US 3634084 A
A photographic process in which multiple low-intensity, discrete light flashes are used to expose a photographic emulsion in order to mitigate the formation of spurious exposed areas in the image which are produced by a single, high-intensity flash exposure. Improved resolution also results from the multiple flash process.
Claims available in
Description (OCR text may contain errors)
United States Patent Inventors John F. Creedon Waverly; Richard W. Kern, Vestal, both of N.Y. Appl. No. 650,945 Filed July 3, 1967 Patented Jan. 11, 1972 Assignee International Business Machines Corporation Armonk, N.Y.
MULTIPLE LIGHT FLASH PHOTOGRAPHIC PROCESS 8 Claims, 10 Drawing Figs.
U.S. Cl 96/46, 96/27 E, 96/36.2
Int. Cl ..G03c11/00, G03c 5/04 Field of Search 96/27, 46, 36.2, 38.4, 45.2
 References Cited UNITED STATES PATENTS 2,763,180 9/1956 Beck 96/45.2 3,451,813 6/1969 Kinney et a1 96/36.2
FOREIGN PATENTS 1,055,073 1/1967 Great Britain 96/36.2
OTHER REFERENCES R. C. lngraham, SCP and Solid State Technology, March 1965, Pp- 33- 34, 39 42.
Aspden, Electronic Flash Exposure Macmillan (1960) pp.'l4, 15, 27- 29 Primary ExaminerN0rman G. Torchin Assistant E.\'aminerJohn Winkelman AttorneysHanifin and Jancin and Norman R. Bardales ABSTRACT: A photographic process in which multiple lowintensity, discrete light flashes are used to expose a photographic emulsion in order to mitigate the formation of spurious exposed areas in the image which are produced by a single, high-intensity flash exposure. Improved resolution also results from the multiple flash process.
PATENTEU JAN 1 I872 3,634,084
SHEET 1 [IF 3 INVENTORS JOHN F. CREEDON RECHARD W. KERN ATTORNEY PATENTED mu 1 1912 34,0 4
sum 2 0r 3 15 FLASHER CONTROL W 14 MED JAN 1' 1 1972 FATE SHEET 3 BF 3 BACKGROUND OF THE INVENTION The invention relates to a photographic process and more particularly to a photographic process for mitigating the formation of spurious exposed areas in the image which isproduced in a light-sensitive emulsion.
As is well known to those skilled in the art, when a photographic emulsion is exposed by an illuminating light which is directed towards a subject, a latent image of the subject is formed in the photographic emulsion. The image appears in the emulsion when the latter is developed. The image is formed from the exposed and unexposed areas of the emulsion. If the emulsion is developed by a conventional or negative developing technique, the exposed areas become darker and the unexposed areas become lighter with respect to each other and is referred to as a negative. The light and dark areas of the negative image correspond to the dark and light areas, respectively, of the subject. If the emulsion on the otherhand is developed by a reversal developing technique, also referred to as a direct positive developing technique, an opposite effect is produced. That is, the exposed areas appear lighter and the unexposed areas appear darker. In this latter case the dark and light areas of the image correspond to the dark and light areas, respectively, of the subject. In either case, if the emulsion layer is on a transparent substrate such as a transparent glass plate or the like, the lighter areas are transparent and the darker areas are opaque. The dark and light areas in turn define and represent the respective contrasting features of the subject which is photographed.
It has been found that spurious exposed areas appear as small spots or regions within the unexposed areas thereby masking or obliterating the actual feature represented by the unexposed areas. The spots are dark if a conventional or negative developing technique is utilized and thus appear as opaque spots in the transparent areas if the emulsion is supported by a transparent substrate. The spots are lighter if a direct positive developing technique is employed and appear as transparent spots in the opaque areas if the emulsion is supported by a transparent substrate. The presence of these spots or regions has been found to be objectionable to the quality of the image produced in the emulsion and the present invention is directed towards elimination and/or mitigation of their formatron.
In the printed circuit and/or integrated circuit manufacturing arts utilizing photoresist techniques, for example, these spots have been found to be highly undesirable. More specifically, as is well known to those skilled in the art, these photoresist techniques employ a photomask which contains a replica of the circuit pattern. The mask is aligned with the photoresist emulsion layer carried on a supporting substrate. The layer is exposed by a light source through the mask so that the pattern image is transferred onto the layer. Thereafter, further operations including inter alia developing, fixing, etching, and/or stripping are performed on the layer to produce the resultant printed and/or integrated circuit. As is customary in this art, the photomask is a reduced photographic recording made from an enlarged artwork of the circuit pattern which is to be finally transferred to the photoresist emulsion layer. Generally, there are usually at least two photographic reductions made. The first reduction is made from the initial artwork and the second reduction is made from the first reduction and so forth. The final reduction serves as the photomask. Heretofore in the prior art, there was no known way to eliminate and/or mitigate the spots produced in the photomask when the latter was being made. Consequently, the image of the spots in the photomask were transferred to the photoresist emulsion layer of the circuit board when exposed by the light source. In turn, when the circuit board was finally processed, it would also contain spurious regions of the material used to form the circuit pattern. As a result, the electrical parameters of the circuit board were adversely affected. For example, the spurious regions of the board caused by the spots of the photomask adversely affects the resistive and/or reactive impedance characteristics of the final circuit board elements and/or were conducive to forming short circuits therebetween and the like. The presence of the aforementioned spurious spots are even more undesirable in the manufacture of microminiature integrated circuits where the dimensions of the spots become comparable in size to the dimensions of the final circuit element(s).
SUMMARY OF THE INVENTION It is an object of this invention to provide a photographic process which produces improved image quality in the photograph recording.
It is another object of this invention to provide a photographic process which mitigates the formation of spurious exposed areas in the image which is produced in a photographic emulsion.
It is another object of this invention to provide a photographic process which enhances the line edge sharpness between contrasting features of the image produced in a photographic emulsion.
Another object of this invention is to provide a photographic process which is particularly amenable but not limited thereto to the manufacture of printed and/or integrated circuits.
Still another object of this invention is to provide a photographic process which produces an improved photomask of the kind used in the manufacture of printed and/or integrated circuits by photoresist techniques.
Still another object of this invention is to provide an improved photographic process which produces a photomask of the aforementioned kind which is relatively free from spurious exposed areas in the image carried thereby.
Still another object of this invention is to provide an improved photographic process which produces a photomask of the aforementioned kind which is relatively free of exposed areas in the image carried thereby and which has an improved line edge sharpness between the contrasting features of the image.
Accordingly, the invention features a photographic process wherein multiple, low-intensity, discrete light flashes are directed at the subject to expose a light-sensitive photographic emulsion and thereby mitigate the formation of spurious exposed areas in the image.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiment of the invention as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view, partially illustrated in perspective, of the apparatus for making from initial artwork the first photoreduction and from which reduction in turn a photomask of the type used in photoresist methods is to be made;
FIG. 2 is a perspective view illustrating in greater detail the first photoreduction of FIG. 1;
FIG. 3 is a schematic view of apparatus which may be used to make a photomask from the first reduction of FIG. 2 in accordance with the principles of the preferred embodiment of the present invention;
FIG. 4 is a perspective view of the undeveloped photomask made by the apparatus of FIG. 3;
FIGS. 5a and 5b are partial enlarged views of the mask of FIG. 4 which are developed by a conventional, i.e., negative, developing technique and by a direct positive developing technique, respectively; 1
FIG. 6a is a photomicrograph illustrating the spurious exposed areas in a developed emulsion of a photomask and which are produced when a known photographic process of the prior art is utilized to make the mask; and
FIGS. 6b-6d are photomicrographs illustrating the successive mitigation of the formation of the spurious areas in the developed photographic emulsions of respective difi'erent photomasks which are made by the photographic process of the present invention.
In the FlGS.,' like elements are designated with similar reference numerals.
DESCRIPTION OF THE PREFERRED EMBODIMENT Accordingly, referring to FIG. 1, the member 1 is the initial artwork from which a first photoreduction 2, shown schematically'thereimis' made by camera '3. By way of example, to fabricate the initial artwork l a light-sensitive emulsion layer 4 is completely'deposited on a planar surface of a glass substrate. The layer 4 is then photographically fixed without exposure. Themember lis then immersed in a water-soluble, red due solution, which iscommercially availablesTo set the dye the member is next immersed in a 2 percent acetic acid solution and then dried. Next, with the aid of a diamond scribing tool and an X-.Y positioner, to which the member 1 is temporarily affixed, the layer 4 is carefully inscribed with the outline of a preselected circuit pattern on a predetermined en-' larged scale. Thereafter, thoseparts of the layer 4 corresponding to the circuit pattern are removed from surface 5 such that l the remaining parts of layer 4 will outline the enlarged circuit pattern which-is formed by the visible parts of the underlying glass surface '5. As is obvious there'is a contrast between the visible parts of theglass surface 5 and the remaining layer 4. For sake of clarity, the circuit pattern is illustrated in FIG. 1 as having a generally'rectangular configuration I with a horizon- V tal dimension L.
colored filter, such as the green filter 21, may be provided to furnish near monochromatic light to the light-sensitive emulsion 19. In addition, a shutter member 22, illustrated schematically, may also be optionally provided. The elements ofapparatus l0 and the members 2 and 9 are juxtaposed with" respect to one another so that exposure of emulsion 19 through member 2 will produce areduced image therein.
According to the principles of the present invention, the v;
emulsion 19 is exposed by multiple, low-intensity, discrete light flashes from the tube 1 1. In practice, the exposure takes,
place in a photographic darkroom or other suitable environ- J ment in which the emulsion 19 will not be sensitive to the ambient light. As a result, a reduced image of the member2is transferred to the emulsion 19 in which theformation of the aforementioned spurious spots are eliminated and/or mitigated. The image of member 2 produced in emulsion I9 is reduced by a predetermined reduction factor which for the particular aforementioned case is a reduction factor of five and one-half times. Thus, in the particular aforementioned casewith a first reduction factor of 12 times and a subsequent reduction factor of five and one-half times, the image in emulsion 19 is 66 times smaller than the initial artwork I.
The member 9 is adapted to be selectively moved relative to the apparatus 10 in a plane which is normal to the optical axis 23 of apparatus 10. For example, this is generally accomplished by temporarilyaffixin'g the member 9 to an X.Y posi-Q tioning table, not shown. The member 9 is moved to different preselected positions where, at each position, the reduced imageof the member 2, or of another similar member(s) hav-; 7 ing a different circuit pattern configuration(s), is photographically transferred to the emulsion l9. In the art, this is referred to as a step-and-repeat operation and allows a multiple array of images to be formed on photomask 9. In accordance with the aforementioned principles of the present invention, the
emulsionl9 at each position isexposed to multiple, low-intensity, discrete light flashes from the lamp 11 so as to transfer a The photoinember 2 has a high resolution, light-sensitive,
emulsion 7, cf. FIG. 2, which initially is completely deposited on a planar surface of a transparent glass plate 8. After exposing the emulsion 7 to the artwork 1 in camera 3, the member 2 is developed by a direct positive developing technique and the resultant reduc'ed imageof artwork l appea'rs in the emulsion 7, asis shown in greater detail in FIG. 2. The circuit pattern thus corresponds to the rectangular configuration ll, FIG. 2,
the dimensions of which correspondto a preselected reducjtion factor. In one such particular case a reduction factor. of
12 is utilized. Consequently, for the given example the" horizontal dimension of the configuration II will bc-onetwellth of the size of the horizontal dimension L of the artjustably controls the number of flashes as well as the intensity of'tube 11. System 12 is connectedto asuitable'energi zing source,.not shown, by conductors 12a. Also included is a condenser lens system 13 which collects the lightfrom tube 11 and provides uniform illumination of the subject, i.c., member .2. ltalso projects the arc of tube filament 14 in the plane of the iris. diaphragm member 15 shown schematically therein. An objective, lens System16, l7, l8 focuses the image of member 2 onto the light-sensitive emulsion 19 which is deposited on a planar surface of the transparent glass plate 20 of photomember 9. In the" preferred embodiment, the emulsion 19 is of the type knownas Kodalith Type 3 emulsion and the member 9 is commercially available as an integral member known as Kodalith Ortho Plates, Type 3. Kodalith is a registered trademark of the Eastman Kodak Company. A
reduced image of the member, e.g., member 2, at that particular position and in which emulsion 19 is eliminated and/or mitigated the formation of the aforementioned spurious spots hereinafter discussed. When the step-and-repeat operation is completed, there will be a discrete image of a circuit pattern in m emulsion 19 at each position to which member 9 is moved which collectively form the composite circuit pattern of the mask. Forsake of clarity, in the emulsion 19 shown in FIG. 4
there is illustrated in outline form the reduced latent image designated III of the pattern configuration ll of mask 2 for only one position of the mask 9. f I
The type of developing technique required for developing photomask 9 depends upon the photoresist emulsion, not shown, to be utilized for the circuit board, not shown. For example, if a positive photoresist emulsion for the circuit board.
is utilized, the opaque parts of themaskare required to form the circuit pattern. Consequently, in this case'the photomask is developed under the aforementioned conventional or nega-- 1 tive developing technique to produce the resultant circuit patternin the circuit board. When the positive photoresist emulsion of the circuit board is subsequently exposed through the.
transparencies of the negative developed photomask to ultraviolet light, the ultraviolet light destroys" the exposed T I areas of the positive photoresist andthe exposed areas are thereafter removed leaving the unexposed areas in the form of the desired circuit pattern. On the other hand, if a negative photoresist is used for the circuit board, the transparent parts of the photomask are required to form the circuit pattern in; the circuit board. Consequently, a direct positive developing h technique is utilized 'to develop the photomask so as to produce the resultant pattern in the circuit board. In this latter case, the ultraviolet exposure of the negative photoresist of I the circuit board through the transparencies of the photomask hardens" the exposedareas of the negative photoresist and the unexposed areas are subsequently removed leaving the hardened" exposed areas in the form of the desired circuit pattern.
Referring now to FIG. 5a, it is assumed that the photomask 9 of FIG. 4 is to be utilized with a circuit board having a positive photoresist emulsion. Consequently, the photomask 9 is developed with a conventional or negative developing technique. As a result, when the mask 9 is developed, the partially illustrated opaque area of the developed emulsion 19 shown in FIG. 5a corresponds to the circuit pattern configuration III. If on the other hand, the photomask 9 of FIG. 4 is to be used with a circuit board having negative photoresist emulsion, the mask 9 is developed with a direct positive developing technique. Thus, referring to FIG. 5b, under the latter circumstances, when the mask 9 is developed, the partially illustrated transparent area of the developed emulsion 19 shown therein corresponds to the circuit pattern configuration Ill.
Referring now to FIG. 6a, it has been found that when a high-intensity flash from tube 11, FIG. 3, is used to expose the photoemulsion 19 of mask 9 according to a known photographic process, undesired exposed areas are produced in the image in the emulsion 19. Thus, as shown by the photomicrograph of FIG. 6a, which is taken of a part of a photomask sample 91 which has been exposed by the known process and developed by a conventional negative developing technique, the spurious exposed areas appear therein as opaque spots 25 in the unexposed region 26. Region 27 corresponds to the exposed area of the photomask which represents part of the circuit pattern configuration. In addition, the line edge sharpness 28 between regions 26 and 27 is very poor.
Referring now to FIG. 6b, when a number of multiple, low intensity, discrete lamp flashes from lamp 11, FIG. 3, were used to expose the photoemulsion 19 of another similar photomask sample 92, partially shown, in accordance with the principle of the present invention, the number of spots 25' diminished and the line edge sharpness 26 improved. As the number of flashes were further increased for each photomask sample, the number of spots further diminished and the line edge sharpness further improved. Thus, as shown in FIG, 6c, for an increased number of flashes used to expose another photomask sample 93, partially shown, of similar construction as that of mask samples 91, 92 the number of spots 25" decreased further and the line edge sharpness 28" further improved.
Referring to FIG. 6a, increasing the number of flashes even more for another photomask sample 94, partially shown, and of similar construction to the mask samples 91-93 provided even further elimination of the spots 25" and further improvement of the line edge sharpness 28". In the four mask samples 91-94, inc., the optical densities of their respective circuit forming areas, i.e., exposed areas 27-27, are kept the same. For sake of clarity and explanation, the microscopic views represented by FIGS. 6a6d were taken in an area corresponding approximately to the area outlined by the broken line circle A, FIG. 5a, of their respective photomask samples 91-94. It should be understood that if the photomask samples 91-94 had been developed by a direct positive developing technique the spurious spots would have appeared as transparent spots or holes in the developed emulsion 19 which surrounds the transparent area representing the circuit pattern configuration. In FIG. 5b, one such spurious transparent spot 25a is illustrated in a greatly exaggerated manner for purposes of clarity and explanation. Using the principles of the present invention, the formation of spurious spots, such as spot 25a, are likewise eliminated and/or mitigated.
It can be readily seen that by utilizing the principles of the present invention the image quality of mask 9 is greatly enhanced. The formation of spurious exposed areas are eliminated and/or mitigated. Consequently, when the mask is used in conjunction with the photoresist emulsion of the circuit board, the conductive or circuit-forming material which would be formed from these spurious spots is likewise eliminated and/or mitigated on the circuit board. Consequently, the tendency of the spurious spots to form a short circuit bridge between adjacent circuit elements on the circuit board is also eliminated and/or mitigated. In addition, by
eliminating and/or mitigating the formation of these spurious spots, the electrical impedance characteristics of the resultant circuit elements of the board is enhanced and crosstalk between the elements is eliminated and/or mitigated. In addition, the improved quality of the photomask provided by the present invention, due to the elimination or mitigation of the spurious spots and/or the improved line edge sharpness characteristic of the photomask image, allows the packaging density of the final circuit to be increased, i.e., the circuit elements may now be more closely compacted and the number thereof increased.
By way of comparison, tables I-IV indicate a typical set of photographic elements and changes in various parameters utilized in making photomask samples similar to those illustrated in FIGS. 6a-6d, respectively. It should be noted that the photographic process utilized to obtain the values indicated in table I, which corresponds to FIG. 6a, was performed in accordance with the principles of a known photographic process and that the photographic process of the present invention was utilized to obtain the values indicated in tables II to IV, which correspond to FIGS. 6b-6d, respectively.
Number of Flashes Intensity per Flash Member 9 Xenon Flash Lamp Type Number of Flashes Intensity per Flash Member 9 Xenon Flash Lamp Type Number of Flashes Intensity per Flash Member 9 Xenon Flash Lamp Type Number of Flashes Intensity per Flash L44 watt-seconds Table II Kodak Kodalith Ortho Plate, Type 3 0.0l44 watt-seconds Table III Kodak Kodalith Ortho Plate, Type 3 0.0036 watt-seconds Table IV Kodak Kodalith Ortho Plate, Type 3 0.00 I 8 watt-seconds While the principles of the present invention have been described in a preferred photographic embodiment for making photomasks of the aforementioned kind, it should be readily recognized by those skilled in the art that the invention is not limited thereto but may be utilized in other photographic processes in which it is desired to improve the image quality and more particularly to eliminate formation of the spurious exposed areas appearing in the unexposed areas of the photographic image, and/or to improve the line edge sharpness characteristic of this image. Thus, for example, the invention may be practiced in photographic processes such as those used in taking commercial, studio, and/or scientific-type photographs.
Thus, while the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.
What is claimed is:
1. In a photographic printing process of the type using a flash lamp system to expose through a transparency a silver halide emulsion, the emulsion having formed therein spurious exposed areas in the unexposed areas of the resultant image produced in the emulsion when exposed by at least one highintensity flash of said system, the improvement comprising the step of:
exposing through said transparency a photographic, silver 7 I halide emulsion by multiple, lower intensity, discrete light energy flashes from said system to mitigate the formation of spurious exposed areas in the unexposed areasof the resultant image produced in the emulsion and improve emulsion, said lower intensity being 0.0144 watt-seconds or less per flash. 6. A photographic printing processaccording to claim wherein said opaque object pattern is a predetennined printed the resolution thereof, saidlower intensity being 0.0144
1 watt-seconds or less per flash. 7 29A photographic 'printingprocess according to claim 1 wherein said transparency is a photographic transparency. 3." A photographic printing process according to claim 2 I wherein said photographic transparency contains an image of an electrical circuit pattern therein and said photographic emulsion is developed to'forma photomask for use in astep and repeat system for the manufacture of integrated circuits.
' 14.:A photographic printing process according to claim 2 wherein said photographic transparency contains an image of an electrical circuit pattern therein and said photographic emulsion is deposited on a transparent substrate to form a photomask for a photoresist operation for themanufacture of printed circuits. V i i 5. In a photographic printing processof the type using a flash-lamp system to expose a silver halideemulsion through a transparent glass member having an opaque object pattern carried thereon; the emulsion having formed therein spurious intensity flash of said system, the improvement comprisingthe steps of:
. disposing said transparent glass member between said system and an unexposed, photographic silver halide emulsion deposited on a transparent substrate of a second member; and V intermittently exposing the photographic emulsion to multiple lower intensity radiant energy flashes from said system for a'predetermined number of exposures to mitigate the formation of spurious exposed areas in the unexposed areas of the resultant image produced in the exposed areas in the unexposed areas of theresultant image produced in the emulsion when exposed by at least one highcircuit configuration.
7. A photographic printing process according to claim-5 wherein said opaque object pattern is a predetermined in.
tegrated circuit configuration. I i 1 8. In a photographic printing process for makingf'a I photoemulsion mask for use in a.step-and epeatsystem for I the manufacture of electrical circuit boards and the'like, said process being of the type using a flash lamp system to expose through a first photographic transparency reduction asilver halide emulsion to form a second photographic transparency reduction, the emulsion having formed therein spurious ex posed, areas in a unexposed areas of thehresultant image produced in the emulsionwhen' exposed by at least one high= intensity flash of said system, the improvement comprising the steps of:
making an initial enlarged artwork of a predetermined cir cuit patterni V making said first photographic transparency reduction" of said circuit pattern from said enlarged artwork; and
making a second photographic transparency reduction from said first photographic reduction including the substeps of disposing said first photographic transparencyreduc-j tion between said system and an unexposed silver halide emulsion exposing said last-mentioned emulsion to multiple, lower intensity, discrete light flashes provided by said system through said first photographic transparency reduction to mitigate the formation of spurious exposed areas in'theunexposed areas of the, resultantirnageproduced in said last-mentioned emulsion, and'develop-. ing and fixing the latent image to providethe second,
photographic reduction comprising said photomask, said lower intensity being 0.0l44 watt-seconds or less per flash. r