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edfas.org 19 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 1 (continued on page 22) of the channel-select gate and floating gate-control gate, and the S/Nvalue remained suitable over the entire image. Consequently, the carrier density in the source line, select gate, floating gate, and channel is readily apparent. It is evident that p-type carriers were present in the floating and select gates while the source line, control gate, and channel contained n-type carriers. The spatial resolution of the SNDM techniquewas assessedby acquiring a profile of the floating gate,moving along the line labeledA-Aʹ. The resulting signal was fitted using a Gaussian function, for which the full width at half maximum equaled the thick- ness of the floating gate, and the average width obtained from six trials was 3.8 nm. Figure 2b shows the resulting profile, inwhich theblack and red lines indicate the experi- mental data and the Gaussian fit. The fitted curve was used to estimate the spatial resolution of this technique between the points associated with signal intensities of 25% and 75%. The average value based on six points was determined to be 1.9 nm, [15] confirming the extremely high resolution of this technique. TIME-RESOLVED SCANNING NONLINEAR DIELECTRIC MICROSCOPY As noted, anovel digital technique termed tr-SNDMhas recently been demonstrated, as an improvement on the previous analog process. [13] Figure 3 provides a diagram of the tr-SNDM instrumentation. [13] With this instrumen- tation, Δ f ( t ) values are acquired over time using a digital demodulation process as opposed to a standard analog demodulator. The signal generated by the SNDM probe (that is, the LC oscillator) in the microwave frequency range is down-converted to lower frequencies and cap- tured using a high-speed digitizer, based on the desired experimental bandwidth. The resulting digital signal is subsequently demodulated in a post-processing step. This digital demodulation provides increased flexibility with regard to the ability to change the demodulation bandwidth. Consequently, the localized capacitance data can be more accurately demodulated compared with an analog device, as a result of the absence of signal distor- tion and fluctuation due to narrow bandwidth frequency characteristics and its aging experienced in a conventional analog frequency demodulator. The use of tr-SNDM as a means of monitoring defects at SiO 2 /4H-SiC interfaces on the basis of the DLTS tech- nique was assessed. DLTS is a useful means of monitor- ing transient capacitance and is commonly employed to examine semiconductors for electrically active defects. [16] DLTS can also determine the physical origins of defects by providing quantitative data regarding defect densities and energy depths. Typically, DLTS is carried out in association with a MOS capacitor and cannot directly determine the spatial distribution of defects. However, prior work has suggested that such information could potentially be obtained. [17] In previous research, the transient capaci- tancewaveformwasmonitored (representinga sumof first to sixth-order Fourier components) using a multichannel lock-in amplifier. This techniquewas termed local-DLTS. [9] (a) Fig. 2 (a) SNDM (d C /d V ) image of a 3D flashmemory cell structure. (b) SNDM profile of the floating gate obtained along the line labeled A-Aʹ. (b)