Towards the application of nanocrystalline metals in MEMS

Article abstract of DOI:10.1002/pssa.200566155

A comparison is presented of the cross-sectional mechanical properties (i.e. hardness and Young's modulus) for electrodeposited microsystem components with conventional polycrystalline and nanocrystalline structures. It is shown that nanocrystalline electrodeposits exhibit considerabily improved crosssectional properties both in terms of uniformity (hardness and Young's modulus) and overall hardness level.

Full text of DOI:10.1002/pssa.200566155

Original
Paper
phys. stat. sol. (a) 203, No. 6, 1259–1264 (2006) / DOI 10.1002/pssa.200566155
Towards the application of nanocrystalline metals in MEMS
Mohammadreza Baghbanan*, 1, Uwe Erb1, and Gino Palumbo2
1
Department of Materials Science and Engineering, University of Toronto, 184 College St., Toronto,
ON, M5S 3E4, Canada
Integran Technologies Inc. 1 Meridian Road, Toronto, Ontario, M9W 4Z6, Canada
2
Received 30 September 2005, revised 21 February 2006, accepted 22 February 2006
Published online 3 May 2006
PACS 61.46.Hk, 62.25.+g, 68.37.Hk, 68.37.Lp, 81.07.Bc, 81.16.–c, 85.85.+j
A comparison is presented of the cross-sectional mechanical properties (i.e. hardness and Young’s
modulus) for electrodeposited microsystem components with conventional polycrystalline and nanocrys-
talline structures. It is shown that nanocrystalline electrodeposits exhibit considerabily improved cross-
sectional properties both in terms of uniformity (hardness and Young’s modulus) and overall hardness le-
vel.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
MicroElectroMechanical Systems (MEMS) have already been used widely in the automotive industry
over the past decade. Considerable growth is expected over the next several years for microcomponent
usage in other areas including medical, biological, aerospace and space exploration, consumer product
and defense applications.
In general, MEMS can be described as intelligent miniaturized systems, with characteristic dimen-
sions ranging from ∼1–1000 µm, that combine sensing and/or actuating functions with processing func-
tions. Such systems are typically multifunctional combining two or more of electrical, mechanical, opti-
cal, chemical, biological, magnetic or other properties. Two of the most common techniques in the
fabrication of Si-based microsystems are bulk micromachining and surface micromachining, while me-
tallic MEMS structures are usually produced through molding processes. In bulk micromachining, mi-
cromechanical structures are created by etching into the bulk of the substrates with either anisotropic
dissolution using alkali solutions or deep reactive-ion etching (DRIE) while surface micromachining is
based on prearranging stacks of functional and sacrificial layers on a substrate with subsequent removal
of the sacrificial layers to produce the final structure [1].
This paper deals with metallic microsystem components which, in most cases, are integrated into the
structure by various forms of electrodeposition or electroless plating processes such as LIGA (German
acronym for Lithographie [lithography], Galvanoformung [electroforming] and Abformung [molding])
or HARMS (high aspect ratio moldings).
1.1 Molding process (LIGA)
LIGA is the most widely used process to fabricate metallic components for MEMS. In this process indi-
vidual microcomponents are made by specialized electroplating techniques. Figure 1 shows schemati-
cally the various steps involved in a typical LIGA process. The advantage of LIGA is that structures can
* Corresponding author: e-mail: m.baghbanan@utoronto.ca, Phone: +1 416 946 3315, Fax: +1 416 946 3316
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M. R. Baghbanan et al.: Towards the application of nanocrystalline metals in MEMS
b. X-ray/UV Exposure
a. Cast PMMA
on substrate
Mask
c. PMMA development
g. Separate
microcomponent
from substrate
d. Electroform
through PMMA
f. Separate Metal
from PMMA
e. Planarize
Microcomponent
Fig. 1 (online colour at: www.pss-a.com) Schematic diagrams showing the various steps in a typical
LIGA process: a) PMMA cast on metal substrate, b) X-ray exposure, c) PMMA development, d) electro-
plating through PMMA mold, e) planarizing microcomponent, f) separating PMMA from plated compo-
nent and substrate and g) final product separated from substrate.
be made of almost any metal that can be electroplated at relatively low stress levels. These include nickel
(Ni), copper (Cu), gold (Au), silver (Ag), cobalt (Co), as well as nickel alloys (typically Ni–Co and
Ni–Fe), with nickel being the most commonly LIGA-processed material [2].
In the manufacture of hybrid microsystems with multifunctional capabilities, electrodeposition is
frequently used to produce metallic components with specific mechanical and physical properties. In this
paper the effect of microstructure on the overall performance and local property variations for both con-
ventional and nanostructured nickel will be discussed. Directionally similar results as shown in this paper
for Ni were also obtained for cobalt and nickel-iron alloy deposits [3].
2 Experiments
Two types of nickel microsamples were synthesized using 1) conventional electrodeposition with poly-
crystalline structure and 2) a newly developed electrodeposition method that results in nanocrystalline
structure. The plating technique used for nanocrystalline deposits involved pulsed-current electrodeposi-
tion in a modified Watts electroplating bath (containing nickel sulfate and nickel chloride) as previously
described for bulk nanocrystalline nickel electrodeposits [4, 5]. Pulse plating can be carried out at current
densities considerably higher than the limiting current density for direct current plating, which promotes
crystal nucleation, i.e. nanostructure formation. The conventional polycrystalline nickel micro-samples
were also deposited from a Watts bath by applying direct current rather than pulsed current [6]. Micro-
structural characterization was performed on these materials in terms of grain size distribution and aver-
age grain size using scanning and transmission electron microscopy (SEM, TEM). The mechanical prop-
erties of the deposits were evaluated using a Shimadzu (DUH-W201) dynamic ultra-micro hardness
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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phys. stat. sol. (a) 203, No. 6 (2006)
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tester with Berkovich nanoindenter. The cyclic loading mode was employed with maximum load of
100 mN and 50–150 mN for hardness and Young’s modulus measurements, respectively.
3 Results and discussion
Nanoindentation results for conventional polycrystalline Ni electrodeposits show (Fig. 2a) low overall
hardness (∼2 GPa) with a gradient in the hardness profile (to about 3.2 GPa) perpendicular to the sub-
strate surface of the deposit. The Young’s modulus measurements results for polycrystalline Ni exhibit
(Fig. 2b) considerable variation throughout the cross-section with values as high as 260 GPa and as low
as 150 GPa. This undesirable property variability can be traced back to microstructural effects [7]. The
microstructure of the conventional electrodeposits (Fig. 3a) shows grain size gradients resulting from the
fine grained to columnar structure transition with increasing component thickness, which explains the
decrease in hardness with increasing distance from the substrate surface. The fine grained to columnar
structure transition in conventional deposits is mainly the result of growth competition in which grains
with low surface energies grow at a faster rate than grains with high surface energies [6]. The variation in
the Young’s modulus is a direct result of the anisotropy of the elastic constants in columnar grains with
different orientations. The elastic moduli for Ni range from 310 GPa in the 〈111〉 direction to 150 GPa in
the 〈100〉 direction [8].
In contrast, the nanostructured Ni deposit shows uniform small grain size throughout the entire cross-
section of the component without the transition from fine to large grained columnar structure (Fig. 3b).
GROWTH DIRECTION
100 µm
100 µm
4
3.5
3
300
250
200
150
100
50
2.5
2
1.5
1
0.5
0
b)
a)
0
0
50 100 150 200 250 300
0
50 100 150 200 250 300 350 400 450
Distance on cross-section (µm)
Distance on cross-section ( m)
µ
Fig. 2 (online colour at: www. pss-a.com) a) Cross-sectional hardness and b) Young’s modulus for a
polycrystalline Ni electrodeposit.
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M. R. Baghbanan et al.: Towards the application of nanocrystalline metals in MEMS
GROWTH DIRECTION
GROWTH DIRECTION
40 nm
20 µm
a)
b)
Fig. 3 Cross-sections of a) electroplated polycrystalline and b) nanocrystalline nickel microcomponents
[9].
Figure 4 shows TEM bright and dark field images as well as a diffraction pattern of the nanostructured
Ni used in this research. By measuring the grain diameter of several hundred grains in diffracting condi-
tion in dark field micrographs, the average grain size of the nanocrystalline material was determined to
be 13 nm.
From the considerable reduction in average grain size, the overall hardness of the deposit increased to
over 6 GPa (Fig. 5a) due to extensive Hall–Petch strengthening (as per Eq. (1)).
K
H = Ho +
,
(1)
d
where H, Ho, K, and d, are the hardness of large grained and nanocrystalline nickel, the Hall–Petch con-
stant and average grain diameter, respectively.
As a result of the elimination of the fine grain to columnar transition, the hardness of nanocrystalline
Ni perpendicular to the substrate is relatively uniform as shown in Fig. 5a. Furthermore, the large varia-
tions in Young’s modulus observed in conventional electrodeposits (Fig. 2b) are virtually eliminated
(Fig. 5b). Instead, the Young’s modulus is relatively uniform in cross-section at around 200 GPa, which
is essentially the same value as expected for conventional polycrystalline nickel with equiaxed grain
shape and random crystallographic texture [10].
These properties of nanocrystalline Ni result in improved performance indicators such as specific
strength and elastic energy storage capacity (i.e. resilience). A comparison between these performance
200
b)
a)
Fig. 4 a) TEM bright field and dark field images and b) electron diffraction pattern of nanocrystalline Ni
with 13 nm average grain size [9].
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
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phys. stat. sol. (a) 203, No. 6 (2006)
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GROWTH DIRECTION
100 nm
100 nm
250
200
150
100
50
8
7
6
5
4
3
2
1
0
b)
0
a)
0
50
100 150 200 250 300
0
10 20 30 40 50 60 70 80
Distance on cross-section ( m)
Distance on cross-se ction ( m)
µ
µ
Fig. 5 (online colour at: www.pss-a.com) a) Cross-sectional hardness and b) Young’s modulus for a nanocrystal-
line Ni electrodeposit.
indicators for conventional and nanocrystalline Ni showed [9] quite remarkable improvements in nano-
crystalline components e.g. a factor of four for specific strength and an order of magnitude for elastic
energy storage capacity.
In addition to the enhancement of mechanical properties, microsystems would also benefit from other
enhanced properties of nanocrystalline metal deposits. For example, it has been previously shown that
grain size reduction in nickel to about 10 nm results in considerable reductions in the coefficient of fric-
tion (by about 50 percent) and 2 orders of magnitude improvement in sliding wear resistance [5]. These
properties are of considerable importance for the tribological performance of microsystems consisting of
moving parts. Similarly, the grain size independent saturation magnetization combined with the in-
creased electrical resistivity at grain sizes less than 100 nm [5] render superior soft magnetic properties at
reduced eddy current losses for magnetic components operating at high frequencies.
4 Conclusions
Cross-sectional analysis of hardness and Young’s modulus was performed on electrodeposited nickel
microsystem components with conventional polycrystalline and newly developed nanocrystalline struc-
tures. While conventional nickel electrodeposits show considerable local variations in these properties,
nanocrystalline deposits exhibit cross-sectional uniformity and considerably enhanced overall hardness
values. The improved properties of nanocrystalline nickel result in substantial enhancements of micro-
system performance indicators such as elastic energy storage capacity and wear resistance.
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Acknowledgements Financial support by the Natural Sciences and Engineering Research Council of Canada
(NSERC) is gratefully acknowledged.
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© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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