Enhanced Surface Metrology

Transcription

1 Enhanced Surface Metrology Russell M. Kurtz RAN Science & Technology, LLC Ryder Nesbitt Hexagon Metrology, Inc.

2 There is a constant search for more accurate measurement Traditionally 3 Parameters COST MEASURMENT SPEED ACCURACY Traditional Tradeoffs Cheap, Fast, Inaccurate Expensive, Slow, Accurate

3 We Present a Method of Adding One More Tradeoff Parameter MEASUREMENT PRECISION Measurement Precision (perpendicular to the surface) vs. Sample Correlation (along the surface)

4 Through application of statistical sampling and curve fitting we can improve precision by approximately the same amount that we increase correlation

5 Theory & Methods POINT CLOUDS The number of points in a 3-D space, each corresponding to a point on the surface of the item

6 Described as Deviation from CAD Three possible deviation causes: 1.The real surface deviation from nominal δ r 2.Consistent measurement inaccuracies δ m 3.Random measurement inaccuracies or other random noise δ n

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8 Our Focus: Noise (δ n ) Reduction defect visible with low noise hidden by moderate noise 10 x 10 mm portion of a surface, nominally spherical, defect height = ~1.5 µm

9 Methods of Noise Reduction A. Arithmetic Mean over a Square Sampling Area M 2 N 2 n N 2 calc 0, 0 w mn meas 0 m, 0 n m M 2 N 2 M 2 m M 2 n N 2 w mn B. Arithmetic Mean over a Circular Region w mn = 1 over the range m 2 + n 2 (M/2) 2, again with M = N This statistical process uses 78.5% as many samples as the square, and is expected to reduce noise by a factor of 0.886M. C. Gaussian Sampling Process over Square Region w mn e 2 m2 n 2 M 2 2

10 Sample Correlation, 5 pixel wide Square Circular Gaussian

11 Targeting Task Performance Model Probability of Identifying a 3-µm Feature with No Noise Reduction

12 Experiments Flat Sample: WLS400 block made of ceramic with a polished white surface finish

13 Experiments Flat Sample: WLS400 Measurement Noise and Actual Surface Deviation Before Statistical Noise Reduction After Statistical Noise Reduction

14 Experiments Sphere Sample: WLS400 Test on a known sphere, whose diameter was specified to be ± in. but whose surface was specified smooth only to ±0.001 in. We scanned a section of the upper half of the sphere, a circular area whose diameter was mm.

15 Experiments Sphere Sample: WLS400 Measurement Noise and Actual Surface Deviation Before Statistical Noise Reduction After Statistical Noise Reduction

16 Experiments Sphere Sample: WLS400 Surface of the sphere with locations measured from its center. Surface color shows deviation from nominal (after noise reduction)

17 Experiments Interferometric Measurement Test on a flat, metallic surface (aluminum), mechanically polished to R A 60. We used a Mach-Zender interferometer to obtain an interferogram covering a square 1 cm on a side. The illumination wavelength was 532 nm.

18 Experiments Interferometric Measurement Original Interferogram A B Original fit to a sinusoid with intensity varying from 0 to 1 C The difference is the noise pattern D After noise reduction

19 Summary & Conclusions We have developed a statistical method for noise reduction of three-dimensional metrology systems. The method involves combination of measurement values over a range of points, and permits weighting the points (enabling the user to, for example, assign confidence values to the various measurements).

20 Summary & Conclusions On the flat surface, we reduced the deviation from nominal by more than a factor of two. The sphere was specified to an accuracy of ±25.4 µm, and we measured that its accuracy was ±15.8 µm. With the interferometric measurement we determined that the surface deviated 16.0% from perfect flatness.

21 Summary & Conclusions These improvements indicate that the statistical surface metrology enhancement method can be useful in detecting small features.

22 Enhanced Surface Metrology Russell M. Kurtz RAN Science & Technology, LLC Ryder Nesbitt Hexagon Metrology, Inc.