Model film thickness, spin coating

Currem field characteristics measured wiih conjugated polymers sandwiched between an indium-tin oxide (ITO) anode and an aluminum cathode are usually hole dominated and are, consequently, appropriate for testing injection/lransport models for the case of unipolar current How. Data shown in Figure 12-1 refer to injection-limited currents recorded on typically 100 nm thick spin-coated films of derivatives of poly(y d/"fi-phenylenevinylene) (PPV) and a planarized poly(/ /" -pheny-leue) employing a Keilhley source measure unit. The polymers were ... 

Fig. 8. X-ray reflection diagram of a thin polystyrene film on float glass [160]. The reflectivity R is plotted against the glancing angle . The film is spin coated from solution. A model fit (dashed line) to the reflectivity data is also shown where the following parameters are obtained film thickness = 59.1 0.1 nm, interface roughness glass-polymer = 0.4 0.1 nm, surface roughness polymer-air = 0.6+1 nm, mean polymer density = 1.05 + 0.01 g/cm-3. The X-ray wavelength is 0.154nm...

IR spectra were obtained on a Model 10MX Nicolet Fourier Transform infrared spectrometer. IR films were spin-coated from polymer solutions in chlorobenzene on either KBr discs or silicon wafers polished on both sides. The samples were baked in vacuum at 90 C for at least 1 hour to ensure solvent removal. Film thicknesses were approximately 1 jtm, sufficient to remove interference fringe effects from the spectra. 

Germanium ATR crystals were coated with thin polymer films by spin-coating techniques. A commercially available spin coater (Headway Research, Inc., Model EC-101) with a specially designed Teflon chuck was used to hold the Ge crystal. For example, Biomer--the medical polyurethane supplied by Ethicon (Somerville, NJ)--was applied in three to four coats in a 1.5 percent solution of cyclohexanone to produce stable films with thickness less than the depth of penetration of the IR field. 

Dissolution Measurement. Resist solutions 1n mixtures of Isoamyl acetate/cyclohexanone/methyl Isobutyl ketone (90 5 5 by volume) were filtered through 0.45 pm disc filters, then spin-coated onto silicon wafers at about 2000 rpm. The coated wafers were prebaked 1n a convection oven at 90°C for 1 hour, then stored 1n a desiccator. The basicities of the alkaline solutions were titrated by a standard HC1 solution with a Fisher Accument pH meter, Model 805 MP. The film thickness 1s about 2 pm. Resist dissolution was measured by a He-Ne laser Interferometer 1n a thermostated bath at the desired temperatures (12.131-... 

Thin solid films of polymeric materials used in various microelectronic applications are usually commercially produced the spin coating deposition (SCD) process. This paper reports on a comprehensive theoretical study of the fundamental physical mechanisms of polymer thin film formation onto substrates by the SCD process. A mathematical model was used to predict the film thickness and film thickness uniformity as well as the effects of rheological properties, solvent evaporation, substrate surface topography and planarization phenomena. A theoretical expression is shown to provide a universal dimensionless correlation of dry film thickness data in terms of initial viscosity, angular speed, initial volume dispensed, time and two solvent evaporation parameters. 

Infrared spectra were recorded on a Perkin-Elmer Model 983G double-beam spectrophotometer in the transmission mode using 3500 A thick PBTMSS films spin-coated and processed on polished NaCl plates. Spectral subtraction and absorbance correction to account for the decreased film thickness were used to isolate the silicon oxide absorption band at about... 

Spin coating is a commonly available process in the IC industry and has been characterized and modeled for a variety of polymer solutions (16). The thickness of spin coated PI films depends... 

Summarizing, two conditions must be fulfilled in order to obtain from the simulations a confinement-induced and thickness-dependent distribution of the end-to-end distance for terminal subchains. First, a chain should be in contact with both interfaces. This happens only when the film thickness becomes comparable to the size of the chains and, obviously, explains why the confinement-induced mode does not exist in the bulk. Second, the interactions at the interfaces should be asymmetric One interface should immobilize the polymer chains, while the second one should only reflect them. This asymmetry could be induced by the nonequivalent preparation of the electrodes in the experiment While one interface is prepared by spin-coating, the other one is prepared by evaporation of aluminium on top of the polymer film (see Section II for details). A similar picture of asymmetry was found in studies on thin PS films, with a preparation procedure identical with ours. For thin PS films capped between two aluminum electrodes a three-layer model was proposed, in which, in addition to a middle-layer having bulk properties, a dead (immobilized) layer and a liquid-like layer were assumed to be present at the interfaces. 

The spin coating technique has attracted interest, since it maintains many aspects of technical catalysts prepared by pore volume or incipient wetness impregnation, and simultaneously allows the interpretation and analysis in a similar way as the more well-defined model systems discussed above [30]. Here, a solution of the desired catalyst precursor is dropped onto a wafer covered with an oxide film, which is spun on a rotor to create a liquid layer of uniform thickness in order to mimic traditional wet impregnation preparation of catalysts. Control of the catalyst loading and particle size is to some degree achieved by varying the rotation speed, concentration, and vapor pressure of the solute. Still the method suffers, however, from many of the drawbacks associated with wet-impregnated model catalysts, which imparts detailed mechanistic studies. 

Resist films 0.5 to 1 ym thick were spin-coated on 3-inch diameter silicon wafers (oxide coated). The polymers were applied from 10% solutions in chlorobenzene, 2-ethoxyethanol, or 2-methoxyethyl acetate using a Headway Research Model EC-101D spinner. Prebaked (1 hr, 80°C, vac.) films were exposed to the e-beam and then extracted with about 4 ml of tetrahydrofuran, THF. About 4 to 8 hrs was allowed for dissolving to take place. Molecular weights were estimated using a Waters Model 201 HPLC equipped with 4 y-Styragel columns of nominal sizes 500, 10, ... 

Bornside et al. [138] have developed a model for spin coating in which evaporation has been analyzed in terms of the mass flux (or mass transfer) from the liquid phase into the adjacent gas phase. Such a mass flux is controlled by a convection-diffusion process that depends on the solution concentration that increases as the solvent leaves the liquid phase. The characteristics of the gas phase in the close vicinity may also have a speciflc effect on the evaporation process. A modifled model based on the equations of Meyerhofer and Bornside was used by Chang et al. [123] to predict the fllm thickness of spin-coated polymers. In their model, two equations were used one to predict the wet fllm thickness, h, after spin coating but before drying and another to determine the final fllm thickness (hf). The film thicknesses that they have theoretically predicted agree well with those experimentally determined, especially in solutions of low polymer concentration. 

In trying to answer these questions it is certainly instructive to examine previous fundamental experimental works in which such thin films have been employed. In particular, we have to mention studies on polymer interdiffusion in which thin polymer films were used for testing theoretical concepts of polymer physics [102-106]. Spin-coating was typically used to prepare the films because this process presents an easy way of obtaining smooth film of precisely controllable thickness, even in the nanometer range. This is an essential criterion for investigating, for example, polymer diffusion across an interface between two films. While these experiments successfully supported the model of chain reptation [102-106], they also indicated some deviations that hinted, for example, at the enrichment of chain ends at surfaces [104, 106]. 

More involved models to relate the thickness to the spinning conditions have been developed [36]. Generally, the polymer molecules within spin-coated films are relatively disordered and order has to be induced after deposition via stretch aligning or more commonly via electrical poling at elevated temperature [37]. 

In addition to the anodized aluminium specimens, commercially available glass plates for microscopic studies and thermally oxidized silicon wafers (20 mm x 20 mm pieces with a hole) were also coated for model experiments. These hydrophilic materials were ultrasonically cleaned in ethanol (analytical grade, 10 min). Thick polymer films on glass plates were formed by solvent evaporation of the polymer solution (10%). The wafers were spin-coated (solution of 1% polymer in 2-butanone, 2000 min 30 s) and heated at 120°C for 1 h. Typical thicknesses of these layers were about 50 nm. 

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