IMPROVED PROCESS FOR HIGH YIELD 3D INCLINED SU-8
STRUCTURES ON SODA LIME SUBSTRATE TOWARDS APPLICATIONSIN OPTOGENETIC STUDIESJonathan T.W. Kuo and Ellis Meng
University of Southern California, Los Angeles, California, USA
ABSTRACTDESIGNWe report an improved process for backside inclined
lithography.
The specific improvement is thecombination of backside inclined exposure and glycerol
medium index matching with a Parylene adhesion layer.
This method allows for high aspect ratio SU-8 inclinedstructures with large surface contact areas on soda lime
substrates for optical applications in optogenetic studies.
Improved yields over the entire wafer were achieved withimproved SU-8 substrate adhesion and perfect gaplesscontact between the masked substrate and SU-8. The
surface roughness of the SU-8 structures is less than 8
nanometers, smooth enough for use as mirrors.

An array of mirrors would address many of thelimitations of current optogenetic studies. Quick temporaland spatial control of light activation are possible sincemultiple points can be photostimulated at once. The array
format improves throughput compared to conventional
methods since there would be no need to translate thesample. Using microfabrication techniques, dense arrays
can be fabricated so as to more closely approach thecomplexity of neural circuitry. This array can then becoupled to a microelectrode array (MEA) platform forsimultaneous optical probing of neurons as well as
electrical recording (Figure 1).

INTRODUCTION
High aspect ratio oblique SU-8 structures can beproduced with inclined lithography which varies theexposure angle [1]. However, significant improvement inthicker SU-8 structures is obtained by backside exposure
through the substrate by leveraging the gapless contact
between the thin film mask deposited on the substrate andSU-8 directly applied over the mask [2]. Backside
inclined exposure has already been used to achieve highaspect ratio microneedles [3]. The achievable angle waslimited to > 54º due to the refractive index mismatch
between air (n = 1), the glass photomask (n = 1.53), andSU-8 (n = 1.67) [4]. To create SU-8 inclined structures
down to 19º, index matching with a glycerol (n = 1.6) bath
was employed (also requires anti-reflection coating on thesilicon wafer substrate) [4]. Tight contact between themask and wafer is required; a gap shifts the inclined
structure due to a longer UV light propagation path [4].
In practice, tight contact can be difficult to establish.
We combine backside inclined exposure withglycerol index matching to achieve high aspect ratio SU-8
structures on glass with large contact area and high yield.
Such structures are useful for optogenetic studies.
Optogenetics is the study of cells that have beentransfected to express ion channels that respond to light[5]. The most prevalent one is Channelrhodopsin-2,ChR2, a cation channel that opens in response to blue
light (~460 nm) [6]. Optogenetic studies are typically
performed with a single optical fiber positioned with amicromanipulator underneath a microscope near a cell ofinterest.
Patch clamping is used to record thephysiological response of the cell. Such setups are limitedin throughput since only one light spot can be applied at atime; this limits the experiments that can be performed.
Using MEMS, optrodes having optical fiber tip walls
coated with gold for simultaneous light stimulation andelectrical recording were fabricated but require precisealignment with micromanipulators [7]. Glass or SU-8
could be used to form waveguides [8-9], however opticalpower loss and having beams of light instead of precise
light spots for ChR2 activation are problematic.

978-1-4673-0325-5/12/$31.00 ©2012 IEEEFigure 1: The output of a high powered LED with
attached condenser lens is directed into an optical fiber.
Emitted light is reflected off the mirror and stimulates
optogenetic cells.
Electrophysiological activity issimultaneous recorded by an MEA. A mirror array
allows for simultaneous photostimulation of multiple
points.
An array of mirrors for directing light emitted fromindividual optical fibers towards optogenetic cells was
fabricated with SU-8 using inclined exposure through thebackside of the substrate facilitated using a glycerol index
matching medium. Inclined exposure creates oblique SU8 structures by controlling the angle at which UV light
exposes the photoresist. Backside exposure enables highaspect ratio structures since there is no gap between themask and the photoresist; minimal diffraction of light isachieved due to this gapless contact [2]. However,inclined exposure has inherent limitations on theachievable angled structure (>54º) due to the index ofrefraction mismatch between air, the glass substrate, andSU-8. Here, we use glycerol (n = 1.6) immersion tocompensate for the index mismatch and allows theachievable angle to be extended down to 19º [4].
SU-8 adheres poorly to glass due to differences inthermal coefficients of expansion and so use of backside
inclined exposure is limited to structures that are removed
from the substrate or possess small contact area. Parylene
C can be used to significantly improve the adhesion of
SU-8 on glass due its high rate of elongation (200%)
which allows it to serve as a thermal stress reliever
between SU-8 and glass [10]. Parylene's index ofrefraction is 1.639 and the material is optically transparent
to UV light (λ = 365 nm) with no measurable attenuation

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MEMS 2012, Paris, FRANCE, 29 January - 2 February 2012

of optical power in thin film format (1-10 µm).
Therefore, Parylene is used as an adhesion layer in thisapplication.
We combine backside inclined exposure through amasked soda lime substrate with a Parylene stress relief
layer and glycerol index matching to create high aspect
angled SU-8 structures with large contact areas and highyields (Figure 2). The achievable angles are determinedby Snell's law. For our application, SU-8 structures of 45ºwere desired; thus θ1 was set at 47.5º.

Post-exposure bake was performed with the same
controlled temperature ramp to 95 ºC from room
temperature as before. The temperature of 95 ºC was heldfor 2 hours and then the temperature was ramped down toroom temperature to avoid thermal shock. The wafer wasdeveloped in PGMEA. Finally, the wafer was rinsed (IPAand DI water) and air dried with N2after development.

Figure 2: (top) Detailed UV light path through thebackside of the photoresist starting first at the glycerol
index matching medium and finally terminating throughthe SU-8 photoresist. (bottom) Derivation showing that
Snell's law is reduced to the refraction between glyceroland SU-8.

FABRICATIONThe entire fabrication process is realized at lowtemperatures (90 ºC) which enables its use on a variety ofpolymer-based substrates and facilitates adoption intemperature sensitive processes (Figure 3). Fabrication on
soda lime wafers began with patterning of AZ 4400
photoresist onto the substrate followed by e-beamdeposition of Ti (100 nm) and subsequent liftoff toproduce the mask. Next, a Parylene adhesion promoter A174 was applied followed by a Parylene C coating (2-4µm). SU-8 2035 photoresist was spun (700 rpm, 15 sec)on at a thickness of 250 µm and allowed to relax for 10
minutes on a level hotplate. An inverted funnel wasplaced over the wafer in order to prevent SU-8 photoresist
redistribution due to disturbances of the ambient air flow.
Softbaking began with a controlled temperature ramp of 5ºC every 3 minutes from room temperature to 95ºC inorder to minimize thermal shock to the thick photoresist.
The temperature was held at 95ºC for 2 hours in order tocompletely softbake the photoresist. The temperature wasthen slowly ramped down from 95ºC to room temperature.
The wafer was inverted and placed on an inclined
wafer holder covered with a non-reflective surface andbent to the desired angle. The mounted wafer was placedin a glycerol bath under a UV lamp and the bath wasallowed to sit for 15 minutes in order to ensure a clear
glycerol solution for UV light transmission. An exposure
dose of 4000 mJ/cm2 was applied and the inclined wafer
holder was taken out of the glycerol bath.

Figure 3: Fabrication process for backside exposure
through soda lime and Parylene with complete immersion
in glycerol index matching bath. The desired angle is set
by a wafer holder and placed in the glycerol solution withthe attached wafer for inclined backside exposure.

EXPERIMENTS AND RESULTS
Fabrication Process and Mechanical Integrity
The device was coated gold and imaged using
scanning electron microscopy (SEM) to verify that thedesired photoresist thickness and angle was achieved(Figure 4). Consistent angled SU-8 structures wereobtained within dies and over the entire wafer, despitethinner SU-8 at the edges (Figure 5). SU-8 has notorious
adhesion issues to soda lime substrates. The use ofParylene as an adhesion layer has greatly aided in SU-8
processing on soda lime (Figure 6) [10-11].
The bond strength between SU-8 to Parylene and theunderlying soda lime substrate was investigated first by ascotch tape test. For reference, SU-8 on soda lime
without a Parylene adhesion layer and Si were tested aswell. As expected, SU-8/Si structures survived the scotch264tape test while SU-8/soda lime did not.
SU8/Parylene/soda lime structures also survived the scotch
tape test demonstrating that use of a Parylene layer
significantly improved adhesion between SU-8 and sodalime substrate.

Figure 4: SEM images of the resulting SU-8 structures
showing that the desired angle of 45º was obtained.
Next, the bond was subjected to a pull test. A string
was attached on one end to the SU-8 with epoxy and theother to an Accu-Weigh scale. Force was slowly
increased until failure or to the maximum force
measurable (350 g or 3.4 N). The bond strength of SU8/Si withstood the maximum force applied indicating abond strength greater than 0.85 N/cm2.
SU8/Parylene/soda lime structures also possessed a bondstrength greater than 0.85 N/cm2. In contrast, SU-8/soda
lime bond failed at 0.1 N/cm2.

Figure 5: Fabrication results (a) without and (b) with aParylene layer between the soda lime glass substrate andSU-8. Delamination of SU-8 across the wafer cannot be
prevented without a Parylene stress relief layer. 100%
yield was achieved over a 3” soda lime wafer with theaddition of a Parylene layer. No delamination wasobserved. (c) Consistent, well-defined angled structures
over the entire 3" wafer were obtained, even at the edges
where the photoresist is thinner.

Optical Quality
The angled SU-8 structures are to be used for opticalapplications in optogenetic studies. As such, the surface
roughness is critical for determining the suitability ofoptical applications. A DekTak profilometer was used tomeasure the surface roughness of the angled SU-8
structure. The SU-8 structure roughness was found to notvary by more than 8 nanometers (Figure 6) which issmooth enough for use as a mirror.
The mirror array device was placed under amicroscope and an optical fiber was placed in the fiber
groove (Figure 7). The optical fiber was butt-coupled to alight source and the emitted light from the optical fiber
was reflected off the mirror and upwards into themicroscope camera. The image was saved and theintensity profile was analyzed with ImageJ. A Gaussian
distribution was seen which is expected since the emitted
light from the optical fiber was not collimated (Figure 8).

The distance between the emitted light and mirror
was minimized and the target above the mirror is expected
to be a maximum distance of 600 m away. The opticalpower loss from light dispersion at this distance isexpected to be 89% [12]. With a standard coupling lossof 30% from a high powered LED to optical fiber; theoptical power seen by the target cells is ~0.89 mW or 28
mW/mm2, more than enough to activate ChR2 ionchannels [12].

CONCLUSION
The technique utilized here improves backside
inclined lithography of SU-8 structures through use of aParylene adhesion layer. SU-8 adhesion to the soda lime
substrate is greatly improved and structures with large
contact areas with the substrate can be fabricated without
delamination.
Consistent inclined angled structures265throughout the entire wafer can be achieved due to thegapless contact between the masked substrate and the SU8 photoresist. An optical device for use in optogenetic
studies has been fabricated utilizing this technique andcharacterized. Future work entails coupling the device toa MEA and photostimulating cells cultured on the MEA.

light source was used for the inclined exposure.

REFERENCES
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Figure 6: Representative surface profile of the resulting
SU-8 angled structure. The variation is less than 8 nm;
thus the structure is suitable for use in opticalapplications.

[6]

[7]

Figure 7: Light is delivered to the mirror by an opticalfiber butt-coupled to a halogen light source.

[8]

[9]

[10]
Figure 8: Representative intensity profile of the reflected
light from the SU-8 mirror structure. The intensity is in a
Gaussian distribution which is as expected for noncollimated light source.

[11]

ACKNOWLEDGEMENTS[12]This work was funded in part by the NSF EAPSI
program (JK) and NSF CAREER grant under Award
Number EEC-0547544. The authors would like to thankDr. Fan-Gang Tseng and members of his Nano-/MicroBio-Opto-Electro-Mechanical Systems and Fluidics Lab
at National Tsing Hua University for their hospitality.
The authors would also like to thank Dr. Donghai Zhu
and members of the USC Biomedical Microsystems
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CONTACT*J.T.W. Kuo, tel: +1-213-821-3949; jonathtk@usc.edu

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