Today’s EUV masks are optimized for maximum reflectivity at 6o angle of incidence to support imaging on the 0.33NA scanners. The High-NA EUV scanner will have an NA of 0.55 and anamorphic optics, as right balance between productivity and keeping the angles on mask level compatible with the current EUV masks. However, for selected use-cases increased Mask-3D effects are observed. The enlarged angular spread on mask for vertical L/S may cause, amongst others, an increased non-telecentricity variation across slit compared to 0.33NA, potentially leading to on-product overlay errors. In this paper, we will show the root-cause of these enhanced M3D effects for High-NA EUV and the role of the EUV mask multilayer. We will show how an ideal EUV multilayer could look like to mitigate these M3D effects and propose a practical embodiment in the form of a RuSi multilayer stack. We will show some first experimental results of an ion beam deposited RuSi test sample, assess the main challenges from manufacturability point of view and what imaging performance can be expected for such a mask multilayer applied for High-NA EUV.
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.
The EUV mask absorber structure is currently ~ 80nm thick TaN-based film, subtractively patterned through resist mask using dry reactive etch. A thin (2-3nm) Ru layer, below the absorber, protects the reflective Mo/Si multilayer, and acts as an etch stop. For future nodes, shadowing from an 80nm thick feature is considered prohibitive. Accordingly, more highly absorbing materials are being investigated, to allow a thinner absorber and reduced 3-D effects. For example, Ni, Pt and Pd are practical materials with EUV extinction coefficient more than double that of TaN, allowing for 25-40 nm absorber thickness. A challenge for Ni, Pt, or Pd--based absorbers is that reactive etch processes are unavailable. We discuss patterning of these materials by physical ion beam etch (IBE), the etch method of choice for these materials in magnetic or electrode applications. We demonstrate IBE etch rates up to ~80-200 nm/minute, implying process times of less than 20s. Via material and ion species optimization, we demonstrate etch selectivity to Ru up to ~ 1.8:1. Using SRIM simulations, we investigate ion damage through the underlying Ru layer, versus ion species and ion energy. Simulation predicts that any damage can be confined within a protective 2.5nm Ru layer, for ion energies of 200-400V. Depth of damage is reduced from ~8 nm to ~ 1.8 nm by reduction of the beam energy from 1200V to 200V. Based on the angular dependence of the IBE rates, we simulate IBE patterning of absorber structures, and demonstrate effective patterning down to 48nm mask half-pitch (~ 24nm wafer pitch for a 5nm node). Etched feature sidewall angles of 81-86o are demonstrated.
For future nodes, TaN-based absorber layers on EUV mask-blanks, may need to be replaced with thinner layers of new material systems. Ni and Co based materials are promising material candidates owing to their high EUV absorption. Ion Beam Etching (IBE) is being explored as an option for patterning these metallic systems that are hard to etch by Reactive Ion Etch. In this work we expand our initial work on the IBE of Ni absorber films to include the role of etch beam energy and alternative etch-masks for both Ni and Co based films. We present experimental film level data such as etch uniformity, angular-dependent etch rates, and surface roughness. We extend the modeling of IBE of line-space patterns, to narrower line widths and various etch-mask materials vis-a-vis side wall angle and CD fidelity, both as a function of beam energy and angle of etch.
Development progress and roadmap, for high-reflective Mo/Si multilayers for EUV mask-blanks, are reviewed. We outline the state-of-the-art in low-defect-density secondary ion beam deposition (IBD), and ongoing hardware development for performance improvement and high-volume manufacturing. We further discuss extension of ion beam technology to later steps in the EUV mask manufacturing: deposition of highly-uniform 2.5 – 3nm Ru capping layers; and patterning of novel Ni absorber structures. IBD-deposited Ru films are demonstrated with uniformity of 0.7% 3σ over a 188mm diameter area. By x-ray reflection with Cu Kα radiation, we measure a film density of 12.4 g/cm3, and a roughness of less than 1.0nm. Deposition rates of ~ 1 – 7 nm/min are demonstrated, implying a capping layer deposition time of 20 seconds – 3 minutes. . For advanced absorber patterning, we discuss Argon ion beam etch (IBE) of Ni films. Ni and Ru IBE etch rates of ~ 8 – 80 nm/min are demonstrated, implying absorber etch times of ~ 30 seconds – 5 minutes. IBE Ni:Ru etch selectivity is 1:1 to 1.3:1, so Ru is not a ‘stopping layer’, etch depth must be controlled by time, and Ni uniformity is a requirement. IBE Ni:Photoresist etch selectivity is 0.8:1 to 1.6:1. We simulate the IBE absorber pattern definition for mask features of half-pitch 96nm (24nm at wafer level). Ion beam incidence angle can be optimized to maintain critical dimension within 6% of the pre-etch value.