Grid electrodes for a display device

Abstract: A magnetic matrix display device has a grid electrode in which differing first grid (G1) and second grid (G2) apertures are used. A sensor element and a second grid electrode are combined as a single structure. Other aspects of the invention relate to the use of a polyimide coating for the grid electrodes, the use of a compliant mounting and adjustment for a grid assembly and the provision of an illuminated border on a magnetic matrix display.

Description: The present invention relates to a magnetic matrix display device and more particularly to grid electrodes for use in such a display. Yet more particularly, the present invention relates to the use of differing first grid (G1) and second grid (G2) apertures in such a display and to a combined sensor element and second grid electrode. A magnetic matrix display of the present invention is particularly although not exclusively useful in flat panel display applications such as television receivers and visual display units for computers, especially although not exclusively portable computers, personal organisers, communications equipment, and the like. Conventional flat panel displays, such as liquid crystal display panels and field emission displays, are complicated to manufacture because they each involve a relatively high level of semiconductor fabrication, delicate materials, and high tolerances. GB Patent Application 2304981 discloses a magnetic matrix display having a cathode for emitting electrons, a permanent magnet with a two dimensional array of channels extending between opposite poles of the magnet, the direction of magnetisation being from the surface facing the cathode to the opposing surface. The magnet generates, in each channel, a magnetic field for forming electrons from the cathode means into an electron beam. The display also has a screen for receiving an electron beam from each channel. The screen has a phosphor coating facing the side of the magnet remote from the cathode, the phosphor coating comprising a plurality of pixels each corresponding to a different channel. In a colour magnetic matrix display, each of the corresponding phosphor pixels may be a group of phosphor elements, each group corresponding to a different channel and each group typically comprising a Red, a Green and a Blue phosphor element. There are first and second deflection anodes for sequentially addressing electron beams emerging from the channels to different ones of the phosphor elements thereby to produce a colour image on the screen. The first and second deflection anodes are arranged as a pair of combs. There are grid electrodes disposed between the cathode and the magnet for controlling the flow of electrons from the cathode into each channel. These control grids comprise a first group of parallel control grid conductors (first grid) extending across the magnet surface in a column direction and a second group of parallel control grid conductors (second grid) extending across the magnet surface in a row direction so that each of the channels is situated at the intersection of a different combination of a row grid conductor and a column grid conductor. In operation, each of the first group of grid conductors are held at one of two fixed potentials, whilst each of the second group are driven to analog voltages that will determine the beam current which will flow. Additionally, the grid drive voltages for certain applications such as a display using a very low beam current or using a high beam current may be outside the range desirable in order to minimise the cost of the grid drivers. The second grid conductors, which are driven by digital to analog converters (DACs), should ideally be capable of being driven at CMOS compatible voltages. Too high a voltage leads to expensive drivers, such as those which are used in Plasma panels. Too low a voltage leads to excessive difficulty in controlling beam current due to sensitivity to electrical noise, DAC linearity and the like. In accordance with the present invention, there is now provided a display device comprising: cathode means for emitting electrons; a permanent magnet; a two dimensional array of channels extending between opposite poles of the magnet; the magnet generating, in each channel, a magnetic field for forming electrons from the cathode means into an electron beam; a screen for receiving an electron beam from each channel, the screen having a phosphor coating comprising a plurality of groups of adjacent pixels facing the side of the magnet remote from the cathode, each corresponding to a different channel; grid electrode means disposed between the cathode means and the magnet for controlling flow of electrons from the cathode means into each channel, the grid electrode means comprising a plurality of parallel row conductors and a plurality of parallel column conductors arranged orthogonally to the row conductors, each channel being located at a different intersection of a row conductor and a column conductor, each intersection having a corresponding aperture in each of the row conductor and the column conductor, the apertures in the row conductors and the column conductors being different in size. The differing sizes of apertures allows the DACs driving the second grid or row conductors to be within a range of voltages which is compatible with the use of CMOS technology for the drivers and also to improve the control of the beam current and reduce sensitivity to electrical noise, DAC linearity and the like. In a preferred embodiment, the apertures in the row conductors are smaller than the corresponding apertures in the column conductors. This increases the voltage sensitivity of the second grid (the row conductors) and allows the use of lower voltage drivers for a given beam current. Additionally, the edges of the first grid and second grid do not precisely coincide meaning that an insulating layer used between the first grid and the second grid may be extended. Preferably, the grid electrode means further comprises a first insulating layer disposed between the row conductors and the column conductors, the first insulating layer having apertures intermediate in size between those of the row conductors and those of the column conductors. The positioning of such an insulating layer can be done with a low accuracy whilst still ensuring that the first and second grid tracks do not short together. Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a simplified cross-sectional view of an example of a prior art Magnetic Matrix Display device; Figure 2 is a cutaway plan view of the example of figure 1; Figure 3 is a view of prior art first and second control grids having equal diameter apertures; Figure 4 is a view of first and second control grids according to the present invention having unequal diameter apertures giving increased voltage sensitivity; Figure 5 is a view of first and second control grids according to the present invention having unequal diameter apertures giving decreased voltage sensitivity; and Figure 6 is a graph of beam current flowing versus second control grid voltage for varying size of second grid aperture. Referring to Figure 1, an example of a magnetic matrix display device 10 comprises a plane cathode 20 facing a plane anode 30. A phosphor coating 150 is disposed on the side of the anode 30 remote from the cathode. A permanent magnet 140 is disposed between the anode 30 and the cathode 20. The magnet 140 is perforated by a two dimensional matrix of channels 160. A grid assembly is disposed between the magnet 140 and the cathode 20. The grid assembly comprises first and second electrically isolated arrays of parallel conductors hereinafter referred to as first grids 71 and second grids 72 respectively. The first grids 71 are arranged orthogonally to the second grids 72 to form a lattice pattern. Apertures are formed in the first grids 71 and the second grids 72. The apertures are located at each intersection of a first grid 71 and a second grid 72. Each aperture is aligned with a different channel 160. The phosphor coating comprises a plurality of pixels each corresponding to a different channel. In a colour magnetic matrix display, each of the corresponding phosphor pixels may be a group of phosphor elements, each group corresponding to a different channel and each group typically comprising a Red, a Green and a Blue phosphor element. Deflection anodes 302,304 are arranged as a pair of combs between the magnet 140 and the anode 30 to sequentially address electron beams emerging from the channels to different ones of the phosphor elements. Referring to Figure 2, column drive circuitry 170 is connected to the first grids 71. Row drive circuitry 180 is connected to the second grids 72. This has the advantage that for a conventional display having a four to three aspect ratio, with more columns than rows, the number of more complex expensive analog drivers is reduced at the cost of having more simple, cheap digital switches. Referring back to figure 1, in operation, the anode 30 is held at a higher potential than the cathode 20. Electrons emitted from the cathode 20 are thus accelerated towards the anode 30. As electrons enter each of the channels 160 in the magnet 140 they are collimated into a dense beam by the magnetic field therein. In operation, admittance of electrons to the channels is selectively controlled via the grid assembly. Each channel 160 is addressable by appropriate voltage signals applied by the row drive circuitry 180 and the column drive circuitry 170 to the corresponding first grid 71 and second grid 72. Electrons are thus selectively admitted or blocked from entering each channel 160, passing through the magnet 140 and reaching the corresponding region of the phosphor coating 150 to generate a pixel of a displayed image on the screen. The pixels of the displayed image are scanned in a refresh pattern. To produce the refresh pattern, a column of pixels is energised by applying an appropriate voltage, via the row drive circuitry 180 to the corresponding second grid 72 with the voltage on the first grids 71 set via the column drive circuitry 170 so that no beam current flows. The voltages on the remaining first grids 72 are set by the column drive circuitry 170 so that no beam current flows for any operating voltage on the second grids 71. The voltages on the second grids 72 are then modulated by row drive circuitry 180 as a function of input video data corresponding to the energised column of pixels. The process is then repeated for the next successive column. The row and column functions are transposed relative to that conventionally used in LCDs, that is the rows are driven by an analog voltage and the columns are switched between two analog levels, however such transposition is not an essential feature of a magnetic matrix display. In some circumstances, the design of a magnetic matrix display may require the grid electrode drive voltages to be kept within a certain range of voltages in order to minimise the cost of the grid electrode drivers. A typical desired range is that of CMOS technology, in which the desired range of output voltages is between zero volts and approximately ten volts. "Conventional" grid apertures are sometimes not sufficient to achieve the desired beam current with these drive voltages. Two specific examples where these "conventional" grid apertures are not sufficient are: 1. A display which is to use a very low beam current, such as, for example, disclosed in co-pending GB Patent Application 9703741.0 (Attorney Docket Reference UK9-96-079). The beam current in such a display will be excessively sensitive to second grid electrode voltage, that is, the DAC driving the second grid electrode is only operating near to zero volts with correspondingly poor voltage quantisation and hence beam current quantisation. 2. A display which is to use a high beam current, such as, for example, a projection display or an avionic display readable in sunlight. In such a display, the second grid electrode voltage required is beyond that attainable from cost-effective CMOS drivers. The second grid electrode voltage required is determined by the transfer characteristics of the magnetic matrix display. Figure 3 shows a conventional magnetic matrix display grid structure in which the apertures in the second grid 72 structure and the first grid 71 structures are equal in diameter. Figure 4 shows a magnetic matrix display grid structure in which the apertures in the first grid are larger than the apertures in the second grid. This has the effect of "unmasking" the second grid and increasing the voltage sensitivity of the second grid, thus allowing lower second grid drive voltages to be used for a given beam current. Figure 5 shows a magnetic matrix display grid structure in which the apertures in the second grid are larger than the apertures in the first grid. This has the effect of "masking" the second grid and decreasing the voltage sensitivity of the second grid, thus reducing the sensitivity of the second grid and improving the voltage quantisation and hence beam current quantisation. A computer simulation of the variations in second grid aperture diameter has been completed. Table 1 below shows the first grid cutoff voltage values measured for a first grid aperture of 250µm and a second grid aperture of discrete values between 250µm and 100 µm. Cutoff values with second grid voltage = 0 volts. Second grid Aperture (µm) 250 200 150 100 Cutoff Voltage (v) -2.625 -2.65 -2.1 -1.875 Table 2 below shows the beam currents obtained with the second grid set at voltages between 0 volts and 5 volts. One volt steps were used in the simulation, except for the case where the first grid aperture was 100µm, where 0.5 volt steps were used from 0 volts to 1 volt. Above this voltage the beam current increased rapidly. Second grid (G2) voltage (from cutoff) vs. beam current Second grid (G2) Aperture (µm) 250 200 150 100 G2=0.0V 2.1nA 2.1nA 2.1nA 2.1nA G2=0.5V - - - - - - - - - - - - - - - - - - - - - 395.4nA G2=1.0V 182.5nA 241.7nA 347.2nA 736.5nA G2=2.0V 322.5nA 439.0nA 604.1nA - - - - - - - G2=3.0V 444.2nA 577.8nA 764.0nA - - - - - - - G2=4.0V 521.4nA 698.8nA 901.9nA - - - - - - - G2=5.0V 625.3nA 770.8nA 1021.4nA - - - - - - - Figure 6 shows the results of Table 2 in graphical form. It should be noted that the nature of the computer simulation is such that the true space charge behaviour in front of the physical cathode is not necessarily modelled. If this were modelled, then an increase in beam current for a given second grid (G2) voltage would be seen, that is the gamma would increase. Thus with increasing beam current, the quantised current drawn from the emitter does not increase (as would be the real case), but instead remains constant, that is, the cathode is operating in a thermally saturation limited mode. Increases in beam current are due to increases in the cathode electron collection area. However, despite this being the case, the effect of changing relative grid hole diameters is clearly demonstrated. Even though there is a "line of sight" path from the cathode to exposed second grid (G2) tracks, electrons are still subject to the strong focusing and collimation effects inherent in the design of the magnetic matrix display. Even at highly positive second grid voltages, no electrons passing through the channels in the magnet collide with the second grid, demonstrating the high display efficiency that can be maintained with this invention. A further advantage of using a second grid aperture smaller than a first grid aperture is that the edges of the first grid and second grid apertures do not precisely coincide. This means that the insulating layer used between the first grid and the second grid can be extended. For example, if a 150µm diameter second grid aperture is used with a 250µm diameter first grid aperture, the aperture in the insulating layer could be, for example, 200µm diameter, thus permitting low accuracy of placement of the insulating layer whilst still ensuring that the first grid and second grid tracks do not short together.

Claims: A display device comprising: cathode means (20) for emitting electrons; a permanent magnet (140); a two dimensional array of channels (160) extending between opposite poles of the magnet; the magnet generating, in each channel, a magnetic field for forming electrons from the cathode means into an electron beam; a screen for receiving an electron beam from each channel, the screen having a phosphor coating (150) comprising a plurality of groups of adjacent pixels facing the side of the magnet remote from the cathode, each corresponding to a different channel; grid electrode means (71, 72) disposed between the cathode means and the magnet for controlling flow of electrons from the cathode means into each channel, the grid electrode means comprising a plurality of parallel row conductors (72) and a plurality of parallel column conductors (71) arranged orthogonally to the row conductors, each channel being located at a different intersection of a row conductor and a column conductor, each intersection having a corresponding aperture in each of said row conductor and said column conductor, characterised in that the apertures in said row conductors and said column conductors are different in size. A display device as claimed in claim 1 wherein said apertures in said row conductors (72) are smaller than said corresponding apertures in said column conductors (71). A display device as claimed in claim 2 wherein said grid electrode means (71, 72) further comprises an insulating layer disposed between said row conductors (72) and said column conductors (71), said insulating layer having apertures intermediate in size between those of said row conductors and those of said column conductors.

Inventors: BEETESON JOHN STUART, KNOX ANDREW RAMSAY, PIETRZAK CHRISTOPHER CARLO, BEETESON, JOHN STUART, KNOX, ANDREW RAMSAY, PIETRZAK, CHRISTOPHER CARLO
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1998
Main classification: H01J31
IPC codes: H01J29