US3609033A

US3609033A - Mask for etching enlargement

Info

Publication number: US3609033A
Application number: US850450A
Authority: US
Inventors: John J Frantzen
Original assignee: Buckbee Mears Co
Current assignee: Buckbee Mears Co
Priority date: 1969-08-15
Filing date: 1969-08-15
Publication date: 1971-09-28
Anticipated expiration: 1988-09-28
Also published as: DE2005151A1; BE744171A; FR2057648A5; JPS501870B1; GB1252713A; NL7009574A

Abstract

A mask has a metallic material, preferably cold-rolled steel, covered by an etchant-resist layer on both sides. On one side the layer has a circular area of diameter X2 void of etchant resist, and on the second side the layer has a circular area of diameter X3 and an annular area of outer diameter X1 both void of etchant resist. The three areas are coaxially aligned.

Description

United States Patent [50] Field ofSearch 355/26, 77,

[72] lnventor John J. Frantzen North Saint Paul, Minn. 125 [21 l 2 1969 Primary Examiner-Samuel S. Matthews [22] 1 e I Assistant Examiner-Michael D. Harris [45] Patented Sept 1971 Attorney-Stryker and Jacobson [73] Assignee Buckbee-Mears Company a St. Paul, Minn.

ABSTRACT: A mask has a metallic material, preferably coldrolled steel, covered by an etchant-resist layer on both sides. [54] MASK FOR Q F ENLARGEMENT On one side the layer has a circular area of diameter X, void of 4 Claims 7 Dram"! etchant resist, and on the second side the layer has a circular [52] US. Cl 355/26, area of diameter X and an annular area of outer diameter X 355/77, 355/ 125 both void of etchant resist. The three areas are coaxially [5 1] Int. Cl 60;"; 21132 aligned.

1. flz y I 4 *h X 2 F /v i m I T i 35 30 PATENTEBsEmmn 3.609033 sum 1 or 2 INVENTOR JOHN J. FRANTZE/V ATTORNEYS l. Fieldof the Invention This invention relates generally to making shadow masks for colored television tubes and, more specifically, for photopn'nting unusual patterns in register that are suitable for etching enlargement in a two-step etching process.

2. Description of the Prior Art In a colored television picture tube, the shadow mask or aperture mask is located between the electron guns at therear of the tube and the phosphor-coated faceplate at the front'or viewing face of the tube. Electron beams pass through the openings or apertures inthe shadow mask and impinge upon a suitable color producing phosphorus dot on the faceplate. Behind each of the openings of the shadow masks are three phosphorus dots, a triad, one dot for each of the three primary colors. During the asse'mbly of the picture tube theshadow mask is used as a mask or master for forming the phosphorus dots on the faceplate of the television tube. This is usually done using photographic techniques which are well known in the industry today.

In order to produce the maximum possible brilliance at the face of the tube when the phosphor is struck by the electron beam but at the same time to focus the beam on the proper triad, it is necessary to have the phosphor dots smaller in diameter than the diameter of the apertures of the shadow inask. To facilitatethe screening of the phosphorus dots with the existing apertures of the shadow mask, it requires a temporary stepping down or reduction in the diameter of the apertures by partially filling the apertures with a second material tha'tis different from the mask material. The, after using the mask for forming the pattern for the phosphorus dots on the faceplate of the tube, the filling material can be removed to enlarge the holes in the aperture mask so thatthe maximum brilliance can be obtained in the television tube.

' Another method of obtaining smaller holes in the shadow mask for the deposition of the phosphorus dots is to etch a set ar first circular opening in the shadow mask which have a diameter suitable for use as a pattern for forming the phosphorus dots on thefaceplate of the television tube. Then after layingthe phosphorus dot pattern on the faceplate of the television tube the holes are subjected to etching enlargement by controllably spraying etchant on the mask.

The present invention comprises the method of printing and compensation for forming a suitable pattern in register in photo resist for producing the etched configuration of a set of "circular openings with a corresponding set of undercut annular areas located around the circular openings. During the two-step etching process a set of larger diameter annular areas which are coaxially located with respect to the first set of circula'r'openings are etched partially through the *mask. Ina l'a'ter etching step the annular area'which was etchedpartially through the first etching step is etched completely through,

thus causingthe material inside the annular area to'drop out leaving a set of largerdiameter openings in the shadow mask.

As the etching pattern does not exactly follow the resist pattern, and as the relative dimensions between the circular openings and the annular areas are quitesmall, it is difficult to uniformly etch away the'material around the circular openings to cleanly remove the annular ring surrounding the central opening.

SUMMARY Briefly, the present'invention comprises a process where the holes in a shadow mask are first etched to produce an opening for laying a phosphorus dot pattern and then etched to remove an annular ring thus leaving the opening enlarged to the set of pinholes and onto a second photosensitive plate and then developing the negative formed on the second photosensitive plate. Next, the negative master plates are placed in optical registration and put into a suitable printing apparatus to print the etching pattern in photoresist on opposite-sides of the mask. In the next step the mask is immersed in an etchant until a first set of central apertures are etched out. next, the mask'is used to lay the phosphorus dot pattern and finally, the mask is immersed in etchant until the material surrounding the aper ture has been removed to provide the enlarged aperture opening of the desired size.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows material used for making the negative pattern on a plate glass;

FIG. 2 shows the frontside patterned image on a suitable glass plate;

FIG. 3 shows the camera setup and the pinhole negative for use with the image that is projected on one side of a master pattern used for making a shadow mask;

FIG. 4 shows the backside patterned image whichis formed on the opposite side of the shadow mask;

FIG. 5 shows a cross-sectional oblique view of the etchantresist pattern located on opposite sides of the material;

FIG. 6 shows the cross-sectional appearance of the mask after etching partially through the mask; and

FIG. 7 shows the cross-sectional appearance of an aperture as the surrounding annulus is etched free from the mask.

DESCRIPTION OF THE PREFERRED METHOD FIG. 1 showsa portion of a suitable opaquematerial for use in making an enlarged pattern for use in the photoprinting process. An example of a suitable opaque material is an opaque material that comes in a roll and is marketed under'the trade name Studnite. Preferably, the operator cuts thepattern at approximately times scale to minimize and reduce errors in forming the pattern. After cutting the;pattem outline in the opaque material, the areas 1.1, 12, and 13 (H6. -l,-).are

carefully stripped away from the backing material. 'Note,the

pattern also includes guidelines 11 located horizontally and vertically to enable the operator to properly align'the pattern with respect to the camera. Next, the materialis contacted'to a first glass plate and then recontacted to a second glass plate 16 (FIG. 2) to put the pattern in negative contrast. The negathe contrast patterns are identified by 110, 12a, and 13a,

Next, the single pattern in negative contrast onglass plate '16 -'is placed in a copyboard holder 17. Similarly, a backside'pattern 27 (FIG. 4) is formed on a glass plate 27.

After placing pattern plate 16 in copyboard holder 17, a suitable source of illumination such as a set of mercury vapor lights 18 are turned on behind the copyboard. At thisti'me negative 8 and pinhole board 9 are not in the camera as.pat-

tern plate 16 has to be aligned prior to exposing the negatives.

respect to the lineup pattern. Typically, the lineup pattern is also a glass plate that has a set of mutually perpendicular lines located in the center of the glass plate. With the lights on behind the copyboard, the image on the copyboard pattern plate is lined up with respect to the center of the lineup pattern in the back of the camera. After obtaining proper alignment of pattern plate 16, the operator fastens pattern plate 16 to prevent movement during the subsequent steps and removes the lineup pattern plate from the back of the camera. In the next step, the lights are turned off and pinhold board 9 is placed into the proper position in the back of the camera (FIG. 3). Next, the operator places a photosensitive plate 8 behind the pinho ld board. Photosensitive plate 8 is positioned so that it lies in a plane parallel to the pinhole board. As the lens has been removed from the camera 10 an image can be projected from the copyboard through each of the pinholes and onto photosensitive plate 8 when light 18 is turned on.

Turning light 18 on produces a set of images on photosensitive plate 8 which are identical in appearance to the copyboard image or pattern plate 16, because they all have been formed from the same copyboard image on pattern plate 16.

In order to obtain the backside pattern for the shadow mask, the same procedure of placing the pattern plate 27 in copyboard holder 17 and aligning pattern plate 27 with respect to a lineup pattern is repeated. Care must be taken to ensure that the spacing of the pattern plates from the pinhole board and the spacing of photosensitive plate from the pinhole board is correct to ensure that the proper reduction in image size is obtained.

In utilizing this process to form a multiple negative aperture pattern from a master frontside pattern and a multiple negative aperture pattern from master backside pattern, it is necessary to utilize either the same pinhole board or an identical pinhole board to obtain multiple negative frontside patterns and multiple negative backside patterns that can be placed in alignment. By utilizing the same pinhole board, it eliminates the possibility of misalignment of registration due to irregularities from pinhole board to pinhole board.

Once the photosensitive plates have been exposed by light projectable through the image on the copyboard and the openings in the pinhole board, the photosensitive plates can be developed. These negatives containing a multiple frontside and backside patterns can be used as a master pattern to mask off the photoresist material on both sides of the mask. However, if the negative master patterns are printed in misalignment, the two-step etching process cannot be satisfactorily accomplished. For example, if there is misalignment so that the circular backside pattern is not coaxial with the annular frontside pattern, it will produce regions where the etchant cannot etch completely through the mask without enlarging the central opening beyond the preferred size. Obviously, such masks are unsuitable for use in colored television industry as the electron beams cannot be focused properly through the apertures unless the apertures are properly sized.

The foregoing has been a description of the process of forming the frontside and backside pattern on suitable negative master patterns. In addition to forming and then printing the patterns in registration on the frontside and backside of the mask, it is necessary to carefully select the relative pattern dimensions to enable the operator to cleanly remove the excess material with the two-step etching process. The aspect of the dimensional compensation for obtaining a pattern having a suitable undercut surrounding region will now be described.

In order to appreciate the difficulties in forming a suitable pattern that can be used in a two-step etching process, an ex ample of the typical dimensions involved in the apertures of the shadow mask are given hereafter, although no limitation is intended thereto. In a typical shadow mask the diameter of the central opening a (FIG. 6) is approximately 0.008 inches. After the shadow mask has been used as a pattern to lay the phosphorus dots on the faceplate, it is necessary to enlarge the central opening b (FIG. 7) to approximately 0.015 inches. FIG. 5 shows a material 30 having a layer of patterned etchant resist 31 on one side and layers of patterned etchant resist 32 and 33 located on the opposite side. Resist layer 31 has a central area 34 which is void of etchant resist and similarly located on the opposite side are two

areas

35 and 36 which are also void of etchant resist. Reference character X designates the diameter of circular area 34 and reference characters X and X designate the respective outermost dimension of area 25 and area 36. Reference character L designates the width of area 25 while reference character T designates the thickness of material 30.

FIG. 6 shows a greatly enlarged aperture '38 having a central opening of diameter a in cross section after the first etching step. Note, the outline of the etched material 30 with the relatively bowl-shaped sides 20 and the smaller

angular areas

21 and 22. Also, note how the etchant has undercut the corners of the resist.

Briefly, in order to obtain a mask having a suitable undercut configuration such as shown in FIG. 6, it is necessary to print the pattern in photoresist so that the outermost dimension Xhd l on one side of the mask is approximately equal to the outermost dimension X on the opposite side of the shadow mask (see FIG. 5).

Also, in order to properly undercut the mask, it is necessary to limit the width of the annular area L to slightly less than one-fourth the thickness T of the shadow mask. Having the dimension L too large causes the material in area 35 to etch completely through to sidewalls 20 before the desired central opening 38 is obtained. Having the width L too small prevents the etchant from etching to the proper depth so that ringlike member 40 (FIG. 7) can be removed in a second etching step while still maintaining the integrity of the final dimension b.

Located in photoresist within the central area having a dimension X is a second central area having a dimension designated by X This diameter is substantially equal to the diameter of the central opening a through the mask (see FIG. 6). During etching the etchant undercuts the resist and generally angles inward to produce the desired central opening of diameter 0 shown in FIG. 6.

In order to etch a typical cold roll steel shadow mask, the operator lowers the shadow mask containing the photoresist pattern, which is shown in FIG. 5, into a solution of ferric chloride etchant. The shadow mask remains in the etchant until the etchant etches away the central circular areas thus leaving an opening of diameter a. This can be detennined by visual inspection and by monitoring the time the mask is in the etchant. When the mask is immersed in etchant, the larger diameter open area 35 etches faster into the shadow mask than the corresponding

smaller areas

35 and 36 which are located on the opposite side of the mask. After the apertures 38 appear uniform throughout the mask, the mask is removed from the etchant and washed to prevent any etchant clinging to the mask from enlarging or further undercutting the mask. In a

typical mask areas

35 and 36 etch approximately onethird of the thickness of the mask while on the opposite side of the mask the center of the larger area 34 etches to a depth of approximately two-thirds the thickness of the mask in the same amount of time. By limiting the width of the annular area L so that it is approximately onefourth of the thickness of the mask and utilizing the bowl-shaped etching pattern, one can produce an area surrounding the opening of thickness c which can be etched away in a later etching step. After the mask has been used to lay the phosphorus dot pattern, the mask is immersed in etchant until the area of thickness c has been etched through (FIG. 7). This causes ringlike member 40 to drop out leaving an operator having an enlarged final diameter b. The resist can now be removed and the mask placed in the television tube.

Although the process has been described in terms of circular openings and circular secondary areas, it is apparent that the process could be also used to print oblong, hexagonal or other shaped openings.

lclaim:

1. An aperture mask suitable for etching enlarging comprising: a metallic material having a thickness T; a first layer of etchant resist located one side of said material and a second layer of etchant resist located on the opposite side of said material; said first layer of etchant resist having a first circular area of diameter X being substantially void of etchant resist; said second layer of etchant resist having a second circular area having a diameter X being substantially void of etchant resist, said second layer of etchant resist having an annular area substantially void of etchant resist, said annular area having an outer diameter X said outer diameter X being substantially equal to the diameter X, of said first circular area.

2. The inventions as described in claim 1 wherein said first circular area, said second circular area and said annular area are located in coaxial alignment. 1

3. The inventions as described in claim 2 wherein the width

Claims

1. An aperture mask suitable for etching enlarging comprising: a metallic material having a thickness T; a first layer of etchant resist located one side of said material and a second layer of etchant resist located on the opposite side of said material; said first layer of etchant resist having a first circular area of diameter X2 being substantially void of etchant resist; said second layer of etchant resist having a second circular area having a diameter X3 being substantially void of etchant resist, said second layer of etchant resist having an annular area substantially void of etchant resist, said annular area having an outer diameter X1, said outer diameter X1 being substantially equal to the diameter X2 of said first circular area.

2. The inventions as described in claim 1 wherein said first circular area, said second circular area and said annular area are located in coaxial alignment.

3. The inventions as described in claim 2 wherein the width of the annular area L is approximately one-fourth the thickness T of said material.

4. Inventions as described in claim 3 wherein said material compromises cold-rolled steel.