JPS6253764B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a flatness measuring method for displaying flatness by receiving, self-scanning, and calculating mainly primary interference fringes by a Fizeau interference or Michelson interference optical system with a photodiode array. It is possible to reduce the flatness to 0.01 economically, easily and reliably.
The purpose is to measure and display with high precision of μm.

Generally, the flatness of an object's surface is measured by using an optical flat under white light or monochromatic light and reading its interference fringe pattern. That is, in Fig. 1 A and B, 1 is an interference fringe pattern formed by optical flat, and the deflection Δp of the interference fringe pattern is read by visual measurement or a scale with respect to the interference fringe pitch P, and is calculated by the following formula. Then calculate the flatness.

Flatness = λ/2n x Δp/P However, in the above formula, λ = interference wavelength, n = interference order, generally, this order n is n = 1, and the resolution that can read △p/P is generally around 20%. Therefore, λ = 0.6 μm (He-
For Ne laser light, if λ = 0.633 μm), the flatness is 0.06 μm (0.6 μm/2 x 20/100 = 0.06
In order to obtain the currently required flatness measurement accuracy of the order of 0.01 μm, accurate high-order interference fringes of n = 3 to 5 are required. need to get it. For this purpose, a powerful light source is required, the precision of the optical system components is high, and the path of the light must be selected so as not to introduce other noises, which makes the equipment expensive and complicated. It is also difficult to adjust and maintain it.

In contrast to the conventional measurement method as described above, the present invention
It is characterized by receiving the primary interference fringes produced by the Fizeau interference or Michelson interference optical system with a photodiode array, self-scanning it, and calculating it to display the flatness with high accuracy on the order of 0.01 μm. The flatness measuring method of the present invention will be explained in detail below.

FIG. 2 shows a block diagram of an embodiment of the measurement method of the present invention, and 2 is a pattern of primary interference fringes produced by Fizeau interference or Michelson interference optical system, and the solid lines constituting the fringe pattern are 3 and dotted line 4
represent the peaks and valleys of the interference fringes, respectively. Reference numeral 5 denotes a photodiode array (image sensor) having a self-scanning function, which is arranged so as to be substantially orthogonal to the interference fringes, and is provided so as to move in parallel on a straight line as shown by the arrow. In the case of this example, the photodiode array has 512 bits (elements) and has a self-scanning function that is equivalent to or higher than the processing speed of 10 kHz of the A/D (analog/digital) converter described later, that is, one scan is approximately 1/20 seconds. It has the following functions. This photodiode array is, for example, 5mm/
It is moved an appropriate distance at a speed of seconds in the direction of the arrow, that is, in the direction in which the interference fringes extend. This distance is selected so that the deflection Δp of the pattern of interference fringes therebetween is sufficiently large compared to the pitch between elements of the photodiode array. In the example of FIG. 2, the interference fringe pattern 2 is moved between a chain line 6 on the left side and a broken line on the right side. As shown in the figure, the interference fringe pattern has a plurality of peaks and valleys, and the photodiode array moves in parallel while capturing the plurality of interference fringes.

While the photodiode array is thus moved in the direction of the arrow, self-scanning is performed at three appropriate positions in the direction of movement. As mentioned above, the self-scanning speed of the photodiode array is sufficiently faster than the movement speed, so there is no need to stop its movement during self-scanning. As shown in the block diagram, the output of each element of the photodiode array is sequentially amplified by an amplifier circuit, peak-held by a peak-hold circuit, and the hold signal value is converted into a digital value by an A/D converter and sent to the microprocessor. The data is read into a setter, which performs arithmetic processing to determine the flatness, and displays this on a display device.

This arithmetic processing will be explained in a little more detail. One self-scanning of the photodiode array provides the microprocessor with data regarding the brightness of the area captured by each of the 512 elements. These data are calculated by calculating the average value of three elements, each element and two adjacent elements, in order to correct variations in the output characteristics of each element.
Processing is performed to set the value of the element located in the center of the two elements. Based on the data for 512 elements obtained by correcting in this way, the elements corresponding to the center of peak 3 (bright fringe) or trough (4) or dark fringe of the interference fringe, that is, the element that takes the maximum value or minimum value, are calculated. Identify the number (defined sequentially with one end of the photodiode array as a reference). The number of such elements is equal to the number of stripes, that is, 4 or 5 in the example of FIG. 2. In general, the peak of the detection signal, that is, the peak 3, may be determined, but the bottom, that is, the valley 4, may be determined in consideration of the saturation of the output of the photodiode.

As described above, the element number can be specified in the 3rd direction of movement.
When performed at the position, Δp is calculated as follows. Now let these three positions be the positions of the chain line 6 and the center, right dashed line in Figure 2, and the element number at the chain line 6 position for the same peak 3 [or valley 4] is (Min) ₁ , and at the center dashed line position. The element number at (Max) and the element number at the dashed line position on the right (Min) ₂
Then, △p=K{(Max)−(Min) ₁ +(Min) ₂ /
2} However, K is determined as the pitch between elements of the photodiode array. Such Δp is calculated as many times as there are peaks or valleys, but it is arbitrary whether one of them is used or the average value or median value is used.

Next, P is determined, for example, at the center dashed line position, each number (MAX), (MAX)' of the element whose number is specified as being located at the center of two adjacent peaks or valleys, respectively. P=K|(MAX)−(MAX)′| This P can also be calculated in multiple numbers, but it is optional whether one of them is used, or the average value or median value is used. It goes without saying that calculations may be made in the same way at the positions of the left and right dashed lines and broken lines.

Note that when calculating ΔP, it is assumed that data separated by λ/4 or more in terms of dimensions are values corresponding to separate peaks or valleys to prevent miscalculations. This takes advantage of the fact that a pattern that is shifted by more than 1/2 of the inter-fringe pitch (λ/2) at the position where the photodiode array performs self-scanning is actually extremely rare.

After determining Δp and P as described above, the flatness is calculated as follows: flatness=λ/2n×Δp/P. As understood from this equation, since Δp and P are used as their ratios, the above-mentioned K is not necessary for the actual calculation of Δp and P, respectively.

As explained above, the measurement method of the present invention identifies peaks or valleys in interference fringes by excluding peaks or valleys that are separated by λ/4 or more. There is no significantly large error in the measurement of degree. By using self-scanning of the photodiode array, for example, even if five interference fringes fall within the 512-bit range of the photodiode array during self-scanning, a resolution of about 100 bits per interference fringe can be obtained. Even if the calculation accuracy of the center of the peak or valley is ±1 bit, the resolution is 2/100 = 2%, so λ = 0.6 μm.
It is possible to display flatness with an accuracy of the order of 0.006 μm (0.6 μm/2 x 0.02 = 0.006 μm), and the flatness measurement of the order of 0.01 μm as described at the beginning,
Display can be achieved simply and reliably.

[Brief explanation of the drawing]

FIGS. 1A and 1B are explanatory diagrams showing patterns of interference fringes by a known optical flat, and FIG. 2 is an explanatory diagram showing a flatness measuring method of the present invention in a block diagram. Below is an explanation of the symbols in the figure. 1: Pattern of interference fringes due to optical flats, 2: Pattern of interference fringes due to Fizeau interference or Michelson engineering, 3: Peaks of interference fringes of pattern 2 (shown as solid lines), 4: Valleys above (dotted lines) ), 5: photodiode array (image sensor), 6: self-scanning start position of photodiode array 5 (represented by a chain line).

Claims (1)

[Claims] 1. A photodiode array arranged to be substantially orthogonal to the interference fringes produced by Fizeau interference or Michelson interference optical system is self-scanned at a plurality of different positions in the direction along the fringes; A microprocessor reads the peaks or valleys of the interference fringe from the output, calculates the pitch P of the interference fringe based on the data of the read peaks or valleys, and calculates the peaks or valleys read at the plurality of different positions. λ/4 inside the valley
A flatness measuring method characterized by calculating and displaying the flatness (λ/2×Δp/P) at the same time as calculating the deflection Δp of the pattern by assuming that the peaks or valleys are the same when the distance is not more than λ=wavelength. .