In a semiconductor manufacturing process, a substrate is supported on a substrate support. Specifically, the substrate is supported on a plurality of burls protruding from the a surface of the substrate support.During use in the semiconductor manufacturing process, the substrate support may be surrounded by air. Oxygen and water within the air can cause the substrate support to undergo oxidation, in which a lop surface of the substrate support is chemically converted into an oxide film. Water in the air around the top surface of the substrate support may come from humidity in the environment surrounding the substrate support, or from areas around the substrate support where water (as immersion fluid) is present. The oxidation process may be accelerated by the presence of electrostatic charges that build up on an underside of the substrate.The oxide film formed is typically softer than the material of the substrate support or the material of the substrate support coating. Relative movement during clamping and unclamping can cause the oxide film to be abrasively removed. This tribo-corrosion process leads to degradation in the flatness of the substrate support.Further, the oxide film is hydrophilic, so in substrate supports used in conjunction with water as an immersion fluid, the water radially outward of the substrate support is attracted into the region between the substrate and the substrate support. This increases adhesive capillary forces,which can cause changes in the pattern of distortion of the substrate (which may be referred to as a distortion fingerprint or wafer load grid (WLG)).It is an object of the present invention to inhibit the formation of an oxide film on the lop surface of the substrate support, to mitigate substrate support flatness drift and changes in the pattern of distortion of the substrate.According to the present invention, there is provided a substrate holding system comprising a substrate support configured to support a substrate, a gas source, and a plurality of conduits,wherein:the substrate support comprises a first port in a central region thereof and a plurality of second ports radially outwards of the first port;the first port and the plurality of second ports are configured to be in fluid communication with the gas source via the plurality of conduits;the gas source is configured to supply an inert gas to a region between the substrate and the substrate support via a first conduit and the first port, and via a second conduit and the plurality of second ports;the substrate holding system is configured such that the inert gas can be supplied to the region between the substrate and the substrate support through the first port or the plurality of second ports; and the substrate holding system is configured to extract gas from the region between the substrate and the substrate support through the plurality of second ports.According to the present invention, there is also provided a lithographic apparatus including a substrate holding system.According to the present invention, there is also provided a method of supporting a substrate on a substrate support, the substrate support comprising a first port in a central region of the substrate support and a plurality of second ports radially outwards of the first port, the method comprising:a substrate loading step in which an inert gas is supplied to a region between the substrate and the substrate support through the plurality of second ports; and a subsequent substrate clamp step in which the inert gas is supplied to the region between the substrate and the substrate support through the first port.According to the present invention, there is also provided a method of manufacturing a device including a method of supporting a substrate.Further embodiments, features and advantages of the present invention, as well as the structure and operation of the various embodiments, features and advantages of the present invention are described in detail below w
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