IMPROVED SELF-SEALING LIQUID ENCAPSULATION IN PARYLENESTRUCTURES BY INTEGRATED STACKABLE ANNULAR-PLATESTICTION VALVEChristian A. Gutierrez and Ellis Meng
University of Southern California, Los Angeles, California, USAABSTRACT
We report on an improved self-sealing structure forParylene-based liquid encapsulation. The specificimprovements are an integrated stiction valve which
reduces overall device footprint by nearly 50% over inplane designs and the use of an annular-plate design in astackable multi-layered configuration for successful longterm liquid encapsulation. We achieve automatic waferlevel liquid entrapment without using adhesives orprocessing at elevated pressures or temperatures. We also
demonstrate the use of electrochemical impedance
measurements as a means for tracking internal liquidvolume.

Figure 1 - Layout and key parameters. Top left: Top-view
indicating key radii. Top right: Partial cross-section
indicating key heights and thicknesses. Bottom row: Fullcross-section images taken through the structure.

INTRODUCTION
Many interesting MEMS applications such as variablefocus liquid lens optics, electrolysis actuators, andelectrowetting devices employ encapsulated liquids [1].
These applications require appropriate fabrication
techniques and structures for long term encapsulation ofsmall liquid volumes. Parylene-based stiction valves havedemonstrated excellent performance in wafer-level sealingof liquids within microchambers [2, 3], but are currently
placed externally in relation to the liquid chamber andactive structures of the device. This configuration
unnecessarily increases the overall device footprint.
Integration of the stiction valve within the active structure
itself was attempted by [4] but liquid retention was limited
to several minutes due to suboptimal valve design. Herewe combine the optimal annular-plate valve design
guidelines derived by [2] in an integrated device to reduceoverall footprint by nearly 50% and achieve automatic
long-term liquid encapsulation.

Figure 2 - Principle of operation. Evaporation throughthe fluidic access ports moves the liquid fronts along theconnecting channels toward the stiction valve. Capillary
forces seal the annular plate of the stiction valve against
the substrate trapping liquid inside the chamber.

DESIGN
Overall device design
The device consists of three main components. 1) anannular-plate stiction valve, 2) peripheral fluidic accessports, and 3) a microchamber. The valve is connected via
short microchannels to the fluidic access ports located on
the periphery and base of the microchamber structure(Figure 1). The chamber is filled by passive soaking at thewafer level in the desired filling solution. Removal fromthe bath and exposure to ambient conditions induces
evaporation through the peripheral access ports. The
evaporating liquid fronts move along the connecting
channels toward the stiction valve. This results in thestiction-induced collapse of the integrated annular-plate,thereby creating a uniform seal against the substrate
trapping the remaining liquid in the chamber above thevalve (Figure 2).
978-1-4244-5763-2/10/$26.00 ©2010 IEEE

Integrated stiction valve design guidelines
The design guidelines for Parylene-based annular stiction
valves were previously investigated [2, 5]. Stiction occurs
when the critical number (Nc) exceeds 1:
Nc =σ ro 4Dh 2p (φ ,υ )(1)

where σ = 2γLAcosθC is the surface energy of the Paryleneliquid interface, D = (Et3/12(1-υ2)) is the flexural rigidityof the plate, γLA is the liquid-air surface tension, θC is the
Parylene contact angle, E is Young’s modulus, t is themembrane thickness, h is the membrane height, υ is
Poisson’s ratio, φ is the ratio ri/ro, and p is a numericalfunction of φ and υ and has been evaluated through the524

use of numerical methods. A combination of parametersthat satisfy the design constraints were selected for thedevice (Table 1). Nc for these valve dimensions is wellabove 1, ensuring that stiction occurs.

respectively (Fig. 3c) . Parylene over the valve center andaccess ports was removed in oxygen plasma (100W,100mT) using a photoresist mask, thereby completing theannular valve structure (ri) and access ports (Fig. 3d, e).

Table 1. Parameters selected for device fabrication

The multi-layered valve-in-chamber sacrificial
photoresist structure and the exposed state of the valve
layer from the previous etching step made it difficult toremove the photoresist etch mask without erosion of theunderlying valve structure. For this reason, the chambersacrificial layer was spun and patterned directly on top of
the etch mask (total of 12μm of photoresist) (Fig. 3f), (rc= 150μm). A 4.2μm Parylene film was then deposited (tc)to enclose the chamber and a final Parylene etching step
re-opened the fluidic access ports (Fig. 3g). Following
sacrificial photoresist removal in acetone, IPA, and DI
water (Fig. 3h), the chambers were filled in a batchprocess by immersion in the desired fluid (DI water).

Parameter
rori
rc
hv
hctvtcDimension (μm)
63-10020-3515021224.2

FABRICATIONThe entire fabrication process is realized at relatively lowtemperatures (90ºC) which enables its use on a variety ofpolymer-based substrates and facilitates adoption intemperature sensitive processes. Standard soda-lime glass
wafers were utilized as the substrate. Fabrication (Figure3) began with optional platinum electrodes patterned on a10μm Parylene film followed by a 1μm Parylene
insulation layer (Fig. 3a, b). The 10μm Parylene film
allows for release of structures and devices from the sodalime substrate for applications where flexibility isrequired. The optional electrodes are exposed to the liquid
within the chamber and serve as an electrical interface forelectrochemical measurement or actuation applications.

Figure 4 - (a) Fabricated device with optional electrodes;
shown while still immersed in DI water. (b) SEM offabricated chamber structure. A thin gold film was used toprevent charging under SEM observation.

EXPERIMENTS AND RESULTS
Stiction valve sealing
Stiction valve sealing and liquid encapsulation capability
were evaluated by using a sample die containing several
identical devices under optical observation. The devices
were filled, as described previously, with DI water. The
die was then removed from the bath and exposed toambient laboratory conditions to initiate fluid
encapsulation (Figure 5).

Figure 3 - Fabrication process shown through B-B’ crosssection designated in Fig. 1. Standard surface
micromachining techniques were utilized in this low
temperature (90ºC) process enabling fabrication on avariety of polymer substrates.

Figure 5 - Optical micrograph of 300μm diameter device
filled with DI water. The chamber is free standing whilethe valve is collapsed and pinned to substrate. The dark
central region indicates the seal between the annularplate and substrate while interference rings indicate
proximity to substrate as plate transitions from contact at
the center to freestanding at the anchored edges.

2μm of photoresist was then spun-on and patterned and2μm of Parylene was deposited forming the valve height(hv), valve radius (ro), and valve membrane thickness (tv),525

Figure 6 - (a) Completely filled chamber with valve immediately after removal from immersion bath. (b) Valved
chamber after 8 hrs of evaporative drying at ambient conditions. Complete valve seal evidenced by interference ringsand darker central annular region. (c) Empty chamber at 9hrs. Valve remains pinned to the substrate. (d) Chamber
without valve immediately after removal from immersion bath. (e) Chamber drying occurs rapidly; liquid evaporation
front appears at 1 hr. (f) At 1.5 hrs, the chamber is completely dry indicating rapid evaporation without a stiction valve.
the ionic conductive path between the electrodes which
varies over time as the chamber deforms to accommodate
liquid volume loss. The phase angle is representative of
liquid volume in the chamber by tracking the contributionof solution resistance to the measured phase. Impedance
measurements were consistent across multiple devices andclearly tracked liquid volume changes over time. By
correlating time stamped optical micrographs withmeasured impedance data, liquid loss was quantitatively
tracked and measured (Figure 7). Phase angle
measurements clearly followed a linear relationship withvolume. As liquid was lost, the phase angle approached-90º, indicating the transition from a mostly resistive
phase angle contribution (i.e. solution present) tocapacitive (i.e. no solution present).

Successful stiction valve sealing was clearly evidenced by
the appearance of a dark central region indicating
conformal contact between the annular plate andsubstrate. Emergence of interference rings between thecentral region and the peripheral plate anchors indicatedproximity to the substrate as the valve plate transitioned
from contact with the substrate to freestanding near theanchors.
Liquid encapsulation
Successful encapsulation of liquid was evidenced by
the appearance of a liquid front within the chamber after
several hours of complete encapsulation. Initially, thechambers appeared uniform due to complete filling andentrapment of liquid within the chamber (Figure 6a). Thiswas followed by the appearance of a liquid front several
hours later as a result of water vapor transmission (Figure6b). Uniformly complete device filling is a unique feature
not demonstrated previously; instead air pockets wereoften observed and thus, liquid entrapment wasincomplete [4, 5]. For comparison, devices without avalve were also fabricated and tested (Figure 6d-f). Within
1.5 hours, valveless devices were completely dry whiledevices with a valve successfully encapsulated liquid up
to 9 hours at ambient conditions (20ºC, ~ 30%RH). The
formation of a liquid front, even in valved devices, clearly
indicated that liquid loss still occurred. Water vapor
transmission through the chamber membrane and not thevalve seal was hypothesized to be the root mechanism ofliquid loss.
Liquid loss mechanism
In order to verify that water vapor transmission throughthe chamber membrane was the primary liquid loss
mechanism, four devices were monitored optically undera microscope and electrochemically by solution
impedancemeasurementsvia
theintegrated
microelectrodes. Electrochemical impedance of theencapsulated liquid was measured (5kHz, 100mVpp) overtime using an Agilent precision LCR meter.

Figure 7 - Top: Impedance magnitude measured overtime. Bottom: Phase angle approached -90º indicating aswitch from resistive solution impedance (wet chamber) tocapacitive impedance (dry chamber).
The change in liquid volume over time (Figure 8)
corresponds to a measured water vapor transmission rate

The impedance magnitude is a complex function of526

(WVTR) of 5.6×10-12 g·μm/μm2·hr through top surface ofthe chamber. This value agrees closely with the previously
reported WVTR value of 5.78 × 10-12 g·μm/μm2·hr (20ºC,30% RH) for Parylene C films [6]. This result confirmsthat the liquid loss mechanism is indeed water vapor
transmission directly through the Parylene membrane andnot the stiction valve seal. Electrochemical impedance
measurements provide a simple and reliable method toquantitatively track internal liquid volumes over time.

encapsulated liquid volumes to measure contact forces
and hydrostatic pressures has been demonstrated withexcellent sensitivity [7]. Furthermore, electrolysis-based
actuation can be fully exploited by this design asincreasing chamber pressure further enforces the seal
between the valve plate and substrate in contrast topreviously reported in-plane designs in which the valve
opened at sufficiently high internal pressures[3, 5].

CONCLUSION
The technique utilized here improves encapsulation
performance by integrating an optimized stiction valve
within a chamber structure. This reduces overall device
footprint by nearly 50% over in-plane approaches, extends
encapsulation lifetime, is performed in batch at the waferlevel, involves no high temperature or pressure steps andis achieved without adhesives. This is accomplished at thecost of only a few additional standard micromachining
processing steps. This improved approach has thepotential to enhance many applications where liquid
encapsulation is required.

ACKNOWLEDGEMENTS

Figure 8 - Liquid volume measured over time (mean ± SD,n=4). The measured WVTR agreed closely with previously
reported values for Parylene films at ambient conditions.
This indicated that the duration of liquid encapsulation
was primarily limited by the water vapor transmission ofthe Parylene film and not valve sealing. Volume wastracked optically and measured using phase angle.

This work was funded in part by the Engineering
Research Centers Program of the NSF under Award
Number EEC-0310723 and the Bill and Melinda Gates
Foundation (CG). The authors would like to thank Dr.
Donghai Zhu and the members of the USC Biomedical
Microsystems Laboratory for their assistance.

DISCUSSION AND APPLICATIONSREFERENCES[1]

The performance-limiting factor for liquid encapsulation
duration is the water vapor transmission rate through asdeposited Parylene films. To further improve
performance, additional techniques can be implemented toenhance encapsulation lifetime (hencapsulation). The
parameters affecting hencapsulation are identified below:

hencapsulation =Mt
WVTR ⋅ SA

[2]

[3]

(2)

where M is mass of water (grams), t is thickness of theParylene membrane (μm), and SA is exposed surface area
available for transmission in (μm2). Increasing membranethickness improves encapsulation lifetime proportionallywhile WVTR and exposed surface area are inverselyproportional. The WVTR of as-deposited Parylene-C
films can be reduced by nearly 50% with additional hightemperature annealing steps [4, 6]. Furthermore,
deposition of thin films such as gold or aluminum can also
be used to reduce the SA available for transmission andprovide a nearly impermeable protective coating [5]. The
use of a combination of these techniques can feasibly
extend encapsulation time for sub-nL volumes (in thecurrent design) out to several days.

[4]

[5]
[6]

[7]

The valve-in-chamber configuration is useful for
applications in electrochemical-based transducers such as
force and pressure sensors. The capability of small
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