JPH01276007A - Surface roughness meter - Google Patents

Abstract

PURPOSE:To exactly measure irregularity of the surface to be measured even when an optical characteristic of the surface to be measured is varied by providing a polariscope by which an irradiation light to the surface to be measured becomes only an S polarized wave against the surface to be measured. CONSTITUTION:As for an incident light beam whose passing range has been regulated 5, its polarization direction becomes 45 deg. against the radial direction, and said incident light is divided into a reflected light beam and a transmission light beam by a beam splitter 3. The surface 1 to be measured is irradiated with the reflected light by an objective lens 2a, and its reflected light is subjected to a phase variation in accordance with irregularity of the surface 1 and a complex index of refraction of the surface 1 and passes through a lens 2a again, and goes into the splitter 3. A reference mirror 4 is irradiated with the transmission light by an objective lens 2b, and the reflected light passes through a lens 2b again and goes into the splitter 3. The mirror 4 is attached to a mirror fine adjustment 28 and its reflected light can vary optical path length, and by a light shielding plate 6 which has been attached to a slide device 26, the reflected light of the mirror 4 is brought to light shielding. A light beam which has passed through the splitter among the light beams from the surface 1 and a light beam which has transmitted through only a prescribed polarization direction by a variable polariscope 7 among the light beams from the mirror 4 are brought to image formation on an image pickup element by an image forming lens 8.

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a surface roughness meter that uses light to measure the surface roughness of materials having conductive surfaces, such as metals and semiconductors. [Conventional technology] Regarding the conventional surface roughness tester using light, please refer to “Contact type method for evaluating the properties of precision machined surfaces” (Editor: (Company)
Japan Society of Precision Machinery Engineers, Publication Date: September 1985) Pages 50 to 1
Discussed on page 12. Measurement methods using light include optical cutting methods, microinterference methods, optical probe methods, and the like. Among these, highly accurate methods include microscopic interference method and some optical probe methods (astigmatism method, critical angle method, etc.). Future methods will use an objective lens to observe the optically reflective surface of the surface, the microinterference method will measure the roughness within the field of view of the objective lens as interference fringes, and the optical probe method will measure the out-of-plane displacement of a single point. Measuring. In addition,
Related to this type of roughness meter is the patent issued in 1983-
No. 728 is mentioned. [Problems to be Solved by the Invention] The above-mentioned conventional technology does not take into account the case where the optical characteristics of the surface to be measured change. When trying to measure , the real surface and the optically reflective surface of a glass surface match, but for a surface with a complex refractive index, such as a metal, the reflected light undergoes a phase change, and the real surface and the optically reflective surface match. will no longer match, and this amount will cause a measurement error. Further, even on the same metal surface, if the complex refractive index changes depending on the location, there is a problem in that measurement errors occur due to the above phenomenon. The purpose of the present invention is to solve the above-mentioned conventional problems and to
An object of the present invention is to provide a surface roughness meter that reduces measurement errors when the complex refractive index of the surface of a semiconductor or the like changes. [Means for solving the problem] The above purpose can be achieved by using only the S-polarized light wave for the surface to be measured out of the light incident and reflected from the surface to be measured, and by keeping the angle of incidence and reflection constant. In addition to reducing the difference between the surface and the optically reflective surface, it also measures the complex refractive index of the surface to be measured,
The above is achieved by correcting the error. [Function] The light reflected from the surface of an object changes depending on its polarization direction, angle of incidence, and refractive index of the object.8Among these, P-polarized light changes greatly depending on the angle of incidence, and in particular, the phase change mentioned in the above problem is the largest. The angle becomes 180 degrees, which greatly increases the difference between the actual surface and the optically reflective surface, which causes measurement errors. But 5. For S-polarized light, the phase change is O at all incident angles in the case of a dielectric material such as glass, and even in the case of a material with a complex refractive index such as a metal, there is a slight phase change determined by the complex refractive index at an incident angle of 0. Although there is a change, it decreases as the incident angle increases, and the incident angle 9
However, at 0 degrees, the phase change is O. In actual measurements, it is not possible to use only the S-polarized light wave with an incident angle of 90 degrees, so some one-constant error remains. Therefore, the complex refractive index of the surface to be measured is measured based on the data of the unevenness of the optically reflective surface of the surface to be measured measured with S-polarized light at a constant angle of incidence, and the difference between the actual surface and the optically reflective surface is calculated. By correcting this, it is possible to accurately measure the unevenness of the actual surface. Also,
The optical properties of the surface to be measured can also be measured from the above complex refractive index. [Example] When light enters the surface of an object, reflection and refraction occur, and this is different if the object is a dielectric or a conductor. The situation will be explained below. Incident light, reflected light, and refracted light all lie within a plane that includes the normal to the object surface, and the component of light that oscillates within this plane is P.
A component of light that vibrates at right angles to one plane of polarized light is called an S-polarized wave. These two components of light are reflected differently depending on the angle of incidence. If the object is a dielectric, the incident angle is set to 01. When the refraction angle is 02, the reflection coefficient Rp of the P polarized light wave and the reflection coefficient R3 of the S polarized light wave are expressed by the following equations. Here, θ2 is expressed by the following equation based on Snell's law, where n is the refractive index of the dielectric. sinθ2=sinθt/n...
(3) Substituting the third equation into the first equation and the second equation, the following equation is obtained. Here, in the fifth equation, under the conditions of n>1 and π/2≧θヱ≧O, both denominators and numerators are positive, so Rs is positive. However, in the fourth equation, the denominator is positive within the above conditions, but the numerator can take positive and negative values including 0, so RP takes positive and negative values including 0 * Polarization at the angle of incidence where Rp = 0 Figure 13 shows the change in Rp e Rs with respect to the incident angle θl, divided into the absolute value of the reflection coefficient and the phase change (n = 1.5).The reflection coefficient of the P-polarized light wave is θl. decreases as the polarization angle increases, and becomes O at the polarization angle. It increases as θ1 further increases, and becomes 1 when θ1=90 degrees. P
The phase change of the polarized light wave is O when θl is smaller than the polarization angle, but when it exceeds the polarization angle, the phase is reversed and becomes 180 degrees. Further, the reflection coefficient of S-polarized light increases as θl increases, and becomes 1 when θ1=90 degrees. The phase change of the S-polarized light wave is always 0, unlike that of the P-polarized light wave. Note that when θ1 = O degree, the reflection coefficient is 0.2 and the phase change is O for both S-polarized light wave and P-polarized light wave, but this is because the incidence is perpendicular and there is no distinction between P-polarized light wave and S-polarized light wave. . For these reasons, in the reflection of light on the dielectric surface, there is no change in phase within a range that does not exceed the polarization angle, so the real surface and the optical reflection surface match, and no measurement error occurs. If the object is conductive, the refractive index n in the dielectric is the complex refractive index n (1+ik) [k is called the absorption coefficient. Also, i can be replaced with ko representing an imaginary number, therefore,
Replace n in the third equation with n (1+ik), the first equation, and the second equation
Substituting into the equation, the reflection coefficient Rp? Rs is the following equation (6)...(7) Here, since the denominator and 9 numerators of equations 6 and 7 are both complex numbers, Rp and Rs are complex numbers. In other words, in the case of reflection from a dielectric material, the phase change is either 0 or 180 degrees, but in the case of reflection from a conductor, various phase changes are possible. Figure 14 shows an example of metal Rp and Rs (n=1.4
4. = 5.23), and similarly to Fig. 13, the changes in Rp and Rs are shown divided into the absolute value of the reflection coefficient and the phase change, and θ1
; When 0, both Rp t and Rs have a reflection coefficient of 0.953.
The phase change is 14.6 degrees. This phase change indicates that the optical reflective surface is moving inside the real surface.
In this case, the value is 12.8 nm (the wavelength of light is 632 nm).
8 nm). Next, as θ1 is increased, Rp and Rs behave differently, and the reflection coefficient of Rp behaves similar to that of a dielectric, decreasing at first, then increasing, and reaching 1 at θ1 = 90 degrees. Become. However, it differs from a dielectric material in that the lowest value of the reflection coefficient is not O. Moreover, the phase change of RP increases as θ1 increases, and when θ1=90 degrees, it becomes 180 degrees, and the difference between the real surface and the optical reflection surface becomes equivalent to 158 nm. On the contrary,
The reflection coefficient of Rs increases with the increase of θ1, and θ1=9
At 0 degrees, it becomes 1. Further, the one phase change decreases as θl increases, and becomes 0 when θ1=90 degrees. Therefore, when using all the light in the objective lens with an optical roughness meter, the reflected light includes P-polarized waves and S-polarized waves from θ1=0 to the angle corresponding to the numerical aperture of the objective lens. The integral value determines the difference between the real surface and the optically reflective surface. Therefore, when the complex refractive index is constant, no measurement error occurs because the above-mentioned difference is constant, but when the complex refractive index changes, the difference changes and a measurement error occurs. Therefore, by using only the S-polarized light wave out of one reflected light and regulating its incident angle within a certain range, the difference between the real surface and the optical reflection surface can be reduced, and measurement errors can be reduced. However, if this continues, errors will remain, so it is necessary to set the complex refractive index of the object surface and correct the above errors. Next, we will show how to measure the bifurcated refractive index by measuring the polarization state of the incident light by linearly polarized light (S-polarized light wave, P-polarized light wave) with a polarization plane of 45 degrees with respect to the plane perpendicular to the surface to be measured, including the incident reflected light. If both have the same amplitude and their phase difference is O), then
The reflected light becomes elliptically polarized light under the influence of the complex refractive index. Now, the first equation and the second equation are expressed as the insulation value rP of the reflection coefficient, I
's and phase changes ΦP and ΦS, the following equation is obtained. Rp=I'p・exp(iΦp)
...(3)Rs=I
” s ・5xp(iΦS) −(9
) Using this, elliptically polarized light is expressed as A-exp(iΔ) (A=I”s/Fp:Δ=ΦP-ΦS). Below, we will explain how to find these A and Δ. Ip-8 when only P polarized light is transmitted among the 11 reflected lights, assuming that the light intensity of P polarized light wave and S polarized light wave is Io.
XS is the light intensity when only polarized waves are transmitted, and I is the light intensity when only the light in the 45 degree direction is transmitted.
4s, a direction 90 degrees off from the direction of I +8, that is, 1
, the light intensity when only the light in the 35 degree direction is transmitted is 11
118, each light intensity becomes the following formula. I p-: rp” ・I o
−(10)I s= r” +2.I o
−(11)I41+=(r”p
"+I's"+2・I'p-r"5-eosΔ)・Io
/2...(12) I 188= D'p"+I' +2-2 ・r'
p-r5-cosΔ)・■o/2 (13) Using this to find A and Δ, the following equation is obtained. A257 stone 7 ... (14) In addition, Δ
can also be determined using only Ipt Is v I41S. Therefore, in the seventh equation showing the reflection state of the S-polarized light wave, the first item of one numerator in the denominator that is affected by the complex refractive index is expressed by the following equation using A and Δ representing elliptically polarized light. 1 + A-exp(iΔ) ... (16)

[7 Therefore, by finding the four light intensities Ip, Is* Is8°I z+se, Equations 14 and 15 are obtained. From Equations 16 and 7, the difference between the real surface of the S-polarized light wave and the optical reflection surface can be found, and by correcting the measurement result of the optical reflection surface, the unevenness of the real surface can be measured without error. Further, when constants n and k that submerge the complex refractive index are calculated from A, Δ and the incident angle θl, the following equations are obtained. k, = tan2 ψ 'e08Δ
−(18) (where φ= a
(retan A) Hereinafter, the present invention will be explained based on Examples. FIG. 1 shows an embodiment of the present invention, where the incident light is S P<,
(The passing range is restricted by the radial polarizer 5, the polarization direction is set at 45 degrees with respect to the radial direction, and the beam splitter 3 separates the reflected light and transmitted light. The reflected light is measured by the objective lens 2a. The reflected light is irradiated onto the surface 1, and the reflected light undergoes a phase change according to the unevenness of the surface 1 to be measured and the optical constant (complex refractive index) of the surface 1 to be measured, passes through the objective lens 2a again, and enters the beam splitter 3. The transmitted light is irradiated onto the reference mirror 4 by the objective lens 2b, and the reflected light passes through the objective lens 2b again and is sent to the beam splitter 3.
The optical path length of reflected light can be varied. Also. A light shielding plate 6 attached to a slide device 26 is provided between the beam splitter 3 and the objective lens 2b, and can shield light reflected from the reference mirror 4 as necessary. The light from the surface to be measured is transmitted through the beam splitter 3, and the light from the reference mirror 4 is beat: l
The light reflected by the Sf liter 3 is transferred to a
Only a predetermined polarization direction is transmitted through the variable polarizer 7; Variable polarizer'7
L: denotes a circular S polarizer 102 and a circular P polarizer 11, which are used for measuring the light intensity according to equations 10 to 13. Spiral polarizers 1, 2, and 13 are installed and can be used depending on the direction of polarization. In order to correspond to the polarization state of the reflected light from the surface to be measured 1, each polarizer had a polarization direction at an angle corresponding to the purpose with respect to a radial direction centered on the optical axis. In addition, in order to regulate the reflection angle of the reflected light used, a light shielding part 14 is provided at the center of each polarizer.
A to 14d are provided. Next, the structure of each polarizer will be described. FIG. 3 shows details of the spiral polarizers 5 and 12, which are linear polarizers 15 and 12.
The light shielding part 14 is made up of a to 15p and a light shielding part 14c.
It can be constructed by arranging fan-shaped linear polarizing plates 15a to 15tsp around c so that their polarization directions are inclined at 45 degrees clockwise with respect to the radial direction. Figure 4 shows a circular S polarizer 10.
This structure also shows fan-shaped linear polarizing plates 16a to 16p arranged around a light shielding part 14a, but the polarization direction thereof is oriented in the tangential direction of the circle. Figure 5 shows circular P polarizer 1.
1, in which fan-shaped linear polarizing plates 17a to 17p are arranged around a light shielding part 14b, but the polarization direction thereof is oriented in the radial direction of the circle. Although the spiral polarizer 13 is not shown, the spiral polarizers 5 and 1
It can be constructed by arranging the two polarization directions with the polarization directions thereof tilted at 45 degrees counterclockwise with respect to the radial direction. These polarizers are simple ones using linear polarizing plates, and although they are not ideal unless the number of fan-shaped divisions is made infinite, they are sufficient for practical use. Figures 6 to 9 show polarizers using other methods; Figures 6 and 7 show circular S polarizers using dielectric polarizing films; Figure 7 is the same, so it is the 6th figure.
The 0-circular prisms 18a and 18b explained in the figure have parallel upper and lower surfaces, inner and outer circumferential surfaces are conical surfaces with an apex angle of 90 degrees, and dielectric polarizing films 19a and 19b are attached to the entire outer circumferential surface. . Two circular prisms 18a and 18b are overlapped with the bottom surfaces of the cones, and when light enters from above, it is totally reflected on the inner circumferential surface of the circular prism 18a and directed toward the outer circumferential surface. Due to the action of the dielectric polarizing film 19a, only the S-polarized light wave is reflected and the P-polarized light wave is transmitted. Therefore, the light incident on the circular prism 18b is only the S-polarized light wave. The light passes through the opposite direction of the circular prism 18a, and finally only the S-polarized light is transmitted. Note that the dielectric polarizing film 19a
, 19b may be omitted. Circular prism 18 if the light transmission radius can change
b can be omitted. FIG. 8 shows a circular P polarizer using a dielectric polarizing film. The circular prisms 20a and 2Qb are on a cylinder whose cross section is an isosceles right triangle, and the two circular prisms 20a and 20b touch on a 45-degree plane. is dielectric polarizing film 1
9 is attached. In this state, when light is incident from above, the dielectric polarizing film 1
9 allows only P-polarized waves to pass through and reflects S-polarized waves, forming a circular P-polarizer. Note that the light shielding portion is also necessary for the polarizers shown in FIGS. 6 to 8. FIG. 9 shows another example of a spiral polarizer, in which S-polarized light waves and P-polarized light waves obtained by a circular S-polarizer 10 and a circular P-polarizer 11 are superimposed. By adjusting the phase of the light, it becomes spirally linearly polarized light. However, this polarizer cannot be used with spiral polarizers 12 and 13. The slide device 26 whose circuit diagram of this embodiment is shown in FIG.
The 9 rotating device [27] and the mirror fine movement device 28 are set in predetermined positions by the control bag [30]. The image signal from the image sensor 9 is digitized by an A/D converter 29 and then taken into a frame memory 31. Note that a necessary number of frame memories 31 are prepared to store a plurality of images. The calculation circuit 32 performs a predetermined calculation for each pixel of the frame memory 31 and outputs a measurement result. Note that the measurement results can also be stored in the frame memory. Next, the measurement method will be explained. First, in measuring the optical measurement surface of the surface to be measured 1, the light shielding plate 6 is placed at the position indicated by the solid line in FIG. interfere above. At this time, the variable polarizer 7 is replaced by a circular S polarizer 10, so that the interference fringes on the image sensor 9 are only those due to S polarized waves. Reference mirror 4 is used to analyze unevenness caused by interference fringes.
is finely moved four times in 1/4 wavelength steps in the optical axis direction by a mirror fine movement device 28, and at each step, the light intensity of each pixel of the interference fringe is measured and the unevenness is calculated by calculation. If the fringe scanning method is used, it can be determined for each pixel of the image sensor 9 with high precision. Next, the measurement of A and Δ using Equations 10 to 15 will be described. First, place the light shielding plate 6 at the position indicated by the broken line in the figure,
Light to the reference mirror 4 is blocked, and only light reflected from the surface to be measured 1 is transmitted to the image sensor 9. In this state, the variable polarizer 7 is rotated, and the transmitted light intensity corresponding to each of the polarizers 10 to 13 is measured for each pixel of the imaging probe 9. From this measured value, calculate equations 14 and 15 for each pixel, and
. Δ, calculate the difference between the real surface of the surface to be measured 1 and the optically reflective surface from Equations 16 and 7, and correct the measurement result of the optically reflective surface using the interference fringes described above. Unevenness can be determined with high precision. Furthermore, by finding n and k for each pixel from equations 17 and 18, the optical constants of the surface to be measured 1 can be measured. FIG. 11 shows another embodiment of the present invention. This is an application of the present invention to the astigmatism method, and a detailed explanation of the astigmatism method will be omitted here. The light that has passed through the beam splitter 3a is split into two by the beam splitter 3b, and the transmitted light is made into only S-polarized waves by a circular S-polarizer 1°.Then, the optical reflection of the surface to be measured 1 is determined by the astigmatism method. Measure the surface. On the other hand, the light reflected by the beam splitter 3b is divided into four lights by beam splitters 30 to 3e, and each light passes through four types of polarizers 10b, 11, and 12°13, and then passes through imaging lenses 8b to 8e. The optical sensors 25a-2
5d, and the light intensity is detected. Inputting the light intensity of optical sensors 25a to 25d whose measurement circuit is shown in FIG. 12,
The error calculation circuit 34 calculates equations 14, 15, 16, and 7 to determine the difference between the actual surface of the surface to be measured 1 and the optical reflective surface. The above-mentioned measurement by the astigmatism method is performed by the astigmatism calculation circuit 33. The measured value of the 0 circuit 33 is given to the subtracter together with the output of the error calculation circuit 34. The subtracter 35 subtracts the error from the measured value obtained by the astigmatism method, and outputs a measurement result of the irregularities of the real surface of the measured surface with little error. In addition, the error calculation circuit 34
It also calculates the equation 18 and outputs the optical constant of the surface to be measured 1. [Effects of the Invention] According to the present invention, measurement errors caused by changes in the difference between the real surface and the optically reflective surface due to changes in the optical characteristics of the surface to be measured can be corrected, so even when the above-mentioned changes occur, the measurement errors of the surface to be measured can be corrected. This has the effect of accurately measuring unevenness.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIGS. 2 to 9
The figures show various polarizers, Figure 10 is a circuit diagram of the embodiment shown in Figure 1, Figures 11 and 12 are diagrams showing other embodiments, Figures 1-3, and FIG. 14 is a diagram showing the light reflection characteristics. DESCRIPTION OF SYMBOLS 1... Surface to be measured, 2... Objective lens, 3... Beam splitter, 5... Spiral polarizer, 7... Variable polarizer. 1 Fig. IS (Separate - Wooden Z3 Mirror Horisu Natsume Kasaraヱ [Non Z Fig. 7 Variable polarization zone 10 Circular 5 Polarization/4 Gloss suppression Z 3 Fig./6' Fig. 5/7 L< Edge 4 Tori Kabuto Taku 6 Hi No. 7 Figure 19 Dielectric, #R Kotatsu Z3 Figure Tobi 9 Figure t9 #@-4 Honi flat Ohame membrane kudzu /ρ Figure 1・) Determination, result output r /1 Figure' fJ/Z Figure Kuzu /3 11U Bian 14 Figure

Claims (1)

[Scope of Claims] 1. In an optical surface roughness meter for measuring the unevenness of a surface to be measured, a polarizer is provided to make the light irradiated onto the surface to be measured only S-polarized. Features an optical surface roughness meter. 2. An optical surface roughness meter for measuring the unevenness of a surface to be measured, which is characterized by being equipped with a polarizer that transmits only S-polarized waves to the surface to be measured among the light reflected from the surface to be measured. Surface roughness meter. 3. An optical surface roughness meter according to claim 1 or 2, characterized in that only light having a constant angle of incidence on the surface to be measured or a constant angle of reflection from the surface to be measured is used. 4. The surface roughness meter according to claim 1, 2 or 3, wherein means is provided for setting a complex refractive index of the surface to be measured, and means for measuring the complex refractive index is provided, so that An optical surface roughness meter that measures the roughness of an actual surface by correcting the difference between the surface and an optically reflective surface, and also measures changes in physical properties of the surface to be measured from the complex refractive index. 5. In the surface roughness meter according to claim 1, 2 or 3, means for keeping the polarization plane of the light incident on the surface to be measured constant;
A means for detecting the elliptical polarization state of the reflected light from the surface to be measured is provided, and the difference between the real surface of the surface to be measured and the optical reflective surface caused by the complex refractive index from the elliptical polarization state is corrected, and the roughness of the real surface is corrected. An optical surface roughness meter characterized by measuring changes in the physical properties of the object to be measured based on the state of elliptically polarized light.