EP0655767A1 - Preventing electron discoloration of glass - Google Patents

Abstract

Glass discoloration by high energy electrons is avoided by applying a thin oxide layer on the surface of the glass that would otherwise be impacted by electrons, which is alkali-free, contains no ions inherently reducible by electron bombardment, and has a thickness larger than the depth of penetration of the electron beam at a given acceleration potential but insufficient color to interfere with desired color coordinate specifications. Suitable thin oxide layers comprise ZnO, SnO₂, In₂O₃ and tin-doped indium oxide.

Description

FIELD OF THE INVENTION
• Method of preventing electron discoloration caused by high energy electron bombardment of glass, and cathode ray tube.
BACKGROUND OF THE INVENTION
• Prolonged exposure of oxide glasses to energetic electrons induces optical absorption, of particular practical significance in cathode ray tubes (CRT,) causing permanent browning of the screen.
• Considerable effort has been expended to alter the panel glass composition in order to minimize this browning effect. Lead oxide exacerbates the browning. However, lead is beneficial because of its high stopping power for x-rays that are produced by the electron beam, and as it imparts desirable physical properties to the glass. In lead-free glasses browning is reduced, but still significant.
• While alkali-free glasses do not brown under an equivalent amount of electron beam dose, they are not suitable for melting and forming the panel glass for CRT.
• United States Patents 3,573,955 and 3,725,710 propose providing a thin, hard, transparent layer containing no more than 1% of easily reduced metal oxides between the window portion of a CRT and a phosphor layer inside the window leaching out lead and ion exchanging potassium ions for sodium, or other means of coating the window to provide the protective layer.
• Recent higher accelerating voltages of 30-40 kV increase browning against which these films are no longer effective. Furthermore, the protective film must adhere to the glass during subsequent CRT production steps, a serious concern if the film must be thicker than about one micron.
• In particular, the continual increase in size from 38 cm (15") windows to 89 cm (35") windows imposes practical limits on the manner in which films are applied. Also, a blank tube undergoes thermal cycling before it becomes a completed CRT. Film thickness must be limited to provide adequate adherence to withstand the differential thermal expansion effect that is encountered during this subsequent thermal cycling.
• The present invention provides a practical method of providing protection, in modern cathode ray tubes, against discoloration due to electron bombardment, and meeting the various concerns described above, and providing a greater flexibility in the procedure for applying a protective film. It further provides a CRT having improved protection against discoloration (browning) due to electron bombardment.
SUMMARY OF THE INVENTION
• The product aspect of the invention resides in a cathode ray tube comprising an envelope having a glass window portion capable of being discolored upon bombardment by high energy electrons, means mounted in the tube to produce a stream of high energy electrons and to direct the stream toward the window, a phosphor layer on the window and a thin oxide film between the window and the phosphor layer, the thin film being alkali-free, free of ions inherently reducible by electron bombardment, and of sufficient thickness and density to prevent any substantial electron penetration to the window.
• The invention further resides in a method of preventing discoloration of glass by high energy electron bombardment which comprises providing a thin oxide layer on the surface of the glass that would otherwise be impacted by electrons, the layer being alkali-free, containing no ions inherently reducible by electron bombardment, having a thickness larger than the depth of penetration of the electron beam at a given acceleration potential, and having insufficient inherent color to interfere with desired color coordinate specification.
BRIEF DESCRIPTION OF THE DRAWINGS
• The single FIGURE in the accompanying drawing is a side view in cross-section. It represents a modified version of a typical cathode ray tube, designated by the numeral 10 and illustrating the present invention.
• The envelope of cathode ray tube 10 includes a window, or faceplate, 12, a funnel portion 14 and a neck portion 16. An electron gun 18, shown diagrammatically, is sealed in neck 16. A phosphor layer 20 has heretofore been applied over the inside surface of window 12. In accordance with the present invention, an oxide film 22 is applied on the inside surface of window 12 intermediate phosphor layer 20 and window 12.
DESCRIPTION OF THE INVENTION
• Since U.S. Patent 3,573,955, a much better understanding of the mechanism for browning of glass by electron beams has developed. As one significant example, it is now known that alkali metal ions play a dominant role in the discoloration. The alkali metal ions are involved in reduction of lead in the glass, a phenomenon involved in creating the browning effect. When the electron beam penetrates the glass, an excess negative charge is produced. The alkali metal ions move inward from the glass surface to stabilize that charge.
• Our invention provides a thin film of a pure oxide of sufficient thickness to substantially block penetration of electrons to the panel glass. The film must not itself be subject to the discoloration mechanism. Nor can it exhibit any significant inherent color.
• There are then six basic requirements for a protective layer to prevent browning. The layer must be:
• 1. Free of alkali metal ions.
• 2. Free of reducible ions.
• 3. Sufficiently lacking in color to permit obtaining the desired color coordinates in the glass.
• 4. Of a thickness such that penetration of the electron beam is minimal.
• 5. Not over about one micron thick in order to maintain good adherence to the glass during subsequent thermal fabricating steps.
• 6. Applied by a method consistent with current commercial panel sizes and fabrication procedures.
• The layer must be alkali-free to ensure that no mobile, positively-charged, ionic species is available to neutralize incoming electrons. Likewise, easily reducible ions, such as lead, titanium, and bismuth, must be absent from the film. This avoids possible direct reduction by electron bombardment.
• It is customary to include minute amounts of the colorant oxides, nickel and cobalt, in the composition of a glass for the faceplate or window of a cathode ray tube. This provides a desired tint in the glass. The protective film must not provide sufficient color to unduly alter this tint. To some extent, however, the glass colorants might be modified to accommodate slight coloration in the protective layer.
• The fourth requirement, minimal electron penetration into the panel glass, insures minimal reduction of lead through the normal browning mechanism. The thickness of the required layer can be roughly estimated from the expression given by W. E. Spear (Proc. Phys. Soc., London, vol. B68, page 991, 1955) for the maximum range of a ballistic electron. This expression is
  
  $\text{R = V²/Bd}$

  

  
  
  where V is the accelerating potential in kilovolts, d is the density of the film and B is a constant. The suggested value of B is 6.2 x 10⁵ kV²-cm/g, although this is to be considered an approximation.
• Table I lists the maximum film thickness values predicted by the Spear formula for representative materials satisfying the first three requirements listed above. These values are determined for current and anticipated accelerating potentials. TABLE I

  Material	Thickness, µm
  	20 kV	30 kV	40 kV
  SiO₂	3	6	11
  Al₂O₃	1.6	3.6	6.4
  ZnO	1.1	2.6	4.6
  SnO₂	0.9	2.0	3.7
  ITO (In₂O₃,ySnO₂)	0.9	2.0	3.7

• These thicknesses are predicted for the range of the ballistic electron which corresponds to the deepest penetration from the surface. Hence, they should be viewed as maximum values. Subsequent actual measurements show that thicknesses of one-half to three-quarters of these values will provide adequate protection. These measurements are made in terms of light transmittance through a film of known thickness after exposure to an actual electron beam.
• ITO is a commonly used designation for a dense, electroconducting film comprised of indium oxide (In₂O₃) doped with a few percent, e.g. 4%, of tin oxide. This material is particularly desirable for present purposes since it may also be applied for conducting purposes to bleed off excess charge. Thus, the art is familiar with the material and methods for its application on glass. An ITO film of about 0.2 micron thickness is customarily used on devices such as LCD panels.
• Test samples were prepared to determine the validity of the estimates. Films were deposited by both plasma-assisted chemical vapor deposition (PCVD) and thermal-assisted chemical vapor deposition. The former can be carried out at 30-100°C, while the latter requires a temperature in the range of 400-500°C. Hence, the former is employed if a substrate is temperature sensitive. However, the latter is preferred since it gives a film with higher density and purity.
• The oxide precursor may be a organo-metallic compound. To produce an alumina film, for example, triethylaluminum, triisobutylaluminum, trimethylamine alane, or aluminum chloride may be employed. In the presence of plasma or heat, and an oxidizing gas, the source compound is oxidized to alumina and organic by-product gases. The latter are exhausted by vacuum while the alumina is deposited on the glass substrate.
• Test samples were prepared in the manner described above. Oxides were deposited on discs cut from a commercial glass used to produce CRT windows. The discs were 4.8 cm (1 7/8") diameter and 11.43 mm thickness, and were ground and polished on both surfaces for film application.
• Testing was carried out in two ways. An actual raster test in a sealed CRT was carried out with a film being exposed for 170 hours at 30 kV with a current of 250 microamperes. While the results with this test were satisfactory, a simpler, although more severe, test was adopted for screening purposes. In this test, an electron microprobe was employed to raster a 200 x 300 µm² area of 20 kV and a current of 400 nanoamperes. This test imposed a higher charge per unit area.
• The results obtained employing the second test procedure are shown in TABLE II below. Four different oxide film materials, in varying depths, were tested. Also, an uncoated glass blank was exposed and measured for comparison. Film thickness is reported in microns (µm). Degree of browning is reported in terms of percent transmittance of the visible spectrum after exposure, as measured by a densitometer. TABLE II

  Material	Thickness µm	Transmittance %
  -	0	46
  SiO₂	1.6	69
  	5.0	100
  Al₂O₃	1.0	69
  ZnO	0.5	80
  	0.9	90
  ITO (In₂O₃,ySnO₂)	0.5	92

• In general, a film thickness that provides a transmittance value of at lest 80% in this test should provide adequate protection. Of course, a transmittance of 90% would be preferable. The results recorded in TABLE II are qualitatively consistent with the predicted values in TABLE I. However, they bear out the earlier suggestion that a film thickness substantially less than the predicted maximum thickness will provide adequate protection. A film thickness necessary to protect to the same level at an accelerating potential of 30 kV would be approximately twice the values given in TABLE II for 20 kV exposure.
• The relationship of the film thickness, and the ease and versatility of deposition, are very significant. When the film thickness is ≦ one micron, the films adhere well. Also, adherence is not influenced by the thermal expansion mismatch between the film and the glass. Moreover, practical methods of coating large panels, such as sol-gel techniques, are limited to thin films. It is considered equally important to find the right film for browning protection, and to provide a practical and inexpensive method of depositing the film on large panels. By employing a film of thickness no greater than about one micron, this is practical to achieve.

Claims (10)

1. In a cathode ray tube comprising an envelope having a glass window portion capable of being discolored upon bombardment by high energy electrons, means mounted in the tube to produce a stream of high energy electrons and to direct the stream at the window, and a phosphor layer on the window, the tube characterized by an oxide film between the window and the phosphor layer, the film being alkali-free, free of ions inherently reducible by electron bombardment, and of sufficient thickness and density to prevent any substantial electron penetration to the window.
2. A cathode ray tube in accordance with claim 1 in which the thin film is free of lead, titanium and bismuth ions.
3. A cathode ray tube in accordance with claim 1 in which the stream of high energy electrons is directed at the window at an accelerating potential of at least 20 kV.
4. A cathode ray tube in accordance with claim 1 in which the thin film is free of any coloration greater than that which, in conjunction with any colorants in the window, will provide a desired tint.
5. A cathode ray tube in accordance with claim 1 in which the thickness of the thin film is sufficient so that, after exposure of a film on a window for 170 hours at 30 kV and a current of 250 microamperes, the transmittance of the window is at least 80%.
6. A cathode ray tube in accordance with claim 1 in which the thin film is composed of an oxide selected from ZnO, SnO₂, In₂O₃ and tin-doped indium oxide.
7. A cathode ray tube in accordance with claim 1 wherein the film thickness is not greater than about one micron.
8. A method for preventing discoloration of glass by high energy electron bombardment which comprises applying an oxide film on the surface of the glass that would otherwise be impacted by electrons, the layer being alkali-free, contains no ions inherently reducible by electron bombardment, has a thickness and/or density such that penetration of electrons to the glass is minimal and has insufficient color to interfere with desired color coordinate specifications.
9. A method in accordance with claim 8 in which the film is applied by plasma-assisted or thermal-assisted chemical vapor deposition, or by coating with a gel and firing, or from a organo-metallic precursor.
10. A method in accordance with claim 8 or 9 in which the film is applied in a thickness not over one micron.

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