**1. Introduction**

368 Recent Advances in Nanofabrication Techniques and Applications

Kubiak, G., Bernardez, L.J., Krenz, K.D., O'Connell, D.J., Gutowski, R. & Todd, A.M., (1996).

Miyamoto, S., Shimoura, A., Amano, S., Fukugaki, K., Kinugasa, H., Inoue, T. & Mochizuki,

Miyamoto, S., Amano, S.: Inoue, T., Nica, P. E., Shimoura, A., Kaku, K., Sekioka, T. &

Mizoguti, H., Abe, T., Watanabe, Y., Ishihara, T., Ohta, T., Hori, T., Kurosu, A., Komori, H.,

Mochizuki, T., Shimoura, A., Amano, S. & Miyamoto, S. (2001). Compact high-average-

Sasaki, A., Nishihara, K., Murakami, M., Koike, F., Kagawa, T., Nishikawa, T., Fujima, K.,

Shimoura, A., Amano, S., Miyamoto, S. & Mochizuki, T. (1998). X-ray generation in cryogenic targets irradiated by 1 m pulse laser. *Appl. Phys. Lett.* 72(2), pp.164-166 Soumagne, G.: Abe, T., Suganuma,T. , Imai, Y., Someya, H., Hoshino, H., Nakano, M.,

sematech.org/meetings/archives/litho/index.htm>

*Lithography*, vol.4, pp.66-71

February 2006

July 2001

pp.5857-5859

March 2005

*Lett.* 86(26), pp.261502-1-261502-3

Debris-free EUVL sources based on gas jets, *OSA TOPS on Extreme Ultraviolet* 

T. (2005). Laser wavelength and spot diameter dependence of extreme ultraviolet conversion efficiency in , 2, and 3 Nd:YAG laser-produced plasmas. *Appl. Phys.* 

Mochizuki, T. (2006). EUV source developments on laser-produced plasmas using cryogenic Xe and Lithium new scheme target, *Proceedings of SPIE, Emerging Lithographic Technologies X*, vol.6151, pp.61513S-1-61513S-10, San Jose, USA,

Kakizaki, K., Sumitani, A., Wakabayashi, O., Nakarai, H., Fujimoto, J. & Endo, A. (2010). 1st generation laser-produced plasma 100W source system for HVM EUV lithography, SO-03, *2010 International Symposium on Extreme Ultraviolet Lithography*, Kobe Japan, October 2010, International Sematech, Available from: <http://www.

power X-ray source by cryogenic target, *Proceedings of SPIE, Applications of X Rays Generated from Lasers and Other Bright Sources II*, vol.4504, pp.87-96, San Diego, USA,

Kawamura, T. & Furukawa, H. (2004). Effect of the satellite lines and opacity on the extreme ultraviolet emission from high-density Xe plasmas. *Appl. Phys. Lett.* 85(24),

Komori, H., Takabayashi, Y., Ariga, T., Ueno, Y., Wada, Y., Endo, A & Toyoda, K.(2005). Laser-produced-plasma light source for EUV lithography, *Proceedings of SPIE, Emerging Lithographic Technologies IX*, vol.5751, pp.822-828, San Jose, USA, Exposure of collector mirrors facing the hot, dense pinch plasma in plasma-based EUV light sources to debris (fast ions, neutrals, off-band radiation, droplets) remains one of the highest critical issues of source component lifetime and commercial feasibility of nanolithography at 13.5-nm. Typical radiators used at 13.5-nm include Xe, Li and Sn. Fast particles emerging from the pinch region of the lamp are known to induce serious damage to nearby collector mirrors. Candidate collector configurations include either multi-layer mirrors (MLM) or single-layer mirrors (SLM) used at grazing incidence. Due to the strong absorbance of 13.5 nm light only reflective optics rather than refractive optics can work in addition to the need for ultra-high vaccum conditions for its transport.

This chapter presents an overview of particle-induced damage and elucidates the underlying mechanisms that hinder collector mirror performance at 13.5-nm facing highdensity pinch plasma. Results include recent work in a state-of-the-art in-situ EUV reflectometry system that measures real time relative EUV reflectivity (15-degree incidence and 13.5-nm) variation during exposure to simulated debris sources such as fast ions, thermal atoms, and UV radiation (Allain et al., 2008, 2010). Intense EUV light and off-band radiation is also known to contribute to mirror damage. For example off-band radiation can couple to the mirror and induce heating affecting the mirror's surface properties. In addition, intense EUV light can partially photoionize background gas used for mitigation in the source device. This can lead to local weakly ionized plasma creating a sheath and accelerating charged gas particles to the mirror surface inducing sputtering. In this overview we will also summarize studies of thermal and energetic particle exposure on collector mirrors as a function of temperature simulating the effects induced by intense off-band and EUV radiation found in EUVL sources. Measurements include variation of EUV reflectivity with mirror damage and in-situ surface chemistry evolution.

In this chapter the details from the EUV radiation source to the collector mirror are linked in the context of mirror damage and performance (as illustrated in Figure 1). The first section summarizes EUV radiation sources and their performance requirements for high-volume manufacturing. The section compares differences between conventional discharge plasma produced (DPP) versus laser plasma produced (LPP) EUV light sources and their possible combinations. The section covers the important subject of high-density transient plasmas and their interaction with material components. The different types of EUV radiators, debris

Irradiation Effects on EUV Nanolithography Collector Mirrors 371

The in-band and off-band radiation generated in these sources is also a critical limitation in operation of these lamps since on average the off-band radiation is converted into heat on nearby plasma-facing components. There are additional challenges in the design of 13.5-nm light sources that include: high-frequency operation limits driven by the need to extract high EUV power at the intermediate focus (IF) and limited by the available high-throughput power of the plasma device (e.g. laser system or discharge electrode system). Additionally, the scaling of debris with EUV power extraction and the limitation of conversion efficiency (CE) with source plasma size also translate into significant engineering challenges to the design of 13.5-nm lithography source design. Figure 1 illustrates, for the case of the DPP configuration, the primary debris-generating sources that compromise 13.5-nm collector mirrors. The first region depicted on the left is defined here as the "transient plasma region". This is the region described earlier with high-density and high-temperature plasma

Fig. 1. Illustration of the various components of EUV 13.5-nm radiation source configuration

In DPP discharge sources material components that make up the electrode system consist of high-temperature, high-toughness materials. Although DPP source design has traditionally used high-strength materials such as tungsten and molybdenum alloys, the extreme conditions in these systems limit the operational lifetime of the electrode. Significant plasma-induced damage is found in the electrode surfaces, which induce degradation and abrasion over time. Figure 2, for example, shows a scanning electron micrograph of a tungsten electrode exposed to a dense plasma focus high-intensity plasma discharge. The key feature in the SEM image is the existence of plasma-induced damage domains that

consisting primarily of three major components: 1) plasma radiator section, 2) debris

effectively have induced melting in certain sections of the electrode surface.

mitigation system and 3) optical collector mirror.

interacting with the electrode surfaces.

distribution, and mitigation sources are outlined. The second section summarizes the various optical collector mirror geometries used for EUV lithography. A brief discussion on the intrinsic damage mechanisms linked to their geometry is included. The third section summarizes in general irradiation-driven mechanisms as background for the reader and its relation to the "quiescent" plasma collector mirrors are exposed in EUV sources. This includes irradiation-driven nanostructures, sputtering, ion mixing, surface diffusion, and ion-induced surface chemistry. The fourth section briefly discusses EUV radiation-driven plasmas as another source of damage to the mirror. These plasmas are a result of using gases for debris mitigation. The fifth section is a thorough coverage of the key irradiationdriven damage to optical collector mirrors and their performance limitations as illustrated in part by Figure 1.

### **2. EUV radiation sources**

There are numerous sources designed to generate light at the extreme ultraviolet line of 13.5-nm. Historically advanced lithography has considered wavelength ranges from hard Xrays up to 157 nm [Bakshi, 2009]. Radiators of 13.5-nm light rely on high-density plasma generation typically based on discharge-produced configurations with magnetically confined high-density plasmas or laser-produced plasmas. Recently, some sources have combined both techniques (Banine 2011). Generation of high-density plasmas to yield temperatures of the order of 10-50 eV require advanced materials for plasma-facing components in these extreme environments in particular discharge-produced plasma (DPP) configurations. This is due to the need of metallic anode/cathode components operating under high-heat flux conditions. Laser-produced plasmas (LPP) benefits from the fact that no nearby electrodes are necessary to induce the plasma discharge. Further details will be described in section 5.1. One challenge in operating EUV lamps at high power is the collected efficiency of photons at the desired exposure wavelength of 13.5-nm. This particular line has a number of radiators with properties that have consequences on EUV source operation. For example radiators at 13.5-nm include xenon, tin and lithium. The latter two are metals and thus their operation complicated by contamination issues on nearby material components such as electrodes and collector mirrors. Further discussion follows in section 2.2 and 2.3. To contend with the various types of debris that are generated in the plasma-producing volume a variety of novel debris mitigation systems (DMS) have been designed and developed for both DPP and LPP configurations.

#### **2.1 Function and material components**

The transient nature of the high-density plasma environment in DPP and LPP systems results in exposure of plasma-facing components to extreme conditions (e.g. high plasma density (~ 1019 cm-3) and temperature (~ 20-40 eV). However, in LPP systems since the configuration is mostly limited by the mass of the radiator and the laser energy supplied to it to generate highly ionized plasma with the desired 13.5-nm light. Both configurations rely on efficient radiators of 13.5-nm light, which include: Li, Sn and Xe. In DPP designs a variety of configurations have been used that include: dense plasma focus, capillary Z-pinch, star pinch, theta pinch and hollow cathode among others. For a more formal description of these high-density plasma sources for 13.5-nm light generation the author refers to the recent publications by V. Bakshi in 2006 and 2009 (Bakshi, 2006; Bakshi, 2009).

distribution, and mitigation sources are outlined. The second section summarizes the various optical collector mirror geometries used for EUV lithography. A brief discussion on the intrinsic damage mechanisms linked to their geometry is included. The third section summarizes in general irradiation-driven mechanisms as background for the reader and its relation to the "quiescent" plasma collector mirrors are exposed in EUV sources. This includes irradiation-driven nanostructures, sputtering, ion mixing, surface diffusion, and ion-induced surface chemistry. The fourth section briefly discusses EUV radiation-driven plasmas as another source of damage to the mirror. These plasmas are a result of using gases for debris mitigation. The fifth section is a thorough coverage of the key irradiationdriven damage to optical collector mirrors and their performance limitations as illustrated in

There are numerous sources designed to generate light at the extreme ultraviolet line of 13.5-nm. Historically advanced lithography has considered wavelength ranges from hard Xrays up to 157 nm [Bakshi, 2009]. Radiators of 13.5-nm light rely on high-density plasma generation typically based on discharge-produced configurations with magnetically confined high-density plasmas or laser-produced plasmas. Recently, some sources have combined both techniques (Banine 2011). Generation of high-density plasmas to yield temperatures of the order of 10-50 eV require advanced materials for plasma-facing components in these extreme environments in particular discharge-produced plasma (DPP) configurations. This is due to the need of metallic anode/cathode components operating under high-heat flux conditions. Laser-produced plasmas (LPP) benefits from the fact that no nearby electrodes are necessary to induce the plasma discharge. Further details will be described in section 5.1. One challenge in operating EUV lamps at high power is the collected efficiency of photons at the desired exposure wavelength of 13.5-nm. This particular line has a number of radiators with properties that have consequences on EUV source operation. For example radiators at 13.5-nm include xenon, tin and lithium. The latter two are metals and thus their operation complicated by contamination issues on nearby material components such as electrodes and collector mirrors. Further discussion follows in section 2.2 and 2.3. To contend with the various types of debris that are generated in the plasma-producing volume a variety of novel debris mitigation systems (DMS) have been

The transient nature of the high-density plasma environment in DPP and LPP systems results in exposure of plasma-facing components to extreme conditions (e.g. high plasma density (~ 1019 cm-3) and temperature (~ 20-40 eV). However, in LPP systems since the configuration is mostly limited by the mass of the radiator and the laser energy supplied to it to generate highly ionized plasma with the desired 13.5-nm light. Both configurations rely on efficient radiators of 13.5-nm light, which include: Li, Sn and Xe. In DPP designs a variety of configurations have been used that include: dense plasma focus, capillary Z-pinch, star pinch, theta pinch and hollow cathode among others. For a more formal description of these high-density plasma sources for 13.5-nm light generation the author refers to the recent

designed and developed for both DPP and LPP configurations.

publications by V. Bakshi in 2006 and 2009 (Bakshi, 2006; Bakshi, 2009).

**2.1 Function and material components** 

part by Figure 1.

**2. EUV radiation sources** 

The in-band and off-band radiation generated in these sources is also a critical limitation in operation of these lamps since on average the off-band radiation is converted into heat on nearby plasma-facing components. There are additional challenges in the design of 13.5-nm light sources that include: high-frequency operation limits driven by the need to extract high EUV power at the intermediate focus (IF) and limited by the available high-throughput power of the plasma device (e.g. laser system or discharge electrode system). Additionally, the scaling of debris with EUV power extraction and the limitation of conversion efficiency (CE) with source plasma size also translate into significant engineering challenges to the design of 13.5-nm lithography source design. Figure 1 illustrates, for the case of the DPP configuration, the primary debris-generating sources that compromise 13.5-nm collector mirrors. The first region depicted on the left is defined here as the "transient plasma region". This is the region described earlier with high-density and high-temperature plasma interacting with the electrode surfaces.

Fig. 1. Illustration of the various components of EUV 13.5-nm radiation source configuration consisting primarily of three major components: 1) plasma radiator section, 2) debris mitigation system and 3) optical collector mirror.

In DPP discharge sources material components that make up the electrode system consist of high-temperature, high-toughness materials. Although DPP source design has traditionally used high-strength materials such as tungsten and molybdenum alloys, the extreme conditions in these systems limit the operational lifetime of the electrode. Significant plasma-induced damage is found in the electrode surfaces, which induce degradation and abrasion over time. Figure 2, for example, shows a scanning electron micrograph of a tungsten electrode exposed to a dense plasma focus high-intensity plasma discharge. The key feature in the SEM image is the existence of plasma-induced damage domains that effectively have induced melting in certain sections of the electrode surface.

Irradiation Effects on EUV Nanolithography Collector Mirrors 373

and NiO from NiCO3, for instance) and copper molybdate (MoCuO4). Dry hydrogen (the dew point temperature is above 20 0C) facilitates the formation of the heterogeneous conglomerates in W-Ni-powders, which do not collapse at sintering or saturate the material (Figure 3a), and spheroidizing of molybdenum particles and re-crystallization through the liquid phase in the conditions of sintering the composite consisting of molybdenum and copper (Figure 3b). For comparison, the structure is shown in Figure 3c obtained from tested W-Ni powders. The structure of the materials was studied by means of scanning electron microscopy (SEM) of the secondary electrons. A variety of materials characterization including surface spectroscopy and X-ray based diffraction is used to assess the condition of the materials after processing with sintering-based techniques. The powder composite materials are so-called pseudo alloys, which provide promising high thermal conductivity

Fig. 3. From left to right, (a) the structure of the W-Cu-Ni-LaB6 pseudo alloy (x540), (b) the

Observations made with secondary mass ion spectrometry (SIMS) on these materials found evidence of hydrogen and beryllium in anode components. Based on these results one can speculate that the hydrogen observed by SIMS after exposing the samples may be caused by that environment, in which the powders are manufactured, sintered, and additionally annealed. In regards to the beryllium observed on the anode surface after exposure to the xenon plasma, one may suppose two possible explanations, each of which requires additional verification. The construction may contain beryllium bronze; or the construction may contain Al203 or BeO based ceramics. Both cases may be the reason for enrichment of

For systems with the absence of the component interactions, the arc xenon plasma impact to the electrode materials does not cause a noticeable change of durability: for MoCuLaB6: HV = 1600-1690 MPa; and for Cu- Al2O3: HV = 660 MPa through the whole height of the anode. In the tungsten and copper based composites, when presence of nickel exists, the mutual dissolution of the elements is increased (W is dissolved in Cu-Ni melt, for instance). At cooling, it may be accompanied by either forming non-equilibrium solid solution, or solidification; which is conformed by the increasing the firmness of the upper part of the anode (3380 MPa compared to 3020 MPa in its lower part). To provide more careful analysis, one should investigate the dependence of electro-conductive composites on heat resistance subject to arc discharges of powerful heat fluxes (up to 107 W/m2). Additional analyses typically conducted include the propagation of cracks, observed on the surface layer of the anode material and deep into the bulk. For that, the precise method of manufacturing is required for further insight on crack development and

structure of the Cu-44%Mo – 1%LaB6 pseudo alloy (x2000), and (c) the structure of "irradiated" W-Cu-Ni pseudo alloy produced by class W-Ni powder (x400).

properties, while displaying sub-unity sputter yields (see Section 4).

the surface samples by these elements during the heating phases.

The second region depicted in Figure 1 is defined as the debris mitigation zone (DMZ). In this region a variety of debris mitigation strategies can be used to contend with the large debris that exists in operation of the DPP source. For example the use of inert gas to slowdown energetic particles that are generated in the pinch plasma region and/or debris mitigation shields that collect macro-scale particulates when using Sn-based radiators in DPP devices. Radiation-induced mechanisms on the surfaces of the DMZ elements also can lead to ion-induced sputtering of DM shield material that eventually is deposited in the nearby 13.5-nm collector mirror. Therefore care is taken to select sputter-resistant materials for the DM shields used such as refractory metal alloys and certain stainless steels. Design of DM shields also involve computational modeling that can aid in identifying appropriate materials depending on the source operation and generation of a variety of debris types such as clusters, ions, atoms, X-rays, electrons and macroscopic dust particles.

Fig. 2. SEM micrographs of a tungsten electrode exposed to high-intensity plasma during the generation of EUV 13.5-m light.

The third region in Fig. 1 consists of the 13.5-nm light collector mirror. The collector mirror has a configuration to optimally collect as much of the 13.5-nm light as possible. Its function is to deliver EUV power in a specified etendue at the intermediate focus (IF) or the opening of the illuminator. This power is in turn dictated by the specification on EUV exposure of the EUV lithography scanner that must be able to operate with 150-200 wafers per hour (wph) at nominal power for periods of 1-2 years without maintenance (so-called highvolume manufacturing, HVM, conditions). This ultra-stringent requirement is one of the primary challenges to EUV lithography today. Since powers of order 200-300 W at the IF need to be sustained for a year or more, materials at the DPP source and those used for collector mirrors will necessarily require revolutionary advances in materials performance. The third region in Figure 1 also depicts what debris the collector mirror is exposed to during the discharge. A distribution of debris energies (i.e. ions), fluxes and masses will effectively affect the mirror surface performance. The third region is also known as the "condenser or collector optics region".

#### **2.2 Selection of electrode materials in DPP EUV devices**

Selection of materials for DPP electrodes depends on the microstructure desired to minimize erosion and maximize thermal conductivity. Figure 3 shows an example of SEM micrographs of materials identified to have promising EUV source electrode properties. The powder composite materials inherited the structural characteristics of the initial powders, determined by the processes of combined restoration of tungsten and nickel oxides (WO3

The second region depicted in Figure 1 is defined as the debris mitigation zone (DMZ). In this region a variety of debris mitigation strategies can be used to contend with the large debris that exists in operation of the DPP source. For example the use of inert gas to slowdown energetic particles that are generated in the pinch plasma region and/or debris mitigation shields that collect macro-scale particulates when using Sn-based radiators in DPP devices. Radiation-induced mechanisms on the surfaces of the DMZ elements also can lead to ion-induced sputtering of DM shield material that eventually is deposited in the nearby 13.5-nm collector mirror. Therefore care is taken to select sputter-resistant materials for the DM shields used such as refractory metal alloys and certain stainless steels. Design of DM shields also involve computational modeling that can aid in identifying appropriate materials depending on the source operation and generation of a variety of debris types

such as clusters, ions, atoms, X-rays, electrons and macroscopic dust particles.

Fig. 2. SEM micrographs of a tungsten electrode exposed to high-intensity plasma during

The third region in Fig. 1 consists of the 13.5-nm light collector mirror. The collector mirror has a configuration to optimally collect as much of the 13.5-nm light as possible. Its function is to deliver EUV power in a specified etendue at the intermediate focus (IF) or the opening of the illuminator. This power is in turn dictated by the specification on EUV exposure of the EUV lithography scanner that must be able to operate with 150-200 wafers per hour (wph) at nominal power for periods of 1-2 years without maintenance (so-called highvolume manufacturing, HVM, conditions). This ultra-stringent requirement is one of the primary challenges to EUV lithography today. Since powers of order 200-300 W at the IF need to be sustained for a year or more, materials at the DPP source and those used for collector mirrors will necessarily require revolutionary advances in materials performance. The third region in Figure 1 also depicts what debris the collector mirror is exposed to during the discharge. A distribution of debris energies (i.e. ions), fluxes and masses will effectively affect the mirror surface performance. The third region is also known as the

Selection of materials for DPP electrodes depends on the microstructure desired to minimize erosion and maximize thermal conductivity. Figure 3 shows an example of SEM micrographs of materials identified to have promising EUV source electrode properties. The powder composite materials inherited the structural characteristics of the initial powders, determined by the processes of combined restoration of tungsten and nickel oxides (WO3

the generation of EUV 13.5-m light.

"condenser or collector optics region".

**2.2 Selection of electrode materials in DPP EUV devices** 

and NiO from NiCO3, for instance) and copper molybdate (MoCuO4). Dry hydrogen (the dew point temperature is above 20 0C) facilitates the formation of the heterogeneous conglomerates in W-Ni-powders, which do not collapse at sintering or saturate the material (Figure 3a), and spheroidizing of molybdenum particles and re-crystallization through the liquid phase in the conditions of sintering the composite consisting of molybdenum and copper (Figure 3b). For comparison, the structure is shown in Figure 3c obtained from tested W-Ni powders. The structure of the materials was studied by means of scanning electron microscopy (SEM) of the secondary electrons. A variety of materials characterization including surface spectroscopy and X-ray based diffraction is used to assess the condition of the materials after processing with sintering-based techniques. The powder composite materials are so-called pseudo alloys, which provide promising high thermal conductivity properties, while displaying sub-unity sputter yields (see Section 4).

Fig. 3. From left to right, (a) the structure of the W-Cu-Ni-LaB6 pseudo alloy (x540), (b) the structure of the Cu-44%Mo – 1%LaB6 pseudo alloy (x2000), and (c) the structure of "irradiated" W-Cu-Ni pseudo alloy produced by class W-Ni powder (x400).

Observations made with secondary mass ion spectrometry (SIMS) on these materials found evidence of hydrogen and beryllium in anode components. Based on these results one can speculate that the hydrogen observed by SIMS after exposing the samples may be caused by that environment, in which the powders are manufactured, sintered, and additionally annealed. In regards to the beryllium observed on the anode surface after exposure to the xenon plasma, one may suppose two possible explanations, each of which requires additional verification. The construction may contain beryllium bronze; or the construction may contain Al203 or BeO based ceramics. Both cases may be the reason for enrichment of the surface samples by these elements during the heating phases.

For systems with the absence of the component interactions, the arc xenon plasma impact to the electrode materials does not cause a noticeable change of durability: for MoCuLaB6: HV = 1600-1690 MPa; and for Cu- Al2O3: HV = 660 MPa through the whole height of the anode. In the tungsten and copper based composites, when presence of nickel exists, the mutual dissolution of the elements is increased (W is dissolved in Cu-Ni melt, for instance). At cooling, it may be accompanied by either forming non-equilibrium solid solution, or solidification; which is conformed by the increasing the firmness of the upper part of the anode (3380 MPa compared to 3020 MPa in its lower part). To provide more careful analysis, one should investigate the dependence of electro-conductive composites on heat resistance subject to arc discharges of powerful heat fluxes (up to 107 W/m2). Additional analyses typically conducted include the propagation of cracks, observed on the surface layer of the anode material and deep into the bulk. For that, the precise method of manufacturing is required for further insight on crack development and

Irradiation Effects on EUV Nanolithography Collector Mirrors 375

Before discussion of collector mirror geometry and configuration a brief background on irradiation-driven mechanism on material surfaces is in order. In DPP EUV devices electrodes at the source are exposed to short (10-20 nsec) high-intensity plasmas leading to a variety of erosion mechanisms. Erosion of the electrodes is dictated by the dynamics of the plasma pinch for configurations such as: dense plasma focus, Z-pinch and capillary. The transient discharge deposits 1-2 J/cm2 per pulse on electrode surfaces. Large heat flux is deposited at corners and edges leading to enhanced erosion. Understanding of how particular materials respond to these conditions is part of rigorous design of DPP electrode systems. Erosion mechanisms can include: physical sputtering, current-induced macroscopic erosion, melt formation, droplet, and particulate ejection [Hassanein et al, 2008]. Erosion at the surface is also governed by the dynamics of how plasma can generate a vapor cloud leading to a self-shielding effect, which results in ultimate protection of the surface bombarded. Determining whether microscopic erosion mechanisms such as: physical sputtering or macroscopic mechanisms such as melt formation and droplet ejection the dominant material loss mechanism remains an open question in DPP electrode design. This is because such mechanisms are inherently dependent on the pinch dynamics and operation of the source. One important consequence of the extreme conditions electrode and collector optics surfaces are exposed is the existence of several irradiation-driven mechanisms that can lead to substantial materials mixing at the plasma-material interface. Bombarment-induced modification of materials can in principle lead to phase transition mechanisms that can substantially change the

Conceptually, the phenomenon of bombardment-induced compositional changes is simplest when only athermal processes exist such as: preferential sputtering (PS) and collisional mixing (CM). Preferential sputtering occurs in most multi-component surfaces due to differences in binding energy and kinematic energy transfer to component atoms near the surface. Collisional mixing of elements in multi-component materials is induced by displacement cascades generated in the multi-component surface by bombarding particles/clusters and is described by diffusion-modified models accounting for irradiation damage. Irradiation can accelerate thermodynamic mechanisms such as Gibbsian adsorption or segregation (GA) leading to substantial changes near the surface with spatial scales of the order of the sputter depth (few monolayers). GA occurs due to thermally activated segregation of alloying elements to surfaces and interfaces reducing the free energy of the alloy system. Typically, GA will compete with PS and thus, in the absence of other mechanisms, the surface reaches a steady-state concentration approaching that of the bulk. However when other mechanisms are active, synergistic effects can once again alter the near-surface layer and complex compositions are achieved. These additional mechanisms include: radiation-enhanced diffusion (RED) due to the thermal motion of nonequilibrium point defects produced by bombarding particles near the surface, radiationinduced segregation (RIS), a result of point-defect fluxes, which at sufficiently high temperatures couples defects with a particular alloying element leading to compositional redistribution in irradiated alloys both in the bulk and near-surface regions. Figure 4 shows the temperature regime where these mechanisms are dominant. All of these mechanisms must be taken under account in the design of proposed advanced materials for the electrodes and the collector optics in addition to considering other bombardment-induced

**4. Irradiation-driven mechanisms on material surfaces** 

mechanical properties of the material accelerating degradation.

propagation. These analyses along with erosion material modeling (discussed in Section 4) are mainly used to dictate materials selection for electrode materials in EUV DPP sources.

#### **2.3 EUV radiators, debris generation and debris mitigation systems**

One particularly important "coupling" effect between the debris mitigation zone region and the collector optics region is the use of inert mitigation gases (e.g. Ar or He) that in turn are ionized by the expanding radiation field and thus generate low-temperature plasma near the collector mirror surface. This phenomenon is briefly discussed in Section 3. Each candidate radiator (e.g. Li, Sn or Xe or any combination) will result in a variety of irradiation-induced mechanisms at the collector mirror surface. For example, if one optimizes the EUV 13.5-nm light source for Li radiators, the energy, flux and mass distributions will be different compared to Sn. Both of these in turn are also different from the standpoint of contamination given that both are metallic impurities and Xe is an inert gas. The former will lead to deposition of material on the mirror surface. In the case of Xe, thermal deposition would be absent however the energetic Xe implantation on the mirror surface could lead to inert gas damage such as surface blistering and gas bubble production for large doses. Debris mitigation systems would have to be designed according to the radiator used.
