Substrate holder, a lithographic apparatus and method of manufacturing devices
A substrate holder (WT) for use in a lithographic apparatus and configured to support a substrate, the substrate holder comprising: a main body ( 20 ) having a main body surface; and a plurality of burls ( 21 ) projecting from the main body surface; wherein each burl has a distal end configured to engage with the substrate; the distal ends of the burls substantially conform to a support plane whereby a substrate can be supported in a substantially flat state on the burls; and a flow control feature ( 22, 22 c, 22 d ) configured to form a gas cushion adjacent the periphery of the substrate holder when a substrate is being lowered onto the substrate holder.
1. A substrate holder for use in a lithographic apparatus and configured to support a substrate, the substrate holder comprising:
a main body having a main body surface;
a plurality of burls projecting from the main body surface; wherein
each burl has a distal end configured to engage with the substrate;
the distal ends of the burls substantially conform to a support plane whereby a substrate can be supported in a substantially flat state on the burls;
a flow control structure configured to form a gas cushion adjacent the periphery of the substrate holder when a substrate is being lowered onto the substrate holder, wherein the flow control structure comprises a raised area at a height closer to the main body surface than the support plane and disposed adjacent a periphery of the substrate holder and wherein the raised area has a distal surface that is one of inclined relative to the support plane, or has a concave surface; and
a seal structure that is provided radially outward of the flow control structure, wherein the seal structure is configured to reduce gas flow into a space below the substrate to thereby reduce an amount of energy required to maintain an under-pressure.
2. The substrate holder of claim 1 , wherein the raised area comprises a ring around substantially the whole periphery of the substrate holder.
3. The substrate holder of claim 1 , wherein the raised area has a width in a radial direction of the substrate holder in the range of from 3 mm to 80 mm.
4. The substrate holder of claim 3 , wherein the raised area has a width in a radial direction of the substrate holder in the range of from 10 mm to 50 mm.
5. The substrate holder of claim 1 , wherein a distance between the distal surface and the support plane is between about 3 μm and about 10 μm.
6. The substrate holder of claim 1 , wherein a distance between the distal surface and the support plane is no more than 50 μm.
7. The substrate holder of claim 1 , further comprising a plurality of through holes in a central region of the substrate holder.
8. The substrate holder of claim 1 , wherein a distance between a distal seal surface of the seal structure and the support plane is less than a distance between the distal surface of the flow control structure and the support plane.
9. A lithographic apparatus for projecting an image onto a substrate, the lithographic apparatus comprising:
a substrate holder according to claim 1 ; and
a clamp system for clamping a substrate to the substrate holder.
10. A method of manufacturing devices using a lithographic apparatus of claim 9 , the method comprising:
loading a substrate onto the substrate holder;
engaging the clamp system; and
exposing a pattern onto the substrate.
11. The substrate holder of claim 1 , wherein the seal structure comprises a pair of concentric walls that project upwardly from an upper surface of the substrate holder, wherein a small gap of about 3 to 10 μm exists between upper seal surfaces of the pair of walls.
12. The substrate holder of claim 1 , wherein the seal structure is further configured to reduce ingress of an immersion liquid into the space below the substrate.