Transcript 60x36 Poster Template - California Institute for

Copper Germanium Nanowires: Synthesis, Fabrication, and Applications
Jimmy Kan, Keith Chan, Zhonglin Wang, Deli Wang
UCSD Department of Electrical and Computer Engineering
Introduction
Resistivity vs. Temperature of Cu3Ge Nanowire Devices
Fabrication and Characterization
•Nanowires were transferred from the growth Si substrate to 1.0cm x 1.0cm SiO2
substrates for placement of contact pads.
•Copper germanium in the ε1-Cu3Ge phase demonstrates considerable advantages over
pure copper in metallization applications.
•A 2.0μm thick layer of Shipley S1818 positive photoresist is spun onto the SiO2 as the
first step in defining the contacts. The substrates are exposed for 15 seconds under a Karl
Suss MA-6 mask aligner and then developed in MP-319 developer for 10 seconds.
•Typical copper germanium films display low resistivity (less than 10μΩ cm as observed by
IBM) over broad temperature ranges while inhibiting the fast interstitial diffusion of copper
atoms into silicon.
Synthesis of Nanowires
•Nanowires were grown on silicon <100> substrates via the vapor-liquid-solid (VLS)
mechanism.
•0.5cm x 1.0cm silicon substrates were sputtered with a 5nm thick film of gold to be used
as a catalyst.
•30nm of a titanium sticking layer and 300nm of gold for the contacts were sputtered onto
the substrates. The contacts are defined after an acetone lift off and wire bonding links the
contacts to a chip for characterization of electrical properties.
•The electrical properties of the device are measured using a Quantum Design Model 7100
Physical Property Measurement System. Current-Voltage measurements are performed
from 345K down to 2K and resistivities are calculated using known nanowire dimensions.
9.5
9
A=1.25x10-9
Results and Discussion
8
500 nm
•The growth parameters for Cu3Ge nanowire growth included a 10 minute ramp to 900° C,
60 minutes of growth at 900° C, and cooling to 400° C before substrates were removed.
The carrier gas flow rate was 70sccm.
4 nm
5
0
-5
7.5
-10
-0.4
7
50
100
150
200
-0.2
0
Voltage (V)
250
0.2
0.4
300
Temperature (K)
A plot of resistivity vs. temperature shows decreasing relative resistivity change as the system is
cooled below 20K.
Applications
g2
•Ultra high purity argon acted as the carrier gas for transport of the vapor to the substrates
downstream.
I-V for Cu3Ge at 345K
μm2
8.5
0
•The substrates were placed 11-13cm downstream in a 1 inch diameter Lindberg Blue tube
furnace with the source material, finely ground germanium and copper powder in a 1:1
molar ratio, placed at the furnace center.
•Nanowire morphologies were examined using a scanning electron microscope. At the
same time, energy dispersive x-ray spectroscopy was performed on the nanowires to
determine composition. Transmission electron microscopy (TEM) imaging was carried out
to determine the crystallinity and for confirmation of the structure and composition.
30μm
10
Current (mA)
•Copper germanium thin films have been researched as a possible candidate for contacts
and interconnects in VLSI applications.
Resistivity (micro-ohm cm)
10.5
g1
D1(g1) ~ 2.87A
D2(g2) ~ 4.04A
a(g1-g2) ~ 112 degree
The electron diffraction pattern and TEM images of the nanowires confirms that the growth is
single crystalline and composition is consistent.
•Remarkably low resistivity and stability against oxidation of Cu3Ge nanowires at all tested
temperatures proves it an ideal candidate for interconnects in VLSI metallization
applications. Other reports have demonstrated this low resistivity in Cu-Ge film systems up
to at least 800K.
•The cryoresistance properties can be amply exploited in development of electron tunnel
junctions which call for materials that easily form ohmic contacts with gold and demonstrate
little or no change in resitivity as temperatures decrease towards 0K.
Scalable Integration through the
Langmuir-Blodgett Method
Chemical Vapor Deposition (CVD)
Setup for Growth of Cu3Ge Nanowires
Transfer aligned NWs to
substrate for use in devices
through lithographic
processing and monolayer
deposition techniques
Wires are ultrasonicated to separate them
from the substrate, coated with an
appropriate amphiphilic surfactant, and are
dissolved in a non-aqueous volatile solvent.
…Ge
Illustration of the growth
process occurring within
the CVD furnace during
VLS growth
A binary alloy phase diagram of Cu-Ge depicts the temperature range for the formation of
Cu3Ge. This stoichiometric ratio is verified in the Cu3Ge nanowires through energy
dispersive x-ray spectroscopy.
•The Cu-Ge nanowires grown were approximately 25-35% germanium according to EDX
data.
1μm
•Other stoichiometric combinations were not achieved when modulating the growth
conditions due to the nature of Cu-Ge phase formation. As can be seen in the Cu-Ge
phase diagram above, other binary phases are not possible within the temperature range
of the nanowire growth. The Copper to Germanium ratio achieved represents a stable,
thermodynamically favorable phase of a Cu-Ge binary compound.
•Analysis of the diffraction patterns reveals that the Cu3Ge grown in nanowire form is
single crystalline and the composition is consistent throughout the wire.
•The Cu3Ge nanowires demonstrate remarkably low resistivity of about 10 μΩ cm at room
temperature.
SEM image of nanowires on Silicon. The wires are generally tapered
and average 400nm in diameter and grow up to 250 μm in length .
•Rate of decrease in resistivity is dramatically reduced as temperatures drop below 20K,
revealing that Cu3Ge nanowires are exceptional cryoresistor candidates.
Compressive surface
pressure is applied to
align the nanowires.
•The Langmuir-Blodgett method can be effectively used with Cu3Ge nanowires for applications
in large area interconnects when registration of individual nanowire-electrode contacts is not
ideal.
•Nanowires can be precisely organized and controlled over large areas for formation of Cu3Ge
nanowire interconnection arrays enabling the assembly and fabrication of a wide variety of
future nanodevices.
Summary
•Cu3Ge nanowires have been successfully synthesized through thermal chemical vapor deposition
and possess high crystal quality as indicated by TEM analysis
•Thin film Cu3Ge’s exceptional interconnection properties are scalable in the form of singlecrystalline nanowires
•Cu3Ge nanowires demonstrate cryoresistor properties as relative resistivity changes decrease as
the system is cooled below 20K
Acknowledgments
•The authors greatly appreciate the assistance of Dr. Xinyu Bao in the fabrication of the
nanowire devices and Dr. Eric Fullerton for providing the PPMS for device characterization.
•The authors would also like to thank CalIT2 for providing a fantastic fellowship program,
funding, and great support of the research.