JPS63163104A - Apparatus for measuring thickness of film - Google Patents

Abstract

PURPOSE:To obtain the title apparatus capable of measuring the thickness of a film with sufficiently high accuracy even in continuous measurement, by providing a resetting means for generating a reset pulse in an image sensor to perform resetting. CONSTITUTION:Since the wavelength incident to each element of an image sensor 11 is predetermined by a spectral means 9, the relation between the number of each element of the sensor 11 and the wavelength is preliminarily stored in the memory of an operation means 13. At the time of measurement, the output of each element of the sensor 11 is successively read and the memory is utilized to calculate wavelengths lambda1, lambda2 from the element number imparting an extreme value within a measuring range and degree difference N is calculated from the difference in the number of extreme values and predetermined operation is performed using the refractive index in the memory to calculate the thickness of an object 4 to be measured. In this case, each pixel output of the sensor 11 is transmitted at every predetermined time T and charge is accumulated in each pixel during that time but, when resetting is performed only for a time t1 by a resetting means 6, charge is not accumulated in each pixel during this period and the charge only corresponding to the time t2 after the release of resetting is accumulated. As mentioned above, when measurement is performed while an accumulation time is limited, the thickness of a film can be stably and continuously measured with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an apparatus for measuring film thickness using optical interference.

[Conventional technology] To measure the film thickness of an object using optical interference,
Light from a light source is projected onto the object to be measured, the transmitted light or reflected light is separated, and interference fringes are detected by a detector to measure the film thickness of the object to be measured.

[Problems to be solved by this invention] However,
If there are slight variations in the refractive index, film thickness, etc. within the measurement spot of the object to be measured, the pattern of interference fringes will vary depending on the difference in film thickness, etc., and when these are overlapped and synthesized at the same time, the pattern shown in Figure 4 is As shown in , this may cause beats or the contrast may become weak, making measurement difficult.

In addition, when measuring a continuously flowing object such as a polymer film by using an image sensor such as a charge accumulation type image carrier COD as a detector, the charge accumulation time is long within one scanning period. When measuring the distance part, the interference fringe pattern that is separated and projected onto the image sensor will differ depending on the difference in the film thickness of the object to be measured, and these will be combined to produce a beat as shown in Figure 3. This may cause contrast to be averaged, weakening the contrast, and making measurement difficult.

In view of the above points, an object of the present invention is to provide a film thickness measuring device that enables measurement with sufficiently high accuracy even in continuous measurement.

[Means for Solving the Problems] This invention projects light from a light source onto an object to be measured, spectrally transmits or reflects the light, detects interference fringes with an image sensor, and detects the object to be measured. In a device that measures film thickness,
This film thickness measuring device is provided with a reset means for changing the reset time of an image sensor.

[Embodiment] FIG. 1 is a configuration explanatory diagram showing an embodiment of the present invention.

In the figure, 1 is a light source, and the light from the light source 1 is projected by a lens 2 through a half mirror 3 onto an object to be measured 4 such as a film, and the light transmitted or reflected from the object to be measured 4 is as follows. In this figure, the light is separated by a spectroscopic means 9 such as a diffraction grating via a half mirror 3, a lens 5, an aperture 7, and a lens 8, and enters an image sensor 11 having a reset function like a one-dimensional COD via a lens 10. . Light corresponding to each wavelength separated by the spectrometer 9 is incident on each element of the image sensor 11, and the intensity of the interference fringe pattern is detected. The output of the image sensor 11 is amplified by an amplifier 12, and a predetermined calculation is performed by a calculation means 13 including a memory, etc., to calculate the film thickness d of the object to be measured 4. Further, a reset means 6 is provided which generates a reset pulse to the image sensor 11 to reset it.

As shown in FIG. 2, parallel light rays L1 and L2 from the light source are reflected on the front and back surfaces of the object to be measured 4 having a film thickness (thickness) d and a refractive index n, and both light rays L1 and L2 are optically When this optical path difference is an integral multiple of the wavelength of light, interference occurs to form interference fringes as shown in FIG. 4.

Assuming that an interference fringe pattern as shown in FIG. 4 is obtained, and for the minimum wavelength λ1 and maximum wavelength λ2 that give the extreme values of the measurement region, the following equation holds, assuming that the orders of interference are m+N and m.

(m+N)λ+ =2nd/cosθ' (1)m
λ2 = 2 nd/cos θ' (2) Find m from equation (2) and substitute it into equation (1) to organize it. When θ' = 0, when the light is projected perpendicularly to the object to be measured 4, cos θ' = 1, and equation (3) is d = N λ1λ2 (4) 2n λ2
−λ1. In this way, the wavelengths λ1, λ2, the order difference between the extreme values N
, from the known refractive index n, the film thickness of the measured object 4 is (3),
It can be found from equation (4).

That is, since the wavelength incident on each element of the image sensor 1 is determined in advance by the spectroscopy means 9, the relationship between each element number of the image sensor 11 and the wavelength is stored in the memory of the calculation means 13.

During measurement, the output of each element of the image sensor 11 is read out sequentially, and as shown in FIG.
, calculate the order difference N from the difference in the number of extreme values, use the refractive index n stored in a memory, etc., perform calculations such as equations (3) and (4), and calculate the film thickness of the object to be measured 4. Find d.

By the way, as shown in FIGS. 4A and 4B, interference fringe patterns shift and overlap due to variations in the film thickness of the object to be measured 4,
There may be weak areas. In order to avoid this, the film thickness is measured as described above from the output for the extreme value when the difference between the local maximum value and the local minimum value is greater than or equal to a predetermined value. This avoids picking up uncertain and weak interference fringes.
The film thickness d of the object to be measured 4 can be measured from strong and reliable interference fringes.

In addition, a charge storage type Wi image element CO is provided in the image sensor 11.
When D is used, if the object to be measured 4 is moving and its thickness etc. deviate, slightly different interference fringe patterns will be incident on each element of the image sensor 11 within one scanning period and will be synthesized.
Overall contrast deteriorates.

For this reason, by applying a reset using the reset means 6, the accumulation time of the image sensor 11 is limited, that is, the measurement time within one scan is limited, thereby reducing the influence of fluctuations in the incident light on the image sensor 11, and reducing the interference. To prevent deterioration of stripe contrast and ensure reliable measurement. As shown in FIG. 3, the output of each pixel of the image sensor 11 is transferred every predetermined time T, during which charge is accumulated in each pixel. However, when the reset means 6 resets the output for time t1, each pixel output during that time is transferred. No charges are accumulated in the pixels, and charges corresponding to the accumulation time of 12 minutes after reset release are accumulated in each pixel. This reset time t1 can be changed and set to an optimal value as desired.

[Effects of the Invention] As described above, in the present invention, the charge accumulation time of the image sensor is limited by the reset means and measurement is performed at predetermined intervals, so that sufficient contrast of interference fringes can be obtained and stability can be achieved. This makes it possible to measure film thickness with high precision.

[Brief explanation of the drawing]

1 and 2 are configuration explanatory diagrams showing one embodiment of the present invention, FIG. 3 is an explanatory diagram of signals, and FIG. 4 is an explanatory diagram of interference fringes.

Claims (1)

[Claims] 1. In an apparatus that projects light from a light source onto an object to be measured, spectrally transmits the transmitted light or reflected light, detects interference fringes with an image sensor, and measures the film thickness of the object to be measured. A film thickness measuring device characterized by being provided with a reset means for changing a reset time.