Method of making printing masks with high energy beams

BACKGROUND OF THE INVENTION

This invention relates to improvements in methods for making printing masks by controlled application of a beam of high-energy emissions to a blank.

There are a variety of applications in which it is desirable to apply a very accurately delineated configuration of printing material to a suitable substrate. One particular application of such a process is in the fabrication of socalled thick film” printed electronic circuits. In making such devices,_ typically a conductor “ink” is applied or printed selectively onto predetermined portions of an appropriate ceramic substrate, and subsequently “fired” onto the substrate. In many cases, requirements of miniaturization or compactness make it desirable to provide such printing with as high a degree of definition as possible, e.g. in the form of one-mil wide stripes separated by three mils or less between the centers of adjacent stripes.

Techniques for attempting such printing are known which utilize screen stencils, made for example of a screen of fine-mesh woven stainless-steel wire or nylon filaments, selectively coated with a mixture of polyvinyl alcohol and polyvinyl acetate to close the mesh openings in the areas where printing is not desired. However, the resolution and accuracy of printing heretofore obtained with such methods has been less than is desirable for some purposes, e.g. a maximum resolution of the order of 5-mil width lines separated by 10 mils between their centers. Better resolution has been obtained using metal foil printing masks made by chemical etching or electro-forming techniques. Typically the total thickness of such a mask is of the order of one to two mils, and the pattern of the desired printed circuit is etched into one side of the mask, half-way through the thickness of the foil, and a pattern of closelyspaced indentations is etched into the mask from the other side to produce perforations through the mask at the bottoms of the grooves. While line resolutions of the order of 2-mil line width and four-mil center-to-center line spacing have been obtained in some instances, the accuracy and reproducibility obtainable with such masks has sometimes been less than desirable, principally because of “undercutting” of the mask material during etching or excessive buildup of material during electro-forming. A further drawback is that the latter method involves photo2 graphic techniques using photoresists for defining the the areas of the mask blank exposed to the etching and/or electroforming operations, and consequent complication and expense in preparing the photographic master through 5 which light is projected to form the pattern-defining image on the photoresist.

It is an object of the present invention to provide a new and useful method for the fabrication of printing masks.

A further object is to provide such a method which is 10 capable of providing grooves or other depressions in a printing mask blank reproducibly and reliably.

It is also an object to provide a method of making a printing mask blank reproducibly and reliably, energy emissions to a mask blank in which the accuracy 15 and reproducibility with which grooves or other depressions are formed in the blank are improved.

SUMMARY OF THE INVENTION

These and other objects and features of the invention 20 are achieved by the provision of a method for making a printing mask by application of a controlled beam of highenergy emissions to a mask blank, in accordance with which method the blank is provided with at least two layers,_ one of which is more readily volatilized than the 25 other in response to impingement by said beam, and the beam is applied to the side of said one layer remote from said other layer to remove at least portions of said one layer -without perforating said other layer, thereby to form a groove or other depression. Preferably the parameters 30 of the beam are controlled so that the depression formed extends to said other layer but does not penetrate it substantially. Additional beam energy may be applied to cut entirely through both layers where a perforation is desired. Using this method, the removal of the material of 35 said one layer tends to be slowed or arrested when the second layer is exposed, and a reproducible depth of groove or other depression therefore more readily obtained, with less criticality of adjustment of the beam parameters such as beam size, beam energy, beam cur40 rent and beam deflection rate.

BRIEF DESCRIPTION OF FIGURES

These and other objects and features of the invention 45 will be more readily understood from a consideration of the following detailed description, taken in connection with the accompany drawings, in which:

FIG. 1 is a plan view illustrating schematically a form of printed-circuit module which may be made by the 50 method of the invention;

FIG. 2 is a sectional view taken along lines 2__2 of

FIG. 1;

FIG. 3 is an elevational sectional view illustrating how such a printed circuit may be formed by means of a 55 stencil mask;

FIG. 4 is a plan view of a stencil mask suitable for use in printing the printed circuit of FIG. 1;

FIG. 5 is a fragmentary plan view of a portion of the mask of FIG. 4;

FIG. 6 is a sectional view taken along lines 6__6 of

FIG. 5;

FIG. 7 is a sectional view taken along lines 7__7 of

FIG. 5;

FIG. 8 is a fragmentary perspective view of a mask 65 blank from which a stencil mask is to be made;

FIG. 9 is a schematic representation, partly in block form, illustrating an arrangement of apparatus suitable for practicing the method of the invention in one of its forms; and

FIGS. 10 and 11 are perspective views showing a portion of a printing mask in various stages of fabrication in accordance with the invention.

3,668 3

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to the drawings by way of example only, FIGS. 1 and 2 show a printed-circuit module comprising a ceramic printed-circuit substrate 10 having adherent to its upper surface a pattern of conductors such as 12 of a conductive “ink,” e.g. a substance which has been applied to the substrate in conformance with a predetermined pattern and thereafter fired to provide suitable electrically-conductive paths on the substrate. Such printed circuit modules and their uses and applications are well known to those skilled in the art of printed circuitry and need not be described further herein.

FIG. 3 illustrates one method of forming such a printed-circuit pattern by placing a stencil mask 14 over the top surface of the substrate 10 and forcing the conductor ink 1'6, which may be in a thin paste, through openings in the stencil onto underlying parts of the substrate by operation of the squeegee 18. The mask 14 has been suitably grooved on the surface adjacent the substrate 10 on the opposite face so that the conductor ink is forced through the apertures and along the grooves into contact with the surface of the substrate 10 where printing is desired. The mask is then removed and the substrate and ink fired in a furnace to form a permanent pattern of conductive paths, as desired. This process, and configurations of the mask suitable for effecting the various desired printing patterns of, are well known in the art and it is an object of the present invention to fabricate such a mask with a high degree of reliability, accuracy and resolution, and without requiring photographic steps.

More particularly, FIGS. 4-7 show in greater detail a form of mask 14 which can readily be made by the method of the invention. In this example the mask 14 may comprise a plurality of grooves such as 20, each extending only partially through the thickness of the mask, and a plurality of spaced-apart apertures such as 22 extending from the bottom of each depression entirely through the remainder of the thickness of the mask to the opposite surface thereof. The grooves such as 20 define regions throughout which ink is applied to the printed-circuit substrate, and the apertures 22 provide the openings through which ink is supplied to these grooves, as discussed above in particular connection with FIG. 3.

As is shown particularly clearly in FIGS. 6 and 7, in accordance with the invention the mask 14 comprises a pair of bonded layers or laminas 26 and 28, the grooves such as 20 extending through the upper lamina 26 and the apertures or perforations extending through the lower lamina 28. The upper lamina 26 is substantially more readily volatilized in response to impingement by a beam of high-energy emissions than is the lower lamina 28. There are a variety of materials suitable for use as these lamina, and for the purpose of the present example it will be assumed that lamina 28 is a nickel foil about 1 mil in thickness and that lamina 26 is a layer of cadmium of about the same thickness electro-plated onto the upper surface of the nickel. The reasons for, and the advantages of, this laminated construction of the mask will be set forth more fully hereinafter.

FIG. 8 shows a portion of the plane laminated mask blank 14 which is used as the starting blank in making the mask stencil, and FIG. 9 illustrates schematically one arrangement of apparatus by which a pattern of grooves and holes suitable for an electronic-circuit printing mask may be provided in this mask blank, rapidly and automatically.

Referring to FIG. 9, a machine bed 30¹ supports a worktable 32 which is controllably positionable with respect to the bed 30 along two mutually orthogonal directions in a horizontal plane. Such motions may be provided by worktable control motor 35 geared to drive the worktable along one direction with respect to a frame 36, and a ,028 .

motor 37 geared to drive frame 36 along the orthogonal direction with respect to the machine bed 30.

On the upper surface of worktable 32 a support block 40 is appropriately fixed in position with respect to the 5 worktable and carries the laminated mask blank 14 on its upper surface. Appropriate clamping arrangements such as 44 may be utilized to clamp the laminated mask material releasably in position on top of the support block 40.

The upper surface of the mask 14 is impinged by a fine focused beam 50 of high-energy electrons from the controllable high-energy electron beam source 52. Electron beam source 52 may be of a type known in the art and commercially available, and which has been used 15 in the prior art for metal working purposes.

Electron beam source 52 includes four control terminals—a focus control terminal 56, a beam-energy control terminal 58, a beam-deflection control terminal 60 and a beam-current control terminal 61, responsive to 20 electrical signals to control, respectively, the focus, energy, angular position and current of the electron beam 50. A programmed control computer 64 is provided with five output control terminals 66, 68, 70, 71 and 72 connected by electrical lines 74, 76, 78, 80 and 81 respec25 tively to terminals 56, 58, 60 and '61 of electron beam source 52 and to a table-position control terminal 84 of the worktable control box 85 from which operation of motors 35 and 37 is controlled by connections not shown. Preferably, the mask blank, worktable system and con30 trol box are enclosed in a chamber 90 in which a partial vacuum is maintained by means of a low-vacuum exhaust system 92, such as is employed in electron-beam milling of large objects.

Control computer 64 is appropriately programmed by ³³ conventional means to provide, at its five output control terminals, appropriate coordinated control signals for controlling the focus, energy, current and position of electron beam 50 and for controlling the position of the mask 14 with respect to the electron beam source 52. The pro⁴⁰ gram incorporated in the control computer 64 may be of any desired form for varying the point of impingement of the electron beam 50 upon the upper surface of the laminated foil 14, as well as for varying the focus, energy and position of the beam all according to a predetermined 45 routine. It is noted that the relative position of the beam 50 with respect to the foil 14 may be altered either by motion of the worktable or by electrical control of beam position, and in general grosser and slower displacements of the relative position of the beam will be accomplished 50 by motion of the worktable while the finer motions required for the rapid tracing-out of desired grooved patterns and for the formation of apertures will normally be provided by electrical deflection of the electron beam.

In general, the rate at which material of the mask is 55 removed depends upon the number and energy of electrons striking a given region of the mask per second and upon the evaporation or volatilization characteristics of the material impinged; the total amount of material removed also depends upon the time for which the beam impinges 60 a given region. Accordingly, more material will be removed from the mask, in general, when the beam dwells in one position for a substantial period of time or moves slowly with respect to the mask; when the energy of the electrons is increased by increasing the accelerating volt65 age to which they are subjected; when the beam current is increased; and when the beam is more sharply focused. By control of these parameters grooves are formed extending only partially through the layer 26 and holes entirely through the material at the bottoms of the 70 grooves are produced by increasing the beam energy, beam current, or beam concentration or by applying the beam for a longer time at a given position where the hole is to be made.

More particularly, as represented in FIG. 10, the beam 75 50 is deflected along the paths on the surface of the upper

3,668 5 lamina 26 in which grooves 20 are to be formed with a deflection speed, beam energy and beam concentration sufficient to assure cutting of the grooves down to the upper face of the lower lamina 28. When the beam is traversing a region in which no groove is to be formed, 5 it is possible for the computer to make the beam energy zero by reducing the accelerating voltage, although alternatively, and generally more easily, the beam current may be cut off by voltage applied to the electron gun from the computer, for example. The material of the lower 10 lamina 28, being much less readily volatilizable than the top mask layer 26, will not be removed at any appreciable rate by the beam conditions used in forming the grooves, and hence sufficient beam energy can be used to assure complete formation of the grooves without danger of re- 15 moving substantial amounts of the underlying lamina 28 or of perforating the latter lamina.

When the grooves have thus been formed, the programmed control computer 64 causes the beam to traverse the grooves again, to dwell at spaced-apart points along 20 the groove, to assume an increased beam e'nergy while at these points, ; and to be substantially blanked out or at least maintained at a lower energy level while travelling between these points. The higher energy beam, while dwelling at the successive spaced-apart points, produces 25 holes through the lower lamina 28 at these points, as illustrated in FIG. 11, thereby providing the desired mask configuration consisting of grooves in the upper surface extending only partly through the mask and holes at spaced intervals along the grooves extending through the 30 remainder of the mask for supplying ink to the grooves during the printing process.

In another form of the. method, the electron beam may be controlled so as to produce the grooves and the perforations during the same scanning of the mask, by 35 providing a speed of beam deflection and a beam energy sufficient to remove only the top lairiina of material where perforations are not required, and by providing a lower deflection speed and/or a higher beam energy while impinging points at which holes are to be produced by pene- 40 tration entirely through both layers. With this procedure it is made entirely certain that the holes and grooves will be accurately aligned, since they are formed in the same scanning of a line by the beam.

The thickness and nature of the materials used for the 45 mask and the parameters of electron-beam energy, concentration, focus and rate of movement, as well as the pattern traversed by the beam may all be varied substantially in different applications. Examples of suitable materials for the upper (first impinged) lamina 26 include 50 zinc, cadmium, indium, tin, bismuth, lead and synthetic plastics;..suitable materials for the lower lamina 28 include nickel and its alloys, copper and its alloys, iron and its alloys, chromium, molybdenum, and tungsten. In a preferred form, the upper layer is cadmium, electroplated 55 onto a lower layer of nickel. In each case the upper layer is readily volatilized under beam conditions which have a relatively smaller effect in volatilizing the material of the lower layer, with the advantages set forth above.

In one specific example of a preferred procedure for 60 forming a mask of a type suitable for effecting printing of electronic circuits, a 1-mil thick foil of nickel is electroplated with a 0.8 mil thick layer of cadmium to provide a laminate foil about 1 inch by 1 inch square, which is then secured to the support block 40 on the worktable 65 32. The electron beam source 52, initially turned off, is turned on automatically by the control computer 64 and deflected along the desired pattern of grooves with a spot size of the beam on the mask surface of about 2 mils in diameter and with a beam energy just sufficient to assure 70 complete removal of the upper-lamina material along the groove, the beam being automatically cut off by signals from the control computer when the beam is moving between portions of the foil in which no groove is to be formed. The resultant grooves extend entirely through 75 ,028 the upper lamina 26 and have a width of approximately 2 mils, and adjacent grooves can be formed with their centers spaced-apart from each other by about 4 mils. The beam, with a spot size typically of about 1.5 mils in diameter, is then automatically caused to traverse the grooves again, dwelling at intervals spaced-apart along the beam path by about 3 mils, with a beam energy at the dwell points sufficient to produce spaced holes extending entirely through the lower lamina 28 at the bottoms of the grooves, as desired. With such a procedure, a pattern comprising grooves covering about half the area of the 1 by 1 square can be produced in two or three minutes. The mask thus produced is then suitable for use in conventional electronic circuit printing.

Another form of beam of high-energy emanations which may be utilized in place of electron beam is a laser light beam of high intensity. Such a beam can be controlled as to its point of impingement by optical means, and its energy, beam width and focus can also be controlled by known means. Accordingly, in another embodiment of the invention, grooves and perforations in a mask suitable for printing of electronic circuits are made by traversing a high-intensity laser beam along the line regions in which grooves are to be produced on the upper surface of a laminate mask, the upper lamina of which is more readily volatilized by the laser than is the lower lamina, after which the laser beam is applied for substantial periods of time at higher intensities to spacedapart points on the bottom of the grooves to produce the desired holes extending through the lower lamina. Suitable structures of apparatus for producing such a laser beam and/or controlling its parameters and position to effect such opertion will be apparent to one skilled in the art. Again, if desired, the formation of the grooves and of the holes may be accomplished on the same scan by suitable control of beam energy and speed.

While the invention has been described with particular reference to specified embodiments in the interest of complete definiteness, it will be understood that it may be embodied in a variety of forms diverse from those specifically shown and described without departing from the scope of the invention as defined by the appended claims.