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Lecture 17: Sensors and Metrology I
SpanosEE290H F05
1
Sensors and Metrology
A Survey
Lecture 17: Sensors and Metrology I
SpanosEE290H F05
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Outline
• General Issues & the SIA Roadmap• Post-Process Sensing (SEM/AFM, placement)• In-Process (or potential in-process) Sensors
– temperature (pyrometry, thermocouples, acoustic waves)– pressure and flow (manometers, momentum devices)– composition (OES, LIF, RGA, mass Spectroscopy Actinometry)– thickness (reflectometry, ellipsometry, scatterometry)– smart-dummy wafers and smart substrates
Lecture 17: Sensors and Metrology I
SpanosEE290H F05
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Introduction
• Sensors (and actuators) are key limiting factors in application of control techniques to semiconductor manufacturing
• sources of difficulty– implementation environment (vacuum, clean facilities, etc.)– perception that in-situ sensors affect process– ex-situ sensors can reduce throughput– cost of ownership– traditional resistance in industry
Lecture 17: Sensors and Metrology I
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General Remarks on Sensors
• modeling is often key part of sensing– physical quantity of interest may not be directly measured (ex:
OES indirectly contains info about etch process state)– thus, sensors are based on a model of the underlying physical
process
sensors = data + model
• signal processing– needed to reduce noise, improve bandwidth– difference between data and information
• problems– sensors require calibration– must account for drift
• other issues- sensor fusion- data compression
Lecture 17: Sensors and Metrology I
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Key Issues in Sensors
• some key tradeoffs– non-invasive vs. invasive– non-destructive vs. destructive– in-situ vs. ex-situ– speed vs. accuracy– noise
• bias (accuracy) vs. precision (repeatability + reproducibility)– a sensor could be inaccurate, (thermocouple readings are off by 4o K)– but the sensor might have good precision, (it is consistently off)– precision is often more important for process control
• modern filtering and estimation methods can be of great use in improved sensing software.
Lecture 17: Sensors and Metrology I
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Accuracy vs. Precision
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SIA Road Map Challenges above 45nm (through 2009)
Lecture 17: Sensors and Metrology I
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SIA Road Map Challenges below 45nm (beyond 2009)
Lecture 17: Sensors and Metrology I
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The 2004 update Metrology Road Map – near term
Lecture 17: Sensors and Metrology I
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The 2004 update Metrology Road Map – long term
Lecture 17: Sensors and Metrology I
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The 2004 update Lithography Metrology Road Map –near term
Lecture 17: Sensors and Metrology I
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The 2004 update Lithography Metrology Road Map – long term
Lecture 17: Sensors and Metrology I
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CD Metrology
• CD-SEM is today’s pre-eminent technique…• Electron yield in interaction volume is a function of
surface topography (Secant effect) and atomic number.• Extracting CD is not so simple...
Lecture 17: Sensors and Metrology I
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Various Edge Detection Algorithms are in use...
Lecture 17: Sensors and Metrology I
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CD-SEM is often calibrated with AFM
Atomic force Microscopy uses either repulsive forces (sub nm proximity), or weaker attractive forces (a few nm away).
Tip tracks surfaces using feedback control.
Shape and size of tip is the critical source of errors.
Standard features are use to calibrate and “de-convolve” the tip profile from the measurements.
AFM is 100-1000 times slower than a CD-SEM.
AFM is sensitive to line-edge roughness.
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CD-SEM vs. AFM standards
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CD-SEM cannot “see” the actual profile, so AFM is used for this purpose...
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AFM/SEM reading of Dense vs. Isolated Lines
dense
iso
AFM SEM
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Comparison for Contact holesAFM SEM
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More CD-SEM vs. AFM comparisons
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Optical CD MeasurementI
• Not very repeatable
• Limited spatial resolution
• Relatively inexpensive
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Electrical CD Measurement
Measurement is very repeatable and fast.Can only be used in conductive layers. Need at least one conductive layer and one insulator.Can be extended for misalignment measurements.
SEMLines
ID: XXNGratingHorz Iso
W/S= 180/1000
GratingVert Iso
W/S= 180/1000
GratingHorz Iso
W/S= 180/360
GratingVert Iso
W/S= 180/360
GratingHorz
MediumW/S = 180/
240
GratingVert MediumW/S = 180/
240
GratingHorz DenseW/S = 180/
180
GratingVert DenseW/S = 180/
180
DUT1Vert
DUT2Vert
DUT3Horz
DUT4Horz VDP
DUT5Horz
DUT6Horz
DUT7Vert
DUT8Vert
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
31
30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
32
SEMLines
ID: XXO
DUT1Vert
DUT2Vert
DUT3Horz
DUT4Horz VDP
DUT5Horz
DUT6Horz
DUT7Vert
DUT8Vert
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
31
30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
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SEMLines
CF_LENS coma/flare
CD_LIN linearity
MEFH
MEF
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Basic SEM Structure
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The Many Modes of SEM
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The Issue of Spatial Resolution
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Chrome on Silicon Example
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Signal Depends Strongly on Material
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Voltage Contrast SEM
Lecture 17: Sensors and Metrology I
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CD-SEM Resolution
Scanning Resolution shown to 1-5nm.
CD metrology on resist has 5-10nm precision.
Other solutions:
ATF, has (theoretically) atomic resolution.
Problem:
What is CD??
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CD-SEM Conclusions
• Accuracy is several (many) nm• Precision (1-σ repeatability + reproducibility) is
1-2nm today• CD-SEM is stand-alone (i.e. expensive)• CD-SEM measurements are available only after
patterning, and data integration with control systems is difficult at best.
• AFM-based calibration will not be possible for trenches less than ~100nm wide.
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State of the Art CD/Imaging-SEM