Precision Fabrication of PDMS Nanofluidic Chips via AFM Tip‑Based Nanomilling

Background

Micro‑ and nanofluidic platforms are pivotal for DNA analysis, cell sorting, protein studies, food safety, and environmental monitoring. As the demand for sub‑100‑nm channels grows—critical for virus detection, nanoparticle manipulation and ion transport studies—accurate control over nanochannel dimensions becomes essential. Traditional fabrication routes such as reactive ion etching, photolithography, FIB, EBL, IL, nanoimprinting, hot embossing, and sacrificial molding each have drawbacks: limited resolution, high cost, or complex mask changes. Ultra‑precision machining methods, especially AFM tip‑based nanomilling, overcome these limitations by offering sub‑nanometer accuracy, high fabrication speed, and minimal environmental requirements.

AFM nanomilling leverages a piezoelectric actuator to drive the tip in a circular trajectory, enabling precise control over the cutting depth and width. Previous studies have demonstrated nanomilling on PMMA and PC, yet their application to PDMS nanofluidic devices remains underexplored. PDMS, with its excellent elastomeric properties and widespread use in microfluidics, is ideal for creating reusable, disposable nanofluidic chips. This work fills the gap by integrating AFM nanomilling with PDMS fabrication to produce controllable nanochannels and evaluating their electrical performance.

Methods

Nanomilling System Setup

The system combines a commercial AFM (Dimension Icon, Bruker) with a piezoelectric actuator (P‑122.01, PI). The actuator’s 1 µm ±‑travel is driven by sinusoidal signals (30–150 V, 100 Hz or 1500 Hz) from a Tektronix AFG1022 amplifier. A 100 nm thick diamond‑coated, 10 nm radius tip (DT‑NCLR, Nanosensors) performs the milling, while a silicon cantilever (68 N/m) records the normal load via a PSD. The PC substrate (15 × 12 × 1 mm, Mw = 35 000) is mounted on an epoxy holder.

a Schematic of the nanomilling setup. b SEM image of the diamond‑coated AFM tip.

Fabrication of Nanochannel and Microchannel Molds

PC nanochannel molds are milled by the AFM system. The tip traces a circular path by driving the actuator at 30–150 V and 90° phase‑shifted x‑ and y‑axis signals. Two drive frequencies (100 Hz, 1500 Hz) and a feed rate of 10 nm (1 µm/s or 15 µm/s) create 80‑µm long nanochannels. Normal loads (17 µN, 25 µN) are set via PSD setpoints, with the tip oriented forward to deposit pile‑ups on both sides and prevent leakage. Single‑scratching controls are also performed for comparison (normal loads 25–58 µN).

Microchannels are fabricated on a Si wafer using SU‑8 photolithography: a 30 µm wide, 50 µm center‑to‑center U‑shaped channel is formed by spin‑coating SU‑82015, UV exposure, and development. The microchannel mold (Fig. 2a4) is then cast in PDMS.

Flowchart of chip fabrication: (a1)–(a6) microchannel steps; (b1)–(b6) nanochannel steps; (c) final bonded chip.

Transfer Printing of Microchannels and Nanochannels

PDMS (Sylgard 184) is cast onto the microchannel mold (ratio 10:1) and cured at 80 °C for 4 h. The resulting replica is peeled and used as the microchannel substrate. For nanochannels, the PC mold is first cast with a pre‑mix ratio (A‑PDMS) of 9:1, 7:1 or 5:1. The cured replica is then pressed against a second PDMS slab (ratio 10:1, 9:1 or 8:1) to transfer the nano‑features. Two‑stage transfer preserves the sub‑100‑nm dimensions while reducing mold damage. Vacuum degassing eliminates bubbles, and a final 95 °C bonding step seals the chip.

Chip Bonding

Oxygen plasma (32 s, 81 W, 1.5 mbar) activates the PDMS surfaces. A custom alignment system—comprising a monocular microscope, precision stage, and holder—ensures precise registration of micro‑ and nano‑channel layers. The bonded assembly is annealed at 95 °C for 20 min, yielding an enclosed, leak‑free device.

a Alignment system. b Final nanofluidic chip.

Results and Discussion

Piezoelectric Actuator Trajectory

Actuator amplitude scales linearly with drive voltage, and higher frequency (1500 Hz) yields larger amplitude. Nanochannel widths ranged from 350 nm to 690 nm, confirming precise dimensional control.

Nanochannel Molds on PC

Single‑scratching shows a monotonic increase in width and depth with higher normal load (Fig. 4a). Nanomilling exhibits a non‑linear relationship: depth initially rises then falls at 100 Hz due to elastic recovery, while at 1500 Hz depth increases sharply as cutting speed surpasses the material’s chip‑forming threshold (Fig. 4d). Widths consistently grow with voltage, with 1500 Hz producing wider channels than 100 Hz for identical loads (Fig. 4c). AFM images (Fig. 5) illustrate symmetric pile‑ups in single‑scratching and asymmetric pile‑ups in nanomilling, consistent with the tip’s circular motion.

First Transfer of Nanochannel Molds

Walls formed from single‑scratching (nanochannel I) and nanomilling (nanochannels II & III) are transferred using PDMS ratios 5:1, 7:1, and 9:1. Walls retain the original depth but widen due to PDMS compliance; the 5:1 ratio yields the widest walls (Fig. 7). This trend reflects the reduced elastic recovery of stiffer PDMS blends.

Second Transfer of Nanochannel Molds

Using the 5:1 wall as the master, a second transfer with PDMS ratios 10:1, 9:1, 8:1 produces final nanochannels (A, B, C) with depths 120 nm, 80 nm, and 60 nm and widths 690 nm, 680 nm, 500 nm, respectively (Fig. 8). Higher stiffness (10:1) yields deeper, wider channels, while softer blends (8:1) recover more, narrowing the features.

Electrical Characterization

Each chip contains four nanochannels and a 1 mM KCl electrolyte. Current–voltage traces (Fig. 9) show that microchannel conductance is identical across chips, while nanochannel conductance varies with geometry. Chip A (60 × 500 nm) exhibits the highest current due to the largest electric double‑layer overlap, whereas chip C (120 × 690 nm) shows the lowest. Chip B demonstrates ohmic‑limiting‑overlimiting behavior at higher fields, indicative of ion transport saturation. These results confirm that the fabricated nanochannels are functional and that dimensional control directly influences device performance.

Conclusions

We have established a scalable, low‑cost method for fabricating sub‑100‑nm PDMS nanofluidic chips using AFM tip‑based nanomilling. The process integrates precision milling, UV lithography, two‑stage PDMS transfer, and plasma bonding, yielding multi‑channel devices with tunable dimensions. Electrical tests validate the functionality of the nanochannels and demonstrate the impact of geometry on ionic conductance. Although the current AFM setup limits channel length to ~80 µm, the approach offers a practical route to high‑performance nanofluidic platforms for biosensing and analytical applications.