Abstract
Colloidal nanosphere monolayers—used as a lithography mask for site-controlled material deposition or removal—offer the possibility of cost-effective patterning of large surface areas. In the present study, an automated analysis of scanning electron microscopy (SEM) images is described, which enables the recognition of the individual nanospheres in densely packed monolayers in order to perform a statistical quantification of the sphere size, mask opening size, and sphere-sphere separation distributions. Search algorithms based on Fourier transformation, cross-correlation, multiple-angle intensity profiling, and sphere edge point detection techniques allow for a sphere detection efficiency of at least 99.8%, even in the case of considerable sphere size variations. While the sphere positions and diameters are determined by fitting circles to the spheres edge points, the openings between sphere triples are detected by intensity thresholding. For the analyzed polystyrene sphere monolayers with sphere sizes between 220 and 600 nm and a diameter spread of around 3% coefficients of variation of 6.8–8.1% for the opening size are found. By correlating the mentioned size distributions, it is shown that, in this case, the dominant contribution to the opening size variation stems from nanometer-scale positional variations of the spheres.
Highlights
Nanosphere lithography (NSL) provides a cost-effective method to fabricate periodic nanopatterns on large surface areas (Hulteen & Van Duyne, 1995; Boneberg et al, 1997; Haginoya et al, 1997; Burmeister et al, 1999)
All three sphere sizes have a coefficient of variation (CV) value of ≤3% specified by the supplier, the results show that the sphere diameter distribution width increases from 2.2 to 3.2% when decreasing the sphere size from 600 to 220 nm
The program enables a correlated analysis of these quantities, that is, the evaluation of interstice size distributions as a function of sphere size and sphere-sphere separation
Summary
Nanosphere lithography (NSL) provides a cost-effective method to fabricate periodic nanopatterns on large surface areas (Hulteen & Van Duyne, 1995; Boneberg et al, 1997; Haginoya et al, 1997; Burmeister et al, 1999). It has been demonstrated that NSL masks can be fabricated in roll-to-roll processes, opening the path to industrial scale applications (Chen et al, 2020). A narrow size distribution of the nanoobjects is desired in order to realize devices with welldefined emission or absorption properties (Haynes & Van Duyne, 2001; Qian et al, 2008). It is important to determine the factors governing the size distribution of the nanoscale openings in the sphere monolayers or double layers, which serve as lithographic masks. In case of close-packed monolayer masks, the mask openings, referred to as interstices, are defined by sphere triples
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