Introduction

Two dimensional materials created ab-initio bythe process of condensation of atoms, molecules orions, called thin films, have unique properties signifi-cantly different from the corresponding bulk materials as a result of their physical dimensions, geometry,non-equilibrium microstructure and metallurgy.These characteristic features of thin films can bedrastically modified and tailored to obtain the desiredproperties. This forms the basis of development of ahost of active and passive thin films devices [1], insuch areas as very large scale integrated (VLSI) cir-cuits, quantum engineering, integrated optics, photo-voltaic conversion and special coatings for surfaceengineering.

The strong relationship of the physical, chemical,metallurgical and electronic structure of thin filmmicro and nanomaterials with the physical propertiesof thin materials has led to the development of a largenumber of microanalytical characterization techni-ques for thin films. The techniques based on X-rayprobe have dominated the field mainly because oftheir simplicity, more reliability, quantitative andnondestructive nature. Of these techniques, X-raydiffraction has played a leading role, as a fundamentaltool for material characterization.

Experimental Techniques

The conventional Bragg-Brentano (also called θ-2θ) X-ray diffraction geometry is not useful for thestudy of ultra thin, graded composition and multi-layered thin films, partly because of poor sensitivityand partly because of the presence of the interferingeffect of the substrate. A useful geometry must bebased on a low angle of incidence of X-ray, so as to be

able to probe the material up to a specific depth. Three geometries have been exploited for the purpose. Thefirst one is the Seemann-Bohlin parafocusing geo-metry [2],[3] while the other two are classified ingracing incidence diffraction (GID) and are based onslight modification of conventional Bragg-Brentanogeometry. These geometries are discussed in thefollowing.

Fig. 1 shows the Seemann-Bohlin parafocusinggeometry. In this geometry the beam focus F, the

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The Rigaku Journal

Vol. 4/ No. 1/ 2 / 1987

CONTRIBUTED PAPERS

CHARACTERIZATION OF THIN FILMS BY GLANCINGINCIDENCE X-RAY DIFFRACTION

R. D. TAREY, R. S. RASTOGI and K. L. CHOPRA*

Thin Film Laboratory, Department of Physics,

Indian Institute of Technology, Hauz Khas,

New Delhi- 110016, India

* Director, Indian Institute of Technology, Kharagpur, India.

Fig. 1 Schematic diagram showing Seemann-Bohlinparafocussing geometry.

sample and the counter lies on the same focusingcircle. The sample should preferably be curved withradius of curvature of the focusing circle. However,for applications to thin films, the radius of focussingcircle is increased considerably in com­parison to thesample size. Seemann-Bohlin geometry has certainimportant advantages for 2θ ≤ 40° which includeshigher intensity without sacrifice of resolution, sim-plification of mechanical design and feasibility ofemploying multiple detectors simultanesouly.

One of the glancing incidence diffraction (GID)geometries shown in Fig. 2 is slight modification ofconventional Bragg-Brentano geometry. As shown infigure, X-rays pass through a suitable slit system andare made to fall on the sample at a glancing angle (α)while the detector on the 2θ axis scans the XRD pat-tern. The diffracted beam optics is modified to aprarallel beam optics and a flat plate monochromator(graphite) is incorporated in the diffracted beam [4].The conventional powder methods are applicable inanalyzing the GID patterns. One can immediately seethe immense potential of this geometry in characteri-

zation of graded and multilayer thin film structures.Fig. 3 shows another GID technique which is capableof analyzing very thin (100 Å) epitaxial layer [5], [6].As shown, X-ray beam irradiates the sample surfaceat a grazing angle slightly larger than the critical angle of total reflection. Diffraction is obtained from latticeplanes perpendicular to the sample surface. Conven-tional θ-2θ scan and radial scan provides informationabout the epitaxial relation, orientational spread,crystallite size and lattice strain.

As can be seen the two GID techniques arecomplementary in nature and are useful for theanalysis of amorphous, polycrystalline and epitaxialthin films.

Experimental Results and Discussion

GID measurements described in this paper wereperformed in our laboratory on a X-ray diffractionsystem (Geigerflex D/MAX-RB) from Rigaku Corp.,Japan. The system is equipped with a 12 kW rotatinganode (CuKα) source (model RU-200B) and a thinfilm attachment with sample rotation facility. Theschematic diagram is shown in Fig. 2. Thin films ofcarbides, nitrides, suicides and amorphous chalco-genides were characterized by GID technique. Someof the interesting examples are given below.

(i) Tungsten carbide (WC)-WC thin films weredeposited on stainless steel (SS) substrates atsubstrate temperature of 500°C by RF reactivemagnetron sputtering of tungsten in presence ofacetylene [7]. Conventional θ-2θ scan on 8.0 µmthick WC film shown in Fig. 4, indicates a fcc B-typestructure. The GID scan at α=5°(Fig. 5) and α=2°(Fig. 6) indicates that as the angle of incidence isdecreased, the peaks corresponding to SS substrateare suppressed. Since no additional peaks other thanfcc B-type WC are observed at α=2°, it can be

Vol. 4 No. 1/2 1987 12

Fig. 4 Conventional X-ray diffraction pattern of WC films

Fig. 2 Schematic diagram of glancing incidence X-raydiffractometer.

Fig. 3 Schematic diagram showing glancing incidence X-ray diffraction for epitaxial films.

inferred that no alloy formation occured at WC/SSinterface.

(ii) Titanium nitride-Titanium nitride [8] films onSS substrates were deposited by cathodic arc plasmadeposition method in presence of nitrogen atmos-phere. Conventional θ-2θ scan and GI D scan at α=2°, respectively, are shown in Fig. 7 and Fig. 8 for a fewmicron thick film. As can be seen, the intensity of thepeaks at 43.5° and 61.5° is enhanced in GID scans incomparison to conventional θ-2θ scan, however theintensity of peak at 36.3° decreases. The enhancement of 43.5° and 61.5° peaks suggests that TiN phase ispresent at the surface while the bulk of the material isa mixture of TiN and Ti2N phases.

(iii) Silicide Films-Refractory metal and nearnoble metal suicides have become a major area ofinterest for VLSI metallization and interconnections[9]. The higher density, higher reliability require-ments of VLSI technology require closer toleranceson electrical and other properties which also putsstringent demands on coating technology and charac-terization. Since different phases are formed whenmetal reacts with silicon, the annealing temperature isan important parameter which decides the formation

of the final phase. The ion beam mixing and rapidthermal annealing techniques are coming intoprominence for their advantages in self alignedsilicide (SALICIDE) technology [10]. Also, theinterest in multicomponent suicides is obvious if onewants to tailor the properties. All this leads to multi-component multiphase characterization problem.

Thin films of CoSi2 in the present study, wereformed by rapid thermal annnealing of 500 Å ofsputter deposited Co film on Si (111) substrates. Theannealing temperatures were in 700°C to 1000°Crange, while the annealing time was 15 seconds. Therelative increase in CoSi2 (111) peak in the GIDspectra of Fig. 9 at increasing temperature indicates apreferential CoSi2 (111 ) orientation on Si (111 ).

Interaction of amorphous alloys of refractory andnear noble metals with Si has been studied by GIDtechnique [11]. Thin films of Mo-Ni alloy weredeposited by co-sputtering of Mo and Ni targets on Si(100). The as deposited films are amorphous innature, Fig. 10(a). The amorphous nature of the filmswas observed to be stable in the temperature range of500-550°C depending upon the cornposition of Moand Ni. However, at 600°C, all compositions leads to

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Fig 5 GID spectra of WC films at an incidence angleα=5°

Fig 6 GID spectra of WC films at an incidence angleα=2°

Fig 8 GID spectra of titanium nitride film at anincidence angle α=2°

Fig 7 Conventional X-ray pattern for titanium nitridefilm.

the phase transformation. The stable phases formedsubsequently were identified as MoSi2 and NiSi2, Fig. 10(b).

(iv) Chalcogenide films-Films of Ge-Se, P-Sechalcogenide glasses doped with Ag, Bi and Sb haveattracted much attention for their applications as in-organic photoresist [12] and optical memory mater-ials [13]. The amorphous films undergo photostruc-tural changes when exposed to a suitable dose ofultra-violet radiation. Bulk glasses of Ge-Se dopedwith Sb were fist prepared by melt quenching tech-nique. Thin films were subsequently evaporated fromthe bulk alloy in vacuum of 10-6 torr [14]. The GIDspectra of as deposited films at an incidence angle(α=5° is shown in Fig. 11 (a). Fig. 11 also shows theGID spectra of the film after suitable ultravioletexposure at the different angles, Figs. 11 (b) and (c). It is evident from the figure that the film is amorphousbefore exposure. On exposure, Sb2Se3 peaks areobserved in GID spectra. The increasing intensity ofSb2Se3 peaks at lower angle of incidence indicateformation of polycrystalline Sb2Se3 at the surfaceonly. GID spectra at an incidence angle of α=5°, Fig.11(c) clearly indicates the superposition of bottom

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Fig. 9 GID spectra of 15 sec. Rapid thermal annealedCo/Si (111) samples at annealing temperatures (a)1000°C (b) 900°C (c) 800°C

Fig. 10 GID spectra of Mo67Ni33 alloy film on Si (100) atincidence angle α=5° (a) as deposited (b) after vacuumannealing at 600°C for 30 min.

Fig. 11 GID spectra for Sb doped Ge-Se films (a) asdeposited (α=2°) (b) uv exposed (α=2°) (c) uv exposed(α=5°) and (d) annealed

amorphous matrix on Sb2Se3 peaks. On annealing theexposed films the Sb2Se3 peaks disappear as seen inthe GID spectra in Fig. 11 (d). These results supported by AES and ESCA studies, suggest evaporation of theultrathin Sb2Se3 films.

Summary

Grazing incidence X-ray diffraction, performedusing a thin film attachment and a 12 kW rotatinganode source, has been found to be an effective andvaluable technique in characterizing a variety ofmulticomponent and multiphase materials in thin filmform. The WC example has clearly ruled out thepossibility of alloy formation at WC/SS interface. Inthe titanium nitride example, conventional XRD onlypointed to Ti2N presence. However, the GID spectrahas clearly brought the depth variation in composition of phases. The identification of Sb2Se3 peaksdemonstrates the surface sensitive nature of the GIDtechnique. The silicide study brings out the capabilityof GID technique in establishing graded multiphasestructures in ultra thin films.

References[1] K. K. Chopra and 1. Kaur, "Thin Film Device Application",

Plenum press, New York, 1 983.

[2] H. P. Klung and L. E. Alexander, "X-ray DiffractionProcedures" John Wiley & Sons, New York, 1 974.

[3] S. S. Lau, W. K. Chu, J. W. Mayer and K. N. Tu, ThinSolid Films, 23 (1974), 205.

[4] Instruction Manual. For Thin Film Attachment(ME210ML2), Rigaku Corp. Japan.

[5] A. Segmuller, "Advances In X-ray Analysis", Vol. 29, C.S. Barrett, J. B. Cohen, J. Faber, R. Jenkins, D. E.Leyden, J. C. Russ and P. K. Predecki, Editors, PlenumPublishing Corp., 1986.

[6] J. H. Souk, A. Segmuller and J. Angiliello, 1. B. M.Research Report, RC 1 241 5, 1 986.

[7] P. K. Srivastava, Ph. D. Thesis, Dept. of Physics, IndianInstitute of Technology, New Delhi, 1986.

[8] K. K. Chopra, V. D. Vankar and R. D. Tarey, unpublishedresults.

[9] S. P. Murarka, "Silicides for VLSI Applications",Academic Press, New York, 1 983.

[10] R. K. Shukla, R. W. Davies and B. W. Tracy, J. Vac. Sci. Technol., B4 (1986), 1344.

[11] R. S. Rastogi, V. D. Vankar and K. K. Chopra, to becommunicated.

[12] A. Kumar, L. K. Malhotra and K. K. Chopra, J. VacuumSci. Technol., A5 (1987), 354.

[13] R. A. Bartolini, Proc. IEEE, Vol. 70 (1982), 589.

[14] A. Kumar, Ph. D. Thesis, Dept. of Physics, IndianInstitute of Technology, New Delhi, 1987.

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