<title>Improved reflectance and stability of Mo/Si multilayers</title>

Abstract

Commercial EUV lithographic systems require multilayers with higher reflectance and better stability then that published to date. Interface-engineered MoISi multilayers with 70% reflectance at 13.5 nm wavelength (peak width of 0.545 nm) and 71% at 12.7 nm wavelength (peak width of 0.49 nm) were developed. These results were achieved with 50 bilayers. These new multilayers consist of Mo and Si layers separated by thin boron carbide layers. Depositing boron carbide on interfaces leads to reduction in silicide formation on the Mo-on-Si interfaces. Bilayer contraction is reduced by 30% implying that there is less intermixing of Mo and Si to form silicide. As a result the Mo-on-Si interfaces are sharper in interface-engineered multilayers than in standard Mo/Si multilayers. The optimum boron carbide thicknesses have been determined and appear to be different for Mo-on-Si and Si-on-Mo interfaces. The best results were obtained with 0.4 nm thick boron carbide layer on the Mo-on-Si interface and 0.25 nm thick boron carbide layer on the Si-on-Mo interface. Increase in reflectance is consistent with multilayers with sharper and smoother interfaces.A significant improvement in oxidation resistance of EUV multilayers has been achieved with ruthenium terminated MoISi multilayers. The best capping layer design consists of a Ru layer separated from the last Si layer by a boron carbide layer. This design achieves high reflectance and the best oxidation resistance in a water vapor (i.e. oxidation) environment. Electron beam exposures of 4.5 hours in the presence of 5~1 0 -~ torr water vapor partial pressure show no measurable reflectance loss and no increase in the oxide thickness of Ru terminated multilayers. Longer exposures in different environments are necessary to test lifetime stability of many years.. I

Kinetic simulation of an extreme ultraviolet radiation driven plasma near a multilayer mirror

“…The mirror is assumed to have a ruthenium surface because in EUV lithography this metal is often applied as a capping layer ͑thick-ness Ϸ1.5 nm͒ to provide a barrier against oxidation of the underlying Mo/ Si stacks. 15 The EUV radiation is partially reflected by the mirror with a reflection coefficient of R ml = 68%, which is typical for Mo/ Si multilayer mirrors. 1,16 The remaining 32% of the radiation is absorbed and is converted primarily to heat.…”

Ab initio Study of the Si Adsorption on Mo(110)

“…The high-reflectance Mo/Si multilayer used in the EUVL system consists of polycrystalline Mo and amorphous Si layers that are separated by an interfacial region [4,5]. In order to obtain high reflectivity, a minimal thickness of Mo/Si interface is required because the intermixing region tends to reduce reflectivity [6].…”

Determination of charge state, energy and angular distributions of tin ions emitted from laser produced plasma based EUV sources.

“…A suitable replacement to these lens systems has been found in molybdenum silicon (Mo/Si) multilayer mirror systems. Due to their reflectivity (∼ 70%) at 13.5 nm [1], this means that source designers must optimise the EUV flux emitted by the source in this range, as it can be reflected 9, 11 or 13 times in the lithography process. Laser produced plasmas (LPPs) can be made to significantly emit at the appropriate wavelengths [2,3].…”