The present invention relates to the fabrication of small three dimensional
structures, particularly to the fabrication of three dimensional circuit structures
used in traveling wave tubes, and most specifically to methods for fabricating helical
circuit structures for use in traveling wave tubes.
BACKGROUND OF THE INVENTION
In traveling wave tubes (TWT's) an electron beam interacts with a
propagating electromagnetic wave to amplify the energy of the electromagnetic wave.
To achieve the desired interaction between the electron beam and the electromagnetic
wave, the electromagnetic wave is propagated through a structure which slows the
axial propagation of the electromagnetic wave and brings it into synchronism with
the velocity of the electron beam. In a TWT, one such so-called slow wave is a helical
coil that surrounds the structure of the electron beam. The kinetic energy in the
electron beam is coupled into the electromagnetic wave, amplifying the wave significantly.
The advantages of such slow wave properties in TWT's are known to those having ordinary
skill in the art.
A wide variety of alternative slow wave structures are known. For
example, those structures disclosed in U.S. Patent Nos. 3,670,196, 4,115,721, 4,005,321,
4,229,676, 2,851,630 and 3,972,005. A number of methods for constructing the helixes
of these structures are known. Common fabrication techniques include winding or
machining. For example, a thin wire or tape of electrically conductive material
may be wound around a mandrel and processed to properly shape the helix to the circular
configuration of the mandrel. However, the process of winding the helix places stress
on the wired tape, creating a helix of limited stability under operating conditions.
Additionally, when heated (for example during annealing or during operation), such
wound helixes do not have dimensional stability (i.e. helices formed in this manner
have a tendency to distort beyond the tolerances required for reliable operation).
Alternatively, a cylindrical helix may be cut into the desired pattern
using electron discharge machining. This process does not produce helices of accurate
dimensions. However, this process tends to produce helices that are embrittled and
subject to cracking.
Although suitable for some purposes, both machining and winding techniques
are subject to serious limitations only capable of reliably manufacturing helixes
of relatively large dimensions. However, when used in high frequency applications
(for example, so-called "Ka-band", "Q-band", "V-band", or "W-band" TWT's) such conventional
techniques do not reliably produce the smaller helixes and circuit structures that
are needed for these high frequency applications. For example, in a TWT operating
in millimeter wavelengths, at frequencies above 20 GHz, conventional techniques
produce TWT circuits that suffer noticeably from mechanical distortion effects and
thermo-mechanical relaxation. At frequencies near, for example, 50 GHz, the circuit
components (including the helix) are so small that conventional manufacturing techniques
can produce satisfactory helixes with only with great difficulty and with often
unpredictable quality. A typical traveling wave circuit element features a coaxial
dielectric support element which is in physical contact with the circuit element.
Due to the effects of mechanical distortion or thermo-mechanical relaxation, conventionally
constructed circuit elements physically distort and become separated from the dielectric
support. This is undesirable. Also, at these frequencies current processes for manufacturing
helixes commonly have a very low product yield. An additional limitation to existing
methods of manufacturing are the inability to produce certain advantageous non-helical
circuit structures. In short, current manufacturing processes produce helices which
are plagued with poor tolerances, dimensional inaccuracies, size limitations, circuit
unreliability, and insufficient robustness to service the needs of high frequency
TWT's. Additionally, a number of non-helical circuit structures have been proposed
by others. The problem with many of these structures is that until now there has
been no satisfactory way to construct them for operation at high frequency.
SUMMARY OF THE INVENTION
Accordingly, it is the feature of this invention to provide methods
and apparatus for constructing small three dimensional circuit structures having
precise physical dimensions to narrow tolerances. It is a further feature of the
invention to construct structures demonstrating high dimensional stability and robustness.
Structures formed in accordance with the present invention also demonstrate improved
thermal performance, reduced rf losses, and increases in overall performance efficiency.
A particular feature of the present invention to provide a methodology for constructing
thermally and dimensionally stable helical circuit elements for use in TWT's to
exacting tolerances at very small dimensions. It is a further feature of the present
invention to provide methods of fabricating heretofore unbuildable circuit elements
as well as methods for constructing such elements.
The principles of the present invention contemplate methods for constructing
thermally and dimensionally stable three-dimensional TWT circuit structures to narrow
tolerances and very small sizes by providing a small hollow preform constructed
of a desired material. A coating of photoresist material is applied to the preform.
The photoresist coating is treated to form a desired pattern in the photoresist
coating such that a portion of the outside surface of the preform is exposed and
another portion of the outside surface of said preform remains covered with the
photoresist pattern. Subsequently, preform material is removed from the exposed
portion of said preform leaving the pattern covered portion in place to create a
preform having a desired shape. After shaping, the photoresist coating is stripped
from said shaped preform, followed by an optional polishing step.
Additionally, the principles of the present invention contemplate
an apparatus for forming small three dimensional circuit structures from preforms
comprising a means for supporting the preform on its axis, a means for rotating
the preform, an exposure source for directing a light beam onto said preform, a
means for shifting said exposure source along said preform, and a means for controlling
said rotating means, said shifting means, and said exposure source to achieve a
predetermined pattern in the preform.
Also, the principles of the present invention as described above contemplate
novel three dimensional structures including a very small helix, a ring bar circuit,
a very small finned ladder circuit structure, and a very small slotted finned ladder
circuit structure as well as traveling wave tubes incorporating these structures.
Other features of the present invention are disclosed or made apparent
in the section entitled "DETAILED DESCRIPTION OF THE INVENTION".
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the present invention, reference is
made to the accompanying drawings in the following "Detailed Description of the
Invention". Reference numbers and letters refer to the same or equivalent parts
of the invention throughout the several figures of the drawings. In the drawings:
DETAILED DESCRIPTION OF THE INVENTION
- FIG. 1A is a block diagram of one embodiment of a device that may be used to
form photoresist patterns in accordance with the principles of the present invention.
- FIG. 1B is a schematic illustration of a device that may be employed to form
photoresist patterns in accordance with the principles of the present invention.
- FIG. 2 is a flowchart illustrating one method of constructing a three dimensional
circuit structure in accordance with the principles of the present invention.
- FIG's 3 and 4 are perspective views of hollow preform shapes for use in accordance
with the principles of the present invention.
- FIG. 5 shows the preform of FIG. 3 after the application of a layer of photoresist.
- FIG. 6 is a top down view of a portion of the preform shown in FIG. 5 showing
a dot caused by an exposure source (e.g. a laser) directed onto a target area of
the preform in accordance with the principles of the present invention.
- FIG. 7 is a top down view as in FIG. 6 after a portion of the preform surface
is treated with an exposure source in accordance with the principles of the present
- FIG. 8 is a top down view as in FIG. 7 after the entire surface of a preform
is treated with an exposure source in accordance with the principles of the present
- FIG. 9 is a schematic side view of a photoresist treated preform in an etch
bath during an etching process in accordance with the principles of the present
- FIG. 10 is a side view of helical circuit structure constructed in accordance
with the principles of the present invention.
- FIG's. 11-15 are perspective views of finned-ladder and slotted finned ladder
circuit structures constructed in accordance with the principles of the present
- FIG. 16 is a cross section view of the embodiment shown in FIG. 15.
- FIG. 17 is a perspective view of a "ring bar" circuit embodiment.
The principles of the present invention may be used to advantageously
construct small three dimensional circuit structures having precise physical dimensions
to exacting tolerances. Furthermore, such structures are free of the mechanical
stresses common to conventionally fabricated structures. Moreover, the structures
of the present invention demonstrate the advantageous features of high dimensional
stability and robustness.
The following description of the presently contemplated best mode
of practicing the invention is not to be taken in a limiting sense, but is made
merely for the purpose of describing the general principles of the invention. The
scope of the invention should be determined with reference to the claims.
Embodiments of the present invention are used to construct helical
circuit structures for use in traveling wave tubes (TWT's) having inside diameters
in the range of about 0.018 inches (18 mils) to about 0.125 inches (125 mils) with
helical wall thicknesses being in the range of about 4-10 mils. The principles of
the present invention have particular usefulness when applied to electrically conductive
and etchable materials including without limitation, copper, molybdenum, tungsten,
and alloys containing these metals. The principles of the present invention are
not confined to the above referenced material but may be applied to any etchable
metal and may also be applied to semiconductor materials or other non-conducting
FIG. 1A is a simplified block diagram depicting an embodiment that
may be used to construct the three-dimensional structures of the present invention.
The apparatus of FIG. 1A includes a means for supporting 105 a preform 10 on an
axis, a means for rotating 106 the preform 10, an exposure source 107 for directing
a light beam onto said preform 10, a means for shifting 108 said exposure source
107 along said preform10, and a means for controlling 100 said rotating means 106,
said shifting means 108, and said exposure source 107 to achieve a predetermined
pattern in the preform 10. The circuit structures disclosed herein may be readily
integrated into traveling wave tubes. The methods for constructing such tubes are
within the skill of one having ordinary skill in the art.
A simplified illustration of an apparatus that may be used in constructing
the three-dimensional structures of the present invention is shown in FIG. 1B. Included
is a photoresist treated preform 10 supported in a pair of chucks 41 and driven
by a controlled motor 45. An optical assembly (also referred to as the exposure
source) 42 mounted upon a guide 46 which facilitates shifting the source 42 longitudinally
(as indicated by the arrows) along the preform 10. The rate of rotation of the preform
10 and the rate of movement of the optical assembly 42 is typically determined by
a controller (not shown). The optical assembly 42 typically includes an optical
source, for example, an ultraviolet (UV) excimer laser such as a LPX 210 manufactured
by Lambda Physik. A wide variety of other lasers known to those having ordinary
skill in the art may be chosen. Additionally, a variety of optical sources may be
used, for example, a Xenon lamp with a focusing lens and a mask. A UV laser is merely
a preferred source due to its coherent radiation and ability to define sharp features
in the photoresist.
The apparatus of FIG. 1B directs an exposing light beam 43 from an
optical source contained within the optical assembly onto a preform 10 which has
already been treated with a layer of photoresist. By controlling the rate at which
the optical assembly (e.g., a laser) 42 is longitudinally shifted (as shown by the
arrows) along the preform 10 and the rate of rotation of a variable speed motor
45 (and thereby the rate of rotation of a preform 10) an exposure pattern may be
formed in the photoresist layer of the preform 10. In addition, by switching the
optical source off and on during exposure, more complicated and discontinuous patterns
may be formed in the photoresist layer. The contour of these patterns are determined
by an encoder pattern which is supplied to the controller 100. Controllers 100 of
the present invention can be a simple mechanically actuated controllers or microprocessor
driven controllers (e.g, computers) or even application specific integrated circuits
(ASIC's). The encoder pattern can be either hardware or software driven and may
be adjusted during processing to accommodate the needs of the manufacturer.
FIG. 2 illustrates one embodiment of a process flow for forming a
helical circuit structure for use in TWT's. A hollow preform is provided (Step 201).
The preform may be of any shape depending on the desired final shape of the structure.
Referring to FIG. 3, a preferred preform 10 is a substantially cylindrical hollow
tube, having an inner diameter ID and an outer diameter OD. The walls 11 of the
tube 10 have a width W. In one preferred embodiment a molybdenum preform 10 has
an inner diameter ID of about 18 mils and an outer diameter OD of about 23 mils.
The walls have a thickness W in the range of about 4-6 mils. Such precision preforms
can be obtained, for example, by forming a larger tube and in a controlled process
drawing the tube down to a nominal size. Then the outside diameter is precision
ground to the needed tolerance and the inside diameter is electron discharge machined
to the precision tolerance required. In widest application the preform 10 may be
constructed of any readily etchable material, but preferred materials include molybdenum
or molybdenum containing alloys, copper or copper containing alloys, stainless steel,
or other etchable metals. A most preferred material being molybdenum.
As illustrated in FIG. 4, other preform shapes may be used to construct
alternative devices. For example, a square preform 20, having a wall 21 width W
may be used.
With reference to FIG's 2 and 5, once a preform 10 of an appropriate
shape and dimension is chosen, the preform 10 is coated with a photoresist material
12 (Step 203). The photoresist may be either a positive or negative photoresist
depending on the needs of the process engineer. It is critical that the outside
surface of the preform 10 be coated with a layer of photoresist material 12.
As illustrated in FIG. 5, a preform 10 has been treated with a layer
of photoresist coating 12. A preferred photoresist is a UV developable photoresist
such as those manufactured by Shipley Company of Marlborough, Massachusetts. However,
other types of photoresist may be used, including negative photoresist and non-UV
Application of the photoresist may be accomplished using a wide range
of techniques, including but not limited to, spraying, dip coating, or types of
spin coating. However, the preferred embodiment uses electrophoretic application
of the photoresist. Electrophoretic application works exceptionally well on three-dimensional
structures. Methods of electrophoretic deposition of photoresist are know to those
with ordinary skill in the art. One such process is outlined in "Electrophoretic
Photoresist Technology: An image of the Future - Today" by D.A. Vidusek; Circuit
World, Vol 15, No. 2, (1989) which is hereby incorporated by reference. The photoresist
coating 12 is applied to a preferred thickness of about 1 mil. Other thicknesses
may be chosen depending on the needs of the process engineer. The resist 12 must
be thick enough so that the preform material is completely etched away before the
photoresist becomes degraded.
Once the photoresist layer 12 is applied, a mask pattern is formed
in the photoresist coating 12. Typically the pattern is formed by optically exposing
the photoresist layer 12 then "developing" the photoresist layer to produce a desired
mask pattern. Optical exposure (Step 205) may be achieved using a wide range of
exposure sources. The particular source chosen is dictated by the needs of the process
engineer based on such factors as desired exposure time, choice of photoresist,
pattern resolution, desired pattern shape, as well as other considerations known
to those having ordinary skill in the art. However, the preferred source is an ultraviolet
(UV) laser. Many other lasers or other light sources may be used, such as UV flash
lamps. The exposure step (Step 205) is accomplished by placing a photoresist treated
preform 10 in an apparatus 40 which will apply a pattern onto the photoresist layer
12. The preform 10 being, for example, a substantially cylindrical hollow tube about
6" in length and having an outer diameter of about 23 mils, is placed in a rotatable
chuck 41, then secured. Once secured the preform 10 is treated with the exposure
source 42. It is advantageous to use a preform 10 having a length longer than the
desired final product. For example, if the final product is a helix of about 4"
in length, then a 6" preform is more than adequate. After being secured in the chuck
41 the preform 10 is rotated while at the same time a laser beam 43 is shifted along
the length of the preform. A laser beam 43 is directed at the preform projecting
a dot onto the photoresist layer. A preferred embodiment uses a laser 43 having
a dot having a diameter of about, 7 mils. A satisfactory pattern may be obtained
in about 60 to 120 minutes.
The preform 10 is positioned on the apparatus 40 such that the light
beam 43 strikes the photoresist layer 12 of the preform 10. The dot produced by
the light beam 43 is moved across the surface of the preform 10, in particular,
shifting along the length of the preform 10 as the preform 10 is rotated enabling
the beam 43 to expose a spiral pattern in the photoresist completely around the
outside of the preform 10. The rotation of the preform 10 and the shifting movement
of the exposure source 42 is determined by the controller 100. The controller 100
uses a pattern forming encoder which can be either hardware or software driven.
The encoder provides instructions which control the rate of rotation of the preform
10 and the rate at which the dot shifts along the length of the preform and whether
the exposure source is turned on or off, as well as other parameters. The encoder
can be set to expose simple spiral patterns or more complex patterns. The encoder
itself can be a simple set of mechanical cams or a more complex encoding apparatus
such as a computer control system. Furthermore, the controller can be interactive,
allowing the operator to adjust the exposure parameters as the photoresist is being
exposed. For example, the controller 100 can be a computer connected to a variable
speed motor 45 and the exposure source 42. The operator can supply further pattern
forming instructions during pattern forming to adjust whether the exposure source
is on, the preform rotation rate, the rate at which the beam moves along the surface
of the preform, etc.
FIG's. 6, 7, and 8 illustrate the exposure effects of a laser beam
43. In FIG. 6 an impinging laser beam projects a dot D onto a target area on the
surface of a preform 10. During the exposure process, the preform 10 is rotated
and the laser source is advanced longitudinally across the preform 10. FIG. 6 shows
an example of a partial exposure pattern 51 formed by the movement of the dot D
across the surface of the preform 10. The exposure area 51 is the region where the
laser beam has exposed the photoresist. Once the entire preform 10 is exposed, a
spiral pattern like that shown in FIG. 8 is formed.
This exposed preform 10 is then developed (Step 207). In a positive
photoresist, the light solublizes the photoresist allowing it to be removed with
the appropriate solvent leaving unexposed photoresist in place. In a negative photoresist
the opposite is true (the light makes the photoresist insoluble) allowing the unexposed
photoresist to be removed. In either case the photoresist forms a desired pattern
on the preform. Reference to FIG. 8 shows a typical pattern used to form a helical
structure in the preform. After coated the photoresist to a preferred thickness
of 1 mil, then exposed to a laser source to form a pattern, the photoresist is developed
using an appropriate solvent. For the UV laser photoresist used in the preferred
embodiment a satisfactory solvent is lactic acid. Development of such photoresists
is known to those having ordinary skill in the art. Typically, such development
times are short on the order of about 1-2 minutes.
Once the preform 10 is developed, leaving a photoresist pattern on
the preform surface, further processing is used to remove preform material from
the areas of the preform not covered with photoresist (Step 209). One preferred
method is by simple chemical etching using enchants optimized to remove the preform
material and having good etch selectivity with the photoresist. As shown in FIG.
9, so-called "wet" etching can be simply accomplished by plugging both ends of the
preform and placing the photoresist patterned preform 10 in container (an etch bath)
70 filled with etchant 71. Both ends of the preform are plugged 72 using, for example,
an elastomer material to prevent entry of the etchant into the interior dimensions
of the preform 10. This allows the etchant to act only on the exposed outside surfaces
of the preform, preventing the etchant from effecting the area of the preform covered
by the photoresist pattern. The preform 10 is preferably suspended in the bath 70
so that all preform surfaces are equally exposed to etchant, enabling even etching
of the preform surface. The etch process may be enhanced further by agitating the
etch bath. The particular etchants used are dependent on the preform material used.
In the case of a molybdenum preform, satisfactory etchants are ferric sulfate, or
ammonium ferric sulfate, or potassium hydroxide etchant solutions. Etching times
of 10-60 minutes are common, for example, using a potassium hydroxide solution on
a molybdenum preform, about 10-15 minutes satisfactorily etches the preform into
the desired shape. The principles of the present invention are not limited to particular
etchants. Especially, with respect to alternative preform materials, other etchants
are commonly used. Additionally, it should be noted that other etching techniques
may be used including, without limitation, plasma etching, ion beam etching, and
reactive ion etching. After the preform has been etched into the desired shape the
preform is removed from the etchant and rinsed using, for example, water followed
by a rinse in acetone and isopropyl alcohol. The photoresist is then removed (Step
211). The photoresist is stripped using processes chemicals known to those having
ordinary skill in the art. Optionally, a polishing step (Step 213) can be added.
For example, the etched preform may be electropolished by placing the etched preform
in a sulfuric acid (H2SO4) solution which is then neutralized
with an ammonium hydroxide (NH4OH) solution to produce a somewhat more
The end result of such a process is the fabrication of a helical structure
80 of preform material such as that shown in FIG. 10. The helical structure 80 has
an outside diameter OD and an inside diameter ID and a plurality of windings each
having a width 81 and thickness 82 and having a distance 83 between the windings.
The following preferred embodiment is in no way intended to limit
the invention but rather intended to illustrate the principles of the invention.
One preferred embodiment is a helix 80 having a length of about 4 inches with a
pitch (# of turns of the helix per inch) of about 50 turns per inch and having a
winding width 81 of about 0.007 inches and having a distance between windings 83
ranging from about 0.0075 inches to about 0.0081 inches of about and having a winding
thickness 82 of about 6 mils. Importantly, the pictured embodiment can be advantageously
varied to accommodate a wide variety of circuit needs. For example, in addition
to varying the pitch, the winding width 81 and distance between windings 83 can
be varied along the length of the helix as needed this includes embodiments where
the pitch, the winding width, and distance between windings vary over the length
of one circuit element. All that needs be done is to provide the appropriate encoder
information to the controller.
The advantage of the methods of the present invention are apparent
in the helix 80 of FIG. 10. First, helixes of such small dimension have not been
constructed. Helixes constructed using conventional methods are limited to constructing
helixes having inside diameters of about 23 mils with outside diameters of about
30 mils or larger. In contrast, the present invention contemplates a helix 80 having
an inside diameter ID of about 18 mils and an outside diameter OD of about 32 mils.
Furthermore, structures fabricated using methods embodied by the present
invention are not subject to the same mechanical stresses present in conventionally
manufactured circuit structures (e.g., those formed using winding processes). These
stresses lead to distortion and dimensional instability in circuit structures so
fabricated. This is easily detected in circuit structures using coaxial dielectric
supports which are intended to remain in physical contact with helical circuit structures
which wind around the supports. Thermal relaxation and distortion effects common
in these conventionally manufactured circuit structures leads to a physical separation
of the circuit structure from the dielectric support. In fact these separations
and distortions are commonly on the order of 5 mils.
In contrast, structures fabricated in accordance with the principles
of the present invention do not demonstrate the dimensional instability which characterizes
conventionally constructed helices. The methods of fabrication and circuit structures
embodying the present invention are not subject to mechanical distortion and dimensional
instability, but rather, demonstrate excellent dimensional stability and do not
become separated from the dielectric support elements even when subject to thermal
stress. In fact, the embodiments of the present invention can easily maintain dimensional
stability wherein the distortion and instability are less than 3 mils. In most cases
the dimensional stability provided by the present invention provides circuit embodiments
wherein the distortion effects are less than a mil.
Additionally, due to the extreme precision attainable with a laser
source, higher tolerances can be attained in the manufacture of such helixes. This
enables greater pitch to be achieved, as well as narrower winding thicknesses 82
and tighter distances between windings 83.
Still more important, TWT circuit shapes and structures which may
previously have existed only in theory are now possible to manufacture. For example,
one family of advantageous structures now manufacturable are so-called "finned ladder"
structures. Such structures are discussed in "Novel High-Grain, Improved-Bandwith,
Finned-Ladder V-Band Traveling-Wave Tube Slow-Wave Circuit Design" by C. Kory and
J. Wilson, IEEE Transactions on Electron Devices, Vol. 42, No. 9 (Sept. 1995) which
is hereby incorporated by reference. Due to manufacturing difficulties no suitable
means exists for reliably fabricating these structures. The present invention may
be used to construct structures of these dimensions.
Referring to FIG. 11 an inner preform 90 comprising a hollow tube
91 constructed of the desired preform material having a plurality of planar fins
92 extending radially therefrom is provided. A preferred embodiment includes a hollow
tube 91 having an inner diameter of about 18 mils and an outer diameter of about
23 mils. The fins are preferably about 6 mils thick and extend radially outward
to contact an outer sleeve preform 95. This basic inner preform 90 is treated and
patterned with photoresist 93 (as shown in FIG. 12). The photoresist may be applied
and patterned using the methods previously discussed herein with respect to the
construction of helical structures. As with the helical embodiment previously discussed,
the ends of the inner preform 90 are plugged with an elastomer and then the inner
preform 90 is etched. Holes may be etched in the tube 91 and slots 94 etched in
the planar fins 92 by any of the methods previously discussed (as shown in FIG.
13). In the pictured embodiment the inner preform 90 is etched to form a series
of coaxial rings 99 positioned having spaces therebetween. The slots 94 etched into
the planar fins 92 correspond to the spaces between the coaxial rings 99. This etched
inner preform 90 is now cooled and slid inside said heated tubular outer sleeve
95. The heat expansion of the outer sleeve preform 95 and the contraction of the
cooled inner preform 90 allow an interference fit to be achieved once a stable equilibrium
temperature is reached, resulting in the fabrication of a so-called "slotted finned-ladder"
slow wave circuit (FIG. 14). The above embodiment is merely an illustration of the
present invention and is not to be taken as limiting the invention, especially with
respect to the precise nature of embodiment dimensions.
A similar structure is shown in FIGS. 15 and 16. They show a traveling-wave
tube circuit having a plurality of hollow cylindrical rings 99. This structure is
formed in a similar fashion to that of FIG 14. i.e., the inner preform is patterned
and etched to the desired shape and interference fitted with the outer sleeve to
complete the circuit. Each ring 99 having an inside surface 100' and an outside
surface 101 and an inside diameter 103 of about 18 mils and an outside diameter
102 of about 23 mils the cylinder wall having a thickness of about 5 mils. The above
embodiment merely illustrates the principles of the present invention and is not
to be taken as limiting the invention, especially with respect to the precise nature
of embodiment dimensions. The rings 99 are positioned such that said rings 99 share
a common axis X. Two planar fins 92 extend radically outward from the rings 99.
Each fin 92 having a proximal end 92p and a distal end 92d is positioned such that
proximal 92p of the fins 92 are in contact with the outside surfaces of said rings
99 extending radically outward from the rings 99. A cylindrical outer sleeve 95
having an inside surface 110 and an outside surface 120 and a diameter 130 larger
than said ring 99 outer diameter 101 is positioned coaxially with said rings 99
and positioned such that the distal ends 92d of said fins 92 are in contact with
the inside surface 110 of said sleeve 95. Again, as with the embodiment of FIG.
14 the inner preform is cooled and slid inside a heated outer sleeve.
Another embodiment advantageously constructed in accordance with the
principles of the present invention is shown in FIG. 17. The pictured embodiment
is a "ring-bar" traveling-wave tube circuit 170 which is related to the family of
helical structures disclosed herein, specifically, a contrawound helix. This structure
is formed in a similar fashion to that of the other previously described structures.
A preform is patterned and etched to the desired shape to complete the circuit.
Each ring 171 having an inside surface 172 and an outside surface 173 and an inside
diameter 174 having a preferred diameter of about 18 mils and a preferred outside
diameter 175 of about 23 mils. The above embodiment merely illustrates the principles
of the present invention and is not to be taken as limiting the invention, especially
with respect to the precise nature of embodiment dimensions. The rings 171 are positioned
such that they share a common axis. The precise spacing 177 between the rings, and
ring width 176 are dependent (as are the other dimensions) on the operating frequency.
Until now circuits such as those described above could not be constructed
at all. Furthermore, the inventors contemplate that the principles of the present
invention may be used to form a variety of other three dimensional structures not
The present invention has been particularly shown and described with
respect to certain preferred embodiments and features thereof. It is to be understood
that the shown embodiments are the presently preferred embodiments of the present
invention and as such are representative of the subject matter broadly contemplated
by the present invention. The scope of the invention fully encompasses other three
dimensional circuit structures not expressly referred to as well as embodiments
which may become obvious to those skilled in the art, and are accordingly to be
limited by nothing other than the appended claims, in which reference to an element
in the singular is not intended to mean "one and only one" unless explicitly stated,
but rather "one or more". All structural and functional equivalents of the elements
of the above-described preferred embodiment that are known or later come to be known
to those of ordinary skill in the art are expressly incorporated herein by reference
and are intended to be encompassed by the present claims. Moreover, it is not necessary
for a device or method to address each and every problem solved by the present invention,
for it to be encompassed by the present claims. Furthermore, no element, component,
or method step in the present disclosure is intended to be dedicated to the public
regardless of whether the element, component, or method step is explicitly recited
in the claims.