Obtaining Accurate Surface Measurements Bruker Nano Surfaces Division Dr. Erik Novak February 2012

Outline

• Overview of precision, stability, accuracy

• Factors affecting data quality and metrology results • Conclusion

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Overview of Precision, Stability, and Accuracy

• Precision – measurement tool dependent/internal characteristic • Resolution?

• Stability – Measurement tool

dependent/internal characteristic influenced by environment and other factors

• Accuracy – Calibrate to an external standard (usually) - how well does my instrument do with respect to “THE TRUTH”?

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Best metrology is had when tools are precise, stable and accurate!

Several Factors Influence Data Quality for Surface Metrology

• Instrument/Environment • Internal mechanics, noise floor of detection mechanism, stability of

hardware and precision of motion (if motion is present) • Calibration • Temperature/vibration considerations

• Test Surface/Sample • Fixturing! • Surface features (structure, heights, slopes) • Surface roughness, form, waviness components

• Analysis and Computations with Data • Measurement produces a representation of the test surface • Filtering and computation methods

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THESE ARE NOT

ALWAYS INDEPENDENT !!!

Accurate Metrology is a Key Component of Product Success for Many Applications

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Precision Machining Optics MEMS and Semiconductor Data Storage

MEMS Cantilevers Microfluidic Channel Knee Implant

Holographic Film Currency Clutch Plate

Cotton Cloth

Instrument Factors – Lateral Calibration is Key to Accuracy

• Lateral Instrument Calibration • With optimized internal mechanics – overall accuracy depends on

calibration of lateral and vertical motion • Lateral calibration can be accomplished several ways:

• Scanning stage (or making single FOV measurement) and measuring periodic sample of known pitch (optical or stylus)

• Measuring a known radius part and ensuring the radius of curvature calculation is correct

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Instrument Factors – Vertical Calibration is Key to Accuracy

• Vertical Instrument Calibration • Measurement accuracy can be verified by setup of measurement of

known height sample (step standard, for example) • The motor steps/unit measurement are computed based on

measurement result for a known step with associated uncertainty • Use a step that is close to the feature heights of interest. • Ensure you calibrate the same way every time

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Bruker ContourGT-X8 Continuously Calibrates for Highest Accuracy

• ContourGT-X8 Offers Continuous Self Calibration • Laser tracks scanner motion by interference with reference signal

reflection • Accuracy traceable to known He-Ne wavelength • Second-level traceable standard

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Sample

Beamsplitter

Illuminator

Reference signal detector(s)

CCD Reference signal module

Laser

Mirror on the scanner

Reference mirror

Measurement Signals

Mirror

• ContourGT-X8 Offers Continuous Self Calibration

• Minimizes impact of irregularity in scan mechanism • Minimized impact of drift of scanner

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Uncertainty in nm of 50 um step measurement

0

50

100

150

200

250

300

Continuous Calibration Without Continous Calibration

Unce

rtain

ty (n

m)

Uncertainty in % of 50 um step measurement

0.00%

0.10%

0.20%

0.30%

0.40%

0.50%

0.60%

Continuous Calibration Without Continous Calibration

Unce

rtain

ty (%

)

Bruker ContourGT-X8 Continuously Calibrates for Highest Accuracy

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Continuous Calibration Reduces Uncertainty in Step Measurement Result

Environmental Factors - Temperature Effects Minimized via Control or Calibration

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• Operating environment control minimizes effects

• Continuous calibration provides excellent correction

Instrument Factors – Slope and Lateral Resolutions Vary With Options

• Key parameters are available for 3D microscopes and stylus that help understand tradeoffs of different instrument options

• Matching the instrument settings to the target is key to obtaining accurate and repeatable results

12

200nm lines

70 degree sloped screw threads

Ra=4 nm Ra=4 nm Ra=4 nm

13

100X

Instrument Factors - Height Capability Can Vary

Ra=4 nm Ra=4 nm Ra=4 nm Ra=4 nm

5X 20X 10X 50X 1X 2.5X

• 3D Microscopes determine a signal peak as you move through focus • Pictures below show signal for a single measurement line as you move through focus • Traces show a smooth surface measured with 3D microscopes using interferometry

(top) and confocal (bottom) technologies

Ra=7nm Ra=12nm Ra=74nm Ra=472nm Ra=4nm

Not Usable

Not Usable

Environmental Factors - Vibration Effects Should Be Understood

• Vibration can cause fringes in WLI based instrument to “print through”

• Typically results in errors of a few 10’s of nm to a few 100’s of nm if severe

• Isolation table or damping mechanism employed to minimize

• Avoid drafting from HVAC units, clean hoods

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Noise in measurements – random noise loses against averaging

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• With random (most) noise, noise will reduce by the square root of the number of averages.

• Averaging can help see finer detail than is otherwise possible. • Difference measurements can tell you the noise floor you are

achieving. • Averaging may not help in loud or high vibration environments • ContourGT-X8 can achieve a 0.015nm noise floor

Ra of Difference Measurement vs. # of averages

00.050.1

0.150.2

0.250.3

0 50 100 150 200 250

# of Averages

Ra

(nm

Smooth surfaces – internal optical reference subtraction is key

• Smooth surfaces with small variation in shape/roughness benefit from instrument reference subtraction

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• Use minimum 4 locations, 4 averages

• Subtracts out common element between measurements from future ones

• Essential for stitching super-smooth objects (wafers, mirrors, etc.)

Smooth surfaces – internal optical reference for spheres works well

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With steep slopes, errors from the optics will have some effect

Effect is typically <80nm For very smooth objects, this error

can affect stitching or certain surface calculations

User can generate a reference using a random ball method – Measure multiple locations on a

sphere of the correct target radius – Average the results – Subtract the base curvature and save

the residual as the reference file – Reduces shape effects to <5nm

Sample Considerations for Accurate Metrology – How Should I Fixture?

• Vacuum is excellent choice where possible

• Provides stability and holds reproducibly if set up with kinematic contacts

Bruker offers quick release dovetail slides with vacuum fixtures for easy on and off handling

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Wide range of crystal structure apparent across PV cell

Sample Considerations for Accurate Metrology – Where Should I Measure?

Data Analysis - Filtering is a critical component of accurate, reproducible results

• 2D Stylus filtering according to ISO 4287/4288 standards

• Filtering separates different

portions of data of interest depending on specific criteria

• Make sure you report data that you care about!

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Robust Gaussian Filtering Better Separates Form from Finish – Leads to More Reproducible Metrology

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-7

-6

-5

-4

-3

-2

-1

0

1

2

0 0.2 0.4 0.6 0.8 1 1.2 1.4

mm

um

Unfiltered Data

Non Robust

Robust

Robust Gaussian Filter Created to Filter Form from Waviness and Roughness Without Surface Distortions

3D filtering analogous critical component of accurate metrology

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• 3D Areal filtering works in analogous way according to ISO 25178-2 standards

• Again, filtering is key to reporting data of interest!

Unfiltered data Waviness + form

Roughness

Filter

3D areal parameters – Accurate results with specialized computations

• 3D extension of R parameters from 2D stylus metrology (Sa, Sq, Sz)

• Skew, kurtosis, bearing area, peak density, slopes are computed

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Ssc: Mean summit curvature

Sds: Summit density

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ISO standard computations enable excellent 2D to 3D correlation as well

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25

Agreement between optical and stylus results is excellent

Stylus profiler; Dektak <2um size tip Single 55 um profile

Optical profiler; Contour GT 115X 0.8 NA objective, XLI Single 55 um profile

Method Ra

Nominal 100 nm

Optical 105 nm

Stylus 108 nm

Know your ‘Standard’! This is a ‘sinusoidal’ standard but deviates greatly from an ideal sine wave

Which is Accurate? Comparing Results Between Systems Creates Challenges

• “New System X measures a part 10nm differently than our old system. How do we offset System X”

• “I measured some parts across the two systems and the correlation is terrible!” • How was each system calibrated? • How do results vary within and across systems of each type? • Can the two systems detect the same features? • Are you examining the same areas on each system? • Do the analysis algorithms on the two systems match? • Is there sufficient range in the values for correlation to be meaningful?

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Veeco WLI Vs Contact Stylus - PEEK y = 0.9663xR2 = 0.9939

0

1000

2000

3000

4000

5000

6000

7000

8000

0 1000 2000 3000 4000 5000 6000 7000 8000Ra nm

Ra

nm

y = 0.8916x - 10.726R² = 0.796

-16

-15

-14

-13

-12

-11

-10

-6 -5 -4 -3 -2 -1 0

Optic

al Va

lue (n

m)

AFM value (nm)

y = 0.7337x - 11.494R² = 0.4111

-16

-15

-14

-13

-12

-11

-10

-6 -5 -4 -3

Optic

al V

alue

(nm

)

AFM value (nm)

27

Standard Error is Used to Evaluate Agreement Between Two Systems

• Good for parts with small range in values compared to the average

• Assumes measurement of the same features

• Two methods are considered agreeable to twice the calculated standard error

• Avoids having to know the true sample standard deviation required by the correlation coefficient

( )( )222

221

2

TsysTsys

TRσσσσ

σ

++=

Correlation coefficient

Standard error: standard deviation of the difference

22

21

2syssysSE σσσ +=

Tσ Is the true variation in the sample set

• Accurate surface metrology depends on many factors

• Verify performance on known samples if possible

• Proper fixturing and filtering are significant contributors to obtaining accurate surface metrology

• Ensure data reported is the data representative of need for test

• Bruker offers instrumentation which provides fast, accurate 2D stylus and 3D optical metrology for virtually all applications needs

• Partner with your metrology provider to ensure proper results!

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Summary

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QUESTIONS? Contact info: erik.novak@bruker-nano.com