Semiconductor Characterization
while low resistance represents high carrier concentrations (p type carrier). Te buffer SiGe layer is initially undoped to limit boron diffusion into the device body. Te buffer layer thins down near the SOI surface. Te device was processed with a lower-dose boron implant to form SDE. Te SSRM image (Figure 11b) clearly shows the SDE junction profile, including its overlap with the gate to form the required overlap capaci- tance for optimum device operation. Careful alignment of the STEM with SSRM image shows that the SD junction profile is outside of the buffer layer, indicating that boron diffused through it. Tis highlights that control of the thermal budget through the process is critical to control of dopant diffusion for optimum device performance. Te dark layer under the gate in the SSRM image corresponds to the c-SiGe layer in the corresponding STEM image. Te resistance contrast indicates that c-SiGe has a lower resistance compared to the underly- ing high resistance of Si n-well and remains doped n-type. Although the SSRM measures carrier profiles in semiconduc- tors, the technique does not distinguish between n-type and p-type regions. SiGe hetero-junction bipolar transistor (HBT). SiGe
bipolar devices have been developed for high-frequency applications, such as mass-market low-cost radar systems for the automotive and drone industries [23]. To achieve high-speed performance, HBTs incorporate SiGe as a base region material with a n-p-n junction width of approximately 50 nm. Optimization of HBT device performance requires
high spatial resolution mapping of the junction profiles and electrostatic potential in the junction. Figure 12 shows sche- matic drawings of SiGe bipolar devices. Figure 12a shows the overall structure of a HBT including the emitter, base, and collector. Figure 12b shows a close-up schematic of the SiGe hetero-junction region highlighted with a yellow dashed oval in Figure 12a. Figure 13a is a 2-D map showing carrier and junction pro-
files of a HBT device by SCM analysis. Te image shows, from top to bottom, n++ emitter, p-type SiGe base, n- collector, and n+ sub-collector. Figure 13b is the vertical 1-D carrier profile taken from the emitter to sub-collector along the center of the region, as illustrated with the yellow arrow in Figure 12b. At
Figure 12: (a) Diagram of
a
hetero-junction bipolar
transistor
(HBT);
(b) enlarged region of the dashed yellow oval region in Figure 12a with p-type SiGe hetero-junction doped with boron.
42
Figure 13: (a) SCM map of a hetero-junction bipolar transistor (HBT); (b) 1-D carrier depth profile through emitter-base-collector junctions in Figure 13a, also illustrated with a yellow arrow in Figure 12a.
www.microscopy-today.com • 2021 May
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