Page 296 - Organic Electronics in Sensors and Biotechnology

Organic Semiconductor Lasers as Integrated Light Sources for Optical Sensors 273

of a chromium layer and a liftoff process, the pattern is transferred into
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the substrate by dry etching. Process B illustrates the possibility of
skipping the process steps of chromium layer deposition and liftoff when
the patterned resist is used as etch mask. In this way a dry etching pro-
cess is applied to directly pattern a silicon wafer substrate, using the
patterned resist as etch mask. A silicon wafer with structured PMMA
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resist is directly used as master in process chain (C). As alternative to
the high-resolution positive resist PMMA, the negative resist hydrogen
silsesquioxane (HSQ) may be used; see process D. HSQ is spin-coated
on a substrate, followed by a prebake step. The resist is then exposed
with the electron beam, resulting in directly written inorganic struc-
tures with silica-like properties. The unexposed areas may be dissolved
in the subsequent development step. The resolution of structures writ-
ten by EBL is determined by the spot size of the electron beam and
secondary effects, affecting the actual size of the nominally exposed
and unexposed areas. The influence of this effect may be reduced by
applying a higher acceleration voltage to the electrons (i.e., higher
energy) and by reducing the resist thickness. PMMA resist may be pat-
terned with minimum lateral structure sizes in the range of 10 nm.
HSQ may also be patterned with minimum lateral structure sizes down
to approximately 10 nm, depending on resist thickness and accelera-
tion voltage.

7.3.2 Master Fabrication: Direct Laser Writing
Conventional direct laser writing is used as a cost saving alternative
to electron beam lithography. Commercially available systems are
2
able to pattern substrates as large as 400 × 400 mm with feature sizes
down to 0.6 μm. In the direct laser writing process a photoresist is
patterned by locally exposing it with a focused laser beam. Common
systems often use a HeCd gas laser source with an emission wave-
length of 442 nm.
An extension to these systems can be made by using a femtosec-
ond laser as light source. 43–46 Titanium-sapphire-based femtosecond
lasers emit in the near infrared (NIR) spectrum at around 800 nm.
Most photoresists are only sensitive in the UV spectral range and can-
not be cured through NIR radiation directly. However, a reaction of
the resist to the NIR radiation is possible with a two-photon absorp-
tion (TPA) process. This means that the simultaneous absorption of
two low-energy photons causes a reaction in the material which nor-
mally can only happen with twice as much of the excitation energy in
a single photon process. This non-linear phenomenon requires very
high irradiation densities. To prevent thermal destruction of the resist,
very short, high-intensity laser pulses are required.
A special feature of the TPA process is the distinct threshold char-
acteristic for the exposure of photoresists. TPA requires a critical radi-
ation dosage at which the process can start. This threshold behavior