A Novel Method for
the Fabrication of
High-Aspect Ratio
C-MEMS Structures
Outline
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Objective
Introduction
Experimental
Results and Discussion
C-MEMS structures with Overhangs
Importance of Baking Source
Self Organization of Resist features in 2-D
Arrays
 3-D Suspended C- MEMS
 Conclusion
Objective
• To give idea about MEMS fabrication
techniques and detail about one recent
work in the field, which could make greater
impacts in the micro fabrication , that is
about a Novel method for the Fabrication of
High aspect ratio C-MEMS Structures
“There is Plenty of Room at the
Bottom”
• Proffesor Richard P.
Feynman gave his famous
lecture titled “There is Plenty
of room at the bottom” on
December 26, 1959, at the
annual meeting of the
American Physical society at
the California Institute of
Technology.
“There is Plenty of Room at the
Bottom”
• In it he described the
enormous amount of space
available on the micro
scale, ‘The entire
encyclopedia could be
written on the head of a
Pin’
Introduction
MEMS
• It refers to devices have a characteristic
length of less than 1mm but more than 1
mm, that combine electrical & Mechanical
Components
• Originated as a result of advances in
Microelectronics and Invention of
Transistors in 1948
• Invention of Integrated Circuit in 1959 by
Kilby boosted the Micro Fabrication
techniques
Typical MEMS and
Microsystems products
Manufacturing Techniques For MEMS
•
•
•
•
Surface Micro Machining
Bulk silicon Micro Machining
Lithography
Lithography,Electro deposition(plating) and
plastic Molding (or its German,
Lithographie, Galvanoformung, Abformung
[LIGA])
• Electro Discharge Machining
Bulk Micromachining
Surface Micromachining
Etching
MEMS-Silicon and Carbon
• Conventional MEMS is mostly based on silicon
• Recently , microfabrication of Carboneous
material has received a lot of attention.
• The different Crystalline structures and
Morphologies enable Carbon, different physical,
chemical, thermal and electrical uses.
• Micro fabrication of Carboneous Materials are
called C- MEMS
MEMS fabrication for Carbon Structures
• Current processing Technology for the
Microfabrication of Carbon structures are
-Focused Ion Beam
-Reactive Ion etching
• They have the following disadvantages
-Time Consuming
-Expensive
Photoresists
• Photoresist
-The photoresistive compound used in
microfabrication is called Photoresist or
simply resist
• Positive Photresist
-It responds to light in such a way as to
make the exposed regions dissolve more
quickly during the development process.
• Negative Photoresist
-It responds in the opposite manner. Unexposed
regions of the resist will dissolve in the
developer While exposed regions remain
behind
In this work
-Photoresist derived C-MEMS structures were
obtained in a two step pyrolysis process in an open
ended quartz-tube furnace
• First
-The samples were baked in a N2 atmosphere
at 3000C for about 40 min first
• Second
-heated in a N2 atmosphere upto 9000C, at
this point the N2 gas is shut off and forming gas
[H2 (5%)/ N2 ] is introduced for one hour
• Then
-the heater is turned off and the sample is
cooled down again in N2 atmosphere to room
temperature [cooling time 10hr(100C/min)]
Schematic of the Process
SU-8 Post arrays before pyrolysis
SU-8 Post arrays after pyrolysis
C-MEMS Structures with Overhangs
• We have seen that how SU-8 Photoresist is used to
create High- Aspect ratio posts with high
resolution and Pattern fidelity
• In this work, Marc.J.Madau et al developed novel
techniques to form
complicated Suspended
C-MEMS by nontraditional
process recipes such as over
exposure, underdevelopment,
directed flow of the developer
and exploitation of surface tension in the
developing Photoresist pattern
Importance of Baking Source
• With Some imposed baking parameters a hard top
layer can easily be observed
• In an Oven the resist is uniformly heated by
convection from all sides.
• Skin formation on the resist surface is often
observed, which reduces further solvent
evporation.
• In general oven-baking for straights SU-8 posts
tend to lead to T-Shaped hats whereas a hot plate
keeps the original shoe more in-tact.
• Oven baking also creates a hard skin on
unexposed resist thereby forcing the developer to
attack the unexposed resist parts faster from the
sides than from top leading to overhangs and
SEM photos of 3-D suspended structures. (a)
and (b): before pyrolysis; (c) and (d): after
pyrolysis.
Self Organisation of the resist features
• Self organized groups of Carbon posts- a feature
most readily observed for high aspect ratio CMEMS arrays with posts higher than 300mm
• The figure shown in (a) demonstrate that posts
aggregation occurred before pyrolysis
• When the Post bake temperature is above the
transition temperature, the unexposed area reflows
and this could enable the posts to move forward
each other
• There is agregation/bunching happened, the mask
patterns are transferred with high fidelity
(a) SU-8 structure before pyrolysis: (b),
(c), and (d) Carbon structure after
pyrolysis.
Collapse of Carbon Posts
• Using too high a pressure on the Nitrogen
drying gun the developed high aspect ratio
post patterns randomly collapsed as shown
in fig (a) Inset.
• The effect caused by the large internal stress
can be alleviated either by reducing the total
exposed arc or by generating discrete smallsized exposed regions.
Remedy
• We can Overcome stress problems and
successfully turn the photoresist structures into CMEMS without cracks, if we use the above
described two step pyrolysis process
• When the developer solution is removed gently
this pulls posts that are tall and close enough
together into symmetric patterns
• The rules controlling the Number of posts per
bunch has not yet been investigated and further
experiments are needed to understand how the
different types of symmetric bunches organize
themselves
3-D Suspended C-MEMS
• We have seen that self Organized bunched CMEMS posts can be made.
• Now, by introducing non traditional lithography
process steps, such as underdevelopment, doping
of the photoresist with Nanoparticles, we can
controll the developing flow direction
• By Using SU-8 Photoresist doped with Fe2O3
nanoparticles, it is found that the formation of
suspended Structures can be controlled much
better
• By proper control of the soft bake, exposure, and
development time, suspended carbon fibers can
also be built as shown
• Investigations are going to make thinner
suspended carbon wires
SEM photos of suspended carbon fibers by
using modified SU-8 photoresist. (a) SU-8
before pyrolysis; (b), (c), and (d): C-MEMS after
pyrolysis.
Complex C-MEMS Structures
• By careful control of the geometrical
distribution of the posts in the array and the
over exposure dose, a wide variety of
suspended and complex C-MEMS
Structures, such as plates and bridges(as
shown in next figure) , can be fabricated
SEM photos of suspended C-MEMS
structures formed from modified SU-8
100.
Improvement of fabrication
• In order to improve patterning quality and
shorten the processing time both immersion
and spray development was used to develop
thick and dense SU-8 arrays
• By actively controlling the spray direction
of developer to attack the exposed resist
from one direction only, we can further
control the resulting patterns to build, for
example, ribbon like SU-8 Structures.
Carbon ribbons after pyrolysis is shown in
the figure
SEM photographs of suspended C-MEMS
ribbons.
C-MEMS microbattery
• The processes described earlier can easily
be extended to two layers or multi layer CMEMS structures
• In the figure shown in the next slide, in the
C-MEMS battery, the current collector for
rows of anode and cathode posts in the
second layer of the carbon
C-MEMS electrodes with underneath carbon-contacts
Conclusion
• The C-MEMS pyrolysis of thick photoresist
provides a powerful new carbon microfabricatin
technique, which has potential application in
Carbon based electronics, sensors, batteries,
microfluidics etc.
• Although there is shrinkage in both height and
width, the C-MEMS structures mostly retain the
original SU-8 photoresist shape
• By careful control of processing parameters and
heating conditions, a variety of complex 3-D
MEMS structures, such as suspended carbon
wires, bridges, plates, self organized bunched
posts(Carbon Flowers) and networks were built
• The photoresist derived high aspect ratio
C -MEMS arrays can be charged and
discharged with Li ions providing a
promising material for microfabrication
solution to current battery miniaturization
problem.
• As the manufacturing techniues for MEMS
are in the initial stages, a detailed analysis
and testing are not possible. Observation
and rule of thumb still persists. So the scope
of engineers to involve in this work is high
References
• Chunlei Wang,Guangyao Jia, Lili H. Taherabadi, and
Marc J. Madou “A Novel Method for the Fabrication of
High-Aspect Ratio C-MEMS Structures” JOURNAL OF
MICROELECTROMECHANICAL SYSTEMS, VOL. 14,
NO. 2, APRIL 2005 pp.347-357.
• Chunlei Wang,Guangyao Jia, Lili H. Taherabadi,
Marc J. Madou, Yuting Yeh and Brauce Dunn” CMEMS structures for the manufacturing of 3-D
Microbatteries”. Electrochemical and solid state
letters. October 19, 2004 pp A435-A438
• W.H. The, U-Dwing, G-Salis, R. harbers, U Dreshler
and R.F Malort, C.G.Smith, H.J .Guntherodt.”SU-8 for
real three dimensional and sub diffraction-limit two
photon microfabrication” Applied Physics letters, Vol
84 November 20, 17 May 2004.
• Marc J Madou, “MEMS Fabrication” The MEMS hand
book, CRC Press pp 16-1 to 16-165.
• Stephen a campBell, “ The Science and
electronics of Microelectronic fabrication”
Oxford Publications pp 182-199.
• F.G.Tseng, Y. J Chaung, and W.K Lin.”A
novel; fabrication method of embedded
microchannels employing simple U.V
dosage control and anti reflection coating”
papers from MEMS 2002 ieee international
conference at Las Vegas, pp 70-72.
• Mohamed Gad-el Hak ”Introduction to
MEMS”. The MEMS Hand Book, CRC
Press, pp 1-1 to 1-5.