US 3669771 A
Description (OCR text may contain errors)
June 13, 1972 L. LERNER $669,771
PROCESS OF ETCHING A SHADOW MASK Filed Jan. 28, 1970 (l4 68BX82 ProporTi0n0I r I2 lomo F l Oxide o o o D e 151. c 0 2nd. c 0 5rd. c D 4th 0 o ecor omze FEZZ Swge stage W T Bloc ken.
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(Time) Size (Fe Hole Size (Dry) lnvenror Morhn L. Lerner Attorney Air Source UnitedStates Patent Office Patented June 13, 1972 3,669,771 PROCESS OF ETCHING A SHADOW MASK Martin L. Lerner, River Forest, Ill., assignor to Zenith Radio Corporation, Chicago, Ill. Filed Jan. 28, 1970, Ser. No. 6,619 Int. Cl. C23f 1/02 US. Cl. 156-8 9 Claims ABSTRACT OF THE DISCLOSURE A shadow mask for a color television tube, having a pattern of apertures dimensioned as required for screening, is further etched after the screening process to attain a hole size larger than the phosphor deposits of the screen. The re-etching occurs in several stages and a densitometer associated with one such stage measures the hole size and derives a voltage used to control the etching time so that the apertures of the re-etched mask are precisely controlled to a desired size.
BACKGROUND OF THE INVENTION 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.
Difficulties have been encountered in the screening of these types of tubes and best results for production on a commercial scale have been achieved with an etch back process. In that process, 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. When this has been accomplished, 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.
The general concept of this etch back process is, of course, well understood; it 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.
Previous efforts have been made to facilitate precision of the etch back process. For example, copending application Ser. No. 811,318, filed Mar. 28, 1969, in the name of Sam H. Kaplan discloses improvements in the etch back process attained by contouring or shaping the holes as originally developed in the shadow mask. In another copending application, Ser. No. 850,408, filed Aug. 15, 1969, in the name of Joseph M. Black, still further improvements are attained by arranging the apertures of the mask uniquely to reduce the processing time of the etch back step. Both of these applications are assigned to the assignee of the present invention.
While these approaches are attractive and useful, 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.
SUMMARY OF THE INVENTION 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.
In one aspect of the invention, 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.
As indicated above, 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. For convenience hereinafter, 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.
It will also be appreciated, especially as the details of the inventive process'are described, that the specific configuration of the apertures of the mask is of no particular consequence. Usually, they are round although they may be hexagonal or rectangular. Any such aperture mask may have its aperture pattern developed and/or enlarged to precisely controlled dimensions through the practice of the subject invention.
BRIEF DESCRIPTION OF THE DRAWING The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identity like elements, and in which:
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.
DESCRIPTION OF THE PREFERRED EMBODIMENT Further to simply the disclosure, 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 oxide coating tends to resist the etchant otherwise employed in forming the aperture pattern in the mask and, therefore, the first workstation 10 of the arrangement of FIG. 1 is an oxide stripper and rinse. In this station, 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.
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,
shown as comprising four stages lla-lld although the number of stages is of no particular importance. Preferably, the mask is subjected to the same etching solution and process in each of the four stages which may individually be of any well known structural arrangement. By way of illustration, 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. In 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. Of course, 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. It is found that 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. These steps are well known and of themselves constitute no part of the pres ent invention. As thus far described, the re-etching arrangement of FIG. 1 is conventional; consideration will now be given to the improvement introduced by the present invention.
More particularly, 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. 2 which represents etching stage wherein a mask 20 in process is shown supported on a carriage 21 in the form of an open frame secured to or constituting part of the work carrying conveyor and through which light from a source 22 collimated by a colliamtor 23 may be directed toward the central area of the mask. 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.
It is convenient to mount photocell 25 within and above the cluster of spray heads 30 through which etching solution delivered from a source (not shown) is admitted through a valve 31 and header 32 to the various spray heads.
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. Accordingly, 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. For the simplest and preferred case, 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. Also during this time lamp 22 is energized and directs a beam of light through aperture mask 20 to photocell 25. Since the starting conditions of the mask are known, because the mask has apertures of preselected size for use in screening, and since the parameters and process times of stages 11a and 1117 are likewise known, 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. If this condition is satisfied, 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.
The curves of FIG. 3 reflect the variable response of mask blanks to the etching process and clearly demon- Strate the need served by the present invention. 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. For the mask represented by curve B 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.
Of course, 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. Experiments have shown that 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. For example, acceptable and consistent readings may be made after the mask has been etched and rinsed with water. It has also been found, however, that equivalent results may be obtained if the measurement is made directly after etching and while the mask is still wet with etching solution. The curve of FIG. 4 indicates that a direct correlation may be made between the hole size of the mask wet with etching solution and the hole size of the mask after it has been dried. Accordingly, a distinct advantage of the described process is that the measurement may be made and the process controls adjusted without removing the mask from the etch workstations.
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.
It has been convenient to describe the invention in the environment of re-etch or etch back simply because the circumstances of that process gave rise to the described solution of precisely controlling the etching process in order to size the apertures of the mask for a black surround tube. The process is, of course, suitable for forming a pattern of apertures in a mask blank while it is in its flattened, imperforate sheet form. This is usually accomplished by arranging resist coatings on both surfaces of the mask with interruptions or openings that represent the apertures to be formed. In such case, 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.
Prior to the development of the process control described herein, the experience of etch back in particular was a variation in hole size of the masks as much as 5 mils.
With the process control of this invention, 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. Of course, 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.
While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the apended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.
1. The process of etching a pattern of apertures of a predetermined size in a shadow mask formed of a material that is subject to attack by a particular etchant which process comprises the following steps:
subjecting said shadow mask, while in a reference condition, to a solution including said etchant for a first etching interval to develop said patern of apertures, if said shadow mask does not have such an aperture pattern in said reference condition, or to enlarge the apertures if said shadow mask has such an aperture pattern in said reference condition but in either case to attain an aperture size which is less than said predetermined size;
directing a stream of energy particles upon one surface of said shadow mask after said etching interval and deriving from energy particles transmitted through the apertures of said shadow mask a control efiect representing the instantaneous size of the apertures in said shadow mask as reflected by the transmissivity of said shadow mask to said particles;
and utilizing said control effect to control a second and subsequent etching of said shadow mask with said etchant to develop said apertures of said pattern substantially to said predetermined size.
2. The etching process in accordance with claim 1 in which said control effect is derived at the completion of said first etching interval, while said shadow mask remains in the etching apparatus, and is utilized to control a subsequent etching step to develop said apertures of said predetermined size.
3. The etching process in accordance with claim 2 in which said first and subsequent etching steps are conducted with the same etching solution;
and in which said control effect is utilized to adjust the etching time of said subsequent etching step.
4. The etching process in accordance with claim 1 in which a solution employed in the processing of said shadow mask has sufficient surface tension to form a film over apertures of said pattern which is at least partially transparent to said energy particles;
and in which a stream of said particular energy is directed to said shadow mask to derive said control effect.
5. The etching process in accordance with claim 4 in which said film serves as a lens that is at least partially transparent to light;
and in which a light beam, having a cross section large with respect to the area of the individual apertures of said shadow mask, is directed upon said shadow mask to derive said control effect.
6. The etching process in accordance with claim 2 in which said etching steps are conducted in at least two enclosed workstations between which said shadow mask is transported;
and in which said 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.
7. Theetching process in accordance with claim 6 in which said 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.
8. 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.
9. The process of enlarging to a predetermined size the apertures of an aperture pattern in a shadow mask formed of a material that is subject to attack by a particular etchant, which process comprises the following steps:
subjecting said shadow mask to an etching solution including said etchant for a first period of time to enlarge the apertures of said shadow mask to a size less than said predetermined size;
at the conclusion of said first etching step measuring the instantaneous size of said enlarged apertures in said shadow mask and deriving therefrom a control effect representing the response of said shadow mask to said etching solution;
and utilizing said 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.
References Cited UNITED STATES PATENTS 2,762,149 9/1956 Mears 156-8 OTHER REFERENCES Photocell Controlled Etcher, by Greene et al., IBM Tech. Disclosure, pp. 582-4, vol. 10, No. 5, October 1967.
JACOB H. STEINBERG, Primary Examiner U.S. Cl. X.R.