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Sensors and Metrology
A Survey
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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
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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
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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
" other issues
 sensors require calibration
- sensor fusion
 must account for drift - data compression
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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.
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Accuracy vs. Precision
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SIA Road Map Challenges above 45nm (through 2009)
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SIA Road Map Challenges below 45nm (beyond 2009)
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The 2004 update Metrology Road Map  near term
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The 2004 update Metrology Road Map  long term
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The 2004 update Lithography Metrology Road Map  near term
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The 2004 update Lithography Metrology Road Map  long term
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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...
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Various Edge Detection Algorithms are in use...
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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
AFM
SEM
dense
iso
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Comparison for Contact holes
AFM SEM
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More CD-SEM vs. AFM comparisons
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Optical CD Measurement
I
" 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.
Grating
Grating Grating Grating Grating Grating Grating Grating
Horz
ID: XXN Horz Dense Vert Dense Vert Medium Horz Iso Vert Iso Horz Iso Vert Iso
Medium
W/S = 180/ W/S = 180/ W/S = 180/ W/S= 180/ W/S= 180/ W/S= 180/ W/S= 180/
W/S = 180/
180 180 240 360 360 1000 1000
240
SEM
Lines
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
SEM
Lines
DUT1 DUT2 DUT3 DUT4 DUT5 DUT6 DUT7 DUT8
32 Vert Vert Horz Horz VDP Horz Horz Vert Vert 31
30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
CF_LENS coma/flare
MEFH
ID: XXO
CD_LIN linearity
MEF
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
SEM
Lines
DUT1 DUT2 DUT3 DUT4 DUT5 DUT6 DUT7 DUT8
32 Vert Vert Horz Horz VDP Horz Horz Vert Vert 31
30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
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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
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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-Ã 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
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