Marco Wedowski

Molecular contamination mitigation in EUVL by environmental control

Abstract EUVL tools operate under vacuum conditions to avoid absorption losses. Under these conditions, the MoSi multilayer mirrors are contaminated, resulting in reduced reflection and thus throughput. We report on experiments on MoSi mirrors exposed to EUV radiation from a synchrotron. To mimic the effects of EUV radiation we also exposed samples using an electron gun. The oxidation rate was found to be ∼0.016 nm/h per mW/mm 2 of EUV radiation under conditions expected for a high throughput EUVL system. This oxidation can to a large extent be suppressed by using smart gas blend strategies during exposure, e.g. using ethanol. A carbon growth rate of 0.25 nm/h was found for a hydrocarbon pressure of 10 −9 mbar Fomblin. We demonstrate that carbonisation can be suppressed by admitting oxygen during electron gun exposure.

EUV time-resolved studies on carbon growth and cleaning

This paper presents first results to assess the feasibility of a cleaning strategy for EUV production tools. The EUV experiments were performed in a dedicated UHV contamination chamber connected to the DLW20 dipole beam line at the PTB laboratories at BESSY II in Berlin. An in-situ reflectometry system has been implemented inside the contamination chamber to allow for real-time detection of mirror reflection changes. The reproducibility of the in-situ reflectometry system has proven to be about 0.03%, allowing the measurement of reflection changes below 0.1%. Cleaning cycles were performed at producation tool power density levels, i,e,, 10-30 mW/mm2 broadband radiation, on capped Mo/Si mirror samples. Carbon was deposited intentionally at ypical hydrocarbon pressures in the 10-8 mbar regime. Cleaning was done at background levels of hydrocarbons and 10-4 mbar molecular oxygen. First results show that cleaning of a mirror at high power densities and typical tool conditions can restore the reflection close to its initial value.

Lifetime testing of EUV optics using intense synchrotron radiation at the PTB Radiometry Laboratory

Degradation of EUV optics during irradiation is a crucial topic as regards lifetime and performance in EUV lithography. To simulate irradiation conditions for future lithography tools, PTB (the German national metrology institute) operates two dedicated beamlines at the electron storage ring BESSY II. Both, undispersed undulator radiation from an EUV optimized undulator as well as focused and filtered bending magnet radiation can be used. Both beamlines provide EUV radiation with power densities of several mW / mm2. A dedicated irradiation chamber with sample load lock and differential pumping allows components such as substrates, multilayer mirrors or filters to be exposed to EUV radiation under different vacuum conditions. At the same laboratory, high-accuracy EUV reflectometry can be performed for proximate assessment of the resulting performance.

Characterization of large off-axis EUV mirrors with high accuracy reflectometry at PTB

CZ SMT AG produced large off-axis EUV mirrors as they are used e.g. in ASMLs alpha demo tools, the predecessor for Extreme Ultraviolet Lithography (EUVL) production tools by ASML. The coating development and a large part of the actual coatings were done by the FOM-Institute. The Physikalisch-Technische Bundesanstalt (PTB) operates an EUV reflectometry facility at the electron storage ring BESSY II for at-wavelength metrology of full-size EUVL optics with a weight of up to 50 kg and a diameter of 550 mm. Critical issues for EUVL mirrors are the high reflectivity close to the theoretical limit, the matching of the period to the operating wavelength of the stepper (13.5 nm) and the imaging properties of the EUV optics. The full multilayer stack needs to be controlled laterally to such extend that the initial sub-nanometre surface figure of the substrate is preserved. The so-called added figure error should not exceed 100 pm in order to ensure faultless imaging at 13.5 nm wavelength. Here, we discuss representative results obtained at large off-axis EUV mirrors. We especially discuss the challenges of measurements at higher local angles of incidence according to the optical design and the accuracy needed in sample alignment for measurement of the coating profiles. PTB has shown excellent reproducibility for measurements of the near normal incidence reflectance of flat homogeneous mirrors over several years. For large off-axis EUV mirrors, measurements have to be done at angles significantly off normal, which dramatically increases the influence of angular alignment errors of the sample on the measured peak wavelength. Furthermore, according to the optical design, these optics have gradients of the coating thickness which require exact knowledge of the measurement position in the mirror coordinates. Extensive studies were done to estimate and validate the uncertainties connected to the sample alignment. Our results clearly show that it is possible to meet and verify the tight specifications for the lateral coating profiles of EUV multilayer mirrors. The non-correctable added figure error is significantly better than required and the overall reflectance of the coatings with a special protective capping layer is 65%.

Prevention of MoSi multilayer reflection loss in EUVL tools

Extreme ultraviolet lithography requires vacuum conditions in the optical train. In order to maintain sufficient energy throughput, reflection reduction of multilayer mirrors due to contamination has to be minimized. We report on oxidation and carbonization experiments on MoSi mirrors under exposure with EUV radiation from a synchrotron. To mimic the effects of EUV radiation we also exposed samples using an electron gun. The oxidation rate was found to be ~0.015 nm/h per mW/mm2 of EUV radiation under vacuum conditions that are typical for a high throughput EUVL system, I.e. 10-6 mbar H2O. This oxidation can to a large extend be suppressed by using smart gas blend strategies during exposure, e.g. using ethanol. A deposition rate of 0.25 nm/h was found when the hydrocarbon pressure of Fomblin was reduced to 10(superscript -9 mbar. We demonstrate that carbonization can be suppressed by admitting oxygen during electron gun exposure.

Irradiation of EUV multilayer optics with synchrotron radiation of a different time structure

Extensive investigations on the lifetime of EUVL optics using synchrotron radiation [1, 2, 3] have been performed at the radiometry laboratory [4] of the Physikalisch-Technische Bundesanstalt (PTB) at the BESSY II electron storage ring in the past. Nevertheless, synchrotron radiation shows a very different time structure as compared to the radiation of EUVL sources to be used in lithography tools. To assess the question, whether the different time structure of the radiation has an impact on the contamination behavior of EUVL optics, an irradiation experiment was performed using synchrotron radiation of different time structure available at the BESSY II electron storage ring: Keeping all other parameters constant, radiation from the normal operation mode of BESSY II, which resembles quasi-cw- illumination, and the special single bunch operation mode, which gives pulsed synchrotron radiation with 1.25 MHz repetition rate were used to irradiate samples in a defined residual gas environment. The reflectance of the samples were measured before and after the illumination to determine the loss in reflectance due to irradiation. Although the time structure of the single bunch mode still differs considerably from those of potential EUVL sources, trends in the contamination behavior could possibly be observed.