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

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

Table 5.4 Comparison of the surface roughness (RMS) and full width at half maximum (FWHM)

for sputtered AlN ﬁlms on various substrates and thin ﬁlms [97]

Substrate Si Si/Mo Si/SiO

Si/Si

Si/SiO

/Ti/Pt

Substrate roughness, R

rms

(Å) 0.94 5.10 0.91 1.27 3.49

AlN roughness, R

rms

(Å) 12 46.3 11.4 16.0 39.7

AlN (0002), FWHM (

◦

) 1.82 5.15 1.78 1.90 2.37

Fig. 5.28 SEM image

illustrating the columnar

growth and smooth texture of

a sputtered AlN ﬁlm

deposited onto Mo on a Si

substrate [96] (Reprinted with

American Vacuum Society)

Table 5.5 Inﬂuence of a Ti seed layer on the sputter deposition of both Mo and AlN ﬁlms [99]

Substrate

Mo deposition

temperature

(

◦

roughness,

rms

(Å)

FWHM (

◦

)

AlN

roughness,

rms

(Å) AlN FWHM (

◦

)

No seed layer 250 3.86 6.2 12.60 3.6

Ti seed (30 nm) 250 2.75 2.1 7.44 1.7

in Fig. 5.29a, increases in oxygen percentage in a Ar/O

mixture leads to decreases

in the surface roughness. However, the FWHM of the [0002] peak increases. One

method of achieving improvements in both the RMS roughness and FWHM is to

use a two-step deposition process [105]. In the process outlined by Lin, Chen, and

Kao [105], the initial step uses a high oxygen percentage (75%) to deposit a smooth

starting layer of ZnO. The second deposition processes s witches to a lower oxygen

percentage (50%) to increase the texture quality of the ﬁlm along with maintaining

accurate stoichiometry. The resulting ﬁlm exhibits a columnar growth pattern along

with an improved surface quality (see Fig. 5.29b).

5 Additive Processes for Piezoelectric Materials 305

(b)(a)

Fig. 5.29 (a) Inﬂuence of oxygen concentration on the texture and surface roughness of RF sput-

tered ZnO thin ﬁlms and (b) an SEM image of a sputtered ZnO ﬁlm deposited using a two-step

5.2.2 Patterning Techniques

In order to enable the full integration potential of AlN and ZnO, selective pattern-

ing capabilities are required including both wet chemical and dry etch processes. A

key aspect of wet chemical processes is their tendency to be very isotropic result-

ing in reasonably large lateral etch rates. However, under many circumstances, wet

etching is preferred to avoid the physical damage and generally low selectivity of

physical etch techniques. Both AlN and ZnO can be removed with wet chemical

etch processes and a list of known etchants have been assembled in Table 5.6.An

important characteristic about ZnO is that its material quality (texture, dielectric

loss, and breakdown strength) are well known to be extremely sensitive to micro-

fabrication processes [107]. As a result care must be taken not only to protect it

during the etching process but also during the remainder of the device fabrication

processes. Protection layers can include the use of silicon dioxide and silicon nitride

barrier layers to name a few.

Table 5.6 Summary of wet etching techniques for AlN and ZnO thin ﬁlms

Etching

parameter AlN AlN AlN AlN ZnO ZnO

Chemical(s) AZ400K

(KOH)

KOH (10%) TMAH H

(85%) HCl (0.1 M) CH

COOH:

(1:1:60)

Mask Apiezon

wax

Ti Cr

Temperature

(

◦

85 RT–85 RT 60 RT RT

Etch rate

(nm/min)

6–1000 6–1500 22 600–3000 300–1200 ∼800

References [108, 109][109, 110][111][112, 113][107, 114,

115]

[107, 116]

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

Fig. 5.30 AlN wet etch rates

in 10 wt% KOH as a function

of solution temperature,

deposition temperature, and

the deposition power of

sputtered AlN [110]

(Reprinted with permission.

The ability of etching AlN has a very strong relationship to the crystalline quality

of the thin ﬁlm. As discussed previously, the crystalline quality is highly dependent

on the deposition parameters. Similarly, the etching rates are linked to the deposition

conditions. The example in Fig. 5.30 illustrates the variance in etching conditions

as a function of solution temperature, RF deposition power, and substrate deposi-

tion temperature. In addition, the lateral etch rates also vary as a function of the

crystalline quality of the ﬁlm (see Fig. 5.31). Another crucial point in the primary

wet etch for sputtered AlN ﬁlms is KOH, the main component in some photoresist

developers [108, 109]. Therefore, special concern must be taken in the selection

of photoresists and developers used during the microfabrication process. Similar to

ZnO ﬁlms, the use of barrier coatings such as silicon dioxide or silicon nitride thin

ﬁlms are common.

Fig. 5.31 SEM images of sputtered AlN following a wet etch in 10 wt% KOH for ﬁlms deposited

at RT and 600

◦

Physical etching of AlN and ZnO by various etching techniques is the choice

for precision deﬁnition of MEMS devices. In general, chlorine plasmas are used for

AlN etching and ﬂuorine plasmas are used for ZnO. A list of common physical etch

techniques for both compounds is given in Table 5.7. In contrast to wet chemical

etching, physical etching leads to nearly vertical sidewalls under the appropriate

etch conditions (see Fig. 5.32). For AlN, the use of hydrogen in the etch chemistries

5 Additive Processes for Piezoelectric Materials 307

Table 5.7 Summary of plasma etch techniques for AlN and ZnO

Etching

parameter AlN AlN AlN ZnO ZnO

System ECR ICP ICP ICP ICP

Chemical(s) Cl

/Ar (5:1) Cl

or Cl

/Ar

RF power (W) 200 500 100–1500 700–1000 300

Bias (V or W) –150 V –100 to –350 V 0 to –100 V 50–200 W

Pressure (Pa) 0.13 0.26 0.26–2 0.4–2 0.26

Etch rate

(nm/min)

10–40 50–700 50–150 410 50

References [117][118, 119][120][121][122]

Fig. 5.32 SEM of an AlN

layer etched using an ICP

plasma using a Cl

/Ar gas

mixture

tends to lead to equal etching rates for the group III and group V components and

leads to smoother etch surfaces [117].

5.2.3 Device-Design Concerns

Designing piezoelectric MEMS with AlN or ZnO requires attention to several key

aspects that affect the overall device performance. The choice of ﬁlm thickness has

a direct impact on the crystalline texture and piezoelectric coefﬁcient. The initial

growth starts with epitaxial matching to the substrate or electrode surface leading

to strained growth with a relatively broad FWHM. For sputtered AlN, as the ﬁlm

thickness increases, the alignment of the crystals improves, reducing the FWHM to

near 1 degree and increasing d

33, f

toward 5 pm/V (see Figs. 5.33 and 5.34)[90,

123].

In most applications, parallel plate conﬁgurations (d

mode) are utilized requir-

ing the use of multilayer composites. As a result, control of residual stress gradients

is imperative and starts with an understanding of the residual stress in the piezoelec-

tric. Similar to the FWHM and piezoelectric coefﬁcient, the residual stress heavily

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

Fig. 5.33 RockingcurveFWHMand d

33, f

as a function of ﬁlm thickness for sputtered AlN on

Si/SiO

Fig. 5.34 d

33, f

as a

function of ﬁlm thickness for

sputtered AlN on

Si/SiO

/Ti/Pt [90] (Reprinted

with permission. Copyright

2004 American Vacuum

Society)

depends on the ﬁlm thickness, primarily a result of high interface stresses as the

piezoelectric is strained to match the underlying layers. As shown in Fig. 5.35,the

stress of a sputtered AlN on Pt changes from compressive to tensile with increas-

ing thickness, for the deposition parameters in [90]. In addition to the stress being

dependent on the ﬁlm thickness, the deposition parameters (RF or DC bias, pres-

sure, gas ﬂow, and temperature) can greatly inﬂuence the residual stress [22, 101,

102, 104]. In many instances, the ﬁlm stress can be controlled over a wide spec-

trum from compressive to tensile stress (see Fig. 5.36 ). In addition, the ﬁlm stress

and the coupling factor have been shown to be interrelated creating a more chal-

lenging situation to achieve both low stress and high piezoelectric properties (see

Fig. 5.37).

5 Additive Processes for Piezoelectric Materials 309

Fig. 5.35 Residual stress as a

function of ﬁlm thickness for

a sputtered AlN ﬁlm on

Si/SiO

/Ti/Pt [90] (Reprinted

with permission. Copyright

2004 American Vacuum

Society)

Fig. 5.36 Residual stress of AlN deposited on Si as a function of substrate bias and substrate

5.2.4 Device Examples

This section provides a sampling of the capabilities of polar thin ﬁlms in MEMS

and how they have been implemented in devices. Examples highlighting some

recent successes are presented, followed by a full case study in AlN contour-mode

resonators and ﬁlters.

The cell phone industry has been driving RF ﬁlter innovation in recent years, pro-

viding challenging requirements for ﬁlter selectivity, power handling, temperature

insensitivity, and ﬁlter ﬁgure of merit (k

eff

Q)[124]. Film (or “free-standing”) bulk

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

Fig. 5.37 The interplay

between the

electromechanical coupling

factor and residual stress at

different deposition pressures

for sputtered AlN ﬁlms on Si

and Si/SiO

[91] (Reprinted

with permission. Copyright

2004 Elsevier)

(a) (b)

Fig. 5.38 Illustration of (a) ﬁlm bulk acoustic resonator (FBAR) structure and (b) solidly mounted

resonator (SMR). B oth devices are typically realized with AlN thin ﬁlms

acoustic resonator and ﬁlter technology now represents a signiﬁcant commercial

success story in RF MEMS. AlN FBAR devices feature extensional thickness-mode

thin-ﬁlm piezoelectric membrane resonators that are suspended above a silicon

substrate (see Fig. 5.38). In addition to the free-standing FBAR structures, the

solidly mounted resonator (SMR) device utilizes a series of acoustic impedance

mismatch layers directly below the resonator to retain acoustic energy within the

transducer.

SMR devices tend to be more robust than the FBAR but have inferior RF per-

formance due to degradation in mechanical quality factor with enhanced acoustic

energy radiation into the substrate. Both SMR and FBAR devices have a number

of advantages over competing technologies such as surface acoustic wave devices

(SAWs), including superior ﬁgure of merit, power handling, electrostatic discharge

insensitivity, and size [125, 126]. Avago Technologies, formally Agilent’s semicon-

ductor products group, has sold hundreds of millions of FBAR-based units including

5 Additive Processes for Piezoelectric Materials 311

(a) (b)

Fig. 5.39 (a) Micrograph of an example FBAR ﬁlter from Avago Technologies Inc. The active

IEEE) and (b) Micrograph of singulated packaged FBAR ﬁlters from Avago [127] (Reprinted with

ﬁlters, duplexers, and multiplexers. The Avago FBAR device (see Fig. 5.39) fab-

rication begins with etching and backﬁlling portions of the silicon substrate with

a sacriﬁcial oxide [125]. Once the wafer is polished, a Mo ﬁlm is deposited and

patterned as an electrode. The AlN ﬁlm is then sputter-deposited and patterned

and followed by the deposition and patterning of the Mo top electrode. Following

additional processing, the structures are released with HF. The Avago devices are

hermetically sealed in a low-cost wafer-level package [127]. The patented process

bonds the FBAR s ilicon substrate with a second silicon wafer housing vias, seals,

and alignment marks. The cap wafer is subsequently thinned with conventional

wafer-thinning. DRIE is then used to open access to the FBAR contacts.

Despite the advantages and success of FBAR devices, there remains a need for

integrating small-scale and low-power multiple-frequency and multiple-standard RF

devices [128]. Cost-effectively integrating large numbers of multiple frequency ﬁl-

ters appears difﬁcult for FBAR devices as they rely on extensional thickness modes

to determine center frequency. In contrast, contour-mode resonators have center fre-

quencies determined by their lithographically deﬁnable lateral dimensions. Great

progress has been made during the last few years in both ZnO and AlN contour

mode resonators [129–132].

High performance extensional contour mode resonators have been demonstrated

in ZnO (see Fig. 5.40). Referring to the research in [ 129], the devices make use

of high-quality single crystal silicon as the bulk of the resonator to provide low

volumetric mechanical losses. Loaded quality factors in vacuum have been demon-

strated with values as high as 11,800 with motional resistances of 1600 . Motional

resistances as low as 600  have also been shown, however, with lower Qs. The

resonators are fabricated with a three-mask process that begins with patterning an

SOI-based single crystal silicon resonator. ZnO ﬁlms are then deposited on the

Cr–Au electroded resonator structures. After an Al top electrode is deposited and

patterned, the ZnO is ﬁnally patterned to provide bottom electrode access.

A number of transduction techniques have been utilized in the actuation of

RF MEMS switches, including electrostatic, electromagnetic, thermomechanical,

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

Fig. 5.40 (a) Illustration and (b) SEM of a ZnO on silicon lateral bulk acoustic resonator [129]

and piezoelectric. The interest in radio and cell-phone frequency compatible AlN-

based ﬁlters has motivated the development of integrated piezoelectric RF MEMS

switches. AlN piezoelectric RF MEMS switches have recently been demonstrated

with good RF performance at these frequencies of interest [133]. These devices

feature an innovative approach to address residual stress deformation, temperature

sensitivity, and the modest piezoelectric coefﬁcients of AlN. A symmetric “dual-

beam” design features pairs of actuators t hat experience similar residual stress and

temperature-induced deformations that minimize variations in the open-state contact

gap despite the mechanical ﬂuctuations (see Fig. 5.41). The device shows immu-

nity to residual stresses, fast switching speeds (∼2 μs), moderate actuation voltage

(∼20 V), and good low frequency isolation (>26 dB) and insertion loss (<0.7 dB) at

2 GHz. These devices have also been successfully cofabricated on the same wafer

as contour-mode AlN RF MEMS resonators. The device fabrication leverages the

process used for AlN contour-mode resonators. Once these features are deﬁned,

Fig. 5.41 (a) SEM image of AlN RF MEMS switch with inset indicating the nanogap contact

location and (b) illustration of principle of operation of “dual-beam” architecture [133] (Reprinted

5 Additive Processes for Piezoelectric Materials 313

including multiple AlN and Pt electrode deposition and patterning steps, an amor-

phous silicon sacriﬁcial layer is deposited and patterned with liftoff. This is followed

by the deposition and patterning of a thick electroplated Au layer to provide RF

transmission lines and unimorph actuator features. The switch is released using a

xenon diﬂuoride dry release to complete the process.

A number of groups are investigating mechanical logic technologies to address

the limitations imposed in CMOS by gate oxide leakage. Static power consump-

tion now rivals dynamic power consumption in CMOS and presents a signiﬁcant

challenge to the continued scaling of CMOS logic devices. Motivated by low-power

nanomechanical logic applications, AlN nanoactuators have recently been demon-

strated [134]. AlN thin ﬁlms with a thickness of 100 nm were implemented in

nanoscale bimorph cantilevered actuators. Displacements of 40 nm were achieved

at 2 V in 18 μm long cantilever devices. The actuators were fabricated using a

ﬁve-mask post-CMOS compatible process similar to the process outlined for the

fabrication of the AlN RF MEMS switch discussed above. A key difference in the

process, however, is the use of a focused ion beam (FIB) tool. The nearly complete

suspended clamped–clamped AlN beam is severed at one anchor with the FIB to

create the cantilevered device seen in Fig. 5.42.

Fig. 5.42 SEM image of

AlN cantilevered bimorph

nanoactuator [134]

(Reprinted with permission.

5.2.5 Case Study

As discussed earlier, there is great interest in developing highly integrated multi-

ple frequency arrays of ﬁlters and resonators for RF applications. Recent progress

in both AlN contour-mode ﬁlters and RF MEMS switches suggests this technol-

ogy could become a viable commercial product [128, 132, 133, 135]. The devices

are generally near-symmetric Pt/AlN/Al composite structures with AlN thicknesses

typically in excess of a micron [131]. The contour vibrational modes are excited