Method and plating bath for depositing a magnetic film

Abstract

A cobalt-iron-boron (CoFeB) film (100) is electrolessly deposited on a substrate (150) using a chloride plating bath. The plating bath may include a primary metal in a concentration of between approximately 0.05 moles per liter and approximately 0.4 moles per liter, a secondary metal in a concentration of between approximately 0.005 moles per liter and approximately 0.04 moles per liter, a complexing agent in a concentration of between approximately 0.15 and approximately 0.8 moles per liter, a pH buffer in a concentration of between approximately 0.5 and approximately 1.5 moles per liter, and a reducing agent in a concentration of between approximately 0.05 and approximately 0.25 moles per liter.

Description

FIELD OF THE INVENTION

The disclosed embodiments of the invention relate generally to magnetic films, and relate more particularly to the deposition of magnetic films with soft magnetic properties.

BACKGROUND OF THE INVENTION

Magnetic recording media, magnetic inductor/transformer circuitry, read/write magnetic recording heads, sensor applications, and other magnetic applications all require materials with suitable magnetic and electrical properties. Such properties are frequently imparted to a material by applying to the material a film or other coating having the appropriate characteristics. Sputtered films and nickel alloys are examples of such coatings. Unfortunately, existing coatings suffer from a variety of drawbacks making them less than desirable. Sputtering and other forms of physical vapor deposition, for example, are typically not compatible with high volume manufacturing, especially for films thicker than approximately one micrometer, due to slow deposition rates and the need for frequent replacement of target materials. Nickel alloy raises safety and environmental concerns because Ni++ is carcinogenic. Permalloy, a nickel-iron compound frequently used as a magnetic material, suffers from eddy current loss during high frequency operation due to its low electrical resistivity. Accordingly, there exists a need for a material exhibiting both soft magnetic properties and high electrical resistivity that is compatible with a high volume manufacturing environment and that does not suffer from safety and other environmental concerns. Ideally, such material should be thermally stable in its magnetic properties and should not require the application of an external magnetic field during deposition.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed embodiments will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying figures in the drawings in which:

FIG. 1 is a cross-sectional view of a material according to an embodiment of the invention that has been applied as a coating or film to a substrate;

FIG. 2 is a flowchart illustrating a method of forming a soft magnetic film according to an embodiment of the invention; and

FIG. 3 is a graph showing certain properties of the material according to an embodiment of the invention.

For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the discussion of the described embodiments of the invention. Additionally, elements in the drawing figures are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of embodiments of the present invention. The same reference numerals in different figures denote the same elements.

The terms “first,” “second,” “third,” “fourth,” and the like in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Similarly, if a method is described herein as comprising a series of steps, the order of such steps as presented herein is not necessarily the only order in which such steps may be performed, and certain of the stated steps may possibly be omitted and/or certain other steps not described herein may possibly be added to the method. Furthermore, the terms “comprise,” “include,” “have,” and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,” “under,” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. The term “coupled,” as used herein, is defined as directly or indirectly connected in an electrical or non-electrical manner. Objects described herein as being “adjacent to” each other may be in physical contact with each other, in close proximity to each other, or in the same general region or area as each other, as appropriate for the context in which the phrase is used. Occurrences of the phrase “in one embodiment” herein do not necessarily all refer to the same embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

In one embodiment of the invention, a cobalt-iron-boron (CoFeB) film is electrolessly deposited on a substrate using a chloride plating bath. In one embodiment, the plating bath comprises a primary metal in a concentration of between approximately 0.05 moles per liter and approximately 0.4 moles per liter, a secondary metal in a concentration of between approximately 0.005 moles per liter and approximately 0.04 moles per liter, a complexing agent in a concentration of between approximately 0.15 and approximately 0.8 moles per liter, a pH buffer in a concentration of between approximately 0.5 and approximately 1.5 moles per liter, and a reducing agent in a concentration of between approximately 0.05 and approximately 0.25 moles per liter.

Embodiments of the invention enable the deposition of a CoFeB alloy film with soft magnetic properties suitable for inductor and other magnetic applications and that is compatible with high-volume silicon process technology. The soft magnetic properties can be obtained without the application of an external magnetic field. Furthermore, the CoFeB film is thermally stable in its magnetic properties up to a temperature of at least approximately 250 degrees Celsius (° C.), which temperature is typical of portions of an integrated process flow in semiconductor manufacturing. In at least certain embodiments, the CoFeB film is superior to cobalt-zirconium-tantalum (CoZrTa) (a material currently used as a soft magnetic film) because CoZrTa requires sputter deposition and is not compatible with high volume manufacturing due to a low deposition rate and the need for frequent replacement of target materials.

By comparison to CoZrTa, CoFeB has a higher deposition rate and, as has been mentioned, does not require a magnetic field during deposition or anneal. Because CoFeB is thermally stable in its magnetic properties, and because CoFeB does not contain nickel (Ni) (and thus does not suffer from the environmental or heath concerns associated with N++ in an electroplating bath), CoFeB films are well-suited for magnetic film applications in high-volume silicon manufacturing.

Referring now to the drawings, FIG. 1 is a cross-sectional view of a material 100 according to an embodiment of the invention that has been applied as a coating or film to a substrate 150. A seed layer 120 lies between substrate 150 and material 100. As an example, seed layer 120 can comprise copper, cobalt, nickel, platinum, palladium, ruthenium, iron, and alloys thereof, in addition to other suitable materials.

Material 100 comprises cobalt (Co), iron (Fe) and boron (B) and is thus referred to as a CoFeB film, as mentioned above. Material 100 has an electrical resistivity, subsequently referred to herein simply as the “resistivity,” of approximately 53 μOhm-cm as deposited and a resistivity of approximately 39 μOhm-cm after an annealing step that will be further discussed below. Material 100 further has an as-deposited hard-axis coercivity that is no greater than approximately 2 Oersted (Oe). After the annealing step the hard-axis coercivity is no greater than approximately 0.5 Oe.

With an electrical resistivity and a coercivity in the ranges given above, the CoFeB film provides good material properties for multiple magnetic applications. The relatively high electrical resistivity may provide the advantage of reducing eddy current losses during high frequency operation compared to conventional magnetic materials, and the relatively low coercivity allows a more immediate response to a change in magnetism, an important quality for materials used in magnetic applications.

It was implied above that material 100 may find utility in various magnetic applications such as magnetic recording heads and recording media, magnetic inductor/transformer circuitry, sensor applications, and the like. Additionally, material 100 can form a part of an on-chip inductor or a part of an integrated silicon voltage regulator (ISVR). Referring still to FIG. 1, material 100, as mentioned, can be thought of as being a fim or coating applied to substrate 150. So applied, material 100 can form a film in a very wide range of thicknesses, such that, for example, in one embodiment material 100 may have a thickness as small as approximately 10 nanometers while in another embodiment material 100 may have a thickness as large as approximately one millimeter. As an example, in the on-chip inductor application mentioned above, material 100 may have a thickness of between approximately 0.1 micrometers and approximately 10 micrometers.

In order to accomplish the electroless deposition, substrate 150 may be placed in a plating solution or plating bath as part of the deposition process. A water-based plating bath according to one embodiment of the invention comprises a primary metal in a concentration of between approximately 0.05 moles per liter and approximately 0.4 moles per liter, a secondary metal in a concentration of between approximately 0.005 moles per liter and approximately 0.04 moles per liter, a complexing agent in a concentration of between approximately 0.15 moles per liter and approximately 0.8 moles per liter, a pH buffer in a concentration of between approximately 0.5 moles per liter and approximately 1.5 moles per liter, and a reducing agent in a concentration of between approximately 0.05 moles per liter and approximately 0.25 moles per liter.

In a particular embodiment, a pH level of the plating bath is between approximately 8.3 and approximately 9.7. In the same or another embodiment, a temperature of the plating bath is between approximately 50 degrees and approximately 85° C. Plating baths exceeding the upper limits of the stated pH and temperature ranges may become unstable. Plating baths with pH values or temperatures below the lower limits of the stated pH and temperature ranges may result in acceptable films, but the deposition process will likely proceed more slowly than when the pH value and temperature are within the stated ranges.

In one embodiment, the primary metal comprises cobalt (II) chloride (CoCl₂), the secondary metal comprises iron (II) sulfate (FeSO₄), the complexing agent comprises citrate, the pH buffer comprises ammonium chloride (NH₄Cl), and the reducing agent comprises dimethylamineborane. It will be appreciated by one of ordinary skill in the art that the plating bath described above may be modified in certain respects yet still produce the CoFeB film that has been described herein. As an example, if the complexing agent were changed from citrate to succinate, while the molar concentration of the complexing agent were maintained, it would be expected that the deposition rate would increase but with the consequence of lower boron content within the resultant film.

The citrate or other complexing agent complexes around the cobalt or other ion and keeps the ion in solution by preventing it from precipitating out of the plating bath. The ammonium chloride or other pH buffer minimizes variation in pH for the plating bath. The dimethylamineborane or other reducing agent acts as a source of electrons so that cobalt can be formed from the cobalt ion present in the plating bath, and furthermore is a source of boron in material 100.

FIG. 2 is a flowchart illustrating a method 200 of forming a soft magnetic film according to an embodiment of the invention. A step 210 of method 200 is to provide a substrate. As an example, the substrate can be similar to substrate 150 that is shown in FIG. 1. In certain embodiments, various underlying active devices and/or circuits may have been prepared on or in the substrate.

A step 220 of method 200 is to form a seed layer or seed layers on the substrate. As an example, the seed layer can be similar to seed layer 120 that is shown in FIG. 1. In one embodiment, step 220 comprises depositing a material comprising a substance selected from the group consisting of copper, cobalt, nickel, platinum, palladium, ruthenium, iron, and alloys thereof. As an example, the material used to form the seed layer may be deposited using a vapor deposition method or an electrochemical method.

A step 230 of method 200 is to electrolessly deposit a CoFeB film (which is the soft magnetic film) over the seed layer using a chloride bath. As an example, the CoFeB film can be similar to material 100 that is shown in FIG. 1. Accordingly, in one embodiment, the CoFeB film has a hard-axis coercivity no greater than approximately 2 Oe and a resistivity no greater than approximately 53 μOhm-cm.

A step 240 of method 200 is an optional step used in one embodiment of the invention. Step 240 is to apply a magnetic field to a surface of the substrate, either during deposition of the CoFeB film or at some other time (such as, for example, during the thermal anneal that is discussed below). Where a magnetic field is used, step 240 can, in some embodiments, comprise applying the magnetic field parallel to or substantially parallel to the surface of the substrate and at a strength of greater than approximately 50 Oe at the substrate surface.

In different embodiments, the CoFeB film is electrolessly deposited in the absence of an external magnetic field. In a particular embodiment, no external magnetic field is applied during any point of method 200, i.e., no external magnetic field is applied at any time during the deposition or subsequent processing of the CoFeB film. In embodiments where no external magnetic field is required, existing plating equipment can be used without modification, whereas a requirement for magnetic field application may necessitate significant equipment modification or new equipment development.

A step 250 of method 200 is to perform a thermal anneal on the CoFeB film. In one embodiment, the thermal anneal may be performed at a temperature of approximately 250° C. for approximately two hours with nitrogen gas (N₂) and with no magnetic field. In a different embodiment, the thermal anneal may be performed at a temperature of approximately 250° C. for approximately two hours in a vacuum and with a magnetic field of approximately 1 Tesla. Other embodiments may use different combinations of the above values of temperature, duration, and environment, both with and without a magnetic field. Still other embodiments may use values of the given or other parameters that are different from those given above, again both with and without a magnetic field.

As implied above, the magnetic properties of the CoFeB film may be defined or improved by the thermal anneal that is performed in step 250 (or another step) of method 200. As an example, and as mentioned above, the CoFeB film may have a resistivity of approximately 53 μOhm-cm and a hard-axis coercivity of approximately 2 Oe prior to the thermal anneal and a resistivity of approximately 39 μOhm-cm and a hard-axis coercivity of approximately 0.5 Oe following the thermal anneal. These properties are illustrated in FIG. 3, which shows hysteresis loops for a CoFeB film electrolessly deposited using a chloride bath according to an embodiment of the invention. Loops for both as-deposited films and annealed films are shown. The annealing was conducted without a magnetic field. It may be observed that the post-annealing coercivity decreases from 2 Oe (as-deposited) to 0.5 Oe. The results demonstrate both soft magnetic properties and thermal stability of the alloy.

The given results for CoFeB may be favorably compared to magnetic properties for other magnetic materials. As an example, such other magnetic materials may have good as-deposited magnetic properties but poor thermal stability. As a particular example, cobalt-tungsten-boron-phosphorus (CoWBP) films show significant degradation in magnetic properties upon annealing at temperatures of 200-250° C.

Returning to FIG. 2, a step 260 of method 200 is to adjust or maintain a pH of the chloride bath such that it is between approximately 8.3 and approximately 9.7 and to adjust or maintain a temperature of the chloride bath such that it is between approximately 50 and approximately 85° C.

Although the invention has been described with reference to specific embodiments, it will be understood by those skilled in the art that various changes may be made without departing from the spirit or scope of the invention. Accordingly, the disclosure of embodiments of the invention is intended to be illustrative of the scope of the invention and is not intended to be limiting. It is intended that the scope of the invention shall be limited only to the extent required by the appended claims. For example, to one of ordinary skill in the art, it will be readily apparent that the method and plating bath discussed herein may be implemented in a variety of embodiments, and that the foregoing discussion of certain of these embodiments does not necessarily represent a complete description of all possible embodiments.

Additionally, benefits, other advantages, and solutions to problems have been described with regard to specific embodiments. The benefits, advantages, solutions to problems, and any element or elements that may cause any benefit, advantage, or solution to occur or become more pronounced, however, are not to be construed as critical, required, or essential features or elements of any or all of the claims.

Moreover, embodiments and limitations disclosed herein are not dedicated to the public under the doctrine of dedication if the embodiments and/or limitations: (1) are not expressly claimed in the claims; and (2) are or are potentially equivalents of express elements and/or limitations in the claims under the doctrine of equivalents.

Claims

1. A method comprising:

providing a substrate;

forming a seed layer on the substrate; and

electrolessly depositing a CoFeB film over the seed layer using a chloride bath.

2. The method of claim 1 wherein:

the CoFeB film has a hard-axis coercivity no greater than approximately 2 Oe and a resistivity no greater than approximately 53 μOhm-cm.

3. The method of claim 1 further comprising:

applying a magnetic field to a surface of the substrate.

4. The method of claim 1 wherein:

the CoFeB film is electrolessly deposited in the absence of an external magnetic field.

5. The method of claim 1 further comprising:

performing a thermal anneal on the CoFeB film.

6. The method of claim 5 wherein:

following the thermal anneal the CoFeB film has a hard-axis coercivity no greater than approximately 0.5 Oe and a resistivity no greater than approximately 39 μOhm-cm.

7. The method of claim 1 further comprising:

adjusting or maintaining a pH of the chloride bath such that it is between approximately 8.3 and approximately 9.7; and

adjusting or maintaining a temperature of the chloride bath such that it is between approximately 50 and approximately 85 degrees Celsius.

8. A plating bath comprising:

a primary metal in a concentration of between approximately 0.05 moles per liter and approximately 0.4 moles per liter;

a secondary metal in a concentration of between approximately 0.005 moles per liter and approximately 0.04 moles per liter;

a complexing agent in a concentration of between approximately 0.15 and approximately 0.8 moles per liter;

a pH buffer in a concentration of between approximately 0.5 and approximately 1.5 moles per liter; and

a reducing agent in a concentration of between approximately 0.05 and approximately 0.25 moles per liter.

9. The plating bath of claim 8 wherein:

a pH level of the plating bath is between approximately 8.3 and approximately 9.7.

10. The plating bath of claim 8 wherein:

a temperature of the plating bath is between approximately 50 degrees and approximately 85 degrees Celsius.

11. The plating bath of claim 8 wherein:

the primary metal comprises cobalt (II) chloride.

12. The plating bath of claim 8 wherein:

the secondary metal comprises iron (II) sulfide.

13. The plating bath of claim 8 wherein:

the complexing agent comprises citrate.

14. The plating bath of claim 8 wherein:

the pH buffer comprises ammonium chloride.

15. The plating bath of claim 8 wherein:

the reducing agent comprises dimethylamineborane.