Rendering SiO₂/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars

Our gas-entrapping microtextured surfaces, or GEMS, they can then trap air on emulsion and liquids, regardless of their surface chemistry. That's why we think that this approach has a tremendous potential for applications that otherwise require perfluorinated coatings. Unlike 3D printing and other conventional manufacturing techniques, photolithography and dry etching allows us to fabricate complex, micro-scale, overhanging re-entrant and doubly re-entrant topographies.

First-time users should use practice wafers and periodically check the etching rates for each type of design before attempting an experiment, as the rate might change with sample size. Fabrication of re-entrant and doubly re-entrant pillars and cavities is a multi-stop process that involves intricate design patterns. Visual demonstration of the microfabrication stuff will help with understanding the protocol.

Start the microfabrication process by creating a new file in an appropriate layout software program. Draw a unit cell comprising of 200 micrometer diagram circle. Copy paste this circle with a center to center distance of 212 micrometers to create an array of circles in a square patch, with a one centimeter squared area.

Next, draw a 100 millimeter diameter circle and place the one centimeter squared square array inside the circle. Replicate this arrangement to create a 4 x 4 grid of square arrays. The features inside the circle will be transferred on to the four inch wafers.

Then export the design file to the desired format for the mass grading system. To clean wafers for the microfabrication, place a four-inch diameter silicon wafer with a 2.4 micrometer thick thermal oxide layer in Piranha solution for ten minutes, before rinsing with deionized water. Then spin the wafer dry under a nitrogen environment.

After drying, use vapor-phase deposition to coat the wafer with hexamethyldisilazane and mount the wafer on to a four-inch vacuum check in a spin coder. Cover the wafer with photoresist and use the spin coder to spread the photoresist uniformly across the surface of the wafer as a 1.6 micrometer thick layer. Bake the photoresist coated on a 110 degree celsius hot plate for two minutes.

Transfer the baked wafer to a direct rating system. Expose the wafer to ultraviolet radiation for 55 milliseconds and transfer the UV exposed wafer into a glass Petri dish containing photoresist developer, to allow the features to develop. After 60 seconds, gently rinse the wafer with deionized water to remove any excess developer, and spin dry the wafer in a nitrogen environment.

After photolithography, transfer the wafer to an inductively coupled plasma reactive ion etching system, that employs a mixture of octafluorocyclobutane and oxygen gases. Run the process for approximately 13 minutes to etch the exposed silica layer. To ensure that the silica layer thickness inside the desired patterns is reduced to zero, use a reflectometer to measure the thickness of the remaining silica and adjust the duration of the subsequent etching period based on the thickness of the silica layers.

After etching the silica layer, transfer the wafer to a deep inductively coupled plasma reactive ion etching system and run this process for five cycles, resulting in an etching depth for silicon equivalent to approximately two micrometers. Clean the wafer with Piranha solution, then rinse and spin dry as demonstrated before. Perform isotropic etching to create an undercut beneath the silica layer with sulfur hexafluoride for 25 seconds, followed by cleaning with Piranha solution rinse and spin dry as demonstrated.

After creating the undercut, use a high-temperature furnace system to grow a 500-nanometer layer of thermal oxide on the wafer. Next, pitch silica vertically downward for three minutes to remove the thermal oxide layer from the cavity bottom, while leaving a silica layer along the side walls that will eventually form the doubly reentrant edge. After etching the excess thermally-grown oxide, repeat five cycles of the Bosch process to deepen the cavities by two micrometers, then clean the wafer with piranha solution, rinse and spin dry as demonstrated.

To create an empty space behind the thermally grown oxide at the mouth of the cavity, isotropically etch the silicon for 150 seconds to obtain the doubly reentrant edge. The amount of time spent in the last isotropic silicon etch must be tuned to create as much space as possible behind the thermally grown oxide without merging the cavities. After creating the doubly reentrant cavities, perform the Bosch process for 160 cycles to increase the depth of the cavities to approximately 50 micrometers.

Clean the wafer in fresh Piranha solution, rinse and spin dry as demonstrated. Transfer the wafer into a vacuum oven at 50 degrees Celsius for 48 hours. The wafer can then be stored in a clean nitrogen flow cabinet.

Here, representative reentrant and doubly reentrant cavities and pillars microfabricated as demonstrated are shown. Silicon dioxide silicon surfaces with arrays of doubly reentrant pillars exhibit apparent contact angles greater than 150 degrees for both water and hexadecane with a minimal contact angle hysteresis. Curiously, when the same silicon dioxide silicon surfaces with arrays of pillars are immersed in the same liquids, they are intruded instantaneously.

In contrast, doubly reentrant cavities entrap air upon immersion in both liquids. Further, confocal microscopy reveals that the overhanging features stabilize the intruding liquids and entrap air inside them. The microfabrication of arrays of pillars surrounded by walls of doubly reentrant profile insulates the stems from wetting liquids, resulting in hybrid microtextures that behave as gas entrapping micro textures.

Using a similar approach, membranes can be designed that would be capable of performing functions of commercial membranes but without using harmful perfluorocarbons, paving the pathway for greener industrial processes. We could investigate the performance of mushroom shaped cavities and pillars in terms of their ability to entrap air under liquids, and also in terms of breakthrough pressures and so on. This protocol involves using a clean room facility, as well as hot plates, flammable and corrosive chemicals.

Therefore safety training and personal protective equipment are required.