1. LightGage™ Frequency Scanning Technology
Corning Tropel
Metrology Instruments
LightGage™ Frequency Scanning Technology
Thomas J. Dunn
26 October 2007

2. Introduction Presentation Outline Introduction
Review of Conventional Interferometry
FSI Technology Description
System Design
Measurement Results and Applications
Conclusion

3. The Challenge
Need to rapidly measure flatness, height, and parallelism of components with following attributes:
Surfaces are discontinuous
Surfaces may be rough
Surfaces may be recessed or part of an assembly A //
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4. The Challenge
Rough surface means no fringes

5. The Solution Corning’s LightGage Interferometer Fast and Easy-to-Use
Full surface and multi-surface
Step heights to 30+ mm A //
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6. LightGage™ Frequency Stepping Interferometer
Corning’s new LightGage™ system is an advanced, full surface, interferometer for measuring form (i.e. flatness), parallelism and depth

7. Introduction Conventional PMI Interferometer
Collect multiple interferograms with camera
Each image collected with different phase (ramp fringes)
Phase can be varied by moving reference surface in small equi-distant steps
Conventional PMI Interferometer

8. Introduction Conventional PMI Interferometer
Interferometric fringe pattern from a spacer is shown. Fringes show deviation from ideal plane
Multiple images are collected as OPD is varied in equi-distant steps
For each pixel we can plot intensity versus step number

9. Introduction Conventional Normal Incidence Interferometry
Measures phase difference from adjacent pixels
Calculates surface height map from expression:
Intensity Variation for two different pixels

10. Introduction PRO’s CON’s Conventional PMI Interferometer
High Resolution and Accuracy
High Speed
CON’s
Works only on smooth surfaces
Cannot measure across discontinuous surfaces (depth, height, etc)
Limited Dynamic Range

11. Technology Overview Frequency Scanning Interferometry
Use a tunable laser source
Change the laser wavelength in equal frequency steps
Collect camera image for each step

12. Technology Overview Frequency Scanning Interferometry
Modulation frequency for each pixel varies
Measure absolute OPD by performing a DFT on each pixel’s modulation function
Measure modulation frequency of each pixel independently

13. Height of each pixel independent of one another
Technology Overview
US Penny
Height of each pixel independent of one another

14. Technology Overview
Modulation Frequency is function of depth

15. Technology Overview Frequency Scanning Interferometry
Laser has a “step-like” tuning behavior
Eliminates frequency errors caused by positioning errors
Allows faster tuning

16. System Design Tunable Laser Source
External Cavity
Tunable Laser Source
Both diode cavity and external cavity have longitudinal laser modes
Desirable to tune the length of the external cavity to mode match the diode cavity
Mode-matching optimizes frequency stability, guarantees equal frequency steps, and optimizes fringe contrast
Diode Cavity
Transmission
Intensity
Frequency
Transmission
Intensity
Frequency

17. System Design Interferometer Head Compact design
Coupled through fiber optic cable
Can be mounted in any orientation
Can be located remotely
Easily integrated for in-line applications

18. System Design Interferometer Head Fizeau Interferometer
Common path minimizes environmental impact
Small form factor
Diffuser disk
Speckle busting
Variable spatial coherence
Camera
TLD
Diffuser
Disk
Object
Fizeau
Plate
Objective
Lens BS

19. Master System Controller
System Design
Interferometer Head
Single side or dual side measurements
Master System Controller
5 meters
Controller (PC)
Sensor
Source Unit
Parts Handling
Sensor

20. LightGage Gen 1 Specifications
Field of View (Circular) 36 mm (1.42 inch), Fixed
Z-Resolution 10 nm (0.40 μinch)
Lateral Resolution mm (0.003 inch)
Measurement Range Up to 36 mm (1.42 inch)
Measurement Time 30 seconds typical
Measurement Method Frequency Scanning Interferometry
Data Points Up to 785,000 pts /measurement

21. LightGage Gen 1 Specifications
Accuracy Repeatability
Flatness 60 nm (2.4 μinch) 20 nm (0.8 μinch)
Parallelism 100 nm (4.0 μinch) 25 nm (1.0 μinch)
Depth/Height** see note 100 nm (4.0 μinch)
** Accuracy/repeatability of height/depth measurement is dependent on part geometry, consult Corning Tropel for more details
Materials: Metals, Polymers, Ceramics, Glass, others
Finishes: Fine-Ground, Lapped, Polished, Superfinished

22. LightGage Measurement Examples
Spacers and Components
(Fuel Injector - Automotive)

23. Flatness and Parallelism of two surfaces
LightGage Measurement Examples
Flatness and Parallelism of two surfaces
(Fuel Pump - Automotive)

24. LightGage Measurement Examples
Form of a precision recessed surface
(HDD – Motor hub)

25. LightGage Measurement Examples
Failure analysis on compressor plates
Mounts
Seal Ring

26. LightGage Measurement Examples
Surfaces on a diffuse powdered metal part (Automotive – Shock Absorber)

27. LightGage Measurement Examples
Form of a precision assembly (HDD – Wavy washer)

28. Conclusion Advantages of Frequency Scanning Interferometry
Measure multiple surfaces simultaneously
Measure diffuse and discontinuous surfaces (many material types)
Fast since it incorporates high-speed data collection with high-speed algorithms
High dynamic range and accuracy