Display device

Abstract: In a flat display device, for example, a luminescent display a spacer structure (5) is provided between two substrates (1, 3). To this end a mask (9, 15) consisting, if necessary, of a plurality of layers is incorporated in a photosensitive material (8,13). After exposure, development and removal of the mask, the desired spacer structure is obtained.

Description: The invention relates to a display device according to the introductory paragraph of Claim 1. Flat display devices of this type are used as display panels in, for example portable computers and in flat panel displays for video applications. A display device of the type as mentioned above is described in EP-A-0.405.262. in this device semi-cylindrical beads, forming spacer parts are located on both sides of a conductive plate. In this way the distance between the substrates is enhanced, preventing possible spark discharge. In operation, electrons emitted by the electron source after having passed holes in the conductive plate impinge on the pixels, which are formed by phosphors. The semi-cylindrical beads on each side of the conductive plate extend in parallel with each other. This implies that between two such beads at the side of the second substrate backscattering of electrons or secondary emission between adjacent pixels may occur. Furthermore the semi-cylindrical beads on the side of both substrates due to their elongated form make evacuation of the device, complicated. One of the objects of the invention is to provide a display device of the type described hereinbefore in which no such backscattering occurs, while evacuation is well possible. To this end a device according to the invention is characterized according to the characterizing part of Claim 1. The closed structure at the location of the pixels prevents scattering of electrons to adjacent pixels, while the apertures between the spacer part adjoining the first substrate ensure a good possibility for evacuation. Backscattering can be further prevented by coating the spacer parts adjoining the second substrate with a conductive layer, through-connected to the second substrate. Using an organic polymer as spacer material makes it possible to define structures having different cross-sections at different heights. In this way a spacer structure can be realised which has its cross-section optimized, dependent on the function to be performed (enclosures protecting the pixels near one substrate, and e.g. column-shaped structures having apertures for evacuation near the other substrate). In USP 4.451.579 spacers are shown each including a pin protruding from one of the substrates (the front plate) and a cylinder protruding from the other substrate, each pin being inserted into one hollow cylinder. At the area of a pixel only pins protrude from the substrate without forming a closed structure around the pixel. In USP 3.771.008 an insulating spacer having apertures between an electron-generating structure and a front-plate is shown. The device shown is based on plasma discharge and shows only single spacers, not being separated in different parts between the front plate and the surface from which electrons emanate. USP 3.855.499 shows a device for alpha-numerical display having different cathode segments, corresponding to display sections provided on a first substrate and phosphors on a second substrate. The second substrate is spaced from the first substrate, bearing a cathode structure. by one single spacing element, surrounding the complete display, so it does not prevent scattering of electrons from one segment to another. A preferred embodiment of a display device according to the invention is characterized in that at least one layer of conducting material is provided between the spacer parts. In this way acceleration grids can be integrated in the spacers, for example, by providing structured metal layers. The device can be manufactured by means of a method similar to the one described in PCT/WO 90.00808. Layers of patternable organic material are provided on a substrate in which layers at least spacer parts are defined photolithographically. The spacer structure is preferably provided by means of sub-layers in which, if necessary, auxiliary masks are provided photolithographically between two sub-layers, while in a plan view the auxiliary masks and the mask on the last provided layer do not overlap each other or overlap each other only partially. Alternatively, after at least one sub-layer has been provided, a part of the spacer can be defined in parts of the patternable material, whereafter this material is provided with a patterned layer of conducting layer of conducting material which in its turn is covered with at least one sub-layer for defining further parts of the spacer. In this way said integrated acceleration grids can be obtained. These and other aspects of the invention will now be described in greater detail with reference to some embodiments and the drawing. Fig. 1 is a diagrammatic representation of a part of a display device according to the invention. Figs. 2 to 7 show diagrammatically the display device of Fig. 1, taken on the line II-II in Fig. 1, during several stages of manufacture. Figs. 8 and 9 show diagrammatically the manufacture of another display device according to the invention. Figs. 10 and 11 show the manufacture of a further device. Fig. 12 shows diagrammatically a further display device according to the invention. Fig. 1 shows a part of a display device according to the invention, comprising a first substrate of, for example, glass or silicon which is provided in this case with a matrix of electron sources 2 (for example, field emitters) which are manufactured in a manner known per se. The pixels 4 which in this example substantially coincide with phosphors provided at the side of the substrate 3 opposite the electron sources 2 are present opposite the electron sources on a second substrate 3 of glass. Although only two pixels 4 are shown, the device actually comprises at least 100,000 to 1,000,000 pixels, dependent on the type of device (monochrome, colour). The substrates 1 and 2 are spaced apart by approximately 500 µm by means of spacers 5. In the relevant example these spacers comprise two parts, namely a first part 5a at the area of the first substrate 1 and a second part 5b at the area of the pixels 4 on the second substrate 3. The parts 5b may extend entirely around a pixel 4. The device shown is driven by causing electrons from the sources 2 to impinge upon the phosphors associated with the pixels 4. Backscattered electrons now impinge upon the parts 5b and thus cannot influence the adjacent pixels. Due to the large distance between the two substrates a comparatively high voltage difference can be applied therebetween (5-10 kV) without any danger of flash-over. The display device can be evacuated by means of the apertures 6 in the spacers 5. The device of Fig. 1 may be manufactured as follows (see Figs. 2 to 7). The manufacture starts from a first substrate 1, for example a semiconductor substrate (silicon or glass in this example) in which or on which electron sources (not shown) are realised, for example field emitters, but semiconductor cathodes as described in Netherlands Patent Application No. 7905470 (PHN 9532) in the name of the Applicant are also possible. A layer 8 of photosensitive polyamide acid or polyamide ester having a thickness of approximately 300 µm is provided on the substrate 1. A suitable polyamide ester is, for example Probimide 348 FC of the firm of Ciba-Geigy. Thin layers (up to approximately 100 µm) can be applied by means of a single spin-coating treatment of the polyamide ester. At the layer thickness used in this case, this polyamide ester is provided in accordance with the method as described with reference to Figs. 8 and 9 or with a suitable tool such as a "spacer knife". To protect the electron sources, a protective coating may be temporarily provided, if necessary. The layer 8 is subsequently covered with a thin layer 9 (approximately 40 nm) of gold in this example on which a layer of positive photoresist 10 is provided. After exposure by means of ultraviolet radiation (shown diagrammatically by means of arrows 11) through a mask 12 which defines the apertures 6, and after development, the parts 10b are removed and the part 10a of the photoresist is left (Fig. 3). With the remaining photoresist as a mask the gold layer 9 is subsequently etched wet-chemically (Fig. 4) in an etchant suitable for this purpose (for example, an aqueous solution of 25% KI and 10% I 2 ). The structure thus produced is coated again with a photosensitive layer 13 of polyamide ester having a thickness of approximately 100 µm (Fig. 5). The assembly is subsequently exposed by means of ultraviolet and visible radiation (shown diagrammatically by means of arrows 14 in Fig. 6) via a mask 15 which defines the parts 5b of the spacers. The wavelength used and the duration of the exposure depend on the light intensity, the material used and the thickness of the layers 8, 13 (for a layer of Probimide 348, with a thickness of approximately 200 µm and exposure by means of the entire Hg spectrum the light intensity is, for example 15 mW/cm 2 for 200 seconds). Since, in a plan view, there are open spaces between the auxiliary mask formed by the layers 9, 10a and the mask 15, the polyamide ester is cured throughout the thickness of the layers 8, 13 at these areas and these parts 5 are left on the subsequent substrate 1 in a subsequent development step. After cleaning, removal of the layers 9, 10a, possible further cleaning steps and a thermal post-treatment, the device of Fig. 7 is obtained. The substrate 1 thus obtained, provided with emitting sources and spacers 5, is then laid on a second substrate 3 of, for example glass and provided with phosphors. After aligning the phosphors with respect to the electron sources, the assembly is sealed along the edges and evacuated. The device of Fig. 1 is then obtained. Figs. 8 and 9 show how spacers having a height of 200 to 1000 µm can be obtained. The polyimide layer 8 is obtained by successively providing sub-layers 8a, 8b, 8c. Each subsequent sub-layer is not provided until the previous sub-layer has acquired a defined layer thickness (for example by means of spin-coating). Subsequently the locations of the spacers to be formed are defined via a mask 15 whereafter the assembly is exposed, developed, cured etc. The spacers 5 thus formed keep the two substrates 1, 3 of Fig. 9 spaced apart by, for example 450µm. In this example no auxiliary masks are used so that the spacers should have a uniform cross-section; in practice the cross-section at the area of the first substrate is usually slightly smaller because a negative photosensitive system is used and because there is light absorption in the layer. Although a device is shown with an electron source for each pixel, the spacers may also be used in other flat display devices such as described in, for example, US-P-4,853,585 (PHN 12.047). Fig. 10 shows the manufacture of another display device partly in a cross-section and partly in a plan view. The method starts again from a substrate 1, for example a glass plate on which a matrix of field emitters is provided. Sub-layers 8a, 8b of polyamide ester are deposited on the substrate 1 in the same way as described hereinbefore. By exposure with ultraviolet radiation cured areas 22 are formed in the sub-layers at the area of parts of the spacers to be formed. The layer thus formed is, however, not yet developed but first covered with a thin metal layer 16 having apertures 17 above the emitters. The metal layer 16 may be provided in advance with the apertures 17, but the pattern of apertures (or any other desired pattern) may also be provided after precipitation of the metal layer by means of etching. Subsequently a layer 8c of polyamide ester is again provided which in its turn is covered with a gold layer 9 patterned by means of etching. Subsequently a layer 13 of polyamide ester is provided whereafter the assembly is exposed with ultraviolet and/or visible radiation via a mask 15. After development, rinsing and possible further treatment, the device of Fig. 11 is obtained. This device has a substrate 1 on which square column-shaped parts 5a of the spacers are present in this example. The other parts of the spacers consist of similar column-shaped parts 5b and parts 5c which are closed along their circumference and which enclose pixels (phosphors) in the ultimate display device. The metal layer 16 which has apertures 7 at the location of field emitters 21 is present on the substrate 1 between the parts 5a and 5b of the spacers. The plate 16 may now function as a common accelerating electrode. To suppress possible backscattering to a further extent, the walls of the closed parts 5c may be coated with a conducting layer which is through-connected to the front plate 3 in, for example, an electrically conducting manner. This can also be achieved by providing a grid which is comparable with the metal layer 16 and by short-circuiting it electrically with the front plate 3. Fig. 11 also shows diagrammatically two field emitters 21. In the present example they form part of a matrix of field emitters which are driven by X lines 18 and Y lines 19 and are mutually insulated by means of an insulation layer 20 at the area of their crossings where the X lines are provided with connection strips 18a. Apertures 7 providing the possibility of providing a vacuum during sealing are present between the parts 5a and between the parts 5b. Finally, Fig. 12 shows a modification in which the closed parts 5b of the spacers have a honeycomb structure. Otherwise, the reference numerals denote the same elements as in the previous Figures. The exiting electron current is shown diagrammatically by means of arrows 23. The invention is of course not limited to the examples shown, but several variations are possible within the scope of the invention. For example, the structure in which the spacers are defined can also be provided on the glass plate with phosphors instead of on the substrate 1. A plurality of metal masks may also be provided between the sub-layers so that, as it were, a part of the electron-optical system is integrated in the spacer(s).

Claims: A display device comprising at least one electron source (2,21) at a first substrate (1) and a second substrate (3) provided with pixels (4), each pixel substantially coinciding with a phosphor, the substrates being spaced apart at a distance of at least 200 µm by spacers (5) comprising at least two spacer parts (5a,5b,5c), characterized in that the spacers have been made of an organic polymer and comprise spacer parts (5a,5b,5c), in which at least the spacer parts (5a) adjoining the first substrate (1) have apertures (6,7) for evacuating the device and the spacer parts (5c) adjoining the second substrate (2), in cross-section parallel to said substrate, form closed structures around each of the pixels. A display device according to Claim 1 characterized in that the spacer parts (5c) adjoining the second substrate (2) are coated with a conductive layer, through-connected to the second substrate. A display device according to Claim 1 or 2 characterized in that each electron source (2,21) on the second substrate (2) is associated with at least three column-shaped spacer parts (5a). A display device according to Claim 1,2 or 3 characterized in that at least one layer of conductive material (16) is provided between the spacer parts (5a,5b,5c).

Inventors: HORNE REMKO, VAN ANDEL MAARTEN ALEXANDER, VAN VEEN GERARDUS NICOLAAS ANN, HORNE, REMKO, VAN ANDEL, MAARTEN ALEXANDER, VAN VEEN, GERARDUS NICOLAAS ANNE
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H01J31
IPC codes: H01J29, H01J9, H01J31, G09F9