We present a comparison of the properties of the Mo/Si multilayers deposited by three techniques: secondary ion beam deposition (IBD), DC magnetron sputtering (MS), and biased-target ion beam deposition (BTIBD). Based on sputter simulations, we demonstrate that the energetics of the depositing adatoms and the intermixing depth depend strongly on the target/beam voltage, process pressure and target-to-substrate distance. XRR measurements indicate that the multilayers deposited by MS are much rougher than those deposited by IBD and BTIBD. SIMS and RBS measurements suggest that the Mo layers have low contamination of Fe and Al indicating insignificant beam overspray onto the target mounting material, as well as minimal Ar implantation. High-resolution TEM analysis shows intermixed layer thickness in the range of 0.3 to 1.9 nm and interfacial roughness in the range of 100 to 500 pm. Optical calculations suggest that the observed level of contamination is expected to have minimal impact on the EUV peak reflectivity. The intermixed layers could result in a 2-5% decrease in peak reflectivity. Further, the optical simulations predict that the 100-200pm range interfacial roughness in IBD and BTIBD cause a <0.5% reduction in peak reflectivity, whereas the 200-500pm range interfacial roughness in MS films is expected to compromise the peak reflectivity by up to 5%. This comparison of a variety of thin film techniques for the deposition of Mo and Si has allowed us to gain a deeper insight into the challenges faced during the fabrication of EUV mask blanks.
The aim of our work is to investigate deposition conditions to further optimize the reflectivity of Mo/Si multilayers (MLs) for reflective coatings of extreme ultraviolet mask blanks. Dark-field transmission electron microscopy (TEM) measurements imply interfacial roughness values of 80 to 150 pm. Bright-field TEM images indicate intermixed layer thicknesses of 0.4 to 1.8 nm. We present reflectivity calculations including these two ML imperfections and compare against prior empirical results. Both interfacial roughness and intermixing are predicted to lower the maximum reflectivity. For example, interfacial roughness of 400 pm lowers the maximum reflectivity by ∼2 % . Smoother interfaces with sub-100 pm allow recovery of ∼1.5 % of the reflectivity. Mo/Si intermixing is predicted to lower the maximum reflectivity by up to 6% relative to an ideal ML. Reflectivity could be recovered by ∼3 % by reducing the intermixing depth by only 20% to 30%. We demonstrate ways to reduce roughness or intermixing by ion beam deposition (IBD). Ion bombardment simulations provide estimates of the atom energy distribution arriving at the mask blank surface during Mo and Si deposition and of stopping depths of each atom into the underlying layer. Key IBD parameters to reduce the deposition energy, and hence the intermixing depth, are summarized: beam voltage and deposition pressure. Lower ion beam voltage or higher pressure can together reduce the intermixing depth by at least 20% to 30%. Bright-field TEM measurements of MLs deposited at various deposition conditions confirm the intermixing predictions.
We investigate the impact of key ion-beam-deposition (IBD) process conditions on the properties of Mo/Si multilayers as reflective coatings for Extreme Ultraviolet (EUV) mask blanks. Dark-field TEM measurements imply interfacial roughness values of 80-90 picometers. Bright-field TEM measurements indicate intermixed layer thicknesses of 0.5 – 1.9nm. We present reflectivity calculations including these two multilayer imperfections and reveal that roughness at this level has insignificant reflectivity impact. However, this level of intermixing could cause a reflectivity drop of ~ 4%. Ion bombardment simulations provide estimates of the atom energy distribution arriving at the mask blank surface during Mo and Si deposition, and of stopping depths of each atom into the underlying layer. Key parameters to modify the deposition energy, and potentially the intermixing depth, are summarized: beam voltage and deposition pressure. Lower ion beam voltage or higher pressure are both predicted to reduce the intermixing depth by 20-30%. Bright-field TEM measurements of multilayers deposited at various deposition conditions confirm the predictions.
As Extreme Ultra Violet lithography (EUVL) is becoming adopted into manufacturing, there is an ongoing need to identify and improve the EUV mask multilayer properties that impact reflectivity. Key properties include the roughness and inter-diffusion depth at the Mo-Si interfaces. During mask usage, on exposure to EUV, the interfaces are impacted during thermal cycling, so interfacial stability is key. We report on the use of X-ray reflectivity (XRR) to probe the interfacial depth and roughness of Mo/Si multilayers deposited via secondary ion beam deposition (IBD). We confirm top-surface roughness by AFM. We measure minimal impact of the underlying substrate on top-surface roughness of Mo-Si multilayer stacks. Mo and Si single-layer roughness are shown to be primarily dependent on deposition angle; with minimal roughness at intermediate angles and significant deterioration beyond a deposition angle of about 60 degrees. We use this angular dependence to systematically vary the interfacial roughness and monitor the impact on the XRR measurement. We demonstrate that XRR, with attention to the Fourier Transform, may also be used to quantify the inter-diffusion depth at the Mo-Si interfaces. We measure inter-diffusion depths of 0.5 - 1.8nm. A simulated model is developed, incorporating both interfacial depth and roughness, and the experimental data are compared with this model. The model could be applied to quantify the impact on the interfaces of: beam energy and flux; incidence angles; gas species and pressure; interfacial treatments; thermal treatment; or mask usage.
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