High‑Throughput Nanopit Fabrication on Polymer Thin Films via AFM Dynamic Plowing Lithography

Background

Nanotechnology’s rapid growth has spurred interest in nanoscale structures for applications ranging from nanoelectromechanical systems to quantum dot fabrication. While conventional lithography techniques (focused ion beam, electron beam, nanoimprint) provide precise control, they are limited by high cost, strict environmental conditions, and slow throughput. In contrast, AFM tip‑based nanolithography offers flexibility and high resolution but has traditionally suffered from low productivity.

Dynamic plowing lithography (DPL) exploits the intermittent contact inherent to tapping‑mode AFM to reduce tip wear and increase material removal rates. By driving the cantilever near its resonance frequency, the tip repeatedly penetrates the sample surface, enabling rapid nanoscale patterning. Previous work has shown DPL’s suitability for thin‑film materials, but its full potential for large‑area, high‑density pit arrays has not been explored.

This study introduces a fast‑scan nanolithography (FSN) approach that harnesses DPL on a commercial AFM platform, achieving high‑throughput pit fabrication on PMMA thin films while preserving tip integrity.

Methods

PMMA (Mw = 120,000) was dissolved in chlorobenzene to 1.25 wt % and spin‑coated on cleaned Si wafers at 6,000 rpm, yielding a film of ~30 nm thickness. Samples were baked at 125 °C for 30 min to remove residual solvent.

All experiments employed a Bruker Dimension Icon AFM with a silicon cantilever (spring constant = 42 N m⁻¹, f₀ ≈ 320 kHz). The tip was diamond‑like‑carbon coated to extend life. Imaging was performed in tapping mode (scan rate = 1 Hz, 256 lines). During DPL, the drive amplitude was raised to V_w (writing) to enhance tip–sample interaction, then returned to V_r (reading) for imaging.

The Nanoman module orchestrated the tip trajectory, enabling precise control of scratching velocity and direction. All experiments were conducted at room temperature.

Results and Discussion

Scratching velocities ranging from 0.1 to 1,000 µm s⁻¹ were tested. At high velocities (≈ 200 µm s⁻¹), isolated pits formed; at medium speeds (≈ 100 µm s⁻¹) pits overlapped, and at low speeds (≈ 50 µm s⁻¹) nanogrooves emerged (Fig. 1b–d). The pit spacing is governed by the scratching speed, as the tip–surface interaction repeats every oscillation cycle (v / f).

Four scratching directions were examined (Fig. 2a): V₁/V₃ along the cantilever’s long axis and V₂/V₄ perpendicular. AFM images (Fig. 3) show that at 200 µm s⁻¹ continuous pits appear regardless of direction, whereas at 100 µm s⁻¹ only occasional deep pits are observed, and at 50 µm s⁻¹ well‑defined grooves with flat bottoms and side pile‑ups are produced.

Depth measurements (Fig. 4) reveal a monotonic decrease with increasing velocity. For velocities below 5 µm s⁻¹, depths are direction‑independent (~ 2 nm), but V₃ shows larger variability due to tip asymmetry. The theoretical spacing between successive tip penetrations (D = v / f) ranges from 0.52 to 2.33 nm, enabling 4,800–5,800 pits per second for velocities of 200–900 µm s⁻¹ (Fig. 5).

The pit‑formation mechanism comprises three stages: (a) elastic deformation where the tip slides with minimal material removal; (b) plastic deformation after ~ 40 penetrations, breaking PMMA chains; (c) pile‑up climbing, where accumulated material is swept away as the tip slides over the mound. The interplay of normal and tangential forces, described by equations (5)–(8), governs the transition between stages.

Using V₁/V₃ directions and a perpendicular feed, arrays of pits were fabricated over 5 µm×5 µm areas. At 400 µm s⁻¹, pits are evenly spaced with ~ 2.5 nm depth (Fig. 7). At 200 µm s⁻¹, pit spacing narrows to ~ 70 nm and shapes approach circular symmetry (Fig. 8a). At 900 µm s⁻¹, spacing expands to ~ 100 nm and pit morphology diverges between directions (Fig. 8b). Fast‑Fourier‑transform (FFT) analysis confirms the inverse relationship between pit density and scratching speed.

Conclusions

Dynamic plowing lithography in tapping mode offers a scalable route to dense nanopit arrays on polymer thin films. A critical scratching speed of 100 µm s⁻¹ distinguishes pit formation from groove creation. Above 5 µm s⁻¹, pit depth is largely independent of direction, while V₃ exhibits reduced depth due to tip asymmetry. By maintaining velocities between 200 and 900 µm s⁻¹, up to 5,800 pits can be generated per second. The process relies on repeated elastic, plastic, and pile‑up stages, requiring 65–80 tip penetrations per pit. The resulting arrays feature 5 µm dimensions, 70 nm spacing, and 2.5 nm depth, demonstrating the method’s suitability for high‑throughput nanofabrication.