HIGH TEMPERATURE BEHAVIOR OF AL THIN FILM
C.-S. Oh1*, K.-S. Park2*, J.-S. Bae3*, H.-J. Lee4&, C.-S. Lee5# and S.-H. Choa6#
* School of Mechanical Engineering, Kumoh National Institute of Technology, GB 730-701, Korea
&
Center for Nanoscale Mechatronics, Korea Institute of Machinery and Materials, DJ 305-343, Korea
#
Samsung Advanced Institute of Technology, GG 449-712, Korea
1
[email protected], 2 [email protected], 3 [email protected],
4
[email protected], 5 [email protected], 6 [email protected]
ABSTRACT
Mechanical properties of thin films play a crucial role in the reliable design of small-scale systems used in MEMS/NEMS and
semiconductor industries. As the applications of thin film become more demanding, there is a need for accurate
characterization of thermal and mechanical properties of the materials that are used in the devices. The aluminium (Al) film is
widely used in many components because of its high electrical conductivity and low cost compared with some noble metals
like gold, silver or platinum. The principal goal of this investigation is to extract two important thermo-mechanical properties,
coefficient of thermal expansion (CTE) and creep behaviour, of a sputtered Al film by adopting a direct strain measurement
technique while heating in a furnace. The in-plane displacement resolution is about 5 nm at the best circumstances. The
o
resolution is dependent on the status of interference fringes. The measured CTE of Al is 26.5 ± 1.38 με/ C, which is a little
o
higher than that of bulk aluminum. It is inferred that the Young’s modulus up to 170 C remain unchanged from the fact that all
o
of the thermal strain curves are very straight. The creep deformation is observed after the temperature of about 180 C under
the stress of 74 MPa. The creep tests are currently being done.
Introduction
Elevated temperatures in the presence of applied stresses may create additional design problems because of thermal
expansion, thermal stresses, and thermal energy causing metallurgical changes to occur. An accurate characterization of
linear CTE of thin films is vital for predicting the thermal stresses, which often result in warpage and failure of a structure. One
material challenge in high temperature applications is the prevention of creep. Creep is a permanent deformation resulting
from an applied stress in a material over a long duration of time.
Several methods have been proposed to measure the CTE for thin film materials such as the X-ray diffraction [1], dual
substrates curvature [2], bilayer microcantilever [3] and laser interferometry [4] techniques. There are merits and demerits for
each method in the view points of heating and strain measurement methods. The direct strain measurement in direct furnace
heating environment will be ideal because it resembles to the standard test methods for bulk materials [5]. Oh and Sharpe [4]
measured the CTE of a polysilicon film by using Joule heating and an ISDG [6]. That was the first direct CTE measurement at
high temperatures. However, there is an inevitable temperature gradient in the test section of a specimen because the
specimen is heated resistively by an electric current passing through it. In addition, a pyrometer, noncontact temperature
sensor, can only measure the average temperature over some area instead of a point temperature.
The aluminum (Al) film is widely used in many components because of its high electrical conductivity and low cost compared
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with some noble metals like gold, silver or platinum. For aluminum, an increase of temperature from 24 C to 150 C is
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sufficient to cause creep over long periods of time. One major drawback to aluminum is their low melting points of 660 C and
corresponding rapid loss in mechanical properties. Elevated temperatures also provide sufficient energy to enable dislocations
to climb, and may activate different slip systems in some metals. These problems can preclude the use of aluminum for many
hot structure applications.
The useful tools in identifying creep mechanisms are microstructural evaluation and the use of deformation maps [7]. A
deformation map developed by Frost and Ashby [8] for pure aluminum is shown in Figure 1. Another technique which sheds
light on the active mechanisms is to determine the activation energy for creep and compare this value to known activation
energies for different mechanisms. In the steady-state regime of the creep relationship, the creep rate is given by a simple
power law:
ε&SS = A1σ e
n
where
ε&SS
⎛ Q app
− ⎜⎜
⎝ RT
⎞
⎟
⎟
⎠
(1)
= steady-state creep rate
A1
= constant for all stresses and temperatures
σ = applied stress
n
= creep stress exponent
Qapp = apparent activation energy for creep
R = universal gas constant
T = absolute creep test temperature
Figure 1. Creep deformation map for pure aluminum
Creep stress exponents and activation energies can be used to identify possible creep mechanisms. Creep strength is
important in considering the life of structures to be exposed to elevated temperatures and stresses for extended periods of
time. Typically, in locations where creep is a potential problem, strict restrictions are placed on the allowable stresses in order
to maintain the creep strain well below creep rupture.
The principal goal of this investigation is to extract two important high temperature properties, coefficient of thermal expansion
(CTE) and creep behavior, of a sputtered Al film by adopting a direct strain measurement technique while heating in a furnace
to induce uniform temperature.
Test Procedures
We have developed techniques and procedures to directly measure the thermal strain in Al tensile specimens at high
o
temperatures. This enables the measurement of both the CTE and the creep behavior at temperatures as high as 300 C.
The test procedures details including a novel specimen releasing technique and the entire test setup incorporating a
homemade miniature furnace are firstly introduced by Oh et al. [9, 10].
A specimen has the shape shown in Figure 2 and are heated directly in a homemade furnace. The specimens are fabricated
through a series of depositions, photolithographies and finally bulk etching from the back side of a Si wafer. The specimen is
530 nm thick and 5 mm wide at its center. There are two Au or Cr markers in the middle of a specimen for the interferometric
strain measurement. A flexible T-type thermocouple is placed very close to a specimen to measure the specimen temperature
accurately.
Figure 2. An aluminum specimen and its preparation procedure
The entire test setup is shown in Figure 3. The system is composed of a homemade furnace and its temperature controller, a
pair of photo diode arrays (PDAs) to detect fringe movements, a He-Ne laser as a light source, a LINE (Laser Interferometric
Nano Extensometer) controller to acquire fringe data, and the control software. The subassembly around a furnace is
enlarged and represented at the right side of Figure 3 to see those components more clearly. This system has three features.
First, the thermal strain can be measured directly, accurately, and precisely without contact. Second, the test procedures can
be applied in tensile test in addition to the current high temperature test. Third, the strain measurement technique is applicable
for not only thin films but also bulk materials without any modifications.
Figure 3. High temperature test setup
The specimen releasing process is schematically illustrated in Figure 4 for clarity. The point is to fix and secure a specimen to
the fixing jig via the safety jig with wax until releasing. If we raise the temperature of the furnace above 65 oC, the wax will be
melted and then the safety jig will be free fell from the fixing jig to let a specimen freestanding. The balancing dummy in the
figure is attached to a specimen to make the freestanding specimen straight and thus to get nice interference fringes.
Strain is directly measured by a LINE system with two markers deposited on the specimen. The more detailed descriptions
regarding the LINE, which is the digital version of an ISDG (Interferometric Strain/Displacement Gage), are given in [11]. The
thermal strain is recorded continuously while heating and cooling between RT and 150 oC for a CTE test and between RT and
200 oC for a creep test with the heating and cooling rate of 1 oC /min, which is proved slow enough to exclude thermal inertia
effect. The applied dead weight is 20 g, which corresponds to the stress of about 74 MPa.
Fixing Jig
(Fixed to
Furnace)
Bolt
Bolt
Fixing Jig
(Fixed to
Furnace)
Balancing
Dummy
LINE(Cr)
Markers
Safety
Jig
Wax
Safety
Jig
Yarn
W
W
Figure 4. Schematic illustration showing the specimen releasing process
Results and Discussion
The thermal strain is recorded upon heating from RT to 150 oC and cooling to RT and is shown in Figure 5. The CTE values
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are 28.0 and 26.3 με/ C upon heating and cooling, respectively. It can be seen in the figure that the two curves show linear
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behavior up to 150 C. This indicates that the elastic modulus is almost unchanged within this temperature range [4]. In
addition, the CTE upon heating is a little higher than that upon cooling.
4000
Thermal Strain, εT [ue]
3000
Al 6-11
W = 20 g
Vertical Setup
1 oC/min
G.L. = 250 μm
o
Cooling αm = 26.3 [με/ C]
2000
o
Heating αm = 28.0 [με/ C]
1000
0
20
40
60
80
100
120
140
160
o
Temperature, T [ C]
Figure 5. Thermal strain upon heating and cooling
Another thermal strain is obtained up to the temperature of 200 oC in lieu of 150 oC and is shown in Figure 6. There are two
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important matters to consider at this point. The thermal strain on heating becomes nonlinear above 160 C and increases very
rapidly. It is considered that this sudden strain increment can be resulted from the following reasons. The Young’s modulus
begins to decrease and the CTE increase around the temperature. In addition, there’s a clear evident that creep occurs
because the thermal strain increases upon initial cooling. The normalized shear stress, σS/μ = 37 MPa / 25 GPa, is about 1.5
-3
o
o
× 10 and the homologous temperature, T/Tm=200 C/660 C, is 0.3. It can be seen that the test conditions correspond to the
low temperature power law creep region from the Figure 1. The CTE values are calculated with the linear portion only and
evaluated as 27.9 and 24.9 με/oC on heating and cooling, respectively. The trend is the same as in the Figure 5. The reason
on this slight difference is not clear right now but we’re working on this.
8000
Thermal Strain, εT [ue]
6000
Al 6-11
W = 20 g
Vertical Setup
1 oC/min
G.L. = 250 μm
4000
o
Cooling αm = 24.9 [με/ C]
2000
o
Heating αm = 27.9 [με/ C]
0
50
100
150
200
o
Temperature, T [ C]
Figure 6. Thermal strain up to 200 oC
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The CTE value of 26.5 ± 1.38 με/ C is obtained through 5 successive measurements with the same specimen. There are two
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important matters to consider at this point. The CTE of Al film is 10 % higher than that of bulk material, which is 24 με/ C in the
temperature range of 20~100 oC [12]. It is assumed that this difference can be originated from the microstructural difference,
which is one of the most dominating parameters. A transmission electron micrograph for an as received Al specimen is shown
in Figure 8 for the reference. More microstructural studies before and after a high temperature test will be followed. In
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addition, all of the curves show linear behavior up to 170 C. This indicates that the elastic modulus is almost unchanged
within this temperature range [4].
5000
αm = 26.5, σα = 1.38 [με/ C]
Al 6-11
W = 20 g
Vertical Setup
o
1 C/min
G.L. = 250 μm
o
Thermal Strain, εT [ue]
4000
3000
2000
1000
α [με/oC]
r2
25.6
28.0
26.3
27.9
24.9
0.995
0.999
0.999
0.999
0.999
0
40
60
80
100
120
140
160
180
o
Temperature, T [ C]
Figure 7. Thermal strain curves for the CTE calculation
200
Figure 8. A TEM micrograph for the as received specimen
Conclusions
The CTE value for a sputtered Al film is evaluated as 26.5 ± 1.38 με/ C up to 170 C through the direct strain measurement
under direct heating in a furnace. There’s an evidence of creep at 200 oC under a rather modest applied stress of 74 MPa.
This study is a logical extension of previous work [4, 9, 10] and is an important contribution to material testing at the microscale. The creep testing is still underway to get the creep stress exponent and the creep activation energy.
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Acknowledgments
This research was supported by a grant (05-K1401-00910) from Center for Nanoscale Mechatronics & Manufacturing, one of
the 21st Century Frontier Research Programs, which are supported by Ministry of Science and Technology, Korea.
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