Single Crystalline Organic Thin Films by Stencil Lithography: Growth and Characterization

Organic semiconductors are important materials in the field of flexible electronics
because their low processing temperatures allow deposition of organic thin films
on plastic foils. Nowadays, the emerging applications in flexible electronics
require high charge carrier mobilities of the organic semiconductors. Among
several routes to achieve this goal, the elimination of the grain boundaries inthe organic thin films has consistently proven itself. Fabrication of the single
crystalline organic thin films, however, presents challenges that often require
processing compromises such as higher substrate temperatures during vacuum
evaporation, solvent contamination during solution growth, etc.
In this thesis we present a novel fabrication technique, stencil lithography, that
can be used to grow single crystalline organic thin films at low substrate
temperatures and with high purity. The stencil lithography is based on
the industrial vacuum thermal evaporation method in combination with
microfabricated shadow masks, called stencils. Using the stencil masks we
are able to reduce the aperture size down to the sub-μm range and limit the
nucleation area until only a single grain is obtained per aperture. We also provide
the models that help designing the aperture size for the single crystalline thin
films and extracting important physical parameters such as the surface diffusion
length of the adsorbed molecules.
In addition to our growth method, we show how the microscopy technique,
differential interference contrast (DIC), can be used to characterize the inplane
crystal orientation with sub-μm lateral resolution and enough sensitivity
even for the thin films with one monolayer. Moreover, our characterization
method provides live image acquisition and can be used with any modern optical
microscope.