WO1992019786A1

WO1992019786A1 - Fabrication process for low loss metallizations on superconducting thin film devices

Info

Publication number: WO1992019786A1
Application number: PCT/US1992/003827
Authority: WO
Inventors: William Levin Olson; Betty Florence Zuck; David Lee Skoglund; Michael James Scharen
Original assignee: Superconductor Technologies, Inc.
Priority date: 1991-05-08
Filing date: 1992-05-08
Publication date: 1992-11-12

Abstract

A method effectively produces patterned metal contacts (34) on a substrate (30) covered with a high temperature superconducting film (32) having good mechanical and electrical properties, especially adhesion and contact resistance, without degrading the microwave properties of the film. In the preferred embodiment, the superconducting film surface is cleaned by chemical etching, preferably with a dilute solution of bromine in methanol. Metal, preferably gold is deposited on the superconductor, preferably by sputtering. After the metal is deposited, it may be optionally patterned utilizing conventional lithographic techniques. Effective metallizations have been made on thallium-containing and YBCO superconductors. Devices result having good adhesion and contact resistance less than or equal to 5 x 10?-6 ohm cm2¿ at 77K. Subsequent annealing reduces contact resistance to less than 1 x 10?-8 ohm cm2¿.

Description

Fabrication Process for Low Loss Metallizations on ffuperconductincr Thin Film Devices

Field of the Invention

This invention relates to effective methods for producing articles from superconducting thallium thin films. More particularly, it relates to methods for providing effective bondable metallization contacts having low loss to superconductive thin films useful for microwave devices.

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 114 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 thallium-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 temperatures range from 90K for TlCaBa₂Cu₂O_χ (the "1122 phase") to 123K for Tl₂Ca₂Ba₂Cu₃O_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 the 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 TlCaBaCu Thin Films by Chemical Deposition, Olson et al, Applied Physics Letters 55, (2), 10 July 1989, pp. 189-190, incorporated herein by reference. Techniques for fabrication of thin film thallium superconductors are described in 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.

In order to make useful electrical devices from the thin film thallium superconductors, it is often necessary to provide one or more electrical contacts to the superconductor. It is particularly desirable to provide a contact with a highly conductive, nonreactive metal such as gold, platinum or silver.

Previously, conventional lift-off techniques were used for formation of gold contacts on superconductors. More particularly, the surface of the superconductor was coated with conventional photoresist, which was then lithographically patterned, exposing the film where the contact metal was to be deposited. The exposed areas of the photoresist were then removed, leaving a mask pattern. Gold was then deposited over both the removed and unremoved portions of the photoresist. Areas of the gold film with the photoresist below it were then removed to accomplish the 'lift off of the gold. Gold remained on the superconducting film in those areas were the photoresist had been removed before deposition to expose the superconductor.

The lift-off technique has produced metallization contacts with mechanical and electrical properties which are less than optimal. From a mechanical standpoint, the amount of adhesion between the gold and superconducting film has been very poor, to at best inconsistent. From an electrical standpoint, the contact resistance has been high, with measurements on the order of magnitude greater than 5 x 10^"4 ohm cm². These inconsistent mechanical and electrical properties are believed to be affected by the exact surface chemistry of the superconducting film surface. Among the various factors which affect the surface are (1) the existence of residual photoresist on the relatively rough high temperature superconductor surface, which can have surface height variations of up to +/- 30% of the film thickness, (2) reactions of the surface with the development chemicals used for the lift off technique and/or reactions of the surface with air, water or cleaning solvents, and (3) the existence of secondary phases of superconductor, particularly in thallium superconductors, which are excluded to the surface of the film during the growth process. Attempts have been made to improve the surface of the films to promote better mechanical and electrical properties. For example, in situ sputter etching has been used to remove photoresist or clean damaged film surfaces. This technique is known to have the negative, feature of degrading the high temperature superconductor film surface by decomposing it to insulating phases.

Other methods of removing surface contamination have been attempted. In particular, efforts have been made to use a positive photoresist compatible chemical etch, such as is common in semiconductor processing. Such an approach would allow cleaning the exposed film surfaces prior to metallization, as well as after photoresist deposition and lithography. However, such chemical etchants are not currently available and efforts to develop this type of etch has proved unsuccessful. Generally, etches which were compatible with the supercon¬ ductor were not compatible with the positive photoresist. and etches compatible with the positive photoresist were not compatible with the superconductor.

Summary of the Invention

A method is provided for effectively producing patterned metal contacts on high temperature superconducting thin films having good mechanical and electrical properties, especially adhesion and contact resistance, without degrading the microwave properties of the film. The surface of the superconducting film is chemically etched, preferably with a dilute solution of bromine in methanol. The metal layer, preferably gold, is deposited on the film, such as by sputtering. If desired, a shadow mask may be used to limit the area of the film on which the gold is deposited. The deposited metal is then patterned with a conventional photolithographic technique using positive photoresist, then utilizing a gold etch on exposed areas to remove undesired gold. Next, the superconducting film is etched, if necessary, preferably with a dilute solution of HCL. Finally, the remaining photoresist is removed.

Optionally, additional photolithographic processing may be utilized by recoating the wafer and exposing the photoresist to the desired pattern. Utilizing this technique, the superconductor may be etched in a pattern different from that utilized for the gold layer.

A principal object of this invention is to provide a method for providing metal contacts on high temperature superconducting films having good adhesion, that is, bondability, and low contact resistance. Another object of this invention is to provide a method for producing patterned gold on thallium or YBCO superconducting films without degrading the microwave properties of the film.

Yet another object of this invention is to provide a process for placing metal contacts on a clean supercon- ducting surface prior to exposure to a photoresist or developer.

Yet another object of this invention is to provide a process for, surface treatment of high temperature superconducting thallium or YBCO thin films without the damaging effects of other surface preparation techniques, such as in situ sputtering or ion milling.

Yet another object of this invention is to provide a method in which the gold is lithographically defined after it is deposited on the superconducting film without degrading the film's microwave properties.

Yet another object of this invention is that annealing need not be used to achieve acceptable properties.

Brief Description of the Drawings

Fig. 1A-F shows a cross-sectional view of the single mask process.

Fig. 2A-D shows a plan view of the single mask process. Fig. 3A-D shows a cross-sectional view of a multiple mask process.

Fig. 4 shows of micrograph of a superconducting resonator with a gold line patterned over the resonating material. Fig. 5 shows the unloaded Q versus device power for the device of Fig. 4.

Detailed Description of the Invention

The basic process of this invention may be understood by considering a single mask process, and then extending to multiple mask processes. The single mask process is shown in Figs. 1 and 2.

It is desirable to begin with a clean superconducting film. In the preferred embodiment, the surface of the film is first rinsed for 10 seconds in each of toluene, acetone, methanol and isopropanol (preferably VLSI grade) . After spraying dry with nitrogen, the wafer is dried at a suitable temperature and time. Preferably, the wafer is baked at 140°C for 30 minutes.

Next, the film is chemically etched with a dilute solution of bromine and methanol (or other suitable solvent) . Exposure of the film for 15 seconds with constant agitation in a 0.5% (by volume) solution of Br₂ in methanol is typically sufficient to provide a clean surface. This cleaning has been found to selectively remove surface segregated secondary phases from thallium films to produce a clean sμperconducting film at its surface. (Vasquez, R.P. and Olson, W. L. , "X-ray Photoelectron Spectroscopy Study of Chemically Etched Thallium Thin Film Surfaces", submitted to Physica C to be published in June, 1991.) Further, this cleaning removes unwanted surface carbonates and hydroxides from the film surface due to the reaction of the film with air, water and/or organic solvents. After removal from the solution, the film is rinsed quickly with dry VLSI grade methanol. Immediately after cleaning, the metal is deposited on the superconducting film. Preferred metals are gold, platinum or silver. Any technique known to the art may be utilized for this purpose, for example, sputtering or evaporation. Optimum adhesion has been shown using sputtering. Typically, the gold is deposited in a thickness from 0.4-1 micron, though thick gold films can be prepared utilizing this technique.

The following sputtering procedure has been used with success on superconducting thallium and YBCO films. First, the now cleaned wafer is placed in an Edwards sputtering machine. The machine is then evacuated, to a pressure of approximately 2 x 10^'6 torr. Next, argon gas is placed in the sputtering machine to give a 2-4 x 10^"3 torr background pressure. For gold, sputtering has been successfully achieved at 500 watts and 0.5 amps. To obtain a film of approximately 7000A given the settings as described, sputtering must be done for approximately 60 seconds. When finished, the sputtering power supply is turned off and the chamber vented with argon gas to atmospheric pressure. Finally, the wafer is removed.

Fig. IA shows a substrate 10 having a superconducting thin film 12 disposed on its surface. Fig. IB shows metal 14 (such as gold) deposited on the superconductor 12 having been defined to a localized area. If desired, the metal 14 coating may be limited through use of a shadow mask (not shown) to the desired areas of the film. Positioning at this step is not critical if subsequent photolithographic definition of the metal is to be used. Fig. 2A shows a plan view of the film 12 with deposited gold regions 14 on the surface. Alternatively, the entire film surface may be coated with metal and the excess material removed as described below.

To obtain better definition, the gold may optionally be lithographically patterned. In this embodiment, the surface of the film 12 coated with gold 14 is coated with positive photoresist (Shipley 1713, 1375). Conventional photoresist coating techniques such as spin coating may be used. For thallium films, Shipley 1713 positive photoresist has used successfully. In the preferred embodiment, the Shipley 1713 photoresist is spin coated at 5000 rpm for 30 seconds, followed by a hot plate bake for 60 seconds at 95°C. The photoresist is then exposed to a Hg lamp to produce the desired pattern, an exposure of 24 seconds to 350 nm light of a fluence of 2.4 mW/cm² has proved sufficient. The resist is then developed for 60 seconds in MF319. The developed film is then rinsed in fresh deionized water and nitrogen dried. The film is then barrel ashed for 2 minutes at 2GHz at high power with an oxygen flow rate of 2.5.

Fig. 1C shows the patterned photoresist 16 disposed upon top of the metal 14, to provide protection from subsequent etching of the regions underneath the photoresist 16. Fig. 2B correspondingly shows the plan view of the photoresist 16 on the metal regions 14, plus a photoresist region 16 disposed directly on the film 12.

When the metal 14 is gold, the gold is then etched, preferably with an iodine gold/silver etch solution. Etching in hot TFS 20 silver etchant (from Transene Q, Rolling, Massachusetts) at 50°C, for 15-20 seconds has proved sufficient to remove sputtered gold films up to 1 micron thick. After etching, the wafer is rinsed in deionized water. If the etching is incomplete, further etching may be done, preferably for 10 seconds followed by rechecking of the film. When sufficiently etched, the wafer is nitrogen dried. Finally, the photoresist is stripped utilizing a solvent such as acetone. After a nitrogen dry, the wafer is barrel ashed for 2 minutes at high power. This final barrel ashing serves to break down the photoresist residue produced by contact with the gold etchant. Fig. IF and 2D show the final resulting structure of a superconductor 12 having a metallization contact 14 disposed thereon. Fig. ID shows a cross-sectional view after the etching of the metal region 14. Fig. 2C shows a plan view after etching, with the photoresist 16 being identified, as it is the uppermost layer at this stage of the process.

The superconducting film may then be etched in areas left unprotected by the photoresist. An effective etchant for thallium superconducting films includes dilute (1:150) solution of HCL, for 10 to 20 seconds. Figs. IE and 2D show the structure after the superconductor 12 has been etched. When etching is completed, the photoresist may be removed. Fig. IF shows the structure after removal of the photoresist. Acetone is a suitable solvent for the Shipley photoresist. The part may then be optionally dried, for example with hepafiltered nitrogen. This technique may be expanded to include further etching steps. One variation of this process is shown in Fig. 3A-D, where a two step lithographic process is used. As described in detail above, the superconducting film surface is cleaned, preferably with a bromine etch. (Gold is then deposited to a desired thickness, for example to 7000A.) The first lithographic etch process then occurs, with the photoresist being deposited and developed to provide protection to the metal which is to remain. Fig. 3A is a cross-sectional view at this step, with a substrate 30, superconducting film 32, gold layer 34 and patterned photoresist 36. The gold is then etched with the gold etchant. Fig. 3B shows the gold 34 etched as defined by the photomask 36. After etching, the first photoresist mask is removed, with a suitable solvent such as acetone.

In a two step lithographic process, the etched device is preferably recoated with photoresist, covering both the gold and the film areas. The desired pattern is then generated lithographically. If the superconductor is to be etched, a dilute solution of HCL is used as described above. Fig. 3C shows a cross-sectional view at this step of the process, with the photoresist 38 disposed over the thallium film 32 not to etched, as well as over the gold 34.

Fig. 3D shows a completed device, having a different patterning of the gold than the superconductor. It will be appreciated by those skilled in the art that the invention disclosure herein may be utilized with any number of processing steps. Particularly, deposition of the gold on the chemically cleaned thallium and YBCO film has been successful in improving mechanical adhesion and reducing electrical contact resistance. Once good mechanical and electrical contact is achieved between the metal and the superconductor, additional processing in subsequent steps may be used at the manufacturers discretion. Experimental Results

To test the impact of this process on the properties of superconducting films, the process was carried out on a superconducting device with the performance properties compared prior to the process and after the process. A superconducting high "Q" microstrip resonator operating at

2.3 GHz was tested and found to have a low power unloaded

Q exceeding 16,000. The resonator was then coated with

2000A thick layer of sputtered gold. The gold was patterned using the above described process to generate a

6 mil. wide line down the center of the resonator. (Fig.

4 shows an optical micrograph of a 2.3 GHz microstrip resonator with a 12 mil. wide gold line deposited directly on the resonator element.) The resonator was measured. It was then retested after the remaining gold was removed by dipping in the commercial Ag etchant solution. The resonator performance for the gold free film is shown by curve number 2 in Fig. 5.

This entire sequence was then repeated again on the device to produce a 12 mil. wide line down the center of the resonator. The final test results for the 2.3 GHz resonator after 4 exposures to the gold etch solution are given in curve number 3 in Fig. 5. Virtually no degradation in the unloaded resonator Q was observed. These results demonstrate the compatibility of the thallium thin film with the gold etch solution. Similar results have been obtained with YBCO films.

The above described process was utilized to prepare gold contact pads on a thallium 2122 superconducting film. Contacts were made by 1 mil. gold wires, which were bonded to the gold pad by thermosonic wedge bonding. The typical contact resistance for bond pads deposited using this procedure is less than 5 x 10^"6 ohm cm² at 77K. Annealing of the films, for example at 450°C for 5 minutes, can reduce the contact resistance to less than 1 x 10^"8 ohm cm². In comparison, the contact resistance for the lift off technique is approximately 500 times greater. Further, repeated bond pull tests on thallium films showed excellent adhesion for 1 miL wire, having a pull strength of 4-8 grams.

The process of this invention was utilized to prepared gold contact pads on YBCO superconducting films. Contacts were made via 1 mil. gold wires, 20 of which were thermosonically attached to the gold pads. In all cases, a pull force of greater than 4 grams was necessary, which is well above the military specification of 3 grams for 1 mil. diameter wire. Specifically, the bond pull results for one film were 5.1 grams with a standard deviation of 0.9 grams, and for a second film of 6.4 grams, with a standard deviation of 1.7 grams. The second film had a contact resistance of 3.5 x 10^"6 ohm cm². A third film had a contact resistance of 3.0 x 10^"7 ohm cm². This process was used to create a 50 ohm microstrip throughline structure consisting of a number of HTSC sections connected by gold bridges through 125 ohmic contacts. The device performed well suggesting a high process yield. The through loss of the transmission line was around ldB at 10 GHz: an average loss of less than O.OldB/contact. This data demonstrates that this process is viable for high performance passive microwave devices. These results confirm that this technique provides excellent mechanical and electrical metallizations for superconducting materials.

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 method for providing metal contacts to superconducting material comprising the steps of: depositing the metal on the superconducting material, and patterning the metal.

2. The method of claim 1 further comprising the step of cleaning the superconductor prior to the deposition step.

3. The method of claim 2 wherein the superconductor is cleaned by chemical etching.

4. The method of claim 3 wherein the chemical etching is done with a solution of bromine and primary or secondary alcohol.

5. The method of claim 1 wherein the deposited metal is gold, platinum, or silver.

6. The method of claim 1 wherein the metal is deposited by sputtering or evaporation.

7. The method of claim 1 wherein the metal is selectively deposited on less than all areas of the superconductor.

8. The method of claim 7 wherein the metal is deposited on less than all areas of the superconductor through use of a shadow mask.

9. The method of claim 1 wherein the metal is photolithographically patterned using a chemical etch.

10. The method of claim 1 wherein the metal is etched after the photolithographic patterning step.

11. A superconducting device comprising: a substrate, a superconducting film deposited on the substrate, and a metal layer deposited directly upon the superconducting film, wherein the contact resistance between the superconducting and the metal layer is less than or equal to 5 x 10^"6 ohm cm² at 77K.

12. The device of claim 11 wherein the adhesion between the metal contact and the film is adequate for gold wire bonding.

13. The superconducting device of claim 11 wherein the superconducting film is a thallium containing superconductor.

14. The superconducting device of claim 11 wherein the superconducting film is a YBCO film.

15. A superconducting device comprising: a substrate, a superconducting film disposed above the substrate, and a metal contact disposed above the superconducting film, wherein the pull strength for a 1 mil. gold wire attached to the metal contact is greater than 3 grams.

16. The superconducting device of claim 15 wherein the superconducting film is a thallium containing superconductor.

17. The superconducting device of claim 15 wherein the superconducting film is a YBCO film.