Refractory metals etching

The refractory metals for which CVD is commonly used to produce free-standing shapes are tungsten, niobium, rhenium, tantalum, molybdenum, and nickelb lb lb l (see Ch. 6). Shapes presently produced include rods, tubes, crucibles, manifolds, ordnance items, nozzles, and thrust chambers. They are usually deposited on a disposable mandrel of copper, molybdenum, or graphite which is subsequently machined off or removed chemically by etching. 

After aluminum, the refractory metals and their silicides have been the subject of the most extensive efforts in metal etching (150-155). Because the fluorides and chlorides of the transition metals and silicon are volatile in the presence of ion bombardment, etch studies have been performed with nearly... 

A needle source consists of a hairpin filament (M80 pm diameter), usuafly of a refractory metal such as tungsten, with a short length of smaller diameter (M25 pm) wire spot-welded to it, Figure lb. The tip of the latter wire, the emitter, is electrochemically etched to a point with a radius of curvature at the apex of 2-5 pm the etching technique for tungsten has been described in detail by others (7,29). As quickly as possible after the assembly is thermally cleaned under vacuum (n<10 " Pa), the emitter is dipped into a molten pool of liquid metal and then withdrawn. If done correctly, the junction formed by the bend in the filament and the emitter wire will hold a small bead of metal, and the emitter will appear shiny from the thin film of metal on its surface. 

A problem encountered when aluminium is used as gate metal for a MIS structure is the significant etching of the aluminium by the bromine solution which is used to form vias or contacts in overlaying ZnS insulator films. To solve this problem, it is proposed in EP-A-0407062 to use refractory metals as the metallization layers of an infrared detector. 

Nitric acid may also be used to treat surfaces of nickel and chrome alloys, in metal etching and in treatment of refractory metals such as zirconium. 

Phosgene was found to be a suitable reagent for the plasma etching of the semiconductor GaAs, but it reacted more slowly than dichlorine [1910], It has also been used, mixed with CO, as an etchant gas for refractory metals and, particularly, their silicides [701a],... 

Plasma can also be effectively applied for etching layers of silicon doped with refractory or rare metals, semiconductors, or via interconnects made with rare or refractory metals. Some of those etching reactions are illustrated in Table 8-4. These processes normally use fluorine-containing gases as a feedstock and have hexafluorides as volatile etching products. However, chlorine is preferred in some specifle cases. Only the most common plasma etching systems are discussed here much more detail on the subject can be found, particularly in the reviews of Flamm (1989) and Orlikovsky (2000). 

Table 8-4. General Chemieal Characteristics of Plasma Etching of Refractory Metals and Some Other Compounds...

Table 4.30. Selected etching and descaling procedures for refractory metals ...

Makrides et al (1967) patented a H2-extraction membrane where refractory metals, such as Ta, Nb and V, were coated with Pd to facilitate H2 ingress and egress and to prevent oxidation of the refractory metal surfaces. They started with commercially available foils. After being etched electrolytically in hydrofluoric acid and washed with acetone, the wet foils were placed in a vacuum chamber where they were further dried by evacuation. After... 

Iron carbide (3 1), Fe C mol wt 179.56 carbon 6.69 wt % density 7.64 g/cm mp 1650°C is obtained from high carbon iron melts as a dark gray air-sensitive powder by anodic isolation with hydrochloric acid. In the microstmcture of steels, cementite appears in the form of etch-resistant grain borders, needles, or lamellae. Fe C powder cannot be sintered with binder metals to produce cemented carbides because Fe C reacts with the binder phase. The hard components in alloy steels, such as chromium steels, are double carbides of the formulas (Cr,Fe)23Cg, (Fe,Cr)2C3, or (Fe,Cr)3C2, that derive from the binary chromium carbides, and can also contain tungsten or molybdenum. These double carbides are related to Tj-carbides, ternary compounds of the general formula M M C where M = iron metal M = refractory transition metal. 

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