UV lithography process for ultra-thick high aspect-ratio SU

Report
Producing Ultra High Aspect
Ratio SU-8 Structures With
Optical Lithography
John D. Williams, Wanjun Wang
Dept. of Mechanical Engineering
Louisiana State University
2508 CEBA
Baton Rouge, LA 70808
Crosses displayed here are 1500 m tall and range in width from 35 to 70 m
High Aspect Ratio Microfabrication

The production of mechanical systems often
requires 3 dimensionality in the design.

To achieve 3-D structures, designers often
transfer complex 2-D patterns deep into a
substrate.

Currently there are three transfer procedures
that yield significant height to width aspect
ratios.



Deep x-ray lithography (aspect ratios >150:1).
Deep silicon etching ( >75:1).
SU-8 UV lithography ( >15:1).
Advantages of High Aspect Ratio
Processes

Provides engineers with the ability to produce
tall mechanical structures.

Allows for the development of fluidic vias and
very narrow diffusers.

Provides the ability to achieve “3-D”
structures on the micro scale.
UV Lithography With SU-8

Optimized for producing MEMS devices.

Spun to thickness' between 10 and 1500 m .

Demonstrated aspect ratios of 25:1 using
UV-lithography.

Best performer to date for thick resist
processing with ultraviolet light.

Can be patterned using a common
broadband contact aligner.
Advantages of SU-8 Processing
for High Aspect Ratio MEMS

Lithography does not require and expensive
light source.

SU-8 processing can be done using common
cleanroom equipment.

3-D structures can be fabricated easily using
multiple exposed layers.

Mature electroplating processes developed
for LIGA processing allow for a wide choice
in material selection.
Disadvantages of SU-8
Processing

Extremely difficult to define proper bake
parameters.

Resist remains “soft” until after exposure.

High concentrations of stress in resist are
present during traditional processing.

Solid polymer is highly self adhesive.

Exposed SU-8 is extremely difficult to
selectively remove.
Current SU-8 Process
Technology

Patterns are currently transferred 1500 m
into resist with aspect ratios of 5:1.

25:1 aspect ratios are commonly presented in
structures between 100 and 400 m tall.

Recent work demonstrates the ability to
achieve 15:1 trenches in 100 m of resist and
50:1 featured patterns in 600 m of resist.
Visual Picture of the State of the
Art in SU-8 UV Lithography

Lin et.al., J. Micromech.
Microeng. 12 (2002) 590-597.


Loechel., J. Micromech.
Microeng.10 (2000) 108-115.
Dentinger et.al., Microelectronics
Engineering. 61-62 (2002) 1001-1007.
Methodologies for Improving the
Aspect Ratio of SU-8 Processes

Chemical modification of the resist.

Addition of high refractive index material
between resist and mask to reduce
diffraction.

Use of selective UV spectrum.


Reduces effects of diffraction.
Eliminates short wavelengths that are absorbed in
the first few microns of the resist leading to pattern
distortion.
Results Achieved Using Process
Improvements

Wavelength filtering
 Ling et.al., Proc. of SPIE. 3999
(2000) 1019-1027.

Before and after diffraction
reduction w/ 365 nm light
 Chuang, Tseng, lin. Microsys.
Tech. 8 (2002) 308-313.

Chemical Modification
 Ruhmann et.al., Proc. of SPIE.
4345 (2001) 502-510.
Our SU-8 Process

SU-8 resist without any modifications

No specific filtering

No diffusive control by added materials
between mask and wafer

Optimized spin and bake procedures

Optimized exposure conditions

Room temperature development in
stagnant fluid
Issues Present in Process

How to coat SU-8 in layers greater than 800
m successful?



What are the proper bake conditions for very
thick resist layers?





Multiple coats for layers over 1100 m.
Maintaining a level surface until after exposure is critical.
Approximately 50min/100 m of resist at 96 C in an oven.
Films greater than 1mm require slightly elevated
temperature if hotplate is used.
Multiple coatings require extra bake time.
Stress reduction obtained by proper cooling of sample.
What is the optimal exposure dose required
to achieve the pattern?

Open field structures require significantly more dose than
holes and closed structures.
Experimental Results

We have greatly reduced the internal stress in
SU-8 films.

We have developed a repeatable procedure
for achieving 1500 m thick layers.

Have established optimal exposure doses for
films 1000, 1200, and 1500 m thick.

Demonstrate the ability to produce open field
structures, including cylinders, with high
aspect ratios.

Demonstrate the ability to pattern holes in
closed structures as deep as 1200 m.
High Aspect Ratio Features
Produced in This Experiment

35 and 50 m wide crosses 1500 m tall.
1150 m Tall Cylinders With
Varying ID and Wall Thickness’
Inner diameters vary from 40 m to 200 m.
 Optical image shows complete development of
the cylinders.
 Cylinder with wall thickness’ less than 30 m
collapsed.

1150 m Tall Cylinders With Min.
Wall Thickness of 50 m

Aspect ratio > 23:1.
 Optical image in corner shows that the
resist was completely developed away
inside the cylinders.
1150 m Tall Crosses 25 m Wide

Aspect ratio
46:1.
 Open field,
free standing
structures
require higher
doses than
cylinders or
hole patterns.
How High of an Aspect Ratio
Can Be Achieved?




50:1 is easily
obtainable.
Here one can see
a 100:1 pattern
(6 m wide and
630 m tall).
A 7 m trench is
also observed from
top to bottom of
the features.
Required new
development
process.

630 um tall patterns. Numbers
represent the width of the
feature on the mask pattern.
Concluding Remarks

We are able to obtain high aspect ratios using
a simple SU-8 lithography process that can
be applied in almost any MEMS laboratory.

We demonstrate, for the first time, the ability
to achieve 100:1 aspect ratios that cannot be
produced using any lithographic technique
other than x-ray lithography.

We believe that the exposure can be
improved simply by using repeatedly
published process modifications.
Acknowledgements

National Science Foundation

NSF Grant ECS-#0104327

Louisiana Space Consortium
(LaSPACE), NASA

Center for Advanced Microstructures and
Devices (CAMD) at Louisiana State
University

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