FABRICATION OF FLEXIBLE POLYMER BIO-MEMS
WITH SUBMICRON FEATURES
Kee Scholten and Ellis Meng
University of Southern California, Los Angeles, California, USAdevices. Additionally, we outline a path towards
parallelization of this method using plasma-enhanced
nano-contact
printing
onParylene
substrates.

ABSTRACT
We present a fabrication method for thin-film
polymer MEMS that achieves an order of magnitudeimprovement in feature size and resolution. Specifically,
we modified the electron beam lithography (EBL) process
for biocompatible poly(chloro-para-xylylene) (ParyleneC), enabling flexible, encapsulated titanium structureswith features as small as 100 nm. The mechanical,
electrical and material properties of these Parylene
nanodevices were investigated, and strategies to further
reduce feature size and achieve batch scale
nanofabrication are discussed.

INTRODUCTION
The proliferation of polymer-based MEMS andmicroelectronics has created a new a platform for
implantable biomedical devices. Common MEMS
materials such as silicon and glass are rigid and therefore
poorly suited for chronic implantation in soft tissues [1];high Young’s moduli materials are likely to induce tissue
damage and consequentially trigger the body’s immune
response, leading to the encapsulation or impediment of
the device by scar tissue. By adapting standard
microfabrication
techniques
tosubstrates
ofbiocompatible polymers, researchers can create flexible,‘soft’ implants which retain the small size and precise
engineering of MEMS technologies. These include
electrode array interfaces to the cortex [2, 3],hippocampus [4], peripheral nerves [5], and retina [6, 7].
Many such devices conform to a common archetype:surface micromachined structures fully encapsulated inbiocompatible polymer (e.g. Parylene, polyimide,PDMS). Unfortunately, achievable resolution (> 5 µm) isconstrained by micropatterning techniques compatible
with polymer substrates. This restriction precludes
polymer-based implants with the component density ofsilicon devices; for example, silicon neural probe arrays
can contain > 100 recording sites [8], while polymer
devices typically have merely 4-16 [9]. However,submicron lithography on polymer substrates is nontrivial; many nanopatterning techniques are incompatible
with polymers which cannot tolerate high-temperatures,
high-intensity radiation, and harsh chemical treatments. In
addition, devices intended as flexible implants require
highly conformal insulation and robust adhesion ofpatterned structures to the polymer base, requirements
strained as components approach the nanoscale.
Only recently have submicron fabrication techniques,ubiquitous among silicon processing, been applied topolymer substrates [10]. Recently we demonstrated anEBL method for patterning free-film Parylene C devices,with submicron electrical structures [11]. Here we
describe an improved method that enables flexible bioMEMS with features as small as 100 nm, in addition tomechanical and electrical characterization of the resulting

978-1-5090-5078-9/17/$31.00 ©2017 IEEEFigure 1 Process flow for submicron fabrication onParylene C by EBL; (a) Contact pads and traces arefabricated on CVD Parylene, (b) PMMA bilayer isdeposited then coated with Cr, and finally patterned byEBL, (c) Ti features are defined by lift-off, (d) Features
are encapsulated in Parylene, and the device is released
with a O2 plasma etch.
.

METHODSElectron Beam Lithography on Parylene CA modified EBL method (Fig. 1) was used inconjunction with lift-off to pattern metal structures on
Parylene substrates. All experiments used clean silicondies (~1-2” sides) coated in 10 µm of Parylene C by wayof room-temperature chemical vapor deposition (CVD);
Silicon served purely as a mechanical support for theParylene base. A resist bilayer was prepared frompolymethylmethacrylate (PMMA, MW: 495k & 950k)
dispersed in anisole. Samples were prepared with resistlayers either 570 nm (400 nm 495k, 170 nm 950k) or 375nm thick (300 nm 495k, 75 nm 950k). Each layer wasdeposited by way of spin-coating, and the samples
subsequently baked at 120 °C for 20 minutes to drive off
residual solvents. Use of higher temperatures
recommended by the manufacturer resulted in stressinduced cracking of the Parylene film. Samples werefinally coated in 15 nm Cr to offset charge accumulation
during e-beam exposure. Features were written at
accelerating voltages of 10-20 kV, exposure dosage of
177 - 600 µC/cm2, with a 30 µm diameter aperture. TheCr layer was etched away using Cr-etchant, and thefeatures developed in a solution of methyl-isobutyl ketone
in isopropanol (1:3 mixture ratio) for approximately 20
seconds. Features were metallized by evaporating Ti (150nm thick), followed by lift-off in acetone. Typicallydevices were then encapsulated in an insulating Parylene
C layer (10 µm).

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MEMS 2017, Las Vegas, NV, USA, January 22-26, 2017

probe. Resistance measurements were also collectedduring mechanical testing: electrical contacts were madeusing wire and conductive silver epoxy, and resistance
was recorded during manual deformation, bending, andtorqueing of the test devices. Devices were also flexedusing a moving stage and stepper motor, while measuringchange in DC resistance as a function of bend angle.
High-Density Neural Probe Prototype
A prototype flexible neural microprobe wasfabricatedusingacombinationofpolymer
micromachining and the above EBL method. The probe
consists of a flexible Parylene shank (2 mm×150 µm)
supporting 32 Ti electrodes (30 µm diameter). Electrodesites were fabricated on a 10 µm thick Parylene base layerusing UV lithography and lift-off. EBL patterning wasthen used to define the connective traces (750 nm widthand spacing, 2 mm long), the critical enabling feature of
the device, and also to pattern the electrode surfaces witharrays of raised Ti structures (400 nm width). Finally theoutline of the device was defined with DRIE and thedevice was peeled from the silicon carrier. A similar
approach was also attempted to produce a Parylene-based
microprobe with 64 electrode sites along a shank ofsimilar dimensions, with more densely spaced connective
traces (400 nm spacing and width).

Figure 2 (a) SEM micrograph of Ti squares of decreasing
size (5 - 0.10 µm), inset shows enlarged view of 100 nmfeatures. (b) SEM micrograph showing the bisected crosssection of Ti wires (400 nm widths and spacing)encapsulated in Parylene C. Intercalary insulation ofParylene C is clearly visible.
Test Structure Fabrication
Three sets of samples were produced to assess thefabrication technique and the properties of the resulting
devices. First: Squares with sides ranging 0.075 - 5 µm
were patterned on Parylene C films using the above EBL
method, and metallized for imaging with scanningelectron microscopy (SEM). These samples were used tomeasure the minimum resolvable pattern the techniquecould produce, and the smallest achievable features which
would adhere to the flexible substrate without
delamination. Second: Rows of parallel metal wires withspacing and widths 0.04 - 5 µm were fabricated through
EBL, encapsulated in Parylene C, then bisected with O2
plasma. Cross-sections of these samples were imaged by
SEM to examine the conformality of CVD Parylene at thesubmicron scale. Third: Conductive test structures werefabricated incorporating straight and serpentine metaltraces with widths and spacing of 0.15 – 5 µm and lengthof 100 µm that spanned larger metal contact pads.
Fabrication entailed patterning Ti alignment marks andcontact pads on a 10 µm thick Parylene C layer using
standard UV lithograph and lift-off, followed bydefinition of submicron structures using EBL. Devices
were encapsulated in Parylene (10 µm), and the contact
pads exposed using a deep reactive O2 etch (DRIE). A
second DRIE step cut-out the devices, which were peeled
off the silicon carrier.

Figure 4 (a) Plot of resistance vs. length-to-width ratio ofpatterned test structures and (c) plot of resistivity vs trace
width. Shaded region spans typical resistivity for thin-film
Ti. Straight (square) and serpentine (circle) traces.

Figure 3 Submicron Ti structures on Parylene C (a) SEM
image of straight 0.15 µm trace and (d) serpentine 0.4 µm
trace with 0.4 µm spacing.

RESULTS
Patterns with features as small as 100 nm and as large
as 5000 nm were lithographically defined on Parylene
substrates through EBL (Fig. 2a). This resolution wasachieved with exposure parameters of 420 µC/cm2 at 20kV, using the ‘thin’ (375 nm) resist bilayer. Reducing theCharacterization & Imaging
Samples were coated with a thin-layer (<5 nm) of Pt,
prior to imaging with SEM, to prevent charge build-up in
the non-conductive Parylene film. Test structures wereprobed for electrical DC resistance using a basic 2-point437

a single polymer shank to the authors’ knowledge.
Connective traces (750 nm pitch and width) werecontinuous without short or break over their 2 mm length,and exhibited the same flexibility as the conductive teststructures. Line resistance was not recorded directly, but
can be extrapolated as ~150 kΩ DC using the results fromelectrical characterization. The raised structures patternedon electrode recording sites increased the effective surface
area of the electrodes by 40%, compensating for thecomparably small geometric surface area of the electrodes(~700 µm2). Fig. 6b shows connective traces from a failed
64-electrode Parylene probe with 400 nm pitch and width.
These devices suffered from several shorts and breaks inconnective traces due to large proximity effects during ebeam exposure.

resist thickness allowed for resolution to be improved
twofold compared with the thicker (570 nm) resist, while
maintaining suitable liftoff profiles. Exposure intensity
above 600 µC/cm2 produced distorted patterns and, insome cases, ablated the underlying Parylene, while
intensity below 240 µC/cm2 was insufficient to develop
patterns. Ti structures with critical dimensions as small as
250 nm could be reliably machined, and exhibited strong
adhesion to Parylene even under repeated deformation.
Parylene insulation was remarkably conformal
despite submicron length-scales. Figure 2b shows
representative SEM images of bisected titanium wires
embedded in Parylene C. The intercalary space between a
set of structures is fully encapsulated, despite the small
pitch and spacing (400 nm). Smaller structures proved
difficult to image with clarity, though there was noindication of poor insulation. This result bodes well forthe use of such devices in implantable bioMEMS, where
good barrier properties is a necessity to minimize water
intrusion or electrical crosstalk.
Flexible conducting traces were successfully andreliably produced with submicron widths (0.25-5 µm) andspacing (0.4-5 µm); traces as thin as 100 nm were
produced though with considerably lower yield.
Representative devices are shown in Figure 3. Linear
traces thinner than 250 nm were observed to break along
their length, while serpentine traces with spacing closer
than 400 nm were frequently distorted due to proximity
effects during exposure. Devices were metallized using
Ti, Au and Pt, with Ti selected for experiments owing tostrong Parylene adhesion and proven biocompatibility.
Submicron structures produced with this method wereflexible and robust; Ohmic characteristics (Fig. 4a) withlow resistivity (patterned Ti: 3.5-6.4 µΩ•m, Fig. 4b) andlow contact resistances (50-100 Ω) were observed.
Resistivity was notably higher than unpatterned Ti films
evaporated to the same thickness (1-2 µΩ•m) [12], andincreased with decreasing width.
Resistance varied immeasurably during manuallyhandling, torsion and bending. Data collected using astepper motor revealed < 0.5% change in resistanceduring bending beyond an angle of 75°, and to a radius ofcurvature below 1 mm (Fig. 5). Sudden and catastrophic
failure was observed at bending radii < 0.2 – 0.3 mm.

Figure 6 Optical micrograph of prototype polymer neuralprobe; (a) complete prototype with 32 electrodes: insets
show SEM micrograph of (left) 32 connective traces with
750 nm pitch and spacing and (right) nanopatterned
electrode site. (b) Section of failed prototype featuring 64
connective traces with 400 nm pitch and spacing,featuring prominent trace break.

DISCUSSION
The high electrical resistivity and low thermal budget
intrinsic to polymers creates several obstacles for
traditional EBL processing. By preparing e-beam resists
at lower temperatures, incorporating a conductive Cr
overcoat, and carefully calibrating exposure dose andvoltage, EBL was successful onvParylene C substrates.
We achieved pattern resolution an order of magnitude
greater than that of typical polymer micromachining withhigh fidelity and good repeatability. Thin-film metal teststructures patterned on Parylene C using EBL were
electrically and mechanically robust down to 250 nm, able
to withstand the deformation and strain expected offlexible devices without mechanical failure or apparent
delamination. Conformality tests were similarly
promising, with CVD insulation effectively coating
submicron structures without voids despite submicron

Figure 5 Representative data from mechanicalcharacterization (700 nm serpentine trace): (left)
fractional change in resistance plotted against bend
angle; (right) side image of bent device during test
A prototype neural microprobe was fabricated (Fig.
6) consisting of a flexible Parylene shank (2 mm × 150µm) supporting 32 electrodes: the highest density array on438

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intercalary spacing.
Though EBL offers high resolution the process isslow and serial; for example, each high-density neuralprobe required 90 minutes of write-time per probe inaddition to arduous pattern alignment. Outside of small
batch production used in research and prototyping, a moretime efficient approach is necessary to achieve a practicalbatch-scale alternative. In on-going work, we areinvestigating a large-area pattern-transfer techniqueadapted from nano-contact printing processes usedpreviously with other polymers [13]. The method isbriefly outlined in Figure 7. First, a negative of thedesired pattern is etched into a glass or oxidized siliconwafer which is coated in Au and a thin Ti adhesion layer.
Then the coated wafer and a Parylene coated sample areexposed briefly to air plasma (20% O2, 80% N2) which
results in exposed hydroxyl surface sites. The two
surfaces are pressed together and heated above 100 °C.
The Ti layer forms a strong bond with the Parylene
surface, and transfers the weakly bound Au pattern to theParylene film. Preliminary results suggest transfer ofhigh-resolution patterns across large-areas is possible withadditional process optimization.

Figure 7 Illustration of nanocontact printing; (a) etchedglass stamp is coated in Au/Ti and plasma treated tocreate hydroxyl sites; (b) metal pattern is transferred toParylene with application of pressure and heat.

CONCLUSION
Electron beam technology is viable technique forpatterning organic and biocompatible polymers withmodifications that account for polymer properties. Here
we have shown pattern resolution of 100 nm is achievableusing thin resist layers and careful calibration of the EBL
recipe for use with Parylene substrates. Thin-film
structures produced with this method, including electricalcomponents, are electrically and mechanically robust
despite the use of a compliant substrate. The combinationof EBL with CVD Parylene C enables the creation offlexible bioMEMS with encapsulated submicroncomponents. This approach has applications for novel
polymer implantable devices, demonstrated here throughthe production of a polymer neural microprobe with highelectrode density. Submicron processing is a necessary
advance for polymer MEMS, and future work will focus
on improvements in scale and parallelization.

ACKNOWLEDGEMENTS
This work was funded by a Postdoctoral Scholar ResearchGrant from the Office of the Provost at USC. The authors
would like to thank Dr. Donghai Zhu of the Keck
Photonics Laboratory for help with fabrication, andmembers of the Biomedical Microsystems Laboratory ofUSC for their assistance.

REFERENCES[1] K. Scholten and E. Meng, "Materials for microfabricatedimplantable devices: a review," Lab on a Chip, vol. 15,
pp. 4256-4272, 2015.
[2] B. Kim, J. Kuo, S. Hara, C. Lee, L. Yu, C. Gutierrez, etal., "3D Parylene sheath neural probe for chronic

CONTACT*E. Meng, tel: +1-213-821-3949; ellis.meng@usc.edu

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