Vacuum release technique

Metallization layers are generally deposited either by CVD or by physical vapor deposition methods such as evaporation (qv) or sputtering. In recent years sputter deposition has become the predominant technique for aluminum metallization. Energetic ions are used to bombard a target such as soHd aluminum to release atoms that subsequentiy condense on the desired substrate surface. The quaUty of the deposited layers depends on the cleanliness and efficiency of the vacuum systems used in the process. The mass deposited per unit area can be calculated using the cosine law of deposition ... 

The sample preparation vacuum line (often called spray-on line ) should allow for (a) controlled mixing of the host gas with the substrate (or with other components that are added to the matrix) by manometric techniques and (b) the controlled release of the gas (mixture) toward the inlet system of the cryostat. These conditions are met by a vacuum line that incoporates a storage bulb for the gas (mixture), inlets for attachment of evacuable containers that allow degassing of the substrate prior to its mixing with the host gas, pressirre gauges that cover suitable ranges, a needle valve that allows the controlled release of the gas, possibly via a flowmeter, and interfaces to the bottles that contain the host gas(es), and to the inlet system that is attached to the vacuum shroud of the cryostat. 

When the pump-and-fill technique is used, a more complex manifold for the distribution of inert gas and vacuum is generally employed, in conjunction with a liquid nitrogen-cooled trap, mechanical vacuum pump, and pressure release bubblers (Fig. 1.2). 

Fig. 7.60 Using the inside-out technique, the piece in question is filled with helium and then placed in a covering of some type that can be evacuated. Any helium that is released within this covering will be detected by the helium leak detector. Note that the tested piece can have either end lifted up so that all surfaces are exposed to the vacuum. From Introduction to Helium Mass Spectrometer Leak Detection, Figs. 3.3 and 3.4, by Varian Associates, Inc. 1980, reproduced with permission.

Techniques of tritium removal from co-deposited layers in next-generation tokamaks, such as ITER, have an important impact on machine operation. Attempts are being made to develop in-situ co-deposit removal techniques that would not overly constrain machine operation, both in terms of T removal and plasma performance recovery after cleanup. In addition to machine operation considerations, the tritium in the co-deposited layers will also have safety implications. During a severe accident, the vacuum vessel of an operating tokamak can be breached. If a significant inventory of tritium in the form of a saturated layer is present, much of this tritium can be released as tritium oxide as the film reacts with oxygen. 

Many silicon hydridesare volatile and reactions are often performed in sealed glass tubes under vacuum. Experience of vacuum techniques is necessary and great caution must be exercised in handling tubes, which can develop high pressures of products. Many of the reactions described involve toxic metal carbonyls and may release CO a well ventilated hood is essential. 

Vacuum-activated release of dissolved gases—In this system, the oily suspension is satnrated with air at atmospheric pressnre. Then the air is released from solntion when the snspension is subjected to vacuum conditions in a floatation nnit. The available amount of air is a limitation of this technique. 

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