Synthesis of Patterned Metal Oxide Nanostructures
Project Overview
Motivation:
This project seeks to develop and understand processes that employ vapor deposition for the synthesis of selfassembled, patterned metal oxide nanostructures. The initial core process under investigation has been
reported by the University of Maryland MRSEC program as part of work conducted in conjunction with Motorola
Corp. (See Review of the State-of-the-Art on this poster) [1-3]. In that work, a continuous, thin, polycrystalline
metal film (40 - 400 nm thick) has been vapor deposited under vacuum on various coating surfaces. The metal
film is subsequently heat treated in a reactive environment (e.g. oxygen) to transform the continuous film into an
array of metal oxide tips.
Various studies have reported that metal oxides can deliver a versatile set of properties including high Tc superconductivity, ferroelectricity, colossal magnetoresistance, and non-linear optical properties [4]. If nanoscale
islands of metal oxide can be created in specific locations on coating surfaces, a large number of new or
improved engineering products can be enabled that benefit society, including:
• Sensor systems [5-6]
• Magnetic devices [10-11]
- imaging arrays
- Spintronics
- environmental monitoring devices
- Non-volatile ferroelectric memory
- pollution measurement
- Magnetic quantum cellular automata
- medical diagnostics
• Field emitters [7-9]
• Camouflage [12]
- flat panel displays
- e-beam pumped semiconductor lasers
- microtriodes and deflection amplifiers
- electric propulsion (Hall thrusters)
- microwave power tubes for communications, electronic warfare, radar
- miniature electron guns
electron beam lithography
high resolution metrology and surface analysis
- ion gauges and quadrupole mass spectrometers
- power electronics
Advisor: James F. Groves
Graduate Student: Yingge Du
Undergraduate Student: Nadine Gergel
Center for Nanoscopic Materials Design
School of Engineering and Applied Science
University of Virginia
nm thick copper films following heat treatment at 800 and 975oC. The heat treatment results show significant
roughening of the film, with the 975oC sample showing the formation of micron sized islands of copper oxide on
silicon. The literature suggests that the higher temperature heat treatment has probably formed one of the two
stable CuO crystal structures [2,15]. At present it is not certain why the lower temperature heat treatment at
800oC did not form island structures. It is possible that the transformation kinetics require more than one hour of
Critical challenges:
This project is using the university’s DVD system for deposition of the metal film. In this project, it is thought that
it would be best to deposit a pure metal layer with no detectable contamination. Then, subsequent heat treating
would introduce the oxygen into the developing nanostructures. It also seems advantageous, for eventual applications, if the metal films can be synthesized on a silicon wafer from which the native oxide has been removed.
Initial auger electron spectroscopy (AES) study of DVD deposited silicon layers [16] reveals the presence of significant amounts of oxygen and carbon contamination in as-deposited films (Figure 9a). (It is anticipated that
similar contamination levels would be found in copper films deposited by the DVD technique.) In addition, AES
of the silicon wafers found measurable amounts of oxygen and carbon on the wafer surface [16], and an attempt
to remove those contaminants via an in-situ heat treatment process generated mixed results that deserve further study (Figure 9b). The AES analysis suggests that the in-situ heat treatment step increased oxygen levels
and decreased carbon levels at the wafer surface. The scan also suggests that carbon and oxygen are codeposited with the silicon. The source of the oxygen and carbon contamination has not yet been identified. For the
processing of these metal oxide nanostructures, the presence of oxygen in the as-deposited metal film might not
be a critical concern although explanations of the island formation phenomenon will need to account for its presence in the as-deposited film. The presence of carbon in the deposited films does appear to be a challenge, and
efforts will be made to identify and eliminate the source of significant carbon contamination. In the long term, the
presence of these levels of contamination in DVD deposits could suggest the need to consider alternate deposition methods.
800oC heat treatment, or the oxide formed could be one of the other three stable phases of copper oxide, particularly one of the two Cu2O structures [15]. Figure 4 shows a top view of the two heat treated copper samples.
Figure 7 AFM optical images show the continued presence of the FIB patterned region even after heat
treatment of the copper covered silicon wafer at 975oC in oxygen for one hour.
Project vision:
As part of the UVa MRSEC program, this project is exploring patterned metal oxide nanostructure growth. Film
synthesis for the project is employing UVA’s directed vapor deposition system to lay down thin continuous metal
layers (Figure 1), and a standard heat treating furnace is being used to develop metal oxide nanostructures.
Figure 10 Auger electron spectroscopy of a) a silicon film deposited at ~100oC with no silicon wafer cleaning
and b) a silicon film deposited at 400oC following an initial temperature excursion to 1000oC [16].
Review of the State of the Art
Figure 3 Copper films prepared via DVD: a) ~50 nm film heat treated in oxygen for one hour at 800oC and b)
~50 nm film heat treated in oxygen for one hour at 975oC.
As noted at the outset, the research that has motivated this project was reported within the past two years by
scientists at the University of Maryland MRSEC (The Center for Oxide Thin Films, Probes and Surfaces) and
Motorola [1-3]. In their three articles the researchers report the synthesis of periodic arrays of various metal
oxide nanostructures, including PdO2, CuO, and Fe2O3 (Figure 11). While the authors suggest various applica-
Figure 8 An AFM scan of FIB region 1 (See Table 1). The FIB modified regions appear to still be faintly visible.
Note that the oxide tip dimensions are significantly reduced from those of Figures 3 and 4.
Figure 1 The University of Virginia’s directed vapor deposition (DVD) system is currently being used as a high
vacuum electron beam physical vapor deposition coater to create thin metal films for this MRSEC
project. The MRSEC-funded thin film deposition monitor allows growing film thickness to be observed
in-situ and reported in real time.
This project will extend beyond the work at Maryland into a central theme of the UVA MRSEC, nanoscale growth
on patterned surfaces. It will utilize modifications of the growth surface before or after metal deposition to influence the spatial development of the oxide structures. In conjunction with other members of the UVA MRSEC
team, this project team will work to develop fundamental understanding of metal oxide nanostructure development through comparison with other experimental results or development of model-based insight.
Project impact upon the MRSEC:
Material Systems
The material systems under study here are different than those utilized in other parts of the UVA MRSEC program, and their investigation provides the entire center with valuable insight into the broader applicability of program results. By drawing the DVD system into the MRSEC, this project provides the entire program with access
to an additional material synthesis platform. This DVD platform can be utilized for this specific project and for
other MRSEC-related fundamental investigations.
Applications
By extending MRSEC surface patterning techniques and understanding to metal oxide material systems, this
project affords opportunities to broaden the program’s impact. As this project demonstrates the synthesis of patterned metal oxide nanostructures, the combination of MRSEC understanding and directed vapor deposition
film synthesis could generate novel material structure / composition combinations that make possible exciting
new applications [13].
Outreach
A number of academic, industrial, and government research organizations around the country have expressed
interest in metal oxide nanostructures. By pursuing this project, the MRSEC opens up opportunities to connect
with the MRSEC program at Maryland, industry researchers at Motorola, and government scientists at the
Pacific Northwest National Lab. Additionally, there could be companies within the Commonwealth of Virginia
such as ITT Night Vision (Roanoke) that can ultimately make use of the developments from this project.
Results to Date
This project is focusing initially upon the synthesis of copper oxide nanostructures on silicon wafer surfaces.
(CuO is the basis of several high-Tc superconductors [14].) The silicon substrates employed in the study are
from 100 mm diameter wafers donated by Motorola. The wafers are packaged, ready for industrial use, and no
special wet or dry cleaning process is currently being employed prior to copper deposition. Within this study, thin
copper films (50 - 150 nm thick) have been prepared using the DVD system configuration shown in Figure 1.
Following deposition, the coated substrates are heated in a tube furnace, and during heat treatment a reactive
gas such as oxygen or nitrous oxide is passed through the tube to facilitate transition to the metal oxide structure. Initial results are shown below for work with unmodified and FIB modified silicon wafer surfaces.
Copper films deposited on smooth silicon wafers:
Atomic force microscopy (AFM) has been employed to characterize the structure of the films deposited and heat
treated for this research. Figure 2 shows the results of AFM analysis on an uncoated silicon substrate employed
for the research and an as-deposited ~100 nm thick copper film. In contrast, Figure 3 shows the structure of ~50
Figure 4 Top view of the two heat treated copper films shown in Figure 3: a) 800oC and b) 975oC.
Copper films deposited on patterned silicon wafers:
Having demonstrated the ability to reproduce results reported in the literature [1-3], the project has begun to
move into an unexplored area of research in which the semiconductor surface patterning abilities of the MRSEC
are being utilized to modify the silicon (or deposited copper) surface to explore control of the location and size of
metal oxide structure growth on the coating surface. As a first investigation of this possibility, a silicon wafer has
been patterned with the focused ion beam prior to deposition of a 50 nm copper layer. The patterning process
utilized is summarized in Table 1 and illustrated in Figure 5.
FIB
Spacing Depth
Modified
Area
1. 30 µm x 30 µm 1 µm x 1 µm
1 µm
5 nm
2. 30 µm x 30 µm 1 µm x 1 µm
2 µm
5 nm
3. 30 µm x 30 µm 1 µm x 1 µm
1 µm
15 nm
Figure 5 FIB patterning dimensions
Following FIB patterning, ~50 nm of copper was deposited on the silicon surface. After deposition, heat treatment was performed for one hour in nitrous oxide at 500oC. The literature has indicated that the use of nitrous
oxide (N2O) or ozone (O3) instead of molecular oxygen can facilitate the formation of metal oxide island struc-
For the next year:
This project has generated an initial set of results demonstrating the synthesis of copper oxide island structures
on the surface of a silicon wafer. At present, it is not possible to control the size and location of tip growth
although the initial results show great promise. During the next year, efforts will be focus upon control over both
tip size and location. To achieve these goals, the following research plans are proposed:
• Study as-deposited copper films created via directed vapor deposition.
XRD, AES, and SEM techniques will be used to generate a better understanding of the crystal structure,
composition, and morphology of thin copper films generated during DVD.
• Explore different MRSEC surface patterning techniques to assess the influence of surface modification
upon island growth structures.
FIB and electrochemical patterning techniques of the MRSEC (Figure 9) will be used to generate silicon
wafer surface modifications that could influence the location and size of metal oxide island structure
growth. The specific geometries of surface modification chosen for this work will ideally be chosen following consultation with other researchers in the MRSEC program who suggest reasons why one surface
modification or another could significant influence the location and dimension of island structure growth.
tures at reduced heat treatment temperatures although no specific information was given about an appropriate
heat treatment temperature for copper in the nitrous oxide environment [3]. The use of a reduced heat treatment
temperature could be important so that the FIB surface modification is not annealed out during oxidation. Figure
6 shows that the 500oC nitrous oxide heat treatment did not lead to the formation of the previously observed
Figure 9 At least two different MRSEC surface modification techniques will be used during the study of metal
oxide nanostructures a) focused ion beam (FIB) etching and b) electrochemical pulse patterning.
To develop metal oxide island structures on the FIB patterned silicon, the sample of Figure 6 was reheated in an
oxygen environment at 975oC for one hour. As Figures 3b and 4b show, these conditions had previously generated micron sized copper oxide island structures on the silicon. Figure 7 shows the new FIB patterned regions of
the silicon wafer after the 975oC heat treatment using the AFM’s optical imaging ability. The patterned regions
are still visible. AFM analysis of the patterned regions has initially proven to be inconclusive (Figure 8). While
the exact location of the FIB patterns is not definitively recognizable, there is some indication that the FIB modication has motivated the development of a different island structure, perhaps even nanoscale rather than micron
scale tips. A more deliberate study must be undertaken before definitive conclusions can be drawn.
Figure 11 Oxidation heat treatment of various metal systems to produce nanostructures: a) Pd at 800oC, b) Pd at
900oC, c) various materials as listed.
tions for these metal oxide nanostructures, including field emission arrays, spintronics, and magnetic memories,
they do not provide a formula for the formation of a well-ordered array of nanoparticles. Indeed the metal oxide
structures formed by this group are micron sized particles, similar to the results shown in Figures 3 and 4 here,
that cannot be grown in specific desired locations on the coating surface. The group has grown tips as tall as 1
µm with spacings ranging from 0.5 – 2 µm. While numerous articles in the literature hint at the opportunities provided to the engineering community by metal oxide nanostructures [4,6,11,12], there appears to be a general
dearth of work in the field, especially when considering self-assembly techniques as the route to metal oxide
nanostructure formation. The Pacific Northwest National Lab (PNNL) does report to have one group that intends
to study the synthesis of metal oxide nanostructures [17].
At PNNL, Scott Chambers and Yong Liang indicate that over the next several years they will focus upon two
sets of metal oxide nanostructure experiments. One set will investigate formation of efficient polarized UV light
emitting device structures synthesized as all epitaxial film heterostructures. The materials to be grown and
investigated are n- and p-type MnxZn1-xO, where x ranges from 0 to ~0.35. Heterostructures of the type Aldoped n-MnxZn1-xO/ZnO/N-doped p-MnxZn1-x O/ZnO (0001), where the thickness of the ZnO layer is 1 to 10
nm, will be grown by plasma-assisted metal organic chemical vapor deposition (MOCVD). The magnetic and
optical properties of these heterostructures will be determined using a wide range of methods, with the ultimate
goal being the fabrication of a surface-emitting LED structure that will generate polarized UV light near the ZnO
bandgap energy of 3.3 eV, at or near room temperature.
In a second set of experiments, the group at PNNL propose to develop structures that will consist of oxide quantum dots (QDs) with potential for a wide range of applications in electronic, optical, chemical, medical, and sensor technologies. In contrast to most QD studies, which are focused on optoelectronic properties and
conventional semiconductor materials, this study is expected to investigate photochemical, ferroelectric, magnetic, and electronic properties of oxide based QDs. The effort builds upon preliminary work at PNNL that has
apparently demonstrated the ability to synthesize, using molecular beam epitaxy (MBE), oxide QD systems that
self organize into different nanostructures and are stable in many environments, including water. The group proposes to investigate interface controlled, self-assembled oxide-QDs (OQDs) and OQD molecules and their
properties important to environmental, energy, and sensor related applications.
References:
island structures. The figure also reveals that 500oC was not a high enough temperature to anneal out the FIB
silicon surface modifications.
Figure 6 FIB analysis of the FIB modified, copper covered silicon wafer reveals that a one heat treatment at
500oC in nitrous oxide is not sufficient to generate the previously observed island structures.
Figure 2 AFM scans of a) an uncoated silicon wafer and b) an as-deposited ~100 nm thick copper film.
Research Plans
• Investigate the growth kinetics of copper oxide island structures on silicon surfaces.
Thin copper films (10 - 100 nm thick) will be deposited onto unpatterned silicon wafers and then heat
treated in different gas environments, at different temperatures, and for different lengths of time. AFM and
SEM analysis will be used to catalogue the dimensions and distribution of island structures generated by
the deposition and heat treatment process. Summary graphs will be generated that show which portions
of the available process space lead to island formation.
Area
Table 1 Dimensions of the FIB modified area
Presentations and recognitions associated with this project:
1. Nadine Gergel received an MRS Undergraduate Research Initiative grant to support her efforts on this
project.
2. To complete her work for the MRS program, Nadine prepared a poster presentation for display at the MRS
Spring Meeting in San Francisco April 1-5, 2002.
3. On June 11, 2002, James Groves will give an invited talk at the AVS New England Annual Symposium on the
MRSEC program in general, highlighting the research of this project.
Remainder of the program through 2005:
Once initial experimental work has demonstrated the ability to control the location and dimension of metal oxide
island structure growth, this project will seek to extend the specific results produced with copper oxide into a
more general understanding of the formation of patterned metal oxide nanostructures.
• Develop a fundamental understanding of the processes that generate metal oxide island nanostructures.
This work could involve both TEM analysis of fabricated samples and, in conjunction with other MRSEC
faculty and students, modeling of the geometries and processes that control nanostructure formation.
• Deposit films other than copper onto silicon and other substrates to provide insight into the ability to generalize the results of this study.
• Consider undertaking a DVD combinatorial synthesis study that identifies metal oxides with new, useful
properties for various future applications.
Ultimately, it is hoped that the work of this project will generate sufficient promise and understanding to motivate
additional funded research either as part of a MRSEC renewal or through independent, related programs.
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