284 BELL SYSTEM TECHNICAL JOURNAL 



leave the cathode as a result of positive ion bombardment, either as a 

 result of single impacts or by cumulative action. This does not seem 

 to hold for all metals and residual gases, and it has been suggested 

 that in some cases sputtering is due to the absorption of radiation pro- 

 duced when the gas ions are stopped at the cathode. 



There has also been a great deal of controversy as to the structure 

 of the sputtered film. Most of the more recent evidence indicates 

 that the films are of a crystalline nature. 



Advantages of Sputtering Over Other Methods 

 OF Metallizing 

 In general, cathode sputtering cannot compete with the more com- 

 mon processes of metallic deposition, but in many special cases it can 

 be employed where other methods are inapplicable. This is especially 

 true (a) when metals are to be deposited upon non-conductors, {b) 

 when the surfaces to be metallized would be injured by contact with 

 chemical solutions or high temperature, (r) when either a very thin 

 continuous metal coat or a very smooth, highly reflecting coat is de- 

 sired, (d) when metals are to be deposited that are very difficult to 

 deposit in any other manner, such as silicon, tellurium, or selenium, 

 (e) when a metal is to be deposited upon another metal far removed 

 from it in the electrochemical series, such as gold or platinum upon 

 aluminum or magnesium. In such cases, electrolytic corrosion is very 

 apt to occur in the presence of moisture or traces of electrolyte. 



Commercial Application 



Cathode sputtering has been used commercially by the Western 

 Electric Company with excellent results in the manufacture of dia- 

 phragms for carbon broadcasting transmitters, illustrated in Fig. 1. 

 This type of microphone consists essentially of a tightly stretched 

 duralumin diaphragm located between two chambers containing gran- 

 ular carbon. The manner in which the double button type of micro- 

 phone is connected to the amplifiers of the broadcasting, public address, 

 or sound picture system is schematically shown in Fig. 2. Those 

 parts of the diaphragm which come in contact with the carbon are 

 covered with gold, a metal which has been found to be ideally suited 

 for microphonic purposes. It is exceedingly important that the gold 

 coating be continuous and remain adherent, for if the carbon should 

 make contact in a number of places with the duralumin, it is quite 

 likely that the microphone would be noisy and therefore unfit for use. 



When the gold spots were formed by the plating process previously 

 used, considerable trouble was experienced because of such defects 



