Title of Invention | "A PROCESS FOR PROVIDING PROTECTIVE ENCAPSULATION OF SUPERCONDUCTOR DEVICES" |
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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 superconductor device. 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 less expensive 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 characteristics. 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 junctions. 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 device. 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 subsequent operation. 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 3 bar. The following examples are given by way of illustration only and should not be construed to limit the scope of invention. Example-1 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. Example-2 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. Example-3 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 encapsulated. Example-4 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 contacts. 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. We Claim: 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. |
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0387-DEL-2004-Abstract-(19-05-2008).pdf
0387-DEL-2004-Claims-(19-05-2008).pdf
0387-DEL-2004-Correspondence-Others-(19-05-2008).pdf
0387-DEL-2004-Form-3-(19-05-2008).pdf
387-DEL-2002-Claims-(10-09-2008).pdf
387-DEL-2002-Claims-(19-08-2008).pdf
387-del-2002-complete specification (granded)-(10-09-2008).pdf
387-DEL-2002-Correspondence-Others-(10-09-2008).pdf
387-DEL-2002-Correspondence-Others-(19-08-2008).pdf
387-del-2002-correspondence-others.pdf
387-del-2002-correspondence-po.pdf
387-DEL-2002-Description (Complete)-(10-09-2008).pdf
387-del-2002-description (complete)-19-05-2008.pdf
387-del-2002-description (complete)-19-08-2008.pdf
387-del-2002-description (complete).pdf
387-DEL-2002-Form-2-(19-08-2008).pdf
Patent Number | 223616 | |||||||||
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Indian Patent Application Number | 387/DEL/2002 | |||||||||
PG Journal Number | 40/2008 | |||||||||
Publication Date | 03-Oct-2008 | |||||||||
Grant Date | 18-Sep-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. | |||||||||
Inventors:
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PCT International Classification Number | B32B 1/00 | |||||||||
PCT International Application Number | N/A | |||||||||
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PCT Conventions:
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