Contribution to the ACTUATOR 2006 conference, Bremen, Germany, June 14 – 16, 2006:

RELIABILITY OF PIEZOELECTRIC MULTILAYER ACTUATORS
P. Pertsch, S. Richter, D. Kopsch, N. Krämer, J. Pogodzik, E. Hennig
PI Ceramic GmbH, Lindenstrasse, 07589 Lederhose, Germany
Abstract:
The reliability of piezoceramic multilayer actuators is an important issue for the extension of their use. Twocompletely different load cases have to be distinguished when reliability parameters of these actuators are specified. One is the AC-driving were the actuators have to generate extreme accelerations. The other one is the pureDC-load which is typical for applications which are taking advantage of the ultrahigh resolution of closed loop
piezoelectric positioning systems.
This paper explains DC- and AC-degradation mechanisms as well as the unique design of the PICMAâ co-fired
actuators which is properly adapted to both load cases. Besides some characteristics of the actuators results of
DC- and AC-reliability investigations are presented.
Keywords: actuator, PZT, piezoelectric, multilayer, co-fired, reliability, lifetime, humiditytime is usually described by a power law with an exponent of -2.5 to -3 [2,3]. An Arrhenius equation isoften applied for the lifetime-temperature-relation.
The observed activation energies are in the order of
1 eV [2]. However, the most severe condition is thehumidity which can easily lower the lifetime bysome orders of magnitude [4].
Increased humidity is not unusual for actuator applications. Clean rooms in the semiconductor industry,
one of the major application fields of nanopositioning equipment, have artificially humidified
air to avoid electrostatic discharging. Another sourceof high humidity are the cooling liquids next to actuator based precise machining equipment.
Looking at the conventional actuator designs (Fig. 1)
the high humidity–DC–signal combination is mostcritical for the open electrode configurations (Fig.
1a,b) were the internal electrodes are just separated
by the active layer thickness and protected by apolymer coating.
The reliability problem is illustrated in Fig. 2 for an
actuator with silver palladium internal electrodes.

Introduction
There are several reasons why piezoelectric multilayer actuators are attractive for a steadily growing
number of applications: low static power consumption, high stiffness, high induced forces, high component mechanical energy density, easy geometricaladaptation due to ceramic technology. But the mostprominent properties are: almost unlimited resolution and ultrahigh accelerations. Whereas the resolution parameter is mainly used in the market of
nanopositioning equipment the acceleration is utilized in applications like fuel injection systems.
Besides lowering the prices extended knowledge ofreliability parameters will promote the furtherspreading of the piezoactuator technology. There isa fundamental difference in the reliability issue between the “resolution”- and the “acceleration”applications: a positioning device has often to stay at
a defined position over long time. Hence a DC-typeof driving signal is typical for such a purpose. On
the contrary a switching AC-signal is characteristic
for fuel injection systems. The paper explains thedifferences between DC- and AC-reliability matters.
The PI Ceramic co-fired multilayer actuators(PICMAâ) were initially developed for positioning
purposes [1]. Later on they were modified in a waythat they can take the very high switching currents
during high acceleration applications. In the second
part of this paper we will describe why thePICMAâ-design is well adapted for both types ofapplications and present some reliability results.

Top view:

polymer coating

Non-termination side view:DC-degradation mechanisms
The degradation of dielectric components under DCdriving conditions is related to the size of the electric field, the temperature and the humidity. For
multilayer actuators the field dependency of the life-termination side

active volume

non-termination side

inactive volume

glue layerd)a)b)c)Fig. 1: Conventional multilayer actuator designs, a)
co-fired all open electrode, b) co-fired open sideelectrode, c) co-fired buried electrode, d) glued chipburied electrode.

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Contribution to the ACTUATOR 2006 conference, Bremen, Germany, June 14 – 16, 2006:

termination layer

Because no polymer coating is able to prevent
moisture penetration [5,6], water is attracted by thefield and ionized by electrolysis. Ag+-ions move asAg(OH) from the anode trough cracks, voids, dissolved secondary phases at the grain boundaries and,especially, on the surface to the cathode [7,8]. There
it is reconverted into metallic silver. As a result silver dendrites grow from the cathode to the anode.
First they lower the resistance later they can cause abreakdown. This phenomenon can be easily studied
by putting a drop of water on an unprotected surfaceof an actuator at its nominal field. It takes just minutes to get a migration (Fig. 2b).
DC-humidity degradation is not just related toAg/Pd-internal electrodes but also actuators withplatinum electrodes tend to degrade at high fields[9]. Some authors just refer to dissolved “ions out ofthe ceramic” which cause the degradation [5,6].
AnodeOH -axial connection

compressive stressvolumetensilestressvolumeEEa)b)c)Fig. 3: Tensile stresses in the inactive actuator volume a) cracking in the termination insulation plane,b) cracking in the buried electrode plane c) actuatorlength dependence of the maximum tensile stress inthe inactive volume [11].

Ag(OH)

HumidityAgPICMA-design
Cathodea)

polymercoating

Fig. 4 shows the PICMAâ-design. On the nontermination surface it has a thin ceramic protection
layer [12] (Fig. 4 d). This inorganic coating can not
be penetrated by the humidity, pretty much as theburied design (Fig. 1 c). The layer is pressed on thesurface already in the green state. It is made of thesame PZT ceramic material as the active layers. Incontrast to the buried design where the insulationwidth has to be at least 200-300 µm for tolerance
reasons, it has uniformly the same thickness as theactive layers. Therefore it is partially accessed by
the scattering field of the internal electrodes (Fig.
4 d) and, consequently, it is partly poled and expands during operation. Hence, the stress as well as
the number of cracks are limited. Effects like significant stress accumulation in the non-termination
insulation volume in the buried electrode design(Fig. 1 c) or stress concentration at the corners in theglued chip design [9] (Fig. 1 d) are avoided.
The second special feature is the slot segmentationof the stack to prevent stress accumulation in thetermination insulation volume [12] (Fig. 4 a). In
contrast to solutions were semi-controlled cracks are
induced by poling these slots are already formed
during sintering. The slot distance of about 2 mm is
well adapted to the minimal stress optimum (Fig.
3c). The slot depth of 300-400 µm matches to thefinding that poling cracks stop growing after 200 µm[13]. Moreover, the slot-layer has double the thickness as the other layers but it is active (Fig. 4 c).
This prevents the slots from further growing.
The third feature is the special slot bypassing contact
stripe layout [14] (Fig. 4 e). Usually the internal
electrodes are contacted by a fired on Ag/Pd-b)

Fig. 2: DC-degradation process in an open silver –palladium electrode actuator a) humidity driven
degradation processes, b) water induced migration
on an actuator surface without polymer protection.
AC-degradation mechanisms
Besides material related AC-degradation phenomena[10] there are also design related issues. All actuator
designs have inactive insulation volumes which are
not field-accessed (Fig. 3). In these volumes tensilestresses are induced because the adjacent active regions expand along the actuator axis.
The higher the tensile stresses the higher is the probability to get uncontrolled cracks in axial direction
which can either cause a dielectric breakdown or justseparate an internal electrode from the termination.
It can be shown that the tensile stresses increase
when the actuator gets higher and the inactive volume gets longer (Fig. 3 c) as well as wider.
One possibility to limit this stress accumulation
could be the segmentation of the actuator by justgluing chips together (Fig. 1 d). Then the stresswould be released partially by the gluing layers. But
this solution has principal drawbacks. Chips as wellas actuators have no precise mechanical dimensions
when they are fired. So either all the single chips
have to be ground before gluing which is expensive
and needs thick passive layers in the design, i.e.
stress concentrations and loss of strain, or they areglued “as fired” which results in non-homogeneous
gluing and termination layers.
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Contribution to the ACTUATOR 2006 conference, Bremen, Germany, June 14 – 16, 2006:

where [9] failures occurred statistically distributedon the stack. We did not observe a local concentration at the corners which would indicate a systematicstress peak in the design.
The PICMAâ suitability for high acceleration applications was tested by cycling the 7x7x36 mm³power-type (Fig. 4 e) at 150 V, 50 µm displacement,150°C, 15 MPa pre-stress. Although the semirectangular rise times were as short as 80 µs and theresulting pulse currents were as high as 20 A
PICMAâ showed no degradation after 5⋅109 cycles.
Another indication of the PICMAâ-reliability is thesignificant reduction in the failure rate of installedacutators. More than 100.000 PICMAâ-actuators
were used in different piezo positioning applicationsin the last four years. The failure rate was reduced
by a factor of 10 to a very low value compared to theopen electrode design actuators.

termination layer. During operation this layer will be
destroyed by cracking after 106-107 cycles [6], atleast at the slots (Fig. 4 e). Therefore these cracks
have to be electrically bypassed. In the PICMAâdesign this is realized by a contact stripe which issoldered on by a controlled machine process. This
contact stripe is not only flexible in terms of acting
low forces to the solder layers by thin lamellas. It
can also be easily adapted to the customer needs.
Active volume
Ceramic insulation
layer
Slots (preventingstress accumulation)
Contact stripe

Slot

Ceramic
insulation
layer

100 µmEc)

Relative frequency sum [%]b)a)Termination layerEd)

test conditions:
100 VDC, 92% RH, 23°C, no pre-stress

9990
50
101,00,180 samples PICMA 5x5x18mm³18 open-electrode extrapolated characteristic lifetime after 28500 hrs. test andsamples, polymer
insulated, character- 11 failed samples: 700,000 hrs.
istic lifetime: 805 hrs.

100Slot
crack

1,000

10,000Time [h]100,000 1,000,000

Fig. 5: DC-lifetime-test of PICMA actuators compared to conventional actuators with an open electrode design.
Conclusions
The design of the co-fired PICMA actuators avoids
the major reliability problems of stacked piezoceramic multilayer actuators for DC-drive under
sever environmental conditions as well as dynamic
pulse type AC-signals. This is ensured by three patented unique design features: 1) ceramic insulation
layer protection, 2) slot segmentation and 3) slot bypassing contact stripe layout.

e)Fig. 4: PICMAâ-design a) non-termination sideview, b) termination side view, c) slot in the nontermination plane, d) insulation layer, e) PowerPICMAâ with termination layer detail (electricalbypassing of the slot crack by the contact stripe).

References

Reliability results
The testing of component reliability is always accompanied by problems like testing for an abnormally long time or an unacceptable large sample
number or finding accelerated test conditions which
include the uncertainty of extrapolation to normal
operating conditions afterwards.
Fig. 5 shows results of a long term DC-test were thePICMAâ-actuators were compared to open electrodedesign actuators under severe conditions. Whereas
all the comparative samples failed after 1,500 hrs.
still 69 of the 80 PICMAâ survived the 92 %relative humidity test after 28,500 hrs. (3.25 years!).
The current extrapolated characteristic lifetime is
700,000 hrs. In contrast to results reported else-[1] Heinzmann, A. et al., ACTUATOR 2002, pp. 403-406.
[2] Nagata, K.; Kinoshita, S., Ferroelectr. 195 (1997), 163-166.
[3] Koh, J.-H.; Jeong, S.-J.; Ha, M.-S.; Song, J.-S., Jap. J. ofAppl. Phys. 43 (2004) 9A, pp. 6212-6216.
[4] Nagata, K.; Thongrueng, J., J. Korean Phys. Soc., 32 (1998),
pp. S1278-S1281.
[5] Yoshikawa, S.; Farrell, M., SPIE Vol. 3985 (2000) 652-659.
[6] Bindig, R.; Helke, G., ACTUATOR 2000, pp. 53-57.
[7] Kanai, H. et al., Ceram. Trans. Ser., 88 (1998), pp. 295-302.
[8] Ling, H.C.; Jackson, A.M., IEEE Trans. Comp., Hybr.,
Manuf. Technol., 1 (1989) 12, pp. 130-137.
[9] Andersen, B. et al., ACTUATOR 2004, pp. 64-67.
[10]Carl, K.; Härdtl, K. H., Ferroelectr., 17 (1978), pp. 473-486.
[11]Lubitz, K. et al. in Setter, N. (Ed.): Piezoelectric materials indevices. EPFL (2002) pp.183-194.
[12]Patent DE 102 34 787 C1.
[13]Weitzing, H., PhD-thesis, TU Hamburg-Harburg (2000).
[14]Patent DE 10 2005 015 405.

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