From MEMS to NEMS with Carbon

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From MEMS to NEMS with Carbon

• Dr. Marc Madou
• UC Irvine
• IMEC, Leuven, June 3, 2004

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Organization of this Talk

• Why carbon as a micromachining material?
• What is C-MEMS/C-NEMS?
• High aspect ratio/high surface area C-MEMS and
  its applications.
• Two-level and three-level C-MEMS
• Suspended C-MEMS structures
• Self assembled C-MEMS
• From MEMS to NEMS carbon nanotubes, Ni
  nanowires and Si nanowires
• Conclusions

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Why carbon as a micromachining material ?

• Wide electrochemical stability window
• Biocompatibility
• Low cost
• Chemically inert
• Easy to derivatize
• Well known for its battery and sensor
  application
• Carbon nanotubes connect via C-MEMS?

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What is C-MEMS/C-NEMS ?

• Focused ion beam (FIB) expensive and time
  consuming
• Reactive ion etching (RIE) oxygen plasma
  etching, isotropic
• Screen printing of commercial carbon inks low
  feature resolution, poor repeatability of the
  carbon composition, widely varying properties of
  the resulting devices
• G.M.Whitesides et al
• soft lithography, glassy carbon microstructures,
  micromolding of a resin such as poly(furfuryl
  alcohol) in an elastomeric mold yields polymeric
  microstructures, the latter are converted to
  free-standing glassy carbon by heat treatment
  (500-1100ºC) in an inert atmosphere
• UCI C-MEMS
• Directly derived from photoresist by pyrolysis

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What is C-MEMS/C-NEMS?

• Photoresists are patterned (e.g, using an
  photolithography) and pyrolyzed in an inert
  environment (e.g., vacuum) to yield carbon films
  and 3D microstructures.

Negative photoresist
Positive photoresist
TEM images
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What is C-MEMS/C-NEMS?
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SU-8/SiO2 or SiN/Si
before
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SU-8/Au(3000Å)/Ti(200Å)/SiO2/Si
before
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SU-8/SiO2/Si
after
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What is C-MEMS/C-NEMS?
Sheet Resistance (Ohm/square)
Temperature (C)
Sheet resistance vs temperature of heat treatment
for AZ-4330 and OCG-825 resists
S.Rnaganathan, M.Madou et.al, Photoresist
derived carbon for microelectromechanical systems
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High aspect ratio/high surface area C-MEMS and
its applications.

• In earlier work we demonstrated that photoresist
  derived carbon electrodes exhibit kinetics
  comparable to glassy carbon for selected
  electrochemical reactions in aqueous and
  nonaqueous electrolytes (Madou et al, JECS).
• More recently we have found that we can
  charge/discharge these C-MEMS arrays.

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Galvanostatic charge/discharge cycle behavior
Carbon film derived from (AZ4620)

• The electrolyte is 1 M LiClO4 in a 11 volume
  mixture of ethylene carbonate (EC) and dimethyl
  carbonate (DMC).
• 0.070 mAh cm-2 for the second and subsequent
  cycles.
• For a fully dense film, this corresponds to
  220 mAh g-1, which is within the range of
  reversible capacities reported for coke.

Anodic peak
Cyclic Voltammetry
Cathodic peak
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High aspect ratio/high surface area C-MEMS and
its applications.

• Lithium-based secondary batteries - high values
  of practical specific energies (150 Whkg-1) and
  energy densities (220 WhL-1)-- vs. gasoline (3000
  Whkg-1).
• Highly ordered graphite, hard carbon and soft
  carbon serve as host materials for lithium
  storage in commercial Li batteries (anode).
• Reported values of energy density are generally
  based on the performance of larger cells with
  capacities of up to several ampere-hours. For
  small microbatteries the achievable power and
  energy densities are diminished because the
  packaging and internal hardware determines the
  size and mass of battery ? New manufacturing
  methods and new materials are needed.

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High aspect ratio/high surface area C-MEMS and
its applications.

• Carbon-microelectromechanical system (C-MEMS)
  technology provides both the material and
  manufacturing solution to this battery
  miniaturization problem.
• We overcome the size and energy density
  deficiencies of 2D batteries by creating three
  dimensional (3D) microelectrode arrays by
  patterning photoresists and converting those
  patterns into new battery and battery array
  designs.

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High aspect ratio/high surface area C-MEMS and
its applications.
CMOS

• High current density on a small foot-print,
• Anodes and cathodes in the same plane (easier to
  manufacture),
• The current collectors and electrode posts are
  all fabricated in the same simple one -step
  process,
• Si substrate is compatible with further CMOS
  integration ?High repeatability of batch
  microfabrication and the C-MEMS material,
• Customized design possible,
• Battery arrays may be stacked using the latest
  space efficient IC packaging techniques (e.g.,
  double sided alignement).

Battery unit
Smart switchable battery arrays baxels are
addressable just like pixels in a serial
arrangement, voltages add up in a parallel
arrangement, currents add up
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Two level and three level C-MEMS
SU-8
SU-8
SU-8
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Two level and three level C-MEMS
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Suspended C-MEMS structures
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Suspended C-MEMS structures
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Suspended C-MEMS structures
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Suspended C-MEMS structures
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Self-assembled C-MEMS structures
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From MEMS to NEMS
C post
Au ball
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SEM images of nanofiber
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Au ball
SEM images of nanofiber
Au ball
Ti
Ti
Au ball
Au ball
Ti
Ti
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TEM photos of typical graphite fibers
For well crystallized graphite d-spacing along
the c axis is 0.334nm
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Possible mechanisms
From MEMS to NEMS
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From MEMS to NEMS
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From MEMS to NEMS
Ni
Cu
Cu
Ni
A typical EDX spectrum identifying the
composition of the wire is Ni
Cu signal comes from Cu-TEM mesh
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Typical TEM images of Ni nanowire
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-
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From MEMS to NEMS

• CATALYST ASSISTED GROWTH
• VAPOUR-LIQUID-SOLID METHODS (VLS)
• SOLID- LIQUID-SOLID METHODS(SLS)

Graphite fibers
Ni nanowires (with carbon present Si wires are
suppressed)
Si nanowires (absence of carbon and on bare Si)
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EDX spectra of Ni particles and Si nanowires
Ni Particle
Si nanowire
NiKa
CuKa
C
O
CuL
CuKß
Si
Cu
NiL
NiKa
CuKa
C
Si
NiL
Cu
CuKß
CuL
Si
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SEM photos of Ni(100Å)/Si at 900oC for 1 hr
Forming gas from 900oC
Forming gas from RT
N2 only
hydrogen
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Forming gas from RT
VSL??
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From MEMS to NEMS

• LIQUID METAL CLUSTER OR CATALYST ACTS AS THE
  ENERGETICALLY FAVORED SITE FOR ABSORBTION OF
  GAS-PHASE REACTANTS (LIEBER, CHARLES, DEPARTMENT
  OF CHEMISTRY AND CHEMICAL BIOLOGY,DEAS,HARVARD
  UNIVERSITY,MA.)

VLS METHOD
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From MEMS to NEMS

• THE NANOWIRE MAY HAVE A CLADDING OF SiO2 AROUND
  THE Si CORE? ATTRIBUTED TO THE RESIDUAL OXYGEN IN
  THE APPARATUS
• 6-20nm NANOWIRES, DEPENDS ON THE MIN. DIAMETER
  OF THE LIQUID-METAL DROPLET THAT CAN BE ACHIEVED
  UNDER EQUILIBRIUM .
• PRESENCE OF A NANOPARTICLE AT THE END OF THE WIRE
  SUGGESTS THE VLS MECHANISM

VLS MECHANISM
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From MEMS to NEMS

• SOURCE IS THE Si SUBSTRATE
• CARRIER GAS IS Ar AND H2

SLS METHOD
DaPENG et. al. , DEPARTMENT OF ELECTRONICS,
PEKING UNIVERSITY,BEIJING, CHINA
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From MEMS to NEMS

• a)Deposition of a thin layer of Ni on the Si
  substrate
• b)Formation of Si-Ni eutectic liquid droplets.
• c) Continuos diffusion of Si atoms through the
  substrate liquid interface into the liquid
  droplets,growth of Nanowires through the liquid
  wire-LS interface
• d)Final state of the SiNW growth.

SLS MECHANISM
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25.4nm
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From MEMS to NEMS

• Mix lt 1 wt of carbon nanofibers into SU-8, and
  after pyrolysis we grow more nano-fibrous
  material on the C posts
• This demonstrates local CVD of carbon during
  pyrolysis (most likely catalyzed by Fe particles)

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Conclusions

• We successfully made high aspect ratio (gt 101)
  carbon posts by pyrolysis from negative
  photoresists in a simple one-step process
• Electrochemical tests demonstrate that these
  C-MEMS electrodes can be charged/discharged with
  Li. A C-MEMS battery approach has the potential
  to solve both manufacturing and materials
  problems all at once
• By careful control of processing parameters and
  heating conditions, a variety of complex 3D
  C-MEMS structures, such as suspended carbon
  wires, bridges, plates, self organized bunched
  posts (carbon flowers) and networks, were built.
• Graphite nanofibers and Ni nanowires were
  obtained in C-MEMS process

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