Gas-purged ionizers and methods of achieving static...

Details Gas-purged ionizers and methods of achieving static... Gas-purged ionizers and methods of achieving static...

2001-10-30

2003-10-21

Jackson, Stephen W. (Department: 2836)

Electricity: electrical systems and devices

Discharging or preventing accumulation of electric charge

By charged gas irradiation

C307S091000, C361S212000, C361S225000, C361S226000, C361S229000

Reexamination Certificate

active

06636411

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to electrical ionizers that produce stable charge-carrier production in gases with varying concentrations of electron attaching components. More particularly, the invention relates to ionizers suited for production test environments of semiconductor devices and component handlers and other environments that might be rendered inert by nitrogen and noble gases.
BACKGROUND ART
In semiconductor component testing, the temperature is normally closely controlled at selected values in the range from −60° C. and +160° C. Cooling is accomplished by the introduction of liquid nitrogen and its cold vapors into a test chamber at ambient pressure. Nitrogen gas which evolves from evaporative cooling is not electron attaching and, as a result, has a profound effect on electrical ionizers both in stability and generation of EMI/RFI.
A primary object of this invention is to produce a balanced amount of negative and positive charge carriers for charge neutralization by injecting a small amount of an electron attaching gas into an electrical ionizer to restore and stabilize negative ion production.
Charge imbalances in semiconductor testing equipment are known to result in electrical discharges that will damage the devices and components being tested.
Accordingly, another object of this invention is to eliminate such charge imbalances in environments where conventional electrical ionizers fail or are difficult to control.
Conventional electrical, x-ray, ultraviolet, and nuclear (radioactive) static eliminators have been used in this application. It has been found that conventional electrical static eliminators were unreliable and those based on ionizing radiation are difficult to control and unacceptable in some markets by their hazardous nature or burdened by licensing requirements.
Ionizers for static eliminators provide positive and negative charges having the mobility needed to be drawn to static (stationary or fixed) electrical charges on surfaces or charged floating conductors. The production of charge carriers is critical to static elimination. Ionization can be achieved by means of ionizing radiation (primarily radioactive, x-ray, and ultraviolet sources) and electrical corona.
The primary processes in ion production are ionization itself and electron attachment. In the ionization process, electrons are separated from a neutral atom or molecule. This action produces positive ions and free electrons.
In a positive corona the ionization process takes place near an electrode region with a positive polarity (a deficiency of electrons). The free electrons that are produced in the ionization process are drawn to this corona electrode (either as free electrons or attached as negative ions). The positive ions have relatively low mobility when compared to the electrons. The positive ions become available for static elimination by providing a gaseous ion current of charge carriers. They also stabilize the ionization process by providing a buffering electric field in the corona region. This stability is aside from the many underlying corona fluctuations and phenomena known for such corona.
Ionization proceeds by similar methods with negative corona. However, the free electrons drift away from the corona electrode at high speed—free electrons typically have a mobility 100-1000 times those of ions. The positive ions that are produced in negative polarity corona are drawn to the nearby negatively charged corona electrode. In order for the corona to be stabilized and for negative ions to be made available for neutralization, the free electrons must attach to neutral atoms or molecules to form negative ions. It is considered known in the prior art that unless only negative charge is to be neutralized, the successful operation of an electrical static eliminator requires an ionizable gas and one that is electron attaching.
High purity nitrogen and the noble gases are not electron attaching. The present invention offers methods to achieve balance in gases with compositions that are dominated by electron non-attaching components, and in chambers with uncontrolled variability of mixtures of electron attaching and non-attaching gases. The present invention is not limited to the case where nitrogen is evaporated to cool component handlers, but is best used in environments where electrical corona is affected by electron non-attaching gases, i.e., gases with large differences in positive and negative carrier mobilities.
The present invention provides low-cost static neutralization in gases where the mobility of corona generated positive and negative carrier species differ greatly or change over time. The stability is achieved by the injection of a small quantity of electron attaching gas, such as air, oxygen, or carbon dioxide, in close proximity to the corona electrode.
The use of air for purging an ionizer has been contemplated for other purposes. R. Mueller, et al. (U.S. Pat. No. 3,111,605) describes a directly coupled static bar with needles centered in orifices. The casing of the static bar is pressurized with air that escapes through the annular spaces around the needles. The bar is intended for use in hazardous areas, where the air is used to keep ignitable vapors away from the ionizing electrodes. Similar inventions were patented by others (see, e.g., Can. Pat. No. 856,917 and W. Ger. Pat. No. 885,450). The Canadian patent was preceded by prior art attempts including extended range (blower-like) applications and blow-off of particles. Others have also considered such externally purged designs for hazardous area use.
One prior effort involves the introduction of a static bar with hollow emitters for extended range use. Another effort used nitrogen-purged ionizing nozzles in a rinser/dryer to control static charge on semiconductor wafers (U.S. Pat. No. 4,132,567). This effort was not successful as a result of instabilities developed in the ionizer when used in nitrogen environments.
U.S. Pat. No. 5,116,583 discloses an air-purged emitter for controlling particle generation in clean rooms. Moisture in air is known to form particulate contaminants when exposed to corona discharge. In the '583 patent, nitrogen, argon, and helium are identified as purge gases. The primary use of the invention in the '583 patent is with dc ionizers. The '583 patent does not recognize the role of electron non-attaching gases in ionizer design and a method of gas injection to achieve ion balance.
U.S. Pat. No. 5,550,703 discloses a particle-free ionization bar with high and low pressure plenums to distribute gases to the emitters. The velocity of gases was then matched to maintain uniform flow with that of superficial flow within the clean room. A need for balanced ionization was identified, but provisions were not incorporated into the device disclosed in the '703 patent to achieve this goal; in other words, no mention is made for the special requirements to achieve balance in electron non-attaching purge gases. Finally, U.S. Pat. No. 5,847,917 describes the use of high velocity gases around emitters to render them contaminant free.
Ionizers based upon electrical corona are basically of three types: direct-coupled alternating current (ac), capacitively-coupled alternating current (ac), and direct current (dc). The dc ionizers can be operated with continuous or pulsed high voltage on the corona electrodes. Ionizers of the ac variety are desirable because the same emitters are used for both positive-and negative-polarity ion generation; thereby, a size reduction is achieved. Also, both polarity carriers are produced at the same distance from the object to be neutralized, and at the same point in space yielding better mixing of ions with the gas stream. Direct current ionizers offer greater control in ion generation and typically have separate positive and negative corona emitters. The present invention is operable for both ac and dc ionizers.
The instability of alternating-current (ac) corona in nitrogen environments provides the most direct evi

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