JPH0357905A - Non-contact measuring apparatus of surface shape - Google Patents

Abstract

PURPOSE:To easily position an optical path correcting lens for measuring a spherical or an a spherical object by displaying the shape of one cross section of object after approximating the same by a quadratic function according to the method of minimum squares from a plurality of interference fringes. CONSTITUTION:A coherent light radiated from a laser source 1 is changed to parallel beams by a pin hole, an objective lens 3 and a collimator lens 4, and further divided into two by a beam splitter 7. One of the light is radiated to an object 6 to be measured through an optical path correcting lens 5, and the reflecting light is returned to the beam splitter 7 through the lens 5. The other light is reflected at the surface of a reference mirror 9 and returned to the beam splitter 7. The light returning from the reference mirror 9 to the beam splitter 7 interferes with the light returning from the object 6 to the beam splitter 7, thereby generating interference fringes. The interference fringes appear as a contour line representing the surface shape of the object 6. The shape of one cross section of the object 6 is approximated by quadratic function approximation by the method of squares from a plurality of interference fringes, which is shown by a display device 19 together with a secondary factor.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is a non-contact surface shape measurement method for measuring the surface of a spherical or aspherical object with ultra-high precision using optical interference. This is related to equipment. [Prior Art] High-precision non-contact surface profile measurement devices using optical interference measurement methods such as the Twyman-Green interferometer have been developed. Among these types of interference measurement methods, the fringe scanning method, which uses multiple interference fringes obtained by gradually changing the optical path length, has particularly high precision, and is used to measure the surfaces of high-precision molds, optical lenses, semiconductor wafers, etc. It is used to measure shape. By the way, since the interferometric method irradiates the surface of the object to be measured with coherent light and uses the interference between the reflected light and the reference light, the light must be incident perpendicularly to the surface of the object to be measured. be. 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, the light is irradiated almost perpendicularly to the surface of the object to be measured. It has been proposed to make this possible (see Japanese Patent Application No. 63258904).The interference fringes generated at this time are obtained by measuring the spherical or aspherical shape of the object as a deviation from the surface shape of the optical path correction lens. becomes.

[Problems to be Solved by the Invention] In the conventional example described above, if the setting of the optical path correction lens is incorrect, the number of interference fringes becomes too large or too small, making it difficult to measure the surface shape. was there. Therefore, in the past, the optical path correction lens was set at the optimal 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 shifted to the positive side or to the minus side.Also, the accuracy of the setting varies depending on the person measuring the measurement, and the error is large. There is a problem. The present invention has been made in view of the above points, and its purpose is to use a non-contact type surface profile measuring device that uses a fringe scanning method using optical interference to measure a spherical or aspherical surface. The purpose is to facilitate the positioning of the optical path correction lens for measuring objects. [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 l that emits coherent light and a light source 1
optical systems 2 to 4 that convert the coherent light from the
a beam splitter 7 that splits the parallel light beam from the optical system 1 into a first light beam and a second light beam; and a beam splitter 7 that reflects the first light beam toward the beam splitter 7. The i constant object 6, the reference mirror 9 that reflects the second light beam toward the beam splitter 7, the optical path correction lens 5 interposed between the measured object eJ6 and the beam splitter, and the reflected light from the reference mirror 9. An optical system (observation surface 8
and a CCD camera 11), a reference mirror drive means (piezoelectric element 10) that moves the position of the reference mirror 9 by a minute amount along the optical path direction, and a plurality of mirrors formed by moving the position of the reference mirror 9 by a minute amount. Calculating means (microprocessor 14
), the cross-sectional shape of the object to be measured 6 is determined from the plurality of interference fringes by at least two
The present invention is characterized by providing means 20 for approximating a quadratic function by multiplication and displaying the quadratic coefficient together with its sign. [Function] In this way, in the present invention, in a non-contact type surface profile measuring device using a fringe scanning method using light interference, one cross-sectional shape of the object to be measured 6 is determined from a plurality of interference fringes to a minimum. Since a means 20 is provided for performing quadratic function approximation using the square method and displaying the quadratic coefficient together with its sign, it is possible to accurately know the direction and amount of positional deviation of the optical path correction lens 5. [Examples] Examples of the present invention will be described below. Figure 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 light by the bin hole 2, objective lens 3, and collimator lens 4,
The beam splitter 7 splits the beam into first and second beams. The first light beam is irradiated onto the object to be measured 6 via the optical path correction lens 5, reflected by the surface of the object to be measured 6, and returned to the beam splitter 7 via the optical path correction lens 5. The optical path correction lens 5 refracts the l-th ray so that the first ray is perpendicularly incident on the surface of the object to be measured 6 and is reflected perpendicularly. When the surface of the object to be measured 6 is convex, the optical path correction lens 5 is a convex lens, and when the surface of the object to be measured 6 is concave, the optical path correction lens 5 is a concave lens. The second beam is reflected from 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 object to be measured 6 to the beam splitter 7 interfere, producing interference fringes. Since the optical path length of the reflected light from the object to be measured 6 differs depending on the surface shape of the object to be measured 6, interference fringes appear as contour lines indicating the surface shape of the object to be measured 6.
If the wavelength of light is λ, adjacent contour lines represent a height change of λ/2. The CCD camera 1 detects this interference pattern on the observation surface 8.
1 makes Fat 5 & and inputs it to the microcomputer ■4. The microcomputer 14 determines the position control amount of the reference mirror 9 and controls the D/A converter 12 and the high voltage amplifier 13 so that the contour lines included in one scanning line of the captured interference fringe image have an appropriate density. A drive signal is given to the piezoelectric element 0 through the piezoelectric element 0 to control the position of the reference mirror 9. Further, the microcomputer 14 calculates and outputs height information of the measurement surface of the object to be measured 6 from a plurality of interference fringes formed by moving the position of the reference mirror 9 by a minute amount. Here, the intensity at the point (x, y) of the interference fringe image is as follows. f (x, y)=Axy+KxyXsin(t+α) In the above equation, α is a phase difference determined by the position of the reference mirror 9. In the fringe scanning method, the points (x, y) of the four interference fringe images obtained when the reference jU 9 is moved by λ/8
The intensity at I 1(x,F)=Axy+KxyXsint1 2(
X, y)=Axy+KxyXsin(t+yr/
2) I, (x,y)=Axy+KxyXsin(t
+ 2π/2)I 4(x+y)=Axy+Kxy
This becomes Xsin(t+3π/2). Then, the height H (x.y) at each point (x.y) of the ebb and flow stripe image can be determined by the following equation. H (x,y)-= (^/4π>xLan-'f(1 2 I 4)/ (I 3
I+)1 The intensities ■1 to ■ at points (x, y) of the four interference fringe images described above are stored in the first to fourth memories 15. The information in the memory l5 is input to the one-section calculating section 16, and is expressed as y=f(x), which indicates the shape of one section passing through the center.
A curve of is obtained. Next, the quadratic approximation coefficient calculation unit 17 calculates the curve of y4(x) using the least squares method to calculate the quadratic leap number y.
Approximate by =ax2+bκ+C. The magnitude and sign of the coefficient a in this quadratic function are displayed on a display 19 via a display driver 18. The above one section calculation section 16, 2
Order approximation coefficient calculation unit l7, display driver 18 and display device 1
The positional deviation detection display stage 20 including 9 is composed of hardware and performs real-time processing. In the above measurement apparatus, if the positioning of the optical path correction lens 5 is accurate, the spherical wave is reflected on the surface of the object to be measured 6, returns along 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 with the same brightness is formed on the observation surface 8. If the optical path correction lens 5 is misaligned, interference fringes formed by interference with the reference light will become concentric fringe images. In other words, if the optical path correction lens 5 is misaligned, a cross section passing through the center due to fringe scanning will have a concave or convex shape. This arc-shaped curve is f=ax2+bx+
By performing least squares approximation using a quadratic function of e, the quadratic coefficient a
The sign and magnitude of the expression indicate the direction and magnitude of the positional shift of the optical path correction lens 5. Furthermore, in a state where there is no positional shift of the optical path correction lens 5, the value of the quadratic coefficient a is theoretically O.
This allows for highly accurate positioning. [Effects of the Invention] As described above, in the present invention, in a non-contact type surface profile measuring device using a fringe scanning method using light interference, one cross section of the object to be measured is determined from the plurality of interference fringes. Since we have provided a means for approximating the shape to a quadratic function using the least squares method and displaying the quadratic coefficient along with its sign, it is possible to accurately determine the direction and amount of positional deviation of the optical path correction lens for measuring spherical or aspherical objects. This has the effect of making it possible to easily determine the position of the optical path correction lens.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a schematic ellipse of an embodiment of the present invention. 1 is a light source, 2 is a bin hole, 3 is an objective lens, 4 is a collimator lens, 5 is an optical path correction lens, 6 is a measured object, 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 calculation unit, 17 is a quadratic approximation coefficient calculation unit, 18 is a display driver, 1
One is the 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 beam that divides the parallel ray from the optical system into a first ray and a second ray. a splitter, an object to be measured that reflects the first beam toward the beam splitter, a reference mirror that reflects the second beam toward the beam splitter, and an optical path correction lens interposed between the object to be measured and the beam splitter. , an optical system for observing interference fringes of the reflected light from the reference mirror and the reflected light from the measured object, a reference mirror driving means for moving the position of the reference mirror by a minute amount along the optical path direction, and a position of the reference mirror. In a non-contact surface profile measuring device, the non-contact type surface profile measuring device includes a calculation means for computing and outputting information regarding the surface profile of the object to be measured from a plurality of interference fringes formed by moving the plurality of interference fringes by a minute amount. A non-contact type surface profile measuring device characterized by comprising means for approximating a cross-sectional shape of a measurement object to a quadratic function by the method of least squares and displaying a quadratic coefficient together with a sign.