Text: Sputtering is not a new concept; the phenomenon was first described in 1852 by Sir William Robert Grove, who referred to the process as "cathode disintegration." The first reported commercial application of sputtering did not take place until 1928. Western Electric Company used cathode disintegration for the manufacture of phonograph records and contacts for microphone transmitters. The process has been refined considerably during the past decade, and its use has been extended to include deposition of dielectric materials. It is widely used in modern industrial production processes for the deposition of dielectric thin films used in microcircuitry. These thin film techniques are rapidly replacing the less controllable vacuum deposition processes used previously. Application of the sputtering process to the manufacture of strain gage pressure transducers was pioneered by CEC in 1977. This major technological advance still offers outstanding performance advantages over all other methods of strain gage construction to date. Sputter deposition takes place in a vacuum. During sputtering or cathode disintegration, molecules of the gage and insulating material are ejected from an electrode held at a negative potential by the impact of positive gas ions bombarding the surface. The ejected molecules strike the target area with kinetic energy several orders of magnitude greater than any other deposition method. The high energy impact of the molecules creates the superior adherence associated with the sputtering process. To obtain the necessary ionization, a gas discharge is maintained between the anode and cathode (target). An inert gas (such as argon) is continuously introduced into the vacuum chamber to provide the discharge. The pressure in the chamber is maintained in the range of 10 to 1,000 microns. The spacing between anode and cathode is typically 2 to 3 inches, with a potential between them in the range of 1,000 to 10,000 volts. In the equilibrium state, an electron is emitted by the cathode and is accelerated toward the anode. The electron, by collision with the argon molecules, produces positive ions that strike the cathode and eject another electron. The gas ions are accelerated with enough energy that bombardment of the cathode actually physically displaces molecules of material. The cathode is composed of the material determined to be ideal for the application. Sputtering permits a virtually unlimited choice of gage and substrate materials. These molecules are accelerated toward the anode and impinge upon it with the force of several thousand electron volts, as opposed to simply condensing on the surface. The basic sputtering process described is called diode sputtering. It can be used for sputtering conductive materials, but cannot be used for dielectrics due to the buildup of a surface charge on the cathode which stops the sputtering process. In order to sputter dielectric materials such as the insulating layer in a thin-film strain gage, RF sputtering is employed. To produce a sputter deposited strain gage pressure transducer, the metallic surface of each strain diaphragm first must be highly polished. An extreme polish is required since the dielectric substrate layer is deposited as a thin film less than one-half thousandth of an inch thick. Any surface defect would penetrate the thin layer and cause a short of the strain gage elements. Following the mechanical polish, the pressure diaphragms are arranged in the sputtering chamber and the system is evacuated. The remaining steps of the manufacturing process take place in a vacuum environment, and thus avoid exposure of the sensor elements to contaminants. The diaphragm surfaces are further cleaned by sputtering-etching a small amount of metal from each active face using a reversed potential. This prepares the surface for the thin insulating layer to be applied by RF sputtering. Next, the dielectric insulating layer is deposited as a thin film over the entire diaphragm surface. No mechanical masking is needed. After the dielectric insulation has been applied, the first cathode is moved aside and the gage cathode (material source) is positioned. Both sources are present in the chamber throughout the procedure, to accomplish deposition of both layers without exposing the sensors to ambient conditions. The excellence of performance of a sputtered gage pressure transducer is primarily its stability with time over a broad temperature range. The gage attachment method eliminates epoxy, solder, or other interface bonding materials; the strain gages exactly follow the strain produced in the pressure diaphragm, precisely reproducing pressure fluctuations. All gage creep, that otherwise might reduce accuracy and stability, is completely eliminated.

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