Patent Number: 
Section: claims

1. An extreme ultraviolet (EUV) radiation source, comprising:a fuel droplet generator configured to provide a plurality of fuel droplets to an EUV source vessel along a first trajectory;a primary laser configured to generate a primary laser beam directed towards the plurality of fuel droplets, wherein the primary laser beam has a sufficient energy to ignite a plasma that emits extreme ultraviolet radiation from the plurality of fuel droplets;a collector mirror that is symmetric about a vertex and that is configured to focus the extreme ultraviolet radiation to an exit aperture of the EUV source vessel located below the collector mirror, wherein the collector mirror is oriented at a first orientation that causes a normal vector extending outward from the vertex of the collector mirror to intersect a vector of a gravitation force by an angle that is less than 90° and further causes a horizontal line extending through the vertex of the collector mirror to overlie an intersection of the first trajectory and the primary laser beam; andwherein the collector mirror has an opening extending through the vertex, and the first orientation of the collector mirror further causes the primary laser beam to extend along the normal vector through the opening from a position overlying the vertex. 2. The EUV radiation source of claim 1, wherein the plurality of fuel droplets are provided to the EUV source vessel along the first trajectory that intersects the normal vector by an angle that is less than 90°. 3. The EUV radiation source of claim 1, further comprising:a pre-pulse laser configured to generate a pre-pulse laser beam, having a lower energy than the primary laser beam, that deforms the plurality of fuel droplets prior to the primary laser beam hitting the plurality of fuel droplets; andwherein the pre-pulse laser beam extends in a first direction that is not parallel to a direction of the primary laser beam, and wherein the direction of the primary laser beam is substantially parallel to an optical axis of the collector mirror. 4. The EUV radiation source of claim 1, further comprising:a fuel droplet collection element located below the fuel droplet generator, wherein the fuel droplet generator and the fuel droplet collection element are aligned along a line that intersects the normal vector by an angle that is less than 90°. 5. The EUV radiation source of claim 4,wherein a topmost point of the collector mirror is arranged above the intersection of the first trajectory and the primary laser beam and is separated from the fuel droplet generator by a first lateral distance, andwherein a bottommost point of the collector mirror is arranged below the intersection of the first trajectory and the primary laser beam and is separated from the fuel droplet collection element by a second lateral distance that is larger than the first lateral distance. 6. The EUV radiation source of claim 1, further comprising:a tin droplet collection element having an angled surface arranged within an interior of the EUV source vessel vertically below the collector mirror, wherein a vertical distance between the angled surface and a bottom of the collector mirror increases as a lateral distance between the angled surface and the bottom of the collector mirror increases. 7. The EUV radiation source of claim 6, further comprising:an intermediate focus unit comprising a cone shaped aperture arranged within the exit aperture between the EUV source vessel and a downstream scanner comprising a plurality of optical elements configured to convey the extreme ultraviolet radiation to a semiconductor workpiece. 8. The EUV radiation source of claim 1, wherein the collector mirror is positioned at a location that is laterally adjacent to and vertically above the intersection of the first trajectory and the primary laser beam. 9. The EUV radiation source of claim 1, wherein the vertex of the collector mirror is arranged vertically above the intersection and a bottommost point of the collector mirror is located vertically below the intersection. 10. The EUV radiation source of claim 1, further comprising:a debris collection element having one or more angled corrugated surfaces extending between the collector mirror and the exit aperture. 11. The EUV radiation source of claim 1, further comprising:a debris collection element having one or more angled corrugated surfaces arranged between the plasma and the EUV source vessel; anda fuel droplet collection element arranged between the debris collection element and the plasma. 12. The EUV radiation source of claim 1, further comprising:a debris collection element having a lower angled surface and an upper angled surface that are configured to collect debris from the plasma, wherein the lower angled surface continuously extends from below the collector mirror to a location that is laterally between the plasma and the exit aperture, and wherein the lower angled surface intersects the vector of the gravitation force by a second angle that is less than 90°. 13. The EUV radiation source of claim 12, wherein the second angle is larger than the angle. 14. A method of generating extreme ultraviolet (EUV) radiation, comprising:providing a plurality of fuel droplets into an EUV source vessel;striking the plurality of fuel droplets with a primary laser beam at an intersection to generate a plasma that emits EUV radiation;focusing the EUV radiation at a focal point using a collector mirror oriented at a first orientation that causes a normal vector extending outward from a vertex of the collector mirror to intersect a vector of a gravitation force by an angle that is less than 90°, wherein the first orientation further causes the vertex to overlie the intersection and the normal vector to further intersect an exit aperture of the EUV source vessel located below the collector mirror; andwherein the collector mirror has an opening extending through the vertex of the collector mirror, and wherein the first orientation of the collector mirror further causes the primary laser beam to extend from a position overlying the vertex and through the opening along the normal vector. 15. The method of claim 14, further comprising:striking the plurality of fuel droplets with a pre-pulse laser beam that deforms the plurality of fuel droplets prior to striking the plurality of fuel droplets with the primary laser beam, wherein the pre-pulse laser beam extends in a direction that is not parallel to a direction of the primary laser beam. 16. The method of claim 14, further comprising:providing the EUV radiation to a semiconductor workpiece via an EUV photomask having a patterned multi-layered reflective surface. 17. An extreme ultraviolet (EUV) radiation source, comprising:a fuel droplet generator configured to provide a plurality of fuel droplets to an EUV source vessel along a first trajectory substantially extending in a same direction as a vector of a gravitation force;a primary laser configured to generate a primary laser beam directed towards the plurality of fuel droplets, wherein the primary laser beam has a sufficient energy to ignite the plurality of fuel droplets to generate a plasma that emits extreme ultraviolet radiation;a collector mirror oriented so that a normal vector extending outward from a vertex of the collector mirror intersects the vector of the gravitation force by an angle that is less than 90°, wherein the normal vector intersects an exit aperture of the EUV source vessel that is located below the collector mirror; andwherein the primary laser beam is configured to intersect the first trajectory at an intersection point that is located vertically below a horizontal line extending through the vertex of the collector mirror and wherein an uppermost point of the exit aperture is located below the intersection point. 18. The EUV radiation source of claim 17,wherein a topmost point of the collector mirror is arranged above the intersection point and is separated from the fuel droplet generator by a first lateral distance, andwherein a bottommost point of the collector mirror is arranged below the intersection point and is separated from a fuel droplet collection element by a second lateral distance that is larger than the first lateral distance. 19. The EUV radiation source of claim 17,wherein the collector mirror has an opening extending through the collector mirror at a location that is offset from the vertex of the collector mirror; andwherein the primary laser beam extends through the opening along a second trajectory that intersects the first trajectory at a substantially perpendicular angle. 20. The EUV radiation source of claim 17, wherein the vertex of the collector mirror is arranged vertically above the intersection point and a bottommost point of the collector mirror is located vertically below the intersection point.