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

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

pMUTs offer the potential of s maller size systems resulting from MEMS fabrica-

tion, higher frequency for improved resolution (targeting 10–100 MHz), and better

impedance matching to air and water [194]. To achieve their full potential, these

arrays require a high electromechanical coupling factor enabling a large bandwidth,

high efﬁciency, and minimal cross-element noise and high aspect ratio structures

[195]. Similar to the low-frequency microphones discussed previously, membrane

structures have been the typical incarnation of the pMUT. An example of such a

device is in Fig. 5.61 in which a 4 μm thick PZT layer was combined with a 10 μm

Si device layer to yield a transducer operating at 16.9 MHz with a quality factor Q

of 25 in air and coupling factor, k

of 3% [195]. One limitation with conventional

micromachining approaches is difﬁculty in achieving ultrahigh aspect ratios neces-

sary for l ow MHz operation. Recent work on using spray-mist deposition of PZT

solutions may be a means of achieving such technology (see [196]).

Fig. 5.61 Example of a pMUT comprised of a Si/SiO

/Ti/TiO

/PZT/Cr/Au fabricated using

Science+Business Media)

Actuation technology continues to be a strong area of development for piezoelec-

tric MEMS including ultrasonic motors, micromirrors, RF devices, and robotics.

Ultrasonic motors using thin-ﬁlm PZT were investigated initially in the middle

1990s as possible replacements for actuators in wrist watches [197, 198]. For this

device, a piezoelectric membrane stator is combined with a set of elastic ﬁns on a

rotor. As the resonant wave is generated within the membrane, it periodically com-

presses the elastic ﬁns resulting in movement in the rotor as the elastic ﬁns relax after

contacting the membrane [199]. Similar to the previously discussed membranes,

they were created using a combination of bulk and surface micromachining tech-

nologies. The device and data shown in Fig. 5.62 were from a membrane patterned

using a KOH etch to create the Si membrane structure. This conﬁguration using a

1 μm PZT ﬁlm was able to achieve nearly 100 rad/s at 5 V, nearly two orders of

magnitude higher velocity compared with an AlN stator of comparable frequency

anda2μmAlNﬁlm.

5 Additive Processes for Piezoelectric Materials 335

Fig. 5.62 (a) Optical micrograph of a PZT membrane conﬁgured for optimal vibration mode using

electrode shaping to create an ultrasonic motor and (b) measurements of the motor angular velocity

Physics)

Piezoelectric MEMS actuators represent a truly enabling actuator technology for

millimeter-scale robotics. The actuation potential of PZT thin-ﬁlm actuators is best

represented in Fig. 5.63. As illustrated, PZT actuators are capable of achieving

forces and displacement values in excess of other common actuation technolo-

gies for a given footprint. This particular actuator technology is being developed

to enable ground mobile millimeter-scale robotics using a thin-ﬁlm lateral actua-

tion technology. This lateral actuation technology is created using manipulation of

the PZT thin ﬁlm relative to the neutral axis position of the beam actuator [16].

Such actuators can achieve mN force levels at micron displacements. Combining

these actuators with single-crystal silicon tethers and end deﬂectors, it is possible to

achieve in-plane rotations of 3–5

◦

for a single element. Cascading these structures

enables 45

◦

in-plane rotations. Rotations at these levels begin to approach reason-

able levels for the creation of biomimetic insect structures using a hexapod gate.

Another area of interest for piezoelectric MEMS actuators is in microﬂight at the

millimeter scale. For this application, actuators are being designed to create both

the ﬂapping and angle of attack motions used for ﬂight by insects [84]. As shown

in Fig. 5.64, wing actuators based on PZT thin ﬁlms have been fabricated using a

combination of surface and bulk micromachining techniques to yield wings capable

of nearly 45

◦

of total angular deﬂection at 5 V at a resonance frequency of 156 Hz.

These wing actuators are approaching the metrics of many millimeter-scale insects

for ﬂight, including resonance frequencies in the 150–250 Hz range. Total angular

deﬂection still requires additional research, as values in the range of 80–180

◦

are

desirable to achieve sustainable ﬂight.

One of the key limitations of electrostatically actuated MEMS switches is the

inability to combine a low actuation voltage with excellent DC and RF performance

(i.e., contact force and insertion loss, respectively) and reliability. Attempts to lower

the actuation voltage with electrostatic switches generally rely on reducing either

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

Fig. 5.63 Force-displacement plot of various actuation technologies for actuators conﬁned to a

500 × 500 μm

area. The straight line marks the target 1 nN-m minimum performance for mm-

(a) (b)

Fig. 5.64 Images of a PbZr

1−x

(PZT) microelectromechanical systems actuated microwing

(2.5 mm length) (a)at0Vand(b) at resonance (156 Hz, 5 Vp-p), showing that resonant behavior

the mechanical stiffness of the released structures or the gap between the mechan-

ical bridge/cantilever and the corresponding biasing pad [200]. Reductions in the

stiffness can limit the restoring force of the switch and can lead to stiction failures

[201]. Decreasing the electrode gap limits the high-frequency RF performance (in

particular, the isolation in the open state for series switches) [202].

The ﬁrst functioning piezoelectric MEMS DC relays were demonstrated by

Gross et al. using in-plane poled PZT ﬁlm actuators [203]. These devices used a

cantilever beam with a supporting elastic layer, a PZT thin ﬁlm as the actuator, and

patterned gold structures to act as the electrodes (see Fig. 5.65). This device operated

5 Additive Processes for Piezoelectric Materials 337

Fig. 5.65 Illustration of a

thin-ﬁlm PZT unimorph

switch developed at Penn

State University [203]

(Reprinted with permission.

Institute of Physics)

Fig. 5.66 SEM image of a

bulk micromachined RF

MEMS series switch using a

thin-ﬁlm PZT actuator to

complete the signal path

[204] (Reprinted with

IEEE)

as low as 20 V with a switching-on time as low as 2 μs. The ﬁrst reported working

RF MEMS switch using PZT thin ﬁlms used bulk micromachining to release a uni-

morph actuator and a suspended transmission line [204, 205]. This design utilized

a cantilever perpendicular to the wave propagation direction along the RF conduc-

tor of the coplanar waveguide (CPW). The switch operates as low as 2.5 V with an

insertion loss better than 0.7 dB and isolation better than –21 dB up to 20 GHz (see

Fig. 5.66).

One limitation with the RF switch actuators described above is the require-

ment for bulk micromachining. Surface micromachining is substantially cheaper

and easier to integrate with other fabrication processes than bulk micromachining

techniques [206]. The development of a surface micromachined, piezoelectric RF

MEMS switch is discussed in full detail in the remainder of this section heavily

leveraging the information described in [29, 182].

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

5.3.5 Case Study on the Design and Processing of a RF MEMS

Switch Using PZT Thin-Film Actuators

The ﬁrst few considerations for the design of a MEMS switch with PZT actuators

are the position of the actuator relative to the microwave transmission line and the

contact conﬁguration. Referencing the description in Section 5.1.3.6, thin-ﬁlm PZT

generates an in-plane compressive strain at large electric ﬁeld values when used in

a parallel plate conﬁguration (d

mode). The resulting direction of actuation for a

thin-ﬁlm actuator is dictated by three main parameters, the magnitude and sense of

the transverse piezoelectric stress constant, e

31,f

induced strain (force), and the posi-

tion of the geometric midplane of the piezoelectric thin ﬁlm relative to the composite

neutral axis (moment arm). If the geometric midplane of the piezoelectric layer lies

above the neutral axis, the induced piezoelectric strain will deﬂect the actuator up

out of the wafer surface. Using this conﬁguration, a switch can be designed with the

electrical contacts of the switch residing above the actuators (see Fig. 5.67).

Fig. 5.67 Cross-section

schematic of a PZT actuator

and electrical contacts

conﬁgured for use as switch

Another important consideration is that the PZT actuator is a multilayer compos-

ite comprised of at least four layers (elastic layer, metal, piezoelectric, metal) with

each layer having signiﬁcantly different coefﬁcients of thermal expansion (CTE)

and residual stress. The net result of such a structure is the potential for signiﬁcant

residual-stress-induced deformations in the unbiased actuator state and thermally

induced deformations from the differences in CTE. The reader is directed to Section

5.3.3 for a description of the residual stress gradient and thermally induced stress

deformations. For proper switch operation, a negative static curvature is desired

and can be reproducibly fabricated using an elastic layer comprised of a three-

layer composite of PECVD, SiO

, and Si

. The negative curvature using an

oxide/nitride/oxide elastic layer for the actuator not only enables switch opera-

tion with a normally open switch state but also ensures that thermally induced

deformations do not cause the switch to open or close inadvertently.

With the design of the actuator complete, the RF transmission line should be

designed to accommodate the actuator with minimal perturbation to the RF signal.

Based on previous discussions on the piezoelectric strain generation in a parallel

plate capacitor conﬁguration, the switch contacts should be conﬁgured above the

5 Additive Processes for Piezoelectric Materials 339

actuators using the vertical motion generated in the actuators. The placement of the

actuators requires a close interaction between the mechanical design and the high-

frequency design aspects of the RF transmission line. By placing the actuators in the

gaps between the RF conductor and ground planes in a coplanar waveguide (CPW)

transmission line, the high dielectric constant PZT combined with the air cavity

beneath the released actuator (created with a XeF

etch) can be compensated with

minor changes in the CPW dimensions (i.e., width of the RF transmission line and

RF-GND gap) so as to maintain a 50  impedance in the switch (see Table 5.13 and

Fig. 5.68). The two actuators residing in the RF gaps can be connected mechanically

with a coupling beam containing a short section of conductor used to complete the

RF circuit. Two very short cantilevered structures anchored to each section of RF

conductor overhang the conductor on the mechanical coupling beam and allow the

switch structure to close the series conﬁgured gap upon actuation. Another design

feature is air bridges that span the RF conductor to electrically connect each of the

actuators. The actuator bias lines allow the actuation voltage to be applied indepen-

dently of the RF conductor. The bias air bridges spanning the RF conductor require

the width of the conductor to be modiﬁed to maintain 50  impedance because of

the capacitive loading on the RF conductor. Thus, the RF conductor is narrowed

underneath the bias air bridges and widened in the regions where the PZT actuators

are located (see Fig. 5.68). It can be compensated with minor changes in the CPW

gaps so as to maintain a 50  impedance.

Table 5.13 Dimensional information for a PZT switch design using a CPW transmission line

Feature Width (μm) Length (μm)

RF conductor (outside of actuators) 75 >150

RF conductor (under air bridges) 50 10

RF gap (between actuators) 40

RF gap (outside of actuator regions) 50

Ground plane 440 660

PZT actuator 40 100, 125, 150

RF cantilever (in) 100 37.5

RF cantilever (out) 75 37.5

RF contact pad (in) 120

RF contact pad (out) 77

Bias air bridges 170 10

Ground straps 18 80

The fabrication of the PZT switch begins with the choice of an appropriate sub-

strate for high-frequency performance and reasonable piezoelectric properties in the

PZT thin ﬁlm. The preferred substrate is silicon because of the wealth of knowledge

regarding fabrication processes as well as previous history with processing PZT thin

ﬁlms on platinized silicon substrates. However, as illustrated in Fig. 5.69, a standard

resistivity (1–30  cm) silicon substrate has extremely poor transmission line loss

at microwave frequencies. Traditional microwave substrates, alumina and sapphire

[202], have transmission line losses less than 0.04 dB/wavelength (see Fig. 5.69).

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

Fig. 5.68 Illustration of the

RF conductor width

modiﬁcations to account for

(a) capacitive loading from

the DC bias air bridges and

(b) the changes to the

capacitance within the RF

gap from the air cavity

underneath the actuators and

CPW transmission line

Fig. 5.69 Coplanar

waveguide transmission line

(T-line) loss as a function of

frequency for various

substrates. Note: The alumina

is a polycrystalline substrate

Even though high-resistivity silicon is not commonly thought of as a microwave sub-

strate, it performs quite well, with a transmission line loss of ∼0.05 dB/wavelength

at 40 GHz. The transmission line loss is reported as dB/wavelength and not dB/mm

because the primary aim of MEMS switches is in distributed RF circuits. As a con-

sequence, the interest is in designing the smallest low-loss circuits such as phase

shifters and tunable antenna receivers. As the microwave wavelength depends on

the dielectric constant of the substrate, other substrates such as quartz and fused sil-

ica usable for transmitting energy point-to-point, where dB/mm is important, require

much larger circuits to maintain a particular wavelength. The result is that the line

loss per wavelength increases. For the following discussion, high resistivity (>10 k

cm) Czochralski-grown (100) silicon substrates are s elected for fabrication of a PZT

switch.

5 Additive Processes for Piezoelectric Materials 341

Table 5.14 Plasma-enhanced chemical vapor deposition parameters for silicon dioxide and

silicon nitride thin ﬁlms deposited using a plasma-Therm 790

Material

SiH

5% He

(sccm) He (sccm)

(sccm) N

(sccm)

Pressure

(mT) Temp (ºC)

Power

(W)

SiO

70 93 390 0 0 900 250 25

90 488 0 160 3.00 900 250 45

The next step in the actuator fabrication is deposition of the stress engineered

elastic layer comprised of SiO

/Si

/SiO

. The previous discussions regarding this

elastic layer combination are based on PECVD-deposited ﬁlms deposited at 250

◦

using the deposition parameters in Table 5.14. One concern with PECVD ﬁlms used

in conjunction with higher temperature processing is the release of trapped hydrogen

within the ﬁlms, especially when using silane (SiH

) as a precursor gas. With the

CSD PZT having a thermal budget of 700

◦

C, any trapped hydrogen within the ﬁlm

must be released prior to deposition of the metal and PZT layers. Following PECVD

deposition, the elastic layer is annealed at 700

◦

CinﬂowingN

(∼5 sccm) for 60 s

to eliminate loosely attached hydrogen ions [207–209]. The hydrogen release also

results in a restructuring of the atomic bonds within the ﬁlm, altering the residual

stress within the PECVD ﬁlms leading the values reported in Table 5.12.

Following the elastic layer, the base metal layer of Ti/TiO

/Pt is deposited,

preferably using sputter deposition to achieve a high degree of (111) texture in the

Pt. The details of the deposition process for achieving a highly textured Pt layer can

be best described in [158]. The process begins with the deposition of Ti followed

by an oxidation anneal to convert the titanium into titanium dioxide conﬁgured in

the rutile crystal structure. The 820 Å Pt layer is then sputter-deposited at 500

◦

With the processing temperature for the PZT being 700

◦

C, the residual stress within

the Pt should be characterized following an anneal at 700

◦

C. The anneal results in

a stress relaxation of the metal thin ﬁlms, with an increase in the tensile stress of

the metal layer (see Fig. 5.56). However, for device fabrication, the anneal is elim-

inated to ensure optimal growth of the PZT thin ﬁlm without the introduction of Pt

hillocks.

The planar depositions of the actuator materials are completed with the deposi-

tion of the active piezoelectric layer, PZT (52/48), followed by a sputter deposition

of Pt. For the devices demonstrated in [29], a 0.5 μm thick PZT layer was deposited

via sol-gel using a 2-methoxyethanol solvent-based solution. Sputtered PZT (52/48)

has also successfully been used to create the same switch actuators. After the PZT,

a0.1μm thick Pt layer is sputter-deposited at 300

◦

The next steps in the switch fabrication include actuator patterning, patterning

the RF transmission lines, creation of air bridge structures, and device release and

are schematically represented in Fig. 5.70. The actuator patterning begins by deﬁn-

ing the area of the top Pt electrode regions using Ar ion-milling. Next, the PZT and

bottom bilayer of Ti/Pt or TiOx/Pt are patterned with ion-milling stopping on the

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

Fig. 5.70 Fabrication process ﬂow schematic for creating a PZT actuated RF MEMS switch: (a)

pattern top Pt, (b) pattern PZT and bottom Pt, (c) open via to bottom Pt, (d) open etch release

trench, (e) thin elastic layer (pink layer is resist), (f) pattern CPW transmission line, (g) deposit

contact dimple metal, (h) pattern sacriﬁcial resist layer, (i) air bridge deposition, patterning, and

release of sacriﬁcial resist, (j)XeF

etch release of actuator [57] (Reprinted with permission.

elastic layer with a small amount of overetch into the top of the elastic layer permit-

ted. The opening via to contact the bottom Pt electrode is completed with either a

combination of ion-milling and wet etching or simply a wet etch using a combina-

tion of H

O:HCl:HF (2:1:0.04). The ﬁnal step required for patterning the actuator

is the opening of the etch trench in the elastic layer surrounding the actuator. This

etch utilizes a combination of CF

,CHF

, and He to remove the elastic layer and

provide a shallow overetch into the silicon substrate.

Prior to deposition of the RF transmission lines, the remaining elastic layer is

thinned using a combination of CF

,CHF

, and He to remove the top layer of

5 Additive Processes for Piezoelectric Materials 343

silicon dioxide, silicon nitride, and 50 nm of the bottom silicon dioxide layer. As

described in [29], this etch is crucial for reducing the parasitic and achieving low

loss transmission lines. The transmission lines are patterned using a metal liftoff

technique with evaporated Ti/Au (20/730 nm). In addition, a separate layer of Au/Pt

(400/100 nm) or Au/Ru (400/100 nm) is deposited to serve as contact dimples for

the switch.

The air bridge structures are created using a combination of a sacriﬁcial pho-

toresist layer using a mild high-temperature bake to round the corners of the resist

followed by metal liftoff of an electron-beam-evaporated 2 μm Au layer. The resist

is then removed using an oxygen plasma to then create the free-standing air bridge

structures for the switch. Prior to the ﬁnal release of the actuators, a short plasma

etch using a combination of CF

,CHF

, and He is used to remove any oxidation

on the exposed silicon surfaces. The ﬁnal device (see Fig. 5.71) etch is completed

using a timed XeF

designed to provide an approximate lateral undercut of 35 μm.

Fig. 5.71 SEM image of a

PZT-actuated RF MEMS

switch [57] (Reprinted with

IEEE)

5.4 Summary

This chapter has introduced the fundamentals of piezoelectric and ferroelectric

materials, provided simple models for the response of thin ﬁlm piezoelectric

devices, and presented an overview of applications for these devices. The mate-

rial deposition and patterning techniques for the polar piezoelectric materials, AlN

and ZnO; and the ferroelectric material PZT have been reviewed. Device design

considerations related to processing have also been addressed for these materials.

Examples and discussions of demonstrated devices and their fabrication processes

have been presented for both polar and ferroelectric materials.

The motivation to overcome the processing challenges with these materials con-

tinues to gain momentum as piezoelectric MEMS devices continue to demonstrate

unique capabilities and in many cases superior device performance. Many of the