Surface phenomena related to mirror degradation in extreme ultraviolet (EUV) lithography

Properties of ultrathin films appropriate for optics capping layers exposed to high energy photon irradiation

Ruthenium capping layers ~2 nm thick are used to protect and extend the lifetimes of Si/Mo multilayer mirrors used in extreme ultraviolet lithography (EUVL) applications. In the present work, we use ultrahigh vacuum surface science methods to address two aspects of Ru surface chemistry: (a) use of atomic hydrogen to remove oxygen from O-covered Ru, and (b) the effects of a model background hydrocarbon gas (methyl me thacrylate, MMA) on the accumulation of carbon on a Ru(10 1 0) single crystal surface. Atomic H is very effective in removing O from Ru even at 300K; the interpretation is that H reacts directly with adsorbed O to make OH, and a subsequent H atom reacts with OH to make H 2 O, which desorbs at 300K. MMA adsorbs strongly on Ru in the first monolayer and dissociates upon heating; H 2 and CO desorption products are seen upon heating from 300 to 600K, while a fractional monolayer of C remains on the surface. Physisorbed multilayers of MMA form at temperatures below ~170K. Thermal desorption of MMA dosed onto O-covered Ru shows that MMA reacts and completely removes an adsorbed O monolayer upon heating. Electron bombardment of MMA/Ru causes polymerization and also can induce accumulation of carbon - all depends on electron fluence, partial pressure of MMA, and substrate temperature. Keywords : Extreme ultraviolet lithography (EUVL), EUV optics contamination, EUV optics lifetime, Ruthenium, Ru(10

Surface chemistry of Ru: relevance to optics lifetime in EUVL

Extreme ultra violet lithography (EUVL) is no longer a future technology but is going to be inserted into mass production of semiconductor devices of 7 nm technology node in 2018. EUVL is an extension of optical lithography using extremely short wavelength (13.5 nm). This short wavelength requires major modifications in the optical systems due to the very strong absorption of EUV light by materials. Refractive optics can no longer be used, and reflective optics is the only solution to transfer image from mask to wafer. This is why we need the multilayer (ML) mirror-based mask as well as an oblique incident angle of light. This paper discusses the principal theory on the EUV mask design and its component materials including ML reflector and EUV absorber. Mask shadowing effect (or mask 3D effect) is explained and its technical solutions like phase shift mask is reviewed. Even though not all the technical issues on EUV mask are handled in this review paper, you will be able to understand the principles determining the performance of EUV masks.

Mask Materials and Designs for Extreme Ultra Violet Lithography

Extreme ultraviolet lithography (EUVL) is a next generation lithographic techniques under development for fabricating semiconductor devices with feature sizes smaller than 32 nm. The optics to be used in the EUVL steppers is reflective optics with multilayer mirror coatings on each surface. The wavelength of choice is 13.5 nm determined by the optimum reflectivity of the mirror coatings. The light source required for this wavelength is derived from a hot-dense plasma produced by either a gas discharge or a laser. This study concentrate only on the laser produced plasma source because of its advantages of scalability to higher repetition rates. The design of a the laser plasma EUVL light source consists of a plasma produced from a high-intensity focused laser beam from a solid/liquid target, from which radiation is generated and collected by a large solid angle mirror or array of mirrors. The collector mirrors have the same reflectivity characteristics as the stepper mirrors. The EUVL light source is considered as the combination of both the hot-dense plasma and the collector mirrors. The EUVL light sources required by the stepper manufacturers must have sufficient EUV output power and long operational lifetimes to meet market-determined chip production rates. The most influential factor in achieving the required EUV output power is the conversion efficiency (CE) of laser input energy relative to the EUV radiation collected. A high CE is demonstrated in a separate research program by colleagues in the Laser Plasma laboratory at CREOL. Another important factor for the light source is the reflectivity lifetime of the collection optics as mirror reflectivity can be degraded by deposition and ablation from the plasma debris. Realization of a high CE but low debris plasma source is possible by

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Debris Characterization And Mitigation Of Droplet Laser Plasma Sources For Euv Lithography

A chemical model of the surface oxidization of a ruthenium capping layer was constructed for the lifetime scaling of the mirrors of EUVL projection optics. It has two steps: the formation of active oxygen from adsorbed water by EUV irradiation, and the diffusion of active oxygen and its reaction with Ru at the interface between Ru oxide and Ru metal. The rate equations for these steps can be solved independently because the time scale is very different. This paper reports results for Step 1. They revealed the following: (1) The predicted dependence of the amount of oxidation on water pressure is linear over a very wide range. That means that water pressure is a good measure of the degree of acceleration. The reported non-linear dependence on water pressure cannot be explained without the influence of other factors, such as the amount of background hydrocarbons. (2) The rate equation showed the dependence on light intensity also to be linear. That is, a theoretical model based on a simple rate equation cannot predict the reported non-linear intensity dependence. So, we calculated the rise in surface temperature caused by irradiation over a long period from the rate equation for one-dimensional thermal conduction and the Lambert-Beer Law for surface photoabsorption. These calculations revealed that irradiation for a long period of time causes the surface temperature to rise, thus reducing the rate of formation of active oxygen. So, light intensity is a limiting factor in acceleration tests. (3) The rate of oxidation is significantly lower for pulse excitation than for quasi-continuous (synchrotron radiation) light when the EUV light source produces narrow pulses at a low repetition rate.

Iwao Nishiyama

Papers

Model of Ru-surface oxidation for the lifetime scaling of EUVL projection optics mirror