Cantilever-like micromechanical sensors - Artek

Rep. Prog. Phys. 74 (2011) 036101 A Boisen et al
(a)
(b)
(c)
(d)
Si substrate
photoresist
Figure 9. Fabrication of silicon-based cantilevers by bulk
micromachining: (a) substrate preparation by thin film deposition;
(b) patterning of cantilevers by photolithography and etching;
(c) cantilever release by etching through the bulk wafer from the
backside and (d) removal of the etch stop layer.
approach, where the suspended structures are defined by
etching from the backside all the way through the wafer, is
called bulk micromachining [94].
The fabrication is based on single crystal silicon wafers
with thicknesses of 350–500 µm. During substrate preparation
thin films of the actual cantilever material such as silicon
nitride, silicon oxide or poly-silicon are deposited. The
thickness of these films defines the final thickness of the
cantilevers. Usually, a three-layered substrate is prepared, see
figure 9(a), where the intermediate material is the so-called
etch stop layer. This material protects the actual device layer
during the release in order to secure a well-defined thickness
and a highly reflecting surface of the cantilever. A common
method is the use of single crystal silicon wafers (1 0 0) with a
built-in silicon oxide film. These wafers are also called siliconon-insulator
(SOI) and can be ordered from a range of suppliers
[95–97]. The cantilevers are defined by UV patterning of
photoresist [98] on the front side of the wafer. For nanoscale
cantilevers, the UV lithography can be replaced by electronbeam
lithography [99]. The resist pattern is transferred to
the device layer by wet etching [98] or reactive ion etching
(RIE) [98], figure 9(b). The etching from the backside is
achieved with potassium hydroxide (KOH) [98] or by deep
reactive ion etching (DRIE) [100], figure 9(c). As a final
step the etch stop layer is removed to release the cantilevers,
figure 9(d).
The process results in free-standing cantilevers, accessible
from both sides of the wafer. This is often advantageous.
Both sides of the cantilever can be easily inspected and the
cantilever can be placed in a liquid or gas flow perpendicular
to the cantilever where the etched holes in the wafer serve
as microfluidic channels. However, the cantilevers are rather
fragile and not well protected. Moreover, etching through the
entire silicon wafer is time consuming.
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In recent years, materials such as silicon carbide [101],
graphene [102] and diamond-like carbon [103] have emerged
as alternatives to silicon, all offering unique chemical and
mechanical properties.
3.2. Polymer-based devices
3.2.1. Materials. Since the late 1990s, an increasing amount
of work has been reported on the fabrication of polymer-based
cantilevers. The main motivation to introduce polymers is that
Young’s modulus typically is two orders of magnitude lower
than for traditional silicon-based materials. Consequently,
the stiffness of the cantilevers is reduced and the sensitivity
increases. Equation (2.19) shows that the cantilever bending
due to surface stress changes is directly proportional to Young’s
modulus of the cantilever material. The increased sensitivity
is achieved if the thickness of the polymer cantilevers is
comparable to the ones fabricated using silicon. Another
important driving force to evaluate various polymers and
fabrication methods is the possibility to reduce costs of raw
material and fabrication.
In 1994, Pechmann and co-workers fabricated the
first polymer microcantilevers with a standard Novolakbased
photoresist as cantilever material [104]. In 1999,
Genolet used the negative epoxy photo-resist SU-8 to define
AFM-cantilevers [105]. Young’s modulus of SU-8 is low
(around 4 GPa [106]) compared with silicon and silicon nitride
(180 and 290 GPa, respectively) which makes it a suitable
candidate for micromechanical structures for surface stress
measurements [107, 108]. In parallel, a large number of wellknown
polymers such as polyimide [109, 110], polystyrene
[111–113], polypropylene [113], polyethylene therephtalate
(PET) [114] and fluoropolymer [115] have been evaluated
for the fabrication of micromechanical sensors. Recently,
new thermoplasts specifically developed for microfabrication
such as parylene [116, 117] and TOPAS® [118, 119] were
introduced as cantilever materials.
3.2.2. Surface micromachining. A typical process scheme
for the fabrication of polymer cantilevers is illustrated in
figure 10. Compared with silicon-based devices, the freestanding
structures are fabricated by building up layers on
the surface of a carrier substrate. This approach is called
surface micromachining [120]. First, a so-called sacrificial
layer is applied, figure 10(a), followed by deposition of a
thin cantilever material. In most cases this is achieved by
spin-coating an organic solution of a polymer. An interesting
alternative is parylene, which is deposited by chemical vapour
deposition using di-para(xylylene) [121].
The most straightforward method to pattern the polymer
cantilever is to use UV-photolithography, figure 10(b). This
requires that the polymer is a photoresist such as SU-8 or
polyimide [105, 107–109]. The thin polymer film defining
the cantilevers is spin-coated, baked and exposed using
standard equipment, which minimizes fabrication time and
costs. An alternative approach is patterning the cantilevers
by a combination of traditional photolithography and polymer
etching. For example, O2-plasma etching has been used