Nanoscale space charge generation in local oxidation nanolithography

Abstract

We have measured the surface potential and the space charge generated during the first stages of atomic force microscopy field-induced oxidation. Space charge densities are about 10 17 cm −3 for oxidation times below 10 ms. In a dry atmosphere, the surface potential is negative. However, in humid air the surface potential could be either positive or negative. This effect is attributed to a screening effect of the water molecules. These results explain and support the use of local oxidation patterns as templates for building molecular architectures. They also establish the space charge build up as an intrinsic feature in local oxidation experiments. © 2010 American Institute of Physics. ͓doi:10.1063/1.3459976͔Atomic force microscopy ͑AFM͒ oxidation nanolithography is a robust and flexible tip-based nanofabrication method [1][2][3][4][5] that is used to fabricate high resolution patterns and nanoscale devices. Thus, templates to build molecular architectures, 6,7 resist masks, 8 nanomechanical resonators, 9,10 or several nanoelectronic devices and transistors [11][12][13][14] have been fabricated by local oxidation. In AFM oxidation, the tip is used as a cathode and the water meniscus formed between the tip and the surface is the source of the oxyanions species. 15 The strong localization of the electrical field lines near the tip apex and the lateral confinement of the oxyanions species within the liquid meniscus give rise to a nanometer-size oxide dot. [16][17][18][19][20] The kinetics of the oxidation process is influenced by the generation of a space charge. [21][22][23] The model by Dagata et al. 21,24 considers two competing mechanisms, a fast oxidation process that applies to the initial stages ͑below 1 s͒ and a slower, indirect process that applies for longer oxidation times and involves space charge. Dubois and Bubendorff's model is based on a charge trapping-detrapping mechanism. 23 Most of the AFM oxidation nanolithography applications involve oxidation times below 1 s. Recently, local oxide patterns have been used as high resolution templates for the growth of functional materials such as protein carriers 7 or single molecule magnets. 25 The underlying mechanism behind the organization of molecular architectures is based on the electrostatic interactions between the charged or polarized molecules and the space charges trapped inside the local oxides. However, there is no experimental evidence that supports the existence of space charges for short oxidation times. In addition, the sign of the charge has not been measured.Here, we perform Kelvin probe force microscopy ͑KPFM͒ experiments to determine the space charge sign and density. The measurements establish the presence of a space charge build up during the earlier stages of the oxidation process ͑below 10 ms͒. The sign of the apparent space charge depends on the relative humidity inside the chamber where the KPFM measurements are performed. In dry conditions ͑N 2 gas environment͒ the observed charge is always negative. However, in the presence of wat...

Single cycle and transient force measurements in dynamic atomic force microscopy

“…Again, a base excited cantilever is simulated and vibrated in the presence of dissipative hysteretic forces as modeled by (5) and (6) and with conservative forces (7) and (8). The assumption of base motion does not affect the generality of the formalism or the concepts developed in this work; modication would lead to a formalism for directly excited cantilevers.…”

“…Force (1) (black lines in Fig. 6a-c) includes short and long range hysteresis forces as modeled with (5) and (6). Force (2) (blue lines in Fig.…”

“…E dis > 0 provided d < a 0 . A second source of dissipation has been implemented employing (6). As discussed, while also hysteretic in nature, the force in (6) Then, since the ability of (2) to recover the forces is not affected by the choice of d on /d off distances, here, without loss of generality, we dene them to be in the long range.…”

“…This is particularly noticeable in the metastable regions such as the long range hysteretic force modeled by (6); note the errors in the area lying between the d on À d off distances there. The source of these errors can be appreciated when trying to employ 50 harmonics to reconstruct the force with the 2-mode model.…”

“…The capacity of the atomic force microscope AFM to image, 1,2 probe 3-5 and manipulate 6,7 matter is a testament to its versatility. The instrument is currently employed by researchers in elds ranging from biology 8,9 to semiconductor theory and devices 10 and in hybrid systems.…”