The Gas Discharge

Usually, the sputtering process takes place in the presence of a gas discharge. Therefore, in the following the aspects of the gas discharge of importance for the sputtering process will be discussed. 

The most simple case is that of a direct voltage gas discharge. The formation of a gas discharge can best be discussed based on current/voltage curves of the phenomenon. Such a curve is presented in Fig. 74. 

Under certain conditions, an avalanche process can now take place. Ions strike the cathode and release secondary electrons there. These are accelerated in the field of the cathode and due to collisions with the residual gas atoms and molecules produce new ions, which are again accelerated towards the cathode and produce new secondary electrons. If a condition is achieved, where the number of produced electrons is sufficient to produce so many ions that they can again produce the same number of electrons, then the discharge is self-maintaining. 

A uniform current intensity distribution can be achieved by further increasing the applied power, finally causing a rise of current and voltage. This range, the so-called abnormal discharge, is used for cathode sputtering. 

In the case of the use of an uncooled cathode, with a current density of about 0.1A cm 2, an additional thermionic emission of electrons takes place. This results in another avalanche effect and since the output impedance of the supply limits the voltage, a discharge with low voltage and high current density commences. 

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The need for auxiliary heating is another factor that must be carefully evaluated. Due to the nature of the thermodynamic process, the gas discharging from an expander is at a much lower temperature than gas discharging from a regulator station operating within the same pressure bounds. If temperatures downstream of the expander are allowed to drop too low, potential problems may arise, such as hydrate formation and material compatibility. 

Adiabatic Head. The height in ft of gas supported at the compressor discharge as the gas discharges into a system at the desired pressure level is the adiabatic head. The compression of the gas column is adiabatic. The temperature and pressure of the compression column will be related by the adiabatic expression. 

The rotary lobe compressor stage is a rather low-pressure machine. These compressors do not compress gas internally in a fixed sealed volume as in other rotaries. The straight lobe compressor uses two rotors that intermesh as they rotate (see Figure 3-79). The rotors are timed by a set of timing gears. The lobe shapes may be involute or cycloidal in form. The rotors may also have two or three lobes. As the rotors turn and pass the intake port, a volume of gas is trapped and carried between the lobes and the housing of the compressor. When the lobe pushes the gas toward the outlet port, the gas is compressed by the back pressure in the gas discharge line. 

Fig. 16.8 Pictures of the gas discharge corona of (a) an apple fruitlet at T-stage and (b) of a slice of a ripe apple. The digitally recorded picture is described by 23 parameters that undergo statistical analyses with learning tool algorithms or methods...

Weibel, F.P, Sadikov, A. and Bigler, C. (2005). First results with the gas discharge visualisation (GDV) method (Kirlian photography) to assess the inner quality of apples , 1st Scientific FQH Conference, FQH, Research Institute of Organic Agriculture (FiBL), CH-5070 Frick, Switzerland, p. 77. 

In a single stage installation, overall collection efficiency is not influenced by the external terminus of the gas discharge tube, whether a round elbow, Fig. 12(a), a mitered elbow, Fig. 12(b), or a volute housing,... 

Example B. Suppose in the previous example that the loss rate had been specified as not to exceed 40 lbs/hr. To minimize the increase in pressure drop accompanying any increase in inlet velocity necessary to reduce Dth to a value which would bring performance up to the desired level, it might be more expedient to instead increase the gas discharge velocity. 

We say the gas discharges under these conditions, meaning that it can conduct, i.e. an electric current can flow through it. 

A simple interpretation of this choking condition is as follows. The gas flows as a result of the pressure difference P — P2. When the gas speed reaches the speed at which a pressure wave propagates relative to the gas, any pressure wave generated will be unable to travel upstream but will remain stationary relative to the pipe. Thus, if the pressure in the reservoir into which the gas discharges is reduced below Pw, the fact cannot be transmitted upstream and so the flow rate will not change. 

In Gases from Volcanoes."—The nature of the gases discharged from volcanoes has been most carefully studied from about the middle of the last century, with the result that the chemical composition of the gas discharged has been determined at many different volcanoes, and at different times at the same volcano. From these inves%ations it would appear that in the more violent discharges there are very considerable amounts of hydrc en, while in the more placid eruptions there is little gas of any description, except steam, generally accompanied by water containing mineral salts. 

A very high voltage is required to operate the gas discharge tubes. Do not come into contact with the source while viewing the tubes. 

Do not work with the gas discharge tubes yourself. Your teacher will demonstrate how it works. 400... 

Your teacher will set up the gas discharge tube apparatus to demonstrate the emission spectra of hydrogen and other elements. 

A disadvantage may be the existence of many spontaneous lines from the gas discharge in gas lasers, or the pumped light from the flashlamp in solid state lasers which is superimposed upon the laser light. 

The advantage of this method is that the population change of definite levels is selective and controllable which allows the separate collision processes to be determined in spite of the fact that many other perturbing processes occur simultaneously in the gas discharge. 

When filling the flask with carbon dioxide, lower the gas discharge tube of the Kipp gas generator or of the cylinder with carbon dioxide to the bottom of the flask. When the flask is filled with the carbon dioxide (how can this be determined ), slowly (why ) extract the gas discharge tube, close the flask opening with the stopper and... 

Heat the retort with the potassium permanganate. When the air is displaced from the retort and the gas discharge tube, introduce the latter into the bottom opening of the gas meter for draining off the water. The water should flow into the sink. Fill the gas meter with oxygen (up to two-thirds of its volume), extract the gas discharge... 

Reduction of a Metal Oxide wear eye protection ). Assemble the apparatus shown in Fig. 44 and check its tightness. Put 40-50 pieces of granulated zinc into a 250-ml flask. Pour anhydrous calcium chloride into U-shaped tube. Put 0.3 g of copper oxide into the wide part of a refractory tube, and a piece of copper gauze or foil (why ) into the gas-discharge tube out of which hydrogen will flow. Pour 100-150 ml of a 20% sulphuric acid solution into the flask. Again check... 

Effect of the Surface Area on the Reaction Rate. Assemble an apparatus as shown in Fig. 45. Introduce 0.1 g of powdered metallic zinc into a 50-ml reaction flask and pour in 10 ml of a 20% sulphu- ric acid solution. Put the end of the gas-discharge tube under the burette and determine the volume of hydrogen that evolves during two to five minutes. Perform the experiment at a constant temperature, shaking the reaction mixture. Why ... 

Pour about 350 ml of distilled water into a flask with a volume of 0.5-1 litre and saturate it with air by shaking it. Fill the flask of the apparatus and the gas-discharge tube with the water saturated with air. Close the flask, making sure that not a single air bubble remains in the flask and in the tube. Put a test tube filled with water onto the free end of the gas-discharge tube immersed in a bath with water and fasten it in a stand (the test tube and the stand are not shown in... 

Preparation of Chlorine by Oxidizing Hydrochloric Acid with Potassium Permanganate. Liquefaction of the Chlorine. Assemble an apparatus as shown in Fig. 54. Spill 10-15 g of potassium permanganate into flask 1. Pour a 37 % hydrochloric acid solution into dropping funnel 2, a saturated sodium chloride solution into cylinder 5, and a little concentrated sulphuric acid into wash bottle 4. Put the end of the gas-discharge tube of the apparatus into test tube 5 cooled outside by solid carbon dioxide (dry ice) wetted with acetone. What is the boiling point of chlorine ... 

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