XRD Line Profile Analysis of Tungsten Thin Films

HRTEM and XRD line profile analysis of tungsten thin films

A. M. Tonejca, I Djerdj,a A. Tonejca and N. Radićb a , andelka @ phy.hr

a Dept. of Physics, Faculty of Science, University of Zagreb, Bijenička 32,

P.O.Box 331, 10002 Zagreb, Croatia

bRuđer Bošković Institute, Bijenička c. 54, P.O.Box 180, 10002, Zagreb,

Croatia;

Tungsten thin films show high electric conductivity, high mechanical strength, good metal barrier performance and fine patternability (1), and are suitable for X-ray masks, X-ray mirrors, interconnects on Si-based devices and metallization layers.

Thin tungsten films were deposited onto glass substrates by DC magnetron sputtering at different substrate temperatures (77 K, 293 K, 523 K) and at different working-gas pressures in the range 0.7-2.8 Pa. Samples were characterized by X-ray diffraction (XRD, Fig. 1e) and subsequently refined with the Rietveld method (2). Because of appearance of preferred orientation in XRD patterns the other methods of line profile analysis were used. Using these methods the diffracting domain sizes from 15 to 45 nm and the root mean square (r.m.s.) microstrains have been determined. The dependence of structural parameters on deposition conditions was investigated as well. It was found that residual stress and lattice parameter are correlated. We suggest that the observed increase of lattice parameters in comparison to the bulk values occurred due to the substitutional incorporation of Ar atoms, and W atoms residing in auto-interstitial positions. The high values of r.m.s. microstrain, 0.251% for b-W and 0.345% for a-W, on average, confirmed this feature. The a phase is predominantly found at low Ar pressure, while an increase in the latter causes the increase of the fraction of the b phase. The substrate temperature also exhibits an impact on the phase composition.

High resolution electron microscopy (HRTEM) and selected area electron diffraction (SAED) of some representative samples ( 77 K, 0.7 Pa ) were analysed in the JEOL JEM 200kV microscope with field emission gun. For HRTEM investigation tungsten films deposited onto glass substrates were prepared by cross- sectional procedure and thinned in the Gatan ion-mill. The preferred orientation was observed in α -W in [110] direction.The presence of α-W and β-W was proved in SAED patterns as well as in FT of the HRTEM images. The residual stresses resulting from the growth of sputtered films originates primarily from the specific energy transfer into the substrate and the growing films. The influence of the residual stresses on XRD and SAED patterns is observed as the preferred orientation in particular direction. In XRD patterns the intensities of some peaks are missing and are not in accordance with the intensities given in ICDD. The preferred orientation in SAED pattern, of the observe sample is given in Fig 1.c in α -[110] / β -[210] direction and α -[110] direction of the Fourier transform ( FT) of original image ( Fig, 1a ) The filtered image displayed in Fig.1b reveals the lattice fringe image ( d110 = 0.238 nm ) of α-W phase. The elongated grains with some intercalated regions resulted from residual stresses in the observed thin film.

We thank dr. Miran Čeh for giving opportunity to perform HRTEM measurements and preparation of samples in his laboratory. Thanks to Mrs M. Gec for her advices and help in preparation cross- sectional samples.

(1) Shen YG, Mai YW, Zhang QC, McKenzie DR, McFall WD, McBride WE. J Appl. Phys 2000;87:1:177-187.

( 2) Djerdj I, Tonejc AM, Tonejc A and Radić N, Vacuum 80 (2005) 151-158.

Fig. 1a Fig.1b

Fig. 1e. Enlarged part of X-ray diffraction patterns of thin tungsten films deposited at different Ar pressure (0.7 Pa, 1.4 Pa, 2.1 Pa, 2.8 Pa) and different substrate temperatures (77 K, 293 K, 523 K).