Abstract:
An inorganic, top-surface, semiconducting dielectric overcoat, having a selected time constant permits electric field charge and dissipation at a selected rate to facilitate particulate material movement over an underlying electrode grid. The coating may be made from nitrides, oxides or oxy-nitrides of silicon, or amorphous silicon. A planarized, wear resistant, chemically stable surface, and minimized inter-electrode build-up are also provided by the overcoat.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to U.S. patent application Ser. Nos. 09/163,893, 09/164,124, 09/164,250, 09/163,808, 09/163,765, 09/163,839, 09/163,954, 09/163,924, 09/163,904, 09/163,799, 09/163,664, 09/164,104, 09/163,825, issued U.S. Pat. Nos. 5,717,986, 5,893,015, 5,968,674, and 5,853,906, and U.S. patent application Ser. No. 09/128,160, each of the above being incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to the field of overcoat materials, and more specifically relates to overcoat materials functioning as relaxation coatings applied to electrode grids. 
     There are known or proposed systems for electrostatically moving or assisting with the movement of fine particulate materials, such as marking material (e.g., toner) and the like. One such system is described in U.S. patent application Ser. No. 09/163,839. 
     According to the teachings of the aforementioned application Ser. No. 09/163,839, a grid of small and closely spaced electrodes are connected to a driver circuit such that a phased d.c. travelling electrostatic wave is established along the grid. Charged particulate material is transported by the electrostatic wave in a desired direction, at a desired velocity. 
     In such a system, it is desirable to provide a planarized surface over which the particulate material may travel. Such a surface eliminates the problem of particulate material becoming trapped between the electrodes. Furthermore, it is desirable to provide a material over the electrodes to provide rapid charge dissipation at a selected time constant. Arcing between electrodes must be prevented. Wear resistance is also a desired attribute of such a layer. Finally, it is important that such a layer be chemically stable. That is, the layer material must not react with the particulate material nor change characteristics in the presence of the operating environment. However, no known material to date has been able to optimize each of these desired attributes. 
     It is known to encapsulate electronic devices, such as integrated circuits, in protective coatings. Such coatings may provide physical protection from scratches, and a moisture barrier between the devices and the ambient environment. However, such materials are generally not used as top-surface dielectrics. Furthermore, such insulation and passivation layers typically have very high resistivities to avoid possible electrical shorts between covered leads. 
     Accordingly, there is a need in the art for a coating which provides a planarized surface, has a selected time constant, is wear resistant, and is chemically stable. 
     SUMMARY 
     The present invention is a novel coating for application over an electrode grid particle mover. The coating is an inorganic material which may be compatible with silicon processing, such as chemical vapor deposition (CVD) and may be incorporated into the production of silicon-based components such as an electrode grid. 
     The coating is a top-surface (that is, not sandwiched between layers) semiconducting dielectric, having a selected time constant to permit electric field charge and dissipation at a selected rate to permit particulate material movement over an underlying electrode grid. 
     According to one embodiment, the coating is a material selected from the group comprising: a nitride, an oxide, and an oxy-nitride of silicon, and amorphous silicon. The coating may be formed by CVD, plasma assisted CVD (PACVD), or other known processing techniques. 
     The time constant of the coating, as determined by the product of the dielectric constant and the resistivity of the material, is preferably between 0.5-100 microseconds (ms). Within this range of time constant, particulate material may be moved from electrode to electrode, across a grid of electrodes, at a speed about 1 to 2 meters per second (m/s). However, the larger the time constant, the slower the speed of movement of the particulate material across the electrode grid. The bulk resistivity of the coating is preferably between 1×10 9  and 1×10 12  ohm·centimeters (Ω·cm). 
     Thus, the present invention and its various embodiments provide numerous advantages discussed above, as well as additional advantages which will be described in further detail below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained and understood by referring to the following detailed description and the accompanying drawings in which like reference numerals denote like elements as between the various drawings. The drawings, briefly described below, are not to scale. 
     FIG. 1 is a cross-sectional illustration of a grid-type particulate particle mover having an overcoat thereon according to the present invention. 
     FIG. 2 is a cross-sectional illustration of a hybrid device, including both an electrode which is part of a particulate transport electrode grid, and a thin film transistor which may be used for driver, clock, logic or other circuitry. 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, numeric ranges are provided for various aspects of the embodiments described, such as pressures, temperatures, thicknesses, etc. These recited ranges are to be treated as examples only, and are not intended to limit the scope of the claims hereof. In addition, a number of materials are identified as suitable for various facets of the embodiments, such as for marking materials, layer materials, etc. These recited materials are also to be treated as exemplary, and are not intended to limit the scope of the claims hereof. 
     With reference now to FIG. 1, there is shown therein in cross-section one embodiment  10  of a grid of electrodes  14  formed on a substrate  12 . Overlying the grid of electrodes  14  is an inorganic overcoat  16  according to the present invention. Other layers (not shown) may form a part of an embodiment of the type shown in FIG. 1, such as interface layers, electrical interconnection layers, etc. In addition, the geometry of an embodiment may vary from that shown in FIG. 1 (although not shown herein). For example electrodes may be formed to have a different profile, and may be formed in differing locations on the substrate. In any case, a traveling electrostatic wave produced by means not shown herein causes particulate material  18  to travel from electrode to electrode in the direction of arrow A. 
     Electrodes  14  are typically constructed of aluminum, although they may be formed of other materials. A common process for the formation of electrodes  14  is magnetron sputter deposition. Two important criteria for the overcoat of the present invention are that (1) the process used to form it not negatively affect the electrodes or substrate, and (2) that the material from which it is formed not negatively interact with the electrodes or the substrate. Thus, according to one embodiment of the present invention, the overcoat  16  is formed of silicon nitride by a plasma-assisted chemical vapor deposition (PACVD) process. PACVD is a low temperature process, the deposition taking place in the range of 300° C., which is well below the approximately 660° C. melting point of aluminum. 
     The desired resistivity of the silicon nitride film may be obtained by controlling the ratio of silicon to nitrogen. In one embodiment of the present invention, the ratio of silicon to nitrogen may be on the order of between 1.35:1.0 and 1.45:1.0, preferably 1.40:1.0. Other ratios, however, may still provide the time constant sought by the present invention. 
     The ultimate thickness of the overcoat layer will depend on the electrode metal thickness. For 0.6 μm thick metal electrodes, an overcoat layer thickness on the order of 0.5 to 1.0 μm will suffice although planarization may not be fully achieved. A layer thickness up to 4.0 μm or thicker may accomplish planarization and still serve to substantially provide the functions described herein. 
     Importantly, the silicon nitride overcoat will have a resistivity on the order of between 1×10 9  and 1×10 12 Ω−cm, and preferably between 1×10 9 Ω−cm and 1×10 10 Ω−cm, or even between 1×10 9 Ω−cm and 5×10 9 Ω−cm. This is significantly lower than the resistivity of a typical silicon nitride insulation or passivation layer, which would be on the order of 10 14  to 10 16 Ω−cm. The time constant (τ) for the overcoat is related to the resistivity (ρ) and the dielectric constant (ε), as: 
     
       
         τ=ρ·ε 
       
     
     A desired time constant for the proper establishment then dissipation of an electric field for particulate transport at a reasonable speed (1 to 2 m/s) is in the range of 0.5-100 ms. The dielectric constant of silicon nitride is in the range of 6 to 9. Thus, it is required that the resistivity be tailored to achieve the desired time constant. However, time constants up to, for example 1 second, are contemplated hereby, with the consequent reduction in particulate material transport speed. Indeed, a desired transport speed may be obtained by properly selecting the time constant of the layer (i.e., adjusting the resistivity). 
     While silicon nitride provides the desired control of resistivity (and hence the desired time constant), and is also compatible with current processes used to form the electrode grid (and potentially other layers and devices), it also provides scratch resistance, serves as a moisture barrier, and has low adhesion to many particulate materials, especially marking materials in marking device embodiments. However, a class of other materials may serve to function well as overcoat materials, providing some or all of the advantages discussed above. For example, an oxide of silicon, an oxy-nitride of silicon, and even amorphous silicon can provide many if not all of the above-mentioned advantages. 
     Thus, while the present invention has been discussed in terms of one embodiment focussing on silicon nitride, it will be apparent to one skilled in the art that various embodiments of a coating for a particulate marking material transport device have been disclosed herein. Furthermore, while embodiments described and alluded to herein are capable providing an adequate overcoat for devices including electrode grids, such as particulate marking material movers, the present invention is not limited to marking material or particle movement, but may find applicability in a variety of other environments requiring an overcoat. For example, the overcoat may be applied over multiple devices to form on a substrate, such as the electrode grid  20  and thin-film transistor  22 , of the embodiment  24  shown in FIG.  2 . Thus, it should be appreciated that the description herein is merely illustrative, and should not be read to limit the scope of the invention nor the claims hereof.