Measuring and Characterizing in the
PRISM Micro/Nano Fabrication
Laboratory
Pat Watson, Mikhail Gaevski, Joe Palmer, and Conrad Silvestre
Goals
• Describe the types of measurements we can make in the
MNFL to characterize your devices and processes
• Describe the basic principles of operation of the
metrology tools in the fab
– Understand capabilities and limits of these techniques
– Design processes/masks with these tools in mind
• Qualify you to use:
–
–
–
–
–
Nanospec reflectometer
Gaertner ellipsometer
Dektak profiler
Olympus and other microscopes/cameras
Plus sign off as “observed” on KLA-Tencor profilometer
• Provide some references for further study
Types of measurements
• Part 1
– Inspection and feature size measurements
• Part 2
– Height/Depth/Step measurements
– Film thickness
– Refractive index
– Film Stress
• Other – electrical conductivity
Inspection and Feature Size
Measurements - Microscopes
• Qualitative (Inspection) - Ensure that:
– a lithography, etch, or liftoff step was processed
correctly
– there is no contamination
• Quantitative – Ensure that:
– a feature dimension is correct
– one mask level is properly aligned to another
Optical microscopes in the MNFL
• Olympus MX40 with
Nomarski contrast and
DP70 camera
• Olympus MX51 with
front/back illumination
and UC30 camera
• Nikon with Fuji camera
• Leitz
• Meiji
Microscope resolution – a grating
• Bragg’s law determines how light
is scattered as plane waves from a
grating:
P  sin q   n  l
nl
q
P
• The Numerical Aperture of the
objective lens, NA=sin(qmax),
indicates whether or not the
scattered plane waves of light
enter the optics
• A grating can be resolved only if at
least two diffracted plane waves
of light scattered from it can be
captured by the objective lens
Pmin 
q
l
NA
P
Microscope resolution and
illumination
Objective Lens
f
q
Off-axis incident
light
P
Microscope resolution and
illumination
• Off axis illumination improves resolving power
• Modified Bragg’s law: P  sin q   P  sin f   nl
sin fmax 
• Cone of light defined by:  
sin( q max )
(partial coherence)
l
• Minimum pitch: Pmin  NA  (1   
The smallest resolvable feature
• For =1, NA=0.8, l=510 nm, Pmin=320 nm
• Minimum feature size is about ½ Pmin, or 160 nm
• In reality, there will be essentially no contrast at
minimum pitch, so observable minimum feature size
is a bit larger, ~200 nm
• The illumination condition is just as important as the
imaging optics when considering resolution
• The accuracy of linewidth measurements is also
about +/- 200 nm
Magnification
• Microscopes are designed with an apparent 25
cm viewing distance of the eye, about the closest
a typical person can focus
• The minimum angle the eye can separate is about
1 minute of arc, or about 75 mm at 25 cm
• For a 10x eyepiece and 100x objective
(mag=1000x), the eye can potentially resolve 75
nm features, better than resolution limit (~200
nm)
• Any higher magnification is “empty”; no more
information is gained by magnifying further
Microscope resolution – an example
• 300 nm thick E-beam resist on Si substrate
• EBL written pattern with gratings from 100 nm
line / 100 nm space to 1000 nm line / 1000nm
space
SEM image of 100 nm
L/S (200 nm pitch)
Low magnification
optical image of gratings
SEM image of 200 nm
L/S (400 nm pitch)
Microscope resolution – an example
400 nm L/S
100x objective, NA 0.8, DP70 camera
100 nm L/S
200nm L/S
Feature Size, 1 mm line
1.5
Feature( x1.0)
Feature( x0.8) 1
Feature( x0.9)
Fback
2
Fback3
Fback4
2
2
0.5
0
1
 0.5
0
x
0.5
1
More illumination tricks - darkfield
imaging
Objective Lens
Angle of
Incidence
greater than NA
• >1, annular illumination, light from specular reflection
does not enter objective lens
• But scattered light from particles can enter optics
Darkfield imaging – an example
InP substrate with 500 nm SiO2
Backside illumination and other stuff
• Backside illumination
on photolithography
microscope – a good
way to characterize
masks and observe
devices on transparent
substrates
• The photolithography
microscope can also
capture video and
stitch images
Nomarski contrast
• Split source light into 2
slightly displaced beams
with orthogonal
polarizations
• Phase shift the two beams
reflected off wafer surface
with a Wollaston Prism
• Reflect light off sample topography further phase
shifts light
• Merge the beams and
select a common
polarization state of each
Back Focal Plane
of Objective Lens
From Mansud Mansuripur , Classical
Optics and its Applications, Cambridge
Nomarski contrast
5
1
2
1
2
4
3
4
3
5
Move
Wollaston
Prism back
and forth to
shift phase of
s and p
waves
Nomarski contrast simulation
4


I  x, y   sin 2  Pa  x, y   Pa x  k x , y  k y 
f 
l


• Example: 10 nm deep steps etched into a
reflective (Si) surface
– 510 nm (green) light source
– Wollaston prism phase difference varied +/-180° in 5°
intervals
• Maximum contrast at j~ 45°
Normaski contrast simulation
Nomarski contrast – Phase shift 180°
Nomarski contrast – Phase shift 90°
Nomarski contrast – Phase shift 45°
Nomarski contrast – Phase Shift 0°
Nomarski contrast – Phase shift -90°
Nomarski contrast – Phase shift 90°
(again)
Nomarski contrast – Phase shift -360°
Nomarski contrast – Surface roughness
Conventional illumination,
focus on bottom of trenches
Nomarski contrast
Microscope - basic operation
Darkfield/Brightfield
Illumination Filters
Nomarski (Wollaston)
Prism
Nomarski Polarizers
Stage Focus (z)
Illumination Intensity
Stage x/y motion
Microscope - basic operation
•
•
•
•
•
•
•
•
Choose contrast technique: brightfield, darkfield, or Nomarski
Set turret to lowest magnification objective (2.5x or 5x)
Lower stage to give adequate clearance between objective and sample surface
Place sample on stage and move it under objective
Turn on illumination and adjust intensity (check apertures)
Adjust eyepiece separation to accommodate your pupil distance
Find best focus through right eyepiece by the adjusting stage height, then rotate
left eyepiece so that its image is as sharp as right
Increase magnification and adjust illumination accordingly
Left Eyepiece
Focus Adjustment
Pupil Distance
Adjustment
Rotate to
Adjust Left
Focus
Move Stage to
Adjust Right
Focus
Camera and software - capturing
images
Objective Lens Selector
Image Capture
Live Image
Exposure Control
Image Buffer
Focus Monitor
Camera and software - measuring
features
Magnifier
Measurement Toolbar
Linescan Result
Measurement Results
Scale Bar
Designing experiments with microscopy in
mind
• Make measurements with
Linewidth Control Features (LCF)
– Make them simple to find and
measure
– Use text and arrows on mask
patterns to identify features
• Minimum resolvable grating
~200 nm L/S
• Precision of linewidth
measurement ~ +/- 200 nm
• Accuracy of linewidth
measurements can also depend
on the materials, sidewalls, local
environment
4.0
Measuring and Characterizing in the
PRISM Micro/Nano Fabrication
Laboratory, Part 2
Pat Watson, Mikhail Gaevski, Joe Palmer, and Conrad Silvestre
Goals
• Describe the types of measurements we can make in the
MNFL to characterize your devices and processes
• Describe the basic principles of operation of the
metrology tools in the fab
– Understand capabilities and limits of these techniques
– Design processes/masks with these tools in mind
• Qualify you to use:
–
–
–
–
–
Nanospec reflectometer
Gaertner ellipsometer
Dektak profiler
Olympus and other microscopes/cameras
Plus sign off as “observed” on KLA-Tencor profilometer
• Provide some references for further study
Types of measurements
• Part 1
– Inspection and feature size measurements
• Part 2
– Height/Depth/Step measurements
– Film thickness
– Refractive index
– Film Stress
• Other – electrical conductivity
Height Measurements
• Large steps: from 50 to >300 mm
– Mitutoyo Digital Micrometer
– Optical Microscope
• Small steps
– From 1 mm to 65 mm: Dektak Profilometer
– From 1 nm to 327 mm: KLA Tencor Profilometer
• 3 ranges: 327 mm, 32 mm, and 6 mm
Mitutoyo digital gauge
Olympus microscope height
measurement
• Select highest practical
magnification – image
will have smallest depth
of focus
• Record height on stage
micrometer at top and
bottom of step
Profilometery
•
•
•
•
Photoresist thickness, resist loss during processing
etch depth
Residues
Deposition thickness
Profilometry in the MNFL
• KLA-Tencor P15
profilometer
• Uses a stylus to measure
contours as specimen
moves underneath on
optical flat
• Precision in nm range
• Maximum step 327 mm
• Accuracy depends on step
height: better than +/-5%
for 100 to 1000 nm range
Profilometry in the MNFL
• Dektak profiler –simple
to use
• Like the KLA-Tencor, it
uses a stylus to
measure contours as
specimen moves
underneath
• Maximum step: 65 mm
• Accuracy depends on
step height, limited by
flatness of background
Profilometry – the stylus
• Stylus shape is a
piece-wise
continuous function:
2

r  r 2  x  xc 

H stylus ( x, xc )  
x  xc  r cosq 




r
1

sin
q


tan q 

x  xc  r cosq 
x  xc  r cosq 
• Treat surface profile
as pair of step
functions:
H profilex   d  H x  w 2  H x  w 2
2q
R
xc
Profilometry example
2.0 mm radius
tip with 45°
cone, KLATencor scan of
32 mm L/S,
etched 21 mm
deep
Profilometry example
Simulation of 21.4
mm deep trench with
vertical walls, 2.0
mm radius tip, and
45° cone (circles),
or 30° cone
(crosses)
Profilometer trace and feature sizes
5mm
8mm
12mm
2.0 mm radius tip with 45° cone angle
Profilometer trace and feature sizes
5mm
8mm
12mm
2.0 mm radius tip with 45° cone angle
Profilometry – basic operation of
KLA-Tencor
• Load sample – note size!
• Camera has a small field
of view, so choose step
features carefully
• Some parameters to start
with:
–
–
–
–
–
–
10 mm/s scan speed
500 Hz sample rate
1 scan avg.
2.0 mg applied force
100 mm range
(20 nm data steps)
Profilometry – basic operation
• User cursor
windows to level
• User cursor
windows to
measure height
differences
• Besides height,
surface roughness
and other
artifacts can help
characterize etch
and deposition
processes
Designing experiments with profilometry in
mind
• Make measurements with
Control Features
– make sure they are large
enough for the stylus to
reach the bottom of a
trench
– build a variety of sizes if
the height to be measured
is unknown
– Make them simple to find
and measure
– Use text and arrows on
mask patterns to identify
features
Thin Film Measurements
• Transparent thin film properties are important
parameters in micro and nanoscale device
fabrcation
– Photoresist and other polymers
– SiO2 and Si3N4
– a-Si on dielectric
• Reflectometry uses thin film optical effects to
measure layer thickness
Reflectometry
• Measure polymer or dielectric layer thickness
on reflective substrate
• Measure one unknown thickness on stack of
known transparent layers
• Measure thickness in small regions (as small
as about 10mm x 10mm)
• With D2 Lamp, measure oxide thickness on Si
in 10 nm regime
Reflectometry in the MNFL
• Nanospec AFT reflectometer
• Spectrometer ranges from 200 to
800 nm measures light intensity
at each wavelength
• 2 light sources, conventional
Halogen lamp and Deuterium
lamp
• Computer and software to fit
data
Spectrometer
Microscope
Reference wafer
Control computer
Principles of reflectometery
• Fresnel coefficients determine E field reflection magnitude and phase at 2
interfaces
• Optical path length through film shifts phase of each pass
• Equations for the multiple reflections form Geometric series
• Absolute square of E field intensity gives Intensity
Reflectometry: SiO2 on Si
• Period of oscillation
identifies optical path
length, n*t
• Thinner films increase
period
• Even films with less
than one period over
optical range can be
analyzed
Reflectometry: SiO2 on Si
• Example, 100 nm SiO2
on Si
• +/- 5% thickness
variation can be
measured if reflectivity
is properly calibrated
Short wavelength lamp option
Reflectometry – basic operation
• Warm up illumination
source! At least 15 minutes
• Choose recipe and objective
lens (F9)
• Move reference wafer
under microscope, focus,
and take reference (F7)
• Move to sample, focus and
take measurement (F10)
• Check fit with graph (F2)
Designing experiments with
reflectometry in mind
• Design easy-to-find features to get repeatable
results
• Design mask levels to keep measured film
stacks simple (1 or at most 2 films)
• Note that complex semiconductor (InGaAsP or
SiGe) stacks can confuse results, so it may be
better to run a monitor wafer along with your
real sample during etch/deposition
Measuring film thickness and refractive
index, ellipsometry
• Ellipsometry measures the difference in
intensity and phase between TE and TM (or s
and p) light reflected from a surface
• It can determine the index of refraction and
film thickness for single layer film (with
caveats)
• It can determine any two parameters for
substrates or for multiple films
Ellipsometry in the MNFL
• Multi-angle
• Multiwavelength
– 632.8 nm
– 800 nm
– 1300 nm
• Spot a few mm
in size
• Automatic
model fit
Ellipsometry


Rp d l f 0 
 
 
r01p f 0  r12p f 0  e
 
 


 2i  d l f 0
1  r01p f 0  r12p f 0  e


 2i  d l f 0
• Reflection equations are
similar to reflectometry
except that the angle of
incidence is not 90°
• Reflections of s and p
polarization states (TE
and TM) are not the
same
• Plane polarized light
incident on a surface
will in general be
elliptically polarized
upon reflection
Ellipsometry
• Measure the ratio of reflection
of p and s polarizations,
Rs
 tan(  )  e
• Solution to thickness of
transparent film on Si is not
unique:
– Thk+N*283 nm for SiO2
– Thk+N*233 nm for Al2O3
– Thk+N*179 nm for Si3N4
• Period:
l
2 n 2film  sin 2 fincidence 
• Using 2 angles or 2
wavelengths breaks
“degeneracy”
200 nm
i
300
Del (degrees)
Rp
Psi and Del with Varying SiO2 Thickness on Si
280 nm
200
0 nm
100 nm
100
0
0
20
40
60
80
Psi (degrees)
Following H. Tompkins, A User’s Guide
to Ellipsometry, Academic Press, 1993
Ellipsometry, 2 angle or 2
wavelength measurements
300
Del (degrees)
200
100
0
0
500
110
3
Thickness (nm)
1.510
3
210
3
• Green:  vs. SiO2 film
thickness at 70° angle of
incidence
• Magenta:  vs. thickness
at 50°
• Red points show
ambiguity:  is the same
for all 100nm + N*283nm
• Blue points at 50°
representing same
thickness as red points
are not at constant 
Ellipsometry – basic operation
• Warm up light source! At
least 5 minutes
• Set angle (typically 70°)
• Align wafer and beam on
stage
• Select program
– Angle
– Wavelength (632.8 nm)
– Guess of film thickness
• Turn on rotating analyzer
• Measure and Calculate
Measuring film stress profilometry
• Measure profile across a 100 mm wafer before
and after depositing a film
• Requires special wafer chuck
• Net wafer bow and film thickness are used to
calculate film stress (Stoney Equation)
E t

6t f  R  1  
2
s
Measuring film stress profilometry
References
• Classical Optics and its Applications, Mansu Mansuripur, Cambridge
University Press, Cambridge, UK, 2002.
• Introduction to Fourier Optics, J. W. Goodman, McGraw-Hill, New York,
1968
• Principles of Optics, M. Born and E. Wolf, Pergamon Press, Oxford, UK,
1980.
• Spectroscopic Ellipsometry and Reflectometry, A User’s Guide, H. G.
Tomkins and W. A. McGahan, John Wiley and Sons, New York, 1999.
• Introduction to Modern Optics, G. R. Fowles, Dover Publications, Mineola,
NY, 1989.
• A User’s Guide to Ellipsometry, H. G. Tompkins, Academic Press, San Diego,
CA, 1993.
• Ellipsometry and Polarized Light, R. M. A. Azzam and N. M. Bashara, North
Holland, Amsterdam, 1987.
• Handbook of Optical Constants of Solids, E. Parik, editor., Academic Press,
Orlando, 1985 (available on-line at Princeton University Library).
Software
• MathCAD
– Great for working with complex numbers
– $113 through University
• R
–
–
–
–
Statistical analysis environment based on “S” and S-plus
Publication quality plots
Easy to script
Free!
• MatLab
– Easy to script
– Free through University