Microfluidics for Biologists Fundamentals and Applications

is one way of circumventing these apparent obstacles. There are adjacent services
and team of professionals who can carry out an operation instead of you. To
communicate your plan for photolithography one needs a properly designed mask
with definite criteria for fabrication. In the following paragraphs few guidelines for
masking will be introduced that will allow you outsourcing of your designs easily.
It is important to know what sizes of masks and substrates are typically used in
the cleanroom to which you are outsourcing. Standard 4 in. wafers are processed
with 5 in. masks; 3 in. wafers usually require 4 in. masks. Once you know the size of
substrate that the process line can handle, maximum lateral features of your design
can be determined. Important is to leave free space of about one centimetre over the
periphery of the wafer. That space is for handling the wafer manually, using
tweezers, without damaging the features close to the periphery. When the design
is fully tested and goes to mass cleanroom robotised production the space can be
reduced. Photolithography serves as the base for more ubiquitous application,
outside of the cleanroom environment. Soft lithography can be achieved in a regular
chemical laboratory due to the advances in photopolymerization chemistry and the
plethora of commercially available photoresists.
The most common negative photoresist used in microfluidics is SU-8 due to its
multiple implementations and well characterised fabrication properties. It is epoxy-
based resist that comes in different viscosities, which facilitate the formation of

Transparency

mask

UV exposure

Silicon wafer

Silicon wafer Silicon wafer

Silicon wafer

PDMS curing

PDMS

chip

Channels

Reservoirs for

media

Inlets for

hydrogel

injection

Developed SU-8

patterns

PDMS

detachment

SU-8 photoresist

abc

def

Fig. 3.3 Illustration of the method of photolithography using SU-8 photoresist. As described from
(a)–(c), SU-8 photopolymer is first coated on a isopropanol cleaned silicon wafer, followed by
aligning the transparency mask (black squarein section (b)) of the desired microfluidic design.
Later, the aligned mask is exposed to UV light, which crosslinks SU-8 in the shape of clear mask
region. After mask removal, the SU-8-coated wafer was washed with SU-8 removal solvent. This
will remove all the unexposed SU-8; thus, leaving behind desired structures on the mask, as
depicted in frame‘(c)’. On this mould, soft lithography can be used by pouring PDMS and curing it
thermally. Upon removal, PDMS will give us the desired dimensioned fluidic network. Reprinted
by permission from Macmillan Publishers Ltd: [Nature Protocols, 2012] [ 5 ]

3 Manufacturing Methods Overview for Rapid Prototyping 89

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