Camera and illumination matching for inspection system

• US 20050052530A1
• Filed: 07/14/2004
• Published: 03/10/2005
• Est. Priority Date: 07/14/2003
• Status: Active Grant

• Current Assignee: Onto Innovation Inc.
• Original Assignee: August Technology Corporation (Onto Innovation Inc.)
• Inventors: Watkins, Cory, Simpkins, Patrick
• Granted Patent: US 7,589,783 B2
• Time in Patent Office: Days
• US Class Current: 348/131
• CPC Class Codes: G01N 21/8806 Specially adapted optical and illumination features
• CPC Class Codes: G01N 21/9501 Semiconductor wafers manufacturing processes per se of semiconductor devices implementing a measuring step H01L22/10
• CPC Class Codes: H04N 7/188 Capturing isolated or intermittent images triggered by the occurrence of a predetermined event, e.g. an object reaching a predetermined position signal generation from motion picture films H04N5/253

Abstract

An inspection tool includes a camera having a gain value and an offset value set to provide a first camera output indicating a first desired average gray value when the camera is illuminated with a precision light source set at a first light level. The inspection tool includes an inspection light source configured to be set at a second light level such that when the inspection light source illuminates a known target, the camera provides a second output indicating a second desired average gray value.

Claims

1. An inspection tool comprising:

a camera having a gain value and an offset value set to provide a first camera output indicating a first desired average gray value when the camera is illuminated with a precision light source set at a first light level; and

an inspection light source configured to be set at a second light level such that when the inspection light source illuminates a known target, the camera provides a second output indicating a second desired average gray value.

2. The inspection tool of claim 1, wherein the camera provides a third output indicating a third desired average gray value when the camera is illuminated with the precision light source set at a third light level.

3. The inspection tool of claim 1, wherein the inspection light source is configured to be set at a third light level such that when the inspection light source illuminates the known target, the camera provides a third output indicating a third desired average gray value.

4. The inspection tool of claim 1, wherein the precision light source is National Institute of Standards and Technology (NIST) traceable.

5. An inspection system comprising:

a first inspection tool comprising a first camera; and

a second inspection tool comprising a second camera matched to the first camera.

6. The inspection system of claim 5, wherein the second camera is configured to be matched to the first camera by adjusting a first gain and first offset of the first camera and a second gain and second offset of the second camera such that an output of the second camera when focused on a target is substantially equal to an output of the first camera when focused on the target.

7. The inspection system of claim 6, wherein the first inspection tool is configured to adjust the first gain and first offset of the first camera by:

stepping a light level setting of a light source through a plurality of light levels;

illuminating a sensor array of the first camera with the light source at each of the plurality of light levels;

determining an average gray value for the sensor array at each of the plurality of light levels;

fitting the plurality of light levels and the average gray value at each of the plurality of light levels to a line in the form y=m*x+b, wherein y is an average gray value, x is a light level, m is a slope of the line, and b is an intercept of the line;

comparing the slope and the intercept to desired values; and

adjusting the first gain and first offset based on the comparison.

8. The inspection system of claim 7, wherein the second inspection tool is configured to adjust the second gain and second offset of the second camera in substantially the same manner as the first inspection tool is configured to adjust the first camera.

9. The inspection system of claim 8, wherein the light source comprises a precision light source.

10. The inspection system of claim 5, wherein the first inspection tool comprises a first light source and the second inspection tool comprises a second light source, and wherein the first inspection tool is illumination matched to the second inspection tool.

11. The inspection system of claim 10, wherein the first inspection tool is configured to be illumination matched to the second inspection tool by:

providing a target of known reflectance in the first inspection tool;

illuminating the target with the first light source;

stepping the first light source through a plurality of light level settings; and

adjusting a first light level at each of the plurality of light level settings until an average gray value of a sensor array of the first camera focused on the target is within a desired range at each of the plurality of light level settings.

12. The inspection system of claim 11, wherein the first inspection tool is configured to be illumination matched to the second inspection tool by:

providing the target of known reflectance in the second inspection tool;

illuminating the target with the second light source;

stepping the second light source through the plurality of light level settings; and

adjusting a second light level at each of the plurality of light level settings until an average gray value of a sensor array of the second camera focused on the target is within the desired range at each of the plurality of light level settings.

13. A method for camera matching, the method comprising:

stepping a light level setting of a precision light source through a plurality of light levels;

illuminating a sensor array of a camera with the precision light source at each of the plurality of light levels;

determining an average gray value for the sensor array at each of the plurality of light levels; and

fitting the plurality of light levels and the average gray value at each of the plurality of light levels to a line in the form y₁=m₁*x₁+b₁, wherein y₁ is an average gray value, x₁ is a light level, m₁ is a slope of the line, and b₁ is an intercept of the line.

14. The method of claim 13, further comprising:

comparing the slope and intercept to target values; and

adjusting the camera based on the comparison.

15. The method of claim 14, wherein adjusting the camera comprises adjusting a gain of the camera.

16. The method of claim 14, wherein adjusting the camera comprises adjusting an offset of the camera.

17. The method of claim 14, wherein adjusting the camera comprises adjusting a gain and offset of the camera.

18. The method of claim 13, further comprising:

determining a gray value for each pixel in the sensor array at each of the plurality of light levels; and

fitting the plurality of light levels and the gray values for each pixel at each of the plurality of light levels to a line for each pixel in the form y₂=m₂*x₂+b₂, wherein y₂ is a gray value, x₂ is a light level, m₂ is a slope of the line, and b₂ is an intercept of the line.

19. The method of claim 14, further comprising:

providing a target of known reflectance;

illuminating the target with an inspection light source;

stepping the inspection light source through a plurality of light level settings; and

adjusting a light level at each of the plurality of light level settings until an average gray value of the sensor array of the camera focused on the target is within a desired range at each of the plurality of light level settings.

20. The method of claim 19, wherein stepping the inspection light source comprises stepping the light source through 255 light level settings.

21. The method of claim 13, wherein stepping the light level setting of the precision light source comprises stepping the light level setting of the precision light source through 255 light levels.