Ultraviolet resistive coated mirror and method of fabrication

• US 5,513,039 A
• Filed: 05/26/1993
• Issued: 04/30/1996
• Est. Priority Date: 05/26/1993
• Status: Expired due to Term

• Current Assignee: Litton Systems Incorporated (Northrop Grumman Corporation), Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Litton Systems Incorporated (Northrop Grumman Corporation)
• Inventors: Louderback, Anthony W., Sun, Ming-Jau, Lu, Samuel, Stewart, Alan F.
• Primary Examiner(s): Dzierzynski, Paul M.
• Assistant Examiner(s): Juba, Jr., John
• Application Number:
  US08/067,385
• Time in Patent Office: 1,070 Days
• Field of Search: 359/359, 359/361, 359/584, 359/586, 359/588, 359/590, 359/589
• US Class Current: 359/584
• CPC Class Codes: G02B 5/0833 comprising inorganic materials only
• CPC Class Codes: G02B 5/0891 Ultraviolet [UV] mirrors apparatus for microlithography exposure G03F7/70; X-ray multilayer structures G21K1/06
• CPC Class Codes: G02B 5/283 designed for the ultraviolet

Abstract

A mirror for reflecting a selected frequency of light includes a plurality of alternating quarter wavelength thick layers of two dielectric materials that are resistive to physical and chemical changes upon exposure to ultraviolet radiation coated upon a substrate. The materials have different refractive indices. The materials preferably are comprised of a mixture of alumina and tantala as the high refractive index layer and alumina as the low refractive index layer.

Claims

1. A mirror for reflecting a selected frequency of light, comprising:

a substrate;

a first set of layers of a first dielectric material comprised of titanium dioxide, TiO₂, and having a refractive index n₁ coated upon the substrate to a thickness equal to one quarter of the wavelength corresponding to the selected frequency in the first dielectric material;

a second set of layers of a second dielectric material comprised of silica, SiO₂ and having a refractive index n₂ coated upon the first set of layers to a thickness equal to one quarter of the wavelength corresponding to the selected frequency in the second dielectric material, the layers of the first and second materials being applied alternately to produce a stack of layers of the first and second materials;

a layer of a third dielectric material that comprises a mixture of SiO₂ and HfO₂ coated upon one layer of the second set of layers; and

a layer of a fourth dielectric material that comprises Al₂ O₃ coated upon the layer of the third dielectric material.

2. The mirror of claim 1 wherein the third dielectric material is approximately 20% SiO₂ and 80% HfO₂.

3. A method for forming a mirror for reflecting a selected frequency of light, comprising the steps of:

forming a first set of layers of a first dielectric material from a material comprising titanium dioxide, TiO₂, and having refractive index n₁ coated on a substrate to a thickness equal to one quarter of the wavelength corresponding to the selected frequency in the first dielectric material; and

forming a second set of layers of a second dielectric material comprising silica, SiO₂, and having refractive index n₂ coated upon the first set of layers to a thickness equal to one quarter of the wavelength corresponding to the selected frequency in the second dielectric material, the layers of the first and second materials being applied alternately to produce a stack of layers of the first and second materials;

forming a layer of a third dielectric material on one layer of the second set of layers from a material comprising a mixture of SiO₂ and HfO₂ ; and

forming a layer of a fourth dielectric material on the layer of the third dielectric material from a material comprising Al₂ O₃.

4. The method of claim 3 including the step of forming the third dielectric material to be approximately 20% SiO₂ and 80% HfO₂.

5. A mirror for reflecting a selected frequency of light, comprising:

a substrate;

a first set of layers of a first dielectric material comprised of titanium dioxide, TiO₂, and having a refractive index n₁ coated upon the substrate to a thickness equal to one quarter of the wavelength corresponding to the selected frequency in the first dielectric material;

a second set of layers of a second dielectric material comprised of silica, SiO₂ and having a refractive index n₂ coated upon the first set of layers to a thickness equal to one quarter of the wavelength corresponding to the selected frequency in the second dielectric material, the layers of the first and second materials being applied alternately to produce a stack of layers of the first and second materials;

a layer of a third dielectric material that comprises a mixture of Al₂ O₃ and HfO₂ ; and

a layer of a fourth dielectric material that comprises Al₂ O₃.

6. The mirror of claim 5 wherein the third dielectric material is 5 to 7 percent Al₂ O₃ and 95 to 93 percent HfO₂.

7. A method for forming a mirror for reflecting a selected frequency of light, comprising the steps of:

forming a first set of layers of a first dielectric material from a material comprising titanium dioxide, TiO₂, and having refractive index n₁ coated on a substrate to a thickness equal to one quarter of the wavelength corresponding to the selected frequency in the first dielectric material; and

forming a second set of layers of a second dielectric material comprising silica, SiO₂, and having refractive index n₂ coated upon the first set of layers to a thickness equal to one quarter of the wavelength corresponding to the selected frequency in the second dielectric material, the layers of the first and second materials being applied alternately to produce a stack of layers of the first and second materials;

forming a layer of a third dielectric material on one layer of the second set of layers from a material comprising a mixture of Al₂ O₃ and HfO₂ ; and

forming a layer of a fourth dielectric material on the layer of the third dielectric material from a material comprising Al₂ O₃.

8. The method of claim 7 including the step of forming the third dielectric material to be between 5 to 7 percent Al₂ O₃ and between 95 to 93 percent HfO₂.