In a first example described herein, there is a sensor for determining a property of an EUV radiation beam provided by an EUV radiation source; wherein the sensor comprises: first and second electrodes, each of the first and second electrodes configured to substantially surround a cross-section of the EUV radiation beam; a voltage source configured to apply a potential difference across the first and second electrodes; and a current sensor configured to measure a current flowing between the first and second electrodes. The first and second electrodes may be configured such that flow of a plasma formed by illumination of a gas by the EUV radiation beam causes a current flow between the first and second electrodes. Beneficially, the measured current may correspond to the property of the EUV radiation beam. Accordingly, the first and second electrodes may be, or operate in a manner similar to, Langmuir probes. The sensor may further comprise a controller configured to determine the property based on the measured current. The controller may be configured to determine the property based on the measure current. Beneficially the sensor may provide the determined property of the EUV radiation beam. The sensor may further comprise a pressure sensor configured to measure a pressure sensor of a gas. The pressure sensor may be a capacitance manometer. The measured pressure may be used to determine the property. In examples comprising the controller, the controller may be configured to determine the property based on the measured current and the measured pressure. The gas may be hydrogen. One or more of the first and second electrodes may be provided within the EUV radiation source or the lithographic apparatus. One or more of the first and second electrodes may be configured to substantially surround the cross-section of the EUV beam substantially at an intermediate focus. In other words, a plane defined by the first (or second) electrode may intersect the intermediate focus. Beneficially, the cross-section of the EUV beam may be relatively small at the intermediate focus and thus the first and second electrodes may smaller than otherwise if provided at a position substantially at an intermediate focus. Alternatively, one or more of the first and second electrodes may be configured to substantially surround the cross-section of the EUV beam away from the intermediate focus. Beneficially, such a sensor be subjected to lower pressure gradients and / or lower thermal loads than otherwise. The property may be a power or energy of the EUV radiation beam. One or more of the first and second electrodes may be annular cylinders. One or more of the first and second electrodes may comprise molybdenum or nickel. The first electrode may be one of a plurality of first electrodes. The second electrode may be one of a plurality of second electrodes. A lithographic system may comprise an EUV radiation source, a lithographic apparatus and the sensor of the first example. The system may further comprise a hydrogen gas source. The hydrogen gas source may be configured to provide hydrogen gas. The hydrogen gas may, for example, provide a cleaning effect by removing contaminants from within the system. The gas (which forms the plasma) may be hydrogen gas. In a second example described herein, there is a method of determining a property of an EUV radiation beam comprising: providing first and second electrodes such that each of the first and second electrodes substantially surrounds a cross-section of the EUV radiation beam; applying, with a voltage source, a potential difference across the first and second electrodes; measuring, with a current sensor, a current flowing between the first and second electrodes, wherein the measured current corresponds to the property of the EUV radiation beam. The method may further comprise measuring, with a pressure sensor, a pressure of gas. The method may further comprise providing one or more of the first and second electr
展开▼
