Radiation generated plasmas:a challenge in modern lithography (2008)

This thesis deals with the study of the properties of plasmas generated by extreme ultra- violet (EUV) radiation and the interaction of these plasmas with multilayer mirrors. The presence of the radiation generated plasma in front of the multilayer mirrors forms a po- tential threat to the lifetime of the optical components of lithography tools. In the plasma boundary layer in front of the mirror, ions are accelerated by the self-generated electric ¯eld towards the mirror. In case these ions gain su±cient velocity, they can damage the mirror surface in a process called physical sputtering. The properties of the plasma are investigated in two ways: by experimental observations and by theoretical considerations. The plasma is far from equilibrium due to the low densities of the plasma constituents and the transient nature of the EUV sources. This puts severe limitations on the plasma diagnostic techniques that can be used to measure the plasma properties. A computer code has been written to obtain better fundamental insight into the dy- namics of the ion acceleration in the plasma boundary layer. The code combines a one- dimensional Particle-In-Cell model with the method of Monte Carlo collisions. In this kinetic plasma model, the motion of the charged particles is followed under the in°uence of their self-generated electric ¯eld. The PIC-MC simulations calculate the °ux and energy distribution of the ions that hit the multilayer mirror. It is found that the energy at which ions hit the mirror is limited by the average electron energy in the plasma. To obtain the damage to the multilayer mirror, the distributions must be convoluted with the sputter yield, which is the number of target atoms removed per incoming ion as a function of the kinetic energy of the ion projectile. The sputter yield shows a threshold behavior. Ions with an impact energy below the sputter threshold energy do not contribute to sputtering. It is found that the damage to the mirror is mainly the result of doubly charged argon ions, whereas singly charged argon ions contribute only for a small amount. The predicted sputter rate is extremely low, typically 1 nm per billion EUV pulses. Furthermore, the impact energy of the ions is in°uenced by the energy of electrons that are emitted from the mirror as a result of the photo-electric e®ect. Most of the emitted 175 photo-electrons are much less energetic than the electrons that are created in the bulk of the plasma by photoionization. The photo-electrons decrease the average electron energy in the plasma, which in turn leads to a smaller electric ¯eld in the plasma boundary layer, and as a result, a lower impact energy of the ions on the multilayer mirror. With the PIC-MC model, a parameter study has been carried out to determine the e®ect of both the pressure and the EUV intensity on the sputter rate. Simulations show that the sputter rate increases monotonically with the intensity. The pressure dependence shows a more complex behavior. Besides argon, also mixtures of argon with hydrogen gas have been simulated with the PIC-MC model. When hydrogen gas is added to the background gas the sputter rate decreases, mainly because the hydrogen acts as an energy sink for the electrons in the plasma as a result of inelastic electron-neutral collisions. To study the plasma-wall interaction, experiments have been performed. To verify the energy dependence of the sputter yield, measurements have been carried out at Ar- gonne National Laboratory, Illinois. Results show good agreement with the semi-empirical formula of Yamamura that is used in the PIC-MC model. In a test facility at ASML, Veldhoven, experiments have been conducted to experimen- tally determine the sputter rate of multilayer mirrors. For this purpose, a collimated beam of EUV radiation is focussed on a mirror sample. With EUV re°ectometry and secondary photo-electron emission measurements, the condition of the top layer of the mirror can be monitored in-situ. In combination with ex-situ X-ray Photo-electron Spectroscopy (XPS) changes to the thickness of the top layer can be measured. The sputter rate measurements are hampered by the EUV radiation induced growth of a thin layer of carbon on top of the mirror. It is shown that with atomic hydrogen cleaning the carbon contamination can be removed from the mirror. During the sputter rate measurements EUV exposures are alternated with atomic hydrogen cleaning cycles. Furthermore, sputter rate measurements have been carried out on a separate experi- mental setup, which has been cleaned to reduce the carbon growth rate. On this setup the sputter rate is too low to be detected. However, if the EUV bandpass ¯lter at the entrance to the measurement chamber is removed strong sputtering as a result of the radiation generated plasma is observed. In conclusion, the investigations in this thesis provide better understanding of the mech- anism of sputtering in EUV radiation generated plasmas. The measurement results are in agreement with the predictions of the PIC-MC simulations that the sputter rate of multi- layer mirrors is extremely low. Moreover, with the addition of hydrogen to the background gas the sputter rate can be even further reduced. The PIC-MC model provides a sound basis for future research on transient phenomena in low-density plasmas. Stralingsgegenereerde Plasma's een uitdaging in de moderne lithography Dit proefschrift gaat over de studie van de eigenschappen van plasma's die ontstaan door extreem ultraviolette (EUV) straling en de interactie van zulke plasma's met multilaag- spiegels. De aanwezigheid van een door straling gegenereerd plasma in de onmiddellijke nabijheid van multilaagspiegels vormt een mogelijke bedreiging voor de levensduur van deze optische onderdelen van lithogra¯emachines. In de grenslaag van het plasma worden ionen versneld richting de spiegel door het zelfgegenereerd elektrische veld. Als deze ionen voldoende snelheid ontwikkelen kunnen zij schade aanrichten wanneer zij botsen met het spiegeloppervlak. Dit proces heet fysisch sputteren. De eigenschappen van het plasma zijn op twee manieren onderzocht: door experimenten te verrichten en door theoretische beschouwingen. Het plasma is ver uit evenwicht vanwege de lage deeltjesdichtheid in het plasma en vanwege de gepulste aard van de EUV bronnen. Dit stelt hoge eisen aan de diagnostische methoden die gebruikt kunnen worden om de eigenschappen van het plasma te meten. Om fundamenteel inzicht te krijgen in de dynamica van het versnellen van ionen in de grenslaag van het plasma is een computerprogramma ontwikkeld. Dit programma combi- neert een 1-dimensionaal Particle-In-Cell model met de methode van Monte Carlo botsin- gen. Dit kinetische plasmamodel simuleert de beweging van geladen deeltjes onder invloed van hun zelfgegenereerd elektrisch veld. Met de PIC-MC simulaties kunnen de °ux- en energieverdeling van de ionen die botsen met de spiegel worden berekend. Hieruit is geble- ken dat de energie waarmee de ionen de spiegel raken wordt begrensd door de gemiddelde energie van de elektronen in het plasma. Om de schade die aan de multilaagspiegels wordt toegebracht te kunnen uitrekenen, moeten de °ux- en energieverdeling worden gewogen met de sputteropbrengst: het aantal atomen dat van de spiegel wordt afgeslagen per inkomend ion als functie van de kinetische energie van het ion. De sputteropbrengst vertoont drempelgedrag. Ionen met een energie onder de drempelenergie dragen niet bij aan het sputterproces. Uit de simulaties is verder gebleken dat de schade aan de spiegels vooral ontstaat door dubbelgeladen argon ionen, terwijl enkelgeladen argon ionen in veel minder mate bijdragen. De voorspelde sputtersnelheid is extreem laag: 1 nanometer per miljard EUV pulsen.