Transcript AlN

Progress report on 'Smart' Die Coatings for Al
Pressure Die Casting
J.J Moore, M. Hasheminiasari, J. Lin
Advanced Coatings and Surface Engineering Laboratory,
Colorado School of Mines, Golden, Colorado
P.Ried
Ried and Associates, LLC, Portage, Michigan
Outline
 Research background.
 Progress in the AlN piezoelectric thin film
deposition using P-CFUBMS
 Progress in the AlN piezoelectric thin film
piezoelectric property measurement
Why a ‘smart’ die coating?
 Typical failure process: increased strain
followed by release of strain on
cracking
 Need for real time
detection/monitoring of strain in
coating system prior to catastrophic
failure
 Need to detect incipient failure at
elevated temp & harsh environment,
e.g., oxidation, corrosion, etc.
 In-service detection of incipient
failure, e.g., cracking of coating
before catastrophic failure of die
coating occurs
Optimized ‘smart’ tribological coating system
working layer
3
2
1
Optimized
Tribological coating
Compositionally
graded
Superlattice
‘Smart’ layer
Thin film electrodes
/adhesion layer
Piezoelectric layer
Die substrate
(Ferritic Nitrocarburized)
Graded CrAlN
CrN/AlN
Graded CrN
Graded CrN
Cr
Cr
Technical examples
of the optimized
tribological layers
How a ‘smart’ die coating works
Piezoelectricty: Electronic Response to Mechanical Stress or
Mechanical Response to Electrical stimulation
The voltage generate in piezoelectric materials is proportional to magnitude of applied
mechanical stress. The proportionality constant is given by the piezoelectric constant, ε 31 , as : V
=E.d = ε3,1.σ1,2 (E= electric field, d= film thickness, σ1,2= stress in x, y direction, 3 is in z direction)
F
-
+
++
+
The actual sensor element might be very small in size. For simplicity, we have assumed a continuous
thin film, but we can also use a more complex thin film sensor network using photolithography.
The selection of the piezoelectric materials
- High piezoelectric constant/response:
Materials with permanent polarisation (non-centro-symmetric crystal):
ferroelectrics (e.g. PbTiO3, Bi4Ti3012, LiNbO3),
& non-ferroelectric wurtzite nitrides (e.g. AlN, GaN)
P
Requires a c-axis oriented film

P
-High curie temperature
TC
Stable at high temperature
- Low dielectric constant to avoid electric
charge breakdown
T
Why ‘AlN’ candidate?
Properties
Current
response
(31,f) [C/m2]
Voltage response
(31,f /o33) [GV/m1]
Coupling
Coefficient
(Kp,f)2 on Si
Curie
Temperature
(Tc) [C]
CTE
() [10-6 K-1]
Pb(ZrxTi1-x)O3
[PZT]
-14.7
-1.2
0.2
~300
7.2
LiNbO3
-5.8
N/A
0.02
1210
11
AlN
-1.0
-10.3
0.11
~1100
4
ZnO
-0.7
-7.2
0.06
N/A
5
• AlN and LiNbO3 are more suitable than traditional PZT for ‘smart’ die coatings
because of their high electrically stable Curie temperatures;
• AlN exhibits low dielectric constant (dielectric constant K33 of 12) and high resistivity
(1016 cm) which lead to its high resistance to the electrical breakdown;
• AlN films exhibit good structural and bonding compatibility to the overall tribological
coating system which is based on nitrides.
Deposition systems (P-CFUBMS)
•
Deposition system:
 Pulsed closed field unbalanced magnetron sputtering (P-CFUBMS);
 The Cr and Al targets were used for sputtering Cr electrode/adhesion layer
and the AlN piezoelectric layer;
 The Al target was pulsed at 100-300 kHz;
 The Al target was powered at 1000 W; the working pressure is 3 mTorr ,
100% N2 flow.
How to obtain the maximum piezoelectric effects in AlN
thin films
Piezoelectric materials have two crystalline configurations:
Generate voltage or electricity under stressinduced structural distortion
Changing the shape of the material under
voltage or electricity
 AlN in the bulk form exhibits no piezoelectric activity. However, when properly oriented on
a compatible substrate, AIN thin films exhibit piezoelectric properties.
 Theoretically, a strong c-axis growth in the AlN, which is related to a (002) preferred
orientation, is recognized to exhibit potential good piezoelectric response.
Can be optimized by controlling the ion energy and ion flux in the plasma
dcMS: effect of TiN layer on ε3,1
Sample no. Architecture
7
AlN/TiN/Ti/Si
Thickness
Processing parameter
Piezoelectric
coeff. (C/m2)
900
DC power: on TiN/Ti/Si substrate
-0.90
5
AlN/Ti/Si
700 nm
DC power: on Ti/Si substrate
-0.74
6
AlN/Pt/Ti/Si
Not measured
DC power: on Pt/Ti/Si substrate
-0.49
TiN interlayer (diffusion barrier) produce best piezoelectric effect: probably due to its
best lattice matching with AlN, leading to reduced leakage current.
3500
3000
AlN/Ti/SS
AlN/SS
Ti/SS
AlN <101>
AlN/TiN/Ti/Si, dcMS unbiased substrate
AlN <002>
4000
Si
1000
SS
Ti <002>
TiNx ?
1500
SS
TiN
Ti
2000
TiNx ?
AlN
Intensity
2500
500
0
30
40
50
2
60
70
80
dcMS: effect of TiN & bias on ε3,1 when using Ti as
electrode/adhesion layer
Sample
no.
Architecture
1
AlN/TiN/Ti/Si
(TiN layer
prevents Ti
diffusion)
900 nm
2
AlN/TiN/Ti/Si
500 nm
3
4
AlN/Ti/Si
AlN/Ti/Si
Thickness
700 nm
470 nm
Processing parameter
Piezoelectric
coefficient (C/m2)
-0.90
-50 V bias (introduces inplane residual stress)/150 (Reported value:
W power
1.05 C/m2)
Same sample, after
annealing
No substrate bias (less
residual stress)
No substrate bias (less
residual stress)
Insertion of TiN layer enhanced piezoelectric constant
Effect of
increased ion
energy (bias)
Effect of
annealing
-0.87
-0.74
-0.14
Effect of
thickness:
(decrease in
conduction leads to
higher piezoelectric
signal in thick
samples)
I-V characteristics: thickness dependence, dcMS
0.01
+
+
+
+
+
+
+
+
+
+
+
2
Current density (A/cm )
Films thinner than 0.4μm show considerably
lower resistance.
The resistance of a film with 0.9μm is 105
times higher.
Implies that the charge leakage could be
surface roughness induced.
The other reason for the high leakage could
be related to porosity & defects in the film.
1E-3
1E-4
Thick film (~0.90 micron)
Thinner film (~0.35 mcron)
1E-5
1E-6
Bre
1E-7
1E-8
High field
region
- - - - - - - - ------cavities
Rough surface ++
+ + + + + +
+ + + +
++
+
High field
region
- - - - - - - - - - -
0.1
1
V (Volts)
10
o
akd
wn
Effects of the pulsing frequency on the preferred (002)
orientation of AlN/Cr films
Measured by XRD:
Rocking curve:
25000
20000
0.98
0.96
Intensity
(002) Orientation Percentage
1.00
0.94
15000
200 kHz
150 kHz
350 kHz
250 kHz
300 kHz
0 kHz
10000
0.92
3mTorr/floating bias
3mTorr/-50V
0.90
5000
0.88
0
0
50
100
150
200
250
300
350
400
6
8
10
12
14
16
18
20
22
Theta
Pulse Frequency [kHz]
Optimized pulsing regimes of 100-200kHz with 1μs reverse time were identified to
achieve high degree of (002) orientation in the AlN/Cr films.
24
AlN films: P-CFUBMS – effect of frequency
pulsed biased
substrate: 100 kHz,
2 S, - 50V
AlN
TiN
Ti
Si substrate
Sample
Architecture Thickness
no.
Processing parameter
Piezoelectric
coeff. (C/m2)
5
AlN/TiN/Ti/Si
800 nm
100 Hz/1microSec/1kW/closed
field/3mT/N2//0 bias/3hr
-0.69
6
AlN/TiN/Ti/Si
800 nm
200 kHz/1microSec/1kW/closed
field/3mT/N2/0 bias/3hr
-0.82
800 nm
300 kHz/1microSec/1kW/close
field/3mT/N2/0bias/3hr
-0.92
7
AlN/TiN/Ti/Si
Increased
ion energy
Why use pulsed magnetron sputtering (PMS)?
 Pulsing the Al target can effectively reduce the arcing problems generated during the AlN
film deposition.
 It was found that the pulsed plasma in the middle frequency range exhibits much higher ion
energy and ion flux than in the dc discharged plasma.
Technical example: CrAlN coating deposition.
Ion energy distribution
C
B
29
+
N2
Intensity [Counts/Sec]
350/1.4 (51%)
350/1.0 (65%)
350/0.4 (86%)
100/5.0 (50%)
100/2.5 (75%)
2.4x10
7
2.0x10
7
1.6x10
7
1.2x10
7
8.0x10
6
4.0x10
6
29
Ion Flux [Counts/Sec]
A
Ion flux change
36
N2
+
+
Ar
27
+
Al
52
+
Cr
0.0
100/1.0 (90%)
0
20
40
60
80
100
120
Ion Energy [eV]
140
100/1.0
100/2.5 100/5.0
350/0.4 350/1.0 350/1.4
160
Pulsing Parameters
Microstructure of AlN coatings deposited under
various pulsing frequencies
Working pressure=3 mTorr,
substrate bias=floating bias
0kHz-dc
Non-columnar
structure
150kHz
200kHz
250kHz
300kHz
Thermal Stability of AlN: P-CFUBMS – 100kHz, 1μs
reverse time, 5mTorr
0.40
0.25
0.4
0.4
DSC
First derivative
Heat flow [mW/mg]
0.3
0.3
0.20
0.30
0.15
o
0.2
1322 C
0.10
0.1
0.2
0.05
0.0
0.1
0.0
-0.1
0.00
-0.2
-0.05
1300
1310
1320
0.35
1330
1340
0.25
0.20
0.15
0.10
1350
0.05
-0.1
0.00
-0.2
exo
0
200
First derivative [mW/mg/min]
0.5
-0.05
400
600
800
1000
1200
1400
Temperature [ C]
o
DSC: no apparent phase change or reaction below about 1318°C.
Thermal Stability of AlN: P-CFUBMS – 100kHz,
1μs reverse time, 5mTorr
After DSC test
Intensity
AlN (002)
XRD analysis confirmed:
 No phase changes of AlN
coatings after 1400 oC DSC
test
 The diffraction peaks of Cr
adhesion layer and Si
substrate disappeared after
1400 oC DSC test due to the
formation of CrSi.
As-deposited
Si
Si
Cr
30
35
40
45
50
55
60
Diffraction angle [2-Theta]
65
Thermal Stability of AlN: P-CFUBMS – 100kHz,
1μs reverse time, 5mTorr
As-deposited
Top-view
After 1400 oC
Cross-sectional
SEM analysis confirmed:
 No obvious grain growth of
AlN coatings after the
annealing test at 1400 oC in
Ar.
 The columnar structure of
AlN coatings was retained
well after the annealing test at
1400 oC in Ar.
Piezoelectric characterization using Capacitance measurements
A-ΔA
A
No voltage applied
C0 
d+Δd
d
voltage applied
Cv 
A
)
A
Cv  C0
d
(1 
)
d
(1 
 0 A
d
 0 ( A  A)
d  d
Cr  1 


A

A
(
) 

C

x
Cv
d


r
, x  33

 , Cr 
C0
d31
d  d ( 1  Cr ) 

Cr  x 1 
Cr
Electrodes
(before voltage applied)
(after voltage applied)
d33= Piezo strain coeff.
AlN
Substrate
Impedance
Analyzer
Piezoelectric Measurements Results (5mtorr, 60%Nitrogen)
9.0E-07
7
10 V
8.0E-07
Charge Constant d33 (pm/v)
6
Capacitance (F)
7.0E-07
6.0E-07
5.0E-07
5V
4.0E-07
3.0E-07
0V
2.0E-07
5
4
3
2
1
1.0E-07
0.0E+00
1000
0
0.E+0
10000
100000
Frequency (Hz)
1000000
2.E+5
4.E+5
6.E+5
Frequency (Hz)
8.E+5
1.E+6
New Setup for Direct Piezoelectric
Measurements
Function
Generator
Actuato
r
AlN
Si Substrate
Cr
Electrodes
Lock-in
Amplifier
Results (Direct Piezoelectric Measurements)
80% Nitrogen (3.0 mtorr)
20% Nitrogen (5.0 mtorr)
3.5
12
CFAlN 6 (5mtorr, 20%Nitrogen)
CFAlN 4( 5mtorr, 80%Nitrogen)
3
10
R² = 0.9996
Piezo-response (mV)
Piezo-response (mV)
R² = 0.9994
2.5
2
1.5
1
8
6
4
2
0.5
0
0
0
5
10
15
20
25
Applied Force (N)
30
35
0
5
10
15
20
25
Applied Force (N)
30
35
CHALLENGES
• Measurement of high temperature thin film
piezoelectric properties
• Incorporating complex piezoelectric thin
film network into overall tribological
coating system using photolithography
• In-situ monitoring of piezoelectric response
under service conditions
Conclusions
• Films of single phase aluminum nitride with (002) orientation were
successfully deposited by DC and pulsed DC reactive magnetron
sputtering.
• Pulsed DC sputtered films yielded the phase formation at much lower
sputtering power and contained in-plane stress.
• There was a charge breakdown problem associated with the films,
which was minimized at higher thickness ≥ 900 nm.
• Optimum AlN/Cr film was identified by degree of (002) orientation
and rocking curve. At present, the optimum parameters are a pressure
of 5mTorr, and pulsing regime of 100-200kHz with 1μs reverse time
• High (002) orientation growth was obtained from P-CFUBMS system.
Proper Cr or TiN/Ti underlayer favored high (002) hexagonal AlN
orientation growth
Future Work
• Optimize P-CFUBMS deposition process correlated with plasma
properties of ion energy & ion flux using Hiden EQP plasma diagnostics
that achieve high temperature piezoelectric properties, minimum residual
stress
• Improve the piezoelectric measurement techniques
• Incorporate piezoelectric thin film sensors into one of ACSEL’s optimized
multi-functional tribological coatings and evaluate ‘smart’ capabilities.
• Prepare the Li-Nb composite target and reactively sputter this target to
produce LiNbO3 piezoelectric thin films & compare with AlN with respect
to deposition & piezoelectric property/performance.