Page 343 - Introduction to Information Optics

328 6. Interconnection with Optics

//. Reactive Ion Etching
RIE processes are combinations of both physical (sputtering) and chemical
etching actions. Basically, the physical component of the etching process
provides better anisotropy on the side walls, but results in inferior surface
quality; whereas, chemical etching tends to produce smooth etched surfaces but
creates curved side walls. The extent of each etching action can be adjusted
through optimizing the RIE conditions to obtain the best result; i.e., to have
both straight side walls and smooth surface quality. To make physical etching
dominant, the conditions can be selected with low pressure, high RF power,
and use of a less reactive ion. Similarly, chemical etching can be enhanced
through increasing the pressure, lowering the RF power, or using more reactive
ions. In our process, the optimized RIE conditions are chamber pressure 15
mTorr, RF power 150 W, and O 2 (etchant gas) flow rate 10-SCCM (cubic
centimeters per minute at standard temperature and pressure). A special holder
is used to hold the sample with a 45° slope angle with respect to the electrode
in the RIE chamber. The ion stream coming down vertically to the sample
bombards the polymer through the windows in the aluminum film at a 45
angle with respect to the substrate. The aluminum mask does not wear down
and protects the other parts against RIE. Two parallel 45° slanted side walls
are formed in the desired position and direction under each opening window
after RIE etching. A Faraday cage is used to cover the sample and the holder
during the etching. A modified Faraday cage having a 45° tilted grid surface,
which can change the direction of the ion stream by 45°, can be used to put
the sample horizontally on the electrode and cover the area to be etched with
the cage, with the tilt surface perpendicular to the direction along which the
micromirror is to be etched. With this method, a larger sample can be
processed and etched more uniformly.
III. Aluminum Removal
In the final step, the aluminum mask is removed through wet etching.

The micromirror couplers fabricated by the above procedures were exam-
ined using a-step and SEM. Usually with the conditions given above, 120
minutes of etching results in a 7-micron etching depth, which is sufficient to
terminate the waveguide layer with a 45° TIR micromirror coupler. The SEM
(scanning electronic microscope) is used to inspect the etching profile and the
surface quality. The sample to be inspected is cut into small pieces to meet the
size requirement of the SEM specimen holder. The sample is tilted to a proper
angle to view the cross section and obtain the information of the etched profile.
The quality of the 45° sloped surface can also be checked through SEM under
a proper viewing angle and magnification. The SEM micrograph of the cross
section of a 45° microcoupler is shown in Fig. 6.28 and a photograph showing
a waveguide mirror coupler acting as a input coupler is shown in Fig. 6.29.