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5.3 Waveguide processing 67
InP
Q(1.25)
InP
First growth
Second growth
InP
Q(1.25)
InP
First growth
Second growth
Figure 5.3: SEM pictures of passive waveguides that have been butt-joint to a passive
waveguide. a) (110) direction b) (-110) direction
5.3 Waveguide processing
This section describes the fabrication of the ridge waveguides in the active-passive layer stack
that was described in Sec. 5.2. A short outline of the processing is as follows: First, the
waveguides are defined in SiN. The RIE step that follows is a three step process: etching of
high contrast waveguides, etching of the InGaAs contact layer from the passive sections. and
etching all waveguides to the target depth. Then the wafer is passivated with SiN and contact
openings are defined. Finally, metal contacts are deposited. These steps are explained in more
detail in the list below. The labels in the list correspond with those in Fig. 5.4.
A) In the first step a 100 nm thick SiN waveguide mask is deposited in a PE-CVD reactor.
B) A layer of positive resist is spun on top of the SiN mask and the waveguide pattern is
defined in the resist using contact lithography. Then the waveguide pattern is transfered
from the resist to the SiN mask with a RIE using CHF .
C) In the first waveguide etch, only the deeply-etched ridge waveguides are etched. The
areas that contain deeply-etched waveguides are defined as openings in positive resist
using lithography. It is the resist that is used as the masking layer in the RIE step that
follows and, therefore, the resist is made more resistant to the RIE plasma by post baking
it. The etch plasma consists of a mixture of CH and H ¦ , and between etch periods, an
O ¦ descumming step removes polymer that is deposited on the mask [92]. In order to
improve the uniformity of the waveguide etch, the etching of InGaAs, InGaAsP and InP
material is done on a 2” InP substrate wafer.
The reasons to start with the deeply-etched guides can be explained using Fig. 5.5, which
shows the waveguides after all the RIE steps. The deeply-etched guides require an additional
etch in the Q(1.25) film of depth
compared to the shallowly-etched guides.
This requires an additional etching time
is the etch rate of the film
, , refractive index=2.0
, where