JPH04351943A - Infrared analysis method - Google Patents

Abstract

PURPOSE:To determine concentration gradient of chemical species on a film surface accurately from a measured value by using infrared rays polarized perpendicularly to an incident surface so that a theoretical value and an experimental value can coincide with each other. CONSTITUTION:A total reflection infrared absorption analysis method is a method to measure concentration gradient in the depth direction from a film surface with regard to chemical species existing in close vicinity to the film surface, and infrared rays polarized perpendicularly to an incident surface is used as the infrared rays to be incident on a film. According to this method, since a parallel electric field amplitude component as a cause of discrepancy in a theoretical value and an experimental value is not contained in the incident light, the theoretical value and the experimental value coincide with each other, so that the concentration gradient of the chemical species on the film surface can be determined accurately from a measured value. In order to carry out the total reflection infrared absorption analysis method, a polarizer is placed on the incident light side of a sample, and the infrared rays to be incident on the sample are polarized perpendicularly to the incident surface. In the case when the polarizer is installed so that the infrared rays to be incident on the sample become perpendicular polarization, there is no particular limitation on its position. Generally, it is supported with a holder and so on in front of the sample.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring chemical species near the surface of a film using total reflection infrared absorption spectrometry (hereinafter abbreviated as ATR method).

0002

BACKGROUND OF THE INVENTION One of the analytical methods widely used at present is chemical analysis using infrared absorption spectroscopy. Measurement of infrared absorption spectra is usually carried out by a so-called transmission method. In the transmission method, a sample is irradiated with infrared light and the transmitted light is observed to measure the infrared light absorption of the sample. For this reason, so that the absorption is within the appropriate transmittance range,
It is necessary to adjust the thickness and concentration of the sample to an appropriate range. Because of this adjustment, the transmission method may be inconvenient in sampling. In addition, the permeation method basically uses a homogeneous sample (e.g., solution, KBr disk, neat).
It is suitable for the analysis of non-uniform samples, but is often inappropriate for investigating the properties of heterogeneous samples or parts of samples.

On the other hand, the ATR method, which utilizes the phenomenon of total internal reflection of infrared light, involves bringing the sample into contact with a crystal with a high refractive index.
The infrared light totally reflected at the interface between the crystal and the sample is measured. Therefore, the absorption intensity can be adjusted by increasing or decreasing the contact area (the number of total reflections). For this reason, it is often advantageous in terms of sampling compared to the transmission method. Furthermore, since a spectrum of the sample surface can be obtained, it can be used as a method for analyzing the sample surface.

Using the ATR method, even if the sample is thick and opaque, an infrared absorption spectrum can be obtained as long as the surface of the sample can be brought into contact with the crystal. Also,
As described later, by adjusting the measurement conditions, it is possible to measure the spectrum of the surface layer at a depth of 0.1 μm to 1 μm from the interface (sample surface), so it is possible to measure the distribution of chemical species along the depth direction from the surface. It can also be used to measure the thickness of thin films. Therefore, the ATR method is an advantageous analysis method for measuring chemical species near the film surface.

As mentioned above, the ATR method is an excellent surface analysis method, but the problem is that the loss of light is greater than that of the transmission method, and the amount of light reaching the detector is 1/5 to 1/10. be. For this reason, conventional infrared spectrometers have a high quality of A.
Measurement using the TR spectrum was not easy. but,
In recent years, the accuracy of infrared spectrometers has improved significantly, and high-quality AT
It has become possible to measure the R spectrum.

Regarding the principle of surface analysis by ATR method,
This will be explained below. The surface of the film to be measured is brought into close contact with a plate-shaped crystal that has a high refractive index and is highly transparent to infrared light, and the film is placed at a certain angle (critical When infrared light is incident on a crystal at an angle (incident angle) greater than or equal to the angle of incidence, the infrared light that enters the crystal travels through the crystal and reaches the interface between the crystal and the film, where total reflection occurs. wake up At this time, since the infrared light slightly penetrates the film surface at the interface between the crystal and the film, the reflected infrared light contains information about the vicinity of the film surface. The depth to which this infrared light penetrates can be changed by changing the refractive index of the crystal and the incident angle of the infrared light, so information (distribution) in the depth (thickness) direction from the film surface can be obtained. .

In actual measurements, in order to increase the absorption intensity, the above crystal is used as a prism, and while the prism is sandwiched between the sample, infrared light is incident from the side of the prism, and the infrared light is applied to the upper and lower interfaces. It is common to observe reflected light that has undergone multiple total reflections from the side opposite to the incident side.

The high refractive index crystal that forms this prism is called an internal reflection element (hereinafter abbreviated as IRE), and is made of KRS-5 (refractive index: 2.4) and ZnSe (refractive index: 2.4). , Si (refractive index: 3.5), Ge (refractive index: 4.0), etc. are generally used.

[0009]J. Opt. Soc. Ame. , vol.
．． 55 (1965) No. 7pp. 851 has A
It has been reported that the TR method can be used to quantify chemical species and the theoretical absorbance of its polarized light spectrum. Also, Macromolecules, vol.
．． 4 (1971) No. 2 pp. 174, the polar groups generated by irradiating the polypropylene surface with ultraviolet rays were measured using the ATR method to determine the depth (thickness) from the surface.
It is described that the direction is quantified. It is also known to measure and evaluate surface film thickness using the ATR method (PerkinElmer Japan FT-IR
Forum '89 materials). In addition, Chapter 4 of ``Material Analysis by Infrared Method'' by Nishikita and Iwamoto (published by Kodansha Scientific) provides a detailed explanation of the ATR method.

The theory of the ATR method will be explained in more detail.

The electric field amplitude E of the infrared light incident on the sample decreases exponentially with the depth of penetration. This relationship is shown in equation 1 below.

0012

[Math 1]

[0013] Here, E0 is the electric field amplitude at the interface,
Z is the depth from the interface. dp is the depth at which the electric field amplitude attenuates to 1/e, and is called the "penetration depth." This d
p is represented by the following two formulas.

[0014]

[Math 2]

Here, λ is the wavelength of the incident light, θ is the angle of incidence,
n1 is the refractive index of the crystal, and n21 is the refractive index of the film relative to the crystal.

As shown in equations 1 and 2, the penetration depth dp of the infrared light incident on the sample from the sample surface is determined by the wavelength λ of the incident light, the angle of incidence θ, the refractive index n1 of the crystal, and the film thickness. depends on the refractive index n21 for the crystal. Therefore, by adjusting these values, you can control the depth to be measured,
The depth distribution of chemical species can be quantified.

Further, the absorbance A in the ATR method is expressed by the following three formulas.

[0018]

[Math 3]

Here, N is the number of reflections, ε is the molecular extinction coefficient of the chemical species of interest, and C(z) is the depth dependence of the concentration of the chemical species.

On the other hand, when the electric field amplitude of the incident light incident on the crystal is 1, the electric field amplitude E at the interface between the sample and the crystal is
0 is expressed by the following 4 equations, assuming that the component perpendicular to the plane of incidence (the plane containing the normal to the interface and the incident ray) is Ey0, and the components parallel to it (parallel polarized light) are Ex0 and Ez0. It is expressed as equation 7.

[0021]

[Math 4]

[0022]

[Math 5]

[0023]

[Math 6]

[0024]

[Math 7]

Therefore, Equation 4 (for vertically polarized light) and Equation 5 (
(for parallel polarized light), formula 6 (for parallel polarized light), or 7
Substituting the equation (for unpolarized light or circularly polarized light) into equation 3, we get
The theoretical value of absorbance A for each polarization state can be calculated. Conversely, from the measured absorbance A, the molecular extinction coefficient ε and C(
z) can be found.

[0026]

[Problem to be solved by the invention] However, when the distribution of chemical species in the depth direction is actually measured using the ATR method as described above, the agreement between the theoretical absorbance value and the experimental value is not always good. In some cases, the experimental value was less than half of the theoretical value. As a result, the value of the molecular extinction coefficient determined from the measurement result by the ATR method is significantly different from the true value determined by the transmission method, resulting in a problem that the reliability of the measurement is low.

In the above-mentioned "Material Analysis by Infrared Method" by Nishikita and Iwamoto, it is stated that since the absorbance of the ATR spectrum with parallel polarized light is always greater than the absorbance of the vertically polarized ATR spectrum, when measuring with non-polarized light, It states that correction is required and the correction formula is shown. However, it is not clear whether the correction formula described therein is effective when the ATR method is used to investigate the distribution of chemical species in the depth direction.

[0028]

[Means for Solving the Problems] As a result of studying the above-mentioned problems, the present inventor found that if the incident infrared light has an electric field amplitude component parallel to the plane of incidence, there is a difference between the theoretical value and the experimental value. The present invention was achieved by discovering that discrepancies occur.

The present invention is a total reflection infrared absorption analysis method for measuring the concentration gradient of chemical species near the film surface in the depth direction from the film surface. Provided is a total internal reflection infrared absorption analysis method characterized by using infrared light polarized perpendicularly to the object.

According to the total internal reflection infrared absorption analysis method of the present invention, the incident light does not contain the parallel electric field amplitude component, which is the cause of the inconsistency between the theoretical value and the experimental value. There is substantial agreement, and the concentration gradient of the chemical species on the film surface can be accurately determined from the measured values.

To carry out the total internal reflection infrared absorption analysis method of the present invention, a polarizer is placed on the incident light side of the sample in the optical system of a conventional ATR measuring device, It is only necessary to polarize the infrared light incident on the IRE) perpendicularly to the plane of incidence. The position of the polarizer is not particularly limited as long as it is installed so that the infrared light incident on the sample becomes vertically polarized light. However, the sample (IRE) is usually supported with a holder or the like in front of it.

[0032] The method of the present invention will be described in detail below, step by step.

As described above, the ATR spectrum is usually obtained by spectroscopically measuring infrared light that is multiple-reflected at the contact interface between the sample and the IRE. Therefore, the IRE is usually sandwiched between samples and set in the optical system. At this time, the number of reflections is adjusted by adjusting the area of the contact interface between the sample and the IRE.

As the IRE, the above-mentioned KRS-5 (refractive index: 2.4), ZnSe (refractive index: 2.4), Si (
In addition to Ge (refractive index: 3.5) and Ge (refractive index: 4.0),
Te (refractive index: 6.3 for vertically polarized light), ZnTe (refractive index: 2.7), GaAs (refractive index: 3.3) Al2O
3 (refractive index: 1.7), MgO (refractive index: 1.7),
AgCl (refractive index: 2.0) and AgBr (refractive index:
2.2) can be mentioned.

ATR that can be used to implement the present invention
The optical system of the measuring device is Wilks type,
ck type, type using semi-cylindrical IRE, cylindrical IR
Types that use E can be mentioned.

The sample attached to the IRE is set in an optical system, a polarizer is placed using a holder just in front of the incident light side of the sample (IRE), and the spectrum is measured using an infrared spectrophotometer.

As the polarizer, one used is normally used for infrared light. That is, the normal infrared region (400~
In the far-infrared region (10 to 4000 cm-1), we use a laminated plate type with stacked silver chloride plates, or a wire grid type based on a material that does not absorb in the infrared region, such as silver bromide. 400 cm-1), it is common to use a wire grid type one based on polyethylene. However, since silver chloride polarizers have problems such as requiring a considerably large space and not being able to obtain a high degree of polarization for highly convergent light beams, it is preferable to use a wire grid type polarizer.

Although the measurement method of the present invention is possible in principle using a conventional dispersive infrared spectrometer,
It is practical to use an R infrared spectrometer.

[0039]

[Example] The present invention will be further explained in the following example.
These do not limit the invention.

[Example 1] The following gelatin dispersion was coated on a cellulose acetate film (support) in the coating amount shown in Table 1, and dried at 70°C for 3 minutes to form five types of gelatin-coated films. It was created.

──────────────────────────
──────────── Coating liquid ──────────────────────────
──────────── Gelatin

7.5g salicylic acid
2g
acetone

500ml methanol

200ml methylene chloride
300ml────
──────────────────────────
───────

Table 1 shows the film thickness of each gelatin calculated from the above coating amount.

[0043]
Table 1────────────────
────────────────────── Application amount (ml/m2)
Film thickness (μm) ────────────────
────────────────────
15

0.087 20

0.115 25

0.
144 34

0.196 50

0.289
──────────────────────────
────────────

[0044] This film is
Each piece was cut into small pieces of 1 cm x 4 cm and brought into close contact with a germanium plate (IRE) with a thickness of 2 mm, and ATR measurement was performed using infrared light that was vertically polarized using a polarizer. As a spectrometer, PerkinElmer 1720X
A Fourier transform infrared spectrophotometer was used. Polarizer is KR
An aluminum grid with S-5 as a substrate was used. The incident angle of the infrared rays was 45°, and the number of scans was 50.

[0045] The absorbance value from the baseline of the absorption at 1650 cm-1 of the obtained spectrum was defined as the absorbance. The obtained spectrum is shown in FIG. 1, and the results are shown in Table 2 below.

On the other hand, when the penetration depth dp was calculated using Equation 2, it was found to be 0.407 μm. Furthermore, since gelatin exists in a layered form on the support, C(z) is represented by the following formula 8.

[0047]

[Math. 8]

[0048] Here, t represents the thickness of the gelatin film. The above dp and C(z) were substituted into Equation 3, and since the incident light was vertically polarized, Equation 4 was used to organize the equation to obtain Equation 9 below.

[0049]

[Math. 9]

FIG. 2 shows a graph in which the measured absorbance of each gelatin-coated film is plotted according to Equation 9 above.

In FIG. 2, a straight line (passing through (1,0)) was determined by regression calculation for each measurement point, and the value of the intercept (ie, 2.26ε) was determined. As a result, a value of 0.26 was obtained as the molecular extinction coefficient ε of gelatin.

[Comparative Example 1] Absorbance was measured in the same manner as in Example 1, except that the measurement was performed using unpolarized infrared light without using a polarizer. Also, as in Example 1, dp, C(z
) was substituted into Equation 3, and since it is non-polarized light, Equation 7 was used to rearrange it to obtain Equation 10 below.

[0053]

[Math. 10]

The absorbance measurement results are shown in Table 2, and a graph plotted according to the above equation 10 is shown in FIG.

The molecular extinction coefficient ε of gelatin determined from the intercept value of the graph in the same manner as in Example 1 was 0.13.

[0056]
Table 2────────────────
──────────────────── Film thickness t (μm) e-4.92t Absorbance of vertically polarized light Absorbance of unpolarized light ────────
──────────────────────────
──── 0.086 0.65
5 0.235 0.3
46 0.116 0.
565 0.278 0
．． 417 0.145
0.490 0.321
0.462 0.196
0.381 0.363
0.534 0.289
0.241 0.412
0.611 ────────────
──────────────────────────

The molecular extinction coefficient ε of gelatin determined by the transmission method is 0.29, which agrees well with the value of 0.26 obtained by measurement using vertically polarized light (Example). On the other hand, the value of 0.13 obtained by measurement using non-polarized light (comparative example) is a value that deviates greatly. From the above results, it can be seen that quantitative performance is improved by performing measurements using vertically polarized light.

[0058]

[Effect of the invention] In the total reflection infrared absorption analysis method of the present invention,
The incident light does not contain the parallel electric field amplitude component, which is the cause of the discrepancy between the theoretical value and the experimental value, which has been a problem in measurements using the ATR method. Therefore, the theoretical value and the experimental value substantially match, and the concentration gradient of the chemical species on the film surface can be accurately determined from the measured value.

[Brief explanation of the drawing]

FIG. 1 is an infrared absorption spectrum of a sample.

FIG. 2 is a graph plotting the relationship between film thickness and absorbance according to a theoretical formula.

[Explanation of symbols]

I Absorption of amide at 1650 cm-2 II For vertically polarized light (Example 1)

Claims (1)

[Claims] 1. A total internal reflection infrared absorption spectrometry method for measuring the concentration gradient of chemical species near the film surface in the depth direction from the film surface, the method comprising: An infrared analysis method characterized by using vertically polarized infrared light.