Nanoscale Chemical Imaging of Block Copolymers
Supramolecular self-assembly of block copolymer (BCP) films is a promising method to extend nanofabrication beyond the limits of optical lithography, readily producing periodic structures with periods of 10-100 nm, and to advance this class of materials there is a pressing need for an analytical technique with a high degree of chemical specificity and spatial resolution. Direct spatio-chemical imaging of the specific domains and defect structures in self-assembled BCP patterns will provide valuable information about the materials, process, and pattern quality. Current investigations in real space are based on electron microscopy (EM) techniques, atomic force microscopy and near-field optics, all with limitations. This set of images of a lamellar BCP, PS-b-P2VP with L0 = 45 nm, provides a demonstration of PiFM’s novel analytical and diagnostic capability on a more complex sample containing separate regions of unguided fingerprint patterns and parallel line patterns formed by DSA. It is common practice to selectively stain the P2VP domain in order to achieve sufficient EM contrast. However, such infiltration or staining techniques (or partial etching) may alter or distort domain shape and/or boundary profiles. With PiFM no additives or modification of the sample are required.
Lamellar region of PS-b-P2VP. PiFM imaging at (A) 1447 cm-1, (B) 1452 cm-1, (C) 1469 cm-1, (D) 1492 cm-1, and (E) 1589 cm-1. The PiFM point spectra taken from the locations shown in (E) were normalized against a Si background. The different spectral response (green curve) of one of the three nodules clearly visible in (A) suggests that a material other than PS and P2VP may be present.
Directed self-assembly (DSA) region of PS-b-P2VP. PiFM images at 1492 cm-1 (left) and 1589 cm-1 (right) are overlaid on the 3D topographical image. These images show that PS domains (blue region in the left image) lie on top (higher topography resulting from the XPS guiding lines) and in the valley (filled in via pitch multiplication) of the ridges and P2VP domains (blue region in the right image) lie on both sides of the ridges as designed.
One of the earliest hyPIR imaging performed with Vista-IR on PS-b-P2VP sample, featured as a supplemental video for our Science Advances paper. The combined image highlights P2VP, PS, and contaminant in blue, green, and red tints. In this hyPIR image, there is a slight shift in the wavenumber (~10 cm-1) due to a calibration error. While the topography is not clear, the phase image correlates well with the P2VP image.