Abstract:
A print head suitable for use in an image forming system is provided having a pair of electrode layers separated by an isolating structure that includes a semiconductor. The presence of the semiconductor, such as a semiconductor layer, extends the life of the print head by reducing degradation of the print head.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to image forming systems, and specifically relates to charged particle emitting print heads utilized in electron beam imaging printing.  
         BACKGROUND OF THE INVENTION  
         [0002]    In an image forming system, such as ionography, or electron beam imaging (EBI), a latent electrostatic image is formed on an imaging dielectric surface by directing beams of charged particles onto the surface. The latent electrostatic image thus formed may then be developed by applying toner particles to the imaging surface that are attracted to those areas of the imaging surface where the electrostatic latent image resides. The toner particles on the imaging surface are then transferred to a receiving member (such as paper) before the imaging surface is cleaned in preparation for a new imaging cycle.  
           [0003]    The source of the beams of charged particles in the image forming system is a print head. Referring to FIG. 1A, a typical print head  10  includes three layers that have electrodes. A first layer includes a plurality of RF-line electrodes  16  separated from a second layer of finger electrodes  12  by a dielectric layer  14 . A third layer is a screen electrode  18  isolated from the finger electrodes by a spacer layer  20 . The surface of both the RF-line electrodes  16  and the finger electrodes  12  can be smoothed by a smoothing dielectric  11 . In thin film structures, the smoothing dielectric is usually SOG (spin on glass). The finger electrodes  12  have finger openings  13 , typically circular, which are generally aligned with the apertures  22  in the screen electrode  18 , as shown in FIG. 1A. The RF line electrodes  16  intersect the finger electrodes  12  where the finger openings  13  are located. If a high voltage is applied to the finger electrodes  12  and the RF-line electrodes  16 , an electrical breakdown of air inside the finger openings  13  occurs.  
           [0004]    Referring to FIG. 1B, a cross-section of a single charge production site of the print head  10  is shown. The electrical breakdown causes formation of gaseous plasma full of charged ions and electrons. While the polarity of particles used for imaging is determined by the polarity of the screen electrode  18  potential with respect to a grounded imaging member  24 , on/off switching of charge emission from the print head  10  is regulated by a potential difference between the screen electrode  18  and the finger electrodes  12 .  
           [0005]    The dielectric layer  14  is typically formed from stoichiometric compounds, such as silicon oxide, silicon nitride, silicon oxy-nitride, aluminum oxide, titanium oxide, boron nitride, etc., or their combination. Electrical conductivity of such materials is very low, about 10 −14  S/cm or less at room temperature.  
           [0006]    A disadvantage of conventional print heads, and especially print heads designed for high density printing, is that the dielectric layer is subject to degradation. In particular, with repeated printing cycles, the plasma generated in the finger openings  13  degrades the dielectric layer.  
           [0007]    Referring to FIG. 2, evidence of the dielectric degradation is shown. Underneath the finger electrode with a circular opening, there can be seen a dielectric layer, which in this particular case is aluminum oxide. The dielectric layer has been subjected to electrical discharges for a time equivalent to printing about 150,000 pages. Significant erosion of the dielectric material can be seen in the amount of dielectric by-products formed in the area around the opening. Such deterioration leads to charge generation reduction and therefore to print quality degradation. Ultimately, such degradation can lead to a full dielectric breakdown of the print head.  
         SUMMARY OF THE INVENTION  
         [0008]    For the aforementioned reasons, there exists in the art a need for an electron beam imaging print head less susceptible to degradation arising from plasma generation.  
           [0009]    The present invention provides a print head for an image forming system that is resistant to erosion. The print head comprises RF-line and finger electrodes separated by an isolating structure containing a dielectric and a semiconductor or resistive material. For example, the isolating structure may include a dielectric coated with a layer of semiconducting material. Typically, the semiconductor utilized in the present invention has a conductivity between about 10 −6  and about 10 −3  S/cm. The semiconductor can be made of a solid solution of a gas in a metal or semiconductor, where the gas includes a hydrogen gas, a nitrogen gas, an oxygen gas, and a halogen gas, or their mixtures. The semiconductor may also include solid solutions of non-metals in a metal, where the non-metals include boron and/or carbon.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1A shows a conventional charge emitting print head with three layers of electrodes separated by dielectric materials.  
         [0011]    [0011]FIG. 1B is a schematic cross-section of a single charge emitting site of the print head illustrated in FIG. 1A.  
         [0012]    [0012]FIG. 2 illustrates the degradation of a dielectric separating finger electrodes from RF-electrodes.  
         [0013]    [0013]FIG. 3 shows a schematic cross-section of a single charge-emitting site of the present invention.  
         [0014]    [0014]FIG. 4 illustrates the reduction of print head degradation as a result of applying the principles of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    Referring to FIG. 3, there is shown a single charge-emitting site of a print head of the present invention. The print head comprises multiple electrode layers. The layers comprise a first electrode layer  34  that includes RF-line electrodes  36 , and a second electrode layer  30  that includes finger electrodes  32  with openings  45 . The first electrode layer  34  and the second electrode layer  30  are separated by an isolating structure  38  that is electrically insulating. The isolating structure  38  includes a dielectric and a semiconductor or a resistive material.  
         [0016]    The isolating structure  38  can include a dielectric layer  38   a  having a conductivity lower than about 10 −14  S/cm. The isolating structure further includes a semiconductor layer  38   b  having a thickness of about 2 micrometers, and an electrical conductivity of between about 10 −6  and about 10 −3  S/cm. Examples of semiconductors that may be used according to the teachings of the present invention include solid solutions of gases, such as hydrogen, nitrogen, oxygen, and halogens, and non-metals, such as carbon and boron, in metals and semiconductors. A distinguishing feature of the materials used in the present invention is a relatively low concentration of dissolved elements as compared with those for stoichiometric compounds.  
         [0017]    The print head further includes a screen electrode  44  with apertures  46  separated from the second electrode layer  30  by a spacer layer  40 . The charges emitted from the print head help form a latent image on an imaging member  50  utilized for forming images on a substrate, such as paper.  
         [0018]    The use of a semiconductor in the isolating structure  38  helps to decrease the degradation of the print head. In operation of the print head, a high frequency voltage is applied to the RF-line electrodes  36  resulting in plasma generation inside the finger openings  45 . Without a semiconductor in the isolating structure  38 , during a half-period of the applied voltage, particles of one polarity bombard the central part of the surface of the dielectric layer  38   a , charging the surface to a voltage almost equal to the voltage of the finger electrodes  32 . Around the charged area, a strong fringing electric field arises causing a local increase of the kinetic energy of opposite polarity particles bombarding the dielectric surface during the next half-period. Such a bombardment causes sputtering of the dielectric layer. Liberated atoms may chemically react with reactive ions and finally create by-products as shown in FIG. 2.  
         [0019]    Including a semiconductor in the isolating structure  38 , however, according to the teachings of the present invention, helps to reduce these by-products. For example, coating the dielectric layer  38   a  with the semiconductor layer  38   b  allows for charge migration inside the upper part of the isolating structure  38 . As the surface of the dielectric layer  38   a  is negatively charged, some electrons migrate toward the surface of the dielectric layer  38   a , as well as laterally. These partially mobile electrons effectively screen the electrical fringing fields and therefore reduce the energy of the impinging positive ions during the next half-period of the applied voltage. To prevent print head degradation, the electrical conductivity of the semiconductor in the isolating structure  38  may be judiciously chosen to accommodate the frequency of the applied voltage and the dimensions of the print head.  
         [0020]    [0020]FIG. 4 illustrates the significant reduction in degradation that occurs if a semiconductor layer  38   b  is used to cover the top side of the dielectric layer  38   a . The opening  45  and surrounding structure shown in FIG. 4 has been subjected to electrical discharges and resulting air plasma for a time equivalent to printing about 340,000 pages. This time is more than twice the time that the dielectric surface of FIG. 2 has been exposed to electrical discharges. In contrast to the opening filled with by-products shown in FIG. 2, where the isolation structure is only a single dielectric layer  14 , the opening  45  in FIG. 4 shows minimal degradation, and is almost free of by-products despite the longer exposure to air plasma. The reduction of the degradation in the print heads of the present invention, having an isolating structure  38  that includes a semiconductor, significantly extends the life of the print head.  
         [0021]    While various aspects of the invention have been set forth by the drawings and the specification, it is to be understood that the foregoing detailed description is for illustration only and that various changes in parts, as well as the substitution of equivalent constituents for those shown and described, may be made without departing from the spirit and scope of the invention as set forth in the appended claims.