X-ray thin-film measurement techniques

1 1. IntroductionThis is the fifth article in the series of X-ray thin-filmmeasurement techniques . The second, third and fortharticles of this series, previously published in the RigakuJournal, describe out-of-plane, high-resolution and in-plane XRD measurements to obtain crystallographicinformation on crystal size, lattice strain and orientationrelationship of a thin-film material. These measurementshave been based on the premise of a crystalline thin the other hand, the X-ray reflectivity (XRR) measurement is not a technique to evaluate diffractionphenomenon. The XRR measurement techniquedescribed in this article is used to analyze X-rayreflection intensity curves from grazing incident X-raybeam to determine thin-film parameters includingthickness, density, and surface or interface article will provide an overview of the principles ofX-ray reflectivity, measurement procedures, and analysismethods.

2 It also discusses the procedural flow frommeasurement to analysis, as well as X-ray reflectivity measurementThe X-ray reflectivity method has the followingcharacteristics:A) It can be used to study a single-crystalline,polycrystalline or amorphous )It can be used to evaluate surface roughness andinterface width (arising from roughness andinterdiffusion) )It can be used to study an opaque film under ) It can be used to determine the layer structure of amultilayer or single-layer )It can be used to measure film thickness fromseveral to 1000 The phenomenon of sample surfaces atgrazing-angle incidenceWhen electromagnetic waves including visiblewavelength are incident onto a sample surface, theseelectromagnetic waves are reflected off it. The incidentelectromagnetic waves generate a specularly reflectedwave, a refracted wave a diffused reflections, as shownin Fig.

3 The case of X-rays, which are incidentelectromagnetic waves, the refractive index of a materialis slightly less than 1. Therefore, the X-rays undergototal reflection when incident on a flat surface of amaterial at a grazing angle is smaller than the criticalangle for total reflection (qc). Thus X-ray reflectivity isrelated to the values of refractive index and X-raywavelength. It should be noted that Cu-KaX-rays isused throughout this 2 shows a calculated X-ray reflectivity curvefor bulk Si and Figure 3 shows the X-ray optics for thecases of the incident angles smaller, equal to, and greaterthan the critical angle for total reflection, qc. As shownin Fig. 2, when an X-ray beam is impinged at a grazingangle on to an ideal flat surface of a material, totalreflection occurs at below the incident angle qc, and theincident X-rays do not penetrate into the material.

4 X-rayreflectivity decreases rapidly with increasing incidentangle, qabove qc. The ratio of specularly reflected X-rays decreases proportionally to Rigaku Journal, 26(2), 20101X-ray thin-film measurement techniquesV. X-ray reflectivity measurementMiho Yasaka*Technical articles* Application Laboratory, Rigaku and refraction of X-rays on curve of Information provided by an X-ray reflectivitymeasurementThe X-ray optics for an X-ray beam onto a flat surfaceof material at grazing angles has been described in theprevious section. In this section, the analysis of theresulting X-ray reflectivity curve to obtain informationon the structural parameters of a thin film is Film thicknessChanges in reflected intensity when a substrate madeof an ideal material is laminated uniformly with asubstance having a different electron density aredescribed below.

5 The observed scattering X-rays are thesum of individual electron scatterings. The intensity ofX-ray reflectivity is calculated from each layer which isconstructed from elemental species and filling rate ofspace. Section 5 will describe total reflection andrefraction in more detail. Figure 4 shows the X-rayreflectivity curve of an Au film deposited on a Sisubstrate. Generally, the Y-axis of X-raw reflectivitycurve is shown in a logarithmic scale of the normalizedintensity of I/I0. A logarithmic scale is used because ofthe wide dynamic range of X-ray reflectivity occurs between the X-rays reflected fromthe surface of the Au film and the interface between theAu film and the Si substrate. As shown in Fig. 4, thereflectivity profile shows oscillations caused by this X-ray interference. These oscillations were first observedin 1931 by Kiessing and is called Kiessing fringes(1).

6 The oscillation depends on the film thickness, and thethicker film, the shorter period of the DensityThe effects of thin-film density on an X-rayreflectivity curve are described below. Figure 5 showsthe reflectivity curves of three 20 nm thick films withdifferent densities deposited on Si substrates. The Au,Cu and SiO2films in Fig. 5 are used as examples for thecases of heavy, medium and light density materials, results indicated that the amplitude of theoscillation and the critical angle for total reflectionprovide information on the density of films. Theamplitude of the oscillation depends on the differencebetween the densities of the film and its substrate, thelarge the difference in the film density, the higher theamplitude of the Surface or interface roughnessFigure 6 shows the reflectivity curves for a Sisubstrate with two different values of surface results shown in Fig.

7 6 indicate that reflected X-rays decrease more rapidly with a larger surfaceroughness. In other words, the larger the roughness of afilm, the faster the decay rate of X-ray the other hand, Figure 7 shows the results ofmeasurement for two types of substrates with differentvalues of interface roughness. The results indicate thatamplitude of the oscillation decreases with increasing2 The Rigaku Journal, 26(2), 2010X-ray thin-film measurement techniques VFig. and refraction of X-rays at material surfacewith the changes in the grazing of Au film on Si reflectivity curves of Au, Cu and SiO2film onSi substrates (film thickness is 20 nm).interface roughness. The term interface roughness includes that physical uneven interface and transitionalboundary layer which continually changes in density. Inboth cases, interface roughness is recognized to acontinuous variation of electric density along thethickness 8 shows the summaries of the effect of filmthickness, density, roughness of surface and interface onthe X-ray reflectivity curve of a thin film deposited on aSi , the X-ray reflectivity technique is a methodfor determining the layer structure of a thin film.

8 Theprinciples of X-ray reflectivity method will be describedin section 5 in greater Useful methods and practices in X-rayreflectivity measurementAn accurate X-ray reflectivity curve is defined byclear appearances of the total reflection critical angle,the period and the amplitudes of the oscillation. It isnecessary to obtain precise information so thatappropriate X-ray optics depending on the film thicknessand sample size are used. In addition, when an X-raybeam is incident on the sample at a grazing angle, astrict having adjustment is Selecting incident opticsThe X-ray reflectivity measurements must accuratelymeasure a wide range of intensities measured at grazingX-ray incident angles. For grazing X-ray incidence, theuse of a well-collimated incident X-ray beam isimportant. Conventional X-ray optics use narrow slits tocollimate the incident X-ray beam.

9 The intensity of theincident X-ray beam is thus very weak, and X-rayreflected curve with a wide dynamic range of intensity isdifficult to obtain. Recently, an X-ray optical systemequipped with a parabolic multilayer mirror and anincident-beam monochrometer has been used to producea high-brightness and parallel incident X-ray angular divergence of an incident X-ray beam canbe controlled by incident X-ray optical , X-ray optics must be selected on the basis offilm thickness. Nowadays, to obtain a high-resolutionand parallel incident X-ray beam, a parabolic multilayermirror and an incident monochrometer are commonlyused. Two X-ray reflectivity curves are compared usingthe X-ray optics system with a multilayer mirror, whichgives the highest intensity among all incident X-rayoptics. The multilayer mirror produces a parallelincident beam with a vertical divergence of.

10 Figure 9 shows the X-ray reflectivity curves fortwo Au films with different thicknesses deposited on Rigaku Journal, 26(2), 20103 Technical articlesFig. reflectivity curves of Si substrates with twodifferent values of surface reflectivity of Si substrate differences withinterface roughness (Film thickness is 20 nm).Fig. provided by X-ray reflectivity of Au films on Si substrates(film thicknesses are 10 and 30 nm).For the 300 nm thick film, oscillations of X-rayreflectivity cannot be observed using the optics systemwith a multilayer mirror. This is because the period ofthe oscillation changes depending on film a larger film thickness, the period of the oscillationis smaller, and the resolution of the X-ray optics used formeasuring X-ray reflectivity must be increased. Table 1provides an overview of the optics used in reflectivitymeasurement and guidelines for film Relationship between incident angle andirradiated widthFor a reflectivity measurement at a low-angle range2q 0 , the incident X-ray beam impinges onto a largearea of the sample surface, even when a narrow incidentslit is used.