SPIE Journal Paper | 22 March 2013
KEYWORDS: Etching, Extreme ultraviolet lithography, Photomasks, Extreme ultraviolet, Multilayers, Line width roughness, Inspection, Photoresist processing, Plasma, Silicon
An overview of extreme ultraviolet lithography (EUVL) mask etch is presented and a EUVL mask etch study was carried out. Today, EUVL implementation has three critical challenges that hinder its adoption: extreme ultraviolet (EUV) source power, resist resolution-line width roughness-sensitivity, and a qualified EUVL mask. The EUVL mask defect challenges result from defects generated during blank preparation, absorber and multilayer deposition processes, as well as patterning, etching and wet clean processes. Stringent control on several performance criteria including critical dimension (CD) uniformity, etch bias, micro-loading, profile control, defect control, and high etch selectivity requirement to capping layer is required during the resist pattern duplication on the underlying absorber layer. EUVL mask absorbers comprise of mainly tantalum-based materials rather than chrome- or MoSi-based materials used in standard optical masks. Compared to the conventional chrome-based absorbers and phase shift materials, tantalum-based absorbers need high ion energy to obtain moderate etch rates. However, high ion energy may lower resist selectivity, and could introduce defects. Current EUVL mask consists of an anti-reflective layer on top of the bulk absorber. Recent studies indicate that a native oxide layer would suffice as an anti-reflective coating layer during the electron beam inspection. The absorber thickness and the material properties are optimized based on optical density targets for the mask as well as electromagnetic field effects and optics requirements of the patterning tools. EUVL mask etch processes are modified according to the structure of the absorber, its material, and thickness. However, etch product volatility is the fundamental requirement. Overlapping lithographic exposure near chip border may require etching through the multilayer, resulting in challenges in profile control and etch selectivity. Optical proximity correction is applied to further enhance the resolution. Other resolution enhancement techniques, such as phase shifting, are also in consideration for EUVL. Phase-shifting will involve partial etching of the multilayer. The trend to use shorter EUV wavelength (e.g., 6.7 nm) for enhancing resolution will use new multilayer and absorber compositions, and will require new etch process development efforts. TaBO/TaBN absorber layers (features down to 40 nm) were etched with vertical profiles, low etch CD bias, and 1.7 nm etch CD uniformity (3σ ). In the light shed application, Mo/Si multilayer etching yielded vertical profiles and high etch selectivity.
