WO1992020092A1

WO1992020092A1 - Passivation coating for superconducting thin film device

Info

Publication number: WO1992020092A1
Application number: PCT/US1992/003832
Authority: WO
Inventors: William Levin Olson; Betty Florence Zuck
Original assignee: Superconductor Technologies Inc
Current assignee: Clearday Inc
Priority date: 1991-05-08
Filing date: 1992-05-08
Publication date: 1992-11-12
Anticipated expiration: 1993-11-08

Abstract

A coating for high temperature superconducting devices consists of polyimide. In the preferred embodiment, the polyimide Probamide 412 is utilized to provide a passivation coating for thallium containing or YBCO superconductors. A substantially planar local structure is formed which may be utilized as the base for further structures, such as metallizations. A method for providing polyimide passivation coating on superconductors comprises generally the steps of coating the superconductor with the polyimide, optionally patterning the polyimide with photolithographic techniques, and curing the polyimide by a baking step.

Description

DESCRIPTION

Passivation Coating For Superconducting Thin Film Device

Field of the Invention

This invention relates to the manufacture of.useful devices from high temperature superconducting thin films.

More particularly, the invention relates to providing an effective passivation coating for thallium and YBCO thin film superconductors.

Background of the Invention

The phenomenon of superconductivity was first ob¬ served by Kamerlingh Onnes in 1911. Superconductivity refers to that state of metals and alloys in which electrical resistivity is zero when the specimen is cooled to a sufficiently low temperature. The temperature at which a specimen undergoes the phase transition from a state of normal electrical resistivity to a superconduct- ing state is known as the critical temperature T_c. Until recently, the critical temperature in known superconduct¬ ing materials was relatively low, requiring expensive cooling apparatus, and often the use of liquid helium.

Starting in early 1986, with the announcement of a superconducting material having a critical temperature of 30K, (See e.g., Bednorz and Muller, Possible High Tc superconductivity in the Ba-La-Cu-0 System, Z.Phys. B- Condensed Matter 64, 189-193 (1986)) materials having successively higher transition temperatures have been announced. Currently, superconducting materials exist which have critical temperatures well in excess of the boiling point of liquid nitrogen, 77K, a relatively inexpensive and simple to use coolant.

Initially, compounds which exhibited super- conductivity at temperatures above 77K were based on the combination of alkaline earth metals and rare earth metals such as barium and yttrium in conjunction with copper, typically referred to as YBCO compounds. See, e.g., u, et al- Superconductivity at 93K in a New Mixed-Phase Y-Ba- Cu-0 Compound System at Ambient Pressure, Phys. Rev. Lett., Vol. 58, No. 9, pp. 908-910 (1987). After the YBCO compounds, compounds containing bismuth were discovered. See, e.g., Maeda, A New High-Tc Oxide Superconductor Without a Rare Earth Element, J.J. App. Phys. 37, No. 2, pp. L209-210 (1988) and Chu, et al. Superconductivity up to 114K in the Bi-Al-Ca-Br-Cu-0 Compound System Without Rare Earth Elements, Phys. Rev. Lett. 60, No. 10, pp. 941- 943 (1988).

Starting in early 1988, thallium based supercon¬ ductors have been prepared, generally where the composi¬ tions have various stoichiometries of thallium, calcium, barium, copper and oxygen. To date, the highest transition temperatures for superconductors have been observed in thallium containing compounds. A number of Tl-based superconducting phases have been identified. See, e.g., G. Koren, A. Gupta and R.J. Baseman, Appl.Phys.Lett. 54, 1920 (1989). The transition tempera¬ tures range from 90K for TlCaBa-Cu_O_χ (the "1122 phase") to 123K for Tl₂Ca₂Ba₂Cu-O_χ (the "2223 phase") . Additionally, a number of different thallium based compounds have been identified, some of which include lead. All of these compounds will be collectively referred to as thallium containing superconductors.

High temperature superconductors have been prepared in a number of forms. The earliest forms were preparation of bulk materials, which were sufficient to determine the existence of the superconducting state and phases. More recently, thin films have been prepared, which have proved useful for making practical superconducting devices. Thin films of thallium and YBCO superconductors have been formed on various substrates. More particularly as to thallium superconductors, the applicant's assignee has successfully produced thin film thallium superconductors which are ^" epitaxial to the substrate. See, e.g. Preparation of Superconducting TICaBaCu Thin Films by Chemical Deposition, Olson et al. Applied Physics Letters 55, (2), 10 July 1989, pp. 189-190, and copending applications: Superconductor Thin Layer Compositions and Methods, SN: 238,919, filed August 31, 1989; Liquid Phase Thallium Processing and Superconducting Products, SN: 308,149, filed February 8; 1989; Controlled Thallous Oxide Evaporation for Thallium Superconductor Films and Reactor Design, SN: 516,078, filed April 27, 1990; and In Situ Growth of Superconducting Films, SN: 598,134, filed October 16, 1990, all incorporated herein by reference.

It is well known in the industry that high temperature superconducting films tend to be thermodynami- cally unstable. The YBCO compounds are unstable in the presence of CO-, water, as well as a variety of common laboratory chemicals. As to thallium superconductors. Applicants have discovered that the 2122 phase of TlCaBaCuO_χ decomposes under the effect of water in a matter of minutes, resulting in a transparent, nonsuperconducting phase. Additionally, various organic materials degrade the films severely.

The use of passivation coatings is known both in the superconducting field, as well as other fields, such as semiconductor electronics. Traditionally, passivation coatings were used for scratch protection and corrosion resistance. Further, passivation coatings served to protect the device during assembly operations, which typically bring the film in contact with substances such as solders, fluxes, degreasing chemicals, encapsulation materials and the like.

Heretofore, no acceptable passivation coatings have been developed for high temperature superconductors with proven use for microwave device applications. This lack of success in producing a successful passivation coating is believed to be attributable to a number of factors: (1) the complex reaction chemistry of the high temperature superconducting materials, (2) the sometimes surprisingly high level of sensitivity of the high temperature superconducting materials to physical vapor deposition ("PVD") process conditions used to deposit passivation coatings, particularly high temperature and the existence of a plasma, (3) films' roughness which gives rise to pinholes, (4) poor adhesion, (5) lack of patternability, (6) dubious high frequency microwave properties of passivation material, and (7) temperature cycle-mechanical stability. Attempts have been made to passivate superconducting films against water corrosion by reacting the films with inorganic ions. Typically, the films are reacted with anionic salts, such as fluorides, sulfates, sulfides or iodides. These salts are reacted with the surface of the film to form a 'inert' barrier layer. While some films so treated have degraded more slowly in water, the impact of such a surface treatment on the surface loss, and accord¬ ingly its impact on the usefulness of the films for microwave applications, is unproved at this time. Attempts to use physical vapor deposition coating techniques, widely used in the semiconductor field, with these materials have proved unsatisfactory. Thin films of high temperature superconductors may have extremely rough surfaces, often with deviations of +/- 30% of the film thickness. This extreme surface roughness results in PVD coatings which are marked by pin holes or microcracks. Further, high temperature superconducting thin films sometimes lack chemical stability when subject to the high temperature and plasma environments often used to deposit PVD coatings.

Further it was the belief of those skilled in the art, prior to the time of this invention, that superconducting YBCO films were extremely reactive to organic materials. Accordingly, organic materials were widely believed to be useless for the purpose of passivation coatings for high temperature superconducting materials. Finally, thallium is a particularly toxic material, and thallium containing superconductors require safety precautions prior to their handling. Providing a passivation coating which is nontoxic to humans, and which effectively reduces the safety risks, is highly desirable.

There has been a persistent and acute need in the field for a passivation coating which is compatible with thallium containing superconductors.

Summary of the Invention A curable organic coating provides an effective passivation coating for superconductors. Particularly, a polyimide formulation has proved to be compatible with YBCO and thallium containing superconductors and to provide an effective passivation coating. In the preferred embodiment, Probamide 412, a preimidized polyimide formulation from Ciba Geigy is used for passivation. In the preferred method of application, the superconducting thin film is cleaned in organic solvents and dried. The Probamide 412 is spin coated on the film. In the preferred embodiment, a coating in the range of 1- 50 microns thick at the center of the film is easily achievable. The deposited coating is then preferably soft baked at 110°C for 15 minutes. Optionally, the coating may be lithographically patterned, the Probamide being photochemically active, being cross-linked by exposure to UV light.

It is a principal object of this invention to provide a passivation coating which improves the life and reliability of high temperatures superconducting devices. It is yet another object of this invention to provide a passivation coating which reduces the possible exposure to humans of the toxic thallium containing superconductors.

It is yet a further object of this invention to provide a dielectric coating for superconducting films having a rough surface, which results in a relatively planar coating suitable for further device processing.

It is a further object of this invention to provide a passivation coating which does not significantly degrade the properties of superconducting devices.

Description of the Drawings

Fig. 1 shows the unloaded Q as a function of device power for a thallium device, before and after coating with polyimide, at both- 5.6 GHz and 16.5 GHz at 77K. Fig. 2 shows the unloaded Q as a function of the device power for a thallium resonator having a polyimide passivation coating processed at various temperatures for various times.

Fig. 3 shows the unloaded quality factor versus device power for a YBCO resonator having a polyimide passivation coating, annealed at various times and temperatures, at 2.4 GHz.

Fig. 4 shows the quality factor versus device power for a thallium resonator which is uncoated as compared to a thallium resonator having a polyimide passivation coating which was cycled 50 times in air from room temperature to 77K.

Detailed Description of the Invention

Broadly speaking, the passivation coating process consists of the steps of (1) preparation of the superconductor wafer (2) deposition of the passivation coating on the wafer, (3) optionally patterning the passivation coating, such as by lithography and (4) optionally post-baking the coating. The passivation coating and process described herein is useful for presently known high temperature superconductors, and is described with specific reference to thallium superconductors and YBCO superconductors.

To provide a passivation coating for a superconducting thin film, the film is first cleaned. Preferably, the film is washed for 10 seconds each in VLSI grade toluene, acetone, methanol, and isopropanol. After washing, the film is dried at a sufficient temperature and time, such as 140^βC for 30 minutes, avoiding extremes of temperature which might cause film damage. Optionally the film may be dried with nitrogen.

The film is then coated with a polyimide, in the preferred embodiment Probamide 412 from Ciba Geigy. Any conventional coating technique may be used. In the preferred embodiment, spin coating is used. A Headway Photoresist Spinner has been successfully employed. Using conventional techniques, the wafer is placed on the photoresist spinner chuck and centered. Then, approximately 60% of the wafer surface is coated with polyimide. The spinner is run at 500 rpm for 5 seconds, followed by a ramp up to 3500 rpm for 40 seconds.

Utilizing this technique, a 3 micron thick coating of

Probamide 412 has been formed at the center of the film.

The film plus coating is preferably soft baked at 110°C for 15 minutes on a temperature regulated hot plate. This soft bajke eliminates the solvents from the film, and prepares the structure for lithography.

Any known lithographic technique consistent with these materials may be employed. For example, contact mask aligners may be employed to pattern the coating.

Specifically, applicants have utilized a Kasper Mask

Aligner, having a Hg lamp with a fluence of 2.1 mW/cm² at

360nm, for exposing the polyimide. In the preferred process, exposure is made for 90 seconds. The film is then placed in 50 mL of QZ3301 developer solution (gamma butyral lactone base solvent) for 1.5 minutes with constant agitation to remove the unexposed areas of polyimide. After removal from the developer, the film is dipped for 40 seconds in 50 mL of fresh developer, followed by emersion for 10 seconds in 50 mL of rinse solution, QZ3312. Immediately following^• removal of the wafer from the rinse solution, the wafer is dried in ultrafiltered dry nitrogen. The wafer is then baked again for 1.5 hours at 140^βC to remove any remaining developer or rinse solvents. Optionally, heating may be done under vacuum to aid drying. At this point, the patterned polyimide layer is now ready for further processing, such as addition of metallization.

If desired, the film may be further heat treated to cross link the polyimide coating for stability at higher process temperatures. Generally, it has been found that temperatures less than 250^βC should be used to avoid damage to the film.

The disclosed passivation process and coating materials have proved to be particularly useful for microwave devices. Significantly, the passivation process and materials do not significantly degrade the performance of microwave devices formed from the superconducting films. Data obtained from devices manufactured according to the procedure of this invention are provided below.

Fig. 1 shows the unloaded quality factor (Q) of a thallium resonator device, both before and after coating with polyimide. By comparing the results at a given frequency before and after coating with the polyimide, it can be seen that there was no substantial impact on the device performance caused by the polyimide passivation coating up to 16.6 GHz. All measurements were made at

77K. The device had a 3 micron thick polyimide coating disposed on it, utilizing the process steps disclosed herein. After coating, the film was postbaked at 110"C for 15 minutes. Fig. 2 shows the unloaded Q as a function of device power for a thallium resonator having a polyimide passivation coating which was processed at various times and temperatures. The measurements were made at 5.6 GHz at 77K. Device performance was not significantly impacted until processing at a temperature of 250^βC for 1 hour.

Fig. 3 shows the unloaded quality factor as a function of device power for a YBCO resonator having a polyimide passivation coating, subject to annealing at various times and temperatures. The YBCO films obtained for the polyimide passivation were grown by in situ laser ablation techniques on lanthanum aluminate substrates. The films were patterned using wet chemical etch techniques to form 5.6 GHz resonators. As in the case for the thallium film results of Fig. 2, there is no significant degradation of properties until processing at 250°C. Fig. 4 shows the quality factor as a function of device power for thallium resonators. The square box with a dot shows the results for a thallium resonator without a passivation coating. The black diamond shows the results for a thallium resonator coated by the process and polyimide coating of this invention, after thermally cycling the device 50 times in air from room temperature to 77K. The sample was cooled by placing it in liquid nitrogen. The sample was then removed and left in air. Significant condensation of water was present on the sample. Nevertheless,, after cycling 50 times, the resonator performance was not degraded as shown by the results graphed in Fig. 4. In marked contrast, an unpassivated film subject to such extensive temperature cycling would have severely degraded. As a further test of the protection of the superconductor provided by the coating of this invention, a film coated in accordance with this invention was subject to hydrochloric acid (HC1) . Significantly, no pinholes, cracks or delaminations occurred during temperature cycling. The film remained intact through repeated acid treatments.

Though the invention has been described with respect to specific preferred embodiments, many variations and modifications may become apparent to those skilled in the art. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.

Claims

Claims:

1. A superconducting device comprising: a substrate, a superconductor with a Q of 2,500 or greater dis- posed on the substrate, and a polyimide coating covering at least some of the superconducting material.

2. The device of claim 1 wherein the superconductor is a thallium containing superconductor.

3. The device of claim 1 wherein the superconductor is a YBCO superconductor.

4. The device of claim 1 wherein the polyimide is in the range of 1-50 microns thick.

5. The structure of claim 1 wherein the coating is approximately 3 microns thick.

6. The structure of claim 1 wherein the coating provides a substantially planar localized surface.

7. The structure of claim 1 wherein the polyimide is preimidized.

8. The structure of claim 1 wherein the polyimide is Probamide 412.

9. The structure of claim 1 wherein the supercon¬ ductor has a Q greater than or equal to 5,000.

10. The structure of claim 1 wherein the supercon- ductor has a Q in excess of 15,000.

11. An improved coating for a superconductor having low resistance to microwaves, the improvement comprising a coating of a polyimide on the superconductor.

12. The improved coating of claim 11 where the polyimide is preimidized.

13. The improved coating of claim 11 where in the preimidized polyimide is Probamide 412.

14. The improved coating of claim 11 where the superconductor is a high temperature copper oxide super- conductor.

15. The improved coating of claim 11 where in the superconductor is a YBCO superconductor.

16. The improved coating of claim 11 where in the superconductor is a thallium containing superconductor.

17. The improved coating for a superconductor of claim 11 where in the Q of a superconductor is greater than or equal to 2500.

18. A superconductive device comprising a high temperature superconductor useful for microwave devices and a passivation coating adjacent the superconductor, formed by the following process: applying a polyimide to the superconductor, and baking the combination at a temperature of 250°C or less.

19. The superconductive device formed by the process of claim 18 where in the baking is at substantially 250°C for 10 minutes.

20. The superconductive device formed by the process of claim 18 where after the bake step, the device is 1ithographed.

21. The superconductive device formed by the process of claim 18 where the lithography is done by photolithography.

22. The superconductive device formed by the process of claim 18 further including the step of a second bake after the lithography step.