Method and apparatus for X-ray and extreme ultraviolet inspection of lithography masks and other objects

• US 6,002,740 A
• Filed: 10/02/1997
• Issued: 12/14/1999
• Est. Priority Date: 10/04/1996
• Status: Expired due to Term

• Current Assignee: The United States of America As Represented By The Secretary of Commerce, Wisconsin Alumni Research Foundation (University of Wisconsin System)
• Original Assignee: Wisconsin Alumni Research Foundation (University of Wisconsin System)
• Inventors: Cerrina, Franco, Lucatorto, Thomas B.
• Primary Examiner(s): Porta, David P.
• Application Number:
  US08/942,757
• Time in Patent Office: 803 Days
• Field of Search: 378/43, 378/34, 378/35, 378/58, 378/62, 250/306, 250/423 P, 382/144, 382/149
• US Class Current: 378/43
• CPC Class Codes: G01B 15/025 by measuring absorption
• CPC Class Codes: G01B 15/06 for measuring the deformation in a solid
• CPC Class Codes: G03F 1/22 Masks or mask blanks for imaging by radiation of 100nm or shorter wavelength, e.g. X-ray masks, extreme ultraviolet [EUV] masks; Preparation thereof
• CPC Class Codes: G03F 1/24 Reflection masks; Preparation thereof
• CPC Class Codes: G03F 1/84 Inspecting
• CPC Class Codes: G03F 7/7065 Defects, e.g. optical inspection of patterned layer for defects
• CPC Class Codes: G03F 7/70666 Aerial image, i.e. measuring the image of the patterned exposure light at the image plane of the projection system

Abstract

Inspection of objects such as X-ray lithography masks is carried out by passing X-rays or extreme ultraviolet light through an object which absorbs in a pattern to provide a patterned X-ray or ultraviolet image which is then directed to a converter. The converter converts the image incident upon it to an image formed by electrons emitted from the converter. The emitted electrons are magnified in an electron microscope and the magnified electron image is displayed by the electron microscope. The visible image may be further digitized and processed by a computer, including long-term storage or display on a computer monitor. X-ray lithography masks may be inspected by passing X-rays through masks of the same type that will be used for lithography so that the magnified image of the X-rays passed through the masks corresponds to the pattern of X-rays that will be incident on a photoresist, allowing accurate inspection of X-ray masks before use.

Claims

1. Apparatus for the inspection of an X-ray mask including an X-ray transparent substrate layer that allows X-rays to pass relatively unimpeded therethrough and X-ray absorbers positioned on the substrate layer in an absorber pattern through which X-rays are passed in a pattern, comprising:

(a) a converter positioned to receive the X-rays in the pattern passed through the X-ray mask and to convert the pattern of X-rays received to a pattern of electrons emitted from the converter with the local intensity directly proportional to the local intensity of the X-rays impinging on the converter;

(b) an electron microscope positioned to receive the electrons emitted from the converter and having electron optics therein to magnify the pattern of electrons and provide a visible display of the magnified pattern;

(c) a camera receiving the visible display and providing an output signal corresponding thereto;

(d) a computer receiving the output signal and digitizing the output signal;

(e) a display connected to the computer to display the digitized image from the computer;

(f) means for storing a digitized as desired pattern of the X-ray mask; and

(g) means for comparing the as designed pattern to the digitized image.

2. The apparatus of claim 1 wherein the converter comprises a thin film phosphor deposited on a membrane to form a photo-emitting cathode.

3. The apparatus of claim 1 wherein the electron microscope comprises a photo-emission electron microscope including an electrostatic imaging column, a microchannel plate image intensifier with the electrostatic imaging column outputting an enlarged photo-emission yield picture of the pattern of electrons emitted from the converter onto the microchannel plate image intensifier, and a phosphorous screen, with the electrons being accelerated by the microchannel plate image intensifier onto the phosphorous screen to convert the photo-emission yield distribution into a visible image.

4. The apparatus of claim 1 including data memory connected to the computer for storage of the digitized image.

5. The apparatus of claim 1 including an X-ray source providing X-rays that are passed through the object.

6. The apparatus of claim 5 wherein the X-ray source is a synchrotron.

7. The apparatus of claim 1 wherein the display comprises a computer monitor.

8. A method for inspection of an X-ray lithography mask having a substrate layer and X-ray absorber material formed on the substrate in a pattern by X-rays, comprising the steps of:

(a) passing X-rays through the X-ray mask such that the X-rays are more greatly absorbed by the absorber material in the pattern than the X-rays passed through the substrate between the absorber material so that the X-rays passed through the X-ray mask define an X-ray image;

(b) directing the X-rays passed through the X-ray mask onto a converter to convert the X-ray image to a corresponding image formed by electrons emitted from the converter; and

(c) magnifying the image of the emitted electrons by an electron microscope and displaying the magnified electron image.

9. The method of claim 8 wherein in the step of passing the X-rays through the object, the X-rays are provided from a synchrotron.

10. The method of claim 8 further comprising the step of digitizing the magnified electron image and storing the digitized image data.

11. The method of claim 8 wherein the X-ray lithography mask is designed to form a pattern on a photoresist layer when the X-ray mask is positioned a selected distance from the photoresist layer and X-rays are passed through the mask onto the photoresist layer, and comprising additionally the step of positioning the X-ray mask at the selected distance from the converter before passing X-rays through the mask and directing the X-rays onto the converter.

12. The method of claim 8 including the additional step of comparing the magnified electron image to an as designed lithography mask pattern.

13. A method for inspection of a lithography mask having material formed thereon for patterning radiation incident on the mask, comprising the steps of:

(a) exposing the mask to radiation to pattern the radiation;

(b) directing the patterned radiation into a converter to convert the patterned radiation to a corresponding patterned image formed by electrons emitted from the converter;

(c) magnifying the image of the emitted electrons by an electron microscope; and

(d) displaying the magnified electron image.

14. The method of claim 13 wherein the lithography mask is an X-ray lithography mask having a substrate layer and an X-ray absorber material formed on the substrate in a pattern and wherein the step of exposing the mask to radiation includes passing X-rays through the X-ray lithography mask.

15. The method of claim 13 wherein the lithography mask is an extreme ultraviolet lithography mask having a pattern of ultraviolet non-reflective materials overcoated on an ultraviolet reflector and wherein the step of exposing the mask to radiation includes directing ultraviolet radiation to impinge on the mask.

16. The method of claim 13 including the additional step of comparing the magnified electron image to an as designed lithography mask pattern.

17. The method of claim 13 wherein the lithography mask is designed to form a pattern on a photoresist layer when the mask is positioned a selected distance from the photoresist layer and the photoresist layer is exposed to radiation via the mask, and comprising additionally the step of positioning the lithography mask at the selected distance from the converter before exposing the mask to radiation and directing the radiation into the converter.