Characterization of ion track-etched conical nanopores in thermal and PECVD SiO2 using small angle X-ray scattering

Abstract

Conical nanopores in amorphous SiO₂ thin films fabricated using the ion track etching technique show promising potential for filtration, sensing, and nanofluidic applications. The characterization of the pore morphology and size distribution, along with its dependence on the material properties and fabrication parameters, is crucial to designing nanopore systems for specific applications. Here, we present a comprehensive study of track-etched nanopores in thermal and plasma-enhanced chemical vapor-deposited (PECVD) SiO₂ using synchrotron-based small-angle X-ray scattering (SAXS). The nanopores were fabricated by irradiating the samples with 89 MeV, 185 MeV, and 1.6 GeV Au ions, followed by hydrofluoric acid etching. We present a new approach for analyzing the complex highly anisotropic two-dimensional SAXS patterns of the pores by reducing the analysis to two orthogonal one-dimensional slices of the data. The simultaneous fit of the data enables an accurate determination of the pore geometry and size distribution. The analysis reveals substantial differences between the nanopores in thermal and PECVD SiO₂. The track-to-bulk etching rate ratio is significantly different for the two materials, producing nanopores with cone angles that differ by almost a factor of two. Furthermore, thermal SiO₂ exhibits an exceptionally narrow size distribution of only 2-4%, while PECVD SiO₂ shows a higher variation ranging from 8% to 18%. The impact of different ion energies on the size of the nanopores was also investigated for pores in PECVD SiO₂ and shows only negligible influence. These findings provide crucial insights for the controlled fabrication of conical nanopores in different materials, which is essential for optimizing membrane performance in applications that require precise pore geometry.

Keywords: SiO2; etched ion tracks; small angle X-ray scattering (SAXS); swift heavy ion irradiation; track-etched nanopores.

Figure 1

Plan-view (a, b) and cross-sectional (c, d) scanning electron microscopy images of nanopores in thermal (a, c) and PECVD (b, d) SiO₂. The thermal and PECVD SiO₂ thin films were irradiated with 1.6 GeV Au ions and subsequently etched in 3% HF for 8.5 min and 6 min respectively. The irradiation fluence was 5 × 10⁸ ions/cm². The fluence was verified by counting nanopores in multiple SEM images, closely matching the expected values. The top-view images (a, b) highlight the circular pore openings, while the cross-sectional views (c, d) reveal conical pore geometry. The cone-angle (β) of the conical pores in thermal SiO₂ (c) is approximately 1.8 times less than that in PECVD SiO₂ (d).

Figure 2

Representative two-dimensional scattering pattern (left) from conical nanopores in thermal SiO₂ illustrating the regions used for horizontal (blue) and vertical (orange) cuts. The sample was irradiated with 1.6 GeV Au ions and etched for 15 mins in 3% HF. Measurements were performed with a tilt angle of the surface normal of ≈20° with respect to the X-ray beam. The corresponding one-dimensional intensity profiles (right) are shown as a function of the magnitude of the scattering vector .

Figure 3

Two-dimensional small-angle X-ray scattering (SAXS) patterns of conical nanopores in thermal (a) and PECVD (b) SiO₂ produced from irradiation of thin film samples with 1.6 GeV Au ions and etching with 3% HF, shown using the same intensity-contrast scale. The arrows in (a) highlight secondary scattering features that are more pronounced in thermal SiO₂ than in the PECVD sample, consistent with a higher polydispersity of pore sizes in the latter.

Figure 4

One-dimensional SAXS profiles of ion-irradiated thermal (a, b) and PECVD (c, d) SiO₂, extracted along the horizontal (a, c) and vertical (b, d) directions. Each pattern corresponds to a different tilt angle (labels in degrees), offset vertically for clarity. The solid lines denote model fits based on conical and core-transition models.

Figure 5

Half cone angle (a), percentage polydispersity (b), and nanopore radius (c) as functions of etching time for conical nanopores in thermal and PECVD SiO₂, irradiated with Au ions at different energies. The shaded regions in (a) and (b) highlight approximate parameter ranges for thermal versus PECVD SiO₂. In (c), solid lines represent linear fits to the data.