Name: rendering sio2/si surfaces omniphobic by carving gas-entrapping microtextures comprising reentrant and doubly reentrant cavities or pillars
Uploaded: 2020-02-11T12:00:00
Duration: 8 min 2 s
Description: Watch this Scientific Journal Video about Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars at JoVE.com

HM acknowledges funding from King Abdullah University of Science and Technology (KAUST).

NOTE: Arrays of reentrant and doubly reentrant cavities and pillars were microfabricated by adapting the multistep protocol for pillars reported by Liu and Kim27. Precautions were taken to minimize the formation of pin residues or particles on our surfaces that could interfere with wetting transitions36.
MICROFABRICATION OF CAVITIES 
Broadly, the protocols for the microfabrication of reentrant and doubly reentrant cavities (RCs and DRCs) consist of two-dimensional layout designing, photolithography, general silica etching, and specific silicon etching, depending on the final feature required37,38,39,40,41.
1. Design
<ol>
	<li>Start the microfabrication process by designing the required pattern in a layout software42. An example of such a software is listed in the Materials List.</li>
	<li>Using the software, create a new file. Draw a unit cell comprising a circle of diameter, D = 200 &#181;m. Copy and paste this circle with a center-to-center distance (pitch) of L = 212 &#181;m to create an array of circles in a square patch of area 1 cm2 (Figure 2).</li>
	<li>Draw a circle of diameter 100 mm (4 inches). Place the 1 cm2 square array inside the circle and replicate it to create a 4 x 4 grid of square arrays. Features inside the circle will be transferred onto the 4-inch wafers (Figure 2).</li>
	<li>Export the design file to the desired format for the mask writing system (e.g., the GDSII format).</li>
</ol>
2. Cleaning of Wafers
<ol>
	<li>Clean a silicon wafer 4 inches in diameter, &#60;100&#62; orientation, and with a 2.4 &#181;m thick thermal oxide layer (see the Materials List), in piranha solution for 10 min. Piranha solution comprises sulfuric acid (H2SO4, 96%): hydrogen peroxide (H2O2, 30%) in a 3:1 volumetric ratio and is maintained at T = 388 K.</li>
	<li>Rinse the wafer with deionized water and spin-dry under nitrogen (N2) environment.</li>
</ol>
3. Photolithography
<ol>
	<li>Coat the wafer with hexamethyldisilazane (HMDS) using vapor-phase deposition to improve adhesion with the photoresist. Refer to Table 1 for the process details.</li>
	<li>Mount the wafer on a 4-inch vacuum chuck in the spin coater. Cover the wafer with the AZ-5214E photoresist. Use the spin coater to spread the photoresist uniformly on the surface as a 1.6 &#181;m-thick layer. Refer to Table 2 for spin coating parameters.</li>
	<li>Bake the photoresist-coated wafer on a hot-plate maintained at 110 &#176;C for 120 s.</li>
	<li>Transfer the wafer to a direct-writing system and expose the wafer to UV radiation for 55 ms (defocus: +5). This step transfers the desired design on the AZ-5214E (used in the positive tone; see Materials List) (Figure 2).</li>
	<li>Place the UV-exposed wafer in a glass Petri dish containing the AZ-726 developer for 60 s for the features to develop. See Materials List for details.</li>
	<li>Remove the wafer from the developer solution and rinse with deionized (DI) water gently to remove excess developer. Spin dry the wafer in a N2 environment. These steps are presented in Figure 3A&#8211;C.</li>
</ol>
NOTE: At the end of this step, design patterns on the wafer can be seen under a standard optical microscope.
4. Anisotropic Etching of Silica (SiO2) Layer
NOTE: The goal of this step is to completely etch away the silica layer (2.4 &#181;m-thick) that was exposed during photolithography to expose the silicon layer underneath.
<ol>
	<li>After photolithography, transfer the wafer to an inductively coupled plasma (ICP) reactive-ion etching (RIE) system that employs a mixture of octafluorocyclobutane (C4F8) and oxygen (O2) gases to etch silica vertically downward (anisotropic etching).</li>
	<li>Run the ICP-RIE process for approximately 13 min to etch the exposed silica layer. Refer to the ICP-RIE parameters in Table 3. During this step, the photoresist layer also gets completely etched away (Figure 3C&#8211;D).</li>
	<li>To ensure that the silica layer thickness inside the desired patterns is reduced to zero, so that the silicon layer is exposed, measure the thickness of the remaining silica using a reflectometer. Adjust the duration of the subsequent etching period based on the thicknesses of the silica layers (especially in and around the patterns).</li>
</ol>
NOTE: A reflectometer was used to measure the thickness of the remaining silica layer43. Alternatively, other tools, such as ellipsometer or an interactive color chart to predict the color of SiO2 and thickness can also be used44,45.
The procedures detailed in steps 1 and 4 are common for both reentrant and doubly reentrant cavities. However, the etching protocols for the silicon layer are different and are described below:
5. Reentrant Cavities
<ol>
	<li>Anisotropic silicon etching
	<ol>
		<li>After etching the silica layer, transfer the wafer to a deep ICP-RIE system to etch silicon. The first step consists of a fluorine-based anisotropic etching method known as the Bosch process that etches silicon vertically downward, creating a straight wall. 
		NOTE: The Bosch process uses C4F8 and sulfur hexafluoride (SF6) gases in the reaction chamber: the C4F8 deposition creates a passivation layer, while the SF6 etches silicon vertically downward. Thus, the Bosch process enables the microfabrication of deep trenches in silicon with high-aspect ratios.</li>
		<li>Run this process for five cycles, which corresponds to an etching depth for silicon equivalent to &#8776; 2 &#181;m. Process parameters are listed in Table 4.</li>
		<li>Clean the wafer in piranha solution for 10 min to remove any remnants of the Bosch process. Rinse the wafer with DI water and spin-dry in a N2 environment (Figure 3E).</li>
	</ol>
	</li>
	<li>Isotropic silicon etching: In order to create the reentrant feature, perform isotropic etching that would create an undercut beneath the silica layer. A 5 &#181;m overhang can be achieved by etching the silicon layer with SF6 for 2 min 45 s (Figure 3F). Refer to Table 5 for the process parameters.</li>
	<li>Anisotropic silicon etching: Once the reentrant features are created, tune the depth of the cavities by the Bosch process (step 5.1). 
	NOTE: To microfabricate cavities with a depth of hc &#8776; 50 &#181;m, 160 cycles of the Bosch process are required (Figure 3G, Table 4).</li>
	<li>Wafer cleaning and storage
	<ol>
		<li>Clean the wafer using piranha solution as described in step 2. After this step, the wafer becomes superhydrophilic, characterized by contact angles of water, &#952;o &#8776; 0&#176;.</li>
		<li>Store the wafer in a glass Petri dish and place inside a clean vacuum oven maintained at T = 323 K and vacuum pressure PVac = 3.3 kPa for 48 h, after which the intrinsic contact angle of the silica layer stabilizes to &#952;o &#8776; 40&#176;.</li>
		<li>Store the samples in a clean cabinet equipped with an outward nitrogen (99%) flow, ready for further characterization.</li>
	</ol>
	</li>
</ol>
6. Doubly Reentrant Cavities
<ol>
	<li>Anisotropic silicon etching: To create doubly reentrant cavities, follow steps 1, 2, 3, 4, and 5.1 (see Figure 4A&#8211;E).</li>
	<li>Isotropic silicon etching 
	In order to create doubly reentrant features, reentrant features must be created first. To achieve that, perform isotropic etching to create an undercut beneath the silica layer. Etch the silicon layer with SF6 for 25 s (Figure 4F). Refer to Table 5 for the process parameters. Subsequently, clean the wafer using piranha solution as described in step 2.</li>
	<li>Thermal oxide growth
	<ol>
		<li>To achieve doubly reentrant features, grow a 500 nm layer of thermal oxide on the wafer, using a high temperature furnace system (Figure 4G).</li>
		<li>Measure the thickness of the oxide layer using a reflectometer. 
		NOTE: The oxidation was carried out by exposing the samples to an environment comprising oxygen (O2) and water vapor, leading to the wet oxidation of silicon in an enclosed environment at temperatures ranging from 800&#8211;1,200 &#176;C.</li>
	</ol>
	</li>
	<li>Silica etching: Carry out the same process as described in the step 4 to etch silica vertically downward for 3 min. As a result of the anisotropic etching, the thermal oxide (500 nm thick silica layer) is etched away from the cavity, but it leaves an &#34;overhang&#34; along the sidewalls that would form the doubly reentrant edge eventually (Figure 4H, Table 3).</li>
	<li>Anisotropic silicon etching: Repeat five cycles of the Bosch process to deepen of the cavities by &#8776; 2 &#181;m (Figure 4I, Table 5). This step is necessary to remove the silicon behind the doubly reentrant feature in the next step. Clean the wafer using piranha solution.</li>
	<li>Isotropic silicon etching: Perform the isotropic etching of silicon for 2 min and 30 s using the process parameters described in Table 4. This step creates an empty space (&#8776;2 &#181;m) behind the thermally-grown oxide at the mouth of the cavity, leading to the doubly reentrant edge (Figure 4J).</li>
	<li>Anisotropic silicon etching: Use the Bosch process recipe (step 5.1) for 160 cycles to increase the depth of the cavities to hc &#8776; 50 &#181;m, (Figure 4K, Table 5).</li>
	<li>Wafer cleaning and storage: Clean the wafer using piranha solution and store as described in step 5.4 above.</li>
</ol>
MICROFABRICATION OF PILLARS 
The design protocol for fabricating reentrant and doubly reentrant pillars and &#34;hybrids&#34; (comprising doubly reentrant pillars surrounded by walls) consists of three key steps: wafer preparation, silica etching, and specific silicon etching. Figure 5A&#8211;C show the top-view of the layout design for reentrant and doubly reentrant pillars, while Figure 5D&#8211;F represent the layout of the hybrid arrays. Select the dark-field option of the UV exposure in order to expose the whole wafer except for the pattern using the same photoresist (AZ5214E) (Figure 6A&#8211;C and Figure 7A&#8211;C). Besides these specificities, the processes for cleaning the wafer (step 2) and etching silica (step 4) are identical.
7. Reentrant Pillars
<ol>
	<li>Anisotropic silicon etching: After photolithography, UV exposure, development, and etching silica with the specificities for pillars described above (steps 1&#8211;4), transfer the wafer to a deep ICP-RIE system to etch the silicon layer using the Bosch process. This step controls the height of the pillars. Use 160 cycles of the Bosch process to achieve pillars of height, hP &#8776; 30 &#181;m (Figure 6E, Table 5). Clean the wafer as described in step 2.</li>
	<li>Isotropic silicon etching: Perform isotropic etching using SF6 for 5 min to create the reentrant edge on the pillars (Figure 6F, Table 4). The resulting length of the overhang is 5 &#181;m.</li>
	<li>Piranha cleaning and storage: Clean the wafer using piranha solution and store as described in step 5.4 above.</li>
</ol>
8. Doubly Reentrant Pillars and Hybrids
<ol>
	<li>Anisotropic silicon etching: After etching SiO2, transfer the wafer to a deep ICP-RIE system to etch the Si under the SiO2 layer. Perform five cycles of the Bosch process that corresponds to an etching depth of &#8776; 2 &#181;m (Figure 7E, Table 4). Subsequently, clean the wafer as described in step (2).</li>
	<li>Isotropic silicon etching: Carry out isotropic etching using SF6 for 16 s to create the reentrant edge (Table 5, Figure 7F). Clean the wafer as described in step 2.</li>
	<li>Thermal oxide growth: Grow 500 nm layer of thermal oxide all over the wafer using a high temperature furnace system as described in step 6.3 (Figure 7G).</li>
	<li>Silica etching: Etch the thermally-grown oxide layer (500 nm thick) for 3 min as described in step 6.4 (Figure 7H, Table 3).</li>
	<li>Anisotropic silicon etching: Repeat 160 cycles of the Bosch process (Table 4) to increase the height of the pillars (Figure 7I). Clean the wafer as described in step 2 above.</li>
	<li>Isotropic silicon etching: Perform isotropic etching of silicon for 5 min using the process parameters as described in Table 4. This step creates the doubly reentrant edge (Figure 7J). The space between pillar stem and doubly reentrant edge is &#8776;2 &#181;m.</li>
	<li>Wafer cleaning and storage: Clean the wafer using piranha solution and store as described in step 5.4 above.</li>
</ol>
Figure 8 represents the list of processes used in microfabricating reentrant and doubly reentrant cavities and pillars.

The authors declare that they have no competing interests.

Here we discuss additional factors and design criteria to help the reader in applying these microfabrication protocols. For cavity microtextures (RCs and DRCs) the choice of pitch is crucial. Thinner walls between adjacent cavities would lead to low liquid-solid interfacial area and high liquid-vapor interfacial area, leading to high apparent contact angles34. However, thin walls could compromise the mechanical integrity of the microtexture, for instance, during handling and characterization; a little over-etching with thin walls (e.g., in step 6.6) could destroy the entire microtexture; under-etching with thin walls could also prevent the development of doubly reentrant features. If the DRC features are not fully developed, their ability to entrap air for long-term might suffer, especially if the liquid condenses inside the cavities26. For this reason, we chose the pitch in our experiments to be L = D + 12 &#956;m (i.e., the minimum wall thickness between the cavities was 12 &#181;m). We also fabricated doubly reentrant cavities with a smaller pitch of L = D + 5 &#956;m, but the resulting surfaces were not homogeneous due to structural damage during microfabrication.
During the etching of the silica layer with C4F8 and O2 in step 4, the prior history of usage or the cleanliness of the reaction chamber could give variable results, despite following the same steps, for instance, in a common user facility such as in most universities. Thus, it is recommended that this step is performed in short time periods, for instance, no more than 5 min each and monitored the thickness of the silica layer by an independent technique, such as reflectometry. For our wafers with a 2.4 &#956;m-thick silica layer, a typical etching routine took 13 min to remove silica completely from the targeted areas (Table 3). Because the photoresist was also etched during the process, this step removed 1 &#956;m of the silica layer that was initially masked by the photoresist. Furthermore, to ensure that the etching rate was as expected, and to avoid cross-contamination from previous etch processes (a common issue in multiuser facilities), silica etching was always preceded by etching a sacrificial wafer as a precautionary step. During the development of the photoresist, the exposed surface might get contaminated with the photoresist&#39;s traces/particles, which could act as (microscopic) masks leading to the formation of pin residues. To avoid this, rigorous cleaning and storage protocols should be followed throughout the microfabrication process36.
Similarly, during the Bosch process, even though the SiO2 layer acts as a mask for the Si-layer underneath, it does get etched during long etching cycles, albeit at slower rates. Thus, the depth of the cavities or the height of the pillars is limited up to the point that the reentrant features will not be compromised. The passivation and etching times during the Bosch process should be tuned to obtain smooth walls. This can be achieved by testing recipes iteratively and observing their effects on samples, for instance, using electron microscopy.
In the case of RPs and DRPs, the longer the duration of isotropic etching, the smaller the diameter of the stem. If the diameter is less than 10 &#956;m, it might lead to mechanical fragility. This limitation should inform the design at the beginning of the microfabrication procedure.
Dry-etching tools commonly available in universities do not have industrial-grade tolerances, leading to spatial non-uniformities in terms of the rate of etching inside the chamber. Thus, the features obtained in the center of the wafer might not be the same as those at the boundary. To overcome this limitation, we used four-inch wafers and concentrated only in the central region.
We also recommend using direct-writing systems instead of using hard-contact masks for photolithography, allowing for rapid changes in design parameters, including feature diameters, pitches and shapes (circular, hexagonal and square), etc.
Obviously, neither SiO2/Si wafers nor photolithography are the desired materials or processes for the mass production of omniphobic surfaces. However, they serve as an excellent model system to explore innovative microtextures for engineering omniphobic surfaces, for instance by biomimetics26,27,34,35,46,47, which can be translated to low-cost and scalable materials systems for applications. It is expected that in the near future, the design principles for GEMs might be scaled up using techniques such as 3-D printing48, additive manufacturing49, and laser micromachining50, among others. Microtextured SiO2/Si surfaces could also be used for templating soft materials29,51. Currently, we are investigating applications of our gas-entrapping surfaces for mitigating cavitation damage47, desalination46,52, and reducing hydrodynamic drag.

Solid surfaces that exhibit apparent contact angles, &#952;r &#62; 90&#176; for polar and nonpolar liquids, such as water and hexadecane, are referred to as omniphobic1. These surfaces serve numerous practical applications, including water desalination2,3, oil-water separation4,5, antibiofouling6, and reducing hydrodynamic drag7. Typically, omniphobicity necessitates perfluorinated chemicals and random topographies8,9,10,11,12. However, the cost, non-biodegradability, and vulnerability of those materials/coatings pose a myriad of constraints, e.g., perfluorinated desalination membranes degrade as the feed-side temperatures are raised, leading to pore-wetting13,14, and perfluorinated/hydrocarbon coatings also get abraded15,16 and degraded by silt particles in the flow streams and cleaning protocols. Thus, there is a need for alternative strategies for achieving the functions of perfluorinated coatings (i.e., entrapping air on immersion in liquids without using water repellent coatings). Therefore, researchers have proposed surface topographies comprised of overhanging (reentrant) features that could entrap air on immersion by microtexturing alone17,18,19,20,21,22,23,24,25. These microtextures come in three types: cavities26, pillars27, and fibrous mats8. Hereafter, we will refer to reentrant features with simple overhangs as reentrant (Figure 1A&#8211;B and Figure 1E&#8211;F) and reentrant features with overhangs that make a 90&#176;-turn towards the base as doubly reentrant (Figure 1C&#8211;D and Figure 1G&#8211;H).
In their pioneering work, Werner et al.22,28,29,30,31 characterized cuticles of springtails (Collembola), soil-dwelling arthropods, and explained the significance of mushroom-shaped (reentrant) features in the context of wetting. Others have also investigated the role of mushroom-shaped hairs in sea-skaters32,33 towards facilitating extreme water repellence. Werner and coworkers demonstrated the omniphobicity of intrinsically wetting polymeric surfaces by carving biomimetic structures through reverse imprint lithography29. Liu and Kim reported on silica surfaces adorned with arrays of doubly reentrant pillars that could repel drops of liquids with surface tensions as low as &#947;LV = 10 mN/m, characterized by apparent contact angles, &#952;r &#8776; 150&#176; and extremely low contact angle hysteresis27. Inspired by these amazing developments, we followed the recipes of Liu and Kim to reproduce their results. However, we discovered that those microtextures would catastrophically lose their superomniphobicity, i.e. &#952;r &#8594; 0&#176;, if wetting liquid drops touched the edge of the microtexture or if there was localized physical damage34. These findings demonstrated that pillar-based microtextures were unfit for applications that required omniphobicity on immersion, and they also questioned the criteria for assessing omniphobicity (i.e., should they be limited to contact angles alone, or if additional criteria are needed).
In response, using the SiO2/Si wafers, we prepared arrays of microscale cavities with doubly reentrant inlets and, and using water and hexadecane as the representative polar and nonpolar liquids, we demonstrated that (i) these microtextures prevent liquids from entering them by entrapping air, and (ii) the compartmentalized architecture of the cavities prevents the loss of the entrapped air by localized defects34. Thus, we have termed these microtextures as &#34;gas-entrapping microtextures&#34; (GEMs). As the next step, we microfabricated GEMs with varying shapes (circular, square, hexagonal) and profiles (simple, reentrant, and doubly reentrant) to systematically compare their performance under immersion in wetting liquids26. We also created a hybrid microtexture comprising arrays of doubly reentrant pillars surrounded by walls with doubly reentrant profiles, which prevented liquids from touching the stems of the pillars and robustly entrapped air on immersion35. Below, we present detailed protocols for manufacturing GEMs on SiO2/Si surfaces through photolithography and etching techniques along with design parameters. We also present representative results of characterizing their wetting by contact angle goniometry (advancing/receding/as-placed angles) and immersion in hexadecane and water.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>AZ-5214 E photoresist</td>
			<td>Merck</td>
			<td>DEAA070796-0W59</td>
			<td>Photoresist, flammable liquid</td>
		</tr>
		<tr>
			<td>AZ-726 MIF developer</td>
			<td>Merck</td>
			<td>10055824960</td>
			<td>To develop photoresist</td>
		</tr>
		<tr>
			<td>Confocal microscopy</td>
			<td>Zeiss</td>
			<td>Zeiss LSM710</td>
			<td>Upright confocal microscope to visualize liquid meniscus shape</td>
		</tr>
		<tr>
			<td>Deep ICP-RIE</td>
			<td>Oxford Instruments</td>
			<td>Plasmalab system100</td>
			<td>Silicon etching tool</td>
		</tr>
		<tr>
			<td>Direct writer</td>
			<td>Heidelberg Instruments</td>
			<td>&#181;PG501</td>
			<td>Direct-writing system</td>
		</tr>
		<tr>
			<td>Drop shape analyzer</td>
			<td>KRUSS</td>
			<td>DSA100</td>
			<td>To measure contact angle</td>
		</tr>
		<tr>
			<td>Hexadecane</td>
			<td>Alfa Aesar</td>
			<td>544-76-3</td>
			<td>Test liquid</td>
		</tr>
		<tr>
			<td>Highspeed imaging camera</td>
			<td>Phantom vision research</td>
			<td>v1212</td>
			<td>To image droplet bouncing</td>
		</tr>
		<tr>
			<td>HMDS vapor prime</td>
			<td>Yield Engineering systems</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Hot plate</td>
			<td>Cost effective equipments</td>
			<td>Model 1300</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrogen peroxide 30%</td>
			<td>Sigma Aldrich</td>
			<td>7722-84-1</td>
			<td>To prepare piranha solution</td>
		</tr>
		<tr>
			<td>Imaris software</td>
			<td>Bitplane</td>
			<td>Version 8</td>
			<td>Post process confocal microscopy images</td>
		</tr>
		<tr>
			<td>Nile Red</td>
			<td>Sigma Aldrich</td>
			<td>7385-67-3</td>
			<td>Fluorescent dye for hexadecane</td>
		</tr>
		<tr>
			<td>Nitrogen gas</td>
			<td>KAUST lab supply</td>
			<td></td>
			<td>To dry the wafer</td>
		</tr>
		<tr>
			<td>Petri dish</td>
			<td>VWR</td>
			<td>HECH41042036</td>
			<td></td>
		</tr>
		<tr>
			<td>Reactive-Ion Etching (RIE)</td>
			<td>Oxford Instruments</td>
			<td>Plasmalab system100</td>
			<td>Silica etching tool</td>
		</tr>
		<tr>
			<td>Reflectometer</td>
			<td>Nanometrics</td>
			<td>Nanospec 6100</td>
			<td>To check remaining oxide layer thickness</td>
		</tr>
		<tr>
			<td>Rhodamine B (Acros)</td>
			<td>Fisher scientific</td>
			<td>81-88-9</td>
			<td>Fluorescent dye for water</td>
		</tr>
		<tr>
			<td>SEM stub</td>
			<td>Electron Microscopy Sciences</td>
			<td>75923-19</td>
			<td></td>
		</tr>
		<tr>
			<td>SEM-Quanta 3D</td>
			<td>FEI</td>
			<td>Quanta 3D FEG Dual Beam</td>
			<td></td>
		</tr>
		<tr>
			<td>Silicon wafer</td>
			<td>Silicon Valley Microelectronics</td>
			<td></td>
			<td>Single side polished, 4&#34; diameter, 500 &#181;m thickness, 2.4 &#181;m thick oxide layer</td>
		</tr>
		<tr>
			<td>Spin coater</td>
			<td>Headway Research,Inc</td>
			<td>PWM32</td>
			<td></td>
		</tr>
		<tr>
			<td>Spin rinse dryer</td>
			<td>MicroProcess technology</td>
			<td>Avenger Ultra -Pure 6</td>
			<td>Dry the wafers after piranha clean</td>
		</tr>
		<tr>
			<td>Sulfuric acid 96%</td>
			<td>Technic</td>
			<td>764-93-9</td>
			<td>To prepare piranha solution</td>
		</tr>
		<tr>
			<td>Tanner EDA L-Edit software</td>
			<td>Tanner EDA, Inc.</td>
			<td>version15</td>
			<td>Layout design</td>
		</tr>
		<tr>
			<td>Thermal oxide growth</td>
			<td>Tystar furnace</td>
			<td></td>
			<td>To grow thermal oxide in patterned silicon wafer</td>
		</tr>
		<tr>
			<td>Tweezers</td>
			<td>Excelta</td>
			<td>490-SA-PI</td>
			<td>Wafer tweezer</td>
		</tr>
		<tr>
			<td>Vacuum oven</td>
			<td>Thermo Scientific</td>
			<td>13-258-13</td>
			<td></td>
		</tr>
		<tr>
			<td>Water</td>
			<td>Milli-Q</td>
			<td>Advantage A10</td>
			<td>Test liquid</td>
		</tr>
	</tbody>
</table>

rendering sio2/si surfaces omniphobic by carving gas-entrapping microtextures comprising reentrant and doubly reentrant cavities or pillars

We present microfabrication protocols for rendering intrinsically wetting materials repellent to liquids (omniphobic) by creating gas-entrapping microtextures (GEMs) on them comprising cavities and pillars with reentrant and doubly reentrant features. Specifically, we use SiO2/Si as the model system and share protocols for two-dimensional (2D) designing, photolithography, isotropic/anisotropic etching techniques, thermal oxide growth, piranha cleaning, and storage towards achieving those microtextures. Even though the conventional wisdom indicates that roughening intrinsically wetting surfaces (&#952;o &#60; 90&#176;) renders them even more wetting (&#952;r &#60; &#952;o &#60; 90&#176;), GEMs demonstrate liquid repellence despite the intrinsic wettability of the substrate. For instance, despite the intrinsic wettability of silica &#952;o &#8776; 40&#176; for the water/air system, and &#952;o &#8776; 20&#176; for the hexadecane/air system, GEMs comprising cavities entrap air robustly on immersion in those liquids, and the apparent contact angles for the droplets are &#952;r &#62; 90&#176;. The reentrant and doubly reentrant features in the GEMs stabilize the intruding liquid meniscus thereby trapping the liquid-solid-vapor system in metastable air-filled states (Cassie states) and delaying wetting transitions to the thermodynamically-stable fully-filled state (Wenzel state) by, for instance, hours to months. Similarly, SiO2/Si surfaces with arrays of reentrant and doubly reentrant micropillars demonstrate extremely high contact angles (&#952;r &#8776; 150&#176;&#8211;160&#176;) and low contact angle hysteresis for the probe liquids, thus characterized as superomniphobic. However, on immersion in the same liquids, those surfaces dramatically lose their superomniphobicity and get fully-filled within &#60;1 s. To address this challenge, we present protocols for hybrid designs that comprise arrays of doubly reentrant pillars surrounded by walls with doubly reentrant profiles. Indeed, hybrid microtextures entrap air on immersion in the probe liquids. To summarize, the protocols described here should enable the investigation of GEMs in the context of achieving omniphobicity without chemical coatings, such as perfluorocarbons, which might unlock the scope of inexpensive common materials for applications as omniphobic materials. Silica microtextures could also serve as templates for soft materials.

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

This work presents microfabrication protocols for achieving cavities and pillars with reentrant and doubly reentrant profiles on SiO2/Si wafers using photolithography and dry etching. Resulting microtextured surfaces demonstrate remarkable liquid repellence, characterized by robust long-term entrapment of air under wetting liquids, despite the intrinsic wettability of silica.

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

We present microfabrication protocols for rendering intrinsically wetting materials repellent to liquids (omniphobic) by creating gas-entrapping ...

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