Enhanced Surface Potential Detection using Frequency Modulation SKPM
Scanning Kelvin probe microscopy (SKPM) to measure work function and local electrical potential of polymer materials
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Scanning Kelvin Probe microscopy (SKPM) allows measuring work function and electrical potential distribution of various materials, to provide a better understanding of nanostructures and CMOS semiconductor devices to improve their performance.[1-2] SKPM has also been used to determine quantitative information such as the charge distribution in polymer materials.[3, 4] Compared to other techniques, SKPM is nondestructive and compatible to ambient conditions. Conventional Amplitude Modulation (AM) SKPM has already allowed remarkable enhancements in the field of device reliability, but it is limited by its signal-to-noise detection ability. For this scope Park Systems recently developed Frequency Modulation FM-SKPM. The results show that FM-SKPM is significantly more sensitive than AM-SKPM in measuring surface potential distribution.
A polymer-patterned array on a silicon substrate was analyzed using a Park NX10 AFM. Two separate scans were conducted to acquire AM-SKPM and FM-SKPM images of the same frame using the same scan parameters and conductive Mikromasch NSC36Cr-Au cantilever. In SKPM, electrostatic and van der Waals forces are present between the AC biased tip and the sample. Separate lock-in amplifiers in the Park NX10 electronics, named lock-in 1 and 2, allow the simultaneous acquisition of topography and EFM data. Lock-in 1 retrieves the topography by analyzing the tip motion caused by the van der Waals interactions, while lock-in 2 obtains electrical information by analyzing the electrostatic force caused by the AC bias applied to the tip. The frequency of the applied AC signal is set small enough (5 - 20 kHz) to avoid interference with the cantilever oscillation (70-330 kHz). In FM-SKPM setup, the NCM phase signal of Lock-in 1 is sent to Lock-in 2 to serve as a source for EFM operation through an additional BNC cable (Fig. 1). Furthermore, a separate DC bias is applied to the cantilever and controlled to create a feedback loop that zeros out the electrical oscillation between tip and sample caused by the AC bias applied to the cantilever. The value of this offsetting DC bias is a measure of the surface potential.
The topography data show that the array was successfully deposited on the substrate, while the surface potential data acquired in SKPM show the surface potential structure.
Data were analyzed using the Park Systems XEI software. The topography and AM-SKPM data in Fig. 2 were acquired simultaneously. The measured peak-to-valley height of the square-like structures was ≈ 150 nm. Both AM-SKPM and FM-SKPM data show a patterned array with an irregularity on the surface (pointed by a red arrow). In the SKPM images, the darker color of the squared features indicates a lower surface potential on the domains than on the substrate. By comparing the results acquired with AM and FM-SKPM, one can easily determine that FM-SKPM has better sensitivity in detecting surface potential variation. In fact, FM-SKPM provided higher resolution with sharper edges of the domains and detected smaller potential variations than AM-SKPM on the same irregular surface.
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1. Lan Fei (2018) Fundamentals of Kelvin Probe Force Microscopy and its applications in the characterization of solar cells. Doctoral Dissertation, University of Pittsburgh.
2. J. Pineda, et al., Electrical Characterization of Semiconductor Device Using SCM and KPFM Imaging. https://www.parksystems.com/index.php/applications/electrical-electronic...
3. J. Gonzalez, et al., Charge distribution from SKPM images, PCCP, Issue 40, 2017.
4. M. Ortuño, et al., Conducting polymers as electron glasses: surface charge domains and slow relaxation, Scientific Reports volume 6, Article number: 21647 (2016).