JPS6350706A - 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 in calculated from the max. and min. wavelengths of a measuring section among wavelengths imparting the max. or min. value of said detector and the number of extreme values therebetween. 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 the max. and min. wavelengths are calculated from element numbers imparting extreme values in a measuring range and degree difference is calculated from the numerical difference between extreme values and predetermined operation is performed based on of 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 the 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 detectors 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,
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,
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 very inconvenient.

In view of the above points, an object of the present invention is to project light onto an object to be measured at two or more angles, and to obtain interference fringes for each of the objects to be measured. This is a film thickness measuring device that detects the refractive index with a detector, determines the refractive index of the object to be measured from the wavelength that gives the extreme value, and calculates the film thickness.

[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 through a half mirror 3, a lens 5, a shutter means 6 such as a chopper, an aperture 7, a lens, and a spectroscopic means 9 such as a diffraction grating. 11.

Light corresponding to each wavelength separated by the spectroscopic means 9 is incident on each element of the image sensor 11, and the intensity of the interference '·\ stripe 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, whereby the film thickness d of the object to be measured 4 is calculated.

As shown in FIG. 2, parallel 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 thickness (thickness) d1 and a refractive index n, and both rays L+ and L2 are reflected on the optical path. They have a difference of 2nd/cos θ', and when this optical path difference is an integral multiple of the wavelength of light, they interfere to form interference fringes as shown in FIG.

Assuming that an interference fringe pattern as shown in FIG. 3 is created,
Regarding the minimum wavelength λ1 and maximum wavelength λ2 that give the extreme values of the measurement region, the following equation holds true, assuming that the respective orders of interference are m + N and m.

(m+N)λ+ = 2 nd/ cosθ' (
1) mλ2 = 2 nd/cosθ' (2) (
Find m from equation 2), substitute it into equation (1), and rearrange it as follows. In this way, wavelengths λ1, λ2, (order difference in production value N
, from the refractive index n determined in advance, the film thickness of the object to be measured 4 is determined from equations (3) and (4).

That is, since the wavelength incident on each element of the detector 11 is determined in advance by the spectroscopy 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.

Then, during measurement, the output of each element of the detector 11 is sequentially read out, and as shown in FIG. 3, the wavelengths λ1 and λ2 are determined using memory from the element number that gives the extreme value within the measurement range.
Find the order difference N from the difference in the number of extreme values, use the refractive index n stored in a memory, etc., and calculate the film thickness d of the object to be measured 4 by performing calculations such as equations (3) and (4). .

By the way, find a field where the refractive index n of the object to be measured 4 is unknown,
Correct automatically.

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 by the detection units 14a and 14b. The light is detected, and interference fringes are detected using a detection system similar to that shown in FIG.

λ1: Minimum wavelength that gives the extreme value of the interference fringe pattern when light is projected and received at angle θa λ2: Maximum wavelength λ1' same as above Minimum wavelength that gives the extreme value of interference pattern (turn) when light is projected and received at two angles θb 2 Same as above Maximum wavelength m 1 : Order of interference at minimum wavelength λ1 when light is projected at angle a e a m2 : Order of interference at maximum wavelength λ2 when light is projected at angle θa Order of interference at 22' n: refractive index, d: film thickness N=m+m2, N'=ITl+'m2',
n = sinθa /sinθa', sinθl)
The following equation holds true using /sin θb'.

m1λ'2 = 2 nd/cosθa' = 2 nd
/ rcho7revised71cho7 =2nd/ -5in 2 &afu n2 (5)
m2λ2=2nd/-s+n2 a/nz(6
)m +'λ+'=2nd/v -3ln2 b/
n2 (7) m 2'λ2'=2nd/-5i
n 2 /n 2 (8) N=lIl+m2 From equations (9) and (10), the following equation is obtained.

From equation (11), since everything other than n is known, n can be found.

Now, for the sake of simplicity, consider the case when θa=Q. Since the right side can be determined by measurement or the like, the value of the refractive index n is determined from this, and the film thickness d is determined from equations (3), (4), etc.

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, the same light sources 1a and 1b and the detection units 14a and 14b are used, and the measurement is performed by changing the angle using an appropriate moving means before measurement, and during actual measurement, for example, the measurement is performed from a vertical method. You can do it like this.

In addition, as shown in Figure 3A and B, in order to avoid measuring objects, the film thickness is calculated as described above from the output for the extreme value when the difference between the maximum value and the minimum value is greater than or equal to a predetermined value. Thereby, the film thickness d of the object to be measured 4 can be measured 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 dynamic image sensor CCD is used as the detector 11, if the object to be measured 4 moves and its thickness etc. deviate, a slightly different interference fringe pattern will appear on the detector. The light enters each of the 11 elements within one scanning period and is synthesized, resulting in poor contrast as a whole.

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

[Effects of the Invention] Since the refractive index can be corrected automatically, accurate film thickness measurement is always possible even if the type of the object to be measured or the refractive index changes. In particular, since the order of interference can be set arbitrarily, the refractive index can be calculated reliably.

[Brief explanation of the drawing]

FIG. 1, FIG. 2, and FIG. 4 are explanatory views showing an embodiment of the present invention, and FIG. 3 is an explanatory view of interference fringes.

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 the transmitted or reflected light from the object to be measured for the light emitted from this light source at two or more angles. A detector that performs spectroscopy using a spectroscopic means and detects interference fringes; Film thickness measurement characterized by comprising: calculation means for calculating the refractive index of the object to be measured by performing a predetermined calculation from the number of extreme values between them, and calculating the film thickness of the object to be measured from this refractive index. Device.