Sample preparation
for ligand-receptor bonds

For all following bond-force measurements, the samples are initially prepared as shown in figure 3.1(a-e). Figure 4.5 presents the design that is used for the optical lithography in step (a). There are two thin ($\approx$ 2$\mu$m wide) and two wide lines ($\approx$ 12$\mu$m) which are always connected by a contact pad at both ends. The outer part of the design, where the conducting lines broaden, was done to avoid problems during the laser lithography. Very thin and long lines were often disconnected or joined with neighbouring lines. This design, with very few corners and broadened lines minimised such problems.

Figure 4.5: Complete design used for all bond-force measurements. The measurement area is magnified.

First, the markers will be collected on the thin lines, before a high current through the wide line tries to rupture the examined bond between marker and surface. The SiO$_2$ protection layer in step (e) is imperative for the subsequent biological surface preparation (see section 4.3). Typically, the design of figure 4.5 is structured three times in a row on one piece of Si-wafer. This wafer piece is then glued with conducting silver paste to an IC-socket, and the structure is connected to the socket by wire bonding.