Jackson M.J. Micro and Nanomanufacturing
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Nanomachining 633
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12.
Micro-and Nanomanufacturing
12.1 Introduction
The future of micro-and nanomanufacturing lies in the ability to
convert micro-and nanofabrication techniques into mass production
manufacturing processes where small-scale products can be eco-
nomically manufactured in a short period of time. This may be
achieved by combining micro-and nanoscale processes, or by com-
bining "top down" and "bottom up manufacturing techniques. The
development of so-called hybrid manufacturing processes is set to
take center stage in the future development of micro-and nanomanu-
facturing processes. This chapter focuses on current developments
in the field of micro-and nanomanufacturing.
12.2 Micromanufacturing
12.2.1 Electroplating
The fabrication of master molds in preceding chapters was fo-
cused on the use of lithographic techniques such as LiGA, deep x-
ray lithography, wet etching, dry etching using plasmas, etc. These
techniques, although widely used, are very slow at producing master
molds. An alternative technique that is proving to be slightly faster
is electroplating that can produce master molds from a variety of
636 Micro- and Nanomanufacturing
materials and is an established way of manufacturing precision
products.
The object to be plated must be metallized and/or masked to de-
fine regions that require the addition of metals to their planar sur-
faces.
Electrodeposition is based on the principle of electrotrolysis,
which means that many metals can be deposited. The process of
electroplating is very simple and can be performed using very basic
equipment such as a laboratory beaker, electrodes, an electrolytic so-
lution, and a power supply. Electroplating can deposit up to 1mm of
metal in a reasonably short timescale. Electrolytic solutions can
contain salts of metals, water, anions, and cations, and the solution
can be buffered to control the pH level.
The density of parts produced using electrodeposition is typi-
cally 1-4% below the theoretical value that accounts for voids, inclu-
sions,
porosity, and impurities. Once electroplated, products are
usually annealed to increase the density close to 100%. The influ-
encing factors of a good quality micropart include a deposited thin
film thickness that can be in the range between 12-18 //m.
Stresses in these parts are caused by the interference of interacting
grains and the use of high current densities when electroplating. The
adhesion of plated films is dependent upon the match between sub-
strate and electrodeposited film, the type of oxide film on the sub-
strate, and on hydrogen embrittlement that can be promoted by the
reduction reaction at the cathode that releases hydrogen into the
electroplating cell. A selection of microscale parts produced by
electroplating is shown in Fig. 12.1. Electroplating can be adapted
very easily to produce microscale parts and mold masters with rela-
tive simplicity and is considered as a future micromanufacturing
process for producing metallic microscale parts and components,
and masters for dies and molds that can be used for the subsequent
molding of polymeric microparts and components.
12.2.2 Casting
Casting is another large-scale manufacturing process that can be
adapted to operate at the microscale. The process of casting micro-
structures is widespread in the academic world but is not widely
Micro- and Nanomanufacturing 637
used for the production of microparts in industry. Casting offers a
low cost solution to the manufacture of planar microstructures with
intricate features and is typically used with silane elastomers such as
polydimethylsiloxane (PDMS). The casting of PDMS is performed
by initially mixing the PDMS elastomer with a curing agent in a
10:1 weight ratio. The pre-polymer mixture must be degassed, typi-
cally at 20-50 mTorr for 1 hr in a desiccator with a vacuum pump.
The critical step is to remove any air bubbles in the mixture. Mixing
PDMS at different ratios with the curing agent will affect the result-
ing mechanical properties, i.e., 10:1 PDMS has E = 750 kPa and 5:1
PDMS has E = 868 kPa.
Ni beams,
150 lam wide,
65|.imhigh
Cu beams,
58
Mm,
thick resist mol
Fig. 12.1. A variety of mold components made from copper and nickel by elec-
trodeposition
The compound is poured on to a master pattern and is allowed to
set for three hours at lOO^C. The master pattern can be silicon-
based material or any other material for that matter. PDMS can also
be cast on to photoresist material such as SU-8 and AZ4620. The
cast elastomer can be peeled off the master pattern and be used
many times to deposit microscale particles that form microscale fea-
tures.
The cast PDMS is typically 1mm thick but can be thicker. It
can be attached to a rigid body so that it can be used as a stamp
638 Micro-and Nanomanufacturing
when stamping microscale features. The fact that the stamp can be
used multiple times makes it a micromanufacturing process that can
be used for depositing micro-(and nano) scale particles, molecules,
etc.
This type of technology can be used to create features for mi-
crofluidic channels simply by casting PDMS layers to act as valves
and switches on the surface of microchannels (Fig. 12.2).
m
w
""ii
nail
Fig. 12.2. Microchannels etched in sihcon with a layer of PDMS deposited to act
as a flow valve to control the flow of liquid in a microfluidic chamber
PDMS can also be cast to form soft tools that are used in micro-
molding processes such as hot embossing. The casting of thin layers
of PDMS can be performed to provide three-dimensional features,
which is especially important in the proper functioning of micro-and
nanofluidic flows. Figure 12.3 shows the casting of thin layers of
PDMS in a three-dimensional microfluidic device.
The versatility of this material has not yet been realized in many
micro-and nanofabrication processes. However, the future devel-
opment of micro-and nanomanufacturing processes will certainly be
aided by the extensive use of PDMS stamps, molds, and tools. The
Micro- and Nanomanufacturing 639
casting of microparts in metals such as zinc and aluminum alloys has
already started in the U.S. castings sector. The complexities of cast-
ing metals before they freeze in the molds they fill will lead to the
development of warm casting processes or hybrid developments that
may include semi-soUd casting of microparts at low temperatures. A
new wave of low temperature castable alloys will be required to
meet these demands.
SU-S PlistoreiiM
/ *
i_V 3^
-^^
Si wkfcr
i
Afsnunum plate Hop)
.j^h^^
Ctotwp
-T^
I. Jo. cl ^1.7 MirnH'ltnromcrh S\si. vol. 9. pp.
76-SI.
2000.
j300nm
•^^ 4
Fig. 12.3. The casting of thin layers of PDMS between sihcon and SU-8 photore-
sist shows how three-dimensional mcirofluidic devices can be manufactured sim-
ply by casting and clamping [1]
12.2.3 Molding
The process of molding microparts individually is extremely wide-
spread at the present time but has yet to be optimized for manufac-
turing microparts. The molding process has rapidly developed in the
area of PDMS molding using silicon masters that have been pro-
duced using techniques such as LiGA, deep x-ray lithography, etch-
ing, etc.
Figure 12.4 shows the process of forming a PDMS molding us-
ing PDMS as a molding that can be used in microfluidic systems. In
640 Micro- and Nanomanufacturing
order to convert the fabrication of PDMS molding to manufacturing
PDMS moldings, the future manufacturing of these moldings will
require the development of injection molding and hot embossing
techniques. Hot embossing is a technique that is rapidly gaining
popularity in producing microscale features in thermoplastic materi-
als.
The three stages of manufacture involve heating the polymer,
embossing the polymer to the shape of the pattern, de-embossing to
produce the molding. The embossing temperature is typically be-
tween 50 and lOO^C. The overall embossing time is approximately
ten minutes. Commercial embossing equipment is shown in Fig.
12.5.
Fabricaiion and $ifani2ai>on oi master
r y r' ' /• .^ I' ^ /• /- r .', i' 'j.T y ^ ^^.r ^ ^ ^
J-
^
A
r y y t'r ^
ts>:\\\s.\i
Pouring
o1
PDMS over master
PDMS
is.v^.^.^.vv^ v. v >. v v Sil \. ^ \ \ s. x
K.
^ v s «. \
PDMS
HL
Curini9f sind releasing of PDMS
i,.
J L
d h
Fig. 12.4. Formation of a PDMS molding from a master pattern. Courtesy of G.
Whitesides, Harvard University
Micro-and Nanomanufacturing 641
1
1
f
f.
: •
it
race fnuir
/
/.
'v^^^vy^^i
••••(
H| Dtix^^iii^
Top
hot plme
Bottom
movable
hot plate
CoiKrotler
Fig. 12.5. Schematic view of hot embossing and commercial embossing equip-
ment
The success of the hot embossing technique is dependent on the
tool or pattern that is being embossed. Hard tools are prepared by
machining, electroplating, dry or wet etching of silicon, or using
LiGA. A comparison of tools created using these technologies is
shown in Table 12.1. Figure 12.6 shows a selection of microscale
products created using hot embossing. The products are produced
on a mass scale and the typical embossing time for producing these
parts was 8 minutes. Typical products produced using this micro-
manufacturing process include lab-on-a-chip products such as
ACLARA's Labcard as shown in Fig. 12.7.
The development of three-dimensional microdevices has
prompted the recent development of hot embossing with soft tools
[2,3].
The process involves coating a pattern with PDMS elastomer,
allowing the PDMS soft tool to set, the using the soft tool to form an
impression in a thermoplastic material by applying a direct force on
to the surface of the polymer via the soft tool during hot embossing.
The schematic outline of the process is shown in Fig. 12.8 with a
scanning electron image of the soft PDMS tool and the embossed
PMMA material in Fig. 12.9.
642 Micro- and Nanomanufacturing
Table 12.1. Comparison
of
hard molding tools produced using various methods
Silicon UV-LIGA LIGA
CNC
SU-8
Aspect
ratio
Feature
size
Lifetime
Stiction
Finish
Time
to
fabricate
Low(~l
to
2)
?5^m
-30
Yes
High (dry)
Low (wet)
Long
(slow etch)
High(~6)
?25^m
-100
No
Moderate
Moderate
Very
high(-lOO)
-0.1
|im
-50
No
Low
(-lOnrn)
Long (slow
growth)
Very
low(-l)
?100|im
>100
No
High
Short
Low(~l
to
2)
?25|Lim
Few tens
Yes
Moderate
Moderate
(Bake time)
250 ^m deep PMMA channel.
An
array
of
600 ^m wide
features with
1
mm center-to-center spacing
ccoHSoe
J. Narasimhan
and
I. Papautsky,
J.
Mcromech.
Mcroeng.,
vol. 14, pp. 96-103,2004.
H. Becker and C. Gamer," Electrophoresis, vol. 21, pp. 12-26,2000.
Fig. 12.6.
Hot
embossed microproducts
[2,3]