Selectively Grown Silicon Nano-Wires for Transistor Devices
Jonathon A.
1 2008
2
1
Brame ,
Nathan
1
Woods ,
Dr. Stephanie A.
2
Getty
Student Internship Program, Goddard Space Flight Center, Department of Physics, Provo, UT, jon.brame@gmail.com
NASA Goddard Space Flight Center, Materials Engineering Branch, Code 541, Greenbelt, MD, Stephanie.A.Getty@nasa.gov
Applications
Abstract
The goal of this project is to fabricate a transistor device using silicon nano-wires (SiNW)
as semiconductors. In order to be able to establish electrical conduct with the nano-wires
the SiNW growth is confined to certain areas that can be contacted using electron-beam
lithography (EBL). This is done by controlling the catalyst deposition through angleevaporation onto pillars on the device. The nanowires then grow from the sidewalls of the
pillars and can be contacted using EBL.
Background
We have been able to successfully fabricate pillar structures on a
Silicon/Silicon Dioxide chip and grow Silicon nanowires from the
sidewalls of these pillars. This ability to grow SiNWs selectively will
allow us to selectively place devices on a substrate as part of a lab-ona-chip ensemble. We are also able to control the amount of gold
catalyst by varying the height of the pillars. The amount of catalyst
determines the length and width of nanowires. This will allow us to
maximize the number of devices as well as control their dimensions.
This device was designed to be the transistor in
a chemical sensing field effect transistor (chemFET) application. Once the placement of a
SiNW transistor device can be controlled, it can
be placed in-line with a micro-fluidics
framework which could deliver liquid at
unknown concentrations across the device. The
charge concentration in the fluid will act as a
variable gate voltage, and once calibrated will
indicate the concentration of chemical in the
fluid.
In combination with a liquid chromatography
device and a mass spectrometer, the chem-FET
device will contribute to the “lab on a chip”
concept for planetary exploration.
Figure (top) A single Silicon
nanowire is used as a
transistor with source (S),
drain (D) and gate (G)
electrodes. (bottom) An IV
curve for the device at top
with several different gate
voltages. This work, done by
Chung at UCLA, showed the
viability of making transistors
out of as-grown SiNWs.
Silicon Nanowires (SiNW) are grown using a goldcatalyzed LPCVD process. At high temperatures in the
growth furnace, the thin-film (~2 nm) gold catalyst beads
up, allowing crystalline Silicon to grow in narrow wires
under the gold “seeds”. Growth temperature is 450° C and
the feedstock gas is Silane (SiH4). The result is a wire
with width from 10-40 nm and length anywhere from 100
nanometers to several micrometers. Since these wires are
made of Silicon, they exhibit semiconductor properties and
can be doped for use in transistor devices (see Chung et
al).
Results
Figure A pillar structure
with a SiNW grown from
the side wall. The pillar
height is 2400Å and the
SiNW dimensions are
about 1 μm long by 20 nm
in diameter.
Figure Future potential missions to Mars
(above) and Saturn’s moon Titan (at left) are
two possible applicationgs where the ChemFET device would be useful. A knowledge
of the chemical composition of the planets
and moons in our solar system will enhance
our ability to expand exploration to places
like these and beyond
Fabrication
Electron beam lithography (EBL) is used to pattern small squares (1μm on a side) on a Si/SiO2 wafer, then a gold layer is deposited using thermal evaporation.
These patterned gold squares are used as an etch stop in a reactive ion etch (O2, CF4) to create “pillars” of oxide 2400Å high. Next gold catalyst for nanowire
growth is angle-evaporated at an angle of 5° (from parallel) onto the sides of the pillars. This confines the growth to the sides of the pillars. Once nanowires are
grown, EBL and a titanium/gold E-beam evaporation are used to pattern contact pads to electrically contact the ends of the wires. Initial testing can be done using
the Silicon substrate as a gate for the nanowire transistor. Eventually the gate voltage will be supplied by passing ions in an ionized fluid flowing over the top of
the transistor.
1
5
2
6
3
4
7
8
Figure
1. Gold is deposited on top of a Silicon/Silicon Dioxide wafer that has
been through a lithography process to expose small squares on the
surface.
2. Once the resist is removed, all that remains are squares of gold 1μm
x1μm.
3. The wafer is put into a reactive ion etcher where fluorine ions
directionally etch away 2400Å of oxide wherever it is not protected
by gold.
4. The gold on top of the pillars is removed using a potasium iodide
etch.
5. Gold catalyst is angle-evaporated onto the sidewalls of the pillars.
6. Silicon nanowires are grown using an LPCVD at 450°C with Silane
as the feedstock gas.
7. Another lithography process exposes the ends of the nanowires, then
gold is evaporated over the entire surface.
8. After removing the resist and excess gold, the nanowires are
contacted at each end by gold leads and the device is ready for
testing.
Total
Survi
ving
Pillars
Pillars
with
no
wires
Pillars
with
one
wire
Pillars
with
two
wires
Pillars
with
three
wires
Pillars
with
>3
wires
Pillars
with
Usable
wires
1200Å
35
15
11
4
3
2
3
2400Å
24
3
11
3
4
3
11
Table 1 Comparison of the
number of pillars with usable
wires at heights of 1200Å and
2400Å. Usable wires are wires
extending more than 500nm
from the side wall. Although the
wires are slightly thicker, the
density of usable devices is
much greater at 2400Å.
Conclusions and Future Work
•
Selectively placing nanowires is possible using pillar-grown
SiNWs.
•
The ideal pillar height for maximum wire density is 2400A.
•
Actual transistor devices are in fabrication and will be tested in
the near future.
•
Once transistors are tested, process will be used to selectively
place SiNWs for use in Chem-FET devices.
Bibliography
•Sung-Wook Chung, Jae-Young Yu, and James R. Heath, “Silicon
nanowire devices” Applied Physics Letters, 2000.
Yi Cui, “High Performance Silicon Nanowire Field Effect Transistors”
Nano Letters, 2003
Acknowledgements
Student Internship Program
Rocky Mountain Space Grant Consortium
BYU- Dr. David Allred
C. Taylor, T. Adachi, V. Mikula