Page 312 - Organic Electronics in Sensors and Biotechnology

Organic Semiconductor Lasers as Integrated Light Sources for Optical Sensors 289

DUV irradiation

Chromium mask
Core

Cladding
Waveguides
FIGURE 7.22 Fabrication of planar waveguide structures in PMMA using DUV
irradiation. (Reprinted with permission from Ref. 96. Copyright 2007 IEEE.)

spin-coated PMMA layers on a carrier with a lower refractive index or
single-material PMMA foils with several hundred micron thickness can
be used. Figure 7.22 illustrates the process.
This technique has several advantages because only a single poly-
mer layer is used, which serves as the substrate and waveguide as
well. No further etching or development steps are required. Wave-
guides can be fabricated using a commercial mask aligner (EVG 620
from EV Group) at a dosage between 3 and 5 J/cm² at 240 nm, lead-
ing to an increase in the refractive index between 0.008 and 0.015. A
waveguide loss between 0.7 and 0.8 dB/cm is obtained at a wave-
length of 1550 nm, which is mainly attributed to material losses. For
visible light of 635 nm wavelength, losses as low as 0.1 dB/cm were
observed.
It has also been demonstrated that it is possible to fabricate pas-
sive optical components such as planar waveguides, splitters, and
couplers using this approach. 94
Certain structures and devices such as sharp bends or resonators
cannot, however, be designed as planar structure, as they require
strong guiding rib structures. To fabricate this type of structure, the
combination of replication by hot embossing and refractive index
modification is required. Figure 7.23 summarizes the processing steps
for the replication of the molded part through (a) hot embossing and

Force
DUV

Heat

(a) (b) (c)
FIGURE 7.23 Process steps for hot embossing of ridge waveguide structures
and deep UV fl ood exposure.