Postdeposition UV-Ozone Treatment: An Enabling Technique to Enhance the Direct Adhesion of Gold Thin Films to Oxidized Silicon

Abstract

We found that continuous films of gold (Au) on oxidized silicon (SiO₂) substrates, upon treatment with ultraviolet (UV)-ozone, exhibit strong adhesion to the SiO₂ support. Importantly, the enhancement is independent of micro- or nanostructuring of such nanometer-thick films. Deposition of a second Au layer on top of the pretreated Au layer makes the adhesion stable for at least 5 months in environmental air. Using this treatment method enables us to large-scale fabricate various SiO₂-supported Au structures at various thicknesses with dimensions spanning from a few hundreds of nanometers to a few micrometers, without the use of additional adhesion layers. We explain the observed adhesion improvement as polarization-induced increased strength of Au^δ-Si^δ+ bonds at the Au-SiO₂ interface due to the formation of a gold oxide monolayer on the Au surface by the UV-ozone treatment. Our simple and enabling method thus provides opportunities for patterning Au micro/nanostructures on SiO₂ substrates without an intermediate metallic adhesion layer, which is critical for biosensing and nanophotonic applications.

Keywords: UV-ozone treatment; adhesion enhancement; gold oxide; gold thin films; oxidized silicon substrates.

Figure 1

Schematic diagram of (a) the fabrication process for patterning periodic Au microdots using a shadow mask and the procedure for quantifying the adhesion strength of (b) Au microdots treated with UV-ozone and (c) Au microdots without the UV-ozone treatment.

Figure 2

Fraction of remaining Au area versus the diameter or area-to-perimeter ratio of Au microdots after the tape test for cases 1–4 (see Table 1). Case 1: ∼13 nm thick Au microdots without the UV-ozone treatment. Case 2: ∼13 nm thick Au microdots treated with UV-ozone for 5 min. Case 3: ∼13 nm thick Au microdots with the 5 min UV-ozone treatment and immersion in DI water for 10 min. Case 4: ∼24 nm thick Au microdots treated with UV-ozone for 5 min.

Figure 3

Fabrication process for patterning well-adhering Au nanostructures supported directly on the surface of oxidized Si wafers, without additional adhesion layers.

Figure 4

Top-view optical images (scale bar: 50 μm) of periodic (a) Au microdots (∼2 μm diameter), (b) Au microlines (∼2 μm width) with the inserted top-view HR-SEM images (scale bar: 5 μm), and top-view HR-SEM images (scale bar: 5 μm) of periodic (c) Au nanodots (∼346 nm diameter), (d) Au nanolines (∼286 nm width) with the inset close-up HR-SEM images (scale bar: 1 μm).

Figure 5

(a) Top-view HR-SEM images (scale bar: 1 μm) of periodic Au nanodots and lines supported directly on the surface of different substrates. (b) AFM images (1 × 1 μm²) of the surface of these substrates with their corresponding surface roughness (Ra).

Figure 6

Schematic diagrams of the mechanism for the adhesion enhancement of (a) ∼13 nm thick Au microdots and (b) ∼24 nm thick Au microdots with the oxidized Si substrate, using the UV-ozone treatment.

Figure 7

Fraction of remaining Au area versus the diameter or area-to-perimeter ratio of Au microdots after the tape test for cases 5–8 (see Table 1). Case 5: ∼13 nm thick Au microdots with the 5 min UV-ozone treatment and immersion in ethanol for 10 min. Case 6: re-treating the sample of case 5 with UV-ozone for 5 min. Case 7: ∼13 nm thick Au microdots treated with UV-ozone for 5 min and stored for 2 weeks. Case 8: stacked-layer Au microdots sonicated at room temperature in ethanol for 2 h.