JOURNAL OF MICROELECTROMECHANICAL SYSTEMS

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Characterization and Modification of Adhesionin Dry and Wet Environments in
Thin-Film Parylene Systems
Jessica Ortigoza-Diaz , Kee Scholten , and Ellis Meng, Fellow, IEEE

Abstract— Parylene C is a thin-film polymer used as a biocompatible barrier layer in biomedical implants and implantable
MEMS; free-film Parylene C may serve as both the substrate
and insulation in polymer-based microdevices, a growing branchof biomedical technology. The adhesion of vapor deposited
Parylene C, particularly when exposed to wet, in vivo environments, is a critical determinant of device lifetime for suchpolymer-based implants. This paper explores several novel
strategies for improving the adhesion of multi-layer Parylene
structures, including thermal annealing and the use of several
chemical interposer layers. Interfacial adhesion of ParyleneParylene and Parylene-platinum-Parylene films was examinedusing a standard T-peel test to quantify adhesion and measurefilm integrity under chronic exposure to saline up to two years.
Improved adhesion and barrier properties in Parylene-Parylene
films resulted from the inclusion of diamond-like carbon andethylene glycol diacrylate layers. Thermal annealing improved
Parylene film integrity in wet environments but was insufficientfor improving the integrity of Parylene-platinum interfaces.
A 100-fold increase in adhesive strength at such interfaces
was achieved using a commercially available adhesion promoter,and the corresponding improvements in resistance to moisturedriven delamination were observed. X-ray diffraction and X-ray
photoelectron spectroscopy results are provided to highlight therole of film morphology and surface composition in adhesion
integrity.
[2018-0076]
Index Terms— Adhesion, AdPro Plus®, annealing, DLC,
EGDA, Parylene, T-peel test, XPS, XRD.

I. I NTRODUCTIONP

ARYLENE C, referred to here as Parylene and known
generically as poly-(para-chloro-xylylene), is a semicrystalline hydrophobic polymer formed as a thin, conformal,
and pinhole-free film using a unique chemical vapor deposition (CVD) technique. Parylene is recognized for its chemical
inertness, electrical resistivity, low moisture permeability, andManuscript received April 4, 2018; revised June 8, 2018; accepted
June 24, 2018. This work was supported by the NSF under Grant EFRI1332394. Subject Editor S. Konishi. (Corresponding author: Ellis Meng.)
J. Ortigoza-Diaz and K. Scholten are with the Biomedical Engineering
Department, University of Southern California, Los Angeles, CA 90089 USA(e-mail: jortigoz@usc.edu; kscholte@usc.edu).
E. Meng is with the Biomedical Engineering Department, University ofSouthern California, Los Angeles, CA 90089 USA, and also with the ElectricalEngineering Department, University of Southern California, Los Angeles,
CA 90089 USA (e-mail: ellis.meng@usc.edu).
This paper has supplementary downloadable material available at
http://ieeexplore.ieee.org, provided by the author.
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2018.2854636

proven biocompatibility. For several decades, thin Parylene
coatings have been used as waterproof insulation for electronics intended for use in harsh environments, a category that
now increasingly includes biomedical implants. Parylene has
been used extensively to coat silicon, metal, and glass surfaces
intended for extended in vivo use, including neural recording
probes [1], coronary stents [2], implantable electronics [3], [4]and dental implants [5]. In addition, Parylene is used as a
substrate material from which to microfabricate biomedical
implants because of its low Young’s modulus and flexibility.
Despite well touted barrier properties, Parylene coatings andthin film Parylene devices exposed to wet environments forextended durations can suffer from moisture permeation anddelamination; this limitation has motivated a significant bodyof work seeking to improve the lifetime and reliability of thesecoatings by increasing the strength of Parylene adhesion tosurfaces of interest. Many strategies have been investigated to
improve Parylene adhesion, including physical modifications(e.g. melting, anchoring, surface roughening), thermal modifications (e.g. annealing), chemical modification (e.g. surface
plasma treatment) and inclusion of chemical interposer layers(e.g. silane A-174 and plasma polymerized adhesion layers)[6]–[8]. An exhaustive search of literature reported techniques,with mechanical tensile and peels strength serving as a proxyfor coating adhesion, is summarized in the supplemental
material.
Due to its widespread use in biomedical implants, Parylene
has emerged as a key material in the growing field of polymerbased biomedical microdevices comprising microelectronic,
microfluidic and micromechanical systems wherein the bulk
structure is composed of the thin-film, flexible polymer.
Examples include polymer-based neural probes [9], [10],cochlear implants [11], [12], retinal electrodes [13], [14],and pressure sensors [15], [16]. Devices frequently feature a
simple symmetric design: a base layer of flexible polymer,
a thin layer of patterned metal (frequently platinum), and a
top, insulating layer of polymer. This approach is motivatedby evidence that reductions in size and mechanical rigidityof an implant can mitigate the physiological foreign body
response, and enable chronic in vivo performance. Thin andflexible polymer-based devices offer an enticing alternative torigid and sharp implants of silicon and metal, and Parylene,
owing to its extensive history in biomedical applications andcompatibility with standard micromachining processes, is aprime choice among available materials [14], [17]–[19].

1057-7157 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMSTABLE IC OMPARISON OF W ET A DHESION F ORCES R EPORTED IN THE L ITERATURE FOR PARYLENE I NTERFACES

As with examples of Parylene coated rigid structures,
thin-film Parylene devices are also subject to delamination
after extended exposure to wet, in vivo conditions. While a few
reports describe Parylene-based thin-film devices functioningin vivo for longer than a year [9], these examples are uncommon. Numerous reports describe moisture intrusion and subsequent delamination of Parylene films over periods of weeks
and months. These failure modes reflect poor adhesion of CVD
Parylene to pre-deposited Parylene films, or at Parylene-metalParylene interfaces [20]–[22]. Adhesion between Parylene
layers is dominated by physical adsorption and, to an extent,chemical reaction with free radicals, whereas adhesion at
Parylene-metal interfaces is typically mediated by a combination of hydrogen bonding and Van der Waals forces [18],[23], [24]. In either case, the formation of chemical bonds
through use of an intermediary linker is possible. For example,
Driesche et al. [25] have reported the use methacrylate-based
coupling agents to support Parylene-gold adhesion. Without
the use of such an adhesion promoter bonds are easily broken
by water molecules, creating a serious problem under in vivo
conditions where the interfaces encounter bodily fluids [23].
Delamination under wet conditions may be induced by water
vapor condensing within voids created by surface particulates present during CVD [4]. Stringent attention to surface
cleanliness may reduce the risk of this phenomenon, but evenwhen deposited under cleanroom conditions, Parylene coatings
still exhibit voids in which water vapor can nucleate andcondense. In addition, there are reports that coating failure canbe hastened by application of electric current across insulated

metal features [20], [22], the action of compounds generated
during inflammatory responses [26], and the presence ofinternal stress between layers [27]. Parylene structure andadhesion can likewise be effected by the chemical and physicaldemands of typical sterilization processes, required for clinicaluse in vivo [28].
While there are many proposed methods for improving
adhesion between Parylene and non-polymeric substrates,
there are relatively few reports describing methods to improve
Parylene-Parylene adhesion [29]–[37] or Parylene-metalParylene adhesion [8], [31], [32], [35]–[43] in thin-film
devices, and robust chronic adhesion remains elusive.
Table 1 shows the techniques reported in the literature for
improving Parylene adhesion to different materials under‘wet’ conditions, referring to chronic exposure to water and/or
saline. Table S1 summarizes an exhaustive search of the literature for methods to improve adhesion under dry conditions. By
far the most common approach is the use of thermal annealing[10], [13], [27], [44]–[48]. The application of temperatureand pressure during annealing can induce polymer-polymer
bonding through entanglement of polymer chains [29], [30],and the high temperature can alter Parylene crystal structure,
limiting the rate of moisture diffusion [27], [49]. Less common
are reports of chemical linkers or adhesion promoting layers
appropriate for Parylene multilayer structures. While silane
A-174 (3-trimethoxysilylpropyl methacrylate) is commonlyused to improve Parylene adhesion to glass and silicon,
the adhesion mechanism relies on the presence of hydroxyl
groups absent from Parylene surfaces, and as such theORTIGOZA-DIAZ et al.: CHARACTERIZATION AND MODIFICATION OF ADHESION IN DRY AND WET ENVIRONMENTS

compound does not promote strong adhesion between Parylene
layers [31]. Also, A-174 cannot be used when sacrificial
photoresist layers are present [50]. Other chemical adhesion
layers have been presented to aid Parylene coating of metals
such as gold through a thiol-linker group [25]. Currently, there
is no commercially available chemical linker demonstrated to
improve Parylene-Parylene bonding. An alternative approachentails the use of non-linking interposer layers; under thisapproach a thin-film is deposited prior to CVD Parylene, which
improves adhesion and/or operational lifetime by modifying
surface energy, providing improved barrier properties (seework with Al2 O3 [37]), or supporting adhesion to anintermediate layer (e.g. thin-film metal). Plasma-polymerizedfilms have been used for years to improve the properties ofParylene coated bulk metal structures [8], [23], [51], though
once again there are scant references to this approach forthin-film polymer-based devices.
There is no commercially available compound, or stateof-the-art protocol, established that adequately improves
Parylene-Parylene thin-film adhesion, beyond the broad recommendation to thermally anneal Parylene films. Several ideas
presented or proposed in the literature, including plasma
treatments and interposer layers [8], [31], [36]–[39], havenot been rigorously compared and most have not been tested
under chronic wet conditions. This represents a critical deficit
in current research, as thin-film polymer-based microdevices
are increasingly proposed for biomedical implantation because
they are soft and flexible, yet moisture intrusion and delamination remain frequently reported failure modes.
This study investigates and characterizes strategies toimprove the adhesion of Parylene-Parylene films and to extend
the operational lifetime of thin-film Parylene devices underchronic exposure to moisture, specifically in a simulated
physiological saline environment. The purpose is to advance
realization of a reliable, generalizable method. This work has
specific application to the creation of polymer-based microdevices intended for chronic in vivo implantation. We compared
a combination of thermal annealing strategies and the use of
several interposer films. Samples of Parylene-Parylene bilayers
were prepared with different adhesion strategies; the adhesivestrength of those interfaces were measured quantitatively under‘dry’ conditions, and as a function of soaking duration inwarm saline (simulated in vivo conditions) for up to two years.
We measured adhesive strength with 180° T-peel test, andrecorded the number of days of exposure in a ‘wet’ environment before each strategy failed. The efficiency of thermal
annealing was examined as function of annealing time, andwe report x-ray diffraction data as an examination of the corresponding morphological change in the Parylene films. Threechemical interposer layers were selected: amorphous diamondlike carbon (DLC), ethylene glycol diacrylate (EGDA), anda proprietary commercial compound sold under the brandname AdPro Plus®. DLC and EGDA have been previously
reported to improve the barrier properties of flexible polymer
films, though there is no quantitative report of their effect onParylene-Parylene adhesion. These materials were included asthey hypothetically offer improved adhesion and/or moisturebarrier properties [52]–[55]. AdPro Plus® is a Parylene-metal

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TABLE II
M ATERIAL C OMBINATIONS

adhesion promoter designed to improve adhesion of noble
metals to Parylene C and is the only promoter designed
specifically for this function to our knowledge. We compared
the adhesive strength and operational lifetime of ParyleneParylene films with interposer layers of DLC and EGDA,
and interposer layers of platinum supported by AdPro Plus®.
Platinum was chosen owing to its widespread use in polymerbased biomedical microdevices, a result of its high biocompatibility and corrosion resistance. For purposes of control wealso examined the adhesion strength and operational lifetimeof Parylene-platinum-Parylene films without AdPro Plus®,
and with and without thermal annealing treatments. Devices
were prepared with large area coverage of platinum films,
an unfavorable scenario compared with functional thin-film
devices, which feature very limited exposed metal.
While the formulation of AdPro Plus® is proprietary andnot disclosed, we provide an empirical examination of its
performance compared with experimental controls: thermal
annealing treatments common in current research, and EGDA
and DLC interposer layers, strategies motivated by recent
academic research. This work represents the most rigorous
examination of strategies to improve adhesion in multi-layer
Parylene-Parylene devices.
II. S AMPLE P REPARATION AND E XPERIMENTAL M ETHODSThis study examined wet and dry adhesion in ParyleneParylene and Parylene-platinum-Parylene film systems.
Table 2 lists the seven material combinations prepared andtested along with corresponding abbreviations used in thisarticle.
A. Sample Fabrication
Test structures for T-peel testing were fabricated using common polymer micromachining techniques. Structures consistedof a well-defined bonded area (4.3 × 3 mm) connected to twoParylene flaps (12 μm thick) each perforated with a 3.3 mmclamping hole (Fig. 1).
Figure 2 illustrates the layer-by-layer process by which samples were prepared. First, Si carrier wafers were dehydrated
at 110 °C then coated in a base layer of Parylene (12 μm)
by CVD (PDS 2010, Specialty Coating Systems (SCS),
Indianapolis, IN) (Fig. 2a). PMP samples were prepared by
sputter depositing platinum (2000 Å, LGA Thin Films, Santa
Clara, CA) through a photoresist mask (2 μm, AZ 5214 E-IR;
Integrated Micro Materials, Argyle, TX) followed by lift-off

4Fig. 1. a) Sample schematic of T-peel test structure, b) test structure in
T-peel pulling apparatus, and c) top and d) lateral views of a sample under
testing.

Fig. 2. Cross-sectional view of the fabrication process for PMP (left) andPP (right) T-peel tests samples.

to define the bonded area. A 4 μm thick film of photoresist (AZ 4400; AZ Electronic Materials, Branchburg, NJ)
was patterned lithographically to create a sacrificial spacer(Fig. 2b), prior to depositing the top Parylene layer (12 μm)
by CVD (Fig. 2c); the sacrificial layer assisted separationof the two Parylene layers containing the clamping regions.
The sample outline and clamping holes were etched using
O2 reactive ion etching (100 W:100 mTorr:5 min cycles;
RIE-80 Plasma Etching System, Oxford Plasma Technology,UK) through a thick photoresist mask (30 μm, AZ 4620;
AZ Electronic Materials, Branchburg, NJ) (Fig. 2d). The
sacrificial photoresist was removed with acetone, and thenthe released samples were rinsed with isopropanol and water(Fig. 2e).
B. Adhesion Layers
1) Ethylene Glycol Diacrylate: Ethylene glycol diacrylate is a cross-linked anchoring layer that can be grafted
to a Parylene surface by initiated chemical vapor deposition(iCVD). This technique allows for thin, uniform films to
be deposited conformally in a single step and without useof solvents [56]. EGDA was selected for investigation asa Parylene-Parylene adhesion layer, owing to recent reports

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS

demonstrating EDGA films as durable and moisture resistant when grafted to Parylene, despite a 30-day soak in
1× phosphate buffered saline (PBS) [52]. PEGDAP samples
were prepared using an iCVD process described in detail elsewhere [52], [56], [57] in which a benzophenone photoinitiator
is used to attach a thin (10-20 nm) EGDA film to the base
Parylene layer. Successful EGDA deposition was confirmed
by a decrease in water-droplet contact angle, indicating amore hydrophilic surface (55° ± 2°, compared to 90° typicalfor native Parylene) [52]. Samples were then processed tocompletion as described in A, with care taken to avoid any
cleaning or surface treatment after EGDA deposition.
2) Diamond-Like Carbon: Diamond-like carbon (DLC) isa conformal, hard, chemically inert, and low friction coating
consisting of an amorphous form of carbon with diamondbonds [53], [58]. Because of its potential as a barrier layer,
this film was also evaluated as a potential adhesion promoter
in Parylene-Parylene films. The 0.23 μm thick film wasdeposited by Morgan Advanced Materials using ion-beamplasma-enhanced CVD with a hydrogen concentration ranging
from 30% to 40%. After receiving the coated wafers, theDLC film was cleaned by O2 plasma, and samples were thenprocessed to completion as described in A.
3) AdPro Plus®: AdPro Plus® is a biocompatible, proprietary adhesion promoter available from Specialty Coating
Services, designed to improve adhesion between Parylene andmetals, such as, platinum, titanium, gold, etc. [21]. Wafers
were sent to SCS for treatment with AdPro Plus® and deposition of the bottom and/or top Parylene layers dependingon the interface, then returned for final processing. Samples
were prepared with AdPro Plus® at each Parylene-platinum
interface, and samples were prepared with the adhesion promoter present at both interfaces. No cleaning or surface
treatment was performed after the promoter deposition to avoid
removal or damage of the AdPro Plus® layer.
C. Thermal Annealing
A subset of test structures was thermally annealed in an
attempt to improve adhesion between layers and reduce moisture permeation, by way of an induced change in crystallinityand pore size. Several annealing durations were evaluated todetermine the effects of annealing on the adhesion of PP and
PMP samples. As reported by Charmet et al. [7], heating
Parylene above its melting point results in an amorphousmorphology which is susceptible to moisture intrusion, thus
samples were annealed at 200 °C for 24, 48 and 72 hours
under vacuum, then cooled to room temperature overnight.
The use of a vacuum was necessary to prevent oxidation of
Parylene, which occurs at temperatures higher than 125 °C in
the presence of oxygen [59]. During the annealing process,flaps of the T-peel structures were separated using a Teflon
film to avoid thermal bonding of the layers.
D. T-Peel Tests
Adhesive strength was measured using a T-peel test basedon ASTM standard D1876-08 [60]. Test structures were peeledapart at 180° using a custom motorized stage while force

ORTIGOZA-DIAZ et al.: CHARACTERIZATION AND MODIFICATION OF ADHESION IN DRY AND WET ENVIRONMENTS

Fig. 3.

Representative raw T-peel test data.

was measured using a 50 g load cell (Omega, Stamford, CT,
USA). Samples were clamped onto the stage with one end
fixed to a stationary post and the other to a movable post
driven by a stepper motor. The motorized stage was driven
at 2 mm/sec to slowly peel apart the interface at the bonded
region (Fig. 1). A characteristic T-peel measurement is shownin Fig. 3. The force registered by the load cell increases as
the bonded area begins to peel (first peak), then stabilizes toa relatively constant force during peeling, and drops to zeroas the interface is fully separated.
The peeling strength (mN/mm) is defined as the mean force
recorded on the load-cell, averaged over the period between
local maximums which denote the start and end of the peeling(marked by dashed red-lines in Fig. 3), divided by the widthof the bonded area. Four samples from each experimentalgroup were tested without exposure to solution (exempting
PDLCP, for which only two samples were measured), and four
samples were tested at each time point in the soaking study(as described in E).
E. Long-Term Soaking Study
Approximately 60 samples of every combination of experimental parameters (14 experimental groups in total) were
immersed in 1× concentration PBS at 37 °C to mimic physiological conditions. The degradation of the bond was measured
by T-peel test at time points 1, 4 and 7 days during the first
week. Then, samples were tested weekly for a month, andthen monthly for 2 months. After completion of a 3-monthsoak, samples were tested every 3 months until the samples
delaminated or until 1 year had elapsed. Then, samples were
tested every 6 months until delamination or until 2 years had
elapsed.
F. XPS
After T-peel tests Parylene interfaces were cleaned andrinsed with isopropanol and deionized water, then analyzed

Fig. 4.

5

XRD 2θ scans of un-annealed and annealed Parylene films.

using X-ray photoelectron spectroscopy (XPS), to determine
fractional atomic composition. The XPS measurements were
performed with a Kratos Axis Ultra DLD instrument (Kratos
Analytical, UK) with a monochromatic Al Kα x-ray source,
and probed the top 5 nm of each surface.
G. XRDThe crystallinity of Parylene films, prior and post annealing, was characterized by X-ray diffraction (XRD), using an
Ultima IV Powder Diffractometer (Rigaku, USA). Glass slides
were coated with 12 μm of Parylene and were cleaned withisopropanol and deionized water prior to measurements. The
scans were measured from 10° to 18° (incident angle) in order
to measure the Parylene diffraction peaks, which are known
to appear around 14° [61].
III. R ESULTS
A. Crystallinity/XRD Analysis
The XRD spectra of un-annealed and annealed Parylene
films for different anneal times is shown in Fig. 4. The
diffraction peaks appeared at 2θ ≈ 14° for all tested samples.
The crystallite size of the Parylene films was calculatedusing Scherrer’s equation, which is given by:
0.9λ(1)
FWHM cosθ
where λ is the wavelength of Cu Kα X-ray source, FWHM
(full width half maximum) is derived from the measured peak,and θ is the Bragg angle (degree of the diffraction peak). The
FWHM values were calculated by fitting a Gaussian function
to the peaks using OriginPro software (Northampton, MA).
The FWHM decreased as anneal time increased, whereas
the crystallite size of un-annealed Parylene increased after a
24-hour anneal and then was unchanged for 48 and 72 houranneal. The percentage of crystallinity is related to the intensity of the Bragg peak as the peak intensity increases theCrystallite size =6

JOURNAL OF MICROELECTROMECHANICAL SYSTEMSTABLE IIIP ROPERTIES OF PARYLENE ( N=1) AT D IFFERENT A NNEAL T IMES

Fig. 5. Failure modes presented at day 0 during testing for a) typical PMAdP
peeled apart sample with no tearing, b) PMAdP sample torn at the interface
and b) 48h-annealed PAdMAdP sample torn at the clamping hole.

crystallite concentration in the polymer increases as well [61].
The percentage is calculated as the ratio of the area of thecrystalline peak to the whole area, amorphous plus crystallized
area [62], [63]. Thus, the film that was annealed for 48 hours
has the greatest percentage of crystallinity.
B. T-Peel Tests
Thermal annealing resulted in dramatic increases in the
peeling force for the majority of the experimental groups,
and increased the stiffness of the Parylene layers. As such,
while most samples peeled apart at the bonded interface
during T-peel testing (Fig. 5a), we also observed two modesof failure: tearing at the interface (Fig. 5b) and at one of theclamping holes (Fig. 5c). This was observed for several PP,
PMP and PEGDAP samples, and therefore the reported values
must be strictly considered a lower bound of peeling strength.
C. Parylene-Parylene Samples
The adhesive strength of PP samples as a function of
annealing time is presented in figure 6a for samples annealed
for a duration of 24, 48, and 72 hours at 200 °C. The
mean force required to peel apart PP samples increased from
∼38 to ∼74 mN/mm following a 24-hour anneal, increased
further to ∼155 mN/mm following a 48 hour anneal, anddiminished for a 72 hour anneal. These samples were notsubject to long-term soaking and thus these experiments arereferred to as dry testing.
Fig. 6b shows how adhesive strength decreased as a functionof total time immersed in saline. Un-annealed samples suffereda catastrophic loss in adhesion following just a single day ofsoaking, and delaminated completely after 4 weeks, whereas
annealed samples exhibited greater moisture resistance andlonger lifetimes. The 48-hour annealed samples notably maintained minimal to no loss in adhesion over 2 years. Owing to
the results of these experiments, a 48-hour anneal was used inall subsequent testing where annealed samples were prepared.

Fig. 6. Average force per unit length required to peel apart annealed andun-annealed samples of Parylene-Parylene layers (mean±SE, n=4): a) peelingstrength as a function of thermal annealing time for dry samples, andb) peeling strength as a function of time soaked in warm saline.

D. Parylene-Platinum-Parylene Samples
The inclusion of metal (specifically platinum) between
Parylene layers resulted in a dramatic decrease in adhesion;
un-annealed PMP samples were peeled apart with just
∼3 mN/mm prior to soaking (Fig. 7a), while soaked
samples delaminated after just 4 days in saline (Fig. 7b).
Annealing significantly increased adhesion (to a peeling forceof ∼22 mN/mm), however, even annealed PMP samples
exhibited weaker adhesion than un-annealed ParyleneParylene samples. Annealed PMP samples exhibited gradualadhesion loss during soaking and delaminated after 3-weeks.
Figure 8 shows the different mechanisms of adhesion failures for annealed (Fig. 8a) and un-annealed (Fig. 8b) PMP
samples, after 3 weeks and 4 days, respectively. In annealed
samples, the metal film delaminated completely from bothbase and top layers of Parylene. Wrinkling of the metal film
demonstrated that significant stress was introduced by theannealing process. In un-annealed samples, the top (second)layer of Parylene consistently separated from the metal film,

ORTIGOZA-DIAZ et al.: CHARACTERIZATION AND MODIFICATION OF ADHESION IN DRY AND WET ENVIRONMENTS

Fig. 7. Average force per unit length required to peel apart annealed andun-annealed samples of Parylene-Parylene (PP) and Parylene-platinumParylene (PMP) layers (mean ± SE, n=4): a) Peeling strength of dry samples;b) Peeling strength as a function of time soaked in warm saline.

Fig. 8. Delaminated a) 48h-annealed and b) un-annealed PMP samples after
a 3-week and 4-day soak in PBS, respectively.

while the adhesion between the metal film and base (first)
layer of Parylene remained intact.
E. Parylene Samples With Adhesion Layers
Un-annealed samples with EGDA and DLC adhesion
layers exhibited improved adhesion, compared to unannealed Parylene-Parylene samples. The mean peelingforce for un-annealed PEGDAP and PDLCP samples was∼99 and ∼58 mN/mm respectively (Fig. 9a). Annealing

7

Fig. 9.
Average force per unit length required to peel apart annealed
and un-annealed samples of Parylene-Parylene (PP), Parylene-Ethylene glycol
diacrylate-Parylene (PEGDAP), and Parylene-Diamond-like carbon-Parylene(PDLCP) layers (mean ± SE, n=4): a) Peeling strength of dry samples;b) Peeling strength as a function of time soaked in warm saline.

improved the adhesion of PDLCP samples marginally but
not for PEGDAP samples; average peeling force for annealed
PDLCP samples increased to ∼68 mN/mm, while the peelingforce decreased to ∼83 mN/mm for annealed PEGDAP
samples (Fig. 9a).
Un-annealed PEGDAP samples exhibited minimal to noloss in adhesion over a period exceeding 54 weeks soaking
in PBS, whereas annealed PEGDAP samples exhibited aloss in adhesion strength after 12 weeks and completelydelaminated after a year in PBS. Un-annealed PEGDAP
samples still retained integrity up to at least 82 weeks ofsimulated in vivo environment (Table 4). The peeling force
for un-annealed and annealed PDLCP samples dropped in
the first 4 days and then it stabilized after a 4-week soak in
PBS up to a month and year for un-annealed and annealed
samples, respectively. Following an 8-weeks soak in PBS,
un-annealed PDLCP samples presented a loss of adhesion andwere completely delaminated after a year. Following T-peel
tests, we observed that delamination consistently occurredat the interface between the top (second) Parylene layer and8

JOURNAL OF MICROELECTROMECHANICAL SYSTEMSTABLE IVS TATUS OF S OAKING S TUDY

TABLE VT IME TO I NTERFACE FAILURE : W ITH & W ITHOUT A DHESION P ROMOTERDLC layer for un-annealed and annealed PDLCP samples.
PDLCP samples were curled after release due to the stress ofthe film. Table 4 summarizes the survival times of samples
subject to soaking until the submission of this manuscript.
F. Platinum Samples With Adhesion Promoter
Figure 10 displays the results of T-peel measurementson ‘dry’ samples of Parylene-platinum-Parylene films with
AdPro Plus® deposited below, above, and on both sides of themetal film. Samples with the adhesion layer deposited prior
to sputtering the platinum film (PAdMP) exhibited very weakadhesion, similar to untreated PMP samples (∼3 mN/mm).
In contrast, samples with the adhesion layer deposited after
sputtering the platinum film (PMAdP), and samples with theadhesion layer deposited both before and after the platinum
film (PAdMAdP), exhibited excellent adhesion (mean T-peelmeasurements of 336 and 143 mN/mm respectively). The
T-peel measurement of un-annealed PMAdP samples was thehighest recorded among all sample combinations. Thermal
annealing of these samples yielded unexpected results,
adhesion strength of PAdMP samples increased to produce
T-peel measurements of ∼52 mN/mm, while adhesion strengthof PMAdP and PAdMAdP decreased to 94 and 87 mN/mm
respectively. Most samples peeled apart during testing and the
top (second) Parylene layer detached from the metal, however,
all un-annealed PMAdP and annealed PMAdP and PAdMAdP
samples tore at the interface and clamping holes, respectively.
Results from soak-testing reflected these same trends, andare compiled in Table 5. Un-annealed PAdMP samples failed
after just 4 days of immersion in saline (similar to untreated
PMP samples), but, if annealed, withstood 3 weeks before
suffering delamination. Un-annealed PMAdP samples exhibited excellent resistance to moisture, but failed after just
2 weeks immersion in saline if annealed. In annealed samples,the metal film delaminated completely from both base (first)

Fig. 10. Average force per unit length required to peel apart annealed andun-annealed samples of Parylene-platinum-Parylene layers with and without
AdPro Plus® adhesion promoter (mean ± SE, n=4). Samples featured AdPro
Plus® between the base Parylene and metal layer (PAdMP), between themetal and top Parylene layer (PMAdP) and on both sides of the metal layer(PAdMAdP): a) Peeling strength of dry samples; b) Peeling strength as afunction of time soaked in warm saline.

and top (second) layers of Parylene while in un-annealed
samples, the top (second) layer of Parylene separated fromthe metal film.
G. XPS Analysis
Table 6 shows the results of XPS, detailing the atomic composition at the failing surface of the platinum interfaces after
peel tests. Samples are presented corresponding to different
time points in the soaking study. The presence of oxygenin platinum containing samples is likely the result of theO2 plasma cleaning process used after metal deposition andprior to the deposition of AdPro Plus®.
The most notable feature of this dataset is the presence andstrength of the platinum signal. In no sample was platinum
detected on the top (second) layer of Parylene following
t-peel testing. Instead, platinum was detected only on thebottom (first) layer of Parylene, with the exception of theORTIGOZA-DIAZ et al.: CHARACTERIZATION AND MODIFICATION OF ADHESION IN DRY AND WET ENVIRONMENTS

TABLE VIATOMIC C OMPOSITION OF THE M ETAL C OMBINATIONS

annealed PAdMP sample for which no platinum was detected;
this is consistent with visual observation that the stressed
platinum film delaminated entirely from both Parylene layers.
This dataset was useful for examining the relative strengthof platinum-Parylene bonds. Notably, the platinum film consistently and preferentially remained on the bottom layer.
Beyond this, variation between results proved too severe to
draw precise conclusions about the surface composition.
IV. D ISCUSSION
Our results confirm both the relatively weak adhesion
between untreated Parylene layers, and the susceptibility of
Parylene coatings to adhesive failure and delamination under
wet conditions. This phenomenon has been noted extensively,
both in literature and anecdotally, yet Parylene remains animportant insulator for biocompatible implants. For free-film
devices, such as those tested here, the susceptibility is exacerbated as both sides of the film offer a conduit for moisturepenetration.
Experimental testing of annealed samples show that the thermal treatment dramatically improves both Parylene-Parylene
adhesion and barrier properties. This is in agreement with priorreports, and the mechanism can be understood as thermallyinduced entanglement of the polymer chains and an increasein crystallinity which reduces moisture permeation. The useof a 48-hour anneal has been commonly reported, again
both in the literature and anecdotally by other researchers,
and the comparison of the 24-hour, 48-hour and 72-hourdatasets shows this protocol is well supported by the data.
While improvements in adhesion and barrier properties areevident after 24 hours, continued annealing more than doubles
the adhesive strength, while the lack of improvement at the
72-hour mark suggests whatever crystallization or morphology
change is occurring is accomplished at the end of 48 hours.
This is further supported by the XRD data (Fig. 3); the sharpest
diffraction peaks were observed for samples annealed for
48-hours, with less crystallinity observed for 24- and 72-hourannealed samples.
We measured incredibly weak adhesion of Parylene
to platinum films, and the thermal annealing method
improved the adhesive strength and barrier properties only

9slightly. Our experiments examined only platinum, andspecifically sputter-deposited platinum, so we are hesitantto generalize to all metals or even platinum films depositedby other methods, however, the likely cause of this weakadherence is the mismatch in surface energy between thehydrophobic Parylene layer and the hydrophilic platinum
layer (in agreement with Hwang et al. [64]), and such amechanism would likely apply to other metals as well.
Despite improvements following annealing, the lifetime ofannealed PMP samples requires further improvement toachieve long term Parylene-based implants. This result is ofserious concern; the majority of both polymer bioMEMS andParylene coated medical devices incorporate conductive metal
layers, and platinum specifically is a common choice.
The use of EGDA and DLC interposer layers increased thelifetime of Parylene-Parylene devices under soaking conditions. Annealing produced minimal improvement, and neither
the inclusion of DLC or EGDA was comparable to annealed
PP in adhesive strength or lifetime (Fig. 9). We will note that
there was a considerable period of time (6 months) between
the DLC deposition and the final Parylene insulation, and thisrisked oxidation or contamination of the DLC surface. DLC
was not tested with a platinum layer as this would require atitanium adhesion layer between interposer layer and platinum,
which would introduce a confounding factor. EDGA was nottested with a platinum layer as the solvents required for metalliftoff were expected to attack the adhesion layer.
AdPro Plus® was chosen for testing because it is advertised exclusively for improving adhesion at Parylene-metalinterfaces. Results of both soak and T-peel tests show adramatic improvement in barrier properties and adhesion for
Parylene-platinum-Parylene samples, but also revealed a critical sensitivity to temperature that may complicate processing.
Un-annealed PMAdP samples exhibited the strongest adhesive
force of any sample tested in this study (Fig. 10), and agreatly extended lifetime (>24 weeks) under saline soaking
conditions. However, annealed PMAdP samples incurred asignificant decrease in both adhesion and lifetime, as didannealed PAdMAdP samples. As the only difference between
the un-annealed and annealed samples was the heat treatment
itself, we are confident that this loss in adhesion can be
ascribed to the action of the temperature on the AdPro Plus®.
Our hypothesis is that the adhesion promoter denatures underhigh temperature, and this is supported by the failure of the
PAdMP samples, which delaminated almost immediately. The
deposition of platinum and associated lithography can drive
samples to temperatures above 100 °C, and it would appear
this is sufficient to disable the adhesion promotor. These results
are further supported by the XPS data; the bottom layer ofthe un-annealed PAdMP exhibits a small platinum signal thatvanishes following annealing, suggesting the platinum film lostadhesion after heat treatment. Conversely, the bottom layerof the un-annealed PMAdP samples exhibit a strong platinum
signal, that increases following annealing, suggesting the platinum film lost adhesion to the top layer after heat treatment.
In all cases, we see that the platinum film is retained on
the bottom layer of Parylene, suggesting stronger adhesion of
metal deposited on Parylene than Parylene deposited on metal.

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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS

We note that prior work by Charmet et al. [7] examined
a combination of thermal treatment (350 °C) and the useof silane A-174 adhesion promotor at the interface between
silicon and CVD Parylene. Their results indicated no decreasein adhesion or insulation integrity with the addition of theheat treatment. The heat sensitivity of AdPro Plus® appears
to be intrinsic to this specific formulation, and is not a general
result of combining thermal treatment with chemical adhesion
layers.
In almost all experiments, adhesion failure occurred
between the top (insulating) layer of Parylene and the bottom layer regardless of material (e.g. base layer Parylene,
platinum, EGDA, DLC). This agrees with previous findings
that, absent the formation of a chemical bond, Parylene
typically adheres poorly as deposited, particularly to smoothsurfaces or materials with significant differences in surface
energy [65], [66]. We also observed very strong adhesion
between deposited platinum and the bottom (base) layer ofParylene. This may be a result of the O2 plasma descum
performed prior to sputtering, which can roughen surface
morphology and induce hydrophilicity in Parylene. Finally, we
also note that the samples tested featured large-area platinum
films, relative to smaller patterned structures commonly usedin devices. Therefore, the presence of PP interfaces between
metal features, such as, traces and electrodes, may improveoverall adhesion of the structure.
V. C ONCLUSIONS
The adhesion of thin (∼10 μm) CVD Parylene films to predeposited layers of Parylene and metal is insufficient for robust
insulation intended for chronic exposure to wet environments.
Thermal annealing of Parylene under vacuum significantly
improves both the barrier properties of the bulk medium andthe adhesion of Parylene-Parylene interfaces, enabling samples
to survive for up to 2 years in saline, and the benefits of
the annealing process are exhausted after 48-hours at 200 °C.
Annealing, however, is insufficient for improving the adhesion
at Parylene-platinum interfaces. Interposing materials, including diamond-like carbon and ethylene glycol diacrylate, canprovide improvements in both the moisture resistance andadhesion at Parylene-Parylene interfaces without reliance onheat treatments. The inclusion of AdPro Plus® between platinum films and insulating Parylene layers improved adhesion
and moisture resistance by an order of magnitude beyond thatof either annealed or un-annealed Parylene-platinum-Parylene
films, however the adhesion proved very sensitive to elevatedtemperatures, and may not be compatible with additional
processing.
Broadly, we note that despite its reported barrier properties,
very thin Parylene layers are susceptible to moisture intrusionand subsequent insulation failure/delamination. Methods
which reduce water vapor permeation, including the use ofmoisture resistant interposer layers and crystallization underthermal annealing, can significantly reduce this effect anddramatically extend the lifetime of Parylene-based or Paryleneinsulated devices exposed chronically to water or saline.
Reducing moisture intrusion alone is insufficient if there issignificant mismatch in surface energy between Parylene andthe target, which in this study included sputtered platinum.
Methods that roughen the Parylene surface, or induce
hydrophilicity, such as O2 plasma exposure, appear tosignificantly improve adhesion. Improving dry and wet
adhesions of Parylene to metal is critical for long-term stabilityof material interfaces in Parylene-based medical implants.
ACKNOWLEDGMENT
The authors thank Dr. Joseph E. Yehoda of Morgan
Advanced Materials for depositing DLC films, Frank Ke and
Dr. Rakesh Kumar of SCS for depositing AdPro Plus® andParylene films, Mark De Luna for depositing EGDA and XPS
measurements and helpful discussions, and the members of
the Biomedical Microsystems Laboratory for their assistance.
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Jessica Ortigoza-Diaz received the B.S. degree
in mechatronics engineering from the National
Autonomous University of Mexico in 2013 and the
M.S. degree in biomedical engineering from the University of Southern California (USC) in 2018, where
she is currently pursuing the Ph.D. degree in biomedical engineering with the Biomedical Microsystems Laboratory. She is currently focusing on thereliability testing of Parylene C. She was a recipientof the USC–CONACyT Graduate School Fellowship.

Kee Scholten received the B.S. degree in applied
physics from the California Institute of Technology
in 2009 and the Ph.D. degree in applied physics
from the University of Michigan in 2014, under
Prof. E. Zellers, with a focus on opto/microfluidic
sensors for gas-phase micro-devices. He is currently a Post-Doctoral Scholar with the Biomedical
Microsystems Laboratory, University of Southern
California. His research explores the development of
micro- and nano-technology for ubiquitous chemicaland biomedical sensing, with a focus on microelectromechanical systems including microfluidic transducers for chemicalsensors, and flexible electrochemical interfaces for neurological sensors.
Ellis Meng (M’02–SM’09–F’16) received the
B.S. degree in engineering and applied science andthe M.S. and Ph.D. degrees in electrical engineering from the California Institute of Technology(Caltech), Pasadena, in 1997, 1998, and 2003,
respectively. Since 2004, she has been with the University of Southern California, Los Angeles, where
she was the Viterbi Early Career Chair with theViterbi School of Engineering and was the Department Chair from 2015 to 2018. She is currently aProfessor of biomedical engineering with the University of Southern California, where she also holds a joint appointment with
the Ming Hsieh Department of Electrical Engineering. Her research interests include bioMEMS, implantable biomedical microdevices, microfluidics,
multimodality integrated microsystems, and packaging. She is a member of
Tau Beta Pi, the Biomedical Engineering Society, the Society of WomenEngineers, and the American Society for Engineering Education. She is afellow of ASME, BMES, and AIMBE. She was a recipient of the Intel Womenin Science and Engineering Scholarship, the Caltech Alumni Association
Donald S. Clark Award, the Caltech Special Institute Fellowship, the NSF
CAREER Award, and the Wallace H. Coulter Foundation Early Career
Translational Research Award. In 2009, she was recognized as one of theTR35 Technology Review Young Innovators under 35.