Integrated Optics - Applied Optics Fundamentals and Device Applications - page 240

a relatively large dielectric constant and be able to be deposited in thin
(1000 Å) pin-hole free layers to facilitate its use as an insulator in capacitors
in electronic or microwave circuits. The insulators should have an index of
refraction less than that of GaAs to prevent stray optical coupling (all of the
commonly-used oxides and nitrides satisfy this requirement). In addition, the
insulating material should be available in high purity form, and the chosen
deposition technique should not introduce significant amounts of any atoms
that can act as dopants in GaAs. For example ion-beam sputter-deposited
SiO 2 layers deposited by thermally excited vacuum vapor deposition would
not be a problem. More than one insulating material might be used on the
same substrate wafer to take advantage of differences in index of refraction,
microwave dielectric constant, or differing processing characteristics such
as chemical etch rate. Nonetheless, the thermal expansion coefficients of all
layers should be adequately matched to provide good adherence.
5.6.6 Photolithography
It is possible to fabricate common optical, electronic and microwave devices
by standard photolithographic techniques. The resists and exposure meth-
ods used for microwave integrated circuit fabrication are essentially the
same as those used to produce optical integrated circuits. The one element
which is an exception is the optical grating used for coupling or feedback.
Spacing of the grating bars can be as small as 0.1 μm, which is within the
resolution of commonly used photoresists, but is beyond the capabilities of
conventional mask alignment machines. If gratings are used in any of the
optical devices it will be necessary to expose the photoresistors for the grat-
ing by interference (sometimes called “holographic”) techniques [31]. Most
other optical, microwave, and electronic devices have minimum dimensions
greater than 1.0 μm, which can be produced by commercially available mask
alignment machines.
5.7 WaveguideBuildingBlockProcessingConsiderations
5.7.1 Introduction
As discussed in a previous section, the buried strip-loaded waveguide
has some inherent advantages over the simple raised rib design. First, the
vertical confinement is such that the mode energy is generally contained
in the waveguide layer and thus only sees the minimal roughness of the
interfacial layer as opposed to the etched sidewalls. Second, a structural
design analysis shows that lateral confinement is achieved with a very
reasonable strip width. Third, from an electrical point of view, the p-n
junction is buried and therefore will exhibit forward electrical breakdown