Method and apparatus for sputtering superconducting thin films of niobium on quarter-wave resonant cavities of copper for accelerating heavy ions

A method of providing a thin film for lining quarter-wave resonant cavities of copper comprises the deposition of a superconducting material, in particular niobium, in the form of a micro-layer having a substantially constant thickness covering both the cylindrical surface of the cavity and its bottom formed of a plane plate, by biased-diode d.c. sputtering through emitting cathodes in a form fitting geometrically the surfaces to be lined.

BACKGROUND OF THE INVENTION 
The present invention relates to acceleration cavities for heavy ions and, 
more particularly, to a method and an apparatus for making the 
acceleration cavities superconductive at radiofrequency. 
The application of superconductivity to acceleration cavities at 
radiofrequency allows beams of particles or heavy ions to be accelerated 
to otherwise unpredictable energy levels with a low power consumption. 
Niobium is a superconducting material which provides the greatest quality 
factor and allows the highest acceleration fields to be reached in an 
accelerating cavity due to its high critical temperature (T.sub.c), its 
high critical field, its low resistivity under normal conditions, and its 
ability to resist surface oxidation. 
The construction of cavities using niobium as the superconducting material, 
however, involves a highly sophisticated technology since the 
superconductivity property of niobium is strongly affected by impurities 
of even diminutive concentration. This is why cavities of copper are 
preferably made superconductive by electrodeposition of lead 
(electroplating), which has, however, a reduced efficiency since 
radiofrequency loss is proportional to the exponential of the ratio 
-T.sub.c /T; and the T.sub.c of niobium is 9.25.degree. K., while the 
T.sub.c of lead is 7.2.degree. K. Moreover, with regard to acceleration 
fields, those provided by niobium are higher than those provided by lead; 
the critical magnetic field H.sub.c of lead is 800 gauss at 0.degree. K. 
whereas that of niobium is 2000 gauss at 0.degree. K. 
Therefore, cavities for heavy ions, and in particular quarter-wave cavities 
or resonators (QWR) of niobium, have been provided only by electron-beam 
soldering of rolled sheets of niobium or composite sheets of copper and 
niobium produced by explosion. The results at radiofrequency confirm the 
absolute superiority of niobium over lead, but are heavily attendant with 
high cost. Moreover, the high number of electron-beam soldering required 
causes residual radiofrequency loss and makes the mechanical construction 
process more complex. 
SUMMARY OF THE INVENTION 
The subject invention seeks to obviate the aforementioned shortcomings by 
providing a method and apparatus for lining copper quarter-wave cavities 
with niobium at low cost while optimizing the performance of the cavities 
at radiofrequency. More particularly, the advantages are achieved by 
biased-diode d.c. sputtering of a thin film of niobium onto the inner 
surface of the copper resonator. 
The sputtering of niobium on copper accelerating cavities for electrons is 
a widely proven technology: niobium cavities provided by sputtering are 
considerably more efficient than the corresponding niobium bulk cavities. 
Such a technique, however, has never been applied to acceleration cavities 
for heavy ions, because they have a significantly more complex geometry 
than acceleration cavities for electrons. That is why a less sophisticated 
technology, such as that of lead electrodeposition on copper, is generally 
accepted for quarter-wave cavities or resonator (QWR) at the cost of 
higher power loss, lower acceleration fields at least by a factor of 2, 
and a higher surface oxidation which results in stability loss. 
According to the present invention, the technique of depositing 
superconducting films of niobium by biased-diode d.c. sputtering is 
extended to quarter-wave resonators of copper for heavy ions by resorting 
to a number of expedients which satisfactorily solve the problems involved 
with such technology including, for instance, the preliminary treatment of 
the inner surface of the cavity to be lined and the use of special 
equipment for sputtering niobium onto the bottom plate of the cavity and 
in the cavity itself using different cathodic arrangements and assuring 
uniform thickness of the deposited film. 
In order to guarantee such results, according to the invention, a cathode 
having an axial symmetry is used for the sputtering within the resonator, 
while a plane cathode is used for the bottom plate. In both cases, the 
geometry of the cathode fits that of the target surface to be lined. 
In the preferred embodiment, the axial symmetry cathode is a cylindrical 
tube having a thickness of about 2 mm. It should be noted that the 
diameter of such tube plays an important role for improving the uniformity 
of the thickness of the deposited film: a thickness uniformity of the 
order of 15% is provided if the diameter of the cathode is the 
arithmatical mean of the outer and inner diameters of the coaxial line of 
the quarter-wave resonant cavity on which the film is to be deposited. 
Another advantage of the selected arrangements is surprisingly based upon 
the fact that the lines of the electric field at the upper edge of the 
cylindrical cathode converge and are quite concentrated so that the upper 
surface of the resonant cavity can be lined without resorting to auxiliary 
devices. Accordingly, the cylindrical cathode is preferably positioned so 
that the distance between its upper tip and the upper surface of the 
cavity is equal to the radial distance between the cathode and the inner 
surface of the cavity. 
Another problem to be solved when using the sputtering technique in QWR is 
suppressing the electronic bombardment of plasma at the end of the central 
shaft of the resonant cavity. The concentration of electrons in this 
region causes an excessive local heating, with the consequence of film 
peelage due to the different thermal expansion coefficients of copper and 
niobium after cooling of the cavity to room temperature. 
Such a problem is solved according to the invention by inserting a steel 
stem at a suitable distance from the end of the central shaft, said stem 
being grounded and acting as a collector of electrons, thus protecting the 
film during sputtering. 
It is noted that in this case the best results are achieved when the 
distance between the stem and the end of the central shaft corresponds to 
the radial distance between the cathode and the side surface of the 
cavity. 
Another problem to be solved is the deposition of film in the holes of the 
beam transit gates. This problem is solved according to the invention by 
using a cylindrical magnetron of smaller diameter than the holes and by 
sputtering the surfaces concerned separately from the cavity. 
Alternatively, a plasma formed at that portion of the cathode facing the 
hole which is then lined by a thermal electron emitter, for instance a 
thin niobium filament brought to high temperature. Another alternative is 
to maintain the continuity of the plasma in front of the hole by using a 
thin grid, still preferably of niobium, placed before said hole and 
grounded. In both alternatives the localized deposition of niobium may 
advantageously be made simultaneously with the whole cavity. 
Particular attention must be paid to avoid the capture of impurities of 
diminutive concentrations within the film of niobium during its growth. As 
already mentioned, the presence of even small amounts of impurities 
strongly reduces both the critical temperature and resistivity ratio of 
niobium (the resistivity ratio is the ratio between the resistivity at 
300.degree. K. and the resistivity at 10.degree. K. and generally 
corresponds to the degree of purity of niobium). 
In order to solve this problem, the method of the present invention 
provides a bottom plate flanged to an annular substrate so as to increase 
the surface area to be lined, and a cathode dimensioned so as to 
accommodate the wider target. In this way, the impurities which would 
otherwise diffuse between cathode and anode are gettered by the film of 
niobium and do not reach the surface exposed to radiofrequency. 
In the case of cavity sputtering, the gettering property of the film is 
utilized by flanging all of the holes of the cavity and by extending the 
latter with a body of the same diameter which will getter in such case any 
impurity which can diffuse through the open bottom of the cavity. 
A further feature of the invention consists in that the cathodes do not 
need any cooling since the insulators of alumina or boron nitride which 
are provided have low degassing rates even at high temperatures under 
considerable cleaning conditions. Moreover, a power supply is used having 
a protection circuit capable of discontinuing the supply of power whenever 
an arc occurs. Thus, it is assured that any arc does not impair the growth 
of the film. 
The cathode configuration is considerably simplified by the lack of 
cooling. 
Further features and advantages of the present invention will be more 
readily apparent from the following description with reference to the 
accompanying drawings which illustrate the preferred embodiment of the 
invention by way of a non-limitative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIGS. 1 through 4, the quarter-wave resonant cavity or 
resonator consists of a hollow cylindrical body 20 with a hollow central 
shaft 21 extending from the upper base. Body 20 is closed at the lower 
side by a bottom plate 22 (FIG. 2). 
The method according to the subject invention provides the use of two 
different cathodes for lining both the surface having axial symmetry and 
the bottom plate 22 having plane symmetry with a superconducting film of 
high-purity niobium. 
The cathode for sputtering inside the resonator has an axial symmetry, for 
example a cylindrical tube 23 shown in FIG. 3, also showing steel stem 25 
acting as a collector of electrons which would otherwise damage the film 
at the end of shaft 21. Cathode 24 for the bottom plate 22 is planar as 
shown in FIG. 2, and the copper substrate cooperates with the target to 
prevent dust or flakes of niobium from precipitating onto the surface. 
Sputtering cathode 24 is a disc of high-purity niobium (RRR 200, Residual 
Resistivity Ratio=R.sub.300.degree. K. /R.sub.10.degree. K.) with a larger 
diameter than the bottom plate 22 which has to be lined and is flanged to 
the annular substrate 35. 
Cathode 24 is supported by an insulating plug 28 of boron nitride, the 
lower end of which is fitted in a through hole of a grounded shield 26 
preferably of aluminum spaced at a distance of 5 mm from cathode 24. Cable 
30 for electrically supplying the cathode is shielded by a steel bellow 32 
and insulators 34 of boron nitride or alumina. 
FIGS. 5 and 6 show two possible alternatives for sputtering the holes of 
the transit gates simultaneously with the cavity. In FIG. 5 a thermal 
emitter of electrons formed of a thin niobium filament 29, which is 
brought to high temperature by a power supply A, is inserted into the hole 
of the transit gate 27. In FIG. 6 a thin grid 31 of niobium connected to 
ground is placed before the hole of the gate facing cathode 23. 
As already mentioned, the copper surface to be treated should be free from 
impurities and foreign matters; therefore, it is necessary that the copper 
cavity be subjected to a hard cleaning treatment before the sputtering. 
To this end, one or more per se known methods such as tumbling, 
electrochemical cleaning, and chemical cleaning can be used. By way of 
example, a suitable treatment consists of a first tumbling phase followed 
by chemical cleaning and then by the introduction of the resonator into an 
ultrahigh vacuum chamber for a hard vacuum degassing at 300.degree. C. 
In FIG. 4 the apparatus for carrying out the method according to the 
invention is schematically shown. It includes a vacuum pedestal 40 with a 
cylindrical support 42 on which the quarter-wave cavity 20 suitably 
flanged to extension 36 is placed. Insulators 44 are disposed between 
supports 42 and extension 36. Stem 25 is located on vacuum pedestal 40 
along the axis of cavity 20. The whole assembly is enclosed in an 
ultrahigh vacuum bell 48. 
Alternatively, in case of an industrial production, the same resonant 
cavity may be used as the vacuum chamber by flanging it directly through 
extension 36 to the vacuum pumping system. 
FIG. 7 shows the graph of the critical superconductivity transition 
temperature (T.sub.c) within the cavity. As is apparent from said graph, 
all of the values sensed in the indicated zones are greater or equal to 
the value of the niobium bulk (T.sub.c =9.25.degree. K.). 
The technique of d.c. biased diode sputtering applied to the very 
particular geometry of the quarter-wave resonator (QWR) allows 
quarter-wave resonator cavities of niobium deposited on copper to be 
provided. 
Such cavities have a number of advantages, among which are: a) their 
reduced cost which is significantly less than niobium bulk and exploded 
niobium bonded to copper cavities; b) the thermal stability of the 
composite system formed of centimeter thick copper substrate bonded to a 
few microns thick layered niobium; c) the easy regeneration of a damaged 
cavity due to the short time needed for stripping the film to be replaced 
and for the sputtering of a new niobium film. 
The present invention illustrates the setup techniques for manufacturing a 
copper cavity sputtered with niobium. Research, and in particular during 
the sample examination tests, have revealed that the final radiofrequency 
results (Q-factor and accelerating field) depend on some technical factors 
which impair the quality of the results if they are neglected, namely: 
1) The sputtering should be as steady as possible. Since plasma is not 
stabilized by a magnetic field and the niobium target is not cooled during 
the deposition, arcs or parasitic discharges may occur. Any current spikes 
damage either the cathode or niobium film, thus impairing locally both the 
adhesion to the substrate and the superconducting property. The 
suppression of any spikes may be accomplished by a) carefully cleaning the 
surfaces of the target and the substrate, b) using a low ripple power 
supply, and c) using an ultrahigh vacuum electrical insulating sleeve with 
minimal portions thereof under potential. In fact, one problem of the d.c. 
biased diode sputtering is that the plasma contacts any surface under 
potential, including the insulating sleeve, as the plasma is not 
restricted by magnetic fields. 
2) There is a cut-off level for argon operating pressure. In fact, if the 
pressure is below such a level, the plasma in the cavity will not be 
continuous, i.e. there will be an annular region within the cathode about 
the central shaft situated at more or less half its length in which there 
will be no plasma (FIG. 8). Of course, the thickness of the film in such a 
region will be much lower than elsewhere in the cavity and, thereby 
reducing superconductivity in this region. 
By increasing the pressure of argon above cut-off level, the hole in the 
plasma is progressively closed until the optional sputtering conditions 
are reached. Of course, the cut-off pressure depends on the geometry of 
the sputtering system and above all on the distance between the inner 
surface of the target and the central shaft of the cavity (the lower the 
target diameter, the higher the operating pressure should be to suppress 
the hole in the plasma). For such reason the cut-off pressure has to be 
determined every time through the examination of the samples introduced 
into the cavity. During the present research it has been noted that the 
argon pressure of 0.2 mbar is sufficient to allow deposition to be carried 
out without experiencing this problem. 
3) The bending radius of the corners between the two coaxial conductors and 
the short circuit plate within the copper cavity impairs considerably the 
final radiofrequency results in the sputtered quarter-wave resonator more 
than any other factor. The cathode configuration provided by the present 
invention is optimized so as to operate with corners as rounded as 
possible. (FIGS. 9A and 9B). 
A copper cavity having a few millimeter bending radius of the corners will 
always provide an uneven thickness of niobium deposition. The film is less 
thick at the corners. Because the deposition rate at the corners is 
extremely lower than elsewhere, the film in such region is also less pure. 
In addition, the high density of film impurities at the corners is 
promoted by sharper corners which result in less active bias. According to 
the cathode configuration and the size of the resonator, and on the base 
of experimental tests, corners having a 20 mm bending radius (FIG. 9A) are 
not problematic. A 30 mm bending radius (FIG. 9B), however, provides the 
best thickness continuity as well as optimum superconducting 
characteristics of the deposited film (of course a 30 mm bending radius 
corresponds to the maximum which can be provided in a cavity as the inner 
shaft has a diameter of 60 mm and the outer cylinder has an inner diameter 
of 180 mm). 
Corners having a smaller bending radius, for example 10 mm, are 
insufficient and result in inadequate film thickness; moreover, two opaque 
rings will appear at the corners connecting the central shaft and outer 
cylinder where the film has a brown, non-reflecting color. In this region, 
the RRR value is about 2, which is too low if a Q-factor higher than 
2.times.10.sup.8 and an accelerating field higher than 3 MV/m is to be 
reached (such values are obtained in cavities made by depositing lead on 
copper). Repeated tests have proved that the presence of such rings 
considerably impairs the efficiency of sputtered Nb/Cu cavities. Niobium 
film in such impaired regions analyzed by electron microscope has a column 
structure with grains of a few tens micron size separated from one another 
by grain boundaries about one micron large. The brown color of such film 
portion may be due to the size of the grain boundaries. 
The ten microns size of niobium grains is due to the high temperature 
reached by the substrate at the end of the deposition process. As the 
cathode is not cooled and the sputtering is carried out under a "closed 
configuration," i.e. with substrate embracing completely about the 
cathode, the temperature of the cavity increases considerably during the 
deposition. Therefore, starting from a substrate temperature of 
300.degree. C., the film thickness is limited by the fact that the 
deposition temperature does not exceed 800.degree. C. 
Though the superconducting characteristics of the film improve with the 
increase of the substrate temperature, if the latter is too high there is 
the disk of a copper diffusion from the substrate through the grain 
boundaries. The presence of copper between niobium grains within the 
coherence distance from the surface causes residual radiofrequency losses 
(not depending on the temperature). 
Satisfactory results have also, however, been achieved in cavities having a 
bending radius of 10 mm by a two-step deposition. Before the normal 
deposition, which is carried out according to the above-described 
criteria, the short circuit plate of the cavity as well as the connecting 
corners are covered by niobium sputtered from a niobium disc placed 
perpendicularly and coaxially to the cavity axis. The temperature of the 
cavity during the first run does not exceed 400.degree. C. The first 
niobium layer probably acts as a diffusion barrier to copper diffusion or 
to the doping of the film by oxygen or hydrocarbons contained in the 
copper. 
4) A further improvement of RF results can be provided by pre-sputtering 
the cathode. In fact, the temperature of niobium target certainly exceeds 
one thousand degrees just after sputtering. As soon as the discharge is 
extinguished, the target begins to cool. During this step the target will 
eject any impurity from the chamber. In the same way the impurities are 
absorbed onto the surface of the target whenever the latter is exposed to 
the atmosphere. 
There are two possible solutions: a) the target is pre-sputtered by a 
moving cathode system in an extension flanged to the cavity and the target 
is then introduced into the cavity where the film deposition takes place; 
b) instead of 1 micron niobium a much thicker layer is deposited so that 
the surface of the film is as pure as possible. 
5) Regarding the beam port problem, the methods outlined above may be 
adapted to cover the entire length of the beam port interior. Of course, 
the RF fields are attenuated in the beam ports so as to be zero at their 
ends. Obviously, the whole length of any beam port need not be perfectly 
superconductive since the length of the beam ports is overestimated. 
Nonetheless, a further solution is needed to cover the interior of the 
beam ports with niobium. Such solution, according to the subject 
invention, is based upon two considerations: there is a hole in the plasma 
just before the beam port which becomes more and more narrow and tends to 
be eliminated a) with increasing pressure, and b) with increasing distance 
between the cathode and wall port. It may then be possible to deposit the 
niobium film even in the interior of the beam ports by operating at high 
argon pressure (range 10.sup.-1 mbar) and by narrowing the target near the 
beam ports by maintaining the diameter of the target and increasing the 
distance between the target and beam port. 
Of course, such technique means that the cavities to be sputtered have all 
the same dimensions. 
6) The research has been carried out with the cavity placed within an 
ultrahigh vacuum system. This means that the provision of vacuum levels 
necessary for sputtering niobium is not trivial; besides it requires a 
long baking time. Therefore, the cavity can be directly flanged to the 
vacuum pump, thus reducing both the volume to be pumped and the surfaces 
to be degassed. 
7) Even if not directly detected by the present invention, there is strong 
evidence to suggest that the best results can be achieved through a 
composite sputtering technique consisting of biased diode plus magnetron. 
Higher deposition rates and a greater stability of the sputtering 
parameters could be provided by assisting the biased-diode discharge with 
an outer magnetic field. 
All of the above technical factors have provided a cavity with a low-field 
Qo of 7e+O8 and an acceleration field of 4 MV/m. As a comparison, the best 
lead cavity produced by the National Laboratories of Legnaro has a maximum 
Qo of 3e+O8 (FIG. 10). Notwithstanding this, since a better result is 
achieved each time after a radiofrequency test, the results are considered 
more and more interesting. The present invention has been illustrated and 
described with reference to a preferred embodiment thereof, but it should 
be understood that modifications can be made by those skilled in the art 
without departing from the scope of the present industrial invention.