Microstructured Silicon sSi Transistor with Doped Contact on Plastics

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Microstructured Silicon (?s-Si) Transistor with
Doped Contact on Plastics
Z.-T. Zhu, E. Menard, D.-Y. Kang, K. Hurley, and
J.A. Rogers - University of Illinois at
Urbana-Champaign
Objectives
Technical Approach

• The materials such as inorganic semiconductors
  for high performance electronics on large area,
  flexible substrate.
• A robust transistor with minor contact effect on
  flexible plastic.
• Short channel devices with high mobility for
  high frequency (25MHz) applications

Spin SOG and cure
Lithography, etch
Doping with SOD
Doped S, D contacts
Device Performance
Technical Accomplishments

• Developed the spin-on dopant process to
  selectively dope the contacts.
• Reduced the contact resistance by an order of
  magnitude compared to undoped case.
• Fabricated the ?s-Si transistors with doped
  contact on PET/ITO substrates

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High Performance Single Crystal Silicon
Transistors on Flexible Plastic Substrates
Z.-T. Zhu, E. Menard, K. Hurley, R. G. Nuzzo,
and J. A. Rogers
University of Illinois at Urbana-Champaign Departm
ent of Materials Science and Engineering,
Department of Chemistry Beckman Institute and
Frederick Seitz Materials Research
Laboratory Urbana, Illinois 61801
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Electronics on Large Area, Flexible Substrates

• High Performance
• Mechanically Flexible and Robust
• Reduced Cost
• Light weight
• Robust
• Roll-to-Roll

PNAS, 98 (9), 4835 (2001). Science, 291, 1502
(2001).
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Our Approach to Flexible Inorganic Semiconductors
(I)

• Top-down approach
• Lithography and etching
• Micron-scale and nanoscale semiconductor objects
• Dry printing or solution casting to flexible
  substrate
• High performance single crystal devices on
  flexible substrate

Etienne et. al., Appl. Phys. Lett, 2004
Microstructured Si (ms-Si)
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Our Approach to Flexible Inorganic Semiconductors
(II)
Etienne et. al., Appl. Phys. Lett, 2004 Sun et.
al, Nano. Lett.. 2004 Lee et. al, unpublished.
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Transistor Basics
Channel Length L 80 mm Channel Width W 800 mm
p-channel pentacene
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Microstructured Single Crystal Silicon
Transistors on Plastic
Etienne et. al., Appl. Phys. Lett, in press.
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Characteristics of ms-Si transistors on
Epoxy/ITO/PET
L100 mm W200 mm
L5 mm W200mm
At short channel, the device is dominated by the
contact resistance between Si and metal electrodes
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Scaling Analysis of Contact Resistance
If the contacts are ohmic, then
L
Source
Drain
Rparasitic
Rchannel
Rparasitic
where
and Rparasitic is independent of L
Measure devices with different L to determine
Rparasitic The reciprocal of the slope of RW vs.
L gives the intrinsic device mobility
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Scaling Analysis of the undoped ms-Si transistors
Significant contact resistance between Ti and
undoped Si source and drain contacts, due to the
Schottky barrier. Need to overcome the problem
for short channel applications.
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Doping Process with Spin-on Dopant (SOD)

• Spin-on dopant process to dope the top Si layer
  of SOI readily.
• Doping level is controlled by SOD concentration
  and drive-in temperature.
• To overcome the contact resistance between metal
  electrodes and ?s-Si.

Spin selected dopant (n or p) solution on SOI
Dopant glass
SOI
Drive-in with Rapid thermal annealing
Dopant glass
Doped SOI
HF solution to remove the dopant glass layer
Doped SOI
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Resistance of Doped-Si Metal Contacts

• Rtotal 2Rc (Rs/W)L
• 100 nm SOI doped with phosphorous spin-on
  dopant _at_950 C for 5 sec.
• Rs 228 5 O/sq, and RcW 1.7 0.05 Ocm .
• phosphorus conc. 1019 cm-3.

Contact resistance reduced by an order of
magnitude compared to undoped case
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ms-Si Transistors with Doped Contacts on Plastics
mS-Si
epoxy
ITO/ Mylar
1. Make mS-Si lift off in HF 2. Dry transfer to
ITO/Mylar 3. Pattern Ti S/D electrodes
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Characteristics of contact-doped ms-Si
transistors on Epoxy/ITO/PET

• ?s-Si transistors with doped contacts are
  fabricated on plastic substrate successfully.
• Device mobility is not dominated by contact
  resistance for short channel devices on plastic.

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Scaling Analysis
Minor contact effects for the contact-doped
microstructured Si transistor on plastics
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Mechanical Flexibility
compression
tension
Etienne et. al., Appl. Phys. Lett, in press.

• The change of the device characteristics and
  mobility is within 20 in most cases under
  tension and compression strains.
• Device performance is still good under 350
  cycles of compression between 0 to 0.98 strains.

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Summary

• We demonstrate a spin-on dopant process for
  contact-doped single crystal silicon transistors
  on plastic substrates.
• Scaling analysis indicates that this process
  yields devices that are not contact limited,
  which creates the possibility for high frequency
  silicon devices on plastic substrates.
• The devices on plastic substrates show a
  remarkably good mechanical flexibility and
  fatigue stability.
• This contact doped ms-Si approach gives a
  potential route to high performance, large area,
  and flexible electronics .

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Acknowledgements
Profess John A. Rogers Professor Ralph . G.
Nuzzo The ms-Si team Funding DARPA MacroE
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Top Gate Devices

• Do not work
• No gate leakage
• Device does not turn off
• Reason ?? Still mystery

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Modified Peanut ms-Si
Cross section
On PDMS
Top view
The advantage here is that high quality thermal
oxide can be grown readily for high performance
top gate devices.
Leftover on wafer
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Flexible Solar Cell