AN
ABSTRACT
OF
THE
THESIS
OF
Michael A. Viliardos for the degree Master of Science in
Mechanical Engineering presented on July 23,
Title:
Thermal Annealing of Mo/Si
1992 .
Multilayers
to Assess
the
Stablity Relevant to Soft X-ray Projection Lithography.
Redacted for Privacy
Abstract approved:
Michael
E.
Kassner
The thermal stability of sputtered Mo/Si multilayers
(ML)
was investigated by annealing at 260-342°C for 0.5-3000
hours.
Two distinct stages of interlayer growth were seen:
primary region of ~3A,
and a slower secondary growth region.
The interlayer growth rate from deposition of Mo-on-Si was
-200 times greater than the growth rate from deposition of
Si-on-Mo in the secondary region.
The interdiffus ion
coefficient, Do~100cm2/s, and activation energy, Ea~2.5eV,
for the Mo-on-Si secondary stage growth are comparable to
values for diffusion of Si in h-MoSi2.
Annealing of the ML
causes the normal incidence reflectivity to decrease.
a
THERMAL ANNEALING OF Mo/Si MULTILAYERS
TO ASSESS
THE STABILTY RELEVANT
SOFT X-RAY
TO
PROJECTION LITHOGRAPHY
by
Michael
A
A.
Viliardos
THESIS
submitted
to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master
of
Science
Completed July 23, 1992
Commencement June
19 93
APPROVED:
Redacted for Privacy
Professor of Mechanical Engineering in charge of major
Redacted for Privacy
Head of department of Mechanical Engineering
Redacted for Privacy
Dean of Graduate Schocgl
Date thesis is presented:
July
Typed by Michael Viliardos for
Michael
23,
1992
A. Viliardos
ACKNOWLEDGMENTS
Special
assistance.
thanks to M.
I
also
wish
my
committee
the
providing the needed data:
and LAXS;
D.
Gaines,
R.
many
people
Financial
T.
study was provided by Lawrence Livermore National
to
Burhanuddin.
members
for
Thanks
S.
thank
W.
Laboratory.
and
to
their
Kennedy,
this
Warnes,
Kassner and B. Rosen for all
who
support
assisted
in
U. Cheng for the HREM micrographs
Spitzer,
M.
for the NIR measurements for BESSY.
Krumrey,
and
P.
Muller
I also wish to thank my
wife and friends for their support and assistance.
TABLE
OF
CONTENTS
INTRODUCTION
1
EXPERIMENTAL
PROCEDURE
7
Specimen Fabrication
X-ray Diffraction
7
10
Heat
11
Treatment
High Resolution Electron Microscopy
12
Normal
14
Incidence Reflectance
RESULTS
20
Unannealed Samples
Annealing Experiments
20
21
Diffusion
Normal Incidence Reflectance
22
25
Stability
26
CONCLUSIONS
41
FUTURE WORK
42
BIBLIOGRAPHY
43
LIST
OF
FIGURES
Figure
Page
1.
Arrhenius plot of log D versus 1/T for the
research of other investigators.
2.
Side and top schematic of the ML
deposition system.
16
3.
Cleavage pattern of ML coated wafer.
18
Side and top schematic of the BESSY
19
4.
6
refleetometer.
5.
HREM micrographs of a) unannealed ML sample
28
compared to those annealed at 316°C for
b)l hour, c)40 hours, d)80 hours.
6.
HREM micrographs of annealed MLs with
similar interlayer growth at different
temperatures and times a) 160h @ 260,
& 298, c)lh @ 316, and d)lh @ 342°C.
7a.
7d.
8.
34
316°C.
Interlayer growth rates of Mo/Si MLs
annealed at
33
298°C..
Interlayer growth rates of Mo/Si MLs
annealed at
32
260°C.
Interlayer growth rates of Mo/Si MLs
annealed at
7c.
b)60h
Interlayer growth rates of Mo/Si MLs
annealed at
7b.
31
35
342°C.
Arrhenius plot of log D versus 1/T for
this study compared with that of other
36
investigations.
9.
Arrhenius plot of log G (growth rate) versus
1/T for the primary surge region.
10a. Measured NIR data of annealed specimens
37
39
at 316°C compared with the "theoretical"
reflectivity.
10b. Measured NIR data of annealed specimens
at 342°C compared with the "theoretical"
reflectivity.
40
LIST
OF
TABLES
Table
1.
£a.g_e_
Summary of previous investigators' annealing
5
experiments.
2.
Listing of the average pre-annealed lambda
17
(A) and gamma (y) for the seven ML samples
produced.
3.
SAXS and HREM results
for the seven
27
unannealed specimens.
4.
HREM results for the annealed ML samples at
260, 298, 316, and 342°C.
29
5.
NIR data for unannealed ML samples and
annealed ML samples at 316 and 3 42°C including
peak reflectivity, peak position, and full
38
width half maximum.
Thermal
Annealing
Stability
of
Mo/Si
Relevant
Multilayers
to
Soft
to
Assess
X-ray
the
Projection
Lithography.
INTRODUCTION
Since
the
on reducing
through
advent
the
the
of
feature
use
of
features
use
soft
of
shorter
x-ray
0.5
the
light.
projection
reflective
possible
by
using
circuitry.
wavelength
microns,
but
like
method
light
in
to
multilayer
two
method
x-rays.
lithography
optics
this
One
(SXPL).
focus
(ML)
the
is
the
is
limited
consist
of
produce
stack.
The bilayer period,
This,
x-rays
is
however,
which
are
Multilayers
alternating materials
or
the
approach
coatings.
typically
a
One
This can be mitigated by
wavelengths,
requires
of
researchers have worked
Visible and UV light has been used to
of
by diffraction of
size
shorter
lithography process.
achieve
computers,
the
built
up
distance
to
from
the center of one material to the center of the next layer of
the same material,
hundred.
several
The
to
of
the ML is
number
upwards of
of
tens of angstroms to several
layers
a hundred.
deposited
number
combinations
index of
component
are
chosen
refraction,
like
to
C,
vary
from
The materials used are a
high atomic number component like Mo,
atomic
can
Si,
maximize
W,
Ru,
B4C,
the
or Cr,
or
Be.
and a low
Material
difference
of
while minimizing the absorption.
While
the reflectivity from a single layer is low (-10^),
reflectivity
from
all
the
the
the
from a ML is high because the reflected fields
boundaries
add
in
phase,
like
a
quarter
wave
stack.
Interfacial
reflectivity
by
reflections.
are
roughness
destroying
Other factors
substrate
roughness,
thickness and density.
affected
because
by
the
they
by
unstable and will
great
interest
chemical
their
of
the
MLs
demonstrate
two
nature,
and
are
and
many
are
the
material
strongly
and
thermodynamically
it
is of
stability of
ways
thermal
layer
to
the
produce
evaporation,
sputtering.
superior
of
materials
any temperature,
methods
deposition,
the
the
the reflectivity
width,
the structural
there
vapor
sputtered
very
interdiffuse at
Some
coherence
influence
between
Currently,
multilayers.
that
affects
Because the reflectivity is
to determine
multilayers.
phase
interfacial
interface
are,
greatly
In
general,
formation
and
consequently higher x-ray reflectivity.
Several
stability
researchers
of
Mo/Si
reflectivity of -70%.
to other types of MLs
et
al.
[2],
relatively
400°C.
and
the
due
to
the
studied
the
theoretical
[1].
maximum
Notable studies include Holloway
et
al.
annealing
[3].
times
These
and
works
involved
temperatures
Holloway,
et al.,
above
interface was wider,
deposited
at
450°C,
on Mo
found that the Mo deposited
and continued to grow even though
interface
Holloway,
et
had
al. ,
no
apparent
detected
MoSi2
interfaces with selected area electron diffraction.
et
thermal
All the works involve sputtered Mo and Si deposited
Si
Also,
recently
This is quite favorable when compared
Stearns
short
on a Si wafer.
on Si
ML
have
al. ,
found
growth
of
the
interlayers
growth.
in
the
Stearns,
characterized
by
amorphization
and
silicidation
at
400°C.
Also,
the
normal
incidence reflectivity of the MLs was degraded with increased
annealing temperatures.
couples
have
interface.
[5],
found
These
Guivarc'h,
summary of
Other researchers working with Mo-Si
growth
of
h-MoSi2
include Cheng,
et
al.
these works
[6],
et
(hexagonal)
al.
[4],
Nechiporenko,
is presented
at
the
et
al.
[7].
A
Gage,
et
Gage,
et
al.
in Table
1.
al., found that Si was the predominant moving species through
the use of markers.
An Arrhenius
plot
of
log D
versus
studies can be seen in Figure 1.
1/T
slope
of
all
the
the various
The activation energies for
the various studies are nearly identical,
the
for
points.
as can be seen by
There
is,
however,
much
variation in the results for diffusion coefficients among the
various
the
studies.
deposited
This
ML
can
structure
deposition parameters.
during
sputtering
interlayers
variations
[8].
are
be
attributed
as
well
as
For instance,
has
been
Other
different
shown
differences
differences
in
in
the
higher argon pressure
to
factors
to
cause
that
measurement
thicker
could
initial
cause
techniques
these
and
data
analysis.
The
purpose
interdiffusion
of
this
behavior
of
study
Mo-Si
is
ML
to
investigate
structures
relevant
the
to
soft x-ray projection lithography over a wide range of times
at relatively low temperature
of
interest
are
composed
of
(260-342°C) .
40
layer
The ML structures
pairs
of
having thicknesses of -30 and 40 A respectively.
Mo
and
Si
These MLs
are designed for an optimum norma], incidence reflectivity of
-65% at 130 A.
in this study the MLs were annealed at 260,
298,
342°C
316,
and
for
times
up
to
3000
hours.
The
objective is to ascertain the low temperature stability by
determining a diffusion coefficient and an activation energy.
Also,
any
stress
Information
extrapolated
stability
of
relaxation
obtained
to
the
Mo/Si
at
involved
these
relevant
ML
for
will
lithography as reflective coatings.
in
identified.
temperatures
temperature
use
be
soft
to
will
be
determine
the
X-ray
projection
Table 1: Summary of previous investigators' annealing experiments
ML
Mo/Si
studies
Holloway, et al. Stearns, et al.
ML period (A)
# of layers
Anneal
temperatures (°C)
Anneal times (h)
Diffusion
coeffecient (m2/s)
Activation
energy (eV)
couples
Cheng, et al.
Gage.et al.
Guivarc'h, et al.
130
1 10
NA
NA
NA
30.5
30
NA
NA
NA
400-500
200-800
560-580
850-1 100
475-1000
30 sec
0.5
0- 1
0-25
0-9
7x10"19
4x10-22
1x10"3
1x10-9
2x10-7
2.0
NR
2.3
2.1
2.4
NA (not applicable)
NR (not reported)
(_n
-5
"I
T
'
'T""
I
|
D
Stearns (thick)
A
Holloway (thick)
•
Holloway (thin)
•o
(Si in h-MoSi2>
o»
Q
Gage
Nechiporenko
Cheng
n
Guivarc'h
♦
-10
•
*faB
E
o
^
-15
A
CD
O
-20
Ea = 2.5 eV
-25
0.5
1.0
1.5
2.0
103/T[K-1]
Figure 1: Arrhenius plot of log D versus 1/T for the research
of other investigators.
7
EXPERIMENTAL
PROCEDURE
Specimen Fabrication
The
multilayers
alternately
polished
thick.
used
sputtering
single
in
Mo
crystal
this
and
(100)
study were
Si
onto
silicon
a
fabricated by
75
wafer
mm
diameter
0.020
inches
There was no attempt to remove the -20 A of native
oxide
prior
purchased
to
deposition
of
the
ML.
The
wafers
from International Wafer Service with a
were
specified
flatness of 5 microns and an RMS roughness of 3-7 A over 200
microns.
The
deposition
system
is
illustrated
consisted of a cylindrical chamber 36
16 inches tall.
in
Figure
2.
It
inches in diameter and
The top was removable via a hydraulic hoist
and was sealed with an o-ring.
The system was
evacuated to
50 mTorr using a two stage Alcatel model 2033 mechanical pump
with an oil trap.
mm
diameter
The chamber was then evacuated using a 200
Helix
cryo-torr
Four 500 W
quartz
chamber
to
facilitate
After
2 0 minute
a
8
high
vacuum
cryogenic
pump.
lamps were placed symmetrically about
the removal
bake,
of water
followed by
from the
an overnight
the
system.
pump,
the
chamber base pressure was typically 5 x 10~8 Torr.
The
about
substrate
its
chamber
followed a planetary motion.
central
central
axis,
axis.
which
The
in
turn
substrate was
It
revolved
mounted
rotated
about
face
down on
a spinner motor assembly which was mounted onto a 2 8.5
diameter
platter
at
assembly
consisted
of
a
radius
a
Canon
of
15
d.c.
inches.
motor
The
the
inch
spinner
CNG26-02301
Y520
which was
tube.
encapsulated in a 0.75
inch I.D.
stainless
steel
One end was sealed with a 2.7 5 inch knife edge flange
and copper gasket.
A ferrofluidic feedthrough MB-250-KN-089
from
Corp.
Ferrof luidics
was
used
rotation to the substrate fixture.
V
and
spun at
bottom of
~4
Hz.
The
the platter,
the targets.
to
transmit
The platter was
motor
The motor operated at 40
substrate was
which was
the
-6
cm
aligned with
from
the
supported by a
the
surface of
central
shaft
which entered the chamber through the lid via a ferrofluidic
feedthrough from Ferrofluidics Corp.,
Platter
rotation
was
provided
compumotor with 100:1 reduction,
part number 54-116298A.
by
a
Parker
A/AX03-135
and controlled with a Schwab
SC386-25U IBM compatible microcomputer.
The
sputtering
magnetrons
located in
180 degrees apart.
toroidal
array
linear array.
of
sources
were
the bottom of
The magnetic
Al-Ni-Co
tube
that
was
effluent
to
the
target was clamped to the
paste
as
an
a
central
larger
than
the
target
It approached within 5 mm of the
This effectively confined the
target
top of
interlayer
electrical conductivity.
the Si
surrounding
Shielding in the shape of a
slightly
bottom of the rotating platter.
silver
the chamber and placed
The inner and outer arrays were separated by a
surrounded each magnetron.
sputtered
planar
field was generated from a
magnets
channel used for water cooling.
square
rectangular
to
area.
A
12.7
x
25.4
cm
each magnetron body using
provide
good
thermal
Each target was 99.99% pure,
target being doped with 10 ppm boron.
and
with
The Mo target
was 6.35 mm thick, while the Si target consisted of a 3.12 mm
Si
plate soldered to a 3.12
mm Mo backing plate using a
melting temperature indium alloy.
surface
of
controlled
power
each
target
was
independently
supplies,
capable
The magnetic
-300
G.
by
Advanced
of
operating
field at the
Each
Energy
in
source
MDX-10K
constant
current or voltage mode at a power up to 5 kw.
the
MLs
current
were
deposited
and voltage
in
from
the
each
constant
supply
was
d.c.
power,
In this study
power
was
low
mode.
monitored
The
by
the
microcomputer.
Argon was provided by boil
through a Matheson model
the
chamber
from
8301
off
from a dewer and passed
thermal purifier.
the bottom at
two
symmetrical
It
entered
points
near
each target. The pressure was maintained at 1.75 mTorr during
deposition,
with
Comptech model
cryogenic
a
variable
CVO-6-5M,
pump.
A
vane-type
placed between
Vacuum General
manometer measured the pressure
throttle
the
model
chamber
80-6B
valve,
and
the
capacitance
and provided adjustments
to
the throttle valve.
The Ar flow was held constant by a Vacuum
General
mass
were
model
KM-4
controller.
then ignited and maintained at
for the
first
Si and Mo
stabilized
platter
was
held
deposition was
targets.
target
and
targets,
for
The
sputtering
levels of
respectively.
5 minutes,
stationary.
The
during which
After
280
the
5
sources
and 110 W
sources
the
were
substrate
minute
period,
initiated by rotating the substrate over the
The platter was
1.10
rpm
over
rotated
the
Mo
at
0.89
target
rpm
and
over
swept
the
out
Si
110
10
degrees
centered
between
the
on
target
the
respective
regions,
The platter made a total of
of the multilayer.
the
targets.
platter
In
moved
at
the
5.00
70°
rpm.
40 revolutions during deposition
The chamber was backfilled with nitrogen
to atmospheric pressure at the end of
the deposition and the
sample was removed.
Seven ML coated wafers were produced for this study and
are listed in Table 2.
The samples had a bilayer spacing (A)
in the range of 69.4 to 70.0 A and a Mo to bilayer thickness
(y) ratio of -0.44.
X-ray Diffraction
The
angle
X-ray
(SAXS)
diffraction
consisted
and large angle
(LAXS).
obtain Bragg diffraction peaks
determined.
obtained
Rotoflex
Information
with
LAXS.
diffractometer
(X=1.542 A)
in
two
types:
small
SAXS was performed to
from which the ML period was
about
SAXS
of
the
was
crystalline
performed
the
phases
using
a
was
Rigaku
0-26 geometry with a Cu Ka
source operating at 20 kV and 20 mA.
The sample
holder consisted of a precision stage driven by piezoelectric
actuators providing tilt and translation.
to
measurement,
parallel
to
incident
beam.
was
X-ray
aligned
beam
The
and
sample
with
the
located
was
then
in
The
sample,
surface
the
of
center
rotated
from
prior
the
of
ML
the
grazing
incidence to -4° in q and the analog results were recorded on
a chart recorder.
with
the
chart
The sample was scanned at 1/8° per minute,
recorder
set
at
a
1
second
time
constant
and
11
at
20 mm/min chart speed.
Throughout the scan,
the detector
count rate was adjusted to maintain each diffraction peak in
the upper half of the chart paper to improve the resolution.
LAXS
Sciences
was
at
performed
Arizona
at
State
the
Center
University.
for
A
Solid
Rigaku
State
Rotoflex
diffractometer with computer control with a Cu Ka (^=1.542 A)
source operating at 50 kV and 30 mA was used.
scanned from 20 to 50° in 20.
The sample was
Scans were repeated 10 times
and combined to increase the signal to noise ratio.
Peat
Treatment
Each prepared wafer was cleaved into ten 12x24 mm pieces
as shown in Figure 3 and individually numbered.
accomplished by
bending
moment
performed
on
scratching the
causing
each
encapsulated in
the
piece
wafer
and
16 mm O.D.
sample edge
to
then
Cleaving was
and applying a
fracture.
they
Pyrex glass
were
tubes
SAXS
was
individually
at
a pressure
less than 10~5 Torr to avoid oxidation.
Heat
treatment
accelerate
the
a
was
performed
diffusion
performed
in
timer was
placed in the
on
process.
Lindberg model
same
51894
The
electrical
from
A
to
was
continuous
circuit
to verify
The temperature of
furnace was monitored using K type thermocouples placed
in close proximity to the encapsulated samples.
were
sample
annealing
furnace.
that power was maintained to the furnace.
the
each
placed
the
in
the
bottom
furnace
of
the
center
furnace
on wire
to
The samples
racks
ensure
~3
a
inches
uniform
12
temperature.
260,
298,
The
316,
samples were annealed at
and 342°C for various times.
temperatures of
Five minutes was
added to the anneal times to allow for heat up of the sample
as
it
was
samples
introduced
were
air
temperature) .
into
the
quenched
The
furnace.
(~5
samples
Once
minutes
were
to
removed
annealed,
cool
from
to
their
the
room
Pyrex
capsules by rapidly heating the end opposite the sample with
a
propane
torch
causing
pressure equalization.
the
sample
facing
cracks
downwards
onto
the
wafer
left
unannealed
determine
if
sample
the
form
which
allowed
for
The tube was then cracked open with
fragments
was
to
to
avoid
surface.
to
be
One
used
encapsulation
distributing
as
sample
a
from
control
process
glass
each
and
affected
to
the
multilayer.
High Resolution Electron Microscopy
The microstructure of
HREM.
The
samples
were
(half of an original
the
sample
pieces
each
were
other.
and
was
the ML samples was studied using
prepared
sample).
then
bonded with
After
A
cut
in
epoxy
drying
mechanically polished to
(HREM)
from
a
x
12
mm
sample
2 mm piece was cleaved from
half
with
lengthwise.
the ML
overnight,
fit
12
the
The
surfaces
two
facing
specimens
were
a slot in a steel holder which
then was inserted into a brass tube and glued in place with
epoxy.
using
The assembly was then sectioned into -0.5 mm pieces
a
diamond
saw.
mechanically polished,
The
individual
on both sides,
to a
sections
thickness
of
were
-120
13
microns.
After polishing,
the sections were dimpled on one
side using a Gatan model 656 dimple grinder until the center
thickness
of
thickness was
the
specimen
was
-25
microns.
determined by back lighting the
observing a dull red appearance.
The
proper
specimen and
The sample was then placed
in an Gatan dual ion mill, model 600, equipped with a liquid
nitrogen cold stage.
The sample was milled from both sides
at 5 kv and 1 mA at an angle of 14 degrees until perforation
was
achieved.
100°C
during
completed,
The
sample
temperature was
the preparation.
Once
maintained below
the preparation was
the sample was screened in a JEM 200 CX at 200 kV
to ensure that the ML structure was visible.
Subsequently,
the specimen was viewed in a JEM 4000EX transmission electron
microscope operated at 400 kv
Electron
Microscopy
microscope
has
a
at
located at the Facility for
Arizona
point
to
State
point
University.
resolution
of
This
1.67
A.
Selected area electron diffraction (SAED) and high resolution
images were obtained for analysis.
HREM analysis
of the individual
was
performed
layers.
to
determine
the
The micrographs were
thickness
digitally
scanned at 300 dots per inch (dpi) using an area about 1.5 by
2.5 inches.
Three to six scans from different regions were
made for each sample.
gray
scale
image.
This converted the picture to a 256
Thickness
Image version 1.04 by National
Services Branch (NIMH).
substrate
[111]
measurements were made
Institute of
using
Health Research
The scale was set using the silicon
direction as
a reference.
Density slices
14
were made in which a range of gray scale values are assigned
a single
color as an aid to measurement.
establish boundaries between each layer.
measurements were made for the Mo,
layers,
thus
providing
15
to
This helped to
For each scan, five
Mo-on-Si,
Si,
and Si-on-Mo
3 0 measurements.
These were
averaged and the standard deviation was determined.
Normal Incidence Reflectance
The
effects
reflectance
of
properties
(NIR)
annealing
were
the
determined
radiation.
These
beamline
the VUV Radiometric
of
on
measurements
ML
soft
using
synchrotron
were performed
Laboratory
Electron Storage ring Synchrotron facility
X-ray
at
using
the
the
Berlin
(BESSY).
The X-
ray wavelength was varied from 110 to 170 A by rotating a
toroidal grating with a resolution of 0.5 A.
Higher order
contributions were
reduced by
using a 1 micron Be
filter.
The
the
was
microns
beam
size
at
sample
reduced
through the use of a circular aperture.
to
200
A 12 x 12 mm sample
was placed in a standard mount which consisted of a circular
aluminum disk with a recessed pocket for each sample, and a
stainless steel
cover to hold the samples in place.
mount held two samples and was attached to
vacuum chamber.
translating the
Each
the holder in the
The holder was capable of rotating and
samples as shown in Figure 4.
The vacuum
chamber was pumped using a turbomolecular and ion pump to the
mid 10-7
Torr range.
from normal.
X-ray reflectance was measured at
0.8°
A channel electron multiplier detector was used
15
and operated in the linear range of the photon counting mode
[9].
A reference scan was first made with the sample out of
the beam.
The
sample was
then placed in the
beam and
the
detector moved such that similar geometries were maintained.
Two or three points were measured on each sample to check for
spatial
variations.
The
reflectivity
was
dividing the sample scan by the reference scan.
obtained
by
16
Side View
Spinner notor assembly
<^J>
• Vacuun chanber
Substrate
platter
Plasna
Substrate
Target
Shielding
n
Magnetron
source
\,
J
Top View
Lanps
Viewport
II
°
\\
Throttle valve
Cryo
Figure 2: Side and top schematic of ML depostion system.
17
Table 2: Listing of the average pre-annealed lambda
gamma (y)
for the seven ML samples produced.
Sample
Lambda
(A)
Gamma
0529A
69.7
0.43
0530A
69.6
0604A
70.0
0.43
0.47
0605A
69.8
69.4
0.47
1014A
69.5
0.45
1015A
69.0
0.46
1011A
0.48
(A) and
18
6
1
2
7
8
3
4
5
9
10
Figure 3:
Cleavage pattarn of ML coated wafer,
19
Side
View
Manipulator
Aperture
stop
r
Sample
n holder
\f=D Detector
Vacuun
chamber
Top View
X-rays
Figure 4: Side and top schematic of the BESSY reflectometer.
20
RESULTS
Unannealed Samples
The
six unannealed
specimens
were
removed
capsules and SAXS was performed on each one.
reported in table 3.
from their
The results are
The period of the ML remained the same
indicating that the ML were unaffected by the encapsulation
process.
errors
Slight
in
position.
initial
sample
in the period can be attributed to
alignment
or
in
determining
the
peak
HREM was performed on the samples to determine the
structure
thicknesses.
Table
shifts
3.
the
ML
and
to
measure
the
layer
The measurements for each sample are listed in
Sample
unannealed
of
0604A9,
specimens
is
which
is
illustrated
representative
in Figure
5.
of
the
It
was
found that the ML consisted of an amorphous layers of silicon
and
crystalline
respectively.
layers
These
of
Mo,
layers
of
were
amorphous region of mixed Mo and Si.
forms
a
thicker
structure of
heavier Mo
greater
deposited Mo
occurring
region
the amorphous
atoms
depth
mixed
Si
and
separated
deposition
deposition
relatively flat and abrupt.
by
a
perhaps
adatoms
[10,
of
the
A,
thin
The Mo on deposited Si
due
to
11] .
to
the
ML.
intermix
The
forms a thin interlayer due to
during
-23.3
and greater momentum of
allowing the Mo
during
-30.5
Si
on
open
the
to a
the
little mixing
The
layers
are
LAXS performed on these samples
show a peak at 40.0° corresponding to the (110) Mo peak.
The
HREM pictures show random crystallites in the Mo layer only.
In the scanned images,
individual atomic planes are easily
21
identified.
annealed
This
is
samples
the
starting
and
is
condition
consistent
characterizations of the microstructure [2,
Annealing
remaining
temperatures
(260,
42
samples
298,
316,
were
and 342°C)
samples were annealed at
hours,
all
with
3,
the
other
10, 12-14].
Experiments
The
Nine
of
260°C
annealed
at
for various
for times
up
four
times.
to
3000
eleven samples were annealed at 2 98°C up to 500 hours,
thirteen samples at 316°C up to 100 hours, and 9 samples at
342°C for times up to 20 hours.
caused a
higher
contraction of
density
the ML period due to
silicides
discussed subsequently.
Annealing of these samples
at
the
interfaces
formation of
which will
be
HREM measurements show the growth of
each interlayer and the reduction in thickness of the pure
materials.
Continued
annealing
of
the
samples
causes
increased interlayer growth and further reduction in the ML
period.
mixture
The
but
interlayer
with
is
initially
an
increased
annealing
times,
becomes crystalline.
this
Mo-Si
region
The thickness is shown as a function of
the annealing time in Figure 5.
at 316°C for 1,
amorphous
These samples were annealed
40, and 90 hours.
The Mo-on-Si interlayer
grew faster than the Si-on-Mo interlayer during annealing.
With sufficient annealing time the ML structure consists of
thin layers of crystalline Mo separated by thick layers of hMoSi2 •
The Si
layer has
interlayer growth.
been completely consumed by
the
Analysis of the LAXS results confirm the
increasing crystallinity of the interlayer region.
A single
peak is observed which shifts with increasing annealing time.
This single peak is composed of a (110) Mo peak at 40.0° and
a (111) h-MoSi2 peak at 41.6°.
decrease of
the
(110)
The shifting is due to the
bcc-Mo peak and the increase of
(111) h-MoSi2 peak with increasing annealing time.
comparison
of
amounts
crystalline regions
of
analysis
results
was
are
the
HREM pictures
performed on
presented
in
layer has been measured,
deviation.
the
Table
confirm the
in the
Visual
increasing
interlayers.
HREM
annealed samples,
and
4.
of
The
along with the
the
thickness
the
each
associated standard
As can be seen, each interlayer grows with time,
eventually consuming the pure Si layers and leaving a thin Mo
layer.
Similar
temperatures
are
interlayer
shown
in
growth
Figure
6.
trends
at
These
different
samples
were
annealed at 260°C for 160 hours,
298°C for 60 hours,
for 1 hour,
It can be seen that the ML
and 342°C for 1 hour.
316°C
structure is similar for specimens having different annealing
temperatures and times.
This suggests that the mechanisms at
work are similar at all the temperatures studied.
Diffusion
The Mo-Si
interlayer growth rate was
plotted as
difference of the square of the interlayer width
w2(0)]
[3] versus time t using the HREM results.
the
[w2(t)
-
The w2 (t)
and the w2(0) refer to the square of the interlayer thickness
at time t and t=0 (unannealed), respectively.
Figure 7 shows
2 5
this
for
(a)
260°C,
'• c)
(b) 298°C,
316°C,
both the thin and thick interlayers.
and
(d)
342°C for
At the temperatures
studied, the thick interlayer had a greater growth rate than
This disparity was also observed in the
the thin interlayer.
study by Holloway et al
In Figure 7,
two
[2].
This will be discussed later.
distinct
regions
can be
seen.
The
initial growth of -3A is observable at all temperatures and
for both the thin and thick interlayers.
growth may
be
different
from the
This rapid primary
secondary growth which
suggests that different kinetics may be relevant.
In
describing the second stage kinetics
interlayers
diffusion
it was
assumed that
limited
with
of
the Mo-Si
the interlayer growth is
constant
concentrations
at
the
juncture of the Mo-Si interlayer and the elemental layers.
Also, the concentration gradient through the interlayer was
assumed to be linear.
annealing
at
time
coefficient D(t')
The Mo-Si interlayer width w(t)
t can be
which
AC
interlayer.
is
to
the
interdif fusion
by the following:
w2(t) = w2(0) + 2 AC
in
related
after
the
change
D(t') dt \
in
(1)
concentration
across
the
Integrating this, assuming D(t') to be constant
(confirmed by the linear plots of [w2(t)
- w2(0)] versus t,
seen in figure 5), the relationship:
D = [w2(t)
is
apparent.
The
- w2(0) ] / 2t ,
(2)
interdiffusion coefficient
described as an Arrhenius equation :
D
= D0 exp
[-Ea/kT],
(3)
can
also be
24
where D0 is the temperature independent diffusion constant
and Ea
is
the
activation
Figure
8,
log D as
second
stage
determined
growth
interlayers
and
energy
rates
is
compared
for
interdiffusion.
from equation
plotted versus
to
the
2 using
1/T
results
for
of
In
the
both
other
investigators.
Linear regression was performed on the data to determine
the slope (Ea) and the intercept
(D0).
The thick interlayer
(Mo-on-Si) diffusion coefficients are -200 times greater than
those
for
energy
the
for
thin
both,
interlayer
however,
temperature range studied.
(Si-on-Mo) .
is
nearly
The activation
identical
over
the
The activation energy was found
to be Ea~2.5eV, and the interdiffusion coefficient was Do~100
cm^/s for the second stage growth of the thick interlayer.
These values are comparable to bulk diffusion values of
h-MoSi2 in Si[4-7] and Mo/Si multilayer interdiffusion [2,3].
The interdiffusion coefficient for the thin interlayer was
found to be Do-0.05 cm2/s.
could
be
attributed
to
a
The difference between the two
chemical
diffusion
could form during the deposition process.
barrier
that
Such a barrier
would have to form as the ML sample passed from one target to
the other.
most
Partial oxidation of the Mo surface would be the
likely
candidate
for
a
chemical
diffusion
barrier.
Recent work at ASU, however, has not detected any impurities
using
energy
dispersive
x-ray
spectroscopy
and
electron
energy loss spectroscopy using a VG-501 STEM operating at 100
kV with an estimated probe size of
10 A and and a minimum
25
resolution of 5 atomic percent oxygen.
A similar plot for the primary surge region can be seen
in Figure 9.
The disparity in growth rates is not seen in
the surge region.
Ea~1.6 eV.
stress
The activation energy for this region is
A possible mechanism for this region could be
relaxation
Normal Incidence Reflectivity (NIR)
Normal incidence X-ray reflectance was performed on the
relevant annealed samples,
was not too great.
i.e. those with a contraction that
More than a few A of contraction shifts
the bandwidth of the mirror, making it useless.
Also,
the
interlayer growth that occurs with this contraction quickly
degrades the reflectivity.
samples measured,
Table 5 has a listing of the
the peak reflectivity,
and peak location.
This data was compared to the theoretical reflectivity using
a matrix method modified to incorporate non-ideal interfaces
[15] .
Theoretical
obtained
using
stoichiometry,
roughness).
fits
of
interlayer
and
by
experimental
NIR
regions
uniform
assuming
of
abrupt
data
comparison of
used
(no
Best fitting of the unannealed data was obtained
the experimental
data for the specimens at
also
MoSi2
interfaces
using optical constants determined by Windt [16].
shows a
were
a
linear
(a)
to
316°C and (b)
concentration
Figure 10
the theoretical
342°C.
gradient
The model
through
the
interface.
This close fit of the model with the data tends
to
the
support
assumption
of
a
linear
gradient
made
in
equation 1.
The NIR of all the samples was found to decrease
with increased annealing time and temperature.
A correlation
between the two temperatures can be drawn because the samples
annealed at
different
times
and temperatures
had similar
decrease in reflectivity.
For example, the samples that had
similar interlayer growth,
like the 0.5h at 342°C and the 5h
at 316°C, had nearly the same decrease and shift in peak
reflectivity.
Stability
To
assess
the
stability
of
the
ML,
one
must
establish the operating temperature of the coatings.
expected to be near ambient temperature [17].
first
This is
From NIR, the
increase in interlayer thickness that degrades the ML is ~3A.
This would require using Ea from the primary surge region.
Using this value and a conservative temperature of 50°C the
time
for
the
calculated.
interface
to
grow
a
given
amount
can
be
The time for a ML to degrade was found to be
much beyond the expected operating life of one year.
Table 3: SAXS and HREM results for the seven unannealed specimens.
SAXS (A)
HREM (A)
As deposited Post encapsu-
Thin
1
Standard
Thick
Standard
Standard
Mo
Standard
Deviation
_Lay_er_
23.16
OevjaLon
0.99 "~
22.59
G.75
Sample
Lambda
0529A4
69.6
69.6
0.0
5.13
0.44
10.11
0.51
30.25
0.99
0530A1
69.5
69.7
-0.2
4.69
0.56
9.65
0.63
31.63
0.64
70.1
70.1
0.0
4.93
0.80
9.97
0.77
30.88
1.31
22.92
69.7
\A2
69.6
0.1
5.85
0.46
10.67
0.80
28.61
1.16
23.28
i ia
0604A9
0605A1
ation Lambda
Difference
interlayer Deviation interlayer Deviation
layer
1011A2
69.6
69.6
0.0
4.56
0.52
9.31
0.89
31.53
1.21
24.13
o.8';
1014A1
69.5
69.5
0.0
4.93
0.49
10.45
0.89
30.21
1.44
23.41
69.2
1.00
69.1
0.1
4.90
0.56
9.58
0.85
30.92
0.99
23.54
0.95
1015A2
:o
W8&W&
'^^^^^T^^^^W^W^'':^-^^'" "
* >»v -
* ^ >•
, >- ;:
/
,
,
> >
fsftG&lKS*,
PSs?»*>
•' ;'; •'^.x>•; s- >••»•>,
Figure 5: HREM micrographs of a) unannealed ML sample compared to those annealed at 316°C
for b) 1 hour,
c) 40 hours,
d) 80 hours.
CO
00
Table 4: HREM results for the annealed ML samples at 260 and 298°C.
Time
Thin
Standard
Sample
interlayer
Deviation
1014A10
5.26
0.55
10.88
0605A2
6.36
0.50
Thick
Standard
Si
Standard
Mo
Standard
Layer
Deviation
Layer
Deviation
0.65
29.30
0.94
22.28
0.59
11.57
0.95
26.98
0.99
22.10
0.90
22.06
1.03
interlayer Deviation
260°C
1h
10h
40h
0605A4
7.09
0.44
12.09
0.93
25.75
1.14
80h
0605A5
7.67
0.58
14.01
0.78
25.09
0.70
20.41
1.34
160h
0605A6
7.68
0.60
14.25
0.44
24.66
0.58
20.42
0.71
500h
1015A6
7.76
0.48
14.42
0.76
22.61
0.90
20.38
0.96
1000h
1015A7
7.80
0.55
14.59
1.37
22.65
0.99
19.80
1.31
2000h
1015A8
7.88
0.50
15.15
0.50
22.47
0.42
19.19
0.44
3000h
1015A9
8.06
0.75
15.78
0.92
21.62
1.31
18.62
1.07
1h
1014A8
5.18
0.55
10.90
0.87
28.82
0.87
22.33
1.24
5h
0529A2
7.20
0.59
12.83
0.59
26.12
0.69
20.66
0.87
298°C
25 h
1014A6
7.71
0.45
13.92
0.42
23.73
0.57
20.55
0.57
60h
0529A3
8.66
0.73
14.20
0.54
22.26
0.87
20.33
1.08
160h
0529A6
9.39
0.76
15.23
0.73
20.07
1.10
20.19
0.93
225h
1014A7
9.41
0.74
18.10
1.33
18.91
1.36
17.67
1.15
312h
1014A3
9.36
0.58
18.97
0.68
18.20
0.95
16.92
0.87
384h
1014A2
9.79
0.74
21.83
2.39
15.44
1.81
15.12
1.21
450h
1014A4
10.38
0.63
24.80
1.70
11.46
2.07
13.36
1.25
500h
1014A5
10.41
0.52
24.21
1.30
10.78
1.09
12.80
1.06
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Figure 6: HREM micrographs of annealed MLs with similar interlayer growth annealed at dif
ferent times and temperatures a) 160h @ 260, b) 60h @ 298, c) lh @ 316, and d) lh @ 342°C.
200
a)
T = 260 °C
o
Thick (Mo-on-SI)
•
Thin (Si-on-Mo)
100
CM
1000
2000
3000
t(hours)
Figure 7a: Interlayer growth rates of Mo/Si MLs annealed at
260
°C.
,
550
,....T—,
r__,
j
,_
i
'
r "• '•>
-r~>—r—
o
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450
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b) T = 298 °C
500
. , ...^—,
O
Thick (Mo-on-Si)
A
Thin (Si-on-Mo)
-
400
«
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350
tM
°<
300
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250
200
CM
150
100
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A
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0
50
I
i
I
i
I
i
•
i
100 150 200 250 300 350 400 450 500 550
t(hours)
Figure 7b:
2 98
°C.
Interlayer growth rates of Mo/Si MLs annealed at
3 4
-i—i—•—i—i—i—i—i—•—|—i—|—i—|—i—|—i—f—,—p
500
c)T = 316°C
400
CM
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Thick (Mo-on-SI)
A
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300
CM
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200
100
b°°
A
A
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A
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0
A
A
20
30
40
50
60
i i • i • i • I
70
80
90
100 110
t (hours)
Figure 7c: Interlayer growth rates of Mo/Si MLs annealed at
316
°C.
1—r-T
....
550
500
450
-
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r
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1
T_.
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350
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8
10
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i
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14
•
16
t(hours)
Figure 7d:
342
°C.
Interlayer growth rates of Mo/Si MLs annealed at
to
CM
E
o
o
Figure 8: Arrhenius plot of log D versus 1/T for this study
compared with that of other investigations.
"<"7
•10
T
>~
2
q
Thick (Mo-on-Si)
a,
Thin (Mo-on-Si)
o
A
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2
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O
O
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A
Ea = 1.6eV
-14
-
•15
1.5
1.6
1.7
1.8
1.9
2.0
103/T[K-1]
Figure 9:
Arrhenius plot of log G (growth rate) versus 1/T
for the primary surge region.
Table 5: NIR data for unannealed ML samples and annealed ML
samples at 316 and 342°C including peak reflectivity, peak
position,
and full width half maximum.
Sample
Lambda
%R
Peak
FWHM
1011A2
69.6
60.0
135.5
5.4
5h @ 316°C
1011A3
66.5
52.9
130.0
4.5
50h @ 316°C
1011A7
64.2
41.0
126.3
3.6
Unannealed
1011A2
69.6
60.0
135.5
5.4
0.5h @ 342°C
1011A9
66.4
49.6
130.0
4.6
Figure
10a
Unannealed
Figure
10b
0.6
\
Data
5h
/
i
Model
/
i\
/
i\
/
0.5
-
Unann ealed
thin
thick
0.4
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//
//
t
50h
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1
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8.7
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thick 14.5
0.3
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5h
o
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4.6
9.3
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Unannealed
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1 \
1
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CD
thin
1
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1
1
1
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1 \
j
- a)T = 316°C
I
1 \
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0.1
0.0
-i
115
120
125
130
135
1
1
r
1
140
1
1
r
145
X-ray Wavelength (A)
Figure 10a: Measured NIR data from annealed ML specimens at 316 °C compared with the
"theoretical" reflectivity.
6
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CD
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41
CONCLUSION
Mo/Si
effective
multilayers
x-ray
(MLs!
reflectors
were
at
a
fabricated
wavelength
that
of
are
-135.4.
Experiments were performed to determine the thermal stability
of these MLs.
at
The annealing experiments on Mo/Si multilayers
temperatures of
distinct
260-342°C for 0.5-3000 hours indicate two
stages of thermally activated
The primary
stage occurs
quickly
interlayer growth.
to ~3A
thick,
while
the
secondary growth is slower and the diffusion coefficient D is
constant with time.
thick interlayer
The interdiffusion coefficients for the
(Mo-on-Si)
are 200 times greater
the thin interlayer (Si-on-Mo).
than for
The secondary stage of the
thick interlayer growth interdiffusion coefficient Do~100
cm2/s, and the activation energy Ea~2.5 eV are comparable to
bulk diffusion of Si in h-MoSi2.
The x-ray reflectance of
the multilayer decreased with increased annealing time and
temperature due to growth of the interlayer region.
42
FUTURK
WORK
The work that remains to be done is extended annealing
at
temperatures
energy
remains
below
2 60r"'C
determine
if
the
activation
Also,
further- investigation into
the primary stage growth remains.
The mechanisms involved in
both stages
constant.
to
have yet
to be
identified,
interlayer grows more than the other.
as well
as why one
43
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