Beam window devices and methods of making same

A beam tube having a hole or "window" covered by a thin, beam permeable membrane is provided with a polymeric ring for minimizing stress concentration in the membrane adjacent the periphery of the hole to relieve stress concentrations that would otherwise occur when the membrane is forced inwardly into the hole by atmospheric or other pressure on the exterior of the tube. The exterior surface of the housing for the beam tube contains means for retaining the polymeric ring in a predetermined location on the exterior surface. The retaining means which retain the polymeric material away from the hole may comprise an annular channel formed in the exterior surface of the front wall, an annular raised inner ridge formed on the exterior surface, an annular raised outer ridge also formed on the exterior surface to define an annular track between the outer and inner ridges, or a ring of flow preventing material on the exterior surface of the housing. In a manufacturing method, a liquid polymer precursor is placed on the exterior surface of the front wall and retained by the retaining means when the membrane, precursor and housing are heated to an elevated bonding temperature so as to cure the polymer precursor to form the polymeric ring retained by the retaining means away from the hole during the bonding step.

FIELD OF THE INVENTION 
The present invention relates to beam tubes having a housing with an 
aperture or "window" in the wall of the housing so that the beam may pass 
out of the housing, and to methods of making such tubes and components 
thereof. 
BACKGROUND OF THE INVENTION 
Beam apparatus typically includes a hollow housing and a "gun" or source of 
energetic electrons or electromagnetic radiation such as X-rays mounted 
within the housing. The interior or the housing is maintained under vacuum 
so as to facilitate generation and direction of the beam. In many types of 
electron beam devices, for example, the electron beam does not pass out of 
the housing. For example, in a common cathode ray tube, the beam acts only 
on a phosphor inside the housing to produce visible light, which in turn 
is transmitted through a transparent wall of the housing. Likewise, in 
certain electron beam material treatment apparatus, such as electron beam 
welding devices and the like, the workpiece to be treated by the electron 
beam is placed into the housing and the housing is then evacuated before 
operation. 
Other types of beam apparatus require that the beam pass out of the tube 
housing. The electron tube housing is provided with an aperture or 
"window" for passage of the electron beam, so that the beam can be 
directed on a workpiece positioned outside of the housing. For example, in 
electron beam sterilization and chemical curing processes, an item to be 
treated is positioned outside of the housing in front of the window and 
treated by the beam. In electron beam printing processes, a document to be 
printed is positioned outside of the housing, in front of the window, and 
treated with an electron beam so as to apply an electrical charge on the 
document in a manner corresponding to the pattern of the desired printing. 
My own U.S. Pat. No. 5,093,602 and PCT International Publication No. 
WO/91/07772 disclose methods and devices in which an electron beam is used 
to promote dispersion of a fluent material such as a liquid, slurry or 
gas-borne powder. In this arrangement, the electron beam is generated 
inside an evacuated housing and passes out of the housing through an 
opening or window so that the beam impinges upon the fluent material. 
The opening or window in the electron tube housing must be covered with a 
membrane which permits passage of the electron beam to the outside of the 
housing, but which blocks passage of air or other fluids into the housing 
so as to preserve the vacuum within the housing. For example, in fluid 
dispension methods according to the aforementioned patent and application, 
the fluid typically is at atmospheric or superatmospheric pressure. Thus, 
the membrane must allow passage of the electron beam out of the tube and 
into the fluid while isolating the interior of the housing from the fluid. 
Membranes utilized for passage of an X-ray or electron beam must meet 
numerous conflicting requirements. The "beam-permeable" membrane must have 
relatively low absorption for the X-rays or electrons so that the beam 
passes through the membrane with little attenuation. As used herein, the 
term "beam permeable" refers to the allowance of electrons or 
electromagnetic radiation such as X-rays to pass through a given object, 
and here in particular, a membrane. This is significant both with respect 
to the power remaining in the beam and with respect to possible effects of 
the beam on the membrane itself. Thus, in electron beam applications where 
the membrane absorbs a substantial fraction of the electrons in the beam, 
the energy imparted by the electrons may heat the membrane to an 
unacceptable degree or otherwise destroy the membrane. The requirement for 
low absorption leads to a strong preference for very thin membranes formed 
from materials having inherently low electron absorptivity, typically 
materials formed from elements having low atomic number. The membrane must 
provide an effective barrier against entry of atmospheric or other 
materials into the interior of the tube housing. It should be 
substantially impermeable to common gases and liquids, and must have 
sufficient physical strength to resist differential pressure encountered 
in service. As the interior of the housing is maintained substantially 
under vacuum, the differential pressure applied to the membrane is 
substantially equal to the absolute pressure prevailing on the outside of 
the housing in the vicinity of the membrane. Where the exterior surface of 
the membrane is exposed to a fluent material under superatmospheric 
pressure, high differential pressures are encountered. The differential 
pressure causes considerable stress in the membrane. Moreover, the fluid 
pressure may fluctuate, and hence the stress applied to the membrane may 
be a fluctuating stress. These factors require that the membrane have 
considerable mechanical strength. 
Further, the tube housing may be subjected to substantial temperature 
changes during manufacture and service. It is normally necessary to 
subject an electron tube to a so-called "bakeout" treatment at elevated 
temperature during manufacture. Typically, the bakeout procedure is 
conducted after the electronic components of the tube such as electrodes, 
coils and the like have been mounted inside of the housing but before the 
housing has been fully sealed. The elevated temperature drives off 
volatile materials from the inside of the housing and from the electronic 
components. The heating and cooling which occurs during the bakeout 
process can induce significant thermal expansion and contraction of the 
membrane and housing, leading to still further stresses. The magnitude of 
such stresses is directly proportional to the difference between the 
coefficients of thermal expansion of the membrane material and the 
coefficient of thermal expansion of the adjacent housing material. 
All of these factors taken together present a significant technical 
challenge. Moreover, in many applications the cost of the electron tube 
structure is of significance. 
Considerable effort has been devoted in the art to the search for an 
electron tube structure and methods of making electron tubes which satisfy 
the foregoing considerations. Neukermans, U.S. Pat. No. 4,468,282 
discloses an electron beam window structure and methods of making the same 
in which a window material such as boron carbide (B.sub.4 C) or other 
similar material is deposited on a substrate by chemical vapor deposition. 
The substrate is then etched to form a hole in alignment with the 
deposited window material. The substrate forms a wall of the electron tube 
housing, and the etched hole constitutes the window opening. VanRalte et 
al, U.S. Pat. No. 3,788,892 forms an opening in the wall of the housing 
and covers that opening with a temporary support film. The window material 
is then deposited in a relatively thin layer over temporary support film. 
The deposited window material extends beyond the periphery of the 
temporary support film, so that the deposited window material bonds with 
the housing wall. After deposition of the window material, the temporary 
support film is removed, dissolving the same. Another reference directed 
to fabrication of electron beam permeable membranes is Japanese Laid-Open 
Patent Publication 2-138900. U.S. Pat. Nos. 3,531,340; 5,030,318; and 
4,228,815 describe fabrication of thin, membrane-like structures for other 
purposes. 
Various attempts have been made to select structural configurations for the 
windows opening, of the housing and associated components so as to 
maximize the pressure resistance of the window. Much of this work has been 
directed to optimization of large area electron beam window structures, 
having a window area (measured in the plane of the membrane) on the order 
of 1 cm.sup.2 or more, and typically 100 cm.sup.2 or more. These 
large-window structures typically incorporate a supporting framework with 
multiple apertures and a unitary membrane extending across the various 
apertures. Structures of this type are described, for example, in U.S. 
Pat. Nos. 4,721,967; 4,333,036 and 4,591,756. A further electron beam 
window structure is shown in U.S. Pat. No. 3,105,916. 
Despite all of this effort in the art heretofore, there have been 
substantial, unmet needs heretofore for improved electron tube structures 
equipped with electron permeable membranes; for improved methods of making 
such structures; and for improved components for use in fabricating such 
structures. 
The invention of the '942 application and the present invention address 
these needs. 
THE '942 INVENTION 
My copending application, U.S. Ser. No. 08/045,942, (the "942 application") 
filed Apr. 12, 1993, now U.S. Pat. No. 5,391,958, discloses a method of 
making an electron beam tube preferably including the steps of placing a 
closure unit including an electron-permeable portion and a polymeric 
material on a surface of a wall of a hollow housing. The placing step is 
conducted so that the electron-permeable portion overlies a hole in the 
wall of the housing and so that the polymeric material is in contact with 
the wall of the housing. The method further includes the step of bonding 
the closure unit and housing to one another to thereby form an assembly so 
that the closure unit seals the hole and so that the closure unit is 
connected to the housing through the polymeric material. Additionally, the 
method includes the steps of baking the assembly at an elevated bakeout 
temperature while evacuating the interior of the housing and cooling the 
so-baked assembly. 
The closure unit may include an electron-permeable membrane and a ring of 
polymeric material formed separately from the membrane. The step of 
placing the closure unit may include the step of placing the membrane and 
the ring so that the membrane overlies the hole in said wall and so that 
the ring surrounds the hole and lies between the membrane and the wall. 
Preferably, the polymeric material has a glass transition temperature and 
the bakeout temperature is above the glass transition temperature of the 
polymeric material. The polymeric ring desirably includes or consists 
essentially of a polymer having appreciable strength at temperatures above 
its glass transition temperature and up to the bakeout temperature. Most 
preferably, the polymeric material consists essentially of polyimide 
having a glass transition temperature less than about 250.degree. C. and 
the bakeout temperature is above about 300.degree. C. 
The polymeric ring serves to hold the membrane in place during the bakeout 
step, and serves as a permanent part of the assembly after the process is 
complete. However, the polymeric ring also serves to absorb any 
differences in thermal expansion during cooling after bakeout. At least 
part of the cooling involves cooling over a range above the glass 
transition temperature of the polymer ring. While the polymer is above its 
glass transition temperature, it is relatively soft and pliable and hence 
can accommodate some movement of the membrane relative to the housing 
wall, so as to compensate for differential thermal expansion of the 
membrane and wall materials. Thus, cooling from the bakeout temperature to 
the glass transition temperature of the polymeric ring does not induce 
appreciable stress in the membrane. The glass transition temperature is 
relatively close to room temperature, typically less than about 
250.degree. C., and therefore cooling from the glass transition 
temperature to room temperature entails only limited amounts of 
differential thermal expansion. Moreover, even below the glass transition 
temperature, the polymeric ring can deflect to some extent and hence can 
mitigate stresses induced by differential thermal expansion at least to 
some degree. 
Although the polymeric ring can deflect at temperatures above its glass 
transition temperature, it still maintains appreciable structural 
strength, sufficient to keep the membrane in position during the bakeout 
step. Particularly when the preferred polymeric materials are employed, 
the structural strength of the polymeric ring is sufficient to permit 
application of differential pressure across the membrane during the 
bakeout cycle. Thus, the exterior of the tube housing may be exposed to 
normal atmospheric pressure whereas the interior of the tube housing is 
connected to a vacuum pump or other suction device through a temporary 
connection port in the housing. The temporary connection port is closed at 
the end of the bakeout cycle. This in turn permits rapid, economical 
handling of the assemblies during the bakeout cycle in mass production 
operations. 
A housing component for a an electron beam tube is also provided in the 
'942 application. A component according to this aspect includes a front 
wall. The front wall has an exterior surface, an interior surface, and a 
hole extending from the exterior surface through the front wall to the 
interior surface. An electron permeable membrane overlies the hole. A ring 
of a polymeric material encircles the hole. The ring is interposed between 
the membrane and the exterior surface of the front wall. The membrane is 
bonded to the ring whereas the ring is bonded to the front wall so that 
the membrane is bonded to the front wall at least partially through the 
ring. In use, the front wall will bound the interior space of the housing 
and the hole in the front wall will constitute the window for passage of 
the electron beam. The component may also include the outer walls of the 
housing. Components according to this aspect can be used in methods as 
discussed above. 
The membrane preferably consists essentially a material selected from the 
group consisting of carbides, nitrides hydrides and oxides of metals 
selected from the group consisting of silicon, aluminum, and boron, and 
combinations of these materials. Boron nitride, boron hydride and 
combinations thereof form one particularly useful set of materials for use 
in the membrane, boron nitride hydride being most preferred. The front 
wall may be formed from essentially any material having requisite 
structural strength and impermeability. It is not necessary to match the 
coefficient of thermal expansion of the membrane precisely. Thus, 
inexpensive, easy-to-work materials such as metals may be used to good 
advantage. 
A further aspect of the '942 invention provides a component for an electron 
beam tube which also has a front wall. Here again, the front wall has 
exterior and interior surfaces and a hole extending from the exterior 
surface the front wall to the interior surface. An electron permeable 
membrane is secured to the exterior surface of the front wall so that the 
membrane overlies the hole, either by means of the polymeric ring 
structure discussed above or by other means. Here again, the front wall 
will serve as the front wall of the electron tube housing. Accordingly, 
when the component is used in an electron tube and the tube is subjected 
to differential pressure conditions in which the ambient pressure on the 
exterior surface exceeds the low subatmospheric pressure within the 
housing, the ambient pressure will urge the membrane inwardly, against the 
exterior surface and into the hole. The ambient pressure will apply stress 
to the membrane. In a component according to this aspect of the invention, 
stress relief means are provided for mitigating stress concentration in 
the membrane adjacent the periphery of the hole. 
This aspect of the '942 invention incorporates the realization that when 
ambient pressure tends to urge the membrane into the hole to the 
interaction between the membrane and the wall at the periphery of the hole 
tends to create a substantial stress concentration in the membrane, and 
the related realization that the strength and service life of the membrane 
may be substantially enhanced by mitigating this stress concentration. 
The hole may have a peripheral surface extending generally parallel to the 
axis of the hole and generally transverse to the exterior surface of the 
front wall. The stress relief means may include a juncture surface merging 
with the peripheral surface and with the exterior surface of the front 
wall, the juncture surface flaring outwardly away from the axis of the 
whole so as to provide a gradual transition between the exterior surface 
of the front wall and the peripheral surface of the hole. Thus, the 
juncture surface may define a radius between the exterior surface and the 
peripheral surface. 
The stress relief means is particularly beneficial where the electron 
permeable membrane is a material housing relatively high elastic modulus, 
such as the carbides, nitrides and hydrides discussed above. Also, 
although the features discussed above may be utilized with components 
having holes of different sizes, they are particularly valuable where the 
hole is less than about 5 mm, and especially less than about 1 mm in 
diameter, and where the membrane is less than about 3 micrometers thick. 
Although the invention not limited by any theory of operation, it is 
believed that relief of the stress concentration at the periphery of the 
hole is particularly important for components of this configuration. 
Yet another aspect of the '942 invention includes electron tubes 
incorporating components according to aspects of the invention discussed 
above. 
SUMMARY OF THE PRESENT INVENTION 
The present invention provides a method of making a beam tube whereby a 
closure unit, including a beam-permeable membrane and a ring of polymeric 
material formed separately from the membrane, is placed on an exterior 
surface of a front wall of a hollow housing defiing an interior therein so 
that the beam-permeable membrane overlies a hole formed in the hollow 
housing and so the polymeric ring is between the membrane and front wall. 
The front wall includes means for retaining the ring of polymeric material 
in a predetermined location on the exterior surface of the front wall of 
the housing and away from the hole during the bonding step. The bonding 
step includes bonding the closure unit and the housing to one another to 
thereby form an assembly so that the closure unit is connected to the 
housing through the polymeric material with the retaining means retaining 
the polymeric material away from the hole during the bonding step. The 
means for retaining can comprise an annular channel formed in the exterior 
surface of the front wall of the housing with the ring of polymeric 
material being at least partially retained in the annular channel. The 
step of placing the polymeric ring can further include the step of placing 
a liquid polymer precursor in the annular channel so that the precursor 
lies between the exterior surface and the membrane, with the bonding step 
including the step of heating the membrane, precursor and housing to an 
elevated bonding temperature to cure the polymer precursor and form the 
polymeric ring disposed at least partially in the annular channel while 
bonding the ring to the membrane and exterior surface. Preferably, the 
annular channel has a maximum depth beneath the exterior surface of the 
front wall of 100 microns or less, and preferably about 50 microns. 
In accordance with another aspect of the invention, the means for retaining 
can comprise an annular raised inner ridge formed on the exterior surface 
of the front wall of the housing so as to encircle the hole and be 
encircled by the polymeric ring to retain the polymeric ring in a 
predetermined position with respect to the hole. In this manner, a liquid 
polymer precursor can be placed on the exterior surface of the front wall 
so as to encircle and surround the inner raised ridge such that in the 
bonding step, the membrane, precursor and housing are heated to an 
elevated bonding temperature to cure the polymeric precursor and form the 
polymeric ring while bonding the ring to the membrane and the front wall. 
Preferably, the raised inner ridge has a maximum height above the exterior 
surface of the front wall of 100 microns or less, and more preferably, 
about 50 microns. An additional raised outer ridge can also be formed on 
the exterior surface of the front wall so as to encircle the inner ridge 
and define an annular track therebetween on the exterior surface to retain 
the polymeric ring. Thus, the liquid polymer precursor can then be placed 
in the annular track for bonding as described above. 
A further aspect of the invention provides that the means for containing 
comprises a flow preventing ring disposed on the exterior surface of the 
front wall of the housing and between the polymeric ring and the hole. In 
this aspect, liquid polymer precursor may be placed on the exterior 
surface of the front wall of the housing so as to encircle and surround 
the flow preventing ring, with the bonding step including the step of 
heating the membrane, precursor and housing so as to cure the polymeric 
precursor and form the polymeric ring while bonding the ring to the 
membrane and the front wall. Preferably, the flow preventing ring is 
formed by coating a portion of the exterior surface of the front wall with 
a diamond layer having a preferred thickness of about 5 microns or less. 
The present invention also provides a forward housing for a beam tube. 
Although the present invention has been described herein as relating to 
electron beam tubes, the beam tube can also be provided for use with 
electromagnetic radiation emitting sources, such as X-rays. The beam tube 
is provided with a front wall with an exterior surface, an interior 
surface and a hole extending inwardly from the exterior surface through 
the front wall to the interior surface. The front wall further includes 
means for retaining the ring of polymeric material in a predetermined 
location on the exterior surface and away from the hole during bonding. As 
described above, the means for retaining may include an annular channel; 
an annular raised inner ridge alone or in combination with an annular 
raised outer ridge to form an annular track therebetween; or a ring of 
flow preventing material or several of these structures. 
The housing may further include a beam-permeable membrane overlying the 
exterior surface and covering the hole, and a ring of polymeric material 
interposed between the membrane and the exterior surface of the front wall 
.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
A tube in accordance with one embodiment disclosed in the '942 application 
incorporates a hollow housing 10. Housing 10 incorporates a rear envelope 
portion 12 formed from a dielectric material, preferably a glass such as a 
borosilicate, soda lime or lead oxide glass of the type commonly used for 
fabrication of electron tube envelopes and for so-called "lamp working" 
processes in the glass industry. Rear portion 12 is generally in the form 
of a cylindrical tube. A hollow tubular temporary port portion 13 projects 
from the cylindrical rear portion. 
The housing 10 further includes a front or forward portion 14 formed from a 
metal. Forward portion 14 and rear portion 12 are sealingly bonded 
together as schematically indicated at 16 so that the forward and rear 
portion cooperatively enclose an interior space 18. Forward portion 14 
defines a front wall 20. 
Conventional electron beam generating, accelerating and focussing 
components schematically indicated at 22 are disposed inside housing 10. 
These components are electrically connected to leads 24. Leads 24 extend 
out of housing 10 through the wall of rear portion 12 at the read end of 
the housing, remote from front wall 20. The leads are provided with 
glass-to-metal seals of the type commonly employed in vacuum tube 
technology. 
Front wall 20 has an interior surface 26 facing towards the interior 18 of 
the housing and an oppositely facing, exterior surface 28 facing away from 
the housing. A hole 30 extends through front wall 20, from exterior 
surface 28 to interior surface 26. As best seen in FIG. 2, hole 30 has an 
axis 32 and a peripheral surface 34 which, in this instance, is generally 
in the form of a surface of revolution about axis 32. As the peripheral 
surface extends from the interior side of the wall 20, adjacent interior 
surface 26 towards exterior surface 28, the peripheral surface extends 
generally in the direction of axis 32 and generally in a direction 
transverse to the plane of exterior surface 28. Front wall 20 further 
defines a juncture surface 36 merging with peripheral surface 34 and with 
front surface 28. Juncture surface 36 is a surface of revolution about 
hole axis 32. The generator of juncture surface 36, is itself a curve with 
a radius of curvature r.sub.1. As further discussed hereinbelow, the 
generator may have either a constant or varying radius of curvature. 
Juncture surface 36 flares outwardly, away from hole 30 and away from axis 
32 so that juncture surface 36 provides a smooth, gradual transition 
between front wall surface 28 and peripheral surface 34. Not only does 
this smooth, gradual transition provide mechanical stress relief as herein 
described, but it also provides electromagnetic stress relief by avoiding 
the presence of sharp edges or corners which would otherwise allow 
development an unwanted electrostatic charge concentration. In the 
particular arrangement illustrated, wall 20 also defines an interior 
transition surface 38 flaring outwardly, away from hole 30 and away from 
axis 32 at the juncture between peripheral surface 34 and interior surface 
26. 
Hole 30 does not have a uniform diameter throughout its entire extent, but 
instead has a minimum diameter at a point along axis 32 about midway 
between surfaces 26 and 28. As used in this disclosure with reference to a 
hole or aperture of non-uniform diameter, the term "minimum transverse 
dimension" should be taken as referring to the diameter of the largest 
rigid sphere which could pass unimpeded through every portion of the hole. 
Where the peripheral wall bounding the hole is substantially in the form 
of a surface of revolution about an axis, such as with hole 30, the 
minimum transverse dimension is simply the minimum diameter of such 
surface of revolution at any point along its axis. The desired minimum 
transverse dimension or diameter will depend, to some extent, on the 
application in which the electron group is to be utilized. For many 
applications, transverse dimensions less than about 10 mm, particularly 
less than about 5 mm, and most preferably about 1 mm can be employed. Such 
dimensions can be employed, for example, in electron tubes for many fluid 
atomization processes according to my aforementioned U.S. Patent and 
International Publication. 
Hole 30 may be formed in front wall 20 by conventional machining processes 
or, more preferably, by etching. For example, a conventional etching 
process in which the front wall is masked and then exposed to an etching 
solution can be employed. Combinations of such processes can also be used. 
For example, the hole can be formed by a machining process such as 
drilling, laser ablation, or the like, and the flaring juncture surface 36 
and interior transition surface 38 may be formed by exposing the front 
wall, with the formed hole to an etchant so that the etchant dissolves 
material from the front wall. Similarly, these surfaces can be formed by 
electro-polishing, i.e., by reverse electroplating in which the front wall 
serves as the cathode and metal is removed. Other conventional 
metalworking processes can also be used to form the hole and the juncture 
surface. 
A polymeric ring 40 overlies the exterior surface 28 of front wall 20 and 
encircles hole 30 and juncture surface 36. As illustrated, ring 40 is a 
thin, generally sheetlike annulus having a thickness, measured in the 
direction transverse to exterior surface 28, many times less than the 
dimensions of the ring in directions parallel to surface 28. The thickness 
t of ring 40 preferably is between about 0.1 .mu. and about 10 .mu., more 
desirably between about 0.1 .mu. and 3.0 .mu., and most desirably between 
about 0.5 .mu. and 1.0 .mu.. The interior dimensions of ring 40 desirably 
are just slightly larger than the dimensions of juncture surface 36. That 
is, the innermost edge of ring 40 should lie just outboard of the location 
where the juncture surface 36 merges into exterior surface 28. The width W 
of ring 40, i.e., the distance between the interior edge of the ring 
adjacent to hole 30 and the exterior edge of the ring remote from the 
hole, measured parallel to exterior surface 28 of wall 20 desirably is at 
least about 0.05 mm, and more preferably between about 0.2 and about 4 mm. 
Where hole 30 and juncture surface 36 are in the form of surfaces of 
revolution about axis 32, ring 40 can be a circular annulus having an 
interior diameter di and exterior diameter do. For example, di may be 
about 1.0 mm to about 3.0 mm, whereas do may be about 5.0 mm to about 7.0 
mm. 
Ring 40 is formed from a polymeric material. As further discussed 
hereinbelow, the material of ring 40 should be capable of bonding to the 
material of front wall 20, and also to the electron-permeable membrane 
incorporated in the apparatus. Further, the material of ring 40 desirably 
has a glass transition temperature below the bakeout temperature to be 
used in forming the election tube. The glass transition temperature 
desirably is as low as possible but above the maximum temperature which 
the ring will reach during storage and/or service of the tube after 
manufacture. The material of ring 40 should have substantial strength 
above its glass transition temperature to withstand the process discussed 
hereinbelow and when heated during bakeout will tend not to form a liquid 
but will rather have a gummy consistency. Ring 40 may include, or consist 
essentially of, polymers selected from the group consisting of polyimides 
and epoxy. 
Polyimides are particularly preferred. An especially preferred polyimides 
is that sold under the designation EL-5010 by National Starch, Inc. of 10 
Finderne Ave., Bridgewater, N.J. The EL-5010 material has a glass 
transition temperature of about 230.degree. C. Ring 40 may be formed by 
die-cutting from a preformed sheet of the desired polymeric material. 
Alternatively, ring 40 may be formed in situ by depositing the polymeric 
materials from solution or suspension, by polymerization in situ or by 
conventional plastics processing techniques such as powder coating or spin 
coating, as further described below. 
A thin, electron-permeable membrane 44 overlies ring 40 and the exterior 
surface 28 of wall 20. Membrane 44 covers wall 30 and juncture surface 36, 
and extends outwardly, away from the hole beyond the outer periphery of 
ring 40. Membrane 40 desirably is formed from a material of high strength 
and relatively low atomic number. The preferred materials generally have 
elastic modulus greater than about 10.sup.12 dynes/cm.sup.2. Preferred 
materials include compounds of carbon and nitrogen and hydrogen with 
metals such as Si, Al and B. Thus, the carbides, nitrides, hydrides and 
oxides of these metals may be employed. Combinations including mixed 
compounds such as nitride hydrides, nitride carbides and carbide hydrides 
may also be employed. SiC, BN, B.sub.4 C, Si.sub.3 N.sub.4, Al.sub.4 
C.sub.3, Al.sub.2 O.sub.3 and other compounds may be employed. A 
particularly preferred compound, however, is boron nitride hydride or 
B.sub.4 NH. Desirably, membrane 40 is less than about 3 micrometers thick. 
The lower limit of membrane thickness is set by the need to provide a 
pinhole-free, substantially impermeable membrane and to provide sufficient 
structural strength to enable the membrane to withstand stresses 
encountered in service. Preferably, the membrane is between about 20 nm 
and about 3 micrometers thick. Membranes between about 0.1 and 1 
micrometers thick are more preferred. Suitable membranes can be formed by 
chemical vapor deposition on a temporary substrate which is then removed 
from the membrane, as by etching after the membrane is formed. Also, 
suitable membranes are commercially available from suppliers including 
Kevex, division of Fisons, Inc., Valencia, Calif. 
In an assembly process, the front portion 14 and rear portion 12 of the 
housing are united with one another, and the electronic components 22 are 
disposed in the interior space 18 enclosed by the front and rear housing 
portions. Then, a coating of a polymer precursor in liquid form is applied 
on the exterior surface 28 of the front wall 20 of front housing portion 
14 so that the coating occupies a ring-like region having the same 
configuration as the desired polymeric ring 40. Thus, the ring-like liquid 
coating region surrounds hole 30. The liquid coating can be applied by any 
suitable coating process such as brushing, roller-coating, silk screen 
coating or the like. Spin coating is particularly preferred. In a spin 
coating process, the liquid precursor is applied to the front surface 28, 
and then front wall 20 is rotated rapidly about axis 32 so as to spread 
the liquid over the front wall. After spreading the liquid, the membrane 
44 is placed over the liquid coating. The front surface of the wall, the 
surface of the membrane or both may be cleaned and/or subjected to other 
surface treatments to promote adhesion. Merely by way of example, where 
the polymeric material includes a polyimide, the surfaces may be 
pretreated with a solution in methanol and water of a so-called "adhesion 
promoter" available from the aforementioned National Starch Company under 
the designation "AP-20". 
The assembly is subjected to conditions which cure the liquid precursor and 
form polymeric ring 40. This curing step also causes the polymeric ring to 
bond to membrane 44 and to the exterior surface 28 of the wall. Normally, 
the curing operation involves heating the assemblage of the membrane, 
liquid coating and front housing portion, typically while maintaining it 
under atmospheric pressure, The precise heating steps used to cure the 
liquid precursor will vary with the composition of the precursor used. 
However, for the preferred polyimide materials, the steps used to cure the 
membrane typically involve heating to about 250-350 degrees C..degree., 
followed by gradual cooling to about room temperature. The membrane 44 and 
polymeric ring 40 cooperatively constitute a closure unit which seals hole 
30. With the membrane bonded in place by the polymeric ring, the entire 
assemblage is subjected to heat, as from a heater 46 (FIG. 1) while 
maintaining the interior space 18 of the housing in communication with a 
vacuum source 48 such as a conventional vacuum pump and cold trap 
assemblage, so as to draw out volatile materials from the interior of 
housing 10, and from electronic parts 20. As illustrated, communication 
between the interior of the housing and vacuum source 48 is maintained 
through temporary port 13. Preferably, the temporary port is connected to 
the vacuum source but the exterior of housing 10 is maintained under 
normal, atmospheric conditions. Thus, there is no need to conduct the 
bakeout procedure inside of a vacuum oven. The assembly is heated to a 
preselected bakeout temperature sufficient to promote relatively rapid 
evaporation of volatile residues within the assembly. Desirably, the 
bakeout temperature is between about 250.degree. and about 450.degree. C., 
and more desirably, about 350.degree. to about 450.degree. C. The elevated 
bakeout temperature is maintained for a time sufficient to allow the 
volatile substances in the assembly to pass out through temporary port 13 
to vacuum source 48, typically about 1 hour or more. After sufficient time 
has elapsed, temporary port 13 is closed by momentarily heating the glass 
wall of housing portion 12 at the temporary port and deforming the wall to 
close the port. Because the ring 40 has substantial structural strength 
even above its glass transition temperature, it holds membrane 44 in place 
during the bakeout step. The polymeric ring is effective to maintain the 
membrane in place as well as to exclude the surrounding atmosphere despite 
the substantial pressure differential between the atmosphere and the low, 
sub-atmospheric pressure in the interior space 18 of housing 10. 
After the bakeout step, the assemblage is cooled, typically to room 
temperature, or to another convenient handling temperature below the glass 
transition temperature of the polymer in ring 40. While the assemblage is 
cooling through the range between the bake-out temperature and the glass 
transition temperature of the polymer in the ring, the polymer of the ring 
can deform and can allow movement of membrane 44 relative to front surface 
28 to an extent sufficient to compensate for differences in thermal 
expansion between membrane 44 and front wall 20. After cooling, an 
auxiliary sealing material 50 may be applied around the periphery of 
membrane 44 so as to further seal the membrane to wall 20 and provide an 
even more reliable seal against entry of foreign substances into interior 
space 18. Although ring 40 provides a good seal against entry of the 
atmosphere or other foreign materials into the interior space, the ring 
need not be the sole barrier to 20 long-term permeability if the auxiliary 
sealing material is employed. Among appropriate auxiliary sealing 
materials which can be used to good advantage are epoxy and urethane. 
During the bake-out process, and in service as well, membrane 44 is 
subjected to differential pressure which tends to urge it inwardly, into 
hole 30. Thus, the pressure of the atmosphere, or the pressure of any 
surrounding fluid will tend to force the membrane into the hole. As 
schematically depicted in FIG. 2, the membrane tends to bow inwardly, into 
hole 30, and hence is forced against front wall 20, so that the inwardly 
directed forces on the membrane are transmitted between the membrane and 
the front wall. Juncture surface 36 permits such force transmission while 
maintaining reasonable levels of stress in those portions of the membrane 
contacting the wall. Thus, because juncture surface 36 flares outwardly in 
a gradual manner, a substantial region of membrane 44 can contact wall 20. 
By contrast, if hole 30 were provided with a sharp, knife edge at its 
intersection with exterior surface 28, membrane 44 could contact wall 20 
only along a very narrow region at the knife edge. Moreover, the flaring 
juncture surface 36 allows the membrane to bend into engagement with the 
wall without substantial wrinkling or distortion. Both of these factors 
tend to reduce the stress in membrane 44 particularly at its juncture with 
wall 20. Thus, a given membrane can withstand a substantially greater 
pressure than would otherwise be the case. Also, resistance of the 
membrane to fluctuating stresses, such as may be encountered where the 
external ambient pressure fluctuates, is greatly enhanced. The reduced 
stress materially reduces the tendency of the membrane material to fatigue 
under the influence of the fluctuating stress. 
The degree of stress reduction achieved by juncture surface 36 will vary 
with the precise configuration of the juncture surface. Some stress 
reduction can be achieved by a juncture surface having an arbitrary, 
constant radius of curvature r.sub.1. Such a constant-radius juncture 
surface corresponds to the surface generated by revolving a segment of a 
circle with radius r1 about axis 32. Preferably, the generator of juncture 
surface 36 is a segment of an ellipse having its major axis parallel to 
front surface 28 and its minor axis parallel to the axis 32 of the hole. 
Thus, the radius of curvature r.sub.1 of the juncture surface increases 
progressively along the juncture surface in the direction away from the 
central axis 32 of the hole. Stated another way, the radius of curvature 
of the juncture surface increases progressively from the point where the 
juncture surface merges with the peripheral wall of the hole to the point 
where the juncture surface merges with front wall 28. The generating curve 
of the juncture surface need not be an ellipse or a circle, but instead 
may have an arbitrary shape calculated to provide the minimum stress in 
the membrane. Such calculations can be performed using stress analysis 
computer programs by assuming a uniform pressure load on the outer surface 
of membrane 44 and taking into account the local deflection of the front 
wall 20 under contact loads transmitted from the membrane, as well as 
friction between the membrane and the juncture surface. By calculating of 
the maximum local stress in the membrane for a particular juncture surface 
generating curve, varying the curve and recalculating the stress 
repetitively, conventional computer techniques for iterative calculation 
can be used to find the optimum generating curve for the juncture surface 
36. 
In use, electronic components 22 are employed to produce an electron beam 
and to focus that beam so that the same passes through hole 30 in forward 
wall 20. The beam passes through that portion of membrane 44 overlying the 
hole. The electron tube can be used for various applications where 
emission of an electron beam outside of the tube is required. A preferred 
use, however, is in dispersion and atomization of fluent materials. 
In the method discussed above, the forward portion 14 and rear portion 12 
of housing 10 are sealed and bonded to one another before assembly of the 
membranes and polymeric ring to the forward portion 14 of the housing. 
Such sealing may incorporate a conventional glass-to-metal seal of the 
type commonly utilized in the electron tube and lamp arts. Alternatively, 
the steps of uniting the front and rear housing portions 12 and 14 and 
sealing the same together, and introducing the electronic components and 
sealing the same in place may be performed after bonding the membrane in 
place. The seal 16 may incorporate a hoop or gasket 52 formed from a 
polymeric material similar to that discussed above with reference to ring 
40. Again, the polymeric material, at temperatures above its glass 
transition temperature but below the bakeout temperature used in the 
fabrication process is flexible but yet has appreciable mechanical 
strength. Where such a hoop is employed, the forward portion 14 and 
rearward portion 12 of the housing 10 can be bonded to one another by 
bonding processes similar to those discussed above. Thus, a liquid 
precursor material as discussed above is placed between the forward 
portion 14 and rearward portion 12 of the housing in the region to be 
occupied by hoop 52. Thus, the liquid precursor may be coated onto one or 
both housing portions before the housing portions are united with one 
another. Alternatively, the precursor may be introduced by capillary 
action into the space between the housing portions while the same are held 
in their assembled position. The liquid precursor is cured to form hoop 52 
in the same way as the precursor is cured to form membrane 44, thereby 
bonding the housing portions to one another. This may be done before or 
after membrane 44 is bonded to the front wall 20 of the forward portion 
14. Preferably, however, both bonding steps are performed simultaneously. 
Thus, the membrane can be positioned on the front wall, and the forward 
portion can be positioned on the rearward portion with the polymeric 
precursor in place to form the membrane 44 and polymeric hoop 52 as 
illustrated. The entire assemblage can be heated to promote curing and 
bonding. The heating step used in the aforementioned bonding operations 
may occur as the assemblage is taken from room temperature to the elevated 
bake-out temperature. Preferably, application of vacuum to the interior of 
the housing is delayed until after the bonding steps are complete, i.e., 
until after the polymeric materials have been cured to solid, coherent 
condition. 
Polymeric hoop 52 permits appreciable relative movement between the 
adjacent surfaces of forward portion 14 and rearward portion 12, and thus 
compensates for differential thermal expansion and contraction occurring 
at temperatures above the glass transition temperature of the polymeric 
material used in the hoop. Here again, because the glass transition 
temperature is relatively close to room temperature, only moderate stress 
will be induced by differential thermal expansion or contraction of 
forward portion 14 and rearward portion 12 over the range between the 
glass transition temperature and room temperature, even where there is a 
substantial difference in thermal coefficient of expansion between these 
two portions. There is, accordingly, no need to provide precise matching 
between the coefficients of thermal expansion. Where this approach is 
employed, forward portion 14 may be fabricated from a relatively 
inexpensive metal such as ordinary steel, stainless steel, copper, 
aluminum or other metal. Where a conventional glass-to-metal seal is 
employed, the formed portion 14 of the housing desirably is formed from a 
material which has a coefficient of expansion close to that of the 
rearward portion. 
Apparatus according to a further embodiment of the '942 application is 
partially depicted in FIG. 3. This apparatus includes a forward portion 
having a front wall 120 with an interior surface 126 and exterior surface 
128. Here again, a polymeric ring 140 is provided between membrane 144 and 
the exterior surface 128 of front wall 120, and the membrane covers a hole 
130 extending through front wall 120. 
Front wall 120 has a preselected wall thickness K in regions remote from 
hole 130 and tapers to a substantially smaller wall thickness L in a 
region 132 immediately surrounding hole 130. Region 132, having this 
lesser wall thickness, is substantially flexible. Flexible region 132 has 
an exterior surface 129 which is flush with the other portions of exterior 
surface 128, whereas the inwardly facing surface of flexible region 132 
(the surface of region 132 facing downwardly in FIG. 3) is substantially 
recessed from the interior surface 126 of the wall. In use, and during the 
aforementioned evacuation and bakeout steps, flexible region 132 can 
deflect inwardly so as to conform with membrane 144 when membrane 144 is 
forced inwardly, into hole 130 by differential pressure. Flexible region 
132 desirably merges gradually into the remainder of wall 120. That is, 
there is no sharp transition in thickness between flexible region 132 and 
the remainder of the wall, but instead, a gradual, progressive increase in 
thickness from the edge 134 immediately adjacent hole 130 to the rest of 
wall 120. At edge 134, flexible wall portion 132 may have essentially zero 
thickness. This configuration can be formed by conventional machining 
processes. Alternatively, it can be fabricated by etching from wall 120 
using an etchant applied to the interior surface 126 of the front wall. 
Preferably, the interior surface is covered by a masking material with a 
hole of approximately the same size as the desired hole minimum diameter 
or minimum transverse dimension d.sub.m. The etchant will progressively 
remove material starting at the interior surface and form the tapering 
wall configuration shown. In other respects, the structure, operation and 
fabrication process are the same as those discussed above with reference 
to FIGS. 1 and 2. 
Apparatus according to a further embodiment of the '942 application is 
partially depicted in FIG. 4. This apparatus includes a forward component 
having a front wall 220 with a hole 230. Hole 230 is provided with an 
outwardly flaring juncture surface 236 similar to the juncture surface 
discussed above with reference to FIG. 2. However, membrane 244 is not 
attached to front wall 220 by means of a deformable polymeric ring. 
Instead, membrane 244 is bonded to a substantially rigid ring 246. Ring 
246 may be composed of silicon, a metallic material or a polymer. 
Preferably, membrane 244 is formed by chemical vapor deposition on a solid 
part (not shown) which is then etched to form an opening 248, and thereby 
form ring 246. Membrane 244 becomes bonded to the solid part during the 
chemical vapor deposition process, and remains attached to the ring when 
the ring is formed by etching. Ring 246 is secured to the exterior surface 
228 of front wall 220 by application of a bonding materials 250 such as 
silver solder, polyimide or epoxy, around the periphery of the ring and 
membrane prior to the bakeout procedure. This arrangement is less 
preferred inasmuch as it does not provide for relief of differential 
thermal expansion and contraction between ring 246 and membrane 244 during 
the bakeout procedure. However, it does provide the benefits of stress 
relief afforded by outwardly flaring juncture surface 236. 
A fabrication process and apparatus in accordance with a further embodiment 
of the '942 application is depicted in FIGS. 5A-5D. In a first stage of 
this fabrication process, the front wall 320 is machined or etched as 
discussed above with reference to FIG. 2 to form a hole 330 with the 
outwardly flaring juncture surface 236. A temporary, filler material 331 
with a low melting temperature is then placed into hole 330. The exterior 
surface 328 of front wall 320, and the filler material 331 are polished to 
form a smooth, continuous, flush surface. A layer 333 of a high 
temperature bonding material such as silver (FIG. 5B) is applied on this 
flush surface. A membrane 344 is then applied by vapor deposition atop 
layer 333. Membrane 344 bonds to layer 333 during the deposition step. A 
peripheral portion of membrane 344 may also bond directly to the exterior 
surface 328 of front wall 320. Temporary filler material 331 is then 
removed, as by heating, leaving the assemblage in the configuration 
illustrated in FIG. 5C. In this configuration, high temperature bonding 
material 333 covers the central portion of membrane 344, in alignment with 
hole 330. 
The assemblage is then exposed to an etchant solution applied from the 
interior surface 326 of wall 320. The etchant is selected so that it 
attacks bonding material 333 but does not substantially attack the 
materials of wall 320 or membrane 344. The etchant passes through hole 330 
and attacks the portion of bonding material layer 333 aligned with the 
hole and with outwardly flaring juncture surface 336. The etchant thus 
progressively removes portions of layer 333, working from the center of 
hole 330 outwardly. After sufficient time has elapsed, the etching process 
is interrupted, leaving the assemblage in the condition illustrated in 
FIG. 5D. Thus, a ring-like structure 335 is formed from layer 333, so that 
membrane 344 is connected to wall 320 through ring-like structure 335 
adjacent the periphery of the membrane and remote from hole 330. However, 
those portions of exterior surface 328 disposed adjacent hole 330, inside 
ring 335 are free of bonding material. Juncture surface 336 is also free 
of bonding material. Thus, when membrane 344 is forced inwardly by 
differential pressure during service, as schematically indicated in FIG. 
5d, it can bear on the juncture surface 336. 
Where layer 333 and ring-like structure 335 are formed from a metallic 
material or other material which remains substantially rigid at all 
temperatures for the bakeout procedure, it does not provide compensation 
for differential thermal expansion or contraction in the same manner as 
discussed above with reference to FIGS. 1-3. However, layer 333 and 
ring-like structure 335 can be formed from a polymeric material as 
discussed above with reference to ring 40, to provide compensation for 
differential thermal expansion. Thus, polyimides and other common polymers 
can be etched in the production scheme contemplated by FIGS. 5A-5D. 
A tube according to another embodiment of the '942 application has a 
forward housing portion defining a front wall 420 (FIG. 6) with a hole 430 
and juncture surface 436 similar to the corresponding components discussed 
above. The closure unit overlying and sealing hole 430 includes a 
polymeric sheet 433 bonded to the exterior surface 428 of the front wall, 
and an additional electron-permeable, gas-impermeable membrane 444 
overlying the polymeric sheet and bonded to the front wall through the 
polymeric sheet. The electron-permeable, gas-impermeable membrane 444 may 
be similar to the membrane 44 discussed above with reference to FIGS. 1 
and 2. Polymeric sheet 433 acts to absorb differences in thermal expansion 
between electron-permeable membrane 444 and front wall 420. This action is 
substantially the same as the action of polymeric ring 40 (FIGS. 1 and 2). 
However, the polymeric sheet of FIG. 6 also extends across hole 430. 
Accordingly, the polymeric sheet 433 should be thin enough that it does 
not substantially impede passage of the electron beam. With the preferred 
polyimide materials, the membrane should be substantially less than about 
0.5 mm (500 micrometers) thick. 
Structures as illustrated in FIG. 6 may be fabricated by bonding a 
separately-formed sheet of polymer to the front wall, and then applying 
the electron-permeable membrane 444 on the polymer sheet. The 
electron-permeable membrane may be formed by chemical vapor deposition on 
the polymeric sheet. Alternatively, the polymeric sheet may be formed in 
situ from liquid polymer on the exterior surface of the front wall by use 
of a temporary filler material similar to that discussed above with 
reference to FIG. 5B. The temporary filler material is removed after 
curing the polymeric sheet. 
Numerous other variations and combinations of the features discussed above 
can be utilized. Merely, by way of example, a front wall having a 
deformable, flexible region as illustrated in FIG. 3 can be utilized to 
provide stress relief at the periphery of the hole in the structures of 
FIGS. 4 and 5. Also, other means for mitigating stress concentration in 
the membrane adjacent the periphery of the hole may be employed. For 
example, a separate cushioning or load distributing body, in the form of a 
relatively small ring of a springlike, compressible material, can be 
interposed between the membrane and the front wall at the periphery of the 
hole. Such a cushioning structure can be formed integrally with the 
polymeric ring used to take up thermal expansion. In yet another variant, 
the membrane can be formed integrally with the polymeric ring. Thus, the 
closure unit used to seal the hole in the front wall may include a unitary 
sheet of an electron-permeable polymeric material similar to the polymeric 
membrane 433 discussed above with reference to FIG. 6, but without the 
additional membrane 444. That polymeric sheet may be placed over the hole 
in the front wall and the periphery of the sheet may be bonded to the wall 
by heating in the manner discussed above. This variant relies solely on 
the polymeric sheet to seal the hole, and requires that the polymeric 
sheet be electron-permeable. Thus, the polymeric sheet constitutes both 
the polymeric material and the electron-permeable portion in the closure 
unit. A polyimide sheet about 0.5 mm thick generally provides sufficient 
mechanical strength and electron permeability. However, the polyimide 
sheet allows gradual permeation of air into the housing and therefore is 
suitable for use as an electron-permeable membrane only for a relatively 
short-lived electron tube. 
Also, although the outwardly flaring juncture surfaces 36 discussed above 
with reference to FIG. 2 are generally in the form of a surface of 
revolution generated by rotation of a curved generator line about the axis 
of the hole, similar results can be approximated by a juncture surface 
defined by one or more conical portions. Where a plurality of conical 
portions are included, the same may include a conical portion of 
relatively small included angle merging with the peripheral surface of the 
hole and another conical portion of larger included angle extending from 
the first conical portion to a juncture with the exterior surface of the 
front wall. Greater numbers of conical portions of progressively 
increasing included angle may be provided. Although the embodiments 
discussed above employ holes of circular cross-section, with peripheral 
and juncture surfaces in the form of surfaces of revolution about an axis, 
other embodiments may include holes of non-circular cross-section. For 
example, the hole in the front wall may be in the form of an elongated 
slot. In this case, the juncture surface would flare outwardly at each 
edge of the slot. 
Referring now to FIG. 7, a beam tube according to one aspect of the present 
invention has a forward housing portion defining a front wall 520 with an 
interior surface 526 and an exterior surface 528. Provided on exterior 
surface 528 and extending into front wall 520 is an annular channel 560 
extending circumferentially around hole 530 extending through front wall 
520. A polymeric ring 540 is disposed between beam-permeable membrane 544 
and exterior surface 528 of front wall 520. Polymeric ring 540 is also 
disposed in annular channel 560 and partially retained therein so as to 
keep polymeric ring 540 in a fixed or predetermined position with respect 
to hole 530. Annular channel 560 has a predetermined maximum depth D.sub.a 
beneath exterior surface 528 of preferably about 100 microns or less, and 
more preferably, about 50 microns. 
Annular channel 560 also has a width W', i.e., the distance between the 
interior edge of the channel adjacent to hole 530 and the exterior edge of 
the channel remote from the hole, measured parallel to exterior surface 
528 of wall 520, desirably at least about 0.05 mm, and more preferably 
between about 0.2 and 4 mm. 
In a method of forming the beam tube in accordance with the present 
invention, polymeric ring 540 is formed by the step of placing a liquid 
polymer precursor in annular channel 560 and between exterior surface 528 
and membrane 544. The bonding step is then carried out and includes the 
step of heating membrane 544, the precursor, and housing portion to an 
elevated bonding temperature so as to cure the polymer precursor and form 
polymeric ring 540 disposed at least partially in annular channel 560 
while bonding polymeric ring 540 to membrane 544 and exterior surface 528 
of front wall 520. In particular, the polymer bonds to the surfaces of 
annular channel 560. The formation of annular channel 560 in front wall 
520 is a highly advantageous feature when forming polymeric ring 540 from 
a liquid polymer precursor as it functions to retain the polymeric 
material away from hole 530, thus preventing any unwanted migration of the 
polymeric material towards and into hole 530 or other unwanted areas on 
exterior surface 528 during the bonding process. 
According to another embodiment of the present invention, FIG. 8 shows a 
forward housing portion defining a front wall 620 with an interior surface 
626 and an exterior surface 628. Provided on exterior surface 628 is an 
annular raised inner ridge 665 extending circumferentially around hole 630 
which extends through front wall 620. A polymeric ring 640 encircles inner 
ridge 665 so that inner ridge 665 retains polymeric ring 640 in a 
predetermined position with respect to hole 630 and, in particular, keeps 
the polymeric material away from the hole. Raised inner ridge 665 has a 
predetermined maximum height H.sub.R above exterior surface 628 of front 
wall 620 of about 50 microns or less, and preferably about 40 microns. 
Preferably, inner ridge 665 is at a distance far enough away from hole 630 
so as to retain polymeric ring 640 and at the same time not otherwise 
interfere with the stress-relieving regions adjacent hole 630. However, 
the radially inward side of inner ridge 665 may also be formed with a 
gentle downward slope so that the inward side of the ring blends gradually 
into the outwardly flowing juncture surface 636 immediately surrounding 
hole 630 so as to assist in providing stress relief at the periphery of 
hole 630. 
In the embodiment illustrated, an annular raised outer ridge 666 is also 
formed on exterior surface 628 of front wall 620 so as to encircle inner 
ridge 665 and define an annular track 667 therebetween. Annular raised 
outer ridge 666 also has a preferred maximum height above exterior surface 
628 of about 50 microns or less and preferably about 40 microns, and 
preferably has a gentle sloping geometry so as not to introduce any 
additional stresses to membrane 644. Annular track 667 also functions to 
retain polymeric ring 640 therein as is with the case of annular channel 
560 as shown in FIG. 7. That is, the inner and outer ridges 665 and 666 
cooperatively constrain the polymer in a manner similar to the walls of 
channel 560 (FIG. 7). 
In a method of forming the beam tube where only raised inner ridge 665 is 
provided, polymeric ring 640 can be formed by placing a liquid polymer 
precursor on exterior surface 628 of front wall 620 so as to encircle and 
surround inner raised ridge 665. In the bonding step, the membrane 644, 
precursor and housing are heated to an elevated bonding temperature so as 
to cure the polymer precursor and form polymeric ring 640 while bonding 
the ring to membrane 644 and exterior surface 628 of front wall 620. 
Referring to FIG. 9, a tube according to yet another aspect of the present 
invention has a forward housing portion defining a front wall 720 with an 
interior surface 726 and an exterior surface 728. Provided on exterior 
surface 728 is a ring of flow preventing material 770 extending 
circumferentially around the periphery of hole 730. Polymeric ring 740 is 
disposed between membrane 744 and surrounds flow preventing ring 770 in a 
manner similar to the surrounding of inner ridge 665 (FIG. 8). 
In a method of forming the beam tube, polymeric ring 740 is formed by 
placing a liquid polymer precursor on exterior surface 728 so as to 
encircle and surround flow preventing ring 770. In the bonding step, 
membrane 744, the precursor and the housing are heated to an elevated 
bonding temperature so as to cure the polymer precursor and form polymeric 
ring 740 while bonding the polymeric ring to membrane 744 and exterior 
surface 728. In this manner, flow preventing ring acts as a non-wetting 
barrier such that the polymeric material will remain in a predetermined 
area on exterior surface 728 and not migrate towards hole 730. Flow 
preventing ring 770 may be formed from any substance which will not be wet 
by the polymer at the elevated bonding temperature and which is otherwise 
innocuous in the electron tube environment. Thus, the non-wetting 
substance should not "outgass" or emit volatile substances. Silicon and 
diamond films are suitable. Thus, the ring can be formed by coating a 
portion of exterior surface 728 of front wall 720 with a diamond layer or 
silicon layer of a few microns thick, preferably 5 microns or less. This 
may be accomplished through known techniques in the art, such as chemical 
vapor deposition, sputtering, etc. 
Numerous other variations and combinations of the features discussed 
previously with respect to retaining the polymeric ring can be utilized. 
For example, flow preventing ring 770 may also be provided on the exterior 
surfaces 528 and 628 of the embodiments shown in FIGS. 7 and 8, and a 
second outer flow preventing ring could be utilized similar to outer ridge 
666 in FIG. 8. Furthermore, although the present invention is preferably 
used with electron beam tubes as described in my '942 application, it may 
also be used with other beam tube applications such as X-ray beam tubes. 
As these and other variations and combinations of the features described 
above can be utilized without departing from the present invention, the 
foregoing description of the preferred embodiments should be taken by way 
of illustration rather than by way of limitation of the present invention 
as defined by the claims.