Chapter 7 – Complementary Experimental Tools  289

which is thus independent of h(0). The thickness used depends on the photoresist and

varies in the range ~0.5 μm up to 100 μm or more.

There are two types of photoresists. A positive resist becomes soluble to the photoresist

developer on exposure to light, whereas a negative resist hardens and becomes insoluble

upon exposure to light. A surface pattern, or nanoimprimpt, is placed on top of the photo­

resist. This consists of a dark printed pattern that acts as a mask normally of glass covered

with chromium put on top of the thin layer of photoresist, which block outs the light in areas

where chromium is present, and so areas of photoresist directly beneath are not exposed

to light and become insoluble to the photoresist developer. The unexposed portion of the

photoresist is dissolved by the photoresist developer (Figure 7.4a).

A popular photoresist in biophysical applications is the negative photoresist SU-​8—​an

epoxy resin so called because of the presence of eight epoxy groups in its molecular structure.

SU-​8 has ideal adhesive properties to silicon-​based substrates and can be spun out to form a

range of thicknesses at the micron and submicron scale, which makes it ideal for forming a

high-​resolution mask on a substrate to facilitate further etching and deposition stages in the

microfabrication process. It is for these reasons that SU-​8 is a popular choice for biological

hybrid MEMS devices, or Bio-​MEMS, for example, for use as miniaturized biosensors on lab-​

on-​a-​chip devices (see Chapter 9).

A typical protocol for generating a surface pattern from SU-​8 involves a sequence of spin-​

coating at a few thousand rpm for ca. 1 min, followed by lower temperature soft baking at

65°C–​95°C prior to exposure of the nanoimprint-​masked SU-​8 to UV radiation, and then a

post bake prior to incubation with the photoresist developer, rinsing, drying, and sometimes

further hard baking at higher temperatures of ~180°C.

FIGURE 7.4  Microfabrication using photolithography. Schematic example of photolithography to engineer patterned

surfaces. (a) A wafer of a substrate, typically silicon based can, for example, be oxidized if required, prior to spin-​coating

in a photoresist, which acts as a sacrificial material during this “lift-​off” process. (b) Exposure to typically long UV light

(wavelength ~400 nm) results in either hardening (positive photoresist) or softening (negative photoresist), such that

the softened/​unhardened photoresist can be removed by the solvent. At this stage, there are several possible options in

the microfabrication process, either involving removal of material (such as the chemical etching indicated) or addition of

material (such as due to vapor deposition, such as here with a surface layer of gold). The final removal of the photoresist

using specific organic results in complexly patterned microfabricated surfaces.