Ghodssi R., Lin P., MEMS Materials and Processes Handbook

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314 R.G. Polcawich and J.S. Pulskamp

through the d

coefﬁcient in c-axis-oriented AlN ﬁlms. In a typical two-port conﬁg-

uration, an input broadband RF signal is transduced into the mechanical domain via

the converse piezoelectric effect by an input transducer. The signal is then largely

ﬁltered by the mechanical frequency response of the structure and subsequently

transduced back into the electrical domain by means of the output transducer. The

input transducer effectively behaves as an actuator whereas the output transducer

behaves as a sensor. Different resonator geometries are utilized to achieve the best

RF performance at various frequencies, including the rectangular plate geometry

(∼10 to ∼100 MHz operation), ring resonators (∼100–400 MHz), and higher-order

contour-mode plate geometries (see Fig. 5.43). These devices have been demon-

strated at frequencies from 20 MHz to about 700 MHz with quality factors as high

as 4300 (230 MHz) and with low motional resistances ranging from 50 to 700 .

Fig. 5.43 Plot illustrating suggested geometries for AlN RF MEMS contour-mode resonators for

These resonators have also been successfully implemented as bandpass ﬁlters

[128]. Electrically coupled resonators, in a ladder conﬁguration, have been demon-

strated with rectangular plate resonators at 93 MHz and with ring resonators at

236 MHz (see Table 5.8). The ﬁlters are comprised of individual series and shunt

one-port resonators conﬁgured as cascaded L networks. Within each L network,

the series resonance of the series resonator is designed to coincide with the par-

allel resonance of the shunt resonator in order to optimally form the passband of

5 Additive Processes for Piezoelectric Materials 315

Table 5.8 Summary of performance of AlN contour-mode resonators [1]

Filter Photograph Data Performance

Electrically coupled ladder [9]

∼ 93.2 MHz

3dB

∼ 305 kHz

%BW

3dB

∼ 0.33%

20dB

∼ 671 kHz

I.L. ∼ –4 dB

Rejection ∼ 27 dB

term

∼ 2k

Electrically coupled ladder [9]

∼ 236.2 MHz

3dB

∼ 605 kHz

%BW

3dB

∼ 0.26%

20dB

∼ 1.8 MHz

I.L. ∼ –7.9 dB

Rejection ∼ 26 dB

term

∼ 1k

316 R.G. Polcawich and J.S. Pulskamp

Table 5.8 (continued)

Filter photograph Data Performance

Mechanically coupled array [10]

∼ 40 MHz

3dB

∼ 392 kHz

%BW

3dB

∼ 0.98%

20dB

∼ 1.1 MHz

I.L. ∼ –1.5 dB

Rejection ∼ 20 dB

term

∼ 1.5 k

Dual mode resonator [1]

∼ 22.4 MHz

3dB

∼ 112 kHz

%BW

3dB

∼ 0.5%

20dB

∼ 358 MHz

I.L. ∼ –4.8 dB

Rejection ∼ 30 dB

term

∼ 2.5 k

5 Additive Processes for Piezoelectric Materials 317

the ﬁlter. The bandwidth of the ﬁlter is related to the difference between the series

resonance of the shunt resonator and the parallel resonance of the series resonator.

This requires a frequency shift between the two resonators, within each L network,

by as much as a few percent and as little as a few tenths of a percent. Piazza, et al.

[128] accomplished this by modifying the geometry of the bottom Pt electrode to

affect mass loading of the composite resonators between the series and shunt devices

within the ﬁlter. These devices show very good RF performance with further gains

expected with optimization. The rectangular plate-based ﬁlters, featuring eight res-

onators, display 4 dB of insertion loss and 27 dB of out-of-band rejection at 93 MHz

with 2 k termination resistances. The ﬁlters based on eight ring resonators display

8 dB of insertion loss and 26 dB of out-of-band rejection at 236 MHz with 1 k

termination resistances.

The fabrication of these AlN contour-mode ﬁlters utilizes a four-mask CMOS

compatible process (see Fig. 5.44). The process begins with the deposition of a

LPCVD l ow-stress nitride buffer layer onto a silicon substrate. A 100 nm RF sput-

tered Pt bottom electrode layer is patterned by liftoff. The AlN ﬁlms are deposited

with a single-module Advanced Modular Sputtering (AMS, Goleta, CA) PVD sput-

ter tool. Bottom electrode vias through the AlN ﬁlm are deﬁned with a wet etch

of heated phosphoric acid. The wet etch provides a modest slope etch proﬁle,

facilitating the ensuing top electrode contact required for properly conﬁguring the

series and shunt resonators of the ﬁlter. A dry etch is used to deﬁne the top Al

electrode. The AlN layer is patterned with a hard mask of low-temperature oxide

and a chlorine-based dry etch. In addition, the top electrode is passivated by a

∼30 nm layer of Nb during the AlN etch step. The Nb l ayer, remaining LTO

hard mask, and the low-stress nitride layer beneath the resonators are removed

during the subsequent CF

-based dry etch. Finally, xenon diﬂuoride is used to

release the devices and to remove any remaining low-stress nitride from beneath the

resonators.

Fig. 5.44 Illustration of the fabrication process used to demonstrate AlN electrically coupled ﬁl-

ters and images of the wet etch bottom electrode via [128] (Reprinted with permission. Copyright

2007 IEEE)

318 R.G. Polcawich and J.S. Pulskamp

5.3 Ferroelectrics: PZT

This section concentrates on the leading ferroelectric material used in thin-ﬁlm

piezoelectric MEMS: lead zirconate titanate (PbZr

1−x

) or PZT. Possessing a

piezoelectric coefﬁcient nearly an order of magnitude larger than its nonferroelectric

counterparts, PZT is the ideal candidate for devices requiring large displacements

or forces. The goals of this section include an analysis of the deposition of these

materials, patterning techniques, identiﬁcation of device design and processing con-

cerns, and ﬁnally a detailed subsection covering examples of PZT devices including

a case study on PZT actuators for RF switching applications.

5.3.1 Material Deposition

PZT thin ﬁlms have been successfully deposited by a wide variety of processes

with a majority of the early development done through various sputtering techniques

with a strong focus on RF or ion-beam deposition using bulk ceramic targets [136–

139]. Current research efforts primarily utilize sputtering [140, 141], metal-organic-

chemical vapor deposition (MOCVD) [142–144], and chemical solution deposition

techniques [145–151] with many of the techniques summarized in Table 5.9.PZT

deposition methods require strict control of the stoichiometry to prevent nucleation

of nonferroelectric ﬂuorite and pyrochlore structures [ 152, 153]. Furthermore, lead

oxide (PbO) becomes highly volatile above 500

◦

C so one must take care to provide

enough excess lead to compensate f or lead loss through the release of PbO during

any high-temperature processing and annealing [154].

A variety of substrates can be used for PZT deposition. However, there are

requirements for both the substrate and/or metallization layers. Growth of PZT is

strongly nucleation controlled and with proper control of the nucleation, crystal

orientation of the ﬁlm can be manipulated [155, 156]. Single crystal substrates (i.e.,

MgO, SrTiO

) can be used to nucleate epitaxial, single-crystal PZT thin ﬁlms [157].

For most MEMS applications, silicon is the substrate of choice. When using Si

it is important to prevent the formation of lead silicide at t he substrate interface by

using buffer layers and/or metal layers. In most instances, a layer of silicon diox-

ide is used as a buffer layer in combination with a bilayer of Ti/Pt to serve as the

base electrode for the PZT deposition. With the lattice constant of platinum being

close to that of PZT (2–3% mismatch for c-axis-oriented PZT), Pt can be an excel-

lent template for (111) oriented PZT ﬁlms as Pt tends to grow with a strong (111)

texture. In order to maximize the texture within the Pt, the preferred material stack

is SiO

/TiO

/Pt [158]. The layer of TiO

serves as a buffer layer and template that

allows a higher degree of texture within the Pt. For detailed information on the

method required for achieving highly textured (111) Pt, the readers are directed to

[158] with one of the key ﬁgures reproduced in Fig. 5.45. Methods of manipulat-

ing the texture of the PZT away from (111) orientation when using Pt layers are

discussed later within this section.

5 Additive Processes for Piezoelectric Materials 319

Table 5.9 Deposition methods and parameters for PZT thin ﬁlms

Deposition parameter DC sputter (reactive) RF sputter MOCVD MOCVD CSD

Target stoichiometry Pb, Zr, Ti (3 metal targets) Pb

1.20

(Zr

0.53

0.47

Excess Pb % 20 10 and 30

RF power (W) 100

Bias (V or W)

Pressure (Pa or T) 1–2 5 T

Gas O

(1–3 sccm) O

(1 L/min) O

Deposition rate (nm/min) 10–20 12

Temperature (

◦

C) 570 followed by post-dep RTA 550–700 550 540 RT→350

→650

References [140][140, 160][144][143][149]

320 R.G. Polcawich and J.S. Pulskamp

Fig. 5.45 Texture, FWHM of

the sputtered Pt (222) peak,

as a function deposition

temperature and after thermal

processing of a PZT ﬁlm

deposited atop the Pt ﬁlm

[158]

There are several metal and/or conducting oxides (i.e., Ru and RuO

and Ir and

IrO

) that can be used in place of Pt. These alternative electrodes can exhibit a higher

resistance to Pb interdiffusion and thus provide a better barrier. Oxide electrodes are

commonly used for the top electrode to reduce ferroelectric fatigue (reduction of

switchable polarization with bipolar cycling). Note, ferroelectric fatigue has very

little dependence on the bottom electrode material. In a vast majority of MEMS

applications, bipolar switching will be avoided with typical operation relying on

unipolar drive (see Section 5.1.3.10).

Sputter deposition of PZT thin ﬁlms has been completed through either reactive

deposition using three metal targets (Pb, Zr, and Ti) or RF sputtering using ceramic

targets. In either case, PZT thin ﬁlms with excellent ferroelectric and piezoelectric

properties can be obtained. One challenge with sputter deposition is accurate con-

trol of the Pb and O content within the ﬁlms. Lead has a higher sputter yield and

higher volatility (i.e., PbO) compared to Zr and Ti. As a result, the lead loss must

be compensated with either higher ﬂuxes sputtered from pure Pb targets or excess

Pb incorporated in ceramic targets. An additional item of note is the substrate tem-

perature during deposition. The use of high temperatures during deposition results

in higher Pb loss and typically requires higher excess lead content targets. If the

substrate temperature can be maintained greater than 600

◦

C, pure perovskite-phase

PZT thin ﬁlms can be achieved. Otherwise, a crystallization anneal is required

on the sputtered ﬁlm to convert the deposited layer into the desired perovskite

phase.

MOCVD processes for PZT thin ﬁlms have been researched as a key technology

for mass production of ferroelectric random access memory (FeRAM) especially for

device architectures requiring exceptional step coverage. For PiezoMEMS applica-

tions, there has been limited information reported on the piezoelectric properties

MOCVD-deposited thin ﬁlms. For a summary of the MOCVD deposition process

readers are directed to [142, 144] for more detail.

Chemical solution deposition (CSD) methods for ferroelectric thin ﬁlms have

been developed based on a variety of methods (i.e., sol-gel and metallorganic

decomposition) and precursors [159]. The CSD techniques can be categorized into

5 Additive Processes for Piezoelectric Materials 321

three main groups: sol-gel, chelating process, and metallorganic decomposition

[149, 150]. The two most widely used approaches are based on a sol-gel approach

using 2-methoxyethanol as a solvent and the inverted mixing order process using

methanol as a solvent. Sol-gel methods have been primarily based on the research

of Budd et al. in 1985 [145].

With the inverted mixing order (IMO) process, the lead, titanium, and zirconium

precursors are combined in the opposite order from those developed in the original

sequential addition process by Budd et al. [145, 160, 161]. In addition, the solvent

used for the IMO process is methanol with slight additions of acetic acid and deion-

ized water for additional solution stability. The use of methanol instead of 2-MOE

enables a less toxic solution preparation and deposition process for the user [159].

The most heavily researched processes for the deposition of PZT ﬁlms uses 2-

MOE as a solvent and lead acetate trihydrate, titanium iso-propoxide, and zirconium

n-propoxide as the precursors [145, 162, 163]. This process has been used to pro-

duce PZT ﬁlms with the highest piezoelectric coefﬁcients to date. A general version

of the 2-MOE-based solution process ﬂow is highlighted in Fig. 5.46.Tobegin,the

lead precursor (lead acetate trihydrate) is dissolved in 2-MOE followed by vacuum

distillation to remove the water. The dehydrated lead acetate is allowed to redissolve

in a portion of the 2-MOE solvent before being mixed with the Zr and Ti precur-

sors. Prior to being added to the lead precursor, the Zr and Ti precursors are mixed

in the desired stoichiometric ratio at room temperature with 2-MOE. The combined

solution is reﬂuxed for 2–3 h at 120

◦

C using a rotary evaporator or comparable

setup. Next, the solution undergoes a short vacuum distillation to remove a small

amount of reaction by-products and adjust the concentration, typically 0.4 M. The

ﬁnal step is to add 4 volume percent of formamide, which serves as a drying control

Fig. 5.46 Schematic process ﬂow for creating a 2-MOE-based solution for PZT thin ﬁlms

322 R.G. Polcawich and J.S. Pulskamp

Fig. 5.47 Deposition and thermal treatment process ﬂow for chemical solution deposition of PZT

thin ﬁlms based on a 2-Methoxyethanol solvent system

agent [164]. Additionally, a small percentage of acetylacetate may be added to the

solution to assist in the long-term stability of the solution [165].

Regardless of the solution chemistry, the deposition processes are quite sim-

ilar and graphically represented in Fig. 5.47. The only discrepancies are minor

changes to the pyrolysis temperature or the crystallization temperature. The depo-

sition begins by placing the solution into a syringe conﬁgured with a 0.1–0.2 μm

PTFE (polytetraﬂuoroethylene) ﬁlter. The solution is statically dispensed onto t he

substrate and then spun to achieve a uniform ﬁlm thickness across the substrate.

Following the spin, the ﬁlm is pyrolyzed on a hot plate to remove the volatile organ-

ics. The spin and pyrolysis steps are repeated as required (i.e., one to four layers)

prior to the crystallization anneal in ﬂowing oxygen. Using a 0.4 Molar solution, a

four-layer spin process typically yields a ﬁnal ﬁlm thickness close to 0.25 μm. The

process is continuously repeated until the desired ﬁlm thickness is achieved.

Regardless of deposition method, there are several key material parameters with

which to judge the quality of the PZT thin ﬁlm. The ﬁrst is the crystalline quality

of the ﬁlm via X-ray diffraction (XRD). The best quality piezoelectric ﬁlms will

possess a high degree of texture in the (100)/(001) orientation (see Fig. 5.48). The

easiest method of assuring a high degree of (100) texture is through templating the

nucleation and growth with a PbTiO

seed layer and proper attention to the amount

of excess lead present during any high-temperature annealing or crystallization pro-

cesses [155, 163]. An example XRD pattern of a highly textured CSD PZT thin ﬁlm

is shown in Fig. 5.49.

Optimizing material quality and properties requires controlling the stoichiomet-

ric gradients through the thickness of the PZT ﬁlm. Figure 5.50 illustrates the

importance of controlling these gradients, as these gradients represent deviations

of the average properties of the ﬁlm from the intended composition. As stated ear-

lier, PZT ﬁlms are strongly nucleation controlled. With heterogeneous nucleation

occurring at the electrode interface, Ti rich compositions nucleate ﬁrst [155]. The

end result is a sawtooth Zr/Ti gradient through the PZT thickness with a Ti rich

layer at the electrode interface and a Zr-rich layer at the ﬁlm surface (see Fig. 5.50)

[149]. To compensate for the gradient, Calame and Muralt developed a process to

5 Additive Processes for Piezoelectric Materials 323

Fig. 5.48 Transverse

piezoelectric stress constant,

31, f

,ofa1μm PZT thin ﬁlm

as a function of Zr/Ti ratio for

both (100)/(001) and (111)

oriented thin ﬁlms [163]

(Reprinted with permission.

Fig. 5.49 –2 X-ray diffraction pattern for PZT (53/47) thin ﬁlms (1–4 μm) using a PbTiO

seed layer to promote a highly textured (100) orientation [163] (Reprinted with permission.

reduce the gradient by manipulating the stoichiometry of each of the four CSD lay-

ers (63, 58, 48, and 43% Zr content) deposited prior to the crystallization step [149].

As shown in Fig. 5.50, the average composition through the entire 2 μm thickness

closely resembles the MPB desired composition of 53/47.