Corning Tropel Metrology Instruments LightGage Frequency Scanning Technology Thomas J. Dunn 6 October 007
Introduction Presentation Outline Introduction Review of Conventional Interferometry FSI Technology Description System Design Measurement Results and Applications Conclusion
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 5.00 // 0.00 A / / 0.00 3
The Challenge Rough surface means no fringes 4
The Solution Corning s LightGage Interferometer Fast and Easy-to-Use Full surface and multi-surface Step heights to 30+ mm A 5.00 // 0.00 A / / 0.00 5
LightGage Frequency Stepping Interferometer Corning s new LightGage system is an advanced, full surface, interferometer for measuring form (i.e. flatness), parallelism and depth 6
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 equidistant steps I = U I = U + U + U x + U x = Ae U ifx = OPD + A e cos ifx ( f ( x x )) 7
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 8
Introduction Conventional Normal Incidence Interferometry Modulation by PMI Measures phase difference from adjacent pixels. Calculates surface height map from expression: λ h = φ π Intensity (Arbitrary units) 0.8 0.6 0.4 0. 0-0. 6 6 Pixel Pixel Frame Numbe r Intensity Variation for two different pixels 9
Introduction Conventional PMI Interferometer PRO s CON s High Resolution and Accuracy High Speed Works only on smooth surfaces Cannot measure across discontinuous surfaces (depth, height, etc) Limited Dynamic Range 0
Technology Overview Frequency Scanning Interferometry Use a tunable laser source Change the laser wavelength in equal frequency steps Collect camera image for each step I = U + U = Ae if n x + A e if n x I = U + U + U U cos ( f ( x x )) n
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 Modulation by PMI Modulation by FSI.. Intensity (Arbitrary units) 0.8 0.6 0.4 0. 0-0. Pixel Pixel Intensity (Arbitrary units) 0.8 0.6 0.4 Pixel 0. Pixel 0 6 6 6 6-0. Frame Numbe r Frame Number
Technology Overview US Penny Height of each pixel independent of one another 3
Technology Overview Modulation Frequency is function of depth 4
Technology Overview Frequency Scanning Interferometry Laser has a step-like tuning behavior Eliminates frequency errors caused by positioning errors Allows faster tuning 8070 LightGage TLD Tuning Characteristics 8068 8066 8064 Mode # 806 8060 8058 8056 8054-0.0 0 0.0 0.04 0.06 0.08 0. 0. 0.4 0.6 0.8 P (mm) 5
System Design Tunable Laser Source Both diode cavity and external cavity have longitudinal laser modes ν = c L 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 Transmission Transmission Intensity Intensity External Cavity Diode Cavity Frequency Frequency 6
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 7
System Design Interferometer Head Fizeau Interferometer - Common path minimizes environmental impact - Small form factor Diffuser disk - Speckle busting - Variable spatial coherence Fizeau Plate Objective Lens Diffuser Disk TLD Object BS Camera 8
System Design Interferometer Head Single side or dual side measurements 5 meters Master System Controller Sensor Controller (PC) Source Unit Parts Handling Sensor 9
LightGage Gen Specifications Field of View (Circular) 36 mm (.4 inch), Fixed Z-Resolution 0 nm (0.40 µinch) Lateral Resolution 0.08 mm (0.003 inch) Measurement Range Up to 36 mm (.4 inch) Measurement Time 30 seconds typical Measurement Method Frequency Scanning Interferometry Data Points Up to 785,000 pts /measurement 0
LightGage Gen Specifications Accuracy Repeatability Flatness 60 nm (.4 µinch) 0 nm (0.8 µinch) Parallelism 00 nm (4.0 µinch) 5 nm (.0 µinch) Depth/Height** see note 00 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
LightGage Measurement Examples Spacers and Components (Fuel Injector - Automotive)
LightGage Measurement Examples Flatness and Parallelism of two surfaces (Fuel Pump - Automotive) 3
LightGage Measurement Examples Form of a precision recessed surface (HDD Motor hub) 4
LightGage Measurement Examples Failure analysis on compressor plates Seal Ring Mounts 5
LightGage Measurement Examples Surfaces on a diffuse powdered metal part (Automotive Shock Absorber) 6
LightGage Measurement Examples Form of a precision assembly (HDD Wavy washer) 7
Conclusion Advantages of Frequency Scanning Interferometry Measure multiple surfaces simultaneously Measure diffuse and discontinuous surfaces (many material types) Fast since it incorporates highspeed data collection with highspeed algorithms High dynamic range and accuracy 8