strengthening and thermal stability of nanoscale metallic
TRANSCRIPT
Strengthening and Thermal Stability of Nanoscale Metallic Multilayers
for Electrodes in Piezoelectric SystemsRachel Schoeppner, David F. Bahr
School of Mechanical and Materials Engineering
Washington State University, Pullman, WA 99164
Technique
SputteringThe Platinum Molybdenum metallic multilayers were deposited using Physical VaporDeposition (PVD) by way of sputtering. This technique allows the layers to be depositeddown to the order of a few nanometers. Sputtering is a process where target materials arebombarded with high-energy particles, in this case ionized Argon gas. The atoms areejected from the target and condense on the substrate to create a thin film.
AnnealingAfter the multilayers were sputtered, they were annealed at 650 C in a vertical furnace for
10 minutes. Annealing helps to relieve stress that develops in the layers from sputtering
and allows the grains on the surface of the Platinum to grow. This is an vital step before the
deposition of the PZT to insure good adhesion and limit the possibility of cracking. In some
cases, oxides formed on the surface. In order to inhibit the growth of oxides, the wafers
were then annealed in an Argon atmosphere. Annealing in an Argon atmosphere proved
successful when the wafer was put into a cold furnace, brought up to temperature , held at
650 C for 10 minutes and then furnace cooled.
Spinning PZTPZT was then deposited onto the annealed wafers to see if the Platinum Molybdenum
multilayers would withstand the PZT processing. The PZT was sol-gel deposited onto the
wafer using a syringe equipped with a filter. The filter is used to avoid particles from getting
onto the wafer. These particles are defects and end up being a source of failure for the
PZT. After spinning one layer the PZT is slowly, to avoid thermal shock, lowered onto a hot
plate at 350 C. After three layers are deposited, the wafer is annealed with a rapid thermal
annealer (RTA) which brings the wafer up to 650 C for 30 seconds. The RTA step is done
every three layers to relieve some internal stress in the PZT . If more than three layers are
spun before going through an RTA cycle to relieve the stress it creates too much stress
between the electrode and PZT layer.
Background
Research has been conducted on the strengthening properties of metallic multilayers with
thicknesses on the order of tens of nanometers. As the thickness of the layers decreases
below about 100nm the strength begins to increase rapidly, peaking at about 3-5nm, then
dropping off slightly as it goes below about 2nm.
Different dislocation slip mechanisms control each step. During the rapid increase in
hardness, down to 50nm thickness, the Hall-Petch (H-P) model (σ~ h^-1/2) accurately
describes the trend. In this regime dislocations pile up at the interface until the stress over
comes the barrier strength at which time the dislocation passes through the layers and the
electrode fails.
For this particular study Platinum and Molybdenum were used because of the possibility of
solid solution strengthening along with strengthening of multilayers.
Lead Zirconate Titanate
Lead Zirconate Titanate (PZT) is a ferroelectric ceramic that is often used in MEMS
devices. The thicker the PZT layer the better the properties. However, as the PZT gets
thicker, the internal stresses also increase. So in order to alleviate internal stresses and
stop cracks from developing in the PZT, it has to be annealed every few layers.
Micro Electo Mechanical Systems (MEMS) are an increasingly important part of
technology and are continuously being improved through research. Research conducted
by Dr. D. J. Morris showed that by bulging the membrane of a MEMS transducer, the
coupling coefficient improves significantly. Currently the device is constructed like the
picture below. Since having an SiO2 support layer takes energy to strain, if that support
layer is removed then there would be less energy required to bulge the membrane thereby
making the coupling coefficient even greater. However, if the SiO2 support layer is to be
removed the bottom electrode would need to be strengthened since the currently used
Platinum electrode is not strong enough.
Figure 5. 20 layer 20nm Pt / 40nm Mo
as deposited
Figure 8. Annealed 40nm/80nm Pt/Mo Figure 9. Annealed 20nm/40nm Pt/Mo
Figure 6. 10 layer 40nm Pt / 80nm Mo
as deposited
Hardness Data
PZT Spinning Results
Figure 3. Pt-Mo phase diagram
Conclusions
Summary• Successful in creating stronger Pt electrodes, however Mo recipe sputtered layers twice
as thick as predicted.
• Metallic multilayers when on the order of less than 50nm are about 2 times stronger than the standard bottom electrode currently used in MEMS devices.
• Discovered new recipe for Molybdenum sputtering for the WSU equipment that deposits predictable layer thicknesses.
• Annealing produced oxides on surface which made it impossible to successfully spin PZT.
Further research• Find correct thicknesses for electrodes that are capable of being annealed at 650C.
• Perform test on bulged membranes to see if they actually are more efficient.
• Test Ti/Pt multilayers instead of Mo/Pt multilayer to see if oxidation is still an issue.
ReferencesZhao M. H.; Fu R.; Lu D.; Zhang T. Y.; Acta Materialia 2002, 50, 4241-4254
Hoagland R.G.; Kurtz R.J.; Henager C.H. Jr.; Scripta Materialia 2004, 50, 775-779
Misra A.; Hirth J.P.; Hoagland R.G.; Acta Materialia 2005, 53, 4817-4824
AcknowledgementsSpecial thanks to my advisor David Bahr, along with Katerina Bellou, John Youngsman and Joshah Jennings for all of their help. Financial support for this work was provided by the National Science Foundation’s Research Experience for Undergraduates Site Program in the Division of Materials Research under grant number DMR 0755055, Characterization of Advanced Materials.
Scanning Electron Microscope
Motivation
The multilayers are clearly visible in the figures above. However after simple
measurements of Figure 6 it was discovered that the Molybdenum was sputtering twice as
fast as originally thought. This made the Molybdenum layer twice as thick than the
Platinum layer which could have assisted in the formation of the oxides shown below.
Different methods of annealing were tested to see which method produced the smoothest
surface.
It was also noted that once the multilayers
were annealed some of the layers seemed
to spherodize. The layers are no longer
uniform might contribute to the decreased
hardness values.
Table 1. Hardness values of various Mo/Pt multilayers
As deposited
H (GPa)
Annealed
H (GPa)
Platinum --- 2.3+/- 0.28
40nm/20nm Mo/Pt 5.36+/-0.31 4.23+/-0.7
80nm/40nm Mo/Pt 4.91+/- 0.27 4.15+/-0.5
35nm/100nm Mo/Pt 3.16+/-0.57 3+/-0.60
45nm/100nm Mo/Pt 3.9+/-0.34 3.22+/-0.5
100nm/100nm Mo/Pt 3.9+/-0.34 3.7+/-0.36
As can be seen from Table 1 there is a general
trend of increased hardness with decreasing layer
thickness. The exception being the 35nm/100nm
sample. Additionally, the annealed 40nm/20nm
and the 80nm/40nm samples are harder than the
thicker layers and twice as strong as the
annealed Platinum.
Figure 10. Argon annealed 40nm/40nm Pt/Mo
Figure 7. 10 layer 40nm Pt / 40nm Mo
as deposited
Figure 2. Flow strength of Cu–Nb multilayers,
estimated as nanoindentation measured hardness
divided by a factor of 2.7, as a function of h0.5 where
h is the individual layer thickness. *
Figure 1. Cross-section of free standing
MEMS transducer.
Figure 4. Schematic of Sputtering process (left) and actual sputtering chamber (right).
At layer thicknesses smaller than ~50nm the
H-P model breaks down since there are
fewer dislocations available in the layer to
pile up. At this point the Orowan model is
prominent (σ~ h^-1 ln(h)) and dislocations
slip by dislocation bowing.
The substrate on which the PZT is spun is of great importance. When the Mo/Pt layers were
initially annealed oxides formed which created a poor surface that the PZT had to grow on.
The rougher the surface, the less likely the PZT is to correctly adhere. As can be seen in
Figure 11, even when the layers were annealed in an oxygen free environment the surface
still showed deposits (middle and right) but not of the same nature as the oxides previously
observed (left). When the substrate is not uniform and smooth the PZT can end up peeling
up or cracking as in the pictures below.
Figure 12. PZT failures on different substrates. Cracks forming on top of a circular deposit (left) cracking of PZT (middle),
peeling of PZT due to inconsistent surface underneath.
Figure 11. Surface of annealed Mo/Pt layers during different conditions. 80nm Mo/ 40nm Pt in
oxygen environment (left), 40nm Mo/ 20nm Pt in Argon environment (middle) and 80nm
Mo/40nm Pt in Argon environment (right).
At about 3-5 nm the hardness peaks. The
peak can be many times harder than either
of the metals and is stronger than the rule of
mixtures predicts. Once the layer
thicknesses is smaller than 2 nm the
strength begins to decrease again.
Not only is there a possibility of solid solution
strengthening but they also do not have the
same crystal structure which results in a
incoherent interface. Platinum has a face
centered cubic (FCC) crystal structure where
as Molybdenum has a body centered cubic
(BCC) crystal structure. The incoherent
interface produces its own barrier to slip which
increases the strength of the multilayer as well.