WO1998050965A1

WO1998050965A1 - Method for preparation of substrates for thin film superconductors and improved devices incorporating substrates

Info

Publication number: WO1998050965A1
Application number: PCT/US1998/007579
Authority: WO
Inventors: Michael M. Eddy; Betty F. Zuck; Gregory G. Firpo
Original assignee: Superconductor Technologies, Inc.
Priority date: 1997-05-02
Filing date: 1998-04-15
Publication date: 1998-11-12
Also published as: WO1998050965A9

Abstract

Improved reproducibility of fabrication of superconducting thin film devices on MgO substrates is accomplished through use of the inventive procedure. As a first step of the process, the surface of the MgO is removed. Preferably, the surface is removed through argon plasma bombardment, whether with low energy or high energy ions. Most preferably, ion milling is utilized, with superior results being obtained by milling at 90° to the surface of the substrate. The second step of the invention is to anneal the MgO substrate. This effects recrystallization of the substrate. In the preferred embodiment, the substrate is annealed at approximately 1050 °C in 17 % dry oxygen, 83 % dry nitrogen for 2.5 hours. Lower temperatures, such as 950 °C, may be utilized, though the anneal times, e.g., three hours or greater, impacts upon the commercial viability of the process. Higher temperatures, e.g., 1150 °C, permit steps in the surface to become unacceptably large, and often permit second phases to migrate to the surface of the substrate. Varying other factors, such as the oxygen partial pressure, change the optimal anneal temperature. Structurally, the resultant surface manifests steps or terraces, which comprise exposed peripheral edges of atomic planes within the substrate. Optionally, additional cleanings steps are performed. Devices fabricated from filling formed with the inventive method result in higher quality and more uniform devices.

Description

DESCRIPTION

METHOD FOR PREPARATION OF SUBSTRATES FOR

THIN FILM SUPERCONDUCTORS AND IMPROVED

DEVICES INCORPORATING SUBSTRATES

Field of the Invention

This invention relates to methods for the preparation of substrates upon which thin film high temperature superconducting structures are formed. More particularly, this invention relates to highly advantageous methods for the preparation of magnesium oxide (MgO) substrates upon which high temperature superconducting films are formed.

Background of the Invention

Various high temperature superconducting materials have been discovered. Generally, these discoveries began in 1986 with the discovery of BaLaCuO systems, (see "Possible High T_c Superconducting in the Ba-La-Cu-O System", Bednorz and

Mueller, V. Phys. B-Condensed Matter No. 64, 189-193 (1986)), which was followed by the discovery of the YBCO materials, (see Wu et al., "Superconductivity at 93K in a New Mixed-Phase YbaCuO Compound System At Ambient Pressure", Physical Review Letters, Vol. 58, No. 9, pp 908-910 (1986)), followed by the discovery of the bismuth materials, (see Maeda et al., Japanese Journal of Applied Physics, Vol. 27, No.

2, pp L209-210 (1988) and Chu et al., "Superconductivity up to Rare-Earth Elements", Physical Review Letters, Vol. 60, No. 10, pp 941-943 (1988)) and the thallium based system such as TICaBaCuO and TIBaCuO, see Sheng and Hermann 332: 138-139 (1988). Other materials have since been discovered. Generally, these materials all include layered copper oxide planes. They will be referred to as a class as the "layered copper oxide" materials or "high temperature superconductors". Often times, these materials contain substitute substituent element.

While early materials were generally formed in bulk form, useful devices have been fabricated from thin film materials. See, e.g., Olson, et al, "Preparation of Superconducting TICaBaCu Thin Films By Chemical Deposition", Appl. Phys. Lett. 55, No. 2, 189-190 (1989). Eddy, "Method for Epitaxial Lift-Off for Oxide Films Utilizing Superconductor Release Layers", U.S. Patent No. 5,527,766, Sun, J.Z., Hammond, R.B., and Scalapino, D.J., "Superconducting Devices Having a Variable Conductivity Device for Introducing Energy Loss", U.S. Patent No. 5,328,893, Eddy,

"Reactor Vessel for Manufacture of Superconducting Films", U.S. Patent No. 5,306,699, and Eddy, M.M., "Epitaxial Thallium High Temperature Superconducting Films Formed Via a Nucleation Layer", U.S. Patent No. 5,508,255, all incorporated herein by reference. Various substrates have been used as supports for thin films of high temperature superconducting material. The typical substrates of choice for the various layered copper oxide materials include lanthanum aluminate (LaA103), magnesium oxide (MgO), strontium titanate (SrTi03), yttria stabilized zirconia and sapphire. These substrates have relative advantages and disadvantages. Generally, an optimal substrate meets some or all of the following criteria. First, the thermal expansion properties of the substrate and the crystal must be compatible. Second, the substrate must have good thermal stability properties throughout the processing temperature range. Third, there must be a relatively good match of the crystal lattice structure between the substrate and the film. Fourth, the diffusion between the substrate and the film is preferably low. Fifth, the substrate preferably has sufficient integrity to survive the rigors of manufacture and real world utilization. Sixth, problematic crystal phenomenon, e.g., twinning (where the crystal changes structure as a function of temperature), should be minimized. Seventh, the dielectric constant and loss tangent must be compatible with the intended use of the device, e.g., for microwave compatible devices a substrate with a low dielectric constant and low loss tangent is highly advantageous. Finally, the cost of the substrate must be considered as it is often significant.

Thus, when making a choice of a substrate, the historical emphasis was on compatibility of the substrate with the growth of a quality film. For example, the concerns of thermal expansion, thermal stability, lattice mismatch, interdiffusion and twinning were of concern. The factors such as dielectric constant and cost were of lesser direct concern, though these factors were critically important to the formation of useful devices, and ultimately, market acceptance.

Important microwave and radio frequency (RF) applications require twin free, low dielectric constant substrates. In addition, it is preferred that the substrates be available in relatively large areas (2 inch rounds or greater) in order to fabricate devices such as narrow band filters at desired frequencies. The use of magnesium oxide as a substrate is highly desirable in that it has a very low loss tangent, an isotropic dielectric constant, minimal or no twinning and a coefficient of thermal expansion closely matched to high temperature superconductors.

However, magnesium oxide is a problematic substrate with which to work. First, magnesium oxide is highly hydroscopic, and absorbs water on its surface, a factor which does not favor growth of epitaxial films. See, e.g., B.J. Kim et al., J. Vac. Sci. Technology, B 12, p 1631 (1994). Second, magnesium oxide is difficult to polish. Third, even after polishing, magnesium oxide is difficult to effectively clean prior to film growth. As a result of these difficulties, inconsistent films often resulted from the growth of a superconductor on a magnesium oxide substrate.

Various approaches were suggested in the art, none of which proved commercially satisfactory. First, annealing of the MgO substrate prior to the growth of the film was shown in some cases to provide improved results. See, e.g., B.H. Moeckly et al, Appl. Phys. Lett., 57, p 1687 (1990). Second, the growth of an intermediate SrTiO3 layer on the surface of the MgO separating it from the superconductor was utilized.

See, e.g., J.T. Cheung, Appl. Phys. Lett. 64, 3180 (1992). This approach was fairly complicated, and the results did not show sufficient reproducibility for commercial requirements. None of these techniques provided a commercially effective solution permitting the commercial utilization of MgO substrates for superconductors. While techniques existed which permitted growth of high temperature superconducting films on MgO which provided extremely good performance, See, e.g., Eddy, M.M., "Epitaxial Thallium High Temperature Superconducting Films Formed Via a Nucleation Layer", U.S. Patent No. 5,508,255, commercial realities favored attempts to reduce the cost of the overall device. Significantly, when inadequate surface preparation of the MgO resulted in a film having suboptimal electrical, superconducting or other properties, it was often the case that the resultant substrate and film were deemed not commercially useful. Accordingly, the investment in both direct cost, such as for the substrate and all of the high temperature superconducting materials constituent components, as well as the significant expenditure of time and money in processing the film, was lost. This great expenditure, and the resultant impact upon commercialization, is further appreciated when it is understood that no practical test short of complete manufacture of the composite structure of film plus substrate, followed by patterning of the film and its electrical testing, is sufficient to accurately predict the ultimate performance of the film. Accordingly, significant investment in both materials cost, and highly skilled labor costs, as well as lost opportunity cost for the utilization of the machines, was incurred in making suboptimal films. Despite the clear desirability of the use of MgO as substrates for commercially produced films, no commercially satisfactory method has been utilized heretofore.

Summary of the Invention

Improved reproducibility of fabrication of superconducting thin film devices on MgO substrates is accomplished through use of the inventive procedure. As a first essential step of the process, the surface of the MgO is removed. Preferably, the surface is removed through argon plasma bombardment, whether with low energy or high energy ions. Most preferably, ion milling is utilized, with superior results being obtained by milling at 90° to the surface of the substrate. The second step of the invention is to anneal the MgO substrate. This effects recrystallization of the substrate.

In the preferred embodiment, the substrate is annealed at approximately 1050°C in 17% dry oxygen, 83% dry nitrogen for 2.5 hours. Lower temperatures, such as 950 °C, may be utilized, though the anneal times, e.g., three hours or greater, impacts upon the commercial viability of the process. Higher temperatures, e.g., 1150°C, permit steps in the surface to become unacceptably large, and often permit second phases to migrate to the surface of the substrate. Varying other factors, such as the oxygen partial pressure, change the optimal anneal temperature. Structurally, the resultant surface manifests steps or terraces, which comprise exposed peripheral edges of atomic planes within the substrate. Optionally, cleaning steps may be utilized prior to the removal of the MgO surface. Preferably, a nonaqueous cleaner, most preferably one which does not absorb water, is utilized. In the preferred embodiment, anhydrous isopropyl alcohol (IP A) is utilized.

Optionally, a postannealing cleaning step may be utilized. The preferred process results in a high degree of repeatability of devices. In this way, significant cost savings may be achieved through avoiding the loss of the materials components, including the substrate and constituent materials of the superconductor, as well as the significant investment in skilled labor for manufacturing the devices. Through use of this method, extremely repeatable devices may be fabricated. Advantageously, microwave devices may be fabricated which have exceptionally high and repeatable Qs, a measure of the microwave performance quality of a device, low and repeatable intermods, relatively sharper transition temperatures and narrower x-ray diffraction rocking curves.

Accordingly, it is an object of this invention to provide an improved method for preparing MgO substrates for use with high temperature superconducting films.

It is yet a further object of this invention to provide a highly repeatable method for fabrication of superconducting devices.

It is yet another object of this invention to provide a commercially viable method for manufacturing useful devices from superconducting thin films.

Brief Description of the Drawings

Fig. 1 is a perspective view of the combination of the substrate and superconducting thin film layer, with a cut-away portion.

Fig. 2 is a process flow for the preferred method of this invention.

Fig. 3 is a cross-sectional drawing of a MgO substrate where the exposed surface is off angle from the crystal planes.

Fig. 4 are atomic force microscopy pictures of the surface of an ion milled, but not yet annealed MgO substrate.

Fig. 5 is an atomic force microscopy picture of an ion milled and annealed MgO substrate. Fig. 6 is a graph showing the unloaded Q of a device manufactured with films formed according to the preferred method of this invention (to the right of A), in comparison to devices manufactured with films formed without use of the preferred method (to the left of A).

Detailed Description of the Invention

Fig. 1 shows a perspective view with a cut-out portion of a substrate 10 and overlying film 12. The film is intended to represent a layered copper oxide or high temperature superconductor. The drawing is not to scale, since the typical thickness of a substrate 10, for the case of MgO, is 0.5 mm. For a thallium containing superconduc- tor film 12, when formed as a thin film, is typically in the range of from about 0.3 to about 0.9 microns. Substrates 10 are typically round in shape and have two substantially planar surfaces. Once the combination of substrate 10 and film 12 is formed, it may then be patterned, such as by photolithographic patterning, and formed into useful devices. Fig. 2 shows a flowchart of the preferred process. Broadly, the method comprises the following steps. First, the substrate is subject to a cleaning step 20, most preferably with anhydrous isopropyl alcohol. Second, the substrate is subject to a surface removal step 22, most preferably by ion milling at 90° to the substrate surface. Third, the substrate is subject to an annealing step 24, preferably at substantially 1050°C for approximately 2.5 hours in a 17% dry oxygen, 83% dry nitrogen atmosphere. Fourth, the substrate is subject to a cleaning step 26, again, preferably with anhydrous isopropyl alcohol. The method of this invention may be practiced by performing the surface removal step 22 and the anneal step 24. The initial cleaning step 20 and final cleaning step 26 are optional and are used only if necessary to clean the substrate. The various steps will now be considered in greater detail.

In the preferred embodiment, it is desirable to start the inventive process with an MgO substrate having an epitaxial polish with root mean square (RMS) roughness less than or equal to 20 A. The initial cleaning step 20 serves to prepare the substrate 10 for the surface removal step 22. For example, if the MgO substrate 10 is obtained from a third party supplier, the cleaning step 20 may serve to remove residue or particle contamination from shipment. Generally, the solution used for the cleaning step 20 should be something that is non-aqueous, which does not absorb water. Since MgO reacts with water, reducing the amount of water in the cleaning solution avoids unnecessary reaction between the MgO substrate and the water. In the preferred method, anhydrous isopropyl alcohol is utilized for the initial cleaning step 20. Alternative cleaning solutions consistent with the goals and objects of this invention, are known to the art, and include, e.g., ethyl alcohol.

The surface removal step 22 serves to remove undesired materials. By way of example, the surface removal step 22 may serve to remove adsorbed materials such as water and carbon dioxide, or other materials such as polishing residue if such material is used in the initial preparation of the substrate 10. Optionally, in addition to removing the non-MgO materials, it may be desired to remove a portion of the MgO surface itself. Doing so ensures that all of the non-MgO materials are removed, and may serve to remove damaged surface layers of the MgO. It should be noted, however, that the use of ion milling itself may result in some damage to the surface of the MgO substrate.

The preferred method of removal of the MgO surface is by plasma bombardment, preferably argon plasma bombardment. This may be done with either low energy (e.g., 300 mA at 100 volts) or high energy (e.g., 500 mA at 2,000 volts) ions. The most preferred method utilizes ion milling of the MgO surface. Applicants have discovered that the use of ion milling at substantially 90° to the substrate surface results in a maximum smoothness of the substrate surface. While Applicants do not wish to be bound by any theory explaining this discovery of optimal results, it is believed that when the ion milling is performed at substantially 90°, that a portion of the argon ions may channel through the surface of the MgO substrate as the incidence of the ions is aligned with the crystal structure of the substrate. Significantly smoother substrate surfaces have been achieved utilizing the substantially 90° ion milling technique, as compared with the conventional 45° angle of incidence milling technique. In the preferred embodiment, the ion milling is performed through use of an ion tech ion beam etch system with an MPS-5000 power supply. The currents, voltages and times of operation in the preferred embodiment for a MgO substrate depend upon the condition in which the substrate is received from the vendor, but typically are as follows: 400 volts, 180 mA, at 90° milling with an argon plasma

As a result of the surface removal step 22, the substrate is relatively smooth, preferably having an RMS roughness less than 25A. Fig. 3 shows a cross-sectional drawing of a substrate 30. The cross-section shows steps or terraces 32. The steps or terraces 32 constitute the terminal ends of planes of atoms in the crystal substrate 30.

These steps or terraces 32 result from the slight deviation of the substrate surface 32

(shown globally as the horizontal dashed line) with the crystalline planes (the perpendicular solid line 36 representing the orthogonal to that plane). This slight deviation is represented by the angle θ in Fig. 3. In the preferred method, the MgO substrates are subject to the specification that the angle θ is less than substantially 1 °.

Ideally, a uniform substrate 30 having a θ of 0 would be optimal, however, in practice, achieving such precision is difficult. Applicants have demonstrated that use of the methods described herein provide superior, reproducible devices while still being able to use substrates which may be obtained in commercial quantities at practical prices.

Fig. 4 is an atomic force microscopy image showing the surface of MgO after ion milling at substantially 90°. The RMS roughness of this surface is 2.58 A, and with an image z range of 2.610 nm.

The annealing step 224 serves to crystallize the surface of the MgO. The preferred conditions for annealing are at substantially 1050°C for 2.5 hours in a 17% dry oxygen, 83% dry nitrogen atmosphere. Other gases may be included, e.g., argon, or may be wholly substituted consistent with the goals and objects of this invention.

There is a range of temperatures in which the annealing may occur. For example, on the relatively high side of the temperature range, annealing at 1150° C may be utilized, though if annealed too long, the steps 34 become relatively large and/or second phases tend to migrate to the surface of the substrate. Alternatively, at the lower end of the temperature range, temperatures such as 950 °C may be utilized, though the anneal times become relatively long, often exceeding three hours, and sometimes fail to result in formation of the desired steps or terraces 34 (Fig. 3). Thus, while 1050°C is the preferred temperature for the process, a range of temperatures exist which are sufficient to result in the crystallization of the substrate, which results in a commercially satisfactory device. Fig. 5 shows an atomic force microscopy image of an MgO substrate after the anneal step.

The post annealing cleaning step 26 is optional. If necessary, the cleaning step 26 may remove any materials which reside on the MgO surface after annealing. Ideally, it is desirable to perform the annealing step 24 in a clean environment so as to avoid the need for the post anneal cleaning step 26. However, if the annealing is not done in such conditions, the optional post anneal cleaning step 26 may be employed. Generally, the considerations for selection of the cleaning solution are as described for the initial cleaning step 20. In the preferred embodiment, the cleaning solution is anhydrous isopropyl alcohol.

The combination of substrates prepared by the inventive method, coupled with the formation of superconducting films on the substrate result in highly reproducible, high performance devices. The preferred superconducting films are those formed of the YBCO family and the thallium containing superconductor family. As shown by the electrical testing results in Fig. 6, the Q of resonators formed on substrates prepared in accordance with the methods of this invention (to the right of label A) are both higher and more uniform compared to those formed on substrates which were not subject to this process (those to the left of the label A). Q is one measure of the microwave performance of devices. There are three main factors which impact upon the Q of the device. First, the superconducting component impacts upon the Q, the better the formation of the superconducting crystalline structure, the better the Q. Second, the lower the loss tangent of the substrate, the better the Q. Third, the lower the loss from the packaging, environment and radiation losses, the better the Q. Generally, the third factor relates principally to the structure or design of the electrical component and its packaging and environment. Since the reciprocal of the total Q is proportional to the sum of the inverse of the three factors mentioned, above, it is important that each of these factors be properly selected and effectively achieved in order to maximize the total Q of the device. Thus, as this relates to the inventive method disclosed here, the effective use of an MgO substrate with its very loss tangent contributes significantly to the overall Q of the commercial device.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity and understanding, it may be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims.

Claims

We Claim:

1. A method for preparing magnesium oxide substrates for use in superconducting devices comprising the steps of: removing the surface of the magnesium oxide substrate by plasma bombardment, removing sufficient material so as to expose a surface substantially free of materials other than the substrate material, followed by the step of, annealing the substrate at a temperature for a time sufficient to crystallize the surface of the substrate.

2. The method for preparing substrates for superconductors of Claim 1 wherein the step of surface removal of the substrate is achieved by plasma bombardment.

3. The method for preparation of substrates for superconductors of Claim 2 wherein the plasma bombardment comprises argon plasma bombardment.

4. The method for preparation of substrates for superconductors of Claim 2 wherein the plasma has the range of 300 mA at 100 volts to 500 mA at 2,000 volts energy for the ions.

5. The method for preparation of substrates for superconductors of Claim 2 wherein the step of removing the surface of the substrate is performed by ion milling.

6. The method for preparing substrates for superconductors of Claim 5 wherein the ion milling is performed at substantially 90┬░ to the surface of the substrate.

7. The method for preparation of substrates for superconductors of Claim 1 wherein the root means square roughness of the surface after the step of removal of material is 25A or less.

8. The method for preparation of substrates for superconductors of Claim

I wherein the annealing step is performed at substantially 1050┬░C.

9. The method for preparation of substrates for superconductors wherein the annealing step is performed in the temperature range from substantially 950┬░C to 1150┬░C.

10. The method for preparation of substrates for superconductors of Claim 9 wherein the anneal temperature is less than 1150┬░C.

11. The method for preparation of substrates for preparation of substrates for superconductors of Claim 1 further including a cleaning step prior to the step of removal of material.

12. The method for preparation of the substrate for superconductors of Claim

I I wherein the cleaning step is performed with anhydrous isopropyl alcohol.

13. The method for preparation of the substrate for superconductors of Claim 11 wherein the cleaning step is performed with ethyl alcohol.

14. The method for preparation of substrates for superconductors of Claim

1 further including the step of cleaning the substrate after the anneal step.

15. The method for preparation of substrates for superconductors of Claim 14 wherein the cleaning step is performed with anhydrous isopropyl alcohol.

16. The method for preparation of substrates for superconductors of Claim 1 further including the step of forming a thin film superconductor on the substrate.

17. The method for preparation of substrates for superconductors of Claim 16 wherein the thin film superconductor is in the YBCO family.

18. The method for preparation of substrates for superconductors of Claim 16 wherein the thin film superconductor is in the thallium superconducting family.

19. The method for preparation of substrates for superconductors of claim 1 wherein the step of removing the surface of the magnesium oxide substrate results in a terraced surface.