Vacuum Metallization of Polymers

For more than 25 years, vacuum metallization of polymer webs with aluminum has been carried out on an industrial scale. Initially used to coat PVC for decorative purposes, the metallization of synthetic materials is today first and foremost used to produce functional coatings [1]. In the course of the past few years, metals other than aluminum have gained increasing importance in the metallization of polymer webs. The development of the metallization process was, and still is, subject to two requirements, namely higher quality and lower cost. Higher quality means ... 

Lipin, Yu.V, Rogachev, A.V, Sydorsky, S.S., Kharitonov, VV (1994), Technology of Vacuum Metallization of Polymer Materials, Belorussian Academy of Engineering and Technology, Gomel, Belarus. 

Vacuum metallizing of polymer films, such as cellulose acetate, butyrate, and Mylar, is performed in essentially the same way. Film rolls are unreeled and rewound during the deposition process to metallize the desired surface. A protective abrasion-resistant coating is then applied to the metallized surface in an automatic coating machine. 

Samples suitable for SEM measurements include most solids which are stable under vacuum (metals, ceramics, polymers, minerals). Samples must be less than 2 cm in diameter. Non-conducting samples are usually coated with a thin layer of carbon or gold in order to prevent electrostatic charging. 

Parylene polymers are not manufactured and sold directly. They are deposited from the vapor phase by a process which in some respects resembles vacuum metalizing. Parylene polymers are formed at a pressure of about 0.1 torr from a reactive dimmer in the gaseous or vapor state. Unlike vacuum metalizing, the deposition is not line of sight, and all sides of an object to be encapsulated are uniformly impinged by the gaseous monomer. Due to the uniqueness of the vapor phase deposition, Parylene polymers can be formed as structurally continuous films from as thin as a fraction of a micrometer to as thick as several mils. 

Commercial spectrometers are usually bakeable, can reach ultrahigh-vacuum pressures of better than 10" Torr, and have fast-entry load-lock systems for inserting samples. The reason for the ultrahigh-vacuum design, which increases cost considerably, is that reactive surfaces, e.g., clean metals, contaminate rapidly in poor vacuum (1 atomic layer in 1 s at 10 Torr). If the purpose of the spectrometer is to always look at as-inserted samples, which are already contaminated, or to examine rather unreactive surfaces (e.g., polymers) vacuum conditions can be relaxed considerably. 

From a reaction engineering viewpoint, semiconductor device fabrication is a sequence of semibatch reactions interspersed with mass transfer steps such as polymer dissolution and physical vapor deposition (e.g., vacuum metallizing and sputtering). Similar sequences are used to manufacture still experimental devices known as NEMS (for nanoelectromechanical systems). 

Metal atoms can be incorporated into polymers using two approaches. For probing new reactions between metal atoms and polymers a small-scale spectroscopic approach, sometimes referred to as the Fluid Matrix Technique (11), is used. The coreactant polymer matrix, containing on the order of 0.5 fll of polymer, is preformed on an optical surface. In the case of viscous fluids such as 2 the material is painted on the substrate and held at temperatures ranging typically from 200 to 270 K. The temperature is chosen to maintain low volatility but retain mobility. Under high vacuum [10 6 torr]... 

For detailed characterization and extensive studies of reactivity, multi-gram quantities are still needed and large-scale metal vapor synthetic routes are necessary. The equipment required for this is well-documented (4) and so will not be described in detail here. The principles are those of the Fluid Matrix Technique except that in order to accommodate 10-100 gram of polymer, the coreactant is contained within a rotating flask which serves to provide a continuously renewed film as metal atoms are produced under high vacuum. 

Si(Li) detectors without Be windows ("windowless") or with thin metal-coated polymer films (Ultra-Thin Window UTW) have become an important peripheral to modern-day AEMs for the qualitative detection of elements with 5vacuum requirements because the removal of the Be window increases the probability of detector contamination (from the specimen or column environment) and consequent degradation of performance [12]. Windowless and UTW Si(Li) detectors are commonly installed with additional airlock mechanisms and only on instruments with acceptable levels of vacuum cleanliness. Thus, design constraints on modern AEMs preclude placement of the UTW detector close to the sample. In addition, loss of detection efficiency at low energies (light-element K-lines with the L-lines of transition metals all conspire to limit windowless or UTW EDS analysis of minerals to a qualitative basis only. 

Metallizers with external unwind and rewind facilities have also become known. Here the web is transported into and out of the vacuum chamber through locks. Because of demanding requirements of instruments and apparatus, these coaters have not gained acceptance for coating of polymer webs. They are successfully used in semi-continuous processes, for example glass coating. 

Preform. A metal screen is made in the shape of the final product. Glass fiber is chopped 2 in. long and sprayed uniformly all over the shaped screen, using vacuum on the back side of the screen to assist the process. A small amount of binder, typically 5 percent of polymer in latex form, is sprayed onto the fiber to hold its shape. It is then removed from the screen, placed in the mold, saturated with an equal weight of liquid resin, and the mold is pressed at 1380 kPa (200 psi) and heated until cured, typically 3-15 min. This early process has been largely replaced by SMC. 

The metal-on-polymer interface has been the most studied Interface as metals can conveniently be deposited by evaporation in situ 1n a controllable fashion in a UHV system (26-33). In the case of polyimide, Cu and Cr have been the most studied metals but other metals including N1, Co, Al, Au, Ag, Ge, Ce, Cs, and Si have been studied. The best experimental arrangement includes a UHV system with a load lock Introduction chamber, a preparation chamber with evaporators, heating capabilities, etc., and a separate analysis chamber. All the chambers are separated by gate valves and the samples are transferred between chambers under vacuum. Alternative metal deposition sources such as organometall1c chemical vapor deposition are promising and such techniques possibly can lead to different interface formation than obtained by metal evaporation(34). 

The evolution and decomposition of metal clusters in the polysiloxanes has been quantified (49), and a diffusion-plus-reaction model for cluster growth at the surface and in the near subsurface region of a polymer film has been developed (SO). Collectively, the studies show that organometallic chemistry at the polymer/vacuum interface can have profound effects on both the dynamics of polymer chains at the surface and the evolution of low nuclearity clusters (SO, 51). 

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