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J. Eur. Opt. Society-Rapid Publ. 22, 33( 2026) Numerous studies have been reported on the optical and structural properties of ALD-grown HfO 2, demonstrating its applicability in optical thin films across a wide spectra range, for instance as anti-reflection coatings at 266 nm, 355 nm, 532 nm and / or 1064 nm [ 31, 33 ], or as high-reflective mirrors at 355 nm and 532 nm [ 32 ]. Nevertheless, comprehensive experimental data on its optical constants over an extended spectral range, particularly toward the vacuum ultraviolet( VUV) remain comparatively scarce.

The present work aims to expand the accessible optical database by providing detailed optical properties of PEALD HfO 2 thin films down to a wavelength of 140 nm. By combining multiple complementary measurement techniques [ 34, 35 ], we extract reliable optical constants from the visible to the VUV range. The resulting dataset provides a solid foundation for the optical design and simulation of HfO 2-based coatings and meta-optical devices.

2 Thin film deposition

Hafnium oxide thin films are commonly deposited by atomic layer deposition using a variety of hafnium precursor chemistries in combination with either thermal or plasma-assisted oxidation steps. In thermal ALD, both inorganic precursors such as hafnium tetrachloride( HfCl 4)[ 36 ] and metal-organic precursors tetrakis( dimethylamido) hafnium( TDMAH), tetrakis( ethylmethylamido) hafnium( TEMAH) [ 37 ], or b-diketonate-based [ 38, 39 ] compounds have been widely employed together with water as the oxidant. While HfCl 4-based processes typically require higher deposition temperatures and may lead to halogen-related impurities, metal-organic precursors enable lower-temperature growth and improved process flexibility. In plasmaenhanced ALD( PEALD), the molecular oxidant is replaced by an oxygen plasma, which enhances ligand removal and film densification [ 40 ]. PEALD processes using TDMAH or TEMAH have been shown to produce dense, stoichiometric HfO 2 films with reduced impurity content at comparatively low substrate temperatures [ 30 – 32 ], making them especially attractive for optical coatings and nanophotonic applications.

Depending on the chosen precursor chemistry and oxidation scheme, the resulting films may exhibit significant differences in growth per cycle, density, impurity concentration, and ultimately optical material properties. The HfO 2 thin films investigated in this study were deposited at 100 ° C using plasma-enhanced atomic layer deposition in an OpAL reactor( Oxford Instruments Plasma Technology). Tetrakis( dimethylamido) hafnium( TDMAH, Hf [ N( CH 3) 2 ] 4) served as the metal precursor, while an oxygen plasma was employed as the oxidizing reactant. Each ALD cycle consisted of a TDMAH precursor pulse( 0.4 s), followed by a purge( 8 s) and pump-down( 2 s) sequence to remove excess precursor and by-products, a gas stabilization period( 3 s) followed by an oxygen plasma exposure( 10 s) and a final purge( 10 s). The details are summarized in Table 1.

The deposition was performed on fused silica substrates in order to enable transmission measurements over a wide

Table 1. ALD parameter for Hafnia deposition using TDMAH and oxygen plasma in an OPAL-tool.

spectral range. To ensure sufficient optical sensitivity in ellipsometry measurements while maintaining negligible interference from the substrate backside, a total film thickness of approximately 200 nm was targeted. The deposition process yielded smooth, homogeneous films with excellent macroscopic uniformity.

3 Optical characterization methods

The optical properties of the HfO 2 films were characterized using a combination of spectroscopic ellipsometry and spectrophotometry to cover a broad spectral range. Vacuum ultraviolet and ultraviolet ellipsometric measurements were performed using a Jobin Yvon UVISEL2 ellipsometer at photon energies ranging from 1.5 to 8.7 eV at a fixed angle of incidence of 70 °. Additional ellipsometric data in the range from 0.6 to 6.5 eV were acquired with a Jobin Yvon UVISEL ellipsometer using multiple angles of incidence between 55 ° and 75 °. Infrared ellipsometry measurements covering the range from 300 to 6500 cm-1 were conducted using a Woollam IR-VASE ellipsometer, again employing variable angles of incidence. Complementary transmission and reflection spectra were recorded using a Perkin Elmer Lambda 1050 spectrophotometer and a Bruker Vertex 80v Fourier-transform infrared spectrometer. For reflectance measurements, illumination from both the film and substrate side was used to enhance sensitivity to interface effects. The experimental data were analyzed using the universal dispersion model implemented in the newAD2 software package [ 34 ]. All relevant parameters, including film thickness, dispersion coefficients, and interface roughness, were treated as free fitting parameters. This approach ensured a consistent description of the optical response across all spectral regions and measurement techniques.

4 Results and discussion

The combined optical analysis yielded film thickness values between 217 and 221 nm, depending on the specific measurement configuration and spot size. This small variation indicates a high degree of thickness uniformity across the