Marking Array Having Improved Corrosion Resistance

This invention relates to improvements in the manner of driving the marking array of an ionographic marking apparatus for extending the lifetime of its modulation electrodes.

In commonly assigned U.S. Pat. No. 4,584,592 issued on Apr. 22, 1986 in the names of Hsing C. Tuan and Malcolm J. Thompson entitled, "Marking Head For Fluid Jet Assisted Ion Projection Imaging Systems", there is disclosed a marking array for use in conjunction with the marking head of an ion projection printer of the type disclosed in commonly assigned U.S. Pat. No. 4,463,363 issued on July 31, 1984 in the names of Robert W. Gundlach and Richard L. Bergen, entitled, "Fluid Jet Assisted Ion Projection Printing". In that printer, an imaging charge is placed upon a moving receptor sheet, such a paper, by means of a linear array of closely spaced minute air streams. Charged particles, comprising ions of a single polarity (preferably positive), are generated in an ionization chamber of the marking head by a high voltage corona discharge and are then transported to and through the exit region of the marking head, where they are electrically controlled, at each image pixel point, by an electrical potential applied to a modulating electrode. Selective control of the modulating electrodes in the array will enable spots of charge and absence of charge to be recorded on the receptor sheet for subsequent development.

A large area marking head for a page-width line printer would typically measure about 8.5 inches wide. A high resolution marking array capable of printing 200 to 400 spots per inch would, therefore, include about 1700 to 3400 conductive metallic modulation electrodes. The entire array measuring on the order of 8.5 inches by 0.7 inches also would include a multiplexed addressing assembly comprising metallic address lines and data lines and amorphous silicon thin film active switching elements. All of these elements would be fabricated upon a single low cost substrate, such as glass.

During the operation of such an ionographic printer there is an outflow of corrosive agents from the ionization chamber. These agents have a propensity to attack the exposed modulation electrodes very rapidly, thereby lowering the operational lifetime of the marking array. Heretofore, the modulation electrodes have been fabricated of inexpensive electrically conductive materials which are compatible with standard thin film deposition techniques and which may be also used for conductive lines and for contacts with the active devices. Typically, this material has been aluminum. It has been observed that aluminum modulation electrodes oxidize rapidly, resulting first in changed electrical characteristics since the aluminum oxide is insulating and not conductive, and finally in catastrophic electrical and mechanical failure as the electrodes are fully converted to the brittle insulating oxide which flakes off the substrate. An inert material, such as gold, has yielded extremely corrosion resistant electrodes but its cost and non-compatibility with the marking head fabrication process has negated its practical use. In a copending patent application, filed concurrently herewith, entitled "Modulation Electrodes Having Improved Corrosion Resistance" in the names of Nicholas K. Sheridon and Henry Sang Jr. (identified by Attorney Docket No. D/87083), there is disclosed a marking array comprising modulation electrodes fabricated of an aluminum/copper alloy having improved corrosion resistance in the ionographic environment.

Therefore, it is the primary object of this invention to protect the modulation electrodes from the corrosive ionographic effluents by imposing a constant low level bias voltage on them relative to the marking head.

The present invention may be carried out, in one form, by providing a marking array in an ionographic marking apparatus wherein a low level voltage is established across the modulation region for inhibiting deterioration of the modulation electrodes.

Other objects and further features and advantages of this invention will be apparent from the following, more particular, description considered together with the accompanying drawings, wherein:

FIG. 1 is a partial cross-sectional elevation view showing the marking head of a fluid jet assisted ion projection printing apparatus,

FIG. 2 is a schematic representation of the marking array used in the FIG. 1 device,

FIG. 3 is a transconductance curve, and

FIG. 4 is an illustration of a test arrangement used to observe the application of bias applied to the modulation electrodes and its effect on corrosion of the electrodes.

With particular reference to the drawings, there is illustrated in FIG. 1 a schematic representation of the marking head 10 of a fluid jet assisted ionographic printing apparatus. Although a more representative embodiment of the present state of the marking head is described in commonly assigned U.S. Pat. No. 4,644,373 issued on Feb. 17, 1987, in the names of Nicholas K. Sheridon and Gerhard K. Sander, and entitled "Fluid Assisted Ion Projection Printing Head", the following description is based on the schematic FIG. 1 form.

Within the housing 10 is an ion generation region including an electrically conductive chamber 12, a corona wire 14 extending substantially coaxially in the chamber, a high potential source 16, on the order of several thousand volts DC, applied to the wire 14, and a reference potential source 18, such as ground, connected to the wall of chamber 12. The corona discharge around the wire creates a source of ions, of a given polarity (preferably positive), which are attracted to the grounded chamber wall and fill the chamber with a space charge.

An axially extending inlet channel 20 delivers pressurized transport fluid (preferably air) into the chamber 12 from a suitable source, schematically illustrated by the tube 22. An axially extending outlet channel 24 conducts the transport fluid from the corona chamber 12 to the exterior of the housing 10, past an ion modulation region 26. As the transport fluid passes through and exits the chamber 12, through outlet channel 24, it entrains a number of ions and moves them into the ion modulation region 26, past ion modulation electrodes 28, on the marking array 29.

Ions allowed to pass completely through and out of the housing 10, through the outlet channel 24, come under the influence of accelerating back electrode 30 which is connected to a high potential source 32, on the order of several thousand volts DC, of a sign opposite to that of the corona source 16. A charge receptor 34 moves over the back electrode 30 and collects the ions upon its surface. Subsequently the latent image charge pattern may be made visible by suitable development apparatus (not shown). Alternatively, a transfer system may be employed, wherein the charge pattern is applied to an insulating intermediate material, such as the dielectric surface of a conductive drum or belt. In such a case, the latent image charge pattern may be made visible by development upon the drum or belt surface and subsequently transferred to an image receptor sheet.

Once the ions have been swept into the outlet channel 24 by the transport fluid, it becomes necessary to render the ion-laden fluid stream intelligible. This is accomplished in the modulation region by individually switching the modulation electrodes 28, between a low voltage source 36 (on the order of ten to twenty volts DC) and a reference potential 37 (which may be ground) by means of a switch 38. The modulation electrode 28 and the grounded opposite wall 40, which bridge the gap across the outlet channel, comprise a capacitor, across which the low voltage potential of source 36, may be applied, when connected through switch 38. Thus, an electric field, extending in a direction transverse to the direction of the transport fluid flow, is selectively established between a given modulation electrode 28 and the grounded opposite wall 40.

"Writing" of a selected spot is accomplished by connecting a modulation electrode to the reference potential source 37, held at about 0 volts, so that the ion "beam", passing between the electrode and its opposite wall, will not be under the influence of a field therebetween and transport fluid exiting from the ion projector, in that "beam" zone, will carry the "writing" ions to accumulate on the desired spot of the image receptor sheet. Conversely, no "writing" will be effected when the modulation electric field is applied to an electrode. This is accomplished by closing switch 38 and applying the low voltage potential of source 36, on the order of about 10 to 20 volts, to the electrode 28 in order to impose upon the electrode a charge of the same sign as the ionic species. The ion "beam" will be repelled and driven into contact with the opposite, electrically grounded, conductive wall 40 where the ions recombine into uncharged, or neutral air molecules. Thus, an image-wise pattern of information is formed by selectively controlling each of the modulation electrodes on the marking array so that the ion "beams" associated therewith either exit or are inhibited from exiting the housing, as desired.

To record high quality pictorial information it is not sufficient to "write" in a binary manner (ON or OFF, black or white) and "writing" with a grey scale is desired. Referring to the transconductance curve of FIG. 3 it can be seen that there is a bell-shaped profile to the relationship between the modulation voltage and the ion output current. At very small and very large modulation voltages, the peak and the tail of the curve, the ion current will be ON and OFF, respectively, over a larger latitude of modulation voltage levels, owing to the relative flatness at these regions of the curve. In the steeply curved portion, variations in the modulation voltage will have a greater effect on the ion output current. It is in this section of the curve that multiple levels of grey are "written". Application of different potential values to the modulation electrodes enables control of the ion output in proportion to applied potential. Therefore, it should be recognized that grey scale printing is dependent upon accurate control of the voltage applied to each electrode, for each desired value. However, it has been observed that when an oxide layer builds up on the modulation electrodes, ions passing through the modulation region will tend to accumulate thereon. Since the accumulated bias does not dissipate rapidly, it will have an adverse effect on accurate control of the actual bias applied to the electrodes because the actual charge will be the sum of the applied charge (desired) and the accumulated charge (residual). Looking at the transconductance curve of FIG. 3, if it is desired to "write" a grey level A with a given electrode, and that electrode had previously been "writing" black or a darker level of grey, at which more ions flowed through the modulation region, some ion accumulation will result and the grey level A' will be "written".

The marking array 29 comprises a large area substrate 42 (represented by the dotted outline in FIG. 2) along one edge of which are formed an array of modulation electrodes (E) 28, a multiplexed data entry or loading circuit, comprising a small number of address bus lines (A) 44 and data bus lines (D) 46, and thin film switching elements 33, on for each electrode. With this array it is possible to directly address each electrode with only the small number of wire bonds needed to interface the electrodes with the external driver circuits 54 and 46.

For simplicity and economy of fabrication over the large area, full page-width head, thin film techniques are used. The switches 38 are preferably amorphous silicon transistors (a-Si:H TFTs), although other materials such as polycrystalline Si, laser annealed Si, CdS, Te, or ZnO may be used. As shown, each modulation electrode 28 is connected to the drain electrode 48 of the thin film transistor by a conductive trace, an address bus line 44 is connected to the gate electrode 50, and a data bus line 46 is connected to the source electrode 52. The low temperature a-Si:H fabrication process allows a large degree of freedom in the choice of substrate materials, enabling the use of inexpensive amorphous materials such as glass, ceramics and possibly some printed circuit board materials. Preferably, the substrate is glass and the modulating electrodes, the address and data buses are aluminum. Aluminum is the material of choice because it is compatible with the a-Si:H processing and makes good contacts with the source, drain and gate electrodes of the a-Si:H TFTs.

However, the aluminum modulation electrodes have been found to oxidize rapidly when used in the ionographic process because they are subjected to the corrosive effluents from the corona chamber 12. Since the other aluminum elements are protected and are not contacted by the effluents they are unaffected. It is the purpose of this invention to retain the above-stated benefits of aluminum as the marking array material of choice while extending the lifetime of the modulation electrodes in the corrosive atmosphere.

We have found that in addition to the ions created by the corona discharge within the chamber 12, there is also ozone and numerous oxides of nitrogen (N.sub.2 O, NO.sub.2, NO) as well as the excited states of these gases which are far more corrosive than their non-activated states. In higher humidity conditions, where water is available, acids of nitrogen are also present. It is likely that the corrosive action is caused by the combined action of the ions and the gases. For example, it is believed that the gas molecules (i.e. ozone and nitrous oxide) initially blanket the surface of the electrodes, but it is not until the ions, moving in the air stream, collide with the surface and displace electrons from the metal that the surface is susceptible to react with the gases blanketed thereon. Then the elctrode surface is rapidly oxidized. We have observed that in about 100 hours the highly corrosive atmosphere completely oxidizes the 1 to 2 micron thick aluminum electrodes. In that inordinately short time the aluminum electrodes embrittle and flake off of the substrate due to the stresses created by the expansion of the aluminum oxide.

Short of the catastrophic electrical failure brought about by the complete oxidation of the modulation electrodes we have also observed a fall-off in grey scale control as oxidation progresses. This phenomenon occurs as an insulating layer of oxide is built up on the electrodes. The insulating layer accumulates charge thereon, so that the net effect of the switching potential imposed on the electrodes is lessened and the accurate control needed for multiple levels of grey is subverted.

Our invention relates to applying a small bias potential, on the order of 1 to 2 volts across the ion modulation region 26 so as to drive the ions away from the modulation electrodes 28 and toward the opposite wall 40. This will prevent collision of the ions against the surface of the electrodes which action abets rapid oxidation, as explained above. It should be understood that the bias potential need be sufficiently small so as not to interfere with the switching action required for "writing" control, whether binary or grey scale. It may be as high as about 10% of the OFF voltage without significant adverse consequences.

The curve of FIG. 3 plots modulation voltage against ion output current for an ideal cause (i.e. it is symmetric about 0 volts). It can be seen that the maximum ion output current is available when 0 volts is applied to the modulation electrodes, or stated another way, when there is no relative bias across the ion modulation region 26. Ion output current is pinched off at a relative bias of about 15 to 20 volts. The area of interest relative to the oxidation prevention bias is the upper portion of the curve where there is very little drop off in ion current for small changes in modulation voltage. Thus, a 10% (about 1.5 to 2.0 volts) bias will be seen to have very little effect on ion output current. It is important, relative to grey scale "writing" control, that the oxidation prevention bias should not intrude into the steeply sloped section of the transconductance curve, but even for binary "writing", diminished ion output in this section will not produce the desired crisp black print quality.

The oxidation prevention potential may be applied in a number of different ways. It may be applied directly to each of the modulation electrodes by selecting the reference potential source 37 to be in the range of 0.75 to 2.5 volts above the bias applied to the opposite wall 40. Alternatively, the reference potential source 18, connected to the head 10 may be negatively biased relative to the bias applied to the modulation electrodes. Relative bias between the control elements across the ion modulation region also can be achieved by virtue of differences in materials electrically connected together.

If the head is made of a first material (e.g., stainless steel) and the modulation electrodes are made of a second material (e.g., aluminum) and these elements are connected to one another by means of a common ground, a potential difference will exist between the two materials. This will be manifested by a bias across the ion modulation region which is straddled by these disparate materials. The potential difference is equal to the difference in work functions of the materials. For example, the stainless steel head may have a work function on the order of 4 electron-volts and the aluminum electrodes may have a work function on the order of 3 electron-volts. Thus, an inherent bias of 1 volt is present across the ion modulation region 26 by virtue of the different materials being connected to a common ground.

In FIG. 4 there is shown a test device marking array 29' whose modulation electrodes 28 have been grouped in segments and shorted together so that it is possible to impose the same bias on all of the electrodes in each segment. Then the test device was run in an ionographic recording apparatus and after about 100 hours operation at room temperature and at room RH the corrosive effects were studied. It can be seen that different biases were imposed on the nine segments, with the exception that the bias imposed on the end segments 1 and 9 was repeated on segments 3 and 7, respectively, because the end segments had to be discounted due to non-uniform ion flow at these locations in the test apparatus. The following lists the bias imposed on each segment and the observations made regarding the condition of the modulation electrodes in each segment: (1) -1.5 volts, end effects; (2) ground, typical array portion, blistering and transparent material; (3) -1.5 volts, severe blistering; (4) -0.75 volts, about the same as segment 2; (5 ) +0.75 volts, decrease in oxide noticed; (6) +1.5 volts, oxide diminishes and vanishes; (7) +3.0 volts, no oxide; (8) +2.25 volts, no oxide; (9) +3.0 volts, end effects.

From the above test and other related observations, we have concluded that a small benefit is initially realized at about +0.75 volts and that no significant further advantage is realized at a bias greater than about +2.5 volts. Any greater bias will adversely interfere with the "writing" characteristics of the array as can be seen in FIG. 3. Although we have discussed our invention primarily in terms of the test arrangement wherein the selected corrosion inhibiting bias is applied to the modulation electrodes. It is important to bear in mind, however, that it is the relative bias across the ion modulation region 26 that affects the rate of oxidation.

We have provided an improvement in the modulation structure of the marking head of an ionographic marking aparatus for increasing the effective lifetime of the modulation electrodes in the highly corrosive atmosphere of such a device. Additionally, by inhibiting the rate of oxidation of the electrodes, more accurate control of the potential applied to them may be achieved over a longer period of time, thus improving grey scale control.