Photolithographic method for making helices for traveling wave tubes and other cylindrical objects

The helix (10) of the slow wave structure of a travelling wave tube is formed on a steel mandrel (12) by depositing the metal (44) of the helix on the mandrel and then coating the deposited metal with a photo resist (46). A laser light beam (24,26), having a cross section in the form of a short line, is focused upon the resist and moved linearly along the axis of the mandrel while the mandrel is rotated. The resulting helical exposure pattern on the photo resist is developed and the remainder of the undeveloped resist is then removed to expose a helical pattern (50,52) of deposited helix metal (44). The latter is subjected to etching processes so as to remove the deposited metal between the turns of the helical resist pattern (54,56), leaving a helix (44) of deposited metal on the mandrel underneath the resist. The resist is then removed and the mandrel etched away to leave the completed helix.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present relates to traveling wave tubes and more particularly concerns 
the helix of the slow wave structure of such tubes and a method of 
manufacture of such helix, or manufacture of other cylindrical objects. 
2. Description of the Related Art 
In travelling wave tubes, a stream of electrons is caused to interact with 
a propagating electromagnetic wave in a manner that amplifies energy of 
the electromagnetic wave. To achieve desired interaction between the 
electron stream and the electromagnetic wave, the latter is propagated 
along a slow wave structure such as an electrically conductive helix that 
is wound about the path of the electron stream. The slow wave structure is 
conveniently explained as providing a path of propagation for the 
electromagnetic wave that is considerably longer than the straight axial 
length of the structure so that the traveling electromagnetic wave may be 
made to propagate axially at nearly the same velocity as the electron 
stream. More accurately described, the wave does not travel along the 
helix but travels along the axis of the helix at a speed much less than 
the speed of light in a vacuum because of boundary conditions imposed by 
the helix. 
Slow wave structures of the helix type may be supported within a tubular 
housing by means of a plurality of longitudinally disposed dielectric rods 
that are circumferentially spaced about the slow wave helix structure. 
Various other means are available for supporting the helix within its 
envelope. 
Typical slow wave structures of the prior art are disclosed in U.S. Pat. 
No. 3,670,196 to Burton H. Smith, U.S. Pat. No. 4,115,721 to Walter Fritz, 
U.S. Pat. No. 4,005,321 to Arthur E. Manoly, U.S. Pat. No. 4,229,676 to 
Arthur E. Manoly, U.S. Pat. No. 2,851,630 to Charles K. Birdsall, and U.S. 
Pat. No. 3,972,005 to John E. Nevins, Jr., et al. 
The helix of the slow wave structure in the prior art is generally 
manufactured by winding or machining techniques. For winding a helix, a 
thin ribbon of an electrically conductive material may be wound around a 
mandrel and processed to properly shape the helix to the circular 
configuration of the mandrel. For machining a helix, a cylinder of helix 
metal may be cut into the desired pattern using electron discharge 
machining. Both winding and machining techniques are limited to 
manufacture of helices of relatively large size. Using such techniques, it 
is exceedingly difficult to fabricate small helices that are needed for 
higher frequency shorter wave tubes. For traveling wave tubes operating in 
the millimeter wave length, at frequencies above about 20 GHz, for 
example, circuit components including the helix are so small that 
conventional manufacturing techniques for the helix result in helices of 
poor dimensional precision. Moreover, yield of such processes is small 
because of the difficulty of handling and operating upon the very small 
parts. Thus, prior manufacturing techniques provide helices that are not 
dimensionally accurate, having poor tolerances, are not of sufficiently 
small diameter and have less dimensional stability, at least in part due 
to distortion arising from removal of the helix from its mandrel. 
Although photolithographic techniques are used in semiconductor and 
flexible cable fabrication, these processes are employed on planar 
surfaces and have not been employed for the manufacture of components of 
traveling wave tube slow wave structures. 
Accordingly, it is an object of this invention to provide hollow 
cylindrical objects, such as helices, and manufacturing methods therefor 
that avoid or minimize above mentioned problems. 
SUMMARY OF THE INVENTION 
In carrying out principles of the present invention, in accordance with a 
preferred embodiment thereof, a mandrel carries a photo resist coating in 
which is formed a pattern of helical turns providing a first pattern 
between the turns of the photo resist and a second mating pattern in 
registration with the turns of the helical photo resist. A pattern of 
helix material is electroformed or sputtered on the mandrel, either in the 
first helical pattern between the turns of the resist, or in the second 
helical pattern, in registration with and beneath the turns of the resist. 
The resist and mandrel are then removed to leave the completed 
electroformed or sputtered helical object.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Methods and apparatus of this invention are applicable to manufacture of 
different types of hollow cylindrical objects, but will be described in 
connection with manufacture of helices for traveling wave tubes for 
purposes of exposition. 
Traveling wave tubes and their slow wave structures are well known and 
disclosed, for example, in the United States patents identified above. The 
helix of a slow wave structure of a traveling wave tube is an electrically 
conductive ribbon having a helical configuration that is supported within 
and spaced from an outer envelope by a number of dielectric rods, blocks 
or other supports. An exemplary helix of such a slow wave structure is 
illustrated in FIG. 1 and generally designated by numeral 10. As 
previously mentioned, the dimensions of the traveling wave tube, including 
its helix, become ever smaller as operating frequencies increase. The 
availability of helices of still smaller dimensions will enable 
manufacture of traveling wave tubes operable at even higher frequencies. 
Thus, a helix for high frequency traveling wave tube operation in the 
order of 20 GHz or more, may have a length of between six and ten inches, 
an inside diameter in the order of 0.060 inches, and a cross-sectional 
dimension of each helix turn between about 0.01 and 0.02 inches on each 
side. These dimensions are provided solely as an example of the small 
sizes of the parts to be made and may be even smaller when parts are made 
by the processes to be described herein. Difficulties of precision 
manufacture of such small parts are apparent, particularly when tolerances 
as low as .+-.1 micrometer are required. Material of the helix is 
electrically conductive and generally a metal such as copper, molybdenum 
or tungsten is used. The helix may be of a soft copper where it is to be 
braised to its supporting dielectric rods. A harder metal such as 
molybdenum or the like is preferred when the helix is to be mounted in the 
tube by coining or pressure contact with its support. 
A simplified illustration of apparatus that may be employed in the 
performing the processes of the present invention is illustrated in FIG. 2 
and includes an elongated mandrel 12 supported in a chuck 14 driven by a 
motor 16. The mandrel is supported in an end support generally indicated 
at 20. An optical assembly of light source 22, focusing lenses 24, and 
masks 26 is mounted upon guide rods 30 for linear travel in a direction 
parallel to the axis of the mandrel 12 and is driven by a screw 34 which 
is rotated by a motor 36 carried on a support 38 that mounts the mandrel 
rotating motor 16. If deemed necessary or desirable, the entire apparatus 
may be enclosed in a housing chamber indicated at 40 to carry out certain 
controlled environment processing to be described below. 
The mandrel is a relatively small diameter elongated cylinder having a 
diameter equal to the desired inner diameter of the helix 10 that is to be 
made by the process. The length of the mandrel is slightly longer than the 
length of the finished helix to enable the mandrel to be held in the 
appropriate tooling 14, 20. As illustrated in FIG. 3, suitable helix metal 
such as copper, molybdenum, or tungsten is deposited upon the mandrel 12, 
completely covering its circumference for the length of the desired helix. 
If copper is to be deposited, it may be coated or electrolytically plated 
upon the mandrel which is preferably made of an electrically conductive 
metal such as stainless steel, for example. The metal coating 44 has a 
thickness equal to the thickness of the desired helix that is to be made. 
Alternatively, the metal coating of the mandrel illustrated in FIG. 3 may 
be applied by chemical vapor deposition or sputtering a metal such as 
molybdenum. In such a case, the mandrel would be rotated by the apparatus 
illustrated in FIG. 1 during the sputtering process so as to obtain a 
uniform thickness of the metal coating. Other coating processes such as 
electroless plating or electrophoretic coating may be employed. If deemed 
necessary or desirable, a number of similar or identical mandrels may be 
coated simultaneously. 
Then, as illustrated in FIG. 4, a coating of a conventional positive or 
negative photo resist material 46 is applied as by spraying, for example, 
to the metal coated mandrel. Again, to assure uniform thickness of the 
photo resist, the mandrel may be rotated during application of the photo 
resist. The photo resist will then be optically exposed, developed, and 
have its exposed portions removed according to well known 
photolithographic processes so as to provide a helical pattern of photo 
resist as illustrated in FIG. 5. This step is performed employing the 
apparatus of FIG. 2 including the traversing optical assembly 22, 24, 26. 
Alternatively, the photo resist can be exposed by ultraviolet light, 
x-rays or electron beams. 
A suitable light source, such as the laser indicated at 22, is focused by 
means of optics 24 through a mask 26 onto the coated mandrel so as to 
expose a short line. The shape of the light beam exiting the mask and 
impinging upon the photo resist is defined by the mask 26. The short line 
of light that is projected on the photo resist extends in a direction 
parallel to the axis of the mandrel and has a length equal to the distance 
(in the direction of the helix axis) between windings of the helix to be 
formed, for a positive resist. If a negative resist is used, the line of 
light has a length equal to the width of a winding, which is preferred if 
the distance between windings is to be varied. With the short line of the 
optical beam impinging upon the photo resist, the mandrel is rotated at a 
fixed speed, and, simultaneously, the entire optical assembly is driven at 
a fixed speed in a linear path precisely parallel to the axis of the 
mandrel so that a helical pattern of the photo resist is exposed to the 
light. Either speed may be varied, as will be explained below, if a 
varying helical pitch is desired. The resist is then developed, and for a 
positive photo resist the exposed portion of the resist is removed, 
leaving a pattern of photo resist as illustrated in FIG. 5. Spaces such as 
spaces 50 and 52 between adjacent resist helical turns 54, 56 define a 
first helical pattern. The turns 54,56 of the resist define a second 
helical pattern of exposed helix metal. 
In the process illustrated in FIGS. 3-5, after removal of the exposed 
portions of the photo resist, exposed areas of the deposited metal 44 are 
then removed by a conventional etching solution. This leaves a helical 
pattern of deposited metal directly beneath the developed photo resist 
helix including its turns 54, 56 so that with the developed helical 
pattern of photo resist subsequently removed or stripped from the mandrel 
the assembly appears as illustrated in FIG. 6. Now the mandrel may be 
removed by a conventional etching process leaving the completed helix 10 
as shown in FIG. 1. 
If deemed necessary or desirable, the mandrel may be reused by first 
coating the mandrel with a suitable release material. For example, a 
coating of tungsten oxide may be used for a tungsten helix wound on a 
tungsten mandrel. The coating is interposed between the mandrel surface 
and the first coating of metal 44. Then, after removing the helical resist 
pattern from the helically etched metal, the interposed release coating 
can be etched away to enable release and removal of the helix from the 
mandrel and to allow the mandrel to be reused. 
FIGS. 7-9 illustrate a modification of the process described above. In this 
arrangement, as shown in FIG. 7, the mandrel 12 is first completely coated 
with a photo resist 58 to a thickness that is equal to or greater than the 
thickness of the desired helix that is to be made. Then, employing the 
apparatus and techniques illustrated and described in connection with FIG. 
2, a light beam, configured in a short line extending axially of the 
mandrel, is caused to impinge on the photo resist and moved in a helical 
pattern along the helical resist by simultaneously rotating the mandrel 
and linearly moving the optical assembly at fixed speeds. In this method, 
the length of the line of light in a direction parallel to the axis of the 
mandrel is equal to the width of the helix winding for positive resist or 
to the distance (measured axially) between adjacent turns of the desired 
helix for a negative resist. The exposed resist is then developed and the 
exposed (or unexposed) material removed to leave a helical pattern of 
resist 63,64 as illustrated in FIG. 8. In this arrangement, it should be 
noted that the resist is effectively formed as a negative pattern so that 
spaces such as 60 and 61 between adjacent turns of resist 63 and 64 define 
the width of the metal helix that is to be manufactured. The width of the 
individual helical turns 63, 64 is defined by the length of the optical 
line that is projected through the optical mask 26 to impinge upon the 
photo resist. Now, as illustrated in FIG. 8, the mandrel exhibits a first 
helical pattern formed by spaces 60, 61 between adjacent turns of the 
resist 63, 64 and a second helical pattern in registration with the 
helical photo resist turns 63, 64. The pattern formed by the spaces 60, 61 
is a positive pattern for the desired metal helix and on this positive 
pattern will be formed the desired metal helix. 
To this end, as illustrated in FIG. 9, the metal of the helix is formed 
upon the mandrel and helical resist pattern to provide the deposited metal 
66, 68, covering the photo resist 63, 64 and deposited metal 70, 72 in the 
spaces between the adjacent turns of the photo resist 63, 64. The 
deposited metal 70,72 forms the helix that is an end product of this 
process. The helix metal may be deposited on the resist covered mandrel in 
the step illustrated in FIG. 9 by any suitable coating process, including 
various types of electroforming or sputtering as previously described. 
That portion of the deposited metal, if any, such as areas indicated at 
66, 68 in FIG. 9 that adhere to the photo resist 63, 64 may then be 
removed together with the helically patterned photo resist. This will 
leave only the helical metal pattern 70,72 on the mandrel 12. The mandrel 
12 is subsequently etched away in the manner previously described, and 
there remains only the completed helix 10. 
Although, in the arrangement of FIGS. 7-9, a standard photo resist material 
has been employed to form the helical pattern on the mandrel, it will be 
readily appreciated that other arrangements can be employed. For example, 
instead of employing a photo resist, the mandrel may be completely coated 
with a layer of inert electrically nonconductive material such as Teflon 
to a thickness equal to or greater than the desired thickness of the 
helix. Then, the Teflon coating may have a helical grooved pattern 
identical to the spaces 60, 61, illustrated in FIG. 8, ablated therein by 
a laser such as an excimer laser. In such an arrangement, the optical 
assembly 22, 24, 26 is replaced by a laser having its beam appropriately 
configured and sized so that when the laser is longitudinally shifted in a 
linear path parallel to the mandrel axis while the mandrel is 
simultaneously rotated, a helical groove is ablated in the Teflon coating 
completely through the Teflon to the mandrel, thus exposing the 
electrically conductive mandrel surface in a positive helical pattern. 
This exposed mandrel surface may then be subjected to electroforming, such 
as electrolytic or electroless plating, for example, to deposit the metal 
that is to form the helix. After removing the mandrel and the Teflon, the 
completed helix remains. In this embodiment the helix metal may be 
applied, alternatively, by sputtering. 
Laser ablation of a helical groove in the Teflon coating has the advantage 
of increased precision of geometry and control of dimensions of the 
configuration of the resulting helix because the laser ablated groove 
dimensions and configurations may be more accurately and precisely 
controlled, and walls of a laser ablated groove may be more precisely 
perpendicular to the mandrel surface. 
The methods and apparatus described above have been discussed in connection 
with the manufacture of a helix for the slow wave structure of a traveling 
wave tube and will provide the advantages of increased precision, 
accuracy, and repeatability with concomitant improved yield and 
performance for manufacture of smaller and smaller helices. Nevertheless, 
the disclosed methods and apparatus may also be applied to manufacture of 
other hollow cylindrical objects, including electrical circuitry to be 
formed on a non-planar surface. Electrical circuits having a configuration 
of generally helical form or other patterns having a non-planar 
configuration may be made by the described processes. Examples of such 
helix derived electrical circuits include ring bar, folded helix, 
contra-wound helix and bifilar helix. Thus, in making a helix derived 
electrical circuit employing the method of FIGS. 7-9, for example, when 
optically exposing the photo resist 58, the light source may be modulated, 
to be turned on and off, according to a predetermined program, while the 
light source is moving parallel to the mandrel axis and the mandrel is 
rotating. For manufacture of electrical circuitry, the beam of the light 
source is caused to be focused to a point, rather than to a line. By 
turning the light source off and on during the relative linear and 
rotational motion of the light source and mandrel and, further, by 
relatively varying rotational speed of the mandrel and linear velocity of 
the optics, a wide variety of patterns may be achieved. 
As mentioned above, to obtain a helix having a uniform pitch throughout its 
length, the rotational speed of the mandrel and the linear velocity of the 
optics relative to the mandrel are both fixed throughout the optical 
exposure. Where it is desired to vary the helix pitch as, for example, to 
decrease helix pitch so as to cause axial velocity of the traveling wave 
of the traveling wave tube to decrease in a manner corresponding to 
decrease of axial velocity of the electron stream, the rotational velocity 
and the translational velocity of the mandrel and optics may be increased 
or decreased respectively. 
There have been disclosed methods and apparatus for manufacture for 
photolithographic manufacture of helices for traveling wave tubes that 
provide for devices of significantly smaller sizes and thus of higher 
frequencies and resulting in greater yield of smaller, more precise, helix 
structures.