U.S. patent number 3,846,661 [Application Number 05/296,366] was granted by the patent office on 1974-11-05 for technique for fabricating integrated incandescent displays.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Alan V. Brown, Frederick Hochberg.
United States Patent |
3,846,661 |
Brown , et al. |
November 5, 1974 |
TECHNIQUE FOR FABRICATING INTEGRATED INCANDESCENT DISPLAYS
Abstract
Incandescent "microfilaments" for integrated display devices,
and the like, are batch fabricated using planar technologies. The
planar incandescent filaments are made of thin films, suspended to
minimize heat conduction losses. Heat losses are additionally
minimized by appropriately shaping the ends of the filaments. By
utilizing planar technologies all filaments of a display device may
be fabricated en masse in a single plane and, individual filaments
may be etched to various shapes and curves thereby obviating the
problems encountered where elements must be strung between support
posts, in a straight line. Typically, a ceramic substrate is first
coated with a layer of reflecting material and etched, if desired,
to the pattern selected for the reflecting surface. Thereafter, a
support material, such as glass, is deposited over the substrate
and reflecting material. Holes are then drilled into the glass and
substrate and filled with conductive material, to thereby form
support posts. A filament material, such as tungsten, is then
deposited over the support layer so as to make conductive contact
with the underlying support posts. By using conventional etching
techniques a filament of desired pattern is then formed between the
posts. Thereafter the support layer may be etched away leaving the
filament suspended between the posts.
Inventors: |
Brown; Alan V. (Yorktown
Heights, NY), Hochberg; Frederick (Yorktown Heights,
NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
26836172 |
Appl.
No.: |
05/296,366 |
Filed: |
October 5, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
138409 |
Apr 29, 1971 |
3715785 |
|
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Current U.S.
Class: |
313/491; 313/510;
315/68; 345/73; 313/494; 315/64 |
Current CPC
Class: |
H01K
3/02 (20130101) |
Current International
Class: |
H01K
3/00 (20060101); H01K 3/02 (20060101); H01j
007/42 () |
Field of
Search: |
;313/18R,109.5,275,277
;315/64,66,68,69 ;340/336 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kominski; John
Assistant Examiner: Dahl; Lawrence J.
Attorney, Agent or Firm: Jordan; John A.
Parent Case Text
This is a division of application Ser. No. 138,409 filed Apr. 29,
1971, now U.S. Pat. No. 3,715,785.
Claims
We claim:
1. An incandescent filament arrangement comprising:
an insulative substrate base member having a lower surface and an
upper surface and having a plurality of conductive support posts
extending at least from said lower surface through said substrate
base member to at least said upper surface, and a body of
incandescent material characterized by being substantially planar
in a plane substantially parallel to and displaced from at least a
portion of said upper surface and extending between and being
integral with at least two of said plurality of conductive support
posts.
2. The incandescent filament arrangement as set forth in claim 1
wherein said body of incandescent material tapers near end of the
said at least two conductive support posts so as to compensate for
end heat conduction losses in said incandescent material and cause
uniform incandescence along said body of incandescent material from
post to post.
3. The incandescent filament arrangement as set forth in claim 2
wherein the said support posts which extend to at least the said
upper surface of said substrate base member project beyond the said
upper surface of said substrate base member.
4. The incandescent filament arrangement as set forth in claim 3
wherein a layer of reflective material is provided between said
body of incandescent material and said substrate base member.
5. The incandescent filament arrangement as set forth in claim 4
wherein said body of incandescent material exhibits over at least a
portion thereof a generally serpentine configuration.
6. The incandescent filament arrangement as set forth in claim 5
wherein said body of incandescent material is channel-shaped along
the path of said serpentine configuration.
7. An integrated incandescent display device comprising:
an insulative substrate base member having a lower surface and an
upper surface and having a plurality of conductive support posts
extending at least from said lower surface through said substrate
base member to at least said upper surface, and a plurality of
incandescent filament segments for displaying information each of
which segment is characterized by being substantially planar in a
plane substantially parallel to and displaced from at least a
portion of said upper surface with respective ones of said segments
extending between and being integral with at least two of said
plurality of conductive support posts.
8. The display cell as set forth in claim 7 wherein each of said
filament segments are of substantially serpentine configuration in
said plane between said posts.
9. The display cell as set forth in claim 8 wherein each of said
filament segments bows slightly in the direction of view.
10. The display cell as set forth in claim 9 wherein said filament
segments taper in said plane as they approach said posts.
Description
BACKGROUND OF THE INVENTION
The present invention relates to incandescent illumination and
display apparatus, and processes for making same. More
particularly, the present invention relates to improved
incandescent filament arrangements and, processes for fabricating
such filament arrangements, particularly as pertains, for example,
to incandescent display devices. In accordance with the principles
of the present invention planar fabrication techniques are employed
to make improved incandescent elements of a planar "microfilament"
variety, which "microfilaments" may readily be utilized in
integrated incandescent illumination and display apparatus.
Heretofore, the filaments of incandescent display cells and
illumination apparatus have typically been fabricated by
individually wiring each filament, of whatever type, to appropriate
support posts. One of the difficulties with such an approach, it
can be seen, resides in the fact that it is highly time-consuming
and expensive. In addition, such an approach clearly imposes a
restraint upon the size and shape of the filament that may,
practically, be fabricated. One commonly used prior art display
apparatus, for example, is manufactured by employing, as individual
incandescent filaments, coiled wire, bonded to posts. It is
evident, from such an arrangement that the required bonding process
is cumbersome and costly and the fabricated device is limited in
size, shape and efficiency.
Not only is the "coiled wire" filament approach difficult to
fabricate but, in addition, the "coiled wire" approach, with
filament bonded between support posts, necessarily requires the
individual filaments to be stretched in a straight-line.
Accordingly, individual filament elements cannot be selectively
configured to individual shapes. Not only are the individual
filament elements constricted to a straight-line shape but, in
addition, when the elements are arranged in a display device,
elaborate arrangements must be made to allow the filaments to
overlap at the terminals thereof in order to minimize gaps in the
configured symbols. This is necessitated, in part at least, by the
fact that end losses, i.e., conduction losses at the filament
terminals, prevents the individual filaments from effectively
illuminating the full length of the filament wire line.
Accordingly, when alphanumeric characters, for example, are made by
the selective illumination of individual filaments, the individual
filaments are arranged to overlap at the terminals thereof so that
illuumination is present the full length of the line segments of
the alphanumeric character, such that the character looks
relatively continuous. Exemplary of such an arrangement is that
described more particularly by P. C. Demarest et al. in U.S. Pat.
No. 3,408,523, issued Oct. 29, 1968.
It can be seen that in the "coiled wire" approach to the
fabrication of incandescent filaments, such as that described by
Demarest et al., cumbersome and complex manufacturing procedures
are necessarily encountered. In addition, it can be seen that
display devices fabricated in accordance with such an approach
necessarily are thick, bulky and inefficient, in that the necessity
of overlapping the filaments requires the filaments to be arranged
in different planes. Reduction of thickness obviously involves
critica tolerances and difficult packaging problems which
significantly effect reliability.
It is therefor, accordingly, an object of the present invention to
provide an improved process for the manufacture of incandescent
filaments.
It is a further object of the present invention to provide an
improved incandescent display device, and improved filament
therefor.
It is still a further object of the present invention to provide an
improved incandescent filament for use as an illumination device in
display cells, and the like.
It is yet a further object of the present invention to provide an
improved alphanumeric display apparatus.
It is yet still a further object of the present invention to
provide a highly effective process for fabricating integrated
incandescent display apparatus.
It is another object of the present invention to provide a process
for fabricating incandescent display devices where all the
filaments of each device are in a single plane and wherein each of
the devices may be shaped to any of a variety of selcted
configurations.
It is still another object of the present invention to provide an
incandescent display device that may be fabricated without the need
for mechanical assembly.
It is yet still another object of the present invention to provide
an improved incandescent display device whereby the incandescent
filaments glow at lower temperatures than heretofore achieved
thereby increasing the lifetime of the filaments. This is
accomplished because the planar filament, in accordance with the
present invention, provides a greater radiating surface area within
the character size employed.
It is yet another object of the present invention to provide an
improved display device and incandescent filament therefor that may
be manufactured economically and which exhibits high reliability
and improved performance.
Although planar fabrication techniques have, in the past, been
applied to a variety of integrated circuit and semiconductor device
construction processes, such techniques have not heretofore been
employed in the fabrication of incandescent filaments, as taught in
accordance with the principles of the present invention.
Typical of the prior art problems confronted with the planar
technology is that involving crossovers in integrated circuits.
Exemplary of the manner in which the latter problem is confronted
is that given by Lepselter in the Bell Laboratories Record, in the
article entitled "New Gold Crossovers Interconnect Integrated
Circuits," February 1968.
In accordance with the principles of the present invention there is
provided a process for either the individual or batch fabrication
of incandescent "microfilaments," using planar technology, whereby
display devices, and the like, may readily be constructed, en masse
without mechanical assembly, in a manner so that all the filaments
are in a single plane. The resultant display device so fabricated
is economical in construction costs, simple, reliable and
efficient, exhibiting individual incandescent elements which may be
shaped to any of a variety of desired configurations and which
operate at reduced average illumination temperatures.
The foregoing and other objects, features and advantages of the
invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a series of steps A-J, exemplary of those that may
be employed in carrying out the process, in accordance with the
principles of the present invention.
FIG. 2 depicts a perspective view of a typical incandescent
filament configuration produced employing the process, in
accordance with the principles of the present invention.
FIG. 3 depicts a series of steps A-F, exemplary of those that may
be employed to fabricate a channel-type filament form, in
accordance with the principles of the present invention.
FIG. 4 depicts a series of steps A-D, akin to those described in
FIG. 1, wherein a channeled substrate is filled to provide a
temporary support and thereafter emptied to leave a free standing
filament to bridge the channel.
FIG. 5 shows a typical 16-segment alphanumeric display cell
fabricated en masse in a single process, in accordance with the
principles of the present invention.
FIG. 6 shows an exemplary 5 .times. 7 matrix array of incandescent
filaments fabricated en masse, in accordance with the principles of
the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
In FIG. 1 is shown a series of steps exemplary of those that may be
employed in carrying out the preferred mode, in accordance with the
principles of the present invention. These steps, designated A-J,
depict the significant features used in carrying out a process to
fabricate a "microfilament" typified by the filament configuration,
shown in perspective in FIG. 2.
As shown in step A of FIG. 1, a substrate 1 of ceramic material,
for example, is first coated with a layer 3 of reflective material.
In this regard it should be noted that substrate 1 may be any of a
variety of supports upon which a reflective material, such as
metal, may be deposited. The layer 3 is to be used as a reflecting
surface for the ultimate incandescent filament to be fabricated. It
should be noted, also, that layer 3 may be deposited by any of a
variety of conventional techniques, such as vapor deposition or
sputtering. Thereafter, layer 3, which may in the typical preferred
mode be chromium or tungsten, is etched to the configuration
desired for this purpose. As shown in step B of FIG. 1, this etched
configuration may take a form akin to that of the incandescent
filament, to ultimately be fabricated. This can be seen more
clearly by reference to FIG. 2 wherein the configuration of
reflector 3 is shown to be the same as incandescent filament 15,
thereabove.
After the etching of reflective layer 3, as shown in step B of FIG.
1, a support layer 5 is deposited thereover, as shown in step C
thereof. The function of layer 5 is to support the subsequently
applied layer of incandescent material, to be fabricated into a
filament. Typically, layer 5 may be glass, such as Corning 7070
glass. It should be noted here that although the process described
with respect to FIG. 1 depicts a single incandescent filament, it
is clear that these steps are merely illustrative of the key steps,
in accordance with the principles of the present invention, and
that these steps are likewise applicable to the simultaneous
fabrication of any number of incandescent filaments. Thus, the
variously described deposition and etching steps could obviously
operate to effect contemporaneous fabrication of any number of
variously formed filaments. In fact, it is a very significant
aspect of the present invention that a display cell, employing
incandescent filaments, can be fabricated en masse.
After support layer 5, which as hereinabove indicated may be glass,
is deposited upon substrate 1 with its configured reflective layer
3, as shown in step C, holes are drilled through the glass layer
and substrate, as shown in step B. These holes may be formed by any
of the variety of techniques. For example, an electron beam
drilling technique may readily be used. Alternatively, the holes
may be machined by mechanical drilling. The holes, as shown at 7
and 9 in step D, are then filled with a conductive material to
thereby form posts, as shown at 11 and 13, in step E. In this
regard, the holes may be filled to form posts by an electroplating
process, whereby the holes become plated with copper, for example.
It is clear that any of a variety of metals may be employed to
fabricate the posts 11 and 13, as shown in step E. The main
requirements for these posts are that they are of sufficient
strength to support the ultimate incandescent filament, to be
suspended therebetween, and that they are conductive.
After conductive support posts 11 and 13 have been fabricated, as
shown in step E of FIG. 1, a layer of incandescent material 15 is
then deposited upon the layer of glass 5 and into conductive
contact with the support posts, as shown in step F. Incandescent
material 15 may be any of a variety of well known incandescent
materials. In the typical preferred embodiment, incandescent layer
15 may be made of tungsten. The incandescent layer may, it is
clear, likewise, be deposited by any of a variety of well known
techniques. For example, layer 15 may be deposited by any one of
the E-gun evaporation techniques, sputtering techniques or CVD
(chemical vapor deposition) techniques. A particular manner in
which selective CVD techniques may be employed to fabricate layer
15 of tungsten will be described hereinafter with respect to FIG.
3.
Afte incandescent material layer 15 has been deposited upon support
layer 5, it is etched to provide the desired filament
configuration, as shown by step G of FIG. 1. As hereinabove
indicated, FIG. 2 shows one possible filament configuration. It
should be noted, that any of a variety of etching techniques may be
employed to etch incandescent layer 15 to the desired
configuration. For example, conventional photo-resist and chemical
etching techniques may readily be employed to appropriately etch
layer 15. It is clear that utilization of photo-resist techniques
allows the fabrication of very minute and intricate filament
patterns. The significance of employing photo-etch techniques will
become more clear hereinafter when it is recognized that very
minute individual filaments are thereby allowed to be fabricated in
a matrix array scheme, otherwise not capable of being fabricated
using conventional mechanical fabrication techniques. It should,
likewise, be noted that these conventional photo-etch techniques
may also be employed to etch the pattern desired in reflective
layer 3.
After the incandescent filament has been etched, in accordance with
step G, a layer of conductive material 17 may then be deposited
upon the opposing surface 19 of substrate 1, and into conductive
contact with support posts 11 and 13, as shown in step H.
Thereafter, conductive layer 17 is etched so that all that remains
is a pair of electrical contacts 21 and 23 in respective conductive
relationship with posts 11 and 13, as shown in step I. These
contacts may be in the form of conductive tabs or wire lines. Layer
17 may be any of a variety of conductive materials, such as copper.
After conductive layer 17 has been etched to form electrical
contacts 21 and 23, as shown in step I, then, support layer 5 is
removed as shown in step J. It is clear that support layer 5 may
readily be removed by using processes such as chemical etching, or
any of a variety of other material removal processes.
FIg. 2 shows a perspective view of a typical incandescent filament
configuration, fabricated in accordance with the principles of the
present invention. It is clear from FIG. 2 that substrate 1 may be
any of a variety of ceramic materials, as indicated with respect to
the description in regard to the steps of FIG. 1. Likewise,
filament 15 may be fabricated from any of a variety of incandescent
materials. It should be recognized that the arrangement shown in
FIG. 2 is merely illustrative of an arrangement that might be
fabricated, in accordance with the principles of the present
invention. Thus, in this regard, filament 15 may take any of a
variety of configurations. Likewise, substrate 1 may be of any
reasonable size and may support as many filaments as this size will
reasonably accommodate. With respect to the fabrication of multiple
filaments on a substrate to create, for example, a display cell or
the like, it should be recognized that the essentials of the steps
enumerated in FIG. 1 remain the same for such fabrication, as they
do for the fabrication of the basic filament shown therein. In
particular, the various steps of depositing, etching, removing and
the like, remain substantially the same whether a single filament
is being fabricated or multiple filaments are being fabricated.
Thus, it is clear that a display cell may readily be created en
masse, using the techniques in accordance with the present
invention.
In FIG. 3 there is shown a series of steps A - F which may be
employed as an alternative to several of the steps shown in FIG. 1.
It should be recognized in this regard that the steps shown in FIG.
1 produce an incandescent filament which is somewhat serpentine in
shape in its planar dimension. Typically, filament 15 in FIG. 2 may
be of the order of 12 microns wide and 1 micron thick while being
suspended 5 mils above substrate 1. It is clear that smaller widths
and thicknesses may be fabricated. However, it is likewise clear
that the rigidity of the filament becomes a factor in any practical
arrangement. In this latter regard, the steps depicted in FIG. 3
illustrate a process that may be employed to make the filament
channel-shaped to thereby increase its rigidity.
The initial prepatory steps required in the process depicted in
FIG. 3 are akin to those of the process depicted in FIG. 1.
Accordingly, it can be seen that step A in FIG. 3 corresponds to
step C in FIG. 1, and is arrived at by the same antecedent steps,
as those shown at A and B in FIG. 1. Likewise, it can be seen that
step B in FIG. 3 corresponds to step D in FIG. 1. However, after
the holes 7 and 9 have been filled so as to create posts 11 and 13,
rather than deposit a layer of incandescent material, as shown in
step F of FIG. 1, a layer of material, which is selectively
responsive to the chemical vapor deposition of the incandescent
material to be employed, is thereafter deposited. This selective
material is shown as layer 25 in step C of FIG. 3 and may comprise
any of a variety of materials which will act to nucleate the vapor
of the incandescent material to be deposited. Thus, where tungsten,
for example, is to be employed as the incandesant material and 7070
glass is employed as the support layer 5, then, layer 25 may be
copper, since copper will nucleate the tungsten vapor while the
7070 glass will act to oblate the tungsten vapor.
After layer 25, which for purposes of illustration and example may
be taken as copper, is deposited on support layer 5, which may be
taken as 7070 glass, the layer of copper, which is to act somewhat
as a mold, is etched to the configuration desired for the ultimate
filament to be fabricated. As shown in step D of FIG. 3, copper
layer 25 is etched to form a copper mold exhibiting a pattern akin
to the somewhat serpentine pattern exhibited by element 15 in FIG.
2. It is clear that this mold pattern must necessarily be somewhat
smaller than the size of the ultimate incandescent filament to be
built-up thereon. It can be seen, with respect to step D, that the
copper mold pattern exhibits a cross-section 27 which is smaller
than the cross-section of posts 11 and 13, and which is centrally
positioned upon the respective posts.
After copper layer 25 has been etched to form a mold, the selective
chemical vapor deposition step is then carried out. Accordingly,
the arrangement shown in step D is inserted into a chemical vapor
deposition chamber whereby it is exposed to a tungsten vapor. In
accordance with such a process the tungsten vapor selectively
deposits upon the copper since the surface of the 7070 glass layer
5 acts to oblate the tungsten vapor, thereby preventing any
significant build-up of tungsten thereon. After the desired
thickness of tungsten has been deposited upon the copper, so as to
form an incandescent filament 29, the chemical vapor deposition
step is terminated and the device is removed from the chamber.
Thereafter, the 7070 glass layer 5 and copper are removed by
etching, leaving, in the same manner as described with respect to
the process of FIG. 1, the incandescent filament suspended between
posts 11 and 13, as shown in step f. However, it can be seen that
the resultant incandescent filament 29 is channel-shaped and,
accordingly, will thereby exhibit added rigidity.
In FIG. 4 there is depicted a series of steps A-D representing a
further alternative scheme for fabricating incandescent filaments
to that depicted in FIG. 1. In the arrangement of FIG. 4, rather
than employ a temporary support layer, as was done in the
previously described processes, a ceramic substrate 31 with a
channel 33 may be employed. Channel 33 may be formed by any of a
variety of conventional material removal techniques, such as,
etching. As in the previously described processes, a pair of holes
7 and 9 are formed in the substrate, as shown in step A.
Thereafter, the holes are filled with conductive material, as
previously described, to create a pair of conductive posts 35 and
37. However, as can be seen from the configuration of the
substrate, these posts are not required to act as sole support for
the incandescent filament to be subsequently fabricated. Channel 33
is also filled with a material, which is to act as a temporary
support. This material may be any of a variety of materials capable
of being selectively removed thereafter by any of a variety of
techniques, such as, chemical etching. Thus, channel 33 may be
filled with a ceramic material, for example, or glass, or metal.
Typically, channel 33 may be filled with a metal such as copper, as
depicted by 39 in step B of FIG. 4.
After channel 33 has been filled, a layer of incandescent material
is then deposited and thereafter etched to form the incandescent
filament, as described in the previous processes. This is shown in
step C of FIG. 4 where incandescent filament 41 is shown exhibiting
a shape akin to that described in regard to FIGS. 1 and 2. After
filament 41 has been etched the body of copper 39 in channel 33 may
then be removed thereby leaving incandescent filament 41 bridged
across the banks of the channel and in conductive contact with
posts 35 and 37. It should be recognized that where support layer 5
in FIG. 1 and the support body 39 in FIG. 4 are made of insulative
material, it is not absolutely necessary that they be removed.
However, it is clear that the incandescent filament under such
circumstances would operate at unnecessarily high power levels.
Accordingly, the better mode is to suspend a substantial portion of
the incandescent filament in free space whereby heat dissipation
may be minimized.
In FIG. 5 there is depicted a conventional 16-segment alphanumeric
display cell exhibiting incandescent filaments fabricated in
accordance with the principles of the present invention. As can be
seen, each segment exhibits a zig-zag or somewhat serpentine
configuration, akin to that previously described. It is clear,
however, that any of the variety of segment configurations may be
employed. The manner of fabricating the 16 segment cell arrangement
of FIG. 5 is the same as that described, for example, in regard to
the process of FIG. 1. However, as is evident, rather than
directing the process steps toward the fabrication of a single
element, the process steps are to be directed toward fabricating
arrays of segments, en masse. Typically, the cell arrangement of
FIG. 5 would measure 150 mils high and 100 mils wide. Thus, in
fabricating an arrangement as shown in FIG. 5, a ceramic substrate
15 slightly in excess of the dimensions for an array of cells
(alphanumeric characters) would initially be chosen. Thereafter a
temporary support layer could be deposited thereupon (not shown)
and holes, at appropriate locations, thereafter drilled
therethrough. Thus, holes would be drilled by electron-beam for
example, to allow fabrication of posts 43 through 83, as depicted
in FIG. 5. After drilling, the holes would be filled and a layer of
tungsten, for example, deposited over the entire surface of the
support layer, as in previously described FIG. 1. Then, the various
filaments, as depicted in FIG. 5, would be configured by using
photo-etch techniques and the support layer thereafter removed. It
can be seen that for purposes of illustration the cross-section of
the various posts is substantially larger than that of the
filaments shown.
A 16-segment alphanumeric cell, akin to that depicted in FIG. 5,
utilizing a tungsten filament with dimensions of the order of 12
microns wide and 1 micron thick, suspended 5 mils above the
substrate by support posts approximately 100 mils apart, typically
operates at a temperature of 1,200.degree. C. Such operating
temperatures provide long-life filaments.
In FIG. 6 there is further depicted another example of an
incandescent display cell, fabricated in accordance with the
principles of the present invention. As can be seen, FIG. 6
provides an array of individual incandescent filament units 85. The
individual filaments, which may be any planar shape, are
selectively addressable whereby selected alphanumeric characters
may be created. Typically, the alphanumeric matrix display of FIG.
6 may be a 5 .times. 7 arrangement, measuring, for example, 150
mils high and 100 mils wide with each incandescent unit located
.025 mils on center. It is clear that with the above proportions
the individual filaments of each unit may be of the order of 0.015
mils, for example.
It is clear that an incandescent display device comprising a matrix
array of individual incandescent filaments, of the variety and
dimensions given above, is not capable of practically being
fabricated by conventional assembly techniques. However, by
employing the planar etching techniques of the present invention,
such a display cell may readily be fabricated en masse.
It should be recognized that although the zig-zag or substantially
serpentine configuration of the planar incandescent filament
described is only one of any of a variety of configurations that
may be fabricated in accordance with the present invention, this
particular configuration represents a compromise between the
strength required to span a post-to-post distance of up to 100 mils
and the electrical driving impedance characteristic required to
match that of the standard logic circuitry employed in the display
driving apparatus. In addition, this configuration provides an
efficient and visually pleasing source of illumination. In this
regard it should be noted that since relatively small support posts
may be fabricated by the present invention, conduction losses at
the ends of the filaments are minimized. To improve this even
further it should be recognized that the filaments may be
fabricated to taper as they approach the posts. Thus, end losses
are substantially reduced.
It has, likewise, been found that tungsten filaments made in the
manner of the process described with respect to FIG. 1, exhibit a
slight bow upwardly from the surface of substrate 1, as shown in
FIG. 2, after the temporary support layer 5 has been removed. This
can be seen to be clearly advantageous when it is recognized that
upon energizing the filament, the filament expands in response to
the heat generated. Accordingly, the slight upward bow assures that
the filament, upon expansion, will move away from the surface of
substrate 1, thereby avoiding possible contact therewith. In this
regard, then, it should be noted that this slight upwardly
extending bow may be introduced into the process by selecting a
material for the temporary support layer which has a coefficient of
thermal expansion significantly less than that of the substrate,
such that the combined layers will bow upwardly at the center
thereof upon cooling, after deposition of the support layer.
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
details may be made therein without departing from the spirit and
scope of the invention.
* * * * *