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Generation of paramagnetic hybrid inorganicorganic thin films.

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Full Paper
Received: 15 January 2010
Accepted: 1 March 2010
Published online in Wiley Interscience: 13 April 2010
(www.interscience.com) DOI 10.1002/aoc.1653
Generation of paramagnetic hybrid
inorganic/organic thin films
Abhinav Bhushana , Huilan Hana , Alex Sutherlandb , Stefanie Boehmec ,
Frank Yaghmaied and Cristina E. Davisa∗
There is a growing interest in developing advanced materials for thin film applications in biology, electronics, photonics
and engineering. We report the development of hybrid inorganic/organic thin films containing nickel, iron and cobalt
paramagnetic materials. By etching the resist in oxygen plasma after processing, most of the organic component of the resist
was removed. The elemental chemical composition of the films was confirmed by energy dispersive X-ray spectroscopy. This
process can potentially lead to patterning paramagnetic thin films containing paramagnetic materials by following standard
c 2010 John Wiley & Sons,
photolithography protocols, obviating the need for a wet or vacuum metal deposition. Copyright Ltd.
Keywords: organometallic; paramagnetic; hybrid thin films; MEMS; nanotechnology
Body
530
With current medical and biological science advances, there is
an increasing need for developing and processing advanced
materials that can be used in interdisciplinary biological, electronic
and engineering research applications.[1 – 6] It is often desirable
that, in addition to uniformity and structure, the thin films have
additional properties such as electrical conductivity or resistivity,
magnetism, etch resistance, and optical transparency.[7 – 9] These
properties can be introduced into a material by design due to
properties innate to film components or these features can be
introduced during the synthesis. Hybrid organometallic films
have recently attracted considerable attention as they have
the advantages of both organic and inorganic materials, which
include ease in processing, superior resistance to chemicals
and excellent mechanical and optical properties.[10 – 14] Typically,
metallic films are deposited either by vacuum deposition
systems or by wet chemistry electrodeposition. Our group has
previously reported the development of hybrid films containing
lead.[10] Here, we report the development of novel photopatternable paramagnetic hybrid inorganic/organic thin films
where a paramagnetic material-containing polymer thin film is
patterned directly onto a substrate such as a silicon wafer. The
material has a photoactive compound that gives it patternable
properties similar to a photoresist. Through a plasma etch, most
of the polymer constituents of the film are removed, which leaves
behind a film that has a higher content of the paramagnetic
material.
Figure 1 shows a schematic of the experimental steps taken to
formulate the resist and prepare the films. Iron (II) acrylate, nickel
(II) 2,4-pentadionoate and cobalt (II) 2,4-pentadionoate were purchased from Gelest (Morrisville, PA, USA). Solvents used in the
resists were propylene glycol monomethyl ether acetate (99%,
Sigma-Aldrich), ethanol (200 proof, Gold Shield), tetrahydrofuran
(THF; >99.9%, inhibitor free, Sigma-Aldrich) and methoxyethanol
(99%, Sigma-Aldrich). Trimethylolpropane triacrylate (Sartomer) was used to form the polymer matrix and 3-(trimethoxysilyl)
Appl. Organometal. Chem. 2010, 24, 530–532
Synthesis of
Chemicals
Lithography
Film
Characterization
Wafer Coating
Exposure
Development
Oxygen Plasma
Figure 1. Schematic showing the steps from synthesis of chemicals to
forming the thin films.
propylmethacrylate (98%, Sigma-Aldrich) was used as a SiO2
precursor. Polyfox TB (Omnova) and 2-aminopropyltriethoxysilane
∗
Correspondence to: Cristina E. Davis, University of California, Department of
Mechanical and Aerospace Engineering, CA 95616, USA.
E-mail: cedavis@ucdavis.edu
a Department of Mechanical and Aerospace Engineering, One Shields Avenue,
University of California, Davis, Calisfornia, USA
b Department of Chemistry, University of California, Davis, Calisfornia, USA
c Department of Chemical Engineering and Materials Science, University of
California, Davis, Calisfornia, USA
d Northern California Nanofabrication Center, University of California, Davis,
Calisfornia, USA
c 2010 John Wiley & Sons, Ltd.
Copyright Paramagnetic hybrid inorganic/organic thin films
UV sensitive organometallic resist
Wafer
A
UV exposure
B
Figure 4. An SEM image of the nickel paramagnetic organometallic film
after oxygen plasma processing.
Develop resist
C
Oxygen plasma
of resist glue, 16.5 g TMPTA and 3.3 g SLA were added. After 2 h
of mixing, the solution was transferred into a class 100 cleanroom
and 6.1 g PAC was added. The solution was stirred for another 2 h
and used for spin coating immediately after. The resist with cobalt
2,4-pentanedionate was exactly the same as that for the nickel
salt.
D
Figure 2. An outline of the steps taken to form the thin film by UV
lithography. (A) The UV-sensitive organometallic resist is spin coated on to
a silicon wafer, (B) which is patterned by UV light. (C) The resist is developed
in MIBK and (D) subjected to oxygen plasma which removes the organic
content of the resist.
(99%, Sigma-Aldrich) were used as the surface leveling agent (SLA)
and the resist glue, respectively. The photoactive compound used
was Irgacure 2022 (Tri-iso, Claremont, CA, USA). Methyl isobutyl
ketone (MIBK, Sigma-Aldrich) was used as the exposed resist
developer.
In a 100 ml opaque bottle, 2 g iron (II) acrylate was added to
18.6 ml PGMEA and 5 ml ethanol and mixed using a magnetic stir
bar. After the salt had completely dissolved, 1 g of resist glue,
5 g TMPTA and 1 g SLA were added while stirring continuously.
After 2 h of mixing, the solution was transferred into a class
100 clean room and 3 g of the photoactive compound was
added. The solution was stirred for another 2 h before using it
for processing. The process for preparing the nickel and cobalt
organometallic resists was slightly different than the one used for
iron. Since both the nickel and cobalt precursor salts had the same
organic chemical functional group, similar reagents were used to
prepare and develop the resist pattern. One gram of nickel 2,4pentanedionate was first completely dissolved in a 14 g mixture of
1 : 1 methoxyethanol and THF. Once the salt was dissolved, 4.1 g
Lithography
A 4 ml aliquot of the photoresist was poured onto a polished
silicon wafer (University Wafer) using a syringe and a 0.1 µm,
25 mm inorganic membrane filter (Anotop 25, Whatman). The
resist was spread at 500 rpm for 3 s and spun at 1500 rpm for
60 s. After a 3 min softbake at 110 ◦ C on a hotplate, the wafer
was exposed using 365 nm, i-line radiation (Karl Suss, MA-4)
in full contact mode. After exposure, the resist that contained
iron was baked at 120 ◦ C for 3 min and developed in MIBK for
6 min, while the nickel and cobalt resists were baked for 3 min
at 140 ◦ C before developing in MIBK for 9 min. After lithography,
the wafers were put in a reactive ion etch chamber (Technics)
to perform oxygen plasma etch at 200 W, 150 Torr chamber
pressure, and 7 sccm oxygen flow rate. The oxygen plasma
removed the organic layer, leaving behind mostly the metallic
constituents of the resist. A schematic of the lithographic process
is shown in Fig. 2. The elemental composition of the thin films
was determined by energy dispersive X-ray spectroscopy (EDX)
(FEI XL30-SFEG). Figure 3 shows the optical microscope images
of the nicely developed patterns of the three films. A scanning
electron microscope image of the nickel organometallic film after
the oxygen plasma is shown in Fig. 4. The EDX elemental analysis
shows that the organometallic films contain oxygen, carbon, silicon
and the metal. The EDX scans of the films of all three metals in
531
Figure 3. Microscope images of the patterns formed in the three films.
Appl. Organometal. Chem. 2010, 24, 530–532
c 2010 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
A. Bhushan et al.
7000
15000
C
10000
5000
O
C
6000
Si
20000
Intensity (CPS)
Intensity (CPS)
25000
5000
4000
3000
O
2000
1000
Ni
0
Fe
Si
0
0
0.5
1.5
1
2
2.5
0
1
0.5
Intensity (CPS)
Energy (keV)
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
1.5
2
Energy (keV)
Si
C
O
Co
0
0.5
1
1.5
2
2.5
3
Energy (keV)
Figure 5. EDX spectra showing the elemental chemical composition of the nickel, cobalt and iron paramagnetic organometallic films.
Fig. 5 show the presence of the metal in the film after the oxygen
plasma.
This approach of realizing metal-rich hybrid films is very
useful as no special equipment is required to prepare the
photosensitive films beyond standard lithography equipment,
and the process to fabricate devices from these materials
is a single, simple lithographic step. Structures from these
materials can be additionally fabricated by simple wet and dry
etching steps, obviating the need for either vacuum deposition
processes such as sputtering and e-beam evaporation or wet
electrochemical deposition. Moreover, the process is flexible,
which allows the possibility of incorporating more than one
type of metal into the film. These heterogeneous materials
can thus be tailored to a wide variety of physical and chemical
properties. A wide variety of application areas in the biomedical
and biological device industry can make use of the micro- and
nano-structured materials and take advantage of their novel
properties.
Conclusions
In summary, hybrid organometallic films containing paramagnetic metals were incorporated in a photoresist, and lithographic patterns were formed into the material. We demonstrate a simple plasma etch procedure to eliminate the majority of the organic components of the hybrid film. EDX
analysis confirms the presence of nickel, iron and cobalt paramagnetic materials in the resist after oxygen plasma. Means
for a complete removal of the organic component of the
hybrid film are being investigated. The material and the
method have the promise of depositing paramagnetic thin
films on substrates for biomedical, electronic and engineering
applications.
Research (C.E.D.), and grant number T32-GM08799 from NIGMSNIH along with an industry/campus supported fellowship under
the Training Program in Biomolecular Technology (T32-GM08799)
at the University of California, Davis (H.H.). The contents of this
manuscript are solely the responsibility of the authors and do not
necessarily represent the official views of the funding agencies.
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Acknowledgments
This project was partially supported by grant number UL1
RR024146 from the National Center for Research for Medical
532
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c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 530–532
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