| Summary: | 博士 === 國立交通大學 === 光電工程學系 === 100 === This study investigates the defect isolation and dopant profiling using secondary electron potential contrast (SEPC). A novel primary electron energy adjustment method is proposed to remedy the imperfections in traditional SEPC method, which uses fixed primary electron energy. For dopant profiling, a novel in situ nano-probe biasing is applied to enhance the SEPC signal, restoring the missing dopant contrast successfully.
First author discusses the application of SEPC is applied to investigate the leakage and high resistance in a metal oxide semiconductor field effect transistor (MOSFET). The contact nodes in an MOSFET can be classified into four categories: the polysilicon gate node, p+/n-well node, n+/p-well node, and, well nodes. Most studies set primary electron beam energy (EPE) at 1 keV and used potential contrast to identify the gate oxide rupture and continuity failures. However, the bright and dark contrast of samples cannot distinguish these four nodes types well. For instance, the contrast of a p+/n-well node and well nodes is bright in scanning electron microscope (SEM). However, a leaky p+/n-well node exhibits the same brightness as the well nodes, an insufficiency of the EPE 1 keV condition for identifying p+/n-well nodes and well nodes. Previous studies indicate that the contrast of SEPC arises from the surface charging effect, which is initiated by the interactions between the primary electron beam and sample. The EPE 1 keV condition results in the positive charging on the sample. Positive charging will set the p+/n-well node in forward bias and leak positive charges into well nodes. Thus, the EPE 1 keV condition cannot be used to distinguish the p+/n-well node and well nodes. This can be solved by setting the p+/n-well node in reverse bias. This study increases the EPE to 5 keV to reverse surface charging from positive to negative. Experimental results demonstrate that the 1 keV and 5 keV EPE conditions can be used to identify these four nodes. Finally, the analytical method was applied to a real failure case and no abnormality under the conventional EPE=1 keV condition was observed. However, the proposed EPE=5 keV can isolate a defect successfully and complete the imperfect conventional method.
The second part of this study discusses the application of SEPC to diode dopant profiling. Since 1967, researchers have observed dopant contrast in SEM image. The dopant contrast arises from built-in potential across the diode. This study also uses this property to identify a p+/n-well junction leakage path in a static random access memory (SRAM). However, for a small bandgap material like silicon, the built-in voltage is as small as 1.12 eV. Dopant contrast is weak and, in the worse case, no contrast is observable. The surface-damaged layer generated by sample preparation is believed to be the cause of dopant contrast reduction, inhibiting the application of SEPC to the integrated circuit (IC) failure analysis. For SEPC enhancement, this study studied the contrast effect under different sample preparation methods. By triggering the diode in the reverse bias condition through in situ nano-probe biasing, that dopant contrast can be restored. The SEPC image was digitalized and quantified for conversion of image contrast to the voltage scale, allowing the identification of the depletion region and electrical junction. The overlap length between the poly silicon gate and p+ region is also depicted by the two-dimensional (2D) imaging. The proposed method can maintain stable voltage conditions in the junction, facilitating the inspection of dopant area by SEM, and the development of an efficient method for examining dopant areas. Experimental results also confirmed the method has promising application in site-specific junction inspection. Finally, the novel method was applied to identified the failure cause of a current mirror mismatch. The inspection method successfully identified a 0.4 µm p-well layer misalignment caused by the mismatch. The experimental split also confirmed that a p-well misalignment exceeding 0.4 µm will cause failure.
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