Lithographic apparatus, substrate holder and method of manufacturing

1. A lithographic projection apparatus comprising:

a radiation system for providing a projection beam of radiation;

a support structure for supporting a patterning device, said patterning device serving to pattern said projection beam according to a desired pattern to form a patterned beam;

a substrate holder comprising protrusions, each of said protrusions having an extremity, said extremities of said protrusions defining a substantially flat plane of support for supporting a substantially flat substrate, said substrate holder provided with a clamp to provide a pressing force for pressing said substrate against said extremities of said protrusions; said protrusions in an edge zone of said substrate holder arranged to provide a substantially flat overhanging of said substrate in relation to said pressing force of said clamp; and

a projection system for projecting said patterned beam onto a target portion of said substrate, wherein said protrusions in an off-edge zone of said substrate holder are distributed so as to provide a substantially equal supporting area for said protrusions in said off-edge zone, said supporting area being defined by a Voronoi diagram distribution associated with at least a plurality of said protrusions.

2. A lithographic apparatus according to claim 1 , wherein said substrate holder is a substantially circular structure for supporting a substantially circular substrate and wherein said protrusions are arranged in substantially concentric circles in a zone away from a central zone of said circular substrate holder; wherein at least some concentric circles are non-equidistant.

3. A lithographic apparatus according to claim 2 , wherein said protrusions are distributed in substantially equilateral triangular patterns.

4. A lithographic apparatus according to claim 1 , wherein center protrusions are arranged according to a central protrusion and a surrounding first row of protrusions is arranged in a regular hexagonal shape.

5. A lithographic apparatus according to claim 4 , wherein a second row of protrusions is arranged to have 12 protrusions oriented substantially symmetrical around said first row of protrusions.

6. A lithographic apparatus according to claim 2 , wherein said substrate holder comprises a plurality of N openings, each of said N openings defining a second edge zone, wherein each concentric circle traversing said N second edge zones comprises an integer number of N protrusions.

7. A lithographic apparatus according to claim 6 , wherein said plurality of N openings is arranged in an N/3 multiple of sets of three symmetrically configured openings.

8. A lithographic apparatus according to claim 6 , wherein said substrate holder comprises a plurality of M notches, each of said M notches defining a third edge zone, wherein each concentric circle traversing said M third edge zones comprises an integer number of M protrusions.

9. A lithographic apparatus according to claim 8 , wherein said plurality of M notches is arranged in an M/3 multiple of sets of three symmetrically configured notches.

10. A lithographic apparatus according to claim 1 , wherein said substrate holder further comprises a wall for providing a leak seal and wherein said clamp is a vacuum system that forms a vacuum pressure between said substrate holder and said substrate.

11. A lithographic apparatus according to claim 10 , wherein said vacuum pressure ranges from 0.1 to 0.9 bar with respect to ambient pressure.

12. A lithographic apparatus according to claim 11 wherein said vacuum pressure ranges from 0.5 to 0.2 bar with respect to ambient pressure.

13. A lithographic apparatus according to claim 10 , wherein a radial distance x between said wall and a circle of protrusions nearest thereto satisfies a relationship 0.3<x/d<0.6, where d is a mutual radial separation of two circles of protrusions nearest to said wall.

14. A lithographic apparatus according to claim 1 , wherein said edge zone is defined by at least one concentric circle of protrusions arranged nearest to an edge.

15. A lithographic apparatus according to claim 1 , wherein said at least a plurality of said protrusions associated with said Voronoi diagram distribution is only located in said off-edge zone.

16. A lithographic apparatus according to claim 1 , wherein said at least a plurality of said protrusions associated with said Voronoi diagram distribution includes protrusions located in said off-edge zone and protrusions located in said edge zone.

17. A substrate holder for a lithographic apparatus, comprising:

protrusions, each of said protrusions having an extremity, said extremities of said protrusions defining a substantially flat plane of support; and

a clamp to provide a pressing force for pressing said substrate against said extremities of said protrusions; said protrusions in an edge zone of said substrate holder arranged to provide a substantially flat overhanging of the substrate in relation to said pressing force,

said protrusions in an off-edge zone of said substrate holder being distributed so as to provide a substantially equal supporting area for said protrusions in said off-edge zone, said supporting areas being defined by a Voronoi diagram distribution.

18. A method of manufacturing a substrate holder for a lithographic apparatus, comprising:

providing a plate for supporting a substantially flat substrate;

distributing a first plurality of supporting protrusions in an off-edge zone of the plate so as to provide a substantial equal supporting area for the first plurality of supporting protrusions, extremities of each protrusion of the first plurality of supporting protrusions defining a substantially flat plane of support;

arranging a second plurality of supporting protrusions in an edge zone of the plate so as to provide a substantially flat overhanging of the substrate in relation to a force pressing the substrate against the projections; and

calculating a supporting area distribution defined by a Voronoi diagram associated with the protrusions and keeping an area deviation of the supporting area distribution to a minimum.

19. A method according to claim 18 , wherein the method further comprises:

a) calculating a height distribution of the substrate in correspondence with a pressure exerted on the substrate to press the substrate against the first plurality of distributions and to deform the protrusions locally in axial directions;

b) analyzing a focus error and an overlay error associated to the height distribution; and

c) redistributing the first plurality of supporting protrusions in areas wherein either or both of the focus and overlay error exceeds a predefined maximum value; and

d) repeating steps a–c in an iterative way.

20. A method according to claim 19 , wherein the height distribution is calculated in correspondence with a finite element analysis of a bending stiffness of the substrate.