Testing Damage Layers in GaAs Wafers
Slicing GaAs wafers from ingots produces damage layers that have to be completely removed by polishing. A non-destructive control of the machining process is desirable. The laser induced surface acoustic waves has been demonstrated to be a method for characterizing the finishing state of such surfaces [1].
Figure 1 presents the laser-acoustic results for a wafer stepwise polished after having been sawn from the GaAs ingot.
Figure 1: Laser-acoustic results for GaAs - wafers with different polishing depth d
For the as-sawn wafer (polishing depth d = 0 µm), the velocity drastically decreases with increasing frequency. This suggests that a layer with a remarkable lower velocity covers the surface. High density of micro-cracks must be assumed causing such a considerable reduction of the stiffness of the surface layer. Polishing the samples results in more gradually descending dispersion curves. The velocity is nearly constant for a polishing depth of 10 µm. This means that all waves independent on the velocity and therefore consequently independent on their penetration depth propagate through the same material.
The results suggest using the slope of the curve as measure for the depth of the damage region. Figure 2 shows the slope dc/df to increase with the polishing depth. Beyond the polishing depth of 10 µm the slope is zero, indicating the damage layer to have been removed.
The results suggest using the slope of the curve as measure for the depth of the damage region. Figure 2 shows the slope dc/df to increase with the polishing depth. Beyond the polishing depth of 10 µm the slope is zero, indicating the damage layer to have been removed.
Figure 2: Slope of the dispersion curve depending on the polishing depth
[1] D. Schneider, R. Hammer, M. Jurisch: Semicond. Sci. Technol. 14(1999)93