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.