Proof-of-Concept for Gas-Entrapping Membranes Derived from Water-Loving SiO₂/Si/SiO₂ Wafers for Green Desalination

Gas entrapping membranes, or GEMs, can robustly entrap air on immersion in wetting liquids. As a result, they achieve this function due their structure, which can be of use, for example, in desalination by membrane distillation. Photolithography allowed us to create complex overhanging architectures on both sides of a silicon wafer resulting in GEMS.

It provided a pathway to fabricate GEMS using conventional micro-fabrication techniques. Proper alignment marks should be placed on the photo mask to achieve vertically aligned posts. We suggest using multi-scale alignment marks with the smallest size of at least four times the pole diameter.

Fabrication of silica GEMS involves intricate design patterns and a multi-step process, and so this will demonstrate enough micro-fabricating steps will help in understanding the protocol. Begin this procedure with design of arrays and mask development as described in the text protocol. Immerse the silicon wafer in a freshly prepared piranha solution.

Maintain at a temperature of 388 Kelvin for 10 minutes. Rinse the wafer with deionized water for two cycles in a wet bench, then dry the wafer under a nitrogen environment in the spin dryer. Expose the wafer to the vapor of HMDS to improve adhesion of the photoresist with the silica surface.

Transfer the wafer onto a vacuum chuck of a spin coater to spin coat the photoresist. Use AZ 5214 photoresist as a negative tone to achieve a 1.6 micron thick film of the photoresist. Bake the photoresist-coated wafer at 105 degrees Celsius on a hot plate for two minutes.

This dries and hardens the photoresist film, which otherwise sticks to the glass mask and causes contamination issues during UV exposure. It also improves adhesion of the photoresist to the silica surface. Expose the wafer under UV exposure for 15 seconds through the chrome mask using a mask-alignment system to achieve the desired design on the photoresist.

Then, bake the realized wafer at 120 degrees Celsius on a hot plate for two minutes. During this step, the exposed negative photoresist film further cross links. As a result, the UV-exposed parts of the photoresist are no longer soluble in the developer solution, while the unexposed areas are soluble.

Further expose the wafer under UV light for 15 seconds in a UV cure system. During this step, photoresist areas that were not previously exposed are exposed, and can later be dissolved in the developer. Next, immerse the wafer in a 50 mL bath of the AZ 726 photoresist developer for 60 seconds to achieve the desired photoresist pattern on the silicon wafer.

Subsequently clean the wafer using deionized water, and further blow dry it with nitrogen gas. Sputter chromium on the wafer for 200 seconds to obtain a 50 nanometer thick chromium layer. The deposition is performed using a magnetron-type DC reactive sputter with a standard two-inch round target source in an argon environment.

Sonicate the sputtered wafer in an acetone bath for five minutes to lift off the remaining photoresist from the wafer, leaving behind the desired features with a chromium hard mask. After rinsing the back side of the wafer with a copious amount of acetone and ethanol, blow dry with a nitrogen gun. Then, repeat the spin coating, baking, and UV-exposure steps on the back side of the wafer.

For UV exposure, use the manual back alignment with crosshair module in the contact aligner to align the desired features on the back side with the front side of the wafer using the alignment marks in the mask. For the back side of the wafer, continue with the sputter and photoresist lift off steps to generate the required design with chromium hard mask on both sides of the wafer. The chromium-covered surface does not undergo etching.

Thus, spots in which chromium is absent on the wafer define the inlets and outlets of the pour. Undergo etching of the exposed silicon dioxide layer on both sides of the wafer by an inductively-coupled plasma reactive ion etcher that employs fluorine and oxygen chemistries. The duration is 16 minutes for each side.

Process the wafer with five cycles of anisotropic etching using the Bosch process to create a notch in the silicon layer. This process is characterized by a flat side wall profile using alternating depositions of octafluorocyclobutane and sulfur hexafluoride gases. By alternating anisotropic etching and polymer deposition, the silicon etches straight down.

This step is performed on each side of the wafer. Next, immerse the wafer in a bath of piranha solution maintained at a temperature of 388 Kelvin for 10 minutes. This removes the polymers deposited in the anisotropic step.

To create the undercut, which yields the reentrant profile, undergo isotropic etch using a sulfur hexafluoride-based recipe for a duration of 165 seconds. This step is performed on each side of the wafer. To perform anisotropic silicon etching, transfer the wafer to a deep inductively-coupled plasma-reactive ion etcher to etch 150 microns of silicon.

Perform 200 cycles of deep etching using the Bosch process. Repeat this step with the back side of the wafer. Now, undergo piranha cleaning of the wafer in the wet bench for 10 minutes to remove polymeric contaminants deposited from the etching process, which ensures uniform etching rates.

Repeat these etching and cleaning steps to realize through pores in the wafer having reentrant inlets and outlets. Remove the chromium from the wafer by immersing in a 100 mL bath of chrome etchant for 60 seconds. After the microfabrication process, clean the wafer with 100 mL of freshly prepared piranha solution in a glass container for 10 minutes.

Then, further blow dry with a 99%pure nitrogen gas pressure gun. Place the samples in a glass Petri dish inside a clean vacuum oven at 323 Kelvin until the intrinsic contact angle of water on smooth silicon dioxide is stabilized at an intrinsic contact angle theta equal to 40 degrees after 48 hours. Store the obtained dry samples, which are the silica GEMs, in a nitrogen cabinet.

Scanning electron micrographs of silica GEMs show a tilted cross-sectional view, a magnified cross-sectional view of a single pore, and magnified views of reentrant edges at the inlets and outlets of the pore. The pores of these GEMs were vertically aligned. Inlet and outlet diameter was 100 microns.

Center-to-center distance between the pores was 400 microns. Separation between the reentrant edges and pore wall was 18 microns, and length of the pores was 300 microns. Shown here are computer-enhanced 3D reconstructions of the air-water interface at the inlets of silica GEMs underwater.

Also shown are cross-sectional views along the white dotted lines. In the case of reentrant cavities, the condensation of water vapor inside the cavities displaced the entrapped air, which caused bulging of the air-water interface upwards and destabilized the system. In contrast, silica GEMs remained free from bulging for a much longer period, even though the rate of heating was similar.

These results were rationalized on the basis of preferential condensation of water vapor from the laser-heated reservoir on the cooled air-water interface of the other side. However, it was not possible to measure the rate of mass transfer in this experimental configuration. Prevent the removal of silica reentrant structures during Bosch process, it is used to add silicon.

It is very crucial to have chromium hard mask. These findings might unlock the potential of common materials for applications that currently require perfluorinated coatings, such as for drag reduction or for suppression and purification.