Abstract:
Air ionization apparatus includes a plurality of air ionizing electrodes connected to a source of high ionizing voltages, and includes a bias source connected to supply bias voltage to a reference electrode positioned near the air ionizing electrodes to alter the field gradients thereabout to selectively enhance production of positive or negative air ions in response to levels and polarity of bias voltage supplied to the reference electrode.

Description:
FIELD OF THE INVENTION 
     This invention relates to electrical circuits for supplying positive and negative air ions, and more particularly to embodiments of air ionizers that operate on alternating current (AC) and include direct current (DC) biasing for promoting substantially zero residual electrostatic charges on target objects. 
     BACKGROUND OF THE INVENTION 
     Air ionizing apparatus that produces both positive and negative air ions can be used to reduce electrostatic charges on various objects such as semiconductor wafers and die during fabrication processes. However, reducing the level of electrostatic charges to the grounded level can be difficult because negative ions are more readily produced and transported through air from an ion generator to the object than positive ions. 
     Conventional AC air ionizers differ from DC or pulse-type ionizers because all emitter points exhibit the same electrostatic field gradient on applied AC voltage at the same time. There are thus no bipolar potentials on spaced emitter points at any given time as with DC air ionizers, so charge neutralization by AC air ionizers over the area of an object tends to be more uniform. However, the swings in voltages attributable to residual charges on surfaces of objects tend to fluctuate with the frequency at which the AC ion generator produces air ions. Controlling high ionizing voltages, for example, via feedback circuitry to diminish the fluctuations, is generally difficult so lower voltages are used and a reference electrode is disposed adjacent each emitter point to develop the necessary electric field gradient sufficient to produce corona. Certain known AC ionizers apply opposite polarities of the AC voltages to one or more pairs of space emitter points to diminish the AC voltage swings on the target object. Other known AC ionizers rely upon such waveform controls as amplitude or pulse-width or phase modulations to achieve ion balance and reduce voltage variations on the target object. 
     SUMMARY OF THE INVENTION 
     In accordance with the illustrated embodiments of the present invention, a reference electrode receives a DC bias voltage as an offsetting potential to alter the mix of positive and negative generated ions. A negative bias voltage is generally required for an isolated system, and for a positive grounded system, but the bias voltage level (and polarity) may have to vary in response to such operating conditions as the ion-generating characteristics of emitter points, and the like, in order to achieve near ground or reference level charge neutralization of a target object. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an AC ionizer with capacitive isolation of emitter points that are positioned near a control electrode; 
     FIG. 2 is a schematic diagram of an AC ionizer with capacitive isolation of emitter points that are positioned behind a control screen; 
     FIG. 3 is a schematic diagram of an air ionizer operable on AC or DC including a source of high ionizing voltage and grouped pairs of emitter points that are connected to respective floating terminals of the source, and that are positioned near a control electrode; 
     FIG. 4 is a schematic diagram of an air ionizer operable on AC or DC including a source of high ionizing voltage and grouped pairs of emitter points that are connected to respective floating terminals of the source, and that are positioned behind a control screen; 
     FIGS. 5 and 6 are embodiments of circuits for developing positive and negative DC bias voltages from AC supplies; and 
     FIGS. 7 and 8 are embodiments of circuits for sensing corona via connection to the control electrodes or screens of the circuit embodiments illustrated in FIGS.  1 - 4 . 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring now to the schematic circuit diagram of FIG. 1, there is shown an array of a plurality of emitter points  9  that are capacitively coupled via capacitors  11  to one terminal of an AC supply  13 . A reference electrode  15  such as a bar or rings, or the like, is disposed in close proximity to the emitter points  9 , and is connected through a bias source  17  to another terminal of the AC supply  13 . The reference electrode  15  promotes high electric field gradients about the emitter points  9  to enhance production of air ions. All emitter points  9  are subjected to the same AC voltage at all times, so no bipolar effect is evident over the area of a target object  10 , and the emitter points  9  remain isolated from ground return via the capacitors  11 . The bias supply  17  may include an adjustable supply of DC bias  19  and resistive coupling  21  to the reference electrode  15  for varying the voltage thereon over a range of about ±150 volts. 
     The DC bias  19  is typically set to provide negative DC bias voltage on the reference electrode  15  to enhance production of positive air ions as a result of the asymmetrical field gradients developed around the emitter points  9  relative to the DC bias over each cycle of the AC supply  13 . A corona detector, as later described herein, is connected  23  to the reference electrode  15  to detect proper level and polarity of DC bias source  17  sufficient to produce corona and associated production of ions. 
     Production of air ions from an AC source significantly reduces bipolar effects of ions impinging upon the area of a target object  10 , but tends to cause swings in the electrostatic potential of the target object  10  at the frequency of the AC source. High frequency sources may be used to attenuate the magnitude of swings in the electrostatic potential of the target object attributable to the time constants and associated lag times of such electrostatic potential being able to change as rapidly as the high frequency of an AC source. However, high frequency high voltage AC sources are more expensive and commonly suffer from recombination of positive and negative ions produced in rapid succession about the emitter points, and, therefore, become ineffective by and about 1-2 KHz. Accordingly, lower frequency, low voltage AC sources are favored by powering an AC air ionizer with the aid of a reference electrode  15  positioned in close proximity to the emitter points  9 . 
     Referring now to the schematic circuit diagram of FIG. 2, there is shown an array of a plurality of emitter points  9  disposed behind a conductive screen  16  as a reference electrode in a circuit otherwise similar to the circuit that is illustrated and described above with reference to FIG.  1 . The screen  16  serves as an isopotential plane which terminates the field gradients about the emitter points  9  and thereby significantly inhibits voltage swings from occurring on the target object  10 . 
     Referring now to the schematic circuit diagram of FIG. 3, there is shown an array of a plurality of emitter points  9  connected in pairs per phase of the ionizing voltage source  14  (AC or DC). Each of the pairs of emitter points  9  is connected to the respective terminal of the source  14  through resistors  18  that limit the current that can flow. The pairs of emitter points per phase (or terminal of opposite polarity) promotes production of ions of both polarities at the same time to significantly diminish the swings of electrostatic potential on the target object  10  under conditions of AC excitation  14 . In this embodiment, the ion-generating circuitry  9 ,  14 ,  18  ‘floats’ relative to a reference level (e.g., has no current return path to ground), and a grounded DC bias source  17  is connected to reference electrode  21  that is positioned closely about the emitter point  9  as a bar or rings, or the like, to enhance the potential gradients about the emitter points  9  suitable for generating air ions from a low voltage source  14 . The DC bias source  17  connected to the reference electrode  21  is variable in amplitude (and polarity) over a range of about ±150 volts to enhance production of positive ions, for reasons as previously described herein. The reference electrode  21  is also connected to a corona detector  23 , as later described herein. 
     Referring now to FIG. 4, there is shown an array of a plurality of emitter points  9  disposed behind a conductive screen or grid  25  that serves as a reference electrode and that is connected to a variable grounded source  17  of DC bias voltage. The emitter points  9  are connected in phased pairs via resistors  18  to a ‘floating’ source (AC or DC)  14  of high ionizing voltage. The potential applied to screen  25  thus alters the symmetry of field gradients per half cycle of ionizing voltages from the AC source  14  to promote greater production of positive air ions in the manner as previously described herein. Reference electrode  25  forms an isopotential plane that terminates the field gradients about the emitter points  9  and significantly diminishes variations in the electrostatic potential on the target object  10  attributable to the AC source  14 . 
     Referring now to the schematic circuit diagram of FIG. 5, there is shown one embodiment of a DC bias circuit  17  for operation on applied AC signal to produce DC output bias voltage that is variable over a range of amplitudes and polarities of about ±150 volts (on applied AC of about 120 volts). Specifically, diode  27  is connected to conduct during positive half cycles of the applied AC voltage to charge up capacitor  29 , and diode  33  is connected to conduct during the positive half cycles to charge up capacitor  31 . During negative half cycles of the applied AC voltage, diode  35  conducts to transfer charge between capacitors  31  and  37  to produce positive and negative voltages across capacitors  29  and  37 , respectively, relative to a reference conductor  41 . A variable level and polarity of voltage at output  43  may be derived through potentiometer  39  connected between the capacitors  29  and  37  for biasing the reference electrodes  15 ,  16 ,  23 ,  25  in the illustrated embodiments, as previously described herein with reference to FIGS. 1-4. 
     Referring now to FIG. 6, there is shown another embodiment of a DC biasing circuit for operation on applied AC signal. Each of the diodes  45 ,  47  is connected in conduction phase opposition to the other diode to charge (and discharge) capacitor  49  during alternate half cycles of the applied AC voltage in proportions determined by the setting of potentiometer  51  which therefore determines the level and polarity of DC bias voltage available at output  53  for application to the reference electrodes  15 ,  16 ,  23 ,  25  in the illustrated embodiments, as previously described herein with reference to FIGS. 1-4. 
     Referring now to FIG. 7, there is shown a schematic circuit diagram of a corona detector for connection to the reference electrodes  15 ,  16 ,  23 ,  25  in the illustrated embodiments as previously described herein with reference to FIGS. 1-4. Specifically, the input terminal  23  couples to a series resonant circuit of capacitor  55  and inductor  57 , the common terminal of which is connected to the base of transistor  59  that is connected as an emitter follower. The resonant circuit may be tuned to a dominant frequency component of noise that is attributable to corona discharge, as sensed by the reference electrode  15 ,  16 ,  23 ,  25 . Transistor  59  exhibits asymmetrical conduction on half cycles of the base signal (that includes a high level resonance component), with resultant charging of the capacitor  61  connected at the output  63 . An indicator such as a Light Emitting Diode (LED) or other utilization circuit (not shown) may be connected to output  63  to provide alarm indication of corona activity in the operating conditions associated with the characteristics of the emitter points  9 , the setting of bias source  17 , and the like. 
     Referring now to FIG. 8, there is shown a schematic circuit diagram of another embodiment of a corona detector in which a first emitter-follower transistor  65  is directly coupled to a second emitter-follower transistor  67 . The first emitter-follower transistor  65  receives base signal at the common connection of the resonant circuit including capacitor  55  and inductor  57 , and exhibits asymmetrical conduction characteristics on alternate half cycles of the base signal, with resultant charging of capacitor  61  connected to the emitter of transistor  65 . The voltage across capacitor  61  is applied to the base of the second emitter-follower transistor  67  which provides a signal on output  63  suitable for energizing an indicator such as an LED or other utilization circuit connected thereto. Such output signal is representative of corona activity in the operating conditions associated with the characteristics of the emitter points  9 . Diode  69  is connected in conduction opposition across the emitter and collector of the first transistor to limit excessive signal levels from destroying one or both transistors  65 ,  67 . 
     Typical values of circuit components are C 55 =100 pFd; L 57 =100 μH; C 61 =1.0 μFd; R 62 =10 KΩ for operation at selected resonant frequency of about 1.6 MHz. 
     Therefore, the circuitry of the present invention promotes more nearly balanced delivery of positive and negative air ions to a target object in response to separate biasing of a reference electrode positioned in proximity to ion-generating emitter electrodes.