Local purge within metrology and inspection systems

1. An apparatus comprising:

an optical metrology or inspection head that produces a light beam that is incident on a substrate to be optically measured and is received by the optical metrology or inspection head after interacting with the substrate;

a chuck for holding the substrate, wherein at least one of the chuck and the optical metrology or inspection head is movable to position the substrate at a plurality of measurement positions with respect to the optical metrology or inspection head; and

a purge gas distribution manifold coupled to a purge gas source, the purge gas distribution manifold having at least one port through which the light beam is transmitted, the purge gas distribution manifold having a bottom surface that is 25 mm or less from a top surface of the substrate and has a plurality of apertures through which purge gas is expelled over the top surface of the substrate, wherein the plurality of apertures are distributed over a surface area that is at least as large as a surface area of the top surface of the substrate to distribute the purge gas over an entirety of the top surface of the substrate at all measurement positions of the substrate with respect to the optical metrology or inspection head.

2. The apparatus of claim 1 , wherein the purge gas comprises an inert gas.

3. The apparatus of claim 2 , wherein the inert gas comprises at least one of clean dry air, nitrogen, argon or a combination thereof.

4. The apparatus of claim 1 , wherein the plurality of apertures in the bottom surface of the purge gas distribution manifold is formed by a porous media.

5. The apparatus of claim 1 , wherein the plurality of apertures in the bottom surface of the purge gas distribution manifold is a pattern of holes.

6. The apparatus of claim 5 , wherein the holes are coupled to distribution grooves.

7. The apparatus of claim 1 , wherein the purge gas distribution manifold is partitioned into a plurality of zones, the purge gas distribution manifold further comprises a plurality of valves coupled to the purge gas source, wherein each zone is associated with at least one valve, each valve being controlled to reduce or stop a flow of purge gas to an associated zone when the substrate is not present within the associated zone.

8. The apparatus of claim 7 , further comprising a plurality of bypass valves associated with each valve, wherein each bypass valve provides a reduced flow of purge gas to maintain a charge of purge gas within an associated zone when an associated valve is shut off.

9. The apparatus of claim 1 , wherein the purge gas distribution manifold is held linearly stationary with respect to the substrate when the at least one of the chuck and the optical metrology or inspection head is moved to position the substrate at the plurality of measurement positions with respect to the optical metrology or inspection head.

10. The apparatus of claim 1 , further comprising a fence surrounding the substrate on the chuck, wherein the fence is held linearly stationary with respect to the substrate when the at least one of the chuck and the optical metrology or inspection head is moved to position the substrate at the plurality of measurement positions with respect to the optical metrology or inspection head.

11. The apparatus of claim 10 , wherein the fence comprises an upper surface that is 10 mm or less from the bottom surface of the purge gas distribution manifold.

12. The apparatus of claim 1 , further comprising a reference chip, wherein the purge gas distribution manifold extends over the reference chip.