Vacuum thermal evaporation

Thermal vacuum evaporation. This method is used for evaporation and the subsequent deposition of various metals. Rather volatile metals such as Ag, Au, Cu, and Pd can be evaporated from heated containers. Evaporation of less volatile metals, in particular, Ti or Mo, occurs by electrical heating of metal filaments or bands [32]. In certain conditions chemical active gases, such as oxygen, sulfur vapors, and others, introduced in evaporation zone react with metal atoms giving semiconductor compounds (for example, oxides, sulfides). 

The heterojunction devices were fabricated by masking the surface, then using a 10 % HNO3 aqueous solution to etch down through 2x2 mm windows to remove ZnO and leave regions of the AlGaN layer exposed. Ohmic contacts to the n-ZnO and / -Alo i2Gao 88N were made by thermal vacuum evaporation of Al and Ni, respectively. ... 

The existing OLED fabrication procedures fall into two major categories (1) thermal vacuum evaporation of the organic layers in small molecular OLEDs, and (2) wet coating techniques of the polymer layers in PLEDs. 

One of the most salient advantages of thermal vacuum evaporation is that it enables fabrication of multilayer devices in which the thickness of each layer can be controled easily, in contrast to spin coating (see below). In addition, 2-dimensional combinatorial arrays of OLEDs, in which two parameters (e.g., the thickness or composition of two of the layers) may be varied systematically across the array, can be relatively easily fabricated in a single deposition procedure.50,12 This combinatorial fabrication greatly enhances the efficiency of systematic device fabrication aimed at optimizing the various parameters. 

Vacuum Deposition. Vacuum deposition, sometimes called vacuum evaporation, is a PVD process in which the material is thermally vaporized from a source and reaches the substrate without coUision with gas molecules in the space between the source and substrate (1 3). The trajectory of the vaporized material is therefore line-of-sight. Typically, vacuum deposition takes place in the pressure range of 10 10 Pa (10 10 torr), depending on the level of contamination that can be tolerated in the resulting deposited film. Figure 3 depicts a simple vacuum deposition chamber using a resistively heated filament vaporization source. 

Photochromic silver—copper haUde films were produced by vacuum evaporation and deposition of a mixture of the components onto a sUicate glass substrate (13). The molar ratio of the components was approximately 9 1 (Ag Cu) and film thicknesses were in the range of 0.45—2.05 p.m. Coloration rate upon uv exposure was high but thermal fade rates were very slow when compared with standard silver haUde glass photochromic systems. 

In Section 13.2, we introduce the materials used in OLEDs. The most obvious classification of the organic materials used in OLEDs is small molecule versus polymer. This distinction relates more to the processing methods used than to the basic principles of operation of the final device. Small molecule materials are typically coated by thermal evaporation in vacuum, whereas polymers are usually spin-coated from solution. Vacuum evaporation lends itself to easy coaling of successive layers. With solution processing, one must consider the compatibility of each layer with the solvents used for coating subsequent layers. Increasingly, multilayered polymer devices arc being described in the literature and, naturally, hybrid devices with layers of both polymer and small molecule have been made. 

Evaporation can be performed directly from reactors or kettles provided that substances are thermally stable. Such evaporation is time consuming because of the low heat-transfer surface area per unit volume. In the case of temperature sensitive materials, the residence time in the evaporator must be short and the temperature should be as low as possible. Consequently, continuous vacuum evaporators with a short residence time should be used to treat such materials. Falling-film (thin-film) evaporators are suitable to perform such operations. A typical falling-film evaporators is shown in Fig. 7.2-14. Centrifugal evaporators are also commonly used. 

Vacuum evaporators are also used to recover plating chemicals. They are closed systems that use steam heat to evaporate water under a vacuum. This results in lower boiling temperature, with a reduction in thermal degradation of the solution. Like atmospheric evaporators, they require low maintenance and are self-operating. A climbing file evaporator is an example of a vacuum evaporator. 

During vacuum evaporation at 60° C of ethyl acetate from a solution of the oxime, a violent explosion occurred. The oxime was found to be thermally unstable above 60°C, and at the m.p., 90°C, an exotherm of over 100°C occurred accompanied by rapid gas evolution. 

Thomas, R. N., Univ. Safety Assoc. Safety News, 1981, 15, 16-17 Failure of the product (0.5 g) to crystallise out from the aqueous DMF reaction liquor led to vacuum evaporation of the solution at 60-70°C. Dining evaporation the mixture exploded violently, shattering the fume cupboard sash of toughened glass. The product may well be thermally unstable, but reaction of DMF with excess warm perchloric acid, possibly in near-absence of water, may also have been involved. 

The method of thermal vacuum deposition has been used to obtain tin and silicon deposit on TEG surface. The main difficulty at metal component deposition on graphite support is to obtain uniform metal coating on the surface of disperse particles. The system of uninterrupted mixing of TEG powder during the material evaporation has been created. Its principle... 

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