JPS6350705A - Apparatus for measuring film thickness - Google Patents

Abstract

PURPOSE:To obtain an apparatus for automatically correcting the refractive index of an object to be measured, by constituting the title apparatus so that beams are projected to the object to be measured at two or more angles and each interference fringe is detected by a detector and the refractive index of the object to be measured is calculated from a wavelength imparting an extreme value. CONSTITUTION:The relation between each element number of an interference fringe pattern detector 11 and a wavelength is preliminarily stored in the memory of an operation means 13. Then, the output of each element of the detector 11 is successively read at the time of measurement and a wavelength imparting the extreme value of each interference pattern in a measuring range is calculating and degree difference is calculated from the numerical difference of extreme values and predetermined operation is performed based on the refractive index from the memory to calculate the film thickness of an object 4. When the refractive index of the object 4 is unknown, especially, when the kind of the object 4 changes and the refractive index thereof changes, beam sources 1a, 1b are arranged so that beams therefrom are projected to the object 4 at different angles thetaa, thetab with respect to the normal line of the measuring position of the object 4 and both beams are detected by detection parts 14a, 14b. Subsequently, a refractive index is calculated according to a predetermined formula and automatically corrected to calculate the film thickness of the object 4.

Description

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

[Prior art] To measure the film thickness of a target using optical interference,
The light from the light source is projected onto the object to be measured, while
To measure the film thickness of the object to be measured, it is necessary to know the refractive index in advance, and if the type of object to be measured changes and the refractive index changes, it is necessary to change the settings of the measurement system accordingly. It was extremely inconvenient.

In view of the above points, an object of the present invention is to provide a film thickness measuring device that automatically corrects the refractive index of an object to be measured.

[Means for Solving the Problems] This invention projects light onto the object to be measured at two or more angles, detects interference fringes at each angle with a detector, and selects the object to be measured from the wavelength that gives the extreme value. The refractive index of the object is determined and the film thickness is calculated. Measured using a film thickness measuring device.

[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 light source 1 is a lens.
2, the light is projected onto the object to be measured 4, such as a film saw, through the half mirror 3, and the light transmitted or reflected by the object to be measured 4 is transmitted through the half mirror 3, lens 5, and shutter such as a chopper in this figure. The light is separated by a spectroscopic means 9 such as a diffraction grating through a means 6, an aperture 7, and a lens 8, and enters a detector 11 of an image sensor such as a CCD through 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 the amplifier 12, and a predetermined calculation is performed by the ringing means 13, whereby the film thickness d of the object to be measured 4 is calculated.

As shown in FIG. 2, parallel rays [1, 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 rays L+, ! 2 has an optical path difference 2 nd/CO3e ', and when this optical path difference is an integral multiple of the wavelength of light, interference occurs to form interference fringes as shown in FIG.

Then, the following formula holds true.

(m+N)λ+ =2nd/cosθ'(1)mλ2
= 2 nd/cos θ' (2) (Determine m from formula 2> and substitute it into formula (1) to organize it. When θ'-〇 is projected perpendicularly to the object to be measured 4, COS θ'- 1, equation (3) becomes: In this way, the wavelengths λ1, λ2, and the order difference N between the extreme values
, from the known refractive index n, the film thickness of the measured object 4 is (3),
It can be generally determined from equation (4).

In other words, since the wavelength incident on each element of the detector 11 is determined in advance by the spectroscopic means 9, the relationship between each element number of the detector 11 and the wavelength is stored in the memory of the calculation means 13.
I will serve you.

Find the wavelengths λ1 and λ2 using (Memo 1 from the element number that gives hesitivity), and find the order difference from the difference due to the extreme value,
Using the refractive index stored in the memory, calculations such as equations (3) and (4) are performed to determine the film thickness d of the object to be measured 4.

By the way, if the refractive index aperture of the object to be measured 4 is unknown, (
3) The film thickness d cannot be determined from formula (4), and especially when the type of object to be measured 4 changes and the 5 refractive index n changes, calculate the refractive index as follows and automatically Correct it accordingly.

As shown in FIG. 4, light sources 1a and 1b project light at different angles θa and θb with respect to the normal to the measurement position of the object to be measured 4, and the light is reflected or transmitted through the object to be measured 4 by the detection units 14a and 14b. The interference fringes are detected using a detection system similar to that shown in FIG.

It is assumed that interference of order m occurs at wavelength λa when light is projected at an angle θa, and at wavelength λb when light is projected at an angle θb, and n = sin θ
a/sinθa', sinθb/sin O'
.

= 2nd/FTTi ■Go] 8" = 2nd/ 1-5in 2θa /n 2 (5
) mλb = 2nd/-5in 20b/n
2 (6) By dividing equations (5) and (6) around each side, the following equation can be found.

When the projection angle θa=Q, sin θa−〇, equation (8) becomes as follows.

In other words, the refractive index n is determined from equations (8) and (9), and (
3>, the film thickness d can be determined from equation (4).

In this way, even if the refractive index n of the object to be measured 4 is unknown, the refractive index n can be easily determined and the film thickness d can be measured.

In addition, as the light sources 1a and 1b and the detection units 14a and 14b,
It is also possible to use the same one and measure it at different angles using an appropriate moving means before caulking measurement, and during actual measurement, measure from the vertical direction, for example.

Note that, as shown in FIGS. 3A and 3B, interference fringe patterns may shift and overlap due to variations in the film thickness of the object to be measured 11, resulting in weak portions. In order to avoid this, the film thickness is measured as described above from the output of the extreme value when the difference between the maximum value and the minimum value (*) of the output for each wavelength of the detector 11 is greater than a predetermined value. This makes it possible to measure the film thickness d of the object to be measured 4 from strong and reliable interference fringes without picking up uncertain and weak interference fringes.

Furthermore, if an image sensor such as a charge storage type diary element CCD is used as the detector 11, the reliability will be slightly deteriorated if the object 4 to be measured moves and its thickness etc. deviate.

For this reason, a shutter means 7 such as a chopper whose sectors are rotated by a motor is used to limit the time for one measurement by continuing the incident light to Vfr, thereby reducing the influence of fluctuations in the incident light on the detector 11, and reducing interference fringes. To prevent the contrast from deteriorating and ensure reliable measurement.

[Effects of the Invention] As mentioned above, this invention determines the refractive index n from the interference fringes at at least two angles, so it is possible to automatically correct the refractive index, and Even if the type of measurement target or refractive index changes, accurate film thickness measurement is always possible.

[Brief explanation of drawings]

FIG. 1, FIG. 2, and FIG. 4 are conjunctive explanatory diagrams showing one embodiment of the present invention, and FIG. 3 is an explanatory diagram of interference fringes. 1・
...Light source, 2.5.8.10... Lens, 3... Half mirror 14... Measured object, 6... Shirt patent applicant Chino Seisakusho Co., Ltd. No. 10 Now No. 20

Claims (1)

[Claims] 1. A light source that emits light at at least two or more angles with respect to the normal to the measurement position of the object to be measured, and a light source that emits light at two or more angles from the object to be measured. A detector that separates transmitted light or reflected light using a spectroscopic means to detect interference fringes, and calculates the refractive index of the object to be measured from the wavelength that gives the extreme value of each interference fringe pattern at two or more angles of this detector. 1. A film thickness measuring device comprising: a calculation means for calculating film thickness. 2. The film thickness measuring device according to claim 1, which includes a case where the projection angle of the light source is set to 0 degrees. 3. The film thickness measuring device according to claim 1 or 2, characterized in that two or more of the light sources and the detectors are provided, and a moving means is provided that allows the angle of light emission and reception to be arbitrary. 4. The film thickness measuring device according to any one of claims 1 to 3, characterized in that an image sensor is used as the detector.