Non-Contact Surface Metrology and Zegage Optical Profiler
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Non‐Contact Surface Metrology and ZeGage Optical Profiler John Fabio Sam Banerjea Zygo Corporation ©2014 Zygo Corporation. All rights reserved. About Zygo • Zygo has 44 years experience in Metrology and Optical Systems. • Two main business divisions: – Metrology Solutions that provide process control for surface shape, surface texture, material characteristics, film thickness and feature position based on Zygo’s expertise in optical interferometry. – Optical Systems and Components ranging from meter-class plano optics to optical assemblies. • Publically traded (NASDAQ: ZIGO) since 1983. • Strong history of innovation and technology leadership with >300 patents issued. 2 How Zygo Measure Surfaces •For –Form – Roughness – Waviness – Structure – Relationships to other surfaces • And we want to do it – Quantitatively – Quickly – Easily – Without damaging the part 3 Benefits of Optical Methods • Full 3D topography scan • Picks up 1M+ data points • No damage to the surface • Fast scan speeds • Lower vertical noise Mechanical stylus 3D Optical metrology 4 A Look at Interferometry Several optical 3D metrology techniques are available today: • Vertical resolution down to nanometers, regardless of magnification • Fields of view from µm** to mm • Lateral feature size down to sub µm • High data density • Non contact, high speed • Machined, polished, burnished and ground surfaces • Complex surface structures From Laser Focus World, 1995 ** 1 micrometer = 39.37 microinches 5 3D Interference Microscopy 6 Interference Microscopy • An interference microscope is a conventional microscope with the addition of a beam splitter and reference mirror • Most interference microscopes are near-equal path systems, using filtered incandescent or LED light sources 7 Scanning the White Light Fringes • The two signals correspond to two camera pixels • In an actual system, there are about a scan million pixels and individual scan signals Signal A scan B A Signal B Height difference 8 Michelson Interference Objective • Equal path (usually) Pupil (location of source image) • Works with extended white light sources • Ideal for low-magnification objectives Reference mirror Object 9 Mirau Objective • Equal path (usually) • Works for white light • Requires an extended or off- axis source (because of central obscuration) • Best for high-magnification Reference mirror objectives (central obscuration) Beam splitter Object 10 A Complete Interference Microscope System • Designed for high precision on small objects • Includes staging, controls and software for surface texture analysis • Basic performance capabilities: – Surface topography repeatability – Lateral resolution – Measurement range – Correlation to stylus measurements – Vibration sensitivity 11 Wavelength and NA Set Optical Lateral Resolution • Sparrow criterion limit = common specification for a microscope • At low mag ( <20X ), the camera can be as important as the optics, reducing the effective resolution by “pixelization” Optical resolution at l = 560 nm Magnification NA Sparrow criterion (micrometers **) 1X 0.03 9.50 2X 0.055 5.18 2.75X 0.08 3.56 5.5X 0.15 1.90 10X 0.30 0.95 20X 0.40 0.71 50X 0.55 0.52 ** 1 micrometer = 39.37 microinches 12 Correlation to Stylus Measurements • Evaluation using a ZeGage interference microscope at Zygo compared to a stylus instrument at UNCC Center for Precision Metrology • Rubert reference specimens – 4 random samples with Ra= 0.02 µm … 0.15 µm – 8 sinusoidal samples with Ra = 0.1µm … 6.4 µm and pitch = 2.5 µm … 135 µm • Halle roughness standards – 2 samples with Ra= 0.024 µm and 0.22 µm 13 Summary of Ra: Random Specimens Sample name Stylus ZeGage, 10X ZeGage, 20X ZeGage, 50X Rubert 501 0.015 µm ‐ 0.016 µm 0.012 µm Rubert 502 0.035 µm ‐ 0.036 µm 0.031 µm Rubert 503 0.067 µm ‐ 0.095 µm 0.076 µm Rubert 504 0.119 µm ‐ 0.150 µm 0.130 µm Halle 2070/03 0.0238 µm 0.0237 µm 0.0239 µm ‐ Halle 2058/01 0.216 µm 0.237 µm 0.215 µm ‐ Graphical Representation Direct Comparison of Stylus and Optical of the 50X Results Profiles at 50X for the 502 Specimen R2=0.9964 14 Summary of Ra: Sinusoidal Specimens Sample Name Pitch Stylus ZeGage with 20X ZeGage with 50X Rubert 543 2.5 µm 0.018 µm ‐ 0.021 µm Rubert 542 8 µm 0.060 µm 0.048 µm 0.051 µm Rubert 529 10 µm 0.097 µm 0.108 µm 0.090 µm Rubert 531 100 µm 0.315 µm 0.316 µm 0.315 µm Rubert 528 50 µm 0.507 µm 0.506 µm 0.515 µm Rubert 530 100 µm 1.009 µm 1.012 µm 1.050 µm Rubert 527 100 µm 2.995 µm 3.025 µm 3.066 µm Rubert 525 135 µm 6.389 µm ‐ 6.362 µm Graphical Representation 2 R =0.9998 of the Above Results R2=1 Direct Comparison of Stylus and Optical Profiles at 20X for 528 Specimen 15 Comparing Scanning Methods Interferometry Stylus scan B A B A scan 16 Comparing Interferometry to Stylus Results Interferometry Interferometry Stylus Stylus 17 Potential Damage by Stylus Measurement Taken on Paper 18 Potential Damage by Stylus Measurement Taken on Paper 19 Common S Parameters Sa = Average Roughness Sq = Root Mean Square Roughness •Sa and Sq represent an overall measure of the texture comprising the surface Plateau-like Surface Sa = 16.03 nm • They are insensitive in differentiating peaks, Sq = 25.4 nm valleys and the spacing of the various texture features • Used to indicate significant deviations in the texture characteristics •Sa is primarily used for machined surfaces Surface with Peaks Sa = 16.03 nm •Sq is typically used to specify optical surfaces Sq = 25.4 nm 20 Common S Parameters Ssk = Skewness Sku = Kurtosis •Ssk and Sku represent symmetry and deviation from an ideal norm Surface with Multiple Peaks Ssk = 3.20 S = 18.71 •Ssk indicates the predominance of peaks or ku valleys •Sku shows inordinately high peaks/ deep valleys •Ssk is useful for honed surfaces and wear conditions Periodic Texture Ssk = 0.16 S = 1.63 •Sku is used to check presence of peaks and ku valleys 21 Common S Parameters Sp = Max Peak Height Sv = Max Valley Depth Spv (or Sz) = Max Height of Surface Printed surface with deep valley structures •Sp, Sv, and Spv are evaluated from the absolute highest and lowest points found on the surface. •Sp is the height of the highest point and Sv is the depth of the lowest point •Sp is used for sliding contact applications •Sv is related to fluid retention, lubrication or coatings Polymer surface prepared with asperities •Spv is the difference between Sp and Sv and is used to Sp = 0.90 µm characterize sealing surfaces and coatings Sv = -15µm 22 S Parameters to R Parameter Correlation Description Stylus ZeMaps Roughness Average Ra Sa Root Mean Square Rq Sq Peak Rp Sp Valley Rv Sv Total Height (Peak to Valley) Rt Spv (or Sz) Material Ratio Rmr Smr1 Smr2 Profile Height Pt PV Skewness Rsk Ssk Kurtosis Rku Sku Max Roughness Depth Rmax IsoSRmax Average Max Height Rz IsoSRz Base Roughness Depth R3z IsoSR3z Bearing Ratio (Core Roughness Depth) Rk MR-T-Sk Bearing Ratio (Reduced Peak Height) Rpk MR-T-Spk Bearing Ratio (Reduced Valley Depth) Rvk MR-T-Svk Bearing Ratio (Peak Material Component) Mr1 MR-T-Smr1 Bearing Ratio (Valley Material Component) Mr2 MR-T-Smr2 Waviness Parameters W Sa Waviness Height Wt PV Note: R parameters defined by ISO 4287, S parameters defined by ISO 25178-2 23 ISO 25178 : Geometric Product Specifications, Surface Texture: Areal • Standard prepared by WG16 of the ISO technical committee TC213. • ISO 25178 means a collection of international standards relating to the analysis of 3D areal surface texture. • First international standard for specification and measurement of 3D surface texture. • Standard describes applicable measurement technologies, calibration methods, and calibration standards/software that are required. • Standard covers non-contact measurement. 24 Form, Roughness and Waviness Segmentation • Segmentation is accomplished using Fourier transform methods… • … or more advanced spline-based methods • Cutoff limits (ls, lc, lf) are defined in standards: ISO 4287, ASME B46.1 (USA), JIS B0601 (Japan) 25 Calibration is Essential for Best Results • Like all metrology tools, coherent scanning interferometer systems should be calibrated for the following: – Accurate XY metrology • Lateral calibration standard – System form bias •SiCFlat – Vertical height scale • Step height standard 26 Specialized Objectives with Long Working Distance • For hard to reach areas, Michelson type objectives are available with 40 mm of working distance • Applications include measurements inside cones and cylinders using an auxiliary optic 3 mm Fold mirror 27 ZeGage Non-Contact Profiler • Entry-level optical evaluation of 3D form and surface finish on precision machined or fabricated surfaces. • Takes measurements of ISO-compliant roughness parameters, form, step-height and material wear. • Higher-precision alternative to traditional contact or touch-probe surface finish gages. • Vibration isolation elements built into the chassis. • Applications found in industrial manufacturing, R&D, quality inspection, failure analysis, wear analysis and more. • Markets served include medical, automotive, consumer electronics, military, and aerospace. 28 ZeGage Highlights • Built for shop floor and lab usage • ≤ 3.5 nm vertical resolution independent of magnification or vertical scan length • SureScan™ Technology eliminates the need for an isolation table • Novel through-the-lens focus aid assists part setup at exactly the area of measurement • Easy to use and learn ZeMaps software with ISO 25178-2* compliant surface parameters • 1X – 50X objectives available * ISO 25178-2:2012 - Geometrical Product Specifications - Surface texture: Areal -- Part 2: Terms, definitions and surface texture parameters 29 ZeGage Design Features System Footprint: Z-Stage based scanner W: 525 mm enables scans up to 20 mm D: 525 mm Internal software Max H: 815 mm controlled focus aid illuminator Sealed Head cover has no manual controls or adjustments ZeMaps software on 64-bit Windows 7 Direct threaded Single objective standard.