Abstract
High-density ruthenium (Ru) thin films were deposited using Ru(EtCp)2 (bis(ethylcyclopentadienyl)ruthenium) and oxygen by thermal atomic layer deposition (ALD) and compared to magnetron sputtered (MS) Ru coatings. The ALD Ru film growth and surface roughness show a significant temperature dependence. At temperatures below 200 °C, no deposition was observed on silicon and fused silica substrates. With increasing deposition temperature, the nucleation of Ru starts and leads eventually to fully closed, polycrystalline coatings. The formation of blisters starts at temperatures above 275 °C because of poor adhesion properties, which results in a high surface roughness. The optimum deposition temperature is 250 °C in our tool and leads to rather smooth film surfaces, with roughness values of approximately 3 nm. The ALD Ru thin films have similar morphology compared with MS coatings, e.g., hexagonal polycrystalline structure and high density. Discrepancies of the optical properties can be explained by the higher roughness of ALD films compared to MS coatings. To use ALD Ru for optical applications at short wavelengths (λ = 2–50 nm), further improvement of their film quality is required.
Highlights
Ultrathin metal films are essential for numerous applications, especially in microelectronics [1], heterogeneous catalysis [2], soft X-ray optics, and sensing
The results obtained in this study indicate that further optimization of atomic layer deposition (ALD) Ru coatings for optical applications is essential to leverage on the ALD capability of conformal coatings on nanostructured substrates or on complex shaped optics
Ruthenium thin films grown by ALD have been evaluated for optical applications
Summary
Ultrathin metal films are essential for numerous applications, especially in microelectronics [1], heterogeneous catalysis [2], soft X-ray optics, and sensing. Smooth and high-density Ru thin films are a preferred solution, for example, as electrodes for dynamic random access memories (DRAM) [4,5,6,7], metal-oxide-semiconductor field-effect transistors (MOSFET) [8], metal-insulator-metal capacitors [9,10], and grazing-incidence soft X-ray mirrors [11]. Since conventional physical vapor deposition (PVD) technologies cannot realize conformal coatings on complex shaped substrates, ALD is being considered as a promising technology. High-efficiency metal wire polarizers for UV spectral range have already been realized based on frequency doubling technique with iridium (Ir) coatings by ALD [12,13].
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