Download the PDF copy of the presentation slides
Transcript of Download the PDF copy of the presentation slides
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
2/28/2012 2
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”?
2/28/2012 3
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
2/28/2012 4
THESE ARE NOT
ALWAYS INDEPENDENT !!!
Accurate Metrology is a Key Component of Product Success for Many Applications
2/28/2012 5
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
2/28/2012 6
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
2/28/2012 7
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
2/28/2012 8
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
2/28/2012 9
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
2/28/2012 10
Continuous Calibration Reduces Uncertainty in Step Measurement Result
Environmental Factors - Temperature Effects Minimized via Control or Calibration
2/28/2012 11
• 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
2/28/2012 14
Noise in measurements – random noise loses against averaging
2/28/2012 15
• 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
2/28/2012 16
• 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
2/28/2012 17
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
2/28/2012 19
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!
2/28/2012 20
Robust Gaussian Filtering Better Separates Form from Finish – Leads to More Reproducible Metrology
2/28/2012 21
-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
2/28/2012 22
• 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
2/28/2012 23
Ssc: Mean summit curvature
Sds: Summit density
2/28/2012 24
ISO standard computations enable excellent 2D to 3D correlation as well
2/28/2012
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?
2/28/2012 26
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!
2/28/2012 28
Summary
www.bruker.com
© Copyright Bruker Corporation. All rights reserved.
QUESTIONS? Contact info: [email protected]