Semiconductor Characterization
potential map in the region of interest without diffraction contrast. Terefore, there is no overlap or projection issue along the vertical direction of Figure 14. In the top of the emitter, the electrostatic potential of n++ is fully saturated toward the bottom of the conduction band (highest in elec- trostatic potential), while the blue p+ region on the two sides of the base contact is at the top of the valence band (lowest in electrostatic potential). Assuming the electrostatic poten- tial difference of these two regions is approximately 1.1 V, and using these two regions of electrostatic potential, one can determine the location of the middle point in the bipolar device along the vertical direction. In the electron holography measurement, the electrostatic
Figure 14: High spatial resolution junction mapping by electron holography for a bipolar SiGe device with 0.2 nm fringe spacing.
the extreme leſt, the profile shows a negative dC/dV signal with low intensity that indicates a highly doped n+ emitter. Te sig- nal is positive in the base region indicating a p-type carrier. Te collector and sub-collector regions have negative dC/dV signals with high and low intensities indicating n- and n+ dop- ing, respectively. Figure 14 is a junction map of a bipolar SiGe device with
0.2 nm fringe spacing by dual lens electron holography. Te middle red layer is the SiGe layer with electrostatic potential different from the Si material. Figure 15 is the junction line profile of Figure 14 from top to bottom, with the line profile position shown as the yellow arrow in Figure 12b. In the line profile (Figure 15), the SiGe layer is the p-type material sand- wiched between n++ on top and n+ at the bottom, with the top part as the emitter, followed by the SiGe layer as the p-type base in the middle, and the bottom part as the collector. If we assume spatial resolution is 3× fringe spacing, the spatial reso- lution of Figure 14 is approximately 0.6 nm for the junction profile. To obtain higher spatial resolution along the verti-
cal line, Si is tilted along the horizontal direction until a white Si image is observed in order to get an electrostatic
potential of the p-type SiGe is different from the p-type Si. In this case, the p-type SiGe shows as red in Figure 14, which is similar in color to the n-type Si. In contrast, the SCM measure- ment is sensitive to carrier type and concentration, but it does not distinguish the material type as Si and SiGe as shown in Figure 13a. In this case the thin layer p-type SiGe in the middle of the device has a similar dC/dV value as the one for the p-type Si on the side (bright white color). It is noted that the device in Figure 13 has a wider n- region than that shown in Figure 14. Tis difference is due to different manufacturing conditions for the two devices. Te electrostatic potential in SiGe, shown as a red curve
in Figure 15, is a function of Ge concentration. Te ampli- tude profile (blue curve) shows that the SiGe layer has lower amplitude despite a higher Ge concentration, an opposite behavior to the electrostatic potential measurement. Te signal-to-noise ratio is lower in amplitude profile compared to the phase profile. Tis indicates that the electrostatic potential profile is more sensitive to Ge concentration varia- tion than the amplitude profile. It has been reported that Ge concentration variation within the SiGe layer can greatly enhance device speed [23]. However, the phase profile is the convolution of electrostatic potential of SiGe and carrier con- centration. To accurately measure Ge concentration variation in such a thin layer structure, dark field electron holography with high spatial resolution can be used to measure the lat- tice constant along the vertical direction [12,13]. Te larger the lattice constant, the higher the Ge concentration. Tis kind of Ge concentration variation characterization in SiGe layers is critical for process development, process matching, and physical defect analysis.
Discussion Te examples presented in this paper show that dual lens
electron holography enables high spatial resolution junction mapping with good signal-to-noise ratio for various applica- tions. Dual lens electron holography can obtain the high- est spatial resolution of 0.6 nm with fringe spacing of 0.2 nm (under the assumption that spatial resolution of an electron hologram is 3× fringe spacing). More industrial applications of the technique have been published [24–26]. One limiting factor to the spatial resolution for electron
Figure 15: Vertical line profile of junction profile in Figure 14 from top to bot- tom as a red curve. The blue curve is the amplitude intensity profile. The line profile position is shown in Figure 12b as the yellow arrow.
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holography is the sample tilt, which causes projection issues. For junction profile analysis, the sample is tilted off zone axis to reduce the diffraction contrast in the sample to obtain an electrostatic potential image. Depending on the tilt and sample
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