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US3669771A - Process of etching a shadow mask - Google Patents

Process of etching a shadow mask Download PDF

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Publication number
US3669771A
US3669771A US6619A US3669771DA US3669771A US 3669771 A US3669771 A US 3669771A US 6619 A US6619 A US 6619A US 3669771D A US3669771D A US 3669771DA US 3669771 A US3669771 A US 3669771A
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United States
Prior art keywords
mask
etching
apertures
size
shadow mask
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Expired - Lifetime
Application number
US6619A
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English (en)
Inventor
Martin L Lerner
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Zenith Electronics LLC
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Zenith Radio Corp
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Publication date
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
    • H01J9/02—Manufacture of electrodes or electrode systems
    • H01J9/14—Manufacture of electrodes or electrode systems of non-emitting electrodes
    • H01J9/142—Manufacture of electrodes or electrode systems of non-emitting electrodes of shadow-masks for colour television tubes

Definitions

  • the subject invention is directed to a process of etching a pattern of apertures of a desired size in an electrode, such as the color-selection electrode or shadow mask, of the current form of shadow-mask color picture tube.
  • the process may be employed to develop an aperture pattern in a mask blank in the initial steps of fabricating the mask and it has equal application to what has become known as etch back.
  • Etch back is that step in the fabrication of a color television picture tube in which an aperture mask that has been employed in photographically printing a phosphor mosaic on the screen of a cathode-ray tube has its holes enlarged to the end that the cross section of the electron beams of such a tube, as determined by the apertures of the mask, exceeds the size of the phosphor deposits on the screen.
  • This relation of beam to dot size is characteristic of both a black-surround and a postdeflection-focus color picture tube.
  • a black surround color tube is the subject of Pat. 3,146,368, issued Aug. 24, 1964, in the name of Joseph P. Fiore et a1. and assigned to the assignee of the present invention. It differs from conventional shadow mask types of cathode-ray tubes in two material respects: (1) each phosphor dot of the screen is surrounded by a material that is absortive of light, and (2) its electron beam diameter is larger than the diameter of the phosphor dots.
  • a post-deflection-focus or acceleration color tube differs from the conventional shadow mask device in that additional beam focusing is introduced after the center of deflection. Because of the added focusing, more of the beam electrons are able to impinge upon the screen than otherwise and it is necessary that the phosphor dots be smaller in size than the apertures of the mask. In the latter respect, it is much like the black-surround tube.
  • the shadow mask is formed and contoured in the conventional way, differing only as to hole size.
  • the apertures of the mask initially have the dimensions required for screening so that the mask may be utilized in conventional manner for photographically printing the green, blue and red phosphors on the screen and, in the case of black surround tubes, for developing holes in a light-absorbing layer covering the screen area and into which phosphor is to be deposited.
  • the mask is subjected to another etching process, namely etch back, in order to enlarge or open up its apertures to the size required for the mask as it is to be finally installed in the tube envelope.
  • this etch back process simply entails re-introducing the mask into a workstation where it is immersed, sprayed or otherwise treated with an etchant solution of sufficient concentration and for a suitable period of time to etch away the walls of the apertures and enlarge them to the size required. It will be appreciated that the final size of the apertures of the mask is critical. If they are too large, problems of color field purity will be experienced with the tube in which the mask is installed. On the other hand, if the apertures are too small in conjunction with misregistration attendant problems of white field purity will occur.
  • the present invention is a still further development which makes possible precision of the etch back process whatever may be the specific shape of the apertures initially developed in the mask. Accordingly, it is an object of the present invention to provide an improved process for etching a pattern of apertures in an electrode or shadow mask.
  • the process of the invention is for etching a pattern of apertures of predetermined size in an electrode formed of a material that is subject to attack by a particular etchant.
  • the process comprises the. following steps. Initially, the electrode is subjected, while in a reference condition, to a solution of that etchant for a first etching interval to develop the desired pattern of apertures, if the electrode does not have such an aperture pattern in its reference condition, or alternatively to enlarge the apertures if the electrode does have such a pattern in its reference condition but in either case this etching step limits the aperture size to a value which is less than the predetermined size ultimately desired.
  • a stream of particles is directed upon the surface of the electrode after the first etching step and a control effect is derived therefrom representing the instantaneous size of the apertures as reflected by their transmissivity to such particles.
  • This control effect is utilized to control a second and subsequent etching of the electrode with the etchant to develop the apertures of the pattern substantially to the desired predetermined size.
  • the electrode is subjected to an etching solution in one stage for a particular time and at the end of that time a control potential is developed which is a measure of the response of the material of the electrode to the etching process.
  • This control potential is used to adjust the processing time of one or more succeeding stages in which the electrode is treated with an etching solution, preferably the same as that employed in the first stage.
  • the control permits precision in the final size of the apertures of the electrode.
  • the structure to be etched is variously referred to as a color-selection electrode, shadow mask or aperture mask employed in a color cathode-ray tube so that, in the three-gun variety, the electron beam issued from each of the three guns is permitted to excite phosphors of only an assigned one of the three colors constituting the mosaic or dot triad type of screen.
  • the structure will be referred to as an aperture mask and also for convenience it will be assumed that the screen is of the type which has circular or dot shaped deposits of phosphor obtained by photographic printing with an aperture mask having a pattern of circular holes arranged in a field corresponding to the shape of the image screen of the tube.
  • the aperture field may be circular or rectangular but usually a rectangular pattern is employed.
  • FIG. 1 is a block diagram of apparatus for performing the inventive process
  • FIG. 2 is a schematic representation of a densitometer arrangement that may be employed in one or more of the etching stages indicated in the arrangement of FIG. 1; while FIGS. 3 and 4 are curves used in explaining the process conducted with the arrangement of FIG. 1.
  • the invention will be described initially in the environment of re-etching or etch back and it will be assumed that an aperture mask having a rectangular pattern of circular apertures has been employed for photographically screening the cathode-ray tube in which the mask is ultimately to be installed.
  • the mask will, therefore, be conventional except that its apertures will have the dimensions required for screening. They may have uniform size or may be graded, being a little larger in the center than at the edges. In other words, the hole diameter decreases gradually with radial distance from the center of the mask.
  • the mask stock is generally cold rolled steel of approximately 7 mil thickness with. apertures in the center having a diameter of about 9 mils.
  • the mask will also have been oxidized or blackened on both surfaces to exhibit the heat-conducting and the lightreflecting properties of a black body. This surface treatment is distinctly preferred where the aperture mask is employed in photographic screening but since the details of that screening are of no moment to the present invention, they need not be considered further herein.
  • the first workstation 10 of the arrangement of FIG. 1 is an oxide stripper and rinse.
  • the oxide coating on the flat surfaces of the mask and on the walls of its apertures is removed by a treatment with hydrochloric acid and a detergent and after the mask has been stripped of its oxide coatings, it is rinsed with water.
  • the mask After leaving workstation 10, the mask is in its reference or starting condition by which is meant for the assumed embodiment of the invention the condition of the mask just prior to etch back. It, therefore, does have a desired aperture pattern that was previously developed in the initial fabrication of the mask.
  • the next step in the process is conducted in a multistage etching system 11,
  • etching apparatus of the general type that may be employed is disclosed in Pat. 2,762,149, issued Sept. 11, 1956 and Pat. 2,822,635, issued Feb. 11, 1958, both in the name of N. B. Mears. Since a multiplicity or succession of etching stages are contemplated, it is convenient to have the mask carried by a step-by-step conveyor through each of the several workstations.
  • Each etching stage is a chamber which may have entrance and exit doors automatically operated by a suitable programmer to permit an aperture mask carried by the conveyor to be introduced into the chamber and to be removed therefrom after a chosen time interval.
  • each chamber there is a cluster or field of spray heads positioned with respect to the rest position of the mask in that stage to direct a uniform flow of an etching solution of suitable concentration over one or both surfaces of the mask.
  • the solution includes an etchant which attacks the mask material and may, for example, be ferric chloride. Best results can be expected if all four etching stages receive etching solution from a common source so that the parameters of the process, as imposed by the etching solution itself, are common to all stages.
  • the final stage 11d leads to an arrangement 12 where the re-etched mask is rinsed, de-carbonized, blackened and finally rinsed with de-ionized water.
  • a mask formed of cold rolled steel may have a carbon film as it emerges from the re-etch stations; this film is removed by a treatment of phosphoric acid.
  • Blackening is undertaken to restore an oxide coating to the surfaces of the mask because it is desirable that the mask have the heat-conducting and light-reflecting properties of a black body as finally installed in the tube.
  • the blackening may be accomplished by heat treatment in an oxidizing atmosphere or this may be accomplished chemically in a salt bath of iron or zinc phosphate.
  • the process of the invention involves directing a stream of particles upon the surface of the mask after a predetermined etching time and deriving therefrom a control effect representing the size of the apertures in the mask as reflected by its transmissivity to such particles.
  • the nature of the particles employed in deriving the desired control effect is subject to considerable variation and may be solid particles or energy particles, that is to say, utilizing the particle analogue of energy as is frequently done especially in dealing with photons and phonons. It is convenient and practical to use light energy, visible or invisible but, of course, sources of visible light quickly suggest themselves.
  • An appropriate arrangement for responding to light to develop a control voltage is referred to in the art as a densitometer and is indicated schematically in FIG.
  • a photocell 25 is positioned directly across from light source 22 at workstation 110 to intercept and measure the amount of light energy transmitted through the mask apertures that are interposed in the light path. It is preferred that the light beam be large in cross section compared to the area of the individual apertures of the mask, a cross section of one square inch being representative of a useful beam size. Air is blown through nozzle 24 to clear away adhered acid.
  • the photocell develops a control effect, specifically a control voltage, dependent upon the quantity of light incident thereon and this control effect is utilized to control the etching of the mask in process in order to develop the apertures thereof to the desired size.
  • Calibration of the densitometer to effect control of the etching process so that the re-etched mask has precisely dimensioned apertures is readily obtained by using, as a reference, the value of voltage developed either when the light path between source 22 and photocell 25 is unencumbered by a mask or in the presence of a standard mask known to have apertures of the proper dimensions. In the pres ence of a mask having smaller dimensions, less light impinges upon cell 25 and the voltage developed is essentially a linear function of incident light.
  • the voltage output of cell 25 represents the measured aperture size of the mask in process and may be utilized to control any subsequent etching that may be necessary to achieve the desired hole size in the mask.
  • This densitometer arrangement for developing a voltage to control the re-etch process is represented in FIG. 1 by the box 13.
  • the control voltage developed in unit 13 is applied to a proportional controller 14 associated with etching stage 110 to adjust at least one parameter thereof for the purpose of controlling the re-etch process.
  • the processing time is controlled since it is desired that the etching solution of all four stages be the same.
  • Unit 14 in response to the control voltage from unit 13 develops an output signal having a duration which varies with the magnitude of the applied control voltage.
  • the signal output of unit 14, by controlling an electrically operable valve such as valve 31 of stage 110, adjusts the etching time of that stage.
  • Proportional controllers of this type are known in the art and a solid state form is available from the Potter Brumfield Division of American Machine & Foundry Company under the designation of a time delay relay.
  • the process carried out in the arrangement of FIG. 1 is one in which the aperture mask, having been stripped of its surface oxide layers at workstation 10, is etched successively with a common etching solution and for equal and known time intervals in stages Ila-11b.
  • the mask is then transferred to stage 110 and, as is common practice with start-stop conveyor systems, a rest time is available in which the doors preferably close over the entrance and exit ports of the stage.
  • lamp 22 is energized and directs a beam of light through aperture mask 20 to photocell 25.
  • the reading of the densitometer or the magnitude of control voltage developed in source 13 reflects the response of the mask blank to the etchant and it further represents the size of the apertures in the mask as reflected by the transmissivity of the mask to light. It is preferred that the processing times of stages 11a and 11b be insufiicient to achieve an aperture size that is required of the mask in its final form; certainly, the final aperature size must not be exceeded in these stages.
  • the densitometer measurement in determining the speed of the etching process in respect of the mask instantaneously in process further determines the amount of continued etching, if any, required to attain the desired size apertures in the mask. This determination is manifest in the voltage applied from source 13 to proportional controller 14 which accordingly adjusts the etching time of stage 110. If desired, the control of unit 14 may also extend to the fourth stage 11d of the etching arrangement as indicated. Since the control voltage is developed in unit 13 before etching is initiated in stage 110, control unit 14 may adjust the time of the third stage. If inadequate processing time is available in stage 11C, further control of the etching may take place in stage 11d.
  • Curve A indicates a mask response that attains an aperture size within an acceptable tolerance range, represented by ordinate limits Min and Max in the process interval of the third stage 110. In this instance proportional controller 14 interrupts the etching of that stage within these limits.
  • an acceptable aperture size is not obtained until the fourth stage 11d is reached and in this case controller 14 permits stage to operate over its total available etching time and adjusts stage 11d to terminate etching therein at the appropriate time.
  • the response of the mask material to the etching process may vary over such wide limits that the condition of curve C can be expected to be experienced. This represents the case where an aperture size Within acceptable limits is attained at the conclusion of the etching process of stage 11b. In such a case the control voltage from source 13, in operating upon controller 14, prevents additional etching of this particular mask in stages 11c and 11d.
  • the densitometer measurement may be made after the mask in process has been wiped dry, removing all solution and liquid from the mask. It is distinctly preferable, and has been found in fact very practicable, to conduct the densitometer measurement while the mask is still wet.
  • consistent densitometer readings may be made after the mask has been wetted by a liquid of such surface tension as to form a film or lens over the mask apertures so long as the solution is at least partially transparent to the light beam employed in the densitometer.
  • acceptable and consistent readings may be made after the mask has been etched and rinsed with water.
  • a multistation etching system of the type shown is attractive for mass production but the invention is not limited in this respect. It may be utilized advantageously where a single workstation is used. In that case, the mask is etched for a partciular process time, measured and then subjected to additional etching at the same workstation if necessary to achieve a precisely controlled aperature size.
  • the etching process is conducted for a sufiicient time interval to at least develop a desired pattern of holes in the mask after which the densitometer measurement is taken to control the further etching in order that the apertures may have a precisely controlled final size.
  • the tolerance range has been drastically reduced and may conveniently be kept within a range of .2 to .5 mil. Moreover, a most attractive saving in cost is made possible in the preferred process of the invention because the densitometer reading is made at a point in the process before the apertures of the mask will have attained the desired final size. This avoids the objection of past practices wherein the measurement of hole size could frequently occur after the maximum allowable size had been exceeded with consequent destruction of the mask.
  • the degree of control may be further extended by providing each of stages 11b, 11c and 11d with its own control arrangement comprising counterparts of units 13 and 14 individual to each such stage.
  • control effect is derived after said shadow mask has been transported from the first to the second of said workstations but before etching takes place in said second station.
  • controleffect is derived by developing a light beam in said workstation and by directing said light beam upon the apertured surface of said shadow mask and measuring within said enclosed workstation the amount of said beam emerging through the apertures of said pattern.
  • the etching process is accordance with claim 6 in which said control effect is utilized to adjust the etching time in the second of said workstations.
  • control eficect to determine the duration of a second and subsequent etching of said shadow mask with said solution to further enlarge said apertures substantially to said predetermined size.

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  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • ing And Chemical Polishing (AREA)
  • Manufacture Of Electron Tubes, Discharge Lamp Vessels, Lead-In Wires, And The Like (AREA)
  • Electrodes For Cathode-Ray Tubes (AREA)
US6619A 1970-01-28 1970-01-28 Process of etching a shadow mask Expired - Lifetime US3669771A (en)

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US661970A 1970-01-28 1970-01-28

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DE (1) DE2100612C3 (de)
GB (1) GB1331814A (de)
NL (1) NL7018858A (de)

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3909311A (en) * 1974-08-05 1975-09-30 Hitachi Ltd Shadow mask for use in color picture tube and method for fabricating same
US3973965A (en) * 1972-05-30 1976-08-10 Tokyo Shibaura Electric Co., Ltd. Making shadow mask with slit-shaped apertures for CRT
US4050821A (en) * 1976-09-27 1977-09-27 Bell Telephone Laboratories, Incorporated Linewidth measurement method and apparatus
EP0004615A1 (de) * 1978-04-10 1979-10-17 Siemens Aktiengesellschaft Verfahren und Vorrichtung zum Prüfen von hochpräzisen Formätzteilen
US5387313A (en) * 1992-11-09 1995-02-07 Bmc Industries, Inc. Etchant control system

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3049595A1 (de) * 1980-12-31 1982-07-08 Albert-Frankenthal Ag, 6710 Frankenthal Baenderstrecke zum transport und zur verlangsamung von falzprodukten

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3973965A (en) * 1972-05-30 1976-08-10 Tokyo Shibaura Electric Co., Ltd. Making shadow mask with slit-shaped apertures for CRT
US3909311A (en) * 1974-08-05 1975-09-30 Hitachi Ltd Shadow mask for use in color picture tube and method for fabricating same
US4050821A (en) * 1976-09-27 1977-09-27 Bell Telephone Laboratories, Incorporated Linewidth measurement method and apparatus
EP0004615A1 (de) * 1978-04-10 1979-10-17 Siemens Aktiengesellschaft Verfahren und Vorrichtung zum Prüfen von hochpräzisen Formätzteilen
US5387313A (en) * 1992-11-09 1995-02-07 Bmc Industries, Inc. Etchant control system

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DE2100612A1 (de) 1972-01-27
DE2100612B2 (de) 1973-06-20
NL7018858A (de) 1971-07-30
GB1331814A (en) 1973-09-26
DE2100612C3 (de) 1974-01-10

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