|Title of Invention||
"A PROCESS FOR PROVIDING PROTECTIVE ENCAPSULATION OF SUPERCONDUCTOR DEVICES"
|Abstract||The present Invention relates to a process for providing protective encapsulation of superconductor devices. This process consists of two steps. In the first step, a coating by conventional spin coating technique of uniform layer of photo resist on top of the superconductor device is carried out. The spun coated photo resist layer is hardened by annealing it at 80-100°C for 30 minutes. In the secon step, the superconductor device is encapsulated inside a copper enclosure in pure Helium gas atmosphere. In the present process of protection, high-Tc device does not come in direct contact with moisture during thermal cycling from room temperature to low temperature such as liquid nitrogen temperature. The encapsulated superconductor device shows excellent stability of device characteristics over a long period of 36 months even after repeated thermal cycling between liquid nitrogen temperature and room temperature.|
|Full Text||The present Invention relates to a process for providing protective
encapsulation of superconductor devices.
More particularly the invention relates to encapsulation of high Tc
High-Tc superconductors are newly discovered superconducting materials and
several of these show superconductivity above liquid nitrogen temperature
whereas the conventional low temperature superconductors show
superconductivity only upto 23 K. This discovery of high-Tc superconductors
provides an opportunity to develop high-Tc devices such as SQUIDs, Microwave
Passive Circuits etc. which can be operated at liquid nitrogen temperature (77K).
High Tc superconductor devices have great potentiality for wide spread
application since liquid nitrogen which is used for cooling these devices are very
Characteristics of high-Tc superconductors are found to deteriorate as the
superconductors comes in contact with moisture or if there is a loss of oxygen.
For long term operation of a high-Tc superconductor device its characteristics
should not change with time and even after several thermal cyclings from liquid
nitrogen temperature to room temperature. This requirement necessitates the
use of some sort of protective layer over the device so that the working life of the
device may be increased.
A few methods have been proposed in the prior art disclosures to protect high-
Tc superconductors and devices. Reference may be made to a Japanese
patent (JP 020246608, Feb. 1990) by Kawasaki et. al. wherein preparation of
silicon oxide, Al-oxide or oxides of a metal have been disclosed for use over the
superconductors as the protection layer. In this disclosure the main drawback
is that the use of the overcoat layer as mentioned is done by means of physical
vapour deposition. This will essentially damage the superconducting properties
and will need an additional thermal treatment step to recover the desired
In an another disclosure (WO 9003265, April, 1990) by Weaver et. al., a
method comprising of evaporation of Al, Si, AI-W or CaF2 in activated oxygen is
explained. In this disclosure again, the use of physical vapour deposition is
made and use has been made of oxygen atmosphere to compensate for any loss
of oxygen during the deposition process. This then again has a drawback of the
inability to retain the initial characteristics of the superconductor due to the need
to create the passivation layer for encapsulation in an oxygen atmosphere.
Reference may be made to yet another disclosure (WO 9009683 Aug 1990) by
Lyon et al, wherein the inventors have described a method for encapsulating the
high Tc superconductor with a solution of metal which is capable of being
oxidised by the superconductors to form a passivation layer. This method has
the serious drawback of disturbing the stoichiometry and oxygen content of a
high Tc superconductor due to the necessary reaction with the metal solution.
To be able to recover the desired superconducting properties, the system
necessarily has to be heat treated again.
In another disclosure by Imai Komiko (JP 04124002 April 1992), preparation of a
protective layer, by fusing and applying a metal(alloy) containing Bi, Pb, Cu or
noble metals, has been described. Deposition of metal layer or an alloy are not
very suitable for superconducting devices since it can mess up with contact
pads. Moreover, it can also affect the characteristics of the devices which use
In some methods top thin layer of high-Tc superconductors are modified to act as
a passivation layer. For example reference maybe made to a disclosure by
Vasquez Richard (US 54501, Feb. 1991) . In this disclosure, surface of a high-
Tc superconductor such as YBCO is passivated by reacting the native Y, Ba, Cu
metal ions with ions such as sulfate or an oxalate. This passivation treatment is
done by dipping the superconductor in dilute aqueous solution.
In another patent disclosure (US 5130295, August 1992) Labib has disclosed
preparation of thin layer of mixed phase of YBa2Cu3O7 on the top of YBCO. This
essentially means repeating the process of preparation of the high Tc system on
top of already prepared device.
In a patent disclosure by Gorsshkov (SU 1832135, August 1993) formation of a
passivation layer by Argon ion bombardment is described to protect Bi-Sr-Ca-
Cu-O film. These methods of the modification of the thin top layer of high-Tc
superconductors are not suitable for thin film high-Tc devices, since such
modifications can affect the characteristics of the device due to modification of
structure and stoichiometry on the surface.
In an European patent (EP 484010, May 1992) Josefowicz et. al. have revealed
that a passivation layer comprising of a first layer of group II oxides such as
MgO and second layer of polymide can be created on top of the device . In this
process after deposition of MgO heat treatments at elevated temperatures are
needed which again does not guarantee the recovery of the superconducting
properties in a post deposition situation.
Preparation of SrTiO3 (STO) layer on high Tc film has also been disclosed in an
US patent (US 657203, September 2000). In this process also STO layer is
prepared at an elevated temperature which may not suit to some high-Tc devices
in much the same way as has been mentioned above in respect of inventions of
the prior art, as disclosed above.
In several of the above mentioned process heating is done at an elevated
temperature in excess of 800 ° C which does not suit to high-Tc devices such as
SQUIDs, Josephson Junction, where, a fine microbridge is fabricated. This is
due to the fact that high temperature treatment can modify the device
properties. Moreover in the above mentioned process, high-Tc devices along
with the protection layer is in direct contact with the liquid nitrogen during
operation at 77 K. This can affect the life of device. Moreover, there is
deposition of moisture on the protection layer during warming up of the device
from 77 K to room temperature. Several thermal cycling can cause cracks in
the protection layer and can also damage the contacts which can result in the
failure of the device. What is required that once a high-Tc device is made the
passivation/encapsulation process should not change the device characteristics.
In the present invention a simple and novel process is described in which no
change in superconducting properties of the device occurs during the process of
protection and encapsulation and long life of high-Tc device is ensured by
avoiding direct contact with external atmosphere.
Main objective of the present invention is to provide a process for providing
protective encapsulation of superconductor devices which obviates the
drawbacks as mentioned above.
Another object of the present invention is to provide a process of protective
encapsulation of high Tc superconductor devices.
Yet another object of the present invention is to provide a process of protective
encapsulation without the need of any post deposition high temperature
treatment and oxygen treatment.
Still another object of the present invention is to provide a process of protective
encapsulation to give superconductor device characteristics stable for at least
36 months with multiple thermal cycling between room temperature and liquid
nitrogen temperature .
A process for providing protective encapsulation of high-Tc superconductor
devices is disclosed. This process consists of two steps. In the first step, a
coating by conventional spin coating technique of uniform layer of photoresist on
top of the superconductor device is carried out. The spun coated photoresist
layer is hardened by annealing it at 80-100°C for 30 minutes. In the second
may be such as helium
In yet another embodiment of the present invention the encapsulated device
obtained may have a stability of at least 36 months with multiple thermal cycling
between room temperature and liquid nitrogen temperature.
In another embodiment of the present invention the stability of the temperature
of the encapsulated device may be achieved in about 1 min.
High-Tc superconductors are very susceptible to moisture and this leads to
deterioration of the characteristics of high-Tc superconductors devices. The
present invention provides a process for protection of high-Tc superconductor
devices which ensures a long life of the device.
The present encapsulation method consist of two steps. In the first step
preferably a photo resist, is coated on the high-Tc superconductor device.
Coating of photo resist is done after the high-Tc device is fabricated and metal
contact pads have been prepared. In order to prepare a uniform layer of photo
resist one drop of photo resist (AZ 1470) is put on the top of the superconducting
film device and is spun using a spinner at a preferred speed of 3000 rpm for
one minute. This leads to a deposition of about one micron thick photo resist
layer on the superconductor device. This device with the photo resist layer is
baked at a temperature in the range of 80-100°C preferably for 30 minutes.
The preferred temperature of baking is 90 °C. This heat treatment hardens the
photo resist layer and also leads to good adherence with the superconducting
During the second step, the device is encapsulated preferably in a copper
cavity in pure Helium gas atmosphere. For the encapsulation, superconducting
device is placed on the bottom of the circular copper cavity (1) of 2 cm diameter
and 1 cm height and contacts are made through vacuum tight feed throughs
which are fixed on the top cover of the cavity box. The cover of the box has two
1" long copper tubes of 2 millimeter diameter. These are used for creating
vacuum in the box and for filling the helium gas respectively. The cover is fixed
on the copper cavity using araldite. The thickness of the araldite is optimally
fixed to give crack free coating for effective sealing. The thickness selected is
critical to avoid any development of cracks during thermal cyclings. The copper
cavity is evacuated through a rotary pump for 30 minutes and helium gas is
flushed in. This process is repeated 2-3 times and finally after filling helium gas
at the pressure of 3 Bar, the copper tubes are sealed by pinching it. The
process described in the present invention do not lead to any change in the
superconducting properties of the encapsulated superconductor device. The
device shows the same characteristics even after several thermal cycling from
room temperature to liquid nitrogen temperature for a long period of operation.
Figure 1 shows result of a high-Tc superconducting quantum interference device
(SQUID) which has been encapsulated following the present process and has
been tested over a period of 36 months. After each cooling to liquid nitrogen
temperature (77K), SQUID peak to peak voltage and SQUID noise at 1 Hz are
measured. These two characteristics; SQUID peak -peak voltage and SQUID
noise are very important parameters of a SQUID device and directly related with
the superconducting properties such as critical current, critical temperature etc.
Even a slight change in the superconducting properties will change the peak to
peak voltage or noise of the SQUID. It is evident from figure 1 that the
characteristics of the high-T c SQUID do not change even after several thermal
cycling over a long period. This shows that the present method is very
successful in protecting high-T c device from degradation.
The scientific principle underlying the scheme of passivation is to prevent
interaction of moisture and reactive gases with the superconductor device. It is
the interaction of these environmental parameters which leads to surface level
changes which result in degradation of the properties and long term usability.
Further, the repeated thermal cyclings between croygenic temperatures and
room temperature lead to generation of thermal shocks to the device which
obviously results in micro mechanical damages to the devices and hence
damage the device itself. Thus, in order to prevent the devices from such
harmful conditions the passivation layers are to be used in conjunction with
housing of the device in a suitable enclosure having adequate means of thermal
conduction between the cryogen and the device for effective cooling for the
device to operate. The use of a photo resist or a polymer for that matter serves
this very purpose in conjunction with the copper cavity having a small amount of
helium gas to maintain the device at low temperature during cooling cycle and
The novelty of the invention lies in the avoidance of use of any high
temperature processing for providing protective coating and use of any gas for
maintaining the superconductor properties for effecting
passivation/encapsulation as in the prior art.
This novelty is achieved by use of spin coating and baking of a thin layer of a
polymer on the superconducting device and further encapsulating the coated
device in an enclosure capable of providing a non reactive oxygen free
atmosphere provided by the presence of an inert gas at pressure not exceeding
The following examples are given by way of illustration only and should not be
construed to limit the scope of invention.
An Bi2 Sr2 Ca2 Cu 3 O x SQUID device with already made electrical contacts
was taken. The device was kept on a Headway photo resist spinner vacuum
chuck and was held in position by vacuum by means of a rotary vacuum pump. A
drop of positive photo resist ( Shipley AZ 1470 photo resist) was put at the
center of the device and spun. The spinning speed was selected for 3000 rpm
for a time of 60 seconds. After the spinner stopped the device was carefully
removed from the spinner chuck by releasing air in the vacuum line and then put
in an oven set at a temperature of 90° C. The baking was done for a time of one
hour. The device so coated had a hardened overcoat of photo resist.
This coated device was then fixed to the bottom of a copper cavity circular in
nature having a diameter of 2 cm and a height of 1 cm. Next the leads of the
device were soldered to the four electrical vacuum feed throughs in a circular
plate of copper of 2 mm thickness and 2 cms. diameter by carefully
keeping the wires intact. This circular copper plate has two inlet pipes which are
used for feeding helium gas and evacuation, respectively. Next, the circular plate
was set on open portion of the cavity and araldite was applied for two times by
means of fine brush. The sealed structure was then allowed to cure at room
temperature for about 24 hours. After the curing a rotary vacuum pump is
connected to one inlet on the circular plate and the other inlet connected to a
helium gas line without the gas pressure . Evacuation is done for about 1/2
hr and the pump is disconnected for a while. Helium is purged in the cavity. Next
the gas line is closed and the vacuum line is opened again to remove the gas.
This sequential operation is done for 3 times when finally the gas is bled under
throttle under vacuum and as the pressure of helium in the cavity is seen to
stabilise at 3 Bar, the two inlets are simultaneously sealed by pinching. This way
the encapsulation was completed.
Peak - peak voltage of the encapsulated SQUID device was used to measure at
a temperature of 77 K. The SQUID Voltage was 5 mV as measured immediately
after the encapsulation. The SQUID voltage was then measured subsequently
after a regular interval of 15 days for seven months. There was no measurable
change in the SQUID voltage of 5 mV.
Peak - peak voltage of the encapsulated SQUID device of example -1 was used
to measure the SQUID voltage. The device was stored in ambient atmosphere
at room temperature for twenty months and then the measurements were taken.
The SQUID Voltage observed was 5 mV as that when the device was first
The encapsulated device was taken to measure the flux noise at a frequency of
1 Hz., immediately after encapsulation. The flux noise was 1 x10"3ct>o/V Hz. The
flux noise was measured for six months at an interval of one month. It was
observed that the value remained the same for the period under mention.
The main advantages of the invention are:
1. The device does not come in direct contact with liquid nitrogen thereby
avoiding any affect of boiling nitrogen on the device and its electrical
2. During the heating cycle to room temperature there is no moisture
condensation on the device.
3. Since helium gas is filled inside the copper enclosure with a gas tight
sealing the device can be cooled even close to liquid helium temperature.
4. The encapsulated superconductor device attains the temperature of
operation in a short interval of about 1 min.
5. The encapsulated superconductor device is capable of use for long
periods of at least 36 months with multiple thermal cycling between room
temperature and liquid nitrogen temperature.
1. A process for providing protective encapsulation of superconductor devices which comprises: spin coating a thin layer of photoresist on the top of a superconductor device (7), hardening the said spun coated photoresist layer by annealing at 80-100° C for a period of 30 minutes, characterized in that encapsulating the said coated device in a copper metal enclosure (1) filled with an inert gas at a pressure upto 3 bar, covered by a copper metal cover plate (4).
2. A process as claimed in claim 1, wherein the superconductor device is a high or low Thermal conductivity device.
3. A process as claimed in claims 1-2, wherein the photoresist layer is positive photo resist or negative photo resist material like conventional polymer is used in spin coating the superconductor device.
4. A process as claimed in claims 1-3, wherein the speed of the spinner in spin coating is in the range of 2500-3000 rpm.
5. A process as claimed in claim 1-4, wherein the inert gas used in the enclosure (1) is helium.
6. A process as claimed in claims 1-5, wherein the stability of the temperature of the encapsulated superconductor device is achieved in 1 minute.
7. A process for providing protective encapsulation of superconductor devices substantially as herein described with reference to the specification and drawings.
|Indian Patent Application Number||387/DEL/2002|
|PG Journal Number||40/2008|
|Date of Filing||28-Mar-2002|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI-110 001, INDIA.|
|PCT International Classification Number||B32B 1/00|
|PCT International Application Number||N/A|
|PCT International Filing date|