micromachines
Review

Techniques and Considerationsin the Microfabrication of Parylene C
Microelectromechanical Systems
Jessica Ortigoza-Diaz 1 , Kee Scholten 1 ID , Christopher Larson 1 ID , Angelica Cobo 1 ,Trevor Hudson 1 , James Yoo 1 ID , Alex Baldwin 1 ID , Ahuva Weltman Hirschberg 1
and Ellis Meng 1,2, *1

2*

Department of Biomedical Engineering, University of Southern California, Los Angeles, CA 90089, USA;jortigoz@usc.edu (J.O.-D.); kscholte@usc.edu (K.S.); larsonce@usc.edu (C.L.); acobo001@gmail.com (A.C.);
tqhudson@usc.edu (T.H.); james.yoo@usc.edu (J.Y.); abbaldwi@usc.edu (A.B.);
aweltman1@gmail.com (A.W.H.)
Ming Hsieh Department of Electrical Engineering, University of Southern California,
Los Angeles, CA 90089, USA
Correspondence: ellis.meng@usc.edu; Tel.: +1-213-740-6952

Received: 31 July 2018; Accepted: 18 August 2018; Published: 22 August 2018


Abstract: Parylene C is a promising material for constructing flexible, biocompatible and corrosionresistant microelectromechanical systems (MEMS) devices. Historically, Parylene C has been
employed as an encapsulation material for medical implants, such as stents and pacemakers, due toits strong barrier properties and biocompatibility. In the past few decades, the adaptation of planar
microfabrication processes to thin film Parylene C has encouraged its use as an insulator, structuraland substrate material for MEMS and other microelectronic devices. However, Parylene C presents
unique challenges during microfabrication and during use with liquids, especially for flexible, thinfilm electronic devices. In particular, the flexibility and low thermal budget of Parylene C require
modification of the fabrication techniques inherited from silicon MEMS, and poor adhesion at
Parylene-Parylene and Parylene-metal interfaces causes device failure under prolonged use in wet
environments. Here, we discuss in detail the promises and challenges inherent to Parylene C andpresent our experience in developing thin-film Parylene MEMS devices.
Keywords: Parylene; microfabrication; MEMS

1. Introduction
1.1. History and Types of Parylene
Parylene is the trade name for poly-(para-xylylene), a class of semicrystalline, hydrophobicpolymers which can be deposited as thin, conformal, pinhole-free films using chemical vapor
deposition (CVD). Parylene was discovered by Michael Mojzesz Szwarc in the late 1940s but was notcommercialized until 1965 following the development of the Gorham deposition process at Union
Carbide [1,2]. Types of Parylene that are commonly available include Parylene N, Parylene C, Parylene
D and Parylene HT (Figure 1). A comparison of the properties of these films can be found in [3].
Parylene N is composed of an aromatic ring with attached methylene groups [4]; it has been used as adielectric and has the slowest deposition rate of the Parylenes [5]. Parylene C features a single chlorine
atom on its benzene ring and is recognized for its chemical inertness, electrical resistivity, low moisturepermeability and proven biocompatibility. Parylene D has two chlorine atoms, resulting in similar
properties to Parylene C with slightly higher temperature resistance. Parylene HT is a fluorinatedMicromachines 2018, 9, 422; doi:10.3390/mi9090422

www.mdpi.com/journal/micromachines

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variant of the Parylene N polymer and is marked by high temperature stability [6]. The two moststability
The twotypes
mostincommon
Parylenetypes inarecommercial
are Parylene
NC
and
C,
common[6].
Parylenecommercial
applications
Parylene applications
N and C, whereas
Paryleneis thewhereasParylene
C is themost common
material
encountered in MEMS devices.
most common
material
encounteredin MEMSdevices.

Figure1.1.Chemical
Chemicalstructure
structureofofParylene
Parylenetypes.
types.
FigureParylene
ParyleneC,
C,referredreferredfrom
fromhere
hereononas
asParylene,
Parylene,isisaaUS
USPharmacopeia
PharmacopeiaclassclassVI
VImaterialmaterial[7],[7],having
havingthehighestbiocompatibility
certificationforaplastic
material.
Forseveraldecades,thin
the highest biocompatibility certification for a plastic material. For several decades, thinParylene
Parylenecoatings
coatings were
were used
used toto createcreate waterproof
waterproof insulation
insulation forfor electronics
electronics intended
intended forfor useuse inin harsh
harsh
environments,acategorythat
increasingly
includesbiomedical
implants.
Parylene
exhibits
environments, a category that increasingly includes biomedical implants. Parylene exhibitslow
low
intrinsic
inaddition
additiontotooptical
opticaltransparency,
mechanicalflexibility
and compatibility
withintrinsic stressstress intransparency,
mechanicalflexibility
and compatibility
with several
severalstandard
micromachining
processes
Parylenebe usedin combination
standard
micromachining
processes[8–11].[8–11].
WhileWhile
Parylenecan becan
usedin combination
with with
rigid
rigid
substrates
suchassiliconandglass,
ithas
been
increasingly
utilizedasaflexible
structural
substrates such as silicon and glass, it has been increasingly utilized as a flexible structural materialmaterialin thefieldgrowingfield of polymer-based
biomedical microelectromechanicalsystemsin the growingof polymer-based
biomedical microelectromechanicalsystems (bioMEMS).
As a(bioMEMS).
As
adynamic
polymermaterial,
Parylene
presents
uniquechallenges
duringdynamic polymer material, Parylene presents unique challenges during microfabrication and use
microfabrication
use whichwhich are furtherand
explored
here.are further explored here.
1.2.
1.2.Thin-FilmThin-FilmParylene
ParyleneDevice
DeviceMicrofabrication
Microfabrication
Parylene-based
are preparedpreparedthroughthrougha acombination
combination
microfabrication
processes
Parylene-based bioMEMS
bioMEMS areof ofmicrofabrication
processes
andandcommonlyconsistof a simple
sandwicha baseoflayerof Parylene,
a thinlayer ofcommonlyconsistof a simple
sandwich
design,design,with a withbase layer
Parylene,a thin layerof patterned
patterned
metal traces
defining
other components,
a top
layer of Parylene.
Figure 2adepicts
metal defining
andtraces
otherandcomponents,
and a topand
layer
of Parylene.
Figure 2 depicts
flexible
adeviceflexiblein which serves
Paryleneservesboth the material
structural
material
andHowever,insulator.devices
However,in device
which Parylene
as boththeasstructuralandinsulator.
maydevicesalso beonsupported
on standard
rigid
substrates
suchsilicon
as glass
andflexibility
silicon when
flexibility
also be may
supported
standard rigid
substrates
such
as glass andwhen
is notrequired.
isRegardlessnot required.
Regardlessof thefinal format, microfabrication
requiresthat Parylene
be supportedof thefinal format,
microfabrication
requires that Parylene
be supported
by rigid
substratebyrigid fabrication,
substrate during
fabrication,withoptional
releaseoftowards
a free film
devicetowards
the end of
duringwithoptional
release
of
a
free
film
device
the
end
of
the
process.
Micromachines 2018, 9, x FOR PEER REVIEW
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the process.
Devices are realized using a combination of three process categories: additive processes(deposition), subtractive processes (etching) and patterning (e.g., photolithography). Parylene isdeposited exclusively through a highly conformal CVD process at room temperature, with typicaldeposition rates on the order of ~2 μm/h. Other deposition methods are critical for polymer MEMS
processing; spin coating is used to deposit photo-patternable polymer layers (photoresist) for use as
an etch mask or shadow mask, or as a sacrificial layer, while physical vapor deposition (PVD)
methods including evaporation and sputtering are used for metal deposition. Etching processes arelimited to dry methods due to Parylene’s high chemical inertness. Reactive ion etching (RIE) anddeep reactive ion etching (DRIE) are oxygen plasma-based techniques commonly used to etch
Parylene.
Parylene devices are predominantly patterned using standard UV lithography processes,
Figure 2. Cross section of a typical device showing insulated and exposed metal features, such as
Figure recently,
2. Cross sectionof a typicaldevicecan
showing
insulated andexposedmetal beamfeatures,such as traceswith
and, morenanoscalefeaturesbe patternedusingelectronlithography
tracesand electrodes,
respectively.
andelectrodes,
respectively.
appropriate protective layers [12] to prevent beam damage.
1.3. State-of-the-Art Parylene-Based Devices
Devices are realized using a combination of three process categories: additive processes
Parylenesubtractive
is used extensively
to coatprintedcircuitboards,(e.g.,wires,photolithography).
MEMS devices, andParylene
biomedical(deposition),processes(etching)
andpatterningis
implants, andless commonly
structuralor substratematerialatfor
electronicand MEMS
deposited
exclusively
throughasa ahighly
conformal
CVD processroom
temperature,
withdevices.
typical
Still, over thelastonseveralyears,
devices
fordepositiona large variety
of applications
been describeddeposition
rates
the orderof many~2 µm/h.
Othermethods
are criticalhave
for polymer
MEMS
in
the
literaturefeaturing
Paryleneaseitherthestructuralor
substratematerial(Figure3). useParyleneprocessing; spin coating is used to deposit photo-patternable polymer layers (photoresist) foras an
has been used as a flexible coiled cable to connect medical implants to external circuitry [13–15]. Radio
frequency powered coils were fabricated with Parylene for wireless transmission (Figure 3a) [16,17]
and more recently a new method to wirelessly transduce electrochemical impedance was developed
using Parylene coils [18]. Parylene microsensors (Figure 3b) were developed to measure intraocularand intracranial pressures [19–21] and Parylene thermal sensors were created to detect small flows[22–25]. Other devices include neurocages for in vitro neural network study [26–28], bellows for drug

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etch mask or shadow mask, or as a sacrificial layer, while physical vapor deposition (PVD) methods
including evaporation and sputtering are used for metal deposition. Etching processes are limited todry methods due to Parylene’s high chemical inertness. Reactive ion etching (RIE) and deep reactive
ion etching (DRIE) are oxygen plasma-based techniques commonly used to etch Parylene. Parylene
devices are predominantly patterned using standard UV lithography processes, and, more recently,
nanoscale
be ofpatterned
lithography
withappropriate
protective
Figurefeatures
2. Cross cansectiona typical using
device electron
showing beam
insulatedand exposedmetalfeatures, suchaslayerstraces[12] andto prevent
beam
damage.
electrodes,
respectively.
1.3.
1.3. State-of-the-Art
State-of-the-Art Parylene-Based
Parylene-Based Devices
Devices
Parylene
Parylene isis usedused extensively
extensively toto coatcoat printed
printed circuit
circuit boards,
boards, wires,wires, MEMS
MEMS devices,
devices, andand biomedical
biomedical
implants,
andlesscommonlyasastructuralor
substratematerialforelectronicandMEMS
devices.
Still,
implants, and less commonly as a structural or substrate material for electronic and MEMS
devices.
overyears,years,
manymany
devices
for a largeof applications
have beenin theStill, theoverlasttheseveral
last several
devices
for a variety
large variety
of applications
havedescribedbeen described
literaturefeaturing
Paryleneaseitherthestructuralor
substratematerial(Figure3).
Parylenehas
been
in the literature featuring Parylene as either the structural or substrate material (Figure 3). Parylene
used
as
a
flexiblecoiledcable
toconnect
medicalimplants
toexternal
circuitry[13–15].
Radio
frequency
has been used as a flexible coiled cable to connect medical implants to external circuitry [13–15]. Radiopoweredwere fabricatedParylenewirelessfor
transmission(Figure 3a)(Figure[16,17] 3a)and[16,17]
more
frequencycoils
poweredcoils were withfabricatedwithfor
Parylenewireless transmission
recentlyanewmethodtowirelesslytransduce
electrochemical
impedancewasdeveloped
using
and more recently a new method to wirelessly transduce electrochemical impedance was developed
Parylenecoils [18].
Parylenemicrosensors(Figure3b)3b)
weredeveloped
using Parylene
coils [18].
Parylenemicrosensors(Figureweredevelopedtotomeasure
measure intraocular
intraocularandintracranial pressurespressures[19–21][19–21]andand
Parylene
thermalsensors
createdto detectand intracranial
Parylene
thermalsensors
werewerecreatedto detectsmall small
flows
flows
[22–25].
Otherdevices
includeneurocages
for
in
vitro
neuralnetwork
study[26–28],bellows[22–25]. Other devices include neurocages for in vitro neural network study [26–28], bellows for drug
fordrug [29–31],
deliveryan[29–31],
an electrochemical
patency
sensor [32], devices
microfluidic
devices[7] anddeliveryelectrochemical
patency sensor[32], microfluidic[7] andelectrothermal
electrothermal
valves
[33].
In
the
field
of
neuralprostheses
Parylene
was
used
to
createbothpenetrating
valves [33]. In the field of neural prostheses Parylene was used to create both penetrating (Figure 3c)(Figureand non-penetrating
microelectrodearrays[40–42]. Microfluidic
channels
were[34–39] 3c)
and[34–39]
non-penetrating
microelectrodearrays [40–42].
Microfluidic
channels were
integrated
integrated
intoneural
Paryleneneuralprobesto injectdrugs[43–45]. Parylene-based
cuffelectrodes,
into Parylene
probesto injectdrugs[43–45].
Parylene-based
cuff electrodes,
which
recordwhich
recordneuralactivity
withinperipheral
nerves,were
alsodeveloped[46–48].
Paryleneneural activity within peripheral nerves, were also developed [46–48]. Parylene spinalspinal
cord
cord
stimulators
also beenrealizedhigh-density
electrode[49,50].
stimulators
havehavealso beenrealized
withwith
high-density
electrodesites sites[49,50].

Figure 3.
Wirelesscoilscoils(Reprinted
from from
Reference[16] withfrom Elsevier);(b) flow,
Figure3. (a)(a)Wireless(Reprinted
Reference[16]permission
with permission
from Elsevier);pressureandpatency
sensorstomonitorhydrocephalus
treatment(©2016IEEE.
Reprinted,
with(b) flow, pressure and patency sensors to monitor hydrocephalus treatment (©2016 IEEE. Reprinted,
permission,from from
Reference[23]);[23]);(c) hippocampal
neuralprobeprobearrayarray(©2017IEEE.IEEE.
Reprinted,
with
with
permission,
Reference(c) hippocampal
neural(©2017
Reprinted,
permission,
fromReference[37]);
and(d)retinalprosthesis
thatmatchesthecurvature
of
the
eye
with permission, from Reference [37]); and (d) retinal prosthesis that matches the curvature of the eye(Reprinted
from
Reference[49]
withpermission
fromElsevier).
Paryleneistransparent
andthe(Reprinted from Reference [49] with permission from Elsevier). Parylene is transparent and the opaque
opaquefeaturesaremetal.
features are metal.

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4 of 25Parylene isalso aa common
common materialmaterial for
Paryleneis alsofor sensory
sensory neuralneural prosthetics,
prosthetics, such
such asas retinal
retinal implants.
implants.
High-density electrodearrays were
were fabricatedfabricated on
on Parylene
Parylene thatthat waswas subsequently
subsequently thermoformed
thermoformed totoHigh-density
electrode arrays
match
the
curvature
of
the
eye
(Figure3d)[8,49,51].
An
origami-likedevicewasfabricatedtomatch
match the curvature of the eye (Figure 3d) [8,49,51]. An origami-like device was fabricated to matchthe curvature
curvature ofof thethe eyeeye without
without thethe useuse ofof heatheat [52].
[52]. Cochlear
arrays, which
which must
must conformconformtheCochlear electrodeelectrode arrays,
to theof theby exploiting
Parylene’s properties
properties as
as aa thin
thin
tothe complex
complex anatomy
anatomy of
the inner
inner ear,
ear, were
were developed
developed by
exploiting Parylene’s
andflexible
substrate[53].
and flexible substrate [53].
Despite growing
Despitegrowing interest,
interest, many
many researchers
researchers reportreport difficulties
difficulties when
when processing
processing Parylene
Parylene MEMS,
MEMS,
as
well
as
variousmodesofmaterial
anddevicefailure.
These
problems
largely
stem
from
a
of
as well as various modes of material and device failure. These problems largely stem fromlacka lack
well-defined
protocolsforthemachining,use
andhandlingofParylene-based
devices.
Commonof well-defined protocols for the machining, use and handling of Parylene-based devices. Common
processing techniques
techniques developed
developed forfor semiconductor
semiconductor materials
materials andand glassglass areare often
often incompatible
incompatible withwithprocessingorganic
polymers,owingtoParylene’s
limitedthermalbudget,gas
permeability,lowmechanicalorganic polymers, owing to Parylene’s limited thermal budget, gas permeability, low mechanicalstrength andand uniqueunique chemicalchemical properties.
properties. While
While many
many ofare surmountable,
surmountable, solutions
solutionsstrengthof these
these obstacles
obstacles areare rarely
rarelydiscussed
discussed
in literature.
the literature.
Here
we present
a compilationcommonencounteredchallenges
arein theHere wepresenta compilationof commonofchallenges
encounteredduring the construction
Parylene
MEMS
and a descriptionof current
best practices
toduringthe constructionof ParyleneofMEMS
anda descriptionof current
best practices
to avoid
avoidissues.
these issues.
these
2. Challenges
2.1.
Thermal Budget
Budget2.1. ThermalOne
One ofof thethe most
most persistentpersistent challenges
challenges ofof Parylene
Parylene isis itsits limited
limited thermal
thermal budget,
budget, aa consequenceconsequenceof
its
thermoplastic
nature;inatmosphere
Paryleneissubjecttooxidationattemperaturesof its thermoplastic nature; in atmosphere Parylene is subject to oxidation at temperatures greater
greater◦ C, glass transition temperature around 90 ◦ C and a melting temperature at 290 ◦ C [10].
thanthan 100
100 °C,glass transition temperature around 90 °C and a melting temperature at 290 °C [10].
These
These temperaturestemperatures areare commonlycommonly encounteredencountered andand eveneven surpassed
surpassed during
during standard
standard siliconsiliconmicromachining
andelectrical
packaging.
For
example,duringphotolithography,
photoresist
mustbemicromachining and electrical packaging. For example, during photolithography, photoresist must◦
soft-baked
at elevatedtemperatures(100–120C) to °C)
removeresidualresidual
solvent andexothermic
reactionsbe soft-baked
at elevatedtemperatures(100–120to removesolvent
and exothermic◦ C [54]. Soldering, plasma
duringtheUV
activationoftheresistcangenerate
temperatures
of
up
to
200
reactions during the UV activation of the resist can generate temperatures of up to 200 °C [54].
etching,
PVD
and othermethods
commonmicromachining
require temperatures
which canrequirecause
Soldering,plasmaetching,
PVD
and toothermethods common
to micromachining
Paryleneto burn,
bubble,or crack(Figuretemperatureswhich
can cause
Parylene
to4).
burn, bubble, or crack (Figure 4).

Figure 4.
4. Images
Images ofof Parylene
Parylene damage
damage due
due toto highhigh heat
heat oror radiation
radiation exposure.
exposure. (a)(a) Cracked
Cracked Parylene
Parylene
Figureafter
development
andsoftbaking
at
115
°C
for
3
min;
and
(b)
air
bubblesappearedunder◦
after development and soft baking at 115 C for 3 min; and (b) air bubbles appeared under thethe film,
film,
during
hard
bake
at
90
°C
for
15
min,
in
regionspreviously
exposedtoUVlightduringlithography.
during hard bake at 90 ◦ C for 15 min, in regions previously exposed to UV light during lithography.

Thermal annealing is a common treatment for improving adhesion between Parylene-Parylene
Thermal annealing is a common treatment for improving adhesion between Parylene-Paryleneinterfaces [55,56]. This process requires high temperature (>200 °C)
compared to Parylene’s glass
interfaces [55,56]. This process requires high temperature (>200 ◦ C) compared to Parylene’s glasstransition temperature and must be performed under vacuum to avoid the effects of oxidationtransition temperature and must be performed under vacuum to avoid the effects of oxidation(browning, wrinkling and becoming brittle) as shown in Figure 5.
(browning, wrinkling and becoming brittle) as shown in Figure 5.

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5 of 25(b)(c)

Figure5. Comparison
Comparison ofand under
under intact
intactFigure 5.
of thermal
thermal annealed
annealed devices
devices inin thethe presence
presence ofof oxygenoxygen (a,b)(a,b) and◦vacuum(c).
Deviceswereheated
at
200
°C
for
48
h.
vacuum (c). Devices were heated at 200 C for 48 h.

2.2.
Parylene Films
Films
2.2. Water
Water Diffusion
Diffusion throughthrough Parylene
Parylene
employed as
as encapsulation
encapsulation for
medical implants,
implants, suchas stents,
stents, pacemakers
pacemakers andandParylene isis employed
for medicalsuch asneuralprobes [57,58][57,58] due
due totoits
itsstrongstrongwater
waterbarrier
barrierproperties.
properties.However,
However,like
likeallallpolymers,
polymers,
Paryleneneural probes
Parylene
is
is
in
fact
permeable
tomoisture
and
gases
[59].
Both
water
and
ions
in
solution(i.e.,
salt)candiffusein fact permeable to moisture and gases [59]. Both water and ions in solution (i.e., salt) can diffuse
throughthin layers
layers ofof Parylene
Parylene inin lessless time
time than
than thethe intended
intended use
use duration.
duration. For
For aa Parylene-based
Parylene-based
through thinbioMEMSor
microdevice
withinsulation
layersonlyafew
microns
thick,
limiting
permeationisbioMEMS or microdevice with insulation layers only a few microns thick, limiting permeationis criticalcriticaltoprevent
electricalshorts,corrosionofencapsulated
components
andcatastrophic
failure.
to prevent electrical shorts, corrosion of encapsulated components and catastrophic failure. The mostThe
most method
effectiveat
methodat preventing
water permeationis theof thermal
annealing
to increase
effective
preventing
water permeationis the use
ofuse
thermalannealing
to increasethethecrystallinity,
thoughthis
onlyservestoslow
permeation,
notprevent
itoutright[56,60].
crystallinity, though this only serves to slow permeation, not prevent it outright [56,60]. Unfortunately,
Unfortunately,
therequantifying
is scarce datascale of particularly
the problem,forparticularly
for theveryfilms
thin
there is scarce datathequantifying
scale of thethe
problem,the very thin
(~µm)
(~μm)
used in Parylene-based
microdevices.
Below
weobservations
describe someobservations andused infilms
Parylene-based
microdevices. Below
we describe
someand measurements
wemeasurements
wehavecollected
onthe
topic.
have collected on the topic.
2.2.1.
Permeability
2.2.1. Water
Water Permeability
Publishedvapor transmission
transmission rate
Parylene canvary quite
quite
Published values
values forfor waterwater vapor
rate (WVTR)(WVTR) inin Parylene
can vary
substantiallybetweensources.
sources.
Specialty
Coating
Systems
[61]
and
Para
[62], two
substantially between
Specialty
Coating
Systems(SCS) (SCS)[61] andPara
Tech[62],Techtwo prominent
2·day,
2
prominent
vendorsfor
Parylenecoatings,
publishWVTR
valuesof
0.0830and
0.0550g·mm/mvendors for Parylene coatings, publish WVTR values of 0.0830 and 0.0550 g·mm/m ·day, respectively,◦
respectively,
at 37et°C.
et al. [63]watermeasuredvapor permeationin thin, small-area
Paryleneat 37 C. Menon
al.Menon[63] measured
vaporwater
permeationin thin, small-area
Parylene structures
structures
for bothandannealed
and un-annealed
films;
WVTR
values from
calculated
fromreportedfor both annealed
un-annealed
films; WVTR
valuescalculated
reported
permeation
permeation
measurements
arelisted
in
Table
1.
WVTR
valuesmaydifferbased
ondeposition
measurements are listed in Table 1. WVTR values may differ based on deposition parameters and may
parameters
andmaybethickness
non-linear
for very thinthroughfilms, astransmission
be non-linear
with
film
forwithveryfilm
thin thickness
films, as transmission
defectsor large throughpores indefects
or
large
pores
in
thin-films
maydominateoverdiffusion-driven
transmission.
thin-films may dominate over diffusion-driven transmission.
In
Table 1,
1, we
we provide
provide ourour own
own WVTRWVTR measurements
measurements forfor annealed
annealed andand unannealed
unannealed films
of
In Tablefilms ofvarying thicknesses.
thicknesses. Films
varying
Films were
were deposited
deposited on
on planar
planar siliconsilicon wafers
wafers using
using aa PDS
PDS LabcoterLabcoter 20102010 (SCS,(SCS,◦
Indianapolis,IN);annealedfilms
were
heated
for
48
h
at
200
°C
under
vacuum.
The
measurements
Indianapolis, IN); annealed films were heated for 48 h at 200 C under vacuum. The measurements
were based
based onon thethe beaker
beaker method
method forfor determination
determination ofof waterwater vapor
vapor permeability
permeability [64].
[64]. Glass
Glass beakers
werebeakers
with ~40~40 mLwithmL ofof deionizeddeionized water
water were
were sealed
sealed with
with Parylene
Parylene (1.7
(1.7 cmcm diameter
diameter circular
circular aperture)
aperture) using
using
marineepoxy.
The
beakers,
kept
at
room
temperatureandapproximately
33%
relative
humidity,marine epoxy. The beakers, kept at room temperature and approximately 33% relative humidity,
were weighedweighed weekly
weekly forfor 22 months
months (mean(mean values
values recorded
recorded inin Table
Table1).
1).
were
Notably,
ourvaluesdiffer
from
Menon
et
al.
despitesimilartemperatures
andandhumidity.
In
Notably, our values differ from Menon et al. despite similar
temperatures
humidity.
agreement
with
Menon
et
al.
we
see
a
significant
decreasein
WVTR
valuesafter
annealingfilms;
forIn agreement with Menon et al. we see a significant decrease in WVTR values after annealing films;
10
15 μm
thick
films
wewe
seeseea 25%
for and
10 and
15 µm
thick
films
a 25%decrease
decreaseininWVTR,
WVTR,iningeneralgeneralagreementagreement with
with reports
reports andandanecdotes
fromresearchers
notingadecreaseinwater
permeation
andanincreasein
effective
lifetime
anecdotes from researchers noting a decrease in water permeation and an increase in effective lifetimeforannealed Parylene/Parylene-coated
Parylene/Parylene-coated devices.
thefor annealed
devices.There
Therewaswasno
no significant
significant decrease
decrease in
in WVTRWVTR forfor theannealedfilm, which
which indicate
indicate thatthat films
films at
at thisthis thickness
thickness have
have permeation
permeation driven
driven by
by defects
defects
annealed 55 μmµm film,
ratherthan
diffusion
throughthebulk.
rather than diffusion through the bulk.

Micromachines 2018, 9, 422

6 of 25

Micromachines
2018, 9,x FORtransmission
PEER REVIEWrate (WVTR) measurements for different un-annealed and annealed
6 of 25
Table 1. Water
vapor

Parylene film thickness.
Table 1. Water vapor transmission rate (WVTR) measurements for different un-annealed andannealed Parylene film thickness.
Thickness
Mean WVTR
Standard
Temperature and Relative
Reference

Reference

Un-annealed
This work
This work

Un-annealed

Annealed
Annealed

Specialty Coating Systems
SpecialtyPara
Coating
Systems
Tech
Para Tech
Un-annealed
Menon et al., Un-annealed
Menon et al.,Annealed
2009
2009
Annealed(µm)

Thickness
5
(µm)
510
15
10
155
510
15
10
15-9
9
9
9

(g·mm/m2 ·day)

Mean WVTR
2·day)
0.0194(g·mm/m
0.01760.01940.01850.01760.01850.01850.01850.01280.01390.01280.01390.08300.08300.05500.05500.05470.05470.02760.0276

Error

Standard
0.0012Error
0.00100.00120.00250.00100.00250.00140.00140.00100.00240.00100.0024 - -Humidity (Mean ± SE)

Temperature and Relative Humidity
20.6 ±(Mean
0.14 ◦±C,SE)
34 ± 3%◦ C, 32 ± 3%20.7±
0.13 °C,
20.6 ± 0.1434 ± 3%◦ C, 33 ± 3%20.8
0.12 °C,
20.7±± 0.1332 ± 3%◦ C,3320.8±± 0.123%3%
20.6
0.14 °C,
33±±◦ C,3320.6±± 0.143%3%
20.7
0.14 °C,
34±±◦ C, 35 ± 3%20.7
0.13 °C,
20.7±± 0.1434 ± 3%◦ C,± 90%0.13 °C,
35 ±F1249
3%3720.7
ASTM
37 ◦°C,37
C, 90%
90%ASTM
ASTMF1249
F1249
37 °C, 90% ASTM F1249

20 ◦ C, 30% ASTM D1653
20 °C, 30% ASTM D1653

2.2.2.
Ion
Permeability
2.2.2.
Ion
Permeability
Paryleneis isknown
barrier properties,
properties,particularly
particularlywhen
when
compared
Parylene
knownforforhaving
havingexcellent
excellent ionic
ionic barriercompared
to to
other
polymers.
tests(ASTM(ASTM
B117-(03)),
reported
no evidenceof corrosion
otherpolymers.Under
Under salt-fog
salt-fog tests
B117-(03)),
SCSSCS
reported
no evidenceof corrosion
or salt or
saltdeposits
deposits
Parylene-coatedPCB
boardsafterof exposure
Mordelt
al. reported
onon
Parylene-coatedPCB
boardsafter
144 144h ofhexposure[61] [61]and andMordelt
et al.etreported
25
25 μmµm thickthicklayers
layersof
ofParylene
Parylenewithstoodwithstood0.9%
0.9%saline
salinesolution
solutiondays
beforebreakdown[65].
forfor
upup
to to
3030
days
beforebreakdown
[65].
Very
layersof Parylene
(<10
μm),
however,susceptible
to ionintrusionoverrelatively
Very
thinthinlayersof Parylene
(<10
µm),
however,
arearestillstillsusceptible
to ionintrusionoverrelatively
short
short
time periods.
InParylene-based
many Parylene-based
microdevices,
intrusiona Parylene
insulation
time
periods.
In many
microdevices,
ionic ionic
intrusion
into intoa Parylene
insulation
layer
can itschangeits dielectric
properties,
thereby increasinglines orcanlayerchangedielectric
properties,
thereby increasing
parasitic parasitic
couplingcoupling
between betweenlines or decreasing
decreasing
shunt
impedance[66,67].
In
thismannerion
permeabilitymayaffectdeviceperformanceshunt impedance [66,67]. In this manner ion permeability may affect device performance even if ionsevenbreach
if ionsthe
neverbreachbarrier.
the Parylene
barrier.that
We theobserved
the useof thermal
annealing
can ofnever
Parylene
We observed
use of that
thermalannealing
can lower
the rate
the rateof iondiffusionacrossa thin Parylene
have notthe
yeteffect.
quantified the effect.
ionlowerdiffusionacrossa thinParylene
memberbut havemember
not yet but
quantified
Delamination/Adhesion
2.3.2.3.
Delamination/Adhesion
Parylenesuffers
suffersfrom
frompoor
poor adhesion
adhesion tosuch
as gold
and andplatinum,aParyleneto itself
itselfandandnoblenoblemetals,
metals,such
as gold
platinum,considerable
drawbackintheimplementationofParyleneforbioMEMS.
The
adhesionofParylenea considerable drawback in the implementation of Parylene for bioMEMS. The adhesion of Parylene
devices, which consist of thin films of Parylene-metal-Parylene sandwiches, is compromised whendevices, which consist of thin films of Parylene-metal-Parylene sandwiches, is compromised when devices
devices are soaked in wet environments [58,60,66,68,69]. Weak adhesion can accelerate catastrophic
are soaked in wet environments [58,60,66,68,69]. Weak adhesion can accelerate catastrophic failure of
failure of Parylene devices; as Parylene films lift off a substrate, voids form in which water vapor canParylene devices; as Parylene films lift off a substrate, voids form in which water vapor can condense,
condense, creating continuous paths of solution that create electrical shorts and drive further
creating continuous paths of solution that create electrical shorts and drive further delamination (Figure 6).
delamination (Figure 6). Several strategies were investigated to improve adhesion of Parylene to
Severalstrategies
investigatedto improveadhesion
Parylene tosurface
substrates
such as siliconandsubstrates
such wereas silicon
and glass,includingmelting,ofanchoring,roughening,thermalglass,
includingmelting,anchoring,
surfaceroughening,thermalannealing,surface
plasmatreatment
andannealing, surface plasma treatment and the inclusion of chemical layers, such as silane A-174 andtheplasmainclusionofchemicallayers,suchassilaneA-174
andplasmapolymerized
adhesionlayers[70–72]polymerized adhesion layers [70–72] but many of these techniques are not applicable orbut
manyof thesefortechniquesapplicableor calibrated
for adhesionof Parylene
Parylene,or thin-film
calibrated
adhesionare
ofnot
Paryleneto Parylene,or thin-film
metalsused in topolymerMEMS
andmetalsusedinpolymerMEMS
andflexibleelectronics.
flexible electronics.

Figure 6. Parylene devices where delamination was noticeable on the macro scale. (a) Parylene layers
Figure 6. Parylene devices where delamination was noticeable on the macro scale. (a) Parylene layers
visibly split from each other; (b) detachment of the top Parylene layer from the metal and bottom
visibly split from each other; (b) detachment of the top Parylene layer from the metal and bottom
Parylene layers; and (c) electrodes and metal traces began to move or slide around between the
Parylene layers; and (c) electrodes and metal traces began to move or slide around between the
Parylene layers.
Parylene layers.

Micromachines 2018, 9, 422

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Micromachines
9, x FORtwo
PEERsurfaces
REVIEW is achieved through chemical and physical interventions.
7 ofAdhesion2018,
between
In25thecase of Parylene-Parylene interfaces, adhesion is dominated by physical adsorption, whereas for
Adhesion between two surfaces is achieved through chemical and physical interventions. In the
Parylene-metal interfaces, adhesion is typically a combination of hydrogen bonding and Van der Waals
case of Parylene-Parylene interfaces, adhesion is dominated by physical adsorption, whereas forforces [56]. Poor adhesion between Parylene and other materials may result from differences in surface
Parylene-metal interfaces, adhesion is typically a combination of hydrogen bonding and Van derenergy at the interface [73] between hydrophobic Parylene and hydrophilic metals, a phenomenon
Waals forces [56]. Poor adhesion between Parylene and other materials may result from differences
exacerbated by surface contamination. The presence of internal stress between layers [60] appears toin surface energy at the interface [73] between hydrophobic Parylene and hydrophilic metals, afurtheraggravate adhesion failure and hasten delamination. Delamination under wet conditions may
phenomenon exacerbated by surface contamination. The presence of internal stress between layers
be [60]induced
by to
water
vapor
condensing
withinvoids
by surface particulatespresent
appears
furtheraggravateadhesion
failureand createdhasten delamination.
Delamination
underduring
wet
CVD
[74],
suggestingthat
surfacecleanlinessiscriticalinmaximizing
adhesion.
Annealingof Parylene
conditions may be induced by water vapor condensing within voids created by surface particulates◦ C under vacuum) improves adhesion. Reports suggest that annealing increases the(48present
h at 200
during
CVD [74], suggesting that surface cleanliness is critical in maximizing adhesion.
entanglement
the polymer
reducing
stressimproves
by recrystallizationof Parylene-Parylene
Annealing ofofParylene(48 hchainsat 200while°C undervacuum)adhesion. Reportssuggest thatinterfaces[57,75].
Interposer
layers havebeen investigatedtoreducing
either modify
surface energy ofannealingincreases
the entanglementof alsothe polymer
chains whilestress theby recrystallizationa coatedmaterial, thereby
improvingadhesion,or alsoto createa barrier against
watervapor,
of Parylene-Paryleneinterfaces[57,75].chemicalInterposer
layers havebeen investigatedto eithermodifyminimizing
intrusionand material,
subsequent
failure[76–78]. Plasma-enhanced
is a amethod
the surfacewaterenergyof a coatedtherebyimprovingchemical adhesion,Parylene
or to create
barrier toagainstadhesion
water vapor,minimizing
water
intrusionand subsequent
failure [76–78].
improveof Parylene
to othersurfacesby cleaning
and modifying
the filmPlasma-enhanced
within the process
Paryleneis aFormethodto improveadhesion
ofto
Paryleneother surfaces
by cleaning
chamber[79].
a detailed
study ofmethods
improvetoadhesionof Parylene
layersandandmodifying
to platinum,
thefilm
withintheprocesschamber[79].
Foradetailedstudyofmethods
toimproveadhesion of
the reader is referred to [56].
Parylenelayers andplatinum,
the reader is referredortoParylene-metal[56].
Delamination
at toeitherthe Parylene-Paryleneinterface can affect device
Delamination
ateithertheParylene-Parylene
orParylene-metalinterfaceaffect device
performance well before catastrophic failure occurs. Many Parylene devices rely canon insulated
tracesperformance
wellbeforecatastrophic
failureoccurs.
Many
Parylenedevices
relyoninsulatedwhich terminate in exposed electrodes; delamination can alter this conductive pathway andtraces
lead towhich terminate in exposed electrodes; delamination can alter this conductive pathway and lead tosignaldrift. These changes can be observed by monitoring the electrical/electrochemical impedance
signal drift. These changes can be observed by monitoring the electrical/electrochemical impedanceof an electrode and trace insulated in Parylene and immersed in water and understood using aof an electrode and trace insulated in Parylene and immersed in water and understood using amodified Randles circuit model [80] (Figure 7). For a perfectly insulated electrode, the circuitmodified Randles circuit model [80] (Figure 7). For a perfectly insulated electrode, the circuit consists
consists of a solution resistance RS , charge transfer resistance Rct and a constant phase elementof a solution resistance RS, charge transfer resistance Rct and a constant phase element Ydl to represent
Ydl to represent the double layer capacitance. Delamination presents an alternate conduction path fromthe double layer capacitance. Delamination presents an alternate conduction path from the electrodethetoelectrodeto thesolution,with Rdelamresistance
representing
resistancethroughand
theconstantinsulation
andelement
constant
the solution,
with
Rdelam representing
throughthe insulation
phase
phase
element
Ydelamthe
representing
capacitancethroughbetween
the insulation
betweenelectrode
trace Aand
Ydelamrepresenting
capacitancethe
throughthe insulationelectrode
trace and
electrolyte.
electrolyte.
A
capacitance
C
is
also
includedtomodelparasitic
capacitances
withinconnecting
wire
capacitance Cwire is also included
to model parasitic capacitances within connecting cables, whichcables,whichcausesa phaseroll-off
at high frequencies.
causesa phaseroll-off
at highfrequencies.

Figure 7. Circuit model of a Parylene-metal-Parylene device with an exposed electrode under chronic
Figure 7. Circuit model of a Parylene-metal-Parylene device with an exposed electrode under chronicsoaking conditions. Rct represents the charge transfer resistance at the exposed electrode surface,soaking conditions. Rct represents the charge transfer resistance at the exposed electrode surface,
while Ydl models the double-layer capacitance at the electrode-electrolyte interface as a constant phase
while Ydl models the double-layer capacitance at the electrode-electrolyte interface as a constantelement. Rdelam and Ydelam represent the resistive and capacitive charge transfer through the Parylene
phase element. Rdelam and Ydelam represent the resistive and capacitive charge transfer through theinsulation; the magnitude of Rdelam decreases as Parylene-Parylene delamination progresses. Rs
Parylene insulation; the magnitude of Rdelam decreases as Parylene-Parylene delamination progresses.
represents solution resistance; Cwire represents parasitic capacitance; WE represents working
Rs represents solution resistance; Cwire represents parasitic capacitance; WE represents working
electrode/exposed electrode; RE represents reference electrode.
electrode/exposed electrode; RE represents reference electrode.

This model was confirmed using electrochemical impedance spectroscopy (EIS) data collectedfrom
platinum
electrodes,
coatedandinsulated withimpedanceParylene Cspectroscopy
films and soaked
37 °C
saline forThismodel wasconfirmed
usingelectrochemical(EIS)indata
collected
from◦2
14 dayselectrodes,(n = 8 electrodes)(Figure8). All withelectrodes
measured× soaked
1500 μmin andwereconstructed
platinumcoated andinsulated
Parylene
C films300
and37 C
salinefor 14 daysfrom2000 Å thickplatinum10300
μm×thickon the samefrom
die.2000EIS Å(n =8 electrodes)(Figure8). All insulated
electrodesbetween
measured
1500Paryleneµm2 and layers
were constructedmeasurements
wererecorded
betweeneach
electrode
and
a
large
platinum
counter
electrodeusing
thick platinum insulated between 10 µm thick Parylene layers on the same die. EIS measurements werea Gamry Reference 600 potentiostat (Gamry Instruments, Warminster, PA, USA), with an Ag/AgCl

Micromachines 2018, 9, 422

8 of 25

recorded
betweenelectrodeand a large platinum counter electrode using a Gamry Reference
Micromachines
2018, 9, xeach
FOR PEER
REVIEW
8 of 25600 potentiostat (Gamry Instruments, Warminster, PA, USA), with an Ag/AgCl electrode used asusedas a reference.
show thatboth solution
resistance anddelamination
aelectrodereference.
Modeling
resultsModelingshow thatresults
both solution
resistanceand delamination
resistancedrop
resistance drop
dramaticallyafterofonly
one dayof soaking,while theinterface capacitances
gradually
dramatically
after
only one day
soaking,while
the interface
capacitances
gradually increaseinincrease inover
magnitude
theThetesting
period.
dramatic, two droporder-of-magnitude
drop inmagnitude
the testingover
period.
dramatic,
twoTheorder-of-magnitude
in delamination resistancedelamination
may
be due toeither thewaterpenetration
through
theofParylene
or theearlymay
be due to resistanceeither water
penetration
throughParyleneor the earlystagesdelamination
andthestagesdropof delaminationand the may
smallbedrop
resistancedue toelectrode,delaminationsmall
in solution resistancedue into solution
delamination
aroundmay
the beexposedwhicharoundwould
the exposed
electrode, effective
which would
increaseincrease
the electrode’s
surfacearea. the electrode’s effective surface area.

Figure 8.
8. TheThe results
results ofof modelingmodeling electrochemicalelectrochemical impedance
impedance spectroscopy
spectroscopy (EIS)(EIS) spectra
spectra between
between aa
Figurethin-film
platinum
electrodeinsulated
between10μmParylenelayersandalarge
platinum
counterthin-film platinum electrode insulated between 10 µm Parylene layers and a large platinum counterelectrodeduringa
14-daysoak
in1×
phosphatebufferedsaline
(PBS)
at
37
°C.
Both
(a)
R
S and (b)◦electrode during a 14-day soak in 1× phosphate buffered saline (PBS) at 37 C. Both (a) RS and
Rdelam
dropdrop
after after
the first
the magnitudes
of boththe(c)
cross-insulation
capacitance(Ycross)(b)
Rdelamthe day,
first while
day, while
the magnitudes
of(c)
both
the cross-insulation
capacitanceand (d)the double layer capacitance (Ydl) steadily increase over the course of the test.
(Y
cross ) and (d) the double layer capacitance (Ydl ) steadily increase over the course of the test.

2.4. Packaging
and ElectricalElectrical Connections
Connections
2.4.
Packaging andElectrical packaging
filmfilm
electrodes(typically
100–200
nm thick)free film
Electrical
packagingofofthinthinelectrodes(typically
100–200
nm on
thick)
on Parylenefree film
baseddevicesis
exceptionallychallenging.
Numerous
problems
arise
from
the
thin
profile,Parylene-based devices is exceptionally challenging. Numerous problems arise fromflexibility
the thinandlimitedthermalbudgetofParylenefilms.
Notably,
Paryleneisfundamentally
incompatible
withprofile, flexibility and limited thermal budget of Parylene films. Notably, Parylene is fundamentallysoldering,
due
to
the
high
temperatures
required(typically
250
°C)
and
the
low
glass
transition◦
incompatible with soldering, due to the high temperatures required (typically 250 C) and the low glass
temperatureof Parylene(~90 °C)[81].
Parylenedevices
arearepoorly◦ C)Similarly,transition
temperatureof Parylene(~90[81]. Similarly,
Parylenedevicespoorlycompatible
compatible withwithconventional
wirebonding,
which
makes
it
difficulttopackage
Parylenedeviceswithsiliconconventional wirebonding, which makes it difficult to package Parylene devices with silicon integrated
integrated
Ball bonding
typicallyrequiresabove
300 °C to achieve
circuits.
Ballcircuits.
bonding typicallyrequirestemperatures
abovetemperatures
300 ◦ C to achieve
thermocompression[82],
thermocompression[82],
risking
thermaldamagetoParylene,
and
even
when
attempted
therisking thermal damage to Parylene, and even when attempted the resulting bond between the wire
resultingbond
the wiredetaches
and thewhen
thin filmon theParylenewhen thetool
retracts,
andthe thinfilmbetween
on the Parylene
the tool
retracts,
sincedetaches
the metal-metalbond
is stronger
since
the
metal-metalbondisstrongerthanthemetal-Parylenebond(Figure9).
than the metal-Parylene bond (Figure 9).
Batch and reversible electrical connections to Parylene devices can be achieved using
zero-insertion force (ZIF) connectors (Hirose Electric Co., Ltd., Tokyo, Japan), which are hinged
friction-force connectors with spring-like copper pins that contact targeted pads when closed. Designed
originally for connecting flat flexible cables (FFC) to PCB, ZIF connectors can be made compatible with

Micromachines 2018, 9, x FOR PEER REVIEW

9 of 25

Micromachines 2018, 9, 422

9 of 25Parylene electronics by mounting Parylene devices on thicker sections of more rigid polymers such aspolyetheretherketone (PEEK). While simple, reversible electrical connections are possible, the contact
pads have large, fixed footprints compared to flip-chip or wire-bonding. Though ZIFs are designed for
repeated connections, it should be noted that after approximately 15 cycles, connections can fail due towear
and tearthroughParylene
Micromachines
2018,
9, x FORthe
PEER
REVIEW [83] substrate by the mechanical pins.
9 of 25
Figure 9. Holes left by gold removal during ultrasonic wire (ball) bonding on a gold thin film on
Parylene substrate.

Batch and reversible electrical connections to Parylene devices can be achieved using zeroinsertion force (ZIF) connectors (Hirose Electric Co., Ltd., Tokyo, Japan), which are hinged frictionforce connectors with spring-like copper pins that contact targeted pads when closed. Designed
originally for connecting flat flexible cables (FFC) to PCB, ZIF connectors can be made compatible
with Parylene electronics by mounting Parylene devices on thicker sections of more rigid polymers
such as polyetheretherketone (PEEK). While simple, reversible electrical connections are possible, thecontactpads
have leftlarge,
fixedremoval
footprints
compared
to wireflip-chip
wire-bonding.
Figure9. Holesby gold
duringultrasonic(ball) or
bonding
on a gold Thoughthin film ZIFs
on areFigurefor
9. Holesleft connections,by gold removal
duringbe
ultrasonic
wire(ball)
bonding on a 15
gold
thin filmondesigned
repeateditshould
noted
that
after
approximately
cycles,connections
Parylene substrate.
can Parylene
fail due substrate.
to wear and tear through the Parylene [83] substrate by the mechanical pins.
Other
methods
includethe useof conductive
epoxiesdevices[84] orcan
adhesivefilms. using
Anisotropic
Batch and reversible
electricalconnections
to Parylene
be achieved
zeroOthermethods
includetheuseofconductive
epoxies[84]
oradhesivefilms.
Anisotropic
conductive
film(ACF),
acommonmaterialforsemiconductor
packaging,comprisesathin
resin ofinsertion force (ZIF) connectors (Hirose Electric Co., Ltd., Tokyo, Japan), which are hinged frictionconductive
film(ACF),
a commonmaterial
semiconductor
packaging,comprisesa thinresin
suspendedpolymerwhich
formpins
afor
unidirectional
conductive
path
under
mild
heat
and
force
connectors
with spheres
spring-likecopperthat
contact targeted
pads
when
closed.
Designedofsuspendedpolymerspheres
which
form
a
unidirectional
conductive
path
under
mild
heat
and
pressure.
ACF
curingtemperaturesarerelativelyhigh(150–200°C)butoxidationofParylenecanbeoriginally for connecting flat flexible cables (FFC) to PCB, ZIF connectors can be made compatible◦ C) but oxidation of Parylene can bepressure.
ACF
curingtemperaturesarerelativelyhigh(150–200
avoidedby applying
theby
film
under aninert atmosphere,
lowering
the ofcuringandwithParyleneelectronics
mounting
Parylenedevices on or
thicker
sections
moretemperaturerigid polymers
avoidedbythe
applying
the film(PEEK).
underaninertsimple,
atmosphere,or loweringthecuring temperatureandadjustingtime required
to cure.
For
high
bond
yield,alignment
betweenthesubstrates
such
as polyetheretherketone
Whilereversible
electricalconnections
are possible,
theisadjustingtheand
time
toof
cure.
For highbond yield,alignment
betweenthe substrates
importantimportantinrequired
the casebuilding
acompared
customjigtooradaptinganexisting
flipThoughchipisbonder,thecontactpads
have
large,
fixed
footprints
flip-chiporwire-bonding.
ZIFs are
and
in
the
case
of
buildingacustomjig
oradaptinganexisting
flip
chip
bonder,thelevelnessandlevelnessfor
andthermo-mechanicalproperties
the bond
headapproximately
are critical. In15
thisway,connections
ACF can be
designed
repeatedconnections, it
should beofnotedthat after
cycles,thermo-mechanicalproperties
of
the
bond
head
are
critical.
In
this
way,
ACF
can
be
successfully
successfully
usedforelectrical
packagingtoParylenedevices
includingASIC
integration
can fail due to wear and tear through the Parylene [83] substrate by the mechanical pins. (Figure 10)
usedfor Inelectrical
packaging
to weParylenedevices
includingASIC
integration(Figure10)with[85–88].
[85–88].
unpublishedresults,
achievedlarge-area
packaging
Parylene
pitch
Other
methods
includetheuseof conductive
epoxies[84] ofor
adhesiveelectronics
films. AnisotropicIn
unpublishedresults,
weachievedlarge-area
packagingofParyleneelectronics
with
pitch
and
width
and width ofcontacts
to (100material
μm pitch)using
ACF (Dexerials
CP13341-18AA,
Dexerials,
Tokyo,
conductive
film(ACF),down
a commonforsemiconductor
packaging,comprisesa thin resinofofcontacts
down
to
(100
µm
pitch)using
ACF(Dexerials
CP13341-18AA,
Dexerials,
Tokyo,
Japan).
Japan).
suspended polymer spheres which form a unidirectional conductive path under mild heat andpressure. ACF curing temperatures are relatively high (150–200 °C) but oxidation of Parylene can be
avoided by applying the film under an inert atmosphere, or lowering the curing temperature andadjusting the time required to cure. For high bond yield, alignment between the substrates isimportant and in the case of building a custom jig or adapting an existing flip chip bonder, thelevelness and thermo-mechanical properties of the bond head are critical. In this way, ACF can be
successfully used for electrical packaging to Parylene devices including ASIC integration (Figure 10)[85–88]. In unpublished results, we achieved large-area packaging of Parylene electronics with pitch
and width of contacts down to (100 μm pitch) using ACF (Dexerials CP13341-18AA, Dexerials, Tokyo,
Japan).

Figure10.
10.Schematic
Schematicrepresentation
representationofofaaParylene-based
Parylene-basedneuralneuralprobe
probeconsisting
consistingofofthree
threePEDOTPEDOTFigure(poly(3,4-ethylenedioxythiophene))-nanostructured
electrodes
and
one
gold
electrodeascontrol.
The(poly(3,4-ethylenedioxythiophene))-nanostructured electrodes and one gold electrode as control.
deviceisanisotropic
conductive
film
(ACF)
bondedontoaflexiblepolyimide
cable,
which
is
then
The device is anisotropic conductive film (ACF) bonded onto a flexible polyimide cable, which issoldered
ontoonto
a pina connector
adaptedto theacquisition
system.
The cross-section
shows
then
solderedpin connector
adaptedtowirelessthe wirelessacquisition
system.
The cross-section
bond bondpads frombondedvia ACFto thetobond
padspadsof theofpolyimide
cable.
Reprinted
from
shows
pads thefromdevice
the device
bondedvia ACFthe bond
the polyimide
cable.
Reprinted[88]
withpermission
fromElsevier.
from [88] with permission from Elsevier.

Figure 10. Schematic representation of a Parylene-based neural probe consisting of three PEDOT(poly(3,4-ethylenedioxythiophene))-nanostructured electrodes and one gold electrode as control. The
device is anisotropic conductive film (ACF) bonded onto a flexible polyimide cable, which is thenMicromachines 2018, 9, 422

10 of 25

Alternatively, another strategy is to directly incorporate integrated circuits with Parylene devices
during the fabrication process to achieve the densest connections. Rodger et al. developed a method toform interconnects between Parylene devices and bare silicon integrated circuit dies during wafer-levelprocessing, by etching through-holes in a carrier wafer, affixing the dies with temporary adhesives,
and then evaporating metal. But because thin-film metal is deposited directly on top of the inserted
chip, this method is extremely sensitive to the planarity of the dies [89]. Chang et al. developed
a method to spread conductive epoxy over a Parylene layer to create interconnects to a silicon die,which was held in place by a PDMS mold, however, this method was prone to shorts between bondpads resulting from bridging by the low viscosity epoxy [90]; this required selective repair using
laser ablation.
2.5. Sterilization
Parylene coatings for biomedical devices and Parylene-based bioMEMS require sterilization
before use in vivo. However, many standard sterilization protocols require application of high heator oxidative stress, which can damage or otherwise change Parylene bulk properties. For example,
an autoclave creates a high temperature and high humidity environment for sterilization, which cancause Parylene bilayers to delaminate and Parylene coatings to lose adhesion [57,91]. Alternative
methods, such as the use of ethylene oxide, have been used successfully without damage to thin-filmParylene devices [36]. Several methods for sterilizing Parylene devices are described in literature
and a brief summary of various methods and the major conclusions drawn regarding the effect onParylene, is compiled in Table 2. Notably, steam autoclaving, one of the most common methods, is also
one of the most destructive. Other potentially damaging methods include electron beam sterilization,
which can cause ionization and a decrease in crystallinity of bulk Parylene and gamma sterilization,
which appears to decrease the adhesion between Parylene and metal.
We previously examined the use of hydrogen peroxide plasma as a means to sterilize
Parylene-based electrochemical sensors. Devices were visibly unchanged following treatment andelectrode characterization, using EIS and cyclic voltammetry, indicated no changes in electrodeproperties following treatment [32].
2.6. Handling
Finally, we note that owing to the thin and flexible nature of Parylene devices, they can bepotentially damaged by rough handling. Electrical traces incorporated into Parylene devices orribbon cables are actually surprisingly robust, able to survive down to a bend radius of 100 µm
and under fatigue testing of up to 100,000 bends [76]. However, we observed that wrinkling or
crumpling of Parylene devices, which can happen inadvertently, can cause destructive creases inthin metal connections. Parylene devices are also very light and can easily be blown away by thenitrogen streams commonly used for cleaning microdevices, or by the venting of a vacuum chamber,or even by exhalation. They are also subject to strong static forces, owing to Parylene’s properties as
an electrical insulator. We recommend handling Parylene devices gingerly, with tweezers, held by
the edge of the device or even by a purpose built Parylene tab designed into the structure. Weaklyadhesive double-sided tape can be used to secure Parylene devices temporarily, during post-processing,packaging or imaging.

Micromachines 2018, 9, 422

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Table 2. Sterilization methods used and their effect on Parylene. “No adverse effects recorded” indicates the sterilization method was used in literature but no adverse
effects on adhesion or bulk properties was recorded, while “n/a” indicates no use of the sterilization method was found in literature.
Sterilization Method

Effect on Bulk Parylene

Effect on Parylene-Parylene
Adhesion

Effect on Parylene to
Metal Adhesion

Effect on Parylene Adhesion to
Other Materials

Reference

Electron beam
ISO 11137

Chemical structure changed: partial
breakage of C-Cl bonds, ionization ofpolymer, crystallinity decrease

No adverse effects recorded

No adverse
effects recorded

n/a[92,93]Gamma radiation
ISO 11137

Recombination, cross-linking (increases
strength and decreases elongation), loss
in bond strengthn/aDecreased adhesion,
causing loss of electricalinsulation capabilities

To silicon wafer:
Crystallinity increased, no change inYoung’s modulus[91,94]Ethylene oxide
ISO 11135

Formation of inorganic chlorides,
reduction of chlorine amount. EtO is
toxic, carcinogenic, flammable, explosive

For sterilization after thermal
annealing, no adverse effects
recorded

Decreased electricalinsulation capabilities(but not as damaging as
gamma sterilization

To glass:
No adverse effects recorded[93,95,96]

Autoclave (steam)ISO 17665

Parylene became brittle and hard,decreased adhesion, changed chemical
stability, did not contaminate

Decreased adhesion

To silicon:
Significant decrease in adhesion,
Parylene crystallinity increased,
electrical stability decreased[57,91,97–99]

H2 O2 Plasma
ISO/NP 22441 *No change in adhesion. Recommended“best suitability for Parylene” (based on
XRD testing), successfully killed bacteria
without degradation of Parylene coating

n/a

n/a

To Silastic:
Parylene withstood implantation andwas unaffected. However, coating
was easily peeled off after
implantation, suggesting poor
Parylene-Silastic adhesion[93,100,101]

Antibiotic coating
No standard

0.5–0.75 mg/mL concentration oftetracycline nanoparticles completely
eradicated E. coli but not aureus bacteria

n/a

n/a

n/a

[102]

Decreased adhesion* NP: the standard is a new proposal and is still under consideration by the ISO.

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3. Micromachining of Parylene Films

Even though processes for silicon were successfully adapted to micromachine Parylene, there is
3. Micromachining of Parylene Films
a lack of well-defined protocols and standards when working with this polymer. Parylene-based
Eventhoughprocesses forwereconstructedsuccessfullyusing
adaptedmicromachine
is a
MEMS
and
microdevices
are silicon
typicallya tocombinationof Parylene,bulk andthere
surfacelackof well-definedand standards
workingwiththisapolymer.
Parylene-based
MEMS
micromachining
andprotocols
photolithography.
In awhen
typicalprocessflow,
foundational
Parylene layerisandmicrodevices
are typicallyconstructedusing a acombinationof withbulk andsurfacemicromachining
depositedon a support
substrate,
almost always
silicon wafer
its nativeoxide
layer intact,
anda typical
processa foundational
Parylene layer
is depositedonusingphotolithography.
CVD. SubsequentInlayersof metalorflow,
polymer
are then depositedand patternedusingasupportsubstrate,
almostalwaysasilicon
wafer
with
its
nativeoxide
layer
intact,using
CVD.
photolithography. Metal layers commonly serve as conductive traces or electrodes and are deposited
Subsequent
layers ofor polymer
are then deposited
andAdditional
patterned polymer
using photolithography.
using evaporation
ormetalsputteringand patterned
by lift-off.
layers include
Metal
layerscommonlyserve
as conductive
traces orelectrodes
and arephotoresist
deposited using
evaporation
additional
Parylenefilms,
which
may be patterned
on-topof sacrificial
patterns
to createorsputtering
andpatternedbylift-off.
Additional
polymerlayersincludeadditional
Parylenefilms,
three-dimensional structures. Structures may be created using O2 plasma etching throughawhich
may bepatterned
on-topof sacrificial
photoresist
patterns
to create three-dimensional
structures.
photoresistmask,
to create
MEMS
componentsusingbulk-micromachining,or to expose
metal
Structures
may be bycreatedusing
O2 plasmaetching
photoresistmask, fromto create
MEMS
electrodes covered
polymer
insulation.
Finally,thethroughcompletea device
is removed
the support
components
bulk-micromachining,or tochallenges
expose metal
by polymer
substrate andusing
packaged.
Below we describe
andelectrodes
solutions covered
encounteredduringinsulation.
each step
Finally,
the complete
device is removed
from the support substrate and packaged. Below we describeof fabrication
for Parylene-based
microdevices.
challenges and solutions encountered during each step of fabrication for Parylene-based microdevices.
3.1. Deposition
3.1. Deposition
Parylene is deposited by CVD, producing a highly uniform and conformal coating [103].
Paryleneis depositedby CVD,a highlyuniformand conformal[103].
Typically
Typically the film
is transparent
andproducing
homogenous;
however,in someinstances coating
Parylenecoatings
may
the
film
is
transparent
andhomogenous;
however,insomeinstancesParylenecoatings
maybemarredbe marred by odd “spherule” inclusions. The macroscopic appearance is hazy and white, sometimesby
odd “spherule”
inclusions.
and white,sometimes
described
as “cloudy”
Parylene.The
Themacroscopic
microscopicappearance
appearanceisishazypresented
in Figure
11. The described
spherules
as“cloudy”
Parylene.
The
microscopic
appearanceispresented
inFigure11.
The
spherules
may bemay be unreacted Parylene monomers that bond to each other in the gas phase prior to deposition,aunreacted
Parylene monomers
that bondbefore
to eachdeposition
other in thegas phasepriorvolume-to-surface-area
to deposition, a result
result of insufficientmolecular collisions
causedby a highofmolecular
collisions
before deposition
causedbythe
a highvolume-to-surface-areaof theof insufficientthe depositionchamber[104]. Increasing
the surface
area ofchamber,by including structures
depositionchamber[104].
Increasing
thesurfaceareaofthechamber,byincluding
structures
witha
with large surface-area such as a mesh, may prevent the formation of these spherules. SCS,
large
surface-areasuch as acoating
mesh, may
prevent
the formation
manufacturerof Parylene
tools,
describes
the cause ofasthesehigh spherules.
depositionSCS,
ratesa manufacturer
and chamberofParylenecoating
tools,
describes
the
cause
as
high
deposition
rates
and
chamberpressure.
pressure.
This
was repeatedly
devices thatthat wereThis phenomenon
phenomenon was
repeatedly observed
observed when
when coating
coating Parylene-based
Parylene-based devices
werepreviously
previously subjectedsubjected toto somesome form
form ofof mechanicalmechanical action,action, such
such asas ultrasonicultrasonic treatment
treatment oror scrubbing
scrubbingduringmetal-liftoff process.
These devices,
devices, when
when insulated
insulated with
with CVD
CVD Parylene,
Parylene, frequently
frequently present
present
during aa metal-liftoff
process. Thesewith
with this
this cloudy
cloudy appearance,
appearance, with
with thethe densest
densest appearances
appearances ofof spherules
spherules along
along thethe edges
edges ofof thin-film
thin-film
metal
structuresoratlocationswherethemechanicalactionwasmostsevere.
Itisunclearmetal structures or at locations where the mechanical action was most severe. It is unclear whether
whethertheseinterfere with
with adhesion
adhesion between
between thethe Parylene
Parylene andand otherother layers
layers andand whether
they
these phenomena
phenomena interfere
whether they
impactimpact thethe integrity
integrity ofof thethe Parylene
Parylene coating.
coating.

Figure 11.
11.(a)(a)Deposited
Deposited
Parylene
incorporating
spherules
whichinresults
in ina appearance
cloudy in
FigureParylenelayerlayer
incorporating
spherules
which results
a cloudy
appearance
whenobserved
by
eye;
and
(b)
magnifiedphotographrevealingthepresencesmallwhen observed by eye; and (b) magnified photograph revealing the presence of small ofclusters
clustersofspherules.
of spherules.

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3.2. Lithographic Processes
Microfabrication commonly
patterningof photoresist
to serve
as anasetch
Microfabricationcommonlyentails
entailsphotolithographic
photolithographic
patterningof photoresist
to serveanmask,mask,
lift-offlift-off
mask,mask,or patterned
sacrificial
layer. Photolithography
involvesinvolves
a series ofetchor patterned
sacrificial
layer. Photolithography
a sub-processes,
series of subsuch as coating,
pre/post
UVbaking,
light exposure
development.
These processes
theprocesses,
such ascoating,baking,pre/post
UV lightand
exposureand development.
Theseinvolve
processes
use of heatradiationandall can compromise
the integritytheof Parylene
involvethe anduse UV
of heatand UV
radiation
and all can compromise
integrity films.
of Parylene films.
For example, the combination of Parylene gas permeability and the off-gassing of photoresist
during UV exposure [54] can lead to the formation of bubbles in Parylene film. Figure 12 shows a
representative image
following
UVUV
exposureof aof20aµm
positive
resist resist
layer
representative
imageofofthis
thisphenomenon,
phenomenon,
following
exposure
20 thickμm thick
positive
on
top
of
a
10
µm
thick
Parylenefilm.
The
phenomenon
tends
to
be
more
pronounced
withthickerlayer on top of a 10 μm thick Parylene film. The phenomenon tends to be more pronounced withresists andhigherdosage ofUV radiation.
thicker
resists
andexposure
higher exposure
dosageof UV radiation.

Figure 12. Observed gas bubbles in a Parylene coated wafer following UV exposure of a photoresist
Figure 12. Observed gas bubbles in a Parylene coated wafer following UV exposure of acoating.
photoresist coating.

Photodegradation of Parylene has been reported in literature through a two-step process
Photodegradation of Parylene has been reported in literature through a two-step process involving
involving direct photolytic processes resulting in the formation of UV and IR absorbing structures,
direct photolytic processes resulting in the formation of UV and IR absorbing structures, followed byfollowed by photo-induced oxidation of the methylene groups and benzene ring [105]. It is also well
photo-induced oxidation of the methylene groups and benzene ring [105]. It is also well known that UV
known that UV radiation can deteriorate the thermal and electrical properties of Parylene films if theradiation can deteriorate the thermal and electrical properties of Parylene films if the doses are large
doses are 2large (>12 J/cm2) [106]. Although the small doses of UV exposure during lithography are(>12 J/cm ) [106]. Although the small doses of UV exposure during lithography are unlikely to reachunlikely to reach the threshold for full photodegradation, it is hypothesized that some combination
the threshold for full photodegradation, it is hypothesized that some combination of minor oxidationof minor oxidation and mechanical/thermal stress may be responsible for the observed phenomenon
and mechanical/thermal stress may be responsible for the observed phenomenon when thick resist
when thick resist films are used. For example, bubbles between the Parylene and substrate may
films are used. For example, bubbles between the Parylene and substrate may appear following theappear following the UV exposure step for photolithography (Figure 12). Piercing the Parylene
UV exposure step for photolithography (Figure 12). Piercing the Parylene bubbles in select non-criticalbubbles in select non-critical areas and the use of a vacuum (after piercing) was found to aid inareas and the use of a vacuum (after piercing) was found to aid in removal of the bubbles.
removal of the bubbles.
Photolithography has long been implemented and thoroughly characterized on silicon andPhotolithography has long been implemented and thoroughly characterized on silicon and glass
glass wafer substrates. In adapting traditional microfabrication techniques to Parylene substrates,
wafer substrates. In adapting traditional microfabrication techniques to Parylene substrates,
photoresist coating protocols for silicon have largely been ported over and calibrated on a case-by-casephotoresist coating protocols for silicon have largely been ported over and calibrated on a case-bybasis. In order to provide a practical guide for achieving desired photoresist thickness on Parylenecase basis. In order to provide a practical guide for achieving desired photoresist thickness on
and to better characterize the process, spin curves were created for two different photoresists on
Parylene and to better characterize the process, spin curves were created for two different
Parylene-coated silicon substrates and compared to those for silicon and glass substrates. Spin curves
photoresists on Parylene-coated silicon substrates and compared to those for silicon and glass
were produced for both AZ P4620 (Integrated Micro Materials, Argyle, TX, USA), a common etch mask
substrates. Spin curves were produced for both AZ P4620 (Integrated Micro Materials, Argyle, TX,
and sacrificial layer resist and AZ 5214E-IR (Integrated Micro Materials, Argyle, TX, USA), a common
USA), a common etch mask and sacrificial layer resist and AZ 5214E-IR (Integrated Micro Materials,
lift-off resist. The spin-coating parameters used are listed in Table 3. Five 100 mm silicon wafers
Argyle, TX, USA), a common lift-off resist. The spin-coating parameters used are listed in Table 3.
coated in 8 µm of Parylene were coated with photoresist at spin rates from 1000 rpm to 5000 rpm
Five 100 mm silicon wafers coated in 8 μm of Parylene were coated with photoresist at spin rates
in 1000 rpm intervals. Five 100 mm prime silicon wafers and glass wafers were also coated at eachfrom 1000 rpm to 5000 rpm in 1000 rpm intervals. Five 100 mm prime silicon wafers and glass wafers
spin rate for comparison. 5 mm-wide strips of photoresist near the wafer center and wafer edge were
were also coated at each spin rate for comparison. 5 mm-wide strips of photoresist near the wafer
swabbed away from the surface using acetone and thickness was measured at these points using acenter and wafer edge were swabbed away from the surface using acetone and thickness was
surface profilometer.
measured at these points using a surface profilometer.

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Micromachines 2018, 9,Tablex FOR 3.
PEER
REVIEW for photoresist spin curves on different surfaces.
Parameters

14 of 25

Table 3. Parameters for photoresist
surfaces.
AZ P4620 spin curves on different
AZ 5214E-IR

Surfaces
Surfaces
Pre-spin
Pre-spin
Spin acceleration
Mainacceleration
spin
Spin
Bake
Main spin
Bake

P4620
AZ 5214E-IR
Silicon,AZ
Parylene-coatedsilicon, glass(100 mm wafers)
Silicon,Parylene-coated(100rpm
mmfor
wafers)500
rpm for 5 s silicon, glass500
8s
500 rpm for 5 s ~1000 rpm/sec
500 rpm for 8 s
1000, 2000, ~1000
3000, 4000,
5000 rpm for 45 s
rpm/sec
5 1000,min at90 ◦3000,
C 4000, 5000 rpm70for
s at
2000,
4590
s ◦C
5 min at 90 °C
70 s at 90 °C−1/2 andTo compare the spin curves of different surfaces, thicknesses were plotted against ωrpm–½
To compare the spin curves of different surfaces, thicknesses were plotted against ωrpm andleast-squares linear regression was performed on each set, followed by testing for coincidence of theleast-squares linear regression was performed on each set, followed by testing for coincidence of theregression
lines(Figurewas foundfoundamong
amongthethespinspincurves
curvesParylene,regression
lines(Figure13).
13).NoNosignificant
significant difference
difference wasonon
Parylene,silicon
and
glass
with
AZ
P4620,
norwith
AZ
5214E-IR(p<0.05).
These
spin
curvesmay
serve
silicon and glass with AZ P4620, nor with AZ 5214E-IR (p < 0.05). These spin curves may serve as asa astarting
point
in
the
practical
designofaParylenemicrofabrication
process.
starting point in the practical design of a Parylene microfabrication process.

Figure 13. Spin curves for (a) AZ P4620 and (b) AZ 5214E-IR resists on silicon, glass and Parylene.
Figure 13. Spin curves for (a) AZ P4620 and (b) AZ 5214E-IR resists on silicon, glass and Parylene.
Error bars represent standard deviation (n = 5).
Error bars represent standard deviation (n = 5).

3.3. Metal Deposition
3.3. Metal Deposition
Metal deposition is a key process in the fabrication of polymer MEMS devices to create
Metal depositiona key
in thefabrication
polymer MEMS
to create
conductive
conductive
elementsissuch
asprocesstraces andelectrodes.
Theofdepositionof metaldevices
on Parylene,
however,iselements
as traces andTheevaporation
deposition canof metal
Parylene,intrinsic
however,rife withrife withsuch
complications.
Bothelectrodes.
sputtering andinduceonsignificant
andis
extrinsic
complications.
sputteringand evaporation
can induce
significant
extrinsic stressstress which Bothcan induce
curvaturein the resulting
free-film
Paryleneintrinsic
devices.and
In addition,
the
which
can induce
curvaturein themetals,
resultingfree-film
Parylenedevices.
In addition, processes,
the deposition
depositionof highmelting pointsuch
as platinum,throughhigh temperaturesuch ofhighmelting pointcan
metals,such
as platinum,throughhighprocesses,
as evaporation,
as evaporation,lead tocrackingof the metal
film orthetemperature
underlying Parylene
duesuchto thermal
stress,resulttoofcracking
mismatchofinthe
themetal
thermalcoefficientsof expansion
and film
stress.
cana leadfilm
or the underlying
Parylenedue
to thermal stress, a result of
Figure14 shows
ancoefficients
image of platinum(2000 Å)depositedon a Parylene coated silicon wafer (10mismatchin thethermalof expansion
andfilm stress.
μmFigurethick Parylene
with stressinduced(2000
cracks.
platinumon
wasevaporated
with silicona Temescal
14 showsfilm),
an imageof platinumÅ)The
depositeda Parylene
coatedwafer
uncooled
stage;
evaporation
waferwithtemperature(10BJD-1800µm thick e-beamParyleneevaporator
film), withusing
stress an
inducedcracks.
The during
platinum
was evaporateda Temescal
surpassed
110 °C
as measured
withmonitorstickers(Omegawafer
Engineering,
Norwalk,CT,
BJD-1800
e-beamevaporator
using
antemperature
uncooled stage;
duringevaporation
temperaturesurpassed
USA)
placed
on
the
back
of
the
wafer.
Changesinthedeposition
rate
and
tool
power
were
insufficient◦
110 C as measured with temperature monitor stickers (Omega Engineering, Norwalk, CT, USA) placed
prevent
Improving
contact rate
betweenthe wafertheinsufficient
stage is difficult,
in
onto
the
back ofcracking.
the wafer.
Changesthein thermal
the deposition
and toolpowerandwere
to prevent
part because the Parylene which coats the back of the silicon wafer serves as an insulator. Breaking
cracking. Improving the thermal contact between the wafer and the stage is difficult, in part because
the deposition into a series of four steps with 15 min pauses between each deposition can prevent
the Parylene which coats the back of the silicon wafer serves as an insulator. Breaking the deposition
cracking with metals such as titanium, evaporated at a lower temperature but is insufficient for
into a series of four steps with 15 min pauses between each deposition can prevent cracking withplatinum. Ultimately, the most reliable method to avoid cracking was to use a tool (CHA Industries
metals such as titanium, evaporated at a lower temperature but is insufficient for platinum. Ultimately,
MARK 40, Fremont, CA, USA) with a larger throw distance between the metal target and the wafer
thestagemost(22″reliablemethodwas
todepositing
use a toolthe(CHA
Industries
40, Fremont,compared
withto8″avoid
for thecracking
Temescal)andplatinumin fourMARK
500 Å steps.
Heat
CA,
USA)
with awith
largerthrow distance
betweenmetal target
and theandwafer(22” comparedwasmeasuredtemperaturemonitorstickersthe
as previously
described
wasstagemaintained
below
with
77 8”
°C.for the Temescal) and depositing the platinum in four 500 Å steps. Heat was measured withtemperature monitor stickers as previously described and was maintained below 77 ◦ C.

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Figure 14. (a) Stress-induced cracking of deposited platinum likely due to excess heat generated

Figure14. (a) Stress-induced
crackingof depositedplatinum
likelyduedueto excessheatheat
generated
duringFigureStress-induced
cracking
oftraces
deposited
platinum
likelyto excessgenerated
during 14.
the (a)process;and (b) cracked
metal
after lift-off.
the during
process;
and(b)crackedmetal
tracesafter
lift-off.
and (b) cracked
metal
after lift-off.
Figure the14. process;(a) Stress-induced
cracking
oftraces
deposited
platinum likely due to excess heat generated
during the process;(b) cracked
metal traces
after lift-off.
Depositing
metal andthroughsputtering
can avoid
thermal stress and cracking but can impart
Depositing
metal
throughsputtering
thermalstresscracking
butseverefilm stress,
warpingtheParylene
filmcanandavoid
forcingreleaseddevicestocurve.
15aimpartis an
Depositing
metal
throughsputtering
can
avoid
thermalstressandandcracking
butFigurecan canimpartseverefilm
warping
the Parylene
andforcingreleaseddevices
to Figurecurve.
Figure15a
is
an of
Depositing
metal
throughsputteringcanthermalstresscracking
butcanimpartimageofa stress,
Parylene
substrate
patterned
withaavoidnegative
profilephotoresist
for metal(AZ
filmsevere
stress,warpingtheParylenefilm
and film
forcingreleaseddevices
to andcurve.
15alift-off
is an
image
image
of
a
Parylene
substrate
patterned
withanegative
profilephotoresist
for
metal
lift-off(AZ
severefilm
stress,warpingtheParylenefilm
andforcingreleaseddevicestocurve.
Figure15a
isan5214E-IR)andsputtercoated
with
2000
Å
of
platinum.
TherippledappearanceistypicalofParylenea Parylene substrate patterned with a negative profile photoresist for metal lift-off (AZ 5214E-IR) andimagecoatedof aandParylene
substrate
patterned
witha negative
profilephotoresist
formetal lift-off(AZ
5214E-IR)coated with
2000 Å theofplatinum.
rippledappearance
is typicalof Parylene
films
bysputtersputtering
highlights
severityofThe
film
stresssputtercoated with
2000 Å ofand
platinum.
The rippled
appearanceis (Figure
typical 15b).
of Parylene films coated by
5214E-IR)and2000 Å
of platinum.
Therippledappearance
is typical
Parylene
films
coatedbysputtersputteringandwithhighlights
theseverity of
film
stress(Figure15b).
Figure16shows
acoatedmicromachined
Parylene-basedneuralprobe
array preparedwithofsputteredsputtering and highlights the severity of film stress (Figure 15b).
filmsFigurecoatedbyhighsputtering
highlights
theseverityasof510
film
stress(Figure
15b).
16
shows
a micromachined
Parylene-basedneuralprobe
arraybypreparedwithsputteredplatinum.
The
level
ofand
intrinsic
stress(estimatedMPaprovided
LGA ThinFilms,
Santa
Figure16 shows
a micromachined
Parylene-basedneuralprobe
probearray
array
preparedwithsputteredFigure16showsamicromachined
Parylene-basedneuralpreparedwithsputteredplatinum.
The
high
level
of
intrinsic
stress(estimated
as
510
MPa
provided
by
LGA
Thin
Films,
Santa
Clara, CA, USA) forces the final device to curve severely, rendering it unusable. By comparison,theplatinum.
TheThehighlevelof theintrinsic
stress(estimatedas 510MPaprovided
byby
LGA
Thin
Films,
Santa
Clara,
platinum.
high
level
of
intrinsic
stress(estimated
as
510
MPa
provided
LGA
Thin
Films,
Santa
Clara,
CA,
USA)
forcesfinal
devicetocurveseverely,
renderingitunusable.
By
comparison,thesame devices are shown prepared with e-beam evaporated platinum, demonstrating reduced
CA,sameUSA) devices
forcesthefinal
deviceto curve
severely,
rendering
it unusable.
By comparison,
the same
devicesClara,CA, USA)
forcesthe prepared
final
deviceto curve
severely,
rendering
it unusable.
By comparison,
the
are
shown
with
e-beamevaporated
platinum,
demonstrating
reducedcurvature.
are shownpreparedwithe-beamevaporated
platinum,
demonstrating
reducedcurvature.
same
devicesareshown
preparedwithe-beamevaporated
platinum,
demonstrating
reducedcurvature.
curvature.

Figure 15. (a) Severe wrinkling/rippling seen in sputtered deposited platinum film results fromcompressivea higher magnitude;seen
and in(b)sputteredwrinkled areas
aroundthe device
metalfeatures.
Figure15. (a)stresses
Severeofwrinkling/rippling
deposited
platinum
film
results
from
Figure 15. (a) Severe wrinkling/rippling seen in sputtered deposited platinum film results from
Figure 15. (a)Severeofwrinkling/ripplingseen
platinum
filmmetal
results
fromcompressive
stressesa higher magnitude;
andin(b)sputteredwrinkled depositedareas aroundthe device
features.
compressive
stressesof ofa higher
wrinkledareas
areasaroundarounddevicemetal
features.
compressive
stressesa highermagnitude;
magnitude;andand (b)(b) wrinkled
thethedevicemetal
features.

Figure 16. (a) Top view; and (b) side view of neural probe arrays after release from silicon carrier
wafer.
Arraysfabricated
with
more
severecurvature
thancarrier
with
Figure 16.
(a) Topview; and(b)sputteredside viewplatinumof neural possessedprobe arrays
after
releasefrom silicon
Figure
16.
(a)
Top
view;
and
(b)
side
view
of
neuralprobe
arraysafter
releasefromsiliconcarrier
platinumdepositedbye-beam.
wafer.
sputteredmoreseverecurvaturethan with
Figure16. Arrays(a) Topfabricatedview; andwith(b) sideview platinumof neural possessedprobe arrays
after
releasefrom siliconcarrier
wafer. Arrays fabricated with sputtered platinum possessed more severe curvature than withplatinumdepositedbye-beam.
wafer. Arrays fabricated with sputtered platinum possessed more severe curvature than with platinum
platinum deposited by e-beam.

deposited by e-beam.

Micromachines 2018, 9, 422
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3.4. Etching
3.4. Etching
Due to the inertness of Parylene it is not practical to etch it chemically. Although there are reportsDueto the inertness
Parylene
it is not practicalto etch it benzonate
chemically. at
Althoughof wetetchingParylene ofusing
chloronapthelene
or benzoyl
150 °C there[107],arethisreports
is an◦ C [107], this is an extremeofwetetchingParyleneusing
chloronaptheleneorbenzoylbenzonateat150
extreme temperature for Parylene and can affect the bulk properties of unetched sections. Thus,
temperature forParylene andaffect thebulk propertiesof unetched
sections.
Thus,
mechanisms
mechanisms
to selectively
etchcanParyleneare limited
to dry techniques.
Parylenecanbe removed
withto selectively
Paryleneare limited
to iondrybeamtechniques.
can beremovedwith oxygen
oxygenplasmaetch
etching[108–111],
reactive
etching Parylene[112], oxygen
reactive
ion etching(RIE)
plasmaetching[108–111],
reactive
ion likebeam
etching[112], oxygen
reactive
ionof
etching(RIE)
andetchinga deepand a deep
reactive
ion etching(DRIE)
methodinvolvingalternating
cyclesoxygenplasmareactive
ion etching(DRIE) likeinvolvingalternating
cyclesmethodof oxygenplasmatoetchingandandfluorocarbon
passivation
layermethod
deposition[108,112–116].
The latteris referredas O2 DRIE.
fluorocarbon
passivation
layer
deposition[108,112–116].
The
lattermethodisreferred
to
as
O
DRIE.
Photoresist etch masks are most commonly used to lithographically pattern Parylene, 2despite
Photoresist
etch(approximately
masks are most commonly
to lithographicallypattern
Parylene,
despitethe low
selectivity
1:1). Otherusedmaterials
including somemetalscan beused theas
low selectivity(approximately
1:1).
Other
includingmetalscanbutbe usedas masking
masking
materials
if a high etchrateandmaterials
good selectivity
aresomerequired[115]
the coating
andmaterialsofifmetal
a highmasksetch rate
goodmany
selectivity
arethermal
required[115] butproblems
the coatingand etchingof
etching
mayandinduceof themismatch
described
above.
metal masks may
induce manyof theduring
thermalproblems
described
Additionally,
re-depositionof metalthemismatch
etch process
is common
and above.
as suchAdditionally,
photoresists
re-depositionof metalduringthe[116,117].
etch process
is common
as of
such
photoresists
remainthe most
remainthe most
commonchoiceAcrossa broad andrange
processes,
AZ P4620,
a positivecommon
choice[116,117].
Across
a
broad
range
of
processes,
AZ
P4620,
apositive-tone
photoresist,tone photoresist, was used as a Parylene etch mask for both O2 RIE and an O2 DRIE process (presented
wasusedtoasetcha Parylene
etchfor bothan Oranging
in [113])
to etch2 RIE andfilm,
2 DRIE process
in [113])a varietyof maskstructures
intoOParylene
from 1 (presented
to 15 μm deep.
Selectivityavarietyofstructures
into
Parylene
film,
ranging
from
1
to
15
µm
deep.
Selectivity
rates
typicallyvary
rates typically vary between 0.9 and 1, for etch rates varying between 0.55 and 0.80 μm/min.
between
0.9
and
1,
for
etch
rates
varying
between
0.55
and
0.80
µm/min.
The most common problem we observe during plasma etching of Parylene is the formation of
The most
commonproblem
we observe
duringplasmaetchingsubstrateof Parylene
the formationofgas bubbles,
either
withinphotoresist
layers or
betweenthe support
andisParylene
coating.
gas
bubbles,either
withinphotoresist
layersor
betweenthe
supportsubstrateandParylenecoating.
The apparent cause is the off-gassing of volatile components of photoresist or remnants of the solventThe
causetheis theoff-gassingof volatilecomponentsof photoresistmay
or remnantsof the solvent
usedapparent
to distributephotoresist.
Minor
off-gassing
from photoresists
be inconsequential,orused to
distribute thewhen
photoresist.
Minorfrom photoresists
may be inconsequential,
or eveneven
unnoticeable,
processing
onoff-gassing
rigid substrates,
however Parylene
is gas-permeable
andunnoticeable,
when processing
rigid
substrates,
however
Parylene
gas-permeable
andevendeform
minor
even
minor volatile
products on
canproducebubbling
when
under isvacuum.
Bubblesmayvolatile
products
cancreatingproduceholes
bubbling
maythedeformphotoresist
masks,
photoresist
masks,
whichwhen
leadunder
to etchvacuum.
damage,Bubbles
or deform
Parylene
itself making
creating
holes
which
lead
to
etch
damage,ordeformtheParylene
itselfmakingfutureprocessing
future processing difficult. Figure 17 shows a representative example photoresist bubbling observed
difficult. Figure
17 shows
a representative
photoresist
bubbling
observed occurs
followinga Oreadily
followinga O2 DRIE
using
80 W RF and example
900 W ICP
power. Thisphenomenon
more
2 DRIE
using 80
W RF
andpower,
900 W high
ICP power.
Thisphenomenon
occursmore readily
when photoresist
using high power,
when
using
high
density,inductively
coupledplasmas.
Very thicklayershighdensity,inductively
coupledplasmas.
Very
thickphotoresist
layers
(>10
µm)
and,
in
particular,(>10 μm) and, in particular, very thick edge beads are more susceptible, likely due to the residual
very
thick
beads
are moresusceptible,
likelydue observedto the residual
solvent not
drivenoff during
solvent
notedgedrivenoff duringsoft-baking
steps.
We alsothe appearanceof gasbubbleswhensoft-baking multi-layer
steps. We also
observed
the featuring
appearanceof gas bubbles whenprocessing
multi-layer
processing
Parylenedevices
three-dimensional
Parylenestructures
definedParylenedevicesfeaturing
Parylenestructures
defined using
sacrificial
layers.
using
sacrificial
layers.
This isthree-dimensionala relatively common
techniquewhere photoresist
is patterned
and
then
This
is ainrelatively
where microfluidic
photoresist isorpatterned
andstructures.
then coatedin Parylene
tocoatedParylenecommon
to createtechnique
Parylene-based
mechanicalThesesacrificial
createParylene-based
microfluidicormechanicalstructures.
These
sacrificial
layersareparticularly
layers are particularly susceptible to forming bubbles during subsequent etch steps, as gas expands
susceptible
to forming
bubbles required
during subsequent
steps,
as gasexpands
rapidly
underthroughthe low
rapidly
under
the low pressure
for plasmaetch
etching,
fasterthan
the rateof diffusionpressurerequiredthe Parylene
film. for plasma etching, faster than the rate of diffusion through the Parylene film.

Figure17. Out-gassing
in bubbling
bubbling observed
observed inin sacrificial
sacrificial photoresist
photoresist
Figure 17.
Out-gassing ofof photoresist
photoresist solvent
solvent results
results in
2 deep reactive ion etching (DRIE).
structures
sandwichedinParylenefollowingO
structures sandwiched in Parylene following O2 deep reactive ion etching (DRIE).

The formation of these gas bubbles appears particularly sensitive to the hard-baking step
The formation of these gas bubbles appears particularly sensitive to the hard-baking step following
following photoresist development. In a simple experiment, Parylene coated 4″ silicon wafers were
photoresist development. In a simple experiment, Parylene coated 4” silicon wafers were masked withmasked with 10 μm thick AZ P4620 and, following development, hard-baked on a hot plate at 90 °C

Micromachines 2018, 9, 422

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Micromachines 2018, 9, x FOR PEER REVIEW

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10 µm thick AZ P4620 and, following development, hard-baked on a hot plate at 90 ◦ C for either 0,2,
12 h. 0,
Following
O2 DRIE
process(approximately
2 µm etch), we
observedcatastrophic
fororeither
2, or 12 h.a brief
Followinga brief
O2 DRIE
process (approximately
2 μmetch), weobserved
bubbling
for bubbling
the sample
for 2 h,bakedwith minimal
or nobubbling
unbaked samples
and samples
catastrophic
forbaked
the sample
for 2 h, withminimalor forno bubblingfor unbaked
baked
for 12 h.
We hypothesize
after 2 h thatof hardvolatiles
diffusethroughthe Parylene
and samples
baked
for 12 h. We that
hypothesize
afterbaking,2 h of hardbaking,volatiles
diffusethroughlayer
andbecometrappedbetweenthe
Paryleneandsiliconwafer
substrate
once
the
wafer
cools.
the Parylene layer and become trapped between the Parylene and silicon wafer substrate oncetheWesuspect
volatiles
arevolatiles
responsible
for later bubbling
the DRIE.
h of hardwafer
cools.theseWe suspect
these
are responsible
for laterinbubblingin By
thecontrast,
DRIE. By12
contrast,
12
bakingmaycompletely
drive off photoresist
volatiles fromthe wafer,whileomitting
hard bakeh of hardbakingmay completely
drive off photoresist
volatiles
from thewafer,
whiletheomitting
theentirely
prevents
diffusionvolatilesof
under
the Parylene.
stronglyWe
recommend
avoiding a
hard bakeentirelyany
prevents
anyofdiffusion
volatiles
under theWe
Parylene.
strongly recommend
hard-bakewhen etching
Parylenethroughphotoresist
If a hard-bakeor isifavoiding astep
hard-bakestep when
etchingParylenethroughmasks.
photoresist
masks. Ifisarequired,hard-bakeresistis intended
for isuse
as a sacrificial
we recommend
a lengthy
hard-bakeat lowhard-bake(<100 ◦ C)
required,or if resist
intendedfor usestructure,
as a sacrificial
structure, we
recommenda lengthy
temperature
under
an inert atmosphereto minimize
thermalstress andoxidation.
at low (<100 °C)
temperature
under an inert
atmosphereto minimize
thermalstress and oxidation.
3.5.
Photoresist Stripping
Stripping
3.5. Photoresist
Generally,
Generally, photoresists
photoresists (whether(whether aa mask
mask oror sacrificial
sacrificial layer)
layer) areare removed
removed with
with acetone
acetone oror otherotherorganic
Inorganic solvents,
as sulfuric
sulfuric acid,
acid, areare used
used when
when photoresist
photoresist cannot
cannot be
be easily
easilyorganic solvents.
solvents. Inorganic
solvents, such
such asremovedsolvents
alone.
Oxygenplasma,a dry method,is also used
to remove
removed withwithorganicorganic
solvents
alone.
Oxygenplasma,a dry method,is alsoused photoresist
to removemasks
but ismasks
often impractical
it removesasParylene
at thesame rate.
shows
residual
AZ
photoresist
but is often as
impractical
it removes
Paryleneat theFigure
same 18
rate.
Figure18 shows
P4620
photoresist
from
the
etch
mask
on
a
platinum
electrodeafter
acetonesoaking.
residual AZ P4620 photoresist from the etch mask on a platinum electrode after acetone soaking.

Figureon metal
metal feature.
feature.
Figure 18.
18. Oxygen
Oxygen plasma
plasma treatedtreated AZ
AZ P4620
P4620 photoresist
photoresist residue
residue on

3.5.1. Sacrificial Layer
3.5.1. Sacrificial Layer
Stripping of sacrificial photoresist used to define features such as channels in Parylene devices
Stripping of sacrificial photoresist used to define features such as channels in Parylene devices cancan be particularly challenging after O2 DRIE process. For example, in our experience, long soak
be particularly challenging after O2 DRIE process. For example, in our experience, long soak duration
duration in acetone (30 h at room temperature) was required to remove sacrificial photoresist usedin acetone (30 h at room temperature) was required to remove sacrificial photoresist used to define ato define a microfluidic channel. Such lengthy soaks may lead to delamination of weakly adhered
microfluidic channel. Such lengthy soaks may lead to delamination of weakly adhered Parylene layers.
Parylene layers. To address both the length soak time and preservation of device integrity, several
To address both the length soak time and preservation of device integrity, several parameters wereparameters were evaluated to accelerate dissolution of AZ P4620 used as a sacrificial layer forevaluated to accelerate dissolution of AZ P4620 used as a sacrificial layer for microfluidic channels
microfluidic channels (26.3 mm in length [48]). The combination of increased acetone volume, heating(26.3 mm in length [48]). The combination of increased acetone volume, heating (40 ◦ C) and agitation(40 °C) and agitation (magnetic stir-bar) significantly reduced soak time, mitigating risk of Parylene(magnetic stir-bar) significantly reduced soak time, mitigating risk of Parylene delamination anddelamination and reducing clearance time.
reducing clearance time.
3.5.2. Oxygen
3.5.2.
Oxygen Plasma
Plasma Exposed
Exposed Photoresist
Photoresist
Photoresist subjectedsubjected tooxygen plasma
Photoresist
to prolonged
prolonged oxygen
plasma can
can become
become crosslinked
crosslinked andand difficult
difficult toto strip
strip
using
acetonealone(Figure19a,b).
This
necessitates
the
use
of
more
aggressive
strippers.
Severalusing acetone alone (Figure 19a,b). This necessitates the use of more aggressive strippers. Several
solutions were
evaluated toremove this
this photoresist
photoresist fromfrom platinum
platinum electrodes
electrodes andand contact
contact pads
pads but
but
solutionswere evaluatedto remove
only piranhapiranha solution
solution andand homemade
homemade stripper
stripper successfully
successfully removed
removed any
any residual
residual photoresist.
photoresist.
only
Upon mixing the Piranha solution (4 parts sulfuric acid (H2SO4) and 1 part 30% hydrogen
peroxide (H2O2)), an exothermic reaction occurs that raises the solution temperature above 100 °C.
Soaking the Parylene device in a piranha solution at temperatures above 60 °C resulted in metaldelamination. Consequently, devices were soaked only after the solution cooled down to 40 °C for 3
min. Although piranha was successful, evidence that Parylene was partially oxidized after thermal

Micromachines 2018, 9, 422

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Upon mixing the Piranha solution (4 parts sulfuric acid (H2 SO4 ) and 1 part 30% hydrogen
peroxide (H2 O2 )), an exothermic reaction occurs that raises the solution temperature above 100 ◦ C.
Soaking the Parylene device in a piranha solution at temperatures above 60 ◦ C resulted in metaldelamination. Consequently, devices were soaked only after the solution cooled down to 40 ◦ C for
Micromachines 2018, 9, x FOR PEER REVIEW
18 of 25
3 min. Although piranha was successful, evidence that Parylene was partially oxidized after thermal
treatmentsuggeststhatfor Parylene-based
Parylene-baseddevices.
devices.On
On
other
hand,
treatmentsuggests
thatthisthismethod
methodisistoo
too aggressive
aggressive forthetheother
hand,thethehomemade
stripper,consisting
of
1
part
Remover
PG(N-methyl-2-pyrrolidone
based)
and1
part
homemade stripper, consisting of 1 part Remover PG (N-methyl-2-pyrrolidone based) and 1 partof AZ
726 (<3%
tetramethylammoniumhydroxidebased developer),successfully
removed removed
photoresistof AZ
726 (<3%
tetramethylammoniumhydroxidebased developer),successfully
residueafter aresidue
15-minafter
soaka at
50 ◦ Csoak
as shown
inas
Figure19c,d.
photoresist
15-minat 50 °Cshown
in Figure 19c,d.

Figure 19. DRIE oxygen plasma exposed AZ P4620 photoresist residue on (a) a platinum electrode;Figure 19. DRIE oxygen plasma exposed AZ P4620 photoresist residue on (a) a platinum electrode;and (b) contact pads. Cleaned (c) platinum electrode; and (d) contact pads with homemade stripper.
and (b) contact pads. Cleaned (c) platinum electrode; and (d) contact pads with homemade stripper.

3.6. Release
3.6. Release
Releasing the device from the support or carrier wafer is the final step in Parylene MEMS
Releasingthe device
fromthethesupportcarrierwafer
is athefinal
stepoxidein Parylene
processing. Most
commonly,supportis aorsilicon
wafer
with
thin
nativelayer andMEMS
theprocessing.
Most
commonly,thesupportisasilicon
wafer
with
a
thin
nativeoxide
layerpeeled
and theParylene adhesion to this material is quite weak. In such cases, Parylene devices can be simply
Paryleneadhesionto thismaterial
is quite
weak. In suchcases,andParylenedevicescan betosimply
off usingtweezers,or thewafer canbe submergedin waterthe devices
allowedfloat topeeled
theoffsurface.
using tweezers,
or thewafer cansubmerged
in waterandandthethedevices
allowedfloat
to theIn some cases,
however,thebe
adhesion
betweenParylene
supportcan be toverystrongsurface.
In someParylene
cases, however,adhesion
andbe
thenearly
supportcan be verystrongand releasing
without thedestroying
thebetween
thin-filmParylene
devices can
impossible.
In many
andreleasing
Parylene
without
destroying
thethin-film
devicescanbenearlyimpossible.
In
manycases this adhesion is deliberate; sacrificial adhesion layers such as aluminum [45,118] or chrome [85]
cases
this
adhesiondeliberate;sacrificial
adhesionsuch
aluminum[45,118]
or chrome
[85]
have
been
used foriscomplex
Paryleneprocessing
andlayersdevicesareas
thenreleased using
KOHor anodic
NaCl[45,118]
etchingfor Al,or chromeetchantand
for devices
Cr [85]. are
Wethenalso released
demonstrated
release
of
have
been
used forcomplex
Paryleneprocessing
using theKOH
or anodic
Parylenedevicesfromsupportsubstrateof polyethylene
terephthalate(PET) by theboiling
the of
NaCl[45,118]
etching
for aAl,or chromeetchant
for Cr [85]. We
also demonstrated
releaseensemble
in
water
for
severalminutes.
In
otherscenarios,theadhesionmaybeunintentional
orParylene devices from a support substrate of polyethylene terephthalate (PET) by boiling the ensemble
We observed
thatdevices annealed
on wafer
200°C for 48 h undervacuumin undesirable.
water for several
minutes.
In Parylene
other scenarios,
the adhesion
mayatbeunintentionalor undesirable.
◦
andbecamevirtually
impossibletoremove.
Insuchsituations,the
onlyremaining
optionmay
be to
We observed that Parylene devices annealed on wafer at 200 C for 48 h under vacuum and became
etch
away
the
underlying
siliconorothercarrier
substrate.
virtually impossible to remove. In such situations, the only remaining option may be to etch away theunderlying silicon or other carrier substrate.
4. Discussion
4. Discussion
Applying complex, high-resolution micromachining processes to Parylene C substrates ispossiblebut complex,
will require
modification ofnearly every protocol
developedfor C
siliconor otherrigid
Applying
high-resolution
micromachining
processes
to Parylene
substrates
is possible
substrates. No single definitive set of guidelines exists but we can describe a useful compilation of
but will require modification of nearly every protocol developed for silicon or other rigid substrates.
best practices.
No single definitive set of guidelines exists but we can describe a useful compilation of best practices.
Efforts should be taken to minimize exposure to heat during every step of the fabrication
process. Avoiding the glass transition temperature (90 °C) of Parylene is typically unrealistic but
avoiding or minimizing the use of high temperature (>100 °C) processes can prevent oxidization,
thermal stress, the formation of gas bubbles, or irreversible changes to Parylene morphology. Hightemperature anneals (200 °C for 48 h in vacuum) are useful to improve moisture barrier properties orMicromachines 2018, 9, 422

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Efforts should be taken to minimize exposure to heat during every step of the fabrication process.
Avoiding the glass transition temperature (90 ◦ C) of Parylene is typically unrealistic but avoiding or
minimizing the use of high temperature (>100 ◦ C) processes can prevent oxidization, thermal stress,
the formation of gas bubbles, or irreversible changes to Parylene morphology. High temperatureanneals (200 ◦ C for 48 h in vacuum) are useful to improve moisture barrier properties or to shape
Parylene by exploiting thermoplasticity, but must be done under vacuum or inert atmosphere andParylene must be allowed to cool slowly to room temperatures over several hours. Rapid heating
above 120 ◦ C on a hotplate or during PVD can introduce destructive stress and must be avoided.
Parylene C is broadly amenable to most photolithography techniques, including electron-beamlithography but users should be aware of the risk of oxidation due to radiation exposure andcomplications from gas transport in Parylene films arising from off-gassing of thick photoresist films.
Avoiding the use of a hard-bake following development, removing thick edge beads and avoiding
high intensity or long duration UV exposure, are all recommended practices. Patterned photoresist
masks have been used with success to transfer high-aspect ratio structures to Parylene substrates using
O2 RIE and DRIE but this method requires very thick photoresist films due to low selectivity during
O2 etching.
Metal deposition onto Parylene films can prove incredibly challenging. Evaporative PVD can
introduce thermal stress and cracking of either the Parylene or metal structures, while sputtering canintroduce film-stress which can warp or wrinkle the Parylene film. These challenges are exacerbatedby high-melting point metals and thicker metal films. The use of cooled stages or heat-sinking mayhelp during evaporative PVD but the insulative nature of Parylene coatings makes such thermal
control difficult. We have observed the best results by using an e-beam deposition system with a throw
distance (distance between target and metal crucible) greater than 20” and splitting depositions ofthick films into multiple steps punctuated with 15 min pauses.
Finally, for those Parylene-based bioMEMS intended for chronic use under wet-saline
environment, we note that the risk of water permeation or delamination remains one of the largest
obstacles. We observed WVTR decreasing significantly, for films thicker than 5 µm, following a
48 h thermal anneal and this is consistent with various literature and anecdotal reports about theadvantages of annealing Parylene devices and the difficulties using very thin Parylene as a moisturebarrier. The large variation in WVTR measurements between different sources prompts a call for more
research into how deposition parameters change barrier properties. New methods to improve Parylene
adhesion to thin-film metal would similarly help increase Parylene-device lifetime in vivo.
5. Conclusions
In conclusion, while applying micromachining to Parylene substrates can be challenging,with attention to the limited thermal budget and polymeric properties of Parylene, most processes
can be successfully transferred to this thin, flexible material. We believe that the observations andrecommendations we list here can serve as the basis for a series of best-practices regarding Parylene
microfabrication. Continuing research is needed, particularly on the adhesion and barrier properties
in the construction of multi-layer devices. By solving these and other challenges Parylene stands tobecome a key material in a new generation of flexible electronic microdevices and polymer MEMS.
Author Contributions: J.O.-D. wrote the paper, designed and performed WVTR experiments and analyzed thedata; K.S. analyzed the WVTR data and assisted with the organization and editing; C.L. wrote the spin curve
section, designed and performed spinning experiments; A.C. contributed to the metal deposition, etching andphotoresist stripping sections; T.H. designed and performed bubbling experiments; J.Y. wrote the packaging
section; A.B. wrote the sterilization section and contributed to the delamination section; A.W.H. contributed to themetal deposition section; and E.M. guided the organization and scope and assisted with editing.
Funding: This research was sponsored in part by the NSF under award numbers CBET-1343193 and EFRI-1332394,
NIH under award number U01 NS099703 and Defense Advanced Research Projects Agency (DARPA) BTO under
the auspices of Doug Weber/Eric Van Gieson through the DARPA Contracts Management Office Cooperative
Agreement HR0011-15-2-0006. In addition, the following fellowship support is acknowledged: USC-CONACyT

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Fellowship (J.O.-D.), Viterbi School of Engineering Ph. D. Merit Fellowship (A.C.), USC Provost Fellowships (C.L.,
J.Y.), and Alfred E. Mann Institute Fellowship (A.B.).
Acknowledgments: The authors would like to thank Donghai Zhu and members of the USC Biomedical
Microsystems Laboratory for their assistance.
Conflicts of Interest: The authors declare no conflict of interest.

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