Biennial Report 2002/03 - Solid-State Electronics Department
4.4.5
171
Microelectronic Patterning Technics for K2SO4/Au NanoCrystals
Student:
Supervisor:
M. Barth
Q. T. Do
Introduction
The nanoparticle coated crystal (NCC) is a very promising candidate for the self-assembled
fabrication of electronic devices at the nanoscale (cf. chapter 4.4.4 and 4.4.6). In this work the
compatibility of NCC to standard technological processes is investigated. In practical the thermal
budget of NCC is given and their compatibility to a complete optical lithography procedure is
shown. Finally, the conductivity of Au-K2SO4 NCC bridging a metal-finger structure is given.
Nanoparticle coated micro-crystals (NCC) with a K2SO4 core and Au nanoparticles were provided
by the University of Strathclyde, Glasgow, in a suspension diluted in propanol at the ratio 7:1. The
process of NCC deposition onto a wafer is described in chapter 4.4.4. For the ease of identifying
specific NCC the wafers were patterned with a numbered Pt test grid. The topology of the NCCs
was measured using an atomic force microscope DI 3000 in tapping mode. The size of the
investigated NCC is a few hundred nanometres long and wide (700-2000 nm) and the height is
about 50-250 nm.
Compatibility to Optical lithography
Au-K2SO4 NCC were deposited on GaAs substrates. In order to identify a specific NCC a
numbering grid on the substrates were fabricated before deposition. The shape and surface of
identical NCC was investigated using scanning force microscope using a DI 3000 in tapping mode.
The following standard lithography procedure was applied: (1) Spinning-on of photo resist at 7000
rpm, annealing on a hot plate at 95°C; (2) Exposure of a pattern using UV light, (3) Development in
one-to-one solution of sodium hydroxide (alkaline) and pure water, and (4) Stripping of the photo
resist using hot Aceton
(b)
(a)
(c)
1 µm
Fig. 1
AFM micro scans of a selected NCC (a) before lithography steps, (b) after the first
lithography step, (c) after the second lithography step
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Biennial Report 2002/03 - Solid-State Electronics Department
The steps 1-4 were repeated twice. The adhesion of the NCC on the wafer is so high that there is
neither a loss of NCC nor a change of position regardless of any treatment tested. A more detailed
analysis of the NCC surface shows even no surface integrity modification or a change of the height
and shape of the NCC. This investigation shows that the NCC withstands all chemical treatments of
a full lithography technology. The NCCs of investigation were covered with photo resist and not
exposed to water or developer which may solve the salt core of the NCC.
Annealing treatment
A set of samples based on Au-K2SO4 NCC on Si-substrates was prepared in order to study the
surface after annealing. The heat treatment was carried-out in a rapid thermal annealer (RTA) at
temperatures ranging from 300°C to 560°C. The roughness is analysed using the AFM in a scan
field of approximately 1 x 1 µm² in the central part of the NCC surface. The roughness data are
plotted in Fig. 2 vs. annealing temperature. Up to temperatures of 500°C almost no modification is
observed and the surface roughness remains at about 1 nm. At temperatures above 500°C the
surface roughness increases rapidly attributed to a clustering of the Au nanoparticles. The AuK2SO4-NCC surface after the annealing experiment is shown in Figure 2a, b. After annealing at
560°C a steeper increase of surface roughness and a significant clustering (fig. 2a,b) indicate that
this is the critical temperature.
(a) 300 °C
0 nm
0 nm
500 nm
500 nm
(b) 560 °C
s u rfa c e ro u g h n e s s [n m ]
0 nm
Fig. 2
0 nm
500 nm
2 .0
NCC B
NCC A
1 .5
1 .0
0 .5
0 .0
(c)
500 nm
as
d e p o s ite d
300
400
500
600
a n n e a lin g te m p e ra tu re [° C ]
Surface of Au-K2SO4 NCC measured with AFM: topology scan of the NCC (a) after
300°C treatment, (b) after560 °C treatment, and (c) development of surface roughness
vs. annealing temperature for two NCC samples A, B.
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The mean radius of the Au nanoparticles used for NCC fabrication has been determined to 1.025 by
high-resolution AFM without tiopronin coating after deposition on a Si wafer. Using the model
from Borel and Buffat [1] the melting-point of a gold nanoparticles at this size can be calculated to
619°C. This analysis indicates that the change of the surface roughness beyond 500 °C is driven by
initial melting of the Au nanoparticles as indicated in dark islands in Fig. 2b.
Au-K2SO4-NCC Current-Transport
In the final part of the work the conductivity of a single Au-K2SO4 NCC was measured. The test
structure consists of 1 µm fingers of 100 nm Au separated by 1 µm spacing. In order to suppress the
leakage current to below 10 pA the structure was realized on a SiO2-coated Si-wafer by optical
contact printing. The Au-K2SO4 NCC was deposited onto the pattern such that it forms a bridge
between two neighbouring contacts (cf Fig. 3a). Finally, the same metallisation was defined and
evaporated such that the NCC is sandwiched between two metallisation layers. The I-V
characteristic (cf. Figure 3b) exhibit a symmetric but somewhat non-linear behaviour with a current
of typically 100 nA at 1 V. It was measured three times over the same NCC giving reproducible
results. This also indicates that the NCC must be photo resist resistant.
0.3
0.2
current [µA]
NCC
0.1
0.0
-0.1
-0.2
-0.3
b
a
Fig. 3
-1.5 -1.0
-0.5
0.0
0.5
1.0
1.5
voltage [V]
Conductivity test of a Au-K2SO4 NCC: (a) metal finger pattern on with a NCC bridging
the contact and (b) I-V characteristic of a single NCC at room-temperature
Conclusions
The Au-K2SO4-NCC sticks on the wafer surface and is resistant against the tested resists. It is fully
compatible to an optical lithography process. The thermal budget of the NCC exceeds 500 °C which
opens-up various process technologies. The conductivity of a single NCC is high enough aim at
electronic devices.
References
[1]
K. Schaber; “Thermodynamik disperser Systeme”, Skriptum, Karlsruhe 2003