Residual Stresses in Tungsten Lines: Analysis of Experimental- (Micro-Raman Spectroscopy, Xrd) and Numerical Results
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109 Mat. Res. Soc. Symp. Proc. Vol. 391 01995 Materials Research Society
SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUES The samples were prepared by growing 100 nm thermal oxide on (100) Si wafers, next 50 nm W was sputtered and this was followed by growing of 0.65 gim highly tensile CVD W in a coldwall system. The chuck temperature was 400 'C and the deposition pressure was 30 Torr. The H2 and WF 6 flows were respectively 7000 sccm and 150 sccm. The tungsten film was patterned and etched to give an array of 8 mm long W-lines with spacing 2 gm and width 1.2 gtm (narrow lines), and an array with spacing 2.5 and width 3.8 gim (wide lines). Another wafer was coated with a uniform W-film. This film was studied by XRD before and after it was separated from the substrate. The latter is necessary to determine the lattice constant of the unstressed film. The XRD measurements were performed on a Siemens goniometer [2]. Stresses were determined by assuming the plane stress condition (ofzz = 0). The unstrained lattice constant, ao, was also deduced, and compared with the one calculated from measurements on the detached film. The values of the Young modulus and of the Poisson coefficient used in the calculations were respectively Ef = 410 GPa and vf = 0.29 for the tungsten, and Es = 130.2 GPa and vs = 0.28 for the silicon. Micro-Raman experiments were performed on a DILOR-XY system using the 457.9 nm laser line of an Argon laser. The incident light was polarised along the width of the tungsten lines, the scattered light was not analysed. The sample was moved with an XY-translation stage in steps of 0.1 .tm and at each position, if possible, a Raman spectrum of silicon was recorded. A Lorentz function was fitted to the silicon-Raman peaks, in order to determine the peak frequency. EXPERIMENTAL RESULTS X-Ray Diffraction The strain-, and the calculated stress tensor components for the film and the two samples are given in Table I [3]. They are referred to the axis system as indicated in Fig. 1. The value of the lattice constant ao, calculated assuming ofz = 0, is 3.1648 A. This value is in agreement with the value measured in the detached film (3.1649 A). These results were discussed in [3]. TABLE I: Strain and stress components in the film and the lines, obtainedfrom XRD. Units of strain: 10-3; units of stress: MPa [3].
exx
Eyy
&-zz
afxx
ofyy
Ofzz
Film:
2.25
2.25
-1.94
1300
1300
0
Lines with width: 3.8 gtm 1.2 gm
-0.040 -0.368
2.194 2.203
-0.876 -0.750
269 122
979 937
0 0
Micro-Raman spectroscopy Fig. 1 shows the result of a scan across the first and the second W-line of the wide-lines array. The Raman shift obtained far from the lines is taken as stress free reference value (0 Rcm-1 in the figure). Upon approaching the first line, the Raman frequency shifts downwards, indicating tensile stress. Under the line, no Raman signal can be obtained. In between the lines, a downward shift is measured, with maximum near the edges of the lines, and minimum at the center (c) in between the lines. In order to make a
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