As and Epitaxial-Growth MnSi Thin Films - OPUS Würzburg

120 A. Fabrication of Four-Terminal Corbino Gated Structure
Fig. A.2: Bridge structures used for the Corbino sturctures. Because of the length of bridges
used, bridges with supporting posts (a) made of cross-linked PMMA (insulating) was used.
However for some structures, bridges (Figure A.2.b.) are stable enough without needing the
extra posts.
A.1 Fabrication Details
The Corbino structure and four-terminal configuration was used for canceling spurious
effects coming from contact resistance. Because of the latter, the conductance fluctuations
can be attributed to scattering and localization processes within the material. The
disadvantage of this structure is that the Hall effect is shorted and cannot be measured
in this geometry. The design of the main four-terminal device is shown in Figure A.1
The device is composed of four rings, where the current is applied in rings R1 and R4
withthevoltagedropmeasured alongringsR2andR3, followingthenormal four-terminal
measurement configuration. These rings are 1µm in width, which necessitate the need of
a suspended metallic connection from the rings to bigger bond pads while avoiding the
conducting substrate. The bridge step refers to the technique by [Borz 05]. The crosslinked
PMMA step was included to ensure that the structures (long bridges) do not short
to the conducting substrate, but has been proven unnecessary because of the stability of
the bridge structures.
The gradient of dose factors are to correct for the proximity effect. Lower dose causes
left-over resist to form clumps under the contacts and increase contact resistances. (Figure
A.1) Another effect of incorrect dose calibration is difficulties in lifting off the metal
in between rings without using an ultra-sonic bath. Because of the closed smooth (no
corners) structure, lifting off metal from this structure is very challenging. A good calibration
of the e-beam dosage would result in a resist profile where the metal can easily