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Journal of Materials
Volume 2013 (2013), Article ID 678361, 20 pages
http://dx.doi.org/10.1155/2013/678361
Review Article

Recent Developments in the X-Ray Reflectivity Analysis for Rough Surfaces and Interfaces of Multilayered Thin Film Materials

Center for Supports to Research and Education Activities, Kobe University, Kobe 657-8501, Japan

Received 31 December 2012; Revised 6 June 2013; Accepted 24 June 2013

Academic Editor: Te-Hua Fang

Copyright © 2013 Yoshikazu Fujii. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

X-ray reflectometry is a powerful tool for investigations on rough surface and interface structures of multilayered thin film materials. The X-ray reflectivity has been calculated based on the Parratt formalism, accounting for the effect of roughness by the theory of Nevot-Croce conventionally. However, in previous studies, the calculations of the X-ray reflectivity often show a strange effect where interference effects would increase at a rough surface. And estimated surface and interface roughnesses from the X-ray reflectivity measurements did not correspond to the TEM image observation results. The strange result had its origin in a used equation due to a serious mistake in which the Fresnel transmission coefficient in the reflectivity equation is increased at a rough interface because of a lack of consideration of diffuse scattering. In this review, a new accurate formalism that corrects this mistake is presented. The new accurate formalism derives an accurate analysis of the X-ray reflectivity from a multilayer surface of thin film materials, taking into account the effect of roughness-induced diffuse scattering. The calculated reflectivity by this accurate reflectivity equation should enable the structure of buried interfaces to be analyzed more accurately.

1. Introduction

X-rays scattered from a material surface at a glancing angle of incidence provide a wealth of information on the structure of the surface layer of materials. X-ray scattering spectroscopy is a powerful tool for investigations on rough surface and interface structures of multilayered thin film materials [123], and X-ray reflectometry is used for such investigations of various materials in many fields [14, 15, 20]. In many previous studies in X-ray reflectometry, the X-ray reflectivity was calculated based on the Parratt formalism [1], coupled with the use of the theory of Nevot and Croce to include roughness [2]. However, the calculated results of the X-ray reflectivity done in this way often showed strange results where the amplitude of the oscillation due to the interference effects would increase for a rougher surface.

Because the X-ray scattering vector in a specular reflectivity measurement is normal to the surface, it provides the density profile solely in the direction perpendicular to surface. Specular reflectivity measurements can yield the magnitude of the average roughness perpendicular to surface and interfaces but cannot give information about the lateral extent of the roughness. In previous studies, the effect of roughness on the calculation of the X-ray reflectivity only took into account the effect of the density changes of the medium in a direction normal to the surface and interface. On the other hand, diffuse scattering can provide information about the lateral extent of the roughness. In contrast to previous calculations of the X-ray reflectivity, in the present analysis, we consider the effect of a decrease in the intensity of penetrated X-rays due to diffuse scattering at a rough surface and rough interface.

In this review, we show that the strange result has its origin in a currently used equation due to a serious mistake in which the Fresnel transmission coefficient in the reflectivity equation is increased at a rough interface, and the increase in the transmission coefficient completely overpowers any decrease in the value of the reflection coefficient because of a lack of consideration of diffuse scattering. The mistake in Nevot and Croce’s treatment originates in the fact that the modified Fresnel coefficients were calculated based on the theory which contains the X-ray energy conservation rule at surface and interface. In their discussion, the transmission coefficients were replaced approximately by the reflection coefficients by the ignoring diffuse scattering term at the rough interface and according to the principle of conservation energy at the rough interface also. The errors of transmittance without the modification cannot be ignored. It is meaningless to try to precisely match the numerical result based on a wrong calculating formula even to details of the reflectivity profile of the experimental result. Thus, because Nevot and Croce’s treatment of the Parratt formalism contains a fundamental mistake regardless of the size of roughness, this approach needs to be corrected. In the present study, we present a new accurate formalism that corrects this mistake and thereby derive an accurate analysis of the X-ray reflectivity from a multilayer surface, taking into account the effect of roughness-induced diffuse scattering. The calculated reflectivity obtained by the use of this accurate reflectivity equation gives a physically reasonable result and should enable the structure of buried interfaces to be analyzed more accurately. This paper is the review article that is edited based on the two research articles of IOP Science [22, 23] and the later study.

2. TEM Observation and X-Ray Reflectivity Measurement for Surfaces and Interfaces of Multilayered Thin Film Materials

The surface and interfacial roughness of the same sample of multilayered thin film material was measured by transmission electron microscopy (TEM) and compared them with those from X-ray reflectivity measurements. The surface sample for examination was prepared as follows; a GaAs layer was grown on by molecular beam epitaxy (MBE). From TEM observations, the thickness of the GaAs layer was 48 nm, the root-mean-square (rms) roughness of the GaAs surface was about 2.8 nm, and the rms roughness of the interface between GaAs and Si was about 0.7 nm. Figure 1 shows a cross-section image of this sample observed by TEM.

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Figure 1: Cross-section image of by TEM observation.

X-ray reflectivity measurements were performed using a Cu-Kα1 X-ray beam from an 18 kW rotating-anode source. Figure 2 shows the measured reflectivity of X-rays (wave length 0.154 nm) from a GaAs layer with a thickness of 48 nm on a silicon wafer. The decrease in signal for angles larger than the total reflection critical angle shows oscillations. These oscillations are caused by interference between X-rays that reflect from the surface of GaAs layer and those that reflect from the interface of the GaAs layer and Si substrate. The characteristics of these oscillations reflect the surface roughness and the interface roughness.

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Figure 2: Measured X-ray reflectivity from a silicon wafer covered with a thin (48 nm) GaAs layer.

3. X-Ray Reflectivity Analysis

In Section 3.1, we consider the calculation of the X-ray reflectivity from a multilayer material by the Parratt formalism [1], and in Section 3.2, the calculation of the X-ray reflectivity when roughness exists in the surface and the interface is considered.

3.1. X-ray Reflectivity from a Multilayer Material with a Flat Surface and Flat Interface

The intensity of X-rays propagating in the surface layers of a material, that is, the electric and magnetic fields, can be obtained from Maxwell’s equations [24]. The effects of the material on the X-ray intensity are characterized by a complex refractive index , which varies with depth. We divide a material in which the density changes continuously with depth into layers with an index . The complex refractive index of the th layer is . The vacuum is denoted as and . The thickness of the th layer is , the thickness of the bottom layer being assumed to be infinite.

The reflectance of an -layer multilayer system can be calculated using the recursive formalism given by Parratt [1]. In the following, we show in detail the process of obtaining Parratt’s expression and, further, show that this expression requires conservation of energy at the interface. We go on to show that the dispersion of the energy by interface roughness cannot be correctly accounted for Parratt’s expression.

Following that approach, let be the refractive index of the th layer, defined as where and are the real and imaginary parts of the refractive index. These optical constants are related to the atomic scattering factor and electron density of the th layer material.

For X-rays of wavelength , the optical constants of theth layer material consisting of atoms per unit volume can be expressed as where is the classical electron radius and and are the real and imaginary parts of the atomic scattering factor of the th element atom, respectively.

We take the vertical direction to the surface as the -axis, with the positive direction pointing towards the bulk. The scattering plane is made the --plane. The wave vector of the th layer is related to the refractive index of the th layer by and, as this necessitates that the -direction components of the wave vector are constant, then the -direction component of the wave vector of the th layer is In the 0th layer, that is, in vacuum, In the th layer, the components of the wave vector are

The electric field of X-ray radiation at a glancing angle of incidence is expressed as

The incident radiation is usually decomposed into two geometries to simplify the analysis, one with the incident electric field parallel to the plane of incidence (-polarization) and one with perpendicular to that plane (-polarization). An arbitrary incident wave can be represented in terms of these two polarizations. Thus, and correspond to -polarization and to -polarization; those components of the amplitude’s electric vector are expressed as The components of the wave vector of the incident X-rays are The electric field of reflected X-ray radiation of exit angle is expressed as where

Because an X-ray is a transverse wave, the amplitude and the wave vector are orthogonal as follows:

We consider the relation of the electric field of X-rays incident at a flat surface from vacuum, the electric field of X-rays propagating in the first layer material, the electric field of X-rays reflected from the surface exit to vacuum, and the electric field of X-rays propagating toward to the surface in the first layer material, as shown in Figure 3.

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Figure 3: Reflected and transmitted X-rays.

The electric fields , in the first layer material below the surface are expressed as where The relation of the amplitudes ,  ,  , and can be found from the continuity equations of the electric fields for the interface between the 0th and 1th layers as follows: Another relation of the amplitudes ,  ,  , and can be found from the continuity equations of the magnetic fields for the interface between the 0th and 1th layers is shown as follows: From the previous equations, these amplitudes are related by the Fresnel coefficient tensor for refraction and the Fresnel coefficient tensor for reflection as follows: Here, the Fresnel coefficient tensor for refraction at the interface between the 0th and 1th layers is given by The Fresnel coefficient tensor for reflection from the interface between the 0th and 1th layers is given by Here, we consider the reflection from a flat surface of a single layer. The reflection coefficient is defined as the ratio of the reflected electric field to the incident electric field at the surface of the material. The reflection coefficient from a single-layer flat surface is equal to the Fresnel coefficient for reflection, as the following shows

In general, when X-rays that are linearly polarized at an angle impinge on the surface at an angle of incidence , the components of the amplitude’s electric vector are expressed as The amplitudes of reflected X-ray radiation are expressed as The X-ray reflectivity is Then where Then, Taking an average for , Then For the reflectivity in the case of -polarized X-rays incident,

Next, we consider the reflection from a flat surface of a multilayer with flat interfaces. We consider the electric field of X-rays propagating in the th layer material, the electric field of X-rays propagating in the th layer material, and the electric field of X-rays reflected from the th layer material at of the interface between the th layer and th layers as shown in Figure 4.

678361.fig.004
Figure 4: Reflection and transmission of X-rays in the th, th, and th layers of a multilayer material.

The electric fields , at the interface between the th layer and th layer and the electric fields , below the interface between the th layer and th layer are expressed as The electric fields of X-rays at the interface between the th layer and th layer can be formally expressed as follows: where is the Fresnel coefficient tensor for reflection from the interface between the and layers, and is the Fresnel coefficient tensor for refraction at the interface between the and layers. In addition, the electric field within the th layer varies with depth as follows: The amplitudes and at the th layer are derived from the previous equations for the interface between the and layers as follows: This relation is expressed by the following matrix: Here, the Fresnel coefficient tensor for refraction at the interface between the th and th layers is given by The Fresnel coefficient tensor for reflection from the interface between the and layers is given by

The amplitudes and of the electric fields , at the th layer and the amplitudes and of the electric fields , at the th layer are related by the following equations: For -polarization, the Fresnel coefficients are Then, the relations between the amplitudes , , , and at the interface of the th and th layers are expressed as follows:

The reflection coefficient is defined as the ratio of the reflected electric field to the incident electric field at the surface of the material and is given by The reflection coefficient of the electric field to the electric field at the interface of th layer and th layer is and the ratio is related to the ratio as follows: Here, from the relation between the Fresnel coefficient for reflection and the Fresnel coefficient for refraction, We can formulate the following relationship: It is reasonable to assume that no wave will be reflected back from the substrate, so that Then, the X-ray reflectivity is simply

3.2. Previous Calculations of X-Ray Reflectivity When Roughness Exists at the Surface and Interface

When the surface and interface have roughness, the Fresnel coefficient for reflection is reduced by the roughness [819]. The effect of the roughness was previously put into the calculation based on the theory of Nevot and Croce [2]. The effect of such roughness was taken into account only through the effect of the changes in density of the medium in a vertical direction to the surface and interface. With the use of relevant roughness parameters like the root-mean-square (rms) roughness of the th layer, the reduced Fresnel reflection coefficient for -polarization is transformed as follows: and the X-ray reflectivity is calculated using the following equation:

Figure 5 shows the result (dots) of a calculation based on these expressions of the reflectivity of X-rays from a GaAs layer with a thickness of 48 nm on Si substrate. The rms roughness of the interface of GaAs and Si was set to 0.7 nm, the value derived from the TEM observations. The rms roughness of the GaAs surface was set to 2.8 nm, the value derived from the AFM measurements. The agreement of the calculated and experimental results in Figure 2 is not good. The calculated result suggests the following: if the value of the surface roughness and the interfacial roughness in the calculation would be made larger, the calculated result will more closely approach the experimental result. In the TEM observation and AFM measurements, one half of the peak to peak value of the interface roughness equates to 1 nm, and that of the GaAs surface is 4 nm. We then recalculated the reflectivity values of this order for the surface roughness and the interface roughness in the calculation. Three calculated results for a roughness of GaAs surface of 3.5 nm, 4 nm, and 4.5 nm, with an interface roughness of 1 nm are shown in Figure 6.

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Figure 5: Calculated (dots) and measured (line) reflectivity from a GaAs layer with a thickness of 48 nm on a Si substrate. The surface roughness is 2.8 nm and the interfacial roughness is 0.7 nm.
678361.fig.006
Figure 6: Calculated (dotted, dashed, and thin lines) and measured (thick line) reflectivity from a GaAs layer with a thickness of 48 nm on a Si substrate. In the calculation, the interface roughness is 1.0 nm. Three calculated results with the roughness of GaAs surface set at 3.5 nm, 4 nm, and 4.5 nm are shown.

Although the calculated results did more closely approach those from experiment, they still showed poor agreement. The ratio of the oscillation amplitude to the value of the reflectivity near an angle of incidence of 0.36° in the calculated reflectivity for the GaAs surface of 4 nm roughness in Figure 6 is larger than that of the reflectivity for a small roughness of 2.8 nm in Figure 2, that is, near an angle of incidence of 0.36° interference effects appear to increase the reflectivity in the case of large roughness. It seems very strange that interference effects would operate in this way.

Figure 7 shows the reflectivity from a GaAs-covered silicon wafer, solid line shows the calculated result in the case of flat surface and flat interface, dashed line shows the calculated result in the case that the surface has an rms roughness of 4 nm, and dotted line shows the equivalent result when the surface and interface both have an rms roughness of 4 nm. In the latter case, the reflectivity curve (dots) decreases more quickly than that in Figure 3. However, the ratio of the oscillation amplitude to the value of the reflectivity does not decrease. It seems unnatural that the effect of interference does not also decrease at a rough surface and interface, because the amount of coherent X-rays should reduce due to diffuse scattering at a rough surface and interface.

678361.fig.007
Figure 7: Calculated reflectivity from a GaAs layer with a thickness of 48 nm on a Si substrate. The solid curve is for a flat surface and a flat interface. The dashed curve is for a surface roughness of 4 nm and a flat interface, while the dotted curve is for a surface roughness of 4 nm and interface roughness of 4 nm.

In the reflectivity curve (dashed line) for a surface roughness of 4 nm and with a flat interface, the ratio of the oscillation amplitude to the size of the reflectivity near an angle of incidence of 0.36° is much larger than the reflectivity of the flat surface in Figure 3. It seems very strange that the interference effects would increase so much at a rough surface.

To probe these effects further, we then recalculated the reflectivity for surface roughness of 3.5 nm, 4 nm, and 4.5 nm, and with a flat interface. Those calculated reflectivity results are shown in Figure 8. The ratio of the oscillation amplitude to the reflectivity near an angle of incidence of 0.36° in calculated reflectivity is larger in all cases than that of the reflectivity in the case of a flat surface in Figure 3.

678361.fig.008
Figure 8: Calculated reflectivity from a GaAs layer with a thickness of 48 nm on a Si substrate. In the calculation, the interface roughness is 0 nm. Three calculated results are shown for a GaAs surface with an rms roughness of 3.5 nm, 4 nm, and 4.5 nm.

For most angles of incidence within this range, the reflectivity of the surface with a roughness of 4 nm is near the mean value of the reflectivity of surfaces with roughnesses of 3.5 nm and 4.5 nm. However, near an angle of incidence of 0.36°, the reflectivity of the surface with a roughness of 4 nm is very much attenuated compared to that same average. It seems very strange that the reflectivity of the average roughness has a value quite different from the mean value of the reflectivity of each roughness, because the value of the roughness is not the value of the amplitude of a rough surface but the standard deviation value of various amplitudes of rough surface.

Figure 9 shows the reflectivity from a tungsten-covered silicon wafer calculated by the theory in use prior to this work. The ratio of the oscillation amplitude to the value of the reflectivity from a surface with an rms surface roughness of 0.3 nm (dashed line) does not decrease near an angle of incidence of 1.8° but increases. This result is strange and not reasonable.

678361.fig.009
Figure 9: X-ray reflectivity from a silicon wafer covered with a thin (10 nm) tungsten film calculated by the theory in use prior to this work. Solid line shows the case of a flat surface. Dashed line shows the case of a surface with an rms surface roughness of 0.3 nm.
3.3. Effect of Roughness on X-Ray Reflectivity of Multilayer Surface

We now consider the previous strange result of the X-ray reflectivity which was calculated based on the Parratt formalism [1] with the use of the Nevot and Croce approach to account for roughness [2]. In that calculation, the X-ray reflectivity is derived using the relation of the reflection coefficient and as follows: where the reduced Fresnel reflection coefficient that takes into account the effect of the roughness is as follows: However, the relationship between the reflection coefficients and was originally derived as the following equation: Here, the following conditional relations between the Fresnel coefficient for reflection and refraction are relevant to the previous equation: then, that is,

The Fresnel coefficients for refraction at the rough interface are derived using the Fresnel reflection coefficient as follows: Therefore, the Fresnel coefficients for refraction at the rough interface are necessarily larger than the Fresnel coefficient for refraction at the flat interface. The resulting increase in the transmission coefficient completely overpowers any decrease in the value of the reflection coefficient. These coefficients for refraction obviously contain a mistake because the penetration of X-rays should decrease at a rough interface because of diffuse scattering. We propose that the unnatural results in the previous calculation of the X-ray reflectivity originate from the fact that diffuse scattering was not considered. In fact (52) contains the X-ray energy conservation rule at the interface as the following identity equation for the Fresnel coefficient:

Here, we consider the energy flow of the X-ray. In electromagnetic radiation, E, H, the energy flow is equal to the Poynting vector where and and are the dielectric and magnetic permeability. The Poynting vector is therefore Then, the Poynting vector that crosses the interface is

The amplitudes and of the electric fields , at the th layer and amplitudes and of the electric fields , at the th layer are related by the following equations: When we can describe the previous equation as From the determinant of the refraction matrix, Then That is, the X-ray energy flow is conserved at the interface. When the Fresnel coefficients at the rough interface obeys the following equations, these coefficients fulfil X-ray energy flow conservation at the interface, and so diffuse scattering was not considered at the rough interface.

This conservation expression should not apply any longer when the Fresnel reflection coefficient is replaced by the reduced coefficient when there is roughening at the interface. Therefore, calculating the reflectivity using this reduced Fresnel reflection coefficient in (50) will incorrectly increase the Fresnel transmission coefficient ; that is, .

The penetration of X-rays should decrease at a rough interface because of diffuse scattering. Therefore, the identity equation for the Fresnel coefficients become where is a decrease due to diffuse scattering. Then, in the calculation of X-ray reflectivity when there is roughening at the surface or the interface, the Fresnel transmission coefficient should be used for the reduced coefficient. Several theories exist to describe the influence of roughness on X-ray scattering [819]. When the surface and interface are both rough, the Fresnel coefficient for refraction has been derived in several theories [1419].

3.4. The Refractive Fresnel Coefficient of a Rough Interface Used in Previous Reflectivity Calculations

Initially, we consider the reduced Fresnel coefficient, which is known as the Croce-Nevot factor. When the -position of the interface of 0th layer and 1th layer fluctuates vertically as a function of the lateral position because of the interface roughness, the relations between theamplitudes , , , and are derived by the use of the Fresnel coefficient tensor for refraction and the Fresnel coefficient tensor for reflection as follows: Then,where ThenWe take the average value of the matrix over the whole area coherently illuminated by the incident X-ray beam. This leads toFor Gaussian statistics of standard deviation value ,Thereforewhere . Then the Fresnel reflection coefficients are reduced as follows: However, the Fresnel refraction coefficients increase as follows:

The modified Fresnel refraction coefficients corresponds to equation (10.29) in p.200 of Holy et al. [14], equation (8.24) in p.242 of Daillant and Gibaud [15], and equation (1.117) in p.29 of Sakurai [20]. However, no one obtained the expression corresponding to . It is peculiar that and are asymmetrical. It comes to cause a different result if 1th layer and 0th layers are replaced and calculated. Therefore this derived should not be used to calculate the reflectivity from rough surfaces and interfaces.

The derived Fresnel refraction coefficients increase. This increase in the transmission coefficient completely overpowers any decrease in the value of the reflection coefficient as the following:

Moreover, if the deformation modulus of is assumed to be , the left side of (80) exceeds unity and therefore (78) is obviously wrong.

In Nevot and Croce’s treatment of the Parratt formalism for the reflectivity calculation including surface and interface roughness [2], the relations of the Fresnel coefficients between reflection and transmission as (52), (68), and (80) were not shown. Furthermore, the modification of the Fresnel coefficients according to Nevot and Croce has been used for only surface and interface reflection. However, the modification of the transmission coefficients has an important role when the roughness of the surface or interface is high, and the effect of diffuse scattering due to that roughness should not be ignored, as shown in (69). The error in Nevot and Croce’s treatment [2] originates in the fact that the modified Fresnel coefficients was calculated based on the Parratt formalism which contains the X-ray energy conservation rule at the surface and interface. In the discussion on pp.767-768 of Nevot and Croce’s [2], their Fresnel coefficients at the rough interface fulfil X-ray energy flow conservation at the interface, and so diffuse scattering was ignored at the rough interface. In their discussion, the transmission coefficients and were replaced approximately by the reflection coefficients and by the ignoring diffuse scattering term, and according to the principle of conservation energy. The reflection coefficient at the rough interface should be expressed as a function of the reflection coefficient and transmission coefficient . However, the reflection coefficient at the rough interface was expressed only by the reflection coefficient , while the transmission coefficient had already been replaced by the reflection coefficient by the ignoring diffuse scattering term in the relationship based on the principle of the conservation of energy. Thus, the reflection coefficient at the rough interface as equation of p.771 in Nevot and Croce [2] had been expressed with the reflection coefficient only, and this results in the equation was also sure to include the conservation of energy.

The resulting increase in the transmission coefficient completely overpowers any decrease in the value of the reflection coefficient at the rough interface. Thus, because Nevot and Croce’s treatment of the Parratt formalism contains a fundamental mistake regardless of the size of the roughness, results using this approach cannot be correct. The size of the modification of the transmission coefficient is one-order smaller than that of reflection coefficient, but the size of transmission coefficient is one-order larger than the reflection coefficient at angles larger than critical angle. Thus, the errors of transmittance without the modification cannot be ignored.

Of course, there are cases where that Nevot and Croce’s treatment can be applied. However, their method can be applied only to the case where there is no density distribution change at all in the direction parallel to the surface on the surface field side, and only when the scattering vector is normal to the surface. A typical example of surface medium to which this model can be applied is one where only the density distribution change in the vertical direction to the surface exists, as caused by diffusion, and so forth. In such a special case, Nevot and Croce’s treatment can be applied without any problem. However, because a general multilayer film always has structure in a direction parallel to the surface field side, Nevot and Croce’s expression fails even when the roughness is extremely small. The use of only Fresnel reflection coefficients by Nevot and Croce is a fundamental mistake that does not depend on the size of the roughness.

3.5. The Refractive Fresnel Coefficient of a Rough Interface Used in New Reflectivity Calculations

To proceed, we therefore reconsider the derivation of the average value of the matrix as the same derivation of (70) when we consider the reduced Fresnel coefficient, which is known as the Croce-Nevot factor.

When the -position of the interface of the 0th layer and 1th layer fluctuates vertically as a function of the lateral position because of the interface roughness, the relations between the electric fields are derived by the use of the Fresnel coefficient tensor for refraction and the Fresnel coefficient tensor for reflection as follows: where Then

Then the amplitudes , , , and are derived as follows: Matrix description of the relations is as follows:ThenWe take the average value of this matrix:For Gaussian statistics of standard deviation value ,Then the Fresnel reflection coefficients are found as follows