Dynamic Focus Adjustment with Optical Height Detection Apparatus in Electron Beam system

• US 20110260055A1
• Filed: 04/21/2010
• Published: 10/27/2011
• Est. Priority Date: 04/21/2010
• Status: Active Grant

• Current Assignee: ASML Netherlands BV (ASML Holding NV)
• Original Assignee: Hermes Microvision, Inc. (ASML Holding NV)
• Inventors: NGUYEN, VAN-DUC, WANG, YI-XIANG, JAU, JACK, CHEN, ZHONGWEI, WANG, JOE
• Granted Patent: US 8,791,414 B2
• Time in Patent Office: Days
• US Class Current: 250/307
• CPC Class Codes: G01B 11/22 for measuring depth
• CPC Class Codes: G01N 2223/32 adjustments of elements during operation
• CPC Class Codes: G01N 2223/3301 beam is modified for scan, e.g. moving collimator
• CPC Class Codes: G01N 2223/3302 object and detector fixed
• CPC Class Codes: G01N 2223/646 flaws, defects
• CPC Class Codes: G01N 23/20058 Measuring diffraction of electrons, e.g. low energy electron diffraction [LEED] method or reflection high energy electron diffraction [RHEED] method
• CPC Class Codes: H01J 2237/216 Automatic focusing methods
• CPC Class Codes: H01J 2237/2482 Optical means
• CPC Class Codes: H01J 2237/2487 using digital signal processors
• CPC Class Codes: H01J 37/21 Means for adjusting the focus adjusting the focus while observing the image by photographic or optical means H01J37/22; means for observing the object or the point of impact on the object in tubes for the localised treatment of materials H01J37/3005
• CPC Class Codes: H01J 37/28 with scanning beams H01J37/268, H01J37/292, H01J37/2955 take precedence

Abstract

The present invention generally relates to dynamic focus adjustment for an image system. With the assistance of a height detection sub-system, present invention provides an apparatus and methods for micro adjusting an image focusing according the specimen surface height variation by altering the field strength of an electrostatic lens between objective lens and sample stage/or a bias voltage applied to the sample surface. Merely by way of example, the invention has been applied to a scanning electron inspection system. But it would be recognized that the invention could apply to other system using charged particle beam as observation tool with a height detection apparatus.

Claims

1. A method for dynamic adjusting a focus of an electron beam image while inspecting a specimen, the method comprising:

providing an electron beam inspection system;

providing an optical illumination flux through a pattern plate and a lens to a surface of the specimen to project a pattern onto the surface of the specimen, the pattern being associated with the pattern plate;

detecting the illumination flux reflected from the surface of the specimen with a detector;

processing information associated with the detected illumination flux;

generating optical images based on a portion of the information associated with the detected illumination flux, a first image including a first image part for the pattern and a second image part for the specimen;

determining a local height variation of the surface of the specimen within an electron beam field of view according to a position variation of the pattern between these optical images;

comparing the local height variation with a depth of focus of an electron beam image;

calculating correction factors to compensate the local height variation respect to the depth of focus of the electron beam image within the field of view;

generating a control signal based on the portion of the information associated with these optical images;

providing the control signal to a controlled objective;

adjusting the focus of the electron beam image of the specimen in response to the control signal that apply to the controlled objective; and

performing an electron beam inspection for the specimen.

2. The method of claim 1, wherein the controlled objective for the image focuses adjustment is the specimen on a sample stage.

3. The method of claim 2, wherein the control signal applies to the specimen is a voltage bias on the specimen surface.

4. The method of claim 3, wherein the calculating scanning correction factors further includes:

image rotation compensation;

image magnification compensation; and

primary beam shift compensation.

5. The method of claim 1, wherein the controlled objective for the image focuses adjustment is a control electrode.

6. The method of claim 5, wherein the control signal applies to the control electrode is a voltage to generate electrostatic field strength.

7. The method of claim 6, wherein the calculating scanning correction factors further includes:

image rotation compensation;

image magnification compensation; and

primary beam shift compensation.

9. The device for dynamic measuring height variation according to claim 7, further comprising an optical system for the optical illuminating source and the detecting unit.

10. The device for dynamic measuring height variation according to claim 7, wherein the means for calculating bias of the specimen surface and scanning compensation includes:

image rotation compensation;

image magnification compensation; and

primary beam shift compensation.

8. A device for dynamic measuring height variation of a specimen to be inspected by a tool, comprising:

an optical illuminating source for emitting light on the specimen;

a detecting unit for receiving light reflected from the specimen;

means for calculating focus of the tool;

means for calculating bias of the specimen surface and scanning compensation; and

means for controlling bias of the specimen surface.

11. An electron beam system, comprising:

an electron source for emitting a primary beam;

a condenser lens for condense the primary beam;

an objective lens for focusing the primary beam on a surface of a specimen; and

means for measuring height variation of the specimen.

12. The electron beam system according to claim 11, wherein the means for measuring height variation of the specimen comprises:

an optical illuminating source for emitting light on the specimen;

a detecting unit for receiving light reflected from the specimen; and

means for varying landing energy of the primary beam.

13. The electron beam system according to claim 12, wherein the means for varying landing energy of the primary beam comprises:

means for calculating focus of the tool;

means for calculating bias of the specimen surface and scanning compensation; and

means for controlling bias of the specimen surface.

14. The electron beam system according to claim 13, wherein the means for calculating bias of the specimen surface and scanning compensation includes:

image rotation compensation;

image magnification compensation; and

primary beam shift compensation.

15. The electron beam system according to claim 12, wherein the means for varying landing energy of the primary beam comprises:

means for calculating focus of the tool;

means for calculating electrostatic strength of a control electrode and scanning compensation; and

means for controlling electrostatic strength of the control electrode.

16. The electron beam system according to claim 15, wherein the means for calculating electrostatic strength of a control electrode and scanning compensation includes:

image rotation compensation;

image magnification compensation; and

primary beam shift compensation.

17. The electron beam system according to claim 11, further comprising an optical system for the optical illuminating source and the detecting unit.

18. A device for adjusting focus of an electron beam in a scanning electron microscope (SEM), said device comprising:

an optical height detection unit for detecting a surface of a specimen to be inspected by the SEM and outputting signals of height variation information;

a SEM focus control unit for receiving the signals of height variation information, calculating focus, and outputting signals of focus adjusting information; and

a SEM focusing unit for focusing the electron beam of the SEM on the surface of the specimen according to the signals of focus adjusting information, wherein said SEM focusing unit is selected from the group consisting of means for adjusting bias of the specimen'"'"'s surface, a control electrostatic lens, and a condenser lens.

19. The device for adjusting focus of an electron beam in a scanning electron microscope according to claim 18, wherein the control electrostatic lens is a plate with a central hole to allow a primary beam of the SEM passing therethrough.