DUT 6 and the beam splitter 7, and an optical system for observing the interference fringes of the reflected light from the reference mirror 9 and the reflected light from the DUT 6 (observation surface 8 and CCD
Camera 11), reference mirror drive means (piezoelectric element 10) for moving the position of reference mirror 9 by a small amount along the optical path direction, and a plurality of interference fringes formed by moving the position of reference mirror 9 by a small amount. In the non-contact type surface profile measuring device including a calculation means (microprocessor 14) for computing and outputting information regarding the surface profile of the DUT 6 from the plurality of interference fringes, A means 20 for displaying a quadratic coefficient together with a sign by approximating a quadratic function by the method of least squares is provided, and the optical path correction lens 5 is positioned so that the quadratic coefficient becomes substantially zero. is there.

[Operation] In the present invention, as described above, in the non-contact type surface profile measuring apparatus using the fringe scanning method by the interference of light, one cross-sectional profile of the DUT 6 can be determined from a plurality of interference fringes. Since the means 20 for displaying the quadratic coefficient together with the sign by performing the quadratic function approximation by the least squares method is provided, the direction and amount of the positional deviation of the optical path correction lens 5 can be accurately known.

[Examples] Examples of the present invention will be described below. FIG. 1 shows a schematic configuration of an embodiment of the present invention. Laser light source 1
The coherent light emitted from is converted into parallel rays by the pinhole 2, the objective lens 3 and the collimator lens 4,
The beam splitter 7 splits the light into first and second light rays. The first light beam is applied to the DUT 6 via the optical path correction lens 5, is reflected by the surface of the DUT 6, and returns to the beam splitter 7 via the optical path correction lens 5. The optical path correction lens 5 refracts the first light ray so that the first light ray is vertically incident on the surface of the DUT 6 and is reflected vertically. When the surface of the DUT 6 is convex, the optical path correction lens 5 is a convex lens, and when the surface of the DUT 6 is concave, the optical path correction lens 5 is a concave lens. The second light ray is reflected by the surface of the reference mirror 9 and returns to the beam splitter 7. The light returning from the reference mirror 9 to the beam splitter 7 and the light returning from the DUT 6 to the beam splitter 7 interfere with each other to generate an interference fringe. Since the optical path length of the reflected light from the DUT 6 differs depending on the surface shape of the DUT 6, the interference fringes appear as contour lines indicating the surface shape of the DUT 6.
When the wavelength of light is λ, adjacent contour lines represent a height change of λ / 2. This interference fringe is picked up by the CCD camera 11 on the observation surface 8 and input to the microcomputer 14. The microcomputer 14 uses the reference mirror 9 so that the contour lines included in one scanning line of the captured interference fringe image have an appropriate density.
Position control amount is determined and a drive signal is given to the piezoelectric element 10 via the D / A converter 12 and the high voltage amplifier 13 to control the position of the reference mirror 9. The microcomputer 14 also calculates and outputs height information of the measurement surface of the DUT 6 from a plurality of interference fringes formed by slightly moving the position of the reference mirror 9.

Here, the intensity at the point (x, y) of the interference fringe image is as follows.

I (x, y) = Axy + Kxy × sin (t + α) In the above equation, α is a phase component determined by the position of the reference mirror 9. In the fringe scanning method, the intensity at the point (x, y) of four interference fringe images obtained by moving the reference mirror 9 by λ / 8 is I ₁ (x, y) = Axy + Kxy × sint I ₂ (X, y) = Axy + Kxy × sin (t + π / 2) I ₃ (x, y) = Axy + Kxy × sin (t + 2π / 2) I ₄ (x, y) = Axy + Kxy × sin (t + 3π / 2). Then, the height H (x, y) at each point (x, y) of the interference fringe image can be obtained by the following equation.

H (x, y) = (λ / 4π) × tan ⁻¹ {(I ₂ −I ₄ ) / (I ₃ −I ₁ )} The intensity at the point (x, y) of the above four interference fringe images. I ₁ ~ I
₄ is stored in the first to fourth memories 15. This memory 1
The information of 5 is input to the one-section calculating unit 16 to obtain a curve of y = f (x) showing the shape of one section passing through the center. Next, the quadratic approximation coefficient calculation unit 17 approximates the curve of y = f (x) by the quadratic function y = ax ² + bx + c by the method of least squares. The magnitude and sign of the coefficient a in this quadratic function are
It is displayed by the display device 19 via the display driver 18. The positional shift detection display means 20 including the one-section calculation unit 16, the second-order approximation coefficient calculation unit 17, the display driver 18, and the display unit 19 is configured by hardware and performs real-time processing.

In the above measuring apparatus, if the optical path correction lens 5 is positioned accurately, the spherical wave is reflected by the surface of the DUT 6, returns through the same optical path, enters the optical path correction lens 5, and returns to the beam splitter 7. This causes interference with the reference light, and theoretically an interference image having the same brightness is formed on the observation surface 8. If the position of the optical path correction lens 5 is deviated, the interference fringes formed by the interference with the reference light become concentric fringe images. That is, if the position of the optical path correction lens 5 is displaced, one cross section passing through the center by the fringe scanning becomes concave or convex. This arc-shaped curve is f = ax ² + bx + c
If the least-squares approximation is performed using the quadratic function of, the sign and size of the quadratic coefficient a indicate the direction and size of the positional deviation of the optical path correction lens 5. Further, in the state where there is no displacement of the optical path correction lens 5, the value of the quadratic coefficient a is theoretically 0, which enables accurate positioning.

[Effects of the Invention] In the present invention, as described above, in the non-contact type surface profile measuring apparatus using the fringe scanning method by the interference of light, one cross section of the object to be measured is obtained from the plurality of interference fringes. Since the means for displaying the quadratic coefficient together with the sign by approximating the quadratic function to the shape by the least-squares method, the direction and amount of the positional deviation of the optical path correction lens for measuring the spherical or non-surface object to be measured are accurate. Therefore, there is an effect that the optical path correction lens can be easily positioned.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of an embodiment of the present invention. 1 is a light source, 2 is a pinhole, 3 is an objective lens, 4 is a collimator lens, 5 is an optical path correction lens, 6 is an object to be measured, 7
Is a beam splitter, 8 is an observation surface, 9 is a reference mirror, 10 is a piezoelectric element, 11 is a CCD camera, 12 is a D / A converter, 13 is a high-voltage amplifier, 15 is a memory, 16 is a one-section operation unit, and 17 is A second-order approximation coefficient calculator, 18 is a display driver, and 19 is a display.

Claims (1)

[Claims] 1. A light source that emits coherent light, an optical system that converts the coherent light from the light source into parallel rays, and a parallel ray from the optical system is divided into a first ray and a second ray. Beam splitter, a DUT that reflects a first light beam toward the beam splitter, a reference mirror that reflects a second light beam toward the beam splitter, and an optical path interposed between the DUT and the beam splitter. A correction lens, an optical system for observing the interference fringes of the reflected light from the reference mirror and the reflected light from the DUT,
Information about the surface shape of the DUT is calculated and output from the reference mirror driving means that moves the position of the reference mirror by a small amount along the optical path direction and the plurality of interference fringes formed by moving the position of the reference mirror by a small amount. In the non-contact type surface profile measuring device including the calculating means, a means for displaying a quadratic coefficient together with a sign by approximating a quadratic function of one cross section of the object to be measured from the plurality of interference fringes by the least square method. And a non-contact type surface profile measuring device, wherein the optical path correction lens is positioned so that the quadratic coefficient is substantially zero.