PARYLENE ENCAPSULATED SUB-MICRON STRUCTURES FOR
IMPLANTABLE BIOMEMS
K. Scholten and E. Meng*
Department of Biomedical Engineering, Viterbi School of Engineering, University of Southern California, USAABSTRACT
We present a method for sub-micron machining of flexible,
thin-film structures fully encapsulated in biocompatible polymer
poly(chloro-p-xylylene) (Parylene C) that improves feature size
and resolution by an order of magnitude compared to prior work. A
low temperature electron beam lithography (EBL) processcompatible with Parylene-coated silicon substrates was developed,
and characterized using patterned Ti structures with criticaldimensions down to 250 nm, including conducting traces,
serpentine resistors, and electrodes with a nano-patterned texture.
Using this newly developed technique, the first flexible, free-filmParylene-Ti-Parylene devices with nanoscale components werefabricated and characterized. One application of these sub-micron
structures encapsulated in Parylene is next generation minimallyinvasive implants. Thus, we also demonstrated a prototype highdensity Parylene-based microelectrode neural probe using ournanopatterning approach.

INTRODUCTION
Polymer MEMS on flexible substrates enable novel
microdevices ideal for minimally invasive biomedical implants.
The typical approach is to employ surface micromachined
structures encapsulated in biocompatible polymers such as
Parylene C, polyimide, or PDMS. The selection of polymeric
substrates over conventional MEMS materials like silicon or glass
conveys several advantages: low elastic moduli, high electricalresistivity, chemical resistivity, optical transparency, andmechanical flexibility. In particular, a major motivation for the useof polymers are their mechanical properties, especially for
applications involving contact with soft tissue, such as retinal orneural microprobes, where rigid substrates can induce tissue
damage due to mechanical mismatch, and subsequently trigger
immune responses that impede performance and device lifetime. In
recent years several successful examples of implantable polymer
bioMEMS have been demonstrated, including implantable neuralprobes [1, 2], flow sensors, and pressure sensors [3, 4].
Despite the many advantages of polymeric micromachined
devices, a major limitation remains - the poor feature resolution (>5µm) achievable with typical micromachining processes that arecompatible with polymer substrates. While sub-micron fabricationis routinely employed on conventional MEMS materials, there has
been limited work transitioning this capability to soft polymer
substrates [5]. Nanoscale patterning has been applied to polymers
to create flexible stamps and shadowmasks for fabrication on othersubstrates [6, 7], but direct sub-micron fabrication on polymer
surfaces, or for polymeric biomedical implants remains atechnology gap [8]. Existing nanolithography protocols involve
exposure of substrate materials to high temperatures and energies
that exceed recommended operating conditions for mostcommonly used MEMS polymers. Methods such as nano-imprint
lithography and self-assembly benefit from their highlyparallelized approach to cover large surface areas and have beenused to pattern repeated arrays of simple structures on polymers [9,10]. However, these techniques place restrictions on material
choice and are pose difficulties in aligning multi-layer patterns,
such as those required for many implantable microdevices.

As such, existing polymer MEMS cannot compete withsilicon devices on feature density or feature size, placing limits on
structures that can be easily produced and the resulting device
capabilities. For example, polymer neural microprobes typicallycontain less than a dozen electrodes for stimulating or recording,whereas the state-of-the-art comprises silicon devices with morethan a hundred electrodes per probe [11]. This limitation precludes
polymer MEMS with the capabilities, complexity or minimal
footprint achievable with conventional materials.
We developed a modified electron beam lithography (EBL)
protocol that enables fabrication of biocompatible polymer MEMSwith sub-micron feature resolution; our design consists of thin,free-film Parylene devices encapsulating titanium structures withfeature size as small as 250 nm. This resolution was achieved withminimal optimization and represents a starting point for achievable
nanoscale resolution. We describe our fabrication process andpresent initial electrical and mechanical characterization, showing
our devices as functional, flexible and robust. Finally, we present ademonstrative application of this technology: a prototype Parylenebased cortical neural microprobe, with a greater number anddensity of recording sites than any prior polymer probe.

Figure 1: Process flow for sub-micron fabrication on Parylene C:(a) Ti contact pads and traces were fabricated on a vapor
deposited
Parylene
filmusingconventional
contact
microlithography, (b) PMMA bilayer (PMMA 495, 950) was
coated with Cr (15 nm) then patterned with e-beam, (c) Ti features
were deposited with lift-off, (d) features were encapsulated in
Parylene contact pads were exposed with plasma etch, and device
was released.

METHODS
Fabrication
The preparation of encapsulated structures (Figure 1) entailed
a combination of conventional UV lithography, polymer
micromachining, and a modified EBL method. First, silicon carrier
wafers were coated in 10 µm of Parylene C through chemicalvapor deposition (CVD). Structures including alignment marks,
contact pads and large electrical traces were patterned
lithographically using a OAI hybralign series 200 mask aligner(OAI, San Joesa, CA), then metallized with titanium using e-beam
evaporation and liftoff. A PMMA resist bilayer was prepared
comprising 495k 2% solids in anisole (400 nm thick) and 950k 6%

solids in anisole (170 nm thick). Residual solvents were driven off
with a 120 °C hotplate bake for 20 minutes following deposition ofeach layer; the comparatively low baking temperature wasnecessary to avoid stress-induced Parylene cracking or polymeroxidation. A conductive chromium layer (10-15 nm) was deposited
above the resist to prevent charge buildup during e-beam exposure,a consequence of the insulative properties of the Parylene layer.
Representative images of the fabrication process development andresults are shown in Figures 2-4.

change to be monitored as a function of bend angle.

Figure 3: Representative SEM images of sub-micron Ti structures
on Parylene: (a) straight 5 µm trace, (b) straight 0.25 µm tracewith inset showing detailed view at higher magnification, (c)serpentine 5 µm trace (5 µm spacing), and (d) serpentine 0.4 µm
trace (0.4 µm spacing).

Figure 2: Sub-micron machining on Parylene C under various
conditions: (a) Parylene cracking induced by baking PMMA film
at 170° C, (b) damage to Parylene film due to uncalibrated e-beam
exposure; and Ti features of decreasing size (5 - 0.25 µm)
patterned on Parylene with (c) insufficient and (d) sufficient dose.
EB dose and voltage were varied (150-900 µC/cm2; 10-20kV) to determine suitable parameters to achieve test patterns
consisting of square arrays (5 & 0.5 µm sides). Then, squares ofdecreasing sizes (5-0.05 µm) were patterned with an optimized
dose (420 µC/cm2; 20 kV) to determine smallest resolvablefeatures for the resist settings used (Fig. 2d). Test structures usedin mechanical and electrical testing were patterned at the optimized
dose, and consisted of straight and serpentine traces with widthsand spacings of 5-0.25 µm. Following exposure, the chromium
layer was dissolved in CR-7 chromium etchant (TranseneCompany Inc., Danvers, MA), and the structures developed in 1:3
ratio of methyl isobutyl ketone to isopropanol (18-30s).
Dose and size calibration structures were successfully
metallized with titanium and the resulting structures examined byscanning electron microscopy. Test structures were metallized withtitanium (150 nm) and then encapsulated with a second CVD
Parylene layer (10 µm). O2 deep reactive ion etching (DRIE) wasused to expose the contact pads, and cut out test devices, whichwere then gently peeled off the silicon carrier to complete releaseof devices. No observable damage to the substrate or structures
resulted from the peeling process.
Mechanical and Electrical Testing
DC resistance measurements were taken across all test traces
using a basic 2-point probe on unstrained devices. For DC
measurements during mechanical testing, electrical contacts weremade using wire and conductive epoxy (silver epoxy, MG
Chemicals, Surrey, CA). Devices were first manually bent andtwisted while monitoring for changes in resistance or failures ofthe sub-micron trace. In a second set of experiments, devices wereflexed using a moving stage and stepper motor, allowing resistance

Figure 4: (a) Photograph of released and encapsulated test
structure (400 nm serpentine Ti trace) with magnified inset. (b)Encapsulated device flexed while resistance is recorded. (c)Fractional change in resistance plotted against bend angle forrepresentative test structure.
Neural Probe Prototype
A prototype cortical neural microprobe was fabricated using
this method as a demonstrative example of a future application of
sub-micron resolution Parylene devices. This microprobe consistedof a flexible Parylene shank (2 mm×150 µm) supporting 32
electrodes, each patterned with arrays of nano-scale ‘bump’structures (400 µm) as a means to increase the effective surface
area. Titanium electrode sites (15 µm radius) were machined on a
Parylene coated silicon wafer along with alignment marks using
standard micromachining. The above EBL method was used topattern the bump structures and connective electrical traces withwidth and spacing of 750 nm. DRIE was used to cut-out the device
shape, before removing it from the silicon carrier.

RESULTS
Careful optimization of the EBL process proved critical forworking with the polymer substrate. The Parylene base layer
exhibited cracking under thermal-induced stress and damage
during high energy ebeam exposure. Subsequently, processing was
tailored to avoid rapid heating or steps above 120 °C, and EBL wasconducted at energies at or below 20 kV, with thick resist bilayers
to assist in absorbing e-beam energies and protect the underlying
Parylene. A metal surface layer proved necessary for highresolution EBL patterning, as without such a layer, surface charges
collected and distorted the beam shape. Chromium was chosen due
to its ease of removal, and low atomic mass which allowed
prepatterned titanium structures to be visualized for alignment.
Exposure doses between 300 and 500 µC/cm2 at 20 kV
produced the smallest structures with most clearly defined features;
structures patterned at lower doses or energies failed to develop
fully while higher doses and energies produced distorted features
or damaged Parylene film. Titanium structures with criticaldimensions as small as 250 nm could be reliably machined, andexhibited strong adhesion to Parylene even under repeated
deformation. Titanium was chosen owing to its demonstrated
adhesion to Parylene and proven biocompatibility, though both
platinum and gold were also tested, with the latter suffering frominsufficient adhesion.
Features below 250 nm were not easily resolved, likely a limitof the achievable aspect ratio of the PMMA bilayer. Test devices
with widths of 5-0.25 µm, and spacing of 5-0.4 µm weresuccessfully and repeatedly fabricated (Fig. 4) and proved flexible
and robust. Serpentine traces with spacing of 0.25 µm sufferedfrom shorts and collapsed traces, perhaps a result of proximity
exposure during patterning. Otherwise, under DC characterization,
traces are Ohmic with no recorded shorts or faults even during
repeated flexion and torsion (bend angle > 90°). Key values frommechanical and electrical testing are compiled in Table I, andFigure 5 displays trace resistance as a function of the length/width
ratio, from which we calculate resistivity of the thin-film metal(3.5-6.4 µΩ·m, compared to bulk values of 0.42 µΩ·m) andcontact resistance (50-100 Ω) at the interface between sub-micron
structures and larger traces. Mean resistance was observed to varyby < 0.5% under flexion down to 500 µm radius of curvature (Fig.
4bc).
Straight

Serpentine

Dimension (nm)

250

400

Resistivity (µΩ m)

6.35

3.54

Contact resistance (Ω)

9848Torsion (°)

> 90

> 90

Flexion (°)

> 90

> 90

Table 1: Tabulated values are derived from electrical andmechanical characterization of straight and serpentine tracesencapsulated in Parylene.
Figure 6 shows images of the prototype neural microprobe.
The completed device had 32 recording sites on a 2 mm shank,more than double the electrode density of any prior polymer neuralprobe, and four times that of prior work done with Parylene C.
This high density was achievable due to the small pitch and tightspacing of the connective electrical connections (750 nm),fabricated here through EBL. Traces extended the full length of the2 mm shank without short or break, and exhibited the same
flexibility as traces in test devices. Line resistance for the traces on
the prototype was not measured, though can be extrapolated as 150kΩ from the regression plotted in Figure 5. Though larger thantypical values of traces measuring several µm wide, at 1 kHz(typical recording frequency for neural probe measurements) thecontribution to impedance will be marginal.

Figure 5 Plot of resistance vs. length-to-width ratio of patterned
test structures: straight (square) and serpentine (circle) traces.
The nano-patterned structures, arrays of raised titanium
bumps in the center of each electrode, served to increase effective
surface area, and lower impedance. Each structure was rectangular,
150 nm tall and 400 nm across, and the arrays were calculated toincrease effective surface area by 40%.

Figure 6 (a) Prototype Parylene-neural probe machined with submicron features including (b) densely packed conductive traces(750 nm linewidth and spacing) and (c) nano-patterned recording
electrodes.

DISCUSSION
Through the use of the fabrication techniques describedabove, flexible Parylene-based devices were produced integrating
electrical traces and structures with pitch and spacing of just a few
hundred nanometers. Devices were flexible and robust, withinsulated traces functioning despite severe and repeated flexion
and torsion. These experiments bode well for the production of a
new generation of polymer MEMS, with higher feature density
and/or smaller footprints, and the highly biocompatible
composition is encouraging for applying this technology toimplants.
High resolution patterning on organic polymers is non-trivial;
as seen here, issues of temperature, radiation intensity, and charge

complicate existing methods developed for semiconductor
applications. But these challenges are not unmanageable, andthrough careful preparation and calibration high resolution
patterning, such as EBL, is achievable with high fidelity and good
repeatability on such unconventional substrates. The resolutions
achieved here were likely limited by the aspect ratio attainablewith our choice of resist, and it is likely nanoscale fabrication ispossible with only slight modification of this technique.
In on-going work, we are exploring other potentialapplications, including free-film devices with electricalcomponents, such as resistors and capacitors, directed written into
the film using dense serpentine traces and interdigitated comb
designs respectively, or high density stimulating electrodes forneural or retinal devices, with large arrays of electrodes smallenough to stimulate single sites of interest. We are cognizant of thescalability limitations of this work. Though EBL offers highresolution, the cost per device in time and money is too high fortechnologies intended for large scale distribution. This technique is
useful for prototype devices intended for research, where the scaleof production is small, or for prototyping novel devices. For otherapplications we are investigating pattern-transfer techniques
compatible with Parylene that will enable batch-scale fabrication.

[7].

[8].

[9].

[10].

[11].

X.-M. Zhao, Y. Xia and G. M. Whitesides, "Softlithographic methods for nano-fabrication", J. Mater.
Chem., 7, 7 (1997).
K. Gonsalves, C. Halberstadt, C. T. Laurencin and L.
Nair, Biomedical nanostructures, John Wiley & Sons,
2007.
S. Aksu, M. Huang, A. Artar, A. A. Yanik, S.
Selvarasah, M. R. Dokmeci and H. Altug, "Flexible
plasmonics on unconventional and nonplanar substrates",
Advanced Materials, 23, 38 (2011).
V. Auzelyte, B. Gallinet, V. Flauraud, C. Santschi, S.
Dutta‐Gupta, O. J. Martin and J. Brugger, "Large‐Area
Gold/Parylene Plasmonic Nanostructures Fabricated by
Direct Nanocutting", Advanced Optical Materials, 1, 1(2013).
K. Seidl, S. Herwik, T. Torfs, H. P. Neves, O. Paul and
P. Ruther, "CMOS-based high-density silicon
microprobe arrays for electronic depth control inintracortical neural recording", Microelectromechanical
Systems, Journal of, 20, 6 (2011).

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

CONCLUSIONS
Despite material challenges electron beam lithography can bemade compatible with organic polymers for MEMS processing.
Through the use of such a technique, and the conformal nature of
CVD Parylene, we have produced thin-film biocompatible devices
with sub-micron features. This approach has applications for novel
polymer implantable devices, demonstrated here through theproduction of a polymer neural microprobe with high electrodedensity. Sub-micron processing is a necessary advance for polymer
MEMS, and future work will need to strive towards matching
those resolutions achievable with silicon and other materials, tomatch their capabilities in mechanically pliant platforms.

ACKNOWLEDGEMENTS
This work was funded by a Postdoctoral Scholar Research Grant
from the Office of the Provost at USC. The authors would like tothank Dr. Donghai Zhu of the Keck Photonics Laboratory for help
with fabrication, and members of the Biomedical Microsystems
Laboratory of USC for their assistance. Travel support wasprovided by OAI.

REFERENCES
[1].

[2].

[3].
[4].
[5].
[6].

B. Kim, J. Kuo, S. Hara, C. Lee, L. Yu, C. Gutierrez, T.
Hoang, V. Pikov and E. Meng, "3D Parylene sheathneural probe for chronic recordings", Journal of neuralengineering, 10, 4 (2013).
D. C. Rodger, A. J. Fong, W. Li, H. Ameri, A. K. Ahuja,C. Gutierrez, I. Lavrov, H. Zhong, P. R. Menon and E.
Meng, "Flexible parylene-based multielectrode array
technology for high-density neural stimulation andrecording", Sensors and Actuators B: chemical, 132, 2(2008).
K.-M. Lin, H. J. Sant, B. K. Ambati and B. K. Gale.
A. Sepúlveda, A. Pontes, J. Viana, R. G. de Villoria, F.
Fachin, B. Wardle and L. A. Rocha, 2011.
K. Nagaraj, K. Sangeeth and G. Hegde, 2014.
C. P. Tan, B. R. Cipriany, D. M. Lin and H. G.
Craighead, "Nanoscale resolution, multicomponent
biomolecular arrays generated by aligned printing withparylene peel-off", Nano letters, 10, 2 (2010).