Hydrophobic wafer

Vapor-phase decomposition and collection (Figs 4.16 to 4.18) is a standardized method of silicon wafer surface analysis [4.11]. The native oxide on wafer surfaces readily reacts with isothermally distilled HF vapor and forms small droplets on the hydrophobic wafer surface at room temperature [4.66]. These small droplets can be collected with a scanning droplet. The scanned, accumulated droplets finally contain all dissolved contamination in the scanning droplet. It must be dried on a concentrated spot (diameter approximately 150 pm) and measured against the blank droplet residue of the scanning solution [4.67-4.69]. VPD-TXRF has been carefully evaluated against standardized surface analytical methods. The user is advised to use reliable reference materials [4.70-4.72]. 

In spite of its name, it uses hydrodynamic drag to exert a removal force on the siuface particles. Brush bristles do not contact the particle or the surface but rather act as oars or paddles that push liquid across the wafer surface, dislodging particles. This technique is effective for particles larger than 1 pm. It is suitable for both hydrophilic and hydrophobic wafers. 

Surface energy was calculated using the Fowkes method [21] to determine the dispersion and polar (or nondispersion) component of Wettability measurements indicate a surface energy close to 20 mJ/m for the hydrophobic wafer (CHj grafted surface), with a polar component equal to zero. This result confirms the hydrophobic character of the grafted wafer. A surface energy equal to 79 mJ/m was obtained for the hydrophilic substrate (polar component = 42 mJ/m ). 

FIGURE 12.2 Friction coefficient of PDMS A and B as a function of normal force in contact with hydrophobic wafer (speed = 25 mm/min). 

Significant differences between hydrophobic and hydrophilic substrates are observed at lower speed, with a low friction for hydrophobic wafer at low normal load. For example, as illnstrated in fig. 12.5 (presenting the friction coefficient of PDMS A versus speed), the friction coefficient of PDMS A (at 25 mm/min and 1N) is equal to 1.04 for the hydrophobic wafer and 1.58 for the hydrophilic wafer (factor equal to 1.5 between the two coefficients). However, when the friction speed is increased, the difference between the friction coefficients of hydrophilic and hydro-phobic systems becomes lower, and both coefficients are identical for high speed, with a value close to 1.3 (for a normal force of 1 N). 

FIGURE 12.5 Evolution of the friction coefficient of PDMS A as a function of speed, in contact with hydrophilic and hydrophobic wafers (F, = 1 N). 

Higher shear rate and stress are indeed able to induce a chain orientation at the PDMS surface [24]. Such an orientation will probably modify the rheological behavior of the polymer interface, and the anisotropy of the confined interfacial layer is able to induce a lower shear resistance. Moreover, it is also necessary to interpret the speed dependence of the friction for both substrates, with an increase of the friction coefficient with speed for the hydrophobic wafer and a decrease for the hydrophilic one. 

In summary, a high shear rate is able to confine and orient the chains of the PDMS surface. The cohesion, or shear resistance, of this layer will be lower (i.e less dissipative) than hydrophilic-wafer/PDMS interactions, explaining the decrease of the friction with speed for the hydrophilic substrate, but greater (more dissipative) than hydrophobic-wafer/PDMS interactions, explaining the increase of the friction with speed for the hydrophilic substrate. Finally, at high speed, shear should occur preferentially within this confined layer (and consequently not exactly at the polymer-substrate interface), leading to similar friction for both substrates. Analysis of the transfer layer (observed by atomic force microscopy) allows a better understanding of the involved mechanisms [25-27],... 

First, the inflnence of the snbstrate chemistry was more significant compared with the case of PDMS, with PS friction coefficients close to 0.40 and 0.15, respectively, for hydrophilic and hydrophobic wafers. The friction coefficient of the hydrophilic wafer was always mnch greater than that of the hydrophobic one, whatever the experimental conditions (speed, normal force). Moreover, the effect of friction speed was totally different in the case of polystyrene compared to PDMS. A slight increase of the friction coefficient with speed conld indeed be observed for both types of substrates, attributed to viscoelastic effects. It was also shown that the normal force had a negligible influence. 

The aim of this study was to illustrate the complex role of the interface through the study of PDMS friction in contact with both hydrophilic and hydrophobic silicon wafers. Experimental results have shown that the friction coefficient of hydrophilic substrates can be either greater than or similar to that of hydrophobic wafers, depending on the friction speed (and normal load). At low speed, a significant difference between the two substrates is observed, but the influence of the substrate chemistry becomes negligible at higher friction speed. 

Fig. 3.13 Schematics of the chemically heterogeneous surfaces fabricated by Gao and McCarthy, Surface a comprises a partially hydrolyzed hydrophilic spot (white) on a hydrophobic silicon wafer (gray) Surface b comprises a superhydrophobic (textured) spot on the hydrophobic wafer (white) d is the diameter for the heterogeneous spot and D is the diameter of the water droplet used in contact angle measurements (Reproduced with permission from [36], Copyright 2007 The American Chemical Society)...

Two main types of catalyst layers are used in PEM fuel cells polyfefrafluo-roethylene (PTFE)-bound catalyst layers and thin-film catalyst layers [3]. The PTFE-bound CL is the earlier version, used mainly before 1990. If confains two components hydrophobic PTFE and Pt black catalyst or carbon-supported Pt catalyst. The PTFE acts as a binder holding the catalyst together to form a hydrophobic and structured porous matrix catalyst layer. This porous structure can simultaneously provide passages for reacfanf gas fransport to the catalyst surface and for wafer removal from fhe cafalysf layer. In fhe CL, the catalyst acts as both the reaction site and a medium for electron conduction. In the case of carbon-supported Pt catalysts, both carbon support and catalyst can act as electron conductors, but only Pt acts as the reaction site. 

Fairweather et al. [204] developed a microfluidic device and method to measure the capillary pressure as a function of fhe liquid water saturation for porous media wifh heferogeneous wetting properties during liquid and gas intrusions. In addition to being able to produce plots of capillary pressure as a function of liquid wafer safuration, their technique also allowed them to investigate both hydrophilic and hydrophobic pore volumes. This method is still in its early stages because the compression pressure and the temperatures were not controlled however, it can become a potential characterization technique that would permit further understanding of mass transport within the DL. 

The structure of this interface determines fhe sfabilify of PEMs, the state of water, the strength of interactions in the polymer/water/ion system, the vibration modes of side chains, and the mobilities of wafer molecules and protons. The charged polymer side chains contribute elastic ("entropic") and electrostatic terms to the free energy. This complicated inferfacial region thereby largely contributes to differences in performance of membranes wifh different chemical architectures. Indeed, the picture of a "polyelectro-lyfe brush" could be more insighttul than the picture of a well-separated hydrophobic or hydrophilic domain structure in order to rationalize such differences. ... 

As much as the nanophase segregated morphology of Nafion has been a controversial issue in the literature over several decades, the need for understanding the structure and distribution of wafer in PEMs has sfimulafed many efforfs in experimenf and theory. Major classifications of water in PEMs distinguish (1) surface and bulk wafer, (2) nonfreezable, freezable-bound, and free wafep and (3) wafer vapor or liquid water. Anofher fype of wafer offen discussed is that associated with hydrophobic regions. 

For very hydrophobic compounds, when is very high (10 ), fhere is evidence from field studies involving fish, birds, and animals fhaf fhere is a bioaccumulation or bioconcentration up to food chain [114]. The pesticide DDT is an example of bioaccumulating chemical with a Kqw of —10 . The thermodynamic model for air/water and 1-ocfanol/wafer acfivify coefficients and the importance of the activity coefficient at infinite dilution measurements have been presented as well [114]. 

The hydrophilic substrate is moved continuously out of the water subphase at constant film pressure. During the upstroke the monolayer is transferred onto the wafer with the headgroups oriented towards the solid substrate and the alkyl chains exposed to the air. This renders the hydrophilic solid surface with a high surface energy of about 50 mN/m (for silicon) to a hydrophobic surface with a relatively low surface energy in the range of 20-30 mN/m. 

Besides PMMA, compression molding was also used to fabricate microstructures on PC chips (1 mm thick). High temperature (188°C) and pressure (11 metric ton pressure applied by a hydraulic press) were used. Before bonding, the hydrophobic channel surface was treated with UV irradiation (220 nm) to increase surface charge, which would assist aqueous solution transport. The molded chip was thermally bonded to another PC wafer. During use, the bonded chip did not yield to a liquid pressure up to 150 psi (134°C, 4 metric ton, 10 min) [938],... 

Fig. 2 AFM images after adsorption of PMBQ, Adsorption on different substrates, PMBQ in water a mica, b silicon-wafer, hydrophil, c silicon-wafer, hydrophob, PMBQ in 1 molff1 KC1 d mica, e silicon-wafer, hydrophil, f silicon-wafer, hydrophob...

Langmuir films prepared at air/water interface are transferred on solid substrates and LB firms are constructed. The morphologies of the LB films and their molecular arrangements depend on the characters of hydrophilic and hydrophobic moieties of amphiphiles and the affinity of the moieties to the substrates. Transition of molecular arrangement in LB films of third generation amphiphilic PAMAM dendrimers with dodecyl terminal groups was elucidated [77]. The LB films were transferred by horizontal lifting onto hydrophobic silicon wafers and were devoted to their structural characterization. Since silicon wafers are ahead treated (that is, oxidized) by an aqueous solution of... 

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