Method and apparatus for modifying a substrate surface of a photolithographic mask

1. A method of modifying a surface of a substrate of an optical component for extreme ultraviolet radiation, the method comprising the step of:

focusing femtosecond light pulses of a laser system onto the substrate so that a plurality of color centers is generated inside the substrate, wherein the color centers are distributed to cause a modification of the substrate surface, and wherein a wavelength of the femtosecond light pulses varies in a range of about 0.3 μm to 3.0 μm.

2. The method of claim 1 , wherein the generated color centers are distributed to cause a compaction of the substrate so that a height of the substrate surface is reduced.

3. The method of claim 1 , further comprising the step of measuring the absorption of the generated color centers to determine the height reduction of the substrate surface.

4. The method of claim 1 , wherein a focal point of the femtosecond light pulses is varied within the substrate and/or the femtosecond light pulses are varied in three dimensions and/or varied focal points are overlapping.

5. The method of claim 1 , wherein a repetition rate for the femtosecond light pulses ranges from about 1 Hz to 100 MHz and/or the femtosecond light pulses have a pulse energy within a range of 0.01 μJ and 10 mJ and/or the femtosecond light pulses have a pulse duration in the range of 10 fs to 10000 fs.

6. The method of claim 1 , wherein the femtosecond light pulses are focused into the substrate through a rear substrate surface so that the generated color centers cause a modification of a front substrate surface opposite to the rear substrate surface and/or the femtosecond light pulses are focused into the substrate through the rear substrate surface so that the generated color centers cause a modification of the rear substrate surface.

7. The method of claim 1 , wherein the laser source comprises:

a. at least one light source for generating light pulses of variable pulse duration, repetition rate and energy;

b. at least one objective for focusing light pulses of a light beam; and

c. at least one scanning unit for scanning the light beam across the substrate surface, wherein the pulse duration, the repetition rate, the energy and/or the focusing are selected such that color centers are generated in the portion of the substrate.

8. The method of claim 7 , wherein the at least one scanning unit is adapted to vary the focal point of the laser beam in beam direction and/or a substrate holder is adapted to hold the substrate and to scan the substrate in two and/or three dimensions with respect to the light beam and or the at least one scanning unit and/or the substrate holder are adapted to scan the laser beam so that the light pulses overlap in three dimension.

9. The method of claim 7 , wherein the at least one light source and the at least one objective are adapted to generate intensities up to about 10 20 W/cm 2 and/or the at least one objective is adapted to generate focal points having a diameter of about 1 μm.

10. The method of claim 1 , wherein the optical component is a mirror for extreme ultraviolet radiation, the mirror comprising:

a. at least one substrate having at least one multi-layer structure on a first side; and

b. at least one transparent conductive coating on a second side of the substrate opposite to the first side.

11. The method of claim 1 , wherein the optical component is a photolithographic mask for extreme ultraviolet radiation, comprising:

a. at least one substrate having at least one multi-layer structure and at least one absorbing structure on a first side; and

b. at least one transparent conductive coating on a second side of the substrate opposite to the first side.

12. The method of claim 10 or claim 11 , wherein the at least one transparent conductive coating comprises indium tin oxide and/or comprises fluorine tin oxide and/or aluminum zinc oxide and/or antimony tin oxide.