The reflowed patterns were transferred with high fidelity into the Si device layer using a low DC-bias (∼ 30 V), inductively coupled plasma reactive-ion etch (ICP-RIE) on an Oxford Plasmalab 100 system with load-lock [33]. The angled mask was prone to erosion during the etch process, and so the ICP-RIE was optimized to minimize roughness caused by mask erosion. The ratio of the flow rates, 12.0 sccm C4F8 to
12.0 sccm SF6, and the 1000 watts ICP and 6 watts RF power were optimized to
etch through the thin SiNx cap and top Si layer in a single 2.5 minute etch, shown in
Fig. 3.5(a). During the 2–3 minute etch, the chamber pressure was held at 15 mTorr with 15 Torr of helium backing on the 20◦C stage. The SF6 etchant provides nearly
isotropic etching of silicon. Conversely, C4F8 results in heavy polymerization on the
etched sidewalls leading to a very anisotropic etch [53, 54]. Etching with too little C4F8, resulted in very jagged but vertical sidewalls because the etch was too chemical
angled sidewalls with occasional larger chunks of silicon missing from the sidewall. This effect was hypothesized to be due to the slower etch rate using large flows of C4F8, which allowed uneven build-up of polymers on the silicon sidewalls. Note that
optimized etches of just SiNx did not possess this problem at high C4F8 flow rates,
allowing the etch to be continually smoothed by increasing C4F8 at the expense of
sidewall verticality. SiNx cap was desirable over SiO2 because of the similarity in
individually optimized Si and SiNx etch recipes, allowing the plasma to be lit only
once. The single well-calibrated dry-etch resulted in improved repeatability, simplified etch optimization, and shorter fabrication times. While this etch produced excellent device performance, the etched sidewalls were less than perfectly vertical at an angle of 75◦. In addition, the optimized etch was found to give similarly excellent results with or without the SiNx cap, allowing for increased design freedom of the process
flow. The SEM micrographs in Figure 3.6 show a device after the optimized dry-etch but with the reflowed resist in place. The noticeable overetch of the silicon into the BOX layer was done to ensure a uniform sidewall.
After the device layer dry-etch, each sample was soaked in tetrachloroethylene (TCE) for 20 minutes to remove the electron-beam resist shown in Fig. 3.6.2 The sample was removed and quickly rinsed in acetone, isopropyl alcohol (IPA), methanol, and DI H2O. After a quick N2 dry, the remaining plasma-hardened organics on the
surface were removed with a high-temperature Piranha clean as described below. If a straight taper probe was to be used to test the optical resonators, an etch-mask surrounding the disks was photolithographically defined and the wafer surrounding the disks etched down several microns, leaving the devices “isolated” on a mesa. The preferred method of isolation was a multi-step wet and dry etching process. First, AZ 5214 photoresist was spun on the sample at 2000 rpm for 60 seconds. Then the edge bead was removed from the sample with a clean wipe soaked in acetone. The sample was then oven-baked for 12 minutes at 100◦C. An additional edge bead removal was accomplished with a UV exposure of 18 seconds while the central device region of the chip remained protected from UV. The outer, and now exposed, resist was developed away in AZ-300 MIF in approximately 60 seconds. At this point, a
2It was empirically determined that TCE is a superior etchant on electron-beam resist while acetone is superior at
(a)
(b)
2 μm
300 nm
resist
BOX
Si disk
Figure 3.6: (a) SEM micrograph of a 5μm radii microdisk after mask reflow and low bias voltage ICP/RIE etching with ZEP mask still in place. (b) Close-up view of the same device showing the smooth sidewalls.
carefully aligned mesa stripe was aligned to the set of devices, ensuring that the edges of each device’s fiber via would be left unprotected by the photoresist. After a second exposure and develop step, the sample was hard baked in an oven for 5 minutes at 180◦C. The temperature of the second bake was enough to cause the resist to reflow, providing intimate contact with the sample, limiting undercutting during subsequent isotropic etching steps. The top ∼ 10μm of the sample was then etched away with dry and wet etches while the devices were protected by the photoresist. First, the aforementioned device layer etch was used to remove the top Si layer from the wafer. Then a 7 minute SiO2 etch removed approximately 1 μm of the 2−3μm BOX layer.
The etch parameters used were 15 sccm of C4F8, 4 sccm of O2, 6 mTorr chamber
pressure, 20 Torr helium backing, 10◦C table temperature, 500 W of ICP power, 80 W of RF power, yielding a DC bias of 190−170 V. The remainder of the BOX was removed with a stirred 5:1 H2O:HF solution for approximately 20 minutes. Finally,
the silicon substrate was etched down several microns with a pure SF6 dry-etch for
10 minutes. The “Si Mesa” etch parameters used were 20 sccm of SF6, 15 mTorr
chamber pressure, 10 Torr helium backing, 15◦C table temperature, 1200 W of ICP power, 10 W of RF power, yielding a DC bias of 37 V. Finishing the BOX etch with a wet-etch prevented silicon pillar formation during the Si Mesa etch because the BOX silica was uniformly removed. Figure 3.7 shows an SEM micrograph of a 4.5 μm radii microdisk after mesa isolation etching and HF undercut (described below). The figure is looking parallel to the direction of fiber placement. The mesa isolation lithography runs left and right in the figure.
After the completion of any lithography plus dry-etch step, a Piranha clean was needed in order to remove the hard baked resist [53]. Inside a 50 mL beaker, 30 mL of H2SO4 was heated to 65◦C on a 160◦C hotplate with magnetic stirbar at 350
rpm. At this point, 7 mL of 30% H2O2 was added to the solution. The exothermic
reaction that resulted self-heated the solution up to the desired operating temperature of 100−110◦C. Once the Piranha solution reached 100◦C, the sample was suspended in the center of the swirling acid with solid fluorinate plastic (teflon-like) tweezers. Suspending the sample was essential because the evolved CO2 gas during the clean
10 μm
Fiber Via
Disk
Isolation Line
Figure 3.7: SEM micrograph of a 4.5 μm radii microdisk after mesa isolation etching and HF undercut.
cleaned for 30−60 minutes in the Piranha during which additional H2O2 was added
to maintain the temperature. The sample was finally rinsed in three separate beakers of DI H2O and subjected to N2.
Following the Piranha etch to remove organic materials, a dilute hydrofluoric acid solution (typically 5:1 H2O:HF) was used to remove the protective SiO2 or SiNx layer
and partially undercut the disk, as seen in Fig. 3.1. The process of undercutting is crucial in order to allow strong fiber coupling with high ideality [55]. The etch time was typically 45−65 minutes depending on the original BOX thickness and disk radius. The sample would be removed from the etchant several times during the etch to chart the undercut’s progress. The undercut pedestal takes on its angular hourglass shape due to a higher etch rate on the wafer UnibondR [8] versus the bulk silica. For
disks of radii greater than 5 μm, the BOX could be fully etched away beneath the disk while retaining a reasonably thick pedestal. After the HF etch, the wafer was rinsed in deionized water and again dried with clean, dry N2. Upon completion of
the processing, the wafers were immediately removed to an N2 purged enclosure for
storage and/or characterization.