Electrochemical Nanolithography on Silicon: An Easy and Scalable Method to Control Pore Formation at the Nanoscale.

Lithography on a sub-100 nm scale is beyond the diffraction limits of standard optical lithography but is nonetheless a key step in many modern technological applications. At this length scale, there are several possible approaches that require either the preliminary surface deposition of materials or the use of expensive and time-consuming techniques. In our approach, we demonstrate a simple process, easily scalable to large surfaces, where the surface patterning that controls pore formation on highly doped silicon wafers is obtained by an electrochemical process. This method joins the advantages of the low cost of an electrochemical approach with its immediate scalability to large wafers.

Toward Lateral Length Standards at the Nanoscale Based on Diblock Copolymers.

The self-assembly (SA) of diblock copolymers (DBCs) based on phase separation into different morphologies of small and high-density features is widely investigated as a patterning and nanofabrication technique. The integration of conventional top-down approaches with the bottom-up SA of DBCs enables the possibility to address the gap in nanostructured lateral length standards for nanometrology, consequently supporting miniaturization processes in device fabrication. On this topic, we studied the pattern characteristic dimensions (i.e., center-to-center distance L0 and diameter D) of a cylinder-forming polystyrene-b-poly( methyl methacrylate) PS-b-PMMA (54 kg mol-1, styrene fraction 70%) DBC when confined within periodic SiO2 trenches of different widths (W, ranging between 75 and 600 nm) and fixed length (l, 5.7 µm). The characteristic dimensions of the PMMA cylinder structure in the confined configurations were compared with those obtained on a flat surface (L0 = 27.8 ± 0.5 nm, D = 13.0 ± 1.0 nm). The analysis of D as a function of W evolution indicates that the eccentricity of the PMMA cylinders decreases as a result of the deformation of the cylinder in the direction perpendicular to the trenches. The center-to-center distance in the direction parallel to the long side of the trenches (L0l) is equal to L0 measured on the flat surface, whereas the one along the short side (L0w) is subjected to an appreciable variation (ΔL0w = 5 nm) depending on W. The possibility of finely tuning L0w maintaining constant L0l paves the way to the realization of a DBC-based transfer standard for lateral length calibration with periods in the critical range between 20 and 50 nm wherein no commercial transfer standards are available. A prototype transfer standard with cylindrical holes was used to calibrate the linear correction factor c(Δx')xx' of an atomic force microscope for a scan length of Δx' = 1 µm. The relative standard uncertainty of the correction factor was only 1.3%, and the second-order nonlinear correction was found to be significant.

Thickness and Microdomain Orientation of Asymmetric PS-b-PMMA Block Copolymer Films Inside Periodic Gratings.

The ordering process of asymmetric PS-b-PMMA block copolymers (BCPs) is investigated on flat SiO2 surfaces and on topographically patterned substrates. The topographic patterns consist of periodic gratings of 10 trenches defined by conventional top-down approaches and subsequently neutralized using a P(S-r-MMA) random copolymer (RCP). When the ordering process is accomplished on a flat surface at a temperature ranging between 180 and 230 °C, cylindrical microdomains perpendicularly oriented with respect to the substrate are observed irrespective of annealing temperature. In contrast, when the ordering process occurs on topographically patterned substrates, different phenomena have to be considered. The simultaneous effect of the flow around the gratings and the BCP flux from the zone located between adjacent trenches (mesa) into the inner part of the trenches results in significant thickness variations of the confined BCP film. Therefore, the amount of BCP inside the trenches depends on the width of the mesa region, which acts as a BCP reservoir. Moreover, within each trench group, the BCP thickness progressively decreases from the external to the central trenches composing the periodic grating. The thickness variation of the BCP film within the trenches strongly affects the ordering process, ultimately leading to different orientations of the microdomains within the trenches. In particular, when the annealing temperature is 190 °C a precise confinement of the BCP within the trenches featuring a perpendicular cylinder morphology is observed. At higher temperatures, mixed or parallel orientations of the microdomains are obtained depending on the width of the trenches in the periodic grating.