Method for determining absolute reflectance of a material in the ultraviolet range

• US RE34,783 E
• Filed: 08/23/1993
• Issued: 11/08/1994
• Est. Priority Date: 02/01/1990
• Status: Expired due to Term

• Current Assignee: Nanometrics Incorporated (Onto Innovation Inc.)
• Original Assignee: Nanometrics Incorporated (Onto Innovation Inc.)
• Inventors: Coates, Vincent J.
• Primary Examiner(s): Dzierzynski, Paul M.
• Assistant Examiner(s): Dunn, Drew A.
• Application Number:
  US08/111,298
• Time in Patent Office: 442 Days
• Field of Search: 250/372, 356/445, 356/448, 356/51
• US Class Current: 250/372
• CPC Class Codes: G01N 2021/1748 Comparative step being essential in the method
• CPC Class Codes: G01N 21/33 using ultraviolet light G01N21/39 takes precedence
• CPC Class Codes: G01N 21/55 Specular reflectivity

Abstract

A method for determining a value of absolute reflectance of a material at .[.a predetermined.]. .Iadd.any .Iaddend.wavelength, in the ultraviolet range from its measured reflectance which includes system losses contributed by optics, illumination sources, detectors, etc. The method involves the measurement of reflectance from a known material such as single crystal silicon whose absolute reflectance is well known, dividing the measured value by the absolute value to obtain a system efficiency coefficient at the known wavelength and then, without changing the illumination or optics, measuring the reflectance of the unknown material and applying this coefficient to this measured value to obtain its absolute value.

Claims

1. A method for determining an absolute reflectance of material from a microscopic measurement of its measured reflectance in the ultraviolet radiation range, said method comprising the steps of:

determining a value of absolute reflectance of a known material at a predetermined wavelength;

measuring the reflectance of said known material to obtain a value of measured reflectance with a microscope illuminated with radiation at said predetermined wavelength;

with said values of absolute reflectance and measured reflectance, calculating an efficiency coefficient representing all absorption and losses caused by the microscope optical system, its reflectance detectors and its illumination system at said predetermined wavelength;

measuring the reflectance of an unknown material to obtain a second value of measured reflectance with said microscope illuminated with said radiation at said predetermined wavelength;

applying said efficiency coefficient to said second value of measured reflectance to obtain a value of absolute reflectance of said unknown material.

2. The method claimed in claim 1 wherein said step of applying includes the step of .[.multiplying.]. .Iadd.dividing .Iaddend.said second value by said efficiency coefficient.

3. The method claimed in claim 2 wherein said microscope is a reflecting microscope.

4. The method claimed in claim 3 wherein said predetermined wavelength is in the ultraviolet radiation range.

5. The method claimed in claim 2 wherein the determined values of absolute reflectance of said known material, said value of measured reflectance of said known material and said value of measured reflectance of said unknown material are stored in a memory of a computer that performs the step of calculating said efficiency coefficient and said value of absolute reflectance of said unknown material.