Protective coating for nonlinear optical crystal
An amorphous layer is used as a protective coating for hygroscopic nonlinear optical crystals. The amorphous layer consists of one or more alkali metal borates and/or alkali earth metal borates. The amorphous layer slows or prevents water and/or oxygen from diffusing into the hygroscopic nonlinear optical crystal, thus simplifying handling, storage and operating environmental requirements. One or multiple additional coating layers may be placed on top of the amorphous layer, with the additional coating layers including conventional optical materials. The thicknesses of the amorphous layer and/or additional layers may be chosen to reduce reflectance of the optical component at one or more specific wavelengths. The coated nonlinear optical crystal is used in an illumination source utilized in a semiconductor inspection system, a metrology system, or a lithography system.
1. A system including:
a light source configured to generate incident light having a wavelength in a range between 100 nm and 300 nm;
a sensor; and
an optical system configured to direct the incident light onto a sample and direct light from the sample to the sensor,
wherein the light source includes a nonlinear optical crystal comprising:
a substrate comprising a hygroscopic nonlinear optical material and configured to convert incident light having a wavelength longer than 300 nm to output light having a wavelength between 100 nm and 300 nm; and
a first amorphous material layer disposed on the substrate,
wherein the first amorphous material layer forms a continuous encapsulating structure surrounding an outer surface of the substrate,
wherein the first amorphous material layer consists essentially of one or more alkali metal borates.
2. The system of claim 1 , wherein the hygroscopic non-linear optical material comprises at least one of a cesium lithium borate (CLBO) crystal or a cesium triborate (CBO) crystal.
3. The system of claim 1 , wherein the one or more alkali metal borates include at least one of lithium triborate (LBO), lithium tetraborate (LB4), cesium lithium borate (CLBO), cesium triborate (CBO), or cesium tetraborate (CB4).
4. The system of any of claim 1 , wherein the first amorphous material layer is configured to minimize a reflectivity of at least one face of the nonlinear optical crystal at one or more of the wavelength of the incident light or the wavelength of the output light.
5. The system of claim 1 , wherein the first amorphous material layer has a thickness in a range between 30 nm and 200 nm.
6. The system of claim 1 , wherein the first amorphous material layer is formed directly on a surface of the substrate.
7. The system of claim 1 , wherein the light source further comprises: at least one optical material layer disposed over the first amorphous material layer, wherein the first amorphous layer and the at least one optical material layer are configured such that at least one of the incident light and the output light passes through the first amorphous material layer, the at least one optical material layer, and the surface of the substrate.
8. The system of claim 7 , wherein the at least one optical material layer comprises an optical material having a refractive index that is lower than a refractive index of the amorphous material layer.
9. The system of claim 7 , wherein said at least one optical material layer comprises at least one of magnesium fluoride, calcium fluoride, aluminum fluoride and silicon dioxide.
10. The system of claim 1 , wherein the system comprises at least one of a semiconductor inspection system or a semiconductor metrology system.
11. The system of claim 1 , wherein the system comprises a lithographic system, the system being configured to expose a pattern on the sample.
12. A system including:
a light source configured to generate incident light having a wavelength in a range between 100 nm and 300 nm; and
an optical system configured to direct said incident light onto a sample,
wherein the light source includes at least one nonlinear optical crystal comprising:
a substrate comprising a hygroscopic non-linear optical material and configured to convert incident light having a wavelength longer than 300 nm to output light having a wavelength between 100 nm and 300 nm;
a first amorphous material layer disposed on the substrate; and
a second optical material layer disposed on a top surface of the first amorphous material layer,
wherein the first amorphous material layer forms a continuous encapsulating structure that surrounds the outer surface of the substrate,
wherein the first amorphous material layer consists essentially of one or more alkali metal borates,
wherein the second optical material layer comprises a second optical material having a refractive index that is lower than a refractive index of the amorphous material layer,
wherein the first amorphous layer and the second optical material layer are configured such that a portion of the output light passes through both the first and second optical material layers to the top surface of the substrate.
13. The system of claim 12 , wherein the system comprises at least one of a semiconductor inspection system or a semiconductor metrology system.
14. The system of claim 12 , wherein the system comprises a lithographic system, the system being configured to expose a pattern on the sample.
15. A crystal assembly comprising:
a nonlinear optical crystal comprising a substrate comprising a hygroscopic nonlinear optical material and configured to convert incident light having a wavelength longer than 300 nm to output light having a wavelength between 100 nm and 300 nm; and
a first amorphous material layer disposed on the substrate of the nonlinear optical crystal,
wherein the first amorphous material layer forms a continuous encapsulating structure surrounding an outer surface of the substrate,
wherein the first amorphous material layer consists essentially of one or more alkali metal borates.
16. A crystal assembly comprising:
a nonlinear optical crystal comprising a substrate comprising a hygroscopic non-linear optical material and configured to convert incident light having a wavelength longer than 300 nm to output light having a wavelength between 100 nm and 300 nm;
a first amorphous material layer disposed on the substrate; and
a second optical material layer disposed on a top surface of the first amorphous material layer,
wherein the first amorphous material layer forms a continuous encapsulating structure that surrounds the outer surface of the substrate,
wherein the first amorphous material layer consists essentially of one or more alkali metal borates,
wherein the second optical material layer comprises a second optical material having a refractive index that is lower than a refractive index of the amorphous material layer,
wherein the first amorphous layer and the second optical material layer are configured such that a portion of the output light passes through both the first and second optical material layers to the top surface of the substrate.
17. A method comprising:
providing a nonlinear optical crystal;
annealing the nonlinear optical crystal;
placing the nonlinear optical crystal in an inert environment above a liquid;
setting the temperature of the liquid to a first desired temperature TI;
setting the temperature of the nonlinear optical crystal to a desired temperature Tc;
lowering the nonlinear optical crystal into the liquid;
forming an amorphous layer on the nonlinear crystal; and
removing the nonlinear optical crystal from the liquid after a predetermined time, wherein the nonlinear optical crystal comprises at least one of a CLBO crystal or a CBO crystal, and the liquid consists essentially of at least one of an alkali metal borate or an alkali earth metal borate.
18. The method of claim 17 , wherein the predetermined time is chosen to result in a thickness of the amorphous layer that reduces the reflectivity of the nonlinear crystal at a selected wavelength.
19. The method of claim 18 , wherein the selected wavelength is between 130 nm and 550 nm.
20. The method of claim 18 , wherein the thickness is in the range of 30 nm to 200 nm.
21. The method of claim 17 , further comprising adjusting the temperature of the liquid after lowering the nonlinear optical crystal into the liquid.
22. The method of claim 17 , further comprising a second coating step, the second coating step comprising forming a second layer on the amorphous layer.
23. The method of claim 22 , wherein the second layer comprises at least one of magnesium fluoride, calcium fluoride, aluminum fluoride or silicon dioxide.