Page 300 - Organic Electronics in Sensors and Biotechnology

Organic Semiconductor Lasers as Integrated Light Sources for Optical Sensors 277

molding tools are nickel alloys, such as nickel-cobalt and nickel-iron.
The tool, a so-called shim, 56, 57 is plated to a thickness of about
200 to 300 μm. As this process does not wear the master tool, basi-
cally an unlimited number of replicas can be fabricated. This process
therefore enables the mass production of nano- and microcomponents
at low cost.
A possible approach toward decreased process times by eliminat-
ing the heat-up and cooldown phase of the hot embossing is the UV
nanoimprint lithography. Here, a transparent imprinting tool is
pressed into a UV-curable liquid polymer or solgel material. The
material is then cured by exposing it to UV radiation. 58, 59
Another replication method used for laser fabrication is the
microcontact printing (μCP) process. 60, 61

7.3.6 Replication: Cast Molding and Photolithography
There is a major difference between imprinting and cast molding.
Instead of applying pressure for the imprinting process, the master is
cast with the material in the latter approach. The transition from
imprinting to casting can be seamless. For example, the step-and-flash
imprint lithography (SFIL) describes the same process steps as the UV
nanoimprint lithography. The sole difference is the pressure applied to
the substrate for the UV-NIL process. Depending on the material the
master is made of, casting by itself can be separated into hard and soft
casting. Therefore the mentioned SFIL is an instance of hard casting
whereas an example of soft casting would be the use of the flexible
PDMS [poly-(dimethylsiloxane)] as the master’s material. 62
Besides the imprinting approaches described above, conventional
63
photolithography can be applied to fabricate laser resonators. The
required feature size can be structured without difficulties using
193 nm immersion lithography. However, these lithography systems
are very complex and expensive compared to imprinting methods. 64, 65
Therefore a high packing density is required for economic reasons
when such equipment is used. As this is given for high-price prod-
ucts such as microprocessors, it is not the case for lab-on-chip devices
where most of the wafer space is filled with large-scale structures
such as fluidic channels and optical waveguides.

7.3.7 Active Layer Deposition
For the manufacturing of an integrated organic laser it is necessary to
selectively deposit the active layer. This typically means that layers
with thicknesses between 130 and 400 nm and areal dimensions
2
down to 500 × 500 μm have to be fabricated. The two main deposi-
tion techniques for organic lasers are spin coating and thermal vapor
deposition. Often only one deposition technique can be used for an
organic material.