Revealing interfacial widths and line-edge roughnesses in cylinder-forming block copolymers using low keV analytical transmission electron microscopy

Abstract text (incl. figure legends and references)

Lithography based on the self-assembly (SA) of block copolymers (BCP) is a competitive alternative to extreme UV lithography applied in semiconductor industries, since it allows to create regular arrays of motives with sub-ten nanometre feature size economically and on large areas [1]. During BCP-SA two blocks of immiscible polymer species, A and B, which are covalently bond to each other in a molecular chain, undergo a microphase separation. Depending on the BCP chain length ratio, this can result in various motives, e.g. hexagonally arranged cylinders of block A in a matrix of block B. For further lithography processes one of the blocks is selectively removed. Since the features after processing are a direct replica of their lithographic mask, the abruptness of the microphase interfaces (interfacial width) in the phase separated BCP film [2] and the line edge roughness (LER) of the resulting mask are crucial but difficult to measure. Previous studies have employed analytical techniques that either only allow to analyse the surface of the BCP mask or average the properties over a large area or provide comparatively low spatial resolution. However, future progress in BCP lithography requires high-resolution information on the internal interfaces of microphases at or below the single-nanometre scale.

Here, we investigate the quality of the BCP masks by employing advanced analytical (scanning) transmission electron microscopy ((S)TEM) at 60 kV in combination with XPS and PM-IRRAS to derive information about the size of the polymer domains, the interfacial width and LER, as well as the chemical changes occurring upon mask development using different polymer removal techniques.

BCP lithography masks are made from cylinder forming BCP consisting of the two polymer species polystyrene (PS) and polymethyl methacrylate (PMMA). Membranes of as-phase-separated as well as developed BCP films are studied, the latter after either wet chemical UV/acetic acid or Ar/O2 plasma treatment. Films are fabricated on a sacrificial SiO₂ substrate which for TEM preparation is removed by HF etching, followed by skimming of the BCP membrane with a Quantifoil TEM grid. Domain sizes, interfacial widths and LERs are calculated from high-resolution ADF STEM images using a self-written MATLAB application which is based on the analysis of angularly resolved radial intensity profiles of individual pores. This allows to monitor the LER of cylindrical mask openings at unprecedented, sub-nm resolution in different stages of the process chain and to detect the presence of pockets of the opposite polymer species protruding through the PS-PMMA interfaces. In addition, energy-filtered TEM element and thickness maps are acquired to analyse changes in chemistry and thickness of the BCP masks and it is shown that Ar/O₂ plasma treatment, while leading to smoother interfaces, also results in oxidised polymer surfaces [3].

[1] K. Brassat, J.K.N. Lindner, Advanced Materials Interfaces 7.5 (2020): 1901565.

[2] J. Bürger et al., Nanomaterials 10.1 (2020): 141.

[3] J. Bürger et al., Advanced Materials Interfaces (2022), accepted.