CoLLOQUE DE PHYSIQUE
Colloque C7, supplement au nolO, Tome 50, octobre 1989
GLANCING INCIDENCE X-RAY STUDIES OF TITANIUM NITRIDE THIN FILMS USING A NEW MULTIPURPOSE LABORATORY SPECTROMETER
R.C BUSCHERT", P.N. GIBSON, W. GISSLER, J. HAUPT and T.A. CRABB
Institute for Advanced Materials, Joint Research Centre of the Commission of the European Communities, IL-Ispra 21020, Italy 'Turner Laboratory, Goshen College, Goshen, IN, U.S.A.
RBsum6
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Un spectrometre B rayon X h incidence rasante utilisant un tube h rayon X standard et un detecteur solide au germanium a BtB construit pour un balayage de diffraction vertical et horizontal, et des mesures de fluorescence sous incidence rasante. 11 a BtB utilise pour Btudier des films minces de nitrure de titane obtenus dans diff6rentes conditions de stoechiomgtrie, compression,dimension des grains, orientation et paramhtre cr istallins. En particulier, des films obtenus B la temperature de l'azote liquide du substrat montrent des etats tr&s comprimes, independants du mattSriau de support. Des films obtenush la temp6rature' ordinaire et au dessus ont des lignes de diffraction plus Btroites, des
orientat ions pr8fgrentielles et des tensions de compression plus faibles. En supprimant et Bliminant la diffusion due au suppprt, la techniq.ue de l'incidence rasante a 6tB tres utile pour dgterminer le caractdre, amorphe et le contenu amorphe de ces films minces obtenus B basse tempirature
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Abstract
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A multipurpose laboratory glancing angle X-rayspectrometer using a standard X-ray tube and a germanium solid state detector has been designed for vertical and horizontal diffraction scans, reflectivity and glancing angle fluorescence measurements. It has been used to study titanium nitride thin films grown under various conditions that vary the stoichiometry, strain, grain size, orientation and lattice parameter. In particular, films grown at
liquid nitrogen substrate temperature show very high compressive stresses, very small grain sizes, random orientation and lattice parameter shifts, independent of substrate material. Films grown at room temperature and above have much narrower diffraction lines, preferred orientation and lower compressive stresses. By suppressing and even eliminating the substrate scattering, the glancing
incidence technique has been very helpful in determining the amorphous-like character or amorphous content of these low temperature thin films
.
Introduct-
Glancing angle incidence X-ray diffract ion [l] , a comparatively new technique, has proved to be very powerful in studying near surface structures such as thin films and interfaces, because of its depth profiling capabilities. Much of the work has been done with either synchrotron or rotating anode X-ray sources. The spectrometer described here uses a standard laboratory X-ray tube and has sufficient diffraction
intensity to obtain spectra in a few hours from thin polycrystalline or amorphous films on var'ious substrates. The basic geometrical arrangement
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989716
and goniometers are flexible enough to permit reflectivity measurements and horizontal and vertical diffraction scans. In addition the solid state Ge detector used to detect the diffracted intensity detects the fluorescence at the same time and this can be analyzed by a multichannel analyzer to give the atomic composition of the same near-surface volume. The
instrument has been used to study titanium nitride thin films grown under widely varying conditions and on various substrates. The measurements have been most helpful in studying strain, grain size, particle orientation and lattice parameter.
Experimental arranqemenk
Figures la and lb show the geometrical arrangements for vertical and
horizontal diffraction scans. A copper target tube with a line focus and an adjustable precision collimator between the source and the sample
(separation 13 cm) provides an incident beam with a vertical divergence down to 0.2 milliradians (half -angle). This is adequate to obtain good reflectivity curves and to accurately control the depth of beam penetration o n flat samples. The X-ray tube together with the incident collimator is mounted on a goniometer that provides motion in the vertical plane (horizontal axis). The incident graze angle between this collimated beam and the horizontal sample surface can be varied by this goniometer adjustment. Since the sample surface always remains horizontal in this geometry, liquid surfaces can be studied. Rapid sample surface alignment is accomplished by a simple optical laser reflection system.
As can be seen in figs. la and lb the small incident graze angle results in a very large diffracting surface area. To achieve reasonable angular resolution requires either a soller collimator or a diffracted beam monochromator such as a bent pyrolitic graphite crystal [2]. Here a soller slit of 0.2 degrees angular divergence and a germanium detector for
monochromation provides narrow lines, low background and reasonable intensity. For accurate angular calibration a small amount of a standard silicon powder is placed on the sample surface in one area. With curve fitting of sample and silicon standard lines and interpolation very accurate lattice parameters can be obtained. The observed silicon line width gives a good indication of the instrument line broadening and it was observed to be 0.2 to 0.3 degrees (FWHM).
Since both the incident beam and sample remain stationary during a vertical scan, highly oriented planes that are parallel to the surface (such as (111) TiN grown on a high temperature substrate) cannot diffract.
For vertical and angled planes a horizontal scan or a combination of
horizontal and vertical scans is required. Additional soller collimators in the incident and also in the diffracted beams are required for good angular resolution. Horizontal scans give diffraction from planes nearly
perpendicular to the surface whereas the vertical scans show diffraction from planes at various angles ( 1 5
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45 degrees) to the surface. This can be useful in detecting non-uniform strains due to the high compressivestresses frequently found in this type of sample.
For precision reflectivity curves [3] strictly monochromatic radiation is necessary. This is achieved by a vertical variable radius bent silicon crystal located at the theta axis of a vertical goniometer centered near where the source is indicated in fig. la. A point X-ray source is mounted
further out on an arm with 2 theta motion. The monchromator is bent to achieve a sharp focus at a narrow slit in front of the detector after reflection from the sample. It was found that this arrangement produces any wavelength desired (from the continuous X-ray tube spectrum) with excellent wavelength resolution and sufficient intensity for good reflectivity curves but not enough intensity for surface diffraction.
The spectrometer is controlled by an IBM
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PC-XT computer with Superior Electric microstepping motors for the various goniometer motions. The detector pulses are timed and counted in a Tecmar Labmaster interface. This makes a very flexible system for on-line real time display and analysis of the data.Sample reparation
Titanium nitride films of various stoichiometry were prepared by
reactive ion beam sputtering at a deposition temperature of 77 K and 300 K
and by reactive RF magnetron sputtering at 300 K from a Ti target. The ion beam was extracted from a 3 cm diameter ion source of Commonwealth
Scientific Corporation, and for the RF sputtering a Leybold-Heraeus
sputtering facility of type 2 400 was used. Prior to deposition the systems were pumped down to a pressure of about lxl0exp-6 mbar
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The ion beam was neutralized by injecting electrons with a hot tungsten filament. Ion beam sputtering was performed with argon ions of an energy of 1200 eV and of a current of 40 mA. RF magnetron sputtering was performed at a power of 180 W in an argon atmosphere of 5 mbar. To vary the stoichiometry of the films nitrogen gas of different partial pressure was added.The films were deposited on glass substrates, which were cleaned in a special cleaning solution in an ultrasonic bath and then rinsed several times in bi-distilled water. Before sputter deposition the substrate was sputter-etched for several minutes. Also the target was cleaned by pre - sputtering.
Relative deposition rates were measured with a fused silica rate measuring system of type Leybold-Heraeus Inficon XTC. An absolute
calibration was made with a mechanical stylus system of the Talystep type covering a part of the substrate with a mask and measuring the step height.
Results
The effectiveness of grazing incidence diffraction in reducing
substrate background scattering and emphasizing thin film signal is shown in figures 2a, 2b and 2c. The sample in this case is TiN of 3000 A
thickness (grown by RF sputtering) on a glass substrate held at room temperature. The copper target X-ray tube was normally run at 30 mA and 35 k V , with the step scans taking 0.5 to 2 hours depending on the statistics desired. The three graze angles are 6 , 1.5 and 0.5 degrees, the latter being close to the calculated critical angle of 0.33 degrees. The
calculated corresponding l/e penetration depths are 9400, 2400 and 780 A.
Most of the subsequent scans were with a graze angle of 1 degree which is a reasonable compromise between intensity and depth of penetration. As can be
s e e n from f i g . 2 a t h e i n t e n s e "amorphous t y p e " o f s p e c t r u m from t h e g l a s s
substrate tends to obscure the weaker TiN film spectrum at a graze angle of 6 degrees. This situation is even worse for the standard Bragg-Brentano geometry where the graze angle is much larger (ranging from 20 to 40 degrees )
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The relatively weak (111) and (200) reflections and the pronounced intensity shift from (311) to (220) reflections as the graze angle is changed from 6 to 1.5 and 0.5 degrees, show that the (111) planes are rather strongly oriented parallel to the surface. Strong (111) orientation of RF sputtered films has been noted by others [4]. The ion beam sputtered films at room temperature and 77 K show mostly random orientation of the crystallites. Very broad diffraction lines of width 2
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4 degrees (FWHM), depending on growth conditions, indicate small grain size and/or high strain in these films. The detailed quantitative determination of the separate grain size and strain effects requires an analysis of the line shape by Fourier profile analysis as suggested originally by Warren and Averbach [S3 or by more recent and simpler methods 161 involvingcombinations of Cauchy (particle size) and Gaussian (strain) functions such a s Voigt and pseudo Voigt functions. The experimental data lie between these functions indicating both particle size and strain broadening.
Assuming only particle size broadening the Scherrer formula gives an average particle size of 80 A. SEM studies (private communication by E.
Lang) of these films indicate particle sizes of 100 A to 200 A . , substantially greater. This indicates both strain and particle size broadening of comparable amounts.
In fig. 3a a typical spectrum of stoichiometric sputtered TIN is shown.
These ion beam sputtered films at low substrate temperature ( 7 7 K) have very small particle size and random orientation. This is presumably due to the very low mobility of the surface atoms at low deposition temperatures.
The residual stress of these films is very high, up to 12 GPa, with the maximum ocurring on the low nitrogen side of stoichiometry as measured by the substrate curvature. The X-ray lattice parameter is of course affected by both composition, stress and defect density. By separately measuring the strain (substrate curvature) and the composition (AES) the true variation of lattice parameter with composition is to be determined.
As the nitrogen composition is reduced to less than about 30 % the structure suddenly shifts as shown in fig. 3b. A further nitrogen reduction t o a residual 3 % results in the titanium spectrum shown in fig. 3c. The broad diffraction lines shown in fig. 3b make it difficult to determine the phase or phases present in this sample. Some authors [7] report the
presence of Ti2N or Ti2N mixed with Ti in this composition regime.
The thinnest polycrystalline film detected so far was a 25 A thick (as detected by AES) Si02 film on an amorphous Si substrate
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The intensity of the Si02 diffraction llne, at a few c.p.s., was readily detectable above the background which in turn could be reduced to 5 - 10 c.p.s. by reducing the graze angle to below the critical angle.In conclusion, then, the spectrometer with its simple and reliable laboratory X-ray source, versatile geometry and moderately high angular resolution has proven to be very useful in studying thin films and near
surface structure in general and TiN films under various growth conditions in particular.
References
[l] W.C. Marra, P. Eisenberger and A.Y. Cho, J. Appl. Phys. =(11), 6927 (1979)
[z]
I.K. Robinson, Phys. Rev. Lett. 50(15), 1145 (1983) [3] H. Kiessig, Ann. Physik 10: 715 <1931)[4] J , - E . Sundgren, Thin Solid Films 128, 21 (1985)
[S] B.E. Warren and B. L. Averbach, J . Appl. Phys. 21,595 (1950) [6] Th.H. De Keijzer, E.J. Mittemeijer and H.C.F. Rozendaal, J. Appl.
Cryst. 16, 309 (1983)
[7] J. Chevall ier , Int
.
Colloquium on Cutting Metals, Saint -Etienne (1979 )DETECTOR
SAMPLE
HORIZONTAL SLITS
SOU
T SAMPLE
Fig.1: Schematic diagram of glancing angle diffraction geometries
F i g . 2 : D i f f r a c t i o n f r o m a 3 0 0 0 A t h i c k TiN f i l m o n a g l a s s s u b s t r a t e a t d i f f e r e n t g r a z e a n g l e s
F i g . 3 : D i f f r a c t i o n f r o m t i t a n i u m n i t r i d e f i l m s d e p o s i t e d u n d e r d i f f e r e n t c o n d i t i o n s . G r a z e a n g l e = 1 d e g r e e .
