CA1281247C - Methods for high tc superconductor film deposition - Google Patents

Abstract

METHODS FOR HIGH Tc SUPERCONDUCTOR FILM DEPOSITION

ABSTRACT OF THE DISCLOSURE

The present invention describes two simple methods to prepare high Tc multi element superconductor films. In the first method, metallic elements (or oxides) are deposited sequentially on substrates in a vacuum system. The film composition is controlled by controlling the amounts of the source materials introduced into the boats. In addition, one or more of the elements (or oxides) are introduced to the films by treating the films in an environment containing such elements. The other method described involves the deposition of films on substrates with arbitrary shape using a non vacuum paint on method.

Description

~124t7 ~C~GROUND O~ THE INVEN~ION

The discovery of high Tc superconductors like Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ca-Ba-Cu-O with resistance transition tem-peratures above the boiling point of liquid nitrogen (about 77 K~
has attracted the world wide attention. The high Tc superconduc-tors have many potential applications in physics, medical sciences, computers and in most of the engineering areas. For some of the applications, superconductors in a bulk form are required (for example in power transmission and magnetic levita-tion). For applications in electronic areas like in electronic devices and circuits, superconductor materials in a thin film form are needed (thickness about 1 micron). Thin superconductor films are needed for interconnects, high frequency transmission lines and for the formation of activP devices like transistors and ~unctions. Deposition of the thin films is usually carried out in a vacuum chamber in order to achieve good crystalline quality. There are several vacuum methods which can be used for such film deposition: sputtering from a single target, sputtering from multiple targets, electron beam co-evaporation, and electron beam sequential evaporation. For all of these methods, except the sputtering from a single target, a precise thickness control or deposition rate control is needed in order to achieve the re-quired film composition for the high Tc superconductivity. The required precise thickness control or deposition rate control often makes the vacuum processes to be complex and expensive. For the high Tc film deposition using the single target sputtering method in a conventional vacuum system, the film composition is often not uniform even over a relatively small substrate area of about 1 cm2. ~rom the above statement, it is clear that for the meth~ds involving more than one source (or target), very precise control over the deposition rate of the elements is needed. For the case of sequential electron beam evaporation, a precise control of thickness of individual layer is required. The precise control of the composition (atomic ratio of Y:Ba:Cu:O, for example, for the Y-Ba-Cu-O compound) is needed in order to achieve the crystalline phase for the high Tc superconductivity.
In the present invention, we describe an improved vacuum method which will allow one to deposit the high Tc superconductor thin films with controlled stoichiometry without the need of deposition rate controllers and thickness controllers. The composition of the final films is controlled simply by controlling the amounts of source materials introduced into the boats during the deposition. The thickness of the films is controlled by the total weight of the source materials and the distance between substrates and boats containing the source materials.

During the vacuum deposition of the high Tc superconductor thin films, it is also beneficial to avoid the presence of one or more of the constituent elements. For example, it is desirable to avoid Tl in the vacuum system during the deposition of the Tl-Ca-Ba-Cu-O films tTl is highly toxic and may contaminate the vacuum ~;~8~247 system and thus the environment)~ In the present invention, a method is also introduced to deposit films without containing initial Tl. The required amounts of Tl for the formation of the superconducting phase are introduced into the films by carrying out a subsequent heat treatment.

For the electronic and power applications, it is also required to deposit the high Tc superconductor films on objects with an arbitrary or irregular shape. For such objects with the arbitrary or irregular shape, it is often difficult to deposit films with the conventional vacuum methods. In the second part of the invention, we describe a paint on method for this purpose.
This paint on method does not require a vacuum system and can be conveniently used to prepare thick films of the high Tc superconductors (thickness more than 10 microns).

OBJECTS AND STATEMENT

An object of the present invention is to provide an improved vacuum evaporation method to deposit thin films of the high Tc superconductors with controlled composition and thus the superconducting properties.

Another object of the invention is to provide a method to incorporate selected elements (or oxides) in the films to achieve superconductivity during a post deposition heat treatment.

2~

Yet another object of the invention is to teach a non vacuum method for the preparation of thick films of the high Tc superconductors on regular or irregular substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a chamber arrangement with two boats to achieve the controlled deposition of the multi element Tl-Ca-Ba-Cu-O films.

Fig. 2 shows a chamber arrangement with four boats to achieve the controlled deposition of the multi element Tl-Ca-Ba-Cu-O films.

Fig. 3 shows a chamber arrangement with one boat to achieve the controlled deposition of the multi element Tl-Ca-Ba-Cu-O films.

Fig. 4 shows a chamber arrangement with three boats to achieve to the controlled deposition of the multi element Tl-Ca-Ba-Cu-O
films.

Fig. 5 shows the cross sectional view of the as-deposited multi layer films using the two boat arrangement shown in Fig. 1.

Fig. 6 shows the cross sectional view of the as-deposited multi layer films using the four boat arrangement shown in Fig. 2.

Fig. 7 shows the furnace arrangement for the heat treatment in an B

l~a~2~

environment containing 2 and Tl or T102.

Fig. 8 illustrates the non vacuum paint on method to produce superconducting thick films on substrates wlth an arbitrary or irregular shape.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to the production of high Tc superconductor thin films and thick films. For the thin superconducting films, Figs. 1, 2, 3 and 4 illustrate several vacuum chamber arrangements to achieve sequential deposition of the films. The source materials, either elements (Tl, Ca, Ba, Cu for example) or oxides (T102, CaO, BaO and CuO), are weighted and put in the boats. One or more boats are adopted to achieve the sequential deposition. In Fig. 1, a system with two boats is shown where Tl, Ca, Ba are put in a Ta boat (1) and Cu in a MO
boat (2). The films are deposited on supportive substrates (3).
The substrates are mounted on a substrate holder (4). The deposition is carried out in a vacuum environment enclosed by a vacuum chamber (5) and pumped by a pumping unit. Substrate heating may be applied during the multi layer film deposition process. Fig. 2 shows a system with four separate boats which can be heated separately to achieve the sequential multi-layer. Boat (2) is for Cu, (6) for Ba, (7) for Ca and (8) for Tl. A single boat system is shown in Fig. 3 where the boat (9) is used to 2~7 evaporate Tl, Ca, Ba and Cu. A system with 3 boats [(10), (11) and (12)] is shown in Fig. 4. The actual elemental sequence of the multilayer film is determined by the sequence of boat heating during the evaporation for the four boat arrangement. For the two boat arrangement, it is determined by the relative melting points of the elements. In all cases, the element Cu is evaporated last in order to cover and to protect the more active elements like Ba and Ca. The deposition is repeated several times on the same set of substrates to obtain the desired thickness of about 1 micron (the actual thickness is determined by the total amounts evaporated). The cross sectional views of the as-deposited films using the two-hoat arrangement is shown in Fig. 5 and is shown in Fig. 6 for the ones evaporated using the four boat arrangement in Fig. 2. As stated before, for the films deposited using the two-boat arrangement, the elemental sequence is determined by the boat heating sequence and the elemental melting points in one boat (1). Therefore, if the process described in Example 1 is carried out, then multi-layer films with the cross section shown in Fig. 5 will be produced. In Fig. 5, the Tl~Ca+Ba layers (13) are not completely uniform due to the difference of the individual vapollr pressure. The material is rich in Tl near the substrate (3) and is rich in Ca away from the substrate (3). For the four boat arrangement, the elemental sequence is determined purely by the heating sequence of the elements. For example, one can first evaporate a Cu layer (14) followed by a Tl layer (15), a Ba layer (16) and a Ca layer (17). This sequence is repeated to obtain the proper thickness of about 1 micron. It is also noted that the actual deposition sequence for Tl, Ca and Ba is not important and may be varied during the experiments. For this method, it is therefore clear that the film stoichiometry is controlled simply by controlling the weights of the materials loaded into the boats during each evaporation. During the evaporation, a fixed amount (or fixed pressure) of oxygen may be allowed in the deposition chamber. After each evaporation, new materials are loaded into the boats. The loading of the source materials is achieved by opening the chamber and then adding the materials directly into the boat or by adding materials with a jig located inside the vacuum chamber without breaking the vacuum. It is important to note that the geometry of the boats should be similar in order to achieve uniform deposition.

After the multi layer film deposition, a high temperature treatment is made on the films in a quartz chamber ((18), Fig~ 7) containing 2 This high temperature treatment is needed for interdiffusion of the elements and thus the formation of the compounds. The presence of 2 in the chamber is needed in order to supply sufficient oxygen to the films (19). It is also noted that a quartz or alumina container (20) with Tl or TlO2 (21) in it is placed inside the heating chamber near the films for the Tl-Ca-Ba-Cu-O compound. During the treatment process, a Tl or TlO2 vapour pressure will be maintained in order to avoid the loss of Tl during the process. After the high temperature treatment, a second treatment at a lower temperature in the same chamber is carried out. The second treatment is needed to adjust 2~7 the content of oxygen and thallium in the final thin films.

For the production of high Tc superconductor thin films containing volatile and toxic elements like Tl, it is beneficial not to include it inside the vacuum chamber during the deposition to minimize the environmental contamination. The film preparation is carried out by first depositing Ca-Ba-Cu-O layers on supportive substrates. After the deposition, the films are treated in the chamber containing both oxygen and thallium as shown in Fig. 7. Longer treatment time should be allowed in order to let sufficient amounts of Tl or T102 to diffuse into the films to form the superconducting compounds.

Alternate method to prepare the Ca-Ba-Cu-O films is sputtering. In this method, a target containing Ca, Ba, Cu and O
is prepared by mixing and pressing weighted amounts of CaC03, BaC03 and CuO. The pressed material is heated at about 900C for 2 hours and then re-ground to form new powder. The new powder is pressed and heated again to forrn the final target. The Ca-Ba-Cu-O
films are sputt:ered on substrates. After the sputtering, the films are heat treated in the furnace containing both oxygen and thallium. Both the oxygen and thallium will diffuse into the films during the treatment process to form the compounds. This process again allows one to obtain high Tc superconductor films.

In many electrical and electronic applications, it is required to form superconductor films on objects with an L2~7 irregular shape. For such objects, the conventional vacuum deposition methods may not be applicable for the film deposition.
In the present invention, a simple paint on method is described.
In this method, oxide powder of the source materials (TlO2, CaO, BaO and CuO for example) are prepared and weighted. The materials are then mixed with a liquid to form a paste. The process is illustrated in Fig~ 8. The paste (22) is applied using a brush (23) or other tool onto the surface of the substrate (24). After the paste application, the film is heated at a relatively low temperature to remove the liquid in the film. The film is then heated in an environment containing oxyyen and/or other elements like thallium. The compounds are formed during the heat treatment process. The thickness of the thick films is about 10 microns or more.

Elemental Tl, Ca, ~a and Cu are first weighted to the desired atomic ratio, for example Tl:Ca:Ba:Cu = 2:2:2:3 or Tl:Ca:Ba:Cu = 2:2:2:4. The actual amounts are: Tl = 614 mg, Ca =
120.4 mg, Ba = 412.62 mg and Cu = 286.35 mg. Tl, Ca and Ba are divided equally into four parts for four separate evaporations while Cu is divided into five parts for five evaporations. The additional Cu layer is intended to serve as the "buffer" layer to the substrate. The evaporation process is described as follows.
One part of Cu is introduced into a Mo boat in the vacuum system.
Substrates of ZrO2, MgO or ZrO2 coated Si substrates are then B

mounted on a substrate holder and loaded into the vacuum system as shown in Fig. 1. The surfaces of the substrates to be deposited should face the source boats. The vacuum chamber is assembled and evacuated by the pumping unit. After the pressure has reached a value below about 10-5 torr, power is applied to the Mo boat to evaporate the first Cu layer. After the first Cu layer evaporation, the chamber is open and one part of Cu is introduced into the same Mo boat and one part of Tl, Ca, and Ba is introduced into a Ta boat (length of the heating zone about 5 cm). The Ta boat should be located near the Mo boat to achieve uniform deposition of all the elements. After proper evacuation of the chamber to a pressure below about 10~5 torr, the Ta boat is first heated to evaporate sequentially Tl, Ba and Ca. Care should be taken to heat the Ta boat gradually in order to minimize the reaction between elements in the Ta boat. The reaction often results in heat which will produce vigorous and non-uniform evaporation. After all three elements have evaporated, the power to the Ta boat is turned off and the power to the Mo boat is turned on to evaporate Cu. The second Cu layer will cover the combined Tl, Ca and Ba layer on the substrates and on the bell jar and parts inside the vacuum jar. Therefore, the active elemental layers are protected and the system contamination is minimized. Once the second Cu has been evaporated completely, the system is allowed to cool for about 10 minutes before opening the chamber to re-load the source materials. Same amounts of Tl, Ca, Ba are introduced into the Ta boat and Cu to the Mo boat for the second evaporation. The above `~'^''~ 1 1 ~8~7 evaporation is repeated for four times. It is noted again that after each of the evaporation, the film is covered with Cu. This top copper layer will minimize unwanted oxidation during the chamber opening operation. In addition, environmental contamination by other active elements like Tl, Ba and Ca also will be minimized.

After the sequential evaporation, the films are removed from the evaporation unit and stored in an evacuated container or in a container with inert atmosphere. Samples are then selected for heat treatment in a horizontal resistively heated furnace with a quartz chamber. The furnace is first set at 830C. The samples and a quartz boat with a piece of Tl in it are put in the cool zone of the furnace tube. Oxygen valve is now open to allow oxygen to flow and to purge the furnace tube. After this, the quartz tube with the samples and Tl piece are pushed in two steps to the central zone of the furnace and treated for about 5 minutes. After this short treatment, the quartz tube with the samples is pulled to the cool zone and the furnace temperature reduced to 730C. A new Tl piece is put in the quartz boat and the samples are treated at the low temperature for a period of 1-3 hours. Finally the samples are removed from the furnace and cooled. The superconducting thin films are now formed on the substrates. It is noted that during the above described heat treatment processes, the quartz boat with the Tl piece is placed inside the quartz reaction tube in the up stream position. This position is beneficial in maintaining a steady Tl or TlOX vapour ~;~8~47 pressure in the sample area.

In this example, we would like to describe the preparation of Tl-Ca-Ba-Cu-O superconducting films from materials without initial Tl. The system used is shown in Fig. 1 where one boat (Mo) is used for Cu and the other (Ta) for Ca and Ba. Pure elemental materials with the following amounts are prepared: Ca =
120.4 mg, Ba = 412.62 mg and Cu = 286.35 mg. The atomic ratio is : Ca:Ba;Cu = 2:2:3 or 2:2:4. Again Ca and Ba are divided equally into four parts and Cu is divided into five parts. One part of Cu in loaded into the Mo boat and the substrates also mounted on the substrate holder. After evacuation to the desired pressure, Cu is evaporated on the substrates. The chamber is now open and one part of Cu is loaded into the Mo boat and Ca and Ba are loaded into the Ta boat. After evacuation, the power to the Ta boat is turned on to deposit Ba and then Ca. After this deposition, Cu is evaporated to cover the entire surface. The above sequential evaporation is repeated for four times to give a total thickness of about 1 micron. It is noted that the film is always covered with Cu after each evaporation and therefore the active elements are protected by the Cu layer. After the evaporation, the films are first treated at 830C in the quartz chamber described in EXAMPLE 1 for about 5 minutes. A second treatment at about 730C
is then carried out with a piece of Tl placed in the quartz boat located in the up stream posikion for a period of about 5-10 B

hours. During the low temperature treatment, it is beneficial to add one or two new pieces of Tl into the quartz boat to maintain the steady Tl or TlOX vapour pressure. After the complete treatment, Tl atoms diffuse into the films to form the superconducting compounds.

This example describes the formation of Tl-Ca-Ba-Cu-O films from rf sputtered Ca-Ba-Cu-O layer samples. High purity powder of CaC03, BaCO3, and CuO are weighted to the following atomic ratio:
Ca:Ba:Cu = 2:2:3. Total weight of the materials is about 40 gm.
The powder is thoroughly mixed and then pressed to form disks with a diameter of 2.5 cm and a thickness of 0.5 cm. The disks are then put into a box furnace set at 900C for a two-hour heat treatment~ After the treatment in room atmosphere, the disks are cooled and then re-ground to form new powder. The new powder is pressed again in a jig to form a disk with a diameter of 5 cm and a thickness of about 2 mm. After a second heat treatment at 900C
for 2 hours, the disk is cooled and then used as the sputtering target. Deposition of Ca-Ba-Cu-O films is carried out in a second vacuum unit with a rf sputtering gun. The distance between the target and the substrates (ZrO2, MgO or ZrO2 coated Si) is about 3 cm and the samples are mounted in position about 2.5 cm from the normal projection of the target center on a sample holder.
The sample holder is water cooled during the deposition. The incident rf power (frequency 13.5 MHz) is set at about 60 watts and an argon + 5~ oxygen mixture is used as the sputtering gas.
The sputtering is carried out for a period of about 5 hours to obtain films with a thickness of about 1 micron~ Smooth and amorphous films are usually obtained using the above process.
After the deposition, the Ca-Ba-Cu-O films are treated in the quartz chamber described in EXAMPLE 2 to diffuse Tl and 2-Superconducting thin films finally can be obtained from the sputtered Ca-Ba-Cu-O films without containing the Tl.

While the invention has been described with reference to Tl-Ca-Ba-Cu-O high Tc superconducting compounds, preparation of other high Tc superconductors like Bi-Sr-Ca-Cu-O and Y-Ba-Cu-O
may well be achieved by the same methods. Therefore, the present invention should not be considered to limit to only one type of high Tc superconductors. In addition, the heat treatment times and the temperature required to formed the high Tc compounds many change with the treatment system. The amounts of source materials required for the sequential evaporation also will vary with the thickness requirement and the distance between the boat and the substrates. The sequential evaporation can be achieved by using a single long boat, two boats, three boats or even four boats (as shown in Figs. 1-4). Finally, it is worthwhile to point out that the present multi-layer sequential evaporation method and the paint-on method are very simple for the production of the high Tc thin films.

Claims (5)

1. A vacuum process for fabricating of high Tc superconducting thin films, said method comprising the steps of:

- sequential depositing of layers of elements or oxides on a supportive substrate in a vacuum chamber;

- controlling the chemical composition of said films by controlling the amounts of individual source materials introduced into the boats.

- controlling the film thickness by controlling the total amounts of the source materials and the distance between the substrates and the boats;

- heat treating said films in an environment containing oxygen to create the superconducting phase.

2. A process as defined in claim 1 wherein said films are chosen from the following combinations: Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ca-Ba-Cu-O.

3. A process as defined in claim 1 further comprising a heat treatment step in an environment containing O2 and Tl or Tl0x to maintain Tl in the Tl-Ca-Ba-Cu-O films.

4. A process as defined in claim 1 wherein said films are treated in an environment containing O2 and Tl or TlO2 to diffuse the Tl into the films without initial Tl to form the superconducting Tl-Ca-Ba-Cu-O compounds.

5. A process as defined in claim 1 wherein said heat treating process is carried out at temperatures in a range from 600 to 1000°C.