Inspecting optical masks with electron beam microscopy

• US 5,665,968 A
• Filed: 02/26/1996
• Issued: 09/09/1997
• Est. Priority Date: 05/27/1992
• Status: Expired due to Term

• Current Assignee: KLA Instruments, Corporation
• Original Assignee: KLA Instruments, Corporation
• Inventors: Brodie, Alan D., Chen, Zhong-Wei, Jau, Jack Y., Meisburger, Dan, Grenon, Brian J.
• Primary Examiner(s): Berman, Jack I.
• Assistant Examiner(s): NGUYEN, KIET TUAN
• Application Number:
  US08/607,191
• Time in Patent Office: 561 Days
• Field of Search: 250/310, 250/306, 250/307
• US Class Current: 250/310
• CPC Class Codes: H01J 2237/2817 Pattern inspection
• CPC Class Codes: H01J 37/28 with scanning beams H01J37/268, H01J37/292, H01J37/2955 take precedence
• CPC Class Codes: H01J 37/3005 Observing the objects or the point of impact on the object

Abstract

There is disclosed an apparatus to scan an electron beam across an optical phase shift mask and automatically inspect the mask to determine the features of the phase shift mask and classification of defects. An electron beam is directed at the surface of a mask for scanning that mask and detectors are provided to measure the secondary and backscattered charged particles from the surface of the mask. The mask is mounted on an x-y stage to provide it with at least one degree of freedom while the mask is being scanned by the electron beam. By analysis of various waveform features in each of the secondary and backscatter electron waveforms obtained from a phase shift mask, various physical features of the mask can be detected, as well as their size and position determined. The thickness of chromium layers can also be determined. In the inspection configuration, there is also a comparison technique for comparing the pattern on the substrate with a second pattern for error detection.

Claims

1. A system to automatically inspect an optical mask, said system comprising:

an evaporative film coating system to apply a conductive coating to a top surface of said optical mask to produce a conductive optical mask;

a grounding strap to connect said conductive coating of said conductive optical mask to electrical ground;

a field emission electron source to provide an electron beam;

a charged particle beam column to deliver and scan said electron beam from said field emission electron source on a top surface of said conductive coating;

a backscatter electron detector to detect backscattered electrons from said conductive optical mask to generate a backscatter electron waveform as said electron beam scans said conductive optical mask; and

a secondary electron detector to detect secondary electrons from said conductive optical mask to generate a secondary electron waveform as said electron beam scans said conductive coating.

2. A method for automatically inspecting an optical mask, said method comprising the steps of:

a. evaporating a conductive coating to a top surface of said optical mask to produce a conductive optical mask;

b. electrically grounding said conductive coating;

c. scanning an electron beam on said top surface of said conductive coating of step b.;

d. detecting backscattered electrons from said conductive coating of step c. to form a backscatter electron waveform; and

e. detecting secondary electrons from said conductive coating of step c. to form a secondary electron waveform.