Consequences of high-frequency operation on EUV source efficiency

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1 Consequences of high-frequency operation on EUV source efficiency Tatyana Sizyuk Center for Materials under Extreme Environment (CMUXE), School of Nuclear Engineering Purdue University, West Lafayette, IN, USA ABSTRACT A potential problem of future extreme ultraviolet (EUV) sources, required for high volume manufacture (HVM) regimes, can be related to the contamination of chamber environment by products of preceding laser pulse/droplet interactions. Implementation of high, 100 khz and higher, repetition rate of EUV sources using Sn droplets ignited with laser pulses can cause high accumulation of tin in the chamber in the form of vapor, fine mist, or fragmented clusters. In this work, the effects of the residual tin accumulation in the EUV chamber in dependence on laser parameters and mitigation system efficiency were studied. The effect of various pressures of tin vapor on the CO 2 and Nd:YAG laser beams propagation and on the size, the intensity, and the resulting efficiency of the EUV sources was analyzed. The HEIGHTS 3D package was used for this analysis to study the effect of residual background pressure and spatial distribution on EUV photon emission and collection. It was found that background pressure in the range of 1-5 Pa does not significantly influence the EUV source produced by CO 2 laser. Larger volume with this pressure conditions, however, can reduce the efficiency of the source. However, an optimized volume of mix with proper density could increase the efficiency of the sources produced by CO 2 lasers. Keywords: EUV, LPP, HVM, Tin vapor, CMUXE, HEIGHTS, Debris mitigation 1. Introduction The success and the cost of the next generation computer chips will depend on the performance of EUV sources and on the duration of the efficient operation and lifetime of these nanolithography devices. While the efficiency of the sources is continuously being improved, their operational cycle is still highly restricted due to the degradation of the optical mirrors as well as necessity of maintaining clean operating chamber environment and components. Current investigations of EUV source production in semiconductor industry are focused on the use of dual-pulse laser produced plasma (LPP) on droplets of liquid tin targets. The main objectives and the challenges in the enhancement of these light sources are related to maximizing the conversion efficiency (CE) of the source as well as to increase components lifetime of the collector optical system. Optimum conditions satisfying both requirement depend on the first laser for target preparation, second laser for EUV photon production, and on the initial droplet target size. The interaction of pre-pulse laser beam with solid/liquid tin droplets results in the 1

2 production of vapor, plasma, and a mist of nano-/micro-fragments. The prevailing concentration of neutrals or ions and size of dust particles agglomeration in the developed mix depend on laser characteristics, mainly pulse duration, intensity, and wavelength. Various experiments showed that picosecond or nanosecond lasers with high intensity can deform or fragment small liquid droplet and expand it to relatively larger volume of mix mist and matter 1,2. Nanosecond lasers with lower intensity can transform small liquid droplets to disk-like targets that allows efficient usage of larger spot size of a second follow up laser 3. The difference in the developed mixed composition affects the rate of tin accumulation in the chamber and determine requirement for the mitigating system. While most of the plasma species and neutrals from preceding iteration can escape the area of the next laser/droplet interactions, the remaining fragmented parts of previous droplets, having much lower velocities 4, can be heated by incoming laser of next iteration and/or absorb fraction of the emitted EUV. For example, spectroscopic, laser-induced-fluorescence analysis showed such possibility of neutrals/fragments remaining after splitting even small, 20 m droplets 5. Various approaches were proposed for the damage mitigation and protection from contamination of the mirror optical collection system by the atomic and ionic debris in EUV chamber. Steady state and pulsed electric fields 6,7, magnetic fields in different configurations 8,9,10,11 were studied and optimized to reduce ions energy and flux to the collecting optics. Additionally, secondary plasma systems 12 were proposed to ionize Sn neutrals which then can be deviated by electric or magnetic fields or a combination of both. Modeling of LPPs in axial magnetic field showed spatial and temporal effects of magnetic field on plasma evolution and ions distribution and suggested that the time-varying magnetic fields as a magnetic pinch would be more efficient for the mitigating purposes 13. Gas flow near the mirror surfaces is currently considered as the main method for in-situ optics cleaning from the deposited Sn debris 8,14. However, buffer gases in the chamber even at relatively low pressure, 1-10 Pa, can absorb in-band and out-band EUV light that results in EUV induced plasma formation and development of various related phenomena 15,16. For example, such plasma can lead to the formation of electric field near mirrors surfaces accelerating Sn ions towards collecting optics 15. Several experimental and modeling studies predicted EUV induced plasma parameters such as electron density and temperature in dependence on gas pressure and EUV pulse energy 17,18. These studies used external source of EUV radiation and estimated the parameters of plasma induced in gas filled chamber without connection to the original EUV created plasma. However, in LPP chamber, EUV induced plasma developed near the mirror system can also be affected by the original plasma created from the target by single- or double-pulse laser beams. Reduced collision and recombination rates at low pressure in the chamber will result in free electrons propagation from the actual source area that can further affect EUV induced plasma properties. These studies require consideration of mixed environment in the chamber and dynamics and interaction of two flows, original plasma and injected gas. Our previous simulation results showed strong dependence of the CE on the properties of mist developed by the short pre-pulse lasers 19. It was shown that, for example, the density in the 2

3 vapor cloud surrounding the micro-fragments has large effect on the source efficiency and resulted in an increase from 2.1% to 3.3% in the CE. Recent experimental results showed that a CE as high as 5% can be achieved using picosecond (ps) lasers for the pre-pulse. Lasers with short pulse duration interacting with 20 m droplets can break the droplet into smaller submicron fragments. This leads to high evaporation and ionization rates of the developed mist particles by the following main laser pulse. Increasing the frequency of droplet generation and laser pulses will lead to accumulation of tin dust in the area surrounding EUV source production. Assessment of this issue and its effect are especially important for high volume manufacture (HVM) and the development of higher power sources where larger droplets are required to produce larger volume of EUV generating plasma. Allowing higher power of EUV source, larger droplets, however, will require higher prepulse laser intensities 3 and will lead to more and larger fragments and mist generation. In this work, the effects of possible accumulation of tin vapor on the EUV source evolution were studied. Modeling of source environment consisting from tin droplets and micro-fragments surrounded by vapor cloud with different pressures was performed using the comprehensive HEIGHTS simulation package. Studying the effect of different vapor volumes and densities around the droplet, we found regimes with transition from the degradation of EUV source efficiency to enhancement of EUV photons emission. Effect of chamber conditions on further fragments/vapor distribution was also analyzed. The maximum source operation frequency and efficiency could be affected and limited by the background environment inherited during source operation. 2. Brief description of modeling approach The HEIGHTS package includes advanced state-of-the art 3-D models for various interactions of laser photons with liquid/vapor/plasma, plumes hydrodynamics, and radiation and thermal processes. We continued to upgrade our HEIGHTS package for the analysis and optimization of EUV sources from LPPs. Our modeling and simulation include all phases of laser-target evolution: from laser/droplet interaction, energy deposition, target vaporization and fragmentation, ionization, plasma hydrodynamic expansion, thermal and radiation energy redistribution, and EUV photons collection as well as detail mapping of photons source location and size. These models were described in several publications 20,21. Brief physics description included in the package are given below. The HEIGHTS package consists of a set of independent modules for describing the main processes taking place in laboratory plasma devices. These integrated processes can be described by the general form of the hydrodynamic equation set for modeling of LPPs with a twotemperature approximation model given by: 3

4 v t v t e t h ei t v e vv p h 0 p h h 0 T T S v ei pi i Ti Qei e e i i rad Q las Here, is the density of plasma; v is the velocity of plasma; p h is the hydrodynamic pressure; eh ee ei ekin is the total energy; e e is the electronic component of the plasma energy, which includes thermal energy of electrons and ionization energy; e i is the ion 2 v component of the plasma energy; and e kin is the kinetic energy of the plasma. Pressure 2 has electron and ion parts ph pe pi. Thermal conduction in the plasma is considered as the combined result of the electron e Te and ion i Ti components, where is the conductivity coefficient and T is the temperature. Laser and plasma radiation processes are represented here as the laser heating source as Q las and flux S rad. The Q ei term is the energy interchange between electrons and ions. Splitting methods are used to separate the hyperbolic and parabolic parts of the above equations that allows implementation of different numerical methods for efficient modeling of hydrodynamic fluxes, heat conduction in plasma, laser heating, and radiation transport. These models for plasmas processes are integrated with models for the description of target evolution which include laser energy absorption/reflection at the surface, heat conduction in material, melting, and evaporation. Radiation transport (RT), one of the critical parts in the modeling of plasma evolution, has two implementations in the HEIGHTS package. These include a direct numerical solution of the RT equation using the Gaussian quadrature method for volume integration along the path of photons, and Monte Carlo models. Both methods were compared regarding the accuracy of produced results and requirements for the solid angle discretization (for the Gaussian quadrature integration) or pseudophotons number (for Monte Carlo integration) 22. The HEIGHTS models continued to be well benchmarked at each interaction physics phase during plasma evolution as well as in the whole integrated LPP systems. The results were extensively benchmarked against experimental studies for the in-band EUV photons production and for debris and ions generation 19. 4

5 3. Plasma and source dynamics at various background conditions To study the effects of the residual tin gas pressure, several background conditions were considered in this analysis. CO 2 laser with 30 ns duration and 100 mj total energy and Nd:YAG laser with 10 ns pulse and 40 mj total energy were used in these simulations. CO 2 laser having 200 m spot size and Nd:YAG laser having 100 m spot size heated 100 m droplet. This resulted in intensities of W/cm 2 and 5x10 10 W/cm 2 respectively. Gaussian temporal and spatial profiles were used in all cases. Much larger spot than droplet size in the case of CO 2 laser was chosen to produce more efficient EUV source that is due to longer duration of this laser and more efficient interaction with expanded plasma. 23. Figure 1 illustrates the modeling approach correspondent to mist cloud which might be formed at the conditions of the high-frequency operation of LPP source. Among the main components of the HEIGHTS package, Monte Carlo radiation transport and Monte Carlo modeling of laser photons interaction with tin droplet, vapor and plasma, include all possible mechanisms of interaction, absorption, reflection and reabsorption. These allows simulating all processes evolution self-consistently, e.g., it takes into account reduction of target heating due to laser absorption in evolving vapor/plasma; at the same time, heating of target by radiation from hot plasma; heating and ionization of vapor cloud around expanding plasma; changing of plume dynamics around the droplet due to high pressure in plasma layer absorbing most of laser photons, that again affects laser photon absorption and reflection. Liquid Tin Laser beam Figure 1. Illustration of mist distribution around the droplet during high-frequency operation of LPP and droplet generator The above approach was used to find conditions affecting EUV source characteristics. Comparison of the results for relatively low pressure, relevant to the pressure of the mitigating gas, and extremely high pressure for EUV devices was done to find the regimes affecting EUV photons generation and collection. Figures 2 (a) and (b) show the difference in the evolution of plasma plume created by CO 2 laser with 30 ns duration from 100 m droplet surrounded by tin 5

6 vapor at pressures of 3 Pa (Fig. 2a) and 300 Pa (Fig. 2b). Such difference in background conditions changes significantly both the density and temperature distribution in the evolving plasma. a) b) Figure 2. Effect of accumulated Sn in focus area on plasma dynamics produced by CO 2 laser: a) 3 Pa background pressure; b) 300 Pa background pressure. White contours show electron density distribution (logarithmic values) Figures 3 (a) and (b) illustrate the effect of the background conditions on the EUV source intensity and shape. In the case of low pressure, combination of relatively high electron temperature of 60 ev and density of ~10 18 cm -3 above the target results in low concentration of EUV producing ions 25 that explains low intensity of EUV production in this area (Fig. 3a). However, at higher pressure conditions (Fig. 3b), large volume with densities of cm -3 and temperatures appropriate for 13.5 nm photons absorption/emission is formed around the actual source near the target. Significant portion of EUV photons emitted in most productive plasma can be absorbed in this volume (Figs. 2b and 3b). Thus, this source has lower intensity and smaller size compared with the source produced at lower pressure. Small portion of collectable EUV photons is produced in the plasma layer formed far above the target with cm -3 electron density and 30 ev temperature (Fig. 2b). However, the intensity of these peripheral sources is much smaller due to two orders of magnitude lower density in comparison with the main source located near the target. Formation of such region in presence of background pressure was discussed in detail previously 25. 6

7 a) b) Figure 3. Effect of background tin on EUV source evolution produced by CO 2 laser: a) at 3 Pa background pressure; b) at 300 Pa background pressure The difference in the CE of the above sources is ~30%. The pressure of 1-5 Pa can be considered as the highest pressure when the background tin vapor does not result in any effects on the source efficiency in comparison with vacuum chamber conditions. Ten times increase in this pressure, for example, to 30 Pa, results in only ~5% decrease in the CE even in the case of relatively large vapor volume expanded up to 1 cm. These are encouraging results considering that very high pressure of 300 Pa would unlikely be established in EUV chamber with efficient mitigating system as well as good vacuum system in the chamber without perturbing the droplet injection system. However, a strong vacuum system in the chamber could affect the target system performance and its stability. Also, depending on the type of the mitigation system, more background gas can be produced. Therefore, it is important to study the limits of the background gas on source performance during the high frequency operation required for future high power devices needed for the HVM. The effect of background conditions also depends on the volume of the surrounding vapor plume. Decrease in the CE can be even at lower pressure if the background volume size is increased 11. Increasing the pressure from 3 Pa to 300 Pa in the volume with around 10 cm layer thickness above the target results in six times drop in the CE. Figures 4 (a) and (b) show simulation results of Nd:YAG laser interaction with droplet surrounded by vapor assuming constant pressure of 300 Pa (Fig. 4a) and pressure gradient with lowest value of 3 Pa and highest value of 300 Pa near the target (Fig. 4b). 7

8 a) b) Figure 4. Effect of background pressure distribution on plasma expansion produced by Nd:YAG laser: a) 300 Pa constant pressure; b) pressure gradient from 300 Pa near the target to 3 Pa at 8 cm above the target (red contours show logarithmic values of electron density) High constant vapor pressure confines plasma around the source increasing EUV reabsorption near the source and in cold relatively dense vapor/plasma above. Comparison of EUV photon absorption (in the range of %) for the two cases considered above is given in Fig. 5. Figure 5 shows high absorption rate in the area surrounding actual EUV source. The source location can be determined by the combination of ev temperature and cm -3 electron density in the case of Nd:YAG produced plasma (Fig. 4). Relatively high EUV absorption in cold layers above, especially in the case of 300 Pa pressure conditions, is related to photoionization of atoms and low charged ions. Lower absorption of photons with energies of ev in plasma with 5-15 ev temperatures can be explained by prevailing concentration of Sn 5+ - Sn 7+ ions in this area which have higher ionization energy than considered EUV photons (94 ev for Sn 5+ ) and do not contribute to these photons emission/absorption process due to EUV transitions 24. a) b) Figure 5. Effect of background tin on EUV photon absorption: a) 300 Pa constant pressure; b) pressure gradient from 300 Pa near the target to 3 Pa at 8 cm above the target (white contours show electron temperature) 8

9 4. Increasing source efficiency by surrounding vapor The above results showed that the efficiency of EUV source can be reduced at higher pressure of the surrounding vapor and at larger vapor volume. However, there are also conditions when the CE of the source can be increased due to the presence of tin vapor surrounding solid/liquid droplets or fragments. Previous studies showed that this effect is different for CO 2 and Nd:YAG lasers 25. Background vapor in a volume with radius of 500 m allows almost two times increase in the CE of the source produced by CO 2 laser. More detailed analysis of plasma evolution at various background and laser conditions allowed identifying the regimes for positive effects of background vapor on the EUV photons emission and collection. These results are presented for single CO 2 laser interaction with 50 μm droplets. Such systems in vacuum chamber conditions produce sources with very low efficiency 26. However, the efficiency can be increased if the proper vapor plume will surround the droplet. Figures 6 and 7 show simulation results for the conditions when small (100 m in diameter) and large (700 m in diameter) volume of vapor surrounded a 50 m droplet. In the case of smaller volume, vapor did not contribute much to the development of EUV source the CE of this source is similar to the CE produced from the droplet in high vacuum conditions. a) b) Figure 6. Plasma density and temperature (white contours) distribution during CO 2 laser interaction with 50 m droplet surrounded by a) small (100 m in diameter) and b) large (700 m in diameter) vapor plumes around the droplet Larger volume of the vapor with atoms concentration of 5x10 17 cm -3, corresponding to pressure of 3.5 kpa, has led to larger EUV source and contributed ~40% of additional EUV output (Fig. 7b). 9

10 a) b) Figure 7. Images of temporal EUV source produced at the peak of laser pulse: a) small (100 m in diameter) and b) large (700 m in diameter) vapor plumes around the droplet Further increase in vapor layer above the droplet allowed achieving source efficiency similar to the efficiency of planar targets created by lasers with large spot size. Figure 8 shows dependence of the source efficiency on the vapor volume indicating the transition to negative effect of background on the CE. Figure 8. CE dependence on background vapor layer around the target in the sources created by Nd:YAG laser The above analysis showed the important effects of the conditions around the droplet on EUV source evolution and production. These studies can be extrapolated to dual-beam systems, when droplet can be split into small fragments and mix created by short pre-pulse laser 27. These studies should be important for the development of higher power sources for HVM when larger targets will be required to produce larger volume of EUV producing plasma and due to increasing difficulty of preparing such larger droplets for the second, CO 2, pulsed laser heating. 10

11 5. CONCLUSION The success and the economic cost of the next generation computer chips will depend on the performance of EUV sources and on the efficient operation and lifetime of these nanolithography devices. While the efficiency of the sources is continuously being improved, their operational cycle is still highly restricted due to the degradation of the optical mirrors as well as necessity of maintaining clean chamber environment and components. These issues will even be more pronounced since the requirements for higher EUV power becomes more demanding. Continued improvement and optimization of EUV lithography technology requires, among others, improving the processes of EUV source development. Increasing the power of sources requires increasing the size of Sn droplets that will lead to more complex processes of target preparation by pre-pulse lasers. It is expected that the source chamber will have some residual gas pressure and fine mist due to the required high frequency operation for the high-volume manufacture (HVM). Strong vacuum system in the chamber could affect the target system, its stability, and cost. In addition, depending on the type of the mitigation system, more background gas can be produced. Therefore, it is important to carefully study the limits of the background gas on overall source performance. Detailed analysis of various mix compositions is required to optimize the source and to reduce debris accumulation in the chamber. The comprehensive HEIGHTS 3D package was used for the analysis of effect of background pressure and volume on EUV photon emission and collection. It was found that background pressure in the range of 1-5 Pa does not significantly influence the EUV source produced by CO 2 laser. Larger volume with this pressure conditions, however, can reduce the efficiency of the source. On the other hand, an optimized volume of mix with proper density could increase the efficiency of the sources produced by CO 2 lasers. ACKNOWLEDGMENTS This work is supported by the National Science Foundation, PIRE Project. We gratefully acknowledge the computing resources provided by the Blues cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory. References 1. S. Fujioka, M. Shimomura, Y. Shimada, S. Maeda, H. Sakaguchi, Y. Nakai, T. Aota, H. Nishimura, N. Ozaki, A. Sunahara, K. Nishihara, N. Miyanaga, Y. Izawa, and K. Mima, Appl. Phys. Lett. 92, (2008). 2. H. Mizoguchi, H. Nakarai, T. Abe, K. M Nowak, Y. Kawasuji, H. Tanaka, Y. Watanabe, T. Hori, T. Kodama, Y. Shiraishi, T. Yanagida, G. Soumagne, T. Yamada, T. Yamazaki, S. Okazaki, and T. Saitou, Proc. SPIE 9422, 94220C (2015). 3. D. Kurilovich, A. L. Klein, F. Torretti, A. Lassise, R. Hoekstra, W. Ubachs, H. Gelderblom, and O. O. Versolato, Phys. Rev. Applied 6, (2016). 11

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