IRRADIATION TECHNOLOGY FOR MANUFACTURABLE SUPERCONDUCTING DEVICES
The sum of award for this research project had been substantially cut compared to our original research
proposal, however, significant progress had been made in the past two years. The main results are summarised
below.
1. Development of in-situ characterisation hardware
After the initial design and engineering drawings of the cold sample stage for in-situ implant experiment, the
mechanical parts were machined promptly in the mechanical workshop. These were assembled shortly with
electrical connections for temperature measurement, controlling heater and leads for in-situ device
characterisation. By using a closed cycled cryogenic pump, the sample stage’s temperature can be set easily at
any temperature between 25 K and 400 K with temperature fluctuation less than 0.5 K. The beam heating up
effect is less than 0.1 K even at low temperature. Two sets of electronics for IV characterisation were
assembled, tested and finally commissioned in June 1999 in both sites, one set in Guildford and another set in
Cambridge. With the computer software developed in Cambridge, all data sampling and measurement process
are automatic. In addition to the characteristic current versus voltage measurement, now we can easily monitor
the time and temperature dependent resistance of sample under ion beam irradiation. This is useful for the
investigation of defect dynamics in irradiated samples. Displayed below is the new sample holder designed for
in-situ low temperature implant.
Figure 1 Sample holder for in-situ low temperature irradiation experiment
2. Room temperature proton implantation
Room temperature proton implants have been carried out systematically in order to collect experimental data on
the damage effects in both YBCO thin films and mask materials. More than 20 batches of proton beam
implantation experiment had been done. A satisfactory linear T c and implant dose correlation had been
established by tilting the incident proton beam by 15 0 with respect to the normal of sample surface. This
provides a good guidance for the design of low temperature implant sample stage and mask structure. The
experiment results reveal that the damage effects depend upon the initial defect status at high irradiation dose
levels. For irradiation with tilting angle by 70 with respect to the normal of sample surface, damage effects
saturated easily, and a non-linear Tc and implant dose behaviour has been recorded, as shown in Fig. 2. The
radiation damage in YBCO thin film samples was also analysed by RBS technique.
100
15 degree
Tc in K
80
7 degree
60
40
20
0
0.0E+00 5.0E+15 1.0E+16 1.5E+16 2.0E+16
Implant dose in ions/cm2
Figure 2 Irradiation dose dependent T c suppression in 70 and 150 tilted experiment for 200 nm YBCO thin films
on LaAlO3 substrate with 50 keV proton beam irradition at room temperature, where the saturation effect of Tc
suppression in 70 tilted implant is clear at high dose
3. Low temperature proton implant with in-situ characterisation
Masked YBCO tracks have been irradiated by 50 keV proton beam at temperatures below T c with a number of
small dose run at 1015 ions/cm2 , in order to monitor the damage build-up and Josephson junction formation
process. The temperature dependent resistance just after the implant and a typical IV curve at 63 K are shown in
Fig. 3. From the resistance data, it is estimated that the barrier layer width is about 100 nm. This is in good
agreement with the simulation results described below. Since April 1999, we have carried out at least 7 batches
of in-situ low temperature implantation experiment with 50 keV proton beam, in addition to on-line studies on
the annealing effect of the defects formed by irradiation process.
3.5
200
3
T=63 K
0
n
2
R A (m )
c
2.5
I (A)
100
2
-100
1.5
-200
-0.2
-0.1
0
0.1
V(mV)
1
15
5 10
0.5
0.2
2
ions/cm
15
2
7.5 10 ions/cm
and 298 K anneal
0
30
a)
40
50
60
70
80
90
T(K)
Figure 3 A typical temperature dependent resistance of a YBCO track after proton beam, with the insert
showing the characteristic IV curve indicating the formation of Josephson weak link. The thermal annealing
effect is also displayed by changes in resistance against temperature.
4. Monte Carlo simulation of 2D implant damage profile in YBCO thin films
Damage build-up in YBCO thin films has been studied by the Monte Carlo simulation code CRYSTAL [1]. The
simulation code CRYSTAL was initially developed for modelling of damage accumulation and channelling
implantation in crystal silicon. It was adapted properly to include the complex orthorhombic crystal structure of
superconducting YBCO. The program is highly parallelised and vectorised. The program uses the well-known
binary collision approximation (BCA) to model the deflection of the trajectories of moving particles. The ion
irradiation is simulated by following the fate of a large number of sequentially generated pseudo-projectiles,
each of which carries an equivalent dose corresponding to a fluence increment obtained by dividing the total
fluence by the number of pseudo-projectiles and scaled for the topography of interest.
Generally Monte Carlo simulation on the irradiation damage profiles is for room temperature, 300 K,
irradiation with 105 trajectories recorded for each simulation normalised to a nominal implantation dose, say
1016 ions/cm2 for proton beam, and 1014 ions/cm2 for oxygen beam. Other doses are used as well in order to
compare with the experimental observations directly. The selection of normalised implant doses is arbitrary, but
they are about to produce similar level of radiation damage and substantial suppression of Tc. Self annealing
effect of damage created by irradiation is excluded at this stage. The idealised structure model for the simulation
is composed of amorphous LaAlO3 substrate with infinite thickness, 200 nm crystalline YBa 2Cu3O7- (YBCO)
thin film, amorphous 30 nm Au and 400 nm Nb masks with a slot opening in the central. The implantation plane
is the bc plane and projection plane is ac plane of YBCO crystal, the normal of the YBCO surface is (001)
direction. For a 50 keV proton beam with 105 pseudo-projectiles, the computation time is about 11 hours on a
250 MHz quad processor SuperSPARC II workstation. Some 200 simulation processes have been carried so far,
some results are highlighted below.
4.1 Radiation damage effects in YBCO thin films
Energy deposit in ev/cc
As expected, the damage build-up inside YBCO thin film is proportional to the dose. The simulated damage
level may over estimate the real damage effects accumulated in the target YBCO thin film, as some of the
defects will be annealed out after irradiation. However, for the experimental conditions adopted here, a linear T c
suppression and normal state resistivity increase against irradiation dose implies that the variation of annealing
across the radiation dose range considered here is negligible.
22.4
22
21.6
21.2
Energy deposit
20.8
0
5
10
15
20
Tilting angle in degree
Figure 4 Beam tilting angle dependent avaerage energy deposit in YBCO derived from Monte Carlo simulation
for 50 keV proton beam irradiation.
Tilting of the incident beam in relative to the normal of YBCO thin film leads to an increase in damage
level by a nominally same dose of proton beam, indicating a strong channelling effect. The critical channelling
angle is less than 50, as shown in Fig. 4. In practical implant experiments, a much large tilting angle, say 15 0,
has been used in order to achieve a linear effective damage build-up against irradiation dose. This fact indicates
that the annealing out rate is very sensitive to the detailed structure of defects formed, thus leading to various
annealing behaviours with different incident beam orientations.
With an identical normal irradiation dose of 1016 protons/cm2, the lower the incident beam energy, the
higher the damage level created inside the YBCO thin films, though inhomogeneous distribution of damage
build-up will be created when the incident beam energy is too low.
4.2 Sidewall effect of High Aspect Ratio Nb Mask
Simple range calculation reveals that a 350 nm thick Nb film should be enough to stop a 50 keV proton beam
completely. However, the gradual opening of a central slot on the Nb mask leads to a significant “leakage” of
incident beam into the YBCO thin film and LaAlO3 substrate covered by Nb mask. To reduce the extended
damage build-up in the masked area, the thickness of Nb mask has to be increased substantially, as illustrated
by simulated results in Fig. 5
Figure 5 (a) top left, simulated net doping distribution with 50 keV proton beam irradiation up to 7.5e16
ions/cm2 for a model with 1200 nm Nb mask; (b) top right, simulated energy deposit profile with 50 keV proton
beam irradiation up to 7.5e16 ions/cm2 for a model with 1200 nm Nb mask; (c) bottom left, simulated net
doping distribution with 50 keV proton beam irradiation up to 1e16 ions/cm2 for a model with 400 nm Nb
mask; and (d) bottom right, simulated energy deposit profile with 50 keV proton beam irradiation up to 1e16
ions/cm2 for a model with 400 nm Nb mask.
As expected, the lateral beam damaged area decreases as the Nb mask thickness increases. As far as
the proton beam implant effect is concerned, a minimum 1200 nm Nb mask layer seems necessary to stop
nearly all the proton implant inside the Nb mask layers, Fig. 5a. It is interesting to point out here that
roughening of the Nb mask slot sides helps to stop the implant ions a little bit more effectively, obviously due to
the effective damping of side interaction cascade. So the simulation with smooth surfaces gives an upper limit
to the implant damage spreading process. For a given irradiation dose, it is also easy to understand that the
damage level is lower in the YBCO target with a thick Nb mask compared to that with a thin Nb mask, as more
scattered proton ions have been stopped by the mask. Ignoring the self-annealing process, the reduction in
damage level can be easily compensated by increasing the implant dose. In fact the nominal dose used for 1200
nm Nb mask structure is 7.5 times higher than that for the structure with only 400 nm Nb mask, in order to give
a comparable damage level in the central area confined by the mask opening width.
Reduction in beam energy can also help to stop the beam penetration, and also to reduce the range of
damage spreading. To compensate the small reduction in damage level just inside the central slot defined area
with decrease in incident beam energy, the normalised implant dose has been increased similarly for the low
energy irradiation.
It is interesting to point out here that the beam divergence is not important from our simulation studies.
This is because incident ions scattered into the mask opening came from a volume of the mask with a dimension
comparable to the range of the ion for the nanometer mask structure considered here.
4.3 Proton beam irradiation versus oxygen beam irradiation
Figure 6 (a) top left, simulated net doping distribution with 50 keV proton beam irradiation up to 1e16 ions/cm 2
for a model with 400 nm Nb mask; (b) top right, simulated energy deposit profile with 50 keV proton beam
irradiation up to 1e16 ions/cm2 for a model with 400 nm Nb mask; (c) bottom left, simulated net doping
distribution with 50 keV oxygen beam irradiation up to 1e14 ions/cm 2 for a model with 400 nm Nb mask; and
(d) bottom right, simulated energy deposit profile with 50 keV oxygen beam irradiation up to 1e14 ions/cm 2 for
a model with 400 nm Nb mask.
Oxygen ion irradiation is more effective in creating damage, but far more oxygen ions are reflected back into
the YBCO film. The simulated results are displayed in Fig. 6. It was anticipated that a 50 keV proton beam
would create a better damage barrier compared to a 50 keV oxygen beam, as the ranges of proton and oxygen
ions in YBCO target are quite different. Unexpectedly, however, the overall damage profiles are very similar
for both proton and oxygen beams. The broadening of damage profile near the YBCO and Nb mask interface is
the consequence of significant sidewall scattering contribution from the mask walls for the proton beam, and the
broadening of the damage profile near the YBCO and substrate interface is possibly due to more low energy
cascade and back reflection from the substrate for the oxygen beam. This also points out that the oxygen beam
is slightly better compared to proton beam for a very thin sample, say 50 nm in thickness. For 200 nm thin
films, the short range of oxygen ions inside YBCO may be a problem in creating homogeneous damage
distribution. In fact, we may benefit more from the more effective stopping of oxygen ions by the mask
structures, thus reduction in sidewall scattering has been achieved.
4.4 Conclusions
The contribution of mask structure is getting more important as the size of mask structure goes down to the
nano-metre scale due to sidewall interaction for high aspect ratio mask structure. This interaction is intrinsic for
the structure model used here, and its contribution towards the damage and dopant distributions can be modified
with changing initial incident beam energy, aspect ratio and surface finishing of the mask structure. It is natural
to anticipate that such an effect will be less important as the width of opening goes up, i.e., the aspect ratio goes
down. Provided the beam energy is high and target film thickness is not very thick, the most prominent effect of
sidewall interaction will be the inhomogeneous distribution of both damage level and net doping concentration
along the lateral direction. So the entire damaged YBCO track can be better described as a series of stripes with
varied superconducting transition temperatures. It is still likely to form a badly damaged central area with
suppressed superconducting transition temperatures or even non-superconducting at all, thus leave a narrow
working temperature and normal state resistance window for the formation of Josephson weak links. This has
been confirmed by our in-situ implant experiments.
5. Workshop and academic visit
From October 1997, we have held five workshops discussing the management and academic programme of this
research grant. These include: the first meeting in 1997 in Cambridge, the second meeting in September 1998 in
Guildford, the third meeting in January 1999 in Cambridge, the fourth meeting in March in Guildford and the
fifth meeting in October 1999 in Guildford. In addition to these group research meetings, the two post-doctoral
research fellows have visited each other’s site frequently for the installation and commission of in-situ
experimental apparatus, preparation of samples and in-situ proton implantation experiment.
Reference
[1] I. Chakarov and R. Webb, Radiat. Eff. Def. Solids 33(1994)447