A hemispherical Langmuir probe array detector for angular resolved measurements on droplet-based laser-produced plasmas

Abstract

In this work, a new diagnostic tool for laser-produced plasmas (LPPs) is presented. The detector is based on a multiple array of six motorized Langmuir probes. It allows to measure the dynamics of a LPP in terms of charged particles detection with particular attention to droplet-based LPP sources for EUV lithography. The system design permits to temporally resolve the angular and radial plasma charge distribution and to obtain a hemispherical mapping of the ions and electrons around the droplet plasma. The understanding of these dynamics is fundamental to improve the debris mitigation techniques for droplet-based LPP sources. The device has been developed, built, and employed at the Laboratory for Energy Conversion, ETH Zürich. The experimental results have been obtained on the droplet-based LPP source ALPS II. For the first time, 2D mappings of the ion kinetic energy distribution around the droplet plasma have been obtained with an array of multiple Langmuir probes. These measurements show an anisotropic expansion of the ions in terms of kinetic energy and amount of ion charge around the droplet target. First estimations of the plasma density and electron temperature were also obtained from the analysis of the probe current signals.

Vacuum ultraviolet spectral emission properties of Ga, In and Sn droplet-based laser produced plasmas

“…The acceleration of ions at fixed pressure in the plasma is linked to charge separation effects such as Double Layer formation (DL) and due to the Coulomb electric field in the plasma. These dynamics are discussed in detail elsewhere [26,28]. For our case it is evident from Fig.…”

“…This is also confirmed by charged particles measurements that were carried out with an array of multiple Langmuir Probes located at different distances and angles from the plasma ignition point. The complete description of the Langmuir Probe array and related experiments can be found in [26]. The probes allowed us to measure from the Time Of Flight (TOF) signals of the ions (i.e.…”

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

“…From the ion current that is measured at close distances from the droplet target (few tens of mm), different plasma parameters are obtained such as the electron and ion density and the electron temperature as a function of angle and radial distance from the plasma. 4 The experiments were performed in the ALPS 2 (second facility of the project applied laser plasma science) facility under the same experimental conditions (laser irradiance, background pressure level, and droplet target size) as the ones for the ion angular mapping obtained using the ESA in the present work. The results reported here were obtained for one of the probes composing the array positioned at 82 with respect to the main laser beam direction and at different distances from the irradiation site (20 mm, 30 mm, and 40 mm).…”

“…It was already shown that the spatial density distribution of the droplet-based plasma is ellipsoidal rather than spherical. 3,4 The angular distributions of the different ion charge states need to be studied at distances to the collector optics, which are of interest to the EUVL community. The goal is to evaluate the debris load on the sensitive optical surfaces.…”

Spectral emission properties of a laser-produced plasma light source in the sub-200 nm range for wafer inspection applications

“…The charge particles emission from the plasma has been studied with multiple arrays of Langmuir probes (LPs) and electrostatic ion energy analyzers. 25,26 The emitted radiation from the plasma is studied from the EUV regions with fast x-ray photodiodes to the visible region with different spectrographs.…”