February_EDFA_Digital

edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 1 18 log devices such as a lock-in amplifier and frequency demodulator. This article explains the basic principles of conventional analog-type SNDM as a technique for the assessment of semiconductors, and discusses experi- mental data obtained from carrier profiling of a 3D flash memory cell as ademonstrationof the performance of this method. Subsequently a novel digital tr-SNDM process is introduced. The performance of tr-SNDM is demonstrated by performing simultaneous local capacitance-voltage analyses, voltage derivative profiling, and deep level tran- sient spectroscopy (DLTS) using this technique. PRINCIPLES AND APPLICATION OF SNDM Figure 1 presents a diagram of the SNDM instrumen- tation. In this system, the contact force between the tip and sample is maintained at a constant value using a commercially available scanning probemicroscopy (SPM) controller, based on the same optical lever technique associated with contact atomic force microscopy (AFM). The SNDM probe comprises a capacitance sensor made from an active, free-running inductance and capacitance (LC) oscillator operating in the gigahertz frequency range and having a conductive cantilever tip. Following contact between the specimen surface and the tip, the probe’s oscillation frequency, f 0 , depends on both the tip-to- sample capacitance, defined as C ts ( t ) = C ts + Δ C ts ( t ), and the static capacitance, C . Note that the latter value includes both stray andbuilt-in capacitance contributions. Changes in the oscillation frequency, Δ f ( t ), will be correlated with variations in C ts (that is, Δ C ts ( t )). The latter term is a func- tion of the ac voltage that is applied between the sample and the tip: V s ( t )= V s,0 cos ω s t . It should be noted that the ac bias voltage is applied from the sample to the tip, meaning that the sample voltage sign is used as the bias voltage sign herein. The terms Δ f ( t ) and Δ C s ( t ) are related as (Eq 1) Thus, the demodulated FM signal generated by the SNDMprobe in conjunctionwith a lock-in amplifier and an FM demodulator is directly proportional to Δ C s ( t ). [14] That is, the SNDM technique is based onmonitoring variations in local capacitance resulting from modulation of the depletion layer below the tip in response to the applied voltage. This signal is related to the frequency shift, which in turn is translated to give a voltage signal via the FM demodulator. The lowest capacitance that canbedetected using the SNDM technique is 2×10 -22 F/√Hz, meaning that the carrier concentrations that SNDM can assess in silicon (Si) samples spans six orders of magnitude, and even an exceptionally low carrier concentration of 5×10 13 cm -3 can be detected with a good signal-to-noise (S/N) ratio. [12] The exceptional sensitivity of this techniquewas dem- onstrated bymeasuring SNDMsignals in the vicinity of the channel and floating gate of a 3D flash memory device. These trials employed a standard analog SNDM device incorporating an extremely sharp diamond tip. The use of this tip together with the very high sensitivity of SNDM to capacitance variations allowed the assessment of dopant impurities on the nanometer scale. Figure 2 presents an image obtained from the memory cell based on the d C /d V signal generated using SNDM. In this image, the dark and bright regions respec- tively correspond to negative (d C /d V < 0) and positive (d C /d V > 0) signal intensities and are associated with n- and p-type conduction, respectively. In this assessment, an exceptionally fine tip having a radius of less than 5 nm was employed to mitigate issues related to the shunting Fig. 1 Diagram showing the basic apparatus and principles of SNDM.