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244 E. Meng et al.
The most common method of etching silicon for making molds over 4 μm deep
is the Bosch process used in a deep reactive ion etching (DRIE) tool. This process is
cyclic, consisting of a brief etch step followed by sidewall passivation. This cyclic
process produces the characteristic sidewall texture shown in the image in Fig. 4.41.
By shortening the cycles, the sidewall becomes smoother. Smoother sidewalls are
normally desirable because roughness will in most cases be accurately replicated
in the plastic during embossing, and can promote adhesion between the plastic and
mold, making demolding more difficult. The roughness of silicon sidewalls can be
reduced by repeated oxidation and strip cycles, but better options include using cry-
oetching and neutral loop discharge (NLD) etching instead of the Bosch process.
Most dry deep silicon etch processes can be tweaked to adjust the sidewall angle. A
slight taper simplifies demolding.
Fig. 4.41 SEM image of silicon mold sidewall etched by Bosch process DRIE
Shallow features can be etched in silicon using conventional RIE processes to
produce smooth sidewalls. The geometries produced by wet anisotropic silicon
etchants (such as KOH, TMAH, and EDP) are relatively easy to emboss.
4.8.4.2 Nickel
Nickel is probably the most widely used mold material for large volume applications
because it displays excellent wear properties. It is also the first choice for aggressive
features and embossing processes due to its high strength and toughness. Nickel can
be machined using conventional milling processes, but for micro- and nanoscale
4 Additive Processes for Polymeric Materials 245
features, it is most commonly electroplated into a mold that determines the final
geometry. The mold is subsequently removed.
X-ray LIGA (German acronym for Lithographie, Galvanoformung, Abformung,
i.e., “lithography, electroplating, and molding”) is the pre-eminent method for fab-
ricating very-high-aspect-ratio nickel molds. The use of X-ray lithography to define
the mold into which the nickel is electroplated, allows the definition of extremely
high-aspect-ratio features with extremely smooth sidewalls (<100 nm sidewall
roughness). The substrate can be scanned and tilted during the X-ray exposure to
make the mold taper. This taper is transferred to the nickel during electroplating,
which aids demolding. Other LIGA variations exist, including UV-LIGA in which
the mold is fabricated using conventional lithography. Molds can be fabricated from
materials other than photoresist, such as silicon or fused silica. It is common to take
a conventional part to be duplicated in plastic and use it as the mold for electroplat-
ing. Note that this process is not straightforward if the aspect ratio of recessed areas
on the plated part exceeds 1:1. See Fig. 4.42.
Fig. 4.42 SEM image of LIGA mold sidewall demonstrating extremely low roughness
4.8.4.3 SU-8
SU-8 is a high-contrast epoxy negative photoresist that can be coated and patterned
in very thick layers. Once the material is fully cross-linked it displays excellent
mechanical properties, chemical resistance, and thermal stability (T
g
=200
◦
C, SU-8
3000 series [463]). Consequently, it can be a relatively rapid method of producing
mold inserts. The wear properties of SU-8 are not ideal for embossing of challeng-
ing features, but for moderate features in low volumes, it can be cost effective and
suitable for rapid prototyping. Grayscale lithography has been demonstrated with
246 E. Meng et al.
Fig. 4.43 High-aspect-ratio SU-8 features
SU-8 to fabricate 3-D microstructures [464]. A concern with the use of SU-8 mold
inserts for aggressive processes is that the adhesion of the features to the substrates
may be insufficient, and may deteriorate with use. See Fig. 4.43.
4.8.5 Conventional Machining of Molds
Inasmuch as the focus of this work is on micro- and nanotechnologies, we have
focused on micro- and nanomachined mold inserts. Hot embossing is not a new
technology; however, it has only recently become widely used on the micro- and
nanoscales. Consequently there is a long history of molds fabricated using conven-
tional machining. Conventional machining has also evolved to be able to produce a
wide variety of features on the micron scale.
4.8.5.1 Milling
High-precision CNC tools can mill parts using very small diameter bits. Although
the diameters of the bits might be too big to machine channels that would be consid-
ered small enough to fall into the MEMS realm, they can remove material from
either side of a wall to make it narrow enough to be considered MEMS scale.
This “wall” feature will form a MEMS scale channel when embossed into plas-
tic. Inasmuch as the pattern is created one feature at a time, the cost is proportional
to the size, number of features, and complexity of the mold.
4 Additive Processes for Polymeric Materials 247
4.8.5.2 Laser
Recent advances in laser and chemically assisted laser micromachining have
expanded the potential application for mold insert fabrication. Historically, feature
roughness has been a concern, but this is not the impediment it used to be.
4.8.5.3 Focused Ion Beam
Focused ion beam (FIB) tools have been used to structure substrates, which
have been subsequently used as molds. This is a beam-based etch, therefore
complex 3-D geometries can be fabricated. It also means that the cost scales lin-
early with the amount of material to be removed. Fine features require a finer
beam, which will cost more per unit volume of material removed. This approach
can provide very rapid turnaround of very small-scale features, but the cost
is high.
4.8.5.4 Fixture of Molds
The effort and expense justified for the mold fixture will depend on the parameters
of the process to be run, as well as the number of parts to be run. The format for
the mold and the substrate will be dictated to a large degree by the embossing tool
to be used; it requires due consideration when preparing to emboss. In most cases
some machining will be required beyond the creation of the mold pattern in order to
emboss and subsequently demold.
4.8.5.5 Release Coatings
The use of mold release agents is relatively mature in injection molding, and has
direct relevance to microscale hot embossing. The mold release agent reduces or
prevents adhesion between the mold and the material being embossed. This is
advantageous as it reduces wear on the mold, and also reduces the possibility of
fouling the mold with fragments of the embossed material.
The direct application of an off-the-shelf release agent risks distorting microscale
geometries due to the granularity of the material, either forming a texture, or
droplets. A custom formulation, application, or dilution may be required. It is also
common to deposit a thin CVD layer of PTFE or a PTFE-like material. It is essen-
tial that the surface be very clean prior to deposition to improve adhesion of the
deposited layer to the mold.
4.8.6 Process Development
There are two common philosophies for process development, both of which are
predicated on the assumption that the material to be embossed is inexpensive relative
248 E. Meng et al.
to the mold. The first approach seeks to protect the mold from mechanical damage
by ensuring that the embossed material is compliant, and the force is low.
For the first-time embosser, the suggested starting temperature is in the range
of 10–20
◦
C above the material T
g
. Low force and short flow time should be used.
What is considered “low force” is very geometry-specific; it depends on the area
of the mold and the size and flow resistance of the features, as well as the depth
of the features. When in doubt start as low as in the 2–5 kN range (for low-aspect-
ratio nanoscale features, this range may even be considered high). Note that molds
made from standard <100> silicon substrates 100 mm in diameter and 500 μm thick
are prone to crack when the force exceeds 8 kN. By keeping the flow time short,
excessive substrate flow is less likely. If the mold is not filling with the substrate
material, consider increasing the mold fill time before increasing the force.
The initial demold temperature should be high (i.e., 5–10
◦
C below T
g
), so that
the substrate is still somewhat compliable. The demold temperature can be reduced
until no evidence of feature stretching or “drawing” is evident (as can be observed
in Fig. 4.44).
Fig. 4.44 Plastic “drawn” by the demolding step. Note that features with high perimeter-to-cross-
sectional area are the worst affected
The second approach seeks to protect the mold from being “fouled” by the sub-
strate material breaking free and staying embedded in the mold. In this case, it is
better to start with low force and low temperature. This will result in the high points
of the mold bearing the embossing force with little accommodation by the sub-
strate, so it is important that the mold not have isolated high points that can be
damaged. The temperature and force should be increased together until some mold
fill is observed, then the focus should be on force to get good mold fill.
4 Additive Processes for Polymeric Materials 249
Fig. 4.45 Bubbles beginning to form due to high emboss temperature
The formation of bubbles in the embossed plastic (Fig. 4.45) is an indicator that
the embossing temperature is high.
4.8.7 Minimum Substrate Thickness
The substrate molded has to be sufficiently rigid that the embossed features are well
enough attached to the substrate that they do not adhere better to the mold and either
stretch or tear free. The material needs to be mechanically sound and either thick
(i.e., thick enough to be rigid with respect to the fixture and the demolding force),
or bonded with good adhesion to a rigid handle layer. Similarly, it is important
to make sure that the protective films often applied to plastics are removed prior
to embossing because in many cases they will bond to the mold and can be very
difficult to remove.
Another consideration for embossing thin plastics is the issue of flow resis-
tance. Essentially the embossed layer is a flow channel, which can be described
by Poiseuille’s law:
R =
12vl
wh
3
,
where: R = flow resistance, v = viscosity, l = length, w = width and h = height.
As the height of the flow channel (layer thickness) is reduced, the flow resistance
increases as a function of the height cubed, so thin layers require excessive force to
achieve reasonable material flow.
250 E. Meng et al.
4.9 Materials Properties
Table 4.6 includes material properties presented in tabular format for some of the
polymers discussed in this chapter. Additional property tables are found in the
preceding text.
Table 4.6 Some material properties for select polymers
Property
Parylene C
[465]
Polyimide
[466]SU-8
LCP
(Vectra
A-950) [452]
BCB
(Cyclotene)
[428]
Young’s modulus
(GPa)
2.8 1.8–8.5 0.7–5.9 [6,
22, 23, 48,
50, 63, 66,
73, 115,
467–471]
Tensile strength
(MPa)
69 100–350 15–120 [22,
23, 73, 471]
30 kpsi 85–87
Yield strength
(MPa)
55 MPa
Poisson’s ratio 0.26–0.33
[22, 472]
0.34
Elongation (%) 200 10–120 1.8–24 [22,
23]
8
Cure temperature
(
◦
C)
250–375
Melting point
(
◦
C)
290 280
Glass transition
temperature
(
◦
C)
235–400 50–240 [2, 5,
20, 22, 23,
110]
> 350
Linear coefficient
of expansion
3.5×10
−5
/
◦
C 3–70 ppm/K 52–452
ppm/K [22,
53]
0–30 ppm/K 52 ppm/K
Acknowledgments This work was supported in part by the Engineering Research Centers
Program of the NSF under Award Number EEC-0310723 (Meng), NSF CAREER grant under
Award Number ECS-0547544 (Meng), NSF CAREER grant under Award Number CMMI-
0239163 (Zhang), NSF grant under Award Number CMMI-0826191 (Zhang), and NSF grant under
Award Number CBET-0933653
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