LAwave

Measuring Single Crystal Properties

The laser-acoustic method can be used for testing the properties of bulk materials. A possible application is testing the correct orientation of single crystals, because the surface wave velocity depends characteristically on the propagation direction.
Figure 1 shows the surface wave velocity versus the propagation direction for a (100) silicon wafer. The [110] direction was used as reference direction.
For non-coated materials, the surface wave velocity does not depend on frequency and, therefore, only one data point is sufficient to describe the velocity in a given direction. This point represents the complete dispersion curve that is a line parallel to the frequency axis for such a homogenous material.

Example01
Figure 1: Surface wave velocity c depending on the propagation direction "alpha" for a (100) silicon wafer

The diagram reveals the regular four-fold symmetry of this wafer. The same velocity value was always measured after having tilted the sample stepwise by 90°.
Not so for the example in the Figure 2 that shows a similar investigation for a (111) wafer.

Example02
Figure 2: Surface wave velocity c depending on the propagation direction "alpha" for a (111) silicon wafer

The plot shows the six-fold symmetry of silicon in the (111) plane, but for a correct orientation, the plot must be completely regular. The measured irregularity proves the crystallographic misorientation of this wafer that obviously was not exactly cut from the silicon ingot.

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