Design and performance of capping layers for extreme-ultraviolet multilayer mirrors

Abstract

Multilayer lifetime has emerged as one of the major issues for the commercialization of extreme-ultraviolet lithography (EUVL). We describe the performance of an oxidation-resistant capping layer of Ru atop multilayers that results in a reflectivity above 69% at 13.2 nm, which is suitable for EUVL projection optics and has been tested with accelerated electron-beam and extreme-ultraviolet (EUV) light in a water-vapor environment. Based on accelerated exposure results, we calculated multilayer lifetimes for all reflective mirrors in a typical commercial EUVL tool and concluded that Ru-capped multilayers have approximately 40x longer lifetimes than Si-capped multilayers, which translates to 3 months to many years, depending on the mirror dose.

Growth and thermal oxidation of Ru and ZrO2 thin films as oxidation protective layers

“…202 Due to the limited access to EUV sources or synchrotron beamlines, similar oxidation is pursued by means of electron beams in presence of water vapor, 201,203 as observed in several studies. 49,128,132,204 However, the process occurring might be somehow different from using EUV radiation, since photons are not present.…”

“…Si forms a thin (~1-2 nm) self-terminated native, stable oxide which does not evolve at atmospheric conditions. 49,50 However, Mo forms a thicker (several nanometers) oxide that continues oxidizing upon air exposure. 8,51 Thus, up to date, multilayer mirrors are normally terminated by Si.…”

“…55 Another example is the use of B4C barriers when Ru is applied as protective layer on Si-terminated MLMs. 49 3. Low surface roughness.…”

“…67 Another example is the EUV-induced oxidation of a Ru-capped MLM, which leads to a loss of reflectance. 49 11. Non-affine surface to external contaminants.…”

“…40 Apart from EUV reflectance simulations, there are several reported studies where the EUV reflectance is experimentally measured, before and after EUV exposure, (or similar ebeam exposure) in a water vapor environment from Si-terminated Mo/Si MLMs, capped with different protective layers (with or without diffusion barriers) such as Ru, 128 Ru/Mo, 128,129 130 Most of these studies not only measure the EUV reflectance upon exposure, but also investigate the EUV-induced oxidation resistance of the protective material (and/or underlying multilayer). 41,49,128,131 In this line, there are other studies reported that test different protective materials against oxidation on a multilayer (or a single Mo or Si layer, mimicking the last layer of the multilayer), not only by EUV (or e-beam) exposure in presence of water, 132 but also by atomic oxygen, 133 by oxygen plasma, 134 or by ambient air. 8 In addition, several of these studies give some insights on the effect of the growth (thickness, 132 conditions 8 ) on the oxidation resistance of the deposited protective layers.…”