Self-aligned double grids for vacuum tubes

Self-aligned double grids for vacuum tubes and methods for making such double grids are provided. The self-aligned double grids are especially suitable for improving the efficiency and performance characteristics of high frequency power amplifier tetrode tubes.

This invention pertains to vacuum tube technology in general and more 
particularly to self-aligned double grids for vacuum tubes and methods for 
making the same. 
The simplest amplifying vacuum tube is the triode consisting of a cathode, 
a control grid and an anode known as the plate. The grid, frequently in 
the form of a wire mesh or screen having a low ratio of wire area to open 
area, is situated in close proximity to the cathode. 
During operation of the tube, the cathode of the vacuum triode is heated to 
the temperature required for thermionic emission of electrons. The plate 
is maintained positive with respect to the cathode and, as a consequence, 
electrons flow from the cathode to the plate. Because of its proximity to 
the cathode, a small negative potential on the grid with respect to the 
cathode can counteract the attractive force of the relatively large 
positive potential of the plate and cause current flow across the tube to 
cease. As the grid is made less negative (more positive), more and more 
electrons will be able to pass through the grid and flow to the positive 
plate. Thus, in this manner, the grid-to-cathode voltage controls the 
current from a power supply connected between the cathode and the plate. 
For a given grid voltage, the plate current increases monotonically with 
the plate voltage. Conversely, at a given plate voltage, the plate current 
increases with increasing grid voltage. 
The electrical properties of the vacuum triode described above are called 
the static characteristics because they are obtained from direct current 
(DC) measurements. It is also a characteristic of vacuum triodes that 
there are small capacitances between the grid and cathode, grid and plate, 
and plate and cathode. Those capacitances are not measured by the direct 
currents used to determine the static characteristics. However, since an 
alternating current, I, is proportional to the capacitance times the 
time-derivative of the voltage (I=C dv/dt), the very small capacitances 
between grid and cathode, grid and plate and plate and cathode may draw 
appreciable current when the tube is operated in an alternating current 
(AC) mode. In fact, those interelectrode capacitances are limiting factors 
in the high frequency performance of tubes especially those operated in 
the microwave region (generally 200 megahertz (MHz) to 100 gigahertz 
(GHz)) of the electromagnetic spectrum. The grid-to-plate capacitance, 
although only a few picofarads, is particularly bothersome because it not 
only decreases the possible amplification by capacitive shorting, but it 
also provides a path whereby some of the output signal of the tube is fed 
back into the input signal of the tube possibly resulting in unwanted 
oscillations. 
One way to reduce the grid-to-plate capacitance is to insert a second grid, 
termed a screen grid, in the vacuum tube between the control grid and the 
plate to provide an electrostatic shield between the control grid and the 
plate. This screen grid is operated with a positive potential of the order 
of the plate voltage. One result of the use of a screen grid, an 
advantage, is that variations in the plate voltage have little effect on 
the potential distribution near the cathode and therefore do not affect 
the plate current. Thus, the relative effect of the grid voltage on the 
plate current is increased and thus the voltage amplification capability 
of the tube is enhanced. 
A large percentage of the electrons leaving the cathode, approximately 25 
to 35% in current high power vacuum tubes, are captured by the screen 
grid, and thus never reach the plate, thereby reducing tube efficiency. 
The capture of electrons by the screen grid is primarily due to the 
attractive effect of the positive screen grid for negative electrons, the 
mutually repulsive effect of electrons which tends to repel them into the 
screen grid, and physical misalignment between the control and screen 
grids. 
Previous attempts to carefully align the control and screen grids to 
improve vacuum tube efficiency have proved difficult, costly and generally 
ineffective. Further, with current state-of-the-art wire mesh grids, 
microphonics, or grid vibrations, operate to counteract the potential 
benefits of carefully engineered and aligned grids. The adverse effects of 
grid vibrations increase with increasing frequency at which the vacuum 
tubes operate and thus are particularly detrimental to the performance of 
high frequency power amplifier tubes such as ultra-high frequency (UHF) 
tetrodes. 
The method of this invention provides self-aligned double grids which may 
be used to improve the efficiency and performance characteristics of 
vacuum tubes. Since the self-aligned grids are physically positioned on 
the same substrate or structure in close proximity to each other, smaller 
grid currents will be required for the double grids of the invention 
compared to conventional grid arrangements thereby improving device 
efficiency. Additionally, inefficiencies in prior art tube designs due to 
physical misalignment of the grids are obviated. Further, by physically 
positioning the grids on the same structure, grid vibrations are largely 
eliminated and performance characteristics are improved. 
Briefly, the method of the invention for producing the self-aligned double 
grids comprises the steps of first making a plurality of cut-out regions 
in at least a portion of a substrate having first and second major 
surfaces and an outer peripheral edge area interconnecting the major 
surfaces and defininf the shape of the substrate. The cut-outs are 
arranged in an array or pattern separated by grid members or a lattice 
work of the material of the substrate within the perimeter of the array or 
pattern. Alumina has been found to be a suitable material for the 
substrate and a laser beam has been found to be an effective means for 
making the cut-outs. Second, a first thin planar layer of a conducting 
material is applied to the first major surface of the substrate and a 
second thin planar layer of a conducting material is applied to the second 
major surface of the substrate in a manner such that the layers on the 
opposite major surfaces of the substrate are electrically isolated. This 
step is effectively accomplished by selectively masking the substrate, 
particularly the interior peripheral edge area of the cut-outs. Sputtering 
has been found to be an effective means for applying the layers and 
tungsten, whose coefficient of thermal expansion is nearly equal to that 
of alumina, a suitable material for the conducting layers. 
Similarly, and briefly, the article of the invention comprises a substrate 
having first and second major surfaces and an outer peripheral edge area 
which interconnects the major surfaces and defines the overall shape of 
the substrate. A plurality of cut-outs, arranged in an array or pattern, 
is located in at least a portion of the substrate. Each cut-out has an 
inner or interior peripheral edge area which interconnects a first 
aperture lying in the plane of the first major surface and a second 
aperture lying in the plane of the second surface. A first thin planar 
layer of a conducting material overlies and is contiguous with the first 
major surface and a second thin planar layer of a conducting material 
overlies and is contiguous with the second major surface. The first and 
second layers of conducting material are electrically isolated from each 
other and do not cover the apertures thereby providing substantially 
identical pairs of open areas in register with each other in the two 
layers.

In FIGS. 1 and 1A there are shown two examples of substrates suitable for 
use in the practice of the invention. Planar substrate 10 of FIG. 1 
typically has first 12 and second 14 major surfaces with outer peripheral 
edge area 16 interconnecting major surfaces 12 and 14. The shape of area 
16 will be dictated by the design of the vacuum tube device (not shown) 
with which the completed self-aligned double grids of the invention will 
be utilized and circles, squares and rectangles are, for example, within 
the contemplation of the invention. Hollow cylindrical substrate 20 of 
FIG. 1A has first or inner 22 and second or outer 24 major cylindrical 
surfaces with outer peripheral edge areas 26 interconnecting cylindrical 
surfaces 22 and 24. The material of the substrate must be capable of 
withstanding thermal cycling between ambient temperature and the operating 
temperature of the vacuum tube device and, as will be described below in 
more detail, must be compatible with the thin planar layer of conducting 
material to be applied to the major surfaces. Alumina has been found to be 
a particularly suitable material. 
An array or pattern of cut-outs is next fabricated in at least a portion of 
the substrate. A laser beam has been found to be an acceptable means for 
fabricating the cut-outs, especially in alumina. In FIG. 2, laser beam 30, 
which has fully penetrated the thickness of substrate 10, is shown in the 
process of making cut-outs 32 by being scanned about the periphery of a 
cut-out to be formed. The heat from laser beam 30 effectively vaporizes 
the material of the substrate about the periphery forming kerf 34 
permitting the material inside kerf 34, termed "punchouts" 36, to fall 
away from substrate 10. If the laser beam has insufficient power in the 
scanning mode to continuously remove the substrate material to form kerf 
34, the laser may be used to form kerf 34 by "drilling" a series of 
overlapping holes about the periphery of the cut-outs to be formed thereby 
freeing punchouts 36. 
Generally, punchouts 36 inside the laser-scanned or laser-drilled periphery 
or kerf 34 will fall away spontaneously from substrate 10 leaving cut-outs 
34 separated by a lattice-work 38 of the material of substrate 10. In the 
event that punchouts 36 do not readily separate from substrate 10, 
substrate 10 may be immersed in an etchant bath (not shown), preferably 
with ultrasonic agitation, in which the material of substrate 10 is 
soluble. The etchant will dissolve any remainining "bridges" between 
punchouts 36 and lattice work 38. Use of the etchant is also advantageous 
in that it tends to smooth away any rough edges along interior peripheral 
edge areas 40 of cut-outs 32. Interior peripheral edge areas 40 
interconnect first 42 and second 44 apertures of cut-outs 32 and lie in 
the planes of first 12 and second 14 major surface of substrate 10, 
respectively. 
An example of a completed array within a portion of substrate 10 is shown 
in FIG. 3. The array of FIG. 3 is a square array, i.e., the spacing 
between orthogonal center-lines 46 and 48, which pass through the 
geometric centers of square cut-outs 32, is the same. The invention, 
however, is not limited to square cut-outs nor square arrays thus allowing 
great flexibility in the design and arrangement of the cut-outs. For 
example, it is within the contemplation of the invention that cut-outs 32 
be circular and that the array be formed by arranging the circular 
cut-outs about the peripheries of a series of concentric circles. The term 
array refers to an arrangement of cut-outs having a regular or repeating 
periodicity. Cut-outs 32 need not be formatted in the rigorous arrangment 
termed an array as a less structured arrangement, a pattern, may be 
desirable. 
The next step is to apply a thin layer of a conducting material to major 
surfaces 12 and 14 of substrate 10. In preparation for this step, interior 
peripheral edge areas 40 and outer peripheral edge area 16 are masked. 
This step is performed to ensure that the layer of conducting material on 
surface 12 is electrically isolated from the layer of conducting material 
on surface 16. 
One masking method that has been found effective is to immerse substrate 10 
into a bath (not shown) of a liquid solution of 15% butyl acetate and 85% 
acetone (by weight). After withdrawal of substrate 10 from the bath, and 
evaporation of the acetone, a continuous solid film 50 is left on surfaces 
12, 14, and 16 and in the interstices of cut-outs 32 as is shown 
schematically in FIG. 4 for the substrate of FIG. 3. 
Shrinkage of butyl acetate film 50 upon drying enabled film 50 to be peeled 
off of major surfaces 12 and 14 from edge area 16 up to the beginning of 
the array structure since the cut-outs at the outer perimeter of the array 
acted as lines of perforations. The remainder of film 50 within the 
perimeter of the array was removed by gentle rubbing with 600 grit silicon 
carbide grinding paper. A desirable feature of butyl acetate film 50 is 
that it pulverizes and breaks into minute pieces rather than smearing as 
would wax or other such compounds. Substrate 10 now has surfaces 12 and 14 
exposed, clean and free from foreign matter while the interstices of 
cut-outs 32, including inner peripheral edge areas 40, and outer 
peripheral edge area 16 are covered and blocked by butyl acetate film 50 
as shown in FIG. 4A. 
A thin planar film of a conducting material is next deposited over surfaces 
12 and 14 of substrate 10. The material of the thin film must be well 
bonded to surfaces 12 and 14 so that mechanical vibrations and thermal 
cycling do not cause delamination of the film from the underlying 
substrate. For alumina with a thermal expansion coefficient of 
6.times.10.sup.-6 .degree.C..sup.-1 tungsten, with a thermal expansion 
coefficient of 4.5.times.10.sup.-6 .degree.C..sup.-1 provides an almost 
perfect thermal expansion match. Tungsten is also attractive since it 
forms a strong bond with alumina. 
Of the various means of depositing tungsten on the alumina substrate, 
sputter deposition is preferred since sputtering results in the best 
bonding between the deposited thin conducting film and the underlying 
substrate. 
The deposited thin films of conducting material typically carry currents in 
the microampere range thus the film thickness is dictated by a need to 
compensate for any erosion over the life of the tube and by a need to 
minimize thickness to minimize mechanical constraint between the film and 
the substrate. Thus, film thickness in the range of from about 0.1 
micrometer to about 25 micrometers is preferred. 
It is preferable to sputter in a series of runs of short duration instead 
of one long continuous run to avoid overheating the substrate thereby 
destroying the butyl acetate film. There is no requirement that the same 
material be deposited on both surfaces 12 and 14. It is also not necessary 
that the thin layers of conducting material be deposited over 100% of 
surfaces 12 and 14. It is only necessary that the conducting material be 
applied within and slightly beyond the perimeter of the array, thus an 
effective means of masking outer peripheral edge area 16 is to place a 
mask having a suitably sized opening to expose the array onto surfaces 12 
and 14 prior to sputtering. 
Substrate 10 with the deposited thin planar conducting layers is next 
soaked in an acetone bath, preferably with ultrasonic agitation, to remove 
both the butyl acetate and overlying tungsten deposit from the interstices 
of the alumina grid structure to complete the fabrication of the 
self-aligned double grid composite structure 60 of the invention shown in 
FIGS. 5 and 6. 
Self-aligned double grid 60 is shown in FIG. 6 schematically in use in 
vacuum tube device 100 which is illustrated in the form of a typical high 
power high frequency microwave tube. Connection 72 is made to thin planar 
conducting layer 62 on surface 12 of substrate 10 facing cathode 74 and 
connection 76 is made to thin planar conducting layer 64 on surface 14 of 
substrate 10 facing anode 78 through vacuum-sustaining enclosure 70. Thus 
layers 62 and 64 
With this construction the periphery of each open area in grid 62 is 
permanently aligned (i.e., in register) with the periphery of an open area 
of substantially identical size and configuration in grid 64 and at the 
same time the grids are electrically isolated from each other. 
In operation, cathode 74 is heated by heater 80 to a temperature sufficient 
to cause thermionic emission of beams of electrons 82 which pass through 
cut-outs 32 in self-aligned double grid 60. In passing through double grid 
60 the electrons in beams 82 are acted upon by control and screen grids 62 
and 64, respectively. Since grids 62 and 64 are aligned, approximately 95% 
of the electrons generated at cathode 74 should reach anode 78. In 
contrast, in the prior art version of tube 100 of FIG. 6A, having separate 
wire mesh control and screen grids 92 and 94, respectively, only about 65% 
of the electrons generated at cathode 74 reach anode 78 due to physical 
misalignment of grids 92 and 94 and the adverse effects of the attractive 
force of positive screen grid 94 for negative electrons and the mutually 
repulsive forces between electrons in beams 82. 
Alternatively, it has been found that the thin layers of conducting 
material may first be applied to major surfaces 12 and 14 by sputter 
deposition to form an assembly and that the array of cut-outs 32 may 
subsequently be formed by laser-scanning or laser drilling through such 
assembly. An advantage of this embodiment of the invention is that it is 
not necessary to conduct the additional steps required to mask interior 
peripheral edge areas 40. Disadvantages, however, are that the conducting 
material tends to reflect the laser beam thus reducing the efficiency of 
the laser machining process and that vaporized conducting material which 
may be redeposited on interior peripheral edge areas 40 may cause short 
circuits between the layers of conducting material on opposite major 
surfaces. Immersion in an etchant bath, which is used to dissolve any 
remaining "bridges" between punchouts 36 and lattice work 38, is also 
helpful with this embodiment in removing stray areas or bits of 
redeposited conducting material. 
In another embodiment useful in such linear beam or "guided-grid" type 
tubes as cathode-ray tubes, at least one cut-out may be laser-drilled 
axially through a solid cylindrical piece of substrate material. This 
would yield the hollow cylindrical substrate of FIG. 1A with which 
cut-outs 26 would normally be formed radially between major cylindrical 
surfaces 12 and 14. Thereafter, thin metallic layers of a conducting 
material would be applied to opposite peripheral edge areas 26. Thus, for 
example, self-aligned pairs of electrodes such as the control electrode 
and screen grid (or accelerator) or the focusing electrode and 
accelerating electrode of a cathoderay tube may advantageously be made on 
single insulating substrates. 
The following Examples are provided by means of illustration, and not by 
way of limitation, to further instruct those skilled in the art of the 
practice of the invention. 
EXAMPLE I 
A 15 by 15 array of square cut-outs 0.020 inch by 0.020 inch separated by 
0.003 inch wide grid members was formed in an 0.8 inch diameter by 0.008 
inch thick alumina wafer by laser drilling a series of overlapping holes 
through the wafer around the perimeters of the square cut-outs with a Nd 
YAG laser (Control Laser Model 512) having 6 watts average power when 
operated in the Q-switched mode at a 3 kHz repetition rate with 19 amps 
lamp current and an 80 mil aperture. 
All of the squares did not fall out of the alumina wafer because of a few 
remaining "bridges" between the grid members and the material 
("punchouts") inside the peripheries of the square cut-outs. These 
punchouts were removed by placing the wafer in a boiling bath of 50% KOH 
for 15 minutes and subjecting the bath to ultrasonic agitation. This 
combination of a slight etch plus the mechanical impulses of the 
ultrasonic waves removed all of the punchouts leaving the desired square 
array of 20 mil square cut-outs separated by 3 mil wide grid members 
between adjacent cut-outs as shown in FIG. 7 at 4.times. and in FIG. 7A at 
10.times.. 
The wafer was next immersed in a liquid solution of 15% butyl acetate and 
85% acetone (by weight), removed from the solution, and allowed to dry. 
Upon subsequent evaporation of the acetone, a solid continuous film of 
butyl acetate was left behind on the surfaces of the wafer and in the 
interstices of the square cut-outs. The butyl acetate film was peeled away 
from the planar surfaces of the wafer beyond the perimeter of the array 
leaving behind an integral coherent film on the 0.003" wide alumina grid 
members. The grid member area of the alumina wafer was gently rubbed with 
600 grit silicon carbide grinding paper to remove the butyl acetate film 
from the grid members. 
The prepared wafer was placed in a sputtering chamber and a 0.1 micron 
thick layer of tungsten was sputtered onto the major surfaces of the 
alumina wafer in a series of five 30-second runs. Prior to commencing the 
sputtering, the area of the wafer beyond the perimeter of the array was 
masked with a metal washer to prevent tungsten from depositing on the 
outer peripheral edge area of the wafer. A series of runs was used instead 
of one long continuous run to avoid overheating the wafer and destroying 
the butyl acetate film. 
The wafer with the deposited tungsten was soaked in acetone for a minute 
and then agitated by ultrasonic waves for one second to remove the butyl 
acetate and overlying tungsten deposit from the interstices of the 
cut-outs. The resulting self-aligned double grid is shown in FIG. 8. The 
resistance between the metal grids on the opposite major surfaces was 
found to exceed 10,000 ohms. 
EXAMPLE II 
The self-aligned double grid of FIG. 9 was produced by first sputter 
depositing a 0.1 micron thick layer of tungsten on both sides of a 0.8 
inch diameter by 0.008 inch thick alumina wafer. The array of 0.020 inch 
by 0.020 inch square cut outs separated by 0.003 inch wide grid members 
shown in FIG. 9 was then laser-drilled using the laser and laser drilling 
parameters of Example I. This grid is particularly suitable for use in 
high frequency, high power vacuum tubes used in such applications as 
microwave transmission and radar. 
While the invention has been particularly shown and described with 
reference to several preferred embodiments thereof, it will be understood 
by those skilled in the art that various changes in form and detail may be 
made therein without departing from the true spirit and scope of the 
invention as defined by the appended claims.