Tungsten is attractive for very large scale integrated device metallization and as absorber for x-ray lithographic masks. To minimize distortions in an x-ray mask, intrinsic stresses in the absorber films have to be low and reproducible. We present the results of a systematic study of the microstructure and stress of rf sputter-deposited W films as a function of deposition parameters, using optical interferometry, scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. Rutherford backscattering spectrometry and Auger electron spectroscopy were used for the chemical analysis of the films. By controlling the nucleating phase and mobility of the adatoms, we have produced W films with low stresses (<±50 MPa). The low-stress films have a 〈110〉 preferred orientation and a bimodal grain size distribution with large, elongated grains surrounded by small equiaxed grains. The lattice parameter and the argon content in W films increased with decreasing argon deposition pressure. It was found that a low base pressure (<10−7 Torr) is necessary to produce stable α-phase W and to eliminate the metastable β phase. Additional seed layers (Cr, SiO2 ) at the Si/W interface resulted in a small improvement in stress control of the W films. The W films were stable (no change in stress) on subsequent anneals at 200 °C for 50 h in vacuum and argon ambients.
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Kola et al. (1991) studied this question.