Effect of substrate and film thickness on the growth, structure, mechanical and optical properties of chromium diboride thin films

Abstract

The growth patterns of chromium diboride thin films deposited by thermal evaporation on borosilicate glass, fused silica, single crystal Si and Cu substrates are reported. It is shown that the adhesion of films is best on Cu, whereas on the other substrate films of thickness of > 200 nm are not stable. Scanning electron micrographs reveal that the films on Cu are marginally dense whereas on the other three substrates there is evidence for microporosity, clustering and three dimensional cracks. The as-deposited films on borosilicate glass substrates were amorphous independent of thickness. The films on fused silica, in contrast, crystallized at 60 nm thickness and showed the formation of boron deficient CrB₂. At higher thickness there was evidence for both CrB₂ and the boron deficient Cr₂B₃ or Cr₃B₄. In the case of films on Si substrate, the presence of both CrB₂ and the boron deficient Cr₂B₃ or Cr₃B₄ is evident. On Cu substrates, up to a thickness of 200 nm only reflections due to the CrB₂ phase are observable. At higher thickness the films consist of both CrB₂ and the boron deficient Cr₂B₃ or Cr₃B₄ phases. Nanoindentation studies reveal strong substrate dependent mechanical behavior. The hardness of the films is highest on fused silica with a value of 13.5 GPa. The highest hardness achieved on borosilicate glass and Cu substrates was 10 GPa. Interestingly, the Young's modulus value on all the substrates is less than 50% of the bulk value, ranging between 60 and 100 GPa. This has been correlated with the presence of microporosity and non-stoichiometry in the films. Spectral transmission studies on the films show that they become opaque on borosilicate glass and fused silica substrates at thicknesses > 150 nm. The reflectance of the Cu substrate is enhanced by 25% at 2500 nm due to the presence of the chromium diboride coatings.