# Patent

## Title
Systems and methods for forming magnetic recording media with improved grain columnar growth for energy assisted magnetic recording

## Abstract

Systems and methods for forming magnetic recording media with improved columnar growth for energy assisted magnetic recording are provided. In one such method, a first sub-layer of a magnetic layer is formed on a substrate, the magnetic layer including a magnetic material and a plurality of non-magnetic segregants, a top surface of the first sub-layer is etched to substantially remove the non-magnetic segregants accumulated on the top surface, and a second sub-layer of the magnetic layer is formed on the first sub-layer.

## Background
FIELD
Aspects of the present invention relate to heat assisted or energy assisted magnetic recording, and, more particularly, systems and methods for forming magnetic recording media with improved columnar growth for heat assisted or energy assisted magnetic recording.
BACKGROUND
Due to the increasing demand for more data storage, heat assisted or energy assisted magnetic recording concepts have been pursued as ways to achieve higher density magnetic recording well over a Terabit/in2 in media design. Among the many available magnetic materials, FePt is often chosen as one of the suitable materials for a magnetic recording layer. This material is shown to have a desired thermal gradient near the Curie point for heat assisted magnetic recording.
To achieve magnetic material (e.g., FePt, FePd) with high densities, non-magnetic segregants (e.g., C, Cr, B, SiO2, TiO2, Cr2O3, Ag, BN, V2O5, ZrO2, Nb2O5, HfO2, Ta2O5, WO3, MgO, B2O3, ZnO, etc.) can be added in order to attain smaller grain sizes of the magnetic material with sufficiently low grain size distributions (e.g., <20%). Carbon has been found to be one of the effective additives which shows the above mentioned properties. However, as the grain sizes get smaller, it becomes difficult to make the magnetic recording layer thicker. For example, in an FePt—C system, a ratio t/D (where t is the thickness, and D is the grain diameter) is found to be limited to approximately 1. This leads to severe reduction in read-back amplitude and hence poor recording performance at high densities. Therefore, it is desirable to improve the performance of existing magnetic recording layers and methods for forming the same.
SUMMARY
Embodiments of the present invention are directed to magnetic recording media with improved columnar growth of the magnetic grains. Embodiments of the present invention are also directed to methods for forming the improved magnetic recording media.
According to an embodiment of the present invention, a method for fabricating a magnetic recording medium is provided. According to the embodiment, a first sub-layer of a magnetic layer is formed on a substrate, the magnetic layer including a magnetic material and a plurality of non-magnetic segregants, a top surface of the first sub-layer is etched to substantially remove the non-magnetic segregants accumulated on the top surface, and a second sub-layer of the magnetic layer is formed on the first sub-layer.
According to another embodiment of the present invention, a magnetic recording medium is provided. According to the embodiment, the magnetic recording medium includes a substrate and a magnetic recording layer on the substrate, the magnetic recording layer including a magnetic material and a plurality of non-magnetic segregants. In the embodiment, the magnetic material includes a plurality of grains having substantially continuous columnar crystal growth. In one embodiment, the magnetic recording layer has a thickness of t, a diameter of the plurality of grains is D, and a ratio of t/D may be greater than 1.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and aspects of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings in which:
FIG. 1 illustrates a cross-sectional functional view of a layer stack of a magnetic recording medium for energy assisted magnetic recording (EAMR) according to an embodiment of the present invention.
FIG. 2 conceptually illustrates a process for fabricating an EAMR medium with an elongated columnar grain structure without the formation of an undesirable layer of segregant particles, according to an embodiment of the present invention.



DETAILED DESCRIPTION
As the present invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the present invention to particular modes of practice, and it is to be appreciated that all changes, equivalents, and substitutes that do not depart from the spirit and technical scope of the present invention are encompassed in the present invention.
FIG. 1 illustrates a layer stack of a magnetic recording medium for energy assisted magnetic recording (EAMR). Referring to FIG. 1, the magnetic recording medium includes a substrate 10, an adhesion layer 20 on the substrate 10, one or more heatsink layers 30 on the adhesion layer 20, one or more under layers 40 on the heatsink layers 30, a magnetic layer 50 on the under layers 40, and a protective coat on the magnetic layer 50. The substrate 10 may be a glass substrate, a metal substrate, or any other suitable substrates. The adhesion layer 20 may include CrTa, NiP, NiTa, or other suitable materials. The one or more heatsink layers 30 may include Cu, W, Ru, Mo, Ag, Au, Cr, Mg, Rh, Be, or alloys thereof, or other suitable materials. The one or more under layers 40 may include MgO, TiN, TiC, AlRu, VC, HfC, ZrC, TaC, NbC, CrC, NbN, CrN, VN, CoO, FeO, CaO, NiO, MnO, or other suitable materials.
In FIG. 1, the magnetic layer 50 includes a first magnetic sub-layer 50a and a second magnetic sub-layer 50b. The second magnetic sub-layer 50b is formed on the first magnetic sub-layer 50a after performing an etching process on the first magnetic sub-layer 50a. The etching process removes an accumulation of segregants such as carbon on a top surface of the first magnetic sub-layer 50a. The accumulation of segregants will be discussed below in more details. Each of the first and second magnetic sub-layers (50a and 50b) may include a magnetic material such as Fe, Co, and/or combinations thereof, according to several embodiments of the present invention. For example, the magnetic material may include FePt, FePd, CoPt, or other suitable materials. In one embodiment, the magnetic layer 50 has a ratio of t/D greater than 1, where t is a thickness of the magnetic layer 50, and D is a diameter of the grains of the magnetic layer 50.
In one embodiment, the magnetic layer 50 includes L10 FePt, and the under layers 40 include MgO on which FePt can grow with the desired texture. The substrate 10 can be a high temperature glass substrate or a metal substrate that facilitates the growth of the layers formed thereon for obtaining a good crystallographic texture growth for L10 FePt. In order to grow the FePt magnetic layer, carbon is added to segregate the grains of FePt because FePt and carbon are immiscible.
While not bound by any particular theory, when an FePt layer is grown to be thicker than a certain thickness (e.g., about 5 nm), a layer of carbon forms on a top surface of the formed FePt layer. The carbon layer will decouple the FePt grains vertically when the FePt layer is grown to be thicker than the certain thickness. It was found that carbon atoms cover the top of the FePt grains after the FePt layer is grown to the certain thickness, thus preventing the columnar growth of the FePt grains. Therefore, when the thickness of the FePt layer is grown to be thicker than, e.g., about 5 nm, an upper FePt layer is separated from a bottom FePt layer by a layer of carbon formation between the upper FePt layer and the bottom FePt layer. This phenomenon makes the epitaxial columnar growth of the FePt layer very difficult above a certain thickness (e.g., 5 nm). As such, the control of grain distributions and magnetic properties of the magnetic layer become difficult.
FIG. 2 conceptually illustrates a process for fabricating an EAMR medium with an elongated columnar grain structure without the formation of an undesirable layer of segregant particles (e.g., C), according to an embodiment of the present invention. Referring to FIG. 2, in a block S100, the process forms a first sub-layer 110 of a magnetic layer (e.g., FePt layer) on a suitable substrate (not shown in FIG. 2), where the magnetic layer includes a magnetic material (e.g., FePt) and non-magnetic segregants (e.g., C, Cr, B, SiO2, TiO2, Cr2O3, Ag, BN, V2O5, ZrO2, Nb2O5, HfO2, Ta2O5, WO3, MgO, B2O3, ZnO, etc.). The first sub-layer 110 is grown to a preselected thickness (e.g., 5 nm). At this preselected thickness, a segregant layer 112 (e.g., carbon layer) forms on the top surface of the first sub-layer 110. The presence of this undesirable segregant layer 112 prevents further vertical columnar growth of grains of the magnetic layer because the segregant layer 112 decouples the magnetic grains vertically.
Still referring to FIG. 2, in block S102, the process etches a top surface of the first sub-layer 110 to substantially remove the segregants accumulated on the top surface. The segregant layer 112 can be etched away by a suitable etching process. During the etching process of the first sub-layer 110, a portion of the first sub-layer 110 proximate to the top surface is removed. The removed portion of the first sub-layer 110 has a concentration of the segregants higher than that of the other portions of the first sub-layer 110. In one embodiment, an inductively coupled plasma (ICP) etching process can be used to etch out the segregant layer 112 to expose the grains of the first sub-layer 110 below. The ICP etching process can be performed with an ICP etch gas mixture selected from the group consisting of Ar, H2, O2, Xe, Ne, N2, and other suitable etching gases. However, the present invention is not limited to the above described etching process, and other suitable etching processes, such as ion milling, sputter etching, reactive ion etching, etc., may be used in various embodiments.
Still referring to FIG. 2, in block S104, after the removal of the segregant layer 112, the process grows a second sub-layer 114 of the magnetic layer on the exposed grains of the etched first sub-layer 110 to increase the total thickness of the resultant magnetic layer. Because the segregant layer 112 has been removed by etching, a thicker magnetic layer 200 can be fabricated (e.g., greater than 5 nm or 7 nm) with continuous columnar growth than if the segregant layer 112 were not removed. In several embodiments of the present invention, the magnetic layer 200 include FePt that can be grown from about 10 nm to about 15 nm in thickness with good epitaxy. According to the embodiment of FIG. 2, the resultant magnetic layer (i.e., FePt layer) can have a ratio of t/D greater than about 1, where t is the thickness of the magnetic layer and D is a diameter of the FePt grains.
In several embodiments, the magnetic layer 200 include FePt that has grain sizes between about 4 nm and about 9 nm in diameter, inclusive. In other embodiments, the magnetic layer 200 include FePt that has grain sizes between 5 nm and about 6 nm, inclusive. In several embodiments, the first sub-layer 110 has a thickness between about 3 nm and about 6 nm, inclusive, and the second sub-layer 114 has a thickness between about 3 nm and about 10 nm, inclusive. In several embodiments, the first sub-layer 110 has a thickness between about 3 nm and about 4 nm, inclusive. It should be appreciated that the above described materials and processes used for forming the EAMR medium are illustrative only, and the present invention is not limited thereto. In several embodiments, the EAMR medium may include other suitable magnetic materials and segregants.
In some embodiments, the above described processes can be used to form additional sub-layers of the magnetic layer 200. For example, after forming the second sub-layer 114, a top surface of the second sub-layer 114 can be etched to substantially remove a second segregant layer (not shown in FIG. 2) accumulated on the top surface of the second sub-layer 114. Then, a third sub-layer (not shown) of the magnetic layer 200 is formed on the second sub-layer 114. In addition, an adhesion layer, a heatsink layer, and an under-layer can be formed between the magnetic layer 200 and the substrate.
In the above described embodiments, the process or method can perform the sequence of actions in a different order. In another embodiment, the process or method can skip one or more of the actions. In other embodiments, one or more of the actions are performed simultaneously or concurrently. In some embodiments, additional actions can be performed.
According to the above described embodiments of the present invention, magnetic layers with smaller grain sizes can be grown thicker to provide sufficient read-back signal and good signal-to-noise ratio (SNR). Also, the above described processes can significantly improve the surface roughness of a magnetic medium.
While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims and their equivalents.

## Claims

1. A method for fabricating a magnetic recording medium to increase a thickness of a magnetic recording layer of the magnetic recording medium, the method comprising:
forming a first sub-layer of the magnetic recording layer on a substrate, the magnetic recording layer comprising a magnetic material and a plurality of non-magnetic segregants, wherein the forming the first sub-layer of the magnetic recording layer causes the non-magnetic segregants to accumulate over the magnetic material and at a top surface of the first sub-layer;
etching the top surface of the first sub-layer to remove substantially all of the non-magnetic segregants accumulated at the top surface; and
forming a second sub-layer of the magnetic recording layer on the first sub-layer.

2. The method of claim 1, wherein the forming of the second sub-layer of the magnetic recording layer comprises continuing a columnar crystal growth of the magnetic recording material from the first sub-layer.

3. The method of claim 1, wherein the etching the top surface of the first sub-layer comprises removing a portion of the first sub-layer proximate to the top surface.

4. The method of claim 3, wherein the portion of the first sub-layer proximate to the top surface has a concentration of the non-magnetic segregants higher than that of the other portions of the first sub-layer.

5. The method of claim 1, wherein the etching is performed by an etching process selected from the group consisting of inductively coupled plasma (ICP) etching, ion milling, sputter etching, reactive ion etching, and combinations thereof.

6. The method of claim 5:
wherein the etching is performed by inductively coupled plasma (ICP) etching; and
wherein the ICP etching is performed with an ICP etch gas mixture selected from the group consisting of Ar, H2, O2, Xe, Ne, and N2.

7. The method of claim 1, wherein the magnetic recording material comprises a material selected from the group consisting of Fe, Co, Ni, and combinations thereof.

8. The method of claim 7, wherein the magnetic recording material comprises FePt.

9. The method of claim 1, wherein the plurality of non-magnetic segregants comprise a material selected from the group consisting of C, Cr, B, SiO2, TiO2, Cr2O3, Ag, BN, V2O5, ZrO2, Nb2O5, HfO2, Ta2O5, WO3, MgO, B2O3, ZnO, and combinations thereof.

10. The method of claim 9, wherein the plurality of non-magnetic segregants comprise carbon.

11. The method of claim 1, wherein the magnetic recording material has grain sizes between about 4 nm and about 9 nm in diameter, inclusive.

12. The method of claim 11, wherein the magnetic recording material has grain sizes between about 5 nm and about 6 nm in diameter, inclusive.

13. The method of claim 1, wherein the first sub-layer has a thickness between about 3 nm and about 6 nm, inclusive, and the second sub-layer has a thickness between about 3 nm and about 10 nm, inclusive.

14. The method of claim 13, wherein the first sub-layer of the magnetic recording layer has a thickness between about 3 nm and about 4 nm, inclusive.

15. The method of claim 1, further comprising:
etching a top surface of the second sub-layer to substantially remove the non-magnetic segregants accumulated at the top surface of the second sub-layer; and
forming a third sub-layer of the magnetic layer on the second sub-layer.

16. The method of claim 1, wherein a ratio of a thickness of the magnetic recording layer to a grain diameter of the magnetic material is greater than 1.

17. The method of claim 1, further comprising forming, between the magnetic recording layer and the substrate, an adhesion layer on the substrate, a heat sink layer on the adhesion layer, and an under-layer on the heat sink layer.

18. The method of claim 1, further comprising forming a protective coating layer directly on the second sub-layer.

19. The method of claim 1, wherein the magnetic recording layer is configured to record information.