Page 204 - Sami Franssila Introduction to Microfabrication

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Moulding and Stamping

Moulding and stamping are age-old techniques that have Terminology in the field of micromoulding and
recently been given new twists by microtechnologies. stamping is not established because the field is new and
The printing industry depends on stamping the inked rapidly expanding. Sometimes the field is known as soft
typeface against paper for transferring the ink. The very lithography, but this really applies to surface stamping
same process has now been adopted in microfabrication, only. Microcontact printing (µCP) is a surface stamping
with sophisticated tools and materials for micrometre method that relies on alkanethiol inks on gold surfaces.
and even nanometre dimensions. Moulding of metals, Hot embossing is the name used for volume stamping
plastics and ceramics can be extended to novel applica- of MEMS structures, and is sometimes referred to as
tions by microfabrication techniques. hot embossing lithography (HEL). The same technique
Thomas Alva Edison used sputtered gold seed is called nanoimprint lithography (NIL) in communities
layer, wax mask and gold electroplating to fabricate that aim at ultimate resolution. The name step-and-stamp
phonograph masters. The technology entered production is used when NIL is performed analogously to step-and-
in 1901 and it could replicate 125 µm pitch (200 repeat lithography, that is, one chip is exposed at a time
grooves/inch), 25 µm thick structures. Electroplating followed by a mechanical movement to fill the wafer
is still a major method for mould-master fabrication. with patterns.
In microfluidic applications, dimensions are not much
smaller than in Edison’s time; in fact, traditional machine
tools could, in principle, be used to fabricate the masters, 18.1 MOULDING
but most often the surface finish is too rough and the
pattern complexity makes machining throughput low but Materials of all classes can be used as moulds: resist
it is useful for quick turnaround time prototyping. mould for electroplated nickel, electroplated nickel
Moulding and stamping have different material flows: mould for PDMS, PDMS mould for ceramics, or single-
in moulding, material is being transported into the mould crystal silicon for polysilicon, diamond and PZT. Of
(Figure 18.1(a)). The traditional method is casting and course, thermal and other limitations apply, but clearly
is still in use in microfabrication: thick polymethyl the choices are many. There is a plethora of variants
methacrylate (PMMA) resists and polydimethyl siloxane of these techniques, and this chapter discusses just the
(PDMS) elastomers are cast. But our usage includes basic issues involved in the replication technologies.
various transport and deposition processes: injection Injection moulding is applied for micrometre dimen-
of thermoplastics, electroplating of metals, CVD of sions in mass manufacturing: molten plastic is injected
polysilicon or diamond or sol-gel of PZT. In stamping, into a mould insert to fabricate compact discs (CDs).
there is no transport of material: the polymeric material, However, from a general microfabrication point of view,
which is on the wafer to begin with, is modified locally CD is an easy application because the aspect ratios are
by the stamp (Figure 18.1(b)). ca. 0.2 only, the pattern density is quite uniform and
Moulding can be further divided into methods that the pattern sizes are not dissimilar. Circular symmetry
use reusable or disposable moulds (Figure 18.2). In with injection from the centre is beneficial for stress
stamping, we can distinguish two cases: 2D-surface minimization.
processes and 3D-volume processes, which have rather Moulding can be continued to further generations:
different requirements for stamp masters. instead of using the moulded piece itself, it can be used

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)