Quantifying Field-Induced Contrast Effects in Photoelectron Emission Microscopy
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Quantifying Field-Induced Contrast Effects in Photoelectron Emission Microscopy K. Siegrist1, V.W. Ballarotto2 and E.D. Williams2, 3 National Institute for Standards and Technology, Gaithersburg, MD 20899 2 Laboratory for Physical Sciences, 8050 Greenmead Dr, College Park, MD 20740 3 Dept. of Physics, University of Maryland, College Park, MD 20742 1
ABSTRACT Samples consisting of electrically isolated titanium lines fabricated on a titanium surface were used to quantify voltage-induced contrast effects in photoelectron emission microscopy (PEEM). Induced contrast effects were observed to extend 6 µm for a -5 V bias applied to a 303 nm tall raised line. We therefore explored, via numerical calculation, the spatial extent of the perturbation to the PEEM accelerating field caused by the bias applied across the step height. The intensity full width at half minimum agreed well with the calculated width defined by the 10% level of lateral field strength. For a line 550 nm tall, a correspondence was found for a calculated width defined by a 5% lateral field strength. It was observed that neighboring structures a few µm away affected the image contrast, for sufficiently strong applied bias. This suggests that effects can easily be induced at distances of 0.5 µm for modest applied voltages, as has been previously observed for structures buried under oxide layers 0.5 µm thick [1]. INTRODUCTION Photoelectron emission microscopy (PEEM) is sensitive to near-surface voltage gradients because of the extremely low initial energy of photoemitted electrons (< 1eV) that form the image [2,3]. The presence of voltage gradients at the surface can alter the imaging conditions enough that dramatic contrast can result. These lateral surface fields can arise simply from surface topography [4,5] or in combination with an externally applied bias, making it difficult to discern the source of the contrast. Thus, it is necessary to quantify this field-induced effect in a controlled manner to establish a basis for quantitative image interpretation. To generate voltage gradients at the sample surface under controlled conditions, we have fabricated layered device structures where the top metal layer is electrically isolated from the bottom metal layer. The spatial extent and magnitude of the surface field effect can then be investigated systematically by controlling the applied bias voltage on structures of varying height, and at varying distance from a neighboring structure. In this paper, we report on a quantitative investigation of field-induce contrast generated on well-characterized Ti/SiO2/Ti structures. EXPERIMENTAL DETAILS Electrically isolated titanium lines were fabricated by successive e-beam evaporation of Ti, SiO2 and Ti on clean 3 inch Si wafers. The tri-layered wafers were then patterned via photolithography and etched back to the bottom Ti layer, leaving sets of isolated raised lines which could be addressed with a different bias voltage. The bottom Ti layer was at least 300 nm thick while thickness of the intervening oxide
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