Andrea Z. Giovannini

Laser-produced plasma light source for extreme-ultraviolet lithography applications

High-brightness extreme-ultraviolet light sources are required for mask inspections and metrology, including mask blank inspection, actinic pattern inspection, and aerial image measurement system to improve yield and lower cost of ownership. Laser-produced plasma (LPP) light sources have the highest potential to achieve the brightness requirements for all the range of mask inspection tools currently foreseen. High brightness of LPP sources (100 to 1000  W/mm2 sr) is the result of a smaller source size ( ∼ 0.1  mm) than that of competing technologies. Since brightness is inversely proportional to the area of the source, smaller source size corresponds with greater brightness and hence greater inspection throughput. At the Laboratory for Energy Conversion of ETH Zurich, a fully operational continuous-running multi-kHz LPP light source has been developed over the last five years and is now undergoing system optimization. Adlyte, a spin-off of ETH Zurich, is working with industry leaders to commercialize this LPP source. Individual subsystem configuration and the physical boundary conditions and limitations that affect power, brightness, stability, and lifetime management are discussed. This integrated system produces a measured brightness of 259  W/mm2 sr. Outlook for the future growth and integration of the source in high-volume manufacturing tools is then discussed.

Three-dimensional extreme ultraviolet emission from a droplet-based laser-produced plasma

The emission of extreme ultraviolet radiation in a 2% bandwidth centered at 13.5 nm (in-band) is measured in one hemisphere. The targets of the laser are tin droplets. In-band emission is measured at angles from the laser axis larger than 120°. Analytical models representing the physical processes are developed and calibrated with the experimental data. In the models two assumptions are investigated, isentropic and isothermal 2D-axisymmetric expansion. The parameters of the models are the density distribution of the plasma and the location where the EUV emission is centered. The parameters are inferred by the calibration of the models with the experimental data. The predictions of the models are validated with experiments where slab targets were used.

Effects of the dynamics of droplet-based laser-produced plasma on angular extreme ultraviolet emission profile

The emission distribution of extreme ultraviolet (EUV) radiation from droplet targets is dependent on the dynamics of the laser-produced plasma. The EUV emission is measured on a 2% bandwidth centered at 13.5 nm (in-band). The targets of the laser are small (sub-50 μm) tin droplets, and the in-band emission distribution is measured for different laser irradiances and droplet sizes at various angular positions. Larger droplets lead to a faster decay of EUV emission at larger angles with respect to the laser axis. A decrease in laser irradiance has the opposite effect. The measurements are used together with an analytical model to estimate plume dynamics. Additionally, the model is used to estimate EUV emission distribution for a desired droplet diameter and laser irradiance.

Clean and stable LPP light source for HVM inspection applications

At the Laboratory for Energy Conversion, ETH Zurich a new tin droplet-based laser-produced plasma source with application in EUV lithography is operational since Q3 2013. The EUV source ALPS II is equipped with a large capacity droplet dispenser and a high power (kW), high repetition rate (>6 kHz) Nd:YAG laser. The new source should address the requirements of high volume manufacturing for different inspection and metrology applications found in EUV lithography. The average source brightness is equal to 350 W/mm2sr. Individual droplet tracking in time and space, which is coupled to a droplet positioning and triggering system helps to increase the pulse-to-pulse EUV emission stability of the source. The lateral droplet stability is on the order of 10-15% of the droplet diameter. The individual droplet triggering yields deviations between the laser trigger and the droplet passage time at the irradiation site of less than 1 us, even for large droplet timing fluctuations (>5%). The in-band EUV radiation is measured with an energy monitor, which is coupled to a fast analog hardware-based integrator. The pulse-to-pulse EUV energy stability for high stability data equals 3% (σ). In the case of window-averaged (0.1 s) data, the EUV stability equals 0.86% (σ). Low stability data is also reported. The large brightness of the presented LPP-based light source can be tuned to adjust the EUV light stability that is required by the inspection tool.

Angular ion species distribution in droplet-based laser-produced plasmas

The angular distribution of the ion species generated from a laser irradiated droplet target is measured. The employed instrument was an electrostatic energy analyzer with differential pumping. Singly and doubly charged ions were detected at an argon ambient gas pressure of 2 × 10−2 mbar. The amount of Sn+ and Sn2+ and their kinetic energy is measured from 45° to 120° from the laser axis. Sn+ expands approximately isotropically, and Sn2+ expansion is peaked towards the incoming laser radiation. The singly charged ion kinetic energy is close to constant over the measurement range, while it decreases by around 30% for Sn2+. A calibrated model of the ion expansion that includes recombinations correctly predicts the mean ion charge distribution. The model is able to qualitatively estimate the influence of the laser wavelength on the mean ion charge distribution. The results show a more pronounced isotropic distribution for shorter wavelengths, and a more forward-peaked distribution for longer wavelengths. The...

Normal incidence collector for LPP sources with integrated debris mitigation

The leading candidate for the manufacture of next generation semi-conductor devices is extreme ultraviolet lithography, with laser-produced plasmas as a candidate 13.5nm light source. A primary challenge for continuous operation is elimination of the debris load on the collector, without compromising the radiation intensity at the intermediate focus. A novel combination of thermal management and debris mitigation is developed in order to reduce the deformation and degradation of collector optics. This patent-pending novel debris mitigation technique enables the continuous operation of the source. The paper shows atomic force microscope measurements of the collector surface deposition. The debris mitigation system run at 50% design operating condition gives 90% debris mitigation effectiveness. The average temperature of the cooled collector is 36.5°C at the design point, with a peak-to-peak difference of 0.8°C. The collector geometry, after being adapted to compensate the thermal deformation, yields a spot size of 30μm at IF. Computational simulations, using ETHs multi-scale computational tools, complement the presented experimental results.

Extreme ultraviolet and out-of-band radiation emission from a tin-droplet-based LPP source

Extreme Ultraviolet Lithography (EUVL) is a leading candidate for the future development of smaller and faster microchips with feature sizes of 32 nm or less. Tin laser-produced plasmas (LPPs) are one of the most promising sources of in-band radiation for EUV lithography and inspection applications. However, ions emitted from these LPPs may cause significant damage to components, specifically the collector optics. Tin-droplet targets have the ability to supply the minimum mass required to generate the EUV radiation, leading to substantial decrease in the amount of generated debris. Absolute intensity measurements of the EUV radiation formed using a droplet target, have been recorded. Measurements were recorded over 2 pi steradian with respect to the plasma. The droplet generator, a fully in-house developed system, was synchronized with a Nd:YAG laser operating at a frequency of 5 kHz. The laser pulse was focused to a power density of approximately 1011 W/cm2 to maximize in-band emission. EUV measurements were recorded at the Intermediate Focus of the High Brightness Source of the Laboratory for Energy Conversion. To complement the EUV measurements, out-of-band measurements were simultaneously recorded with a calibrated spectrometer from 300 to 1000 nm.

Self-confined plasma in a magneto-plasma compressor and the influence of an externally imposed magnetic field

The spatial profile of a magnetically self-confined plasma formed by a magneto-plasma compressor is measured in radial and axial direction with an array of Faraday cups, and radiation with a wavelength as short as extreme ultraviolet is detected. A Gaussian distribution approximates the radial profile of the ion current density. Along the axis, the ion current density remains approximately constant up to 2.25 inner anode radii from the electrodes end, before diverging. The plasma stream is further compressed by the addition of an external magnetic field. The external magnetic field is measured and is close to the self-induced magnetic field. The measurements of the profile show an increase in the ion current density after the inlet into the external magnetic field. The ion current density at 3.25 inner anode radii from the end of the electrodes doubles, and the width of the plasma stream decreases. Estimations show that the increase of the ion current density is also due to an increase of around 25% of the mean ion charge, hence to a temperature increase of around 50% enhancing photon emission at shorter wavelength. A design guideline is presented to dimensionalize the amplitude of the external magnetic field depending on the wanted temperature increase in the plasma stream.

Rarefied flow expansion in linear aerospikes

The formation of the kinetic boundary layer (KBL) on diverging surfaces is studied experimentally and computationally. The diverging surfaces are chosen to follow profiles commonly used for linear aerospikes, in order to study the KBL formation in an ideal and two-dimensional flow expansion bounded only on one side. Experiments with different operating conditions at low Re number (from 132 to 2826) and in the transition regime (Kn number from 6.4 to 0.42) are used to evaluate the thickness of the KBL. The ambient pressure, the ratio between the stagnation and surface temperature, the roughness and the shape of the surface are parametrically varied maintaining the same pressure ratio between the total and the ambient pressures. Simulations are validated and used to study the influence of gas-surface interaction on formation of the KBL and to quantify the non-equilibrium state of the flow field. It is shown that the topography of the surface does not influence the growth of the KBL, but the flow can be tail...

EUV brightness, spot size, and contamination measurements at the intermediate focus

The next generation of semi-conductor devices will be manufactured using extreme ultraviolet lithography with a laser-produced plasma as a candidate 13.5nm light source. A primary challenge, particularly for metrology tools, is the stability and the brightness of the generated EUV at the intermediate focus. In the experimental facility at ETH a novel collecting system is studied to optimize brightness and stability, and to avoid contamination after the intermediate focus. Different experimental studies are shown to confirm the designs success for both the EUV beam quality and lack of contamination after the intermediate focus.