This invention relates to thin film high voltage electrographic writing
heads for recording information on a recording medium and, in particular, to improvements
in the control of the phenomenon known as "flare" or "flaring" that occurs upon
electrode discharge in electrographic writing processes. The invention is especially
concerned with an electrographic writing head for forming discrete electrostatic
charges on a recording medium moved in a plane relative to said head comprising
a substrate, a plurality of spatially disposed electrode lines formed on said substrate,
and a writing nib formed at the end of each of said electrode lines and having
its writing tip lying along an edge of said substrate.
It is known in the electrographic writing head art to employ for electrographic
writing a plurality of spatially disposed conductive electrode lines deposited
on an insulating substrate which terminate in a nib or stylus. The nibs may be
of any metal suitably formed using for example photolithographic and electroforming
techniques such as copper, nickel or tungsten, or may be polysilicon formed on
a silicon or ceramic substrate. Examples of such electrographic writing head structures
are disclosed in US-A-4,356,501 and 4,415,403. These thin film electrographic writing
head structures also have included driving logic and circuitry integrally fabricated
upon the same head substrate, such as the multiplexed driving circuit, high voltage
and low voltage thin film transistors and accompanying address and data bus lines.
Low voltage address lines operate to selectively address nibs or groups of nibs
for discharging by applying a high voltage to the stylus via its connected high
voltage thin film transistor. Such an arrangement is shown in US-A-4,588,997. Pulse
forming circuit arrangements may include a R-C network between a voltage source
and the nibs for the purpose of providing lower address voltages to the nibs and
facilitating the supply of voltage to the nibs to cause a sufficient discharge
for latent image writing. Examples of such networks are shown in US-A-4,030,107,
4,359,753 and 4,466,020.
One of the problems encountered in this technology is that the discharge
from the nibs is not always uniform so that the latent image spots created on the
recording medium are nonuniform in shape and enlarged or irregular in size compared
to other latent image spots. This phenomenon is known in the art as "fare" or "flaring".
Flare is detrimental to the quality of printed or plotted images on the recording
medium because the spot sizes formed on the recording medium on discharge of the
nibs are not uniform and flare out in an irregular pattern. Also, arcing across
nibs to the recording medium further causes such enlargement and destructive disfiguration
of the uniformity of spot size. To prevent flaring from occurring, limiting resistors
have been placed in the driving logic or in the electrode lead lines leading to
the nibs to limit the flow of current to the nibs and prevent such arcing and spot
size irregularity. Examples of resistance that particularly function in this manner
are disclosed in Russian patent publication No. 611,173 and US-A-4,415,403, which
respectively illustrate limiting resistors 3 and 82 in electrode lead lines to
stylus or nib 1 and 88.
However, the problem of flaring still prevails in the art in spite
of the utilization of such limiting resistors. Flaring still occurs and spot sizes,
while being more uniform in size, still remain with ragged edges and nonuniform
The present invention is intended to overcome this problem, and provides
an electrographic writing head of the kind specified which is characterised by
an impedance formed in each of said nibs to reduce the intercoupling capacitance
effect between adjacently disposed nibs to thereby prevent flaring from occurring
on the deposition of charge from said writing tips onto the recording medium.
According to this invention, flaring can be substantially eliminated
or significantly reduced, forming uniform latent image spots by providing resistance
in the nibs per se or in an area in the nibs close to the nib ends. Flaring is
caused by an excessive form of discharge due to the energy stored by the capacitance
that inherently exists between spatially adjacent nibs. Upon discharge of an adjacent
nib, the energy stored in this capacitance is also discharged resulting in an arc
discharge which is uncontrolled by any impedance intended for current limiting
as taught in the prior art. By incorporating impedance at or adjacent to the nib
end, current limiting is imposed upon the inherent capacitance between adjacent
nibs to eliminate or substantially reduce the ability of an associated nib to flare
thereby improve the writing quality of the electrographic writing head.
Another advantage achieved by the use of impedance at the nib is that
in the event of an electrical short circuit between neighboring nibs or an inadvertent
connection of nibs of significantly different potential, the resulting flow of
current therebetween will be sufficiently small so that no damage will occur to
the electrode structure or their driving logic which damage would be catastrophic
preventing further use of the electrographic writing head.
An electrographic writing head in accordance with the invention will
now be described, by way of example, with reference to the accompanying drawings,
- Figure 1 is a schematic illustration of the circuit arrangement known in the
- Figure 2 is a schematic illustration of the circuit arrangement comprising this
- Figure 3 is a perspective view of an embodiment of a nib structure for an electrographic
writing head of this invention.
- Figure 4 is a perspective view of another embodiment of a nib structure for
an electrographic writing head of this invention.
- Figure 5 is a perspective view of still another embodiment of a nib structure
for an electrographic writing head of this invention.
Reference is now made to Figure 1 wherein there is shown a typical
circuit arrangement to the electrode nibs of an electrographic writing head exclusive
of voltage sources and drivers. Only three nibs and their accompanying circuit
arrangement are shown for purposes of simplicity, as there are several hundreds
of nibs across the head. The circuit arrangement for each nib 10 in the head comprises
a pulse forming circuit containing a capacitor 12 and a load resistor or impedance
14 connected together to nib 10. Impedance 14 is required generally due to the
large capacitance value of capacitor 12. The R-C time constant of this arrangement
is selected to provide a sufficiently quick response time and duration to conclude
with a pulse that will provide Paschen voltage breakdown in the gap 15 between the
end of nib 10 and recording medium 18 resulting in a discharge and deposition of
a writing spot on the surface of the recording medium. Limiting resistor 16 is
included for current limiting to prevent arcing resulting in enlargement and nonuniform
alteration of the writing spot. This type of arrangement is generally shown, for
example, in US-A-4,359,753 and 4,415,403, supra.
In spite of the use of limiting resistors 16, the problem of flaring
persists so that it is clear that the use per se of such resistors 16 in the lead
line to the nibs is not sufficient to prevent flaring to a degree that writing
resolution is improved to an acceptable level.
The solution to the problem is by, first, proper isolation and identification
of the source of the problem. Examination of the electrical characteristics of
the writing head electrode geometry indicates that, due to the very close spacing
of the nibs 10, the intercoupling capacitance 20 therebetween is quite large, for
example, on the order of 1 to 5 pf. This capacitance is sufficiently large and
representative of an energy store near the point of electrode or nib discharge
to provide additional energy on nib discharge. Since the capacitance is in line
between nibs 10, the discharge geometry resulting on recording medium 18 will be
materially effected and will have flares extending toward adjacent nibs 10. As a
result, an irregular shaped writing spot will be formed in spite of the presence
of limiting resistor 16.
This flaring can be substantially eliminated by employing an impedance,
such as resistance 22, in nib 10, as illustrated in Figure 2, preferably either
close to the writing end of the nib 10 or at the writing end of nib 10. Resistor
22 represents local impedance at or in proximity to the source of discharge and
charge deposition so that the effect of stored energy in the form of intercoupling
capacitance 20 between nibs is very small and, therefore, is effectively eliminated
and, as a result, is effective in substantially eliminating conventional nib flare.
Experiments have shown that the value of resistor 22 is chosen to be several megohms,
typically between 50-1000 megohms, although this value may be even larger. This
value, however, cannot be made too large as the discharge speed of adjacent nibs
will be effected due to a large RC time constant between adjacent nibs. In other
words, the time response of a nib will be effected by the RC time constant with
neighboring nibs. Also, the interelectrode capacitance between nibs along the full
length of the electrodes up to driving circuit 13 has been measured and found to
be on the order of 0.1 pf.
In Figure 2, driving circuit 13 may comprise any circuitry known in
the art for driving nibs 10 including the capacitor/resistor network in Figure
1. Such known circuits include thin film semiconductor drivers, resistor network,
capacitor network, semiconductor integrated circuit, discrete nib drivers or commutator
The principal concept in reducing the formation of flare is to reduce
the amount of energy stored at nibs 10 due to the existing intercoupling capacitance
20. This capacitance functions as a store of energy that provides the energy to
increase the extent of flaring on discharge. By reducing this energy store, a large
energy dump cannot occur, which would be productive of flaring.
Figures 3-5 relate to particular geometries for inclusion of resistance
22 in nib 24 or close to the end of nib 24. In each of these three enlarged figures,
only the nibs 24 are shown exclusive of their lead lines as patterned on support
25, e.g. a fiberglass substrate. Nibs 24 may be, for example, about 1µm thick and
comprised of a strip of Al on a very thin Cr layer for substrate adhesion. The
Cr layer may be, for example, twenty times thinner than the A1 layer. Resistance
26 may be comprised of n+ amorphous silicon. Resistance 26 may also be an oxide
of Al, Ni or Co. In Figure 3, resistance 26 in each nib 24 is positioned adjacent
to the writing end 28. However, resistance 26 is proportionately very close to
the end of each nib 24. In Figure 4, resistance 26 is positioned at the writing
end of each of the nibs 24. In Figure 5, resistance 26 in each nib 24 is positioned
adjacent to the nib writing end 28A, as in the case of Figure 3. However, in this
embodiment, the nib writing ends 28A are thinner to further reduce the intercoupling
capacitance at this point 20 between adjacent nibs 24. With the combination of
nib resistance 26 and thin sheet writing nibs 28A, the intercoupling capacitance
is substantially eliminated due to reduced cross sectional area of the nib at this
In the preferred embodiment, it has been found that the nib writing
end 28 may be about 1 µm thick and about 0.43 mm long to provide a sufficiently
long wear length. The range of thickness for nib 28 or 28A (or resistance nib 26
in the case of Figure 4) may be about 0.5 µm to 5 µm. In the Figure 5 embodiment,
the thickness of nib writing end 28A may be, for example, 0.5 µm.
The lower limit of nib thickness is governed by catastrophic damage
to the nib end due to disintegration upon application of a high voltage and subsequent
discharge, unless it is possible to reduce the energy delivered to the nib and
still obtain a suitable write discharge. However, there is a limit to how far the
voltage can be reduced and still obtain a suitable write discharge. Further, a
nib that is too thin will not have sufficient mechanical contact with the recording
The upper limit of nib write end thickness is governed by a thickness
that is too large providing too much capacitance and defeating the purposes sought
after in this invention.
In summary, the intercoupling capacitance between writing nibs in
an electrographic head can be effectively eliminated to significantly reduce nib
flaring by placing resistance at the nib writing tip or adjacent to the nib writing
tip. The effectiveness can be further enhanced by reducing the thickness of the
nib writing end.
While the invention has been described in conjunction with a few specific
embodiments, it is evident to those skilled in the art that many alternative, modifications
and variations will be apparent in light of the foregoing description.