METHOD OF POST-ETCHING A MECHANICALLY TREATED SUBSTRATE

Abstract: Method of providing a pattern of apertures and/or cavities in, for example, a glass duct plate of a plasma-addressed liquid crystal display, in which first a mechanical treatment is performed (for example, by means of powder blasting) and then a wet-chemical etching treatment is performed to render the walls of the ducts microscopically less rough so that the optical disturbance is reduced and the glass becomes clearer again.

Description: Method of post-etching a mechanically treated substrate. The invention relates to a method of providing a pattern of apertures and/or cavities in a surface of a substrate, using a mechanical treatment. Such a pattern is usually provided by means of powder blasting, grinding, lapping, sawing, drilling or another mechanical treatment. If this method is used to provide cavities (ducts) in the (generally glass) duct plate of a PALC display (as known, for example, from WO 96/29689-A1, US Patent ...), the problem occurs that the treated part of the surface (the inner side of the ducts) is microscopically rough after performing the mechanical treatment, so that the generated image does not have a sufficiently satisfactory contrast when the display incorporating the plate is being used. It is an object of the invention to provide a method in which the walls of apertures and cavities obtained by a mechanical treatment can be processed in such a way that the surface is microscopically less rough after the treatment. To this end, the invention is characterized in that, after performing the mechanical treatment, a wet-chemical etching process is performed which renders the walls of the apertures and cavities microscopically less rough. More particularly, a mechanically treated duct plate can be rendered microscopically smooth enough by means of the wet-chemical etching process, so that it functions satisfactorily in a PALC (plasma-addressed liquid crystal) display. The method may also be used to advantage for making cavities which constitute the cells of a cell plate or ducts of a duct plate of a plasma display (PDP), for example, a high-resolution plasma display. The bottom and/or walls of the cells or ducts are preferably microscopically smooth because it is then easier to provide them with a layer, for example, an electrode. In accordance with an embodiment, the mechanical treatment is a powder blasting process using a powder blast-resistant mask. This process has the advantage that very fine patterns can be made. In accordance with a further embodiment, the substrate is coated with an etch-resistant layer, for example, a chromium layer before it is mechanically treated. This has the advantage that the method can also be used if the part of the surface which is not mechanically treated must not come into contact with the etching liquid. The etch-resistant layer is provided before the mechanical treatment takes place, so that the walls of apertures or cavities that have been made will not be coated. In accordance with a further embodiment, an etchant containing HF is used as a wet-chemical etchant. This may be, for example, pure HF, HF with acids such as HNO 3 and HF with NH»F. It is possible to use a large number of etchant concentration variants. Another possible etchant is an etchant containing HBF 4 . Also in this case, different concentrations are possible. An advantage of HF is that it etches rapidly. An advantage of HBF 4 is that there is less deposit. Another advantage is that it does not etch too rapidly so that the process can be better controlled. A combination of HF and HBF 4 may be used, for example, in a ratio of 1: 10. Then, the advantages of both etchants can be combined. It is alternatively possible to use a concentrated lye solution as an etchant. A drawback is the slow etching rate. Dependent on the substrate to be etched (for example, a given type of glass), a given etchant in a given concentration is most suitable. A specific embodiment of the method according to the invention comprises the steps of providing a chromium layer on a glass surface providing a blast-resistant mask on the chromium layer producing at least a jet of abrasive powder particles directing the jet onto the surface of the mask which is present on the surface to be treated performing a relative movement between the jet and the surface with the mask removing the blast-resistant mask wet-chemical etching with a glass-dissolving agent, with the chromium layer protecting the subjacent surface - chromium-etching for the purpose of removing the chromium layer. In a further specific embodiment of the method, a plate is completely immersed in an etching bath. This simplifies the method considerably. If it is not necessary to perform a strong etching treatment or if the untreated surface does not need to be smooth, then this immersion treatment in an etching bath may be used without any further measures. If more stringent requirements are imposed on the untreated side, then this side can be provided with an etch-resistant layer before the plate is immersed into the etching bath. The substrate is preferably made of glass, and 2 to 200 micrometers is etched off the surface. Then the treated glass is completely clear again and gives a very good contrast. More particularly, 5 to 100 micrometers is etched off. Then the glass is clear enough for use in a display. The invention is particularly practical for making a pattern of cavities, particularly parallel ducts or cells. The invention is also applicable in practice for treating brittle materials such as ferrite, particularly when they are provided with a pattern by means of powder blasting. Ferrite is used, inter alia, in magnetic heads for video recorders. The invention is also applicable in practice for treating silicon or other semiconductor materials, particularly at small material dimensions, as is desired in electronic uses, where it is important that the material is microscopically smooth enough. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. Similar components in the Figures have identical reference numerals. In the drawings: Fig. 1 is a diagrammatic cross- section of a plate system comprising a duct plate for a PALC display. Fig. 2 is a diagrammatic cross-section of a part of a glass surface after several steps of the method according to the invention. Fig. 3 is a diagrammatic cross-section of a part of a substrate during consecutive steps of an embodiment of the method according to the invention, in which no mask is used. Fig. 4 is a diagrammatic cross-section of a part of a substrate possibly provided with layers during consecutive steps in an embodiment of the method according to the invention, in which a mask is used. Fig. 5 is a diagrammatic cross-section of a part of a substrate possibly provided with layers during consecutive steps in an embodiment of the method according to the invention, in which a protective layer and a mask are used. Fig. 6 is a diagrammatic cross-section of a part of a plate provided with layers during consecutive steps in an embodiment of the method according to the invention, in which a mask and a protective layer are used. Fig. 1 is a diagrammatic cross-section of a plate system for an embodiment of a PALC display. Ducts (3) are situated between walls (2) of the duct plate (1). In this case, the ducts have a substantially angular cross-section. However, ducts having a rounded cross-section may alternatively be made. In this case, the ducts are relatively deep. They may also be made relatively shallow. Two transparent closure plates (4, 5) enclosing an LCD liquid (6) are situated above the duct plate. In order to obtain an image, light is sent through the plate system. The double arrow indicates the direction of the light path. The duct plate is preferably made by blasting ducts into a substantially flat plate. However, the ducts may alternatively be obtained by means of another mechanical treatment. Fig. 2 is a diagrammatic cross-section of a glass surface after a first (I) and a second (II) step of the process according to the invention. To simplify the Figure, the ratios between the distances d and b are not the same as in the real situation. I. After powder blasting, the glass has a rough surface. It is dull. The disturbed layer has tapering pits (12), for example, terminating in a crack having a typical maximum thickness (D) which, dependent on the substrate and the method, varies between 2 and 50 micrometers, particularly between 5 and 20 micrometers. This typical depth of the cracks is dependent on the size of the powder particles and the blasting rate. For small cracks (of the order of 5-25 μ ) the total depth (d) of the pits is generally twice as large as the depth of the crack. The width (b) of the pits at the surface is generally much smaller than 1 micrometer. The width may often be of the order of, or smaller than, the wavelength of visible light. Due to the treatment, mechanical tensions often also occur in the glass. When light is sent through the plate, these causes result in diffusion and consequent depolarization of the light. When a plate for a PALC display is provided with ducts by means of powder blasting, the dullness of the glass and the occurring depolarization is a problem when the plate is used. It has a poorer transparency and there is a reduction of the maximum contrast and the maximum black. When the two polarization filters of the PALC display are crossed, light still passes through it. II. After post-etching, the disturbed layer, i.e. the layer in which the pits are present, is etched off to a depth of 10-40 micrometers. The maximum depth (D) of the crack is still substantially 5-20 micrometers because 10-40 micrometers is not only etched off the peaks but also off the dips. The total maximum depth (d) becomes smaller. After etching for a long period, the depth of the cracks (D) will also be smaller. However, since etching is isotropic, very sharp peaks are completely etched off and the pits become larger, even if a short etching treatment is performed. The peaks are less sharp. The microscopical roughness is then smaller so that the glass is less dull and there is less diffusion of light and thus less depolarization. These plates have a good quality. The quality can even be improved by etching for a longer period and removing, for example, 100 micrometers or more. When 200 micrometers is removed, the plate is almost completely clear. Then, also the macroscopical roughness, the maximum depth (d) of the pits has become very small. The time required for this treatment is, however, correspondingly longer so that the production time increases. If the etching treatment is continued too long, the surface of the larger pits then produced may be attacked, which leads again to smaller pits. When a stronger etchant is used, the etching rate is larger and a shorter etching time is required to etch equally deep. When the etching time is shorter, deviations in etching depth with respect to a desired etching depth may increase. If the etching treatment lasts a few seconds too long or too short, the result will be a greater deviation in micrometers of the desired etching depth when etching in a strong etchant than when etching in a weaker etchant. Consequently, etching rates which are not too high are preferred for patterns for which small dimensional tolerances are required. A number of measuring results of contrast tests will be given by way of example. In one experiment, a pattern of parallel ducts was powder-blasted in plates of barium-containing borosilicate glass by means of Al 2 O 3 powder having a particle diameter of approximately 20-25 μm, at a rate of about 100 m/s. At such a high blasting rate, the glass will become very dull. Subsequently, the plates were post-etched under different circumstances. Finally, the maximum contrast was measured. This was done by placing the plate successively between parallel and crossed polarizers and by sending light through the assembly in both cases and by dividing the measured light currents (the largest light current was divided by the smallest light current). The light current is understood to mean the electric current (in microampere) through a photosensitive cell which was exposed to the transmitted light. If the maximum light current is referred to as Imax and the minimum light current is referred to as Imin, then the maximum contrast is given by Imax/Imin. Some results will be given below. Measuring series 1: * etchant: 25% HBF 4 temperature: 40 °C etching time (minutes) maximum contrast (dimensionless) 0 40 15 130 30 240 60 660 120 > 1000 Measuring series 2: * etchant: 12.5 % HF * temperature: 25 °C etching time (minutes) maximum contrast (dimensionless) 0 30 4 200 8 600 15 > 1000 Measuring series 3: * etchant: 20% HF * temperature: 25 °C etching time (minutes) maximum contrast (dimensionless) 0 30 2 300 4 700 8 > 1000 15 > 1000 Measuring series 4: * etchant: 50% HF * temperature: 25 °C etching time (minutes) maximum contrast (dimensionless) 0 35 0.5 900 1 > 1000 4 > 1000 Fig. 3 is a diagrammatic cross-section of a part of a substrate (6) during consecutive steps of an embodiment of the method according to the invention, in which no mask is used. The Figure consecutively shows the substrate (6): I - at the start of the process II - after mechanical treatment, in which cavities (7) were made, one of which is shown in the Figure m - after wet-chemical etching, by which less rough cavities (7') were obtained from the cavities (7). The mechanical treatment may be, for example, sawing, lapping, drilling, filing, grinding and particularly powder blasting. The substrate may be any material which can be mechanically post-processed and chemically etched. Different characteristic damage patterns and pit depths may be obtained in the different mechanical treatments. The etchant and the etching rate are adapted thereto. For deeper pits, for example, a longer etching time is preferably used. For narrow patterns, an etchant which is not too strong is preferably used for the etching treatment. Fig. 4 is a diagrammatic cross-section of a part of the substrate (6) possibly provided with a blasting mask (8) during consecutive steps in an embodiment of the method, in which a mask is used. The Figure consecutively shows the substrate: I - at the start of the process, without a coating II - with a blasting mask (8) with apertures (9) provided on it III - after powder blasting, by which cavities (7) were obtained IV - after wet-chemical etching, by which less rough cavities (7') were obtained from the cavities (7). The blasting mask may be, for example a metal mask or a polyurethane mask. Fig. 5 is a diagrammatic cross-section of a part of a substrate (6) possibly provided with an etch-resistant protective coating (10) and a possible blasting mask (8) during consecutive steps in an embodiment of the method according to the invention, in which a mask and an etch-resistant protective coating are used. The Figure consecutively shows the substrate (6): I - at the start of the process, without a coating II - with an etch-resistant protective coating (10) on it m - with a blasting mask (8) IV - after powder blasting, at which cavities (7) were obtained V - after the blasting mask was removed VI - after wet-chemical etching, by which less rough cavities (7') were obtained from the cavities (7) VH - after removal of the protective coating. The protective coating (10) protects the part of the surface coated thereby against the influence of the chemical etchant. It may be a metal coating, for example, a chromium coating. In this case, this coating can be removed by means of, for example, chromium etching when it is no longer necessary. The coating may alternatively consist of another type of material, for example, a polymer. Fig. 6 is a diagrammatic cross-section of a part of a glass plate (11) provided with layers during consecutive steps in an embodiment of the method according to the invention. The Figure consecutively shows the plate: I - at the start of the process, without a coating π - with an etch-resistant protective coating (10) on the side which was not mechanically treated during this process HI - with a blasting mask (8) provided with apertures (9) on it IV - after powder blasting, by which cavities (7) were obtained V - after wet-chemical etching, by which less rough cavities (7') were obtained from the cavities (7) VI - after removal of the protective coating. The protective coating (10) may be, for example, a metal coating of chromium or another material such as a polyurethane. It is not always necessary to protect one side of the plate. In some cases, protection is not necessary. A plate can be post-etched by immersing it, for example, into an etching bath. In summary, the invention relates to a method of providing a pattern of apertures and/or cavities in, for example, a glass duct plate of a plasma-addressed liquid crystal display, in which first a mechanical treatment is performed (for example, by means of powder blasting) and then a wet-chemical etching treatment is performed to render the walls of the ducts microscopically less rough so that the optical disturbance is reduced and the glass becomes clearer again.

Claims: CLAIMS: 1. A method of providing a pattern of apertures and/or cavities in a surface of a substrate, using a mechanical treatment, characterized in that, after performing the mechanical treatment, a wet-chemical etching process is performed which renders the walls of the apertures and cavities microscopically less rough. 2. A method as claimed in Claim 1, characterized in that the mechanical treatment is a powder blasting process, using a powder blast-resistant mask. 3. A method as claimed in Claim 1, characterized in that the etchant contains HF (hydrogen fluoride). 4. A method as claimed in Claim 1, characterized in that the etchant contains HNO 3 . 5. A method as claimed in Claim 1, characterized in that the surface is coated with an etch-resistant layer before the mechanical treatment is performed, which etch- resistant layer remains present during the wet-chemical etching process. 6. A method as claimed in Claim 1, characterized in that the substrate is a plate which is immersed into an etching bath after the mechanical treatment. 7. A method as claimed in Claim 6, characterized in that one side of the plate is provided with a protective coating before it is immersed into the etching bath. 8. A method as claimed in Claim 1, characterized in that the substrate is made of glass and in that a layer of 2-200 micrometers is etched off. 9. A method as claimed in Claim 1, characterized in that the substrate is made of glass and in that a layer of 5-100 micrometers is etched off. 10. A method as claimed in Claim 1, characterized in that the cavities are ducts. 11. A method as claimed in Claim 10, characterized in that the substrate is a plate for a display. 12. A method as claimed in Claim 11, characterized in that the plate is a duct plate for a plasma-addressed liquid crystal display. 13. A method as claimed in Claim 11, characterized in that the plate is a duct plate for a plasma display.

Inventors: BUSIO JOHANNES MARIA MARCUS, IN T VELD FREDERIK HENDRIK, LARSEN POUL KJERPY, POSTMA LAMBERTUS, SPIERINGS GIJSBERTUS ADRIANUS, VAN DER PUTTEN JAN BAPTIST PET, BUSIO, JOHANNES, MARIA, MARCUS, IN'T VELD, FREDERIK, HENDRIK, LARSEN, POUL, KJERPY, POSTMA, LAMBERTUS, SPIERINGS, GIJSBERTUS, ADRIANUS, CORNELUS, MARIA, VAN DER PUTTEN, JAN, BAPTIST, PETRUS, HENRIKUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: C03C15
IPC codes: C03C15, C03C19, C23F1, G02F1