# Patent

## Title
Near field transducer using dielectric waveguide core with fine ridge feature

## Abstract

An apparatus for energy assisted magnetic recording of a storage disk includes a plurality of dielectric waveguide cores disposed near an air bearing surface of a magnetic recording device. Each waveguide core has a fine ridge feature on a first surface of the waveguide core and configured to receive incident light energy from an energy source. A near field transducer (NFT) is formed at the air bearing surface for focusing light energy received from the waveguide core and transmitting the focused light energy onto the storage disk surface to generate a heating spot. The NFT includes at least one plasmonic metal element disposed above the fine ridge features of the waveguide cores to form an interface for delivering propagating surface plasmon polaritons (PSPPs) to the air bearing surface. Each fine ridge feature is configured with a width approximately equivalent to a width of the heating spot.

## Background
CROSS-REFERENCE TO RELATED APPLICATION(S)
This application claims the benefit of U.S. Provisional Application Serial No. 62/010,057 filed on Jun. 10, 2014, which is expressly incorporated by reference herein in its entirety.


BACKGROUND
High density storage disks are configured with layers of materials that provide the required data stability for storage. The magnetic properties of the media may be softened when writing to the disk to assist changing the bit state. Energy Assisted Magnetic Recording (EAMR) device or Heat Assisted Magnetic Recording (HAMR) technology provides heat that is focused on a nano-sized bit region when writing onto a magnetic storage disk, which achieves the magnetic softening. A light waveguide directs light from a laser diode to a near field transducer (NFT). The NFT focuses the optical energy to a small spot on the target recording area which heats the magnetic storage disk during a write operation. Inefficiencies in the NFT can have a negative impact on the power budget of the laser diode and the EAMR/HAMR system lifetime. Higher NFT efficiency allows for lower laser power demand, relieving EAMR/HAMR system requirement on the total optical power from the laser source, and results in less power for parasitic heating of the EAMR/HAMR head resulting for improved reliability.
In the NFT, plasmonic metal can be used to interface with an energized dielectric waveguide for propagating surface plasmon polaritons (SPPs), which carry out the nano-focusing function beyond the light's diffraction limit. High quality plasmonic metals rely on high density free electrons which have weak mechanical robustness, and are susceptible to damages caused by thermal or mechanical stresses present in an EAMR head. Under these stresses, the service lifetime of the EAMR/HAMR device is limited to NFT failure occurring at the plasmonic metal part having fine (nano-sized) features, such as at a ridge or a pin.



BRIEF DESCRIPTION OF THE DRAWINGS
Various aspects of the present invention will now be presented in the detailed description by way of example, and not by way of limitation, with reference to the accompanying drawings, wherein:
FIG. 1 shows a diagram of an exemplary hard disk drive;
FIG. 2 shows a diagram of an exemplary embodiment of a dielectric waveguide core pair, each waveguide core having a fine ridge feature to form an interface with a nearby near field transducer (NFT) element for heat assisted magnetic recording on a storage disk;
FIG. 3 shows a diagram of an alternative exemplary embodiment with a fine ridge feature on the dielectric waveguide core being offset from the centerline of the waveguide core; and
FIG. 4 shows a diagram of an alternative exemplary embodiment for in which the waveguide core has two fine ridge features.



DETAILED DESCRIPTION
The detailed description set forth below in connection with the appended drawings is intended as a description of various exemplary embodiments and is not intended to represent the only embodiments that may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the embodiments. However, it will be apparent to those skilled in the art that the embodiments may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the embodiments. Acronyms and other descriptive terminology may be used merely for convenience and clarity and are not intended to limit the scope of the embodiments.
The various exemplary embodiments illustrated in the drawings may not be drawn to scale. Rather, the dimensions of the various features may be expanded or reduced for clarity. In addition, some of the drawings may be simplified for clarity. Thus, the drawings may not depict all of the components of a given apparatus.
Various embodiments will be described herein with reference to drawings that are schematic illustrations of idealized configurations. As such, variations from the shapes of the illustrations as a result of manufacturing techniques and/or tolerances, for example, are to be expected. Thus, the various embodiments presented throughout this disclosure should not be construed as limited to the particular shapes of elements illustrated and described herein but are to include deviations in shapes that result, for example, from manufacturing. By way of example, an element illustrated or described as having rounded or curved features at its edges may instead have straight edges. Thus, the elements illustrated in the drawings are schematic in nature and their shapes are not intended to illustrate the precise shape of an element and are not intended to limit the scope of the described embodiments.
The word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Likewise, the term “embodiment” of an apparatus or method does not require that all embodiments include the described components, structure, features, functionality, processes, advantages, benefits, or modes of operation.
As used herein, the term “about” followed by a numeric value means within engineering tolerance of the provided value.
In the following detailed description, various aspects of the present invention will be presented in the context of an interface between a waveguide and a near field transducer used for heat assisted magnetic recording on a magnetic storage disk.
FIG. 1 shows a hard disk drive 111 including a disk drive base 114, at least one rotatable storage disk 113 (e.g., such as a magnetic disk, magneto-optical disk), and a spindle motor 116 attached to the base 114 for rotating the disk 113. The spindle motor 116 typically includes a rotating hub on which one or more disks 113 may be mounted and clamped, a magnet attached to the hub, and a stator. At least one suspension arm 108 supports at least one head gimbal assembly (HGA) 112 that holds a slider with a magnetic head assembly of writer and reader heads. A ramp assembly 100 is affixed to the base 114, and provides a surface for tip of the suspension arm 108 to rest when the HGA 112 is parked (i.e., when the writer and reader heads are idle). During a recording operation of the disk drive 111, the suspension arm 108 rotates at the pivot 117, disengaging from the ramp assembly 100, and moves the position of the HGA 112 to a desired information track on the rotating storage disk 113. During recording, the slider is suspended by the HGA 112 with an air bearing surface of the slider that faces the rotating disk 113, allowing the writer head to magnetically alter the state of the storage bit. For heat assisted magnetic recording, a near field transducer (NFT) on the air bearing surface may couple light energy from a waveguide to produce a heating spot on the rotating disk 113 for magnetically softening the bit space.
FIG. 2 shows a diagram of an exemplary embodiment of an NFT 200 and waveguide core arrangement. The NFT 200 may be formed by a plasmonic metal element 205, which may be aligned with the air bearing surface (ABS) 210 of a slider. The surface of the recording medium 113 (e.g., a rotating storage disk) is exposed to the ABS 210. As the slider flies over the recording media 113, a cushion of air is maintained between the slider and the disk. In this embodiment, two dielectric waveguide (WG) cores 211, 212 are arranged to each carry light energy toward the ABS 210. The light energy may be generated by a common laser diode source (not shown) that may be split in half by a splitter (not shown). The dielectric waveguide cores 211, 212 may be of equal length to ensure that the combined energy wave at the ABS 210 is in substantial phase alignment for constructive interference and maximum energy emission to the surface of the recording medium 113. Alternatively, dielectric waveguide cores 211, 212 may be of unequal length such that the incident energy waves may have a particular phase difference that optimizes constructive interference and maximum energy magnitude at the ABS 210. The two waveguide cores 211, 212 are substantially linear and converge at a junction near the ABS 201 at an interior angle between 0 and 180 degrees, (e.g., approximately 90 degrees as shown in FIG. 2).
As shown in FIG. 2, each waveguide core 211, 212 has a fine ridge feature 201, 202. along the surface of the waveguide core 211, 212 facing the plasmonic metal element 205. For illustrative purpose, the plasmonic metal element 205 is depicted as transparent to reveal the fine ridge features 201, 202 below. The optical energy from the fine ridge feature 201, 202 of the dielectric waveguide cores 211, 212 in proximity with the plasmonic metal element 205 energizes propagating surface plasmon polaritons (PSPPs) along the plasmonic metal surface toward the ABS. The fine ridge feature 201, 202 of the waveguide core 211, 212 may be configured to have a width substantially less than the width of the waveguide core 211, 212. For example, the ridge feature 201, 202 may be about 10-70 nm wide compared to waveguide core 211, 212 having a width of about 300-500 nm. Also, the height of the fine ridge feature may be about 10-70 nm for example. Because the fine ridge feature 201, 202 is formed on the dielectric waveguide core 211, 212, which is made of a material more robust than plasmonic metal, the reliability and durability of the NFT 200 may be improved.
The dielectric material of the waveguide core 211, 212 and the fine ridge feature 201, 202 may be Ta2O5 for example. The material of the plasmonic metal element 205 may be a gold alloy, for example. Other examples of plasmonic metals that may be used to form the metal bar element include silver or copper alloys. A gap (e.g., 20 nm) may exist between the plasmonic metal element 205 and the fine ridge feature 201, 202 of the dielectric waveguide core 211, 212. Alternatively, the gap may be omitted, and the plasmonic metal element 205 may directly contact the fine ridge feature 201, 202, at least for a portion of the interface between the fine ridge feature 201, 202 and the plasmonic metal element 205. The two dielectric waveguide cores 211, 212 and the entire plasmonic metal element 205 may be encapsulated by a silicon oxide material.
The fine ridge features 201, 202 may be configured as shown in FIG. 2, converging at a junction above the junction of the dielectric waveguide cores 211, 212. The junction of fine ridge feature 201, 202 may occur on a common plane. The junction of fine ridge feature 201, 202 may be formed near at the ABS, with an extension 203 arranged perpendicular to the ABS 210 as shown in FIG. 2. Alternatively, the ABS 210 may be formed directly at the junction of the fine ridge features 201, 202, thus eliminating the extension 203. The precision of the delivered heating spot size at the recording disk 113 surface may be based on the exposed width of the fine ridge feature extension 203, or exposed junction 201, 202 at the ABS 210. There may be a direct correspondence between the width dimension of the exposed fine ridge feature 203 and the heating spot size. Alternatively, other factors may affect the heating spot size, such as the length of the waveguide cores 211, 212, which may affect the constructive interference of the incident light energy at the ABS 210. The target size of the heating spot on the recording disk 113 is dependent on the track size as the slider flies over the track, which may be between 10-70 nm wide for example. The size of the heating spot also depends on the distance between the ABS 210 and the disk 113. The focus of the heating spot may be optimized by minimizing the gap distance.
The thickness of the plasmonic metal element 205 is not a significant factor in achieving the precise nano-sized heating spot, and therefore the thickness may be configured according to providing adequate heat transfer for controlling the peak temperature in the NFT 200. Also, having the fine ridge feature 201, 202 of the waveguide core 211, 212 serving as the energy focusing feature for PSPP activity, may allow the plasmonic metal element 205 to have relaxed shape and size requirements (i.e., length and width dimensions) for which a fine feature is not necessary. The plasmonic metal element 205 configuration is determined by a shape and size necessary to interact with the fine ridge features 201, 202 (i.e., a footprint that covers the fine ridge features 201, 202). As an example, the size of a semi-circle shaped plasmonic metal element 205 may be 1000 nm in diameter and greater than 100 nm in thickness. While shown in FIG. 2 as a semi-circle shape, the plasmonic metal element 205 may be configured in shapes other than a semi-circle, such as a rectangular or polygonal block.
The NFT 200 configuration as shown in FIG. 2 having two fine ridge features 201, 202 serving as PSPP elements may provide approximately twice as much electrical field magnitude compared with a NFT configuration having only a single fine ridge feature with plasmonic metal element at the ABS 210, driven by a common total input power in the waveguide system. The constructive interference produced by the two PSPP elements allows improved efficiency of energy delivery from the laser diode source, which translates to longer service life of the EAMR/HAMR device. To optimize the efficiency of the two PSPP element configuration, each PSPP element 201, 202 is configured with a length L which is an integer multiple of coupling length Lc from dielectric waveguide core to the PSPP element 201, 202 (e.g., for Lc of 1200 nm, the length of the PSPP element should be about a(1200 nm) where a is an integer value). With the PSPP element having a length L approximately equivalent to aLc ensures that the maximum energy transfer propagates from the PSPP element 201, 202 at the ABS. If the length of the PSPP element deviates from aLc, some of the energy wave may be lost to the dielectric waveguide core 211, 212.
The NFT 200 does not have to be limited to two interfering PSPP elements as shown in FIG. 2. In an alternative embodiment, N (a positive integer) PSPP elements interfere at the ABS 210, which may provide approximately N times increase of the electrical field magnitude, driven by a common total input power in the waveguide system. The value of N can increase beyond 2 or 3, until other parasitic interferences within the three dimensional layout the EAMR head becomes a limiting factor. For N>3, the PSPP elements may be arranged in a three dimensional configuration. The plasmonic metal element 205 would also conform to a corresponding three dimensional configuration to provide the PSPP interface with the fine ridge features for the NFT 200.
FIG. 3 shows an alternative exemplary embodiment in which the fine ridge feature 202 of the dielectric waveguide core is offset from the centerline of the waveguide core. Hence, the fine ridge feature 202 may be fabricated with a deviation from the centerline of the waveguide core 212 and still interface with the plasmonic metal element 205 for a functional PSPP interface. In a similar manner, the fine ridge feature 201 may be formed offset from the centerline of the waveguide core 211. In this alternative embodiment, the fine ridge features 201, 202 converge at a junction point near the ABS 210 as in FIG. 2, either with an extension 203, or at the ABS 210 without the extension 203.
The number of PSPP elements may vary with respect to the number of fine ridge features on dielectric waveguide cores. FIG. 4 illustrates an example where the waveguide core 202 has two fine ridge features 202 disposed side-by-side below the plasmonic metal element 205.
The various aspects of this disclosure are provided to enable one of ordinary skill in the art to practice the present invention. Various modifications to exemplary embodiments presented throughout this disclosure will be readily apparent to those skilled in the art, and the concepts disclosed herein may be extended to other devices. Thus, the claims are not intended to be limited to the various aspects of this disclosure, but are to be accorded the full scope consistent with the language of the claims. All structural and functional equivalents to the various components of the exemplary embodiments described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112(f) unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”

## Claims

1. An apparatus for energy assisted magnetic recording of a storage disk, comprising:
a plurality of dielectric waveguide cores disposed near an air bearing surface of a magnetic recording device, each waveguide core comprising a fine ridge feature on a first surface of the waveguide core and configured to receive incident light energy from an energy source; and
a near field transducer formed at the air bearing surface for focusing light energy received from the waveguide core and transmitting the focused light energy onto the storage disk surface to generate a heating spot, comprising:
at least one plasmonic metal element disposed above the fine ridge features of the waveguide cores to form an interface for delivering propagating surface plasmon polaritons (PSPPs) to the air bearing surface;

wherein each fine ridge feature is configured with a width approximately equivalent to a width of the heating spot.

2. The apparatus of claim 1, wherein the interface comprises a gap between at least one plasmonic metal element and the fine ridge features of the waveguide cores.

3. The apparatus of claim 1, wherein
each of the waveguide cores and corresponding fine ridge features is substantially linear and comprises a first end and a second end, and
the first ends of all fine ridge features are connected together at a junction point near the air bearing surface with at least a portion of the connected fine ridge features is exposed on the air bearing surface.

4. The apparatus of claim 3, wherein the junction point of the first ends further comprises a fine ridge feature extension configured perpendicular to the air bearing surface and exposed on the air bearing surface.

5. The apparatus of claim 1, wherein the at least one plasmonic metal element is configured as a single element arranged to cover all of the fine ridge elements, the plasmonic metal element having a variable thickness and a substantially flat surface facing the fine ridge elements.

6. The apparatus of claim 4, wherein the at least one plasmonic metal element is configured as a semi-circular disk having a straight edge aligned with the air bearing surface.

7. The apparatus of claim 1, wherein the fine ridge feature is offset from the centerline of the waveguide core.

8. The apparatus of claim 1, wherein at least one of the waveguide cores comprises at least two fine ridge features on a common waveguide core surface.

9. The apparatus of claim 1, wherein the width of the heating spot is approximately equivalent to a width of physical space on the storage disk reserved for a single data bit.

10. The apparatus of claim 1, wherein the waveguide cores and the plasmonic metal element are encapsulated by a silicon oxide material.

11. The apparatus of claim 1, wherein at least three waveguide cores and corresponding fine ridge features are arranged in a three dimensional configuration to interface with the at least one plasmonic metal element at the air bearing surface.

12. A magnetic hard disk drive, comprising:
a rotatable magnetic storage disk;
a laser diode;
a plurality of dielectric waveguide cores disposed near an air bearing surface of a magnetic recording device, each waveguide core comprising a fine ridge feature on a first surface of the waveguide core and configured to receive incident light energy from an energy source; and
a near field transducer formed at the air bearing surface for focusing light energy received from the waveguide core and transmitting the focused light energy onto the storage disk surface to generate a heating spot, comprising:
at least one plasmonic metal element disposed above the fine ridge features of the waveguide cores to form an interface for delivering propagating surface plasmon polaritons (PSPPs) to the air bearing surface;


wherein each fine ridge feature is configured with a width approximately equivalent to a width of the heating spot.