ES2929230A1 - PROCEDURE FOR DIRECT OBTAINING THE EXTINCTION COEFFICIENT OF LOW-ABSORPTION MATERIALS (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Procedure for directly obtaining the extinction coefficient of low absorption materials, comprising the steps of providing a sample by depositing two sheets (3, 4) on a substrate (1), one thicker (X1) and one thicker (X1). X2) smaller than a material (2) with low absorption, provide a beam of light (5) with a polarization p at a working wavelength, which hits the sheets (3, 4) following the Brewster angle θB , create a relative movement with a certain frequency between the light beam (5) and the sample in such a way that the light beam (5) moves between the two sheets (3, 4), obtain a transmittance difference (ΔT) between the two sheets (3, 4), and derive from this the extinction coefficient k. (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

PROCEDURE FOR DIRECT OBTAINING THE EXTINCTION COEFFICIENT

OF LOW ABSORPTION MATERIALS

OBJECT OF THE INVENTION

The object of the present invention is a procedure to obtain the extinction coefficient of materials with low absorption, deposited in the form of a sheet on two areas in a sample, so that the sheet thickness differs between the two areas. The difference in transmittance is an additional optical function that can be measured directly with the disclosed procedure, which can be useful in calculating both the refractive index and the extinction coefficient of a studied material. The disclosed procedure is particularly suitable for measuring an extinction coefficient that is low and therefore cannot be accurately measured with standard spectrophotometers and ellipsometers.

BACKGROUND OF THE INVENTION

The extinction coefficient k of a sheet material, together with its refractive index n, is an indispensable parameter for designing optical coatings. For this reason, techniques are required to measure it with low uncertainty.

The extinction coefficient of a sheet material can be derived from photometric as well as ellipsometric measurements. A typical sample consists of a substrate coated with a sheet of the material whose extinction coefficient needs to be measured.

In principle, both reflectance and transmittance depend on the extinction coefficient of the thin film material. However, when the extinction coefficient of the sheet material is low, the dependence of k on reflectance and/or transmittance becomes less than the uncertainty of the reflectance and/or transmittance measurements and the refractive index of the sheet. sheet, such that reflectance and transmittance depend mainly only on the refractive index of the material in foil and the refractive index and extinction coefficient of the substrate, and is little dependent on the k value of the foil material.

The same problem is observed when trying to obtain/measure a small extinction coefficient from ellipsometric measurements; for the same type of sample, the ellipsometry parameters have an almost undetectable dependence on the low extinction coefficient of a fairly transparent sheet material. The coefficient k is connected to the absorption coefficient a through:

4nk

a = — <1> where X represents the vacuum wavelength of the radiation. The existing small dependence, to which reference has been made, that ellipsometric measurements have on a low extinction coefficient of the sheet material, results in a high error when this parameter is determined from such measurements.

Calorimetry is a non-optical technique used to measure small extinction coefficients, mainly of massive materials, rather than sheets. Calorimetry measures the heating of the sample due to the absorption of light of the specific wavelength incident on the sample.

But again, a low extinction coefficient of the sheet material results in heating of the sample that is difficult to detect. Laser calorimetry has been used to detect the absorption of crystals with low extinction coefficients; however, lasers are often available at particular wavelengths, which is not suitable for covering the extended spectrum.

For a relatively thin sheet with a low absorption or extinction coefficient, measuring its extinction coefficient requires distinguishing between heating generated on the sheet and on the substrate. The dynamics of the heating of the slide and the sample must be measured and interpreted, and extra parameters such as the specific heat of the material must be known.

A third group of techniques for measuring low extinction coefficients involves optical procedures called modulation spectroscopy. These involve the derivative of an optical function, such that the resulting function increases the dependence on a low extinction coefficient. For this reason, the derivative of the spectrum with respect to wavelength has been inserted into some spectrophotometers to measure the derivative of reflectance and transmittance.

A difficulty for the interpretation of these results arises from the fact that, superimposed with the derivative of the optical function, the derivative of the spectral dependence of the source, the optics and the detector also contribute to the observed signal. Removing the last part from the first may not be easy in general.

Other modulation spectroscopy techniques that provide the derivatives of some optical response function with respect to an external modulation, such as thermoreflectance, piezorreflectance, electroreflectance, photoreflectance or rotoreflectance, have been shown to be successful. Interpretation of measurements with these techniques may require a deeper understanding of the effect of the stimulus on the electronic bands of the material, which may not be available with most materials.

Among the modulation spectroscopy techniques, double beam spectroscopy uses two samples of the same material, but of different lengths, which are successively introduced between a source and a detector, and the difference in detector readings in the two cases is a measure of the difference in transmission loss of the two samples. This procedure is intrinsic to massive samples, since only absorption differences and not interferences are taken into account in this technique, which is necessary for the case of sheets.

DESCRIPTION OF THE INVENTION

The present invention discloses a procedure to directly obtain the extinction coefficient (and therefore the absorption coefficient) of a low absorption material (a low absorption material refers to a material whose internal transmittance is close to 100%), where the material is in the form of a sheet deposited on a substrate.

For low absorption materials, extracting the extinction coefficient from photometric (reflectance and transmittance) and/or standard ellipsometric measurements is difficult.

The present procedure is based on the measurement of the transmittance difference over two areas in a sample, both covered with sheets with the studied material whose extinction coefficient k is the parameter to be calculated.

The two areas have different sheet thicknesses, one of which may be zero, ie said area may remain uncoated. The transmittance difference is measured by relative movement of the light beam and sample between its two areas.

In a first embodiment of the method, for any angle of incidence and polarization, it provides an estimate of the derivative of the transmittance with respect to the thickness of the sheet. In a second embodiment of the method, for a given angle of incidence and polarization state of the incident light beam, the measured transmittance difference is proportional to the extinction coefficient k of the material when the material has relatively low light absorption, and the proportionality factor is known a priori from geometric parameters, the wavelength of the incident radiation, and the refractive index of the material.

In greater detail about the procedure, it comprises a first step of measuring the transmittance differences. Two alternative first steps are proposed to measure the transmittance difference in a sample that has two different areas with different thicknesses of the studied material.

A first alternative involves oscillation of the light beam or the sample: a light beam of a given wavelength successively traverses the gap between the two areas. The same first stage applies when one area is foiled with a given thickness and the other area is uncoated.

The oscillation of the light beam between the two areas can be obtained either by oscillating the sample with respect to a fixed light beam, or by causing the light beam to oscillate with respect to a fixed sample. This sample is called the main sample.

The oscillation amplitude must be large enough so that the entire light beam is successively contained in each area. The direction of the oscillation is preferably close to perpendicular to the plane defined by the direction of light propagation and the boundary line between the two areas. The swing center is preferably close to the line separating the two areas.

A second alternative to the first stage for the light beam to pass through the two areas of the sample is by rotating the sample about a perpendicular axis while holding the incident light beam stationary, so that the axis of rotation is located approximately cutting the line that separates the two areas.

In any case, for the first embodiment of the method, which provides an estimate of the derivative of the transmittance with respect to the thickness of the sheet, any angle of incidence and polarization of the light beam can be used. For the second embodiment of the method, which provides an exact transmittance difference, the light beam follows a certain angle of incidence (Brewster angle) and polarization state p.

A second stage of the method consists in measuring the transmitted intensity modulated signal obtained by oscillation or rotation. Regardless of the embodiment, two techniques can be used.

A first technique uses a lock-in amplifier that is locked to the oscillation or rotation frequency of the sample, such that the oscillation or rotation frequency is the lock-in reference frequency and therefore background noise is removed. with frequencies different from the reference signal and its harmonics.

The lock-in amplifier provides a digital or analog output with a rectified value that is proportional to the difference in intensity between the two areas of the show. Therefore, the signal provided by the lock-in amplifier is a direct detection of the difference in intensity of the beam of light transmitted over the two areas.

A second technique, applicable only for the oscillating sample, consists of measuring the transmitted signal with a detector at a single wavelength or with a spectrometer over a spectral range. Two measurements are made, one at each edge of the linear oscillation, and the two measurements are subtracted. To reduce the uncertainty of this difference, particularly when the difference is small, the step is repeated as many times as necessary and the mean is calculated.

Consider the case where the difference in shell thickness in the two areas, or the shell thickness in the covered area only, is small enough to omit second-order and second-order Taylor series expansion terms higher in the transmittance-versus-thickness function. In this assumption, the measured differences of the transmitted intensity become proportional to the derivative of the transmittance with respect to the thickness of the sheet:

dr_ [J(x1) - J(x2)]/J0

dx x 1- x 2 ''

where (X1) and (X2) are the two thicknesses of the sheet, where X1>X2, l(X1) and l(X2) are the intensities transmitted in the sheets, and where is the incident intensity. The derivative of the reflectance is obtained for the desired angle of incidence and polarization of the incident beam.

The direct measurement of said derivative through the direct measurement of [I(xi)-I(x ² )] provides, in the first embodiment of the invention, a new function that can be used to determine the refractive index and the coefficient extinction of the sheet material, which are parameters that define the interaction between light and a sheet of material studied.

The calculation of the refractive index and/or the extinction coefficient uses said derivative either alone or in combination with the usual reflectance and transmittance functions. The calculation procedure of the first embodiment of the procedure it is compatible with standard procedures, for example, modifying an initial refractive index and/or extinction coefficient by trial and error until the best fit to the measured reflectance and/or transmittance is obtained. The first embodiment of the method adds the transmittance derivative to the standard reflectance and/or transmittance functions as supplementary functions to be fitted, either in combination or not in combination with the usual reflectance and transmittance functions.

In particular, in the first embodiment, the derivative of transmittance with respect to the thickness of the sheet material xm must be calculated. The transmittance is given by:

with:

where Ip and Is are the radiation intensities of the beam, with the electric vector parallel and perpendicular, respectively, to the plane of incidence. P and s then refer to functions for these same components of the electric vector. Ts and Tp are given by:

s,ps,p tOm\ 2 \t™\ 2 \tso\ 2 \exp ( .Pm )\2

O^ms^sO

I1 + r0mrmsexP ( -2Pm)\2

where the indices 0, m, and s refer to the air surrounding the sample, the sheet material, and the substrate, respectively, and |z|2 represents the modulus of the complex number z.

^s,p s,p 2

^R 50 lrso

s,V _ _ r ^ s r ^ e x y j l p m )

smO l r ^ r ^ e x p i i P m )

^{q S , p} _ ^srp

ri t P = ^{]s,p ,} ks,P'í k = 0m,ms,s0

J V + «k

s,p

t ljjk = 1 rjk > jk = 0m,ms,s0

qsj = ( rij ikj^cosdj , j = 0 ,m,s

p_( nj ikj) ;

-, j = 0 ,m,s

eos di

2nixjqj

Pl = ,J = m,s

xm is the thickness of the sheet and xs is the thickness of the substrate.

The angles of radiation propagation in air, d0, in the sheet material, d m, and in the substrate material, d s, satisfy:

n0sind0 = ( nm ikm)sindm = ( ns iks)sinds

where 0m and ds are in general complex numbers, while 00 is a real number that is equal to the angle of incidence of the light beam incident on the sample. Both km and even more ks must be small numbers for the transmittance to be measurable, so both 0m and 0S must have a small imaginary part.

In the above example, an incoherent superposition of successively reflected beams on the substrate surfaces has been assumed. In the above example, a coherent superposition of successively reflected beams on the surfaces of the sheet has been assumed.

The second embodiment of the method comprises a third stage, which consists in the direct calculation of the extinction coefficient from the transmittance differences.

The difference in transmittance over the two areas with different sheet thicknesses can be used to calculate the extinction coefficient of the sheet material at the following conditions. The following measurement conditions allow the direct measurement of k of the sheet material: when the measurement angle is the Brewster angle of the sheet material and the light beam is incident on the sample with polarization p, i.e. with the electric field vibrating in the plane of incidence. As a reminder, the Brewster angle applies only to materials with low absorption and therefore a low extinction coefficient.

At this angle of incidence and polarization, the signal provided by the lock-in amplifier, which is equal to the difference of the intensity transmitted in the two areas, or the subtracted signal, when the signal is measured by a detector in the two areas of the sample, is directly proportional to the following term:

exp ^{4 nkx 2}

Xsindg — exp ^{4 nkx 1}

_Xsindg (3)

where 0 ^b represents the Brewster angle of the sheet material at the working wavelength. The particular case of X2=0 also satisfies the previous formulas; this case simplifies the requirements for the sample used with this technique, since the sheet material needs to be deposited only on one of the two areas of the substrate, without the need for a sheet on the other area.

Measuring the incoming beam at the Brewster angle and also with polarization p simplifies the problem, since the effect of optical interferences, which are present at any other angle and polarization, is avoided.

Measurements at the Brewster angle ds of the sheet material require prior knowledge of the refractive index n of the sheet material at the desired wavelength, which are related as 6S(A,)=arctan[n(A,)] . For materials with low absorption, the refractive index can easily be obtained by ellipsometry or by conventional photometry.

The measured transmittance difference can be normalized to the single transmittance measured in one of two areas of the sample, such as the area with the thinnest sheet X ² . When measuring with a lock-in amplifier, you must also measure the absolute transmittance of the sample area with a sheet thickness of x2 and will be called T2. When measured with a standard detector or spectrometer on the two sample areas, T2 is equal to the intensity measured on the thinnest (or uncoated) foil area of the sample.

The normalized transmittance difference is equal to:

<4>

<4>

where ^{A x = xi - x 2} represents the difference in thickness of the sheet between the two sample areas, which also includes the case x2=0. To obtain a positive sign, the transmittance in the thicker sheet is subtracted from the transmittance in the thinner sheet, k can be obtained from this expression since all other parameters are known and AT/T2 is obtained from the measurement of AT and T2. The Eq. (4) You need not assume that the difference Ax in the thickness of the sheet is small. When k is low (which is a requirement for a material to have a Brewster angle) such that it holds:

k « ^XsyndB

_2nAx' (5)

the exponential function in Eq. (4) can be expanded to first order, and simplifies to:

AT4nkAx

T2 XsíndB

where k are obtained directly:

For example, for a visible wavelength of 550 nm, a material with a refractive index of 1.6 and a thickness difference of 50 nm, to satisfy inequality (5), k must be much less than 1.48. , which is approximately three or more orders of magnitude greater than the usual values of the extinction coefficient of transparent materials. Thus, under these conditions of Brewster angle measurement and polarization p, and for virtually any value of k (consistent with the material having a Brewster angle), k is proportional to a measured transmitted intensity difference of according to Eq. (6). But if k does not satisfy inequality (5), Eq. (4) also allows us to calculate k, since all the other parameters are known.

To correct for deviations in the signal over time, which can complicate the measurement of small differences in intensity, light intensity can be monitored. This can be done by normalizing the intensity difference with a light intensity measurement, which is measured independently of the intensity difference. This normalization must also be done when measuring intensity in an area of the sample, such as T ² .

Additionally, the second embodiment of the method may further comprise a step for removing the oscillation trace in the transmittance difference measurement with the lock-in amplifier.

When measured with a lock-in amplifier, the specific oscillation of the light sample system can result in a signature in the measured signal. This includes the fact that it takes a finite fraction of the oscillation period for the light beam to traverse the boundary between the two sample areas; when the light beam is crossing said boundary, the measured signal evolves progressively (as opposed to instantaneously) from the signal in one area of the sample to the signal in the other area.

Other difficulties in interpreting the measurements can arise from the asymmetry or anharmonicity of the oscillation. To avoid calculating the effect of this footprint to interpret the results, the footprint can be removed by normalization if T ² is measured with an oscillatory motion identical to that used to measure AT.

Let's see how to measure T2 with an oscillatory or rotational movement identical to that used to measure AT. In the oscillatory movement, the incident light beam is made to cross, successively, the separation between the area corresponding to the thickness of the sheet x2 with transmittance T2 and an opaque mask with transmittance T=0 (P3). In the rotary movement, a mask is placed in front of or behind the sample to cover one of the areas but not the other.

The same frequency (both oscillatory and rotary motion) and amplitude (oscillatory motion) are used as when measuring the difference in transmittance between the areas of thicknesses xi and x2. The signal provided by the lock-in amplifier is now proportional to the difference T2-0=T2 and this measurement implies the same footprint as when measuring the difference in intensity transmitted between the two areas with two different thicknesses of the material, so that the ratio of the two measurements cancels out the footprint effect. This stage will be referred to as the sheet masking stage. The sample used in the sheet mask step may be the main sample or otherwise a second sample prepared for this purpose.

Therefore, when measuring T2 with the sheet mask stage, the normalized function AT/T2 is obtained by dividing the two transmitted intensity difference measurements. With this stage the trace of the oscillation is cancelled.

DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and to help a better understanding of the characteristics of the invention, according to a preferred example of its practical embodiment, a set of drawings is attached as an integral part of said description where, For illustrative and non-limiting purposes, the following has been represented:

Figure 1.- Shows a schematic view of a sample with two areas with different sheet thickness, where an incident light beam crosses periodically between both areas.

Figure 2.- Shows a schematic view of a sample with two areas with different sheet thickness, where the sample rotates with respect to an incident light beam.

Figure 3.- Shows a schematic view of a sample with two areas with different sheet thicknesses and an opaque mask added next to the area with the thinnest sheet.

Figure 4.- Shows the normalized transmittance difference (left axis, line with triangles) and the extinction coefficient k (right axis, line with circles) of a fluoride sheet as a function of wavelength.

PREFERRED EMBODIMENT OF THE INVENTION

Described below, with the help of figures 1 to 4, a preferred embodiment of the procedure to directly obtain the extinction coefficient of low absorption materials by means of a beam of light (5) directed at a sample (1), in where a material (2) has been deposited, forming two areas (3, 4) of different thickness (X1,X2).

Particularly, a first embodiment of the method provides an estimate of the derivative of the transmittance with respect to the thickness of the sheet. In the first embodiment, any angle of incidence and polarization of the light beam (5) can be used. A second embodiment of the procedure is also described, which provides an exact difference in transmittance, and in which the light beam (5) has to follow an angle of incidence (Brewster angle) and a determined p -state of polarization.

Both embodiments of the method comprise a first step of measuring the transmittance differences. Two alternatives are proposed to measure the difference in transmittance or reflectance in the sample (1).

The stage involves the oscillation of the beam of light (5) or of the sample (1): the beam of light (5) of a given wavelength is successively made to cross the border between the two areas (3, 4) of the sample.

The oscillation of the light beam (5) between the two areas (3, 4) can be obtained either by oscillating the sample (1) with respect to a fixed light beam (5), or by making the light beam (5 ) oscillates with respect to a fixed sample (1).

The amplitude of the oscillation must be large enough so that the entire area of the light beam (5) is successively contained in each area (3, 4) of the sample. The direction of oscillation lies close to the perpendicular to the plane defined by the direction of light propagation and the boundary line between the two areas (3, 4) of the sample. The center of the oscillation is preferably close to the line separating the two areas (3, 4) of the sample.

As can be seen in figure 1, the sample (1) has two areas (3, 4) with different sheet thicknesses X1 and X2, which crosses the light beam (5) periodically. In Figure 1, the sample (1) or the light beam (5) is oscillated so that the incident light beam (5) periodically crosses the boundary line between the two areas (3, 4) of the sample between P1 and P2.

The X axis is perpendicular to the sample surface (1). Y and Z are contained in the surface of the sample (1). XZ is the plane of incidence. The electric vector is contained in XZ. The P1-P2 oscillation is preferably close to being parallel to the Y axis.

A second alternative for the light beam (5) to cross the two areas (3, 4) of the sample is presented in Figure 2, and is achieved by rotating the sample (1) with respect to a perpendicular axis while moving. it keeps the incident light beam (5) static, so that the axis of rotation is located approximately cutting the line that separates the two areas (3, 4) of the sample.

In this case, the sample (1) also presents two areas (3, 4) with different sheet thicknesses X1 and X2, which the light beam (5) periodically crosses. The sample (1) is rotated with respect to the light beam (5). The axis of rotation is perpendicular to the surface of the sample (1) and crosses the sample (1) near the boundary line between the two areas (3, 4).

In the second embodiment of the procedure, to directly measure the extinction coefficient from the transmittance difference measurements, the light beam (5) incidents the sheet (2) at the Brewster angle 0B with polarization p, according to it is represented both in figure 1 and in figure 2. In the first embodiment, the light beam (5) hits the sheet (2) at any angle and with any polarization.

A second stage of the method consists in measuring the modulated signal of the transmitted intensity obtained by oscillation or rotation. Regardless of the performance of the procedure, two techniques can be used to perform the second stage.

In a first technique, a lock-in amplifier is used that is synchronized with the oscillation or rotation frequency of the sample (1), such that the oscillation or rotation frequency is the lock-in reference frequency, and thus this mode removes background noise with frequencies different from the reference signal and its harmonics.

The lock-in amplifier provides a digital or analog output with a rectified value that is proportional to the difference in intensity in the two areas (3, 4) of the sample. Therefore, the signal provided by the lock-in amplifier is a direct detection of the difference in intensity of the transmitted light beam (6) in the two areas (3, 4) of the sample.

A second technique, applicable only for the oscillating sample, consists in measuring the transmitted signal (6) with a detector in a single wavelength or with a spectrometer in a spectral range. Two measurements are made, one at each edge of the linear oscillation, and the two measurements are subtracted. To reduce the uncertainty of this difference, particularly when the difference is small, the step is repeated as many times as necessary and the mean is calculated.

The procedure comprises a third stage, which consists in the direct calculation of the extinction coefficient from the difference in transmittance, obtained in previous stages.

Additionally, the second embodiment of the method can further comprise a step for removing the oscillation trace in the transmittance difference measurement with the lock-in amplifier, presented in Figure 3.

An opaque mask (9) is added to the sample (1) side with the thinner sheet (X2). By moving the sample (1) or the light beam (5), the light beam (5) is oscillated between the sample (1) and the mask (9) such that the light beam (5) periodically crosses the limit line between the material (2) and the mask (9), marked as P2 and P3 in figure 3.

An example of the determination of the extinction coefficient of a fluoride sheet is presented, by applying the second embodiment of the procedure. Measuring the extinction coefficient of a material in the far ultraviolet (UVL, wavelength in the range 100-200 nm) involves inherent difficulties compared to the visible. Even though for most materials the extinction coefficient in the UVL is higher than in the visible, there are extra complications to measure in the UVL, such as the need to operate in a vacuum and the limited availability and moderate output of lamps. , monochromators, detectors, polarizers, etc.

A fluoride sheet (2) was deposited on a transparent substrate (1) so that half of the substrate (1) was coated and the other half remained uncoated. The sample was introduced into a UVL reflectometer from the GOLD group (Thin Film Optics Group) and its transmittance was measured at various wavelengths corresponding to spectral lines of a deuterium lamp.

At each wavelength, the AT transmittance difference between the two areas (3, 4) was measured using a lock-in amplifier. A Rochon polarizer was used to polarize the light linearly in the plane of incidence. Sample (1) was placed in the Brewster angle of the fluoride sheet. The Brewster angle was obtained from a previous determination of the refractive index of fluoride, which had been obtained from reflectance measurements.

Figure 4 shows the measured AT/T ratio (normalized transmittance difference) (left axis, dashed line with triangles) for a fluoride foil as a function of wavelength, and the extinction coefficient k (right axis, solid line with circles) obtained from said measurement using Eq. (7):

^{k = X s índ B Í} AA (7) 4rcAx T2

Tu and Te represent the transmittance measured in the uncoated and coated area, respectively. AT represents Tu-Tc and AT/T represents (Tu-Tc)/Tu. The thickness of the fluoride sheet was measured to be 40 nm, so the inequality in Eq. (5) was widely satisfied:

The smooth wavelength dependence of k suggests that AT/T is a suitable function for the determination of k.

Claims (11)

1.- Procedure for directly obtaining the extinction coefficient of materials (2) with low absorption, where this comprises the stages of: - providing a sample by depositing on a substrate (1) a sheet (3) of a thickness (X1), where the sheet (3) is made of a low absorption material (2), and leaving a region of the substrate (1) not covered by the foil (3), - provide a beam of light (5) with a polarization p at a working wavelength, which hits the sheet (3) following the Brewster angle dB of the sheet (3) in relation to an X axis perpendicular to the sheet (3), - creating a relative movement with a determined frequency between the light beam (5) and the sample in such a way that the light beam (5) moves between the sheet (3) and the uncoated region of the substrate (1), - obtaining a transmittance difference ( AT) between the sheet (3) and the uncoated region of the substrate (1) by measuring a modulated signal of transmitted intensity (6) of the light beam (5), the signal being proportional to:

where k is the extinction coefficient, A the working wavelength, and (X1) the thickness of the sheet (3), - normalizing the difference in transmittance ( AT) to a single transmittance ( T2) measured in the region of the substrate (1) not covered by the sheet (3), equating the normalized transmittance to:

obtain the extinction coefficient k as:

2. The method according to claim 1, wherein the sample is provided by depositing on the substrate (1) two sheets (3, 4) in two adjacent areas, one of a greater thickness (X1) and one of a thickness (X2 ) minor, with the two sheets (3, 4) made of a low absorption material (2), and where the procedure comprises the steps of: - provide a beam of light (5) with a polarization p at a working wavelength, which falls on the sheets (3, 4) following the Brewster angle dB of the sheet (3, 4) in relation to an axis X perpendicular to the sheets (3, 4), - create a relative movement with a determined frequency between the beam of light (5) and the sample in such a way that the beam of light (5) moves between the two sheets (3, 4), - Obtain a transmittance difference ( AT) between the two sheets (3, 4) by measuring a modulated signal of transmitted intensity (6) of the light beam (5), the signal being proportional to:

where k is the extinction coefficient, A the working wavelength, and (X1, X2) the thickness of the sheets (3, 4), - normalize the transmittance difference ( AT) to a single transmittance ( T2) measured in the thinnest sheet (3, 4), equating the normalized transmittance to:

being Ax=X1-X2, - obtain the extinction coefficient k as:

3. - The method according to claim 1, wherein the relative movement between the light beam (5) and the sample is achieved by oscillating the sample with respect to a fixed light beam (5). 4. - The method according to claim 1, wherein the relative movement between the light beam (5) and the sample is achieved by oscillating the light beam (5) with respect to a fixed sample. 5. - The method according to claim 1, wherein the relative movement between the light beam (5) and the sample is achieved by rotating the sample with respect to an axis perpendicular to the sheets (3,4) at the same time that the beam of light (5) remains static. 6. - The method according to claim 1, wherein the measurement of the transmitted intensity modulated signal (6) is performed using a lock-in amplifier synchronized with the frequency of the relative movement of the sample. 7. - The method according to claim 3 or 4, wherein the measurement of the transmitted intensity modulated signal (6) is achieved using a detector at a single wavelength. 8. - The method according to claim 3 or 4, wherein the measurement of the transmitted intensity modulated signal (6) is achieved using a spectrometer in a spectral range. 9. - The method according to claim 6, wherein it additionally comprises a step to eliminate a trace of the oscillation in the measurement of the transmittance difference with the lock-in amplifier, which in turn comprises the steps of: - providing an opaque mask (9) with zero transmittance located on one side of the sheets (3, 4), - creating a relative movement between the beam of light (5) and the sample, making the beam of light (5) cross between the sheet with a thickness (X2) smaller with a transmittance T ² and the opaque mask (9), - measure a difference in transmittance (AT) between both sheets (3, 4), - obtain a normalized transmittance as AT/T ² . 10. - Procedure to obtain information on the optical properties of the sheets (3, 4), where it comprises the stages of: - providing a sample by depositing on a substrate (1) two sheets (3, 4) in two adjacent areas, one of a greater thickness (X1) and one of a thickness (X2) smaller, where at least one of the sheets (3, 4) is made of a low absorption material (2), - provide a beam of light (5) with an incident intensity, which falls on the sheets (3, 4), - create a relative movement between the light beam (5) and the sample in such a way that the light beam (5) moves between the two blades (3, 4), - measure a transmitted intensity (6) on each blade (3. 4), - Obtain a derivative of the transmittance with respect to the thickness (X1, X2) of the sheet (3, 4), following the equation:

where (X1) and (X2) are the two thicknesses of the sheet (3, 4), where X1>X2, l(X1) and l(X2) are the intensities transmitted (6) in the sheets (3, 4), e lo is the incident intensity. 11. The process according to claim 10, wherein it additionally comprises the steps of: - calculate a theoretical derivative of the transmittance with respect to the thickness of the sheet (3, 4), - obtain a difference between the theoretical derivative of transmittance and the measured derivative of transmittance — dx , and - find a value of the refractive index and/or the extinction coefficient that minimizes the difference between the theoretical derivative of the transmittance and the measured derivative of the transmittance — dx .