JPH01270603A - Infrared thickness meter - Google Patents

Abstract

PURPOSE:To prevent the generation of a measuring error even when the incident angle of P-polarized beam is shifted from a Brewster angle, by providing a polarizing irradiation means constituted of a cuesta prism and a polarizer, a beam detector and an operation means. CONSTITUTION:The collimation beam from a beam source 3 is brought to two parallel beams by a cuesta prism and P-polarized beam is separated by a polarizer 5 and S-polarized beam is separated by a polarizer 6. The beam from the prism 4 is set so as to be incident to an object 1 to be measured at a Brewster angle. A beam detector 9 detects the P-polarized and S-polarized beams condensed by a lens 8 and an operation means stores not only a relational curve of the incident angle of an S-polarized beam component and the intensity of reflected beam but also a relational expression of the transmission intensity of P-polarized beam expressed by the functions of an incident angle, an angle of refraction and a film thickness. The incident angle and angle of refraction at the time of measurement are calculated on the basis of the relational curve and the measuring signal of S-polarized beam and substituted for the relational expression of P-polarized beam and the film thickness of the object to be measured is calculated on the basis of the measuring signal of the P-polarized beam. By this method, no measuring error appears even when the surface state of the object 1 to be measured is changed and the incident angle of the P-polarized beam is shifted from the Brewster angle.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an infrared thickness meter that transmits infrared rays through a sheet-like object to be measured and measures the thickness of the material from its absorbance.

<Prior art> Conventionally, for example, in Japanese Patent Application Laid-Open No. 60-224002, light from a light source is passed through a polarizer to extract P-polarized light, which is incident on a measured object at Brewster's angle, the transmitted light is reflected, and then An apparatus is disclosed that transmits light through the object to be measured and determines the thickness of the object based on the absorbance of the transmitted light.

As shown in FIG. 4(a), P-polarized light 7. When the incident angle φ is set to Brewster's angle and the sheet-shaped object 1 is made incident on the object 1, the reflectance of P-polarized light is zero and the total light amount becomes the object t*
1, the light reflected by the mirror 2 is incident on the object to be measured 1, so that training with a large signal component can be performed with little optical loss.

However, since the object 1 to be measured is moving, its surface condition is not constant, and the incident angle φ of the P-polarized light cannot always be maintained at the Brewster's angle. FIG. 4<b) shows this state. When the surface condition of the sheet-like object 1 changes and the angle of incidence φ of the Pi beam 1p deviates from the 1-star angle,
Reflection occurs at the interface between the air and the object to be measured 1, and the reflected light component becomes optical loss. In the case of JP-A No. 60-224002, the light passes through the object 1 to be measured twice, so that the optical loss is doubled.

This light loss is determined by the detector to be light absorption in the object to be measured 1, and becomes a cause of error.

<Problem to be Solved by the Invention> The technical problem to be solved by the present invention is to make P1 polarized light incident on the object to be measured at the Brewster angle, and calculate the thickness of the object to be measured from the absorbance of the transmitted light. The object of the present invention is to prevent measurement errors from occurring even when the incident angle of P1m light deviates from the Brewster's angle due to a change in the surface condition of the object to be measured, in an infrared thickness meter for determining the thickness of the object.

Means for Solving the Problems> The configuration of the present invention is that, in the infrared thickness gauge, a, the light from the light source, the Pi light, and the S [shoulder tip] are separated, and these lights are directed to the object to be measured at the same angle of incidence. B. Polarized light detection means for detecting the S@ light component that is not reflected by the object to be measured and the P1 flat shoulder portion that has passed through the object to be measured; C. Relationship curve between incident angle and reflected light intensity,
Also, the incident angle, the refraction angle, and a relational expression of the transmitted light intensity of P-polarized light expressed as a function of the film thickness are stored, and the incident angle at the time of measurement is determined based on the relational curve and the measurement signal of the SIN light. and the refraction angle, and convert these into the above P
A calculation means for calculating the film thickness of the object to be measured by substituting it into the relational expression of the IN light and performing an uncover operation on the measurement signal of the P iti light is provided, and the P-polarized light is The purpose is to prevent the influence of the reflected light component from appearing on the measurement results when the angle deviates from the star angle.

Function> The above technical means works as follows. That is, the light from the light source is converted into two parallel beams using, for example, a Kestal prism, and a polarizer is placed in each optical path to produce P m light, S (
Separate tn light. These lights are made incident on the sheet-like object to be measured at Brewster's angle.

The l) polarized light component transmitted through the object to be measured and the S-polarized light component reflected by the object to be measured are separated by a mechanical separation means using, for example, a chopper, and each is measured as a light intensity by a detector. The transmitted light intensity of the P-polarized light component depends on Fresnel's equation, and is expressed as a function of the incident angle, refraction angle, and film thickness. The reflected light intensity of the S-polarized light component is expressed by Fresnel's equation as a function of the incident angle and the refraction angle. on the other hand,
When the refractive index of the transparent material is known, the angle of incidence and the angle of refraction can be related by Snell's equation.

Prior to measurement, the incident angle to the object to be measured is set to, for example, a 2-star angle, and the reflected light intensity of the S polarized light component is measured. Based on this, a calibration curve between the incident angle and the reflected light intensity of S-polarized light is obtained. During measurement, if the surface condition of the object to be measured changes and the incident angle deviates from the Brewster angle, the S
The reflected light intensity of the polarized light component changes. The incident angle is determined from this reflected light intensity using the calibration curve, the refraction angle is determined based on this, the incident angle and the refraction angle are substituted into the relational expression for the transmitted light intensity of the P1m light, and the calculation is performed. Determine the film thickness of the object to be measured.

<Example> The present invention will be described in detail below with reference to the drawings. FIG. 1 is an exploded perspective view showing an optical system of an apparatus according to an embodiment of the present invention. In the figure,
Elements that are the same as those in FIG. 4 are given the same reference numerals. 3 is a laser light source, 4 is a Kesku prism light source 3
The collimated light is made into two parallel beams. Polarizers 5 and 6 are placed in the optical path of these 1,000-line lights. For example, the polarizer 5 separates the P14m light I-3, and the polarizer 6 separates the S-polarized light Lp. The light from the Kester prism 4 is set to be incident on the object to be measured 1 at Brewster's angle. 7 is a rotary chopper that separates the P-polarized light component that has passed through the object to be measured 1, been reflected by the mirror 2, and passed through the object to be measured 1 again, and the S-polarized component that has been reflected by the object to be measured 1; Holes 7, 7 for component detection, hole 7° for Sj4 light component detection.

b 7d is provided and rotated in the direction of the arrow to alternately detect the P-polarized light component and the S-polarized light component. 8 is a condensing lens, 9
is a photodetector that detects the P polarized light component and the S Im light component collected by this lens.

Next, the operation of the apparatus having such a configuration will be explained with reference to the explanatory diagram of FIG. FIGS. 2(a, 2) show a state in which the surface of the object to be measured 1 has not changed, and FIG. 2(b) shows a state in which the surface of the object to be measured 1 has changed by an angle η relative to the mirror 2.

P-polarized light t-P and S-polarized light 7 which are not incident on the object to be measured 1 from the Kester prism 4 in FIG. 1 through the polarizers 5 and 6.
8 is incident on the object to be measured 1 at an incident angle φ at Brewster's angle when the surface is not changing as shown in FIG. 2(a).

Of these, the transmittance of P-polarized light Lp is 1.1. passes through the object 1 to be measured twice by the mirror 2, so if it is not absorbed within the object 1 to be measured, 'f' + P -j < SI n 2φ - S jn according to Fresnel's formula
2 χ) , /(Sin2 (φ+Z)C'O82(
φ−χ))]2...(1) (Tap, χ: refraction angle). In the case of incidence at Brewster's angle, there is the relationship φ1χ-(π/2)...(2), so equation (1) is 1゛1P-[(Sin2φ-3jn2χ)/Co52
(φ-χ)]' = 13).

Next, when the surface of the object to be measured 1 is tilted at an angle η with respect to the mirror 2 (Fig. 2 (b)), since the incident angle φ of the P-polarized light L P is incident at a angle other than Brewster's angle, the air and the object to be measured are Reflection at the interface with 1t, P, R2P.

R3, R4p are generated and are represented below.

R+ p −(t, a R2(φ1η−χ1))/(
t, an 2 (φ 10 η − t χ 1
) )...(4) (However, χ1: refraction angle relative to the incident angle φ1η) R2P
= jtan' (χ, -9S-77>j /(t,
an2 (χ1+φ+η)) ...(5) R3p = (jan' (φ-7722))/(t
an2 (φ-η+χ, )) ... (6) (However, χ2: refraction angle with respect to the incident angle φ-η) R4p
−(t, an 2(χ2−φ10η))/it, an
'(χ2+φ-η)) ...(7) Reflectance (R) and transmittance (T) are generally R + 'T''=1
...(8), therefore, P
Polarization, transmission of '4s 'I' 2p is 'T', 2 -+1-R, ・R2 ・R3P ・P
P PR, IP+
... is given by (9). When light absorption within the object to be measured 1 is taken into consideration, the following results are obtained.

T2 P −((Ao −K) ・d/Cosχ1
)-(1-P, + ・R2P 'R3P'R
a p )...(10) (However, Ao = incident light intensity, d: film thickness of the object to be measured 1, K
: constant) On the other hand, the reflection intensity R18 of S (b+lights 1 and 8 follows Fresnel's formula, R13=At; I-(S in2f$ Z>
l,/'(Sin2(φ+χ))co...(]1) It has the characteristics as shown in FIG. When the refractive index n of the object to be measured 1 is known, the incident angle φ is set to the Brewster angle corresponding to the refractive index, and the S-polarized light I78 is made incident on the object to be measured 1. A curve is specified from the detected optical elasticity A at that time, and a calibration curve of R18 is obtained. Since the curve itself does not change, a calibration curve can be specified at one point 4 (1J constant). Using this calibration curve, the incident angle φ at that time can be determined from the reflected light intensity of the 59jl light I78.

On the other hand, the angle of incidence φ and the angle of refraction χ have the following relationship according to Snell's formula: no-8inφ=n, -3inχ...(12) From this, χ can be found (where nO: refractive index of air) . As a result, 6, η, χ in equations (4) to (7)
1. χ2 is found, R+, R2p.

R3 and Rd are found. Furthermore, ( A
O' K ) can be found by performing measurements using a material whose film thickness is known. By substituting these values into equation (10), the film thickness d of the object to be measured 1 can be determined from the detected intensity of the transmitted light 2P of the P-polarized light 1-1.

<Effects of the Invention> According to the present invention, the surface condition of the object to be measured changes and P
No measurement error occurs even if the incident angle of polarized light is less than the blister angle.

[Brief Description of the Drawings] 1], 1- Fig. 1 is an exploded perspective view showing an optical system of an apparatus according to an embodiment of the present invention;
FIGS. 2 and 3 are explanatory diagrams for explaining the operation of the apparatus according to the embodiment of the present invention, and FIG. 4 is an explanatory diagram for explaining the drawbacks of the conventional apparatus.

Claims (1)

[Scope of Claims] P-polarized light is incident on a sheet-like object to be measured at Brewster's angle, the transmitted light is reflected by a mirror and transmitted through the object again, and the absorbance of the transmitted light is determined based on the absorbance of the object. An infrared thickness meter for determining the film thickness of a film, comprising: a. polarized light irradiation means for separating P-polarized light and S-polarized light from light from a light source and irradiating these lights onto the object to be measured at the same angle of incidence; b. a polarization detection means for detecting an S-polarized light component reflected by the body and a P-polarized light component transmitted through the measured object; c. a relationship curve between the incident angle of the S-polarized light component and the reflected light intensity; A relational expression for transmitted light intensity of P-polarized light expressed as a function of angle, refraction angle, and film thickness is stored, and the incident angle and refraction angle at the time of measurement are calculated based on the relational curve and the measurement signal of S-polarized light. and substituting these into the relational expression of the P-polarized light, and calculating the film thickness of the object to be measured based on the measurement signal of the P-polarized light, and An infrared thickness meter characterized in that when the P-polarized light deviates from the Brewster's angle, the influence of the reflected light component does not appear on the measurement results.