Online proceedings - EDA Publishing Association

In our work, we proposed an innovated dry process for
PZT-electrode fine pattern fabrication [12]. This process use
conventional ICP-RIE system and Ar/SF 6 mixed gas as the
etchant. AFM images of the PZT film before and after the
dry-etching revealed that the grain-boundary and the shape
of the PZT crystallites were more identifiable when using
lower Ar concentration in Ar/SF 6 mixed etchant. It suggested
that the etching is reactive-physical combined process. As
shown in Fig. 3, the highest PZT etching-rate (58 nm/min)
and the best etching-selectivity (PZT:Pt=1.14) was achieved
at 66.7% of Ar in Ar/SF 6 mixture. We also confirmed that the
remanent polarization and the dielectric constant of the PZT
film were 16.5 μC/cm 2 and 1019 before the etching, 15.5
μC/cm 2 and 1013 after the etching. It demonstrated that the
proposed dry-etching process did not degrade the properties
of the PZT film. As shown in insert of Fig. 3, a
PZT-electrode fine pattern with the feature size of 2 μm was
successfully obtained by this process.
2.3 Low temperature bonding of PZT film
For most of the MEMS and IC devices, process
temperature of more than 400 °C is fatal. However, to obtain
well-crystallized and (100)-oriented PZT film by sol-gel
method, high annealing temperature in the range of 600–750
°C is usually required. Even the transforming temperature of
PZT perovskite-phase (~530 °C) is beyond that of most
MEMS and ICs can withstand. Although adding modifiers
into PZT solution [13], or using seeding layers to enhance
PZT nucleation [14] has been reported effective to reduce the
PZT annealing temperature, their benefits to piezoelectric
MEMS fabrication are limited. Z.Wang et al. proposed the
bonding of bulk PZT with silicon wafer, and then thin down
the PZT to less than 10 μm by using chemical mechanical
polishing [15]. This method is complicated, time-consuming,
and high-cost. Therefore, it is hard to be used for MEMS
mass production.
Ti: 50 nm
Cr: 50 nm
PZT film: 1 μm
Pt/Ti: 200/50 nm
SiO 2 : 2 μm
Si: 500 μm
Si: 400 μm
4-inch
11-13
May 2011, Aix-en-Provence, France
Au film 100/300 nm
Au film: 300 nm
film low temperature integration on silicon substrate. We use
Au film as the intermediate layer. Fig. 4 shows schematic
view of the sample and its SAM image after bonded at 150
°C. Die-shear test revealed that the bonding strength is ~75
MPa at the bonding pressure of 325 MPa [16]. This
technique effectively avoids the damage of PZT high
temperature annealing to other MEMS components and ICs.
It also enables the fabrication of piezoelectric material,
piezoelectric MEMS components, and ICs separately by
different wafers, and then bonded them together for
integrated MEMS/ICs mass production.
2.4 Energy dissipation in piezoelectric MEMS device
Piezoelectric MEMS device offers great self-actuation
and self-sensing capabilities, but it always suffers from low
device performance in sensitivity. To investigate the energy
dissipation mechanism, various cantilevers with different
layers are designed and fabricated, which includes SiO 2
elastic layer, Pt/Ti bottom electrode layer, PZT film, Ti/Pt/Ti
upper electrode layer, and SiO 2 top electric passivation layer.
The measured quality-factors (Q-factor) of those cantilevers
were analyzed by theoretical calculation. Fig. 5 summarized
the difference between measured Q-factors and theoretical
calculated Q-factors. It clearly revealed that the energy
dissipation by the PZT film and the multi-layered structure is
extremely large. It is comparable to air dumping under
atmospheric pressure and becomes dominating under
reduced pressures [17].
Q-factor decreases to calculated value (%)
10
0
-10
-20
-30
-40
-50
-60
-70
SiO 2
SiO 2
+Ti/Pt
SiO 2
+Ti/Pt+PZT+Ti/Pt/Ti+SiO 2
SiO 2
+Ti/Pt+PZT
Length: 200 μm
Length: 250 μm
Length: 300 μm
1 2 3 4 5
Number of structure layer
Fig.5 Q-factor of the cantilever decreased with the increasing of structure
layers, especially after PZT film integration.
Fig. 4 PZT bonding on silicon substrate by surface activated bonding (SAB)
using Au film as the intermediate layer: schematic view of the sample (left)
and SAM images of the sample bonded at 150 °C (right).
To promote the PZT application in case >400 °C
temperature cannot been used, surface activated bonding
(SAB) was introduced in our work for the first time for PZT
Based on above results, a piezoelectrically-actuated
micro cantilever [18] and a piezoelectrically-transduced disk
resonator [19] were designed and fabricated in our work as
resonant-based ultra-sensitive mass sensor for human
healthcare and other applications. The device fabrication was
done by 4-inch SOI wafers using above large area PZT film
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