Air ionization is the most effective method of eliminating static charges on nonconductive materials and isolated conductors. Air ionizers generate large quantities of positive and negative ions in the surrounding atmosphere, which serve as mobile carriers of charge into the air.
As ions move through the air, they are attracted to oppositely charged particles and surfaces. Neutralization of electrostatically charged surfaces can be rapidly achieved through this process.
Air ionization may be performed using electrical ionizers, which generate ions in a process known as corona discharge. Electrical ionizers produce air ions by intensifying an electric field around a sharp point until it overcomes the dielectric strength of the surrounding air. Negative corona occurs when electrons are flowing from the electrode into the surrounding air. Conversely, the flow of electrons from the air molecules into the electrode produces positive corona.
To achieve the maximum possible reduction in static charges from an ionizer of a given output, the unit must produce equal amounts of positive and negative ions. That is, the output of the ionizer must be balanced. If it is out of balance, the isolated conductor and insulators can become charged, rendering the ionizer ineffective.
Controlling the balance or offset voltage of an ionizer in a given environment is becoming increasingly important. Many state-of-the-art semiconductor devices are susceptible to ESD at voltage potentials below 100 volts. Ionizer balance must be correspondingly controlled to avoid product losses and malfunctions.
Pulsed DC Ionizers
Pulsing ionization systems offer good discharge times, the measure of rate of charge neutralization. They are the systems of choice in environments with poor or inadequate airflow such as laminar flow environments in clean rooms, inside semiconductor wafer-handling equipment, and when long distances must be covered such as overhead room ionization systems.
However, most prior pulsing systems did not attempt to limit offset voltage during pulse-mode operation. As a result, pulse times and output levels had to be carefully selected to achieve the desired discharge time without producing excessive offset voltage swing levels.
Offset voltage must be maintained within acceptable limits so device damage does not occur. The sometime objectionable offset voltage swings generated in a pulse-mode system are a result of the positive and negative pulses where only one polarity of ionization is provided.
The resulting stream of ionization creates swings of offset voltages that can be measured on an isolated conductor. To limit the swing, the end-user is forced to adjust the output of the pulse ionization system to a lower level or select a pulse time that achieves the same result. In either case, discharge times become longer, which is an undesirable side effect.
Peak Reduction Technology
Meeting the increasing demands in clean-room environments requires a method of controlling the offset voltage in pulsed systems without negatively affecting the discharge times. The system also must provide performance consistent with the requirements in the International Technology Roadmap for Semiconductors (ITRS) that defines the offset voltage and surface charging goals to 2020.
Peak reduction technology comprises a method of controlling the overlap of the pulsed outputs, both positive and negative, and determining an overlap that satisfies the offset voltage goal without greatly affecting the discharge time.
Evolution of Ionizing Equipment
Ionizing equipment has evolved over the years to provide the maximum benefit in clean-room applications, especially for the semiconductor industry, which prohibits air-assisted ionizers due to the laminar flow environment. The evolution started with AC ionizers, then progressed to steady-state (DC), to pulsed DC, and now to pulsed DC with peak reduction technology.
AC
AC ionizers use an alternating high-voltage supply connected to emitters. All emitters receive both positive and negative voltage, producing positive and negative ions from each pin (Figure 1). Air assist is critical with this arrangement because so much ion-to-ion recombination occurs.
• Discharge Times: At long distances without air assist, discharge times are poor due to substantial ion recombination and because only a portion of the overall cycle generates ions.
• Offset Voltages: The balance typically is very good because an abundance of positive and negative ions is always at the target surface.
Figure 1. Graphical Representation of an AC Ionizer
Steady-State
Steady-state or DC ionizers have separate sets of emitters connected to either positive or negative supplies. The high positive and negative voltages stay on simultaneously and constantly in this configuration (Figure 2).
• Discharge Times: Dramatically better discharge times are realized vs. AC ionizers without air assist. Not as much, but some recombination occurs.
• Offset Voltages: The balance for DC ionizers typically is not as good as AC, but both positive and negative ions do reach the target surface concurrently, which tends to lower the overall voltage offset.
Figure 2. Graphical Representation of a DC Ionizer
Pulsed DC
The pulsed DC ionizer is similar to steady-state with a separate positive and negative supplies, but on/off pulses are applied to the emitters on an alternate basis (Figure 3).
• Discharge Times: The discharge times can be substantially better than steady-state ionizers because less recombination occurs.
• Offset Voltages: Voltage offsets are greater than steady-state. Less positive and negative ions are available concurrently at the target surface. Pulsed DC ionizers with off time exaggerate the peaks by separating +/- ions in time.
Figure 3. Graphical Representation of a Pulsed DC Ionizer
Pulsed DC With Peak Reduction Technology
The pulsed DC with a peak reduction ionizer configuration is similar to pulsed DC, but the opposite-polarity supply stays on past each transition in the bipolar pulsing (Figure 4). As a result, the target surface receives both positive and negative ions concurrently during the transitions, reducing the offset peaks substantially.
• Discharge Times: Depending upon the percentage of pulse overlap applied, discharge times can be very close to standard pulsed DC operation.
• Offset Voltages: Offset voltages are reduced dramatically.
Figure 4. Graphical Representation of a Pulsed DC Ionizer With Peak Reduction
Summary
Air ionization technology has made continual advances to meet the increasing demands of clean-room environments. Most recently, pulsed DC with peak reduction technology has provided an important variable to adjust in pushing the performance envelope. Specifically in the semiconductor industry, peak reduction ionization technology enables compliance with stringent discharge times with only 15-V/cm voltage offsets in most applications, putting the performance already equal to the ITRS Roadmap in the year 2020.
About the Authors
John Gorczyca is director of product development for Simco Ionization for Electronics Manufacture and Simco Industrial Static Control. He has more than seven years of research and design experience in the field of static control and an additional 10 years of research and development work in applications in various fields. Mr. Gorczyca performed graduate research at Carnegie Mellon University and obtained a master's in electrical and computer engineering. 215-997-3415, e-mail: jgorczyca@esimco.com
Roger J. Peirce is director of technical services for Simco Ionization for Electronics Manufacture. Mr. Peirce started his career in 1970 at Bell Labs and co-founded Voyager Technologies in 1983 to design ESD test equipment. Then for 20 years before joining Simco, he was affiliated with ESD Technical Services, a consulting company he founded in 1986. 215-997-3430, e-mail: rpeirce@esimco.com
Brad Williford is global semiconductor OEM accounts manager for Simco's semiconductor ionization products and has more than 10 years of experience in the semiconductor capital equipment and materials markets. Previously, he served as Southeast account manager for Semitool and as Eastern Region technical marketing manager for Asahi Glass Electronic Materials. Mr. Williford received a B.S.M.E and an M.B.A. from Virginia Tech. 919-567-0145, e-mail: bwilliford@esimco.com
Simco Electronics Division, 2257 N. Penn Rd., Hatfield, PA 19440
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