Electricity: electrical systems and devices – Electric charge generating or conducting means
Patent
1987-12-03
1989-10-31
Hix, L. T.
Electricity: electrical systems and devices
Electric charge generating or conducting means
361235, 361231, H01T 2300, H05F 306
Patent
active
048781495
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a device for generating ions in gas streams for reducing electrostatic charges which, on sensitive products, such as e.g. microchips, films, magnetic plates, laser storage plates and printed circuit boards, in the case of an uncontrolled discharge lead to destruction or increased particle deposition.
2. Background of the Invention
In the manufacture of highly integrated semi-conductor components, with laser and magnetic storage plates and with other products having microstructures in the resolution range of one micrometer and less, both particle contamination and uncontrolled, electrical discharges lead to considerable quality losses. The term microstructures here also covers sensitive plastic films or surfaces in general, in which the deposition of micro particles lead to quality losses. Electro-static charges are the cause of the damage. Such manufacturing processes, for example, take place in clean rooms, whose air is prefiltered to a very high level and flows through the clean room in a low turbulence, piston-like displacement flow. The air flowing into such clean rooms can be filtered to such a high level that virtually no particles pass via, the air flow, into the clean room. The particles produced during manufacture, largely result from the production process itself or are caused by the operating personnel. The device, according to the invention, can also be operated at restrictive work places or stations with specially produced air flow.
The charges are produced by friction, electrostatic induction, or capacitive processes and are unavoidable during the movement of the product, particularly on insulating surfaces. Charge densities can occur, which lead to voltages of several thousand volts. These charged surfaces, by means of electrostatic forces, increasingly attract aerosols, particularly charged aerosols.
In the case of surfaces charged with 500 V, there is approximately a 20X particle deposition compared with a neutral surface. However, such surface charges can be discharged in uncontrolled manner over the microstructures, which can either be destroyed by an electric breakdown or by high current densities. Sensitive metal oxide semiconductor structures on silicon chips can be destroyed by discharges of voltages of around 50 V.
The charging of insulating surfaces on the product and increased particle deposition can be prevented through the air flow containing ions having a positive and negative sign. Thus, charges are compensated both on airborne particles and on the product surfaces. There can be no uncontrolled discharges over the microstructures. Surface discharges are reduced by a controlled discharge over air ions. In the case of electrostatically sensitive products a uniform distribution of positive and negative ions is particularly important.
For generating positive and negative air ions, it is known to use the Townsend gas discharge in the non-uniform electrical field of needle points of wires. A device for generating ions on points is disclosed by U.S. Pat. No. 1,356,211, while DE-OS No. 28 09 054 describes a device for generating ions on wires. In the vicinity of the points or wire surface a discharge zone is formed with an extension of approximately 0.5 mm, in which the gas molecules are ionized. With increasing distance from the discharge zone the speed decreases as a result of the field which is becoming ever weaker. A condition which must be fulfilled for ensuring that the ions can be carried away by the air stream is that their speed is the non-uniform field drops to a value which is lower than the air speed. For igniting a gas discharge on highly curved surfaces a voltage of 6 to 7 kV is necessary. When operating such ionizers with a voltage of approximately 10 kV, the speed of the ions decreases within 50 to 100 cm to a value below 1 m/sec. The standard air flow rate at clean work stations is approximately 0.5 m/sec. It becomes clear from what has been stated hereinbefore that for the distribution of
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Sebald Thomas
Stiehl Hans-Henrich
Basile Andrew R.
Hanlon, Jr. William M.
Hix L. T,.
Rutledge D.
Sorbios Verfahrenstechnische Gerate und GmbH
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