Optical lithography

Optical lithography is used extensively in this thesis. All conducting lines, contact pads and TMR test structures are made with optical lithography. In principle, the lithography always consists of the following steps: a) spin-coating the resist on top of the sample, b) annealing the resist, c) exposing desired parts to light of a specific wavelength and d) developing the resist. These steps transfer a given structure to the resist on top of the sample. Depending on the resist type, the resist is preserved in the exposed area (negative resist) or in the non-exposed area (positive resist). The resist protects some parts of the sample surface, and on the other part thin films can be deposited or can be etched with Ar$^+$-ions. Afterwards the resist is removed by suitable solvents.

The positive photoresist AR-P 5350 from ALLRESIST GmbH is used for all optical lithography. It is spin-coated at 4000rpm (or 6000rpm for mask lithography) for 30secs and annealed for 30min at 92. Then, the resist is exposed with a laser lithography or a UV-mask lithography system. Afterwards, it is developed for 45sec with the ALLRESIST developer AR 300-35 (mixed 2:1 with H$_2$O). Because of the special undercut profile of this resist, the real structures are about 1$\mu$m wider than specified in the design file. After all necessary steps, the resist is removed with the ALLRESIST remover AR 300-70 in an ultrasonic bath for 15mins.

The used laser lithography system is a DWL 66 from HEIDELBERG INSTRUMENTS GmbH with a laser from MELLES GRIOT and a 4mm write head. The laser has an output of 90mW at 442nm. A built in camera easily allows the alignment to already existing structures. The minimum possible size of the structures with the used photoresist is 1$\mu$m. This laser lithography system is also used to fabricate UV-masks that can be used with the UV-mask lithography process. For such masks, the design is inverted and structured on a glass substrate. To block the UV-light, a thick tantalum layer is sputtered in the created holes. After lift-off, the mask is complete and can be used for very quick parallel optical lithography.

Figure 2.3: Design of the TMR standard mask.

0.5

For one-step lithography with
structure sizes larger than 5$\mu$m, a UV-mask lithography system from THERMO ORIEL is used. The homogeneous UV light source with
300W exposes the sample through the mask that lies on top of the sample. The exposure time is between 3 and 5secs, depending on the age of the UV-lamp. Figure 2.3 shows the design of the used TMR standard mask with squares that define areas of 90000$\mu$m$^2$, 40000$\mu$m$^2$, 10000$\mu$m$^2$, 506.25$\mu$m$^2$ and 56,25$\mu$m$^2$.