Broadway, D.

NASA Technical Reports Server

X-ray optical thin film deposition and analysis capability at 
NASA MSFC
D. Broadway
11/8/2017 D. Broadway NASA MSFC 1
https://ntrs.nasa.gov/search.jsp?R=20170011231 2019-08-31T01:21:26+00:00Z
Talk Outline
11/8/2017 D. Broadway NASA MSFC 2
• Optical thin film coating capability at MSFC
• Deposition system
• X-ray reflectometer (XRR)
• A method for the deposition of broadband coatings
• Thin film stress measurement:
• Ex-situ example
• In-situ
• Example: Stress behavior in polycrystalline materials during film growth
• Current optical methods of in-situ measurement
• Limitations, sensitivity
• New method of in-situ stress measurement using fiber optic 
displacement sensor
• Two embodiments: circular, cantilever-substrate
• Sensitivity
• Repeatability performance
• Device validation
• Effect of material interfaces on film stress: Ir/B4C, Ir/Si, Mo/Si, Mo/B4C,…
• Multilayers to compensate stress in x-ray optical coatings?
• W/Si example
X-ray optical thin-film coatings
• Single or multilayers thin-films
• Layer thicknesses
in the range of tens to hundreds of 
angstroms (Å)
• Multilayers contain hundreds of 
layer pairs
• Layer thickness can be numerically
designed to ellicit a specific spectral 
response
• Optical performance dominated by
interface roughness/diffusion
• Lateral and in-depth thickness 
gradients enables:
 collimating and focusing optics
 bandpass filters, notch filters, etc.
Roughness influence on first order reflectivity 
θ, deg.
1.70 1.75 1.80 1.85 1.90
R
,
%
0.0
0.2
0.4
0.6
0.8
σ = 3 Å
σ =3.3 Å
σ = 3.5 Å
σ = 3.7 Å
σ = 4.2 Å
W/Si, N=200, Γ=0.25, d=25Å
11/8/2017 D. Broadway NASA MSFC 4
2)cos()(22)cos()(2
2)cos()(22)cos()(2
010
010
θλθλ
θλθλ
−+−
−−−
=
+−−
+−−
−
jNjN
jNjN
jN
nn
nn
s
r
2)cos()(22)cos()(2
)(
)(
2)cos()(22)cos()(2 1)(
)(
001
1
00
1
θλθλ
λ
λ
θλθλ
λ
λ
−+−
−−−
=
−+−
+−
−
−+−
+−
−
−
















jNjN
jN
jN
jNjN
jN
jN
jN
nn
n
n
nn
n
n
p
r
)(
)cos(12 2
1
2
0
λ
θ
λ
piβ
+−
−
−
−=
jN
jN
jN
n
h
jN
jN
i
ejSjNr
i
ejSjNr
jS −
−
−
+
+
−
=
+ βνν
βνν
ν
2
1
2
1
*ννν SSR =
ps RRR
2
1
2
1
+=
X-ray reflectivity from the multilayer
2)sin(
2
228
0
0
θ
λ
σpi−
= err
Modified Fresnel coefficient:
d ≡ hA+hB
The X-ray Reflectometer System
• High flux rotating anode source 
• Cu target 8 keV X-rays  
• 5-35 kV, 10-150 mA
• Series of Tungsten slits to reduce
beam size on sample 
• High speed silicon drift detector 
• Amptek
• (Energy res. of 0.14 keV at 5.9 keV) 
• Precision sample alignment and 
positioning
• Custom control + real-time data 
collection software (LabVIEW)
• Vacuum chuck sample mount
• No monochromator due high detector resolution
11/8/2017 D. Broadway NASA MSFC 5
Recently developed XRR system at MSFC
• Interlaboratory study conducted to ensure quality 
of XRR measurements 
Gurgew, Broadway, Gubarev, Ramsey
Thin Film Stress
• The stress can be compressive or tensile.
• Various components of stress:
 Intrinsic, , which is related to the film’s microstructure. 
 Thermal stress, ∆ ,which arises due to the difference in the linear 
expansion coefficient between the film and substrate and the difference 
between substrate temperature, , during deposition and subsequent cooling 
to room temperature:
 Extrinsic, 	
, that results due to external forces applied to the film substrate 
system such as bending of the substrate to produce a figured optic.
• The film stress can be enormous for some materials (i.e. GPa’s)
• The curvature,, of the deformed substrate is proportional to the product of film 
stress and film thickness,, through a constant that describes the geometric and 
mechanical properties of the substrate (Stoney’s Equation)--namely, the 
substrate’s thickness, , and biaxial modulus, 


:
 


6 1  

∆      ∆
N/m
10/27/2014 D. Broadway NASA MSFC 6
• In controlling film stress, the aim is to manipulate the energy of the 
sputtered atoms and influence the adatom mobility at the film 
surface.
• For a given material, stress is highly process dependent for 
magnetron sputtering and influenced by deposition conditions:
 Gas Pressure (stress reversal)
 Deposition Rate (cathode power)
 Substrate Temperature
 Substrate bias
 There is a trade-off between film stress and film quality (i.e. 
roughness, density).
 Generally, the deposition conditions needed to achieve good X-
ray reflectivity result in high film stress.
Thin Film Stress (Cont’d))
10/27/2014 D. Broadway NASA MSFC 7

  ∆
Tension (+)
Substrate
adhesive adhesive
Concave substrate deformation
Adhesive force between “film” and substrate
  ∆
Compression (-)
Convex substrate deformation
Stress/Spring Model

The film will delaminate if the stress is greater than the adhesion between the film as substrate
10/27/2014 D. Broadway NASA MSFC 8
Detrimental effects of high film stress:
High compressive stress (buckling) High tensile stress (cracking)
• Substrate deformation
o Of particular concern for grazing incidence X-ray optics since the stress can alter the precise 
geometrical figure and degrade its focusing or collimating properties. 
 Significant technological challenge for the next generation of lightweight X-ray space 
telescopes like Lynx:
 The desire to achieve sub-second resolution has motivated deposition techniques
to correct substrate figure errors which rely on a very low stress film (ie. A few MPa) 
 Substrates are only 10’s of microns thick.
 The X-ray reflective Ir layer is highly stressed (~4 GPa)
• Cracking, buckling and delamination
If the force per unit length due to the stress in the film exceeds the adhesive force, delamination 
of the film will occur.
“telephone chord” propagation
10/27/2014 D. Broadway NASA MSFC 9
Ex-situ measurement of thin film stress
!"#$%&'	)$: 	 


6 1  

κ +
,-
,.
 /#$"
!0%12/3	4%5#163"2#$	#,%:
7  
4


8
0 deg 90 deg
κx=0.0212 and κy=0.0214 m-1
Tallysurf stylus profilometer
Si <111>
Current methods of optical in-situ thin film stress measurement:
10/27/2014 D. Broadway NASA MSFC 11
Multi-beam stress sensor (MOSS):
Micro cantilever:
Minimum detectable stress ∆σhf:
• Ranges from 0.5-50 MPa*nm depending on 
method and substrate ( i.e. geometry and mechanical 
properties)
• MOSS is 50 MPa*nm for 100 µm thick silicon substrate
Draw backs with current optical methods:
• Requires external optical access to the substrate
through angled viewports
• Limited to specific deposition geometries
• Complex
• Requires the use of opaque substrates such as
crystalline silicon.
• Film side is measured which can result in destructive 
interference effects when measuring transparent films.
These methods determine the substrate curvature by various 
optical means from which the integrated stress is calculated 
from the Stoney Eqn.:
 


6 1  9

New approach to in-situ stress measurement
11/8/2017 D. Broadway NASA MSFC 12
New approach to in-situ stress measurement:
• Utilizes a high resolution (i.e. 5nm) vacuum 
compatible fiber optic displacement sensor.
• Curvature determined from out-of-plane 
displacement measurement of the substrate.
• Uses double-side polished substrate.
• Same arrangement can be used for thermal annealing.
• Glass substrates can be utilized.
• Easily implemented into existing deposition systems.
• Very sensitive method.
 
4
3

1  9

4

<
Stoney Eqn. for cantilever:
Stoney Eqn. for circular substrate:
 

<
3 1  9 =

Ongoing work:
• Adapting to rotating substrates
• Adapting to curved (i.e. segmented) substrates
D.M. Broadway, U.S. Patent 9,601,391 (Granted March 2017).
D.M. Broadway, U.S. Patent Application 15/425,740 (Filed February 2017).
Pending publication in Review of Scientific Instruments
Method 1: Method 2:
Minimum detectable integrated stress, ∆ 
11/8/2017 D. Broadway NASA MSFC 13
δ, nm
-3 -2 -1 0 1 2 3
N
o
r
m
a
l
i
z
e
d
 
F
r
e
q
u
e
n
c
y
 
D
i
s
t
r
i
b
u
t
i
o
n
0.00
0.02
0.04
0.06
0.08
0.10
0.12
3σ = 2.15 nm
110 µm thick Si <111>
Ds= 25.4 mm 
δ, nm
-30 -20 -10 0 10 20 30
N
o
r
m
a
l
i
z
e
d
 
F
r
e
q
u
e
n
c
y
 
D
i
s
t
r
i
b
u
t
i
o
n
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
3σ = 30.3 nm
100 µm thick Schott D263 cantilever-substrate
L=37.5 mm
∆  
4
3

1  9

4

∆< ∆  

∆<
3 1  9 =

• The minimum detectable stress is limited by the combined ambient vibrational background of the substrate 
and electronic noise of the displacement sensor.
• The sensitivity further depends on the mechanical and geometric properties of the substrate.
• The cantilever approach is more sensitive to a given integrated stress but is also more sensitive to 
vibrational noise—compensating effect.
• The cantilever approach is advantageous because it is flexible in its orientation and easily adapted to 
various deposition geometries.
15 MPa*nm 9 MPa*nm
Sensitive enough to measure stress in x-ray multilayers
Stress evolution in polycrystalline films
11/8/2017 D. Broadway NASA MSFC 14
Depends on:
Substrate temperature
Argon pressure
Mass of sputtered atoms
Substrate bias
Surface energyNucleation & island growth
Island coalescence
Low surface roughnessSurface roughness 
increases with
film thickness
Volmer-Weber Growth Mode
11/8/2017 D. Broadway NASA MSFC 15
Argon pressure, mTorr
0 5 10 15 20 25
I
n
s
t
a
n
t
a
n
e
o
u
s
 
i
n
t
r
i
n
s
i
c
 
s
t
r
e
s
s
,
 
G
P
a
-4
-3
-2
-1
0
1
2
Cr
Ni
Ir
Scaling of the critical pressure with film density
ρfilm, g/cm
3
6 8 10 12 14 16 18 20 22 24
A
r
g
o
n
 
c
r
i
t
i
c
a
l
 
p
r
e
s
s
u
r
e
,
 
m
T
o
r
r
0
5
10
15
20
25
Cr
Ni
Ir
Results are consistent with D.W. Hoffman, Internal stress of 
sputtered Chromium, Thin Solid Films, 40 (1977) 355-363
, 
,
 

3> 1  

1

,<
,"
Stress reversal in polycrystalline films (steady state)
Instantaneous stress:
• Efficient for parametrizing the stress
• Independent of film thickness in the 
steady-state regime of film growth
• Substrate in thermal equilibrium
• State of stress ( i.e. tensile or compressive) at low 
pressure is strongly influenced by substrate 
temperature for low density metals like chromium.
• Therefore, the state of stress will depend on the heat 
transfer mechanisms of the substrate (i.e. how it is mounted)
for a given deposition system.
Ts~70oC
Sensitivity at the transition pressure (circular substrate)
Stress reversal in Cr with argon pressure
has been measured with the instrument.
Consistent with the previous work of 
Hoffman (i.e. stress reversal).
Measurement sensitivity is better 
than resolution in the control of 
Argon pressure.
1.62 mTorr 1.60 mTorr
Reduction of Ir film stress with argon pressure optimization
Ar pressure, mTorr
0 5 10 15 20 25
σ
i
,
 
G
P
a
-4
-3
-2
-1
0
1
2
hf = 30.0 nm
hf = 20.0 nm
hf = 10.0 nm
hf , nm
0 5 10 15 20 25
σ
i
h
f
 
,
 
N
/
m
-60
-40
-20
0
20
1.6 mTorr
5.0 mTorr
15.0 mTorr
19.0 mTorr
24.0 mTorr
Type I
Type II
Argon pressure, mTorr
0 5 10 15 20 25
σ
t
o
t
,
 
G
P
a
-4
-3
-2
-1
0
1
2
σ
r
m
s
,
 
Å
2
4
6
8
10
12
14
Total stress
Surface roughness
• Surface roughness is to high at
the argon critical pressure of ~22 mTorr
• We can achieve low stress and lower
argon pressure by exploiting the zero stress
that occurs shortly after island coalescence.
• The use of in-situ stress measurement
allows use to tune the stress to within a
few MPa 
hf, nm
0 2 4 6 8 10 12 14
σ
h
f
,
 
N
/
m
-10
-8
-6
-4
-2
0
2
4
6
8
cooling time, min.
0 2 4 6 8 10
σtot = -2.89 x 10-3 GPa
Reduction in the total 
stress by 3 orders
of magnitude (i.e. to -
2.89 MPa)
Good adhesion
Promising result: 5Å 
RMS roughness
Further reduction in the 
roughness is
possible through 
optimization of Ar 
pressure
Near-zero stress in Iridium (15.0 mTorr)
11/8/2017 D. Broadway NASA MSFC 19
Refined in-situ stress sensor
In-situ stress of single layer thin films
11/8/2017 D. Broadway NASA MSFC 20
hf, nm
0 3 6 9 12 15
σ
h
f
,
 
M
P
a
*
n
m
*
1
0
3
-14
-12
-10
-8
-6
-4
-2
0
2
4
t, sec.
0 30 60 90 120 150
W
hf, nm
0 3 6 9 12 15 18
-25
-20
-15
-10
-5
0
5
t, sec.
0 30 60 90 120 150
a-Si
X Data
0 2 4 6 8 10 12 14
Y
 
D
a
t
a
0
2
4
6
8
10
12
14
16
X Data
0 30 60 90 120 150
Cr
mass, mg
0 20 40 60 80
δ
m
,
µ
m
-200
-150
-100
-50
0
Calculated
Measured
Es=72.9 GPa
<? 
26A.
B
8
3=  .
Calibration masses placed on cantilever 
tip used to validate substrate modulus 
and linear range of the sensor:
Type IType II
Ts~27-30oC, 2.5 mTorr Ar
100µm thick Schott D263 
Device performance
11/8/2017 D. Broadway NASA MSFC 21
t, sec.
0 100 200 300 400
δ
,
 
µ
m
-40
-20
0
20
-199 MPa
3.38 nm
WSi
W Si
θ, deg.
0.0 0.5 1.0 1.5 2.0 2.5 3.0
R
,
 
%
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
Model
Measured
t, seconds
225 230 235 240
δ
,
 
µ
m
-25
-20
-15
-10
-5
Run #1
Run #2
Run #3
Average
∆δWi
∆δSii
∆ε
bilayer index, i
5 10 15 20
σ
h
f
,
 
M
P
a
*
n
m
*
1
0
3
-6
-4
-2
0
2
4
6
W
Si
±2.5 % run-to-run repeatability
±0.5% within run repeatability
Γ=0.38
d=3.38nm
σ=0.40nm
11/8/2017 D. Broadway NASA MSFC 22
t, sec.
0 20 40 60 80
δ
,
 
µ
m
0
5
10
15
20
25
Mo on D263
Mo on B4C
t, sec.
0 20 40 60 80
δ
,
 
m
m
-20
-15
-10
-5
0
B4C on Mo
Effect of material interfaces on the film stress (Mo-based)
t, sec.
0 20 40 60 80
δ
,
 
µ
m
0
5
10
15
20
25
Mo on D263
Mo on Si
t, sec.
0 10 20 30 40 50
δ
,
 
µ
m
-20
-15
-10
-5
0
Si on Mo
Effect of material interfaces on the film stress (Ir-based)
11/8/2017 D. Broadway NASA MSFC 23
t, sec.
0 10 20 30
δ
,
 
µ
m
-25
-20
-15
-10
-5
0
5
10
Ir on D263
Ir on Si
t, sec.
0 10 20 30 40 50
δ
,
 
µ
m
-25
-20
-15
-10
-5
0
Si on Ir
t, sec.
0 10 20 30
δ
,
 
µ
m
-25
-20
-15
-10
-5
0
5
10
Ir on D263
Ir on B4C
t, sec.
0 20 40 60 80
δ
,
 
µ
m
-14
-12
-10
-8
-6
-4
-2
0
B4C on Ir
11/8/2017 D. Broadway NASA MSFC 24
Effect of material interfaces on the film stress (W, Cr-based)
t, sec.
0 20 40 60 80 100
δ
,
 
µ
m
-10
0
10
20
30
40
50
W on D263
W on Si
t, sec.
0 10 20 30 40 50
δ
,
 
µ
m
-20
-15
-10
-5
0
Si on W
t, sec.
0 20 40 60 80
δ
,
 
µ
m
0
10
20
30
40
50
Cr on Si <100>
Cr on B4C
t, sec.
0 20 40 60 80
δ
,
 
µ
m
-14
-12
-10
-8
-6
-4
-2
0
B4C on Cr
Multilayers to compensate integrated stress
11/8/2017 D. Broadway NASA MSFC 25
• Currently single layer films (i.e. Cr) with tensile stress are used as one technique to compensate the 
integrated stress in x-ray optical coatings to near-zero:
• The columnar microstructure of metal films in tension results in increasing surface roughness as the film 
thickness increases—thereby limiting the method’s applicability.
• The increased surface roughness can severely degrade the optical coating’s performance; particularly for 
high energy broadband multilayers.
• Multilayers interrupt the columnar growth so roughness doesn’t increase with film thickness (for Glass & Si)
 C	
 DD  EE  + 0
DD G 0
EE H 0
 H 0
Stress compensating ML: Cr/B4C, Mo/B4C, …
Future work:
• Influence of substrate temperature and 
deposition rate.
• Impact to total deposition time.
• Optimization of layer thicknesses.
• Surface roughness characterization.
• Addition of N to B4C based ML’s to 
increase dep. rate and smooth 
interfaces.
Cr
Ir
B4C
Example:
t, min.
0 5 10 15 20 25
M
P
a
*
n
m
*
1
0
3
-20
0
20
40
60
In-situ stress in W/Si multilayers
11/8/2017 D. Broadway NASA MSFC 26
Ts~30oC
Schott D263
ξW~0.13 nm/sec
ξSi~0.18 nm/sec
W
Si
W
W
Si
Si
d ~ 6.8 nm
Γ ~ 0.38
W
W W
Si Si
Si
d ~ 3.4 nm
Γ ~ 0.38
Si
W
Si
W
Si
d ~ 7.3 nm
Γ~0.80
σ ~ 0.45 nm
6X104 MPa*nm
Conclusions
• We have introduced a novel method for the in-situ measurement of film stress using a 
fiber optic displacement sensor. 
• The device is less complex than other current optical methods and easily 
implemented into an existing deposition system.
• The device’s sensitivity is 0.009 N/m (9 MPa*nm) for a 100 µm thick glass substrate.
• This sensitivity is capable of detecting changes in stress due to small changes in 
deposition parameters such as argon process pressure (i.e. ±0.02 mTorr).
• The sensitivity can easily detect changes in the integrated stress in the individual 
layers of multilayer films of sub-nanometer thickness.
• The in-situ stress measured with the device is in good qualitative agreement with the 
known behavior of metals films (i.e. stress reversal, Volmer-Weber growth).
• We presented the influence of the material interfaces on the evolution of the film 
stress for several material pairs including: Mo/Si, Mo/B4C, W/Si, Ir/B4C,…
• We have proposed a new stress compensating method that utilizes multilayers
– This method might be applicable to balance the stress in broadband multilayer that are more 
than a micron in total thickness.
– More investigation is needed to study the impact to the total deposition time through 
optimization of the layer thicknesses
11/8/2017 D. Broadway NASA MSFC 27


X-Ray Optical Thin Film Deposition and Analysis Capability at NASA MSFC

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server