Integrated Optics - Applied Optics Fundamentals and Device Applications - page 248

5.9.2 Photolithography and Waveguide Fabrication
The first LiNbO 3 waveguides reported were produced via the outdiffusion
of lithium oxide from the crystal lattice from photolithographically defined
regions. Although this method was effective at changing the refractive index
of selected regions to define the waveguide it tended to produce guides with
high losses, and it was only possible to produce multimode waveguides [44].
The high losses have been attributed to lattice imperfections resulting from
the outdiffusion [45]. The first devices formed via titanium indiffusion were
reported in 1974 [46] and represented a substantial improvement over the
existing technology. Devices based on the Ti diffusion process also provide
greater possible changes in the index of refraction and less in-plane scatter-
ing [47] compared to the lithium oxide out diffused devices.
The fabrication of waveguides in LiNbO 3 materials via Ti diffusion is
a relatively straightforward process and employs the sequence shown in
Figure 5.20 [48]. After surface polishing, areas are defined on the LiNbO 3
surface where waveguides are desired, typically by a photolithographic
process. Next Ti is deposited over the entire substrate; then, the resist is
removed, lifting off the deposited Ti from all but the defined waveguide
areas. The Ti is then diffused into the crystal at high temperatures under
controlled conditions. Precise descriptions of these individual steps, their
significance with regard to device performance, and ongoing research in
the related photolithography and diffusion physics will be discussed in the
following sections.
The first step in the processing of LiNbO 3 for waveguides is surface prepa-
ration. The crystal surface must be very flat and low in camber in order to
enable the subsequent fabrication of devices with good yields. One of the
advantages of LiNbO 3 over the other electro-optic materials arises from the
fact that it is a strong nonhydroscopic crystal which is easily polished [49].
Another advantage is its availability. Currently high quality substrates of
LiNbO 3 up to 8 cm long are commercially available [50] so there are cost
advantages tied to the use of this material. There are several other ferro-
electric materials which offer similar or even slightly better properties [51]
but when cost and availability are taken into account LiNbO 3 is the preferred
choice [52].
After polishing, the LiNbO 3 surface is ready for liquid photoresist applica-
tion. These photoresists are polymeric solutions which are sensitive to cer-
tain types of (typically UV) radiation. The polished surface can be coated
with resist via spray or dip application, but spin coating is the most common
method. In spin coating the substrate is turned very rapidly, up to 5000 rpm,
on a turntable as a precisely measured amount of photoresist is poured onto
the center of the spinning substrate. This results in a thin, very uniform
thickness coating across the surface of the substrate. These resist solutions
are only 20%-33% solid so there is a drying step required to drive off all the
carrier solvents.