Photoconductivity measurements of implanted samples

Implantation of B and P are used for many applications in the microelectronic industry, but so far there was no method available to check the homogeneity of these implantations without contacting the samples and changing their properties due to the necessary annealing steps. The difficulties so far were that the implanted area is usually at a depth of only several µm and that the implanted doses are very low. MDP enables to characterize these implanted samples with a high resolution and a good differentiation between different implanted doses.

Fig. 1: implanted P dosis with a depth of only 2 µm
Fig. 2: measured photoconductivity of the sample with different implanted P doses

In this case not the lifetime, but the photoconductivity or signal height is the most sensitive parameter for detecting inhomogeneity in implantations. It depends strongly on the resistivity and the lifetime itself.

In the MDPmap and MDPingot equipment it is possible to integrate up to 4 lasers with different wavelength. Furthermore it is possible to measure with different pulse length from a very short pulse of only 100 ns, where no carrier diffusion takes place to a pulse length of several ms, where the carriers diffuse into the sample depth. Hence by varying the laser wavelength and the pulse length, it is possible to measure with different penetration.

In this case a 660 nm laser with a pulse length of 100 ns was chosen; hence a penetration depth of approximately 4 µs was achieved. Figure 1 shows the implanted P doses in the measured Cz-Si sample and figure 2 demonstrates how the different doses can be distinguished by photoconductivity measurements.