Method for high-precision multi-layered thin film deposition for deep and extreme ultraviolet mirrors

• US 5,911,858 A
• Filed: 02/18/1997
• Issued: 06/15/1999
• Est. Priority Date: 02/18/1997
• Status: Expired due to Term

• Current Assignee: Sandia Corporation (Martin Marietta Corporation)
• Original Assignee: Sandia Corporation (Martin Marietta Corporation)
• Inventors: Ruffner, Judith Alison
• Primary Examiner(s): Nguyen, Nam
• Assistant Examiner(s): VERSTEEG, STEVEN H
• Application Number:
  US08/801,883
• Time in Patent Office: 847 Days
• Field of Search: 204/192.13, 204/192.15, 204/192.23, 204/192.26, 204/192.27, 204/192.28
• US Class Current: 204/192.27
• CPC Class Codes: C23C 14/044 using masks to redistribute rather than totally prevent coating, e.g. producing thickness gradient
• CPC Class Codes: C23C 14/547 using optical methods
• CPC Class Codes: C23C 14/568 Transferring the substrates through a series of coating stations C23C14/562 takes precedence
• CPC Class Codes: G02B 5/0891 Ultraviolet [UV] mirrors apparatus for microlithography exposure G03F7/70; X-ray multilayer structures G21K1/06
• CPC Class Codes: G03F 7/70958 Optical materials or coatings, e.g. with particular transmittance, reflectance or anti-reflection properties

Abstract

A method for coating (flat or non-flat) optical substrates with high-reflectivity multi-layer coatings for use at Deep Ultra-Violet ("DUV") and Extreme Ultra-Violet ("EUV") wavelengths. The method results in a product with minimum feature sizes of less than 0.10-μm for the shortest wavelength (13.4-nm). The present invention employs a computer-based modeling and deposition method to enable lateral and vertical thickness control by scanning the position of the substrate with respect to the sputter target during deposition. The thickness profile of the sputter targets is modeled before deposition and then an appropriate scanning algorithm is implemented to produce any desired, radially-symmetric thickness profile. The present invention offers the ability to predict and achieve a wide range of thickness profiles on flat or figured substrates, i.e., account for 1/R² factor in a model, and the ability to predict and accommodate changes in deposition rate as a result of plasma geometry, i.e., over figured substrates.

Claims

1. A method for fabricating high-precision thin film coatings for multi-layered mirrors in a deposition chamber for use with a light source at a desired wavelength of operation, comprising the steps of:

selecting a first target material and a second target material to be deposited on at least one optical substrate, each of the first and second target materials having a suitable refractive index N at the desired wavelength of operation, the refractive index N being defined by N=n+ik, where n is a real component, i is the square root of negative one, and k is an imaginary component;

calibrating the thickness deposition profiles for the first target material and the second target material;

modeling a desired thickness deposition profile for the first target material and the second target material; and

depositing a plurality of layers of a first thin film layer of the first target material and a second thin film layer of the second target material on the at least one optical substrate to produce at least one multi-layered mirror, the plurality of layers of the first thin film layer and the second thin film layer being deposited at a predetermined pressure, the plurality of layers of the first thin film layer and the second thin film layer having a predetermined number, thickness, and thickness gradient, wherein the predetermined thickness gradient is determined using a thin film coating design program that optimizes reflectance by maintaining a constant optical phase shift for each of the first thin film layer and the second thin film layer, wherein the optical phase shift is determined by δ

=(4π

dN cosθ

)/(λ

)±

π

, where θ

is the optical phase shift, d is the thickness of each of the first thin film layer and the second thin film layer, N is the refractive index of each of the first thin film layer and the second thin film layer, λ

is the desired wavelength of operation, and θ

is the angle of incidence of incoming light from the light source relative to the normal.

2. The method of claim 1, wherein the at least one optical substrate is non-flat.

3. The method of claim 1, wherein the at least one optical substrate is flat.

4. The method of claim 1, further comprising the step of controlling the lateral and vertical thickness of each of the first thin film layer and the second thin film layer of the plurality of layers by scanning the position of the at least one optical substrate with respect to a sputter target during deposition.

5. The method of claim 1, further comprising the step of:

performing an adequate pre-sputter of the first target material and the second target material at a predetermined pressure to remove impurities from the surfaces of the first target material and the second target material.

6. The method of claim 5, wherein said step of performing the adequate pre-sputter and said step of depositing a plurality of layers are performed using DC sputtering power for the first target material and the second target material.

7. The method of claim 6, wherein the DC sputtering power is pulsed-DC sputtering power.

8. The method of claim 1, wherein said step of depositing a plurality of layers comprises the steps of:

sputter depositing the first thin film layer and the second thin film layer;

measuring actual thickness and uniformity of the first thin film layer and the second thin film layer;

comparing actual thickness of the first thin film layer and the second thin film layer to the desired thickness of the first thin film layer and the second thin film layer; and

determining and implementing a scale factor for subsequent depositions.

9. The method of claim 1, wherein the first thin film layer and the second thin film layer are alternately deposited.

10. The method of claim 9, wherein the first thin film layer is silicon.

11. The method of claim 10, wherein the second thin film layer is molybdenum.

12. The method of claim 1, wherein the second target material is selected from the group consisting of Mo, Al₂ O₃, and combinations thereof.

13. The method of claim 1, wherein the first target material is selected from the group consisting of Si, SiO₂, MgF₂, CaF₂, Be, B, C, B₄ C, and combinations thereof.

14. The method of claim 1, further comprising the step of determining an optimum deposition pressure for the deposition chamber.

15. The method of claim 14, further comprising the step of determining an optimum deposition rate.

16. The method of claim 15, further comprising the step of determining an optimum deposition profile of the first target material and the second target material.

17. The method of claim 16, wherein said step of modeling comprises the step of:

accounting for a predetermined thickness gradient of the first thin film layer and the second thin film layer.

18. The method of claim 1, further comprising the step of accounting for a 1/R² dependence of the deposition rate, where R is the normal distance from a target to the at least one optical substrate at any given point on the optical substrate, and for dependence of deposition rate on a plasma geometry at a plurality of points along the at least one optical substrate, the plasma geometry changing in response to the shape of the at least one optical substrate.

19. The method of claim 1, wherein said step of depositing comprises the step of using a computer-based calibration, modeling, and deposition program.

20. The method of claim 1, further comprising the steps of:

using X-ray reflectometry at a predetermined wavelength to obtain a plurality of results and modeling the plurality of results to determine a bi-layer period, a thickness of each of the plurality of layers, and an interfacial roughness and to enable detection of a decoupled signal that results from variations in the thickness of each of the plurality of layers;

determining the thickness of the plurality of layers at a plurality of points on the at least one optical substrate to assess thickness gradient accuracy;

optimizing deposition times and deposition profiles that are used to deposit the plurality of layers in said step of depositing; and

measuring an ultimate reflectance of the at least one multi-layered mirror using the light source at the desired wavelength of operation.

21. The method of claim 20, wherein the light source is a synchrotron radiation source.

22. The method of claim 20, wherein the light source is a laser plasma source.

23. The method of claim 20, wherein the desired wavelength of operation is 13.4-nm.

24. The method of claim 20, wherein the desired wavelength of operation is 193-nm.

25. The method of claim 20, wherein the desired wavelength of operation is 248-nm.

26. The method of claim 1, further comprising the step of positioning a stationary circular aperture in-between the light source and the at least one optical substrate to restrict the lateral extent of a desired deposition profile.

27. The method of claim 1, wherein the desired wavelength of operation is greater than 150-nm, the first target material being selected such that the real component n of the refractive index N of the first target material has a lower value than the real component of the refractive index of the second target material.

28. The method of claim 1, wherein the desired wavelength of operation is less than 150-nm, the first target material being seleceted such that the imaginary component k of the first target material has a lower value than the imaginary component k of the second target material.