Lithographic apparatus comprising an internal sensor and a mini-reactor, and method for treating a sensing surface of the internal sensor

1. A lithographic apparatus comprising:

a projection system constructed and arranged to project a beam of radiation onto a target portion of a substrate;

an internal sensor having a sensing surface; and

a mini-reactor movable with respect to the sensor, the mini-reactor comprising

an inlet for a hydrogen containing gas,

a hydrogen radical generator, and

an outlet for a hydrogen radical containing gas,

the mini-reactor being constructed and arranged to create a local mini-environment comprising hydrogen radicals substantially only to the sensing surface to treat the sensing surface,

wherein the mini-reactor comprises a reactor chamber with a chamber opening, and wherein an internal surface of the reactor chamber comprises a first material that does substantially not react with hydrogen and hydrogen radicals, the first material having a hydrogen radical surface recombination coefficient of less than or equal to about 0.02, and wherein an external part of the mini-reactor comprises a second material having a hydrogen radical surface recombination coefficient of greater than about 0.02; and

a cooling element constructed and arranged to cool at least part of the mini-reactor.

2. The lithographic apparatus according to claim 1 , wherein the mini-reactor is movable with respect to the sensor in order to create the local mini-environment substantially only to the sensing surface during use of the lithographic apparatus.

3. The lithographic apparatus according to claim 1 , wherein the mini-reactor is movable with respect to the sensor in order for the mini-reactor to be brought in a vicinity of the sensor to create the mini-environment.

4. The lithographic apparatus according to claim 1 , wherein the sensor comprises a sensor selected from the group consisting of a wafer stage sensor, a mask stage sensor, a detector sensor, a collector sensor, a collector frame sensor, an illuminator sensor, an illuminator frame sensor, a position sensor, and an energy sensor.

5. The lithographic apparatus according to claim 1 , wherein the mini-reactor is arranged to purge the sensing surface of the sensor.

6. The lithographic apparatus according to a claim 1 , wherein the sensing surface has a sensing surface area or a sensing surface cross-sectional area, wherein the reactor chamber opening has a chamber opening area, wherein the ratio of the reactor chamber opening area and the sensing surface area or the sensing surface cross-sectional area is in the range of about 0.2-2, and wherein the reactor chamber opening area is substantially equal to or larger than the sensing surface area or the sensing surface cross-sectional area.

7. The lithographic apparatus according to claim 1 , further comprising a treatment sensor arranged to evaluate a result of the treatment.

8. The lithographic apparatus according to claim 1 , wherein the first material comprises quartz or Si 3 N 4 .

9. The lithographic apparatus according to claim 1 , wherein the second material comprises aluminum.

10. The lithographic apparatus according to claim 1 , wherein the cooling element is constructed and arranged to cool the internal surface of the reactor chamber.

11. The lithographic apparatus according to claim 1 , wherein the first material is a coating provided to walls of the reactor chamber.

12. The lithographic apparatus according to claim 1 , wherein an external edge of a wall of the reactor chamber facing the sensing surface, in use, comprises the second material.

13. The lithographic apparatus according to claim 12 , wherein the second material of the external edge is a coating.