Laser-acoustic results were compared with two alternative mechanical test methods for thin films, the membrane deflection and the indentation technique [1].
For the membrane deflection technique, the film must be prepared as membrane. The substrate is removed by etching. The free-standing film is placed in a vacuum chamber. The pressure in the chamber is reduced. This causes the membrane to bend. The bending is measured by an interferometric method.
The comparison tests were performed with polysilicon films. The film thickness was 460 nm. The substrate was fused silicon that had to be removed for the membrane test. The samples were annealed to study a possible effect of residual stresses. Figure 1 shows the film modulus depending on the annealing temperature, measured with both the laser-acoustic and the deflection technique. The difference between the results was not higher than 5 GPa. This is a deviation of less than 4 % and within the uncertainty of both methods.

Figure 1: Young’s modulus of polysilicon films versus the annealing temperature, measured with the laser-acoustic and the membrane deflection technique.
Membrane deflection results from Dr. Maier-Schneider, TU Berlin

[1] D. Schneider, B. Schultrich, H.-J. Scheibe, H. Ziegele, M. Griepentrog, Thin Solid Films 332 (1998) 157

Micro-indentation is a well-known method for mechanical characterization of hard coatings. The Young’s modulus of a film can be gained from the unloading curve of the indentation test. Comparative tests were performed with TiN, TiCN and CrN. The films were deposited on steel by magnetron sputtering. Film thickness was in the range from 0.8 to 2.3 µm. Figure 2 shows the Young’s moduli ELA and Eind measured with the laser-acoustic technique and the micro-indenter Fischerscope H100VP-B. The indentation test was performed with a load of 20 mN to make sure that the effect of the substrate can be excluded.
The tests were repeated five times with both test methods. Figure 2 displays the main values and the standard deviations. It shows a remarkable agreement of the results obtained with both techniques that is revealed by a slope of 1.01 and a correlation coefficient of R = 0.995. These comparative studies required that the tests were performed with care. The load cell and displacement pick-up of the micro-indenter were directly calibrated. The zero point and the effect of the shape of the indenter tip were corrected. The laser-acoustic method needs the thickness and density of the film as an input parameter for the fit procedure. The thickness was determined with a Calotester. The generally accepted values of "rho"TiN = 5.4 g/cm3, "rho"CrN = 6.18 g/cm3 and "rho"TiCN = 5.6 g/cm3 were put in for the density of the film materials.

Figure 2: Comparison of Young’s moduli ELA determined by the laser-acoustic technique and by micro-indentation Eind for films of TiN, TiCN and CrN deposited on steel.
Results from Dr. M. Griepentrog, BAM Berlin