From "petal effect" to "lotus effect"

From "petal effect" to "lotus effect" on the highly flexible Silastic S elastomer microstructured using a fluorine based RIE process Christophe Frankiewicz 1 , Farzam Zoueshtiagh 1 , Abdelkrim Talbi 1 , Jérémy Streque 1 , Philippe Pernod and Alain Merlen 2

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IEMN - EC Lille - PRES Universite Lille Nord de France, Avenue Poincaré, 59652 Villeneuve d’Ascq 2 ONERA, Chemin de la Huniere, 91123, Palaiseau E-mail: [email protected] Abstract. A fluorine-based reactive ion etching (RIE) process has been applied on a new family of silicone elastomer named "Silastic S" for the first time. Excellent mechanical properties are the principal advantage of this elastomer. The main objective of this study was (i) to develop a new process with an electrodeposited thin Nickel (Ni) layer as a mask to obtain a more precise pattern transfer for deep etching (ii) to investigate the etch rates and the etch profiles obtained under various plasma conditions (gas mixture ratios and pressure). The resulting process exhibits etch rates that range from 20 µm/h to 40 µm/h. The process was optimized to obtain anisotropic profiles of the edges. Finally, it is shown that (iii) the wetting contact angle could be easily modified with this process from 103◦ to 162◦ , with an hysteresis that ranges from 2◦ to 140◦ . The process is, at present, the only reported solution to reproduce the "petal effect" (high contact angle hysteresis value) on a highly flexible substrate. A possibility to control the contact angle hysteresis from the "petal effect" to the "lotus effect" (low contact angle hysteresis value) has been investigated to allow a precise control on the required energy to pin or unpin the contact line of water droplets. This opens multiple possibilities to exploit this elastomer in many microfluidics applications.

From "petal effect" to "lotus effect" on the highly flexible Silastic S elastomer microstructured using a fluorine based RIE p 1. Introduction Flexible elastomers are gaining considerable use in a wide range of applications including medical, industrial, electronics and microfluidics. In modern MEMS (MicroElectroMechanical Systems) technology, PDMS (Polydimethylsiloxane) is one of the most used biomaterial for the development of microfluidic components for medical applications such as microchannel, micropumps, and fluid reservoirs [1, 2, 3]. PDMS is also well suited for flexible microwave antennas [4] and stretchable electronic circuits [5, 6]. Flexible actuators are, in many cases, made from PDMS elastomer, due to its superior mechanical deformation under electric or magnetic force [7, 8, 9], but much less stretchable and resistant to tear deformation than the hereby study "Silastic S" silicone elastomer. More precisely, Silastic S has a 4 times elongation at break compared to PDMS according to Dow Corning [10] (see table 1), that we also experimentally checked here (see section 2).

Durometer [ShoreA] Viscosity [Pa.s] Elongation [%] Tear Strength [kN/m] Tensile Strength [MPa]

PDMS

Silastic S

48 4 140 2.6 7.1

26 12.8 850 24.5 6.9

Table 1. Mechanical comparison of Silastic S with PDMS (Dow Corning data)

During the last two decades, the main process used to design these structures was based on a PDMS molding process [5, 11]. Recently, higher resolution patterns were obtained by Reactive Ion-Etching (RIE). PDMS etching was firstly studied by Garra et al. [1] who developed a dry etching process that uses standard RIE equipment and gases. Their conclusion revealed that a mixture of 75% of CF4 and 25% of O2 enhances the etch rate, α, up to α ≈ 20 µm/h. This process was later improved by the use of SF6 /O2 mixture, with the highest α also obtained with 25% O2 [12, 13, 14]. The variation of the chamber pressure has also been studied [12, 13, 14] and it was found that its increase from 50 mT up to 150 mT could lead in an increase in α, whereas higher pressures decreased it. The etched patterns were obtained with a lithographically defined aluminium mask [1, 12] , or with glass dummy wafers [13, 14]. The surface roughness is always highly affected by this etching process [1]. Indeed, the wetting properties of rigid superhydrophobic surfaces has also raised great interest in the last decades [19, 22, 23]. Conventionally, the wettability of a surface can be modified by engineering the physical aspect of the surface (or texture) or by chemically modifying the surface energy [27, 28]. Texture alteration or modification is usually obtained by roughening the substrate or structuring the surface with pillars or cones like structures [30, 31]. For chemical modification, low surface energy materials are generally coated to modify the liquid affinity with the surface [29]. These methods are however much less frequently applied on flexible substrate owing to delicate process issues. Additionally, recent studies have shown that hierarchical multiscales that combines micro and nano structures decrease the contact angle hysteresis (CAH) [33, 27]. In this paper, mechanical properties and dry etching process are investigated on a new elastomeric material commercially available as Dow Corning "Silastic S". A new process with an electrodeposited nickel mask, allowing better results is defined (higher etch rate, better control of the profiles with high aspect ratios). Relevant

From "petal effect" to "lotus effect" on the highly flexible Silastic S elastomer microstructured using a fluorine based RIE p parameters such as the etchant gas ratio, the chamber pressure and the RadioFrequency (RF) power were studied to define the optimum etch rate α. Finally, wettability and contact angle hysteresis of these etched substrates are investigated to obtain superhydrophobicity with a possibility to control the contact angle hysteresis from 2◦ ("lotus effect") to 140◦ ("petal effect"). 2. Dry etching of Silastic S 2.1. Experimental

Elastomer thickness [µm]

The silicone rubber base was mixed according to Dow Corning instructions (weight ratio 10:1 base:curing agent), degassed in a vacuum chamber and then spin-coated on a 3 inch silicon wafer to obtain a 40 µm layer thick. Note that the evolution of the Silastic layer thickness has been determined as a function of the rotation speed of the spin-coater in a prior experiment (figure 1-a). The wafers were subsequently cured during 10 minutes at 100◦ C. The samples were then left aside in the clean room environment for 24 hours so that mechanical properties were fully established (figure 1-b, compared to PDMS with Dow Corning data). We have investigated the mechanical behavior of both Silastic S and the PDMS under traction (figure 1-c) and found it to be in good agreement with the manufacturer data. For these traction tests, samples were prepared to obtain a 2 mm thickness and were then stretched to rupture. These tests were only made to give an estimated comparison, for more details on mechanical properties of elastomers under traction tests at the microscale, recent studies have been carried, e.g. [39]. It can be underlined that this elastomer exhibits a much higher elongation at break compared to PDMS (figure 1-c) which is necessary for applications such as tactile actuators [9] and many other MEMS. As another example, traction forces exerted by cells have been determined using deflection of PDMS micropillars [37]. According to this study, the forces are proportional to F ∝ E∆x where E is the Young modulus and ∆x the lateral elongation of the pillars. Silastic S has a Young modulus (E= 0.97 Mpa after Shore-A conversion) 3/4 times lower than PDMS (E= 1.33 MPa) and thus, for an identical force applied on the pillars, the ∆x could be 4/3 times higher, resulting in a more accurate measurement. The deformation of PDMS micropillars was also used to dynamically measure the shear forces in a pipe flow [38]. A similar way of thought could be carried to conclude on the high potentiality to use the Silastic S elastomer as a more accurate alternative to PDMS for deformation measurements of flexible structures.

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220 Silastic S 200 180 160 140 120 100 80 60 40 20 1000 1500 2000 2500 3000 3500 4000 4500 5000 Spin-coating speed V [r/min]

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Figure 1. Silastic S elastomer properties: (a) experimentally measured thickness evolution with spin-coating speed (V [r/min]), acceleration duration A = 1 s to reach full rotational spin-coating speed V, Time t = 40 s (b) Elongation at break for PDMS and Silastic S against mechanical stress experimentally measured on a test specimen thickness of 2 mm, the results are comparable to Dow Corning data shown on table 1

From "petal effect" to "lotus effect" on the highly flexible Silastic S elastomer microstructured using a fluorine based RIE p

(a)

(b) Figure 2. (a) Schematic illustrations of the process to define etch profiles before RIE of the Silastic S: 1) Silastic S spin-coating + Cr-Au sputtering + AZnlof 2020 Spin-coating, 2) Removal of insulated photoresist 3) electrodeposited Nickel mask deposit 4) Removal of leftover photoresist + Cr-Au ion etching (b) SEM image between step 3 and 4.

First etching tests were carried out with a standard process, that is to say a thin layer of aluminium was sputter-deposited (≤ 200 nm to avoid cracks [25, 7]), followed by a photoresist spincoating (AZ 1518) and a photolithography step to define the patterns to etch. The elastomer was etched in a Oxford Plasmalab 80 plus Reactive Ion Etcher. The process pressure was varied from 50 to 200 mT, the RF power from 100 W to 300 W and the etchant SF6 /O2 gas mixture from 0% to 100% SF6 . The total flow was kept constant at 70 sccm. A step height measurement was completed with a profilometer after each samples etching experiment to evaluate the etch rate as well as the surface roughness. It is important to note that each etching experiment has been carried by alternating 10 min. etching phases and 10 min. rest phases to prevent high temperatures effects (e.g. melting...). Thereafter, the etching time refers to the sum of the etching phases times. The aluminium mask intended to protect the un-etched surface from the etchant gas was however completely etched after about 20 minutes (etching time). Therefore, a new process allowing deep etching was developed (figure 2-a). One of the main advantage of this present process is that it remains compatible with standard micromachining along with high resolution of the order of micrometers. First, a Cr-Au (Chromium-Gold) layer is sputter-deposited on a 3 inch silicon wafer where a 40 µm thick layer of Silastic has been coated beforehand. Then a negative photoresist AZnlof 2020 was spin coated to obtain a 2 µm thick layer. The mask used during the photoresist insolation step was designed with a large variety of shapes and sizes such as circles, high aspect ratio rectangles and squares with dimensions ranging from 2 µm to 15000 µm. The parameters of the electrodeposition device were defined to obtain a 1 µm Ni layer. Electrodeposition is preferred to sputtering to obtain a homogeneous thick layer (figure 2-b). The photoresist and the Cr-Au layer are then removed using PG remover and an ion-polishing system, respectively. The samples are then etched 60 minutes (total etching time) in the reactive ion etcher. Finally, an orthophosphoric acid (H3 PO4 ) solution is used to remove the nickel layer.

From "petal effect" to "lotus effect" on the highly flexible Silastic S elastomer microstructured using a fluorine based RIE p 35

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Figure 3. Etch rate evolution with (a) mixture composition at a constant pressure of 100 mT (b) chamber pressure at a constant mixture ratio (SF6 /02 65/35% Vol.) (c) RF power at a constant pressure of 100 mT, with a constant mixture ratio (SF6 /02 75/25% Vol.) for a typical 100 µm width profile. Each symbol displays the results of a single sample measurement

2.2. Results and discussion In the present work, as depicted in figure 3-a, the highest etch rate α of Silastic is obtained for a 65/35% SF6 /O2 etchant gas mixture. In the case of PDMS, Garra et al. [1] and Szmigiel et al. [12] measured the highest α value with 80/20% SF6 /O2 mixture while Knizikevicius [17] analytically estimated it to be 73/27% SF6 /O2 for Si etching. In our process, as the main chemical reaction is identical to that in a PDMS or Si plasma (i.e. Si+4F → SiF4 ), the highest etch rate is found to be close to these two mixture proportions (80/20% and 73/27% SF6 /O2 ) while the observed difference can be associated to the oxygen radicals, reacting with the methylgroups to form CO, CO2 and H2 O. During the tests, a strong dependance of α with the pattern width was also observed. The etching is faster for larger profiles since there is less backscattering and a higher aspect ratio between width and depth. For profiles larger than 1000 µm, α appears to reach a maximum value. The etch rate evolution with the chamber pressure is shown on figure 3-b. At 50 mT pressure, α is higher for larger profiles with a maximum value of ≈ 25 µm/h for patterns widths ≥ 1000 µm. For a chamber pressure set at 100 mT, the discrepancy between small and large profiles in α is minimum. This discrepancy however increases again for higher chamber pressure, up to a maximum value obtained for the maximum chamber pressure studied here: the process is more sensitive to the size when there is less sputtering. Note that two different processes are exhibited in these plasmas

From "petal effect" to "lotus effect" on the highly flexible Silastic S elastomer microstructured using a fluorine based RIE p where physical and chemical etching are always linked. The lower the pressure, the higher the ions energy is, thus enhancing physical etching. Here, at 100 mT, both ions energy and chemical reactions enhance the etching, resulting in a high value of α. After that, ions energy is decreasing significantly and thus is lowering the values of α until chemical reactions balance this loss and increase the etch rate α again. The etch rate evolution with RF power was also studied. The evolution is nearly linear (figure 3-c) for a plasma carried out at a 100 mT chamber pressure and with a gas mixture of 65/35% SF6 /O2 . As expected, ions with more energy can etch the 100 µm test profiles faster.

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(d) Figure 4. SEM images of a sample etched with the previously defined optimized parameters gas mixture ratio of 65/35% SF6 /O2 , RF power of 300 W, chamber pressure. (a) deep etching of a 100 µm side length square at 100 mT during 50 minutes (total etching time), (b-c-d) high aspect ratio structures obtained at 50 mT with various time (cuboids l. x w. x h. 80x10x10 µm, holes Ø4 µm h. 20 µm, pilars Ø4 µm h. 10 µm)

In summary, best profile edges along with deep and fast etching can be achieved with a chamber pressure at 100 mT, gas mixture ratio of 65/35% SF6 /O2 , RF power of 300 W (figure 4-a). Here, the obtained edges profiles are nearly straight and their straightness can be improved by lowering the chamber pressure (figure 4b). Such technique can be used to generate very high aspect ratio microholes and micropillars (figure 4-c,d) which could find applications in low frequency phononic engineering (sound filters or vibrations isolators) or microfluidics (hydrophobic surface), respectively.

From "petal effect" to "lotus effect" on the highly flexible Silastic S elastomer microstructured using a fluorine based RIE p 3. Superhydrophobicity from Silastic S In this section, we investigate the perspective to generate superhydrophobic surfaces on the highly flexible Silastic S elastomer using the etching process mentioned above. Indeed, the RIE on Silastic S is accompanied by the generation of random micro structures (figure 4 a-d). Below, we will show that this resulted roughness exhibits superhydrophobic characteristics which grants the present process highly interesting aspects. First, the measurements show that superhydrophobicity could be achieved with a single step etching process to reach a static contact angle of 155◦ . It will also be seen that the highest CAH obtained here (140◦ ) is the highest CAH ever obtained on a flexible substrate. This is of interest for reproducing highly sticking superhydrophobic surfaces similarly to the red rose petal [32]. Second, a combination of patterned surface along with either RIE etching or with a perfluorodecyltrichlorosilane (FDTS - C10 H4 F17 SiCl3 ) layer coating can lead to hierarchical scale. This provides grounds for superhydrophocity with low CAH, similarly to hierarchical structures obtained on a lotus leaf [34]. 3.1. Very high CAH

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Figure 5. SEM images of the surface roughness for samples etched with a gas mixture ratio of 65/35% SF6 /O2 , a RF power of 300 W and a chamber pressure of (a) 40 mT for 10 minutes, (b) 100 mT and (c) 150 mT for 30 minutes. (d) Measured roughness as a function of chamber pressure for samples etched 30 minutes. Ra is the arithmetic average of absolute values, Rz is the average distance between the lowest valley and highest peak

The RIE etching process modifies the roughness of the Silastic S due to random ion collisions and chemical reactions. Indeed, the lower the chamber pressure, the higher the roughness, as shown on fig 5. Note that these roughness measurements were

From "petal effect" to "lotus effect" on the highly flexible Silastic S elastomer microstructured using a fluorine based RIE p Contact angles

Static

Advancing

Receding

Hysteresis

Silastic S

103 113 115 123 130 128

105 115 130 125 135 140

75 60 55 40 50 50

30 55 75 85 85 90

Sample 1 : 25/75 % SF6 /O2 Sample 2 : 35/65 % SF6 /O2 Sample 3 : 50/50 % SF6 /O2 Sample 4 : 65/35 % SF6 /O2 Sample 5 : 75/25 % SF6 /O2

Table 2. Contact angles of samples etched with various mixture composition at 75 mT, 300 W

obtained using a 3D microscope Hirox KH 8700 (the step height parameter was set at 0.1 µm). According to the result, we chose to set the pressure chamber at its lowest possible value where the substrate is most roughened (justification discussed thereafter) and, thus, where it is more likely for a droplet to experience a Cassie-Baxter state (also known as the fakir state, for which the droplet sits on top of the emerging microstructures). Samples were prepared following previous discussions: the elastomer was mixed following Dow Corning instructions again, degassed in a vacuum chamber, spin-coated on a silicon wafer to obtain a 50 µm thickness layer (figure 1-a) and cured at 100◦ C for 20 minutes before being etched 10 minutes. Static and dynamic CA (advancing and receding) were then measured using the sessile drop technique and a goniometer with a 5 µL droplet. The advancing and receding CA have been measured by dispensing or pumping water with an electronic syringe pump at a very low flow rate (Capillary number was Ca ≈ 10−7 ). According to table 2, it appears that the mixture proportion does not play a significant role on the contact angle values and correspondingly on the surface roughness. Adding SF6 in the plasma only has a slightly change in the surface roughness that could be explained by the fact that molecules of SF6 are heavier than those of O2 : increasing their number in the plasma mixture causes a more severe attack. It is also shown on figure 6, that CAH increases with samples etched at lower pressures. In fact, a decrease in pressure increases the mean free path of ions and slows down the collision processes. It is important to note that a lower mean free path also implies more chemical reaction of radicals instead of pure ion collisions. The surface is consequently much smoother with processes carried out at high pressures 5. The surface roughness directly influences the CAH and controls the droplet’s impalement. In the present section, Wenzel state (for which the droplet is impaled on the structure) was systematically observed due to the low height and small width of the microstructures randomly generated by such etching process (figure 6). The contact line is consequently always pinned resulting in a very high hysteresis angle. The highest CAH obtained here is 140◦ with a static CA of 155◦ leading us to consider this surface as superhydrophobic. High hysteresis angles together with high CA are usually observed in nature on a rose petal [32] or on garlic and scallion leaves [35]. The single step process described here is, at present, the only reported solution to reproduce on a flexible substrate the "petal effect" where these high CA and CAH coexist.

From "petal effect" to "lotus effect" on the highly flexible Silastic S elastomer microstructured using a fluorine based RIE p 160 140 120

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Figure 6. Contact angles measured after 10 minutes etch at 75/25% SF6 /O2 as a function of the chamber pressure. Each symbol displays the results of a single sample measurement

3.2. Tunable CAH Given the micrometer size of the structure, it is also possible to consider this process as a second step to obtain multiscales surfaces on micropillars or microbumps. Recent studies on PDMS [26] and on silicon [33] show that a pillared surface allows high contact angles for superhydrophobic states and that adding a second hierarchical scale on the top of pillars decreases significantly the contact angle hysteresis. Here, similar observation is made where, from a first scale bump structure with diameters ranging from 5 to 40 µm, a substantial increase in the static CA is measured. The second hierarchical scale can be obtained either chemically with a silane coating (that result in decreasing the surface energy) or physically with the previous etching process (that result in modifying the surface texture). The study shows that CAH could be as low as 2◦ up to 96◦ in either Cassie impregnating state or Cassie state also known as the "lotus effect" [27]. The process to obtain the bump structures and the second hierarchical scale is as follows. First, the etching process with an electrodeposited Ni mask is performed (figure 2, RIE step parameters: 40 mT, 300 W, 75/25% SF6 /O2 ) on three substrates to obtain the bump structures depicted on figure 7-a. The Ni layer is then removed using an orthophosphoric acid. From these three samples, the surface roughness and/or surface energy of two of them is enhanced by two different means: (i) one sample is etched 10 more minutes in the plasma etcher (40 mT, 300 W, 75/25% SF6 /O2 ) (Figure 7-b), (ii) the second sample is chemically coated by dipping it for 2 hours in a diluted 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS - C10 H4 F17 SiCl3 ) solution in n-hexane (C6 H14 ) precursor. This coating was prepared in a vacuum dried glove box. However, we forced the polymerization of FDTS in aggregate form [36] on the substrate surface by opening the container to air during one minute. This procedure increased drastically the roughness of the original sample as shown on figure 7-c. Table 8 shows the measured contact angles obtained from the three above samples and for different bump sizes.


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(c) Figure 7. SEM images of micro bumps obtained with the Ni mask process (a) originally (b) after a 10 minutes plasma etching (c) after an FDTS coating


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2h3d8 3 h 4 d 11.5 7 h 12 d 20 9 h 15 d 40 30 h 15 d 80

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155 / 42 160 / 40 160 / 12 162 / 7 160 / 43

160 / 10 160 / 9 160 / 6 162 / 2 160 / 10

Figure 8. Contact angles of samples etched with various dimensions parameters for the hexagonal bump network.

The first conclusion is that roughening the bumps, and in particular their tops after removing the Ni layer, results in an increase of the static CA up to 162◦ showing superhydrophobic characteristics. This was expected as observed in previous cited studies. The second hierarchical etching step enhance greatly the hysteresis down to 2◦ showing that this elastomer must be considered as a candidate surface for the well-studied "lotus effect". This is a consequence of the low chamber pressure plasma that generates pikes on the surface and thus changes the Wenzel to a Cassie-Baxter wetting state. Additionally, the FDTS surface coating in forms of aggregates allows to tune precisely the CAH with values that range in the middle of the two uncoated wafers (see table 8). Given that the fluorinated tail functional group is not necessarily in contact with the water droplet under an aggregate form, the surface energy is much higher on such surfaces compared to a single monolayer. This had been shown in past [36] but never used to tune the contact angle on a flexible substrate. Finally, we have demonstrated that the superhydrophobicity of Silastic S elastomer may be of considerable interest in microfluidic either to study the "petal" or the "lotus" effects on highly flexible substrate. It has also been shown that the CAH could be obtained in a wide range between [2◦ ; 140◦ ] for a superhydrophobic surface (CA > 150◦ ). This could be used to precisely control the required energy to pin or unpin water droplets opening multiple possibilities to exploit this elastomer in many various applications. 4. Conclusions The elastomer Silastic S has been etched with an etch rate up to 40 µm/h. The most important results which are discussed in this study are four-fold. First, the process was optimized with the relevant parameters: the chamber pressure (100 mT), the RF power (300 W) and the mixture proportion using an SF6 /O2 plasma (65/35% SF6 /O2 ). Second, a new process involving a Nickel mask, obtained by electrodeposition, was described and optimized. It was shown that it allows to obtain a high resolution of the profiles, on the order of a micrometer, applicable to many other elastomers and polymers. Third, superhydrophobicity was obtained with static contact angle up to 162◦ . A one step process has been optimized to obtain very high hysteresis reproducing the "petal effect". This was reported for the first time on a highly flexible substrate. Fourth, the contact angle hysteresis could range from 2◦ to 140◦

From "petal effect" to "lotus effect" on the highly flexible Silastic S elastomer microstructured using a fluorine based RIE p allowing a direct control of the required energy to pin or unpin water droplets on this substrates. This opens multiple possibilities to exploit this elastomer in various microfluidics applications. References [1] [2] [3] [4] [5]

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[16]

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