Page 301 - Organic Electronics in Sensors and Biotechnology

278 Chapter Seven

Obligatory for every thin-film deposition is a clean substrate. The
possible cleaning techniques are often limited by the substrate’s resis-
tance to chemical treatment. If there are no limitations, a cleaning cycle
with acetone, isopropanol, and deionized water rinsing will remove
most residues. Treating the substrate subsequently with a plasma
cleaning step (e.g., argon/oxygen plasma) removes remaining organic
compounds on the surface and additionally enhances the wettability.
For the spin-coating process, the material has to be dissolved in a
suitable solvent and applied to the substrate. The substrate is then
rotated at up to 4000 rpm, causing the superfluous liquid to be spun
out. After evaporation of the solvent, a thin solid film remains.
Although being of impressive simplicity, this technique is problem-
atic when patterning of the layer is required. Due to the good solubility
of organic materials, traditional concepts used for inorganic materials
such as masking and etching are not applicable.
In contrast to spin coating, the vapor deposition of the active
material is ideal for producing patterned laser areas. To vapor-deposit
organic materials, a controlled thermal evaporation source inside a
vacuum chamber is needed. Typically the source consists of a cruci-
ble, a heating wire, and a thermocouple element. The crucible holds the
organic material and is wrapped with the heating wire. The thermo-
couple element measures the temperature of the crucible according to
which the current through the heating wire is regulated. Usually the
deposition rate is determined by an oscillating crystal detector placed
above the evaporation source. The best position for the detector is
close to the substrate. However, this becomes problematic when two
materials have to be evaporated simultaneously, e.g., for guest-host
systems such as Alq :DCM. In this case it has to be ensured that the
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deposition rate of the dopant can be measured without being influ-
enced by the host source.
For calibration and a check of the evaporation conditions, a sur-
face roughness meter can be used to determine the layer thickness
after the deposition. For measuring the dopant ratio of the film, the
amplified spontaneous emission (ASE) wavelength gives feedback in
the case of optical gain materials. This wavelength is independent of
the film thickness given that the film forms a waveguide. Addition-
ally, absorption and transmission spectra can be used.
For coevaporation the control should be automated to ensure repro-
ducible evaporation conditions and deposition results. For an Alq :DCM
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laser with a typical doping concentration of around 3% DCM, the depo-
sition rates for the two materials are on the order of 0.3 and 0.01 nm/s,
respectively. The ratio has to be held constant for more than 15 min,
assuming a desired layer thickness of about 300 nm.
To deposit only defined areas of the substrate, a mask with cut-
outs is fixed close to the substrate. This enables the coating of specific
single-laser fields. The change of masks also allows the deposition of
different materials on different areas. This is of particular interest for